Negative electrode piece and battery

Abstract

The present invention relates to the field of batteries, and specifically relates to a negative electrode piece and a battery comprising the negative electrode piece. The negative electrode piece comprises a negative electrode current collector and a negative electrode coating on at least one surface of the negative electrode current collector; a first portion in the negative electrode coating comprises a covering material comprising an Na element and a silicon-based material, where the mass ratio of Na to Si is b; the ratio of the depth of a recessed portion of the outer surface of the negative electrode coating to the thickness of the negative electrode coating is d, and the following relationship is satisfied: 6.37 × b0.5 - 0.43 ≤ d ≤ 6.37 × b0.5 - 0.13. The negative electrode piece has relatively low expansion during charge and discharge cycling of a battery.

Description

Negative plate and battery Technical Field

[0001] The present disclosure relates to the field of batteries, and in particular to a negative electrode sheet and a battery comprising the negative electrode sheet. Background Art

[0002] The specific capacity of silicon-based materials is much higher than that of currently commercialized graphite materials. Therefore, adding a certain amount of silicon-based materials to the negative electrode coating can significantly improve the energy density of lithium-ion batteries. However, silicon-based materials undergo significant volume expansion and contraction during the lithium insertion and removal process, especially under high-rate charging conditions, which accelerates capacity decay and seriously affects the battery's cycling performance.

[0003] Therefore, it is very necessary to invent a battery with excellent cycle performance.

[0004] Summary of the Invention

[0005] The present invention aims to overcome the aforementioned problems of the prior art and provides a negative electrode sheet and a battery comprising the negative electrode sheet. The negative electrode sheet of the present invention has a low expansion rate during the battery charge and discharge cycle, and the battery of the present invention has excellent cycle performance under high-rate charging conditions.

[0006] According to a first aspect of the present disclosure, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode coating on at least one side of the negative electrode current collector; the negative electrode coating comprises a first portion and a second portion, the first portion being adjacent to the negative electrode current collector; the first portion comprises a silicon-based material and a covering, the silicon-based material being dispersed in the covering, the covering comprising a Na element, and in the first portion, a ratio of the mass of the Na element to the mass of the Si element is b; the outer surface of the negative electrode coating comprises a concave portion; a ratio of the depth of the concave portion to the thickness of the negative electrode coating is d, satisfying 6.37×b 0.5 -0.43≤d≤6.37×b 0.5 -0.13.

[0007] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0008] Through the above technical solution, the present disclosure has at least the following advantages compared with the prior art:

[0009] (1) The negative electrode sheet disclosed herein concentrates the silicon-based material in the first portion of the negative electrode coating on the side close to the negative electrode current collector, which can reduce the lithium ion density on the surface of the silicon-based material, improve the uniformity of lithium insertion in the silicon-based material, and prevent the phenomenon of excessive lithium insertion in local areas and excessive expansion;

[0010] (2) The negative electrode sheet disclosed herein disperses the silicon-based material in a covering containing a certain amount of Na element, which can form a complete and uniform protective layer on the surface of the silicon-based material, further regulating the difficulty of lithium insertion on the surface of the silicon-based material, while providing a high-strength and high-elastic bonding network to inhibit the continuous increase of the gaps around the silicon-based material;

[0011] (3) The negative electrode sheet disclosed in the present invention is provided with a recess on the outer surface of the negative electrode coating to increase the storage capacity of the electrolyte in the negative electrode coating, while shortening the liquid phase lithium ion transmission distance of the negative electrode active material and avoiding the distance between the recess and the silicon-based material being too close, thereby improving the cycle stability and service life of the battery under fast charging conditions.

[0012] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG1 is a schematic cross-sectional view of a negative electrode sheet in an example of the present disclosure.

[0014] FIG2 is a schematic cross-sectional view of a negative electrode sheet in an example of the present disclosure.

[0015] FIG3 is a schematic diagram showing a concave hole on a negative electrode sheet in an example of the present disclosure.

[0016] FIG4 is a schematic diagram showing a groove on a negative electrode sheet in an example of the present disclosure.

[0017] FIG5 is a schematic diagram showing the width of a groove in an example of the present disclosure.

[0018] FIG6 is a schematic diagram showing the spacing of grooves in an example of the present disclosure.

[0019] FIG7 is a schematic diagram showing the distribution of recesses in an example of the present disclosure. DETAILED DESCRIPTION

[0020] The following describes the specific embodiments of the present disclosure in detail. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0021] In a first aspect, the present disclosure provides a negative electrode sheet, which may include a negative electrode current collector and a negative electrode coating on at least one side of the negative electrode current collector. FIG1 is a schematic cross-sectional view of a negative electrode sheet in an example of the present disclosure, wherein FIG1(a) shows a case where the negative electrode coating is set on one side; FIG1(b) and FIG1(c) show cases where the negative electrode coating is set on both sides. In FIG1(a), the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode coating 2 on one side of the negative electrode current collector 1; in FIG1(b) and FIG1(c), the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode coating 2 on both sides of the negative electrode current collector 1.

[0022] In the present disclosure, the negative electrode coating layer may include a first portion and a second portion, and the first portion may be adjacent to the negative electrode current collector.

[0023] In one example, the negative electrode coating includes a first portion and a second portion, wherein the first portion is adjacent to the negative electrode current collector, and the second portion is adjacent to a side of the first portion away from the negative electrode current collector.

[0024] In one example, the negative electrode coating layer consists of the first portion and the second portion, wherein the first portion is adjacent to the negative electrode current collector, and the second portion is a portion of the negative electrode coating layer excluding the first portion.

[0025] In the present disclosure, the first portion may include a silicon-based material and a covering, the silicon-based material being dispersed in the covering, and the outer surface of the negative electrode coating may have a concave portion. FIG2 is a cross-sectional schematic diagram of a negative electrode sheet in an example of the present disclosure. In FIG2 , the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode coating 2 on both sides of the negative electrode current collector 1. The negative electrode coating 2 includes a first portion 21 and a second portion 22. The first portion 21 is adjacent to the negative electrode current collector 1, and the second portion 22 is adjacent to a side of the first portion 21 away from the negative electrode current collector 1. The first portion 21 includes a silicon-based material 211 and a covering 212. The silicon-based material 211 is dispersed in the covering 212. The outer surface of the negative electrode coating 2 has a concave portion 3.

[0026] In the present disclosure, the outer surface of the negative electrode coating refers to the surface of the negative electrode coating away from the negative electrode current collector.

[0027] In the present disclosure, the cover includes Na element, and in the first portion, the ratio of the mass of Na element to the mass of Si element is b; the ratio of the depth of the concave portion to the thickness of the negative electrode coating is d, which satisfies 6.37×b 0.5 -0.43≤d≤6.37×b 0.5 -0.13.

