CN113488741B - Asymmetric diaphragm based on para-aramid, preparation method and application - Google Patents
Asymmetric diaphragm based on para-aramid, preparation method and application Download PDFInfo
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- 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
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- 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
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- 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
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- 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
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Abstract
The invention belongs to the technical field of preparation of lithium battery diaphragms, and discloses an asymmetric diaphragm based on para-aramid, a preparation method and application thereof, wherein the asymmetric diaphragm based on para-aramid comprises a porous structure layer and a compact structure layer directly positioned on the porous structure layer, the compact structure layer and the porous structure layer are both formed by para-aramid nano-fibers and are integrally formed, so that the problems that a coating is easy to fall off and the like are effectively avoided. The invention can especially obtain the asymmetric diaphragm with the thermal stability of 200 ℃ and the porosity of 60 percent by improving the structure, the composition, the corresponding preparation method, the process design and the like of the diaphragm.
Description
Technical Field
The invention belongs to the technical field of lithium battery diaphragms, and particularly relates to an asymmetric diaphragm based on para-aramid fiber, a preparation method and application.
Background
In recent years, the application field of lithium batteries is becoming wider, and the increasing miniaturization and weight reduction of mobile electronic devices have put higher and higher demands on the energy density of lithium batteries. The lithium-sulfur battery has extremely high theoretical specific capacity (1675mAh g) -1 ) And theoretical energy density (2600Wh kg) -1 ) (Advanced science,2018,5,1700270), and the anode active material is elemental sulfur with abundant reserves, so that the anode active material is low in cost and is expected to become a next-generation commercial lithium battery. However, the use of lithium-sulfur batteries still faces some technical difficulties, in particular soluble long-chain polysulfidesThe shuttling effect of (c) tends to cause the following serious problems: (1) the active material of the positive electrode is continuously lost, so that the cycle capacity of the battery is rapidly reduced; (2) the polysulfide shuttled to the lithium negative electrode corrodes the lithium negative electrode, aggravates the growth of lithium dendrite, and seriously affects the safety of the battery. How to inhibit the shuttling of polysulfide and greatly improve the cycle capacity and safety of lithium-sulfur batteries become the focus of current research.
The separator is one of the core components of the battery, which prevents direct contact between the positive and negative electrodes while providing a transport path for lithium ions. Due to the large pore diameter, the commercial polyolefin separator (pore diameter of 20-60 nm) cannot effectively inhibit the shuttling of polysulfide (diameter of about 1-2 nm) in the lithium sulfur battery. In addition, the commercialized polyolefin separator has the following problems: (1) the nonpolar polyolefin separator has poor affinity with polar electrolyte and low porosity, so that the nonpolar polyolefin separator has low absorption rate and retention rate on the electrolyte, and the coulombic efficiency of the battery is low; (2) the polyolefin diaphragm has poor thermal stability, is easy to shrink when heated, and is easy to cause short circuit caused by the contact of the positive electrode and the negative electrode in the battery, thereby causing safety accidents. Multifunctional interlayer modification, surface grafting modification and surface coating modification on the surface of the polyolefin diaphragm are common methods for improving the above problems. Among them, surface coating modification is the most widely used method in the field of battery separators due to low cost and easy mass production. For example, patent CN105826580A discloses coating a modified SPEEK/PP/FCB asymmetric membrane on the surface of a polyolefin membrane; CN109546056A discloses an aramid phase-change coated separator. The modified layer coated on the surface of the polyolefin separator allows free lithium ion transmission, and can inhibit the shuttling of polysulfide. However, such surface-coated modified layers also suffer from the following major drawbacks: (1) after the membrane is soaked in the electrolyte for a long time, the modified layer is easy to fall off from the surface of the polyolefin membrane; (2) the modified layer cannot fundamentally solve the problems of poor thermal stability and low porosity of the polyolefin diaphragm.
Some of the prior art has been concerned with the disadvantages of coatings. For example, patent CN107221628A discloses a method for preparing a lithium battery separator, and a lithium battery, which specifically uses polyacrylonitrile as a main material and uses nanocellulose as an additive to prepare a three-layer lithium battery separator with a finger-shaped through hole structure. Although the finger-shaped through hole structure has high porosity and can store a large amount of electrolyte, the distribution of lithium ions cannot be effectively homogenized, and uniform lithium deposition is not facilitated; meanwhile, the structure with three porous layers cannot inhibit the shuttle of polysulfide.
