CN114388981A - Electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity and preparation method thereof - Google Patents

Electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity and preparation method thereof Download PDF

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CN114388981A
CN114388981A CN202111504452.XA CN202111504452A CN114388981A CN 114388981 A CN114388981 A CN 114388981A CN 202111504452 A CN202111504452 A CN 202111504452A CN 114388981 A CN114388981 A CN 114388981A
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ionic conductivity
membrane
nanofiber
lithium battery
electrospun
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CN114388981B (en
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魏真真
邢建欣
李和钦
樊文暄
赵燕
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Suzhou University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
    • H01M50/457—Separators, membranes or diaphragms characterised by the material having a layered structure comprising three or more layers
    • D—TEXTILES; PAPER
    • D04—BRAIDING; LACE-MAKING; KNITTING; TRIMMINGS; NON-WOVEN FABRICS
    • D04H—MAKING TEXTILE FABRICS, e.g. FROM FIBRES OR FILAMENTARY MATERIAL; FABRICS MADE BY SUCH PROCESSES OR APPARATUS, e.g. FELTS, NON-WOVEN FABRICS; COTTON-WOOL; WADDING ; NON-WOVEN FABRICS FROM STAPLE FIBRES, FILAMENTS OR YARNS, BONDED WITH AT LEAST ONE WEB-LIKE MATERIAL DURING THEIR CONSOLIDATION
    • D04H1/00—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres
    • D04H1/70—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres
    • D04H1/72—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres the fibres being randomly arranged
    • D04H1/728—Non-woven fabrics formed wholly or mainly of staple fibres or like relatively short fibres characterised by the method of forming fleeces or layers, e.g. reorientation of fibres the fibres being randomly arranged by electro-spinning
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/44—Fibrous material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/491—Porosity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/494—Tensile strength
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/497—Ionic conductivity
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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  • General Chemical & Material Sciences (AREA)
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Abstract

The invention relates to an electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity and a preparation method thereof, wherein the electrospinning lithium battery diaphragm is formed by compounding at least 3 layers of nanofiber membranes, and the orientation degrees of every two adjacent layers of nanofiber membranes are different; the preparation method comprises the following steps: firstly, spinning layer by layer on the same receiving roller by utilizing an electrostatic spinning technology to prepare a plurality of layers of nanofiber membranes, simultaneously controlling electrostatic spinning process parameters to enable the orientation degrees of every two adjacent layers of nanofiber membranes to be different, drying the plurality of layers of nanofiber membranes, and then rolling to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity. The method has simple process; the product of the invention has good tensile resistance and high ionic conductivity, and can improve the safety performance and electrochemical performance of the lithium ion battery to a great extent.

Description

Electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity and preparation method thereof
Technical Field
The invention belongs to the technical field of lithium ion batteries, and relates to an electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity and a preparation method thereof.
Background
Lithium ion batteries are a new generation of energy storage systems due to their high energy density, high rate of charge and discharge, low self-discharge rate, long cycle life, and environmental friendliness. The diaphragm is used as an important part in the lithium ion battery, although the diaphragm does not directly participate in internal electrochemical reaction, the diaphragm is placed between a positive electrode and a negative electrode, the two electrodes can be prevented from being directly contacted, the internal short circuit is avoided, lithium ions are allowed to migrate in the charging and discharging process, and the diaphragm plays an important role in the safety and the service performance of the battery. The electrostatic spinning technology can produce a non-woven membrane with a large surface area and an interconnected porous structure, so that a nanofiber membrane obtained by electrospinning has high porosity and excellent electrolyte absorption capacity, the ionic conductivity and the electrochemical performance of a lithium ion battery can be remarkably improved, the nanofiber membrane is widely used for preparing the lithium ion battery membrane, the lower tensile strength of the electrospun membrane even cannot meet the requirement of the battery assembly process, and the improvement of the mechanical performance becomes a key field for further research on the electrospun membrane, so that the improvement of the mechanical performance of the membrane and the retention (or improvement) of the electrochemical performance of the electrospun membrane have important significance for the development of the electrospun lithium ion battery membrane.
Generally, methods for improving the mechanical properties of the electrospun membrane mainly include introducing high-performance fibers, adding auxiliary materials, performing post-treatment on the membrane, and the like. Document 1(Cross linked Polyimide Nanofiber Membrane Prepared via Amponia Pretreatment and Its Application as As an upstream thermal stage Separator for Li-Ion Batteries [ J].Journal of the Electrochemical Society,2017,164(6) A1328-A32.) the high-performance fiber Polyimide (PI) is introduced to prepare the nanofiber membrane of the lithium ion battery, and the tensile breaking strength and the ionic conductivity of the prepared nanofiber membrane are greatly improved. Although the high-performance polymer materials are selected to effectively improve the mechanical and electrochemical properties of the electrospun membrane, most of the high-performance polymer materials are difficult to process into a membrane simply due to the high rigidity of the segments, so that the introduction of new high-performance materials faces great resistance. Document 2 (High-string, thermal stable nylon 6,6 composite nanofibrillar dispensers for lithium-ion batteries [ J)]Journal of Materials Science,2017,52(9):5232-2And SiO2The tensile breaking strength and the ionic conductivity of the nanofiber membrane diaphragm obtained by electrospinning of the inorganic nanoparticles are greatly improved, and the method for adding the auxiliary material can effectively improve the mechanical property and the electrochemical property of the electrospun diaphragm, but in the process of assembling the battery, the nanoparticles are likely to fall off from the diaphragm and enter the electrolyte, so that the performance of the battery is deteriorated. The pressure and heat treatment is a common post-treatment method for improving the mechanical property of the nanofiber membrane at present. Document 3(Partially oxidized polyacrylonitrile nanoparticles as a thermally stable separator for lithium batteries [ J)]Polymer,2015,68(335-43.) sequentially subjecting the electrospun PAN nanofiber membrane to pressurization and heat treatment to finally obtain a nanofiber membrane with a cross-linked structure, wherein the mechanical properties of the nanofiber membrane are greatly improved, but the pores of the electrospun membrane are reduced due to the pressurization and heat treatment, so that the permeability of electrolyte is poor, and the ionic conductivity and the corresponding electrochemical properties are reduced.
