Silicon negative electrode plate, preparation method thereof and lithium ion battery

Abstract

The invention provides a silicon negative electrode plate, a preparation method thereof and a lithium ion battery. The silicon negative electrode plate comprises a conductive layer and an active layer which are sequentially stacked on the surface of a current collector, the conductive layer is made of a carbon nano tube and graphene, and the mass ratio of the carbon nano tube to the graphene is (0.5-2): 1; and the active layer is made of a silicon negative electrode material. The conductive layer comprising the carbon nanotubes and the graphene is arranged between the silicon-containing active layer and the current collector, and the carbon nanotubes and the graphene form a three-dimensional porous conductive network structure, so that the conductivity of the silicon negative electrode plate is remarkably improved, the stripping force of the silicon negative electrode plate is improved, and the service life of the silicon negative electrode plate is prolonged. The volume change caused by silicon expansion / shrinkage in the charging and discharging process is effectively buffered, and the stripping force, the cyclic expansion rate, the first coulombic efficiency and the cycle performance of the silicon negative plate are synchronously improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, specifically to the negative electrode of lithium-ion batteries, and particularly to a silicon negative electrode sheet, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] The commercial negative electrode material of lithium-ion batteries is mainly graphite (theoretical specific capacity: 372 mAh / g), but its capacity has approached the theoretical limit and it is difficult to meet the requirements for high energy density in fields such as electric vehicles and portable electronic devices. The capacity improvement of other carbon materials (such as hard carbon and soft carbon) or metal oxides (such as TiO2) is limited. Therefore, researchers have turned to materials with higher capacities. Silicon (Si) has become the most promising negative electrode material due to its unique physical and chemical properties. It has an ultra-high theoretical specific capacity of 3579 mAh / g, which is about 10 times that of graphite.

[0003] However, during the use of silicon negative electrodes, there are a series of problems. For example, during the charge and discharge process, the alloying reaction between silicon particles and lithium ions brings a high lithium intercalation amount while accompanied by a volume expansion of up to 300%. Compared with the volume expansion of 12% during lithium intercalation in graphite, the relatively large volume expansion of silicon particles will cause cracks in the electrode sheet, and the active material will fall off from the current collector. In addition, the conductivity of silicon negative electrodes is poor. Silicon is a typical semiconductor material, and its conductivity at room temperature is only 1×10 -3 S / cm, which is far lower than the conductivity of graphite of about 10 3 S / cm. Its material properties themselves result in low electron transfer efficiency. Moreover, with the detachment and intercalation of lithium ions, the repeated expansion / contraction of silicon particles will damage the electrode structure, further exacerbating the charge transfer hysteresis. Also, during the charge and discharge process, the repeated expansion / contraction of silicon negative electrodes will cause the continuous rupture and formation of the negative electrode SEI film, making the electrolyte continuously contact the silicon-carbon negative electrode interface, consuming the electrolyte and generating a thick but uneven SEI film (rich in insulating components such as LiF and Li2CO3), resulting in low Coulomb efficiency and increased side reactions.

[0004] CN119191304A discloses a method for improving the volume expansion effect of silicon-based negative electrodes. In this invention, a porous carbon-coated silicon material is first prepared by an in-situ synthesis method, and then the porous carbon-coated silicon material is prepared into a graphene-coated silicon material by an electrostatic self-assembly method; the graphene-coated silicon material is used as the negative electrode active material. The graphene-coated silicon material prepared by this invention can effectively improve the conductivity of the composite material and inhibit its volume expansion during the charge and discharge process, thereby improving the electrochemical performance.

[0005] CN119742334A discloses a modified silicon anode material, which includes a core and a shell layer. The core includes silicon anode particles and a carbon material layer coated on the surface of the silicon anode particles, and the shell layer is a polyacrylic acid layer, which is coated on the surface of the carbon material layer. This invention can effectively improve the problem of large expansion rate of the silicon anode material, which is beneficial to improving the performance of the battery.

[0006] CN115842129A discloses a polyimide for a silicon-based anode binder of a lithium battery and a silicon-based anode. The polyimide for the silicon-based anode binder of the lithium battery is obtained by polycondensation and imidization of a diamine containing a sulfonic acid group, a diamine containing a linear segment structure and a dianhydride. This invention uses the polyimide to maintain the structural stability of the silicon-based active material, which can effectively improve the expansion problem of the silicon-based anode and improve the cycle performance of the lithium battery.

[0007] Existing technologies usually modify the silicon anode material itself, which has certain limitations. Therefore, it is of great significance to provide a silicon anode application method with universality and industrial application potential. Summary of the Invention

[0008] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a silicon anode sheet, its preparation method and a lithium ion battery. By providing a conductive layer including carbon nanotubes and graphene between the silicon-containing active layer and the current collector, the carbon nanotubes and graphene form a three-dimensional porous conductive network structure, significantly improving the conductivity of the silicon anode sheet. The cycle performance and the first Coulomb efficiency are effectively improved, and the peel strength of the silicon anode sheet is increased, effectively buffering the volume change caused by the expansion / contraction of silicon during charge and discharge. The expansion rate of the silicon anode sheet during the cycle is significantly reduced.

[0009] To achieve the purpose of this invention, the following technical solutions are adopted:

[0010] In the first aspect, the present invention provides a silicon anode sheet, which includes a conductive layer and an active layer stacked in sequence on the surface of the current collector;

[0011] The material of the conductive layer includes carbon nanotubes and graphene, and the mass ratio of the carbon nanotubes to the graphene is (0.5 - 2):1;

[0012] The material of the active layer includes a silicon anode material.

