A thin film and its preparation method and battery

Abstract

A film and a preparation method thereof and a battery, wherein the film contains the following components: a thermoplastic resin and a lyophilic modifier. The lyophilic modifier is added to modify the thermoplastic resin, thereby significantly improving the lyophilicity of the film and reducing the crystallinity. The modified film has excellent lyophilic and liquid-retaining properties, which is beneficial to improving the battery performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a thin film and a preparation method thereof, and a battery. Background Art

[0002] The diaphragm is an important component of lithium-ion batteries. Its function is to isolate the positive and negative electrodes and prevent short circuits between the positive and negative electrodes. It also has an ion conduction function, allowing the electrochemical reaction to proceed smoothly. The diaphragm has an important influence on the cycle performance, rate performance and safety performance of the battery. In order to improve the lyophilic and liquid-retaining properties of the diaphragm, there are many studies on the coating modification of the diaphragm. However, there is a problem of poor adhesion between the coating and the base film in the coating, and the coating has the risk of falling off, which ultimately affects the performance of the battery. Summary of the invention

[0003] According to the first aspect, an embodiment provides a film, which contains the following components: a thermoplastic resin and a lyophilic modifier.

[0004] According to the second aspect, an embodiment provides a method for preparing the film of the first aspect, wherein the preparation method comprises dry single drawing.

[0005] According to the third aspect, an embodiment provides a battery, wherein the battery comprises the film according to the first aspect.

[0006] According to a film and its preparation method and battery of the above embodiment, the thermoplastic resin is modified by adding a lyophilic modifier, which significantly improves the lyophilicity of the film and reduces the crystallinity. The modified film has excellent lyophilic and liquid-retaining properties, which is beneficial to improving battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 A comparison diagram of the dynamic changes of a thin film electrolyte according to an embodiment.

[0008] Figure 2 A comparison chart of surface liquid absorption rates of an embodiment. DETAILED DESCRIPTION

[0009] The present invention is further described in detail below by specific embodiments in conjunction with the accompanying drawings. Wherein similar elements in different embodiments adopt associated similar element numbers. In the following embodiments, many detailed descriptions are for making the present application better understood. However, those skilled in the art can easily recognize that some features can be omitted in different situations, or can be replaced by other elements, materials, methods. In some cases, some operations related to the present application are not shown or described in the specification, this is to avoid the core part of the present application being overwhelmed by too much description, and for those skilled in the art, it is not necessary to describe these related operations in detail, and they can fully understand the related operations according to the description in the specification and the general technical knowledge in the art.

[0010] In addition, the features, operations or characteristics described in the specification can be combined in any appropriate manner to form various implementations. At the same time, the steps or actions in the method description can also be interchanged or adjusted in a manner that is obvious to those skilled in the art. Therefore, the various sequences in the specification and the drawings are only for the purpose of clearly describing a certain embodiment and are not meant to be a required sequence, unless otherwise specified that a certain sequence must be followed.

[0011] The serial numbers assigned to the components herein, such as "first", "second", etc., are only used to distinguish the objects described and do not have any order or technical meaning. The "connection" or "coupling" mentioned in this application include direct and indirect connections (couplings) unless otherwise specified.

[0012] As used herein, "room temperature" refers to 23°C ± 2°C.

[0013] According to the first aspect, in one embodiment, a film is provided, the film comprising the following components: a thermoplastic resin and a lyophilic modifier.

[0014] In one embodiment, the lyophilic modifier includes but is not limited to at least one of maleic anhydride (CAS registration number: 108-31-6, also known as maleic anhydride), silane, polystyrene, styrene, polyvinylidene fluoride, polyethylene terephthalate (CAS registration number: 25038-59-9, also known as polyethylene terephthalate), titanate, aluminate, phosphate, borate or their copolymers and blends.

[0015] In one embodiment, the silane includes but is not limited to monosilane (SiH 4 ), disilane (Si 2 H 6 ) etc.

[0016] In one embodiment, the titanate includes but is not limited to at least one of triisostearoyl titanate, isopropyl dioleyl titanate, isopropyl tri(dioctylphospho) titanate, and the like.

[0017] In one embodiment, the aluminate includes but is not limited to at least one of triisopropyl aluminate, tribenzyl aluminate, and the like.

