JPH08102323A - Positive electrode material for lithium ion secondary battery and its manufacture - Google Patents

Abstract

PURPOSE: To provide a lithium ion secondary battery with high energy density and less capacity drop even if charge/discharge cycles are repeated for a long time. CONSTITUTION: A positive electrode material for a lithium ion secondary battery has cubic system spinel structure and is represented by a molecular formula of LixMn2 Oy (1.1×1.5, 4.2<y<=4.4). The positive electrode material has a lattice constant (a) of 8.16-8.18Å and a true density (f) of 3.1-3.5g/cm<3> . The positive electrode material is manufactured by mixing Mn2 O3 as a raw material of manganese and LiOH.H2 O as a raw material of lithium, crushing the mixture, then heat-treating the mixture powder in the air at 700-1000 deg.C.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a positive electrode material for a lithium ion secondary battery and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, with the progress of electronic technology, various electronic devices have been reduced in size and weight, and as a result, the storage space of batteries in the devices has been inevitably reduced and narrowed. To accommodate the reduction of battery storage space,
It is necessary to reduce the size of the battery, and it is desired to develop a battery having high energy density and long life.

A nickel-cadmium battery has been put into practical use as a rechargeable secondary battery, but the energy density of this battery is not satisfactory, and there is a drawback that the battery contains cadmium. The battery that was introduced to the market next to the Ni-Cd battery was a nickel-hydrogen type that can rapidly charge and discharge a large current at the same time, and at the same time, a lithium secondary battery was also realized. When a lithium secondary battery was charged during use, lithium became crystals of dendrite, which leaked between the electrodes and caused an explosion explosion, resulting in discontinuation of production. Nickel-hydrogen batteries have the drawbacks of being heavy and having a low energy density per volume.

Lithium-ion secondary batteries have recently received attention as batteries having high safety and high energy density because they do not use metallic lithium in the negative electrode, and mass production has started as a result of many technological developments.

[0005]

Materials such as LiCoO ₂ and LiNiO ₂ having an α-NaFeO ₂ structure and LiMn ₂ O _{4 having} a cubic spinel structure are known as 4V class positive electrode materials for lithium ion secondary batteries. There is. However, when these materials are used for the positive electrode of a lithium secondary battery, repeated charging and discharging causes a reduction in capacity early, and it is difficult to obtain practical level characteristics. Therefore, it is desired to improve the charge / discharge characteristics of the 4V class positive electrode material of the lithium ion secondary battery.

Among the above three compounds, LiCoO ₂ having good charge and discharge characteristics is frequently used, but cobalt, which is the main component, is produced in a very limited number of countries.
It is expensive and can be difficult to obtain depending on the political situation. LiCoO ₂ is manufactured by adding Li ₂ CO ₃ and xCo (O
H) ₂ · yCoCO ₃ After mixing, 800 ° C ~ 900
Although it is performed by firing at ° C, in this manufacturing method, an oxygen-deficient compound that can achieve a high energy density, that is, L
iCoO _{2 -x} is difficult to generate.

On the other hand, LiMn₂O_FourIts manufacturing
Abundant raw materials, low price, and low manufacturing cost
Therefore, various studies have been made for practical use. LiM
n₂O _FourIs MnO₂And Li₂CO₃Manganic acid like
Compound and lithium salt are mixed, crushed, and then heat treated.
It is easy to synthesize, and the molar ratio of lithium to manganese is generally
A stoichiometric amount of 1: 2 was produced. This substance
Has poor discharge capacity and cycle characteristics, which are the basis of battery characteristics.
It wasn't practical.

The present invention solves the above-mentioned problems in a lithium ion secondary battery. It is possible to obtain a battery having a long life with a very small decrease in capacity even if charging and discharging are repeated for a long time at a high energy density. An object of the present invention is to provide a positive electrode material for a lithium ion secondary battery that can be manufactured and a method for producing the same.

[0009]

The positive electrode material for a lithium ion secondary battery of the present invention has a cubic spinel structure and a molecular formula L.
i _X Mn ₂ O _Y (1.1 ≦ X ≦ 1.5, 4.2 <Y ≦
4.4). The positive electrode material for a lithium ion secondary battery has a lattice constant a in the range of 8.16Å to 8.18Å and a true density ρ of 3.1 g / cm ^{3 to} 3.5.
It is characterized in that it is in the range of g / cm ³ .

This positive electrode material for a lithium ion secondary battery uses Mn ₂ O ₃ as a manganese raw material and LiOH · H ₂ O as a lithium raw material, and after mixing and pulverizing the raw materials, 700 ° C. to 1000 ° C. in the atmosphere. It is produced by heat treatment with C.

