JPH11121040A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent peeling of an electrode active material and suppress the deterioration of the cycle characteristics by covering the internal electrode body, which is formed by winding a positive electrode plate and a negative electrode plate via a separator film so as not to be in contact with each other, with an elastic body of electrolyte resistance and by storing it in a metal battery case. SOLUTION: It is preferable that an elastic body is a heat-shrinkable tube and a polymer having such properties of absorbing nonaqueous electrolyte and expanding. The heat-shrinkable tube is preferable to be one of ethylene- propylene rubber, chloroprene rubber, nitrile-butadiene rubber, vinyl chloride resin, and bridging polyethylene. An internal electrode body 20 which is formed by winging a positive electrode 11 and a negative electrode 12 via a separator film 13 so as not to be in contact with each other is inserted in an aluminium battery case 14 via an insulating sheet so as not to be in direct contact with each other. The heat shrinkage tube 15 is used as the insulating sheet so as to apply compressive stress on the internal electrode body 20 at all times, has drawing parts 16 formed therein, and fixed to the battery case 14.

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 having a low internal resistance of a battery, high charge / discharge efficiency, and low cycle deterioration. Also, the present invention relates to a lithium secondary battery that prevents charge / discharge failures and failures caused by external forces such as vibration and rotation caused by transportation and operation of a moving body.

[0002]

2. Description of the Related Art In recent years, with the rapid miniaturization of electronic devices such as communication devices and personal computers, lithium-ion rechargeable batteries capable of charging / discharging with a large energy density have been used as power sources for these devices. Secondary batteries are attracting attention. In addition, lithium secondary batteries have attracted attention as motor driving batteries for electric vehicles and power storage power sources, and early commercialization of large-capacity lithium secondary batteries is expected.

[0003] The basic characteristics of a lithium secondary battery, including the battery voltage, largely depend on the positive electrode active material. The characteristics include a high electrode potential, a large electric capacity, and a high electrochemical activity. It is desired that the compound be insoluble in an electrolytic solution, be chemically stable, and be electronically conductive. And, as a substance showing such properties,
LiCoO ₂ and LiMn ₂ O ₄ are mainly studied.

On the other hand, the negative electrode active material has a large electric capacity (lithium ion capacity), a good charge / discharge cycle property, a low discharge voltage (high battery voltage) and flatness, and a low packing density. It is required to be large, and as a practical material exhibiting such characteristics, mainly a carbonaceous material is being studied.

[0005] Further, the electrolyte is required to be an aprotic solvent that does not react with the electrode active material and especially lithium, and to have a high dielectric constant and a low viscosity in order to be a highly ionic conductive solution. Here, when a carbonaceous material having a high degree of graphitization is used for the negative electrode active material, propylene carbonate, which is widely used in lithium primary batteries, reacts with propylene carbonate and generates gas upon charging. Therefore, a solution based on ethylene carbonate is used. On the other hand, for a material having a low degree of graphitization, a mixed solution containing an organic solvent such as ethylene carbonate, propylene carbonate, diethyl carbonate, γ-butyrolactone, or dimethoxyethane is used.

Further, the electrolyte to be added to the selected organic solvent for the electrolytic solution has a high solubility in the electrolytic solution in order to enhance ionic conductivity, exists as dissociated ions, and is chemically compatible with the electrode active material. It is required to have the property of producing a stable electrolytic solution that does not react, and it is mainly used for LiPF ₆ and LiBF ₄
Alternatively, a Lewis acid salt such as LiAsF ₆ is preferably used.

As a structure of an internal electrode body of a lithium secondary battery using each of the above members, a wound type shown in FIG. 2 is generally employed. This is because the ionic conductivity of a non-aqueous electrolyte used for a lithium secondary battery is about two orders of magnitude lower than the ionic conductivity of an aqueous electrolyte used for a conventional secondary battery. This is because, in order to obtain the same charge / discharge performance as a battery using an aqueous electrolyte, it is necessary to make the electrode thinner and increase the electrode facing area.

