JP2016189321A - Positive electrode active material for lithium ion secondary battery, positive electrode for lithium ion secondary battery and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for lithium ion secondary battery having high rate characteristics, and to provide a positive electrode for lithium ion secondary battery and a lithium ion secondary battery.SOLUTION: A positive electrode active material for lithium ion secondary battery has active material particles containing one or more kinds of compound containing Li and a transition metal, and a coating layer covering at least a part of the surface of the active material particles. The coating layer is composed of at least one kind of graphene and multilayer graphene. In the Raman spectrum of the coating layer, there are G band (peak of 1530-1630 cm) and 2D band (peak of 2650-2750 cm), and the intensity of the 2D band (2D/G) standardized by the intensity of the G band satisfies the relation 0.05≤2D/G.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material for lithium ion secondary batteries having high rate characteristics, a positive electrode for lithium ion secondary batteries, and a lithium ion secondary battery using the same.

In recent years, lithium ion secondary batteries, which are attracting attention as power sources for mobile devices and large-sized devices such as electric vehicles, are required to further improve rate characteristics and energy density. Lithium ion secondary batteries are mainly composed of a positive electrode, a negative electrode, an electrolytic solution, a separator, and the like, and in particular, improvement in rate characteristics can be expected by improving the characteristics of the positive electrode.
For example, Patent Documents 1 and 2 report a technique for reducing the internal resistance of the positive electrode by coating the surface of the positive electrode active material particles with a conductive material. Patent Document 3 improves the rate characteristics by controlling the carbonization degree (electron conductivity) of the coating layer by the intensity ratio of the G band and D band of the Raman spectrum of the active material containing carbon on the particle surface.
However, with the diversification of lithium ion secondary batteries, further improvement in rate characteristics is required.

JP 2007-173134 A Special table 2014-510997 gazette JP 2012-234766 A

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide a lithium ion secondary battery having high rate characteristics.

In order to achieve the above goal, the positive electrode active material according to the present invention has active material particles containing at least one compound containing Li and a transition metal, and a coating layer covering at least a part of the surface of the active material particles. and, the covering layer comprises at least one of the graphene or multilayer graphene, in the Raman spectrum of the coating layer, and G band (peak appearing at ^{1530cm -1 ~1630cm} -1 derived from the stretching vibration of C-C bonds), Intensity of 2D band having a 2D band (a peak appearing at 2650 cm ⁻¹ to 2750 cm ⁻¹ derived from inelastic scattering of phonons twice between Dirac cones in the Brillouin zone) and normalized by the intensity of the G band (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int .

  By using the positive electrode active material according to the present invention, a lithium ion secondary battery having improved rate characteristics can be provided. When the Raman spectrum of the active material in which at least a part of the surface of the active material particles is coated with graphene or multilayer graphene satisfies the above conditions, the conductivity of the good graphene or multilayer graphene itself and the good graphene or multilayer graphene This is presumably because the conductivity between the active material and the active material is compatible.

In the Raman spectrum of the coating layer of the positive electrode active material according to the present invention, the intensity of the 2D band (2D _int / G _int ) normalized by the intensity of the G band satisfies 0.1 ≦ 2D _int / G _int . desirable.
When these conditions are satisfied, the conductivity of the graphene or multilayer graphene itself and the conductivity between the graphene or multilayer graphene and the active material are further improved, so that the rate characteristics are further improved.

In the positive electrode according to the present invention, at least a part of the active material particle surface is coated with graphene or multilayer graphene, and the G band and 2D band of the Raman spectrum of the coating layer satisfy 0.05 ≦ 2D _int / G _int. Contains substances.

  By using the positive electrode according to the present invention, a positive electrode with improved rate characteristics can be obtained.

  The lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode includes the positive electrode active material described above.

  By using the lithium ion secondary battery according to the present invention, a lithium ion secondary battery with improved rate characteristics can be provided.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material which has a high rate characteristic, the positive electrode using the same, and a lithium ion secondary battery can be provided.

It is a schematic cross section of the lithium ion secondary battery of this embodiment.

