Negative electrode sheet and lithium ion battery

Abstract

The application provides a negative electrode sheet and a lithium ion battery. The negative electrode sheet comprises 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 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 particle size of the first active material is smaller than the particle size of the second active material, and the particle size of the first active material is larger than the particle size of the first polymer microsphere; the particle size of the first polymer microsphere is smaller than the particle size of the second polymer microsphere, and the particle size of the second polymer microsphere is smaller than the particle size of the second active material. The application utilizes the size difference of coating pores formed by the particle size gradient of the active material, matches the polymer microspheres with different particle sizes, and fills the active material pores. The battery prepared by the application has excellent rate and cycle performance, can ensure a small full charge rebound without increasing the existing electrode volume space, and can improve 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 lithium ion battery. BACKGROUND

[0002] Lithium ion 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 3C, energy storage, power battery and other fields. With the rapid development of energy storage technology, higher requirements are put forward for lithium ion 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, the problems of slow reaction kinetics and insufficient mechanical properties limit its development. However, the design of thick electrode of lithium ion 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 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, higher rate performance can be achieved by hierarchical gradient design of pore size gradient. The hierarchical gradient design of pore size gradient is that the double-layer coating is formed by stacking active materials with different particle sizes, and the pore size is different. The larger the pore size, 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 pore size will reduce the volumetric energy density of the battery and also reduce the electronic conductivity of the electrode. For the electrode with gradient distribution of pores, it is generally believed that the electrode with gradually reduced pores 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 active material particles with different particle sizes to form different pore sizes to achieve better ion transport performance. However, the improvement of battery rate performance and cycle performance in 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 lithium ion battery, which improves by adding polymer microspheres with different particle sizes into the double-layer coated negative electrode sheet with different porosities.

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

[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 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 particle size of the first active material is smaller than the particle size of the second active material, and the particle size of the first active material is larger than the particle size of the first polymer microsphere; the particle size of the first polymer microsphere is smaller than the particle size of the second polymer microsphere, and the particle size of the second polymer microsphere is smaller than the particle size of the second active material.

[0009] Further, the particle size D 50 of the first active material is 7-12 μm, the particle size D 50 of the second active material is 13-20 μm, the particle size D 50 of the first polymer microsphere is 20-100 nm, and the particle size D 50 of the second polymer microsphere is 200-800 nm. Preferably, the particle size D 50 of the first polymer microsphere is 35-70 nm, and the particle size D 50 of the second polymer microsphere is 300-680 nm.

[0010] Further, the weight percentage of the first polymer microsphere in the first coating layer is 0.1-0.5%, and the weight percentage of the second polymer microsphere in the second coating layer is 0.1-0.3%.

[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 (100-200):(10-30):(5-20):(10-40);

[0012] The second polymer microsphere comprises a second copolymer, the second copolymer comprising 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 (100-200):(10-30):(5-20):(10-40).

[0013] The first and second copolymers of the present application, the first and third structural units each independently comprise a structural unit obtained by polymerization of an acrylic monomer, the acrylic monomer comprising at least one of methyl acrylate, ethyl acrylate, isobutyl acrylate, t-butyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, bisacrylamide, phenyl methacrylate, n-hexyl acrylate.

[0014] The first and second copolymers of the present application, the second and fourth structural units each independently comprise a structural unit obtained by polymerization of a vinyl aromatic monomer, the vinyl aromatic monomer comprising at least one of styrene, β-methyl styrene, 4-methoxyl styrene, 2-methyl styrene.

[0015] The first and second copolymers of the present application, the first and third functional structural units 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, diacetone acrylamide.

[0016] The first and second copolymers of the present application, the second and fourth functional structural units 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, polypropylene glycol dimethacrylate.

[0017] Further, in preparing the first copolymer, in addition to adding the monomers corresponding to the first, second, first functional and second functional structural units respectively, a first crosslinking agent, a first polymerizable emulsifier and a first initiator are added, and the weight ratio of the monomer corresponding to the second functional structural unit to the first crosslinking agent, the first polymerizable emulsifier and the first initiator is (10-40):(2-5):(1-3):(0.5-3).

[0018] Similarly, in preparing the second copolymer, in addition to adding the monomers corresponding to the third, fourth, third functional and fourth functional structural units respectively, a second crosslinking agent, a second polymerizable emulsifier and a second initiator are added, and the weight ratio of the monomer corresponding to the fourth functional structural unit to the second crosslinking agent, the second polymerizable emulsifier and the second initiator is (10-40):(2-5):(1-3):(0.5-3).

[0019] The first crosslinking agent and the second crosslinking 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.

[0020] 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.

[0021] The first initiator and the second initiator are each independently selected from at least one of azobisisobutyronitrile, azobisisobutyramidine hydrochloride, azobisisopropylimidazoline hydrochloride, dimethyl azobisvalerate, ammonium persulfate, sodium persulfate, or potassium persulfate.

[0022] In a second aspect, the present application provides a lithium ion battery, comprising a positive electrode sheet, a separator, an electrolyte, and the negative electrode sheet of the first aspect.

[0023] Compared with the prior art, the present application has the following beneficial effects.

[0024] The present application utilizes the size difference of the coating pores formed by the particle size gradient of the active material, matches the polymer microspheres of different particle sizes, and fills them into the pores of the active material. The first coating layer close to the current collector adopts the first active material of smaller particle size due to its better electronic conductivity, ensures a higher packing density to form a smaller pore structure, and thus needs to match the first polymer microspheres of smaller particle size. The second coating layer away from the current collector needs to select the second active material of larger particle size to form a larger pore structure due to its poor electronic conductivity away from the current collector and to ensure the rapid transmission of lithium ions. Since the packing density of large particles is lower, a larger pore structure is formed, and larger particles of the second polymer microspheres need to be filled. In this way, the lithium ions can be more quickly extracted from the inside of the negative electrode and then embedded into the positive electrode side under the thick electrode system.

