JP2017199670A - Positive electrode material for lithium ion battery, method for manufacturing the same, positive electrode for lithium ion battery, and lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material for a lithium ion battery, by which a lithium ion battery with high-temperature storage suitability good enough to suppress battery degradation can be obtained without marring output characteristics.SOLUTION: A positive electrode material for a lithium ion battery comprises positive electrode active material particles coated with functionalized graphene. The coverage of the positive electrode active material particles by the functionalized graphene is 80% or more on average. The ratio of functionalization determined from an expression (1) below based on measurements according to X-ray photoelectron spectrometry on the positive electrode material is 0.3-1.3. Functionalization ratio=[(Peak area based on C-O single bond)+(Peak area based on C=O double bond)+(Peak area based on COO bond)]/(Peak area based on C-C, C=C and C-H bonds) (1).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode material for a lithium ion battery and a method for producing the same.

  Compared to conventional nickel cadmium batteries and nickel metal hydride batteries, lithium-ion batteries can be made smaller and lighter as high-voltage, high-energy density batteries. Information-related mobile communications such as mobile phones and laptop computers Widely used in electronic equipment. As one of the means to solve environmental problems in the future, it is expected that the use will be expanded to in-vehicle applications mounted on electric vehicles and hybrid electric vehicles, and industrial applications such as electric tools. High output is eagerly desired.

  A lithium ion battery is configured by arranging a positive electrode and a negative electrode having an active material capable of reversibly removing and inserting lithium ions, and a separator separating the positive electrode and the negative electrode in a container and filling a non-aqueous electrolyte. .

For the positive electrode, a positive electrode active material for a lithium ion battery (hereinafter sometimes referred to as a positive electrode active material or an active material), a conductive agent and a binder containing a binder are applied to a metal foil current collector such as aluminum. It is pressure-molded. Current positive electrode active materials include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and a ternary system in which nickel is partially substituted with manganese and cobalt (LiNi _x Mn _y Co _1-xy O ₂ ), Ternary system partially substituted with cobalt-aluminum (LiNi _x Co _y Al _1-xy O ₂ ), spinel-type lithium manganate (LiMn ₂ O ₄ ), and complex oxides of lithium and transition metals ( Hereinafter, a powder of lithium metal oxide) may be used relatively well. In recent years, highly safe olivine (phosphoric acid) has attracted attention, and in particular, lithium iron phosphate (LiFePO ₄ ) containing iron, which is a resource-rich and inexpensive material, has begun to be put into practical use. A lithium manganese phosphate (LiMnPO ₄ ) having a high energy density has attracted attention as a next-generation active material. In addition, metal oxides such as V ₂ O ₅ and metal compounds such as TiS ₂ , MoS ₂ , and NbSe ₂ are also used.

  Also, the negative electrode is formed by applying an electrode material containing an active material, a conductive additive and a binder to a metal foil current collector such as copper as in the case of the positive electrode, and in general, as the active material of the negative electrode, Lithium alloys such as metallic lithium, Li-Al alloy, Li-Sn, silicon compounds containing SiO, SiC, SiOC, etc. as basic constituent elements, conductive polymers doped with lithium, such as polyacetylene and polypyrrole, lithium ions in the crystal Intercalation compounds incorporated into the carbon material, carbon materials such as natural graphite, artificial graphite, and hard carbon are used.

In the positive electrode active material currently in practical use, LiNi _0.8 Co _0.15 Al _0.05 O _{2 in} which a part of Ni atoms of lithium nickelate (LiNiO ₂ ) is substituted with Co atoms and Al atoms, since the ternary part of Ni atoms were replaced by Mn atoms and Co atoms _{(LiMn x Ni y Co 1-} x-y O 2) price, properties and balanced in terms of safety, these cathode active Substances are commonly used.

However, despite such attempts to stabilize the structure, battery degradation problems such as capacity degradation and increased internal resistance still exist when used under severe conditions such as repeated use or use at high temperatures. ing. Such battery deterioration is considered to be caused by a decomposition reaction of the electrolytic solution on the surface of the positive electrode active material, resulting in a high-resistance deterioration layer. For example, when LiNi _0.8 Co _0.15 Al _0.05 O ₂ is used as the active material, divalent Ni that is inactive on the surface is generated and grown by the cycle test and the storage test, It has been reported that electronic and ionic conductivity is reduced.

In order to suppress such battery deterioration, Non-Patent Document 1 discloses a method of covering the active material particle surface with an insulating layer such as Al ₂ O ₃ , AlPO ₄ , and LiAlO ₂ . Patent Documents 1 and 2 and Non-Patent Document 2 disclose a technique of reducing after mixing graphene oxide and a positive electrode active material.

JP 2013-93316 A International Publication No. 2014/115669

J. et al. Cho, Y. et al. J. et al. Kim, and B.B. Park, Chemistry of Materials, 12 (12), 3788 (2000). Qin Z. , Et al. Journal of Materials Chemistry, 2011, 22, 21144

  Since the coating with the insulating layer described in Non-Patent Document 1 is poor in electronic conductivity and ion conductivity, aluminum oxide is unavoidably reduced in output characteristics as a compensation for suppressing battery deterioration.

  On the other hand, in Patent Document 1 and Non-Patent Document 2, particles are obtained by mixing graphene oxide and an active material, drying, and reducing the dried body. However, in any case, since the graphene oxide is reduced by heating in a reducing atmosphere or an inert atmosphere at a high temperature of 500 ° C. to 800 ° C., the active material is covered with graphene having high electron conductivity. . If it does so, while electronic conductivity will become high, there exists a tendency for ionic conductivity to become scarce.

  In Patent Document 2, for the purpose of imparting conductivity to an active material having low conductivity, a precursor particle is produced by solid-phase mixing graphene oxide powder and olivine-based active material particles with a ball mill, and in the air Reduction is performed at 150 ° C to 250 ° C. According to this method, when olivine-based active material particles are used, the output characteristics are improved because the electron conductivity and the ionic conductivity are compatible. However, since a ball mill is used for mixing, when the granulated cathode particles are used, the granulated cathode particles are crushed and the specific surface area increases, and the graphene defects tend to increase. Problems remain in the effect of suppressing the decomposition reaction of the liquid.

  The objective of this invention is providing the positive electrode material for lithium ion batteries from which the lithium ion battery which has the favorable high temperature storage characteristic by which battery deterioration was suppressed, without impairing output characteristics.

The present invention for solving the above problems is a positive electrode material for a lithium ion battery in which positive electrode active material particles are coated with functionalized graphene,
An average coverage of the positive electrode active material particles by the functionalized graphene is 80% or more;
It is a positive electrode material for a lithium ion battery having a functionalization ratio of 0.3 or more and 1.3 or less obtained from the following formula (1) from a measurement value of the positive electrode material measured by X-ray photoelectron spectroscopy.
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on C—H bond) (1)

  By producing a lithium ion battery using the positive electrode material for a lithium ion battery of the present invention, a lithium ion battery having good high-temperature storage characteristics and cycle characteristics can be obtained without impairing output characteristics.

It is a figure which shows the result of the peak derived from carbon, and peak fitting when a positive electrode material is measured by X-ray photoelectron spectroscopy according to the measurement example 2.

