JP2017117754A - Binder composition for secondary battery electrode, secondary battery electrode mixture, secondary battery electrode and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a binder composition for a secondary battery electrode having high adhesion and electrochemical stability.SOLUTION: The binder composition for an electrode includes a graft copolymer having in its main chain a repeating unit represented by the formula (I) and a repeating unit represented by the formula (II). (G is a graft chain represented by the formula (a))SELECTED DRAWING: Figure 1

Description

  The present invention relates to a binder composition for a secondary battery electrode, a secondary battery electrode mixture, a secondary battery electrode, and a secondary battery.

  In recent years, as various electronic devices have been reduced in size and weight, secondary batteries used as power sources for these electronic devices have been required to have various performances such as higher capacity, smaller size, and lighter weight. Lithium ion secondary batteries are widely used because they have advantages such as high voltage, long life, and high energy density.

  In order to improve the performance of these lithium ion secondary batteries, research on each constituent material constituting the lithium ion secondary battery has been actively conducted, and many proposals have been made regarding binder materials for electrodes (binders for electrodes). Has been made. The basic properties required for the binder material for electrodes include the function of binding the particles constituting the electrodes such as electrode active material particles and conductive auxiliary particles (binding properties), and the electrode mixture as a current collector. First, the function of adhering (adhesiveness) is mentioned. In addition, along with the recent increase in capacity and voltage of lithium secondary batteries, electrode binder materials have many properties such as flexibility and high electrochemical stability, in addition to higher binding and adhesion than before. The characteristics are required. By using a binder material that satisfies these characteristics, it is possible to maximize the active material content in the electrode mixture, increase the density and thickness of the electrode mixture layer, and increase the capacity of the secondary battery. It becomes.

  As typical materials used for the binder material for electrodes, vinylidene fluoride-based polymers (PVDF-based polymers) mainly composed of vinylidene difluoride and fluorine-based monomers are used. Roughly divided into acrylate polymers composed of acrylic monomers. Although the PVDF polymer is excellent in electrochemical stability and ionic conductivity, it has a defect that the content of polar functional groups is lower than that of the acrylate polymer, resulting in poor binding and adhesion. Moreover, the PVDF polymer used for the electrode binder material is usually designed as a crystalline polymer in order to avoid excessive electrolyte swelling and electrolyte dissolution. Such a crystalline polymer is difficult to increase in flexibility as compared with an amorphous acrylic polymer.

  On the other hand, an acrylate polymer having more polar functional groups than a PVDF polymer is easily oxidatively decomposed in a battery environment. In addition, an electrode using an acrylate polymer has a lower ion conductivity and a higher resistance than a PVDF polymer. As a result, the electrode using the acrylate polymer has a problem in that the life and output performance of the battery are lowered.

  As a method for solving these problems, a composite of PVDF polymer and acrylate polymer can be considered. As one of them, Patent Document 1 proposes a method of forming an interpenetrating network structure (IPN (interpenetrating polymer network) structure) between a PVDF polymer and an acrylate polymer. Also, in Patent Document 2 and Patent Document 3, acrylate polymer is grafted to PVDF polymer by performing atom transfer radical polymerization of acrylate monomer from PVDF polymer chain. A technique to make it has been proposed.

JP 2014-89970 A JP2015-106564A JP 2014-186895 A

  However, the composite polymer having an IPN structure proposed in Patent Document 1 often has a significantly lower melting point or loses crystallinity in some cases as compared with a pure PVDF polymer. In addition, such a composite polymer has a high concern that characteristics such as oxidation resistance and heat resistance derived from the crystallinity of the PVDF polymer are impaired.

  In addition, the polymers described in Patent Document 2 and Patent Document 3 retain crystallinity, which is one of the characteristics of the PVDF polymer, because the PVDF polymer is a main chain skeleton. Therefore, the polymers described in these documents are theoretically considered to be ideal binder materials in terms of heat resistance and electrochemical stability.

  However, as a result of studies by the present inventors, it has been found that it is difficult to grow a long-chain graft chain having a number average degree of polymerization of acrylate units exceeding 30 from a PVDF polymer. When the chain length of the grafted acrylate polymer is short, the mechanical properties of the PVDF polymer of the main chain are not significantly improved, and the flexibility of the whole polymer and the adhesiveness as a binder are increased. It is also difficult to improve.

Further, the polymers described in Patent Document 2 and Patent Document 3 cannot sufficiently exhibit their characteristics due to problems caused by the production method as described below.
First, in the production method described in the same document, there is a concern about the by-product of a single chain of an acrylate polymer having low electrochemical stability. In the method described in this document, it is necessary to carry out the polymerization reaction at a high temperature of 80 ° C. or higher in order to initiate the polymerization from the C—Cl bond in the PVDF polymer chain having a low radical initiation ability. Is likely to cause homopolymerization of the acrylate monomer.

  Next, in the production method described in the same document, a metal halide is used as a polymerization catalyst. In particular, in the production method described in Patent Document 2, a PVDF polymer having a main chain substituted with chlorine, bromine or iodine is used. In this case, as a result of atom transfer radical polymerization, it is inevitable that a C—Cl or C—Br bond is formed at the terminal of the acrylate graft chain. Thereby, the electrochemical stability of the obtained polymer is lowered.

  Furthermore, it is difficult to completely remove transition metals (for example, copper and iron) of the polymerization catalyst from the obtained polymerization product. If copper and iron, which are paramagnetic metals, remain in the battery component, reliability during long-term use of the battery is impaired.

  Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a binder composition for secondary battery electrodes having high adhesion and high electrochemical stability, and It is in providing the used secondary battery electrode mixture, a secondary battery electrode, and a secondary battery.

  In order to solve the above problems, according to an aspect of the present invention, a graft containing a repeating unit represented by the following formula (I) and a repeating unit represented by the following formula (II) in the main chain: There is provided a binder composition for a secondary battery electrode comprising a copolymer.

Where
G is a graft chain represented by the following formula (a),

R ¹ and R ³ are each independently a hydrogen atom or a linear or branched alkyl group or fluoroalkyl group having 1 to 20 carbon atoms,
R ² and R ⁴ independently represent a hydrogen atom, a linear or branched alkyl group having 1 to 20 carbon atoms, a hydroxyalkyl group, a fluoroalkyl group or an epoxyalkyl group, or a polyalkylene An oxide (polyalkylene oxide) chain;
R ⁵ is an end group of the graft chain,
m is an arbitrary natural number.

  The graft copolymer according to this aspect has a strong adhesive force and a high electrochemical stability.

  Here, the graft copolymer may further contain a repeating unit represented by the following (III).

In the above formula,
R ⁶ is a hydrogen atom, a fluorine atom, a fluoroalkyl group or a trifluoromethoxy group.

  The binder composition for secondary battery electrodes according to this viewpoint has high adhesion and high electrochemical stability.

  Furthermore, the graft copolymer may contain 10 to 70% by mass of the graft chain based on the graft copolymer.

  The binder composition for a secondary battery electrode according to this viewpoint has higher adhesion and higher electrochemical stability.

  Further, the graft copolymer may have a number average polymerization degree of repeating units in the graft chain of 30 or more.

  The binder composition for secondary battery electrodes according to this viewpoint has even higher adhesion.

  The graft copolymer has a number average degree of polymerization of 1000 or more of a main chain comprising the repeating unit represented by the formula (I) and the repeating unit represented by the formula (II). Also good.

  The binder composition for secondary battery electrodes according to this point of view has much higher electrochemical stability.

  Further, the graft copolymer may not have a bond between a bromine atom or a chlorine atom and a carbon atom.

  The binder composition for secondary battery electrodes according to this point of view has much higher electrochemical stability.