  An example of a preferred embodiment of a lithium ion secondary battery according to the present invention will be described in detail with reference to the drawings. However, the lithium ion secondary battery of the present invention is not limited to the following embodiments. In addition, the dimensional ratio of drawing is not restricted to the ratio of illustration.

(Lithium ion secondary battery)
An electrode and a lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG. The lithium ion secondary battery 100 mainly includes a laminate 40, a case 50 that accommodates the laminate 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 40. Although not shown, the electrolytic solution is housed in the case 50 together with the laminate 40.

  The laminated body 40 is configured such that the positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32. The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 20 and the negative electrode 30 are collectively referred to as electrodes 20 and 30, and the positive electrode current collector 22 and the negative electrode current collector 32 are collectively referred to as current collectors 22 and 32. The positive electrode active material layer 24 and the negative electrode active material The material layer 34 is collectively referred to as the active material layers 24 and 34.

(Positive electrode active material layer)
The positive electrode active material layer 24 is mainly composed of a positive electrode active material, a binder, and, if necessary, a conductive agent.

(Positive electrode active material)
The positive electrode active material has active material particles containing one or more compounds containing Li and a transition metal, and a coating layer that covers at least a part of the surface of the active material particles, and the coating layer is at least graphene or multilayer graphene It made one, in the Raman spectrum of the coating layer, ^(the peak of 1530cm ^-1 ~1630cm -1) G band, D band ^(peak of 1300cm ^-1 ~1400cm -1) and 2D band ^{(2650cm -1 ~2750cm} -1 Peak)
The intensity (2D _int / G _int ) of the 2D band normalized by at least the intensity of the G band satisfies 0.05 ≦ 2D _int / G _int .

If the intensity of the 2D band normalized by the G band satisfies 0.05 ≦ 2D _int / G _int , the graphene or multilayer graphene forming the coating layer has high electron conductivity, and the irregularities on the surface of the active material particles Therefore, the rate characteristic is improved.

In addition, when the intensity of the 2D band normalized by the intensity of the G band (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int ≦ 0.4, not only the rate characteristics, It is preferable because the stability of the paint is improved and the coating property is improved.

  Here, graphene is a substance of a monoatomic layer having a structure in which six-membered rings of carbon atoms are spread on a plane. Multilayer graphene is a substance having a structure in which a plurality of graphenes are stacked, and a multilayer graphene having a thickness of 100 nm or less.

(Coating layer)
The covering layer preferably satisfies the condition that the thickness of the material forming the covering layer is 20 nm or less from the viewpoint of the crystallinity of the material constituting the covering layer and the adhesion to the active material particles.

More preferably, the intensity of the 2D band (2D _int / G _int ) normalized by the intensity of the G band satisfies 0.1 ≦ 2D _int / G _int .
When the graphene or multilayer graphene forming the coating layer satisfies the above conditions, the conductivity of the graphene or multilayer graphene itself, the conductivity between the graphene or multilayer graphene and the active material particles are improved, and Li insertion and desorption from the active material is also achieved. This is preferable because it becomes easy and rate characteristics are further improved.

  Furthermore, it is preferable that the intensity (Dint / Gint) of the D band normalized by the intensity of the G band is 0.6 ≦ Dint / Gint ≦ 1.2.

  When the D band intensity normalized by the G band satisfies 0.6 ≦ Dint / Gint, the adhesion between graphene or multilayer graphene and the active material particles is improved, and by satisfying Dint / Gint ≦ 1.2 The crystallites constituting the graphene or multilayer graphene have a size sufficient for the graphene or multilayer graphene to exhibit good conductivity.

