Negative electrode sheet and lithium ion battery

Abstract

The application discloses a negative electrode sheet and a lithium ion battery, and belongs to the technical field of lithium ion batteries. 3 The negative electrode active material comprises a first negative electrode active material and a second negative electrode active material; the negative electrode active material satisfies A-B=0.01-0.75 g / cm 3 ; A is the limit compaction density of the first negative electrode active material, and B is the limit compaction density of the second negative electrode active material; in the negative electrode active material layer, along the direction away from the current collector, the negative electrode active material layer comprises n coating layers, 3<=n<=10, and n is an integer; along the direction away from the current collector, the content of the first negative electrode active material in the n coating layers gradually decreases, and the content of the second negative electrode active material gradually increases. The application can avoid the problems of local black spots or lithium precipitation at the anode interface, and simultaneously improve the cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a negative electrode sheet and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy, high working voltage, wide working temperature range, small volume, light weight, long storage life and the like, and have become a new generation of green and environmentally friendly batteries. With the continuous development of lithium ion battery related technologies, new demands have been put forward for energy density, battery life, safety and battery cost.

[0003] With the increase of the thickness of the lithium ion secondary battery electrode sheet and the increase of the compaction density, the single-layer mechanical mixing coating method in the prior art has been difficult to meet the requirements of energy density and safety performance. Such coating method can cause overpressure of the electrode sheet, poor wettability, poor safety and high risk of thermal runaway.

[0004] Although the emerging double-layer coating technology can solve the above problems, due to the difference in viscosity and surface tension of the two slurries, a large contact / diffusion impedance can be generated between the layers, which is not conducive to the transmission of lithium ions, resulting in reduced rate performance of the lithium ion battery and easy lithium precipitation during the cycle process. SUMMARY

[0005] Therefore, the technical problem to be solved by the present application is to overcome the defects of low rate performance of the lithium ion battery and easy lithium precipitation during the cycle process in the prior art, so as to provide a negative electrode sheet and a lithium ion battery.

[0006] To this end, the present application provides the following technical solutions.

[0007] In a first aspect, the present application provides a negative electrode sheet, comprising a current collector and a negative active material layer arranged on at least one surface of the current collector;

[0008] The negative active material layer comprises a negative active material, and the negative active material comprises a first negative active material and a second negative active material;

[0009] The negative active material satisfies: A-B = 0.01-0.75 g / cm 3 ;

[0010] A is the limiting compaction density of the first negative active material, and B is the limiting compaction density of the second negative active material;

[0011] In the negative active material layer, along the direction away from the current collector, the negative active material layer comprises n coating layers, 3

[0012] The content of the first negative electrode active material gradually decreases and the content of the second negative electrode active material gradually increases in the n coating layers in a direction away from the current collector.

[0013] In a possible implementation, the thickness of the first layer to the (n+1) / 2 layer gradually decreases and the thickness of the (n+1) / 2 layer to the n layer gradually increases in the n coating layers in a direction away from the current collector; the coating layer close to the current collector is the first layer and the coating layer away from the current collector is the n layer.

[0014] Preferably, the thickness of the (n+1) / 2 layer is h (n+1) / 2 = 0, the thickness of the first layer is h1 = the thickness of the n layer is hn. n ;

[0015] Preferably, h1≤130μm.

[0016] In a possible implementation, D50 i gradually increases in the n coating layers in a direction away from the current collector; D50 i is the average particle size of the negative electrode active material in the i layer.

[0017] Preferably, D50 n - D501≤5μm.

[0018] In a possible implementation, X-Y≥3mAh / g is satisfied.

[0019] X is the gram capacity of the first negative electrode active material and Y is the gram capacity of the second negative electrode active material.

[0020] Preferably, the gram capacity of the first negative electrode active material is >355mAh / g.

[0021] Preferably, the gram capacity of the second negative electrode active material is ≤354mAh / g.

[0022] In a possible implementation, M-N≥2 is satisfied.

[0023] M is the OI value of the first negative electrode active material and N is the OI value of the second negative electrode active material.

[0024] Preferably, the OI value of the first negative electrode active material is 4-8.

[0025] Preferably, the OI value of the second negative electrode active material is 2-5.

[0026] In a possible implementation, the negative electrode active material includes one or more of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, titanium-based material, silicon-based material, tin-based material, nitride and metal lithium.

[0027] In a possible implementation, the thickness H of the negative active material layer is < 400 μm;

[0028] In a possible implementation, the limit compaction density of the first negative active material is 1.75-1.95 g / cm 3 ;

[0029] In a possible implementation, the limit compaction density of the second negative active material is 1.2-1.78 g / cm 3 ;

[0030] In a possible implementation, the particle size Dv50(1) of the first negative active material is 5-15 μm;

[0031] In a possible implementation, the particle size Dv50(2) of the second negative active material is 8-20 μm.

