JP2019175631A - Positive electrode for lithium ion secondary battery and lithium ion secondary battery using the same - Google Patents

Abstract

To provide a positive electrode active material for a secondary battery having high cycle characteristics at high potential, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the same.SOLUTION: There is provided a positive electrode 101 for a lithium ion secondary battery including a lithium metal composite oxide as a positive electrode active material and a carbon material represented by the following general formula (1). The general formula (1): LiC(1). (x>12 in the above general formula (1))SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  In recent years, electronic devices such as mobile phones and personal computers have been rapidly reduced in size and cordless, and there is an increasing demand for secondary batteries having a small size, light weight, and high energy density as power sources for driving these devices. Under such circumstances, a lithium ion secondary battery having a large charge / discharge capacity and a high energy density has attracted attention.

To increase the energy density of lithium ion secondary batteries, it is urgent to develop positive electrode active materials with high potential and high capacity. Conventionally, the search and development of a positive electrode active material exhibiting a high potential has led to the appearance of a battery voltage of around 4 V, which has attracted attention. For example, lithium transition metal composite oxides mainly composed of nickel or cobalt such as Li _x NiO ₂ (0 <x ≦ 1.0) and Li _x CoO ₂ (0 <x ≦ 1.0) have high potential and stability. The most promising in terms of sex and long life. Among these, Li _a Ni _x Co _y Al _z O _2, which is a positive electrode active material mainly composed of lithium nickelate (LiNiO ₂ ), where a, x, y, and z are 0.9 ≦ a ≦ 1. .3, 0.3 ≦ x ≦ 0.95, 0.01 ≦ y ≦ 0.50, 0.001 ≦ z ≦ 0.70, and x + y + z between x, y and z = 1)) is a positive electrode active material exhibiting a relatively high potential, and is expected to have a high discharge capacity and an increased energy density.

  It is known that these positive electrode active materials can draw more capacity when used at a higher potential of 4.4 V or higher.

  However, when these positive electrode active materials are used at a higher potential of 4.4 V or higher, there is a problem that the cycle characteristics deteriorate due to side reactions with the electrolytic solution.

  In order to solve such a problem, a method of coating the surface of the positive electrode active material with an oxide film or carbon has been proposed. For example, Patent Document 1 reports that safety and cycle characteristics are improved by mixing a vanadium phosphate coated with carbon and a lithium nickel composite oxide.

JP 2013-84566 A

  However, the method of Patent Document 1 does not sufficiently suppress the deterioration of the lithium nickel composite oxide, and further improvement in cycle characteristics at a high potential is demanded as the lithium ion secondary battery is diversified.

  The present invention has been made in view of the above-described problems of the prior art, and provides a lithium ion secondary battery having high cycle characteristics at a high potential.

In order to solve the above problems, a lithium ion secondary battery according to the present invention is characterized by containing a lithium metal composite oxide as a positive electrode active material and a carbon material represented by the following general formula (1).
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)

  According to this, the electric potential in the vicinity of the carbon material represented by the general formula (1) is lower than the electric potential in the vicinity of the positive electrode active material particles. The presence of the carbon material represented by the general formula (1) near the surface of the positive electrode active material particle makes the potential near the surface of the positive electrode active material particle lower than the inside of the positive electrode active material particle, and the positive electrode active material at a high potential. The reaction between the particle surface and the electrolytic solution and the release of oxygen from the positive electrode active material can be suppressed, and the cycle characteristics at a high potential can be improved.

  The carbon material in the coating layer preferably contains at least one of graphene, multilayer graphene, and graphite.

  According to this, it is suitable as a carbon material represented by the above general formula (1), and more effectively lowers the potential of the surface of the positive electrode active material, and more efficiently improves cycle characteristics at a high potential. it can.

  The ratio of the carbon material to the total mass of the positive electrode active material is preferably 0.1% by mass to 10% by mass.

  According to this, it is suitable as the addition amount of the carbon material represented by the general formula (1), and the cycle characteristics at a high potential can be improved more efficiently.

  The carbon material is preferably present as a coating layer covering at least a part of the surface of the positive electrode active material particles.

  According to this, it is suitable as a containing form of the carbon material represented by the general formula (1), and the cycle characteristics at a high potential can be improved more efficiently by being formed as a coating layer.

