JP4969982B2 - Carbonated polyvinyl alcohol, process for producing the same, and gel electrolyte - Google Patents

Description

  The present invention relates to a novel polyvinyl alcohol derivative that can be used in a polymer electrolyte and a method for producing the same, and also relates to a polymer gel electrolyte using the polyvinyl alcohol derivative.

  In recent years, application of a polymer solid electrolyte that does not leak electrolyte has been studied as an electrolyte for lithium secondary batteries. However, if the polymer solid electrolyte is used as it is, there is a problem that the ionic conductivity is low.

In order to improve the ionic conductivity of a polymer solid electrolyte, a gel electrolyte in which a polymer is swollen with an electrolytic solution has been developed. In order to apply such a gel electrolyte to a secondary battery, the gel electrolyte has a high ionic conductivity and at the same time serves as a separator between the positive electrode and the negative electrode. It is necessary to have. As an example of improving the mechanical strength, there is a method described in Patent Document 1 in which a conductive polymer is applied to a porous substrate membrane and swollen with an electrolytic solution to form a gel electrolyte. This is a technique for ensuring the strength by the porous substrate film, but has a problem that it is inferior in safety because it has poor electrolyte retention and can leak when the battery is damaged. Therefore, development of a gel electrolyte that does not use such a porous substrate membrane has been performed. A gel electrolyte in which a polymer is swollen with an electrolyte without using such a porous substrate membrane tends to be insufficient in strength. In order to increase the strength of the gel electrolyte, the proportion of the polymer may be increased. However, the ionic conductivity decreases as the proportion of the electrolyte decreases. On the contrary, if the amount of the electrolytic solution is increased, the ionic conductivity is improved, but the strength is lowered, and the electrolyte retaining ability of the gel electrolyte is exceeded, and problems such as bleeding of the electrolytic solution also occur. In the case of a battery using a gel polymer electrolyte, ionic conductivity is an important factor that directly affects battery performance, and electrolyte oozing is an important factor that directly affects battery safety. Gel electrolytes have ionic conductivity. It is required to be good and free from leakage.
JP 2002-241533 A

  As described above, the gel electrolyte is required to have good ion conductivity and at the same time no leakage, and as a polymer for gel electrolyte that does not use a porous substrate membrane, it is highly polar. There is also a need for high-strength polymers. However, in general, there is a problem in workability during production due to instability during monomer synthesis, gelation during polymerization, etc., and there is also a problem in cost, so it is difficult to produce a highly polar polymer. .

  Accordingly, an object of the present invention is to provide a novel highly polar polymer that is easy to manufacture and has good ion conductivity and electrolyte retention when applied to a gel electrolyte. Another object of the present invention is to provide a gel electrolyte having both good ionic conductivity and electrolyte retention.

In the present invention, a hydroxyl group (hereinafter referred to as OH group) of polyvinyl alcohol (hereinafter sometimes abbreviated as PVA) having 20 to 90 mol% of vinyl acetate units and 10 to 80 mol% of vinyl alcohol units. At least part of the group represented by —OC (═O) OR (where R is an alkyl group having 12 or less carbon atoms which may have a substituent, and an ether bond is introduced into the alkyl chain) A carbonated polyvinyl alcohol having a structure substituted with a substituted group or an aromatic group.
The carbonated polyvinyl alcohol of the present invention preferably has a structure in which 50 mol% or more of the hydroxyl groups are substituted with a group represented by the —OC (═O) OR.
In another embodiment of the carbonated polyvinyl alcohol of the present invention, a part of hydroxyl groups of polyvinyl alcohol having 20 to 90 mol% vinyl acetate units and 10 to 80 mol% vinyl alcohol units is converted to —OC (═O). Substituted with a group represented by OR (R represents an alkyl group having 12 or less carbon atoms which may have a substituent, a group in which an ether bond is introduced into an alkyl chain, or an aromatic group). Furthermore, it is carbonated polyvinyl alcohol having a structure in which part or all of the remaining hydrogen atoms of the hydroxyl group (hereinafter sometimes referred to as H) are substituted with an isocyanate compound residue.
The present invention is also a gel electrolyte using the above carbonated polyvinyl alcohol.
The present invention further includes polyvinyl alcohol having 20 to 90 mol% vinyl acetate units and 10 to 80 mol% vinyl alcohol units, and a halogenated formate represented by XCOOR (R may have a substituent). A carbonated polyvinyl alcohol characterized by reacting with an alkyl group having a carbon number of 12 or less, a group having an ether bond introduced into an alkyl chain, or an aromatic group, and X is a halogen atom. It is a manufacturing method.

