JP3128143B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly, to improvement of a positive electrode active material.

[0002]

2. Description of the Related Art Titanium disulfide, vanadium pentoxide, manganese oxide and the like have been proposed as positive electrode active materials for lithium secondary batteries. Attention has been paid.

[0003] When the 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 LiMn ₂ O
It has been proposed to use a lithium-manganese composite oxide in which lithium (Li) is introduced into a manganese oxide, such as ₄ , for example.

Here, the lithium manganese composite oxide that has been proposed so far will be described as follows.

First, LiMn ₂ O ₄ (for example, US Pat. No. 4,507,371) and recently proposed ones such as Li ₂ Mn ₄ O ₉ and Li ₄ Mn ₅ O ₁₂ [for example, Mat, Res, Bull . , 25 , p657 (1
990)].

Then, lithium hydroxide (LiOH) and chemical manganese dioxide are mixed at a molar ratio of Li to Mn (manganese) of 90:10 to 30:70 and calcined to obtain Li.
MnO ₂ containing ₂ MnO ₃ (JP-A-63-1140)
No. 64).

Recently, a cubic compound represented by the chemical formula of LiMn ₃ O ₆ has been proposed [for example,
EMKI KAGAKU, 58 (5), p477 (19
90)].

This substance is composed of lithium nitrate (LiNO ₃ )
And manganese dioxide (MnO ₂ ) obtained by mixing and firing at a molar ratio of Li: Mn of 1: 3, and MnO ₂ containing Li ₂ MnO ₃ is Li / M
The synthesis methods such as the n molar ratio, the lithium salt to be used, and the synthesis temperature are different, and the X-ray diffraction images are also different.

The LiMn ₃ O ₆ has a content of 2.0 to
It has been confirmed that charge / discharge can be performed between 3.5 V (vs. Li), 170 to 180 mAh / g at the initial stage, and 1 after the cycle.
It shows a discharge capacity of 60 mAh / g and a theoretical discharge capacity of 20 mAh / g.
It is said to be 0 mAh / g. These are Li _{1 + x} Mn ₃ O in which Li is inserted into LiMn ₃ O _6.
6 (x> 0). However, this discharge capacity is 1
60 mAh / g and the discharge capacity of manganese dioxide (about 25
0 mAh / g).

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having a large charge / discharge capacity by solving the problem that LiMn ₃ O ₆ has only a small charge / discharge capacity. With the goal.

[0011]

SUMMARY OF THE INVENTION The present invention provides a LiMn ₃ O
_{When 6} is charged to a potential of 3.7 V or more with respect to Li, L
Li forming the lattice of iMn ₃ O ₆ can be omitted, and unlike the past, Li _1-y Mn ₃ O ₆ (0 <y
It has been found that the region <1) can also be used for charging and discharging, and the above object has been achieved.

That is, LiMn ₃ O
₆ to the Li insertion region (that is, Li _{1 + x} Mn ₃ O
_In the present invention, in addition to this, the Li _1-y Mn ₃ O ₆ region is also used for charging and discharging, and the region available for charging and discharging is increased to increase the charging and discharging capacity. It was.

In the Li _{1 -y} Mn ₃ O ₆ region, the potential is high and the energy density is improved. By removing Li, the potential can be increased to about 4.3 V with respect to Li.

[0014] Electrochemical extraction of Li has been performed by LiMO ₂ based materials such as LiCoO ₂ and LiNiO ₂ and Li ₂ Mn ₄ O ₉ , among those having a spinel structure.
LiMn ₂ O ₄ except for Li ₄ Mn ₅ O ₁₂ has been reported, but LiMn ₃ O ₆ has not been reported.

In the present invention, LiMn ₃ O _{6 is} converted to LiMn ₃ O _6.
Can be performed electrochemically or chemically. To electrochemically extract Li from LiMn ₃ O ₆ ,
LiMn ₃ O ₆ may be charged up to 3.7 V or more with respect to Li. To chemically remove Li from LiMn ₃ O ₆ , for example, LiMn ₃ O ₆ may be added to a sulfuric acid aqueous solution having a pH of about 2 in a sulfuric acid aqueous solution. What is necessary is just to soak for a time.

For the negative electrode, lithium, a lithium alloy, lithium carbon or the like is used. And, as the lithium alloy, for example, lithium-aluminum, lithium-lead, lithium-indium, lithium-gallium,
Various lithium alloys, such as lithium-indium-gallium, which can be used for a lithium secondary battery can be used.

The electrolyte includes, for example, LiCF ₃ SO ₃ ,
LiClO ₄ , LiPF ₆ , LiBF ₄ , LiC ₄ F ₉
One or more of the electrolytes, such as SO _3, may be used, for example, 1,2-dimethoxyethane, 1,2-diethoxyethane, 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 is used.