[0032] In a possible implementation, the surface of the negative active material layer away from the current collector is provided with a groove region.

[0033] In a possible implementation, the depth h of the groove region satisfies: 0.05h n ≤ h ≤ h n .

[0034] In a possible implementation, the groove region comprises a plurality of linear grooves and / or a plurality of holes;

[0035] Preferably, the line width of the linear groove is 10-150 μm;

[0036] Preferably, the line depth of the linear groove is 3-30 μm;

[0037] Preferably, the hole diameter of the hole is 20-200 μm;

[0038] Preferably, the hole depth of the hole is 5-50 μm.

[0039] The plurality of refers to one or more.

[0040] In a second aspect, the application provides a lithium ion battery comprising the negative electrode sheet.

[0041] The technical scheme of the application has the following advantages:

[0042] 1. The application provides a negative electrode sheet, comprising a current collector and a negative active material layer arranged on at least one surface of the current collector; the negative active material layer comprises a negative active material, the negative active material comprises a first negative active material and a second negative active material; the limit compaction density of the first negative active material - the limit compaction density of the second negative active material = 0.01-0.75 g / cm3; in the negative active material layer, along the direction away from the current collector, the negative active material layer comprises n coating layers, 3≤n≤10, n is an integer; along the direction away from the current collector, the content of the first negative active material in the n coating layers gradually decreases, and the content of the second negative active material gradually increases. 3 ; the limit compaction density of the first negative active material is larger and the gram capacity is higher, and the second negative active material is a negative active material with better kinetics.

[0043] The first negative active material with larger limit compaction density and higher gram capacity is a negative active material with higher energy density, and the second negative active material with smaller limit compaction density and lower gram capacity is a negative active material with better kinetics. It is difficult for a single negative active material to meet the requirements of battery kinetics and high energy density, and the use of two kinds of negative active materials with high kinetics and high gram capacity can effectively solve the above problems.

[0044] The application adopts gradient coating technology, which can improve the uniformity of the first negative active material and the second negative active material per unit area, avoid the problem of local black spots or lithium precipitation at the anode interface caused by single-layer mechanical mixing coating. At the same time, the second negative active material with good kinetics is placed more in the surface layer of the negative electrode sheet, which is more conducive to the improvement of cycle stability. Compared with the double-layer coating technology, the gradient coating technology can further improve the uniformity of the distribution of the two kinds of negative materials in the thickness direction, reduce the difference in viscosity and surface tension of the slurry between different layers, and reduce the transmission / diffusion resistance between adjacent two layers. BRIEF DESCRIPTION OF DRAWINGS

[0045] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed to be used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0046] Figure 1 It is a schematic diagram of a negative electrode sheet;

[0047] Figure 2 It is a schematic diagram of a negative electrode sheet surface groove area.

[0048] Reference signs:

[0049] 1-current collector; 2-negative active material layer; 3-groove area; Z-direction away from the current collector. Detailed Implementation

[0050] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0051] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0052] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active material layer disposed on at least one surface of the current collector;

[0053] The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a first negative electrode active material and a second negative electrode active material;

[0054] The negative electrode active material satisfies the following condition: AB = 0.01–0.75 g / cm³. 3 ;

[0055] A is the ultimate compaction density of the first negative electrode active material, and B is the ultimate compaction density of the second negative electrode active material.

[0056] In the negative electrode active material layer, along the direction away from the current collector, the negative electrode active material layer includes n coating layers, where 3≤n≤10, and n is an integer;

[0057] Along the direction away from the current collector, the content of the first negative electrode active material in the n coating layers gradually decreases, while the content of the second negative electrode active material gradually increases.

[0058] In one possible implementation, the formula y is satisfied. i =aln(i)+b;

[0059] In the formula, i is the i-th layer of n coating layers, y i The percentage of the first negative electrode active material in the i-th layer out of the total mass of the negative electrode active material in the i-th layer;

[0060] Preferably, y1 = 1, y n =0; Among the n coating layers, the coating layer closest to the current collector is the first layer, and the coating layer furthest from the current collector is the nth layer.

[0061] The total mass of the negative electrode active material refers to the sum of the masses of the first negative electrode active material and the second negative electrode active material in the i-th layer.

[0062] Ultimate compaction density refers to the maximum compaction a material can withstand, typically determined by observing whether SEM particles break or electrode sheets fracture during the rolling process. The recommended compaction density for materials is slightly lower than the ultimate compaction density by 0.02 to 0.05.

[0063] By adjusting the proportions of the first and second anode active materials, efficient channels for ions and electrons are constructed, reducing lithium-ion diffusion resistance and slowing down battery capacity decay. This is achieved by controlling the proportions of active materials in different coating layers to meet specific requirements. i =aln(i)+b, achieving a second negative electrode active material with good and stable outermost dynamics, and a first negative electrode active material with high energy density and high compaction at the innermost layer, thus meeting the core requirements of high safety performance, high energy density, and fast charging. The content ratio y of the first and second negative electrode active materials is... i / (1-y i The range is (0, +∞).