[0028] In one example, 6.37×b 0.5 -0.34≤d≤6.37×b 0.5 -0.22.

[0029] During the battery charge and discharge cycle, silicon-based materials repeatedly undergo huge volume expansion and contraction. This behavior, on the one hand, can lead to the rupture of the SEI film on the surface of the silicon-based material, causing continuous interfacial side reactions, consuming active lithium ions and the electrolyte that acts as a liquid phase transport. At the same time, a layer of side reactants that hinder the transmission of electrons and ions is covered on the surface of the silicon-based material, causing the battery capacity to decay too quickly and the rate performance to drop significantly. On the other hand, it can cause the gaps around the silicon-based material to continue to increase, reducing the physical contact surface and contact points between active materials, and reducing the filling degree of the electrolyte, thereby increasing the difficulty of electron and ion transmission within the negative electrode coating, further damaging the battery's rate performance. Under high-rate charging conditions, the lithium insertion of the silicon-based material becomes more uneven, the degree of lithium insertion in some areas is too high, and the local expansion further increases, causing the battery performance to deteriorate rapidly.

[0030] The inventors of the present disclosure have systematically discovered that the cycling performance of silicon-doped lithium-ion batteries can be improved by increasing the stability of the SEI film on the surface of silicon-based materials, enhancing the elasticity and strength of the physical bonding network, and reducing the heterogeneity of lithium insertion in silicon-based materials. Based on this, the inventors of the present disclosure propose:

[0031] First, the silicon-based material is concentrated in the first part of the negative electrode coating near the negative electrode current collector (the inner side of the negative electrode coating). In this way, when the battery is charged, lithium ions will preferentially embed in the second part outside the negative electrode coating. Then, they will be transported to the surface of the silicon-based material in the first part through the tortuous pore channels storing electrolyte and / or the solid-phase channels with different orientations of negative electrode active materials (such as graphite particles) in the second part. As a result, the lithium ion density on the surface of the silicon-based material is much lower than that on the outer surface of the negative electrode coating. This can prevent the phenomenon of uneven lithium embedding in the silicon-based material under high-rate charging conditions of the battery.

[0032] Secondly, the silicon-based material is distributed in the covering, which includes a certain amount of Na element. When the mass ratio of the Na element to the Si element is within a specific range, the covering can evenly and fully wrap the silicon-based material to form a stable interface protection layer, reduce the speed of lithium ion insertion into the silicon-based material, improve the uniformity of lithium insertion, and effectively inhibit the rupture and repeated growth of the SEI film on the surface of the silicon-based material. At the same time, the covering has a strong bonding force with the silicon-based material, forming a high-strength and high-elastic bonding network, which prevents the continuous increase of the internal voids of the negative electrode coating after repeated charge and discharge;

[0033] Furthermore, a recess is provided on the outer surface of the negative electrode coating away from the negative electrode current collector, and the ratio of the depth of the recess to the thickness of the negative electrode coating satisfies a certain relationship. This can not only shorten the lithium ion transmission distance of the active material in the second part of the negative electrode coating when lithium is inserted, thereby enhancing the high-rate charging capability of the battery, but also increase the storage capacity of the electrolyte in the negative electrode coating, thereby improving the cycle life, and prevent the distance of lithium ions diffusing from the outer surface of the negative electrode coating to the silicon-based material from becoming too short, thereby ensuring the uniformity of lithium insertion in the silicon-based material.

[0034] In the present disclosure, 0.009≤b≤0.023, e.g., 0.009, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, or 0.023.

[0035] The inventors of the present disclosure discovered that when b<0.009, the Na element content is too low, which may cause the covering outside the silicon-based material to be unable to effectively reduce the insertion rate of lithium ions, thereby causing the lithium insertion uniformity of the silicon-based material to deteriorate; when b>0.023, the Na element content is too high, which may cause the dispersion of the covering outside the silicon-based material to decrease, and the integrity and uniformity to deteriorate, or it may increase the difficulty of lithium ion insertion into the silicon-based material, causing the charging performance of the battery to be significantly affected.

[0036] In one example, 0.012≤b≤0.019.

[0037] The inventors of the present disclosure further discovered that when b is in the range of 0.012-0.019, the integrity and uniformity of the covering outside the silicon-based material are better, which makes the lithium insertion uniformity of the silicon-based material higher. At the same time, the difficulty of lithium ions penetrating the covering is moderate, which can further improve the cycle stability of the battery.

[0038] In the present disclosure, in the first part, the mass content of the Na element and the mass content of the Si element can be tested by the following method: drying the negative electrode sheet until the negative electrode coating is loose, peeling off or scraping off the negative electrode coating on the surface (the second part), then soaking the negative electrode sheet in deionized water to completely remove the remaining negative electrode coating (the first part), evaporating the powder to obtain the mass content of the Na element by the inductively coupled plasma (ICP) method. The mass content of the Si element is measured by the X-ray energy dispersive spectroscopy (EDS) method.

[0039] In the present disclosure, 0.25≤d≤0.75, e.g., 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, or 0.75.

[0040] The inventors of the present disclosure found that when d<0.25, the depth of the recess is too small, the amount of electrolyte that can be stored is limited, and the cycle life of the battery cannot be effectively improved. At the same time, the effect on the lithium ion transmission distance of the second part of the negative electrode coating is small, and the rate performance of the battery cannot be effectively improved; when d>0.75, the depth of the recess is too large, and lithium ions can reach the vicinity of the silicon-based material through the electrolyte in the recess, reducing the difficulty of lithium insertion in the silicon-based material, which may lead to increased lithium insertion unevenness in the silicon-based material, thereby causing excessive local expansion.

[0041] In one example, 0.4≤d≤0.64.

[0042] In the present disclosure, the second portion may include a carbon-based material.The recess may be located in the second portion.

[0043] In the present disclosure, the ratio of the shortest distance from any point on the inner wall of the recess to the silicon-based material in the first portion to the thickness of the negative electrode coating is a, 0.1≤a≤0.45, for example 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 or 0.45.

[0044] The inventors of the present disclosure discovered that when a is in a specific range, the depth of the recess can be ensured to be moderate, which not only meets the requirements of increasing the electrolyte storage capacity and enhancing the fast charging capability of the second part of the negative electrode coating, but also prevents excessive interference with the uniformity of lithium insertion in the silicon-based material.

[0045] In one example, 0.15≤a≤0.32.

[0046] In the present disclosure, the shortest distance from any point on the inner wall of the recess to the silicon-based material in the first part can be tested by the following method: randomly cut the negative electrode sheet, use a scanning electron microscope (SEM) to measure, and obtain the shortest distance between the inner wall of the recess and the silicon-based material in the area. The values ​​corresponding to 10 recesses are measured cumulatively and the average value is taken.

[0047] In the present disclosure, the shortest distance from any point on the inner wall of the recess to the silicon-based material in the first part can be 6μm-30μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm or 30μm.