<Positive electrode material for lithium ion battery>
The positive electrode active material particles for lithium ion batteries in the present invention can be used without particular limitation as long as they act as positive electrodes in lithium secondary batteries. For example, a layered oxide active material, a lithium excess active material, a spinel positive electrode active material such as lithium manganate (LiMn ₂ O ₄ ), a metal oxide active material such as V ₂ O ₅ , TiS ₂ , MoS ₂ , A metal compound-based active material such as NbSe ₂ or an olivine-based active material such as lithium iron phosphate or lithium manganese phosphate can be used. The present invention can be suitably applied to substances.

The type of the layered oxide-based active material is not particularly limited. However, LiCoO ₂ , LiNiO ₂ , Li (Ni _x Co _y ) O ₂ (where x + y = 1) from the capacity and output, and the results as a positive electrode material for a lithium ion battery. _{, Li (Ni x Co y Al} z) O 2 ( provided that _{x + y + z = 1)} , Li (Ni x Mn y Co z) O 2 ( provided that x + y + z = 1) , Li (Ni x Mn y) O 2 ( provided that x + y = _{1), Li 2 MnO 3 -Li} (Ni x Mn y Co z) O 2 ( provided that x + y + z = 1) , can be suitably used. Among them, the positive electrode material containing Ni has a high electron conductivity of the active material itself, a high capacity, but is poor in stability and has a large influence on the surface deterioration of the positive electrode active material. Therefore, the technique of the present invention is more suitably applied. it can.

  Further, the active material particles in the present invention include Na, Mg, K, Ca, Sc, Ti, V, Cr, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru as doping element metals. Rh, Pd, Ag, Cd, In, Sn, Cs and Ba may contain one or more metal elements selected from the group consisting of less than 10% by weight based on the active material particles.

Graphene generally refers to a sheet of sp ² bonded carbon atoms having a thickness of 1 atom (single-layer graphene). In this specification, a material having a flaky shape in which single-layer graphene is stacked is also included. Called graphene. Similarly, functionalized graphene and graphene oxide, which will be described later, are also referred to as including a laminated flake-like substance.

  Functionalized graphene refers to graphene in which a part of the graphene graphite structure is modified with a functional group such as a hydroxyl group, a carboxyl group, a ketone group, or an epoxy group. In the present invention, it is preferable to use graphene having a functionalization rate of 0.3 or more as the functionalized graphene.

  The functionalization rate is determined by X-ray photoelectron spectroscopy. In X-ray photoelectron spectroscopy, when a sample containing carbon is measured, a peak derived from carbon is detected in the vicinity of 284 eV, but it is known that when carbon is bonded to oxygen, it shifts to a higher energy side. Yes. Specifically, peaks based on C—C bonds, C═C bonds, and C—H bonds, in which carbon is not bonded to oxygen, are detected in the vicinity of 284 eV without shifting, and in the case of C—O bonds, in the vicinity of 286 eV, In the case of C = O bond, it shifts to around 287.5 eV, and in the case of COO bond, it shifts to around 288.5 eV. Therefore, a signal derived from carbon is detected in a form in which respective peaks around 284 eV, 286 eV, 287.5 eV, and 288.5 eV are superimposed. By separating the peak into each component by peak fitting, each peak area can be calculated, and it can be understood how much carbon is functionalized. Signals also appear near 286e and 290.5eV based on the graphite component. This signal is fitted as a component based on CC, C = C and CH bonds.

That is, the functionalization rate is
Functionalization rate = [(peak area based on C—O bond) + (peak area based on C = O bond) + (peak area based on COO bond)] / (C—C, C = C and C—H Peak area based on binding)
It is a numerical value defined by.

  If the size in the direction parallel to the graphene layer of the functionalized graphene of the positive electrode material of the present invention is too small, the contact resistance between the functionalized graphene increases, resulting in poor electron conductivity. Moreover, it becomes difficult to coat the functionalized graphene on the positive electrode active material particles. Therefore, the functionalized graphene in the present invention needs to be larger than a certain level. The size in the direction parallel to the graphene layer is preferably 1.0 μm or more, more preferably 5.0 μm or more, and further preferably 10 μm or more. Here, the size in the direction parallel to the graphene layer means the average of the maximum diameter and the minimum diameter when observed from a direction perpendicular to the plane direction of the graphene, and is randomly measured for 10 graphenes on the positive electrode active material particles. Mean the average value.

  In the positive electrode material of the present invention, the decomposition reaction of the electrolytic solution can be suppressed by coating the positive electrode active material particles with functionalized graphene, and the reaction can be suppressed as the average coverage is higher. In the present invention, the coating satisfies L> T when the particle diameter (median diameter) of positive electrode active material particles described later is L (μm) and the average coating thickness of functionalized graphene described later is T (μm). It means that functionalized graphene exists on the surface of the positive electrode active material particles in the state. In the present invention, the positive electrode active material particles need to be coated with functionalized graphene with an average coverage of 80% or more, and more preferably with an average coverage of 90% or more. The average coverage in this specification refers to the surface of the positive electrode material after observing the surface of the positive electrode material or the positive electrode containing the positive electrode material with a scanning electron microscope (SEM) and considering the observed image as a two-dimensional planar image. It is the value which calculated the ratio for which a graphene coating part accounts. Specifically, one of the positive electrode materials is magnified and observed by SEM, and the area of the graphene-coated portion in the area of the positive electrode active material particles is calculated after regarding the observed image as a two-dimensional planar image. This operation is performed for 50 positive electrode active material particles selected at random, and the average value is defined as the average coverage.

  The positive electrode material of the present invention has a functionalization rate of 0.3 or more and 1.3 or less. If the functionalization rate is too high, the ion conductivity increases, but the graphite structure collapses and the electron conductivity deteriorates. In addition, the reaction between the functional group and the electrolytic solution is likely to occur, and the high temperature storage characteristics are deteriorated. Therefore, in order to maintain electron conductivity, the functional grouping rate needs to be 1.3 or less. On the other hand, if the functionalization rate is too low, the electron conductivity is increased by being covered with graphene having high electron conductivity, but the ion conductivity tends to be poor. From these viewpoints, the functionalization rate is preferably 0.5 or more, and more preferably greater than 0.8. The functionalization rate is preferably 0.9 or less.

  Furthermore, a positive electrode material having both output characteristics and high-temperature storage characteristics can be obtained by a combination of conditions having a high average coverage and a high functional grouping ratio. When the average coverage is 90% or more and the functionalization rate is greater than 0.8 and less than or equal to 1.3, particularly high temperature storage characteristics can be achieved.

  The method for measuring the functionalization rate of the positive electrode material is as described above.

  FIG. 1 is a diagram showing the peak derived from carbon and the results of peak fitting when the positive electrode material is measured by X-ray photoelectron spectroscopy according to Measurement Example 2. Each component indicated by a solid line is a functionalized carbon component, and each component indicated by a dotted line is a component based on C—C, C═C, and C—H bonds.

The positive electrode material of the present invention has the intensity (I (D)) of the D band (near 1360 cm ⁻¹ ) and the intensity (I (G)) of the G band (near 1590 cm ⁻¹ ) of the Raman spectrum measured by laser Raman spectroscopy. ) Ratio (I (D) / I (G)) is preferably 0.7 or more and 1.7 or less. I (D) / I (G) is more preferably 0.8 or more and 1.1 or less, and further preferably 0.8 or more and 0.9 or less.

