JP2775754B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a chargeable / dischargeable non-aqueous electrolyte secondary battery expected to be used as a power source for various small electronic devices.

[Conventional technology]

Conventionally, a non-aqueous electrolyte battery using a light metal such as lithium or a lithium alloy as a negative electrode active material and using a non-aqueous electrolyte as an electrolyte has a high energy density, low self-discharge, and low storage stability. Because of its superiority, it is widely used as a power supply for electronic wristwatches, pacemakers, or a memory backup power supply.

Non-aqueous electrolyte batteries currently in practical use are primary batteries, but in recent years there has been an increasing demand for the development of high-capacity non-aqueous electrolyte secondary batteries that can be charged and discharged as a power source for various electronic devices.

Generally, as a negative electrode active material of a non-aqueous electrolyte secondary battery,
Metallic lithium, lithium alloy (eg, lithium-aluminum alloy), conductive polymer doped with lithium ion (eg, polyacetylene, polypyrrole, etc.),
Further, an interlayer compound or the like using lithium ions as a guest is used, and a non-aqueous electrolyte obtained by dissolving an electrolyte in an organic solvent is used as an electrolyte.

On the other hand, various materials have been proposed as the positive electrode active material so far.
As described in 54836, TiS ₂ , MoS ₂ , NbSe ₂ , V ₂ O
_{5 and the} like.

The discharge process of a non-aqueous electrolyte secondary battery using these materials proceeds as lithium ions of the negative electrode are occluded (doped) between the layers of the positive electrode active material. The process proceeds when lithium ions are released (undoped) from the interlayer to the negative electrode. That is, charging and discharging are repeated through the process in which lithium ions of the negative electrode enter and exit between the layers of the positive electrode active material.

However, in the above-described positive electrode active material, lithium ions gradually become difficult to be released during the repetition of the charging / discharging process, so that the discharge capacity gradually decreases and the cycle life is shortened. In addition, since these positive electrode active materials are expensive, manufacturing a large-capacity battery increases the cost and cannot compete with nickel-cadmium batteries, which are currently the mainstream of secondary batteries.

Therefore, as a positive electrode active material with less deterioration in discharge capacity due to charge / discharge cycles, excellent cycle life characteristics, and excellent economic efficiency, the applicant of the present application has previously described, for example,
61-257479 discloses a material mainly composed of LiMn ₂ O ₄ .

In such a non-aqueous electrolyte secondary battery, the electric conductivity of the non-aqueous electrolyte is low. Therefore, in order to endure a battery with a large current, it is necessary to reduce the internal resistance of the battery by increasing the electrode area as much as possible. is necessary. For example, as the positive electrode current collector, nickel, iron, stainless steel, titanium, aluminum, and the like have been conventionally used because of relatively excellent corrosion resistance in the battery and low cost. In particular, aluminum has excellent corrosion resistance and electrical conductivity at high voltage,
This is advantageous because a lightweight and large-area foil can be easily obtained. For example, Japanese Patent Application Laid-Open No.
A non-aqueous secondary battery using a μm aluminum foil as a positive electrode current collector is disclosed.

[Problems to be solved by the invention]

By the way, lithium is used as a negative electrode active material,
A negative electrode of a non-aqueous electrolyte secondary battery using a material mainly composed of Mn ₂ O ₄ as a positive electrode active material usually has an excess having a capacity of two to three times or more of a positive electrode capacity in order to secure a sufficient life. Amount of lithium is used. For this reason, if this non-aqueous electrolyte secondary battery is continuously forcibly discharged to a capacity equal to or greater than the positive electrode capacity, lithium that cannot be accommodated between the layers due to doping precipitates on the positive electrode surface, penetrates through the separator, and short-circuits with the negative electrode. There is a risk of fire. In particular, in a cylindrical battery in which a negative electrode and a positive electrode are wound and housed in a cylindrical outer can via a separator, the proportion of the electrode active material in a limited volume is increased to increase the capacity. From the point of view,
In recent years, formation of a large amount of a positive electrode active material on a thin current collector and reduction in thickness of a separator have been performed, and prevention of the above-described accident has become an essential issue.

Therefore, an object of the present invention is to solve the above-described problems and to provide a non-aqueous electrolyte secondary battery with high safety, high capacity, and long life.

