A negative electrode sheet, a method for manufacturing the same, and a lithium ion battery

Abstract

The present application relates to a kind of negative plate and its preparation method and lithium ion battery, the negative plate includes current collector and the active material layer coated on the surface of current collector;The active material layer includes at least one composite negative active material layer;The composite negative active material layer includes the graphite layer and silicon carbon layer stacked;The silicon carbon layer is arranged on the surface of current collector.The present application uses the graphite layer and silicon carbon layer to carry out the composite negative active material layer of layer, so that negative plate not only has the high energy density of silicon negative electrode material, simultaneously utilize the characteristics of small graphite rebound, provide the release space in thickness for negative plate;Graphite layer is placed in the upper layer of silicon carbon layer, and the expansion of silicon negative electrode material in lower layer is inhibited in structure.

Description

Technical Field

[0001] This invention belongs to the field of negative electrode preparation technology, and relates to a negative electrode, and more particularly to a negative electrode, its preparation method, and a lithium-ion battery. Background Technology

[0002] Low energy density, poor cycle performance, and poor fast charging performance are the main problems of lithium-ion batteries.

[0003] Low energy density severely restricts the driving range of electric vehicles. Although some lithium battery companies are researching the use of silicon-carbon materials as lithium-ion anodes to improve the energy density of lithium-ion batteries, the volume of silicon-based materials expands by 100%-300% during the charging and discharging process of lithium-ion batteries, causing the cell thickness to exceed the standard. At the same time, the continuous contraction and expansion will cause the silicon-carbon anode material to become powdery, which seriously affects the battery life.

[0004] Poor cycle performance is reflected in the peel strength of the electrode. During the electrode drying process, the SBR binder and carbon black migrate to the surface as the solvent evaporates, causing the binder particles to accumulate in the upper layer of the electrode. There is very little binder distribution in the current collector and coating area, which affects the adhesion between the binder and the foil, resulting in a decrease in the peel strength of the electrode and an increase in the internal resistance of the electrode.

[0005] While using a fast-charging graphite system can achieve fast charging of batteries, it also brings new problems, such as low battery energy density, which cannot meet the long-range requirements of electric vehicles.

[0006] CN 111446415A provides a silicon anode sheet, its preparation method, and its uses. The silicon anode sheet includes a negative electrode current collector and an active material layer on one or both surfaces of the current collector, as well as a functional layer on the surface of the active material layer. The functional layer includes carbon fibers, metal fibers, and conductive adhesive. The active material layer includes a negative electrode active material selected from silicon materials. The method involves first coating an active material layer onto the surface of the current collector, drying it, and then further coating it with the functional layer. After drying and rolling, the silicon anode sheet is obtained. The carbon fibers in the functional layer reduce the resistivity of the silicon anode sheet and enhance its liquid absorption capacity, reducing aging time and improving production efficiency. The addition of metal fibers in the functional layer significantly reduces the resistivity of the silicon anode sheet, enhances its toughness, and effectively prevents powder shedding.

[0007] CN 113764622A discloses a method for preparing a low-expansion lithium-ion battery silicon-carbon anode sheet. The method involves compounding porous nano-silicon and long-tube carbon nanotubes in a citric acid solution to form a Si-CNT precursor, which is then ground, mixed, and sintered with artificial graphite and sucrose to obtain silicon-carbon powder. The obtained silicon-carbon powder, aqueous composite binder, graphene, dispersant, and pure water are dispersed in a specific ratio to form a slurry, which is then coated onto both sides of a metal foil. After drying, the low-expansion lithium-ion battery silicon-carbon anode sheet is obtained. This invention uses porous nano-silicon compounded with one-dimensional carbon nanotubes to form a linear network structure on the outside and inside of the larger-pore-diameter porous nano-silicon, thereby effectively suppressing the volume expansion of the silicon-carbon powder during charge-discharge cycles. The selected high-adhesion aqueous composite binder, through the network formed by ester bonds, can better contact the silicon-carbon powder, improving the adhesion between the silicon-carbon powder and other powders, and between the slurry and the foil, further reducing the expansion of the silicon-carbon anode sheet and improving the cycle life of the battery.

