阅读说明：本技术 一种低内应力锂离子电池隔膜及制备方法 (Low-internal-stress lithium ion battery diaphragm and preparation method thereof ) 是由 赵洪亮 刘涛涛 沈亚定 于 2021-09-01 设计创作，主要内容包括：本发明提供一种低内应力锂离子电池隔膜的制备方法,在聚乙烯原料中加入添加剂,匹配相应的工艺设计,来达到降低隔膜内应力的目的；聚乙烯原料选用超高分子量聚乙烯、高密度聚乙烯、聚苯乙烯混合而成；在超高分子量聚乙烯、高密度聚乙烯、白油混熔时,添加聚苯乙烯、增强纤维和成核剂进行混熔,从配方设计上降低可能存在的微观内应力；聚苯乙烯为改性聚苯乙烯；将横拉烘箱的结构设计为单独控制7个加热区的温度,按照由中心对称向两侧递增的方式对隔膜进行加热；本发明制备的低内应力锂离子电池隔膜是聚乙烯微孔隔膜,厚度为5-20μm,孔隙率在30-50％,透气值在130-300,孔径为0.01-0.1μm,热收缩低于3,拉伸强度高于2000MPa。(The invention provides a preparation method of a low internal stress lithium ion battery diaphragm, which is characterized in that an additive is added into a polyethylene raw material, and a corresponding process design is matched, so that the purpose of reducing the internal stress of the diaphragm is achieved; the polyethylene raw material is prepared by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; when the ultrahigh molecular weight polyethylene, the high density polyethylene and the white oil are mixed and melted, the polystyrene, the reinforcing fiber and the nucleating agent are added for mixing and melting, and the possible microscopic internal stress is reduced from the formula design; the polystyrene is modified polystyrene; the structure of the transverse pulling oven is designed to independently control the temperature of 7 heating zones, and the diaphragm is heated in a mode of increasing from central symmetry to two sides; the low internal stress lithium ion battery diaphragm prepared by the invention is a polyethylene microporous diaphragm, the thickness is 5-20 μm, the porosity is 30-50%, the ventilation value is 130-300, the pore diameter is 0.01-0.1 μm, the thermal shrinkage is less than 3, and the tensile strength is higher than 2000 MPa.)

1. A preparation method of a low internal stress lithium ion battery diaphragm is characterized by comprising the following steps: the manufacturing method comprises the following steps:

(1) mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

2. The preparation method of the low internal stress lithium ion battery separator according to claim 1, characterized in that: the polyethylene raw material in the step (1) is prepared by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the molecular weight of the ultra-high molecular weight polyethylene is 1000000-4000000, and the molecular weight of the high density polyethylene is 300000-800000; the additives are reinforcing fibers and nucleating agents.

3. The preparation method of the low internal stress lithium ion battery separator according to claim 2, characterized in that: in the step (2), the content of the components in percentage by weight is 65-75% of white oil, 10-15% of ultrahigh molecular weight polyethylene, 10-15% of high density polyethylene, 0.1-1% of polystyrene, 0.1-1% of reinforcing fiber and 0.1-1% of nucleating agent.

4. The preparation method of the low internal stress lithium ion battery separator according to claim 1, characterized in that: in the step (2), the rotating speed of the screw of the extruder is 30-40rpm, the extrusion temperature is 150-250 ℃, and the temperature of the casting sheet roller is 15-40 ℃.

5. The preparation method of the low internal stress lithium ion battery separator according to claim 1, characterized in that: the stretching temperature of the longitudinal stretching in the step (3) is 50-120 ℃, and the stretching ratio of the longitudinal stretching is 5-9.

6. The preparation method of the low internal stress lithium ion battery separator according to claim 1, characterized in that: the primary transverse stretching temperature in the step (4) is 90-140 ℃; the secondary transverse stretching temperature in the step (6) is 120-140 ℃, the secondary transverse stretching ratio is 1.2-2, and the heat setting temperature in the step (7) is 80-100 ℃.