[Means for solving the problem]

The present inventors have conducted studies to achieve the above object, and as a result, using aluminum as the material of the positive electrode current collector, and making this thickness enough to alloy an excessive amount of lithium, The inventors have found that a battery with high safety can be obtained by forming an appropriate amount of the positive electrode active material thereon, and have reached the present invention.

That is, the non-aqueous electrolyte secondary battery according to the present invention, a negative electrode comprising an excess amount of lithium with respect to the positive electrode capacity, LiMn ₂ O
_A non-aqueous electrolyte secondary battery in which a positive electrode in which a positive electrode active material layer mainly composed of ₄ is formed on both surfaces of an aluminum current collector and a non-aqueous electrolyte is housed in an outer can together with the non-aqueous electrolyte, The thickness T (μm) of the current collector is expressed by the following formula (I) T ≧ 3.442 (3862B-1.6A) (I) [However, in the formula, A was discharged at a discharge end voltage of 2.0 V for 2 hours. Discharge capacity per unit area of the positive electrode (mAH /
cm ² ), B is the lithium weight per unit area of the negative electrode (g / c
m ² ). ] And the total thickness C (μ) of the positive electrode active material layers formed on both surfaces on the aluminum current collector.
m) is represented by the following formula (II) and formula (III) when 10 ≦ T <30: C ≦ 2T + 120 (II) When 30 ≦ T ≦ 40, C ≦ −2T + 240 (III) Is what you do.

Here, in order to improve the safety of the nonaqueous electrolyte secondary battery, and to realize a high capacity and a long life, the thickness of the aluminum current collector and the thickness of the positive electrode active material layer formed thereon are required. Must be properly selected.

First, when discussing the thickness of the aluminum current collector, from the viewpoint of achieving high capacity, it is necessary that the aluminum current collector be formed sufficiently thin and have a large area. However, when the aluminum current collector is too thin, the aluminum current collector breaks due to insufficient strength at the time of a winding operation in a battery manufacturing process, resulting in a disadvantage such as a short circuit. On the other hand, from the viewpoint of alloying an excessive amount of lithium, it is necessary that the aluminum current collector has a certain thickness. However, if the aluminum current collector is too thick, the number of windings must be reduced within the limited volume of the outer can, and therefore the electrode area cannot be increased. And the output cannot be increased.

The present inventors conducted an experiment using a two-hour rate discharge to determine the thickness of the aluminum current collector satisfying the contradictory conditions. As a result, the maximum amount of lithium that can be doped into the positive electrode active material was determined. Was found to be 1.6 times the discharge capacity required to achieve a normal battery end-of-discharge voltage of 2.0V. Now, the discharge capacity per unit area of the positive electrode (mAH / cm ² ) and the weight of lithium per unit area of the negative electrode (g / cm ² ) when discharging at a 2-hour rate with a discharge end voltage of 2.0 V are denoted by B. Then, the thickness T (μm) of the aluminum current collector required for alloying all the excess lithium with the aluminum current collector in the nonaqueous electrolyte secondary battery is obtained by the following equation (I).

T ≧ 3.442 (3862B-1.6A) (I) In the formula, 3862B on the right side indicates the discharge capacity of lithium per unit area. Therefore, the term (3862B-1.6A) indicates the excess lithium per unit area of the electrode. Shows the discharge capacity.

On the other hand, when discussing the thickness of the positive electrode active material layer, it is necessary that the positive electrode active material layer be formed to be somewhat thick on the aluminum current collector in order to achieve high capacity. However, if the thickness is too large, there is a risk of peeling off from the aluminum current collector during the winding operation in the battery manufacturing process. Therefore, the thickness must be appropriately set according to the thickness of the aluminum current collector. The present inventors experimented by varying the thickness T (μm) of the aluminum current collector and the total thickness C (μm) of the positive electrode active materials formed on both surfaces on the aluminum current collector. As a result, it was found that peeling of the positive electrode active material layer did not occur when the conditions represented by the following formulas (II) and (III) were satisfied.

