JP2001093577A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery using a layered rock structure lithium-transition metal composite oxide as a positive electrode active substance, which has improved cycle characteristic by maintaining a pressure for pressing an electrode body at a suitable level. SOLUTION: The lithium secondary battery includes an electrode body 10, which is formed by depositing a sheet-shaped positive electrode containing lithium-transition metal composite oxide of a layered rock structure, on a sheet- shaped negative electrode, while interposing a separator therebetween. Also, the lithium secondary battery includes electrode body-pressing means serving to press the electrode body 10 toward the deposited direction of the positive electrode and the negative electrode at a pressure of 5 kg/cm2.

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 utilizing a phenomenon of inserting and extracting lithium ions.

[0002]

2. Description of the Related Art With the downsizing of personal computers, video cameras, mobile phones, and the like, high-performance batteries are required, and because of their high energy density, in the fields of information-related equipment and communication equipment, Lithium secondary batteries have already been put to practical use and have come into widespread use. Rechargeable batteries are generally
Good cycle characteristics are required in which the capacity is not significantly reduced even by repeated charging and discharging, and particularly high cost lithium secondary batteries require higher cycle characteristics.

A lithium secondary battery has an electrode body in which a positive electrode using a lithium transition metal composite oxide or the like as a positive electrode active material and a negative electrode using a carbon material or the like as a negative electrode active material are laminated with a separator interposed therebetween. It has a general configuration. The electrode body is impregnated with a non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent. During charging, lithium ions are separated from the positive electrode active material and occluded in the negative electrode active material. Lithium ions are released from the negative electrode active material and occluded in the positive electrode active material.

A lithium transition metal composite oxide or the like serving as a positive electrode active material or a carbon material or the like serving as a negative electrode active material repeatedly undergoes volume changes such as expansion and contraction as lithium is absorbed and desorbed. Due to this volume change, not only a phenomenon such as dropping of the active material from the electrode occurs, but in particular, in the case of the lithium transition metal composite oxide, the secondary particles become finer due to this volume change, and the conductive material inside the positive electrode becomes conductive. Sex is hindered. For these reasons, the capacity of the lithium secondary battery is reduced.

As a technique for maintaining good cycle characteristics of a lithium secondary battery, for example, as disclosed in Japanese Patent Application Laid-Open No. H4-294071, an electrode body is pressurized in its laminating direction to remove active material and to make it finer. Techniques for preventing a decrease in conductivity have been studied.

[0006]

The technique disclosed in the above publication
The operation is LiV with spinel structure._ThreeO₈As positive electrode active material
It is a technique when it is used, 1kg / cm^TwoRatio of above
Pressurization is performed with a relatively small pressure. At present
Is a LiCoO crystal having a layered rock salt structure._TwoEtc.
Using lithium transition metal composite oxide as positive electrode active material
Lithium secondary batteries are the mainstream, with layered rock salt structures
Lithium using a transition metal composite oxide as a positive electrode active material
According to a re-test conducted by the present inventors on the secondary battery,
1kg / cm ^TwoWith a moderate pressure, the cycle characteristics
The effect has not been sufficiently obtained.

The crystal structure of the layered rock-salt structure lithium transition metal composite oxide is as follows: a lithium layer composed of lithium atoms, a transition metal layer composed of transition metal atoms, and a lithium layer composed of oxygen atoms.
The lithium layer is sandwiched between two oxygen layers. When lithium is released from the crystal structure due to charging, the distance between the oxygen layers increases due to electrostatic repulsion of the oxygen layers present on both sides of the lithium layer. As a result, the lithium transition metal composite oxide undergoes volume expansion. That is, unlike the lithium-transition metal composite oxide having a spinel structure, extremely large volume expansion and contraction are repeated, and the fineness of the secondary particles becomes severe. Therefore, in a lithium secondary battery using a layered rock salt structure lithium transition metal composite oxide as a positive electrode active material, 1 kg
At a relatively small pressure of at least / cm ² , the finely divided secondary particles cannot be sufficiently adhered to each other, and the cycle characteristics are not significantly improved.

