JP3440638B2 - Non-aqueous electrolyte secondary battery - Google Patents

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 improvement of a negative electrode.

[0002]

2. Description of the Related Art As electronic devices have become smaller and lighter, there has been a growing demand for smaller and lighter batteries as their 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, there is a problem that dendritic lithium (dendrites) is generated during charging, and this dendrite grows and penetrates the separator during repeated charging and discharging of the battery, causing an internal short circuit in the battery. , To the present, it has not been completely resolved.

As a 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 a lithium as a negative electrode, but also in this case. 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 and short-circuits the battery and causes a rapid temperature rise like the lithium metal negative electrode. It is hard to say that it was solved because it occurred.

On the other hand, as a solution to the above problems,
Batteries using carbon for the negative electrode have been proposed. A battery using carbon as a negative electrode of a non-aqueous electrolyte secondary battery is 1986.
Annual Meeting of the 27th Battery Symposium P. 97 or 198
7th Annual 28th Battery Symposium Summary P. 201, vanadium pentoxide, manganese dioxide, or chromium oxide is used as a positive electrode active material, and lithium as an active material is supported on carbon of a negative electrode by an electrochemical method outside the battery system. It is said that. 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-shaped 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 is adopted.

[0005] Recently, the 1993 33rd Battery Symposium Abstracts P. In 83, as a power source for electronic equipment, Li is used for the positive electrode.
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. The 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 carbon of the negative electrode, and lithium is not deposited on the negative electrode due to 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 the lithium-containing oxide LiCoO ₂ is used for the positive electrode and the active material lithium is supplied from the positive electrode.
It is not necessary to support lithium on the negative electrode by a special treatment method.

Not only the above LiCoO ₂ but also LiNi is used as the positive electrode active material of the 4V class lithium ion secondary battery.
O ₂ , LiMn ₂ O ₄ , LiFeO ₂ , or these C
The materials in which o, Ni, Mn, and Fe are partially replaced with other metal elements have been studied so far. Also, as carbon as the negative electrode material, initially, so-called amorphous carbon such as coke, pyrolytic carbon, or low temperature fired products of various organic substances has been mainly studied, but absorption and release of lithium as the active material. Recently, highly crystalline carbon, so-called graphite-based carbon, has been attracting attention from the viewpoint of ability.

In Japanese Patent Laid-Open No. 4-115457, it is said that a graphite material composed of easily graphitizable spherical particles as a 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 1 g of carbon.
On the other hand, it shows a very large value of 372 mAh. Nevertheless, it was the Journal of Electrochemical that was not initially adopted as a negative electrode for lithium-ion secondary batteries.
As reported in Society 117, No. 2 (1970) p. 222, when propylene carbonate, which is widely used as one of the solvent components of electrolytes in non-aqueous electrolyte primary batteries, is used, its solvent molecule Was decomposed on the surface of graphite, and the intercalation reaction of lithium into graphite was not carried out smoothly. In contrast, the 1992 59th Electrochemical Convention Abstracts P. 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. Is not limited to carbon, but if it is powder, it is greatly influenced by its shape.

Considering the shape of carbon powder,
It is roughly divided into four types: block, scale, and fibrous. In a lithium-ion battery, usually, a mixed paste of carbon and a binder is applied to both sides 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.
Of the above four shapes, scale-like carbon has the best filling property. That is, in the other three types of carbon, the shape of the particles does not change even if the electrode plate is dried and then rolled, and the particles are simply densely packed, but in the scale-like carbon, the particles are oriented in the same direction by rolling. Therefore, the tightness is higher and the filling property is also higher. Therefore, it can be said that natural graphite or artificial graphite having a scaly powder shape is the most excellent material for the carbon negative electrode material from the viewpoint of the ability to insert and extract lithium and the filling property of carbon powder.

In particular, natural graphite can be used as long as it has been treated with a purity of 99% or more without any difference due to the difference in the place of production. Further, as a typical flake-shaped artificial graphite, coal pitch or petroleum pitch is graphitized, and artificial graphite manufactured by Lonza Co. or Nippon Graphite Co., Ltd. may be mentioned.

