JPH07134988A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a lithium secondary battery excellent in a high rate charge/ discharge property and a high temperature resistant storage property by improving a particle diameter or a specific surface area of the specific spherical graphite in a negative electrode active material. CONSTITUTION:A nonaqueous electrolyte secondary battery is provided with a positive electrode consisting of oxide containing lithium and a rechargeable negative electrode mainly consisting of graphite powder. The graphite powder, to which lithium can be intercalated, is a spherical substance, and also it is optically anisotropic granular substance having a lamella structure consisting of monophase, and in this granular formed by graphitizing a mesophase small spherical body generated in a heat treatment process of pitch at a low temperature, spacing of a 002 face (d002) by a wide angle X-ray diffraction method is 3.36-3.40Angstrom , while a specific surface area by a BET method is 0.7-5.0m<2>/g.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
In particular, it relates to the negative electrode.

[0002]

2. Description of the Related Art In recent years, portable electronic devices for consumer use,
Cordless is advancing rapidly. Along with this, there is an increasing demand for a small-sized, lightweight secondary battery having a high energy density, which serves as a power source for driving those devices. From this point of view, non-aqueous secondary batteries, especially lithium secondary batteries, have great expectations as batteries having high voltage and high energy density, and their development is urgently needed. Conventionally, manganese dioxide, vanadium pentoxide, titanium disulfide, etc. have been used as a positive electrode active material of a lithium secondary battery. A battery was composed of these positive electrode, lithium negative electrode and organic electrolyte. However, generally, in a secondary battery using lithium metal for the negative electrode, problems such as an internal short circuit due to dendrite-like lithium generated at the time of charging and a side reaction between an active material and an electrolytic solution are major obstacles to making the secondary battery. Furthermore, no one has been found to be satisfactory in high-rate charge / discharge characteristics and over-discharge characteristics.

In recent years, the safety of lithium batteries has been pointed out severely, and it is very difficult to ensure safety in battery systems using lithium metal or lithium alloy for the negative electrode. Recently, a new type of electrode active material utilizing an intercalation reaction of a layered compound has been attracting attention, and an interlayer compound is considered as an electrode material for a secondary battery. In particular, a carbon material capable of intercalating / deintercalating Li ions is promising as a negative electrode material for a lithium secondary battery, and its development is being actively conducted. On the other hand, since a carbon material is used for the negative electrode, LiCoO ₂ or LiNi, which is a compound having a higher voltage and containing lithium, is used as the positive electrode active material.
It has been proposed to use a complex oxide in which O ₂ and further Co and Ni are partially replaced with other elements.
However, although such a non-aqueous battery can obtain a high energy density, it is difficult to obtain a high output density as compared with an aqueous battery. This is largely due to the ionic conductivity of the electrolytic solution, and the ionic conductivity of the non-aqueous electrolytic solution is 100 times less than that of the aqueous solution.

As a method for solving these problems, it is conceivable to increase the electrode area, that is, to use a thin and large-area electrode plate. In particular, a manufacturing method in which a metal foil, which is a current collector, is coated with a paste in which an electrode active material is dispersed in a binder solution or a dispersion medium and dried, is relatively well known. It is possible to obtain an area plate.

[0005]

Various carbon materials have been studied as the carbon material for the negative electrode. Low crystalline graphite exhibits excellent cycle reversibility, but the amount of lithium that can be intercalated is low. However, the high energy density cannot be expected because the electrode potential is relatively noble. On the other hand, a highly crystalline graphite material gives a relatively high capacity, but since the graphite is repeatedly expanded and contracted in the c-axis direction due to the intercalation / deintercalation reaction of lithium, and has a high capacity. The amount of change is large and the cycle characteristics are unfavorable.

In addition, in the electrode manufacturing method as described above,
Apply the pace of graphite powder to both sides of the current collector, for example, copper foil,
When a negative electrode plate is manufactured by a method such as drying, it is easy to obtain a thin, large-area electrode plate, but the physical properties of the graphite powder, especially the particle size or specific surface area, have an extremely large effect on battery characteristics. Especially, the discharge capacity of the battery, the high rate charge / discharge characteristics, and the high temperature resistant storage characteristics are affected. On the other hand, regarding the particle size of graphite powder, it has been conventionally proposed to regulate the average particle size. However, regulation of only the average particle size is not sufficient for solving the problem, because it may include a large amount of particles and small particles that are larger than necessary.