The three modification methods are limited by raw materials and preparation processes, so that a simple and effective method is sought to simultaneously improve the mechanical property and the electrochemical property of the electrospinning membrane, which is of great importance.
Disclosure of Invention
The invention aims to solve the problems in the prior art and provides an electrospun lithium battery diaphragm (namely an electrospun nanofiber diaphragm for a lithium ion battery) with high tensile strength and high ionic conductivity and a preparation method thereof.
The invention aims to prepare the electrospinning lithium battery diaphragm with composite orientation gradient (namely, the orientation degrees of every two adjacent layers of nanofiber membranes are different) by utilizing a simple electrostatic spinning technology so as to realize the bidirectional improvement of the electrospinning lithium battery diaphragm in the aspects of electrochemical performance and mechanical performance. The oriented structure can be obtained by simply controlling the rotating speed of an electrostatic spinning receiving roller, for example, an oriented PAN nanofiber membrane is prepared in document 4(Anisotropic semi-aligned PAN @ PVdF-HFP separator for Li-ion batteries [ J ]. Nanotechnology,2020,31 (43)), and the tensile breaking strength and high ionic conductivity of the oriented PAN nanofiber membrane are greatly improved relative to a random (i.e. non-oriented) nanofiber membrane, but the document does not study the specific relation between the orientation degree and mechanical properties and ionic conductivity, and does not relate to the structural design of orientation gradient.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity is formed by compounding at least 3 layers of nanofiber membranes, the orientation degrees of every two adjacent layers of nanofiber membranes are different, and the porosity of every two adjacent layers of nanofiber membranes is correspondingly changed due to the different orientation degrees of every two adjacent layers of nanofiber membranes. The orientation structure breaks the original random state of the fibers (all directions exist), and the number and the size of pores formed by the overlapping of the fibers are changed.
As a preferred technical scheme:
according to the electrospun lithium battery separator with high tensile strength and high ionic conductivity, the at least 3 layers are 3 layers or 5 layers, and the average fiber diameter of each layer of fiber membrane is 180.44-312.08 nm.
The electrospun lithium battery separator with high tensile strength and high ionic conductivity has the following orientation degree of each layer of nanofiber membrane: the method comprises the following steps of (1) increasing in a one-way mode, or decreasing in a one-way mode, or increasing in a two-way mode from the middle layer to two sides (the electrospinning lithium battery diaphragm is preferably an odd layer to ensure that the structure is symmetrical when the substitution degree increases in a two-way mode from the middle layer to the two sides), or decreasing in a two-way mode from the middle layer to the two sides, wherein the orientation degree is not more than 54% at most, and the orientation degree is not less than 27% at least; the difference of the orientation degrees of the two adjacent layers of nanofiber membranes is more than or equal to 3 percent.
According to the electrospun lithium battery separator with high tensile strength and high ionic conductivity, all nanofiber membranes are electrostatic spinning membranes of polymers.
According to the electrospun lithium battery separator with high tensile strength and high ionic conductivity, the polymer is polyacrylonitrile, polyurethane, polyvinylidene fluoride or polyvinyl alcohol, and the polymer has good affinity and electrochemical stability to a liquid electrolyte.
According to the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity, the thickness of the electrospun lithium battery diaphragm is 60-73 micrometers, the porosity is 85.9-93%, the tensile breaking strength is 9.95-19.5 MPa, and the ionic conductivity is 2.49-3.09 ms/cm; the ion conductivity of the existing commercial membrane is only about 0.7ms/cm, and the tensile breaking strength of a pure random PAN or PVDF membrane is only 1-5 MPa.
The invention also provides a preparation method of the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity, which comprises the steps of firstly utilizing an electrostatic spinning technology to spin layer by layer on the same receiving roller to prepare a plurality of layers of nanofiber membranes, simultaneously controlling electrostatic spinning process parameters to ensure that the orientation degrees of every two adjacent layers of nanofiber membranes are different, drying the plurality of layers of nanofiber membranes and then rolling to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity; the number of layers of the multilayer nanofiber membrane is at least 3.
