JP3066086B2 - Non-aqueous secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention can absorb and release lithium.
Negative electrode using an active material as an active material, and LiCoO _TwoThe active material
Non-aqueous secondary batteries with positive electrodes
The present invention relates to a non-aqueous secondary battery having improved electrical characteristics.

[0002]

2. Description of the Related Art A secondary battery of this type has a high voltage and a high energy density, and has been actively studied in recent years.
As a whole, various materials have been proposed as positive and negative electrode active materials. Specifically, lithium metal has been proposed as a negative electrode active material. However, when lithium metal is used, repetition of charge / discharge generates dendrites and fines, resulting in short-circuiting inside the battery and deterioration of cycle characteristics. In consideration of such problems, a lithium alloy as a negative electrode active material,
Carbon materials and metal oxides have been proposed.

On the other hand, as positive electrode active materials, Li-containing manganese dioxide, molybdenum trioxide, vanadium pentoxide, L
iCoO ₂ , sulfide of titanium or niobium and the like have been proposed, and some of them have been put to practical use. Among these positive electrode active materials, LiCoO ₂ has a potential of Li / Li ⁺ during discharge.
Is very promising because it shows an extremely high potential of 4 V and a large discharge capacity.

[0004]

However, the above L
A battery using iCoO ₂ as a positive electrode active material has a problem that charge / discharge cycle characteristics deteriorate when charge / discharge at a deep depth is repeated. In particular, the potential of LiCoO ₂ is Li / Li ⁺ (electrode potential of the dissolution and deposition reaction of lithium).
However, if the battery is discharged to 2 V or less, the charge-discharge cycle characteristics are significantly deteriorated.

The present invention has been made in view of such circumstances, and has as its object to provide a non-aqueous secondary battery having excellent overdischarge characteristics.

[0006]

In order to achieve the above object, the present invention provides a nonaqueous system comprising a negative electrode using a material capable of absorbing and releasing lithium as an active material and a positive electrode using LiCoO ₂ as an active material. In the secondary battery, the electrode potential of the positive electrode is L
Before the battery voltage becomes 2 V or less with respect to i / Li ⁺ , the battery voltage becomes 0 V
The capacitance ratio between the positive electrode and the negative electrode is set so that

[0007]

The inventor of the present invention repeated various experiments and found that the electrode potential of LiCoO ₂ was ₂ to Li / Li ⁺ .
It has been found that when overdischarge occurs to V or less, the crystal structure of LiCoO ₂ changes, and as a result, the reversibility of charge and discharge is impaired. Therefore, the battery voltage becomes 0 V before the electrode potential of the positive electrode becomes 2 V or less with respect to Li / Li ⁺ as described above, that is, the electrode potential of the positive electrode becomes Li / Li ⁺ .
If the capacity ratio between the positive electrode and the negative electrode is set so that the electrode potential of the negative electrode becomes the same as the electrode potential of the positive electrode before the voltage becomes 2 V or less with respect to ⁺ , discharge occurs until the electrode potential of the positive electrode becomes 2 V or less. Can be prevented. Therefore, even when charging and discharging are repeated, LiCo used as the positive electrode active material is used.
Since the change in the crystal structure of O ₂ can be prevented, the reversibility of LiCoO ₂ can be maintained.

For example, a case where a carbon material is used as a negative electrode active material and LiCoO ₂ is used as a positive electrode active material will be described below with reference to FIGS. 4 and 5. FIG. 4 is a graph showing a change in the negative electrode potential at the time of discharging (when undoping lithium), and FIG. 5 is a graph showing a change in the positive electrode potential at the time of discharging. 4 and 5, at the time of discharging, the negative electrode potential changes from 0 V to 3 V with respect to Li / Li ⁺ , while the positive electrode potential changes from 4 V to 0 V with respect to Li / Li ⁺ . In this case, the capacitance ratio between the positive electrode and the negative electrode is set so that the electrode potential of the negative electrode becomes equal to the electrode potential of the positive electrode before the electrode potential of the positive electrode becomes 2 V or less with respect to Li / Li ⁺ . Specifically, the positive electrode potential is 4V to 2V
Is set to be larger than the negative electrode capacity until the negative electrode potential changes from 0V to 2V. With such a setting, the battery voltage becomes 0 V before the electrode potential of the positive electrode becomes 2 V, so that LiCoO ₂ used as the positive electrode active material was used.
Is not overdischarged. Therefore, LiCoO ₂
Can be maintained reversibly.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS.
This will be described below. FIG. 1 is a cross-sectional view of a non-aqueous secondary battery according to an example of the present invention, in which a positive electrode 1 mainly composed of LiCoO ₂ , a negative electrode 2 mainly composed of coke, and this negative electrode 2 were brought into contact. An electrode group 4 composed of a lithium foil 11 and a polypropylene separator 3 interposed between the positive electrode 1 and the negative electrode 2 or the lithium foil 11 is spirally wound. The electrode group 4 is disposed in the negative electrode can 6.
The negative electrode 2 is connected to the negative electrode 2 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.
Is provided with a coil spring 9 therein. The coil spring 9 is configured such that when the internal pressure inside the battery rises abnormally, it is pressed in the direction of arrow A and the gas inside is released to the atmosphere. The positive electrode cap 8 and the positive electrode 1 are connected by a positive electrode lead 10. The lithium foil 11 is configured to be taken into coke when the electrolyte is injected into the negative electrode can 6. The battery dimensions were 14.2 mm in diameter and 50.0 in height.
mm.