[0011]

A lithium ion secondary battery using the positive electrode material for a lithium ion secondary battery of the present invention as a positive electrode exhibits stable charge / discharge characteristics with a very small decrease in capacity. LiMn ₂ O _{4, which} is known as one of the conventional positive electrode materials, is JCPDS
(Thejoint committee on po
(wer diffraction standards)
According to the card, the lattice constant a is 8.248Å, the true density ρ
Is 4.281 g / cm ³ . On the other hand, the positive electrode material for a lithium ion secondary battery of the present invention has a molecular formula of Li _X Mn ₂ O.
_Y (1.1 ≦ X ≦ 1.5, 4.2 <Y ≦ 4.4), the lattice constant a is in the range of 8.16Å to 8.18Å, and the true density ρ is 3 .1 g / cm ^{3 to} 3.5 g / c
It is within the range of m ³ .

Comparing these, the positive electrode material for a lithium ion secondary battery of the present invention has a higher ratio of lithium and oxygen to manganese in the crystal than the conventional LiMn ₂ O ₄ , a small lattice constant a, and a true density ρ. It can be seen that is significantly smaller. It is considered that this is because the lattice constant a becomes small because the Li atom substitutes for the site where the Mn atom originally exists, and the ionic radius of the substituted Li atom is smaller than the ionic radius of the Mn atom. It is considered that the true density ρ is extremely small because the Li atom or oxygen atom having a smaller mass than the Mn atom is substituted at the position of the Mn atom that should exist in the unit cell of the crystal.

The charge / discharge reaction of the lithium ion secondary battery is
It occurs when lithium ions intercalate in the crystals of the positive electrode material. The crystal structure of the positive electrode material greatly affects the battery characteristics. From this, the positive electrode material for a lithium ion secondary battery of the present invention has a very stable cubic spinel structure in which lithium ions are extremely easily diffused and are not easily broken even with repeated charge / discharge cycles. It is considered to be a thing.

The reason for this is to speculate that a larger amount of Li and oxygen atoms in the crystal than the stoichiometric amount increases the amount of Li atoms involved in diffusion, while the excess oxygen atoms continue to retain the spinel structure. It is considered to contribute to the formation of the van der Waals layer. By using Mn ₂ O ₃ having vacancy due to lack of Mn atoms and LiOH.H ₂ O, which is less reactive than carbonate, as a starting material, the invasion and diffusion reaction of Li into the vacancy further progresses, and Molecular Formula of the Invention Li _X Mn ₂ O _Y (1.1 ≦ X ≦ 1.5, 4.2 <Y ≦
Even in 4.4), the cubic spinel structure can be maintained.

[0015]

EXAMPLES Examples of the present invention will be described in detail below. Predetermined amounts of Mn ₂ O ₃ as a manganese raw material and LiOH.H ₂ O as a lithium raw material were precisely weighed, thoroughly mixed and crushed in an agate mortar, and then placed in an alumina boat and heat-treated in the atmosphere. It was The heating temperature at this time was 700 ° C., and the heating time was 12 hours. In this way, a Li-Mn compound having a molar ratio of lithium to manganese of 1.33: 2 was synthesized.

As a comparative example, the same treatment was carried out and the molar ratios of lithium and manganese were 1: 2 and 0.86: 2.
The following Li-Mn compound was synthesized. Next, an electrode was created using these three types of Li-Mn compounds to assemble a cell. Li-Mn compound, carbon black, and Teflon powder were precisely weighed so that the weight ratio was 90: 5: 5,
After sufficiently kneading, the mixture was rolled into a sheet and pressed onto a stainless mesh current collector to prepare a positive electrode. On the other hand, as a negative electrode, a 1 mm thick lithium foil was pressure-bonded to a stainless net-like current collector similar to the positive electrode. These positive electrode and negative electrode are stacked via a separator and immersed in an electrolytic solution together with a reference electrode to form a three-electrode cell. As the reference electrode, an electrode made of lithium, which is the same as the negative electrode, was used. The electrolytic solution was prepared by dissolving 1 mol of lithium hexafluoroarsenate as an electrolyte in a solution of 1 dm ^{3 in} which propylene carbonate and dimethoxyethane were mixed at a ratio of 1: 1.

