JP2007242282A - Battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery having a negative electrode containing in particular a graphite-based material which has a high energy density. <P>SOLUTION: The battery 1 is constituted of a current collector 14 and a negative electrode 10 which consists of active material layers 15 and 16 having as a main material a granule body adhering a plurality of natural graphite and in which the bulk density of the granule body is 0.20 g/cm<SP>3</SP>or more and 0.70 g/cm<SP>3</SP>or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a battery, particularly a nonaqueous electrolyte secondary battery having a negative electrode containing a graphite-based material.

With the development and widespread use of portable electronic devices such as personal computers (PCs) and mobile phones, the so-called two power sources of these electronic devices can be repeatedly charged and discharged and are suitable for resource saving. Secondary batteries are widely used.
In this secondary battery, a graphite material that uses an intercalation reaction of lithium (Li) between graphite layers, or a carbonaceous material that applies lithium insertion / extraction to pores is used as the negative electrode active material. So-called lithium ion secondary batteries have been developed and widely used.

In recent years, the demand for the capacity of the secondary battery has become stronger than before due to the increase in power consumption and the prolonged usage time due to the high performance of the electronic device. In order to cope with this trend, demand for higher energy density is increasing. In order to meet these requirements, research on non-aqueous electrolyte secondary batteries has been promoted, and among them, lithium ion secondary batteries have been proposed as high-performance non-aqueous electrolyte secondary batteries.
Conventionally, in this lithium ion secondary battery, the active material layer constituting the negative electrode has been composed of natural graphite having a scaly shape. This natural graphite has a capacity of 360 to 370 mAh / g which is close to the theoretical capacity (372 mAh / g) due to its high crystallinity. Therefore, it is considered preferable to use a structure filled with natural graphite at a high volume density, rather than using artificial graphite, which is generally considered to have a capacity of about 330 to 350 mAh / g.

In natural graphite, a plurality of layers in which carbon atoms form a network structure and spread in a planar shape are laminated, so that the shape in the thickness direction is defined and grown in a lump shape. That is, the surface where the interlayer is exposed is called an edge surface. When this natural graphite is used for the negative electrode of a battery, lithium ions are intercalated from the edge during charging and diffused between the stacked surfaces, and during discharge, the lithium ions are deintercalated and released from the edge. The
However, natural graphite has a problem that load characteristics and cycle characteristics are lower than artificial graphite which does not have a clear edge surface due to its high crystallinity. This is because artificial graphite has low crystallinity and does not have a clear edge surface, so the entire surface of the graphite particles acts even during intercalation / deintercalation, whereas natural graphite However, intercalation / deintercalation is not performed, and the layered direction of the layer having the above-described network structure is perpendicular to the electrode surface so that it is likely to be in a so-called sleeping state, so that the edge surface faces outward (positive electrode side). Due to difficult things.

As a configuration for improving the load characteristics and cycle characteristics of natural graphite constituting the negative electrode, the edge surface is directed in a different direction from the plane parallel to the electrode by, for example, magnetic field orientation or spheroidization of the graphite particles, for example, standing vertically In addition, a method for improving isotropicity has been proposed (see, for example, Patent Document 1).
According to the configuration in which the edge surface is directed in a different direction with respect to the negative electrode surface, it is not necessary for lithium ions released from the positive electrode side during charging, that is, intercalation, to wrap around the edge surface avoiding a mesh-like plane. Since the structure easily receives lithium ions from the edge surface, the acceptability is improved.

However, even if such means are used, natural graphite originally has a small number of edge surfaces, so it is difficult to say that the lithium ion acceptability is sufficiently high, and a battery with higher lithium ion acceptability is required. Yes.
JP 2002-114572 A

  The present invention has been made in view of such problems, and an object of the present invention is to provide a battery having high energy density and improved load characteristics and cycle characteristics.

The battery according to the present invention is a battery provided with an electrolyte together with a positive electrode and a negative electrode, wherein the negative electrode contains a plurality of natural graphite granules, and the bulk density of the granules is 0.20 g / cm. ³ or more and 0.70 g / cm ³ or less.

According to the battery of the present invention, the negative electrode contains a plurality of natural graphite granules, and the bulk density of the granules is 0.20 g / cm ³ or more and 0.70 g / cm ³ or less. Therefore, a battery having a high energy density and improved load characteristics and cycle characteristics can be configured.