[0008] While the above technical solutions have suppressed silicon expansion and reduced resistance to some extent, they have not improved the peel strength of the electrode or the fast charging performance.

[0009] Therefore, how to improve the peel strength of the negative electrode and enhance its fast-charging performance while suppressing silicon expansion and reducing resistance is a problem that urgently needs to be solved in the field of negative electrode fabrication technology. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a negative electrode sheet, its preparation method, and a lithium-ion battery. The negative electrode active material layer is a composite of a graphite layer and a silicon-carbon layer, enabling the negative electrode sheet to not only possess the high energy density of silicon negative electrode materials but also utilize the low rebound characteristic of graphite, providing a space for thickness expansion. By controlling the graphite layer to be placed on top of the silicon-carbon layer, the expansion of the silicon negative electrode material in the lower layer is structurally suppressed.

[0011] To achieve this objective, the present invention adopts the following technical solution:

[0012] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector and an active material layer coated on the surface of the current collector; the active material layer comprising at least one composite negative electrode active material layer;

[0013] The composite negative electrode active material layer includes a graphite layer and a silicon-carbon layer stacked together;

[0014] The silicon-carbon layer is disposed on the surface of the current collector.

[0015] This invention employs a composite negative electrode active material layer of graphite layer and silicon carbon layer, which enables the negative electrode sheet to not only have the high energy density of silicon negative electrode material, but also utilizes the small rebound characteristic of graphite to provide space for thickness release of the negative electrode sheet; by controlling the graphite layer to be placed on the upper layer of silicon carbon layer, the expansion of silicon negative electrode material in the lower layer is suppressed in the structure.

[0016] Preferably, the raw materials for the graphite layer include graphite, binder, and conductive agent.

[0017] Preferably, the mass ratio of graphite: binder: conductive agent is (90-98):(1-6):(1-5), for example, it can be 90:5:5, 95:2:3, 92:3:5, 91:5:4 or 98:1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] Preferably, the graphite includes any one or a combination of at least two of artificial graphite, natural graphite, or modified natural graphite. Typical but non-limiting combinations include combinations of artificial graphite and natural graphite, combinations of natural graphite and modified natural graphite, combinations of artificial graphite and modified natural graphite, or combinations of artificial graphite, natural graphite, and modified natural graphite.

[0019] The graphite provided by this invention is a fast-charging graphite, which improves fast-charging performance.

[0020] The properties of the fast-charging graphite obtained by this invention include:

[0021] (1) Fast-charging graphite is graphite nano-sized, which reduces the particle size of graphite, shortens the lithium ion insertion path, and improves the fast-charging performance of graphite.

[0022] (2) The lithium-ion intercalation channel was increased and the graphite surface was coated and modified. Through surface coating, the specific surface area of ​​graphite was reduced, the active sites on the graphite surface were reduced, and the high current charging and discharging capability of graphite was improved.

[0023] Preferably, the adhesive comprises styrene-butadiene rubber and / or carboxymethyl cellulose.

[0024] Preferably, the conductive agent comprises conductive carbon black.

[0025] Preferably, the D of the graphite 50 The value is 18 to 22 μm, for example, it can be 18 μm, 19 μm, 20 μm, 21 μm or 22 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the initial capacity of the graphite is 357 mAh / g or higher, for example, 357 mAh / g, 360 mAh / g, 365 mAh / g, 370 mAh / g or 400 mAh / g, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0027] Preferably, the maximum compacted density of the graphite is 1.6–1.8 g / cm³. 3 For example, it could be 1.65 g / cm³ 3 1.68g / cm 3 1.7g / cm 3 1.75g / cm 3 Or 1.78g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0028] Preferably, the raw materials for the silicon-carbon layer include graphite, silicon oxide, binder, and conductive agent.

[0029] Preferably, the mass ratio of graphite:siloxane:binder:conductive agent is (10-90):(5-90):(0.5-3):(0.55-6), for example, it can be 45:46:3:6, 20:75:2:3, 36:63:0.5:0.5, 10:86:2:2 or 90:5:3:2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] In this invention, the content of binder and conductive agent in the silicon carbon layer is higher than that in the graphite layer, with the difference reaching 50 wt%. During the coating and baking process, the binder and conductive agent in the silicon carbon layer will flow to the graphite layer, so that the binder and conductive agent are evenly distributed in the electrode, reducing the impact of flow on peel strength and resistivity.