A charge / discharge cycle test was conducted on the cells thus assembled having three kinds of positive electrodes. Current density 1.0mA / cm ² , voltage range 4.3V
Charging / discharging was repeated at /3.0V. The result is shown in FIG. In FIG. 1, curve 1 shows the cycle characteristics when the Li—Mn compound of this example (the molar ratio of lithium and manganese is 1.33: 2) is used for the positive electrode, and curve 2 shows the molar ratio of lithium and manganese of 1 : 2 shows the cycle characteristics when using the Li-Mn compound for the positive electrode, and curve 3 shows the cycle characteristics when using the Li-Mn compound with a molar ratio of lithium and manganese of 0.86: 2 for the positive electrode. .

As a result of X-ray structural analysis of the above-mentioned three kinds of Li-Mn compounds, it was confirmed that they had a cubic spinel structure. Furthermore, the lattice constant a of these Li-Mn compounds was determined from the X-ray diffraction pattern. The density ρ was obtained using a pycnometer, and the amount of oxygen in the compound was obtained from the weight change before and after the heat treatment.

Table 1 shows the values of the lattice constant a, the density ρ, and the amount of oxygen in the compound. The amount of oxygen is shown by the molar ratio of oxygen to manganese.

[0020]

[Table 1]

As is clear from the cycle characteristic curve of FIG. 1, in the cell using the Li-Mn compound of the present embodiment as the positive electrode, the decrease in capacity upon repeated charge and discharge was significantly small, and 10 cycles were compared with the first cycle. The discharge capacity retention rate of the eyes was 91.3%. In addition, it can be seen from Table 1 that the Li-Mn compound of this example has a smaller lattice constant a and a smaller density ρ than the Li-Mn compound of the comparative example.

As another comparative example, L is obtained by using a manganese raw material and a lithium raw material which are generally used in the conventional manufacturing method.
An i-Mn compound was synthesized. MnO as manganese raw material
₂ , Li ₂ CO ₃ was used as a lithium raw material, the molar ratio of lithium to manganese was set to 1: 2, and a Li—Mn compound was synthesized by the same method as in the example. went. The result is shown in FIG. Curve 4 shows the cycle characteristics. It was confirmed by X-ray structural analysis that this Li-Mn compound had a cubic spinel structure. The cell using the Li-Mn compound as the positive electrode had a discharge capacity retention ratio of 3.4% at the 10th cycle with respect to the first cycle.

As yet another comparative example, a Li-Mn compound was synthesized using the manganese raw material used in the production method of the present invention and the lithium raw material generally used in the conventional production method. Using Mn ₂ O ₃ as a manganese raw material and Li ₂ CO ₃ as a lithium raw material and synthesizing three kinds of Li—Mn compounds having different molar ratios of lithium and manganese in the same manner as in the example, a tripolar cell was prepared. Then, a charge / discharge cycle test was performed. The result is shown in FIG. In FIG. 3, the curve 5
Is Li- in which the molar ratio of lithium to manganese is 1.33: 2.
Cycle characteristics when using a Mn compound for the positive electrode, curve 6 shows Li-Mn in which the molar ratio of lithium and manganese is 1: 2.
The cycle characteristics when the compound is used for the positive electrode, curve 7 is Li-M in which the molar ratio of lithium and manganese is 0.86: 2.
The cycle characteristics when an n compound is used for the positive electrode are shown.

It was confirmed by X-ray structural analysis that these Li-Mn compounds had a cubic spinel structure. The discharge capacity retention ratio at the 10th cycle with respect to the 1st cycle of the cell using the Li-Mn compound in which the molar ratio of lithium and manganese having the smallest capacity decrease was 1: 2 as the positive electrode was 69.4%. In still another comparative example, a manganese raw material generally used in the conventional manufacturing method and a lithium raw material used in the manufacturing method of the present invention are used to obtain Li.
-Mn compound was synthesized.

Using MnO ₂ as a manganese raw material and LiOH.H ₂ O as a lithium raw material, three types of Li having different molar ratios of lithium and manganese were prepared in the same manner as in the example.
After synthesizing the —Mn compound, a charge / discharge cycle test was performed using a 3-electrode cell. FIG. 4 shows the results. In FIG. 4, curve 8 shows a molar ratio of lithium to manganese of 1.33:
The cycle characteristics when the Li-Mn compound of No. 2 is used for the positive electrode, the curve 9 is the cycle characteristics when the Li-Mn compound of which molar ratio of lithium and manganese is 1: 2 is used for the positive electrode, and the curve 10 is the lithium and manganese. The molar ratio of 0.8
The cycle characteristic when a 6: 2 Li-Mn compound is used for the positive electrode is shown.

