Puncture-resistant diaphragm and efficient preparation method

Abstract

The invention discloses a puncture-resistant diaphragm and an efficient preparation method, and belongs to the field of battery diaphragms. The method sequentially comprises the steps of extrusion, sheet casting, first longitudinal stretching, first transverse stretching, extraction, drying, synchronous double stretching, heat setting and rolling. The synchronous stretching comprises secondary transverse stretching and secondary longitudinal stretching. According to the novel high-puncture-resistance diaphragm and the preparation method, the problems of abrasion of an extraction bearing and diaphragm breakage in extraction at a high linear speed are solved, the problem of friction damage in the preparation process of the diaphragm is solved, and the production line speed is greatly increased. Compared with a traditional diaphragm, the diaphragm prepared by the invention has high puncture strength, high compression strength and low short circuit risk, and the safety is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of battery separators, and in particular to a puncture-resistant separator and an efficient preparation method thereof. Background Art

[0002] Since lithium batteries may have problems such as negative electrode lithium dendrites, pole piece burrs, foreign particles, and external force impact during processing and use, the puncture resistance of the diaphragm is crucial to the safety performance of the battery.

[0003] As the market develops, the prices of lithium batteries and diaphragm materials are gradually decreasing, and product cost control is crucial. Among various measures, increasing the production line speed is the most effective method. In the existing technology, due to limitations such as extraction equipment, the maximum line speed of the diaphragm production line does not exceed 90m / min. During the preparation of the diaphragm, rollers are required for longitudinal stretching or traction. As the thickness of the diaphragm becomes thinner, the rollers will cause damage to the diaphragm, such as scratches, resulting in an increased risk of short circuits. Summary of the invention

[0004] The present invention provides a new diaphragm preparation method in view of the above technical problems, which can prepare a highly puncture-resistant diaphragm at a high line speed, and the diaphragm has a specific puncture strength of ≥60gf / μm.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A method for preparing a puncture-resistant diaphragm, the method comprising the following steps: extrusion, sheet casting, first longitudinal stretching, first transverse stretching, extraction, drying, synchronous double stretching, heat setting and winding; the synchronous double stretching includes a second transverse stretching and a second longitudinal stretching;

[0007] Among them: the stretching ratio meets the following conditions:

[0008] 1.5≤L MD1 ≤4,1.5≤L MD2 ≤5,1.5≤L TD1 ≤5,1.5≤L TD2 ≤5;

[0009] 7≤[(L MD1 *L MD2 ) 2 +(L TD1 *L TD2 ) 2 ] 1 / 2 ≤20;

[0010] 4≤L MD1 *L TD1 ≤15;

[0011] Among them, L MD1is the first longitudinal stretching ratio, L MD2 is the second longitudinal stretching ratio, L TD1 is the first transverse stretching ratio, L TD2 is the second transverse stretching ratio.

[0012] Preferably, the above process also includes the following constraints:

[0013] 0.01 / s≤L MD1 / t MD1 ≤3 / s

[0014] 0.01 / s≤L TD1 / t TD1 ≤3 / s;

[0015] Among them, t MD1 is the time corresponding to the first longitudinal stretching, t TD1 It is the time corresponding to the first transverse stretching.

[0016] Preferably, the above process also includes the following constraints:

[0017] 0.005 / s≤L MD2 / t MD2 ≤1.5 / s

[0018] 0.005 / s≤L TD2 / t TD2 ≤1.5 / s;

[0019] Among them, t MD2 is the time corresponding to the second longitudinal stretching, t TD2 is the corresponding time of the second transverse stretching.

[0020] Preferably, the above process also includes the following constraints:

[0021] The synchronous stretching outlet must meet the following conditions:

[0022] Chain clip center distance ≤80mm;

[0023] 40m / min≤Extraction speed≤150m / min;

[0024] Winding speed ≥150m / min.

