JPH10154510A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which is equipped with a positive electrode containing positive electrode active material which has less deterioration resulting from repeated charging and discharging, that is, less cycle deterioration than that of conventional lithium manganese compound oxide. SOLUTION: In this lithium secondary battery, the storage and discharge of lithium ions between a positive electrode 2 containing positive electrode active material and a negative electrode 4 containing negative electrode active material is conducted. In this case, the positive electrode active material is composed of a lithium manganese iron spinel compound which is expressed by a general formula Li1+x Mn2-x-y Fey O4 , and (x) and (y) are in an area inside a polygon specified by specific points on a two-dimensional plane expressed by rectangular coordinates of (x) and (y).

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 a lithium secondary battery in which a positive electrode active material is improved.

[0002]

2. Description of the Related Art A lithium ion secondary battery having a positive electrode containing a lithium-cobalt composite oxide as a positive electrode active material and a negative electrode containing a carbonaceous material such as coke has been employed as a main power source for cordless phones and personal computers. . However, since the lithium-cobalt composite oxide is relatively expensive, attention has been paid to switching to a nickel- and manganese-based lithium-based composite oxide that is resource-rich and inexpensive.

[0003] Among manganese oxides, manganese dioxide composed only of manganese and oxygen has poor reversibility and poor charge / discharge characteristics. For example, lithium salts are introduced into manganese oxide such as LiMn ₂ O _4. It has been proposed to use a lithium manganese composite oxide having a modified spinel crystal structure as a positive electrode active material (US Pat. No. 4,507,507).
371).

However, among the lithium manganese composite oxides, LiMn ₂ O ₄ having a spinel structure with a stoichiometric composition in which the element ratio of Li / Mn is 、 is, for example, M.P.
M. "Journal published by Thackeray
of the Electrochemical So
vol. 142, no. 8, 2558-
As described in the article on page 2563, there is a problem that the charge / discharge capacity is greatly reduced due to repetition (cycle) of charge / discharge. Therefore, as disclosed in the article, the element of Li / Mn is used. Composition with slightly higher Li concentration than stoichiometric composition with 1/2 ratio (Li _1.03 Mn _1.97 O ₄ )
By increasing the degree of oxidation of manganese to 3.5 or more by selecting a lithium manganese composite oxide having a stoichiometric composition or adding another ion such as Mg or Zn to the above stoichiometric composition, the cycle characteristics are improved. Improvements are being considered.

Also, US Pat. No. 5,316,877 discloses Li _1.05 Mn _1.95 O ₄ and Li _1.1 Mn _1.9 O _4.
LiMn ₂ O ₄ spinel compounds having a stoichiometric composition such as
It is disclosed that a lithium manganese composite oxide to which i is added has good cycle characteristics.

Further, US Pat. No. 5,425,932 discloses a composition (L) having a Li concentration higher than a stoichiometric composition.
It is disclosed that a lithium manganese composite oxide having i _1.05 Mn ₂ O ₄ ) has high capacity and improved cycle characteristics.

[0007] However, the M.I. M. Thac
In the lithium manganese composite oxide described in Keray's paper, although the cycle characteristics are improved up to about 10 cycles by adding 0.03 mol of Li ions more than the stoichiometric composition, the results in the case of further repetition are obtained. Is not listed.

Further, in the examples of US Pat. No. 5,316,877, test examples at 10 cycles or more are described, but no long-term cycle characteristics are shown.

Further, Li _1.05 Mn ₂ O ₄ described in US Pat. No. 5,425,932 has a higher initial capacity than that of US Pat. No. 5,316,877. However, the capacity slightly decreases as the number of cycles increases. However, thereafter, the capacity up to 120 cycles hardly changes.

As described above, the lithium manganese composite oxides disclosed in the above three prior arts show improvement in charge / discharge cycle characteristics as compared with the lithium manganese composite oxide having a stoichiometric composition, but are not always sufficient.

