JP2000323173A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of preventing a reduction of a discharge capacity caused by a repeating charge and discharge without causing a capacity reduction per unit volume and a generation of an internal short. SOLUTION: This secondary battery is provided with a generation element 3 in which a positive electrode in which a positive electrode active substance layer is formed on at least one surface of a current-collector and a negative electrode 6 in which a negative electrode active substance layer is formed on at least one surface of the current-collector are interposed through a separator 5, and a nonaqueous electrolyte solution. In this case, the separator 5 has a lower hardness than any hardness of the positive electrode active substance layer and the negative electrode active substance layer and has a porosity of 70% or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, mobile communication devices, notebook computers, palmtop computers, integrated video cameras,
In order to reduce the size and weight of electronic devices such as portable CD (MD) players and cordless mobile phones, small and large-capacity batteries are required as power supplies for these electronic devices.

[0003] As batteries widely used as power supplies for the electronic equipment, primary batteries such as alkaline manganese batteries and secondary batteries such as nickel cadmium batteries and lead-acid batteries are known. Among them, non-aqueous electrolyte secondary batteries that use lithium composite oxide as the positive electrode active material and carbonaceous materials that occlude and release lithium as the negative electrode are small, lightweight, have high single-cell voltage, and have high energy density. Is attracting attention.

[0004] The non-aqueous electrolyte secondary battery described above requires a smaller area of the positive electrode active material and the negative electrode active material than a water-based secondary battery to extract a large current because the non-aqueous electrolyte has low ionic conductivity. Need to be bigger. For this reason, a thin sheet-like positive electrode and a negative electrode are laminated with a separator interposed therebetween, and this is wound into a cylindrical or oblong spiral shape to form a power generating element. The thin sheet-like positive electrode and negative electrode are generally manufactured by a method in which a metal foil as a current collector carries a layer of a mixture containing a positive electrode active material and a negative electrode active material, respectively.

In a non-aqueous electrolyte secondary battery, the power generation element is housed in a container for the purpose of holding the electrolyte, electrical insulation, shape retention, and the like, and the non-aqueous electrolyte is stored in the container in which the power generation element is housed. It has a structure in which a positive electrode, a negative electrode and a separator constituting the power generating element are injected and a non-aqueous electrolyte is impregnated.

[0006]

In the above-described non-aqueous electrolyte secondary battery, generally, when charging and discharging are repeated a number of times, the capacity that can be discharged gradually decreases. The operating time is shortened. Various causes have been considered for the decrease in the discharge capacity. One cause is that
Since the carbonaceous material used for the negative electrode active material expands during charging and contracts during discharging, the structure of the negative electrode active material layer collapses due to repeated volume changes, and the carbonaceous material falls off the current collector, The phenomenon that the electrical connection between the materials is reduced is considered. In addition, the lithium composite oxide used for the positive electrode active material also expands during charging and contracts during discharging, so that the structure of the positive electrode active material layer collapses and is added to increase the active material itself or conductivity. It is conceivable that the carbon particles fall off and the capacity decreases.

In order to prevent the active material layer from collapsing, for example, the types and amounts of carbon particles added to the active material layer and the resin added as a binder are variously changed, or the active material layer is changed. Investigations have been made to change the conditions for formation. However, it has been difficult to effectively prevent the active material layer from collapsing even if such measures are taken.

On the other hand, it is considered that the collapse of the structure of the active material layer is caused by the fact that pressure generated when the active material expands due to charging is accumulated inside the power generating element without escaping. In order to avoid this, it is considered to release the pressure by a method such as providing a space inside the power generating element or using a porous material having a high porosity for the separator. However, in these methods, since the volume of the secondary battery is increased, the charge / discharge capacity per unit volume is reduced. In particular, in the latter method (to increase the porosity of the separator), there is a possibility that irregularities on the surface of the active material layer penetrate the separator and cause an internal short circuit. When the thickness of the separator is increased to avoid an internal short circuit, the charge / discharge capacity per unit volume similarly decreases.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of preventing a reduction in discharge capacity due to repetition of charging and discharging without causing a reduction in capacity per unit volume and occurrence of an internal short circuit. Is what you do.