[0025] In some more specific technical solutions: the method comprises the following steps:

[0026] (1) Extrusion: paraffin oil and polyethylene raw materials are mixed in an extruder, heated and melted, and then extruded through a slit die to obtain a polyethylene-paraffin oil mixed melt with a uniform phase;

[0027] The extrusion temperature of the extruder is 180°C-230°C, and the mass ratio of polyethylene to paraffin oil is 1.5:8.5-4:1;

[0028] (2) Slab casting: The melt obtained in step (1) is quenched at a temperature of 10-50° C. to obtain a thick sheet with a thickness of 0.4 mm to 3 mm;

[0029] (3) Longitudinal stretching: The thick sheet obtained in step (2) is subjected to the first longitudinal stretching, with a stretching ratio of 2-4 times, a stretching time of 2-10 seconds, and a stretching temperature of 40°C to 100°C;

[0030] (4) Transverse stretching: The oil film obtained by the first longitudinal stretching is subjected to the first transverse stretching at a temperature in the range of 70-130° C. to obtain a porous oil film; the transverse stretching ratio is 2-4 times, and the stretching time is 3-10 s;

[0031] (5) Extraction: extracting the paraffin oil from the porous oil film obtained above using an extractant;

[0032] (6) Drying: The extracted membrane is allowed to completely evaporate the extractant;

[0033] (7) Synchronous biaxial stretching: After drying, the film is stretched synchronously in the transverse and longitudinal directions to obtain a film;

[0034] The conditions for synchronous stretching in the transverse and longitudinal directions are as follows:

[0035] The transverse stretching ratio is 2-5, the longitudinal stretching ratio is 2-5; the second biaxial stretching temperature is 95℃~142℃, the stretching time is 2-15s, and the longitudinal linear speed at the second biaxial stretching outlet is ≥150m / min,

[0036] Optimum: The longitudinal linear speed of the secondary biaxial stretching outlet is ≥ 280 m / min, and the stretching time is 5-10 s;

[0037] (8) Heat setting:

[0038] The film obtained in step (7) is heat-set in a synchronous stretching machine to obtain a lithium battery separator with high puncture strength.

[0039] In the above method: the polyethylene in step (1) is ultra-high molecular weight polyethylene with a molecular weight of 600,000 to 5,000,000 and a melting point of 132° C. to 145° C.

[0040] In the above method: in step (1), the mass ratio of polyethylene to paraffin oil is 1-3:5-10.

[0041] In the above method: in step (5), the extraction temperature is 10 to 35° C., preferably 15 to 25° C.; the extraction time is 1 to 5 min, preferably 2 to 3 min.

[0042] In the above method: in step (6), the drying temperature is 20-70°C, preferably 30-45°C, and the drying time is 4-30s, preferably 5-20s.

[0043] A puncture-resistant diaphragm is prepared by the above method.

[0044] As a preferred technical solution, the heat setting in step (8) includes two parts: heat fixing and heat shrinking. The purpose of the heat fixing is to improve the crystallinity of the diaphragm under stress, thereby improving its thermal stability.

[0045] The purpose of heat shrinkage is to shrink the diaphragm to a certain extent under high temperature, so as to further reduce the heat shrinkage rate of the diaphragm. The temperature used for heat fixation is 105℃~142℃, and the time is 1s-40s. During the heat shrinkage process, the shrinkage rate is 3%~30%, the heat shrinkage temperature is 100℃~140℃, and the heat shrinkage time is 3s~40s.

[0046] Beneficial effects of the present invention:

[0047] The present invention provides a new high puncture resistance diaphragm and preparation method, which overcomes the problems of extraction bearing wear and extraction film breakage at high line speed, overcomes the friction damage problem in the diaphragm preparation process, and achieves a significant increase in production line speed. Compared with traditional diaphragms, the diaphragm prepared by the present invention has high puncture strength, high compressive strength, low short circuit risk, and greatly improved safety. DETAILED DESCRIPTION

[0048] The present invention is further described below in conjunction with embodiments, but the protection scope of the present invention is not limited thereto:

[0049] Table 1 Parameter constraints of Examples 1 to 2

[0050]

[0051]

[0052] Embodiment 1:

[0053] (1) Extrusion:

[0054] First, paraffin oil and polyethylene (melting point 136.5°C, viscosity-average molecular weight 1.47 million; manufacturer Yanshan Petrochemical, product model MIII-1) were processed using a twin-screw extruder at a weight ratio of 3:1, melted and plasticized at 200°C to obtain a mixed melt.