[0011]

SUMMARY OF THE INVENTION The present invention provides a lithium ion secondary battery provided with a positive electrode containing a positive electrode active material which is less deteriorated by repeated charge / discharge than conventional lithium manganese composite oxides, that is, has less cycle deterioration. What you want to do.

[0012]

SUMMARY OF THE INVENTION A lithium secondary battery according to the present invention is characterized in that lithium ions are absorbed and released between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material during charging and discharging. In the secondary battery, the positive electrode active material is represented by a general formula Li _{1 + x} Mn _2-xy Fe _y O ₄ , wherein x and y are the following points A to A on a two-dimensional plane where x and y are orthogonal coordinates. It is characterized by comprising a lithium manganese iron spinel compound in a region inside the polygon defined by D.

Point A (x1 = 0.030, y1 = 0.050) Point B (x2 = 0.110, y2 = 0.050) Point C (x3 = 0.050, y3 = 0.200) Point D (X4 = 0.030, y4 = 0.200) In the lithium secondary battery according to the present invention, the negative electrode active material is made of at least one carbonaceous material selected from coke, spherical mesophase carbon, and fibrous mesophase carbon. Is preferred.

In the lithium secondary battery according to the present invention, the positive electrode active material has a general formula Li _{1 + x} Mn _2-xy Fe
It is represented by _y O _4, x and y are x, following point A in the two-dimensional plane and y orthogonal coordinate, E, to consist of lithium manganese iron spinel compound in the interior area of the triangle defined by F Is more preferred.

Point A (x1 = 0.030, y1 = 0.050) Point E (x5 = 0.0075, y5 = 0.050) Point F (x6 = 0.030, y3 = 0.150)

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lithium secondary battery according to the present invention will be described in detail with reference to FIG. For example, a positive electrode 2 is housed in a positive electrode can 1 made of stainless steel.
The separator 3 is disposed on the positive electrode 2. The non-aqueous electrolyte in which the electrolyte is dissolved in the inducing solvent is impregnated and held in the separator 3. The negative electrode 4 is connected to the separator 3
Is placed on top. At the opening of the positive electrode can 1, a negative electrode can 6 is provided via an insulating gasket 5. By caulking the negative electrode can 6 and the positive electrode can 1, the negative electrode can 6 is inserted into the positive electrode can 1 and the negative electrode can 6. The positive electrode 2, the separator 3, and the negative electrode 4 are sealed.

Next, the positive electrode 2, the negative electrode 4, the separator 3, and the non-aqueous electrolyte will be described in detail. (1) Positive electrode 2 This positive electrode 2 is represented by a general formula Li _{1 + x} Mn _2-xy Fe _y O ₄ , where x and y are the following in a two-dimensional plane where x and y are orthogonal coordinates shown in FIG. It is produced by pressure molding a mixture containing a lithium manganese iron spinel compound in a region inside the polygon defined by points A to D, a conductive auxiliary, and a binder. .

Point A (x1 = 0.030, y1 = 0.050) Point B (x2 = 0.110, y2 = 0.050) Point C (x3 = 0.050, y3 = 0.200) Point D (X4 = 0.030, y4 = 0.200) The composition of x and y in the above general formula is such that when the values of x and y are larger than the straight line connecting point B and point C shown in FIG. May be smaller. Also, when the value of y is larger than the straight line connecting the points C and D shown in FIG. 2, the discharge capacity may be reduced. When the value of y is smaller than the straight line connecting the points A and B shown in FIG.
When the value of x is smaller than the straight line connected by the above, the charge / discharge cycle characteristics may be inferior.

In particular, the general formula Li _{1 + x} Mn _2-xy Fe _y O
₄ , where x and y are from a lithium manganese iron spinel compound in a region inside a triangle defined by points A, E, and F on a two-dimensional plane having x and y as orthogonal coordinates shown in FIG. More preferably.

