JP2008088329A - Carbonated cellulose compound, process for producing the same, and gel electrolyte - Google Patents

Abstract

【選択図】なしThe present invention provides a novel cellulosic compound that is easy to produce and has good ionic conductivity and electrolyte retention when applied to a gel electrolyte.
SOLUTION: At least a part of hydroxyl groups of a hydroxyalkyl cellulose having a repeating unit represented by the following formula (1) (the definition of each symbol is as defined in the specification) is represented by -OC (= O) A carbonated cellulose compound having a structure substituted with a group represented by OR ⁸ (the definition of R ^{8 is} as defined in the specification).

[Selection figure] None

Description

  The present invention relates to a novel cellulose compound 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 cellulose compound.

  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. However, 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.

  Under such circumstances, various polymers have been examined as polymers for gel electrolytes, and in Patent Document 1, a cellulose compound was used as a gel electrolyte excellent in ion conductivity and shape retention. Gel electrolytes have been proposed. However, there are many hydroxyl groups in the cellulose compound, and these hydroxyl groups react with Li ions and take in Li ions, so that there is a problem that battery characteristics are deteriorated.

On the other hand, Patent Document 2 discloses a compound in which a hydroxyl group of cellulose is carbonated, and Patent Document 3 discloses a compound in which a hydroxyl group of starch having a structure close to cellulose is carbonated. However, these references do not describe any use of these carbonated compounds in gel electrolytes for lithium secondary batteries. In Patent Documents 2 and 3, a highly polar amide solvent or the like is used for the production of these carbonated compounds. This is because starch and cellulose have low solubility in nonpolar solvents, and it has been considered that reaction in nonpolar solvents is difficult. When using a highly polar amide solvent, etc., it is difficult to carry out operations such as washing with water and concentrating the solvent in the purification after the reaction. As a result, the purification process becomes complicated and the carbonated compound can be easily obtained. There was a problem that it could not be manufactured.
JP-A-8-225626 JP-A-5-65301 JP 2004-359737 A

  In view of the above circumstances, an object of the present invention is to provide a novel cellulosic compound that is easy to produce and has good ion conductivity and electrolyte retention when applied to a gel electrolyte. To do. Another object of the present invention is to provide a gel electrolyte having both good ionic conductivity and electrolyte retention.

  The present invention provides the following formula (1)

[Wherein n represents an integer of 2 or more, R ^{1 to} R ⁶ each independently represents a hydrogen atom or a group represented by (R ⁷ O) _m H (R ⁷ represents a lower alkylene group, m represents an integer of 1 or more. ] The hydroxyalkyl cellulose which has a repeating unit represented by [Hereinafter, it is also called hydroxyalkyl cellulose (1). ] At least a part of the hydroxyl group (hereinafter sometimes referred to as OH group) is a group represented by —OC (═O) OR ⁸ (R ⁸ is an optionally substituted carbon). This is a carbonated cellulose-based compound having a structure substituted with an alkyl group, an alkyl group, a group in which an ether bond is introduced into an alkyl chain, or an aromatic group.
The carbonated cellulose compound 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 ⁸ .
In another embodiment of the carbonated cellulose compound of the present invention, a part of the hydroxyl group of the hydroxyalkyl cellulose (1) is substituted with a group represented by —OC (═O) OR ⁸ (R ⁸ has a substituent. And an alkyl group having 12 or less carbon atoms, a group in which an ether bond is introduced into the alkyl chain, or an aromatic group.) And a part or all of hydrogen atoms of the remaining hydroxyl group Is a carbonated cellulose compound having a structure in which is substituted with an isocyanate compound residue.
The present invention is also a gel electrolyte using the above carbonated cellulose compound.
The present invention further relates to a hydroxyalkyl cellulose (1) and a halogenated formate represented by XCOOR ⁸ (R ⁸ is an alkyl group having 12 or less carbon atoms which may have a substituent, an ether in the alkyl chain. A group into which a bond is introduced or an aromatic group, and X represents a halogen atom.) And at least one selected from the group consisting of tetrahydrofuran, benzene, toluene, dialkyl ether, dialkyl carbonate, and acetic acid alkyl ester It is a manufacturing method of the carbonated cellulose compound characterized by making it react using as a solvent.