[0018] In one embodiment, the phosphate ester includes but is not limited to at least one of phosphate monoester, phosphate diester, phosphate triester, and the like.

[0019] In one embodiment, the borate ester includes but is not limited to at least one of 3-(tert-butoxycarbonylamino)phenylboronic acid, isopropanol pinacol borate, vinylboronic acid pinacol ester, and the like.

[0020] In one embodiment, the thermoplastic resin includes but is not limited to polyolefin.

[0021] In one embodiment, the polyolefin includes but is not limited to at least one of polyethylene, polypropylene, poly-1-butene, polypentene, and the like.

[0022] In one embodiment, the film is a three-layer structure, including two surface layers and a middle layer located between the two surface layers, the surface layers contain a lyophilic modifier, and the middle layer contains a thermoplastic resin.

[0023] In one embodiment, the three-layer structure includes a first surface layer, an intermediate layer, and a second surface layer in sequence, and the thickness of the intermediate layer accounts for 20 to 90% of the total thickness of the three layers, preferably 30 to 60%;

[0024] In one embodiment, the thickness of the two skin layers is equal.

[0025] In one embodiment, the thickness ratio is first surface layer: middle layer: second surface layer=3:4:3.

[0026] In one embodiment, the film can be used as a high-performance multilayer lithium battery separator, which is composed of at least three layers, wherein the middle layer is a thermoplastic resin layer, and the surface layer is a thermoplastic resin layer modified by a lyophilic substance. After the modified substance is added, the lyophilicity is improved and the crystallinity is reduced. The modified separator has excellent lyophilic and liquid-retaining properties, which is beneficial to improving battery performance.

[0027] In one embodiment, the surface layer and the middle layer are compounded by dry single drawing.

[0028] In one embodiment, the dry single drawing includes casting, annealing, compounding, and stretching.

[0029] In one embodiment, the two surface layers independently contain the following components: thermoplastic resin and lyophilic modifier. Each surface layer contains thermoplastic resin and lyophilic modifier. During the preparation process, the thermoplastic resin and lyophilic modifier do not chemically react, but are physically mixed and stretched into a film.

[0030] In one embodiment, the weight average molecular weight of the thermoplastic resin contained in the middle layer is greater than the weight average molecular weight of the thermoplastic resin contained in the surface layer.

[0031] In one embodiment, the weight average molecular weight of the thermoplastic resin contained in the surface layer is ≥ 300,000, preferably 300,000 to 500,000. The weight average molecular weight of the thermoplastic resin contained in the surface layer includes but is not limited to 300,000, 310,000, 320,000, 330,000, 340,000, 350,000, 360,000, 370,000, 380,000, 390,000, 400,000, 410,000, 420,000, 430,000, 440,000, 450,000, 460,000, 470,000, 480,000, 490,000, 500,000, etc.

[0032] In one embodiment, the weight average molecular weight of the thermoplastic resin contained in the middle layer is 500,000 to 600,000, preferably 530,000 to 550,000. The weight average molecular weight of the thermoplastic resin contained in the middle layer includes but is not limited to 500,000, 510,000, 520,000, 530,000, 540,000, 550,000, 560,000, 570,000, 580,000, 590,000, 600,000, etc.

[0033] In one embodiment, the melt index of the thermoplastic resin contained in the surface layer is 0.5 to 5 g / 10 min, preferably 0.5 to 4.0 g / 10 min, more preferably 0.5 to 3.0 g / 10 min, and even more preferably 0.5 to 2.0 g / 10 min. The melt index of the thermoplastic resin contained in the surface layer includes but is not limited to 0.5g / 10min, 0.6g / 10min, 0.7g / 10min, 0.8g / 10min, 0.9g / 10min, 1.0g / 10min, 1.1g / 10min, 1.2g / 10min, 1.3g / 10min, 1.4g / 10min, 1.5g / 10min, 1.6g / 10min, 1.7g / 10min, 1.8g / 10min, 1.9g / 10min, 2.0g / 10min, 3.0g / 10min, 4.0g / 10min, 5.0g / 10min and the like.