The average thickness (D _B ) of the coating layer with respect to the average particle diameter (D _A ) of the positive electrode active material particles preferably satisfies 0.001 ≦ D _B / D _A ≦ 0.5.

  According to this, it is suitable as the coating layer thickness with respect to the particle diameter of the positive electrode active material particles, and the cycle characteristics at a high potential can be improved more efficiently.

The average thickness of the covering layer is preferably 10 nm ≦ D _B ≦ 500 nm.

  By using the positive electrode active material layer according to the above aspect, a lithium ion secondary battery having high cycle characteristics at a high potential can be provided.

It is a cross-sectional schematic diagram of the lithium ion secondary battery concerning this embodiment.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments according to the invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Lithium ion secondary battery>
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery 1 according to the present embodiment. As shown in FIG. 1, the lithium ion secondary battery 1 includes a power generation element 10 and an exterior body 20. The power generation element 10 is impregnated with an electrolytic solution. The exterior body 20 prevents the electrolytic solution from leaking to the outside, and prevents external air and moisture from reaching the power generation element 10.

(Power generation element)
The power generation element 10 includes a positive electrode 101, a negative electrode 102, and a separator 103. In the power generation element 10 illustrated in FIG. 1, a pair of a positive electrode 101 and a negative electrode 102 are opposed to each other with a separator 103 interposed therebetween. Here, description is made based on an example in which the positive electrode 101 and the negative electrode 102 are one layer at a time, but the number of stacked layers is not limited.

  The positive electrode 101 includes a positive electrode current collector 101A and a positive electrode active material layer 101B formed on the positive electrode current collector 101A. The negative electrode 102 includes a negative electrode current collector 102A and a negative electrode active material layer 102B formed on the negative electrode current collector 102A. The separator 103 is located between the negative electrode active material layer 102B and the positive electrode active material layer 101B.

<Positive electrode>
The positive electrode 101 according to the present embodiment includes a positive electrode current collector 101A, a positive electrode active material layer including a positive electrode active material, a positive electrode binder, a positive electrode conductive additive, and a carbon material represented by the following general formula (1). 101B. The carbon material may be present as a coating layer that covers at least a part of the surface of the positive electrode active material particles.
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)
(Positive electrode current collector)
The positive electrode current collector 101A may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 101B is mainly composed of a positive electrode active material, a positive electrode binder, a positive electrode conductive additive, a carbon material represented by the following general formula (1), and a positive electrode additive. The carbon material may be present as a coating layer that covers at least a part of the surface of the positive electrode active material particles.
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)

(Positive electrode active material)
As the positive electrode active material, lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a counter anion (for example, PF ₆ ⁻ ) of the lithium ion are reversibly performed. If it can be made to advance, it will not specifically limit, A well-known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and chemical formula: LiNi _x Co _y Mn _z M _a O ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, M is a composite represented by one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Metal oxide, lithium vanadium compound Li _a (M) _b (PO ₄ ) _c (where M = VO or V, and 0.9 ≦ a ≦ 3.3, 0.9 ≦ b ≦ 2.2, 0 .9 ≦ c ≦ 3.3), olivine-type LiMPO ₄ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr or VO) , lithium titanate _(Li 4 T _{5 O 12), LiNi x Co} y Al z O 2 ( composite metal oxide such as 0.9 <x + y + z < 1.1) can be mentioned.

(Binder for positive electrode)
As the positive electrode binder, the positive electrode active materials are bonded together, and the positive electrode active material layer 101B and the positive electrode current collector 101A are bonded. The binder is not particularly limited as long as it can be bonded as described above. For example, fluorine resin such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide A resin, a polyamideimide resin, or the like may be used. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene, polythiophene, and polyaniline. Examples of the ion conductive conductive polymer include those obtained by combining a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as LiClO ₄ , LiBF ₄ , and LiPF _6. It is done.

  The content of the binder in the positive electrode active material layer 101B is not particularly limited, but when added, the content is preferably 0.5 to 5 parts by mass with respect to the mass of the positive electrode active material.

(Conductive aid for positive electrode)
The conductive auxiliary agent for positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 101B, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, and conductive oxides such as ITO.

