JP2019116705A - Modified cellulose fiber, dispersion liquid, porous film, electricity storage element and method of manufacturing porous film - Google Patents

Abstract

To provide a modified cellulose fiber easy to satisfactorily disperse in a dispersion liquid and capable of stably forming a porous film, a dispersion liquid containing the modified cellulose fiber, a porous film formed using the dispersion liquid, an electricity storage element provided with the porous film, and a method of manufacturing a porous film using the dispersion liquid.SOLUTION: It is about a cellulose fiber used for forming a porous film by coating and drying. A hydrogen atom of a hydroxyl group in a part of hydroxyl groups that an unmodified cellulose fiber has is subjected to modification treatment of substituting a hydrogen atom of the hydroxyl group by a specific hydrophobic group, or a cellulose fiber is subjected to modification treatment so that a modified cellulose fiber emerges from pure water when mixing 0.1 g of the modified cellulose with 10 ml of pure water, successively shaking for 30 sec., and further standing still for 30 sec.SELECTED DRAWING: None

Description

  The present invention provides a modified cellulose fiber suitably used for the formation of a porous membrane, a dispersion containing the above-mentioned modified cellulose fiber, a porous membrane formed using the above-mentioned dispersion, and the above-mentioned porous membrane And a method of manufacturing a porous film using the above-described dispersion liquid.

  Conventionally, various porous membranes are used in applications such as filters. In recent years, porous membranes have also been applied to separators for secondary batteries such as lithium batteries.

  As a porous film used as a separator, for example, a porous film containing a fiber material in the form of a fiber assembly formed so as to adhere to an electrode has been proposed (see Patent Document 1).

Unexamined-Japanese-Patent No. 2015-191871

  The porous film having a function as a separator described in Patent Document 1 is manufactured using a slurry containing cellulose fibers. However, when a coated film made of a slurry containing cellulose fibers is dried, it is difficult to form a porous film having a desired opening diameter and porosity due to the formation of hydrogen bonds between the cellulose fibers. There is. In addition, cellulose fibers are often difficult to disperse well in various dispersion media such as water and organic solvents. If the dispersion state of the cellulose fibers is not uniform, the uniformity of the film thickness and the opening diameter of the porous film formed using the slurry is often insufficient. When the film thickness and the opening diameter of the porous film are not uniform, problems such as lithium dendrite formation easily occur due to the bias of the electric field, and it is difficult to obtain a high performance electrochemical device.

  The present invention has been made in view of the above problems, and is easily dispersed in a dispersion, and a modified cellulose fiber capable of stably forming a porous film, and the modified cellulose Provided are a dispersion containing fibers, a porous film formed using the above-mentioned dispersion, an electric storage device provided with the above-mentioned porous film, and a method for producing a porous film using the above-mentioned dispersion. With the goal.

  The present inventors have modified a cellulose fiber used for forming a porous film by coating and drying, in which the hydrogen atom in the hydroxyl group in a part of the hydroxyl group possessed by the unmodified cellulose fiber is substituted with a specific hydrophobic group 0.1 g of denatured cellulose fiber is mixed with 10 mL of pure water, then shaken for 30 seconds, and then allowed to stand for another 30 seconds, so that the denatured cellulose fiber is denatured so that it floats up to pure water. It has been found that the above problems can be solved by the application, and the present invention has been completed.

That is, a first aspect of the present invention is a modified cellulose fiber used for forming a porous membrane,
The modified cellulose fiber is an organic group having 3 or more carbon atoms, in which a hydrogen atom in a hydroxyl group in a part of the hydroxyl groups of the unmodified cellulose fiber is bonded to an oxygen atom derived from the hydroxyl group by an O-C bond, and / or A fiber substituted with an organosilyl group,
It is a modified cellulose fiber.

A second aspect of the present invention is a modified cellulose fiber used for forming a porous membrane, which comprises:
It is a modified cellulose fiber in which 0.1 g of modified cellulose fiber is mixed with 10 mL of pure water, then shaken for 30 seconds, and left to stand for another 30 seconds, whereby the modified cellulose fiber floats up with respect to the pure water.

A third aspect of the present invention is a dispersion used to form a porous membrane, comprising:
A dispersion liquid is a dispersion liquid containing the modified cellulose fiber concerning the 1st aspect or the 2nd aspect, and an organic solvent.

  A fourth aspect of the present invention is a porous membrane obtained by drying a coating film comprising the dispersion liquid according to the third aspect.

  A fifth aspect of the present invention is a storage element including the porous film according to the fourth aspect.

The sixth aspect of the present invention is
Applying the dispersion according to the third aspect on a substrate to form a coated film;
And drying the coated film to form a porous film.

  According to the present invention, it is easy to be well dispersed in a dispersion, and modified cellulose fiber capable of stably forming a porous film, a dispersion containing the modified cellulose fiber, and the above-mentioned dispersion It is possible to provide a porous film formed using a liquid, an electric storage device provided with the above-mentioned porous film, and a method of manufacturing a porous film using the above-mentioned dispersion liquid.

«Modified cellulose fiber»
The modified cellulose fiber is a modified cellulose fiber used to form a porous membrane. The formation of the porous film will be described in detail in the description of the dispersion described later.

  A preferred embodiment of the modified cellulose fiber is an organic group having 3 or more carbon atoms in which a hydrogen atom in the hydroxyl group in a part of the hydroxyl group of the unmodified cellulose fiber is bonded to an oxygen atom derived from the hydroxyl group by an O-C bond. And / or a modified cellulose fiber which is a fiber substituted with an organosilyl group. Hereinafter, this modified cellulose fiber is also referred to as "first modified cellulose fiber".

  In another preferred embodiment of the modified cellulose fiber, 0.1 g of the modified cellulose fiber is mixed with 10 mL of pure water, followed by shaking for 30 seconds, and when left still for 30 seconds, the modified cellulose fiber is the pure water Is a modified cellulose fiber that floats up against Hereinafter, this modified cellulose fiber is also referred to as a "second modified cellulose fiber".

  The above-mentioned modified cellulose fiber according to the preferred embodiment is easily and well dispersed in an organic solvent. For this reason, when using the dispersion liquid containing said modified cellulose fiber, formation of the porous film which is uniform in film thickness and opening diameter is easy.