As a preferred technical scheme:
according to the preparation method of the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity, the following steps are taken to control the electrostatic spinning process parameters so that the orientation degrees of every two adjacent nanofiber membranes are different: controlling the transverse moving speed to be 0m/s (the electrostatic spinning device can be provided with a transverse moving mechanism or not, and when the transverse moving mechanism is provided, the transverse moving speed is required to be controlled to be 0m/s), and adjusting the rotating speed of the receiving roller to ensure that the orientation degrees of every two adjacent layers of nanofiber membranes are different; the traversing speed is controlled to be 0m/s, the larger the rotating speed of the receiving roller is, the more the number of fibers in the rotating direction of the receiving roller (namely the tangential direction of the roller) is, the larger the orientation degree is, the orientation structure can influence the pore diameter and the number of pores of the nanofiber membrane, and further influence the porosity, and the larger the orientation degree is, the higher the porosity is.
According to the preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity, the electrostatic spinning process parameters are as follows: the concentration of the spinning solution is 10 wt%, the temperature is 20-30 ℃, the relative humidity is 30-50%, the distance from an injection needle to a receiving roller is 10-15 cm, the spinning applied voltage is 10-18 kV, the injection speed is 0.6-0.8 ml/h, the traversing speed is 0m/s, the rotating speed variation range of the receiving roller is 500-2500 rpm, and the spinning time variation range is 1-2 h; the drying temperature is 60-80 ℃, and the rolling pressure is 1-3 MPa; the porosity can be reduced by pressurization, but in combination with a specific experiment, the nanofiber membrane has strong electrostatic adsorption without pressurization, which is not beneficial to the smooth detachment of the membrane from the receiving paper and the smooth assembly of the battery.
The principle of the invention is as follows:
the invention obtains the multilayer oriented gradient composite diaphragm with different orientation degrees of every two adjacent layers of nanofiber membranes by controlling the rotating speed of the electrostatic spinning receiving roller, and further obtains the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The reason why the rotation speed of the electrostatic spinning receiving roller causes the different orientation degree of each two adjacent layers of nanofiber membranes is that: the increase in the rotational speed of the receiving drum increases the number of fibers aligned in a range of directions received by the drum, i.e., the degree of fiber orientation increases.
The different orientation degrees of each two adjacent nanofiber membranes can cause the electrospun lithium battery separator to have high tensile strength because: the increase in the degree of orientation of the single-layer film causes an increase in the force in the direction of orientation of the film, while the force in the direction not oriented is reduced accordingly, because the amount of fibers spun by electrospinning per unit time is constant, the degree of orientation increases, the amount of fibers in a specific direction increases, the force in the corresponding direction increases, but the amount of fibers in the other directions decreases, resulting in the film as a whole exhibiting an increase in the force in the direction of orientation and a reduction in the force in the direction of non-orientation.
Assuming that there are two films in an extreme case, namely, a random nanofiber film (i.e. all fibers are arranged in a completely random manner) as shown in fig. 5 and an oriented nanofiber film (i.e. all fibers are arranged in a regular manner) as shown in fig. 6, the forces of the fibers in all directions are balanced in the random nanofiber film of fig. 5 due to the random ordering of the fibers, and the difference between the forces in the non-oriented direction and the forces in the oriented direction is huge due to the absence of bonding points between the fibers in the oriented nanofiber film of fig. 6, a schematic view of a nanofiber composite film obtained by controlling the difference of the orientation degrees of each two adjacent layers of nanofiber films is shown in fig. 7, and since the forces of the nanofiber films are mainly the sum of the resultant force in the fiber axial direction (the resultant force is larger when the orientation degree is larger) and the force at the bonding points between the fibers, it can be seen that the bonding points between the fibers are obviously increased by the bonding of the oriented nanofiber film and the random nanofiber film, the mechanical property of the fiber membrane in all directions is enhanced; when the fiber film is subjected to tension, the random nanofiber film is mainly subjected to the contact points between fibers; the oriented nanofiber membrane is mainly born by the axial direction of the fibers, after the oriented nanofiber membrane and the fibers are compounded to obtain the nanofiber composite membrane, when the nanofiber composite membrane is stretched (the stretching can be in any direction, but is not limited to the axial stretching), the nanofiber composite membrane not only has the axial fibers, but also has contact points among the fibers to cooperatively bear stretching force, so that the tensile strength is obviously improved, namely, the stress points of the fiber membrane designed by orientation gradient are more.
