JP3130531B2 - Non-aqueous solvent secondary battery - Google Patents

Description

The present invention relates to an improvement in a positive electrode active material in a non-aqueous solvent secondary battery using a carbonaceous material as a negative electrode support.

[Related Art] In recent years, with the development of electronic equipment, there has been a demand for the development of a secondary battery that is small, lightweight, has a high energy density, and can be repeatedly charged and discharged.

As this type of secondary battery, those using lithium or a lithium alloy as a negative electrode active material, and using oxides such as molybdenum, vanadium, titanium, and niobium, sulfides, and selenides as a positive electrode active material are known. I have.

Recently, spinel-type LiMn with improved and improved cycle characteristics of manganese oxide with high energy density
Studies on ₂ O ₄ and other lithium manganese composite oxides are being actively conducted.

These lithium manganese oxide as a positive electrode active material,
In a battery system using lithium as a negative electrode active material, the dissolution / precipitation reaction of lithium, which is a negative electrode active material, is repeated by repeating the cycle, and a problem that a needle-like lithium dendrite deposit is formed on a lithium substrate in due course. Occurs. Therefore, in the battery system, the battery life is regulated by the crystal structure collapse that gradually progresses in the positive electrode active material, the generation of dendrites and the decomposition reaction of the solvent on the negative electrode side, and the life of more than 500 cycles and long-term reliability It is very difficult to manufacture a battery having a characteristic.

On the other hand, an electrode active material utilizing intercalation or doping of a layered compound having a different reaction form from these manganese oxides has attracted attention. Since these electrode active materials do not cause complicated chemical reactions during charge and discharge reactions, they are expected to have extremely excellent charge / discharge cycle characteristics. Above all, those using a carbonaceous material as a support have attracted attention. This carbonaceous material is used as a negative electrode support, and LiCoO ₂ / LiNiO ₂ or Ti is used as a positive electrode active material.
A battery system using S ₂ and MoS ₂ has been proposed.

However, when a carbonaceous material is used as the negative electrode active material, TiS ₂ ,
When a metal chalcogen compound such as MoS ₂ is used as the positive electrode active material, the electromotive force is small (1.0 to 1.2 V). Therefore, as the positive electrode active material, LiCoO ₂ showing an average operating voltage of about 3.5 V,
LiNiO ₂ , LiCo _x Ni _(1-x) O _{2 and the} like have been studied.

[Problems to be Solved by the Invention] However, when the carbonaceous material is used as the negative electrode carrier, Li in the positive electrode active material is doped into the carbonaceous material in the initial charging reaction in the battery system, but in the subsequent discharge reaction, Only about 70 to 80% of the total Li amount doped is de-doped from the carbon material, leaving lithium that does not participate in the reaction on the negative electrode side, and insufficient Li on the positive electrode side, and the capacity retention ratio in subsequent cycles is reduced. There is a problem that it is greatly reduced. The cause of the incomplete undoping is the result of the study by the present inventors,
It is considered that Li bonded to carbon exists in amorphous parts in the carbon material.

The present invention has been made in order to improve such problems, when using a carbonaceous material as a negative electrode support, the amount of lithium remaining on the negative electrode side in the initial charge and discharge is contained in the positive electrode active material in advance, and the cycle It is to obtain a specific excellent positive electrode active material.

[Means for Solving the Problems] That is, the non-aqueous solvent secondary battery of the present invention uses a carbonaceous material as a negative electrode carrier and a general formula LiMO as a positive electrode active material.
₂ (wherein M is one or more elements selected from Co, Ni, Fe and Mn) electrochemically or chemically doped with lithium.

LiMO ₂ used in the present invention is generally synthesized by the following method. That is, lithium, Co, Ni, Fe or one or more of transition metal carbonates, nitrates, sulfates, hydroxides and the like selected from two or more selected from Mn,
They are obtained by mixing these in a stoichiometric ratio and firing. In addition, a carbon salt is preferable as a starting material. The firing temperature varies slightly depending on the starting material, but is usually 600-1,000
In the temperature range of 600C, preferably in the range of 600-800C.

As a method for doping LiMO ₂ with lithium ions, an electrochemical or chemical method can be used. First, as an electrochemical method, a conductive agent and a binder are added to the active material obtained by the above method to produce a plate-shaped positive electrode used for a normal cylindrical battery. An electric current is applied to this in a specific water solvent using metallic lithium as a counter electrode, and lithium is doped in the positive electrode. The doping amount of lithium is represented by the general formula Li _{1 + x} MO ₂ (where M is 0.1 ≦ x ≦ 0.3 as described above)
Is preferable. This is because if x is less than 0.1, the amount of lithium migrating into the negative electrode is small, and the capacity deterioration will increase in subsequent charge / discharge cycles. This causes the capacity to deteriorate due to charge and discharge cycles. The lithium-doped LiMO ₂ thus obtained is used for battery assembly after washing with a solvent.

