JPH04332482A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To prevent precipitation of lithium in a negative pole, and obtain good charge/discharge cycles for a battery comprising a negative pole formed of cokes, a positive pole, and nonaqueous electrolyte including lithium salt. CONSTITUTION:A battery is composed of a negative pole 1 and a positive pole 2 formed of cokes, and nonaqueous electrolyte obtained by dissolving lithium salt in nonaqueous solvent. The quantity of the electrolyte is set to 3.0mul or more and less than 5.5mul per mAh of a discharge capacity of the battery. When the electrolyte quantity per mAh of the discharge capacity is less than 3.0mul, a volume or the negative pole 1 to get in contact with the electrolyte is reduced to precipitate lithium on the surface of the negative pole at the time of charging to generate an inner shortcircuit. On the other hand, if the electrolyte quantity exceeds 5.5mul, a inconvenience of leak of the electrolyte in a process of caulking a battery cell 5 occurs. By setting the electrolyte quantity to be in a proper range, therefore, precipitation of lithium can be prevented, and good charge/discharge cycles can be achieved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode.

0002

2. Description of the Related Art The remarkable progress in electronic technology in recent years has led to successive miniaturization and weight reduction of electronic devices. Along with this, batteries as mobile power sources are becoming smaller and smaller.
Light weight and high energy density are required.

[0003] Conventionally, aqueous solution secondary batteries such as lead batteries and nickel-cadmium batteries have been mainstream as secondary batteries. However, although these batteries have excellent cycle characteristics, they are not fully satisfactory in terms of battery weight, energy density, and self-discharge.

[0004]Recently, therefore, research and development of non-aqueous electrolyte secondary batteries using lithium or lithium alloys as negative electrodes has been actively conducted as a secondary battery to replace the above-mentioned nickel-cadmium batteries and the like. This non-aqueous electrolyte secondary battery has high energy density, little self-discharge, and is lightweight, making it suitable as a mobile power source.

However, in this non-aqueous electrolyte secondary battery, lithium crystals grow in a dendrite shape during charging due to repeated charge/discharge cycles, and this crystal-grown lithium reaches the positive electrode, causing an internal short circuit. Because of this potential, a sufficient cycle life cannot be obtained, which is a major obstacle to practical application.

[0006] Therefore, a non-aqueous electrolyte secondary battery using a carbon material for the negative electrode has been proposed as a non-aqueous electrolyte secondary battery with high energy density and long cycle life. This non-aqueous electrolyte secondary battery takes advantage of the fact that carbon intercalation compounds of lithium can be easily formed electrochemically, and when such a non-aqueous electrolyte secondary battery is charged, Lithium supported on a carbon material, lithium in the crystal structure of a positive electrode active material, lithium dissolved in an electrolyte, or the like is doped between the carbon layers of the negative electrode. and,
The carbon material doped with lithium functions as a lithium electrode, and lithium is released from between the carbon layers during discharge.

[0007] This nonaqueous electrolyte secondary battery has a high energy density, and unlike the above-mentioned nonaqueous electrolyte secondary battery that uses lithium as a negative electrode, lithium crystal growth does not occur, so repeated charging and discharging cycles are not required. withstand For this reason,
It is expected to be used as a power source for devices with relatively high power consumption such as video cameras, laptops, and personal computers, and studies are being conducted on the type of carbon material that will become the negative electrode and the structure of the electrode.

[0008] For example, the electrode structure includes a negative electrode,
A spiral electrode structure, in which a stacked electrode consisting of four layers of separator, positive electrode, and separator are laminated in this order, is wound many times in a spiral shape is used because it has a large electrode area and can withstand use under heavy loads.

[0009] Furthermore, coke such as petroleum coke and pitch coke, pyrolytic carbon, and glassy carbon have been proposed as carbon materials for the negative electrode. It is attracting attention because it can be produced simply by firing pitch in an inert gas and is available at low cost.

0010

[Problems to be Solved by the Invention] However, in a nonaqueous electrolyte secondary battery using a carbonaceous material for the negative electrode as described above, the discharge capacity decreases as the charge/discharge cycle progresses. Good cycle characteristics may not be maintained for long periods of time. One of the factors that influences this reduction in discharge capacity is the volume of the nonaqueous electrolyte (amount of nonaqueous electrolyte) contained in the nonaqueous electrolyte secondary battery.