[0034] In one embodiment, the melt index of the thermoplastic resin contained in the middle layer is 0.5-5 g / 10 min, preferably 0.5-4.0 g / 10 min, and more preferably 0.5-3.0 g / 10 min. The melt index of the thermoplastic resin contained in the middle layer includes but is not limited to 0.5 g / 10 min, 0.6 g / 10 min, 0.7 g / 10 min, 0.8 g / 10 min, 0.9 g / 10 min, 1.0 g / 10 min, 1.1 g / 10 min, 1.2 g / 10 min, 1.3 g / 10 min, 1.4 g / 10 min, 1.5 g / 10 min, 1.6 g / 10 min, 1.7 g / 10 min, 1.8 g / 10 min, 1.9 g / 10 min, 2.0 g / 10 min, 3.0 g / 10 min, 4.0 g / 10 min, 5.0 g / 10 min, and the like.

[0035] In one embodiment, in the two surface layers, the mass of the thermoplastic resin accounts for 20% to 90% of the total mass of the thermoplastic resin and the lyophilic modifier, including but not limited to 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, etc., preferably 80%.

[0036] In one embodiment, the thickness of the film is 6-40 μm, preferably 6-30 μm, and more preferably 10-30 μm.

[0037] In one embodiment, the contact angle of the film is less than 20°.

[0038] In one embodiment, the air permeability of the film is 100-600s / 100mL; the porosity is 30-60%; the tensile strength in the MD direction is ≥1200kgf / cm 2 ;Tensile strength in TD direction ≥100kgf / cm 2 ; The heat shrinkage rate in the longitudinal direction (MD direction) is ≤10%, and the heat shrinkage rate in the transverse direction (TD direction) is ≤5%; the puncture strength is 200-1000g; the elastic modulus is 400-850kgf / cm 2 MD refers to the machine direction, i.e. the longitudinal direction, and TD refers to the direction perpendicular to the machine direction, i.e. the transverse direction.

[0039] In one embodiment, the film is a separator for a battery, which is mainly used to isolate the positive and negative electrodes of the battery to prevent the positive and negative electrodes from short-circuiting.

[0040] In one embodiment, the battery includes but is not limited to a lithium ion battery and a sodium ion battery.

[0041] According to a second aspect, a method for preparing the film of the first aspect is provided, the method comprising dry single drawing.

[0042] In one embodiment, the dry single drawing includes casting, annealing, compounding, and stretching.

[0043] In one embodiment, the casting step includes feeding the raw materials for preparing the middle layer and the surface layer into an extruder respectively, melting and extruding, making the extruded materials contact the surface of a chilled roller, and pulling them into a film. The extruder can be a conventional extruder in the art.

[0044] In one embodiment, the extrusion temperature is 140-280°C.

[0045] In one embodiment, the melt is filtered and then extruded from a die head, and the die head temperature is 150-155°C.

[0046] In one embodiment, the temperature of the chilled roller is 60-120°C.

[0047] In one embodiment, the annealing temperature is 135-155°C.

[0048] In one embodiment, the annealing time is 8 to 24 hours.

[0049] In one embodiment, during compounding, the middle layer film and the surface layer film are compounded to form a composite film.

[0050] In one embodiment, the composite forms a three-layer structure, including two surface layers and a middle layer located between the two surface layers.

[0051] In one embodiment, the stretching process includes cold drawing preheating, cold drawing, hot drawing preheating, hot drawing, and heat setting in sequence.

[0052] In one embodiment, the cold drawing preheating temperature is 40-120°C, the cold drawing temperature is set to 40-90°C, and the speed ratio is 1.1-2.0; the hot drawing preheating section temperature is 120-160°C, and the hot drawing preheating speed ratio is 1.5-4.0; the hot drawing section temperature is 125-165°C, and the speed ratio is 1.5-5.5; the heat setting temperature is 130-170°C, and the speed ratio is 0.5-2.0.

[0053] In one embodiment, the process further includes rolling up and placing the stretched film in a static state.

[0054] In one embodiment, the standing time is 8 to 36 hours. The main function of standing is to release the internal stress.

[0055] In one embodiment, after the static state, a heat treatment is further included. The main function of the heat treatment is to release the internal stress for a second time.

[0056] In one embodiment, the temperature of the heat treatment is 40-120°C.

[0057] In one embodiment, the heat treatment time is 4 to 12 hours.

[0058] In one embodiment, the process further includes layering and slitting the film obtained by stretching.

[0059] According to the third aspect, in one embodiment, a battery is provided, wherein the battery comprises the film according to the first aspect, and the film is used to isolate the positive and negative electrodes of the battery.

