Negative electrode sheet, and preparation method therefor and use thereof

Abstract

Disclosed in the present application are a negative electrode sheet, and a preparation method therefor and the use thereof. The negative electrode sheet comprises a current collector and a composite active substance layer arranged on at least one surface of the current collector, wherein the composite active substance layer comprises a first active substance layer and a second active substance layer, which are sequentially arranged on the surface of the current collector, with the second active substance layer being arranged on the surface, away from the current collector, of the first active substance layer. The first active substance layer comprises a first binder, and the first binder comprises a low-swelling binder; and the second active substance layer comprises a second binder, and the second binder comprises at least one of a high-swelling binder and an acrylic acid derivative multipolymer, wherein the volume swelling ratio of the low-swelling binder is 25-40%, and the volume swelling ratio of the high-swelling binder is 60-85%.

Description

Negative electrode sheet and preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 2024104311639 filed on April 10, 2024 with the China Patent Office, the whole content of the above application is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium battery, in particular to a negative electrode sheet and a preparation method and application thereof. BACKGROUND

[0003] Electric vehicles driven by lithium ion batteries are one of the promising zero-emission transportation tools to solve air pollution and energy crisis problems. In recent years, the application scenarios of lithium ion batteries have increased, and the use frequency has also become higher and higher. TECHNICAL PROBLEM

[0004] Lithium ion batteries will expand or shrink in structure due to the deintercalation of lithium during the charging and discharging process. When charging the lithium ion battery, the process of intercalation of lithium (such as graphite negative electrode, hard carbon negative electrode, etc.) or alloying intercalation of lithium (such as silicon-based negative electrode, lithium metal negative electrode, etc.) occurs on the negative electrode side. Therefore, the negative electrode material will generally expand in volume as the depth of lithium intercalation increases, for example, the graphite negative electrode will generally produce a volume expansion of 10%-15%, and the silicon-based negative electrode can produce a volume expansion of up to 300%. This will affect the decline of the kinetic performance, cycle performance and high-temperature storage performance of the lithium battery to some extent. TECHNICAL SOLUTION

[0005] In a first aspect, the present application provides a negative electrode sheet, which adopts the following technical scheme:

[0006] A negative electrode sheet, comprising a current collector and a composite active material layer arranged on at least one surface of the current collector, the composite active material layer comprising a first active material layer and a second active material layer arranged in sequence on the surface of the current collector; the second active material layer is arranged on the surface of the first active material layer away from the current collector;

[0007] The first active material layer comprises a first binder, the first binder comprises a low swelling binder, and the low swelling binder comprises a low swelling styrene-butadiene-based binder; the monomer composition of the low swelling styrene-butadiene-based binder comprises at least one of butadiene and styrene;

[0008] The mass fraction of the first binder in the first active material layer is 1%-3%;

[0009] The second active material layer comprises a second binder, the second binder comprises at least one of a high-swell binder, an acrylic derivative multi-copolymer; the high-swell binder comprises at least one of a high-swell styrene-butadiene type binder and a high-swell styrene-acrylate type binder, the monomer composition of the high-swell styrene-butadiene type binder comprises at least one of butadiene and styrene, and the monomer composition of the high-swell styrene-acrylate type binder comprises at least one of styrene and acrylate; and the monomer composition of the acrylic derivative multi-copolymer comprises at least one of acrylic acid, an acrylic acid salt, an acrylate, and acrylonitrile.

[0010] The mass ratio of the second binder in the second active material layer is 0.8%-2%;

[0011] The volume swell rate of the low-swell binder is 25%-40%, and the volume swell rate of the high-swell binder is 60%-85%.

[0012] In a second aspect, the application provides a preparation method of a negative electrode sheet, which adopts the following technical scheme:

[0013] A preparation method of a negative electrode sheet comprises the following steps:

[0014] S1: mixing, stirring a first conductive agent, a first dispersing agent, a first active material, and a first solvent to obtain a first active slurry; and mixing, stirring a second conductive agent, a second dispersing agent, a second active material, and a second solvent to obtain a second active slurry.

[0015] S2: coating the first active slurry and the second active slurry on at least one surface of the current collector, wherein the first active slurry is coated on the surface of the current collector, and the negative electrode sheet is obtained by drying and cold pressing.

[0016] In a third aspect, the application provides a lithium ion battery, which adopts the following technical scheme:

[0017] The lithium ion battery comprises a positive electrode sheet, a separator, an electrolyte, and the negative electrode sheet as described above. Advantages

[0018] The swelling rate of the low-swelling binder = (the mass or volume of the low-swelling binder after swelling in the electrolyte - the initial mass or volume of the low-swelling binder) / the initial mass or volume of the low-swelling binder x 100%; specifically, the volume swelling rate of the low-swelling binder in the present application = (the volume of the low-swelling binder after swelling in the electrolyte - the initial volume of the low-swelling binder) / the initial volume of the low-swelling binder x 100%; the volume swelling rate of the high-swelling binder in the present application = (the volume of the high-swelling binder after swelling in the electrolyte - the initial volume of the high-swelling binder) / the initial volume of the high-swelling binder x 100%.

[0019] The low-swelling binder has a relatively low swelling rate in the electrolyte, and by using the low-swelling binder in the first active material layer close to the current collector, the low-swelling binder can still maintain good volume stability during the cycling of the lithium battery, can maintain the adhesion between the negative active material layer and the current collector, and will not cause the first active material layer to fall off or peel off from the current collector, thereby improving the cycling stability of the composite active material layer and further improving the cycling performance of the lithium battery; and due to the volume stability of the low-swelling binder, the active particles in the active material layer can be closely together during the cycling of the lithium battery, and there will be no large pores between the particles, which is conducive to the electron conduction in the first active material layer, thereby improving the rate performance of the lithium battery.

