JPH08180878A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To improve the cycle characteristic of a lithium secondary battery without losing mass productivity. CONSTITUTION: This lithium secondary battery uses a lithium-contained compound as a positive electrode active material and a carbonaceous material capable of doping and dedoping lithium as a negative electrode active material. In such a lithium secondary battery, a metal fiber sheet material is used as a negative electrode current collector 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery using a lithium-containing compound as a positive electrode active material and a carbonaceous material capable of doping and dedoping lithium as a negative electrode active material. More specifically, the present invention relates to a lithium secondary battery that achieves higher cycle characteristics by using a metal fiber sheet material for a negative electrode current collector.

[0002]

2. Description of the Related Art With the progress of electronic technology in recent years, small portable electronic devices such as VTRs with built-in cameras, mobile phones, laptop computers, etc. have been developed. There is a strong demand for the development of energy density secondary batteries.

As a secondary battery that meets such demands,
In recent years, lithium non-aqueous electrolyte secondary batteries have been developed and some of them have been put on the market.

As a negative electrode active material for such a lithium non-aqueous electrolyte secondary battery, metallic lithium is not used,
A carbonaceous material that can be doped and dedoped with lithium is used, and a strip-shaped copper foil having a thickness of 8 to 30 μm is used as a negative electrode current collector. This improves safety, suppresses deterioration of cycle characteristics, reduces self-discharge, and suppresses memory effect as compared with the case where metallic lithium is used for the negative electrode.

Further, as the positive electrode active material, LiCoO ₂
Using a lithium composite oxide with a high redox potential, such as
A strip-shaped aluminum foil having a thickness of 10 to 30 μm is used as the positive electrode current collector. This improves the output voltage of the secondary battery.

As the non-aqueous electrolyte, a high dielectric constant solvent such as propylene carbonate or ethylene carbonate and 1,
LiClO ₄ , LiAsF ₆ , LiP in a mixed solvent with a low-viscosity solvent such as 2-dimethoxyethane (DME).
F ₆ , LiBF ₄ , LiCF ₃ SO ₃ or LiN
A solution in which a supporting electrolyte such as (CF ₃ SO ₂ ) ₂ is dissolved is used.

By the way, a secondary battery for use as a power source for portable electronic equipment has excellent heavy load characteristics such that it can be discharged with a relatively large current or can be rapidly charged, and can be used outdoors in winter. Since it is also expected to be used, it is desired that it has excellent low-temperature characteristics. However, the non-aqueous electrolyte used in a lithium secondary battery has a conductivity of 1 compared to an aqueous electrolyte that has been used in nickel cadmium secondary batteries and lead batteries that have been used as secondary batteries for a long time. Since there is only about / 40, there is a problem that the heavy load characteristic and the low temperature characteristic become insufficient.

To solve this problem, an electrode current collector of 50 μm
It has been proposed to improve the heavy load characteristics and the low temperature characteristics without deteriorating the discharge capacity by thinning the electrodes by using a metal foil having a thickness of m or less, thereby increasing the effective reaction area of the battery. (Battery Technology, 3 , page 100 (1991)).

When the electrode current collector made of this thin metal foil is applied to a lithium non-aqueous electrolyte secondary battery, the electrode body used has a so-called winding structure. Such a spirally wound electrode body is usually manufactured by coating a positive electrode mixture containing a positive electrode active material on both sides of a metal thin film electrode current collector and a negative electrode on both sides of another metal thin film electrode current collector. It is prepared by winding a negative electrode coated with a negative electrode mixture containing an active material and a separator such as a porous polypropylene film around the negative electrode.

[0010]

However, the lithium non-aqueous electrolyte secondary battery using such a metal thin film electrode current collector is still deteriorated in cycle characteristics, that is, the battery capacity decreases as charging and discharging are repeated. It was a problem. This is probably because lithium is not uniformly doped or dedoped into the carbonaceous material of the negative electrode during charge / discharge of the battery.

To solve this problem, it is proposed that the amount of active material on the outer peripheral side of the current collector of the negative electrode or the positive electrode of the wound electrode body is made larger than that on the inner peripheral side to improve cycle characteristics. (JP-A-4-12471). However, it is necessary to make the thickness of the active material layer on the outer peripheral side thicker than the thickness of the active material layer on the inner peripheral side, which complicates the coating operation of the active material and reduces the mass productivity of the battery.

The present invention is intended to solve the above problems of the prior art, and uses a lithium-containing compound as a positive electrode active material and a carbon material capable of doping and dedoping lithium as a negative electrode active material. An object of the present invention is to improve cycle characteristics of a lithium secondary battery by means suitable for mass production.