<First Modified Cellulose Fiber>
As described above, in the first modified cellulose fiber, the hydrogen atom in the hydroxyl group in part of the hydroxyl group of the unmodified cellulose fiber has 3 or more carbon atoms bonded to the oxygen atom derived from the hydroxyl group by the O-C bond A fiber substituted with an organic group and / or an organosilyl group of
It is a modified cellulose fiber.
Hereinafter, the substituent which substitutes the hydrogen atom in the hydroxyl group which unmodified | non-denatured cellulose fiber has is described also as "O-substituent."

  When the O-substituent in the first modified cellulose fiber is an organic group having 3 or more carbon atoms which is bonded to an oxygen atom derived from a hydroxyl group by an O-C bond, the type of the organic group is the object of the present invention It is not particularly limited as long as it does not inhibit

The carbon atom number of the organic group as an O-substituent is 3 or more, and 4 or more is preferable. The carbon atom number of the organic group as the O-substituent may be, for example, 6 or more, 8 or more, 10 or more, or 15 or more.
The number of carbon atoms of the organic group as the O-substituent is not particularly limited as long as the object of the present invention is not impaired. 50 or less are preferable, for example, 30 or less are more preferable, and, as for the carbon atom number of the organic group as O- substituent, 20 or less is especially preferable.

  The organic group as the O-substituent may be a hydrocarbon group, or a group containing a carbon atom and a hetero atom other than a hydrogen atom. Examples of the hetero atom include an oxygen atom, a nitrogen atom, a sulfur atom, a phosphorus atom, a halogen atom, a boron atom, and a silicon atom.

The structure of the organic as the O-substituent is not particularly limited, and may be linear, branched, cyclic or a combination of these structures. The O-substituent may be an aliphatic group or an aromatic group. The O-substituent may contain one or more unsaturated bonds.
A linear or branched O-substituent, which does not contain a cyclic structure, has few steric hindrances between O-substituents and it is easy to introduce the O-substituent into modified cellulose fibers. Chain organic groups are preferred.

Examples of the organic group containing a hetero atom include alkoxyalkyl group, cycloalkyloxyalkyl group, aryloxyalkyl group, aralkyloxyalkyl group, alkoxyaryl group, cycloalkoxyaryl group, aryloxyaryl group, aralkyloxyaryl group, Examples thereof include an acyl group, an alkoxycarbonyl group, a cycloalkoxycarbonyl group, an aryloxycarbonyl group, an aralkyloxycarbonyl group, a halogenated alkyl group, a halogenated cycloalkyl group, a halogenated aryl group, and a halogenated aralkyl group.
Among these, alkoxyalkyl groups, cycloalkyloxyalkyl groups, aryloxyalkyl groups, aralkyloxyalkyl groups, alkoxyaryl groups, aryloxyaryl groups, aralkyloxyaryl groups, acyl groups, and halogenated alkyl groups are preferable.

  Preferred examples of the alkoxyalkyl group include 2-methoxyethyl group, 3-methoxy-n-propyl group, 4-methoxy-n-butyl group, 5-methoxy-n-pentyl group, 6-methoxy-n-hexyl group Group, 7-methoxy-n-heptyl group, 8-methoxy-n-octyl group, 9-methoxy-n-nonyl group, 10-methoxy-n-decyl group, 2-ethoxyethyl group, 3-ethoxy-n group -Propyl group, 4-ethoxy-n-butyl group, 5-ethoxy-n-pentyl group, 6-ethoxy-n-hexyl group, 7-ethoxy-n-heptyl group, 8-ethoxy-n-octyl group, Examples include 9-ethoxy-n-nonyl and 10-ethoxy-n-decyl.

  Preferred examples of the cycloalkyloxyalkyl group include 2-cyclopentyloxyethyl group, 3-cyclopentyloxy-n-propyl group, 4-cyclopentyloxy-n-butyl group, 5-cyclopentyloxy-n-pentyl group, 6 -Cyclopentyloxy-n-hexyl group, 7-cyclopentyloxy-n-heptyl group, 8-cyclopentyloxy-n-octyl group, 9-cyclopentyloxy-n-nonyl group, 10-cyclopentyloxy-n-decyl group, 2-cyclohexyloxyethyl group, 3-cyclohexyloxy-n-propyl group, 4-cyclohexyloxy-n-butyl group, 5-cyclohexyloxy-n-pentyl group, 6-cyclohexyloxy-n-hexyl group, 7-cyclohexyl group Oxy-n-heptyl group, 8-cycle Hexyloxy -n- octyl group, 9-cyclohexyloxy -n- nonyl group, and a 10-cyclohexyloxy -n- decyl groups.

  Preferred examples of the aryloxyalkyl group include 2-phenoxyethyl group, 3-phenoxy-n-propyl group, 4-phenoxy-n-butyl group, 5-phenoxy-n-pentyl group, 6-phenoxy-n- group Examples include hexyl, 7-phenoxy-n-heptyl, 8-phenoxy-n-octyl, 9-phenoxy-n-nonyl and 10-phenoxy-n-decyl.

  Preferred examples of the aralkyloxyalkyl group include 2-benzyloxyethyl group, 3-benzyloxy-n-propyl group, 4-benzyloxy-n-butyl group, 5-benzyloxy-n-pentyl group, 6- Benzyloxy-n-hexyl group, 7-benzyloxy-n-heptyl group, 8-benzyloxy-n-octyl group, 9-benzyloxy-n-nonyl group, and 10-benzyloxy-n-decyl group It can be mentioned.

  Preferred examples of the alkoxyaryl group include p-methoxyphenyl group, m-methoxyphenyl group, o-methoxyphenyl group, p-ethoxyphenyl group, m-ethoxyphenyl group, o-ethoxyphenyl group, p-n- Propyloxyphenyl group, m-n-propyloxyphenyl group, o-n-propyloxyphenyl group, p-n-butyloxyphenyl group, m-n-butyloxyphenyl group, and o-n-butyloxyphenyl group Can be mentioned.

  Preferred examples of the aryloxyaryl group include p-phenoxyphenyl group, m-phenoxyphenyl group, and o-phenoxyphenyl group.

  Preferable examples of the aralkyloxyaryl group include p-benzyloxyphenyl group, m-benzyloxyphenyl group and o-benzyloxyphenyl group.

As a halogen atom contained in a halogenated alkyl group, a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom are preferable, and a fluorine atom is more preferable.
In the halogenated alkyl group, at least a part of the hydrogen atoms of the alkyl group may be substituted by a halogen atom, and all hydrogen atoms of the hydrogen atoms of the alkyl group may be substituted by a halogen atom.