[0055] (2) Casting:

[0056] The melt extruded through the casting die was cooled on a casting roller at 20° C. to obtain a thick sheet with a thickness of 1.5 mm.

[0057] (3) Longitudinal stretching:

[0058] The thick sheet obtained in step (2) was longitudinally stretched at a stretching temperature of 45° C., a stretching ratio of 3 times (the size before stretching was 3 times that after stretching), and a stretching time of 10 seconds.

[0059] (4) Transverse stretching:

[0060] The porous oil film after longitudinal stretching was transversely stretched at 75°C with a stretching ratio of 3 times and a stretching time of 10s. After stretching, a porous oil film with a low thickness and not easy to break was obtained.

[0061] (5) Extraction:

[0062] The porous oil film was extracted with dichloromethane to remove the paraffin oil in the film. The extraction temperature was 25° C. and the extraction time was 2 min.

[0063] (6) Drying:

[0064] The extracted film was dried at 40°C for 7 seconds.

[0065] (7) Synchronous biaxial stretching:

[0066] The dried film was biaxially stretched using a biaxial stretching machine at a stretching temperature of 125° C., a stretching time of 7 seconds, a longitudinal ratio of 4 times, a transverse ratio of 4 times, and a longitudinal linear velocity of 300 m / min at the stretching outlet.

[0067] (8) Heat setting

[0068] The film was heat-set in the heat-setting section of the synchronous stretching machine at a heat-setting temperature of 136° C. for 7 seconds; and then heat-shrunk at 133° C. for 7 seconds with a shrinkage rate of 15%.

[0069] After the above steps, a diaphragm with high puncture resistance is obtained.

[0070] Embodiment 2:

[0071] (1) Extrusion:

[0072] First, paraffin oil and polyethylene (melting point 136.7° C., viscosity-average molecular weight 3 million; manufacturer Yanshan Petrochemical, product model MIII-1) were processed using a twin-screw extruder at a weight ratio of 8.5:1.5, melt-plasticized at 210° C., and obtained a mixed melt.

[0073] (2) Casting:

[0074] The mixed melt was extruded through a casting die and cast at a surface temperature of 20°C to obtain a thick sheet with a thickness of 1.43 mm.

[0075] (3) Longitudinal stretching:

[0076] The thick sheet obtained in step (2) is firstly longitudinally stretched at a temperature of 45° C., a ratio of 3 times, and a time of 10 seconds.

[0077] (4) Transverse stretching:

[0078] The porous oil film after longitudinal stretching was transversely stretched at 75°C with a stretching ratio of 2.5 times and a stretching time of 10s. After stretching, a porous oil film with a low thickness and not easy to break was obtained.

[0079] (5) Extraction:

[0080] The porous oil film was extracted with dichloromethane to remove the paraffin oil in the film. The extraction temperature was 25° C. and the extraction time was 2 min.

[0081] (6) Drying:

[0082] The extracted film was dried at 40°C for 7 seconds.

[0083] (7) Synchronous biaxial stretching:

[0084] The dried film was biaxially stretched using a biaxial stretching machine at a stretching temperature of 125° C., a stretching time of 7 seconds, a longitudinal ratio of 3.5 times, a transverse ratio of 3.5 times, and a longitudinal speed of 300 m / min at the stretching outlet.

[0085] (8) Heat setting

[0086] The film was heat-set at 136° C. for 7 seconds and then heat-shrunk at 133° C. for 7 seconds with a shrinkage rate of 15%.

[0087] After the above steps, the lithium battery separator with high puncture resistance as described in Example 2 is obtained.

[0088] Comparative Example 1:

[0089] The main difference between Comparative Example 1 and the embodiment is that the main steps of preparing the diaphragm are extrusion → casting → simultaneous biaxial stretching → extraction → drying → heat setting.

[0090] (1) Extrusion: Paraffin oil and polyethylene raw materials are mixed in an extruder, heated and melted, and then extruded through a slit die to obtain a polyethylene-paraffin oil mixed melt with a uniform phase.

[0091] The extrusion temperature used by the extruder is 180°C-230°C.

[0092] The mass ratio of raw materials, polyethylene to paraffin oil, is 1.5:8.5 to 4:1, preferably 1.5:8.5.

