JP2015159050A - Li battery material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coating material capable of achieving reduction both in irreversible capacity and battery resistance, and a coated negative electrode material.SOLUTION: The present invention in order to solve the problem has the following characteristics. In a lithium ion secondary battery including a positive electrode, a negative electrode and an electrolyte, the negative electrode has a negative electrode active material, the negative electrode active material is coated with a coating material represented by formula (1), and a coating amount of the coating material is 0.01 wt.% or more and 4 wt.% or less with respect to the negative electrode active material.

Description

  The present invention relates to a material for a Li battery.

  In recent years, materials for Li batteries have been actively developed. It is known that a negative electrode for a Li battery has a high reducing activity of the electrolytic solution and causes problems such as an increase in irreversible capacity and a reduction in battery capacity due to decomposition and deposition of the electrolytic solution due to the potential of the negative electrode. For this reason, the technique which coat | covers a negative electrode active material and prevents precipitation of electrolyte solution has been developed until now.

JP 2009-76433 A Special table 2003-525957

  However, although the conventional coating material can prevent the electrolytic solution from being deposited, there is a problem that the lithium ion conductivity and the electron transfer property are low and the output of the battery cannot be increased. In order to reduce battery resistance, it is considered effective to introduce a polymer having a functional group with high Li ion dissociation in the side chain.

  Patent Document 1 discloses a technique using a polymer of lithium styrenesulfonate as a coating material for a negative electrode active material. However, lithium styrene sulfonate has a high ability to capture lithium and there is room for improvement in lithium ion conductivity.

  Patent Document 2 discloses a technique of using a fluoroether-substituted aromatic compound for a matrix polymer of gel electrolyte. In order to improve the lithium ion conductivity, the structure of the coating material, the coating concentration, the coating method, etc. are considered to be important.

  Accordingly, an object of the present invention is to provide a coating material and a coated negative electrode material that can achieve both irreversible capacity and reduction in battery resistance.

  The features of the present invention for solving the above-described problems are as follows. In a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolytic solution, the negative electrode has a negative electrode active material, and the negative electrode active material is coated with a coating material represented by the formula (1). The amount of the lithium ion secondary battery is 0.01 wt% or more and 4 wt% or less with respect to the negative electrode active material.

(In the formula (1), R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are any of an alkyl group, a halogen group and hydrogen. Z in the formula (1) is an aromatic group, X is hydrogen, an alkali metal, or an alkaline earth metal, m in the formula (1) is the number of repeating units of the CF ₂ group, and B in the formula (1) is a polar functional group. X and y are copolymer composition ratios, and x / (x + y) satisfies 0 <x / (x + y) ≦ 1.

  The present invention can provide a Li battery having a small irreversible capacity and low battery resistance. 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.

  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, a battery container (that is, a battery can) 13, a positive electrode current collecting tab 14, a negative electrode current collecting tab 15, an inner lid 16, It comprises an internal pressure release valve 17, a gasket 18, a positive temperature coefficient (PTC) resistance 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 resistance 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 case 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 case 13 is selected from materials that are corrosion-resistant to the nonaqueous electrolyte, such as aluminum, stainless steel, and nickel-plated steel. Further, when the battery container 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 container 13 or alloying with lithium ions in the portion in contact with the nonaqueous electrolyte. Thus, the material of the battery container 13 is selected.