[0025] The use of polymer microspheres of different particle sizes to fill the double-layer coated coating can improve the wettability of the negative electrode material to the electrolyte, so that the gap and interface between the negative electrode active material particles have sufficient electrolyte. In the charging process, the lithium ions extracted from the positive electrode can be more quickly migrated to the surface of the negative electrode. In the discharging process, the stress generated by the reduction of the interlayer spacing of the lithium ions embedded in the graphite negative electrode structure can cause the electrolyte absorbed by the polymer microspheres to be partially released.

[0026] The monomers of the acrylic structural unit with a higher proportion and a larger swelling rate are introduced into the polymer microspheres, the impregnation, dispersion, suspension capacity and stability of the active substance are increased, and the other structural unit monomers have a higher affinity with the active substance, the slurry stability is better, and the pole piece peeling strength is higher. By optimizing and controlling the proportion of the copolymer structural unit and the functional unit in the polymer microspheres, and the structural unit and the functional unit each have different effects and functions, the electrolyte swelling degree can be ensured to be high, and the structure is not damaged, when filled into the gap between particles with different porosities and different pore sizes, the full charge rebound of the battery is small, and the rate and cycle life of the battery are improved.

[0027] The double-coated thick negative pole piece designed in the application adopts active substances with different particle sizes to form different pore sizes, and fills polymer microspheres with different particle sizes into the double-coated coating, so that the prepared battery has excellent performance, the existing pole piece volume space is not increased, the full charge rebound is small, and the rate and cycle performance of the battery are improved. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a structural schematic diagram of the negative pole piece in some embodiments of the application.

[0029] In the figure: 1-collector, 2-first coating, 2.1-first active substance, 2.2-first polymer microspheres, 3-second coating, 3.1-second active substance, 3.2-second polymer microspheres. DETAILED DESCRIPTION

[0030] In order to make the technical problems, technical solutions and beneficial effects of the application clearer and more apparent, 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 are within the protection scope of the application.

[0031] It should be noted that the particle size in the application is represented by the volume average particle size (also known as the median particle size), which refers to the particle size value corresponding to the cumulative volume distribution percentage of 50%, which is tested by a laser particle size analyzer. 50 .

[0032] In the application, as known by those skilled in the art in the field of chemical synthesis, each structural unit represents the corresponding structural part in the obtained polymer after the monomer participates in the polymerization reaction. The weight ratio of each structural unit is the weight ratio of the monomer providing each structural unit.

[0033] 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 in sequence from the side of the current collector outward. Figure 1 As shown, the negative electrode sheet comprises a current collector 1 and a first coating layer 2 and a second coating layer 3 arranged in sequence from the side of the current collector 1 outward. The first coating layer 2 comprises a first active material 2.1 and a first polymer microsphere 2.2, and the second coating layer 3 comprises a second active material 3.1 and a second polymer microsphere 3.2. The particle size of the first active material 2.1 is smaller than the particle size of the second active material 3.1, and the particle size of the first active material 2.1 is larger than the particle size of the first polymer microsphere 2.2; the particle size of the first polymer microsphere 2.2 is smaller than the particle size of the second polymer microsphere 3.2, and the particle size of the second polymer microsphere 3.2 is smaller than the particle size of the second active material 3.1.

[0034] The use of polymer microspheres with different particle sizes in the double-layer coated negative electrode sheet with different porosities can improve the wettability of the electrolyte to the negative electrode material, so that the electrolyte has sufficient space in the gap and interface between the negative electrode active material particles, and the lithium ions removed from the positive electrode can be transferred to the surface of the negative electrode more quickly during the charging process. The stress generated by the shrinking of the interlayer spacing due to the removal of lithium ions embedded in the negative electrode structure during the discharging process can cause the electrolyte absorbed by the polymer microspheres to be partially released.

[0035] In some embodiments, the particle size D 50 of the first active material is 7-12 μm, the particle size D 50 of the second active material is 13-20 μm, the particle size D 50 of the first polymer microsphere is 20-100 nm, and the particle size D 50 of the second polymer microsphere is 200-800 nm.

[0036] The coating layer porosity size difference formed by the particle size gradient of the active material matches the polymer microspheres with different particle sizes, which are filled between the active material pores. The first coating layer close to the current collector uses the first active material with a smaller particle size and matches the first polymer microsphere with a smaller particle size, which can ensure a higher packing density to form a smaller pore structure under better electronic conductivity; while the second coating layer close to the separator selects the second active material with a larger particle size and fills the second polymer microsphere with a larger particle size, which has a lower packing density to form a larger pore structure, ensuring the rapid transmission of lithium ions. In this way, the lithium ions in the thick electrode system can be removed from the negative electrode more quickly and then embedded into the positive electrode side.

[0037] More specifically, the particle size D 50may be 7 μm, 7.5 μm, 8.1 μm, 8.6 μm, 9 μm, 9.7 μm, 10.2 μm, 10.9 μm, 11.3 μm, or 12 μm, etc.; the particle size D of the second active substance 50 may be 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm; the particle size D of the first polymer microsphere 50 may be 20 nm, 27 nm, 35 nm, 42 nm, 50 nm, 60 nm, 71 nm, 82 nm, 90 nm, or 100 nm, etc., preferably 35-70 nm; the particle size D of the second polymer microsphere 50 may be 200 nm, 260 nm, 300 nm, 370 nm, 420 nm, 500 nm, 560 nm, 600 nm, 680 nm, 730 nm, or 800 nm, etc., preferably 300-680 nm.

[0038] In some embodiments, the weight percentage of the first polymer microsphere in the first coating layer is 0.1-0.5%; the weight percentage of the second polymer microsphere in the second coating layer is 0.1-0.3%.

[0039] The first coating layer close to the side of the current collector comprises, in addition to the first active substance and the first polymer microsphere, a first conductive agent and a first binder. For example, the first active substance can be graphite, the first conductive agent can be SP, and the first binder can be CMC+SBR; the weight ratio of the first active substance, the first conductive agent, CMC, SBR, and the first polymer microsphere in the first coating layer is (94.9-95.3):1.0:1.2:2.4:(0.1-0.5).

[0040] The second coating layer close to the side of the separator comprises, in addition to the second active substance and the second polymer microsphere, a second conductive agent and a second binder. For example, the second active substance can be graphite, the second conductive agent can be SP, and the second binder can be CMC+SBR; the weight ratio of the second active substance, the second conductive agent, CMC, SBR, and the second polymer microsphere in the second coating layer is (95.7-95.9):1.0:1.2:1.8:(0.1-0.3).