Disclosure of Invention
In view of the above defects or improvement requirements of the prior art, the present invention aims to provide an asymmetric membrane based on para-aramid, a preparation method and an application thereof, wherein the problems that a coated membrane coating is easy to fall off and the like can be effectively avoided by improving the structure and the composition of the membrane, the corresponding preparation method, the process design and the like. The asymmetric diaphragm in the invention is composed of a compact structure layer and a porous structure layer, wherein the compact structure layer and the porous structure layer are tightly connected and integrally formed to form an asymmetric integral structure, the compact structure layer and the porous structure layer are both only composed of the same material of para-aramid nanofiber, and because the compact structure layer and the porous structure layer are integrally formed, strong intermolecular force is generated between the two structure layers, and the compact structure layer can hardly fall off from the porous structure layer. Asymmetric membranes with thermal stability of up to 200 ℃ and porosity of up to 60% can be obtained in particular on the basis of the invention. The asymmetric diaphragm provided by the invention is applied to the lithium-sulfur battery, so that shuttling of polysulfide can be physically blocked, and the transmission of lithium ions is not influenced, thereby effectively improving the cycle capacity of the lithium-sulfur battery.
In order to achieve the above object, according to one aspect of the present invention, there is provided an asymmetric membrane based on para-aramid, including a porous structure layer and a dense structure layer directly on the porous structure layer, wherein the dense structure layer and the porous structure layer are both made of para-aramid nanofibers and are integrally formed.
According to another aspect of the present invention, the present invention provides a method for preparing the above asymmetric membrane based on para-aramid, which is characterized by comprising the following steps:
(1) mixing and stirring a para-aramid raw material, strong base, a proton transfer agent and dimethyl sulfoxide to obtain a casting solution;
(2) coating the casting solution obtained in the step (1) on a smooth substrate to obtain a wet film attached to the substrate;
(3) standing the wet film obtained in the step (2) together with a substrate to evaporate part of solvent components in the wet film to obtain a semi-dry film, so as to obtain the semi-dry film attached to the substrate;
(4) soaking the semi-dry film obtained in the step (3) and the substrate in a coagulating bath, carrying out phase transformation on the semi-dry film in the coagulating bath to obtain a semi-dry film-coagulating bath mixture, and removing the semi-dry film-coagulating bath mixture from the substrate, thereby obtaining the semi-dry film-coagulating bath mixture with an asymmetric structure; the soaking time is 1-2 h;
(5) soaking the semi-dry membrane-coagulation bath mixture obtained in the step (4) in deionized water to remove strong base and dimethyl sulfoxide, thereby obtaining an asymmetric wet membrane; wherein, when the proton transfer agent is a non-aqueous proton transfer agent, the soaking can also remove the non-aqueous proton transfer agent;
(6) and (4) drying the asymmetric wet film obtained in the step (5) to obtain the asymmetric diaphragm based on the para-aramid.
According to another aspect of the present invention, there is provided a method for preparing the above asymmetric membrane based on para-aramid, comprising the steps of:
(S1) mixing and stirring the para-aramid raw material, strong base, proton transfer agent and dimethyl sulfoxide to obtain a casting solution;
(S2) coating the casting solution obtained in the step (S1) on a smooth substrate to obtain a wet film attached to the substrate;
(S3) immersing the wet film obtained in the step (S2) together with a substrate in a coagulating bath, wherein the wet film is phase-converted in the coagulating bath to obtain a wet film-coagulating bath mixture, and is released from the substrate, thereby obtaining a wet film-coagulating bath mixture having an asymmetric structure; the soaking time is 1-2 h;
(S4) soaking the wet membrane-coagulation bath mixture obtained in the step (S3) in deionized water to remove strong alkali and dimethyl sulfoxide, thereby obtaining an asymmetric wet membrane; wherein, when the proton transfer agent is a non-aqueous proton transfer agent, the soaking can also remove the non-aqueous proton transfer agent;
(S5) drying the asymmetric wet film obtained in the step (S4) to obtain the asymmetric membrane based on the para-aramid.
As a further preferable aspect of the present invention, in the step (1) or the step (S1), the para-aramid raw material is one or more of para-aramid pulp, chopped para-aramid fibers having a length of 2mm to 10mm, and para-aramid fiber filaments having a length of not less than 10 mm;
the strong base is one of potassium tert-butoxide or potassium hydroxide;
the proton transfer agent is one of methanol, ethanol, n-butanol and water;
the mass ratio of the para-aramid nanofibers to the strong base to the proton transfer agent is 1:1: 1-1: 2: 2;
the mass ratio of the para-aramid nano-fiber to the dimethyl sulfoxide is 1: 95-5: 85;
the stirring is carried out in a container with a reflux function at the temperature of 10-80 ℃, and the stirring time is 0.5-168 hours.