[0013] By providing a conductive layer including carbon nanotubes and graphene between the silicon-containing active layer and the current collector, the present invention forms a continuous in-plane conductive network with graphene, significantly reducing the interfacial resistance of the electrode and optimizing electron transport. Carbon nanotubes build a three-dimensional conductive network in the conductive layer, further improving its electron conductivity. The two cooperate with each other to form a conductive layer with a dense porous structure, significantly enhancing the conductivity of the silicon negative electrode sheet, improving the cycling performance and the initial Coulomb efficiency of the silicon negative electrode sheet. At the same time, the two-dimensional structure formed by graphene can effectively wrap silicon particles to reduce swelling, and the high mechanical strength of carbon nanotubes and the formed three-dimensional conductive network can further disperse the volume stress change of silicon swelling, inhibiting swelling. The two cooperate synergistically, improving the peel strength of the silicon negative electrode sheet and effectively buffering the volume change caused by silicon swelling / shrinking during charge and discharge, and significantly reducing the swelling rate of the silicon negative electrode sheet during cycling.

[0014] By adjusting the mass ratio of carbon nanotubes to graphene to be (0.5 - 2):1, the present invention realizes the synchronous improvement of the peel strength, cyclic swelling rate, initial Coulomb efficiency, and cycling performance of the silicon negative electrode sheet.

[0015] Preferably, the material of the conductive layer further includes a binder, and the mass ratio of the binder to the total mass of carbon nanotubes and graphene is (0.15 - 0.5):1.

[0016] Preferably, the areal density of the conductive layer is 1 mg / cm 2 - 4 mg / cm 2 .

[0017] Preferably, the areal density of the active layer is 5 mg / cm 2 - 10 mg / cm 2 .

[0018] In a second aspect, the present invention provides a method for preparing the silicon negative electrode sheet as described in the first aspect, the preparation method including:

[0019] Coating a conductive layer slurry on the surface of the current collector, and sequentially performing first drying and first rolling to obtain the conductive layer; coating an active layer slurry on the surface of the conductive layer, and sequentially performing first drying and second rolling to obtain the silicon negative electrode sheet;

[0020] The conductive layer slurry includes carbon nanotubes and graphene; the mass ratio of the carbon nanotubes to the graphene is (0.5 - 2):1;

[0021] The active layer slurry includes a silicon negative electrode material.

[0022] In the preparation method provided by the present invention, the first rolling of the conductive layer is beneficial to reducing the thickness of the battery cell, increasing the energy density of the battery cell and improving the electronic conductivity, and can improve the adhesion between the conductive layer with a porous structure composed of graphene and carbon nanotubes and the current collector, helping to form a dense porous structure conductive layer, which is more conducive to buffering the volume change during cycling and suppressing expansion. At the same time, the dense and uniform three-dimensional conductive network can improve the conductive performance.

[0023] By regulating the structure of the silicon negative electrode sheet, according to the specific structural characteristics of the silicon negative electrode sheet, corresponding adjustments are made on the basis of the existing preparation process. Without complex preparation processes, the problems of poor conductivity and high expansion rate of the silicon negative electrode sheet during charge and discharge are effectively improved, and the stripping force, cyclic expansion rate, first Coulomb efficiency and cyclic performance of the silicon negative electrode sheet are synchronously improved.

[0024] Preferably, the pressure of the first rolling is 3 MPa - 10 MPa.

[0025] Preferably, after the first drying and before the first rolling, the thickness of the conductive layer is 10 μm - 20 μm.

[0026] Preferably, the pressure of the second rolling is 5 MPa - 10 MPa.

[0027] Preferably, the second rolling is carried out twice.

[0028] Preferably, after the second rolling, the thickness of the active layer is 60 μm - 130 μm.

[0029] Preferably, the preparation method of the conductive layer slurry includes: performing a first mixing on carbon nanotubes and graphene to obtain a conductive layer powder; and performing a second mixing on the conductive layer powder and a binder solution to obtain the conductive layer slurry.

[0030] Preferably, in the binder solution, the mass ratio of the binder to the mass of the conductive layer powder is (0.15 - 0.5):1.

[0031] Preferably, the first mixing includes a first stirring.

[0032] Preferably, the revolution rate of the first stirring is 15 rpm - 18 rpm, and the dispersion rate is 500 rpm - 1000 rpm.

[0033] Preferably, the time of the first mixing is 0.5 h - 1 h.

[0034] Preferably, the second mixing includes a second stirring.

[0035] Preferably, the revolution speed of the second stirring is 20 rpm - 30 rpm, and the dispersion speed is 2000 rpm - 3000 rpm.

[0036] Preferably, the second mixing is carried out under vacuum.

[0037] Preferably, the time of the second mixing is 5 h - 10 h.

[0038] Preferably, the viscosity of the conductive layer slurry is 2000 mPa·s - 5000 mPa·s.

[0039] Preferably, the preparation method of the active layer slurry includes: performing a third mixing on the negative electrode active material, the conductive agent and the binder to obtain an active layer powder; and performing a fourth mixing on the active layer powder, the binder solution and water to obtain the active layer slurry.

[0040] Preferably, the mass ratio of the binder in the negative electrode active material, the conductive agent and the binder solution is (91 - 96):(1 - 2):(3 - 7).

[0041] Preferably, the third mixing includes a third stirring.