[0064] a and b are constants, which can be determined by converting y1 and y2. n The result is obtained by substituting into the formula.

[0065] For example:

[0066] The negative electrode active material layer consists of 5 coating layers, i.e., n=5; y1=1, y5=0 are defined, meaning that the negative electrode active material in the first layer is the first negative electrode active material, and the negative electrode active material in the fifth layer is the second negative electrode active material. Calculations yield a=-0.621, b=1, which satisfies:

[0067] y i = -0.621ln(i)+1.

[0068] In one possible implementation, along the direction away from the current collector, the thickness of the first coating layer to the (n+1) / 2nd coating layer gradually decreases, and the thickness of the (n+1) / 2nd coating layer to the nth coating layer gradually increases;

[0069] Preferably, the thickness h of the i-th layer i Satisfy: h i =di 2 +ei+f;

[0070] Preferably, h (n+1) / 2 =0, h1=h n ;

[0071] Preferably, h1 ≤ 130 μm.

[0072] In order to prevent the cycle stability from being unable to be better balanced due to the outermost high-dynamics second negative active material being too thin, the thickness of the outermost coating layer (the second negative active material content is the highest) is set to be larger; in order to maximize the overall energy density and rate performance of the battery cell, the thickness of the innermost coating layer with high compaction and high capacity (the first negative active material content is the highest) is set to be larger. When the thicknesses of the layers in the negative active material layer satisfy h i = di 2 + ei+ f, the thicknesses of the first layer and the n-th layer of the negative active layer can be moderate, which can ensure high dynamics and further improve the energy density of the battery, and greatly improve the overall energy density and safety performance of the battery cell.

[0073] d, e, and f are constants, which can be obtained by substituting h (n+1) / 2 and h n into the formula.

[0074] Optionally, the negative active material layer includes n coating layers, when n is even, h (n+1) / 2 = 0; when n is odd, h (n+1) / 2 ≥ 5 μm.

[0075] Exemplarily:

[0076] The negative active material layer includes 5 coating layers, i.e. n = 5; h3 = 5 μm, h1 = h5 = 100 μm are limited, d = 23.75, e = -142.5, and f = 218.75 are calculated, i.e. h1 = 100 μm, h2 = 23.75 μm, h3 = 5 μm, h4 = 23.75 μm, and h5 = 100 μm are satisfied.

[0077] h i = 23.75i 2 - 142.5i + 218.75

[0078] In a possible implementation, along the direction away from the current collector, the D50 i of the n coating layers gradually increases; the D50 i of the negative active material in the i-th layer is the particle size corresponding to the cumulative volume distribution of 50%, which can be obtained by laser particle size testing. Gradually increasing the particle size of the negative active material away from the current collector side can delay the consumption of the negative electrode film-forming additive in the electrolyte due to the too small particle size of the surface layer negative active material, thereby delaying the problem of black spot lithium precipitation of the battery cell caused by electrolyte bridge breaking.

[0079] Preferably, D50 i = gi+ h; in the formula, g > 0 and h > 0;

[0080] Preferably, D50 n - D50 1≤ 5 μm;

[0081] g, h are constants, calculated by substituting D50 n and D501 into the formula. Exemplary:

[0082] The negative active material layer comprises 5 coating layers, i.e. n = 5; D501 = 10 μm, D50 n = 15 μm, g = 1.25, h = 8.75 are calculated, i.e. satisfy:

[0083] D50 i = 1.25i + 8.75.

[0084] In a possible implementation, the negative active material comprises one or more of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, titanium-based material, silicon-based material, tin-based material, nitride and metallic lithium;

[0085] In a possible implementation, the thickness H of the negative active material layer is < 400 μm.

[0086] In a possible implementation, the limit compaction density of the first negative active material is 1.75-1.95 g / cm 3 ;

[0087] In a possible implementation, the limit compaction density of the second negative active material is 1.6-1.78 g / cm 3 ;

[0088] In a possible implementation, X-Y ≥ 3 mAh / g is satisfied;

[0089] X is the gram capacity of the first negative active material, and Y is the gram capacity of the second negative active material;

[0090] Preferably, the gram capacity of the first negative active material is > 355 mAh / g;

[0091] Preferably, the gram capacity of the second negative active material is ≤ 354 mAh / g;

[0092] In a possible implementation, the first negative active material is a graphite material, the gram capacity of the first negative active material is 357-360 mAh / g, preferably 357-360 mAh / g; the second negative active material is a graphite material, the gram capacity of the second negative active material is 330-354 mAh / g, preferably 350-354 mAh / g.

