JP2020119805A - Negative electrode material for lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, and lithium-ion secondary battery - Google Patents

Abstract

To provide a negative electrode material for a lithium ion secondary battery capable of producing a lithium ion secondary battery excellent in initial efficiency and discharge capacity, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.SOLUTION: There is provided a negative electrode material for a lithium ion secondary battery which has an R value of Raman spectroscopy of 0.7 or more and a value obtained by dividing the R value by Nspecific surface area (R value/Nspecific surface area) of 0.2 or less.SELECTED DRAWING: None

Description

The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

BACKGROUND ART Lithium-ion secondary batteries have been widely used for electronic devices such as notebook PCs, mobile phones, smartphones, and tablet PCs because of their small size, light weight, and high energy density. _2. Description of the Related Art In recent years, hybrid electric vehicles (HEV) and the like, which combine a gasoline engine and a battery, have become popular against the backdrop of environmental problems such as global warming due to CO ₂ emission. Recently, it is also used in electric vehicles (EVs) that run on batteries alone, power sources for storing electric power, and the like, and there is a demand for further increasing the capacity of lithium ion secondary batteries.

A carbon material such as graphite is widely used as a negative electrode material of a lithium ion secondary battery. When graphite is used as the negative electrode material, lithium ions are consumed in the decomposition of the electrolytic solution due to the reaction with graphite during the initial charge, and the initial efficiency (initial discharge capacity/initial charge capacity) tends to decrease.

It has been reported that coating graphite with amorphous carbon is effective as a method of suppressing decomposition of the electrolytic solution due to reaction with graphite (see, for example, Patent Document 1). In the method described in Patent Document 1, graphite as a core and a carbon precursor are mixed and fired to coat the graphite with amorphous carbon.

JP, 2014-44950, A

Covering graphite with amorphous carbon is an effective means for suppressing excessive decomposition of the electrolytic solution and improving the initial efficiency, but this method improves the discharge capacity of the lithium ion secondary battery. It is not possible.

In order to meet the above-mentioned demand for higher capacity of the lithium-ion secondary battery, it is required for the negative electrode material to increase the discharge capacity in addition to the initial efficiency.
In view of the above problems, one aspect of the present invention is a negative electrode material for a lithium-ion secondary battery, a negative electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery capable of producing a lithium-ion secondary battery having excellent initial efficiency and discharge capacity. The purpose is to provide.

Means for solving the above problems include the following embodiments.
<1> A lithium ion secondary having an R value of Raman spectroscopy of 0.7 or more and a value obtained by dividing the R value by the N ₂ specific surface area (R value/N ₂ specific surface area) of 0.2 or less. Negative electrode material for batteries.
<2> The negative electrode material for a lithium ion secondary battery according to <1>, wherein the R value is 1.6 or less.
<3> The negative electrode material for a lithium ion secondary battery according to <1> or <2>, which is graphite particles having an amorphous carbon layer on at least a part of the surface.
<4> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <3>, which has a volume average particle diameter (D ₅₀ ) of 6 μm to 40 μm.
<5> In the differential thermal analysis, in the heat treatment process in an oxygen atmosphere, the number of exothermic peaks observed in the range of 300° C. to 900° C. is 1, and any one of <1> to <4> The negative electrode material for the lithium ion secondary battery described.
<6> In the thermogravimetric measurement, the weight loss rate in the range of 300° C. to 900° C. is less than 0.1% in the heat treatment process in a nitrogen atmosphere. <1> to <5>. Negative electrode material for lithium-ion secondary batteries.
<7> 0 CO ₂ adsorption amount obtained from CO ₂ adsorption amount measured at ℃ is 0.30cm ³ /g~1.50cm ³ / g, according to any one of <1> to <6> Negative electrode material for lithium-ion secondary batteries.
<8> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <7>, which has an average interplanar spacing (d ₀₀₂ ) of 0.337 nm or less determined by an X-ray diffraction method.
<9> A negative electrode for a lithium ion secondary battery, comprising a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of <1> to <8>, and a current collector.
<10> A lithium ion secondary battery including the negative electrode for a lithium ion secondary battery according to <9>.

According to one aspect of the present invention, there are provided a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent initial efficiency and discharge capacity. It

Is a diagram showing a temperature program of the pretreatment performed in CO ₂ adsorption measurement.

Hereinafter, modes for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and ranges thereof, and does not limit the present invention.

