JP7650458B2 - Conductive agent for electrodes, composition for electrodes, and electrodes for lithium ion batteries - Google Patents

Description

The present invention relates to an electrode conductive agent, an electrode composition, and an electrode for a lithium ion battery.

In recent years, small, lightweight, and high-capacity lithium-ion batteries have been used in mobile phones, digital cameras, and other devices. Lithium-ion batteries are also being developed as large secondary batteries for use in electric vehicles, but there is a demand for higher battery capacity, shorter charging times, and improved battery life in order to extend the driving range while maintaining safety and reliability.

The electrodes used in lithium-ion batteries are positive or negative electrodes in which a mixture containing an electrode active material, a conductive agent, and a binder is applied to the surface of a metal foil current collector. In the positive electrode, lithium-containing metal composite oxides and the like are used as the active material, and active materials with a higher nickel content in the metal composite oxide are being investigated in order to improve the effective capacity of the battery.

However, while increasing the nickel content in the metal composite oxide improves the effective capacity of the battery, it also increases the interfacial resistance between the current collector and active material, and the volume resistance between the active materials, causing a potential drop. Furthermore, if charging is continued in a state where a potential drop has occurred, active oxygen is generated by a reaction between oxygen derived from the active material and alkaline components. This results in corrosion of the current collector foil, shortening the battery life.

Electrodes made of graphene or exfoliated graphite have also been proposed to improve the conductivity of lithium-ion batteries (Patent Documents 1 and 2).
For example, Patent Document 1 discloses a lithium ion secondary battery element using laminated graphene powder as a conductive agent, the laminated graphene powder having an intensity ratio of I _D peak/I _G peak of 0.3 to 2.8 measured by Raman spectroscopy. In Patent Document 1, the laminated graphene powder is produced by subjecting graphite to a special surface treatment such as oxidation and reduction in order to enhance the dispersibility of the laminated graphene powder in an organic solvent. Patent Document 2 discloses a secondary battery electrode forming material using flaky graphite as a conductive agent.

However, even with these conventional methods, it is necessary to simultaneously increase the nickel content in the active material to improve battery capacity while suppressing potential drop and corrosion of the current collector due to the generation of active oxygen. There is a demand for further shortening charging time and further improving battery life.

Special table 2014-505002 publication JP 2014-182873 A

The object of the present invention is to provide an electrode conductive agent, an electrode composition, and an electrode for lithium ion batteries that can suppress potential drop and corrosion of the current collecting foil even when the nickel content ratio in the active material is increased, thereby achieving high battery capacity, shortened charging time, and improved battery life.

As a result of intensive research into solving the above problems, the inventors discovered that by incorporating exfoliated graphite having specific characteristics into a composition for electrodes in lithium ion batteries, the above object of the present invention can be achieved without subjecting the graphite to special surface treatment, and thus completed the present invention.

That is, the present invention provides a conductive agent for an electrode of a lithium ion battery, the conductive agent comprising exfoliated graphite (excluding an active material) having the following characteristics (1) to (3) :
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship: [G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) The half-width (G-FWHM) of the G band (1580 cm ⁻¹ ) of the Raman spectrum measured by Raman spectroscopy using an argon laser is 15 to 22 cm ⁻¹ .
(3) The thickness of the basal plane of the exfoliated graphite particles is 5 to 40 nm as measured by a field emission scanning electron microscope (FE-SEM).

The present invention also provides a composition for an electrode of a lithium ion battery, comprising exfoliated graphite having the above characteristics (1) to (3) (excluding the active material) and a binder.
The present invention also provides an electrode for a lithium ion battery, comprising a current collector coated with the electrode composition of the present invention.
The present invention also provides a method for producing an electrode for a lithium ion battery, which comprises applying the electrode composition of the present invention onto a current collector.
Furthermore, the present invention provides a lithium ion battery comprising the electrode of the present invention.

By using the specific ultra-thin graphite of the present invention as a conductive agent, it is possible to significantly reduce the volume resistivity and produce an electrode composition with significantly improved discharge capacity without the need for special surface treatment of the graphite as in the conventional method. Therefore, even if the nickel content ratio in the active material is increased, the potential drop can be suppressed, and as a result, corrosion of the current collector foil can be suppressed. Therefore, it is possible to provide a conductive agent for electrodes, an electrode composition, and an electrode for lithium-ion batteries that can achieve high battery capacity, shortened charging time, and improved battery life.

