POLYOLEFIN MICROPOROUS FILM AND PREPARATION METHOD THEREOF

Abstract

The present invention relates to a separator film in a lithium battery, more specifically relates to a polyolefin microporous film and a preparation method thereof. The polyolefin microporous film has a porosity above 30% and below 65%, a median pore size above 10 nm and below 60 nm, a stress-strain (σ-ε) curve integral in Machine Direction (MD) and Transverse Direction (TD) directions simultaneously fulfilling the following definition:

E=∫.sub.0.sup.εσ(ε)dε>150 J/m.sup.2; and the largest pore size, the smallest pore size and the median pore size fulfilling the following definition: 0.9<P<1.2, wherein P=(the largest pore size−the smallest pore size)/the median pore size. The polyolefin microporous film according to the present invention has high tenacity, small pore sizes, concentrated distribution of pore sizes, great stability for coiling, and enabling high number of battery cycles. When used as a separator film, battery manufacturing safety and the service life of the battery being made can be increased.

Claims

1. A polyolefin microporous film, characterized by having the following characteristics: a. porosity above 30% and below 65%; b. a median pore size above 10 nm and below 60 nm; c. stress-strain (σ-ε) curve integral in Machine Direction (MD) and Transverse Direction (TD) directions simultaneously meet the following definition:
E=∫.sub.0.sup.εσ(ε)dε>150 J/m.sup.2; d. largest pore size, smallest pore size and the median pore size meet the following definition: 0.9<P<1.2, wherein P=(the largest pore size−the smallest pore size)/the median pore size.

2. The polyolefin microporous film of claim 1, having the following characteristics: a. the stress-strain (σ-ε) curve integral in MD and TD directions simultaneously meet the following definition:
E=∫.sub.0.sup.εσ(ε)dε>250 J/m.sup.2; b. the largest pore size, the smallest pore size and the median pore size meet the following definition: 0.99<P<1.01, wherein P=(the largest pore size−the smallest pore size)/the median pore size; c. the median pore size is above 25 nm and below 35 nm.

3. A preparation method of a polyolefin microporous film, characterized by the step of mixing polyolefin resins and additives to form a mixture, extruding the mixture, and forming the mixture into a cast sheet at 0° C. or below.

4. The preparation method of claim 3, wherein a cast sheet forming temperature is 0° C. to −80° C.

5. The preparation method of claim 3, wherein the polyolefin resins refer to a mixture of two or more kinds of polyolefin resins with viscosity average molecular weight from 0.4 million to 5 million.

Description

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] Preferred ways of implementing the present invention will be described in detail below with reference to the embodiments. Before we proceed, a person skilled in the art should understand that the technical terms appearing in the description and the claims should not be limited to the interpretations according to an ordinary dictionary. Instead, the inventors should be allowed to appropriately define technical terms for the best explanation of the present invention, and these defined technical terms should be interpreted based on the concept and implication of the technical aspects of the present invention. Accordingly, the description below is only intended to be preferred examples for illustrative purpose, and should not limit the scope of the present invention. Therefore, modifications and other alternative configurations that achieve the same technical effects may be possible given that they are not deviated from the spirit and scope of the present invention.

[0022] A preparation method of polyolefin microporous film according to the present invention will be described below. However, given that the characterizing features of the present invention are met, there are no limitations as to the specific types and ratios of polyolefin, plasticizers, antioxidants and other additives being used; the use and if use, the types, of extracting agent; the forms and methods of mixing and extruding, film casting, stretching, and thermoforming; the specific techniques of coiling and dividing; and the use of continuous production method or not.

[0023] The preparation method of the polyolefin microporous film according to the present invention mainly comprises the following steps:

[0024] Mixing polyolefin resins and additives to form a mixture; extruding the mixture which flows out from a die; cooling the mixture to below 0° C. by a cooling agent to form a cast sheet; performing at least one first stretch of the cast sheet in at least one direction; passing the stretched cast sheet through at least one extracting tank to extract plasticizers; after extraction, performing at least one second stretch of the cast sheet in at least one direction; finally, performing relax thermoforming treatment to obtain a stable final product.