  The electrode group is housed in the battery container 13, the negative electrode current collecting tab 15 is connected to the inner wall of the battery container 13, and the positive electrode current collecting tab 14 is connected to the bottom surface of the battery lid 20. The electrolyte is injected into the battery container interior 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 container 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 MxO ₂ (however, at least selected from the group consisting of M = Co, Ni, Fe, Cr, Zn, Ti) 1 type, x = 0.01 to 0.2), Li ₂ Mn ₃ MO ₈ (however, M = at least one selected from the group consisting of 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 (where little is selected from the group consisting of M = Ni, Fe, Mn Both _{one, 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 = 0.01 to 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, or a metal foam plate 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 forming. 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 in the present invention comprises a negative electrode active material, a binder, and a current collector. As the negative electrode active material, an easily graphitized material obtained from natural graphite, petroleum coke, coal pitch coke, etc. is heat-treated at a high temperature of 2500 ° C. or higher, and mesophase carbon or amorphous carbon, carbon fiber, and lithium are alloyed. A metal or a material having a metal supported on the surface of carbon particles is used. For example, a metal or alloy selected from lithium, silver, aluminum, tin, silicon, indium, gallium, and magnesium. Further, the metal or an oxide of the metal can be used as a negative electrode active material. Furthermore, lithium titanate can also be used.
<Coating material>
The negative electrode active material can be prevented from reacting with the electrolytic solution and the negative electrode active material by coating with the coating material represented by the formula (1). Here, the coating amount of the coating material is an important value for obtaining the effect of the present application. The coating amount is 0.01 wt% or more and 4 wt% or less with respect to the negative electrode active material, preferably 0.1 wt% or more and 3 wt% or less, particularly preferably 0.3 wt% or more and 2 wt% or less. When the coating amount of the coating material of the present invention is large, the ionic conductivity at the negative electrode interface is lowered, so that the resistance is increased. Further, if the coating amount is large, the SEI produced is adversely affected, which is considered to adversely affect the battery resistance. On the other hand, if the coating amount is small, the number of parts that come into contact with the electrolytic solution increases and the irreversible capacity increases. In the present invention, it has been found that the effect of the present application can be enhanced by adjusting the coating amount.

In the formula (1), R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are any of an alkyl group, a halogen group and hydrogen. Carbon number of an alkyl group is 1-10. Z in the formula (1) is either an aromatic group or a hydrocarbon group having 1 to 10 carbon atoms. As the aromatic group, for example, a phenylene group or a naphthylene group can be used, and a phenylene group is preferable. Z may have a functional group, and the functional group may be substituted with an electron withdrawing group such as halogen. When Z is not present, the — (CF ₂ ) _m SO ₃ X group and the polymer main chain are directly connected. X is composed of hydrogen, an alkali metal, or an alkaline earth metal. From the viewpoint of battery performance, alkali metals are preferable, and sodium and lithium are particularly preferable. M in the formula (1) is the number of repeating units of the CF ₂ group. m is 1 or more and 20 or less, preferably 1 or more and 10 or less. Particularly preferred is 2. The presence of a CF ₂ group having a high electron-withdrawing property adjacent to the sulfo group reduces the electron density of the sulfo group, resulting in an increase in the degree of dissociation of Li ions. It is considered that the increase in the degree of dissociation facilitates the movement of lithium ions and decreases the resistance of the battery. In addition, adjusting m can increase the density of sulfo groups present in the polymer, and this is thought to affect the reduction in battery resistance.

Equation (1) - (CF _{2) m} SO ₃ X in CF ₂ is for drawing electrons from SO _3, it is possible to reduce the electron density on the sulfo group. For this reason, the lithium ion capturing power of the sulfo group is reduced, and lithium ions can be efficiently delivered.

  B in the formula (1) is a polar functional group, and a functional group having a hydroxyl group, a carboxyl group, a sulfo group, an amino group, or a phosphate group is preferably used.

By copolymerizing a monomer having —O (CF ₂ ) _m SO ₃ X and a monomer having B, a higher irreversible capacity reduction effect can be obtained. This is presumably because the interaction between the polymer and the negative electrode active material (particularly carbon-based) is increased by copolymerizing the monomer having B, and the covering property is improved.

In the formula (1), the segment having —O (CF ₂ ) _m SO ₃ X can be produced by polymerizing the monomer represented by the formula (2).

In the formula (2), R ₁ to R ₃ , Z, and X are the same as those in the formula (1). The monomer of formula (2) can be prepared by the following synthesis method. Hereinafter, a method for synthesizing the monomer of the formula (7) will be exemplified as a specific example of the formula (2).

First, the compound of formula (3) is dissolved in tetrahydrofuran. In order to prevent polymerization, BrCF ₂ CF ₂ Br is added dropwise so as to be 1.1 equivalents to the formula (3) while blowing dry air. The solution is then heated to 50 ° C. and stirred for 3 hours to obtain formula (4). Na ₂ (S ₂ O ₄ ) is added to the formula (4) in the presence of NaHCO ₃ and heated again to 50 ° C. to obtain the formula (5). Next, Br ₂ is added to the formula (5) so that the amount of Br ₂ is 1.1 equivalent to the formula (5), thereby obtaining the formula (6). Formula (7) is obtained by adding lithium hydroxide dissolved in water to Formula (6) and stirring. Formula (7) is separated and purified by column chromatography.