In the present disclosure, the term “process” includes not only a process independent from other processes but also the process if the purpose of the process is achieved even when the process cannot be clearly distinguished from the other process. ..
In the present disclosure, the numerical range indicated by using "to" includes the numerical values before and after "to" as the minimum value and the maximum value, respectively.
In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other stages. .. Further, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.
In the present disclosure, each component may include a plurality of types of corresponding substances. When there are multiple types of substances corresponding to each component in the composition, the content rate or content of each component is the total content rate or content of the multiple types of substances present in the composition unless otherwise specified. Means quantity.
In the present disclosure, a plurality of types of particles corresponding to each component may be included. When a plurality of types of particles corresponding to each component are present in the composition, the particle size of each component means a value for a mixture of the plurality of types of particles present in the composition, unless otherwise specified.
In the present disclosure, the term “layer” or “film” means that when the region in which the layer or film is present is observed, in addition to being formed over the entire region, only in a part of the region. The case of being formed is also included.
In the present disclosure, the term “laminate” refers to stacking layers, two or more layers may be combined and two or more layers may be removable.

<Negative electrode material for lithium-ion secondary battery>
The negative electrode material for a lithium ion secondary battery (hereinafter, also simply referred to as “negative electrode material”) of the present invention has an R value of Raman spectroscopic measurement of 0.7 or more, and a value obtained by dividing the R value by the N ₂ specific surface area. (R value/N ₂ specific surface area, hereinafter also referred to as “R value per area”) is 0.2 or less.

As a result of the study by the present inventors, it was found that a lithium ion secondary battery excellent in initial efficiency and discharge capacity can be obtained by using a negative electrode material satisfying the above conditions. Although the reason is not clear, for example, when the R value of the negative electrode material is 0.7 or more, the decomposition of the electrolytic solution is sufficiently suppressed, the decrease in the initial efficiency is suppressed, and the R value per area is When it is 0.2 or less, it is considered that the lithium ion insertion site of the negative electrode material is sufficiently secured and a sufficient discharge capacity is obtained.

R value of the negative electrode material in the present disclosure, in the Raman spectrum obtained in Raman spectroscopy, and intensity Ig of the maximum peak in the vicinity of 1580 cm ^-1, the intensity ratio of the intensity Id of the maximum peak in the vicinity of 1360 cm ^-1 (Id / Ig). Here, the peak appearing near 1580 cm ^-1, generally a peak identified as corresponding to the graphite crystal structure, means a peak observed for example ^{1530cm -1 ~1630cm} -1. Moreover, the peak appearing in the vicinity of 1360 cm ⁻¹ is usually a peak identified as corresponding to the amorphous structure of carbon, and means, for example, a peak observed at 1300 cm ^{−1 to} 1400 cm ⁻¹ .

In the present disclosure, Raman spectroscopic measurement of the negative electrode material is performed by using a laser Raman spectrophotometer (NRS-1000, JASCO Corporation) and irradiating the sample plate on which the negative electrode material is set to be flat with laser light. I do. The measurement conditions are as follows.
Laser light wavelength: 532nm
Wave number resolution: 2.56 cm ^-1
Measuring range: 1180 cm ^{-1 to} 1730 cm ^-1
Peak Research: Background Removal

The R value of the negative electrode material is an index indicating the degree of graphitization on the surface of the negative electrode material. When the R value of the negative electrode material is 0.7 or more, it is considered that the decomposition of the electrolytic solution due to the reaction with the negative electrode material is sufficiently suppressed and the initial efficiency is improved.

The method of adjusting the R value of the negative electrode material is not particularly limited. For example, when the negative electrode material is graphite particles having an amorphous carbon layer on at least a part of the surface, the R value tends to be increased by increasing the ratio (adhesion amount) of the amorphous carbon layer to the graphite particles. is there.

From the viewpoint of improving the initial efficiency, the R value of the negative electrode material is preferably 0.75 or more, more preferably 0.8 or more, and further preferably 0.85 or more.

The R value of the negative electrode material is preferably 1.6 or less. If the R value is 1.6 or less, for example, an excessive increase in the specific surface area due to the local attachment of amorphous carbon having a large specific surface area to the surface of the graphite particles as the negative electrode material is sufficiently suppressed. The initial efficiency tends to be secured.

From the viewpoint of ensuring the initial efficiency, the R value of the negative electrode material is more preferably 1.4 or less, and further preferably 1.2 or less.