  The carbonated cellulose-based compound of the present invention is easy to produce, and when applied to a gel electrolyte, a gel electrolyte having good ion conductivity and electrolyte retention is obtained. Therefore, a lithium secondary battery including an electrolyte using the carbonated cellulose-based compound has good ionic conductivity and no leakage problem. Since the production method of the present invention facilitates purification of the carbonated cellulose compound, the carbonated cellulose compound is more effective than the conventional method using a highly polar amide solvent used for the production of a similar compound. Manufacturing is easy.

  First, the hydroxyalkyl cellulose (1) which is the base of the structure of the carbonated cellulose compound of the present invention will be described.

  In Formula (1), n is an integer greater than or equal to 2, Preferably it is 2000-26000, More preferably, it is 2000-10000.

R ⁷ represents a lower alkylene group, and the lower alkylene group means an alkylene group having 1 to 4 carbon atoms, preferably an alkylene group having 1 to 3 carbon atoms. The lower alkylene group may be linear or branched, and examples include a methylene group, an ethylene group, a trimethylene group, a propylene group (—CH ₂ CH (CH ₃ ) —), a tetramethylene group, and the like.
R ⁷ is most preferably a propylene group because of easy procurement of raw materials.
Note that the chemical structures of R ⁷ of the hydroxyalkyl cellulose (1) need not all be the same. R ⁷ may be not only one type but also two or more types within the above definition.

  m is an integer greater than or equal to 1, Preferably it is 1-10.

R ^{1 to} R ⁶ each independently represent a hydrogen atom or a group represented by — (R ⁷ O) _m H. Since the group represented by — (R ⁷ O) _m H contains oxygen, the carbonated cellulose compound of the present invention is more polar than the carbonated mere (unmodified) cellulose. It becomes high and it becomes excellent in the characteristic as a battery.

Since hydroxyalkyl cellulose (1) is hydroxyalkyl cellulose, not all of R ^{1 to} R ⁶ in all repeating units are hydrogen atoms. Further, in the equation (1), one is in the repeating unit are six groups represented by R ¹ to R ⁶ is present, R ¹ to R ⁶ represent groups may be different among the repeating units. For example, in one repeating unit, R ¹ may be a hydrogen atom, and in another repeating unit, R ¹ may be a group represented by — (R ⁷ O) _m H. The ratio of the group represented by — (R ⁷ O) _m H to all R ^{1 to} R ⁶ in all repeating units is preferably 50 to 100 mol%. Within this range, because the solubility of the carbonated cellulose compound in the solvent is improved during the carbonated reaction, the reactivity is improved, and the carbonated cellulose compound becomes flexible and the polarity increases. , Battery characteristics become higher.

The carbonated cellulose compound of the present invention has a structure in which at least a part of the OH group of the hydroxyalkyl cellulose (1) is substituted with a group represented by —OC (═O) OR ⁸ . By introducing the carbonate structure into the cellulosic compound, the ionic conductivity of the cellulosic compound can be increased as compared with the conventional modified cellulosic compound.

Here, R ⁸ 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 ⁸ may be linear, branched or cyclic, and is methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec- Butyl, tert-butyl, pentyl, isopentyl, sec-pentyl, tert-pentyl, neopentyl, cyclopentyl, hexyl, isohexyl, cyclohexyl, heptyl, octyl, 2-ethylhexyl Examples thereof include a group, a decyl group, an undecyl group, and a dodecyl group.

The group in which an ether bond is introduced into the alkyl chain having 12 or less carbon atoms represented by R ⁸ may be linear, branched or cyclic, and includes a methoxyethyl group, a methoxypropyl group, a methoxybutyl group, Examples thereof include alkoxyalkyl groups such as ethoxyethyl 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 ⁸ include aryl groups such as phenyl group and toluyl group.

The alkyl group having 12 or less carbon atoms, the group having an ether bond introduced into the alkyl chain, or the aromatic group represented by R ⁸ has a substituent within the range where the total carbon number is 12 or less. May be. 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 ⁸ is preferably an alkyl group having 12 or less carbon atoms and a group having an ether bond introduced into the alkyl chain, and particularly preferably a group having an ether bond introduced into the alkyl chain from the viewpoint of improving the polarity.

Note that the R ⁸ chemical structures of the carbonated cellulose compound of the present invention need not all be the same. R ⁸ may be not only one group within the above definition but also two or more groups within the above definition.