As another chemical method, LiMO ₂ obtained by the above synthesis method is immersed in a lithium compound solution, doped with lithium ions, and then
By performing heat treatment at ~ 450 ° C, lithium-doped LiMO ₂ can be obtained. Examples of the lithium compound used herein include a lithium organic compound such as an organic solvent solution of n-butyllithium, sec-butyllithium, and tert-butyllithium; and an aqueous solution of a lithium salt such as lithium oxalate or lithium nitrate which decomposes at a low heating temperature. Can be used. When a lithium organic compound is used, LiMO ₂
After being immersed in a solution and reacted, it is washed with a solvent and used as a positive electrode active material. On the other hand, when a lithium salt is used, similarly after immersion, 300 to 450 ° C., preferably 300 to 400 ° C.
For doping with lithium and using as a positive electrode active material.

As the carbonaceous material as the negative electrode carrier, a carbonaceous material obtained by firing an organic compound is preferably used to improve battery characteristics. The organic compound serving as a raw material of the carbonaceous material is not particularly limited as long as it is a commonly used one. For example, a phenol resin, particularly a novolak resin, and polyacrylonitrile can be used. As the carbonaceous material, Japanese Patent Application No. Hei.
Those having the characteristics of an organic compound fired body as shown in JP-A-283086 are particularly preferred.

In the non-aqueous solvent secondary battery of the present invention, a binder may be used to form a positive electrode and a negative electrode. Examples of the binder include a terpolymer of ethylene-propylene-cyclic diene, polytetrafluoroethylene, polyacrylic acid, and polyacrylates.

Examples of the electrolyte of the non-aqueous electrolyte used in the non-aqueous solvent secondary battery of the present invention include lithium salts such as LiPF ₆ , LiClO ₄ , LiBF ₄ , and LiCF ₃ SO ₃ . Examples of the solvent for the electrolyte include propylene carbonate (PC), ethylene carbonate (EC), tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, and 1,2-dimethoxyethane (DME). These solvents are one or two
A mixture of propylene carbonate and 1,2-dimethoxyethane, a mixed solvent of ethylene dicarbonate and 2-methyltetrahydrofuran, and a mixture of ethylene carbonate and ethylene carbonate, from the viewpoint of prolonging the charge / discharge cycle life, And 1,2-dimethoxyethane, and a mixed solvent of propylene carbonate and ethylene carbonate are preferred.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to the drawings by way of examples and comparative examples.

Example 1 Commercially available lithium carbonate and cobalt carbonate were weighed out so that the molar ratio of Li and Co became Li / Co = 1.10, and thoroughly mixed in a mortar. This mixture was placed in an alumina crucible and heat-treated at 800 ° C. for 6 hours in an electric furnace.
The obtained fired product was cooled and pulverized again, and was similarly subjected to a heat treatment at 800 ° C. for 6 hours. Thereafter, it was sufficiently washed with distilled water to wash away unreacted alkali components. This product was identified as LiCoO ₂ by the powder X-ray method.

90% by weight of this product, 7% by weight of acetylene black as a conductive material and 3% by weight of a terpolymer of ethylene-propylene-cyclic diene as a binder are kneaded in hexane to prepare a slurry-like positive electrode mixture. And the thickness of this positive electrode mixture
After coating and air-drying on a 15 μm stainless substrate, pressurizing to a constant thickness, then heating and drying at 200 ° C. for 10 hours to obtain a plate-like positive electrode having a 0.26 mm thick positive electrode mixture layer. Manufactured.

This positive electrode and a lithium metal negative electrode are wound, and a mixed solvent of propylene carbonate (PC) and 1,2-dimethoxyethane (DME) (volume ratio of 50:50) is used as an electrolyte, and lithium hexafluorophosphate (LiPF ₆ ) is used as an electrolyte. It was immersed in an electrolytic solution containing 0.5 molar, 3 at a current density of 1 mA / cm ² to a positive electrode area
Discharging 60 mAh of electricity yielded Li _1.3 CoO ₂ . The positive electrode was taken out of the electrolytic solution and washed with a 1,2-dimethoxyethane solvent to obtain a positive electrode of the present invention. These series of operations were performed in an argon gas glove box.

On the other hand, the carbonaceous material serving as the negative electrode carrier is obtained by sintering a novolak resin at 950 ° C. in a nitrogen atmosphere, and then 2,000 ° C.
And carbonized by heating to a powder having a mean particle size of 10 μm.

A terpolymer of ethylene-propylene-cyclic diene as a binder was dissolved in hexane, and carbonaceous material: binder = 97: 3
And a slurry-like negative electrode mixture was prepared. The slurry was applied on a stainless steel substrate having a thickness of 10 μm and dried to form a negative electrode mixture layer having a thickness of 0.2 mm.