For example, if the amount of non-aqueous electrolyte in the battery is small, there will be a portion of the electrode that does not come into contact with the non-aqueous electrolyte. Since this portion does not participate in the charge/discharge reaction, the current density of the electrode is higher than when the entire electrode is in contact with the electrolyte. As a result, during charging, lithium is likely to be supplied in excess of the lithium doping capacity of the negative electrode, which causes metallic lithium to precipitate on the negative electrode, penetrate the separator, and reach the positive electrode, causing an internal short circuit. I will do it.

The present invention has been proposed in view of the above-mentioned conventional circumstances, and provides a non-aqueous electrolyte secondary battery that is unlikely to cause internal short circuits even after repeated charging and discharging and exhibits good cycle characteristics. The purpose is to provide

[0013]

[Means for Solving the Problems] In a battery, only the portions of the negative electrode and the positive electrode that are actually in contact with the electrolyte participate in the charge/discharge reaction. Therefore, in a non-aqueous electrolyte secondary battery that uses coke, which is a carbon material, as the negative electrode, if the portion of the negative electrode that is in contact with the non-aqueous electrolyte is small relative to the discharge capacity, lithium will accumulate in the negative electrode during charging. Unable to carry it all,
Metallic lithium is deposited on the surface of the negative electrode, causing an internal short circuit. In order to prevent such internal short circuits, the part of the negative electrode that is in contact with the electrolyte must have a volume sufficient for the discharge capacity. It is believed that it is important to appropriately adjust the

The non-aqueous electrolyte secondary battery of the present invention was proposed from this viewpoint, and includes a negative electrode mainly composed of coke, a positive electrode, and a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent. In a non-aqueous electrolyte secondary battery comprising an aqueous electrolyte, the amount of the non-aqueous electrolyte is 1 m of discharge capacity of the battery.
It is characterized by being 3.0 μl or more and 5.5 μl or less per Ah.

In the nonaqueous electrolyte secondary battery of the present invention, the amount of nonaqueous electrolyte is 3.0 μl to 5.0 μl per 1 mAh of discharge capacity from the viewpoint of charge/discharge cycle characteristics and workability during manufacturing.
The volume is 5 μl. That is, when the amount of nonaqueous electrolyte per 1 mAh of discharge capacity is less than 3.0 μl, lithium is deposited on the surface of the negative electrode as the charge/discharge cycle progresses, causing an internal short circuit. On the other hand, if the amount of non-aqueous electrolyte per 1 mAh exceeds 5.5 μl, there will be operational inconveniences such as the electrolyte overflowing from the battery can when the battery can is caulked.

Examples of coke constituting the negative electrode in the non-aqueous electrolyte secondary battery include petroleum coke,
Examples include pitch coke and coal coke. These cokes have a (002) plane spacing d002 of 3.
40≦d002≦3.55. These cokes are obtained by calcining petroleum pitch, coal pitch, etc. in vacuum or inert gas.

Furthermore, it is also possible to use a carbonaceous material in which the amount of lithium doped is increased by adding a phosphorus compound or a boron compound when carbonizing petroleum pitch or the like.

On the other hand, it is preferable to use a material containing a sufficient amount of lithium as the positive electrode material, such as a composite metal oxide represented by the general formula LiMO2 (where M represents at least one of Co and Ni). lithium-containing interlayer compounds, etc. are used. Among these, Li
CoO4, LiCo0.8 Ni0.2 O2 are
It is advantageous in obtaining high voltage, high energy density, and good cycle characteristics.

Further, as the electrolytic solution, for example, an electrolytic solution prepared by using a lithium salt as an electrolyte and dissolving it in an organic solvent is used. Examples of the organic solvent here include, but are not limited to, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, Single or mixed solvents such as 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, and propionitrile can be used, and any conventionally known electrolytes can be used. LiClO2
, LiAsF6, LiPF6, LiBF4, LiB
(C6 H5)4, LiCl, LiBr, CH3
There are SO3 Li, CF3 SO3 Li, etc.

[0020]

[Operation] The non-aqueous electrolyte secondary battery of the present invention uses inexpensive coke as the negative electrode. Therefore, it can be manufactured at low cost and productivity can be improved. In addition, in the above non-aqueous electrolyte secondary battery, the amount of non-aqueous electrolyte is 1 mAh, which has a discharge capacity of 1 mAh.
3.0 μl or more and 5.5 μl or less per volume. In a non-aqueous electrolyte secondary battery that uses coke as the negative electrode, if the amount of non-aqueous electrolyte per unit discharge capacity is within the above range, the volume of the electrode that actually functions as an electrode (the volume of the electrode that is in contact with the electrolyte) is volume) is sufficient for the discharge capacity. Therefore, during charging, good cycle characteristics can be obtained without lithium being deposited on the surface of the negative electrode.

