WO2020110943A1

WO2020110943A1 - Lithium ion secondary battery negative electrode and lithium ion secondary battery

Info

Publication number: WO2020110943A1
Application number: PCT/JP2019/045806
Authority: WO
Inventors: 大輔原田; 浩文井上; 大輔香野
Original assignee: 昭和電工株式会社
Priority date: 2018-11-26
Filing date: 2019-11-22
Publication date: 2020-06-04

Abstract

The present invention relates to a lithium ion secondary battery negative electrode that comprises a mixture layer that includes a negative electrode active material. The density of the mixture layer is 1.51–1.90 g/cm³. In a log differential pore volume distribution for the mixture layer as measured by mercury intrusion, there is a pore diameter D1 that has a maximum value at 0.50–2.00 µm. The negative electrode active material includes artificial graphite and natural graphite. The artificial graphite is 1–40 mass% of the total of the artificial graphite and the natural graphite. The compressive strength of the artificial graphite is 20–100 MPa, and the compressive strength of the natural graphite is 0.1–15 MPa. The lithium ion secondary battery negative electrode has high energy density and high large-current discharge characteristics.

Description

Negative electrode for lithium-ion secondary battery and lithium-ion secondary battery

The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery. More specifically, the present invention relates to a negative electrode for a lithium ion secondary battery, which is formed by using artificial graphite and natural graphite as a negative electrode active material and has excellent large current characteristics, and a lithium ion secondary battery.

❖ Lithium-ion secondary batteries are used as power sources for portable electronic devices, and in recent years have also been used as power sources for power tools and electric vehicles. In electric vehicles such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), high charge/discharge cycle characteristics should be maintained for more than 10 years, and large current load characteristics sufficient to drive high power motors. And to have a high energy density in order to extend the cruising range. In particular, a plug-in hybrid vehicle (PHEV) has a smaller battery capacity than an EV, and a trickle charge must be performed by driving a motor with a low capacity battery.

Previously, carbon-based materials such as graphite materials have been mainly used as the negative electrode material, but recently, metal-based negative electrode materials have also been developed. However, there are many problems such as cycle life and stability, and many problems still remain.

Especially in automobile applications such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), long-term cycle characteristics for more than 10 years and large current load characteristics for driving a high power motor are mainly required characteristics. In addition, high energy density is required to extend the cruising range, which is severe compared to mobile applications.

In this lithium-ion secondary battery, generally, a lithium salt such as lithium cobalt oxide is used for the positive electrode active material, and a carbon-based material such as graphite material is used for the negative electrode active material.

Carbon-based materials are roughly classified into graphite materials having a high graphitization degree and amorphous carbon materials having a low graphitization degree, but both of them can be used as a negative electrode active material because they are capable of lithium insertion/desorption reaction. it can.

Graphite materials include natural graphite and artificial graphite. It is known that natural graphite is formed into spherical particles. Lithium-ion secondary batteries using natural graphite have some performance required as a power supply for portable electronic devices, but have reached the performance required as a power supply for electric vehicles and electric tools. Absent. However, due to its high degree of graphitization, a capacity equivalent to the theoretical battery capacity and a high mixture layer density can be obtained, and it is widely used as a negative electrode material mainly for mobile applications.

On the other hand, artificial graphite is known as mesocarbon spheres and graphitized products such as petroleum pitch, coal pitch, petroleum coke, and coal coke. These artificial graphites show excellent charge/discharge performance that satisfies the requirements of large currents and ultra long-term cycle characteristics of large batteries, while the degree of graphitization is lower than that of natural graphite and satisfies the requirements of capacity and mixture layer density. Has not reached.

The so-called hard carbon and the negative electrode material using soft carbon as the amorphous carbon material have excellent characteristics against large currents, and also have relatively good cycle characteristics. However, the energy density is too low and the price is very expensive, so it is used only for some special large batteries.

The battery performance greatly depends on the type of negative electrode material as described above, but improvement of battery characteristics by controlling the electrode structure has also been studied.

Patent Document 1 discloses a negative electrode for a lithium ion battery in which pore-shaped artificial graphite is used and the pore size in the electrode is regulated.

Patent Document 2 discloses a multi-layered negative electrode having a plurality of layers in the horizontal direction with respect to the current collector foil and having different densities depending on the layers.

Further, it has been verified that a graphite material is mixed and the characteristics of artificial graphite and natural graphite are utilized, and Patent Document 3 discloses a negative electrode in which two or more kinds of graphite, artificial graphite and natural graphite, are mixed. ..

Japanese Patent No. 3329162 Japanese Patent No. 5900113 (US Pat. No. 9,508,979) Japanese Patent No. 6154380

With the conventional technology as described above, sufficient characteristics that satisfy both the energy density of the electrode mixture layer and the output of the battery have not been obtained.

In the negative electrode described in Patent Document 1, since the graphite material is flaky graphite, the graphite is strongly oriented with respect to the current collector foil, the vertical diffusion of lithium ions is poor, and a high main force cannot be obtained. At that time, since expansion and contraction are repeated in one direction, there is a problem that the charge/discharge cycle life is insufficient.

Regarding the negative electrode having the multilayer structure described in Patent Document 2, the diffusion of lithium ions in the high-density layer is rate-determining, and a sufficient output cannot be obtained.

Patent Document 3 discloses a negative electrode for a lithium-ion battery in which a graphite material is composed of artificial graphite and natural graphite, but has a drawback that the mixture layer density is low and the capacity is low because of a high ratio of artificial graphite. Therefore, a battery satisfying the energy density cannot be obtained.

