Negative electrode sheet and secondary battery

Abstract

The application provides a negative electrode sheet and a secondary battery. The negative electrode sheet comprises a current collector and a first coating layer and a second coating layer arranged in turn from the side close to the current collector outward, the first coating layer comprises a first active material and a first polymer microsphere, and the second coating layer comprises a second active material and a second polymer microsphere; the swelling rate of the first polymer microsphere is less than the swelling rate of the second polymer microsphere, and the compaction density of the first coating layer is greater than the compaction density of the second coating layer. The application adopts double-layer coating coating layers with different compaction densities to form different pore sizes, fills the polymer microspheres with different swelling rates into the double-layer coating coating layers, ensures that the prepared battery has excellent performance, does not increase the existing electrode sheet volume space, ensures a small full-charge rebound, and thus improves the rate and cycle performance of the battery.

Description

TECHNICAL FIELD

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

[0002] Secondary batteries have the advantages of high energy density, large output power, long service life, wide working temperature, low self-discharge rate, environmental friendliness, etc., and have been widely applied in the fields of 3C, energy storage, power batteries, etc. With the rapid development of energy storage technology, higher requirements are put forward for secondary batteries, which not only have high energy density, but also meet the multiple demands of fast charging and cycle life.

[0003] Although increasing the electrode thickness is a direct and effective means to improve the energy density of the battery, problems such as slow reaction kinetics and insufficient mechanical properties limit its development. However, the design of a thick electrode of a secondary battery is a complex multi-objective optimization process, which needs to comprehensively evaluate multiple parameters including energy density, power density, cycle life, safety, electrochemical performance and cost, etc. to successfully build a lithium battery thick electrode with high energy density, high safety and long life. Therefore, the hierarchical microstructure of the electrode sheet needs to be finely designed, especially for thick electrode sheets, the charge transfer and electrochemical reaction kinetics are often limited, and the rate performance is poor, and the capacity utilization is low.

[0004] At present, this problem can be improved by hierarchical gradient design of porosity. The porosity distribution gradient refers to the hierarchical structure with more than two layers in the thickness direction of the electrode. The same active material is usually selected by different compactions to form different porosities. Generally, the larger the porosity, the smaller the pore tortuosity, the higher the effective diffusion coefficient and effective conductivity of lithium ions, and the smaller the ion impedance. However, too high porosity will reduce the volumetric energy density of the battery and also reduce the electronic conductivity of the electrode. For the porosity distribution gradient electrode, it is generally believed that the electrode with gradually reduced porosity or porosity from the separator to the current collector can reduce the ion resistance without compromising the electronic conductivity and volumetric energy density, thereby improving the rate performance of the battery.

[0005] The double-layer coating process generally selects different compaction densities for the upper and lower coatings to form different porosities to achieve optimal ion transport performance. However, the improvement of the rate performance and cycle performance of the battery under the thick electrode system in practical application is limited. SUMMARY

[0006] In order to solve the problems of poor rate performance and low capacity utilization of the battery prepared by the existing double-layer coated thick negative electrode sheet, the application provides a negative electrode sheet and a secondary battery, which improves by adding polymer microspheres with different swelling rates into the double-layer coated negative electrode sheet with different porosities.

[0007] The present application is realized by the following technical solutions.

[0008] In a first aspect, the present application provides a negative electrode sheet, comprising a current collector and a first coating layer and a second coating layer arranged outward from the side of the current collector in sequence, the first coating layer comprising a first active material and a first polymer microsphere, and the second coating layer comprising a second active material and a second polymer microsphere; the swelling rate of the first polymer microsphere is less than the swelling rate of the second polymer microsphere, and the compaction density of the first coating layer is greater than the compaction density of the second coating layer.

[0009] Further, the swelling rate of the first polymer microsphere is 500% to 1000%, the swelling rate of the second polymer microsphere is 1100% to 2000%, the compaction density of the first coating layer is 1.6 to 1.7 g / cm 3 , and the compaction density of the second coating layer is 1.5 to 1.59 g / cm 3 .

[0010] Further, the weight percentage of the first polymer microsphere in the first coating layer is 0.1 to 0.3%, and the weight percentage of the second polymer microsphere in the second coating layer is 0.1 to 0.3%.

[0011] Further, the first polymer microsphere comprises a first copolymer, the first copolymer comprising a first structural unit, a second structural unit, a first functional structural unit and a second functional structural unit, and the weight ratio of the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit is (50 to 150):(30 to 50):(20 to 40):(25 to 50).

[0012] The second polymer microsphere comprises a third structural unit, a fourth structural unit, a third functional structural unit and a fourth functional structural unit, and the weight ratio of the third structural unit, the fourth structural unit, the third functional structural unit and the fourth functional structural unit is (150 to 300):(10 to 30):(5 to 20):(5 to 25).

[0013] Further, the first structural unit and the third structural unit each independently comprise a structural unit obtained by polymerization of an acrylic monomer, and the acrylic monomer comprises at least one of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-iso-octyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, bisacrylamide, phenyl methacrylate and n-hexyl acrylate.

[0014] Further, the second structural unit and the fourth structural unit each independently comprise a structural unit obtained by polymerization of a vinyl aromatic monomer, the vinyl aromatic monomer comprising at least one of styrene, beta-methyl styrene, 4-methoxystyrene, and 2-methyl styrene.

[0015] Further, the first functional structural unit and the third functional structural unit each independently comprise a structural unit obtained by polymerization of an acrylamide monomer, the acrylamide monomer comprising at least one of N-hydroxyethyl acrylamide, N-(isobutoxymethyl) acrylamide, and diacetone acrylamide.

[0016] Further, the second functional structural unit and the fourth functional structural unit each independently comprise a structural unit obtained by polymerization of an acrylate-polyol, the acrylate-polyol comprising at least one of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) diacrylate, and polypropylene glycol dimethacrylate.

[0017] In a second aspect, the present application provides a secondary battery comprising a positive electrode sheet, a separator, an electrolyte, and the negative electrode sheet of the first aspect.

[0018] Further, the secondary battery has a full charge bounce rate of less than 22%.