[0031] Preferably, the graphite includes any one or a combination of at least two of artificial graphite, natural graphite, or modified natural graphite. Typical but non-limiting combinations include combinations of artificial graphite and natural graphite, combinations of natural graphite and modified natural graphite, combinations of artificial graphite and modified natural graphite, or combinations of artificial graphite, natural graphite, and modified natural graphite.

[0032] Preferably, the silicon oxide comprises silicon suboxide.

[0033] Preferably, the raw material for the silicon-carbon layer also includes a thickener.

[0034] Preferably, the thickener comprises polyacrylic acid.

[0035] Preferably, the mass percentage of the thickener relative to the mass of the silicon carbide layer raw material is 1 to 10 wt%, for example, it can be 1.5 wt%, 2 wt%, 5 wt%, 8 wt% or 9 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] The present invention adds a thickener to the silicon-carbon layer, preferably polyacrylic acid, wherein the oxygen-containing groups of polyacrylic acid can form with the surface of silicon suboxide, thereby mitigating the volume expansion of the silicon anode material.

[0037] Preferably, the adhesive comprises styrene-butadiene rubber.

[0038] Preferably, the conductive agent comprises conductive carbon black and / or carbon nanotubes.

[0039] Preferably, the mass percentage of the carbon nanotubes to the silicon-carbon layer raw material is 0.05 to 1 wt%, for example, it can be 0.55 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt% or 0.9 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] This invention incorporates carbon nanotubes as a conductive agent into a silicon-carbon layer. In addition to increasing the conductivity of the electrode, it can also act as a framework in the silicon-carbon layer, mitigating volume expansion.

[0041] Preferably, the D of the graphite 50 The value is 16–19 μm, for example, it can be 16.5 μm, 17 μm, 17.5 μm, 18 μm or 18.5 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] Preferably, the initial capacity of the graphite is 360mAh / g or higher, for example, 360mAh / g, 365mAh / g, 370mAh / g, 375mAh / g, or 380mAh / g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the maximum compacted density of the graphite is 1.9 g / cm³. 3 The above, for example, could be 1.9 g / cm³. 3 1.95g / cm 3 2g / cm 3 2.05g / cm 3 2.1g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0044] Preferably, the silicon-oxygen D50 The value is 13 to 17 μm, for example, it can be 13.5 μm, 14 μm, 15 μm, 16 μm or 16.5 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] Preferably, the initial capacity of the silicon oxide is 500 mAh / g or higher, for example, 500 mAh / g, 505 mAh / g, 510 mAh / g, 515 mAh / g, or 520 mAh / g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] Preferably, the maximum compaction density of the silicon-oxygen compound is 1.7 g / cm³. 3 The above, for example, could be 1.7 g / cm³. 3 1.75g / cm 3 1.8g / cm 3 1.85g / cm 3 1.9g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0047] Preferably, the thickness ratio of the graphite layer to the silicon carbide layer is 1:(1 to 9), for example, it can be 1:2, 1:3, 1:5, 1:7 or 1:8, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the areal density of the negative electrode sheet is 160–180 g / m³. 2 For example, it could be 162g / m 2 165g / m 2 170g / m 2 175g / m 2 Or 178g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0049] In a second aspect, the present invention provides a method for preparing the negative electrode sheet according to the first aspect, the method comprising the following steps:

[0050] The raw materials and solvents for the graphite layer are mixed to obtain a graphite layer slurry; the raw materials and solvents for the silicon-carbon layer are mixed to obtain a silicon-carbon layer slurry.

[0051] The graphite layer slurry and the silicon-carbon layer slurry are coated, with the silicon-carbon layer slurry coated on the surface of the current collector and the graphite layer slurry coated on the surface of the silicon-carbon layer slurry. After drying, the negative electrode sheet is obtained.