The high ionic conductivity of the electrospun lithium battery separator caused by the different orientation degrees of each two adjacent layers of nanofiber membranes is caused by the following three reasons:
(1) the orientation structure breaks the original random state of the fiber (all directions exist), the number and the size of pores formed by overlapping the fiber and the fiber are changed, and the larger the orientation degree is, the larger the porosity is; therefore, the larger the number of lithium ions passing through the separator per unit time, i.e. the lithium ion migration rate, is, the different the porosity of each two adjacent membranes of the gradient composite structure, and therefore the lithium ion migration rate in each two adjacent membranes of the gradient composite structure is also different, and the lithium ion migration rate varies in a manner including: from slow to fast (the porosity of each corresponding layer of nanofiber membrane is increased in a unidirectional way), from fast to slow to fast (the porosity of each corresponding layer of nanofiber membrane is increased in a bidirectional way from the middle layer to two sides), from slow to fast to slow (the porosity of each corresponding layer of nanofiber membrane is decreased in a bidirectional way from the middle layer to two sides); the change of the lithium ion migration rate can influence the lithium ion migration rate and show an enhanced effect, so that the ionic conductivity of the electrospun lithium battery diaphragm is influenced;
(2) because the orientation degrees of the fibers of the two adjacent layers of the composite membrane are different, the probability that the fiber directions of the adjacent layers of the composite membrane are different is far greater than the probability that the fibers of the upper layer and the lower layer in the fiber layers are different under the condition of constant rotating speed of the roller, so that the probability that the micropores of the middle layer are covered by the fibers of the upper layer and the lower layer is increased by the composite membrane (even if the porosity of the nanofiber membrane of each layer is increased from the middle layer to two sides in a bidirectional increasing mode, the probability that the micropores of the middle layer are covered by the fibers of the upper layer and the lower layer is increased, if the rotating speed of the roller is kept unchanged, the probability that the fibers between the layers are overlapped with the fibers is higher, but under the composite structure, the orientation of the two adjacent layers is different, the probability that the fibers of the upper layer cover the pores of the middle layer is increased, so that a small container is formed by covering the pores of the fibers of the middle layer is increased and decreased, a container is formed by a plurality of micropores which are equivalent to the middle layer, so that the capability of the diaphragm for retaining the liquid electrolyte is increased, and the ionic conductivity is increased;
(3) the rotating speed of the roller is relatively high, the average diameter of the fibers shows a tendency of decreasing along with the increase of the rotating speed of the roller, and the specific surface area of the fibers is increased due to the decrease of the average diameter of the fibers, so that the area of the fibers contacting the liquid electrolyte is increased, and the ionic conductivity of the fibers is improved.
In conclusion, the electrospun lithium battery diaphragm is a multilayer composite diaphragm, an interlocking structure is formed among all the layers of the multilayer composite diaphragm, so that the liquid retaining capacity of the diaphragm is improved, the ionic conductivity and other electrochemical properties of the diaphragm can be improved, and the force of the nanofiber membrane with high orientation degree in the orientation direction and the non-orientation direction is effectively balanced by the composite structure with different orientation gradients, so that the tensile breaking strength of the composite diaphragm in all directions is superior to that of a single-layer oriented membrane.
Advantageous effects
(1) The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity is simple in process and low in cost;
(2) the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity has good tensile breaking strength and ionic conductivity, and has excellent application prospect.
Drawings
FIG. 1 is a stress-strain diagram of an electrospun lithium battery separator with high tensile strength and high ionic conductivity prepared in examples 1-5;
FIG. 2 is a stress-strain diagram of the nanofiber membrane for a single-layer lithium ion battery prepared in comparative examples 1-5;
FIG. 3 is a resistance diagram of the electrospun lithium battery separator with high tensile strength and high ionic conductivity prepared in examples 1-5;
FIG. 4 is a resistance diagram of the nanofiber membrane for single-layer lithium ion batteries prepared in comparative examples 1 to 5;
FIG. 5 is a schematic of a random nanofiber membrane;
FIG. 6 is a schematic view of an oriented nanofiber membrane;
fig. 7 is a schematic view of a nanofiber composite membrane.
Detailed Description
The invention will be further illustrated with reference to specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and such equivalents may fall within the scope of the present invention as defined in the appended claims.
The orientation degree test method comprises the following steps: the most basic method for describing the fiber orientation is to measure the direction state of the fibers in a region by using probability distribution, and the principle is to count the included angles between all the fibers in a selected region and a reference line and to describe the orientation degree of the fibers by using the probability of the fibers appearing between specific angles beta and y, and the specific operation steps are as follows: the rotation direction of the receiving roller is defined as a reference direction, then an included angle between 100 fibers and the base line is measured, an included angle range (generally from several degrees to 20 degrees) is defined, the proportion of the fibers in the included angle range to the total measured fiber number (100) is calculated to be the orientation degree, and the selected included angle range is 0-20 degrees.
Tensile breaking strength: using a universal material testing machine to obtain the tensile breaking strength of the sample, wherein the width of the sample is 10mm, the length of the sample is 40mm, the tensile speed is 5mm/min, the clamping distance is 20mm, each sample is tested for 5 times, and the average value is taken as the tensile breaking strength of the sample; the lengthwise direction of the sample corresponding to each of the examples and comparative examples was the same as the reference direction (i.e., the receiving roller rotating direction).
Porosity: the porosity of the membrane is measured by adopting a n-butanol absorption test, the membrane is cut into small round pieces with the diameter of 19mm before measurement, and the calculation formula of the porosity is as follows:
Figure BDA0003402796410000061
wherein Wd and Ww are the weight of the membrane in g before and after 2 hours immersion in n-butanol; rhobIs the density of n-butanol, in g/cm3；VdIs the volume of the nanofiber membrane before being soaked in n-butyl alcohol, and the unit is cm3；
Ionic conductivity: the ionic conductivity of the separator was studied by recording the Electrochemical Impedance Spectroscopy (EIS) of the separator, the impedance measurement being carried out by maintaining a frequency range of 50Hz to 500Hz and an amplitude of 5mV, the ionic conductivity σ being calculated as follows:
σ＝d/(Rb·S)；
wherein d is the thickness of the separator in cm; rbIs the bulk resistance, in units of Ω; s is the area of the symmetrical electrode in cm2。
Example 1
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 28 ℃, the relative humidity is 37%, the distance between an injection needle and a receiving roller is 12cm, the spinning applied voltage is 15kV, the injection speed is 0.6ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 2500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(4) and drying the 3 layers of composite nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1.5MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 60 mu m, the porosity of 93 percent, the tensile breaking strength of 19.5MPa and the ionic conductivity of 3.09 ms/cm.