Point A (x1 = 0.030, y1 = 0.050) Point E (x5 = 0.0075, y5 = 0.050) Point F (x6 = 0.030, y3 = 0.150) As the agent, any material may be used as long as it does not cause a chemical change in the battery. For example, natural graphite (scale graphite, flake graphite, earth graphite), artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber or metal powder (copper, nickel, aluminum, silver, etc.), metal fiber, polyphenylene derivative Or a mixture thereof may be used. In particular, it is preferable to use a mixture of graphite and acetylene black as the conductive agent. The conductive agent is used in an amount of 1 to 50% by weight, more preferably 2 to 3% by weight based on the lithium manganese iron spinel compound.
It is desirable to add 0% by weight. When the carbon or graphite is used as the conductive agent, it is preferable to add 2 to 15% by weight based on the lithium manganese iron spinel compound.

Examples of the binder include starch, polyvinyl alcohol, carboxycellulose, hydroxypropyl alcohol, regenerated cellulose, diacetylcellulose, polyvinyl chloride, polyvinylpyrrolidone, tetrafluoroethylene, polyvinylidene fluoride,
One or more selected from polysaccharides such as ethylene-propylene-diene polymer (EPDM), sulfonated ethylene-propylene-diene polymer, styrene butadiene rubber, polybutadiene, fluororubber, and polyethylene oxide, thermoplastic resins, and polymers having rubber elasticity; Mixtures can be used. However, when a compound having a functional group that reacts with lithium, such as a polysaccharide, is used, it is preferable to add a compound having an isocyanate group to inactivate the functional group.

The positive electrode is allowed to contain a filler composed of fibers such as olefin polymers such as polypropylene and polyethylene, glass and carbon.
Such a filler is preferably blended in an amount of 30% by weight or less based on the lithium manganese iron spinel compound.

(2) Negative Electrode 4 The negative electrode 4 is produced by pressure-forming a mixture comprising a negative electrode active material, a conductive agent and a binder.

As the negative electrode active material, for example, artificial graphite, natural graphite, pyrolytic carbon, coke, resin fired body, mesophase small sphere, mesophase pitch and the like can be used. In particular, it is preferable to use at least one carbonaceous material selected from coke, spherical mesophase carbon, and fibrous mesophase carbon.

As the conductive material, for example, acetylene black, carbon black or the like can be used. Examples of the binder include styrene-butadiene latex (SBR), carboxymethyl cellulose (CM)
C), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), vinylidene fluoride-hexafluoropropylene copolymer, fluorine Vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer,
Polytrifluoroethylene (PTrFE), vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, or the like can be used.

(3) Separator 3 The separator 3 is made of, for example, an olefin polymer such as polypropylene or polypropylene, a glass fiber, a polyethylene-alumina fiber, a fluoropolymer, a cellulose polymer, a polyimide, or a nylon. Among these, polypropylene and polypropylene having organic solvent resistance and hydrophobicity are preferred, and polypropylene is most preferred.

The separator 3 is in the form of a microporous sheet, a nonwoven fabric, a woven fabric, or the like. The porosity of the microporous separator is preferably 35 to 39% as a value according to ASTM D2873. The microporous separator has a maximum diameter of 0.05 to 0.15 μm in the long diameter by SEM observation,
Preferably, it has a minor diameter of up to 0.01 μm. A more preferable pore diameter is 0.1 to at most in the long diameter.
It is 0.14 μm and the maximum diameter is 0.03 μm at the maximum.

The separator has a thickness of 20 to 30 μm.
m, more preferably about 25 μm. Another physical property of the separator is that the air permeation resistance is AS.
25 to 60 sec / 10m as determined by TM D726
1, ASTM D1204, less than 5% by ASTM D1204, 90 ° C., 60 minutes, tensile strength is ASTM D882.
MD value of about 1055 kg / cm ² , TD value of 8
It is preferably about 4 kg / cm ² .