In order to solve this problem, the inventors have found out the following with respect to the particle size and specific surface area of the graphite powder as a result of various experiments. For example, it is said that the graphite powder does not have a perfect spherical shape when the particle size is small, particularly when it is 3 μm or less. In this case, even if it is graphitized at a high temperature, the crystallinity is low, and only a layer structure with an undeveloped layer is obtained. Therefore, the amount of lithium that can be intercalated or deintercalated is small, and thus 100 to 150
Only a capacity of about mAh / g can be obtained. Also,
When the particle size is too large, when a thin electrode plate is prepared by the above method, a gap is likely to be formed between the particles and the packing density cannot be increased. If the density is forcibly increased by rolling, the mixture will peel off from the current collector, and there is a problem that the battery cannot be constructed. On the other hand, with respect to the specific surface area, generally, the smaller the reaction area is, the larger the polarization becomes, which is disadvantageous in terms of high rate charge / discharge characteristics. However, the larger the specific surface area, the larger the number of particles with a smaller particle size, or the shape of the powder is not spherical, and the surface has more irregularities and the surface activity becomes higher than necessary, so the battery was stored at high temperature. In this case, there is a problem that it reacts with the electrolytic solution, and gas is generated to cause deterioration.

[0008]

In order to solve these problems, the present invention relates to a graphite powder, which is a spherical substance capable of intercalating lithium and which is optically anisotropic and has a single phase. Is a granular material having a lamellar structure, which is obtained by graphitizing mesophase spherules generated in a heat treatment process at a low temperature of pitch, and has a 002 plane spacing (d ₀₀₂ ) of 3.36 by a wide-angle X-ray diffraction method. Angstrom or higher 3.4
It is 0 angstrom or less, and the specific surface area measured by the BET method is 0.7 to 5.0 m ² / g. Here, the graphite powder has a diameter of 3 μm or less in a volume ratio of 30% or less, and 1
Those having a volume ratio of 5 μm or more are preferably 20% or less by volume.

[0009]

The graphite granules, which are optically anisotropic and have a lamellar structure consisting of a single phase, are typically graphitized by heat-treating the mesophase spheres generated during the carbonization process of pitch at high temperature. Spherical mesocarbon micro beads (MCM
B), which is Vol. 6, No. 5 (1973).
Year). Furthermore, the degree of graphitization of this MCMB is an important factor, and in terms of physical properties of carbon, the interplanar spacing (d ₀₀₂ ) of 002 planes is 3.36 to 3.
The 42 Angstrom one gives good results. By the way, also in this MCMB, when the graphitization temperature is low, a pseudographitic state in which d ₀₀₂ is 3.43 angstroms or more is obtained, and in this case, the capacity is small like other pseudographite materials and the carbon electrode This is not preferable because the electric potential as is noble. In order to obtain preferable d ₀₀₂ , the heat treatment temperature for graphitization in an inert gas atmosphere is 200.
The temperature must be 0 ° C or higher, and the graphitization degree is almost saturated at 2800 to 3000 ° C. It should be noted that heat treatment at 3000 ° C. or higher does not cause any change in physical properties and is rather uneconomical. That is, it is considered that the heat treatment temperature for graphitization of MCMB is preferably 2000 ° C. to 3000 ° C. in an inert gas atmosphere. More preferable heat treatment conditions are a temperature of about 2800 ° C. and a treatment time of 10 to 14 hours.

Originally, this carbon material is a graphitizable carbon material, and becomes a substantially spherical graphite granular material that retains the original shape of the mesophase spherules even when heat-treated at a high temperature.
Mesophase spheres have an average particle size of 1 to 80 μm.
Although the spherical particles are as small as m, the average particle size of the graphite particles after heat treatment is almost the same. As a manufacturing method for obtaining fine mesophase small spheres, there is a method of selecting spherical particles generated in a molten pitch obtained by heat-treating the pitch at a low temperature of about 300 to 500 ° C. in the heat treatment for carbonizing the pitch. More preferably, the pitch is 3 at about 400 ° C.
Hold for ~ 6 hours and select the spherical particles produced under these conditions.