  According to the other viewpoint of this invention, the secondary battery electrode mixture containing the said binder composition for secondary battery electrodes is provided.

  The secondary battery electrode mixture according to this aspect has a strong adhesive force and high electrochemical stability, and is excellent in durability.

  According to the other viewpoint of this invention, the secondary battery electrode containing the said binder composition for secondary battery electrodes is provided.

  The secondary battery electrode by this viewpoint is produced using the said binder composition for secondary battery electrodes which has high electrochemical stability and strong adhesive force. Therefore, the durability of the secondary battery can be improved by producing a secondary battery using the secondary battery electrode.

  According to the other viewpoint of this invention, a secondary battery provided with said secondary battery electrode is provided.

  The secondary battery according to this viewpoint has high durability.

  As described above, according to the present invention, a binder composition for a secondary battery electrode having high adhesion and high electrochemical stability, and a secondary battery electrode mixture, a secondary battery electrode, and a secondary battery electrode using the same. A secondary battery can be provided.

It is a sectional side view which shows the structure of the lithium ion secondary battery which concerns on embodiment of this invention. It is a graph which shows the differential scanning calorimetry result of the graft copolymer as an example in the binder composition for secondary battery electrodes which concerns on embodiment of this invention.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

Prior to the description of the secondary battery electrode binder composition, secondary battery electrode mixture, secondary battery electrode, and secondary battery according to the present embodiment, the graft copolymer contained in the secondary battery electrode binder composition The coalescence and the manufacturing method thereof will be described.
<1. Graft copolymer>
First, the graft copolymer contained in the binder composition for secondary battery electrodes according to the present embodiment will be described.

  The graft copolymer includes a repeating unit represented by the following formula (I) and a repeating unit represented by the following formula (II) in the main chain.

Ｒ^１およびＲ^３は、独立して、水素原子または炭素数１〜２０の直鎖もしくは分岐状のアルキル基もしくはフルオロアルキル基であり、
Ｒ^２およびＲ^４は、独立して、水素原子、炭素数１〜２０の直鎖もしくは分岐状のアルキル基、ヒドロキシアルキル基、フルオロアルキル基もしくはエポキシアルキル基、またはポリアルキレンオキシド鎖であり、
Ｒ^５は、前記グラフト鎖の末端基であり、
ｍは、任意の自然数である。 Where
G is a graft chain represented by the following formula (a),

R ¹ and R ³ are each independently a hydrogen atom or a linear or branched alkyl group or fluoroalkyl group having 1 to 20 carbon atoms,
R ² and R ⁴ are each independently a hydrogen atom, a linear or branched alkyl group having 1 to 20 carbon atoms, a hydroxyalkyl group, a fluoroalkyl group or an epoxyalkyl group, or a polyalkylene oxide chain;
R ⁵ is an end group of the graft chain,
m is an arbitrary natural number.

  Since the graft copolymer has a vinylidene fluoride skeleton in the main chain, it is excellent in electrochemical stability and ion conductivity. On the other hand, since the graft copolymer has an acrylate side chain as a graft chain, it has excellent adhesion to the current collector and binding force of the active material. In addition, due to the acrylate side chain, the graft copolymer also has high flexibility. Further, the graft copolymer can be produced by a method described later, and it is not necessary to use a halogen atom or a transition metal element at the time of graft polymerization. For this reason, in the graft copolymer, bonds such as C—Cl, C—Br, and C—I are preferably excluded. Moreover, the transition metal element derived from a catalyst is not contained in the composition containing the obtained graft copolymer. Therefore, the graft copolymer sufficiently exhibits the characteristics of the vinylidene fluoride skeleton and the acrylate side chain. As a result, when such a graft copolymer is used for an electrode as a binder for a secondary battery electrode, the durability of the electrode under high voltage is improved, and consequently the cycle characteristics and durability of the secondary battery are improved. To do.

In R ¹ and R ³ , examples of the linear or branched alkyl group having 1 to 20 carbon atoms include a methyl group, an ethyl group, an n-propyl group, and an n-butyl ( n-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, n-nonyl group n-nonyl) group, n-decyl group, n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group n-tetradecyl group, n-pentadecyl group, n-hexadecyl group Linear alkyl groups such as n-heptadecyl group, n-octadecyl group, n-nonadecyl group, n-icosyl group, and isopropyl (isopropyl) ) Group, isobutyl group, sec-butyl group, tert-butyl group, 1-methylbutyl group, 2-methylbutyl group, 3 -Methylbutyl (3-methylbutyl) group, 1,1-dimethylpropyl (1,1-dimethylpropyl) group, 1,2-dimethylpropyl (1,2-dimethylpropyl) group, 1-ethylpropyl (1-e thylpropyl group, 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, 4-methylpentyl group, 1, 1-dimethylbutyl (1,1-dimethylbutyl) group, 1,2-dimethylbutyl (1,2-dimethylbutyl) group, 2,2-dimethylbutyl (2,2-dimethylbutyl) group, 2,3-dimethylbutyl ( 2,3-dimethylbutyl) group, 3,3-dimethylbutyl (3,3-dimethylbutyl) group, 1-methyl-1-ethylpropyl group, 2-methyl-1-ethyl Propyl (2-methyl-1-ethylpropyl) group, 1-methyl-2-ethylpropyl (1-methyl-2-ethylpropyl) group, 1,1,2-trimethylpropyl (1,1,2-trimethylpropyl) group, Examples include branched alkyl groups such as 1,2,2-trimethylpropyl (1,2,2-trimethylpropyl) group.

  In addition, examples of the linear or branched fluoroalkyl group having 1 to 20 carbon atoms include those in which one or more hydrogen atoms of the above-described alkyl group are substituted with fluorine atoms. More specifically, the linear or branched fluoroalkyl group having 1 to 20 carbon atoms may be a mono, di or perfluoromethyl group, mono, di, tri, tetra or perfluoro. Linear groups such as ethyl (mono, di, tri, tetra or perfluoroethyl) groups, mono, di, tri, tetra, penta, hexa or perfluoropropyl (mono, di, tri, tetra, penta, hexa or perfluoropropyl) groups Fluoroalkyl group, mono-, di-, tri-, tetra-, penta-, hexa- or perfluoroisopropyl (mono, di, tri, tetra, penta, hexa or perfluoroisopropyl) group, mono , Di, tri, tetra, penta, hexa, hepta, octa or perfluoroisobutyl (mono, di, tri, tetra, penta, hexa, hepta, octa or perfluoroisobutyl), mono, di, tri, tetra, penta, hexa , Hepta, octa or perfluoro-sec-butyl (mono, di, tri, tetra, penta, hexa, hepta, octa or perfluoro-sec-butyl) group, mono, di, tri, tetra, penta, hexa, hepta, Branched fluoroalkyl groups such as octa- or perfluoro-tert-butyl (mono, di, tri, tetra, penta, hexa, hepta, octa or perfluoro-tert-butyl) groups Le group.

  Moreover, although carbon number of the alkyl group and fluoroalkyl group mentioned above should just be in the range mentioned above, Preferably it is 1-10, More preferably, it is 1-5, More preferably, it is 1-3.

R ¹ and R ³ are not particularly limited as long as they are the groups described above, but are preferably independently a hydrogen atom or a linear or branched alkyl group or fluoroalkyl group having 1 to 10 carbon atoms, It is more preferably a hydrogen atom or a linear alkyl group or fluoroalkyl group having 1 to 5 carbon atoms, more preferably a hydrogen atom, a methyl group or a trifluoromethyl group, and a hydrogen atom or a methyl group. Even more preferred.

The alkyl group and fluoroalkyl group in R ² and R ⁴ can be the same as the groups in R ¹ and R ³ described above.

