JP2002270225A - Lithium secondary battery - Google Patents

Abstract

(57) [Summary] [Problem] A lithium ion secondary battery is 400 Wh / L.
With an increase in capacity beyond that described above, it becomes incompatible with a conventional electrolyte volume configuration. SOLUTION: When the total amount of the non-aqueous electrolyte in the battery is defined as Q, Q is a positive electrode using lithium cobalt oxide as a main positive electrode active material, a negative electrode using graphite as a main negative electrode active material, and the polymer film. , The space between the electrode and the polymer film, the space between the side surface of the group consisting of the electrode and the polymer film and the inner wall of the battery case, and the space in the battery (upper and lower portions of the group) Vy (Vx + 0.4V
y) ≦ Q ≦ (Vx + 0.8Vy).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-capacity lithium secondary battery having improved battery characteristics, particularly cycle characteristics, by optimizing the amount and characteristics of electrolyte solution in the battery of the lithium secondary battery. is there.

[0002]

2. Description of the Related Art The charge and discharge characteristics and cycle characteristics of a lithium secondary battery largely depend on the type of active material of the positive electrode and the negative electrode, the type of the electrolytic solution, and the like. The relationship between the voids is also an important factor.

It has been proposed that a lithium secondary battery having excellent charge / discharge characteristics be obtained by configuring a battery by defining the amount of electrolyte per 1 Ah of battery capacity as the amount of electrolyte in a lithium secondary battery. (Japanese Unexamined Patent Publication No.
2311973 and JP-A-2000-285959).

In a stacked battery, the amount of electrolyte per volume in the battery is regulated, and a separator having a high liquid retention rate is used, so that the battery is free from the power generation elements (the positive electrode, the negative electrode and the negative electrode which are power generation elements). In order to reduce the ratio of the electrolyte present in a state (not permeated into the separator) and to suppress the leakage of the electrolyte due to an increase in the internal pressure of the battery, the liquid retention rate of the separator is set to 1.5 g / cc or more. There has also been proposed a configuration in which a battery is configured with an amount of electrolyte solution per volume of 0.2 cc or more and less than 0.4 cc (Japanese Patent Laid-Open No. 7-28)
No. 2818).

Further, in a lithium secondary battery using a negative electrode made of an organic fired body, a rise in internal pressure of gas is suppressed, liquid leakage due to an increase in internal pressure, battery breakage, etc. are prevented, and a long-life and highly reliable battery is obtained. For this purpose, it has been proposed to provide a space of 0.3 cc or more per 1 Ah of capacity in a container (Japanese Patent No. 2,646,657).

[0006] In addition, in a non-aqueous electrolyte secondary battery using a positive electrode made of lithium manganese oxide, a positive electrode,
It has been proposed that when the sum of the void volumes calculated from the porosity of each of the negative electrode and the separator is 1, the amount of the non-aqueous electrolyte is 0.8 to 1.5. JP-A-2000-294294).

[0007]

In recent years, the capacity of lithium secondary batteries has been increased, and research and development of batteries having a volume capacity density exceeding 400 Wh / L have been promoted.

As one of the techniques for increasing the capacity of the battery, it has been promoted to increase the density of the mixture in the electrode plate and to pack power generating elements into the battery volume as much as possible. In this case, the space that does not become a power generation element per capacity needs to be reduced as much as possible, and the porosity in the electrode plate group is also reduced.

[0009] As described above, when the porosity in the electrode plate group is reduced, a method of defining the liquid amount per volume (Japanese Patent Laid-Open No. 9-23)
In 1973 and JP-A-2000-285959, the amount of free liquid not involved in charge and discharge increases,
Not suitable for high capacity.

[0010] Conversely, it has been proposed to eliminate the free state by using a separator having a high liquid retention rate as in the configuration described in Japanese Patent Application Laid-Open No. 7-282818. A thin and low porosity material must be used and cannot be applied.

The structure described in Japanese Patent No. 2,646,657, in which a void of 0.3 cc or more per 1 Ah is provided in a container is effective when the anode is an organic fired body, but has a high capacity. In a battery, a graphite-based material having a small irreversible capacity has to be used as the negative electrode active material in order to increase the capacity, and thus cannot be applied.

Further, when the sum of the void volumes calculated from the porosity of each of the positive electrode, the negative electrode, and the separator described in JP-A-2000-294294 is 1, the amount of the nonaqueous electrolyte is 0 .8 to 1.5,
When the positive electrode is made of lithium manganese oxide, it is effective for suppressing lithium consumption and suppressing capacity deterioration. However, in a high capacity battery, the positive electrode active material is also required to have a high discharge capacity because of the high capacity. Therefore, a lithium cobalt oxide having a high energy density and a high weight energy density must be used and cannot be applied.