[0019] Compared with the prior art, the present application has the following beneficial effects.

[0020] The present application aims to form different porosities by coating different compaction densities in two layers, and match the two-layer coating design by adding polymer microspheres with different swelling rates. The first coating layer close to the current collector has a larger compaction density and fills the first polymer microspheres with a smaller swelling rate due to its better electronic conductivity. The second coating layer away from the current collector has a lower compaction density and fills the polymer microspheres with a larger swelling rate due to its poor electronic conductivity, in order to ensure the rapid transmission of lithium ions.

[0021] The use of polymer microspheres with different swelling rates in the two-layer coating with different porosities can improve the wettability of the negative electrode material to the electrolyte, and make the gap and interface between the negative electrode active material particles have sufficient electrolyte. In the charging process, the lithium ions released from the positive electrode can migrate to the negative electrode surface more quickly. In the discharging process, the stress generated by the shrinking of the interlayer spacing of the lithium ions embedded in the negative electrode structure can cause the electrolyte absorbed by the polymer microspheres to be partially released.

[0022] The double-layer coated thick negative plate designed in the application adopts double-layer coating layers with different compactness to form different pore sizes, and fills polymer microspheres with different swelling rates into the double-layer coated coating layers, so that the prepared battery has excellent performance, the full charge rebound is small without increasing the existing volume space of the plate, and the rate and cycle performance of the battery are improved. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects solved by the application clearer, the application will be further described in detail below in combination with specific embodiments. It should be understood that the embodiments described herein are part of the embodiments of the application, rather than all the embodiments, and are used to explain the application, but not to limit the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0024] In the application, each structural unit represents a corresponding structural moiety of a monomer in a polymer obtained after the monomer participates in a polymerization reaction, as known to those skilled in the art in the field of chemical synthesis. The weight ratio of each structural unit is the weight ratio of the monomer providing the structural unit.

[0025] In a first aspect, the application provides a negative plate, comprising a current collector and a first coating layer and a second coating layer arranged outward from the side close to the current collector in sequence, the first coating layer comprising a first active material and a first polymer microsphere, and the second coating layer comprising a second active material and a second polymer microsphere; the swelling rate of the first polymer microsphere is less than the swelling rate of the second polymer microsphere, and the compactness of the first coating layer is greater than the compactness of the second coating layer.

[0026] In some embodiments, the swelling rate of the first polymer microsphere is 500% to 1000%, the swelling rate of the second polymer microsphere is 1100% to 2000%, the compactness of the first coating layer is 1.6 to 1.7 g / cm 3 , and the compactness of the second coating layer is 1.5 to 1.59 g / cm 3 .

[0027] In some embodiments, the weight percentage of the first polymer microsphere in the first coating layer is 0.1 to 0.3%, and the weight percentage of the second polymer microsphere in the second coating layer is 0.1 to 0.3%.

[0028] The first coating layer close to the side of the current collector comprises, in addition to the first active material and the first polymer microspheres, a conductive agent and a binder. For example, the first active material can be graphite, the conductive agent can be SP, and the binder can be CMC+SBR; the weight ratio of the first active material, the conductive agent, CMC, SBR, the first polymer microspheres in the first coating layer is (94.9-95.1):1.0:1.2:2.4:(0.1-0.3).

[0029] The second coating layer close to the side of the separator comprises, in addition to the second active material and the second polymer microspheres, a conductive agent and a binder. For example, the second active material can be graphite, the conductive agent can be SP, and the binder can be CMC+SBR; the weight ratio of the second active material, the conductive agent, CMC, SBR, the second polymer microspheres in the second coating layer is (95.7-95.9):1.0:1.2:1.8:(0.1-0.3).

[0030] In some embodiments, the first polymer microspheres comprise a first copolymer, the first copolymer comprises a first structural unit, a second structural unit, a first functional structural unit and a second functional structural unit, and the weight ratio of the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit is (50-150):(30-50):(20-40):(25-50).

[0031] The second polymer microspheres comprise a third structural unit, a fourth structural unit, a third functional structural unit and a fourth functional structural unit, and the weight ratio of the third structural unit, the fourth structural unit, the third functional structural unit and the fourth functional structural unit is (150-300):(10-30):(5-20):(5-25).

[0032] More specifically, in the first polymer microspheres, the weight ratio of the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit is 50:30:20:25, 55:30:25:30, 60:30:25:30, 65:30:25:35, 70:35:25:30, 75:30:25:30, 80:35:25:30, 85:40:25:35, 90:40:25:35, 95:41:26:36, 100:40:26:35, 105:35:25:35, 110:35:25:35, 115:35:25:30, 120:30:25:30, 125:35:30:30, 130:35:35:30, 135:40:40:35, 140:40:45:40, 145:45:40:45, 150:50:40:50, etc.

[0033] The weight ratio of the third structural unit, the fourth structural unit, the third functional structural unit and the fourth functional structural unit in the second polymer microsphere is 150:10:5:5, 160:10:5:5, 170:10:6:6, 180:12:6:6, 190:13:7:8, 200:15:8:9, 210:16:8:10, 220:16:8:10, 230:18:10:13, 245:18:13:16, 260:20:13:18, 270:23:15:20, 280:25:15:23, 290:28:18:24, 300:30:20:25, etc.

[0034] In the copolymer of the present application, the first structural unit and the third structural unit each independently comprise a structural unit obtained by polymerization of an acrylic monomer, and the acrylic monomer comprises at least one of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, bisacrylamide, phenyl methacrylate, n-hexyl acrylate.

[0035] In the copolymer of the present application, the second structural unit and the fourth structural unit each independently comprise a structural unit obtained by polymerization of a vinyl aromatic monomer, and the vinyl aromatic monomer comprises at least one of styrene, β-methylstyrene, 4-methoxystyrene, 2-methylstyrene.

[0036] In the copolymer of the present application, the first functional structural unit and the third functional structural unit each independently comprise a structural unit obtained by polymerization of an acrylamide monomer, and the acrylamide monomer comprises at least one of N-hydroxyethyl acrylamide, N-(isobutoxymethyl) acrylamide, diacetone acrylamide.

