JPH10199506A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a new lithium secondary battery by employing a sulfur compound of iron expressed by a general formula Fe1-x S (0<=x<=1) composed of such components as FeS, FeS1+x , Fe2 S3 and FeS, etc., as a positive pole or a negative pole. SOLUTION: When a sulfur compound of iron expressed by a general formula Fe1-x S (0<=x<=1) is used as the positive pole of a lithium secondary battery, at least one kind is used, as a negative pole, out of either one of lithium metal, lithium alloy and a substance capable of storing/releasing lithium previously including such lithium as having lower oxidation/reduction potential than that of the iron sulfur compound in the general formula. In addition, when it is used in the negative pole, a substance capable of staring/releasing lithium previously including such lithium as having higher oxidation-reduction potential than that of an iron sulfur compound in the general formula is used in a positive pole. Thereby, it becomes possible that a discharge capacity per unit weight of positive pole active material is large and has a very high percentage discharge capacity even at the 20-th cycle can be maintained relative to the initial capacity.

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 using iron sulfide for a positive electrode or a negative electrode. More specifically, the present invention relates to a general formula, Fe _1-x S (0 ≦ x <1). The present invention relates to a lithium secondary battery using a sulfur compound of iron for a positive electrode or a negative electrode.

[0002] Lithium secondary batteries are batteries that are positioned as the most promising batteries among new type secondary batteries.

[0003]

2. Description of the Related Art Since iron compounds are abundant and inexpensive in terms of resources as raw materials, application to electrode active materials has been attempted for a long time.

For example, studies have been made on the application of iron disulfide (FeS ₂ ) and iron sulfide (FeS), which are sulfur compounds of iron, to lithium primary batteries.

When FeS ₂ or FeS is used for the positive electrode of a lithium primary battery, although the problem of corrosiveness remains, the theoretical discharge capacity per unit weight is larger than other positive electrode materials, and the conventional aqueous primary Because of the compatibility with the battery voltage of the battery, FeS ₂ has already been put to practical use as a positive electrode for a lithium primary battery.

However, the studies so far have been limited to the application to the positive electrode for a lithium primary battery, and no studies have been made on the positive electrode or the negative electrode of a lithium secondary battery.

[0007] In addition, application of iron oxide to a negative electrode of a lithium secondary battery is being studied.

[0008] For example, Fe ₂ O ₃ and Fe ₃ O ₄ are exemplified, but none of them have been put to practical use until now, since the irreversible capacity in the first cycle is extremely large. As described above, despite the fact that iron as a raw material is abundant and inexpensive in terms of resources, despite its promising application to electrode active materials, those in which iron compounds have been put to practical use as actual batteries have It is limited to FeS ₂ for lithium primary batteries, and development of new lithium secondary batteries using iron compounds is desired.

On the other hand, lithium batteries have recently become more important as lightweight, small-sized, high-performance batteries with high energy density. Particularly, with the rapid spread of portable small-sized devices for personal use such as mobile phones and personal computers. Accordingly, development of a lithium secondary battery as a drive power source for these devices has been desired.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel lithium secondary battery using an iron compound.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to propose a new lithium secondary battery using an iron compound, and as a result, have found that the general formula, Fe _1-x S (0 ≦ x < The fact that the sulfur compound of iron represented by 1) can be applied to the positive electrode or the negative electrode of a lithium secondary battery, and that a novel lithium secondary battery that has never existed before can be constructed by using the positive electrode or the negative electrode. As a result, the present invention has been completed.

[0012]

The present invention will be specifically described below.

The lithium secondary battery of the present invention has a general formula:
An iron sulfur compound represented by Fe _1-x S (0 ≦ x <1) is used.

As the sulfur compound of iron, FeS, FeS _{1 + x} having a non-stoichiometric composition, and iron sulfide such as Fe ₂ S ₃ are generally known, and the sulfur compound of iron used in the lithium secondary battery of the present invention is known. The compound represented by the general formula: Fe _1-x S (0 ≦ x <
The compound is not particularly limited as long as the compound satisfies the relation of 1), and may be a mixture or a composite of at least one of the above three types of iron sulfide.

F is exemplified as a component constituting a sulfur compound of iron represented by the general formula, Fe _1-x S (0 ≦ x <1).
eS, FeS _{1 + x} , Fe ₂ S ₃ and FeS ₂ are compounds having the following properties.