7. The preparation method of the low internal stress lithium ion battery separator according to claim 1, characterized in that: transversely stretching the primary transverse stretching in the step (4) and the secondary transverse stretching in the step (6) by using a transverse stretching oven; seven air ports are formed in the upper row of hot air ports and the lower row of hot air ports of the transverse-pulling oven in the TD direction, the seven air ports are correspondingly divided into 7 heating areas, the temperature of each area can be independently controlled, and the diaphragm in the transverse-pulling oven is heated in a mode that the temperature is gradually increased from the central symmetry to the two sides.

8. The preparation method of the low internal stress lithium ion battery separator according to claim 2, characterized in that: the additives in the step (1) are reinforcing fibers, nucleating agents and graphene oxide quantum dots; the reinforced fiber is nano fiber crystal, and the polystyrene is modified polystyrene.

9. The method for preparing the low internal stress lithium ion battery separator according to claim 8, wherein the method comprises the following steps: the preparation steps of the modified polystyrene are as follows: ultrasonically stirring potassium persulfate, styrene and divinylbenzene, reacting for 7-9h at 68-72 ℃, washing with centrifugal water and filtering; then adding into sulfuric acid solution, stirring at 30-50 ℃, washing with water to neutrality, filtering, adding into mixed solution of ethanol, n-heptane and deionized water, reacting at 68-72 ℃ for 8-10h, centrifuging, washing with alcohol, filtering, and vacuum drying to obtain the modified polystyrene.

10. A low internal stress lithium ion battery diaphragm is characterized in that: prepared by the method of any one of the preceding claims 1 to 9, wherein the membrane has a thickness of 5 to 20 μm, a porosity of 30 to 50%, a permeability of 130 to 300 and a pore size of 0.01 to 0.1 μm.

Technical Field

The invention relates to the field of battery diaphragms, in particular to a low-internal-stress lithium ion battery diaphragm and a preparation method thereof.

Background

A lithium ion battery is a secondary battery that operates by movement of lithium ions between a positive electrode and a negative electrode; lithium ion batteries are used in mobile phones and notebook computers used in daily life, and are generally called as lithium batteries; the electrode of the battery generally adopts a material containing lithium element, and is representative of modern high-performance batteries.

The lithium ion battery is composed of a positive electrode material, a negative electrode material, electrolyte and a diaphragm. The diaphragm plays a role in isolating the positive pole and the negative pole, is an important component of the lithium ion battery, so that the lithium ion battery can achieve the effect of limiting the rise of current under the condition of overcharge or temperature rise, and the explosion caused by the short circuit of the battery is prevented, so that the diaphragm has higher requirements on the mechanical property and the thermal strength of the battery diaphragm.

The internal stress of the battery diaphragm is overlarge, so that the shrinkage of the battery diaphragm is large, the size of the battery diaphragm is reduced, the diaphragm is wrinkled or rolled to form a rib, an inverted triangle and the like, and the deformation of the membrane surface of the battery diaphragm is poor, so that the subsequent slitting of the battery diaphragm and the winding of a battery are influenced; the defects can cause the production yield of the battery diaphragm to be greatly reduced, the performance and the safety of the lithium battery are finally affected, the yield of the battery is reduced, the coiling is difficult, and even the danger of short circuit and explosion of the battery is generated.

Disclosure of Invention

The invention aims to provide a low-internal-stress lithium ion battery diaphragm and a preparation method thereof, which aim to solve the problems in the prior art.

In order to solve the technical problems, the invention provides the following technical scheme:

the preparation method of the low internal stress lithium ion battery diaphragm comprises the following steps:

(1) mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally stretched film in the step (3) to obtain a primary transversely stretched film;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the film extracted in the step (5) to obtain a film subjected to secondary transverse stretching;

(7) heat setting: performing heat setting treatment on the film subjected to secondary transverse pulling in the step (6) to obtain a heat-set film;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Further, the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the molecular weight of the ultrahigh molecular weight polyethylene is 1000000-4000000, and the molecular weight of the high density polyethylene is 300000-800000; the ultrahigh molecular weight polyethylene and the high density polyethylene are selected because the molecular weight is large, the acting force and the entanglement degree among macromolecular chains are increased along with the increase of the molecular weight, so that the internal stress of the diaphragm is reduced, and the polystyrene is dispersed in a similar bead shape in a PE continuous phase, so that the internal stress can be dispersed and relieved along a spherical surface, and the purpose of reducing the internal stress is achieved.

