JPH10255801A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery with a long life time, in which capacity decrease is very small even if changing and discharging is repeated for a long time in a high energy density and a high temperature or if it is preserved at a high temperature. SOLUTION: This lithium secondary battery is equipped with nonaqueous electrolyte and a positive electrode while having lithium or a substance which can occlude and discharge lithium for a negative electrode. In this case, for the positive electrode active substance, a substance which is lithium manganese oxide with a spinel structure and of which pre-edge peak strength appeared in the neighbourhood of 6540eV in a X-ray absorbing end neighbouring structure (XANES) spectrum of a MnK absorbing end when standardized by an absorption coefficient under 6740eV, increases gradually along with deintercadation of lithium, in a X-ray absorption spectrum measured by X-ray absorbing fine structure analysis (XAFS) method, is adopted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in characteristics of a nonaqueous secondary battery using a spinel lithium manganese oxide as a positive electrode active material.

2. Description of the Related Art As electronic devices become smaller and lighter, there is an increasing demand for high energy density secondary batteries as power sources. In recent years, lithium ion secondary batteries using LiCoO _2, which can provide a voltage as high as about 4 V with respect to metallic lithium, as a positive electrode active material and using a carbon material as a negative electrode active material have come to be marketed. However, LiCoO ₂ is a material having a low production of Co, so that the cost of the raw material Co compound is high, and there is concern about stable supply. Therefore, development of an alternative material has been desired. As one example, LiMn ₂ O ₄ has been proposed. Since LiMn ₂ O ₄ is abundant in raw materials, inexpensive, and inexpensive in production, various studies have been made toward practical use. Generally, LiMn ₂ O ₄ is prepared by mixing MnO ₂ and Li ₂ CO ₃ such that the molar ratio of lithium to manganese is 1: 2.
_It can be easily synthesized by a method of mixing manganese oxide and lithium salt as described in ₃ , and then subjecting it to heat treatment. However, this material has a problem that cycle characteristics, which are the basics of battery characteristics, are poor.

In order to solve this problem, for example,
No. 24043 discloses that the lattice constant a of a crystal is 8.22.
It has been proposed to use the following LiMn ₂ O ₄ as an active material. Japanese Patent Application Laid-Open No. Hei 6-187993 proposes a lithium-excess composition in order to increase the oxidation state of manganese. Also, Patent Publication No. 2058834
No., a part of Mn in LiMn ₂ O ₄ was changed to Co, Cr,
It is proposed to substitute Fe. They propose to improve the lithium manganese oxide by focusing on the crystal lattice and the oxidation number of manganese to improve the cycle characteristics. Although the effect of improving the cycle characteristics at room temperature can be expected, the decrease in capacity due to storage at high temperatures and charge / discharge cycles is still large and insufficient.

SUMMARY OF THE INVENTION The present invention is to solve such a problem of a lithium ion secondary battery, and is capable of repeatedly charging and discharging at a high energy density and at a high temperature for a long period of time, or even when stored at a high temperature. Is extremely low,
It is an object of the present invention to provide a positive electrode material for a lithium ion secondary battery capable of obtaining a long-life battery.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and shows an X-ray absorption spectrum measured by X-ray absorption fine structure analysis (XAFS) method of 6750 eV. Found that the pre-edge peak intensity appearing at around 6540 eV in the X-ray absorption near edge structure (XANES) spectrum of the MnK absorption edge when normalized by the absorption coefficient in the above is deeply related to the high temperature characteristics of the secondary battery. It is. Hereinafter, the present invention will be described in detail. As described above, proposals for improving the characteristics of the spinel-type lithium manganese oxide generally involve stabilizing the crystal by increasing the average oxidation number to more than 3.5.
The present inventors have conducted thorough studies on manganese elements and high-temperature characteristics based on the recognition that the proposed active materials are insufficient for high-temperature characteristics. That is, spinel-type lithium manganese oxides with different synthesis conditions such as firing conditions and compositions were synthesized, and after testing at high temperature, the electronic state of manganese was examined in detail by XAFS measurement. If the capacity is extremely low even after repeated storage or storage at a high temperature, the structure near the X-ray absorption edge of the MnK absorption edge (XANE) determined from the X-ray absorption spectrum measured by the X-ray absorption fine structure analysis (XAFS) method
S) It was found that the pre-edge peak intensity appearing around 6540 eV of the spectrum gradually increased with lithium deintercalation (release).

