Polypropylene film for composite current collector and preparation method and application thereof

Abstract

The present application relates to the field of current collector electronic technology materials, and particularly relates to a polypropylene film for composite current collector, a preparation method and application thereof.The polypropylene film for composite current collector comprises a three-layer structure, and from outside to inside, the three-layer structure is sequentially a first surface layer, a core layer and a second surface layer; the first surface layer and the second surface layer are composed of a copolymer or grafting material and dendritic polyamide PAMAM and high stereoregularity polypropylene; the core layer is composed of medium stereoregularity polypropylene, nano metal oxide particles and conductive material.The polypropylene film is formed by synchronous bidirectional stretching on a linear motor synchronous stretching line (LISIM line).The polypropylene film for composite current collector has the characteristics of high bonding strength with metal, excellent mechanical properties, good heat resistance and low internal resistance, and is particularly suitable for application in a composite current collector.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of current collector electronic technology materials, and particularly relates to a polypropylene film for composite current collectors, a preparation method and application thereof. BACKGROUND

[0002] The four main parts of a lithium ion battery are positive electrode material, negative electrode material, separator and electrolyte. In addition to the four main parts, the current collector used to store the positive and negative electrode materials is also an important part of the lithium battery.

[0003] The current collector has the important functions of bearing active materials (bearing property) and collecting micro-current (conductivity). Thinness and functionalization are the main development directions of the current collector. However, there is a natural contradiction between the mechanical strength and the thickness requirement of copper / aluminum foil, and the thinning processing link will further increase its cost. Therefore, the composite copper / aluminum foil becomes a new scheme for the light and thin current collector.

[0004] The composite current collector is a "sandwich" structure, the inner layer is a polymer macromolecule layer (such as PET, PP or PI), and the two sides are metal conductive layers (such as Al or Cu). In the prior art, the inner layer polymer is mostly polypropylene, which has the advantages of low cost, small density and acid and alkali resistance compared with terephthalate glycol (PET) and polyimide (PI), but the material itself has defects such as low strength, poor heat resistance and poor metal bonding strength, which need to be further modified in material and process to expand the application.

[0005] In the prior art, the composite current collector has the advantages of high safety, high specific energy, long service life, low cost and the like compared with the single metal foil current collector, but also has defects such as complex process, low yield and high internal resistance. Polypropylene has the advantages of low cost, small density and acid and alkali resistance compared with terephthalate glycol (PET) and polyimide (PI), but the material itself has defects such as low strength, poor heat resistance and poor metal bonding strength, which need to be further modified in material and process to expand the application.

[0006] CN 115000418 A discloses a coating-free composite current collector, which uses a modified polypropylene film containing carboxyl and / or ester groups on the molecular chain, so that the collector can be directly compounded with the plating layer. Although the polypropylene film is grafted with carboxyl and / or ester groups, the adhesion property with metal is improved, but since the polypropylene film is only grafted with carboxyl and / or ester groups, the intermolecular space is loose, and the overall thermal stability and tensile strength of the material are affected. In addition, active groups are introduced on the surface of the material to improve the surface energy of the material, but these active groups are not stable, and with the free movement of the molecular chain, the active groups gradually turn to adjacent molecules or groups, even in the material, the system tends to be stable, and the surface energy will decrease over time, which will also cause the adhesion of the metal layer to decrease over time.

[0007] Therefore, there is a need for a polypropylene film substrate for a composite current collector that can enhance the bonding strength with the metal layer while ensuring its thermal stability and tensile strength. SUMMARY

[0008] To solve the problems mentioned in the above background, the present application provides a polypropylene film for a composite current collector, which comprises a three-layer structure, from outside to inside, a first surface layer, a core layer, and a second surface layer.

[0009] The first surface layer and the second surface layer are composed of a copolymer or graft of a copolymer containing carboxylic anhydride bonds and ester bonds, dendritic polyamide PAMAM, and high crystallinity polypropylene.

[0010] The core layer is composed of medium crystallinity polypropylene mixed with nano metal oxide particles and conductive material.

[0011] On the basis of the above technical solution, further, the first surface layer and the second surface layer are composed of 1wt%-5wt% of a copolymer or graft of a copolymer containing carboxylic anhydride bonds and ester bonds, 5wt%-15wt% of dendritic polyamide PAMAM, and 80wt%-94wt% of high crystallinity polypropylene.

