A negative electrode binder with a core-shell structure, and a preparation method and application thereof

Abstract

The application provides a negative electrode binder with a core-shell structure and a preparation method and application thereof. The negative electrode binder with the core-shell structure comprises a core layer copolymer and a shell layer copolymer coated on the surface of the core layer copolymer; monomers of the core layer copolymer comprise styrene monomers, butadiene monomers and first acrylic ester monomers; and monomers of the shell layer copolymer comprise a combination of styrene monomers, butadiene monomers, acrylonitrile monomers and second acrylic ester monomers. The negative electrode binder provided by the application not only has good flexibility, dispersibility and bonding strength, but also is beneficial to inhibiting the volume expansion of negative electrode active material generated in the charging and discharging process, thereby improving the cycle performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of adhesive materials technology, specifically relating to a core-shell structured negative electrode adhesive, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries are widely used due to their advantages such as high energy density, high volumetric energy density and long cycle life. Their applications include consumer electronics such as laptops, mobile phones or digital products, as well as power batteries for electric vehicles or energy storage power stations.

[0003] As an important component of the electrode in lithium-ion batteries, the binder mainly plays the role of adhering the electrode active material and conductive agent to the surface of the current collector, providing strong adhesion to maintain the integrity of the electrode structure, thereby ensuring that the lithium-ion battery can cycle normally. Therefore, the quality of its performance will directly affect the electrochemical performance of the battery.

[0004] Currently, commercially available negative electrode binders mainly include styrene-butadiene rubber (SBR) binders and acrylic (PAA) binders. On the one hand, SBR binders have relatively weak bonding strength, resulting in poor inhibition of graphite active materials and silicon-carbon negative electrode materials, and are also not conducive to the uniform dispersion of negative electrode active materials and conductive agents. However, SBR binders have good flexibility, are less prone to powder shedding during electrode slitting and winding, and have greater swelling capacity in the electrolyte, which is beneficial for improving the rate performance of the battery. On the other hand, acrylic binders have high bonding strength, can suppress the rebound of active materials, and have good long-term cycle performance. They are also beneficial for dispersing negative electrode active materials and conductive agents to form a good conductive network and promote electron conduction. However, acrylic binders have greater hardness and electrode brittleness, and are prone to powder shedding during electrode slitting, affecting the long-term cycle performance of the battery.

[0005] Existing technologies disclose methods for improving the aforementioned negative electrode binders. Researchers mostly use physical blending of the two. The performance of the blended polymer depends on the miscibility at the molecular level of the system, i.e., the effect generated by the interaction between functional groups, such as van der Waals forces, dipole interactions, or hydrogen bonds. However, it is easy to produce inhomogeneity and poor consistency of the prepared electrode sheets, as well as the problem of local overcharging, which in turn affects the overall performance of the battery.

[0006] Therefore, there is an urgent need in this field to develop a negative electrode binder material that not only has good bonding strength and dispersion performance, but also has long-term stable cycling performance during battery cycling and is not easy to fall off. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a core-shell structured negative electrode binder, its preparation method, and its applications. The negative electrode binder provided by this invention not only possesses good flexibility, dispersibility, and bonding strength, but also helps to suppress the volume expansion of the negative electrode active material during charging and discharging, thereby improving the battery's cycle performance.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a negative electrode binder having a core-shell structure, the negative electrode binder having a core-shell structure comprising a core layer copolymer and a shell layer copolymer coated on the surface of the core layer copolymer;

[0010] The monomers of the core layer copolymer include styrene monomers, butadiene monomers, and first acrylate monomers;

[0011] The monomers of the shell copolymer include a combination of styrene monomers, butadiene monomers, acrylonitrile monomers, and second acrylate monomers.

[0012] This invention provides a negative electrode binder with styrene-butadiene rubber copolymer as its main component. It is copolymerized by adding specific types of monomers that promote the uniform dispersion of the negative electrode active material and conductive agent, as well as acrylate monomers. This not only retains the flexibility of the styrene-butadiene rubber copolymer binder but also prevents powder shedding during electrode slitting. Simultaneously, it improves the dispersion ability of the negative electrode binder for the negative electrode active material and enhances the uniformity of the distribution of the negative electrode active material and conductive agent in the negative electrode, thereby forming a good conductive network. The consistency of the prepared negative electrode is also improved, thus enhancing the rate performance of the battery. Furthermore, the addition of acrylate monomers enhances the mechanical strength of the prepared negative electrode binder, which helps suppress the volume expansion of the negative electrode active material during charge and discharge, thus improving the battery's cycle performance.

[0013] Preferably, the first acrylate monomer and the second acrylate monomer each independently comprise any one or a combination of at least two of the following: acrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-propyl acrylate, cyclohexyl acrylate, phenyl methacrylate, lauryl acrylate, dodecyl acrylate, hexadecyl acrylate, sulfonyl acrylate, methacrylic acid, methyl methacrylate, n-butyl methacrylate, sulfonyl methacrylate, lauryl acrylate, glycidyl methacrylate, tert-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, vinyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, phenyl methacrylate, dodecyl methacrylate, or hexadecyl methacrylate.

[0014] Preferably, the mass ratio of the core copolymer to the shell copolymer in the core-shell structured negative electrode binder is (0.5-2):1, for example, it can be 0.5:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, etc.

[0015] In this invention, by adjusting the mass ratio of the core copolymer to the shell copolymer, the synthesized polymer is ensured to possess both good strength to suppress the expansion of the negative electrode active material during charging and discharging, and good flexibility, while also exhibiting good dispersibility for the negative electrode material and graphite, ensuring the formation of a good conductive network. A mass ratio that is too low will lead to increased rigidity of the synthesized polymer, resulting in a brittle negative electrode sheet and powder shedding during electrode slitting. Conversely, a mass ratio that is too high will result in poor polymer strength, poor suppression of electrode expansion during charging and discharging, and poor dispersibility for both the negative electrode material and the conductive material, failing to form a good conductive network.

