CN111630686A

CN111630686A - Composite diaphragm, preparation method thereof and lithium battery comprising composite diaphragm

Info

Publication number: CN111630686A
Application number: CN201880000769.XA
Authority: CN
Inventors: 程文广; 陈秀峰
Original assignee: Shenzhen Senior Technology Material Co Ltd
Current assignee: Shenzhen Senior Technology Material Co Ltd
Priority date: 2018-07-03
Filing date: 2018-07-03
Publication date: 2020-09-04
Also published as: WO2020006671A1

Abstract

The application provides a composite diaphragm, a preparation method thereof and a lithium battery comprising the composite diaphragm. The composite diaphragm comprises a diaphragm layer substrate and a functional diaphragm layer compounded on the surface of the diaphragm layer substrate, wherein the functional diaphragm layer is prepared from nano ceramic with oleophylic surface, polyethylene and pore-foaming agent, and the diaphragm layer substrate is prepared from nano ceramic with oleophylic surface, copolymerized polypropylene, low-melting point polyethylene and pore-foaming agent. The composite diaphragm has the advantages of high liquid absorption and retention rate, small average pore diameter and high void ratio, has excellent comprehensive performance, and has great industrial value and wide market prospect.

Description

Composite diaphragm, preparation method thereof and lithium battery comprising composite diaphragm

Technical Field

The application relates to the technical field of batteries, in particular to a composite diaphragm, a preparation method thereof and a lithium battery comprising the composite diaphragm.

Background

Lithium ion batteries play a significant role in daily life. Lithium ion batteries have become the first choice for numerous portable electronic devices and electric vehicles due to their high energy density, long cycle life, safety, no pollution, and fast charge and discharge. As an important component of lithium ion batteries, lithium ion battery separators are receiving increasing attention. The performance of the diaphragm influences the capacity, safety performance and cycle performance of the lithium ion battery, and has a decisive effect on the comprehensive performance of the lithium ion battery.

At present, commercial lithium battery separators are mainly polyolefin single-layer films or multi-layer films, but polyolefin separators have poor affinity with electrolyte and poor liquid retention capacity.

Content of application

The composite diaphragm has the advantages of high liquid absorption and retention rate, small average pore diameter and high porosity, so that the composite diaphragm has excellent comprehensive performance.

The application aims to provide a preparation method of the composite diaphragm, and the composite diaphragm with better quality can be prepared by the preparation method.

The present application also provides a lithium ion battery comprising the composite separator, which has the advantages of the composite separator.

In order to achieve at least one of the above objects, the present application provides the following technical solutions:

a composite diaphragm comprises a diaphragm layer substrate and a functional diaphragm layer compounded on the surface of the diaphragm layer substrate, wherein the functional diaphragm layer is prepared from nano ceramic with oleophylic surface, polyethylene and pore-foaming agent, and the diaphragm layer substrate is prepared from nano ceramic with oleophylic surface, copolymerized polypropylene, low-melting point polyethylene and pore-foaming agent.

A preparation method of the composite membrane comprises the following steps:

melting and plasticizing the mixed nano ceramic with oleophylic surface, polyethylene and pore-foaming agent to obtain a first melt;

melting and plasticizing the mixed nano ceramic with oleophylic surface, the copolymerization type polypropylene, the pore-foaming agent and the low-melting point polyethylene to obtain a second melt;

compounding the first melt and the second melt and then extruding to obtain a casting film;

and extracting paraffin oil after the cast film is stretched in two directions.

A lithium ion battery comprises a positive electrode, a negative electrode and the composite diaphragm or the composite diaphragm prepared by the preparation method.

