JP2001257003A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery, with a carbon material used as a negative electrode and lithium manganate as a positive electrode, which has an excellent charge/discharge characteristics with large capacity and high energy density with little irreversible capacity even at high temperature, with reduced cycle capacity deterioration of the negative and positive electrodes. SOLUTION: To attain above purpose, a positive electrode is made to include a soluble boron compound in its electrolyte, the boron compound including at least either B2O3, H3BO3, HBO2, H2B4O7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly, to a positive electrode active material used for a non-aqueous electrolyte battery. .

[0002]

2. Description of the Related Art LiCoO ₂ and LiNi are currently used as a positive electrode active material of a lithium secondary battery exhibiting an operating voltage of 4 V class.
Lithium-containing oxides having an α-NaFeO ₂ structure such as O ₂ and lithium-containing oxides having a spinel structure such as LiMn ₂ O ₄ are used. Among them, LiMn ₂ O ₄ having a spinel structure is a positive electrode active material with low manufacturing cost and excellent safety. On the other hand, a lithium metal, a lithium alloy, a carbon material, or the like is used as the negative electrode active material.
Among the carbon materials, if graphite is used, especially graphitized graphite, for example, lithium manganate is used for the positive electrode, a flat operating battery voltage can be obtained, and thus there is an advantage that the operating time of various portable devices can be extended. .

However, the LiMn ₂ O ₄ used in the positive electrode, when using a halogen-containing lithium salt such as LiPF ₄ in the electrolytic solution, the lithium salt reacts with trace water, hydrohalic acids such as hydrofluoric acid Occurs. This hydrohalic acid dissolves LiMn ₂ O ₄ of the positive electrode, forms a high-resistance film such as MnF _{2 on} the carbon surface of the negative electrode, and causes a reduction in cycle performance.

On the other hand, in order to improve the cycle performance of lithium manganate materials, Japanese Patent Application Laid-Open No. 4-233161 discloses
JP-A-5-21067, JP-A-6-187793
Japanese Patent Application Laid-Open Publication No. H11-163873 discloses a technique in which part of Mn of lithium manganate having a spinel structure is replaced with another element. However, especially when batteries are left at high temperatures,
Mn is eluted into the electrolyte, and the high resistance MnF ₂
It did not solve the above-mentioned problem that precipitation occurred.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and in a battery using a carbon material for a negative electrode and lithium manganate for a positive electrode, deterioration of cycle capacity of the negative electrode and the positive electrode is reduced. It is an object of the present invention to provide a lithium secondary battery which is improved, has a high capacity and a high energy density even at a high temperature, and has excellent charge / discharge cycle characteristics with little irreversible capacity.

[0006]

In order to solve the above-mentioned problems, the present invention provides a lithium secondary battery using a lithium-containing manganese oxide for a positive electrode, wherein the positive electrode contains a boron compound soluble in an electrolytic solution. It is a lithium secondary battery characterized by the above-mentioned. Further, the boron compound is represented by B
A lithium secondary battery comprising a boron compound containing at least one selected from ₂ O ₃ , H ₃ BO ₃ , HBO ₂ , and H ₂ B ₄ O ₇ .

When a carbon material is used for the negative electrode, the reaction of absorbing and releasing lithium into and from the carbon material is largely governed by the state of the film formed between the electrolyte and the carbon surface. Explaining the lithium metal negative electrode as a model, if a lithium metal surface has a dense and highly ion-conductive coating, it exhibits excellent battery characteristics. The rate characteristics and cycle characteristics deteriorate.
Here, the former film component is lithium carbonate, lithium oxide, or the like, and the latter film component is lithium fluoride or MnF _2.
And so on. The same can be said for a coating formed on the surface of a carbon material. That is, one of the factors that increase the interfacial resistance of the carbon material is that the carbon material surface
For example, a film having low ionic conductivity such as lithium fluoride and MnF ₂ may be formed.

The present inventors have conducted research on the above points, and as a result, the positive electrode contains a boron compound soluble in an electrolytic solution, and the boron compound is composed of B ₂ O ₃ , H ₃ BO ₃ , HB
By including at least one selected from O ₂ and H ₂ B ₄ O _7, it was found that the cycle life performance was significantly improved.