[0048] In the present disclosure, in the negative electrode coating, the ratio of the mass content of the Na element to the mass content of the Si element is c, 0.01≤c≤0.025, for example, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, 0.023, 0.024 or 0.025.

[0049] In one example, 0.014≤c≤0.021.

[0050] In the present disclosure, the mass content of the Na element and the mass content of the Si element in the negative electrode coating can be measured by the following method: the negative electrode sheet is immersed in deionized water to completely remove the negative electrode coating, and the powder is evaporated to dryness, and the mass content of the Na element is measured by the ICP method. The mass content of the Si element is measured by the EDS method.

[0051] In the present disclosure, the thickness of the negative electrode coating may be 40 μm to 80 μm, for example, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm or 80 μm.

[0052] In one example, the thickness of the negative electrode coating is 50 μm-70 μm.

[0053] In the present disclosure, the thickness of the negative electrode coating refers to the thickness of the negative electrode coating on one side of the negative electrode current collector.

[0054] In the present disclosure, the depth of the recess may be 10 μm-70 μm, for example, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm or 70 μm.

[0055] In one example, the depth of the recess is 20 μm-45 μm.

[0056] The inventors of the present disclosure have discovered that when the depth of the recess is within a specific range, it can better store the electrolyte, while improving the lithium insertion dynamics of the second part, and ensuring that the lithium insertion process of the first part is relatively gentle and uniform, which can further improve the cycle stability of the battery.

[0057] In this disclosure, the depth of the recess has the conventional meaning in the art, and is generally considered to refer to the vertical distance from the lowest point of the recess to the surface of the negative electrode coating. The depth of the recess can be measured by cutting the negative electrode sheet perpendicular to the surface of the negative electrode current collector to expose a cross-section, observing the cross-section using an SEM or optical microscope, and measuring the depth of the recess.

[0058] In the present disclosure, the recess may include a recessed hole and / or a groove.

[0059] In the present disclosure, the recessed portion may include a recessed hole. FIG3 is a schematic diagram of recessed holes on a negative electrode sheet in an example of the present disclosure, wherein FIG3(a) is a schematic top view of the negative electrode sheet, FIG3(b) is a schematic cross-sectional view along the dashed line in the length direction of FIG3(a), and FIG3(c) is a schematic cross-sectional view along the dashed line in the width direction of FIG3(a). In FIG3, the outer surface of the negative electrode coating has a plurality of recessed holes.

[0060] In the present disclosure, the recessed portion may include a groove. FIG4 is a schematic diagram of a groove on a negative electrode sheet in an example of the present disclosure, wherein FIG4(a) is a schematic top view of the negative electrode sheet, FIG4(b) is a schematic cross-sectional view along the dashed line in the length direction of FIG4(a), and FIG4(c) is a schematic cross-sectional view along the dashed line in the width direction of FIG4(a). In FIG4, the outer surface of the negative electrode coating has a groove.

[0061] In the present disclosure, the pore size of the concave pores may be 30 μm-100 μm, for example, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm or 100 μm.

[0062] In one example, the diameter of the concave hole is 40 μm-80 μm.

[0063] The inventors of the present disclosure have found that when the pore diameter of the concave pores is within a specific range, the concave pores can be prevented from being blocked by the generation of side reactants, and the increase of local lithium insertion heterogeneity can be prevented, thereby improving the cycle stability of the battery.

[0064] In the present disclosure, the pitch of the recessed holes may be 100 μm-400 μm, for example, 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, 250 μm, 260 μm, 270 μm, 280 μm, 290 μm, 300 μm, 310 μm, 320 μm, 330 μm, 340 μm, 350 μm, 360 μm, 370 μm, 380 μm, 390 μm or 400 μm.

[0065] In one example, the pitch of the concave holes is 200 μm-300 μm.

[0066] The inventors of the present disclosure have found that when the spacing of the concave holes is within a specific range, the excessive damage to the surface structure of the negative electrode coating caused by too small a spacing can be reduced, while avoiding the decrease in lithium insertion uniformity caused by too large a spacing, thereby improving the cycle stability of the battery.

[0067] In the present disclosure, the shape of the projection of the concave hole on the surface of the negative electrode coating can be regular or irregular, and both can achieve good results. The "aperture" and "spacing" have conventional meanings in the art: the "aperture" can generally be understood as, when the shape of the projection of the concave hole on the surface of the negative electrode coating is circular, the aperture is the diameter of the circle; when the shape of the projection of the concave hole on the surface of the negative electrode coating is non-circular, the aperture is the equivalent diameter of the diameter of a circle with the same area. The "spacing" refers to the shortest distance between the edges of two adjacent concave holes.

[0068] In the present disclosure, the pore size can be measured by SEM, and the spacing can be measured by microscope or SEM.

[0069] In the present disclosure, there is no special limitation on the method of making the concave holes. The concave holes can be made by laser drilling or physical drilling, both of which can achieve good results.

[0070] In the present disclosure, the width of the groove may be 20 μm-90 μm, for example, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm or 90 μm.

[0071] In one example, the width of the groove is 30 μm-60 μm.

[0072] The inventors of the present disclosure have found that when the width of the groove is within a specific range, the groove can be prevented from being blocked by the generation of side reactants, and the increase of local lithium insertion heterogeneity can be prevented, thereby improving the cycle stability of the battery.

[0073] In the present disclosure, the pitch of the grooves may be 800 μm-2200 μm, for example 800 μm, 900 μm, 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm, 2000 μm, 2100 μm or 2200 μm.

[0074] In one example, the pitch of the grooves is 1200 μm-1800 μm.

[0075] The inventors of the present disclosure have found that when the spacing of the grooves is within a specific range, it can reduce excessive damage to the surface structure of the negative electrode coating due to too small a spacing, while avoiding the decrease in lithium insertion uniformity caused by too large a spacing, thereby improving the cycle stability of the battery.

[0076] In the present disclosure, when the groove is distributed along the length direction of the negative electrode sheet, the length of the groove may be less than the length of the negative electrode coating, or may be equal to the length of the negative electrode coating; when the groove is distributed along the width direction of the negative electrode sheet, the length of the groove may be less than the width of the negative electrode coating, or may be equal to the width of the negative electrode coating.

