JP3048808B2 - Non-aqueous electrolyte secondary battery - Google Patents

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 have become more portable.
Cordless technology is rapidly advancing. Accordingly, there is an increasing demand for a small, lightweight, and high-energy-density secondary battery that serves as a power supply for driving these devices. From such a viewpoint, a non-aqueous secondary battery, particularly a lithium secondary battery, is expected to be a battery having a high voltage and a high energy density, and its development is urgently required. Conventionally, manganese dioxide, vanadium pentoxide, titanium disulfide, and the like 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, in general, in a secondary battery using lithium metal for the negative electrode, problems such as an internal short circuit due to dendritic lithium generated at the time of charging and a side reaction between the active material and the electrolytic solution are major obstacles to the formation of the secondary battery. Further, no satisfactory charge / discharge characteristics or overdischarge characteristics have been found.

In recent years, the safety of lithium batteries has been strictly pointed out, 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 attracted attention, and an interlayer compound has been 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 pursued. On the other hand, with the use of a carbon material for the negative electrode, the positive electrode active material has a higher voltage and is a compound containing lithium, such as LiCoO ₂ or LiNi.
It has been proposed to use a composite oxide obtained by substituting O ₂ , and further a part of Co and Ni with another element.
However, in such a non-aqueous battery, although a high energy density can be obtained, it is difficult to obtain a high output density as compared with an aqueous battery. This is largely attributable to the ionic conductivity of the electrolytic solution. At present, the non-aqueous electrolytic solution has an ionic conductivity of 1/100 or less as compared with the aqueous solution.

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

[0005]

Various carbon materials for the negative electrode have been studied in the past. Low-crystalline graphite exhibits excellent cycle reversibility, but has a low lithium content that can be intercalated. And high energy density cannot be expected because the electrode potential is relatively low. On the other hand, a highly crystalline graphite material gives a relatively high capacity, but the expansion and contraction of the graphite in the c-axis direction are repeated by the intercalation / deintercalation reaction of lithium, and the capacity is high. The amount of change is also large, resulting in undesirable cycle characteristics.

In the above-described electrode manufacturing method,
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 property value of the graphite powder, particularly the particle size or specific surface area, has an extremely large effect on battery characteristics. In particular, the discharge capacity, high rate charge / discharge characteristics, and high temperature storage characteristics of the battery are affected. On the other hand, it has been conventionally proposed to regulate the average particle size of the graphite powder. However, the regulation of only the average particle size is not sufficient to solve the problem, because it is possible to include a large number of particles larger or smaller than necessary.

[0007] As a result of various experiments, the present inventors have found the following with respect to the particle size and specific surface area of graphite powder. For example, it is said that when the particle size is small, particularly when the particle size is 3 μm or less, the graphite powder does not have a perfect spherical shape. In this case, even if it is graphitized at a high temperature, the crystallinity is low, and only those having an undeveloped layer structure can be obtained. Therefore, the amount of lithium that can be intercalated and deintercalated is small,
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-described method, a gap is easily formed between the particles, and the packing density cannot be increased. If the density is forcibly increased by rolling, there is a problem that the mixture is separated from the current collector and a battery cannot be formed. On the other hand, with respect to the specific surface area, generally, the smaller the reaction area, the smaller the reaction area and the larger the polarization, which is disadvantageous in terms of high-rate charge / discharge characteristics. However, as the specific surface area increases, the number of particles having a small particle size increases, or the shape of the powder is not spherical, but the surface has many irregularities and the activity of the surface becomes higher than necessary. In such a case, there is a problem that a reaction occurs with the electrolytic solution and gas is generated to deteriorate the battery.