Further, the additives in the step (1) are reinforcing fibers and nucleating agents;

further, in the step (2), the content of the components in percentage by weight is 65-75% of white oil, 10-15% of ultrahigh molecular weight polyethylene, 10-15% of high density polyethylene, 0.1-1% of polystyrene, 0.1-1% of reinforcing fiber and 0.1-1% of nucleating agent; when the ultrahigh molecular weight polyethylene, the high density polyethylene and the white oil are mixed and melted, the polystyrene, the reinforcing fiber and the nucleating agent are added in a certain proportion for mixing and melting, so that the possible microscopic internal stress is reduced from the formula design;

the reinforced fiber can be entangled with a large number of macromolecular chains, so that the internal stress of the diaphragm is reduced; sorbitol is selected as the nucleating agent, so that a plurality of small spherulites can be formed in the diaphragm, the internal stress is dispersed, and the effect of reducing the internal stress of the diaphragm is achieved.

Further, the screw rotating speed of the extruder is 30-40rpm, the extrusion temperature is 150-250 ℃, and the temperature of the casting sheet roller is 15-40 ℃.

Further, the stretching temperature of the longitudinal stretching is 50-120 ℃, and the stretching ratio of the longitudinal stretching is 5-9.

Further, the temperature of primary transverse stretching is 90-140 ℃; the secondary transverse stretching temperature is 120-140 ℃, the secondary transverse stretching ratio is 1.2-2, the molecular activation energy is increased at high temperature, and the molecular chain structure is conveniently oriented under the action of transverse stretching force, so that a better stretching effect is obtained in the TD direction.

Further, transversely stretching in the transverse stretching oven in the step (4) and transversely stretching in the transverse stretching oven in the step (6) for the second time;

the transverse-pulling oven is designed to be provided with seven air ports in the TD direction, the seven air ports are correspondingly divided into 7 heating zones, the temperature of each zone can be independently controlled, the diaphragm in the transverse-pulling oven is heated in a mode of increasing from central symmetry to two sides, necking during heating is reduced, stretching uniformity of the diaphragm in the TD direction is improved, and therefore internal stress caused by uneven stretching is reduced;

in the one-time transverse stretching process in the step (4), the diaphragm sequentially passes through 7 heating zones of a transverse-pulling oven, wherein the temperatures of the 7 heating zones are 112 ℃, 112.5 ℃, 113 ℃, 113.5 ℃, 113 ℃, 112.5 ℃ and 112 ℃;

in the secondary transverse stretching process in the step (6), the diaphragm sequentially passes through 7 heating zones of a transverse pulling oven, and the temperatures of the 7 heating zones are 132 ℃, 132.5 ℃, 133 ℃, 133.5 ℃, 133 ℃, 132.5 ℃ and 132 ℃.

Further, the heat setting temperature is 80-100 ℃.

The low internal stress lithium ion battery diaphragm prepared by the invention is a polyethylene microporous diaphragm, the thickness is 5-20 μm, the porosity is 30-50%, the ventilation value is 130-300, the pore diameter is 0.01-0.1 μm, the thermal shrinkage is less than 3, and the tensile strength is higher than 2000 MPa.

Further, in the step (1), the additives are graphene oxide quantum dots, reinforcing fibers and nucleating agents;

further, the reinforced fiber is nano fiber crystal, the polystyrene is modified polystyrene, and the preparation steps of the modified polystyrene are as follows: ultrasonically stirring potassium persulfate, styrene and divinylbenzene, reacting for 7-9h at 68-72 ℃, washing with centrifugal water and filtering; then adding into sulfuric acid solution, stirring at 30-50 ℃, washing with water to neutrality, filtering, adding into mixed solution of ethanol, n-heptane and deionized water, reacting at 68-72 ℃ for 8-10h, centrifuging, washing with alcohol, filtering, and vacuum drying to obtain the modified polystyrene.