[0010]

A non-aqueous electrolyte secondary battery according to the present invention for achieving the above object has at least one of a positive electrode having a positive electrode active material layer formed on at least one surface of a current collector and a current collector. A power generating element in which a negative electrode having a negative electrode active material layer formed on one surface thereof is stacked via a separator, and a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte, wherein the separator is
It has a hardness lower than any of the hardnesses of the positive electrode active material layer and the negative electrode active material layer, and has a porosity of 70% or less.

In the non-aqueous electrolyte secondary battery having such a configuration, the pressure generated by the expansion of the positive electrode active material and the negative electrode active material during charging is controlled by a flexible separator having a hardness lower than the hardness of the positive and negative electrode active material layers. Can be absorbed by deformation. For this reason, structural collapse of the positive electrode active material layer and the negative electrode active material layer can be prevented. By setting the porosity of the separator to 70% or less, an internal short circuit between the positive and negative electrodes can be prevented. Therefore, it is possible to obtain a long-life non-aqueous electrolyte secondary battery in which a reduction in discharge capacity due to repetition of charging and discharging is prevented without causing a reduction in capacity per unit volume and occurrence of an internal short circuit.

Another non-aqueous electrolyte secondary battery according to the present invention comprises a positive electrode having a positive electrode active material layer formed on at least one surface of a current collector and a negative electrode having a negative electrode active material layer formed on at least one surface of the current collector. A power generating element laminated with a separator interposed therebetween, and a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte, wherein the separator has a structure in which two or more material layers having different hardnesses are laminated. And, the material layer in contact with the positive electrode active material layer has a hardness lower than the hardness of the positive electrode active material layer, and the material layer in contact with the negative electrode active material layer has a hardness lower than the hardness of the negative electrode active material layer. It is characterized by the following.

In the non-aqueous electrolyte secondary battery having such a configuration, the separator has a structure in which two or more material layers having different hardnesses are laminated, and the material layer (the first material layer) in contact with the positive electrode active material layer is used. Since the material layer has a hardness lower than the hardness of the positive electrode active material layer, the pressure generated by the expansion of the positive electrode active material during charging can be absorbed by the first material layer. On the other hand, since the material layer (second material layer) of the separator in contact with the negative electrode active material layer has a hardness lower than the hardness of the negative electrode active material layer, the pressure generated by the expansion of the negative electrode active material during charging is reduced by the second material layer. Can be absorbed. As a result, structural collapse of the positive electrode active material layer and the negative electrode active material layer can be prevented.

[0014] Further, 70 between the first and second material layers.
By arranging the third material layer having a porosity of not more than 0.1% to constitute the separator, an internal short circuit between the positive and negative electrodes can be prevented.

Therefore, it is possible to obtain a long-life non-aqueous electrolyte secondary battery in which a decrease in discharge capacity due to repetition of charging and discharging is prevented without causing a reduction in capacity per unit volume and occurrence of internal short circuit.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A non-aqueous electrolyte secondary battery (for example, a cylindrical non-aqueous electrolyte secondary battery) according to the present invention will be described in detail with reference to FIG.

A bottomed cylindrical container 1 made of, for example, stainless steel has an insulator 2 disposed at the bottom. Power generation element 3
Are stored in the container 1. The power generating element 3
Has a structure in which a laminated body in which a positive electrode 4, a separator 5 and a negative electrode 6 are laminated in this order is spirally wound so that the negative electrode 6 is located outside.