The N ₂ specific surface area (m ² /g) of the negative electrode material is not particularly limited as long as the R value per area of the negative electrode material is 0.2 or less. From the viewpoint of ensuring a sufficient reaction area of the negative electrode material, the N ₂ specific surface area (m ² /g) of the negative electrode material is preferably a value such that the R value per area of the negative electrode material is 0.18 or less, It is more preferable that the R value per area of the negative electrode material is 0.16 or less.

The N ₂ specific surface area (m ² /g) of the negative electrode material may be, for example, 20 m ² /g or less, 17 m ² /g or less, or 12 m ² /g or less.
The N ₂ specific surface area (m ² /g) of the negative electrode material may be, for example, 0.5 m ² /g or more, 2 m ² /g or more, or 5 m ² /g or more. Good.

In the present disclosure, the N ₂ specific surface area of the negative electrode material can be determined using the BET method from the adsorption isotherm obtained by the nitrogen adsorption measurement at 77K.

The method for adjusting the N ₂ specific surface area (m ² /g) of the negative electrode material is not particularly limited. For example, it can be adjusted by the particle size, particle shape, surface condition, etc. of the negative electrode material. As a specific method, a method of heat-treating the negative electrode material to increase the N ₂ specific surface area and the like can be mentioned.

The negative electrode material is preferably graphite particles having an amorphous carbon layer on at least a part of the surface. When the negative electrode material is graphite particles having an amorphous carbon layer on at least a part of the surface, it is possible to more effectively improve the initial efficiency and the discharge capacity of the lithium ion secondary battery using the negative electrode material.

When the negative electrode material is graphite particles having an amorphous carbon layer on at least a part of the surface, examples of the graphite particles include spherical natural graphite, natural graphite such as flake graphite, and artificial graphite, but are not particularly limited. ..
Natural graphite is preferable from the viewpoint of mass productivity, and spherical natural graphite is more preferable from the viewpoint of suppressing particles from being oriented in a certain direction when pressed to increase the density of the electrode.

The amorphous carbon layer is not particularly limited as long as it exists on at least a part of the surface of the graphite particles. From the viewpoint of effectively suppressing the decomposition of the electrolytic solution, the amorphous carbon layer is preferably in a state of covering the graphite particles.

The method for forming the amorphous carbon layer on the surface of the graphite particles is not particularly limited. For example, it may be carried out by heat-treating a mixture of an organic compound (carbon precursor) which can be changed into carbon by heat treatment and graphite particles, and melting, vaporizing or sublimating the organic compound. Alternatively, it may be carried out by a chemical vapor deposition method using a carbon precursor which is gaseous or easily vaporizable.

Specific examples of the carbon precursor include low molecular weight compounds such as carboxylic acid, high molecular weight compounds such as phenol resin, styrene resin, polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate and polybutyral, ethylene heavy end pitch, coal pitch. , Petroleum pitch, coal tar pitch, asphalt cracking pitch, PVC pitch produced by pyrolyzing polyvinyl chloride (PVC), pitches such as naphthalene pitch produced by polymerizing naphthalene in the presence of a super strong acid, starch , And polysaccharides such as cellulose. The carbon precursors may be used alone or in combination of two or more.

A carboxylic acid is preferably used as the carbon precursor. As a result of the study conducted by the present inventors, it was found that when carboxylic acid is used as the carbon precursor, the discharge capacity is more improved than when other materials are used as the precursor. Although the reason is not clear, for example, when a carboxylic acid is used, the degree of decrease in the specific surface area of the negative electrode material is small when an R value equivalent to that when other materials are used is obtained. May have some effect on the surface condition of the negative electrode material. Furthermore, it is conceivable that the amorphous carbon layer formed by using the carboxylic acid exhibits a property similar to that of hard carbon, which has low crystal orientation and has many lithium ion insertion/removal sites.

In the present disclosure, the carboxylic acid means a compound having at least one carboxy group in the molecule, and a compound further having a hydroxy group (hydroxy acid), a compound further having an amino group (amino acid) and the like are also included in the carboxylic acid. .. The carboxylic acids may be used alone or in combination of two or more.

The number of carboxy groups contained in the carboxylic acid is not particularly limited, but it is preferably 1 to 3, and more preferably 1 or 2.