Preferable examples of the halogenated alkyl _{group, - (CF 2) 2 CF} 3, - (CF 2) 3 CF 3, - (CF 2) 4 CF 3, - (CF 2) 5 CF 3, - (CF _{2) 6 CF 3, - (} CF 2) 7 CF 3, - (CF 2) 8 CF 3, - (CF 2) 9 CF 3, -CH 2 CF 2 CF 3, -CH 2 (CF 2) 2 CF ₃ , -CH ₂ (CF ₂ ) ₃ CF ₃ , -CH ₂ (CF ₂ ) ₄ CF ₃ , -CH ₂ (CF ₂ ) ₅ CF ₃ , -CH ₂ (CF ₂ ) ₆ CF ₃ , -CH ₂ (-CH ₂ ) CF ₂ ) ₇ CF ₃ , -CH ₂ (CF ₂ ) ₈ CF ₃ , -CH ₂ CH ₂ CF ₃ , -CH ₂ CH ₂ CF ₂ CF ₃ , -CH ₂ CH ₂ (CF ₂ ) ₂ CF ₃ ,- CH ₂ CH ₂ (CF ₂ ) ₃ CF ₃ , —CH ₂ CH ₂ ( CF ₂ ) ₄ CF ₃ , —CH ₂ CH ₂ (CF ₂ ) ₅ CF ₃ , —CH ₂ CH ₂ (CF ₂ ) ₆ CF ₃ , —CH ₂ CH ₂ (CF ₂ ) ₇ CF ₃ , and —CH ( CF ₃ ) ₂ can be mentioned.

  Preferred examples of the acyl group will be described later.

When the organic group as the O-substituent is a hydrocarbon group, the structure of the hydrocarbon group is not particularly limited. Preferred examples of the hydrocarbon group include an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group and the like.
Preferred examples of the hydrocarbon group will be described later.

Preferred examples of the organosilyl group include the following formula (A1):
-SiR ^a1 R ^a2 -(-O-SiR ^a1 R ^a2- ) _p -R ^a3 (A1)
And a group represented by
R ^a1 , R ^a2 and R ^a3 are each independently a hydrocarbon group having 1 to 6 carbon atoms, and p is an integer of 0 or more.

Each of R ^a1 , R ^a2 and R ^a3 is preferably independently an alkyl group having 1 or more and 4 or less carbon atoms, or a phenyl group. It is more preferred that R ^a1 , R ^a2 and R ^a3 are all methyl groups.
In the formula (A1), the upper limit of p is not particularly limited as long as the object of the present invention is not impaired. p is preferably an integer of 0 or more and 35 or less, and more preferably an integer of 0 or more and 10 or less.

Preferred specific examples of the organosilyl group include, for example, trimethylsilyl group, triethylsilyl group, tripropylsilyl group, triisopropylsilyl group, tert-butyldimethylsilyl group, and triphenylsilyl group.
Further, a group represented by the above formula (A1), in which R ^a1 , R ^a2 and R ^a3 are all methyl groups and p is an integer of 1 or more and 35 or less is also preferable as the organosilyl group.

The organic group as an O-substituent is easy to synthesize and obtain modified cellulose fiber, and the dispersibility of the modified cellulose fiber at the time of preparation of the dispersion is particularly good, as represented by the following formula (1):
-(CO) _n -R ¹ (1)
(In the formula (1), when n is 0 or 1 and n is 0, R ¹ is a hydrocarbon group having 3 or more carbon atoms, and when n is 1, R ¹ is , A hydrocarbon group having 2 or more carbon atoms.)
The group represented by is preferable. The group represented by Formula (1) is a hydrocarbon group when n is 0, and is an acyl group when n is 1.
That is, the organic group as the O-substituent is preferably a hydrocarbon group or an acyl group.

R ¹ is not particularly limited as long as it is a hydrocarbon group having a predetermined number of carbon atoms. Preferred examples of R ¹ include an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group and the like.

When R ¹ is an alkyl group, preferred examples of the alkyl group include n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group, n-pentyl group, n -Hexyl group, n-heptyl group, n-octyl group, 2-ethylhexyl group, n-nonyl group, n-decyl group, n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group, n -A pentadecyl group, n-hexadecyl group, n-octadecyl group, n-nonadecyl group, and an icosyl group are mentioned.
When R ¹ is a cycloalkyl group, preferred examples of the cycloalkyl group include cyclopropyl group, cyclobutyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group, cycloheptyl group, cyclooctyl group, cyclononyl group, and cyclodecyl group. Groups are mentioned.
When R ¹ is an aryl group, preferable examples of the aryl group include a phenyl group, a tolyl group, a naphthyl group and a biphenylyl group.
When R ¹ is an aralkyl group, preferable examples of the aralkyl group include benzyl group, phenethyl group, naphthalen-1-ylmethyl group, and naphthalen-2-ylmethyl group.

  The method for introducing the O-substituent described above to the unmodified cellulose fiber is not particularly limited. O-substituents are introduced into unmodified cellulose fibers according to various known methods.

When the bond between the O-substituent and the oxygen atom is an ether bond, the O-substituent is introduced into the unmodified cellulose fiber by a well-known ether synthesis method such as Williamson's ether synthesis method.
When the bond between the O-substituent and the oxygen atom is an ester bond, O-- is obtained by a known acylation method using an acid halide (preferably acid chloride) of an organic acid, an acid anhydride of an organic acid, or the like. Substituents are introduced into the unmodified cellulose fiber.
When the O-substituent is an organosilyl group, the organosilyl group is introduced to the unmodified cellulose fiber by, for example, a known silylation reaction using halosilane, alkoxysilane or the like.

The unmodified cellulose fiber may be natural cellulose isolated from natural products or synthetic cellulose synthesized chemically, but natural cellulose is preferred because it is inexpensive and available in large quantities. . Natural cellulose is isolated from, for example, plants, animals, bacteria-positive gels and the like. Specific examples of the natural cellulose include softwood pulp, hardwood pulp, non-wood pulp, bacterial cellulose (BC), cellulose isolated from sea squirt, cellulose isolated from seaweed and the like. Examples of non-wood-based pulps include cotton-based pulps (for example, cotton linters, cotton lint and the like), straw pulps, bagasse pulps and the like. These may be used in combination of 2 or more types.
Among these, softwood pulp, hardwood pulp and non-wood pulp such as cotton linters, cotton lint, straw pulp and bagasse pulp are preferable.

