JP2000072444A - Lithium manganate and organic electrolyte secondary battery using the same - Google Patents

Abstract

[PROBLEMS] To improve storage characteristics and cycle life characteristics of an organic electrolyte secondary battery having a positive electrode mainly composed of lithium manganate. Kind Code: A1 Abstract: A lithium manganate capable of electrochemically inserting and desorbing lithium by charge and discharge, wherein Ag or an Ag oxide is fixed on the surface of the lithium manganate particles and used as a positive electrode active material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to lithium manganate and an organic electrolyte secondary battery using the same.

[0002]

2. Description of the Related Art Organic electrolyte secondary batteries typified by lithium secondary batteries are mainly used in portable devices such as VTR cameras, notebook computers and mobile phones, taking advantage of their high energy density. . Particularly in recent years, lithium ion secondary batteries using a carbon material capable of occluding and releasing lithium for a negative electrode have become widespread. The internal structure of this battery is usually wound as shown below. That is,
An active material is applied to a metal foil to form a positive electrode and a negative electrode, wound through a separator, the wound material is stored in a cylindrical can serving as a container, and after pouring the electrolyte, the cap is removed. We attach and seal.

[0003] The carbon material used as the negative electrode active material is in a state in which lithium has been completely released at the time of battery assembly, that is, in a discharged state. Thus, the active material also discharged state positive electrode such as lithium cobaltate (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganese oxide (LiMn ₂ O
₄ ) etc. are used. Then, it becomes possible to function as a battery by the initial charge, and a lithium ion secondary battery is manufactured. However, since the above-mentioned positive electrode active material does not have sufficient electron conductivity, it is used for the positive electrode as a mixture obtained by mixing a conductive powder and a binder, which are inexpensive and stable in a battery, such as graphite or carbon black, as a conductive agent. Is done.

Among the above-mentioned positive electrode active materials, a lithium secondary battery using lithium manganate having a spinel crystal structure has a higher stability by heating than a battery using lithium cobaltate or lithium nickelate. Are better. Therefore, batteries using lithium manganate as the active material for the positive electrode have attracted attention as high-safety batteries that are suitable for large-sized lithium secondary batteries for power storage, electric vehicles, and the like. In addition, lithium manganate having a spinel structure in the crystal structure causes the crystal to repeatedly expand and contract due to insertion and desorption of lithium during the charge / discharge reaction. Then, when charge and discharge are repeated, the structure of the positive electrode active material layer is collapsed, so that the electron conductivity is reduced and the charge and discharge cycle life is shortened. Furthermore, when lithium manganate is used as the positive electrode active material, Mn is contained in the organic electrolyte.
Dissolves, causing a decrease in the charge / discharge cycle life characteristics of the battery and a decrease in the storage characteristics.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to improve the cycle life characteristics and storage characteristics of an organic electrolyte secondary battery using lithium manganate.

[0006]

According to a first aspect of the present invention, there is provided a lithium manganate capable of electrochemically inserting and desorbing lithium by charge and discharge.
Ag or Ag is added to the surface of the lithium manganate particles.
In the second invention, the mole percentage of the element relative to lithium manganate is 0.5 to 10%, and in the third invention, the charge and discharge are performed. Lithium manganate capable of electrochemically inserting and desorbing lithium by the use of lithium manganate having Ag or an oxide of Ag fixed on the surface of particles of the lithium manganate as a positive electrode. The fourth invention is characterized in that the mole percentage of the element with respect to lithium manganate is 0.5 to 10%.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Lithium manganate Lithium manganate used for the positive electrode active material was prepared as follows. A 1M aqueous solution of manganese sulfate is dissolved in a 0.5M aqueous solution of sulfuric acid and kept at 93 ± 2 ° C., graphite is used as a counter electrode (cathode), and titanium (Ti) is used as a working electrode (anode).
Using a plate, manganese dioxide was precipitated on the working electrode by subjecting the working electrode to electrolytic oxidation at a current density of 1 A / dm ² . The manganese dioxide was peeled off from the working electrode, washed thoroughly with water, dried and then crushed in an automatic mortar for 30 minutes.
An electrolytic manganese dioxide powder (EMD) was obtained. Then EM
D and lithium carbonate, a commercially available high-purity reagent, were weighed so that the stoichiometric ratio of lithium manganate in the metal element composition ratio was obtained, mixed until uniform, filled in an alumina plate, and placed in an air stream at 600 ° C. Preliminary firing was carried out at a temperature of 10 ° C. for 10 hours. After cooling this powder to room temperature, it was pulverized in an automatic mortar to disaggregate the secondary particles. This powder was filled into an alumina dish in the same manner as in the pre-baking, and the powder was placed in an air stream at 750 ° C. for 12 hours.
Main firing was carried out for a period of time. After cooling to room temperature, the mixture was pulverized in an automatic mortar and sieved to remove particles having a particle size of 70 μm or more. The obtained lithium manganate was LiP by ICP analysis.
It was confirmed to be Mn ₂ O _4, and it was confirmed that the composition had a substantially desired composition ratio.