[0077] In the present disclosure, the projection of the groove on the negative electrode coating includes 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 width direction of the negative electrode sheet. Figure 5 shows a schematic diagram of the width of the groove in an example of the present disclosure, where the two long sides of the groove in Figures 5(a) to 5(c) are straight lines, and the two long sides of the groove in Figure 5(d) are curved lines. In FIG5(a) and FIG5(b), the two long sides are arranged in parallel. Therefore, in the width direction of the negative electrode sheet, the vertical distance from any point on one long side to the other long side is equal. In this case, the width of the groove is the vertical distance L from any point on one long side to the other long side in the length direction or width direction of the negative electrode sheet. In FIG5(c), the two long sides of the groove are straight lines, but are not arranged in parallel. Therefore, the distance from any point on one long side to the other long side in the width direction is not equal. In this case, the width of the groove can be averaged, that is, 50 points are selected at equal distances on one long side based on the length of the side. That is, the distance between each point is equal. Selecting points in this way can make the calculation result more accurate). The width L corresponding to each point is measured, and the average value is taken to obtain the width of the groove. In Figure 5(d), the two long sides are curved. Therefore, the distance from any point on one long side to the other long side in the width direction is not equal. At this time, the width of the groove can also be averaged. That is, 50 points are randomly selected on one long side (because the two long sides in Figure 5(d) are curved, there is no relationship between the two long sides in Figure 5(c) . Therefore, 50 points can be randomly selected for measurement). The width L corresponding to each point is measured, and the average value is taken to obtain the width of the groove.

[0078] In the present disclosure, the spacing of the grooves refers to the average distance between the two adjacent long sides of two adjacent grooves on the negative electrode coating in the length direction or width direction of the negative electrode sheet. As shown in Figure 6, a schematic diagram of the spacing of the grooves in an example of the present disclosure is shown, wherein Figure 6 (a) shows the case where the two adjacent long sides are straight and parallel, Figure 6 (b) shows the case where the two adjacent long sides are straight and non-parallel, and Figure 6 (c) shows the case where the two adjacent long sides are curved. In Figure 6 (a), the two adjacent long sides are straight and parallel. Therefore, in the width direction, the vertical distance from any point on one long side to the other long side is equal. At this time, the spacing of the grooves is the distance M from any point on one long side to the other long side in the width direction; in Figure 6 (b), the two adjacent long sides are straight, but not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. At this time, the spacing of the grooves can be averaged, that is, on a long side, with the length of the side as the reference, 50 points are selected at equal distances (that is, the distance between each point is equal, Selecting points in this way can make the calculation results more accurate), measure the width M corresponding to each point, and take the average value to get the groove spacing; in Figure 6(c), the two adjacent long sides are curved. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the groove spacing can also be averaged, that is, 50 points are randomly selected on one long side (since the two long sides in Figure 6(c) are curved, there is no relationship between the two long sides in Figure 6(b), so 50 points can be randomly selected for measurement), measure the width M corresponding to each point, and take the average value to get the groove spacing.

[0079] In the present disclosure, the width and spacing of the grooves can be measured by SEM testing.

[0080] In the present disclosure, the aperture of the recessed hole can be consistent in the thickness direction of the negative electrode sheet, or can gradually decrease with increasing depth; the width and length of the groove can be consistent in the thickness direction of the negative electrode sheet, or can gradually decrease with increasing depth.

[0081] In the present disclosure, a surface of the first portion on a side away from the negative electrode current collector may have a protrusion.

[0082] In one embodiment, the surface of the first portion away from the negative electrode current collector has a protrusion (as shown in FIG2 ). The protrusion is formed by the shape and stacking of the silicon-based particles in the first portion. This protrusion can increase the contact area between the silicon-based particles and the second portion, improve the uniformity of lithium ion embedding into the silicon-based particles, and effectively suppress the problem of excessive expansion caused by excessive local lithium embedding in the silicon-based particles, thereby improving the cycle stability of the negative electrode sheet.

[0083] In the present disclosure, the median particle size Dv50 of the silicon-based material can be 3μm-15μm, for example, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm or 15μm.

[0084] The inventors of the present disclosure have discovered that by limiting the median particle size Dv50 of the silicon-based material to a specific range, the specific surface area of ​​the silicon-based material can be ensured to be within an appropriate range, thereby reducing the side reactions between the silicon-based material and the electrolyte and improving the cycle stability of the battery.

[0085] In one example, the median particle size Dv50 of the silicon-based material is 5 μm-12 μm.

[0086] In the present disclosure, the median particle size Dv50 of the silicon-based material can be measured by conventional methods in the art, such as laser particle size measurement method and Malvern particle size tester.

[0087] In the present disclosure, the average particle size of the silicon-based material can be 3μm-15μm, for example, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm or 15μm.

[0088] In one example, the average particle size of the silicon-based material is 5 μm-12 μm.

[0089] In the present disclosure, the average particle size of the silicon-based material can be measured using conventional methods in the art. For example, using a SEM, a cross-sectional image of the negative electrode sheet along the thickness direction is selected. At least 20 silicon-based material particles are selected from the image, and the particle size of each silicon-based material particle is measured and the average value is calculated. For accuracy, the average value of 5 operations is used as the test result.

[0090] In the present disclosure, the second part may include or exclude the Na element, and the mass content of the Na element in the first part is higher than the mass content of the Na element in the second part.

[0091] The inventors of the present disclosure have discovered that when the mass content of the Na element in the first part is higher than the mass content of the Na element in the second part, the effect of the volume expansion and contraction of the silicon-based material during the lithium insertion and delithiation process on the negative electrode sheet can be reduced, the structural stability of the negative electrode sheet can be improved, and the cycle stability of the battery can be improved and the thickness expansion rate of the battery can be reduced. The reason may be that: when the mass content of the Na element in the first part is lower than or equal to the mass content of the Na element in the second part, the cover cannot form a uniform and complete protective layer on the surface of the silicon-based material due to the low mass content of the Na element in the first part; or the mass content of the Na element in the second part is too high, which will hinder the transmission and diffusion of lithium ions in the second part, adversely affecting the lithium insertion kinetics of the negative electrode coating, resulting in poor cycle performance of the battery.

[0092] In the present disclosure, based on the total mass of the first part, the content of the silicon-based material may be 30wt%-70wt% (for example, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt% or 70wt%), and the content of the covering may be 30wt%-70wt% (for example, 70wt%, 65wt%, 60wt%, 55wt%, 50wt%, 45wt%, 40wt%, 35wt% or 30wt%).

[0093] The inventors of the present disclosure have discovered that the silicon-based material and the covering have specific contents, so that the covering can better regulate the difficulty of lithium insertion of the silicon-based material and inhibit the expansion and contraction of the silicon-based material.

[0094] In one example, based on the total mass of the first portion, the content of the silicon-based material is 45 wt%-60 wt%, and the content of the covering is 40 wt%-55 wt%.

[0095] In the present disclosure, the cover may include a first binder and a first conductive agent. Based on the total mass of the cover, the content of the first binder may be 30 wt% to 99.9 wt% (e.g., 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, 99 wt%, 99.5 wt%, or 99.9 wt%), and the content of the first conductive agent may be 0.1 wt% to 70 wt% (e.g., 70 wt%, 65 wt%, 60 wt%, 55 wt%, 50 wt%, 45 wt%, 40 wt%, 35 wt%, 30 wt%, 25 wt%, 20 wt%, 15 wt%, 10 wt%, 5 wt%, 1 wt%, 0.5 wt%, or 0.1 wt%).

