Negative electrode sheet, method for manufacturing the same, lithium ion battery, and electric device

Abstract

This application relates to a negative electrode sheet and its preparation method, a lithium-ion battery, and an electrical device. The negative electrode sheet includes a current collector and a negative electrode active layer covering at least one surface of the current collector; the negative electrode active layer includes an intermediate layer and an outer layer sequentially stacked on the surface of the current collector; the intermediate layer is made of silicon with a silicon mass percentage greater than 50%, and the outer layer is made of a first conductive carbon material. In the negative electrode active layer, the intermediate layer is mainly made of silicon, which can fully utilize the advantage of silicon's high specific energy, giving the negative electrode sheet a higher overall energy density; the outer layer is made of a first conductive carbon material, which can effectively suppress the formation of an unstable solid electrolyte interface layer by electrolyte reaction, thus delaying battery capacity decay.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a negative electrode sheet and its preparation method, a lithium-ion battery, and an electrical device. Background Technology

[0002] Traditional lithium-ion batteries typically use graphite as the anode material, but graphite anode materials have a low energy density, reaching only 374 mAh·g. -1 This cannot meet the growing market demand for high-energy-density batteries. Research shows that each silicon atom can accommodate 4.4 lithium atoms to form L... i22 Si5 has an energy density as high as 4200 mAh·g -1 Silicon is a promising anode material. However, during lithium-ion battery cycling, silicon anodes readily react with the electrolyte to form an unstable solid electrolyte interface layer, leading to increased interfacial impedance between the anode and the electrolyte and accelerating battery capacity decay. Summary of the Invention

[0003] Based on this, this application provides a negative electrode sheet that can effectively suppress the formation of an unstable solid electrolyte interface layer by electrolyte reaction, and can delay the capacity decay of the battery, as well as its preparation method, lithium-ion battery, and power device.

[0004] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active layer covering at least one surface of the current collector;

[0005] The negative electrode active layer comprises an intermediate layer and an outer layer sequentially stacked on the surface of the current collector;

[0006] The intermediate layer is made of silicon and the mass percentage of silicon is greater than 50%, and the outer layer is made of a first conductive carbon material.

[0007] Optionally, the intermediate layer is made of a mixture of silicon and a first conductive carbon material.

[0008] Optionally, the first conductive carbon material includes at least one of graphite, carbon nanotubes, and graphene.

[0009] Optionally, the intermediate layer has at least two layers, and as the distance from the surface of the current collector increases, the mass percentage of silicon in each intermediate layer gradually decreases, while the mass percentage of the first conductive carbon material gradually increases.

[0010] Optionally, the negative electrode active layer further includes an inner layer, which is disposed between the current collector and the intermediate layer. The inner layer is made of a second conductive carbon material and the mass percentage of the second conductive carbon material is greater than 50%.

[0011] Optionally, the inner layer is made of a mixture of silicon and a second conductive carbon material.

[0012] Optionally, the second conductive carbon material includes at least one of graphite, carbon nanotubes, and graphene.

[0013] Optionally, the thickness ratio of the inner layer, the middle layer, and the outer layer is (0.1~0.5):(0.1~5):(0.1~0.5).

[0014] Optionally, the total thickness of the negative electrode active layer is 10 nm to 900 nm.

[0015] Optionally, the total thickness of the negative electrode active layer is 300nm~900nm.

[0016] Optionally, the thickness of the current collector is 1 μm to 100 μm.

[0017] Optionally, the thickness of the current collector is 10 μm to 100 μm.

[0018] Optionally, the current collector may be made of copper and / or stainless steel.

[0019] Secondly, this application provides a method for preparing a negative electrode sheet as described in the first aspect above, comprising the following steps:

[0020] The negative electrode active layer is formed on the surface of the current collector.

[0021] Optionally, the negative electrode active layer is formed on the surface of the current collector by physical vapor deposition.

[0022] Optionally, the conditions for forming the intermediate layer by physical vapor deposition include:

[0023] (1) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A;

[0024] (2) The coating time for each intermediate layer is 0.1h to 24h;

[0025] And / or, the conditions for forming the outer layer by physical vapor deposition include:

[0026] (1) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A;

[0027] (2) The coating time is 0.1h~24h.

[0028] Optionally, the intermediate layer is a mixture of silicon and a first conductive carbon material, and the conditions for forming the intermediate layer by physical vapor deposition further include:

[0029] (1) The silicon target and the first conductive carbon material target are deposited simultaneously, and the deposition bias voltage of the silicon target is higher than that of the first conductive carbon material target.

[0030] (2) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A.