[0041] In some embodiments, 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, the weight ratio of the first structural unit, the second structural unit, the first functional structural unit, and the second functional structural unit being (100-200):(10-30):(5-20):(10-40);

[0042] The second polymer microspheres comprise a second copolymer, the second copolymer 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 (100-200):(10-30):(5-20):(10-40).

[0043] More specifically, the weight ratio of the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit is 100:16:6:15, 105:17:6:16, 110:17:6:17, 115:17:7:17, 120:17:7:17, 125:17:7:17, 130:17:7:17, 135:17:7:17, 140:18:8:18, 150:18:10:20, 160:18:10:22, 170:18:13:25, 177:18:13:25, 183:18:13:25, 190:18:13:25, 195:18:13:25, 200:28:18:37, 200:30:20:40, etc. Similarly, the weight ratio of the third structural unit, the fourth structural unit, the third functional structural unit and the fourth functional structural unit is 100:16:6:15, 105:17:6:16, 110:17:6:17, 115:17:7:17, 120:17:7:17, 125:17:7:17, 130:17:7:17, 135:17:7:17, 140:18:8:18, 150:18:10:20, 160:18:10:22, 170:18:13:25, 177:18:13:25, 183:18:13:25, 190:18:13:25, 195:18:13:25, 200:28:18:37, 200:30:20:40, etc.

[0044] In the first copolymer, the first structural unit and the second structural unit are main structures, the first structural unit is relatively more and the second structural unit is relatively less; the first functional structural unit and the second functional structural unit are functional units for functional adjustment, the first functional structural unit is relatively less and the second functional structural unit is relatively more. Similarly, in the second copolymer, the third structural unit and the fourth structural unit are main structures, the third structural unit is relatively more and the fourth structural unit is relatively less; the third functional structural unit and the fourth functional structural unit are functional units for functional adjustment, the third functional structural unit is relatively less and the fourth functional structural unit is relatively more.

[0045] The first structural unit and the third structural unit in the copolymer of the present application each independently comprise a structural unit obtained by polymerization of an acrylic monomer, wherein the acrylic monomer comprises at least one of methyl acrylate, ethyl acrylate, isobutyl acrylate, t-butyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, bisacrylamide, phenyl methacrylate, and n-hexyl acrylate.

[0046] The second structural unit and the fourth structural unit in the copolymer of the present application each independently comprise a structural unit obtained by polymerization of a vinyl aromatic monomer, wherein the vinyl aromatic monomer comprises at least one of styrene, β-methylstyrene, 4-methoxystyrene, and 2-methylstyrene.

[0047] The first functional structural unit and the third functional structural unit in the copolymer of the present application each independently comprise a structural unit obtained by polymerization of an acrylamide monomer, wherein the acrylamide monomer comprises at least one of N-hydroxyethyl acrylamide, N-(isobutoxymethyl)acrylamide, and diacetone acrylamide.

[0048] The second functional structural unit and the fourth functional structural unit in the copolymer of the present application each independently comprise a structural unit obtained by polymerization of an acrylate-polyol, wherein the acrylate-polyol comprises at least one of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) diacrylate, and poly(propylene glycol) dimethacrylate.

[0049] The acrylic structural unit is mainly used for adjusting the electrolyte swelling degree 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 with the active material; and the acrylate-polyol structural unit can form a crosslinked network structure with the crosslinking agent, which is used for stabilizing the volume change of the negative electrode material caused by the expansion and contraction during the charging and discharging process after the copolymer of the polymer microspheres absorbs the liquid, and is also used for inhibiting the full charge rebound rate of the negative electrode sheet.

[0050] If 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, which increases the full charge rebound of the battery, and the large rebound causes the active coating and the foil to partially fall off in the later stage of the cycle, thereby reducing the cycle life. If the acrylic structural unit is too little, it will reduce the adhesion to the active material, and at the same time reduce the swelling degree of the polymer microspheres, which reduces the migration rate of lithium ions in the negative electrode during the charging and discharging process, thereby affecting the lithium precipitation of the battery under high-rate fast charging conditions and the cycle life of the battery.

[0051] If the proportion of the vinyl aromatic structural unit is too high, the glass transition temperature of the polymer microspheres will increase, which will cause the electrode sheet to crack easily during the processing and baking process, resulting in hard and brittle electrode sheet, reduced peel strength, and problems such as material falling during rolling and die cutting. Moreover, too high a proportion will reduce the swelling rate of the polymer microspheres, which will affect the rate and cycle of the battery. If the proportion of the vinyl aromatic structural unit is too low, it will not be able to maintain the good spherical structure of the polymer microspheres, which will also reduce the peel strength of the electrode sheet.

[0052] If the proportion of the acrylamide functional unit is too high, the glass transition temperature of the polymer microspheres will increase, which will cause the electrode sheet to crack easily during the processing and baking process, resulting in hard and brittle electrode sheet, reduced peel strength, and problems such as material falling during rolling and die cutting. Moreover, too high a proportion will reduce the swelling rate of the polymer microspheres, which will affect the rate and cycle of the battery. If the proportion of the acrylamide functional unit is too low, it will reduce the adhesion between the active material, which will affect the lithium precipitation of the battery under high-rate fast charging conditions and the cycle life of the battery.

[0053] If the proportion of the acrylate-polyol functional unit is too high, it can inhibit the full charge rebound of the battery to a certain extent, but at the same time it will cause the glass transition temperature of the polymer microspheres to increase, the processing performance to deteriorate, and the peel strength of the electrode sheet to decrease, which will cause lithium precipitation under fast charging conditions and affect the cycle performance of the battery. If the proportion of the acrylate-polyol functional unit is too low, it will not be able to ensure the formation of a certain three-dimensional spatial network structure with the crosslinking agent, which will cause the electrode sheet to rebound under full charge conditions, and further cause lithium precipitation under fast charging conditions.

[0054] In some specific 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 added, 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 (10-40):(2-5):(1-3):(0.5-3).