(Positive electrode active material coating layer)
The positive electrode active material coating layer according to this embodiment includes a carbon material represented by the following general formula (1).
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)

  The potential near the carbon material represented by the general formula (1) is lower than the potential near the positive electrode active material particles. By the presence of the carbon material represented by the general formula (1) near the surface of the positive electrode active material particles, the potential near the surface of the positive electrode active material particles becomes lower than the inside of the positive electrode active material particles. From this, the effect of improving the cycle characteristics at a high potential can be obtained by suppressing the reaction between the surface of the positive electrode active material particles and the electrolytic solution at a high potential and the release of oxygen from the positive electrode active material.

  In the positive electrode active material coating layer according to this embodiment, the carbon material in the coating layer preferably further includes at least one of graphene, multilayer graphene, and graphite.

  According to this, it is suitable as a carbon material into which lithium can be inserted, and the potential on the surface of the positive electrode active material can be more efficiently lowered and the cycle characteristics at a high potential can be more efficiently improved.

The ratio of these carbon materials and Li can be identified by quantifying the amounts of Li and C in LiC _x , respectively. The amount of each component can be identified by a known analysis method, for example, the amount of Li can be identified by ICP emission spectroscopy, and the amount of C can be identified by a carbon / sulfur analyzer (CS meter).

  In the positive electrode active material coating layer according to this embodiment, the ratio of the carbon material to the total mass of the positive electrode active material is preferably 0.1% by mass to 10% by mass.

  According to this, it is suitable as an addition amount of the carbon material, and the cycle characteristics at a high potential can be improved more efficiently.

  The amount of these carbon materials added can be identified by a known analysis method such as a carbon / sulfur analyzer (CS meter).

  In the positive electrode active material coating layer according to this embodiment, the carbon material is preferably present as a coating layer that covers at least a part of the surface of the positive electrode active material particles.

  According to this, it is suitable as a containing form of the carbon material represented by the general formula (1), and the cycle characteristics at a high potential can be improved more efficiently by being formed as a coating layer.

In the cathode active material coating layer according to the present embodiment, the average thickness (D _B ) of the coating layer with respect to the average particle diameter (D _A ) of the cathode active material particles is 0.001 ≦ D _B / D _A ≦ 0.5. It is preferable to satisfy.

  According to this, it is suitable as the coating layer thickness with respect to the particle diameter of the positive electrode active material particles, and the cycle characteristics at a high potential can be improved more efficiently.

In the positive electrode active material coating layer according to this embodiment, the average thickness of the coating layer is preferably 10 nm ≦ D _B ≦ 500 nm.

The thickness of the coating layer can be identified by a known analysis method such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The identification method uses the following method. A cross-sectional image of the positive electrode active material layer 101B is taken, and arbitrary 100 particles are selected. Observed from the center of the positive electrode active material at this time, the average value of the distance from the center to the surface is D _A / 2, and the average value of the distance from the surface of the positive electrode active material to the outermost layer of the coating layer is the positive electrode active material an average thickness D _B of the coating layer.

<Negative electrode>
(Negative electrode current collector)
The negative electrode current collector 102A only needs to be made of a conductive material. For example, a metal foil such as aluminum, copper, nickel, or titanium can be used.

(Negative electrode active material layer)
The negative electrode active material layer 102B is mainly composed of a negative electrode active material, a negative electrode binder, and a negative electrode conductive additive.

(Negative electrode active material)
The negative electrode active material used for the negative electrode active material layer 102B may be any compound that can occlude and release (precipitate / dissolve, alloy or non-alloy) lithium ions, and a known negative electrode active material can be used. Examples of the negative electrode active material include carbon materials such as lithium metal, graphite capable of occluding and releasing lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, Metals that can be combined with lithium such as aluminum, silicon, tin, silicon oxide SiO _x (0 <x <2), amorphous compounds mainly composed of oxides such as tin dioxide, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.

  When lithium metal is used in the negative electrode 102, lithium metal may not be present before initial charging (if the lithium metal is not present before initial charging, the battery is in a discharged state and is safe during battery manufacture). In this case, lithium metal is deposited on the negative electrode current collector 102A during charging, and the lithium metal deposited during discharging is eluted. This layer containing lithium metal can be regarded as the negative electrode active material layer 24. In preparation for the shortage of the amount of lithium metal contributing to the discharge, a lithium metal foil may be provided on at least a part of the negative electrode current collector 22 before the initial charging.