The embodiment of the application provides a composite diaphragm, a preparation method thereof and a lithium ion battery with the composite diaphragm, and the composite diaphragm has the beneficial effects that:

the copolymerization type polypropylene and the low-melting point polyethylene are introduced into the film layer matrix in the composite diaphragm, and the copolymerization type polypropylene can have a good interface effect and improve the compatibility of each component in the film layer matrix; the low-melting-point polyethylene has a lower melting point, and the temperature can be kept in a lower range in the preparation process, so that a film substrate with small pore diameter and high porosity can be formed. In addition, the nano ceramics with oleophylic surfaces are introduced into the functional membrane layer and the membrane substrate, and the nano ceramics have smaller particle size, so that the surface areas of the functional membrane layer and the membrane substrate layer are increased, the functional membrane layer and the membrane substrate can be better combined, the adsorption of the composite diaphragm on electrolyte in the lithium battery can be increased, the liquid absorption and retention rate of the composite diaphragm is improved, and the ionic conductivity is improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments or the prior art of the present application, the drawings used in the description of the embodiments or the prior art will be briefly described below.

FIG. 1 is a scanning electron microscope atlas of a composite septum provided in a first embodiment;

FIG. 2 is a scanning electron microscope atlas of a composite septum provided in a second embodiment;

FIG. 3 is a scanning electron microscope atlas of a composite septum provided in a third embodiment;

FIG. 4 is a scanning electron microscope atlas of a composite septum provided in a fourth embodiment;

FIG. 5 is a scanning electron microscope atlas of a composite septum provided in a fifth embodiment;

FIG. 6 is a scanning electron microscope atlas of a composite septum provided in a sixth embodiment;

FIG. 7 is a scanning electron microscope atlas of a composite septum provided in a seventh embodiment;

FIG. 8 is a scanning electron microscope atlas of a composite septum provided in an eighth embodiment;

fig. 9 is a scanning electron microscope atlas of a composite septum provided in a ninth embodiment;

fig. 10 is a scanning electron microscope atlas of the composite separator provided in the comparative example.

Detailed Description

Embodiments of the present application will be described in detail below with reference to examples, but those skilled in the art will appreciate that the following examples are only illustrative of the present application and should not be construed as limiting the scope of the present application. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The embodiment provides a composite diaphragm, which comprises a diaphragm substrate and a functional diaphragm compounded on the surface of the diaphragm substrate, wherein the functional diaphragm is prepared from nano-ceramics with oleophylic surface, polyethylene and a pore-foaming agent, and the diaphragm substrate is prepared from nano-ceramics with oleophylic surface, copolymerized polypropylene, low-melting-point polyethylene and a pore-foaming agent.

The low-melting-point polyethylene and the copolymerization type polypropylene are introduced into the film layer matrix in the composite diaphragm, and the copolymerization type polypropylene can have a better interface effect and improve the compatibility of each component in the film layer matrix; the low-melting-point polyethylene has a lower melting point, and the temperature can be kept in a lower range in the preparation process, so that a film substrate with small pore diameter and high porosity can be formed. In addition, the nano ceramics with oleophylic surfaces are introduced into the functional membrane layer and the membrane substrate, and the nano ceramics have smaller particle size, so that the surface areas of the functional membrane layer and the membrane substrate layer are increased, the functional membrane layer and the membrane substrate can be better combined, the adsorption of the composite diaphragm on electrolyte in the lithium battery can be increased, the liquid absorption and retention rate of the composite diaphragm is improved, and the ionic conductivity is improved.

The composite diaphragm may be formed by combining a functional film layer on one surface of a film substrate, or by combining functional film layers on both the upper and lower surfaces of a film substrate. The number of the functional film layers and the number of the film layer substrates can be prepared according to the needs.

In some embodiments, a method of making a composite separator includes: and melting and plasticizing the mixed nano ceramic with oleophylic surface, polyethylene and pore-foaming agent to obtain a first melt.

In order to ensure that the first melt obtained after melting and plasticizing all the components is uniform, firstly, the nano ceramic with oleophylic surface and the polyethylene are fully mixed, and then, the mixture is fully mixed with the pore-foaming agent. Wherein the mass of the nano ceramic with oleophylic surface is 1-12% of the mass of the powder of the functional membrane layer raw material. In some embodiments, the mass of the surface-oleophilic nano-ceramic is 3% to 8% of the mass of the powder of the functional membrane layer raw material.

In this embodiment, the porogen in the first melt comprises paraffin oil, and after the paraffin oil is extracted, a stable pore structure can be formed. It should be noted that, since the paraffin oil does not belong to powder, the mass of the powder of the functional film layer herein refers to the sum of the mass of the nano ceramic and the polyethylene with oleophilic surfaces. Additionally, in some embodiments, the polyethylene in the first melt comprises high molecular weight polyethylene or ultra high molecular weight polyethylene.