The present inventors have found that a negative electrode having excellent load characteristics and cycle characteristics can be formed by pulverizing and pulverizing scaly natural graphite and bonding a plurality of natural graphites to form a granulated body. This has led to the present invention.
In particular, once pulverized, the ratio of edges per unit volume can be increased, resulting in a negative electrode active material having more edges per unit volume than conventional natural graphite, and lithium ion intercalation and deintercalation. The structure is suitable for the

Embodiments of the present invention will be described below with reference to the drawings.
In the present embodiment, a so-called element-wound cylindrical battery as shown in a perspective view with a part cut in FIG. 1A will be described as an example.

In the battery 1 according to this embodiment, a wound body 3 is disposed around a center pin 4 (not shown) inside a cylindrical battery can 2 having an open upper surface, and is sealed by a battery lid 5. Have a configuration.
The battery can 2 is composed of, for example, a nickel-plated iron can, and the wound body 3 is wound in the same manner through the wound separators 7 and 8 containing the electrolyte, and the positive electrode 9 and the negative electrode 10. Are arranged opposite to each other.

The battery lid 5 is provided with a safety valve mechanism 8 together with a thermal resistance element (Positive Temperature Coefficient; PTC element) inside, and is attached to the battery can 2 by caulking through a gasket or the like. . That is, the inside of the battery can 1 is sealed by the battery can 2 and the battery lid 5.
The safety valve mechanism 18 is electrically connected to the battery lid 5 via a heat-sensitive resistor element. When the internal pressure of the battery becomes a certain level or more due to an internal short circuit or external heating, for example, a built-in disk The plate is reversed and the electrical connection between the battery lid 5 and the wound body 3 is cut. Here, the heat-sensitive resistance element is to prevent abnormal heat generation due to a large current by limiting the current by increasing the resistance value when the temperature rises.

FIG. 1B schematically shows a cross-sectional structure of the wound surface of the wound body 3 in the present embodiment.
In the present embodiment, the wound body 3 has a configuration in which a strip-like (thin plate-like) positive electrode 9 and a negative electrode 10 are arranged to face each other via separators 7 and 8 containing an electrolyte, and these are wound.
For example, a positive electrode lead made of aluminum and a negative electrode lead (not shown) made of nickel, for example, are connected to the positive electrode 9 and the negative electrode 10 of the wound body 3, and the positive electrode lead is welded to the safety valve mechanism 18 to be connected to the battery lid 5. The negative electrode lead is directly welded and electrically connected to the battery can 2.

Here, the positive electrode 9 is, for example, a positive electrode current collector 11 having a first main surface 11a serving as an inner surface and a second main surface 11b serving as an outer surface in a winding structure. The material layer 12 and the outer-surface positive electrode active material layer 13 are formed on the second main surface 11b side, respectively.
Both the inner surface positive electrode active material layer 12 and the outer surface positive electrode active material layer 13 do not necessarily have to be provided, but are preferably selected and configured according to the intended battery configuration and characteristics. The positive electrode current collector 11 can be made of, for example, aluminum, nickel, stainless steel, or the like.

The inner surface positive electrode active material layer 12 and the outer surface positive electrode active material layer 13 include a positive electrode active material, and may include a conductive agent such as a carbonaceous material and a binder such as polyvinylidene fluoride as necessary. Good. As the positive electrode active material, for example, a lithium-containing metal composite oxide composed of lithium and a transition metal, for example, represented by the general formula LixMO ₂ containing a sufficient amount of lithium (Li) is preferable. This is because the lithium-containing metal composite oxide can generate a high voltage and has a high density, so that it is possible to further increase the capacity of the secondary battery. M is one or more transition metals, and for example, at least one of the group consisting of cobalt, nickel, and manganese is preferable. x varies depending on the charge / discharge state of the battery and is usually a value in the range of 0.05 ≦ x ≦ 1.10. Specific examples of such a lithium-containing metal composite oxide include LiCoO _{2 and} LiNiO ₂ . In addition, any 1 type may be used for a positive electrode active material, and 2 or more types may be mixed and used for it.

In addition, since the battery 1 according to this embodiment is intended to achieve a high capacity, the positive electrode active material layers 12 and 13 of the positive electrode 9 are repeatedly charged and discharged in a steady state (for example, about 5 times charging / discharging). In addition, it is necessary to include Li corresponding to a charge / discharge capacity of 250 mAh or more per 1 g of the negative electrode carbonaceous material, and it is desirable to include Li corresponding to a charge / discharge capacity of 300 mAh or more. More preferably, it contains a minute amount of Li.
Note that it is not always necessary to supply Li from the positive electrode active material layers 12 and 13. For example, in a solvent described later, Li corresponding to a charge / discharge capacity of 250 mAh or more per 1 g of carbonaceous material exists in the battery system. It ’s fine. The amount of Li is appropriately selected according to the measurement of the discharge capacity of the battery.

