JP5810032B2 - Positive electrode protective agent for lithium ion secondary battery, positive electrode material for lithium ion secondary battery, non-aqueous electrolyte for lithium ion secondary battery, lithium ion secondary battery, and production method thereof - Google Patents

Description

  The present invention relates to a positive electrode protective agent for a lithium ion secondary battery, a positive electrode material for a lithium ion secondary battery, a non-aqueous electrolyte for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing them.

  In recent years, development of lithium ion secondary batteries has been actively promoted. Patent Document 1 discloses an overcharge inhibitor using an overcharge inhibitor that uses a polymer having a polymerizable monomer having a functional group that undergoes electropolymerization at least at a potential of 4.3 to 5.5 V based on a lithium metal standard as a repeating unit. A technique for increasing the internal resistance of a battery at the same time as reacting with a positive electrode having a high potential to undergo electropolymerization is disclosed. Patent Document 2 discloses an electrode protective film containing a polymer having a structural unit (I), or an arm part containing a polymer having a structural unit (I), and a core part containing an organic group having a branched chain. A technique for providing a more practical electrode protective film having an excellent ability to protect an electrode is disclosed by an electrode protective film containing a star polymer having the above.

JP2011-159550A JP 2010-198888 A

  On the other hand, the battery to which the overcharge inhibitor described in Patent Document 1 is added has a problem that gas is generated inside the battery and the battery can swells when subjected to a high temperature storage test. These causes are considered to be caused by decomposition of a part of the electrolytic solution and the overcharge inhibitor.

  Moreover, the electrode protective agent described in Patent Document 2 has little action of suppressing the decomposition of the electrolytic solution, and it is difficult to solve problems such as high-temperature storage characteristics.

  In order to solve these problems, it is necessary to form a film having high durability and high ion conductivity at the electrode interface. Therefore, the present invention has been made to develop a positive electrode protective agent for a lithium ion secondary battery that reacts within the operating potential of the positive electrode, forms a highly durable film on the electrode, and improves cycle characteristics.

  The features of the present invention for solving the above-described problems are as follows.

  The positive electrode protective agent for lithium ion secondary batteries containing the copolymer which has a repeating unit represented by the following (Formula 1) and a repeating unit represented by the following (Formula 2).

(In (Formula 1), Z ₁ is a residue of a polymerizable functional group. R ₁ contains an aromatic functional group, and the potential based on the lithium metal is within the operating potential of the lithium ion secondary battery. _The copolymer is electropolymerized by 1. n ₁ is an integer equal to or greater than 1. In (Formula 2), Z ₂ is a residue of a polymerizable functional group, which reacts with water to form a metal ion and a complex. (The dehydrating functional group that forms the complex-forming functional group to be formed is formed in the main chain of Z _2. n ₂ is an integer of 1 or more.)

  In the above, the copolymer is a positive electrode protective agent for a lithium ion secondary battery having a repeating unit represented by the following (formula 3).

(In (Formula 3), Z ₃ is a residue of a polymerizable functional group. R ₃ is a highly polar highly polar organic group. N ₃ is an integer of 1 or more.)

In the complexation functional group, -OR, -SR, -COOR, or -SO ₃ R (R is, H, alkali metal, alkaline earth metal or an alkyl group.) Is either 1 or more Positive electrode protective agent for lithium ion secondary batteries.

In the above, Z ₂ is formed on the main chain in the repeating unit formed by polymerizing the functional group formed on the main chain in the repeating unit formed by polymerizing the maleic anhydride derivative or the maleimide derivative. The positive electrode protective agent for lithium ion secondary batteries which is a functional group.

In the above, R ₁ is a positive electrode protective agent for a lithium ion secondary battery, which is any one or more of a naphthyl group, an anthracene group, and a tetracene group.

  In the above, the positive electrode protective agent for lithium ion secondary batteries whose copolymer is (Formula 4) or (Formula 5).

(In (Formula 4) and (Formula 5), R _{1 to} R ₉ are H or an alkyl group. R ₆ and R ₇ may or may not be present. Is directly connected to the main chain of the copolymer, where x, y, and z are functional group composition ratios, and x + y + z = 1, 0 <x <1, 0 <y <1, and 0 <z <1. , N ₃ is 0, z = 0 and x + y = 1, E is a substituent of an aromatic functional group, n is the number of substituents, and C is a highly polar high Polar organic group.)

  The positive electrode protective agent for lithium ion secondary batteries whose copolymer is (Formula 6) or (Formula 7) in the above.

(In (Formula 6) and (Formula 7), R _{1 to} R ₁₀ are H or an alkyl group. R ₆ and R ₇ may or may not be present. .x directly linked to the main chain of the copolymer, y, z is a composition ratio of functional groups, x + y + z = 1,0 <x <1,0 <y <1,0 <z <1 and is .n ₃ When 0 is 0, z = 0 and x + y = 1, E is a substituent of an aromatic functional group, n is the number of substituents, and C is a highly polar organic group having high polarity. .)

  The positive electrode material for lithium ion secondary batteries which has a positive electrode active material and said positive electrode protective agent for lithium ion secondary batteries adhering to a positive electrode active material.

  The electrolyte solution for lithium ion secondary batteries containing said positive electrode protective agent for lithium ion secondary batteries.

  In the above, the copolymer is an electrolyte solution for a lithium ion secondary battery that is contained in an amount of 0.01 wt% to 5 wt% with respect to the electrolyte solution for a lithium ion secondary battery.

  The lithium ion secondary battery containing said electrolyte solution for lithium ion secondary batteries, a positive electrode, and a negative electrode.

