Control methods for the ionic conductivity of temperature-responsive polymers

Abstract

This invention provides a novel method for controlling ionic conductivity using a polymer solution that dissolves in an organic solvent at low temperatures to form a transparent solution and undergoes phase separation upon heating; specifically, a temperature-responsive polymer that induces lower critical solution temperature (LCST) type phase separation. The method allows the ionic conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, wherein the salt dissociates into ionic components, to change with temperature. Preferably, the temperature-responsive polymer is one that exhibits LCST type phase separation.

Description

Technical Field

[0001] This invention relates to a method for controlling the ionic conductivity of a temperature-responsive polymer. More specifically, it relates to a method for controlling ionic conductivity, characterized by heating a solution containing a temperature-responsive polymer, a salt, and an organic solvent, wherein the salt dissociates into ionic components (hereinafter referred to as a temperature-responsive ionic solution), to induce phase separation in a temperature region higher than the lower critical solution temperature (hereinafter sometimes abbreviated as LCST), thereby reducing the ionic conductivity. Background Technology

[0002] Ion conductors have attracted much attention in recent years for various applications such as fuel cells, lithium-ion batteries, and chemical sensors. For example, Patent Document 1 discloses a method of using them as gas sensors. Furthermore, Patent Document 2 discloses a method of using them as lithium-ion batteries, and Patent Document 3 discloses a method of using them as storage devices.

[0003] As the utilization of ion conductors has expanded, attempts have also been made to control the ion conductivity of ion conductors. For example, Patent Document 4 discloses controlling the movement of lithium ions in a lithium-ion battery through the closing function of a separator, an attempt that has been implemented many times before. However, since the closing function of the separator depends on the shrinkage and melting function of the separator, there are problems such as the inability to freely design the temperature at which the closing function is performed.

[0004] Furthermore, stimulus-responsive polymers whose properties or forms change in response to temperature or pH changes are considered suitable for use as various sensors or functional materials. For example, the polymer represented by the following structural formula (a) is known as a polymer that undergoes so-called LCST-type phase separation; it dissolves in water at low temperatures to form a transparent solution and undergoes phase separation when heated.

[0005]

[0006] Furthermore, Non-Patent Document 1 discloses a polymer that dissolves in an ionic liquid (ionic solvent) at low temperatures to form a transparent solution and undergoes phase separation upon heating. However, in applications as parts for electrical appliances such as sensors, components containing water or ionic solvents with high dielectric constants are required, as are components containing organic solvents with low dielectric constants. However, prior reports on polymers exhibiting LCST-type phase separation in organic solvents are very few, and attempts to use temperature-responsive polymers to control ionic conductivity in solutions containing organic solvents are unknown.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2020-060452

[0010] Patent Document 2: Japanese Patent Application Publication No. 2023-146966

[0011] Patent Document 3: Japanese Patent Application Publication No. 2020-031178

[0012] Patent Document 4: Japanese Patent Application Publication No. 2023-099131

[0013] Non-patent literature

[0014] Non-patent literature 1: Takayuki Ueki and Masayuki Watanabe, “Lower critical solution temperature behavior of linear polymers in ionic liquids and corresponding volume phase transitions of polymer gels”, Langmuir, American Chemical Society, 2007, Vol. 23, No. 3, pp. 988-990 (Masayuki Watanabe et al., Langmuir (2007), 23, 3, 988-990). Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] The present invention was made in view of the situation of the prior art, and its object is to provide a method for controlling ion conductivity using a polymer solution that dissolves in an organic solvent at low temperature to form a transparent solution and undergoes phase separation if heated, i.e. a temperature-responsive polymer that causes LCST-type phase separation.

[0017] Technical means to solve the problem

[0018] To solve the aforementioned problem, the inventors conducted repeated and diligent research, resulting in the completion of this invention. Specifically, this invention has the following structure.

[0019] [1] A method for controlling ion conductivity, wherein the ion conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt and an organic solvent, wherein the salt dissociates into ionic components, changes according to temperature.

[0020] [2] According to the method for controlling ion conductivity described in [1], the temperature-responsive polymer is a polymer that exhibits phase separation at the lower critical solution temperature.

[0021] [3] According to the method for controlling ion conductivity as described in [1] or [2], wherein the temperature-responsive polymer is a polymer that exhibits phase separation at a lower critical solution temperature in a temperature-responsive ion solution.

[0022] [4] The method for controlling ion conductivity according to any one of [1] to [3], wherein the organic solvent comprises a carbonate-based organic solvent.

[0023] [5] The method for controlling ion conductivity according to any one of [1] to [4], wherein the salt is a lithium-containing salt.