[0012] The core layer is composed of 1wt%-5wt% of nano metal oxide particles, 1wt%-5wt% of conductive material, and 90wt%-98wt% of medium crystallinity polypropylene.

[0013] On the basis of the above technical solution, further, the copolymer or graft containing carboxylic anhydride bonds and ester bonds is at least one of ethylene vinyl alcohol copolymer (EVOH), polypropylene maleic anhydride graft (PP-g-MAH), and polypropylene graft glycidyl methacrylate (PP-g-GMA).

[0014] On the basis of the above technical scheme, further, the dendritic polyamide amine PAMAM is selected from at least one of PAMAM 1.0 generation, PAMAM 2.0 generation, PAMAM 3.0 generation, PAMAM 4.0 generation, PAMAM 5.0 generation, PAMAM 6.0 generation, PAMAM 7.0 generation, PAMAM 8.0 generation, PAMAM 9.0 generation, PAMAM 10.0 generation.

[0015] On the basis of the above technical scheme, further, the nano metal oxide particles are at least one of nano MgO, nano Al2O3, nano CaO, nano ZnO.

[0016] On the basis of the above technical scheme, further, the conductive material is at least one of carbon nanotubes, nanometer carbon fibers, graphene, carbon black.

[0017] On the basis of the above technical scheme, further, the high isotacticity polypropylene is polypropylene with isotacticity greater than 97%.

[0018] On the basis of the above technical scheme, further, the thickness of the first surface layer and the second surface layer is 0.5-1 mu m, and the thickness of the core layer is 2-5 mu m.

[0019] On the basis of the above technical scheme, further, the corona value of the first surface layer and the second surface layer after corona treatment is greater than or equal to 48 nN / m.

[0020] The application also provides a preparation method of the polypropylene film substrate for the composite current collector, comprising the following steps:

[0021] The granules of the first surface layer, the core layer and the second surface layer are respectively put into auxiliary extrusion 1, main extrusion and auxiliary extrusion 2 according to the proportion, and the three-layer structure cast sheet of the first surface layer, the core layer and the second surface layer is obtained through melt co-extrusion and extremely cold air shower cooling shaping, wherein the temperature from the extruder to the die head is 230-270 DEG C, and the temperature of the extremely cold roller is 80-120 DEG C.

[0022] The obtained cast sheet is subjected to preheating, magnetic suspension linear motor synchronous stretching, shaping, corona, winding, curing, slitting and packaging, and the double-stretched polypropylene film for the composite current collector is obtained.

[0023] The preheating temperature is 170-185 DEG C, the stretching temperature is 168-178 DEG C, the stretching ratio MD is 5-9 times, the stretching ratio TD is 5-9 times, the shaping temperature is 173-185 DEG C, and the curing condition is 30-40 DEG C / 72h.

[0024] The application further provides a use of the polypropylene film for composite current collector in preparation of a composite current collector.

[0025] The polypropylene film for composite current collector provided by the application has the advantages that the dendritic polyamide PAMAM and the copolymer or graft of the polymer containing carboxylic anhydride bond and ester bond are mixed in the outer layer of the film, the dendritic polyamide PAMAM and the copolymer or graft can ensure the bonding strength of the polypropylene film and the metal, and at the same time, the long-term effectiveness of the bonding strength can be ensured, and the heat resistance and tensile strength of the film can be improved. In addition, the core layer is made of the medium-regulation polypropylene mixed with the nano metal oxide particles and the conductive material, the nano metal oxide particles and the conductive material can not only reduce the internal resistance of the film, but also can be closely combined with the dendritic polyamide PAMAM, and further improve the thermal stability of the film and the strength of the whole film. The selected medium-regulation polypropylene can further save the material cost and expand the richness of the material selection. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A film layer structure schematic diagram of the polypropylene film for composite current collector provided by the application.

