Negative plate and secondary battery

Abstract

The invention relates to the technical field of batteries, and discloses a negative plate and a secondary battery, the negative plate comprises a negative current collector and a negative coating located on at least one surface of the negative current collector, the surface of the negative coating is provided with a concave part, the depth of the concave part is H1 [mu] m, H1 is more than or equal to 5 and less than or equal to 65, the negative coating comprises a negative active material, and the negative active material comprises a positive electrode active material and a negative electrode active material. The negative electrode active material comprises silicon-carbon composite particles, the average sphericity of the silicon-carbon composite particles is epsilon, the epsilon satisfies 0.5 < = epsilon < = 1, the median diameter Dv50 of the silicon-carbon composite particles is phi [mu] m, and the phi and H1 satisfy 0.4 < = H1 / phi < = 10. According to the negative plate provided by the invention, the expansion of the silicon-carbon composite particles in the third direction Z and the surface impedance of the silicon-carbon composite particles can be reduced, the dynamic performance of the silicon-carbon composite particles can be improved, meanwhile, the stress among the silicon-carbon composite particles can be uniformly distributed, the expansion among the silicon-carbon composite particles is relieved, and the service life of the negative plate is prolonged. And moreover, the silicon-carbon composite particles can be in full contact with electrolyte, so that cyclic lithium precipitation and expansion are improved, and the service life of the lithium ion battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a negative electrode sheet and a secondary battery. Background Art

[0002] With the rapid development of lithium-ion battery technology, people have put forward higher requirements for the energy density, fast charging ability, and charge-discharge rate of lithium-ion batteries. Fast-charging lithium batteries with high energy density are also the development trend of consumer lithium-ion batteries. However, with the increase in energy density, the thickness of the positive and negative electrode sheets of lithium batteries becomes thicker and the transmission distance of lithium ions becomes longer. In addition, after the charging speed increases, lithium ions are more likely to accumulate on the surface of the electrode sheet during the charge-discharge process, causing congestion on the surface of the electrode sheet. Lithium ions cannot quickly embed into the bottom material far from the surface layer of the electrode sheet and close to the current collector of the electrode sheet, resulting in ineffective utilization of the active material. In addition, potential changes cause lithium deposition on the negative electrode surface in the later stage of cycling, especially for the negative electrode sheet doped with silicon negative electrodes. With the increase in the thickness of the electrode sheet, lithium deposition and swelling are more serious, seriously affecting the service life of lithium-ion batteries. Summary of the Invention

[0003] In view of this, the present invention provides a negative electrode sheet and a secondary battery to improve the problems of cyclic lithium deposition and swelling of lithium-ion batteries.

[0004] In a first aspect, the present invention provides a negative electrode sheet, comprising:

[0005] A negative electrode current collector, and a negative electrode coating located on at least one surface of the negative electrode current collector. A concave portion is provided on the surface of the negative electrode coating, and the depth of the concave portion is H1 μm, where H1 satisfies 5 ≤ H1 ≤ 65. The negative electrode coating includes a negative electrode active material, and the negative electrode active material includes silicon-carbon composite particles. The average sphericity of the silicon-carbon composite particles is ε, where ε satisfies 0.5 ≤ ε ≤ 1. The median particle size Dv50 of the silicon-carbon composite particles is and H1 satisfies

[0006] In an optional embodiment, the satisfies

[0007] and / or, the concave portion includes a concave pit, and the pore diameter of the concave pit is A1 μm, where A1 satisfies 20 ≤ A1 ≤ 135; and / or, the spacing between the concave pits is L1 μm, where L1 satisfies 50 ≤ L1 ≤ 1000;

[0008] and / or, the concave portion includes a groove, the groove has an extension length on the surface of the negative electrode coating, the width of the groove is A2 μm, where A2 satisfies 20 ≤ A2 ≤ 135; and / or, the spacing between the grooves is L2 μm, where L2 satisfies 200 ≤ L2 ≤ 3000.

[0009] In an alternative embodiment, cross-sectional characterization is performed within a range of ψ μm around the recess, where ψ satisfies 50 ≤ ψ ≤ 100, and the number of silicon-carbon composite particles is N, where N satisfies 2 ≤ N ≤ 10.

[0010] In an alternative embodiment, cross-sectional characterization is performed within a range of ψ μm around the recess, where ψ satisfies 50 ≤ ψ ≤ 100, the breakage rate of the silicon-carbon composite particles is less than or equal to 10%, the number of silicon-carbon composite particles with damaged surfaces is less than or equal to 10, and the damaged volume of the silicon-carbon composite particles does not exceed 30% of the silicon-carbon composite particles themselves.

[0011] In an alternative embodiment, the total thickness of the negative electrode coating is H2 μm, where H2 satisfies 35 ≤ H2 ≤ 70;

[0012] The negative electrode coating includes a first coating and / or a second coating;

[0013] When the negative electrode coating includes a first coating and a second coating, the second coating is located on at least one surface of the negative electrode current collector, and the first coating is located between the negative electrode current collector and the second coating;

[0014] Wherein, the thickness of the first coating is H3 μm, the thickness of the second coating is H4 μm, H3 and H4 satisfy 1 / 4 ≤ H3 / H4 ≤ 4, and H3 + H4 = H2.

[0015] In an alternative embodiment, when the negative electrode coating includes a first coating, the negative electrode active material includes silicon-carbon composite particles; based on the mass of the first coating, the mass content of the silicon-carbon composite particles is 2% to 90%;

[0016] And / or, when the negative electrode coating includes a first coating and a second coating, the negative electrode active materials of the first coating and the second coating include silicon-carbon composite particles; based on the mass of the first coating, the mass content of the silicon-carbon composite particles is 2% to 90%; based on the mass of the second coating, the mass content of the silicon-carbon composite particles is 2% to 90%;

[0017] And / or, when the negative electrode coating includes a first coating and a second coating, the negative electrode active material of the first coating includes silicon-carbon composite particles; based on the mass of the first coating, the mass content of the silicon-carbon composite particles is 2% to 90%;

[0018] And / or, when the negative electrode coating includes a first coating and a second coating, the negative electrode active material of the second coating includes silicon-carbon composite particles; based on the mass of the second coating, the mass content of the silicon-carbon composite particles is 2% to 90%.

[0019] In an alternative embodiment, the distance between the bottom of the recess and the negative electrode current collector is H5 μm, where H5 satisfies 0 < H5 ≤ 5.

[0020] In an alternative embodiment, the areal density of the negative electrode sheet is σ mg / cm 2 , where σ satisfies 7 ≤ σ ≤ 22;

[0021] and / or, the surface resistance of the negative electrode sheet is R mΩ, where R satisfies 15 ≤ R ≤ 100.

[0022] In an alternative embodiment, the Raman spectrum of the silicon-carbon composite particles satisfies having a first characteristic peak at 470 cm -1 ~520 cm -1 , having a second characteristic peak at 1300 cm -1 ~1400 cm -1 , and having a third characteristic peak at 1520 cm -1 ~1620 cm -1 ; the ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak satisfies 0.001 ≤ I ≤ 0.2;

[0023] and / or, the XRD diffraction pattern of the silicon-carbon composite particles satisfies having a first diffraction peak at 27.4° to 29.4°, and having a second diffraction peak at 42.4° to 44.4°; the full width at half maximum W1 of the first diffraction peak and the full width at half maximum W2 of the second diffraction peak satisfy 3° ≤ W1 + W2 ≤ 35°.