[0024] [6] The method for controlling ion conductivity according to any one of [1] to [5], wherein the temperature-responsive ion solution contains less than 30% by weight of a temperature-responsive polymer.

[0025] The effects of the invention

[0026] This invention provides a very simple method for controlling ionic conductivity in temperature-responsive ionic solutions using temperature-responsive polymers. Attached Figure Description

[0027] [ Figure 1 ] Figure 1 It is a graph showing the relationship between the measured temperature and the rate of change of ionic conductivity.

[0028] [ Figure 2 ] Figure 2 Poly(CEVE-alt-pMeOBzA), synthesized in Example 1, is the precursor of the temperature-responsive polymer used in this invention. 1 H-NMR spectrum.

[0029] [ Figure 3 ] Figure 3 Poly([Me2Im][BF4]-alt-pMeOBzA), the temperature-responsive polymer synthesized in Example 1 and used in this invention, is... 1 H-NMR spectrum. Detailed Implementation

[0030] The present invention will now be described, but the present invention is not limited to the specific examples or embodiments described in the form used to carry out the invention, as long as it does not depart from the spirit of the present invention.

[0031] The present invention is configured as a method for controlling ion conductivity, wherein the ion conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, wherein the salt dissociates into ionic components, changes according to temperature, and the ion conductivity of the dissociated ions is controlled by the phase transition of the temperature-responsive polymer based on temperature.

[0032] As an example of control, for instance, at temperatures lower than the LCST of a temperature-responsive polymer having an LCST, the ions of the dissociated salt can move freely. However, if the solution temperature rises and reaches a temperature higher than the LCST of the temperature-responsive polymer having an LCST, the movement of the dissociated salt ions is restricted by the phase-separating temperature-responsive polymer, thereby reducing ionic conductivity. This method of controlling ionic conductivity is particularly preferred when used as an electrolyte in lithium-ion batteries. If the lithium-ion battery experiences abnormal heating and reaches a temperature higher than the LCST of the temperature-responsive polymer having an LCST, the ionic conductivity decreases, preventing abnormal heating of the lithium-ion battery. In this case, the temperature response of the temperature-responsive polymer can occur reversibly or irreversibly. In the case of reversible phase separation, if the temperature of the lithium-ion battery drops, it can be reused, thus allowing for repeated use. In the case of irreversible phase separation, since the battery is completely prevented from re-entering an abnormally heated state, the battery can be used more safely.

[0033] Regarding the phase separation mechanism of temperature-responsive polymers, any phase separation mechanism is possible as long as the movement of the dissociated ions can be restricted. For example, it can occur in a globular state or through the interaction of polymers to form new network polymers.

[0034] <Temperature-responsive polymers>

[0035] The temperature-responsive polymers used in this invention are not particularly limited as long as they are dissolved in a solution containing an organic solvent, but are preferably temperature-responsive polymers with LCST.

[0036] As temperature-responsive polymers with LCST, the following are preferably listed: acrylamide polymers and copolymer derivatives, acrylate polymers and copolymer derivatives, methacrylate polymers and copolymer derivatives, vinyl ether polymers and copolymer derivatives, and vinyl polymers and copolymer derivatives. Vinyl ether polymers and copolymer derivatives and vinyl polymers and copolymer derivatives are particularly preferred.

[0037] Regarding the molecular weight of the temperature-responsive polymer, there are no particular limitations as long as it dissolves in a solution containing an organic solvent. However, the number average molecular weight of the temperature-responsive polymer is preferably between 1,000 and 500,000. From the viewpoint of obtaining sufficient temperature responsiveness, a number average molecular weight of 1,000 or more is preferred. Furthermore, from the viewpoint of solubility in the solution containing the organic solvent or processability based on the optimal viscosity of the dissolved solution, a number average molecular weight of 500,000 or less is preferred.

[0038] The weight-average molecular weight (WMWM) / number-average molecular weight can be selected based on whether the control of ionic conductivity changes rapidly or slowly. A WMWM / number-average molecular weight of 1.25 or less can rapidly induce LCST-type phase separation. The smaller the WMWM / number-average molecular weight value, i.e., the narrower the molecular weight distribution and the closer it is to monodispersity, the more rapidly the change from transparent solution to opaqueness due to phase separation at a specified temperature occurs, leading to a more rapid LCST-type phase separation. Therefore, from this perspective, a WMWM / number-average molecular weight value closer to 1.0 is preferred when a rapid change in ionic conductivity is desired; however, considering ease of manufacture, the lower limit for the WMWM / number-average molecular weight is approximately 1.01. Conversely, a WMWM / number-average molecular weight of 2.0 or higher can induce LCST-type phase separation at a slower rate. A higher weight-average molecular weight (WMW / NUMBER) ratio, i.e., a wider molecular weight distribution, results in a more gradual and slower change from a transparent solution to an opaque solution due to phase separation caused by heating at a specified temperature. This leads to a more decelerated LCST-type phase separation. Therefore, from this perspective, a higher WMW / NUMBER ratio is preferred when a slow change in ionic conductivity is desired. However, considering ease of manufacturing, the upper limit for WMW / NUMBER is approximately 10.

