A multi-layer co-extruded cross-linked network solid electrolyte membrane and preparation method thereof

Abstract

The present invention provides a multi-layer co-extruded cross-linked network solid electrolyte diaphragm and a preparation method thereof, belonging to the technical field of diaphragms. The multi-layer co-extruded cross-linked network solid electrolyte diaphragm is a five-layer co-extruded structure of outer layer / middle layer / inner layer / middle layer / outer layer. The raw materials of the middle layer include high melt index homopolymer polypropylene, a cross-linking agent and an initiator. The raw materials of the inner layer are low melt index homopolymer polypropylene and a polymer solid electrolyte. The problems of low porosity and small pore size of the traditional diaphragm, large interface internal resistance in semi-solid / solid batteries, difficulty in suppressing pole piece expansion, and low ion conductivity are solved. The strength is enhanced by cross-linking the polymer film, the middle layer uses high melt index polypropylene to ensure the fluidity of the cross-linking agent and the initiator, and the outer layer and the inner layer solid electrolyte are polymerized to form a cross-linked network. The outer layer is separated from the oxide solid electrolyte and the diaphragm pore size is enlarged. The oxide solid electrolyte is added to the outer layer to provide mobile Li ions and improve the ion conductivity.

Description

Technical Field

[0001] The invention belongs to the technical field of diaphragms, and in particular to a multi-layer co-extruded cross-linked network solid electrolyte diaphragm and a preparation method thereof. Background Art

[0002] The new energy industry is one of the strategic emerging industries. The research of new battery materials is listed as a priority and it is important to promote technological development. At present, liquid lithium-ion batteries are used on a large scale, but they have problems such as low energy density and poor safety. In contrast, semi-solid / solid batteries have the advantages of high safety, wide electrochemical window and high energy density. Therefore, the development of semi-solid / solid electrolytes is a key path to achieve the "dual carbon goals". As an important component, the battery separator plays an important role in isolating the positive and negative electrodes and storing electrolytes. The separator material contains a microporous structure, and the insulating olefin polymer material is mainly used as a barrier. Its performance is related to key properties such as capacity, cycle performance and charge and discharge current density. The mainstream preparation methods of the separator are divided into dry uniaxial stretching separator, dry biaxial stretching separator and wet biaxial stretching separator. With the development of semi-solid / solid, new requirements are put forward for the separator, such as large pores and high ionic conductivity, which are conducive to electrolyte transport and filling. Those skilled in the art are in urgent need of developing a composite separator for semi-solid batteries and a preparation method thereof to meet existing usage needs and performance requirements. Summary of the invention

[0003] In view of the above problems, the present invention aims to provide a multi-layer co-extruded cross-linked network solid electrolyte membrane and a preparation method thereof.

[0004] The present invention is achieved through the following technical solutions:

[0005] A multi-layer co-extruded cross-linked network solid electrolyte separator, the multi-layer co-extruded cross-linked network solid electrolyte separator is a five-layer co-extruded structure of outer layer / middle layer / inner layer / middle layer / outer layer, the outer layer raw materials include low melt index homopolymer polypropylene, oxide solid electrolyte and polymer solid electrolyte, the middle layer raw materials include high melt index homopolymer polypropylene, a cross-linking agent and an initiator, and the inner layer raw materials are low melt index homopolymer polypropylene and a polymer solid electrolyte;

[0006] The mass ratio of low melting index homopolymer polypropylene, polymer solid electrolyte and oxide solid electrolyte in the outer layer is 50-75:20-40:5-10; the mass ratio of low melting index homopolymer polypropylene and polymer solid electrolyte in the inner layer is 50-70:30-50.

[0007] The preparation method of the multi-layer co-extruded cross-linked network solid electrolyte membrane is to mix the components of the surface layer, the middle layer and the inner layer in proportion and then perform multi-layer co-extrusion, cool the cast sheet, perform heat treatment, longitudinal cold stretching, longitudinal hot stretching, heat setting, and traction and winding to obtain the multi-layer co-extruded membrane.