(Binder for negative electrode)
In order to bond the negative electrode active material and the negative electrode active material, the negative electrode active material and the conductive additive, and the negative electrode active material and the negative electrode current collector 102A, a binder is added to the negative electrode active material layer 102B. Properties required for the binder include not being dissolved or extremely swollen in the electrolytic solution, being resistant to reduction, and having good adhesion. As the binder used for the negative electrode active material layer 102B, polyvinylidene fluoride (PVDF) or a copolymer thereof, polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole ( PBI), styrene butadiene rubber (SBR), carboxymethyl cellulose, polyacrylic acid (PA) and copolymers, cross-linked metal ions of polyacrylic acid (PA) and copolymers, polypropylene grafted with carboxylic anhydride (PP ) And polyethylene (PE), or a mixture thereof. Of these, polyamideimide is preferable. Polyimide is added as a precursor polyamic acid, and is heat-treated after electrode formation to become polyimide.

  The binder content in the negative electrode active material layer 102B is not particularly limited, but is preferably 1% by weight to 15% by weight based on the total weight of the negative electrode active material, the conductive additive and the binder, and 3% by weight to More preferably, it is 10% by weight. When the amount of the binder is too small, there is a tendency that a negative electrode having sufficient adhesive strength cannot be formed. On the other hand, if the amount of the binder is too large, the binder is generally electrochemically inactive and thus does not contribute to the discharge capacity, and it tends to be difficult to obtain a sufficient volume or weight energy density. The content of the conductive additive in the negative electrode active material layer 102B is not particularly limited, but when a conductive additive is added, it is usually preferably 0.5% by weight to 20% by weight with respect to the negative electrode active material. More preferably, the content is 1% by weight to 12% by weight.

(Conductive aid for negative electrode)
There is no limitation in particular as a conductive support agent for negative electrodes, The thing similar to the conductive support agent for positive electrodes described above can be used.

<Separator>
The separator 103 only needs to be formed of an electrically insulating porous structure, for example, a single layer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, polyester, and Examples thereof include a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polypropylene.

<Electrolyte>
The electrolytic solution is impregnated in the power generation element 10. As the electrolytic solution, an electrolyte solution containing a lithium salt or the like (electrolyte aqueous solution, non-aqueous electrolyte solution using an organic solvent) can be used.

(solvent)
In the non-aqueous electrolyte solution, an electrolyte is dissolved in a non-aqueous solvent, and a cyclic carbonate and a chain carbonate may be contained as a non-aqueous solvent.

  As cyclic carbonate, what can solvate electrolyte can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate can reduce the viscosity of the cyclic carbonate. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like may be mixed and used.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

(Electrolytes)
Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF _{2 SO 2) 2, LiN (} CF 3 SO 2) (C 4 F 9 SO 2), LiN (CF 3 CF 2 CO) 2, lithium salts such as LiBOB (lithium bis oxalate borate) may be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of the degree of ionization.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte solution is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. In addition, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

(Exterior body)
The exterior body 20 seals the power generating element 10 and the electrolytic solution therein. The outer package 20 prevents leakage of the electrolytic solution to the outside, penetration of moisture and the like into the battery 1 from the outside, and the like.

  As illustrated in FIG. 1, the exterior body 20 includes a heat-sealing resin layer 201, a metal layer 202, and a heat-resistant resin layer 203 in order from the power generation element 10. As a material of the heat sealing resin layer 201, polyolefin such as polyethylene and polypropylene can be used. As a material of the metal layer 202, aluminum, stainless steel, or the like can be used. As a material of the heat resistant resin layer 203, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide (PA), or the like can be used.

(Terminal)
The terminals 30 and 31 are made of a conductive material such as aluminum or nickel. One of the terminals is a positive electrode terminal 30 and the other is a negative electrode terminal 31. One end (inner end) of the terminals 30 and 31 is connected to the power generation element 10, and the other end (outer end) extends to the outside of the exterior body 20. The two terminals 30 and 31 may extend in the same direction or may extend in different directions. The positive electrode terminal 30 is connected to the positive electrode current collector 101A, and the negative electrode terminal 31 is connected to the negative electrode current collector 102A. The connection method is not particularly limited, and welding, screwing, or the like can be used.

<Method for manufacturing lithium ion secondary battery>
The lithium ion secondary battery 1 according to this embodiment can be manufactured, for example, by the following method.

(Production method of positive electrode)
The manufacturing method of the positive electrode 101 includes a composite process, a slurry preparation process, an electrode application process, and a rolling process.