[0020] The high-swelling binder has a relatively high electrolyte absorption property in the electrolyte compared with the low-swelling binder, and by using the high-swelling binder in the second active material layer away from the current collector, the adsorption amount of the electrolyte in the second active material layer can be improved, and under the action of the fully infiltrated electrolyte, the acrylic derivative multi-copolymer in the second active material layer can react with the solvated ions in the electrolyte to generate a SEI film with uniform thickness and good stability, thereby reducing the DCR value of the second active material layer, which is conducive to improving the conductivity of the second active material layer, alleviating the decrease in the tightness between the active particles caused by the swelling of the high-swelling binder due to the absorption of the electrolyte, avoiding the increase in the ion impedance in the second active material layer, and further improving the kinetic performance of the lithium battery. In addition, the increase in the adsorption amount of the electrolyte in the second active material layer can also improve the wettability of the electrolyte to the first active material layer close to the current collector to a certain extent, and since the low-swelling binder is used in the first active material layer, the first active material layer can still maintain good volume stability after being infiltrated by the electrolyte from the second active material layer, thereby improving the ion and electron transport performance of the composite active material layer, which is conducive to improving the kinetic performance of the lithium battery. BRIEF DESCRIPTION OF DRAWINGS

[0021] FIG. 1 is a structural schematic diagram of the negative electrode sheet of the present application.

[0022] Fig. 2 is a schematic diagram of the adhesive film used in the adhesive swelling rate test.

[0023] Legend of reference signs:

[0024] 1. current collector; 2. composite active material layer; 21. first active material layer; 22. second active material layer. Embodiment of the present application

[0025] In some embodiments, the low-swelling styrene-butadiene-based adhesive comprises low-swelling styrene-butadiene rubber; the high-swelling styrene-butadiene-based adhesive comprises high-swelling styrene-butadiene rubber, the high-swelling styrene- propylene-based adhesive comprises high-swelling styrene-propylene rubber; the acrylic salt comprises at least one of sodium acrylate, lithium acrylate.

[0026] In some embodiments, when the second adhesive comprises both the high-swelling adhesive and the acrylic derivative multi-copolymer, the mass ratio of the high-swelling adhesive to the acrylic derivative multi-copolymer is 1:1.5-2.5.

[0027] In some embodiments, the thickness of the composite active material layer is 100-300 μm.

[0028] In some embodiments, the ratio of the thickness of the first active material layer to the thickness of the second active material layer is (1-1.5):1.

[0029] By controlling the ratio of the thickness of the first active material layer to the thickness of the second active material layer to be within the above range, the content of the adhesive in each active material layer can be reasonably regulated, and the cooperation effect of the first adhesive and the second adhesive can be optimized, thereby significantly improving the kinetic performance and cycle performance of the lithium battery.

[0030] When the thickness of the first active material layer is too thick while the thickness of the composite active material layer remains unchanged, although the interface adhesion between the first active material layer and the current collector is strong, and the active material layer does not peel off from the surface of the current collector, the entry ability of the electrolyte into the part of the first active material layer close to the current collector decreases, which will increase the ion transfer impedance of the first active material layer and reduce the specific energy of the first active material layer to some extent; in addition, the thickness of the second active material layer is too thin at this time, and the content of the high-swelling adhesive contained therein remains unchanged, the proportion of the high-swelling adhesive in the whole negative electrode sheet is low, and the absorption capacity of the electrolyte is limited, which will cause the kinetic performance to be unsatisfactory.

[0031] When the thickness of the first active material layer is too thin, the interface adhesion between the first active material layer and the current collector decreases, and the first active material layer is prone to peeling off from the current collector; in addition, the thickness of the second active material layer is too thick, and when the high-swelling binder absorbs the electrolyte, the volume expansion caused thereby will destroy the stability of the second active material layer, which is not conducive to the structural stability of the second active material layer, and will inevitably cause the decrease of the cycle stability of the battery.

[0032] In some embodiments, the first active material layer further comprises a first active material, and the first active material comprises at least one of graphite and silicon-containing graphite; the second active material layer further comprises a second active material, and the second active material comprises at least one of graphite and silicon-containing graphite.

[0033] In some embodiments, the mass percentage of each component in the first active material layer is as follows: the first active material is 93.5%-98.4%, the first conductive agent content is 0%-1.5%, the first dispersing agent or first thickening agent content is 0.6%-2%, and the first binder content is 1%-3%.

[0034] In some embodiments, the mass percentage of each component in the second active material layer is as follows: the second active material is 93.5%-98.6%, the second conductive agent content is 0%-1.5%, the second dispersing agent or second thickening agent content is 0.6%-2%, and the second binder content is 0.8%-2%.

[0035] In some embodiments, the first dispersing agent and the second dispersing agent comprise at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose; the first conductive agent and the second conductive agent comprise at least one of conductive carbon black, conductive graphite, and carbon nanotubes; and the first solvent and the second solvent comprise water.

[0036] In some embodiments, the first active slurry is prepared by a method comprising the following steps:

[0037] A1: mixing the first dispersing agent with water, stirring and dispersing to obtain a first dispersing glue solution with a solid content of 1%-2%;

[0038] A2: mixing the first active material, the first conductive agent, and 30wt%-50wt% of the first dispersing glue solution, stirring and dispersing to obtain a first kneading slurry;

[0039] A3: mixing 50wt%-70wt% of the first dispersing glue solution with the first kneading slurry, stirring and dispersing to obtain a first pretreated slurry;

[0040] A4: mixing the first binder with the first pretreatment slurry, stirring and dispersing to obtain the first active slurry.

[0041] In some embodiments, the second active slurry is prepared by a method comprising the following steps:

[0042] B1: mixing the second dispersant with water, stirring and dispersing to obtain a second dispersing glue solution with a solid content of 1%-2%;

[0043] B2: mixing the second active material, the second conductive agent, and 30wt%-50wt% of the second dispersing glue solution, stirring and dispersing to obtain a second kneading slurry;

[0044] B3: mixing 50wt%-70wt% of the second dispersing glue solution with the second kneading slurry, stirring and dispersing to obtain a second pretreatment slurry;

[0045] B4: mixing the second binder with the second pretreatment slurry, stirring and dispersing to obtain the second active slurry.

[0046] In some embodiments, when the second binder comprises a high-solubility binder and an acrylic derivative multi-copolymer, the steps of B3 and B4 are as follows:

[0047] B3: mixing 50wt%-70wt% of the second dispersing glue solution, the second kneading slurry, and the acrylic derivative multi-copolymer, stirring and dispersing to obtain a second pretreatment slurry;

[0048] B4: mixing the high-solubility binder with the second pretreatment slurry, stirring and dispersing to obtain the second active slurry.

