CN114094284A

CN114094284A - Novel cross-linked diaphragm and preparation method thereof, battery and electronic equipment

Info

Publication number: CN114094284A
Application number: CN202111356632.8A
Authority: CN
Inventors: 邱长泉; 李堃; 彭锟; 宫晓明; 蔡裕宏; 虞少波; 庄志; 程跃
Original assignee: Wuxi Enjie New Material Technology Co ltd
Current assignee: Suzhou Greenpower New Energy Materials Co ltd
Priority date: 2021-11-16
Filing date: 2021-11-16
Publication date: 2022-02-25
Anticipated expiration: 2041-11-16
Also published as: WO2023087735A1

Abstract

The invention relates to the field of battery diaphragms, and particularly discloses a novel cross-linked diaphragm which comprises an ultraviolet cross-linked upper surface layer, a core layer and an ultraviolet cross-linked lower surface layer; the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer comprises a cross-linking agent and a photoinitiator, and the core layer comprises a polyolefin composition; the thickness of the novel cross-linked diaphragm is 0.5-12 mu m; the thickness of the ultraviolet cross-linked upper surface layer and the ultraviolet cross-linked lower surface layer is 20-80% of that of the novel cross-linked diaphragm; the membrane breaking temperature of the novel cross-linked membrane is 191-215 ℃. By combining the polyolefin composition with a crosslinking agent and a photoinitiator in a specific manner, a novel crosslinked separator having high toughness, ultra-high rupture temperature, high puncture strength, and relatively high tensile modulus and tensile strength can be formed, which facilitates processing of the separator in the cell process. The novel cross-linked diaphragm provided by the invention is beneficial to improving the mechanical abuse resistance and heat abuse resistance of the battery.

Description

Novel cross-linked diaphragm and preparation method thereof, battery and electronic equipment

Technical Field

The invention relates to the field of battery diaphragms, in particular to a novel cross-linked diaphragm and a preparation method thereof, a battery and electronic equipment.

Background

Lithium ion batteries are currently commercialized and widely used secondary power sources. In lithium ion batteries, the separator is a porous, electrochemically inert medium between the positive and negative electrodes, which does not participate in the electrochemical reaction, but is critical to the safety performance of the cell. Currently used polyolefin separators may have some drawbacks. For example, the ductility of the separator is not good, which may cause the separator to be punctured when the cell is mechanically misused. As another example, the closed cell temperature of the separator is high, so that the electrochemical path is more difficult to cut off when the cell overheats. In another example, the membrane breaking temperature of the membrane is low, so that the membrane is melted when the battery cell is overheated. The defects easily cause the damage of the diaphragm and the formation of short-circuit points between the anode and the cathode, thereby causing potential safety hazards.

Disclosure of Invention

In view of the above, it is desirable to provide a battery separator having both mechanical abuse resistance and thermal abuse resistance to solve the technical problem.

In order to achieve the purpose, the technical scheme of the invention is realized as follows:

the invention aims to provide a novel cross-linked diaphragm, which comprises an ultraviolet cross-linked upper surface layer, a core layer and an ultraviolet cross-linked lower surface layer; the thickness of the novel cross-linked diaphragm is 0.5-12 mu m; the thickness of the ultraviolet cross-linked upper surface layer and the ultraviolet cross-linked lower surface layer is 20-80% of that of the novel cross-linked diaphragm; the novel cross-linked diaphragm has a diaphragm breaking temperature of 191-215 ℃ and a closed pore temperature of 95-150 ℃.

The thickness of the novel crosslinked separator is preferably 0.5 to 12 μm, more preferably 1 to 9 μm, even more preferably 3 to 9 μm, and even more preferably 5 to 6 μm. It is noted that reducing the thickness of the novel crosslinked separator is advantageous for improving the efficiency of ion transport in the cell and for improving the energy density of the battery (the thickness of the novel crosslinked separator may be, for example, 9 μm or less). The thickness of the novel cross-linked diaphragm is improved, self-discharge in the battery core is reduced, the isolation capability of the novel cross-linked diaphragm is improved, and the safety of the battery is improved (the thickness of the novel cross-linked diaphragm can be more than 3 microns).

Further, the crosslinking density of the novel crosslinked diaphragm is 12-72%.

Further, the difference between the closed pore temperature and the membrane breaking temperature of the novel cross-linked membrane is 49-103 ℃.

Further, the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer comprises a cross-linking agent and a photoinitiator; the thickness of the ultraviolet cross-linked upper surface layer and the ultraviolet cross-linked lower surface layer is 20-75% of the thickness of the novel cross-linked diaphragm.

Further, the crosslinking agent is a bifunctional or polyfunctional monomer with an olefinic double bond, including at least one of: 1, 6-hexanediol diacrylate, neopentyl glycol diacrylate, divinylbenzene, bismaleic acid diacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, triallyl isocyanurate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate.

Still further, the photoinitiator is a free radical polymerization photoinitiator or a cationic polymerization photoinitiator comprising at least one of: benzoin, benzoin bis methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether, diphenylethanone, alpha-dimethoxy-alpha-phenylacetophenone, alpha-diethoxyacetophenone, alpha-hydroxyalkylphenone, alpha-aminoalkylphenone, aroylphosphine oxide, bisbenzoylphenylphosphine oxide, benzophenone, 2, 4-dihydroxybenzophenone, Michler's ketone, thiopropoxythioxanthone, isopropylthioxanthone, diaryliodonium salts, triaryliodonium salts, alkyliodonium salts, isopropylbenzocene ferrocenehexafluoro phosphate.

Further, the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer has an ultraviolet wavelength range of 230-350 nm when cross-linked by ultraviolet irradiation.

Further, the ultraviolet radiation time of the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer is 0.2-1 s when the ultraviolet radiation cross-linking is carried out.

Further, the ultraviolet radiation power of the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer is 1-4 Kw when the ultraviolet radiation cross-linking is carried out.

Further, the core layer comprises a polyolefin composition; the polyolefin composition has a viscosity average molecular weight in the range of 30X 10⁴～1000×10⁴In the meantime.

Still further, the polyolefin composition comprises a first polyethylene, a second polyethylene, both selected from any one of the following: polyethylene, polyethylene-propylene copolymer, derivative of polyethylene-propylene copolymer, polyethylene-butene copolymer, derivative of polyethylene-butene copolymer, polyethylene-hexene copolymer, derivative of polyethylene-hexene copolymer, polyethylene-octene copolymer, derivative of polyethylene-octene copolymer, polystyrene-ethylene-styrene copolymer, derivative of polystyrene-ethylene-styrene copolymer, polystyrene-ethylene-butene-styrene copolymer, derivative of polystyrene-ethylene-butene-styrene copolymer, polyethylene-hydrogenated oligocyclopentadiene, derivative of polyethylene-hydrogenated oligocyclopentadiene, polyethylene oxide, derivative of polyethylene oxide, and polyethylene oxide, Polypentene-ethylene copolymers, derivatives of polypentene-ethylene copolymers, polyhexene-ethylene copolymers, derivatives of polyhexene-ethylene copolymers, polymethylpentene-ethylene copolymers, derivatives of polymethylpentene-ethylene copolymers.

Furthermore, the polyolefin composition also comprises polypropylene or derivatives thereof, wherein the enthalpy delta Hm of the polypropylene or the derivatives thereof is 55-85J/g, and the density is more than or equal to 0.9g/cm³。

Further, the polyolefin composition has a viscosity average molecular weight in the range of 110X 10⁴～500×10⁴In the meantime.

Further, the novel crosslinked separator satisfies at least one of:

the transverse and longitudinal extensibility is less than or equal to 120 percent;

the transverse and longitudinal tensile moduli are both more than or equal to 2000 MPa;

the ratio of the longitudinal tensile modulus to the transverse tensile modulus is more than or equal to 0.9;

the transverse and longitudinal heat shrinkage rates are less than or equal to 1.8 percent at 120 ℃;

the porosity is 20% -85%;

the puncture strength is 650-1400 gf;

the transverse tensile strength and the longitudinal tensile strength are both more than or equal to 2000kgf/cm²；

The air permeability is less than or equal to 172s/100cc/5 mu m.

Here, the novel crosslinked separator preferably has a bidirectional elongation at break of 120% or less, more preferably 50 to 120%, and even more preferably 80 to 120%.

It is advantageous to increase the bidirectional elongation at break of the novel crosslinked separator to reduce the safety problem caused by expansion and contraction of the battery wound body during charge and discharge (the bidirectional elongation at break of the novel crosslinked separator may be 50% or more, for example). The novel crosslinked separator has reduced two-way elongation at break, and is advantageous for improving mechanical strength and thermal stability (the two-way elongation at break of the novel crosslinked separator may be, for example, 120% or less).

Optionally, the novel crosslinked separator has a longitudinal and transverse (MD/TD) tensile strength of greater than or equal to 2000kgf/cm²It is further preferable that the novel crosslinked separator has a tensile strength in the machine direction and transverse direction (MD/TD) of not less than 3000kgf/cm²More preferably, the novel crosslinked separator has a tensile strength in the machine direction and transverse direction (MD/TD) of 4000kgf/cm or more²Most preferably, the novel crosslinked separator has a tensile strength in the machine and transverse directions (MD/TD) of not less than 4500kgf/cm²。

Optionally, the ratio of the longitudinal direction/transverse direction (MD/TD) tensile strength of the novel crosslinked separator is preferably 0.9 or more, more preferably 0.95 to 1.2, and further preferably 0.96 to 1.16.

Optionally, the novel cross-linked membrane has a tensile modulus in the machine direction and transverse direction (MD/TD) of not less than 3000MPa, more preferably not less than 3500MPa, still more preferably not less than 4000MPa, and most preferably not less than 4500 MPa.

Optionally, the ratio of the longitudinal direction/transverse direction (MD/TD) tensile modulus of the novel cross-linked membrane is preferably 0.9-1.2, and more preferably 0.91-1.1.

It should be noted that the tensile modulus of the separator is provided to facilitate winding of the cell and processing of the coating.

The novel cross-linked membrane preferably has a closed pore temperature of 95-150 ℃, more preferably 140-145 ℃, even more preferably 140-142 ℃, and even more preferably 95-121 ℃.

It should be noted that, increasing the closed pore temperature of the novel cross-linked membrane is beneficial to preventing the battery from melting in the normal use process, and is further beneficial to improving the thermal stability of the battery (the closed pore temperature of the novel cross-linked membrane is preferably above 140 ℃). The reduction of the closed-cell temperature of the novel crosslinked separator is advantageous for improving the safety of the battery (the closed-cell temperature of the novel crosslinked separator is preferably 150 ℃ or lower, more preferably 145 ℃ or lower, and still more preferably 142 ℃ or lower).

The novel crosslinked separator preferably has a film breaking temperature of 191 ℃ or higher, more preferably 191 to 215 ℃, and further preferably 209 to 215 ℃.

It is noted that increasing the membrane rupture temperature of the novel crosslinked separator is advantageous for improving the safety of the battery under high temperature conditions (for example, in a thermally abnormal environment, the membrane rupture temperature of the novel crosslinked separator is preferably 209 ℃.

The novel crosslinked separator preferably has an air permeability of 100 to 172s/100cc, more preferably 150 to 172s/100cc, still more preferably 150 to 163s/100cc, and yet more preferably 150 to 155s/100 cc.

The novel crosslinked separator has an improved air permeability and is advantageous for reducing the self-discharge defect rate (the air permeability of the novel crosslinked separator may be, for example, 100s/100cc or more). The reduction of the air permeability of the novel crosslinked separator is advantageous for improving the ion transport efficiency in the electric core (the air permeability of the novel crosslinked separator may be, for example, 172s/100cc or less).

Here, the porosity of the novel crosslinked separator is preferably 20% to 30%.

Here, the puncture strength is preferably 400 to 1400gf, more preferably 450 to 1400gf, even more preferably 500 to 1400gf, even more preferably 600 to 1400gf, and most preferably 650 to 1400 gf. The puncture strength of the novel cross-linked diaphragm is improved, the safety of the novel cross-linked diaphragm is favorably improved, and the winding of the battery cell and the processing of the coating are favorably realized.

