JP2840357B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery having a negative electrode made of lithium or a lithium alloy, a positive electrode, and a separator interposed between the positive and negative electrodes. In particular, it relates to improvement of a positive electrode.

2. Description of the Related Art Molybdenum trioxide, vanadium pentoxide, titanium or niobium sulfide has been proposed as a positive electrode active material for a secondary battery of this type, and some of them have been put to practical use.

On the other hand, manganese dioxide and fluorocarbon are known as typical examples of a positive electrode active material of a non-aqueous primary battery, and these have already been put to practical use. In particular, manganese dioxide has the advantage of being excellent in preservability, abundant in resources, and inexpensive.

In view of such a background, it is considered useful to use manganese dioxide as the positive electrode active material of the non-aqueous secondary battery. However, manganese dioxide has difficulty in reversibility and has a problem in charge / discharge cycle characteristics.

Therefore, the applicant of the present application has proposed to use manganese dioxide containing Li ₂ MnO ₃ as a positive electrode active material as described in JP-A-63-114064 in order to suppress the above-mentioned disadvantages when using manganese dioxide. is suggesting. With such a structure, since the crystal structure has a reversibility with respect to intrusion and desorption of lithium ions, improvement in cycle characteristics is recognized.

On the other hand, lithium is used as the negative electrode of the secondary battery. However, in this case, during discharge, lithium, which is the negative electrode active material, dissolves as ions, and during charging, the reverse reaction causes a reaction to deposit as metal lithium on the negative electrode. Therefore, there has been a problem that the internal short circuit is caused by reaching the positive electrode.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 52-5423, a negative electrode using a lithium-aluminum alloy has been proposed. This is because in the case of lithium alone, when lithium is ionized and eluted by discharge, the negative electrode surface becomes uneven, and during subsequent discharge lithium concentrates intensively on the protrusions and grows in a dendritic manner. Therefore, in the case of a lithium-aluminum alloy, in order to restore lithium to form an alloy with aluminum serving as a base of the negative electrode during charging,
This is because there is an advantage that the dendritic growth of lithium can be suppressed.

However, even in this case, when the charge / discharge cycle is repeated, finely divided lithium metal is generated on the alloy surface, and the utilization rate of lithium decreases. In addition, since the electrolytic solution is absorbed by the finely divided negative electrode, the amount of the electrolytic solution between the positive and negative electrodes decreases, and the cycle characteristics deteriorate.

Problems to be Solved by the Invention By the way, when the battery is charged and discharged, an electrode effect is concentrated on a peripheral portion due to an edge effect. Therefore, the dendritic growth and pulverization of lithium easily occur particularly in the peripheral portion of the negative electrode. Therefore, in order to improve the cycle characteristics of the battery, it is necessary to suppress the edge effect.

The present invention has been made in view of such a situation, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of suppressing dendritic growth and pulverization of lithium and improving cycle characteristics.

Means for Solving the Problems In order to achieve the above object, the present invention provides a nonaqueous electrolyte secondary battery having a negative electrode made of lithium or a lithium alloy, a positive electrode, and a separator interposed between the positive and negative electrodes. Wherein the active material having a large discharge capacity is used at the center of the plane of the positive electrode plate, and the active material having a small discharge capacity is used at the periphery of the plane of the positive electrode plate.

Operation With the above configuration, the discharge capacity per unit area of the positive electrode is smaller at the periphery of the plane than at the center of the plane of the cathode plate. Therefore, the active material utilization rate in the negative electrode is smaller in the peripheral portion than in the central portion. As described above, the active material utilization rate can be reduced in the peripheral portion of the negative electrode where the current tends to concentrate due to the edge effect, so that the growth of dendritic crystals of lithium and the pulverization of the lithium alloy in the peripheral portion of the negative electrode are suppressed. Will be.

Further, since the discharge capacity is reduced only in the peripheral portion, the discharge capacity of the battery is not reduced much.

Embodiment An embodiment of the present invention will be described below with reference to FIGS.