Comparative example 1
A preparation method of a nanofiber membrane for a single-layer lithium ion battery comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are basically the same as those of the embodiment 1, and the difference is only that the rotating speed of a receiving roller is 500rpm, and the spinning time is 5 hours;
(2) and drying the nanofiber membrane at the temperature of 60 ℃, and then rolling under the pressure of 1MPa to obtain the nanofiber membrane for the single-layer lithium ion battery.
The finally prepared nanofiber membrane for the single-layer lithium ion battery has the thickness of 67 mu m, the average diameter of the fibers of 312.08nm, the orientation degree of 27 percent, the porosity of 79.4 percent, the tensile breaking strength of 5.6MPa and the ionic conductivity of 1.86 ms/cm.
In contrast to example 1, the tensile rupture strength and the ionic conductivity of the nanofiber membrane for a single-layer lithium ion battery prepared in comparative example 1 are much lower than those of example 1, because the nanofiber membrane for a single-layer lithium ion battery prepared in example 1 is a nanofiber membrane with an orientation gradient composite structure, and compared with the random nanofiber membrane in comparative example 1, not only is the number of fibers in the orientation direction increased, so that the resultant force in the orientation direction is increased, but also the increase of the bonding points between the fibers plays a role in increasing the mechanical strength of the nanofiber membrane. The increase of the ionic conductivity is attributed to the fact that the fiber diameter in the partial nanofiber layer under the gradient composite structure is thinned, so that the specific surface area of the whole fiber membrane is increased, and the contact area of the whole fiber membrane and the liquid electrolyte is increased; the change of the pores of each layer of the gradient composite structure increases the penetration rate of lithium ions; the interlocking structure between layers of the gradient composite membrane increases the ability to retain liquid electrolyte.
Comparative example 2
A preparation method of a nanofiber membrane for a single-layer lithium ion battery is basically the same as that of comparative example 1, except that the rotating speed of a receiving roller is controlled to be 1000 rpm.
The finally prepared nanofiber membrane for the single-layer lithium ion battery has the thickness of 63 mu m, the average diameter of the fibers of 272.65nm, the orientation degree of 33 percent, the porosity of 82.6 percent, the tensile breaking strength of 8.47MPa and the ionic conductivity of 2 ms/cm.
The tensile rupture strength and the ionic conductivity of the nanofiber membrane for the single-layer lithium ion battery prepared in comparative example 2 are much lower than those of example 1, compared with example 1, because the nanofiber membrane for the single-layer lithium ion battery prepared in example 1 is a nanofiber membrane with an orientation gradient composite structure, and compared with the micro-oriented nanofiber membrane in comparative example 2, not only is the number of fibers in the orientation direction increased, so that the resultant force in the orientation direction is increased, but also the increase of the bonding points between the fibers plays a role in increasing the mechanical strength of the nanofiber membrane. The increase of the ionic conductivity is attributed to the fact that the fiber diameter in the partial nanofiber layer under the gradient composite structure is thinned, so that the specific surface area of the whole fiber membrane is increased, and the contact area of the whole fiber membrane and the liquid electrolyte is increased; the change of the pores of each layer of the gradient composite structure increases the penetration rate of lithium ions; the interlocking structure between layers of the gradient composite membrane increases the ability to retain liquid electrolyte.
Compared with comparative example 1, the tensile rupture strength and the ionic conductivity of comparative example 2 are much greater than those of comparative example 1, because the increase of the orientation degree of the fibers not only improves the tensile rupture strength of the nanofiber membrane, but also improves the porosity of the nanofiber membrane, thereby improving the ionic conductivity.
Comparative example 3
A preparation method of a nanofiber membrane for a single-layer lithium ion battery is basically the same as that of comparative example 1, except that the rotating speed of a receiving roller is controlled to be 1500 rpm.
The finally prepared nanofiber membrane for the single-layer lithium ion battery has the thickness of 69 micrometers, the average diameter of the fibers is 241.37nm, the orientation degree is 38%, the porosity is 83.3%, the tensile breaking strength is 9.39MPa, and the ionic conductivity is 2.13 ms/cm.
The tensile rupture strength and the ionic conductivity of the nanofiber membrane for the single-layer lithium ion battery prepared in comparative example 3 are much lower than those of example 1, compared with example 1, because the nanofiber membrane for the single-layer lithium ion battery prepared in example 1 is a nanofiber membrane with an orientation gradient composite structure, not only the number of fibers in the orientation direction is increased so that the resultant force in the orientation direction is increased, but also the increase of the bonding points between the fibers plays a role in increasing the mechanical strength thereof, compared with the oriented nanofiber membrane in comparative example 3. The increase of the ionic conductivity is attributed to the fact that the fiber diameter in the partial nanofiber layer under the gradient composite structure is thinned, so that the specific surface area of the whole fiber membrane is increased, and the contact area of the whole fiber membrane and the liquid electrolyte is increased; the change of the pores of each layer of the gradient composite structure increases the penetration rate of lithium ions; the interlocking structure between layers of the gradient composite membrane increases the ability to retain liquid electrolyte.
Compared with comparative example 2, the tensile rupture strength and the ionic conductivity of comparative example 3 are much greater than those of comparative example 2, because the increase of the orientation degree of the fibers not only improves the tensile rupture strength of the nanofiber membrane, but also improves the porosity of the nanofiber membrane and further improves the ionic conductivity.