Specific examples of the carboxylic acid include the following compounds.
(1) Saturated fatty acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, lauric acid, tridecylic acid, palmitic acid, stearic acid (2) Acrylic acid, methacrylic acid, oleic acid, linoleic acid, α-linolenic acid, etc. Unsaturated fatty acids (3) Saturated dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid and adipic acid (4) Unsaturated dicarboxylic acids such as maleic acid and fumaric acid (5) Benzoic acid, isophthalic acid, terephthalic acid Aromatic carboxylic acids such as acid and salicylic acid

Among the carboxylic acids, isophthalic acid, fumaric acid, lauric acid and stearic acid are preferable, and isophthalic acid and stearic acid are more preferable.

In the differential thermal analysis, the negative electrode material preferably has one exothermic peak observed in the range of 300° C. to 900° C. during the heat treatment process in an oxygen atmosphere. The fact that the number of exothermic peaks observed under this condition is one means that the peak derived from amorphous carbon is extremely small, and that the proportion of amorphous carbon in the negative electrode material is extremely small. ..

The higher the crystallinity of the negative electrode material, the higher the energy density. Since the higher the crystallinity of graphite is, the more stable it is against oxidation, the exothermic peak is preferably observed in the range of 700°C to 900°C, and more preferably around 800°C.

In the thermogravimetric measurement, the negative electrode material preferably has a weight reduction rate of less than 0.1% in the range of 300° C. to 900° C. during the heat treatment process in a nitrogen atmosphere. It is considered that the smaller the weight reduction rate under this condition is, the greater the degree of conversion of the above-mentioned carbon precursor into carbonaceous matter is, and the more excellent the characteristics as the negative electrode material are. The weight reduction rate is more preferably less than 0.07%, further preferably less than 0.03%.

In the present disclosure, the differential thermal analysis and thermogravimetric measurement of the negative electrode material are measured using a differential thermogravimetric simultaneous measurement apparatus (for example, TG/DTA6200 manufactured by Seiko Instruments Inc.). Regarding the differential thermal analysis, a sample of the negative electrode material placed in a platinum sample container is heat-treated in an oxygen atmosphere at 25° C. to 950° C., and the difference in temperature change from α-alumina as a reference is defined as differential heat (DTA). .. For thermogravimetric measurement, the value obtained by dividing the weight reduction amount of the sample in the nitrogen atmosphere in the same temperature range by the weight of the sample before the heat treatment and multiplying by 100 is taken as the weight reduction rate (%).

The volume average particle size of the negative electrode material is not particularly limited. For example, it is preferably 6 μm to 40 μm, more preferably 8 μm to 30 μm, and further preferably 10 μm to 25 μm. When the volume average particle diameter of the negative electrode material is 6 μm or more, aggregation of particles when the negative electrode material is mixed with another material to form a negative electrode material composition is suppressed, and good coatability tends to be obtained. is there. On the other hand, when the volume average particle size of the negative electrode material is 40 μm or less, the diffusion distance of lithium from the surface of the negative electrode material to the inside does not become too long, and the input/output characteristics of the lithium ion secondary battery are favorably maintained. It is in.

In the present disclosure, the volume average particle diameter of the negative electrode material is 50% when the cumulative volume distribution curve is drawn from the smaller diameter side in the particle diameter distribution of the negative electrode material measured by the laser diffraction/scattering method. It is a particle diameter (D ₅₀ ). The volume average particle diameter of the negative electrode material is measured by, for example, a laser diffraction particle size distribution analyzer (for example, SALD-3000J manufactured by Shimadzu Corporation) in a state where the negative electrode material is dispersed in purified water containing a surfactant. be able to.

Negative electrode material is preferably CO ₂ adsorption amount of CO ₂ was determined by adsorption measurements at 0 ℃ is 0.30cm ³ /g~1.50cm ³ / g.
If the amount of CO ₂ adsorbed on the negative electrode material measured under the above conditions is 0.30 cm ³ /g or more, the resistance associated with the insertion and desorption of lithium ions tends to decrease to 1.50 cm ³ /g or less. And, there is a tendency that the deterioration of capacity when held in a charged state can be suppressed.

In the present disclosure, the measurement of the CO ₂ adsorption amount of the negative electrode material is performed using BELSORP-miniII (manufactured by Microtrac Bell Co., Ltd.). Prior to the measurement, the temperature program shown in FIG. 1 is pretreated and degassed. The CO ₂ adsorption amount is calculated by dividing the CO ₂ adsorption volume when the relative pressure P/P ₀ =0.9806 by the sample weight.