[0052] Preferably, the solid content of the graphite layer slurry is 50-60 wt%, for example, it can be 51 wt%, 52 wt%, 54 wt%, 56 wt%, 58 wt% or 59 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] Preferably, the solid content of the silicon carbide slurry is 35-40 wt%, for example, it can be 35.5 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, or 39.5 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0054] Preferably, the coating includes single-layer coating and / or double-layer coating.

[0055] The single-layer coating refers to coating a silicon-carbon layer first, and then coating a graphite layer; the double-layer coating refers to using a double-layer coating technique to simultaneously coat a silicon-carbon layer and a graphite layer.

[0056] The process of coating the silicon-carbon layer and graphite layer in a single layer may involve one or multiple coating operations.

[0057] Thirdly, the present invention provides a lithium-ion battery comprising a negative electrode as described in the first aspect.

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] (1) The present invention uses a negative electrode active material layer composed of graphite layer and silicon carbon layer, so that the negative electrode not only has the high energy density of silicon negative electrode material, but also utilizes the small rebound of graphite to provide a space for thickness release of the negative electrode; by controlling the graphite layer to be placed on the upper layer of silicon carbon layer, the expansion of silicon negative electrode material in the lower layer is suppressed in the structure.

[0060] (2) The content of binder and conductive agent in the silicon carbon layer of the present invention is higher than that in the graphite layer. During the coating and baking process, the binder and conductive agent of the silicon carbon layer will flow to the graphite layer, which reduces the influence of flow on peel strength and resistivity. Detailed Implementation

[0061] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0062] Example 1

[0063] This embodiment provides a negative electrode sheet, which includes a current collector and an active material layer coated on the surface of the current collector; the active material layer includes a composite negative electrode active material layer; the composite negative electrode active material layer includes a graphite layer and a silicon-carbon layer stacked thereon; the silicon-carbon layer is disposed on the surface of the current collector, the graphite layer is disposed on the surface of the silicon-carbon layer, the thickness ratio of the graphite layer to the silicon-carbon layer is 1:1, and the areal density of the negative electrode sheet is 176 g / m³. 2 .

[0064] According to the mass percentage, the raw material of the graphite layer is modified natural graphite: conductive carbon black: carboxymethyl cellulose: styrene-butadiene rubber = 96.8:1.3:1.1:0.8, wherein the modified natural graphite (MD-4, saffron graphite) is a fast-charging graphite, and the D of the modified natural graphite is... 50 The modified natural graphite has a thickness of 20 μm, an initial capacity of 357 mAh / g, and a maximum compaction density of 1.7 g / cm³. 3 .

[0065] According to the mass percentage, the raw materials of the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 49.2:40.5:2:0.1:2:6.2, and the D of the modified natural graphite is... 50 The thickness is 17.5 μm, the initial capacity of the modified natural graphite is 360 mAh / g, and the maximum compaction density of the modified natural graphite is 1.9 g / cm³. 3 The D of the silicon suboxide 50 The silica has a thickness of 15 μm, an initial capacity of 500 mAh / g, and a maximum compaction density of 1.7 g / cm³. 3 .

[0066] The method for preparing the negative electrode sheet includes the following steps:

[0067] The raw materials for the graphite layer are mixed with water to obtain a graphite layer slurry with a solid content of 55 wt%; the raw materials for the silicon carbide layer are mixed with water to obtain a silicon carbide layer slurry with a solid content of 37 wt%.

[0068] The graphite slurry and the silicon-carbon slurry are coated using a double-layer coating technique. The silicon-carbon slurry is coated on the surface of the current collector, and the graphite slurry is coated on the surface of the silicon-carbon slurry. After drying, the negative electrode is obtained.

[0069] Example 2

[0070] This embodiment provides a negative electrode sheet, which includes a current collector and an active material layer coated on the surface of the current collector; the active material layer includes a composite negative electrode active material layer; the composite negative electrode active material layer includes a graphite layer and a silicon-carbon layer stacked thereon; the silicon-carbon layer is disposed on the surface of the current collector, the graphite layer is disposed on the surface of the silicon-carbon layer, the thickness ratio of the graphite layer to the silicon-carbon layer is 1:5, and the areal density of the negative electrode sheet is 160 g / m³. 2 .