Iron (II) sulfide is composed of pure FeS and FeS
Any compound having a non-stoichiometric composition represented by _{1 + x} .

Pure FeS is made by heating reduced iron and distilled sulfur in a vacuum sealed vessel at 1000 ° C. for 24 hours. It can also be obtained by adding ammonium sulfide to an aqueous iron (II) salt solution. Its properties are colorless to light brown crystals.

Nonstoichiometric FeS _{1 + x} is a compound that naturally occurs as pyrrhotite. In addition, it is industrially produced by melting iron powder and sulfur in a crucible, and its properties are gray-black flakes or lumps.

A compound having a non-stoichiometric composition may be oxidized in the air in a wet state to separate sulfur and form a brown basic sulfate.

Fe ₂ S ₃ is a black powder obtained by adding a sufficient amount of ammonium sulfide to an iron (III) salt solution, and may release sulfur in humid air to become a hydrate of an oxide. .

FeS ₂ is produced as ores as marcasite (mineral name: marcasite) and pyrite (mineral name: pyrite). Of these, the one that functions as a battery is that it has a high packing density and good electron conductivity. It is a pyrite having a rock salt type crystal structure. Although pyrite is synthesizable, it only functions as a battery material from naturally occurring ores.

A general formula used for the lithium battery of the present invention,
When an iron sulfur compound represented by Fe _1-x S (0 ≦ x <1) is used for an electrode of a lithium secondary battery, there is no particular limitation. In addition to the heat treatment performed as a pretreatment, a normal heat treatment as a secondary battery material may be performed after the heat treatment is performed in advance.

The method of performing the heat treatment in advance is not particularly limited as long as the material composition can maintain the relationship represented by the general formula, Fe _1-x S (0 ≦ x <1).

In the lithium secondary battery of the present invention, when an iron sulfur compound represented by the general formula, Fe _1-x S (0 ≦ x <1) is used for the positive electrode of the lithium secondary battery, It is necessary to use at least one of lithium metal, a lithium alloy, and a substance capable of inserting and extracting lithium, which previously contains lithium having a lower oxidation-reduction potential than the sulfur compound of iron of the present invention.

In the lithium secondary battery of the present invention, when an iron sulfur compound represented by the general formula Fe _1-x S (0 ≦ x <1) is used for the negative electrode, the positive electrode of the present invention It is necessary to use a lithium-containing substance capable of inserting and extracting lithium which has a higher oxidation-reduction potential than the iron sulfur compound.

Examples of the lithium alloy include lithium / aluminum alloy, lithium / tin alloy, and lithium / tin alloy.
A lead alloy or the like is exemplified.

The substance capable of inserting and extracting lithium containing lithium in advance is not particularly limited.
₂ O ₄ , LiMnO ₂ , LiCoO ₂ , LiNiO ₂ ,
Oxides such as lithium-containing transition metal oxides such as LiFeO ₂ and composites of these compounds, carbonaceous materials such as graphite and graphite, iron oxides such as FeO, Fe ₂ O ₃ and Fe ₃ O ₄ , CoO and Co Examples include compounds in which cobalt oxide such as ₂ O ₃ and Co ₃ O ₄ is doped with lithium in advance. Note that application of these substances containing lithium in advance and capable of inserting and extracting lithium to the negative electrode or the positive electrode is restricted by the oxidation-reduction potential.

The electrolyte of the lithium secondary battery of the present invention is not particularly limited. For example, carbonates such as propylene carbonate, diethyl carbonate, and dimethyl carbonate; sulfolane such as sulfolane and dimethyl sulfoxide;
Lactones such as butyl lactone, and ethers such as dimethoxyethane in at least one organic solvent, such as lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, etc. A solution in which at least one kind of lithium salt is dissolved, an inorganic or organic lithium ion conductive solid electrolyte, or the like can be used.

A battery shown in FIG. 1 was constructed using the iron compound composite of the present invention for a positive electrode or a negative electrode.

In the drawing, 1: lead wire for positive electrode, 2:
Positive electrode collecting mesh, 3: Positive electrode, 4: Separator, 5:
Negative electrode, 6: negative electrode current collector mesh, 7: negative electrode lead wire,
8: Container.