[0011]

However, when not only scaly graphite but also graphite powder having a uniform particle size is applied to achieve a constant packing density, the gap between the graphite particles is unreasonable. As a result, there will be a high packing density and a low packing density. For this reason, the high packing density hinders the movement of lithium ions during charging and discharging, and the high rate discharge characteristics are significantly deteriorated.

When flake-shaped natural graphite or artificial graphite is used as the negative electrode material, the orientation is large, so that the filling property is improved by rolling, but the filling property is too high and the voids are reduced. , It hinders the movement of lithium ions due to charge and discharge. In addition, most of these shapes have an ab plane area of the crystal structure and are involved in the electrochemical reaction.
Since the area in the axial direction is small, there is a problem that the low temperature discharge characteristic and the high rate discharge characteristic are poor.

Therefore, when the particles of graphite powder are made small and the area in the c-axis direction is made large in order to increase the electrode reaction area, the above characteristics are improved, but conversely, when the battery becomes hot. The lithium ion-occluded graphite and the electrolytic solution rapidly cause an exothermic reaction, resulting in poor safety.

[0014] The present invention has been made to solve the above problems, by adjusting the arrangement of the graphite powder particles were mixed different graphite powder particle size, to improve the high rate discharge characteristics, and excellent safety Another object is to provide a non-aqueous electrolyte secondary battery.

[0015]

In order to solve these problems, the present invention is a non-aqueous electrolyte secondary battery comprising a negative electrode made of graphite and a positive electrode made of a lithium-containing oxide,
At least 2 graphite powders having different average particle diameters as the negative electrode
One kind of graphite powder (a) has an average particle size of more than 10 μm and less than 20 μm, and the other one kind of graphite powder (b) has an average particle size. Particle size is greater than 1 μm and 10 μm
Smaller than the average particle size of the graphite powder (a) by 4.3 μm or more , and a graphite mixed material
Of which graphite powder (b) accounts for 10 to 60% by weight.
There is . In particular, the graphite powder is preferably scale-shaped artificial graphite or natural graphite.

[0016]

In the negative electrode structure of the present invention, since graphite particles having different particle diameters are mixed and used, when graphite particles having a small particle diameter enter the gap between the particles of large graphite and the particles have a constant packing density. However, since a uniform packing density can be obtained by adjusting the arrangement of the graphite powder particles, the movement of lithium ions can be carried out uniformly without hindrance.

Further, in order to improve the high rate discharge characteristics, simply Unlike to use small particle size graphite, the reaction area of the electrode was adjusted by mixing the different graphite particle sizes, the battery is a high temperature It is possible to provide a safe non-aqueous electrolyte secondary battery that does not cause a sudden exothermic reaction that occurs when the battery becomes full.

[0018]

【Example】

[Embodiment 1] The present invention will be described in detail below with reference to the drawings shown in the embodiments.

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 plate group, in which the positive electrode and the negative electrode are spirally wound a plurality of times with a separator interposed therebetween 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. Insulating rings 7 are provided on the upper and lower portions of the electrode plate group 4, respectively. Below, positive,
The negative electrode plate and the like will be described in detail.

The positive electrode is a mixture of Li ₂ CO ₃ and Co ₃ O ₄ ,
3 parts by weight of acetylene black and 7 parts by weight of a fluororesin binder were mixed with 100 parts by weight of LiCoO ₂ powder synthesized by firing at 900 ° C. for 10 hours, and suspended in an aqueous carboxymethyl cellulose 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 = 3.36Å, Lc = 1000Å, BET
100 parts by weight of surface area by the method = 8.2 m ² / g) was mixed with 5 parts by weight of styrene / butadiene rubber and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. Then, this paste is applied to both sides of a copper foil having a thickness of 0.02 mm, dried and rolled to have a thickness of 0.19 mm and a width of 40 mm.
mm and a length of 280 mm.

Then, a lead made of aluminum is attached to the positive electrode plate and a lead made of nickel is attached to the negative electrode plate, and the lead is spirally wound through a polyethylene porous film having a thickness of 0.025 mm, a width of 45 mm and a length of 730 mm. It was delivered to a battery case with a diameter of 14.0 mm and a height of 50 mm.
The electrolytic solution contained 1 mol / liter of a solvent prepared by mixing ethylene carbonate (hereinafter abbreviated as EC) and diethyl carbonate (hereinafter abbreviated as DEC) methyl propionate (hereinafter abbreviated as MP) at a volume ratio of 30:50:20. A solution of LiPF ₆ was used, which was poured and then sealed to prepare a battery, which was named battery A. Here, the battery specifications were a nominal voltage of 3.6 V and a nominal capacity of 550 mAh.