As a further preferable aspect of the present invention, in the step (2) or in the step (S2), the smooth substrate is specifically a smooth glass substrate, a smooth polytetrafluoroethylene substrate, or a smooth stainless steel substrate;
the coating is preferably knife coating;
the thickness of the wet film formed by coating is preferably 100 to 500 μm.
In a further preferred aspect of the present invention, in the step (3), the standing is performed in an oven at 30 to 80 ℃ for 0 to 60 min.
As a further preferred aspect of the present invention, in the step (4) or in the step (S3), the coagulation bath is one or more selected from deionized water, ethanol, methanol, and n-hexane.
In a further preferred embodiment of the present invention, in the step (5) or the step (S4), the soaking time is 48 to 168 hours; and in the soaking process, the deionized water is replaced every 12 hours or less.
In a further preferred embodiment of the present invention, the drying in the step (6) or the step (S5) is natural drying at room temperature, freeze drying, or CO 2 Any one or more of supercritical drying and hot-pressing drying.
According to another aspect of the invention, the invention provides the application of the asymmetric diaphragm based on para-aramid as a diaphragm in a lithium-sulfur battery, wherein the dense structural layer of the asymmetric diaphragm based on para-aramid faces a sulfur positive electrode, and the porous structural layer faces away from the sulfur positive electrode.
Through the technical scheme, compared with the prior art, the asymmetric diaphragm comprises a compact structure layer and a porous structure layer, the compact structure layer and the porous structure layer are tightly connected and integrally formed to form an asymmetric integral structure, the compact structure layer and the porous structure layer are both formed by the para-aramid nanofiber, and due to the integral formation, strong intermolecular force is generated between the two structure layers, the compact structure layer can hardly fall off from the porous structure layer, and the problem that a modified layer coated on the surface of the existing diaphragm, such as a polyolefin diaphragm, is easy to fall off is solved. In addition, in the asymmetric diaphragm obtained by the invention, the porous structure layer has high porosity and is rich in polar groups, so that a large amount of electrolyte can be absorbed, and the coulomb efficiency of the battery is effectively improved; meanwhile, the porous structure layer is a three-dimensional continuous pore structure layer, so that a large number of paths are provided for rapid migration of lithium ions, and lithium ion distribution is homogenized, thereby being beneficial to obtaining more uniform lithium deposition; when the asymmetric diaphragm is applied to the lithium-sulfur battery, the shuttle of polysulfide can be physically blocked by the compact structure layer, and the transmission of lithium ions is not influenced, so that the cycle capacity of the lithium-sulfur battery is effectively improved. In addition, the asymmetric diaphragm in the invention also has better thermal stability.
Specifically, the asymmetric diaphragm based on the para-aramid fiber comprises a compact structural layer and a porous structural layer, wherein the two structural layers are made of the same material, namely the para-aramid nanofiber, and compared with the prior art, the asymmetric diaphragm based on the para-aramid fiber can achieve the following beneficial effects:
(1) the asymmetric diaphragm in the invention is only made of one material, namely the para-aramid nano-fiber;
(2) under the assistance of strong base, proton transfer agent and dimethyl sulfoxide, para-aramid is used as a raw material to prepare a nano fibrous aramid polymer, namely aramid nano fiber; preparing an asymmetric membrane by taking aramid nano-fibers as a main material; according to the preparation method, dimethyl sulfoxide is used as a solvent to construct a membrane casting solution, and if other solvents are used, a nano-fibrous aramid polymer cannot be obtained; the casting solution is formed by stirring, and the raw material para-aramid (with thicker diameter) is used for dispersing the nano-fibrous aramid polymer (with thinner diameter) in the casting solution in the stirring process;
(3) the preparation of the asymmetric membrane is based on an evaporation-solvent exchange phase inversion method; when the casting film liquid wet film coated on the substrate is not stood and is directly soaked in the coagulating bath, the dimethyl sulfoxide solvent and the coagulating bath are rapidly exchanged and diffused with each other at the interface of the casting film liquid/the coagulating bath, the surface viscosity of the aramid nano-fiber solution at the interface is rapidly increased, and a compact layer is formed by phase conversion at the same time; the fast formed compact layer slows down the exchange rate of the solvent and the coagulating bath in the casting solution wet film, and the slow solvent exchange forms a spongy porous structure layer in the casting solution. When the wet film of the casting solution coated on the substrate is kept still (for example, kept still for a period of time at 30-80 ℃), the solvent on the upper layer is continuously evaporated, and the concentration and viscosity of the surface layer are increased; and after the casting film liquid wet film with the upper solvent partially evaporated is soaked in a coagulating bath, the degree of compactness and the thickness of a compact structure layer obtained by phase conversion are increased. It can be seen that the thickness of the dense layer can be controlled by standing, for example, the longer the standing time is, the higher the surface concentration is, the thicker the dense layer is;