  The manufacturing method of the positive electrode protective agent for lithium ion secondary batteries containing the copolymer which has a repeating unit represented by the following (Formula 1), and a repeating unit represented by the following (Formula 2).

(In (Formula 1), Z ₁ is a residue of a polymerizable functional group. R ₁ contains an aromatic functional group, and the potential based on the lithium metal is within the operating potential of the lithium ion secondary battery. _The copolymer is electropolymerized by 1. n ₁ is an integer equal to or greater than 1. In (Formula 2), Z ₂ is a residue of a polymerizable functional group, which reacts with water to form a metal ion and a complex. (The dehydrating functional group that forms the complex-forming functional group to be formed is formed in the main chain of Z _2. n ₂ is an integer of 1 or more.)

  According to the present invention, the high-temperature storage characteristics of the battery can be improved. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

The figure which represents typically the internal structure of the battery which concerns on one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

  In the specification, numerical ranges indicated using “to” indicate ranges including numerical values described before and after “to” as the minimum value and the maximum value, respectively.

  FIG. 1 is a diagram schematically showing the internal structure of a battery according to an embodiment of the present invention. A battery 1 according to an embodiment of the present invention shown in FIG. 1 includes a positive electrode 10, a separator 11, a negative electrode 12, that is, a battery can 13, a positive electrode tab 14, a negative electrode tab 15, an inner lid 16, an internal pressure release valve 17, a gasket 18, It is composed of a positive temperature coefficient (PTC) PTC element 19, a battery lid 20, and an axis 21. The battery lid 20 is an integrated part composed of the inner lid 16, the internal pressure release valve 17, the gasket 18, and the PTC element 19. A positive electrode 10, a separator 11, and a negative electrode 12 are wound around the shaft center 21.

  The separator 11 is inserted between the positive electrode 10 and the negative electrode 12 to produce an electrode group wound around the axis 21. As the axis 21, any known one can be used as long as it can support the positive electrode 10, the separator 11, and the negative electrode 12. In addition to the cylindrical shape shown in FIG. 1, the electrode group has various shapes such as a laminate of strip electrodes, or a positive electrode 10 and a negative electrode 12 wound in an arbitrary shape such as a flat shape. Can do. The shape of the battery can 13 may be selected from shapes such as a cylindrical shape, a flat oval shape, a flat oval shape, and a square shape according to the shape of the electrode group.

  The material of the battery can 13 is selected from materials that are corrosion resistant to the non-aqueous electrolyte, such as aluminum, stainless steel, and nickel-plated steel. Further, when the battery can 13 is electrically connected to the positive electrode 10 or the negative electrode 12, the material is not deteriorated due to corrosion of the battery can 13 or alloying with lithium ions in the portion in contact with the nonaqueous electrolyte. Thus, the material of the battery can 13 is selected.

  The electrode group is housed in the battery can 13, the negative electrode tab 15 is connected to the inner wall of the battery can 13, and the positive electrode tab 14 is connected to the bottom surface of the battery lid 20. The electrolyte is injected into the battery can 13 before the battery is sealed. As a method for injecting the electrolyte, there are a method of adding directly to the electrode group in a state where the battery cover 20 is released, or a method of adding from an injection port installed in the battery cover 20.

  Thereafter, the battery lid 20 is brought into close contact with the battery can 13 to seal the entire battery. If there is an electrolyte inlet, seal it as well. As a method for sealing the battery, there are known techniques such as welding and caulking.

  The lithium ion battery according to an embodiment of the present invention can be manufactured, for example, by disposing the following negative electrode and positive electrode facing each other via a separator and injecting an electrolyte. The structure of the lithium ion battery according to an embodiment of the present invention is not particularly limited. Usually, the positive electrode and the negative electrode and the separator separating them are wound into a wound electrode group, or the positive electrode, the negative electrode, and the separator are combined. A stacked electrode group can be formed by stacking.

<Positive electrode>
The positive electrode 10 includes a positive electrode active material, a conductive agent, a binder, and a current collector. Illustrative examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2−x M _x O ₂ (however, selected from the group consisting of M = Co, Ni, Fe, Cr, Zn, Ti) At least one selected from the group consisting of x = 0.01-0.2), Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Ni, Cu, Zn), Li _{1− x} A _x Mn ₂ O ₄ (where A = Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, at least one selected from the group consisting of x, 0.01 to 0.1), LiNi _1-x M _x O ₂ (however, at least one selected from the group consisting of M = Co, Fe, and Ga, x = 0.01 to 0.2), LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , _{LiCo 1-x M x O 2} ( provided that at least one selected from the group consisting of M = Ni, Fe, Mn _{x = 0.01~0.2), LiNi 1-} x M x O 2 ( however, M = Mn, Fe, Co , Al, Ga, Ca, at least one selected from the group consisting of Mg, x = 0 0.01-0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO _{4 and the} like.

  The particle size of the positive electrode active material is usually defined so as to be equal to or less than the thickness of the mixture layer formed from the positive electrode active material, the conductive agent, and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the positive electrode active material powder, the coarse particles can be removed in advance by sieving classification or wind classification to produce particles having a thickness of the mixture layer thickness or less. preferable.

  In addition, since the positive electrode active material is generally oxide-based and has high electrical resistance, a conductive agent made of carbon powder for supplementing electrical conductivity is used. Since both the positive electrode active material and the conductive agent are usually powders, a binder can be mixed with the powders, and the powders can be bonded together and simultaneously bonded to the current collector.