As a method for adding a boron compound to a positive electrode, a lithium-containing manganese oxide, which is a positive electrode active material, is added with H
₃ How to create the electrodes from a mixture of BO ₃ and the like. However, H ₃ BO ₃ contains a large amount of hydrogen atoms that react with lithium, and may cause irreversible side reactions in the battery. Therefore, heat treatment of the positive electrode at a temperature of 100 ° C. to 140 ° C. or higher is required. Is preferred. By the heat treatment, H ₃ BO ₃ is considered to change the HBO ₂ and H ₂ B ₄ O ₇ and the like.

Further, the heat treatment temperature is set at 300.degree.
It is more preferable to set the temperature to not less than ℃ or more. In this case, it is considered that H ₃ BO ₃ changes its form to B ₂ O ₃ , and the side reaction is further suppressed.

Further, the heat treatment temperature is set at 400.degree.
C is most preferable. In this case, H ₃ BO ₃ becomes B ₂ O ₃
And a part of B ₂ O ₃ further contributes to a reaction in which Mn of the lithium-containing manganese oxide substitutes for B.

However, when heat treatment is performed in an electrode state containing an organic substance such as a binder, the heat treatment temperature is preferably set to 100 ° C. to 200 ° C. in order to avoid deterioration of the organic substance such as the binder.

Since B ₂ O ₃ easily reacts with the moisture in the air, it is preferable that the heat treatment is performed as late as possible in the manufacturing process.

However, in practice, since the amount of the boron compound required to obtain the effects of the present invention is very small, the battery performance is hardly influenced by the composition of the boron compound.

[0015]

DETAILED DESCRIPTION OF THE INVENTION The present invention is not limited by the following description of the starting material of the active material, the production method, the positive electrode, the negative electrode, the electrolyte, the shape of the separator and the battery, and the like.

The negative electrode carbon material used in the battery of the present invention may be any carbon material capable of occluding and releasing lithium. For example, when the plane spacing (d ₀₀₂ ) estimated by X-ray diffraction is 0.3354-0.3369 nm. And carbon particles having a crystallite size (Lc) of 10 nm or more in the c-axis direction are preferable.

As the positive electrode active material, Mn of LiMn ₂ O ₄
An oxide obtained by substituting the site with another element may be used. The other elements include Li, B, V, Al, Ni, Co,
Elements such as Mg, Cr, and Tb can be suitably substituted.

As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. In terms of organic solvent resistance and hydrophobicity, polypropylene,
Sheets, microporous membranes, nonwoven fabrics and the like made from olefin polymers such as polyethylene, polyvinylidene fluoride, polytetrafluoroethylene and the like are used. The pore size of the separator is in a range generally used for a battery, and is, for example, 0.01 to 1 μm. Similarly, the thickness is in the range generally used for batteries, for example, 20 to 40 μm.

Examples of the electrolyte supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , which exhibit high lithium ion conductivity.
LiOSO ₂ CF ₃ or the like can be used. These fluorinated electrolyte salts are usually contained in a non-aqueous electrolyte at a concentration of 0.6M to 2.0M.
M, preferably dissolved in a concentration of 0.8M to 1.2M.

As the electrolyte solvent, it is preferable to use a combination of a high dielectric constant solvent and a low viscosity solvent. Examples of the high dielectric constant solvent include ethylene carbonate (E
C) and cyclic carbonates such as propylene carbonate (PC) are preferred. These high dielectric constant solvents may be used alone or in combination of two or more. Examples of the low-viscosity solvent include chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC), and lactones such as γ-butyrolactone. These low viscosity solvents may be used alone or in combination of two or more.

[0021]

The present invention will be described below in further detail with reference to examples.

(Invention Battery 1) Lithium acetate dihydrate,
Manganese (II) acetate tetrahydrate was mixed such that the element ratio of Li: Mn was 1.10: 1.9, and this was added to acetic acid, stirred while applying heat, and completely dissolved. . Next, the acetic acid was evaporated to obtain a mixed salt. The mixed salt was calcined in air at 500 ° C. and then calcined at 850 ° C. After firing, the powder was pulverized to obtain a powder. When the powder was analyzed by X-ray diffraction, it was confirmed that lithium manganate having a spinel structure was obtained. H ₃ BO ₃ was added to the particles at a weight ratio of 0.1% of lithium manganate, and the mixture was stirred. Using the powder as a positive electrode active material, a coin-type lithium battery having a capacity of 17 mAh shown in FIG. 1 was prototyped as follows.