(4) further, the thickness of the dense structure layer can be controlled by controlling the temperature and time during the standing process from the wet film to the semi-dry film, for example, in example 2, the wet film is placed at 80 ℃ for 30min, the solvent in the upper layer is continuously evaporated, the concentration and viscosity of the surface layer are increased, the degree of densification and the thickness of the dense layer are increased, and finally the dense structure layer with the thickness as high as 2 μm can be obtained; the pore diameter of the porous structure layer can be regulated and controlled by the concentration of the initial membrane casting solution and the temperature and time in the standing process of converting the wet membrane into the semi-dry membrane, for example, the higher the concentration of the membrane casting solution is, the higher the temperature in the step of converting the wet membrane into the semi-dry membrane is, the longer the time is, and the smaller the pore diameter is;
(5) compared with the diaphragm (such as a polyolefin diaphragm with the modified surface) prepared by adopting a coating process in the prior art, the compact structure layer and the porous structure layer in the asymmetric diaphragm are tightly connected by the para-aramid nanofiber, the compact structure layer and the porous layer are formed by simultaneously converting in a coagulating bath, and the compact structure layer hardly falls off from the porous structure layer;
(6) in the asymmetric diaphragm obtained by the method, the porous structure layer is a three-dimensional continuous spongy pore structure layer, so that a large number of paths are provided for the rapid migration of lithium ions;
(7) in addition, the concentration of the aramid nano-fiber in the casting solution can be preferably 1 wt% -5 wt%, and the porous structure layer has ultrahigh porosity and is rich in polar groups, so that a large amount of electrolyte can be absorbed;
(8) the porous structure layer in the asymmetric diaphragm has narrow pore size distribution in the in-plane direction and the continuous porous structure layer with the pore diameter smaller than 100nm, so that the uniform lithium ion distribution is facilitated, and more uniform lithium deposition is further facilitated;
(9) when the asymmetric diaphragm is applied to the lithium-sulfur battery, the compact structure layer can effectively physically obstruct shuttling of polysulfide (with the size of about 1-2 nm) and does not influence the transmission of lithium ions, so that the cycle capacity of the lithium-sulfur battery is effectively improved;
(10) since the aramid nanofibers have excellent thermal stability, the asymmetric membrane of the present invention has excellent thermal stability, and the thermal shrinkage rate at 200 ℃ is almost 0.
In addition, compared with the prior art CN107221628A, the invention adopts the para-aramid nano-fiber to form an integrated asymmetric structure, and based on an evaporation-solvent exchange phase conversion method, two different structures are simultaneously prepared by using one raw material, thereby simplifying the preparation steps of the asymmetric membrane; in addition, because the nano fibrous aramid polymer is used as a main material, in the obtained asymmetric diaphragm, the porous structure layer has a three-dimensional continuous spongy pore structure, and the three-dimensional spongy pore structure is more favorable for homogenizing lithium ion distribution compared with a finger-shaped pore structure, so that the lithium deposition is favorably stabilized, and the safety performance of the lithium battery is finally improved.
Drawings
Fig. 1 is an SEM image of the surface of the dense structural layer of the p-aramid asymmetric membrane obtained in example 4. The scale in the figure represents 10 μm.
Fig. 2 is an SEM image of the surface of the porous structure layer of the p-aramid asymmetric separator obtained in example 4. The scale in the figure represents 10 μm.
Fig. 3 is a sectional SEM image of the dense structural layer side of the p-aramid asymmetric membrane obtained in example 4. The scale in the figure represents 1 μm.
Fig. 4 is a cycle capacity curve of the asymmetric separator and the commercial polyethylene separator obtained in example 4, respectively assembled in a lithium sulfur battery.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the respective embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The asymmetric diaphragm based on the para-aramid fiber comprises a compact structure layer and a porous structure layer, wherein the compact structure layer and the porous structure layer are both formed by para-aramid nanofiber, and the compact structure layer and the porous structure layer are tightly connected by the para-aramid nanofiber.