Further, the characteristic feature of the graphite granules is that the graphite particles are composed of a very uniform single phase. For example, when the cross section of the particles is viewed with a polarizing microscope, it is a uniform phase with almost no boundaries. Is observed. In particular,
Since it has high crystallinity and a single phase, the potential is base and the potential change is flat in the charge and discharge reaction of the negative electrode, which is also advantageous in terms of battery voltage characteristics. Incidentally, in the case of this graphite granule, the intercalation and deintercalation reactions of lithium are 0.0
It progresses at an extremely low potential of 5 to 0.20V. This graphite is an anisotropic graphite having a well-ordered crystal phase, and further has a lamellar structure, that is, a structure in which the crystal phase perpendicular to the c-axis in the spherical particles is oriented like a stack of discs, Intercalation of Li results in the insertion of lithium between the crystal phases. That is, because of such a lamellar structure, lithium can be penetrated at all locations on the particle surface, and an extremely smooth intercalation reaction that can secure a high capacity can be realized. further,
In addition to the fact that such an ideal intercalation reaction can be realized, the graphite granules are almost spherical, and physical distortion is unlikely to occur in the particles due to expansion and contraction of graphite in the c-axis direction. The shape is also considered to be a factor that can maintain excellent cycle reversibility.

Next, when a thin, large-area graphite electrode is obtained by the above-mentioned manufacturing method, the graphite powder having a particle diameter of 3 μm or less and a volume ratio of 30% or less and 15 μm or more is used. By using a material having a volume ratio of 20% or less and a specific surface area by BET method of 0.7 to 5.0 m ² / g, the packing density is high, and the high rate charge / discharge characteristics and the high temperature storage characteristics are excellent. It is possible to obtain a plate. The binder to be added is preferably used in a state in which fine particle powder is dispersed in a solution, and the dispersant may be an aqueous solution or a non-aqueous solvent, but an aqueous solution is more preferable in the manufacturing process. Boarding is ready. Further, when the graphite mixture is made into a paste, a slight amount of a thickening agent such as carboxymethyl cellulose can be added.

As the electrolytic solution, a conventionally known electrolytic solution can be used, but when a graphite material is used for the negative electrode, propylene carbonate is not preferable because it tends to cause a decomposition reaction during charging and generate gas. It can be said that ethylene carbonate, which is a simple cyclic carbonate, is suitable because it hardly causes a side reaction as in the case of propylene carbonate. However, since ethylene carbonate has a very high melting point and is solid at room temperature, it is difficult to use it as a single solvent. Therefore, it is preferable to use a mixed solvent in which an aliphatic carboxylic acid ester such as 1,2-dimethoxyethane or diethyl carbonate having a low melting point and a low viscosity is mixed. Further, as the lithium salt which is soluble in these solvents, any conventionally known ones such as lithium hexafluorophosphate, lithium borofluoride, lithium hexafluoroarsenate and lithium perchlorate can be used.

On the other hand, for the positive electrode, LiCoO ₂ , LiNiO _{2, LiFeO 2} and L _, which are compounds containing lithium ions, are used.
iMn ₂ O ₄ or the like can be used. These composite oxides can be easily obtained, for example, by using a carbonate or oxide of lithium or cobalt as a raw material, and mixing and firing them according to the target composition. Of course, similar synthesis can be performed when other raw materials are used. Among them, LiCoO ₂ has the largest chargeable / dischargeable capacity and is chemically stable in the electrolytic solution. Usually, the firing temperature is set between 650 ° C and 1200 ° C.