[0013]

The present inventor has found that the above object can be achieved by using a metal fiber sheet material as a negative electrode current collector, and has completed the present invention.

That is, according to the present invention, in a lithium secondary battery using a lithium-containing compound as a positive electrode active material and a carbonaceous material capable of doping and dedoping lithium as a negative electrode active material, a metal is used as a negative electrode current collector. Provided is a lithium secondary battery using a fiber sheet material.

The present invention will be described in detail below.

In the present invention, a metal fiber sheet material is used as the negative electrode current collector. Thereby, the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved without sacrificing mass productivity. The reason for this will be described with reference to FIG. 1 (cross-sectional view of the negative electrode in the present invention) and FIG. 2 (cross-sectional view of a conventional negative electrode).

That is, as shown in FIG. 2, an outer peripheral active material layer 2 and an inner peripheral active material layer 3 containing a carbonaceous material capable of being doped and dedoped with lithium on both surfaces of a negative electrode current collector 1 made of a metal foil. In the conventional negative electrode 20 formed with and, the active material layers 2 and 3 on both surfaces thereof are completely separated by the negative electrode current collector 1. Therefore, it is considered that the doping / dedoping of lithium with respect to the negative electrode is not uniform in the outer peripheral active material layer 2 and the inner peripheral active material layer 3, and the cycle characteristics are deteriorated.

On the other hand, in the case of the negative electrode 10 of the present invention, as shown in FIG. 1, the negative electrode active material also enters the inside of the negative electrode current collector 1 made of a metal fiber sheet material. Outer peripheral active material layer 2 and inner peripheral active material layer 3
And are integrated. Therefore, it is considered that the doping and dedoping of lithium with respect to the negative electrode is performed uniformly on the outer peripheral active material layer 2 and the inner peripheral active material layer 3, and the cycle characteristics are improved.

Further, by using the metal fiber sheet material as the negative electrode current collector, lithium can be uniformly doped / dedope in the outer peripheral active material layer 2 and the inner peripheral active material layer 3, so that the metal fiber can be used. It is not necessary to change the thicknesses of the outer peripheral active material layer 2 and the inner peripheral active material layer 3 formed on both sides of the sheet material. Therefore, the negative electrode can be manufactured with high mass productivity as in the conventional case without requiring complicated operations.

The metal fiber sheet material used in the present invention is not limited as long as a metal that does not form an alloy with lithium is used. For example, nickel fiber,
A material obtained by directly molding a metal or alloy fiber such as copper fiber or stainless fiber into a sheet shape, or a non-woven fabric obtained by dispersing these metal fibers in an organic binder, molding the dispersion into a sheet shape and sintering. Sheet material can be used. Further, a sheet material obtained by plating such a sheet material with a metal or the like having better conductivity than its constituent metal, for example, a material obtained by nickel-plating a stainless fiber sheet material can also be used. .
Further, a sheet material obtained by plating a carbon fiber sheet with a metal such as nickel, or a sheet material made of synthetic fiber or glass fiber and subjected to electroless nickel plating can be used.

The thickness and porosity of the metal fiber sheet material of the present invention, the fiber diameter and fiber length of the metal fiber to be used, the type of plating metal, the plating method, the plating thickness, etc., depend on the electrolytic solution of the battery. It can be appropriately determined according to the type of organic solvent or electrolyte, the type of active material of the positive electrode or the negative electrode, and the purpose of use of the battery.

In the present invention, a carbonaceous material capable of doping and dedoping lithium is used as the negative electrode active material. Examples of such carbonaceous materials include pyrolytic carbons, cokes (pitch cokes, needle cokes, petroleum cokes, etc.), graphites, glassy carbons, organic polymer compound fired products (phenolic resins, furan resins, etc.). Carbonized by firing at an appropriate temperature), carbon fiber, activated carbon and the like. Among them, the (002) plane spacing is 3.70 angstroms or more and the true density is 1.70 g /
less than cc and 70 by differential thermal analysis in air flow
A carbonaceous material having no exothermic peak at 0 ° C or higher can be preferably used.

In the present invention, a lithium-containing compound that can be charged and discharged is used as the positive electrode active material. Preferably, a lithium composite oxide mainly composed of Li _x MO ₂ (wherein M represents one or more kinds of transition metals, and 0.05 ≦ x ≦ 1.10) can be used. This allows
It is possible to improve the energy density of the battery.
It is preferable to use at least one of Co, Ni, and Mn as the transition metal M, and as such a lithium composite oxide, LiCoO ₂ , LiNiO ₂ ,
_{LiNi y Co 1-y O 2} , LiMn 2 O 4 , etc. can be preferably exemplified.