The positive electrode 1 comprises the above mixed powder, acetylene black and polytetrafluoroethylene powder in a weight ratio of 8%.
The mixture was mixed at 5: 10: 5, and toluene was added and kneaded well. This was formed into a sheet having a thickness of 0.8 mm by a roller press. Next, the sheet was punched into a circle having a diameter of 16 mm, and heat-treated at 90 ° C. for 40 hours under reduced pressure to obtain a positive electrode 1. The positive electrode 1 was used by being pressed against a positive electrode can 4 having a positive electrode current collector 6.

In the negative electrode 2, artificial graphite and polytetrafluoroethylene powder were mixed at a weight ratio of 95: 5, and toluene was added and kneaded sufficiently. This was formed into a sheet having a thickness of 0.1 mm by a roller press. Next, the sheet was punched into a circle having a diameter of 16 mm,
After drying for 5 hours, negative electrode 2 was obtained. The artificial graphite has an average particle size of 6 μm and a plane spacing (d ₀₀₂ ) determined by X-ray diffraction.
0.337 nm and a crystallite size (Lc) of 55 nm in the c-axis direction were used. The negative electrode 2 was used by being pressed against a negative electrode can 5 provided with a negative electrode current collector 7.

The electrolytic solution is prepared by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.
LiPF ₆ dissolved at a concentration of 1 mol / l was used. As the separator 3, a microporous film made of polypropylene was used. Using the positive electrode 1, the negative electrode 2, the separator 3, and the electrolytic solution, a coin-type lithium battery having a diameter of 20 mm and a thickness of 1.6 mm was produced. This battery is referred to as Battery 1 of the invention.

(Inventive Battery 2) A coin-type lithium battery was prototyped in the same manner as in Inventive Battery 1, except that the positive electrode was heat-treated under reduced pressure at 100 ° C. for 40 hours. This battery is referred to as Battery 2 of the invention.

(Battery 3 of the Present Invention) A coin-type lithium battery was prototyped in the same manner as Battery 1 of the present invention, except that the positive electrode was heat-treated under reduced pressure at 140 ° C. for 40 hours. This battery is referred to as Battery 3 of the invention.

(Inventive Battery 4) A coin-type lithium battery was prototyped in the same manner as in Inventive Battery 1, except that the positive electrode was heat-treated at 300 ° C. for 40 hours under reduced pressure. This battery is referred to as Battery 4 of the invention.

(Batteries 5 to 8 of the Present Invention) Batteries were produced in the same manner as the batteries of the present invention 1 to 4 except that the negative electrode was made of metal lithium having a thickness of 0.1 mm instead of artificial graphite.
The batteries are referred to as batteries 5 to 8 of the present invention, respectively.

(Comparative Battery 1) A coin-type lithium battery was prototyped in the same manner as Battery 1 of the present invention except that H ₃ BO ₃ was not added to the positive electrode. This battery is referred to as Comparative Battery 1.

(Comparative Battery 2) The positive electrode was heated at 300 ° C. under reduced pressure for 4 hours.
A coin-type lithium battery was prototyped in the same manner as Comparative Battery 1 except that the heat treatment was performed for 0 hour. This battery is referred to as Comparative Battery 1.

(Comparative Batteries 3 and 4) Batteries were produced in the same manner as Comparative Batteries 1 and 2, except that 0.1-mm thick metallic lithium was used instead of artificial graphite for the negative electrode. These batteries are referred to as comparative batteries 3 and 4, respectively.

In order to observe the behavior of the positive electrode close to that of a single electrode, a charge / discharge test was performed using batteries 5 to 8 of the present invention and comparative batteries 3 and 4 using metallic lithium for the negative electrode. Charging is constant current charging with a current of 0.05 mA and a final voltage of 4.2 V.
The discharge was a constant current discharge with a current of 0.05 mA and a final voltage of 3.0 V. The test temperature was 25 ° C. Table 1 shows the ratio of the discharge capacity to the charge capacity in the first cycle as the initial charge / discharge efficiency. For reference, the form of the boron compound estimated from the heat treatment temperature of the positive electrode is also shown in Table 1.

