Composite negative electrode sheet and preparation method and application thereof

Abstract

A composite negative electrode sheet comprises a current collector and a negative electrode active material layer arranged on at least one side of the current collector, the negative electrode active material layer comprising a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer arranged in sequence along the thickness direction of the current collector; the first negative electrode active material layer contains a first negative electrode active material comprising primary particles and secondary particles; the second negative electrode active material layer contains a second negative electrode active material comprising primary particles and secondary particles; the third negative electrode active material layer contains a third negative electrode active material comprising primary particles; and the composite negative electrode sheet satisfies the formula: 30%≤ω1×η1+ω2×η2+ω3×η3≤70%. The application realizes good mutual interaction of the whole through multi-layer coating and regulation of specific condition parameters of each layer, and the product obtained is applied in a battery, which can significantly improve the high-temperature performance under the condition that the fast-charging ability of the battery is not significantly different.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, and more particularly to a composite negative electrode sheet and a preparation method and application thereof. BACKGROUND

[0002] In recent years, with the continuous progress of the battery industry, people's demand for high temperature and fast charging performance of batteries is getting higher and higher. However, the improvement direction of high temperature and fast charging is generally opposite, so how to realize the balance of high temperature and fast charging has become a technical problem to be solved by the technical personnel in the field.

[0003] The negative electrode sheet is an important component in the structure of the battery and has an important influence on the performance of the battery. With the progress of technology, one of the improvement methods of the electrode sheet level is the double-layer coating, which has attracted more and more attention. Although the technical means of double-layer coating can improve the fast charging capacity of the battery, the negative electrode material with better fast charging capacity is placed on the upper layer of the electrode sheet and contacts with the electrolyte, which leads to the deterioration of the high temperature performance. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a composite negative electrode sheet and a preparation method and application thereof, which realizes the significant improvement of the high temperature performance under the condition that the difference of the fast charging capacity of the battery is not large through the multi-layer coating method (three layers and more).

[0005] The present application provides a composite negative electrode sheet, which comprises a current collector and a negative active material layer arranged on at least one side of the current collector, the negative active material layer comprising a first negative active material layer, a second negative active material layer and a third negative active material layer arranged in sequence along the thickness direction of the current collector; the first negative active material layer comprises a first negative active material, the first negative active material comprising primary particles and secondary particles; the second negative active material layer comprises a second negative active material, the second negative active material comprising primary particles and secondary particles; and the third negative active material layer comprises a third negative active material, the third negative active material comprising primary particles.

[0006] The composite negative electrode sheet satisfies the relationship formula as shown in formula (I);

[0007] 30%≤ω1×η1+ω2×η2+ω3×η3≤70% Formula (I);

[0008] In formula (I), ω1 is the weight ratio of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass ratio of the secondary particles in the first negative electrode active material, ω2 is the weight ratio of the second negative electrode active material layer in the negative electrode active material layer, η2 is the ratio of the secondary particles in the second negative electrode active material, ω3 is the weight ratio of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the ratio of the secondary particles in the third negative electrode active material.

[0009] Preferably, 30%≤ω1≤50%, 30%≤ω2≤50%, and 5%≤ω3≤20%.

[0010] Preferably, η1≤50%, η2≥50%, and η3 is 0.

[0011] Preferably, the first negative electrode active material is one or more of artificial graphite and natural graphite; the second negative electrode active material is one or more of artificial graphite and natural graphite; and the third negative electrode active material is artificial graphite.

[0012] Preferably, in the first negative electrode active material layer, part of the first negative electrode active material is coated with an amorphous carbon layer, and the mass ratio of the first negative electrode active material coated with the amorphous carbon layer to the first negative electrode active material in the first negative electrode active material layer is ≤30%.

[0013] Preferably, in the second negative electrode active material layer, part of the second negative electrode active material is coated with an amorphous carbon layer, and the mass ratio of the second negative electrode active material coated with the amorphous carbon layer to the second negative electrode active material in the second negative electrode active material layer is ≥50%, and the average thickness of the amorphous carbon layer is ≥0.3 μm.

[0014] Preferably, the thickness of the third negative electrode active material layer accounts for 5%-15% of the thickness of the active material layer.