Further, one side or two sides of the novel cross-linked diaphragm are provided with a coating, and the coating comprises one or more of an organic coating, an inorganic coating and an organic/inorganic composite coating.

Still further, the inorganic coating comprises a ceramic coating comprising at least one of: alumina, silica, titania, zirconia, zinc oxide, barium oxide, magnesium oxide, beryllium oxide, calcium oxide, thoria, aluminum nitride, titanium nitride, boehmite, apatite, aluminum hydroxide, magnesium hydroxide, barium sulfate, boron nitride, silicon carbide, silicon nitride, cubic boron nitride, hexagonal boron nitride, mesoporous molecular sieves (MCM-41, SBA-15), pearl mica layers.

Still further, the organic coating includes at least one of: polyvinylidene fluoride coatings, vinylidene fluoride-hexafluoropropylene copolymer coatings, polystyrene coatings, aramid coatings, polyacrylate or its modifications coatings, polyester coatings, polyarylate coatings, polyacrylonitrile coatings, aromatic polyamide coatings, polyimide coatings, polyethersulfone coatings, polysulfone coatings, polyetherketone coatings, polyetherimide coatings, polybenzimidazole coatings, polydopamine.

Further, the organic/inorganic composite coating layer may be prepared by mixing the above-described inorganic coating layer with an organic coating layer.

The invention also aims to provide a battery, which comprises a positive electrode, a negative electrode, an electrolyte and any one of the novel crosslinked separators.

The invention also provides electronic equipment which comprises a shell, a display screen, a circuit board assembly and the battery, wherein the display screen, the circuit board assembly and the battery are accommodated in the shell, and the battery supplies power to the display screen and the circuit board assembly.

The invention also provides a mobile device which comprises the battery.

The invention aims to provide a preparation method of the novel cross-linked diaphragm, which comprises the following steps:

s1, pre-irradiating the polyolefin composition with an irradiation dose of 0.1-1 Mrad to obtain a pre-irradiated polyolefin composition;

s2, mixing the raw material mixture containing the pre-irradiated polyolefin composition, the cross-linking agent, the photoinitiator and the pore-forming agent, and extruding the mixture from a screw extruder to form a gel sheet;

s3, carrying out biaxial stretching on the gel sheet, and then removing the pore-forming agent in the gel sheet;

s4, performing heat setting on the gel sheet, wherein the heat setting comprises low-rate stretching and retracting operations;

s5, crosslinking the gel sheet by ultraviolet irradiation surface layer;

s6, rolling and cutting the gel sheet to form the novel cross-linked diaphragm.

Further, the polyolefin composition described in S1 includes one or more of polyethylene, polyethylene copolymer, polypropylene or polypropylene derivative which differ in viscosity-average molecular weight.

Further, the S2 raw material mixture also comprises a polyolefin emulsion.

Furthermore, the polyolefin emulsion is polyethylene wax emulsion, the solid content of the polyolefin emulsion is 10-70%, the melting point of solid particles is 80-135 ℃, more preferably 80-120 ℃, further preferably 80-90 ℃, and further preferably 110-120 ℃.

Here, the melting point of the solid particles is selected in relation to the cell closing temperature, and the upper limit is preferably lower than the melting point of different polyolefin separators, for example, the upper limit is preferably 135 ℃, so that when the polyolefin separator is UHMWPE, the melting point is about 135 ℃, and then the polymer emulsion with the melting point lower than 135 ℃ can form a coating layer with the cell closing temperature lower than that of the PE separator, thereby improving the overall safety performance of the battery. Other temperatures depend on the different polyolefin materials and preference when one wants to lower the membrane's closed cell temperature even further.

Further, in S1, a cobalt source or an electron accelerator is used for pre-irradiation.

Here, the pre-irradiation is to use an electron accelerator to perform low-dose irradiation on the polyolefin composition material before mixing and extrusion, so that the polyethylene macromolecules generate a small amount of free radicals under the action of high-energy rays. Thus, the originally stable polyolefin chain ends can be activated, and can be quickly crosslinked under the extremely short ultraviolet exposure, so that the crosslinking degree can be controlled and deepened; and because the intermolecular free radical density is lower in the subsequent processing processes including mixing extrusion, stretching extraction, annealing and the like, spontaneous crosslinking is not enough, and the influence of the rise of the melt viscosity on the processability is avoided. In general, the method can reduce the processing difficulty, shorten the subsequent ultraviolet crosslinking time, control the crosslinking degree, improve the production efficiency and improve the mechanical property and the film breaking temperature.

Further, the area after stretching/the area before stretching in the biaxial stretching in S2 is 10 to 50 times.

Further, in S3, the low-magnification stretching is 1-3 times, and the stretching temperature of the low-magnification stretching is 105-135 ℃; the retraction ratio of the retraction operation is 0.5-10%.

Further, the wavelength range of the ultraviolet light in S4 is between 230 nm and 350 nm.

Further, the ultraviolet radiation time in S4 is 0.2-1S.

Further, the ultraviolet radiation power in S4 is 1-4 Kw.

Further, the weight of the cross-linking agent is between 1 and 20 parts and the weight of the photoinitiator is between 0.1 and 2 parts based on 100 parts of the polyolefin composition.

The weight of the cross-linking agent is more preferably between 1 and 15 parts, further preferably between 1 and 10 parts, further preferably between 1 and 2 parts, and most preferably between 1 and 1.5 parts; the weight of the photoinitiator is more preferably 0.5-1.5 parts, and still more preferably 0.7-1.2 parts.

The proportion of the cross-linking agent in the raw material mixture of the novel cross-linked diaphragm is reduced (or the proportion of the initiator in the raw material mixture of the novel cross-linked diaphragm is improved), which is favorable for further improving the cross-linking efficiency of the polyolefin microporous membrane and reducing the processing difficulty. In addition, the novel cross-linked diaphragm is beneficial to preventing wrinkling and creep deformation, and the tensile modulus of the novel cross-linked diaphragm is beneficial to being improved.

The proportion of the cross-linking agent in the raw material mixture of the novel cross-linked diaphragm is improved (or the proportion of the polyolefin composition in the raw material mixture of the novel cross-linked diaphragm is reduced), so that the possibility of insufficient cross-linking density of the novel cross-linked diaphragm is favorably reduced, and the mechanical property of the novel cross-linked diaphragm is favorably improved. And in addition, the membrane breaking temperature of the novel cross-linked membrane is favorably improved.

In the application, the proportion of the polyolefin composition is optimized, so that the novel cross-linked diaphragm can be improved to obtain relatively good elongation, stiffness and tensile modulus.

Further, the polyolefin composition comprises a polyethylene, the polyolefin being selected from any one of the following: polyethylene, polyethylene-propylene copolymer, derivative of polyethylene-propylene copolymer, polyethylene-butene copolymer, derivative of polyethylene-butene copolymer, polyethylene-hexene copolymer, derivative of polyethylene-hexene copolymer, polyethylene-octene copolymer, derivative of polyethylene-octene copolymer, polystyrene-ethylene-styrene copolymer, derivative of polystyrene-ethylene-styrene copolymer, polystyrene-ethylene-butene-styrene copolymer, derivative of polystyrene-ethylene-butene-styrene copolymer, polyethylene-hydrogenated oligocyclopentadiene, derivative of polyethylene-hydrogenated oligocyclopentadiene, polyethylene oxide, derivative of polyethylene oxide, and polyethylene oxide, Polypentene-ethylene copolymers, derivatives of polypentene-ethylene copolymers, polyhexene-ethylene copolymers, derivatives of polyhexene-ethylene copolymers, polymethylpentene-ethylene copolymers, derivatives of polymethylpentene-ethylene copolymers.

Furthermore, the density of the polyethylene is preferably 0.85-0.99 g/cm³。

The density of the polyethylene is more preferably 0.91 to 0.97g/cm3, and still more preferably 0.92 to 0.95g/cm 3.

Here, by changing the density of the polyethylene, the compatibility of the polyethylene with polypropylene (when the polyolefin composition described below contains polypropylene) can be changed. In addition, optimizing the density of the polyethylene is also beneficial to optimizing the delamination degree of the polyethylene or polypropylene.

Because of the different viscosity-average molecular weights, the polyethylene has different strengths and elongations and can have good toughness and processability. Therefore, the separator prepared by polyethylene containing the cross-linking agent and the photoinitiator and having a certain cross-linking degree can simultaneously have relatively high elongation, excellent strength and thermal stability. Screening for polyethylene of suitable viscosity average molecular weight can balance the risk of reduced elongation due to crosslinking.

The proportion of the cross-linking agent in the polyolefin composition of the novel cross-linked diaphragm is improved, the possibility of uneven and bad ultraviolet radiation of the novel cross-linked diaphragm is favorably reduced, the number of melt crystal points in the novel cross-linked diaphragm is reduced, and the quality of the novel cross-linked diaphragm is favorably improved. And in addition, the membrane breaking temperature of the novel cross-linked membrane is favorably improved.

Specifically, the polyolefin composition further comprises polypropylene or derivatives thereof, wherein the enthalpy delta Hm of the polypropylene or the derivatives thereof is preferably 55-85J/g, more preferably 60-80J/g, and the density is preferably 0.9g/cm³Above, more preferably 0.91g/cm³The above.

Here, the polyethylene in the polyolefin composition may be blended with polypropylene, which may be interpenetrated within the polyethylene to form relatively fine crystals, rather than large platelets. This is advantageous for improving the overall performance of the novel crosslinked separator.

By optimizing the enthalpy delta Hm of the polypropylene, the heat resistance stability of the polyolefin microporous membrane and the compatibility with polyethylene materials are improved.

By adding polypropylene with enthalpy delta Hm within the range of 55-85J/g into the novel cross-linked diaphragm, the tensile strength is favorably improved, and the problems of overhigh flexibility and lower stiffness of the diaphragm surface are solved; it is advantageous to reduce the possibility of edge overhang, wrap around misalignment, bending, wrinkling, etc. during cutting or attaching of the coating.

In addition, in the high-temperature shearing and melting stage, the polyethylene with relatively high viscosity-average molecular weight can be cut and inserted among polypropylene molecular chains, so that the phenomenon of incompatibility of polyethylene and polypropylene can be improved. In addition, relatively uniform phase separation characteristics may be formed during the co-extrusion process (e.g., during the casting process), which may be advantageous to prevent the separator thickness from deviating too much.

Further, the raw material mixture may further include at least one of: antioxidant: such as phenols, amines, phosphites, thiodipropionates, etc.; a stabilizer: such as sodium stearate, calcium stearate, magnesium stearate, zinc stearate, etc.; antistatic agents, radiation light absorbers, light stabilizers, nucleating agents, inorganic particles, and the like.

Further, the raw material mixture may further include a thermoplastic resin other than the polyolefin.

Further, the raw material mixture may further include at least one of: linear low density polyethylene, branched polyethylene, polymethyl methacrylate, polyvinylidene fluoride, polyacrylonitrile, and the like.

The invention has the following beneficial effects:

according to the invention, the polyolefin composition, the cross-linking agent and the photoinitiator are combined in a specific manner, so that a novel cross-linked diaphragm with high toughness, ultrahigh film breaking temperature and high puncture strength can be formed; according to tests, the novel cross-linked diaphragm with the thickness of 5-6 microns provided by the application can have the diaphragm breaking temperature of 191-215 ℃ and the hole closing temperature of 95-121 ℃. Compared with the diaphragm made of uncrosslinked polyethylene, the novel crosslinked diaphragm has relatively high diaphragm breaking temperature and a large safety range of 49-103 ℃. And has a relatively high tensile modulus and tensile strength, which facilitates processing of the separator in the cell process (e.g., facilitates avoiding problems of edge protrusion, winding deviation, bending, wrinkling, etc. due to high elongation characteristics). The novel cross-linked diaphragm provided by the invention is beneficial to improving the mechanical abuse resistance and heat abuse resistance of the battery.