[0018]

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

Example 1 LiMn ₃ O ₆ was synthesized by sufficiently pulverizing and mixing lithium nitrate (LiNO ₃ ) and electrolytic manganese dioxide (MnO ₂ ), followed by heat treatment. This synthesis was performed as follows.

Lithium nitrate (LiNO ₃ ) and manganese dioxide (MnO ₂ ) were mixed at a molar ratio of Li: Mn = 1: 3. This is heat-treated at 350 ° C. for 20 hours in air to obtain L
iMn ₃ O ₆ was synthesized.

The LiMn ₃ O ₆ obtained as described above
Was used as a positive electrode active material, and phosphorous graphite as an electron conduction aid and polytetrafluoroethylene as a binder were mixed at a ratio of 100: 50: 3 (weight ratio) to prepare a positive electrode mixture. A mold was filled with 40 mg of this positive electrode mixture, pressed at 1 t / cm ² into a disk having a diameter of 10 mm, 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, reference numeral 1 denotes the above positive electrode, and reference numeral 2 denotes a negative electrode made of disc-shaped lithium having a diameter of 14 mm.

Reference numeral 3 denotes a separator made of a microporous polypropylene film, and reference numeral 4 denotes an electrolyte absorber made of a nonwoven polypropylene fabric. Reference numeral 5 denotes a stainless steel positive electrode can, and reference numeral 6 denotes a positive electrode current collector made of a stainless steel net.
Reference numeral 7 denotes a negative electrode can made of stainless steel and having a surface plated with nickel.

Reference numeral 8 denotes a negative electrode current collector made of a stainless steel mesh, which is spot-welded to the inner surface of the negative electrode can 7, and the negative electrode 2 is pressed to the negative electrode current collector 8 made of the stainless steel net. ing. Reference numeral 9 denotes a polypropylene-made annular gasket. An electrolyte obtained by dissolving 0.6 mol / l of LiCF ₃ SO ₃ in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane at a volume ratio of 1: 1 is injected into the battery. Have been.

The battery of Example 1 was charged with a charging current of 0.392.
The battery was charged and discharged with a discharge current of 0.785 mA at a voltage of 3.9 to 2.0 V. That is, the positive electrode active material LiMn ₃
Li _1-y Mn ₃ O _{6 from} which Li of O ₆ has been removed (0 <y <1)
Region and Li _{1 + x} Mn ₃ O ₆ (x>
The battery was charged and discharged over the region 0).

The relationship between the number of charge / discharge cycles of the battery of Example 1 and the capacity at the time of discharging per unit weight is shown in Table 1 together with the case of Comparative Example 1 described later.

Comparative Example 1 A battery was prepared in the same manner as in Example 1, and the battery was charged with a charge current of 0.392 mA and a discharge current of 0.32 mA.
It was charged and discharged between 3.5 and 2.0 V at 785 mA as before. That is, Li _{1 + x} Mn ₃ O ₆ (x>
Charge / discharge was performed only in the region 0).

Table 1 shows the number of charge / discharge cycles and the discharge capacity per unit weight of the battery of Comparative Example 1. The capacity at the time of discharge per unit weight was calculated based on the weight of the charged positive electrode. The start of charge / discharge is from the discharge direction.

[0029]

[Table 1]

As shown in Table 1, Example 1 has a larger capacity than Comparative Example 1 in the second and subsequent cycles. It should be noted here that the lithium amount (210 mAh / g) equal to or greater than the lithium amount (176 mAh / g) inserted in the first cycle of discharging in Example 1 was removed during charging. That is, it is clear from this that Example 1 also uses the region of Li _1-y Mn ₃ O ₆ (0 <y <0).

In Comparative Example 1, the theoretical discharge capacity was less than the above-mentioned theoretical discharge capacity of 200 mAh / g in all the cycles. However, after the second cycle of Example 1, the theoretical discharge capacity exceeded the previously reported theoretical discharge capacity value. Value.

Lithium (Li) in lithium composite oxide
As described above, in addition to Li _1-y Mn ₃ O ₆ of the present invention, Li _1-x CoO ₂ , Li
_1-x Mn ₂ O ₄ has been reported.

These crystal structures are the same as those of the LiMn _{3 of} the present invention.
O ₆ is said to be cubic with Li and Mn arranged in octahedral positions, whereas LiCoO ₂ is hexagonal with a layered structure, and LiMn ₂ O ₄ is cubic with spinel structure (Li Are tetrahedral positions), which are different from each other.

Comparing the voltages, it is found that the Li _1-y Mn of the present invention
_As shown in FIG. 2, ₃ O ₆ is used for charging and discharging at 3.9 to 2.0 V, whereas Li _1-x CoO ₂ is used for 4.6 to 3. It has a higher potential of 9V.
However, since Li _1-x CoO ₂ contains Co (cobalt), the cost is high, and since the potential is high, oxidation of the current collector and the electrolytic solution is also a serious problem.