[0071] According to the mass percentage, the raw materials of the graphite layer are artificial graphite: conductive carbon black: carboxymethyl cellulose: styrene-butadiene rubber = 96.3:1.6:1.1:1, wherein the artificial graphite (BFC-18, BTR graphite) is a fast-charging graphite, and the D of the artificial graphite... 50 The thickness is 18 μm, the initial capacity of the artificial graphite is 360 mAh / g, and the maximum compaction density of the artificial graphite is 1.6 g / cm³. 3 .

[0072] According to the mass percentage, the raw materials of the silicon-carbon layer are artificial graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 49.5:40.5:2:0.1:1.7:6.2, and the D of the artificial graphite is... 50 The thickness is 16 μm, the initial capacity of the artificial graphite is 360 mAh / g, and the maximum compaction density of the artificial graphite is 1.9 g / cm³. 3 The D of the silicon suboxide 50 The silica has a thickness of 13 μm, an initial capacity of 500 mAh / g, and a maximum compaction density of 1.7 g / cm³. 3 .

[0073] The method for preparing the negative electrode sheet includes the following steps:

[0074] The raw materials for the graphite layer are mixed with water to obtain a graphite layer slurry with a solid content of 50 wt%; the raw materials for the silicon carbide layer are mixed with water to obtain a silicon carbide layer slurry with a solid content of 35 wt%.

[0075] The graphite slurry and the silicon-carbon slurry are coated in a single layer, with the silicon-carbon slurry coated on the surface of the current collector and the graphite slurry coated on the surface of the silicon-carbon slurry. After drying, the negative electrode is obtained.

[0076] Example 3

[0077] This embodiment provides a negative electrode sheet, except that the active material layer includes two composite negative electrode active material layers, and the rest of the structural components are the same as those in Embodiment 1.

[0078] Example 4

[0079] This embodiment provides a negative electrode sheet, except that the raw materials of the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 49.2:42.4:1.3:0.1:0.8:6.2, and the rest are the same as in Example 1.

[0080] Example 5

[0081] This embodiment provides a negative electrode sheet, except that the raw materials of the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 49.2:43:1:0.1:0.5:6.2, and the rest are the same as in Example 1.

[0082] Example 6

[0083] This embodiment provides a negative electrode sheet, except that the modified natural graphite is replaced with conventional artificial graphite (MD-1, saffron graphite), and the rest of the structural components are the same as in Example 1.

[0084] Example 7

[0085] This embodiment provides a negative electrode sheet, except that the raw materials of the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: carboxymethyl cellulose = 49.2:40.5:2:0.1:2:6.2, and the polyacrylic acid is replaced with an equal mass of carboxymethyl cellulose, the remaining structural components are the same as in Example 1.

[0086] Example 8

[0087] This embodiment provides a negative electrode sheet. Except for the silicon-carbon layer, which is made of modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 44.4:40.5:2:0.1:2:11, the other structural components are the same as in Example 1.

[0088] Example 9

[0089] This embodiment provides a negative electrode sheet, except that the raw materials for the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: styrene-butadiene rubber: polyacrylic acid = 49.2:40.5:2.1:2:6.2, and the rest are the same as in Example 1.

[0090] Comparative Example 1

[0091] This comparative example provides a negative electrode sheet, which includes a current collector and an active material layer coated on the surface of the current collector; the active material layer includes a graphite layer, the thickness of which is the same as the thickness of the active material layer in Example 1, and the areal density of the negative electrode sheet is 176 g / m³. 2 .

[0092] According to the mass percentage, the raw material of the graphite layer is modified natural graphite: conductive carbon black: carboxymethyl cellulose: styrene-butadiene rubber = 96.8:1.3:1.1:0.8, wherein the modified natural graphite (MD-4, saffron graphite) is a fast-charging graphite, and the D of the modified natural graphite is... 50 The modified natural graphite has a thickness of 20 μm, an initial capacity of 357 mAh / g, and a maximum compaction density of 1.7 g / cm³. 3 .