Further, as described above, in the case of the layered rock salt structure, it expands due to charging of the lithium secondary battery. When a carbon material is used for the negative electrode active material, the carbon material also expands due to charging. Therefore, during charging, both the positive electrode and the negative electrode expand, and the expansion of the electrode body becomes remarkable. Since a separator is interposed between the positive electrode and the negative electrode, and the separator generally uses a porous sheet, the separator is compressed and thinned by expansion of the positive electrode and the negative electrode, and as a result, the lamination thickness of the electrode body itself is also reduced. Become thin. In the technology described in the above publication, from the structure of using a normal leaf spring for the pressing member,
Even if the pressing force is initially set to 1 kg / cm ² , since the lamination thickness of the electrode body becomes small, after repeating the charging and discharging, only the pressing force lower than the initially set 1 kg / cm ² can be obtained. Also in this respect, the effect of improving the cycle characteristics was insufficient.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems of a lithium secondary battery. In a lithium secondary battery using a layered rock salt structure lithium transition metal composite oxide as a positive electrode active material, an electrode assembly is added. It is an object of the present invention to provide a lithium secondary battery having good cycle characteristics by making the pressure to be applied appropriate.

[0010]

The lithium secondary battery of the present invention comprises a sheet-shaped positive electrode containing a lithium transition metal composite oxide having a layered rock salt structure as a positive electrode active material, and a sheet-shaped negative electrode. And an electrode body pressing means for constantly pressing the electrode body with a pressure of 5 kg / cm ² or more in the stacking direction of the positive electrode and the negative electrode. .

That is, in the lithium secondary battery of the present invention,
Unlike a conventional lithium secondary battery, the pressure for pressing the electrode body is set to a relatively large value of 5 kg / cm ² or more,
In addition, by constantly applying the pressure in the stacking direction of the electrode body, the active material falls off due to expansion and contraction, and the inside of the positive electrode due to the miniaturization of the secondary particles of the lithium transition metal composite oxide as the positive electrode active material is caused. The decrease in conductivity can be suppressed more sufficiently, and the cycle characteristics become extremely good.

Although the present invention relates to a lithium secondary battery having an electrode body pressurizing means, in other words, a sheet-like material containing a lithium transition metal composite oxide having a layered rock-salt structure as a positive electrode active material has a crystalline structure. A method for charging and discharging a lithium secondary battery having an electrode body formed by laminating a positive electrode and a sheet-shaped negative electrode with a separator interposed therebetween, wherein the electrode body is always 5 kg / cm in the laminating direction of the positive electrode and the negative electrode. A charging / discharging method for a lithium secondary battery in which charging / discharging is performed while applying pressure at ^two or more pressures may also be used.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention comprises an electrode body and an electrode body pressurizing means for pressing the electrode body as main components. The electrode body includes a sheet-like positive electrode and a sheet-like negative electrode, and a separator sandwiched therebetween. In the present lithium secondary battery, components other than the positive electrode active material and the electrode body pressurizing means are not particularly limited, and various types of lithium secondary batteries can be obtained by combining already known components. . Hereinafter, one embodiment of the lithium secondary battery of the present invention will be described with respect to each component.

<Positive Electrode> The positive electrode is configured to include a lithium transition metal composite oxide having a layered rock salt structure as a positive electrode active material. This layered rock salt structure lithium transition metal composite oxide has a basic composition of LiCoO ₂ , LiNiO ₂ , LiMnO by using transition metals such as Co, Ni, and Mn.
Lithium-cobalt composite oxide, lithium-nickel composite oxide, lithium-manganese composite oxide, etc. represented by ₂ etc. can be used. In addition to those having these basic compositions, in order to improve the characteristics as a positive electrode active material, a material in which a transition metal site is replaced with another element, a material in which a Li site is replaced with an element such as an alkali metal, or the like is used. Can also. In addition, these layered rock salt-structured lithium transition metal composite oxides may be used alone as a positive electrode active material, or may be used as a mixture of two or more.

Of these, lithium-cobalt composite oxides are easy to synthesize, most stable, have good cycle characteristics, and are the positive electrode active material which is the mainstream of current lithium secondary batteries. Therefore, when giving priority to cycle characteristics, it is preferable to use a lithium-cobalt composite oxide. However, Co as a constituent element is very expensive, and the cost of a lithium battery is high. On the other hand, in the lithium manganese composite oxide, since the constituent element Mn is inexpensive, the cost as the positive electrode active material is reduced. Therefore, when giving priority to the cost of the lithium secondary battery, it is desirable to use a lithium manganese composite oxide as the positive electrode active material.

The lithium-nickel composite oxide has the advantage of a large capacity and is not as expensive as the lithium-cobalt composite oxide in terms of cost, and is expected to be used as a positive electrode active material instead of the lithium-cobalt composite oxide.
However, since the volume change due to insertion and extraction of lithium is relatively large, the cycle characteristics are slightly inferior. However, in the lithium secondary battery of the present invention, the purpose is to improve the cycle characteristics by pressurizing the electrode body, and since the effect of this pressurization can be expected most, the lithium nickel composite oxide was used. If the battery capacity is large,
A well-balanced lithium secondary battery with excellent cycle characteristics.