In R ² and R ⁴ , examples of the linear or branched hydroxyalkyl group having 1 to 20 carbon atoms include groups in which one or more hydrogen atoms of the alkyl group described above are substituted with a hydroxyl group.

  Examples of such a hydroxyalkyl group include a hydroxymethyl group, a 1- or 2-hydroxyethyl group, a 1-, 2- or 3-hydroxy-n-propyl (1). -, 2- or 3-hydroxy-n-propyl) group, 1-, 2-, 3- or 4-hydroxy-n-butyl (1-, 2-, 3- or 4-hydroxy-n-butyl) group , 1-, 2-, 3-, 4-or 5-hydroxy-n-pentyl (1-, 2-, 3-, 4-or 5-hydroxy-n-pentyl) groups and the like, And 1- or 2-hydroxyisopropyl group, 1-, 2- or 3- Droxyisobutyl (1-, 2-or 3-hydroxyisobutyl) group, sec-butyl (1-, 2-or 3-hydroxy-sec-butyl) group, 1-hydroxymethylpropyl (1-hydroxymethylpropyl) group, 1- Alternatively, branched hydroxyalkyl groups such as a 2-hydroxy-tert-butyl (1-or 2-hydroxy-tert-butyl) group can be mentioned.

In R ² and R ⁴ , examples of the linear or branched epoxyalkyl group having 1 to 20 carbon atoms include groups in which one or more hydrogen atoms of the above-described alkyl group are substituted with an epoxy group. It is done.

  Examples of such an epoxyalkyl group include an epoxymethyl group (glycidyl group), 1- or 2-epoxyethyl group, 1-, 2-, or 3- Epoxy-n-propyl (1-, 2-or 3-epoxy-n-propyl) group, 1-, 2-, 3- or 4-epoxy-n-butyl (1-, 2-, 3-or 4- epoxy-n-butyl) group, 1-, 2-, 3-, 4- or 5-epoxy-n-pentyl (1-, 2-, 3-, 4- or 5-epoxy-n-pentyl) group, etc. A straight-chain epoxyalkyl group, and a 1- or 2-epoxyisopropyl group, 1-, 2- or 3-epoxy Cyisobutyl (1-, 2-or 3-epoxyisobutyl) group, 1-, 2- or 3-epoxy-sec-butyl (1-, 2-or 3-epoxy-sec-butyl) group, 1-epoxymethylpropyl ( Examples thereof include branched epoxyalkyl groups such as a 1-epoxymethylpropyl group and a 1- or 2-epoxy-tert-butyl group.

In R ² and R ⁴ , as the polyalkylene oxide chain, an unsubstituted polyalkylene oxide chain such as a polyethylene oxide chain, a polypropylene oxide chain, and a butylene oxide chain, and one or more hydrogen atoms thereof are substituted with an alkyl group. Substituted polyalkylene oxide chains.

R ² and R ⁴ are not particularly limited as long as they are the groups described above, but are independently preferably a linear or branched alkyl group or a fluoroalkyl group having 1 to 10 carbon atoms, a hydrogen atom or It is more preferably a linear or branched alkyl group having 1 to 10 carbon atoms, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert- A butyl group or an n-pentyl group is more preferable, and a methyl group, an ethyl group, an n-propyl group, an n-butyl group, or an n-pentyl group is still more preferable.

R ⁵ is a terminal group of the graft chain. R ⁵ corresponds to a terminal group in the production process of the acrylate polymer to be a graft chain, and thus is an atomic group derived from a polymerization initiator or a hydrogen atom generated by chain transfer during polymerization.

In R ⁵ , examples of the atomic group derived from the polymerization initiator include an alkyl group, an alkoxy group, a phenyl group, a sulfonic acid group, an isobutyronitrile group, an alkyl isobutyrate group, an alkyl carbonate group, a phenyl carbonate group, 2-methylbutanenitrile group, 2,4-dimethylpentanenitrile group, 4-methoxy-2,4-dimethylpentanenitrile group, cyclohexanecarbonitrile group, isobutylimidazine group, 2-isopropyl-4,5-dihydro-1H -Imidazole group, alkyl-1,1-divinylbenzene group, 2,2-diphenylacetonitrile group, naphthalene group, alkyl 2-phenylacetate group, alkylborane group, alkyl (alkoxy) -borane group and the like. Examples of the carbon number and specific examples of the “alkyl” group and “alkoxy” group in the atomic group include those described above.

  Although m will not be specifically limited if it is arbitrary natural numbers, For example, 5-1000, Preferably it is 15-500, More preferably, it is 30-100.

  In the above formula (a), the number average degree of polymerization of the graft chain G is not particularly limited, but may be, for example, 20 or more, preferably 30 or more, more preferably 30 to 60. Thus, by making the graft chain G relatively long, the adhesion force of the binder to the current collector and the binding force of the active material by the graft copolymer can be further improved. Further, the flexibility of the binder can be further improved.

  The polymerization mode of the acrylic polymer of the graft chain represented by the above formula (a) is not particularly limited, and the acrylic polymer is a homopolymer composed of an acrylate monomer, a random copolymer, It can be an alternating copolymer or a block copolymer.

  Moreover, the graft copolymer may contain other repeating units in addition to the repeating unit represented by the above formula (I) and the repeating unit represented by the formula (II). Examples of such a repeating unit include a repeating unit represented by the following formula (III).

In the above formula,
R ⁶ is a hydrogen atom, a fluorine atom, a fluoroalkyl group or a trifluoromethoxy group.

R ⁶ is preferably a hydrogen atom, a fluorine atom, a linear or branched fluoroalkyl group having 1 to 5 carbon atoms or a trifluoromethoxy group, and preferably a hydrogen atom, a fluorine atom or a trifluoromethyl group. More preferably, it is a trifluoromethyl group.

  The graft copolymer which concerns on this embodiment is content of the repeating unit represented by Formula (I), the repeating unit represented by Formula (II), and the repeating unit represented by following formula (III). Is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more based on the graft copolymer.

  Particularly preferably, the graft copolymer according to the present embodiment consists essentially of a repeating unit represented by the formula (I) and a repeating unit represented by the formula (II), or represented by the formula (I). Or a repeating unit represented by the formula (II), or essentially a repeating unit represented by the formula (I), a repeating unit represented by the formula (II) and the following formula (III) Or a repeating unit represented by the formula (I), a repeating unit represented by the formula (II), and a repeating unit represented by the following formula (III).

  The number average degree of polymerization of the main chain comprising the repeating unit represented by the formula (I) and the repeating unit represented by the formula (II) of the graft copolymer is not particularly limited, but is preferably Is 1000 or more, more preferably 1,500 to 15,000, and still more preferably 2,000 to 13,000.

  Further, the molar ratio of the repeating unit represented by the formula (I) and the repeating unit represented by the formula (II) in the graft copolymer is not particularly limited. It can be 90.0: 10.0, preferably 99.5: 0.5 to 95.0: 5.0.

  The polymerization mode of the graft copolymer is not particularly limited, and can be, for example, random copolymerization, alternating copolymerization, or block copolymerization.

  Moreover, the weight average molecular weight of the graft copolymer is not particularly limited, but is preferably 100,000 to 5,000,000, and more preferably 200,000 to 3,000,000.

  The graft copolymer preferably contains 10 to 70% by mass, more preferably 15 to 60% by mass of the graft chain G with respect to the graft copolymer. As a result, the adhesiveness to the current collector caused by the graft chain G and the binding property of the active material are particularly excellent, and the electrochemical stability of the graft copolymer is particularly excellent. it can.

  The graft copolymer preferably does not have a bond between a bromine atom or a chlorine atom and a carbon atom, more preferably a bond between a halogen atom other than a fluorine atom and a carbon atom. Thereby, the electrochemical stability of the graft copolymer can be made particularly excellent.