  For the coating material of the present invention, a homopolymer of x = 1 in formula (1) can also be used, and a copolymer consisting of x and y can also be used. By copolymerization, a higher irreversible capacity reduction effect is exhibited. As the monomer to be copolymerized, a monomer having a structure of formula (8) to formula (12) is preferably used.

  X in Formula (8), Formula (9), Formula (10), Formula (11), and Formula (12) is an alkali metal or H. From the viewpoint of electrochemical stability, an alkali metal, particularly lithium or sodium, is Preferably used.

  When formula (7) is used as the coating material of formula (1), the copolymer is represented by formula (13).

In the formula (13), R ₄ , R ₅ and R ₆ are any of an alkyl group, a halogen group and hydrogen. When formulas (8) to (12) are used as monomers to be copolymerized with formula (7), the segment having B is represented as follows.

  X in Formula (14), Formula (15), Formula (16), Formula (17), and Formula (18) is an alkali metal or H. From the viewpoint of electrochemical stability, an alkali metal, particularly lithium or sodium, is Preferably used.

  In the present invention, a polymer obtained by homopolymerizing each of the formulas (8) to (12) and a polymer obtained by homopolymerizing the formula (7) can be mixed and used as a coating material.

  As the polar functional group B, a functional group having a hydroxyl group, a carboxyl group, a sulfo group, an amino group, or a phosphate group, for example, the formula (19), the formula (20), the formula (21), the formula (22), and the formula (23). Can be used. These have a particularly high affinity with a carbon-based negative electrode active material.

  X in the formula (19), the formula (20), the formula (21), the formula (22), and the formula (23) is an alkali metal or H. From the viewpoint of electrochemical stability, an alkali metal, particularly lithium or sodium Are preferably used.

(Polymerization method)
As described above, the coating material of the present invention can be produced by polymerizing monomers. 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 initiator blending amount 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.

(Polymer structure)
In the present invention, the polymer structure of the formula (1) is not limited to a linear structure, and may be a branched structure, a crosslinked structure, or a dendrimer structure. From the viewpoint of workability when coating the polymer on the negative electrode active material, a polymer having a linear structure is preferably used. The polymerization mode when monomers are copolymerized is not particularly limited as long as a polymer can be formed, and examples thereof include random copolymerization, alternating copolymerization, block copolymerization, and graft copolymerization.

(Molecular weight of polymer)
The number average molecular weight of the polymer of the formula (1) is 1,000 to 5,000,000, preferably 5,000 to 1,000,000. If the number average molecular weight is too small, the coating material is dissolved in the electrolytic solution, so that the effect of the present invention is reduced. On the other hand, if the number average molecular weight is too large, agglomeration or the like is likely to occur during coating, resulting in a decrease in productivity. Therefore, it is important to adjust the number average molecular weight.

(Copolymerization composition ratio)
In the formula (1), x and y are the ratio of the segment having —O (CF ₂ ) _m SO ₃ X and the segment having B. x and y are also polymerization ratios of the monomer having —O (CF ₂ ) _m SO ₃ X and the monomer having B. By changing this composition ratio, the affinity with the negative electrode active material, the lithium ion conductivity Can be adjusted. The ratio x / (x + y) of the ratio x to y in the formula (1) is 0 <x / (x + y) ≦ 1, preferably 0.1 ≦ x / (x + y) ≦ 0. 9, particularly preferably 0.25 ≦ x / (x + y) ≦ 0.75. By controlling x / (x + y), it is possible to provide a Li battery that can achieve both irreversible capacity and low resistance.

(Coating method of coating material)
As a coating method, it is preferable to dissolve the polymer of the formula (1) in a solvent, add a negative electrode active material to the solution, stir, then dry the solvent and coat. By coating the covering material on the negative electrode active material in advance, the resistance to electrolytic solution is increased when applied to the battery, and the effect of the present application is enhanced because the covering material remains in the negative electrode active material. The solvent is not particularly limited as long as the polymer dissolves, but a protic solvent such as water and ethanol, an aprotic solvent such as N-methylpyrrolidone, and a nonpolar solvent such as toluene and hexane are preferably used.