When a lithium nickel composite oxide is used, a stoichiometric composition represented by a composition formula LiNiO ₂ can be used. Further, in order to improve the cycle characteristics and the like of the secondary battery, a battery in which part of the Ni site is replaced with another element can be used. Among those substituted with other elements, the composition formula LiNi _x M1 _y M2 _z O ₂ (M1 is C
at least one selected from o and Mn; M2 is Al;
At least one selected from B, Fe, Cr, and Mg; x
+ Y + z = 1; 0.5 <x <0.95; 0.01 <y <
0.4; 0.001 <z <0.2).

This LiNi _x M1 _y M2 _z O ₂ is obtained by partially substituting Ni sites with two or more kinds of elements M1 and M2 having different roles. If the substitution ratio is defined by the ratio of leaving Ni without substitution, that is, the value of x in the composition formula, 0.5 <x <0.95. If x ≦ 0.5, not only a layered rock salt structure but also a second phase such as a spinel structure is generated. If x ≧ 0.95, the substitution effect is too small. This is because a battery having the desired good cycle characteristics cannot be formed. Note that 0.
More preferably, the range is 7 <x <0.9.

The element M1 selected from Co and Mn mainly serves to stabilize the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the lithium secondary battery are more favorably maintained, and particularly, deterioration of the battery capacity due to charge / discharge at high temperature and storage at high temperature is suppressed. In order to sufficiently exert the effect of improving the cycle characteristics, the substitution ratio of M1, that is, the value of y in the composition formula, is set to 0.01 <y <0.4. y
When ≦ 0.01, the crystal structure of the formed secondary battery is not sufficiently stabilized, so that the cycle characteristics are not good.
If ≧ 0.4, the crystallinity of the lithium-nickel composite oxide is undesirably reduced. It is more preferable that 0.1 <y <0.3. Further, the replacing element M1 is Co
Is more desirable. In Co, while suppressing the capacity reduction due to the element substitution, the obtained composite oxide Li
This is because (Co, Ni) O ₂ is an all-solid solution type and has an advantage of minimizing a decrease in crystallinity.

The element M2 selected from Al, B, Fe, Cr and Mg mainly serves to suppress the decomposition reaction of the active material due to the release of oxygen and to improve the thermal stability. Due to this role, the substitution ratio of M2, that is, the value of z in the composition formula, is set to 0.001 <z <0.2. z ≦
In the case of 0.001, a sufficient effect on safety cannot be obtained, and in the case of z ≧ 0.2, the capacity of the positive electrode decreases, which is not preferable. In addition, 0.01 <z <0.
It is more preferably set to 1. Further, the substituting element M2
It is more preferable to use Al. This is because Al has an advantage of minimizing a decrease in capacity while improving thermal stability.

For example, to manufacture a layered rock salt structure lithium nickel composite oxide represented by the composition formula LiNi _x Co _y Al _z O ₂ , use LiOH · H ₂ O, Ni (O
This can be synthesized by mixing predetermined amounts of H) ₂ , Co ₃ O ₄ , and Al (OH) ₃ and baking them in an oxygen stream at a temperature of about 850 ° C. for about 20 hours.

The positive electrode is prepared by mixing a powder of the above-mentioned lithium transition metal composite oxide, which is a positive electrode active material, with a conductive material and a binder, and adding an appropriate solvent to form a paste-like positive electrode mixture. It can be coated and dried on the surface of a current collector made of a metal foil such as aluminum, and if necessary, compressed to increase the electrode density to form a sheet. The conductive material is for ensuring the electrical conductivity of the positive electrode, and may be one or a mixture of two or more of carbon material powders such as carbon black, acetylene black, and graphite. The binding agent plays a role of binding the active material particles and the conductive material particles, and may be a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent in which the active material, the conductive material, and the binder are dispersed.

By the above-mentioned means, a sheet-like positive electrode formed by forming a positive electrode mixture containing a positive electrode active material into a layer,
The lithium secondary battery can be cut into appropriate dimensions according to the size, shape, and the like of the lithium secondary battery to be created. In addition, a current collecting lead or the like may be attached to the positive electrode for current collection from the positive electrode to the outside.

<Negative electrode> As the negative electrode active material, metallic lithium,
A lithium alloy or the like can be used. A sheet-shaped negative electrode can be produced by attaching metal lithium or the like to a negative electrode current collector in a foil shape or a sheet shape. However, when such metal lithium or the like is used for the negative electrode, there is a possibility that dendrite is deposited on the surface of the negative electrode due to repeated charging and discharging, and there is a concern about the safety of the secondary battery. Therefore, when considering the safety of the lithium secondary battery, it is desirable to use a carbon material capable of inserting and extracting lithium as the negative electrode active material.

