Anti-overcharge fast-charging negative electrode plate, preparation method thereof and soft package battery

Abstract

The invention discloses an anti-overcharge fast-charging negative electrode plate, a preparation method thereof and a soft package battery, and the anti-overcharge fast-charging negative electrode plate comprises a negative electrode current collector; the silicon-carbon mixing layer is arranged on the surface of the negative electrode current collector, raw materials comprise a first negative electrode active material and silicon carbon, and the doping amount of the silicon carbon in the silicon-carbon mixing layer is 1-10 wt%; the hard carbon mixing layer is arranged on the surface of the silicon carbon mixing layer, the raw materials comprise a second negative electrode active material and hard carbon, and the doping amount of the hard carbon in the hard carbon mixing layer is 1-15 wt%. According to the prepared soft package battery, under the high-rate charging condition, the peak intensity ratio of LiC6 to LiC12 in an XRD spectrum of a negative plate when the battery is charged to a specific voltage can be obviously increased, the soft package battery has better fast charging capacity under the high rate, over-charging is not prone to occurring, and the safety performance is improved.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and particularly to a fast-charging negative electrode sheet for preventing overcharging, a preparation method thereof, and a soft-pack battery. Background Art

[0002] With the wide application of lithium-ion batteries in the fields of electric vehicles, rail transit, energy storage, etc., the demand for high-energy-density and long-cycle-life lithium-ion battery technologies is increasing continuously. With the increase in energy density, the charging rate of the battery has also attracted more and more attention. However, with the increase in the charging rate, overcharging of the positive electrode and lithium plating on the negative electrode are likely to occur under high voltage and large current, leading to safety accidents. Therefore, it is urgent to improve the safety of fast-charging battery cores.

[0003] Soft-pack batteries and square-shell batteries are two common types of batteries in the new energy industry. Their essential differences in design will result in different safety characteristics, such as overcharge protection devices.

[0004] The square-shell battery can adopt the following several overcharge prevention mechanisms:

[0005] 1) Current interrupt device (CID): When the internal pressure is too high, the positive electrode lead is cut off, or when the current is too large, the current-limiting sheet of the positive electrode tab is burned out.

[0006] 2) Overcharge safety device (OSD): During overcharging, the pressure generated inside the battery causes the OSD to trigger an internal short circuit, and the large current instantly fuses the fuse to cut off the battery circuit.

[0007] However, for soft-pack batteries, due to the characteristics of the shell material and structure, it is difficult to integrate a similar pressure release device. Moreover, soft-pack batteries are prone to expansion and deformation during overcharging, making the traditional overcharge prevention design scheme applicable to square-shell batteries difficult to be directly applied to soft-pack batteries. Summary of the Invention

[0008] Therefore, the technical problem to be solved by the present invention is to overcome the problem that soft-pack batteries are prone to overcharging under high-rate charging conditions in the prior art, which in turn affects the safety performance, so as to provide a fast-charging negative electrode sheet for preventing overcharging, a soft-pack battery, and a preparation method thereof that can effectively prevent soft-pack batteries from overcharging under large current.

[0009] A fast-charging negative electrode sheet for preventing overcharging, comprising:

[0010] A negative electrode current collector;

[0011] A silicon-carbon blended layer is provided on the surface of the negative electrode current collector. The raw materials include a first negative electrode active material and silicon-carbon, and the doping amount of silicon-carbon in the silicon-carbon blended layer is 1-10 wt%.

[0012] A hard carbon blended layer is provided on the surface of the silicon-carbon blended layer. The raw materials include a second negative electrode active material and hard carbon, and the doping amount of hard carbon in the hard carbon blended layer is 1-15 wt%.

[0013] As an example: the doping amount of silicon-carbon in the silicon-carbon blended layer can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, etc.; the doping amount of hard carbon in the hard carbon blended layer can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, etc.