The following are specific examples:
example 1
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(S1) mixing the chopped para-aramid fiber, potassium hydroxide, water and dimethyl sulfoxide, wherein the mass ratio of the para-aramid fiber, strong base (namely potassium hydroxide), proton transfer agent (namely water) and dimethyl sulfoxide is 1:1:1:97, and continuously stirring for 168 hours at 10 ℃ to obtain a casting solution;
(S2) scraping the casting solution obtained in the step (S1) onto a smooth glass substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 100 μm;
(S3) directly soaking the wet film obtained in the step (S2) in deionized water without standing, wherein the wet film is subjected to phase conversion in the deionized water to obtain a wet film-deionized water mixture (the wet film-deionized water mixture is an asymmetric film primary product) and is peeled off from the glass substrate, and the soaking time is 1 h;
(S4) soaking the wet membrane-deionized water mixture obtained in the step (S3) in deionized water for 48 hours to remove strong base and dimethyl sulfoxide, and replacing the deionized water every 12 hours to obtain an asymmetric wet membrane;
(S5) naturally drying the asymmetric wet film obtained in the step (S4) at room temperature to obtain the asymmetric diaphragm based on the para-aramid.
Taking a glass substrate as a reference, in the preparation process, the surface which is not in contact with the substrate forms a compact structure layer after phase inversion, and the surface which is in contact with the substrate forms the surface of a porous structure layer after phase inversion.
Further, the prepared asymmetric separator based on para-aramid may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Example 2
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid fiber filaments, potassium hydroxide, ethanol and dimethyl sulfoxide, wherein the mass ratio of the para-aramid fibers, strong base (namely potassium hydroxide), proton transfer agent (namely ethanol) and dimethyl sulfoxide is 5:5:5:85, and continuously stirring for 30min at 80 ℃ to obtain a casting solution; the vessel, which is operated with agitation, has a condensing assembly that allows for reflux to avoid loss of lower boiling point reagents such as ethanol, and the following examples are similar.
(2) Scraping the casting solution obtained in the step (1) on a smooth polytetrafluoroethylene plate substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 500 microns;
(3) putting the wet film obtained in the step (2) and a polytetrafluoroethylene plate together at 80 ℃ for 30min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in methanol, carrying out phase conversion on the semi-dry film in the methanol to obtain a semi-dry film-methanol mixture, and falling off from the substrate, wherein the soaking time is 1.5 h;
(5) soaking the semi-dry film-methanol mixture in the step (4) in deionized water for 48h to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 8h to obtain an asymmetric wet film;
(6) and (5) freeze-drying the asymmetric wet film obtained in the step (5) to obtain the asymmetric diaphragm based on the para-aramid.
Further, the prepared asymmetric separator based on para-aramid may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Example 3
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid pulp, potassium hydroxide, n-butanol and dimethyl sulfoxide, wherein the mass ratio of the para-aramid, strong base (namely potassium hydroxide), proton transfer agent (namely n-butanol) and dimethyl sulfoxide is 2:3:3:92, and continuously stirring for 60min at 70 ℃ to obtain a casting solution;
(2) scraping the casting solution obtained in the step (1) on a smooth stainless steel plate substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 100 micrometers;
(3) putting the wet film obtained in the step (2) and a stainless steel plate together at 60 ℃ for 3min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in ethanol, phase-converting the semi-dry film in the ethanol to obtain a semi-dry film-ethanol mixture, and dropping off the semi-dry film-ethanol mixture from the substrate, wherein the soaking time is 1 h;
(5) soaking the semi-dry film-ethanol mixture obtained in the step (4) in deionized water for 72 hours to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 12 hours to obtain an asymmetric wet film;
(6) the asymmetric wet film obtained in the step (5) is utilized by utilizing the prior known CO 2 And (4) supercritical drying to obtain the asymmetric membrane based on the para-aramid.
Further, the prepared p-aramid-based asymmetric separator may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Example 4
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid pulp, potassium tert-butoxide, methanol and dimethyl sulfoxide, wherein the mass ratio of para-aramid, strong base (potassium tert-butoxide), proton transfer agent (methanol) and dimethyl sulfoxide is 3:3:3:91, and continuously stirring for 60min at 70 ℃ to obtain a casting solution;
(2) scraping the casting solution obtained in the step (1) on a smooth glass substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 200 mu m;
(3) putting the wet film obtained in the step (2) and glass together at 60 ℃ for 0.5min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in deionized water, performing phase conversion on the semi-dry film in the deionized water to obtain a semi-dry film-deionized water mixture, and dropping off the semi-dry film-deionized water mixture from the substrate, wherein the soaking time is 2 hours;
(5) soaking the semi-dry film-deionized water mixture in the step (4) in deionized water for 72h to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 12h to obtain an asymmetric wet film;
(6) and (4) drying the asymmetric wet film obtained in the step (5) at a high temperature and under a hot pressure (100 ℃, 10kPa) to obtain the asymmetric diaphragm based on the para-aramid.
