Silicon-carbon negative electrode sheet, preparation method thereof, and lithium ion battery

Abstract

The present disclosure provides a silicon-carbon negative electrode sheet, a preparation method thereof, and a lithium ion battery. The silicon-carbon negative electrode sheet comprises a high-capacity silicon-carbon active layer, an inner conductive buffer constraint layer, and an outer conductive buffer constraint layer. The outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer. The rigid protective shell is used to reduce the contact reaction rate of the high-capacity silicon-carbon active layer with the electrolyte and to limit the expansion of the high-capacity silicon-carbon active layer. The inner conductive buffer constraint layer and the outer conductive buffer constraint layer both comprise high-compaction high-capacity graphite (compaction density ≥ 1.70 g / cm 3 , specific capacity ≥ 360 mAh / g). The porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer. The silicon-carbon negative electrode sheet not only ensures the stability of the SEI film interface, but also realizes good restriction of the silicon-carbon expansion of the high-capacity silicon-carbon active layer, reduces the overall deformation of the silicon-carbon negative electrode sheet and the silicon particle crushing phenomenon, provides a good roll release space for the silicon-carbon negative electrode sheet, and ensures the preparation of a high-capacity thin silicon-carbon negative electrode sheet.

Description

Technical Field

[0001] This disclosure relates to the field of silicon-carbon anode technology, and in particular to a silicon-carbon anode and its preparation method, as well as a lithium-ion battery. Background Technology

[0002] Silicon-carbon anode sheets are lithium-ion battery anode sheets made of silicon and carbon materials. They can significantly improve the energy density of lithium-ion batteries and better meet the ever-changing market demands.

[0003] Although silicon-carbon anode sheets have a high energy density, their volume expansion rate after lithium intercalation reaches 300%. In addition, most silicon-carbon anode sheets currently use anode slurry directly coated onto the current collector, as in patent CN 119581488A. This leads to the problem that the active material of the anode slurry is prone to detaching from the current collector after multiple cycles, resulting in severe performance degradation of lithium-ion batteries.

[0004] Some scholars have attempted to use the tiered structure silicon-carbon anode sheet described in patent CN 113782712 A. This tiered structure of carbon layer slurry B and slurry A addresses the problem of easy detachment between the active material and the current collector in the anode slurry to some extent. Alternatively, they have tried the structure of the silicon-carbon anode sheet described in CN 202737032U, which uses a buffer layer, such as graphite, between the current collector and the active layer to address the issue of silicon-carbon detachment. However, the high-capacity silicon-carbon active layer of these anode sheets is directly exposed to the electrolyte. This causes the exposed silicon anode surface to regenerate a solid electrolyte interphase (SEI) film with the electrolyte, which then covers the original SEI film, easily forming a thick and uneven SEI film, making it difficult to effectively ensure the stability of the SEI interface. Summary of the Invention

[0005] The purpose of this disclosure is to overcome the shortcomings of the prior art and provide a silicon-carbon anode sheet that ensures the stability of the SEI film interface, effectively limits the silicon-carbon expansion of the high-capacity silicon-carbon active layer, reduces the overall deformation and silicon particle breakage of the silicon-carbon anode sheet, and provides a better rolling pressure release space for the silicon-carbon anode sheet, thus ensuring the preparation of a high-capacity thin silicon-carbon anode sheet, its preparation method, and a lithium-ion battery.

[0006] The purpose of this disclosure is achieved through the following technical solution: A silicon-carbon anode sheet includes a current collector and at least one sandwich-type buffer conductive structure. The sandwich-type buffer conductive structure includes an inner conductive buffer constraint layer, a high-capacity silicon-carbon active layer, and an outer conductive buffer constraint layer. The inner conductive buffer constraint layer and the high-capacity silicon-carbon active layer are sequentially disposed on the current collector. The outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer. The rigid protective shell is used to reduce the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte, and also to limit the expansion of the high-capacity silicon-carbon active layer. Both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer comprise high-compact, high-capacity graphite, with a compaction density ≥ 1.70 g / cm³. 3 Specific capacity ≥360mAh / g; The porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer.

[0007] In one embodiment, the porosity of the inner conductive buffer constraint layer is 30%-40%.

[0008] In one embodiment, the inner conductive buffer constraint layer comprises, by mass parts, the following materials in the following mass parts: Graphite 94-97 parts; The first conductive agent is 1.5 to 3.0 parts; The first adhesive is 1.5 to 3.0 parts.

[0009] In one embodiment, the porosity of the outer conductive buffer constraint layer is 20%-30%.

[0010] In one embodiment, the outer conductive buffer constraint layer comprises, by mass parts, the following materials in the following mass parts: Graphite 94-97 parts; The first conductive agent is 1.5 to 3.5 parts; The first adhesive is 1.5 to 2.8 parts.

[0011] In one embodiment, the first conductive agent comprises at least one of conductive carbon black and single-walled carbon nanotubes; and / or, The first adhesive comprises at least one of sodium carboxymethyl cellulose and styrene-butadiene rubber.

[0012] In one embodiment, the high-capacity silicon-carbon active layer comprises the following materials in parts by mass: 92-97 parts silicon-carbon; The second conductive agent is 0.1 to 0.3 parts; The second adhesive is 1.0 to 2.5 parts.

[0013] Among them, the second conductive agent includes at least one of conductive carbon black and single-walled carbon nanotubes.

[0014] In one embodiment, the thickness of the high-capacity silicon-carbon active layer satisfies the relationship Ι: 0.3×(a + b) < y < 0.5×(a + b), where a is the thickness of the inner conductive buffer constraint layer, b is the thickness of the outer conductive buffer constraint layer, and y is the thickness of the high-capacity silicon-carbon active layer.

[0015] A method for preparing a silicon-carbon negative electrode sheet includes the following steps: Obtain inner conductive buffer constraint slurry, high-capacity silicon-carbon slurry, and outer conductive buffer constraint slurry respectively; Coat the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry on the current collector in sequence to form a sandwich buffer conductive structure, and obtain a silicon-carbon negative electrode sheet A; Perform multiple rollings on the silicon-carbon negative electrode sheet A to obtain the silicon-carbon negative electrode sheet described in any of the above embodiments.

[0016] A lithium-ion battery includes the silicon-carbon negative electrode sheet described in any of the above embodiments.

[0017] Compared with the prior art, the present disclosure has at least the following advantages: 1) Since the inner conductive buffer constraint layer and the high-capacity silicon-carbon active layer are sequentially arranged on the current collector, and the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer, the formed sandwich buffer conductive structure has a "sandwich" sandwich structure, so that the electrolyte does not directly contact the high-capacity silicon-carbon active layer, thereby reducing the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte, ensuring that the growth of the SEI film during the cycle does not show the phenomenon of being thick and uneven, and further better ensuring the stability of the SEI film interface; also, because the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer, and both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer include high-compaction high-capacity graphite, the SEI film formed by the reaction of the outer conductive buffer constraint layer with the electrolyte will be thinner, more stable, and have better conductivity than the SEI film formed on the surface of traditional silicon-carbon, further ensuring the stability of the SEI film interface, effectively reducing the loss of active substances in the high-capacity silicon-carbon active layer, and being beneficial to improving the first Coulomb efficiency and cycle retention rate of the lithium-ion battery.