In Raman spectroscopy, the carbon material has a peak (G band peak) near 1590 cm ⁻¹ based on the graphite structure. The peak position of the G band peak shifts to a higher energy side as the number of defects in the graphite structure increases. Since the positive electrode material of the present invention uses functionalized graphene, the peak position is preferably on a relatively high energy side, and specifically, the peak position is preferably 1600 cm ⁻¹ or more. In this specification, even when shifted to 1600 cm ⁻¹ or more, it is called a G band.

  I (D) / I (G) is an evaluation index of the degree of defects. If the functionalization rate is comparable, it means that the larger the I (D) / I (G), the more graphene defects, and the lithium ion conductivity is improved by passing lithium ions through the defects. If it is too large, the decomposition reaction of the electrolytic solution tends to occur at the defective portion. On the other hand, if I (D) / I (G) is too small, electron conductivity tends to be high, but ion conductivity tends to be poor.

  In addition, all the Raman spectroscopy measurements in the present invention are results obtained by using an argon ion laser as an excitation laser and an excitation wavelength of 514.5 nm.

  The average coating thickness of the functionalized graphene of the positive electrode material of the present invention is preferably 100 nm or less, more preferably 50 nm or less, particularly preferably 20 nm or less, because if it is too thick, the ionic conductivity decreases and the output characteristics are impaired. is there. The coating thickness in the present invention is that the positive electrode material is kneaded with an epoxy resin, applied to a PET film, and then cured and resin-embedded, and the whole film is milled by an ion milling device to obtain resin and positive electrode material particles. A sample with the cross section taken out is measured as a sample. This sample is observed using a transmission electron microscope, and the average value of the thicknesses of the thickest part and the thinnest part of the graphene present on the surface of the positive electrode material is defined as the coating thickness. An average value obtained by arbitrarily measuring 10 positive electrode active material particles is defined as an average coating thickness.

  If the particle size of the positive electrode material in the present invention is too small, it tends to agglomerate when preparing the electrode paste, which may cause problems such as difficulty in preparing the electrode coating film. Therefore, the particle diameter of the positive electrode material is preferably 0.5 μm or more, more preferably 1.0 μm or more, and further preferably 3.0 μm or more. On the other hand, if the particle diameter is too large, it takes time for the electrolytic solution to penetrate into the inside, and the ionic conductivity tends to deteriorate. Therefore, the particle diameter is preferably 20 μm or less, more preferably 15 μm or less, and further preferably 10 μm or less. Here, the particle diameter of the positive electrode material refers to the median diameter measured by a laser diffraction / scattering apparatus. The measurement is performed by adjusting the transmittance to 75% to 95% in an aqueous dispersion.

  In the positive electrode material of the present invention, the value obtained by dividing the size of the functionalized graphene in the direction parallel to the graphene layer by the particle diameter (median diameter) of the positive electrode active material particles is 0.05 to 3.0. Is preferably 0.3 or more and 2.0 or less, more preferably 0.5 or more and 1.5 or less. If it is less than 0.05 or exceeds 3.0, the exposure of the active material increases, so that the effect of suppressing the decomposition reaction of the electrolytic solution on the surface of the positive electrode active material becomes weak.

  If the element ratio (oxidation degree) of the oxygen atom to the carbon atom of the positive electrode material in the present invention is too high, the ion conductivity increases, but the graphite structure collapses and the electron conductivity deteriorates. In addition, the reaction between the functional group and the electrolytic solution is likely to occur, and the high temperature storage characteristics are deteriorated. Therefore, in order to maintain electronic conductivity, the degree of oxidation is preferably 0.6 or less, and more preferably 0.5 or less. On the other hand, if the degree of oxidation is too low, the electron conductivity is increased by being covered with graphene having high electron conductivity, but the ion conductivity tends to be poor. From these viewpoints, the degree of oxidation is preferably 0.3 or more, and more preferably 0.4 or more. In the present invention, the degree of oxidation is a value obtained by measuring the positive electrode material by X-ray photoelectron spectroscopy.

<Positive electrode for lithium ion battery>
The positive electrode for a lithium ion battery of the present invention (hereinafter sometimes simply referred to as “positive electrode”) is a mixture layer containing the positive electrode material for a lithium ion battery of the present invention and, if necessary, a conductive additive and a binder. It is what has.

  It is preferable that the mixture layer further includes a conductive additive having high electron conductivity. Conductive aids include low functionalized graphene, graphite, carbon fiber, carbon black, acetylene black, carbon materials such as carbon nanofiber, metal materials such as copper, nickel, aluminum or silver, or powders and fibers of these mixtures. Carbon black or a fiber-shaped carbon nanofiber in which the connection of primary particles called a structure is developed is more preferable because it improves the conductivity in the thickness direction of the electrode. When the amount of the conductive auxiliary is too small, the electron conductivity is deteriorated, and when the amount is too large, the ratio of the active material per mixture layer is reduced. Therefore, the content of the conductive auxiliary is 0.50 in the positive electrode material of the present invention. It is preferably 5.0% by weight, and more preferably 0.75 to 1.5% by weight.

  The mixture layer of the positive electrode for a lithium ion battery usually contains a binder. Examples of the binder contained in the mixture layer include polysaccharides such as starch, carboxymethylcellulose, hydroxypropylcellulose, regenerated cellulose, and diacetylcellulose, thermoplastic resins such as polyvinyl chloride, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene, and polypropylene; Fluoropolymers such as tetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, butadiene rubber, fluororubber and other polymers having rubber elasticity, polyimide Precursor and / or polyimide resin, polyamideimide resin, polyamide resin, polyacrylic acid, sodium polyacrylate, acrylic resin, polyacrylonitrile, if It can be mentioned polyether such as polyethylene oxide. When the amount of the binder is too small, the binding strength is weakened, and when the amount is too large, the resistance is increased. Therefore, the content of the binder is 0.50 to 5.0% by weight in the positive electrode material of the present invention. Is preferable, and it is more preferable that it is 0.75 to 1.5 weight%.

  Usually, a positive electrode for a lithium ion battery has a mixture layer as described above formed on a current collector. As the current collector, a metal foil or a metal mesh is preferably used, and an aluminum foil is particularly preferably used.

  In addition, when analyzing the physical property of the positive electrode material for lithium ion batteries which exists in a positive electrode, it carries out as follows. First, the battery is disassembled in an Ar glove box, the positive electrode is washed with dimethyl carbonate, and vacuum-dried for 1 hour in the side box of the Ar glove box. Next, the mixture layer is peeled off using a spatula, the obtained powder is dissolved in N-methylpyrrolidone (NMP), and filtered to obtain a filtered material (a positive electrode material for a lithium ion battery, a conductive aid, NMP). Separate into filtrate (NMP, etc.). The obtained filtrate is vacuum-dried, NMP is added in an amount 5 times the weight of the filtrate again, and a strong shearing force such as Filmix (registered trademark) 30-30 (Primics) or wet jet mill is applied. The positive electrode material for lithium ion batteries and the conductive aid are separated using an apparatus. Lithium ion battery is obtained by subjecting the obtained processed product to a sieve having a pore diameter that can pass only the smaller one of the size of the conductive auxiliary agent obtained by SEM observation and the particle diameter of the positive electrode material for lithium ion battery. Separate the positive electrode material and the conductive additive. The physical properties of the positive electrode material for a lithium ion battery thus isolated can be analyzed.