[0004] On the other hand, in the case where the capacity is clearly reduced by the high-temperature cycle or high-temperature storage, that is, in the case where the high-temperature characteristics are deteriorated, the pre-edge peak intensity appearing around 6540 eV is once uncharged due to the deintercalation of lithium. It has been found that the peak intensity becomes larger as the state becomes smaller than the state and lithium is further deintercalated, thereby completing the present invention. The pre-edge peak appearing before the MnK absorption edge is generally between 1 s and 3 s of K-shell electrons.
It is said that it appears due to absorption due to the transition to d or ligand distortion. Although the details of the lithium manganese oxide of the present invention are unknown, it is presumed that the electronic state of Mn and the state of Mn atoms based on the coordination structure are slightly different from general lithium manganese oxide. You. That is, the present invention relates to a secondary battery comprising a nonaqueous electrolyte and a positive electrode, wherein the positive electrode active material is a lithium manganese oxide having a spinel structure, wherein the negative electrode is lithium or a substance capable of inserting and extracting lithium. In the X-ray absorption spectrum measured by the fine structure analysis (XAFS) method, MnK when normalized by the absorption coefficient at 6750 eV
The pre-edge peak intensity that appears near 6540 eV in the X-ray absorption near edge structure (XANES) spectrum at the absorption edge increases with lithium deintercalation.
It is a non-aqueous secondary battery characterized by the above.

Here, measurement of general X-ray absorption fine structure analysis (XAFS) will be described. Although light such as IR and UV is absorbed by the substance, X-rays are also absorbed by the substance without exception, and the energy of the absorbed amount is converted into photoelectrons, fluorescent X-rays, and heat. At this time, part of the photoelectrons generated by the absorption of X-rays is reflected as structural information on the amount of X-ray absorption by scattering and interference by a plurality of atoms. That is, by monitoring the amount of X-ray absorption, information on the atomic structure can be obtained. When a substance is placed on an X-ray beam line, the X-ray irradiated on the substance (incident X-ray: I
0) Intensity and X-rays that have passed through the substance (transmission X-rays: It)
From the intensity, the X-ray absorption amount (X-ray absorption coefficient) of the substance is calculated. When the X-ray energy (wavelength) is changed while monitoring the increase / decrease of the X-ray absorption coefficient and the X-ray absorption spectrum is measured, a sharp rise (absorption edge) of the X-ray absorption coefficient is observed at a certain energy position. Since the energy position of the absorption edge is unique to an element, if structural information can be extracted in the energy region near the absorption edge, it means that the information is information unique to the element.

When the X-ray absorption spectrum is measured with sufficient accuracy in the energy region near the absorption edge of a certain element of interest,
Large structural vibration accompanied by attenuation is observed in the energy region of several tens eV from the absorption edge. This is called X-ray absorption near edge stru
cture), which mainly contains information on the electronic state and three-dimensional structure of the element of interest. Further, in the energy region of several hundred eV on the higher energy side than XANES, fine structural vibration accompanied by similar attenuation is observed. This is called X-ray absorption fine structure (EXAFS: Extended X
-ray absorption Fine structure) and contains information about the local structure (interatomic distance and coordination number) near the element of interest. In recent years, the above-mentioned XANES and EXAFS
Are collectively called XAFS. By the way, in order to search for high-performance active materials in lithium-ion secondary batteries, etc., the local crystal structure of each active material with different charge / discharge amount and the details of the structural change due to charge / discharge are measured and clarified. It is thought that doing so will be an effective guide. In order to perform such measurement by XAFS, a method of preparing an active material charged and discharged to a predetermined charge / discharge amount in advance and transmitting X-rays to the active material can be considered. However, in such a measurement method, when the active material is taken out from the closed cell for charge and discharge and moved to the XAFS measurement cell, the physical properties of the active material may be changed due to careless contact with air or the like. As a result, accurate measurement may not be performed.

Further, it is necessary to perform charging and discharging of the active material in another place, and it is necessary to prepare various kinds of active materials having different charging and discharging amounts. Was. The present inventors have made it possible to eliminate the inconvenience by enabling the charge and discharge of the active material in a sealed cell, and at the same time, to the local crystal structure and the charge and discharge of each active material having different charge and discharge amounts. An X-ray absorption fine structure measurement cell of a battery material capable of easily and accurately measuring the details of the accompanying structural change was manufactured. In the present invention, XAFS measurement is performed using the above-mentioned cell while charging and discharging various spinel-type lithium manganese oxides, and the MnK-absorption edge near-X-ray absorption edge structure (XANES) spectrum obtained from the X-ray absorption spectrum As a result of a detailed investigation of the relationship between the pre-edge peak intensity appearing at around 6540 eV and the battery characteristics of the active material, it was found that there was a strength relationship suitable for high-temperature characteristics. In the measurement of the X-ray absorption fine structure (XAFS) in the present invention, an X-ray energy of 6490 eV to 67 was used by using a Si (111) channel cut monochromator as a spectral crystal.
Scanning was performed between 50 eV, and transmission was performed. The integration time was 2 seconds / point. The angle was corrected by setting the K absorption edge of Cu of 8980.3 eV to 12.7185 degrees.