Examples of the active material containing one or more compounds including Li and a transition metal include lithium cobaltate represented by LiCoO ₂ , lithium manganate represented by LiMnO ₂ , and Li _x Ni _(1-yz) Co _y Al. _{z O 2 [0.05 ≦ x ≦} 1.2,0 <y ≦ 0.5,0 <z ≦ 0.5, y + z ≦ 0.5] or _{Li x Ni (1-y-} z) Co y Mn _{z O 2 [0.05 ≦ x ≦} 1.2,0 <y ≦ 0.5,0 <z ≦ 0.5, y + z ≦ 0.5] lithium nickelate represented by the represented by LiFePO ₄ Examples thereof include lithium iron phosphate, lithium manganese phosphate represented by LiMnPO ₄ , lithium vanadium phosphate represented by LiVOPO ₄ , Li ₃ V ₂ (PO ₄ ) _{3, and the} like. Among them, Li _x Ni _(1-yz) Co _y Al _z O ₂ [0.05 ≦ x ≦ 1.2, 0 <y ≦ 0.5, 0 <z ≦ 0.5, y + z ≦ 0.5 ] Is preferable because of its high capacity.
Needless to say, a plurality of compounds of the above-described examples may be mixed. In the case of mixing, at least the graphene or multilayer graphene is Li _x Ni _(1-yz) Co _y Al _z O ₂ [0.05 ≦ x ≦ 1.2, 0 <y ≦ 0.5, 0 <z. It is preferable to coat the compound represented by ≦ 0.5, y + z ≦ 0.5].

  Here, the type of the active material according to the present embodiment can be identified by analysis of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, or the like. Among these, X-ray diffraction is preferable.

  The active material preferably constitutes secondary particles. When constituting secondary particles, the average particle size of the secondary particles of the active material is preferably 500 nm to 50 μm. If the average particle size is within this range, the proportion of the active material increases due to the decrease in the amount of graphene or multilayer graphene necessary to form a sufficient coating layer on the surface of the active material particles, and the inside and outside of the active material particles The conduction of electrons and Li ions is improved, and the capacity of the positive electrode is improved.

The Raman spectrum of the coating layer formed on the surface of the active material particles by graphene or multilayer graphene according to the present embodiment can be measured by, for example, a laser Raman microscope.
In the laser Raman microscope, for example, an argon ion laser can be used as a light source. A known Raman measurement apparatus using an argon ion laser may be used. For example, the measurement may be performed using an argon ion laser having a wavelength of 514.532 nm.

The measured Raman spectra are G band, D band and 2D band (peaks appearing at 1300 cm ⁻¹ to 1400 cm ⁻¹ derived from elastic scattering and inelastic scattering of phonons once between Dirac cones in the Brillouin zone) and wave number range ^{(700cm -1 ~1100cm -1, 1800cm -1} ~2500cm -1 and ^{2850cm -1 ~3100cm} -1) excluding the 2D band performs fitting using a quadratic function with respect to, the secondary function The baseline is the intensity of the G band and 2D band. Further, the measurement is repeated several times, and the measurement is repeated until the standard deviation of the noise with respect to the G band intensity becomes 5% or less.

(Positive electrode current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Positive electrode binder)
The binder binds the active materials to each other and binds the active material to the current collector 22. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And the like.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFPPTFE-based) Fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride Ride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber It may be used vinylidene fluoride-based fluorine rubbers such as rubber (VDF-CTFE-based fluorine rubber).

  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. In this case, since the binder also functions as a conductive material, it is not necessary to add a conductive material. Examples of the ion conductive conductive polymer include those obtained by combining a polymer compound such as polyethylene oxide and polypropylene oxide with a lithium salt or an alkali metal salt mainly composed of lithium.

(Conductive agent)
By forming a coating layer of graphene or multilayer graphene on the surface of the active material particles, the battery exhibits high rate characteristics without adding a conductive agent during electrode preparation. However, rate characteristics are further improved by adding a small amount of a carbon material having a small particle diameter such as carbon black or acetylene black at the time of preparing the coating. This is presumably because the carbon material having a small particle size enhanced the electrical conductivity at the contact between the coating layers of graphene or multilayer graphene.

(Negative electrode active material layer)
The negative active material layer 34 is mainly composed of a negative electrode active material, a binder, and, if necessary, a conductive agent.