When 10 ≦ T <30 C ≦ 2T + 120 (II) When 30 ≦ T ≦ 40 C ≦ −2T + 240 (III) The range represented by the above formulas (II) and (III) is the second
It can be illustrated as shown. In this figure, the vertical axis represents the thickness T (μm) of the aluminum current collector, and the horizontal axis represents the total thickness C (μm) of the positive electrode active material layers formed on both surfaces of the aluminum current collector. These conditions are based on the assumption that the thickness of the aluminum current collector is 10 to 40 μm. If the thickness is less than 10 μm, the mechanical strength is insufficient. If the thickness is more than 40 μm, the volume of the aluminum current collector occupying the inside of the battery is reduced. Is relatively large, the battery capacity is reduced, and it is not practical.

As described above, in the non-aqueous electrolyte secondary battery according to the present invention, the thickness of the aluminum current collector is selected as described above, so that the area of the positive electrode can be increased as long as the battery capacity is not reduced. Have been. However, with the increase in the area, the resistance of the current collector itself, the positive electrode lead connected thereto, and the safety valve may not be ignored. Therefore, when the positive electrode lead and the safety valve are made of aluminum having a low electric resistivity similarly to the current collector and they are connected to each other by ultrasonic welding, the battery capacity can be further improved.

The above conditions can be applied to a non-aqueous electrolyte secondary battery having a shape other than the cylindrical shape.

In order to form the positive electrode active material layer on the aluminum current collector as described above in the actual battery manufacturing process, the positive electrode active material is kneaded together with a conductive agent and a binder, and is added onto the aluminum current collector. For example, a method of pressure molding or a method of dispersing in a suitable solvent together with a conductive agent or a binder and applying the dispersion may be employed. At this time, graphite, acetylene black, metal powder and the like are used as the conductive agent, and polyvinylidene fluoride, polyethylene, polyacrylonitrile and the like are used as the binder.

The positive electrode thus produced is wound together with a negative electrode made of lithium foil via a separator, immersed in a non-aqueous electrolyte, and accommodated in an outer can. The non-aqueous electrolyte at this time is not particularly limited, and a conventionally known organic solvent and an electrolyte appropriately combined can be used.

Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane,
1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-
Examples include dioxolan, 4-methyl-1,3-dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole and the like.

Further, as the electrolyte, LiClO ₄ , LiAsF ₆ , LiPF ₆ , L
iBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ L
i and the like can be used alone or in combination.

Further, a conventionally known insulating material can be used as the separator, and examples thereof include polypropylene, polytetrafluoroethylene, polyethylene, and polyacetal.

[Action]

In the non-aqueous electrolyte secondary battery as described above, the mechanism by which excess lithium that cannot be occluded between the layers of the positive electrode active material LiMn ₂ O ₄ alloys with the aluminum current collector is roughly considered as follows. Have been. In the early stages of alloying,
Lithium-aluminum alloys exhibit an (α + β) phase, whose potential is about 0.3 V lower than lithium. However, when the lithium content gradually increases and exceeds 52 atomic% in the alloy composition, a β phase appears. In this β phase, the diffusion rate of lithium is greatly reduced, and the potential approaches the potential of lithium. From such electrochemical facts, it is suggested that when the lithium content in the lithium-aluminum alloy exceeds 52 atomic%, lithium starts to precipitate on the surface.

In the present invention, the aforementioned formula (I) derived experimentally is used.
Thus, the thickness of the aluminum current collector required to alloy excess lithium can be determined. If a non-aqueous electrolyte secondary battery is manufactured according to this result, precipitation of lithium on the surface of the positive electrode can be prevented,
It is possible to prevent accidents such as short circuit and fire.

In the present invention, the thickness of the positive electrode active material layer on the surface of the aluminum current collector is given by the formulas (II) and (III) according to the thickness of the aluminum current collector. If a non-aqueous electrolyte secondary battery is manufactured according to this result, peeling of the positive electrode active material layer does not occur even when the electrode is wound.

Thus, by applying the present invention, it is possible to provide a non-aqueous electrolyte secondary battery with high capacity, high safety and high reliability.

〔Example〕

Hereinafter, examples in which the present invention is applied to a so-called explosion-proof nonaqueous electrolyte secondary battery will be described based on experimental results.

First, regarding the configuration of the explosion-proof non-aqueous electrolyte secondary battery,
This will be described with reference to FIG.