[0015] Preferably, the particle size Dv50 of the third negative electrode active material is 5 μm-10 μm, and the particle size Dv50 of the second negative electrode active material is greater than the particle size Dv50 of the third negative electrode active material.

[0016] The application also provides a preparation method of the composite negative electrode sheet.

[0017] The first negative electrode slurry, the second negative electrode slurry, and the third negative electrode slurry are prepared respectively.

[0018] Coating the first negative electrode slurry on a current collector to form a first negative electrode active material layer; coating the second negative electrode slurry on the first negative electrode active material layer to form a second negative electrode active material layer; and finally coating the third negative electrode slurry on the second negative electrode active material layer to form a third negative electrode active material layer, thereby obtaining a composite negative electrode sheet.

[0019] The application further provides a battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet is the composite negative electrode sheet as described in the technical solution.

[0020] The application provides a composite negative electrode sheet, comprising a current collector and a negative electrode active material layer arranged on at least one side of the current collector, wherein the negative electrode active material layer comprises a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer arranged in sequence along the thickness direction of the current collector; the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprising primary particles and secondary particles; the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprising primary particles and secondary particles; the third negative electrode active material layer comprises a third negative electrode active material, the third negative electrode active material comprising primary particles; and the composite negative electrode sheet satisfies the following relationship: 30%≤ω1×η1+ω2×η2+ω3×η3≤70%; wherein ω1 is the weight ratio of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass ratio of the secondary particles in the first negative electrode active material, ω2 is the weight ratio of the second negative electrode active material layer in the negative electrode active material layer, η2 is the ratio of the secondary particles in the second negative electrode active material, ω3 is the weight ratio of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the ratio of the secondary particles in the third negative electrode active material. Compared with the prior art, the application realizes good mutual interaction as a whole by means of multi-layer coating and regulation of specific condition parameters of each layer, and the composite negative electrode sheet obtained by the application can significantly improve the high-temperature performance under the condition that the fast-charging capacity of the battery is not significantly different. DETAILED DESCRIPTION

[0021] The technical solutions of the application will be described clearly and completely below in combination with the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0022] The present application provides a composite negative electrode sheet, comprising a current collector and a negative electrode active material layer arranged on at least one side of the current collector, the negative electrode active material layer comprising a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer arranged in sequence along the thickness direction of the current collector; the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprising primary particles and secondary particles; the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprising primary particles and secondary particles; and the third negative electrode active material layer comprises a third negative electrode active material, the third negative electrode active material comprising primary particles.

[0023] The composite negative electrode sheet satisfies a relationship as shown in formula (I);

[0024] 30%≤ω1×η1+ω2×η2+ω3×η3≤70% Formula (I);

[0025] In formula (I), ω1 is the weight ratio of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass ratio of the secondary particles in the first negative electrode active material, ω2 is the weight ratio of the second negative electrode active material layer in the negative electrode active material layer, η2 is the ratio of the secondary particles in the second negative electrode active material, ω3 is the weight ratio of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the ratio of the secondary particles in the third negative electrode active material.

[0026] In the present application, the composite negative electrode sheet comprises a current collector and a negative electrode active material layer arranged on at least one side of the current collector, the negative electrode active material layer comprising a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer arranged in sequence along the thickness direction of the current collector.

[0027] The present application does not have special restrictions on the type and source of the current collector, and any current collector known to those skilled in the art for preparing a negative electrode sheet can be used.

[0028] In the present application, the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprising primary particles and secondary particles; the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprising primary particles and secondary particles; and the third negative electrode active material layer comprises a third negative electrode active material, the third negative electrode active material comprising primary particles; on this basis, the composite negative electrode sheet satisfies a relationship as shown in formula (I);

[0029] 30%≤ω1×η1+ω2×η2+ω3×η3≤70% Formula (I);

[0030] In formula (I), ω1 is the weight ratio of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass ratio of the secondary particles in the first negative electrode active material, ω2 is the weight ratio of the second negative electrode active material layer in the negative electrode active material layer, η2 is the ratio of the secondary particles in the second negative electrode active material, ω3 is the weight ratio of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the ratio of the secondary particles in the third negative electrode active material.