[0039] The content of the temperature-responsive polymer is preferably 30% by weight or less, more preferably 5% by weight or less, relative to the temperature-responsive ionic solution. From the viewpoint of the stability of the inherent properties of the temperature-responsive ionic solution or its ease of use in application, the content is preferably 30% by weight or less. For example, if added to the electrolyte of a lithium-ion battery, the ionic conductivity of the electrolyte before addition can be maintained by the optimal viscosity of the electrolyte. Furthermore, the content of the temperature-responsive polymer is preferably 0.01% by weight or more, more preferably 0.1% by weight or more, relative to the temperature-responsive ionic solution. From the viewpoint of the control effect brought about by the phase transition of the temperature-responsive polymer, the content is preferably 0.01% by weight or more.

[0040] <Salt>

[0041] The salts used in this invention can be used without particular restrictions as long as they are soluble in a solution containing an organic solvent.

[0042] Regarding the salt content, from the viewpoint of the rate of change in ionic conductivity, a relatively high content is preferred. Specifically, relative to a temperature-responsive ionic solution, the salt content is preferably 0.1 mol / L or more, more preferably 0.5 mol / L or more, and particularly preferably 1.0 mol / L or more. From the viewpoint of sufficient change in ionic conductivity, the salt content is preferably 0.1 mol / L or more relative to a temperature-responsive ionic solution. Furthermore, from the viewpoint of ease of preparation of a temperature-responsive ionic solution, the salt content is preferably 3 mol / L or less, more preferably 1.5 mol / L or less relative to a temperature-responsive ionic solution. From the viewpoint of salt solubility, the salt content is preferably 3 mol / L or less relative to a temperature-responsive ionic solution.

[0043] Salt can be used alone or in combination with two or more types.

[0044] When used in lithium-ion batteries, the preferred salt is a lithium-containing salt. Specifically, examples include: lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, and lithium difluoro(oxalate)borate.

[0045] <Organic solvent>

[0046] The organic solvents used in this invention can be any conventionally known organic solvents without limitation. Generally, it is not a problem to use organic solvents that are purchased as reagents, but it is more preferable to use organic solvents with high purity, such as those for battery use. One organic solvent can be used alone, or two or more can be used in combination. For example, considering applications in lithium-ion batteries, from the viewpoint of salt solubility, carbonate-based, ether-based, or ester-based organic solvents are preferred. Specifically, the preferred materials are: ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate, ethylene ethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, 1,3-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl acetone, ethyl acetone, methyl difluoroacetone, ethyl trifluoroacetone, methyl propionate, ethyl propionate, propyl propionate, and vinyl acetone.

[0047] Alternatively, commercially available reagents pre-dissolved in salt can be used.

[0048] Example

[0049] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0050] (Example 1)

[0051] (Synthesis of precursors for temperature-responsive polymers)

[0052] Synthesis of Poly(CEVE-alt-pMeOBzA)

[0053] The poly(2-chloroethyl vinyl ether-alt-p-methoxybenzaldehyde) synthesized in the examples (hereinafter referred to as poly(CEVE-alt-pMeOBzA)) is a polymer represented by the following formula. Furthermore, n in the formula is an integer from 30 to 85.

[0054]

[0055] In a nitrogen-purified three-necked flask, 34 mL of dehydrated toluene, 3.8 g of purified 2-chloroethyl vinyl ether, 4.8 g of anisaldehyde, and 4.7 g of 1,4-dioxane were added. The resulting solution was cooled to below 5°C, and 1.0 mL of a 0.20 mol / L hexane solution of ethanesulfonic acid was added, followed by stirring in an ice bath for 5 minutes. The solution was then cooled to below -70°C, and 1.0 mL of a 0.20 mol / L gallium chloride solution was added, maintaining the temperature below -70°C while stirring until homogeneous to initiate the polymerization reaction. After 7 hours, 25 mL of methanol with a trace amount of ammonia was added to stop the reaction.

[0056] The solution, after the reaction was stopped, was brought to room temperature, and 100 mL of dichloromethane was added. The solution was then washed with deionized water until neutral. After dehydration, the solvent and other substances were removed by vacuum distillation, yielding 8.9 g of a pale yellow oil.

