JP3103101B2 - Lithium secondary battery and method of manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a lithium secondary battery and a method for manufacturing the same, and more particularly, to an improvement in a positive electrode active material thereof.

[Conventional technology]

As positive electrode active materials for lithium secondary batteries, titanium disulfide, vanadium pentoxide, manganese oxide, and the like have been proposed, but recently, manganese oxide, which is abundant in resources and inexpensive, has attracted particular attention.

For this manganese oxide, manganese dioxide, which is composed of only manganese and oxygen has a problem in reversibility, because the charge-discharge characteristics is poor, for example, as such as LiMn ₂ O _4,
It has been proposed to use a lithium-manganese composite oxide in which lithium has been introduced into a manganese oxide (for example, US Pat. No. 4,507,371).

Therefore, the present inventors synthesized LiMn ₂ O ₄ according to the method described in the above-mentioned US Patent No. 4,507,371,
When it was used as a positive electrode active material for a lithium secondary battery and charged and discharged with a lithium anode in a voltage range of 3.5 to 2.0 V, it was found that the charge and discharge capacity was unexpectedly small.

Further, 0.26e ⁻ / Me [0.2 atom per manganese (Mn) atom]
It has also been reported that when a battery is charged / discharged with a relatively large capacity of 6 electrons (e ⁻ )], deterioration is increased [Electrochemistry, 57, (6), p533 (1989)].

[Problems to be solved by the invention]

The present invention, as described above, solves the problem that it was not possible to obtain a lithium secondary battery having good charge / discharge characteristics with conventionally used manganese dioxide or LiMn ₂ O _{4 and} has excellent charge / discharge characteristics. An object is to provide a lithium secondary battery.

[Means for solving the problem]

The present invention provides a Li / M by heat-treating lithium acetate or lithium acetate dihydrate and manganese dioxide.
When n = 3/5 to 5/5 (molar ratio), the lattice constant a ₀ = 8.15Å (0.815
The above object has been achieved by synthesizing a lithium manganese oxide that can be indexed by a cubic system of (nm) and using this as a positive electrode active material of a lithium secondary battery.

That is, by obtaining a lithium manganese composite oxide having a small lattice constant of lattice constant a ₀ = 8.15 °, Mn
^Reduced the generation rate of ³⁺ , suppressed the structural change from cubic to tetragonal based on Mn ³⁺ , improved the charge / discharge characteristics, and obtained a lithium secondary battery with excellent charge / discharge characteristics It is.

With the above Li / Mn = 3/5 to 5/5 (molar ratio), the lattice constant a ₀ = 8.15
The characteristics of the lithium manganese composite oxide that can be indexed by the cubic crystal of Å and the characteristics of the conventionally used LiMn ₂ O ₄ will be described in detail while comparing them with those of the conventional LiMn ₂ O ₄ .

Li / Mn used as a positive electrode active material in the present invention = 3/5 to
Lithium-manganese composite oxide that can be indexed by a cubic system with a lattice constant a ₀ = 8.15 ° at 5/5 (molar ratio) (hereinafter simply referred to as “lithium-manganese composite oxide of the present invention”)
Has an X-ray diffraction pattern similar to that of the conventional LiMn ₂ O _4, and both belong to the cubic system. The difference is the lattice constant. Table 1 shows the substances belonging to the cubic system among lithium manganese composite oxides that have been reported so far.

As shown in Table 1, the lithium manganese composite oxide of the present invention has a lattice constant of 8.15 °, which is smaller than that of LiMn ₂ O ₄ . In addition, Li reported so far
_It has a smaller lattice constant than ₄ Mn ₅ O _{12 and the} like, and has the smallest lattice constant among the lithium manganese composite oxides shown in Table 1.

Among the lithium manganese oxides listed in Table 1, only the LiMn ₂ O ₄ whose charge / discharge characteristics were measured as a positive electrode active material for a lithium secondary battery was the other one [that is, with parentheses (parentheses). Are not measured at all.

It is presumed that the lithium manganese composite oxide of the present invention exhibits superior charge / discharge characteristics as compared with LiMn ₂ O ₄ for the following reason.