[0037] In the copolymer of the present application, the second functional structural unit and the fourth functional structural unit each independently comprise a structural unit obtained by polymerization of an acrylate-polyol, and the acrylate-polyol comprises at least one of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, polypropylene glycol dimethacrylate.

[0038] The acrylic structural unit is mainly used for adjusting the electrolyte swelling rate of the copolymer; the vinyl aromatic structural unit is mainly used for stabilizing the structure of the copolymer; the polar functional group of the acrylamide functional monomer unit is mainly used for further increasing the affinity with the electrolyte and improving the binding force between the active material; and the crosslinking network structure formed by the acrylate-polyalcohol structural unit and the crosslinking agent is used for stabilizing the volume change of the polymer microsphere copolymer after liquid absorption due to the expansion and contraction of the negative electrode material in the charging and discharging process, and is also used for inhibiting the full charge rebound rate of the negative electrode sheet.

[0039] The present application mainly increases or reduces the swelling rate of the polymer microsphere by adjusting the amount of acrylic acid, so as to obtain polymer microspheres with different swelling rates, i.e., first polymer microspheres with a swelling rate of 500% to 1000% and second polymer microspheres with a swelling rate of 1100% to 2000%.

[0040] If the amount of the acrylic structural unit is too much, the polymer microspheres swell too much due to the good affinity between the acrylic structural unit and the electrolyte, the full charge rebound of the battery is increased, and the active coating and the foil are partially separated after the cycle, which reduces the cycle life. If the amount of the acrylic structural unit is too small, the adhesion between the active material and the polymer microspheres is reduced, and the swelling rate of the polymer microspheres is reduced, which reduces the migration rate of lithium ions in the negative electrode during the charging and discharging process, and thus the lithium precipitation of the battery under the condition of large rate and fast charging and the cycle life of the battery are affected.

[0041] If the proportion of the vinyl aromatic structural unit is too high, the glass transition temperature of the polymer microspheres is increased, the electrode sheet is easily cracked during the processing and baking process, the electrode sheet is hard and brittle and the peeling strength is reduced, the material is dropped during the rolling and die cutting process, and the rate and cycle of the battery are affected. If the proportion of the vinyl aromatic structural unit is too low, the good spherical structure of the polymer microspheres cannot be maintained, and the peeling strength of the electrode sheet is also reduced.

[0042] If the proportion of the acrylamide functional unit is too high, the glass transition temperature of the polymer microspheres is increased, the electrode sheet is easily cracked during the processing and baking process, the electrode sheet is hard and brittle and the peeling strength is reduced, the material is dropped during the rolling and die cutting process, and the rate and cycle of the battery are affected. If the proportion of the acrylamide functional unit is too small, the binding force between the active material and the polymer microspheres is reduced, the electrode sheet is easily dropped during the large rate and fast charging of the battery, and the cycle life of the battery is affected.

[0043] If the proportion of the acrylic ester-polyol functional unit is too high, although the full charge rebound of the battery cell can be inhibited to a certain extent, the glass transition temperature of the polymer microspheres will increase, the processing performance will be poor, the strip strength of the pole piece will decrease, and the pole piece edge will be prone to material falling under fast charging conditions, thereby affecting the cycle performance of the battery cell. If the proportion of the acrylic ester-polyol functional unit is too low, it cannot ensure the formation of a certain three-dimensional spatial network structure with the crosslinking agent, which leads to a large rebound of the pole piece under full charge conditions, and further leads to material falling of the pole piece under fast charging conditions.

[0044] In some embodiments, when preparing the first copolymer, in addition to adding the monomers corresponding to the first structural unit, the second structural unit, the first functional structural unit, and the second functional structural unit, respectively, a first crosslinking agent, a first polymerization type emulsifier, and a first initiator are also included, and the weight ratio of the monomer corresponding to the second functional structural unit to the first crosslinking agent, the first polymerization type emulsifier, and the first initiator is (25-50):(3-5):(2-3):(1-3).

[0045] More specifically, the weight ratio of the monomer corresponding to the second functional structural unit of the first copolymer to the first crosslinking agent, the first polymerization type emulsifier, and the first initiator can be 25:3:2:1, 25:3:2:1.5, 30:3:2:1.5, 30:3.5:2:2, 35:3.5:2.5:2, 35:4:2.5:2.5, 40:4:2.5:2.5, 40:5:2:2, 45:4:2.5:2.5, 45:5:3:3, 50:5:2.5:2, 50:5:3:3, etc.

[0046] Similarly, when preparing the second polymer microspheres, in addition to adding the monomers corresponding to the third structural unit, the fourth structural unit, the third functional structural unit, and the fourth functional structural unit, respectively, a second crosslinking agent, a second polymerization type emulsifier, and a second initiator are also included, and the weight ratio of the monomer corresponding to the fourth functional structural unit to the second crosslinking agent, the second polymerization type emulsifier, and the second initiator is (5-25):(2-3):(1-2):(0.5-2).

[0047] More specifically, the weight ratio of the monomer corresponding to the fourth functional structural unit of the second polymer microspheres to the second crosslinking agent, the second polymerization type emulsifier, and the second initiator can be 5:2:1:0.5, 7:2:1.5:1, 9:2:1:0.5, 10:2:1.5:1, 13:2:1:1, 15:2.2:1:1, 17:2.5:2:1.5, 19:2:1.5:1.5, 21:2:2:1.5, 23:2.5:2:1.5, 24:3:2:2, 25:3:2:1.5, etc.

[0048] The first cross-linking agent and the second cross-linking agent are each independently selected from at least one of diethylene glycol dimethacrylate, ethylene glycol diacrylate, tetra(ethylene glycol) diacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, neopentyl glycol diacrylate, 1,6-hexanediol dimethacrylate, and pentaerythritol tetraacrylate.

[0049] The first polymeric emulsifier and the second polymeric emulsifier are each independently selected from at least one of an allyl polyoxyethylene ether, an ammonium salt of an allyl polyoxyethylene ether sulfonic acid, a sodium salt of a vinyl sulfonic acid, or a polyoxyethylene nonylphenol phosphate ester.