The container 1 contains an electrolytic solution. The insulating paper 7 having a central portion opened is placed above the electrode group 3 in the container 1. The insulating sealing plate 8
The sealing plate 8 is disposed at the upper opening of the container 1 and caulked in the vicinity of the upper opening inward.
Is fixed to the container 1 in a liquid-tight manner. The positive terminal 9 is
The insulating sealing plate 8 is fitted at the center. Positive lead
One end of the cathode 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9.
Connected to each other. The negative electrode 6 is connected to the container 1 as a negative terminal via a negative lead (not shown).

The positive electrode has a structure in which a positive electrode active material layer containing an active material is formed on at least one surface of a current collector.

Examples of the current collector include a plate of aluminum, nickel or stainless steel, and a mesh of aluminum, nickel or stainless steel.

As the positive electrode active material, various oxides,
For example, manganese dioxide, lithium manganese composite oxide,
Examples include lithium-containing nickel oxide, lithium-containing cobalt compound, lithium-containing nickel-cobalt oxide, lithium-containing iron oxide, lithium-containing vanadium oxide, and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (L
iNiO ₂ ), lithium manganese oxide (LiMn ₂ O)
₄ or LiMnO ₂ ) is preferable because a high voltage can be obtained.

The positive electrode active material layer preferably contains a binder and a conductive agent.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). it can.

Examples of the conductive agent include acetylene black, carbon black, graphite and the like.

The negative electrode has a structure in which a negative electrode active material layer containing an active material is formed on at least one surface of a current collector.

The negative electrode active material is not particularly limited.
Metallic lithium, lithium alloy, or coke, carbon fiber, graphite, mesophase pitch-based carbon, pyrolytic gas phase carbonaceous material, resin that reversibly occupy / release, or intercalate / disintercalate lithium ions during charge / discharge Examples thereof include carbonaceous materials such as fired bodies.

The positive electrode active material layer preferably contains, for example, a binder.

The binder preferably contains, for example, a binder such as polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene copolymer, styrene-butadiene rubber, and carboxymethyl cellulose.

The non-aqueous electrolyte has a composition in which an electrolyte is dissolved in a non-aqueous solvent.

As the electrolyte, for example, lithium perchlorate
(LiClO_Four), Lithium borofluoride (LiBF_Four),
Lithium hexafluorophosphate (LiPF₆), Arsenic hexafluoride
Lithium (LiAsF)₆), Trifluoromethanesulfo
Lithium phosphate (LiCF_ThreeSO _Three), LiN (CF_ThreeS
O_Two)_TwoEtc. can be used.

Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate; cyclic esters such as γ-butyrolactone; tetramethylsulfolane, dimethylsulfoxide, N-methylpyrrolidone, dimethylformamide and derivatives thereof. And other non-aqueous solvents; and the like. These non-aqueous solvents can be used in the form of one kind or a mixture of two or more kinds. In addition, dimethyl carbonate, methyl ethyl carbonate, chain carbonate such as diethyl carbonate and acetonitrile, ethyl acetate, methyl acetate, these non-aqueous solvents
By mixing a solvent such as toluene and xylene, the viscosity of the non-aqueous electrolyte can be reduced.

The concentration of the electrolyte in the non-aqueous solvent is as follows:
It is preferable to set it to 0.5 mol / L or more.

The separator has the following (1),
It has the structure described in (2).

(1) Separator The separator has a hardness lower than that of each of the positive electrode active material layer and the negative electrode active material layer, and
% Or less.

The separator is made of polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-
It is made of a microporous membrane made of a butene copolymer or a woven or nonwoven fabric having fibers of these materials. In particular, a microporous membrane is preferred.

The hardness of the separator before assembling the battery may be determined, for example, by using a hardness tester (trade name, manufactured by Elionix Inc .; EN).
The difference between the hardness of the positive electrode active material layer and the hardness of the negative electrode active material layer when T-1100) is used is preferably 0.2 to 40.

The hardness of the separator after being used after being incorporated in a battery can be determined, for example, by measuring the hardness of the positive electrode active material layer and the negative electrode active material layer when using a hardness tester (trade name: ENT-1100, manufactured by Elionix Inc.). The difference with respect to the hardness is preferably 0.1 to 30.

