Method for controlling ion conductivity using temperature-responsive polymer

Abstract

Provided is a new method for controlling ion conductivity using a polymer solution that dissolves in an organic solvent at a low temperature to form a transparent solution, and undergoes phase separation when the temperature increases, more specifically, using a temperature-responsive polymer that undergoes lower critical solution temperature (LCST)-type phase separation.　This method for controlling ion conductivity changes, according to the temperature, the ion conductance of a temperature-responsive ionic solution which contains a temperature-responsive polymer, a salt, and an organic solvent, and in which the salt is dissociated into ionic components. Preferably, the temperature-responsive polymer is a polymer that exhibits lower critical solution temperature-type phase separation.

Description

Control of ionic conductivity using temperature-responsive polymers

[0001] The present invention relates to a method for controlling ionic conductivity using a temperature-responsive polymer, and more specifically to a method for controlling ionic conductivity, characterized in that a solution containing a temperature-responsive polymer, a salt, and an organic solvent, in which the salt is dissociated into ionic components (hereinafter abbreviated as a temperature-responsive ionic solution), is heated to cause phase separation in a temperature range higher than a lower critical solution temperature (hereinafter abbreviated as LCST), thereby reducing the ionic conductivity.

[0002] Ionic conductors are used in a variety of applications, including fuel cells, lithium-ion batteries, and chemical sensors, and have attracted attention in recent years. For example, Patent Document 1 discloses their use as gas sensors. Patent Document 2 discloses their use as lithium-ion batteries, and Patent Document 3 discloses their use as memory devices. As the uses of ionic conductors expand, attempts have been made to control the ionic conductivity of ionic conductors. For example, Patent Document 4 discloses controlling the movement of lithium ions in lithium-ion batteries using the separator's shutdown function, and many such attempts have been made in the past. However, the separator's shutdown function has problems, such as the inability to freely design the temperature at which shutdown occurs because it is caused by the shrinkage and melting of the separator film. Furthermore, stimuli-responsive polymers, which change their properties and morphology in response to temperature changes, pH changes, and other factors, are being considered for use in various sensors and functional materials. For example, the polymer represented by the following structural formula (a) is known to dissolve in water at low temperatures to form a clear solution, but undergoes phase separation upon heating, a so-called LCST-type phase separation.

[0003] Furthermore, Non-Patent Document 1 discloses a polymer that dissolves in an ionic liquid (ionic solvent) at low temperatures to form a clear solution, and undergoes phase separation when heated. However, for use as components in electrical appliances such as sensors, components made of water or ionic solvents with high dielectric constants are required, but components made of organic solvents with low dielectric constants are also required. However, there have been very few reports on polymers that exhibit LCST-type phase separation in organic solvents, and no attempts have been known to use temperature-responsive polymers to control ionic conductivity in solutions containing organic solvents.

[0004] JP 2020-060452 A JP 2023-146966 A JP 2020-031178 A JP 2023-099131 A

[0005] Takayuki Ueki and Masayuki Watanabe, "Lower Critical Solution Temperature Behavior of Linear Polymers in Ionic Liquids and the Corresponding Volume Phase Transition of Polymer Gels," Langmuir, American Chemical Society, 2007, Vol. 23, No. 3, pp. 988-990 (Watanabe, M. et al, Langmuir, (2007), 23, 3, 988-990).

[0006] The present invention has been made in view of the above-mentioned circumstances of the prior art, and an object of the present invention is to provide a 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, i.e., a temperature-responsive polymer that causes LCST-type phase separation.

[0007] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. That is, the present invention has the following features.

[0008] [1] A method for controlling ionic conductivity, which comprises changing the ionic conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, in which the salt is dissociated into ionic components, depending on the temperature.

[0009] [2] The method for controlling ionic conductivity according to [1], wherein the temperature-responsive polymer is a polymer that exhibits lower critical solution temperature phase separation.

[0010] [3] The method for controlling ionic conductivity according to [1] or [2], wherein the temperature-responsive polymer is a polymer that exhibits lower critical solution temperature phase separation in a temperature-responsive ionic solution.

[0011] [4] The method for controlling ionic conductivity according to any one of [1] to [3], wherein the organic solvent includes a carbonate-based organic solvent.