Further, the mixed nano ceramic with oleophylic surface, the copolymerization type polypropylene and the low-melting point polyethylene are melted and plasticized to obtain a second melt.

Similarly, in order to make the second melt after melting and plasticizing the components uniform, the nano ceramic with oleophylic surface, the copolymer polypropylene and the low-melting point polyethylene are fully mixed and then fully mixed with the pore-foaming agent. Wherein, the mass of the nano ceramic with oleophylic surface is 1-12% of the mass of the powder of the film substrate raw material. In some embodiments, the mass of the surface oleophilic nano-ceramic is 3% to 8% of the mass of the powder of the membrane layer substrate raw material. Likewise, the porogen in the second melt comprises paraffin oil, and after the paraffin oil is extracted, a stable pore structure can be formed. The mass of the powder of the film substrate raw material refers to the sum of the mass of the nano ceramic, the copolymerization type polypropylene and the low-melting point polyethylene with oleophylic surfaces.

In addition, the mass of the copolymerization type polypropylene is 5-15% of the mass of the powder in the film layer matrix raw material. The copolymerization structural unit of the copolymerization type polypropylene is ethylene or butylene, and the molecular weight of the structural unit is 5-15% of the molecular weight of the molecular chain main chain structure of the copolymerization type polypropylene. Alternatively, in some embodiments, the molecular weight of the structural unit is 8% to 12% of the molecular weight of the molecular chain backbone structure of the co-polymerized polypropylene. Because the copolymerization type polypropylene is obtained by copolymerizing ethylene or butylene and propylene, the copolymerization type polypropylene has better interface effect and can improve the compatibility of each component in the film substrate.

In some embodiments, the co-polypropylene is an ethylene-based co-polypropylene having an ethylene content of 8.6%, a molecular weight of about 25 million, and a melt index of 0.56.

The low-melting point polyethylene has a lower melting point lower than 130 ℃, and the temperature can be kept in a lower range in the preparation process, so that a film substrate with a small aperture, a high porosity and a wider closed-pore film breaking temperature window can be formed. In addition, the low melting point polyethylene has a molecular weight higher than 30 ten thousand.

The surface oleophilic nano-ceramics in the first melt and the second melt are both obtained by modifying the nano-ceramics by using a silane coupling agent.

Wherein the nano ceramic is selected from one or more of nano aluminum oxide, nano titanium oxide, nano silicon oxide, nano zirconium oxide and nano zinc oxide. In some embodiments, the nanoceramic has a particle size of 5-200 nm.

The silane coupling agent is selected from one or more of vinyltrimethoxysilane, methacryloxypropyltrimethoxysilane and vinyltriethoxysilane.

In some embodiments, the surface modification is performed by dispersing the nano ceramic particles in a silane coupling agent, and then washing and drying are performed to obtain the nano ceramic particles with oleophilic surfaces. In some embodiments, the modification temperature is 40-100 ℃ and the modification time is 2-12 h.

The nano ceramic has smaller particle size, increases the surface area of the functional film layer and the film substrate layer, can ensure that the functional film layer and the film substrate are better combined, can increase the adsorption of the composite diaphragm to electrolyte in a lithium battery, improves the liquid absorption and retention rate of the lithium battery, and improves the ionic conductivity. And the modified nano ceramic has oleophilic property, so that the modified nano ceramic is more easily and uniformly dispersed with other components.

Further, compounding the obtained first melt and the second melt and then extruding to obtain a casting film; and extracting paraffin oil after the cast film is stretched in two directions.

In some embodiments, the raw material of the functional film layer is melted and plasticized in an extruder to obtain a first melt, and the first melt is then conveyed to the middle runner of the three-layer die head through a melt pump, a filter and a metering pump. The raw material of the film layer matrix is melted and plasticized in another extruder to obtain a second melt, the second melt is conveyed to an upper flow passage and a lower flow passage of the three-layer die head through a melt pump, a filter and a metering pump, and the thicknesses of the film layer matrix and the functional film layer can be adjusted by controlling the melt flow rates of the upper flow passage and the lower flow passage and the middle flow passage. And then extruding and compounding through a three-layer die to obtain the casting film.