<2. Method for producing graft copolymer>
Next, the manufacturing method of the graft copolymer mentioned above is demonstrated.
The method for producing a graft copolymer according to the present embodiment uses the following formula (V) for a copolymer containing a repeating unit represented by the following formula (I) and a repeating unit represented by the following formula (IV). And a step of graft-polymerizing the compound represented by (1) by a coupling reaction.

In the above formula,
R ^{1 to} R ⁵ and m are as described above,
A is a hydrogen atom or a monovalent organic or inorganic cation,
X is Cl, Br or I.

Further, the monovalent organic or inorganic cation in A is not particularly limited, and examples thereof include inorganic cations such as Na ⁺ , K ⁺ and Li ⁺ , and organic cations such as ammonium and triethylammonium.

  The copolymer containing the repeating unit represented by the above formula (I) and the repeating unit represented by the above formula (IV) is another repeating unit, for example, a repeating unit represented by the above formula (III). Units may be included.

  The copolymer containing the repeating unit represented by the above formula (I) and the repeating unit represented by the above formula (IV) can be obtained by a known method, for example, a radical polymerization method of the corresponding monomer. . Similarly, the precursor of the compound represented by the formula (V) can be obtained by a known method, for example, a radical polymerization method and an ionic polymerization method of the corresponding monomer. In any method, the copolymer and the compound may be modified as appropriate.

  The solvent in the above step is not particularly limited as long as various substrates and catalysts can be dissolved, and a known solvent can be appropriately used.

  Further, the catalyst for the coupling reaction in the above step is not particularly limited, but an organic amine compound is preferably used. As such an organic amine compound, 1,8-diazabicyclo [5.4.0] -7-undecene (1,8-Diazabicyclo [5.4.0] -7-undecene), 1,5-diazabicyclo [ 4.3.0] -5-nonene (1,5-Diazabiccyclo [4.3.0] -5-nonene), 1,5,7-triazabicyclo [4.4.0] dec-5-ene (1,5,7-Triazabiccyclo [4.4.0] deca-5-ene), triethylamine, diisopropylamine, diisopropylethylamine and the like.

  Further, the amount of catalyst for the coupling reaction in the above step is not particularly limited, but a copolymer containing the repeating unit represented by the above formula (I) and the repeating unit represented by the above formula (IV). Preferably it is 0.5-2.0 mol equivalent with respect to the carboxyl group in Formula (IV) in a inside, More preferably, it is 0.8-1.2 mol equivalent.

  Moreover, it is preferable not to use the catalyst containing a transition metal as a catalyst for the coupling reaction in the said process. Thereby, it is prevented that a transition metal remains in the composition containing the obtained graft copolymer, and the reliability in the long-term use of the secondary battery finally manufactured can be improved.

The reaction time of the coupling reaction in the above step is not particularly limited, but can be, for example, 30 minutes to 48 hours, preferably 1 to 24 hours.
Moreover, the reaction temperature of the coupling reaction in the above step is not particularly limited, but may be, for example, 10 to 80 ° C., preferably 20 to 60 ° C.

  According to the manufacturing method of the graft copolymer which concerns on this embodiment demonstrated above, the graft copolymer which concerns on this embodiment can be manufactured efficiently and with a sufficient yield. In addition, the graft copolymer produced by the method for producing a graft copolymer according to the present embodiment does not include a bond between a halogen atom other than a fluorine atom and a carbon atom. Furthermore, in the method for producing a graft copolymer according to this embodiment, it is not necessary to use a catalyst containing a transition metal, and a graft copolymer composition in which no transition metal remains can be produced. Therefore, the graft copolymer obtained by the method for producing a graft copolymer according to the present embodiment can prevent deterioration of the performance of the secondary battery due to the presence of halogen atoms other than fluorine atoms and transition metals. is there.

  Moreover, according to the method for producing a graft copolymer according to the present embodiment, a graft copolymer having a relatively long graft chain, for example, a graft chain having a number average degree of polymerization of 30 or more can be produced. Therefore, when the method for producing a graft copolymer according to this embodiment is used, the degree of freedom in designing the graft copolymer is high. On the other hand, in the methods described in Patent Documents 2 and 3, it was difficult to introduce such a long graft chain.

<3. Binder composition for secondary battery electrode>
Next, the binder composition for secondary battery electrodes according to this embodiment will be described. The binder composition for secondary battery electrodes according to this embodiment can be used as a binder for electrodes of secondary batteries, for example, lithium ion secondary batteries.

  Moreover, the binder composition for secondary battery electrodes which concerns on this embodiment contains the graft copolymer mentioned above. The graft copolymer described above is excellent in adhesion, binding power and electrical stability. Therefore, the binder composition for secondary battery electrodes according to the present embodiment can be applied to both the positive electrode and the negative electrode. In particular, the binder composition for a secondary battery electrode according to the present embodiment has both high adhesion and high flexibility due to the acrylate graft chain and electrical stability, oxidation resistance and rigidity due to the polyvinylidene fluoride main chain. It is preferably applied to the positive electrode because it is expressed.

  In addition, the binder composition for secondary battery electrodes can also contain other binder materials in addition to the graft copolymer described above. As such a binder material, such a binder material may be, for example, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, or styrene-butadiene rubber. , Acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, nitrocellulose, nitrocellulose, etc. In addition, various other known binders can be used.

  However, the binder composition for a secondary battery electrode preferably contains mainly the above-mentioned graft copolymer, more preferably essentially consists of the above-mentioned graft copolymer, and the above-mentioned graft copolymer. More preferably, it consists of. Thereby, the characteristic of the above-mentioned graft copolymer is effectively exhibited in the secondary battery manufactured.

  More specifically, the binder composition for a secondary battery electrode comprises 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass of the graft copolymer in the binder composition for a secondary battery. More preferably, it is 95% by mass or more.

<4. Secondary battery electrode mixture>
Next, the secondary battery electrode mixture according to this embodiment will be described.
The secondary battery electrode mixture according to the present embodiment includes an active material and the above-described graft copolymer. Hereinafter, the secondary battery electrode mixture will be described separately for the positive electrode mixture and the negative electrode mixture.

(Positive electrode mixture)
The positive electrode mixture according to the present embodiment includes a positive electrode active material and a binder.

The positive electrode active material is, for example, a solid solution oxide containing lithium, but is not particularly limited as long as the material can electrochemically occlude and release lithium ions. The solid solution oxide is, for example, Li _a Mn _x Co _y Ni _z O ₂ (1.150 ≦ a ≦ 1.430, 0.45 ≦ x ≦ 0.6, 0.10 ≦ y ≦ 0.15,. 20 ≦ z ≦ 0.28), LiMn _x Co _y NizO ₂ (0.3 ≦ x ≦ 0.85, 0.10 ≦ y ≦ 0.3, 0.10 ≦ z ≦ 0.3), LiMn1 _{. 5} Ni _0.5 O ₄ .

  Although content of the positive electrode active material in a positive electrode mixture is not specifically limited, For example, 90.0-99.8 mass%, Preferably, it is 92.5-99.5 mass%, More preferably, it is 95.0- It can be 99.0% by weight.

  A binder is the binder composition for two battery electrodes mentioned above containing a graft copolymer. Further, the content of the binder in the positive electrode mixture is not particularly limited, but is, for example, 0.1 to 5.0% by mass, preferably 0.2 to 3.0% by mass, more preferably 0.3 to It can be 1.5% by weight.

  The positive electrode mixture may contain a conductive agent. The conductive agent is, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, or the like, but is not particularly limited as long as it is intended to increase the conductivity of the positive electrode.

Further, the content of the conductive material in the positive electrode mixture is not particularly limited, but is, for example, 0.05 to 10.0 mass%, preferably 0.1 to 6.0 mass%, more preferably 0.2. It can be -3.0 mass%.