[0093] (2) Slab casting: The melt obtained above was quenched at 20°C to obtain a slab with a thickness of 1.5 mm.

[0094] (3) Biaxial stretching: Simultaneous biaxial stretching, stretching ratio 7 times, stretching time 15s, stretching temperature 125℃

[0095] (4) The porous oil film is extracted with dichloromethane to remove the paraffin oil in the film. The extraction temperature is 25° C. and the extraction time is 2 min.

[0096] (5) Drying:

[0097] The extracted film was dried at 40°C for 7 seconds.

[0098] (5) Horizontal stretching

[0099] The film was stretched transversely at a transverse stretching ratio of 1.5, a stretching temperature of 125° C., a stretching time of 8 s, and a stretching speed of 60 m / min.

[0100] (7) Heat setting:

[0101] The film obtained above was heat-set under certain temperature conditions, with the heat-setting temperature being 135° C. and the heat-setting time being 6 seconds, to obtain the diaphragm described in Comparative Example 1.

[0102] Comparative Example 2:

[0103] (1) Extrusion:

[0104] First, paraffin oil and polyethylene (melting point 136.5°C, viscosity-average molecular weight 1.47 million; manufacturer Yanshan Petrochemical, product model MIII-1) were processed using a twin-screw extruder at a weight ratio of 3:1, melted and plasticized at 200°C to obtain a mixed melt.

[0105] (2) Casting:

[0106] The melt extruded through the casting die was cooled on a casting roller at 20° C. to obtain a thick sheet with a thickness of 1.5 mm.

[0107] (3) Longitudinal stretching:

[0108] The porous oil film was longitudinally stretched at a temperature of 45° C., a stretching ratio of 3 times (the size before stretching was 3 times that after stretching), and a stretching time of 10 seconds.

[0109] (6) Transverse stretching:

[0110] The porous oil film after longitudinal stretching was transversely stretched at 65°C, with a stretching ratio of 3 times and a stretching time of 10s. After stretching, a porous oil film with a low thickness and not easy to break was obtained.

[0111] (5) Extraction:

[0112] The porous oil film was extracted with dichloromethane to remove the paraffin oil in the film. The extraction temperature was 25° C. and the extraction time was 2 min.

[0113] (6) Drying:

[0114] The extracted film was dried at 40°C for 7 seconds.

[0115] (7) Longitudinal stretching:

[0116] The dried film was longitudinally stretched by a longitudinal stretching machine at a stretching temperature of 125° C., a stretching time of 7 s, a stretching ratio of 4 times, and a stretching outlet speed of 300 m / min.

[0117] (8) Transverse stretching:

[0118] A transverse stretching machine was used for transverse stretching, with a stretching temperature of 125°C, a transverse ratio of 4 times, and a stretching time of 7s.

[0119] (8) Heat setting

[0120] The film was heat-set in the heat-setting section of the synchronous stretching machine at a heat-setting temperature of 136° C. for 7 seconds, and then heat-shrunk at 133° C. for 7 seconds with a shrinkage rate of 15%.

[0121] Performance testing:

[0122]

[0123]

[0124] The results show:

[0125] Example 2 increases the mass proportion of paraffin oil and polyethylene, making the gram weight relatively small, reduces the stretching ratio to relatively increase the thickness, and reduces the specific strength while maintaining a high puncture strength.

[0126] Comparative Example 1 does not have step 7, resulting in a lower puncture strength than Example 1 at the same thickness. Comparative Example 2 is an asynchronous double-pulling process after extraction, and its puncture strength is still worse than that of Example 1.

[0127] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a puncture-resistant diaphragm, characterized in that: The steps of the method are extrusion, sheet casting, first longitudinal stretching, first transverse stretching, extraction, drying, synchronous double stretching, heat setting and winding; the synchronous double stretching includes second transverse stretching and second longitudinal stretching; Among them: the stretching ratio meets the following conditions: 1.5≤L MD1 ≤4,1.5≤L MD2 ≤5,1.5≤L TD1 ≤5,1.5≤L TD2 ≤5; 7≤[(L MD1 *L MD2 ) 2 +(L TD1 *L TD2 ) 2 ] 1 / 2 ≤20; 4≤L MD1 *L TD1 ≤15; Among them, L MD1 is the first longitudinal stretching ratio, L MD2 is the second longitudinal stretching ratio, L TD1 is the first transverse stretching ratio, L TD2 is the second transverse stretching ratio.