"Composite process"
In the compounding step, the positive electrode active material and the coated carbon material are mixed while applying a shearing force, and the carbon material is uniformly formed on the surface of the positive electrode active material while increasing the density of the composite particles to such an extent that the carbon material does not deteriorate. To attach.

"Slurry preparation process"
Next, the composite particles composed of the positive electrode active material and the coated carbon material are mixed with a binder and a solvent according to the type thereof, for example, a solvent such as N-methyl-2-pyrrolidone in the case of PVDF, to prepare a slurry. To do.

"Electrode fabrication process"
The above-mentioned slurry is applied onto the positive electrode current collector 101A using a method appropriately selected from known methods such as a doctor blade, a slot die, a nozzle, and a gravure roll. The amount of active material supported on the positive electrode can be adjusted by adjusting the amount of coating and the line speed. Subsequently, the solvent in the slurry applied on the positive electrode current collector 101A is removed. The removal method is not particularly limited, and the positive electrode current collector 101A coated with the slurry may be dried, for example, at 60 ° C. to 150 ° C.

"Rolling process"
Finally, rolling is performed by a roll press, and the positive electrode 101 is completed. At this time, a higher electrode density can be obtained by heating the roll and softening the binder. The roll temperature is preferably in the range of 100 ° C to 200 ° C.

(Production method of negative electrode)
The negative electrode 102 can be produced by a slurry production process, an electrode application process, and a rolling process, excluding the above-described compounding process. Note that each step can be manufactured under the same conditions as the positive electrode 101.

  Finally, the power generation element 10 is manufactured by laminating the positive electrode 101 and the negative electrode 102 with the separator 103 interposed therebetween.

  Then, the terminals 30 and 31 are welded to the positive electrode current collector 101A and the negative electrode current collector 102A, respectively, by a known method.

(Method for manufacturing exterior body)
Next, the outer package 20 is produced. As a method for producing the outer package 20, first, respective films that become the heat-sealing resin layer 201, the metal layer 202, and the heat-resistant resin layer 203 are prepared. Next, adhesion between each layer is performed. For adhesion, an electrolyte-resistant adhesive (such as urethane adhesive) can be used. In particular, when reliability is required, an acid (maleic anhydride) -modified resin having adhesiveness to the resin layers 201 and 203 itself is used without using an adhesive between the resin layers 201 and 203 and the metal layer 202. It is preferable to use it. Then, the laminate film for an exterior body obtained by laminating each layer is folded at the center in the longitudinal direction with the heat sealing resin layer 201 inside, and the left and right ends in the longitudinal direction are heat sealed at 200 ° C. Thus, the outer package 20 is produced.

  Then, the produced power generation element 10 is sealed in the exterior body 20, and an electrolytic solution is injected into the exterior body 20. The lithium ion secondary battery 1 is completed by heat-sealing the opening of the outer package 20.

  As mentioned above, although preferred embodiment which concerns on this invention was described, this invention is not limited to the said embodiment.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Preparation of positive electrode)
LiCoO _{2 having} an average particle diameter of 10 μm as a positive electrode active material, and graphene prepared so that Li: C = 1: 24 as a covering carbon material of x = 24 in the following general formula (1) with respect to the positive electrode active material LiCoO ₂ Using 1.2 parts by mass and using a meso-fusion made by Hosokawa Micron inclined at 10 degrees, the composite was performed under the condition of 2500 rpm. When the obtained composite particles were observed with an SEM, the particle size (D _A ) of LiCoO ₂ was about 10 μm, and the thickness (D _B ) of the coating layer was about 30 nm. At this time, the ratio of the average particle diameter (D _A ) of the positive electrode active material particles and the thickness (D _B ) of the coating layer is D _B / D _A = 0.003. Next, 85 parts by mass of the positive electrode active material LiCoO ₂ on which the carbon coating layer described above is formed, 5 parts by mass of carbon black, and 10 parts by mass of PVDF are dispersed in N-methyl-2-pyrrolidone (NMP), and the positive electrode active material layer A slurry for formation was prepared. This slurry was applied to one surface of an aluminum metal foil having a thickness of 20 μm so that the applied amount of the positive electrode active material was 9.0 mg / cm ² and dried at 100 ° C. to form a positive electrode active material layer. Then, it pressure-molded with the roller press and produced the positive electrode.
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)

(Preparation of negative electrode)
90 parts by mass of natural graphite, 5 parts by mass of carbon black, and 5 parts by mass of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a negative electrode active material layer. The slurry was applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. Then, it pressure-molded with the roller press and produced the negative electrode.