[0024] In a second aspect, the present invention further provides a secondary battery, including: a positive electrode sheet and a separator, and the negative electrode sheet as described above, and the negative electrode sheet, the positive electrode sheet and the separator are used to form an electric core.

[0025] Beneficial effects:

[0026] 1. For the negative electrode sheet provided by the present invention, by regulating the depth H1 of the concave portion of the negative electrode coating to satisfy: 5 ≤ H1 ≤ 65, not only can more buffer space be given to the silicon-carbon composite particles, reducing the expansion of the silicon-carbon composite particles in the third direction Z, but also the electrolyte can fully infiltrate the negative electrode coating, reducing the surface impedance of the silicon-carbon composite particles and improving their kinetic performance, enabling lithium ions to quickly embed into the side of the negative electrode coating close to the negative electrode current collector, making the negative electrode active material fully and effectively utilized; by regulating the average sphericity ε of the silicon-carbon composite particles to satisfy: 0.5 ≤ ε ≤ 1, in cooperation with the depth H1 of the concave portion, uniform distribution of stress between the silicon-carbon composite particles can be achieved, alleviating the expansion between the silicon-carbon composite particles, reducing the fragmentation of the silicon-carbon composite particles and the damage of the SEI film, and improving the cycle stability; by controlling the value of the median particle size Dv50 of the silicon-carbon composite particles, so that and H1 satisfy The greater the number of silicon-carbon composite particles exposed on the surface of the recess, the more silicon-carbon composite particles can come into sufficient contact with the electrolyte, reducing the surface impedance of the silicon-carbon composite particles and improving their kinetic performance. At the same time, the recess can absorb more expansion of the silicon-carbon composite particles, reducing the expansion of the silicon-carbon composite particles in the third direction Z, thereby improving cyclic lithium plating and expansion and extending the service life of the lithium-ion battery. Therefore, by comprehensively controlling the depth H1 of the recess of the negative electrode coating, the average sphericity ε of the silicon-carbon composite particles, and the median particle size Dv50 of the silicon-carbon composite particles value such that 5 ≤ H1 ≤ 65, 0.5 ≤ ε ≤ 1, it is possible to comprehensively regulate the expansion of the silicon-carbon composite particles, the different depths of the recess, the stress distribution on the silicon-carbon composite particles, and the contact between the silicon-carbon composite particles and the electrolyte, achieving the purpose of improving lithium plating and expansion of the battery and enhancing the cycling performance.

[0027] 2. The secondary battery of the present invention includes the negative electrode sheet of the first aspect. Therefore, the secondary battery includes all the beneficial effects of the negative electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is a cross-sectional view of a negative electrode sheet of an embodiment of the present invention along the third direction;

[0030] Figure 2 It is a top view of a negative electrode coating of a negative electrode sheet having a pit according to an embodiment of the present invention;

[0031] Figure 3 It is a top view of a negative electrode coating of a negative electrode sheet having a groove according to an embodiment of the present invention;

[0032] Figure 4 It is a schematic cross-sectional characterization diagram of a negative electrode sheet of an embodiment of the present invention when the negative electrode coating includes a first coating;

[0033] Figure 5 It is a first cross-sectional characterization diagram of a negative electrode sheet of an embodiment of the present invention when the negative electrode coating includes a first coating and a second coating;

[0034] Figure 6 It is a second cross-sectional characterization diagram of a negative electrode sheet of an embodiment of the present invention when the negative electrode coating includes a first coating and a second coating;

[0035] Figure 7 Schematic diagram of the third cross-section characterization of a negative electrode sheet according to an embodiment of the present invention when the negative electrode coating includes a first coating and a second coating.

[0036] Explanation of reference numerals:

[0037] 10. Negative electrode current collector; 11. Negative electrode coating; 111. First coating; 112. Second coating; 113. Silicon-carbon composite particles; 12. Recess; 121. Pit; 122. Groove.

[0038] X - First direction; Y - Second direction; Z - Third direction. Detailed implementation manners

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] The following combines Figures 1 to 7 , and describes the embodiments of the present invention.

[0041] According to an embodiment of the present invention, on the one hand, a negative electrode sheet is provided, including:

[0042] A negative electrode current collector 10, and a negative electrode coating 11 located on at least one surface of the negative electrode current collector 10. As shown in Figure 1 , a recess 12 is provided on the surface of the negative electrode coating 11. The depth of the recess 12 is H1 μm, and H1 satisfies 5 ≤ H1 ≤ 65. The negative electrode coating 11 includes a negative electrode active material. As shown in Figure 4 , the negative electrode active material includes silicon-carbon composite particles 113. The average sphericity of the silicon-carbon composite particles 113 is ε, and ε satisfies 0.5 ≤ ε ≤ 1. The median particle size Dv50 of the silicon-carbon composite particles 113 is and H1 satisfies

[0043] It should be noted that the arrow "X" in the accompanying drawings represents the first direction X. The "first direction X" refers to the width direction of the electrode sheet and / or the second largest dimension direction of the electrode sheet; the arrow "Y" in the accompanying drawings represents the second direction Y. The "second direction Y" refers to the length direction of the electrode sheet and / or the largest dimension direction of the electrode sheet; the arrow "Z" in the accompanying drawings represents the third direction Z. The "third direction Z" refers to the thickness direction of the electrode sheet and / or the smallest dimension direction of the electrode sheet; the first direction X, the second direction Y, and the third direction Z are perpendicular to each other pairwise.

[0044] When the sphericity of the silicon-carbon composite particles 113 is low, the silicon-carbon material has an irregular shape. During the charge and discharge process, adjacent silicon-carbon composite particles 113 are squeezed against each other in the first direction X and / or the second direction Y, resulting in a relatively large stress on the silicon-carbon composite particles 113 in the third direction Z and uneven stress distribution among the particles in the coating.