As described above, the electrolyte having the optimum viscosity and having the optimum viscosity permeates the positive and negative electrodes and the pores in the polymer film and the space between the electrode and the polymer film which are the power generation elements in the battery. There is a problem that the technology for forming the optimum amount of electrolyte solution is not sufficient.

[0014] As described above, the present invention has been made to solve the above problems, and has as its object to secure a high capacity and a long life of a lithium secondary battery having a small void, in which an electrolyte is composed of a positive electrode, a negative electrode and a polymer film. By optimally filling the voids in the space and the space between the electrode and the polymer film that serves as the separator, the ion conductivity of lithium ions in the electrode plate and between the electrode plate and the polymer film is improved, and it is a power generation element. A lithium secondary battery that can make maximum use of the positive electrode active material and the negative electrode active material can be provided.

[0015]

In order to achieve the above object, in the lithium secondary battery of the present invention, when the total amount of electrolyte in the non-aqueous electrolyte is Q, Q is the positive electrode, and the negative electrode is the negative electrode. And the total pores Vx of the polymer film, the space between the electrode and the polymer film, and the space between the side surface of the electrode group consisting of the electrode and the polymer film and the inner wall of the battery case, and the inner space of the battery ( (Vx + 0.4Vy) ≦ Q ≦ (Vx + 0.8Vy).

By making the structure of the invention as described above, a high capacity and long life lithium secondary battery can be obtained. In the battery of the present invention, even when the electrode group expands due to repetition of charge and discharge, there is no liquid withering, and there is no extra free liquid. Therefore, the battery is suitable for high capacity and has little liquid leakage.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION A lithium secondary battery according to claim 1 of the present invention comprises a positive electrode using lithium cobalt oxide as a main positive electrode active material and a negative electrode using natural graphite or artificial graphite as a main negative electrode active material. A porous polymer membrane disposed between them, a non-aqueous electrolyte, and a volume capacity density of 4
In the lithium secondary battery of 00 Wh / L or more, when the total amount of the electrolyte in the nonaqueous electrolyte is Q, Q is the total pore Vx of the positive electrode, the negative electrode, and the polymer film, and And the space between the side surfaces of the group consisting of the electrodes and the polymer film and the inner wall of the battery case, and the space inside the battery (upper and lower parts of the group) Vy, (Vx + 0.4 Vy ) ≦ Q ≦ (Vx + 0.8Vy).

In the battery of the present invention, the positive electrode is made of a high-voltage, high-capacity lithium-cobalt oxide as a main active material in order to increase the capacity.

In the negative electrode of the present invention, natural graphite or artificial graphite having a small irreversible capacity and a high capacity is used as a main negative electrode active material.

As the porous polymer membrane serving as the separator, a conventionally known porous polymer membrane may be used, but a thin, highly safe polyethylene or the like is used.

As for the non-aqueous electrolyte, as non-aqueous solvents, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) can be used. A mixture of two or more chain carbonates is preferred. As the electrolyte salt, a conventionally known lithium salt such as LiPF ₆ or LiBF ₄ can be used.

In the present invention, the space between the electrode and the polymer film and between the side surface of the group consisting of the electrode and the polymer film and the inner wall of the battery case and the inner space of the battery with respect to the total amount Q of the electrolyte solution. (Upper and lower parts of the group) By defining the amount of the electrolytic solution with respect to Vy, a high capacity and long life lithium secondary battery can be obtained. Here, the total electrolyte Q is (Vx +
If it is smaller than 0.4 Vy), the liquid absorption due to the expansion of the electrode plate group when the battery is repeatedly charged / discharged cannot be replaced, and the solution will wither. On the other hand, if it is larger than (Vx + 0.8 Vy), the amount of liquid in a free state not involved in charge / discharge increases, which is unsuitable for increasing the capacity and also increases the risk of liquid leakage.

The lithium secondary battery according to a second aspect of the present invention is the lithium secondary battery according to the first aspect, wherein the volume of the positive electrode is V1, the volume of the negative electrode is V2, and the volume of the polymer film is V1. V3, the porosity of the positive electrode is β1, the porosity of the negative electrode is β2, and the porosity of the polymer film is β3, and the total pore volume of the positive electrode, the negative electrode, and the polymer film is The group space volume Vx is calculated as Vx = V1β1 + V2β2 + V3β3, and the electrolytic solution permeates the group space volume Vx.