[0096] The inventors of the present disclosure have discovered that when the first binder and the first conductive agent in the cover have specific contents, the expansion and contraction of the silicon-based material can be further suppressed without affecting the conductive properties of the negative electrode sheet.

[0097] In one example, based on the total mass of the cover, the content of the first binder is 55 wt %-80 wt %, and the content of the first conductive agent is 20 wt %-45 wt %.

[0098] In the present disclosure, the first binder may include a sodium-containing binder and optionally a non-sodium-containing binder.

[0099] In the present disclosure, the “optional sodium-free binder” means that the first binder may or may not include a sodium-free binder.

[0100] In the present disclosure, the sodium-containing binder may include at least one of sodium carboxymethyl cellulose, sodium polyacrylate and sodium alginate. The sodium-free binder may include at least one of polyacrylic acid, lithium polyacrylate, carboxymethyl cellulose, lithium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polytetrafluoroethylene, polypropylene, styrene-butadiene rubber (SBR) and epoxy resin.

[0101] In the present disclosure, the first conductive agent may include at least one of carbon black, carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, and graphene.

[0102] In the present disclosure, based on the total mass of the negative electrode coating, the content of the silicon-based material may be 2wt%-15wt%, for example, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, 10.5wt%, 11wt%, 11.5wt%, 12wt%, 12.5wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt% or 15wt%.

[0103] In one example, based on the total mass of the negative electrode coating, the content of the silicon-based material is 4 wt % to 10 wt %.

[0104] In the present disclosure, based on the total mass of the negative electrode coating, the content of the carbon-based material may be 65 wt%-95 wt%, for example, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt% or 95 wt%.

[0105] In one example, based on the total mass of the negative electrode coating, the content of the carbon-based material is 75 wt % to 90 wt %.

[0106] In the present disclosure, based on the total mass of the negative electrode coating, the content of Si element can be 1wt%-9wt%, for example, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt% or 9wt%.

[0107] In one example, based on the total mass of the negative electrode coating, the content of Si element is 2 wt % to 6 wt %.

[0108] In the present disclosure, based on the total mass of the negative electrode coating, the content of Na element may be 150 ppm-1000 ppm, for example, 150 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm or 1000 ppm.

[0109] The inventors of the present disclosure have found that when the mass content of the Na element in the negative electrode coating is within a specific range, the integrity and uniformity of the covering in the first part can be made higher, while the transport kinetics of lithium ions inside the negative electrode coating can be moderate, thereby making the battery have higher cycle stability.

[0110] In the present disclosure, the silicon-based material may include at least one of silicon-carbon, silicon-oxygen, nano-silicon, and a silicon alloy. The terms "silicon-carbon" and "silicon-oxygen" have conventional meanings in the art, with silicon-oxygen generally referring to oxides containing silicon, and silicon-carbon referring to composite materials containing silicon and carbon.

[0111] In one example, the silicon-based material includes at least one of silicon oxygen and silicon carbon.

[0112] In one example, the silicon carbon includes a material in which silicon or partially oxidized silicon is filled (including partially filled or completely filled) in pores of porous amorphous carbon or porous crystalline carbon.

[0113] In the present disclosure, based on the total mass of the silicon-based material, the content of Si element can be 30wt%-70wt%, for example, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt% or 70wt%.

[0114] The inventors of the present disclosure have discovered that when the content of Si element in the silicon-based material is limited to a specific range, the stability of the silicon-based material can be enhanced, thereby further improving the structural stability of the negative electrode sheet.

[0115] In one embodiment, based on the total mass of the silicon-based material, the content of Si element is 40 wt%-65 wt%.

[0116] In the present disclosure, the carbon-based material may include at least one of artificial graphite, natural graphite, mesocarbon microsphere graphite, soft carbon, and hard carbon.

[0117] In one example, the carbon-based material includes artificial graphite.

[0118] In the present disclosure, the second part may further include a second binder and a second conductive agent. The second binder may include at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polytetrafluoroethylene, polypropylene, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, sodium alginate, SBR, and epoxy resin. The second conductive agent may include at least one of carbon black, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0119] In the present disclosure, the surface density of the first portion may be 1 mg / cm 2 -1.8mg / cm 2 , for example 1 mg / cm 2 , 1.1mg / cm 2 , 1.2mg / cm 2 , 1.3mg / cm 2 , 1.4mg / cm 2 , 1.5mg / cm 2 , 1.6mg / cm 2 , 1.7mg / cm 2 or 1.8 mg / cm 2 .

[0120] In the present disclosure, the compacted density of the first part may be 0.5 g / cm 3 -1.5g / cm 3 , for example 0.5g / cm 3 , 0.6g / cm3 , 0.7g / cm 3 , 0.8g / cm 3 , 0.9g / cm 3 , 1g / cm 3 , 1.1g / cm 3 , 1.2g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 or 1.5g / cm 3 .

[0121] In one embodiment, the first portion has a compacted density of 0.8 g / cm 3 -1.3g / cm 3 .

[0122] The inventors of the present disclosure have discovered that when the surface density and compaction density of the first part are within a specific range, the distribution uniformity of the silicon-based material can be prevented from being reduced due to too little silicon-based material or too low a compaction density. The phenomenon of excessive expansion due to too much silicon-based material or reduced contact area between the silicon-based material and the second part due to too high a compaction density can also be avoided, thereby improving the cycle stability of the battery.

[0123] In the present disclosure, the surface density of the second part can be 6 mg / cm 2 -12mg / cm 2 , for example 6 mg / cm 2 、6.5mg / cm 2 , 7mg / cm 2 , 7.5mg / cm 2 , 8mg / cm 2 、8.5mg / cm 2 , 9mg / cm 2 , 9.5mg / cm 2 、10mg / cm 2 、10.5mg / cm 2 、11mg / cm 2 、11.5mg / cm 2 or 12 mg / cm 2 .

[0124] In one embodiment, the surface density of the second portion is 7 mg / cm 2 -11mg / cm 2 .

[0125] The inventors of the present disclosure have discovered that when the surface density of the second part is within a specific range, the excessive decrease in lithium ion transfer kinetics in the second part due to too high a surface density can be avoided, and the inability to effectively control the lithium insertion process of the silicon-based material in the first part due to too low a surface density can also be prevented, thereby improving the cycle stability of the battery.

[0126] In the present disclosure, the compaction density of the negative electrode coating can be 1.2 g / cm 3 -1.85g / cm 3 , for example 1.2 g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 or 1.85g / cm 3 .

[0127] In the present disclosure, the compacted density and the areal density can be obtained by testing using conventional methods in the art.