The present invention has the following configurations.
[1] the anode active material and a negative electrode for a lithium ion secondary battery comprising a mixture layer comprising the density of the mixture layer is at 1.51 g / cm ³ or more 1.90 g / cm ³ or less, the mercury intrusion In the Log differential pore volume distribution of the mixture layer measured by the method, there is a pore diameter D1 having a maximum value at 0.50 μm or more and 2.00 μm or less, and the negative electrode active material contains artificial graphite and natural graphite, The mixing ratio of the artificial graphite is 1% by mass or more and 40% by mass or less with respect to the total of the artificial graphite and the natural graphite, the compressive strength of the artificial graphite is 20 MPa or more and 100 MPa or less, and the compressive strength of the natural graphite is A negative electrode for a lithium ion secondary battery, which has a pressure of 0.1 MPa or more and 15 MPa or less.
[2] The negative electrode for a lithium ion secondary battery as described in 1 above, further having a pore diameter D2 having a maximum value in the pore volume distribution of 0.10 μm or more and 0.25 μm or less.
[3] In the pore volume distribution, the ratio of the pore volume P1 of the pore diameter D1 to the pore volume P2 of the pore diameter D2 (pore volume P1/pore volume P2) is 2.1 or more, or 1 3. The negative electrode for a lithium ion secondary battery described in 2 above.
[4] The negative electrode for a lithium ion secondary battery according to any one of 1 to 3 above, wherein the artificial graphite is an artificial graphite having a carbon coating layer on the surface.
[5] The negative electrode for a lithium ion secondary battery according to any one of 1 to 4 above, wherein the natural graphite is natural graphite having a carbon coating layer on the surface.
[6] The artificial graphite has an intensity ratio ID/IG of a peak intensity (ID) in the range of 1300 to 1400 cm ⁻¹ and a peak intensity (IG) in the range of 1580 to 1620 cm ⁻¹ measured by Raman spectroscopy. 6. The negative electrode for a lithium ion secondary battery according to any one of 1 to 5 above, wherein the R value is 0.05 or more and 0.50 or less.
[7] The natural graphite has an intensity ratio ID/IG of a peak intensity (ID) in the range of 1300 to 1400 cm ⁻¹ and a peak intensity (IG) in the range of 1580 to 1620 cm ⁻¹ measured by Raman spectroscopy. 7. The negative electrode for a lithium ion secondary battery according to any one of 1 to 6 above, wherein the R value is 0.05 or more and 0.50 or less.
[8] The negative electrode for a lithium ion secondary battery according to any one of 1 to 7 above, wherein the average interplanar spacing d002 of the artificial graphite by an X-ray diffraction method is 0.3359 nm or more and 0.3370 nm or less.
[9] The artificial graphite has a 50% particle diameter (D50) in a volume-based cumulative particle size distribution of 4 μm or more and 30 μm or less, and the natural graphite has a D50 of 10 μm or more and 30 μm or less. Negative electrode for lithium-ion secondary battery.
[10] When the BET specific surface area of the artificial graphite is 0.5 m ² /g or more and 7.0 m ² /g and the BET specific surface area of the natural graphite is 1.0 m ² /g or more and 7.0 m ² /g or less. 10. The negative electrode for a lithium ion secondary battery according to any one of 1 to 9 above.
[11] The negative electrode for a lithium ion secondary battery according to any one of the above 1 to 10, wherein the artificial graphite is obtained by heat-treating petroleum-based coke and/or coal-based coke at 2500° C. or higher.
[12] A lithium-ion secondary battery including the negative electrode described in any one of 1 to 11 above.

According to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery having a high energy density and a large current discharge characteristic.

Hereinafter, embodiments of the present invention will be described in detail.
[1] Negative Electrode for Lithium Ion Secondary Battery The negative electrode for a lithium ion secondary battery of the present invention has a negative electrode mixture layer (hereinafter referred to as a mixture layer).
[1-1] Density of Mixture Layer The density of the mixture layer of the negative electrode in one embodiment of the present invention is preferably 1.51 g/cm ³ or more. When the density is 1.51 g/cm ³ or more, a battery having high energy density can be obtained. From the same viewpoint, 1.53 g/cm ³ or more is more preferable, and 1.55 g/cm ³ or more is further preferable. The mixture layer density is preferably 1.90 g/cm ³ or less. When the density is 1.90 g/cm ³ or less, there are sufficient pores in the mixture layer, so that the electrolyte solution sufficiently penetrates into the mixture layer and the diffusion of lithium ions is improved, resulting in high current discharge. The characteristics are improved. From the same viewpoint, it is more preferably 1.80 g/cm ³ or less, still more preferably 1.70 g/cm ³ or less.

[1-2] Pore Volume Distribution of Mixture Layer The negative electrode mixture layer in one embodiment of the present invention has a specific pore volume distribution. That is, when the Log differential pore volume distribution (hereinafter, simply referred to as pore volume distribution) of the mixture layer is evaluated by the mercury intrusion method, the mixture layer has a maximum pore diameter D1 of 0.50 μm or more. Preferably present. When the pore diameter D1 is 0.50 μm or more, lithium ions are smoothly transported in the mixture layer, and good large current discharge characteristics can be obtained. From the same viewpoint, it is more preferable that the pore diameter D1 is 0.51 μm or more. The pore size D1 is preferably 2.00 μm or less. When the pore diameter D1 is 2.00 μm or less, large local pores do not exist in the negative electrode, uniform pores are formed, and good large current discharge characteristics can be obtained. From the same viewpoint, the pore diameter D1 is more preferably 1.40 μm or less, further preferably 1.10 μm or less.