The internal electrode body 1 is configured such that a positive electrode plate 2 and a negative electrode plate 3 are wound via a separator 4 so that a large-area positive electrode plate 2 and the like can be accommodated in a metal cylindrical container.
In the case of such an internal electrode body 1, the number of lead wires 5 from each of the electrode plates 2 and 3 may be at least one, and if it is desired to reduce the current collection resistance from each of the electrode plates 2 and 3 However, since the number of lead wires may be increased, the internal structure of the battery is not complicated, and there is an advantage that the battery can be easily assembled.

The internal electrode body 1 is housed in a metal battery case such that the electrode plates and the lead wires do not come into contact with each other when the battery is manufactured. At this time, one electrode lead wire is connected to the battery case, and the battery case may be electrically connected to an external output terminal as a conductive path.
An insulating sheet is inserted between the internal electrode body 1 and the inner surface of the battery case so that the internal electrode body 1 does not directly contact the battery case.

[0010]

SUMMARY OF THE INVENTION In the above-described lithium secondary battery, for example, the LiC of the positive electrode is increased due to the occlusion and release of lithium during repeated charging and discharging for a long time.
When both the oO ₂ particles and the carbon particles of the negative electrode expand and contract, there is a problem that contact between the particles deteriorates and the internal resistance of the internal electrode body increases. In addition, since the metal foil of each electrode plate expands due to heat generated due to internal resistance during charge and discharge, LiCoO 2 applied to each electrode plate is expanded.
Contact between _two particles or carbon particles worsens,
There is a problem that the internal resistance increases.

In order to solve such a problem, it has been practiced to add a fibrous carbon material to these particles to improve the current collecting ability of the electrode plate. However, since the binder is required, the electrode plate becomes bulky and the absolute capacity of the battery is reduced.

Japanese Patent Application Laid-Open No. 7-73899 describes that an internal electrode body in which a positive electrode plate and a negative electrode plate are alternately stacked with a separator interposed therebetween is made of a heat-expandable resin or a liquid-expandable resin filled with a gas. There is disclosed a lithium secondary battery having a structure in which the resin is sandwiched and accommodated in a battery case, and a compression stress is applied to the internal electrode body by expansion of the resin. When such a heat-expandable resin is used, when the battery temperature is high, the electrode plates and the like are compressed, but when the battery is not used, the adhesion between the electrode plates is reduced. It is presumed that good characteristics are not always obtained in the entire period. Furthermore, when a liquid-swelling resin is used, sufficient pressure cannot be applied to the internal electrode body because the hardness of the resin generally decreases after expansion. Therefore, there is a problem that sufficient expansion characteristics of the resin may not be obtained in the relationship between the compressive force received from the internal electrode body and the liquid absorbing property.

In a large-capacity lithium secondary battery for an electric vehicle, the internal electrode body is sufficiently fixed in a battery case because the battery itself receives an external force such as vibration or rotation during driving of the vehicle. If not, the internal electrode body may move or shift in the battery case, which may cause a charge / discharge failure such as a short circuit or disconnection inside the battery or an overdischarge accident. Therefore, the battery structure must avoid such a situation.

[0014]

Means for Solving the Problems The present invention has been made in view of the above-described problems of charge / discharge characteristics, cycle deterioration, and use safety of a large-capacity lithium secondary battery. That is, according to the present invention, an internal electrode body formed by winding a positive electrode plate and a negative electrode plate via a separator film so that the positive electrode plate and the negative electrode plate do not come into direct contact with each other is a metal battery. A lithium secondary battery housed in a case and using a non-aqueous electrolyte solution, wherein the internal electrode body is covered with an elastic body having an electrolyte resistance. You.

Here, in the lithium secondary battery of the present invention, a heat-shrinkable tube or a polymer having a property of absorbing and expanding a non-aqueous electrolyte is preferably used as an elastic body. As such a heat-shrinkable tube, a tube made of any of ethylene propylene rubber, chloroprene rubber, nitrile butadiene rubber, vinyl chloride resin, and crosslinked polyethylene is preferably used. Also,
It is preferable that the body of the battery case is subjected to drawing processing so that the internal electrode body hardly moves in the battery case.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the lithium secondary battery according to the present invention, since the internal electrode body is always subjected to a compressive stress from the outside by a heat shrink tube or the like, contact of the electrode active material particles applied to the electrode plate is prevented. Since the battery is kept dense and the separation of the electrode active material particles is avoided, the internal resistance of the battery is small, and the cycle deterioration is suppressed. Furthermore, since the internal electrode body is stably held in the battery case by the drawn portion provided in the battery case, it exhibits good resistance to external forces such as vibration and rotation during driving of the vehicle as a power source for an electric vehicle. . Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments.