[0093] In a possible implementation, the first negative active material is a combination of a graphite material and a silicon material; the gram capacity of the silicon material is 1600-3000 mAh / g, preferably 1700-1800 mAh / g.

[0094] In a possible implementation, M-N≥2 is satisfied;

[0095] M is the OI value of the first negative electrode active material, and N is the OI value of the second negative electrode active material;

[0096] Preferably, the OI value of the first negative electrode active material is 4-8;

[0097] Preferably, the OI value of the second negative electrode active material is 2-5;

[0098] The OI value (Orientation Index) is a parameter describing the degree of order of material arrangement. The OI value is calculated by analyzing the X-ray diffraction (XRD) spectrum, which involves the ratio of the diffraction signals of the (002) and (004) crystal planes to the diffraction signal of the (110) crystal plane.

[0099] Graphite materials with high OI values and high orientation degrees generally have better electrical conductivity and thermal stability, thereby improving the performance of lithium ion batteries. With decreasing orientation degrees, the ability to charge at high rates gradually improves.

[0100] In a possible implementation, the particle size Dv50(1) of the first negative electrode active material is 5-15 μm;

[0101] In a possible implementation, the particle size Dv50(2) of the second negative electrode active material is 8-20 μm.

[0102] In a possible implementation, the first negative electrode active material supports 2C and below rate performance;

[0103] In a possible implementation, the second negative electrode active material supports 2.5C and above rate performance;

[0104] In a possible implementation, the artificial graphite as the first negative electrode active material is prepared from needle coke as a raw material;

[0105] In a possible implementation, the artificial graphite as the second negative electrode active material is prepared from petroleum coke as a raw material.

[0106] In a possible implementation, the surface of the negative electrode active material layer away from the current collector is provided with a groove region.

[0107] By forming a concave groove area on the surface of the pole piece corresponding to the lithium precipitation area, the pore structure of the negative pole piece can be improved, the overall electrolyte infiltration effect and speed of the pole piece can be increased, the liquid storage space can be increased, the liquid storage capacity of the electrolyte during long cycle process can be improved, the problems of poor electrolyte infiltration and insufficient electrolyte in the later cycle of the high compaction high energy density system battery can be effectively solved, thereby improving the power of the battery, improving the cycle life and capacity retention rate of the battery. At the same time, the negative pole piece is provided with a groove area, the mechanical strength of the groove area is reduced, and the elasticity is good, which can effectively alleviate the volume expansion caused by excessive compaction of the negative pole, thereby effectively alleviating the expansion of the negative pole during charging and discharging, improving the safety and life of the lithium ion battery. It can also enhance mass transfer, reduce ion impedance, and shorten the transmission distance of lithium ions.

[0108] In a possible implementation, the depth h of the groove area satisfies: 0.05h n <h≤h n .

[0109] The depth h of the groove area satisfies the above limitation, which can obviously improve the interface problem, and at the same time avoid the influence of the too deep groove area on the protection of the outermost second negative active material to the inner negative active material, further improving the safety performance of the battery.

[0110] In a possible implementation, the groove area is prepared by laser wire drawing;

[0111] Preferably, the line width is 10-150 μm;

[0112] Preferably, the line depth is 3-30 μm;

[0113] In a possible implementation, the groove area is prepared by laser wire drawing;

[0114] Preferably, the hole diameter is 20-200 μm;

[0115] Preferably, the hole depth is 5-50 μm.

[0116] In a possible implementation, the groove area is prepared by laser wire drawing;

[0117] Embodiment 1

[0118] The embodiment provides a preparation method of a lithium ion battery, comprising:

[0119] (1) preparing a negative pole piece:

[0120] The first negative active material and the second negative active material are both graphite, and the parameters of the two kinds of graphite are shown in Table 1:

[0121] Table 1

[0122]

[0123] A-B = 0.17 g / cm in this embodiment 3 , X-Y = 10 mAh / g, M-N = 2.

[0124] The negative active material layer 2 is divided into 6 coating layers (as shown in Figure 1 ) along the direction away from the current collector 1, h1 = h6 = 100 μm, h 3.5 = 0 μm are defined, y1 = 1, y6 = 0 are defined, and the mass content of the first negative active material in each coating layer satisfies:

[0125] y i = -0.558ln(i) + 1.

[0126] That is, y1 = 1, y2 = 0.61, y3 = 0.39, y4 = 0.23, y5 = 0.10, y6 = 0.

[0127] The thickness of each coating layer satisfies: h i = 16i 2 - 112i + 196.

[0128] That is, h1 = 100 μm, h2 = 36 μm, h3 = 4 μm, h4 = 4 μm, h5 = 36 μm, h6 = 100 μm.

[0129] The D50 i of the n coating layers gradually increases along the direction Z away from the current collector. D501 = 13 μm, D506 = 17 μm are defined, and the thickness of each coating layer satisfies D50 i = 0.8i + 12.2.