[0055] Similarly, in preparing the second copolymer, 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, a second crosslinking agent, a second polymerizable emulsifier and a second initiator are also added, and the weight ratio of the monomer corresponding to the fourth functional structural unit to the second crosslinking agent, the second polymerizable emulsifier and the second initiator is (10-40):(2-5):(1-3):(0.5-3).

[0056] More specifically, the weight ratio of the monomer corresponding to the second functional structural unit to the first crosslinking agent, the first polymerizable emulsifier and the first initiator can be 10:2:1:0.5, 10:2:1:1, 15:2:1:1, 20:2:1:0.5, 20:2:1:1, 25:2.5:1:1, 25:2:1:0.5, 30:2:1:1, 30:3:1.5:1.5, 35:3:1.5:1.5, 35:3:2:1.5, 35:4:3:2, 40:2:2:1, 40:3:2:1, 40:4:2:2, 40:5:2:1, 40:5:3:2, 40:5:3:3, etc. The weight ratio of the monomer corresponding to the fourth functional structural unit to the second crosslinking agent, the second polymerizable emulsifier and the second initiator can be 10:2:1:0.5, 10:2:1:1, 15:2:1:1, 20:2:1:0.5, 20:2:1:1, 25:2.5:1:1, 25:2:1:0.5, 30:2:1:1, 30:3:1.5:1.5, 35:3:1.5:1.5, 35:3:2:1.5, 35:4:3:2, 40:2:2:1, 40:3:2:1, 40:4:2:2, 40:5:2:1, 40:5:3:2, 40:5:3:3, etc.

[0057] The first crosslinking agent and the second crosslinking 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, pentaerythritol tetraacrylate.

[0058] The first polymerizable emulsifier and the second polymerizable emulsifier are each independently selected from allyl polyoxyethylene ether, ammonium salt of allyl polyoxyethylene ether sulfonic acid, sodium vinyl sulfonate or polyoxyethylene nonylphenol phosphate.

[0059] The first initiator and the second initiator are each independently selected from azobisisobutyronitrile, azobisisobutyramidine hydrochloride, azobisisopropylimidazoline hydrochloride, dimethyl azobis isobutyrate, ammonium persulfate, sodium persulfate or potassium persulfate.

[0060] The first polymer microsphere of the present application comprises a first copolymer, and the preparation method comprises the following steps:

[0061] (1) In the environment of inert gas, first polymerization type emulsifier, first crosslinking agent and deionized water are mixed according to the proportion, the weight ratio of deionized water to monomer corresponding to the first structural unit is (3-5):2, and stirring is carried out at a speed of 500-1000 rpm for 1-3 h; then monomers corresponding to the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit are added according to the aforementioned weight ratio and mixed, and stirring is carried out at a speed of 100 r / min for 2-5 h to obtain a pre-emulsion;

[0062] (2) Then the mixture of deionized water and first initiator is slowly added to the pre-emulsion, and mixing reaction is carried out at a speed of 100 r / min, wherein the weight ratio of deionized water to initiator is 100:(2-5), the reaction temperature is 70-90℃, and the reaction time is 8-20 h to obtain a polymer emulsion with a solid content of 40-50%. The solid particles in the polymer emulsion are the first polymer microspheres.

[0063] Similarly, the second polymer microsphere of the present application comprises a second copolymer, and the preparation method comprises the following steps:

[0064] (1) In the environment of inert gas, second polymerization type emulsifier, second crosslinking agent and deionized water are mixed according to the proportion, the weight ratio of deionized water to monomer corresponding to the third structural unit is (3-5):2, and stirring is carried out at a speed of 500-1000 rpm for 1-3 h; then monomers corresponding to the third structural unit, the fourth structural unit, the third functional structural unit and the fourth functional structural unit are added according to the aforementioned weight ratio and mixed, and stirring is carried out at a speed of 100 r / min for 2-5 h to obtain a pre-emulsion;

[0065] (2) Then the mixture of deionized water and second initiator is slowly added to the pre-emulsion, and mixing reaction is carried out at a speed of 100 r / min, wherein the weight ratio of deionized water to initiator is 100:(2-5), the reaction temperature is 70-90℃, and the reaction time is 8-20 h to obtain a polymer emulsion with a solid content of 40-50%. The solid particles in the polymer emulsion are the second polymer microspheres.

[0066] The preparation method and the particle size control method of the copolymer are known to those skilled in the art or can be known by those skilled in the art according to the prior art. For example, the particle size of the polymer microspheres can be controlled by controlling the solid content of the pre-emulsion, the stirring speed and stirring time, the temperature and time of the polymerization reaction and the like during the synthesis process. Generally, the higher the stirring speed, the smaller the particle size of the polymer microspheres; the longer the stirring time, the smaller the particle size of the polymer microspheres; and the longer the reaction time, the smaller the particle size of the polymer microspheres. The monomers used in the preparation process of the first polymer microspheres and the second polymer microspheres and the amount thereof are basically the same, and different particle sizes of the polymer microspheres are obtained by controlling the reaction temperature, time and speed and the like.

[0067] More specifically, in step (1), the stirring speed is controlled at 700-1000 rpm for 2-3 h before the mixed monomers are added, and the stirring speed is controlled at 100 r / min for 3-5 h after the mixed monomers are added; in step (2), the reaction temperature is controlled at 80-90℃, and the reaction time is controlled at 13-20 h, so as to obtain a polymer emulsion with a solid content of 40-50%, and the solid particles in the polymer emulsion are the first polymer microspheres with a particle size D 50 of 20-100 nm.

[0068] In step (1), the stirring speed is controlled at 500-700 rpm for 1-2 h before the mixed monomers are added, and the stirring speed is controlled at 100 r / min for 2-3 h after the mixed monomers are added; in step (2), the reaction temperature is controlled at 70-80℃, and the reaction time is controlled at 8-12 h, so as to obtain a polymer emulsion with a solid content of 40-50%, and the solid particles in the polymer emulsion are the second polymer microspheres with a particle size D 50 of 200-800 nm.

[0069] In a second aspect, the present application provides a lithium ion battery comprising a positive electrode sheet, a separator, an electrolyte and the negative electrode sheet of the first aspect.