[0128] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0129] In the present disclosure, the components of the battery other than the negative electrode sheet (such as the positive electrode sheet, separator, and electrolyte, etc.) can be conventionally selected in the art.

[0130] In one example, the battery further includes a positive electrode, a separator, and an electrolyte.

[0131] In the present disclosure, the positive electrode sheet may include a positive electrode current collector and a positive electrode coating on at least one side of the positive electrode current collector. The positive electrode coating may include a positive electrode active material. The positive electrode active material may be a conventional choice in the art, for example, including at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, and a lithium-rich manganese-based material.

[0132] In the present disclosure, the positive electrode coating may further include a positive electrode binder and a positive electrode conductive agent. The positive electrode binder may include a binder conventionally used in the art, for example, including at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose and SBR. The positive electrode conductive agent may include a conductive agent conventionally used in the art, for example, including Super P.

[0133] In the present disclosure, based on the total mass of the positive electrode coating, the content of the positive electrode active material can be 80wt%-99wt% (for example, 80wt%, 85wt%, 90wt%, 95wt% or 99wt%), the content of the positive electrode binder can be 0.5wt%-10wt% (for example, 10wt%, 7.5wt%, 5wt%, 2.5wt% or 0.5wt%), and the content of the positive electrode conductor can be 0.5wt%-10wt% (for example, 10wt%, 7.5wt%, 5wt%, 2.5wt% or 0.5wt%).

[0134] In the present disclosure, the separator may include a separator conventionally used in the art, such as a polyethylene film.

[0135] In the present disclosure, the electrolyte can be a conventional choice in the art, for example, the electrolyte includes an organic solvent and an electrolyte salt. The organic solvent is, for example, selected from ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), butylene carbonate (BC), 1,3-propane sultone (PS), dimethyl fluorocarbonate, fluoroethylene carbonate (FEC), fluoroethyl methyl carbonate, ethyl propyl carbonate (EPC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC) and methyl propyl carbonate (MPC). The electrolyte salt is, for example, selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPF2O2) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0136] In the present disclosure, the battery can be assembled in accordance with conventional methods in the art.

[0137] It should be noted that the numerical expressions such as "first" and "second" in the present disclosure are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0138] The present disclosure will be described in detail below through examples. The examples described in this disclosure are only a portion of the examples of the present disclosure, not all of the examples. Based on the examples in this disclosure, all other examples obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this disclosure.

[0139] In the following examples, unless otherwise specified, all materials used were commercially available analytical grade.

[0140] The following group I of examples is used to prepare the negative electrode sheet of the present disclosure.

[0141] Example I1

[0142] Prepare the negative electrode sheet as follows:

[0143] (1) A silicon-based material (silicon carbon, a material formed by completely filling the pores of porous amorphous carbon with silicon, wherein the average pore size of the pores of the porous amorphous carbon is 2 nm, the median particle size Dv50 of the porous amorphous carbon is 7 μm, the median particle size Dv50 of the silicon carbon is 7 μm, the average particle size of the silicon carbon is 7 μm, and the weight content of the Si element in the silicon carbon is 48 wt %), carbon black, sodium carboxymethyl cellulose (the mass content of the Na element in the sodium carboxymethyl cellulose is 6.4 wt %), lithium carboxymethyl cellulose, styrene-butadiene rubber, and deionized water are uniformly stirred in a mass ratio of 52:16:6:10:16:400 to obtain a first negative electrode slurry;

[0144] (2) artificial graphite, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, styrene-butadiene rubber, and deionized water were uniformly stirred in a mass ratio of 97:0.8:0.1:1:1.1:150 to obtain a second negative electrode slurry;

[0145] (3) Prepare the first negative electrode slurry with a surface density of 1.4 mg / cm 2 Evenly coat on both sides of the copper foil with a thickness of 6μm, dry and roll-press to a compaction density of 0.99g / cm 3 The second negative electrode slurry is prepared with a surface density of 9.2 mg / cm 2 Uniformly coating the dried copper foil coated with the first negative electrode slurry on both sides, and drying;

[0146] (4) Roll the electrode sheet obtained in step (3) until the compaction density of the negative electrode coating is 1.73 g / cm 3 ; Holes are punched on the surface in a square distribution form (as shown in Figure 7, a schematic diagram of the distribution of the concave parts in an example of the present disclosure), the pore diameter of the concave holes is 60μm, the spacing is 250μm, and the depth of the concave holes is 31μm; cutting and welding the pole ears to obtain negative electrode sheets, wherein the mass content of silicon-based materials in the negative electrode coating is 7wt%, and the mass content of carbon-based materials is 84wt%.

[0147] Example 12

[0148] The method was carried out in accordance with Example I1, except that the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and styrene-butadiene rubber were mixed in a mass ratio of 52:16:7.1:8.9:16; wherein the surface density of the first negative electrode slurry was 1.59 mg / cm 2 , compacted density is 1.15g / cm 3 The surface density of the second negative electrode slurry is 10.43 mg / cm 2 ; The pore diameter of the concave holes is 78μm, the spacing is 205μm, and the depth is 44μm.

[0149] Example 13

[0150] The method was carried out in accordance with Example I1, except that the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and styrene-butadiene rubber were mixed in a mass ratio of 52:16:5.2:10.8:16; wherein the surface density of the first negative electrode slurry was 1.15 mg / cm 2 , compacted density is 0.81g / cm 3 The surface density of the second negative electrode slurry is 7.58 mg / cm 2 ; The pore diameter of the concave holes is 41μm, the spacing is 295μm, and the depth is 21μm.

[0151] Example 14 Group

[0152] This group of examples is carried out with reference to Example I1, except that the ratio b of the mass content of the Na element to the mass content of the Si element in the first part is changed, specifically:

[0153] In Example I4a, the mass ratio of the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and styrene-butadiene rubber in the first part is 52:16:8.3:7.7:16;

[0154] In Example I4b, the mass ratio of the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and styrene-butadiene rubber in the first part is 52:16:4:12:16.

[0155] Example 15 Group

[0156] This group of examples is carried out with reference to Example I1, except that the depth of the punching is changed. Specifically:

[0157] In Example I5a, the depth of the recessed hole is 40 μm;

[0158] In Example I5b, the depth of the recessed holes is 23 μm.

[0159] Example 16 group

[0160] This group of examples was carried out with reference to Example I1, except that the ratio a of the shortest distance from any point on the inner wall of the recess to the silicon-based material in the first portion to the thickness of the negative electrode coating was changed, specifically:

[0161] In Example I6a, the first negative electrode slurry was applied, dried, and rolled to a compaction density of 0.64 g / cm 3 ;

[0162] In Example 16b, the first negative electrode slurry was applied and then dried and rolled to a compaction density of 1.38 g / cm 3 .