[0031] In the present application, when the total ratio of the secondary particles (ω1×η1+ω2×η2+ω3×η3) in the active material layer is less than 30%, the electrode sheet is prone to have excessive rebound, the battery has large swelling force, and the battery performance is deteriorated; when the total ratio of the secondary particles (ω1×η1+ω2×η2+ω3×η3) in the active material layer is higher than 70%, the electrode sheet has large rebound in the transverse direction, and the electrode sheet is prone to wrinkle. Therefore, by regulating the total ratio of the secondary particles in each layer by formula (I), the present application can realize good overall interaction, and the obtained composite negative electrode sheet has optimal performance. In the preferred embodiments of the present application, 42.5%≤ω1×η1+ω2×η2+ω3×η3≤46%.

[0032] In the present application, 30%≤ω1≤50%, 30%≤ω2≤50%, and 5%≤ω3≤20%; η1≤50%, η2≥50%, and η3 is 0.

[0033] In the present application, ω1 is preferably 35%-50%, and can be 35%, 43%, 45%, 46%, or 50% in particular; η1 can be 50%, 40%, 30%, 20%, 10%, or 0% in particular; the tap density of the secondary particles is lower than that of the primary particles, and the processing difficulty is large; when the ratio of the secondary particles in the first negative electrode active material is too high, the electrode sheet is prone to have insufficient peeling force and electrode sheet demolding.

[0034] In the present application, ω2 is preferably 35%-50%, and can be 35%, 43%, 45%, 46%, or 50% in particular; η2 can be 50%, 60%, 70%, 80%, 90%, or 100% in particular; the second layer active material has high secondary particle content, which can ensure the fast charging capability of the second negative electrode active material layer and realize the fast charging capability of the composite negative electrode sheet.

[0035] In the present application, ω3 is preferably 8%-15%, and can be 8%, 10%, 14%, or 15% in particular; η3 is 0, i.e., the third negative electrode active material contains only primary particles; because the surface reactivity of the primary particles is low and the high-temperature performance is good, the contact between the electrolyte and the second negative electrode active material can be reduced, and the overall high-temperature performance can be improved.

[0036] In the present application, the first negative electrode active material is preferably one or more of artificial graphite and natural graphite; the second negative electrode active material is preferably one or more of artificial graphite and natural graphite; and the third negative electrode active material is preferably artificial graphite. In the present application, the third negative electrode active material layer is in contact with the electrolyte, and the purpose is to reduce the contact of the first to second negative electrode active material layers with the electrolyte. Natural graphite has many internal defects and poor high-temperature performance, and is not suitable for the third negative electrode active material layer, so the third negative electrode active material is preferably artificial graphite, and there is no such limitation for the first negative electrode active material layer to the second negative electrode active material layer.

[0037] In the present application, in the first negative electrode active material layer, part of the first negative electrode active material is coated with an amorphous carbon layer on the surface. The mass ratio of the first negative electrode active material coated with an amorphous carbon layer to the first negative electrode active material in the first negative electrode active material layer is preferably ≤ 30%, and can be 30%, 20%, or 10%. In the present application, the first negative electrode active material layer is in contact with the current collector. The amorphous carbon layer has more defects on the surface, and the active sites are increased, which is beneficial to improve the fast charging capacity. When the coating ratio is higher, the peeling force between the current collector and the electrode sheet is poor, and the electrode sheet is difficult to demold.

[0038] In the present application, in the second negative electrode active material layer, part of the second negative electrode active material is coated with an amorphous carbon layer on the surface. The mass ratio of the second negative electrode active material coated with an amorphous carbon layer to the second negative electrode active material in the second negative electrode active material layer is preferably ≥ 50%, and can be 50%, 60%, 70%, 80%, 90%, or 100%. The average thickness of the amorphous carbon layer is preferably ≥ 0.3 μm. In the present application, the second negative electrode active material layer plays a role in rapid transmission and embedding of lithium ions, and the fast charging capacity is required to be high. At least part of the surface is coated with an amorphous carbon layer to improve the fast charging capacity. At the same time, when the carbon layer is too thin, the improvement of the fast charging capacity is not obvious.