[0008]

SUMMARY OF THE INVENTION In order to solve these problems, the present invention provides a spherical powder capable of intercalating lithium as an graphite powder, which is optically anisotropic and has a single phase. This is a granular material having a lamellar structure, which is obtained by graphitizing mesophase small spheres generated in a heat treatment process at a low temperature of pitch, and has a 002 plane spacing (d ₀₀₂ ) of 3.36 determined by a wide-angle X-ray diffraction method. Angstrom and above 3.4
0 angstrom or less and 0.7 to 5.0 m ² / g in specific surface area measurement by the BET method. Here, the graphite powder having a diameter of 3 μm or less has a volume ratio of 30% or less and 1% or less.
Those having a size of 5 μm or more preferably have a volume ratio of 20% or less.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Graphite particles having an optically anisotropic and lamellar structure consisting of a single phase are typically graphitized by heat-treating mesophase small spheres generated during the carbonization process of pitch at a high temperature. Spherical mesocarbon microbeads (MCM
B), which is the journal of the Petroleum Institute of Japan, Vol. 6, No. 5, (1973).
Year). Further, the degree of graphitization of the MCMB is an important factor, and the plane spacing (d ₀₀₂ ) of the 002 plane is 3.36 to 3.36 in terms of physical parameters of carbon.
42 angstroms gives good results. Incidentally, also in this MCMB, when the graphitization temperature is low, a quasi-graphitic state having d ₀₀₂ of 3.43 Å or more can be obtained. In this case, the capacity is small like other pseudo-graphite materials, Is not preferred because the potential becomes noble. In order to obtain a 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. The heat treatment at 3000 ° C. or higher does not involve a change in physical properties and is rather uneconomical. That is, it is considered that the heat treatment temperature for graphitization of MCMB is preferably set to 2000 ° C. to 3000 ° C. in an inert gas atmosphere. More preferred heat treatment conditions are a temperature of about 2800 ° C. and a treatment time of 10 to 14 hours.

[0010] Originally, this carbon material is a graphitizable carbon material, and becomes almost spherical graphite granules which maintain the shape of the original mesophase spherules even when subjected to heat treatment at a high temperature.
The mesophase microspheres have an average particle size of 1 to 80 μm.
Although the particles are spherical particles as small as m, the average particle size of the graphite particles after the heat treatment is almost the same. As a method for producing fine mesophase spheres, there is a method of selecting spherical particles generated in a molten pitch obtained by heat-treating a pitch at a low temperature of about 300 to 500 ° C. in a carbonization heat treatment process for the pitch. More preferably, the pitch should be 3 at about 400 ° C.
Keep for ~ 6 hours and select the spherical particles generated under these conditions.

Further, the characteristic of the graphite particles is that the graphite particles consist of a very uniform single phase. For example, when the cross section of the particles is viewed with a polarizing microscope, the particles have a uniform phase with almost no boundaries. It is observed that In particular,
Since it is highly crystalline and has a single phase, the potential is low and the potential change becomes flat in the charge / discharge reaction of the negative electrode, which is advantageous from the viewpoint of battery voltage characteristics. By the way, in the case of this graphite granule, the lithium intercalation and deintercalation reactions are 0.0
It proceeds at a very low potential of 5 to 0.20V. This graphite is an anisotropic graphite having an orderly crystal phase, and further has a lamellar structure, that is, a structure in which crystal phases perpendicular to the c-axis in spherical particles are oriented as if disks are stacked, Intercalation of Li results in insertion of lithium between the crystal phases. That is, because of such a lamellar structure, lithium can enter at all places on the particle surface, and an intercalation reaction that can ensure extremely smooth and high capacity can be realized. further,
In addition to realizing such an ideal intercalation reaction, the graphite granules are almost spherical, and physical distortion is hardly generated in the particles due to expansion and contraction of graphite in the c-axis direction. It is considered that the shape is also a factor that can maintain excellent cycle reversibility.

Next, when a thin, large-area graphite electrode is to be obtained by the above-described manufacturing method, the graphite powder having a diameter of 3 μm or less has a particle diameter of 30% or less and 15 μm or more in volume ratio. By using one having a volume ratio of 20% or less and a specific surface area of 0.7 to 5.0 m ² / g by the BET method, an electrode having a high packing density, excellent high-rate charge / discharge characteristics and high-temperature storage characteristics is obtained. It is possible to obtain a board. The binder to be added is preferably used in a state in which fine particle powder is dispersed in a solution. The dispersing agent may be an aqueous solution or a non-aqueous solvent. It is ready for boarding. When the graphite mixture is pasted, a slight amount of a thickening agent such as carboxymethylcellulose may be added.