[0050] The first initiator and the second initiator are each independently selected from at least one of azobisisobutyronitrile, azobisisobutyramidine hydrochloride, azobisisopropylimidazoline hydrochloride, dimethyl azobis-2-methylpropionate, ammonium persulfate, sodium persulfate, or potassium persulfate.

[0051] The first polymer microspheres and the second polymer microspheres of the present application have the same structural units of the copolymer, but the weight ratio of the structural units of the copolymer is different; the same preparation method can be used, and the corresponding monomers are added according to the corresponding weight ratio of the structural units, so that the corresponding first polymer microspheres and the second polymer microspheres are obtained. The preparation method of the first polymer microspheres and the second polymer microspheres can include the following steps:

[0052] (1) In an inert gas environment, first, the polymeric emulsifier, the cross-linking agent, and deionized water are mixed in proportion, and the weight ratio of deionized water to acrylic monomer is (4-5):3, and stirred at a speed of 1000 rpm for 1-5 h; then the acrylic monomer, the vinyl aromatic monomer, the acrylamide monomer, and the acrylic ester-polyol mixture are added according to the aforementioned weight ratio, and stirred at a speed of 200 r / min for 3-8 h to obtain a pre-emulsion;

[0053] (2) Then, the mixture of deionized water and initiator is slowly added to the pre-emulsion, and the weight ratio of deionized water to initiator is 50:(1-3), and the mixing reaction is carried out at a speed of 100 r / min, the reaction temperature is 70-90℃, and the reaction time is 5-15 h, to obtain a polymer emulsion with a solid content of 40-50%, and the solid particles in the polymer emulsion are first polymer microspheres or second polymer microspheres.

[0054] In a second aspect, the present application provides a secondary battery including a positive electrode sheet, a separator, an electrolyte, and the negative electrode sheet of the first aspect.

[0055] In some specific embodiments, the full charge bounce rate of the secondary battery is less than 22%.

[0056] The specific embodiments of the present application will be further explained by the following examples and comparative examples. Specifically, the negative electrode sheet and the secondary battery disclosed by the present application will be explained.

[0057] The reagents, materials and instruments used in the following description are conventional reagents, conventional materials and conventional instruments, which are commercially available, and the reagents involved can also be synthesized by conventional synthesis methods. The methods in the examples are conventional methods in the art, unless otherwise specified.

[0058] The monomers meeting the present application can be commercially available.

[0059] Example 1

[0060] 1) Preparation of polymer microspheres

[0061] In an inert gas environment, 2.5 g of allyl polyoxyethylene ether, 4 g of pentaerythritol tetraacrylate and 120 g of deionized water were mixed and stirred at 1000 rpm for 3 h; then 90 g of methyl acrylate, 40 g of styrene, 30 g of N-(isobutoxymethyl) acrylamide, 35 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were mixed and stirred at 200 r / min for 5 h to obtain a pre-emulsion; then 100 g of deionized water and 2 g of azobisdimethylamino formamide hydrochloride were slowly added to the pre-emulsion, mixed at 100 r / min, the reaction temperature was 85℃, and the reaction time was 10 h, to obtain a polymer emulsion with a solid content of 40%; the solid particles in the polymer emulsion were first polymer microspheres, and the swelling rate of the first polymer microspheres was 700%.

[0062] In an inert gas environment, 2.5 g of allyl polyoxyethylene ether, 4 g of pentaerythritol tetraacrylate and 120 g of deionized water were mixed and stirred at 1000 rpm for 3 h; then 90 g of methyl acrylate, 40 g of styrene, 30 g of N-(isobutoxymethyl) acrylamide, 35 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were mixed and stirred at 200 r / min for 5 h to obtain a pre-emulsion; then 100 g of deionized water and 2 g of azobisdimethylamino formamide hydrochloride were slowly added to the pre-emulsion, mixed at 100 r / min, the reaction temperature was 85℃, and the reaction time was 10 h, to obtain a polymer emulsion with a solid content of 40%; the solid particles in the polymer emulsion were first polymer microspheres, and the swelling rate of the first polymer microspheres was 700%.

[0063] 2) Preparation of negative electrode sheet

[0064] 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min. Then, 950g of the first active material, graphite, was added and stirred at 1000 rpm for 15 min. After high-speed dispersion at 4000 rpm for 120 min, 24g of SBR and 1.5g of the first polymer microspheres were added. The mixture was then vacuum defoamed and stirred at 1000 rpm for 30 min. Water was added to adjust the viscosity to 4000 mP·s, resulting in a negative electrode slurry with a solid content of approximately 50% containing the first polymer microspheres. The negative electrode slurry was then coated onto both sides of the negative electrode current collector copper foil to form the first coating, with a single-sided coating surface density of 55 g / m². 2 After drying, the material is cold-pressed, and the compacted density of the first coating after cold pressing is 1.65 g / cm³. 3 .

[0065] 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min. Then, 958g of graphite (the second active material) was added, and the mixture was stirred at 1000 rpm for another 15 min. Following this, the mixture was dispersed at 4000 rpm for 120 min. Then, 18g of SBR and 1.5g of the second polymer microspheres were added. The mixture was then vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was added to adjust the viscosity to 4200 mP·s, resulting in a negative electrode slurry with a solid content of approximately 50% containing the second polymer microspheres. The negative electrode slurry was then coated onto the surface of the first coating to form a second coating. The surface density of the single-sided coating was 55 g / m². 2 After drying and cold pressing, the compacted density of the second coating is 1.55 g / cm³. 3 After further processing such as slitting, the negative electrode sheet is obtained.

[0066] 3) Secondary battery preparation

[0067] Preparation of positive electrode sheet: The prepared LFP series positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil, then dried, and then subjected to cold pressing, slitting and other processes to obtain the positive electrode sheet.

[0068] The negative electrode, separator (PE film), and positive electrode of this invention are wound in sequence, with the separator positioned between the positive and negative electrode to act as a separator, thus forming an electrode assembly. The electrode assembly is placed in an outer packaging, injected with commercially available electrolyte, and sealed. After processes such as electrolyte injection, formation, and venting, a secondary battery is obtained.