In the mixture layer of the negative electrode in one embodiment of the present invention, it is preferable that the pore volume distribution has a pore diameter D2 that exhibits a maximum value with a pore diameter smaller than the pore diameter D1. The pore diameter D2 is more preferably 0.10 μm or more. When the pore diameter D2 is 0.10 μm or more, the large current discharge characteristics tend to be improved. From the same viewpoint, the pore diameter D2 is more preferably 0.11 μm or more, and most preferably 0.12 μm or more. The pore diameter D2 is preferably 0.25 μm or less. When the pore diameter D2 is 0.25 μm or less, the large current discharge characteristics tend to be improved. From the same viewpoint, the pore diameter D2 is more preferably 0.20 μm or less, and most preferably 0.18 μm or less.

In the pore volume distribution of the mixture layer, the ratio (P1/P2) of the pore volume P1 in the pore diameter D1 to the pore volume P2 in the pore diameter D2 is preferably 2.1 or more. When the ratio is 2.1 or more, the number of pores that are rarely used for charge and discharge is reduced, which contributes to improvement of the electrode density. From the same viewpoint, the ratio is more preferably 2.2 or more.

[1-3] Composition of Mixture Layer The mixture layer of the negative electrode in one embodiment of the present invention contains a negative electrode active material, and the negative electrode active material contains both artificial graphite and natural graphite.
The mixture layer can contain a conductive auxiliary agent. By including the conductive auxiliary agent, conductivity is imparted to the electrode and the battery life is improved. The conductive auxiliary agent is not particularly limited, and examples thereof include acetylene black, Ketjen Black (registered trademark), carbon fiber and the like. The content of the conductive additive is preferably 0.1 to 15% by mass in the mixture layer.

The blending ratio of the artificial graphite is preferably such that the ratio of the artificial graphite to the total of the artificial graphite and the natural graphite is 1% by mass or more. When the proportion of the artificial graphite is 1% by mass or more, appropriate pores are formed around the artificial graphite, and the large current discharge characteristics are improved. From the same viewpoint, the proportion of artificial graphite is more preferably 3% by mass or more, further preferably 5% by mass or more. The proportion of artificial graphite is preferably 40% by mass or less. When the proportion of the artificial graphite is 40% by mass or less, even if the artificial graphite having a high compressive strength is mixed, the mixture layer is well filled by the pressure molding of the negative electrode, and the energy density of the negative electrode is improved. From the same viewpoint, the proportion of artificial graphite is more preferably 35% by mass or less, further preferably 30% by mass or less.

The artificial graphite and the natural graphite may have a carbon coating layer on their particle surfaces. By having the carbon coating layer, the edge surface of graphite is reduced and the initial Coulombic efficiency is improved. More preferably, the carbon coating layer is an amorphous carbon layer. By having the amorphous carbon layer, the large current discharge characteristics can be improved.

The method for coating the surface of artificial graphite and natural graphite is not limited, but for example, an organic compound is attached to the surface of graphite particles, and the graphite is coated with amorphous carbon by firing in an inert atmosphere at a temperature range of 900 to 1500°C. There is a method.
The organic compound is not particularly limited, but petroleum pitch, coal pitch, phenol resin, polyvinyl alcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin and epoxy resin can be used.
The calcination equipment is not particularly limited, but a heat treatment apparatus such as a rotary kiln, a roller hearth kiln, or an electric tubular furnace can be used.

As a procedure, an organic compound is mixed with a solvent to form a liquid, which is mixed and kneaded with graphite, and then the solvent is volatilized and a baking treatment is performed, whereby the surface of the graphite particles can be coated with a carbon coating layer. Alternatively, a method in which the organic compound and the graphite particles are simply mixed with each other and the mixture is heat-treated may be used.

[1-4] Compressive Strength of Graphite The artificial graphite contained in the negative electrode in one embodiment of the present invention preferably has a compressive strength of 20 MPa or more. When the compressive strength of the artificial graphite is 20 MPa or more, the artificial graphite maintains its shape even in the pressure molding of the negative electrode, so that the cycle characteristics tend to be improved. From the same viewpoint, the compressive strength is more preferably 30 MPa or more, further preferably 35 MPa or more. The compressive strength of the artificial graphite is preferably 100 MPa or less. When the compressive strength is 100 MPa or less, the density of the mixture layer tends to be high, and damage to natural graphite during pressure molding of the negative electrode can be suppressed and deterioration of cycle characteristics can be suppressed. From the same viewpoint, the compressive strength is more preferably 80 MPa or less, further preferably 60 MPa or less.

The compressive strength of the natural graphite contained in the negative electrode in one embodiment of the present invention is preferably 0.1 MPa or more. This is the lower limit of detection. The compressive strength of natural graphite is preferably 15 MPa or less. When the compression strength is 15 MPa or less, the mixture layer is well filled by pressure-molding the negative electrode, and the energy density of the negative electrode tends to increase. From the same viewpoint, the compressive strength is more preferably 14 MPa or less, further preferably 13 MPa or less.

▼ Compressive strength is measured with a micro compression tester. The measurement is performed at 10 points, and the average value of the calculated compressive strengths is taken as the compressive strength of the material.

[1-5] Average Particle Diameter of Graphite The 50% particle diameter (D50) in the volume-based cumulative particle size distribution of the artificial graphite contained in the negative electrode in one embodiment of the present invention is preferably 4 μm or more. If the D50 of the artificial graphite is 4 μm or more, the number of active sites on the particle surface is small, and side reactions are reduced when the artificial graphite is used as a battery material. From the same viewpoint, D50 is more preferably 5 μm or more, and most preferably 6 μm or more. The D50 of the artificial graphite is preferably 30 μm or less. When D50 is 30 μm or less, the electric resistance of the electrode becomes small and the large current discharge characteristic is improved. From the same viewpoint, D50 is more preferably 25 μm or less, and most preferably 23 μm or less.