The carbonated cellulose compound 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 ^8. However, the OH group generally deteriorates battery characteristics. In addition, since it adversely affects the flexibility of the carbonated cellulose compound, the OH group is preferably substituted by 50% or more by a group represented by —OC (═O) OR ⁸ , and 70% More preferably, the above substitution is performed.

The hydroxyalkyl cellulose (1) has 6 OH groups in one repeating unit, and the number and position of OH groups substituted with a group represented by —OC (═O) OR ⁸ between each repeating unit. May be different. For example, in one repeating unit, all six OH groups are substituted with a group represented by —OC (═O) OR ⁸ , and in another repeating unit, all three OH groups are all —OC (═O). It may be substituted with a group represented by OR ⁸ , and in another repeating unit, a structure in which six OH groups remain as they are without substitution may be used.

Although there is no restriction | limiting in particular about the manufacturing method of the carbonated cellulose type compound of this invention, Hydroxyalkyl cellulose (1) and halogenated from the ease of manufacture (especially the availability of a solvent, the ease of handling). Formic acid ester (R ⁸ is as defined above, X represents a halogen atom) and at least one selected from the group consisting of tetrahydrofuran, benzene, toluene, dialkyl ether, dialkyl carbonate, and acetic acid alkyl ester as a solvent It is preferable to be used as a reaction. The reaction is preferably performed 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. R ⁸ carbonate cellulosic compounds of the present invention, since derived from R ⁸ halogenated formic acid ester, the R ⁸ halogenated formic ester, select the R ⁸ to be introduced into the carbonate cellulosic compounds of the present invention do it.

  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 1 to 5 with respect to 1 mol of OH groups of the raw hydroxyalkyl cellulose (1). Mole is preferable, and 1 to 3 mol is more preferable. 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.

  Hydroxyalkyl cellulose (1) can be obtained by synthesis by a known method. For example, it can be produced by reacting cellulose with an epoxy compound such as ethylene oxide or propylene oxide. Moreover, it is also possible to obtain as a commercial item.

  In the production method of the present invention, at least one selected from the group consisting of tetrahydrofuran, benzene, toluene, dialkyl ether, dialkyl carbonate, and acetic acid alkyl ester is used as the solvent. These are solvents capable of dissolving a carbonated cellulose compound, although they have poor solubility in hydroxyalkyl cellulose. In the prior art, a highly polar amide solvent such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, dimethylimidazolidinone, and the like has been used as a solvent for carbonateation of unmodified cellulose and starch. However, since these solvents are high polarity and high boiling point solvents, it has been difficult to carry out water washing separation, solvent concentration and the like in purification after the reaction. However, in the production method of the present invention, in the reaction, even if the raw material hydroxyalkyl cellulose is not dissolved before the reaction, a solvent capable of dissolving the carbonated cellulose compound (that is, the solvents listed above) is used. Thus, it was revealed that a carbonated cellulose compound can be obtained with high reactivity. By the selection of this solvent, the production method of the present invention has a simpler purification process than the conventional method, and the production of the carbonated cellulose compound is facilitated.

As the dialkyl ether, those having 6 or less carbon atoms are preferable, and diethyl ether is more preferable.
As the dialkyl carbonate, those having 6 or less carbon atoms are preferable, and dimethyl carbonate and diethyl carbonate are more preferable.
As the alkyl acetate, those having 6 or less carbon atoms are preferable, for example, methyl acetate, ethyl acetate, propyl acetate, butyl acetate and the like, and ethyl acetate is more preferable.

  The amount of the solvent used is preferably 2 to 100 g with respect to 1 g of the raw material hydroxyalkyl cellulose (1).

  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 hydroxyalkyl cellulose (1).

  The reaction temperature and time may be appropriately determined depending on the amount of the OH group, halogenated formate and solvent, etc. of the hydroxyalkyl cellulose used, but the reaction temperature is preferably 0 to 50 ° C., and the reaction time is 3-72 hours are preferred.

  From the reaction mixture, the desired carbonated cellulose compound 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 a carbonated cellulose type compound can be isolated.

  When an OH group remains in the carbonated cellulose compound of the present invention, the OH group generally replaces the hydrogen atom (H) of the OH group with an isocyanate compound residue in order to reduce battery characteristics. It is 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 replace H of the OH group of the carbonated cellulose compound 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 cellulose compound 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. A triisocyanate compound added with 3 moles of hexamethylene diisocyanate is preferred.