Comparative example 4
A preparation method of a nanofiber membrane for a single-layer lithium ion battery is basically the same as that of comparative example 1, except that the rotating speed of a receiving roller is controlled to be 2000 rpm.
The finally prepared nanofiber membrane for the single-layer lithium ion battery has the thickness of 67 mu m, the average fiber diameter of 216.11nm, the orientation degree of 51 percent, the porosity of 84.2 percent, the tensile breaking strength of 11.69MPa and the ionic conductivity of 2.22 ms/cm.
The tensile rupture strength and the ionic conductivity of the nanofiber membrane for the single-layer lithium ion battery prepared in comparative example 4 are much lower than those of example 1, compared with example 1, because the nanofiber membrane for the single-layer lithium ion battery prepared in example 1 is a nanofiber membrane with an orientation gradient composite structure, and compared with the oriented nanofiber membrane in comparative example 4, not only is the number of fibers in the orientation direction increased, so that the resultant force in the orientation direction is increased, but also the increase of the bonding points between the fibers plays a role in increasing the mechanical strength of the nanofiber membrane. The increase of the ionic conductivity is attributed to the fact that the fiber diameter in the partial nanofiber layer under the gradient composite structure is thinned, so that the specific surface area of the whole fiber membrane is increased, and the contact area of the whole fiber membrane and the liquid electrolyte is increased; the change of the pores of each layer of the gradient composite structure increases the penetration rate of lithium ions; the interlocking structure between layers of the gradient composite membrane increases the ability to retain liquid electrolyte.
Compared with comparative example 3, the tensile rupture strength and the ionic conductivity of comparative example 4 are much greater than those of comparative example 3, because the increase of the orientation degree of the fibers not only improves the tensile rupture strength of the nanofiber membrane, but also improves the porosity of the nanofiber membrane and further improves the ionic conductivity.
Comparative example 5
A preparation method of a nanofiber membrane for a single-layer lithium ion battery is basically the same as that of comparative example 1, except that the rotating speed of a receiving roller is controlled to be 2500 rpm.
The finally prepared nanofiber membrane for the single-layer lithium ion battery has the thickness of 55 microns, the average diameter of the fibers is 180.44nm, the orientation degree is 54%, the porosity is 85.5%, the tensile breaking strength is 12.98MPa, and the ionic conductivity is 2.42 ms/cm.
The tensile break strength and the ionic conductivity of the nanofiber membrane for the single-layer lithium ion battery prepared in comparative example 5 are much lower than those of example 1, compared with example 1, because the tensile break strength of the nanofiber membrane for the single-layer lithium ion battery prepared in example 1 is enhanced by the increase of the bonding points between the fibers compared with the oriented nanofiber membrane in comparative example 5. The increase of the ionic conductivity is attributed to the fact that the fiber diameter in the partial nanofiber layer under the gradient composite structure is thinned, so that the specific surface area of the whole fiber membrane is increased, and the contact area of the whole fiber membrane and the liquid electrolyte is increased; the interlocking structure between layers of the gradient composite membrane increases the ability to retain liquid electrolyte.
Compared with comparative example 4, the tensile rupture strength and the ionic conductivity of comparative example 5 are much greater than those of comparative example 4, because the increase of the orientation degree of the fiber not only improves the tensile rupture strength of the nanofiber membrane, but also improves the porosity of the nanofiber membrane and further improves the ionic conductivity.
The stress-strain diagram of the nanofiber membrane for the single-layer lithium ion battery prepared in the comparative examples 1 to 5 is shown in fig. 2, and the force of the nanofiber membrane in the orientation direction gradually increases with the increase of the orientation degree.
The bulk impedance diagram of the nanofiber membrane for the single-layer lithium ion battery prepared in the comparative examples 1 to 5 is shown in fig. 4, and the bulk resistance of the nanofiber membrane is gradually reduced with the increase of the orientation degree, so that the ionic conductivity is gradually increased.
Example 2
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 25 ℃, the relative humidity is 45%, the distance between an injection needle and a receiving roller is 12cm, the spinning applied voltage is 15kV, the injection speed is 0.6ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(4) and drying the 3 layers of nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 67 mu m, the porosity of 88.4 percent, the tensile breaking strength of 10.25MPa and the ionic conductivity of 2.87 ms/cm.
Comparing example 2 with example 1, it can be seen that the change of tensile rupture strength of the membrane and the change of porosity and ionic conductivity can be caused by changing the orientation combination sequence and the orientation degree of the multilayer nanofiber membrane.
Comparing example 2 with comparative example 1, it can be seen that the oriented composite of the inventive multilayer nanofiber membrane significantly improves the tensile break strength and ionic conductivity of the membrane.
Example 3
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 23 ℃, the relative humidity is 42%, the distance between an injection needle and a receiving roller is 12cm, the spinning applied voltage is 15kV, the injection speed is 0.6ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(2) directly spinning layer by layer on the same receiving roller to prepare a second layer of nanofiber membrane under the condition that the first layer of nanofiber membrane is not uncovered; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1000rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 33%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 38%;
(4) under the condition of not uncovering the product of the step (3), directly and continuously spinning on the same receiving roller to prepare a fourth layer of nano fiber film; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2000rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 51%;
(5) directly spinning layer by layer on the same receiving roller to prepare a fifth layer nanofiber membrane under the condition of not uncovering the product of the step (4); wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(6) and drying the 5 layers of nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 67 mu m, the porosity of 86.4 percent, the tensile breaking strength of 9.95MPa and the ionic conductivity of 2.49 ms/cm.
