Composite negative plate as well as preparation method and application thereof

Abstract

The composite negative electrode plate comprises a current collector and negative electrode active material layers arranged on at least one surface of the current collector, and the negative electrode active material layers comprise a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer which are sequentially arranged in the thickness direction of the current collector; the first negative electrode active material layer comprises a first negative electrode active material, and 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, and 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, and the third negative electrode active material comprises primary particles; the composite negative plate meets the formula: 30% < = omega 1 * eta 1 + omega 2 * eta 2 + omega 3 * eta 3 < = 70%. According to the invention, through multi-layer coating and regulation and control of specific condition parameters of each layer, good overall interaction is realized, and when the obtained product is applied to a battery, the high-temperature performance can be remarkably improved under the condition that the difference of the fast charging capacity of the battery is not large.

Description

Technical Field

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

[0002] In recent years, with the continuous progress of the battery industry, people have higher and higher demands for high temperature and fast charging performance of batteries. However, the improvement directions of high temperature and fast charging are generally opposite, so how to achieve both high temperature and fast charging has become a technical problem that technicians in this field need to solve urgently.

[0003] The negative electrode is an important component of the battery structure and has a significant impact on battery performance. With the advancement of technology, double-layer coating, one of the means of improving the electrode layer, has received more and more attention. Although the technical means of double-layer coating can improve the fast charging capability of the battery, it places the negative electrode material with better fast charging capability on the upper layer of the electrode in contact with the electrolyte, resulting in deterioration of high temperature performance. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a composite negative electrode sheet and its preparation method and application, which, through a multi-layer coating method (three layers or more), achieves significant improvement in high temperature performance while maintaining little difference in battery fast charging capability.

[0005] The present invention 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, and 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, and 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, and the third negative electrode active material comprises primary particles;

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

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

[0008] In formula (I), ω1 is the weight proportion of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass proportion of the secondary particles in the first negative electrode active material, ω2 is the weight proportion of the second negative electrode active material layer in the negative electrode active material layer, η2 is the proportion of the secondary particles in the second negative electrode active material, ω3 is the weight proportion of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the proportion of 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, the mass ratio of the second negative electrode active material with the amorphous carbon layer coated on the surface 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% to 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 to 10 μm; the particle size Dv50 of the second negative electrode active material is larger than the particle size Dv50 of the third negative electrode active material.

[0016] The present invention also provides a method for preparing the composite negative electrode sheet described in the above technical solution, comprising the following steps:

[0017] preparing a first negative electrode slurry, a second negative electrode slurry, and a third negative electrode slurry respectively;

[0018] The first negative electrode slurry is coated on the current collector to form a first negative electrode active material layer; the second negative electrode slurry is coated on the first negative electrode active material layer to form a second negative electrode active material layer; finally, the third negative electrode slurry is coated on the second negative electrode active material layer to form a third negative electrode active material layer to obtain a composite negative electrode sheet.

[0019] The present invention also 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 described in the above technical solution.

[0020] The present invention 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 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 The composite negative electrode sheet satisfies the following relationship: 30%≤ω1×η1+ω2×η2+ω3×η3≤70%; wherein ω1 is the weight proportion of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass proportion of the secondary particles in the first negative electrode active material, ω2 is the weight proportion of the second negative electrode active material layer in the negative electrode active material layer, η2 is the proportion of the secondary particles in the second negative electrode active material, ω3 is the weight proportion of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the proportion of the secondary particles in the third negative electrode active material. Compared with the prior art, the present invention achieves better overall interaction by multi-layer coating and regulating the specific condition parameters of each layer. The obtained composite negative electrode sheet is applied to the battery, which can significantly improve the high temperature performance when the battery fast charging capacity is not much different. DETAILED DESCRIPTION

[0021] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The present invention 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, and 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, and 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, and the third negative electrode active material comprises primary particles;

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

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

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

[0026] In the present invention, the composite negative electrode sheet includes a current collector and a negative electrode active material layer arranged on at least one side of the current collector, and the negative electrode active material layer includes 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 invention has no special restrictions on the type and source of the current collector, and a current collector for preparing negative electrode sheets well known to those skilled in the art can be used.