The carbon materials that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon,
Further, there may be mentioned an organic compound fired body such as a phenol resin, and a graphitizable carbon powder such as coke.
The carbon material used as the negative electrode active material has its own advantages,
The selection may be made according to the characteristics of the lithium secondary battery to be manufactured.

Of these, natural and artificial graphites have the advantage of being capable of forming a lithium secondary battery having a large capacity (high energy density) and good power characteristics because of its high true density and excellent conductivity. There is. When a lithium secondary battery utilizing this advantage is manufactured, it is desirable that the graphite used has high crystallinity, the (002) plane spacing d ₀₀₂ is 3.4 ° or less, and the crystallite thickness in the c-axis direction. It is preferable to use one having Lc of 1000 ° or more. In addition, artificial graphite is, for example, 2800 ° C.
It can be manufactured by heat treatment at the above high temperature. In this case, the easily graphitizable carbon as a raw material includes coke and optically anisotropic small spheres (mesocarbon microbeads: MCMB) obtained in the process of heating pitches at about 400 ° C. .

[0027] Graphitizable carbon is generally obtained by using tar pitch obtained from petroleum or coal as a raw material.
MCMB, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, and the like. An organic compound fired body such as a phenol resin can also be used. Since graphitizable carbon is an inexpensive carbon material, it can be a negative electrode active material that can constitute a lithium secondary battery that is excellent in cost. Among them, coke has the advantages of low cost and relatively large capacity, and considering this point,
It is desirable to use coke. When coke is used, it is preferable to use one having a (002) plane spacing d ₀₀₂ of 3.4 ° or more and a crystallite thickness Lc in the c-axis direction of 30 ° or less.

The non-graphitizable carbon is a so-called hard carbon, and is a carbon material having a structure close to an amorphous state represented by glassy carbon. Generally, it is a material obtained by carbonizing a thermosetting resin, and does not develop a graphite structure even when the heat treatment temperature is increased. The non-graphitizable carbon has the advantages of high safety and relatively low cost. In view of this, it is desirable to use non-graphitizable carbon as the negative electrode active material. Specifically, for example, a phenol resin fired body, a polyacrylonitrile-based carbon fiber, pseudo isotropic carbon, a furfuryl alcohol resin fired body, or the like can be used. More preferably, (00
2) It is preferable to use those having a plane spacing d ₀₀₂ of 3.6 ° or more and a crystallite thickness Lc in the c-axis direction of 100 ° or less.

The above-mentioned graphite, easily graphitizable carbon, hardly graphitizable carbon and the like can be used singly or as a mixture of two or more kinds. As an embodiment in which two or more kinds are mixed, for example, as an embodiment in which the above-mentioned lithium-nickel composite oxide is used as the positive electrode active material, while securing safety during overcharge, the lithium-nickel composite oxide as the positive electrode active material is used. Graphite and non-graphitizable carbon materials such as non-graphitizable carbon and easily graphitizable carbon were used for the purpose of improving the cycle characteristics by limiting the amount of lithium absorbed and desorbed from the material. A case where they are mixed can be exemplified. When a mixture of graphite and a non-graphitized carbonaceous material is used for the negative electrode active material, the mixture ratio of both is
What is necessary is just to determine by the balance between cycle characteristics and discharge capacity.

When a carbon material is used as the negative electrode active material, the negative electrode was formed into a paste-like negative electrode mixture by mixing a binder into a powder of the carbon material and adding an appropriate solvent as needed. Like the positive electrode, the negative electrode mixture is applied to the surface of a current collector made of metal foil such as copper, dried, and then, if necessary, formed by increasing the density of the negative electrode mixture by pressing or the like. As a binder,
Like the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride or the like can be used, and as a solvent, an organic solvent such as N-methyl-2-pyrrolidone can be used.

By the above means, a sheet-like negative electrode formed by forming a negative electrode mixture containing a negative electrode active material in a layered form,
As in the case of the positive electrode, the lithium secondary battery can be cut into appropriate dimensions according to the size, shape, and the like of the lithium secondary battery to be produced. Also, the attachment of a current collecting lead or the like for current collection from the negative electrode to the outside can be performed in the same manner as in the case of the positive electrode.