FIG. 1 is a graph showing the results of measuring the initial discharge capacity of coin cells produced using electrode compositions 1 and 3 produced in Example 1 and Comparative Example 2. FIG. 1 is a graph showing the results of measuring the discharge capacity after 80 cycles when coin cells were produced using electrode compositions 1 and 3 produced in Example 1 and Comparative Example 2. FIG. 13 is a graph showing the results of measuring the initial discharge capacity of coin cells produced using electrode compositions 4 and 5 produced in Example 2 and Comparative Example 3. FIG. 13 is a graph showing the results of measuring the discharge capacity after 80 cycles when coin cells were produced using electrode compositions 4 and 5 produced in Example 2 and Comparative Example 3. FIG. 13 is a graph showing the results of measuring the initial discharge capacity of coin cells produced using electrode compositions 6 and 7 produced in Example 3 and Comparative Example 4. FIG. 13 is a graph showing the results of measuring the discharge capacity after 80 cycles when coin cells were produced using electrode compositions 6 and 7 produced in Example 3 and Comparative Example 4. FIG. 1 is a cross-sectional view illustrating a lithium ion battery produced using an electrode composition 1 (Example 1) of the present invention.

A typical embodiment for implementing the present invention is described in detail below, but the present invention is not limited to the following embodiment.

(exfoliated graphite)
The present invention is characterized by using exfoliated graphite having the following characteristics (1) and (2) as an electrode conductive agent for a lithium secondary battery.
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship: [G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) The full width at half maximum (G-FWHM) of the G band (1580 cm ^-1 ) of the Raman spectrum measured by Raman spectroscopy using an argon laser is 15 to 22 cm ^-1 .
In the present invention, the conductive agent means a material that contributes to the conductivity of an electrode, and the exfoliated graphite of the present invention has the function of further reducing the resistance of the electrode and further improving the conductivity.

(Feature (1): Band intensity ratio of Raman spectrum)
When the exfoliated graphite of the present invention is measured by Raman spectroscopy using an argon ion laser (excitation wavelength 532 nm), the intensity ratio of the G band (1580 cm ^-1 ) and the D band (1360 cm ^-1 ) in the Raman spectrum satisfies the following relationship:
[G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8

Here, the intensity ratio of the G band (1580 cm ⁻¹ ) and the D band (1360 cm ⁻¹ ) in the Raman spectrum indicates the crystallinity of the graphite (basal plane).
In the present invention, the band intensity ratio (G band/D band) can be calculated by plotting a straight line between the baseline of the G band (1500-1650 cm ⁻¹ ) and the baseline of the D band (1300-1400 cm ⁻¹ ) in a Raman spectrum measured under conditions of an output of 0.5 mW, an exposure time of 5 Hz (0.2 s), and 1000 scans (accumulation), and then determining the peak heights of the G band and D band from the baseline and applying the results to the following formula:
Band intensity ratio = G band peak height / D band peak height

The band intensity ratio (G band/D band) of the exfoliated graphite of the present invention is measured in accordance with Japanese Industrial Standard JIS K 0137-2010, and specifically, from the viewpoint of maintaining the crystallinity of the exfoliated graphite, the lower limit is preferably 8.2 or more, 8.4 or more, 8.6 or more, more preferably 8.8 or more, and particularly preferably 9.0 or more, and the upper limit is preferably 25 or less, 23 or less, 20 or less, more preferably 15 or less, and particularly preferably 13 or less. The upper and lower limit values of the band intensity ratio can be combined in any combination.

(Feature (2): Raman spectrum G band half-width (G-FWHM))
The exfoliated graphite of the present invention has a half-width (G-FWHM) of the G band (1580 cm ^-1 ) in the Raman spectrum of 15 to 22 cm ^-1 when measured by Raman spectroscopy using an argon ion laser (excitation wavelength 532 nm). Here, the half-width (G-FWHM) of the G band (1580 cm ^-1 ) means the crystallinity of the graphite.

In the present invention, the half width of the G band (G-FWHM) is measured in accordance with Japanese Industrial Standard JIS K 0137-2010 "General Rules for Raman Spectroscopic Analysis." Specifically, in a Raman spectrum measured under conditions of an output of 0.5 mW, an exposure time of 5 Hz (0.2 s), and 1000 scans (accumulation), a baseline (1500-1650 cm ^-1 ) of the G band is drawn as a straight line, and then the peak height of the G band from the baseline is determined, and the bandwidth at 50% of the determined peak height can be calculated as the half width.

The lower limit of the half width at half maximum (G-FWHM) of the G band of the exfoliated graphite of the present invention is preferably 15.0 or more, 15.5 or more, or 16.0 or more, more preferably 16.5 or more, and particularly preferably 17.0 or more, and the upper limit is 21.5 cm ^-1 or less, 21.0 cm ^-1 or less, more preferably 20.5 cm ^-1 or less, and particularly preferably 20.0 cm ^-1 or less. The upper and lower limits of the half width at half maximum (G-FWHM) of the G band can be combined in any desired manner.