[0018] 2) Furthermore, due to the strong adhesion provided by the inner and outer conductive buffer constraint layers, especially with the inner conductive buffer constraint layer having a higher porosity than the outer conductive buffer constraint layer, the rigid protective shell effectively restricts the expansion of the high-capacity silicon-carbon active layer in the middle. This ensures that the expanding high-capacity silicon-carbon active layer in the middle can quickly transfer internal stress to the inner and outer conductive buffer constraint layers, effectively preventing excessive stress concentration at the interface between the high-capacity silicon-carbon active layer and the inner or outer conductive buffer constraint layer. This reduces particle breakage and crack propagation in the high-capacity silicon-carbon active layer. Thus, while effectively restricting the silicon-carbon expansion of the high-capacity silicon-carbon active layer, it also significantly reduces silicon particle displacement and the overall deformation of the silicon-carbon anode sheet, thereby reducing the overall deformation and silicon particle breakage of the silicon-carbon anode sheet. It also reduces the amount of the second adhesive used in the high-capacity silicon-carbon active layer, improving the utilization rate of the active material. 3) Due to the compaction density of high-density graphite under high pressure is ≥1.70 g / cm³ 3 Specific capacity ≥360mAh / g; This is beneficial for forming a rigid, high-capacity protective shell. In this way, while meeting the requirements for preparing high-capacity silicon-carbon anode sheets, it can also effectively limit the silicon expansion of the high-capacity silicon-carbon active layer, effectively avoiding the problem that low-compacted graphite cannot effectively limit silicon expansion, and also avoiding the problem that low-capacity graphite cannot be used to prepare high-capacity silicon-carbon anode sheets.

[0019] 4) Because the porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer, the inner conductive buffer constraint layer near the current collector has a higher buffer space. This allows the inner conductive buffer constraint layer to preferentially buffer the stress caused by the silicon expansion of the high-capacity silicon-carbon active layer. The inner conductive buffer constraint layer with higher porosity can buffer more silicon expansion, which reduces the deformation probability of the outer conductive buffer constraint layer itself and also reduces the expansion rate of the high-capacity silicon-carbon active layer to the outer conductive buffer constraint layer, further ensuring the stability of the SEI film. Moreover, the inner conductive buffer constraint layer can form a continuous and stable electron conduction network with the high-capacity silicon-carbon active layer and the outer conductive buffer constraint layer. This ensures that even if the internal high-capacity silicon-carbon active layer undergoes some structural changes, such as detachment or collapse, electrons can still be smoothly transmitted through the outer conductive buffer constraint layer, avoiding interruption of the electron conduction path and effectively preventing the problem of a sudden drop in lithium-ion battery capacity due to obstructed electron transport. Furthermore, since the porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer, it ensures that the silicon particles in the high-capacity silicon-carbon active layer will not break under high rolling pressure conditions, providing better rolling pressure release space for the silicon-carbon anode sheet, thereby ensuring the preparation of a high-capacity thin silicon-carbon anode sheet. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this disclosure and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a photograph of the silicon-carbon negative electrode sheet disassembled after cycling in Embodiment 1 of the present invention. Detailed Implementation

[0022] To facilitate understanding of this disclosure, a more complete description will be given below with reference to the accompanying drawings, which illustrate preferred embodiments of the present disclosure. However, this disclosure can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to enable a more thorough and complete understanding of the disclosure.

[0023] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly attached to the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] To better understand the technical solutions and beneficial effects of this disclosure, the following detailed description is provided in conjunction with specific embodiments: An embodiment of a silicon-carbon anode sheet includes a current collector and at least one sandwich-type buffer conductive structure. The sandwich-type buffer conductive structure includes an inner conductive buffer constraint layer, a high-capacity silicon-carbon active layer, and an outer conductive buffer constraint layer. The inner conductive buffer constraint layer and the high-capacity silicon-carbon active layer are sequentially disposed on the current collector. The outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer. The rigid protective shell is used to reduce the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte, and also to limit the expansion of the high-capacity silicon-carbon active layer. Both the inner and outer conductive buffer constraint layers comprise high-compacted high-capacity graphite with a compaction density ≥ 1.70 g / cm³. 3 The specific capacity is ≥360mAh / g; the porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer.

[0026] It is understood that, since the inner conductive buffer constraint layer and the high-capacity silicon-carbon active layer are sequentially disposed on the current collector, and the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer, the resulting sandwich-type buffer conductive structure has a "sandwich" structure. This prevents the electrolyte from directly contacting the high-capacity silicon-carbon active layer, thereby reducing the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte. This ensures that the SEI film growth does not exhibit uneven thickness during cycling, thus better protecting the... This ensures the stability of the SEI film interface. Furthermore, since the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer, and both the inner and outer conductive buffer constraint layers include high-capacity graphite, the SEI film formed by the reaction of the outer conductive buffer constraint layer with the electrolyte is thinner, more stable, and has better conductivity than the SEI film formed on the traditional silicon-carbon surface. This further ensures the stability of the SEI film interface, effectively reduces the loss of active material from the high-capacity silicon-carbon active layer, and is beneficial to improving the first coulombic efficiency and cycle retention rate of lithium-ion batteries.

[0027] It can also be understood that, due to the strong adhesion provided by the inner and outer conductive buffer constraint layers, especially with the inner conductive buffer constraint layer having a higher porosity than the outer conductive buffer constraint layer, the rigid protective shell can effectively limit the expansion of the high-capacity silicon-carbon active layer in the middle. This ensures that the expanding high-capacity silicon-carbon active layer in the middle can quickly transfer internal stress to the inner and outer conductive buffer constraint layers, effectively preventing excessive stress concentration at the interface between the high-capacity silicon-carbon active layer and the inner or outer conductive buffer constraint layers. This reduces particle breakage and crack propagation in the high-capacity silicon-carbon active layer. Thus, while achieving good limitation of silicon-carbon expansion in the high-capacity silicon-carbon active layer, it also significantly reduces silicon particle displacement and overall deformation of the silicon-carbon anode sheet, thereby reducing overall deformation and silicon particle breakage. Furthermore, it reduces the amount of second adhesive used in the high-capacity silicon-carbon active layer, improving the utilization rate of the active material. This is understandable, given that high-density, high-compact graphite has a compaction density ≥1.70 g / cm³. 3 With a specific capacity ≥360mAh / g, it is beneficial to form a rigid high-capacity protective shell. In this way, while meeting the requirements for preparing high-capacity silicon-carbon anode sheets, it can also achieve better restriction of silicon expansion in the high-capacity silicon-carbon active layer, effectively avoiding the problem that low-compacted graphite cannot achieve better restriction of silicon expansion, and also avoiding the problem that low-capacity graphite cannot be used to prepare high-capacity silicon-carbon anode sheets.

[0028] It should be noted that although adding an outer conductive buffer confinement layer can effectively suppress the high-capacity silicon-carbon active layer, both the outer and inner conductive buffer confinement layers have a compaction density ≥1.70 g / cm³. 3 High-capacity graphite with a specific capacity ≥360mAh / g is prone to stress concentration and insufficient expansion space in the outer and inner conductive buffer constraint layers located on both sides of the high-capacity silicon-carbon active layer due to excessive rigidity, which makes the outer and inner conductive buffer constraint layers prone to deformation.

[0029] Therefore, in this disclosure, by controlling the porosity of the inner conductive buffer constraint layer to be greater than that of the outer conductive buffer constraint layer, the inner conductive buffer constraint layer near the current collector side has a higher buffer space. This allows the inner conductive buffer constraint layer to preferentially buffer the stress caused by silicon expansion of the high-capacity silicon-carbon active layer. The inner conductive buffer constraint layer with higher porosity can buffer more silicon expansion, which reduces the deformation probability of the outer conductive buffer constraint layer itself and also reduces the expansion rate of the high-capacity silicon-carbon active layer towards the outer conductive buffer constraint layer, further ensuring the stability of the SEI film. Moreover, the inner conductive buffer constraint layer can form a continuous and stable electron conduction network with the high-capacity silicon-carbon active layer and the outer conductive buffer constraint layer. This ensures that even if the internal high-capacity silicon-carbon active layer undergoes some structural changes, such as detachment or collapse, electrons can still be smoothly transmitted through the outer conductive buffer constraint layer, avoiding interruption of the electron conduction path and effectively preventing the problem of a sudden drop in lithium-ion battery capacity due to obstructed electron transport.