[0045] For the negative electrode sheet provided by the present invention, when the depth H1 of the concave portion of the negative electrode coating is too large, stress concentration is likely to occur in the concave portion area, leading to cracking or peeling of the negative electrode coating, thereby shortening the cycle life of the battery; when the depth H1 of the concave portion of the negative electrode coating is too small, insufficient buffer space cannot be provided for the silicon-carbon composite particles, resulting in an increase in the expansion of the silicon-carbon composite particles in the third direction Z. At the same time, the electrolyte cannot fully infiltrate the negative electrode coating, especially the negative electrode coating on the side close to the negative electrode current collector, and lithium ions cannot be quickly embedded into the bottom material far from the surface layer of the electrode sheet and close to the electrode current collector, resulting in more serious lithium deposition and expansion; therefore, by controlling the depth H1 of the concave portion 12 of the negative electrode coating 11 to satisfy: 5 ≤ H1 ≤ 65, with the unit of H1 being μm, not only can more buffer space be provided for the silicon-carbon composite particles 113, reducing the expansion of the silicon-carbon composite particles 113 in the third direction Z, but also the electrolyte can fully infiltrate the negative electrode coating 11, reducing the surface impedance of the silicon-carbon composite particles and improving their kinetic performance, enabling lithium ions to quickly embed into the side of the negative electrode coating 11 close to the negative electrode current collector 10, and making full and effective use of the negative electrode active material. When the average sphericity ε of the silicon-carbon composite particles is too small, it will lead to uneven stress distribution on the surface of the silicon-carbon composite particles, especially in the area of the deeper concave portion, which increases the risk of local stress concentration, making the silicon-carbon composite particles more likely to break and pulverize, and is also not conducive to the stability of the solid electrolyte interface (SEI) film on the surface of the silicon-carbon composite particles, thereby affecting the cycle performance; in the present invention, by making the average sphericity ε of the silicon-carbon composite particles 113 satisfy: 0.5 ≤ ε ≤ 1, in combination with the depth H1 of the concave portion, uniform distribution of stress among the silicon-carbon composite particles 113 can be achieved, alleviating the expansion among the silicon-carbon composite particles 113, reducing the breakage of the silicon-carbon composite particles and the damage of the SEI film, and improving the cycle stability. When the median particle size Dv50 of the silicon-carbon composite particles is small, more electrolyte decomposition and SEI film formation will occur, consuming active lithium ions and affecting the cycle life of the battery; when the median particle size Dv50 of the silicon-carbon composite particles is too large, a larger volume change will occur during the charge and discharge process. At this time, if the depth H1 of the concave portion is too small, the concave portion cannot absorb more expansion of the silicon-carbon composite particles, easily causing particle breakage and pulverization of the electrode material, thereby reducing the cycle life of the battery. And the present invention controls the value of and H1 to satisfy When When the value of H1 is fixed, it can make the value less than a specific value, the smaller the value, so that the number of silicon-carbon composite particles 113 exposed on the surface of the concave part is larger. Furthermore, more silicon-carbon composite particles 113 can be in full contact with the electrolyte, reducing the surface impedance of the silicon-carbon composite particles 113 and improving their kinetic performance; when When the value of H1 is fixed, it can make the value greater than a specific value, the larger the value, the greater the expansion. At the same time, the concave part 12 absorbs more expansion of the silicon-carbon composite particles 113, reducing the expansion of the silicon-carbon composite particles 113 in the third direction Z, thereby improving cyclic lithium deposition and expansion and prolonging the service life of the lithium-ion battery.

[0046] Therefore, by comprehensively controlling the depth H1 of the concave part of the negative electrode coating, the average sphericity ε of the silicon-carbon composite particles, and the median particle size Dv50 of the silicon-carbon composite particles values, making them satisfy 5 ≤ H1 ≤ 65, 0.5 ≤ ε ≤ 1, it is possible to comprehensively regulate the expansion of the silicon-carbon composite particles, the different depths of the concave part, the stress distribution on the silicon-carbon composite particles, and the contact situation between the silicon-carbon composite particles and the electrolyte, achieving the purpose of improving lithium deposition and expansion of the battery and enhancing the cycling performance.

[0047] As an example, H1 can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or within the range composed of any two of the above values; ε can be 0.5, 0.6, 0.7, 0.8, 0.9, 1 or within the range composed of any two of the above values; can be 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or within the range composed of any two of the above values.

[0048] Preferably, 0.85 ≤ ε ≤ 1.

[0049] In an alternative embodiment, satisfy Preferably, and / or,

[0050] In some embodiments, as shown in Figure 2 the figure, the concave part 12 includes concave pits 121, and a plurality of concave pits 121 are arranged at intervals along the first direction X and / or the second direction Y; the aperture of the concave pit 121 is A1 μm, and A1 satisfies 20 ≤ A1 ≤ 135; and / or, the spacing between the concave pits 121 is L1 μm, and L1 satisfies 50 ≤ L1 ≤ 1000;

[0051] and / or, see Figure 3 As shown, the concave portion 12 includes a groove 122, and the groove 122 has an extension length on the surface of the negative electrode coating 11, such as Figure 3 As shown, the groove 122 can extend along the first direction X on the surface of the negative electrode coating 11, and the plurality of grooves 122 are arranged at intervals along the second direction Y. The width of the groove 122 along the second direction Y is A2 μm, and A2 satisfies 20≤A2≤135; the spacing of the groove 122 along the second direction Y is L2 μm, and L2 satisfies 200≤L2≤3000. In the manufacturing process, by making holes in the negative electrode sheet, such as Figure 2 As shown in FIG. 1 , pits 121 may be formed on the surface of the negative electrode coating 11. Figure 1 As shown in , the cross-sectional shape of the pit 121 along the third direction Z can be a cone-like structure, that is, the surface aperture of the pit 121 is larger than the bottom aperture of the pit 121, and the depth of the pit 121 is 5μm to 65μm. For the pit 121, if the aperture of the pit 121 is too small, it is not only easy to cause insufficient buffer space for the silicon-carbon composite particles 113 in the first direction X and / or the second direction Y, causing the negative electrode sheet to expand along the third direction Z, but also not conducive to the electrolyte to more quickly and fully infiltrate the negative electrode coating 11, limiting the actual capacity of the battery, therefore, A1 needs to satisfy A1≥20; if the aperture of the pit 121 is too large, it is easy to cause excessive loss of negative electrode active materials and reduce the energy density of the battery cell, therefore, A1 also needs to satisfy A1≤135. If the spacing between the pits 121 is too small, the pits 121 are too dense, which will not only easily cause excessive loss of negative electrode active materials and reduce the energy density of the battery cell, but also easily reduce the structural stability of the negative electrode sheet. Therefore, L1 needs to satisfy L1≥50; if the spacing between the pits 121 is too large, the pits 121 are too sparse, which will easily limit the transmission path of lithium ions during the charging and discharging process, resulting in reduced ion transmission efficiency and limiting the actual capacity of the battery. Therefore, L1 must also satisfy L1≤1000.

[0052] In the manufacturing process, the negative electrode sheet can also be wired, such as Figure 2 As shown in FIG. 1 , a groove 122 may also be formed on the surface of the negative electrode coating 11, such as Figure 1 As shown in , the cross-sectional shape of the groove 122 along the third direction Z can be a cone-like structure, that is, the surface aperture of the groove 122 is larger than the bottom aperture of the groove 122, such as Figure 2As shown, the edge of the groove 122 is an irregular curved line, and the depth of the groove 122 is 5 μm to 65 μm. For the groove 122, if the width of the groove 122 is too small, it is not only easy to cause insufficient buffer space for the silicon-carbon composite particles 113 in the first direction X and / or the second direction Y, resulting in the expansion of the negative electrode sheet along the third direction Z, but also not conducive to the electrolyte wetting the negative electrode coating 11 faster and more fully, restricting the actual capacity of the battery. Therefore, A2 needs to satisfy A2≥20; if the width of the groove 122 is too large, it is easy to cause excessive loss of the negative electrode active material and reduce the energy density of the battery cell. Therefore, A2 also needs to satisfy A2≤135; if the spacing of the grooves 122 is too small and the grooves 122 are too dense, it is not only easy to cause excessive loss of the negative electrode active material and reduce the energy density of the battery cell, but also easy to reduce the structural stability of the negative electrode sheet. Therefore, L2 needs to satisfy L2≥200; if the spacing of the grooves 122 is too small and the grooves 122 are too sparse, it is easy to limit the transmission path of lithium ions during charge and discharge, resulting in a decrease in ion transmission efficiency and restricting the actual capacity of the battery. Therefore, L2 also needs to satisfy L2≤3000.