[0025] Specifically, the polyolefin resins used in the present invention may be homopolymers such as polyethylene, polypropylene, polybutene, and mixtures thereof, or may be copolymers such as ethylene, propylene, butene, 4-methyl-1-pentene, and mixtures thereof, or may be mixtures of two or more of said homopolymers or copolymers. With a view to forming a film easily from a polymer melt, a viscosity average molecular weight of the polymer is 0.4 million or more, preferably 0.5 million or more, and more preferably 0.6 million or more, and most preferably 0.7 million or more. With a view to processing the polymer melt easily, the viscosity average molecular weight of the polymer is 5 million or less, preferably 4 million or less, more preferably 3 million or less, and most preferably 2 million or less.

[0026] The additives of the present invention include plasticizers, antioxidants and other additives; plasticizers can be liquid or solid hydrocarbon mixtures such as white oil and solid paraffin; or organic acid esters such as dioctyl phthalate (DOP); or inorganic acid esters such as trioctyl phosphate (TOP); or organic substances that can form a homogeneous solution or melt with polyolefin resins below the boiling point. One or two types or more of antioxidants can be used to protect the polyolefin resins and the plasticizers from degradation and deterioration, and at the same time render a film surface of the final separator film being made to be flat, smooth and free of defects.

[0027] According to the mixing and extruding methods of the present invention, the polyolefin resins, the plasticizers, the antioxidants and the other additives can be premixed by using a mixer and then being put into an extruder to perform melt extrusion. Alternatively, the polyolefin resins, the plasticizers, the antioxidants and the other additives are fed in through the same or different feeding ports via a single screw, dual screws or multi-screws used alone or in combination, and then directly being mixed and extruded.

[0028] Film casting and forming methods of the present invention is to trigger thermally induced phase separation between the polyolefin polymer and the plasticizer by cooling through the use of an agent, and then to form a cast sheet having a thickness as practically required by film casting. The cooling agent is any substance that can lower the temperature of the cast sheet to below 0° C., such as liquefied air, nitrogen, carbon dioxide; refrigerants such as coolant; or highly thermal conductive substances such as metals, ceramics, graphite, and graphene. During the process when the cast sheet is cooled to form a two-phase structure, the plasticizers are in a dispersed phase and will undergo a nucleation stage and a growth stage. Nucleation is facilitated by providing stronger supercooling in an initial stage, in other words, there are more number of pores being formed. Given a fixed porosity, the formed pores have small sizes. At the same time, low temperature inhibits the growth of dispersed phase, in other words, it is difficult for the pores to grow bigger and the differentiation of the pore sizes is small, therefore, a structure with small pore sizes and concentrated distribution (small varieties) of pore sizes is obtained. Such a net-shape structure is more stable, avoids defects caused by large pores and uneven force distribution caused by wide distribution of pore sizes. Therefore, the film according to the present invention has high tenacity. The cast sheet forming temperature is preferably 0 to −80° C., and m ore preferably −35° C. to −80° C.

[0029] The stretching method used in the present invention is to perform at least one first stretch and at least one second stretch both in at least one direction before and after extraction respectively. Stretching can be synchronous stretching or asynchronous stretching, or a combination of synchronous stretching and asynchronous stretching. Stretching can be continuous or discontinuous.

[0030] Extraction method used in the present invention is to provide one or more extraction tanks in a transporting direction of the stretched (first stretch) cast sheet according to concentration gradient, and eventually residual extracting agent on the film surface or inside the film is removed through a drying air box with a low negative pressure. The extracting agent can be any solvent such as dichloromethane and chloroform, which dissolves together with the plasticizers but does not dissolve polyolefin.

[0031] The relax thermoforming method used in the present invention is to perform at least one relax thermoforming treatment in at least one direction of the cast sheet.

