JPH02278657A - Lithium secondary battery and its manufacturing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Industrial Application Field The present invention relates to a secondary battery with good performance such as high energy density, low self-discharge rate, and long cycle life.

(2) Prior art] The use of baltic oxide for the positive electrode of lithium (Li) secondary batteries has been described by K., Mizushjma et al., Mat.
Rcs.

Bull, Vol. 1.5. p, 783.1980
It has been reported in etc.

However, when these cobalt oxides are used in lithium secondary batteries, they are manufactured as lithium cobalt oxides, so γ-type products cannot be obtained, and they have poor performance in both cycle life and energy density. I haven't gotten anything.

On the other hand, J. MOLENDA et al.
te tonics 12 (1984) and others, γ-type cobalt oxide was produced from sodium salt and CO3O4 and applied to non-aqueous secondary batteries using Na negative electrodes.

(3) Problems to be solved by the invention There are various crystal structures of cobalt oxide, such as α-type, β-type, γ-type, etc., and any of these structures can be used as the positive electrode of a secondary battery. Electrode performance varies greatly depending on the type of electrode used.

In addition, when applying cobalt oxide to the positive electrode of a lithium secondary battery, conventionally one manufactured from Li salt and Co salt was used, so only the α-type could be obtained, resulting in high capacity and long cycle life. I haven't gotten something that lasts a long time.

(4) Means for Solving the Problems In order to obtain a cathode material that improves electrode performance, the present inventors conducted intensive research on the structure and manufacturing method of cobalt oxide, and found that Na salt and Co3O4 It is a sodium cobalt oxide produced from a mixture of the following, and has a particularly γ-type crystal structure, that is, one Na or Co atom and 6 oxygen atoms, with the Na or Co atom sandwiched between them, and the coordination of the oxygen atoms is 3. The inventors discovered that a structure exhibiting a rectangular prism shape is excellent as a positive electrode material for lithium secondary batteries, and arrived at the present invention.

There are no particular restrictions on the method for producing the sodium cobalt oxide having the above structure, but for example, by thoroughly mixing a sodium salt, sodium oxide, or sodium peroxide with cobalt oxide, and under a low humidity condition, A method in which the reaction is performed by slow firing in a dry air or dry oxygen atmosphere is preferred.

However, if the firing temperature is 600° C. or lower, it is difficult to form a γ structure. In order to obtain a sodium cobalt oxide having a γ-type structure, it is necessary to perform firing at a temperature of 700° C. or higher. The firing time varies depending on the amount of synthesis, workability, etc., but 10 hours or more is appropriate. There is no particular limit to the heating rate and cooling rate for the above firing, but in order to efficiently produce a γ-type structure, avoid rapid heating and cooling, and range from 1°C/min to 10°C/min. Preferably, the temperature is raised and cooled.

The atomic ratio of Na and Co in the sodium cobalt oxide is determined by the atomic ratio of Na and Co in the mixed raw material before firing. If the Na/Co atomic ratio is less than 0.65, cobalt oxide with no electrode activity will remain in the product as an unreacted product, reducing the electrode activity.

In addition, if the Na/Co atomic ratio is set to 0.65 or more, no unreacted cobalt oxide remains, but if it exceeds 0.90, the raw material of the Na compound may remain, or the crystal structure may change significantly, resulting in an excellent Electrode material cannot be obtained.

Therefore, it is necessary to suppress the Na/Co atomic ratio to 0.65 or more and 0.90 or less, which approximately corresponds to the range in which a crystallographically stable γ-type structure can be obtained. In particular, in order to maintain high capacitance density and make an electrode with recirculation property, 0.70 to 0.8
A range of 5 is preferred.

The reason for this is not clear, but by coordinating sodium ions, which are difficult to be released electrically or chemically, in the sodium cobalt oxide, the change in shape of the crystal structure due to charging and discharging is softened, and until a high potential is reached. It is presumed that this is because even if oxidized, electrode destruction, that is, structural deterioration is unlikely to occur.

The electrode reaction when the electrode of the present invention is used as a positive electrode of a Li-based secondary battery is considered to follow equation (1).

(However, X and y are positive numbers of 1 or less.) The range of x+y in reversible NaCoOZ is 0.5 to 0.1, and formula (1) is expressed as its maximum value. When shown, ...... (2) and Equation (2) are obtained.

Therefore, in order to operate in the maximum usable range, the Na/Co atomic ratio of the sodium cobalt oxide in the charged state must be 0.5.

On the other hand, as mentioned above, in order to obtain a sodium cobalt oxide having a γ structure, the Na/Co atomic ratio must be 0.65 or more and 0.90 or less.