[0069] Example 2

[0070] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that the compaction density of the first coating is 1.60 g / cm³ during the preparation of the negative electrode sheet. 3The compaction density of the second coating is 1.50 g / cm³. 3 Everything else is the same.

[0071] Example 3

[0072] The negative electrode and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that, in the preparation process of the negative electrode, the compaction density of the first coating is 1.70 g / cm³. 3 The compaction density of the second coating is 1.59 g / cm³. 3 Everything else is the same.

[0073] Example 4

[0074] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that in the preparation of the negative electrode sheet, the amount of graphite added as the first active material in the first coating is 951g, and the amount of the first polymer microspheres added is 3g; in the second coating, the amount of graphite added as the second active material is 959g, and the amount of the second polymer microspheres added is 3g. All other steps are the same.

[0075] Example 5

[0076] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that in the preparation of the negative electrode sheet, the amount of graphite added as the first active material in the first coating is 949g, and the amount of the first polymer microspheres added is 1g; in the second coating, the amount of graphite added as the second active material is 957g, and the amount of the second polymer microspheres added is 1g. All other steps are the same.

[0077] Example 6

[0078] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that the monomers, crosslinking agents, polymerization emulsifiers, and initiators selected in the preparation of the first and second polymer microspheres are different: the acrylic monomer is tert-butyl acrylate, the vinyl aromatic monomer is 2-methylstyrene, the acrylamide monomer is N-hydroxyethylacrylamide, the acrylate-polyol monomer is poly(ethylene glycol) diacrylate, the crosslinking agent is diethylene glycol dimethacrylate, the polymerization emulsifier is allyl polyoxyethylene ether sulfonate ammonium salt, and the initiator is dimethyl azobisisobutyrate. All other steps are the same.

[0079] Example 7

[0080] The polymer microspheres, negative electrode, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that the swelling ratios of the first and second polymer microspheres differ due to the different weight ratios of the selected monomers. All other steps are the same.

[0081] The specific preparation steps for the first and second polymer microspheres are as follows:

[0082] In an inert gas environment, 2g of allyl polyoxyethylene ether, 3g of pentaerythritol tetraacrylate, and 80g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 50g of methyl acrylate, 30g of styrene, 20g of N-(isobutoxymethyl)acrylamide, and 25g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 50g of deionized water and 1g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres with a swelling rate of 500%.

[0083] In an inert gas environment, 1g of allyl polyoxyethylene ether, 2g of pentaerythritol tetraacrylate, and 205g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 150g of methyl acrylate, 10g of styrene, 5g of N-(isobutoxymethyl)acrylamide, and 5g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 25g of deionized water and 0.5g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling rate of 1100%.

[0084] Example 8

[0085] The polymer microspheres, negative electrode, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that the swelling ratios of the first and second polymer microspheres differ due to the different weight ratios of the selected monomers. All other steps are the same.

[0086] The specific preparation steps for the first and second polymer microspheres are as follows:

[0087] In an inert gas environment, 3g of allyl polyoxyethylene ether, 5g of pentaerythritol tetraacrylate, and 200g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 150g of methyl acrylate, 50g of styrene, 40g of N-(isobutoxymethyl)acrylamide, and 50g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 100g of deionized water and 3g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres with a swelling rate of 1000%.

[0088] In an inert gas environment, 2g of allyl polyoxyethylene ether, 3g of pentaerythritol tetraacrylate, and 400g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 300g of methyl acrylate, 30g of styrene, 20g of N-(isobutoxymethyl)acrylamide, and 25g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 60g of deionized water and 2g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling rate of 2000%.

[0089] Example 9

[0090] The polymer microspheres, negative electrode, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that the monomers selected and their weight ratios are different in the preparation of the first and second polymer microspheres, and the swelling rates of the first and second polymer microspheres are different. All other aspects are the same.

[0091] The specific preparation steps for the first and second polymer microspheres are as follows:

[0092] In an inert gas environment, 3g of allyl polyoxyethylene ether, 5g of pentaerythritol tetraacrylate, and 160g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 120g of lauryl acrylate, 35g of 4-methoxystyrene, 30g of diacetone acrylamide, and 40g of polyethylene glycol dimethacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 100g of deionized water and 3g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres, and the swelling ratio of the first polymer microspheres was 850%.

[0093] In an inert gas environment, 2g of allyl polyoxyethylene ether, 3g of pentaerythritol tetraacrylate, and 300g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 225g of lauryl acrylate, 30g of 4-methoxystyrene, 15g of diacetone acrylamide, and 18g of polyethylene glycol dimethacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 60g of deionized water and 1.8g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling ratio of 1600%.

[0094] Example 10

[0095] Polymer microspheres, a negative electrode sheet, and a secondary battery were prepared according to most of the steps in Example 1. The main differences between this example and Example 1 are: in the preparation of the polymer microspheres, the swelling ratio of the first polymer microsphere is 900%, and the swelling ratio of the second polymer microsphere is 1700%; in the preparation of the negative electrode sheet, the addition amounts of the first and second polymer microspheres are different, and the compaction density of the first coating is 1.66 g / cm³. 3 The compaction density of the second coating is 1.53 g / cm³. 3 Everything else is the same. Details are as follows:

[0096] 1) Preparation of polymer microspheres

[0097] In an inert gas environment, 2g of polyoxyethylene nonylphenol phosphate, 4g of ethylene glycol diacrylate, and 180g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 135g of ethyl acrylate, 35g of β-methylstyrene, 30g of N-(isobutoxymethyl)acrylamide, and 30g of polypropylene glycol dimethacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 100g of deionized water and 3g of ammonium persulfate was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres with a swelling rate of 900%.

[0098] In an inert gas environment, 2g of polyoxyethylene nonylphenol phosphate, 3g of ethylene glycol diacrylate, and 320g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 240g of ethyl acrylate, 30g of β-methylstyrene, 15g of N-(isobutoxymethyl)acrylamide, and 18g of polypropylene glycol dimethacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 60g of deionized water and 1.8g of ammonium persulfate was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling ratio of 1700%.