[0015]

The present invention will be described in detail below with reference to its examples. [Embodiment 1] FIG. 1 is a vertical sectional view of a cylindrical battery used in this embodiment. However, only a part of the electrode plate group is shown in cross section. In the figure, reference numeral 1 denotes a battery case formed by processing a steel plate having organic electrolyte resistance. Reference numeral 2 is a sealing plate which incorporates a safety valve and also serves as a positive electrode terminal. 3 is an insulating packing. Reference numeral 4 denotes an electrode plate group, in which the positive electrode and the negative electrode are spirally wound a plurality of times with the separator interposed therebetween and are housed in the case 1. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. 7
Insulating rings 8 and 8 are provided on the upper and lower portions of the electrode plate group 4, respectively. The positive and negative electrode plates will be described below.

The positive electrode is manufactured as follows. First, L
LiCoO ₂ powder synthesized by firing a mixture of i ₂ CO ₃ and Co ₃ O ₄ at 900 ° C. was mixed with an aqueous dispersion of acetylene black and polytetrafluoroethylene, and this was added to an aqueous solution of carboxymethylcellulose. Suspend into a paste. Next, this paste is applied to both sides of an aluminum foil having a thickness of 0.03 mm, dried, and rolled to obtain an electrode plate having a thickness of 0.19 mm, a width of 40 mm and a length of 250 mm. The negative electrode is manufactured as follows. First, mesocarbon micro beads (hereinafter abbreviated as MCMB) made from mesophase spherules produced in a heat treatment process at a low temperature of pitch were heat-treated at 2800 ° C. to be graphitized, and a powder having d ₀₀₂ of 3.37 angstroms. Was mixed with an aqueous dispersion of styrene-butadiene rubber, and the mixture was suspended in an aqueous solution of carboxymethyl cellulose to form a paste. The particle size distribution is measured by a laser diffraction particle size distribution measuring device (SALD manufactured by Shimadzu Corporation).
-2000) and a 0.01% aqueous surfactant solution was used as the dispersion medium. Next, the above paste is applied to both sides of a copper foil having a thickness of 0.02 mm, dried and rolled to a thickness of 0.20 to 0.22 mm, a width of 42 mm, and a length of 285.
mm plate. Table 2 shows the packing density of the negative electrode when using MCMB adjusted to each particle size.

Leads are attached to the positive electrode and the negative electrode, respectively, and they are spirally wound with a polyethylene separator interposed therebetween and housed in a battery case having a diameter of 14.0 mm and a height of 50 mm. The electrolyte contains ethylene carbonate and 1,2
-Using 1 mol / l of LiPF ₄ dissolved in a solvent in which dimethoxyethane is mixed at a volume ratio of 50:50. After pouring this electrolyte solution into the above battery case and sealing it, a test battery is obtained. The test battery was evaluated by performing constant current charging with a final voltage of 4.1 V at 100 mA in an environment of 20 ° C., and discharging was a discharge current of 100 mA and a final discharge voltage of 3.
A constant current discharge of 0 V was performed. The result is shown in FIG.

[0018]

[Table 1]

[0019]

[Table 2]

From the results of the packing density of the electrode plates shown in Table 2, Battery C using a negative electrode containing 25% of powder having a diameter of 15 μm or more.
Has a low packing density in the electrode plate. This is probably because there are gaps between the particles. Further, when trying to increase the density further by rolling, slip occurs between the mixture and the copper foil as the current collector due to strain due to rolling,
The mixture was peeled off and it became impossible to construct a battery. As shown in FIG. 2, the battery E using the negative electrode in which the proportion of powder of 3 μm or less is 35% is smaller in discharge capacity than other batteries. The reason for this is 3
Since the layer structure of powders of μm or less is not developed even when graphitized at high temperature, too many lithium ions cannot be intercalated, or even if intercalated, they cannot be deintercalated during discharge. It is thought that it will disappear.

In FIG. 3, each battery was charged at 100 mA at room temperature with a final voltage of 4.1 V, and then discharged at a final voltage of 3.0 V at 100 mA and 1000 mA, respectively. (10
00), the capacity Cap (10
The ratio to 0) is shown. According to the results shown in FIG. 3, Battery C using a negative electrode containing 25% of powder having a diameter of 15 μm or more
Are remarkably inferior in high rate discharge characteristics. It is presumed that the reason for this is that when the amount of powder having a diameter of 15 μm or more increases, the reaction area decreases and a gap is formed between the mixture to reduce the current collecting property, resulting in increased polarization. The high-rate charge characteristic was also similar to the discharge characteristic. From the above results, the particle size of the graphite powder is 30% or less by volume and 1
It can be seen that it is optimal that the volume ratio of those having a thickness of 5 μm or more is 20% or less.