Examples will be shown below as specific examples of the present invention, but the present invention is not limited by these examples.

[0032]

【Example】

Example 1 [Structure of Battery] In Example 1, a mixture of FeS (reagent first grade), polytetrafluoroethylene as a conductive agent and acetylene black (trade name: TAB-2) was used in a weight ratio of 2:
1 was mixed. 75 mg of the mixture is 1 ton / cm
^At a pressure of ² , 20mmφ mesh (SUS 316)
After being molded into a pellet on top, at 200 ° C for 5 hours,
A vacuum drying treatment was performed. This is used for the positive electrode in FIG. 1, the negative electrode in FIG. 1 is a lithium piece cut out of a lithium foil, and the electrolyte is lithium hexafluorophosphate at a concentration of 1 mol / dm ³ . A solution of ethylene carbonate and diethyl carbonate dissolved in a mixed solvent having a volume ratio of 1: 2 was impregnated into a separator shown in FIG. 1 to form a battery having a sectional area of 2.5 cm @ 2 shown in FIG.

[Evaluation] The battery prepared by the above method was used in a
The battery was discharged at a constant current of 4 mA / cm ² until the battery voltage reached 1.2 V. A flat discharge curve was shown, a discharge capacity of 400 mAh was obtained per weight of the positive electrode active material, and a discharge capacity of 80% of the initial capacity was maintained even at the 20th cycle. Also,
The charge / discharge coulomb efficiency in the first cycle was 90%.

Example 2 As Example 2, a positive electrode was previously applied to the positive electrode at a temperature of 110 ° C. in air.
A battery was prepared and evaluated in the same manner as in Example 1, except that a battery heat-treated for 2 hours was used. Initially, a discharge capacity of 420 mAh per weight of the positive electrode active material was obtained, and the discharge capacity of 90% of the initial capacity was maintained even in the 20th cycle. The charge / discharge coulombic efficiency in the first cycle was 85%.

Example 3 As Example 3, a positive electrode was previously applied to the positive electrode at a temperature of 220 ° C. in air.
A battery was prepared and evaluated in the same manner as in Example 1, except that a battery heat-treated for 2 hours was used. Initially, a discharge capacity of 450 mAh was obtained per weight of the positive electrode active material, and the discharge capacity was maintained at 70% of the initial capacity even at the 20th cycle. The charge / discharge coulombic efficiency in the first cycle was 90%.

Example 4 In Example 3, a mixture of FeS and a conductive agent used for the positive electrode in Example 1 was used for the negative electrode in FIG. 1, and a commercially available lithium manganese spinel was used in the positive electrode in FIG. (LiMn ₂ O
₄ ) was mixed with a mixture of a conductive agent, polytetrafluoroethylene and acetylene black (trade name: TAB-2) at a ratio of 2: 1 by weight, and 75 mg of the mixture was mixed with 1 ton /
At a pressure of ² cm ² , a mesh of 20 mmφ (SUS 31
6) A battery was formed and evaluated in the same manner as in Example 1 except that a pellet was formed thereon and then subjected to a vacuum drying treatment at 200 ° C. for 5 hours.

Initially, a discharge capacity of 110 mAh per weight of the positive electrode active material was obtained.
% Discharge capacity was maintained.

Comparative Example 1 As Comparative Example 1, a battery was constructed and evaluated in the same manner as in Example 4 except that Fe ₂ O ₃ was used as the negative electrode active material.

Initially, a discharge capacity of only 50 mAh was obtained per weight of the positive electrode active material.
Only 0% was maintained. The charge / discharge coulombic efficiency in the first cycle was 60%.

[0040]

As described above, the general formula, Fe
_{By using} a sulfur compound of iron represented by _1-xS (0 ≦ x <1) for a positive electrode or a negative electrode of a lithium secondary battery, a novel lithium secondary battery can be formed. Proposing a new lithium secondary battery is an industrially useful finding.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a battery constituted by an example and a comparative example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode lead wire 2 positive electrode current collecting mesh 3 positive electrode 4 separator 5 negative electrode 6 negative electrode current collecting mesh 7 negative electrode lead wire 8 container

Claims (1)

[Claims] 1. A lithium secondary battery using a sulfur compound of iron represented by the general formula, Fe _1-x S (0 ≦ x <1) for a positive electrode or a negative electrode.