[0014] Furthermore, the thickness of the hard carbon blended layer is 40-60 μm; as an example: the thickness of the hard carbon blended layer can be 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, etc.

[0015] And / or, the thickness of the silicon-carbon blended layer is 70-80 μm; as an example: the thickness of the silicon-carbon blended layer can be 70 μm, 72 μm, 74 μm, 76 μm, 78 μm, 80 μm, etc.

[0016] Furthermore, the first negative electrode active material and / or the second negative electrode active material are each independently selected from one or more of natural graphite, artificial graphite, soft carbon, and mesophase carbon microspheres;

[0017] And / or, the particle size of the first negative electrode active material is 10-20 μm; as an example: the particle size of the first negative electrode active material can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc.

[0018] And / or, the particle size of the second negative electrode active material is 3-8 μm; as an example: the particle size of the second negative electrode active material can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc.

[0019] Furthermore, both the silicon-carbon blended layer and the hard carbon blended layer further include a binder and a conductive agent.

[0020] Preferably, the binder includes one or more of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyacrylic acid (PAA), styrene-butadiene rubber (SBR), and polyimide (PI);

[0021] And / or, the conductive agent includes one or more of carbon black (SP), Ketjen black, carbon nanotubes (CNT), and graphene.

[0022] The present invention also provides a method for preparing the above-mentioned fast-charging negative electrode sheet, including:

[0023] Preparing a hard carbon blended slurry from the raw materials of the hard carbon blended layer, and preparing a silicon-carbon blended slurry from the raw materials of the silicon-carbon blended layer;

[0024] Coating the silicon-carbon blended slurry on the negative electrode current collector and drying to obtain a negative electrode sheet with a silicon-carbon blended layer;

[0025] Coating the hard carbon blended slurry on the surface of the silicon-carbon blended layer and drying to obtain the fast-charging negative electrode sheet.

[0026] Furthermore, the present invention also provides a soft-pack battery, including a positive electrode sheet and a negative electrode sheet, and the negative electrode sheet is the above-mentioned fast-charging negative electrode sheet.

[0027] Even further, the positive electrode sheet includes:

[0028] A positive electrode current collector;

[0029] A high-nickel polycrystalline layer, disposed on the surface of the positive electrode current collector, and the nickel content in the high-nickel polycrystalline layer is 80 wt% - 95 wt%; as an example: the nickel content in the high-nickel polycrystalline layer can be 80 wt%, 82 wt%, 84 wt%, 86 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt%, etc.;

[0030] A medium-nickel single-crystalline layer, disposed on the surface of the high-nickel polycrystalline layer, and the nickel content in the medium-nickel single-crystalline layer is 60 wt% - 79 wt%; as an example: the nickel content in the medium-nickel single-crystalline layer can be 60 wt%, 62 wt%, 64 wt%, 66 wt%, 68 wt%, 70 wt%, 72 wt%, 74 wt%, 76 wt%, 79 wt%, etc.

[0031] Preferably, the raw materials in the high-nickel polycrystalline layer include a high-nickel polycrystalline ternary positive electrode material;

[0032] The raw materials in the medium-nickel single-crystalline layer include a medium-nickel single-crystalline ternary positive electrode material.

[0033] More preferably, the total thickness of the high-nickel polycrystalline layer and the medium-nickel single-crystalline layer is 100 - 400 μm; the ratio of the thickness of the high-nickel polycrystalline layer to the thickness of the medium-nickel single-crystalline layer is (5 - 8):(2 - 5); that is, the thickness of the high-nickel polycrystalline layer can be 50 - 320 μm; and / or, the thickness of the medium-nickel single-crystalline layer can be 20 - 200 μm.