[0031] Optionally, the negative electrode active layer further includes an inner layer, wherein the material of the inner layer comprises a second conductive carbon material and the mass percentage of the second conductive carbon material is greater than 50%, and the conditions for forming the inner layer by physical vapor deposition include:

[0032] (1) The second conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A;

[0033] (2) The coating time is 0.1h~24h.

[0034] Optionally, the inner layer is a mixture of silicon and a second conductive carbon material, and the conditions for forming the inner layer by physical vapor deposition further include:

[0035] (1) The silicon target and the second conductive carbon target are deposited simultaneously, and the deposition bias voltage of the second conductive carbon target is higher than that of the silicon target.

[0036] (2) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A.

[0037] Thirdly, this application also provides a lithium-ion battery comprising a negative electrode as described in the first aspect above.

[0038] Fourthly, this application also provides an electrical device comprising the lithium-ion battery described in the third aspect above.

[0039] The negative electrode sheet provided in this application has a negative electrode active layer covering the current collector. The negative electrode active layer includes a middle layer and an outer layer stacked sequentially. The outer layer is located on the side furthest from the current collector and is made of a first conductive carbon material. It is used for direct contact with the electrolyte and can effectively suppress the formation of an unstable solid electrolyte interface layer by electrolyte reaction, avoiding an increase in the interface impedance between the negative electrode and the electrolyte, thereby helping to reduce battery capacity decay. The middle layer is located between the current collector and the outer layer and is mainly made of silicon. It can fully utilize the high specific energy of silicon, giving the negative electrode sheet a higher overall energy density. Detailed Implementation

[0040] To facilitate understanding of this application, a more comprehensive description of the application will be provided below with reference to embodiments. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0041] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0043] The terms “first,” “second,” “third,” “fourth,” etc. (if present), as used in this application, are used to distinguish similar objects and are for descriptive purposes only. They are not necessarily used to describe a specific order or sequence, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as “first” or “second” may explicitly or implicitly include at least one of that feature. In the description of this application, “multiple” means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0044] In this application, terms such as "preferred," "better," "more suitable," and "ideal" are used only to describe implementation methods or embodiments with better effects, and should be understood not to limit the scope of protection of this application.

[0045] In this application, terms such as "further," "even more," and "particularly" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0046] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active layer covering at least one surface of the current collector;

[0047] The negative electrode active layer comprises an intermediate layer and an outer layer sequentially stacked on the surface of the current collector;

[0048] The middle layer is made of silicon, with silicon accounting for more than 50% of the mass, and the outer layer is made of a first conductive carbon material.

[0049] The negative electrode sheet provided in this application has a negative electrode active layer covering the current collector. The negative electrode active layer includes a middle layer and an outer layer stacked sequentially. The outer layer is located on the side furthest from the current collector and is made of a first conductive carbon material. It is used for direct contact with the electrolyte and can effectively suppress the formation of an unstable solid electrolyte interface layer by electrolyte reaction, avoiding an increase in the interface impedance between the negative electrode and the electrolyte, thereby helping to reduce battery capacity decay. The middle layer is located between the current collector and the outer layer and is mainly made of silicon. It can fully utilize the high specific energy of silicon, giving the negative electrode sheet a higher overall energy density.

[0050] In some embodiments, the intermediate layer is made of a mixture of silicon and a first conductive carbon material. The inventors discovered that while using silicon as the primary material for the intermediate layer can significantly improve the battery's energy density, the silicon undergoes significant volume expansion during lithium-ion insertion and extraction from the silicon anode. This expansion can lead to silicon pulverization failure and affect its contact with the current collector or electrolyte, with volume expansion reaching up to 300%. Therefore, a mixture of silicon and a first conductive carbon material is used as the intermediate layer material. The first conductive carbon material has a relatively loose structure and a large deformation capacity, providing ample expansion space for the silicon volume expansion. This helps suppress silicon pulverization failure and improves its contact with the current collector or electrolyte.

[0051] In some embodiments, the first conductive carbon material includes at least one selected from graphite, carbon nanotubes, and graphene. Preferably, the first conductive carbon material is graphite.

[0052] In some embodiments, the intermediate layer comprises at least two layers. As the distance from the current collector surface increases, the mass percentage of silicon in each intermediate layer gradually decreases, while the mass percentage of the first conductive carbon material gradually increases. As the distance from the current collector surface increases, the intermediate layer gradually approaches the outer layer, which is in direct contact with the electrolyte. With the wetting of the material, the electrolyte gradually permeates from its interface with the outer layer into the interior of the outer layer and even into the intermediate layer. From the current collector surface towards the outer layer, the mass percentage of the first conductive carbon material in the mixture of silicon and the first conductive carbon material in each intermediate layer gradually increases in a gradient. This helps to further suppress the reaction between silicon and the electrolyte to form an unstable solid electrolyte interface layer, thereby reducing the interface impedance between the negative electrode and the electrolyte and slowing down battery capacity decay. Furthermore, the gradual increase in the mass percentage of the first conductive carbon material in the mixture of silicon and the first conductive carbon material results in greater electronic conductivity, which is beneficial for improving the alloying process of lithium ion insertion or extraction from silicon in the negative electrode.