Comparing example 3 with example 1, it can be seen that changes in tensile break strength and porosity and ionic conductivity of the film can be caused by changing the oriented composite sequence, number of layers and degree of orientation of the multilayer nanofiber film.
Comparing example 3 with comparative example 1, it can be seen that the oriented composite of the inventive multilayer nanofiber membrane significantly improves the tensile break strength and ionic conductivity of the membrane.
Example 4
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 26 ℃, the relative humidity is 40%, the distance between an injection needle and a receiving roller is 12cm, the spinning applied voltage is 15kV, the injection speed is 0.6ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 2500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(2) directly spinning layer by layer on the same receiving roller to prepare a second layer of nanofiber membrane under the condition that the first layer of nanofiber membrane is not uncovered; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 38%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(4) under the condition of not uncovering the product of the step (3), directly and continuously spinning on the same receiving roller to prepare a fourth layer of nano fiber film; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 38%;
(5) under the condition of not uncovering the product of the step (4), directly and continuously spinning on the same receiving roller to prepare a fifth layer of nano fiber film; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(6) and drying the 5 layers of nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1.5MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 60 mu m, the porosity of 91.4 percent, the tensile breaking strength of 13.85MPa and the ionic conductivity of 2.91 ms/cm.
Comparing example 4 with example 1, it can be seen that the change of tensile breaking strength of the film and the change of porosity and ionic conductivity can be caused by changing the orientation compounding order, the number of layers and the orientation degree of the multilayer nanofiber film.
Comparing example 4 with example 3, it can be seen that the change of the tensile rupture strength of the membrane and the change of the porosity and the ionic conductivity can be caused by changing the orientation compounding order and the orientation degree of the multilayer nanofiber membrane.
Comparing example 4 with comparative example 1, it can be seen that the oriented composite of the inventive multilayer nanofiber membrane significantly improves the tensile break strength and ionic conductivity of the membrane.
Example 5
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyacrylonitrile in N, N-dimethylformamide to form a spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 27 ℃, the relative humidity is 38%, the distance between an injection needle and a receiving roller is 12cm, the spinning applied voltage is 15kV, the injection speed is 0.6ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 38%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 54%;
(4) under the condition of not uncovering the product of the step (3), directly and continuously spinning on the same receiving roller to prepare a fourth layer of nano fiber film; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 38%;
(5) under the condition of not uncovering the product of the step (4), directly and continuously spinning on the same receiving roller to prepare a fifth layer of nano fiber film; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 500rpm, and the spinning time is 1 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyacrylonitrile, and the orientation degree is 27%;
(6) and drying the 5 layers of nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1.5MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 60 mu m, the porosity of 92.8 percent, the tensile breaking strength of 13.1MPa and the ionic conductivity of 2.94 ms/cm.
Comparing example 5 with example 1, it can be seen that the change of tensile breaking strength of the film and the change of porosity and ionic conductivity can be caused by changing the orientation compounding order, the number of layers and the orientation degree of the multilayer nanofiber film.
Comparing example 5 with example 3, it can be seen that the change of the tensile rupture strength of the membrane and the change of the porosity and the ionic conductivity can be caused by changing the orientation compounding order and the orientation degree of the multilayer nanofiber membrane.
Comparing example 5 with comparative example 1, it can be seen that the oriented composite of the inventive multilayer nanofiber membrane significantly improves the tensile break strength and ionic conductivity of the membrane.
The stress-strain diagram of the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity prepared in the embodiments 1-5 is shown in fig. 1, the tensile breaking strength of the nanofiber membrane can be greatly enhanced through reasonable orientation gradient structure design, and the tensile breaking strength of the composite membrane is stronger than that of a pure random nanofiber membrane no matter what the design structure is.
The resistance diagram of the electrospun lithium battery separator with high tensile strength and high ionic conductivity prepared in the embodiments 1-5 is shown in fig. 3, and the orientation gradient structure design can effectively reduce the bulk resistance of the nanofiber membrane, so that the ionic conductivity is improved.
Example 6
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyurethane in a mixed solution of N, N-dimethylformamide and tetrahydrofuran to form a spinning solution with the concentration of 15 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 20 ℃, the relative humidity is 30%, the distance between an injection needle and a receiving roller is 10cm, the spinning applied voltage is 10kV, the injection speed is 0.7ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 2500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyimide, and the orientation degree is 54%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2000rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyimide, and the orientation degree is 52%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyimide, and the orientation degree is 38%;
(4) and drying the 3 layers of nanofiber membranes at the temperature of 60 ℃, and then rolling under the pressure of 1MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 69 micrometers, the porosity of 85.9%, the tensile breaking strength of 15.6MPa and the ionic conductivity of 2.47 ms/cm.