(Preparation of electrolyte)
Were mixed so that EC / DEC = 3/7 by volume and this dissolved LiPF ₆ at a concentration of 1 mol / L. Thereafter, to this solution, lithium difluorophosphate (LiPO ₂ F ₂ ) was added as an additive so as to have a concentration of 1.0 × 10 ⁻² mol / L, thereby preparing an electrolytic solution.

(Production of evaluation lithium-ion secondary battery)
The positive electrode and negative electrode produced above and a separator made of a polyethylene microporous film were sandwiched between them to be put in an aluminum laminate pack. The electrolytic solution prepared above was injected into this aluminum laminate pack and then vacuum-sealed to prepare a lithium ion secondary battery for evaluation of Example 1.

(Measurement of capacity retention after 500 cycles)
About the lithium ion secondary battery for evaluation produced above, using a secondary battery charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.), the charge rate is 1.0 C (when constant current charging is performed at 25 ° C. in 1 hour) Charging is performed until the battery voltage reaches 4.5 V, which is generally a high potential, by constant current charging (current value at which charging ends), and the battery voltage is 2.8 V by constant current discharging at a discharge rate of 1.0 C. Discharge was performed until Detecting the discharge capacity after the charge and discharge ends, to determine the battery capacity to Q ₁ before the cycle test.

The battery determined battery capacity Q ₁ above, until again using a secondary battery charge and discharge test device, a 4.5V battery voltage is generally a high potential at a constant current charging charge rate 1.0C The battery was charged and discharged until the battery voltage reached 3.0 V by constant current discharge at a discharge rate of 1.0 C. The charge / discharge was counted as one cycle, and 500 cycles of charge / discharge were performed. Then, to detect the discharge capacity after 500 cycles of charge and discharge termination, to determine the battery capacity Q ₂ after 500 cycles.

From the capacitor _Q 1, _{Q 2} obtained above, according to equation (2), the capacity retention ratio was obtained E after 500 cycles. The obtained results are shown in Table 1.
E = Q ₂ / Q ₁ × 100 (2)

[Examples 2 to 5]
Rotational speed of the appropriate mechanofusion the kind and particle diameter of the positive electrode active material, by changing the processing time, and controls the D _B, except that D B _/ D _A changes as shown in Table 1, Example 1 Similarly, a lithium ion secondary battery was produced. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

[Examples 6 to 12]
As shown in Table 1, a lithium ion secondary battery was produced in the same manner as in Example 1 except that a carbon material in which the value of x of LiC _x was changed in the general formula (1) was used. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

"Examples 13 to 16"
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the carbon material forming the coating layer was changed as shown in Table 1. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

[Examples 17 to 23]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the amount of the coated carbon material was changed as shown in Table 1. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed. The amount of the coated carbon material is indicated by a ratio (% by weight) to the total weight of the positive electrode active material and the carbon material. The mechanofusion rotational speed, by changing the processing time, it has become possible to keep the D _B to a constant value.

[Examples 24-31]
Using the positive electrode active material having a different average particle diameter (D _A) as shown in Table 1, the rotational speed of mechanofusion, by changing the processing time, to control the thickness of the covering layer (D _B), D B _A lithium ion secondary battery was produced in the same manner as in Example 1 except that / DA was changed. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

[Examples 32-39]
_A lithium ion secondary battery as in Example 1 except that positive electrode active materials having various average particle diameters (D _A ) were used as shown in Table 1 and the thickness (D _B ) of the coating layer was controlled. Was made. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed. It should be noted that D _B / D _A can be maintained at a constant value by changing the rotation speed and processing time of mechanofusion.

[Example 40]
As shown in Table 1, a lithium ion secondary battery was produced in the same manner as in Example 1 except that the composite treatment was not performed in the composite step of the positive electrode active material and the coated carbon material. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

"Comparative Examples 1 and 2"
As shown in Table 1, a lithium ion secondary battery was produced in the same manner as in Example 1 except that a carbon material that did not satisfy the general formula (1) and was not layered carbon was used as the covering carbon material. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

“Comparative Example 3”
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a carbon material that did not satisfy the general formula (1) was used as the covering carbon material as shown in Table 1. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

“Comparative Example 4”
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the coated carbon material was not used and composite was not performed as shown in Table 1. And the charge / discharge test of 500 times was performed on the above-mentioned conditions, and the capacity | capacitance maintenance factor was computed.