[0053] In some embodiments, the negative electrode active layer further includes an inner layer disposed between the current collector and the intermediate layer. The inner layer is made of a second conductive carbon material, and the mass percentage of the second conductive carbon material is greater than 50%. As the layer that is in direct contact with the surface of the current collector, the inner layer, whose material is mainly composed of the second conductive carbon material, is beneficial to improving the bonding performance between the negative electrode active layer and the current collector, preventing the negative electrode active layer from detaching from the current collector. Moreover, the second conductive carbon material has good electronic conductivity, and direct contact with the current collector further enhances the conductivity of the negative electrode.

[0054] In some embodiments, the inner layer is a mixture of silicon and a second conductive carbon material. In addition to carbon, the inner layer also contains silicon, which helps to improve the high energy density of the battery. Furthermore, the presence of carbon effectively suppresses silicon pulverization failure, thereby improving the contact capability between the inner layer and the current collector.

[0055] In some embodiments, the second conductive carbon material includes at least one of graphite, carbon nanotubes, and graphene. Preferably, the second conductive carbon material is graphite.

[0056] In some embodiments, the thickness ratio of the inner layer, the middle layer, and the outer layer is (0.1~0.5):(0.1~5):(0.1~0.5). Controlling the thickness ratio of the inner layer, the middle layer, and the outer layer within the above-mentioned suitable range is beneficial to fully utilize the function of each layer and produce the best overall effect.

[0057] In some embodiments, the total thickness of the negative electrode active layer is 10 nm to 900 nm. Controlling the total thickness of the negative electrode active layer within this range ensures that the negative electrode has a high specific capacity and maintains a high capacity retention rate during lithium battery cycling. Furthermore, this thickness can be achieved in the manufacturing process. Understandably, within the above thickness range, a larger thickness results in a higher energy density for the negative electrode, but also increases the manufacturing difficulty. Understandably, the total thickness of the negative electrode active layer can be, for example, but not limited to, 10 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, etc. Preferably, the total thickness of the negative electrode active layer is 300 nm to 900 nm.

[0058] In some embodiments, the thickness of the current collector is 1 μm to 100 μm. If the current collector thickness is too small, the manufacturing process is difficult, but the current collector does not contribute to battery activity. If the thickness is too large, it will reduce the overall energy density of the negative electrode. Therefore, it is preferable to control the thickness of the current collector within the range of 1 μm to 100 μm. Understandably, the thickness of the current collector can be, for example, but not limited to, 1 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, etc. Preferably, the thickness of the current collector is 10 μm to 100 μm.

[0059] In some embodiments, the current collector is made of copper and / or stainless steel. Both copper and stainless steel, as conductive metals, have good conductivity, ensuring good conductivity when used as the current collector. Preferably, copper is the conductive metal. It is understood that in other embodiments, the current collector may also include other conductive polymers or other materials.

[0060] Secondly, this application provides a method for preparing a negative electrode sheet as described in any of the embodiments of the first aspect above, comprising the following steps:

[0061] A negative electrode active layer is formed on the surface of the current collector.

[0062] Understandably, in this application, the negative electrode active layer can be formed on the surface of the current collector in any way, such as ball milling, co-precipitation, deposition, etc. The negative electrode active layer formed in any way can enable the negative electrode sheet to have both high specific capacity and high capacity retention rate during lithium battery cycling.

[0063] In some embodiments, a negative electrode active layer is formed on the surface of the current collector using physical vapor deposition. The inventors discovered that negative electrode active layers prepared by traditional methods such as ball milling and co-precipitation have poor bonding between silicon and other composite materials such as conductive carbon materials in the negative electrode active layer, thus affecting the Li... +During the insertion / extraction process, the reciprocating expansion of silicon in the negative electrode can easily cause the interface between silicon and other composite materials to peel off, leading to a sharp decrease in capacity. Furthermore, the negative electrode active layer prepared by the above methods contains a low amount of silicon material with high specific energy, resulting in a lower actual energy density of the negative electrode. In contrast, physical vapor deposition (PVD), compared to other traditional methods such as ball milling and co-precipitation, allows for more uniform and dense material deposition within the layer, and also results in stronger interlayer bonding. For example, for an intermediate layer composed of a mixture of silicon and a first conductive carbon material, PVD ensures a more uniform and well-bonded mixture of silicon and the first conductive carbon material, making it less prone to separation due to silicon's reciprocating expansion. For instance, at the interface between the intermediate and outer layers, the strong bonding between them allows for a tight bond, preventing peeling. Moreover, PVD can produce a negative electrode active layer with a high silicon content, resulting in a negative electrode with a higher energy density.