<Method for producing positive electrode material for lithium ion battery>
In the method for producing a positive electrode material for a lithium ion battery according to the present invention, the positive electrode active material particles for a lithium ion battery are coated with graphene oxide by an operation of liquid-phase mixing the graphene oxide dispersion and the positive electrode active material particles, followed by drying. Preparing the precursor particles, and reducing the precursor particles so that the functionalization rate obtained by the following formula (1) from the measured value by X-ray photoelectron spectroscopy is 0.3 or more and 1.3 or less Including the step of.
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on C—H bond) (1)
Graphene oxide can be produced by a known method. Commercially available graphene oxide may be purchased. Oxidized graphite (graphite) is also called graphite oxide, but in this specification, it is included in graphene oxide. When graphite is oxidized, the distance between the graphite layers becomes long, and it has a peak at 9.0 ° to 13.0 ° by X-ray diffraction measurement.

  The graphite used as the raw material for graphene oxide may be either artificial graphite or natural graphite, but natural graphite is preferably used. The number of meshes of the raw graphite is preferably 20000 or less, and more preferably 5000 or less.

  The modified Hammers method is preferable as a method for producing graphene oxide. An example is given below. Using graphite (for example, graphite powder) as a raw material, concentrated sulfuric acid, sodium nitrate and potassium permanganate are added and reacted at 25-50 ° C. with stirring for 0.2-5 hours. Then deionized water is added to dilute to obtain a suspension, which is subsequently reacted at 80-100 ° C. for 5-50 minutes. Finally, hydrogen peroxide and deionized water are added and reacted for 1 to 30 minutes to obtain a graphene oxide dispersion. The obtained graphene oxide dispersion is filtered and washed to obtain a graphene oxide dispersion.

  As an example of the ratio of each reactant, the ratio of graphite powder, concentrated sulfuric acid, sodium nitrate, potassium permanganate and hydrogen peroxide is 10 g: 150 to 300 ml: 2 to 8 g: 10 to 40 g: 40 to 80 g. When adding concentrated sulfuric acid, sodium nitrate and potassium permanganate, use an ice bath to control the temperature. When adding hydrogen peroxide and deionized water, the mass of deionized water is 10 to 20 times the mass of hydrogen peroxide.

Since defects in graphene oxide affect the lithium ion conductivity of the positive electrode material for lithium ion batteries, it is preferable to use those having many defects. The ratio of the intensity of the intensity of the D band is specifically measured by laser Raman spectroscopy ^{(1360cm -1) (I (D} )) and G band ^{(1590cm -1) (I (G} )) (I (D) / I (G)) is preferably 0.6 or more and 1.0 or less.

  The functionalization rate of graphene oxide is preferably an appropriate functionalization rate because it affects the functionalization rate of functionalized graphene after the reduction treatment and the coating state of the positive electrode active material. Specifically, the functionalization rate of graphene oxide is preferably 0.3 or more and 1.3 or less.

  In addition, the higher the element ratio (degree of oxidation) of oxygen atoms to carbon atoms in graphene oxide, the dispersibility in the polar solvent is improved due to the repulsion between the functional groups in the polar solvent. There is a tendency to be coated. On the other hand, after the layered oxide positive electrode active material is coated with graphene oxide, the degree of oxidation is preferably 0.8 or less so that only graphene oxide can be reduced by low-temperature heating. The oxidation degree is a value obtained by measuring the positive electrode material by X-ray photoelectron spectroscopy.

  The functionalization rate and the degree of oxidation of graphene oxide can be adjusted by changing the amount of oxidizing agent used for the oxidation reaction of graphite. Specifically, the higher the amount of sodium nitrate and potassium permanganate used in the oxidation reaction, the higher the degree of oxidation, and the lower the amount, the lower the degree of oxidation. The weight ratio of sodium nitrate to graphite is not particularly limited, but is preferably 0.2 or more and 0.8 or less. The ratio of potassium permanganate to graphite is not particularly limited, but is preferably 1 or more and 4 or less.

  The precursor particles are preferably prepared by repeating the operation of mixing the graphene oxide dispersion in the liquid phase with the positive electrode active material particles and then drying the mixture several times as necessary.

  In this operation, first, graphene oxide powder and a polar solvent such as NMP are mixed using a mixer such as Filmix (registered trademark) 30-30 (Primix) to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion must be such that electrostatic repulsion due to the functional group of graphene oxide is more likely to occur than the force of aggregation of different graphene oxides, so the solid content concentration is 1.0%. The following is preferable, 0.5% or less is more preferable, and 0.2% or less is more preferable.

  And the obtained graphene oxide dispersion liquid and positive electrode active material particles are mixed. As an apparatus for mixing, an apparatus capable of applying a shearing force is preferable, and examples thereof include a planetary mixer, Philmix (registered trademark) (Primics), a self-revolving mixer, a planetary ball mill, and the like.

  Further, after mixing the graphene oxide dispersion and the positive electrode active material particles, the active material particles are collected by using filtration or sieving and dried. The drying method is not particularly limited, and a drying machine capable of raising the temperature to a temperature at which the solvent can be removed or higher may be used. For example, after raising the temperature of the inert oven to 80 ° C. or higher, the sample is charged and heated for about 40 minutes. The method of doing is mentioned. If it is desired to perform treatment more uniformly and in a large amount, for example, a batch type rotary kiln may be used.

  When the collected active material particles are observed with an SEM, and the average coverage is less than 50%, by mixing and drying with graphene oxide dispersion a plurality of times, precursor particles with a high coverage are obtained. Manufacturing conditions can be set.

  It is also preferable as a method for producing the precursor particles that the positive electrode active material particles are brought into contact with the silane coupling agent before or when the graphene oxide dispersion is mixed with the positive electrode active material particles. If this method is used, it becomes possible to coat the active material particles thinly and efficiently with graphene oxide.

  Here, the silane coupling agent is a reaction group (hydrolyzable group) that can be expected to react with or interact with organic substances in one molecule, and easily forms a hydroxyl group by hydrolysis to chemically bond with an inorganic material. It is an organosilicon compound having both reactive groups (reactive functional groups) that are easily chemically bonded to organic materials. As the silane coupling agent, those having an alkoxysilyl group as a hydrolyzable group and a vinyl group, an epoxy group or an amino group as a reactive functional group are preferably used.

The silane coupling agent used in this method is preferably one in which the shortest distance between the hydrolyzable group and the reactive functional group is 10 atoms or less in that the active material particles and graphene oxide are more easily adhered. For example, in the case of 3-aminopropyltrimethoxysilane (H ₂ NC ₃ H ₆ Si (OCH ₃ ) ₃ ) described later as a preferred silane coupling agent, the Si atom of methoxysilane which is a hydrolyzable group and a reactive functional group When the N atoms of the amino group as a group are connected with the shortest distance, it is considered that they are arranged at a distance corresponding to three carbon atoms.

  Examples of the silane coupling agent having a shortest distance of 10 atoms or less that are preferably used in this method include the following.

  Examples of the silane coupling agent having a vinyl group and an alkoxysilyl group include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, allyltrimethoxysilane, vinyltris (2-methoxyethoxy) silane, and p-styryltrimethoxysilane. , 3-acryloxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, and 3-methacryloxypropylmethyldiethoxysilane.

  Examples of the silane coupling agent having an epoxy group and an alkoxysilyl group include 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 3-glycidoxy. Examples thereof include propylmethyldiethoxysilane, 2- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, 3-isocyanatepropyltrimethoxysilane having an isocyanate group and alkoxysilane, and 3-isocyanatepropyltriethoxysilane.