[0042] Preferably, the revolution speed of the third stirring is 15 rpm - 25 rpm, and the dispersion speed is 500 rpm - 1000 rpm.

[0043] Preferably, the time of the third mixing is 0.5 h - 1 h.

[0044] Preferably, the fourth mixing includes a fourth stirring.

[0045] Preferably, the revolution speed of the fourth stirring is 20 rpm - 30 rpm, and the dispersion speed is 2000 rpm - 3000 rpm.

[0046] Preferably, the fourth mixing is carried out under vacuum.

[0047] Preferably, the time of the fourth mixing is 1 h - 5 h.

[0048] Preferably, the viscosity of the active layer slurry is 3000 mPa·s - 8000 mPa·s.

[0049] Preferably, the solid content of the active layer slurry is 40% - 45%.

[0050] In a third aspect, the present invention provides a lithium-ion battery, which includes the silicon negative electrode sheet as described in the first aspect, or includes the silicon negative electrode sheet prepared by the preparation method described in the second aspect.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] (1) By providing a conductive layer including carbon nanotubes and graphene between the silicon-containing active layer and the current collector, graphene forms a continuous in-plane conductive network, and carbon nanotubes construct a three-dimensional conductive network in the conductive layer. The two cooperate with each other to form a conductive layer with a dense porous structure, realizing the synchronous improvement of the peel force, cyclic swelling rate, initial Coulomb efficiency, and cycling performance of the silicon negative electrode sheet.

[0053] (2) By regulating the structure of the silicon negative electrode sheet and making corresponding adjustments based on the specific structural characteristics of the silicon negative electrode sheet on the basis of the existing preparation process, without complex preparation processes, the problems of poor conductivity and high swelling rate of the silicon negative electrode sheet during charge and discharge are effectively improved, realizing the synchronous improvement of the peel force, cyclic swelling rate, initial Coulomb efficiency, and cycling performance of the silicon negative electrode sheet. Specific Embodiments

[0054] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in this application are intended to cover non-exclusive inclusion.

[0056] In the description of this application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order, or primary-secondary relationship of the indicated technical features. In the description of the embodiments of this application, "a plurality" means more than two unless otherwise specifically defined.

[0057] In a specific embodiment, the present invention provides a silicon negative electrode sheet, which includes a conductive layer and an active layer sequentially stacked on the surface of a current collector;

[0058] The material of the conductive layer includes carbon nanotubes and graphene, and the mass ratio of the carbon nanotubes to graphene is (0.5 - 2):1;

[0059] The material of the active layer includes a silicon negative electrode material.

[0060] In the present invention, a conductive layer including carbon nanotubes and graphene is provided between the silicon-containing active layer and the current collector. The graphene forms a continuous in-plane conductive network, significantly reducing the interfacial resistance of the electrode and optimizing electron transport. The carbon nanotubes construct a three-dimensional conductive network in the conductive layer, further improving its electron conductivity. The two cooperate with each other to form a conductive layer with a dense porous structure, significantly enhancing the conductivity of the silicon negative electrode sheet and improving the cycling performance and first Coulombic efficiency of the silicon negative electrode sheet. At the same time, the two-dimensional structure formed by the graphene can effectively wrap the silicon particles to reduce swelling, and the high mechanical strength of the carbon nanotubes and the formed three-dimensional conductive network can further disperse the volume stress change of the silicon expansion, inhibiting the expansion. The two cooperate synergistically, resulting in an increase in the peel force of the silicon negative electrode sheet and effectively buffering the volume change caused by the silicon expansion / contraction during the charge and discharge process. The expansion rate of the silicon negative electrode sheet during the cycling process significantly decreases.

[0061] In the present invention, by regulating the mass ratio of carbon nanotubes to graphene, the synchronous improvement of the peel force, cyclic expansion rate, first Coulombic efficiency, and cycling performance of the silicon negative electrode sheet is achieved. In the conductive layer, the mass ratio of carbon nanotubes to graphene is (0.5 - 2):1. For example, it can be 0.5:1, 0.7:1, 0.9:1, 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, or 2:1, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0062] In the present invention, the type of the current collector is not particularly limited and can be reasonably selected according to the application scenario of the silicon negative electrode sheet. Exemplarily, the current collector can be a copper current collector, an aluminum current collector, or a composite current collector.

[0063] In some embodiments, the conductive layer further includes a binder. The ratio of the mass of the binder to the total mass of carbon nanotubes and graphene is (0.15 - 0.5):1. For example, it can be 0.15:1, 0.2:1, 0.25:1, 0.3:1, 0.35:1, 0.4:1, 0.45:1, or 0.5:1, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0064] In the silicon negative electrode sheet provided by the present invention, the type of the binder in the conductive layer is not particularly limited, and any binder applicable to lithium-ion batteries can be selected. Exemplarily, the binder in the conductive layer can be any one or a combination of at least two of CMC (sodium carboxymethyl cellulose), SBR (styrene-butadiene rubber), PVDF (polyvinylidene fluoride), or PAA (polyacrylic acid), and PAA is preferred.

[0065] The higher the areal density of the conductive layer, the better the improvement effect on the conductivity and peel strength of the silicon negative electrode sheet and the overall battery cell, and the better the improvement effect on the electronic conductivity, cycle retention rate, and cycle expansion rate. However, if the areal density of the conductive layer is too high, it will instead cause a decrease in the capacity of the silicon negative electrode sheet. At the same time, the too-high areal density of the negative electrode sheet will increase the thickness of the electrode sheet, thereby reducing its electronic conductivity, resulting in a decrease in the volume energy density and mass energy density of the battery cell based on the silicon negative electrode sheet of the present invention.