[0093] Comparative Example 2

[0094] This comparative example provides a negative electrode sheet, which includes a current collector and an active material layer coated on the surface of the current collector; the active material layer includes a silicon-carbon layer, the thickness of which is the same as the thickness of the active material layer described in Example 1, and the areal density of the negative electrode sheet is 176 g / m³. 2 .

[0095] According to the mass percentage, the raw materials of the silicon-carbon layer are modified natural graphite: silicon suboxide: conductive carbon black: conductive carbon nanotubes: styrene-butadiene rubber: polyacrylic acid = 49.2:40.5:2:0.1:2:6.2, and the D of the modified natural graphite is... 50 The thickness is 17.5 μm, the initial capacity of the modified natural graphite is 360 mAh / g, and the maximum compaction density of the modified natural graphite is 1.9 g / cm³. 3 The D of the silicon suboxide 50 The silica has a thickness of 15 μm, an initial capacity of 500 mAh / g, and a maximum compaction density of 1.7 g / cm³. 3 .

[0096] Comparative Example 3

[0097] This comparative example provides a negative electrode sheet, except that the graphite layer is disposed on the surface of the current collector and the silicon-carbon layer is disposed on the surface of the graphite layer, and the remaining structural components are the same as those in Example 1.

[0098] The obtained negative electrode sheets were used to assemble lithium-ion batteries according to GB31241-2014, and performance tests were conducted.

[0099] The test results are shown in Table 1.

[0100] Table 1

[0101]

[0102]

[0103] The following conclusions can be drawn from Table 1:

[0104] (1) As can be seen from Examples 1-3, the present invention uses a negative electrode active material layer composed of graphite layer and silicon carbon layer, so that the negative electrode sheet not only has the high energy density of silicon negative electrode material, but also utilizes the small rebound of graphite to provide a release space in thickness for the negative electrode sheet; by controlling the graphite layer to be placed on the upper layer of silicon carbon layer, the expansion of silicon negative electrode material in the lower layer is suppressed in the structure.

[0105] (2) As can be seen from the comparison between Examples 4 and 5 and Example 1, when the content of binder and conductive agent in silicon carbon layer is equal to or less than that in graphite layer, the peel strength and resistance of electrode are low. This is because during the electrode coating and baking process, the flow of binder and conductive agent to the outer layer will affect the peel strength and resistance of electrode. Setting the content according to the inner and outer gradient of graphite layer and silicon carbon layer can effectively reduce the influence of flow and floating.

[0106] (3) As can be seen from the comparison between Example 6 and Example 1, when conventional graphite is used, the resulting negative electrode sheet has poor fast charging performance and is prone to serious lithium plating.

[0107] (4) As can be seen from the comparison between Examples 7 and 8 and Example 1, adding 0.05 to 10 wt% thickener to the silicon-carbon layer of the present invention is beneficial to improving the performance of the negative electrode sheet. This is because the thickener forms hydrogen bonds with the surface of the silicon-oxygen material, which inhibits volume expansion.

[0108] (5) As can be seen from the comparison between Example 9 and Example 1, the use of carbon nanotubes as the conductive agent in the silicon-carbon layer of the present invention is beneficial to improving the performance of the negative electrode sheet. This is because carbon nanotubes increase the conductivity and play a skeleton role, which can suppress the volume expansion of silicon.

[0109] (6) As can be seen from the comparison between Comparative Examples 1-3 and Example 1, compared with the structure of a single graphite layer / silicon carbon layer or an outer silicon carbon layer and an inner graphite layer, the present invention adopts a negative electrode active material layer composed of a graphite layer and a silicon carbon layer, so that the negative electrode not only has the high energy density of silicon negative electrode material, but also utilizes the small rebound characteristic of graphite to provide a space for thickness release of the negative electrode; by controlling the graphite layer to be placed on the upper layer of the silicon carbon layer, the expansion of silicon negative electrode material in the lower layer is suppressed in the structure.