[0060] In one embodiment, the battery comprises a lithium battery.

[0061] In one embodiment, the film provided by the present invention includes a resin porous film with thermoplastic resin as the main component and a surface layer containing a lyophilic modified substance. The high-performance multilayer film is prepared by a dry single-drawing process and has excellent liquid absorption and retention properties.

[0062] Example 1

[0063] In this embodiment, the raw material used to prepare the middle layer of the film is polypropylene, with a weight average molecular weight of 500,000 and a melt index of 0.5 g / 10 min.

[0064] The raw materials used to prepare the surface layer of the film are polypropylene and maleic anhydride. By mass, polypropylene:maleic anhydride=60:40. The weight average molecular weight of polypropylene is 400,000 and the melt index is 1.2g / 10min.

[0065] The film preparation methods of Examples 1 to 7 are as follows:

[0066] 1. Casting: The thermoplastic resin particles and modified substances are fed into a single screw extruder (one extruder, two melting and plasticizing channels, used to melt the surface layer raw materials and the middle layer raw materials respectively) through a feeding system, and melt extruded at an extrusion temperature of 140-280°C. After primary filtration and secondary filtration, the melt passes through a three-layer diverter, enters the die head, and is extruded to form a three-layer structure having two surface layers and an intermediate layer between the two surface layers. Both surface layers are maleic anhydride modified polypropylene. The middle layer is polypropylene, the die temperature is 150-155°C, and the melt passes through a high-speed traction chilling roller after coming out of the die. The roller temperature is set to 60-120°C, and the pressure of the negative pressure wind hood is adjusted to make the high-temperature melt fit tightly against the chilling roller. The distance from the melt to the die is L = 0.5-3 cm (the distance is adjustable, and the distance is different for products of different specifications). After the high-speed traction film passes through the online thickness gauge and the online defect detector, the edge bait is cut off and then enters the winding system for winding. This process completes the initial crystallization of the material.

[0067] 2. Annealing (one-time baking): Put the rolled film into a film cart and put it into a heat treatment oven in groups for baking. The heat treatment temperature is 135-155°C and the heat treatment time is 8-24 hours. After baking, push the film cart out of the oven and cool it in a natural environment.

[0068] 3. Compounding: Compound the cooled film to prepare for multi-layer stretching. Compounding means laminating the film, usually 16 to 20 large layers, each large layer has a three-layer structure, the three-layer structure is two surface layers and a middle layer between the two surface layers, both surface layers are maleic anhydride modified polypropylene, and the middle layer is polypropylene. The compounding step is to improve the efficiency of subsequent stretching.

[0069] 4. Stretching: The composite film roll is sent to the stretching machine for stretching and hole forming. The stretching section is divided into five functional areas according to the functional area: cold drawing preheating, cold drawing, hot drawing preheating, hot drawing and heat setting. The cold drawing preheating temperature is 40-120℃, the cold drawing temperature is set to 40-90℃, and the cold drawing speed ratio is 1.1-2.0; the hot drawing preheating section temperature is 120-160℃, the hot drawing preheating speed ratio is 1.5-4.0; the hot drawing section temperature is 125-165℃, the hot drawing speed ratio is 1.5-5.5; the heat setting temperature is 130-170℃, and the heat setting speed ratio is 0.5-2.0. After stretching, the film enters the winding system, and the winding tension is set to 5-50N. After stretching, the product performance has been finalized, and the product can be layered, split into a single large layer structure before compounding, or split into a stacked structure with fewer large layers, and then cut into different specifications (usually including thickness, width, and length) as required.

[0070] 5. Standing: Let the rolled film stand for 8 to 36 hours to release internal stress.

[0071] 6. Heat treatment (secondary baking): After standing, put it back into the oven to release the internal stress for the second time. The oven temperature is 40-120℃ and the time is 4-12 hours.

[0072] 7. Test: Take samples to test the basic physical and chemical properties and elastic modulus of the diaphragm. The elastic modulus value can reflect the rigidity and flexibility of the material. The larger the modulus value, the less likely the material is to deform.

[0073] Embodiments 2 to 7

[0074] Examples 2 to 7 are carried out with reference to Example 1, and the differences are shown in Tables 1 and 2. The differences include different thicknesses of the prepared diaphragms, the specific thicknesses are shown in Table 2, and the specific process parameters of each example are shown in Table 1.