The negative electrode material preferably has an average interplanar spacing d ₀₀₂ of 0.337 nm or less, which is determined by an X-ray diffraction method. Regarding the value of the average interplanar spacing d ₀₀₂ , 0.3354 nm is the theoretical value of the graphite crystal, and the energy density tends to increase as it approaches this value. Therefore, when the average surface spacing d ₀₀₂ of the negative electrode material is 0.337 nm or less, both the initial charge and discharge efficiency and the energy density of the lithium ion secondary battery tend to be excellent.

The average interplanar spacing d ₀₀₂ of the negative electrode material was determined by irradiating the sample that is the negative electrode material with X-rays (CuKα rays), and from the diffraction profile obtained by measuring the diffraction line with a goniometer, the diffraction angle 2θ=24° to 2
It can be calculated using the Bragg equation from the diffraction peak corresponding to the carbon 002 plane that appears near 7°.

<Negative electrode for lithium-ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present disclosure includes a negative electrode material layer including the above-described negative electrode material for a lithium ion secondary battery, and a current collector. The negative electrode for a lithium-ion secondary battery may include other components, if necessary, in addition to the above-described negative electrode material layer including the negative electrode material and the current collector.

The negative electrode for a lithium-ion secondary battery is prepared, for example, by kneading a negative electrode material and a binder together with a solvent to prepare a negative electrode material composition in a slurry form, and applying this on a current collector to form a negative electrode material layer. It can be manufactured by forming the negative electrode material composition into a sheet shape, a pellet shape, or the like and integrating it with a current collector. The kneading can be performed using a dispersing device such as a stirrer, a ball mill, a super sand mill, a pressure kneader, or the like.

The binder used for preparing the negative electrode material composition is not particularly limited. For example, styrene-butadiene copolymer, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, hydroxyethyl acrylate, ethylenically unsaturated carboxylic acid ester such as hydroxyethyl methacrylate, Acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid and other ethylenically unsaturated carboxylic acids, and polyvinylidene fluoride, polyethylene oxide, polyepiclohydrin, polyphosphazene, polyacrylonitrile, etc. Large polymer compounds are included.

When the negative electrode material composition contains a binder, the amount thereof is not particularly limited. For example, the amount may be 0.5 parts by mass to 20 parts by mass based on 100 parts by mass of the total amount of the negative electrode material and the binder.

The negative electrode material composition may include a thickener. As the thickener, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid or a salt thereof, oxidized starch, phosphorylated starch, casein and the like can be used.

When the negative electrode material composition contains a thickener, the amount thereof is not particularly limited. For example, it may be 0.1 to 5 parts by mass with respect to 100 parts by mass of the negative electrode material.

The negative electrode material composition may include a conductive auxiliary material. Examples of the conductive auxiliary material include carbon materials such as carbon black, graphite and acetylene black, and oxides and nitrides having conductivity.

When the negative electrode material composition contains a conductive additive, the amount thereof is not particularly limited. For example, it may be 0.5 to 15 parts by mass with respect to 100 parts by mass of the negative electrode material.

The material of the current collector is not particularly limited and can be selected from aluminum, copper, nickel, titanium, stainless steel and the like. The state of the current collector is not particularly limited and can be selected from foil, perforated foil, mesh and the like. Further, a porous material such as porous metal (foamed metal), carbon paper or the like can also be used as the current collector.

When the negative electrode material composition is applied to the current collector to form the negative electrode material layer, the method is not particularly limited, and a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, Known methods such as a doctor blade method, a comma coating method, a gravure coating method, and a screen printing method can be adopted. After applying the negative electrode material composition to the current collector, the solvent contained in the negative electrode material composition is removed by drying. Drying can be performed using, for example, a hot air dryer, an infrared dryer, or a combination of these devices. You may perform a rolling process as needed. The rolling treatment can be carried out by a method such as flat plate press or calender roll.

When the negative electrode composition formed into a shape such as a sheet or a pellet is integrated with the current collector to form the negative electrode material layer, the integration method is not particularly limited. For example, it can be performed by a roll, a flat plate press, or a combination of these means. The pressure for integration is preferably, for example, about 1 MPa to 200 MPa.