Drawings

Fig. 1 is a schematic structural diagram of the novel cross-linked membrane provided by the invention.

Description of the element reference numerals

100 core layer

101 UV-crosslinked top skin layer

102 uv cross-linked lower skin layer

Detailed Description

The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

Prior to describing the embodiments of the present invention, technical terms appearing in the present invention will be described.

Separator (Separator): the dielectric can be used for separating the positive electrode and the negative electrode of the battery core and preventing the positive electrode and the negative electrode from being in direct contact with each other to cause short circuit. The basic characteristics of the separator are porosity (channels that provide ion transport) and insulation (prevention of electrical leakage). The separator may include a separator substrate and a separator coating.

Separator substrate (Base separator): may refer to the polyolefin microporous membrane portion of the separator. The separator substrate may be used alone in the cell. The separator base material can provide the porosity and the insulation. The novel cross-linked diaphragm is a diaphragm substrate.

Separator coating layer (Separator coating layer): may refer to a thin layer attached to the separator substrate. The membrane coating may be attached to the membrane substrate by means of additive manufacturing. The separator coating layer may be used to enhance the performance of the separator, for example, to improve heat resistance, adhesion, and the like of the separator.

A battery cell (core or cell) may refer to a portion of a battery having a power storage function. The cell may include a positive electrode and a negative electrode.

Heat abuse (Heat ab μ se): which may refer to abuse of the cell in terms of heat (or high temperature). The cells may be tested for thermal abuse using a hot box (e.g., baking the cells using a high temperature (> 130 ℃).

Mechanical abuse (mechanical ab μ se): may refer to mechanical abuse of the electrical core. The cells may be tested for mechanical abuse using a needle test, impact test, or the like.

Elongation (Elongation): which may also be referred to as elongation at break, may refer to the percentage increase in length at which the diaphragm is snapped off relative to the initial length. Specifically, the separator may be subjected to a tensile test under specific conditions, and the increase in the length of the separator divided by the initial length of the separator at the time the separator is just pulled apart may be used to characterize the elongation. A larger value of the elongation means that the separator is less likely to be pulled apart, and the elongation is better. The elongation may be divided into a machine direction (MD, i.e., in the long side direction of the separator) elongation and a transverse direction (TD, perpendicular with respect to the MD, i.e., in the short side direction of the separator) elongation.

Tensile modulus (Tensile mode μ l μ s): it may refer to tensile strength under a certain stretching condition, i.e., a ratio of force per unit length required for the separator in the stretching direction to a sectional area of the separator. The tensile modulus may be divided into a longitudinal direction (MD, i.e., in the long-side direction of the separator) tensile modulus and a transverse direction (TD, perpendicular to the MD, i.e., in the short-side direction of the separator) tensile modulus.

Tensile strength (Tensile strength): can refer to the critical strength value of the diaphragm plastic deformation, and can represent the maximum bearing capacity of the diaphragm under the uniform stretching condition. Tensile strength may refer to the stress resulting from dividing the maximum load force experienced by the membrane by the initial cross-sectional area of the membrane at the time the membrane is just pulled apart. The tensile strength is divided into a machine direction (MD, i.e., in the long-side direction of the separator) tensile strength and a transverse direction (TD, perpendicular to the MD, i.e., in the short-side direction of the separator) tensile strength.

Puncture strength (P μ nct μ re strength): it can mean that a spherical steel needle with the diameter of 1.0mm is adopted to prick the septum at the speed of 300 +/-10 mm/min, and the force required by the steel needle to penetrate the septum is the puncture strength of the septum.

Heat shrinkage (Heat shrinkage): it may refer to the dimensional change rate of the separator in the longitudinal/transverse (machine direction MD, i.e., in the long-side direction of the separator; transverse direction TD, perpendicular to MD, i.e., in the short-side direction of the separator) direction before and after heating. The test method of the thermal shrinkage rate may include: measuring a dimension of the separator in a longitudinal/transverse (MD/TD) direction; placing a separator having a dimension in a longitudinal/transverse (MD/TD) direction in an incubator; heating the constant temperature box to a specific temperature; the dimension of the separator in the longitudinal/transverse (MD/TD) direction after heating was measured.

Viscosity average molecular Weight (Viscosity-average molecular μ lar Weight): can be one of the common methods for expressing the molecular weight of polymers. The polymer may have a polydispersity, and the polymer molecular weight is generally referred to as the average molecular weight of the polymer. Various types of average molecular weights can be obtained by various molecular weight averaging methods. The molecular weight of the polymer obtained by the dilute solution viscosity method can be viscosity average molecular weight.

Molecular weight distribution (distrib μ tion of molecular μ lar weight): the ratio of weight average molecular weight to number average molecular weight or the ratio of viscosity average molecular weight to weight average molecular weight.

Porosity (Porosity): may refer to the percentage of the pore volume in the separator to the total volume of the separator. The porosity P satisfies:

where V may be the total volume of the membrane, m may be the mass of the membrane, and ρ may be the skeletal density (or true density) of the membrane.

Air permeability (G μ rley): may refer to the extent to which the membrane allows gas to pass. The gas permeability can be obtained by measuring the time required for a unit gas volume (100cc) to permeate a separator at a specific pressure (0.05 MPa).

Pore size (Hole size): may refer to the diameter of the through hole in the septum. The pore diameter is obtained by testing with a pore diameter analyzer.

Crystallinity (Crystallinity): can be obtained by Differential Scanning Calorimetry (DSC) test. The crystallinity of the polyolefin separator may be obtained by: calculating the integral of a melting endothermic curve of the polyolefin diaphragm in the process from the beginning of heating to the generation of heat transition enthalpy to obtain a melting enthalpy value (the unit is Joule (J)); the enthalpy of fusion is divided by the mass (g) of the sample to obtain the mass normalized enthalpy of fusion (Δ Hs) of the polyolefin membrane. The mass normalized melting enthalpy (Δ Hs) is then divided by the melting enthalpy (Δ Hf) of 100% crystalline polyolefin to obtain the crystallinity X (%) of the polyolefin separator.

Closed cell temperature (Obt μ rate tempreat μ re): it may refer to the temperature at which the membrane begins to melt and block a portion of the pores that were previously formed during the heating process.

Membrane rupture temperature (R μ pt μ re temperat μ re): it may refer to the temperature at which the membrane melts to such an extent that rupture occurs, resulting in a local or global short circuit.

Embodiments of the present invention provide a separator substrate. The separator substrate may be a microporous membrane, including a polyolefin composition. Specifically, the separator substrate may include a polyolefin composition, a crosslinking agent, and a photoinitiator, wherein the crosslinking agent is not crosslinkable by heat radiation.

The separator substrate may be a porous insulating material. The pores of the separator substrate are permeable to lithium ions (the pores of the separator substrate may be a transport channel for lithium ions).

The separator substrate may include, for example, a polyolefin-based material. The separator substrate may also be referred to as a polyolefin porous separator substrate. The polyolefin group material provides chemical inertness, electrochemical inertness, porosity, electronic insulation and the like for the diaphragm. In addition, according to the performance of the separator described above, the separator base material is required to have performance such as high ductility, high rupture temperature, and low closed cell temperature as a main component of the separator. The polyolefin-based material may include Polyethylene (PE), for example.

It should be noted that the combination of the polyolefin composition, the crosslinking agent and the photoinitiator is advantageous for optimizing the mechanical properties and the heat resistance of the separator substrate.

In certain embodiments, the polyolefin composition comprises at least one of: polyethylene, polyethylene-propylene copolymer, derivative of polyethylene-propylene copolymer, polyethylene-butene copolymer, derivative of polyethylene-butene copolymer, polyethylene-hexene copolymer, derivative of polyethylene-hexene copolymer, polyethylene-octene copolymer, derivative of polyethylene-octene copolymer, polystyrene-ethylene-styrene copolymer, derivative of polystyrene-ethylene-styrene copolymer, polystyrene-ethylene-butene-styrene copolymer, derivative of polystyrene-ethylene-butene-styrene copolymer, polyethylene-hydrogenated oligocyclopentadiene, derivative of polyethylene-hydrogenated oligocyclopentadiene, polyethylene oxide, derivative of polyethylene oxide, and polyethylene oxide, Polypentene-ethylene copolymers, derivatives of polypentene-ethylene copolymers, polyhexene-ethylene copolymers, derivatives of polyhexene-ethylene copolymers, polymethylpentene-ethylene copolymers, derivatives of polymethylpentene-ethylene copolymers.

The copolymer may cause problems such as unstable film formation and difficulty in controlling the molecular weight distribution, compared with the homopolymer.

The propylene copolymer may comprise an ethylene-propylene block copolymer and/or a random copolymer. Preferably, the proportion of the ethylene-propylene block copolymer in the propylene copolymer is higher than the proportion of the random copolymer in the propylene copolymer. Specific reasons may include that the melting point of the ethylene-propylene block copolymer is generally higher than that of the random copolymer.

The separator may also include a separator coating in the event that the performance of the separator substrate is insufficient or to be improved. The membrane coating can be attached to one side or two sides of the membrane substrate, so that the membrane has the properties of high ductility, high membrane breaking temperature, low closed pore temperature and the like. In addition, the separator coating may also have other properties, such as having relatively high adhesion, and the like.

The separator coating layer may include an organic coating layer, an inorganic coating layer, and/or an organic-inorganic composite coating layer.

The inorganic coating may comprise a ceramic coating. The ceramic coating may include at least one of: alumina, silica, titania, zirconia, zinc oxide, barium oxide, magnesium oxide, beryllium oxide, calcium oxide, thoria, aluminum nitride, titanium nitride, boehmite, apatite, aluminum hydroxide, magnesium hydroxide, barium sulfate, boron nitride, silicon carbide, silicon nitride, cubic boron nitride, hexagonal boron nitride, graphite, graphene, mesoporous molecular sieves (MCM-41, SBA-15), and the like.

The organic coating may include at least one of: polyvinylidene fluoride coatings, vinylidene fluoride-hexafluoropropylene copolymer coatings, polystyrene coatings, aramid coatings, polyacrylate or its modifications coatings, polyester coatings, polyarylate coatings, polyacrylonitrile coatings, aromatic polyamide coatings, polyimide coatings, polyethersulfone coatings, polysulfone coatings, polyetherketone coatings, polyetherimide coatings, polybenzimidazoles.

The organic-inorganic composite coating can be prepared by mixing the inorganic coating and the organic coating.

Embodiments of the present invention also provide a lithium ion secondary battery whose core components may include a positive electrode material, a negative electrode material, an electrolyte, a separator, and corresponding communication accessories and circuits. The positive electrode material and the negative electrode material can release lithium ions to realize the storage and release of energy. The electrolyte may be a transport carrier of lithium ions between the positive electrode material and the negative electrode material. The anode material and the cathode material are main energy storage parts of the lithium ion secondary battery, and can embody the energy density, the cycle performance and the safety performance of the battery core. The separator is permeable to lithium ions, but is not conductive by itself, so that the separator can separate the positive electrode material and the negative electrode material to prevent short circuit between the positive electrode material and the negative electrode material.

The positive electrode material may include a positive electrode current collector and a positive electrode active material disposed on the positive electrode current collector. The positive active material includes, but is not limited to, lithium composite metal oxides (such as lithium nickel cobalt manganese oxide, etc.), polyanionic lithium compounds LiMx (PO4) y (M is Ni, Co, Mn, Fe, Ti, V, x is greater than or equal to 0 and less than or equal to 5, y is greater than or equal to 0 and less than or equal to 5), and the like.

The negative electrode material may include a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector. The negative active material includes, but is not limited to, at least one of the following: metallic lithium, lithium alloys, lithium titanate, natural graphite, artificial graphite, MCMB, amorphous carbon, carbon fiber, carbon nanotubes, hard carbon, soft carbon, graphene oxide, silicon carbon compounds, silicon oxy compounds, and silicon metal compounds.