[0053] By adopting the above design for the recess 12, on the one hand, it can provide sufficient buffer space for the silicon-carbon composite particles 113 in the first direction X and / or the second direction Y, reduce the expansion of the silicon-carbon composite particles 113 in the third direction Z. On the other hand, it is conducive to the electrolyte wetting the negative electrode coating 11 faster and more fully, and at the same time ensures sufficient transmission paths for lithium ions during charge and discharge, improves the ion transmission efficiency, and ensures the actual capacity of the battery. On the other hand, it can avoid excessive loss of the negative electrode active material, thereby taking into account the energy density of the battery cell.

[0054] As an example, A1 can be 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 135 or within the range composed of any two of the above values; L1 can be 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 or within the range composed of any two of the above values; A2 can be 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 135 or within the range composed of any two of the above values; L2 can be 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000 or within the range composed of any two of the above values.

[0055] In some embodiments, please refer to Figure 4As shown, cross-sectional characterization is performed within a range of ψ μm around the recess 12, that is, within the range from the surface of the recess 12 to ψ μm around the recess 12, the number of silicon-carbon composite particles is N, where ψ satisfies 50 ≤ ψ ≤ 100 and N satisfies 2 ≤ N ≤ 10; when 50% of the diameter of the silicon-carbon composite particle is within the defined cross-sectional test range, the particle is considered to be within the defined range and counts as one silicon-carbon composite particle; the surfaces of some silicon-carbon composite particles can be tangent to the inner wall of the recess 12 or directly exposed within the recess 12.

[0056] It should be noted that within the defined cross-sectional test range around the recess 12, the number of silicon-carbon composite particles is related to the content of silicon-carbon composite particles in the negative electrode coating 11. The more the content of silicon-carbon composite particles in the negative electrode coating 11, the more the number of silicon-carbon composite particles within the defined cross-sectional test range around the recess 12; at the same time, the number of silicon-carbon composite particles is also related to the depth of the recess 12. The greater the depth of the recess 12, the more the number of silicon-carbon composite particles.

[0057] During the charge and discharge process, lithium ions preferentially embed into the silicon-carbon composite particles on the upper surface layer of the negative electrode coating along the electrolyte and are transmitted to the inside of the negative electrode coating through the silicon-carbon composite particles on the surface layer. If the number of surface silicon-carbon composite particles is small, the transmission channels for lithium ions are few, which easily causes the aggregation and congestion of lithium ions on the surface of the negative electrode coating. When the areal density is larger, the thickness of the surface layer on the negative electrode coating where lithium ions can easily embed is smaller, and the thickness within 50 - 100 μm of the negative electrode coating generally belongs to the surface layer thickness where lithium ions can easily embed. Therefore, controlling the number of silicon-carbon composite particles within the range from 0 μm to ψ μm, where ψ satisfies 50 ≤ ψ ≤ 100, can meet the transmission requirements of the lithium ion transmission channels.

[0058] That is, by adopting the above design, especially for lithium-ion batteries with silicon-containing negative electrode sheets under a large areal density, it can effectively ensure the number of silicon-carbon composite particles within the surface layer where lithium ions can easily embed, thereby ensuring sufficient lithium ion transmission channels, avoiding the aggregation and congestion of lithium ions on the surface of the electrode during the charge and discharge process, ensuring that lithium ions can quickly embed into the bottom material far from the surface layer of the electrode and close to the electrode current collector, ensuring the effective utilization of the active material, avoiding the problem of lithium metal deposition on the negative electrode surface in the later stage of cycling, and at the same time, the existence of the recess 12 can convert the expansion part of the silicon-carbon composite particles in the direction perpendicular to the surface of the electrode into the expansion in the horizontal direction of the electrode, which can not only improve the material utilization rate but also improve the problems of lithium metal deposition and the expansion of the silicon-containing negative electrode sheet.

[0059] As an example, ψ can be 50, 60, 70, 80, 90, 100 or within the range composed of any two of the above values; N can be 2, 3, 4, 5, 6, 7, 8, 9 or 10;

[0060] In some embodiments, please refer toFigure 4 As shown, cross-section characterization is performed within a range of ψ μm around the recess 12, where ψ satisfies 50 ≤ ψ ≤ 100, the breakage rate of the silicon-carbon composite particles is less than or equal to 70%, the number of silicon-carbon composite particles with damaged surfaces is less than or equal to 10, and defects such as holes or missing parts remain on the surfaces of the damaged silicon-carbon composite particles. The damaged volume of the silicon-carbon composite particles does not exceed 40% of the silicon-carbon composite particles themselves.

[0061] It should be noted that the silicon-carbon composite particles can be photographed by a Scanning Electron Microscope (SEM). Cross-section characterization is performed within a range of ψ μm around the recess 12, that is, within the range from 0 μm on the surface of the recess 12 to ψ μm around the recess 12. The total number of all silicon-carbon composite particles is denoted as N, and the number of damaged silicon-carbon composite particles is denoted as P. Among them, the damaged silicon-carbon composite particles refer to those with defects such as holes or missing parts on their surfaces. Then, the breakage rate of the silicon-carbon composite particles is P / N. "The damaged volume of the silicon-carbon composite particles does not exceed 40% of the silicon-carbon composite particles themselves" refers to a single silicon-carbon composite particle. For the evaluation of the damaged volume of the silicon-carbon composite particles, an image of the sample section can be obtained through SEM imaging, and then damaged areas such as holes and cracks can be marked within the defined range. Then, the area of each damaged area can be measured through image analysis software. The damaged volume can be estimated by measuring the damaged areas of multiple cross-sections and extrapolating to the entire particle volume. For complex damage morphologies, the damaged volume can be directly calculated using three-dimensional reconstruction data. The specific measurement method is not specifically limited here.

[0062] By adopting the above design, when the breakage rate of the silicon-carbon composite particles exceeds 70%, it will lead to the loss of active substances and directly cause a rapid decay of the battery capacity. Therefore, by controlling the breakage rate of the silicon-carbon composite particles, it can ensure that the silicon-carbon composite particles can normally exert their capacity, without damaging their electrochemical performance, and avoid the problem of local delamination. At the same time, by controlling the silicon-carbon composite particles to have a certain damaged volume, and the damaged volume does not exceed 40% of the silicon-carbon composite particles themselves, defects can be left on the surface of the spherical silicon. On the one hand, it can absorb its own expansion, and on the other hand, it can make the electrolyte more fully infiltrate the silicon negative electrode material, and a dense Solid Electrolyte Interface (SEI) layer can also be formed inside it, which is beneficial to the improvement of expansion and the enhancement of the cycle life.