<Electrode Body> The electrode body is formed by laminating the above-mentioned sheet-like positive electrode and the above-mentioned sheet-like negative electrode via a separator. The lamination method can be roughly classified into two. One of them is an electrode body 10 constituting a so-called prismatic battery as schematically shown in FIG. 1, using a plurality of sheet-like positive electrodes 1 and a sheet-like negative electrode 2 with a separator 3 interposed therebetween. Is a method of superimposing multiple times alternately (hereinafter, referred to as
This is referred to as “superimposed electrode body”). The other is an electrode body 10 constituting a so-called cylindrical battery as schematically shown in FIG. 2, in which a strip-shaped positive electrode 1 and a strip-shaped negative electrode 2 are respectively connected to one.
Each of the sheets and two strip-shaped separators 3 were used.
And a negative electrode 2 in which separators are sandwiched one by one and wound in a roll around a winding core or the like (hereinafter referred to as a “wound electrode body”). Call).
The lithium secondary battery of the present invention can be applied to a secondary battery having any of a stacked type and a wound type electrode body. As shown in the enlarged cross-sectional view of FIG. 1, the sheet-like positive electrode 1 has a positive electrode mixture layer 1b containing a positive electrode active material on both surfaces of a positive electrode current collector 1a.
However, in the sheet-shaped negative electrode, the sheet-shaped negative electrode 2 has the negative electrode mixture layers 2b containing the negative electrode active material formed on both surfaces of the negative electrode current collector 2a.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds a non-aqueous electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

In a lithium secondary battery, a non-aqueous electrolyte is used to secure the movement of lithium ions between the positive electrode and the negative electrode, and the electrode body is impregnated with the non-aqueous electrolyte. The non-aqueous electrolyte is a solution in which a lithium salt as an electrolyte is dissolved in an organic solvent.As the organic solvent, an aprotic organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ -Butyrolactone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran,
One kind of dioxolane, methylene chloride, etc.
Mixtures of more than one species can be used. The electrolyte to be dissolved is LiI, LiClO ₄ , LiAs.
Lithium salts such as F ₆ , LiBF ₄ , LiPF ₆ , and LiN (CF ₃ SO ₂ ) ₂ can be used.

<Electrode Body Pressing Means> The electrode body pressing means, which is a characteristic part of the lithium secondary battery of the present invention, applies the electrode body to the lamination direction of the positive electrode and the negative electrode at a pressure of at least 5 kg / cm ^{2 at} all times. Press. The stacking direction of the positive electrode and the negative electrode is a direction perpendicular to the sheet-shaped positive electrode and the negative electrode in the case of a superposed electrode body, and the electrode body itself is pressed so as to be crushed in that direction. In the case of a wound electrode body, pressure is applied in the radial direction of the electrode body to tighten the electrode body.

The pressing mechanism always applies 5 kg / cm ² to the electrode body.
Anything that can apply the above pressure may be used.
The method is not particularly limited. For example, in the case of a superimposed electrode body, rigid pressing members are arranged on both sides of the electrode body, and the pressing members are tightened using fasteners such as bolts and nuts so that the interval between the pressing members is reduced. The body may be pressurized. Further, the electrode body may be pressurized by pressing each or one of the pressing members with a force such as hydraulic pressure or air pressure.
Further, the electrode body may be arranged so that the lamination direction of the electrode body is vertical, a weight may be placed on the electrode body, and the electrode body may be pressed by gravity.

As clarified by experiments,
Since the separator interposed between the positive electrode and the negative electrode is a porous body, the separator becomes thinner by continuously pressing the electrode body. When the thickness of the separator is reduced, the thickness of the electrode body (the length of the lamination method) is reduced accordingly. For example, when the above-described tightening by the fastening material is adopted as the pressing mechanism, when the thickness of the electrode body is reduced, the tightening force decreases, and the pressing force of the electrode body also decreases. Pressure is 5kg /
Before dropping below cm ² , it becomes necessary to retighten the fastener.

In order to absorb the decrease in the pressing force due to such thinning of the separator and to always pressurize at a predetermined pressure, the electrode body pressing means is of a type that generates the pressing force by a spring. It is desirable to adopt it. There are pressure members other than springs that can maintain a predetermined pressure, but springs have advantages such as a simple mechanism and excellent strength.

As an example of the electrode body pressing means, FIG. 3 schematically shows an electrode body pressing means for pressing the superposed electrode body with a compression coil spring. The electrode body pressing means 20 includes two pressing members 21, 22 sandwiching the electrode body 10, four columns 23 each having one end fixed to the pressing member 21 and inserted through a hole 22 a provided in the pressing member 22, Each of the ends is formed of a column head 23 a and four compression coil springs 24 abutting against the pressing member 22 so that the column 23 is inserted. Electrode body 10
Is pressed in the laminating direction of the sheet-like positive and negative electrodes by the force of a compression coil spring. Such an electrode body pressurizing means using a spring requires a required 5 kg / kg even if the thickness of the electrode body is reduced to some extent by thinning the separator.
It is easy to design so that a pressing force of not less than cm ² can be maintained.