[0028] In the present invention, 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; on this basis, the composite negative electrode sheet satisfies the relationship shown in formula (I);

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

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

[0031] In the present invention, when the total proportion of secondary particles in the active material layer (ω1×η1+ω2×η2+ω3×η3) is less than 30%, it is easy to cause excessive rebound of the pole piece, large battery expansion force, and deterioration of battery performance; when the total proportion of secondary particles in the active material layer (ω1×η1+ω2×η2+ω3×η3) is higher than 70%, the pole piece rebounds greatly in the lateral direction, causing the pole piece to wrinkle. Therefore, the present invention regulates the total proportion of secondary particles in each layer through the above formula (I), which can achieve overall good interaction and optimize the performance of the obtained composite negative electrode sheet. In a preferred embodiment of the present invention, 42.5%≤ω1×η1+ω2×η2+ω3×η3≤46%.

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

[0033] In the present invention, the ω1 is preferably 35% to 50%, specifically 35%, 43%, 45%, 46%, 50%; the η1 is specifically 50%, 40%, 30%, 20%, 10%, 0%; the secondary particles have lower compaction than the primary particles, and are difficult to process. When the proportion of secondary particles in the first negative electrode active material is too high, it is easy to cause insufficient peeling force of the pole piece and demolding of the pole piece.

[0034] In the present invention, the ω2 is preferably 35% to 50%, specifically 35%, 43%, 45%, 46%, 50%; the η2 can specifically be 50%, 60%, 70%, 80%, 90%, 100%; the second layer of active material secondary particles has a high 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 invention, the ω3 is preferably 8% to 15%, specifically 8%, 10%, 14%, and 15%; the η3 is 0, that is, the third negative electrode active material contains only primary particles, because its surface reaction activity is low and the high temperature performance is good, which can reduce the contact between the electrolyte and the second negative electrode active material and improve the overall high temperature performance.

[0036] In the present invention, 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; the third negative electrode active material is preferably artificial graphite. In the present invention, the third negative electrode active material layer is in contact with the electrolyte in order to reduce the contact between the first to second negative electrode active material layers and the electrolyte, and natural graphite has many internal defects and poor high temperature performance, and is not suitable for the third negative electrode active material layer. Therefore, the third negative electrode active material is preferably artificial graphite, while the first to second negative electrode active material layers are not subject to such restrictions.

[0037] In the present invention, 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 preferably ≤30%, specifically 30%, 20%, 10%. In the present invention, the first active material layer is in contact with the current collector, the surface-coated amorphous carbon has more defects, the active sites are increased, and it is beneficial to improve the fast charging capability; when the coating ratio is higher, it is easy to cause a difference in peeling force with the current collector, and the pole piece is demolded.

[0038] In the present invention, 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 preferably ≥50%, specifically 50%, 60%, 70%, 80%, 90%, 100%; the average thickness of the amorphous carbon layer is preferably ≥0.3μm. In the present invention, the second negative electrode active material layer plays a role in rapid transmission and embedding of lithium ions, and its fast charging capability is required to be high, and at least part of the surface needs to be coated with an amorphous carbon layer to improve the fast charging capability; at the same time, when the coated carbon layer is too thin, the improvement of the fast charging capability is not obvious.

[0039] In the present invention, the thickness d3 of the third negative electrode active material layer preferably accounts for 5% to 15% of the total thickness d of the active material layer, and more preferably 5.3% to 14.7%; in a preferred embodiment of the present invention, 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 invention, when the thickness of the third negative electrode active material layer is too thick, the transmission of lithium ions is affected; when the third negative electrode active material layer is too thin, the processing is difficult and the effect of isolating the electrolyte is poor, and the performance improvement is not obvious; therefore, within the range of 5 to 15%, the comprehensive performance is optimal.