The internal electrode body used in the lithium secondary battery of the present invention has the same winding type structure as that of FIG. 2 described above, and FIG. 1 shows the internal electrode in a plane including the winding axis. A sectional view near the body and the battery case was shown. As the positive electrode plate 11, one obtained by applying lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material and carbon powder to an aluminum foil having excellent electrochemical corrosion resistance with an electrolytic solution is preferably used.

However, since cobalt is generally expensive, it is inferior in battery characteristics such as electromotive force as LiCoO ₂ as a positive electrode active material, but it is also possible to use inexpensive lithium manganate (LiMn ₂ O ₄ ) or the like. Which positive electrode active material is used is determined by the application, use conditions, cost, and the like of the battery. Further, the carbon powder is added to impart conductivity to the positive electrode active material, and acetylene black, graphite powder, or the like can be used. In addition, it is preferable to use a high-purity material for the aluminum foil constituting the positive electrode plate 11 in order to prevent a decrease in battery performance due to corrosion due to an electrochemical reaction of the battery.

On the other hand, as the negative electrode plate 12, a material obtained by applying an amorphous carbonaceous material such as soft carbon or hard carbon or a carbonaceous powder such as natural graphite to a copper foil as an anode active material is preferably used. Here, as with the aluminum member used for the positive electrode, it is preferable to use a high-purity material for the copper foil used as the negative electrode plate 12 in order to withstand corrosion due to an electrochemical reaction.

As the material of the separator film 13, a material having a three-layer structure in which a lithium ion permeable polyethylene film having micropores is sandwiched between porous lithium ion permeable polypropylene films is preferably used. When the battery temperature rises, the polyethylene film softens at about 130 ° C. and the micropores are crushed to move lithium ions, which also serves as a safety mechanism that suppresses the battery reaction. Between the separator film 13 and the positive and negative electrode plates can be prevented by sandwiching the separator film 13 with polypropylene having a softening temperature higher than that of polyethylene.

In this way, the positive electrode plate 11 and the negative electrode plate 12 are
The inner electrode body 20 wound so that the inner electrode body does not contact with each other is inserted into the battery case 14 made of aluminum. At this time, in order to avoid direct contact between the inner electrode body 20 and the battery case 14, Then, an insulating sheet is inserted between the internal electrode body 20 and the battery case 14.

In the present invention, an elastic body having resistance to an electrolytic solution is used as the insulating sheet. Particularly, a battery is easily manufactured, and a state where a compressive stress is always applied to the internal electrode body 20 is provided. Therefore, the heat-shrinkable tube 15 is preferably used. That is, the wound internal electrode body 20
Is inserted into the heat-shrinkable tube 15 and heat-treated appropriately, whereby the heat-shrinkable tube 15 is sufficiently shrunk so that a compressive stress acts on the internal electrode body 20. In this way, it is possible to increase the density of the internal electrode body 20 and increase the absolute capacity of the battery, to avoid the occurrence of poor contact due to expansion and contraction of the electrode active material particles, and to reduce the internal resistance of the battery to a low level. Can be maintained.

As such a heat-shrinkable tube 15,
Those composed of ethylene propylene rubber, chloroprene rubber, nitrile butadiene rubber, vinyl chloride resin, and crosslinked polyethylene are preferably used. In these heat-shrinkable tubes 15, the shrinkage rate can be controlled by the temperature, and since they have flexibility and elasticity even after shrinkage, they can be easily inserted into the battery case 14.