(Negative electrode mixture)
The negative electrode mixture according to the present embodiment includes a negative electrode active material and a binder.

Examples of the negative electrode active material include graphite active material (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), fine particles of silicon or tin or oxides thereof and graphite active material. , Silicon or tin fine particles, alloys based on silicon or tin, and titanium oxide compounds such as Li ₄ Ti ₅ O ₁₂ . The oxide of silicon is represented by SiO _x (0 ≦ x ≦ 2). Examples of the negative electrode active material include metallic lithium and the like in addition to these.

  Although content of the negative electrode active material in a negative electrode mixture is not specifically limited, For example, 90.0-99.5 mass%, Preferably, it is 92.5-99.5 mass%, More preferably, it is 95.0- It can be 99.0% by weight.

  A binder is the binder composition for two battery electrodes mentioned above containing a graft copolymer. Further, the content of the binder in the negative electrode mixture is not particularly limited, but is, for example, 0.1 to 5.0% by mass, preferably 0.2 to 3.0% by mass, and more preferably 0.3 to It can be 1.5% by weight.

<5. Secondary Battery and Secondary Battery Electrode>
(Configuration of lithium ion secondary battery)
Next, the configuration of the lithium ion secondary battery 10 according to the present embodiment will be described with reference to FIG. A lithium ion secondary battery 10 shown in FIG. 1 is an example of a secondary battery according to the present invention. Hereinafter, the graft copolymer described above will be described as being used as a binder in the separator 40.

The lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, a separator 40, and a nonaqueous electrolytic solution. The charge ultimate voltage (redox potential) of the lithium ion secondary battery 10 is, for example, 4.3 V (vs. Li ^/ Li ⁺ ) or more and 5.0 V or less, particularly 4.5 V or more and 5.0 V or less. The form of the lithium ion secondary battery 10 is not particularly limited. That is, the lithium ion secondary battery 10 may be any one of a cylindrical shape, a square shape, a laminate shape, a button shape, and the like.

  The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductor as long as it is a conductor, and is made of, for example, aluminum, stainless steel, nickel-coated steel, or the like.

  The positive electrode active material layer 22 is composed of the positive electrode mixture described above. Moreover, when the graft copolymer mentioned above is contained in the negative electrode active material layer 32 mentioned later, the positive electrode active material layer 22 may be comprised with a well-known positive electrode mixture.

  The positive electrode active material layer 22 is produced, for example, by the following manufacturing method. That is, first, a positive electrode mixture is prepared by dry-mixing a positive electrode active material, a conductive agent, and a binder. Next, a positive electrode mixture slurry is prepared by dispersing the positive electrode mixture in an appropriate organic solvent, and this positive electrode mixture slurry is applied onto the current collector 21, dried and rolled to produce a positive electrode active slurry. A material layer is created.

  The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 may be any conductor as long as it is a conductor, for example, aluminum, stainless steel, nickel-plated steel, or the like.

  The negative electrode active material layer 32 is composed of the negative electrode mixture as described above. Moreover, when the positive electrode active material layer 22 contains the graft copolymer mentioned above, you may be comprised with a well-known negative electrode mixture.

  The negative electrode active material layer 32 is produced by the following manufacturing method, for example. That is, first, a negative electrode mixture is prepared by dry-mixing a negative electrode active material and a binder. Next, a negative electrode mixture slurry is prepared by dispersing the negative electrode mixture in an appropriate solvent, and the negative electrode mixture slurry is applied onto the current collector 31, dried and rolled to thereby produce a negative electrode active material. Layer 32 is created.

  The separator 40 includes a base material 40a and a coating layer 40b. The base material 40a is not particularly limited and may be any material as long as it is used as a separator for a lithium ion secondary battery. As the base material 40a, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the resin constituting the substrate 40a include polyolefin resins typified by polyethylene, polypropylene, and the like, polyethylene terephthalate, and polybutylene terephthalate. Polyester resin, PVDF, vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene (Tetrafluorethylen ) Copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride- Ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene-tetrafluoroethylene- (tetrafluoroethylene-tetrafluoroethylene) hexafluoropropylene copolymer, vinylidene fluoride-ethylene (ethy) ene) - can be exemplified tetrafluoroethylene (tetrafluoroethylene) copolymers.

  The coating layer 40b includes inorganic particles and a binder. As an inorganic particle which concerns on this embodiment, the inorganic particle which has high heat resistance is mentioned, for example. Specific examples of such inorganic particles include oxides of silicon, aluminum, magnesium, titanium, and hydroxides thereof. By including the inorganic particles having high heat resistance in the separator 40, the cycle characteristics of the lithium ion secondary battery 10 are improved. The particle size of the inorganic particles is not particularly limited, and is not particularly limited as long as it is a particle size of the inorganic particles used in the lithium ion secondary battery 10.

  The binder holds inorganic particles in the coating layer 40b, that is, in the separator 40. As such a binder, the binder used for the positive electrode active material layer 22 and various well-known binders are mentioned.

  The separator 40 is produced by the following manufacturing method, for example. That is, first, an inorganic particle dispersion and a binder solution are prepared. The solvent of the inorganic particle dispersion is not particularly limited as long as it can disperse the inorganic particles. The solvent of the inorganic particle dispersion is preferably the same as the solvent of the binder solution. The solvent of the binder solution is not particularly limited as long as the binder can be dissolved. Examples of such a solvent include acetone and N-methylpyrrolidone (NMP). And a slurry is produced by mixing an inorganic particle dispersion liquid and a binder solution. The slurry may optionally add the same solvent as the binder solution to adjust the concentration of the inorganic particles and the binder. Moreover, you may further add another kind of binder to a slurry. And the coating layer 40b is produced by developing (for example, coating) slurry on the base material 40a and drying. The substrate 40a may be immersed in the slurry. The separator 40 is produced by these steps. In FIG. 1, the coating layer 40b is formed only on the surface of the substrate 40a, but the coating layer 40b may also be formed in the pores of the substrate 40a.

  As the non-aqueous electrolyte, the same non-aqueous electrolyte as conventionally used for lithium secondary batteries can be used without any particular limitation. The nonaqueous electrolytic solution has a composition in which an electrolyte salt is contained in a nonaqueous solvent. Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, vinylene carbonate (vinyl carbonate), and the like. ); Cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate chain carbonates such as ethyl carbonate; chain esters such as methyl formate, methyl acetate, butyric acid methyl; tetrahydrofuran (tetrahydrofuran) or derivatives thereof; 1,3- Ethers such as dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl diglyme; Nitriles such as acetonitrile and benzonitrile e) class; dioxolane or a derivative thereof; ethylene sulfide, sulfolane, sultone, or a derivative thereof alone or a mixture of two or more thereof, etc. It is not limited to.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiPF _6-x (CnF _{2n + 1} ) _x [where 1 <x <6, n = 1 or 2], LiSCN, LiBr, Inorganic ions including one kind of lithium (Li), sodium (Na) or potassium (K) such as LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN Salt, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (nC ₄ H ₉ ) ₄ NClO ₄ , (n-C ₄ H ₉ ) ₄ NI, (C ₂ H ₅ ) ₄ N— maleate, (C ₂ H ₅ ) ₄ N-benzoate, (C ₂ H ₅ ) ₄ N-phthalate, lithium stearyl sulfonate, octyl sulphonic acid ), Organic ionic salts such as lithium dodecylbenzenesulfonate (dodecyl benzene sulphonic acid), and the like. These ionic compounds can be used alone or in admixture of two or more. The concentration of the electrolyte salt may be the same as that of the nonaqueous electrolytic solution used in the conventional lithium secondary battery, and is not particularly limited. In this embodiment, a nonaqueous electrolytic solution containing an appropriate lithium compound (electrolyte salt) at a concentration of about 0.8 to 1.5 mol / L can be used.