As shown in FIG. 4, Li _1-x Mn ₂ O ₄ is about 4 V when Li (lithium) is removed, and about 3 V when Li is inserted. It is difficult to use both areas.

On the other hand, the Li _1-y Mn ₃ O _{6 of the} present invention
Does not show as distinct a two-step discharge as Li _1-x Mn ₂ O ₄ ,
Both Li-extracting and Li-inserting regions can be used. Further, in the present invention, Li _1-y Mn ₃
O ₆ can be used at a lower potential than Li _1-x CoO ₂ , so that there is little problem such as oxidation of the current collector.

FIG. 2 shows the battery 5 of Example 1 of the present invention.
It is a figure which shows the charge / discharge curve in the cycle time. FIG. 3 shows Li _x Co in US Pat. No. 4,302,518.
Is a graph showing changes in _1.01 O ₂ of the open circuit voltage due to the change in x (vs. Li). Figure 4 shows the 29th Battery Symposium Abstracts, p1
36 is a diagram showing a change in open-circuit voltage (vs. Li) with a change in _x of Li _x Mn ₂ O _{4 at} No. 36 (1988).

In the examples, in synthesizing LiMn ₃ O ₆ , lithium nitrate (LiNO ₃ ) was used as a lithium salt, but instead lithium carbonate (Li ₂ CO ₃ )
Lithium hydroxide monohydrate (LiOH.H ₂ O) or the like may be used.

In order to obtain pure LiMn ₃ O ₆ , Li /
The Mn molar ratio is reduced to 1/3, and the firing temperature is set to 300 to 350.
It is preferable that the synthesis is performed at a low temperature for a short time at a temperature of 1 to 20 hours. The firing is performed in an atmosphere containing oxygen. In this case, the oxygen concentration is preferably 20% or less.

The above-mentioned DEMKI KAGAK
U, 58 (5), p477 (1990), LiMn
It has been reported that it is easier to synthesize nitric acid (HNO ₃ ) to synthesize ₃ O ₆ , but to bake it. However, if appropriate conditions are selected as described above, pure nitric acid can be obtained without adding nitric acid. Li
Mn ₃ O ₆ can be synthesized.

As the lithium salt, any of lithium nitrate, lithium carbonate, and lithium hydroxide monohydrate can be used. However, when lithium hydroxide monohydrate is used, depending on the synthesis conditions, Since there is a possibility that something other than LiMn ₃ O ₆ may also be formed, it is preferable to use lithium nitrate or lithium carbonate in order to obtain pure LiMn ₃ O ₆ .

In Example 1, Li _1-y Mn ₃ O ₆ was electrochemically extracted from LiMn ₃ O ₆ to make Li _1-y Mn ₃ O ₆ , but Li can be extracted from LiMn ₃ O ₆ by acid treatment. In the case of this acid treatment, LiMn ₃ O ₆ may be immersed in a sulfuric acid aqueous solution having a pH of about 2 for 5 to 6 hours.

In Example 1, electrolytic manganese dioxide was used as manganese dioxide in the synthesis of LiMn ₃ O ₆ , but chemical manganese dioxide may be used instead.

[0044]

As described above, according to the present invention, Li
Mn ₃ O ₆ was replaced with Li _1-y Mn ₃ O ₆ (0 <
By using up to the region of y <1), a lithium secondary battery having a large charge / discharge capacity could be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a lithium secondary battery of the present invention.

FIG. 2 is a diagram showing a charge / discharge curve at the fifth cycle of the battery of Example 1 of the present invention.

FIG. 3 shows L in US Pat. No. 4,302,518.
open circuit voltage (vs. Li) with change in x of ixCo _1.01 O ₂
FIG.

FIG. 4 The 29th Battery Symposium Abstracts, p136 (198
FIG. 8 is a diagram showing a change in open circuit voltage (vs. Li) with a change in x of Li _X Mn ₂ O ₄ in 8).

[Explanation of symbols]

 1 positive electrode 2 negative electrode 3 separator

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirotaka Shima 1-88 Ushitora, Ibaraki-shi, Osaka Hitachi Maxell, Ltd. (56) References JP-A-2-270269 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) H01M 4/02 H01M 4/58 H01M 10/40

Claims (1)

(57) [Claims] 1. Lithium, lithium alloy or lithium alloy
In a lithium secondary battery using carbon as a negative electrode, LiMn ₃ O ₆ is used as a positive electrode active material, and LiMn ₃ O ₆
A lithium secondary battery characterized in that Li inside is electrochemically or chemically removed.