[0049] In some embodiments, the mass of the first active slurry is denoted as t1, the mass of the second active slurry is denoted as t2, the mass of the first binder is denoted as m1, and the mass of the second binder is denoted as m2. The first binder and the second binder satisfy m1 / (t1+t2) > m2 / (t1+t2). Due to the layered slurry preparation, the first binder in the first active slurry near the current collector part will float up during the subsequent drying process, which will cause the adhesion between the first active material layer near the current collector and the current collector to deteriorate, affecting the cycle performance of the lithium battery. By controlling the content of the first binder to be higher than that of the second binder, the deterioration of the adhesion between the composite active material layer and the current collector due to the floating of the first binder can be alleviated, thereby improving the cycle performance of the lithium battery.

[0050] In some embodiments, the mass of the first binder is denoted as m1, the mass of the second binder is denoted as m2, and m1, m2 satisfy 5%≤(m1-m2) / m2×100%≤50%.

[0051] By controlling the content of the first binder and the second binder to satisfy the above value range, the situation that a large amount of the first binder floats to the second active material layer during the drying process due to the excessively high content of the first binder can be avoided, and the low-swelling binder will affect the adsorption of the electrolyte by the second active material layer.

[0052] Example 1

[0053] 1. Preparation of the negative electrode sheet

[0054] Preparation of the first active slurry:

[0055] According to the mass ratio of 93.9%:1.5%:2%:2.6%, graphite particles, conductive carbon black, lithium carboxymethyl cellulose, and a low-swelling styrene-butadiene-based binder (low-swelling styrene-butadiene rubber) were weighed.

[0056] A1: The lithium carboxymethyl cellulose was mixed with water, stirred at a speed of 20 rpm, and dispersed at 2000 rpm for 3 h to obtain a first dispersion glue with a solid content of 1%-2%;

[0057] A2: The graphite particles, conductive carbon black, and 40wt% of the first dispersion glue were mixed, stirred at a speed of 15 rpm, and dispersed at 1000 rpm for 1 h to obtain a first kneaded slurry;

[0058] A3: 60wt% of the first dispersion glue was mixed with the first kneaded slurry, stirred at a speed of 20 rpm, and dispersed at 2000 rpm for 2 h to obtain a first pretreated slurry (fineness ≤30 μm, viscosity 1500-4500 mpa.s);

[0059] A4: The low-swelling binder (the volume swelling rate of the low-swelling styrene-butadiene rubber is 35%) was mixed with the first pretreated slurry, stirred at a speed of 20 rpm, and dispersed at 700 rpm for 0.8 h to obtain a first active slurry.

[0060] Preparation of the second active slurry:

[0061] According to the mass ratio of 94.5%:1.5%:2%:2%, graphite particles, conductive carbon black, sodium carboxymethyl cellulose, and a second binder were weighed, the second binder including a high-swelling styrene-butadiene-based binder (high-swelling styrene-butadiene rubber) and an acrylic derivative multi-copolymer (containing acrylic acid in the monomer composition) with a mass ratio of 1:2.

[0062] B1: mixing sodium carboxymethyl cellulose with water, stirring at a rotation speed of 20 rpm and dispersing at 2000 rpm for 3 h to obtain a second dispersion glue solution with a solid content of 1%-2%;

[0063] B2: mixing graphite particles, conductive carbon black and 40wt% of the second dispersion glue solution, stirring at a rotation speed of 15 rpm and dispersing at 1000 rpm for 1 h to obtain a second kneading slurry;

[0064] B3: mixing 60wt% of the second dispersion glue solution, acrylic derivative multi-copolymer with the second kneading slurry, stirring at a rotation speed of 20 rpm and dispersing at 2000 rpm for 2 h to obtain a second pretreatment slurry (fineness≤30μm, viscosity 1500-4500mpa.s);

[0065] B4: mixing high-swelling styrene-butadiene-based binder (volume swelling rate of high-swelling styrene-butadiene rubber is 70%) with the second pretreatment slurry, stirring at a rotation speed of 20 rpm and dispersing at 700 rpm for 0.8 h to obtain a second active slurry.

[0066] The first binder and the second binder satisfy (m1-m2) / m2x100%=30%;

[0067] The first active slurry and the second active slurry are simultaneously coated on one surface of the copper foil using a double-layer coating machine, and after drying, the above coating operation is repeated on the other surface of the copper foil, and after drying, cold pressing and slitting are performed to obtain the negative electrode sheet.

[0068] 2. Preparation of positive electrode sheet

[0069] The positive electrode slurry is prepared as follows: NCA positive electrode active material, conductive agent acetylene black and binder PVDF are added into a vacuum stirrer in a mass ratio of 97.9%:0.9%:1.2% for mixing, and then solvent NMP is added into the mixed slurry, and the mixed slurry is stirred under the action of the vacuum stirrer until it becomes uniform, thereby obtaining the positive electrode slurry of the present embodiment. The above positive electrode slurry is uniformly coated on both surfaces of the positive electrode current collector aluminum foil, and after drying at room temperature, it is transferred to an oven for further drying. After drying in the oven, a positive electrode sheet semi-finished product is obtained, and then cold pressing and slitting are performed on the positive electrode sheet semi-finished product to obtain the positive electrode sheet to be assembled.

[0070] 3. Assembly of lithium ion battery

[0071] A commercially available polyethylene film is used as the separator of the lithium ion battery, and an electrolyte suitable for a 4.2V (upper limit of charging voltage) voltage system battery is used as the electrolyte. The above positive electrode sheet, negative electrode sheet and separator are subjected to a winding process to obtain a bare cell, and the bare cell is subjected to packaging, liquid injection, standing, formation and capacity distribution to obtain a finished battery.

[0072] The thickness of the first active material layer in the negative electrode sheet in this embodiment is 100 pm; and the thickness of the second active material layer is 100 pm.

[0073] Example 2

[0074] 1. Preparation of the negative electrode sheet

[0075] Preparation of the first active slurry:

[0076] The graphite particles, conductive carbon black, sodium carboxymethyl cellulose, and low-swell styrene-butadiene-based binder (low-swell styrene-butadiene rubber) are weighed according to the mass ratio of 96.3%:0.7%:1.5%:1.5%.