The performance of the separator itself should be advantageous for the lithium ion secondary battery to achieve good charge and discharge performance.

For example, in order to stably and reliably separate the positive electrode material and the negative electrode material, the separator should have strength and ductility to avoid being punctured, i.e., the separator should have some resistance to mechanical abuse.

In another example, the lithium ion secondary battery itself may generate heat during charge and discharge. At higher temperatures, the separator should also have a relatively high stability, i.e. the separator should have a certain resistance to heat or abuse. On one hand, the membrane breaking temperature of the membrane can be relatively high, and the membrane is not easy to melt when the battery core is overheated. On the other hand, the closed cell temperature of the separator may be relatively low, and the electrochemical path between the positive electrode material and the negative electrode material may be relatively easily cut by the separator when the cell is overheated.

Here, the separator in the embodiment of the present invention is the novel crosslinked separator according to the present invention.

The embodiment of the present invention further provides an electronic device, where the electronic device may be a terminal consumer product or a 3C electronic product (computer (comp μ ter), communication (comm μ communication), or consumer (cons μmer), such as a mobile phone, a mobile power supply, a portable device, a tablet computer, an electronic reader, a notebook computer, a digital camera, a vehicle-mounted device, a wearable device, an earphone, and the like.

The electronic device is a mobile phone as an example. The mobile phone comprises a shell, a display screen and a circuit board assembly. Specifically, the housing includes a bezel and a rear cover. The frame encircles in the periphery of display screen and encircles the periphery at the back lid. The cavity formed among the display screen, the frame and the rear cover can be used for placing the circuit board assembly. In one example, both the display screen and the circuit board assembly may be disposed on the housing. The handset may also include a power supply for powering the circuit board assembly. The power source can be a lithium ion secondary battery, and the novel cross-linked diaphragm is used in the battery.

The embodiment of the invention also provides a manufacturing method of the diaphragm base material, which comprises the following steps:

(1) pre-irradiating the polyolefin composition with the irradiation dose of 0.1-1 Mrad to obtain a pre-irradiated polyolefin composition;

(2) the mixture containing the pore former, the pre-irradiated polyolefin composition, the crosslinking agent and photoinitiator, the polyethylene wax emulsion is mixed and extruded from a screw extruder to form a gel sheet.

The description of the polyolefin composition, the polyethylene wax emulsion, the crosslinking agent and the photoinitiator can be referred to the above description and will not be repeated herein.

Step (2) is explained in detail below.

The step (2) may be simply referred to as an extrusion casting process. The extrusion casting step may be specifically a step of kneading, extruding, casting, and cooling a mixture containing the polyolefin composition, the crosslinking agent, the photoinitiator, and the pore-forming agent by a screw extruder to form a gel sheet.

In one possible example, the mixture may be mixed by a counter-current blender, a twin-shaft blade blender, a twin-pot blender, or the like.

In one possible example, the mixture may be subjected to high-temperature mixing by a single-screw extruder or a twin-screw extruder. To obtain a relatively good extrusion result, the viscosity average molecular weight in the polyolefin composition is 500X 10⁴In the following cases, a twin screw extruder is preferred.

The temperature of the extruder is preferably 150 to 300 ℃, more preferably 160 to 260 ℃, and further preferably 170 to 230 ℃.

It is advantageous to increase the temperature of the extruder to improve the melt plasticizing efficiency (the temperature of the extruder is preferably 150 ℃ or higher, more preferably 160 ℃ or higher, and still more preferably 170 ℃ or higher). The temperature of the extruder is advantageously reduced to prevent oxidative decomposition of the polyolefin composition, photoinitiator and crosslinker mixture (the temperature of the extruder is preferably 300 ℃ or less, more preferably 260 ℃ or less, and even more preferably 230 ℃ or less).

The method for forming the cast sheet may be, for example, a rolling method, a free sheet method or the like.

The thickness of the gel sheet is preferably 200 to 700 μm, and more preferably 250 to 550 μm.

The increase in the thickness of the gel sheet is advantageous for increasing the mechanical strength of the separator base material (the thickness of the gel sheet is preferably 200 μm or more, and more preferably 250 μm or more). The thickness of the gel sheet is reduced, which is beneficial to improving the crosslinking density of the diaphragm substrate in the ultraviolet light irradiation process (the thickness of the gel sheet is preferably less than 700 μm, and more preferably less than 550 μm).

The high-temperature melt casting cooling method may be, for example, a direct contact cooling method such as air cooling, water cooling, oil cooling, or bringing a casting sheet into contact with a cooling roll. In view of controlling the thickness of the gel sheet and improving the uniformity of the separator substrate, the embodiment of the present invention preferably employs a method of cooling by contact with a cooling roller.

The purpose of adding the porogen to the polyolefin composition, crosslinker and photoinitiator mixture is to increase the plasticity of the polyolefin mixture. The pore former may, for example, include at least one of: hydrocarbon organic solvents (such as paraffin wax), 2-ethylhexyl phthalate, dibutyl phthalate, alkyl sulfonate, butyl benzyl phthalate, and diisononyl phthalate. Liquid paraffin is preferred for the embodiments of the present invention.

Alternatively, the porogen may be miscible with the polyolefin composition at any ratio (i.e., an organic solvent that forms a homogeneous phase) under high temperature conditions.

The proportion of the polyolefin composition, the crosslinking agent and the photoinitiator in the mixture is preferably 10-50 parts, more preferably 12-30 parts, and even more preferably 15-25 parts.

It is noted that increasing the ratio of the polyolefin composition among the polyolefin composition, the crosslinking agent and the photoinitiator is advantageous for improving the moldability and the processability thereof (the ratio of the polyolefin composition may be, for example, 15 parts or more). The ratio of the polyolefin composition in the polyolefin composition, the crosslinking agent and the photoinitiator is reduced, and the pore-forming property is advantageously improved (the ratio of the polyolefin composition may be 95 parts or less, for example).

The proportion of the pore-forming agent in the mixture is preferably 50-90 parts, and more preferably 75-85 parts. The addition of a pore former to the mixture is advantageous in providing a relatively complete pore structure in addition to enhancing the plasticizing ability of the polyolefin composition.

Optionally, the mixture further comprises inorganic particles. The inorganic particle may be a pore former.

If inorganic particles are used in step (2) and at least a portion of the inorganic particles are removed in the final product, it is advantageous to obtain a relatively high porosity and thus an increased ion transport efficiency. The manner of removing the inorganic particles may be, for example, using a liquid that can dissolve the inorganic particles.

If the inorganic particles are used in step (2) and at least a part of the inorganic particles are retained in the final product, it is advantageous to improve the stability (e.g., improve the mechanical properties, heat resistance, etc.) and polarity (i.e., improve the affinity of the separator substrate for the electrolyte) of the separator substrate.

The inorganic particles may include, for example, at least one of: alumina, silica, titania, zirconia, zinc oxide, barium oxide, magnesium oxide, beryllium oxide, calcium oxide, thoria, aluminum nitride, titanium nitride, boehmite, apatite, aluminum hydroxide, magnesium hydroxide, barium sulfate, boron nitride, silicon carbide, silicon nitride, cubic boron nitride, hexagonal boron nitride.

The size of the inorganic particles can affect the uniformity of mixing. The particle diameter of the inorganic particles is preferably within a range of 5 to 300nm, more preferably within a range of 10 to 100nm, and further preferably within a range of 20 to 50 nm.

Optionally, the mixture may also include an antioxidant.

To reduce oxidative decomposition of the polyolefin composition, the crosslinker and the photoinitiator mixture, an antioxidant may be added to the mixture. That is, a mixture comprising the polyolefin composition, the crosslinking agent, the photoinitiator, and the antioxidant is kneaded and extruded in a screw extruder.

In one possible example, the antioxidant is preferably present in the mixture in a ratio of 0.1 to 5 parts, more preferably 0.2 to 2 parts.

(3) The gel sheet was biaxially stretched.

The step (3) may be simply referred to as a stretching step. The stretching step may be a step of stretching the gel sheet in a biaxial direction.

The biaxial stretching method may be, for example, asynchronous stretching (sequential biaxial stretching using a combination of a differential roll stretcher and a rail chain tenter, i.e., stretching in the first axial direction first and then stretching in the second axial direction), synchronous stretching (simultaneous stretching using a biaxial tenter, i.e., stretching in the first axial direction and the second axial direction simultaneously). Asynchronous stretching is beneficial to improving the stretching forming efficiency.

The area draw ratio (transverse draw ratio, longitudinal draw ratio, or post-draw area/pre-draw area) in the drawing step is preferably 10 to 200 times, more preferably 15 to 150 times, and still more preferably 20 to 70 times.

The reduction of the area stretch ratio in the stretching step is advantageous in increasing the elongation of the separator base material (the elongation of the separator base material may be 50 times or less, for example). Increasing the area stretch ratio in the stretching step is advantageous in increasing the porosity or the porosity transmittance and in improving the thickness uniformity of the separator base material (the thickness of the separator base material may be 10 times or more, for example).

The stretching temperature in the stretching step should be selected with reference to the solid content of the polyolefin composition (the solid content may be the mass percentage of the remaining part of the polyolefin composition after drying under specified conditions in the total amount).

The stretching temperature in the stretching step is preferably 60 to 110 ℃, more preferably 63 to 108 ℃, and further preferably 65 to 106 ℃.

It is advantageous to increase the stretching temperature in the stretching step to prevent cold stretching due to an excessively low stretching temperature and further to prevent a relatively large stress concentration due to insufficient activation of molecular chains (i.e., a relatively large degree of curing) (the stretching temperature in the stretching step may be, for example, 60 ℃. The stretching temperature in the stretching step is lowered to improve the pore structure of the separator (the stretching temperature in the stretching step may be, for example, 110 ℃.

(4) And removing the pore-forming agent in the gel sheet.

The step (4) may be simply referred to as a pore-forming agent removing process. The pore-forming agent removing process may specifically remove the pore-forming agent in the gel sheet by an extractant. The extractant may dissolve the porogen (which may be a good solvent for the porogen) but is incompatible with the polyolefin material (i.e., the extractant may not dissolve the polyolefin material). The extractant may, for example, comprise at least one of: halogenated hydrocarbons (such as dichloromethane, N-hexane, cyclohexane, etc.), acetone, tetrahydrofuran, ethanol, N-methylpyrrolidone, etc. In the present example, the extractant is preferably dichloromethane.

The pore-forming agent can be removed by immersing the gel sheet in an extracting agent, or spraying an extracting agent on the gel sheet to extract the plasticizer, and finally drying the extracted gel sheet.

(5) And (4) ultraviolet irradiation crosslinking.

The step (5) may be simply referred to as a crosslinking step. The ultraviolet irradiation crosslinking specifically refers to that under a certain temperature condition, ultraviolet high-energy irradiation is carried out on the film, and a crosslinking agent is activated under the catalysis condition of a photoinitiator, so that a chemical three-dimensional network crosslinking is generated on an irradiation layer. The radiation may comprise high energy ultraviolet light having a wavelength in the range below 350 nm. The crosslinking agent may include, for example, at least one of: 1, 6-hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), divinylbenzene, bismaleic acid diacrylate, trimethylolpropane triacrylate (TMP-TA), trimethylolpropane trimethacrylate (TMPTMA), triallyl isocyanurate (TAIC-A), pentaerythritol triacrylate (PETA), pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, and the like; the photoinitiators are divided into free-radical and cationic polymerization photoinitiators, such as benzoin and derivatives (benzoin, benzoin dimethyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether), benzils (diphenylethanone, α -dimethoxy- α -phenylacetophenone), alkylbenzophenones (α, α -diethoxyacetophenone, α -hydroxyalkylphenone, α -aminoalkylphenone), acylphosphorus oxides (aroylphosphine oxide, bisbenzoylphenylphosphine oxide), benzophenones (benzophenone, 2, 4-dihydroxybenzophenone, Michler's ketone), one or more of thioxanthones (thiopropoxythioxanthone, isopropylthioxanthone), diaryliodonium salts, triaryliodonium salts, alkyliodonium salts, cumeneferrocenium hexafluorophosphate, etc.