Further, the mass ratio of the potassium persulfate to the styrene to the divinylbenzene is 0.1:10:0.05, and the mass ratio of the ethanol to the distilled water to the n-heptane is 5:4: 1; the cross-linked polystyrene nano-microspheres are prepared from potassium persulfate, styrene and divinylbenzene, are uniformly monodisperse, have the particle size of 400-500nm, are added with sulfuric acid for sulfonation treatment, sulfonate groups are grafted on the surfaces and the interiors of the microspheres by sulfonation to increase the hydrophilicity, and are swelled by ethanol and n-heptane to form porous microsphere structures, so that the internal stress of the diaphragm can be reduced;

according to the invention, strong electrostatic coupling effect is generated among the nanofiber crystal, the graphene oxide quantum dots and the modified polystyrene nano microspheres, and the strong electrostatic coupling effect plays a synergistic effect on changing the interface property and promoting stress transmission, so that the stress is dispersed under the combined action, the stress of the diaphragm is greatly reduced, the elastic modulus of the diaphragm is improved, the membrane surface deformation of the diaphragm is improved, and the finished product rate of battery winding is improved;

the porous flaky micro-nano structure, the modified polystyrene nano-microsphere structure and the porous structure of the nanofiber crystal of the graphene oxide quantum dot play a synergistic role in dispersing stress, the risks of diaphragm size shrinkage and diaphragm surface wrinkling caused by excessive internal stress are reduced, the quality of the diaphragm is improved, the porous flaky micro-nano structure, the modified polystyrene nano-microsphere structure and the porous structure of the nanofiber crystal of the graphene oxide quantum dot play roles in increasing the tortuosity of oxygen gas and water molecule entering routes, the synergistic effect is played on greatly improving the thermal stability of the diaphragm, and the service life of a battery is prolonged.

The invention has the beneficial effects that:

the preparation of the battery diaphragm adopts the ultra-high molecular weight polyethylene and the high density polyethylene because the molecular weight is large, and the acting force and the entanglement degree between macromolecular chains are increased along with the increase of the molecular weight, so that the internal stress of the diaphragm is reduced, and the polystyrene is dispersed in a PE continuous phase in a similar bead shape, so that the internal stress can be dispersed and relieved along a spherical surface, and the purpose of reducing the internal stress of the battery diaphragm is achieved;

when the ultrahigh molecular weight polyethylene, the high density polyethylene and the white oil are mixed and melted, the polystyrene, the reinforcing fiber and the nucleating agent are added in a certain proportion for mixing and melting, so that the possible microscopic internal stress is reduced from the formula design;

the structural design of a transverse-pulling oven is improved, so that seven air ports in total are arranged on the upper row and the lower row of hot air ports of the transverse-pulling oven in the TD direction, the transverse-pulling oven is correspondingly divided into 7 heating zones, the temperature of each zone is independently controlled, a diaphragm in the transverse-pulling oven is heated in a mode of increasing from central symmetry to two sides, necking during heating is reduced, stretching uniformity of the diaphragm in the TD direction is improved, and internal stress of the diaphragm of the battery is reduced;

the graphene oxide quantum dots are added, the reinforcing fibers are made of nanofiber crystals, the polystyrene is made of modified polystyrene, strong electrostatic coupling effect can be generated among the nanofiber crystals, the graphene oxide quantum dots and the modified polystyrene nanospheres, synergistic effect is achieved in changing interface properties and promoting stress transmission, stress is dispersed under the combined effect, stress of the diaphragm is greatly reduced, elastic modulus of the diaphragm is improved, and service life of the battery is prolonged;

the low internal stress lithium ion battery diaphragm prepared by the invention is a polyethylene microporous diaphragm, the thickness is 5-20 μm, the porosity is 30-50%, the ventilation value is 130-300, the pore diameter is 0.01-0.1 μm, the thermal shrinkage is less than 3, and the tensile strength is higher than 2000 MPa;

the lithium ion battery diaphragm prepared by the method disclosed by the invention has the advantages that the internal stress of the diaphragm is greatly reduced, the size shrinkage and the diaphragm surface wrinkle of the diaphragm caused by the overlarge internal stress are avoided, and the diaphragm surface deformation is improved, so that the battery winding yield is improved, the short circuit risk of the lithium ion battery is reduced, the battery cycle number is increased, and the service life is prolonged.

Drawings

Fig. 1 is a schematic structural diagram of a horizontal pulling oven area of the invention.