  Examples of the silane coupling agent having an amino group and an alkoxysilyl group include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3- (2-aminoethyl) aminopropyltrimethoxysilane, N- (2- Aminoethyl) -3-aminopropyltrimethoxysilane, N- (2-aminoethyl) -3-aminopropylmethyldimethoxysilane. A silane coupling agent having an amino group and an alkoxysilyl group is particularly preferable because it can easily form a strong bond with both the hydroxy group on the active material surface and the functional group containing an oxygen atom on the graphene surface. Used.

  The timing for contacting the silane coupling agent is not particularly limited, and the silane coupling agent may be mixed with the positive electrode active material particles in advance at the same time as mixing the positive electrode active material particles and graphene oxide. The silane coupling agent may be mixed with the graphene oxide dispersion after the silane coupling agent treatment.

  In the silane coupling agent treatment of the positive electrode active material particles, a chemical bond including a siloxane bond is formed between the positive electrode active material and the silane coupling agent. That is, the hydrolyzable group of the silane coupling agent is hydrolyzed to form a silanol group, and after making a relatively weak bond such as a hydrogen bond with the hydroxy group on the active material surface, a dehydration condensation reaction occurs, A strong siloxane bond is formed.

  In the silane coupling agent treatment of the positive electrode active material particles, it is preferable to perform heat treatment by bringing the positive electrode active material for a lithium ion battery and the silane coupling agent into contact with each other. The heat treatment method is not particularly limited, and a dryer that can raise the temperature to 80 ° C. or higher may be used. For example, after raising the temperature of the inert oven to 80 ° C. or higher and lower than 150 ° C., the sample is charged and heated for about 40 minutes. A method is mentioned. If it is desired to perform treatment more uniformly and in a large amount, for example, a batch type rotary kiln may be used.

  As a method of bringing the positive electrode active material particles into contact with the silane coupling agent, it is preferable to mix both. The mixing method is not particularly limited, and examples thereof include a method in which the silane coupling agent is directly dissolved in a solvent such as alcohol and then mixed with the active material particles. As an apparatus for mixing, an apparatus capable of applying a shearing force is preferable, and examples thereof include a planetary mixer, Philmix (registered trademark) (Primics), a self-revolving mixer, a planetary ball mill, and the like. Moreover, the method of making active material particle and a silane coupling agent contact can also be used by spraying the solution which dissolved the silane coupling agent on the active material.

  The precursor particles thus obtained are preferably washed as necessary. As the cleaning operation, for example, NMP is added to the precursor particles, and after ultrasonic treatment, the particles are collected and dried using filtration or sieving having pores smaller than the particle diameter of the precursor particles and dried. Can be mentioned. The graphene oxide aggregated by the washing operation can be removed, and precursor particles with a more uniform particle diameter and a thin coating state can be obtained.

  The positive electrode material of the present invention has a functionalization ratio of 0. 0 obtained by the above formula (1) from the measured value of X-ray photoelectron spectroscopy of the precursor particles obtained by combining the graphene oxide and the positive electrode active material particles. It can manufacture by reducing so that it may become 3 or more and 1.3 or less.

  In a general thermal reduction method of graphene oxide, reduction is performed at a high temperature of 500 ° C. or higher in an inert gas atmosphere or a reducing gas atmosphere. However, if the reduction is performed under such conditions, graphene oxide is excessively reduced, and the active material surface is covered with graphene having high electron conductivity, so that the electron conductivity increases, but the ion conductivity tends to be poor. is there. Therefore, when using the thermal reduction method, it is preferable to perform the reduction treatment in the air in a temperature environment of 150 ° C. or higher and 300 ° C. or lower. In particular, since the layered oxide positive electrode active material has low heat resistance and high electron conductivity on the surface of the positive electrode active material itself, the reduction treatment is preferably performed in a temperature environment of 180 ° C. or higher and 250 ° C. or lower.

  Another technique for reducing graphene oxide is to use a reducing agent. The reducing agent here is limited to a substance that exists in a liquid or solid state at room temperature, and does not include a reducing gas. The reduction method using a reducing agent is suitable for maintaining the functionalization rate in graphene because the reduction does not proceed as much as thermal reduction under controlled atmosphere.

  Examples of the reducing agent include organic reducing agents and inorganic reducing agents. Examples of the organic reducing agent include aldehyde reducing agents, hydrazine derivative reducing agents, and alcohol reducing agents.

  Among organic reducing agents, alcohol-based reducing agents are preferable because they can be reduced relatively gently. Suitable alcohol reducing agents include methanol, ethanol, propanol, isopropyl alcohol, butanol, benzyl alcohol, phenol, catechol, ethanolamine, dopamine, ethylene glycol, propylene glycol, diethylene glycol, and the like, particularly benzyl alcohol, catechol. Dopamine is preferred.

  Examples of inorganic reducing agents include sodium dithionite, potassium dithionite, phosphorous acid, sodium borohydride, and hydrazine. Among inorganic reducing agents, sodium dithionite and potassium dithionite are functional groups. It is preferably used because it can be reduced while maintaining the target.

  When a layered oxide active material is used as the active material, the layered oxide active material is easily modified with a reducing agent, and thus a method of reducing graphene oxide at 80 ° C. to 150 ° C. in air is preferable. On the other hand, since an olivine-based active material is stable and hardly denatured by a reducing agent, a technique of reducing using a reducing agent is suitable.

  In the reduction of graphene oxide, the method of thermal reduction at 150 ° C. to 300 ° C. in the above air may be combined with the method of reduction with a reducing agent, and the combination makes it easier to control the functional group ratio of graphene.

(Measurement example 1: average coverage, size of functionalized graphene, average coating thickness)
The positive electrode material is observed with a scanning electron microscope (Hitachi, S-5500) at a magnification of 3,000 to 400,000, and the observation image is regarded as a two-dimensional planar image, and then graphene coating occupies the area of the positive electrode active material particles The average coverage was calculated from the area of the part. This operation was performed for 50 positively selected positive electrode active material particles, and the average value was defined as the average coverage.

  Further, for 10 graphenes on a plurality of randomly selected positive electrode active material particles, the maximum diameter and the minimum diameter in the direction parallel to the graphene layer were measured, and the average value was the size of the functionalized graphene.

  The positive electrode material was kneaded with an epoxy resin, applied to a PET film, cured, and embedded in the resin. The whole film was milled by an ion milling device (Hitachi, IM4000), and cross sections of the resin and positive electrode material particles were taken out to prepare a measurement sample. This sample was observed using a transmission electron microscope, and the average value of the thicknesses of the thickest and thinnest portions of the graphene existing on the surface of the positive electrode material was defined as the coating thickness, and this was arbitrarily set to 10 positive electrode actives. The average value measured for the substance particles was taken as the average coating thickness.

(Measurement Example 2: Functionalization rate, degree of oxidation)
Quantera SXM (manufactured by PHI) was used as an X-ray photoelectron measuring device. Excitation X-rays were monochromatic Al K _α1,2 rays (1486.6 eV), the X-ray diameter was 200 μm, and the photoemission angle was 45 °.

The functionalization rate was determined from the peak shift of the peak based on the narrow scan carbon atom. Specifically, a peak based on a carbon atom includes a peak near 284 eV based on a C═C bond, a C—H bond, a peak near 286 eV based on a C—O bond, and a 287.5 eV based on a C═O bond. The peak was separated into four components, a peak in the vicinity and a peak in the vicinity of 288.5 eV based on the COO bond, and the area ratio of each peak was determined by the following formula (1).
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on C—H bond) (1)
The ratio (oxidation degree) of the functionalized graphene to the carbon atom of the oxygen atom was obtained from the peak area of the oxygen atom in the wide scan and the peak area of the carbon atom.