The negative electrode density of the negative electrode material is not particularly limited. For ^example, it is preferably ^{1.1g / cm 3 ~1.8g / cm 3} , more preferably from ^{1.2g / cm 3 ~1.7g / cm 3} , 1.3g / cm 3 ~1. More preferably, it is 6 g/cm ³ . Negative electrode density is 1.1
When it is g/cm ³ or more, the increase of electronic resistance is suppressed and the capacity tends to be increased, and when it is 1.8 g/cm ³ or less, the deterioration of rate characteristics and cycle characteristics is suppressed. There is.

<Lithium-ion secondary battery>
A lithium ion secondary battery of the present disclosure includes the above-described negative electrode for lithium ion secondary battery. In one embodiment, the lithium ion secondary battery includes a negative electrode for a lithium ion secondary battery, a positive electrode, and an electrolytic solution.

The positive electrode can be obtained by forming a positive electrode layer on the current collector in the same manner as in the method for producing the negative electrode described above. As the current collector, it is possible to use a metal or alloy such as aluminum, titanium, and stainless steel in the form of a foil, a perforated foil, a mesh, or the like.

The positive electrode material used for forming the positive electrode layer is not particularly limited. Examples thereof include metal compounds (metal oxides, metal sulfides, etc.) capable of doping or intercalating lithium ions, and conductive polymer materials. More specifically, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), these complex oxides (LiCo _x Ni _y Mn _z O ₂ , x+y+z=1), Complex oxide containing additional element M′ (LiCo _a Ni _b Mn _c M′ _d O ₂ , a+b+c+d=1, M′: Al, Mg, Ti
, Zr or Ge), spinel type lithium manganese oxide (LiMn ₂ O ₄ ), lithium vanadium compound, V ₂ O ₅ , V ₆ O ₁₃ , VO ₂ , MnO ₂ , TiO ₂ , MoV ₂ O ₈ , TiS ₂ , _{V 2 S 5, VS 2,} MoS 2, MoS 3, Cr 3 O 8, Cr 2 O 5, olivine-type _{LiMPO 4 (M: Co, Ni} , Mn, Fe) lithium-containing compounds such as polyacetylene, polyaniline, polypyrrole , Conductive polymers such as polythiophene and polyacene, and porous carbon. The positive electrode material may be a single material or a combination of two or more materials.

The electrolytic solution is not particularly limited, and for example, a lithium salt as an electrolyte dissolved in a non-aqueous solvent (so-called organic electrolytic solution) can be used.
Examples of the lithium salt include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , and LiSO ₃ CF ₃ . The lithium salt may be used alone or in combination of two or more.
As the non-aqueous solvent, ethylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, cyclohexylbenzene, sulfolane, propane sultone, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1, 2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, trimethyl phosphate ester, triethyl phosphate ester and the like can be mentioned. The non-aqueous solvent may be one type alone or two or more types.

The states of the positive electrode and the negative electrode in the lithium ion secondary battery are not particularly limited. For example, the positive electrode and the negative electrode and, if necessary, the separator disposed between the positive electrode and the negative electrode may be wound in a spiral shape or may be stacked in a flat plate shape.

The separator is not particularly limited, and for example, a resin non-woven fabric, a cloth, a microporous film, or a combination thereof can be used. Examples of the resin include resins containing polyolefin such as polyethylene and polypropylene as a main component. Due to the structure of the lithium ion secondary battery, the separator may not be used when the positive electrode and the negative electrode do not come into direct contact with each other.

The shape of the lithium ion secondary battery is not particularly limited. For example, a laminate type battery, a paper type battery, a button type battery, a coin type battery, a laminated type battery, a cylindrical type battery and a square type battery can be mentioned.

The lithium-ion secondary battery of the present embodiment is excellent in discharge capacity, and therefore is required to have an extended cruising range. It is used in plug-in hybrid electric vehicles (PHEV) and EVs, large-capacity industrial storage batteries, and the like. Is suitable as

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

<Example 1>
(1) Preparation of Negative Electrode Material 100 parts by mass of spherical natural graphite (volume average particle diameter: 10 μm) and 10 parts by mass of isophthalic acid as a carbon precursor were mixed to obtain a mixture. This mixture was subjected to heat treatment (heating in Ar atmosphere from 25° C. to 900° C. at a heating rate of 200° C./hour, holding at 900° C. for 1 hour, and then natural cooling). The sample after the heat treatment was disintegrated with a mixer, and the sieved portion (350 mesh, opening 45 μm) was used as a negative electrode material.