The D50 of the natural graphite contained in the negative electrode in one embodiment of the present invention is preferably 10 μm or more. When the D50 of natural graphite is 10 μm or more, the number of active sites on the particle surface is small, so that side reactions are reduced when used as a battery material. From the same viewpoint, D50 is more preferably 11 μm or more, and most preferably 12 μm or more. Since natural graphite has a higher surface activity than artificial graphite, the lower limit of the particle size that can be used becomes large. The D50 of natural graphite is preferably 30 μm or less. When D50 is 30 μm or less, the electric resistance of the electrode can be reduced, and the large current discharge characteristic is improved. From the same viewpoint, 25 μm or less is more preferable, and 20 μm or less is most preferable.

D50 is measured by measuring the volume-based particle size distribution with a laser diffraction type particle size distribution measuring device, and obtaining the particle size with a cumulative 50%.

[1-6] Specific Surface Area of Graphite The artificial graphite contained in the negative electrode in one embodiment of the present invention preferably has a BET specific surface area of 0.5 m ² /g or more. When the BET specific surface area of the artificial graphite is 0.5 m ² /g or more, the contact area with the electrolytic solution is increased and the characteristics are improved. From the same viewpoint, BET specific surface area is more preferably equal to or greater than 1.0 m ² / g, most preferably at least 1.5 m ² / g. The BET specific surface area of artificial graphite is preferably 7.0 m ² /g or less. When the BET specific surface area of the artificial graphite is 7.0 m ² /g or less, the surface activity of the particles is suppressed, and there is a tendency that the deterioration of Coulomb efficiency and the deterioration of cycle characteristics due to decomposition of the electrolytic solution are suppressed. From the same viewpoint, BET specific surface area less, more preferably 6.0 m ² / g, most preferably at most 5.0 m ² / g.

The BET specific surface area of the natural graphite contained in the negative electrode in one embodiment of the present invention is preferably 1.0 m ² /g or more. When the BET specific surface area of the natural graphite is 1.0 m ² /g or more, the contact area with the electrolytic solution increases and the large current discharge improves. From the same viewpoint, BET specific surface area is more preferably equal to or greater than 1.5 m ² / g, most preferably at least 2.0 m ² / g. The BET specific surface area of natural graphite is preferably 7.0 m ² /g or less. When the BET specific surface area is 7.0 m ² /g or less, the surface activity of the particles is suppressed, and there is a tendency to suppress the deterioration of Coulomb efficiency and the deterioration of cycle characteristics due to decomposition of the electrolytic solution. From the same viewpoint, BET specific surface area less, more preferably ^{^{5.0m 2 / g, 3.0m 2 /}} g or less is most preferred.

Since natural graphite has higher surface activity than artificial graphite, the lower limit of the preferable BET specific surface area is often large.
The BET specific surface area can be measured by the method described in Examples.

[1-7] d002 of graphite
In the artificial graphite contained in the negative electrode in one embodiment of the present invention, the average interplanar spacing d002 of (002) planes by X-ray diffraction is preferably 0.3359 nm or more. When d002 is 0.3359 nm or more, the compressive strength of graphite tends to increase. From the same viewpoint, d002 is more preferably 0.3360 nm or more. The artificial graphite d002 is preferably 0.3370 nm or less. When d002 is 0.3370 nm or less, the crystallinity of graphite is high, the space in which lithium ions can be intercalated is increased, and the capacity is increased. From the same viewpoint, d002 is more preferably 0.3367 nm or less, and most preferably 0.3364 nm or less.

D002 of natural graphite contained in the negative electrode in one embodiment of the present invention is preferably 0.3354 nm or more. When d002 of natural graphite is 0.3354 nm or more, the compressive strength of graphite tends to increase. The d002 of natural graphite is preferably 0.3357 nm or less. When d002 is 0.3357 nm or less, the crystallinity of graphite is high, and thus a battery having a large discharge capacity can be obtained.

d002 can be measured using the known powder X-ray diffraction (XRD) method (Michio Inagaki et al., Japan Society for the Promotion of Science, 117th Committee materials, 117-121-C-5 (1972), Michio Inagaki). , "Carbon", 1963, No. 36, pp. 25-34), Iwashita et al. , Carbon vol. 42 (2004), p. 701-714).

[1-8] R Value of Graphite The artificial graphite contained in the negative electrode in one embodiment of the present invention preferably has an R value of 0.05 or more. When the R value of the artificial graphite is 0.05 or more, insertion and desorption of lithium ions between the graphite layers are facilitated, and the rapid chargeability when used as an electrode material of a secondary battery is improved. From the same viewpoint, the R value is more preferably 0.08 or more. The R value of artificial graphite is preferably 0.50 or less. When the R value is 0.50 or less, the increase in compressive strength is suppressed, the graphite is easily filled, and the energy density is increased. From the same viewpoint, the R value is more preferably 0.30 or less, and most preferably 0.20 or less.

The R value of the natural graphite contained in the negative electrode in one embodiment of the present invention is preferably 0.05 or more. When the R value of the natural graphite is 0.05 or more, insertion and desorption of lithium ions between the graphite layers are facilitated, and the rapid chargeability when used as an electrode material for a secondary battery is improved. From the same viewpoint, the R value is more preferably 0.12 or more, and most preferably 0.20 or more. The R value of natural graphite is preferably 0.50 or less. When the R value is 0.50 or less, the increase in compressive strength is suppressed, the graphite is easily filled, and the energy density is increased. From the same viewpoint, the R value is more preferably 0.40 or less, most preferably 0.30 or less.

In the present specification, the R value is an intensity ratio ID between a peak intensity (ID) in the range of 1300 to 1400 cm ^-1 and a peak intensity (IG) in the range of 1580 to 1620 cm ^-1 measured by Raman spectroscopy. /IG, indicating that the higher the R value, the lower the crystallinity.

