Patent Application
20170271637
This application claims priority to Korean Patent Application No. 10-2016-0031247, filed on Mar. 16, 2016, and all the benefits accruing therefrom under 35 U.S.C. §119, the content of which in its entirety is herein incorporated by reference.
1. Technical Field
The present invention relates to a method of manufacturing a porous polymer membrane and, more particularly, a porous polymer membrane, which has properties suitable for use as a separator for a secondary battery.
2. Description of the Related Art
A porous material, especially a membrane having nano-sized pores, is receiving attention these days in the fields of nano technology, biotechnology, and environmental technology due to various applications including gas storage, filtering, battery separation, water treatment, water purification, etc.
The porous polymer membrane of the invention has properties suitable for use in a separator of a secondary battery. A lithium secondary battery, which is a typical example of a secondary battery, is configured such that an anode, a cathode, and a porous separator disposed therebetween are assembled. The separator, which is interposed between the two electrodes of the battery, functions to prevent internal shorting due to direct contact between the cathode and the anode, and plays an important role as an ion path in the battery and in improving the safety of the battery. In particular, since lithium ions pass through the pores of the separator, the inner pore structure of the separator is regarded as very important.
When a porous polymer membrane is used as a separator for a battery, the inner structure thereof may be easily changed through the use of various polymer materials and functional materials. Hence, such a membrane is known to be a very good candidate for a separator for a lithium secondary battery.
A separator using a porous polymer membrane may be currently manufactured through various processes, such as thermally induced phase separation, phase inversion, track-etching, etc. However, these methods are problematic because the manufacturing process is complicated and expensive processing is required to achieve mass production. Due to such problems, the use of a porous polymer membrane as a battery separator is still difficult. With the goal of utilizing a porous polymer membrane as a battery separator, there is a need to develop a method of manufacturing a polymer membrane having nano-sized pores, which is simple, has low manufacturing costs and good energy efficiency, and is environmentally friendly.
With regard to patents related to porous polymer membranes, Korean Patent Application Publication No. 10-2014-0071094 discloses a method of manufacturing a porous polyolefin membrane, comprising the steps of extruding a polyolefin resin to form an extrusion film, annealing the extrusion film, and uniaxially drawing the extrusion film to form a porous membrane.
Also, Korean Patent No. 10-1536062 discloses a method of manufacturing a microporous polymer membrane, comprising the steps of extruding a resin composition to prepare a precursor film, annealing the precursor film, uniaxially drawing and then thermosetting the annealed film to form a microporous membrane, and photo-crosslinking the microporous membrane with UV light.
Also, Korean Patent Application Publication No. 10-2011-0026609 discloses a method of manufacturing a porous membrane, comprising the steps of kneading a resin mixture comprising 50 to 95 wt % of a polyolefin polymer, 5 to 50 wt % of a polyketone polymer, resulting from terpolymerization of carbon monoxide, unsaturated ethylene and propylene compounds, and the remainder of a plasticizer, melt-extruding the mixture to form a sheet, drawing the sheet in a longitudinal direction and a transverse direction to form a film, and extracting the plasticizer from the film and thermosetting the film.
Also, Korean Patent No. 10-1464430 discloses a method of manufacturing a microporous polymer membrane, comprising the steps of extruding a polymer compound or a composition including the same to form a precursor film, annealing the precursor film, low-temperature drawing the annealed film in a temperature range from a temperature −70° C. lower than the glass transition temperature of the polymer compound to a temperature +70° C. higher than the glass transition temperature of the polymer compound, high-temperature drawing the low-temperature-drawn film at an inclined angle of 20 to 65° in a temperature range from a temperature −40° C. lower than the melting temperature of the polymer compound to the melting temperature of the polymer compound, and alternately laminating two or more high-temperature-drawn films.
Conventional methods of forming pores in polymer membranes are mostly performed in a manner such that pores are formed through photoexposure or extraction of the plasticizer in the drawn polymer membrane, and thus the drawing process is required, the manufacturing processing is complicated, and it is very difficult to control the pore size and the porosity. Hence, in the case where a conventionally manufactured porous polymer membrane is used as a battery separator, the efficiency of the battery may be deteriorated.
Korean Patent Application Publication No. 10-2014-0071094
Korean Patent No. 10-1536062
Korean Patent Application Publication No. 10-2011-0026609
Korean Patent No. 10-1464430
Accordingly, the present invention has been made keeping in mind the problems encountered in the related art, and the present invention is intended to provide a novel method of forming fine pores, for example, nano-sized pores, in a polymer membrane.