[0034] As an example: the total thickness of the high-nickel polycrystalline layer and the medium-nickel single-crystalline layer can be 100μm, 150μm, 200μm, 250μm, 300μm, 350μm, 400μm, etc.; the thickness of the high-nickel polycrystalline layer can be 50μm, 70μm, 100μm, 120μm, 150μm, 170μm, 200μm, 220μm, 250μm, 270μm, 300μm, 320μm; the thickness of the medium-nickel single-crystalline layer can be 20μm, 50μm, 70μm, 100μm, 120μm, 150μm, 170μm, 200μm.

[0035] The above ternary material is lithium nickel cobalt manganese oxide (NCM) or lithium nickel cobalt aluminate (NCA).

[0036] An electrical device includes the above-mentioned soft-pack battery. The electrical device can be a vehicle or the like.

[0037] The technical solution of the present invention has the following advantages:

[0038] 1. A fast-charging negative electrode sheet for preventing overcharging provided by the present invention includes a negative electrode current collector; a silicon-carbon mixed layer disposed on the surface of the negative electrode current collector, the raw materials including a first negative electrode active material and silicon-carbon, and the doping amount of silicon-carbon in the silicon-carbon mixed layer being 1-10 wt%; a hard-carbon mixed layer disposed on the surface of the silicon-carbon mixed layer, the raw materials including a second negative electrode active material and hard-carbon, and the doping amount of hard-carbon in the hard-carbon mixed layer being 1-15 wt%. Through the research of the present invention, it is found that under the condition of high-rate charging, the soft-pack battery is prone to the following failure modes: 1) The lithium insertion ability of the negative electrode is insufficient under a large negative electrode current, resulting in a lithium ion concentration difference on the surface of the negative electrode. Those lithium ions that are not inserted into the negative electrode in time directly combine with the electrons transmitted from the negative electrode material, continuously generating lithium metal (lithium dendrites) on the surface of the negative electrode. Since the thickness of the lithium dendrites continues to increase and then pierces the separator, an internal micro short circuit is caused, the internal temperature rises, and a safety hazard is caused; 2) Since lithium dendrites are continuously generated on the surface of the negative electrode, the potential of the negative electrode sheet will not continue to drop at this time, resulting in that the positive-negative electrode potential difference always cannot reach the charging upper limit voltage, and the positive electrode sheet continues to over-discharge lithium, making it easier for the microscopic layered structure to collapse, releasing oxygen, and having a violent side reaction with the electrolyte, continuously generating a large amount of gas and heat; Finally, when the above situations occur simultaneously, the battery cell will undergo thermal runaway, affecting the safety performance. And when graphite is fully lithiated, that is, when each carbon atom corresponds to one lithium ion, the formed compound is LiC6, and LiC 12 is an intermediate product, and the finally fully lithiated graphite material will generate LiC6 product. Therefore, the peak intensity ratio of LiC6 to LiC 12 in the XRD pattern can be used to represent LiC6 and LiC 12The proportion in the negative electrode sheet. When the content of fully inserted LiC6 in the negative electrode sheet at high rate is higher, that is, when the peak intensity ratio of LiC6 to LiC 12 in the XRD pattern is larger, it indicates that more lithium is inserted at the cut-off voltage, suggesting better fast charging ability and less lithium plating at high rate. In the present invention, the first negative electrode active material in the layer adjacent to the negative electrode current collector is selected to be silicon carbide blended, which can ensure the energy density of the battery; while the second negative electrode active material in the layer far from the negative electrode current collector is selected to be hard carbon blended. Since the hard carbon has a lower lithium plating potential than graphite, it can reduce the lithium plating potential of the overall negative electrode coating, delaying the occurrence of lithium plating under high rate charging conditions of the negative electrode and allowing more lithium ions to have time to insert into the negative electrode; therefore, the present invention provides a fast charging negative electrode sheet with a silicon carbide blending layer with a specific silicon carbide blending amount set adjacent to the negative electrode current collector and a hard carbon blending layer with a specific hard carbon blending amount set far from the negative electrode current collector. Compared with the soft package battery prepared from a conventional negative electrode sheet, the soft package battery prepared using it can significantly increase the peak intensity ratio of LiC6 to LiC 12 in the XRD pattern of the negative electrode sheet when charging to a specific voltage under high rate charging conditions, has better fast charging ability at high rate, is less likely to be overcharged, and improves safety performance.