[0016] Preferably, the thickness of the core layer copolymer is 40nm-80nm, for example, it can be 40nm, 45nm, 50nm, 55nm, 60nm, 70nm, 73nm, 78nm, etc.

[0017] In this invention, by adjusting the thickness of the core copolymer, the polymer exhibits excellent flexibility, reducing the brittleness of the prepared electrode and preventing powder shedding during electrode slitting. Too low a thickness results in insufficient polymer flexibility, leading to a brittle electrode. Conversely, too high a thickness reduces polymer strength and decreases its ability to suppress electrode expansion during charging and discharging.

[0018] Preferably, the thickness of the shell copolymer is 30nm-60nm, for example, it can be 30nm, 35nm, 40nm, 42nm, 50nm, 55nm, 57nm, 60nm, etc.

[0019] In this invention, by adjusting the thickness of the shell copolymer, the polymer achieves high strength and good dispersibility for both the negative electrode material and the conductive material. If the thickness is too low, the polymer will have poor strength, resulting in poor suppression of the volume expansion of the negative electrode material during charging and discharging, and insufficient dispersion of both materials, which is detrimental to the formation of a good conductive network. Conversely, if the thickness is too high, the electrode will become more brittle, leading to powder shedding during electrode slitting and affecting battery performance.

[0020] In a second aspect, the present invention provides a method for preparing a negative electrode binder having a core-shell structure according to the first aspect, the method comprising the following steps:

[0021] (1) Add styrene monomer, butadiene monomer and first acrylate monomer to an aqueous solution containing the first emulsifier, mix and heat once, add the first initiator to carry out a reaction once, raise the temperature to carry out a second reaction after the first reaction, and obtain seed emulsion after cooling.

[0022] (2) The seed emulsion and the aqueous solution of the second emulsifier obtained in step (1) are heated twice, and styrene monomer, butadiene monomer, acrylonitrile monomer and second acrylate monomer are added for pre-emulsification. After the pre-emulsification is completed, the aqueous solution of the second initiator is added for three reactions. After the three reactions are completed, the temperature is raised for four reactions. After cooling, the negative electrode binder with the core-shell structure is obtained.

[0023] Preferably, in step (1), the first emulsifier comprises 0.5-2 parts by weight (e.g., 0.5, 0.8, 1, 1.5, 2, etc.), the water comprises 200-300 parts by weight (e.g., 200, 220, 250, 280, 300, etc.), and the styrene monomer comprises 45-55 parts by weight (e.g., 45, 48, 50 parts). The butadiene monomer is 45-50 parts by weight (e.g., 45, 46, 47, 48, 49, 50, etc.), the first acrylate monomer is 1-5 parts by weight (e.g., 1, 2, 3, 4, 5, etc.), and the first initiator is 0.5-2 parts by weight (e.g., 0.5, 0.8, 1, 1.5, 2, etc.).

[0024] In this invention, by adjusting the weight proportions of the above components, the seed emulsion polymer is made to have both a certain strength and good flexibility.

[0025] Preferably, the first emulsifier in step (1) includes any one or a combination of at least two of the following: sodium dodecyl sulfate, sodium octadecyl sulfate, sodium dodecylbenzene sulfonate, sodium dioctyl and succinate sulfonate, sodium fatty alcohol ether sulfate, sulfonate of ethoxylated fatty acid methyl ester, sodium α-alkenyl sulfonate, sodium secondary alkyl sulfonate, lauryl ether phosphate, or isooctyl ether phosphate.

[0026] Preferably, the first acrylate monomer in step (1) includes any one or a combination of at least two of the following: acrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-propyl acrylate, cyclohexyl acrylate, phenyl methacrylate, lauryl acrylate, dodecyl acrylate, hexadecyl acrylate, sulfonyl acrylate, methacrylic acid, methyl methacrylate, n-butyl methacrylate, sulfonyl methacrylate, lauryl acrylate, glycidyl methacrylate, tert-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, vinyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, phenyl methacrylate, dodecyl methacrylate, or hexadecyl methacrylate.

[0027] Preferably, a surfactant and a first regulator may be added before mixing in step (1).

[0028] Preferably, the surfactant comprises fatty acid glycerides and / or fatty acid ester sulfonates.

[0029] Preferably, the surfactant is present in 0.1-1 parts by weight, for example, 0.2 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.75 parts, 0.8 parts, 0.9 parts, etc.

[0030] Preferably, the first regulator comprises tert-dodecyl mercaptan and / or n-dodecyl mercaptan.

[0031] Preferably, the first regulator is 0.1-1 parts by weight, for example, 0.1 parts, 0.2 parts, 0.5 parts, 0.8 parts, 1 part, etc.

[0032] Preferably, the mixing in step (1) is carried out under an inert atmosphere.

[0033] Preferably, the mixing rate in step (1) is 50-250 rpm, more preferably 100-200 rpm, for example, 50 rpm, 80 rpm, 100 rpm, 120 rpm, 150 rpm, 180 rpm, 200 rpm, 250 rpm, etc.

[0034] Preferably, the heating in step (1) is carried out under stirring.

[0035] Preferably, the temperature of the first heating in step (1) is 60-70℃, for example, it can be 60℃, 62℃, 65℃, 68℃, 70℃, etc.

[0036] Preferably, the first initiator in step (1) includes a peroxide initiator and / or a reducing agent initiator.

[0037] Preferably, the peroxide initiator includes any one or a combination of at least two of hydrogen peroxide, peracetic acid, ammonium persulfate, potassium persulfate, or sodium persulfate.

[0038] Preferably, the reducing initiator includes any one or a combination of at least two of the following: ferrous salt, sodium sulfite, sodium bisulfite, alcohol, or dimethylaniline.