[1-9] Method for Producing Graphite The artificial graphite contained in the negative electrode in one embodiment of the present invention is obtained by pulverizing a raw material such as petroleum coke or coal coke into a predetermined size and graphitizing the obtained pulverized product. It can be manufactured by processing. When non-acicular petroleum pitch coke is used as a raw material, graphite having high compressive strength tends to be obtained, which is preferable. The artificial graphite is preferably non-aggregated particles. With the non-aggregated structure, intra-particle peeling hardly occurs due to repeated expansion and contraction due to charge and discharge, and high temperature cycle characteristics and high temperature storage characteristics are excellent. In addition, the non-aggregated type means a state in which the primary particles are not converted into the secondary particles.

Graphitization is performed by heating the raw material at high temperature. The heating temperature is preferably 2500°C or higher, more preferably 2900°C or higher, and most preferably 3000°C or higher. By setting the heating temperature to 2500° C. or higher, graphite crystals develop and a high discharge capacity can be obtained. The heating temperature is preferably 3300°C or lower. When the heating temperature is 3300° C. or lower, sublimation of carbon is suppressed, and a product can be obtained in high yield.

The natural graphite contained in the negative electrode in one embodiment of the present invention is preferably spherical natural graphite. Natural graphite is usually scaly, and its edge surface is exposed. The edge surface easily reacts with the electrolytic solution, which causes a decrease in initial capacity efficiency. Therefore, it is preferable that the scaly natural graphite is processed into a spherical mass shape to reduce the edge surface to obtain spherical natural graphite.

The method of spheroidizing is not limited, but it is possible to use a method of using an impact type crusher, placing natural graphite in the air flow to collide with the wall surface in the device, and bending the ends to spheroidize. Further, it is also possible to use a method of pulverizing natural graphite and sieving only spherical particles, or a method of processing into a spherical shape by grinding. Examples of the apparatus include a jet mill, a hybridization system (registered trademark: manufactured by Nara Machinery Co., Ltd.), a mechanofusion system (registered trademark: manufactured by Hosokawa Micron Co., Ltd.), and novirta (registered trademark: manufactured by Hosokawa Micron Co., Ltd.). ..

[2] Method for Producing Negative Electrode for Lithium Ion Secondary Battery In the negative electrode for lithium ion secondary battery in one embodiment of the present invention, the mixture layer is formed on a current collector. Examples of the current collector include a foil of nickel or copper, a mesh, or the like.

A method of manufacturing a negative electrode according to an embodiment of the present invention includes a step of obtaining a negative electrode paste containing a negative electrode active material, a step of applying the paste to a current collector, and a step of drying and press-molding the paste on the current collector. Including steps. In this case, the mixture layer is a material layer pressure-molded on the current collector.

The negative electrode paste preferably contains a negative electrode active material, a binder, and a dispersion medium. Further, a conductive auxiliary agent can be included as an optional component. The negative electrode paste can be obtained by kneading the negative electrode active material, the binder, the dispersion medium, and the conductive auxiliary agent as an optional component. The negative electrode paste can be formed into a sheet shape, a pellet shape, or the like.

Examples of the binder include polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and polymer compounds having high ionic conductivity. Examples of the polymer compound having a high ionic conductivity include polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile and the like. The amount of the binder used is preferably 0.5 to 20 parts by mass with respect to 100 parts by mass of the negative electrode active material.

The dispersion medium is not particularly limited, but examples thereof include N-methyl-2-pyrrolidone, dimethylformamide, isopropanol, water and the like. In the case of a binder that uses water as the dispersion medium, it is preferable to use a thickening agent such as carboxymethyl cellulose together. The amount of the dispersion medium is adjusted so that the viscosity is such that it can be easily applied to the current collector.

The method for applying the obtained negative electrode paste to the current collector is not particularly limited, and it can be applied by, for example, a doctor blade or a bar coater. The coating thickness of the paste is preferably 50 to 200 μm. If the coating thickness is too large, it becomes impossible to store the negative electrode in a standardized battery container, and the internal resistance of the battery increases due to an increase in the diffusion distance of lithium ions.
The applied paste is dried and then pressure-molded. The pressure molding can be performed using a molding method such as roll pressing or press pressing. The pressure at the time of pressure molding is preferably about 100 MPa to about 300 MPa (about 1 to 3 t/cm ² ).

[3] Lithium Ion Secondary Battery The lithium ion secondary battery in one embodiment of the present invention uses the above negative electrode as its negative electrode. The lithium ion secondary battery has a structure in which a positive electrode and the negative electrode are immersed in an electrolytic solution or an electrolyte.

In the positive electrode of the lithium ion secondary battery, a lithium-containing transition metal oxide is usually used as the positive electrode active material, and preferably at least selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W. An oxide mainly containing one kind of transition metal element and lithium, wherein a compound having a molar ratio of lithium to the transition metal element of 0.3 to 2.2 is used, more preferably V, Cr, Mn, A compound which is an oxide mainly containing at least one transition metal element selected from Fe, Co and Ni and lithium and in which the molar ratio of lithium to the transition metal is 0.3 to 2.2 is used. In addition, you may contain Al, Ga, In, Ge, Sn, Pb, Sb, Bi, Si, P, B etc. in the range of less than 30 mol% with respect to the mainly existing transition metal. Among the above positive electrode active materials, the general formula Li _x MO ₂ (M is at least one of Co, Ni, Fe and Mn, x=0.02 to 1.2) or Li _y N ₂ O ₄ ( N contains at least Mn, and it is preferable to use at least one material having a spinel structure represented by y=0.02 to 2.