[0077] A1: The sodium carboxymethyl cellulose and lithium carboxymethyl cellulose are mixed with water, stirred at a speed of 30 rpm, and dispersed at 3000 rpm for 1.5 h to obtain a first dispersion glue solution with a solid content of 1%-2%;

[0078] A2: The graphite particles, conductive carbon black, and 50wt% of the first dispersion glue solution are mixed, stirred at a speed of 25 rpm, and dispersed at 1500 rpm for 0.5 h to obtain a first kneaded slurry;

[0079] A3: 50wt% of the first dispersion glue solution is mixed with the first kneaded slurry, stirred at a speed of 30 rpm, and dispersed at 3000 rpm for 1 h to obtain a first pretreated slurry (fineness ≤30 pm, viscosity 1500-4500 mpa.s);

[0080] A4: The low-swell styrene-butadiene-based binder (the volume swell rate of the low-swell styrene-butadiene rubber is 25%) is mixed with the first pretreated slurry, stirred at a speed of 25 rpm, and dispersed at 1000 rpm for 0.5 h to obtain a first active slurry.

[0081] Preparation of the second active slurry:

[0082] The graphite particles, conductive carbon black, sodium carboxymethyl cellulose, and second binder are weighed according to the mass ratio of 97%:0.8%:1.2%:1%, and the second binder includes a high-swell styrene-butadiene-based binder (high-swell styrene-butadiene rubber) and an acrylic derivative multi-copolymer (the monomer composition contains lithium acrylate) with a mass ratio of 1:1.5.

[0083] B1: The sodium carboxymethyl cellulose is mixed with water, stirred at a speed of 30 rpm, and dispersed at 3000 rpm for 1.5 h to obtain a second dispersion glue solution with a solid content of 1%-2%;

[0084] B2: The graphite particles, conductive carbon black, and 50wt% of the second dispersion glue solution are mixed, stirred at a speed of 25 rpm, and dispersed at 1500 rpm for 0.5 h to obtain a second kneaded slurry;

[0085] B3: 50wt% of the second dispersion glue solution, acrylic derivative multi-copolymer (volume swelling rate 80%) is mixed with the second kneading slurry, stirred at 30 rpm and dispersed at 3000 rpm for 1 h to obtain the second pretreatment slurry (fineness ≤ 30 μm, viscosity 1500-4500 mpa.s);

[0086] B4: The high swelling styrene-butadiene-based binder (volume swelling rate of high swelling styrene-butadiene rubber is 85%) is mixed with the second pretreatment slurry, stirred at 25 rpm and dispersed at 1000 rpm for 0.5 h to obtain the second active slurry.

[0087] The first binder and the second binder satisfy (m1-m2) / m2x100%=50%;

[0088] The first active slurry and the second active slurry are simultaneously coated on one surface of the copper foil using a double-layer coating machine, and after drying, the above coating operation is repeated on the other surface of the copper foil, and after drying, cold pressing and cutting to obtain the negative electrode sheet.

[0089] The negative electrode sheet in this embodiment has a first active material layer with a thickness of 150 μm; and a second active material layer with a thickness of 150 μm.

[0090] Example 3

[0091] 1. Preparation of the negative electrode sheet

[0092] Preparation of the first active slurry:

[0093] According to the mass ratio of 98.1%:0.2%:0.6%:1.1%, silicon-doped graphite particles, carbon nanotubes, lithium carboxymethyl cellulose, and low swelling styrene-butadiene-based binder (low swelling styrene-butadiene rubber) are weighed.

[0094] A1: The lithium carboxymethyl cellulose is mixed with water, stirred at 15 rpm and dispersed at 1500 rpm for 6 h to obtain a first dispersion glue solution with a solid content of 1%-2%;

[0095] A2: The silicon-doped graphite particles, carbon nanotubes, and 30wt% of the first dispersion glue solution are mixed, stirred at 10 rpm and dispersed at 500 rpm for 2 h to obtain a first kneading slurry;

[0096] A3: 70wt% of the first dispersion glue solution is mixed with the first kneading slurry, stirred at 15 rpm and dispersed at 1500 rpm for 3 h to obtain a first pretreatment slurry (fineness ≤ 30 μm, viscosity 1500-4500 mpa.s);

[0097] A4: The low-swell styrene-butadiene binder (volume swell rate of low-swell styrene-butadiene rubber is 40%) is mixed with the first pretreatment slurry, stirred at a rotation speed of 15 rpm and dispersed at 400 rpm for 1 h to obtain the first active slurry.

[0098] Preparation of the second active slurry:

[0099] The silicon-doped graphite particles, carbon nanotubes, lithium carboxymethyl cellulose, and the second binder are weighed according to the mass ratio of 98.2%:0.2%:0.6%:1%, and the second binder includes a high-swell styrene-butadiene binder (volume swell rate of high-swell styrene-butadiene rubber is 60%) and an acrylic derivative multi-copolymer (monomer composition contains sodium acrylate) with a mass ratio of 1:2.5.

[0100] B1: The lithium carboxymethyl cellulose is mixed with water, stirred at a rotation speed of 15 rpm and dispersed at 1500 rpm for 6 h to obtain a second dispersion glue with a solid content of 1%-2%;

[0101] B2: The silicon-doped graphite particles, carbon nanotubes, and 30wt% of the second dispersion glue are mixed, stirred at a rotation speed of 10 rpm and dispersed at 500 rpm for 2 h to obtain a second kneading slurry;

[0102] B3: 70wt% of the second dispersion glue, the acrylic derivative multi-copolymer (volume swell rate is 65%) and the second kneading slurry are mixed, stirred at a rotation speed of 15 rpm and dispersed at 1500 rpm for 3 h to obtain a second pretreatment slurry (fineness ≤30μm, viscosity 1500-4500mpa.s);

[0103] B4: The high-swell styrene-butadiene binder (volume swell rate of high-swell styrene-butadiene rubber is 60%) is mixed with the second pretreatment slurry, stirred at a rotation speed of 15 rpm and dispersed at 400 rpm for 1 h to obtain the second active slurry.

[0104] The first binder and the second binder satisfy (m1-m2) / m2x100%=10%;

[0105] The first active slurry and the second active slurry are simultaneously coated on one surface of the copper foil using a double-layer coating machine, and after drying, the above coating operation is repeated on the other surface of the copper foil, and after drying, cold pressing and cutting are performed to obtain the negative electrode sheet.

[0106] In the negative electrode sheet of the present embodiment, the thickness of the first active material layer is 50μm; and the thickness of the second active material layer is 50μm.