[0039] 2. A fast charging negative electrode sheet of the present invention, the first negative electrode active material in the layer close to the negative electrode current collector is large particle graphite blended with silicon carbide, which has a high specific energy while increasing the porosity of the lower layer of slurry, enabling better wetting performance of the electrolyte.

[0040] 3. A soft package battery provided by the present invention, in addition to including the above fast charging negative electrode sheet, further optimizes the structure of the positive electrode sheet. The positive electrode sheet in the present invention includes: a positive electrode current collector; a high nickel polycrystalline layer disposed on the surface of the positive electrode current collector, and the nickel content in the high nickel polycrystalline layer is 80 wt% - 95 wt%; a medium nickel single crystal layer disposed on the surface of the high nickel polycrystalline layer, and the nickel content in the medium nickel single crystal layer is 60 wt% - 79 wt%. Excessive delithiation starts from the surface of the positive electrode sheet, that is, it first occurs on the side far from the positive electrode current collector. The layer far from the positive electrode current collector is the medium nickel single crystal layer, and this medium nickel single crystal layer uses a single crystal material with good overcharge - particle cracking properties, which has good anti - particle cracking characteristics under overcharge conditions and can effectively avoid the above problems of the positive electrode sheet. At the same time, the layer of material close to the current collector is a high nickel polycrystalline material, which has a high specific energy. Description of the Drawings

[0041] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0042] Figure 1 It is a schematic structural diagram of the fast - charging negative electrode sheet in Embodiment 1 of the present invention;

[0043] Figure 2 It is a schematic structural diagram of the positive electrode sheet in Embodiment 1 of the present invention;

[0044] Reference numerals:

[0045] 11 - negative current collector, 12 - silicon - carbon mixed layer, 13 - hard - carbon mixed layer;

[0046] 21 - positive current collector, 22 - high - nickel polycrystalline layer, 23 - medium - nickel single - crystal layer. Specific embodiments

[0047] The following embodiments are provided to better further understand the present invention. They are not limited to the best - mode embodiments, and do not limit the content and protection scope of the present invention. Any product obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts, which is the same as or similar to the present invention, falls within the protection scope of the present invention.

[0048] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "side", "upper", "lower", "top", "bottom", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation to the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0049] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the internal communication of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0050] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0051] Example 1

[0052] A fast-charging negative electrode sheet for preventing overcharging, as Figure 1 shown, includes a negative electrode current collector 11, a silicon-carbon mixed layer 12, and a hard-carbon mixed layer 13 arranged in sequence. The negative electrode current collector 11 is a copper foil with a thickness of 8 μm, the thickness of the hard-carbon mixed layer 13 is 40 μm, and the thickness of the silicon-carbon mixed layer 12 is 80 μm.

[0053] The hard-carbon mixed slurry for forming the hard-carbon mixed layer 13 includes artificial graphite, hard carbon, SP (conductive carbon black), SBR (styrene-butadiene latex), and CMC (carboxymethyl cellulose) with a mass ratio of 93.8:3:0.4:1.6:1.2. The solvent in the hard-carbon mixed slurry is water, and the solid content of the hard-carbon mixed slurry is 50%; the particle size D50 of the artificial graphite is 6 μm.

[0054] The silicon-carbon mixed slurry for forming the silicon-carbon mixed layer 12 includes artificial graphite, silicon-carbon, SP (conductive carbon black), SBR (styrene-butadiene latex), and CMC (carboxymethyl cellulose) with a mass ratio of 94.8:2:0.4:1.6:1.2. The solvent in the silicon-carbon mixed slurry is water, and the solid content of the silicon-carbon mixed slurry is 50%; the particle size D50 of the artificial graphite is 15 μm.