[0039] In the present application, the thickness d3 of the third negative electrode active material layer is preferably 5% to 15% of the total thickness d of the active material layer, and more preferably 5.3% to 14.7%. In the preferred embodiment of the present application, the thickness d1 of the first negative electrode active material layer is preferably 50 μm to 73 μm, the thickness d2 of the second negative electrode active material layer is preferably 56 μm to 78 μm, and the thickness d3 of the third negative electrode active material layer is preferably 8 μm to 22 μm. In the present application, when the thickness of the third negative electrode active material layer is too thick, the transmission of lithium ions is affected. When the thickness of the third negative electrode active material layer is too thin, the processing difficulty is large, the effect of isolating the electrolyte is poor, and the performance improvement is not obvious. Therefore, in the range of 5 to 15%, the comprehensive performance is the best.

[0040] In the present application, the particle size Dv50 of the third negative electrode active material is preferably 5-10 mu m, more preferably 5.2-7.5 mu m; the particle size Dv50 of the second negative electrode active material is preferably larger than that of the third negative electrode active material; in the preferred embodiment of the present application, the particle size Dv50 of the second negative electrode active material is preferably 10.2-14.2 mu m. In the present application, the third negative electrode active material layer uses small-particle material, the small-particle material has small particle size and low ion migration tortuosity, and the fast charging capability of the battery can be considered; and the second negative electrode active material layer has larger particle size, smaller OI value and better active material isotropy, which is beneficial to the improvement of the fast charging capability.

[0041] The present application realizes good overall interaction by the multi-layer coating method (three layers or more) and the regulation of specific condition parameters of each layer, and the composite negative electrode sheet obtained by the method can significantly improve the high-temperature performance under the condition that the fast charging capability of the battery is not significantly different.

[0042] In the present application, if the above multi-layer coating method is three layers or more, the nth negative electrode active material layer (n≥4) is further arranged on the third negative electrode active material layer, and meanwhile, the nth negative electrode active material layer is limited to contain only primary particles, and the first layer to the (n-1)th layer contain both primary particles and secondary particles, which can also meet the core idea (relationship and arrangement of active materials in each layer) of the present application, and will not be described here.

[0043] The present application also provides a preparation method of the composite negative electrode sheet.

[0044] The first negative electrode slurry, the second negative electrode slurry and the third negative electrode slurry are prepared respectively.

[0045] The first negative electrode slurry is coated on the current collector to form the first negative electrode active material layer; the second negative electrode slurry is coated on the first negative electrode active material layer to form the second negative electrode active material layer; and the third negative electrode slurry is coated on the second negative electrode active material layer to form the third negative electrode active material layer, thereby obtaining the composite negative electrode sheet.

[0046] In the present application, the preparation method of the first, second and third negative electrode slurries can use the preparation method of the negative electrode slurry known to those skilled in the art, which specifically includes: mixing negative electrode active material, conductive agent, binder and solvent uniformly to obtain a negative electrode slurry; wherein the negative electrode active material is the first, second and third active materials in the above technical solution, which will not be described here; the conductive agent is preferably conductive carbon black; and the types and sources of the binder and the solvent are not particularly limited in the present application, and the binder and the solvent used for preparing the negative electrode sheet known to those skilled in the art can be used.

[0047] In the present application, the mass ratio of the negative active material, the conductive agent, the binder and the solvent is preferably (90-99):(0.5-1.5):(1-1.5):(1.5-2), and more preferably 95.5:1:1.5:2.

[0048] In the present application, the coating process is preferably specifically as follows: the above negative electrode slurry is coated on at least one surface of the current collector and the corresponding coated layer to form a corresponding active material layer, and then the coated layer is dried, cold-pressed, slitted and die-cut to obtain the composite negative electrode sheet.

[0049] The present application does not have special restrictions on the type and source of the current collector, and any current collector known to those skilled in the art for preparing a negative electrode sheet can be used; the preparation process and specific operations, such as mixing, coating, drying, cold-pressing, slitting and die-cutting, can be carried out using the technical means known to those skilled in the art for preparing a negative electrode sheet, and the present application does not have special restrictions thereon.