  Although the carbonated PVA of the present invention is a high-polarity vinyl polymer, there is no problem in terms of work and cost during production, it is easy to produce, and when applied to a gel electrolyte, A gel electrolyte having good conductivity and electrolyte retention is obtained. Therefore, the lithium secondary battery including the electrolyte using the carbonated PVA has good ionic conductivity and does not have a problem of leakage. The production method of the present invention is useful as a method for producing a highly polar vinyl polymer.

  First, the PVA having 20 to 90 mol% of vinyl acetate units and 10 to 80 mol% of vinyl alcohol units as a group of the carbonated PVA structure of the present invention will be described. The PVA can be obtained by synthesizing and saponifying polyvinyl acetate by a conventional method, and can also be obtained as a commercial product.

  The amount of the vinyl alcohol unit of PVA is preferably 20 to 80 mol% from the viewpoint of carbonation reactivity and ionic conductivity. In addition, about the quantity of said vinyl acetate unit and vinyl alcohol unit of PVA, it is the quantity when the sum total of a vinyl acetate unit and a vinyl alcohol unit is 100 mol%. Therefore, when PVA consists of vinyl acetate units and vinyl alcohol units, the amount of vinyl alcohol units is equal to the degree of saponification.

  The PVA may contain monomer units other than vinyl acetate units and vinyl alcohol units as long as the gist of the present invention is not impaired.

  The carbonated PVA of the present invention has a structure in which at least a part of the OH group of the above PVA is substituted with a group represented by —OC (═O) OR.

  Here, R is an alkyl group having 12 or less carbon atoms which may have a substituent, a group in which an ether bond is introduced into an alkyl chain, or an aromatic group.

  The alkyl group having 12 or less carbon atoms represented by R may be linear, branched or cyclic, and is a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl. Group, tert-butyl group, pentyl group, isopentyl group, sec-pentyl group, tert-pentyl group, neopentyl group, cyclopentyl group, hexyl group, isohexyl group, cyclohexyl group, heptyl group, octyl group, 2-ethylhexyl group , Decyl group, undecyl group, dodecyl group and the like.

  The group represented by R in which an ether bond is introduced into an alkyl chain having 12 or less carbon atoms may be linear, branched or cyclic, and is a methoxyethyl group, a methoxypropyl group, a methoxybutyl group, an ethoxy group. Examples thereof include alkoxyalkyl groups such as ethyl group, ethoxypropyl group and ethoxybutyl group, and polyoxyalkylene groups such as ethoxyethoxyethyl.

  Examples of the aromatic group having 12 or less carbon atoms represented by R include aryl groups such as phenyl group and toluyl group.

  The alkyl group having 12 or less carbon atoms represented by R, the group in which an ether bond is introduced into the alkyl chain, or the aromatic group has a substituent within the range where the total carbon number is 12 or less. Also good. Examples of the substituent include alkyl groups having 4 or less carbon atoms (eg, methyl group, ethyl group, isopropyl group, n-propyl group, isobutyl group, n-butyl group), methoxyethyl group, methoxybutyl group, and the like. Is mentioned.

  R is preferably a group having 12 or less carbon atoms (preferably 6 or less carbon atoms) in which an ether bond is introduced into an alkyl group or an alkyl chain. In particular, from the viewpoint of improving polarity, an ether bond is introduced into an alkyl chain. Groups are preferred.

  The R chemical structures of the carbonated PVA need not all be identical. R may be not only one group within the above definition but also two or more groups within the above definition.