It is preferable to use several sheets of the separator. In this case, the number of bonded sheets is most preferably two. The separator allows glow discharge treatment, corona discharge treatment, plasma discharge treatment, and the like to be performed as necessary. If necessary, a polyethylene oxide-based ion conductive membrane may be bonded.

(4) Nonaqueous Electrolyte This nonaqueous electrolyte has a composition in which an electrolyte is dissolved in a nonaqueous solvent. Examples of the electrolyte include lithium borofluoride (LiBF ₄ ) and lithium hexafluorophosphate (LiPF
₆ ), lithium perchlorate (LiClO ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium aluminum chloride (LiAlCl), LiSbF ₆ , LiB ₁₀
Cl ₁₀ , lithium lower aliphatic carboxylate, LiCl, L
One or more lithium salts selected from iBr, LiI, lithium chloroporan, and lithium 4-phenylborate can be mentioned.

Examples of the non-aqueous solvent include propylene carbonate, propylene carbonate derivatives, ethylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyl lactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrohydrofuran and tetrahydrofuran derivatives , Dimethylsulfoxide, dioxolan, 1,3-dioxolan, dioxolane derivative, formaldehyde, domethylfomermaldehyde, acetonitrile, nitromethane, methyl formate,
Selected from aprotic organic solvents such as methyl acetate, methyl propionate, ethyl propionate, triester phosphate, trimetaxymethane, sulfolane, 3-methyl-2-oxozolinedinone, diethyl ether, and 1,3-propane sultone. Or a mixture of two or more. When a mixture (mixed solvent) of propylene carbonate, ethylene carbonate and butylene carbonate, and 1.2-dimethoxyethane and diethyl carbonate is used, the ratio of the former / the latter is 0.4 / 0.6 to 0.6 / 0. .4 is preferable.

The non-aqueous electrolyte preferably contains at least ethylene carbonate and lithium hexafluorophosphate. Particularly, at least one electrolyte selected from trifluoromethanesulfonic acid, lithium lithium perchlorate and lithium hexafluorophosphate is dissolved in a mixed solvent of propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane and / or diethyl carbonate. Are preferred.

The amount of the electrolyte dissolved in the non-aqueous solvent is as follows:
It is desirably 0.5 to 1.5 mol / l. The amount of the non-aqueous electrolyte added to the battery is not particularly limited,
The amount may be determined depending on the amounts of the positive electrode active material and the negative electrode active material (carbonaceous material) and the size of the battery.

According to the present invention described above, the general formula Li
_{1 + x} Mn _2-xy Fe _y O ₄ , that is, a composition in which a part of the Mn site is replaced by Li and Fe, and x and y in a two-dimensional plane where x, y are orthogonal coordinates By providing a positive electrode including a positive electrode active material made of a lithium manganese iron spinel compound in a region inside a polygon defined by a specific point, a lithium ion secondary battery having a long charge / discharge cycle life can be obtained.

In particular, a positive electrode made of a lithium manganese iron spinel compound in which x and y are in an area inside a triangle defined by a specific point narrower than the area of the polygon on a two-dimensional plane where x and y are orthogonal coordinates. By using a positive electrode containing an active material, a lithium ion secondary battery having a longer charge / discharge cycle life can be obtained.

When a negative electrode containing at least one carbonaceous material selected from coke, spherical mesophase carbon and fibrous mesophase carbon is used, a lithium ion secondary battery with further improved charge / discharge cycle life can be obtained. it can.