[0039] Preferably, the first initiator is added dropwise in step (1).

[0040] Preferably, the temperature of the first reaction in step (1) is 60-70℃, for example, 60℃, 62℃, 65℃, 68℃, 70℃, etc.; the time is 3-6h, for example, 3h, 4h, 5h, 6h, etc.; preferably 62-68℃, and the time is 4-5h.

[0041] Preferably, the temperature of the secondary reaction in step (1) is 70-80℃, for example, 70℃, 72℃, 75℃, 78℃, 80℃, etc.; the time is 4-7h, for example, 4h, 5h, 6h, 7h, etc.; preferably 74-80℃, and the time is 5-7h.

[0042] Preferably, the cooling temperature in step (1) is no higher than 30°C.

[0043] Preferably, the cooling process in step (1) further includes filtration.

[0044] Preferably, by weight, the seed emulsion in step (2) comprises 10-15 parts (e.g., 10, 11, 12, 13, 14, 15, etc.), the second emulsifier comprises 0.5-5 parts (e.g., 0.5, 0.8, 1, 2, 5, etc.), the styrene monomer comprises 35-45 parts (e.g., 35, 38, 40, 42, 45, etc.), and the butadiene monomer comprises 15-2... 5 parts by weight (e.g., 15, 18, 20, 22, 25, etc.), 20-30 parts by weight of acrylonitrile monomer (e.g., 20, 22, 25, 28, 30, etc.), 15-25 parts by weight of second acrylate monomer (e.g., 15, 18, 20, 22, 25, etc.), and 100-200 parts by weight of water (e.g., 100, 120, 150, 180, 200, etc.).

[0045] Preferably, the second emulsifier in step (2) includes any one or a combination of at least two of the following: sodium dodecyl sulfate, sodium octadecyl sulfate, sodium dodecylbenzene sulfonate, sodium dioctyl and succinate sulfonate, sodium fatty alcohol ether sulfate, sulfonate of ethoxylated fatty acid methyl ester, sodium α-alkenyl sulfonate, sodium secondary alkyl sulfonate, lauryl ether phosphate, or isooctyl ether phosphate.

[0046] Preferably, the secondary heating in step (2) is carried out under stirring.

[0047] In this invention, the stirring rate for secondary heating is 50-250 rpm, preferably 100-200 rpm.

[0048] Preferably, the second acrylate monomer in step (2) includes any one or a combination of at least two of the following: acrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-propyl acrylate, cyclohexyl acrylate, phenyl methacrylate, lauryl acrylate, dodecyl acrylate, hexadecyl acrylate, sulfonyl acrylate, methacrylic acid, methyl methacrylate, n-butyl methacrylate, sulfonyl methacrylate, lauryl acrylate, glycidyl methacrylate, tert-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, vinyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, phenyl methacrylate, dodecyl methacrylate, or hexadecyl methacrylate.

[0049] Preferably, before the pre-emulsification in step (2), any one or a combination of at least two of the following can be added: a second regulator, an antifoaming agent, or a preservative.

[0050] Preferably, the total weight of the defoamer and preservative is 0.5-1 parts, for example, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1 part, etc.

[0051] Preferably, the second regulator comprises tert-dodecyl mercaptan and / or n-dodecyl mercaptan.

[0052] Preferably, the second regulator is 0.1-1 parts by weight, for example, 0.1 parts, 0.2 parts, 0.5 parts, 0.8 parts, 0.9 parts, 1 part, etc.

[0053] Preferably, the defoamer includes polyether defoamers and / or polyether-modified silicone defoamers.

[0054] Preferably, the preservative is any one or a combination of at least two of propylparaben, benzoic acid, or sodium benzoate.

[0055] Preferably, the pre-emulsification in step (2) is carried out under stirring.

[0056] In this invention, the stirring rate of the pre-emulsification in step (2) is 300-500 rpm, preferably 350-450 rpm, for example, it can be 300 rpm, 320 rpm, 350 rpm, 380 rpm, 400 rpm, 420 rpm, 450 rpm, 500 rpm, etc.

[0057] Preferably, the pre-emulsification time in step (2) is 0.5-2h, more preferably 1-1.5h, for example, it can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.8h, 2h, etc.

[0058] In this invention, by adjusting the stirring rate and stirring time of pre-emulsification, if the stirring rate is too fast or the stirring time is too long, the stabilizing effect of the polymer emulsion will be destroyed; if the stirring rate is too slow or the stirring time is too short, the pre-emulsification may be insufficient.

[0059] Preferably, the second initiator in step (2) includes a peroxide initiator and / or a reducing initiator.

[0060] Preferably, the peroxide initiator includes any one or a combination of at least two of hydrogen peroxide, peracetic acid, ammonium persulfate, potassium persulfate, or sodium persulfate.

[0061] Preferably, the reducing initiator includes any one or a combination of at least two of the following: ferrous salt, sodium sulfite, sodium bisulfite, alcohol, or dimethylaniline.

[0062] Preferably, the aqueous solution of the second initiator is added dropwise in step (2).

[0063] Preferably, the temperature of the three reactions in step (2) is 70-80℃, for example, 70℃, 72℃, 75℃, 78℃, 80℃, etc.; the time is 4-7h, for example, 4h, 3.5h, 4h, 4.5h, 5h, 6h, 7h, etc., preferably 75-80℃ and 4.5-6h.

[0064] Preferably, the temperature of the four reactions in step (2) is 80-90℃, for example, 80℃, 82℃, 85℃, 88℃, 90℃, etc.; the time is 4-6h, for example, 4h, 5h, 6h, etc., preferably 85-90℃ and 5-6h.

[0065] Preferably, the cooling temperature in step (2) is no higher than 30°C.

[0066] Preferably, the cooling process in step (2) further includes filtration.