[0055] The specific preparation method of the above negative electrode sheet is as follows:

[0056] First, coat the silicon-carbon mixed slurry on the negative electrode current collector 11, and after drying, form a silicon-carbon mixed layer 12 with a thickness of 80 μm. Then, coat a layer of hard-carbon mixed slurry on the surface of the silicon-carbon mixed layer 12, and after drying, form a fast-charging negative electrode sheet with a hard-carbon mixed layer 13 having a thickness of 40 μm.

[0057] A soft-pack battery includes an electrolyte and an electric core. The electric core is prepared by winding a positive electrode sheet, a separator, a negative electrode sheet, and a separator arranged in sequence from the inside to the outside.

[0058] The negative electrode sheet in the electric core is the above-mentioned fast-charging negative electrode sheet; the positive electrode sheet in the electric core is as Figure 2 shown, includes a positive electrode current collector 21, a high-nickel polycrystalline layer 22, and a medium-nickel single-crystalline layer 23 arranged in sequence. The positive electrode current collector 21 is an aluminum foil with a thickness of 15 μm, the thickness of the high-nickel polycrystalline layer 22 is 100 μm, and the thickness of the medium-nickel single-crystalline layer 23 is 50 μm.

[0059] The high-nickel polycrystalline slurry for forming the high-nickel polycrystalline layer 22 is prepared by mixing lithium nickel cobalt manganese oxide, SP (conductive carbon black), PVDF (polyvinylidene fluoride) and the solvent N-methylpyrrolidone (NMP) in a ratio of 97.5:1:1.5. The solid content of the high-nickel polycrystalline slurry is 50%; the nickel content of the lithium nickel cobalt manganese oxide is 85%.

[0060] The medium-nickel single-crystalline slurry for forming the medium-nickel single-crystalline layer 23 is prepared by mixing lithium nickel cobalt manganese oxide, SP (conductive carbon black), PVDF (polyvinylidene fluoride) and the solvent N-methylpyrrolidone (NMP) in a ratio of 97.5:1:1.5. The solid content of the medium-nickel single-crystalline slurry is 50%; the nickel content of the lithium nickel cobalt manganese oxide is 68%.

[0061] The specific preparation method of the above positive electrode sheet is as follows:

[0062] First, the high-nickel polycrystalline slurry is coated on the positive electrode current collector 21, and after drying, a high-nickel polycrystalline layer 22 with a thickness of 100 μm is formed. Then, a layer of medium-nickel single-crystalline slurry is coated on the surface of the high-nickel polycrystalline layer 22, and after drying, a positive electrode sheet with a medium-nickel single-crystalline layer 23 with a thickness of 50 μm is formed.

[0063] Example 2

[0064] The difference from Example 1 is that the D50 of artificial graphite in the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 8 μm, and the D50 of artificial graphite in the silicon-carbon blending layer 12 is 20 μm, and the others are exactly the same as Example 1.

[0065] Example 3

[0066] The difference from Example 1 is that the D50 of artificial graphite in the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 3 μm, and the D50 of artificial graphite in the silicon-carbon blending layer 12 is 10 μm, and the others are exactly the same as Example 1.

[0067] Example 4

[0068] The difference from Example 1 is that the D50 of artificial graphite in both the hard carbon blending layer 13 and the silicon-carbon blending layer 12 of the fast-charging negative electrode sheet is 10 μm, and the others are exactly the same as Example 1.

[0069] Example 5

[0070] The difference from Example 1 is that the content of hard carbon in the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 2%, and the content of silicon-carbon in the silicon-carbon blending layer 12 is 2%, and the others are exactly the same as Example 1.

[0071] Example 6

[0072] The difference from Example 1 is that the content of hard carbon in the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 15 wt%, and the content of silicon carbide in the silicon carbide blending layer 12 is 10 wt%. Others are exactly the same as in Example 1.