[0070] 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 lithium ion battery disclosed by the present application will be described.

[0071] In the following description, the reagents, materials and instruments used 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.

[0072] The monomers meeting the present application can be commercially available.

[0073] Example 1

[0074] 1) Preparation of polymer microspheres

[0075] In an inert gas environment, 2 g of allyl polyoxyethylene ether, 3 g of pentaerythritol tetraacrylate and 375 g of deionized water were mixed and stirred at 900 rpm for 2 h; then 150 g of methyl acrylate, 20 g of styrene, 10 g of N-(isobutoxymethyl) acrylamide, 25 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were mixed and stirred at 100 r / min for 5 h to obtain a pre-emulsion; then a mixture of 60 g of deionized water and 3 g of azobisdimethylamino formamide hydrochloride was slowly added to the pre-emulsion, mixed at 100 r / min, the reaction temperature was 85℃, and the reaction time was 18 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 particle size D 50 was 40 nm.

[0076] In an inert gas environment, 2 g of allyl polyoxyethylene ether, 3 g of pentaerythritol tetraacrylate and 375 g of deionized water were mixed and stirred at 600 rpm for 1.5 h; then 150 g of methyl acrylate, 20 g of styrene, 10 g of N-(isobutoxymethyl) acrylamide, 25 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate were mixed and stirred at 100 r / min for 2.5 h to obtain a pre-emulsion; then a mixture of 60 g of deionized water and 3 g of azobisdimethylamino formamide hydrochloride was slowly added to the pre-emulsion, mixed at 100 r / min, the reaction temperature was 75℃, 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 second polymer microspheres, and the particle size D 50 was 600 nm.

[0077] 2) Preparation of negative electrode sheet

[0078] 12 g of sodium carboxymethyl cellulose (CMC), 10 g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min, then 951 g of first active material graphite with a particle size D 50 of 8 μm was added, stirred at 1000 rpm for 15 min, then high-speed dispersed at 4000 rpm for 120 min, then 24 g of SBR and 3 g of first polymer microspheres with a particle size D 50 of 40 nm were added, vacuum defoaming stirred at 1000 rpm for 30 min, then water was added to adjust the viscosity to 4500 mP·s, and the solid content was about 50%, to obtain a negative electrode slurry containing the first polymer microspheres. Then, the negative electrode slurry was coated on both sides of the negative electrode current collector copper foil to form a first coating layer, and the single-sided coating area density was 60 g / m 2 , dried, and ready for use.

[0079] 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) were stirred at 1000rpm for 15min, and then 958.5g of particles with a diameter of D were added. 50 The second active material, graphite with a particle size of 15 μm, was stirred at 1000 rpm for 15 min, then dispersed at 4000 rpm for 120 min. Finally, 18 g of SBR and 1.5 g of D particle size were added. 50 The second polymer microspheres, with a diameter of 600 nm, were vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was then added to adjust the viscosity to 4480 mP·s, resulting in a negative electrode slurry containing approximately 50% of 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 60 g / m². 2 After drying and cold pressing, the density is 1.6 g / cm³. 3 After processes such as cutting and slitting, the negative electrode sheet is obtained.

[0080] 3) Lithium-ion battery manufacturing

[0081] 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.

[0082] 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 degassing, a lithium-ion battery is obtained.

[0083] Example 2

[0084] Polymer microspheres, negative electrode sheets, and lithium-ion secondary batteries were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that during the preparation of the negative electrode sheet, the particle size D of the first active material in the first coating is... 50 The particle size D of the first polymer microsphere is 12 μm. 50 The particle size D of the second active material in the second coating is 70 nm. 50 The particle size D of the second polymer microspheres is 20 μm. 50 It is 680nm.

[0085] Example 3

[0086] The negative electrode and lithium-ion secondary battery were prepared according to most of the steps in Example 1. The main difference between this example and Example 1 is that during the preparation of the negative electrode, the particle size D of the first active material in the first coating was adjusted. 50 The particle size D of the first polymer microsphere is 7 μm.50 is 35 nm; the particle size D of the second active material in the second coating layer is 300 nm. 50 is 13 pm, the particle size D of the second polymer microspheres is 300 nm. 50 is 300 nm.

[0087] Example 4

[0088] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, and the main difference between this example and Example 1 is that: in the preparation process of the negative electrode sheet, the amount of the first active material graphite added in the first coating layer is 949 g, and the amount of the first polymer microspheres added is 5 g; the amount of the second active material graphite added in the second coating layer is 957 g, and the amount of the second polymer microspheres added is 3 g.

[0089] Example 5

[0090] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, and the main difference between this example and Example 1 is that: in the preparation process of the negative electrode sheet, the amount of the first active material graphite added in the first coating layer is 953 g, and the amount of the first polymer microspheres added is 1 g; the amount of the second active material graphite added in the second coating layer is 959 g, and the amount of the second polymer microspheres added is 1 g.

[0091] Example 6

[0092] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, and the main difference between this example and Example 1 is that: in the preparation process of the negative electrode sheet, the amount of the first active material graphite added in the first coating layer is 953 g, and the amount of the first polymer microspheres added is 1 g; the amount of the second active material graphite added in the second coating layer is 957 g, and the amount of the second polymer microspheres added is 3 g.

[0093] Example 7

[0094] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, and the main difference between this example and Example 1 is that: in the preparation process of the negative electrode sheet, the amount of the first active material graphite added in the first coating layer is 949 g, and the amount of the first polymer microspheres added is 5 g; the amount of the second active material graphite added in the second coating layer is 959 g, and the amount of the second polymer microspheres added is 1 g.

[0095] Example 8

[0096] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, the main difference between this example and Example 1 is that the selected monomers in the preparation of the polymer microspheres are different: the acrylic monomer is tert-butyl acrylate, the vinyl aromatic monomer is 2-methylstyrene, the acrylamide monomer is N-hydroxyethyl acrylamide, the acrylate-polyol is poly(ethylene glycol) diacrylate, the crosslinking agent is diethylene glycol dimethacrylate, the polymeric emulsifier is ammonium salt of allyl polyoxyethylene ether sulfonate, and the initiator is dimethyl 2,2'-azobis(2-methylpropionate).