[0030] Furthermore, since the porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer, it ensures that the silicon particles in the high-capacity silicon-carbon active layer will not break under high rolling pressure conditions, providing better rolling pressure release space for the silicon-carbon anode sheet, thereby ensuring the preparation of a high-capacity thin silicon-carbon anode sheet.

[0031] In one embodiment, the porosity of the inner conductive buffer constraint layer is 20%-30%, and in another embodiment, the porosity of the outer conductive buffer constraint layer is 30%-40%, to ensure that the inner and outer conductive buffer constraint layers can form a suitable gradient porosity. This ensures that the rigid protective shell has the dual function of suppressing silicon expansion space and releasing space for higher rolling pressure, thereby ensuring the preparation of high-capacity thin silicon-carbon anode sheets.

[0032] It should be noted that, since traditional silicon-carbon anode sheets have disclosed a graphite buffer layer, some researchers have attempted to add an additional buffer layer directly on the other side of the high-capacity silicon-carbon active layer to better suppress silicon in the high-capacity silicon-carbon active layer. However, because traditional graphite buffer layers consist only of graphite and some binders, when combining graphite buffer layers with conventional methods in traditional silicon anode sheets, the high contact resistance between graphite particles can lead to a break in the conductive network, resulting in low utilization of the active material in the high-capacity silicon-carbon active layer in the middle, leading to low capacity retention after multiple cycles. Therefore, some researchers have attempted to use the carbon coating structure described in patent CN 113782712 A, directly replacing graphite with highly conductive graphene to improve the conductivity of the buffer layer. However, due to the high specific surface area of ​​graphene (2600 m²),... 2 / g) It easily adsorbs electrolyte, resulting in an excessively thick SEI film, which cannot adequately guarantee the stability of the SEI film interface; in addition, due to the weak van der Waals forces between graphene sheets, it is impossible to effectively suppress the silicon expansion of the high-capacity silicon-carbon active layer.

[0033] Therefore, in this disclosure, by simultaneously introducing a first conductive agent into both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer, and since the first conductive agent includes conductive carbon black and single-walled carbon nanotubes, the added first conductive agent enables the inner conductive buffer constraint layer, the outer conductive buffer constraint layer, and the high-capacity silicon-carbon active layer to form a continuous conductive network. In this way, without affecting the good suppression of silicon expansion of the high-capacity silicon-carbon active layer by the inner and outer conductive buffer constraint layers, the conductivity of the sandwich buffer conductive structure is also improved, thereby improving the utilization rate of the active material of the high-capacity silicon-carbon active layer, and ensuring that the formed SEI film is thinner and more stable, thus better ensuring the stability of the SEI film interface.

[0034] It is also understandable that, because graphene is less flexible than conductive carbon black and single-walled carbon nanotubes (i.e., more rigid), using graphene as the first conductive agent cannot ensure that the rigid protective shell simultaneously fulfills the dual functions of suppressing silicon expansion space and releasing space under high rolling pressure. Therefore, in one embodiment, the first conductive agent includes at least one of conductive carbon black and single-walled carbon nanotubes to ensure that the added first conductive agent has better flexibility, which can better compensate for the high rigidity of high-compact, high-capacity graphite, thereby ensuring that the rigid protective shell simultaneously fulfills the dual functions of suppressing silicon expansion space and releasing space under high rolling pressure.

[0035] Specifically, in one embodiment, the inner conductive buffer constraint layer comprises the following materials in parts by weight: 94-97 parts graphite; 1.5-3.0 parts first conductive agent; and 1.5-3.0 parts first binder, to ensure that an inner conductive buffer constraint layer with a porosity of 20%-30% and good conductivity is prepared.

[0036] In one embodiment, the outer conductive buffer constraint layer comprises, by mass parts, the following materials: 94-97 parts graphite; 1.5-3.5 parts first conductive agent; and 1.5-2.8 parts first binder, to ensure that an outer conductive buffer constraint layer with a porosity of 30%-40% and good conductivity is prepared. In particular, in conjunction with the use of the inner conductive buffer constraint layer, it ensures that the conductivity of the sandwich buffer conductive structure is improved without affecting the good suppression of silicon expansion of the high-capacity silicon-carbon active layer by the inner and outer conductive buffer constraint layers, thereby improving the utilization rate of the active material of the high-capacity silicon-carbon active layer, and ensuring that the formed SEI film is thin and stable, thus better ensuring the stability of the SEI film interface.

[0037] It should be noted that although controlling the porosity of the inner conductive buffer constraint layer to be greater than that of the outer conductive buffer constraint layer can reduce the expansion rate of silicon expansion from the high-capacity silicon-carbon active layer to the outer conductive buffer constraint layer to some extent, it will increase the peeling rate between the fluid and the inner conductive buffer constraint layer. Some researchers have tried to improve the above-mentioned technical problems by increasing the amount of adhesive used, but too much adhesive will affect the conductivity of the inner conductive buffer constraint layer.

[0038] Therefore, in this disclosure, the first conductive agent of the inner conductive buffer constraint layer is set to single-walled carbon nanotubes, and the first conductive agent of the outer conductive buffer constraint layer is set to conductive carbon black. This makes the flexibility of the inner conductive buffer constraint layer higher than that of the outer conductive buffer constraint layer. In this way, without increasing the amount of adhesive used, the peeling rate between the inner conductive buffer constraint layer and the current collector can be improved, while the conductivity of the inner conductive buffer constraint layer is also guaranteed, which is beneficial for constructing a highly conductive and continuous conductive network.

[0039] It is also understandable that, since the first conductive agent of the outer conductive buffer constraint layer is set to conductive carbon black, the surface of the conductive carbon black is relatively smooth and chemically inert. The SEI film generated by the reaction with the electrolyte is thinner and more uniform, which better ensures the interfacial stability of the SEI film. This effectively reduces the rate of side reactions at the SEI film interface, reduces the loss of active materials, and improves the first coulombic efficiency and cycle retention of lithium-ion batteries.

[0040] In one embodiment, the first adhesive includes at least one of sodium carboxymethyl cellulose and styrene-butadiene rubber to ensure that the high-capacity silicon-carbon active layer can be well suppressed and restricted by the inner conductive buffer constraint layer and the outer conductive buffer constraint layer using a conventional first adhesive.

[0041] In one embodiment, the first adhesive is a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber.

[0042] In one embodiment, the high-capacity silicon-carbon active layer comprises the following materials in parts by mass: 92-97 parts silicon-carbon; 0.1-0.3 parts second conductive agent; and 1.0-2.5 parts second binder, to ensure the preparation of a high-capacity silicon-carbon active layer. Furthermore, since the second conductive agent includes at least one of conductive carbon black and single-walled carbon nanotubes, it is beneficial for the high-capacity silicon-carbon active layer to form a continuous conductive network with the inner conductive buffer constraint layer and the outer conductive buffer constraint layer, thereby ensuring the preparation of a high-capacity thin silicon-carbon anode sheet.

[0043] It is understandable that by setting the amount of the first conductive agent used in both the inner and outer conductive buffer constraint layers to be higher than the amount of the second conductive agent used in the high-capacity silicon-carbon active layer, and by setting the amount of the first conductive agent used in the inner conductive buffer constraint layer to be less than the amount of the first conductive agent used in the outer conductive buffer constraint layer, the amount of the first conductive agent used in the inner conductive buffer constraint layer is kept low. This ensures that the inner conductive buffer constraint layer is more flexible and has less expansion resistance. This not only further reduces the peeling rate between the inner conductive buffer constraint layer and the current collector, but also promotes the preferential release of the high-capacity silicon-carbon active layer to the inner conductive buffer constraint layer, reducing the cracking of the outer conductive buffer constraint layer due to excessive rigidity.