(Negative electrode active material)
The negative electrode active material should just be a compound which can occlude / release lithium ion, and can use the well-known negative electrode active material for lithium ion batteries. Examples of the negative electrode active material include carbon materials that can occlude and release lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and the like, aluminum, silicon And particles containing a metal that can be combined with lithium such as tin, an amorphous compound mainly composed of an oxide such as silicon dioxide and tin dioxide, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). . It is preferable to use graphite having a high capacity per unit weight and relatively stable.

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Negative electrode conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotube, carbon material, fine metal powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and fine metal powder, and conductive oxide such as ITO. It is done.

(Negative electrode binder)
The binder used for a negative electrode can use the same thing as a positive electrode.

(Separator)
The separator 10 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.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution has an electrolyte dissolved in a nonaqueous solvent, and may contain a cyclic carbonate and a chain carbonate as a nonaqueous solvent.

  The cyclic carbonate is not particularly limited as long as it can solvate the electrolyte, and a known cyclic carbonate can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate is not particularly limited as long as the viscosity of the cyclic carbonate can be reduced, and a known chain carbonate can be used. 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.

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 Lithium salts such as CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can 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 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. Moreover, 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.

(Method for producing positive electrode active material)
The positive electrode active material according to the present embodiment can be produced by the following coating layer forming step.

(Coating layer forming process)
In the coating layer forming step, a coating layer of graphene or multilayer graphene can be formed on the surface of the active material particles. The method for forming the coating layer is not particularly limited, but an existing method for forming a coating layer on the particle surface, such as a mechanochemical method using mechanical energy such as friction or compression, or a spray drying method in which a coating liquid is sprayed on the particle is used. be able to. Among these, the mechanochemical method is preferable because it can form a coating layer that is uniform and has good adhesion.

  As a specific example of the manufacturing apparatus of the mechanochemical method, an apparatus such as a mechanofusion apparatus or a planetary mill can be used. As an example of a specific apparatus for the spray drying method, a spray dryer or the like can be used. Among these, a mechanochemical method in which shear stress is applied to the powder is preferable, and an example of the apparatus is a mechanofusion apparatus.

  The intensity ratio of the G band and the 2D band in the coating layer on the surface of the active material particle by graphene or multilayer graphene depends on the defect density or the number of layers of graphene or multilayer graphene contained in the coating layer. For example, when a coating layer is formed using a mechanochemical method in which a shear stress can be applied to the powder, the angle of the processing apparatus, the number of revolutions, the processing time, the amount of material input are adjusted, and the heat treatment after the coating layer is formed is appropriately adjusted. Thus, the intensity of the G band and 2D band of the Raman spectrum can be adjusted.

(Method for manufacturing electrodes 20 and 30)
Next, a method for manufacturing the electrodes 20 and 30 according to the present embodiment will be described.

  The active material, binder and solvent are mixed. A conductive material may be further added as necessary. As the solvent, for example, water, N-methyl-2-pyrrolidone or the like can be used. The mixing method of the components constituting the paint is not particularly limited, and the mixing order is not particularly limited. The paint is applied to the current collectors 22 and 32. There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

  Subsequently, the solvent in the paint applied on the current collectors 22 and 32 is removed. The removal method is not particularly limited, and the current collectors 22 and 32 to which the paint is applied may be dried, for example, in an atmosphere of 80 ° C. to 150 ° C.

  Then, the electrode on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way is subjected to a press treatment by a roll press device or the like as necessary. The linear pressure of the roll press can be set to 1000 kgf / cm, for example.

  Through the above steps, an electrode in which the electrode active material layers 24 and 34 are formed on the current collectors 22 and 32 is obtained.

(Method for producing lithium ion secondary battery)
Then, the manufacturing method of the lithium ion secondary battery which concerns on this embodiment is demonstrated. The method for manufacturing a lithium ion secondary battery according to the present embodiment includes a positive electrode 20 containing the active material, a negative electrode 30, a separator 10 interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte solution containing a lithium salt. And a step of enclosing the outer body 50 in the exterior body 50.