If the porosity of the separator exceeds 70%, an internal short circuit may occur due to unevenness on the surface of the positive and negative electrode active material layers. More preferably, the porosity of the separator is 30 to 60%.

(2) Separator This separator has a structure in which two or more material layers having different hardnesses are laminated, and the material layer (first material layer) in contact with the positive electrode active material layer is formed of the positive electrode active material layer. The material layer (second material layer) having a hardness lower than the hardness of the material layer and being in contact with the negative electrode active material layer has a hardness lower than the hardness of the negative electrode active material layer. For such a separator, a material layer having a hardness in accordance with the hardness of the positive and negative electrode active material layers can be selected, so that the pressure generated by the expansion of the positive and negative electrode active materials can be effectively absorbed and these positive and negative electrode active material layers can be selected. It is possible to prevent structural collapse.

Each material layer of the separator is made of polyethylene, polypropylene, ethylene-propylene copolymer,
It is made of a microporous membrane made of an ethylene-butene copolymer or a woven or nonwoven fabric having fibers of these materials.
In particular, a microporous membrane is preferred.

The hardness of the first material layer before assembling the battery may be determined, for example, by using a hardness tester (trade name, manufactured by Elionix Inc .; EN)
The difference in hardness of the positive electrode active material layer when (T-1100) is used is preferably 0.2 to 40.

The hardness of the second material layer before assembling the battery may be determined, for example, by using a hardness tester (trade name, manufactured by Elionix Inc .; EN)
The difference in hardness of the negative electrode active material layer when (T-1100) is used is preferably 0.2 to 40.

The hardness of the first material layer after being used by being incorporated in a battery is, for example, a difference from the hardness of the positive electrode active material layer when a hardness tester (trade name: ENT-1100 manufactured by Elionix Inc.) is used. Is preferably 0.1 to 30.

The hardness of the second material layer after being used after being incorporated in a battery is, for example, a difference from the hardness of the negative electrode active material layer when a hardness tester (trade name: ENT-1100 manufactured by Elionix Inc.) is used. Is preferably 0.1 to 30.

The porosity of at least one of the material layers of the separator is preferably 70% or less.

In the separator, the first and second
It is preferable to interpose a third material layer having a porosity of 70% or less between the material layers. In particular, in order to reduce the hardness of the first and second material layers, that is, to increase the flexibility,
When the porosity exceeds 70%, it is preferable to interpose a third material layer having a porosity of 70% or less between the material layers in order to prevent an internal short circuit between the positive and negative electrodes. More preferably, the porosity of the third material layer is 30 to 60.
%.

The separator can be used in a state in which the respective material layers are integrated by pressure bonding, joined using an adhesive, or simply stacked.

[0048]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the above-mentioned non-aqueous electrolyte secondary battery shown in FIG.

Example 1 <Preparation of Positive Electrode> First, 5% by weight of acetylene black was added to LiCoO ₂ powder as an active material, and a 5% dimethylformamide solution of polyvinylidene fluoride resin was added to this mixture to form a paste. Prepared. This paste is width 5
A positive electrode was prepared by applying the solution to one side of a 5 mm, 20 μm thick aluminum foil and drying to form a 90 μm thick active material layer.

The hardness of the obtained active material layer of the positive electrode was measured using a hardness tester (trade name: ENT-110, manufactured by Elionix Inc.).
It was 10.0 when measured using 0). The porosity and the average pore diameter of the positive electrode active material layer were measured by a mercury porosimetry, and the porosity was 43% and the average pore diameter was 0.5 μm.

<Preparation of Negative Electrode> First, a 5% dimethylformamide solution of polyvinylidene fluoride resin was added to mesophase pitch-based carbon fiber to prepare a paste containing the carbon fiber at 60% by weight. This paste was applied to one side of a copper foil having a width of 57 mm and a thickness of 12 μm, and dried to a thickness of 90 μm.
The negative electrode was produced by forming an active material layer of m.