[0163] Example 17 group

[0164] This group of examples was carried out with reference to Example I1, except that the thickness of the negative electrode coating and the depth of the concave pores were changed (the ratio of the depth of the concave pores to the thickness of the negative electrode coating was controlled to be approximately 0.5). Specifically:

[0165] In Example 17a, the surface density of the first negative electrode slurry is set to 1.71 mg / cm 2 , so that the surface density of the second negative electrode slurry is 11.22 mg / cm 2 , so that the depth of the concave hole is 38μm;

[0166] In Example 17b, the surface density of the first negative electrode slurry is set to 1.00 mg / cm 2 , so that the surface density of the second negative electrode slurry is 6.46 mg / cm 2 , so that the depth of the concave hole is 22μm.

[0167] Example 18 group

[0168] This group of examples is carried out with reference to Example I1, except that the diameter or spacing of the concave holes is changed. Specifically:

[0169] In Example I8a, the pore diameter is increased to 90 μm;

[0170] In Example I8b, the pore size was reduced to 35 μm;

[0171] In Example I8c, the spacing is increased to 350 μm;

[0172] In Example 18d, the spacing is reduced to 150 μm.

[0173] Example 19

[0174] The method was carried out in accordance with Example I1, except that grooves were formed on the surface of the negative electrode coating at equal intervals. The projection of the grooves on the negative electrode coating was rectangular, the width of the grooves was 45 μm, the spacing between the grooves was 1500 μm, the depth of the grooves was 31 μm, and the length of the grooves was 78.2 mm, wherein the length of the grooves was equal to the width of the negative electrode coating.

[0175] Example I10

[0176] The same procedure was carried out as in Example I1, except that the silicon-based material was replaced with a silicon-oxygen material of equal mass, wherein the mass content of Si element in the silicon-oxygen material was 55 wt %.

[0177] The specific parameters of Examples I1-I10 are shown in Tables 1-1 and 1-2, wherein the surface of the first part away from the negative electrode current collector in all examples has a protrusion, and the mass content of the Na element in the first part is higher than the mass content of the Na element in the second part.

[0178] Table 1-1

[0179] Table 1-2

[0180] Comparative Example D1

[0181] The same process was carried out as in Example I1, except that no pore formation was performed.

[0182] Comparative Example D2

[0183] The method was carried out in accordance with Example I1, except that the ratio b of the mass content of the Na element to the mass content of the Si element in the first part was changed to 0.0244, and the mass ratio of the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and styrene-butadiene rubber in the first part was 52:16:9.5:6.5:16, which did not meet the requirement of 6.37×b. 0.5 -0.43≤d≤6.37×b 0.5 -0.13.

[0184] Comparative Example D3

[0185] The method was carried out in accordance with Example 19, except that the ratio b of the mass content of the Na element to the mass content of the Si element in the first part was changed to 0.0244, and the mass ratio of the silicon-based material, carbon black, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and styrene-butadiene rubber in the first part was 52:16:9.5:6.5:16, which did not meet the requirement of 6.37×b. 0.5 -0.43≤d≤6.37×b 0.5 -0.13.

[0186] Comparative Example D4

[0187] The method is carried out in accordance with Example I1, except that the depth of the concave hole is changed to 48 μm, and the 6.37×b 0.5 -0.43≤d≤6.37×b 0.5 -0.13.

[0188] Test Case I

[0189] (1) Concave hole depth test

[0190] The punched negative electrode sheets prepared in the examples and comparative examples were subjected to a concave hole depth test. The specific test method is as follows:

[0191] Through the ion beam milling method, the negative electrode sheet was cut perpendicularly to the surface to expose the cross section of the negative electrode sheet. Using SEM, 50 recessed holes were randomly selected on the cross section of the entire negative electrode sheet. The depth of each recessed hole was measured and the average value was taken. The results are recorded in Table 1-2.

[0192] (2) Aperture test of concave holes

[0193] The punched negative electrode sheets prepared in the examples and comparative examples were subjected to pore size testing. The specific testing method is as follows:

[0194] Using SEM, 50 concave holes were randomly selected on the entire negative electrode sheet, and the diameter of each concave hole was measured. The average value was taken and the results were recorded in Table 1-2.

[0195] (3) Concave hole spacing test

[0196] The perforated negative electrode sheets prepared in the examples and comparative examples were tested for the concave hole spacing. The specific testing method is as follows:

[0197] Using SEM, 50 concave holes were randomly selected on the entire negative electrode sheet, and the distance between two adjacent concave holes was measured. The minimum value was taken and the results were recorded in Table 1-2.

[0198] (4) Groove depth test

[0199] The grooved negative electrode sheets prepared in the examples and comparative examples were subjected to a groove depth test. The specific test method is as follows:

[0200] Through the ion beam milling method, the negative electrode sheet was cut perpendicularly to the surface and the length direction of the groove to expose the cross section of the negative electrode sheet. Using SEM, 20 grooves were randomly selected on the cross section of the entire negative electrode sheet. The depth of each groove was measured and the average value was taken. The results are recorded in Table 1-2.

[0201] (5) Groove width test

[0202] The groove width test was performed on the negative electrode sheets after groove formation prepared in the embodiment and the comparative example. The specific test method is as follows:

[0203] Using SEM, 20 grooves were randomly selected on the entire negative electrode sheet, and the groove width of each groove was measured. The average value was taken and the results were recorded in Table 1-2.

[0204] (6) Test of groove spacing

[0205] The groove spacing of the negative electrode sheets after groove formation prepared in the embodiment and the comparative example was tested. The specific testing method is as follows:

[0206] Using SEM, 20 grooves were randomly selected on the entire negative electrode sheet, and the distance between two adjacent grooves was measured. The minimum value was taken and the results were recorded in Table 1-2.

[0207] The following II group of examples are used to prepare the battery of the present disclosure

[0208] Example III

[0209] Prepare the battery as follows:

[0210] (1) Preparation of positive electrode sheet

[0211] Lithium cobalt oxide, carbon black and polyvinylidene fluoride were mixed in a mass ratio of 96:2:2, N-methylpyrrolidone (solid content of 60wt%) was added and stirred to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on both sides of an aluminum foil with a thickness of 9μm, dried, cut and welded to obtain a positive electrode sheet with a compaction density of 4.1g / cm 3 , the surface density is shown in Table 2.

[0212] (2) Preparation of electrolyte

[0213] EC, PC, PP, LiPF6, FEC and PS were mixed evenly in a mass ratio of 12:12:47:15:10:4.

[0214] (3) Preparation of batteries

[0215] The negative electrode sheet prepared in Example I1, the positive electrode sheet prepared in step (1) and the separator (polyethylene film, thickness of 8 μm) are stacked in order, so that the separator is located between the positive electrode sheet and the negative electrode sheet and plays an isolating role. The battery cell is wound to obtain a battery cell, which is placed in an aluminum-plastic film shell. After drying the moisture, the electrolyte prepared in step (2) is injected. After the steps of packaging, aging, formation, secondary sealing, and sorting, a battery is obtained.