[0063] In some embodiments, please refer to Figure 5 As shown, the total thickness of the negative electrode coating 11 is H2 μm, where H2 satisfies 35 ≤ H2 ≤ 70;

[0064] The negative electrode coating 11 includes a first coating 111 and / or a second coating 112;

[0065] When the negative electrode coating 11 includes the first coating 111 and the second coating 112, the second coating 112 is located on at least one surface of the negative electrode current collector 10, and the first coating 111 is located between the negative electrode current collector 10 and the second coating 112;

[0066] Wherein, the thickness of the first coating 111 is H3 μm, the thickness of the second coating 112 is H4 μm, H3 and H4 satisfy 1 / 4 ≤ H3 / H4 ≤ 4, and H3 + H4 = H2.

[0067] It should be noted that if the thickness of the first coating 111 is too small, it will not only easily affect the overall structural stability of the negative electrode coating 11, resulting in cracks or delamination in the first coating 111, but also easily lead to uneven distribution of the SEI layer formed by the first coating 111, affecting the stability of the SEI layer, which is not conducive to the improvement of expansion and the enhancement of cycle life. Therefore, H3 needs to satisfy H3 / H4 ≥ 1 / 4, H3 + H4 = H2, and H2 satisfies 35 ≤ H2 ≤ 70; if the thickness of the first coating 111 is too large, it will not only easily increase the lithium ion penetration time, and at the same time may cause some active materials not to participate in the electrochemical reaction fully, reducing the utilization rate of the negative electrode active material, but also easily lead to uneven distribution of the SEI layer formed by the first coating 111, affecting the stability of the SEI layer, which is not conducive to the improvement of expansion and the enhancement of cycle life. Therefore, H3 also needs to satisfy H3 / H4 ≤ 4, H3 + H4 = H2, and H2 satisfies 35 ≤ H2 ≤ 70. For the thickness requirement of the second coating 112, the principle is similar to that of the first coating 111 and will not be elaborated here.

[0068] By adopting the above design, on the one hand, the overall structural stability of the negative electrode coating 11 can be ensured, avoiding cracks or delamination; on the other hand, the formed SEI layer can be ensured to be evenly distributed, improving the stability of the SEI layer, which is conducive to the improvement of expansion and the enhancement of cycle life; on the third hand, the utilization rate of the negative electrode active material can be improved.

[0069] As an example, 35 ≤ H2 ≤ 70 can be 35, 40, 45, 50, 55, 60, 65, 70 or within the range composed of any two of the above values; H3 / H4 can be 1 / 4, 2 / 4, 3 / 4, 1, 2, 3, 4 or within the range composed of any two of the above values.

[0070] Preferably, 3 / 7 ≤ H3 / H4 ≤ 7 / 3.

[0071] In some embodiments, please refer to Figure 4As shown, when the negative electrode coating 11 includes the first coating 111, the negative electrode active material includes silicon-carbon composite particles 113; based on the mass of the first coating 111, the mass content of the silicon-carbon composite particles 113 is 2% to 90%;

[0072] and / or, please refer to Figure 5 As shown, when the negative electrode coating 11 includes the first coating 111 and the second coating 112, the negative electrode active materials of the first coating 111 and the second coating 112 include silicon-carbon composite particles 113; based on the mass of the first coating 111, the mass content of the silicon-carbon composite particles 113 is 2% to 90%; based on the mass of the second coating 112, the mass content of the silicon-carbon composite particles 113 is 2% to 90%;

[0073] and / or, please refer to Figure 6 As shown, when the negative electrode coating 11 includes the first coating 111 and the second coating 112, the negative electrode active material of the first coating 111 includes silicon-carbon composite particles 113; based on the mass of the first coating 111, the mass content of the silicon-carbon composite particles 113 is 2% to 90%;

[0074] and / or, please refer to Figure 7 As shown, when the negative electrode coating 11 includes the first coating 111 and the second coating 112, the negative electrode active material of the second coating 112 includes silicon-carbon composite particles 113; based on the mass of the second coating 112, the mass content of the silicon-carbon composite particles 113 is 2% to 90%.

[0075] By adopting the above design for the negative electrode coating 11, combined with the depth of the concave portion being H1, the average sphericity of the silicon-carbon composite particles being ε, and the median particle size Dv50 of the silicon-carbon composite particles being the design can avoid the aggregation and congestion of lithium ions on the surface of the electrode during charge and discharge, ensure that lithium ions can be quickly embedded into the bottom material far from the surface layer of the electrode and close to the electrode current collector, ensure the effective utilization of the active material, avoid the problem of lithium deposition on the surface of the negative electrode in the later stage of cycling, and at the same time, the existence of the concave portion 12 can convert the expansion part of the silicon-carbon composite particles in the direction perpendicular to the surface of the electrode into expansion along the horizontal direction of the electrode, which can not only improve the material utilization rate but also improve the problems of lithium deposition and the expansion of the silicon negative electrode sheet.

[0076] As an example, the mass content of the silicon-carbon composite particles in the first coating can be 2%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or within the range composed of any two of the above values, and the mass content of the silicon-carbon composite particles in the second coating can be 2%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or within the range composed of any two of the above values.

[0077] Furthermore, the depth to which the concave portion 12 extends into the second coating layer 112 is 30% to 65% of the thickness of the second coating layer 112, so as to ensure as much as possible that the silicon active material is not lost after the concave portion 12 is formed.

[0078] As an example, the depth to which the concave portion extends into the second coating layer is 30%, 40%, 50%, 60%, 65% of the thickness of the second coating layer or within the range formed by any two of the above values.

[0079] In some embodiments, please refer to Figure 1 As shown, the distance between the bottom of the concave portion 12 and the negative electrode current collector 10 is H5 μm, and H5 satisfies 0 < H5 ≤ 5.

[0080] Through the above design, damage to the negative electrode current collector 10 during the manufacturing process is avoided.

[0081] As an example, H5 can be 0.1, 0.5, 1, 2, 3, 4, 5 or within the range formed by any two of the above values.

[0082] In some embodiments, the areal density of the negative electrode sheet is σ mg / cm 2 , and σ satisfies 7 ≤ σ ≤ 22;

[0083] And / or, the sheet resistance of the negative electrode sheet is R mΩ, and R satisfies 15 ≤ R ≤ 100.

[0084] The sheet resistance of the negative electrode sheet is R mΩ, and by satisfying 15 ≤ R ≤ 100, the kinetics of the negative electrode sheet are effectively improved, and the risk of polarization improvement and cyclic delamination is reduced.

[0085] As an example, σ can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or within the range formed by any two of the above values; R can be 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 or within the range formed by any two of the above values.