[0008] 2. 1. Positive Electrode FIG. 1 is a sectional view of a cylindrical lithium secondary battery embodying the present invention. Reference numeral 1 denotes a positive electrode current collector, which is an aluminum foil having a thickness of 20 μm. The plane size is 50 mm × 450 mm. Reference numeral 2 denotes a positive electrode active material layer, which is composed of a positive electrode active material lithium manganate having a specific metal immobilized on the particle surface, graphite as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder. Positive electrode active material layer 2
Is described in detail below. Lithium manganate (average particle size about 20 μm), graphite (average particle size about 0.5 μm)
m), PVDF is sufficiently mixed at a weight ratio of 80:10:10, and an appropriate amount of NMP as a dispersion solvent is added thereto, and the mixture is sufficiently kneaded and dispersed to form a slurry. This slurry is applied to both surfaces of the positive electrode current collector 1 by roll-to-roll transfer, dried, and then pressed. The thickness of the positive electrode is 21
3 ± 3 μm, and the density of the positive electrode active material layer 2 was 2.6 g /
cm ³ .

3. The negative electrode 3 is a negative electrode current collector and is a copper foil having a thickness of 10 μm. The plane size is 50 mm × 490 mm. The negative electrode active material for absorbing and releasing lithium ions is composed of amorphous carbon (trade name: CARBOTRON P, manufactured by Kureha Chemical Industry Co., Ltd.) and polyvinylidene fluoride (PVDF) as a binder. A detailed method for forming the negative electrode active material layer 4 will be described. Carbotron P and PVDF are mixed at a weight ratio of 90:10, and an appropriate amount of NMP serving as a dispersion solvent is added thereto, sufficiently kneaded and dispersed to form a slurry. The kneaded material is applied to the negative electrode current collector 3 by roll-to-roll transfer, dried, and then pressed. The thickness of the negative electrode is 132 ± 3 μm, and the density of the negative electrode active material layer is about 1.0 g / cm ³ .

[0010] 4. Battery 5 is a separator, which is a 25 μm-thick microporous polyethylene film. It is wound so that the separator 5 is arranged between the positive electrode and the negative electrode, and inserted into the negative electrode can 6. Then, the tab terminal which has been welded to the negative electrode current collector in advance is welded to the negative electrode can 6. The positive electrode tab terminal 8 is welded to the positive electrode current collector 1 in advance, and then welded to the positive electrode cap 7. Next, 5 ml of the electrolytic solution is injected into the negative electrode can 6. The electrolyte solution includes propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and vinylene carbonate (V) in a volume ratio of 30: 50: 15: 5.
In the mixed solvent of C), LiPF ₆ is dissolved at 1 M / l. The positive electrode cap 7 is placed on the upper part of the negative electrode can, and the upper part of the negative electrode can is caulked via the insulating gasket 9 to seal the battery. Here, in the positive electrode cap 7,
A current interrupt mechanism (pressure switch) that operates in response to an increase in battery internal pressure and a valve mechanism that operates at a higher pressure than the current interrupt mechanism are incorporated. In this embodiment, the operating pressure is 9 k
gf / cm ² current interrupting mechanism and operating pressure 20 kgf / c
An m ² valve mechanism was used.

5. Charge / discharge cycle life test The prepared battery was subjected to a charge / discharge cycle test under the following conditions.
The capacity retention rate was calculated from the ratio of the discharge capacity after 100 cycles to the initial discharge capacity. Charging conditions: constant voltage charging (4.2V), limited current 320m
A, 8 h, 50 ° C., discharge conditions: constant current discharge (1 A), final voltage 2.8 V, 5
At 0 ° C., a pause of 10 minutes was provided between charging and discharging.

6. Storage test After the prepared battery was charged and discharged under the following conditions to check the initial discharge capacity, the battery was charged again and stored at 50 ° C for one month, and the remaining capacity retention rate and the recovery capacity retention rate were measured. .
The remaining capacity retention is the ratio of the capacity to the initial capacity when discharging after storage, and the recovery capacity retention is the ratio of the discharge capacity when charging and discharging after checking the remaining capacity to the initial discharge capacity. It is. Charging conditions: constant voltage charging (4.2V), limited current 320m
A, 8 h, 25 ° C., discharge conditions: constant current discharge (1 A), final voltage 2.5 V, 2
At 5 ° C., a pause of 10 minutes was provided between charging and discharging.