(Measurement Example 3: I (D) / I (G))
Raman measurement was performed using Raman T-64000 (Jobin Yvon / Ehime Bussan). The beam diameter is 100 μm, the light source is an argon ion laser (wavelength: 514.5 nm), the intensity (I (D)) of the D band (near 1360 cm ⁻¹ ) and the G band (near 1590 cm ⁻¹ ) of the obtained Raman spectrum. ) Intensity (I (G)) ratio (I (D) / I (G)).

(Measurement Example 4: Discharge capacity and load characteristics)
An electrode was prepared as follows. 100 parts by weight of the positive electrode material prepared in each example and comparative example, 5 parts by weight of acetylene black (Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive additive, and polyvinylidene fluoride # 7200 (Kureha) as a binder 5 parts by weight) and 50 parts by weight of N-methylpyrrolidone as a solvent were mixed with a planetary mixer (Primics) to obtain an electrode paste. The electrode paste was applied to one side of an aluminum foil (thickness: 15 μm) using a doctor blade and dried at 80 ° C. for 30 minutes to obtain an electrode plate. The electrode mixture layer after drying (excluding the aluminum foil) was 40 μm.

The produced electrode plate was cut out to a diameter of 15.9 mm to be a positive electrode, a lithium foil cut to a diameter of 16.1 mm and a thickness of 0.2 mm was used as a negative electrode, and a cell guard # 2400 (manufactured by Celgard) separator cut to a diameter of 17 mm was used as a LiPF. ₆ ethylene carbonate containing 1M: diethyl carbonate = 7: 3 solvent as an electrolytic solution, to prepare a 2032 type coin battery was set in a charge-discharge tester (manufactured by Toyo system Co., Ltd. TOSCAT-3100), the electrochemical evaluation went.

In charge / discharge measurement,
When the active material is LiMnPO ₄ , the upper limit voltage is 4.4V, the lower limit voltage is 2.7V,
When the active material is LiFePO ₄ , the upper limit voltage is 4.0V, the lower limit voltage is 2.5V,
When the active material is LiMn ₂ O ₄ , the upper limit voltage is 4.3V, the lower limit voltage is 2.7V,
When the active material is LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , the upper limit voltage is 4.2V and the lower limit voltage is 3.0V.
When the active material is other than the Li-rich active material, the charge rate is fixed at 1C, the discharge rate is 3 times at 1C, then 3 times at 3C, and at the third discharge at each rate. Was defined as the discharge capacity. The ratio of the 3C discharge capacity to the 1C discharge capacity obtained by measurement (3C discharge capacity / 1C discharge capacity) was taken as the load characteristic.

  When the active material is a Li-rich active material, the battery is charged with a constant current to an upper limit voltage of 4.5 V at a current rate of 0.1 C, and then repeatedly charged and discharged to 2.0 V twice, followed by a lower limit of the upper limit voltage of 4.8 V The battery was charged and discharged three times at a voltage of 2.0 V, and the capacity at the third discharge was defined as the discharge capacity. The ratio of the discharge capacity with an upper limit voltage of 4.8 V to the discharge capacity with an upper limit voltage of 4.5 V obtained by measurement was taken as load characteristics.

(Measurement Example 5: High-temperature storage characteristics)
After performing the test of Measurement Example 4, constant current charging was performed at 1 C up to the upper limit voltage, and then constant voltage charging was performed at the upper limit voltage. The end current of the constant voltage charge was 0.01C. Thereafter, the battery was removed from the test machine (charged state), and stored at 60 ° C. for 1 week using an inert oven (DN411I manufactured by Yamato Chemical Co., Ltd.). One week later, the battery was taken out, transferred into an Ar glove box (manufactured by Miwa Seisakusho), disassembled, and only the positive electrode was taken out. Using the positive electrode taken out, as a cell guard # 2400 (manufactured by Celgard) separator cut into a diameter of 17 mm, metallic lithium was used as the counter electrode, and LiPF ₆ / EC + DMC (LI-PASTE1 manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte. Then, a coin cell was produced again, and the discharge capacity was measured under the same conditions as in Measurement Example 4. Again, the capacity at the third discharge at each rate was taken as the discharge capacity. The ratio (%) of the 1C discharge capacity (after storage) measured in this measurement example to the 1C discharge capacity (before storage) measured in Measurement Example 4 was defined as the high-temperature storage characteristics.

  When the active material was a Li-rich active material, a coin cell was similarly created again, and the discharge capacity was measured under the same conditions as in Measurement Example 4 to obtain the discharge capacity after storage. The ratio (%) of the discharge capacity (after storage) of the upper limit voltage 4.5V measured in this measurement example to the discharge capacity (before storage) of the upper limit voltage 4.5V measured in Measurement Example 4 was taken as the high temperature storage characteristics.

(Synthesis Example 1-1: Synthesis of graphene oxide powder)
2000 mesh natural graphite powder (Shanghai Ichiho Graphite Co., Ltd. median diameter 20 μm) was used as a raw material. To 10 g of natural graphite powder in an ice bath, 220 ml of 98% concentrated sulfuric acid, 5 g of sodium nitrate, and 30 g of potassium permanganate were added and mechanically stirred for 1 hour, and the temperature of the mixture was kept at 20 ° C. or lower. The above mixed solution was taken out from the ice bath and reacted with stirring in a 35 ° C. water bath for 4 hours, and then a suspension obtained by adding 500 ml of ion-exchanged water was further reacted at 90 ° C. for 15 minutes. Finally, 600 ml of ion-exchanged water and 50 ml of hydrogen peroxide were added, and the reaction was performed for 5 minutes to obtain a graphene oxide dispersion. This was filtered, the metal ions were washed with a dilute hydrochloric acid solution, the acid was washed with ion-exchanged water, and the washing was repeated until the pH became 7, thereby producing a graphene oxide gel. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.3, I (D) / I (G) was 0.990, and the oxidation degree was 0.69.

(Synthesis Example 1-2: Synthesis of graphene oxide powder)
A graphene oxide gel was produced in the same manner as in Synthesis Example 1-1 except that the ratio of the amount of sodium nitrate and potassium permanganate to 70% of that in Synthesis Example 1 was set to 70%. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The graphene oxide obtained had a functionalization ratio of 0.6, I (D) / I (G) of 0.6, and an oxidation degree of 0.30.

(Synthesis Example 1-3: Synthesis of functionalized graphene powder)
1 g of 2000-mesh natural graphite powder (Shanghai Isho Graphite Co., Ltd., median diameter 20 μm) and sodium chloride 20 g were mixed in a mortar for 10 to 15 minutes, then washed with water and dried. The dried graphite powder and 23 ml of concentrated sulfuric acid were mixed in a flask for 24 hours at room temperature. Thereafter, the mixture was heated to 40 ° C. with stirring, and 100 mg of sodium nitrate was added. Next, stirring was continued, and 500 mg of potassium permanganate was added little by little while keeping the temperature at 45 ° C. or lower so as not to cause thermal runaway, and held for 30 minutes. After 3 ml of water was added and waited for 5 minutes, another 3 ml of water was added and waited for 5 minutes, and 40 ml of water was added and waited for 15 minutes. Finally, 140 ml of ion-exchanged water and 10 ml of hydrogen peroxide were added, and the reaction was performed for 5 minutes to obtain a graphene oxide dispersion. This was filtered, the metal ions were washed with a dilute hydrochloric acid solution, the acid was washed with ion-exchanged water, and the washing was repeated until the pH became 7, thereby producing a graphene oxide gel. This graphene oxide gel was freeze-dried to obtain graphene oxide powder.