As the electrolytic solution, a conventionally known electrolytic solution can be used. However, when a graphite material is used for the negative electrode, propylene carbonate is not preferred because it tends to undergo a decomposition reaction at the time of charging and generate gas. It can be said that ethylene carbonate, which is a simple cyclic carbonate, is suitable because it hardly involves side reactions as in the case of propylene carbonate. However, ethylene carbonate has a very high melting point and is a solid at ordinary temperature, so that 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, which is a solvent having a low melting point and a low viscosity, is mixed. As the lithium salt dissolved in these solvents, any of conventionally known lithium salts such as lithium hexafluorophosphate, lithium borofluoride, lithium hexafluoroarsenate and lithium perchlorate can be used.

On the other hand, on the positive electrode, compounds containing lithium ions, such as LiCoO ₂ , LiNiO _{2, LiFeO 2} ,
iMn ₂ O ₄ or the like can be used. These composite oxides can be easily obtained, for example, by using lithium or cobalt carbonates or oxides as raw materials, mixing them according to the desired composition, and firing them. Of course, the synthesis can be performed in the same manner even 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]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. Embodiment 1 FIG. 1 is a longitudinal 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 obtained by processing a steel sheet having resistance to an organic electrolytic solution. Reference numeral 2 denotes a sealing plate incorporating a safety valve and also serving as a positive electrode terminal. 3 is an insulating packing. Reference numeral 4 denotes an electrode group, in which the positive electrode and the negative electrode are spirally wound a plurality of times via a separator and housed in the case 1. A positive electrode lead 5 is drawn from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn from the negative electrode and connected to the bottom of the battery case 1. 7
Reference numerals 8 denote insulating rings 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
An aqueous dispersion of acetylene black and polytetrafluoroethylene was mixed with LiCoO ₂ powder synthesized by calcining a mixture of i ₂ CO ₃ and Co ₃ O ₄ at 900 ° C., and this was added to an aqueous solution of carboxymethyl cellulose. Suspend into a paste. Next, this paste is applied on both sides of an aluminum foil having a thickness of 0.03 mm, dried, and then 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 microbeads (hereinafter, abbreviated as MCMB) made from mesophase small spheres generated in a heat treatment process at a low temperature of pitch are heat-treated at 2800 ° C. and graphitized, and powder having d ₀₀₂ of 3.37 angstroms. And a particle size adjusted as shown in Table 1, mixed with an aqueous dispersion of styrene-butadiene rubber, and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. The particle size distribution was measured using a laser diffraction type particle size distribution analyzer (SALD manufactured by Shimadzu Corporation).
-2000), and a 0.01% aqueous surfactant solution was used as a dispersion medium. Next, the above paste was applied to both sides of a copper foil having a thickness of 0.02 mm, dried, and then rolled to obtain 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 MCMB adjusted to each particle size was used.

Leads are attached to the positive and negative electrodes, respectively, and spirally wound through a polyethylene separator, and housed in a battery case having a diameter of 14.0 mm and a height of 50 mm. As the electrolyte, ethylene carbonate and 1,2 were used.
A solution obtained by dissolving 1 mol / l of LiPF ₄ in a solvent in which dimethoxyethane is mixed at a volume ratio of 50:50 is used. This electrolyte solution is injected into the above battery case and then sealed to form a test battery. The evaluation of the test battery was performed under a 20 ° C. environment at a constant current of 100 mA and a cutoff voltage of 4.1 V, and the discharge was performed at a discharge current of 100 mA and a cutoff 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 in Table 2, the battery C using the negative electrode containing 25% of a powder having a diameter of 15 μm or more was used.
Has a low packing density in the electrode plate. This is thought to be due to the formation of gaps between the particles. In addition, when trying to further increase the density by rolling, slip occurs due to distortion caused by rolling between the mixture and the copper foil as a current collector,
The mixture was exfoliated, making it impossible to construct a battery. As shown in FIG. 2, the battery E using the negative electrode in which the ratio of the powder having a particle size of 3 μm or less is 35% has a smaller discharge capacity than the others. The reason is 3
Even if the powder below μm is graphitized at a high temperature, the layer structure is not developed, so that too much lithium ions cannot be intercalated, or even if it is intercalated, it cannot be deintercalated during discharge. It is thought to be gone.