[0044] It is understood that in this disclosure, the high-capacity silicon-carbon active layer comprises the following materials in parts by weight: 92-97 parts silicon-carbon; 0.1-0.3 parts second conductive agent; 1.0-2.5 parts second binder; the outer conductive buffer constraint layer comprises the following materials in parts by weight: 94-97 parts graphite; 1.5-3.5 parts first conductive agent; 1.5-2.8 parts first binder; and the inner conductive buffer constraint layer comprises the following materials in parts by weight: 94-97 parts graphite; 1.5-3.0 parts first conductive agent; 1.5-3.0 parts first binder. This ensures that the amount of first conductive agent used in both the inner and outer conductive buffer constraint layers is higher than the amount of second conductive agent used in the high-capacity silicon-carbon active layer, and the amount of first conductive agent used in the inner conductive buffer constraint layer is less than the amount of first conductive agent used in the outer conductive buffer constraint layer, thus forming a gradient conductive agent.

[0045] In one embodiment, the second adhesive is a mixture of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid.

[0046] It should be noted that by setting the first conductive agent of the inner conductive buffer constraint layer as single-walled carbon nanotubes, the first conductive agent of the outer conductive buffer constraint layer as conductive carbon black, and the usage amounts of the first conductive agents of both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer being higher than that of the second conductive agent of the high-capacity silicon-carbon active layer, and the usage amount of the first conductive agent of the inner conductive buffer constraint layer being less than that of the first conductive agent of the outer conductive buffer constraint layer, the peeling rate between the inner conductive buffer constraint layer and the current collector can be well reduced. However, since the high-capacity silicon-carbon active layer expands preferentially towards the inner conductive buffer constraint layer, the silicon expansion of the high-capacity silicon-carbon active layer after cycling is likely to squeeze the current collector out of shape, exacerbating the expansion rate of the silicon-carbon negative electrode sheet.

[0047] Therefore, in one embodiment, by setting the thickness of the high-capacity silicon-carbon active layer to satisfy the relational expression Ι: 0.3×(a + b) < y < 0.5×(a + b), where a is the thickness of the inner conductive buffer constraint layer, b is the thickness of the outer conductive buffer constraint layer, and y is the thickness of the high-capacity silicon-carbon active layer, on the one hand, the thickness ratios of the inner conductive buffer constraint layer, the high-capacity silicon-carbon active layer, and the outer conductive buffer constraint layer are better coordinated. Especially in conjunction with the porosity of the inner conductive buffer constraint layer being greater than that of the outer conductive buffer constraint layer, and the usage amounts of the first conductive agents of both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer being higher than that of the second conductive agent of the high-capacity silicon-carbon active layer, and the usage amount of the first conductive agent of the inner conductive buffer constraint layer being less than that of the first conductive agent of the outer conductive buffer constraint layer, in this way, it is ensured that the distributions of the inner conductive buffer constraint layer, the outer conductive buffer constraint layer, and the high-capacity silicon-carbon active layer are more suitable, enabling the stress of the silicon expansion of the high-capacity silicon-carbon active layer to diffuse well towards both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer, so as to ensure that the inner conductive buffer constraint layer and the outer conductive buffer constraint layer do not show the phenomenon of outward extrusion deformation when absorbing the stress of the high-capacity silicon-carbon active layer, thereby reducing the expansion rate and deformation rate of the silicon-carbon negative electrode sheet; at the same time, the thicknesses of the inner conductive buffer constraint layer, the outer conductive buffer constraint layer, and the high-capacity silicon-carbon active layer are also better coordinated, ensuring that a thinner inner conductive buffer constraint layer and outer conductive buffer constraint layer can achieve the dual functions of inhibiting the silicon expansion space and releasing the space for higher roll pressing of the high-capacity silicon-carbon active layer, which is beneficial to preparing a high-capacity thin silicon-carbon negative electrode sheet.

[0048] It can also be understood that when the thickness of the high-capacity silicon-carbon active layer is less than 0.3×(a + b), it is impossible to prepare a high-capacity thin silicon-carbon negative electrode sheet. When the thickness of the high-capacity silicon-carbon active layer is greater than 0.5×(a + b), on the one hand, it is not conducive to the preparation of a high-capacity thin silicon-carbon negative electrode sheet. On the other hand, the too thick high-capacity silicon-carbon active layer is likely to cause the inner conductive buffer constraint layer and the outer conductive buffer constraint layer to be extruded and deformed outward when the silicon expands, resulting in a large expansion rate or deformation problem of the silicon-carbon negative electrode sheet. Therefore, in the present disclosure, by controlling the thickness of the high-capacity silicon-carbon active layer to be 0.3×(a + b) < y < 0.5×(a + b), in this way, on the premise of ensuring the preparation of a high-capacity thin silicon-carbon negative electrode sheet, it is also ensured that the inner conductive buffer constraint layer and the outer conductive buffer constraint layer do not show the phenomenon of outward extrusion and deformation, reducing the expansion rate and deformation rate of the silicon-carbon negative electrode sheet.

[0049] The present disclosure also provides a method for preparing a silicon-carbon negative electrode sheet, including the following steps: First, respectively obtain an inner conductive buffer constraint slurry, a high-capacity silicon-carbon slurry, and an outer conductive buffer constraint slurry; then, sequentially coat the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry on a current collector to form a sandwich buffer conductive structure, obtaining a silicon-carbon negative electrode sheet A; finally, perform multiple rollings on the silicon-carbon negative electrode sheet A to obtain the silicon-carbon negative electrode sheet described in any one of the above embodiments.

[0050] A method for preparing a silicon-carbon negative electrode sheet according to an embodiment includes the following part or all of the steps: S101. Respectively obtain an inner conductive buffer constraint slurry, a high-capacity silicon-carbon slurry, and an outer conductive buffer constraint slurry to prepare for subsequent coating.

[0051] In one embodiment, in terms of parts by mass, 94 parts - 97 parts of graphite, 1.5 parts - 3.0 parts of a first conductive agent, and 1.5 parts - 3.0 parts of a first binder are mixed to obtain an inner conductive buffer constraint slurry.

[0052] In one embodiment, in terms of parts by mass, 92 parts - 97 parts of silicon-carbon, 0.1 parts - 0.3 parts of a second conductive agent, and 1.0 parts - 2.5 parts of a second binder are mixed to obtain a high-capacity silicon-carbon slurry.

[0053] In one embodiment, in terms of parts by mass, 94 parts - 97 parts of graphite, 1.5 parts - 3.5 parts of a first conductive agent, and 1.5 parts - 2.8 parts of a first binder are mixed to obtain an outer conductive buffer constraint slurry.

[0054] In one embodiment, the graphite in the inner conductive buffer constraint slurry and the outer conductive buffer constraint slurry is high-compaction high-capacity graphite with a compaction density ≥ 1.70 g / cm 3The specific capacity is ≥360mAh / g to ensure that the inner and outer conductive buffer constraint layers can effectively limit the silicon expansion of the high-capacity silicon-carbon active layer.

[0055] In one embodiment, the silicon carbon in the high-capacity silicon carbon slurry is high-capacity silicon carbon with a specific capacity of 500 mAh / g-700 mAh / g and a compaction density of 1.5 g / cm³. 3 -1.7g / cm 3 This is to ensure the preparation of high-capacity thin silicon-carbon anode sheets.