[0008] The positive electrode active material of the present invention can be synthesized by the following method. For example, it can be obtained by mixing a lithium compound and a manganese compound and then performing a heat treatment. The lithium compound is not particularly limited, for example, lithium hydroxide, lithium carbonate, lithium nitrate,
Examples thereof include lithium oxide, lithium chloride, lithium sulfate, lithium acetate, lithium iodide, and alkyl lithium. Particularly, lithium hydroxide, lithium carbonate, and lithium nitrate are preferable. The manganese compound is not particularly limited. For example, electrolytic manganese dioxide (EMD), chemically synthesized manganese dioxide (CMD), Mn ₂ O ₃ , MnOOH
And manganese oxides and hydroxides such as Mn ₃ O ₄ . Particularly, electrolytic manganese dioxide, chemically synthesized manganese dioxide, and MnOOH are preferable. The mixing ratio of Li and Mn (Li / Mn ratio) varies depending on the starting compound and the heating conditions, but is usually preferably 0.52 to 0.75. The heat treatment conditions are slightly different depending on the starting compound, but can be obtained by heat treatment at 600 to 950 ° C. As a heating atmosphere, nitrogen, argon, air, oxygen, or a mixed gas thereof can be used. The average particle diameter of the positive electrode active material in the present invention is preferably in the range of 5 to 50 μm, more preferably 5 to 30 μm.

The negative electrode material used in the present invention is not particularly limited as long as it is a substance capable of inserting and extracting lithium reversibly. For example, metallic materials such as metallic lithium and aluminum, carbon materials, carbon materials, graphite, Graphite-like compounds, metal oxides, metal nitrides, and the like can be used. The lithium ion transfer medium used in the present invention is not particularly limited, such as an electrolyte solution in which a lithium salt is dissolved in an aprotic organic solvent or a solid in which a lithium salt is dispersed in a polymer matrix, a semi-solid, or a mixture of both. And lithium salts include, for example, lithium perchlorate, L
iBF ₄ , LiPF ₆ , LiAsF ₆ , CF ₃ SO ₃ L
i, (CF ₃ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₃ C
Li, lithium organic carboxylate, lithium fluorocarboxylate, lithium polymer sulfonate, lithium polymer carboxylate, or the like can be used. Further, as the aprotic organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, organic carbonates such as methyl ethyl carbonate, butyrolactone, propiolactone, ethyl acetate, butyl acetate, propyl acetate, ethyl propionate, propionate Organic ethers such as aliphatic organic esters such as butyl acid, glyme, diglyme, triglyme, tetrahydrofuran, dioxane, diethyl ether and silicone oil; organic amines such as pyridine and triethylamine; organic nitriles such as acetonitrile and propionitrile; It contains at least a part of a simple substance or a mixture, and may contain other aprotic organic solvents such as benzene, toluene, xylene, deca. Mixed use of aromatic hydrocarbons such as benzene, hexane, pentane, decane, etc., alkyl esters such as phenol, catechol and bisphenol, aromatic esters and halogenated hydrocarbons such as chloroform, carbon tetrachloride and dichloromethane. It is also possible.

Next, examples of the polymer matrix include aromatic polyethers such as polyethylene oxide, polytetramethylene oxide, polyvinyl alcohol, and polyvinyl butyral; polyethylene sulfide;
Aliphatic polythioethers such as polypropylene sulfide, aliphatic polyesters such as polyethylene succinate, polybutylene adipate and polycaprolactone, polyethyleneimine, polyimide and precursors thereof can be used. As the separator used in the present invention, for example, a porous sheet of a polyolefin resin such as polyethylene or polypropylene, or a nonwoven fabric thereof can be used. In addition, the battery of the present invention corresponds to claim 1
Except for using a lithium manganese oxide having a spinel structure having the following characteristics as a positive electrode active material, it may have the same configuration as a conventionally known lithium secondary battery (metal lithium secondary battery or lithium ion secondary battery). it can.

Embodiments of the present invention will be described below. Various lithium manganese oxides A to H were prepared by applying various raw materials and firing conditions as shown in Table 1 as the positive electrode active material.