[0037]

Embodiments of the present invention will be described below in detail. (Example 1) First, lithium carbonate (Li ₂ CO ₃ ), electrolytic manganese dioxide (MnO ₂ ), and iron oxide (Fe ₂ O ₃ )
With a molar ratio of lithium, manganese and iron of 1.050: 1.
A mixed powder was prepared by weighing 10 g (total amount) so as to be 90: 0.050 and pulverizing and mixing in an alumina mortar for 30 minutes. Subsequently, this mixed powder was filled in an alumina boat, and the boat was placed in an electric furnace and placed in an air atmosphere.
The temperature was raised to 500 ° C. over 2 hours, and this temperature was maintained for 24 hours. Subsequently, the temperature was raised to 800 ° C. over 1.2 hours, and after maintaining this temperature for 24 hours, 2 ° C./min.
By cooling to 600 ° C. at a rate of, a lithium manganese iron composite oxide was synthesized.

The obtained lithium manganese iron composite oxide was subjected to X-ray diffraction using CuKα radiation. As a result, it was confirmed that all the X-ray diffraction peaks measured at a diffraction angle of 2 ° in the range of 10 ° to 90 ° show X-ray diffraction peaks belonging to lithium manganese iron spinel having a cubic crystal structure. . The lattice index a was calculated at a higher angle side than the X-ray diffraction peak whose plane index is represented by (511). As a result, a = 0.823 nm.

Next, 80 parts by weight of the lithium manganese iron spinel (positive electrode active material), 17 parts by weight of acetylene black as a conductive material, and 3 parts by weight of polytetrafluoroethylene powder as a binder were weighed. Subsequently, after mixing the positive electrode active material and acetylene black for 20 minutes using an automatic mortar, the polytetrafluoroethylene powder was added and mixed for about 20 minutes until the polytetrafluoroethylene was sufficiently fiberized. Subsequently, the mixture was rolled for 0.25 to 0.27 m using a roll press.
m, and pressure-bonded to a stainless steel net as a current collector. Thereafter, an extra active material sheet was removed from the current collector so that the active material portion attached to the current collector became 10 mm × 10 mm, and a Ti lead was discharge-welded to produce a positive electrode.

Next, the negative electrode and the positive electrode, in which a lithium metal foil is press-bonded to a nickel mesh to which a nickel lead is welded, are opposed to each other via a glassy separator, and these are brought together with a reference electrode into a glass container containing an electrolytic solution. To assemble the measurement cell. The reference electrode had a structure in which a lithium metal foil was pressure-bonded to a stainless steel mesh, and was immersed in the container in proximity to the positive electrode. The electrolyte used had a composition in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (mixing volume ratio: 1: 1) at 1.0 mol / l. .

Example 2 First, lithium hydroxide monohydrate (LiOH.H ₂ O) and electrolytic manganese dioxide (Mn)
O ₂ ) and iron oxide (Fe ₂ O ₃ ) were weighed in an amount of 10 g (total amount) such that the molar ratio of lithium, manganese, and iron was 1.033: 1.867: 0.100, and 30 g was weighed in an alumina mortar.
The mixture was crushed and mixed for minutes to prepare a mixed powder. Subsequently, the mixed powder was filled in an alumina boat, and the boat was placed in an electric furnace and heated to 500 ° C. in an air atmosphere over 2 hours, and this temperature was maintained for 24 hours. Continued,
The temperature was raised to 800 ° C. in 1.2 hours,
After holding for a period of time, a lithium manganese iron composite oxide was synthesized by cooling to 600 ° C. at a rate of 2 ° C./min.

The obtained lithium manganese iron composite oxide was subjected to X-ray diffraction using CuKα radiation. As a result, it was confirmed that all the X-ray diffraction peaks measured at a diffraction angle of 2 ° in the range of 10 ° to 90 ° show X-ray diffraction peaks belonging to lithium manganese iron spinel having a cubic crystal structure. . The lattice index a was calculated at a higher angle side than the X-ray diffraction peak whose plane index is represented by (511). As a result, a = 0.823 nm.

Next, a positive electrode was produced in the same manner as in Example 1 using the above-mentioned lithium manganese iron spinel as a positive electrode active material. Thereafter, a measurement cell was assembled in the same manner as in Example 1 using the positive electrode.