[0066] In some embodiments, the areal density of the conductive layer is 1 mg / cm 2 -4 mg / cm 2 , for example, it can be 1 mg / cm 2 , 1.2 mg / cm 2 , 1.4 mg / cm 2 , 1.6 mg / cm 2 , 1.8 mg / cm 2 , 2 mg / cm 2 , 2.2 mg / cm 2 , 2.4 mg / cm 2 , 2.6 mg / cm 2 , 2.8 mg / cm 2 , 3 mg / cm 2 , 3.2 mg / cm 2 , 3.4 mg / cm 2 , 3.6 mg / cm 2 , 3.8 mg / cm 2 or 4 mg / cm 2 , including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable. Preferably, it is 2.5 mg / cm 2 -3 mg / cm 2 .

[0067] In some embodiments, the areal density of the active layer is 5 mg / cm 2 -15 mg / cm 2 , for example, it can be 5 mg / cm 2 , 6 mg / cm 2 , 7 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , 11 mg / cm 2 , 13 mg / cm 2 , 14 mg / cm 2 or 15 mg / cm 2 , including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0068] In another specific embodiment, the present invention provides a method for preparing a silicon negative electrode sheet as described in the foregoing specific embodiment, and the preparation method includes:

[0069] Coating a conductive layer slurry on the surface of the current collector, and sequentially performing first drying and first rolling to obtain the conductive layer; coating an active layer slurry on the surface of the conductive layer, and sequentially performing first drying and second rolling to obtain the silicon negative electrode sheet;

[0070] The conductive layer slurry includes carbon nanotubes and graphene; the mass ratio of the carbon nanotubes to the graphene is (0.5 - 2):1;

[0071] The active layer slurry includes a silicon negative electrode material.

[0072] In the preparation method provided by the present invention, performing the first rolling on the conductive layer is beneficial to reducing the thickness of the battery cell, improving the energy density of the battery cell and the electronic conductivity, and can improve the adhesion between the conductive layer with a porous structure composed of graphene and carbon nanotubes and the current collector, helping to form a tight porous structure conductive layer, which is more conducive to buffering the volume change during cycling and suppressing expansion. At the same time, the tight and uniform three-dimensional conductive network can improve the conductive performance.

[0073] By regulating the structure of the silicon negative electrode sheet, according to the specific structural characteristics of the silicon negative electrode sheet, corresponding adjustments are made on the basis of the existing preparation process. Without complex preparation processes, the problems of poor conductivity and high expansion rate of the silicon negative electrode sheet during charge and discharge are effectively improved, and the simultaneous improvement of the peel strength, cyclic expansion rate, first Coulomb efficiency, and cycling performance of the silicon negative electrode sheet is achieved.

[0074] In some embodiments, the pressure of the first rolling is 3 MPa - 10 MPa, for example, it can be 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, or 10 MPa, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0075] In some embodiments, after the first drying and before the first rolling, the thickness of the conductive layer is 10 μm - 20 μm, for example, it can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0076] In some embodiments, the pressure of the second rolling is 5 MPa - 10 MPa, for example, it can be 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, or 10 MPa, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0077] In some embodiments, the second rolling is performed twice.

[0078] By subjecting the silicon negative electrode sheet after coating the active layer to the second rolling, the present invention is conducive to improving the adhesion between the porous conductive layer and the upper silicon-carbon active material layer, enhancing the encapsulation of silicon particles, effectively dispersing the volume stress change of silicon expansion, suppressing expansion, reducing the expansion rate during cycling, and improving the cycling performance.

[0079] In some embodiments, after the second rolling, the thickness of the active layer is 60 μm - 100 μm, and can be, for example, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm or 100 μm, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0080] In some embodiments, the temperature of the first drying is 80°C - 100°C, and can be, for example, 80°C, 85°C, 90°C, 95°C or 100°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable. During the first drying process, circulating air is also applied to accelerate the volatilization of the solvent, and the frequency of the circulating air is 20 Hz - 30 Hz, and can be, for example, 20 Hz, 22 Hz, 24 Hz, 26 Hz, 28 Hz or 30 Hz.

[0081] In some embodiments, the temperature of the first drying is 80°C - 110°C, and can be, for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C or 110°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable. During the first drying process, circulating air is also applied to accelerate the volatilization of the solvent, and the frequency of the circulating air is 25 Hz - 30 Hz, and can be, for example, 21 Hz, 22 Hz, 23 Hz, 24 Hz or 25 Hz.

[0082] In some embodiments, the method for preparing the conductive layer slurry includes: performing a first mixing of carbon nanotubes and graphene to obtain a conductive layer powder; and performing a second mixing of the conductive layer powder and a binder solution to obtain the conductive layer slurry.

[0083] In some embodiments, in the binder solution, the mass ratio of the binder to the conductive layer powder is (0.15 - 0.5):1. For example, it can be 0.15:1, 0.2:1, 0.25:1, 0.3:1, 0.35:1, 0.4:1, 0.45:1 or 0.5:1, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable. The solvent of the binder solution includes water or NMP. The present invention does not make special limitations and can be reasonably selected according to the type of binder.

[0084] In some embodiments, the first mixing includes a first stirring.

