JPH1021898A - Lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lithium battery capable of large-current charging and discharging by employing an active substance of a positive electrode as a mixed valence complex in a lithium battery made of positive and negative electrodes and non-aqueous electrolyte containing a lithium ion that are capable of performing reversible electrochemical reaction with the lithium ion. SOLUTION: In a lithium battery made of positive and negative electrodes and a non-aqueous electrolyte that are capable of performing reversible electrochemical reaction with a lithium ion, an activated substance of this positive electrode is employed as a mixed valence complex. As the mixed valence complex, a transient metal with its different oxidation state is preferably employed, iron ruthenium, osmium, chromium, manganese, nickel, cobalt or the like is preferably employed for this transient metal, and as the mixed valence complex, Prussian blue (KFe [Fe (CN)6 ], (NH4 ) Fe [Fe (CN)6 ], NaFe [Fe (CN)6 ]) is employed. Thus, a lithium secondary battery with its charging and discharging capacity and energy density per positive electrode active substance weight, its long cyclic service life, and its high stability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery, and more particularly to a positive electrode active material for a non-aqueous electrolyte lithium battery.

[0002]

2. Description of the Related Art In recent years, electronic devices such as a camera-integrated VTR, a CD player, a notebook computer, and a portable telephone have been reduced in size and weight and have been portable, and the development of a high energy density battery as a power source has been awaited. I have. Batteries that meet these demands have higher voltages and higher voltages than secondary batteries such as nickel-cadmium batteries, nickel-metal hydride batteries, and lead-acid batteries.
A lithium secondary battery having a high energy density is expected.

[0003] As a positive electrode active material of a lithium secondary battery satisfying these demands, a material capable of reversibly electrochemically reacting with lithium ions, such as titanium, molybdenum, tungsten, and niobium capable of inserting and extracting lithium. Oxides and sulfides of metals such as vanadium, manganese, iron, chromium, nickel, cobalt, and selenium have been proposed. (Japan MRS Symposium Q “Chromogenic Materials” Preliminary Lectures, 10 pages, 199
6 years). In particular, cobalt showing a high potential with respect to lithium,
Many studies have been made on composite oxides such as nickel and manganese, including their crystal structures, and some have been put to practical use as lithium secondary batteries.

[0004]

However, in a lithium secondary battery using the above positive electrode active material,
For example, although the composite oxide of cobalt and the composite oxide of manganese have a high potential of about 4 V with respect to the negative electrode, the capacity per unit weight of the positive electrode active material capable of inserting and extracting lithium ions is not so high as about 120 mAh / g. . Further, although the capacity per weight of the nickel composite oxide is as high as about 180 mAh / g, it has problems such as deterioration of the active material, deterioration of discharge characteristics, and short cycle life. Has not been reached.

The present invention solves the above-mentioned problems of the prior art, and has a high capacity per weight of the positive electrode active material, a high energy density, a large charge / discharge capacity, a charge / discharge with a large current, and a long cycle life. It is an object to provide a lithium secondary battery using a positive electrode active material.

[0006]

The present invention provides a lithium battery comprising a positive electrode capable of reversible electrochemical reaction with lithium ions, a negative electrode, and a lithium ion-containing nonaqueous electrolyte, wherein the active material of the positive electrode is a mixed atom. A lithium battery, which is a multivalent complex.

In the present invention, the mixed valence complex used as the positive electrode active material is a complex containing an element having a different oxidation state in the molecule.
A three-dimensional mixed valence complex of a metal having a valence of valence, particularly a mixed valence complex of a transition metal having a different oxidation state is suitable. As this transition metal, iron, ruthenium, osmium, chromium, manganese, nickel, cobalt, vanadium, tungsten, titanium, gold, silver, copper and cerium are preferably used, and among them, iron, ruthenium,
Osmium, chromium, manganese, nickel and cobalt are preferred, and iron, ruthenium and osmium are particularly preferred. Mixed valence cyano complexes of these transition metals are preferably used, and mixed valence complexes in which at least a part of these cyano groups are substituted with carbonyl, ammonium, fluorine, OH ₂ , NO group or NO ₂ group are also used. Can be.