  For the current collector of the positive electrode 10, an aluminum foil having a thickness of 10 to 100 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like is used. . In addition to aluminum, materials such as stainless steel and titanium are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A positive electrode slurry in which a positive electrode active material, a conductive agent, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, or a spray method, and then the organic solvent is dried and applied by a roll press. The positive electrode 10 can be produced by pressure molding. In addition, a plurality of mixture layers can be laminated on the current collector by performing a plurality of times from application to drying.

<Negative electrode>
The negative electrode 12 includes a negative electrode active material, a binder, and a current collector. When high rate charge / discharge is required, a conductive agent may be added. Examples of the negative electrode active material that can be used in the present invention include graphite and non-graphitic carbon, metals such as aluminum, silicon, and tin, and alloys thereof, lithium-containing transition metal nitride Li _(3-x) M _x N, silicon The lower oxide Li _x SiO _y (0 ≦ x, 0 <y <2) and the tin lower oxide Li _x SnO _y are selected from materials that form an alloy with lithium or materials that form an intermetallic compound. be able to.

  The material of the negative electrode active material is not particularly limited and can be used other than the above materials. However, when a part of the material such as a material having a large expansion and contraction is selected, if the range used by the negative electrode is too large, Resistance rise may be large. In this case, it is preferable to confirm whether or not the negative electrode potential is below a certain level as a condition for changing the battery voltage.

  However, it is preferable to contain graphite, and the graphite has a graphite interlayer distance (d002) of 0.335 nm or more and 0.338 nm or less. By including such graphite in the negative electrode 12, since the potential curve of graphite has a stage structure, the cycle characteristics of the lithium ion secondary battery can be further improved. The graphite used for the negative electrode 12 is natural graphite, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor grown carbon fiber, pitch-based carbonaceous material, needle coke capable of chemically occluding and releasing lithium ions. , Petroleum coke, and polyacrylonitrile-based carbon fiber. The graphite interlayer distance (d002) can be measured using XRD (X-Ray Diffraction Method) or the like.

  The non-graphite carbon used for the negative electrode 12 is a carbon material excluding the above graphite, and can occlude or release lithium ions. This includes a carbon material having a graphite layer spacing of 0.34 nm or more, which changes to graphite by high-temperature heat treatment at 2000 ° C. or more, a 5-membered or 6-membered cyclic hydrocarbon, and a cyclic oxygen-containing material. Amorphous carbon materials synthesized by pyrolysis of organic compounds are included.

As described above, a material that forms an alloy with lithium or a material that forms an intermetallic compound may be added to the negative electrode 12 having a voltage change rate different from that of the positive electrode 10 as a third negative electrode active material. Examples of the third negative electrode active material include metals such as aluminum, silicon, and tin and alloys thereof, lithium-containing transition metal nitrides Li _(3-x) M _x N, and lower oxides of silicon Li _x SiO _y ( 0 ≦ x, 0 <y <2), and the lower oxide Li _x SnO _{y of} tin. There is no restriction | limiting in particular in the material of a 3rd negative electrode active material, It can utilize also other than said material.

  Since the negative electrode active material generally used is a powder, a binder is mixed with the negative electrode active material so that the powders are bonded together and simultaneously bonded to the current collector. In the negative electrode 12 included in the battery according to the present embodiment, it is desirable that the particle size of the negative electrode active material be equal to or less than the thickness of the mixture layer formed from the negative electrode active material and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the negative electrode active material powder, the coarse particles may be removed in advance by sieving classification or wind classification, and particles having a thickness of the mixture layer thickness or less may be used. preferable.

  For the current collector of the negative electrode 12, a copper foil having a thickness of 10 to 100 μm, a copper perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like is used. In addition to copper, materials such as stainless steel, titanium, or nickel are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A negative electrode slurry in which a negative electrode active material, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, a spray method, or the like, and then the organic solvent is dried and pressure-molded by a roll press. Thereby, the negative electrode 12 can be produced. Moreover, it is also possible to form a multilayer mixture layer on a current collector by carrying out a plurality of times from application to drying.

<Separator>
The separator 11 is inserted between the positive electrode 10 and the negative electrode 12 produced by the above method to prevent a short circuit between the positive electrode 10 and the negative electrode 12. The separator 11 can be a polyolefin polymer sheet made of polyethylene, polypropylene, or the like, or a two-layer structure in which a polyolefin polymer and a fluorine polymer sheet typified by tetrafluoropolyethylene are welded. It is. A mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 11 so that the separator 11 does not shrink when the battery temperature increases. Since these separators 11 need to allow lithium ions to permeate during charging and discharging of the battery, they can be used for lithium ion batteries as long as the pore diameter is generally 0.01 to 10 μm and the porosity is 20 to 90%. .

<Electrolyte>
As a representative example of the electrolyte that can be used in an embodiment of the present invention, lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte, a solvent obtained by mixing dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate with ethylene carbonate, Alternatively, there is a solution in which lithium borofluoride (LiBF ₄ ) is dissolved. The present invention is not limited to the type of solvent and electrolyte, and the mixing ratio of solvents, and other electrolytes can be used.

  Examples of non-aqueous solvents that can be used in the electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2 -Methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphate triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2- There are nonaqueous solvents such as oxazolidinone, tetrahydrofuran, 1,2-diethoxyethane, chloroethylene carbonate, or chloropropylene carbonate. Other solvents may be used as long as they do not decompose on the positive electrode 10 or the negative electrode 12 incorporated in the battery of the present invention.