[0021]

EXAMPLES Preferred examples of the present invention will be described below based on experimental results.

Experimental Example 1 FIG. 1 shows a cylindrical non-aqueous electrolyte secondary battery prepared in this example.

First, the negative electrode 1 was manufactured as follows. Petroleum pitch was used as a starting material and was calcined to obtain coarse pitch coke. This coarse-grained pitch coke was pulverized into a powder with an average particle size of 40 μm, and then this powder was re-fired at 1000° C. in an inert gas to remove impurities to obtain a coke material powder. The coke material powder thus obtained was used as a negative electrode active material carrier, and 90 parts by weight of this coke material powder and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a negative electrode mixture. . This negative electrode mixture is dispersed in the solvent N-methyl-2-pyrrolidone to form a slurry (paste).
I made it. Next, this negative electrode mixture slurry was applied to both sides of a negative electrode current collector, which is a strip-shaped copper foil with a thickness of 10 μm, and after drying the solvent, compression molding was performed using a roller press machine to form a strip-shaped negative electrode 1.
I got it. The film thickness of the negative electrode mixture after molding is 10 mm on both sides.
The width of the strip-shaped negative electrode is 41.5 mm,
The length was 635 mm.

Next, the positive electrode 2 was produced as follows. By mixing 0.5 mole of lithium carbonate and 1 mole of cobalt carbonate and calcining the mixture in air at 900°C for 5 hours, L
iCoO2 was obtained. This LiCoO2 was used as a positive electrode active material, and 91 parts by weight of this LiCoO2, 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder were combined to form a positive electrode mixture. This positive electrode mixture was dispersed in a solvent N-methylpyrrolidone to form a slurry (paste). Next, apply this positive electrode mixture to a thickness of 2
It was uniformly coated on both sides of a positive electrode current collector, which is a strip-shaped aluminum foil of 0 μm, and dried. After drying, compression molding was performed to obtain a strip-shaped positive electrode 2. The thickness of the mixture film after molding is 8 on both sides.
The strip positive electrode had a width of 39.5 mm and a length of 605 mm.

The strip-shaped negative electrode 1, the strip-shaped positive electrode 2, and the separator 3 made of a microporous polypropylene film with a thickness of 25 μm and a width of 44 mm were formed into four layers in the order of negative electrode, separator, positive electrode, and separator. This four-layered laminated electrode body was wound in a spiral shape many times along its length with the negative electrode inside, and the final end of the outermost separator was fixed with tape to produce a spiral electrode. . In addition, the inner diameter of the hollow part at the center of this spiral electrode is 3.5
mm, and the outer diameter was 19.7 mm.

The spiral electrode produced as described above was housed in a nickel-plated iron battery can 5. Also,
Insulating plates 4 are arranged on both the upper and lower sides of the spiral electrode, and in order to collect current from the negative and positive electrodes, an aluminum positive electrode lead is led out from the positive electrode current collector and attached to the battery lid 7, and a nickel negative electrode lead 11 is connected to the negative electrode. It was led out from the current collector and welded to the battery can 5. After that, LiPF6 was placed in a mixed solvent of equal volume of propylene carbonate and diethyl carbonate in the battery can 5.
2.0 ml of a non-aqueous electrolyte dissolved at a rate of 1 mol/l
injected to impregnate the spiral electrode.

Then, the battery can 5 was caulked through an insulating sealing gasket whose surface was coated with asphalt, thereby fixing the battery lid 7 and maintaining the airtightness inside the battery. As described above, a cylindrical nonaqueous electrolyte secondary battery (battery A) having a diameter of 20 mm and a height of 50 mm was produced.

Experimental Examples 2 to 7 Non-aqueous electrolyte secondary batteries (
Batteries B to G) were produced.

[0029] In the case of battery G, non-aqueous electrolyte overflowed onto the battery lid during the process of caulking the battery can during assembly of batteries A to G.

Batteries A to G manufactured in this way
, the charge and discharge cycles were repeated, the discharge capacity at the 10th cycle (initial capacity) and the discharge capacity at the 100th cycle were measured, and the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 10th cycle (initial capacity) was calculated. The capacity retention rate was calculated and the cycle characteristics were evaluated. At this time, the charge/discharge cycle is performed by setting the charging upper limit voltage to 4.1V, charging at a constant current of 1A for 2 hours, and then discharging to a final voltage of 2.75V at a constant load of 6.0Ω. went.