Using the positive and negative electrodes thus obtained, an AA (AA) size non-aqueous solvent secondary battery as shown in FIG. 1 was assembled. That is, the non-aqueous solvent secondary battery 1 includes the cylindrical stainless steel container 3 having the bottom, on which the insulator 2 is disposed, and which also serves as the negative electrode terminal. The container 3 contains an electrode group 4. The electrode group 4 has a structure in which a strip formed by laminating a negative electrode 5, a separator 6, and a positive electrode 7 in this order is spirally wound so that the negative electrode 5 is located outside.
The separator 6 is formed of a polypropylene porous film impregnated with an electrolytic solution. The electrolyte is
A mixed solvent of propylene carbonate and 1,2-diethoxyethane (volume ratio of 50:50) contains lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte at a 0.5 molar concentration. Above the electrode group 4 in the container 3, an insulating plate 8 having an opening at the center is arranged. An insulating sealing body 9 is air-tightly fixed to the container 3 at the upper opening of the container 3. The central opening of the insulating sealing plate 8 has a positive terminal
10 are fitted. The positive electrode terminal 10 is connected to the positive electrode 7
Is connected via a positive electrode lead 11. The negative electrode 5 is connected to the container 3 serving as a negative electrode terminal via a negative electrode lead (not shown).

Example 2 LiCoO ₂ was obtained in the same manner as in Example 1. This was immersed in a commercially available n-butyllithium hexane solution for 2 days to chemically dope lithium. The obtained active material was passed through and thoroughly washed with hexane. This operation was performed in an argon gas glove box. The lithium-doped positive electrode active material obtained by this method had a composition formula of Li _1.25 CoO ₂ . Using this active material, a plate-shaped positive electrode was prepared by the method described in Example 1, and thereby a cylindrical battery similar to that of Example 1 was prepared.

Comparative Example A battery having the same configuration as that of Example 1 was produced except that LiCoO ₂ synthesized in Example 1 was used as a positive electrode active material as it was.

The three types of non-aqueous solvent secondary batteries of Examples 1 and 2 and Comparative Example thus prepared were subjected to a constant temperature of 20 ° C., 100 m
Charge / discharge evaluation was performed in a voltage range from 4.3 V to 3.0 V at a constant current of A. The result is shown in FIG. In the figure, A is the battery of Example 1, B is the battery of Example 2, and C is the discharge capacity retention rate curve of the battery of Comparative Example. As is clear from FIG. 2, the non-aqueous solvent secondary batteries of Examples 1 and 2 have a large amount of lithium undoped from the positive electrode in the initial charging reaction. As a result, lithium ions that no longer participate in charge and discharge can be replenished on the negative electrode side in advance, so that the drop in battery capacity at the beginning of the cycle is reduced and the discharge capacity retention rate when the number of cycles is increased is also improved. It has discharge reversibility.

In the present embodiment, the LiCoO ₂ has been described as an example as the positive electrode active material, Other _{LiNiO 2, LiFeO 2, LiCO y} N
i _(1-y) O ₂ , LiCo _y Mn _(1-y) O ₂ , LiCo _y Fe _(1-y) O _2, etc.
y represents the number of <y <1).

[Effect of the Invention] The non-aqueous solvent secondary battery of the present invention uses a carbonaceous material as a negative electrode carrier, and uses LiMO ₂ electrochemically or chemically doped with lithium as a positive electrode active material. Since the lithium ions remaining in the negative electrode carrier in the initial charge / discharge reaction are doped in the positive electrode in advance, the drop in the initial battery capacity is small,
Further, the cycle characteristics are improved. Therefore, according to the present invention, an excellent nonaqueous solvent secondary battery having an improved capacity and a long life can be obtained.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing a non-aqueous solvent secondary battery of Example 1, and FIG. 2 is a change in discharge capacity with respect to the number of charge / discharge cycles in the non-aqueous solvent secondary batteries of Examples 1, 2 and Comparative Example. FIG. DESCRIPTION OF SYMBOLS 1 ... Non-aqueous solvent secondary battery, 2 ... Insulator, 3 ... Stainless steel container, 4 ... Electrode group, 5 ... Negative electrode, 6 ... Separator, 7 ... Positive electrode, 8 ... Insulating plate, 9 ...... Insulated sealing plate, 10
… Positive terminal, 11… Positive lead. A: Discharge capacity retention rate curve of battery of Example 1 B: Discharge capacity retention rate curve of battery of Example 2 C: Discharge capacity retention rate curve of battery of comparative example

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-3-74062 (JP, A) JP-A-4-58469 (JP, A) JP-A-3-127454 (JP, A) JP-A-3-127454 201368 (JP, A) JP-A-4-123769 (JP, A) JP-A-4-147573 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/36-10 / 40 H01M 4/02-4/04 H01M 4/36-4/62

Claims (1)

(57) [Claims] A carbonaceous material is used as a negative electrode support, and a general formula LiMO ₂ (where M is one or more selected from Co, Ni, Fe and Mn) is used as a positive electrode active material. A non-aqueous solvent secondary battery characterized in that a lithium-containing composite oxide represented by the following formula (1) is electrochemically or chemically doped with lithium.