In addition, examples of the _{electrolyte, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or imide salts such as lithium represented by lithium trifluoromethane sulfonimide, multi There are different types of lithium salts. A nonaqueous electrolytic solution obtained by dissolving these salts in the above-mentioned solvent can be used as a battery electrolytic solution. An electrolyte other than this may be used as long as it does not decompose on the positive electrode 10 and the negative electrode 12 included in the battery according to the present embodiment.

Furthermore, an ionic liquid can be used. For example, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), a mixed salt of lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), triglyme and tetraglyme, a cyclic quaternary ammonium cation (N-methyl) -N-propylpyrrolidinium is exemplified) and an imide-based anion (bis (fluorosulfonyl) imide is exemplified), and a combination that does not decompose at the positive electrode 10 and the negative electrode 12 is selected, and according to this embodiment. Can be used for batteries.

<Positive electrode protective agent>
The positive electrode protective agent in one Embodiment of this invention contains the copolymer which has a repeating unit represented by the following (Formula 1)-(Formula 3). The copolymer may be composed only of a repeating unit represented by (Formula 1), a repeating unit represented by (Formula 2), a repeating unit represented by (Formula 3), or another repeating unit. Units may be included. The positive electrode protective agent may be composed only of a copolymer, or may contain components other than the copolymer.

  In order to form a structure in which a copolymer or a copolymer is electrolytically polymerized on the surfaces of the positive electrode and the negative electrode, a positive electrode protective agent may be added to the electrolytic solution, or the positive electrode before forming a slurry solution in the positive electrode A positive electrode protective agent may be added to the active material, or a positive electrode protective agent may be mixed together with a binder or the like when a slurry-like solution is produced in the positive electrode. When the positive electrode protective material is attached to the positive electrode active material before preparing the slurry-like solution in the positive electrode, the positive electrode active material is added with the positive electrode protective agent.

In (Formula 1), Z ₁ is a residue of a polymerizable functional group. R ₁ contains an aromatic functional group, and the copolymer is electrolytically polymerized by R ₁ with a potential based on the lithium metal being equal to or lower than the operating potential of the positive electrode. n ₁ is an integer of 1 or more.

In (Formula 2), Z ₂ is a residue of a polymerizable functional group, and a dehydrating functional group that forms a complex-forming functional group that reacts with water to form a complex with a metal ion is present in the main chain of Z _2. It is formed. n ₂ is an integer of 1 or more.

In (Formula 3), Z ₃ is a residue of a polymerizable functional group, and R ₂ is a highly polar organic group having high polarity. n ₃ is an integer of 0 or more. The repeating unit represented by (Formula 3) may not be included in the copolymer so that n ₃ is 0 or more. When the copolymer includes a repeating unit represented by (Formula 3), n ₃ is 1 or more.

The polymerizable functional group in Z ₁ and Z ₃ is not particularly limited as long as it causes a polymerization reaction, but is not limited to a cyclic ether group such as an oxirane group or octacene group; An organic group having an unsaturated triple bond such as an organic group having a heavy bond, an acetylenyl group, or a propargyl group is preferably used. In particular, an organic group having an unsaturated double bond such as a vinyl group, an acryloyl group, or a methacryloyl group is preferably used. Different polymerizable functional groups may be used as Z ₁ and Z ₃ , or the same polymerizable functional group may be used.

Regarding R ₁ , the aromatic functional group in one embodiment of the present invention is a functional group having 20 or less carbon atoms that satisfies the Huckle rule. Specifically, cyclohexyl benzyl group, biphenyl group, phenyl group, and its condensate naphthyl group, anthryl group, phenanthryl group, triphenylene group, pyrene group, chrysene group, naphthacene group, picene group, perylene group, pentaphene Group, pentacene group, acenaphthylene group and the like. As R ₁ , one or more of these aromatic functional groups may be used. Some of these aromatic functional groups may be substituted. The aromatic functional group may contain an element other than carbon in the aromatic ring. Specifically, elements such as S, N, Si, and O are used.

In R ₁ , the aromatic compound introduced into the copolymer is electrolytically polymerized within the normal operating potential of the positive electrode. Since the operating potential varies depending on the positive electrode active material, it is important to select an aromatic compound suitable for the positive electrode active material to be used. Working potential of the positive electrode of a lithium ion secondary battery, generally, since a 3.0~4.3Vvs.Li / Li ^+, aromatic functionality R ₁ is to react in 4.3Vs.Li/Li ⁺ less It is desirable to be a group. Usually, in the positive electrode active material (LiCoO ₂ ) using 4.3 V vs. Li / Li ⁺ (potential based on lithium metal reference) as an upper limit, the aromatic functional group in R ₁ is a cyclohexylbenzyl group, biphenyl group, naphthyl group. Anthracene group and tetracene group are preferable, and a naphthyl group, anthracene group, and tetracene group are particularly preferable.

In Z ₂ , any one or more of —OR, —SR, —COOR, and —SO ₃ R (R is H, an alkali metal, an alkaline earth metal, or an alkyl group) can be given as the complex-forming functional group. It is done. When these functional groups are complexed with the positive electrode active material, the catalytic activity for oxidative decomposition of the electrolyte solution of the positive electrode active material is reduced, so that the oxidative decomposition of the electrolyte solution can be suppressed. Specifically, Z ₂ is formed on the main chain in the repeating unit formed by polymerizing the functional group formed on the main chain in the repeating unit formed by polymerizing the maleic anhydride derivative or the maleimide derivative. Functional groups and the like. When the maleic anhydride derivative or maleimide derivative is polymerized, the repeating unit reacts with water to form a carboxyl group and a complex-forming functional group, and exhibits a dehydration effect and a complex-forming functional group that suppresses oxidative degradation. Done at the same time. Without lithium carbonate, water is not generated, and the amount of lithium carbonate is very small. Therefore, when a certain amount of lithium carbonate has reacted, the reaction for generating water is almost stopped. Therefore, there is little possibility that the carboxyl group after dehydration will generate water again.