In some embodiments of the present invention, the polyethylene is converted into an excited state by absorbing ultraviolet energy with the addition of a photoinitiator, hydrogen is abstracted from a polyethylene chain to generate a free radical, and the initiator is added to accelerate crosslinking between polyethylene molecular chains to form a three-dimensional network-structured bulk macromolecule. The structure is non-fusible, loses fluidity, obviously improves heat resistance and has good high-temperature dimensional stability. Because chemical chain bridges are erected among the molecular chains, the physical and mechanical properties are improved, particularly the tensile strength, the rigidity, the wear resistance, the creep resistance and the high-temperature-resistant dimensional stability are improved, and the corresponding elongation at break is reduced. Since the molecular chain of the crosslinked portion is restricted, but it is possible to make a slight vibration near the original position to cancel the impact energy, the impact strength is also improved accordingly.

In the invention, due to the poor ultraviolet penetration capability, only the surface layer of the polyethylene diaphragm can be crosslinked, and the closed pore performance of the inner layer of polyethylene is reserved. The thickness of the crosslinked layer is controlled by controlling the wavelength of ultraviolet rays.

In the invention, the reduction of the wavelength of ultraviolet radiation is beneficial to further improving the crosslinking efficiency of the polyolefin microporous membrane. In addition, the method is favorable for preventing the diaphragm base material from wrinkling and creeping and improving the tensile modulus of the diaphragm base material.

(6) The gel sheet is heat set.

The step (6) may be simply referred to as a heat-setting step. The heat setting process may refer to a low-rate stretching and retracting operation performed on the gel sheet under a certain temperature condition to release stress accumulated in the gel sheet in the previous process, thereby facilitating improvement of the thermal stability of the gel sheet.

The low-ratio stretching (i.e., stretching in the heat-setting step) may specifically mean stretching at a stretching ratio of 3.0 times or less.

It is advantageous to reduce the stretch ratio in the heat-setting step and improve the elongation properties of the gel sheet (the stretch ratio in the heat-setting step is preferably 2.5 times or less, more preferably 2 times or less). It is advantageous to increase the stretch ratio in the heat-setting step to complete the pore structure of the gel sheet (the stretch ratio in the heat-setting step is preferably 1 or more, more preferably 1.2 or more).

The stretching temperature (setting temperature) for the low-ratio stretching is preferably 105 to 135 ℃, more preferably 105 to 130 ℃, and still more preferably 108 to 129 ℃.

It is advantageous to lower the stretching temperature in the heat-setting step to lower the crystallinity of the gel sheet (the stretching temperature in the heat-setting step is preferably 135 ℃ or lower). The stretching temperature in the heat setting step is increased to prevent stress concentration and microcracks from occurring in the gel sheet (the stretching temperature in the heat setting step is preferably 105 ℃ or higher).

The retracting operation may specifically refer to the relaxation of the gel sheet by the retracting track, leaving the gel sheet relaxed or in a semi-free state.

The reduction of the retraction ratio in the retraction operation is advantageous for preventing excessive relaxation, and is further advantageous for increasing the porosity of the gel sheet, and is advantageous for improving the ion transport efficiency (the retraction ratio in the retraction operation is preferably 10% or less, more preferably 4.5% or less, and further preferably 3% or less).

The retraction ratio of the retraction operation is increased, which is beneficial to reducing the internal stress of the gel sheet and improving the thermal shrinkage of the gel sheet (thermal shrinkage can refer to the shrinkage phenomenon under the action of the membrane stress at high temperature) (the retraction ratio of the retraction operation is preferably more than 0.5%, and more preferably more than 1%).

(7) And (5) rolling and slitting.

The step (7) may specifically be to roll and cut the gel sheet. Through the step (7), the separator substrate or the separator provided in the embodiment of the present application can be obtained.

The execution order and the number of execution times of the steps (1) to (7) may not be limited in the embodiment of the present invention.

For example, preferably, the execution order of steps (1) to (7) may be: (1) - (2) - (3) - (4) - (5) - (6) - (7). And (3) is executed before the step (4), which is favorable for perfecting the pore structure of the diaphragm and improving the mechanical strength of the diaphragm.

For another example, the execution sequence of steps (1) to (7) may be: (1) - (2) - (4) - (3) - (5) - (6) - (7).

For another example, the execution sequence of steps (1) to (7) may be: (1) - (2) - (4) - (3) - (6) - (5) - (7).

Step (3) (i.e., the stretching process) may be performed before or after step (4) (i.e., the pore-forming agent removing process), or simultaneously with the process before step (4), or simultaneously with the process after step (4).

The specific embodiment of the invention also provides a manufacturing method of the lithium ion battery. The principle is that the novel cross-linked diaphragm is arranged between a positive electrode material and a negative electrode material (for example, the novel cross-linked diaphragm is assembled according to the sequence of the positive electrode material-diaphragm-negative electrode material or the negative electrode material-diaphragm-positive electrode material); winding a layered member including a positive electrode material, a separator, and a negative electrode material to obtain a wound body; loading the wound body into a battery case; and injecting an electrolyte.

In one possible example, the positive electrode material may be obtained by: mixing a positive electrode active material (such as lithium cobaltate), a conductive agent (such as conductive carbon black, S [ mu ] per-P, SP), a binder (such as polyvinylidene fluoride, polyvinylidene fluoride [ mu ] oxide, PVDF) in a solvent (such as N-methyl pyrrolidone, NMP) according to a mass ratio of 97:1.5:1.5 to form positive electrode slurry; uniformly coating the positive electrode slurry on two sides of a plate (such as an aluminum foil) by coating equipment; drying the anode slurry on the plate by using an oven to remove the solvent; and carrying out cold pressing, splitting and tab welding on the positive electrode material on the plate.

In one possible example, the anode material may be obtained by: mixing a negative electrode active material (such as artificial graphite), a thickening agent (such as carboxymethyl cellulose (CMC)), a binder (such as Styrene Butadiene Rubber (SBR)) in a solvent (such as deionized water) according to a mass ratio of 97:1.3:1.7 to form negative electrode slurry; uniformly coating the negative electrode slurry on two sides of a plate (such as copper foil) by coating equipment; drying the negative electrode slurry on the plate by using an oven to remove the solvent; and carrying out cold pressing, splitting and tab welding on the negative electrode material on the plate.

In one possible example, the membrane may be obtained by: and coating a membrane coating on the surface of the membrane substrate. In the embodiment of the present invention, the thickness of the separator coating layer may be, for example, 0.5 μm to 10 μm.

For example, the separator coating may include an inorganic coating (e.g., a ceramic coating) and an organic coating (e.g., an oily PVDF coating) disposed on the inorganic coating. Among them, the ceramic coating is advantageous in that the heat resistance of the separator can be improved. The PVDF coating has certain adhesive property, and can improve the adhesive force between the diaphragm and the anode material (or between the diaphragm and the cathode material), so that the diaphragm can be more tightly adhered to the anode material or the cathode material, the hardness of the battery cell is further improved, and the passing rate of a needling test of the battery cell is improved. If a bonding gap exists between the diaphragm and the anode material or the cathode material, the hardness of the battery cell is not favorable, and the passing rate of a needling test of the battery cell is also not favorable.

As another example, the separator coating layer may include only an organic coating layer or an organic/inorganic hybrid coating layer, i.e., may be directly coated on the surface of the separator substrate.

The positive electrode material, the diaphragm and the negative electrode material are wound together to form a bare cell. The storage capacity of the bare cell can reach 3.8Ah, and the working voltage of the bare cell can be 3.0-4.43V.

The bare cell is subjected to processes of packaging, baking, liquid injection, formation and the like, and a finished product of the lithium ion battery can be prepared.

In the following examples and comparative examples, the performance parameters were determined as follows:

1. viscosity average molecular weight and molecular weight distribution

a. Sampling: completely dissolving polyolefin materials in organic solvents such as decalin, tetrahydrofuran and the like to prepare a solution with the concentration of 0.5-1.5 mg/mL, standing for a period of time at room temperature, and filtering by adopting a semipermeable membrane.

b. And (3) testing: the viscosity of the polyolefin material was measured by Gel Permeation Chromatography (GPC) at 135 ℃ and the viscosity average molecular weight Mv was calculated from the viscosity [ η ] obtained by the following calculation:

[η]＝6.77×10^-4Mv^0.67

wherein the viscosity average molecular weight of the polypropylene can be calculated according to the following formula:

[η]＝1.10×10^-4Mv^0.8

c. data processing: and (3) drawing a distribution curve of viscosity and viscosity average molecular weight, and reading the molecular weight distribution.

Alternatively, the polyolefin material may be tested several times and the arithmetic mean calculated (calculation of the arithmetic mean is advantageous for reducing the differences introduced by the measurement system).

2. Film thickness

The first method is as follows:

a. sampling:

taking 1X 10 from the diaphragm³mm²Sample (sample area may be, for example,. gtoreq.1.5X 10³mm²) The number of test points depends on the diaphragm (usually not less than 10 points).

b. And (3) testing: the test was carried out by means of a ten-thousandth thickness measuring instrument at 23. + -. 2 ℃.

c. Data processing: and (4) measuring the thickness of each test point and taking the arithmetic mean value.

The second method comprises the following steps:

a. sampling:

for products with width < 200 mm: determining a point every 40mm +/-5 mm along the longitudinal direction (MD), wherein the number of the test points is not less than 10, the number of the test points can be determined according to the width of the diaphragm, and the distance between the measurement starting point and the edge part is not less than 20 mm;

for products with width larger than or equal to 200 mm: and determining a point every 80mm +/-5 mm along the Transverse Direction (TD), wherein the number of the test points is not less than 10, the number of the test points can be determined according to the width of the diaphragm, and the distance from the measurement starting point to the edge part is not less than 20 mm.

b. And (3) testing: each test point is tested by a thickness measuring instrument under the condition of 23 +/-2 ℃, the diameter of a measuring surface is between 2.5mm and 10mm, and the load applied to the test sample by the measuring surface is between 0.5N and 1.0N.

3. Porosity (%)

The first method is as follows:

a. sampling: taking 1X 10 from the diaphragm⁴mm²And (3) sampling.

b. And (3) testing: porosity was measured by density method.

c. Data processing:

the porosity P of the sample as a whole can be calculated by the following formula:

where m can be the sample mass (e.g., obtained by an analytical balance), the skeleton density ρ can be the material true density of the sample, and V can be the volume of the sample.

The second method comprises the following steps:

a. sampling: rectangular test specimens 1 are cut out by means of a 237X 170mm template sampler. The sample is cut as far away as possible from the edge of the membrane (e.g., more than 50mm from the edge of the membrane).

b. And (3) testing: the porosity is measured by densitometry, which involves measuring n (n may be, for example, 9 or more) points of the sample, which may be distributed in an equidistant lattice.

c. Data processing: the porosity Pi of each point can be calculated by the following formula:

wherein mi is the mass of each point, ρ is the skeleton density of the sample (which can be calculated according to the material ratio), and Vi is the total volume of each point (which can be calculated according to the length, width and thickness of the sample);

4. air permeability (s/100cc)

The first method is as follows:

a. sampling: a sample with the diameter of more than or equal to 28mm is cut from the diaphragm.

b. And (3) testing: the test was carried out according to the method specified in JIS P8117-2009. The method specifically comprises the following steps: setting the pressure of the cylinder driving pressure reducing valve to be 0.25MPa, testing the pressure to be 0.05MPa, and selecting JIS according to the testing standard.

c. Data processing: and randomly cutting 6 samples from the full width of the diaphragm, respectively recording the air resistance value of each sample, and calculating the arithmetic mean value of each sample.