In this way, the wound body 30 in which the internal electrode body 20 is fastened by the heat-shrinkable tube 15 is inserted into the battery case 14. At this time, the outer diameter of the wound body 30 and the inner diameter of the battery case 14 It is preferable that there is no gap between them. Therefore, it is preferable to insert the wound body 30 into the battery case 14 such that the outer surface of the heat-shrinkable tube 15 is appropriately compressed. However,
Due to the variation in the size of the manufactured wound body 30, the adhesion between the battery case 14 and the wound body 30 may be insufficient, and in this case, the wound body 30 An improper fixation will result.

Therefore, for example, since the heat-shrinkable tube 15 made of a material such as ethylene propylene rubber has a property of absorbing a small amount of the electrolytic solution and expanding, the electrolytic solution is injected after the wound body 30 is inserted into the battery case 14. When the battery is manufactured, the heat-shrinkable tube 15 eventually expands by absorbing the electrolytic solution, the adhesion to the inner surface of the battery case 14 is enhanced, and the wound body 30 is securely fixed in the battery case 14. The reaction causes the internal electrode body 20 to be tightened. Thus, since the wound body 30 is fixed in the battery case 14 so as to be compressed by the battery case 14, the movement of the wound body 30 in the battery case 14 is prevented.

At this time, a suitable composition is appropriately selected in consideration of the characteristics of the positive electrode active material and the negative electrode active material for the electrolytic solution supplied to the internal electrode body 20. Lithium is added to a mixed solution containing propylene carbonate and diethyl carbonate.
Obtained by dissolving a PF ₆ or LiBF ₄ is employed. In addition,
When using the heat-shrinkable tube 15 having an electrolyte absorbing property, it is preferable to adjust the electrolyte concentration of the electrolytic solution in consideration of the absorbed amount.

As described above, the wound body 30 is the battery case 1
In a state where the battery case 14 is compressed and inserted into the battery case 4, by further performing drawing from the outside to the inside of the battery case 14, the winding body 30 can be securely fixed in the battery case 14. The formed drawn portion 16
In (2), the uneven portion is formed on the inner surface of the battery case 14 in such a manner that the heat-shrinkable tube 15 is compressed and deformed, so that the wound body 30 is more stably held in the battery case 14.

Such a drawn portion 16 may be formed in an annular shape along the outer periphery of the battery case 14 or may be formed so as to be dotted with protrusions. It is preferable to form the wound body 30 at the location from the viewpoint of stable holding of the wound body 30. Further, by providing such a drawn portion 16, even the above-described ethylene propylene rubber or chloroprene rubber having a smaller expansion property due to an electrolytic solution than ethylene propylene rubber can be used without any problem. It is also possible to use a heat-shrinkable tube 15 made of a non-expandable material.

[0029]

Hereinafter, examples of the present invention will be described.
LiMn ₂ O ₄ as a positive electrode active material mixed with acetylene black for improving conductivity is applied to an aluminum foil to produce a positive electrode plate. On the other hand, as a negative electrode plate, soft carbon which is a negative electrode active material is used. Was applied to a copper foil. A wound body having a structure shown in FIG. 2 was produced by using a nonwoven fabric made of polypropylene as a separator and separating the produced positive electrode plate and negative electrode plate by the separator. Next, the wound body was inserted into a heat-shrinkable tube made of ethylene propylene rubber, and inserted into a battery case. Then, an electrolyte prepared by dissolving 1 mol% of LiPF ₆ in propylene carbonate (PC) was injected into the battery. The case was sealed to produce a battery.

On the other hand, as a comparative example, it has the same structure as the above-mentioned example, but instead of the heat-shrinkable tube, as a material for securing electrical insulation between the battery case and the wound body, the A battery using a polypropylene separator having no liquid absorbing swelling property and low elasticity was manufactured.

The battery capacity of each of the produced batteries at the time of initial full charge was about 100 Wh (rated capacity of 25 Ah at a full charge voltage of 4 V). The cycle of discharging these batteries at a discharge rate of 0.2 C to a depth of discharge (DOD) of 90% and then fully charging them was repeated, and the discharge capacity in each cycle was measured.