The hardness of the active material layer of the obtained negative electrode was measured using a hardness tester (trade name, manufactured by Elionix Inc .; ENT-110).
0) was 2.7. Also,
When the porosity and the average pore diameter of the negative electrode active material layer were measured by a mercury intrusion method, the porosity was 39% and the average pore diameter was 3 μm.

<Non-aqueous electrolyte> Lithium hexafluorophosphate (LiPF ₆ ) was dissolved at 1 mol / L in a mixed solvent in which ethylene carbonate and methyl ethyl carbonate were mixed at a volume ratio of 1: 2 to prepare a non-aqueous electrolyte. Prepared.

Next, a porosity of 45% is provided between the positive and negative electrodes.
An average pore diameter of 0.3 μm, a hardness of 2.4 [measured using a hardness tester (trade name: ENT-1100, manufactured by Elionix Inc.)], a 25 μm-thick polyethylene microporous membrane, and a winding machine Spirally wound with a diameter of 17 mm
Was produced. Subsequently, this power-generating element was made of an iron-bottomed cylindrical container with a nickel-plated surface (diameter 1).
8 mm, length 65 mm), and after injecting the non-aqueous electrolyte, an insulating sealing plate having a positive terminal is placed in the opening of the container, and the vicinity of the upper opening of the container is caulked inward. Then, the non-aqueous electrolyte secondary battery having the structure shown in FIG. 1 was assembled.

(Example 2) A porosity of 75%, an average pore diameter of 0.4 µm, and a hardness of 1.3 [measured using a hardness tester (trade name, manufactured by Elionix Inc .; ENT-1100)],
Porosity of 45%, average pore diameter of 0.2 μm, hardness of 1 between the first and second polyethylene microporous membranes having a thickness of 7.5 μm.
1.3 [hardness tester (brand name manufactured by Elionix Inc .;
ENT-1100)] and a 10 μm-thick third polyethylene microporous membrane was interposed and integrated to produce a 25 μm-thick separator. A non-aqueous electrolyte secondary battery having the above-described structure shown in FIG. 1 was assembled in the same manner as in Example 1 except that this separator was used.

(Comparative Example 1) Porosity of 40 as separator
%, Average pore diameter 0.09 μm, hardness 7.6 [hardness tester (trade name, manufactured by Elionix Inc .; ENT-1100)]
And a non-aqueous electrolyte secondary battery having the above-described structure shown in FIG. 1 was assembled in the same manner as in Example 1 except that a microporous polyethylene membrane having a thickness of 25 μm was used.

(Comparative Example 2) Porosity of 62 as separator
%, Average pore diameter 0.07 μm, hardness 15.3 [hardness tester (trade name, manufactured by Elionix Inc .; ENT-110)
0), except that a 25 μm-thick polyethylene microporous membrane was used.
A non-aqueous electrolyte secondary battery having the structure shown in FIG. 1 was assembled.

(Comparative Example 3) Porosity of 75 as separator
%, Average pore diameter 0.4 μm, hardness 1.3 [hardness tester (brand name manufactured by Elionix Inc .; ENT-1100)
And a non-aqueous electrolyte secondary battery having the above-described structure shown in FIG. 1 was assembled in the same manner as in Example 1 except that a microporous polyethylene membrane having a thickness of 25 μm was used.

The obtained Examples 1 and 2 and Comparative Examples 1 to 3
Current value of the non-aqueous electrolyte secondary battery of 1400 mAh
After the voltage reaches 4.2 V, the constant current charging of
The charging is performed for a total of 3 hours while controlling the current value so as to maintain the voltage of 2 V, and thereafter, the constant current discharging at the current value of 1400 mAh is performed, and the discharging is terminated when the voltage reaches 3.0 V. The relationship between the number of charge / discharge repetitions and the discharge capacity retention ratio (the first discharge capacity was set to 100%) was examined. The result is shown in FIG.