[0067] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, the negative electrode active material layer comprising the negative electrode binder with a core-shell structure as described in the first aspect.

[0068] Fourthly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the negative electrode comprises the negative electrode according to the third aspect.

[0069] Compared with the prior art, the present invention has the following beneficial effects:

[0070] This invention provides a core-shell structured negative electrode binder, which is copolymerized by adding specific types of monomers that promote the uniform dispersion of negative electrode active materials and conductive agents, as well as acrylate monomers. This not only retains the flexibility of styrene-butadiene rubber copolymer binders but also prevents powder shedding during electrode slitting. Simultaneously, it improves the dispersion ability of the negative electrode binder for the negative electrode active materials and enhances the uniformity of the distribution of negative electrode active materials and conductive agents within the negative electrode sheet, thereby forming a good conductive network. The consistency of the prepared negative electrode sheet is also improved, thus enhancing the rate performance of the battery. Furthermore, the addition of acrylate monomers enhances the mechanical strength of the prepared negative electrode binder, which helps suppress the volume expansion of the negative electrode active material during charge and discharge, thus improving the battery's cycle performance. Detailed Implementation

[0071] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0072] Example 1

[0073] This embodiment provides a negative electrode binder with a core-shell structure, which includes a core copolymer and a shell copolymer coating the surface of the core copolymer; the monomers of the core copolymer include styrene monomer, butadiene monomer and methyl acrylate monomer; the monomers of the shell copolymer include styrene monomer, butadiene monomer, acrylonitrile monomer and cyclohexyl acrylate monomer.

[0074] In the core-shell structured negative electrode binder, the mass ratio of the core copolymer to the shell copolymer is 1:1, the thickness of the core copolymer is 70 nm, and the thickness of the shell copolymer is 40 nm.

[0075] This embodiment also provides a method for preparing the above-mentioned core-shell structured negative electrode binder, which includes the following steps:

[0076] (1) In a nitrogen atmosphere, according to the weight of 1 part sodium dodecyl sulfate emulsifier (250 parts water), 50 parts styrene monomer, 48 parts butadiene monomer, 2 parts methyl acrylate monomer and 0.5 parts tert-dodecyl mercaptan regulator were added to an aqueous solution containing 1 part sodium dodecyl sulfate emulsifier. The mixture was mixed and heated once at a rate of 150 rpm. The temperature of the first heating was 65°C. 1 part ammonium persulfate initiator was added dropwise and the reaction was carried out at 65°C for 4.5 h. After the first reaction was completed, the temperature was raised to 76°C for a second reaction for 6 h. After cooling to 25°C, the mixture was filtered to obtain the seed emulsion.

[0077] (2) In a nitrogen atmosphere, 12 parts by weight of the seed emulsion obtained in step (1) and 1.25 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier were heated twice at a rate of 150 rpm. During the heating period, 1.25 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier, 40 parts by weight of styrene monomer, 20 parts by weight of butadiene monomer, 25 parts by weight of acrylonitrile monomer, 20 parts by weight of cyclohexyl acrylate monomer and 0.5 parts by weight of tert-dodecyl mercaptan regulator were added and pre-emulsified at a rate of 400 rpm for 1.2 h. The weight of water was 150 parts. After the pre-emulsification, an aqueous solution of ammonium persulfate initiator (1 part by weight of ammonium persulfate initiator) was added dropwise and the reaction was carried out three times at 77 °C for 4 h. After the addition was completed, the reaction was continued for 1.2 h. Then the temperature was raised to 87 °C and the reaction was carried out four times for 5 h. After cooling to 25 °C, the negative electrode binder with the core-shell structure was obtained by filtration.

[0078] Example 2

[0079] This embodiment provides a negative electrode binder with a core-shell structure, which includes a core copolymer and a shell copolymer coating the surface of the core copolymer; the monomers of the core copolymer include styrene monomer, butadiene monomer and methyl acrylate monomer; the monomers of the shell copolymer include styrene monomer, butadiene monomer, acrylonitrile monomer and cyclohexyl acrylate monomer.

[0080] In the core-shell structured negative electrode binder, the mass ratio of the core copolymer to the shell copolymer is 0.8:1, the thickness of the core copolymer is 65 nm, and the thickness of the shell copolymer is 45 nm.

[0081] This embodiment also provides a method for preparing the above-mentioned core-shell structured negative electrode binder, which includes the following steps:

[0082] (1) In a nitrogen atmosphere, according to the weight percentage, 48 parts of styrene monomer, 47 parts of butadiene monomer, 1.5 parts of methyl acrylate monomer and 0.3 parts of tert-dodecyl mercaptan regulator were added to an aqueous solution containing 0.8 parts of sodium dodecyl sulfate emulsifier (220 parts of water) and mixed and heated once at a rate of 120 rpm. The temperature of the first heating was 62°C, and 0.8 parts of ammonium persulfate initiator were added dropwise. The reaction was carried out at 62°C for 5 hours. After the first reaction was completed, the temperature was raised to 74°C for a second reaction for 7 hours. After cooling to 25°C, the seed emulsion was obtained by filtration.

[0083] (2) In a nitrogen atmosphere, 11 parts by weight of the seed emulsion obtained in step (1) and 0.75 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier were heated twice at a rate of 100 rpm. During the heating period, 0.75 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier, 38 parts by weight of styrene monomer, 18 parts by weight of butadiene monomer, 22 parts by weight of acrylonitrile monomer, 18 parts by weight of cyclohexyl acrylate monomer and 0.3 parts by weight of tert-dodecyl mercaptan regulator were added and pre-emulsified at a rate of 350 rpm for 1 h. The weight of water was 120 parts. After the pre-emulsification, the aqueous solution of ammonium persulfate initiator (0.8 parts by weight of ammonium persulfate initiator) was added dropwise and the reaction was carried out three times at 75°C for 4.5 h. After the addition was completed, the reaction was continued for 1.5 h. Then the temperature was raised to 85°C and the reaction was carried out four times for 6 h. After cooling to 25°C, the negative electrode binder with the core-shell structure was obtained by filtration.