[0073] Example 7

[0074] The difference from Example 1 is that the thickness of the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 60 μm, and the thickness of the silicon carbide blending layer 12 is 140 μm. Others are exactly the same as in Example 1.

[0075] Example 8

[0076] The difference from Example 1 is that the thickness of the hard carbon blending layer 13 of the fast-charging negative electrode sheet is 30 μm, and the thickness of the silicon carbide blending layer 12 is 50 μm. Others are exactly the same as in Example 1.

[0077] Example 9

[0078] The difference from Example 1 is that in the positive electrode sheet, the polycrystalline nickel cobalt manganese lithium oxide with a nickel content of 80 wt% and the single-crystalline nickel cobalt manganese lithium oxide with a nickel content of 60 wt% are respectively used in the high-nickel polycrystalline layer 22 and the medium-nickel single-crystalline layer 23. The thickness of the high-nickel polycrystalline layer 22 is 120 μm, and the thickness of the medium-nickel single-crystalline layer 23 is 30 μm. Others are exactly the same as in Example 1.

[0079] Example 10

[0080] The difference from Example 1 is that in the positive electrode sheet, the high-nickel polycrystalline ternary material with a nickel content of 95 wt% and the medium-nickel single-crystalline ternary material with a nickel content of 79 wt% are respectively used in the high-nickel polycrystalline layer 22 and the medium-nickel single-crystalline layer 23. The thickness of the high-nickel polycrystalline layer 22 is 75 μm, and the thickness of the medium-nickel single-crystalline layer 23 is 75 μm. Others are exactly the same as in Example 1.

[0081] Example 11

[0082] The difference from Example 1 is that in the positive electrode sheet, there is only the high-nickel polycrystalline layer 22, and the thickness of the high-nickel polycrystalline layer 22 is 140 μm;

[0083] In the negative electrode sheet, the hard carbon blending slurry for forming the hard carbon blending layer 13 includes artificial graphite, hard carbon, SP (conductive carbon black), SBR (styrene-butadiene latex), and CMC (carboxymethyl cellulose) with a mass ratio of 93.8:3:0.4:1.6:1.2. The solvent in the hard carbon blending slurry is water, and the solid content of the hard carbon blending slurry is 50%; the particle size D50 of the artificial graphite is 6 μm.

[0084] The silicon-carbon blending slurry for forming the silicon-carbon blending layer 12 includes artificial graphite, silicon-carbon, SP (conductive carbon black), and PAA (polyacrylic acid) with a mass ratio of 91.6:5:0.4:3. The solvent in the silicon-carbon blending slurry is water, and the solid content of the silicon-carbon blending slurry is 50%; the particle size D50 of the artificial graphite is 15 μm.

[0085] Other conditions are exactly the same as those in Example 1.

[0086] Comparative Example 1

[0087] The difference from Example 1 is that in this comparative example, the positions of the hard carbon blending layer 13 and the silicon-carbon blending layer 12 are interchanged. Specifically, the silicon-carbon blending layer 12 is provided on the surface of the negative electrode current collector 11, and the hard carbon blending layer 13 is provided on the surface of the silicon-carbon blending layer 12. Other conditions are the same as those in Example 1.

[0088] Comparative Example 2

[0089] The difference from Example 1 is that in this comparative example, a hard carbon blending layer 13 with a thickness of 120 μm is provided on the surface of the negative electrode current collector 11. The hard carbon blending slurry for forming the hard carbon blending layer 13 includes artificial graphite, hard carbon, SP (conductive carbon black), SBR (styrene-butadiene latex), and CMC (carboxymethyl cellulose) with a ratio of 91.8:5:0.4:1.6:1.2. The solvent in the hard carbon blending slurry is water; other conditions are the same as those in Example 1.