[0057] The crude product was repeatedly purified by reprecipitation to obtain 4.4 g of white solid.

[0058] The obtained solid through Figure 2 The hydrogen nuclear magnetic resonance spectrum shown is ( 1 H-nuclear magnetic resonance, 1 The compound was identified as poly(CEVE-alt-pMeOBzA) by ¹H NMR spectroscopy. The number-average molecular weight of the compound is 9,883, and the weight-average molecular weight / number-average molecular weight ratio is 1.63. The following shows... 1 The apparatus and analytical conditions for H-NMR spectroscopy were as follows. The weight-average molecular weight and number-average molecular weight were calculated using gel permeation chromatography (GPC) under the following conditions for polystyrene.

[0059] ( 1 H-NMR)

[0060] Device: JNM-ECZ500R / S1 (500 MHz) manufactured by Nippon Electron Ltd.

[0061] Solvent: Deuterated chloroform

[0062] Measurement temperature: room temperature

[0063] Internal standard substance: Tetramethylsilane

[0064] (GPC measurement conditions)

[0065] Pump: Nexera X2 LC-30AD manufactured by Shimadzu Corporation

[0066] Mobile phase: Tetrahydrofuran

[0067] Measurement temperature: 35℃

[0068] Flow rate: 0.5 mL / min

[0069] Tube Column: 1 Shodex GPCLF-804 manufactured by Resonac (stock) (formerly Showa Denko (stock)).

[0070] Detector: RID-20A

[0071] (Synthesis of temperature-responsive polymers)

[0072] Synthesis of Poly([Me2Im][BF4]-alt-pMeOBzA)

[0073] The poly(ethyl vinyl ether [dimethylimidazolium][tetrafluoroborate]-alt-p-methoxybenzaldehyde) synthesized in the examples (hereinafter referred to as Poly([Me2Im][BF4]-alt-pMeOBzA)) is the polymer represented by the following formula. Furthermore, n in the formula is an integer from 30 to 85.

[0074]

[0075] In a nitrogen-replaced three-necked flask, 4.4 g of the synthesized poly(CEVE-alt-pMeOBzA) with a number-average molecular weight of 9,883 and a weight-average molecular weight / number-average molecular weight of 1.63, 100 mL of dehydrated N,N-dimethylformamide, 9.9 g of 1,2-dimethylimidazole, and 15.4 g of sodium iodide were added, and the internal temperature was raised to 80°C. Subsequently, the mixture was stirred while maintaining an internal temperature of approximately 80°C, and cooled to room temperature after 72 hours.

[0076] A small amount of water was added to the obtained reaction solution to completely dissolve the solid. The solution was then placed into a cellulose tube for dialysis. The cellulose tube containing the solution was placed in a beaker filled with ion-exchange water as a buffer, and dialysis was performed while stirring slowly. The ion-exchange water as a buffer was replaced as appropriate, and dialysis was performed for a total of 24 hours. In this manner, an aqueous solution of a polymer (poly([dimethylimidazolium][iodide]-alt-p-methoxybenzaldehyde), poly([Me2Im][I]-alt-pMeOBzA)) with iodide anion as the counterion was obtained.

[0077] A sodium tetrafluoroborate aqueous solution (prepared by dissolving 4.5 g of sodium tetrafluoroborate in 30 mL of ion-exchange water) was added dropwise to the poly([Me2Im][I]-alt-pMeOBzA) aqueous solution obtained above. The addition was stopped at the point when a brown clump appeared at the bottom, and the solution was placed into a cellulose tube. The cellulose tube containing the solution was placed in a beaker filled with ion-exchange water, and dialyzed for a total of 24 hours while stirring slowly. Afterward, the contents of the cellulose tube were transferred to a flask, and the solvent was removed by vacuum distillation. The resulting light brown solid was vacuum dried at 50°C for 6 hours to obtain 1.9 g of light brown solid. The obtained solid was then... Figure 3 shown 1 H-NMR spectroscopy confirmed it to be poly([Me2Im][BF4]-alt-pMeOBzA). The following shows... 1 Apparatus and analytical conditions for H-NMR spectroscopy.

[0078] ( 1 H-NMR)

[0079] Device: JNM-ECZ500R / S1 (500 MHz) manufactured by Japan Electronics Co., Ltd.