[0008] Furthermore, the extrusion temperature during the multi-layer co-extrusion is 200-230°C, the die temperature is 180-210°C, the casting temperature is 50-80°C, the heat treatment temperature is 110-130°C, and the heat treatment time is 4-12h; the longitudinal cold stretching temperature is 40-70°C, and the stretching ratio is 1.2-1.5; the longitudinal hot stretching temperature is 130-150°C, and the stretching ratio is 1.5-3.0; the heat setting is 130-145°C, and the heat setting time is 1-5min.

[0009] Furthermore, the isotacticity of the low melt index homopolymer polypropylene in the outer layer and the inner layer is not less than 98%, and the melt index at 230°C is 0.5-2g / 10min; the polymer solid electrolyte in the outer layer and the inner layer is one or more of polyethylene oxide, polyacrylonitrile, polyvinyl alcohol, and polymethyl methacrylate.

[0010] Furthermore, the oxide solid electrolyte in the outer layer is one or more of LLTO and LLZO, and the particle size of the oxide solid electrolyte is 50 to 100 nm.

[0011] LLTO (lithium lanthanum titanate / lithium lanthanum titanate, Li 0.33 La 0.56 TiO3), LLZO (lithium zirconium oxide / lithium lanthanum zirconate, Li7La3Zr2O 12 ).

[0012] Furthermore, the mass ratio of the high melt index homopolymer polypropylene, the crosslinking agent and the initiator in the intermediate layer is 92-98:1-5:1-3.

[0013] Furthermore, the isotacticity of the high melt index homopolymer polypropylene in the intermediate layer is not less than 98%, and the melt index at 230°C is 10-20g / 10min; the crosslinking agent in the intermediate layer is divinylbenzene or polyethylene glycol diacrylate, and the initiator in the intermediate layer is diisopropyl peroxide or benzoyl peroxide.

[0014] The present invention uses low melt index homopolymer polypropylene in the outer layer and the inner layer. The low melt index homopolymer polypropylene has a higher molecular weight and correspondingly has a higher strength, which can improve the tensile and puncture performance of the diaphragm. When added to the outer layer and the inner layer, it can support the overall solid electrolyte membrane. As a medium for Li ion conduction in the battery, the polymer solid electrolyte needs to have a higher ion migration number and ion conductivity, and excellent electrochemical stability. Compared with inorganic ceramic-based solid electrolytes, the interface between the polymer solid electrolyte and the electrode has better contact, which can reduce the interface impedance inside the battery. Adding the polymer solid electrolyte to polypropylene and forming a cross-linked network structure by cross-linking can improve the elasticity and interface contact performance of the polymer, so that the electrolyte membrane can withstand the volume expansion generated by the electrode during the charge and discharge process and inhibit the growth of dendrites, and always maintain a low interface impedance in the long cycle of the battery, thereby improving the stability of the solid-state battery. The oxide solid electrolyte added to the outer layer has higher ionic conductivity and chemical stability on the one hand, and is also a provider of Li ions in the battery, and can form mobile Li ions in the solid electrolyte.

[0015] The present invention uses high melt index homopolymer polypropylene in the middle layer. Compared with low melt index homopolymer polypropylene, high melt index homopolymer polypropylene has a lower molecular weight and thus has better fluidity, which helps the crosslinking agent and initiator of the middle layer to migrate to the outer layer and the inner layer. The crosslinking agent and the initiator play the role of crosslinking the polymer solid electrolyte to form a network. When the outer layer / middle layer / inner layer converge at the die head, the crosslinking agent and the initiator of the middle layer will migrate to the outer layer and the inner layer, triggering the crosslinking of the polymer solid electrolyte, forming a crosslinked network structure that can penetrate the entire electrolyte membrane, and improving the ionic conductivity of the electrolyte membrane.