(Average particle size of exfoliated graphite)
The exfoliated graphite of the present invention usually has an average particle size in the range of 0.5 to 20 μm when measured by a laser diffraction/scattering method. Here, the average particle size refers to the average value of the longest diameter of the laminated graphene sheet plane in the primary particles. The average particle size can be measured using, for example, a Microtrac MT3000II series (manufactured by Microtrac Bell Co., Ltd.) after the exfoliated graphite is sufficiently dispersed.

In the present invention, the average particle size of the exfoliated graphite measured as described above is preferably 0.8 to 15 μm, more preferably 1.0 to 10 μm, and even more preferably 1.5 to 5 μm, from the viewpoint that the particles serving as the conductive agent are arranged randomly rather than in a single line, thereby reducing both the plane resistance and the penetration resistance.

(Thickness of exfoliated graphite particles)
The exfoliated graphite of the present invention preferably has a basal plane of the particle having a thickness in the range of 5 to 50 nm as measured by a field emission scanning electron microscope (FE-SEM). In the present invention, the basal plane refers to a layering plane when graphene is layered to form graphite. The thickness of the basal plane of the exfoliated graphite particles of the present invention can be determined by measuring an image of the exfoliated graphite with an FE-SEM and measuring the thickness of 10 arbitrarily selected particles with a scale, and averaging the measured values.

The thickness of the exfoliated graphite particles of the present invention measured in this manner is preferably 5 to 40 nm, more preferably 5 to 30 nm, and particularly preferably 10 to 15 nm, from the viewpoint that even if the flake graphite is physically thinned to the limit by cleavage and crushing, the particles do not have the properties of single-layer graphene.
Ordinary graphite has a structure in which many layers of graphene (0.335 nm thick) are stacked, and the thickness is usually 500 to 2000 nm. The exfoliated graphite of the present invention is graphite obtained by exfoliating ordinary graphite, and usually has a structure in which 10 to 100 layers of graphene are stacked.

The exfoliated graphite of the present invention preferably has a full width at half maximum (FWHM) measured by a field emission scanning electron microscope (FE-SEM) in the range of 5 to 25 nm. Here, the full width at half maximum (FWHM) refers to the difference between two points xb and xa where g(x) is half the extreme value h when a local response curve g(x) = f(x) - b(x) is considered in the vicinity of a point where f(x) is an extreme value in a response curve y = f(x ₎ of y representing a particle size distribution to _x .
The full width at half maximum FWHM of the exfoliated graphite of the present invention is preferably 8 to 22 cm ^-1 , and particularly preferably 10 to 20 cm ^-1 , from the viewpoint that the basal plane is not damaged.

The exfoliated graphite of the present invention is preferably one that does not have a reactive functional group selected from the group consisting of a hydroxyl group, a carbonyl group, and a methine group on its surface. Conventional laminated graphene powders produced by subjecting graphite to special surface treatments such as oxidation and reduction have reactive functional groups on their surface, but the exfoliated graphite of the present invention does not have such reactive functional groups, and yet has good dispersibility in organic solvents, can significantly reduce volume resistivity, and can significantly improve discharge capacity.

The exfoliated graphite of the present invention can be produced by shear-pulverizing and delaminating ordinary graphite using a pulverizer with a wet bead mill method. Zirconia or alumina balls can be used as the beads used in the pulverizer, and the diameter of the beads is usually 0.05 to 10 mm, preferably 0.1 to 5 mm, and more preferably 0.3 to 5 mm. The pulverization time is usually 10 minutes to 10 hours, preferably 20 minutes to 7 hours, and more preferably 30 minutes to 5 hours for a volume of 30 to 100 liters.

When performing the grinding process using the wet bead mill method, it is preferable to have a suitable dispersant present in order to sufficiently increase dispersibility. As the dispersant, it is preferable to use polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), etc. When carboxymethyl cellulose (CMC) is present as a dispersant in the grinding process, the amount of carboxymethyl cellulose (CMC) used should be 0.2 to 15.0 parts by weight per 100 parts by weight of graphite to be ground.

The exfoliated graphite produced by this grinding method has a very low volume resistivity of 15.0 Ω·cm or less, preferably 10.0 Ω·cm or less, more preferably 8.0 Ω·cm or less, and even more preferably 7.0 Ω·cm or less, when the volume resistivity is measured in accordance with Japanese Industrial Standard JIS K 7194 using a commercially available volume resistivity measuring device.

When ultra-thin exfoliated graphite having the above characteristics (1) and (2) is added to an electrode composition, a physical conductive path is easily formed between the current collector and the active material, reducing the interface resistance compared to when a conventional conductive agent such as graphene or graphite is added, and the current flow between the active materials is improved, reducing the volume resistance and suppressing the potential drop. As a result, the generation of active oxygen can be suppressed, and corrosion of the current collector foil can be suppressed.