[0084] Example 3

[0085] This embodiment provides a negative electrode binder with a core-shell structure, which includes a core copolymer and a shell copolymer coating the surface of the core copolymer; the monomers of the core copolymer include styrene monomer, butadiene monomer and methyl acrylate monomer; the monomers of the shell copolymer include styrene monomer, butadiene monomer, acrylonitrile monomer and cyclohexyl acrylate monomer.

[0086] In the core-shell structured negative electrode binder, the mass ratio of the core copolymer to the shell copolymer is 1.2:1, the thickness of the core copolymer is 75 nm, and the thickness of the shell copolymer is 35 nm.

[0087] This embodiment also provides a method for preparing the above-mentioned core-shell structured negative electrode binder, which includes the following steps:

[0088] (1) In a nitrogen atmosphere, according to the weight percentage, 52 parts of styrene monomer, 49 parts of butadiene monomer, 3 parts of methyl acrylate monomer and 0.8 parts of tert-dodecyl mercaptan regulator were added to an aqueous solution containing 1.8 parts of sodium dodecyl sulfate emulsifier (280 parts of water) and mixed and heated once at a rate of 180 rpm. The temperature of the first heating was 68°C, and 1.5 parts of ammonium persulfate initiator were added dropwise. The reaction was carried out at 68°C for 4 hours. After the first reaction was completed, the temperature was raised to 78°C for a second reaction for 5 hours. After cooling to 25°C, the seed emulsion was obtained by filtration.

[0089] (2) In a nitrogen atmosphere, 14 parts by weight of the seed emulsion obtained in step (1) and 1.75 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier were heated twice at a rate of 200 rpm. During the heating period, 1.75 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier, 42 parts by weight of styrene monomer, 22 parts by weight of butadiene monomer, 28 parts by weight of acrylonitrile monomer, 22 parts by weight of cyclohexyl acrylate monomer and 0.8 parts by weight of tert-dodecyl mercaptan regulator were added and pre-emulsified at a rate of 450 rpm for 1.5 h. The weight of water was 180 parts. After the pre-emulsification, the aqueous solution of ammonium persulfate initiator (the weight of ammonium persulfate initiator was 1.5 parts) was added dropwise and the reaction was carried out three times at 80 °C for 3.5 h. After the addition was completed, the reaction was continued for 1 h. Then the temperature was raised to 90 °C and the reaction was carried out four times for 5 h. After cooling to 25 °C, the negative electrode binder with core-shell structure was obtained by filtration.

[0090] Example 4

[0091] This embodiment provides a negative electrode binder with a core-shell structure, which includes a core copolymer and a shell copolymer coating the surface of the core copolymer; the monomers of the core copolymer include styrene monomer, butadiene monomer and methyl acrylate monomer; the monomers of the shell copolymer include styrene monomer, butadiene monomer, acrylonitrile monomer and cyclohexyl acrylate monomer.

[0092] In the core-shell structured negative electrode binder, the mass ratio of the core copolymer to the shell copolymer is 0.5:1, the thickness of the core copolymer is 40 nm, and the thickness of the shell copolymer is 60 nm.

[0093] This embodiment also provides a method for preparing the above-mentioned core-shell structured negative electrode binder, which includes the following steps:

[0094] (1) In a nitrogen atmosphere, according to the weight of 0.5 parts of sodium dodecyl sulfate emulsifier (200 parts of water), 45 parts of styrene monomer, 45 parts of butadiene monomer, 1 part of methyl acrylate monomer and 0.1 parts of tert-dodecyl mercaptan regulator were added to an aqueous solution containing 0.5 parts of sodium dodecyl sulfate emulsifier. The mixture was mixed and heated once at a rate of 80 rpm. The temperature of the first heating was 60°C. 0.5 parts of ammonium persulfate initiator were added dropwise and the reaction was carried out at 60°C for 6 hours. After the first reaction was completed, the temperature was raised to 70°C for a second reaction for 7 hours. After cooling to 25°C, the mixture was filtered to obtain the seed emulsion.

[0095] (2) In a nitrogen atmosphere, 10 parts by weight of the seed emulsion obtained in step (1) and 0.25 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier were heated twice at a rate of 50 rpm. During the heating period, 0.25 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier, 35 parts by weight of styrene monomer, 15 parts by weight of butadiene monomer, 20 parts by weight of acrylonitrile monomer, 15 parts by weight of cyclohexyl acrylate monomer and 0.1 parts by weight of tert-dodecyl mercaptan regulator were added and pre-emulsified at a rate of 300 rpm for 0.5 h. The weight of water was 100 parts. After the pre-emulsification, an aqueous solution of ammonium persulfate initiator (0.5 parts by weight of ammonium persulfate initiator) was added dropwise and the reaction was carried out three times at 70 °C for 5 h. After the dropwise addition was completed, the reaction was continued for 2 h. Then the temperature was raised to 80 °C and the reaction was carried out four times for 6 h. After cooling to 25 °C, the negative electrode binder with the core-shell structure was obtained by filtration.

[0096] Example 5

[0097] This embodiment provides a negative electrode binder with a core-shell structure, which includes a core copolymer and a shell copolymer coating the surface of the core copolymer; the monomers of the core copolymer include styrene monomer, butadiene monomer and methyl acrylate monomer; the monomers of the shell copolymer include styrene monomer, butadiene monomer, acrylonitrile monomer and cyclohexyl acrylate monomer.