[0016] The multilayer co-extruded cross-linked network solid electrolyte membrane of the present invention constructs a cross-linked solid electrolyte network in the membrane by adding an oxide solid electrolyte into a conventional polypropylene membrane, thereby improving the ionic conductivity of the entire electrolyte membrane. At the same time, the continuous phase of the original polypropylene membrane and the continuous phase formed by the cross-linked oxide solid electrolyte will also separate between the continuous phases during the stretching process, thereby expanding the pore size of the membrane. A larger pore size helps the transmission of lithium ions in the pores and improves the ion conductivity of the solid electrolyte membrane.

[0017] Beneficial effects of the present invention:

[0018] Compared with the prior art, the multilayer co-extruded cross-linked network solid electrolyte diaphragm provided by the present invention solves the problems of low porosity and small pore size of traditional diaphragms, large interface internal resistance and difficulty in suppressing pole piece expansion when applied to semi-solid / solid batteries, and further improves the ionic conductivity and high-power charge and discharge performance of the battery. In the pore structure of the traditional polypropylene diaphragm, the present invention forms a solid electrolyte membrane with a cross-linked network by cross-linking the polymer electrolyte diaphragm. Low melt index polypropylene is used in the outer and inner layers to provide strength, and high melt index polypropylene is used in the middle layer to ensure that the cross-linking agent and initiator have fluidity, and the polymer solid electrolyte of the outer and inner layers is polymerized to form a cross-linked network. Polypropylene and oxide solid electrolytes will phase separate during the stretching process, further expanding the pore size of the diaphragm. The oxide solid electrolyte added to the outer layer acts as a provider of Li ions in the battery, and can form mobile Li ions in the solid electrolyte, while improving the ionic conductivity of the electrolyte membrane. DETAILED DESCRIPTION

[0019] The present invention is described below with specific examples, but is not intended to be limiting of the present invention.

[0020] Raw materials: 50nm LLZO inorganic solid electrolyte Ningbo Fengli New Energy; low melt index homopolymer polypropylene with isotacticity not less than 98% and melt index at 230°C of 2g / 10min, polymer solid electrolyte polymethyl methacrylate in the outer and inner layers. High melt index homopolymer polypropylene with isotacticity not less than 98% and melt index at 230°C of 10g / 10min; crosslinker in the middle layer is divinylbenzene, and initiator in the middle layer is diisopropylbenzene peroxide. Example 1

[0021] (1) 75 parts of low melt index homopolypropylene, 20 parts of polymer solid electrolyte and 5 parts of oxide solid electrolyte are measured by electronic scale, put into a mixing bin for mixing, and then put into a first twin-screw extruder; 98 parts of high melt index homopolypropylene, 1 part of cross-linking agent and 1 part of initiator are measured by electronic scale, put into a mixing bin for mixing, and then put into a second twin-screw extruder; 70 parts of low melt index homopolypropylene and 30 parts of polymer solid electrolyte are measured by electronic scale, put into a mixing bin for mixing, and then put into a third twin-screw extruder;

[0022] (2) The temperatures of the first, second and third extruders are adjusted to 210°C. After melting, the material extruded by the first twin-screw extruder is filtered as the outer layer, the material extruded by the second twin-screw extruder is used as the middle layer, and the material extruded by the third twin-screw extruder is used as the inner layer. After compounding in a three-layer co-extrusion die, the die temperature is adjusted to 200°C.

[0023] (3) The melt extruded from the die is cooled at 70°C and a multi-layer composite thick sheet is obtained at a pulling speed of 30 m / min;

[0024] (4) heat treating the thick slice at 120°C for 8 hours;

[0025] (5) The heat-treated thick sheet is firstly subjected to longitudinal cold stretching at a temperature of 60°C with a stretching ratio of 1.3, and then subjected to longitudinal hot stretching at a temperature of 140°C with a stretching ratio of 2.0.

[0026] (6) The stretched sheet was heat-set at 140°C for 3 minutes.