[0130] That is, D501 = 13 μm, D502 = 13.8 μm, D503 = 14.6 μm, D504 = 15.4 μm, D505 = 16.2 μm, D506 = 17 μm.

[0131] In the first layer (i.e. the layer close to the current collector) of the n coating layers, the mass ratio of the first negative active material, the second negative active material, the conductive agent, and the binder is 98.2:0:0.6:1.2; in the 2nd layer, the mass ratio is 60.2:38.0:0.6:1.2, in the 3rd layer, the mass ratio is 38.0:60.2:0.6:1.2, in the 4th layer, the mass ratio is 22.2:76.0:0.6:1.2, in the 5th layer, the mass ratio is 10.0:88.2:0.6:1.2, and in the 6th layer, the mass ratio is 0:98.2:0.6:1.2.

[0132] The slurries of the respective coating layers are prepared respectively and are coated on the negative current collector copper foil in sequence in the direction Z away from the current collector.

[0133] The above product is then subjected to rolling and slitting treatment. Wire is punched on the surface of the negative active material layer to obtain a groove area 3 as shown in FIG. 3, wherein the center distance between two adjacent wires is 150 μm, the wire depth is 5 μm, and the wire width is 15 μm, thereby obtaining a negative electrode sheet. Figure 2

[0134] (2) Preparation of a positive electrode sheet:

[0135] The NCM613 ternary material particles are used as the positive active material, and the conductive agent and the binder are added into the stirring tank in a mass ratio of 97.4:1.4:1.2 to prepare a positive electrode slurry. The positive electrode slurry is coated onto an aluminum foil by using a coating machine, dried at a temperature of 100 °C, and then subjected to rolling and slitting, thereby finally obtaining a positive electrode sheet.

[0136] The above-prepared positive electrode sheet and negative electrode sheet and a separator are wound together to form a battery cell. Subsequently, the battery cell is packaged with an aluminum plastic film, dried by baking to remove moisture, and then injected with an electrolyte. After being subjected to formation, two-sealing and other treatments, a lithium ion battery is prepared.

[0137] Example 2

[0138] This example is basically the same as Example 1, except that the negative active material layer includes three coating layers.

[0139] At this time, the mass content of the first negative active material in each coating layer satisfies:

[0140] y i =-0.91ln(i)+1.

[0141] That is, y1=1, y2=0.37, and y3=0.

[0142] The thickness of each coating layer satisfies: i =95i 2 -380i+385.

[0143] That is, h1=100 μm, h2=5 μm, and h3=100 μm.

[0144] The D50 i of the n coating layers gradually increases in the direction away from the current collector. The D501=13 μm, the D503=17 μm, and the thickness of each coating layer satisfies: i =2i+11.

[0145] That is, D501=13 μm, D502=15 μm, and D503=17 μm.

[0146] ​In the first layer (the layer closest to the current collector) of n coating layers, the mass ratio of the first negative electrode active material, the second negative electrode active material, the conductive agent, and the binder is 98.2:0:0.6:1.2; in the second layer, the mass ratio is 36.3:61.9:0.6:1.2; and in the third layer, the mass ratio is 0:98.2:0.6:1.2.

[0147] Example 3

[0148] This embodiment is basically the same as Embodiment 1, except that the negative electrode active material layer includes 10 coating layers.

[0149] At this point, the mass content of the first negative electrode active material in each coating layer satisfies the following:

[0150] y i = -0.434ln(i)+1.

[0151] That is, y1=1, y2=0.70, y3=0.52, y4=0.40, y5=0.30, y6=0.22, y7=0.16, y8=0.10, y9=0.05, y 10 =0.

[0152] The thickness of each coating layer satisfies: h i =4.9383i 2 -54.321i+149.38.

[0153] That is, h1=100μm, h2=60.5μm, h3=30.9μm, h4=11.1μm, h5=1.2μm, h6=1.2μm, h7=11.1μm, h8=30.9μm, h9=60.5μm, h 10 =100μm.

[0154] Along the direction away from the current collector, D50 in n coating layers i Gradually increase. Limit D501 to 13 μm, D50 10 =17μm, and the thickness of each coating layer meets D50. i =0.4444i+12.556.

[0155] That is, D501 = 13μm, D502 = 13.4μm, D503 = 13.9μm, D504 = 14.3μm, D505 = 14.8μm, D506 = 15.2μm, D507 = 15.7μm, D508 = 16.1μm, D509 = 16.6μm, D50 10 =17μm.