In addition, the present invention is intended to provide a method of manufacturing a porous polymer membrane satisfying the properties necessary for a battery separator.
In addition, the present invention is intended to provide a method of manufacturing a porous polymer membrane, which obviates a drawing process, unlike conventional techniques, has a simple manufacturing process, and makes it possible to control a pore size and porosity.
The present invention provides a method of manufacturing a porous polymer membrane using water pressure, comprising: preparing a membrane from a mixed solution comprising a polymer, a metal salt and a solvent, and forming pores in the membrane by applying water pressure to the membrane.
The pore size and the porosity of the polymer membrane may be controlled depending on the magnitude of the water pressure.
The water pressure may range from 2 to 8 bar.
The porous polymer membrane may be formed on a porous support.
The polymer may include, but is not limited to, any one selected from poly(2-hydroxypropyl methacrylate), poly(2-ethyl-2-oxazoline), poly(acrylamide-co-acrylic acid), polymethacrylamide, polyacrylamide, poly(3-chloro-2-hydroxypropyl-2-methacryloxyethyldimethylammonium chloride), poly(acrylamide-co-2-methacryloxyethyltrimethylammonium bromide), poly(2-methacryloxyethyltrimethylammonium bromide), poly(2-vinyl-1-methylpyridinium bromide), poly(N-vinylpyrrolidone), poly(vinylamine hydrochloride), poly(l-lysine hydrobromide), poly(2-vinylpyridine), poly(4-vinylpyridine), poly(ethylene oxide-b-propylene oxide), poly(allylamine), poly(styrenesulfonic acid-co-maleic acid) sodium salt, poly(methacrylic acid), poly(ethylene-co-acrylic acid), poly(acrylic acid), poly(ethyl acrylate-co-acrylic acid), isotactic polypropylene, poly(vinyl methyl ether), poly(vinyl phosphoric acid) sodium salt, poly(styrenesulfonic acid), poly(N-vinyl acetamide), poly(N-vinyl acetamide-co-sodium acrylate), poly(N-methyl N-vinyl acetamide) homopolymer, poly(n-butyl acrylate-co-2-methacryloxyethyltrimethylammonium bromide), poly(vinylsulfonic acid), poly(N-vinylpyrrolidone-co-vinyl acetate), poly(styrenesulfonic acid-co-maleic acid), cellulose hydroxyethyl ether, cellulose methyl hydroxyethyl ether, poly(ethylene oxide), poly(vinyl acetate), poly(vinyl alcohol), poly(diallyldimethylammonium chloride), poly(maleic acid), poly(l-glycerol methacrylate), poly(butadiene-co-maleic acid), and poly(vinylphosphonic acid).
The metal salt may include, but is not limited to, any one selected from aluminum nitrate nonahydrate, ammonium cerium(IV) nitrate, ammonium nitrate, barium nitrate, beryllium nitrate, calcium nitrate hydrate, calcium nitrate tetrahydrate, cerium(III) nitrate hexahydrate, cesium nitrate, chromium(III) nitrate nonahydrate, cobalt(II) nitrate hexahydrate, copper(II) nitrate hemi(pentahydrate), iron(III) nitrate nonahydrate, lead(II) nitrate, lithium nitrate, lutetium(III) nitrate hydrate, magnesium nitrate hexahydrate, manganese(II) nitrate hydrate, mercury(I) nitrate dihydrate, mercury(II) nitrate monohydrate, mercury(II) nitrate solution, nickel(II) nitrate hexahydrate, palladium(II) nitrate dihydrate, palladium(II) nitrate hydrate, palladium(II) nitrate, potassium nitrate, ruthenium(III) nitrosyl nitrate, silver nitrate, sodium nitrate, titanium nitrate, zinc nitrate hexahydrate, nickel(II) chloride, nickel(II) chloride hexahydrate, nickel(II) acetate tetrahydrate, nickel sulfide, nickel(II) sulfate hexahydrate, nickel(II) nitrate hexahydrate, nickel boride, nickel(II) sulfate, nickel phosphide, nickel(II) acetylacetonate, nickel(II) perchlorate hexahydrate, nickel(II) bromide, nickel(II) hydroxide, nickel(II) bromide hydrate, nickel(II) phthalocyanine, nickel(II) trifluoromethanesulfonate, nickel(II) hexafluoroacetylacetonate hydrate, nickel(II) sulfate heptahydrate, ammonium nickel(II) sulfate hexahydrate, nickel carbonate basic hydrate, nickel(II) chloride hydrate, nickel(II) sulfamate tetrahydrate, nickel(II) carbonate hydroxide tetrahydrate, nickel(II) fluoride, Nickel(II) bromide trihydrate, nickel(II) oxalate dihydrate, nickel(II) octanoate hydrate, and nickel(II) cyclohexane butyrate.