  On the other hand, the negative electrode 10 is, for example, a negative electrode current collector 14 having a first main surface 14a that is an inner surface and a second main surface 14b that is an outer surface in a winding structure, and an inner surface negative electrode active material on the first main surface 14a side. The layer 15 is formed with the outer-surface negative electrode active material layer 16 on the second main surface 14b side.

  The separators 7 and 8 are for preventing a short circuit due to physical contact between the positive electrode 9 and the negative electrode 10 while allowing lithium ions to pass. For example, the separators 7 and 8 are made of a microporous polyolefin film such as a polyethylene film or a polypropylene film. It is configured. In order to ensure safety, the separators 7 and 8 preferably have a function of blocking the current by closing the holes by heat melting at a predetermined temperature (for example, 120 ° C.) or higher to cut off the current.

  Further, the separators 7 and 8 are impregnated with an electrolytic solution (not shown). The electrolytic solution in the present embodiment includes, for example, a nonaqueous solvent and a lithium salt that is an electrolyte salt dissolved in the solvent, and may include various additives as necessary.

  Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3. -Dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propylnitrile, anisole, acetate or propionate.

In addition, any 1 type of these may be used for a solvent, and 2 or more types may be mixed and used for it.
When mixing and using, the structure which added asymmetrical chain carbonates, such as methyl ethyl carbonate and methyl propyl carbonate, as a 2nd component especially using ethylene carbonate as a main solvent is suitable. Furthermore, a mixed solvent of methyl ethyl carbonate and dimethyl carbonate can be used. In this case, the volume ratio to be mixed is preferably in the range of ethylene carbonate: second component solvent = 7: 3 to 3: 7. Moreover, when using the above-mentioned mixed solvent as a 2nd component solvent, it is preferable to make a volume ratio into the range of methyl ethyl carbonate: dimethyl carbonate = 2: 8-9: 1.
Addition of the second component suppresses decomposition of the main solvent, improves conductivity and improves current characteristics, reduces freezing point of electrolyte and improves low temperature characteristics, reaction with lithium metal The effect that safety | security falls and safety is improved is acquired.

As the electrolyte, LiPF ₆ is suitable, but any of those used in this type of battery can be used. For example, as a lithium salt, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ), LiCl or LiBr. Among these, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ or LiN (C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ) is preferable, and LiPF ₆ or LiBF ₄ is particularly preferable. Any one of the lithium salts may be used, or a mixture of two or more may be used.

Next, as a battery manufacturing method according to this embodiment, first, an example of a manufacturing method of the negative electrode 10 will be described.
First, natural graphite is pulverized using a hammer mill, pin mill, ball mill, jet mill or the like. For example, when a hammer mill is used, it is preferable to perform pulverization for 20 minutes or more at a rotational speed of 4000 to 5000 rpm. In addition, as a method for supplying and discharging graphite particles, it is desirable to carry out by bringing the graphite particles into the air stream. Further, in order to obtain fine powdery graphite particles by pulverization, it is necessary to apply such a relatively large impact force. Therefore, the degree of pulverization is considered to be substantially proportional to the treatment time.
Subsequently, a binder solution is added to the pulverized graphite particles. The binder solution used, as a _binder, derivatives of starch and C _{6 H} 10 _{O 5} as a basic _structure, C _{6 H} 10 _{O 5} a basic structure to viscous _{polysaccharide,} a C _{6 H} 10 _{O 5} as a basic structure One or more selected from the group consisting of water-soluble cellulose derivatives, polyuronides, and water-soluble synthetic resins, or polyvinyl alcohol, styrene butadiene rubber, etc. are adjusted to a predetermined concentration and used as a solution. The solution concentration is preferably about 0.02 to 2%.
Subsequently, the binder solution containing the pulverized graphite particles is subjected to low-speed stirring to perform granulation. For this treatment, for example, a stirring mixer with a spray dryer can be used, and the binder solution and the graphite particles are granulated for a predetermined time to obtain spherical granulated graphite particles. Thereafter, the particles are dried while being stirred as they are by a spray dryer, and thus the finished particles are discharged in a shape maintaining a spherical shape. The dryer temperature is preferably 80 ° C to 110 ° C. Further, the granulated graphite particles in this embodiment are obtained by subjecting the granulated graphite particles to a firing treatment under nitrogen or an inert gas to burn off the binder. In addition, it is preferable to perform this baking process for the baking temperature of about 500 to 1000 degreeC, and the baking time of about 1 to 12 hours.
In this way, styrene butadiene rubber (SBR) is added as a binder to the granulate obtained as a substantially spherical shape, and carboxymethyl cellulose (CMC) is mixed as a thickener to prepare a negative electrode mixture, which is dispersed in water. A paste slurry is obtained. The slurry constituting the battery according to the present embodiment is obtained by applying the slurry onto a thin plate (band-shaped) copper foil current collector, drying at, for example, 80 ° C., and press-molding according to the target volume density. The negative electrode 10 of the invention is obtained.