[0027] (7) After pulling and measuring the thickness of the sheet in step (6), the sheet is rolled up to obtain the low-temperature resistant lithium battery microporous membrane of the present invention, wherein the thickness of the middle layer is 10% of the total thickness, the thickness of the inner layer is 30% of the total thickness, and the thickness of the outer layer is 25% of the total thickness. Example 2

[0028] The same method as in Example 1 is used, except that in step (1), the outer layer comprises 70 parts of low melt index homopolymer polypropylene, 20 parts of polymer solid electrolyte and 10 parts of oxide solid electrolyte. Example 3

[0029] The same method as in Example 2 is used, except that in step (1), the outer layer comprises 50 parts of low melt index homopolymer polypropylene, 40 parts of polymer solid electrolyte and 10 parts of oxide solid electrolyte. Example 4

[0030] The same method as in Example 3 is used, except that in step (1), the intermediate layer is 92 parts of high melt index homopolymer polypropylene, 5 parts of a crosslinking agent and 3 parts of an initiator. Example 5

[0031] Raw materials: LLTO inorganic solid electrolyte Ningbo Fengli New Energy LLTO-N2-50; the isotacticity of low melt index homopolymer polypropylene in the outer layer and the inner layer is not less than 98%, and the melt index at 230°C is 0.5g / 10min; polymer solid electrolyte polymethyl methacrylate; the isotacticity of high melt index homopolymer polypropylene is not less than 98%, and the melt index at 230°C is 10g / 10min; the cross-linking agent in the middle layer is Zhengdan Chemical divinylbenzene, and the initiator in the middle layer is organic peroxides such as diisopropyl peroxide and benzoyl peroxide.

[0032] The multi-layer co-extruded cross-linked network solid electrolyte separator is a five-layer co-extruded structure of outer layer / middle layer / inner layer / middle layer / outer layer, the thickness of the outer layer is 30% of the total thickness, the thickness of the middle layer is 20% of the total thickness, and the thickness of the inner layer is 50% of the total thickness. The outer layer raw materials include low melt index homopolymer polypropylene, LLTO oxide solid electrolyte and polymer solid electrolyte, the middle layer raw materials include high melt index homopolymer polypropylene, crosslinking agent and initiator, and the inner layer raw materials are low melt index homopolymer polypropylene and polymer solid electrolyte; the preparation method of the multi-layer co-extruded cross-linked network solid electrolyte separator is to mix the components of the surface layer, middle layer and inner layer in proportion and then perform multi-layer co-extrusion, the ratio of low melt index homopolymer polypropylene, polymethyl methacrylate in the outer layer; polymer solid electrolyte and oxide solid electrolyte is 50:20:5, The ratio of low melt index homopolymer polypropylene and polymer solid electrolyte in the inner layer is 50:30, the extrusion temperature during multi-layer co-extrusion is 200°C, the die temperature is 180°C, the casting temperature is 50°C, the heat treatment temperature is 110°C, and the heat treatment time is 4h; the longitudinal cold stretching temperature is 40°C, and the stretching ratio is 1.2; the longitudinal hot stretching temperature is 130°C, and the stretching ratio is 1.5; the heat setting is 130°C, and the heat setting time is 1min. After the casting is cooled, it is subjected to heat treatment, longitudinal cold stretching, longitudinal hot stretching, heat setting, and traction and winding to obtain the multi-layer co-extruded diaphragm. Example 6

[0033] Raw materials: LLZO inorganic solid electrolyte Ningbo Fengli New Energy LLZO-G3-100; the isotacticity of low melt index homopolymer polypropylene in the outer and inner layers is not less than 98%, and the melt index at 230°C is 2g / 10min; polymer solid electrolyte polyacrylonitrile; the isotacticity of high melt index homopolymer polypropylene is not less than 98%, and the melt index at 230°C is 20g / 10min; the cross-linking agent in the middle layer is Yantai Houde Polymer PEG200DMA polyethylene glycol diacrylate, and the initiator in the middle layer is benzoyl peroxide.