In this battery, a battery in which a negative electrode (2) and a positive electrode (3) are wound via a separator (4) in an outer can (1) is housed as a power generator. In addition, the outermost periphery of the power generator serves as a negative electrode (2) and is in contact with the inner peripheral surface of the outer can (1), and the separator (4) is impregnated with a non-aqueous electrolyte. From the positive electrode (3), a positive electrode lead (3a) is provided for connection with a safety valve described later. An insulating plate (5) is provided above the power generator. A small hole (not shown) through which the positive electrode lead (3a) is inserted is formed in the center of the insulating plate (5). Above the insulating plate (5), there is provided a disc-shaped safety valve (7) held via a gasket (6) fitted on the inner peripheral portion of the outer can (1). A cover (8) serving as a positive electrode terminal of the battery is disposed above the safety valve (7). The safety valve (7) and the lid (8) are disposed in contact with each other at the peripheral portion, and the positive electrode (2) and the lid (8) are connected to the positive electrode lead (3a) and the safety valve (7). Connected through. In addition, the safety valve (7) has a groove (7
a) is formed, and when the pressure in the outer can (1) rises, the groove (7a) is torn and the gas inside escapes to the outside through the gas insertion hole (8a).

A non-aqueous electrolyte secondary battery having the above-described configuration was prepared by the following steps.

First, 86.9 g (1 mol) of commercially available manganese dioxide and 18.5 g (0.25 mol) of lithium carbonate are sufficiently mixed in a mortar, and calcined at 450 ° C. for 1 hour in air on an alumina boat to become a positive electrode active material. LiMn ₂ O ₄ was prepared.

Next, using this LiMn ₂ O ₄ , a positive electrode active material paste was prepared by wet mixing according to the following composition.

LiMn ₂ O ₄ (Positive electrode active material) 87 parts by weight Graphite (conductive agent) 10 parts by weight Polyvinylidene fluoride (binder) 3 parts by weight N-methyl-2-pyrrolidone (dispersant) Proper amount Positive electrode active material paste as described above Using
The results of an experimental study of the thickness of the aluminum current collector and the thickness of the positive electrode active material layer will be described.

Experimental Example 1 In this experimental example, the total thickness of the positive electrode active material layers formed on both surfaces of the aluminum current collector (hereinafter, referred to as both-side coating thickness) C is fixed, and the thickness of the aluminum current collector is set. T
It is an examination of whether or not the cylindrical lithium battery ignites when the value is changed.

First, the positive electrode active material paste was uniformly applied to 30 μm and 40 μm thick aluminum current collectors to a thickness of 80 μm on one side, that is, 160 μm on both sides to form a positive electrode.

Next, each of the above positive electrodes was wound in a roll shape with a lithium foil having a thickness of 80 μm via a separator made of polypropylene, and was plated with nickel and was made of iron.
3.5 mm) and impregnated with a propylene carbonate solution in which LiPF ₆ was dissolved at a concentration of 1 mol / ml, and sealed therein. As shown in FIG. 1, a cylindrical lithium battery L having an outer diameter of 13.8 mm and a height of 42 mm and a cylindrical lithium battery L A lithium battery M was prepared.

In such a cylindrical lithium battery, the above formula (I)
Where A = 5.86 (mAH / cm ² ), B = 0.0043 (g / cm ² ), and the calculated thickness T of the aluminum current collector is 25.
μm or more. Therefore, each of the above cylindrical lithium batteries satisfies this condition.

On the other hand, for comparison, the thickness of the aluminum
Similarly, a cylindrical lithium battery N having a length of m was prepared.

In addition, the thickness of the positive electrode active material layer in each of these cylindrical lithium batteries satisfies the above-mentioned formulas (II) and (III), and therefore, there is no fear of peeling off from the aluminum current collector.

Table 1 shows the results of conducting a constant current forced discharge at a current density of 1 mA / cm ² for each of these batteries and examining the presence or absence of ignition.

As is clear from Table 1, no ignition occurred when the thickness T of the aluminum current collector was 30 μm or 40 μm, but ignition occurred when the thickness T was 20 μm. This ignition was caused by the fact that excess lithium could not be completely alloyed due to an insufficient amount of aluminum, and lithium was deposited on the surface of the positive electrode.