The natural cellulose fiber may be treated to increase the surface area such as beating in order to increase the reaction efficiency at the time of modification.
It is also preferable to use natural cellulose fibers which have been stored in a wet state without being dried after isolation from natural products and purification. When used, the aggregate of microfibrils is in a state of being easily swollen, so that the reaction efficiency in modification is good, and it is easy to be finely divided in preparation of the dispersion.
Furthermore, the natural cellulose fiber may be a dissolving pulp that has been subjected to purification to reduce lignin, hemicellulose and the like by a known method. Also, highly purified cellulose to the extent that it can be used for the production of filter paper conforming to JIS standard (JIS P 3801: 1995, filter paper (for chemical analysis)) is used as unmodified cellulose fiber to be subjected to modification. It is also preferred to use.

When the number of hydroxyl groups in the above-mentioned unmodified cellulose fiber is N ₁ and the number of hydroxyl groups in the modified cellulose fiber is N ₂ in the first modified cellulose fiber, the following formula:
Degree of substitution = (N ₁ −N ₂ ) / N ₁
It is preferable that the degree of substitution calculated by the above is 0.4 or less. 0.3 or less is preferable and 0.2 or less is more preferable.
The lower limit of the degree of substitution is not particularly limited as long as the object of the present invention is not impaired. The lower limit of the degree of substitution is, for example, preferably 0.01 or more, more preferably 0.03 or more, and particularly preferably 0.05 or more.

  By setting the range of the degree of substitution in this manner, it is possible to achieve a balance between the improvement of the dispersibility and the formation of pores with a good shape when the porous membrane is obtained.

The degree of substitution can be calculated from the FT-IR spectrum of the first modified cellulose fiber when the O-substituent is an acyl group. More specifically, based on the FT-IR spectrum, the degree of substitution can be determined while appropriately preparing a calibration curve according to the method described in JP-A-2014-148629.
In addition, even when the O-substituent is not an acyl group, the amount of the substituent can be calculated by measuring the ¹ H-NMR spectrum.

The method of adjusting the degree of substitution in the first modified cellulose fiber is not particularly limited. Typically, the degree of substitution is adjusted by adjusting the amount of reagent used to introduce the O-substituent.
In addition, the degree of substitution can be adjusted also by the degree of refinement and crushing of the unmodified cellulose fiber. This is because the amount of hydroxyl groups exposed to the surface of the unmodified cellulose fiber increases as the unmodified cellulose fiber is highly refined.

  The first modified cellulose fiber described above is used for the preparation of a dispersion described later.

<Second Modified Cellulose Fiber>
As mentioned above, the second modified cellulose fiber is a modified cellulose fiber used for forming a porous membrane,
It is a modified cellulose fiber in which 0.1 g of modified cellulose fiber is mixed with 10 mL of pure water, then shaken for 30 seconds, and left to stand for another 30 seconds, whereby the modified cellulose fiber floats up with respect to pure water.

Modification of the second modified cellulose fiber is not limited to chemical modification as in the first modified cellulose fiber. Examples of modification methods other than chemical modification include physical modification treatment in which the surface of unmodified cellulose fiber is coated with an amphiphilic substance having a hydrophilic site and a hydrophobic site.
In this case, since the surface of the unmodified cellulose fiber is usually hydrophilic, the amphiphilic substance adheres to the surface of the cellulose fiber in a state where the hydrophilic portion of the amphiphilic substance faces the surface side of the cellulose fiber. As a result, the hydrophobic sites of the amphiphilic substance are exposed on the surface of the modified cellulose fiber after treatment.
Such amphiphilic substances include, for example, cellulose nanofibers crushed by collision of a high pressure jet stream described in WO 2007/088974.

  As the second modified cellulose fiber, chemically modified modified cellulose fiber described for the first modified cellulose fiber is preferable because adjustment of the degree of hydrophobicity is easy and the like.

  The second modified cellulose fiber satisfying the above conditions is easily dispersed well in the organic solvent. For this reason, in the case of using the dispersion liquid containing the second modified cellulose fiber, it is easy to form a porous film having a uniform film thickness and opening diameter.

<< Dispersion liquid used for formation of porous membrane >>
The dispersion liquid used for formation of the porous membrane of this embodiment is used in order to form the porous membrane containing the above-mentioned modified cellulose fiber.
The formation of the porous film using the above-mentioned dispersion liquid typically includes the steps of applying the dispersion liquid on a substrate to form a coating film, and drying the coating film.
Hereinafter, in the specification of the present application, “dispersion liquid” is “dispersion liquid used to form a porous membrane” unless otherwise specified.

  The dispersion contains modified cellulose fibers and an organic solvent. When the dispersion contains the above-mentioned modified cellulose fiber, the modified cellulose fiber is well and stably dispersed in the organic solvent in the dispersion. For this reason, it is easy to form a porous film with a high porosity using a dispersion liquid.

The viscosity of the dispersion at room temperature (20 ° C.) is preferably 5 cp to 500 cp, more preferably 10 cp to 400 cp, still more preferably 30 cp to 300 cp, and particularly preferably 50 cp to 200 cp.
The viscosity of the dispersion can be measured using an E-type viscometer.
When the viscosity of the dispersion is within the above range, the coatability of the dispersion is good, and the modified cellulose fiber is well dispersed in the dispersion, and the dispersion stability of the modified cellulose fiber is It is good.
The viscosity of the dispersion can be adjusted by adjusting the solid concentration of the dispersion, changing the type of organic solvent, or adjusting the dispersion diameter of the modified cellulose fiber in the dispersion. In addition, a well-known viscosity modifier may be added to the dispersion to adjust the viscosity of the dispersion, as long as the characteristics of the porous film formed using the dispersion are not significantly adversely affected.

  Hereinafter, the formation of the porous membrane, which is the method of using the dispersion, the essential or optional components contained in the dispersion, and the method of preparing the dispersion will be described.

<Formation of porous membrane>
As described above, typically, the formation of a porous membrane using the above-mentioned dispersion liquid is a step of applying the dispersion liquid on a substrate to form a coating film, and drying the coating film to form a porous membrane And forming.

The substrate is not particularly limited as long as the dispersion can be applied. Examples of the substrate include films, sheets, and substrates made of glass, metal, or resin such as polyethylene terephthalate or polycarbonate.
As described later, a porous film formed using a dispersion is preferably used as a separator in a storage element. In this case, the substrate is an electrode, more specifically, an electrode for a storage element. Thus, when a dispersion is applied to the electrode surface to form a porous film, a separator closely attached to the electrode surface is formed as a porous film, and an electrode complex composed of the electrode and the separator is obtained.
The electrode may be a positive electrode or a negative electrode.