The SEM observation results of the obtained negative electrode are shown in FIGS. 2A to 2D.
As shown in FIG. 2A, on the surface of the negative electrode 10 constituting the battery 1 according to this embodiment, a large number of substantially spherical granules formed by adhesion of a plurality of natural graphites are provided to form an active material layer. You can confirm that
As shown in FIG. 2B, these granulated bodies have irregularities formed on the surface to increase the specific surface area, and, as shown in FIGS. 2C and 2D, finer particles that are smaller than these irregularities. It can be seen that a large number of pores are formed on the surface and the structure can contribute to the promotion of lithium ion intercalation.

<Example>
Examples of the battery according to the present invention will be described below.

First, the first embodiment will be described.
Chinese natural graphite sample powder was prepared as natural graphite, and this sample powder was pulverized using a hammer mill.
The degree of pulverization was defined by the time of this pulverization treatment. [Table 1] shows the relationship between the pulverization time by pulverization of the sample, the specific surface area and the particle size (volume average particle size; D50). The specific surface area was measured by the BET method. At this time, the adsorbate was nitrogen, the degassing temperature was 120 ° C., and the degassing time was 30 minutes. From the results of [Table 1], it was confirmed that the longer the pulverization time, the more finely pulverized.
Note that when the processing time is 20 minutes to 40 minutes, the specific surface area becomes 8.9 m ² / g or more 15.2 m ² / g or less, particularly preferably. In addition, since the particle size is too large if the specific surface area is less than 8.9 m ² / g, the specific surface area is too large if the specific surface area exceeds 15.2 m ² / g. It is considered undesirable because the adhesive strength decreases. That is, the numerical value of D50 is preferably 8.0 μm or more and 15.0 μm or less. This is because if the average particle diameter of natural graphite is within this numerical range, the average particle diameter of the finally obtained granulated body can be 18 μm or more and 30 μm or less. In addition, when the average particle diameter of a granulated body is smaller than 18 micrometers, since a specific surface area is large, it is thought that efficiency falls in the initial charge of the produced battery. On the other hand, when the average particle diameter of the granulated body is larger than 30 μm, it is considered that lithium (Li) diffusion in graphite becomes rate-limiting in the produced battery, and Li acceptability is lowered.

Next, a second embodiment will be described.
The natural graphite subjected to the pulverization treatment described in the first embodiment for 30 minutes was granulated using a stirring mixer with a spray dryer. In the granulation treatment, 30 g of a 1.0% aqueous solution of carboxymethyl cellulose (CMC) was added to 50 g of graphite fine powder, and granulation was performed by stirring and mixing. The dryer drying temperature was 110 ° C.
The granules obtained by this granulation treatment were evaluated by specific surface area measurement, particle size distribution measurement, and sphericity measurement.
The specific surface area was measured by the BET method. At this time, the adsorbate was nitrogen, the degassing temperature was 120 ° C., and the degassing time was 30 minutes.
The particle size distribution was measured with a Horiba laser diffraction / scattering particle size distribution measuring apparatus.
The sphericity measurement was performed using a particle image analyzer FPIA3000 manufactured by SYSMEX CORPORATION. Put graphite particles in a beaker, add 50 ml of purified water, dispersion medium poly (oxyethylene) -octylphenyl ether (manufactured by Wako Pharmaceutical Co., Ltd.), use ultrasonic wave at 300 W for 3 seconds as a sample, and start measurement by instrument suction did. In the image analysis, when the area of each projected image of natural graphite particles was replaced with an equivalent circle, the circumferential diameter was defined as L, the actual perimeter was defined as 1, and L / l was defined as the circularity. Here, if the number of particles having a circularity of 0.85 or less (number basis) is small and the circularity (volume basis) is also a high value, it is determined that the shape is close to a sphere.