[0056] S102. The inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry are sequentially coated onto the current collector to form a sandwich buffer conductive structure, thereby obtaining a silicon-carbon negative electrode A.

[0057] In one embodiment, when the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry are sequentially coated onto the current collector, the following specific steps are included: first, the inner conductive buffer constraint slurry is coated onto one side of the current collector for the first time, and then dried for the first time to form the inner conductive buffer constraint layer; then, the high-capacity silicon-carbon slurry is coated onto the inner conductive buffer constraint layer and dried a second time to form the high-capacity silicon-carbon active layer; immediately afterward, the outer conductive buffer constraint slurry is coated onto the high-capacity silicon-carbon active layer and then dried a third time to obtain the silicon-carbon negative electrode A.

[0058] It is understandable that by coating the outer conductive buffer constraint slurry onto the high-capacity silicon-carbon active layer, the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer. This creates a "sandwich" structure in the resulting sandwich-like conductive buffer structure, preventing the electrolyte from directly contacting the high-capacity silicon-carbon active layer. This reduces the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte, ensuring that the SEI film growth does not exhibit uneven thickness during cycling, thus better guaranteeing the stability of the SEI film interface. Furthermore, since the outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer, and both the inner and outer conductive buffer constraint layers include high-compacted high-capacity graphite, the SEI film formed by the reaction between the outer conductive buffer constraint layer and the electrolyte is thinner, more stable, and has better conductivity than the SEI film formed on the traditional silicon-carbon surface. This further guarantees the stability of the SEI film interface, effectively reducing the loss of active material from the high-capacity silicon-carbon active layer, which is beneficial for improving the initial coulombic efficiency and cycle retention rate of lithium-ion batteries.

[0059] It can also be understood that since the rigid protective shell formed by the inner conductive buffer constraint layer and the outer conductive buffer constraint layer can play a good inhibitory role on the high-capacity silicon-carbon active layer, the dependence of the high-capacity silicon-carbon active layer on the second adhesive is effectively reduced, the total usage amount of the second adhesive of the high-capacity silicon-carbon active layer is relatively low, and the utilization rate of the active material of the high-capacity silicon-carbon active layer is improved.

[0060] In one embodiment, the temperature of the first drying is 70°C - 100°C, the time is 8 min - 10 min, especially in combination with the temperature of the second drying being 90°C - 100°C, the time being 5 min - 8 min, and the temperature of the third drying being 80°C - 110°C, the time being 3 min - 5 min, so as to form a sandwich buffer conductive structure on the current collector, and at the same time ensure that the probability of cracking or yellowing of the dried sandwich buffer conductive structure does not occur, which is beneficial to preparing a highly conductive and continuous sandwich buffer conductive structure.

[0061] It can be understood that the operator can calculate the required coating thicknesses of the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry to be 40 μm - 60 μm, 25 μm - 60 μm, and 40 μm - 60 μm respectively according to the thickness of the high-capacity silicon-carbon active layer satisfying the relationship Ι: 0.3×(a + b) < y < 0.5×(a + b).

[0062] Specifically, in one embodiment, the coating thickness when the inner conductive buffer constraint slurry is first coated on one side of the current collector is 40 μm - 60 μm.

[0063] In one embodiment, the coating thickness of the high-capacity silicon-carbon slurry coated on the inner conductive buffer constraint layer is 25 μm - 50 μm.

[0064] In one embodiment, the coating thickness of the outer conductive buffer constraint slurry coated on the high-capacity silicon-carbon active layer is 40 μm - 60 μm, especially in combination with subsequent multiple rolling conditions, which is beneficial to the thickness of the high-capacity silicon-carbon active layer of the prepared silicon-carbon negative electrode sheet satisfying the relationship Ι: 0.3×(a + b) < y < 0.5×(a + b).

[0065] In one embodiment, the current collector is a copper foil with a thickness of 6 μm - 8 μm.

[0066] S103. The silicon-carbon anode sheet A is subjected to multiple rolling processes to obtain the silicon-carbon anode sheet as described in any of the above embodiments. It is understood that a single rolling process would cause the silicon particles in the high-capacity silicon-carbon active layer to break due to excessive instantaneous stress. Therefore, by performing multiple rolling processes on the silicon-carbon anode sheet A, the stress can be gradually released, effectively avoiding the problem of the silicon particles in the high-capacity silicon-carbon active layer breaking due to excessive instantaneous stress caused by a single rolling process.

[0067] It can also be understood that multiple rolling processes can promote close contact between the inner conductive buffer constraint layer, the high-capacity silicon-carbon active layer, and the outer conductive buffer constraint layer, reducing interlayer voids, improving interfacial adhesion, lowering interfacial resistance, and optimizing electron and ion conduction paths. Furthermore, multiple rolling processes can gradually compact each layer of material to the target density (e.g., high-compacted high-capacity graphite with a compacted density ≥1.70 g / cm³). 3 This enhances the mechanical strength of the rigid protective shell of the "sandwich" core structure and improves the ability to constrain the expansion of the high-capacity silicon-carbon active layer.

[0068] It should be noted that, to ensure that the overall thickness of the silicon-carbon anode sheet finally prepared in this disclosure does not vary significantly, some researchers choose high rolling pressure conditions. This allows for close contact between the inner conductive buffer confinement layer, the high-capacity silicon-carbon active layer, and the outer conductive buffer confinement layer. On the one hand, this reduces the contact resistance between these layers, which is beneficial for preparing a highly conductive and continuous conductive network. On the other hand, it reduces the thickness of each layer, which is beneficial for preparing a thinner high-capacity silicon-carbon anode sheet. Furthermore, it increases the compaction density of the silicon-carbon anode sheet, thereby improving the initial coulombic efficiency of the high-capacity silicon-carbon anode sheet. However, high rolling pressure conditions can cause silicon particles to easily break.

[0069] To address this, some scholars have attempted to use at least two batch rolling processes, but using more than two batch rolling processes would affect the production efficiency of the silicon-carbon anode sheet. Therefore, in this disclosure, the number of rolling processes is two. In one embodiment, the step of performing a second rolling process on the silicon-carbon anode sheet A includes the following specific steps: performing a first rolling process on the silicon-carbon anode sheet A to achieve rolling operation on 90% of the silicon-carbon anode sheet A; then, performing a second rolling process on the silicon-carbon anode sheet A after the first rolling process to achieve rolling operation on 100% of the silicon-carbon anode sheet A. In this way, while ensuring high production efficiency, it is also beneficial to prepare high-capacity thin silicon-carbon anode sheets.

[0070] It is also understandable that the appropriate gradient porosity formed by the greater porosity of the inner conductive buffer constraint layer than the outer conductive buffer constraint layer can provide better release space for the rolling operation. In particular, the first conductive agent of the inner conductive buffer constraint layer is a single-walled carbon nanotube and the first conductive agent of the outer conductive buffer constraint layer is conductive carbon black, which makes the added first conductive agent have good flexibility. This can better compensate for the problem of high rigidity of high-compact, high-capacity graphite, and ensure that the silicon particles are not easily broken under high rolling conditions.