[0099] 2) Preparation of negative electrode sheet

[0100] 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min. Then, 951g of the first active material, graphite, was added and stirred at 1000 rpm for 15 min. The mixture was then dispersed at 4000 rpm for 120 min. Next, 24g of SBR and 2.5g of the first polymer microspheres were added. The mixture was then vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was added to adjust the viscosity to 4000 mP·s, resulting in a negative electrode slurry with a solid content of approximately 50% containing the first polymer microspheres. The negative electrode slurry was then coated onto both sides of the negative electrode current collector copper foil to form the first coating, with a single-sided coating surface density of 55 g / m². 2 After drying, the material is cold-pressed, and the compacted density of the first coating after cold pressing is 1.66 g / cm³. 3 .

[0101] 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min. Then, 959g of graphite (the second active material) was added and stirred at 1000 rpm for 15 min. The mixture was then dispersed at 4000 rpm for 120 min. Next, 18g of SBR and 2g of the second polymer microspheres were added. The mixture was then vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was added to adjust the viscosity to 4200 mP·s, resulting in a negative electrode slurry with a solid content of approximately 50% containing the second polymer microspheres. The negative electrode slurry was then coated onto the surface of the first coating to form a second coating with a single-sided coating surface density of 55 g / m². 2 After drying and cold pressing, the compacted density of the second coating is 1.53 g / cm³. 3 After further processing such as slitting, the negative electrode sheet is obtained.

[0102] Comparative Example 1

[0103] The main difference between this comparative example and Example 1 is that the negative electrode sheet has only one coating layer with a surface density of 110 g / m³. 2 (To ensure the same cell design and volumetric energy density as much as possible, a single coating with an area density of 110 g / m² is used.) 2 The coating consists of only one type of polymer microsphere with a low swelling ratio and a high compaction density; specifically, the coating includes polymer microspheres with a swelling ratio of 700%, and the compaction density of the coating is 1.65 g / cm³. 3 Polymer microspheres with a swelling ratio of 700% were prepared according to the preparation method of the first polymer microsphere in Example 1; and a compaction density of 1.65 g / cm³ was prepared according to the preparation method of the first coating in Example 1. 3 The coating is applied to obtain the negative electrode sheet, and the subsequent preparation of the secondary battery is the same as in Example 1.

[0104] Comparative Example 2

[0105] The main difference between this comparative example and Example 1 is that the negative electrode sheet has only one coating layer with a surface density of 110 g / m³. 2 (To ensure the same cell design and volumetric energy density as much as possible, a single coating with a surface density of 110 g / m² is used.) 2 The coating consists of only one type of polymer microsphere with a large swelling ratio and a low compaction density; specifically, the coating includes polymer microspheres with a swelling ratio of 1500%, and the compaction density of the coating is 1.55 g / cm³. 3 Polymer microspheres with a swelling ratio of 1500% were prepared according to the preparation method of the second polymer microsphere in Example 1; and a compaction density of 1.55 g / cm³ was prepared according to the preparation method of the second coating in Example 1. 3 The coating is applied to obtain the negative electrode sheet, and the subsequent preparation of the secondary battery is the same as in Example 1.

[0106] Comparative Example 3

[0107] The polymer microspheres and negative electrode were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that the upper and lower layers near the current collector and away from the current collector in Example 1 are interchanged; that is, the first coating near the current collector uses the second polymer microspheres with a swelling ratio of 1500% from Example 1, and the compaction density of the first coating is 1.55 g / cm³. 3 The second coating, located away from the current collector, uses first polymer microspheres with a swelling ratio of 700% as described in Example 1. The compaction density of the second coating is 1.65 g / cm³. 3 The subsequent preparation of the secondary battery was the same as in Example 1.

[0108] Comparative Example 4

[0109] Polymer microspheres and negative electrode sheets were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that the polymer microspheres with different swelling ratios in the two coating layers near and away from the current collector in Example 1 were interchanged. That is, the first coating near the current collector uses the second polymer microsphere with a swelling ratio of 1500% from Example 1, and the second coating away from the current collector uses the first polymer microsphere with a swelling ratio of 700% from Example 1. The compaction density of the first coating and the compaction density of the second coating are the same as in Example 1; the subsequent preparation of the secondary battery is also the same as in Example 1.

[0110] Comparative Example 5

[0111] The polymer microspheres and negative electrode sheet were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that the negative electrode sheet was prepared according to the method in Example 1, and the compaction densities of the two coating layers were interchanged. That is, the compaction density of the first coating layer near the current collector side is 1.55 g / cm³. 3 The compaction density of the second coating on the side furthest from the current collector is 1.65 g / cm³. 3 The swelling ratios of the first polymer microspheres in the first coating and the second polymer microspheres in the second coating are the same as in Example 1; the subsequent preparation of the secondary battery is also the same as in Example 1.

[0112] Comparative Example 6

[0113] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that, in the preparation of the negative electrode sheet, the weight ratio of the first polymer microspheres in the first coating is higher, the amount of graphite added as the first active material in the first coating is 950g, and the amount of the first polymer microspheres added is 5g. All other steps are the same.

[0114] Comparative Example 7

[0115] The polymer microspheres, negative electrode, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that, in the preparation of the negative electrode, the weight proportion of the second polymer microspheres in the second coating is higher, the amount of graphite added as the second active material in the second coating is 954g, and the amount of the second polymer microspheres added is 6g. All other steps are the same.

[0116] Comparative Example 8

[0117] The polymer microspheres, negative electrode sheet, and secondary battery were prepared according to most of the steps in Example 1. The main difference between this comparative example and Example 1 is that: in the preparation of the negative electrode sheet, the mass proportion of the first polymer microsphere in the first coating is higher, and the mass proportion of the second polymer microsphere in the second coating is higher; the amount of graphite added as the first active material in the first coating is 950g, and the amount of the first polymer microsphere is 5g; in the second coating, the amount of graphite added as the second active material is 954g, and the amount of the second polymer microsphere is 6g. All other steps are the same.

[0118] Comparative Example 9

[0119] The polymer microspheres and negative electrode were prepared according to most of the steps in Example 7. The main difference between this comparative example and Example 7 is that the first coating near the current collector side uses first polymer microspheres with a swelling ratio of 400%, and the second coating away from the current collector side uses second polymer microspheres with a swelling ratio of 950%. The subsequent preparation of the secondary battery is the same as in Example 7.