(Electrode Composition)
A composition (conductive paste) containing the exfoliated graphite of the present invention, a binder, and, if necessary, a conductive agent other than the exfoliated graphite of the present invention and a solvent can be used as an electrode composition for a lithium ion battery. As the binder, a binder that has been conventionally used in the manufacture of electrodes, such as polyvinylidene fluoride (PVDF), polyimide (PI), acrylic emulsion, and styrene-butadiene rubber (SBR), can be used. As the conductive agent other than the exfoliated graphite of the present invention, carbon black and carbon nanotubes (CNT) can be mentioned, and among them, carbon black is preferable from the viewpoint of purity and conductivity. As the solvent, an organic solvent such as N-methylpyrrolidone (NMP) or an aqueous solvent containing water can be used.
Such a conductive paste not containing an active material can be used as an electrode composition to be mixed with an active material when producing an electrode for a lithium ion battery.

When used as a conductive paste, the concentration of exfoliated graphite in the electrode composition is usually 1 to 30% by weight, preferably 2 to 20% by weight, and more preferably 2 to 15% by weight. The weight ratio of the conductive agent other than exfoliated graphite to exfoliated graphite is usually 5:95 to 50:50, preferably 10:90 to 45:55, and more preferably 10:90 to 30:70. The concentration of the binder in the electrode composition is usually 1 to 20% by weight, preferably 2 to 10% by weight, and more preferably 2 to 9% by weight.

The electrode paste obtained by adding positive or negative active material to the conductive paste can be used as an electrode composition (mixture) to be applied to a current collector and dried to adhere when producing an electrode for a lithium-ion battery.

In the present invention, the positive electrode active material may be _a Li-containing composite oxide represented by the composition formula _LixMO2 or _LiyM2O4 (M is a transition element; 0<x≦1, 0<y≦2). Specific examples of the Li-containing composite oxide include _LiCoO2 , _LiMnO2 , _LiNiO2 , _LiCrO2 , and _LiMn2O4 . Since the present invention can suppress a potential drop even if the content ratio of nickel in the active material is increased, it is preferable _to use a lithium-containing _metal composite oxide containing nickel and represented by the composition _{formula LiNixCoyMnzO2} (wherein x, y, and z are the molar ratios of Ni, Co, and _Mn , respectively, x+y+z=1, and 0.5≦x≦0.9) as the positive electrode active material. Among these, according to the present invention, a lithium-containing metal composite oxide in which the molar ratio x of Ni in the above composition formula is in the range of 0.5 to 0.9, preferably 0.6 to 0.8, can be used.

In the present invention, examples of the negative electrode active material include carbon compounds such as graphite, hard carbon, and coke, silicon, tin alloys, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc. Among these, graphite and lithium titanate are preferred from the viewpoint of safety.

The composition of the electrode composition when used as an electrode paste is usually 0.2 to 5.5 parts by weight, preferably 0.3 to 5.0 parts by weight, and more preferably 0.4 to 4.0 parts by weight, of exfoliated graphite per 100 parts by weight of electrode active material, usually 0.2 to 5.5 parts by weight, preferably 0.3 to 3.5 parts by weight, and more preferably 0.5 to 3.0 parts by weight of conductive agent other than exfoliated graphite, and usually 1.0 to 9.5 parts by weight, and more preferably 1.5 to 7.8 parts by weight of binder. The concentration of exfoliated graphite in the electrode composition is usually 0.2 to 15% by weight, preferably 0.5 to 12% by weight, and more preferably 0.7 to 9% by weight, calculated as solid content. In the electrode composition, the weight ratio of the conductive agent other than exfoliated graphite to exfoliated graphite is usually 5:95 to 50:50, preferably 10:90 to 45:55, and more preferably 10:90 to 30:70.

The electrode paste (electrode composition) can be prepared, for example, by mixing a solution of PVDF dissolved in an organic solvent such as NMP, or a suspension of acrylic emulsion or carboxymethyl cellulose (CMC) dispersed in water, with a positive or negative electrode active material, the exfoliated graphite of the present invention, and, if necessary, a conductive agent other than the exfoliated graphite of the present invention.

(electrode)
The electrode for a lithium ion battery of the present invention can be prepared by depositing an electrode composition (electrolytic paste) on a current collector in the same manner as in conventional electrodes, except that the exfoliated graphite of the present invention is used as a conductive agent. For example, a slurry-like electrode composition (electrolytic paste) in which each component is dispersed in an organic solvent such as NMP or an aqueous solvent containing water is applied to a metal current collector by a doctor blade method, and the solvent is dried to prepare a positive electrode or a negative electrode. As in conventional cases, metals such as aluminum and copper are preferably used as the current collector.