  Table 1 shows the conditions of Examples 1 to 40 and Comparative Examples 1 to 4 and the obtained 500 cycle capacity retention ratio E (%).

  From Table 1, Examples 1-40 are all the comparative examples 1 and 2 which did not use the layered carbon material which satisfy | fills the said General formula (1) as a covering carbon material, and the said general formula (1) as a covering carbon material. In comparison with Comparative Example 3 in which no carbon material satisfying) was used and in Comparative Example 4 in which no coating carbon material was used and no composite was performed, the capacity retention rate after 500 cycles was improved, By using a layered carbon material satisfying the above general formula (1) as the carbon material and performing the composite, a synergistic effect by forming the coating layer was clarified. From the results of Examples 1 to 5, even if any positive electrode active material is used, there is an effect that the capacity maintenance rate after 500 cycles is further improved by combining the layered carbon material satisfying the general formula (1). It was confirmed that it was obtained.

  From the results of Examples 6 to 12, it was confirmed that by using a carbon material satisfying the general formula (1) as the covering carbon material, an effect of further improving the capacity retention rate after 500 cycles was obtained.

  From the results of Example 1 and Examples 13 to 16, it was confirmed that the effect of improving the capacity retention rate after 500 cycles can be obtained by using graphene, multilayer graphene, and graphite among the layered carbons. It was.

From the results of Examples 17 to 23, it was confirmed that by optimizing the amount of the coated carbon material, an effect of further improving the capacity retention rate after 500 cycles was obtained. Furthermore, from the results of Examples 24 to 31, by optimizing the _D B / _{D A} is the ratio of the average particle size of the positive electrode active material _{(D A)} and the thickness of the covering layer _{(D B),} 500 cycles It was confirmed that the effect of further improving the capacity maintenance rate later was obtained.

From the results of Examples 32-39, it was confirmed that by optimizing the thickness (D _B ) of the coating layer, an effect of further improving the capacity retention rate after 500 cycles was obtained.

  From the results of Examples 1 and 40, it was confirmed that an effect of further improving the capacity retention rate after 500 cycles was obtained by forming a coating layer by combining the positive electrode active material and the above-described coated carbon material.

  From Table 1, it was found that the lithium ion secondary battery according to this example can achieve high cycle characteristics at a high potential. On the other hand, in the lithium ion secondary battery according to the comparative example, the cycle characteristics at a high potential were lowered.

  The present invention provides a positive electrode active material for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the same, having high cycle characteristics at a high potential.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Electric power generation element 101 Positive electrode 101A Positive electrode collector 101B Positive electrode active material layer 102 Negative electrode 102A Negative electrode collector 102B Negative electrode active material layer 103 Separator 20 Exterior body 201 Heat-fusion resin layer 202 Metal layer 203 Heat resistant resin Layer 30 Positive terminal 31 Negative terminal

Claims (7)

A positive electrode for a lithium ion secondary battery comprising a lithium metal composite oxide as a positive electrode active material and a carbon material represented by the following general formula (1).
General formula (1):
LiC _x (1)
(In the general formula (1), x> 12)   2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the carbon material includes at least one of graphene, multilayer graphene, or graphite, which is layered carbon.   The lithium ion secondary battery positive electrode according to any one of claims 1 to 2, wherein a ratio of the carbon material to a total mass of the positive electrode active material is 0.1 mass% to 10 mass%.   The lithium ion secondary battery positive electrode according to any one of claims 1 to 3, wherein the carbon material is present as a coating layer that covers at least a part of the surface of the positive electrode active material particles. 5. The average thickness (D _B ) of the coating layer with respect to the average particle diameter (D _A ) of the positive electrode active material particles satisfies 0.001 ≦ D _B / D _A ≦ 0.5. The lithium ion secondary battery positive electrode as described in any one of these. The lithium ion secondary battery positive electrode according to claim 4, wherein an average thickness of the coating layer is 10 nm ≦ D _B ≦ 500 nm.   The lithium ion secondary battery which has a positive electrode as described in any one of Claims 1 thru | or 5, a negative electrode, a separator, and electrolyte.

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