2. The method for preparing a puncture-resistant diaphragm according to claim 1, characterized in that: The above process also includes the following constraints: 0.01 / s≤L MD1 / t MD1 ≤3 / s 0.01 / s≤L TD1 / t TD1 ≤3 / s; Among them, t MD1 is the time corresponding to the first longitudinal stretching, t TD1 It is the time corresponding to the first transverse stretching.

3. The method for preparing a puncture-resistant diaphragm according to claim 1, characterized in that: The above process also includes the following constraints: 0.005 / s≤L MD2 / t MD2 ≤1.5 / s 0.005 / s≤L TD2 / t TD2 ≤1.5 / s; Among them, t MD2 is the time corresponding to the second longitudinal stretching, t TD2 is the corresponding time of the second transverse stretching.

4. The method for preparing a puncture-resistant diaphragm according to claim 1, characterized in that: The above process also includes the following constraints: The synchronous stretching outlet must meet the following conditions: Chain clip center distance ≤80mm; 40m / min≤Extraction speed≤150m / min; Winding speed ≥150m / min.

5. The method for preparing a puncture-resistant diaphragm according to claim 1, characterized in that: The method comprises the following steps: (1) Extrusion: paraffin oil and polyethylene raw materials are mixed in an extruder, heated and melted, and then extruded through a slit die to obtain a polyethylene-paraffin oil mixed melt with a uniform phase; The extrusion temperature of the extruder is 180°C-230°C, and the mass ratio of polyethylene to paraffin oil is 1.5:8.5-4:1; (2) Slab casting: The melt obtained in step (1) is quenched at a temperature of 10-50° C. to obtain a thick sheet with a thickness of 0.4 mm to 3 mm; (3) Longitudinal stretching: The thick sheet obtained in step (2) is subjected to the first longitudinal stretching, with a stretching ratio of 2-4 times, a stretching time of 2-10 seconds, and a stretching temperature of 40°C to 100°C; (4) Transverse stretching: The oil film obtained by the first longitudinal stretching is subjected to the first transverse stretching at a temperature in the range of 70-130° C. to obtain a porous oil film; the transverse stretching ratio is 2-4 times, and the stretching time is 3-10 s; (5) Extraction: extracting the paraffin oil from the porous oil film obtained above using an extractant; (6) Drying: The extracted membrane is allowed to completely evaporate the extractant; (7) Synchronous biaxial stretching: After drying, the film is stretched synchronously in the transverse and longitudinal directions to obtain a film; The conditions for synchronous stretching in the transverse and longitudinal directions are as follows: The transverse stretching ratio is 2-5, the longitudinal stretching ratio is 2-5; the second biaxial stretching temperature is 95℃~142℃, the stretching time is 2-15s, and the longitudinal linear speed at the second biaxial stretching outlet is ≥150m / min, Optimum: The longitudinal linear speed of the secondary biaxial stretching outlet is ≥ 280 m / min, and the stretching time is 5-10 s; (8) Heat setting: The film obtained in step (7) is heat-set in a synchronous stretching machine to obtain a lithium battery separator with high puncture strength.

6. The preparation method according to claim 5, characterized in that: The polyethylene in step (1) is ultra-high molecular weight polyethylene with a molecular weight of 600,000 to 5,000,000 and a melting point of 132° C. to 145° C.

7. The preparation method according to claim 5, characterized in that: In step (1), the mass ratio of polyethylene to paraffin oil is 1-3:5-10.

8. The preparation method according to claim 5, characterized in that: In step (5), the extraction temperature is 10-35° C., preferably 15-25° C.; the extraction time is 1-5 min, preferably 2 min-3 min.

9. The preparation method according to claim 5, characterized in that: In step (6), the drying temperature is 20-70° C., preferably 30-45° C., and the drying time is 4-30 seconds, preferably 5-20 seconds.

10. A puncture-resistant diaphragm, characterized in that: The puncture-resistant diaphragm is prepared by the method described in any one of claims 1 to 9.

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