In the present invention, the interfacial resistance between the current collector and the active material can be further reduced by using a composition containing exfoliated graphite having the above characteristics (1) and (2) as a primer composition for an electrode of a lithium ion battery. In the present invention, the primer or primer layer is an adhesive layer applied between the current collector and the electrode composition layer (electrode mixture layer). Specifically, a primer composition containing another conductive agent such as carbon black and the exfoliated graphite of the present invention is applied to the current collector and dried to form a primer layer on the current collector, and then the electrode composition (electrolytic paste) of the present invention described above is applied to the primer layer, thereby producing an electrode for a lithium ion battery having a lower volume resistivity of the interfacial resistance and high adhesion between the current collector and the electrode mixture layer.
In the primer composition of the present invention, the content of exfoliated graphite relative to other conductive agents is usually 10 to 1000 parts by weight, preferably 15 to 750 parts by weight, and more preferably 20 to 600 parts by weight. The thickness of the primer layer is usually 0.5 to 10 μm, and preferably 0.5 to 5.0 μm. When preparing an electrode for a lithium ion battery, if an electrode composition containing exfoliated graphite of the present invention is used, the primer composition does not need to contain exfoliated graphite of the present invention, and the primer composition may contain a conductive agent other than the exfoliated graphite of the present invention, such as carbon black, or a known conductive agent.

A lithium ion secondary battery can be produced by combining the above-mentioned positive electrode and negative electrode with a separator separating the two electrodes, a positive electrode lead, a negative electrode lead, a positive electrode external terminal, and a negative electrode can. As the electrolyte, an electrolytic solution or a solid electrolyte can be used. As the electrolytic solution, an organic electrolyte solution in which LiPF ₆ is dissolved in a mixed solvent of cyclic carbonate esters such as ethylene carbonate (EC) or propylene carbonate (PC) and chain carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC) can be used. As the solid electrolyte, an inorganic solid electrolyte such as an oxide or sulfide system, or an organic solid electrolyte such as a polymer system can be used.

The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples.

[Production of exfoliated graphite]
(Production Examples 1 to 3)
High purity graphite (J-SP-α manufactured by Nippon Graphite Industries Co., Ltd., average particle size: 6.1 μm) was pulverized by a wet bead mill method using a pulverizer (NVM-30) manufactured by Imex Co., Ltd. under conditions 1 to 3 shown in Table 1 below.

The Raman spectrum band intensity ratio (G band/D band), half-width of the G band (G-FWHM), average particle size, particle thickness, and volume resistivity of the exfoliated graphite obtained by grinding processing using conditions 1 to 3 were measured under the conditions below and the results are shown in Table 2.

(1) Band intensity ratio (G band/D band)
Measurement was performed in accordance with JIS K 0137-2010 "General Rules for Raman Spectroscopic Analysis." That is, in a Raman spectrum measured by Raman spectroscopic analysis using an argon laser under conditions of an output of 0.5 mW, an exposure time of 5 Hz (0.2 s), and 1000 scans (accumulation), the baseline of the G band (1500-1650 ^cm-1 ) and the baseline of the D band (1300-1400 cm ^-1 ) were each drawn as straight lines, and the peak heights of the G band and D band from the baseline were determined, and the band intensity ratio (G band/D band) was calculated by applying the results to the following formula.
Band intensity ratio = G band peak height / D band peak height

(2) G-band full width at half maximum (G-FWHM)
Measurement was performed in accordance with JIS K 0137-2010 "General Rules for Raman Spectroscopic Analysis." That is, in a Raman spectrum measured under conditions of an output of 0.5 mW, an exposure time of 5 Hz (0.2 s), and 1000 scans (accumulation), a baseline (1500-1650 cm ^-1 ) of the G band was created as a straight line, and then the peak height of the G band from the baseline was determined, and the bandwidth at 50% of the determined peak height was calculated as the half-width.

(3) Average particle size (μm)
The average longest diameter of the stacked graphene sheet planes in the primary particles was measured by a laser diffraction/scattering method using a Microtrac MT3000II series (manufactured by Microtrac Bell Co., Ltd.).

(4) Particle thickness (nm)
An image of exfoliated graphite was measured using an FE-SEM, and the thicknesses of 10 arbitrarily selected particles were measured with a scale, and the average value was calculated.

(5) Volume resistivity (Ω·cm) was measured in accordance with JIS K 7194 using a commercially available volume resistivity measuring device.

[Preparation and evaluation of electrode composition]
(Example 1, Comparative Examples 1 and 2)
Lithium cobalt manganese composite oxide ( _{LiNi0.8Co0.1Mn0.1O2} ) as a positive _electrode active material, carbon black (acetylene black Li250 manufactured by Denka Co. _, Ltd.) as a conductive agent, exfoliated graphite manufactured in Manufacturing Example 3 (condition 3), and PVDF# ₇₃₀₀ (manufactured by Kureha Co., Ltd.) as a binder were dispersed in an NMP solution so that the solid weight ratio of the active material, carbon black (conductive agent), exfoliated graphite (conductive agent), and binder was 94.4:1.3:2.3:2.0 to form a slurry (solid concentration 65% by weight), and an electrode composition 1 (Example 1) was prepared. The concentration of exfoliated graphite in the electrode composition at this time was 1.5% by weight.