The mixed valence complex is preferably a compound represented by the formula

## STR1 ## ^{_{Q + x M 4 [R (}} CN) 6 · A - y] 3 (1) ( wherein Q ⁺ is a monovalent cation, M is iron (Fe), ruthenium (Ru), and osmium ( Os), at least one selected from the group consisting of iron, ruthenium, and osmium; A ^- is a monovalent anion; x is 0 or 4
The following positive number, y is a positive number equal to or less than 0 or 1. ).

M and R may be different elements or the same kind of element. The monovalent cation Q ⁺ is preferably an alkali metal ion or an ammonium ion. The monovalent anion A ⁻ is preferably an anion or Cl ⁻ ion or ClO ₄ ⁻ ion in the electrolytic solution. The valences of x and y and the valences of M and R vary depending on the oxidation / reduction state of M and R in the compound, that is, the state of charge and discharge in the electrolytic solution. x and y are usually zero, and take a non-zero value when the battery is operated.

For example, if M and R are each Fe ³⁺
And Ru ^II (the high spin ion Fe is replaced by Fe
³⁺ , Fe ²⁺ , and low spin ions Ru by Ru ^III , Ru ^II
And when the divalent Ru ions are in the most oxidized state and all become Ru ^III (corresponding to the time when the positive electrode of the battery is completely charged), the above chemical formula 1 becomes

[ ^Image Omitted] Fe ³⁺ ₄ [Ru ^III (CN) ₆ .A ⁻ ] ₃ Here, A ⁻ is an anion in the electrolytic solution, for example, Cl ⁻ . Conversely, when the trivalent Fe ions are in the most reduced state and all become Fe ²⁺ (corresponding to the time when the positive electrode of the battery is completely discharged), the above chemical formula 1

## STR3 ## Q ⁺ ₄ Fe 2 ⁺ ₄ [Ru ^II (CN) ₆ ] ₃

When the monovalent cation Q ⁺ operates as the positive electrode of the battery, lithium ions in the electrolyte are located at this ion Q ⁺ . Therefore, when the monovalent cation Q ⁺ is an alkali metal or ammonium ion other than lithium, which becomes an impurity and contributes to shortening the life of the battery, the monovalent cation Q ⁺ is lithium, or Most preferably, x is zero. Likewise the 1 dianion A ^- when operates as a positive electrode of the battery, the anion A in the electrolyte solution ^- is located at the ion Q ^+. Thus 1 dianion A ^- when is other than anions in the electrolyte, since this will contribute to shorten the life of the battery becomes impurities, 1 dianion A ^- Yin in the electrolyte Most preferably, it is an ion or y is zero.

A compound in which at least a part of the cyano group of the above formula, for example, one of the six cyano groups is substituted with a carbonyl, ammonium, fluorine, OH ₂ , NO group or NO ₂ group can also be used. .

Well-known mixed valence complexes include Prussian blue (KFe [Fe (CN) ₆ ],
(NH ₄ ) Fe [Fe (CN) ₆ ], NaFe [Fe (C
N) ₆ ]), which can be suitably used in the present invention. In addition, instead of iron (Fe), a ruthenium (R
Ruthenium purple using u) and osmium purple using osmium (Os) can also be suitably used in the present invention. A part of the cyano group of Prussian blue, ruthenium purple, and osnium purple may be substituted with a carbonyl group, an ammonium group, or the like. Also, a part of Prussian blue divalent iron Fe ^II is cobalt,
It may be replaced by ruthenium, osmium, nickel, silver, or gold, and Prussian blue trivalent iron Fe
Part of ³⁺ may be replaced by cobalt or copper.

Prussian blue is obtained as a precipitate when an aqueous solution containing trivalent iron ions (eg, FeCl ₃ ) and an aqueous solution containing ferrocyanide ions (K ₄ Fe ^II (CN) ₆ : K is a cation) are mixed. This Prussian blue has a crystal structure of a face-centered cubic lattice having a lattice constant of about 10.2 angstroms, and divalent and trivalent iron ions are alternately present at each lattice point, and a cyano group is bridged between them. , Insoluble in water. When this is used as a positive electrode material of a lithium secondary battery, the cation L
The following charge transfer reaction is performed by reversible occlusion and release of i.

[0015]

Embedded image Fe ³⁺ ₄ [Fe ^II (CN) ₆ ] ₃ + 4e ⁻ + 4Li
⁺ ← → Li ⁺ ₄ Fe ²⁺ ₄ [Fe ^II (CN) ₆ ] ₃ Here, the reaction in the ← direction indicates charging, and the reaction in the → direction indicates discharging.