[0075] Table 1

[0076]

[0077]

[0078] The basic physical and chemical performance indicators of the high-performance multilayer lithium-ion battery separators of Examples 1 to 7 are shown in Table 2.

[0079] Table 2

[0080]

[0081] The thickness test method refers to "GB / T 6672-2001 Mechanical measurement method for thickness of plastic films and sheets". The measurement is carried out using a Mahr thickness gauge with a flat contact head. The instrument is calibrated and reset before measurement, and the contact surface is kept clean. A point is measured every 5 cm along the TD direction of the film, and the average value of the five points is the thickness.

[0082] Air permeability test: refer to GB / T 458-2008 Determination of air permeability of paper and paperboard, take 5 samples and test them with an air permeability meter, and take the average value of the measurement as the air permeability value of the sample to be tested.

[0083] The porosity test method is as follows: After the membrane is completely immersed in the electrolyte for 1 hour, it is taken out, the electrolyte on the surface of the membrane is absorbed with absorbent paper, and then weighed. The porosity p is calculated as follows: p = (ρ c -ρ e ) / ρ c *100%.ρ c is the density of the diaphragm after being soaked in the electrolyte, ρ e It is the density of the diaphragm when it is not soaked in electrolyte.

[0084] The tensile strength test method is carried out in accordance with GB / T 1040-2006 Test of tensile properties of plastics. Use a sample cutter to take 5 samples of 100mm×20mm in the MD and TD directions of the film, clamp them on the tensile fixture of the universal tensile machine, and the fixture breaks the sample at a speed of 100mm / min. The software will automatically process and obtain the tensile strength of the sample. The average value of the 5 points is its tensile strength.

[0085] The heat shrinkage test method is carried out in accordance with GB / T 12027-2004 Test method for dimensional change rate of plastic film and sheet upon heating. Five samples of 100 mm × 100 mm or larger are taken along the MD and TD directions of the film. The actual size of the sample is measured. The sample is then sandwiched between two A4 papers. After the oven temperature stabilizes, the sample is placed in the oven and heated at 120°C for 1 hour. The size after heating is measured and the shrinkage rate is calculated. The average value of the five points is the heat shrinkage.

[0086] The puncture strength test method is carried out in accordance with GB / T 21302-2007 General Rules for Composite Films and Bags for Packaging. Five circular samples with a diameter greater than 40 mm are taken along the TD direction of the film and clamped in the puncture fixture. The flat-headed needle moves downward at a speed of 100 mm / min. The maximum force that pierces the sample is the puncture strength. The average value of the five points is the puncture strength.

[0087] Elastic modulus: Refer to the standard "GBT 1040.1-2006 Test of tensile properties of plastics" and prepare samples, test and calculate the elastic modulus according to the corresponding standard.

[0088] Diaphragm contact angle test method: drop the electrolyte onto the surface of the diaphragm sample, use a video contact angle meter to test the contact angle of the diaphragm and the electrolyte, repeat 5 times and take the average value.

[0089] The thermal shrinkage was tested by the following method: the battery separators were cut into samples with a length of 10 cm in the longitudinal direction MD and 10 cm in the transverse direction TD, and placed in an oven preheated to 150°C. After 30 minutes, the samples were taken out and their length and width were measured with a vernier caliper. The thermal shrinkage was calculated according to the following formula:

[0090] Longitudinal heat shrinkage = (longitudinal length of sample before heat treatment - longitudinal length of sample after heat treatment) / longitudinal length of sample before heat treatment;

[0091] Transverse heat shrinkage = (transverse length of sample before heat treatment - transverse length of sample after heat treatment) / transverse length of sample before heat treatment.

[0092] The test method of the liquid absorption rate and liquid retention rate of the diaphragm is as follows: Weigh a film with a size of 10×10 cm, and record the mass as W 0 , immersed in the electrolyte at room temperature, let it stand for 2 hours, then take out the film, absorb the electrolyte on the surface of the film with filter paper and weigh it, the mass is recorded as W 1 After being placed at room temperature for 10 minutes, the weight is weighed again and the mass is recorded as W 2 . Calculated as follows:

[0093] Liquid absorption rate = (W 2 -W 0 ) / W 0 ;

[0094] Liquid retention rate = (W 1 -W 2 ) / (W 1 -W 0 ).