  The graphene oxide obtained had a functionalization rate of 0.4, I (D) / I (G) of 0.410, and an oxidation degree of 0.3.

(Synthesis Example 1-4: Synthesis of graphene oxide powder)
A graphene oxide gel was produced in the same manner as in Synthesis Example 1-1 except that the ratio of the amount of sodium nitrate and potassium permanganate to graphite was 400% of Synthesis Example 1. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.5, I (D) / I (G) was 1.1, and the oxidation degree was 0.85.

(Synthesis Example 1-5: Synthesis of graphene oxide powder)
A graphene oxide gel was prepared in the same manner as in Synthesis Example 1-1 except that natural graphite powder (median diameter 10.0 μm) was used. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.3, I (D) / I (G) was 0.92, and the degree of oxidation was 0.71.

(Synthesis Example 1-6: Synthesis of Graphene Oxide Powder)
A graphene oxide gel was prepared in the same manner as in Synthesis Example 1-1 except that natural graphite powder (median diameter: 5.0 μm) was used. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.4, I (D) / I (G) was 0.90, and the oxidation degree was 0.75.

(Synthesis Example 1-7: Synthesis of graphene oxide powder)
A graphene oxide gel was produced in the same manner as in Synthesis Example 1-1 except that natural graphite powder (median diameter: 3.0 μm) was used. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.3, I (D) / I (G) was 0.91, and the oxidation degree was 0.76.

(Synthesis Example 1-8: Synthesis of Graphene Oxide Powder)
A graphene oxide gel was produced in the same manner as in Synthesis Example 1-1 except that natural graphite powder (median diameter 1.0 μm) was used. The graphene oxide powder was obtained by freeze-drying the graphene oxide gel.

  The functionalization rate of the obtained graphene oxide was 1.3, I (D) / I (G) was 0.92, and the degree of oxidation was 0.91.

(Synthesis Example 2-1: Synthesis of positive electrode active material particles for lithium ion battery)
Ni, Co, and Al nitrates are adjusted to a stoichiometric ratio (Ni: Co: Al = 0.8: 0.15: 0.05) to make a uniform solution, and then adjusted to pH = 9 with aqueous ammonia. After settling, it was washed and dried at 150 ° C. for 6 hours. Thereafter, Li ₂ CO ₃ was mixed and pulverized according to the molar ratio, and then fired at 750 ° C. for 12 hours to synthesize LiNi _0.8 Co _0.15 Al _0.05 O ₂ particles. The median diameter of the obtained active material particles was 10.0 μm.

(Synthesis Example 2-2: Synthesis of Li-rich active material particles)
Ni, Mn, Co and Li acetates are weighed at a molar ratio in terms of composition ratio, dissolved in pure water together with acetate and equimolar citric acid to prepare a citric acid complex solution, and precursors are spray-dried. Got. Next, the obtained precursor was pre-fired at 400 ° C., pulverized, formed into pellets, and finally fired at 900 ° C. in the atmosphere. Thus, 0.5Li ₂ MnO ₃ -0.5Li (Ni _1/3 Co _{1 /} A Li-rich active material having a composition ratio of ₃ Mn _1/3 ) O ₂ was obtained. The median diameter of the obtained active material particles was 10.0 μm.

Example 1
29.85 g of NMP was added to 0.15 g of graphene oxide synthesized according to Synthesis Example 1-1, and mixed using Filmix (registered trademark) 30-30 type (Primics) to obtain a graphene oxide dispersion. Thereafter, 3 g of active material particles of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore, layered oxide active material particles that are granules having a median diameter of 10 μm) are added to the dispersion, and 30 g It was processed for 60 seconds at a rotational speed of 40 m / s (shear speed: 40,000 per second) with a -30 type (Primics). After the treatment, the settled particles and the supernatant liquid were separated by a sieve having a pore diameter of 5 μm, and then only the particles were dried at 80 ° C. for 1 hour using an inert oven (DN411I, manufactured by Yamato Chemical Co., Ltd.). The dried particles were added to the supernatant again, and the steps of mixing and drying were repeated a total of 5 times to obtain precursor particles.

  The precursor particles were heated in air at 150 ° C. for 6 hours using an oven to reduce graphene oxide and obtain a positive electrode material.

(Example 2)
In Example 1, a positive electrode material was prepared in the same manner except that the mixing and drying were performed four times in total when the precursor particles were produced, and that heating was performed in air at 200 ° C. using an oven.

(Example 3)
In Example 1, after obtaining the graphene oxide dispersion, 0.05 g of aminopropyltriethoxysilane (APTS) was added, and mixing and drying were performed three times in total when preparing the precursor particles. A positive electrode material was prepared in the same manner except that it was heated in air at 200 ° C. using an oven.

Example 4
In Example 1, after obtaining the graphene oxide dispersion, 0.05 g of aminopropyltriethoxysilane was added, mixing and drying were performed twice when preparing the precursor particles, and the oven A positive electrode material was prepared in the same manner except that it was heated in air at 190 ° C.

(Example 5)
A positive electrode material was prepared in the same manner as in Example 1 except that a graphene oxide dispersion was obtained using graphene oxide synthesized according to Synthesis Example 1-3, and that the reduction conditions were heating in air at 300 ° C. for 6 hours. did.

(Example 6)
A positive electrode material was produced in the same manner as in Example 1, except that the mixing and drying were performed a total of 7 times when the precursor particles were produced, and that the reduction conditions were heating in air at 150 ° C. for 6 hours. did.

(Example 7)
When the amount of NMP added was 14.85 g, it was carried out except that the mixing and drying were performed a total of 8 times when producing the precursor particles, and the reduction conditions were heating for 6 hours in 200 ° C. air. A positive electrode material was produced in the same manner as in Example 1.

(Example 8)
A positive electrode material was prepared in the same manner as in Example 1 except that the reducing condition was a nitrogen atmosphere at 120 ° C.

Example 9
The point of using graphene oxide synthesized according to Synthesis Example 1-2, the point of adding 0.05 g of aminopropyltriethoxysilane after obtaining the graphene oxide dispersion, and heating under reducing conditions at 180 ° C. in air A positive electrode material was produced in the same manner as in Example 1 except for the points described above.

(Example 10)
The positive electrode material was prepared in the same manner as in Example 1 except that the active material particles were changed to the NCA active material particles obtained in Synthesis Example 2-1, and that the reduction conditions were heated in air at 250 ° C. for 6 hours. Was made.

(Example 11)
The same method as in Example 1 except that the active material particles were changed to the Li-rich active material particles obtained in Synthesis Example 2-2 and the reduction conditions were heated in air at 250 ° C. for 6 hours. Thus, a positive electrode material was produced.

Example 12
The positive electrode was prepared in the same manner as in Example 1 except that the active material particles were changed to LCO active material (LiCoO ₂ manufactured by Hosen Co., Ltd.) and the reduction conditions were heated in air at 250 ° C. for 6 hours. The material was made.

(Example 13)
Same as Example 1 except that the active material particles were LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore, layered oxide active material particles that are granules with a median diameter of 20 μm). Thus, a positive electrode material was produced.