FIG. 3 shows the measurement results. The rate of decrease in the discharge capacity of the battery of the example in which the wound body was held using the heat-shrinkable tube in the present invention was smaller than that of the battery of the comparative example using the conventional polypropylene separator,
Good charge / discharge characteristics were obtained. Further, in any of the batteries, the decrease in the discharge capacity is large in the initial stage of the test in which the number of cycles is less than 40, and the gradient of the characteristic decrease is small after the number of cycles of 40. Characteristically, the characteristic degradation gradient is small, and the characteristic degradation gradient decreases as the number of cycles increases. This suggests that even if the number of tests is further increased, the discharge capacity does not significantly decrease, and that the lithium secondary battery of the present invention has more excellent durability than the conventional lithium secondary battery. It has battery characteristics.

Next, at the end of 100 cycles, the battery was disassembled and the internal structure was inspected. As a result, in the battery of the comparative example using the polypropylene separator,
When a part where the gap between the positive electrode plate and the negative electrode plate is opened is observed, and when the wound body is unwound and the surface of the electrode plate is observed, a part of the electrode active material is a metal foil which is a substrate of the electrode plate. A portion which was peeled off from or swelled up from was observed. In contrast, in the battery of the embodiment using the heat-shrinkable tube,
No abnormality was observed in the shape of the wound body, and there was no peeled portion of the electrode active material and the like inside the wound body, and a portion where the electrode active material was raised was recognized, but the occurrence location was the comparative example. And the area was very small.

Therefore, in the lithium secondary battery of the present invention, the formation of the peeled-off region of the electrode active material and the defective region of the electrical contact state which are not involved in the battery reaction by the charge / discharge test is suppressed, It is considered that a decrease in battery capacity and an increase in internal resistance of the battery were suppressed, and a decrease in battery characteristics was suppressed.

[0035]

As described above, according to the lithium secondary battery of the present invention, the internal electrode body is always subjected to the compressive stress from the heat-shrinkable tube, and the contact of the electrode active material particles applied to the electrode plate is high. In addition, since the electrode active material particles are prevented from peeling off, the internal resistance of the internal electrode body is small, and a remarkable effect of suppressing cycle deterioration is exhibited. In addition, since the wound body (internal electrode body) is stably held in the battery case, when used as a battery for an electric vehicle, the internal electrode is also exposed to external force such as vibration or rotation acting on the battery during driving of the vehicle. An excellent effect of preventing a failure or an accident such as overdischarge due to a short circuit of an electrode inside the battery caused by the body moving inside the battery case and disconnection inside the battery is prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a structure near an internal electrode body and a battery case of a lithium secondary battery of the present invention.

FIG. 2 is a perspective view showing a structure of an internal electrode body of a conventional lithium secondary battery.

FIG. 3 is a graph showing changes in discharge capacity of the lithium secondary battery of the present invention and a conventional lithium secondary battery.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal electrode body, 2 ... Positive electrode plate, 3 ... Negative electrode plate, 4 ... Separator, 5 ... Lead wire, 11 ... Positive electrode plate, 12 ... Negative electrode plate, 1
3 ... separator film, 14 ... battery case, 15 ... heat shrinkable tube, 16 ... drawn part, 20 ... internal electrode body,
30 ... wound body.

Claims (5)

[Claims] An internal electrode body formed by winding a positive electrode plate and a negative electrode plate through a separator film so that the positive electrode plate and the negative electrode plate do not directly contact each other is housed in a metal battery case. A lithium secondary battery using a non-aqueous electrolyte, wherein the internal electrode body is covered with an electrolyte-resistant elastic body. 2. The lithium secondary battery according to claim 1, wherein the elastic body is a heat-shrinkable tube. 3. The lithium secondary battery according to claim 1, wherein the elastic body is a polymer having a property of absorbing a non-aqueous electrolyte and expanding. 4. The lithium secondary battery according to claim 2, wherein said heat shrinkable tube is made of any one of ethylene propylene rubber, chloroprene rubber, nitrile butadiene rubber, vinyl chloride resin, and crosslinked polyethylene. 5. The lithium secondary battery according to claim 1, wherein the body of the battery case is subjected to drawing.