[0034]

[Table 1]

Using the batteries 1 to 4 of the present invention and the comparative batteries 1 and 2, a charge / discharge test was performed. Charging was performed at a constant current of 1 mA and a final voltage of 4.2 V, and discharging was performed at a current of 1 m.
A, constant current discharge with a final voltage of 3.0 V was performed. The test temperature was 25 ° C. Table 2 shows the results of the discharge capacity at the fifth cycle. Further, the number of cycles at the time when the discharge capacity was reduced to 80% of the initial value was measured and indicated as a cycle life. For reference, the form of the boron compound estimated from the heat treatment temperature of the positive electrode is also shown in Table 1.

[0036]

[Table 2]

In Tables 1 and 2, there is almost no difference between the batteries in the initial charge / discharge efficiency and the discharge capacity at the fifth cycle. This is because the added boron compound is 0.1.
This is probably because the amount was as small as 1%, which is not an amount that would affect battery performance.

Further, as is apparent from Table 2, the battery of the present invention to which the boron compound was added exhibited excellent cycle performance as compared with the comparative battery. When the batteries 1 to 4 of the present invention are compared with respect to the cycle life, the higher the heat treatment temperature is, the longer the cycle life is, but it cannot be said that the difference shows a clear difference. This is because the boron compound is effective in dissolving in the electrolytic solution regardless of the composition, and it is considered that the boron compound does not depend on the composition of each boron compound or the shape of the particles.

Next, the battery 1 of the present invention after the cycle life was confirmed
To 4, and the electrolyte, the positive electrode and the negative electrode were taken out. The positive electrode and the negative electrode were washed with dimethylene carbonate, and then dried under vacuum to obtain a test sample. After dissolving these test samples with nitric acid and filtering the residue, the components were analyzed by ICP-issued spectroscopy. As a result, boron was detected more in the electrolyte and the negative electrode than in the positive electrode.

The negative electrodes taken out from the batteries 1 to 4 of the present invention and the comparative batteries 1 and 2 were subjected to argon etching by X-ray photoelectron spectroscopy (XPS),
Analyzed in the depth direction, it was compared abundance of MnF _2, the amount of MnF ₂ detected from the negative electrode of the present battery, compared to the amount of MnF ₂ detected from the negative electrode of the comparison battery, much It was a small amount.

[0041]

As described above, the cycle characteristics were improved by using the positive electrode active material of the present invention. Although this mechanism is not always clear, it is considered as follows. That is, the boron compound added to the positive electrode is dissolved in the electrolytic solution and reacts with a free acid such as hydrogen fluoride. As a result, the concentration of free acid in the battery decreases, so that the dissolution of manganese by free acid is suppressed. As a result, it is considered that MnF ₂ which is a high resistance coating component is not formed on the surface of the carbon negative electrode.

Although the composition of the boron compound when the boron compound is dissolved and the mechanism of the reaction with the hydrofluoric acid are not clear, boron is finally deposited on the carbon surface of the negative electrode together with a film of the decomposition product of the electrolytic solution. The above results indicate that the coatings on the boron-containing carbon surface have low resistance, unlike coatings such as MnF ₂ , and that the resistance does not increase easily even after repeated charge / discharge cycles. Understand from.

In the above embodiment, the lithium secondary battery using artificial graphite as the negative electrode material has been described.
The effects of the present invention have been confirmed when other negative electrode materials are used.

As described above, according to the present invention, by adding a boron compound to lithium manganate, which is the main constituent material of the positive electrode active material, the cycle characteristics can be improved without lowering the capacity. In addition, since these materials are excellent in safety and inexpensive, they are excellent methods for reforming the cathode material, and the resulting batteries have high capacity,
High energy density and excellent charge / discharge cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a sectional view of a battery of the present invention.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiro Fujii 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka Inside Yuasa Corporation (72) Inventor Hiroshi Yufu 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka No. F-term in Yuasa Corporation (reference) 5H029 AJ02 AJ03 AJ05 AK03 AL07 AL12 AM03 AM04 AM05 AM07 BJ03 DJ08 EJ05 HJ02 5H050 AA02 AA07 AA08 BA16 BA17 CA09 CB08 CB12 DA02 DA09 EA12 HA02

Claims (2)

[Claims] 1. A lithium secondary battery using a lithium-containing manganese oxide for a positive electrode, wherein the positive electrode contains a boron compound soluble in an electrolytic solution. 2. The method according to claim 1, wherein said boron compound is B ₂ O ₃ , H ₃ BO ₃ ,
HBO _2, H ₂ B ₄ lithium secondary battery according to claim 1, wherein the from O ₇ selected a boron compound comprising at least one or more.