The above lithium composite oxide includes lithium carbonate, nitrate, oxide or hydroxide, and carbonate, nitrate such as cobalt, manganese or nickel,
It can be prepared by pulverizing and mixing an oxide or a hydroxide according to a desired composition and firing the mixture in an oxygen atmosphere at a temperature range of 600 to 1000 ° C.

In the present invention, a non-aqueous electrolyte solution prepared by dissolving a supporting electrolyte in a non-aqueous organic solvent may be filled between both electrodes. As the organic solvent for forming the non-aqueous electrolyte, it is possible to use an organic solvent such as conventionally used in lithium non-aqueous electrolyte secondary batteries, for example, propylene carbonate, ethylene carbonate, 1, Two
-Dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyl-tetrahydrofuran, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate and the like can be used, and two or more kinds may be mixed and used.

The supporting electrolyte is an electrolyte used as a supporting electrolyte of a conventional lithium non-aqueous electrolyte secondary battery, for example, LiClO ₄ , LiAsF ₆ , Li.
BF ₄ , LiPF _{6 and the} like can be used alone or in admixture of two or more.

The non-aqueous electrolytic solution used in the present invention is not limited to a liquid state and may be a solid state. In this case, a known solid electrolyte can be used.

The shape of the lithium secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape and a button shape can be used. Further, when the battery is a sealed type, in order to ensure higher safety, it is preferable to provide a device that shuts off the current in accordance with an increase in the internal pressure of the battery when an abnormality occurs during overcharging.

[0029]

In the lithium secondary battery of the present invention, a metal fiber sheet material is used as the negative electrode current collector. For this reason,
The negative electrode active material layers on both surfaces of the negative electrode current collector can be integrated. Therefore, it becomes possible to uniformly dope and dedope lithium. Moreover, it is possible to manufacture the negative electrode without lowering the mass productivity.

[0030]

EXAMPLES The present invention will be described in detail below with reference to the following examples and FIG. Here, FIG. 3 is a cross-sectional view of a battery of an example or a comparative example.

Examples 1 to 5 and Comparative Examples 1 to 2 (1) Preparation of Negative Electrode In order to obtain a negative electrode active material, first, petroleum pitch was used as a starting material, and 10 to 20% of a functional group containing oxygen was introduced therein. (Oxygen crosslinking) and then firing at 1000 ° C. in an inert gas stream to obtain a non-graphite carbonaceous material having properties close to those of a glassy carbon material.

When X-ray diffraction of this non-graphite carbonaceous material was conducted, the spacing between (002) planes was 3.76 angstroms and the true density was 1.58 g / cc.

The obtained non-graphite carbonaceous material was pulverized into a powder having an average particle size of 10 μm, and 90% by weight of this powder was mixed with 10% by weight of polyvinylidene fluoride (PVDF) as a binder to prepare a negative electrode mixture. Was prepared. This negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to form a slurry.

Next, the obtained slurry is uniformly applied to both sides of a 20 μm thick negative electrode current collector 31a obtained by forming nickel fibers into a sheet together with a binder and sintering, and after drying, rolls A band-shaped negative electrode 31 was obtained by compression molding with a pressing machine.

(2) Preparation of Positive Electrode To obtain a positive electrode active material, first, lithium carbonate and cobalt carbonate were mixed so that the Li / Co ratio was 1 and calcined in air at 900 ° C. for 5 hours. LiCoO ₂ (according to the JCPDS card as a result of X-ray diffraction measurement) was obtained.

91 wt% of the obtained LiCoO ₂ , 6 wt% of graphite as a conductive agent, and 3 wt% of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a positive electrode mixture. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to form a slurry. Next, this slurry was uniformly applied to both surfaces of a 20 μm-thick strip-shaped aluminum foil as the positive electrode current collector 32a, dried, and compression-molded with a roll press machine to obtain a strip-shaped positive electrode 32.

(3) Preparation of lithium non-aqueous electrolyte secondary battery The obtained positive electrode 32, negative electrode 31, and separator 33 composed of a microporous polypropylene film having a thickness of 25 μm are laminated in this order, and a large number are centered on the center pin 34. The spirally wound electrode body was produced by winding.