First, the lithium manganese composite oxide of the present invention is LiMn ₂ O
Lattice constant is smaller than ₄ . This is considered to be because Mn ⁴⁺ having a smaller radius is contained more than Mn ³⁺ (LiMn ₂ O ₄ has Mn ³⁺ and Mn ⁴⁺ in half each). LiMn ₂ O ₄
Li invades its crystal structure with discharge, and Li ₁₊ χMn
₂ O ₄ (0 ≦ x ≦ 1). ^Along with this, Mn ³⁺ is generated, and a structural change from cubic to tetragonal occurs due to the Jahn-Teller effect of Mn ³⁺ , and this structural change is considered to deteriorate charge / discharge characteristics.

On the other hand, since the lithium manganese composite oxide of the present invention has a large amount of Mn ⁴⁺ , the ratio of Mn ³⁺ generated with discharge is low.
It is considered that the amount is smaller than that of LiMn ₂ O ₄ , so that the deterioration of the charge / discharge characteristics is suppressed, and the charge / discharge characteristics become better than that of LiMn ₂ O ₄ .

By analogy with this idea, the lithium manganese composite oxide of the present invention has the smallest lattice constant among the lithium manganese composite oxides shown in Table 1, and therefore has the best charge-discharge characteristics. It is thought that there is.

Next, the point that the lithium manganese composite oxide of the present invention is different from the lithium manganese composite oxide reported so far from the viewpoint of the synthesis method will be described.

First, LiMn ₂ O _{4 in} Table 1 is lithium carbonate (Li ₂ CO ₃ )
And Mn ₂ O ₃ by heat treatment at 900 ° C. Li _4/3 Mn _5/3 O ₄ described below is obtained by _converting an oxide or carbonate of lithium (Li) and manganese (Mn) to 95% in an oxidizing atmosphere.
It is reported to be synthesized by heat treatment at 0 ° C. for several hours. For the next Li ₂ Mn ₄ O ₉ and Li ₄ Mn ₅ O ₁₂ , the details of the synthesis method are not clear, but they are lithium hydroxide monohydrate (LiOH · H ₂ O) as a lithium salt. Alternatively, using lithium carbonate, γ-Mn
O _2, using a _{β-MnO 2, Mn 2 O} 3, mixtures thereof 400-90
It is described that it is synthesized by heat treatment at 0 ° C. In addition, in the X-ray diffraction image of the substance indicated in the literature as Li ₄ Mn ₅ O ₁₂ , small peaks were observed at diffraction angles 2θ = 22 ° and around 65.5 °, and it was considered that Li ₂ MnO ₃ was mixed. Can be In any case, as shown in Table 1, these Li ₂ Mn ₄ O ₉ and Li ₄ Mn ₅ O ₁₂ have different lattice constant values from the lithium manganese composite oxide of the present invention, and are different materials. Is evident. In addition, a lithium manganese composite oxide having a structure similar to a spinel structure having a manganese valence of 3.80 to 3.90 has been reported (JP-A-2-37665).
However, the lithium-manganese composite oxide of the present invention has a manganese valence of 3.9 or more, and the X-ray diffraction image is different from the lithium-manganese composite oxide described in JP-A-2-3766. Both are different substances.

Further, lithium hydroxide (LiOH) and manganese dioxide (MnO ₂ ) are mixed at a Li / Mn ratio of 10/90 to 67/33 (molar ratio) and heat-treated at 300 to 430 ° C to form a lithium manganese composite oxide. A synthesis method has also been reported (JP-A-63-114064).
Publication, Battery Technical Committee Material 1-1, etc.). In this synthetic method, unlike the cubic system, Li and ₂ MnO _3, has been reported to be composite of γ-βMnO ₂ shifted slightly lower angle, the present invention and a substance obtained by this synthesis The X-ray diffraction image is clearly different from that of the lithium manganese composite oxide, and both are different substances.

On the other hand, the lithium manganese composite oxide of the present invention is synthesized by using a low melting point lithium salt such as lithium acetate or lithium acetate dihydrate as a lithium salt, and subjecting this to a heat treatment with manganese dioxide. Since the lithium salt having a low melting point is used as the temperature during the heat treatment as described above, the target lithium manganese composite oxide can be synthesized at a low temperature of 300 to 500 ° C. If the heat treatment temperature is lower than 300 ° C., the removal of moisture tends to be incomplete, and if it exceeds 500 ° C., the heat treatment itself is possible, but impurity peaks are likely to appear. The heat treatment time is usually 2 to 40 hours, and an oxidizing atmosphere containing oxygen is employed as the atmosphere during the treatment. In the synthesis, lithium acetate or lithium acetate dihydrate and manganese dioxide are mixed at Li / Mn = 3/5 to 5/5 (molar ratio). The Li / Mn (molar ratio) at the time of the synthesis is maintained almost as it is even in the synthesized lithium manganese oxide. L
The i / Mn (molar ratio) is particularly preferably around Li / Mn = 4/5 (molar ratio), and the ratio of lithium is Li / Mn = 3/5 (molar ratio).
If the diameter is smaller, unreacted manganese dioxide tends to be mixed. The ratio of lithium is Li / Mn = 5/5 (molar ratio)
If it is larger, Li ₂ MnO ₃ that does not contribute to charging and discharging tends to be mixed.