The evaluation results in this example are shown in [Table 2] to [Table 4].
At the stirring device rotation speed of 1000 rpm, as shown in [Table 2], no change was confirmed for all parameters.
At the stirring device rotation speed of 2000 rpm, as shown in [Table 3], the specific surface area decreased and D50 increased with time. From this, it is considered that the fine powders are adhered and granulated.
Further, as the time passed, it was confirmed that the degree of spheroidization increased, and it was found that the shape was spheroidized by friction between the aggregates accompanying stirring.
Moreover, when the bulk density of the granulated body for each treatment time was measured, the bulk density was 0.20 g / cm ³ or more and 0.70 g / cm ³ or less from the numerical values of specific surface area, volume average particle diameter, and circularity. It was confirmed that is preferable. By selecting the bulk density within this numerical range, particles having a circularity of 0.85 or less can be made 20% or less on the basis of the number, and in particular, the bulk density is 0.52 g / cm ³ or more and 0.70 g / cm. If it is ³ or less, the specific surface area can be further increased to 6.5 m ² / g or more and 8.7 m ² / g or less. When the specific surface area exceeds 10 m ² / g, the initial charge / discharge efficiency decreases, which is not preferable.
In addition, in the production of an actual battery, this granulated body forms an active material layer through other processes such as firing as shown in an example in a third embodiment to be described later. It was confirmed that the bulk density hardly changed at least by firing.
Further, at a stirring device rotational speed of 4000 rpm, as shown in [Table 4], since the rotational speed was too high, the fine powder was further pulverized and did not reach granulation.

Next, a third embodiment will be described.
As the battery material, it is particularly preferable to use graphite powder having a particle size of about 20 to 30 microns. Therefore, from the result of the second example described above, in this example, the agitation apparatus is rotated in the granulation process. Granules obtained at a speed of 2000 rpm and a stirring time of 30 min were used.
This granulated body was fired for 12 hours in a nitrogen atmosphere, and the change in specific surface area depending on the firing temperature was measured. The specific surface area was measured by the BET method with the adsorbate being nitrogen, the degassing temperature being 120 ° C., and the degassing time being 30 minutes.
The measurement results are shown in [Table 5].
After firing, the specific surface area increased due to volatilization of CMC. Since CMC inhibits lithium intercalation and deintercalation, it is preferable that CMC is not present in the granulated body as much as possible. Therefore, among the firing temperatures at which the granulated body has a specific surface area of 6.5 m ² / g or more and 8.7 m ² / g or less, the one having a firing temperature of 400 ° C. or more at which CMC is considered to have completely volatilized constitutes the battery. Therefore, it is considered preferable.

  In addition, when the pore diameter was measured, as shown in FIG. 3, in the granulated body in the present embodiment, pores having a smaller diameter (about 0.2 μm) are formed, whereas conventional natural graphite is formed. Since only pores having a larger diameter (about 0.9 μm) are confirmed in the granule in the present embodiment, more pores are formed and the battery is charged and discharged. Intercalation / deintercalation is considered to be promoted.

Next, a fourth embodiment will be described.
Based on the results of the third embodiment described above, in this embodiment, a granulated body that has been subjected to a baking treatment at a temperature of 400 ° C. is prepared, and 2% by weight of styrene butadiene rubber ( SBR) was added and 1.0% by weight of CMC was mixed as a thickener to prepare a negative electrode mixture, which was dispersed in water to form a slurry (paste). Thereafter, the slurry is applied on a copper foil having a thickness of 15 μm serving as a negative electrode current collector, passed through a drying furnace at 80 ° C., a granulated body is formed on the current collector, and further compression-molded. A negative electrode having a volume density negative electrode active material layer was formed.
In this example, since the negative electrode active material layer was composed of fired granules, the X-ray diffraction of the electrode after pressing was measured in order to examine its orientation.
In the case of a highly crystalline carbon material such as graphite, the C-axis tends to be oriented in a direction normal to the electrode surface, and the strong peak on the (002) plane due to the orientation is 26.5 ° ( It is known that it is observed near 2θ). Further, the peak of the (110) plane, which becomes large when it is nearly perpendicular to the hexagonal mesh plane (layer plane) electrode plane of graphite, is seen at around 77.5 ° (2θ). The relative intensity ratio I ₍₁₁₀₎ / I ₍₀₀₂₎ of each peak was defined as the degree of orientation, and the ease of orientation (orientation) of the carbon material was evaluated by comparing this value.
In [Table 6], the peak intensity ratio of (110 face) / (002 face) relating to this granulated body (after firing and pressing) is the same as that of natural graphite (before pulverization) used in the first embodiment described above. It is shown in comparison with the natural graphite used in the second example (after pulverization; stirring device rotation speed 2000 rpm, stirring time 30 min) and the granulated body used in the third example (before firing).
Here, the orientation direction of the granule, i.e. the ratio of the graphite in the X-ray diffraction (002) plane diffraction peak intensity I and ₍₀₀₂₎ of the (110) intensity of the diffraction peak of the plane I ₍₁₁₀₎ I _{(110 )} / I ₍₀₀₂₎ is preferably 0.5 or more. If it is less than 0.5, it is not preferable because it is a condition for increasing the grain size without imparting isotropy by increasing the grain size.
As shown in [Table 6], it is recognized that the (110 plane) / (002 plane) strength ratio increases and anisotropy decreases due to granulation, and the strength ratio is 0.5 or more. Was confirmed. From this result, it is considered that the isotropic surface is improved because the edge surface faces in multiple directions, and the lithium ion acceptability is improved.