[0098] In the core-shell structured negative electrode binder, the mass ratio of the core copolymer to the shell copolymer is 2:1, the thickness of the core copolymer is 80 nm, and the thickness of the shell copolymer is 30 nm.

[0099] This embodiment also provides a method for preparing the above-mentioned core-shell structured negative electrode binder, which includes the following steps:

[0100] (1) In a nitrogen atmosphere, according to the weight of 2 parts of sodium dodecyl sulfate emulsifier (300 parts of water), 55 parts of styrene monomer, 50 parts of butadiene monomer, 5 parts of methyl acrylate monomer and 1 part of tert-dodecyl mercaptan regulator were added to an aqueous solution containing 2 parts of sodium dodecyl sulfate emulsifier. The mixture was mixed and heated once at a rate of 250 rpm. The temperature of the first heating was 70°C. 2 parts of ammonium persulfate initiator were added dropwise and the reaction was carried out at 70°C for 3 hours. After the first reaction was completed, the temperature was raised to 80°C for a second reaction for 4 hours. After cooling to 25°C, the mixture was filtered to obtain the seed emulsion.

[0101] (2) In a nitrogen atmosphere, 15 parts by weight of the seed emulsion obtained in step (1) and 2.5 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier were heated twice at a rate of 250 rpm. During the heating period, 2.5 parts by weight of the aqueous solution of sodium dodecyl sulfate emulsifier, 45 parts by weight of styrene monomer, 25 parts by weight of butadiene monomer, 30 parts by weight of acrylonitrile monomer, 25 parts by weight of cyclohexyl acrylate monomer and 1 part by weight of tert-dodecyl mercaptan regulator were added and pre-emulsified at a rate of 500 rpm for 2 hours. The weight of water was 200 parts. After the pre-emulsification, an aqueous solution of ammonium persulfate initiator (2 parts by weight of ammonium persulfate initiator) was added dropwise and the reaction was carried out three times at 80°C for 3 hours. After the addition was completed, the reaction was continued for 1 hour. Then the temperature was raised to 90°C and the reaction was carried out four times for 4 hours. After cooling to 25°C, the negative electrode binder with the core-shell structure was obtained by filtration.

[0102] Example 6

[0103] The difference between this embodiment and Embodiment 1 is that the mass ratio of the core copolymer to the shell copolymer in the core-shell structured negative electrode binder is 0.4:1, while all other aspects are the same as in Embodiment 1.

[0104] Example 7

[0105] The difference between this embodiment and Embodiment 1 is that the mass ratio of the core copolymer to the shell copolymer in the core-shell structured negative electrode binder is 2.5:1, while all other aspects are the same as in Embodiment 1.

[0106] Example 8

[0107] The difference between this embodiment and Embodiment 1 is that the weight of methyl acrylate monomer in step (1) is 0.5 parts, while all other parts are the same as in Embodiment 1.

[0108] Example 9

[0109] The difference between this embodiment and Embodiment 1 is that the weight of methyl acrylate monomer in step (1) is 10 parts, while all other parts are the same as in Embodiment 1.

[0110] Example 10

[0111] The difference between this embodiment and embodiment 1 is that in step (2), the weight of the seed emulsion is 5 parts and the weight of the cyclohexyl acrylate monomer is 10 parts, while the rest are the same as in embodiment 1.

[0112] Example 11

[0113] The difference between this embodiment and embodiment 1 is that the weight of the seed emulsion in step (2) is 20 parts and the weight of the cyclohexyl acrylate monomer is 30 parts, while the rest are the same as in embodiment 1.

[0114] Comparative Example 1

[0115] The only difference between Comparative Example 1 and Example 1 is that methyl acrylate monomer is not added to the monomer of the core layer copolymer; otherwise, they are the same as in Example 1.

[0116] Comparative Example 2

[0117] The only difference between Comparative Example 1 and Example 1 is that cyclohexyl acrylate monomer is not added to the monomer of the shell copolymer; otherwise, they are the same as in Example 1.

[0118] Application Example 1-11 and Comparative Application Example 1-2

[0119] Preparation of negative electrode sheet: According to the weight parts, 95 parts of graphite, 1 part of the above negative electrode binder, 1 part of sodium carboxymethyl cellulose, 1 part of conductive carbon black and 100 parts of deionized water are stirred and mixed evenly using a planetary mixer to obtain a slurry with suitable viscosity. The slurry is coated on the surface of copper foil using a coating machine. After baking and rolling, the negative electrode sheet is obtained.

[0120] Preparation of positive electrode sheet: According to the weight parts, 96.8 parts of positive electrode active material, 1.2 parts of polyvinylidene fluoride binder, 2.0 parts of conductive carbon black and 100 parts of N-methylpyrrolidone were stirred and mixed evenly to obtain a slurry with suitable viscosity. The slurry was coated on the surface of aluminum foil using a coating machine. After baking and rolling, the positive electrode sheet was obtained.

[0121] The preparation method of lithium-ion batteries is as follows:

[0122] The above-mentioned negative electrode, positive electrode and separator are assembled into a lithium-ion coin cell. LiPF6 is dissolved at a concentration of 1 mol / L in an electrolyte with a volume ratio of EC / DEC / EMC = 2:3:1. After electrolyte injection, a lithium-ion battery is obtained.

[0123] Test conditions

[0124] The lithium-ion batteries provided in Application Examples 1 to 11 and Comparative Application Examples 1 to 2 were subjected to performance tests, and the test methods are as follows:

[0125] (1) Cyclic performance:

[0126] a. Charge at a constant current of 0.5C until the voltage reaches 4.2V, then charge at a constant voltage of 4.2V until the current drops to 0.05C, at which point the charging ends.