[0040] In the present invention, the particle size Dv50 of the third negative electrode active material is preferably 5μm to 10μm, more preferably 5.2μm to 7.5μm; the particle size Dv50 of the second negative electrode active material is preferably greater than the particle size Dv50 of the third negative electrode active material; in a preferred embodiment of the present invention, the particle size Dv50 of the second negative electrode active material is preferably 10.2μm to 14.2μm. In the present invention, the third negative electrode active material layer uses small-particle materials, which have small particle sizes and low ion migration tortuosity, and can take into account the fast charging capability of the battery; while the second negative electrode active material layer has a larger particle size, a smaller OI value, and better isotropy of the active material, which is conducive to the improvement of fast charging capability.

[0041] The present invention achieves better overall interaction through multi-layer coating (three layers or more) and regulation of specific condition parameters of each layer. The obtained composite negative electrode sheet is used in the battery, which can significantly improve the high temperature performance without much difference in the fast charging capability of the battery.

[0042] In the present invention, if the above-mentioned multi-layer coating method is more than three layers, it is preferred to further arrange an nth negative electrode active material layer (n≥4) on the third negative electrode active material layer. At the same time, it is limited that the nth negative electrode active material layer contains only primary particles, and the first layer to the n-1th layer all contain primary particles and secondary particles. On this basis, the core idea of ​​the present invention (relationship and the setting of active materials in each layer) can also be met, and will not be repeated here.

[0043] The present invention also provides a method for preparing the composite negative electrode sheet described in the above technical solution, comprising the following steps:

[0044] preparing a first negative electrode slurry, a second negative electrode slurry, and a third negative electrode slurry respectively;

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

[0046] In the present invention, the preparation method of the first, second and third negative electrode slurries can adopt the preparation method of negative electrode slurries well known to those skilled in the art, specifically comprising: uniformly mixing the negative electrode active material, the conductive agent, the binder and the solvent to obtain the negative electrode slurry; wherein the negative electrode active material is the first, second and third active substances in the above technical scheme, and the present invention will not repeat them here; the conductive agent is preferably conductive carbon black; the present invention has no special restrictions on the type and source of the binder and the solvent, and the binder and solvent for preparing the negative electrode sheet well known to those skilled in the art can be used.

[0047] In the present invention, the mass ratio of the negative electrode 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 invention, the coating process is preferably as follows: applying the negative electrode slurry to at least one surface of the current collector and the corresponding coated layer to form a corresponding active material layer, and then drying, cold pressing, slitting, and die-cutting to obtain the composite negative electrode sheet.

[0049] The present invention has no special restrictions on the type and source of the current collector, and the current collector for preparing negative electrode sheets well known to those skilled in the art can be used; the preparation process and specific operations, such as mixing, coating, drying, cold pressing, slitting, die-cutting, etc. can be implemented using technical means for preparing negative electrode sheets well known to those skilled in the art, and the present invention has no special restrictions on this.

[0050] The present invention also 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 described in the above technical solution.

[0051] In the present invention, after the negative electrode sheet is prepared, the present invention prepares a full battery with the obtained negative electrode sheet, positive electrode sheet, separator, and electrolyte. The present invention has no special restrictions on the specific selection, preparation method, and assembly process of the positive electrode sheet, separator, and electrolyte. On the basis of meeting the above-mentioned negative electrode sheet requirements, the positive electrode sheet, separator, and electrolyte that can obtain a full battery that are well known to those skilled in the art can be used and assembled in a conventional manner.

[0052] In a preferred embodiment of the present invention, the preparation process of the positive electrode plate is specifically as follows:

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

[0054] In a preferred embodiment of the present invention, the isolation membrane is a PE porous polymer film.

[0055] In a preferred embodiment of the present invention, the electrolyte is a 1 mol / L to 2 mol / L lithium salt solution, preferably a 1.2 mol / L LiPF 6 The organic solvent is a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 3:5:2.