[0086] In some embodiments, the silicon-carbon composite particles 113 include a hierarchical porous carbon material, silicon nanoparticles dispersed in the pores of the hierarchical porous carbon material, and an amorphous carbon layer coated on the surface of the hierarchical porous carbon material;

[0087] The Raman spectrum of the silicon-carbon composite particles 113 satisfies having a first characteristic peak at 470 cm -1 ~520 cm -1 and having a second characteristic peak at 1300 cm -1 ~1400 cm -1 and having a second characteristic peak at 1520 cm-1 ~1620 cm -1 has a third characteristic peak; the ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak satisfies 0.001 ≤ I ≤ 0.2;

[0088] and / or, the XRD diffraction pattern of the silicon-carbon composite particles 113 satisfies having a first diffraction peak at 27.4° to 29.4° and a second diffraction peak at 42.4° to 44.4°; the full width at half maximum W1 of the first diffraction peak and the full width at half maximum W2 of the second diffraction peak satisfy 3° ≤ W1 + W2 ≤ 35°.

[0089] It should be noted that the Raman spectrum of the silicon-carbon composite particles 113 satisfies having a first characteristic peak at 470 cm -1 ~520 cm -1 which indicates that the negative electrode active material contains crystalline silicon and amorphous silicon. Crystalline silicon has high electrical conductivity and good mechanical properties, and amorphous silicon can provide additional lithium storage sites and help improve the electrochemical performance of the material. The Raman spectrum of the silicon-carbon composite particles 113 satisfies having a second characteristic peak at 1300 cm -1 ~1400 cm -1 which represents the degree of disorder of carbon in the silicon-carbon composite particles. At 1520 cm -1 ~1620 cm -1 has a third characteristic peak, which represents the degree of order of carbon in the silicon-carbon composite particles. By satisfying 0.001 ≤ I ≤ 0.2 for the ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak, on the one hand, it can improve the electrical conductivity, form an effective conductive network, and ensure the efficient transmission of lithium ions and electrons. On the other hand, it can inhibit the volume expansion of silicon, thereby enhancing the cycle stability and rate performance. On the third hand, it can provide mechanical support through the carbon material to prevent the silicon-carbon composite particles from expanding and contracting excessively during charge and discharge, thereby enhancing the energy density of the battery on the premise of ensuring the cycle life.

[0090] It should be noted that the XRD diffraction pattern of the silicon-carbon composite particles 113 satisfies having a first diffraction peak at 27.4° to 29.4°, and the first diffraction peak corresponds to the characteristic peak of silicon; having a second diffraction peak at 42.4° to 44.4°, and the second diffraction peak corresponds to the characteristic peak of carbon. By satisfying 3° ≤ W1 + W2 ≤ 35° for the full width at half maximum W1 of the first diffraction peak and the full width at half maximum W2 of the second diffraction peak, the silicon and carbon in the silicon-carbon composite particles have an appropriate degree of amorphousness, which is beneficial to the transmission of lithium ions in the silicon-carbon composite particles.

[0091] As an example, I can be 0.001, 0.005, 0.01, 0.05, 0.1, 0.15, 0.2, or within the range formed by any two of the above values; W1 + W2 can be 3°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, or within the range formed by any two of the above values.

[0092] The Raman spectrum of the above silicon-carbon composite particles 113 was directly detected by a Raman spectrometer; the XRD diffraction pattern of the silicon-carbon composite particles can be obtained by, for example, using the X-ray diffraction method, and tested using a Shimadzu XRD-6100 type X-ray diffractometer. The sample amount for testing is 0.5 g / cm 2 , with the Kα line of Cu as the incident X-ray, the working voltage of the X-ray source is 40 kV, the test power is 2 kW, with 2θ as the abscissa and the unit is °, the signal intensity as the ordinate, the test range is 10° - 80°, the scanning rate is 4° / min, and the data sampling interval is 0.02°.

[0093] The present invention also provides a secondary battery, including: a positive electrode sheet and a separator, and the negative electrode sheet as described above. The negative electrode sheet, the positive electrode sheet, and the separator are used to form an electric core.

[0094] Examples and Comparative Examples

[0095] A secondary battery, the specific preparation process is as follows:

[0096] 1. Preparation of the positive electrode sheet: Lithium cobaltate LiCoO, conductive carbon black SP, and polyvinylidene fluoride PVDF are fully stirred and mixed in an appropriate amount of N-methylpyrrolidone solvent according to a weight ratio of 96:2:2 to obtain an active material slurry. After diluting the conductive layer slurry with water to a solid content of 50%, it is coated on a positive current collector (aluminum foil) to obtain a current collector coated with an active material layer; after drying and cold pressing, a positive electrode sheet is obtained.

[0097] 2. Preparation of the negative electrode sheet: Mix 97% of negative electrode active material, 0.5% of lithium carboxymethyl cellulose (CMCLi), 1.5% of binder styrene-butadiene rubber (SBR), and 1% of conductive carbon black (SP), and obtain a uniformly dispersed mixture through high-speed stirring. Then use water as a solvent to make a negative electrode slurry, and the solid content in the negative electrode slurry is 50 wt%. The negative electrode coating includes a first coating 111 and / or a second coating 112. When the negative electrode coating only contains the first coating 111, the negative electrode active material in the first coating 111 includes graphite and silicon-carbon composite particles 113, and the mass ratio of the silicon-carbon composite particles 113 in the negative electrode coating is Q. When the negative electrode coating includes the first coating 111 and the second coating 112, the negative electrode active material in both the first coating 111 and the second coating 112 includes graphite and silicon-carbon composite particles 113. The mass ratio of the silicon-carbon composite particles 113 in the first coating 111 is Q1, and the mass ratio of the silicon-carbon composite particles 113 in the second coating 112 is Q2. The average sphericity of the silicon-carbon composite particles 113 is ε, and the Dv50 of the silicon-carbon composite particles 113 is

[0098] During coating, coat the negative electrode slurry on a copper foil with a thickness of 5 μm to form a wet film, and obtain a single-sided negative electrode with a total thickness of H2 μm after drying. After coating both sides in the same way, form a negative electrode sheet with a recess 12 through baking, rolling, and punching. The surface resistance R of the negative electrode sheet is 50 mΩ; the single-sided surface density σ of the negative electrode sheet is 10 mg / cm 2 。

[0099] The process of punching to form the recess 12 is as follows: Place the rolled negative electrode sheet in a laser drilling machine, and use high-energy laser to etch the surface of the negative electrode. Control the power of the laser and the etching time to adjust the aperture (A1 μm), depth (H1 μm), and spacing (L1 μm) of the pit 121 formed by punching, or the width (A2 μm), depth (H1 μm), and spacing (L2 μm) of the linear groove 122 formed by punching. The distance between the bottom of the recess 12 and the negative electrode current collector 10 is H5 μm.

[0100] 3. Preparation of the lithium-ion battery: Stack the positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator placed between the positive electrode and the negative electrode to play an isolation role, and wind to obtain a bare battery core. Place the bare battery core in an outer package, perform vacuum drying, inject electrolyte, and encapsulate. Obtain the lithium-ion battery through processes such as formation, degassing, and trimming.

[0101] Among them, 7-μm PE is used as the separator. The electrolyte includes a solvent composed of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) and LiPF6. The weight ratio of PC, EC, and DEC in the solvent is 1:1:1, and the concentration of LiPF6 is about 1.15 mol / L.