In addition, an electrode composition 2 (Comparative Example 1) was prepared in the same manner as in the preparation of electrode composition 1, except that the exfoliated graphite was changed to ordinary graphite (J-SP-α manufactured by Nippon Graphite Industries Co., Ltd.).
Furthermore, an electrode composition 3 (Comparative Example 2) was prepared in the same manner as in the preparation of the electrode composition 1, except that no exfoliated graphite (conductive agent) was used and the solids weight ratio of the active material, conductive agent, and binder was 93.0:3.5:3.5.

The volume resistivity of electrode compositions 1 to 3 prepared as described above was measured and the results are shown in Table 3 below.

From the results in Table 3, it was confirmed that the volume resistivity increases in the case of electrode composition 2 (Comparative Example 1), which uses ordinary graphite and carbon black as the conductive agents, compared to electrode composition 3 (Comparative Example 2), which does not use graphite as the conductive agent, whereas the volume resistivity can be significantly reduced in the case of electrode composition 1 (Example 1), which uses the exfoliated graphite and carbon black of the present invention as the conductive agents.

Coin cells were prepared using electrode compositions 1 and 3 in accordance with the coin cell CR2032 specified in the Japanese Industrial Standard JIS C 8515. The initial discharge capacity and the discharge capacity after 80 cycles were measured at an upper limit voltage of 4.3 V and a lower limit voltage of 2.7 V. The results are shown in Figures 1 and 2, respectively.

From the results in Figure 1, it was confirmed that when electrode composition 3 (Comparative Example 2) was used, the initial discharge capacity was 187 mAh/g, whereas when electrode composition 1 (Example 1) was used, the initial discharge capacity was 195 mAh/g, and that the initial discharge capacity of electrode composition 1 (Example 1) was increased by approximately 4.3% compared to when electrode composition 3 (Comparative Example 2) was used.

From the results in FIG. 2 , it was confirmed that when electrode composition 3 (Comparative Example 2) was used, the discharge capacity after 80 cycles was 155 mAh/g, whereas when electrode composition 1 (Example 1) was used, the discharge capacity was 163 mAh/g. In other words, when electrode composition 1 (Example 1) was used, the discharge capacity after 80 cycles was increased by approximately 5.2% compared to when electrode composition 3 (Comparative Example 2) was used.
These results show that the use of an organic solvent-based positive electrode composition containing the exfoliated graphite of the present invention as a conductive agent significantly increases the discharge capacity and improves the cycle characteristics.

(Example 2 and Comparative Example 3)
Lithium iron phosphate (LFP) as a positive electrode active material, carbon black (acetylene black Li250 manufactured by Denka Co., Ltd.) as a conductive agent, exfoliated graphite manufactured in Manufacturing Example 3 (condition 3), and acrylic emulsion and carboxymethyl cellulose (CMC) as a binder were dispersed in ion-exchanged water so that the solids weight ratio of the active material, carbon black (conductive agent), exfoliated graphite (conductive agent), and binder was 94.4:1.3:2.3:2.0 to prepare a slurry (solids concentration 55% by weight), and an electrode composition 4 (Example 2) was prepared. In addition, an electrode composition 5 (Comparative Example 3) was prepared in the same manner as the electrode composition 4, except that the conductive agent was changed to only carbon black (acetylene black Li250 manufactured by Denka Co., Ltd.).

In accordance with coin cell CR2032 specified in the Japanese Industrial Standard JIS C 8515, a primer composition containing exfoliated graphite and carbon black as conductive agents and acrylic resin as adhesive was applied to a current collector and dried to form a primer layer on the current collector. Coin cells were then produced using electrode compositions 4 and 5 as the positive electrode composition, and the initial discharge capacity and discharge capacity after 80 cycles were measured at an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V. The results are shown in Figures 3 and 4, respectively.

From the results in Figure 3, it was confirmed that when electrode composition 5 (Comparative Example 3) was used, the initial discharge capacity was 118 mAh/g, whereas when electrode composition 4 (Example 2) was used, the initial discharge capacity was 133 mAh/g, indicating that electrode composition 4 (Example 2) had an initial discharge capacity that was increased by approximately 12.7% compared to when electrode composition 5 (Comparative Example 3) was used.

From the results in FIG. 4 , it was confirmed that when electrode composition 5 (Comparative Example 3) was used, the discharge capacity after 80 cycles was 108 mAh/g, whereas when electrode composition 4 (Example 2) was used, the discharge capacity was 130 mAh/g. In other words, when electrode composition 4 (Example 2) was used, the discharge capacity after 80 cycles was increased by approximately 20.4% compared to when electrode composition 5 (Comparative Example 3) was used.
These results show that the use of an aqueous positive electrode composition containing the exfoliated graphite of the present invention as a conductive agent significantly increases the discharge capacity and improves the cycle characteristics.