In some embodiments, the extrusion mass ratio of the first melt to the second melt is from 1:9 to 9: 1. In some embodiments, the extrusion mass ratio of the first melt to the second melt is from 2:8 to 8: 2. In some embodiments, the extrusion mass ratio of the first melt to the second melt is from 4:6 to 6:4 and in some embodiments, the thickness of the cast film is from 1 to 2 mm. In some embodiments, the extrusion mass ratio of the first melt in the upper and lower runners is 1:2 to 2: 1.

It should be noted that the number of layers of the composite separator may be adjusted by replacing the die according to the number of layers required for the composite separator. For example, when only two films are needed, a two-layer die can be substituted, when a 4-layer die is needed, a four-layer die can be substituted, and so on.

After the cast film is obtained, the cast film is required to be subjected to biaxial stretching, and a thinned sample with a porous structure can be obtained. Wherein the two-way stretching ratio is 2-10 times. In some embodiments, the ratio of biaxial stretching is 4 to 8. In addition, it should be noted that biaxial stretching refers to transverse stretching and longitudinal stretching.

And extracting paraffin oil after the cast film is stretched in two directions. Wherein the solvent used for extracting the paraffin oil comprises dichloromethane. And after extracting the paraffin oil, drying and shaping the casting film to obtain the composite diaphragm. Wherein the thickness of the composite diaphragm is 5-60 mu m, the porosity of the composite diaphragm is 30-60%, and the aperture size of the composite diaphragm is 10-300 nm.

In some embodiments, the drying temperature is 50-90 ℃ and the drying time is 1-3 min; the setting treatment temperature is 90-120 ℃, and the treatment time is 10-120 s. In some embodiments, the drying temperature is 60-70 ℃ and the drying time is 1-3 min; the setting temperature is 105-110 ℃, and the processing time is 30-80 s.

A lithium ion battery comprises a positive electrode, a negative electrode and the composite diaphragm.

Wherein the composite diaphragm is arranged between the anode and the cathode. The composite diaphragm has better liquid absorption and retention capacity, so that the lithium ion battery has better ionic conductivity. The composite diaphragm has smaller aperture, better porosity and wider closed-cell film breaking temperature window, namely low closed-cell temperature and high film breaking temperature, can quickly close the cell, has excellent comprehensive performance, and can improve the safety performance of the lithium ion battery.

In the present embodiment, the types of the positive electrode, the negative electrode, and the electrolyte solution are not particularly limited, and those of the positive electrode, the negative electrode, and the electrolyte solution of a lithium ion battery known to those skilled in the art may be used.

In embodiments of the present application, the active material of the positive electrode includes, but is not limited to, LiFePO₄、LiMn₂O₄、LiCoPO₂And LiNiO₂One or more of (a). The negative active material includes, but is not limited to, one or more of graphite, hard carbon, lithium titanate, and soft carbon.

The features and properties of the present application are described in further detail below with reference to examples:

first embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of the modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a second extruder. The first extruder and the second extruder fully melt and plasticize materials, and the melt extrusion flow rates of the two extruders are the same. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.2mm, carrying out bidirectional stretching with longitudinal stretching multiplying power and transverse stretching multiplying power of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 70 ℃/60s and shaping at 110 ℃/30s to obtain the composite diaphragm with the average thickness of about 20 mu m.

Second embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 6: 4. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.3mm, carrying out bidirectional stretching with longitudinal stretching multiplying power and transverse stretching multiplying power of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/30s to obtain the composite diaphragm with the average thickness of about 22 mu m.

Third embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize materials, and the melt extrusion flow ratio of the two extruders is 4: 6. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.1mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/40s to obtain the composite diaphragm with the average thickness of about 19 mu m.

Fourth embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize materials, and the melt extrusion flow ratio of the two extruders is 3: 7. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.4mm, carrying out bidirectional stretching with longitudinal stretching multiplying power and transverse stretching multiplying power of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/40s to obtain the composite diaphragm with the average thickness of about 25 mu m.