[0027] REFERENCE NUMERALS

[0028] 10 - first surface layer; 20 - core layer; 30 - second surface layer. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the embodiments of the application clearer, the technical scheme in the embodiments of the application will be described clearly and completely below in conjunction with the drawings in the embodiments of the application. Obviously, the described embodiments are some embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application. In the description of the application, it should be noted that the orientations or positional relationships indicated by the terms “center”, “longitudinal”, “transverse”, “upper”, “lower”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inner”, “outer” and the like are based on the orientations or positional relationships shown in the drawings, and are only for the purpose of facilitating the description of the application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements indicated must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the application. In addition, the terms “first” and “second” are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0030] The present application provides the following examples and comparative examples of polypropylene film:

[0031] Example 1

[0032] The composite current collector polypropylene film is a three-layer structure, as shown in the figure, the structure of the film from outside to inside is first skin layer 10, core layer 20, second skin layer 30; Figure 1

[0033] The first skin layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0 and 87wt% homopolypropylene with isotacticity of 98%;

[0034] The core layer is composed of 3wt% nano-Al2O3, 3wt% carbon black and 94wt% homopolypropylene with isotacticity of 96%;

[0035] The first skin layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0 and 87wt% homopolypropylene with isotacticity of 98%.

[0036] Its preparation method:

[0037] The granules of the first skin layer, the core layer and the second skin layer are respectively put into auxiliary extrusion 1, main extrusion and auxiliary extrusion 2 according to the proportion, and through melt co-extrusion and extremely cold air shower cooling, a three-layer structure cast sheet composed of the first skin layer, the core layer and the second skin layer is obtained. The temperature from the extruder to the die head is 230-270℃, and the extremely cold roller temperature is 80-120℃ (within this temperature range, the cast sheet is cooled to make the surface of the cast sheet material crystallize and coarsen, and through the coarsening treatment, the adhesion of the film surface can be further increased).

[0038] The cast sheet prepared above is subjected to preheating, magnetic suspension linear motor synchronous stretching, setting, corona treatment, winding, aging, slitting and packaging processes to obtain a double-stretched polypropylene film for composite current collector. The preheating temperature is 170-185℃, the stretching temperature is 168-178℃, the stretching ratio MD is 5-9 times and the stretching ratio TD is 5-9 times, the setting temperature is 173-185℃, and the aging condition is 30-40℃ / 72h.

[0039] The corona value after corona treatment is ≥48nN / m, and the corona treatment can increase the surface energy of the film and improve its adhesion.

[0040] Example 2

[0041] The composite current collector polypropylene film is a three-layer structure, the structure of the film from outside to inside is first skin layer, core layer, second skin layer;

[0042] ​The first skin layer consists of 5% PP-g-MAH, 15% PAMAM4.0 and 80% homopolypropylene with isotacticity of 98%;

[0043] The core layer consists of 5wt% nano Al2O3, 5wt% carbon black and 90wt% homopolypropylene with isotacticity of 96%;

[0044] The second skin layer consists of 5wt% PP-g-MAH, 15wt% PAMAM4.0 and 80wt% homopolypropylene with isotacticity of 98%.

[0045] The preparation method is the same as that of Example 1.

[0046] Example 3

[0047] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first skin layer, the core layer and the second skin layer from outside to inside;

[0048] The first skin layer consists of 1wt% PP-g-MAH, 5wt% PAMAM4.0 and 94wt% homopolypropylene with isotacticity of 98%;

[0049] The core layer consists of 1wt% nano Al2O3, 1wt% carbon black and 98wt% homopolypropylene with isotacticity of 96%;

[0050] The second skin layer consists of 1wt% PP-g-MAH, 5wt% PAMAM4.0 and 94wt% homopolypropylene with isotacticity of 98%.

[0051] The preparation method is the same as that of Example 1.

[0052] Example 4

[0053] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first skin layer, the core layer and the second skin layer from outside to inside;

[0054] The first skin layer consists of 3wt% EVOH, 10wt% PAMAM1.0 and 87wt% homopolypropylene with isotacticity of 99%;

[0055] The core layer consists of 3wt% nano MgO, 3wt% graphene and 94wt% homopolypropylene with isotacticity of 96%;

[0056] The second skin layer consists of 3wt% EVOH, 10wt% PAMAM1.0 and 87wt% homopolypropylene with isotacticity of 99%.

[0057] The preparation method is the same as that of Example 1.

[0058] Comparative Example 1

[0059] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first surface layer, the core layer, and the second surface layer from outside to inside;

[0060] The first surface layer is composed of 100wt% homopolypropylene with an isotacticity of 96%;

[0061] The core layer is composed of 100wt% homopolypropylene with an isotacticity of 96%;

[0062] The second surface layer is composed of 100wt% homopolypropylene with an isotacticity of 96%.

[0063] The preparation method is the same as that of Example 1.