[0216] Comparative Example DD Group

[0217] The same procedure was followed as in Example II, except that the negative electrode sheet was replaced with the negative electrode sheet prepared in the comparative example of Group D.

[0218] Test Case

[0219] (1) Capacity retention test

[0220] The batteries prepared from the examples of group II and the comparative examples of group DD were tested for capacity retention. The specific test method is as follows:

[0221] The test temperature is 25℃, discharge at 0.5C to 3V, and let it stand for 10 minutes; charge at 1C constant current to 3.83V, charge at constant voltage to 0.05C, and let it stand for 10 minutes;

[0222] Charge at a constant current of 3C to 4.45V, charge at a constant voltage of 0.05C, and let it stand for 10 minutes; discharge at 0.5C to 3V, and let it stand for 10 minutes; perform a 600-cycle test using this charge and discharge procedure, and divide the capacity of the 600th discharge by the capacity of the first discharge as the 600th capacity retention rate. The results are recorded in Table 2.

[0223] (2) Thickness expansion rate test

[0224] The batteries prepared in Group II of Examples and Comparative Examples were subjected to thickness expansion rate tests. The specific test method is as follows:

[0225] The test temperature is 25℃, discharge at 0.5C to 3V, and let it stand for 10 minutes; charge at 1C constant current to 3.83V, charge at constant voltage to 0.05C, let it stand for 10 minutes, and measure the battery thickness, which is the initial thickness;

[0226] Charge at a constant current of 3C to 4.45V, charge at a constant voltage of 0.05C, and let it stand for 10 minutes; discharge at 0.5C to 3V, and let it stand for 10 minutes; perform a 600-cycle test using this charge and discharge procedure, and perform a full-charge thickness test every 100 cycles. The growth rate of this thickness relative to the initial thickness is the thickness expansion rate of the battery, and the results are recorded in Table 2.

[0227] Table 2

[0228] As can be seen from Table 2, the battery prepared using the negative electrode sheet disclosed herein has improved cycle capacity retention and reduced cycle thickness expansion compared to the comparative example.

[0229] The preferred embodiments of the present disclosure are described in detail above, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed by the present disclosure and fall within the scope of protection of the present disclosure.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet comprises a negative electrode current collector and a negative electrode coating on at least one side of the negative electrode current collector; the negative electrode coating comprises a first part and a second part, the first part is adjacent to the negative electrode current collector; the first part comprises a silicon-based material and a covering, the silicon-based material is dispersed in the covering, the covering comprises a Na element, and in the first part, the ratio of the mass of the Na element to the mass of the Si element is b; the outer surface of the negative electrode coating has a concave portion; the ratio of the depth of the concave portion to the thickness of the negative electrode coating is d, satisfying 6.37×b 0.5 -0.43≤d≤6.37×b 0.5 -0.

13.

2. The negative electrode sheet according to claim 1, wherein: 6.37×b 0.5 -0.34≤d≤6.37×b 0.5 -0.22; Preferably, 0.009≤b≤0.023; Preferably, 0.25≤d≤0.

75.

3. The negative electrode sheet according to claim 1 or 2, wherein: 0.012≤b≤0.019； And / or, 0.4≤d≤0.

64.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The second portion is adjacent to a side of the first portion away from the negative electrode current collector; And / or, the negative electrode coating consists of the first part and the second part, the first part is adjacent to the negative electrode current collector, and the second part is the part of the negative electrode coating except the first part; Preferably, the second portion comprises a carbon-based material, and the recess is located in the second portion.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The ratio of the shortest distance from any point on the inner wall of the concave portion to the silicon-based material in the first portion to the thickness of the negative electrode coating is a, 0.1≤a≤0.45; preferably, 0.15≤a≤0.32; And / or, the shortest distance from any point on the inner wall of the recess to the silicon-based material in the first portion is 6 μm-30 μm.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein: The surface density of the first part is 1 mg / cm 2 -1.8mg / cm 2 , compacted density is 0.5g / cm 3 -1.5g / cm 3 ; The surface density of the second part is 6 mg / cm 2 -12mg / cm 2 ; The compaction density of the negative electrode coating is 1.2 g / cm 3 -1.85g / cm 3 ; Preferably, the compacted density of the first part is 0.8 g / cm 3 -1.3g / cm 3 The surface density of the second part is 7 mg / cm 2 -11mg / cm 2 .

7. The negative electrode sheet according to any one of claims 1 to 6, wherein: In the negative electrode coating, the ratio of the mass of the Na element to the mass of the Si element is c, 0.01≤c≤0.025; preferably, 0.014≤c≤0.

021.

8. The negative electrode sheet according to any one of claims 1 to 7, wherein: The thickness of the negative electrode coating is 40 μm-80 μm; preferably 50 μm-70 μm; And / or, the depth of the recess is 10 μm-70 μm, preferably 20 μm-45 μm.

9. The negative electrode sheet according to any one of claims 1 to 8, wherein: The concave portion includes concave holes, the pore diameter of the concave holes is 30 μm-100 μm, and the spacing between the concave holes is 100 μm-400 μm; And / or, the concave portion includes grooves, the width of the grooves is 20 μm-90 μm, and the interval between the grooves is 800 μm-2200 μm.

10. The negative electrode sheet according to claim 9, wherein: The aperture of the concave holes is 40 μm-80 μm, and the spacing between the concave holes is 200 μm-300 μm; And / or, the width of the groove is 30 μm-60 μm, and the spacing between the grooves is 1200 μm-1800 μm.

11. The negative electrode sheet according to any one of claims 1 to 10, wherein: The length of the groove is less than or equal to the length of the negative electrode coating; And / or, the length of the groove is less than or equal to the width of the negative electrode coating.

12. The negative electrode sheet according to any one of claims 1 to 11, wherein: The surface of the first portion away from the negative electrode current collector has a protrusion; And / or, the mass content of the Na element in the first part is higher than the mass content of the Na element in the second part.

13. The negative electrode sheet according to any one of claims 1 to 12, wherein: The median particle size Dv50 of the silicon-based material is 3 μm-15 μm; preferably 5 μm-12 μm; And / or, the average particle size of the silicon-based material is 3 μm-15 μm; preferably 5 μm-12 μm.

14. The negative electrode sheet according to any one of claims 1 to 13, wherein: Based on the total mass of the negative electrode coating, the content of Si element is 1wt%-9wt%; preferably 2wt%-6wt%; Preferably, based on the total mass of the negative electrode coating, the content of Na element is 150ppm-1000ppm.

15. A battery, characterized in that: The battery comprises the negative electrode sheet according to any one of claims 1 to 14.

Images