[0032] A porosity of the polyolefin microporous film prepared by the invention is above 30% and below 65%; a median pore size is above 10 nm and below 60 nm. To obtain high working efficiency and safety of the battery, the porosity is preferably 35%˜60%, more preferably 40%˜55%, most preferably 45%˜50%; the median pore size is preferably 15 nm˜50 nm, more preferably 20 nm to 40 nm, and most preferably 25 nm to 35 nm.

[0033] To attain dimensional stability of the separator during the coiling process, the tenacity in MD and TD directions is E>150 J/m.sup.2, preferably E>170 J/m.sup.2, more preferably E>200 J/m.sup.2, and still even more preferably selected as E>230 J/m.sup.2, and most preferably E>250 J/m.sup.2.

[0034] In view of pore size distribution and prolonging the services of battery cycles, the pore size distribution should be 0.9<P<1.2, preferably 0.93<P<1.1, more preferably 0.95<P<1.05, still even more preferably 0.97<P<1.02, most preferably 0.99<P<1.01.

[0035] Test Methods for Various Physical Properties According to the Present Invention

[0036] (1) Viscosity Average Molecular Weight M.sub.v

[0037] The test for intrinsic viscosity is performed by apparatus model number Julabo AVS370. According to ISO 1628-3:2010, the solvent is dekalin, the temperature is 135° C. Measure the intrinsic viscosity of polyethylene [η], and then convert the measured value to viscosity average molecular weight according to Margolies's equation.

M.sub.v=5.37×10.sup.4×[η].sup.1.49

[0038] (2) Porosity (%)

[0039] Use a sample cutter to obtain 50 mm×50 mm samples from the polyolefin microporous film, and measure an average porosity of 20 samples according to surface density using the following formula.

Porosity (%)=[1−ρ.sub.surface÷(ρ×d)]×100

[0040] wherein ρ.sub.surface is the surface density of the polyolefin microporous film, ρ is a density of polyolefin resins, and d is a thickness of the polyolefin microporous film.

[0041] (3) Stress-Strain (σ-ε) Curve

[0042] The test for tensile strength is carried out using an Electronic Universal Material Testing Machine (model no.: XJ830) manufactured by Shanghai Xiangjie Instrument Technology Company from China, sample size is 150 mm×15 mm, clamping distance is 50 mm, stretching speed is 50 mm/min, ambient temperature is 25° C., and humidity is 30%˜60%. The stress-strain (σ-ε) curve integral in MD (machine direction) and TD (transverse direction) directions, as shown below, reflect the tenacity of the separator film by showing the energy absorption ability of the polyolefin microporous film when subject to external forces, thereby avoiding the deviation caused by differences in elastic modulus:

E=∫.sub.0.sup.εσ(ε)dε

[0043] (4) Distribution of Pore Sizes

[0044] The apparatus used for pore size test is the Capillary Flow Porometer (CFP-1500AE) from Porous Materials, Inc (PMI), a surface tension of an infiltration solution is 15.9 Dynes/cm; the median pore size (ϕ.sub.mean) is obtained from a “half-dry curve” according to “dry-wet process”. Both the maximum pore size and the minimum pore size can be obtained from the pore size distribution graph obtained from this test. The maximum pore size (ϕ.sub.max) is the last value in the pore size distribution graph data, which is the pore size corresponding to the bubble point; the minimum pore size (ϕ.sub.min) is the first value in the obtained pore size distribution graph data. Therefore, pore size distribution (P) can be calculated by the following formula:

P=(ϕ.sub.max−ϕ.sub.min)/ϕ.sub.mean

[0045] (5) Services of Battery Cycles

[0046] Based on GB/T 18287-2013, measure the remaining capacity (%) of the battery after 500 battery cycles.

[0047] (6) Coiling Stability

[0048] Coiling a sheet of separator films cut into width of 600 mm with coiling tension 100N and coiling speed 100 m/min. Taking the inner diameter as an origin, measure a horizontal offset distance of the outer diameter after coiling for 3000 m, and record the average value of 10 parallel samples.