[0012] [5] The method for controlling ionic conductivity according to any one of [1] to [4], wherein the salt is a lithium-containing salt.

[0013] [6] The method for controlling ionic conductivity according to any one of [1] to [5], wherein the temperature-responsive ionic solution contains 30% by weight or less of a temperature-responsive polymer.

[0014] According to the present invention, a very simple method for controlling ionic conductivity in a temperature-responsive ionic solution using a temperature-responsive polymer is provided.

[0015] 1 is a graph showing the relationship between the measurement temperature and the rate of change in ionic conductivity. １ 3 is a H-NMR spectrum of Poly([Me2Im][BF4]-alt-pMeOBzA), which is the temperature-responsive polymer used in the present invention synthesized in Example 1. １ 1H-NMR spectrum.

[0016] The present invention is described below, but is not limited to the specific examples and examples described in the detailed description without departing from the spirit and scope of the present invention. The present invention provides a method for controlling ionic conductivity, which changes the ionic conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, in which the salt is dissociated into ionic components, depending on the temperature. The ionic conductivity of the dissociated ions is controlled by the temperature-dependent phase transition of the temperature-responsive polymer. As an example of this control, for example, at temperatures below the LCST of a temperature-responsive polymer having an LCST, the dissociated salt ions can move freely. However, when the solution temperature rises above the LCST of the temperature-responsive polymer, the phase-separated temperature-responsive polymer restricts the movement of the dissociated salt ions, thereby reducing the ionic conductivity. This method for controlling ionic conductivity is particularly useful when used as an electrolyte for a lithium-ion battery. When a lithium-ion battery experiences abnormal heat generation and the temperature exceeds the LCST of the temperature-responsive polymer having an LCST, the ionic conductivity decreases, thereby preventing the abnormal heat generation of the lithium-ion battery. In this case, the temperature response of the temperature-responsive polymer may be reversible or irreversible. If phase separation occurs reversibly, the lithium-ion battery can be used again when the temperature drops, allowing for repeated use. If phase separation occurs irreversibly, the battery is completely prevented from re-generating abnormally, allowing for safer use. The temperature-responsive polymer may be phase-separated in any manner that can restrict the movement of dissociated ions. For example, the polymer may be precipitated in a globular state, or a new network polymer may be formed through the interaction between polymers.

[0017] <Temperature-Responsive Polymer> The temperature-responsive polymer used in the present invention is not particularly limited as long as it dissolves in a solution containing an organic solvent, but is preferably a temperature-responsive polymer having an LCST. Preferred examples of temperature-responsive polymers having an LCST include acrylamide-based polymers and copolymer derivatives, acrylate-based polymers and copolymer derivatives, methacrylate-based polymers and copolymer derivatives, vinyl ether-based polymers and copolymer derivatives, and vinyl-based polymers and copolymer derivatives, with vinyl ether-based polymers and copolymer derivatives and vinyl-based polymers and copolymer derivatives being particularly preferred. The molecular weight of the temperature-responsive polymer is not particularly limited as long as it dissolves in a solution containing an organic solvent, but the number-average molecular weight of the temperature-responsive polymer is preferably 1,000 to 500,000. From the perspective of achieving sufficient temperature responsiveness, a number-average molecular weight of 1,000 or more is preferred. Furthermore, from the perspective of solubility in a solution containing an organic solvent and ease of handling due to the optimal viscosity of the dissolved solution, a number-average molecular weight of 500,000 or less is preferred. The weight-average molecular weight / number-average molecular weight ratio can be selected depending on whether the ionic conductivity control is to be changed rapidly or slowly. A polymer having a weight-average molecular weight / number-average molecular weight ratio of 1.25 or less can induce LCST-type phase separation at a rapid rate. The smaller the weight-average molecular weight / number-average molecular weight ratio, i.e., the narrower and more monodisperse the molecular weight distribution, the more rapidly the phase separation from a transparent solution to an opaque solution occurs at a given temperature, resulting in faster LCST-type phase separation. Therefore, from this perspective, if a rapid change in ionic conductivity is desired, a weight-average molecular weight / number-average molecular weight ratio closer to 1.0 is preferable. However, considering ease of manufacture, the lower limit of the weight-average molecular weight / number-average molecular weight is approximately 1.01. Furthermore, a polymer having a weight-average molecular weight / number-average molecular weight ratio of 2.0 or more can induce LCST-type phase separation at a slower rate. The larger the weight-average molecular weight / number-average molecular weight ratio, i.e., the wider the molecular weight distribution, the more slowly the phase separation from a transparent solution to an opaque solution occurs at a given temperature, resulting in slower LCST-type phase separation.Therefore, from this perspective, if a gradual change in ionic conductivity is desired, a larger weight-average molecular weight / number-average molecular weight ratio is preferable; however, considering ease of production, the upper limit of the weight-average molecular weight / number-average molecular weight ratio is approximately 10. The content of the temperature-responsive polymer is preferably 30 wt% or less, more preferably 5 wt% or less, relative to the temperature-responsive ionic solution. From the viewpoint of the stability of the inherent performance of the temperature-responsive ionic solution and ease of use in applications, a content of 30 wt% or less is preferable. For example, when added to the electrolyte of a lithium-ion battery, the optimal viscosity of the electrolyte is thought to maintain the ionic conductivity inherent in the electrolyte prior to addition. Furthermore, the content of the temperature-responsive polymer is preferably 0.01 wt% or more, more preferably 0.1 wt% or more, relative to the temperature-responsive ionic solution. From the viewpoint of the control effect due to the phase transition of the temperature-responsive polymer, a content of 0.01 wt% or more is preferable.