[0097] Example 9

[0098] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, the main difference between this example and Example 1 is that the amount (weight ratio) of the selected monomers in the preparation of the polymer microspheres is different: 150 g of methyl acrylate, 10 g of styrene, 5 g of N-hydroxyethyl acrylamide, 10 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, 2 g of pentaerythritol tetraacrylate, 1 g of allyl polyoxyethylene ether, 0.5 g of azobisdimethylammonium hydrochloride and 385 g of deionized water.

[0099] Example 10

[0100] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, the main difference between this example and Example 1 is that the selected monomers and their amounts (weight ratio) in the preparation of the polymer microspheres are different: 150 g of lauryl acrylate, 30 g of 4-methoxystyrene, 20 g of diacetone acrylamide, 40 g of polyethylene glycol dimethacrylate, 5 g of tetraethylene glycol dimethacrylate, 3 g of polyoxyethylene nonylphenol phosphate, 3 g of ammonium persulfate and 435 g of deionized water.

[0101] Comparative Example 1

[0102] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery were prepared according to most of the steps of Example 1, the main difference between this example and Example 1 is that the selected monomers in the preparation of the polymer microspheres are different: the acrylic monomer is tert-butyl acrylate, the vinyl aromatic monomer is 2-methylstyrene, the acrylamide monomer is N-hydroxyethyl acrylamide, the acrylate-polyol is poly(ethylene glycol) diacrylate, the crosslinking agent is diethylene glycol dimethacrylate, the polymeric emulsifier is ammonium salt of allyl polyoxyethylene ether sulfonate, and the initiator is dimethyl 2,2'-azobis(2-methylpropionate).

[0103] The specific steps for preparing the negative electrode sheet are as follows: 12 g of sodium carboxymethyl cellulose (CMC), 10 g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min, then 951 g of active material graphite with a particle size D 50 8 μm was added, stirred at 1000 rpm for 15 min, then high-speed dispersed at 4000 rpm for 120 min, then 24 g of SBR and 3 g of polymer microspheres with a particle size D 50The polymer microspheres, 40 nm in size, were vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was then added to adjust the viscosity to 4205 mP·s, resulting in a negative electrode slurry with a solid content of approximately 50%. The negative electrode slurry was then coated onto both sides of the copper foil used as the negative electrode current collector, with a single-sided coating surface density of 120 g / m². 2 (To ensure the same cell design and volumetric energy density as much as possible, a single coating with a surface density of 120g / m² is used.) 2 The compacted density after drying and cold pressing is 1.6 g / cm³. 3 After processes such as cutting and slitting, the negative electrode sheet is obtained.

[0104] Comparative Example 2

[0105] Polymer microspheres, negative electrode sheets, and lithium-ion secondary batteries 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 slurry includes small-diameter active material graphite and polymer microspheres with a mixture of small and large diameters, and the negative electrode sheet has only one coating layer.

[0106] The specific steps for preparing the negative electrode sheet are as follows: 12g of sodium carboxymethyl cellulose (CMC) and 10g of conductive carbon black (SP) are stirred at 1000rpm for 15min, and then 951g of particles with a diameter of D are added. 50 The active material, graphite, with a particle size of 8 μm, was stirred at 1000 rpm for 15 min, then dispersed at 4000 rpm for 120 min. Finally, 24 g of SBR and 3 g of polymer microspheres (of which 2 g represents the particle size D) were added. 50 Polymer microspheres with a diameter of 40 nm, 1 g is a particle size D 50 The negative electrode slurry, consisting of 600 nm polymer microspheres, was vacuum-defoamed and stirred at 1000 rpm for 30 min. Water was then added to adjust the viscosity to 4185 mP·s, resulting in a solid content of approximately 50%. The negative electrode slurry was then coated onto both sides of the copper foil used as the negative electrode current collector, with a single-sided coating surface density of 120 g / m². 2 (To ensure the same cell design and volumetric energy density as much as possible, a single coating with a surface density of 120g / m² is used.) 2 After drying and cold pressing, the density is 1.6 g / cm³. 3 After processes such as cutting and slitting, the negative electrode sheet is obtained.

[0107] Comparative Example 3

[0108] The polymer microspheres, negative electrode sheet and lithium ion secondary battery were prepared according to most of the steps of Example 1, the main difference between this comparative example and Example 1 is that the particle size gradient of the negative electrode sheet is different although there are two coating layers; in the first coating layer close to the current collector side, the particle size D50 of the first negative electrode active material is 15 μm, and the particle size D50 of the first polymer microspheres is 600 nm; in the second coating layer close to the separator side, the particle size D50 of the second negative electrode active material is 8 μm, and the particle size D50 of the second polymer microspheres is 40 nm. The specific steps for preparing the negative electrode sheet are as follows:

[0109] After 12 g of sodium carboxymethyl cellulose (CMC) and 10 g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min, 958.5 g of the second active material graphite with a particle size D 50 of 8 μm was added, and after high-speed dispersion at 4000 rpm for 120 min, 18 g of SBR and 1.5 g of the second polymer microspheres with a particle size D 50 of 40 nm were added. After vacuum defoaming stirring at 1000 rpm for 30 min, the viscosity was adjusted to 4301 mP·s, and the solid content was about 50%. The negative electrode slurry containing the second polymer microspheres was then coated on the surface of the first coating layer to form a second coating layer, and the single-sided coating layer density was 60 g / m 2 . After drying, cold pressing, and other processes, the negative electrode sheet was obtained.

[0110] After 12 g of sodium carboxymethyl cellulose (CMC) and 10 g of conductive carbon black (SP) were stirred at 1000 rpm for 15 min, 958.5 g of the second active material graphite with a particle size D 50 of 8 μm was added, and after high-speed dispersion at 4000 rpm for 120 min, 18 g of SBR and 1.5 g of the second polymer microspheres with a particle size D 50 of 40 nm were added. After vacuum defoaming stirring at 1000 rpm for 30 min, the viscosity was adjusted to 4301 mP·s, and the solid content was about 50%. The negative electrode slurry containing the second polymer microspheres was then coated on the surface of the first coating layer to form a second coating layer, and the single-sided coating layer density was 60 g / m 2 . After drying, cold pressing, and other processes, the negative electrode sheet was obtained. 3

[0111] Comparative Example 4

[0112] ​The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery are prepared according to most of the steps of Example 1, and the main difference between this comparative example and Example 1 is that the mass ratio of the first polymer microspheres in the first coating is too high during the preparation of the negative electrode sheet, and the amount of the first active material graphite added in the first coating is 946 g, and the amount of the first polymer microspheres added is 8 g.