Further, the positive electrode active material _{_{_{Li y M a D 1-a}}} O 2 (M is Co, Ni, Fe, at least one of Mn, D is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag , W, Ga, In, Sn, Pb, Sb, Sr, B, and P, at least one kind other than M, y=0.02 to 1.2, a=0.5 to 1.). , Or Li _z (N _b E _1-b ) ₂ O ₄ (N is Mn, E is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag, W, Ga, In, Sn, Pb, It is particularly preferable to use at least one of Sb, Sr, B and P, and at least one of the materials having a spinel structure represented by b=1 to 0.2 and z=0.02 to 2.

_{_{_{Specifically, Li x CoO 2, Li x}}} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1- bO _{_{_{_{2, Li x Mn 2 O 4}}}} , Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2-c O ₄ (where x=0.02 to 1.2, a=0.1 to 0.9, b=0.8 to 0.98, c=1.6 to 1.96, z=2.01 Up to 2.3.). The most preferred lithium-containing transition metal _{_{_{oxides, Li x CoO 2, Li x}}} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1 _-b O _z (x=0.02 to 1.2, a=0.1 to 0.9, b=0.9 to 0.98, z=2.01 to 2.3). The value of x is a value before the start of charging/discharging and increases/decreases due to charging/discharging.

The average particle size of the positive electrode active material is not particularly limited, but is preferably 0.1 to 50 μm. The volume of particles of 0.5 to 30 μm is preferably 95% or more. It is further preferable that the volume occupied by the particle group having a particle diameter of 3 μm or less is 18% or less of the total volume, and the volume occupied by the particle group having 15 μm or more and 25 μm or less is 18% or less of the total volume. The particle size here is calculated by the volume-based cumulative particle size distribution in the particle size distribution measurement by the laser diffraction method, and the average particle size is the particle size at a cumulative 50%.
The specific surface area is not particularly limited, but is preferably 0.01 to 50 m ² /g by the BET method, and particularly preferably 0.2 to 10 m ² /g. The pH of the supernatant when 5 g of the positive electrode active material is dissolved in 100 ml of distilled water is preferably 7 or more and 12 or less.

The electrolytic solution used for the lithium ion secondary battery is not particularly limited. For example, lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , CH ₃ SO ₃ Li and CF ₃ SO ₃ Li may be used, for example, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, Examples include so-called organic electrolytes dissolved in non-aqueous solvents such as diethyl carbonate, propylene carbonate, butylene carbonate, acetonitrile, propionitrile, dimethoxyethane, tetrahydrofuran, γ-butyrolactone, and so-called solid or gel polymer electrolytes. ..

Also, it is preferable to add a small amount of an additive that shows a decomposition reaction when the lithium-ion secondary battery is first charged, to the electrolytic solution. Examples of the additive include vinylene carbonate, biphenyl, propane sultone, and the like. The addition amount is preferably 0.01 to 5% by mass.

A separator can be provided between the positive electrode and the negative electrode of the lithium ion secondary battery. Examples of the separator include a nonwoven fabric containing polyolefin such as polyethylene and polypropylene as a main component, a cloth, a microporous film, or a combination thereof.

Note that there is no restriction on the selection of the necessary members for battery configuration other than the above.

Hereinafter, examples of the present invention will be specifically described. It should be noted that these are merely examples for explanation and do not limit the present invention.
The negative electrode active material evaluation methods, the negative electrode preparation methods and evaluation methods, the battery preparation methods and evaluation methods, and the graphite materials used in Examples and Comparative Examples are as follows.

[1] Evaluation of Negative Electrode Active Material [1-1] Compressive Strength The compressive strength was measured by a micro compression tester MCT-510 manufactured by Shimadzu Corporation. A sample was placed on the lower indenter, single particles close to D50 were selected from the optical microscope field, and a compression test was performed in which the particles were pressed by the upper indenter. The compressive strength of the sample was calculated from the equation of St=2.8 P/πd ² (St: compressive strength (MPa), P: test force (N), d: particle diameter (mm)). The measurement was performed at 10 points, and the average value of the calculated compressive strengths was taken as the compressive strength of the material. The standard size of the upper indenter is 50 μm, but when measuring a sample of 10 μm or less, an indenter of 20 μm was selected.

[1-2] Raman spectroscopic analysis NRS-5100 of JASCO Corporation was used as a laser Raman spectroscopic device, and measurement was performed at an excitation wavelength of 532.36 nm.
The intensity ratio ID/IG of the peak intensity (ID) in the range of 1300 to 1400 cm ^-1 and the peak intensity (IG) in the range of 1580 to 1620 cm ^-1 measured by Raman spectroscopy is calculated and defined as the R value. ..

[1-3] Surface spacing d002
Fill a glass sample plate (sample plate window 18×20 mm, depth 0.2 mm) with a mixture of the sample and standard silicon (made by NIST) in a mass ratio of 9:1 under the following conditions: Was measured at.
XRD device: Rigaku's SmartLab (registered trademark)
X-ray type: Cu-Kα ray Kβ ray removal method: Ni filter X-ray output: 45 kV, 200 mA
Measuring range: 24.0 to 30.0 deg.
Scan speed: 2.0 deg. /Min.
The Gakushin method was applied to the obtained waveform to determine the value of the surface spacing d002.

[1-4] 50% particle size (D50)
The 50% particle diameter (D50) was measured in a volume-based particle diameter distribution using a Malvern Mastersizer (registered trademark) as a laser diffraction particle size distribution analyzer.

[1-5] BET Specific Surface Area Using NOVA2200e manufactured by Quantachrome as a BET specific surface area measuring device, 3 g of a sample was put into a sample cell (9 mm×135 mm), and dried at 300° C. for 1 hour under a vacuum condition. , Measurement was performed. N ₂ was used as the gas for measuring the BET specific surface area.