The apparatus used for application of the dispersion is not particularly limited. As a coating device, for example, a contact transfer type coating device such as a roll coater, a reverse coater, or a bar coater, or a non-contact type coating device such as a curtain flow coater, a die coater, a slit coater, or a spray may be used.
The dispersion liquid of the present embodiment can be applied even when unevenness is present on the surface of a substrate such as an electrode. As described above, when unevenness is present on the surface of the substrate, a coating film having a uniform thickness can be easily formed. Therefore, as a coating method, a method using a spray, for example, a rotary atomization type coating device is preferable. As a coating apparatus of a rotation atomization system, the apparatus described in Unexamined-Japanese-Patent No. 2013-115181 can be used, for example.

  The thickness of the coating film is not particularly limited, and is appropriately adjusted in consideration of the thickness of the finally obtained porous film. The preferred thickness of the porous membrane will be described later.

  Next, the coating film formed by the above method is dried to remove the organic solvent and the like from the coating film. The drying method is not particularly limited, but a method of heating a coating film on a substrate such as an electrode is preferable. The heating of the coating film may be performed under atmospheric pressure or under reduced pressure. The heating temperature is not particularly limited, and in consideration of the boiling point of the organic solvent, the heating temperature is appropriately set within a temperature range in which thermal degradation of the porous film does not occur.

<Fiber material>
The dispersion contains the above-mentioned modified cellulose fiber as a fiber material. The above-mentioned modified cellulose fiber is well dispersed in the organic solvent in the dispersion, and the dispersion stability of the dispersion is good. For this reason, the dispersion can be used to form a porous membrane having an opening with a preferred small pore size.

  The dispersion may contain fiber materials other than the above-mentioned modified cellulose fiber, as long as the object of the present invention is not impaired. The fiber material other than the modified cellulose fiber may be an inorganic fiber or an organic fiber.

  Preferred examples of the inorganic fibers include micro glass (glass fibers with a small diameter) and rock wool. Preferred examples of the organic fiber include cellulose fiber materials such as unmodified cellulose fibers, cellulose esters such as carboxymethyl cellulose and cellulose acetate, and lignocellulose, fiber materials of neutral mucopolysaccharides such as chitin and chitosan, Examples thereof include synthetic resin-based fiber materials composed of polyamides such as aliphatic nylon and aromatic nylon (aramid), polyolefins such as polyethylene and polypropylene, vinylon, polyester, polyimide, polyamide imide, polyvinylidene fluoride and the like. With respect to synthetic resin-based fiber materials, fine fibers can be obtained, for example, by a method such as electro spinning.

  70 mass% or more is preferable, as for content of the above-mentioned modified cellulose fiber in the fiber material contained in a dispersion liquid, 80 mass% or more is more preferable, 90 mass% or more is still more preferable, 95 mass% or more is especially preferable And 100% by mass are most preferable.

  The volume average particle diameter of the modified cellulose fiber in the dispersion is not particularly limited, but it has a point that the modified cellulose fiber is easily dispersed in the dispersion, a desired porosity, and / or a desired average pore diameter. From the viewpoint of easily forming a porous film, the thickness is preferably 5 μm to 100 μm, more preferably 5 μm to 80 μm, still more preferably 5 μm to 75 μm, particularly preferably 5 μm to 75 μm, and most preferably 5 μm to 60 μm.

The number average fiber diameter of the modified cellulose fiber in the dispersion is preferably 2 nm to 500 nm, more preferably 2 nm to 100 nm, and particularly preferably 3 nm to 80 nm. When the number average fiber diameter of the modified cellulose fiber is within the above range, the modified cellulose fiber can be stably and favorably dispersed in the dispersion, and the desired porosity and / or desired using the dispersion It is easy to form a porous membrane having an average pore diameter.
1000 nm or less is preferable and, as for the largest fiber diameter of the modified cellulose fiber in a dispersion liquid, 500 nm or less is more preferable. When the maximum fiber diameter of the modified cellulose fiber is within the above range, the modified cellulose fiber is difficult to settle in the dispersion, and the modified cellulose fiber is stably dispersed in the dispersion.

The number average fiber diameter and the maximum fiber diameter of the modified cellulose fiber in the dispersion can be measured, for example, by the following method. First, dilution or concentration is performed as necessary to adjust the solid content concentration of the dispersion to 0.05% by mass or more and 0.1% by mass. The solid solution concentration-adjusted dispersion is cast on a hydrophilized carbon film-coated grid to obtain a transmission electron microscope (TEM) observation sample.
When the dispersion contains fibers having a large fiber diameter, a scanning electron microscope (SEM) image of the surface cast onto glass may be observed.
Then, observation with an electron microscope image is performed at a magnification of 5000 times, 10000 times, or 50000 times depending on the size of the fibers to be configured. An axis of an arbitrary image width in the vertical and horizontal directions is assumed in the image obtained by observation with an electron microscope, and the sample and observation conditions (such as magnification) are adjusted so that 20 or more fibers intersect with the axis. Then, after obtaining an observation image that satisfies this condition, a random axis is drawn for each image in the vertical and horizontal directions for each image, and the fiber diameter of fibers intersecting with the axis is visually read. In this way, images of at least three non-overlapping surface portions are taken with an electron microscope, and the fiber diameter values of fibers intersecting each other on two axes are read (thus, at least 20 x 2 x 3 = 120) Information on the fiber diameter of the book is obtained). The maximum fiber diameter and the number average fiber diameter are calculated from the fiber diameter data thus obtained.

<Organic solvent>
The dispersion contains an organic solvent. The type of the organic solvent is not particularly limited as long as a dispersion containing the modified cellulose fiber dispersed in a desired state can be prepared. The dispersion may contain a combination of two or more organic solvents as the organic solvent.

The boiling point of the organic solvent is preferably 70 ° C. or more and 250 ° C. or less, more preferably 90 ° C. or more and 200 ° C. or less, and particularly preferably 110 ° C. or more and 150 ° C. or less.
When an organic solvent having a boiling point within such a range is used, excessive composition change of the dispersion due to volatilization of the organic solvent hardly occurs, whereby the dispersion state of the modified cellulose fiber in the dispersion is also stable, and porous It is easy to remove the organic solvent from the coated film when forming the film.
The boiling point of the organic solvent is the boiling point at atmospheric pressure.