[0107] Example 4

[0108] 1. Preparation of the negative electrode sheet

[0109] Preparation of the first active slurry:

[0110] Graphite particles, conductive carbon black, carboxymethyl cellulose lithium, and low-swell styrene-butadiene-based binder (low-swell styrene-butadiene rubber) were weighed according to a mass ratio of 97.5%:0.05%:0.75%:1.7%.

[0111] A1: Sodium carboxymethyl cellulose was mixed with water, stirred at a rotation speed of 20 rpm, and dispersed at 2000 rpm for 3 h to obtain a first dispersion glue solution with a solid content of 1%-2%;

[0112] A2: Graphite particles, conductive carbon black, and 40 wt% of the first dispersion glue solution were mixed, stirred at a rotation speed of 15 rpm, and dispersed at 1000 rpm for 1 h to obtain a first kneading slurry;

[0113] A3: 60 wt% of the first dispersion glue solution was mixed with the first kneading slurry, stirred at a rotation speed of 20 rpm, and dispersed at 2000 rpm for 2 h to obtain a first pretreatment slurry (fineness ≤30 μm, viscosity 1500-4500 mpa.s);

[0114] A4: The low-swell styrene-butadiene-based binder (low-swell styrene-butadiene rubber with a volume swell rate of 35%) was mixed with the first pretreatment slurry, stirred at a rotation speed of 20 rpm, and dispersed at 700 rpm for 0.8 h to obtain a first active slurry.

[0115] Preparation of a second active slurry:

[0116] Graphite particles, conductive carbon black, carboxymethyl cellulose lithium, and high-swell styrene-acryl-based binder (high-swell styrene-acryl rubber) were weighed according to a mass ratio of 97.5%:0.05%:0.75%:1.7%.

[0117] B1: Sodium carboxymethyl cellulose was mixed with water, stirred at a rotation speed of 20 rpm, and dispersed at 2000 rpm for 3 h to obtain a second dispersion glue solution with a solid content of 1%-2%;

[0118] B2: Graphite particles, conductive carbon black, and 40 wt% of the second dispersion glue solution were mixed, stirred at a rotation speed of 15 rpm, and dispersed at 1000 rpm for 1 h to obtain a second kneading slurry;

[0119] B3: 60 wt% of the second dispersion glue solution was mixed with the second kneading slurry, stirred at a rotation speed of 20 rpm, and dispersed at 2000 rpm for 2 h to obtain a second pretreatment slurry (fineness ≤30 μm, viscosity 1500-4500 mpa.s);

[0120] B4: The high-swell styrene-acrylic binder (volume swell rate of high-swell styrene-acrylic rubber is 70%) is mixed with the second pretreatment slurry, stirred at a rotating speed of 20 rpm and dispersed at 700 rpm for 0.8 h to obtain the second active slurry.

[0121] The first binder and the second binder satisfy (m1-m2) / m2x100%=50%;

[0122] The first active slurry and the second active slurry are simultaneously coated on one surface of the copper foil using a double-layer coating machine, and after drying, the above coating operation is repeated on the other surface of the copper foil, and after drying, cold pressing and slitting are performed to obtain the negative electrode sheet.

[0123] The negative electrode sheet in the embodiment has a first active material layer with a thickness of 60 μm and a second active material layer with a thickness of 40 μm.

[0124] Example 5

[0125] 1. Preparation of the negative electrode sheet

[0126] Preparation of the first active slurry:

[0127] The silicon-doped graphite particles, carbon nanotubes, lithium carboxymethyl cellulose and low-swell styrene-butadiene binder (low-swell styrene-butadiene rubber) are weighed according to the mass ratio of 96.8%:0.3%:0.9%:2%.

[0128] A1: The lithium carboxymethyl cellulose is mixed with water, stirred at a rotating speed of 15 rpm and dispersed at 1500 rpm for 6 h to obtain a first dispersion glue with a solid content of 1%-2%;

[0129] A2: The silicon-doped graphite particles, carbon nanotubes and 30 wt% of the first dispersion glue are mixed, stirred at a rotating speed of 10 rpm and dispersed at 500 rpm for 2 h to obtain a first kneading slurry;

[0130] A3: 70 wt% of the first dispersion glue is mixed with the first kneading slurry, stirred at a rotating speed of 15 rpm and dispersed at 1500 rpm for 3 h to obtain a first pretreatment slurry (fineness ≤30 μm, viscosity 1500-4500 mpa.s);

[0131] A4: The low-swell styrene-butadiene binder (volume swell rate of low-swell styrene-butadiene rubber is 35%) is mixed with the first pretreatment slurry, stirred at a rotating speed of 15 rpm and dispersed at 400 rpm for 1 h to obtain the first active slurry.

[0132] Preparation of the second active slurry:

[0133] The silicon-doped graphite particles, carbon nanotubes, lithium carboxymethyl cellulose, and acrylic derivative multi-copolymer (containing acrylate in its monomer composition) are weighed according to a mass ratio of 96.8%:0.3%:0.9%:2%.

[0134] B1: The lithium carboxymethyl cellulose is mixed with water, stirred at a rotation speed of 15 rpm, and dispersed at 1500 rpm for 6 h to obtain a second dispersion glue with a solid content of 1%-2%;

[0135] B2: The silicon-doped graphite particles, carbon nanotubes, and 30 wt% of the second dispersion glue are mixed, stirred at a rotation speed of 10 rpm, and dispersed at 500 rpm for 2 h to obtain a second kneading slurry;

[0136] B3: 70 wt% of the second dispersion glue is mixed with the second kneading slurry, stirred at a rotation speed of 15 rpm, and dispersed at 1500 rpm for 3 h to obtain a second pretreatment slurry (fineness ≤30 μm, viscosity 1500-4500 mpa.s);

[0137] B4: The acrylic derivative multi-copolymer (volume swelling rate 75%) is mixed with the second pretreatment slurry, stirred at a rotation speed of 15 rpm, and dispersed at 400 rpm for 1 h to obtain a second active slurry.

[0138] The first binder and the second binder satisfy (m1-m2) / m2×100%=20%;

[0139] The first active slurry and the second active slurry are simultaneously coated on one surface of the copper foil using a double-layer coating machine, and after drying, the above coating operation is repeated on the other surface of the copper foil, and after drying, cold pressing and slitting are performed to obtain the negative electrode sheet.