Table 1 shows the initial capacity of each battery, the amount of nonaqueous electrolyte injected, and the amount of nonaqueous electrolyte per 1 mAh when the capacity of the initial capacity is 1000 mAh.

[Table 1]

Further, FIG. 2 shows the relationship between the amount of electrolyte per 1 mAh of battery discharge capacity and the capacity retention rate after 100 cycles.

As is clear from FIG. 2, the batteries (Battery A, Battery B) have an electrolyte volume of less than 3.0 μl per mAh.
However, the capacity retention rate is low and sufficient cycle characteristics cannot be obtained. On the other hand, the amount of electrolyte per 1mAh is 3.0μ
1 or more batteries (Batteries C to Batteries G) have a capacity retention rate of 85
%, good cycle characteristics can be obtained. , among these, the amount of electrolyte per 1 mAh is 3.7 m
Batteries with a capacity of 1 or more (Batteries D to Batteries G) have a capacity maintenance rate of 9
It exceeds 0%, and better cycle characteristics are achieved.

Therefore, from these results, in order to obtain good cycle characteristics in a non-aqueous electrolyte secondary battery using coke as a negative electrode, the amount of non-aqueous electrolyte per 1 mAh of discharge capacity must be 3.0 mAh or more. It was shown that it is necessary. However, if the amount of electrolyte is too large, there will be an inconvenience that the electrolyte will overflow from the battery can when the battery can is caulked. Therefore, from the viewpoint of cycle characteristics and workability, the amount of non-aqueous electrolyte should be 3.0 μl to 5 μl.
．． Preferably, the volume is 0 μl.

Next, in order to make the above results more reliable, 100 non-aqueous electrolyte secondary batteries each having the same configuration as Batteries A to G were prepared, and each non-aqueous electrolyte secondary battery was We investigated the incidence of internal short circuits during charging of secondary batteries. FIG. 3 shows the relationship between the amount of electrolyte per 1 mAh of battery discharge capacity and the internal short circuit occurrence rate.

As can be seen from FIG. 3, when the amount of electrolyte per 1 mAh is 2.5 ml or more, the rate of occurrence of internal short circuit is low, and furthermore, in batteries with the amount of electrolyte per 1 mAh of 3.0 ml or more, internal short circuit occurs. No occurrence occurred, and more favorable results were obtained.

Although the battery of this example was a cylindrical non-aqueous electrolyte secondary battery using a spirally wound electrode body, the present invention is not limited thereto; for example, It may be of a rectangular cylinder type or the like, and can also be applied to a button-type or coin-type non-aqueous electrolyte secondary battery.

[0038]

[Effects of the Invention] As is clear from the above explanation, the nonaqueous electrolyte secondary battery of the present invention uses inexpensive coke as the negative electrode, so manufacturing costs are low and high productivity can be achieved. It will be done. In addition, since the amount of non-aqueous electrolyte is regulated to be an appropriate amount for the discharge capacity, internal short circuits due to precipitation of lithium that cannot be supported on the negative electrode are prevented, and a good charge-discharge cycle is achieved. characteristics can be obtained. Therefore, according to the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery that is excellent in productivity, energy density, and cycle life and is suitable as a power source for devices applied to laptops, personal computers, etc. becomes.

[Brief explanation of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing an example of a non-aqueous electrolyte secondary battery of the present invention.

FIG. 2 is a characteristic diagram showing the relationship between the amount of non-aqueous electrolyte per 1 mAh of discharge capacity of a non-aqueous electrolyte secondary battery and the battery capacity maintenance rate.

FIG. 3 is a characteristic diagram showing the relationship between the amount of non-aqueous electrolyte per 1 mAh of discharge capacity of a non-aqueous electrolyte secondary battery and the internal short circuit occurrence rate.

[Explanation of symbols]

1...Negative electrode 2...Positive electrode 3...Separator 4...Insulating board 5...Battery can 6... Sealing gasket 7...Battery cover 9... Negative electrode current collector 10... Positive electrode current collector 11... Negative lead 12...Positive lead

Claims (1)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode mainly composed of coke, a positive electrode, and a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent, wherein the non-aqueous electrolyte is The amount is 3.0 per 1mAh of battery discharge capacity.
A non-aqueous electrolyte secondary battery characterized in that the electrolyte is at least μl and at most 5.5 μl.