Example 2 A battery was constructed and tested in the same manner as in Example 1 except that MCMB having different specific surface areas shown in Table 3 was used as the carbon material of the negative electrode. The specific surface area is measured by the BET one-point method measuring device (manufactured by Nikkiso Co., Ltd. 4
200 type Microtrac beta soap automatic surface area meter)
Went by.

[0023]

[Table 3]

In FIG. 4, each battery was charged at 100 mA at room temperature with a final voltage of 4.1 V, and then discharged at a final voltage of 3.0 V at 100 mA and 1000 mA, respectively, and the capacity Cap when discharged at 1000 mA was measured. (10
00), the capacity Cap (10
The ratio to 0) is shown. The smaller the specific surface area,
It can be seen that the high-rate discharge characteristics are inferior, and especially battery F is extremely inferior. The reason for this is that the smaller the specific surface area,
It is speculated that this may be because the reaction area decreases and the polarization during discharge increases. Further, in FIG. 5, each battery was stored in a charged state at 60 ° C. for 1 month, and then subjected to constant current charging at 100 mA at room temperature to a final voltage of 4.1 V, and at 100 mA to a final voltage of 3.0 V. The ratio of the discharge capacity at the 5th cycle when current discharge is performed to the discharge capacity at the time of charge / discharge under the same conditions before storage is shown. From this result, it can be seen that the battery using the powder having the larger specific surface area has a larger deterioration rate, and particularly the battery K has a significantly larger capacity deterioration rate due to storage.

Although the reason for this is not certain, it is considered that the powder having a large specific surface area has a small particle size or has a large number of irregularities instead of a spherical shape. In this case, it is presumed that the surface becomes excessively active, and when the battery is stored at a high temperature, it reacts with the electrolytic solution to deteriorate the battery. Further, when the particle size is small, the layer structure is undeveloped, and it is presumed that the layer structure is destroyed and the battery is deteriorated by storing lithium in an intercalated state. From the above results, the specific surface area of the powder is 0.7 to 5.0 m ^2.
It can be said that / g is optimal. Although LiCoO ₂ was used for the positive electrode in Examples 1 and 2, almost the same effect was obtained when the above-mentioned other lithium-containing oxide was used, although a slight difference in capacity was observed.

[0026]

As is clear from the above description of the embodiments, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having high capacity, high energy density and excellent storage characteristics. it can.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a cylindrical battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing a comparison of discharge curves in Example 1.

3 is a diagram showing a comparison of high rate discharge characteristics in Example 1. FIG.

FIG. 4 is a diagram showing a comparison of high rate discharge characteristics in Example 2.

5 is a diagram showing a comparison of storage characteristics in Example 2. FIG.

[Explanation of symbols]

 1 Battery Case 2 Sealing Plate 3 Insulating Packing 4 Electrode Plate Group 5 Positive Electrode Lead 6 Negative Electrode Lead 7 Insulating Ring 8 Insulating Ring

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Takai 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A positive electrode using a lithium-containing oxide as an active material, a rechargeable negative electrode mainly containing graphite powder, and a non-aqueous electrolyte, wherein the graphite powder is a spherical substance capable of intercalating lithium. And is optically anisotropic and has a lamellar structure consisting of a single phase, and this granular material is a graphitized mesophase sphere produced during the heat treatment process at a low temperature of the pitch. And the interplanar spacing (d ₀₀₂ ) of the 002 plane by the wide-angle X-ray diffraction method is 3.36 angstroms or more and 3.40 angstroms or less, and B
A non-aqueous electrolyte secondary battery having a specific surface area of 0.7 to 5.0 m ² / g according to the ET method. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the graphite powder having a diameter of 3 μm or less has a volume ratio of 30% or less and the graphite powder having a diameter of 15 μm or more has a volume ratio of 20% or less. .