<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 an electrolyte solution that can be used in an embodiment of the present invention, a solvent obtained by mixing ethylene carbonate with dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte, 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.

  When a solid polymer electrolyte (polymer electrolyte) is used, an ion conductive polymer such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyhexafluoropropylene, and polyethylene oxide can be used for the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 11 can be omitted.

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

<Battery system>
The Li battery using the negative electrode active material found in the present application has an excellent property of low resistance. Therefore, heat generation due to the internal resistance of the battery can be suppressed when the battery is used. Therefore, since the battery cooling mechanism can be simplified, it is useful not only for small batteries for portable devices but also for large batteries for in-vehicle use.

  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.

<Synthesis Method of Coating Material Polymer>
Monomer and tetrahydrofuran as a reaction solvent were added to the reaction vessel. Further, AIBN was added as a polymerization initiator to the solution. The concentration of the polymerization initiator was added so as to be 4 wt% with respect to the total amount of monomers. Then, the polymer was synthesized by heating the reaction solution at 60 ° C. for 3 hours.

<Method for producing positive electrode>
A positive electrode active material (LiCoO ₂ ), a conductive agent (SP270: graphite manufactured by Nippon Graphite Co., Ltd.), and a polyvinylidene fluoride binder were mixed at a ratio of 85: 7.5: 7.5% by weight, and N-methyl-2-pyrrolidone was mixed. The mixture was charged and mixed to prepare a slurry solution. 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 of coating negative electrode active material>
Graphite was used as the negative electrode active material. The coating was performed by adding graphite to the polymer aqueous solution and stirring, and then distilling off the water. The coated graphite was used after removing aggregates with a sieve.

<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 produce a negative electrode.

<Negative electrode evaluation method>
The produced negative electrode was punched into a circle having a diameter of 15 mm to prepare an electrode. The evaluation cell was configured by using Li metal as a negative electrode and a counter electrode, inserting a separator between the negative electrode and Li metal, and adding an electrolyte thereto. The evaluation cell was charged at a current density of 0.72 m / cm ² up to a preset lower limit voltage. The discharge was performed at a current density of 0.72 mA / cm ² up to a preset upper limit voltage. The lower limit voltage was 0.01V and the upper limit voltage was 1.5V. The irreversible capacity was obtained from the difference between the charge capacity and the discharge capacity.

<Evaluation method of DC resistance>
The positive electrode and the negative electrode were punched into a circle having a diameter of 15 mm to prepare an electrode. The small battery was configured by inserting a separator between the positive electrode and the negative electrode, and adding an electrolytic solution thereto. The small 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 3.0V. The discharge capacity obtained in the first cycle was taken as the initial capacity of the battery. Then, it charged to 50% of initial capacity, and measured DC resistance.

Example 1
The polymer (A) was synthesized using the formula (7) as a monomer. Moreover, the negative electrode active material was coat | covered using the said polymer. The coating amount was 0.5 wt%. A negative electrode single electrode was prepared and the irreversible capacity was measured. The irreversible capacity was 23.5 mAhg- ¹ . Next, a small battery was produced, and DC resistance was measured. The DC resistance was 10.7Ω.

(Example 2)
In Example 1, it was carried out similarly to Example 1 except having made the polymer amount into 0.3 wt%. As a result of the evaluation, the irreversible capacity was 23.9 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The direct current resistance was 10.6Ω.

(Example 3)
Example 1 was the same as Example 1 except that the polymer amount was 2.0 wt%. As a result of the evaluation, the irreversible capacity was 23.3 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The DC resistance was 10.9 Ω.

Example 4
A polymer (B) was synthesized using monomers of formula (7) and formula (10) as monomers. In addition, the mol ratio of Formula (7) and Formula (10) was set to 75:25. An irreversible capacity was measured by producing a single negative electrode. The irreversible capacity was 23.0 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The DC resistance was 10.9 Ω.

(Example 5)
Polymer (C) was synthesized using monomers of formula (7) and formula (10) as monomers. In addition, the mol ratio of Formula (7) and Formula (10) was 50:50. A negative electrode single electrode was prepared and the irreversible capacity was measured. The irreversible capacity was 22.8 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The direct current resistance was 11.1Ω.