[0080] Solvent: Dimethyl sulfoxide (DMSO)-d6

[0081] Measurement temperature: room temperature

[0082] Internal standard substance: Tetramethylsilane

[0083] (Preparation of temperature-responsive ionic solutions)

[0084] A base solution of dissolved acetone (dehydrated) was prepared by dissolving lithium bis(fluorosulfonyl)imide (manufactured by Kishida Chemical Co., Ltd.) as a salt in acetone (dehydrated) manufactured by Kanto Chemical Co., Ltd., at a concentration of 1 mol / L. A temperature-responsive ionic solution was then prepared by adding Poly([Me2Im][BF4]-alt-pMeOBzA) as a temperature-responsive polymer to the base solution at a concentration of 0.4% by weight.

[0085] (Methods for measuring ionic conductivity)

[0086] Regarding the ionic conductivity of the temperature-responsive ionic solution, the area is: 0.785 cm². 2 Two planar electrodes are positioned facing each other at a distance of 0.5 cm, with a temperature-responsive ionic solution filling the space between them. The resistance R (unit: Ω) between the electrodes is measured by AC impedance spectroscopy. The ionic conductivity (unit: S / cm) is calculated using the measured resistance R according to equation (1).

[0087]

[0088] As the conditions for measuring AC impedance, the measurement was performed at a frequency of 1 MHz to 10 Hz and an amplitude of 100 mV. The value of the real part of the low-frequency end of the semicircle in the Nyquist plot (cole-cole plot) obtained by the measurement was set as the resistance: R.

[0089] To determine the temperature dependence of ionic conductivity, measurements were performed at each of the following temperatures: 10°C, 20°C, 30°C, 40°C, and 50°C. The behavior resulting from temperature increases was also measured. The electrochemical apparatus and measurement conditions used for the Nyquist plot at these locations are shown below.

[0090] (Drawn by Nyquist)

[0091] Device: PARSTAT PMC1000 manufactured by Princeton Applied Research

[0092] Frequency: 1 MHz to 10 Hz

[0093] Amplitude: 100 mV

[0094] Temperature: 10℃, 20℃, 30℃, 40℃, 50℃

[0095] (Examples 2-4, Comparative Example 1)

[0096] Except for changes to the solvent type of the temperature-responsive ionic solution as shown in Table 1, the ionic conductivity was measured using the same procedure as in Example 1.

[0097] [Table 1]

[0098]

[0099] (Evaluation of the temperature dependence of ionic conductivity)

[0100] In order to evaluate the temperature dependence of ionic conductivity, the rate of change of ionic conductivity is calculated according to equation (2).

[0101]

[0102] The obtained rate of change of ionic conductivity is shown in Figure 1 . Figure 1 The results of the temperature dependence evaluation of ionic conductivity obtained from measurements conducted in Examples 1-4 and Comparative Example 1 are plotted as graphs showing the rate of change of ionic conductivity for each measurement temperature. That is, Figure 1 It is a graph showing the relationship between the measured temperature and the rate of change of ionic conductivity.

[0103] Generally, as temperature increases, the viscosity of a solution decreases, making it easier for ions to move. Therefore, ionic conductivity gradually increases with rising temperature. In Examples 1-4, ionic conductivity increased from 10°C to 20°C, but decreased at 40°C and 50°C. This is because the temperature-responsive polymer with LCST undergoes phase separation above the LCST, thereby inhibiting ion movement. In Examples 1-4, the temperature-dependent ionic conductivity was controlled.

[0104] On the other hand, in Comparative Example 1, as the temperature rose, the ionic conductivity gradually increased, and the ionic conductivity was not controlled.

[0105] Industrial availability

[0106] According to the present invention, a very simple method for controlling ion conductivity in a temperature-responsive ion solution using a temperature-responsive polymer is provided. Therefore, the ion conductivity control method of the present invention can be used in various sensors such as fuel cells, lithium-ion batteries, or chemical sensors.

Claims

1. A method for controlling ionic conductivity, wherein the ionic conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, wherein the salt dissociates into ionic components, changes according to temperature.

2. The method for controlling ion conductivity according to claim 1, wherein, Temperature-responsive polymers are polymers that exhibit phase separation at the lower critical solution temperature.

3. The method for controlling ion conductivity according to claim 1, wherein, Temperature-responsive polymers are polymers that exhibit phase separation at the lower critical solution temperature in temperature-responsive ionic solutions.

4. The method for controlling ion conductivity according to any one of claims 1 to 3, wherein, Organic solvents include carbonate-based organic solvents.

5. The method for controlling ion conductivity according to any one of claims 1 to 3, wherein, The salt is a lithium-containing salt.

6. The method for controlling ion conductivity according to any one of claims 1 to 3, wherein, The temperature-responsive ionic solution contains less than 30% by weight of a temperature-responsive polymer.

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