[0140] The negative electrode sheet in this embodiment has a first active material layer with a thickness of 120 μm and a second active material layer with a thickness of 100 μm.

[0141] Example 6

[0142] This embodiment differs from Example 1 in that the composite active material layer has a thickness of 400 μm, and the remaining parts are consistent with Example 1.

[0143] Example 7

[0144] This embodiment differs from Example 1 in that the thickness ratio of the first active material layer to the second active material layer is 1:0.5, and the remaining parts are consistent with Example 1.

[0145] Example 8

[0146] The embodiment differs from example 1 in that the thickness ratio of the first active material layer to the second active material layer is 1:2; the rest is consistent with example 1.

[0147] Example 9

[0148] The embodiment differs from example 1 in that in the process of preparing the first active slurry and the second active slurry, the mass m1 of the first binder, the mass m2 of the second binder, m1, m2 satisfy (m1-m2) / m2x100%=1%; the rest is consistent with example 1.

[0149] Example 10

[0150] The embodiment differs from example 1 in that in the process of preparing the first active slurry and the second active slurry, the mass m1 of the first binder, the mass m2 of the second binder, m1, m2 satisfy (m1-m2) / m2x100%=60%; the rest is consistent with example 1.

[0151] Comparative example 1

[0152] The comparative example differs from example 1 in that in the comparative example, low-swell styrene-butadiene rubber is used in the first active material layer and the second active material layer;

[0153] Specifically, an equal weight (here, "equal weight" refers to the weight after conversion of solid content) of low-swell styrene-butadiene rubber is used instead of the high-swell styrene-butadiene rubber used in the second active slurry; the rest is consistent with example 1.

[0154] Comparative example 2

[0155] The comparative example differs from example 1 in that in the comparative example, high-swell styrene-butadiene rubber is used in the first active material layer and the second active material layer;

[0156] Specifically, an equal weight of high-swell styrene-butadiene rubber is used instead of the low-swell styrene-butadiene rubber used in the first active slurry; the rest is consistent with example 1.

[0157] Comparative example 3

[0158] The comparative example differs from example 1 in that in the comparative example, the binder used in the first active material layer and the second active material layer is a mixed binder material obtained by mixing low-swell styrene-butadiene binders and high-swell styrene-butadiene binders in a mass ratio of 1:1;

[0159] Specifically, in the process of preparing the first active slurry, the mixed binder material described above is used instead of the low-swelling styrene-butadiene-based binder in Example 1; in the process of preparing the second active slurry, the mixed binder material described above is used instead of the high-swelling styrene-butadiene-based binder in Example 1; the rest is consistent with Example 1.

[0160] Comparative Example 4

[0161] The difference between this comparative example and Example 1 is that the volume swelling rate of the styrene-butadiene-based binder in the first active material layer is 20%, and the volume swelling rate of the styrene-butadiene-based binder in the second active material layer is 100%; the rest is consistent with Example 1.

[0162] Comparative Example 5

[0163] The difference between this comparative example and Example 1 is that the volume swelling rate of the styrene-butadiene-based binder in the first active material layer is 50%, and the volume swelling rate of the styrene-butadiene-based binder in the second active material layer is 55%; the rest is consistent with Example 1.

[0164] Comparative Example 6

[0165] The difference between this comparative example and Example 1 is that the second active material layer in the negative electrode sheet is arranged on the surface of the copper foil, the first active material layer is arranged on the surface of the second active material layer away from the copper foil, and the first binder and the second binder satisfy (m2-m1) / m1x100%=20%; the rest is consistent with Example 1.

[0166] Test method

[0167] I. Volume swelling rate test of high-swelling binder and low-swelling binder

[0168] Take the low-swelling styrene-butadiene-based binder (low-swelling styrene-butadiene rubber) as an example: test the swelling rate of the low-swelling styrene-butadiene rubber (SBR) in the above examples and comparative examples, and the specific test steps are as follows:

[0169] (1) Take a certain volume (10-30 mL) of SBR emulsion and place it in a container for film formation (in this application, a plastic container with a bottom diameter of 8 cm and a groove height of 1 cm is used), and the amount of SBR emulsion can be adjusted according to the size of the bottom area of the container. The selection principle of the plastic container is that the SBR does not adhere to the bottom of the container after the film is dried, and it is easy to peel off the film;

[0170] (2) Place the container with SBR emulsion on a horizontal table and dry naturally for 48-96 hours to form a film, or place it horizontally in an oven for 12-48 hours to form a film, and the oven temperature is set in the range of 35°C-50°C;

[0171] (3) After the SBR film is formed, it is taken out of the container and placed in an oven at 50-80°C for 2-5h to continue baking, so as to completely dry the moisture in the SBR film;

[0172] (4) The SBR film is cut into three straight-angle trapezoids of the same size or different sizes according to the shape (straight-angle trapezoid or other non-isosceles trapezoid) of FIG. 2 using a cutting knife or a small knife or scissors, and the size can be appropriately adjusted according to the size of the prepared SBR film;

[0173] (5) As shown in FIG. 2, the planar size of the film is measured in the order of 1, 2 and 3 using a size measuring instrument or a profile measuring instrument with a sensor or laser: the upper base ab of the trapezoid is denoted as o1, o2 and o3, the lower base cd of the trapezoid is denoted as p1, p2 and p3, the height ac of the trapezoid is denoted as q1, q2 and q3, and the height bh of the trapezoid is denoted as r1, r2 and r3;

[0174] (6) As shown in FIG. 2, the thickness of the film at different positions is measured in the order of 1, 2 and 3 using a measuring instrument: the thickness of point a is denoted as j1, j2 and j3, the thickness of point b is denoted as k1, k2 and k3, the thickness of point c is denoted as u1, u2 and u3, the thickness of point d is denoted as l1, l2 and l3, the thickness of the midpoint e between ab is denoted as w1, w2 and w3, the thickness of the midpoint f between ac is denoted as x1, x2 and x3, the thickness of the midpoint g between cd is denoted as y1, y2 and y3, and the thickness of the midpoint i between bd is denoted as z1, z2 and z3;