[0090] Comparative Example 3

[0091] The difference from Example 1 is that in this comparative example, only the silicon-carbon blending layer 12 is provided on the surface of the negative electrode current collector 11. The silicon-carbon blending slurry for forming the silicon-carbon blending layer 12 includes artificial graphite, silicon-carbon, SP (conductive carbon black), and PAA (polyacrylic acid) with a ratio of 90.9:6:0.3:2.8. To ensure that the energy density matches that of the positive electrode, the thickness of the silicon-carbon blending layer 12 is set to 100 μm, and other conditions are the same as those in Example 1.

[0092] Comparative Example 4

[0093] The difference from Example 11 is that in this comparative example, only the silicon-carbon blending layer 12 is provided on the surface of the negative electrode current collector 11. The silicon-carbon blending slurry for forming the silicon-carbon blending layer 12 includes artificial graphite, silicon-carbon, SP (conductive carbon black), and PAA (polyacrylic acid) with a ratio of 93.3:3:0.3:2.8. To ensure that the energy density matches that of the positive electrode, the thickness of the silicon-carbon blending layer 12 is 100 μm, and other conditions are the same as those in Example 1.

[0094] Test Example

[0095] 1. Detection of the peak intensity ratio of LiC6 and LiC 12 in the XRD pattern of the negative electrode sheet.

[0096] The fast-charging negative electrodes of the above-mentioned examples and comparative examples were tested to obtain the XRD patterns of the fast-charging negative electrodes fully charged at a 6C rate, and the peak intensity ratio of LiC6 to LiC 12 in the XRD pattern of the negative electrode was calculated. 6C rate charging: Assuming the battery capacity is x Ah, charging at a 6C rate means charging with a large current of I = 6 * x A. The specific process is as follows:

[0097] The soft-pack batteries of the examples and comparative examples were fully charged to a specific voltage of 5 mV at a 6C rate to obtain the XRD patterns of the negative electrodes. In the XRD pattern, there is a characteristic peak of LiC6 at 2θ = 32 - 35°, with a peak intensity of c1; there is a characteristic peak of LiC 12 at 2θ = 33 - 37°, with a peak intensity of c2; the ratio c1 / c2 is the peak intensity ratio of LiC6 to LiC 12 in the XRD pattern. The peak intensity ratios corresponding to each example and comparative example are shown in Tables 1 - 3 below.

[0098] Table 1

[0099]

[0100]

[0101] Table 2

[0102] Peak intensity ratio Example 1 0.88 Comparative Example 1 0.33 Comparative Example 2 0.67 Comparative Example 3 0.45

[0103] Table 3

[0104] Peak intensity ratio Peak intensity ratio Example 11 0.93 Comparative Example 4 0.53

[0105] From the results of Tables 1 - 3 above, it can be seen that: setting a silicon-carbon blending layer 12 with added hard carbon on the side away from the negative electrode current collector can improve the fast-charging ability of the electrode (the proportion of graphite in the fully charged state at a high rate is high), and the smaller the graphite particle size and the thinner the coating thickness, the more significant the effect; optimizing the nickel content and thickness in the positive electrode hardly reduces the fast-charging ability.

[0106] 2. Detection of the cracking degree of the active particles of the positive electrode.

[0107] The specific detection process is as follows:

[0108] The positive electrodes of Example 1 and Examples 9 - 11 were obtained and assembled into a half-cell with lithium metal; by increasing the charging voltage to overcharge it to a specified SOC, denoted as x% SOC (x > 100), the battery was disassembled after discharging, and the surface electrolyte was washed away with dimethyl carbonate (DMC); the electrode was placed under a scanning electron microscope (SEM) and observed at a magnification of 10,000 times and a scale of 5 μm; the number of cracked particles and the total number of particles in the viewing angle were counted, and the cracking ratio = the number of cracked particles / the total number of particles.

[0109] The results of the cracking proportion under overcharge for different SOCs are shown in Table 4 below.