[0085] In some embodiments, the revolution rate of the first stirring is 15 rpm - 18 rpm. For example, it can be 15 rpm, 16 rpm, 17 rpm or 18 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable. The dispersion rate is 500 rpm - 1000 rpm. For example, it can be 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm or 1000 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0086] In some embodiments, the time of the first mixing is 0.5 h - 1 h. For example, it can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h or 1 h, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0087] In some embodiments, the second mixing includes a second stirring.

[0088] In some embodiments, the revolution rate of the second stirring is 20 rpm - 30 rpm. For example, it can be 20 rpm, 22 rpm, 24 rpm, 26 rpm, 28 rpm or 30 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable. The dispersion rate is 2000 rpm - 3000 rpm. For example, it can be 2000 rpm, 2200 rpm, 2400 rpm, 2600 rpm, 2800 rpm or 3000 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0089] In some embodiments, the second mixing is carried out under vacuum.

[0090] In some embodiments, the time of the second mixing is 5 h - 10 h, for example, it can be 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0091] In some embodiments, during the second mixing, circulating water is also turned on to control the temperature of the slurry at 20°C - 25°C.

[0092] In some embodiments, the viscosity of the conductive layer slurry is 2000 mPa·s - 5000 mPa·s, for example, it can be 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s or 5000 mPa·s, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0093] In some embodiments, the method for preparing the active layer slurry includes: performing a third mixing on the negative electrode active material, the conductive agent and the binder to obtain an active layer powder; and performing a fourth mixing on the active layer powder, the binder solution and water to obtain the active layer slurry.

[0094] In some embodiments, the mass ratio of the binder in the negative electrode active material, the conductive agent and the binder solution is (91 - 96):(1 - 2):(3 - 7), for example, it can be 91:2:7, 92:1.8:6.2, 93:1.6:5.4, 94:1.4:4.6, 95:1.2:3.8, 96:1:3, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0095] In the silicon negative electrode sheet provided by the present invention, the types of the conductive agent and the binder in the active layer are not particularly limited, and the conductive agent and the binder applied in the lithium ion battery can be selected. Exemplarily, the binder in the active layer can be any one or a combination of at least two of SP (conductive carbon black), carbon nanotubes or graphene. The binder in the active layer can be any one or a combination of at least two of CMC (sodium carboxymethyl cellulose), SBR (styrene-butadiene rubber), PVDF (polyvinylidene fluoride) or PAA (polyacrylic acid), and preferably PAA. The solvent of the binder solution includes water or NMP, which is not particularly limited in the present invention and can be reasonably selected according to the type of the binder.

[0096] In some embodiments, the third mixing includes a third stirring.

[0097] In some embodiments, the revolution rate of the third stirring is 15 rpm - 25 rpm. For example, it can be 15 rpm, 16 rpm, 17 rpm, 18 rpm, 19 rpm, 20 rpm, 21 rpm, 22 rpm, 23 rpm, 24 rpm or 25 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable. The dispersion rate is 500 rpm - 1000 rpm. For example, it can be 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm or 1000 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0098] In some embodiments, the time of the third mixing is 0.5 h - 1 h. For example, it can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h or 1 h, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0099] In some embodiments, the fourth mixing includes a fourth stirring.

[0100] In some embodiments, the revolution rate of the fourth stirring is 20 rpm - 30 rpm. For example, it can be 20 rpm, 22 rpm, 24 rpm, 26 rpm, 28 rpm or 30 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable. The dispersion rate is 2000 rpm - 3000 rpm. For example, it can be 2000 rpm, 2200 rpm, 2400 rpm, 2600 rpm, 2800 rpm or 3000 rpm, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0101] In some embodiments, the fourth mixing is carried out under vacuum.

[0102] In some embodiments, the time of the fourth mixing is 1 h - 5 h. For example, it can be 1 h, 2 h, 3 h, 4 h or 5 h, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0103] In some embodiments, during the second mixing, circulating water is also turned on to control the slurry temperature at 20°C - 25°C.

[0104] In some embodiments, the viscosity of the active layer slurry is 3000 mPa·s - 8000 mPa·s. For example, it can be 3000 mPa·s, 4000 mPa·s, 5000 mPa·s, 6000 mPa·s, 7000 mPa·s, or 8000 mPa·s, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0105] In some embodiments, the solid content of the active layer slurry is 40% - 45%. For example, it can be 40%, 41%, 42%, 43%, 44%, or 45%, including but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0106] In yet another embodiment, the present invention provides a lithium-ion battery, which includes the silicon negative electrode sheet as described in the previous embodiment, or includes a silicon negative electrode sheet prepared by the preparation method described in another specific embodiment.

[0107] Example 1

[0108] This example provides a silicon negative electrode sheet, which includes a conductive layer and an active layer sequentially disposed on the surface of the current collector; the areal density of the conductive layer is 2 mg / cm 2 , the mass ratio of carbon nanotubes to graphene in the conductive layer is 1.5:1, and the mass ratio of PAA to the total mass of carbon nanotubes and graphene is 0.3:1; the areal density of the active layer is 8 mg / cm 2 , in the active layer, by mass, graphite: silicon: SP: CMC: PAA is 88:5:1.5:1.5:4.