Further, as shown in (Table 1), the average particle size is 1
7.8 μm (d002 = 3.36Å, Lc = 1000
Å, graphite powder having a surface area by the BET method of 8.2 m ² / g) and an average particle diameter of 2.6 μm (d002 = 3.36 Å, L
c = 1000Å, surface area by BET method = 23.2 m ²
Batteries B to K were produced in the same manner as above except that the mixing ratio of the graphite powder of / g) was changed.

A high rate discharge test (2C discharge: 30 minutes rate) was performed using these batteries A to K. Charge and discharge conditions are as follows: charge current 110 mA, charge end voltage 4.2 V, discharge current 1100 mA, discharge end voltage 3.0 at an ambient temperature of 20 ° C.
I went as V. Further, these batteries were put into a charged state under the same charge condition, and then an external short circuit test was conducted. Table 1 shows the results of these tests.

[0025]

[Table 1]

From (Table 1), it was found that the 2C discharge capacity was remarkably increased by mixing 10% by weight or more of the graphite powder having a small average particle size, and the 2C discharge capacity became almost constant when mixed by 40% by weight. However, in a safety test due to an external short circuit, a rapid temperature rise did not occur up to 40% by weight or less of graphite powder having a small average particle size, but when 40% by weight or more was mixed, the temperature rapidly increased with the ratio. It got bigger.

Therefore, the ratio of the small particle size is 10
It is preferably ˜40% by weight.

Comparative Example Next, as a comparative example, a battery was prepared in the same manner as in the above-mentioned example except that a single graphite powder having an average particle size varied as shown in (Table 2) was used for the negative electrode. Then, these are referred to as batteries L to N.

Then, using these batteries L to N, a high rate discharge test and an external short circuit test were conducted in the same manner as in Example 1. Table 2 shows the results of these tests.

[0030]

[Table 2]

From Table 2, when only the average particle diameter of the graphite powder is changed, the 2C discharge capacity is increased by decreasing the particle diameter and increasing the surface area. The rate of sudden temperature rises increased. For this reason, the safety was lowered, and a battery excellent in both high rate discharge characteristics and safety could not be obtained.

Further, the effect of mixing graphite powder having a small particle size will be described in detail. (FIG. 2) shows the 2C discharge capacity and the ignition rate at the time of external short circuit with respect to the surface areas of the batteries A to N of the example and the comparative example.

From FIG. 2, it can be confirmed that the safety by the external short circuit test depends on the surface area of the graphite powder used for the negative electrode. However, the 2C discharge capacity does not depend only on the surface area of the graphite powder, but simply by mixing the graphite powder having a large average particle diameter with the graphite powder having a small average particle diameter as in the battery of this example, the graphite powder having A discharge capacity larger than that of the battery of Comparative Example in which the average particle diameter was changed was obtained.

This is because in a high rate discharge such as 2C discharge, not only the reaction area but also whether or not lithium ions can be easily moved affects.

When graphite powder having a uniform particle size is applied to the negative electrode, a gap is created between the graphite particles, and if the packing density is kept constant, the gap will be forcedly packed, and the filling will occur. There are high density areas and low density areas. For this reason, the high packing density hinders the movement of lithium ions during charging and discharging, and the high rate discharge characteristics are significantly deteriorated.

However, when graphite particles having different particle diameters are mixed and used, graphite having a small particle diameter enters the gap between the large graphite particles and the particles, and a uniform packing density is obtained even when the packing density is constant. Therefore, the movement of lithium ions during charge / discharge is performed uniformly without being hindered.

However, when graphite powder having an average particle size of 20 μm or more is used for the negative electrode, it is difficult to coat and roll it on the current collector because the particle size is too large.
When graphite having a thickness of 10 μm or less is used, a rapid temperature rise is likely to occur at the time of an external short circuit.
It is desirable to use at least one type of graphite having a size of μm.