Further, the prepared asymmetric separator based on para-aramid may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Fig. 1 shows the surface of the p-aramid asymmetric membrane dense structure layer obtained in example 4 (the surface which is not in contact with the substrate during the preparation process), and it can be seen that the surface of the dense structure layer is smooth and flat and has no pores.
Fig. 2 shows the surface (surface directly contacting with the substrate during the preparation process) of the porous structure layer of the p-aramid asymmetric membrane obtained in example 4.
FIG. 3 is an SEM image of a cross section of the compact structure layer side of the asymmetric p-aramid membrane obtained in example 4, wherein the region indicated by the arrow is a compact structure layer only several tens of nanometers thick; on the right side of the dense structure layer in the figure is a porous structure layer having an average pore diameter of 24.7nm (measured by a specific surface area tester).
Fig. 4 is a cycle capacity curve of the p-aramid asymmetric separator obtained in example 4 and a commercial polyethylene separator (obtained from new materials, jiang, hubei, 16 μm thick, wet double-drawn monolayer polyethylene separator) assembled in a lithium-sulfur battery, respectively; when the aramid asymmetric diaphragm is assembled in a lithium-sulfur battery, the dense structural layer should face the sulfur positive electrode. It can be seen that the lithium sulfur battery using the para-aramid asymmetric separator obtained in example 4 has a more stable cycle capacity than the lithium sulfur battery using a commercial polyethylene separator.
In addition, the asymmetric diaphragm obtained by the embodiment has the thermal stability as high as 200 ℃ and the porosity as high as 60 percent through detection.
Example 5
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid pulp, potassium tert-butoxide, methanol and dimethyl sulfoxide, wherein the mass ratio of para-aramid, strong base (potassium tert-butoxide), proton transfer agent (methanol) and dimethyl sulfoxide is 3:6:6:85, and continuously stirring for 30min at 80 ℃ to obtain a casting solution;
(2) scraping the casting solution obtained in the step (1) on a smooth glass substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 100 micrometers;
(3) putting the wet film obtained in the step (2) and glass together at 80 ℃ for 60min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in n-hexane, wherein the semi-dry film is phase-converted in the n-hexane to obtain a semi-dry film-n-hexane mixture, and the semi-dry film is peeled off from the substrate, and the soaking time is 2 hours;
(5) soaking the semi-dry film-n-hexane mixture obtained in the step (4) in deionized water for 168 hours to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 12 hours to obtain an asymmetric wet film;
(6) and (4) naturally drying the asymmetric wet film obtained in the step (5) at room temperature to obtain the asymmetric diaphragm based on the para-aramid.
Further, the prepared p-aramid-based asymmetric separator may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Example 6
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid pulp, potassium tert-butoxide, methanol and dimethyl sulfoxide, wherein the mass ratio of para-aramid, strong base (potassium tert-butoxide), proton transfer agent (methanol) and dimethyl sulfoxide is 2:4:4:90, and continuously stirring for 60min at 70 ℃ to obtain a casting solution;
(2) scraping the casting solution obtained in the step (1) on a smooth glass substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 300 microns;
(3) putting the wet film obtained in the step (2) and glass together at 60 ℃ for 60min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in n-hexane, wherein the semi-dry film is phase-converted in the n-hexane to obtain a semi-dry film-n-hexane mixture and is dropped off from the substrate, and the soaking time is 1.5 h;
(5) soaking the semi-dry film-n-hexane mixture obtained in the step (4) in deionized water for 168 hours to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 12 hours to obtain an asymmetric wet film;
(6) and (4) naturally drying the asymmetric wet film obtained in the step (5) at room temperature to obtain the asymmetric diaphragm based on the para-aramid.