  Maleimide derivatives have nitrogen atoms in the molecule. Nitrogen atoms are easily oxidized because they have lone pairs. Therefore, a maleic anhydride derivative having no nitrogen atom is preferred.

  The number average molecular weight (Mn) of the copolymer is 50000000 or less, preferably 1000000 or less. More preferably, it is 100000 or less. By using a copolymer having a low number average molecular weight, a decrease in battery performance can be suppressed. The number average molecular weight of the copolymer is measured by GPC (Gel permeation Chromatography).

  Specific examples of the copolymer include a copolymer having a repeating unit represented by the following (formula 4) to (formula 5).

  The repeating unit corresponding to x in (Formula 4) and (Formula 5) corresponds to the repeating unit represented by (Formula 1), and the repeating unit corresponding to y is the repeating unit represented by (Formula 2). Correspondingly, the repeating unit corresponding to z corresponds to the repeating unit represented by (Formula 3).

In one embodiment of the present invention, R _{1 to} R ₉ in (Formula 4) and (Formula 5) are H or an alkyl group. R ₆ and R ₇ may or may not be present. If they are not present, the succinic acid group is directly connected to the polymer main chain. Considering that the amount of the functional group that exhibits the effect according to the embodiment of the present invention is relatively reduced, it is preferable that R ₆ and R ₇ do not exist. Instead of the aromatic functional group present in the repeating unit corresponding to x, the above aromatic functional group may be introduced into R ₆ and R ₇ .

  E in (Formula 4) and (Formula 5) is a substituent of an aromatic functional group, and n is the number of substituents. E is preferably H, an alkyl group, a hydroxyl group, a carboxyl group, a sulfo group, or a halogen group from the viewpoint of the electrolytic polymerization reaction of an aromatic functional group. The presence of an electron-withdrawing functional group such as a sulfo group, a carboxyl group, or a halogen group increases the oxidation potential, and the presence of an electron-donating group such as an alkyl group or a hydroxyl group decreases the oxidation potential. The functional group of E is appropriately selected depending on the positive electrode used.

C in (Formula 4) and (Formula 5) is a highly polar organic group having high polarity. C is the same as R _{2 in} (Formula 3). By selecting a highly polar organic group, solubility in the electrolyte solution is improved, ion conductivity at the electrode interface is improved, and battery performance is improved. Specifically, C is an alkylene oxide group, a cyano group, an amino group, a sulfo group, or a carboxyl group, and an alkylene oxide group is preferably used from the viewpoint of electrochemical stability. In the alkylene oxide group, an ethylene oxide group having —CH ₂ CH ₂ O— as a structural unit is preferably used from the viewpoint of ionic conductivity and electrochemical stability. Further, these functional groups may be directly connected to the main chain, or may be present in a compound having a polymerizable functional group such as a methacryl group, an acrylic group, or a styryl group.

x, y, and z are functional group composition ratios, and x + y + z = 1, 0 <x <1, 0 <y <1, and 0 <z <1. However, when n ₃ is 0, z = 0 and x + y = 1. x is preferably 0.1 to 0.4. When there is too much x, it will become difficult to melt | dissolve in electrolyte solution. Moreover, when there is too little x, it will become difficult to produce electropolymerization and the effect which concerns on one Embodiment of this invention will fall. z is preferably 0.4 or more. If z is too small, dissolution in the electrolytic solution and ionic conductivity at the interface may be impaired. y is determined according to the amount of x and z, and is preferably 0 to 0.1.

  Other specific examples of the copolymer include a copolymer having a repeating unit represented by the following (formula 6) to (formula 7).

In one embodiment of the present invention, R _{1 to} R ₁₀ in (Formula 6) and (Formula 7) are H or an alkyl group. R ₆ and R ₇ may or may not be present. If they are not present, the succinic acid group is directly connected to the polymer main chain.

  E in (Formula 6) and (Formula 7) is a substituent of an aromatic functional group, n is the number of substituents, and is the same as E and n in (Formula 4) and (Formula 5). C in (Expression 6) and (Expression 7) is a highly polar organic group having the same polarity as C in (Expression 4) and (Expression 5).

  In (Formula 6) and (Formula 7), x, y, and z are functional group composition ratios, and are the same as x, y, and z in (Formula 4) and (Formula 5).

  In one embodiment of the present invention, the copolymer is a compound obtained by polymerizing a polymerizable compound. In the present invention, both a polymerizable compound and a copolymer can be used, but from the viewpoint of electrochemical stability, the polymerizable compound is polymerized in advance, and the copolymer is prepared and then purified. It is preferable to use a copolymer. The polymerization may be any of conventionally known bulk polymerization, solution polymerization, and emulsion polymerization. The polymerization method is not particularly limited, but radical polymerization is preferably used. In the polymerization, a polymerization initiator may or may not be used, and a radical polymerization initiator is preferably used from the viewpoint of easy handling. The polymerization method using a radical polymerization initiator can be carried out in a temperature range and a polymerization time which are usually performed. The blending amount of the initiator in the present invention is 0.1 wt% to 20 wt%, preferably 0.3 wt% or more and 5 wt% with respect to the polymerizable compound.