Next, a battery can 35 made of iron plated with nickel is prepared, and an insulating plate 36a is provided on the inner bottom of the can 35.
Was placed. The above-mentioned spiral electrode body was housed in the battery can 35, and the insulating plate 36b was placed thereon. Then, in order to collect the current of the negative electrode 31, one end of a negative electrode lead 37 made of nickel is pressure-bonded to the negative electrode 31 and the other end is a battery can 35.
Welded to. Further, in order to collect the current of the positive electrode 32, one end of a positive electrode lead 38 made of aluminum is attached to the positive electrode 32, and the other end of the thin plate 39 for current interruption for interrupting the current according to the internal pressure of the battery and the PTC element 40. It was electrically connected to the battery lid 41 via the.

Next, an electrolyte solution in which LiPF ₆ as a supporting electrolyte was dissolved at a rate of 1 mol / l in a mixed solvent of equal volume of propylene carbonate and diethyl carbonate was injected into the battery can 35. Then, the battery can 3 is inserted through the insulating sealing gasket 42 coated with asphalt.
5 was crimped, the battery lid 41 was fixed, and a cylindrical lithium nonaqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced.

Example 2 A cylindrical lithium non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 1 except that a 20 μm thick copper fiber sheet material was used as the negative electrode current collector 31a.

Example 3 A cylindrical lithium non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that a 20 μm thick stainless fiber sheet material was used as the negative electrode current collector 31a.

Example 4 Cylindrical lithium non-aqueous solution was used in the same manner as in Example 1 except that a 40 μm-thick plated stainless steel fiber sheet material obtained by plating a stainless fiber sheet material was used as the negative electrode current collector 31a. An electrolyte secondary battery was produced.

Example 5 As the negative electrode current collector 31a, the same procedure as in Example 1 was carried out except that a plated carbon fiber sheet material obtained by plating a carbon fiber sheet material having a thickness of 40 μm with nickel was used. A water electrolyte secondary battery was produced.

Comparative Example 1 A cylindrical lithium non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that a carbon fiber sheet material having a thickness of 40 μm was used as the negative electrode current collector 31a.

Comparative Example 2 A cylindrical lithium non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that a 20 μm thick copper foil was used as the negative electrode current collector 31a.

(4) Evaluation Examples 1 to 5 and Comparative Examples 1 to 1 produced as described above
Regarding the cylindrical lithium non-aqueous electrolyte secondary battery of No. 2, first, charging was performed under the conditions of a charging voltage of 4.20 V, a charging current of 1500 mA, a charging time of 2.5 hours, and a charging temperature of 23 ° C. 500mA, final voltage 2.75
The discharge was performed under the conditions of V and a discharge temperature of 23 ° C. This charge / discharge cycle was repeated, and the capacity retention rate at the 300th cycle relative to the battery capacity at the initial second cycle was determined. The results are shown in Table 1 (results of cycle life test).

[0047]

[Table 1] As can be seen from Table 1, the cycle characteristics of the batteries of the examples were very good. On the other hand, the battery of Comparative Example had insufficient cycle characteristics.

[0048]

According to the present invention, a lithium secondary battery using a lithium-containing compound as a positive electrode active material and a carbonaceous material capable of doping and dedoping lithium as a negative electrode active material can be manufactured with high mass productivity. It can be manufactured and the cycle characteristics can be improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a negative electrode used in the present invention.

FIG. 2 is a cross-sectional view of a conventional negative electrode.

FIG. 3 is a cross-sectional view of batteries manufactured in Examples and Comparative Examples.

[Explanation of symbols]

 31 Negative Electrode 31a Negative Electrode Current Collector 32 Positive Electrode 32a Positive Electrode Current Collector 33 Separator 34 Center Pin 35 Battery Can 36a, 36b Insulation Plate 37 Negative Electrode Lead 38 Positive Electrode Lead 39 Current Cutoff Thin Plate 40 PTC Element 41 Battery Lid 42 Seal Gasket

Claims (4)

[Claims] 1. A lithium secondary battery using a lithium-containing compound as a positive electrode active material and a carbonaceous material capable of doping and dedoping lithium as a negative electrode active material, wherein a metal fiber sheet material is used as a negative electrode current collector. A lithium secondary battery characterized by being used. 2. The lithium secondary battery according to claim 1, wherein the metal fiber constituting the metal fiber sheet material is at least one of nickel fiber, copper fiber and stainless fiber. 3. The lithium secondary battery according to claim 1, wherein the metal fiber sheet material is metal-plated. 4. The lithium secondary battery according to claim 1, wherein the metal fiber sheet material is a carbon fiber sheet metal-plated.