According to the experiment, when the porous polyethylene sheet was used as the separator, the electrode body was 5 kg / cm.
It was clarified that the separator was thinned by about 20% by repeating the charge and discharge by applying a pressure of ² to ²⁰ kg / cm ² . In consideration of this, the effective stroke (pressing allowance) of the electrode body pressing means needs to be larger than [separator thickness × number of stacked separators × 0.2]. Further, when the above-described spring is used as a member that generates a pressing force, the spring itself is always set according to the change in length, considering that the pressing force changes due to a change in the length of the spring. It is necessary to have a spring constant so that pressure can be applied. Although it depends on the spring constant, in consideration of general characteristics of the spring, it is desirable to adopt a spring having a relatively small spring constant and to design the effective stroke of the electrode body pressing means as large as possible.

FIG. 4 schematically shows an example of an electrode body pressing means for pressing a wound electrode body. The electrode body pressing means 20 is formed by forming the spring material itself into a roll shape having an inner diameter smaller than the outer diameter of the wound electrode body 10, and inserts the electrode body 10 in a state where the inner diameter is expanded. Then, by releasing the expanded state, the electrode body 10 is pressed so as to be tightened by the elastic force of the electrode body pressing means 20.

In the case of a lithium secondary battery, the electrode body needs to be sealed. When the electrode body pressurizing means is provided, a method of inserting the electrode body together with the electrode body pressurizing means into a battery case or the like, and sealing the battery case to seal the electrode body can be adopted. May be sealed in a flexible (deformable) battery case, and the electrode body may be pressed from the outside of the battery case by an electrode body pressing means.

FIG. 5 shows an example of a more specific embodiment of the lithium secondary battery of the present invention. In the lithium secondary battery of the present embodiment, a cell assembly in which the electrode body 10 is hermetically sealed in a flexible battery case 30 is defined as one cell, and the plurality of cells are arranged in the electrode stacking direction in the electrode body 10. This is a lithium secondary battery of a type in which the electrode assembly is pressurized by pressing the cell assembly in the electrode stacking direction by the electrode assembly pressurizing means 20. The electrode body pressing means 20 includes pressing members 21 and 22 which also serve as a battery cover and hold the cell assembly, a plurality of columns 23 connecting the pressing members 21 and 22, and a plurality of And a compression coil spring. The support 23 has a bolt shape, is inserted through a hole 22 provided in the pressing member 22, and is screwed to the pressing member 21 with a screw at one end thereof. The pressing member 22 can be moved in a method of laminating the electrode body by using the support 23 as a guide. The compression coil springs 24 are arranged in such a manner that the struts 23 communicate with each other inside the hole 22 of the pressing member 22, and both ends thereof are respectively supported by the struts 23.
Step 23b of the head 23a of the hole and the hole 22a of the pressing member 22
And is kept in a compressed state. The pressing member 21 and the pressing member 22 are applied with a force to expand the compression coil spring 24 in a direction approaching each other, and the cell assembly sandwiched between the pressing member 21 and the pressing member 22 is pressed. Thus, each electrode body 10 is pressurized by the electrode lamination method.

As described above, since a plurality of electrode bodies can be integrally pressurized, even a larger lithium secondary battery can be an efficient electrode pressurizing means. In addition, by incorporating a pressing mechanism using a compression coil spring in a pressing member also serving as a battery cover, there is an advantage that the dead space of the battery can be reduced even with an electrode body pressing unit having a long effective stroke. The pressing member includes
In addition to metal materials, resins such as polypropylene can be used, and polyethylene or the like can be used for a flexible battery case that seals the electrode body.

[0045]

EXAMPLE In order to confirm the effect of improving the cycle characteristics of a lithium secondary battery by pressing the electrode body in the direction of lamination of the electrodes, lithium secondary batteries with variously changed pressures were produced, and those secondary batteries were manufactured. Was subjected to a charge / discharge cycle test. This test and its evaluation are described below as examples.