Embodiment 1

[0049] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are dry-mixed with 0.5 wt % antioxidant 1010 respectively in proportions of 2.5 wt %, 9 wt %, and 18 wt % respectively, and then added in with 70 wt % white oil respectively; and then all mixtures are transferred to a dual-screw extruder, with a mixing temperature of 220° C., and a cast sheet forming temperature of −35° C.

[0050] Next, the cast sheet is introduced into an asynchronous biaxial stretching machine for biaxial stretching. The stretching conditions are MD stretching ratio of 10, TD stretching ratio of 8.8, and stretching temperature of 110° C.

[0051] Then the stretched cast sheet is immersed in a dichloromethane extraction tank; after paraffin oil is extracted, the cast sheet is dried by dry air to remove the dichloromethane on the surface and in the interior of the polyethylene microporous film.

[0052] After that, introduce the cast sheet into a TD stretching machine with a thermoforming temperature of 127° C., and a relaxation rate of 0.95.

[0053] After that, introduce the cast sheet into a MD stretching machine with a thermoforming temperature of 127° C., and a relaxation rate of 0.95.

[0054] Finally, perform coiling to obtain the polyethylene microporous film. The physical properties of the obtained polyethylene microporous film are listed in Table 2.

Embodiment 2

[0055] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are used in proportions of 1 wt %, 8.5 wt % and 15 wt % respectively; the proportion of the antioxidant 1010 is 0.5 wt %; the proportion of the added white oil is 75 wt %; the MD stretching ratio and the TD stretching ratio are both 15. Except for the above, other conditions and steps are the same as embodiment 1.

Embodiment 3

[0056] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are used in proportions of 2 wt %, 9.5 wt % and 18 wt % respectively; the cast sheet forming temperature is −80° C. Except for the above, other conditions and steps are the same as embodiment 1.

Embodiment 4

[0057] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are used in proportions of 2 wt %, 9.5 wt % and 28 wt % respectively; the proportion of the antioxidant 1010 is 0.5 wt %; the proportion of the added white oil is 60 wt %; the cast sheet forming temperature is −80° C. Except for the above, other conditions and steps are the same as embodiment 1.

Embodiment 5

[0058] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are used in proportions of 1.5 wt %, 8 wt % and 15 wt % respectively; the proportion of the antioxidant 1010 is 0.5 wt %; the proportion of the added white oil is 75 wt %; the MD stretching ratio and the TD stretching ratio are both 15. Except for the above, other conditions and steps are the same as embodiment 1.

Embodiment 6

[0059] The cast sheet forming temperature is 0° C. Except f or the above, other conditions and steps are the same as embodiment 1.

Embodiment 7

[0060] Polyethylene homopolymers with viscosity average molecular weights of 3.5 million, 1.5 million, and 0.5 million respectively are used in proportions of 3 wt %, 8 wt % and 13.5 wt % respectively; the proportion of the antioxidant 1010 is 0.5 wt %; the proportion of the added white oil is 75 wt %; the cast sheet forming temperature is 0° C.; the MD stretching ratio and the TD stretching ratio are both 15. Except for the above, other conditions and steps are the same as embodiment 1.

Comparative Example 1

[0061] Polyethylene resins with viscosity average molecular weights of 1.5 million and 0.5 million respectively are used in proportions of 9.5 wt % and 20 wt % respectively; the MD stretching ratio and the TD stretching ratio are both 7. Except for the above, other conditions and steps are the same as embodiment 1.

Comparative Example 2

[0062] A polyethylene resin with viscosity average molecular weight of 0.5 million is used in a proportion of 29.5 wt %. The cast sheet forming temperature is 50° C.; the MD stretching ratio and the TD stretching ratio are both 6. Except for the above, other conditions and steps are the same as embodiment 1.