Example 3 First, lithium carbonate (Li ₂
CO ₃ ), electrolytic manganese dioxide (MnO ₂ ), and iron oxide (Fe ₂ O ₃ ) such that the molar ratio of lithium, manganese, and iron becomes 1.050: 1.850: 0.100.
g (total amount) was weighed and pulverized and mixed in an alumina mortar for 30 minutes to prepare a mixed powder. Subsequently, the mixed powder was filled in an alumina boat, and the boat was placed in an electric furnace and heated to 500 ° C. in an air atmosphere over 2 hours, and this temperature was maintained for 24 hours. Subsequently, the temperature was raised to 800 ° C. over 1.2 hours, the temperature was maintained for 24 hours, and then cooled at a rate of 2 ° C./min to 600 ° C. to synthesize a lithium manganese iron composite oxide.

The obtained lithium manganese iron composite oxide was subjected to X-ray diffraction using CuKα radiation. As a result, it was confirmed that all the X-ray diffraction peaks measured at a diffraction angle of 2 ° in the range of 10 ° to 90 ° show X-ray diffraction peaks belonging to lithium manganese iron spinel having a cubic crystal structure. . The lattice index a was calculated at a higher angle side than the X-ray diffraction peak whose plane index is represented by (511). As a result, a = 0.823 nm.

Next, a positive electrode was produced in the same manner as in Example 1, except that the lithium manganese iron spinel was used as a positive electrode active material. Thereafter, a measurement cell was assembled in the same manner as in Example 1 using the positive electrode.

Examples 4 to 12 Lithium carbonate (Li ₂
CO ₃ ), electrolytic manganese dioxide (MnO ₂ ), and iron oxide (Fe ₂ O ₃ ) as raw materials, and nine kinds of lithium having the composition, crystal structure, and lattice constant shown in Table 1 below in the same manner as in Example 1. Manganese iron spinel was synthesized.

Next, nine kinds of positive electrodes were produced in the same manner as in Example 1 using each of the lithium manganese iron spinels as the positive electrode active material. Thereafter, nine types of measuring cells were assembled in the same manner as in Example 1 using these positive electrodes. Table 1 below also shows the composition, crystal structure, and lattice constant of the lithium manganese iron spinel synthesized in Examples 1 to 3 described above.

[0049]

[Table 1]

Comparative Example 1 First, lithium carbonate (Li ₂
10 g of CO ₃ ), electrolytic manganese dioxide (MnO ₂ ), and iron oxide (Fe ₂ O ₃ ) such that the molar ratio of lithium, manganese, and iron becomes 1.000: 1.90: 0.100.
A lithium manganese iron composite oxide was synthesized in the same manner as in Example 1 except that (the total amount) was weighed.

The obtained lithium manganese iron composite oxide was subjected to X-ray diffraction using CuKα radiation. As a result, it was confirmed that all the X-ray diffraction peaks measured at a diffraction angle of 2 ° in the range of 10 ° to 90 ° show X-ray diffraction peaks belonging to lithium manganese iron spinel having a cubic crystal structure. . The lattice index a was calculated at a higher angle side than the X-ray diffraction peak whose plane index is represented by (511). As a result, a was 0.824 nm.

Next, a positive electrode was prepared in the same manner as in Example 1 using each of the above lithium manganese iron spinels as a positive electrode active material. Thereafter, using this positive electrode in Example 1
The measurement cell was assembled in the same manner as in the above.

Comparative Examples 2 to 9 Lithium carbonate (Li ₂ C)
O _3), electrolytic manganese dioxide (MnO ₂₎ and iron oxide (Fe ₂ O ₃₎ as a raw material composition shown in Table 2 in the same manner as in Example 1, the crystal structure, 8 having a lattice constant
A kind of lithium manganese iron spinel was synthesized.