FIG. 3 shows that each battery was charged at 100 mA at room temperature at a cut-off voltage of 4.1 V, and then discharged at a cut-off voltage of 3.0 V at 100 mA and 1000 mA, respectively, and the capacity Cap when discharged at 1000 mA. (10
00) when discharged at 100 mA.
0). 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 was used.
Have remarkably inferior 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 becomes smaller, a gap is formed between the mixture and the current collecting property is reduced, and the polarization is increased. In addition, the same result as the discharge characteristic was obtained for the high-rate charge characteristic. From the above results, the graphite powder having a diameter of 3 μm or less is 30% or less by volume and 1% or less by volume.
It can be seen that it is optimal that the volume ratio 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 as shown in Table 3 was used as the carbon material for the negative electrode. The specific surface area was measured using a BET one-point method measuring instrument (Nikkiso Co., Ltd. 4
200 type micro track beta soap automatic surface area meter)
Was performed.

[0023]

[Table 3]

FIG. 4 shows that each battery was charged at room temperature at 100 mA at a cut-off voltage of 4.1 V, and then discharged at a cut-off voltage of 3.0 V at 100 mA and 1000 mA, respectively, and the capacity Cap when discharged at 1000 mA. (10
00) when discharged at 100 mA.
0). The smaller the specific surface area,
It can be seen that the high rate discharge characteristics are inferior, and in particular, the battery F is extremely inferior. The reason is that the smaller the specific surface area,
It is presumed that the reaction area is reduced and the polarization at the time of discharge is increased. Further, FIG. 5 shows that each battery is stored in a charged state at 60 ° C. for one month, and then subjected to constant current charging at 100 mA at a normal temperature to a final voltage of 4.1 V, and constant voltage charging at 100 mA to a final voltage of 3.0 V. The ratio to the discharge capacity when the charge and discharge were performed under the same conditions before the storage of the discharge capacity at the fifth cycle when the current discharge was performed is shown. From this result, it can be seen that the battery using the powder having a larger specific surface area has a higher rate of deterioration, and particularly the battery K has a significantly higher 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 the shape is not spherical but has many irregularities. In this case, it is assumed that the surface becomes excessively active, and when the battery is stored at a high temperature, it reacts with the electrolytic solution to cause deterioration of the battery. When the particle size is small, the layer structure is not developed, and it is presumed that the layer structure is destroyed by storing lithium in an intercalated state and the battery is deteriorated. From the above results, the specific surface area of the powder was 0.7 to 5.0 m ^2.
/ G is optimal. In Examples 1 and 2, LiCoO ₂ was used for the positive electrode. However, when the above-mentioned other lithium-containing oxide was used, almost the same effect was obtained although a slight difference in capacity was observed.

[0026]

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

[Brief description of the drawings]

FIG. 1 is a longitudinal 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.

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

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring 8 Insulation ring

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toru Takai 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (56) References JP-A-4-342958 (JP, A) JP-A-5-205 74453 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 4/02 H01M 4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. A positive electrode comprising a lithium-containing oxide as an active material, a rechargeable negative electrode comprising mainly graphite powder and a non-aqueous electrolyte, wherein said graphite powder is a spherical substance capable of intercalating lithium. Yes, optically anisotropic granular material with a lamellar structure consisting of a single phase, and this granular material is a graphitized mesophase sphere generated during the heat treatment process of pitch at a low temperature. And the plane spacing (d ₀₀₂ ) of the 002 plane measured by the wide-angle X-ray diffraction method is 3.36 angstroms or more and 3.40 angstroms or less.
The specific surface area by the ET method is 0.7 to 5.0 m ² / g.
And the graphite powder having a diameter of 3 μm or less has a volume ratio of 30% or less.
The graphite powder containing 15 μm or more and containing
A non-aqueous electrolyte secondary battery containing 0% or less . 2. The method according to claim 1, wherein the non-aqueous electrolyte is ethylene carbonate.
And 1,2-dimethoxyethane or diethyl carbonate
The non-aqueous electrolyte secondary battery according to claim 1, wherein you comprising a mixed solvent consisting of over bets.