[0050] The present application also provides a battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator film, wherein the negative electrode sheet is the composite negative electrode sheet described in the above technical solution.

[0051] In the present application, after the negative electrode sheet is prepared, the obtained negative electrode sheet is combined with a positive electrode sheet, a separator film and an electrolyte to prepare a full battery. The present application does not have special restrictions on the specific selection, preparation method and assembly process of the positive electrode sheet, the separator film and the electrolyte, and any positive electrode sheet, separator film and electrolyte known to those skilled in the art that can obtain a full battery can be used according to the conventional assembly method as long as the above requirements for the negative electrode sheet are met.

[0052] In the preferred embodiment of the present application, the preparation process of the positive electrode sheet is specifically as follows:

[0053] The positive electrode active material, the conductive agent and the binder are mixed uniformly in the solvent at a mass ratio of (90-99):(1-2):(1-2) to obtain a positive electrode slurry; and the above positive electrode slurry is sequentially coated, dried, cold-pressed and slitted to obtain a positive electrode sheet.

[0054] In the preferred embodiment of the present application, the separator film is a PE porous polymer film.

[0055] In the preferred embodiment of the present application, the electrolyte is a 1 mol / L-2 mol / L lithium salt solution, preferably a 1.2 mol / L LiPF6 organic solution; and the organic solvent is a mixture of ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) at a volume ratio of 3:5:2.

[0056] Finally, the preparation method of the full battery is preferably specific as follows: arranging a positive electrode sheet, a separator film and a negative electrode sheet in sequence, placing a layer of the separator film in the middle of each pair of positive and negative electrodes, and winding to obtain a bare battery cell; placing the bare battery cell in an outer packaging shell, and then injecting the prepared electrolyte into the dried bare battery, and then obtaining the lithium ion battery through vacuum packaging, standing, formation, shaping and other processes.

[0057] The application provides a composite negative electrode sheet, which comprises a current collector and a negative active material layer arranged on at least one side of the current collector, wherein the negative active material layer comprises a first negative active material layer, a second negative active material layer and a third negative active material layer arranged in sequence along the thickness direction of the current collector; the first negative active material layer comprises a first negative active material, and the first negative active material comprises primary particles and secondary particles; the second negative active material layer comprises a second negative active material, and the second negative active material comprises primary particles and secondary particles; the third negative active material layer comprises a third negative active material, and the third negative active material comprises primary particles; and the composite negative electrode sheet satisfies the following relationship: 30%≤ω1×η1+ω2×η2+ω3×η3≤70%; wherein ω1 is the weight ratio of the first negative active material layer in the negative active material layer, η1 is the mass ratio of the secondary particles in the first negative active material, ω2 is the weight ratio of the second negative active material layer in the negative active material layer, η2 is the ratio of the secondary particles in the second negative active material, ω3 is the weight ratio of the third negative active material layer in the negative active material layer, and η3 is the ratio of the secondary particles in the third negative active material. Compared with the prior art, the composite negative electrode sheet is obtained by the multi-layer coating method and the regulation of specific conditions of each layer, and the composite negative electrode sheet has good interaction as a whole, so that the high-temperature performance of the battery can be significantly improved under the condition that the fast-charging ability of the battery is not significantly different.

[0058] In order to further illustrate the application, the following examples are used for detailed description. In the following examples of the application, the specific conditions not marked are carried out according to the conventional conditions or the conditions recommended by the manufacturer, and the reagents or instruments not marked with the manufacturer are conventional products that can be purchased in the market.

[0059] Examples 1-13 and Comparative Examples 1-5

[0060] Battery preparation process:

[0061] Preparation of the positive electrode sheet: lithium iron phosphate, polyvinylidene fluoride (PVDF), and conductive carbon black (Super-P) were mixed in a mass ratio of 97:2:1, and after stirring, a positive electrode slurry was formed. The positive electrode slurry was coated on an aluminum foil, dried in an oven, and then prepared into a positive electrode sheet by rolling and slitting.

[0062] The negative electrode sheet of each example and each comparative example in Table 1 was used. The preparation of the negative electrode sheet included the following steps: preparation of a first negative electrode slurry, preparation of a second negative electrode slurry, preparation of a third negative electrode slurry, and coating of the slurry.