As described above, the lithium manganese composite oxide of the present invention has lithium as lithium or lithium acetate.
Use low melting point lithium salt called dihydrate, and 300 ~ 5
In that it can be synthesized at a temperature as low as 00 ° C., it differs from the conventional lithium manganese composite oxide in the method of synthesis.

Further, as described above, the lattice constant a ₀ = Li / Mn = 3/5 to 5/5 (molar ratio) used as the positive electrode active material in the present invention.
The lithium manganese composite oxide that can be indexed by the 8.15Å cubic system is different from other cubic lithium manganese composite oxides such as LiMn ₂ O _{4 in} that the lattice constant is small. Is considered to be a factor indicating good charge / discharge characteristics.

〔Example〕

Next, the present invention will be described more specifically with reference to examples.

Example 1 Lithium acetate dihydrate (CH ₃ COOLi.2H ₂ O) and chemical manganese dioxide (MnO ₂ ) were subjected to a heat treatment at 375 ° C. to form a lattice with Li / Mn = 4/5 (molar ratio). A lithium manganese composite oxide having a constant a ₀ = 8.15 ° and indexable by cubic system was synthesized. The above synthesis was performed as follows.

After weighing lithium acetate dihydrate (CH ₃ COOLi 2H ₂ O) and chemical manganese dioxide (MnO ₂ ) so that the ratio of Li / Mn = 4/5 (molar ratio), crush it with a mortar. And mixed. This was heat-treated at 375 ° C. for 20 hours in oxygen gas.

Using the above lithium manganese composite oxide as a positive electrode active material, phosphorous graphite as an electron conduction aid, and polytetrafluoroethylene as a binder were mixed at a ratio of 100: 20: 5 (weight ratio). A positive electrode mixture was prepared. Filling the positive electrode mixture in a mold, 10mm diameter at 1t / cm ^2, a thickness of about 0.3mm
, And then heat-treated at 250 ° C to obtain a positive electrode.

Using this positive electrode, a button-type lithium secondary battery shown in FIG. 1 was produced.

In FIG. 1, (1) is the above positive electrode, and (2)
Is a disk-shaped negative electrode of lithium having a diameter of 14 mm.
(3) is a separator made of a microporous polypropylene film, and (4) is an electrolyte absorber made of a polypropylene nonwoven fabric. (5) is a stainless steel positive electrode can, (6) is a positive electrode current collector made of stainless steel mesh,
(7) is a negative electrode can made of stainless steel and having its surface plated with nickel. (8) is a negative electrode current collector made of a stainless steel net, which is spot-welded to the inner surface of the negative electrode can (7), and the negative electrode (2) is a negative electrode current collector made of the stainless steel net. It is crimped on (8). (9) is a circular ring gasket made of polypropylene. In this battery, an electrolyte obtained by dissolving 0.6 mol / dissolved LiCF ₃ SO ₃ in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane at a volume ratio of 1: 1 is injected. Have been.

Comparative Example 1 Lithium carbonate (Li ₂ CO ₃ ) and Mn ₂ O ₃
LiMn ₂ O ₄ was synthesized by heat treatment at 00 ° C. The above synthesis was performed as follows.

After weighing lithium carbonate (Li ₂ CO ₃ ) and Mn ₂ O _{3 so} that Li / Mn = 1/2 (molar ratio), they were mixed while pulverized in an agate mortar. This was placed in an Ar / O ₂ = 80/20 argon-oxygen gas mixture at 650 ° C. for 8 hours, followed by 24 hours at 900 ° C.
Heat treated for hours.

When the X-ray diffraction image of the product after the heat treatment was measured, it showed the same pattern as the known X-ray diffraction image of LiMn ₂ O ₄ .