[0095] Figure 1 It is a comparison chart of the contact angles of an ordinary PP (polypropylene) film (a commercial product of Shenzhen ZTE New Materials Technology Co., Ltd., model ZT16400) and the high-performance multilayer lithium-ion battery separator of Example 1. The thickness of the PP (polypropylene) film and the separator of Example 1 are both 20 μm.

[0096] from Figure 1 It can be found that the high-performance multi-layer lithium-ion battery separator absorbs almost all of the energy in about 10ms, while the ordinary PP film still has no obvious absorption at 40ms.

[0097] Figure 2 It is a comparison chart of surface liquid absorption rate. The left picture is the liquid absorption chart of the existing PP (polypropylene) film, and the right picture is the liquid absorption chart of the high-performance multilayer lithium-ion battery separator of Example 1. Figure 2 The picture was taken 1 minute after the dripping. It can be seen that after 1 minute, the existing PP (polypropylene) film in the left picture still only absorbs a small amount of electrolyte, while the high-performance multilayer lithium-ion battery separator in the right picture almost completely absorbs the electrolyte, proving that the liquid absorption performance of the high-performance multilayer lithium-ion battery separator of Example 1 is significantly better than that of the existing PP (polypropylene) film.

[0098] from Figure 1 , Figure 2 It can be found that when the electrolyte is dripped onto the membrane using an ordinary dropper, after the electrolyte contacts the diaphragm, the high-performance membrane of Example 1 absorbs the electrolyte almost instantly, while the electrolyte on the ordinary PP membrane is basically not absorbed.

[0099] Figure 1 , 2 The electrolyte used in the 6 , EC (ethylene carbonate), EMC (ethyl methyl carbonate), DMC (dimethyl carbonate), EC, EMC, DMC are solvents. LiPF in the electrolyte 6 The concentration is 1 mol / L, and by volume, EC:EMC:DMC=1:1:1.

[0100] The above specific examples are used to illustrate the present invention, which is only used to help understand the present invention and is not intended to limit the present invention. For those skilled in the art, according to the concept of the present invention, some simple deductions, modifications or substitutions can be made.

Claims

1. A film, characterized in that: The film is used to isolate the positive and negative electrodes of the battery. The film has a three-layer structure, including two surface layers and an intermediate layer located between the two surface layers. The two surface layers are independently composed of a thermoplastic resin and a lyophilic modifier. The intermediate layer contains a thermoplastic resin, and the weight average molecular weight of the thermoplastic resin contained in the intermediate layer is 500,000 to 600,000; the lyophilic modifier contains maleic anhydride, silane, polystyrene, styrene, polyvinylidene fluoride, polyethylene terephthalate, titanate, aluminate, phosphate, At least one of boric acid esters or their copolymers and blends; the thermoplastic resin comprises polyolefin; wherein the lyophilic modifier is physically mixed with the thermoplastic resin to modify the thermoplastic resin; in the surface layer, the mass of the thermoplastic resin accounts for 20% to 90% of the total mass of the thermoplastic resin and the lyophilic modifier; the contact angle of the film is less than 20°; the air permeability of the film is 100 to 600 s / 100 mL, and the longitudinal tensile strength is ≥1200 kgf / cm 2 ; Transverse tensile strength ≥100kgf / cm 2 .

2. The film according to claim 1, characterized in that The silane comprises at least one of monosilane and disilane; And / or, the titanate comprises at least one of triisostearoyl titanate, isopropyl dioleyl titanate, and isopropyl tri(dioctylphospho) titanate; And / or, the aluminate comprises at least one of triisopropyl aluminate and tribenzyl aluminate; And / or, the phosphate ester comprises at least one of a phosphate monoester, a phosphate diester, and a phosphate triester; And / or, the borate ester comprises at least one of 3-(tert-butoxycarbonylamino)phenylboronic acid, isopropyl pinacol borate, and vinyl pinacol borate.

3. The film according to claim 1, characterized in that The polyolefin comprises at least one of polyethylene, polypropylene, poly-1-butene and polypentene.

4. The film according to claim 1, characterized in that The three-layer structure includes a first surface layer, a middle layer, and a second surface layer in sequence, and the thickness of the middle layer accounts for 20% to 90% of the total thickness of the three layers.

5. The film according to claim 4, characterized in that The thickness of the middle layer accounts for 30~60% of the total thickness of the three layers.