[0064] Comparative Example 2

[0065] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first surface layer, the core layer, and the second surface layer from outside to inside;

[0066] The first surface layer is composed of 3wt% PP-g-MAH and 97wt% homopolypropylene with an isotacticity of 98%;

[0067] The core layer is composed of 3wt% nano-Al2O3, 3wt% carbon black, and 94wt% homopolypropylene with an isotacticity of 96%;

[0068] The second surface layer is composed of 3wt% PP-g-MAH and 97wt% homopolypropylene with an isotacticity of 98%.

[0069] The preparation method is the same as that of Example 1.

[0070] Comparative Example 3

[0071] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first surface layer, the core layer, and the second surface layer from outside to inside;

[0072] The first surface layer is composed of 10wt% PAMAM4.0 and 90wt% homopolypropylene with an isotacticity of 98%;

[0073] The core layer is composed of 3wt% nano-Al2O3, 3wt% carbon black, and 94wt% homopolypropylene with an isotacticity of 96%;

[0074] The second surface layer is composed of 10wt% PAMAM4.0 and 90wt% homopolypropylene with an isotacticity of 98%.

[0075] The preparation method is the same as that of Example 1.

[0076] Comparative Example 4

[0077] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first surface layer, the core layer, and the second surface layer from outside to inside;

[0078] The first surface layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0, and 87% homopolymer polypropylene with a isotacticity of 98%;

[0079] The core layer is composed of 3wt% nano-Al2O3 and 97wt% homopolymer polypropylene with a isotacticity of 96%;

[0080] The second surface layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0, and 87wt% homopolymer polypropylene with a isotacticity of 98%.

[0081] The preparation method is the same as that of Example 1.

[0082] Comparative Example 5

[0083] The composite current collector polypropylene film is a three-layer structure, and the structure of the film is sequentially the first surface layer, the core layer, and the second surface layer from outside to inside;

[0084] The first surface layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0, and 87% homopolymer polypropylene with a isotacticity of 98%;

[0085] The core layer is composed of 3wt% carbon black and 97wt% homopolymer polypropylene with a isotacticity of 96%;

[0086] The second surface layer is composed of 3wt% PP-g-MAH, 10wt% PAMAM4.0, and 87wt% homopolymer polypropylene with a isotacticity of 98%.

[0087] The preparation method is the same as that of Example 1.

[0088] The various performance tests of each example and comparative example are carried out, the test items and test standards are shown in Table 1, and the test results are shown in Table 2.

[0089] Table 1 Test item table

[0090]

[0091] Table 2 Test result table

[0092]

[0093]

[0094] As shown in Table 2, compared with Comparative Examples 1-5, the polypropylene film for composite current collector of Examples 1-4 has high adhesion strength with metal, excellent mechanical properties, good heat resistance and low internal resistance, and is particularly suitable for application in composite current collector.

[0095] In the formula, the initial surface tension and the aluminizing adhesion are used to evaluate the adhesion strength of the film.

[0096] Specifically, the first surface layer, the core layer and the second surface layer in Comparative Example 1 are all prepared from homopolypropylene in the prior art. As shown in the above test results, the existing polypropylene film has a large gap in adhesion strength, resistance, thermal stability and tensile strength compared with the technical solution of the present application.

[0097] In Comparative Example 2, the difference from Example 1 is that PAMAM 4.0 in the first surface layer and the second surface layer is replaced with an equal amount of 98% homopolypropylene, and the other components are the same as in Example 1. As shown in the above test results, the film only uses grafted modification of polypropylene without adding dendritic polyamide for use together, and the adhesion, thermal stability and tensile strength are all decreased compared with the examples of the present application.

[0098] In Comparative Example 3, the difference from Example 1 is that PP-g-MAH in the first surface layer and the second surface layer is replaced with an equal amount of 98% homopolypropylene, and the other components are the same as in Example 1. As shown in the above test results, only dendritic polyamide is used, and since the adhesion of dendritic polyamide itself is not good, the adhesion needs to be combined with a copolymer or graft containing carboxylic anhydride bond and ester bond to achieve better results, and the thermal stability of the film is also affected.

[0099] In Comparative Example 4, the difference from Example 1 is that carbon black in the core layer is replaced with an equal amount of 96% homopolypropylene, and the other components are the same as in Example 1. The overall strength and thermal stability of the film are decreased compared with the present application.

[0100] In Comparative Example 5, the difference from Example 1 is that nano-Al2O3 in the core layer is replaced with an equal amount of 96% homopolypropylene, and the other components are the same as in Example 1. The overall thermal stability of the film is decreased.