The content ratio of the polymer to the metal salt may be set such that the metal salt is used in an amount of 0.01 to 0.6 mol relative to 1 mol of a polymer repeating unit.
In addition, the present invention provides a battery separator, comprising a porous polymer membrane manufactured by the above method.
According to the present invention, the pores in the polymer membrane can be formed by removing the metal salt therefrom or increasing the size of spaces in weakened portions of the polymer due to the plasticization effect of the metal salt through water-pressure treatment. The porous polymer membrane of the invention can be manufactured while controlling the pore size and porosity thereof in a comparatively simply manner such that a polymer membrane is formed using a solution process and is then subjected to water-pressure treatment.
In particular, the porous polymer membrane manufactured by the method of the present invention can be useful as a battery separator, thus achieving high battery efficiency, as can be confirmed experimentally.
The above and other features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
The present invention addresses a method of manufacturing a porous polymer membrane. The porous polymer membrane, manufactured by the method of the present invention, may be provided in any form, such as a flat type, a hollow fiber type, etc., and may be used alone or in combination with another structural element such as a support or the like. In particular, such a membrane is useful as a separator for a battery.
The present invention addresses a method of manufacturing a porous polymer membrane using water pressure, comprising the steps of preparing a membrane from a mixed solution of a polymer, a metal salt, and a solvent and forming pores in the membrane by applying water pressure to the membrane.
Below is a description of individual steps of the method.
Preparing Membrane
In the step of preparing the membrane, a mixed solution of a polymer, a metal salt, and a solvent is formed into a flat membrane or a hollow fiber membrane through a typical process. The mixed solution of a polymer, a metal salt, and a solvent is subjected to spin casting, knife casting or spinning, after which the solvent is removed through a drying process such as room-temperature drying, heat drying, or vacuum drying, thus enabling the formation of a solid membrane.
In the present invention, any kind of polymer may be used so long as it is typically formed into a film. Various polymers may be used in the present invention, as will be described in the following Test Examples. Thus, examples of the polymers are not limited within the scope of the present invention.
In the present invention, any metal salt may be used, and the kind of metal salt is not limited. Through the following Test Examples, various metal salts can be seen to be useful in the present invention. In the present invention, the metal salt may exhibit a plasticization effect within the polymer, and thus polymer chains in the membrane are made loose. While the metal salt escapes from the membrane due to the water pressure, pores are formed. Hence, the kind of metal salt is not particularly limited.
The porous polymer membrane of the present invention may be provided in the form of a film on a porous support, for example, a support having micro-sized pores, or alternatively may be provided as a film without a porous support.
Forming Pores
In the step of forming the pores, upon water-pressure treatment, the size of spaces in the polymer chains, which are weakened due to plasticization of the metal salt, is increased, and moreover, the metal salt escapes from the membrane, whereby pores may result.
In the present invention, the metal salt is solvated by the solvent component remaining in the polymer membrane, and may thus be formed into an ion aggregate larger than free ions or ion pairs. While such an ion aggregate is removed from the polymer membrane by means of the water pressure, pores having a uniform and linear shape are found to result. In the present invention, the term “water pressure” is used to have a meaning including not only water pressure using pure water, but also water pressure using an aqueous solution in which various chemical components are dissolved in water.
A better understanding of the present invention is given through the following Test Examples.
Using an SEM, observed were pores of a membrane manufactured from CA (Cellulose Acetate) dissolved in pure acetone (“neat CA”), a membrane manufactured from CA dissolved in a mixed solvent of acetone/water (w/w 8:2), and a membrane (without water-pressure treatment) manufactured from a solution comprising a metal salt Ni(NO3)2.6H2O and CA dissolved in a mixed solvent of acetone/water (w/w 8:2).
A solution of CA/Ni(NO3)2.6H2O at a weight ratio of 1:0.23 dissolved in a solvent (a mixed solvent of acetone/water, w/w 8:2) was cast and dried, thus manufacturing a membrane, which was then tested for its flux depending on isostatic water pressure. In Comparative Example, neat CA was tested, and the flux was measured using a mechanical flowmeter.
As shown in
With reference to
In order to understand the plasticization effect of the metal salt Ni(NO3)2.6H2O in the CA chains during the water-pressure treatment, FT-IR was measured using a VERTEX 70 FT-IR spectrometer (Bruker Optics Inc.).