[0056] Finally, the preparation method of the full battery is preferably as follows: arrange the positive electrode plate, the isolation film, and the negative electrode plate in order, place a layer of isolation film between each pair of positive and negative electrodes, and wind them to obtain a bare cell; place the bare cell in an outer packaging shell, and then inject the prepared electrolyte into the dried bare cell, and obtain a lithium-ion battery after vacuum packaging, standing, formation, shaping and other processes.

[0057] The present invention 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 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 The composite negative electrode sheet satisfies the following relationship: 30%≤ω1×η1+ω2×η2+ω3×η3≤70%; wherein ω1 is the weight proportion of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass proportion of the secondary particles in the first negative electrode active material, ω2 is the weight proportion of the second negative electrode active material layer in the negative electrode active material layer, η2 is the proportion of the secondary particles in the second negative electrode active material, ω3 is the weight proportion of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the proportion of the secondary particles in the third negative electrode active material. Compared with the prior art, the present invention achieves better overall interaction by multi-layer coating and regulating the specific condition parameters of each layer. The obtained composite negative electrode sheet is applied to the battery, which can significantly improve the high temperature performance when the battery fast charging capacity is not much different.

[0058] In order to further illustrate the present invention, the following examples are provided for detailed description. In the following examples of the present invention, if no specific conditions are specified, the reaction is carried out according to conventional conditions or conditions recommended by the manufacturer, and if no manufacturer is specified for the reagents or instruments used, all of them are conventional products that can be purchased commercially.

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

[0060] Battery preparation process:

[0061] Preparation of positive electrode sheets: Lithium iron phosphate, polyvinylidene fluoride (PVDF), and conductive carbon black (Super-P) are mixed in a mass ratio of 97:2:1, and stirred to form a positive electrode slurry. The positive electrode slurry is coated on aluminum foil, dried in an oven, and then rolled and slit to obtain positive electrode sheets.

[0062] The negative electrode sheet adopts the negative electrode sheet in each embodiment and each comparative example in Table 1. The preparation of the negative electrode sheet includes the following steps: preparing a first negative electrode slurry, preparing a second negative electrode slurry, preparing a third negative electrode slurry, and coating 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) are placed in a stirring tank at a mass ratio of 95.5:1.5:2:1, and pure water is added and stirred evenly to obtain the first negative electrode slurry.

[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) are placed in a stirring tank at a mass ratio of 95.5:1.5:2:1, and pure water is added and stirred evenly to obtain the second negative electrode slurry.

[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) are placed in a stirring tank at a mass ratio of 95.5:1.5:2:1, and pure water is added and stirred evenly to obtain the third negative electrode slurry.

[0066] Coating slurry: The first negative electrode slurry is uniformly coated on the current collector to obtain a first negative electrode active material layer, the second negative electrode slurry is 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 is 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 active material layers are controlled according to Table 1.

[0067] Table 1

[0068]

[0069]

[0070]

[0071] Isolation membrane: PE porous polymer film is used as the isolation membrane.

[0072] Preparation of electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 3:5:2, and then fully dried lithium salt LiPF 6 It is dissolved in a mixed organic solvent at a ratio of 1.2 mol / L to prepare an electrolyte.

[0073] Preparation of full battery: Arrange the positive electrode sheet, the separator, and the negative electrode sheets in the above embodiments and comparative examples in order, place a separator between each pair of positive and negative electrodes, and wind them to obtain a bare cell. Place the bare cell in an outer packaging shell, inject the prepared electrolyte into the dried bare cell, and obtain a lithium-ion battery after vacuum packaging, standing, forming, shaping and other processes.