[0120] The specific preparation steps for the first and second polymer microspheres are as follows:

[0121] In an inert gas environment, 2g of allyl polyoxyethylene ether, 3g of pentaerythritol tetraacrylate, and 80g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 30g of methyl acrylate, 30g of styrene, 20g of N-(isobutoxymethyl)acrylamide, and 25g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 50g of deionized water and 1g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres with a swelling rate of 400%.

[0122] In an inert gas environment, 1g of allyl polyoxyethylene ether, 2g of pentaerythritol tetraacrylate, and 205g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 130g of methyl acrylate, 10g of styrene, 5g of N-(isobutoxymethyl)acrylamide, and 5g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 25g of deionized water and 0.5g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling ratio of 950%.

[0123] Comparative Example 10

[0124] The polymer microspheres and negative electrode were prepared according to most of the steps in Example 8. The main difference between this comparative example and Example 8 is that the first coating near the current collector side uses first polymer microspheres with a swelling ratio of 1100%, and the second coating away from the current collector side uses second polymer microspheres with a swelling ratio of 2150%. The subsequent preparation of the secondary battery is the same as in Example 8.

[0125] The specific preparation steps for the first and second polymer microspheres are as follows:

[0126] In an inert gas environment, 3g of allyl polyoxyethylene ether, 5g of pentaerythritol tetraacrylate, and 200g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 160g of methyl acrylate, 50g of styrene, 40g of N-(isobutoxymethyl)acrylamide, and 50g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 100g of deionized water and 3g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were first polymer microspheres, and the swelling ratio of the first polymer microspheres was 1100%.

[0127] In an inert gas environment, 2g of allyl polyoxyethylene ether, 3g of pentaerythritol tetraacrylate, and 400g of deionized water were first mixed and stirred at 1000 rpm for 3 hours. Then, 310g of methyl acrylate, 30g of styrene, 20g of N-(isobutoxymethyl)acrylamide, and 25g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were added and mixed, and stirred at 200 rpm for 5 hours to obtain a pre-emulsion. Then, a mixture of 60g of deionized water and 2g of azobisisobutyramidine hydrochloride was slowly added to the pre-emulsion, and the mixture was stirred at 100 rpm for 10 hours at a reaction temperature of 85℃ to obtain a polymer emulsion with a solid content of 40%. The solid particles in the polymer emulsion were second polymer microspheres with a swelling ratio of 2150%.

[0128] Performance testing

[0129] The polymer microspheres and negative electrode sheets prepared in the above examples and comparative examples were tested respectively. The swelling rate of the polymer microspheres, the viscosity stability of the negative electrode slurry during the preparation of the negative electrode sheet, the rebound rate of the negative electrode sheet, and the peel strength were tested. The test results of the negative electrode slurry and negative electrode sheet are shown in Table 1.

[0130] [Swelling Rate Test]: After drying the emulsion containing polymer microspheres at room temperature, it was then transferred to a vacuum oven at 100℃ and dried for 24 hours to prepare a dry, bubble-free film, weighed W1. The film was placed in a sealed test bottle, and a lithium salt electrolyte (formulation: EC:EMC:DEC=3:2:5, 1mol / L LiPF6) was poured in, with the electrolyte volume not less than 25 times that of the test sample. The sealed bottle was placed in a 60℃ oven and heated continuously for 72 hours. After heating, it was removed and weighed W2. The swelling rate of the polymer microspheres was then calculated as (W2-W1) / W1×100%.

[0131] [Negative Electrode Sheet Peel Strength Test]: The coated and rolled negative electrode sheet was subjected to a tensile test with a range of 20N. The negative electrode sheet was cut into pieces 20cm long and 3cm wide. 3M double-sided tape was attached to the steel plate. The coated side of the negative electrode sheet was fixed to the tape on the steel plate with the coating side facing down. After rolling back and forth 5 times with a 2.5kg roller, the coating and copper foil were peeled off. The copper foil side was clamped by the upper plate. The tensile test was conducted at a speed of 100mm / min and a temperature of 90°C. The data of the stable tensile section was recorded as the peel strength (N / m) of the negative electrode sheet after rolling.

[0132] Table 1 Performance Tests

[0133]

[0134] The secondary batteries prepared in the above examples and comparative examples were subjected to cell performance tests, including full-charge rebound rate, cell DCIR, rate performance, and cycle performance. The test results are shown in Table 2.

[0135] [Full Charge Rebound Rate Test]: After the battery cells have been fully charged, they are fully charged again at 0.5C under constant current and constant voltage. The cells are then placed in a dry room (relative humidity 0.5%) for full charge disassembly. The thickness t1 of the disassembled negative electrode is measured immediately, and the thickness t2 is measured after 24 hours. The full charge rebound rate = (t1-t2) / t2×100%. Ten points are taken for each negative electrode, and the average value of the ten points is calculated.

[0136]

Cell DCIR Test

[0137] [Rate Performance Test - Charging Performance]: Disassemble the battery at 3C full charge to check for lithium plating on the negative electrode. Charge the battery at 3C constant current and constant voltage to 3.65V, cut off at 0.05C, and disassemble the cell in a dry room to check for lithium plating on the negative electrode interface.

[0138] [Rate Performance Test - Discharge Performance]: The battery was discharged at 1C to 2.75V and then left to stand for 10 minutes; it was then charged at 1C constant current and constant voltage to 3.65V, cut off at 0.05C, and left to rest for 10 minutes; the fully charged battery was then discharged at constant current of 0.33C, 0.5C, 1C, 2C, and 3C to 2.5V, and the discharge capacity at different rates was recorded; the rate performance is reflected by the discharge capacity retention rate at different rates, and the ratio of the discharge capacity at 3C to the discharge capacity at 0.33C is calculated.