The second method comprises the following steps:

a. sampling: 6 square specimens were cut out by means of a 100X 100mm template sampler. The sample is cut as far away as possible from the edge of the membrane (e.g., more than 50mm from the edge of the membrane). Each sample was evenly distributed on the membrane (i.e. the full width of the membrane was averaged to give 6 zones, and 1 sample was cut in each of these 6 zones).

c. Data processing: the air resistance value of each sample is recorded separately, and the arithmetic mean of the air resistance values of the 6 samples is calculated.

5. Puncture strength

The first method is as follows:

a. sampling: and intercepting a sample with the diameter of more than or equal to 45mm from the microporous membrane.

b. And (3) testing: fix the sample on anchor clamps centrally, the test needle is diameter 1 mm's sphere (the material is sapphire), ensures that the sample extends to or surpasses the edge of clamping disk in all directions, confirms that the sample is fixed in on the cyclic annular anchor clamps completely, does not have the phenomenon of skidding. During the test, the membrane was punctured and the machine speed was set at 300 ± 10mm/min until the puncture bat completely ruptured the specimen and the puncture resistance was the maximum force recorded during the test.

c. Data processing: and randomly cutting 6 samples in full width, respectively recording the puncture strength values of the samples, and calculating the arithmetic mean value of the puncture strength values of the samples.

The second method comprises the following steps:

a. sampling: rectangular test specimens of 6 size were cut out by means of a 237X 170mm template sampler. The sample is cut as far away as possible from the edge of the membrane (e.g., more than 50mm from the edge of the membrane). Each sample was evenly distributed on the membrane (i.e. the full width of the membrane was averaged to give 6 zones, and 1 sample was cut in each of these 6 zones).

b. And (3) testing: the test was carried out according to the method specified in the standard ASTM D4833-07. The method specifically comprises the following steps: the test needle head is a spherical needle head with the diameter of 1mm (made of sapphire); fixing the sample on a clamp in the middle, ensuring that the sample extends to or exceeds the edge of a clamping disc in each direction, and confirming that the sample is completely fixed on the annular clamp without slipping; during testing, the speed of the machine is set to be 300 +/-10 mm/min, and the diaphragm is punctured until the test needle head completely breaks the sample; puncture resistance is the maximum force recorded during the test.

c. Data processing: the puncture strength of each sample was recorded separately and the arithmetic mean of the puncture strengths of these 6 samples was calculated.

6. Tensile strength and elongation

The first method is as follows:

a. sampling: on the whole width sample, the diaphragm is cut according to the MD direction and the TD direction respectively to obtain a plurality of strip-shaped samples with the length being more than or equal to 50mm and the width being about 15 +/-0.1 mm (when the MD is tested, the width of the sample can be along the TD direction of the diaphragm, the length of the sample can be along the MD direction of the diaphragm, and when the TD is tested, the width of the sample can be along the MD direction of the diaphragm, and the length of the sample can be along the TD direction of the diaphragm).

b. And (3) testing: and (3) stretching by using a stretcher, wherein the distance between the clamps can be 100 +/-5 mm until the sample is pulled apart, and the stretching speed can be 100 +/-1 mm/min.

c. Data processing: the tensile strength, elongation, of each sample was recorded separately.

The second method comprises the following steps:

a. sampling: rectangular test specimens of 6 size were cut out by means of a 237X 170mm template sampler. The sample is cut as far away as possible from the edge of the membrane (e.g., more than 50mm from the edge of the membrane). Each sample was uniformly distributed on the separator (i.e., the full width of the separator was divided equally in the MD, TD directions of the separator to give 6 zones, and 1 sample was cut out in each of the 6 zones). And then, cutting a strip sample with the length of more than or equal to 150mm and the width of 15 +/-0.1 mm by using a sampling instrument.

b. And (3) testing: the measurement was carried out according to the method specified in GB/T1040.3-2006. The method specifically comprises the following steps: the distance between the clamps can be 100 +/-5 mm, and the stretching speed can be 100 +/-1 mm/min.

c. Data processing: the tensile strength, elongation, and arithmetic mean of the 6 samples were recorded separately for each sample.

7. Tensile modulus

The first method is as follows:

b. And (3) testing: and (3) stretching by using a stretcher, wherein the distance between the clamps can be 100 +/-5 mm, the stretching speed can be 25 +/-1 mm/min, the strain of the starting point can be set to be 0.05%, and the strain of the ending point can be set to be 0.5%.

c. Data processing: the tensile modulus can be calculated by a regression slope method, and the value of the tensile modulus can be equal to the slope of least squares regression linear fitting of a stress-strain curve in Mpa (refer to GB/T1040.1-2018) in an interval of 0.05-0.25% of strain.

The second method comprises the following steps:

8. Pore diameter

a. Sampling: a round sample with a diameter of 15mm was taken with a corresponding tool, and then soaked in a glass dish containing the test solution with tweezers.

b. And (3) testing: the bubble point method was used for the test. The sample was placed in the sample cap and tested according to standard ASTM F316-2011 according to the procedure for a pore size analyzer. Compressed air may be used at low pressure, which may be 80 psi; low purity nitrogen gas can be used at high pressure, the pressure is more than or equal to 350 psi.

c. Data processing: and (4) deriving a test report of the pore size and the pore size distribution of the sample according to the test result.

9. Heat shrinkage at 120 DEG C

a. Sampling: 6 specimens were randomly cut out over the full width. The specific sampling of each sample may include: cutting 100mm along the MD direction of the diaphragm; when the TD direction of the separator is greater than 100mm, the length of the test sample in the TD direction may be 100 mm; when the TD direction of the microporous membrane is less than 100mm, the length of the test sample in the TD direction may be practically defined.

b. And (3) testing: marking the longitudinal and transverse marks of the sample, and measuring and recording the longitudinal and transverse dimensions of each sample; heating the electric heating thermostat to 120 ℃; the sample is flatly placed in the paper jacket layer, and the sample has no folding, wrinkling, adhesion and other conditions; placing a paper sleeve (the number of layers can be 10 for example) holding a sample in a constant-temperature oven flatly in the middle (the door opening time is not more than 3s for example); heating the sample to 120 ℃ by an electric heating thermostat for 1 h; the sample was taken out and cooled to room temperature, and the longitudinal length and the transverse length were measured.

c. Data processing:

calculate the heat shrinkage for each sample:

T＝(L0-L)/L0×100％，

where T may be a sample thermal shrinkage ratio (%), L0 may be a length (mm) of the sample before heating, and L may be a length (mm) of the sample after heating. The arithmetic mean of the sample heat shrinkage was calculated.

10. Measurement of melting Point (. degree. C.) and crystallinity (%)

a. Sampling: the membrane sample was weighed on a balance with an accuracy of 0.01 mg. The mass of the diaphragm sample should be between 5mg and 10 mg. The mass difference between the parallel samples should be within ± 2 mg.

b. And (3) testing: adopting a Differential Scanning Calorimeter (DSC), testing in an N2 atmosphere, firstly heating to a temperature higher than the melting point of the polyolefin by 10 ℃/min and within 30 ℃, preserving heat for 3min to obtain the primary heating crystallinity of the polyolefin, then cooling to a temperature less than or equal to 40 ℃ by 10 ℃/min and preserving heat for 3min, secondly heating to a temperature higher than the melting point of the polyolefin by 10 ℃/min and within 30 ℃ to obtain the secondary heating crystallinity of the polyolefin, and directly reading the melting point temperature.

c. Data processing: calculating the area under the melting endothermic curve (from the beginning of the heating cycle to the generation of the enthalpy of thermal transition) (i.e. integrating the melting endothermic curve) to obtain the value of melting enthalpy, the unit of which is Joule (J); the mass normalized melting enthalpy (Δ Hs) of the sample can be obtained by dividing the melting enthalpy value by the sample mass (g). The crystallinity X (%) of the sample can then be calculated according to the following formula:

degree of crystallinity X (%) -. mass normalized melting enthalpy of sample (Δ Hs)/melting enthalpy of 100% crystalline polyethylene (Δ Hf) × 100%,

wherein the melting enthalpy (Δ Hf) of 100% crystalline polyethylene is 293.8J/g.

11. Needle stick test

a. Sampling: each group took 5 power conversion system (pcs) batteries and marked the center of the cell.

b. And (3) testing: charging the battery cell to a limit voltage of 4.43V at 25 +/-3 ℃ according to a constant current of 1.2A, and then charging at a Constant Voltage (CV) of 4.43V until the current is reduced to 0.025C; after full charge, testing within 12-24 h; and (3) penetrating a steel nail into the central part of the battery core at the speed of 150mm/s at the temperature of 25 +/-3 ℃ until the steel nail penetrates through the battery core, keeping the steel nail for 10min, and withdrawing the steel nail. The diameter of the steel nail is 2.45 +/-0.06 mm, the length of the steel nail is 45 +/-2.5 mm, and the length of the tip can be between 2mm and 4.9 mm.

c. Data processing: observing the experimental phenomenon, judging that the acupuncture does not catch fire or explode after the acupuncture, and judging that the acupuncture passes.

12. Thermal shock test at 130 ℃

a. Sampling: 5pcs of cells were used for each group.

b. And (3) testing: charging the battery cell to a limit voltage of 4.43V at 25 +/-3 ℃ according to a constant current of 1.2A, and then charging at a Constant Voltage (CV) of 4.43V until the current is reduced to 0.025C; after full charge, testing within 12-24 h; heating the battery cell from the initial temperature of 25 +/-3 ℃ in a convection mode or a circulating hot air box, wherein the temperature change rate can be 5 +/-2 ℃/min; heating to 130 + -2 deg.C, and maintaining for 30 min.

c. Data processing: and observing an experimental phenomenon, and judging that the temperature is passed without fire or explosion after the temperature is raised.

13. Thermal shock test at 140 ℃

a. Sampling: 5pcs of cells were used for each group.

b. And (3) testing: charging the battery to a limit voltage of 4.43V at 25 + -3 deg.C with a constant current of 1.2A, and then charging at a Constant Voltage (CV) of 4.43V until the current is reduced to 0.025C; after full charge, testing within 12-24 h; heating the battery from the initial temperature of 25 +/-3 ℃ by a convection mode or a circulating hot air box, wherein the temperature change rate can be 5 +/-2 ℃/min; heating to 140 + -2 deg.C, and maintaining for 30 min.

14. Method for testing closed pore temperature and membrane breaking temperature

A sample of a separator having a length of 30mm and a width of 30mm was cut out, sealed in a metal container having an anode and a cathode, and then injected with a test electrolyte and sealed. The electrolyte for testing was prepared by the following method: lithium salt LiPF6 was mixed with a nonaqueous organic solvent (ethylene carbonate (EC), diethyl carbonate (DEC), Propylene Carbonate (PC), Propyl Propionate (PP), Vinylene Carbonate (VC)) at a mass ratio of 20: 30: 20: 28: 2, and a mass ratio of 8: 92 to prepare a solution.

And connecting the metal bin with a resistance recorder. And (3) putting the metal bin into a 200 ℃ oven, and recording the change of the resistance of the sample in the metal bin along with the temperature. The temperature corresponding to the increase in the resistance of the sample to 1000 ohms was recorded as the closed cell temperature. The temperature at which the resistance of the sample decreased again to 1000 ohms as the temperature increased was recorded as the rupture temperature.

15. Method for testing crosslinking thickness

The first method is as follows:

the mass ratio of the decahydronaphthalene before and after dissolving the diaphragm is the thickness percentage of the cross-linked layer;

the second method comprises the following steps:

the ratio of the increase in the tensile strength after crosslinking to the increase in the tensile strength after complete crosslinking is the thickness percentage of the crosslinked layer.

[ example 1]

The novel crosslinked separator (hereinafter referred to as a separator substrate) provided in example 1 had a viscosity-average molecular weight of 110X 10⁴The polyethylene accounts for 100 parts by weight of the cross-linking agent, the photoinitiator and the antioxidant in the base material of the diaphragm. The crosslinking agent is triallyl isocyanurate, the proportion of the crosslinking agent in the diaphragm base material is 1 part by weight, the photoinitiator is benzophenone, and the proportion of the crosslinking agent in the diaphragm base material is 0.7 part by weight. The antioxidant is 3- (3, 5-di-tert-butyl-4-hydroxyphenyl) isooctyl acrylate, and the proportion of the antioxidant in the diaphragm base material is 0.3 part by weight.