[0109] The preparation method of the silicon negative electrode sheet includes:

[0110] (1) Prepare the conductive layer slurry: Mix carbon nanotubes and graphene according to the mass ratio by a double planetary mixer, and then stir at a revolution rate of 16 rpm and a dispersion rate of 800 rpm for 0.8 h. After stirring is completed, scrape the mixer wall and the stirring paddle, add PAA according to the mass ratio, and add water. Turn on the vacuum and the circulating water, control the temperature of the conductive layer slurry within a certain range, and stir at a revolution rate of 25 rpm and a dispersion rate of 2500 rpm for 7 h. After stirring is completed, test its viscosity to be 3000 mPa·s.

[0111] (2) Preparation of the active layer slurry: Mix graphite, silicon, and SP according to the mass ratio using a double planetary mixer, and then stir at a revolution speed of 20 rpm and a dispersion speed of 700 rpm for 0.75 h. After stirring, scrape the mixer wall and the stirring paddle, add CMC and PAA according to the mass ratio, and add water to control the solid content at 43%. Turn on the vacuum and the circulating water, control the temperature of the conductive layer slurry within a certain range, and stir at a revolution speed of 25 rpm and a dispersion speed of 2500 rpm for 3.5 h. After stirring, test its viscosity to be 5000 mPa·s.

[0112] (3) Preparation of the conductive layer: Coat the conductive layer slurry prepared in step (1) on the surface of the copper current collector, and dry it under the conditions of 90°C and 25 Hz circulating air. After drying, the coating thickness is 15 μm, and roll pressing is carried out under a pressure of 7 MPa to obtain the conductive layer.

[0113] (4) Preparation of the silicon negative electrode sheet: Coat the active layer slurry prepared in step (2) on the surface of the conductive layer prepared in step (3), and dry it under the conditions of 95°C and 28 Hz circulating air. Roll press twice under a pressure of 8 MPa. After roll pressing, the thickness of the active layer is 80 μm to obtain the silicon negative electrode sheet.

[0114] Example 2

[0115] This example provides a silicon negative electrode sheet, which includes a conductive layer and an active layer arranged in sequence on the surface of the current collector; the surface density of the conductive layer is 1 mg / cm 2 , and the mass ratio of carbon nanotubes to graphene in the conductive layer is 0.5:1, and the mass ratio of PAA to the total mass of carbon nanotubes and graphene is 0.15:1; the surface density of the active layer is 5 mg / cm 2 , and in the active layer, by mass, graphite:silicon:SP:CMC:SBR is 90:1:2:2:5.

[0116] The preparation method of the silicon negative electrode sheet includes:

[0117] (1) Preparation of the conductive layer slurry: Mix carbon nanotubes and graphene according to the mass ratio using a double planetary mixer, and then stir at a revolution speed of 15 rpm and a dispersion speed of 500 rpm for 0.5 h. After stirring, scrape the mixer wall and the stirring paddle, add PAA according to the mass ratio, and add water. Turn on the vacuum and the circulating water, control the temperature of the conductive layer slurry within a certain range, and stir at a revolution speed of 20 rpm and a dispersion speed of 2000 rpm for 7 h. After stirring, test its viscosity to be 2000 mPa·s.

[0118] (2) Preparation of the active layer slurry: Mix graphite, silicon, and SP according to the mass ratio using a double planetary mixer, and then stir at a revolution speed of 15 rpm and a dispersion speed of 500 rpm for 0.5 h. After stirring, scrape the mixer wall and the stirring paddle, add CMC according to the mass ratio, and add water. Control the solid content at 40%. Turn on the vacuum and the circulating water, control the temperature of the conductive layer slurry within a certain range, stir at a revolution speed of 20 rpm and a dispersion speed of 2200 rpm for 1 h, add SBR, and continue stirring for 0.5 h. After stirring, test its viscosity to be 3000 mPa·s.

[0119] (3) Preparation of the conductive layer: Coat the conductive layer slurry prepared in step (1) on the surface of the copper current collector, and dry it under the condition of circulating air at 80°C and 20 Hz. After drying, the coating thickness is 10 μm, and roll press it under a pressure of 3 MPa to obtain the conductive layer.

[0120] (4) Preparation of the silicon negative electrode sheet: Coat the active layer slurry prepared in step (2) on the surface of the conductive layer prepared in step (3), and dry it under the condition of circulating air at 85°C and 25 Hz. Roll press it twice under a pressure of 5 MPa. After rolling, the thickness of the active layer is 60 μm to obtain the silicon negative electrode sheet.

[0121] Example 3

[0122] This example provides a silicon negative electrode sheet, which includes a conductive layer and an active layer arranged in sequence on the surface of the current collector; the surface density of the conductive layer is 4 mg / cm 2 , and the mass ratio of carbon nanotubes to graphene in the conductive layer is 2:1, and the mass ratio of PAA to the total mass of carbon nanotubes and graphene is 0.5:1; the surface density of the active layer is 10 mg / cm 2 , and in the active layer, by mass, graphite:silicon:SP:CMC:PAA is 86:10:1:1:2.

[0123] The preparation method of the silicon negative electrode sheet includes:

[0124] (1) Preparation of the conductive layer slurry: Mix carbon nanotubes and graphene according to the mass ratio using a double planetary mixer, and then stir at a revolution speed of 18 rpm and a dispersion speed of 1000 rpm for 1 h. After stirring, scrape the mixer wall and the stirring paddle, add PAA according to the mass ratio, and add water. Turn on the vacuum and the circulating water, control the temperature of the conductive layer slurry within a certain range, stir at a revolution speed of 30 rpm and a dispersion speed of 3000 rpm for 10 h. After stirring, test its viscosity to be 5000 mPa·s.