Next, eight kinds of positive electrodes were produced in the same manner as in Example 1 using each of the lithium manganese iron spinels as the positive electrode active material. Thereafter, eight kinds of measurement cells were assembled in the same manner as in Example 1 using these positive electrodes. Table 2 below also shows the composition, crystal structure, and lattice constant of the lithium manganese iron spinel synthesized in Comparative Example 1 described above.

[0055]

[Table 2]

The measurement cells of Examples 1 to 12 and Comparative Examples 1 to 9 were stored in a thermostatic chamber maintained at 20 ° C. at a current of 1 mA / cm 2.
^The battery was charged at a constant current density of ² and stopped when the voltage reached 4.3 V. After a pause of 10 minutes, the battery was discharged at a constant current density of 1 mA / cm ² and the voltage reached 3.0 V. The discharge is terminated at the time when the
A cycle test was performed in which a pause was performed for a minute. The results of Examples 1 to 3 by such a charge / discharge cycle test are shown in FIG. 3, the results of Examples 4 to 8 are shown in FIG. 4, the results of Examples 9 to 12 are shown in FIG. The results are shown in FIG. 6 and the results of Comparative Examples 5 to 9 are shown in FIG.

As is apparent from FIGS. 3 to 7, the general formula Li
_{1 + x} Mn _2-xy Fe _y O ₄ , where x and y are inside the polygon defined by points A to D on the two-dimensional plane where x and y are orthogonal coordinates shown in FIG. The measurement cells of Examples 1 to 12 provided with the positive electrode containing the lithium manganese iron spinel in the region as the positive electrode active material had a large discharge capacity, and almost no deterioration in the discharge capacity due to the charge / discharge cycle was observed. It can be seen that it has.

On the other hand, the measurement cells of Comparative Examples 1, 3, and 5 had initial discharge capacities of 106 mAh / g and 121 mAh, respectively.
Although it shows high values of mAh / g and 119 mAh / g, the discharge capacity gradually decreases as the number of cycles increases. Further, Comparative Examples 2, 4, and 6 to 9 can maintain a stable discharge capacity irrespective of the increase in the number of cycles, but have a smaller initial discharge capacity than the measurement cells of Examples 1 to 12.

(Examples 13 and 14) Cathode active material 8 composed of lithium manganese iron spinel synthesized in Examples 1 and 3
0 parts by weight, 17 parts by weight of acetylene black as a conductive material, and 3 parts by weight of polytetrafluoroethylene powder as a binder were weighed. Subsequently, each of the positive electrode active materials and acetylene black was subjected to 2
After mixing for 0 minutes, the polytetrafluoroethylene powder was added and mixed for about 20 minutes until the polytetrafluoroethylene was sufficiently fiberized. Subsequently, this mixture was spread into a sheet of 0.27 mm by a roll press machine, and pressed on a stainless steel net as a current collector. Then, the active material portion attached to the current collector was 20 mm × 20 mm.
An excess of the active material sheet was removed from the current collector so as to have a thickness of 2 mm, and two types of positive electrodes were produced by discharge welding Ti leads. The positive electrode active material composed of the lithium manganese iron spinel contained in the positive electrode was 0.2 g.

A commercially available average diameter of 9 μm and an average length of 20
97 parts by weight of a μm mesophase carbon fiber powder and 3 parts by weight of a polytetrafluoroethylene powder as a binder were weighed. Subsequently, the mesophase carbon fiber powder and the polytetrafluoroethylene were mixed for about 20 minutes until they were sufficiently fiberized. Subsequently, the mixture was rolled with a roll press to a thickness of about 0.15.
The sheet was stretched into a sheet having a thickness of 2 mm, and pressed on a stainless steel net as a current collector. Thereafter, an extra active material sheet was removed from the current collector so that the active material portion attached to the current collector became 20 mm × 20 mm, and a Ti lead was discharge-welded to produce a negative electrode.