[0064] In some embodiments, the conditions for forming the intermediate layer by physical vapor deposition include:

[0065] (1) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A.

[0066] (2) The coating time for each intermediate layer is 0.1h~24h.

[0067] Furthermore, the intermediate layer is a mixture of silicon and a first conductive carbon material, and the conditions for forming the intermediate layer by physical vapor deposition also include:

[0068] (1) The silicon target and the first conductive carbon target are deposited simultaneously, and the deposition bias voltage of the silicon target is higher than that of the first conductive carbon target.

[0069] (2) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A.

[0070] Under the above conditions, the mass ratio of silicon and the first conductive carbon material can be effectively controlled. Understandably, when there are at least two intermediate layers, the intermediate layers of each layer are deposited sequentially in a direction gradually moving away from the current collector surface, and the physical vapor deposition process conditions for each layer can be adjusted as needed.

[0071] In some embodiments, the conditions for forming the outer layer by physical vapor deposition include:

[0072] (1) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A.

[0073] (2) The coating time is 0.1h~24h.

[0074] In some embodiments, the negative electrode active layer further includes an inner layer, the inner layer being composed of a second conductive carbon material with a mass percentage greater than 50%, and the conditions for forming the inner layer by physical vapor deposition include:

[0075] (1) The second conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A.

[0076] (2) The coating time is 0.1h~24h.

[0077] Furthermore, the inner layer is a mixture of silicon and a second conductive carbon material, and the conditions for forming the inner layer by physical vapor deposition also include:

[0078] (1) The silicon target and the second conductive carbon target are deposited simultaneously, and the deposition bias voltage of the second conductive carbon target is higher than that of the silicon target.

[0079] (2) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A.

[0080] Under the above conditions, parameter control can effectively control the mass ratio of silicon and the second conductive carbon material, so that the second conductive carbon material is dominant in the mixture of silicon and the second conductive carbon material in the inner layer.

[0081] Furthermore, the conditions for physical vapor deposition also include: first, evacuating the vacuum chamber to a background vacuum of 10... -8 ~0.1Pa, after introducing Ar gas, the pressure is controlled at 10. -3 Pa ~ 10 Pa.

[0082] Thirdly, this application also provides a lithium-ion battery, including the negative electrode sheet as described in any of the embodiments of the first aspect above.

[0083] Fourthly, this application also provides an electrical device comprising the lithium-ion battery described in the third aspect above. The electrical device may include, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc.

[0084] The following are specific examples.

[0085] Example 1

[0086] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and prepare a graphite target with a carbon content of 99.99% as the first and second conductive carbon material targets; polish the surface of a 10µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the background vacuum of the vacuum chamber to less than 1×10⁻⁶. -2 Pa, introduce Ar gas and control the pressure at 0.1 Pa, then turn on the arc ion plating power supply.

[0087] (1) Forming the inner layer of the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 55V, and the deposition current to 15A; set the graphite target deposition bias voltage to 100V, the deposition voltage to 100V, and the deposition current to 20A.

[0088] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 6 hours, forming a 120nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 32% and 68%, respectively.

[0089] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 270V, the deposition voltage to 55V, and the deposition current to 15A; set the graphite target deposition bias voltage to 50V, the deposition voltage to 100V, and the deposition current to 20A.

[0090] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 14h, forming a first intermediate layer with a thickness of 570nm. The mass ratio of silicon and graphite in the first intermediate layer is 87% and 13%, respectively.

[0091] (3) Forming the second intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 300V, the deposition voltage to 55V, and the deposition current to 15A; set the graphite target deposition bias voltage to 100V, the deposition voltage to 100V, and the deposition current to 20A.

[0092] By means of physical vapor deposition, silicon target and graphite target are simultaneously deposited on the surface of the first intermediate layer formed in step (2) for coating. The coating time is 6 hours to form a second intermediate layer with a thickness of 160 nm. The mass ratio of silicon and graphite in the second intermediate layer is 69% and 31%, respectively.

[0093] (4) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 50V, the deposition voltage to 100V, and the deposition current to 20A;

[0094] The graphite target was deposited onto the surface of the second intermediate layer formed in step (3) by physical vapor deposition for 1.5 hours to form an outer layer with a thickness of 40 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0095] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 890nm.

[0096] Example 2

[0097] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of an 80µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2×10⁻⁶. -5 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0098] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 320V, the deposition voltage is 60V, and the deposition current is 10A; the graphite target deposition bias voltage is set to 500V, the deposition voltage is 105V, and the deposition current is 25A.