[0175] (7) The three samples are placed in glass bottles or aluminum bottles with a capacity of 10-50mL (for example, if the sizes of the three samples are the same, they need to be placed in three sample bottles respectively, and if the sizes are different, they can be placed in the same sample bottle), then the containers are filled with lithium ion battery electrolyte, and sealed, and the electrolyte contains at least one of the solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and propyl propionate;

[0176] (8) The glass bottle or aluminum bottle sealed with the sample and electrolyte is placed in an oven for baking, the baking temperature is set at 60-80°C, and the baking time is 1-7d, which is adjusted according to the temperature, that is, the higher the temperature, the shorter the baking time;

[0177] (9) After high-temperature soaking, the SBR film is transferred to room temperature for cooling and taken out, and the electrolyte on the surface of the SBR film is wiped dry with a dust-free paper;

[0178] (10) As shown in Figure 2, the sample is measured by a measuring instrument according to the order of 1, 2, 3 before soaking, and the planar size of the adhesive film is measured: the upper base ab of the trapezoid is recorded as O1, O2, O3, the lower base cd of the trapezoid is recorded as P1, P2, P3, the height ac of the trapezoid is recorded as Q1, Q2, Q3, and the height bh of the trapezoid is recorded as R1, R2, R3;

[0179] (11) As shown in Figure 2, the sample is measured by a measuring instrument according to the order of 1, 2, 3 before soaking, and the thickness of the adhesive film at different positions is measured: the thickness of point a is recorded as J1, J2, J3, the thickness of point b is recorded as K1, K2, K3, the thickness of point c is recorded as U1, U2, U3, the thickness of point d is recorded as L1, L2, L3, the thickness at the midpoint e of ab is recorded as W1, W2, W3, the thickness at the midpoint f of ac is recorded as X1, X2, X3, the thickness at the midpoint g of cd is recorded as Y1, Y2, Y3, and the thickness at the midpoint i of bd is recorded as Z1, Z2, Z3;

[0180] II. Rate performance test

[0181] The lithium batteries in the above examples and comparative examples were subjected to rate performance test, and the specific test steps were as follows: at 25℃, the battery was charged at 3C constant current to 4.25V, and then discharged at 3C constant current to 2.80V, and the discharge capacity was recorded as C2; 3C rate discharge retention rate = C2 / C1 x 100%.

[0182] III. Cycle performance test

[0183] The lithium batteries in the above examples and comparative examples were subjected to cycle performance test, and the specific test steps were as follows: at 25℃, the voltage range was set to 2.5-4.3V, and the prepared battery was charged and discharged at 1C / 1C rate in the range of 2.5-4.3V, and when the battery was cycled for 500 times, the capacity retention rate was recorded.

[0184] IV. High temperature storage performance test

[0185] The lithium batteries in the above examples and comparative examples were subjected to high temperature storage performance test, and the specific test steps were as follows:

[0186] (1) First, the lithium battery was placed at 25℃ for 30min; charged to 4.2V at a constant current of 0.5C, and further charged at a constant voltage of 4.2V until the current was 0.5C; then the lithium battery was discharged at a constant current of 0.5C to 1.5V, and the discharge capacity at this time was recorded as C0S;

[0187] (2) The lithium battery was stored at 60°C for 30 days, and after the storage was completed, the lithium battery was discharged to 1.5V at a constant current of 0.5C; then the lithium battery was charged to 4.2V at a constant current of 0.5C, and further charged to a current of 0.5C at a constant voltage of 4.2V; then the lithium battery was discharged to 1.5V at a constant current of 0.5C, and the discharge capacity at this time was recorded as C30S.

[0188] The storage capacity recovery rate (%) of the lithium battery after being stored for 30 days = C30S / C0S x 100%.

[0189] Table 1

[0190]

[0191] In combination with Examples 1-3, Comparative Examples 1-4 and Table 1, it can be seen that by applying the low-swelling binder in the first active material layer close to the current collector and the high-swelling binder and / or acrylic derivative multi-copolymer in the second active material layer away from the current collector, the low-swelling binder can cooperate with the high-swelling binder and / or acrylic derivative multi-copolymer, so that the lithium ion battery can have excellent kinetic performance, cycle performance and high-temperature storage performance.

[0192] When only the low-swelling binder is used (Comparative Example 1), the kinetic performance of the lithium battery is significantly decreased; when only the high-swelling binder is used (Comparative Example 2), because the swelling rate of the high-swelling binder in the electrolyte is large, the thickness of the battery cell is also greatly expanded, which causes the cycle performance and high-temperature storage performance of the lithium battery to be significantly deteriorated; when the low-swelling binder and the high-swelling binder are mixed (Comparative Example 3), the kinetic performance and cycle stability of the lithium battery are both decreased, and the cooperation effect of the low-swelling binder and the high-swelling binder is not significant.

[0193] When the volume swelling rate of the low-swelling binder in the first active material layer is too low and the volume swelling rate of the high-swelling binder in the second active material layer is too high, the volume expansion of the high-swelling binder will affect the tightness of the active particles in the second active material layer, increase the ion transfer impedance, and the adsorption of the electrolyte by the second active material layer is significantly increased due to the influence of the high-swelling binder, but it cannot be absorbed by the first active material layer close to the current collector in time, further aggravating the instability of the second active material layer, and the kinetic performance of the lithium battery is significantly decreased. When the volume swelling rate of the low-swelling binder in the first active material layer is too high and the volume swelling rate of the high-swelling binder in the second active material layer is too low, the adhesion between the first active material layer and the current collector is decreased, and the adsorption capacity of the electrolyte by the second active material layer is decreased, which is not conducive to the absorption of the electrolyte by the first active material layer close to the current collector, and reduces the kinetic performance and energy density of the lithium battery.

[0194] When the positions of the first active material layer and the second active material layer are interchanged (Comparative Example 6), due to the decrease in the liquid absorption amount and the liquid retention amount of the low-swelling binder to the electrolyte, the second active material layer will be less infiltrated by the electrolyte, not to mention the first active material layer far from the electrolyte, thereby reducing the electron transport and ion transport in the composite active material layer, and the high-swelling binder will affect the adhesion between the first active material layer and the current collector interface due to its poor volume stability. The above arrangement of the first active material layer and the second active material layer will significantly reduce the kinetic performance, cycle performance and high-temperature cycle performance of the lithium battery.

