JPH09129232A - Nonaqueous electrolytic secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic secondary battery with excellent high rate discharge characteristic by specifying the volume of specified pores and the mean pore diameter in the layer of a carbon material. SOLUTION: A nonaqueous electrolytic secondary battery has a negative carbon material layer in which the volume of pores having a diameter of 0.1-10μm in the measurement of the pore distribution by a mercury penetration method is 80% or more based on the total pore volume, and the mean pore diameter is 0.5μm or more but 1.5μm or less. The diffusion of lithium ions is not obstructed even in high rate charge/discharge. A nonaqueous electrolytic secondary battery with excellent high rate charge/discharge characteristic can be obtained.

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 an electrode structure used for the negative electrode thereof.

[0002]

2. Description of the Related Art As electronic devices have become smaller and lighter, there has been an increasing demand for smaller and lighter batteries as power sources. Above all, non-aqueous electrolyte secondary batteries using lithium metal for the negative electrode have been greatly expected because of their large theoretical energy density. However, when lithium metal is used for the negative electrode, dendritic lithium (dendrites) is generated during charging, and the dendrites grow and penetrate the separator during repeated charging and discharging of the battery, causing an internal short circuit of the battery, Furthermore, in such extreme cases, there are problems such as the ignition of the battery, and so far it has not been completely solved.

As means for solving this problem, various attempts have been made to use not a lithium metal alone but an alloy of a low melting point metal such as aluminum, lead, indium, bismuth and cadmium and lithium as a negative electrode. As the battery is charged and discharged, the alloy becomes finer as it repeatedly occludes and releases lithium in the alloy, and this fine alloy penetrates the separator, causing short circuit and ignition of the battery, similar to the lithium metal negative electrode. It is hard to say that it was solved because of doing.

On the other hand, as a solution to the above problem,
A battery using carbon for the negative electrode has been proposed. The battery using carbon as the negative electrode of the non-aqueous electrolyte secondary battery was 1986.
27th Battery Symposium Abstracts P. 97 or 198
7th 28th Battery Symposium Abstracts P. 201, vanadium pentoxide, manganese dioxide, and chromium oxide are used as the positive electrode active material, and a method of supporting lithium as the active material in the negative electrode is performed by an electrochemical method outside the battery system. Has been done. Among them, a battery using vanadium pentoxide for the positive electrode and carbon for the negative electrode has been put into practical use as a coin-type battery mainly used for memory backup applications, etc., and as a method for supporting lithium on the negative electrode, lithium metal and carbon are used in the battery. The method of making electrical contact with is taken.

Recently, the summary of the 33rd Battery Symposium in 1992, P.S. At 83, the positive electrode is Li
A cylindrical battery using CoO ₂ and carbon for the negative electrode has been proposed, and it is reported that the capacity of 70% or more of the initial capacity was maintained even after 1200 cycles in deep charge / discharge. At present, this battery system is put to practical use by each company as a 4V class lithium ion secondary battery. A characteristic of this battery system is that the charge / discharge reaction of the negative electrode is a reaction of occluding and releasing lithium ions in the negative electrode carbon, so that lithium does not deposit on the negative electrode during charging, and therefore dendrite does not occur, which is good. It has the advantage that excellent cycle characteristics can be obtained. At the same time, another feature of this battery system is that a lithium-containing oxide called LiCoO ₂ is used for the positive electrode, and the active material lithium is supplied from the positive electrode. Therefore, lithium is supported on the negative electrode by the above-described method. There is no need.

As the positive electrode active material of the 4V class lithium ion secondary battery, not only LiCoO ₂ but also LiNi
O ₂ , LiMn ₂ O ₄ , LiFeO ₂ , or these C
O, Ni, Mn, and Fe in which other metal elements are partially substituted have been studied. Initially, the negative electrode carbon has been mainly focused on so-called amorphous carbon such as coke, pyrolytic carbon, or low temperature fired products of various organic materials. Therefore, recently, highly crystalline carbon, so-called graphite-based carbon has been attracting attention. JP-A-4-1
In 15457, it is said that a graphite material composed of easily graphitizable spherical particles as the negative electrode exhibits excellent characteristics. C ₆ Li, which is an intercalation compound of graphite and lithium, has been known for a long time, and when electrochemically absorbing and desorbing lithium (intercalation, deintercalation), the theoretical capacity is 372 mAh per 1 g of carbon. It shows a very large value. Nonetheless, it was Journal that was not originally adopted as the negative electrode of the lithium-ion secondary battery.
As reported in l of Electrochemical Society 117, No. 2 (1970) P. 222, using propylene carbonate, which is currently widely used as one of the solvent components of the electrolytic solution in the non-aqueous electrolytic solution primary battery, The solvent molecules were decomposed on the surface of graphite, and the intercalation reaction of lithium into graphite was not carried out smoothly. On the other hand, the abstracts of the 59th Electrochemical Conference of 1992, P.A. In 238, this problem is reported to be solved by using ethylene carbonate as a solvent component of the electrolytic solution as a main component. Since then, natural graphite and various artificial graphites have been studied as negative electrodes for lithium ion secondary batteries, and at present, graphite-based negative electrodes have become the mainstream.