[0018] <Salt> The salt used in the present invention can be used without any particular limitation as long as it dissolves in a solution containing an organic solvent. A higher salt content is preferable from the viewpoint of the rate of change in ionic conductivity. Specifically, the salt content relative to the temperature-responsive ionic solution 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 relative to the temperature-responsive ionic solution is preferably 0.1 mol / L or more. Furthermore, from the viewpoint of ease of preparation of the temperature-responsive ionic solution, the salt content relative to the temperature-responsive ionic solution is preferably 3 mol / L or less, more preferably 1.5 mol / L or less. From the viewpoint of salt solubility, the salt content relative to the temperature-responsive ionic solution is preferably 3 mol / L or less. One type of salt may be used alone, or two or more types may be used in combination. When used in a lithium-ion battery, the salt is preferably a lithium-containing salt. Specific 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.

[0019] <Organic Solvent> Conventionally known organic solvents can be used in the present invention without any restrictions. Generally, there is no problem in purchasing and using commercially available reagents, but highly pure organic solvents such as battery grade are more preferable. One type of organic solvent can be used alone, or two or more types can be mixed. For example, when considering application to lithium ion batteries, carbonate, ether, or ester organic solvents are preferred from the viewpoint of salt solubility. Specifically, preferred are ethylene carbonate (hereinafter abbreviated as EC), propylene carbonate (hereinafter abbreviated as PC), vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate (hereinafter abbreviated as DEC), ethyl methyl carbonate (hereinafter abbreviated as EMC), γ-butyrolactone, 1,3-dioxolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme, triglyme, tetraglyme, methylacetone, ethylacetone, methyldifluoroacetone, ethyltrifluoroacetone, methylpropionate, ethylpropionate, propylpropionate, vinylacetone, etc. Alternatively, commercially available reagents in which the salt is dissolved in advance may be used.