On the other hand, it is difficult to control the average particle size to 1 μm or less, and even if such an average particle size is obtained by pulverization, the yield is poor and the cost is disadvantageous. It is desirable to use at least one type of 10 μm graphite.

In this embodiment, the average particle size is 17.8.
A small particle graphite powder having an average particle diameter of 2.6 μm was mixed with the graphite powder of μm, but the average particle diameter of the small particle graphite powder may be 5.3 μm or 7.8 μm. Also,
The same effect can be obtained when the average particle size of the large particle graphite powder is 12.1 μm.

In this embodiment, LiCoO is used as the positive electrode.
₂ was used, but Li that is a compound containing lithium ions
NiO ₂ and LiMn ₂ O _{4 and} further Co, Ni, or part of Mn may be replaced with other elements such as Co, Mn, and F.
The same effect can be obtained when a complex compound substituted with e, Ni, or the like is used. The composite oxide, for example, using a carbonate or oxide of lithium or cobalt as a raw material,
It can be easily obtained by mixing and firing depending on the target composition, and can be similarly synthesized even when other raw materials are used. Usually, the firing temperature is set between 650 ° C and 1200 ° C.

Although a conventionally known electrolyte can be used as the electrolyte, when a graphite material is used for the negative electrode, propylene carbonate (hereinafter abbreviated as PC) tends to cause a decomposition reaction during charging and generate gas. Therefore, it can be said that ethylene carbonate (EC), which is a similar cyclic carbonate, used in the present example is hardly accompanied by side reactions as in the case of PC. However, since EC has a very high melting point and is a solid at room temperature, it is difficult to use it as a single solvent. Therefore, it is preferable to use a mixed solvent of 1,2-dimethoxyethane, diethyl carbonate (DEC), which is a low-melting point and low-viscosity solvent, and an aliphatic carboxylic acid ester such as methyl propionate (MP). . Although lithium hexafluorophosphate was used as the Li salt dissolved in these solvents in the present embodiment, conventionally known ones such as lithium borofluoride, lithium hexafluoroarsenate, and lithium perchlorate were used. Even in this case, the same effect can be obtained.

[0042]

As is apparent from the above description, the present invention uses at least 2 graphite powders having different average particle diameters in the negative electrode.
One kind of graphite powder (a) has an average particle size of more than 10 μm and less than 20 μm, and the other one kind of graphite powder (b) has an average particle size. Particle size is greater than 1 μm and 10 μm
Less than, and said graphite powder rather small than 4.3μm than the average particle diameter of (a), and black graphite admixture in
Since the proportion of the lead powder (b) is 10 to 60% by weight , a uniform packing density can be obtained even when the packing density is constant, so that the migration of lithium ions is not hindered. It is possible to provide a non-aqueous electrolyte secondary battery excellent in high rate discharge characteristics and safety.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of non-aqueous electrolyte secondary batteries in Examples and Comparative Examples of the present invention.

FIG. 2 is a diagram showing a relationship between a surface area of 2 C discharge capacity and an ignition rate at an external short circuit.

[Explanation of symbols]

1 battery case 2 Seal plate 3 insulating packing 4 electrode group 5 Positive lead 6 Negative electrode lead 7 Insulation ring

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiromi Nagata 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Akikatsu Morita, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-7-37618 (JP, A) JP-A-8-180873 (JP, A) JP-A-8-83610 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/00-4/62

Claims (3)

(57) [Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode made of graphite, a positive electrode made of a lithium-containing oxide, and a non-aqueous electrolyte, wherein the negative electrode is at least two types of graphite powders having different average particle sizes. The mixed graphite mixture is used, and one of them has an average particle diameter of 10 μm.
The average particle size of the other one type of graphite powder (b) is larger than 1 μm and smaller than 20 μm.
Less than 0 .mu.m, and, rather small than 4.3μm than the average particle diameter of the graphite powder (a), and graphite mixed material
Of which graphite powder (b) accounts for 10 to 60% by weight.
A non-aqueous electrolyte secondary battery. 2. The graphite powder (b) has an average particle diameter of 2.6 μm.
The non-aqueous electrolyte secondary battery according to claim 1, which has a size of m or more and 7.8 μm or less. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the graphite powder is flake-shaped artificial graphite or natural graphite.