  The carbonated PVA of the present invention has a structure in which at least a part of the OH group is substituted with a group represented by —OC (═O) OR. However, the OH group generally deteriorates battery characteristics, The OH group is preferably substituted by 50% or more of the group represented by —OC (═O) OR, and preferably 60% or more. More preferably, it is more preferably 80% or more.

  The degree of polymerization of the carbonated PVA of the present invention is not particularly limited, but is preferably 100 to 10,000.

  The production method of the carbonated PVA of the present invention is not particularly limited, but from the viewpoint of ease of production, PVA having 20 to 90 mol% vinyl acetate units and 10 to 80 mol% vinyl alcohol units and represented by XCOOR. It is preferable to react with a halogenated formate ester (R is as defined above, and X represents a halogen atom). The reaction is preferably performed in a solvent in the presence of an alkaline compound having no active hydrogen.

  Examples of the halogen atom represented by X in the halogenated formate include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom, and a chlorine atom is preferable. Since R of the carbonated PVA of the present invention is derived from R of the halogenated formate, R to be introduced into the carbonated PVA of the present invention may be selected as R of the halogenated formate.

  The halogenated formate can be synthesized by a known method or can be obtained as a commercial product. The amount of the halogenated formate used is preferably 1 to 5 moles relative to 1 mole of OH groups of the raw material PVA. ~ 3 mol is more preferred. When the amount is less than 1 mol, sufficient carbonateation is not performed, and when the amount is more than 5 mol, impurities such as unreacted products and by-products increase, which is not preferable.

  As the solvent, a solvent capable of dissolving carbonated PVA may be used except a nitrogen-containing solvent composed of N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylimidazolidinone. In this reaction, it was revealed that even when the raw material PVA is not dissolved before the reaction, the carbonated PVA can be obtained with good reactivity by using a solvent in which the carbonated PVA is soluble. Specific examples of the solvent include tetrahydrofuran, toluene, ethyl acetate and the like. Since these exemplified solvents are easily available and are low polarity and low boiling point solvents, it is easy to perform water washing separation, solvent concentration, etc. in purification after the reaction. The amount of the solvent used is preferably 2 to 20 g with respect to 1 g of the raw material PVA.

  Examples of the alkaline compound having no active hydrogen include pyridine, triethylamine, tributylamine, 1,4-diazabicyclo [2,2,2] octane, and the like. The amount of the alkaline compound used is preferably 0.9 to 1.5 molar equivalents relative to 1 mole of OH groups of the raw material PVA.

  The reaction temperature and time may be appropriately determined depending on the amount of OH group, halogenated formate and solvent, etc. of the PVA used, but the reaction temperature is preferably 0 to 60 ° C., and the reaction time is 3 to 72 hours is preferred.

  From the reaction mixture, the desired carbonated PVA can be purified and isolated according to a conventional method. For example, if necessary, a solvent that separates from water such as toluene is added, and then washed with a saturated saline solution. Subsequently, after drying an organic layer with magnesium sulfate etc., a solvent is removed and it can dry, and carbonated PVA can be isolated.

  When an OH group remains in the carbonated PVA of the present invention, the OH group can generally replace the hydrogen atom (H) of the OH group with an isocyanate compound residue in order to reduce battery characteristics. preferable.

  Here, the isocyanate compound residue is formed by the reaction of the isocyanate group of the isocyanate compound with the OH group (a hydrogen atom is added to the nitrogen atom of the —N═C═O group of the isocyanate compound, and an OH group is added to the carbon atom). A group derived from an isocyanate compound). Therefore, in order to substitute H of the OH group of the carbonated PVA with an isocyanate compound residue, for example, the OH group and the isocyanate compound may be reacted under dry conditions. The isocyanate compound residue may be a residue of a polyfunctional isocyanate compound, and at this time, the carbonated PVA is crosslinked.

  Examples of the isocyanate compound include toluene diisocyanate, hexamethylene diisocyanate, and a triisocyanate compound obtained by adding 3 mol of these diisocyanates to 1 mol of trimethylolpropane. Among them, 1 mol of trimethylolpropane can be exemplified. A triisocyanate compound added with 3 moles of hexamethylene diisocyanate is preferred.