[0139] [Cyclic Test]: After placing the battery in a constant temperature test chamber at 45℃±2℃ for 1 hour, charge it to 3.65V using a 1C constant current and constant voltage, cut off the current at 0.05C, and then discharge it to 2.5V using a 1C constant current. Record the discharge capacity. Repeat the above steps 500 times for this charge-discharge cycle. Measure the discharge capacity Q1 at the first cycle and Q at the 500th cycle. 500 ; Calculate the capacity retention rate Q=Q after 500 cycles. 500 / Q1*100%.

[0140] Table 2 Performance Tests of Secondary Batteries

[0141]

[0142] As shown in Tables 1 and 2, the peel strength of the negative electrode sheet produced by this invention can reach 13~15 N / m, ensuring that the coating does not fall off during the processing and has a good bonding effect; the battery produced has a low cell full charge rebound (i.e. 20%~22%), a discharge capacity retention rate of >82% at 3C, and ensures a capacity retention rate of >87% after 500 cycles at 1C at 45℃.

[0143] As can be seen from the test results of Examples 1-10 and Comparative Examples 1-2, if there is only one coating layer, although each coating layer contains polymer microspheres, lithium will be deposited when the battery is charged at 3C, the DC internal resistance during discharge is too high, the battery rate performance is poor, and the capacity utilization rate is low.

[0144] As can be seen from the test results of Examples 1-3, Examples 6-10, and Comparative Examples 3-5, in the double-layer coating of the negative electrode sheet, the swelling rate and coating compaction density of the polymer microspheres are within the range disclosed in this invention, indicating better performance. The change in the weight ratio of the monomers selected in the first and second polymer microspheres has a significant impact on the swelling rate of the polymer microspheres. The weight ratio of the monomers selected in the first and second polymer microspheres is within the range disclosed in this invention, indicating better performance. By introducing a higher proportion of acrylic structural unit monomers with a larger swelling rate into the polymer microspheres, the wetting and dispersion energy suspension stability of the active material are increased. Simultaneously, the introduction of monomers from other structural units results in higher affinity with the active material, thereby increasing the electrode sheet peel strength. By optimizing the ratio of monomers from two structural units to monomers from two functional structural units, and considering that each of the four monomers has different effects and functions, a high electrolyte swelling rate can be ensured without structural damage. When filled into the interparticle gaps with differences in porosity and pore size, the battery cell exhibits less full-charge rebound and improves the battery cell rate and cycle life.

[0145] As can be seen from the test results of Examples 4-5 and Comparative Examples 6-8, in the double-layer coating of the negative electrode sheet, the weight ratio of polymer microspheres in the coating is within the range disclosed in this invention, resulting in better performance. By optimizing the weight ratio of active material to polymer microspheres, the wetting and dispersion energy of the active material is increased, resulting in higher affinity with the active material, better stability of the negative electrode slurry, higher peel strength of the negative electrode sheet, and lower electrode rebound. This leads to a smaller full-charge rebound in the prepared battery cell and improves the battery cell rate and cycle life.

[0146] As can be seen from the test results of Example 7 and Comparative Example 9, when there are too few acrylic structural units, the adhesion to the active material will be reduced, and the swelling rate of the polymer microspheres will also be reduced. This will lead to a decrease in the migration rate of lithium ions inside the negative electrode during the charging and discharging process, which will make the cell prone to lithium plating under high-rate fast charging conditions and will also affect the cell's cycle life.

[0147] As can be seen from the test results of Example 8 and Comparative Example 10, when there are too many acrylic structural units, the good affinity between acrylic structural units and electrolyte leads to an excessive swelling rate of polymer microspheres, which increases the full-charge rebound of the battery cell. Furthermore, the large rebound causes the active coating and foil to partially fall off in the later stages of the cycle, resulting in a reduction in cycle life.

[0148] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and a first coating and a second coating sequentially disposed from the side closest to the current collector outwards. The first coating includes a first active material and a first polymer microsphere, and the second coating includes a second active material and a second polymer microsphere. The compaction density of the first coating is 1.6~1.7 g / cm³. 3 The swelling ratio of the first polymer microspheres is 500%~1000%, and the compaction density of the second coating is 1.5~1.59 g / cm³. 3 The swelling rate of the second polymer microspheres is 1100%~2000%; The first polymer microspheres include a first copolymer, the first copolymer including a first structural unit, a second structural unit, a first functional structural unit and a second functional structural unit, and the second polymer microspheres include a third structural unit, a fourth structural unit, a third functional structural unit and a fourth functional structural unit; The first and third structural units each independently include structural units obtained by polymerization of acrylic monomers; the second and fourth structural units each independently include structural units obtained by polymerization of vinyl aromatic monomers; the first and third functional structural units each independently include structural units obtained by polymerization of acrylamide monomers; and the second and fourth functional structural units each independently include structural units obtained by polymerization of acrylate-polyols.

2. The negative electrode sheet according to claim 1, characterized in that, The first polymer microspheres account for 0.1-0.3% of the weight in the first coating; the second polymer microspheres account for 0.1-0.3% of the weight in the second coating.

3. The negative electrode sheet according to claim 1, characterized in that, The weight ratio of the first structural unit, the second structural unit, the first functional structural unit, and the second functional structural unit is (50~150):(30~50):(20~40):(25~50); the weight ratio of the third structural unit, the fourth structural unit, the third functional structural unit, and the fourth functional structural unit is (150~300):(10~30):(5~20):(5~25).

4. The negative electrode sheet according to claim 1, characterized in that, The acrylic monomers include at least one of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, octadecyl acrylate, methyl methacrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, bisacrylamide, phenyl methacrylate, and n-hexyl acrylate.

5. The negative electrode sheet according to claim 1, characterized in that, The vinyl aromatic monomers include at least one of styrene, β-methylstyrene, 4-methoxystyrene, and 2-methylstyrene.

6. The negative electrode sheet according to claim 1, characterized in that, The acrylamide monomers include at least one of N-hydroxyethyl acrylamide, N-(isobutoxymethyl)acrylamide, and diacetone acrylamide.

7. The negative electrode sheet according to claim 1, characterized in that, The acrylate-polyol class includes at least one of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, and polypropylene glycol dimethacrylate.

8. A secondary battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and a negative electrode as described in any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that, The full-charge rebound rate of the secondary battery is less than 22%.

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