The reason for this is that the charge / discharge characteristics of the battery are caused by the state of filling of the electrolyte in the power generating element, and it is necessary that a sufficient amount of the electrolyte fills the pores of the electrode and the pores of the polymer film. If the electrolyte solution is sufficient, the positive electrode and the negative electrode transfer lithium ions via the separator and the speed of charge transfer between particles in the electrodes, that is, the mobility of diffusion of lithium ions is improved.

In this case, in a high-capacity battery exceeding 400 Wh / L, it is preferable that the group volume Vg occupies 90% or more of the space in the battery in that the power generating elements are packed in the battery as much as possible.

At this time, the porosity β1 of the positive electrode and the porosity β2 of the negative electrode are 10% or more and 50% or less as measured by a mercury porosimeter, and the porosity β3 of the polymer film is Is preferably 30% or more and 50% or less as measured by a mercury porosimeter.

The reason is that when β1 and β2 are less than 10%, the liquid circulation is poor, and when β1 and β2 are more than 50%, it is not suitable for increasing the capacity. Further, if β3 is less than 30%, the high-rate charge / discharge characteristics deteriorate, and if it exceeds 50%, safety such as overcharge characteristics and internal short circuit prevention cannot be maintained.

The lithium secondary battery according to a fifth aspect of the present invention is the lithium secondary battery according to the first aspect, wherein the space between the electrode and the polymer membrane and between the group side surface and the inner wall of the battery case and the inside of the battery are provided. The space (upper and lower parts of the group) Vy is a space Vcell in the battery, and when the volume of the lead, tape and frame which are members existing in the battery is Vp, Vy = Vcell−Vg−Vp, The space Vy is filled with an electrolytic solution. The reason for defining the electrolyte that fills the space between the electrode and the polymer film and between the side surface of the group and the inner wall of the battery case and the space Vy in the battery (upper and lower parts of the group) is as follows.

Although there is almost no gap in design between the electrode and the polymer film, it is considered that the electrolyte passes through some gap when the electrolyte is penetrated into each power generating element. It is also considered that the gap between the group and the battery inner wall has a complementary relationship. If no electrolyte is present in these gaps, a space is formed between the electrode and the polymer film, and the diffusion of lithium ions during high-rate charging and discharging is hindered. Therefore,
It is necessary that the space between the electrode and the polymer film and the space between the side of the electrode plate group and the inner wall of the battery case are filled with the electrolyte.

In order to maintain good charge / discharge characteristics in the battery, the electrolytic solution is present in the space inside the battery existing in the upper and lower spaces not serving as the power generating element. The liquid must be present. This is because the positive electrode active material and the negative electrode active material repeatedly expand and contract in volume due to exchange of lithium ions, and the space inside the battery fluctuates slightly. And the lower part) must be filled with electrolyte.

When the electrolyte becomes larger than 0.8 in the space between the electrode and the polymer film and between the side surface of the group and the inner wall of the battery case and the space (upper and lower part of the group) Vy in the battery, charging and discharging are performed. The amount of liquid in a free state that is not involved increases, which is not suitable for increasing the capacity, and also increases the risk of liquid leakage.

The lithium secondary battery according to a sixth aspect of the present invention is the lithium secondary battery according to any one of the first to fifth aspects, wherein the viscosity of the electrolyte is 0.5 cp or more and 5 c or less.
p or less, and the contact angle θ with respect to the positive electrode, the negative electrode and the surface of the polymer film is 0 <θ <80 °.

When the viscosity exceeds 5 cp, it is difficult to fill the pores in the electrode plate with the liquid. If it is less than 0.5 cp, there is no problem in terms of the sufficiency of the electrolytic solution, but the conductivity is substantially lowered and the battery characteristics are deteriorated.

Further, even if the viscosity is within the above range,
When the contact angle is 80 ° C. or more, it is difficult to fill the holes.

In this case, from the viewpoint of battery characteristics, the lithium salt dissolved in the solvent of the electrolytic solution is 0.5 mol / mol.
It is preferably from 1 to 2.0 mol / l.

[0036]

<Experiment 1> FIG. 1 shows a structure of a lithium secondary battery sealed with an outer package made of a metal material of the present invention. Figure 1
In the figure, 1 is a positive electrode, 2 is a negative electrode, 3 is a microporous polyethylene film as a polymer film separating the positive electrode and the negative electrode, 4 is a positive electrode lead, 5 is a negative electrode lead, and 6 is a frame. Polymer membrane 3,
The negative electrode 2 and the positive electrode 1 are ultimately housed in the battery case 7 in a state of being entirely wound. Further, the inside of the battery is sealed by the sealing plate 8.