(Example 14)
Example 1 except that the active material particles were LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore, layered oxide active material particles that are granules having a median diameter of 5.0 μm). In the same manner as above, a positive electrode material was produced.

(Example 15)
Obtaining a graphene oxide dispersion using graphene oxide synthesized according to Synthesis Example 1-5, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore, granulated with a median diameter of 1.0 μm) A positive electrode material was produced in the same manner as in Example 1 except that the active material particles were layered oxide active material particles.

(Example 16)
A positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide dispersion was obtained using the graphene oxide synthesized according to Synthesis Example 1-6.

(Example 17)
A positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide dispersion was obtained using the graphene oxide synthesized according to Synthesis Example 1-7.

(Example 18)
A positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide dispersion was obtained using the graphene oxide synthesized according to Synthesis Example 1-8.

(Example 19)
Example 1 except that the active material particles were LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore, layered oxide active material particles that are granules with a median diameter of 0.5 μm). In the same manner as above, a positive electrode material was produced.

(Comparative Example 1)
To 0.24 g of graphene oxide powder synthesized according to Synthesis Example 1-1, 3 g of active material particles of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore) and 3 g of NMP were added, and the obtained paste was used. A 30-30 type (Primics Co., Ltd.) was used for 60 seconds at a rotational speed of 40 m / s (shear rate: 40000 per second). After the treatment, it was dried at 80 ° C. for 1 hour using an inert oven (manufactured by Yamato Chemical Co., DN411I). Thereafter, it was dried at a temperature of 100 ° C. in a vacuum atmosphere at a pressure of 0.01 MPa to produce a dried body. Next, the dried product is pulverized to prepare a powder mixture. The powder mixture is heated in a vacuum atmosphere at a pressure of 0.01 MPa or less at a temperature of 300 ° C. for 6 hours to reduce graphene oxide, The material was made.

(Comparative Example 2)
A positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide was reduced by heating the precursor particles in air at 600 ° C. for 6 hours.

(Comparative Example 3)
In Example 1, a positive electrode material was produced in the same manner as in Example 1 except that mixing and drying were performed only once when producing the precursor particles.

(Comparative Example 4)
In Example 1, the point using the graphene oxide synthesized according to Synthesis Example 1-2, the point where the mixing and drying were performed a total of three times when producing the precursor particles, and the composite particle precursor in the air at 500 ° C. A positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide was reduced by heating for 6 hours.

(Comparative Example 5)
In Example 1, the point using the graphene oxide synthesized according to Synthesis Example 1-4, the point where mixing and drying were performed a total of two times when producing the precursor particles, and the composite particle precursor were 300 ° C. in air The positive electrode material was produced in the same manner as in Example 1 except that the graphene oxide was reduced by heating for 6 hours.

(Comparative Example 6)
In Example 1, the point of using graphene oxide synthesized according to Synthesis Example 1-1, the point that mixing and drying were performed twice when preparing the precursor particles, and the reducing conditions were 6 hours in air at 80 ° C. A positive electrode material was produced in the same manner as in Example 1 except that the heating was performed.

(Comparative Example 7)
0.06 g of graphene oxide powder prepared in (Synthesis Example 1), 3 g of active material particles of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NCM523 manufactured by Umicore), 0.1 g of water, and zirconia balls Seven (diameter 1 cm) were put in a zirconia container (12 ml), and mixed with a planetary ball mill (manufactured by Fritsch, model P-5) at 300 rpm for 6 hours to obtain precursor particles. The precursor particles are heated in air at 150 ° C. for 6 hours using an inert oven (manufactured by Yamato Chemical Co., DN411I) to reduce graphene oxide, and NMC active material particles-highly functionalized graphene composite electrode material Obtained.

  Table 1 shows the results of measurement of various physical properties and battery performance evaluations according to Measurement Examples 1 to 5 for the positive electrode materials prepared in each Example and Comparative Example.

Claims (16)

A positive electrode material for a lithium ion battery in which positive electrode active material particles are coated with functionalized graphene,
An average coverage of the positive electrode active material particles by the functionalized graphene is 80% or more;
A positive electrode material for a lithium ion battery, having a functional grouping ratio of 0.3 or more and 1.3 or less determined by the following formula (1) from a measurement value of the positive electrode material measured by X-ray photoelectron spectroscopy.
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on C—H bond) (1) The ratio (I (G)) of the intensity (I (D)) of the D band (near 1360 cm −1) and the intensity (I (G)) of the G band (near 1590 cm −1) measured by laser Raman spectroscopy of the positive electrode material. The positive electrode material for a lithium ion battery according to claim 1, wherein D) / I (G)) is 0.7 or more and 1.7 or less. The positive electrode material for a lithium ion battery according to claim 2, wherein the I (D) / I (G) is 0.8 or more and 1.1 or less. The positive electrode material for a lithium ion battery according to claim 1, wherein the functionalized graphene has a size in a direction parallel to the graphene layer of 1 μm or more. The positive electrode material for a lithium ion battery according to any one of claims 1 to 4, wherein an average coverage of the positive electrode active material particles by the functionalized graphene is 90% or more. The positive electrode material for a lithium ion battery according to any one of claims 1 to 5, wherein the functionalization rate of the positive electrode material is 0.8 or more and 1.3 or less. The positive electrode material for a lithium ion battery according to claim 1, wherein a coating thickness of the functionalized graphene is 100 nm or less. The positive electrode material for a lithium ion battery according to any one of claims 1 to 7, wherein the positive electrode active material particles are layered oxide active material particles. The positive electrode material for a lithium ion battery according to claim 8, wherein the positive electrode active material particles are layered oxide active material particles containing Ni. The value obtained by dividing the size of the functionalized graphene in the direction parallel to the graphene layer by the particle diameter of the positive electrode active material particles is 0.05 or more and 3.0 or less. Positive electrode material for lithium ion batteries. The positive electrode for lithium ion batteries containing the positive electrode material for lithium ion batteries in any one of Claims 1-10. The lithium ion battery which uses the positive electrode for lithium ion batteries of Claim 11. A step of preparing precursor particles in which the positive electrode active material particles for lithium ion batteries are coated with graphene oxide by an operation of drying after mixing the graphene oxide dispersion and the positive electrode active material particles;
A step of reducing the precursor particles so that a functionalization rate obtained by the following formula (1) from a measurement value by X-ray photoelectron spectroscopy measurement is 0.3 or more and 1.3 or less;
The manufacturing method of the positive electrode material for lithium ion batteries which has.
Functionalization rate = [(peak area based on C—O single bond) + (C = peak area based on O double bond) + (peak area based on COO bond)] / (C—C, C = C and Peak area based on C—H bond) (1) The method for producing a positive electrode material for a lithium ion battery according to claim 13, wherein the step of reducing the precursor particles is performed in a temperature environment of 150 ° C. or higher and 300 ° C. or lower. The method for producing a positive electrode material for a lithium ion battery according to claim 13 or 14, wherein the precursor particles are produced by repeating the operation of drying after mixing the graphene oxide dispersion and the positive electrode active material particles a plurality of times. . The lithium ion battery according to any one of claims 13 to 15, wherein the positive electrode active material particles are brought into contact with a silane coupling agent before or when the graphene oxide dispersion is mixed with the positive electrode active material particles. Manufacturing method of positive electrode material.

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