[2] Preparation of Negative Electrode When plural kinds of graphite are used, graphite was mixed in a V-blender at a predetermined mixing ratio to obtain a negative electrode active material. To the obtained negative electrode active material 96.5 g, 0.5 g of carbon black as a conductive auxiliary agent, 1.5 g of carboxymethyl cellulose (CMC) as a thickener, and water were added appropriately, and the viscosity was adjusted by stirring and mixing, 3.8 g of an aqueous dispersion containing styrene-acrylic polymer fine particles (Polysol (registered trademark), Showa Denko KK) having a solid content ratio of 40% was added, stirred and mixed to obtain a slurry-like dispersion having sufficient fluidity. Was produced.
The prepared dispersion liquid was applied onto a copper foil having a thickness of 20 μm using a roll coater so as to be uniform, and dried to obtain an electrode. The density of the dried electrode was adjusted by roll pressing to obtain a battery evaluation negative electrode. The mixture layer of the electrode was adjusted to 10 mg/cm ² .

[3] Pore Volume of Negative Electrode The pore volume was measured by mercury porosimetry using a mercury porosimeter Autopore IV9520 manufactured by Shimadzu Corporation. In the measurement using a mercury porosimeter, the penetration amount V of mercury entering the pores of the negative electrode is measured while the pressure P is increased stepwise, and the horizontal axis represents the pore diameter (μm) and the rate of change of the penetration amount of mercury (ΔV /ΔP) was plotted on the vertical axis by a logarithmic plot. However, the contact angle of mercury was 130°, the surface tension was 485 mN/m, and the relationship between the pore diameter and the pressure of the pores was calculated by approximating 180/pressure=pore diameter.

[4] Preparation of Battery [4-1] Preparation of Positive Electrode 92 g of Li ₃ Ni _1/3 Mn _1/3 Co _1/3 , 3 g of carbon black (manufactured by TIMCAL) as a conduction aid and 1 g of vapor grown carbon fiber (VGCF (registered trademark), manufactured by Showa Denko KK) and 4 g of polyvinylidene fluoride (PVdF) as a binder were stirred and mixed while appropriately adding N-methyl-pyrrolidone to prepare a slurry dispersion liquid.
The prepared dispersion liquid was applied onto an aluminum foil having a thickness of 20 μm by a roll coater, dried, and then pressure-molded by a roll press. The mixture layer of the obtained positive electrode was 19 mg/cm ² , and the mixture layer density was 3.1 g/cm ³ .

[4-2] Preparation of Electrolyte Solution As a non-aqueous solvent, ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were mixed in a volume ratio of 3:7, and lithium hexafluorophosphate (LiPF ₆ ) was used as an electrolyte salt. What was dissolved at 0.0 mol/L was used as an electrolytic solution.

[4-3] Battery Preparation The negative electrode and the positive electrode were punched out to obtain a negative electrode piece and a positive electrode piece having an area of 20 cm ² . An Al tab was attached to the Al foil of the positive electrode piece, and a Ni tab was attached to the Cu foil of the negative electrode piece. A microporous polypropylene film was sandwiched between a negative electrode piece and a positive electrode piece, and in that state, it was packed in an aluminum laminate. Then, 500 μL of the electrolytic solution was injected therein. Then, the opening was sealed by heat fusion to manufacture a battery for evaluation (designed capacity 50 mAh).

[5] Evaluation of Battery [5-1] Discharge Capacity and Energy Density The battery was charged at a cutoff current value of 1 mA at 5 mA in CC (constant current) and CV (constant voltage) modes with an upper limit voltage of 4.2 V.
With the lower limit voltage of 2.8 V, 0.1 C (5 mA) discharge was performed in CC mode, and the discharge capacity was measured. The energy density is the product of the discharge capacity and the electrode density.

[5-2] Large current discharge test The cell was charged at 5 mA in CC and CV modes with an upper limit voltage of 4.2 V and a cutoff current value of 1 mA, and then discharged at 3.0 C (150 mA) in CC mode at a lower limit voltage of 2.8 V. The discharge capacity at 3.0 C was calculated. The discharge capacity ratio at 3.0 C was calculated based on the 0.1 C discharge capacity (0.1 C=about 5 mA).
(3.0C discharge capacity ratio (%))=(3.0C discharge capacity (mAh))/(0.1C discharge capacity (mAh))×100

[5-3] Cycle characteristics Charging was performed at a cutoff current value of 2.5 mA at 1 C (1 C=50 mA) in CC and CV modes with an upper limit voltage of 4.2 V.
2C discharge was performed in CC mode with a lower limit voltage of 2.8V.
Under the above conditions, 500 cycles of charge and discharge were repeated. The discharge capacity after 500 cycles was measured. The ratio of the discharge capacity at 500 cycles to the initial discharge capacity was calculated and used as the discharge capacity retention rate.
(Discharge capacity retention rate after 500 cycles (%))=(discharge capacity at 500 cycles)/(initial discharge capacity)×100

[6] Graphite material [6-1A] Artificial graphite A
As for the artificial graphite A, non-needle petroleum coke was crushed to D50=6 μm and heat-treated at 3000° C. in an induction heating furnace under an argon atmosphere to obtain graphite. It is non-aggregated artificial graphite produced by mixing parts by mass and firing it at 1200° C. in an inert atmosphere in an electric tubular furnace so as to subject the surface of particles to an amorphous carbon coating treatment. Its compression strength is 38 MPa, D50 is 6 μm, BET specific surface area is 4.5 m ² /g, average surface spacing d002 is 0.3361 nm, and R value is 0.17.