Further, the prepared asymmetric separator based on para-aramid may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
Example 7
The preparation method of the asymmetric membrane based on the para-aramid fiber in the embodiment comprises the following steps:
(1) mixing para-aramid fiber filaments, potassium hydroxide, methanol and dimethyl sulfoxide, wherein the mass ratio of the para-aramid fibers, strong base (namely potassium hydroxide), proton transfer agent (namely methanol) and dimethyl sulfoxide is 5:5:5:85, and continuously stirring for 120min at 50 ℃ to obtain a casting solution;
(2) scraping the casting solution obtained in the step (1) on a smooth polytetrafluoroethylene plate substrate to obtain a wet film attached to the substrate, wherein the thickness of the wet film is controlled to be 400 microns;
(3) putting the wet film obtained in the step (2) and a polytetrafluoroethylene plate together at 50 ℃ for 10min to obtain a semi-dry film;
(4) soaking the semi-dry film obtained in the step (3) in methanol, carrying out phase conversion on the semi-dry film in the methanol to obtain a semi-dry film-methanol mixture, and dropping off the semi-dry film-methanol mixture from a substrate, wherein the soaking time is 2 hours;
(5) soaking the semi-dry film-methanol mixture in the step (4) in deionized water for 48h to remove strong base, proton transfer agent and dimethyl sulfoxide, and replacing the deionized water every 8h to obtain an asymmetric wet film;
(6) and (5) freeze-drying the asymmetric wet film obtained in the step (5) to obtain the asymmetric diaphragm based on the para-aramid.
Further, the prepared p-aramid-based asymmetric separator may be used in a lithium sulfur battery; also, the dense structural layer of the asymmetric separator faces the sulfur positive electrode when the lithium sulfur battery is assembled.
The para-aramid raw materials used in the above examples are para-1414 aramid chopped fibers, para-1414 aramid filaments, and para-1414 aramid pulp of zhejiang xuntai new materials ltd (of course, dupont kevlar chopped fibers and the like may also be used as para-aramid raw materials). In addition, in the above embodiment, the placing operation may be performed in an oven.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (7)
1. The preparation method of the asymmetric membrane based on the para-aramid is characterized by comprising a porous structure layer and a compact structure layer directly positioned on the porous structure layer, wherein the compact structure layer and the porous structure layer are both formed by para-aramid nanofibers and are integrally formed;
the preparation method comprises the following steps:
(1) mixing and stirring a para-aramid raw material, strong base, a proton transfer agent and dimethyl sulfoxide to obtain a casting solution; the para-aramid raw material is specifically one or more of para-aramid pulp, short para-aramid fibers with the length of 2-10 mm and para-aramid fiber filaments with the length of not less than 10 mm; the strong base is one of potassium tert-butoxide or potassium hydroxide; the proton transfer agent is one of methanol, ethanol, n-butanol and water; wherein the mass ratio of the para-aramid nano-fiber to the strong base to the proton transfer agent is 1:1: 1-1: 2: 2; the mass ratio of the para-aramid nano-fiber to the dimethyl sulfoxide is 1: 95-5: 85; the mass ratio of the strong base to the dimethyl sulfoxide is 1: (14-97);
(2) coating the casting solution obtained in the step (1) on a smooth substrate to obtain a wet film attached to the substrate;
(3) standing the wet film obtained in the step (2) together with a substrate to evaporate part of solvent components in the wet film to obtain a semi-dry film, so as to obtain the semi-dry film attached to the substrate;
(4) soaking the semi-dry film obtained in the step (3) and the substrate in a coagulating bath, and allowing the semi-dry film to be phase-converted in the coagulating bath to obtain a semi-dry film-coagulating bath mixture, and allowing the semi-dry film and the substrate to fall off, so as to obtain a semi-dry film-coagulating bath mixture with an asymmetric structure; the soaking time is 1-2 h; the coagulating bath is one or more of deionized water, ethanol, methanol and n-hexane;
(5) soaking the semi-dry membrane-coagulation bath mixture obtained in the step (4) in deionized water to remove strong base and dimethyl sulfoxide, thereby obtaining an asymmetric wet membrane; wherein, when the proton transfer agent is a non-aqueous proton transfer agent, the soaking can also remove the non-aqueous proton transfer agent;
(6) and (4) drying the asymmetric wet film obtained in the step (5) to obtain the asymmetric diaphragm based on the para-aramid.
2. The preparation method according to claim 1, wherein in the step (3), the standing is performed in an oven at 30-80 ℃ for 0-60 min.