[0101] The polypropylene film for composite current collector prepared by the present application can be applied in the preparation of composite current collector technology. The composite current collector is a sandwich structure, and the polypropylene film for composite current collector prepared by the present application can be used as the middle layer, and there is a layer of metal foil on each side of the middle layer, and the metal foils on both sides are one of aluminum foil or copper foil.

[0102] In addition, those skilled in the art should understand that, although there are many problems in the prior art, each embodiment or technical solution of the present application can only be improved in one or several aspects, and it is not necessary to solve all the technical problems listed in the prior art or background art at the same time. Those skilled in the art should understand that what is not mentioned in a claim should not be regarded as a limitation on the claim.

[0103] Although the terms such as first polyamide skin layer, polyamide core layer, second polyamide skin layer and the like are used more frequently herein, the possibility of using other terms is not excluded. The use of these terms is only for the convenience of describing and explaining the essence of the present application; any additional limitation is contrary to the spirit of the present application; the terms "first", "second" in the description and claims of the embodiments of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0104] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can still be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A polypropylene film for a composite current collector, characterized by: The three-layer structure comprises a first surface layer, a core layer, and a second surface layer from outside to inside; The core layer is made of medium degree polypropylene mixed with nano metal oxide particles and conductive material; The first surface layer and the second surface layer are composed of 1 wt%-5 wt% ethylene vinyl alcohol copolymer, polypropylene maleic anhydride graft, polypropylene graft glycidyl methacrylate, 5 wt%-15 wt% dendritic polyamide PAMAM, and 80 wt%-94 wt% high degree polypropylene; The core layer is composed of 1 wt%-5 wt% nano metal oxide particles, 1 wt%-5 wt% conductive material, and 90 wt%-98 wt% medium degree polypropylene.

2. The polypropylene film for composite current collector according to claim 1, characterized in that: The dendritic polyamide PAMAM is at least one of PAMAM 1.0 generation, PAMAM 2.0 generation, PAMAM 3.0 generation, PAMAM 4.0 generation, PAMAM 5.0 generation, PAMAM 6.0 generation, PAMAM 7.0 generation, PAMAM 8.0 generation, PAMAM 9.0 generation, and PAMAM 10.0 generation.

3. The polypropylene film for a composite current collector according to claim 1, characterized by: The nano metal oxide particles are at least one of nano MgO, nano Al2O3, nano CaO, and nano ZnO; and the conductive material is at least one of carbon nanotube, nano carbon fiber, graphene, and carbon black.

4. The polypropylene film for a composite current collector according to claim 1, characterized by: The high degree polypropylene is polypropylene with a degree of isotacticity greater than 97%.

5. The polypropylene film for a composite current collector according to claim 1, characterized by: The thickness of the first surface layer and the second surface layer is 0.5-1 μm, and the thickness of the core layer is 2-5 μm.

6. The polypropylene film for a composite current collector according to claim 1, characterized by: The corona value of the first surface layer and the second surface layer after corona treatment is greater than or equal to 48 nN / m.

7. The method for preparing a polypropylene film for composite current collectors according to any one of claims 1-6, characterized in that, The method comprises the following steps: The granules of the first surface layer, the core layer, and the second surface layer are respectively put into auxiliary extrusion 1, main extrusion, and auxiliary extrusion 2 according to the proportion, and then melt co-extrusion and extremely cold air shower cooling are performed to obtain a three-layer structure cast piece of the first surface layer, the core layer, and the second surface layer; wherein the temperature from the extruder to the die head is 230-270℃, and the temperature of the extremely cold roller is 80-120℃; The obtained cast piece is subjected to the processes of preheating, magnetic suspension linear motor synchronous stretching, setting, corona, winding, aging, slitting, and packaging to obtain a double-stretched polypropylene film for composite current collector; The preheating temperature is 170-185℃, the stretching temperature is 168-178℃, the stretching ratio MD is 5-9 times, the stretching ratio TD is 5-9 times, the setting temperature is 173-185℃, and the aging condition is 30-40℃ / 72h.

8. Use of the polypropylene film according to any one of claims 1 to 6 for the production of a composite current collector, characterized in that: The polypropylene film for composite current collector is an intermediate layer, and each side of the intermediate layer has a metal foil, and the metal foils on the two sides are one of aluminum foil and copper foil.

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