The separator provided in example 1 was obtained through the steps (1) to (7) described in the detailed description.

Specifically, the specific content of step (1) includes: pre-irradiating the high molecular weight polyethylene by using a cobalt source or an electron accelerator to obtain the pre-irradiated high molecular weight polyethylene (the specific parameters of the irradiation dose are shown in tables 1-4).

Specifically, the specific content of step (2) includes: premixing the polyethylene, the triallyl isocyanurate, the benzophenone and the isooctyl 3- (3, 5-di-tert-butyl-4-hydroxyphenyl) acrylate by using a double-shaft blade mixer to obtain a premix; introducing nitrogen into the feeding machine and the double-screw extruder in advance, and then feeding the premix into the double-screw extruder through the feeding machine; preheating liquid paraffin by an oil feeding pump (the preheating temperature of the liquid paraffin is 40 ℃, wherein the viscosity of the liquid paraffin at 40 ℃ can be 28-35 centistokes, centistokes is a kinematic viscosity unit and can be abbreviated as cst, 1cst is 2/s with 1 mm), and feeding the liquid paraffin into a double-screw extruder, wherein the extrusion amount is controlled to be 75-100 kg; the temperature of melt mixing can be 170 ℃, and the screw rotating speed can be 28r/min (rpm, rev μ positions per min μ te); the solid contents of the mixture of polyethylene, triallyl isocyanurate and benzophenone during the mixing process were set by adjusting the delivery rate of the oil feed pump (see table 1). Finally obtaining a blend; the blend was extruded through a T die and cast cooled to obtain a gel sheet (thickness of gel sheet as in tables 1-4).

Specifically, the specific content of step (3) includes: the gel sheet was placed in an asynchronous stretching machine and biaxially stretched (specific parameters of biaxial stretching are shown in tables 1 to 4).

Specifically, the specific content of step (4) includes: the stretched gel sheet was extracted with methylene chloride to remove the liquid paraffin in step (2).

Specifically, the specific content of step (5) includes: and (3) carrying out ultraviolet high-energy irradiation on the extracted gel sheet (the specific parameters of ultraviolet crosslinking are shown in tables 1-4).

Specifically, the specific content of step (6) includes: and (3) carrying out heat setting on the gel sheet after the ultraviolet crosslinking (the specific parameters of the heat setting are shown in the table 1-4).

Specifically, the specific content of step (7) includes: and continuously slitting and winding the heat-set gel sheet.

Other specific parameters of example 1 are referenced in table 1 below.

[ example 2]

Example 2 provides a separator including a separator substrate and a separator coating layer.

The raw material of the diaphragm substrate except the cross-linking agent was 1.5 parts, and the other raw materials were selected and contained in the same manner as in example 1.

The diaphragm coating is a heat-resistant coating and a bonding coating. The heat-resistant coating is Al₂O₃. The bond coat is an oily PVDF.

The separator provided in example 2 can be obtained by the steps (1) to (6) described above and the following step (8). For the details of step (1) to step (6), reference may be made to the above-mentioned embodiment 1, and detailed description is not necessary here.

And (8) arranging a diaphragm coating on the rolled gel sheet (namely the diaphragm substrate).

The specific content of step (8) may include: sending the diaphragm base material into a coating device, and coating a heat-resistant coating on the diaphragm base material by adopting a micro-gravure coating mode (namely, carrying out primary coating); conveying the diaphragm containing the heat-resistant coating into a drying box, and drying the diaphragm by adopting hot air (namely performing primary drying); the method comprises the steps of coating an adhesive coating on the surface of a diaphragm coated with a heat-resistant coating by adopting an oil system micro-gravure coating mode (namely, carrying out secondary coating), conveying the diaphragm containing the adhesive coating into a drying box, drying the diaphragm by adopting hot air (namely, carrying out secondary drying), and putting the diaphragm after the secondary drying into a winding device for winding to obtain a diaphragm finished product.

Other specific parameters of example 2 are referenced in table 1 below.

[ example 3]

Example 3 provides a separator comprising a separator substrate and a separator coating.

The raw materials of the diaphragm substrate were selected and contained in the same manner as in example 1, except that the content of the crosslinking agent in the diaphragm substrate was 2.0 parts.

The manufacturing method and other specific parameters of the separator provided in example 3 can be referred to in example 2 and table 1, and detailed description thereof is not necessary.

[ example 4]

Example 4 provides a separator including a separator substrate and a separator coating.

The raw material selection and the proportion of the separator substrate were the same as those in example 2.

The manufacturing method and other specific parameters of the separator provided in example 4 can be referred to in example 2 and table 1, and detailed description thereof is not necessary.

[ example 5]

Example 5 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 5 can be referred to in example 2 and table 1, and detailed description thereof is not necessary.

[ example 6]

Example 6 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 6 can be referred to in example 2 and table 1, and detailed description thereof is not necessary. In example 6, a single-sided heat-resistant coating and an aqueous PVDF micro-gravure coating were used.

TABLE 1 specific parameters of examples 1-6

[ example 7]

Example 7 provides a separator comprising a separator substrate.

The raw materials of the diaphragm substrate are selected from the same example 2 except that the polyethylene wax emulsion with the melting point of the solid particles of 110-120 ℃ is added.

The manufacturing method and other specific parameters of the separator provided in example 7 can be referred to in example 2 and table 2, and detailed description thereof is not necessary.

[ example 8]

Example 8 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 8 can be referred to in example 2 and table 2, and detailed description thereof is not necessary.

[ example 9]

Example 9 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 9 can be referred to example 2 and table 2, and detailed description thereof is not necessary. The specific processing parameters of example 9 are different from those of example 2 (including, for example, extrusion amount, thickness of the gel sheet, and stretch ratio of biaxial stretching).

[ example 10]

Example 10 provides a separator including a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 10 can be referred to in example 2 and table 2, and detailed description thereof is not necessary. It should be noted that the specific processing parameters of example 10 are different from the specific processing parameters of example 2 (including, for example, polymer solids content, etc.).

[ example 11]

Example 11 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 11 can be referred to example 2 and table 2, and detailed description thereof is not necessary. The specific processing parameters of example 11 are different from those of example 2 (including, for example, extrusion amount, thickness of the gel sheet, and stretch ratio of biaxial stretching).

[ example 12]

Example 12 provides a separator comprising a separator substrate and a separator coating.

The manufacturing method and other specific parameters of the separator provided in example 12 can be referred to in example 11 and table 2, and detailed description thereof is not necessary.

TABLE 2 specific parameters for examples 7-12

[ examples 13 to 15]

Examples 13-15 provide a separator including a separator substrate and a separator coating.

Examples 13-15 provide methods for making the separator and other specific parameters that can be found in example 11 and table 3 and need not be described in detail herein.

[ examples 16 to 18]

Examples 16-18 provide a separator that includes a separator substrate and a separator coating.

The raw materials of the diaphragm substrate are selected and added with the polyethylene wax emulsion with the melting point of the solid particles of 80-90 ℃, and the selection and the proportion of the rest are the same as those in the embodiment 2.

Examples 16 to 18 provide a method for manufacturing a separator and other specific parameters, which can be found in example 11 and table 4 and will not be described in detail.

Comparative examples 1 to 3

Comparative examples 1-3 provide separators comprising a separator substrate and a separator coating.

Comparative example 1 provides a separator having a viscosity average molecular weight of 110X 10⁴The polyethylene of (1).

The separator provided in comparative example 1 was obtained by the following embodiment.

Specifically, the specific content of step (1) includes: introducing nitrogen into polyethylene in a feeding machine and a double-screw extruder in advance by using a double-shaft blade mixer, and then feeding the polyethylene into the double-screw extruder through the feeding machine; preheating liquid paraffin by an oil feeding pump (the preheating temperature of the liquid paraffin is 40 ℃, wherein the viscosity of the liquid paraffin at 40 ℃ can be 28-35 centistokes, centistokes is a kinematic viscosity unit and can be abbreviated as cst, 1cst is 2/s with 1 mm), and feeding the liquid paraffin into a double-screw extruder, wherein the extrusion amount is controlled to be 75-100 kg; the temperature of melt mixing can be 170 ℃, and the screw rotating speed can be 28r/min (rpm, rev μ positions per min μ te); the solid content of the polyethylene during the mixing was set by adjusting the delivery rate of the oil feed pump (see table 3). Finally obtaining a blend; the blend was extruded through a T-die and cast cooled to obtain a gel sheet (thickness of gel sheet as in table 3).

Specifically, the specific content of step (2) includes: the gel sheet was set in an asynchronous stretcher and subjected to biaxial stretching (specific parameters of biaxial stretching are shown in table 1).

Specifically, the specific content of step (3) includes: the stretched gel sheet was extracted with methylene chloride to remove the liquid paraffin in step (1).

Specifically, the specific content of step (4) includes: the gel sheet after uv crosslinking was heat-set (specific parameters of heat-set are shown in table 1).

Specifically, the specific content of step (5) includes: and continuously slitting and winding the heat-set gel sheet.

Other specific parameters of comparative example 1 are referred to in table 3 below.

Comparative examples 2 to 6 provide a method of manufacturing the separator and other specific parameters, which are referred to in comparative example 1 and table 3, and detailed description thereof is not necessary.

Comparative examples 7 to 9

Comparative examples 7-9 provide separators comprising a separator substrate and a separator coating.

The raw material selection and the proportion of the diaphragm substrate were the same as those of comparative example 1.

The manufacturing method and other specific parameters of the separators provided in comparative examples 7 to 9 can be referred to example 11 and table 4, and detailed description thereof is not necessary.

The membrane rupture temperatures of the 5 μm membranes of examples 1 to 6 were all above 175 ℃.

The embodiment 1-3 shows that the improvement of the proportion of the cross-linking agent is beneficial to the improvement of the cross-linking density of the diaphragm, and after the proportion of the cross-linking agent is more than 1.5, the cross-linking density of the diaphragm has an inflection point; further improving the proportion of the cross-linking agent, possibly generating the problem of uneven melt plasticization, reducing the film surface quality and also possibly improving the closed pore temperature of the diaphragm. This means that, for the overall performance of the separator, the higher the proportion of crosslinking agent is, the better.

It can be seen from examples 4-6 that the ultraviolet wavelength has a crucial effect on the crosslinking density, the longer the ultraviolet wavelength is, the stronger the radiation capability is, the more the energy is carried, but the weaker the penetration capability is, only the surface of the diaphragm can be crosslinked, the core layer of the diaphragm is not crosslinked, the smaller the crosslinking density of the diaphragm is, the crosslinking density of the diaphragm is difficult to increase, and further the high-temperature thermal stability of the diaphragm is poor. However, the low-temperature closed-cell temperature of the diaphragm is relatively low because only the superficial polyolefin of the surface layer is crosslinked and the core layer still maintains the low-melting-point performance of the polyolefin.

It can be seen from examples 6 to 7 and 12 that the coating process of the separator (e.g., adjustment of conditions such as coating materials and systems) can affect the cell performance index of the separator, and the uniformity of the separator performance can be improved.

It can be seen from examples 7 to 9 that the crosslinking density of the separator is significantly increased by increasing the irradiation time, and the crosslinking density of the separator can be controlled by controlling the irradiation time. And simultaneously, the mechanical property and the thermal stability of the diaphragm are improved. However, the closed cell temperature of the separator also increases, which is disadvantageous in terms of the safety of the battery.

As can be seen from examples 10 to 12, increasing the power of the high-pressure mercury lamp can further increase the crosslinking density and crosslinking rate of the separator, and thus a suitable separator can be obtained by matching the power and linear velocity of the high-pressure mercury lamp. However, as the power of the high-pressure mercury lamp is increased, the heat generated by the mercury lamp has a large influence on the appearance of the diaphragm, and the quality of the diaphragm is reduced. And with the increase of the crosslinking density, the fracture elongation of the diaphragm is sharply reduced, and the needling pass rate of the battery cell is further influenced.