[0110] In summary, this invention employs a composite negative electrode active material layer of graphite and silicon-carbon layers, which enables the negative electrode sheet to not only possess the high energy density of silicon negative electrode materials, but also utilizes the low rebound characteristic of graphite to provide a release space in terms of thickness for the negative electrode sheet; by controlling the graphite layer to be placed on top of the silicon-carbon layer, the expansion of silicon negative electrode materials in the lower layer is structurally suppressed.

[0111] This invention illustrates the detailed process equipment and process flow through the above embodiments. However, this invention is not limited to the detailed process equipment and process flow described above, meaning that this invention does not necessarily depend on the detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials for the product of this invention, addition of auxiliary components, and selection of specific methods, all fall within the protection scope and disclosure scope of this invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and an active material layer coated on the surface of the current collector; the active material layer includes at least one composite negative electrode active material layer; The composite negative electrode active material layer includes a graphite layer and a silicon-carbon layer stacked together; The silicon carbide layer is disposed on the surface of the current collector; The raw materials for the graphite layer include graphite: binder: conductive agent in a mass ratio of 96.3: 2.1: 1.6; the initial capacity of the graphite is 357 mAh / g or higher. The raw materials for the silicon-carbon layer include graphite, silicon oxide, binder, and conductive agent in a mass ratio of 49.5:40.5:1.7:2.1; the initial capacity of the silicon oxide is 500 mAh / g or more; and the initial capacity of the graphite is 360 mAh / g or more. The content of binder and conductive agent in the silicon carbon layer is higher than that in the graphite layer.

2. The negative electrode sheet according to claim 1, characterized in that, The graphite in the graphite layer includes any one or a combination of at least two of artificial graphite, natural graphite, or modified natural graphite. The D of the graphite in the graphite layer 50 The thickness is 18~22μm; The maximum compacted density of the graphite in the graphite layer is 1.6~1.8 g / cm³. 3 .

3. The negative electrode sheet according to claim 1, characterized in that, The graphite in the silicon-carbon layer includes any one or a combination of at least two of artificial graphite, natural graphite, or modified natural graphite. The silicon oxide includes silicon suboxide.

4. The negative electrode sheet according to claim 1, characterized in that, The conductive agent in the silicon-carbon layer includes conductive carbon black and / or carbon nanotubes. The carbon nanotubes account for 0.05 to 1 wt% of the mass of the silicon-carbon layer raw material.

5. The negative electrode sheet according to claim 1, characterized in that, The graphite of the silicon-carbon layer D 50 The thickness is 16~19μm; The maximum compaction density of the graphite in the silicon-carbon layer is 1.9 g / cm³. 3 above.

6. The negative electrode sheet according to claim 1, characterized in that, The silicon-oxygen D 50 It is 13~17μm; The maximum compaction density of the silicon-oxygen compound is 1.7 g / cm³. 3 above.

7. The negative electrode sheet according to claim 1, characterized in that, The raw materials for the silicon-carbon layer also include a thickener; The thickener accounts for 6.2 wt% of the silicon carbide layer raw material by mass percentage.

8. The negative electrode sheet according to claim 7, characterized in that, The thickener includes polyacrylic acid.

9. The negative electrode sheet according to claim 1, characterized in that, The thickness ratio of the graphite layer to the silicon carbide layer is 1:(1~9); The areal density of the negative electrode is 160~180 g / m³. 2 .

10. A method for preparing a negative electrode sheet according to any one of claims 1-9, characterized in that, The preparation method includes the following steps: The raw materials and solvents for the graphite layer are mixed to obtain a graphite layer slurry; the raw materials and solvents for the silicon-carbon layer are mixed to obtain a silicon-carbon layer slurry. The graphite layer slurry and the silicon-carbon layer slurry are coated, with the silicon-carbon layer slurry coated on the surface of the current collector and the graphite layer slurry coated on the surface of the silicon-carbon layer slurry. After drying, the negative electrode sheet is obtained.

11. The preparation method according to claim 10, characterized in that, The solid content of the graphite layer slurry is 50~60 wt%; The solid content of the silicon carbide slurry is 35-40 wt%. The coating includes single-layer coating and / or double-layer coating.

12. A lithium-ion battery, characterized in that, The lithium-ion battery contains a negative electrode as described in any one of claims 1-9.

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