It was confirmed by X-ray structural analysis that these Li-Mn compounds had a cubic spinel structure. The discharge capacity retention rate at the 10th cycle was 86.0% with respect to the 1st cycle of the cell using the Li-Mn compound having the smallest capacity decrease of 0.86: 2 in the molar ratio of lithium and manganese as the positive electrode. . As can be seen by comparing FIG. 1, FIG. 2, FIG. 3, FIG. 4, and Table 1, Li—Mn in which the molar ratio of lithium to manganese in this example is 1.33: 2.
The compound has a lattice constant a of 8.169Å and a density ρ of 3.2.
8 g / cm ³ , and a cell using this as a positive electrode shows excellent battery characteristics in which the capacity is not significantly reduced even after repeated charge and discharge. This characteristic greatly exceeds the characteristic of LiMn ₂ O ₄ known as a conventional positive electrode material.

[0027]

As described above, by using the positive electrode material for a lithium ion secondary battery of the present invention, the capacity is remarkably reduced even when charging and discharging are repeated for a long time at a high energy density, and the battery has a long life. Can be obtained. Further, this positive electrode material can be synthesized very easily by the production method of the present invention.

[Brief description of drawings]

Fig. 1 Mn ₂ O _{3 as} a manganese raw material and L as a lithium raw material
The compounds synthesized using the IOH · H ₂ O cell used in the positive electrode is a characteristic diagram of a charge-discharge cycle.

FIG. 2 shows MnO _{2 as} a manganese raw material and Li as a lithium raw material.
FIG. 3 is a characteristic diagram of charge / discharge cycles of a cell using a compound synthesized using ₂ CO ₃ as a positive electrode.

FIG. 3: Mn ₂ O _{3 as} a manganese raw material and L as a lithium raw material
FIG. 3 is a characteristic diagram of charge / discharge cycles of a cell in which a compound synthesized using i ₂ CO ₃ is used as a positive electrode.

FIG. 4 MnO _{2 as} a manganese raw material and Li as a lithium raw material
The compounds synthesized using the OH · H ₂ O cell used in the positive electrode is a characteristic diagram of a charge-discharge cycle.

[Explanation of symbols]

1 Cycle characteristic curve when using the Li-Mn compound of the Example of this invention for a positive electrode 2 Cycle characteristic curve when using the Li-Mn compound of a comparative example for a positive electrode 3 Li-Mn compound of a comparative example was used for a positive electrode Cycle characteristic curve when used 4 Cyclic characteristic curve when Li-Mn compound of Comparative Example is used for positive electrode 5 Cycle characteristic curve 6 when Li-Mn compound of Comparative Example is used for positive electrode 6 Li-Mn of Comparative Example Cycle characteristic curve when compound is used for positive electrode 7 Cycle characteristic curve when Li-Mn compound of comparative example is used for positive electrode 8 Cycle characteristic curve when Li-Mn compound of comparative example is used for positive electrode 9 Comparative example Cycle characteristic curve when the Li-Mn compound of No. 10 is used for the positive electrode 10 Cycle characteristic curve when the Li-Mn compound of Comparative Example is used for the positive electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazutomi Yamamoto 3-3-33 Asahigaoka, Hino-shi, Tokyo Inside the Hino Research Center, Furukawa Machinery Co., Ltd. (72) Kenji Nakano Joban Shimo-Funao-cho, Iwaki City, Fukushima Prefecture Des. 23-6 No. 23, Furukawa Battery Co., Ltd. Iwaki Plant (72) Inventor Toru Mangahara Piles, Joban Shimo-Funao-cho, Iwaki City, Fukushima Prefecture Des. No. 23-6 No. 23 Furukawa Battery Co., Ltd. Iwaki Plant (72) Inventor Tanno Satoshi No. 23-6, Hashidashi, Joban Shimo-Funao-cho, Iwaki City, Fukushima Prefecture Furukawa Battery Co., Ltd., Iwaki Plant

Claims (4)

[Claims] 1. A cubic spinel structure having a molecular formula of Li _X Mn
₂ O _Y (1.1 ≦ X ≦ 1.5, 4.2 <Y ≦ 4.4) A positive electrode material for a lithium ion secondary battery. 2. The positive electrode material for a lithium ion secondary battery according to claim 1, wherein the lattice constant is in the range of 8.16Å to 8.18Å. 3. The true density is 3.1 g / cm ^{3 to} 3.5 g /
The positive electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode material has a cm ³ range. 4. Mn ₂ O ₃ is used as a manganese raw material, LiOH.H ₂ O is used as a lithium raw material, and the raw materials are mixed and pulverized, and then heat-treated at 700 ° C. to 1000 ° C. in the atmosphere. The method for producing a positive electrode material for a lithium ion secondary battery according to claim 1, claim 2, or claim 3.