Fifth embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 7: 3. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.5mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 6 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/30s to obtain the composite diaphragm with the average thickness of about 30 mu m.

Sixth embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 2: 8. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.3mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/40s to obtain the composite diaphragm with the average thickness of about 22 mu m.

Seventh embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 8: 2. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.4mm, carrying out bidirectional stretching with longitudinal stretching multiplying power and transverse stretching multiplying power of 7 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/60s and shaping at 105 ℃/30s to obtain the composite diaphragm with the average thickness of about 25 mu m.

Eighth embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow rates of the two extruders are the same. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.5mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 6 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/70s and shaping at 108 ℃/40s to obtain the composite diaphragm with the average thickness of about 30 mu m.

Ninth embodiment

2500g of alumina particles with the average particle size of 60nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain the modified alumina particles, wherein the mass concentration of vinyltrimethoxysilane in the ethanol solution is 10%. 2000g of copolymer polypropylene powder with the content of ethylene structural units being 7.8 percent, 1000g of modified alumina particles and 27000g of low-melting polyethylene powder with the molecular weight being 100 ten thousand are mixed evenly and then are put into a first extruder with 70000g of paraffin oil; then adding 1500g of modified nano alumina particles into 28500g of polyethylene powder with the average molecular weight of 60 ten thousand, uniformly mixing, and putting the mixture and 70000g of paraffin oil into a first extruder. The first extruder and the second extruder fully melt and plasticize materials, and the melt extrusion flow ratio of the two extruders is 4: 6. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.5mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 6 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/70s and shaping at 105 ℃/40s to obtain the composite diaphragm with the average thickness of about 30 mu m.

Tenth embodiment

2500g of titanium oxide particles with the average particle size of 40nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain modified titanium oxide particles, wherein the mass concentration of vinyltriethoxysilane in the ethanol solution is 10%. 1500g of copolymerized polypropylene powder with 5 percent of ethylene structural unit content, 1500g of modified titanium oxide particles and 27000g of low-melting-point polyethylene powder with the molecular weight of 100 ten thousand are mixed uniformly and then put into a first extruder with 70000g of paraffin oil; 2400g of the modified nano titanium oxide particles are added into 27600g of polyethylene powder with the average molecular weight of 60 ten thousand and mixed evenly, and then the mixture and 70000g of paraffin oil are put into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 1: 9. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 2mm, carrying out bidirectional stretching with longitudinal stretching multiplying power and transverse stretching multiplying power of 4 times and 10 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 50 ℃/70s and shaping at 90 ℃/40s to obtain the composite diaphragm with the average thickness of about 30 mu m.

Eleventh embodiment

2500g of silicon oxide particles with the average particle size of 40nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain modified silicon oxide particles, wherein the mass concentration of vinyl triethoxysilane in the ethanol solution is 10%. 2700g of copolymer polypropylene powder with 5 percent of ethylene structural unit content, 300g of modified silicon oxide particles and 27000g of low-melting polyethylene powder with the molecular weight of 100 ten thousand are mixed evenly and then put into a first extruder with 70000g of paraffin oil; then 900g of modified nano silicon oxide particles are added into 29100g of polyethylene powder with the average molecular weight of 60 ten thousand, mixed evenly and then put into a first extruder together with 70000g of paraffin oil. The first extruder and the second extruder fully melt and plasticize materials, and the melt extrusion flow ratio of the two extruders is 3: 7. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. Compounding the melt, extruding out alkane to obtain cast film with thickness of 1mm, longitudinal and transverse stretching in two directions of 2 and 6 times, extracting paraffin oil with dichloromethane, drying at 70 deg.c/70 s and setting at 100 deg.c/40 s to obtain composite diaphragm with average thickness of 30 microns.