Here, with respect to the organic solvent, the term δp [unit: (MPa) ^0.5 ] of energy due to dipolar interaction regarding the Hansen solubility parameter is preferably 11.0 or less, and is 10.0 or less. Is more preferable, and particularly preferably 9.0 or less.
The modified cellulose fiber contains polarizable O-H bonds and C-O bonds. Therefore, dipolar interaction is important in order to disperse the modified cellulose fiber well.
For this reason, it is considered that the dispersion of the modified cellulose fiber is easy when the term δp of the energy due to the dipole interaction of the organic solvent is within the above-mentioned predetermined range.

  As the organic solvent, for example, glycol, glycol monoalkyl ether, glycol alkyl ether acetate, lactone, linear or cyclic ketone, alkane monool, and aprotic polar solvent can be preferably used.

  Preferred specific examples of the organic solvent of the above-mentioned type include glycols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol (DPG), triethylene glycol and tripropylene glycol; ethylene glycol monomethyl ether, ethylene glycol monoethyl Ether, ethylene glycol mono n-propyl ether, propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether, propylene glycol mono n-propyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono n-propyl ether, dipropylene Glycol monomethyl ether (MFDG), dipropylene glycol Monoethyl ether, dipropylene glycol mono n-propyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol mono n-propyl ether, tripropylene glycol monomethyl ether (MFTG), tripropylene glycol monoethyl Ethers, and glycol monoalkyl ethers such as tripropylene glycol mono n-propyl ether; 3-methoxybutyl acetate (MA), ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol mono n-propyl ether acetate, propylene Glycol monomethyl ether acetate (PGMEA), propylene glycol Diethyl ether acetate, propylene glycol mono n-propyl ether acetate, diethylene glycol monomethyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol mono n-propyl ether acetate, dipropylene glycol monomethyl ether acetate (DPMA), dipropylene glycol monoethyl ether acetate , Dipropylene glycol mono n-propyl ether acetate, triethylene glycol monomethyl ether acetate, triethylene glycol monoethyl ether acetate, triethylene glycol mono n-propyl ether acetate, tripropylene glycol monomethyl ether acetate, tripropylene glycol monoethyl ether Glycol and ether alkyl ether acetates such as tripropylene glycol mono n-propyl ether acetate; γ-butyrolactone (GBL), α-methyl-γ-butyrolactone, γ-valerolactone, γ-caprolactone, γ-laurolactone, δ- Valerolactone and lactones such as hexanolactone; dimethyl carbonate, diethyl carbonate, di-n-butyl carbonate, ethylene carbonate, 1,2-propylene carbonate, and carbonates such as 1,3-propylene carbonate; 2-hydroxy- Ethyl 2-methylpropionate, methyl 3-methoxypropionate, ethyl 3-methoxypropionate, methyl 3-ethoxypropionate, ethyl 3-ethoxypropionate, ethyl ethoxyacetate Ethyl hydroxyacetate, methyl 2-hydroxy-3-methylbutanoate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl formate, isopentyl acetate, n-butyl propionate, ethyl butyrate, Other esters such as n-propyl butyrate, isopropyl butyrate, n-butyl butyrate, methyl pyruvate, ethyl pyruvate, n-propyl pyruvate, methyl acetoacetate, ethyl acetoacetate, and ethyl 2-oxobutanoate; methyl ethyl ketone, Linear or cyclic ketones such as diethyl ketone, 2-pentanone, 2-hexanone, 3-hexanone, methyl isobutyl ketone, acetylacetone, cyclopentanone, cyclohexanone, and cycloheptanone; methanol, ethanol, n-propanol, isopropyl Alpanols such as lopanol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-pentyl alcohol, n-hexyl alcohol, cyclohexyl alcohol, and n-heptyl alcohol; N, N-dimethylformamide (DMF), N, N-diethylformamide, N, N-dimethylacetamide (DMAc), N, N-diethylacetamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N, N, N ' N'-Tetramethylurea (TMU), N, N, N ', N'-Tetraethylurea, N-Methylcaprolactam, 1,3-Dimethyl-2-Imidazolidinone (DMI), Dimethylsulfoxide, Hexamethylphos Holic triamide and aceto Aprotic polar solvents tolyl; toluene, xylene, and aromatic hydrocarbons and mesitylene.

Among the above preferred embodiments, particularly preferred organic solvents are shown in the following table together with the value of δp.

The amount of the organic solvent used is not particularly limited, and is appropriately determined in consideration of the viscosity of the dispersion, the coatability of the dispersion, the amount of water in the dispersion, and the like.
Typically, the amount of the organic solvent used is such that the solid content concentration of the dispersion is 0.1% by mass to 10% by mass, preferably 0.5% by mass to 5% by mass.

<Water>
The dispersion may contain water as long as the object of the present invention is not impaired. However, the amount of the organic solvent in the dispersion is preferably as small as possible from the viewpoint of the dispersibility of the above-mentioned modified cellulose fiber. The amount of water in the dispersion is typically preferably 5% by mass or less based on the total amount of the dispersion. When the water content in the dispersion is 5% by mass or less, it is easy to simultaneously achieve the desired porosity and the desired average pore diameter in the porous film formed using the dispersion.
The water content in the dispersion is preferably 3% by mass or less, more preferably 1% by mass or less, particularly preferably 0.5% by mass or less, and most preferably 0.1% by mass or less, based on the total amount of the dispersion. .
By adjusting the amount of water in the dispersion liquid, when forming a porous film having a desired porosity and a desired average pore diameter, control of temperature conditions becomes loose.
This water content can be measured, for example, by a known method such as the Karl Fischer method.

<Other ingredients>
The dispersion may contain the modified cellulose fiber described above and other components other than the organic solvent as long as the object of the present invention is not impaired. As another component, surfactant, an antifoamer, a leveling agent etc. are mentioned.

<Preparation Method of Dispersion>
The dispersion is prepared by mixing modified cellulose fiber, an organic solvent, and, if necessary, water and other components.
The modified cellulose fiber may be used in a solid state or may be used in a dispersed state in a dispersion medium. The modified cellulose fiber is preferably used in a dispersed state in a dispersion medium, since the modified cellulose fiber is easily dispersed in the resulting dispersion.

When the modified cellulose fiber is solid, a dispersion can be prepared by mixing the solid modified cellulose fiber with a desired amount of organic material. In order to finely disperse the modified cellulose fiber in the dispersion liquid to a desired degree, usually, after the modified cellulose fiber and the organic material are mixed, the dispersion treatment is performed.
When the modified cellulose fiber is dispersed in a medium containing water, the medium containing water may be replaced with the organic solvent until the water content in the dispersion becomes 5% by mass or less.
As a method of replacing the medium containing water with the organic solvent, a method of adding the organic solvent after distilling a part or the whole of the medium containing water, or settling the modified cellulose fiber in the container by a centrifugal separator After that, discard the supernatant containing water, and then disperse the modified cellulose fiber precipitated in the organic solvent, filter the modified cellulose fiber with a filter, wash it with the organic solvent, and disperse it again. .