[0071] It is also understandable that the rigid protective shell formed by the inner and outer conductive buffer constraint layers can effectively suppress the silicon expansion of the high-capacity silicon-carbon active layer, thus reducing the waste of active material in the high-capacity silicon-carbon active layer. This ensures that a smaller amount of the second conductive agent used in the high-capacity silicon-carbon active layer is sufficient to meet the conductivity requirements of the high-capacity thin silicon-carbon anode sheet, thereby reducing the amount of the second conductive agent used in the high-capacity silicon-carbon active layer. In particular, the amount of the first conductive agent used in both the inner and outer conductive buffer constraint layers is higher than that used in the first conductive layer. The amount of the second conductive agent used in the high-capacity silicon-carbon active layer, and the amount of the first conductive agent used in the inner conductive buffer constraint layer being less than that used in the outer conductive buffer constraint layer, ensures that the inner conductive buffer constraint layer, the outer conductive buffer constraint layer, and the high-capacity silicon-carbon active layer can form a gradient conductive structure. This ensures that the inner conductive buffer constraint layer, the high-capacity silicon-carbon active layer, and the outer conductive buffer constraint layer form a continuous and stable electron conduction network, allowing the sandwich buffer conductive structure to quickly complete lithium delithiation and lithium insertion, which is beneficial for preparing high-capacity thin silicon-carbon anode sheets. Furthermore, the sandwich buffer conductive structure with a gradient conductive structure ensures that the inner conductive buffer constraint layer has good flexibility. Combined with the fact that the porosity of the inner conductive buffer constraint layer is greater than that of the outer conductive buffer constraint layer, the inner conductive buffer constraint layer can preferentially buffer the stress caused by silicon expansion in the high-capacity silicon-carbon active layer, effectively reducing the deformation probability of the outer conductive buffer constraint layer.

[0072] In one embodiment, the conditions for the first rolling are as follows: the rolling pressure is 2.5 MPa - 3.0 MPa, and the rolling speed is 5 m / min - 30 m / min; in particular, in combination with the conditions for the second rolling: the rolling pressure is 3.0 MPa - 5.0 MPa, and the rolling speed is 5 m / min - 20 m / min, to ensure a better rolling effect on each layer under higher rolling conditions, so as to ensure that the thickness of the high-capacity silicon-carbon active layer of the prepared silicon-carbon negative electrode sheet satisfies the relationship Ι: 0.3×(a + b) < y < 0.5×(a + b). In this way, on the premise of ensuring the preparation of a high-capacity thin silicon-carbon negative electrode sheet, it also ensures that the inner conductive buffer constraint layer and the outer conductive buffer constraint layer do not show the phenomenon of outward extrusion deformation, reducing the expansion rate and deformation rate of the silicon-carbon negative electrode sheet.

[0073] In one embodiment, before the second rolling of the silicon-carbon negative electrode sheet A after the first rolling, the silicon-carbon negative electrode sheet A after the first rolling is also left standing for 1 h - 2 h to ensure stress release after rolling and avoid the rebound of the silicon-carbon negative electrode sheet A.

[0074] The above method for preparing a silicon-carbon negative electrode sheet first obtains the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry respectively; then the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry are sequentially coated on the current collector to form a sandwich buffer conductive structure, obtaining the silicon-carbon negative electrode sheet A. Since the formed sandwich buffer conductive structure has a "sandwich" sandwich structure, the electrolyte will not directly contact the high-capacity silicon-carbon slurry, thereby reducing the contact reaction rate between the high-capacity silicon-carbon slurry and the electrolyte, ensuring that the growth of the SEI film does not show a thick and uneven phenomenon during the cycle, and further ensuring the stability of the SEI film interface; also, since both the outer conductive buffer constraint slurry and the inner conductive buffer constraint slurry include high-compaction high-capacity graphite, the SEI film formed by the reaction of the outer conductive buffer constraint slurry with the electrolyte will be thinner, more stable, and have better conductivity than the SEI film formed on the surface of traditional silicon-carbon, further ensuring the stability of the SEI film interface, effectively reducing the loss of active substances in the high-capacity silicon-carbon active layer, and being beneficial to improving the first Coulomb efficiency and cycle retention rate of the lithium-ion battery. Finally, the silicon-carbon negative electrode sheet A is rolled multiple times to obtain the silicon-carbon negative electrode sheet described in any of the above embodiments, ensuring that under higher multiple rolling conditions, the sandwich buffer conductive structure can provide a better rolling release space to better buffer the internal stress of higher rolling, effectively avoiding the problem that the silicon particles in the high-capacity silicon-carbon active layer do not break under higher rolling conditions, so as to ensure the rapid preparation of a high-capacity thin silicon-carbon negative electrode sheet, not only improving the production efficiency of the silicon-carbon negative electrode sheet, but also ensuring the preparation of a high-capacity thin silicon-carbon negative electrode sheet with high conductivity and continuity.

[0075] Furthermore, the rigid protective shell formed by the inner and outer conductive buffer constraint layers can effectively suppress the high-capacity silicon-carbon active layer, thereby effectively reducing the dependence of the high-capacity silicon-carbon active layer on the second binder. This results in a relatively low total amount of the second binder used in the high-capacity silicon-carbon active layer, thus improving the utilization rate of the active material in the high-capacity silicon-carbon active layer.

[0076] This disclosure also provides a lithium-ion battery, including the silicon-carbon anode sheet described in any of the above embodiments. Since the silicon-carbon anode sheet has a sandwich buffer conductive structure, the sandwich buffer conductive structure can not only better stabilize the SEI film on the surface of the silicon-carbon anode sheet, but also better limit the silicon-carbon expansion of the high-capacity silicon-carbon active layer, and reduce the overall deformation of the silicon-carbon anode sheet and the silicon particle breakage phenomenon, so as to ensure that the expansion rate of the lithium-ion battery is small after multiple cycles, and also improve the first coulombic efficiency and cycle performance of the lithium-ion battery.

[0077] In one embodiment, the lithium-ion battery is a cylindrical lithium-ion battery. A cylindrical lithium-ion battery includes a cell.

[0078] In one embodiment, the battery cell includes a positive electrode, a separator, and a silicon-carbon negative electrode. Furthermore, the separator is a double-sided aqueous PVDF ceramic separator to ensure tight contact between the silicon-carbon negative electrode and to prevent cell expansion.

[0079] In one embodiment, the wound cell is preheated and pressed at 85°C for 30 seconds to 2 minutes to bond the separator to the positive electrode and the silicon-carbon negative electrode to obtain the cell.

[0080] The following are some specific examples. Where %, it refers to a percentage by weight. It should be noted that the following examples do not exhaustively list all possible scenarios, and unless otherwise specified, the materials used in the following examples are commercially available.

[0081] Example 1 (1) Pulping: (1.1) 96.14 kg of high-capacity high-compact graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3 A conductive buffer confinement slurry with an inner layer was obtained by mixing 1.5 kg of SWCNT (single-walled carbon nanotubes), 1.0 kg of CMC (sodium carboxymethyl cellulose), and 0.51 kg of SBR (styrene-butadiene rubber) with a specific capacity of 360 mAh / g (stirring speed 30 r / min, time 180 min). (1.2) 96.76 kg of high-capacity silicon-carbon (manufacturer: high-capacity cubic meters, model: Q700, compaction density: 1.5 g / cm³) was compacted. 3A high-capacity silicon-carbon slurry was prepared by mixing 0.8 kg of CMC (sodium carboxymethyl cellulose), 0.05 kg of SWCNT (single-walled carbon nanotubes), 0.2 kg of SP (conductive carbon black), 0.3 kg of SBR (styrene-butadiene rubber), and 0.5 kg of PAA (polyacrylic acid) at a stirring speed of 30 r / min for 180 min. (1.3) 96.14 kg of high-capacity high-compact graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3 2.0 kg SP (conductive carbon black), 1.2 kg CMC (sodium carboxymethyl cellulose), and 0.8 kg SBR (styrene-butadiene rubber) were mixed (stirring speed 30 r / min, time 180 min) to obtain an outer conductive buffer constraint slurry; (2) Coating: (2.1) The inner conductive buffer constraint paste is first coated on one side of a 6µm copper foil (the initial coating thickness is 40µm, denoted as a), and then dried for the first time (temperature 100℃, time 3min) to form an inner conductive buffer constraint layer on one side of the copper foil; (2.2) A high-capacity silicon-carbon slurry was coated onto the inner conductive buffer constraint layer (coating thickness of 36 μm, satisfying 0.4 × (a + b)), and then dried twice (temperature 90 °C, time 5 min) to form a high-capacity silicon-carbon active layer; (2.3) The outer conductive buffer constraint slurry is coated on the high-capacity silicon-carbon active layer (coating thickness is 50 μm, denoted as b), and then dried three times (temperature 100℃, time 3 min) to form the outer conductive buffer constraint layer, thus obtaining silicon-carbon anode A; (3) Roller pressing The silicon-carbon anode sheet A was subjected to a first rolling process (pressure 2.5 MPa, rolling speed 8 m / min) to achieve 90% rolling of the silicon-carbon anode sheet A; after a 2-hour settling period, a second rolling process (pressure 3.8 MPa, rolling speed 10 m / min) was performed to achieve 100% rolling of the silicon-carbon anode sheet A, resulting in a silicon-carbon anode sheet (with a porosity of 35% for the inner conductive buffer constraint layer and 25% for the outer conductive buffer constraint layer).