[0127] b. Let it sit for 10 minutes;

[0128] c. Discharge to 3V with a constant current of 0.5C;

[0129] d. Let it sit for 10 minutes;

[0130] e. Repeat the above charge-discharge cycle 500 times;

[0131] (2) Ratio performance:

[0132] a. Charge at a constant current of 0.5C until the voltage reaches 4.2V, then charge at a constant voltage of 4.2V until the current drops to 0.05C, at which point the charging ends.

[0133] b. Discharge at a constant current of 0.2C to 3.0V, then stop discharging, let stand for 10 minutes, and record the discharge capacity;

[0134] c. Charge at a constant current of 0.5C until the voltage reaches 4.2V, then charge at a constant voltage of 4.2V until the current drops to 0.05C, at which point the charging ends.

[0135] d. Discharge the battery at 0.5C, 1C, 2C, and 3C respectively, following the steps described above.

[0136] The test results are shown in Table 1.

[0137] Table 1

[0138]

[0139]

[0140] The data in Table 1 illustrates that the core-shell structured negative electrode binder provided by this invention not only retains the flexibility of the styrene-butadiene rubber copolymer binder, but also prevents powder shedding during the electrode cutting process. At the same time, it improves the dispersion ability of the negative electrode binder on the negative electrode active material and the uniformity of the distribution of the negative electrode active material and conductive agent in the negative electrode sheet to a certain extent, thereby forming a good conductive network. The consistency of the prepared negative electrode sheet is also improved, thereby improving the cycle performance and rate performance of the battery.

[0141] Compared with Application Example 1, Application Examples 6-7 show cases where the mass ratio of the core copolymer to the shell copolymer is too low and too high, respectively. This indicates that by adjusting the mass ratio of the core copolymer to the shell copolymer, the synthesized polymer electrode can be guaranteed to have both good strength, suppress the expansion of the negative electrode active material during charging and discharging, and good flexibility. At the same time, it has good dispersibility for the negative electrode material and graphite, ensuring the formation of a good conductive network. Application Examples 8-9 show cases where the weight fraction of methyl acrylate monomer in step (1) is too low and too high, respectively. Application Examples 10-11 show cases where the weight fraction of seed emulsion and the weight fraction of cyclohexyl acrylate monomer in step (2) are too low and too high, respectively. This indicates that by controlling the weight fraction of the above components, the prepared negative electrode binder can further have good performance.

[0142] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A negative electrode binder with a core-shell structure, characterized in that, The negative electrode binder with a core-shell structure includes a core copolymer and a shell copolymer coated on the surface of the core copolymer. The monomers of the core layer copolymer include styrene monomers, butadiene monomers, and first acrylate monomers; The monomers of the shell copolymer include styrene monomers, butadiene monomers, acrylonitrile monomers, and second acrylate monomers; The mass ratio of the core copolymer to the shell copolymer in the core-shell structured negative electrode binder is (0.8~1.2):1; The first acrylate monomer is any one or a combination of at least two of the following: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-propyl acrylate, cyclohexyl acrylate, phenyl methacrylate, lauryl acrylate, dodecyl acrylate, hexadecyl acrylate, methyl methacrylate, n-butyl methacrylate, lauryl methacrylate, glycidyl methacrylate, dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, vinyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, dodecyl methacrylate, or hexadecyl methacrylate.

2. The negative electrode binder with a core-shell structure according to claim 1, characterized in that, The second acrylate monomer includes any one or a combination of at least two of the following: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-propyl acrylate, cyclohexyl acrylate, phenyl methacrylate, lauryl acrylate, dodecyl acrylate, hexadecyl acrylate, methyl methacrylate, n-butyl methacrylate, lauryl methacrylate, glycidyl methacrylate, dimethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, vinyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, dodecyl methacrylate, or hexadecyl methacrylate.

3. The negative electrode binder with a core-shell structure according to claim 1, characterized in that... The thickness of the core copolymer is 40-80 nm.

4. The negative electrode binder with a core-shell structure according to claim 1, characterized in that, The thickness of the shell copolymer is 30-60 nm.

5. A method for preparing a negative electrode binder with a core-shell structure according to any one of claims 1-4, characterized in that, The method includes the following steps: (1) Add styrene monomer, butadiene monomer and first acrylate monomer to an aqueous solution containing the first emulsifier, mix and heat once, add the first initiator to carry out a reaction once, raise the temperature to carry out a second reaction after the first reaction is completed, and obtain seed emulsion after cooling; (2) The seed emulsion and the aqueous solution of the second emulsifier obtained in step (1) are heated twice, and styrene monomer, butadiene monomer, acrylonitrile monomer and second acrylate monomer are added for pre-emulsification. After the pre-emulsification is completed, the aqueous solution of the second initiator is added for three reactions. After the three reactions are completed, the temperature is raised for four reactions. After cooling, the negative electrode binder with the core-shell structure is obtained.

6. The method according to claim 5, characterized in that, Based on the weight parts, in step (1), the first emulsifier is 0.5-2 parts by weight, water is 200-300 parts by weight, styrene monomer is 45-55 parts by weight, butadiene monomer is 45-50 parts by weight, the first acrylate monomer is 1-5 parts by weight, and the first initiator is 0.5-2 parts by weight.

7. The method according to claim 5, characterized in that, In step (1), the first emulsifier includes any one or a combination of at least two of the following: sodium dodecyl sulfate, sodium octadecyl sulfate, sodium dodecylbenzene sulfonate, sodium dioctyl succinate sulfonate, sodium fatty alcohol ether sulfate, sulfonate of ethoxylated fatty acid methyl ester, sodium α-alkenyl sulfonate, sodium secondary alkyl sulfonate, lauryl ether phosphate, or isooctyl ether phosphate.

8. The method according to claim 5, characterized in that, The surfactant and the first regulator are added before the first mixing in step (1).

9. The method according to claim 8, characterized in that, The surfactants include fatty acid glycerides and / or fatty acid ester sulfonates.