By the method shown below, the R value of the obtained negative electrode material, the N ₂ specific surface area, the CO ₂ adsorption amount, the volume average particle diameter (D ₅₀ ), and the differential thermal analysis were performed. The results are shown in Table 1.

[Measurement of R value]
R value of the negative electrode material is subjected to Raman spectroscopic measurement under the conditions described above, in the obtained Raman spectrum, the intensity Ig of the maximum peak in the vicinity of 1580 cm ^-1, the intensity ratio of the intensity Id of the maximum peak in the vicinity of 1360 cm ^-1 It was calculated as (Id/Ig).

[Measurement of N ₂ specific surface area]
The N ₂ specific surface area of the negative electrode material was measured by a multipoint method for nitrogen adsorption at a liquid nitrogen temperature (77K) using a high-speed specific surface area/pore distribution measuring device (FlowSoap II 2300, Tokai Riki Co., Ltd.). And calculated by the BET method.

[Measurement of CO ₂ adsorption amount]
The amount of CO ₂ adsorbed on the negative electrode material was measured under the above-mentioned conditions. As a pretreatment for the measurement, degassing was performed according to the temperature program shown in FIG. The CO ₂ adsorption amount was calculated by dividing the CO ₂ adsorption volume when the relative pressure P/P ₀ =0.9806 by the sample weight.

[Measurement of volume average particle size]
The volume average particle diameter of the negative electrode material was measured by the laser diffraction/scattering method under the conditions described above.

[Differential thermal analysis]
The differential thermal analysis of the negative electrode material was performed under the above-mentioned conditions, and the number of exothermic peaks observed in the range of 300°C to 900°C was examined.

(2) Preparation of lithium-ion secondary battery An aqueous solution (CMC concentration: 2 mass%) of CMC (carboxymethyl cellulose, Daiichi Kogyo Seiyaku Co., Ltd., Serogen WS-C) as a thickener was used with respect to 98 mass parts of the prepared negative electrode material. ) Was added so that the solid content of CMC was 1 part by mass, and kneading was performed for 10 minutes. Next, purified water was added so that the total solid content concentration of the negative electrode material and CMC was 40% by mass to 50% by mass, and kneading was performed for 10 minutes. Then, an aqueous dispersion (SBR concentration: 40% by mass) of SBR (BM400-B, Nippon Zeon Co., Ltd.) was added as a binder so that the solid content of SBR was 1 part by mass and mixed for 10 minutes. To prepare a paste-like negative electrode material composition. This negative electrode material composition was applied to an electrolytic copper foil having a thickness of 11 μm with a comma coater whose clearance was adjusted so that the amount applied per unit area was 4.5 mg/cm ² , to form a negative electrode layer. Then, the electrode density was adjusted to 1.2 g/cm ³ with a hand press. The electrolytic copper foil on which the negative electrode layer was formed was punched into a disk shape having a diameter of 14 mm to prepare a sample electrode (negative electrode).

The prepared sample electrode (negative electrode), separator, and counter electrode (positive electrode) were placed in a coin-type battery container in this order, and an electrolytic solution was injected into the coin-type lithium ion secondary battery. As the electrolytic solution, LiPF ₆ dissolved in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (the volume ratio of EC and MEC is 3:7) to a concentration of 1.0 mol/L. It was used. Metal lithium was used as the counter electrode (positive electrode). A polyethylene microporous membrane having a thickness of 20 μm was used as the separator.

[Evaluation of initial charge/discharge characteristics]
Using the produced lithium ion secondary battery, the initial charge/discharge characteristics were evaluated by the following method.
(1) It was charged to 0 V (V vs. Li/Li ⁺ ) with a constant current of 0.48 mA, and then, constant voltage charging was performed with 0 V until the current value reached 0.048 mA. The capacity at this time was defined as the initial charge capacity.
(2) After a 30-minute rest time, discharging was performed at a constant current of 0.48 mA to 1.5 V (V vs. Li/Li ⁺ ). The capacity at this time was defined as the initial discharge capacity.
(3) The initial charge/discharge efficiency was calculated from the charge/discharge capacity calculated in (1) and (2) above using the following (formula 1).
Initial charge/discharge efficiency (%)=initial discharge capacity (mAh/g)/initial charge capacity (mAh/g)×100 (Equation 1)