The polyolefin composition, the cross-linking agent and the photoinitiator are mixed according to a specific mode, the traditional wet diaphragm manufacturing process is adopted, and proper ultraviolet rays are adopted for radiation, so that the cross-linking of the polyolefin on the surface of the diaphragm is realized, the internal core layer can keep the low-temperature hole closing performance, and the diaphragm with high diaphragm breaking temperature, low hole closing temperature, high puncture strength and good high-temperature thermal stability can be formed.

TABLE 3 concrete parameters of examples 13 to 15 and comparative examples 1 to 3

It can be seen from embodiments 13 to 15 that increasing the thickness of the crosslinked separator can increase the tensile strength and elongation of the separator, and at the same time, increase the thermal shock pass rate of the separator in the battery cell, so that a suitable separator application scenario can be obtained by matching the thickness of the separator.

As can be seen from comparative examples 1-3, the conventional diaphragm has similar closed cell temperature under different thicknesses, the closed cell temperature of the uncrosslinked polyethylene diaphragm is between 138 ℃ and 141 ℃, and the rupture temperature is between 150 ℃ and 160 ℃, and compared with the embodiment, the diaphragm with excellent rupture characteristics can be obtained by controlling the crosslinking density of the polyethylene.

It can be seen from comparative examples 4-6 that, in the case of the crosslinked separator not subjected to pre-irradiation, the mechanical properties were inferior to those of the irradiated separator but stronger than those of the uncrosslinked base film. The closed pore temperature and the film breaking temperature are lower than those of the pre-irradiation diaphragm and higher than those of the uncrosslinked base film.

TABLE 4 specific parameters of examples 16-18 and comparative examples 4-6

It can be seen from examples 16 to 18 that increasing the molecular weight of polyethylene can increase the tensile strength and elongation of the separator, and improve the film breaking temperature, and the higher the molecular weight, the lower the crosslinking density, and the higher the film breaking temperature, so that a suitable application scenario of the separator can be obtained by matching the molecular weight of polyethylene.

It can be seen from comparative examples 7 to 9 that in the case of producing conventional separators with different polyethylene molecular weights, the pore closing temperature is positively correlated with the molecular weight, and separators having excellent rupture characteristics can be obtained by controlling the polyethylene crosslink density as compared with the examples.

According to tests, the novel cross-linked diaphragm with the thickness of 5-6 microns provided by the invention can have the film breaking temperature of 191-236 ℃ and the hole closing temperature of 95-121 ℃. Compared with a non-crosslinked diaphragm, the diaphragm provided by the embodiment of the application has relatively high diaphragm breaking temperature and relatively wide closed-cell diaphragm breaking platform. Moreover, the membrane has relatively high tensile modulus and tensile strength, which is beneficial to processing of the membrane in a cell process (for example, beneficial to avoiding the problems of edge protrusion, winding deviation, bending, wrinkling and the like caused by high elongation characteristics) and the needling passing rate of the cell. The diaphragm provided by the application is beneficial to improving the mechanical abuse resistance and heat abuse resistance of the battery, and the comprehensive performance of the diaphragm is very excellent.

The data in the data table are protected and supported reasonably based on the problem of reserving decimal digits, for example, the thickness of the novel crosslinked diaphragm is 5-6 μm. The crosslinking agent ratio in the present invention is a ratio of parts by weight of the crosslinking agent to parts by weight of the polyolefin composition.

The above matters related to the common general knowledge are not described in detail and can be understood by those skilled in the art.

The present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. A novel cross-linked membrane is characterized in that: comprises an ultraviolet cross-linked upper surface layer, a core layer and an ultraviolet cross-linked lower surface layer; the ultraviolet cross-linked upper surface layer or the ultraviolet cross-linked lower surface layer comprises a cross-linking agent and a photoinitiator, and the core layer comprises a polyolefin composition; the thickness of the novel cross-linked diaphragm is 0.5-12 mu m; the thickness of the ultraviolet cross-linked upper surface layer and the ultraviolet cross-linked lower surface layer is 20-80% of that of the novel cross-linked diaphragm; the membrane breaking temperature of the novel cross-linked membrane is 191-215 ℃.

2. The novel crosslinked separator according to claim 1, characterized in that: the polyolefin composition comprises a first polyethylene and a second polyethylene, wherein the first polyethylene and the second polyethylene are selected from any one of the following: polyethylene, polyethylene-propylene copolymer, derivative of polyethylene-propylene copolymer, polyethylene-butene copolymer, derivative of polyethylene-butene copolymer, polyethylene-hexene copolymer, derivative of polyethylene-hexene copolymer, polyethylene-octene copolymer, derivative of polyethylene-octene copolymer, polystyrene-ethylene-styrene copolymer, derivative of polystyrene-ethylene-styrene copolymer, polystyrene-ethylene-butene-styrene copolymer, derivative of polystyrene-ethylene-butene-styrene copolymer, polyethylene-hydrogenated oligocyclopentadiene, derivative of polyethylene-hydrogenated oligocyclopentadiene, polyethylene oxide, derivative of polyethylene oxide, and polyethylene oxide, Polypentene-ethylene copolymers, derivatives of polypentene-ethylene copolymers, polyhexene-ethylene copolymers, derivatives of polyhexene-ethylene copolymers, polymethylpentene-ethylene copolymers, derivatives of polymethylpentene-ethylene copolymers.

3. The novel crosslinked separator according to claim 1, characterized in that: the polyolefin composition also comprises polypropylene or derivatives thereof, wherein the enthalpy delta Hm of the polypropylene or the derivatives thereof is 55-85J/g, and the density is more than or equal to 0.9g/cm³。

4. The novel crosslinked separator according to claim 1, wherein the crosslinking agent is a bifunctional or polyfunctional monomer with an ethylenic double bond comprising at least one of: 1, 6-hexanediol diacrylate, neopentyl glycol diacrylate, divinylbenzene, bismaleic acid diacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, triallyl isocyanurate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate.

5. The novel crosslinked separator of claim 1, wherein the photoinitiator is a free radical polymerization photoinitiator or a cationic polymerization photoinitiator comprising at least one of: benzoin, benzoin bis methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether, diphenylethanone, alpha-dimethoxy-alpha-phenylacetophenone, alpha-diethoxyacetophenone, alpha-hydroxyalkylphenone, alpha-aminoalkylphenone, aroylphosphine oxide, bisbenzoylphenylphosphine oxide, benzophenone, 2, 4-dihydroxybenzophenone, Michler's ketone, thiopropoxythioxanthone, isopropylthioxanthone, diaryliodonium salts, triaryliodonium salts, alkyliodonium salts, isopropylbenzocene ferrocenehexafluoro phosphate.

6. The novel crosslinked separator according to claim 1, characterized in that: the thickness of the novel cross-linked diaphragm is 3-9 mu m; the thickness of the ultraviolet cross-linked upper surface layer and the ultraviolet cross-linked lower surface layer is 20-75% of that of the novel cross-linked diaphragm; the closed pore temperature is 95-150 ℃.

7. The novel crosslinked separator of claim 6, wherein: the thickness of the novel cross-linked diaphragm is 5-6 mu m; the core layer also comprises a polyolefin emulsion; the closed pore temperature of the novel cross-linked diaphragm is 95-121 ℃.

8. The novel crosslinked separator according to claim 1, characterized in that: the crosslinking density of the novel crosslinked diaphragm is 12-72%; the difference value between the closed pore temperature and the membrane breaking temperature of the novel cross-linked membrane is 49-103 ℃.

9. The novel crosslinked separator according to claim 1, characterized in that: the novel cross-linked membrane has the air permeability of 150-155 s/100 cc.

10. The novel crosslinked separator according to claim 1, characterized in that: the transverse and longitudinal elongation of the novel cross-linked diaphragm is 80-120%.

11. The novel crosslinked separator according to claim 1, characterized in that: the transverse and longitudinal tensile strength of the novel cross-linked diaphragm is more than or equal to 2000kgf/cm²And the transverse tensile modulus and the longitudinal tensile modulus are both more than or equal to 2000 Mpa.

12. The novel crosslinked separator according to claim 1, characterized in that: the novel cross-linked membrane has a puncture strength of 650 to 1400 gf.

13. The novel crosslinked separator according to claim 1, characterized in that: the transverse and longitudinal heat shrinkage rates of the novel cross-linked diaphragm are less than or equal to 1.8% at 120 ℃.

14. The novel crosslinked separator according to claim 1, characterized in that: the polyolefin composition has a viscosity average molecular weight in the range of 30X 10⁴～1000×10⁴In the meantime.

15. The novel crosslinked separator according to claim 1, characterized in that: and one side or two sides of the novel cross-linked diaphragm are provided with coatings, and the coatings comprise one or more of organic coatings, inorganic coatings and organic/inorganic composite coatings.

16. The novel crosslinked separator of claim 15, wherein the inorganic coating comprises a ceramic coating comprising at least one of: alumina, silica, titania, zirconia, zinc oxide, barium oxide, magnesium oxide, beryllium oxide, calcium oxide, thoria, aluminum nitride, titanium nitride, boehmite, apatite, aluminum hydroxide, magnesium hydroxide, barium sulfate, boron nitride, silicon carbide, silicon nitride, cubic boron nitride, hexagonal boron nitride, mesoporous molecular sieves, pearl mica layers.

17. The novel crosslinked separator of claim 15, wherein the organic coating comprises at least one of: polyvinylidene fluoride coatings, vinylidene fluoride-hexafluoropropylene copolymer coatings, polystyrene coatings, aramid coatings, polyacrylate or its modifications coatings, polyester coatings, polyarylate coatings, polyacrylonitrile coatings, aromatic polyamide coatings, polyimide coatings, polyethersulfone coatings, polysulfone coatings, polyetherketone coatings, polyetherimide coatings, polybenzimidazole coatings, polydopamine.

18. A battery comprising a positive electrode, a negative electrode, an electrolyte and the novel crosslinked separator according to any one of claims 1 to 17.

19. An electronic device, characterized in that: comprising a housing, a display screen housed in the housing, a circuit board assembly, and the battery of claim 18, the battery supplying power to the display screen and the circuit board assembly.

20. The preparation method of the novel cross-linked membrane is characterized by comprising the following steps:

s5, crosslinking the gel sheet by ultraviolet irradiation surface layer;

s6, rolling and cutting the gel sheet to form the novel cross-linked diaphragm.

21. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the polyolefin composition of S1 comprises one or more of polyethylene, polyethylene copolymer, polypropylene or polypropylene derivative having different viscosity average molecular weights.

22. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the S2 raw material mixture also comprises a polyolefin emulsion.

23. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the weight of the cross-linking agent is 1-1.5 parts and the weight of the photoinitiator is 0.7-1.2 parts based on 100 parts of the polyolefin composition.

24. The method for producing the novel crosslinked separator according to claim 22, characterized in that: the polyolefin emulsion is polyethylene wax emulsion, the solid content of the polyolefin emulsion is 10-70%, and the melting point of solid particles is 80-120 ℃.

25. The method for producing the novel crosslinked separator according to claim 20, characterized in that: in S1, a cobalt source or an electron accelerator is used for pre-irradiation.

26. The method for producing the novel crosslinked separator according to claim 20, characterized in that: in S3, the area after stretching/the area before stretching is 10 to 50 times in the biaxial stretching.

27. The method for producing the novel crosslinked separator according to claim 20, characterized in that: s4, stretching the film by 1-3 times at a low-magnification stretching temperature of 105-135 ℃; the retraction ratio of the retraction operation is 0.5-20%.

28. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the wavelength range of the ultraviolet rays in S5 is between 230 nm and 350 nm.

29. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the ultraviolet radiation time in S5 is 0.2-1S.

30. The method for producing the novel crosslinked separator according to claim 20, characterized in that: the ultraviolet radiation power in S5 is 1-4 Kw.