  Examples of radical polymerization initiators include t-butyl peroxypivalate, t-hexyl peroxypivalate, methyl ethyl ketone peroxide, cyclohexanone peroxide, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, 2-bis (t-butylperoxy) octane, n-butyl-4,4-bis (t-butylperoxy) valerate, t-butyl hydroperoxide, cumene hydroperoxide, 2,5-dimethylhexane-2, 5-di Hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, α, α'-bis (t-butylperoxy m-isopropyl) benzene, 2,5-dimethyl-2,5-di ( t-butylperoxy) hexane, 2,5 -Organic peroxides such as dimethyl-2,5-di (t-butylperoxy) hexane, benzoyl peroxide, t-butylperoxypropyl carbonate, 2,2'-azobisisobutyronitrile, 2,2'- Azobis (2-methylbutyronitrile), 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), 1, 1 ' -Azobis (cyclohexane-1-carbonitrile), 2- (carbamoylazo) isobutyronitrile, 2-phenylazo-4-methoxy-2,4-dimethyl-valeronitrile, 2,2-azobis (2-methyl-N -Phenylpropionamidine) dihydrochloride, 2,2'-azobis [N- (4-chlorophenyl) -2-methylpropionamidine] Hydrochloride, 2,2'-azobis [N-hydroxyphenyl] -2- [methylpropionamidine] dihydrochloride, 2,2'-azobis [2-methyl-N- (phenylmethyl) propionamidine] dihydrochloride 2,2'-azobis [2methyl-N- (2-propenyl) propionamidine] dihydrochloride, 2,2'-azobis (2-methylpropionamidine) dihydrochloride, 2,2'-azobis [N -(2-Hydroxyethyl) -2-methylpropionamidine] dihydrochloride, 2,2'-azobis [2- (5-methyl-2-imidazolin-2-yl) propane] dihydrochloride, 2, 2 ' -Azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (4,5,6,7-tetrahydro-1H-1,3-diazepine-2- Il) Lopan] dihydrochloride, 2,2'-azobis [2- (3,4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (5-hydroxy -3, 4, 5, 6-tetrahydropyrimidin-2-yl) propane] dihydrochloride, 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane } Dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane], 2,2'-azobis {2-methyl-N- [1,1-bis (hydroxymethyl)- 2-hydroxyethyl] propionamide}, 2,2'-azobis {2methyl-N- [1,1-bis (hydroxymethyl) ethyl] propionamide}, 2,2'-azobis [2-methyl-N- (2-hydroxyethyl) Propionamide], 2,2'-azobis (2-methylpropionamide) dihydrate, 2,2'-azobis (2,4,4-trimethylpentane), 2,2'-azobis (2-methylpropane), dimethyl And azo compounds such as 2,2′-azobisisobutyrate, 4,4′-azobis (4-cyanovaleric acid), and 2,2′-azobis [2- (hydroxymethyl) propionitrile]. .

  In the present invention, the addition amount of the copolymer is very important. If the additive is too much, the battery characteristics are deteriorated. The addition amount is 0.01 wt% or more and 5 wt% or less with respect to the electrolytic solution, preferably 0.05 wt% or more and 3 wt% or less, and particularly preferably 0.1 wt% or more and 0.5 wt% or less.

  In the present invention, in order to exhibit the effect, it is preferable that a product obtained by electrolytic polymerization of the compounds of (Formula 1) to (Formula 7) on the electrode is present. When an electropolymerized product is present, it is formed on the electrode in a state where the electrolytic polymer and the copolymer are mixed. Moreover, although electropolymerization occurs at the positive electrode, the electropolymerization product generated at the positive electrode may move to the negative electrode and exist on the negative electrode. Battery performance is further improved by forming a film on the negative electrode. The electropolymerized product can be analyzed by TOF-SIMS.

〔Example〕
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to these Examples. The results of this example are summarized in Table 1.

<Polymer synthesis method>
A monomer was added to the reaction vessel, and a polymerization initiator was added. AIBN was used as the polymerization initiator. The concentration of the polymerization initiator was added so as to be 1 wt% with respect to the total amount of monomers. Thereafter, the polymer was purified by washing the polymer with hexane.

<Method for producing positive electrode>
A positive electrode active material, a conductive agent (SP270: graphite manufactured by Nippon Graphite Co., Ltd.), and a polyvinylidene fluoride binder were mixed at a ratio of 85:10:10 wt%, and charged into N-methyl-2-pyrrolidone to be mixed. A solution was made. The slurry was applied to a 20 μm thick aluminum foil by a doctor blade method and dried. The mixture application amount was 200 g / m ² . Then, it pressed and produced the positive electrode.

<Method for producing negative electrode>
Polyvinylidene fluoride was mixed with graphite at a ratio of 95: 5% by weight, and further mixed with N-methyl-2-pyrrolidone to prepare a slurry solution. The slurry was applied to a copper foil having a thickness of 10 μm by a doctor blade method and dried. The mixture was pressed so that the bulk density was 1.5 g / cm ³ to prepare a negative electrode.

<Measurement of oxidation potential: cyclic voltammetry (CV)>
The electrolyte solution was prepared by adding a positive electrode protective agent to the electrolyte solution at 0.5 wt%. A cell made of working electrode: platinum, counter electrode, reference electrode: Li metal was prepared, and an electrolytic solution was added thereto. An electrolytic solution was added to the cell, and the potential was swept in the range of 3.0 V vs. Li / Li ^{+ to} 4.5 V vs. Li / Li ⁺ . The sweep rate was 5 mV / sec.