A button-type lithium secondary battery was manufactured in the same manner as in Example 1 except that the above LiMn ₂ O ₄ was sufficiently pulverized and used as a positive electrode active material.

Next, the battery of Example 1 and the battery of Comparative Example 1 were charged and discharged at a charge current of 0.392 mA and a discharge current of 0.785 mA between a voltage of 3.5 to 2.0 V.

Table 2 shows the relationship between the number of charge / discharge cycles and the charge / discharge capacity of the battery of Example 1 and the battery of Comparative Example 1.

As shown in Table 2, the battery of Example 1 had a larger charge / discharge capacity than the battery of Comparative Example 1 at any cycle number.

FIG. 2 shows an X-ray diffraction image of the lithium manganese composite oxide used as the positive electrode active material in Example 1.
Parentheses in FIG. 2 indicate surface indices (hkl). The measurement was performed using a Cukα ray at an applied voltage of 50 kV and an applied current of 150 mA. Table 3 shows the measured values of the diffraction angle (2θ / degree) of each peak and the calculated values when calculated as a cubic crystal with a lattice constant a ₀ = 8.15 °.

As shown in FIG. 2 and Table 3, the lithium manganese composite oxide used as the positive electrode active material in Example 1 was calculated as a cubic crystal having an actually measured diffraction angle of a ₀ = 8.15 °. And the lattice constant a ₀ = 8.
Each peak could be explained as a 15 ° cubic crystal, and no other peaks were observed.

In Example 1, the lithium-manganese composite oxide was synthesized at Li / Mn = 4/5 (molar ratio), but Li / Mn (molar ratio).
May be in the range of Li / Mn = 3/5 to 5/5 (molar ratio).
The heat treatment was performed at 375 ° C., but the heat treatment temperature was 300 to 50.
The temperature may be within the range of 0 ° C.

In the first embodiment, chemical manganese dioxide is used as manganese dioxide. Alternatively, electrolytic manganese dioxide or manganese dioxide obtained by thermally decomposing manganese carbonate may be used. Although lithium acetate dihydrate is used as the lithium salt, lithium acetate may be used instead. Further, in Example 1, lithium was used for the negative electrode, but instead, a lithium alloy such as a lithium-aluminum alloy or a lithium compound such as lithium-coke may be used. In addition to the electrolyte used in Example 1, for example, LiClO ₄ , LiPF ₆ , LiB
One or more electrolytes such as F ₄ , LiC ₄ F ₉ SO ₃
For example, 1,2-dimethoxyethane, 1,2-diethoxyenta, propylene carbonate, ethylene carbonate,
An organic electrolyte dissolved in a single solvent such as γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, or a mixture of two or more solvents may be used.

〔The invention's effect〕

As described above, in the present invention, as the positive electrode active material, Li / Mn = 3/5 to 5/5 (molar ratio) and a lattice constant a ₀ = 8.15Å.
By using a lithium manganese composite oxide that can be indexed by a cubic system, a lithium secondary battery having better charge / discharge characteristics as compared with the case where LiMn ₂ O ₄ is used as a positive electrode active material could be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of the lithium secondary battery of the present invention. FIG. 2 is a view showing an X-ray diffraction image of a lithium manganese composite oxide used as a positive electrode active material in Example 1. (1) Positive electrode, (2) Negative electrode, (3) Separator

Continuation of the front page (56) References JP-A-63-210028 (JP, A) JP-A-1-272051 (JP, A) JP-A-2-139860 (JP, A) JP-A-3-84874 (JP) , A) JP-A-4-169065 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02-4/04 H01M 10/40

Claims (3)

(57) [Claims] 1. A lithium secondary battery using lithium, a lithium alloy or a lithium compound for a negative electrode, wherein the positive electrode active material is Li / Mn = 3/5 to 5/5 (molar ratio) and the lattice constant a ₀ = 8.15Å. A lithium secondary battery characterized by using a lithium manganese composite oxide that can be indexed by cubic crystals. 2. A lithium secondary battery according to claim 1, wherein the lithium manganese composite oxide according to claim 1 is synthesized by heat-treating lithium acetate or a mixture of lithium acetate dihydrate and manganese dioxide. Production method. 3. The method according to claim 1, wherein the mixing ratio of lithium acetate or lithium acetate dihydrate to manganese dioxide is Li / Mn = 3/5 to 5/5 (molar ratio), and the heat treatment temperature is 300 to 500 ° C. Item 3. A method for producing a lithium secondary battery according to Item 2.