[Example I]

FIG. 1 is a half sectional view of a non-aqueous electrolyte secondary battery according to the present invention, and a positive electrode 1 using manganese dioxide containing Li ₂ MnO ₃ as an active material, a negative electrode 2 using lithium as an active material, An electrode group 4 composed of a polypropylene separator 3 interposed between the positive and negative electrodes 1 and 2 is spirally wound. The electrode group 4 is arranged in a negative electrode can 6, and the negative electrode can 6 and the negative electrode 2 are connected by a negative electrode lead 5. A positive electrode cap 8 is attached to the upper opening of the negative electrode can 6 via a packing 7, and the positive electrode cap 8 and the positive electrode 1 are connected by a positive electrode lead 9. The negative electrode can 6 and the negative electrode lead 5 and the positive electrode cap 8 and the positive electrode lead 9 are fixed by spot welding.

Here, it has been experimentally confirmed that in the Li ₂ MnO ₃ -containing manganese dioxide used as the positive electrode active material, the discharge capacity decreases as the Li ₂ MnO ₃ content increases. Therefore, in the positive electrode 1 of this embodiment, Li ₂ MnO
_{By making} the content of ₃ different, the discharge capacity of the central portion of the positive electrode plate and the discharge capacity of the peripheral portion of the flat plate are made different. Hereinafter, a specific method for forming the positive electrode 1 will be described.

(Method of preparing positive electrode mixture having large discharge capacity) First, LiOH and chemical manganese dioxide were mixed in a weight ratio of 10:90.
And then heat-treated in the air (temperature: 375 ° C.) for 20 hours to produce Li ₂ MnO ₃ -containing manganese dioxide as a positive electrode active material. Next, 80% by weight of this positive electrode active material, 10% by weight of acetylene black as a conductive material, and
Add 10 wt% of PTFE dispersion. Subsequently, these were kneaded to prepare a positive electrode mixture having a large discharge capacity.

The positive electrode mixture thus prepared is hereinafter referred to as (a ₁ ) mixture.

(Preparation method for positive electrode mixture with small discharge capacity) Except for mixing LiOH and chemical manganese dioxide in a weight ratio of 20:80, a mixture is prepared in the same manner as the above (a ₁ ) mixture. did.

The positive electrode mixture thus prepared is hereinafter referred to as (a ₂ ) mixture.

Here, both the (a ₁ ) mixture and the (a ₂ ) mixture are Li ₂ M
Although manganese dioxide containing nO ₃ is used as the active material, L
The content of i ₂ MnO ₃ is increasingly towards (a ₁₎ a fixed combination than (a ₂₎ mixture. Therefore, at the time of producing the battery, the portion using the (a ₁ ) mixture has a larger discharge capacity than the portion using the (a ₂ ) mixture.

(Method for producing an electrode using the above two mixtures) First, a stainless lath body (length 100 mm, width 40 mm) 11 as a positive electrode current collector shown in FIG. The mixture (a ₂ ) rolled into a sheet is placed. Next, the work is rolled so that the total thickness becomes 0.8 mm. Next, the mixture (a ₂ ) protruding outside the lath body 11 is removed, and the lath body 11 of the mixture (a ₂ ) crimped to both lath surfaces as shown in FIG. Within 5mm from the periphery (a
₂ ) Remove the mixture (a ₂ ) at other parts (center), leaving only the mixture. Thereafter, the mixture (a ₁ ) previously rolled into a sheet is placed on both surfaces of the lath body 11,
The work is rolled again so that the total thickness becomes 0.8 mm. Thereafter, as shown in FIG. 3 (c), the mixture around the lath body 11 was removed, and a vacuum heat treatment was performed at 250 ° C. to produce the positive electrode 1.

In the positive electrode 1 thus manufactured, the content of Li ₂ MnO ₃ is large (the discharge capacity is small) in the periphery of the flat surface of the positive electrode plate.
(A ₂ ) While the mixture is placed, the center of the plane of the positive electrode plate is
Li ₂ MnO ₃ content is relatively small (discharge capacity is small)
(A ₁₎ so that the mixture is placed.

On the other hand, as the negative electrode 2, a 0.4 mm-thick lithium plate cut into a length of 110 mm and a width of 40 mm was used. As an electrolytic solution, a mixed solvent of propylene carbonate and 1,2-dimethoxyethane was used as a perchloric acid. Lithium dissolved at a rate of 1 mol / was used.

The battery fabricated in this manner is hereinafter referred to as (A) battery.

[Comparative Example I]

Wherein on both sides of the lath (a ₁₎ a fixed combination only crimping, other to produce a rolled to the positive electrode, the battery was fabricated in the same manner as described above in Example I.