The discharge capacity per unit volume and the number of occurrences of internal short-circuits when the same charge / discharge was performed as described above for the non-aqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 (Out of 100) were examined. The results are shown in Table 1 below.

[0061]

[Table 1]

As is apparent from FIG. 2, the secondary batteries of Example 1 and Comparative Example 3 using a separator made of a microporous polyethylene film having a hardness smaller than the hardness of the positive electrode active material layer and the negative electrode active material layer, The secondary battery of Example 2 in which first and second polyethylene microporous films having hardnesses smaller than the hardness of the positive electrode active material layer and the negative electrode active material layer are disposed on both surfaces of the third polyethylene microporous film having a low ratio. Is a secondary battery of Comparative Example 1 using a separator made of a polyethylene microporous membrane having a hardness greater than the hardness of the negative electrode active material layer, and a hardness that is greater than the hardness of each of the positive electrode active material layer and the negative electrode active material layer It can be seen that the decrease in discharge capacity due to repetition of charging and discharging is smaller than that of the secondary battery of Comparative Example 2 using a separator made of a polyethylene microporous membrane having the following.

On the other hand, as is apparent from Table 1, the secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 have higher discharge capacities per unit volume than Comparative Example 3.

The secondary batteries of Examples 1 and 2 and Comparative Example 1 have high reliability without any internal short circuit. On the other hand, in FIG. 2 described above, it can be seen that the secondary battery of Comparative Example 3 in which the decrease in the discharge capacity due to the repetition of charge / discharge is low in the number of occurrences of internal short-circuits and low in reliability.

In the above embodiment, a cylindrical non-aqueous electrolyte secondary battery has been described as an example. However, the present invention can be similarly applied to a square or flat non-aqueous electrolyte secondary battery.

[0066]

As described above in detail, according to the present invention, high performance and high reliability without reducing the discharge capacity due to repetition of charging / discharging without causing a reduction in capacity per unit volume and occurrence of internal short circuit. The non-aqueous electrolyte secondary battery of the present invention can be provided.

[Brief description of the drawings]

FIG. 1 is a half-mount diagram showing a non-aqueous electrolyte secondary battery according to the present invention.

FIG. 2 is a diagram showing the relationship between the number of charge / discharge repetitions and the discharge capacity retention ratio in the secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Container, 3 ... Power generation element, 4 ... Positive electrode, 5 ... Separator, 6 ... Negative electrode 8 ... Insulating sealing plate.

Claims (3)

[Claims] 1. A power generating element in which a positive electrode having a positive electrode active material layer formed on at least one surface of a current collector and a negative electrode having a negative electrode active material layer formed on at least one surface of the current collector are laminated with a separator interposed therebetween. A nonaqueous electrolyte secondary battery including an electrolyte, wherein the separator has a hardness lower than any of the hardness of the positive electrode active material layer and the hardness of the negative electrode active material layer, and
% Non-aqueous electrolyte secondary battery having a porosity of not more than 0.1%. 2. A power generating element in which a positive electrode having a positive electrode active material layer formed on at least one surface of a current collector and a negative electrode having a negative electrode active material layer formed on at least one surface of the current collector are laminated with a separator interposed therebetween. A nonaqueous electrolyte secondary battery including an electrolyte, wherein the separator has a structure in which two or more material layers having different hardnesses are stacked, and the material layer in contact with the positive electrode active material layer is A non-aqueous electrolyte secondary battery having a hardness lower than the hardness of the positive electrode active material layer, and wherein the material layer in contact with the negative electrode active material layer has a hardness lower than the hardness of the negative electrode active material layer. 3. The separator according to claim 1, wherein a material layer having a porosity of 70% or less is disposed between a material layer in contact with the positive electrode active material layer and a material layer in contact with the negative electrode active material layer. The non-aqueous electrolyte secondary battery according to claim 2.