[0125] (2) Preparation of the active layer slurry: Mix graphite, silicon, and SP in a mass ratio using a double planetary mixer, and then stir at a revolution rate of 25 rpm and a dispersion rate of 1000 rpm for 1 h. After stirring, scrape the walls of the mixer and the stirring paddle, add CMC and PAA according to the mass ratio, and add water to control the solid content at 45%. Turn on the vacuum and circulating water, control the temperature of the conductive layer slurry within a certain range, and stir at a revolution rate of 30 rpm and a dispersion rate of 3000 rpm for 5 h. After stirring, measure its viscosity to be 8000 mPa·s.

[0126] (3) Preparation of the conductive layer: Coat the conductive layer slurry prepared in step (1) on the surface of the copper current collector, and dry it under the conditions of circulating air at 100 °C and 30 Hz. After drying, the coating thickness is 20 μm, and roll pressing is carried out under a pressure of 10 MPa to obtain the conductive layer.

[0127] (4) Preparation of the silicon negative electrode sheet: Coat the active layer slurry prepared in step (2) on the surface of the conductive layer prepared in step (3), dry it under the conditions of circulating air at 110 °C and 30 Hz, and roll press it twice under a pressure of 10 MPa. After roll pressing, the thickness of the active layer is 100 μm to obtain the silicon negative electrode sheet.

[0128] Example 4

[0129] This example provides a silicon negative electrode sheet. Compared with Example 1, except that the surface density of the conductive layer is 0.5 mg / cm 2 otherwise, the rest are the same as in Example 1.

[0130] Example 5

[0131] This example provides a silicon negative electrode sheet. Compared with Example 1, except that the surface density of the conductive layer is 5 mg / cm 2 otherwise, the rest are the same as in Example 1.

[0132] Example 6

[0133] This example provides a silicon negative electrode sheet. Compared with Example 1, for the silicon negative electrode sheet, except that during the preparation process, during the process of preparing the conductive layer in step (3), the roll pressing pressure is 2 MPa, the rest are the same as in Example 1.

[0134] Example 7

[0135] This example provides a silicon negative electrode sheet. Compared with Example 1, for the silicon negative electrode sheet, except that during the preparation process, during the process of preparing the conductive layer in step (3), the roll pressing pressure is 12 MPa, the rest are the same as in Example 1.

[0136] Example 8

[0137] This embodiment provides a silicon negative electrode sheet. Compared with Embodiment 1, except that in the preparation process, during the preparation of the conductive layer in step (3), the first rolling is not performed, the rest are the same as in Embodiment 1.

[0138] Embodiment 9

[0139] This embodiment provides a silicon negative electrode sheet. Compared with Embodiment 1, except that in the preparation process, after coating the active layer slurry in step (4), only one rolling is performed, the rest are the same as in Embodiment 1.

[0140] Comparative Example 1

[0141] This comparative example provides a silicon negative electrode sheet. Compared with Embodiment 1, except that it does not have a conductive layer, the rest are the same as in Embodiment 1.

[0142] Comparative Example 2

[0143] This comparative example provides a silicon negative electrode sheet. Compared with Embodiment 1, except that the mass ratio of carbon nanotubes to graphene in the conductive layer is 0.3:1, the rest are the same as in Embodiment 1.

[0144] Comparative Example 3

[0145] This comparative example provides a silicon negative electrode sheet. Compared with Embodiment 1, except that the mass ratio of carbon nanotubes to graphene in the conductive layer is 2.5:1, the rest are the same as in Embodiment 1.

[0146] Performance test:

[0147] First, test the peel strength of the silicon negative electrode sheets provided in all the above embodiments and comparative examples. The test method is as follows:

[0148] First, stick the tape (such as 3M Scotch tape) on the surface of the negative electrode coating, and roll it back and forth with a roller for 5 - 10 minutes to ensure complete contact and no bubbles; then use a universal material testing machine to peel the tape along the 180° direction at a constant rate (usually 50 mm / min), and record the maximum force value during the peeling process.

[0149] Then, match the silicon negative electrode sheets provided in all the above embodiments and comparative examples with a lithium cobalt oxide positive electrode sheet with a voltage of 4.45 V or more to assemble a battery. Charge and discharge are tested under the test regime at 25°C. The charge and discharge process includes: constant current and constant voltage charging at 1C to 4.45 V, and then constant current discharging at 1C to 2 V, and test the initial discharge specific capacity, first Coulomb efficiency, 100 - cycle capacity retention rate, and 100 - cycle expansion rate.

[0150] The test results of the peel strength, initial discharge specific capacity, first Coulomb efficiency, 100 - cycle capacity retention rate, and 100 - cycle expansion rate are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] According to the test data in Table 1, compared with Comparative Example 1, in Examples 1 to 9, by providing a conductive layer including carbon nanotubes and graphene between the silicon-containing active layer and the current collector, and regulating the mass ratio of carbon nanotubes to graphene, the peel force, cyclic swelling rate, first Coulomb efficiency, and cyclic performance of the silicon negative electrode sheet were all synchronously improved.

[0155] According to the test data of Examples 4 - 5 and Example 1, if the areal density of the conductive layer is too small, the conductivity of the silicon negative electrode sheet cannot be effectively improved, and the expansion of silicon particles cannot be inhibited; if the areal density of the conductive layer is too large, the thickness of the silicon negative electrode sheet will increase, the conductivity will decrease, and the energy density of the battery will decrease, that is, whether the areal density of the conductive layer is too large or too small, the synchronous improvement of the peel force, cyclic swelling rate, first Coulomb efficiency, and cyclic performance of the silicon negative electrode sheet cannot be achieved.