<Production method of laminated battery>
A polyolefin separator was inserted between the positive electrode and the negative electrode to form an electrode group. Thereto, an electrolytic solution was injected. Thereafter, the battery was sealed with an aluminum laminate to produce a battery. Then, the battery was initialized by repeating charging / discharging 3 cycles. What is the voltage range? The voltage was 2.5V to 4.2V.

<Evaluation method of laminated battery>
<Cycle evaluation>
The battery was charged at a current density of 0.72 mA / cm ² up to a preset upper limit voltage. The discharge was performed at a current density of 0.72 mA / cm ² up to a preset lower limit voltage. The upper limit voltage was 4.2V and the lower limit voltage was 2.5V. The discharge capacity obtained in the first cycle was taken as the initial capacity of the battery. In the cycle test, the current density was 2.2 mA / cm ² and the voltage range was cycled from 2.5 V to 4.2 V. Cycle characteristics were obtained by dividing the discharge capacity after 50 cycles by the initial capacity.

<Evaluation of gas generation>
Separately, a laminated battery was prepared and initialized. After initialization, the battery was charged to 4.2 V and stored in a constant temperature bath at 80 ° C. for 1 week. The amount of gas generation was defined by taking the difference in volume of the laminated battery before and after heating.

2-Vinylnaphthalene (0.4 mol, 61.6 g), maleic anhydride (0.1 mol, 9.8 g), diethylene glycol monomethyl methacrylate (0.5 mol, 94 g) were added to the solvent dimethyl carbonate (4 mL). Then, AIBN was added as a polymerization initiator so that it might become 4 wt% with respect to the whole quantity of a monomer, and the reaction solution was prepared. Thereafter, the reaction solution was heated at 60 ° C. for 3 hours. Then, the solvent was distilled off and copolymer A was obtained by performing reprecipitation with hexane. Then, when the copolymer A was measured by ¹ NMR, x, y, and z were 40:10:50. Next, the oxidation potential of the polymer A was measured. The oxidation potential was 4.15 V vs. Li ^/ Li ⁺ .

The laminate battery was evaluated. The electrolyte solution used in the lamination was a solution in which the copolymer A was dissolved to a concentration of 0.2 wt%. The electrolytic solution was prepared by adding LiPF ₆ to the electrolyte salt and adding 1.0 wt / L to the solvent (EC / DMC / EMC = 1: 1: 1 vol ratio) and further adding 2 wt% of vinylene carbonate. used.

  The initial capacity was 50 mAh and the cycle characteristics were 95. In addition, the amount of gas generated during high temperature storage was 0.10 mL.

In the same manner as in Example 1, the same examination was performed except that 1-vinylnaphthalene was used as the monomer. The polymer produced using 1-vinylnaphthalene was copolymer B. The oxidation potential was 4.15 V vs. Li ^/ Li ⁺ .

  The initial capacity was 50 mAh and the cycle characteristics were 94. In addition, the amount of gas generated during high temperature storage was 0.10 mL.

In the same manner as in Example 1, evaluation was performed in the same manner as in Example 1 except that the monomer composition ratio was changed to the ratio of Example 3 in Table 1. This copolymer was referred to as Copolymer C. The oxidation potential was 4.35 V vs. Li ^/ Li ⁺ .

  The initial capacity was 48 mAh and the cycle characteristics were 90. In addition, the amount of gas generated during high temperature storage was 0.20 mL.

  In Example 1, it carried out like Example 1 except having made the amount of polymers into 5 wt%.

  The initial capacity was 45 mAh and the cycle characteristics were 85. Further, the amount of gas generated during high-temperature storage was 0.23 mL.

  In Example 1, it carried out like Example 1 except having made the quantity of the polymer 0.01 wt%.

  The initial capacity was 50 mAh and the cycle characteristics were 93. In addition, the amount of gas generated during high temperature storage was 0.15 mL.

  In Example 1, it carried out like Example 1 except using maleimide instead of maleic anhydride.

  The initial capacity was 48 mAh and the cycle characteristics were 93. In addition, the amount of gas generated during high temperature storage was 0.19 mL.

In Example 1, except changing the LiCoO ₂ as a positive electrode active material was performed in the same manner as in Example 1.

  The initial capacity was 48 mAh and the cycle characteristics were 95. The amount of gas generated during high-temperature storage was 0.09 mL.

In Example 1, except changing the LiMn ₂ O ₄ as the positive electrode active material was performed in the same manner as in Example 1.

  The initial capacity was 43 mAh and the cycle characteristics were 94. In addition, the amount of gas generated during high temperature storage was 0.30 mL.

[Comparative Example 1]
In Example 1, it investigated similarly to Example 1 except not adding an additive.

  The initial capacity was 50 mAh and the cycle characteristics were 84. In addition, the amount of gas generated during high temperature storage was 0.25 mL.

[Comparative Example 2]
In Example 7, it examined like Example 7 except not adding an additive.

  The initial capacity was 47 mAh and the cycle characteristics were 90. In addition, the amount of gas generated during high temperature storage was 0.15 mL.

[Comparative Example 3]
In Example 8, it investigated similarly to Example 8 except not adding an additive.

  The initial capacity was 43 mAh and the cycle characteristics were 88. The amount of gas generated during high temperature storage was 0.38 mL.

<Discussion>
Examples 1-6 have better cycle characteristics and less gas generation than Comparative Example 1 using the same positive electrode active material as Examples 1-6. Example 7 has better cycle characteristics and less gas generation than Comparative Example 2 using the same positive electrode active material as Example 7. Example 8 has better cycle characteristics and less gas generation than Comparative Example 3 using the same positive electrode active material as Example 8.