Example 7
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyvinylidene fluoride in N, N-dimethylformamide to form spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 24 ℃, the relative humidity is 40%, the distance between an injection needle and a receiving roller is 13cm, the spinning applied voltage is 16kV, the injection speed is 0.8ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 2500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinylidene fluoride, and the orientation degree is 53%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2000rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinylidene fluoride, and the orientation degree is 50%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinylidene fluoride, and the orientation degree is 36%;
(4) and drying the 3 layers of nanofiber membranes at the temperature of 70 ℃, and then rolling under the pressure of 2MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 73 microns, the porosity of 87.8 percent, the tensile breaking strength of 16.2MPa and the ionic conductivity of 2.54 ms/cm.
Example 8
The preparation method of the electrospinning lithium battery diaphragm with high tensile strength and high ionic conductivity comprises the following specific steps:
(1) dissolving polyvinyl alcohol in deionized water to form spinning solution with the concentration of 10 wt%, and preparing a first layer of nanofiber membrane by adopting an electrostatic spinning technology;
the electrostatic spinning process parameters are as follows: the temperature is 30 ℃, the relative humidity is 50%, the distance between an injection needle and a receiving roller is 15cm, the spinning applied voltage is 18kV, the injection speed is 0.8ml/h, the traversing speed is 0m/s, the rotating speed of the receiving roller is 2500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinyl alcohol, and the orientation degree is 49%;
(2) under the condition of not uncovering the first layer of nanofiber membrane, directly and continuously spinning on the same receiving roller to prepare a second layer of nanofiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 2000rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinyl alcohol, and the orientation degree is 45%;
(3) under the condition of not uncovering the product of the step (2), directly and continuously spinning on the same receiving roller to prepare a third layer of nano fiber membrane; wherein, the electrostatic spinning process parameters are basically the same as the step (1), and the difference is only that the rotating speed is 1500rpm, and the spinning time is 2 h;
the prepared nanofiber membrane is an electrostatic spinning membrane of polyvinyl alcohol, and the orientation degree is 33%;
(4) and drying the 3 layers of nanofiber membranes at the temperature of 80 ℃, and then rolling under the pressure of 3MPa to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity.
The finally prepared electro-spinning lithium battery diaphragm with high tensile strength and high ionic conductivity has the thickness of 68 mu m, the porosity of 86.9 percent, the tensile breaking strength of 13.9MPa and the ionic conductivity of 2.62 ms/cm.

Claims (9)

1. The electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity is characterized by being formed by compounding at least 3 layers of nanofiber membranes, wherein the orientation degrees of every two adjacent layers of nanofiber membranes are different.
2. The electrospun lithium battery separator with high tensile strength and high ionic conductivity of claim 1, wherein the at least 3 layers are 3 or 5 layers, and the average fiber diameter of each layer of fiber membrane is 180.44-312.08 nm.
3. The electrospun lithium battery separator of high tensile strength and high ionic conductivity of claim 1 wherein the degree of orientation of each layer of nanofiber membrane is: the orientation degree is increased in a one-way mode or decreased in a one-way mode or increased in a two-way mode from the middle layer to two sides or decreased in a two-way mode from the middle layer to two sides, the orientation degree is not more than 54% at most, and the orientation degree is not less than 27% at least; the difference of the orientation degrees of the two adjacent layers of nanofiber membranes is more than or equal to 3 percent.
4. The electrospun lithium battery separator of high tensile strength and high ionic conductivity of claim 1 wherein all of the nanofiber membranes are electrospun membranes of a polymer.
5. The electrospun lithium battery separator of high tensile strength and high ionic conductivity of claim 4 wherein the polymer is polyacrylonitrile, polyurethane, polyvinylidene fluoride or polyvinyl alcohol.
6. The electrospun lithium battery separator with high tensile strength and high ionic conductivity according to claim 1, wherein the electrospun lithium battery separator has a thickness of 60-73 μm, a porosity of 85.9-93%, a tensile breaking strength of 9.95-19.5 MPa, and an ionic conductivity of 2.49-3.09 ms/cm.
7. The preparation method of the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity is characterized in that firstly, a plurality of layers of nanofiber membranes are prepared on the same receiving roller by means of layer-by-layer spinning through an electrostatic spinning technology, meanwhile, electrostatic spinning process parameters are controlled to enable the orientation degrees of every two adjacent layers of nanofiber membranes to be different, and then the plurality of layers of nanofiber membranes are dried and rolled to obtain the electrospun lithium battery diaphragm with high tensile strength and high ionic conductivity; the number of layers of the multilayer nanofiber membrane is at least 3.
8. The method for preparing the electrospun lithium battery separator with high tensile strength and high ionic conductivity according to claim 7, wherein the step of controlling the electrostatic spinning process parameters to enable the orientation degrees of every two adjacent nanofiber membranes to be different refers to that: the traversing speed is controlled to be 0m/s, and the rotating speed of the receiving roller is adjusted to ensure that the orientation degree of each two adjacent layers of nanofiber membranes is different.
9. The method for preparing the electrospun lithium battery separator with high tensile strength and high ionic conductivity according to claim 8, wherein the electrostatic spinning process parameters are as follows: the concentration of the spinning solution is 10 wt%, the temperature is 20-30 ℃, the relative humidity is 30-50%, the distance from an injection needle to a receiving roller is 10-15 cm, the spinning applied voltage is 10-18 kV, the injection speed is 0.6-0.8 ml/h, the traversing speed is 0m/s, the rotating speed variation range of the receiving roller is 500-2500 rpm, and the spinning time variation range is 1-2 h; the drying temperature is 60-80 ℃, and the rolling pressure is 1-3 MPa.
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