On the other hand, the requirements for the negative electrode of the battery include the ability of the carbon itself to occlude and release lithium, and the filling property of how much carbon can be packed in the limited volume of the battery. Not only carbon but also powder is greatly influenced by its shape.
Considering the shape of carbon powder, it is roughly classified into four types: granular, lump-like, scale-like, and fibrous. In a lithium ion battery, usually, a mixed paste of carbon and a binder is applied to both or one side of a metal thin film which is a current collector, and the electrode plate is dried and then appropriately rolled to form an electrode. Among the four shapes, scale-like carbon has the best filling property. That is, even if the electrode plate is dried and rolled, the shape of the carbon particles does not change, and the carbon of the other three types is simply densely packed. Since it is oriented in the direction of, the closeness becomes higher and the filling property becomes higher. Therefore, it can be said that natural or artificial graphite having a scaly powder shape is the most excellent carbon negative electrode material from the viewpoint of lithium occlusion / release capacity and carbon powder filling ability.

However, in the case of natural graphite, it is necessary to control the material by the variation of the material due to the difference in the place of production or the special treatment for removing a large amount of impurities. It can be said that flake-shaped artificial graphite is the most excellent.
Typical scale-like artificial graphite is obtained by graphitizing coal pitch or petroleum pitch, and includes artificial graphite manufactured by Lonza or Nippon Graphite.

[0009]

However, there is one problem to be solved when using flake-shaped artificial graphite as a negative electrode material. Certainly, scaly graphite has a high degree of orientation, so the packing property increases by rolling, but on the contrary, the packing property increases too much, and the voids in the electrode are reduced, and when the electrode of the battery is formed, the diffusion of lithium ions And the capacity during high-rate charging / discharging decreases. To solve this,
In Japanese Patent Laid-Open No. 6-267590, the pore size of the negative electrode is 0.1.
It has been proposed that the volume of pores in the range of -10 μm be 80% or more of the total volume of pores. However, even with this configuration, if the average diameter of the pores is less than 0.5 μm or greater than 1.5 μm, the high rate charge / discharge characteristics of the secondary battery for practical use are not satisfied.

The present invention is intended to solve such a problem, and secures appropriate pores between carbon particles required for diffusion of lithium ions, thereby making it difficult for capacity reduction to occur at high rate charge / discharge. It is a thing.

[0011]

Means for Solving the Problems A carbon material capable of inserting and extracting lithium ions is used for a negative electrode, and a pore diameter of 0.1 is measured in a negative electrode mixture by a pore size distribution measurement by a mercury penetration method.
The volume occupied by pores in the range of 10 μm is 80% or more with respect to the total pore volume, and the average pore diameter is 0.5.
The present invention provides a non-aqueous electrolyte secondary battery having a thickness of from 1 μm to 1.5 μm.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION When flake graphite powder is applied on a current collector, the graphite crystal has a property that its C axis is oriented parallel to the vertical direction of the current collector. Due to the orientation of the graphite crystals, the pores for diffusion of lithium ions in the direction perpendicular to the current collector become small, and the capacity decreases at the time of high-rate charge / discharge.

In the negative electrode constitution of the present invention, the average diameter of pores in the negative electrode mixture after coating is 0.5 to 1.5 μm by changing the water content of the pasty negative electrode carbon material before coating. And Lithium ions are more likely to diffuse in the direction perpendicular to the current collector, and the capacity is less likely to decrease during high-rate charge / discharge.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a vertical sectional view of the cylindrical battery used in this embodiment. In the figure, 1 is a battery case formed by processing a stainless steel plate resistant to organic electrolyte, 2 is a sealing plate provided with a safety valve, and 3 is an insulating packing. Reference numeral 4 denotes an electrode group, in which a positive electrode and a negative electrode are spirally wound a plurality of times via a separator and housed in the battery case 1. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2. A negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode plate group 4, respectively. Less than,
The positive and negative plates will be described in detail.

The positive electrode is a mixture of Li ₂ CO ₃ and Co ₃ O ₄ ,
100 parts by weight of LiCoO ₂ powder synthesized by firing at 900 ° C. for 10 hours, 3 parts by weight of acetylene black and 7 parts by weight of fluororesin binder were mixed and suspended in an aqueous carboxymethylcellulose solution to form a paste. . Apply this paste to both sides of 0.03mm thick aluminum foil,
After drying, it is rolled to a thickness of 0.18 mm, a width of 38 mm, and a length of 2.
The plate was 40 mm.