  When Example 2 and Example 6, which are the same except for b, are compared, Example 2 has better cycle characteristics and less gas generation. This is probably because maleic anhydride having no lone electron pair and having no nitrogen atom is used in Example 2.

  When Example 1 and Example 3 which are the same except mol% of a, b, and c are compared, Example 1 has better cycle characteristics and less gas generation. This is considered to be because the ratio of a is 0.1 to 0.4 in Example 1 and the ratio of c is 0.4 or more.

  When Example 1 and Example 5 that are the same except for the polymer concentration of the copolymer are compared, Example 1 has better cycle characteristics and less gas generation. This is probably because the polymer concentration in Example 1 is higher than that in Example 5.

DESCRIPTION OF SYMBOLS 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery can 14 Positive electrode tab 15 Negative electrode tab 16 Inner cover 17 Internal pressure release valve 18 Gasket 19 PTC element 20 Battery cover 21 Axle core

Claims (11)

The positive electrode protective agent for lithium ion secondary batteries containing the copolymer which has a repeating unit represented by the following (Formula 1) and a repeating unit represented by the following (Formula 2).
(In (Formula 1),
Z ₁ is a residue of a polymerizable functional group.
R ₁ is at _{least one} of a naphthyl group, an anthracene group, and a tetracene group, and the copolymer is electropolymerized by R _{1 when the} potential based on the lithium metal is within the operating potential of the lithium ion secondary battery.
n ₁ is an integer of 1 or more.
In (Formula 2),
Z ₂ is a residue of a polymerizable functional group, and a dehydrating functional group that forms a complex-forming functional group that reacts with water to form a complex with a metal ion is formed in the main chain of Z ₂ .
n ₂ is an integer of 1 or more. ) In claim 1,
The said copolymer is a positive electrode protective agent for lithium ion secondary batteries which has a repeating unit represented by the following (Formula 3).
(In (Formula 3),
Z ₃ is a residue of a polymerizable functional group.
R ₃ is a highly polar organic group having a high polarity.
n ₃ is an integer of 1 or more. ) In claim 1 or 2,
The complex forming functional group, -OR, -SR, -COOR, or -SO ₃ R (R is, H, alkali metal, alkaline earth metal or an alkyl group.) Of lithium ion is either 1 or more Positive electrode protective agent for secondary batteries. In any one of Claims 1 thru | or 3,
Z ₂ is a functional group formed on the main chain in the repeating unit formed by polymerizing the maleic anhydride derivative, or a functional group formed on the main chain in the repeating unit formed by polymerizing the maleimide derivative. The positive electrode protective agent for lithium ion secondary batteries which is group. In any one of Claims 1 thru | or 4 ,
The positive electrode protective agent for lithium ion secondary batteries whose said copolymer is (Formula 4) or (Formula 5).
(In (Formula 4) and (Formula 5),
R _{1 to} R ₉ are H or an alkyl group.
R ₆ and R ₇ may or may not be present. If they are not present, the succinic acid group is directly connected to the main chain of the copolymer.
x, y, and z are functional group composition ratios, and x + y + z = 1, 0 <x <1, 0 <y <1, and 0 <z <1. However, when n ₃ is 0, z = 0 and x + y = 1.
E is a substituent of the aromatic functional group.
n is the number of substituents.
C is a highly polar organic group having a high polarity. ) In any one of Claims 1 thru | or 4 ,
The positive electrode protective agent for lithium ion secondary batteries whose said copolymer is (Formula 6) or (Formula 7).
(In (Expression 6) and (Expression 7),
R _{1 to} R ₁₀ are H or an alkyl group.
R ₆ and R ₇ may be present or absent, and when not present, the maleimide group is directly connected to the main chain of the copolymer.
x, y, and z are functional group composition ratios, and x + y + z = 1, 0 <x <1, 0 <y <1, and 0 <z <1. When n ₃ is 0, z = 0 and x + y = 1.
E is a substituent of the aromatic functional group.
n is the number of substituents.
C is a highly polar organic group having a high polarity. ) A positive electrode active material;
A positive electrode material for a lithium ion secondary battery comprising the positive electrode protective agent for a lithium ion secondary battery according to any one of claims 1 to 6 , which is attached to the positive electrode active material. The electrolyte solution for lithium ion secondary batteries containing the positive electrode protective agent for lithium ion secondary batteries in any one of Claims 1 thru | or 6 . In claim 8 ,
The lithium ion secondary battery electrolyte is contained in the copolymer in an amount of 0.01 wt% to 5 wt% with respect to the lithium ion secondary battery electrolyte. The lithium ion secondary battery containing the electrolyte solution for lithium ion secondary batteries of Claim 9 , and a positive electrode and a negative electrode. The manufacturing method of the positive electrode protective agent for lithium ion secondary batteries containing the copolymer which has a repeating unit represented by the following (Formula 1), and a repeating unit represented by the following (Formula 2).
(In (Formula 1),
Z ₁ is a residue of a polymerizable functional group.
R ₁ is at _{least one} of a naphthyl group, an anthracene group, and a tetracene group, and the copolymer is electropolymerized by R _{1 when the} potential based on the lithium metal is within the operating potential of the lithium ion secondary battery.
n ₁ is an integer of 1 or more.
In (Formula 2),
Z ₂ is a residue of a polymerizable functional group, and a dehydrating functional group that forms a complex-forming functional group that reacts with water to form a complex with a metal ion is formed in the main chain of Z ₂ .
n ₂ is an integer of 1 or more. )