Next, the positive electrode and the negative electrode were opposed to each other with a glassy separator interposed therebetween, and this was immersed in a glass container containing an electrolyte to assemble two types of model batteries. As the electrolyte, a mixed solvent of lithium hexafluorophosphate (LiPF ₆ ) of ethylene carbonate and ethyl methyl carbonate (mixing volume ratio 1:
A composition having a composition of 1.0 mol / l dissolved in 1) was used.

Comparative Example 10 A positive electrode was manufactured in the same manner as in Example 13 using the positive electrode active material composed of lithium manganese iron spinel synthesized in Comparative Example 1. Thereafter, a model battery was assembled using the positive electrode in the same manner as in Example 13.

Examples 13 and 14 and Comparative Example 1 Obtained
0 mA in a constant temperature room maintained at 20 ° C.
The charging / discharging was performed with the respective currents. The charge was cut off at 4.3 V, and after a pause of 10 minutes, the discharge was started.
The charge-discharge was cut off at 0 V and paused for 10 minutes. In such a charge / discharge cycle test, the discharge capacity of each model battery at the initial discharge and the discharge capacity at 10 cycles were examined. The results are shown in Table 3 below.

[0064]

[Table 3]

[0065]

As described in detail above, according to the present invention, a lithium ion secondary battery having a large discharge capacity at the time of initial charging and a long charge / discharge cycle life can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a button-type lithium secondary battery according to the present invention.

FIG. 2 shows a general formula Li _{1 + x} Mn _2-xy used in the present invention.
FIG. 3 is a diagram showing x and y regions of a lithium manganese iron spinel compound represented by Fe _y O ₄ .

FIG. 3 is a diagram showing charge / discharge cycle characteristics of the measurement cells of Examples 1 to 3 of the present invention.

FIG. 4 is a diagram showing charge / discharge cycle characteristics of measurement cells of Examples 4 to 8 of the present invention.

FIG. 5 is a diagram showing charge / discharge cycle characteristics of measurement cells of Examples 9 to 12 of the present invention.

FIG. 6 is a diagram showing charge / discharge cycle characteristics of the measurement cells of Comparative Examples 1 to 4.

FIG. 7 is a diagram showing charge / discharge cycle characteristics of measurement cells of Comparative Examples 5 to 9.

[Explanation of symbols]

 1 ... Positive electrode can, 2 ... Positive electrode, 4 ... Negative electrode, 6 ... Negative electrode can.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Sumitomo Yajima 3-4-1-10 Minamishinagawa, Shinagawa-ku, Tokyo Toshiba Battery Corporation

Claims (3)

[Claims] 1. A lithium ion secondary battery in which lithium ions are inserted and extracted between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material during charging and discharging, wherein the positive electrode active material has a general formula Li _{1 + x} Mn _2-xy Fe _y O
Represented by _4, and characterized in that x and y consisting of x, lithium manganese iron spinel compound in the interior area of a polygon defined by the following points A~D in the two-dimensional plane and the orthogonal coordinate y Rechargeable lithium battery. Point A (x1 = 0.030, y1 = 0.050) Point B (x2 = 0.110, y2 = 0.050) Point C (x3 = 0.050, y3 = 0.200) Point D (x4 = 0.030, y4 = 0.200) 2. The lithium secondary battery according to claim 1, wherein the negative electrode active material is made of at least one carbonaceous material selected from coke, spherical mesophase carbon, and fibrous mesophase carbon. 3. The positive electrode active material has a general formula of Li _{1 + x} Mn.
Lithium manganese iron spinel represented by _2-xy Fe _y O ₄ , wherein x and y are in a region inside a triangle defined by points A, E, and F on a two-dimensional plane where x and y are orthogonal coordinates. The lithium secondary battery according to claim 1, comprising a compound. Point A (x1 = 0.030, y1 = 0.050) Point E (x5 = 0.075, y5 = 0.050) Point F (x6 = 0.030, y3 = 0.150)