  For example, the positive electrode 20 including the active material described above, the negative electrode 30 and the separator 10 are stacked, and the positive electrode 20 and the negative electrode 30 are heated and pressed with a press tool from a direction perpendicular to the stacking direction. 20, the separator 10 and the negative electrode 30 are brought into close contact with each other. Then, for example, a lithium ion secondary battery can be manufactured by putting the laminate 40 into a bag-shaped outer package 50 prepared in advance and injecting a non-aqueous electrolyte solution containing the lithium salt. Instead of injecting the nonaqueous electrolyte solution containing the lithium salt into the outer package, the laminate 40 may be impregnated in advance with the nonaqueous electrolyte solution containing the lithium salt.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same function and effect. Are included in the technical scope.

Example 1
(Preparation of positive electrode)
Li _1.0 Ni _0.83 Co _0.14 Al _0.03 O _2.0 (hereinafter referred to as NCA) having an average secondary particle size of 15 μm as an active material containing Li and a transition metal, and an average thickness We measure multilayer graphene with a length of 8 nm and an average length of major axis and minor axis of 15 μm at a mass ratio of 100: 2 and use a Hosokawa micron mechanofusion inclined at 5 degrees for 20 minutes at a rotational speed of 3500 rpm. A coating layer of multilayer graphene was formed on the NCA surface. Further, the NCA on which the coating layer was formed was annealed in a vacuum atmosphere at 400 ° C. for 30 minutes to adjust the intensity of the G spectrum and 2D band of the Raman spectrum. A slurry was prepared by dispersing 97.5% of the positive electrode active material powder and 2.5% of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The obtained slurry was coated on an aluminum foil having a thickness of 15 μm, dried at a temperature of 120 ° C. for 30 minutes, and then pressed using a roll press apparatus at a linear pressure of 1000 kgf / cm to obtain a positive electrode.

(Measurement of multilayer graphene coating layer inside the positive electrode)
The coating state of the coating layer with multilayer graphene on the NCA particle surface was measured using a transmission electron microscope (TEM), a scanning electron microscope (SEM), a laser Raman microscope, a cross section polisher, and an ion milling device. A sample for measurement was prepared by cutting the positive electrode and polishing the cut surface with a cross section polisher and an ion milling device.
Moreover, the Raman spectrum of the coating layer by the multilayer graphene on the NCA particle surface was measured from the electrode surface using a laser Raman microscope.

  By observing the surface of the positive electrode and the cross section of the positive electrode with SEM, EDX, and TEM, it was confirmed that a uniform multilayer graphene coating layer was formed on the NCA particle surface. The thickness of the coating layer was measured, and the average thickness of the coating layer was found to be 190 nm.

  It was confirmed by the Raman mapping measurement of the cross section of the positive electrode with a laser Raman microscope that a coating layer of multilayer graphene was formed on the NCA particle surface.

(Preparation of negative electrode)
As negative electrode active material, 90 parts by mass of natural graphite powder and 10 parts by mass of PVDF were dispersed in NMP to prepare a slurry. The obtained slurry was coated on a copper foil having a thickness of 15 μm, dried under reduced pressure at a temperature of 140 ° C. for 30 minutes, and then pressed using a roll press apparatus to obtain a negative electrode.

(Nonaqueous electrolyte solution)
A non-aqueous electrolyte solution was prepared by dissolving LiPF ₆ at a concentration of 1.0 mol / L and LiBF ₄ at a concentration of 0.1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). The volume ratio of EC to DEC in the mixed solvent was EC: DEC = 30: 70.

(Separator)
A polyethylene microporous membrane having a thickness of 20 μm (porosity: 40%, shutdown temperature: 134 ° C.) was prepared.

(Production of battery)
The positive electrode, the negative electrode, and the separator were laminated to constitute a power generation element, and a battery cell of Example 1 was produced using this and the non-aqueous electrolyte.

(C rate)
The current density for charging or discharging a battery cell in one hour is called 1C. In the following, the current density during charging or discharging is expressed using a constant multiple of the C rate (for example, a current density half of 1C is 0.5C). Expressed as)

(Measurement of rate characteristics)
Using the produced battery cell of Example 1, constant current charging was performed until the voltage reached 4.2 V (vs. Li ^/ Li ⁺ ) at a current density of 0.1 C, and the current density was further increased to 0.05 C. Constant voltage charging was performed at 4.2 V (vs. Li ^/ Li ⁺ ) until the voltage decreased, and the charge capacity was measured.