A method for manufacturing the power generating element of this battery will be described. The positive electrode 1 was prepared by mixing acetylene black as a conductive agent and PVDF as a binder into a positive electrode active material LiCoO ₂ to form a paste, and then applying this to a collector made of Al foil.
After drying and rolling, it was obtained by cutting to a predetermined size. A positive electrode lead 4 was welded to the positive electrode 1 at the end of the current collector.

The negative electrode 2 is prepared by mixing carboxymethylcellulose as a thickener and a binder as a paste into a main graphite powder to form a paste, applying the paste to a current collector made of Cu foil, and drying and rolling. And cut to size. A negative electrode lead 5 was welded to the negative electrode 2 at the end of the current collector.

Reference numeral 3 denotes a commercially available polyethylene microporous membrane serving as a polymer membrane separator. A microporous film 3 was placed between the positive electrode 1 and the negative electrode 2 so as to overlap each other, and was wound into an ellipse as shown in FIG.

After storing the wound power generating element,
The positive electrode lead 4 and the negative electrode lead 5 were inserted into the exterior body 8 (30485 size) in a state where the distal ends thereof protruded outside. The stress ratio of the group inserted here was set to 92% based on the ratio of the sum of the cross-sectional area of the electrode plate, the cross-sectional area of the separator, and the cross-sectional area of the battery exterior.

After injecting a predetermined amount of electrolyte from the opening of the battery case 8 containing the power generating element, the sealing plate 9 is sealed to complete the battery. The electrolyte used was a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l in a solvent in which EC and EMC were mixed at a volume ratio of 1: 1.

The design capacity of this battery is 800 mAh.
And

First, the porosity was calculated from the volume of the pores in the electrode plate based on the volume of the mixture part, the true density of the mixture component, and the weight ratio, and the vacant volume was derived. In this embodiment, the volume of the positive electrode is 2.00 c
c and the mixture volume was 1.73 cc. From the measurement by the mercury porosimeter, the porosity of the positive electrode mixture was 20.
%, And the pore volume was 0.35 cc.

The volume of the negative electrode was 2.05 cc, the volume of the negative electrode mixture was 1.89 cc, and the porosity of the negative electrode mixture was 32% as measured by mercury porosimetry. Was calculated to be 0.58 cc.

The volume of the polymer film is 0.56 cc.
And a porosity of 36 from the measurement result of the mercury porosimeter.
%was gotten. This gave a pore volume of 0.20 cc. Accordingly, the pore volume of the electrode plate in the group and the space in the separator was 1.13 cc.

The space between the electrode and the polymer film and between the side surface of the group and the inner wall of the battery case and the space inside the battery (upper and lower parts of the group) Vy is the space inside the battery Vcell is 5.92 c
c, the volume of the battery internal member 0.17 cc, the volume of the positive electrode, the volume of the negative electrode, and the volume of the separator 1.14c
c was obtained.

Therefore, the amount of the electrolyte is adjusted by the pore volume Vx1.13 cc of the space in the electrode plate and the separator in the group, the space between the electrode and the polymer film, the space between the group side surface and the inner wall of the battery case, and the inside of the battery. Voids (top and bottom of group) Vy1.
The total amount of the electrolyte solution was adjusted to a range of 0.4 to 0.8 times 14 cc, and the solution was injected into the batteries as shown in Table 1 to obtain batteries A to D.

Further, an electrolyte volume per battery capacity of 3.0 cc / Ah was adopted, and the battery capacity was changed from 800 mAh to 2.4 cc. This battery was designated as battery E. However, in practice, the liquid could not be injected into the battery, and the battery could not be manufactured.

[Charge / Discharge Cycle Test] These batteries were subjected to charge / discharge measurement in an environment at room temperature of 20 ° C.

The results of the examples are summarized in Table 1.

[0051]

[Table 1]

As shown in Table 1, the total amount of electrolyte in the battery was Q
Are all the holes Vx of the positive electrode, the negative electrode, and the polymer film.
And the space between the electrode and the polymer film and between the side surface of the group consisting of the electrode and the polymer film and the inner wall of the battery case, and the space inside the battery (upper and lower portions of the group) Vy, Good charge / discharge characteristics were obtained by holding an electrolytic solution satisfying the formula of (Vx + 0.4Vy) ≦ Q ≦ (Vx + 0.8Vy) in the battery.

In the battery A, the capacity is significantly deteriorated due to the shortage of the electrolyte. In the battery D, although the capacity retention ratio is good, the amount of waste electrolyte is large, and there is a possibility of leakage.