[0113] Comparative Example 5

[0114] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery are prepared according to most of the steps of Example 1, and the main difference between this comparative example and Example 1 is that the mass ratio of the second polymer microspheres in the second coating is too high during the preparation of the negative electrode sheet, and the amount of the second active material graphite added in the second coating is 954 g, and the amount of the second polymer microspheres added is 6 g.

[0115] Comparative Example 6

[0116] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery are prepared according to most of the steps of Example 1, and the main difference between this comparative example and Example 1 is that the mass ratio of the first polymer microspheres in the first coating is too high, and the mass ratio of the second polymer microspheres in the second coating is too high during the preparation of the negative electrode sheet; the amount of the first active material graphite added in the first coating is 946 g, and the amount of the first polymer microspheres added is 8 g; the amount of the second active material graphite added in the second coating is 954 g, and the amount of the second polymer microspheres added is 6 g.

[0117] Comparative Example 7

[0118] The polymer microspheres, the negative electrode sheet and the lithium ion secondary battery are prepared according to most of the steps of Example 1, and the main difference between this comparative example and Example 1 is that the monomers selected and their amounts (weight ratio) are different during the preparation of the polymer microspheres: 80 g of lauryl acrylate, 20 g of styrene, 10 g of N-(isobutoxymethyl) acrylamide, 25 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, 3 g of pentaerythritol tetraacrylate, 2 g of allyl polyoxyethylene ether, 1.5 g of ammonium persulfate and 230 g of deionized water.

[0119] Comparative Example 8

[0120] The polymer microspheres, negative electrode sheet and lithium ion secondary battery were prepared according to most of the steps of Example 1, and the main difference between the present comparative example and Example 1 was that the monomers selected and their amounts (weight ratio) were different in the preparation process of the polymer microspheres: 230 g of lauryl acrylate, 20 g of styrene, 10 g of N-(isobutoxymethyl) acrylamide, 25 g of polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, 3 g of pentaerythritol tetraacrylate, 2 g of allyl polyoxyethylene ether, 1.5 g of ammonium persulfate and 605 g of deionized water.

[0121] Performance test

[0122] The negative electrode slurry prepared in the above examples and comparative examples was configured to have a viscosity range of 4000-4500 mP·s (3# rotor 30 revolutions) for coating, and the negative electrode sheet after rolling was tested for rebound rate and peel strength after rolling. The test results are shown in Table 1.

[0123] Negative electrode sheet rebound rate test: the coated negative electrode sheet was rolled to ensure a compaction density of 1.6 g / cm 3 The thickness of the negative electrode sheet at this time was tested with a micrometer, and the initial thickness was recorded. After the negative electrode sheet was placed in a humidity-proof cabinet in a constant temperature room for 24 h, the thickness was measured as the cold-rolled thickness after 24 h. The negative electrode sheet rebound rate was (cold-rolled thickness-initial thickness) / initial thickness x 100%, and 10 points were taken from each negative electrode sheet to calculate the average of the 10 points.

[0124] Negative electrode sheet peel strength test: the negative electrode sheet after coating and rolling was cut into a length of 20 cm and a width of 3 cm, and 3M double-sided tape was attached to the steel plate. The negative electrode sheet was fixed on the tape with the coating side down, and a 2.5 kg roller was rolled back and forth 5 times. Then the coating and copper foil were torn apart. The copper foil side was clamped on the plate, and the tensile strength was recorded as the peel strength of the negative electrode sheet after rolling (N / m) at a speed of 100 mm / min and a temperature of 90°C.

[0125] The lithium ion batteries prepared in the above examples and comparative examples were respectively tested for cell performance, including full charge rebound rate, cell DCIR, rate performance, cycle performance, etc. The test results are shown in Table 1.

[0126] Full charge rebound rate test: the cells were fully charged at 0.5 C, and then were disassembled in a dry room (relative humidity 0.5%) for full charge disassembly. The thickness of the negative electrode sheet after disassembly was immediately tested as t1, and the thickness of the negative electrode sheet after 24 h was tested as t2. The full charge rebound rate was (t1-t2) / t2 x 100%. 10 points were taken from each negative electrode sheet to calculate the average of the 10 points.

[0127] Cell DCIR test: at 25℃, the battery is fully charged at 1C and rested for 1h, then discharged at 1C to SOC 50%, rested for 30min, and the voltage V1 at this time is recorded; the battery is discharged at 1C for 10s, and the voltage V2 at this time is recorded; the discharge DCIR is calculated as (V1-V2) / 1C.

[0128] Rate performance test-charging performance: 3C full charging disassembly to see if the negative electrode lithium is analyzed, the battery is charged at 3C constant current and constant voltage to 3.65V, 0.05C cutoff, and the negative electrode interface is disassembled in the dry room to see if the lithium is analyzed.

[0129] Rate performance test-discharge performance: the battery is discharged at 1C to 2.75V, and then rested for 10min; 1C constant current and constant voltage charging to 3.65V, 0.05C cutoff, and rested for 10min; the full battery is discharged at 0.33C, 0.5C, 1C, 2C and 3C constant current to 2.5V, and the discharge capacity at different rates is recorded; the rate performance is reflected by the discharge capacity retention rate at different rates, which is calculated as the ratio of the discharge capacity at each rate to the discharge capacity at 0.33C.

[0130] Cycle test: after the battery is placed in a constant temperature test room at 45℃±2℃ for 1h, it is charged at 1C constant current and constant voltage to 3.65V, the cutoff current is 0.05C, and it is discharged at 1C current to 2.5V, and the discharge capacity is recorded; the above steps are repeated for 500 times of charging and discharging cycles, and the discharge capacity Q1 at the first cycle and the discharge capacity Q500 at the 500th cycle are measured. 500 The capacity retention rate Q after 500 cycles is calculated as Q=Q 500 / Q1*100%.