[0099] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 1 hour, forming a 30nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 44% and 56%, respectively.

[0100] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 60V, and the deposition current to 10A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 105V, and the deposition current to 25A.

[0101] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 24h, forming a first intermediate layer with a thickness of 610nm. The mass ratio of silicon and graphite in the first intermediate layer is 77% and 23%, respectively.

[0102] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 200V, the deposition voltage to 100V, and the deposition current to 15A;

[0103] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 4 hours to form an outer layer with a thickness of 105 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0104] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 745nm.

[0105] Example 3

[0106] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and prepare a graphite target with a carbon content of 99.99% as the first and second conductive carbon material targets; polish the surface of a 20µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the background vacuum of the vacuum chamber to less than 5 × 10⁻⁶. -8 Pa, introduce Ar gas and control the pressure at 0.2 Pa, then turn on the arc ion plating power supply:

[0107] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 100V, the deposition voltage is 55V, and the deposition current is 20A; the graphite target deposition bias voltage is set to 420V, the deposition voltage is 105V, and the deposition current is 15A.

[0108] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 0.1 h, forming an inner layer with a thickness of 8 nm. The mass percentages of silicon and graphite in the inner layer were 21% and 79%, respectively.

[0109] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 500V, the deposition voltage to 55V, and the deposition current to 20A; set the graphite target deposition bias voltage to 10V, the deposition voltage to 105V, and the deposition current to 15A.

[0110] By physical vapor deposition, silicon target and graphite target are simultaneously deposited onto the surface of the inner layer formed in step (1) for coating. The coating time is 10h, forming a first intermediate layer with a thickness of 330nm. The mass ratio of silicon and graphite in the first intermediate layer is 96% and 4%, respectively.

[0111] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 300V, the deposition voltage to 100V, and the deposition current to 20A;

[0112] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 1.5 hours to form an outer layer with a thickness of 35 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0113] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 373nm.

[0114] Example 4

[0115] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of a 60µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to less than 0.1 Pa, introduce Ar gas and control the pressure at 5 × 10⁻⁶ Pa. -3 Pa, turn on the arc ion plating power supply:

[0116] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 200V, the deposition voltage is 58V, and the deposition current is 16A; the graphite target deposition bias voltage is set to 450V, the deposition voltage is 95V, and the deposition current is 20A.

[0117] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 3 hours, forming a 170nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 38% and 62%, respectively.

[0118] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 58V, and the deposition current to 16A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 95V, and the deposition current to 20A.

[0119] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 17h, forming a first intermediate layer with a thickness of 430nm. The mass ratio of silicon and graphite in the first intermediate layer is 80% and 20%, respectively.

[0120] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 50V, the deposition voltage to 100V, and the deposition current to 15A;

[0121] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 2 hours to form an outer layer with a thickness of 60 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0122] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 660nm.

[0123] Example 5

[0124] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and prepare a graphite target with a carbon content of 99.99% as the first and second conductive carbon material targets; polish the surface of a 30µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2 × 10⁻⁶. -8 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0125] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 60V, the deposition voltage is 55V, and the deposition current is 15A; the graphite target deposition bias voltage is set to 150V, the deposition voltage is 102V, and the deposition current is 15A.

[0126] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 0.5 hours, forming a 20 nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 41% and 59%, respectively.

[0127] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 200V, the deposition voltage to 55V, and the deposition current to 20A; set the graphite target deposition bias voltage to 50V, the deposition voltage to 95V, and the deposition current to 20A.

[0128] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 3h, forming a first intermediate layer with a thickness of 100nm. The mass ratio of silicon and graphite in the first intermediate layer is 59% and 41%, respectively.

[0129] (3) Forming the second intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 500V, the deposition voltage to 60V, and the deposition current to 20A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 95V, and the deposition current to 15A.

[0130] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the first intermediate layer formed in step (2) for coating. The coating time is 10h, forming a second intermediate layer with a thickness of 310nm. The mass ratio of silicon and graphite in the second intermediate layer is 96% and 4%, respectively.

[0131] (4) Forming the third intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 400V, the deposition voltage to 60V, and the deposition current to 15A; set the graphite target deposition bias voltage to 10V, the deposition voltage to 95V, and the deposition current to 20A.

[0132] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the second intermediate layer formed in step (3) for coating. The coating time is 6 hours, forming a first intermediate layer with a thickness of 205nm. The mass ratio of silicon and graphite in the first intermediate layer is 89% and 11%, respectively.

[0133] (5) Forming the fourth intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 300V, the deposition voltage to 55V, and the deposition current to 15A; set the graphite target deposition bias voltage to 50V, the deposition voltage to 100V, and the deposition current to 20A.