6. The film according to claim 4, characterized in that The thickness of the two skins is equal; and / or, the thickness ratio is first surface layer: middle layer: second surface layer = 3:4:3; And / or, the surface layer and the middle layer are compounded by dry single drawing; the dry single drawing includes casting, annealing, compounding and stretching.

7. The film according to claim 4, characterized in that In the two surface layers, the mass of the thermoplastic resin accounts for 80% of the total mass of the thermoplastic resin and the lyophilic modifier; and / or the weight average molecular weight of the thermoplastic resin contained in the intermediate layer is greater than the weight average molecular weight of the thermoplastic resin contained in the surface layer; And / or the weight average molecular weight of the thermoplastic resin contained in the intermediate layer is 530,000 to 550,000.

8. The film according to claim 4, characterized in that The weight average molecular weight of the thermoplastic resin contained in the surface layer is ≥ 300,000.

9. The film according to claim 8, characterized in that The weight average molecular weight of the thermoplastic resin contained in the surface layer is 300,000 to 500,000.

10. The film according to claim 4, characterized in that The melt index of the thermoplastic resin contained in the surface layer is 0.5~5g / 10min.

11. The film according to claim 10, characterized in that The melt index of the thermoplastic resin contained in the surface layer is 0.5~4.0 g / 10min.

12. The film according to claim 11, characterized in that The melt index of the thermoplastic resin contained in the surface layer is 0.5~3.0 g / 10min.

13. The film according to claim 4, characterized in that The thermoplastic resin contained in the middle layer has a melt index of 0.5 to 5 g / 10min.

14. The film according to claim 13, characterized in that The thermoplastic resin contained in the middle layer has a melt index of 0.5 to 4.0 g / 10 min.

15. The film according to claim 14, characterized in that The thermoplastic resin contained in the middle layer has a melt index of 0.5 to 3.0 g / 10min.

16. The film according to claim 1, characterized in that The film has a thickness of 6-40 μm, a porosity of 30-60%, a puncture strength of 200-1000 g, and an elastic modulus of 400-850 kgf / cm 2 .

17. A method for preparing a thin film according to any one of claims 1 to 16, characterized in that: The preparation method includes dry single drawing, which includes a casting step, an annealing step, a compounding step, and a stretching step. In the casting step, the raw materials for preparing the middle layer and the surface layer are respectively put into the extruder, melt-extruded, and the extruded materials are contacted with the surface of a chilled roller to form a film by traction; in the compounding step, a three-layer structure is formed by compounding, including two surface layers and an middle layer located between the two surface layers.

18. The preparation method according to claim 17, characterized in that: Extrusion temperature is 140~280℃; and / or, after the melt is filtered, it is extruded from a die head, and the die head temperature is 150-155° C.; and / or, the chill roll temperature is 60-120°C; and / or, the annealing temperature is 135-155° C.; And / or, the annealing time is 8 to 24 hours.

19. The preparation method according to claim 17, characterized in that: During stretching, the steps include cold drawing preheating, cold drawing, hot drawing preheating, hot drawing and heat setting.

20. The preparation method according to claim 19, characterized in that: The cold drawing preheating temperature is 40~120℃, the cold drawing temperature is set to 40~90℃, and the speed ratio is 1.1~2.0; the hot drawing preheating section temperature is 120~160℃, and the hot drawing preheating speed ratio is 1.5~4.0; the hot drawing section temperature is 125~165℃, and the speed ratio is 1.5~5.5; the heat setting temperature is 130~170℃, and the speed ratio is 0.5~2.

0.

21. The preparation method according to claim 17, characterized in that: The method also includes rolling up and standing the stretched film.

22. The preparation method according to claim 21, characterized in that: The standing time is 8~36h.

23. The preparation method according to claim 21, characterized in that: After standing, heat treatment is also included.

24. The preparation method according to claim 23, characterized in that: The temperature of heat treatment is 40~120℃.

25. The preparation method according to claim 23, characterized in that: The heat treatment time is 4 to 12 hours.

26. The preparation method according to claim 17, characterized in that: The method also includes layering and slitting the film obtained by stretching.

27. A battery, characterized in that: The battery comprises the thin film according to any one of claims 1 to 16.

28. The battery according to claim 27, characterized in that The batteries include lithium-ion batteries and sodium-ion batteries.

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