Twelfth embodiment

2500g of zinc oxide particles with the average particle size of 40nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain modified zinc oxide particles, wherein the mass concentration of methacryloxypropyltrimethoxysilane in the ethanol solution is 10%. 4800g of copolymerized polypropylene powder with 15% of butene structural unit content, 3200g of modified zinc oxide particles and 32000g of polyethylene powder with low melting point and molecular weight of 100 ten thousand are mixed uniformly, and then the mixture and 70000g of paraffin oil are put into a first extruder; 3600g of the modified nano zinc oxide particles are added into 26400g of polyethylene powder with the average molecular weight of 60 ten thousand and mixed evenly, and then the mixture and 70000g of paraffin oil are put into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 2: 8. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. The melt is extruded after compounding, dichloromethane obtains cast films with the thickness of about 1.5mm, the cast films are stretched in two directions with the longitudinal stretching ratio and the transverse stretching ratio of 4 times and 8 times respectively, then dichloromethane is used for extracting paraffin oil in the stretched films, and the composite diaphragms with the average thickness of about 30 mu m are obtained after drying at 90 ℃/70s and shaping at 120 ℃/40 s.

Thirteenth embodiment

2500g of zirconia particles with the average particle size of 40nm are dispersed in 30L of ethanol solution for surface modification, washing and drying to obtain modified silica particles, wherein the mass concentration of vinyl triethoxysilane in the ethanol solution is 10%. 6000g of copolymerized polypropylene powder with 10 percent of ethylene structural unit content, 2000g of modified zirconia particles and 32000g of polyethylene powder with low melting point and molecular weight of 100 ten thousand are mixed uniformly, and then the mixture and 70000g of paraffin oil are put into a first extruder; 3600g of the modified nano zirconia particles are added into 26400g of polyethylene powder with the average molecular weight of 60 ten thousand and mixed evenly, and then the mixture and 70000g of paraffin oil are put into a first extruder. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 2: 8. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. The melt is extruded after compounding, dichloromethane obtains cast films with the thickness of about 1.5mm, the cast films are stretched in the longitudinal direction and the transverse direction with the stretching ratios of 6 times and 8 times respectively, then dichloromethane is used for extracting paraffin oil in the stretched films, and the composite diaphragms with the average thickness of about 30 mu m are obtained after drying at the temperature of 80 ℃/70s and shaping at the temperature of 110 ℃/40 s.

Comparative example

30000g of polyethylene powder with the molecular weight of 100 ten thousand is mixed evenly and then put into a first extruder together with 70000g of paraffin oil; 30000g of polyethylene powder having an average molecular weight of 60 ten thousand were mixed uniformly and then fed into a first extruder together with 70000g of paraffin oil. The first extruder and the second extruder fully melt and plasticize the materials, and the melt extrusion flow ratio of the two extruders is 5: 5. The melt of the first extruder is conveyed to the middle runner of the three-layer die head through the first filter, the first melt pump and the first metering pump, the melt of the first extruder is conveyed to the upper and lower runners of the three-layer die head through the second filter, the second melt pump and the second metering pump, and the melt is evenly distributed to the upper and lower runners. And extruding the melt after compounding to obtain a casting film with the thickness of about 1.2mm, carrying out bidirectional stretching with longitudinal stretching magnification and transverse stretching magnification of 6 times and 8 times respectively, extracting paraffin oil in the stretched film by using dichloromethane, drying at 60 ℃/70s and shaping at 105 ℃/40s to obtain the composite diaphragm with the average thickness of about 20 mu m.

Test examples

1. The composite separators prepared in examples 1 to 9 and comparative example were tested for the pore closing temperature, rupture temperature, air permeability, average pore diameter, porosity, liquid absorption and retention capacity, longitudinal tensile strength and transverse tensile strength, and the results are shown in tables 1 and 2.

(1) The method for testing the closed pore temperature and the membrane rupture temperature comprises the following steps:

cutting a composite diaphragm with the size of 50mm multiplied by 50mm, sealing the composite diaphragm in a mould filled with electrolyte, putting the mould filled with a composite diaphragm sample into an oven, setting the temperature of the oven to be 200 ℃, heating at the speed of 5 ℃/min, and recording the change value of the internal resistance of the grinding tool along with the temperature in the heating process. Wherein, the closed pore temperature is the initial temperature when the internal resistance value begins to obviously rise; the film breaking temperature is the temperature when the resistance value returns to the measuring range of the resistance meter.