[0134] By means of physical vapor deposition, silicon target and graphite target are simultaneously deposited on the surface of the third intermediate layer formed in step (4) for coating. The coating time is 2 hours, forming a second intermediate layer with a thickness of 130 nm. The mass ratio of silicon and graphite in the second intermediate layer is 74% and 26%, respectively.

[0135] (6) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 50V, the deposition voltage to 100V, and the deposition current to 20A;

[0136] The graphite target was deposited onto the surface of the fourth intermediate layer formed in step (5) by physical vapor deposition for 1 hour to form an outer layer with a thickness of 25 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0137] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 790nm.

[0138] Example 6

[0139] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of a 100µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to less than 0.1Pa, introduce Ar gas and control the pressure at 10Pa, and turn on the arc ion plating power supply:

[0140] (1) Forming the inner layer of the negative electrode active layer: Set the silicon target deposition bias voltage to 0V, the deposition voltage to 59V, and the deposition current to 18A; set the graphite target deposition bias voltage to 500V, the deposition voltage to 105V, and the deposition current to 23A.

[0141] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 0.3 hours, forming a 15nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 7% and 93%, respectively.

[0142] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 310V, the deposition voltage to 55V, and the deposition current to 15A; set the graphite target deposition bias voltage to 40V, the deposition voltage to 100V, and the deposition current to 15A.

[0143] By physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 21h, forming a first intermediate layer with a thickness of 590nm. The mass ratio of silicon and graphite in the first intermediate layer is 84% ​​and 16%, respectively.

[0144] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 100V, the deposition voltage to 100V, and the deposition current to 20A;

[0145] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 6 hours to form an outer layer with a thickness of 180 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0146] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 785nm.

[0147] Example 7

[0148] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of an 80µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2×10⁻⁶. -5 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0149] (1) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 60V, and the deposition current to 10A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 105V, and the deposition current to 25A.

[0150] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 24h, forming a first intermediate layer with a thickness of 610nm. The mass ratio of silicon and graphite in the first intermediate layer is 77% and 23%, respectively.

[0151] (2) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 200V, the deposition voltage to 100V, and the deposition current to 15A;

[0152] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 4 hours to form an outer layer with a thickness of 105 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0153] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 715nm.

[0154] Example 8

[0155] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of an 80µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2×10⁻⁶. -5 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0156] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 470V, the deposition voltage is 60V, and the deposition current is 10A; the graphite target deposition bias voltage is set to 120V, the deposition voltage is 105V, and the deposition current is 25A.

[0157] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 2 hours, forming an inner layer with a thickness of 80 nm. The mass percentages of silicon and graphite in the inner layer were 71% and 29%, respectively.

[0158] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 60V, and the deposition current to 10A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 105V, and the deposition current to 25A.

[0159] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 24h, forming a first intermediate layer with a thickness of 610nm. The mass ratio of silicon and graphite in the first intermediate layer is 77% and 23%, respectively.

[0160] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 200V, the deposition voltage to 100V, and the deposition current to 15A;

[0161] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 4 hours to form an outer layer with a thickness of 105 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0162] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 795nm.

[0163] Comparative Example 1

[0164] Prepare a silicon target with a silicon content of 99.99%; polish the surface of a 10µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 1×10⁻⁶. -2 Pa, introduce Ar gas and control the pressure at 0.1 Pa, then turn on the arc ion plating power supply.

[0165] The silicon target deposition bias voltage was set to 50V, the deposition voltage to 55V, and the deposition current to 15A. The silicon target was deposited onto the surface of the copper foil current collector by physical vapor deposition for 20 hours. After the deposition was completed, the coated product was removed from the physical vapor deposition equipment to obtain a silicon anode with a thickness of 900nm.

[0166] Comparative Example 2

[0167] Prepare a graphite target with a carbon content of 99.99%; polish the surface of a 10µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 1×10⁻⁶. -2 Pa, introduce Ar gas and control the pressure at 0.1 Pa, then turn on the arc ion plating power supply.

[0168] The graphite target deposition bias voltage was set to 50V, the deposition voltage to 100V, and the deposition current to 20A. The graphite target was deposited onto the surface of the copper foil current collector by physical vapor deposition for 27 hours. After the deposition was completed, the coated product was removed from the physical vapor deposition equipment to obtain a graphite anode with a thickness of 900nm.

[0169] Comparative Example 3

[0170] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of an 80µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2×10⁻⁶. -5 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0171] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 320V, the deposition voltage is 60V, and the deposition current is 10A; the graphite target deposition bias voltage is set to 500V, the deposition voltage is 105V, and the deposition current is 25A.

[0172] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 1 hour, forming a 30nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 44% and 56%, respectively.