Therefore, before incorporating a sodium cobalt oxide manufactured with an Na/Co atomic ratio of 0.65 or more and 0.90 or less into a battery, it is recommended to oxidize it in advance so that the Na/Co atomic ratio becomes 0.5. is desirable.

There are no particular limitations on the method of oxidizing the sodium cobalt oxide, but examples include chemical oxidation with an appropriate oxidizing agent, electrochemical oxidation, and the like.

In order to use the positive electrode active material of the present invention as an electrode, since the sodium cobalt oxide is electrically conductive, it may be press-molded as it is, but the specific surface area and porosity of the electrode may be reduced to a certain extent. It is often better to add carbon black or the like to maintain the electrode strength, and to add a binder to further improve electrode strength.

There are no particular limitations on the negative electrode used in the secondary battery of the present invention; for example, (j) Li metal, (ii) Li metal alloy, (iii) Li metal or Li metal alloy and conductive polymer or carbon black. An example is a complex with.

Li metal alloys include Li/A, Q alloy, Li/Hg
Examples include alloys, Lj/Sn alloys, Li/Pb alloys, and alloys of three or more metals including Li metal used in these alloys.

A composite of Li metal or Li metal alloy and conductive polymer or conductive polymer refers to a homogeneous mixture, laminate, and base component of Li metal or Li metal alloy and conductive polymer or carbon black. It means a modified form of which is modified with other components.

The supporting electrolyte of the electrolytic solution of the secondary battery of the present invention is a Li salt.

As a typical anion component of the supporting electrolyte, 5OC
FBF and BH3 (however, R is 3 3'
4.

(alkyl group or aryl group having 1 to 10 carbon atoms), and the like.

A specific example of a Li salt as a supporting electrolyte is LiPF.
LiSbF6. LiCfl04,.

4' L i A s F a , CF a S Oa L
l , L t B F 4 .

L i B (Bu) L i B (EL)2(
Bu)2 and the like may be mentioned, but are not necessarily limited to these.These Li salts may be used alone or in a mixture of two or more.

Examples of organic solvents used alone or in combination as a solvent for the electrolyte of the secondary battery of the present invention include lactone solvents such as γ-butyl lactone, carbonate solvents such as propylene carbonate, ethylene carbonate, ether solvents, Examples include dimethoxyethane and tetrahydrofuran.

In the present invention, if necessary, a porous membrane made of synthetic resin such as polyethylene or polypropylene or natural fiber paper may be used as the diaphragm.

There is no particular restriction on the shape of the secondary battery of the present invention, and any of the shapes such as button shape, coin shape, thin plate shape, or cylindrical shape can be produced.

(5) Example Hereinafter, an example in which the present invention is applied to a coin-type secondary battery will be described.

Example 1 Na2O2 and Co3O4 were mixed well at a Na2O2/Co3O4 molecular weight ratio of 1.26 (Na/Co atomic ratio of 0.84), formed into a pellet, and heated under dry oxygen at 4% per minute. The temperature was raised to 740℃ at a rate of 740℃.
It was baked at 40°C for 24 hours. Next, the baked product was allowed to cool naturally and was brought to room temperature, and then crushed well in a mortar to make it fine.
To 100 parts by weight of sodium cobalt oxide, 3 parts by weight of carbon black and 2 parts by weight of EPDM (ethylene propylene rubber) as a binder were added, and after mixing well, a diameter It was press molded to have a thickness of 15 mm.

This positive electrode is used as a counter electrode with Li metal foil, and as an electrolyte with lll1oΩ
A Teflon test cell shown in FIG. 1 was assembled using a propylene carbonate solution of L t A s F 6 of /Ω, and a current value of 1 mA was applied to the Teflon test cell. Charged to lV.

Using the above method, a pre-charged positive electrode and a Li metal foil punched to a diameter of 15 mm were used as the negative electrode, and the electrolyte contained 1.0I.
LiA in a mixed solvent of propylene carbonate and ethylene carbonate (volume ratio 1:1)
A coin-shaped battery (standard: 201B) shown in FIG. 2 was assembled using the dissolved sFs.

In the figure, 5a is a positive electrode, 9 is a nickel wire mesh, 10 is a polypropylene nonwoven fabric, 11 is a polypropylene microporous film, 12 is an insulating backing, and 3b is a negative electrode.

This battery has a constant current of 2 within the range of 2.5V to 4.1■
．． After repeated charging and discharging at 5 mA, the discharge capacity, cycle life, and self-discharge rate when left at 40° C. for 10 days after the 100th cycle of charging were investigated.