[0110] Table 4

[0111]

[0112] The present invention further optimizes the structure and composition of the positive electrode sheet. By increasing the charging voltage to overcharge it to 120% SOC, the degree of particle cracking can be controlled to 3-10%, preferably 3-5%; when overcharged to 140% SOC, the degree of particle cracking can be controlled to 8-15%, preferably 8-10%.

[0113] Obviously, the above embodiments are merely examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A fast-charging negative electrode sheet for preventing overcharging, characterized in that, Comprising: A negative electrode current collector; A silicon-carbon blended layer disposed on the surface of the negative electrode current collector, the raw materials including a first negative electrode active material and silicon-carbon, and the doping amount of silicon-carbon in the silicon-carbon blended layer being 1-10 wt%; A hard carbon blended layer disposed on the surface of the silicon-carbon blended layer, the raw materials including a second negative electrode active material and hard carbon, and the doping amount of hard carbon in the hard carbon blended layer being 1-15 wt%.

2. The fast-charging negative electrode sheet according to claim 1, wherein The thickness of the hard carbon blended layer is 30-60 μm; And / or, the thickness of the silicon-carbon blended layer is 50-140 μm.

3. The fast-charging negative electrode sheet according to claim 1 or 2, wherein Both the first negative electrode active material and the second negative electrode active material are selected from one or more of natural graphite, artificial graphite, soft carbon, and mesocarbon microbeads; And / or, the particle size of the first negative electrode active material is 10-20 μm; And / or, the particle size of the second negative electrode active material is 3-8 μm.

4. The fast-charging negative electrode sheet according to claim 1 or 2, characterized in that The raw materials of the silicon-carbon blended layer and the hard carbon blended layer further include a binder and a conductive agent.

5. The fast-charging negative electrode sheet according to claim 4, characterized in that, The binder includes one or more of polyvinylidene fluoride, carboxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, styrene-butadiene rubber, and polyimide; And / or, the conductive agent includes one or more of carbon black, Ketjen black, carbon nanotubes, and graphene.

6. The preparation method of the fast-charging negative electrode sheet according to any one of claims 1-5, characterized in that, Comprising: Preparing a hard carbon blended slurry using the raw materials of the hard carbon blended layer, and preparing a silicon-carbon blended slurry using the raw materials of the silicon-carbon blended layer; Coating the silicon-carbon blended slurry on the negative electrode current collector and drying to obtain a negative electrode sheet with a silicon-carbon blended layer; Coating the hard carbon blended slurry on the surface of the silicon-carbon blended layer and drying to obtain a fast-charging negative electrode sheet.

7. A soft-pack battery includes a positive electrode sheet and a negative electrode sheet, characterized in that, The negative electrode sheet is the fast-charging negative electrode sheet according to any one of claims 1-5.

8. The soft-pack battery according to claim 7, wherein, The positive electrode sheet includes: A positive electrode current collector; A high-nickel polycrystalline layer disposed on the surface of the positive electrode current collector, and the nickel content in the high-nickel polycrystalline layer is 80 wt%-95 wt%; A medium-nickel single-crystalline layer disposed on the surface of the high-nickel polycrystalline layer, and the nickel content in the medium-nickel single-crystalline layer is 60 wt%-79 wt%.

9. The soft-pack battery according to claim 8, wherein, The raw materials in the high-nickel polycrystalline layer include a high-nickel polycrystalline ternary positive electrode material; The raw materials in the medium-nickel single-crystalline layer include a medium-nickel single-crystalline ternary positive electrode material; And / or, the total thickness of the high-nickel polycrystalline layer and the medium-nickel single-crystalline layer is 100-400 μm; the ratio of the thickness of the high-nickel polycrystalline layer to the thickness of the medium-nickel single-crystalline layer is (5-8):(2-5).

10. An electrical equipment, characterized in that, Including the soft-pack battery according to any one of claims 7-9.

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