Here, the cylindrical non-aqueous secondary battery having the above structure was manufactured as follows. First, cobalt carbonate and lithium carbonate are mixed at a ratio such that the molar ratio of Co and Li becomes 1: 1 and then heat-treated in air at 900 ° C. for 20 hours. Thereby, LiCoO ₂ powder (positive electrode active material powder) is produced. Next, this LiCo
O ₂ powder, acetylene black as a conductive agent, and a fluororesin dispersion as a binder are mixed at a weight ratio of 90: 6: 4 to prepare a positive electrode mixture. Next, the positive electrode mixture was rolled on a current collector formed of a lath plate made of aluminum, and subjected to a vacuum treatment at 250 ° C. for 2 hours to produce a positive electrode 1. The discharge capacity until the electrode potential of the positive electrode 1 was reduced to 2 V with respect to Li / Li ⁺ was 400 mAh.

On the other hand, in parallel with this, coke of 400 mesh or less and a fluororesin dispersion as a binder are mixed at a weight ratio of 95: 5 to prepare a negative electrode mixture. Next, the negative electrode mixture was rolled on a current collector made of a stainless steel lath plate, and subjected to a vacuum treatment at 250 ° C. for 2 hours to prepare a negative electrode 2. In addition, this negative electrode 2
The discharge capacity until the electrode potential of Li / Li ⁺ increased from 0 V to 2 V with respect to Li / Li ⁺ was 380 mAh, and the discharge capacity until the electrode potential increased from 0 V to 3 V was 450 mAh.

Next, a lithium foil 11 equivalent to 50 mAh is brought into contact with the negative electrode 2, and then the positive and negative electrodes 1, 2, the separator 3 and the lithium foil 11 are spirally wound via a separator 3 to form an electrode group 4. After that, the electrode group 4 is inserted into the negative electrode can 6. Li at a rate of 1 mol / liter
PC (propylene carbonate) with ClO ₄ dissolved
Was injected into the negative electrode can 6, and the negative electrode can 6 was sealed with the positive electrode cap 8 to produce a cylindrical non-aqueous secondary battery.

The battery fabricated in this manner is hereinafter referred to as (A) battery. [Comparative Example] The discharge capacity until the electrode potential of the positive electrode decreased to 2 V with respect to Li / Li ⁺ was set to 350 mAh, while the electrode potential of the negative electrode increased from 0 V to 2 V with respect to Li / Li ⁺ . Is set to 430 mAh, and the discharge capacity is set to 160 mA.
A battery was fabricated in the same manner as in the above example except that a lithium foil corresponding to h was contacted .

The battery fabricated in this manner is hereinafter referred to as (X) battery. [Experiment 1] Discharge characteristics in the first cycle of the battery (A) of the present invention and the battery (X) of the comparative example were examined. The results are shown in FIG. The charging and discharging conditions are as follows.
After charging to a battery voltage of 4.3 V at 0 mA, a discharge current of 1 mA
The condition is that the battery is discharged to 0 V at 00 mA.

As is apparent from FIG. 2, (A) of the present invention
It is recognized that the battery and the battery (X) of the comparative example have substantially the same discharge characteristics. [Experiment 2] The charge / discharge cycle characteristics of the battery (A) of the present invention and the battery (X) of the comparative example were examined. The results are shown in FIG. The charge and discharge conditions are the same as those in Experiment 1.

As is apparent from FIG. 3, (A) of the present invention
The battery has more charge / discharge cycle characteristics than the battery (X) of the comparative example.
It is recognized that the properties have been dramatically improved. this is,
In the battery (A) of the present invention, the positive electrode potential is changed from 4V to 2V.
Until the negative electrode potential goes from 0V to 2V.
Is set larger than the negative electrode capacity at
Can be prevented from dropping to 2 V or less, and LiCoO_TwoPossible
Can maintain inversion. On the other hand, the battery (X) of the comparative example
Then, the positive electrode capacity until the positive electrode potential changes from 4V to 2V
From the negative electrode capacity until the negative electrode potential changes from 0V to 2V.
Is also set small, so that the positive electrode potential is 2 V during overdischarge.
To LiCoO _TwoChanges the crystal structure of
It is thought to be caused by

In the above embodiment, coke is used as the negative electrode active material. However, the present invention is not limited to this.
It is needless to say that the same effect as described above can be obtained even with a substance capable of occluding and releasing lithium (for example, lithium alloy and metal oxide).

[0018]

As described above, according to the present invention, it is possible to prevent discharge until the electrode potential of the positive electrode becomes 2 V or less. Therefore, even when charge and discharge are repeated, the reversibility of LiCoO _{2 as} the positive electrode active material can be maintained, and the effect of dramatically improving charge and discharge characteristics at a deep depth can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a non-aqueous secondary battery according to an example of the present invention.

FIG. 2 is a graph showing first cycle discharge characteristics of a battery (A) of the present invention and a battery (X) of a comparative example.

FIG. 3 is a graph showing charge / discharge cycle characteristics of a battery (A) of the present invention and a battery (X) of a comparative example.

FIG. 4 is a graph showing a change in negative electrode potential during discharging.

FIG. 5 is a graph showing a change in a positive electrode potential during discharging.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 11 Lithium foil

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (1)

(57) [Claims] 1. A non-aqueous secondary battery comprising a negative electrode using a material capable of occluding and releasing lithium as an active material and a positive electrode using LiCoO ₂ as an active material, wherein the electrode potential of the positive electrode is Li / Li ⁺ Before the battery voltage becomes 0 V or less,
A non-aqueous secondary battery, wherein the capacity ratio between the positive electrode and the negative electrode is set so as to be V.