<Structure of Lithium Secondary Battery>
In the secondary battery, the composition formula LiNi_0.8Co_0.15
Al_0.05O_TwoLayered rock salt structure lithium nickel represented by
Complex oxide (manufactured by Fuji Chemical: LINILITE CA)
-5) was used. The positive electrode is LiNi_0.8Co_0.15A
l_{0 .05}O_Two85 parts by weight of acetylene
10 racks (HS-100 manufactured by Denki Kagaku Kogyo)
Part, polyvinylidene fluoride as binder (Kureha Chemical Industry)
Manufactured by KF Polymer Co., Ltd.).
-2-Pyrrolidone was added to obtain a paste-like positive electrode mixture,
This positive electrode mixture was used as a 20 μm thick aluminum foil current collector.
Apply on both sides, then dry and roll press
Thus, a sheet-like product was produced. The sheet-like positive electrode
Size is 50mm x 50mm, formed on the current collector surface
The thickness of the positive electrode mixture layer is 40 μm per side.
Was.

As the negative electrode active material, graphitized mesophase microspheres (manufactured by Osaka Gas Chemicals: MCMB25), which are artificial graphite, are used.
-28) was used. The negative electrode was prepared by mixing 95 parts by weight of the graphitized mesophase spherules with 5 parts by weight of polyvinylidene fluoride (KF polymer manufactured by Kureha Chemical Industry Co., Ltd.) as a binder, and adding an appropriate amount of N-methyl-2-pyrrolidone. A negative electrode mixture in the form of was obtained, and this negative electrode mixture was applied to both surfaces of a copper foil current collector having a thickness of 10 μm, dried, and then roll-pressed to produce a sheet. The size of the sheet-shaped negative electrode was 52 mm × 52 mm, and the thickness of the negative electrode mixture layer formed on the current collector surface was 50 μm per side.

The lithium secondary battery used for the test was configured as shown in FIG. 6 using the above-mentioned sheet-like positive electrode and the above-mentioned sheet-like negative electrode. The electrode body 10 includes one positive electrode 1, two negative electrodes 2, and two separators 3 each made of a porous polyethylene sheet (manufactured by Tonen Talpis) having a thickness of 25 μm. , Separator 3,
The negative electrode 2 was laminated in this order. Electrode body pressing means 20
Is a pressing member 2 having a concave portion in which the electrode body is inserted in the center.
1 and a pressing member 22 having a convex portion fitted into the concave portion
, A column 23 using bolts, and a compression coil spring 24
The electrode body 10 is pressurized in the same manner as described in the above embodiment. The compression coil spring 24 is 10 m
m and a spring constant such that a pressure of 90% or more of the initial pressing force can be applied to the electrode body 10 even when the separator 3 is thinned by 20% by the charge / discharge cycle. . The pressing member 2
An O-ring 25 is provided at a fitting portion between the concave portion 1 and the convex portion of the pressing member 22 so that the electrode body 10 is sealed. Further, the electrode body 10 is impregnated with a non-aqueous electrolyte obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 1: 1.

Several types of lithium secondary batteries having different pressures on the electrode body were manufactured. As described above, when the 20% separator is thinned, each is 1 kg / cm ² ,
2 kg / cm ^2, a 5kg / cm ^2, 20kg / cm pressure at the electrode body ² to set the initial pressure to pressurize the respective secondary batteries Sample No. 1, No. 2, No.
3, No. 4 was obtained. For comparison, a secondary battery having an initial pressure of 5 kg / cm ² was prepared by directly tightening with a bolt without using a compression coil spring. No. 5 secondary battery.

<Charge / discharge cycle test>
First, condition the rechargeable lithium battery.
went. Conditioning conditions are at a temperature of 20 ° C.
Lower, current density 0.2 mA / cm ^TwoCharge upper limit current with constant current
Charge to 4.1V, then current density 0.2mA / c
m^TwoThat discharges to a discharge lower limit voltage of 3.0 V at a constant current of
And Next, the conditioning is finished
A charge / discharge cycle test was performed on these secondary batteries. Filling
The conditions for the discharge cycle test are above the actual operating temperature range of the battery.
Current density under the high ambient temperature of 60 ° C
Degree 2mA / cm^TwoCharge current up to 4.1V at constant current
Charge, then current density 2mA / cm^TwoDischarge at constant current
What discharges to the lower limit voltage 3.0V is one cycle,
This cycle was repeated 500 times.

In this charge / discharge cycle test, the initial discharge capacity per unit weight of the positive electrode active material of each secondary battery, the discharge capacity after 500 cycles, the initial DC resistance, and the
The DC resistance after the 00 cycle was measured, and the capacity retention rate and the DC resistance rise rate were determined. In addition, the value obtained based on the calculation formula of (average charge voltage-average discharge voltage) / (charge / discharge current × 2) was adopted as the DC resistance. Table 1 below
Table 2 shows the initial discharge capacity per unit weight of the positive electrode active material of each secondary battery, the discharge capacity after 500 cycles, the initial DC resistance and the DC resistance after 500 cycles, the capacity retention ratio, and the DC resistance rise rate.