(Example 6)
A polymer (D) was synthesized using monomers of formula (7) and formula (10) as monomers. In addition, the mol ratio of Formula (7) and Formula (10) was 25:75. An irreversible capacity was measured by producing a single negative electrode. The irreversible capacity was 22.0 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The direct current resistance was 11.4Ω.

(Example 7)
Polymer (E) was synthesized using monomers of formula (7) and formula (9) as monomers. In addition, the mol ratio of Formula (7) and Formula (10) was 25:75. A negative electrode single electrode was prepared and the irreversible capacity was measured. The irreversible capacity was 22.0 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The direct current resistance was 11.4Ω.

(Comparative Example 1)
In Example 1, it evaluated similarly to Example 1 except not using a coating | covering material. As a result, the irreversible capacity was 25.4 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The DC resistance was 11.5Ω.

(Comparative Example 2)
In Example 1, it evaluated similarly to Example 1 except having made the coating amount into 5 wt%. As a result, the irreversible capacity was 26.2 mAhg ⁻¹ . Next, a small battery was produced, and DC resistance was measured. The direct current resistance was 13.0Ω.

  In order to obtain a sufficient output when producing a lithium ion secondary battery, the direct current resistance is preferably less than 13Ω. Since Comparative Example 1 does not use a coating material, the initial resistance is a low value of 11.5Ω. However, lithium ions are trapped due to formation of precipitates by decomposition of the electrolytic solution on the negative electrode active material due to the reaction between the negative electrode active material and the electrolytic solution, and the value of the irreversible capacity is high. On the other hand, in Examples 1-7, since the coating | covering material was added appropriately, decomposition | disassembly of electrolyte solution can be suppressed. Further, since the covering material used in the examples has high lithium ion conductivity, the DC resistance can be suppressed to less than 13Ω even when 2% of the covering material is used. However, it was found that Comparative Example 2 using 5% of the coating material was not preferable because the DC resistance exceeded 13Ω. From these results, it was found that the coating amount of the coating material is preferably 0.01 wt% or more and 4 wt% or less with respect to the negative electrode active material.

It was found that when the coating material produced by formula (7) alone was used, the direct current resistance was low and an excellent battery could be produced. When the monomer of the formula (10) was copolymerized with the formula (7), it was found that although the direct current resistance was inferior to that of the homopolymerization, a reduction in the irreversible capacity could be realized. This is considered to be because the affinity between the coating material and the negative electrode active material is improved by the segment of the formula (10).

Claims (5)

In a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte solution,
The negative electrode has a negative electrode active material,
The negative electrode active material is coated with a coating material represented by the formula (1),
The covering amount of the covering material is a lithium ion secondary battery that is 0.01 wt% or more and 4 wt% or less with respect to the negative electrode active material.
(In the formula (1), R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are any of an alkyl group, a halogen group and hydrogen. Z in the formula (1) is an aromatic group, X is hydrogen, an alkali metal, or an alkaline earth metal, m in the formula (1) is the number of repeating units of the CF ₂ group, and B in the formula (1) is a polar functional group. X and y are copolymer composition ratios, and x / (x + y) satisfies 0 <x / (x + y) ≦ 1. In claim 1,
The lithium ion secondary battery in which the aromatic group is either a phenylene group or a naphthylene group. In claim 1 or claim 2,
The said covering material is a lithium ion secondary battery represented by Formula (13).
(In the formula (13), R ₄ , R ₅ and R ₆ are any one of an alkyl group, a halogen group and hydrogen. B in the formula (1) is a polar functional group. It is a polymerization composition ratio, and x / (x + y) satisfies 0 <x / (x + y) ≦ 1.) In any one of Claims 1 thru | or 3,
The lithium ion secondary battery, wherein the polar functional group is a functional group having a hydroxyl group, a carboxyl group, a sulfo group, an amino group, or a phosphate group. In claim 4,
The said polar functional group is a lithium ion secondary battery which is either Formula (19), Formula (20), Formula (21), Formula (22), or Formula (23).
(X in Formula (19), Formula (20), Formula (21), Formula (22), and Formula (23) is an alkali metal or H.)