10. The method according to claim 8, characterized in that, The surfactant is 0.1-1 parts by weight.

11. The method according to claim 8, characterized in that, The first regulator includes tert-dodecyl mercaptan and / or n-dodecyl mercaptan.

12. The method according to claim 8, characterized in that, The first regulator is 0.1-1 parts by weight.

13. The method according to claim 5, characterized in that, The mixing in step (1) is carried out under an inert atmosphere.

14. The method according to claim 5, characterized in that, The mixing rate in step (1) is 50-250 rpm.

15. The method according to claim 14, characterized in that, The mixing rate in step (1) is 100-200 rpm.

16. The method according to claim 5, characterized in that, The heating in step (1) is carried out under stirring.

17. The method according to claim 5, characterized in that, The temperature of the first heating in step (1) is 60-70℃.

18. The method according to claim 5, characterized in that, In step (1), the first initiator includes a peroxide initiator and / or a reducing agent initiator.

19. The method according to claim 18, characterized in that, The peroxide initiator includes any one or a combination of at least two of hydrogen peroxide, peracetic acid, ammonium persulfate, potassium persulfate, or sodium persulfate.

20. The method according to claim 18, characterized in that, The reducing agent initiator includes any one or a combination of at least two of the following: ferrous salts, sodium sulfite, sodium bisulfite, alcohols, or dimethylaniline.

21. The method according to claim 5, characterized in that, The method of adding the first initiator in step (1) is dropwise addition.

22. The method according to claim 5, characterized in that, The temperature of the first reaction in step (1) is 60-70℃ and the time is 3-6h.

23. The method according to claim 22, characterized in that, The temperature of the first reaction in step (1) is 62-68℃ and the time is 4-5h.

24. The method according to claim 5, characterized in that, The temperature of the secondary reaction in step (1) is 70-80℃ and the time is 4-7h.

25. The method according to claim 24, characterized in that, The temperature of the secondary reaction in step (1) is 74-80℃ and the time is 5-7h.

26. The method according to claim 5, characterized in that, The cooling temperature mentioned in step (1) is no higher than 30°C.

27. The method according to claim 5, characterized in that, The cooling process described in step (1) also includes filtration.

28. The method according to claim 5, characterized in that, Based on the weight parts, the seed emulsion mentioned in step (2) consists of 10-15 parts by weight, the second emulsifier consists of 0.5-5 parts by weight, the styrene monomer consists of 35-45 parts by weight, the butadiene monomer consists of 15-25 parts by weight, the acrylonitrile monomer consists of 20-30 parts by weight, the second acrylate monomer consists of 15-25 parts by weight, and the water consists of 100-200 parts by weight.

29. The method according to claim 5, characterized in that, In step (2), the second emulsifier includes any one or a combination of at least two of the following: sodium dodecyl sulfate, sodium octadecyl sulfate, sodium dodecylbenzene sulfonate, sodium dioctyl succinate sulfonate, sodium fatty alcohol ether sulfate, sulfonate of ethoxylated fatty acid methyl ester, sodium α-alkenyl sulfonate, sodium secondary alkyl sulfonate, lauryl ether phosphate, or isooctyl ether phosphate.

30. The method according to claim 5, characterized in that, The secondary heating described in step (2) is carried out under stirring.

31. The method according to claim 5, characterized in that, Before pre-emulsification as described in step (2), add any one or a combination of at least two of the following: a second regulator, an antifoaming agent, or a preservative.

32. The method according to claim 31, characterized in that, The total weight of the defoamer and preservative is 0.5-1 parts by weight.

33. The method according to claim 31, characterized in that, The second regulator includes tert-dodecyl mercaptan and / or n-dodecyl mercaptan.

34. The method according to claim 31, characterized in that, The second regulator is 0.1-1 parts by weight.

35. The method according to claim 31, characterized in that, The defoamer includes polyether defoamers and / or polyether-modified silicone defoamers.

36. The method according to claim 31, characterized in that, The preservative is any one or a combination of at least two of propylparaben, benzoic acid, or sodium benzoate.

37. The method according to claim 5, characterized in that, The pre-emulsification described in step (2) is carried out under stirring.

38. The method according to claim 5, characterized in that, The pre-emulsification time in step (2) is 0.5-2 hours.

39. The method according to claim 38, characterized in that, The pre-emulsification time in step (2) is 1-1.5h.

40. The method according to claim 5, characterized in that, In step (2), the second initiator includes a peroxide initiator and / or a reducing agent initiator.

41. The method according to claim 40, characterized in that, The peroxide initiator includes any one or a combination of at least two of hydrogen peroxide, peracetic acid, ammonium persulfate, potassium persulfate, or sodium persulfate.

42. The method according to claim 40, characterized in that, The reducing agent initiator includes any one or a combination of at least two of the following: ferrous salts, sodium sulfite, sodium bisulfite, alcohols, or dimethylaniline.

43. The method according to claim 5, characterized in that, The method of adding the aqueous solution of the second initiator in step (2) is dropwise addition.

44. The method according to claim 5, characterized in that, The temperature of the three reactions in step (2) is 70-80℃ and the time is 4-7h.

45. The method according to claim 44, characterized in that, The temperature of the three reactions in step (2) is 75-80℃ and the time is 4.5-6h.

46. ​​The method according to claim 5, characterized in that, The temperature of the four reactions in step (2) is 80-90℃ and the time is 4-6h.

47. The method according to claim 46, characterized in that, The temperature of the four reactions in step (2) is 85-90℃ and the time is 5-6h.

48. The method according to claim 5, characterized in that, The cooling temperature in step (2) is no higher than 30°C.

49. The method according to claim 5, characterized in that, The cooling process described in step (2) also includes filtration.

50. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode binder with a core-shell structure as described in any one of claims 1-4.

51. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the negative electrode includes the negative electrode according to claim 50.

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