[0020] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0021] Example 1 Synthesis of Thermoresponsive Polymer Precursor Synthesis of Poly(CEVE-alt-pMeOBzA) Poly(2-chloroethyl vinyl ether-alt-paramethoxybenzaldehyde) (abbreviated as poly(CEVE-alt-pMeOBzA) in this specification) synthesized in this example is a polymer represented by the following formula: where n is an integer of 30 to 85. A three-neck flask purged with nitrogen was charged with 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. 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. The mixture was stirred in an ice bath for 5 minutes. The mixture was then cooled to below -70°C, and 1.0 mL of a 0.20 mol / L gallium chloride solution was added. The mixture was stirred uniformly while maintaining the temperature below -70°C, allowing the polymerization reaction to proceed. After 7 hours, the reaction was terminated by adding 25 mL of methanol containing a trace amount of aqueous ammonia. The solution after the reaction was returned to room temperature, 100 mL of dichloromethane was added, and the mixture was washed with ion-exchanged water until neutral. After dehydration, the solvent was removed by distillation under reduced pressure, yielding 8.9 g of a pale yellow oil. The resulting crude product was repeatedly purified by reprecipitation to yield 4.4 g of a white solid. The resulting solid was the same as that shown in Figure 2. １ The H-NMR spectrum confirmed that the compound was poly(CEVE-alt-pMeOBzA). The number-average molecular weight of this compound was 9,883, and the weight-average molecular weight / number-average molecular weight ratio was 1.63. １ The H-NMR spectrum measuring device and analysis conditions are shown below. The weight average molecular weight and number average molecular weight are values ​​calculated in terms of polystyrene using gel permeation chromatography (GPC) under the following conditions.

[0022] ( １ H-NMR) Apparatus: JNM-ECZ500R / S1 (500 MHz) manufactured by JEOL Ltd. Solvent: deuterated chloroform Measurement temperature: room temperature Internal standard substance: tetramethylsilane (GPC measurement conditions) Pump: NexeraX2 LC-30AD manufactured by Shimadzu Corporation Mobile phase: tetrahydrofuran Measurement temperature: 35°C Flow rate: 0.5 mL / min Column: Shodex GPC LF-804 x 1 manufactured by Resonac Inc. (formerly Showa Denko K.K.) Detector: RID-20A

[0023] (Synthesis of Temperature-Responsive Polymer) Synthesis of Poly([Me2Im][BF4]-alt-pMeOBzA) Poly(ethyl vinyl ether [dimethylimidazolium][tetrafluoroborate]-alt-paramethoxybenzaldehyde) (abbreviated herein as poly([Me2Im][BF4]-alt-pMeOBzA)) synthesized in the examples is a polymer represented by the following formula: where n is an integer from 30 to 85. A nitrogen-purged three-neck flask was charged with 4.4 g of the poly(CEVE-alt-pMeOBzA) synthesized above, having a number-average molecular weight of 9,883 and a weight-average molecular weight / number-average molecular weight ratio of 1.63, 100 mL of dehydrated N,N-dimethylformamide, 9.9 g of 1,2-dimethylimidazole, and 15.4 g of sodium iodide, and the internal temperature was raised to 80°C. The mixture was then stirred while maintaining an internal temperature of approximately 80°C, and after 72 hours, cooled to room temperature. A small amount of water was added to the resulting reaction solution to dissolve all solids, and the resulting solution was then placed in a cellulose tubing for dialysis. The cellulose tubing containing the solution was placed in a beaker filled with ion-exchanged water as a buffer, and dialysis was performed with gentle stirring. The ion-exchanged water buffer was replaced as needed, and dialysis was performed for a total of 24 hours. In this way, an aqueous solution of a polymer (poly([Me2Im][I]-alt-pMeOBzA)) with iodine anions as counterions was obtained. An aqueous solution of sodium tetrafluoroborate (aqueous solution prepared by dissolving 4.5 g of sodium tetrafluoroborate in 30 mL of ion-exchanged water) was added dropwise to the aqueous solution of poly([Me2Im][I]-alt-pMeOBzA) obtained above. The addition was stopped when a brown mass appeared at the bottom, and the solution was placed in a cellulose tube. The cellulose tube containing the solution was placed in a beaker filled with ion-exchanged water, and dialysis was carried out for a total of 24 hours while gently stirring. Thereafter, the contents of the cellulose tube were placed in a flask, and the solvent and the like were removed by distillation under reduced pressure. The obtained light brown solid was vacuum dried at 50°C for 6 hours, yielding 1.9 g of a light brown solid. The obtained solid was similar to that shown in Figure 3. １ The H-NMR spectrum confirmed that the product was poly([Me2Im][BF4]-alt-pMeOBzA).１ The H-NMR spectrum measuring device and analysis conditions are shown below. １ H-NMR) Apparatus: JNM-ECZ500R / S1 (500 MHz) manufactured by JEOL Ltd. Solvent: DMSO-d6 Measurement temperature: room temperature Internal standard substance: tetramethylsilane

[0024] (Preparation of Temperature-Responsive Ion Solution) A base acetone solution containing the salt (hereinafter referred to as the base solution) was prepared by dissolving lithium bis(fluorosulfonyl)imide (Kishida Chemical Co., Ltd.) as a salt in acetone (dehydrated) (Kanto Chemical Co., Ltd.) to a concentration of 1 mol / L. Poly([Me2Im][BF4]-alt-pMeOBzA) as a temperature-responsive polymer was added to the base solution to a concentration of 0.4 wt % to prepare a temperature-responsive ion solution.