Comparative Example 3

[0063] The cast sheet forming temperature is 50° C. Except for the above, other conditions and steps are the same as embodiment 1.

INDUSTRIAL APPLICABILITY

[0064] The present invention obtains a polyolefin microporous film with high tenacity and concentrated pore size distribution and also a preparation method thereof, by changing the proportions of different grades of ultra-high molecular weight polyethylene, the proportions of high-density polyethylene resins, the MD and TO stretching ratios, as well as the cast sheet forming temperature. The obtained polyolefin microporous film has improved machinability in a lithium-ion battery, and enhances the safety and prolongs the service life of the lithium-ion battery.

TABLE-US-00001 TABLE 1 Comparisons between the implementations of the embodiments of the present invention and those of the comparative examples TD MD stretching stretching cast machine TD machine MD viscosity sheet thermo- stretching thermo- stretching average mixing forming MD TD stretching forming machine forming machine molecular temp. temp. stretching stretching temp. temp. relaxation temp. relaxation weight formula ° C. ° C. ratio ratio ° C. ° C. ratio ° C. ratio Embodiment 1 3.5 million + 0.5 wt % 220° C. −35° C. 10 8.8 110° C. 127° C. 0.95 127° C. 0.95 1.5 million + antioxidant 0.5 million 1010 + (2.5 wt %\ 70 wt % 9 wt %\ white oil 18 wt %) Embodiment 2 3.5 million + 0.5 wt % — — 15 15 — — — — — 1.5 million + antioxidant 0.5 million 1010 + (1 wt %\ 75 wt % 8.5 wt %\ white oil 15 wt %) Embodiment 3 3.5 million + — — −80° C. — — — — — — — 1.5 million + 0.5 million (2 wt %\ 9.5 wt %\ 18 wt %) Embodiment 4 3.5 million + 0.5 wt % — −80° C. — — — — — — — 1.5 million + antioxidant 0.5 million 1010 + (2 wt %\ 60 wt % 9.5 wt %\ white oil 28 wt %) Embodiment 5 3.5 million + 0.5 wt % — — 15 15 — — — — — 1.5 million + antioxidant 0.5 million 1010 + (1.5 wt %\ 75 wt % 8 wt %\ white oil 15 wt %) Embodiment 6 — — —  0° C. — — — — — — — Embodiment 7 3.5 million + 0.5 wt % —  0° C. 15 15 — — — — — 1.5 million + antioxidant 0.5 million 1010 + (3 wt %\ 75 wt % 8 wt %\ white oil 13.5 wt %) Comparative 1.5 million + — — — 7 7 — — — — — example 1 0.5 million (9.5 wt %\ 20 wt %) Comparative 0.5 million — —  50° C. 6 6 — — — — — example 2 (29.5 wt %) Comparative — — —  50° C. — — — — — — — example 3 “—” means the same as embodiment 1

TABLE-US-00002 TABLE 2 Physical properties of polyethylene microporous film Battery offset capacity distance of Max. Median Min. Pore size after 500 outer diameter Porosity MD tenacity TD tenacity pore size pore size pore size distribution battery after coiling No. (%) E (J/m.sup.2) E (J/m.sup.2) (nm) (nm) (nm) P cycles 3000 m Embodiment 1 47 311 323 45 30 15 1.00 95 0 Embodiment 2 56 162 178 68 44 20 1.09 90 0.2 Embodiment 3 37 234 213 34 22 11 1.05 92 0.1 Embodiment 4 31 278 297 23 14 10 0.93 97 0.1 Embodiment 5 52 222 194 58 39 20 0.97 95 0.2 Embodiment 6 44 250 229 43 27 12 1.15 88 0.2 Embodiment 7 63 157 163 85 55 20 1.17 80 0.3 Comparative 29 140 137 80 50 17 1.3 70 0.7 example 1 Comparative 41 127 150 70 42 22 1.25 73 0.5 example 2 Comparative 43 257 243 81 41 27 1.32 91 0.3 example 3