[0052]

[Table 1]

As apparent from Table 1 above, the compression coil
Sample No. which has a spring and is always pressurized above the set pressure 1
-No. Comparing the secondary battery of No. 4 shows that the pressing force is 1 kg /
cm ^Two, 2kg / cm^TwoNo. 1 and No. Two
Pressing force is 5kg / cm compared to rechargeable battery^Two, 20kg /
cm^TwoNo. 3 and No. 3 The secondary battery of 4 is 5
Even after the charge / discharge cycle of 00 cycles, the DC resistance rises
Low, high capacity retention of over 90%, cycle
It can be seen that the secondary battery has good characteristics. Therefore,
5kg / cm^TwoElectrode body in the electrode stacking direction.
Pressure is effective in improving cycle characteristics.
It can be recognized.

Further, a compression coil spring has a
No. which can be pressurized with a pressure of not less than ² cm ² . No. 3 which has no compression coil spring and whose pressure decreases with the charge / discharge cycle No. 3 Comparing the secondary batteries of No. 5
No. which can always be pressurized with a pressure of 5 kg / cm ² or more. It can be seen that the secondary battery of No. 3 has a low rise in DC resistance, shows a high capacity retention ratio, and has better cycle characteristics. Therefore, it can be confirmed that applying pressure at a pressure equal to or higher than the preset pressure is effective for improving the cycle characteristics.

[0055]

The present invention provides a lithium secondary battery using a layered rock salt structure lithium transition metal composite oxide as a positive electrode active material.
The electrode body is always 5 kg / c in the laminating direction of the positive electrode and the negative electrode.
It is configured to include an electrode body pressurizing means for pressurizing with a pressure of m ² or more. With such a structure, the lithium secondary battery of the present invention has a positive electrode due to the fall of the active material due to expansion and contraction and the miniaturization of the secondary particles of the lithium transition metal composite oxide as the positive electrode active material. A reduction in the internal conductivity can be sufficiently suppressed, and a lithium secondary battery having extremely good cycle characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 schematically shows a superposed electrode body that can be employed in a lithium secondary battery of the present invention.

FIG. 2 schematically shows a wound electrode body that can be employed in the lithium secondary battery of the present invention.

FIG. 3 schematically illustrates an example of an electrode body pressing unit that presses a superimposed electrode body with a compression coil spring that can be employed in the lithium secondary battery of the present invention.

FIG. 4 schematically shows an example of an electrode body pressing means for pressing a wound electrode body that can be employed in the lithium secondary battery of the present invention.

FIG. 5 shows an embodiment of the lithium secondary battery according to the present invention, which has a plurality of electrode bodies and is provided with an electrode body pressing means capable of integrally pressing them.

FIG. 6 schematically shows a configuration of a lithium secondary battery subjected to a charge / discharge cycle test.

[Explanation of symbols]

 1: Positive electrode 2: Negative electrode 3: Separator 10: Electrode body 20: Electrode body pressurizing means

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor, Takeshi Sasaki 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories Co., Ltd. (72) Inventor Yuichi Ito, Nagakute-cho, Nagakute-cho, Aichi County, Aichi Prefecture 41, Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Yoshio Ukyo 41, Toyota-Chuo Research Laboratory, Nagakute-cho, Aichi-gun, Aichi, Japan F-term (reference) 5H003 AA04 AA06 BB05 BC06 BD01 BD03 5H014 AA02 AA06 EE10 HH01 HH08 5H029 AJ05 AJ11 AK03 AL06 AL07 AL12 AM02 AM07 BJ02 BJ21 DJ17 EJ01 HJ02 HJ15

Claims (3)

[Claims] An electrode body formed by laminating a sheet-shaped positive electrode containing a lithium transition metal composite oxide having a layered rock salt structure as a positive electrode active material, and a sheet-shaped negative electrode with a separator interposed therebetween. A lithium secondary battery comprising: an electrode body pressing means for constantly pressing the electrode body in the stacking direction of the positive electrode and the negative electrode at a pressure of 5 kg / cm ² or more. 2. The lithium secondary battery according to claim 1, wherein the electrode body pressurizing means generates the pressing force by a spring. 3. The lithium transition metal composite oxide according to claim 1, wherein
Formula LiNi_xM1_yM2 _zO_Two(M1 is selected from Co and Mn
M2 is Al, B, Fe, Cr, M
g + x + y + z = 1;
5 <x <0.95; 0.01 <y <0.4; 0.001
<Z <0.2) lithium nickel composite oxide
The lithium secondary battery according to claim 1 or 2, wherein
pond.