  Various additives may be added to the nonaqueous electrolytic solution. Examples of such additives include a negative electrode action additive, a positive electrode action additive, an ester additive, a carbonate ester additive, a sulfate ester additive, a phosphate ester additive, and a borate ester additive. Additive, acid anhydride additive, electrolyte additive and the like. Any one of these may be added to the non-aqueous electrolyte, or a plurality of types of additives may be added to the non-aqueous electrolyte.

(Method for producing lithium ion secondary battery)
Next, a method for manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is produced as follows. First, a slurry is prepared by dispersing a mixture of a positive electrode active material, a conductive agent, and a binder in a solvent (for example, N-methyl-2-pyrrolidone). Next, the positive electrode active material layer 22 is produced by spreading (for example, coating) the slurry on the current collector 21 and drying the slurry. The coating method is not particularly limited. Examples of the coating method include a knife coater method and a gravure coater method. The following coating steps are also performed by the same method. Next, the positive electrode active material layer 22 is pressed by a press machine. Thereby, the positive electrode 20 is produced.

  The negative electrode 30 is also produced in the same manner as the positive electrode 20. First, a slurry is prepared by dispersing a mixture of a negative electrode active material and a binder in a solvent (eg, N-methyl-2-pyrrolidone, water). Next, the negative electrode active material layer 32 is produced by spreading (for example, coating) the slurry on the current collector 31 and drying the slurry. Next, the negative electrode active material layer 32 is pressed by a press. Thereby, the negative electrode 30 is produced.

  The separator 40 is produced by the following manufacturing method, for example. That is, first, an inorganic particle dispersion and a binder solution are prepared. And a slurry is produced by mixing an inorganic particle dispersion liquid and a binder solution. The slurry may optionally add the same solvent as the binder solution to adjust the concentration of the inorganic particles and the binder. And the coating layer 40b is produced by developing (for example, coating) slurry on the base material 40a and drying. The substrate 40a may be immersed in the slurry. The separator 40 is produced by these steps.

  Next, an electrode structure is produced by sandwiching the separator 40 between the positive electrode 20 and the negative electrode 30. Next, the electrode structure is processed into a desired shape (for example, a cylindrical shape, a square shape, a laminate shape, a button shape, etc.) and inserted into a container of the shape. Next, by injecting the electrolytic solution having the above composition into the container, each pore in the separator is impregnated with the electrolytic solution. Thereby, a lithium ion secondary battery is produced.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples.

<1. Binder resin synthesis>
[Synthesis Example 1 (Example 1)]
(Synthesis of acrylic acid-modified polyvinylidene fluoride (PVDF) homopolymer)
Into a 2 L reaction vessel, 1 kg of pure water and 0.35 g of carboxymethylcellulose: 0.35 g were put, and nitrogen substitution was performed. Next, after adding 0.5 g of acrylic acid and 500 g of vinylidene fluoride (VDF) monomer, a 75 mass% isododecane solution of t-amyl perpivalate was introduced, and 1.7 g was introduced. The temperature was raised to 50 ° C. While continuously supplying a 2% aqueous solution of acrylic acid, stirring was continued for 12 hours with the system pressure kept constant. After stopping the heating and releasing the pressure until reaching atmospheric pressure, the reaction product was washed with water and dried to obtain an acrylic acid-modified PVDF homopolymer.

The product had the following composition, weight average molecular weight and melting point.
The composition of the product was ¹ H nuclear magnetic resonance ( ¹ H NMR), the weight average molecular weight was measured by gel permeation chromatography (GPC), and the melting point was differential scanning calorimetry (DSC). To obtain. The same applies to other examples and comparative examples.
Composition: VDF / acrylic acid = 98.8 / 1.2 (mol%)
Weight average molecular weight: 1,000,000
Melting point: 165 ° C

(Synthesis of polybutyl acrylate 1)
In a reaction vessel, n-butyl acrylate: 150 g, tetrabutylammonium iodide: 10.8 g and 2-iodo-2-methylpropanenitrile (2-Iodo-2-) (methylpropionitile): 5.7 g was added and stirred, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After reacting for 24 hours, the reaction solution was cooled to room temperature, and 150 g of hexane was added to the reaction solution. After extracting the product, the solvent was distilled off under reduced pressure to obtain 1: 130 g of polybutyl acrylate having a C-I bond at the terminal end (number average molecular weight = 4024, molecular weight distribution = 1). .27).

(Synthesis of graft copolymer A)
Acrylic acid-modified PVDF homopolymer synthesized by the above method: 90 g, polybutyl acrylate 1:82 g and N-methyl-2-pyrrolidone: 1410 g were put in a reaction vessel, and stirred at 60 ° C. After the solid of the polymer was dissolved to form a uniform solution, the inside of the reaction vessel was purged with nitrogen, and 1,8-diazabicyclo [5.4.0] -7-undecene: 3.3 g was added to react. Was started. After reacting for 12 hours, it was cooled to room temperature.

  When a part (1 cc) of the obtained reaction solution was diluted and GPC measurement was performed, no peak was observed other than the main peak seen in the region having a weight average molecular weight of 500,000 or more. All the polybutyl acrylate used for the synthesis was reacted with the acrylic acid-modified PVDF homopolymer, and it was confirmed that no single chain of polybutyl acrylate remained in the product.

The solid produced by dropping the reaction solution into ethanol was dried under vacuum to obtain 140 g of graft copolymer A.
In ¹ H NMR measurement of the acetone-d ₆ solution of the product, a peak of σ = 12.5 ppm derived from the carboxy group proton of the acrylic acid-modified PVDF homopolymer of the raw material disappeared, and σ = 4 The synthesis of the graft copolymer was confirmed because a peak derived from the proton adjacent to the ester group generated by the coupling reaction between PVDF acrylic acid and polybutyl acrylate termination (CI bond) was observed at 8 ppm. did. Further, the graft copolymer A had the following composition.
Composition: polyvinylidene fluoride / polybutyl acrylate = 62/38 (mass%) ( ¹ H NMR)
Melting point: 160 ° C. (DSC)

  FIG. 2 shows DSC curves of the raw material acrylic acid-modified PVDF homopolymer and the coupling product graft copolymer A. In FIG. 2, the DSC curve of the graft copolymer after coupling as in the raw material PVDF homopolymer shows a peak due to crystallization and crystal melting, and it can be seen that the crystallinity is retained after grafting. . In addition, the temperature increase curve of the graft copolymer A shows a clear glass transition near −35 ° C. compared to that of the raw PVDF homopolymer, and the polybutyl acrylate chain is grafted. Show.

[Synthesis Example 2 (Example 2)]
(Synthesis of polybutyl acrylate 2)
A reaction vessel was charged with n-butyl acrylate: 150 g, tetrabutylammonium iodide: 21.6 g and 2-iodo-2-methylpropanenitrile: 11.4 g, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After making it react for 24 hours, it cooled to room temperature and added hexane: 150g to the reaction liquid. After extracting the product, the solvent was distilled off under reduced pressure to obtain 2: 125 g of polybutyl acrylate having a C—I bond at the terminal end (number average molecular weight = 2008, molecular weight distribution = 1.40).