The negative electrode is graphite powder (average particle size 17.8 μm).
m, d002 = 3.36 °, Lc = 1000 °, BET
100 parts by weight of the surface area by the method of 8.2 m ² / g) and 2 parts by weight of styrene / butadiene rubber were mixed and suspended in 100 parts by weight of a 1% by weight carboxymethylcellulose aqueous solution to form a paste having a viscosity of 1300 cP. And apply this paste on both sides of copper foil of 0.02mm thickness,
After drying, it is rolled to a thickness of 0.19 mm, a width of 40 mm, and a length of 2.
The plate was 80 mm.

Aluminum leads are attached to the positive electrode plate, and nickel leads are attached to the negative electrode plate. The leads are spirally wound with a polyethylene porous film having a thickness of 0.025 mm, a width of 45 mm, and a length of 730 mm interposed therebetween. It was delivered to a battery case with a diameter of 14.0 mm and a height of 50 mm.
In the electrolytic solution, 1 mol / liter of LiPF ₆ was dissolved in a solvent prepared by mixing ethylene carbonate (hereinafter, EC), diethyl carbonate (DEC), and methyl propionate (hereinafter, MP) in a volume ratio of 30:50:20. A battery A was used, which was then injected and sealed to obtain a battery A. Here, the battery specifications were a nominal voltage of 3.6 V and 550 mAh.

When the negative electrode was produced, the same as above except that the water content was changed and the paste viscosity was changed as shown in Table 1 by adding water to the paste-like negative electrode carbon material before coating. Then, a negative electrode plate and a battery were produced, and these were named batteries B to E.

[0020]

[Table 1]

The negative plates used in batteries A to E are a to e,
It was cut into a width of 20 mm and a length of 100 mm, and the pore distribution was measured by the mercury porosimetry method. The pore distribution diagram is shown in (FIG. 2).
The average pore diameter increased as the viscosity of the paste before coating decreased. In addition, the volume percentage of pores in the diameter range of 0.5 to 10 μm is determined by any of the negative electrode plates a to e.
It was 80% or more in the carbon material layer.

The average pore diameter estimated from the pore distribution of each negative electrode plate is summarized in (Table 1). A low rate discharge test (0.2 C discharge; 110 mA) and a high rate discharge test (2 C discharge; 1100 mA) were performed using the batteries A to E. The charging / discharging conditions were as follows: charging at a current of 110 mA at 20 ° C. to a voltage of 4.2 V and discharging to a voltage of 3.0 V.

The results are also shown in (Table 1). 2C / in the table
0.2C is the capacity retention rate for 0.5 hour rate discharge to 5 hour rate discharge. As shown in (Table 1), when the average pore diameter of the pore distribution is 0.5 μm or more, sufficient voids for diffusion of lithium ions can be secured between graphite particles, and 82% or more at high rate discharge. Capacity could be maintained. However, when the average pore diameter of the pore distribution was larger than 1.5 μm, the state of electrical bonding between the graphite particles was deteriorated, the electron conductivity of the negative electrode was lowered, and the capacity was lowered.

If the average diameter of the pore distribution is less than 0.5 μm, it becomes impossible to secure the pores required for the diffusion of lithium ions between the graphite particles, and the capacity retention rate during discharge is 32%. Remarkably decreased.

Therefore, the average pore diameter of the pore distribution between the graphite particles in the negative electrode carbon material layer is 0.5 μm or more and 1.5 μm or more.
m or less. More preferably, 1.
It is 0 μm or more and 1.5 μm or less.

In this example, the flake artificial graphite powder was used as the graphite powder, but the same results were obtained with the flake natural graphite powder and spherical graphite powder. Further, regarding the solvent of the non-aqueous electrolyte, in this embodiment, EC and D
A mixed solvent of EC and MP was used, but carbonic acid esters such as propylene carbonate (PC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC), ethers such as tetrahydrofuran (THF), etc., alone or in combination. Similar effects were obtained when a mixture of more than one type was used.

[0027]

INDUSTRIAL APPLICABILITY As described above, the non-aqueous electrolyte secondary battery of the present invention has pores in the range of 0.1 to 10 μm in diameter measured by mercury porosimetry in the negative electrode carbon material layer. It occupies 80% or more of the total pore volume and has an average pore diameter of 0.5 μm or more and 1.5 μm or less, so that diffusion of lithium ions is not hindered even at high rate charge / discharge. . Therefore, a non-aqueous secondary battery having an extremely high rate charge / discharge characteristic can be obtained, which is very useful for practical application of the non-aqueous secondary battery.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery used in this example.

FIG. 2 is a diagram showing pore distribution in a negative electrode carbon material layer.

[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

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hide Koshina 1006 Ojidoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A positive electrode using a lithium-containing oxide, and a negative electrode provided with a layer made of a carbon material capable of inserting and extracting lithium ions, wherein a layer made of the carbon material is formed by a mercury intrusion method. Pore distribution measurement, diameter 0.1-1
The volume of pores in the range of 0 μm is 80% or more of the total pore volume, and the average pore diameter is 0.5 μm.
A non-aqueous electrolyte secondary battery having a size of not less than 1.5 μm. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon material capable of inserting and extracting lithium ions is flake graphite.