3. The preparation method of the asymmetric membrane based on the para-aramid is characterized in that the asymmetric membrane based on the para-aramid comprises a porous structure layer and a compact structure layer directly positioned on the porous structure layer, wherein the compact structure layer and the porous structure layer are both formed by para-aramid nano-fibers and are integrally formed;
the preparation method comprises the following steps:
(S1) mixing and stirring the para-aramid raw material, strong base, proton transfer agent and dimethyl sulfoxide to obtain a casting solution; the para-aramid raw material is specifically one or more of para-aramid pulp, short para-aramid fibers with the length of 2-10 mm and para-aramid fiber filaments with the length of not less than 10 mm; the strong base is one of potassium tert-butoxide or potassium hydroxide; the proton transfer agent is one of methanol, ethanol, n-butanol and water; wherein the mass ratio of the para-aramid nano-fiber to the strong base to the proton transfer agent is 1:1: 1-1: 2: 2; the mass ratio of the para-aramid nano-fiber to the dimethyl sulfoxide is 1: 95-5: 85; the mass ratio of the strong base to the dimethyl sulfoxide is 1: (14-97);
(S2) coating the casting solution obtained in the step (S1) on a smooth substrate to obtain a wet film attached to the substrate;
(S3) immersing the wet film obtained in the step (S2) together with the substrate in a coagulating bath, in which the wet film is phase-converted to obtain a wet film-coagulating bath mixture and is released from the substrate, thereby obtaining a wet film-coagulating bath mixture having an asymmetric structure; the soaking time is 1-2 h; the coagulating bath is one or more of deionized water, ethanol, methanol and n-hexane;
(S4) soaking the wet membrane-coagulation bath mixture obtained in the step (S3) in deionized water to remove strong alkali and dimethyl sulfoxide, thereby obtaining an asymmetric wet membrane; wherein, when the proton transfer agent is a non-aqueous proton transfer agent, the soaking can also remove the non-aqueous proton transfer agent;
(S5) drying the asymmetric wet film obtained in the step (S4) to obtain the asymmetric membrane based on the para-aramid.
4. The production method according to claim 1 or 3, wherein in the step (1) or in the step (S1),
the stirring is carried out in a container with a reflux function at the temperature of 10-80 ℃, and the stirring time is 0.5-168 hours.
5. The production method according to claim 1 or 3, wherein in the step (2) or in the step (S2), the smooth substrate is specifically a smooth glass substrate, a smooth polytetrafluoroethylene substrate, or a smooth stainless steel substrate;
the coating is blade coating;
the thickness of the wet film formed by coating is 100-500 mu m.
6. The method according to claim 1 or 3, wherein in the step (5) or the step (S4), the soaking time is 48-168 hours; and in the soaking process, the deionized water is replaced every 12 hours or less.
7. The method according to claim 1 or 3, wherein the drying in step (6) or in step (S5) is natural drying at room temperature, freeze drying, CO 2 Any one or more of supercritical drying and hot-pressing drying.
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| JPH1140131A (en) * | 1997-07-24 | 1999-02-12 | Du Pont Teijin Advanced Paper Kk | Battery separator |
| CN205723721U (en) * | 2016-06-16 | 2016-11-23 | 沧州明珠隔膜科技有限公司 | A kind of from fire-retardant lithium ion battery separator |
| CN111211276A (en) * | 2020-01-20 | 2020-05-29 | 哈尔滨工业大学 | Preparation method and application of large-scale porous aramid microfiber diaphragm |
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| JP2010254731A (en) * | 2009-04-21 | 2010-11-11 | Sumitomo Chemical Co Ltd | Coating liquid |
| FR2992461B1 (en) * | 2012-06-25 | 2014-07-25 | Commissariat Energie Atomique | SUPERCONDENSER DEVICE ARRANGEMENT, SUPERCONDENSER DEVICE COMPRISING THE ARRANGEMENT, METHOD OF MANUFACTURING AN ARRANGEMENT |
| CN104979515B (en) * | 2015-07-29 | 2017-05-10 | 沧州明珠隔膜科技有限公司 | Aramid fiber polymer coated lithium ion battery membrane and preparation method thereof |
| CN109148790A (en) * | 2017-06-28 | 2019-01-04 | 微宏动力系统（湖州）有限公司 | A kind of porous septum, preparation method and lithium battery |
| CN107452921B (en) * | 2017-07-20 | 2019-06-18 | 清华大学 | A kind of preparation method of p-aramid fiber nanofiber composite lithium ion cell diaphragm |
| CN110112351A (en) * | 2019-05-21 | 2019-08-09 | 清华大学 | A kind of preparation method of p-aramid fiber lithium ion battery separator |
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| JPH1140131A (en) * | 1997-07-24 | 1999-02-12 | Du Pont Teijin Advanced Paper Kk | Battery separator |
| CN205723721U (en) * | 2016-06-16 | 2016-11-23 | 沧州明珠隔膜科技有限公司 | A kind of from fire-retardant lithium ion battery separator |
| CN111211276A (en) * | 2020-01-20 | 2020-05-29 | 哈尔滨工业大学 | Preparation method and application of large-scale porous aramid microfiber diaphragm |
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