The neat CA polymer matrix showed a characteristic IR peak at 3500 cm−1, corresponding to the hydroxyl group of the CA polymer. On the other hand, the membrane including the metal salt Ni(NO3)2.6H2O showed a representative absorption band for CA/Ni(NO3)2.6H2O at 3400 cm−1. This is because the large number of H2O molecules in Ni(NO3)2.6H2O increased the OH absorption peak intensity. When the CA polymer sample was treated at a water pressure of 8 bar, the 3400 cm−1 peak of the CA/Ni(NO3)2.6H2O was shifted to 3500 cm−1, which means that a considerable amount of Ni(NO3)2.6H2O escaped from the polymer matrix due to high water pressure.
Another reason why the pore size and the porosity are increased in the CA polymer matrix was confirmed using a mercury porosimeter.
Referring to
In order to evaluate changes in pores depending on the water-pressure treatment, water pressure was applied at 2 bar (
In order to evaluate the thermal stability of the porous polymer matrix of the present invention, TGA testing was performed.
As for the neat CA membrane and the CA/Ni(NO3)2.6H2O (1:0.23) membrane after water-pressure treatment at 8 bar, about 90 wt % thereof was decomposed at about 300° C. On the other hand, the CA/Ni(NO3)2.6H2O membrane not subjected to water-pressure treatment was decomposed at 80% in the temperature range of 200 to 350° C., and the remaining 20% was decomposed in the range of 350 to 550° C. The boiling point of Ni(NO3)2.6H2O is known to be 136.7° C. The reason why 80% of the CA/Ni(NO3)2.6H2O membrane not subjected to water-pressure treatment was decomposed in the range of 200 to 350° C. is that Ni(NO3)2.6H2O in the membrane was degraded and removed. However, the thermal stability of the membrane subjected to water-pressure treatment was increased compared to that of the CA/Ni(NO3)2.6H2O membrane not subjected to water-pressure treatment. This is deemed to be because the CA/Ni(NO3)2.6H2O was removed from the membrane through water-pressure treatment. Furthermore, the CA/Ni(NO3)2.6H2O, which had remained in a small amount even after water-pressure treatment, was degraded and removed at about 400° C.
Battery-grade 1.3 M LiPF6-EC/DEC having 10 wt % of an FEC electrolyte was used to constitute both a symmetric cell and a half cell. A 300 μm-thick sheet of pure Li metal and a 5 μm-thick sheet of Li metal on a piece of Cu foil were purchased from Wellcos. The cathode components comprising LTO, PVDF, and super P (8:1:1) were dissolved in NMP and then applied on a piece of Cu foil. The cathode was dried in a vacuum oven at 80° C. for 12 hr. A PS (Celgard 2400) separator was used for Comparative Example. The 2032 coin cells were used for symmetric-cell and half-cell measurement. The coin cells were pressurized using a crimping machine (Hohsen Corp). The battery cells were manufactured in an Ar-charged glove box (<0.1 ppm O2 and H2O).
As shown in
As shown in
As shown in
Individual polymer membranes were manufactured by the method of the invention using a CA polymer and metal salts having various metal cations, the anions of which were fixed to nitric acid, and were then subjected to water-pressure treatment at water pressure ranging from 3 bar to 8 bar to form pores, followed by flux measurement. As such, the mass ratio of CA polymer to metal salt was 1:0.23. The results are shown in Table 1 below.
As is apparent from the above results, the porous polymer membrane of the invention can be confirmed to be manufactured using all metal salts having various metal cations.
Individual polymer membranes were manufactured by the method of the invention using a CA polymer and metal salts having various anions, the metal cations of which were fixed to nickel, and were then subjected to water-pressure treatment at water pressure ranging from 3 bar to 8 bar, followed by flux measurement. As such, the mass ratio of CA polymer to metal salt was 1:0.23. The results are shown in Table 2 below.
As is apparent from the above results, the porous polymer membrane of the invention can be confirmed to be manufactured using all metal salts having various anions.
Individual polymer membranes were manufactured by the method of the invention using a nickel(II) nitrate hexahydrate as a metal salt and various polymers, and were then subjected to water-pressure treatment at water pressure ranging from 3 bar to 8 bar to form pores, followed by flux measurement. As such, the mass ratio of polymer to metal salt was 1:0.23. The results are shown in Table 3 below.
As is apparent from the above results, the porous polymer membrane of the invention can be confirmed to be manufactured using various polymers.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2016-0031247 | Mar 2016 | KR | national |