(Example 3 and Comparative Example 4)
Natural graphite CGB-10 (manufactured by Nippon Graphite Industry Co., Ltd.) as a negative electrode active material, carbon black (acetylene black Li250 manufactured by Denka Co., Ltd.) as a conductive agent, exfoliated graphite manufactured in Production Example 3 (condition 3), and BM-400B (manufactured by Zeon Corporation) and carboxymethyl cellulose (CMC) as a binder (binder) were dispersed in ion-exchanged water so that the solids weight ratio of the active material, carbon black (conductive agent), exfoliated graphite (conductive agent), and binder was 89.5:0.6:3.4:6.5 to obtain a slurry (solids concentration 50% by weight), and electrode composition 6 (Example 3) was prepared. In addition, electrode composition 7 (Comparative Example 4) was prepared in the same manner as electrode composition 6, except that the conductive agent was changed to only carbon black (acetylene black Li250 manufactured by Denka Co., Ltd.).

Coin cells were prepared using electrode compositions 6 and 7 in accordance with the coin cell CR2032 specified in the Japanese Industrial Standard JIS C 8515. The initial discharge capacity and the discharge capacity after 80 cycles were measured at an upper limit voltage of 3.5 V and a lower limit voltage of 0.05 V. The results are shown in Figures 5 and 6, respectively.

From the results in Figure 5, it was confirmed that when electrode composition 7 (Comparative Example 4) was used, the initial discharge capacity was 340 mAh/g, whereas when electrode composition 6 (Example 3) was used, the initial discharge capacity was 357 mAh/g, indicating that electrode composition 6 (Example 3) had an initial discharge capacity that was increased by approximately 5.0% compared to when electrode composition 7 (Comparative Example 4) was used.

From the results in FIG. 6 , it was confirmed that when electrode composition 7 (Comparative Example 4) was used, the discharge capacity after 80 cycles was 305 mAh/g, whereas when electrode composition 6 (Example 3) was used, the discharge capacity was 328 mAh/g. In other words, when electrode composition 6 (Example 3) was used, the discharge capacity after 80 cycles was increased by about 7.5% compared to when electrode composition 7 (Comparative Example 4) was used.
These results show that the use of an aqueous negative electrode composition containing the exfoliated graphite of the present invention as a conductive agent significantly increases the discharge capacity and improves the cycle characteristics.
The types of electrode compositions, types of conductive agents, and discharge capacities prepared in the above Examples 1 to 3 and Comparative Examples 1 to 4 are summarized in Table 4 below.

By using the exfoliated graphite of the present invention, an electrode for a lithium ion battery and a lithium ion battery including this electrode can be produced, for example, in the following manner.
[Preparation of electrodes]
(1) Preparation of Positive Electrode The electrode composition 1 is applied to one side of an aluminum foil serving as a positive electrode current collector by a doctor blade method, and the NMP is dried at 120° C. in an oven to prepare a positive electrode 1 .

(2) Preparation of Negative Electrode Carbotron P (manufactured by Kureha Corporation) as hard carbon serving as a negative electrode active material, Acetylene Black Li250 (manufactured by Denka Co., Ltd.) as a conductive agent, and PVDF #9100 (manufactured by Kureha Corporation) were dispersed in an NMP solution so that the solids weight ratio of the active material, conductive agent, and binder was 91:1:8 to prepare a slurry (solids concentration: 50% by weight). The slurry was then applied to one side of a copper foil serving as a negative electrode current collector by a doctor blade method, and the NMP was dried at 120° C. in an oven to prepare a negative electrode.

[Preparation of lithium-ion battery]
An electrolyte solution is prepared by dissolving _LiPF6 at a ratio of 1 mole/liter in an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
A cylindrical first battery MP1 is fabricated using the positive electrode, negative electrode, and electrolyte prepared as described above (battery dimensions: diameter 14.2 mm; length 50.0 mm). Note that a microporous thin film made of ion-permeable polypropylene (manufactured by Polyplastics Co., Ltd., product name "Celgard 3401") is used as a separator.

Figure 7 is a cross-sectional view of the first battery MP1 that was fabricated, which is composed of a positive electrode 1, a negative electrode 2, a separator 3 that separates these two electrodes, a positive electrode lead 4, a negative electrode lead 5, a positive electrode external terminal 6, and a negative electrode can 7. The positive electrode 1 and the negative electrode 2 are housed in the negative electrode can in a spirally wound state with the separator 3 filled with electrolyte, the positive electrode 1 is connected to the positive electrode external terminal 6 via the positive electrode lead 4, and the negative electrode 2 is connected to the negative electrode can 7 via the negative electrode lead 5, so that the chemical energy generated inside the first battery MP1 can be taken out as electrical energy to the outside.