[0016]

Embedded image Fe ³⁺ ₄ [Fe ^II (CN) ₆ ] ₃ -3e ⁻ + 3A ⁻
← → Fe ³⁺ ₄ [Fe ^III (CN) ₆ .A ⁻ ] ₃ where A ⁻ represents an anion in the electrolyte, the reaction in the ← direction represents charging, and the reaction in the → direction represents discharging. .

In the present invention, by using the mixed valence complex as the positive electrode active material, the charge / discharge capacity per unit weight of the positive electrode active material and the energy density are high, charging / discharging at a large current is possible, and the cycle life is extended. The reason why the effect is generated is considered as follows.

In general, a positive electrode active material of a lithium battery has a host structure having a site where lithium ions can be reversibly injected / extracted, and can be associated with redox. On the other hand, characteristics such as having a high potential are required. Many transition metal oxides have been studied as materials meeting these requirements.

For example, a composite oxide of lithium and manganese or cobalt has a close-packed crystal structure, and is considered to have a large number of sites and to be capable of large-capacity charge / discharge. Therefore, lithium can be synthesized only in the state where lithium is buried in those sites in advance, and lithium buried in those sites is difficult to move, so there are not many sites where lithium can reversibly enter and exit, so the actual charge and discharge capacity is not so much Not big. Some substances have relatively large charge / discharge capacities, such as composite oxides of lithium and nickel, but it is difficult to ensure stability when used as a positive electrode active material, and it has not yet been put to practical use. Further, a substance such as tungsten oxide, which is a close-packed and thermodynamically stable substance, has a low potential with respect to lithium, and a very large energy density is not obtained.

The mixed valence complex according to the present invention satisfies all the characteristics required for the above-mentioned positive electrode active material, and has a large ligand constituting the complex, and is connected to a large structure like a zeolite. Easy to make a gap. Since lithium ions diffuse in the gap, high-speed charging and discharging can be performed as compared with the conventional transition metal oxide. In addition, although the degree of filling as a crystal is low, it is not necessary to be a complex compound with lithium. For example, water-insoluble Prussian blue (Fe ^{3 +} ₄ [Fe ^II (CN) ₆ ] ₃ ) which is a typical mixed valence complex is used. In the crystal structure of (2), two kinds of iron ions having different valences, a high spin iron ion Fe ³⁺ and a low spin iron ion Fe ^II , are alternately present at lattice points via a cyano group, and Fe ³⁺ is a CN group. On the nitrogen side, Fe ^II is a face-centered cubic arranged on the carbon side, and has a lattice constant larger than the lattice constant of ordinary inorganic crystals.
It has a lattice constant of 0.2 angstroms, and zeolite pores of 3.2 angstroms in diameter are present on each face of the 1/8 lattice. 4 out of every 8 grids for a total of 4
Since the space in each lattice is an actual site where lithium ions are implanted / extracted, the number of sites is large, and it is considered that a large charge / discharge capacity can be obtained. Further, depending on the state of the mixed valence complex, anions of the electrolytic solution may be injected / extracted at an oxidation-reduction potential higher than the oxidation-reduction potential involved in normal charge / discharge, so that a larger charge / discharge capacity is obtained. Can be In addition, almost no deterioration such as generation of irreversible substances due to repeated charge and discharge is observed, and the battery is a cathode material that is extremely stable and safe from overcharging and discharging.

In the present invention, when Prussian blue, whose production method is industrially established as a pigment, is used as a positive electrode active material, an inexpensive lithium secondary battery can be provided. Prussian blue is an iron compound that is abundant as a resource, and can provide a lithium secondary battery with low environmental pollution.

The mixed valence complex as the positive electrode active material of the present invention is usually produced in the form of a powder having a particle size of 0.02 to 0.2 μm. As a mode for using this as a positive electrode, after mixing an appropriate amount of a fluorine-based resin as a binder and an electron conductivity imparting agent such as acetylene black, the mixture is formed into a plate shape to obtain a positive electrode. it can. The mixed valence complex can be formed into a thin film by, for example, a known electrolytic thin film synthesis method or an electroless deposition method, so that a metal material (for example, platinum, nickel, stainless steel or the like and a substrate coated with an ITO conductive film), a semiconductor, A non-porous or porous sheet-like or flexible film-like positive electrode substrate made of a material having electron conductivity such as graphite is coated with a mixed valence complex in a thin film form, for example, 0.05 to 5 μm. It can also be formed to a thickness of. Alternatively, a thin film of a mixed valence complex, which is a positive electrode active material, may be formed on the surface of each fiber of a positive electrode substrate in which a metal fiber or a graphite fiber is formed into a web shape.