  Alternatively, when an OH group remains in the carbonated PVA of the present invention, the OH group may be esterified by a conventional method.

  When the carbonated PVA of the present invention is applied to a gel electrolyte, it is possible to obtain a gel electrolyte having both good ionic conductivity and electrolyte retention. The carbonated PVA of the present invention is also excellent in flexibility.

  In one embodiment, the gel electrolyte includes the carbonated PVA of the present invention and an electrolytic solution, and the electrolytic solution includes an electrolyte salt and a solvent.

Examples of the electrolyte _{salt, LiPF 6, LiBF 4, LiN} (C 2 F 5 SO 2) 2, LiAsF 6, LiSbF 6, LiAlF 4, LiGaF 4, LiInF 4, LiClO 4, LiN (CF 3 SO 2) 2 , LiCF ₃ SO ₃ , LiSiF ₆ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) and other lithium metal salts.

  Examples of the solvent constituting the electrolytic solution include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and linear carbonates such as dimethyl carbonate (DMC) and diethyl carbonate (DEC).

  What is necessary is just to produce the said gel electrolyte according to a conventional method. For example, it can be produced by applying and drying the carbonated PVA solution of the present invention to form a film and immersing it in an electrolytic solution containing an electrolyte salt. In the case of adding an isocyanate compound, the isocyanate compound may be cast while maintaining the fluidity.

  The electrolyte using the carbonated PVA can be applied to a lithium secondary battery, and the lithium secondary battery has good ionic conductivity and no leakage problem.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.

(Synthesis of carbonated PVA)
Example 1
A vacuum stirrer, a condenser with a calcium chloride tube and a nitrogen inlet tube were attached to a 300 mL three-necked flask, and the atmosphere in the flask was replaced with nitrogen. Under a dry nitrogen atmosphere, 90 g of tetrahydrofuran, LM-15 (PVA, manufactured by Kuraray Co., Ltd., 35 mol%) 16.1 g and 9.5 g of pyridine were added. This solution was cooled to about 0 to 10 ° C. with an ice-cooled bath, and 20 g of methoxybutyl chloroformate was slowly added dropwise with uniform stirring. After the entire amount was added dropwise, the temperature was slowly raised to 20 ° C. and stirring was continued for 48 hours.

  While stirring this solution, 200 mL of toluene was added, washed with 200 mL of saturated brine three times, and then the separated organic layer (upper liquid) was separated. After drying the organic layer with magnesium sulfate, the solvent was removed and dried to obtain a rubbery carbonated PVA. As a result of analysis by NMR, 85% of the OH groups of the raw material PVA were substituted with carbonate residues (groups represented by —OC (═O) OR).

Example 2
Carbonated PVA was obtained in the same manner as in Example 1 except that 23 g of 2-ethylhexyl chloroformate was used instead of 20 g of methoxybutyl chloroformate in Example 1. As a result of analysis by NMR, 85% of the OH group of the raw material PVA was substituted with a carbonate residue.

Example 3
Example 1 was conducted except that 16.1 g of L-10 (PVA, Kuraray, saponification degree: 72 mol%) was used instead of 16.1 g of LM-15 (PVA, Kuraray, saponification degree: 35 mol%) in Example 1. In the same manner as in Example 1, carbonated PVA was obtained. As a result of analysis by NMR, 65% of the OH group of the raw material PVA was substituted with a carbonate residue.

Example 4
Polyvinyl acetate (polymerization degree 1500) 50g was added to 1N sodium hydroxide 70% ethanol aqueous solution 1000mL, and saponification reaction was performed at 50 degreeC for 3 hours. The saponification degree of the obtained PVA was 15%. This PVA was carbonated in the same manner as in Example 1 to obtain carbonated PVA. As a result of NMR analysis, 98% of the OH groups of the raw material PVA were substituted with carbonate residues.

Comparative Example 1
A saponification reaction of polyvinyl acetate was carried out in the same manner as in Example 4 except that the saponification time in Example 4 was changed to 1 hour. The saponification degree of the obtained PVA was 5%. This PVA was carbonated in the same manner as in Example 1 to obtain carbonated PVA. As a result of NMR analysis, 98% of the OH groups of the raw material PVA were substituted with carbonate residues.