[0074] Performance Testing:

[0075] (1) 45℃ cycle life test: Using the Xinwei charge and discharge tester, first place the battery in a 45℃ incubator and leave it for 6 hours to allow the internal temperature of the battery to reach thermal equilibrium with the incubator temperature; then charge and discharge, the charge and discharge process is: 1C constant current charge to 3.65V, then constant voltage charge to 0.05V, leave it for 10 minutes, then 1C discharge to 2.5V, leave it for 10 minutes; 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 / C0×100%, that is, the SOH of the battery at this time; record the number of cycles when the battery reaches 80% SOH.

[0076] (2) 4C fast charging capability test: Using the Xinwei charge and discharge tester, first place the battery in a 25°C incubator and leave it for 6 hours to allow the internal temperature of the battery to reach thermal equilibrium with the incubator temperature; then charge and discharge, the charge and discharge process is: 0.33C constant current charging to 3.65V, then constant voltage charging to 0.05V, leave it for 10 minutes, then 0.33C discharge to 2.5V, leave it for 10 minutes, cycle three times, record the discharge capacity of the last cycle and mark it as C0; then charge to 3.65V at a constant current rate of 4C0, then charge to 0.05C0 at a constant voltage, and discharge to 2.5V at a constant current of 1C0, repeat 50 times, turn on the machine in a fully charged state, disassemble the battery, and observe the lithium precipitation at the negative electrode interface;

[0077] The judgment criteria are: slight lithium deposition (lithium deposition area does not exceed 10% of the negative electrode area), moderate lithium deposition (lithium deposition area accounts for 10% to 50% of the negative electrode area), and severe lithium deposition (lithium deposition area accounts for more than 50% of the negative electrode area).

[0078] The test results are shown in Table 2 below.

[0079] Table 2

[0080] 45℃ cycle life / time 4C fast charging capability Comparative Example 1 1800 Slight lithium deposition 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 Embodiment 11 2840 Moderate lithium precipitation Example 12 2720 Moderate lithium precipitation Embodiment 13 1740 No lithium precipitation

[0081] Experimental results show that the composite negative electrode sheet and battery provided by the present invention have the following advantages: both the fast charging capability and high temperature performance (45°C cycle life) of the battery are taken into account.

[0082] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite negative electrode sheet, characterized in that: The invention comprises 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, and 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, and 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, and the third negative electrode active material comprises primary particles; The composite negative electrode sheet satisfies the relationship shown in formula (I); 30%≤ω1×η1+ω2×η2+ω3×η3≤70% Formula (I); In formula (I), ω1 is the weight proportion of the first negative electrode active material layer in the negative electrode active material layer, η1 is the mass proportion of the secondary particles in the first negative electrode active material, ω2 is the weight proportion of the second negative electrode active material layer in the negative electrode active material layer, η2 is the proportion of the secondary particles in the second negative electrode active material, ω3 is the weight proportion of the third negative electrode active material layer in the negative electrode active material layer, and η3 is the proportion of secondary particles in the third negative electrode active material.

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

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

4. The composite negative electrode sheet according to claim 1, characterized in that: 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 in that: 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 in that: In the second negative electrode active material layer, part of the second negative electrode active material is coated with an amorphous carbon layer, the mass ratio of the second negative electrode active material with the amorphous carbon layer coated on the surface 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 ≥0.3μm.

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

8. The composite negative electrode sheet according to claim 1, characterized in that: The particle size Dv50 of the third negative electrode active material is 5 μm to 10 μm; the particle size Dv50 of the second negative electrode active material is larger than the particle size Dv50 of the third negative electrode active material.

9. A method for preparing a composite negative electrode sheet according to any one of claims 1 to 8, characterized in that: The following steps are involved: preparing a first negative electrode slurry, a second negative electrode slurry, and a third negative electrode slurry respectively; The first negative electrode slurry is coated on the current collector to form a first negative electrode active material layer; the second negative electrode slurry is coated on the first negative electrode active material layer to form a second negative electrode active material layer; finally, the third negative electrode slurry is coated on the second negative electrode active material layer to form a third negative electrode active material layer to obtain a composite negative electrode sheet.

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

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