[0156] The mass ratio of the first negative electrode active material, the second negative electrode active material, the conductive agent, and the binder in the first layer (i.e., the layer close to the current collector) of the n coating layers is 98.2:0:0.6:1.2; the mass ratio in the second layer is 68.7:29.5:0.6:1.2, the mass ratio in the third layer is 51.4:46.8:0.6:1.2, the mass ratio in the fourth layer is 39.1:59.1:0.6:1.2, the mass ratio in the fifth layer is 29.6:68.6:0.6:1.2, the mass ratio in the sixth layer is 21.8:76.4:0.6:1.2, the mass ratio in the seventh layer is 15.3:82.9:0.6:1.2, the mass ratio in the eighth layer is 9.6:88.6:0.6:1.2, the mass ratio in the ninth layer is 4.6:93.6:0.6:1.2, and the mass ratio in the tenth layer is 0:98.2:0.6:1.2.

[0157] Example 4

[0158] This example is basically the same as Example 1, except that the parameters of the first negative electrode active material and the second negative electrode active material are shown in Table 2.

[0159] Table 2

[0160]

[0161] In this example, A-B = 0.55 g / cm 3 , X-Y = 6 mAh / g, and M-N = 2.3.

[0162] Example 5

[0163] This example is basically the same as Example 1, except that the parameters of the first negative electrode active material and the second negative electrode active material are shown in Table 3.

[0164] Table 3

[0165]

[0166]

[0167] In this example, A-B = 0.02 g / cm 3 , X-Y = 6 mAh / g, and M-N = 2.

[0168] Example 6

[0169] This example is basically the same as Example 1, except that in this example, y1 = 1 and y6 = 0.1 are defined, and the mass content of the first negative electrode active material satisfies:

[0170] y i= -0.502ln(i) + 1.

[0171] That is, y1 = 1, y2 = 0.65, y3 = 0.45, y4 = 0.30, y5 = 0.19, y6 = 0.1.

[0172] Example 7

[0173] This example is basically the same as Example 1, except that in this example, y1 = 0.9, y6 = 0, and the mass content of the first negative electrode active material satisfies:

[0174] y i = -0.502ln(i) + 0.9.

[0175] That is, y1 = 0.9, y2 = 0.55, y3 = 0.35, y4 = 0.20, y5 = 0.09, y6 = 0.

[0176] Example 8

[0177] This example is basically the same as Example 1, except that in this example, y1 = 1, y2 = 0.8, y3 = 0.7, y4 = 0.6, y5 = 0.5, y6 = 0.4.

[0178] Example 9

[0179] This example is basically the same as Example 1, except that in this example, y is defined as h1 = h6 = 80 μm, h 3.5 = 0 μm.

[0180] The thickness of each coating layer satisfies: h i = 12.8i 2 - 89.6i + 156.8.

[0181] That is, h1 = 80 μm, h2 = 28.8 μm, h3 = 3.2 μm, h4 = 3.2 μm, h5 = 28.8 μm, h6 = 80 μm.

[0182] Example 10

[0183] This example is basically the same as Example 1, except that in this example, y is defined as h1 = h6 = 60 μm, h 3.5 = 0 μm.

[0184] The thickness of each coating layer satisfies: h i = 9.6i 2 - 67.2i + 117.6.

[0185] That is, h1 = 60 μm, h2 = 21.6 μm, h3 = 2.4 μm, h4 = 2.4 μm, h5 = 21.6 μm, and h6 = 60 μm.

[0186] Example 11

[0187] This embodiment is basically the same as Embodiment 1, except that h1 = 100 μm, h2 = 70 μm, h3 = 30 μm, h4 = 50 μm, h5 = 90 μm, and h6 = 100 μm.

[0188] Example 12

[0189] This embodiment is basically the same as Embodiment 1, except that in this embodiment, D501 is limited to 15μm, D506 to 20μm, and the thickness of each coating layer satisfies D50... i = i+14.

[0190] That is, D501 = 15μm, D502 = 16μm, D503 = 17μm, D504 = 18μm, D505 = 19μm, and D506 = 20μm.

[0191] Example 13

[0192] This embodiment is basically the same as Embodiment 1, except that in this embodiment, D501 is limited to 5μm, D506 to 10μm, and the thickness of each coating layer satisfies D50... i = i + 4.

[0193] That is, D501 = 5μm, D502 = 6μm, D503 = 7μm, D504 = 8μm, D505 = 9μm, and D506 = 10μm.

[0194] Example 14

[0195] This embodiment is basically the same as Embodiment 1, except that in this embodiment, D501 = 13μm, D502 = 15μm, D503 = 15.8μm, D504 = 16μm, D505 = 16.5μm, and D506 = 17μm.

[0196] Example 15

[0197] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the line depth of the wires drilled on the surface of the negative electrode active material layer is 5μm.

[0198] Example 16

[0199] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the line depth of the wires drilled on the surface of the negative electrode active material layer is 100μm.

[0200] Example 17

[0201] This example is basically the same as Example 1, except that in this example, the wire depth for wire bonding on the surface of the negative active material layer is 150 pm.