[0063] Preparation of the first negative electrode slurry: the first active material artificial graphite, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black (Super-P) were placed in a stirring kettle in a mass ratio of 95.5:1.5:2:1, and after stirring uniformly with pure water, the first negative electrode slurry was obtained.

[0064] Preparation of the second negative electrode slurry: the second active material artificial graphite, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black (Super-P) were placed in a stirring kettle in a mass ratio of 95.5:1.5:2:1, and after stirring uniformly with pure water, the second negative electrode slurry was obtained.

[0065] Preparation of the third negative electrode slurry: the third active material artificial graphite, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black (Super-P) were placed in a stirring kettle in a mass ratio of 95.5:1.5:2:1, and after stirring uniformly with pure water, the third negative electrode slurry was obtained.

[0066] Coating of the slurry: the first negative electrode slurry was uniformly coated on the current collector to obtain a first negative electrode active material layer, the second negative electrode slurry was coated on the surface of the first negative electrode active material layer to form a second negative electrode active material layer, and the third negative electrode slurry was coated on the surface of the second negative electrode active material layer to form a third negative electrode active material layer; the weight and thickness distribution of the three layers of active material layers were controlled according to Table 1.

[0067] Table 1

[0068]

[0069]

[0070]

[0071] Separation film: a PE porous polymer film was used as a separation film.

[0072] Preparation of electrolyte: ethylene carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC) were mixed in a volume ratio of 3:5:2, and then fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent at a proportion of 1.2 mol / L to prepare the electrolyte.

[0073] Full battery preparation: the positive electrode sheet, the separator, the negative electrode sheet in each example and each comparative example were arranged in order, a layer of separator was placed between each pair of positive and negative electrodes, and then wound to obtain a bare battery cell. The bare battery cell was placed in an outer packaging shell, and the prepared electrolyte was injected into the dried bare battery. After vacuum packaging, standing, formation, shaping and other processes, a lithium ion battery was obtained.

[0074] Performance test:

[0075] (1) 45℃ cycle life test: using a new Wei charge-discharge tester, first put the battery in a 45℃ oven, stand for 6h, so that the battery internal and oven temperature reach thermal equilibrium; then charge and discharge, the charge-discharge process is: 1C constant current charging to 3.65V, then constant voltage charging to 0.05V, standing for 10min, then 1C discharging to 2.5V, standing for 10min; record the discharge capacity at this time as the first cycle discharge capacity C0, record the capacity of each cycle during the battery cycle, the capacity of the nth cycle is Cn, the capacity retention rate of the nth cycle battery = Cn / C0x100%, that is, the SOH of the battery at this time; record the cycle number when the battery reaches 80% SOH.

[0076] (2) 4C fast charging capacity test: using a new Wei charge-discharge tester, first put the battery in a 25℃ oven, stand for 6h, so that the battery internal and oven temperature reach thermal equilibrium; then charge and discharge, the charge-discharge process is: 0.33C constant current charging to 3.65V, then constant voltage charging to 0.05V, standing for 10min, then 0.33C discharging to 2.5V, standing for 10min, cycle three times, record the discharge capacity of the last cycle and mark it as C0; then charge at a rate of 4C0 to 3.65V, then constant voltage charge to 0.05C0, 1C0 constant current discharge to 2.5V, repeat 50 times, and the battery is disassembled to observe the lithium precipitation of the negative electrode interface under the full power state.

[0077] The judgment standard is: slight lithium precipitation (lithium precipitation area does not exceed 10% of the area of the negative electrode sheet), moderate lithium precipitation (lithium precipitation area accounts for 10%-50% of the area of the negative electrode sheet), and severe lithium precipitation (lithium precipitation area accounts for more than 50% of the area of the negative electrode sheet).

[0078] The test results are shown in the following table 2.