Comparative Example 2
Except that 16.1 g of CM-318 (PVA, Kuraray, saponification degree 90 mol%) was used instead of 16.1 g of LM-15 (PVA, Kuraray, saponification degree 35 mol%) in Example 1, In the same manner as in Example 1, carbonated PVA was obtained. As a result of analysis by NMR, 30% of the OH groups of the raw material PVA were substituted with carbonate residues.

(Evaluation of electrolyte)
To the 10% dimethylformaldehyde solution of carbonated PVA obtained in Examples 1 to 4 and Comparative Examples 1 to 2, a trifunctional isocyanate compound equivalent to the OH group of the carbonated PVA (1 mol of trimethylolpropane and hexamethylene) was added. Triisocyanate compound added with 3 mol of diisocyanate; Nippon Polyurethane, Coronate HL) was mixed. This was developed on a petri dish made of PTFE, dried, and then heated at 50 ° C. for 5 days to obtain a polymer film having a thickness of 50 μm. 0.5 g of this polymer film was immersed in a 1M LiPF ₆ EC / DMC (1/2) solution and allowed to stand at room temperature for 2 days. Thereafter, the liquid on the surface was wiped off to obtain a gel electrolyte film.

As Comparative Example 3, 0.1 g of a porous polyolefin film (porosity of about 35%, thickness of about 25 μm) was immersed in a 1M LiPF ₆ EC / DMC (1/2) solution, and left at room temperature for 2 days. An electrolyte film obtained by wiping off the liquid on the surface was prepared.

  The conductivity of these electrolyte films and the presence or absence of liquid leakage were evaluated by the following methods. The results are shown in Table 1.

(Conductivity measurement)
The electrolyte films of Examples and Comparative Examples were sandwiched by applying a load of 10 g with a platinum plate having a diameter of 10 mm in an argon glove box, and using an impedance measuring device (Princeton Applied Research 263A potentiostat and 5210 Lock in amplifier). Then, the real component impedance of the intersection of the arc on the high frequency side and the straight line on the low frequency side was obtained by the complex impedance method at room temperature (23 ° C.), and the conductivity σ (S / cm) was calculated by the following equation.
σ = d / (R · A)
d: Sample thickness (cm), R: Impedance (Ω), A: Sample cross-sectional area (cm ² )

(Evaluation of leakage)
When the 10 g load was removed, the presence or absence of liquid outflow on the electrode surface was visually determined.

  As is apparent from Table 1, when polyvinyl alcohol having a vinyl alcohol unit amount outside the scope of the present invention was used (Comparative Examples 1 and 2), no leakage was observed, but sufficient electrical conductivity was obtained. I couldn't. On the other hand, when the porous polyolefin film was used (Comparative Example 3), although the electrical conductivity was good, liquid leakage was observed. On the other hand, when the carbonated PVA of the present invention was used (Examples 1 to 4), there was no leakage and the conductivity was good.

  The carbonated PVA of the present invention is suitable for a gel electrolyte such as a lithium secondary battery. Moreover, the production method of the present invention is useful as a production method of a highly polar vinyl polymer.

Claims (3)

  A part of the hydroxyl group of polyvinyl alcohol having 20 to 90 mol% of vinyl acetate units and 10 to 80 mol% of vinyl alcohol units is represented by a group represented by —OC (═O) OR (where R represents a substituent). An alkyl group having a carbon number of 12 or less, a group in which an ether bond is introduced into the alkyl chain, or an aromatic group, which may have, and a part of hydrogen atoms of the remaining hydroxyl group or Carbonated polyvinyl alcohol having a structure in which the whole is substituted with an isocyanate compound residue. A gel electrolyte using the carbonated polyvinyl alcohol according to claim 1 .   Polyvinyl alcohol having 20 to 90 mol% of vinyl acetate units and 10 to 80 mol% of vinyl alcohol units, and halogenated formate represented by XCOOR (R is an optionally substituted carbon number of 12 or less. And a group in which an ether bond is introduced into an alkyl chain, or an aromatic group, and X represents a halogen atom).