As a result, the discharge capacity for each cycle is a maximum of 17.0mAh.
(This corresponds to 0.40 electric equivalents per atom of Co), the cycle life was 205 times, and the self-discharge rate was 5.3%.

Example 2 An alloy of lithium and lead (Li/Pb atomic ratio 3.5) was used as the negative electrode.
) was pressure-molded to a diameter of 15 mm, and 1,5
moΩ/ρ propylene carbonate, ethylene carbonate and 1,2-dimethoxyethane (volume ratio 1:
A coin cell was assembled in the same manner as in Example 1, except that LiPF6 dissolved in a mixed solvent of 1:2) was used.

This battery 2. The battery performance was investigated in the same manner as in Example 1, except that the battery was operated within the range of 1V to 4.0V, and the maximum discharge capacity was 1G, 2mAh (corresponding to 0.39 electric equivalents per 1 atom of Co). ), the cycle life was 250 times, and the self-discharge rate was 8.3%.

Example 3 The same positive electrode as in Example 1 was used, while the negative electrode was made by mixing 100 parts by weight of a lithium and lead alloy (Li/Pb atomic ratio 2.7) well crushed in a mortar, 210 parts by weight of carbon bluff,
As a binder, 2.5 parts by weight of EPDM (ethylene propylene rubber) fine powder dissolved in xylene was added and mixed, and after removing the xylene under reduced pressure, the above mixture contained a nickel wire mesh current collector. It was obtained by press molding to a diameter of 15 mm.

In addition, L h is added to the electrolyte so that the concentration is 1.5 mol/ρ.
A coin cell similar to that in Example 1 was assembled using P Fe dissolved in 2-methyl-tetrahydrofuran. Battery performance was examined in the same manner as in Example 1, except that this battery was operated within the range of 2.1V to 3.8V.

As a result, the maximum discharge capacity was 15.0mAh (Co 1
(equivalent to 0.36 electric equivalents per atom), the cycle life was 243 times, and the self-discharge rate was 6.7%.

Comparative Example I Implementation 1 except that the atomic ratio of Na/Co was changed to 0.50
] A positive electrode was manufactured in exactly the same manner as in Example 1, and a coin-type battery was assembled using the same negative electrode and electrolyte as those used in the example. In addition, X-ray diffraction and elemental analysis of the positive electrode active material after firing revealed that in addition to the sodium cobalt oxide produced in the γ type, about 20% of unreacted Co3 was found.
O4 remained.

When the battery performance of this battery was examined in the same manner as in Example 1, the maximum discharge capacity was 12.0 mAh, the cycle life was 140 times, and the self-discharge rate was 6.3%.

Comparative Example 2 Example 1 except that the atomic ratio of Na/Co was changed to 1.00.
A coin-type battery was assembled in exactly the same manner as above, and the battery performance was examined.

As a result, the maximum discharge capacity was 14.3 mAh, the cycle life was 103 times, and the self-discharge rate was 73%.

Comparative Example 3 Example except that LiC002 obtained by thoroughly mixing Li Co and Co CO3 at a molar ratio of 2 in a mortar and then firing in dry air at a firing temperature of 900°C for 24 hours was used as the positive electrode. Assemble the coin cell as in 1,
We investigated battery performance.

As a result, although the maximum discharge capacity was 16.0 mA h, the capacity decreased significantly with each repetition of charging and discharging, and the cycle life was 75 times.

(6) Effects of the Invention As mentioned above, the secondary battery of the present invention has excellent performance such as high energy density, long cycle life, and low self-discharge rate. Since it is possible to produce batteries of various shapes, such as coin-shaped, the contribution to this industry is extremely large.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a Teflon test cell used to preoxidize the positive electrode. FIG. 2 is a longitudinal cross-sectional view of a coin-shaped secondary battery used to examine battery performance in Examples. 1... Platinum lead wire for negative electrode 2... Platinum wire mesh current collector for negative electrode 1 (3... Negative electrode 3b... Negative electrode 4, porous polypropylene diaphragm 5... Positive electrode 5a... Positive electrode 6...Platinum wire mesh current collector for positive electrode 7...Platinum lead wire for positive electrode 8...Polytetrafluoroethylene container 9...Nickel wire mesh 1O...Polypropylene nonwoven fabric 11-=Polypropylene micro Porous Spy 12・
・Insulation packing room

Claims (2)

[Claims] (1) A lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, characterized in that the positive electrode uses sodium cobalt oxide. (2) The lithium secondary battery according to claim 1, wherein the sodium cobalt oxide of the positive electrode contains 70% or more of a γ-type crystal structure.