Next, a fifth embodiment will be described.
In this example, natural graphite used in the first example (before pulverization), natural graphite used in the second example (after pulverization; stirring device rotation speed 2000 rpm, stirring time 30 min), and in the third example, Using the used granulated body (before firing) and the granulated body obtained in the fourth example (after firing and pressing), a negative electrode was prepared by the method described in the fourth example, and the electrode The specific surface area was measured.
The produced electrodes were each pressed to a volume density of 1.60 g / cm ³ , cut to a length of 1 mm and a width of 10 mm, packed in a test tube for specific surface area measurement, and the specific surface area was measured by the BET method. At that time, the adsorbate was nitrogen, the degassing temperature was 80 ° C., and the degassing time was 40 minutes. In addition, since the volatilization temperature of the CMC binder is about 110 ° C., the deaeration temperature was selected to be lower 80 ° C. in order to examine the influence of firing with higher accuracy.
Table 7 shows the heating temperature and the electrode specific surface area at each heating temperature. From this result, it can be confirmed that the specific surface area of the electrode is increased by the pulverization of graphite, but the specific surface area is further increased for those fired after granulation, and the specific surface area becomes 0.52 or more and 1.10 or less. I was able to confirm. This is thought to be due to the pores in the granulated particles formed during CMC binder volatilization.

Next, a sixth embodiment will be described.
In this example, natural graphite obtained in the first example (before pulverization), natural graphite obtained in the second example (after pulverization), and granulated material obtained in the third example (before firing) The negative electrode was produced by the method shown in the fifth example using each sample with the granulated body (after firing and pressing) obtained in the fourth example, and the embodiment described above using each negative electrode A cylindrical non-aqueous electrolyte secondary battery (diameter 18 mm, height 65 mm) of the cylindrical non-aqueous electrolyte secondary battery described in 1 was manufactured and subjected to a cycle test.

A specific battery manufacturing method in the present embodiment will be described.
First, to each sample (natural graphite or granulated material) corresponding to the first to fourth examples, 2% by weight of styrene butadiene rubber (SBR) is added as a binder, and 1% by weight of carboxyl as a thickener. Methyl cellulose (CMC) is mixed to prepare a negative electrode mixture, dispersed in water to form a slurry (paste-like), and the negative electrode mixture slurry finally becomes a negative electrode current collector. Then, the thin plate-like negative electrode was produced by drying and compression molding.
Subsequently, a mixture of lithium carbonate 0.5 molar and cobalt 1 mole carbonate, after obtaining the LiCoO ₂ was fired for 5 hours in a 900 ° C. in air, grinding the LiCoO _2, obtained by a laser diffraction method 50 A LiCoO ₂ powder having a% cumulative particle size of 15 μm was prepared. Thereafter, 91 parts by weight of a mixture comprising 95 parts by weight of LiCoO ₂ powder and 5 parts by weight of lithium carbonate powder, 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder were prepared to prepare a positive electrode mixture. Then, it was dispersed in N-methylpyrrolidone to form a slurry (paste). The positive electrode mixture slurry was uniformly applied to both surfaces of a 20 μm-thick strip-shaped aluminum foil that finally became the positive electrode current collector, and dried and compression molded to produce a thin plate-shaped positive electrode.
Subsequently, the prepared negative electrode and positive electrode and a separator made of a microporous polypropylene film having a thickness of 25 μm were laminated in the order of the negative electrode, the separator, the positive electrode, and the separator, and then wound many times to form a spiral wound with an outer diameter of 18 mm. A rotating body was produced.
This spiral wound body is housed in a nickel-plated iron battery can, insulating plates are provided on both the upper and lower surfaces of the spiral electrode, and the aluminum positive electrode lead is led out from the positive electrode current collector to the battery lid. The nickel negative electrode lead was led out from the negative electrode current collector and welded to the bottom of the battery can.
An electrolytic solution in which LiPF ₆ was dissolved in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate at a ratio of 1 mol / dm ³ was poured into the battery can in which the wound body was housed. Thereafter, the battery lid was caulked through an insulating sealing gasket whose surface was coated with asphalt, thereby fixing the safety valve device having a current interruption mechanism, the PTC element, and the battery lid, and maintaining the airtightness in the battery.