[0079] Table 2

[0080] 45 °c cycle life / times 4c fast charging capability comparative example 1 1800 slight lithium precipitation comparative example 2 1560 no lithium precipitation comparative example 3 1650 no lithium precipitation comparative example 4 1770 no lithium precipitation comparative example 5 2930 moderate lithium precipitation example 1 2380 no lithium precipitation example 2 2410 no lithium precipitation example 3 2460 no lithium precipitation example 4 2590 no lithium precipitation example 5 2530 no lithium precipitation example 6 2560 no lithium precipitation example 7 2630 no lithium precipitation example 8 2500 no lithium precipitation example 9 1840 no lithium precipitation example 10 2380 moderate lithium precipitation example 11 2840 moderate lithium precipitation example 12 2720 moderate lithium precipitation example 13 1740 no lithium precipitation

[0081] The experimental results show that the composite negative plate and the battery provided by the application have the following advantages: the battery charging capacity and high temperature performance (45 DEG C cycle life) are considered.

[0082] The above description of disclosed embodiments enables one of ordinary skill in the art to make or use the application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite negative electrode, characterized in that, The composite negative electrode sheet comprises a current collector and a negative electrode active material layer arranged on at least one side of the current collector, the negative electrode active material layer comprises, in sequence along the thickness direction of the current collector, a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer; the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprises primary particles and secondary particles; the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprises primary particles and secondary particles; the third negative electrode active material layer comprises a third negative electrode active material, the third negative electrode active material comprises primary particles; The composite negative electrode sheet satisfies a relationship as shown in formula (I); 30%≤ω1×η1+ω2×η2+ω3×η3≤70% Formula (I); In formula (I), ω1 is the weight ratio of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass ratio of the secondary particles in the first negative electrode active material, ω2 is the weight ratio of the second negative electrode active material layer in the negative electrode active material layer, η2 is the ratio of the secondary particles in the second negative electrode active material, ω3 is the weight ratio of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the ratio of the secondary particles in the third negative electrode active material.

2. The composite negative electrode sheet according to claim 1, characterized by 30%≤ω1≤50%, 30%≤ω2≤50%, 5%≤ω3≤20%.

3. The composite negative electrode sheet according to claim 1, characterized by η1≤50%, η2≥50%, and η3 is 0.

4. The composite negative electrode sheet according to claim 1, characterized by The first negative electrode active material is one or more of artificial graphite and natural graphite; the second negative electrode active material is one or more of artificial graphite and natural graphite; and the third negative electrode active material is artificial graphite.

5. The composite negative electrode sheet according to claim 1, characterized by In the first negative electrode active material layer, part of the first negative electrode active material is coated with an amorphous carbon layer, and the mass ratio of the first negative electrode active material coated with the amorphous carbon layer to the first negative electrode active material in the first negative electrode active material layer is ≤30%.

6. The composite negative electrode sheet according to claim 1, characterized by In the second negative electrode active material layer, part of the second negative electrode active material is coated with an amorphous carbon layer, and the mass ratio of the second negative electrode active material coated with the amorphous carbon layer to the second negative electrode active material in the second negative electrode active material layer is ≥50%, and the average thickness of the amorphous carbon layer is ≥0.3 μm.

7. The composite negative electrode sheet according to claim 1, characterized by The thickness of the third negative electrode active material layer accounts for 5%-15% of the thickness of the active material layer.

8. The composite negative electrode sheet according to claim 1, characterized by The particle size Dv50 of the third negative electrode active material is 5 μm-10 μm, and the particle size Dv50 of the second negative electrode active material is greater than the particle size Dv50 of the third negative electrode active material.

9. A method for producing the composite negative electrode sheet according to any one of claims 1 to 8, characterized by, The method comprises the following steps: Preparation of first negative electrode slurry, second negative electrode slurry and third negative electrode slurry respectively; The first negative electrode slurry is coated on the current collector to form the first negative electrode active material layer; the second negative electrode slurry is coated on the first negative electrode active material layer to form the second negative electrode active material layer; and the third negative electrode slurry is coated on the second negative electrode active material layer to form the third negative electrode active material layer, thereby obtaining the composite negative electrode sheet.

10. A battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, characterized by The negative electrode sheet is the composite negative electrode sheet according to any one of claims 1 to 8. The negative electrode sheet is the composite negative electrode sheet according to any one of claims 1 to 8.

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