[0195] In combination with Examples 1, 4-5 and Table 1, it can be seen that when the binder in the second active material layer is only a high-swelling binder or only an acrylic derivative multi-copolymer, the kinetic performance and energy density of the lithium battery decrease slightly, which may be due to the fact that although the high-swelling binder can improve the infiltration of the second active material layer and the first active material layer by high adsorption to the electrolyte, the volume expansion of the high-swelling binder in the second active material layer will be greater than the volume expansion of the low-swelling binder in the first active material layer, which will affect the ion transfer impedance of the second active material layer to some extent; and when only the acrylic derivative multi-copolymer is used, although the acrylic derivative multi-copolymer can reduce the DCR value of the second active material layer, the infiltration of the electrolyte into the first active material layer close to the current collector is low at this time, which not only affects the ion transfer impedance, but also affects the exertion of the energy density of the lithium battery to some extent.

[0196] In combination with Examples 1-5, Example 6 and Table 1, it can be seen that when the thickness of the composite active material layer is too high, the low-swelling binder can slightly weaken the effect of the high-swelling binder and / or the acrylic derivative multi-copolymer in improving the kinetic performance, cycle performance and high-temperature storage performance of the lithium battery, which is because when the composite active material layer is thick, the amount of high-swelling binder in the second active material layer is also large, and the thickness of the second active material layer is also high, which is more likely to cause large expansion of the second active material layer during the cycle process.

[0197] In combination with Example 1, Examples 7-8 and Table 1, it can be seen that when the thickness ratio of the first active material layer to the second active material layer is too large, the total content of the low-swelling binder used in the first active material layer is higher than that used in Example 1, the rate discharge capacity retention of the lithium battery decreases, but the high-temperature storage performance is slightly improved; on the contrary, when the thickness ratio of the first active material layer to the second active material layer is too small, the total content of the high-swelling binder used in the second active material layer is higher than that used in Example 1, the rate discharge capacity retention of the lithium battery is slightly improved, but the high-temperature storage performance shows a downward trend.

[0198] In combination with Example 1, Examples 9-10 and Table 1, it can be seen that when the content of the first binder in the composite active material layer is 1% higher than that of the second binder in the composite active material layer, the problem of floating of the first binder during the forming process cannot be effectively alleviated, which will affect the adhesion between the first active material layer and the current collector interface and reduce the cycle performance of the lithium battery; when the content of the first binder in the composite active material layer is 60% higher than that of the second binder in the composite active material layer, the content of the high-swelling binder in the second active material layer is low, which is not conducive to the infiltration of the electrolyte to the composite active material layer, and is not conducive to the conduction of electrons and ions, which will affect the kinetic performance of the lithium battery.

Claims

1. A negative electrode sheet comprising a current collector and a composite active material layer disposed on at least one surface of the current collector, the composite active material layer comprising a first active material layer and a second active material layer sequentially disposed on the surface of the current collector; the second active material layer being disposed on a surface of the first active material layer distal from the current collector; The first active material layer includes a first binder, the first binder includes a low swelling binder, the low swelling binder includes a low swelling styrene butadiene binder; the monomer composition of the low swelling styrene butadiene binder includes at least one of butadiene and styrene; The mass proportion of the first binder in the first active material layer is 1%-3%; The second active material layer includes a second binder, the second binder including at least one of a high swelling binder and an acrylic acid derivative multipolymer; the high swelling binder includes at least one of a high swelling styrene-butadiene binder and a high swelling styrene-acrylic binder, the monomer composition of the high swelling styrene-butadiene binder includes at least one of butadiene and styrene, the monomer composition of the high swelling styrene-acrylic binder includes at least one of styrene and acrylic acid ester; the monomer composition of the acrylic acid derivative multipolymer includes at least one of acrylic acid, acrylic acid salt, acrylic acid ester, and acrylonitrile; The mass proportion of the second binder in the second active material layer is 0.8%-2%; in, The volume swelling rate of the low swelling binder is 25%-40%, and the volume swelling rate of the high swelling binder is 60%-85%.

2. A negative electrode sheet according to claim 1, wherein: The low swelling styrene butadiene adhesive includes low swelling styrene butadiene rubber; the high swelling styrene butadiene adhesive includes high swelling styrene butadiene rubber; the high swelling styrene acrylic adhesive includes high swelling styrene acrylic rubber; the acrylate includes at least one of sodium acrylate and lithium acrylate.

3. A negative electrode sheet according to claim 1, wherein: When the second binder includes both the high swelling binder and the acrylic acid derivative multipolymer, the mass ratio of the high swelling binder to the acrylic acid derivative multipolymer is 1:1.5-2.

5.

4. The negative electrode sheet according to claim 1, wherein: The thickness of the composite active material layer is 100-300 μm.

5. The negative electrode sheet according to claim 1, wherein: The ratio of the thickness of the first active material layer to the thickness of the second active material layer is (1-1.5):

1.

6. The negative electrode sheet according to claim 1, wherein: The first active material layer further includes a first active material, which includes at least one of graphite and silicon-containing graphite; the second active material layer further includes a second active material, which includes at least one of graphite and silicon-containing graphite.

7. A method for preparing a negative electrode sheet according to any one of claims 1 to 6, comprising the following steps: S1: mixing and stirring a first conductive agent, a first dispersant, a first active material, the first binder, and a first solvent to obtain a first active slurry; mixing and stirring a second conductive agent, a second dispersant, the second active material, the second binder, and a second solvent to obtain a second active slurry; S2: coating the first active slurry and the second active slurry on at least one surface of the current collector, wherein: The first active slurry is coated on the surface of the current collector, dried, and cold-pressed to obtain the negative electrode sheet.

8. The method for preparing a negative electrode sheet according to claim 7, wherein: The mass of the first active slurry is recorded as t1, the mass of the second active slurry is recorded as t2, the mass of the first binder is recorded as m1, the mass of the second binder is recorded as m2, and the first binder and the second binder satisfy m1 / (t1+t2)>m2 / (t1+t2).

9. The method for preparing a negative electrode sheet according to claim 8, wherein: The mass of the first binder is recorded as m1, and the mass of the second binder is recorded as m2, and m1 and m2 satisfy 5%≤(m1-m2) / m2×100%≤50%.

10. A lithium-ion battery comprising a positive electrode sheet, a separator, an electrolyte and the negative electrode sheet according to any one of claims 1 to 6.

Images