(2) Air permeability test method:

composite membrane samples were selected to measure the average time required to permeate 100mL of gas.

(3) Average pore size and porosity test methods:

and (3) testing by adopting a capillary flow meter, namely breaking the wetted composite diaphragm by adopting inert gas, measuring the pressure value of gas outflow, and calculating to obtain the aperture parameter.

(4) The method for testing liquid absorption and retention capacity comprises the following steps:

cutting a 150mm multiplied by 150mm composite diaphragm sample, marking the sample, weighing m1, soaking the composite diaphragm sample in electrolyte for 1h, taking out the sample, and wiping the electrolyte on the surface of the composite diaphragm until the granular electrolyte can not be seen by naked eyes. Weighing the dried composite membrane sample by weight m2, spreading the weighed composite membrane flatly, and weighing the composite membrane by weight m 3. Wherein, the liquid absorption rate is (m2-m1)/m1 is 100%; the liquid retention rate is (m3-m1)/m1 is 100%.

(5) Test methods for longitudinal tensile strength and transverse tensile strength:

cutting 5 composite diaphragm samples of 150mm multiplied by 15mm, using an electronic universal tensile testing machine to measure the longitudinal and transverse tensile strength of the composite diaphragm samples, and taking the average value of the measured values after the experiment is completed.

Table 1 various performance tests of composite separators of examples 1 to 9 and comparative example

As can be seen from the results in table 1, the liquid absorption capacity and the liquid retention capacity of the composite separators of examples 1 to 9 are superior to those of the composite separator of the comparative example, which shows that the composite separators prepared by the processes and raw materials of the examples of the present application have better liquid absorption capacity and liquid retention capacity. In addition, the average pore diameter of the composite membranes of examples 1 to 9 is smaller than that of the composite membranes of comparative examples, and the porosity of the composite membranes of examples 1 to 9 is larger than that of the composite membranes of comparative examples, which shows that the composite membranes prepared by the processes and raw materials of the examples of the present application have smaller pore diameter and higher porosity, and the composite membranes of the examples of the present application can realize rapid pore closing. The film breaking temperatures of the composite diaphragms of examples 1 to 9 and the comparative example are equal to each other, and the film breaking temperatures of the composite diaphragms of examples 2 to 4, 6, 8 and 9 are higher than the film breaking temperature of the composite diaphragm of the comparative example, which is found by comparing the film breaking temperatures of the composite diaphragms of examples 1 to 5 and 7 with the film breaking temperature of the composite diaphragm of the comparative example, and therefore the composite diaphragms prepared by the processes and raw materials of the examples have wider closed-pore film breaking temperature windows and are beneficial to rapid opening closing of the composite diaphragms.

2. The composite separators of examples 1 to 9 and comparative example were tested under a scanning electron microscope at a scale of 1 μm, a voltage of 10KV, a Working Distance (WD) of 9.5mm, and a magnification of 10 KX. The test results are shown in fig. 1-10.

And (4) analyzing results: referring to fig. 1-10, it is found by comparing fig. 1-9 and fig. 10 that the pore diameter of the composite membrane of fig. 1-9 is smaller than that of the composite membrane of fig. 10, and the pore structure of fig. 1-9 is more uniform than that of fig. 10, further illustrating that the composite membrane of the embodiment of the present application has smaller pore diameter, which is beneficial for achieving fast pore closing.

While particular embodiments of the present application have been illustrated and described, it would be appreciated that many other changes and modifications can be made without departing from the spirit and scope of the application. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this application.

Industrial applicability

The application provides a composite diaphragm, a preparation method thereof and a lithium battery comprising the composite diaphragm. The composite diaphragm provided by the embodiment of the application has the advantages of high liquid absorption and retention rate, small average pore diameter and high porosity, has excellent comprehensive performance, and has great industrial value and wide market prospect.