Detailed Description

The technical solutions in the present invention will be described clearly and completely with reference to the following embodiments of the present invention, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It should be noted that if directional indications such as up, down, left, right, front, and back … … are involved in the embodiment of the present invention, the directional indications are only used to explain a specific posture, such as a relative positional relationship between components, a motion situation, and the like, and if the specific posture changes, the directional indications also change accordingly. In addition, technical solutions between various embodiments may be combined with each other, but must be realized by a person skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination should not be considered to exist, and is not within the protection scope of the present invention.

The technical solutions of the present invention are further described in detail with reference to the following specific examples, which should be understood as merely illustrative and not limitative.

Example 1

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the additives are reinforcing fiber and sorbitol; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

white oil, ultrahigh molecular weight polyethylene, high density polyethylene, polystyrene, reinforcing fiber and sorbitol in a ratio of 7.3:1.32:1.32:0.02:0.02:0.02, respectively;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film;

the longitudinal stretching temperature is 105 ℃, and the longitudinal stretching ratio is 7.5;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film;

the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching;

the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film;

the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Example 2

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the additives are reinforcing fiber and sorbitol; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

white oil, ultrahigh molecular weight polyethylene, high density polyethylene, polystyrene, reinforcing fiber and sorbitol in a ratio of 7.3:1.28:1.27:0.05:0.05:0.05 respectively;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film;

the longitudinal stretching ratio is 7.5, and the longitudinal stretching temperature is 105 ℃;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film;

the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching;

the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film; the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Example 3

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the additives are reinforcing fiber and sorbitol; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

white oil, ultrahigh molecular weight polyethylene, high density polyethylene, polystyrene, reinforcing fiber and sorbitol in a ratio of 7.3:1.23:1.23:0.08:0.08:0.08 respectively;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film; the longitudinal stretching ratio is 7.5, and the longitudinal stretching temperature is 105 ℃;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film; the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching; the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film; the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Example 4

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and modified polystyrene; the additive is as follows: nano-fiber crystals, sorbitol, graphene oxide quantum dots; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

preparation of modified polystyrene:

ultrasonically stirring potassium persulfate, styrene and divinylbenzene with the mass ratio of 0.1:10:0.05, reacting for 9 hours at 68 ℃, washing with centrifugal water, and filtering; then adding the mixture into a sulfuric acid solution, stirring at 30 ℃, washing with water to be neutral, filtering, adding a mixed solution of ethanol, n-heptane and deionized water with a mass ratio of 5:1:4, reacting for 10 hours at 68 ℃, centrifuging, washing with alcohol, filtering, and drying in vacuum to obtain modified polystyrene;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

the ratio of white oil to ultrahigh molecular weight polyethylene to high density polyethylene to modified polystyrene to nano-crystalline fiber to the ratio of sorbitol to graphene oxide quantum dots to the white oil to the ultra-high molecular weight polyethylene to the nano-crystalline fiber to the graphene oxide quantum dots is 7.3:1.23:1.23:0.08:0.08: 0.08;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film; the longitudinal stretching ratio is 7.5, and the longitudinal stretching temperature is 105 ℃;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film; the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching; the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film; the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Example 5

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and modified polystyrene; the additive is as follows: nano-fiber crystals, sorbitol, graphene oxide quantum dots; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

preparation of modified polystyrene:

ultrasonically stirring potassium persulfate, styrene and divinylbenzene with the mass ratio of 0.1:10:0.05, reacting for 8 hours at 70 ℃, washing with centrifugal water, and filtering; then adding the mixture into a sulfuric acid solution, stirring at 42 ℃, washing with water to be neutral, filtering, adding a mixed solution of ethanol, n-heptane and deionized water with a mass ratio of 5:1:4, reacting for 9 hours at 70 ℃, centrifuging, washing with alcohol, filtering, and drying in vacuum to obtain modified polystyrene;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet; the ratio of white oil to ultrahigh molecular weight polyethylene to high density polyethylene to modified polystyrene to nano-crystalline fiber to the ratio of sorbitol to graphene oxide quantum dots to the white oil to the ultra-high molecular weight polyethylene to the nano-crystalline fiber to the graphene oxide quantum dots is 7.3:1.23:1.23:0.08:0.08: 0.08;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film; the longitudinal stretching ratio is 7.5, and the longitudinal stretching temperature is 105 ℃;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film; the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching; the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film; the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