After the modified cellulose fiber and the organic solvent are mixed by the above method or the like, preferably, a dispersion treatment is performed.
Preferred examples of the dispersing device used for the dispersing treatment include a ball mill, bead mill, sand mill, two-roll, three-roll, roll mill, colloid mill, jet mill, kneader, homogenizer and the like.
The particle diameter (dispersion diameter, volume average particle diameter) of the modified cellulose fiber in the dispersion liquid and the particle diameter distribution can be adjusted by adjusting the dispersion conditions.

«Porous membrane»
The porous film is a film obtained by drying a coating film made of the above-mentioned dispersion liquid. The dispersion, which is a material for forming the porous membrane, is as described above, and the method for forming the porous membrane is as described for the dispersion.

The porosity of the porous membrane is not particularly limited, and is appropriately determined in consideration of the use of the porous membrane. When a porous film is used as a separator of a storage element, it is easy to move ions such as lithium ions favorably in the porous film, and the mechanical strength of the porous film is good. The porosity of the film is preferably 5% by volume to 80% by volume, more preferably 8% by volume to 70% by volume, and particularly preferably 12% by volume to 60% by volume.
The porosity can be adjusted by adjusting the dispersion diameter of the modified cellulose fiber in the dispersion, the solid content concentration of the dispersion, the drying condition of the coating film at the time of forming the porous film, and the like.
The porosity of the porous membrane can be measured by a mercury porosimeter.

The average pore size of the porous membrane is not particularly limited, and is appropriately determined in consideration of the use of the porous membrane. From the viewpoint of balance between ion permeability and short circuit risk, the average pore diameter of the porous membrane is preferably 0.02 μm or more and 0.5 μm or less, more preferably 0.02 μm or more and 0.45 μm or less, and 0.02 μm or more and 0.4 μm or less Is further more preferable, and 0.02 μm or more and 0.35 μm or less is particularly preferable.
The average pore diameter of the porous membrane can be adjusted by adjusting the dispersion diameter of the modified cellulose fiber in the dispersion, the solid content concentration of the dispersion, the drying conditions of the coating film when forming the porous membrane, and the like.
The average pore size of the porous membrane can be measured by a mercury porosimeter.

  The thickness of the porous membrane is not particularly limited, and may be appropriately determined in consideration of the use of the porous membrane. When a porous film is used as a separator of a storage element, it is preferable that the porous film be thinner because it can be charged at high speed and is easy to manufacture a high capacity storage element. Specifically, the thickness of the porous film is preferably 1 μm to 100 μm, and more preferably 2 μm to 50 μm.

  The shape of the porous membrane is not particularly limited. As described above, after the dispersion liquid is applied, the coating film is dried to form a porous film, so that it is possible to manufacture a porous film having any shape along the shape of the surface to be coated.

  As described above, the porous film described above is particularly preferably used as a separator of a storage element.

«Method for producing porous membrane»
The method for producing a porous membrane is typically
Applying a dispersion on a substrate to form a coating film;
Drying the coated film to form a porous film.
The details of each of the above steps are as described above. The detailed configurations of the dispersion liquid and the modified cellulose fiber are as described above.
As a base material to which the dispersion liquid is applied, as described above, an electrode, particularly an electrode for a storage element is suitably used.

«Storage element»
The storage element may have the conventionally known known configuration relating to the storage element other than the provision of the separator made of the porous film described above.
The separator made of the above-mentioned porous film, for example, constitutes an element of a storage element as an electrode complex complexed with an electrode.

Examples of the storage element include a battery or capacitor such as a lithium ion secondary battery, a polymer lithium battery, an aluminum electrolytic capacitor (capacitor), an electric double layer capacitor, a lithium ion capacitor and the like.
The separator made of a porous membrane is preferably used in a secondary battery, and more preferably used in a lithium ion secondary battery.
As the storage element described above, a lithium battery or a lithium ion battery is preferable.

The configuration of the storage element can be completely the same as that of the conventional battery except that an electrode complex, preferably a composite of both, is used as a unit cell layer, as the separator and the electrode. The structure of the storage element is not particularly limited, and examples include a stacked type, a cylindrical type, a square type, and a coin type.
For example, a lithium ion secondary battery as a storage element can be provided with a unit battery layer in which a porous film (separator) in an electrode complex is impregnated with an electrolytic solution.

  In addition, a capacitor as a storage element, for example, an electric double layer capacitor can also include a unit cell in which a porous film (separator) in an electrode assembly is impregnated with an electrolytic solution.

  In a lithium ion secondary battery or an electric double layer capacitor, for example, a plurality of unit battery layers or unit cells are stacked or wound to form an element, and then the element is housed in an outer package and a current collector is an external electrode After being connected to each other and impregnated with a conventionally known electrolytic solution, the outer packaging material can be sealed.

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples. However, the scope of the present invention is not limited to the examples shown below.

Example 1
Advantech filter paper (5C) 1.00 g (6.17 mmol) was dispersed in 15 mL of pyridine while disintegrating fibers. After adding 5.29 g (24.68 mmol) of hexanoic acid anhydride to pyridine, reflux was performed at 120 ° C. for 2 hours to denature the cellulose fiber. The modified cellulose fiber in the reaction solution was washed sequentially with water, 0.5 M hydrochloric acid, an aqueous sodium hydroxide solution, and water, and then water attached to the modified cellulose fiber was replaced with propylene glycol monomethyl ether (PGME).

  From the slurry after the second water washing, an amount of slurry corresponding to 0.1 g of modified cellulose fiber was taken. The separated slurry was diluted with pure water so as to be 10 mL of pure water with respect to 0.1 g of modified cellulose fiber. The diluted slurry was shaken for 30 seconds and then allowed to stand for 30 seconds. After standing, the modified cellulose fiber floated near the water surface.

  Propylene glycol monomethyl ether is added to the obtained slurry of modified cellulose fibers in propylene glycol monomethyl ether to adjust the solid content concentration to 2.0 mass%, and then Nikkato is added to the liquid containing the modified cellulose fibers. Dispersion treatment was carried out by a bead mill using beads of diameter 0.5 (mm) manufactured under the conditions of circumferential velocity 9 m / sec, bead filling rate 80% for 20 minutes to obtain a dispersion.