[0102] The expansion rate %, lithium deposition situation, and cycling capacity retention rate / % of the batteries of the prepared examples and comparative examples were tested. The specific test process is as follows:

[0103] At 25 °C, within the charge-discharge window of 4.5 V to 3.0 V, 1C / 1C charge-discharge cycling was carried out. The test process is as follows. First, a constant current of 1C was used to charge to 4.2 V, then a constant voltage charge was applied until the cut-off current of 0.05C, and finally a constant current of 1C was used to discharge to 2.5 V. Such cycling tests were carried out 800 times. The ratio of the discharge capacity of the last cycle to the first discharge capacity was calculated to obtain the cycling capacity retention rate / %. At the same time, the thickness H1 of the battery in the last cycle and the thickness H0 of the battery in the first cycle were obtained, and the calculated value of (H1 - H0) / H0 was obtained. This calculated value is the expansion situation / %. The battery after cycling was disassembled to observe the lithium deposition situation on the negative electrode sheet, which was divided into no lithium deposition, slight lithium deposition, and severe lithium deposition according to the degree of lithium deposition.

[0104] Combined with Table 1 below, through several groups of test examples, at the depth H1 of the concave portion 12, the average sphericity ε of the silicon-carbon composite particles 113, the median particle size Dv50 of the silicon-carbon composite particles 113, With different total thicknesses H2 of the negative electrode coating 11 and corresponding changes in the distance H5 between the bottom of the concave portion 12 and the negative electrode current collector 10; the cycling capacity retention rate, cycling lithium deposition and expansion problems of the secondary battery provided by the embodiments of the present invention, as well as the number N, the number P of the surface-damaged ones, the breakage rate, and the damaged volume of the silicon-carbon composite particles 113 within the range of ψ in the negative electrode sheet were verified. The verification results are shown in Table 2.

[0105] Table 1

[0106]

[0107] In the examples and comparative examples of Table 1 above, the negative electrode coating 11 in the negative electrode sheet only includes the first coating 111. Based on the mass of the first coating 111, the mass content Q of the silicon-carbon composite particles 113 is 50%; the concave portion 12 formed by punching is a concave pit 121, the aperture A1 of the concave pit 121 is 70 μm, and the pitch L1 is 300 μm; the ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak in the Raman spectrum of the silicon-carbon composite particles 113 is 0.1. In the XRD diffraction pattern of the silicon-carbon composite particles 113, the sum of the half-peak widths W1 of the first diffraction peak and the half-peak width W2 of the second diffraction peak is 25°.

[0108] Table 2

[0109]

[0110]

[0111] In Table 2 above, through the data comparison between the examples and the comparative examples, it can be seen that comprehensively controlling the depth H1 of the concave portion 12 satisfies 5 ≤ H1 ≤ 65, the average sphericity ε of the silicon-carbon composite particles 113 satisfies 0.5 ≤ ε ≤ 1, and By the mutual cooperation of the depth H1 of the concave portion 12 and the average sphericity ε of the silicon-carbon composite particles 113, the expansion rate can be effectively reduced and the lithium deposition can be reduced, so that the cycle capacity retention rate is significantly improved. By comparing Example 1, 4, 6, 8-9, 12 with other examples, it can be seen that controlling ε, H5 within the range values can effectively control the number N of the silicon-carbon composite particles 113 within the ψ range in the negative electrode sheet, the number P of the surfaces damaged, the breakage rate, and the damaged volume are all within the scope of the present invention, thereby significantly reducing the expansion rate and the lithium deposition situation, and further significantly improving the cycle capacity retention rate.

[0112] Combined with Table 3 below, through several groups of test examples, the cycle capacity retention rate, the cycle lithium deposition and expansion problems of the secondary battery provided in Examples 15-26 of the present invention, as well as the number N of the silicon-carbon composite particles 113 within the ψ range in the negative electrode sheet, the breakage rate, the number of damaged surfaces, and the damaged volume are verified. The verification results are shown in Table 4. In the examples of Table 3 below, the aperture A1 of the concave pit 121, the spacing L1 of the concave pits 121, the width A2 and the spacing L2 after replacing the concave pit 121 with a groove 122, the surface density σ of the negative electrode sheet, the surface resistance R of the negative electrode sheet, the ratio I in the Raman spectrum of the silicon-carbon composite particles 113, W1 + W2 in the XRD diffraction pattern of the silicon-carbon composite particles 113, and the mass content Q of the silicon-carbon composite particles 113 in the first coating 111 are not

[0113] Table 3

[0114]

[0115] In the examples of Table 3 above, the negative electrode coating 11 in the negative electrode sheet only includes the first coating 111, the depth H1 of the concave portion 12 is 32.5 μm, the average sphericity ε of the silicon-carbon composite particles 113 is 0.9, the median particle size Dv50 of the silicon-carbon composite particles 113 is φ = 13 μm, the total thickness H2 of the negative electrode coating 11 is 35 μm, and the distance H5 between the bottom of the concave portion 12 and the negative electrode current collector 10 is 2.5 μm.

[0116] Table 4

[0117]

[0118] In Table 4 above, from the data of the examples, it can be seen that by comprehensively controlling the aperture A1 of the pit 121, the spacing L1 of the pits 121, the width A2 and the spacing L2 after replacing the pits 121 with grooves 122, the areal density σ of the negative electrode sheet, the surface resistance R of the negative electrode sheet, the ratio I in the Raman spectrum of the silicon-carbon composite particles 113, W1 + W2 in the XRD diffraction pattern of the silicon-carbon composite particles 113, and the mass content Q of the silicon-carbon composite particles 113 in the first coating 111 within the range values of the present invention, the effect of effectively achieving no lithium deposition and having a low expansion rate can be achieved, so that the battery of the present invention has a good cycle capacity retention rate.

[0119] Combined with Table 5 below, through several sets of test examples, the cycle capacity retention rate, the problems of cycle lithium deposition and expansion of the secondary battery provided in Examples 27 - 33 of the present invention are verified, as well as the number N of the silicon-carbon composite particles 113 within the range of ψ in the negative electrode sheet, the number P of the surfaces damaged, the breakage rate, and the damaged volume. The verification results are shown in Table 6. In the examples of Table 5 below, the negative electrode coating includes a first coating 111 and a second coating 112. The negative electrode active materials in the first coating 111 and the second coating 112 both include graphite and silicon-carbon composite particles 113. The thickness H3 of the first coating 111, the mass ratio Q1 of the silicon-carbon composite particles 113 in the first coating 111, the thickness H4 of the second coating 112, and the mass ratio Q2 of the silicon-carbon composite particles 113 in the second coating 112 are different.