[0013]

EXAMPLES Example 1 Silver nitrate (AgN
The above-mentioned lithium manganate was dispersed in an aqueous solution of O ₃ ), water was evaporated at about 90 ° C. with stirring, the remaining powder was dried at 120 ° C. for about 24 hours, and then placed in an alumina baking dish. Heat treatment was performed at 400 to 500 ° C. in an air stream. The concentration and amount of the aqueous solution of silver nitrate were adjusted so that the molar percentage of Ag with respect to lithium manganate was 5%, and lithium manganate having Ag immobilized on the surface was obtained. In addition, it was confirmed by ICP analysis that Ag was approximately 5% by mol% with respect to lithium manganate. The cylindrical lithium secondary battery described above was manufactured using this powder.

(Comparative Example 1) Ag or A
A cylindrical lithium secondary battery was manufactured using lithium manganate, to which g oxide was not fixed, as a positive electrode active material.

Table 1 shows that the batteries of Example 1 and Comparative Example 1 were used.
The capacity retention rate at the time of the 00 cycle is shown in percentage. As is clear from Table 1, the capacity of the battery of Comparative Example 1 in the charge / discharge cycle is significantly deteriorated, whereas the capacity of the battery of Example 1 is suppressed. Ag or A on the surface
It is considered that when lithium manganate in which g oxide is fixed is used for the positive electrode, dissolution of the Mn component in the organic electrolyte solution is suppressed regardless of storage or charge / discharge state.

[0016]

[Table 1]

(Examples 2 to 6) The amount of Ag was 0.2, 0.5,
The content was adjusted to 1, 10, and 15%, and lithium manganate having Ag immobilized on the surface of lithium manganate was used as a positive electrode active material. The amount of Ag to be immobilized was adjusted by the concentration and amount of the aqueous solution of silver nitrate.

Table 2 shows the capacity retention rates of the batteries of Examples 1 to 5 after storage, and the capacity retention rates at the 100th cycle. When the amount of Ag is less than 0.5 mol%, the capacity retention ratio is greatly deteriorated. On the other hand, when the amount of Ag increases, deterioration of capacity maintenance is suppressed, but when the mole percentage exceeds 15% and becomes 15 mole%, the initial discharge capacity is greatly reduced. When the amount of Ag is large, the initial discharge capacity is reduced because the amount of the active material whose Ag, which is not directly involved in the lithium charge and discharge reactions is increased, is reduced, and the active material surface is covered with Ag. This is probably because insertion and desorption of lithium into the substance did not proceed smoothly, and internal resistance due to charge transfer and diffusion increased.

[0019]

[Table 2]

The method of immobilizing Ag or an Ag oxide on the surface of lithium manganate is not limited to the above embodiment, but may be other methods such as a method of forming a chemical precipitate, a plating method, and a CVD method. There are a sputtering method, a mechanofusion method, a thermal decomposition method, and the like, and the same effect as the present invention was shown.

[0021]

The active material for a positive electrode according to the present invention is a lithium manganate capable of electrochemically inserting and desorbing lithium by charging and discharging, wherein Ag or Ag oxide is fixed on the surface. And The organic electrolyte lithium secondary battery using lithium manganate for the positive electrode has an extremely large effect in that capacity deterioration can be suppressed during high-temperature storage or charge / discharge cycles.

[Brief description of the drawings]

FIG. 1 is a sectional view of a cylindrical organic electrolyte lithium secondary battery embodying the present invention.

[Explanation of symbols]

1 is a positive electrode current collector, 2 is a positive electrode active material layer, 3 is a negative electrode current collector,
4 is a negative electrode active material layer, 5 is a separator, 6 is a negative electrode can, 7 is a positive electrode cap, 8 is a positive electrode tab terminal, and 9 is a gasket.

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Claims (4)

[Claims] 1. A lithium manganate capable of electrochemically inserting and removing lithium by charging and discharging, wherein Ag or an oxide of Ag is fixed on the surface of the lithium manganate particles. Lithium manganate. 2. The lithium manganate according to claim 1, wherein the mole percentage of the element relative to lithium manganate is 0.5 to 10%. 3. A lithium manganate capable of electrochemically inserting and removing lithium by charging and discharging, wherein the lithium manganate particles have Ag or an oxide of Ag fixed on the surface of the lithium manganate particles. Is used as a positive electrode. 4. The organic electrolyte secondary battery according to claim 3, wherein the mole percentage of the element relative to lithium manganate is 0.5 to 10%.