As the negative electrode active material in the lithium secondary battery of the present invention, a lithium metal or lithium alloy capable of reversibly occluding and releasing lithium ions, a carbonaceous material or a tin oxide material can be used.

As the non-aqueous electrolyte of the lithium secondary battery of the present invention, a non-aqueous electrolyte obtained by dissolving a lithium salt such as lithium perchlorate or LiPF ₆ as a solute in a solvent usually at a concentration of about 1 mol / L is used. As the anion, those having a small size, for example, Cl ion are preferably used. As the solvent, propylene carbonate, chain esters such as ethylene carbonate, γ-lactones such as γ-butyrolactone, chain ethers such as ethoxymethoxyethane, cyclic ethers such as tetrahydrofuran, nitriles such as acetonitrile, and The mixed solvent can be used.

Although the case where the cathode active material of the mixed valence complex is applied to a lithium secondary battery has been described above, the cathode active material in the present invention can also be applied to a lithium primary battery. However, in the lithium primary battery, MnO _{2 and} other materials are used as the negative electrode.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the following examples, which, however, are not intended to limit the scope of the invention.

Example 1 2.5 mmol of hypophosphorous acid was added to an aqueous solution of an equal amount of 20 mmol of ferric chloride hexahydrate and 20 mmol of potassium ferricyanide.
The resulting precipitate was taken out, placed in a thermostat at 60 ° C., kept for 48 hours, and dried to obtain a powdery water-insoluble Prussian blue having a particle size of about 0.1 μm. This Prussian Blue 80
To the parts by weight, 20 parts by weight of powdered acetylene black as a conductivity-imparting agent and 0.1 parts by weight of pelletized polytetrafluoroethylene as a binder were added and kneaded to obtain a mixture having a diameter of 0.05 mm. A stainless steel wire net was used as a core and pressed onto the core to obtain a sheet having a thickness of about 0.1 mm. This was used as a positive electrode, and made of lithium metal having a thickness of 0.1.
mm sheet as a negative electrode and a separator having a thickness of 0.
Using a 2 mm polypropylene non-woven fabric, a coin-type battery using LiClO ₄ dissolved in propylene carbonate at a concentration of 1 mol / L as an electrolyte was prepared in a glow box sufficiently substituted with argon. The charge / discharge characteristics of this battery were measured using an automatic battery charge / discharge device, and the current density (per unit area of the electrode) at the time of charging and discharging was 5%.
The measurement was performed while maintaining a constant value of 00 μA / cm ² . The result is shown in FIG. In the figure, the horizontal axis represents the charge capacity and the discharge capacity (mAh / g) per unit weight of the positive electrode active material, and the vertical axis represents the terminal voltage (V) of the battery. Initial charge characteristic 1 and discharge characteristic 4, charge characteristic 2 and discharge characteristic 5 in the 50th charge / discharge cycle, and charge / discharge cycle 1
A charge characteristic 3 and a discharge characteristic 6 at the 00th time are shown.

It is considered that the positive electrode active material at the time of completion of the charge / discharge is almost in the state similar to the following compound. When charging is completed--Fe ³⁺ ₄ [Fe ^II (CN) ₆ ] _{3 When} discharging is completed--Li ⁺ ₄ Fe ²⁺ ₄ [Fe ^II (CN) ₆ ] ₃

As apparent from FIG. 1, a high voltage of about 3.0 V can be obtained by using Prussian blue as the positive electrode active material, a charge / discharge capacity of about 90 mAh / g, and about 230 mW.
It can be seen that a lithium secondary battery having a cycle life of 100 times or more at an energy density of h / g can be obtained. further,
The battery of the present invention can be measured at a high current density of 500 μA / cm ^{2, and} a current several times that of the lithium secondary battery of Comparative Example 1 described later was observed. Thus, it can be seen that a lithium secondary battery capable of high-speed charge and discharge can be obtained by using Prussian blue as the positive electrode active material.