The battery fabricated in this manner is hereinafter referred to as (X ₁ ) battery.

(Comparative Example II)

A battery was produced in the same manner as in Example I above, except that only the mixture (a ₂ ) was pressed and rolled on both surfaces of the lath body to produce a positive electrode.

The battery fabricated in this manner is hereinafter referred to as (X ₂ ) battery.

[Experiment]

(A) the battery of the present invention and the (X ₁ ) battery of the comparative example,
(X ₂ ) The cycle characteristics of the battery were examined, and the results are shown in FIG. The experimental conditions were as follows: charge current 50 mA, end voltage
After charging to 4.0 V, the condition is that the battery is discharged to a final voltage of 2.0 V with a discharge current of 50 mA.

As shown in FIG. 3, (A) the battery has a high discharge capacity at the beginning of the cycle and a long cycle life,
The (X ₁ ) battery has a high discharge capacity at the beginning of the cycle but a short cycle life, and the (X ₂ ) battery has a low discharge capacity from the beginning of the cycle.

This is because (A) in the positive electrode of the battery, the discharge capacity per unit area of the peripheral portion is smaller than that of the central portion because the Li ₂ MnO ₃ content is larger in the peripheral portion of the positive plate than in the central portion of the planar plate. . Therefore, in the negative electrode opposed to the positive electrode, current concentration in the peripheral portion is suppressed, and as a result, it is possible to suppress the growth and pulverization of lithium dendritic crystals in the peripheral portion of the negative electrode. On the other hand, in the (X ₁ ) battery, since the Li ₂ MnO ₃ content is small in all parts (regardless of the peripheral part and the central part), current concentration occurs in the peripheral part, and the lithium dendrites in the peripheral part. Crystal growth and pulverization occur. In addition, in the (X ₂ ) battery, since the content of Li ₂ MnO ₃ is large in all parts, it is considered that the effect that the discharge capacity per unit area becomes smaller than the current concentration in the peripheral part becomes larger. Can be

In the above example, two kinds of manganese dioxide having different contents of Li ₂ MnO ₃ are used as the positive electrode active material,
The present invention is not limited to this, and the same effects as described above can be obtained if a positive electrode is manufactured using two types of positive electrode active materials having different discharge capacities.

Further, the range in which the positive electrode active material having a small discharge capacity is arranged is preferably set as follows in order to prevent the discharge capacity of the entire positive electrode from being reduced. That is, in the case of a rectangular electrode plate used for a cylindrical battery or a prismatic battery, the distance from the peripheral edge to the inner end is within 1/4 of the length of the short side of the electrode plate, and the circular electrode used for the flat battery is not used. In the electrode plate, it is desirable that the distance from the outer peripheral end to the inner end of the circle is set within 1/4 of the diameter of the circle. Further, in the case of electrodes other than those described above, it is preferable to set according to the above criteria.

In addition, although lithium is used for the negative electrode in the above-described embodiment, it goes without saying that a similar effect can be obtained even if a lithium alloy is used.

Effect of the Invention As described above, according to the present invention, it is possible to suppress the growth of lithium dendrite crystals and pulverization of lithium alloy in the negative electrode peripheral portion, and it is possible to maintain a high discharge capacity, There are effects such as that the cycle characteristics of the non-aqueous electrolyte secondary battery can be dramatically improved.

[Brief description of the drawings]

FIG. 1 is a half sectional view showing an example of the non-aqueous electrolyte secondary battery of the present invention, FIG. 2 is a perspective view showing a method for producing a positive electrode, and FIG. 9 is a graph showing the cycle characteristics of the (X ₁ ) battery and the (X ₂ ) battery. 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Separator.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01M 4/00-4/04 H01M 4/36-4/62 H01M 10/36-10/40

Claims (1)

(57) [Claims] 1. A non-aqueous electrolyte secondary battery having a negative electrode made of lithium or a lithium alloy, a positive electrode, and a separator interposed between the positive and negative electrodes, wherein the active material of the positive electrode is a flat surface of a positive electrode plate. A non-aqueous electrolyte secondary battery characterized in that an active material having a large discharge capacity is used in a central portion and an active material having a small discharge capacity is used in a peripheral portion of a plane of a positive electrode plate.