Experimental Example 2 In this experimental example, the thickness T of the aluminum current collector and the coating thickness C of both surfaces of the positive electrode active material were variously changed to determine whether or not the positive electrode active material layer was peeled off on the aluminum current collector. It has been examined.

First, a positive electrode active material paste was prepared in the same manner as in Experimental Example 1 and uniformly applied to both surfaces of an aluminum current collector having a thickness of 5 to 40 μm to a thickness of 100 to 200 μm as shown in Table 2. Thus, a positive electrode was prepared.

Next, each of the above positive electrodes was wound in a roll shape around a 4 mm diameter winding core together with a lithium foil having a thickness of 80 μm via a separator made of polypropylene, and the occurrence of breakage of the positive electrode and peeling of the positive electrode active material layer was examined. .

For comparison, the case where the thickness T of the aluminum current collector was set to 50 μm, which is larger than the preferable range in the present invention, was also examined.

Table 2 shows the results. In the table, the mark “○” indicates that neither the breaking of the positive electrode nor the peeling of the positive electrode active material layer occurs, and the mark “X” indicates that at least one of them occurs.

According to this experiment, first, when the thickness T of the aluminum current collector is 5 μm, the strength of the aluminum current collector itself is insufficient, so that the coating thickness C on both sides of the positive electrode active material is not sufficient.
Also, the positive electrode was broken. In the case where the thickness T of the aluminum current collector is 10 to 50 μm, the breaking of the positive electrode or the peeling of the positive electrode active material layer is caused by the coating thickness C on both sides of the positive electrode active material
Depends on. When the thickness T of the aluminum current collector is 50 μm, the physical inconvenience as described above does not occur if the thickness C of both surfaces of the positive electrode active material is 140 μm or less. It cannot be said that it is a typical thickness.

Therefore, in the present invention, the preferable thickness T of the aluminum current collector is 10 to 40 μm, and the suitable thickness C of the double-sided coating of the positive electrode active material is in the range indicated by oblique lines in FIG.

〔The invention's effect〕

As is clear from the above description, in the nonaqueous electrolyte secondary battery according to the present invention, the thickness of the aluminum current collector is appropriately selected according to the amount of lithium used for the negative electrode. Therefore, even when forced discharge occurs, ignition due to precipitation of excess lithium does not occur, and the safety of the nonaqueous electrolyte secondary battery is improved. Further, since the thickness of the positive electrode active material layer formed on the aluminum current collector is also appropriately selected according to the thickness of the aluminum current collector,
It is possible to provide a high-capacity and highly reliable nonaqueous electrolyte secondary battery without causing breakage or peeling of the positive electrode active material layer in the manufacturing process.

[Brief description of the drawings]

FIG. 1 is a sectional view of a non-aqueous electrolyte secondary battery to which the present invention is applied. FIG. 2 is an explanatory diagram showing the relationship between the thickness T of the aluminum current collector limited in the present invention and the thickness C of coating on both sides of the positive electrode active material. 1 ... outer can 2 ... negative electrode 3 ... positive electrode 3a ... positive electrode lead 4 ... separator 7 ... safety valve

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masayuki Kageyama 1-1, Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture 1-1 Sony Energy Tech Koriyama Plant (58) Field surveyed (Int.Cl. ⁶ H01M 4/02 H01M 10/40

Claims (1)

(57) [Claims] 1. A non-aqueous electrolyte comprising a negative electrode comprising an excessive amount of lithium with respect to the positive electrode capacity and a positive electrode comprising a positive electrode active material layer mainly composed of LiMn ₂ O ₄ formed on both sides of an aluminum current collector. And the thickness T (μm) of the aluminum current collector is represented by the following formula (I) T ≧ 3.442 (3862B-1.6A) (I) [ Where A is the discharge capacity per unit area of the positive electrode (mAH /
cm ² ), B is the lithium weight per unit area of the negative electrode (g / c
m ² ). ] And the total thickness C (μ) of the positive electrode active material layers formed on both surfaces on the aluminum current collector.
m) is represented by the following formula (II) and formula (III) when 10 ≦ T <30: C ≦ 2T + 120 (II) When 30 ≦ T ≦ 40, C ≦ −2T + 240 (III) Non-aqueous electrolyte secondary battery.