[0131] Table 1: performance test results

[0132]

[0133] From the test results in Table 1, the peel strength of the negative electrode sheet prepared by the application can reach 13-15N / m, ensuring that the coating does not fall off during processing, has good adhesion effect, and has a low sheet rebound rate (i.e. 3%-5%); the battery prepared has a low full cell rebound (i.e. 20%-22%), a small discharge DC resistance (i.e. <46%), no lithium analysis at 3C charging, good rate performance, discharge capacity retention rate >83% at 3C, and capacity retention rate >83% after 500 cycles at 1C high temperature cycle.

[0134] As can be seen from the test results of Examples 1-10 and Comparative Examples 1-2, if the size of the coating layer pore formed by the particle size gradient of the active material does not match the particle size of the polymer microspheres, the negative electrode sheet has a high rebound rate, the battery is prone to lithium precipitation at 3C, the discharge DC internal resistance is high, the rate performance of the battery is poor, the discharge capacity retention rate is reduced, and the cycle life of the battery is reduced.

[0135] As can be seen from the test results of Examples 1-3 and Comparative Example 3, in the double-layer coating of the negative electrode sheet, the particle sizes of the first active material, the first polymer microspheres, the second active material, and the second polymer microspheres are within the range disclosed in the present application, and the performance is better. Adding polymer microspheres with different particle sizes to the double-layer coated negative electrode sheet with different porosities can improve the wettability of the negative electrode material to the electrolyte, make the interstitial space and interface of the negative electrode active material particles have sufficient electrolyte, and make the peeling strength of the negative electrode sheet higher. Without increasing the existing volume space of the sheet, the full charge rebound rate is small, the discharge DC internal resistance is low, the charging lithium precipitation at 3C is improved, and the rate and cycle performance of the battery are improved.

[0136] As can be seen from the test results of Examples 4-7 and Comparative Examples 4-6, in the double-layer coating of the negative electrode sheet, the weight ratio of the polymer microspheres in the coating is within the range disclosed in the present application, and the performance is better. By optimizing the weight ratio of the active material to the polymer microspheres, the polymer microspheres are more fully filled into the pores of the negative electrode main material, the peeling strength of the negative electrode sheet is higher, the negative electrode sheet rebound is lower, so that the prepared battery cell has a small full charge rebound and a low discharge DC internal resistance, and the rate and cycle life of the battery cell are improved.

[0137] As can be seen from the test results of Examples 8-10 and Comparative Examples 7-8, the monomers selected for the polymer microspheres and the weight ratio of the monomers are within the range disclosed in the present application, and the performance is better. By introducing a higher proportion of monomers with a larger swelling rate and an acrylic structural unit, the wettability, dispersion, suspension capacity, and stability of the active material are increased, and other structural unit monomers with higher affinity to the active material are introduced. The peeling strength of the sheet is higher. Moreover, if the proportion of the acrylic structural unit is too small, the battery cell is prone to lithium precipitation under the condition of large rate and fast charging. By optimizing the proportion of the two structural unit monomers and the two functional structural unit monomers, and the four monomers each having different effects and functions, a high electrolyte swelling degree can be ensured, and the structure is not damaged. When filled into the interstitial space with different porosities and pore sizes, the battery cell has a small full charge rebound and a low discharge DC internal resistance, and the rate and cycle life of the battery cell are improved.

[0138] The application is further described in detail with specific examples, but it should be understood that the specific description herein should not be construed to limit the scope of the application, and various modifications made by those skilled in the art after reading the description are within the scope of the application.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet comprises 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 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 particle size D50 of the first active material is 7-12 mu m, the particle size D50 of the first polymer microsphere is 20-100 nm, the particle size D50 of the second active material is 13-20 mu m, and the particle size D50 of the second polymer microsphere is 200-800 nm. The first polymer microsphere comprises a first copolymer, and the first copolymer comprises a first structural unit, a second structural unit, a first functional structural unit and a second functional structural unit; the second polymer microsphere comprises a second copolymer, and the second copolymer comprises a third structural unit, a fourth structural unit, a third functional structural unit and a fourth functional structural unit. The first structural unit and the third structural unit each independently comprise a structural unit obtained by polymerization of an acrylic monomer, the second structural unit and the fourth structural unit each independently comprise a structural unit obtained by polymerization of a vinyl aromatic monomer, 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 second functional structural unit and the fourth functional structural unit each independently comprise a structural unit obtained by polymerization of an acrylate-polyol.

2. The negative electrode sheet according to claim 1, characterized by The particle size D of the first polymer microspheres is 35-70 nm 50 The particle size D of the second polymer microspheres is 300-680 nm 50 The particle size D of the second polymer microspheres is 300-680 nm 3. The negative electrode sheet according to claim 1, characterized by The weight percentage of the first polymer microsphere in the first coating layer is 0.1-0.5%, and the weight percentage of the second polymer microsphere in the second coating layer is 0.1-0.3%.

4. The negative electrode sheet according to claim 1, wherein The weight ratio of the first structural unit, the second structural unit, the first functional structural unit and the second functional structural unit is (100-200):(10-30):(5-20):(10-40), 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 (100-200):(10-30):(5-20):(10-40).

5. The negative electrode sheet according to claim 4, characterized by 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 and n-hexyl acrylate.

6. The negative electrode sheet according to claim 4, wherein The vinyl aromatic monomer comprises at least one of styrene, beta-methylstyrene, 4-methoxystyrene and 2-methylstyrene.

7. The negative electrode sheet according to claim 4, wherein The acrylamide monomer comprises at least one of N-hydroxyethyl acrylamide, N-(isobutoxymethyl) acrylamide and diacetone acrylamide.

8. The negative electrode sheet according to claim 4, wherein The acrylate-polyol comprises at least one of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate and polypropylene glycol dimethacrylate.

9. A lithium-ion battery, characterized by The battery comprises a positive electrode sheet, a separator, an electrolyte and the negative electrode sheet according to any one of claims 1-8.

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