In this embodiment, the anion of the electrolyte is C
lO ₄ ^- a and hardly enters the pores of about 3.2 angstroms which is the gap of the unit cell of the Prussian blue, but the anion of the electrolyte solution and to use those dissolved in propylene carbonate LiCl as electrolyte Cl ^- Higher voltage (approximately 4
V), as well as higher high charge / discharge capacity (about 200 mAh
/ G) and a high energy density. At this time, it is considered that the positive electrode active material at the time of completion of charge / discharge is almost in a state close to the next compound. When charging is completed ^-- Fe ³⁺ ₄ [Fe ^III (CN) ₆ .Cl ⁻ ] _{3 When} discharging is completed ^-- Li ⁺ ₄ Fe ²⁺ ₄ [Fe ^II (CN) ₆ ] ₃

Comparative Example 1 A lithium secondary battery was fabricated in the same manner as in Example 1 except that Li _x CoO ₂ was used as the cathode material instead of the positive electrode used in Example 1. The charge / discharge characteristics were measured, and as a result, all of the voltage, charge / discharge capacity, energy density, high-speed charge / discharge, and cycle life were lower than those in Example 1.

[0032]

As described above, by using a mixed valence complex having a larger number of sites per unit cell as a positive electrode active material as compared with a conventional metal oxide at sites capable of reversibly storing and releasing lithium ions, A lithium secondary battery with high charge / discharge capacity and energy density per active material weight, charge / discharge at a large current, a long cycle life, and high stability can be provided.

Further, for example, Prussian blue is a mixed valence complex of iron which is abundant in resources, and can provide a lithium secondary battery using a positive electrode active material which is inexpensive and has low environmental pollution. Further, since Prussian blue has electrochromic properties, the state of charge and discharge can be viewed as a color change of the positive electrode, and the state of charge and discharge can be monitored.

Also, when applied to a lithium primary battery, high discharge capacity, high energy density, and high current discharge performance are expected.

[Brief description of the drawings]

FIG. 1 is a graph showing battery characteristics of the present invention.

[Explanation of symbols]

 1. Initial charging characteristics 2. Charging / discharging 50th charging characteristics 3. Charging / discharging 100th charging characteristics 4. Initial discharging characteristics 5. 50th charging / discharging 50th discharging characteristics 6. Discharge characteristics at the 100th charge / discharge cycle

Claims (8)

[Claims] 1. A lithium battery comprising a positive electrode capable of reversible electrochemical reaction with lithium ions, a negative electrode, and a lithium ion-containing non-aqueous electrolyte, wherein the active material of the positive electrode is a mixed valence complex. Lithium battery. 2. The lithium battery according to claim 1, wherein the mixed valence complex is a mixed valence complex of a transition metal having a different oxidation state. 3. The mixed valence complex is at least one metal selected from the group consisting of iron, ruthenium, osmium, chromium, manganese, nickel, cobalt, vanadium, tungsten, titanium, gold, silver, copper and cerium. The lithium battery according to claim 1, which is a mixed valence complex of 4. The mixed valence complex is a mixed valence cyano complex of the metal or at least a part of these cyano groups is carbonyl, ammonium, fluorine, OH ₂ , NO.
The lithium battery according to claim 3, which is a mixed valence complex substituted with a group or a NO ₂ group. Wherein said mixed-valence complex is formula 1 ## STR1 ## ^{_{Q + x M 4 [R (}} CN) 6 · A - y] 3 (1) ( wherein Q ⁺ is a monovalent cation, M is At least one selected from the group consisting of iron, ruthenium, and osmium, R is at least one selected from the group consisting of iron, ruthenium, and osmium; A ^- is a monovalent anion; x is 0 or a positive number of 4 or less, and y is a positive number of 0 or 1 or less, or at least a part of the cyano group of the above formula is a carbonyl, ammonium, fluorine, OH ₂ , NO group or lithium battery according to claim 4 which is a compound substituted with NO ₂ group. 6. The mixed valence complex is a compound represented by the above chemical formula 1, wherein Q ⁺ is an alkali metal ion or an ammonium ion, and M and R are both iron. The lithium battery according to 1. 7. The mixed valence complex is a compound represented by the above formula 1, wherein Q ^{+ in the} formula 1 is lithium or x is zero, and M and R in the formula 1 are The lithium battery according to claim 5, wherein both are iron. 8. The lithium battery according to claim 5, wherein the mixed valence complex is Prussian blue.