Subsequently, after resting for 5 minutes, constant current discharge was performed at a current density of 0.1 C until the voltage reached 2.5 V (vs. Li ^/ Li ⁺ ), and the discharge capacity was measured. The current density was calculated by setting 1 C to 190 mAh with respect to 1 g of active material mass.

  The rate characteristics of the battery cells were measured by changing the current density during charging and discharging to 0.3 C, 0.5 C, 1 C, and 0.1 C and repeating the above charging and discharging procedure.

(Examples 2-7, Comparative Examples 1-4)
In Examples 2 to 7 and Comparative Examples 1 to 4, the treatment conditions when forming the coating layer on the active material surface with multilayer graphene by Hosokawa Micron mechanofusion and the heat treatment conditions after the coating layer formation were changed. Positive electrodes having different Raman spectra were produced, and battery cells were produced and evaluated in the same manner as in Example 1. In Comparative Examples 3 and 4, a coating layer using carbon black was formed on the surface of the active material, the processing conditions and the heat treatment conditions after forming the coating layer were changed, and positive electrodes having different Raman spectra of the coating layer were produced. Battery cells were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

From Table 1, a graphene or multilayer graphene coating layer is formed on at least a part of the NCA particle surface, and in the Raman spectrum of the coating layer, the intensity of the 2D band normalized by the intensity of the G band (2D _int / G _int ) However, when 0.05 ≦ 2D _int / G _int is satisfied, it can be seen that the rate characteristics are improved even with an electrode manufactured with a similar composition ratio.
In addition to the range of intensity of each band of the Raman spectrum, it can be seen that when 0.1 ≦ 2D _int / G _int is satisfied, the rate characteristics are further improved and the energy density of the battery is further improved. It can also be seen that the coating film on the surface of the active material particles shows higher rate characteristics when multilayer graphene is used than carbon black.

(Examples 8 to 11 and Comparative Example 5)
In Examples 8 to 11 and Comparative Example 5, the positive electrode active material is changed to lithium cobaltate whose secondary particles have an average particle diameter of 20 μm, and the coating layer on the active material surface with multilayer graphene is formed by mechanofusion made by Hosokawa Micron. A positive electrode having a different Raman spectrum of the coating layer was produced by changing the treatment conditions at the time and the heat treatment conditions after the coating layer was formed, and a battery cell was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2.

From Table 2, a graphene or multilayer graphene coating layer is formed on at least a part of the lithium cobaltate (Li _1.0 Co _1.0 O _2.0 ) particle surface, and in the Raman spectrum of the coating layer, the G band When the intensity of the 2D band normalized by intensity (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int , rate characteristics are improved even for electrodes made with the same composition ratio I understand.
In addition to the range of intensity of each band of the Raman spectrum, it can be seen that the rate characteristic is further improved when 0.1 ≦ 2D _int / G _int is satisfied.

(Examples 12 to 15, Comparative Example 6)
In Examples 12 to 15 and Comparative Example 6, the positive electrode active material was a ternary positive electrode active material (Li _1.0 Ni _1/3 Mn _1/3 Co _1/3 O having an average secondary particle size of 10 μm. _2.0 ), and the processing conditions when forming the coating layer on the active material surface with multilayer graphene by meso-fusion made by Hosokawa Micron and the heat treatment conditions after forming the coating layer are changed, and the positive electrode having a different Raman spectrum of the coating layer A battery cell was produced in the same manner as in Example 1 and evaluated. The results are shown in Table 3.

From Table 3, a coating layer of graphene or multilayer graphene is formed on at least a part of the ternary positive electrode active material (Li _1.0 Ni _1/3 Mn _1/3 Co _1/3 O _2.0 ) particle surface, In the Raman spectrum of the coating layer, when the intensity of the 2D band normalized by the intensity of the G band (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int , the same composition ratio was used. It can be seen that the rate characteristic is improved even with the electrode.
In addition to the range of intensity of each band of the Raman spectrum, it can be seen that the rate characteristic is further improved when 0.1 ≦ 2D _int / G _int is satisfied.