[0025] (Method for measuring ionic conductivity) The ionic conductivity of the temperature-responsive ionic solution was measured using a 0.785 cm ２ The two planar electrodes were placed facing each other at a distance of 0.5 cm, and the space between them was filled with a temperature-responsive ionic solution. The resistance R (unit: Ω) between the electrodes was measured by an AC impedance method. Using the measured resistance R, the ionic conductivity (unit: S / cm) was calculated from equation (1). The measurement conditions for AC impedance were a frequency of 1 MHz to 10 Hz and an amplitude of 100 mV. The real part of the end of the semicircle on the low frequency side in the Nyquist plot (Cole-Cole plot) obtained by the measurement was taken as the resistance: R. To measure the temperature dependence of ionic conductivity, measurements were made at temperatures of 10°C, 20°C, 30°C, 40°C, and 50°C, and behavior due to temperature increase was measured. The electrochemical measurement apparatus and measurement conditions used to measure the Nyquist plot here are shown below. (Nyquist Plot) Apparatus: PARSTAT PMC1000 manufactured by Princeton Applied Research Frequency: 1 MHz to 10 Hz Amplitude: 100 mV Temperature: 10°C, 20°C, 30°C, 40°C, 50°C

[0026] Examples 2 to 4, Comparative Example 1 Ion conductivity was measured in the same manner as in Example 1, except that the solvent type of the temperature-responsive ionic solution was changed as shown in Table 1.

[0027] [Table 1]

[0028] (Evaluation of Temperature Dependence of Ionic Conductivity) To evaluate the temperature dependency of ionic conductivity, the rate of change in ionic conductivity was calculated using formula (2). The obtained ionic conductivity change rate is shown in Figure 1. Figure 1 shows the results of the evaluation of the temperature dependence of ionic conductivity obtained by measurements in Examples 1 to 4 and Comparative Example 1, plotting the ionic conductivity change rate against each measurement temperature. That is, Figure 1 is a graph showing the relationship between measurement temperature and ionic conductivity change rate. Normally, as the temperature increases, the viscosity of the solution decreases, making ions more mobile. Therefore, ionic conductivity gradually increases with increasing temperature. In Examples 1 to 4, ionic conductivity increases when the temperature is changed from 10°C to 20°C, but decreases at 40°C and 50°C. This is because the temperature-responsive polymer with an LCST undergoes phase separation above the LCST, suppressing ion migration. In Examples 1 to 4, the ionic conductivity changed with temperature was controlled. On the other hand, in Comparative Example 1, ionic conductivity gradually increased with increasing temperature, and ionic conductivity was not controlled.

[0029] According to the present invention, a very simple method for controlling ionic conductivity in a temperature-responsive ionic solution using a temperature-responsive polymer is provided. Therefore, the method for controlling ionic conductivity of the present invention can be used in various sensors such as fuel cells, lithium ion batteries, and chemical sensors.

Claims

1. A method for controlling ionic conductivity by changing the ionic conductivity of a temperature-responsive ionic solution containing a temperature-responsive polymer, a salt, and an organic solvent, in which the salt is dissociated into ionic components, depending on the temperature.

2. The method for controlling ionic conductivity according to claim 1, wherein the temperature-responsive polymer is a polymer that exhibits lower critical solution temperature phase separation.

3. The method for controlling ionic conductivity according to claim 1, wherein the temperature-responsive polymer is a polymer that exhibits lower critical solution temperature phase separation in a temperature-responsive ionic solution.

4. The method for controlling ionic conductivity according to any one of claims 1 to 3, wherein the organic solvent includes a carbonate-based organic solvent.

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

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

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