<Example 2>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the compounding amount of isophthalic acid was changed to 5 parts by mass in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 3>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the compounding amount of isophthalic acid was changed to 20 parts by mass in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 4>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the compounding amount of isophthalic acid was changed to 50 parts by mass. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 5>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the carbon precursor was changed to 10 parts by mass of fumaric acid. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 6>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the carbon precursor was changed to 10 parts by mass of lauric acid in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 7>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the carbon precursor was changed to 10 parts by mass of stearic acid. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Example 8>
A negative electrode material and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the volume average particle size of the spherical natural graphite was changed to 37 μm in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Comparative Example 1>
Using 100 parts by mass of spherical natural graphite (volume average particle diameter: 10 μm) as a negative electrode material, a lithium ion secondary battery was produced in the same manner as in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Comparative example 2>
100 parts by mass of spherical natural graphite (volume average particle diameter: 10 μm) was subjected to heat treatment (holding in air at 500° C. for 1 hour, and then naturally cooled). Using the obtained sample as a negative electrode material, a lithium ion secondary battery was produced in the same manner as in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

<Comparative example 3>
100 parts by mass of spherical natural graphite (volume average particle diameter: 10 μm) and 5 parts by mass of coal tar pitch (softening point: 98° C., residual carbon ratio: 50% by mass) as a carbon precursor are mixed to obtain a mixture. It was Next, the mixture was heat-treated (in Ar atmosphere, the temperature was raised from 25° C. to 900° C. at a heating rate of 200° C./hour, held at 900° C. for 1 hour, and then naturally cooled). The heat-treated sample was crushed with a mixer, and a sieve (350 mesh, mesh size 45 μm) was used as a negative electrode material to prepare a lithium ion secondary battery in the same manner as in Example 1. Table 1 shows the results of evaluating the characteristics of the negative electrode material and the lithium ion secondary battery in the same manner as in Example 1.

As shown in the results of Table 1, lithium ion secondary prepared by using the negative electrode material of the example having an R value of Raman spectroscopy of 0.7 or more and an R value per area of 0.2 or less. The battery was manufactured by using the negative electrode material of Comparative Examples 1 and 2 having an R value of Raman spectroscopy of less than 0.7 or the negative electrode material of Comparative Example 3 having an R value per area of more than 0.2. It excels in both initial efficiency and discharge capacity compared to the secondary battery.

The negative electrode material of the example obtained through the step of heat-treating the mixture of spherical natural graphite and the carbon precursor has an increased R value as compared with the negative electrode materials of comparative examples 1 and 2 which do not pass through the step. From the results, it is understood that the amorphous carbon layer is formed on the surface of the spherical natural graphite. Further, the negative electrode material of the example using carboxylic acid as the carbon precursor has a lower R value per area than the negative electrode material of comparative example 3 using coal tar pitch as the carbon precursor.

Claims (10)

A negative electrode for a lithium ion secondary battery, which has an R value of Raman spectroscopy of 0.7 or more and a value obtained by dividing the R value by the N ₂ specific surface area (R value/N ₂ specific surface area) of 0.2 or less. Material. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the R value is 1.6 or less. The negative electrode material for a lithium ion secondary battery according to claim 1, which is a graphite particle having an amorphous carbon layer on at least a part of the surface. The negative electrode material for a lithium ion secondary battery according to claim 1, which has a volume average particle diameter (D ₅₀ ) of 6 μm to ₄₀ μm. The lithium according to any one of claims 1 to 4, wherein in the differential thermal analysis, the number of exothermic peaks observed in the range of 300°C to 900°C is one during the heat treatment in an oxygen atmosphere. Negative electrode material for ion secondary batteries. In the thermogravimetric measurement, the lithium ion according to any one of claims 1 to 5, wherein a weight loss rate in a range of 300°C to 900°C is less than 0.1% in a heat treatment process in a nitrogen atmosphere. Negative electrode material for secondary batteries. 0 CO ₂ adsorption amount obtained from CO ₂ adsorption amount measured at ℃ is 0.30cm ³ /g~1.50cm ³ / g, the lithium ion according to any one of claims 1 to 6 Negative electrode material for secondary batteries. The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 7, wherein the average interplanar spacing (d ₀₀₂ ) determined by X-ray diffraction is 0.337 nm or less. A negative electrode for a lithium ion secondary battery, comprising a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 8 and a current collector. A lithium ion secondary battery including the negative electrode for a lithium ion secondary battery according to claim 9.