[0156] According to the test data of Examples 6 - 8 and Example 1, if the conductive layer is not subjected to the first rolling or the pressure of the first rolling is too large or too small, a dense and uniform porous structure conductive layer cannot be obtained, the electronic conductivity cannot be effectively improved, the volume change during cycling cannot be buffered to inhibit expansion, resulting in a decrease in the performance of the battery.

[0157] According to the test data of Example 9 and Example 1, if the active layer is only rolled once, the adhesion between the conductive layer and the active layer cannot be effectively improved, the expansion of silicon particles cannot be effectively inhibited, resulting in a poor peel force of the silicon negative electrode sheet, and the problem of the active layer peeling off from the current collector cannot be effectively improved.

[0158] According to the test data of Comparative Examples 2 - 3 and Example 1, if the mass ratio of carbon nanotubes to graphene is too large or too small, the continuous in-plane conductive network formed by graphene and the three-dimensional conductive network constructed by carbon nanotubes in the conductive layer cannot effectively cooperate with each other, and the synchronous improvement of the peel force, cyclic swelling rate, first Coulomb efficiency, and cyclic performance of the silicon negative electrode sheet cannot be achieved.

[0159] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A silicon negative electrode sheet, characterized in that, The silicon negative electrode sheet includes a conductive layer and an active layer that are sequentially stacked on the surface of the current collector; The material of the conductive layer includes carbon nanotubes and graphene, and the mass ratio of the carbon nanotubes to the graphene is (0.5 - 2):1; The material of the active layer includes a silicon negative electrode material.

2. The silicon negative electrode sheet according to claim 1, wherein, The material of the conductive layer further includes a binder, and the mass ratio of the binder to the total mass of the carbon nanotubes and graphene is (0.15 - 0.5):

1.

3. The silicon negative electrode sheet according to claim 1 or 2, characterized in that The areal density of the conductive layer is 1 mg / cm 2 - 4 mg / cm 2 ; and / or, the areal density of the active layer is 5 mg / cm 2 - 15 mg / cm 2 .

4. A method for preparing the silicon negative electrode sheet according to any one of claims 1-3, characterized in that, The preparation method includes: Coating a conductive layer slurry on the surface of the current collector, and sequentially performing first drying and first rolling to obtain the conductive layer; coating an active layer slurry on the surface of the conductive layer, and sequentially performing first drying and second rolling to obtain the silicon negative electrode sheet; The conductive layer slurry includes carbon nanotubes and graphene; the mass ratio of the carbon nanotubes to the graphene is (0.5 - 2):1; The active layer slurry includes a silicon negative electrode material.

5. The preparation method according to claim 4, characterized in that, The pressure of the first rolling is 3 MPa - 10 MPa; And / or, the pressure of the second rolling is 5 MPa - 10 MPa; And / or, the second rolling is performed twice; And / or, after the second rolling, the thickness of the active layer is 60 μm - 130 μm.

6. The preparation method according to claim 4, characterized in that, The preparation method of the conductive layer slurry includes: Performing first mixing on the carbon nanotubes and graphene to obtain a conductive layer powder; performing second mixing on the conductive layer powder and a binder solution to obtain the conductive layer slurry.

7. The preparation method according to claim 6, characterized in that, In the binder solution, the mass ratio of the binder to the mass of the conductive layer powder is (0.15 - 0.5):1; And / or, the first mixing includes first stirring; the revolution rate of the first stirring is 15 rpm - 18 rpm, and the dispersion rate is 500 rpm - 1000 rpm; And / or, the time of the first mixing is 0.5 h - 1 h; And / or, the second mixing includes second stirring; the revolution rate of the second stirring is 20 rpm - 30 rpm, and the dispersion rate is 2000 rpm - 3000 rpm; And / or, the second mixing is performed under vacuum; And / or, the time of the second mixing is 5 h - 10 h; And / or, the viscosity of the conductive layer slurry is 2000 mPa·s - 5000 mPa·s.

8. The preparation method according to claim 4, characterized in that, The preparation method of the active layer slurry includes: Performing third mixing on the negative electrode active material, the conductive agent and the binder to obtain an active layer powder; performing fourth mixing on the active layer powder, the binder solution and water to obtain the active layer slurry.

9. The preparation method according to claim 8, wherein The mass ratio of the negative electrode active material, the conductive agent to the binder in the binder solution is (91 - 96):(1 - 2):(3 - 7); And / or, the third mixing includes third stirring; the revolution rate of the third stirring is 15 rpm - 25 rpm, and the dispersion rate is 500 rpm - 1000 rpm; And / or, the time of the third mixing is 0.5 h - 1 h; And / or, the fourth mixing includes fourth stirring; the revolution rate of the fourth stirring is 20 rpm - 30 rpm, and the dispersion rate is 2000 rpm - 3000 rpm; And / or, the fourth mixing is performed under vacuum; And / or, the time of the fourth mixing is 1 h - 5 h; And / or, the viscosity of the active layer slurry is 3000 mPa·s - 8000 mPa·s; And / or, the solid content of the active layer slurry is 40% - 45%.

10. A lithium-ion battery, characterized in that, The lithium ion battery includes the silicon negative electrode sheet according to any one of claims 1 - 3, or the silicon negative electrode sheet prepared by the preparation method according to any one of claims 4 - 9.

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