[0173] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 50V, the deposition voltage to 60V, and the deposition current to 10A; set the graphite target deposition bias voltage to 0V, the deposition voltage to 105V, and the deposition current to 25A.

[0174] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 24h, forming a first intermediate layer with a thickness of 610nm. The mass ratio of silicon and graphite in the first intermediate layer is 77% and 23%, respectively.

[0175] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 640nm.

[0176] Comparative Example 4

[0177] Prepare a silicon target with a silicon content of 99.99%; both the first and second conductive carbon materials are graphite, and a graphite target with a carbon content of 99.99% is prepared as the first and second conductive carbon material targets; polish the surface of an 80µm thick copper foil current collector; place the copper foil current collector in a physical vapor deposition apparatus, and turn on the mechanical pump and molecular pump to evacuate the vacuum chamber to a level less than 2×10⁻⁶. -5 Pa, introduce Ar gas and control the pressure at 10. -3 Pa, turn on the arc ion plating power supply:

[0178] (1) Forming the inner layer of the negative electrode active layer: The silicon target deposition bias voltage is set to 320V, the deposition voltage is 60V, and the deposition current is 10A; the graphite target deposition bias voltage is set to 500V, the deposition voltage is 105V, and the deposition current is 25A.

[0179] Using physical vapor deposition, silicon and graphite targets were simultaneously deposited onto the surface of a copper foil current collector for coating. The coating time was 1 hour, forming a 30nm thick inner layer. The mass percentages of silicon and graphite in the inner layer were 44% and 56%, respectively.

[0180] (2) Forming the first intermediate layer in the negative electrode active layer: Set the silicon target deposition bias voltage to 10V, the deposition voltage to 60V, and the deposition current to 10A; set the graphite target deposition bias voltage to 100V, the deposition voltage to 105V, and the deposition current to 25A.

[0181] By means of physical vapor deposition, silicon target and graphite target are deposited simultaneously on the surface of the inner layer formed in step (1) for coating. The coating time is 24h, forming a first intermediate layer with a thickness of 600nm. The mass ratio of silicon and graphite in the first intermediate layer is 31% and 69%, respectively.

[0182] (3) Forming the outer layer in the negative electrode active layer: Set the graphite target deposition bias voltage to 200V, the deposition voltage to 100V, and the deposition current to 15A;

[0183] The graphite target was deposited onto the surface of the first intermediate layer formed in step (2) by physical vapor deposition for 4 hours to form an outer layer with a thickness of 105 nm. The mass percentages of silicon and graphite in the outer layer were 0% and 100%, respectively.

[0184] After the coating is completed, the coated product is removed from the physical vapor deposition equipment to obtain a negative electrode sheet with a medium thickness of 735nm.

[0185] The negative electrode sheets prepared in Examples 1 to 8 and Comparative Examples 1 to 4 were assembled into coin-type lithium-ion batteries, and the cycle performance of the batteries was tested at 0.1C and 0.5C. The test results are shown in Table 1 below.

[0186] Table 1 Battery performance test results

[0187]

[0188] As shown in Table 1, compared to Comparative Examples 1-4, the negative electrode sheets prepared in Examples 1-8, when assembled into coin-type lithium-ion batteries, exhibit superior overall performance. Furthermore:

[0189] Compared with Comparative Examples 1 and 2, Example 1 uses copper foil current collectors of the same specifications. The negative electrode active layer of Example 1 consists of an inner layer, a middle layer, and an outer layer stacked sequentially. The outer layer is made of graphite, which is used for direct contact with the electrolyte. This effectively suppresses the formation of an unstable solid electrolyte interface layer due to electrolyte reaction, avoiding an increase in the impedance at the negative electrode and electrolyte interface, thus reducing battery capacity decay. The middle layer is mainly made of silicon, which has high specific energy and higher energy density. Therefore, Example 1 exhibits superior overall electrical performance, including first-cycle discharge specific capacity, first-cycle efficiency, and cycle retention rate. Comparative Example 1's negative electrode active layer only includes high-energy silicon. Although it has a high first-cycle discharge specific capacity, it is more prone to capacity decay, resulting in low capacity retention after 50 cycles. Comparative Example 2's negative electrode active layer only includes graphite, which effectively suppresses the formation of an unstable solid electrolyte interface layer due to electrolyte reaction. While it maintains good battery capacity, its energy density is very low.

[0190] Compared to Example 2, Example 7 did not have an inner layer, therefore the battery performance was lower than that of Example 2. Compared to Example 2, although Example 8 had an inner layer, the graphite mass ratio of the inner layer was less than 50%, and the inner layer thickness of Comparative Example 3 was thicker than that of Example 2. Therefore, the first-cycle efficiency and capacity retention rate after 50 cycles of the battery in Comparative Example 3 were also somewhat reduced.