[0034] The multi-layer co-extruded cross-linked network solid electrolyte separator is a five-layer co-extruded structure of outer layer / middle layer / inner layer / middle layer / outer layer, the thickness of the outer layer is 60% of the total thickness, the thickness of the middle layer is 10% of the total thickness, and the thickness of the inner layer is 30% of the total thickness. The raw materials of the outer layer include low melt index homopolymer polypropylene, LLZO oxide solid electrolyte and polyacrylonitrile polymer solid electrolyte, the raw materials of the middle layer include high melt index homopolymer polypropylene, a cross-linking agent and an initiator, and the raw materials of the inner layer are low melt index homopolymer polypropylene and a polymer solid electrolyte; the preparation method of the multi-layer co-extruded cross-linked network solid electrolyte separator is to mix the components of the surface layer, the middle layer and the inner layer in proportion and then perform multi-layer co-extrusion, the ratio of low melt index homopolymer polypropylene, polymer solid electrolyte and oxide solid electrolyte in the outer layer is 75:40:10, and the ratio of low melt index homopolymer polypropylene, polymer solid electrolyte and oxide solid electrolyte in the inner layer is 75:40:10. The ratio of medium-low melt index homopolymer polypropylene to polymer solid electrolyte is 70:30, the extrusion temperature during multi-layer co-extrusion is 230°C, the die temperature is 210°C, the casting temperature is 80°C, the heat treatment temperature is 130°C, and the heat treatment time is 12h; the longitudinal cold stretching temperature is 70°C, and the stretching ratio is 1.5; the longitudinal hot stretching temperature is 150°C, and the stretching ratio is 3.0; the heat setting is 145°C, and the heat setting time is 5min. After the casting is cooled, it is subjected to heat treatment, longitudinal cold stretching, longitudinal hot stretching, heat setting, and traction and winding to obtain the multi-layer co-extruded diaphragm.

[0035] Comparative Example 1

[0036] The same method as in Example 1 is used, except that in step (1), no oxide solid electrolyte is added to the outer layer, and 80 parts of low melting point homopolymer polypropylene and 20 parts of polymer solid electrolyte are added.

[0037] Comparative Example 2

[0038] The same method as in Example 1 is used, except that in step (1), no polymer solid electrolyte is added to the outer layer and the inner layer.

[0039] Comparative Example 3

[0040] The same method as in Example 1 is used, except that the intermediate layer is not included in step (1).

[0041] The porosity, average pore size, maximum pore size, ion conductivity and liquid absorption of the coated separators prepared in Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0042] Table 1

[0043]

[0044] It can be concluded from the test results of the embodiments and comparative examples shown in Table 1 that the multilayer co-extruded cross-linked network solid electrolyte diaphragm provided by the present invention can effectively improve the porosity, pore size and liquid absorption rate of the diaphragm, while having a higher ionic conductivity. Compared with the prior art, the multilayer co-extruded cross-linked network solid electrolyte diaphragm provided by the present invention solves the problem that the traditional diaphragm has low porosity and small pore size, and cannot accommodate solid electrolytes when applied to semi-solid / solid batteries, and further improves the ionic conductivity and high-power charge and discharge performance of the battery. In the pore structure of the traditional polypropylene diaphragm, the present invention forms a solid electrolyte membrane with a cross-linked network by cross-linking the polymer electrolyte diaphragm. Low melt index polypropylene is used in the outer and inner layers to provide strength, and high melt index polypropylene is used in the middle layer to ensure that the cross-linking agent and initiator have fluidity, and the polymer solid electrolyte of the outer and inner layers is polymerized to form a cross-linked network. Polypropylene and oxide solid electrolytes will phase separate during the stretching process, further expanding the pore size of the diaphragm. The oxide solid electrolyte added in the outer layer acts as a provider of Li ions in the battery, and can form mobile Li ions in the solid electrolyte, while improving the ionic conductivity of the electrolyte membrane.

[0045] Note: The multi-layer co-extruded cross-linked network solid electrolyte separator prepared by the present invention was cut into A4 size and tested for various properties. The test items are as follows:

[0046] (1) Average thickness

[0047] Use a micrometer to measure the thickness of the sample at different locations and calculate the average value.

[0048] (2) Porosity

[0049] The average density of the sample was tested using an electronic balance, and the porosity of the sample was calculated based on the theoretical density. Three samples were tested and their average value was calculated.