[0120] Table 5

[0121] H3 / μm H4 / μm H3 / H4 Q1 / ％ Q2 / ％ Example 27 23.3 11.7 2 30 70 Example 28 10.5 24.5 0.4 30 70 Example 29 24.5 10.5 2.3 30 70 Example 30 7 28 0.3 90 2 Example 31 28 7 4 2 90 Example 32 17.5 17.5 1 0 90 Example 33 17.5 17.5 1 90 0

[0122] In the examples of Table 5 above, the depth H1 of the recess 12 in the negative electrode sheet is 32.5 μm, the average sphericity ε of the silicon-carbon composite particles 113 is 0.9, the median particle size Dv50 of the silicon-carbon composite particles 113 is φ = 13 μm, the total thickness H2 of the negative electrode coating 11 is 35 μm, the distance H5 between the bottom of the recess 12 and the negative electrode current collector 10 is 2.5 μm, the recess 12 formed by punching is a pit 121, the aperture A1 of the pit 121 is 70 μm, the spacing L1 is 300 μm, the ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak in the Raman spectrum of the silicon-carbon composite particles 113 is 0.1, and in the XRD diffraction pattern of the silicon-carbon composite particles 113, the sum of the full width at half maximum W1 of the first diffraction peak and the full width at half maximum W2 of the second diffraction peak is 25°.

[0123] Table 6

[0124]

[0125]

[0126] In Table 6 above, from the data of the examples, it can be seen that when the negative electrode coating includes the first coating 111 and the second coating 112, the negative electrode active materials in the first coating 111 and the second coating 112 both include graphite and silicon-carbon composite particles 113; by controlling the thickness H3 of the first coating 111, the mass ratio Q1 of the silicon-carbon composite particles 113 in the first coating 111, the thickness H4 of the second coating 112, and the mass ratio Q2 of the silicon-carbon composite particles 113 in the second coating 112 within the range values of the present invention, the number N of the silicon-carbon composite particles 113, the number P of the surfaces damaged, the breakage rate, and the damaged volume within the ψ range in the negative electrode sheet can be effectively controlled within the range of the present invention, thereby significantly reducing the swelling rate and the lithium deposition situation, and enabling the battery of the present invention to have a better cycle capacity retention rate.

[0127] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A negative electrode sheet, characterized in that: include: A negative electrode current collector (10), and a negative electrode coating (11) located on at least one surface of the negative electrode current collector (10), wherein a concave portion (12) is arranged on the surface of the negative electrode coating (11), wherein the depth of the concave portion (12) is H1 μm, and H1 satisfies 5≤H1≤65, wherein the negative electrode coating (11) comprises a negative electrode active material, wherein the negative electrode active material comprises silicon-carbon composite particles (113), wherein the average sphericity of the silicon-carbon composite particles (113) is ε, and ε satisfies 0.5≤ε≤1, and wherein the median particle size Dv50 of the silicon-carbon composite particles (113) is φ μm, and φ and H1 satisfy 0.4≤H1 / φ≤10.

2. The negative electrode sheet according to claim 1, characterized in that: The φ satisfies 6≤φ≤20; and / or, the concave portion (12) comprises a pit (121), the aperture of the pit (121) is A1 μm, and A1 satisfies 20≤A1≤135; and / or, the spacing of the pit (121) is L1 μm, and L1 satisfies 50≤L1≤1000; And / or, the recess (12) includes a groove (122), the groove (122) has an extension length on the surface of the negative electrode coating (11), the width of the groove (122) is A2μm, A2 satisfies 20≤A2≤135; and / or, the spacing of the grooves (122) is L2μm, L2 satisfies 200≤L2≤3000.

3. The negative electrode sheet according to claim 2, characterized in that: A cross-sectional characterization is performed within a range of ψ μm around the concave portion (12), wherein ψ satisfies 50≤ψ≤100, and the number of the silicon-carbon composite particles (113) is N, and N satisfies 2≤N≤10.

4. The negative electrode sheet according to claim 2, characterized in that: Cross-sectional characterization is performed within a range of ψμm around the recess (12), wherein ψ satisfies 50≤ψ≤100, the breakage rate of the silicon-carbon composite particles (113) is less than or equal to 70%, the number of damaged surfaces of the silicon-carbon composite particles (113) is less than or equal to 10, and the damaged volume of the silicon-carbon composite particles (113) does not exceed 40% of the silicon-carbon composite particles (113) themselves.

5. The negative electrode sheet according to claim 1, characterized in that: The total thickness of the negative electrode coating (11) is H2 μm, and H2 satisfies 35≤H2≤70; The negative electrode coating (11) comprises a first coating (111) and / or a second coating (112); When the negative electrode coating (11) comprises the first coating (111) and the second coating (112), the second coating (112) is located on at least one surface of the negative electrode current collector (10), and the first coating (111) is located between the negative electrode current collector (10) and the second coating (112); The thickness of the first coating (111) is H3 μm, the thickness of the second coating (112) is H4 μm, H3 and H4 satisfy 1 / 4≤H3 / H4≤4, and H3+H4=H2.

6. The negative electrode sheet according to claim 5, characterized in that: When the negative electrode coating (11) includes the first coating (111), the negative electrode active material includes the silicon-carbon composite particles (113); the mass content of the silicon-carbon composite particles (113) is 2% to 90% based on the mass of the first coating (111); When the negative electrode coating (11) includes the first coating (111) and the second coating (112), the negative electrode active materials of the first coating (111) and the second coating (112) include the silicon-carbon composite particles (113); based on the mass of the first coating (111), the mass content of the silicon-carbon composite particles (113) is 2% to 90%; based on the mass of the second coating (112), the mass content of the silicon-carbon composite particles (113) is 2% to 90%; When the negative electrode coating (11) includes the first coating (111) and the second coating (112), the negative electrode active material of the first coating (111) includes the silicon-carbon composite particles (113); the mass content of the silicon-carbon composite particles (113) is 2% to 90% based on the mass of the first coating (111); And / or, when the negative electrode coating (11) includes the first coating (111) and the second coating (112), the negative electrode active material of the second coating (112) includes the silicon-carbon composite particles (113); and the mass content of the silicon-carbon composite particles (113) is 2% to 90% based on the mass of the second coating (112).

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The distance between the bottom of the recess (12) and the negative electrode current collector (10) is H5 μm, and H5 satisfies 0<H5≤5.

8. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The surface density of the negative electrode sheet is σmg / cm 2 , σ satisfies 7≤σ≤22; And / or, the surface resistance of the negative electrode sheet is R mΩ, and R satisfies 15≤R≤100.

9. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The Raman spectrum of the silicon-carbon composite particles (113) satisfies the condition at 470 cm -1 ~520cm -1 The first characteristic peak is at 1300cm -1 ~1400cm -1 There is a second characteristic peak at 1520cm -1 ~1620cm -1 having a third characteristic peak; a ratio I of the intensity of the first characteristic peak to the intensity of the third characteristic peak satisfies 0.001≤I≤0.2; And / or, the XRD diffraction spectrum of the silicon-carbon composite particles (113) satisfies the conditions that it has a first diffraction peak at 27.4°-29.4° and a second diffraction peak at 42.4°-44.4°; and the half-width W1 of the first diffraction peak and the half-width W2 of the second diffraction peak satisfy 3°≤W1+W2≤35°.

10. A secondary battery, characterized in that: include: A positive electrode sheet and a separator, and a negative electrode sheet as claimed in any one of claims 1 to 9, wherein the negative electrode sheet, the positive electrode sheet and the separator are used to form a battery cell.

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