(Synthesis of graft polymer B)
The procedure was the same as the graft copolymer synthesis of Example 1 except that polybutyl acrylate 2 synthesized by the above method was used instead of polybutyl acrylate 1.
The obtained graft copolymer B had the following composition.
Composition: polyvinylidene fluoride / polybutyl acrylate = 81/19 (mass%) (calculated from ¹ H NMR)
Melting point: 162 ° C

[Synthesis Example 3 (Example 3)]
(Synthesis of polybutyl acrylate 3)
A reaction vessel was charged with n-butyl acrylate: 80 g, tetrabutylammonium iodide: 21.6 g and 2-iodo-2-methylpropanenitrile: 11.4 g, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After making it react for 24 hours, it cooled to room temperature and added hexane: 150g to the reaction liquid. After extracting the product, the solvent was distilled off under reduced pressure to obtain 3:70 g of polybutyl acrylate (number average molecular weight = 1200, molecular weight distribution = 1.45).

(Synthesis of graft copolymer C)
The procedure was the same as the graft copolymer synthesis of Example 1 except that polybutyl acrylate 3 synthesized by the above method was used instead of polybutyl acrylate 1.
The obtained graft copolymer C had the following composition.
Composition: polyvinylidene fluoride / polybutyl acrylate = 90/10 (mass%) (calculated from ¹ H NMR)
Melting point: 163 ° C

[Synthesis Example 4 (Example 4)]
(Synthesis of polybutyl acrylate 4)
A reaction vessel was charged with n-butyl acrylate: 150 g, tetrabutylammonium iodide: 8.1 g, and 2-iodo-2-methylpropanenitrile: 2.85 g, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After making it react for 24 hours, it cooled to room temperature and added hexane: 150g to the reaction liquid. After extracting the product, the solvent was distilled off under reduced pressure to obtain 4: 125 g of polybutyl acrylate (number average molecular weight = 8500, molecular weight distribution = 1.20).

(Synthesis of graft copolymer D)
The procedure was the same as the graft copolymer synthesis of Example 1 except that polybutyl acrylate 3 synthesized by the above method was used instead of polybutyl acrylate 1.
The obtained graft copolymer D had the following composition.
Composition: polyvinylidene fluoride / polybutyl acrylate = 48/52 (% by weight) (calculated from ¹ H NMR)
Melting point: 155 ° C

[Synthesis Example 5 (Example 5)]
(Synthesis of acrylic acid-modified PVDF copolymer)
Pure water: 1 kg and carboxymethyl cellulose: 0.35 g were put into a 2 L reaction vessel, and nitrogen substitution was performed. Next, 0.5 g of acrylic acid, 485 g of VDF monomer and 15 g of hexafluoropropene (HFP) monomer were added, followed by a 75 wt% isododecane solution of t-amyl perpivalate: 7 g was introduced, and the temperature inside the system was raised to 50 ° C. Stirring was continued until the system pressure decreased by 10 bar from the initial pressure after the internal temperature reached 50 ° C., then the heating was stopped and the pressure was released until the atmospheric pressure was reached. The reaction product was washed with water and dried to obtain an acrylic acid-modified PVDF copolymer.
The product had the following composition and molecular weight.
Composition: VDF / HFP / acrylic acid = 96 / 2.8 / 1.2 (mol%)
Molecular weight: 1,000,000 (weight average)
Melting point: 150 ° C

(Synthesis of graft copolymer E)
The same procedure as in the graft copolymer synthesis of Example 1 was performed except that the acrylic acid-modified PVDF copolymer synthesized by the above method was used instead of the acrylic acid-modified PVDF homopolymer.
The obtained graft copolymer E had the following composition.
Composition: polyvinylidene fluoride-hexafluoropropylene / acrylic acid = 64/36 (mass%) (calculated from ¹ H NMR)
Melting point: 145 ° C

[Synthesis Example 6 (Example 6)]
(Synthesis of polyethyl acrylate)
In a reaction vessel, ethyl acrylate: 120 g, tetrabutylammonium iodide: 10.8 g, and 2-iodo-2-methylpropanenitrile: 5.7 g were added and stirred, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After making it react for 24 hours, it cooled to room temperature and 150 g of hexane was added to the reaction liquid. After extracting the product, the solvent was distilled off under reduced pressure to obtain 110 g of polyethyl acrylate (number average molecular weight = 5,000, molecular weight distribution = 1.30).

(Synthesis of graft copolymer F)
The procedure was the same as the graft copolymer synthesis of Example 1 except that polyethyl acrylate synthesized by the above method was used instead of polybutyl acrylate 1.
The obtained graft copolymer F had the following composition.
Composition: polyvinylidene fluoride / polyethyl acrylate = 72/28 (mass%) (calculated from ¹ H NMR)
Melting point: 162 ° C (calculated from DSC)

[Synthesis Example 7 (Example 7)]
(Synthesis of polybutyl acrylate 5)
A reaction vessel was charged with n-butyl acrylate: 330 g, tetrabutylammonium iodide: 8.1 g, and 2-iodo-2-methylpropanenitrile: 2.85 g, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 110 ° C. to initiate the reaction. After making it react for 36 hours, it cooled to room temperature and added hexane: 300g to the reaction liquid. After the product was extracted, the solvent was distilled off under reduced pressure to obtain 4: 210 g of polybutyl acrylate (number average molecular weight = 25,000, molecular weight distribution = 1.20).

(Synthesis of graft copolymer G)
The procedure was the same as that of the graft copolymer synthesis of Example 1 except that polybutyl acrylate 5 synthesized by the above method was used instead of polybutyl acrylate 1.
The obtained graft copolymer G had the following composition.
Composition: polyvinylidene fluoride / polybutyl acrylate = 30/70 (mass%) (calculated from ¹ H NMR)
Melting point: 142 ° C

[Synthesis Example 8 (Comparative Example 1)]
(Synthesis of graft copolymer H)
In a reaction vessel, an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride-co-chlorotrifluoroethylene (weight average molecular weight = 60,0000): 100 g (polymer solid content: 10 g), n-butyl acrylate: 50 g, bipyridine 30 g, Cu (I) Cl: 6.33 g and Cu (II) Cl ₂ : 0.86 g were added and stirred, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 140 ° C. to initiate the reaction. After reacting for 24 hours, it was cooled to room temperature.

  When a part (1 cc) of the obtained reaction solution was diluted and GPC measurement was performed, in addition to the main peak seen in the region with a weight average molecular weight of 500,000 or more, the peak was also found in the region with a weight average molecular weight of 100,000 or less. Was observed. Therefore, it was confirmed that the polybutyl acrylate single chain was also by-produced separately from the target graft copolymer by the above reaction.

  The solid produced by adding the reaction solution dropwise to ethanol was dried under vacuum to obtain 105 g of graft copolymer H (polyvinylidene fluoride-co-chlorotrifluoroethylene / polybutyl acrylate = 95/5 (mass). %ratio)).

[Synthesis Example 9 (Comparative Example 2)]
(Synthesis of graft copolymer I)
In a reaction vessel, N-methyl-2-pyrrolidone solution of polyvinylidene fluoride-co-chlorotrifluoroethylene: 100 g (polymer solid content 10 g), n-butyl acrylate: 100 g, N, N, N ′, N ′, N ″ -pentamethyldiethylenetriamine: 40 g, Cu (I) Cl: 9.50 g and Cu (II) Cl ₂ : 1.30 g were added and stirred, and the reaction vessel was sealed with a rubber septum. After the inside of the reaction vessel was purged with nitrogen, the temperature was raised to 140 ° C. to initiate the reaction. After reacting for 24 hours, it was cooled to room temperature. The solid produced by adding the reaction solution dropwise to ethanol was dried under vacuum to obtain 109 g of graft copolymer I (polyvinylidene fluoride-co-chlorotrifluoroethylene / polybutyl acrylate = 90/10 (mass). %ratio)).