Example 6

(1) Mixing materials: preparing a polyethylene raw material and an additive, and then mixing and stirring the polyethylene raw material and the additive to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and modified polystyrene; the additive is as follows: nano-fiber crystals, sorbitol, graphene oxide quantum dots; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

preparation of modified polystyrene:

ultrasonically stirring potassium persulfate, styrene and divinylbenzene with the mass ratio of 0.1:10:0.05, reacting for 7 hours at 72 ℃, washing with centrifugal water, and filtering; then adding the mixture into a sulfuric acid solution, stirring at 50 ℃, washing with water to be neutral, filtering, adding a mixed solution of ethanol, n-heptane and deionized water with a mass ratio of 5:1:4, reacting for 8 hours at 72 ℃, centrifuging, washing with alcohol, filtering, and drying in vacuum to obtain modified polystyrene;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

the ratio of white oil to ultrahigh molecular weight polyethylene to high density polyethylene to modified polystyrene to nano-crystalline fiber to the ratio of sorbitol to graphene oxide quantum dots to the white oil to the ultra-high molecular weight polyethylene to the nano-crystalline fiber to the graphene oxide quantum dots is 7.3:1.23:1.23:0.08:0.08: 0.08;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film; the longitudinal stretching ratio is 7.5, and the longitudinal stretching temperature is 105 ℃;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film; the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching; the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film; the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

In examples 1 to 6, the primary transverse stretching in step (4) and the secondary transverse stretching in step (6) were both transversely stretched using a transverse stretching oven shown in fig. 1; seven air ports are formed in the upper row of hot air ports and the lower row of hot air ports of the transverse-pulling oven in the TD direction, the seven air ports are correspondingly divided into 7 heating areas, the temperature of each area can be independently controlled, and the diaphragm in the transverse-pulling oven is heated in a mode that the temperature is gradually increased from the central symmetry to the two sides.

Comparative example

(1) Mixing materials: preparing a polyethylene raw material, and mixing and stirring to obtain a mixture;

the polyethylene raw material is formed by mixing ultrahigh molecular weight polyethylene, high density polyethylene and polystyrene; the molecular weight of the ultra-high molecular weight polyethylene is 1500000, and the molecular weight of the high density polyethylene is 500000;

(2) extruding: mixing the mixture obtained in the step (1) with pore-foaming agent white oil, and then extruding to obtain a molten-state extruded sheet;

the ratio of white oil to ultrahigh molecular weight polyethylene to high density polyethylene is 7.3:1.32: 1.32;

the screw speed of the extruder is 35rpm, the extrusion temperature is 200 ℃, and the casting temperature is 22 ℃;

(3) longitudinal stretching: longitudinally stretching the molten extruded sheet in the step (2) to obtain a longitudinally-stretched film;

the longitudinal stretching temperature is 105 ℃, and the longitudinal stretching ratio is 7.5;

(4) primary transverse stretching: carrying out primary transverse stretching on the longitudinally-stretched film in the step (3) to obtain a primary transversely-stretched film;

the primary transverse stretching temperature is 113.5 ℃, and the primary transverse stretching ratio is 8.7;

(5) and (3) extraction: passing the film subjected to the primary transverse pulling in the step (4) through an extraction solvent to remove white oil in the film, so as to obtain an extracted film;

(6) secondary transverse stretching: performing secondary transverse stretching on the extracted film in the step (5) to obtain a film subjected to secondary transverse stretching;

the secondary transverse stretching temperature is 133.5 ℃, and the secondary transverse stretching ratio is 1.6;

(7) heat setting: performing heat setting treatment on the film subjected to the secondary transverse pulling in the step (6) to obtain a heat-set film;

the heat setting temperature is 100 ℃;

(8) winding: and (4) winding the film subjected to heat setting in the step (7) on a winding core.

In the comparative example, the primary transverse stretching in step (4) and the secondary transverse stretching in step (6) were both transversely stretched using a commercially available transverse stretching oven.