[6-1B] Artificial graphite B
The artificial graphite B is a non-aggregated artificial graphite produced by crushing non-acicular petroleum coke to D50=22 μm and heat-treating at 3000° C. in an induction heating furnace under an argon atmosphere. Its compressive strength is 52 MPa, D50 is 22 μm, BET specific surface area is 2.5 m ² /g, average surface spacing d002 is 0.3360 nm, and R value is 0.09.

[6-1C] Artificial graphite C
The artificial graphite C is a non-aggregated artificial graphite produced by crushing acicular petroleum coke into D50=12 μm and heat-treating at 3000° C. in an induction heating furnace under an argon atmosphere. Its compressive strength is 5 MPa, D50 is 12 μm, BET specific surface area is 2.0 m ² /g, average surface spacing d002 is 0.3357 nm, and R value is 0.07.

[6-1D] Artificial graphite D
The artificial graphite D is a non-aggregated artificial graphite produced by crushing acicular petroleum coke into D50=6 μm and heat-treating it at 3000° C. in an induction heating furnace in an argon atmosphere. Its compression strength is 4 MPa, D50 is 6 μm, BET specific surface area is 6.1 m ² /g, average surface spacing d002 is 0.3357 nm, and R value is 0.08.

[6-1E] Artificial graphite E
The artificial graphite A was obtained by crushing non-acicular petroleum coke to D50=18 μm and heat-treating it at 3000° C. in an induction heating furnace under an argon atmosphere to obtain graphite. A non-aggregated artificial graphite produced by mixing parts by mass and firing at 1200° C. in an electric tubular furnace in an inert atmosphere to perform a carbon coating treatment on the particle surface. Its compressive strength is 110 MPa, D50 is 18 μm, BET specific surface area is 1.2 m ² /g, average surface spacing d002 is 0.3371 nm, and R value is 0.14.

[6-2A] Natural graphite A
For natural graphite A, 100 parts by mass of spheroidized natural graphite is mixed with 2 parts by mass of petroleum-based pitch, and the particles are subjected to amorphous carbon coating treatment by firing at 1200° C. in an inert atmosphere in an electric tubular furnace. It is an agglomerated spheroidized natural graphite that is produced by applying it. Its compression strength is 12 MPa, D50 is 12 μm, BET specific surface area is 2.4 m ² /g, average surface spacing d002 is 0.3356 nm, and R value is 0.27.

[6-2B] Natural graphite B
For natural graphite B, 100 parts by mass of spheroidized natural graphite is mixed with 5 parts by mass of petroleum pitch, and the particles are subjected to an amorphous carbon coating treatment by firing at 1200° C. in an inert atmosphere in an electric tubular furnace. It is an agglomerated spheroidized natural graphite that is produced by applying it. Its compressive strength is 21 MPa, D50 is 13 μm, BET specific surface area is 1.5 m ² /g, average surface spacing d002 is 0.3356 nm, and R value is 0.43.

Examples 1-5, Comparative Examples 1-15:
Electrodes and batteries were produced using the types and amounts shown in Table 1 as artificial graphite and natural graphite, and their respective characteristics were evaluated. The results are shown in Tables 1 and 2.

Claims

Negative electrode active material A negative electrode for a lithium ion secondary battery comprising a mixture layer comprising the density of the mixture layer is at 1.51 g / cm 3 or more 1.90 g / cm 3 or less, measured by mercury porosimetry In the Log differential pore volume distribution of the prepared mixture layer, a pore diameter D1 having a maximum value is present at 0.50 μm or more and 2.00 μm or less, and the negative electrode active material contains artificial graphite and natural graphite. The blending ratio of the artificial graphite is 1% by mass or more and 40% by mass or less, the compressive strength of the artificial graphite is 20 MPa or more and 100 MPa or less, and the compressive strength of the natural graphite is 0.1 MPa. A negative electrode for a lithium-ion secondary battery, which is 15 MPa or less.
The negative electrode for a lithium ion secondary battery according to claim 1, further comprising a pore diameter D2 having a maximum value in the pore volume distribution of 0.10 μm or more and 0.25 μm or less.
In the pore volume distribution, the ratio of the pore volume P1 of the pore diameter D1 to the pore volume P2 of the pore diameter D2 (pore volume P1/pore volume P2) is 2.1 or more. 2. The negative electrode for a lithium ion secondary battery according to 2.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the artificial graphite is an artificial graphite having a carbon coating layer on the surface.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the natural graphite is a natural graphite having a carbon coating layer on the surface.
The artificial graphite has an intensity ratio ID/IG of a peak intensity (ID) in the range of 1300 to 1400 cm -1 and a peak intensity (IG) in the range of 1580 to 1620 cm -1 measured by Raman spectroscopy. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, which has a value of 0.05 or more and 0.50 or less.
The natural graphite has an intensity ratio ID/IG of a peak intensity (ID) in the range of 1300 to 1400 cm -1 and a peak intensity (IG) in the range of 1580 to 1620 cm -1 measured by Raman spectroscopy. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6, which has a value of 0.05 or more and 0.50 or less.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein an average d-spacing d002 of the artificial graphite by an X-ray diffraction method is 0.3359 nm or more and 0.3370 nm or less.
9. The lithium according to claim 1, wherein the artificial graphite has a 50% particle diameter (D50) in the volume-based cumulative particle size distribution of 4 μm or more and 30 μm or less, and the natural graphite has a D50 of 10 μm or more and 30 μm or less. Negative electrode for ion secondary batteries.
The BET specific surface area of the artificial graphite is 0.5 m 2 /g or more and 7.0 m 2 /g, and the BET specific surface area of the natural graphite is 1.0 m 2 /g or more and 7.0 m 2 /g or less. The negative electrode for a lithium ion secondary battery according to any one of 1 to 9.
The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 10, wherein the artificial graphite is obtained by heat-treating petroleum-based coke and/or coal-based coke at 2500°C or higher.
A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 11.