<3. Production of lithium ion secondary battery>
(Preparation of positive electrode)
LiCo ₂ O ₃ (surface Al ₂ O ₃ treated product): 98% by mass, acetylene black: 1% by mass, in Examples 1 to 7 and Comparative Examples 1 and 2 as binders (binder compositions for secondary battery electrodes) 5% by mass N-methyl-2-pyrrolidone solution of the prepared graft copolymer or resin composition: 20% by mass and N-methyl-2-pyrrolidone for viscosity adjustment dilution were mixed with a planetary mixer, An agent slurry was prepared. Next, the slurry was applied and dried on the aluminum current collector foil so that the coating amount (area density) after drying was 25.0 mg / cm ² , and then the mixture layer density was 4. The positive electrode was produced by pressing to 0 g / cc.

Moreover, the acrylic acid modified polyvinylidene fluoride homopolymer (Comparative Example 3) obtained in Synthesis Example 1 was used as a binder to obtain a positive electrode in the same manner as described above.
(Anode production)
Artificial graphite: 96% by mass, acetylene black: 2% by mass, styrene butadiene copolymer (SBR) binder: 1% by mass, carboxymethyl cellulose (CMC): 1% by mass are mixed with a planetary mixer for further viscosity adjustment. Water was added to the negative electrode mixture slurry. Next, the slurry was coated and dried on the copper foil so that the mixture application amount (area density) after drying was 9.55 mg / cm ² , and then the mixture layer density was 1.65 g / min with a roll press. It pressed so that it might become cc, and produced the negative electrode.

(Cell production)
As a positive electrode cut into a circle with a diameter of 1.3 cm, a coating separator cut into a circle with a diameter of 1.8 cm, a negative electrode cut into a circle with a diameter of 1.55 cm, and a spacer in a stainless steel coin outer container with a diameter of 2.0 cm A copper foil having a thickness of 200 μm cut into a circle having a diameter of 1.5 cm was superposed in this order. Next, 150 μL of an electrolytic solution (1.4 M LiPF ₆ ethylene carbonate / diethyl carbonate / fluoroethylene carbonate = 10/70/20 mixed solution (volume ratio)) was added to the container. Subsequently, a stainless steel cap was put on the container through a polypropylene packing, and the container was sealed with a caulking device for producing a coin battery. This produced the lithium ion secondary battery.

<Adhesion evaluation method>
A 1.5 cm wide adhesive tape (cello tape No. 405 manufactured by Nichiban Co., Ltd.) is attached to a coating separator fixed on a stainless steel plate, and 180 ° peeling is performed using a peeling tester (SHIMAZU EZ-S manufactured by Shimadzu Corporation). Peel strength was measured. The evaluation results are summarized in Table 1.
<Flexibility evaluation method>
The produced positive electrode plate was cut into a 2 cm × 10 cm strip. A cylinder having a diameter of 0.5 cm was arranged at a position 5 cm from the end of the cut strip. While winding the strip around the cylinder, the both ends of the strip were gripped and the strip was bent 180 °. The folded strip was returned to its original position, and it was visually confirmed whether or not a crack occurred at the position where the cylinder was wound (position 5 cm from the end of the strip). A case where no cracks occurred, A a portion where cracks occurred, and B a portion where cracks occurred over the entire 2 cm width of the strip.

<Evaluation of cycle life>
The lithium ion secondary battery produced by the above method was charged and discharged five times at 0.2 C at 25 ° C. and a final charge voltage of 4.4 V, and then a 1.0 C charging / discharging cycle was repeated 200 times. The discharge capacity retention rate (percentage) was calculated by dividing the discharge capacity after 200 cycles at 1.0 C by the discharge capacity after 1 cycle at 1.0 C. The larger the capacity retention rate, the better the cycle life. The evaluation results are summarized in Table 1.

<Evaluation of preservation test>
The lithium ion secondary battery produced by the above method was charged and discharged five times at 0.2 C at 25 ° C. and an end-of-charge voltage of 4.4 V. The battery was charged to 4.4 V at 0.5 C and stored at 60 ° C. for 2 weeks. After allowing to cool to room temperature, discharge to 3V at 0.5C, and charge capacity when charged at 0.5C to 4.4V by 0.5C charge capacity before storage at 60 ° C. Percentage) was calculated. The larger the capacity maintenance rate, the better the high voltage tolerance. The evaluation results are summarized in Table 1.

  When the graft copolymers of Examples 1 to 7 are used as the positive electrode binder, compared to the case of using the binders of Comparative Examples 1 to 3, the adhesion to the base material, the flexibility of the electrodes, and the high voltage The battery life was excellent. On the other hand, those using the graft copolymers of Comparative Example 1 and Comparative Example 2 as the binder were inferior in adhesion and flexibility as compared to this example, and in particular, the cycle characteristics and storage characteristics of the battery were greatly reduced. It was. The graft polymer listed in this example has a higher degree of polymerization (and chain length) of the acrylic polymer of the graft chain than the graft copolymers of Comparative Example 1 and Comparative Example 2, and the ratio of the acrylic graft chain Therefore, it is considered that high adhesion and high flexibility were expressed. Furthermore, the graft copolymers of the present invention listed in Examples 1 to 6 are polymers like the graft copolymers synthesized by atom transfer radical polymerization from the PVDF-co-CTFE main chain skeleton of Comparative Examples 1 and 2. Since it does not contain a C—Br bond and does not contain copper catalyst impurities, it has high electrochemical stability, suggesting that decomposition under high voltage is suppressed.

  As described above, by using the binder composition for a secondary battery electrode according to the present invention, it is possible to achieve both high adhesion and flexibility of the electrode, and to improve the battery life under high voltage. . Thereby, the energy density of the electrode can be improved.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

  For example, in the above embodiment, the binder composition for a secondary battery electrode according to this embodiment is used for a lithium ion secondary battery, but the binder for a secondary battery electrode according to this embodiment is used for another type of secondary battery. A composition may be used.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 20 Positive electrode 30 Negative electrode 40 Separator 40a Base material 40b Coating layer

Claims (9)

The binder composition for secondary battery electrodes containing the graft copolymer in which the main chain contains the repeating unit represented by the following formula (I) and the repeating unit represented by the following formula (II).

Where
G is a graft chain represented by the following formula (a),

R ¹ and R ³ are each independently a hydrogen atom or a linear or branched alkyl group or fluoroalkyl group having 1 to 20 carbon atoms,
R ² and R ⁴ are a hydrogen atom, a linear or branched alkyl group having 1 to 20 carbon atoms, a hydroxyalkyl group, a fluoroalkyl group or an epoxyalkyl group, or a polyalkylene oxide chain,
R ⁵ is an end group of the graft chain,
m is an arbitrary natural number. The binder composition for secondary battery electrodes according to claim 1, wherein the graft copolymer further includes a repeating unit represented by the following (III).

In the above formula,
R ⁶ is a hydrogen atom, a fluorine atom, a fluoroalkyl group or a trifluoromethoxy group.   The binder composition for secondary battery electrodes according to claim 1 or 2, wherein the graft copolymer includes 10 to 70 mass% of the graft chain with respect to the graft copolymer.   The binder composition for secondary battery electrodes according to any one of claims 1 to 3, wherein the number average degree of polymerization of the repeating units in the graft chain is 30 or more.   The number average degree of polymerization of the main chain of the graft copolymer including the repeating unit represented by the formula (I) and the repeating unit represented by the formula (II) is 1000 or more. The binder composition for secondary battery electrodes in any one of Claims 1-4.   The said graft copolymer is a binder composition for secondary battery electrodes as described in any one of Claims 1-5 which does not have the coupling | bonding of a bromine atom or a chlorine atom, and a carbon atom.   The secondary battery electrode mixture containing the binder composition for secondary battery electrodes as described in any one of Claims 1-6.   The secondary battery electrode containing the binder composition for secondary battery electrodes as described in any one of Claims 1-6.   A secondary battery comprising the secondary battery electrode according to claim 8.

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