[0082] Example 2 (1) Pulping: (1.1) 97 kg of high-capacity high-compact graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3A conductive buffer confinement slurry with an inner layer was obtained by mixing 3.0 kg of SWCNT (single-walled carbon nanotubes), 1.25 kg of CMC (sodium carboxymethyl cellulose), and 1.51 kg of SBR (styrene-butadiene rubber) with a specific capacity of 360 mAh / g (stirring speed 30 r / min, time 180 min). (1.2) 96.76 kg of high-capacity silicon-carbon (manufacturer: high-capacity cubic meters, model: Q700, compaction density: 1.5 g / cm³) was compacted. 3 A high-capacity silicon-carbon slurry was obtained by mixing 1.0 kg CMC (sodium carboxymethyl cellulose), 0.05 kg SWCNT (single-walled carbon nanotubes), 0.25 kg SP (conductive carbon black), 0.5 kg SBR (styrene-butadiene rubber), and 1.0 kg PAA (polyacrylic acid) at a stirring speed of 30 r / min for 180 min. (1.3) 97 kg of high-capacity high-pressure compacted graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3 3.5 kg SP (conductive carbon black), 1.2 kg CMC (sodium carboxymethyl cellulose), and 1.6 kg SBR (styrene-butadiene rubber) were mixed (stirring speed 30 r / min, time 180 min) to obtain an outer conductive buffer constraint slurry; (2) Coating: (2.1) The inner conductive buffer constraint paste is first coated on one side of a 6µm copper foil (the initial coating thickness is 50µm, denoted as a), and then dried for the first time (temperature 100℃, time 3min) to form an inner conductive buffer constraint layer on one side of the copper foil; (2.2) A high-capacity silicon-carbon slurry was coated onto the inner conductive buffer constraint layer (coating thickness of 44 μm, satisfying 0.4 × (a + b)), and then dried twice (temperature 90 °C, time 5 min) to form a high-capacity silicon-carbon active layer; (2.3) The outer conductive buffer constraint slurry is coated on the high-capacity silicon-carbon active layer (coating thickness is 60 μm, denoted as b), and then dried three times (temperature 100℃, time 3 min) to form the outer conductive buffer constraint layer, thus obtaining silicon-carbon anode A; (3) Roller pressing The silicon-carbon anode sheet A was subjected to a first rolling process (pressure 3.0 MPa, rolling speed 25 m / min) to achieve 90% rolling of the silicon-carbon anode sheet A; after a 2-hour settling interval, a second rolling process was performed (pressure 5.0 MPa, rolling speed 20 m / min) to achieve 100% rolling of the silicon-carbon anode sheet A, resulting in a silicon-carbon anode sheet (with a porosity of 40% for the inner conductive buffer constraint layer and 30% for the outer conductive buffer constraint layer).

[0083] Example 3 (1) Pulping: (1.1) 94.0 kg of high-capacity high-compact graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3 A conductive buffer confinement slurry with an inner layer was obtained by mixing 1.6 kg of SWCNT (single-walled carbon nanotubes), 1.25 kg of CMC (sodium carboxymethyl cellulose), and 0.25 kg of SBR (styrene-butadiene rubber) with a specific capacity of 360 mAh / g (stirring speed 30 r / min, time 180 min). (1.2) 92.0 kg of high-capacity silicon-carbon (manufacturer: high-capacity cubic meters, model: Q700, compaction density: 1.5 g / cm³) was compacted. 3 A high-capacity silicon-carbon slurry was obtained by mixing 0.5 kg CMC (sodium carboxymethyl cellulose), 0.05 kg SWCNT (single-walled carbon nanotubes), 0.10 kg SP (conductive carbon black), 0.1 kg SBR (styrene-butadiene rubber), and 0.4 kg PAA (polyacrylic acid) at a stirring speed of 30 r / min for 180 min. (1.3) 94.0 kg of high-capacity high-compact graphite (manufacturer: Zichen, model: G49, compaction density: 1.7 g / cm³) was compacted. 3 (Specific capacity 360mAh / g); 1.8kg SP (conductive carbon black), 1.2kg CMC (sodium carboxymethyl cellulose), and 0.3kg SBR (styrene-butadiene rubber) were mixed (stirring speed 30r / min, time 180min) to obtain an outer conductive buffer constraint slurry; (2) Coating: (2.1) The inner conductive buffer constraint paste is first coated on one side of a 6µm copper foil (the initial coating thickness is 40µm, denoted as a), and then dried for the first time (temperature 100℃, time 3min) to form an inner conductive buffer constraint layer on one side of the copper foil; (2.2) A high-capacity silicon-carbon slurry was coated onto the inner conductive buffer constraint layer (coating thickness of 32 μm, satisfying 0.4 × (a + b)), and then dried twice (temperature 90 °C, time 5 min) to form a high-capacity silicon-carbon active layer; (2.3) The outer conductive buffer constraint slurry is coated on the high-capacity silicon-carbon active layer (coating thickness is 40 μm, denoted as b), and then dried three times (temperature 100℃, time 3 min) to form the outer conductive buffer constraint layer, thus obtaining silicon-carbon anode sheet A; (3) Roller pressing The silicon-carbon anode sheet A is subjected to a first rolling process (pressure 2.5 MPa, rolling speed 5 m / min) to achieve 90% rolling of the silicon-carbon anode sheet A; after a 2-hour settling interval, a second rolling process is performed (pressure 3.0 MPa, rolling speed 5 m / min) to achieve 100% rolling of the silicon-carbon anode sheet A, resulting in a silicon-carbon anode sheet (with a porosity of 30% for the inner conductive buffer constraint layer and 20% for the outer conductive buffer constraint layer).

[0084] Comparative Example 1 The difference from Example 1 is that step (2.3) is omitted, while the rest remains the same.

[0085] Comparative Example 2 The difference from Example 1 is that step (2.1) is omitted, while the rest remains the same.

[0086] Comparative Example 3 The difference from Example 1 is that the coating thickness of 44um in step (2.2) is replaced with a coating thickness of 66um to satisfy 0.6×(a+b), and the rest remain unchanged.

[0087] Comparative Example 4 The difference from Example 1 is that the coating thickness of 44um in step (2.2) is replaced with a coating thickness of 22um to satisfy 0.2×(a+b), while the rest remain unchanged.

[0088] Comparative Example 5 The difference from Example 1 is that the first rolling (pressure 3.0 MPa, rolling speed 35 m / min) and the second rolling (pressure 5.0 MPa, rolling speed 20 m / min) in step (3) are replaced with the first rolling (pressure 0.5 MPa, rolling speed 8 m / min) and the second rolling (pressure 1.0 MPa, rolling speed 8 m / min). The resulting silicon-carbon anode sheet has a porosity of 45% for the inner conductive buffer constraint layer and 35% for the outer conductive buffer constraint layer. The rest remains unchanged.