<Experiment 2> [Study on Salt Concentration in Electrolyte Solution] Using a battery having the same configuration as in Experiment 1, the amount of the electrolyte solution was set to 2.00 c for studying the salt concentration in the electrolyte solution.
As c, the lithium salt concentration is 0.4 mol / l, 0.5
mol / l, 1.0 mol / l, 1.5 mol / l,
A battery was formed by injecting 2.0 mol / l and 2.5 mol / l. These batteries were designated as batteries F to K in the order of the lithium salt concentration. These batteries were carried out under exactly the same conditions as the charge / discharge measurement of Example 1.

The results of Example 2 are summarized in (Table 2).

[0056]

[Table 2]

As shown in Table 2, the batteries G to J have a high capacity retention ratio. It was confirmed that when the salt concentration was low, the diffusion of lithium ions was poor, and when the ion concentration was high, the viscosity of the electrolyte solution was increased and the permeability of the electrolyte solution was poor, so that the charge / discharge characteristics were deteriorated. At this time, the viscosity was 5 cp or more. Also,
The measurement of the viscosity in the present invention was carried out using a measuring method of JIS K717.

In this embodiment, the porosity of the separator is 41%. However, the same effect as in this embodiment can be obtained when the porosity is in the range of 30% to 50%.

[0059]

As described above, by defining the amount of the electrolyte and the concentration of the lithium salt contained therein, the amount of the electrolyte suitable for a high-density battery having a small amount of voids can be obtained, and a high capacity and a long life can be obtained. Characteristics can be secured. From these two points, the present invention can provide a lithium ion secondary battery having excellent charge / discharge characteristics.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a lithium secondary battery of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode 2 negative electrode 3 polyethylene microporous film 4 positive electrode lead 5 negative electrode lead 6 frame 7 case 8 sealing plate

Continued on the front page (72) Inventor Yoshiaki Nitta 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. ) Inventor Satoshi Kuranaka 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. EA23 EA24 GA28 HA00 HA07 HA09 HA10 HA19

Claims (7)

[Claims] 1. A positive electrode using lithium cobalt oxide as a main positive electrode active material, a negative electrode using natural graphite or artificial graphite as a main negative electrode active material, a porous polymer film interposed therebetween, a non-aqueous Equipped with an electrolyte and has a volume capacity density of 4
In a lithium secondary battery of 00 Wh / L or more, when the total amount of the electrolyte in the non-aqueous electrolyte is Q, Q is the total pores Vx of the positive electrode, the negative electrode, and the polymer film;
The sum V of the space between the electrode and the polymer film, the space between the side surface of the electrode group consisting of the electrode and the polymer film and the inner wall of the battery case, and the space in the battery (upper and lower parts of the group)
A lithium secondary battery represented by y, wherein (Vx + 0.4Vy) ≦ Q ≦ (Vx + 0.8Vy). 2. The volume of the positive electrode is V1, the volume of the negative electrode is V2, the volume of the polymer film is V3, the porosity of the positive electrode is β1, the porosity of the negative electrode is β2, When the porosity of the membrane is β3, the group space volume Vx, which is the total pore volume of the positive electrode, the negative electrode, and the polymer film, is calculated as Vx = V1β1 + V2β2 + V3β3, and the group space volume Vx is filled with the electrolyte. The lithium secondary battery according to claim 1, wherein the lithium secondary battery has penetrated. 3. The lithium secondary battery according to claim 2, wherein Vg occupies 90% or more of the internal space of the battery when the volume including the holes of the electrode group is Vg. 4. The porosity β1 of the positive electrode and the porosity β2 of the negative electrode are 10% or more and 50% or less as measured by a mercury porosimeter, and the porosity β3 of the polymer film is a mercury porosimeter. 30 at the value measured by
The lithium secondary battery according to claim 2, wherein the content is not less than 50% and not more than 50%. 5. The space between the electrode and the polymer film and between the side of the group and the inner wall of the battery case and the space Vy in the battery (upper and lower parts of the group) are defined as a space Vcell in the battery and exist in the battery. 2. The lithium secondary battery according to claim 1, wherein Vy = Vcell−Vg−Vp, where Vp is the volume of the lead, the tape, and the frame, which are members. 3. 6. The electrolyte solution has a viscosity of 0.5 cp or more and 5 c or more.
The lithium secondary battery according to claim 1, wherein a contact angle θ with respect to the positive electrode, the negative electrode, and the surface of the polymer film is 0 <θ <80 °. 7. The lithium secondary battery according to claim 6, wherein the amount of the lithium salt dissolved in the solvent of the electrolyte is 0.5 mol / l or more and 2.0 mol / l or less.