[0202] Example 18

[0203] This example is basically the same as Example 1, except that in this example, the negative electrode tab is not provided with a groove area (i.e. no wire bonding).

[0204] Comparative Example 1

[0205] This comparative example is basically the same as Example 1, except that the negative electrode tab is prepared by mechanical mixing and single-layer coating.

[0206] Comparative Example 2

[0207] This comparative example is basically the same as Example 1, except that the negative electrode tab is prepared by double-layer coating technology. Two coating layers are provided on the negative current collector, in the first layer, the mass ratio of the first negative active material, conductive agent and binder is 98.2:0.6:1.2; in the second layer, the mass ratio of the second negative active material, conductive agent and binder is 98.2:0.6:1.2.

[0208] Test Example

[0209] 1. Limiting compaction density test:

[0210] Limiting compaction density = surface density / thickness of the material

[0211] (1) The electrode tab dried after coating was cut into an electrode tab with a specification of 300 mm x 250 mm using a paper cutter, and the thickness of the electrode tab at three different positions was measured, and the electrode tab with a thickness error < 3 pm was selected as the experimental tab;

[0212] (2) Using a circular sampler, three electrode tabs with an area of 100 cm 2 and current collectors were cut from different positions of the original electrode tab described in step (1), and the weights of the cut electrode tab and current collector were measured, and the average value was taken as M0 and M1, and the surface density of the electrode tab was calculated according to the formula: S = (M0-M1) / 100cm 2 ;

[0213] (3) Set the linear speed of the double-roller mill to 2.0 m / min, and set the roll pressure to start at 1.3 MPa. After the pressure is stable, the electrode sheet selected in step (1) is put in for rolling. The roll pressure is increased by 0.65 MPa in turn until waves appear on both sides of the electrode sheet, and vertical lines appear in the middle of the electrode sheet. At this time, it is the limit compaction density of the material in this system;

[0214] (4) Measure the thickness of the electrode sheet at three different positions after rolling under different pressures using a micrometer, and record them as L1, L2, and L3. If the difference between any two of the three values is 3 μm or more, the sheet is considered unqualified. Record the thickness of the qualified electrode sheet, take the average value, and record it as Li. Then measure the thickness of the current collector after rolling using a micrometer, and record it as Lo. The formula for calculating the limit compaction density is P = S / (Li-Lo).

[0215] 2. Furnace temperature test:

[0216] Discharge the battery at 0.2C to the cut-off voltage of 3.0V in an environment of 25℃±3℃; stand for 5 min; charge at 0.5C constant current and constant voltage to the upper limit voltage, with a cut-off current of 0.05C; test the full-state voltage, internal resistance, and thickness at 25℃±3℃, and take pictures before testing. Place the full battery into a test box, and heat the test box at a temperature rise rate of (5±2)℃ / min. When the temperature in the box reaches 140℃±2℃, maintain the temperature for 60 min. After the test is completed, test the voltage and internal resistance, and take pictures after testing. If there is no fire or explosion, it is considered to pass the furnace temperature test.

[0217] 3. Cycle test:

[0218] Perform cycle test for 1000T under the condition of 25℃±5℃, and the test conditions are 2C-3.5V(1C cut-off) to 1C-3.65V(0.05C cut-off) / 0.5C-2.5V. Monitor the discharge capacity of each cycle online, and calculate the capacity retention rate.

[0219] 4. Disassemble the full battery after 300T and 600T cycles, and observe whether black spots appear on the anode interface.

[0220] 5. The test method for the rate performance of the battery is:

[0221] 25±5℃, 10min; 0.2C discharge to 3.0V, 10min; certain rate charge (0.2C / 0.3C / 0.4C…), cut-off current 0.05C, 10min, 0.5C discharge to 3.0V; 10min. After 30 cycles of the above steps, the battery was disassembled at full charge. The anode interface was observed. If the anode plate was golden yellow without abnormality and no lithium precipitation, the battery supported the corresponding rate performance. If there were gray / brown / white particles covering on the interface, the battery precipitated lithium, and at this time the battery did not support the corresponding rate performance. The maximum rate of the battery without lithium precipitation was the rate performance of the battery. The rate performance was good when the rate performance was 1.5 or more. The battery with good rate performance could be the standard for the battery to be shipped.

[0222] The test results are shown in Table 4.

[0223] Table 4

[0224]

[0225]

[0226] As can be seen from the comparison of the examples and the comparative examples in Table 1, the application can avoid the problems of local black spots or lithium precipitation at the anode interface, improve the cycle stability, and significantly improve the rate performance of the lithium ion battery.

[0227] As can be seen from the comparison of Example 1, Example 6, Example 7 and Example 8, the formula y i = aln(i) + b can further improve the cycle performance and rate performance of the battery.