[0089] Comparative Example 6 The difference from Example 1 is that the first rolling (pressure 3.0 MPa, rolling speed 35 m / min) and the second rolling (pressure 5.0 MPa, rolling speed 20 m / min) in step (3) are replaced with the first rolling (pressure 4.5 MPa, rolling speed 2 m / min) and the second rolling (pressure 7 MPa, rolling speed 5 m / min). The resulting silicon-carbon anode sheet has a porosity of 15% for the inner conductive buffer constraint layer and 10% for the outer conductive buffer constraint layer. The rest remains unchanged.

[0090] The silicon-carbon negative electrode sheets, lithium cobalt oxide positive electrode sheets, and double-sided aqueous PVDF ceramic separators prepared in the above Examples 1-3 and Comparative Examples 1-6 were wound to form battery cells, and then assembled into cylindrical lithium-ion batteries. The initial Coulomb efficiency, cycle performance, swelling rate of the cylindrical lithium-ion batteries, and the resistance of the silicon-carbon negative electrode sheets before cycling and the swelling condition of the silicon-carbon negative electrode sheets after cycling were detected to obtain the experimental data in the following table: Among them, the detection method for the initial Coulomb efficiency: Electrochemical workstation; The detection method for the cycle performance: Electrochemical workstation; The detection method for the swelling rate: Use a micrometer to measure the thickness of the electrode sheet before and after cycling; The detection method for the resistance of the silicon-carbon negative electrode sheet before cycling: Cut the electrode sheet into a square size of 4 cm × 8 cm, then place the electrode sheet under two probes. The two probes are connected to a resistor through two poles. Rotate the handle of the test device, and the probes are pressed against the electrode sheet by a stable pressure. The pressure is controlled by a pressure gauge. After reaching a certain pressure, read the resistance data of the resistor. This data is the relative value of the electrode sheet resistance; The detection method for the swelling condition of the silicon-carbon negative electrode sheet after cycling: Disassemble the cylindrical lithium-ion battery and visually observe the swelling condition of both sides of the silicon-carbon negative electrode sheet and calculate the swelling rate.

[0091] Table 1 As can be seen from the above table, in Examples 1-3, by reasonably setting the structures and components of each layer of the outer conductive buffer constraint layer, high-capacity silicon-carbon active layer, and inner conductive buffer constraint layer, the exposed outer conductive buffer constraint layer can form a thin and uniform SEI film, effectively ensuring the stability of the SEI film interface. And the rigid protective shell formed by the outer conductive buffer constraint layer and the inner conductive buffer constraint layer can better limit the high-capacity silicon-carbon active layer. At the same time, with the porosity of the outer conductive buffer constraint layer being 20%-30%, the porosity of the inner conductive buffer constraint layer being 30%-40%, and the thickness of the high-capacity silicon-carbon active layer satisfying the relationship Ι: 0.3×(a + b) < y < 0.5×(a + b), to ensure that the formed rigid protective shell can simultaneously meet the dual functions of inhibiting the silicon expansion space and the release space of higher rolling pressure, so as to ensure that the silicon particles in the high-capacity silicon-carbon active layer under higher rolling pressure will not be broken, ensuring the preparation of high-capacity thin silicon-carbon negative electrode sheets, and also reducing the usage amount of the second binder in the high-capacity silicon-carbon active layer, that is, the total usage amount of the second binder in the high-capacity silicon-carbon active layer is reduced to 1.0 part - 2.5 parts. Among them, the comprehensive index of Example 1 is the best. Further, please refer to Figure 1 The physical picture of the silicon-carbon negative electrode sheet disassembled after cycling, showing that there is no obvious deformation and swelling on the surface of the silicon-carbon negative electrode sheet.

[0092] The embodiments described above are merely illustrative of several implementations of this disclosure, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the disclosed patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this disclosure, and these all fall within the protection scope of this disclosure. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A silicon-carbon negative electrode, comprising a current collector and at least one sandwich-type buffer conductive structure, characterized in that... , The sandwich buffer conductive structure includes an inner conductive buffer constraint layer, a high-capacity silicon-carbon active layer, and an outer conductive buffer constraint layer. The inner conductive buffer constraint layer and the high-capacity silicon-carbon active layer are sequentially disposed on the current collector. The outer conductive buffer constraint layer covers the high-capacity silicon-carbon active layer and forms a rigid protective shell with the inner conductive buffer constraint layer. The rigid protective shell is used to reduce the contact reaction rate between the high-capacity silicon-carbon active layer and the electrolyte, and the rigid protective shell is also used to limit the expansion of the high-capacity silicon-carbon active layer. Both the inner conductive buffer constraint layer and the outer conductive buffer constraint layer comprise high-compact, high-capacity graphite, with a compaction density ≥ 1.70 g / cm³. 3 Specific capacity ≥360mAh / g; The porosity of the inner conductive buffer constraint layer is greater than the porosity of the outer conductive buffer constraint layer.

2. The silicon-carbon anode sheet according to claim 1, characterized in that... , The porosity of the inner conductive buffer constraint layer is 30% - 40%.

3. The silicon-carbon anode sheet according to claim 2, characterized in that... , By mass fraction, the inner conductive buffer constraint layer includes the following mass fractions of each material: Graphite 94 parts - 97 parts; The first conductive agent 1.5 parts - 3.0 parts; The first binder 1.5 parts - 3.0 parts.

4. The silicon-carbon anode sheet according to claim 1, characterized in that... , The porosity of the outer conductive buffer constraint layer is 20% - 30%.

5. The silicon-carbon anode sheet according to claim 4, characterized in that... , By mass fraction, the outer conductive buffer constraint layer includes the following mass fractions of each material: Graphite 94 parts - 97 parts; The first conductive agent 1.5 parts - 3.5 parts; The first binder 1.5 parts - 2.8 parts.

6. The silicon-carbon anode sheet according to claim 3 or 5, characterized in that... , The first conductive agent includes at least one of conductive carbon black and single-walled carbon nanotubes; and / or, The first binder includes at least one of sodium carboxymethylcellulose and styrene-butadiene rubber.

7. The silicon-carbon anode sheet according to claim 1, characterized in that... , The high-capacity silicon-carbon active layer includes the following mass fractions of each material: Silicon-carbon 92 parts - 97 parts; The second conductive agent 0.1 parts - 0.3 parts; The second binder 1.0 parts - 2.5 parts; Among them, the second conductive agent includes at least one of conductive carbon black and single-walled carbon nanotubes.

8. The silicon-carbon anode sheet according to claim 1, characterized in that... , The thickness of the high-capacity silicon-carbon active layer satisfies the relational expression Ι: 0.3×(a + b) < y < 0.5×(a + b), where a is the thickness of the inner conductive buffer constraint layer, b is the thickness of the outer conductive buffer constraint layer, and y is the thickness of the high-capacity silicon-carbon active layer.

9. A method for preparing a silicon-carbon negative electrode, characterized in that... , It includes the following steps: Obtain the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry respectively; Coat the inner conductive buffer constraint slurry, the high-capacity silicon-carbon slurry, and the outer conductive buffer constraint slurry on the current collector in sequence to form a sandwich buffer conductive structure, and obtain the silicon-carbon negative electrode sheet A; Perform multiple rollings on the silicon-carbon negative electrode sheet A to obtain the silicon-carbon negative electrode sheet according to any one of claims 1 - 8.

10. A lithium-ion battery, characterized in that... , It includes the silicon-carbon negative electrode sheet according to any one of claims 1 - 8.

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