In this example, the cycle test was performed by charging at a maximum charging voltage of 4.2 V and a charging current of 1 A for 2.5 hours and by a constant current discharge of 500 mA. [Table 8] shows the capacity at the initial stage of the cycle, the capacity retention ratio after 100 cycles with respect to the initial cycle, and the capacity retention ratio at 2.0 C with respect to the capacity at 0.1 C discharge.
From the results shown in [Table 8], the cylindrical test battery having the negative electrode configured using the granulated body according to the present embodiment is the cylindrical test having the negative electrode configured using conventional natural graphite. Improvement in cycle characteristics and load characteristics (2.0 C) were observed as compared with the batteries for use.

Next, a seventh embodiment will be described.
In this example, natural graphite used in the first example (before pulverization), natural graphite used in the second example (after pulverization), and granulated material used in the third example (before firing). And about each sample with the granulated body (after baking and press) obtained in 4th Example, the compressive strength was measured by Shimadzu Corporation micro compression tester MCT-W500J. In addition, the test force which deform | transforms 10% with respect to a particle diameter was prescribed | regulated as intensity | strength. The measurement results are shown in [Table 9].
From the results shown in [Table 9], the strength before volatilization of the binder was as hard as 20 MPa, whereas it decreased to 0.9 MPa after the volatilization of the binder. As a result, in the negative electrode using the granulated body according to the present embodiment that cavities are formed in the particles due to the volatilization of the binder, the penetration efficiency of the electrolytic solution is improved and the movement of lithium ions is promoted. It was confirmed that
In addition, as shown in FIG. 4 in more detail, the natural graphite gradually displaces as the test force increases, whereas in the granulated body in the present embodiment, stepwise. It was also confirmed that the displacement progressed. This is considered to be because the specific surface area is further increased in the granulated body due to the formation of cavities inside.

  As is clear from the description of the above embodiments and examples, the granulation body satisfies the sphericity and specific surface area defined in the present invention, and a paste containing the granulation body is applied to the current collector. The negative electrode corresponding to the intensity ratio and the specific surface area according to the X-ray diffraction method (110 plane / 002 plane) is considered to have a good electrode structure. By using this negative electrode, cycle characteristics are improved and high energy is obtained. It is possible to provide a nonaqueous electrolyte secondary battery having a high density and a long life.

That is, in the battery according to the present invention, the active material layer constituting the negative electrode has a granulated body in which edges are oriented in random directions and pores are appropriately formed. Intercalation / deintercalation is promoted to enable smooth charge / discharge, thereby improving load characteristics.
In addition, by improving acceptability, not only load characteristics but also lithium metal precipitation associated with charge / discharge cycles is suppressed, and therefore cycle characteristics are expected to be improved.

In addition, in the case of the method described in Patent Document 1 described above, since firing is not performed after granulation in production, the obtained granulated body also has poor penetration of the electrolytic solution, and there is a portion that cannot contribute to charge / discharge inside the graphite. It may occur.
However, according to the present invention, since the firing is performed by burning out the binder at a relatively low temperature of 400 ° C., the obtained granule also has excellent electrolyte permeability, and the entire graphite can contribute to charge and discharge. A battery having a negative electrode active material layer can also be configured.

As described above, the embodiments and examples of the battery according to the present invention have been described. However, the materials used in the description and the numerical conditions such as the amount, the processing time, and the dimensions are only suitable examples, and each used for the description. The dimensional shape and arrangement relationship in the figure are also schematic. That is, the present invention is not limited to this embodiment.
For example, in the above-described embodiment, the case where the firing is performed in a nitrogen atmosphere has been described as an example. However, the firing may be performed in another inert gas atmosphere such as argon gas.
Further, in the above-described embodiment, the cylindrical secondary battery has been described as an example. However, the present invention is not limited to this, and other shapes using an exterior member such as a coin type, a button type, a thin type, a large size, or a laminate film can be used. The present invention can be variously modified and changed such that the present invention can be similarly applied to a secondary battery having another structure such as a laminated battery or a laminated structure.