[0050] (3) Aperture

[0051] The pore size of the samples was tested using a pore size analyzer. Three samples were tested and their average pore size and maximum pore size were calculated.

[0052] (4) Air permeability

[0053] The air permeability of the five-layer co-extruded microporous membrane was tested using an air permeability tester. Five samples were tested and their average value was calculated.

[0054] (5) Ionic conductivity

[0055] The diaphragm was assembled into a button cell, the electrolyte was 1M dimethyl lithium hexafluorophosphate solution, and the ionic conductivity of the diaphragm was tested.

[0056] The above is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. All equivalent changes and modifications made according to the content of the present invention are included in the patent scope of the present invention.

Claims

1. A multi-layer co-extruded cross-linked network solid electrolyte membrane, characterized in that: The multi-layer co-extruded cross-linked network solid electrolyte membrane is a five-layer co-extruded structure of outer layer / middle layer / inner layer / middle layer / outer layer. The preparation method comprises mixing the components of the surface layer, the middle layer and the inner layer in proportion and then subjecting the mixture to multi-layer co-extrusion. The extrusion temperature during the multi-layer co-extrusion is 200-230° C. and the die head temperature is 180-210° C. The outer layer raw materials include low melt index homopolypropylene, oxide solid electrolyte and polymer solid electrolyte, the middle layer raw materials include high melt index homopolypropylene, crosslinking agent and initiator, the mass ratio of high melt index homopolypropylene, crosslinking agent and initiator in the middle layer is 92-98:1-5:1-3; the isotacticity of the high melt index homopolypropylene in the middle layer is not less than 98%, and the melt index at 230°C is 10-20g / 10min; the crosslinking agent in the middle layer is divinylbenzene or polyethylene glycol diacrylate, and the initiator in the middle layer is diisopropyl peroxide or benzoyl peroxide; the inner layer raw materials are low melt index homopolypropylene and polymer solid electrolyte; the mass ratio of low melt index homopolypropylene, polymer solid electrolyte and oxide solid electrolyte in the outer layer is 50-75:20-40:5-10; the mass ratio of low melt index homopolypropylene and polymer solid electrolyte in the inner layer is 50-70:30-50.

2. A method for preparing a multi-layer co-extruded cross-linked network solid electrolyte membrane as claimed in claim 1, characterized in that: The method comprises the following steps: mixing the components of the surface layer, the middle layer and the inner layer in proportion and subjecting them to multi-layer co-extrusion; cooling the cast sheet and subjecting it to heat treatment, longitudinal cold stretching, longitudinal hot stretching, heat setting, and traction and winding to obtain the multi-layer co-extruded diaphragm.

3. The method for preparing a multi-layer co-extruded cross-linked network solid electrolyte membrane according to claim 2, characterized in that: The casting temperature is 50-80°C, the heat treatment temperature is 110-130°C, and the heat treatment time is 4-12h; the longitudinal cold stretching temperature is 40-70°C, and the stretching ratio is 1.2-1.5; the longitudinal hot stretching temperature is 130-150°C, and the stretching ratio is 1.5-3.0; the heat setting is 130-145°C, and the heat setting time is 1-5min.

4. The method for preparing a multi-layer co-extruded cross-linked network solid electrolyte membrane according to claim 2, characterized in that: The isotacticity of the low melt index homopolymer polypropylene in the outer layer and the inner layer is not less than 98%, and the melt index at 230° C. is 0.5 to 2 g / 10 min.

5. The method for preparing a multi-layer co-extruded cross-linked network solid electrolyte membrane according to claim 2, characterized in that: The polymer solid electrolyte in the outer layer and the inner layer is one or more of polyoxyethylene, polyacrylonitrile, polyvinyl alcohol, and polymethyl methacrylate.

6. The method for preparing a multi-layer co-extruded cross-linked network solid electrolyte membrane according to claim 2, characterized in that: The oxide solid electrolyte in the outer layer is one or more of LLTO and LLZO, and the particle size of the oxide solid electrolyte is 50 to 100 nm.

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