[0228] As can be seen from the comparison of Example 1, Example 9, Example 10 and Example 11, the formula h i = di 2 + ei + f can further improve the cycle performance and rate performance of the battery.

[0229] As can be seen from the comparison of Example 1, Example 12, Example 13 and Example 14, the formula D50 i = gi + h can further improve the cycle performance and rate performance of the battery.

[0230] As can be seen from the comparison of Example 1, Example 15, Example 16 and Example 17, the formula 0.05h n < h ≤ h n can further improve the cycle performance and rate performance of the battery.

[0231] As can be seen from the comparison of Example 1 and Example 18, the negative plate provided with the groove area can further improve the cycle performance and rate performance of the battery.

[0232] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments are not required to be enumerated. The obvious changes or variations derived from the above description are still within the protection scope of the application.

Claims

1. A negative electrode sheet, characterized by, The negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises a first negative electrode active material and a second negative electrode active material; A is the limit compaction density of the first negative electrode active material, and B is the limit compaction density of the second negative electrode active material; The negative active material satisfies: A-B = 0.01 to 0.75 g / cm 3 ; The negative electrode active material layer comprises n coating layers in the direction away from the current collector, 3≤n≤10, and n is an integer; In the n coating layers, the content of the first negative electrode active material gradually decreases, and the content of the second negative electrode active material gradually increases in the direction away from the current collector; In the n coating layers, the thickness of the first layer to the (n+1) / 2 layer gradually decreases, and the thickness of the (n+1) / 2 layer to the n layer gradually increases in the direction away from the current collector; the coating layer close to the current collector is the first layer, and the coating layer away from the current collector is the n layer; h1≤130 μm. When n is even, the thickness h of the (n+1) / 2 layer （n+1） / 2 = 0.

2. The negative electrode sheet according to claim 1, characterized by Thickness of the 1st layer h1 = thickness of the nth layer h n .

3. The negative electrode sheet according to claim 2, characterized by X-Y≥3 mAh / g is satisfied; 4. The negative electrode sheet according to claim 1, wherein D50 of the n coating layers in a direction away from the current collector i gradually increases; D50 i is the first i average particle size of the negative active material in the layer.

5. The negative electrode sheet according to claim 4, characterized by D50 n - D50 1 ≤ 5 μm.

6. The negative electrode sheet according to claim 1, wherein X is the gram capacity of the first negative electrode active material, and Y is the gram capacity of the second negative electrode active material. The gram capacity of the first negative electrode active material is >355 mAh / g.

7. The negative electrode sheet according to claim 6, characterized by The gram capacity of the second negative electrode active material is ≤354 mAh / g.

8. The negative electrode sheet according to claim 6, characterized by M-N≥2 is satisfied; 9. The negative electrode plate of claim 1, wherein, M is the OI value of the first negative electrode active material, and N is the OI value of the second negative electrode active material. The OI value of the first negative electrode active material is 4-8.

10. The negative electrode sheet according to claim 9, wherein The OI value of the second negative electrode active material is 2-5.

11. The negative electrode sheet according to claim 9, wherein At least one of the following conditions is satisfied:

12. The negative electrode sheet according to any one of claims 1 to 11, wherein (1) the negative electrode active material comprises one or more of natural graphite, artificial graphite, hard carbon, soft carbon, graphene, titanium-based materials, silicon-based materials, tin-based materials, nitrides and metallic lithium; (2) the thickness H of the negative electrode active material layer is <400 μm; (5) the particle size Dv50(1) of the first negative electrode active material is 5-15 μm; (3) the first negative electrode active material has an ultimate compaction density of 1.75 to 1.95 g / cm 3 ; (4) the second negative electrode active material has an ultimate compaction density of 1.2 to 1.78 g / cm 3 ; (6) the particle size Dv50(2) of the second negative electrode active material is 8-20 μm. The surface of the negative electrode active material layer away from the current collector is provided with a groove area.

13. The negative electrode sheet according to any one of claims 1 to 11, wherein The groove area comprises a plurality of linear grooves and / or a plurality of holes; the plurality refers to one or more.

14. The negative electrode sheet according to claim 13, wherein The depth h of the groove region satisfies: 0.05h n ≤ h ≤ 0.2h n , h n is the thickness of the nth layer.

15. The negative electrode sheet according to claim 13, wherein The linear groove has a line width of 10-150 μm.

16. The negative electrode sheet according to claim 15, wherein The linear groove has a line depth of 3-30 μm.

17. The negative electrode sheet according to claim 15, wherein The hole has a hole diameter of 20-200 μm.

18. The negative electrode sheet according to claim 15, wherein The hole has a hole depth of 5-50 μm.

19. The negative electrode sheet according to claim 15, wherein The negative electrode sheet comprises the negative electrode active material layer.

20. A lithium-ion battery, characterized by, ​

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