And (3) performance testing:

the thickness of the diaphragm is measured by a Mark thickness gauge;

the porosity is calculated by a weighing method;

the air permeability value is tested by a Gurley air permeability instrument;

the thermal shrinkage is thermal shrinkage in the stretching direction, and during testing, the dimension change of the diaphragm is tested after the diaphragm is placed in an oven at 105 ℃ and baked for 1 hour, and the thermal shrinkage is calculated by the following formula: heat shrinkage ═ (original size-size after baking)/original size X100%;

the tensile strength was measured using a tensile test instrument; specific data are shown in table 1;

	thickness (μm)	Porosity (%)	Air permeability value	Pore size (mum)	Thermal shrinkage	Tensile Strength (MPa)
							Example 1	8.2	39	185	0.06	1.8	2051
Example 2	8.1	42	182	0.08	2.1	2040
							Example 3	8.3	41	182	0.05	1.6	2060
Example 4	8.1	46	175	0.03	0.6	2120
							Example 5	8.2	50	168	0.01	0.5	2131
Example 6	8.3	48	170	0.02	0.8	2111
							Comparative example	8.2	30	260	0.2	3.2	1821

TABLE 1

The membrane prepared in the embodiment 1-6 has the thickness of 5-20 μm, the porosity of 30-50%, the air permeability of 130-300, the pore diameter of 0.01-0.1 μm, the thermal shrinkage of less than 3 and the tensile strength of more than 2000 MPa; examples 1, 2 and 3 compared with the comparative example, in examples 1 to 3 in which polystyrene, reinforcing fiber and sorbitol were added, and the transverse-drawing oven shown in fig. 1 was used for both primary drawing and secondary drawing, the data obtained on porosity, air permeability, pore diameter, thermal shrinkage and tensile strength were superior to the comparative example, and it was found that the battery separators of examples 1 to 6 prepared according to the present invention had excellent mechanical properties;

judging the condition of the internal stress of the diaphragm by the deformation of the membrane surface of the diaphragm after 14 days after the diaphragm is cut;

the results of the membrane post-deformation measurements of examples 1-6 are shown in Table 2;

one station

Two working positions

Three-station

Four-station

Five stations

Six stations

Seven stations

Example 1

OK

OK

OK

OK

OK

OK

OK

Example 2

OK

OK

OK

OK

OK

OK

OK

Example 3

OK

OK

OK

OK

OK

OK

OK

Example 4

OK

OK

OK

OK

OK

OK

OK

Example 5

OK

OK

OK

OK

OK

OK

OK

Example 6

OK

OK

OK

OK

OK

OK

OK

Comparative example

OK

Not OK

OK

OK

Not OK

OK

OK

TABLE 2

Example 1-6 calculation of average percent defective of diaphragm after slitting in later deformation:

cutting 6 large rolls in total, and winding 7 finished rolls in total after each roll is cut;

the total number of the rolls with poor deformation at the later stage of the winding film surface is as follows: volume 0;

and (3) cutting and rolling the total number of finished rolls: volume 6 × 7 — 42;

the average reject ratio of the later deformation of the winding membrane surface is as follows: 0/42 ═ 0%;

calculating the average defect rate of the diaphragm after slitting of the comparative example in the later period of deformation:

cutting 1 large roll in total, and winding 7 finished rolls in total after each roll is cut;

the total number of the rolls with poor deformation at the later stage of the winding film surface is as follows: 2, coiling;

the average reject ratio of the later deformation of the winding membrane surface is as follows: 2/7 ═ 28%;

the membrane surface deformation is OK after 14 days of the membrane subjected to slitting and rolling in the examples 1-6, the average reject ratio of the late deformation of the rolled membrane surface is 0%, the membrane surface deformation is not OK after 14 days of the membrane subjected to slitting and rolling in the comparative example, and the average reject ratio of the late deformation of the rolled membrane surface is 28%, so that the internal stress of the membrane prepared in the examples 1-6 is lower through comparison, namely the membrane prepared by the method is a low-stress membrane.

The above description is only an embodiment of the present invention, and is not intended to limit the scope of the present invention, and all modifications and equivalents made by the present specification and drawings, or directly/indirectly applied to other related technical fields, are included in the scope of the present invention.