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

  When the carbonated cellulose compound 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. Moreover, the carbonated cellulose compound of the present invention is excellent in flexibility.

  In one embodiment, the gel electrolyte includes the carbonated cellulose compound 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 a solution of the carbonated cellulose compound 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.

  An electrolyte using the carbonated cellulose-based compound 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 cellulose compounds)
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, 10 g of hydroxypropylcellulose (HPC-L manufactured by Nippon Soda) and 9.5 g of pyridine were added. This solution was cooled to about 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 cellulose compound. As a result of analysis by NMR, 75% of the OH groups of the raw material hydroxypropylcellulose were substituted with carbonate residues (groups represented by —OC (═O) OR ⁸ ).

Example 2
A carbonated cellulose compound 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, 80% of OH groups of the raw material hydroxypropylcellulose were substituted with carbonate residues.

(Evaluation of electrolyte)
To the 10% dimethylformaldehyde solution of the carbonated cellulose compound obtained in Examples 1 and 2, a trifunctional isocyanate compound equivalent to the OH group of the carbonated cellulose compound (1 mole of trimethylolpropane and hexamethylene diisocyanate) was added. Triisocyanate compound added with 3 mol; made by 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 1, 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 between platinum plates having a diameter of 10 mm with a load of 10 g in an argon glove box, and an impedance measurement device (Princeton Applied Research 263A potentiostat and 5210 Lock in amplifier) was used. 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.

  From Table 1, it can be seen that if the carbonated cellulose-based compound of the present invention is applied to a gel electrolyte, it becomes a gel electrolyte with no leakage and good conductivity.

  The carbonated cellulose compound of the present invention is suitable for a gel electrolyte such as a lithium secondary battery. In addition, the production method of the present invention is useful because it can easily produce a carbonated cellulose compound.

Claims (5)

Following formula (1)

[Wherein n represents an integer of 2 or more, R ^{1 to} R ⁶ each independently represents a hydrogen atom or a group represented by (R ⁷ O) _m H (R ⁷ represents a lower alkylene group, m represents an integer of 1 or more. At least a portion of the hydroxyl group of a hydroxyalkyl cellulose having a repeating unit represented by], -OC (= O) group represented by OR ⁸ (R ⁸ is an optionally substituted carbon atoms 12 or less, a carbonated cellulose compound having a structure substituted with an alkyl group, a group in which an ether bond is introduced into an alkyl chain, or an aromatic group. 50 mol% or more the -OC (= O) carbonate cellulosic compound according to claim 1, wherein the structure is substituted with a group represented by OR ⁸ of the hydroxyl groups. Following formula (1)

[Wherein n represents an integer of 2 or more, R ^{1 to} R ⁶ each independently represents a hydrogen atom or a group represented by (R ⁷ O) _m H (R ⁷ represents a lower alkylene group, m represents an integer of 1 or more. A part of the hydroxyl group of the hydroxyalkylcellulose having a repeating unit represented by the formula: —OC (═O) OR ⁸ (R ⁸ is optionally substituted 12 carbon atoms) The following alkyl group, a group in which an ether bond is introduced into the alkyl chain, or an aromatic group is substituted.) Further, some or all of the hydrogen atoms of the remaining hydroxyl groups are substituted with isocyanate compound residues. Carbonated cellulose compound having the above structure.   A gel electrolyte using the carbonated cellulose compound according to any one of claims 1 to 3. Following formula (1)

[Wherein n represents an integer of 2 or more, R ^{1 to} R ⁶ each independently represents a hydrogen atom or a group represented by (R ⁷ O) _m H (R ⁷ represents a lower alkylene group, m represents an integer of 1 or more. A hydroxyalkyl cellulose having a repeating unit represented by XCOOR ⁸ and a halogenated formic acid ester represented by XCOOR ⁸ (R ⁸ is an alkyl group or alkyl chain having 12 or less carbon atoms which may have a substituent. An ether group-introduced group or an aromatic group, and X represents a halogen atom.) And at least one selected from the group consisting of tetrahydrofuran, benzene, toluene, dialkyl ether, dialkyl carbonate, and acetic acid alkyl ester A method for producing a carbonated cellulose compound, characterized by reacting using a seed as a solvent.