The obtained dispersion liquid was coated on a glass substrate using a spray, and then dried at 80 ° C. for 3 minutes to dry the coated film, to obtain a porous film with a thickness of 6 μm.
The porosity (volume%) and the average pore diameter (μm) of the obtained porous film were measured with a mercury porosimeter. Table 2 shows the porosity and the average pore diameter of the porous membrane.

[Examples 2 to 4]
A dispersion was obtained in the same manner as in Example 1 except that the organic solvent was changed to an organic solvent of the type described in Table 2.
The volume average particle diameter of the modified cellulose fiber in the dispersion obtained in Examples 2 to 4 is described in Table 2.
The dispersion obtained was used for coating using a spray, and in the same manner as Example 1, a porous film was obtained.
The porosity (volume%) and the average pore diameter (μm) of the obtained porous film were measured with a mercury porosimeter. Table 2 shows the porosity and the average pore diameter of the porous membrane.

[Example 5]
In Example 5, a dispersion was obtained in the same manner as in Example 1 except that hexanoic acid anhydride was changed to 7.25 g (30.08 mmol) of benzoic acid anhydride.
The volume average particle diameter of the modified cellulose fiber in the dispersion obtained in Example 5 is described in Table 2.
The dispersion obtained was used for coating using a spray, and in the same manner as Example 1, a porous film was obtained.
The porosity (volume%) and the average pore diameter (μm) of the obtained porous film were measured with a mercury porosimeter. Table 2 shows the porosity and the average pore diameter of the porous membrane.
In Example 5, when obtaining a dispersion liquid, the water slurry of modified cellulose fiber is fractionated similarly to Example 1, and it is the same as that of Example 1 in the test which shakes modified cellulose fiber in pure water Did. The modified cellulose fiber obtained in Example 5 floated near the water surface after shaking and standing.

[Examples 6 to 7]
Example 6 is the same as Example 1 except that hexanoic acid anhydride is changed to 3.90 g (24.68 mmol) of butyric acid anhydride to make the solid concentration of the dispersion 1.5% by mass. A dispersion was obtained.
Example 7 is the same as Example 1 except that hexanoic acid anhydride is changed to 3.90 g (24.68 mmol) of isobutyric acid anhydride, and the solid content concentration of the dispersion is 1.5% by mass. To obtain a dispersion.
Here, the volume average particle diameter of the modified cellulose fiber in the dispersion obtained in Example 6 is 33.5 μm, and the volume average particle diameter of the modified cellulose fiber in the dispersion obtained in Example 7 is 32.1 μm. Met.
In Examples 6 to 7, when obtaining the dispersion liquid, the water slurry of the modified cellulose fiber is separated as in Example 1, and the modified cellulose fiber is shaken in pure water in the same manner as in Example 1. Test was conducted. The modified cellulose fibers obtained in Examples 6 to 7 all floated near the water surface after shaking and standing.
Moreover, the film was formed like Example 1 using the dispersion liquid obtained in Examples 6-7. It has been confirmed that these membranes also have appropriate porosity.

なお、表２中の有機溶剤の名称について以下の通りである。
ＰＧＭＥ：プロピレングリコールモノメチルエーテル
ＰＭＡ−Ｐ：プロピレングリコールモノメチルエーテルアセテート
ＭＡ：３−メトキシブチルアセテート
ＢＡ：酢酸ブチル

The names of the organic solvents in Table 2 are as follows.
PGME: propylene glycol monomethyl ether PMA-P: propylene glycol monomethyl ether acetate MA: 3-methoxybutyl acetate BA: butyl acetate

  According to this embodiment, a modified cellulose fiber is provided which is easy to disperse well in the dispersion. In addition, since the cellulose fibers can be well dispersed in the dispersion as described above, pores can be stably formed when the membrane is formed.

Claims (15)

Modified cellulose fibers used to form a porous membrane,
The modified cellulose fiber is an organic group having 3 or more carbon atoms, in which a hydrogen atom in the hydroxyl group is partially bonded to an oxygen atom derived from the hydroxyl group by an O-C bond, in a part of the hydroxyl group possessed by unmodified cellulose fiber And / or fibers substituted with organosilyl groups,
Modified cellulose fiber. The modified cellulose fiber is a fiber substituted with the organic group,
The organic group is represented by the following formula (1):
-(CO) _n -R ¹ (1)
(In the formula (1), when n is 0 or 1 and n is 0, R ¹ is a hydrocarbon group having 3 or more carbon atoms, and when n is 1, R ¹ is , A hydrocarbon group having 2 or more carbon atoms.)
The modified cellulose fiber according to claim 1, which is a group represented by When the number of hydroxyl groups in the unmodified cellulose fiber is N ₁ and the number of hydroxyl groups in the modified cellulose fiber is N ₂ , the following formula:
Degree of substitution = (N ₁ −N ₂ ) / N ₁
The modified cellulose fiber of Claim 1 or 2 whose substitution degree calculated by these is 0.4 or less. Modified cellulose fibers used to form a porous membrane,
A modified cellulose fiber in which 0.1 g of the modified cellulose fiber is mixed with 10 mL of pure water, shaken for 30 seconds, and left to stand for another 30 seconds, the modified cellulose fiber floats up with respect to the pure water. A dispersion used to form a porous membrane,
A dispersion comprising the modified cellulose fiber according to any one of claims 1 to 4 and an organic solvent.   The dispersion liquid of Claim 6 whose boiling point of the said organic solvent is 70 degreeC or more and 250 degrees C or less.   The dispersion according to claim 5 or 6, wherein the porous membrane formed by applying the dispersion to an electrode and being formed is used as a separator.   The dispersion according to any one of claims 5 to 7, wherein the viscosity at room temperature (20 ° C) is 5 cp or more and 1500 cp or less.   The dispersion according to any one of claims 5 to 8, wherein the volume average particle diameter of the modified cellulose fiber in the dispersion is 5 μm or more and 100 μm or less.   The porous film formed by drying the coating film which consists of a dispersion liquid of any one of Claims 5-9.   The porous membrane according to claim 10, wherein the porosity is 20% by volume or more and 80% by volume or less.   The porous membrane according to claim 10 or 11, wherein the average pore size is 0.02 μm or more and 0.5 μm or less.   An electrical storage element provided with the porous film of any one of Claims 10-12. A process of applying the dispersion according to any one of claims 5 to 9 on a substrate to form a coated film;
And drying the coated film to form a porous film.   The method for producing a porous membrane according to claim 14, wherein the substrate is an electrode.