[0191] Compared with Example 2, Comparative Example 4 did not have a 100% graphite outer layer. The silicon in the middle layer would come into direct contact with the electrolyte, forming an unstable solid electrolyte interface layer. This increased the impedance at the interface between the negative electrode and the electrolyte. As a result, the battery capacity of Comparative Example 4 decayed faster and had a low capacity retention rate after 50 cycles.

[0192] Compared with Example 2, Comparative Example 5 has a lower silicon content than 50% in the intermediate layer material, resulting in an excessively low silicon content in the negative electrode sheet, thus leading to a low specific capacity of the battery.

[0193] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0194] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector and a negative electrode active layer covering at least one surface of the current collector; The negative electrode active layer includes an intermediate layer and an outer layer sequentially stacked on the surface of the current collector, and the negative electrode active layer also includes an inner layer disposed between the current collector and the intermediate layer; The inner layer is a mixture of silicon and a second conductive carbon material, and the second conductive carbon material accounts for more than 50% by mass. The intermediate layer is a mixture of silicon and a first conductive carbon material, with silicon accounting for more than 50% by mass. There are at least two intermediate layers. As the distance from the surface of the current collector increases, the mass percentage of silicon in each intermediate layer gradually decreases, while the mass percentage of the first conductive carbon material gradually increases. The outer layer is made of a first conductive carbon material.

2. The negative electrode sheet according to claim 1, characterized in that, The first conductive carbon material includes at least one of graphite, carbon nanotubes, and graphene.

3. The negative electrode sheet according to claim 1, characterized in that, The second conductive carbon material includes at least one of graphite, carbon nanotubes, and graphene.

4. The negative electrode sheet according to claim 1, characterized in that, The thickness ratio of the inner layer, the middle layer, and the outer layer is (0.1~0.5):(0.1~5):(0.1~0.5).

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that, The total thickness of the negative electrode active layer is 10nm~900nm.

6. The negative electrode sheet according to claim 5, characterized in that, The total thickness of the negative electrode active layer is 300nm~900nm.

7. The negative electrode sheet according to any one of claims 1 to 4, 6, characterized in that, The thickness of the current collector is 1μm to 100μm.

8. The negative electrode sheet according to claim 7, characterized in that, The thickness of the current collector is 10μm to 100μm.

9. The negative electrode sheet according to any one of claims 1 to 4, 6, and 8, characterized in that, The current collector is made of copper and / or stainless steel.

10. A method for preparing a negative electrode sheet as described in any one of claims 1 to 9, characterized in that, Includes the following steps: The negative electrode active layer is formed on the surface of the current collector.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The negative electrode active layer is formed on the surface of the current collector by physical vapor deposition.

12. The method for preparing the negative electrode sheet according to claim 11, characterized in that, The conditions for forming the intermediate layer by physical vapor deposition include: (1) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A; (2) The coating time for each intermediate layer is 0.1h to 24h; And / or, the conditions for forming the outer layer by physical vapor deposition include: (1) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A; (2) The coating time is 0.1h~24h.

13. The method for preparing the negative electrode sheet according to claim 12, characterized in that, The intermediate layer is a mixture of silicon and a first conductive carbon material, and the conditions for forming the intermediate layer by physical vapor deposition further include: (1) The silicon target and the first conductive carbon material target are deposited simultaneously, and the deposition bias voltage of the silicon target is higher than that of the first conductive carbon material target. (2) The first conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A.

14. The method for preparing the negative electrode sheet according to claim 11, characterized in that, The negative electrode active layer further includes an inner layer, wherein the inner layer is composed of a second conductive carbon material and the mass percentage of the second conductive carbon material is greater than 50%, and the conditions for forming the inner layer by physical vapor deposition include: (1) The second conductive carbon material target deposition bias voltage is 0V~500V, the deposition voltage is 95V~105V, and the deposition current is 15A~25A; (2) The coating time is 0.1h~24h.

15. The method for preparing the negative electrode sheet according to claim 14, characterized in that, The inner layer is a mixture of silicon and a second conductive carbon material, and the conditions for forming the inner layer by physical vapor deposition also include: (1) The silicon target and the second conductive carbon target are deposited simultaneously, and the deposition bias voltage of the second conductive carbon target is higher than that of the silicon target. (2) The silicon target deposition bias voltage is 0V~500V, the deposition voltage is 50V~60V, and the deposition current is 10A~20A.

16. A lithium-ion battery, characterized in that, It includes the negative electrode sheet as described in any one of claims 1 to 9.

17. An electrical device, characterized in that, It includes the lithium-ion battery of claim 16.

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