From the results of Tables 3, 4, and Figures 1 to 6, it was confirmed that electrode composition 1 (Example 1) using the exfoliated graphite of the present invention as a conductive agent can significantly reduce the volume resistivity, and that when a lithium ion secondary battery is produced using electrode composition 1 (Example 1), electrode composition 4 (Example 2), and electrode composition 6 (Example 3), the discharge capacity is significantly increased and the cycle characteristics are also improved. Therefore, by producing an electrode and a lithium ion battery using an electrode composition containing exfoliated graphite of the present invention, it is possible to suppress the potential drop even if the content ratio of nickel in the active material is increased, and as a result, corrosion of the current collector foil can be suppressed. This makes it possible to provide a lithium ion battery that can achieve high battery capacity, shortened charging time, and improved battery life.

1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode lead 5 Negative electrode lead 6 Positive electrode external terminal 7 Negative electrode can

Claims (14)

A conductive agent for an electrode of a lithium ion battery, comprising exfoliated graphite (excluding an active material) having the following characteristics (1) to (3) :
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship: [G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) The half-width (G-FWHM) of the G band (1580 cm ⁻¹ ) of the Raman spectrum measured by Raman spectroscopy using an argon laser is 15 to 22 cm ⁻¹ .
(3) The thickness of the basal plane of the exfoliated graphite particles is 5 to 40 nm as measured by a field emission scanning electron microscope (FE-SEM). 2. The conductive agent according to claim 1 , wherein the average particle size of the exfoliated graphite is 0.5 to 20 μm as measured by a laser diffraction/scattering method. 3. The conductive agent according to claim 1, wherein the exfoliated graphite does not have a reactive functional group selected from the group consisting of a hydroxyl group, a carbonyl group, and a methine group on the surface thereof. A conductive agent for a positive electrode of a lithium ion battery, the conductive agent comprising exfoliated graphite having the following characteristics (1) and (2):
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship:
[G band (1580 cm ^-1 ) strength/D band (1360 cm ^-1 ) Intensity]≧8
(2) The G band of the Raman spectrum (1580 cm ) measured by Raman spectroscopy using an argon laser ^-1 ) half-width (G-FWHM) is 15-22 cm ^-1 is An electrode composition for a lithium ion battery, comprising exfoliated graphite (excluding an active material) having the following characteristics (1) to (3) , and a binder:
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship: [G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) The half-width (G-FWHM) of the G band (1580 cm ⁻¹ ) of the Raman spectrum measured by Raman spectroscopy using an argon laser is 15 to 22 cm ⁻¹ .
(3) The thickness of the basal plane of the exfoliated graphite particles is 5 to 40 nm as measured by a field emission scanning electron microscope (FE-SEM). The electrode composition according to claim 5 , further comprising carbon black. The electrode composition according to claim 5 or 6 , further comprising an electrode active material. The electrode composition according to claim 7 , which contains 0.2 to 5.5 parts by weight of exfoliated graphite per 100 parts by weight of the electrode active material. A composition for an electrode of a lithium ion battery, comprising exfoliated graphite having the following characteristics (1) and (2), an electrode active material, and a binder,
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship:
[G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) A composition for an electrode, wherein the electrode active material has a half-width (G-FWHM) of the G band (1580 cm ^-1 ) of a Raman spectrum measured by Raman spectroscopy using an argon laser of 15 to 22 cm ^-1 , and is a lithium-containing metal composite oxide represented by the composition formula LiNi _x Co _y Mn _z O ₂ (wherein x, y and z are the molar ratios of Ni, Co and Mn, respectively, x + y + z = 1 , and 0.5 ≤ x ≤ 0.9). The electrode composition according to any one of claims 5 to 9 , wherein the electrode composition is a mixture composition for an electrode or a primer composition for an electrode. An electrode for a lithium ion battery, comprising a current collector coated with the electrode composition according to any one of claims 5 to 10 . An electrode for a lithium ion battery, comprising a composition for an electrode for a lithium ion battery, the composition comprising exfoliated graphite having the following characteristics (1) and (2) and a binder, and coated on a current collector,
(1) The band intensity ratio of the Raman spectrum measured by Raman spectroscopy using an argon laser satisfies the following relationship:
[G band (1580 cm ⁻¹ ) intensity/D band (1360 cm ⁻¹ ) intensity]≧8
(2) The electrode is a positive electrode , and the half-width (G-FWHM) of the G band (1580 cm ^-1 ) in the Raman spectrum measured by Raman spectroscopy using an argon laser is 15 to 22 cm ^-1 . A method for producing an electrode for a lithium ion battery, comprising depositing the electrode composition according to any one of claims 5 to 10 on a current collector. A lithium ion battery comprising the electrode according to claim 11 or 12.