1A and 1B are a schematic configuration diagram showing a configuration of an example of a battery according to the present invention, with a part cut away, and a schematic cross-sectional view of a wound body as a main part. A to D are SEM photographs each showing a state of a negative electrode active material in an exemplary configuration of a battery according to the present invention. It is a schematic diagram which shows the measurement result of the particle size distribution of the granulated body of the negative electrode active material in an example structure of the battery which concerns on this invention, and the particle size distribution of the natural graphite of the negative electrode active material of the conventional battery. It is a schematic diagram which shows the result of the compression test with respect to the granulated body of the negative electrode active material in the structure of an example of the battery which concerns on this invention, and the compression test with respect to the natural graphite of the negative electrode active material of the conventional battery.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Battery, 2 ... Battery can, 3 ... Winding body, 4 ... Center (center pin), 5 ... Battery cover, 6 ... Gasket, 7, ... Separator , 8 ... separator, 9 ... positive electrode, 10 ... negative electrode, 11 ... positive electrode current collector, 11a ... first main surface (inner surface), 11b ... second main surface (outer surface) ), 12 ... inner surface positive electrode active material layer, 13 ... outer surface positive electrode active material layer, 14 ... negative electrode current collector, 14a ... first main surface (inner surface), 14b ... second main Surface (outer surface), 15 ... inner surface negative electrode active material layer, 16 ... outer surface negative electrode active material layer, 17 ... positive electrode lead, 18 ... safety valve mechanism, 19 ... heat sensitive resistance element, 20 ..Insulating plates

Claims (16)

A battery comprising an electrolyte together with a positive electrode and a negative electrode,
The negative electrode contains a plurality of natural graphite granules,
A bulk density of the granulated body is 0.20 g / cm ³ or more and 0.70 g / cm ³ or less. Wherein the negative electrode active material, BET specific surface area determined by (Brunauer-Emmett-Teller) method is cell according to claim 1, characterized in that a 0.52 m ² / g or more 1.10 m ² / g . ^2. The battery according to claim 1, wherein the granulated body has a specific surface area measured by a BET method of 6.5 m ² / g or more and 8.7 m ² / g or less. ^2. The battery according to claim 1, wherein a specific surface area of the natural graphite measured by a BET method is 8.9 m ² / g or more and 15.2 m ² / g or less. X-ray diffraction after the dried product obtained by applying a paste made of the granulated body, the binder constituting the negative electrode, and the thickening material to the metal thin film and drying the paste according to the conditions for producing the negative electrode 2. The battery according to claim 1, wherein the strength ratio between the 110 plane and the 002 plane according to the method is 0.5 or more. 2. The battery according to claim 1, wherein an average particle diameter of natural graphite constituting the granulated body is 8.0 μm or more and 15.0 μm or less. The battery according to claim 1, wherein the granulated body has a compressive strength of 0.5 MPa or more and 2.0 MPa or less. The granulated body is
As the _binder, water-soluble cellulose derivatives and viscous _{polysaccharide,} C _{6 H} 10 _{O 5} the basic structure of derivatives of starch which the C _{6 H} 10 _{O 5} as a basic _structure, a C _{6 H} 10 _{O 5} as a basic structure, polyuronide 2. The battery according to claim 1, wherein the battery is produced using one or more selected from the group consisting of water-soluble synthetic resins, polyvinyl alcohol, and styrene-butadiene rubber. When the area of each image of the granule is replaced with an equivalent circle, the equivalent circle diameter is L, the actual perimeter is l, and L / l is defined as the circularity, the circularity is 0. The battery according to claim 1, wherein 85 or less particles are 20% or less based on the number. When the area of each image of the granule is replaced with an equivalent circle, the equivalent circle diameter is L, the actual perimeter is l, and L / l is defined as the circularity. The battery according to claim 1, wherein the circularity is 0.925 or more. The battery according to claim 1, wherein the granulated body is fired under nitrogen or an inert gas. The battery according to claim 1, wherein the granulated body is produced by stirring granulation. The battery according to claim 1, wherein the carbon material capable of occluding and releasing lithium ions is the natural graphite particles. The battery according to claim 1, wherein the natural graphite is scaly. The battery according to claim 1, wherein the battery is a secondary battery including a non-aqueous electrolyte. The battery according to claim 1, wherein the positive electrode and the negative electrode are arranged to face each other with a separator including at least a part of the electrolyte.