(Examples 16 to 19, Comparative Example 7)
In Examples 16 to 19 and Comparative Example 7, the positive electrode active material was changed to spinel manganese (Li _1.0 Mn _2.0 O _4.0 ) having an average particle size of secondary particles of 15 μm, and active by multilayer graphene A battery cell having the same Raman spectrum of the coating layer was produced by changing the processing conditions when forming the coating layer on the material surface by mechanofusion made by Hosokawa Micron and the heat treatment conditions after the coating layer was formed. Were prepared and evaluated. The results are shown in Table 4.

From Table 4, a graphene or multilayer graphene coating layer is formed on at least part of the spinel manganese (Li _1.0 Mn _2.0 O _4.0 ) particle surface, and the intensity of the G band in the Raman spectrum of the coating layer When the intensity of the 2D band normalized by (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int , the rate characteristics are improved even for electrodes made with the same composition ratio. As a result, it can be seen that the energy density of the battery is improved.
In addition to the range of intensity of each band of the Raman spectrum, it can be seen that the rate characteristic is further improved when 0.1 ≦ 2D _int / G _int is satisfied.

(Examples 20 to 23, Comparative Example 8)
In Examples 20 to 23 and Comparative Example 8, the positive electrode active material was changed to lithium iron phosphate (LiFePO ₄ ) whose secondary particles had an average particle diameter of 5 μm, and the coating layer on the active material surface with multilayer graphene was manufactured by Hosokawa Micron The positive electrode having a different Raman spectrum of the coating layer was prepared by changing the processing conditions when forming by mechanofusion and the heat treatment conditions after forming the coating layer, and a battery cell was prepared and evaluated in the same manner as in Example 1. . The results are shown in Table 5.

From Table 5, a graphene or multilayer graphene coating layer is formed on at least a part of the lithium iron phosphate (LiFePO ₄ ) particle surface, and the intensity of the 2D band normalized by the intensity of the G band in the Raman spectrum of the coating layer When (2D _int / G _int ) satisfies 0.05 ≦ 2D _int / G _int , rate characteristics are improved even with electrodes manufactured with the same composition ratio, and as a result, the energy density of the battery is improved. I understand that.
In addition to the range of intensity of each band of the Raman spectrum, it can be seen that the rate characteristic is further improved when 0.1 ≦ 2D _int / G _int is satisfied.

  As is apparent from the evaluation results, it can be confirmed that the example shows a higher rate characteristic than the comparative example.

DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 32 ... Negative electrode collector, 34 ... Negative electrode Active material layer, 40 ... power generation element, 50 ... exterior body, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (5)

An active material particle containing one or more compounds containing Li and a transition metal, and a coating layer covering at least a part of the surface of the active material particle,
The covering layer comprises at least one of the graphene or multilayer graphene, in the Raman spectrum of the coating layer, ^(the peak of 1530cm ^-1 ~1630cm -1) G band, and D band ^(peak of 1300cm ^-1 ~1400cm -1) Having a 2D band (peaks between 2650 cm ^{−1 and} 2750 cm ⁻¹ ),
A positive electrode active material for a lithium ion secondary battery, wherein a 2D band intensity (2D _int / G _int ) normalized by at least a G band intensity satisfies 0.05 ≦ 2D _int / G _int . 2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the intensity of the 2D band (2D _int / G _int ) normalized by the intensity of the G band satisfies 0.1 ≦ 2D _int / G _int. Active material. 3. The lithium according to claim 1, wherein the intensity (D _int / G _int ) of the D band normalized by the intensity of the G band satisfies 0.6 ≦ D _int / G _int ≦ 1.2. Positive electrode active material for ion secondary battery.   The positive electrode for lithium ion secondary batteries using the positive electrode active material for lithium ion secondary batteries as described in any one of Claims 1-3. A lithium ion secondary comprising the positive electrode for a lithium ion secondary battery according to claim 3, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. battery.

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