Claims

The composite diaphragm is characterized by comprising a diaphragm layer substrate and a functional diaphragm layer compounded on the surface of the diaphragm layer substrate, wherein the functional diaphragm layer is prepared from nano ceramic with oleophylic surface, polyethylene and a pore-foaming agent, and the diaphragm layer substrate is prepared from nano ceramic with oleophylic surface, copolymerized polypropylene, low-melting-point polyethylene and a pore-foaming agent.
The composite separator according to claim 1, wherein the surface-oleophilic nano-ceramic is obtained by surface modification of a nano-ceramic with a silane coupling agent.
The composite separator according to claim 2, wherein the silane coupling agent is selected from one or more of vinyltrimethoxysilane, methacryloxypropyltrimethoxysilane, and vinyltriethoxysilane.
The composite separator according to claim 2, wherein the nanoceramic is selected from one or more of nano-alumina, nano-titania, nano-silica, nano-zirconia, and nano-zinc oxide.
The composite separator according to claim 2 or 4, wherein the nano-ceramic has a particle size of 5 to 200 nm.
The composite separator according to any one of claims 1 to 4, wherein said polyethylene in said functional film layer comprises high molecular weight polyethylene.
The composite membrane of any one of claims 1-4, wherein the porogen comprises a paraffin oil.
The composite separator according to any one of claims 1 to 4, wherein the copolymerization structural unit of the copolymerization polypropylene is ethylene or butylene, and the molecular weight of the structural unit is 5 to 15% of the molecular weight of the molecular chain main chain structure of the copolymerization polypropylene.
The composite separator of claim 1, wherein said low-melting polyethylene has a molecular weight higher than 30 ten thousand and a melting point lower than 130 ℃.
The composite separator according to claim 1 or 8, wherein the mass of the copolymer polypropylene is 5% to 15% of the mass of the powder in the film layer base material.
The composite separator according to claim 10, wherein the mass of the copolymer polypropylene is 8-12% of the mass of the powder in the film layer base material.
The composite separator according to claim 1, 2 or 4, wherein the mass of the surface-oleophilic nano-ceramic of the membrane-layer substrate is 1-12% of the mass of the powder in the membrane-layer substrate raw material.
The composite separator of claim 12, wherein the mass of the surface-oleophilic nano-ceramic of the membrane-layer substrate is 3% to 8% of the mass of the powder in the membrane-layer substrate raw material.
The composite separator according to claim 1, 2 or 4, wherein the mass of the surface-oleophilic nano-ceramic of the functional membrane layer is 1-12% of the mass of the powder in the membrane layer base material.
The composite separator according to claim 14, wherein the mass of the surface-oleophilic nano-ceramic of the functional membrane layer is 3-8% of the mass of the powder in the membrane layer base material.
The composite separator according to any one of claims 1 to 15, wherein the thickness of the composite separator is 5 to 60 μm.
The composite separator of any one of claims 1-16, wherein said functional film is laminated to the upper and lower surfaces of said film substrate.
A method of making a composite separator as defined in any one of claims 1 to 17, comprising:

melting and plasticizing the mixed nano ceramic with oleophylic surface, the polyethylene and the pore-foaming agent to obtain a first melt;

melting and plasticizing the mixed nano ceramic with oleophylic surface, the copolymerized polypropylene, the pore-foaming agent and the low-melting-point polyethylene to obtain a second melt;

extruding the first melt and the second melt after compounding to obtain a casting film;

and extracting the paraffin oil after the cast film is subjected to biaxial tension.
The method of manufacturing a composite separator according to claim 18, wherein the solvent used to extract the paraffinic oil comprises dichloromethane.
The method for producing a composite separator according to claim 18, wherein the biaxial stretching is performed at a stretch ratio of 2 to 10 times.
The method for preparing the composite separator according to claim 18, wherein the extrusion mass ratio of the first melt to the second melt is 1:9 to 9: 1.
The method for producing a composite separator according to claim 18, wherein the thickness of the casting film is 1 to 2 mm.
The method of claim 18, wherein the method further comprises, after extracting the paraffin oil from the slurry: and drying and shaping the casting film.
The method for preparing the composite separator according to claim 23, wherein the drying temperature is 50 to 90 ℃ and the setting temperature is 90 to 12 ℃.
A lithium ion battery comprising a positive electrode, a negative electrode and the composite separator according to any one of claims 1 to 17 or the composite separator prepared by the preparation method according to any one of claims 18 to 24.