Example 1 Synthesis of a positive electrode active material was performed using Li ₂ CO ₃ as a lithium compound and electrolytic manganese dioxide (EMD) as a manganese compound.
Mix at an atomic ratio of 0.50, and in air (oxygen concentration 20 vo
1%) at 850 ° C. for 20 hours. Thereafter, Li ₂ CO ₃ was added so that the final composition became Li _1.08 Mn _1.92 O _4, and the mixture was further heated at 650 ° C. for 20 hours to obtain a lithium manganese oxide. The obtained product was analyzed by X-ray diffraction and chemical analysis to obtain a spinel-structured Li.
_1.080 was Mn _1.916 O _4. Next, the positive electrode active material 10
8 parts by weight of carbon powder as a conductive agent and 3.2 parts by weight of polyvinylidene fluoride as a binder are added to 0 parts by weight.
Paste with N-methyl-2-pyrrolidone,
It was applied to a current collector of aluminum foil, dried and pressed to obtain a positive electrode. Subsequently, the positive electrode and the lithium metal of the counter electrode are provided with an X-ray transmission hole in an argon dry box controlled to a moisture content of 1 ppm or less, and the positive electrode and the lithium counter electrode face each other on the X-ray optical path. Carbonate (EC) and diethyl carbonate (D) in which 1.0 mol / l of LiPF6 is dissolved in a closed X-ray absorption fine structure measurement cell capable of charge / discharge operation
The mixed solution of EC) was injected and used for measurement.

In the X-ray absorption fine structure (XAFS), while monitoring the charge / discharge electricity amount in the measurement cell, a predetermined amount of lithium is inserted and removed, and the energy of the X-ray is 6490-6750.
While scanning up to eV, the absorption coefficient was measured sequentially for a predetermined amount of lithium desorption, and an X-ray absorption spectrum was obtained. X
In the X-ray absorption spectrum measured by the X-ray absorption fine structure analysis (XAFS) method, the structure near the X-ray absorption edge of the MnK absorption edge when normalized by the absorption coefficient at 6750 eV (XAN
ES) The range of the spectrum from 6530 to 6580 eV is shown in FIG. FIG. 2 shows an enlarged portion of the pre-edge portion. In FIG. 2, 1 is the pre-edge peak of the uncharged active material, 2 is the pre-edge peak of the active material obtained by deintercalating 0.25 Li by charging, and 3 is the de-interpolating of 0.67 Li by charging. The pre-edge peak of the calmed active material is shown. From this, 6540 eV
It was found that the pre-edge peak intensity appearing in the vicinity gradually increased with lithium deintercalation. (The behavior of the pre-edge accompanying the deintercalation of lithium is defined in Table 1 as pre-edge type R.) Subsequently, using the positive electrode produced above, a battery was produced by the following procedure. As the negative electrode, 5 parts by weight of flaky graphite, 1 part by weight of carboxymethyl cellulose as a binder, and 2 parts by weight of styrene-butadiene rubber were added to 100 parts by weight of graphitized mesocarbon fiber as an active material, and purified water was used. Paste, apply to the current collector of copper foil,
What was dried and pressed was used.

Next, a separator made of a microporous film made of polyethylene was interposed between the above-mentioned positive electrode and negative electrode, laminated one another, and wound many times to produce a spiral electrode body. Further, the spiral electrode body was housed in a nickel-plated iron battery container. Connect the negative electrode lead terminal to the inner bottom of the battery container by spot welding,
The positive electrode lead terminal was similarly connected to the battery sealing plate. Next, 1.0 mol / liter of LiP was placed in the battery can.
A mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) in which F6 is dissolved is injected, and the battery container and the battery sealing plate are caulked and sealed via a polypropylene packing, and have an outer diameter of 17 mm and a height of 50 mm. Was manufactured. This battery was charged and discharged at a charge / discharge current of 670 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 2.7 V. The discharge capacity at that time was 670
mAh. This battery was charged for 20
Charge and discharge were repeated under the temperature conditions of ° C and 60 ° C, respectively.
The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 95% and 83%, respectively.

Further, the battery prepared above was put in a high-temperature bath at 85 ° C. in a charged state of 4.2 V and stored for 24 hours. In this state, the battery was discharged to 2.7V. Discharge capacity is 470 mAh
And the capacity retention ratio (the ratio of the discharge capacity at 85 ° C. to the discharge capacity at room temperature before storage) was 70%. Next, the battery was taken out of the high-temperature bath, returned to room temperature, charged again to 4.2 V, and then discharged to 2.7 V. The discharge capacity is 56
0 mAh, and the recovery rate was 84%. Example 2 The synthesis of the positive electrode active material was performed by mixing LiOH · H ₂ O as a lithium compound and electrolytic manganese dioxide (EMD) as a manganese compound at an atomic ratio of Li / Mn = 0.50, and mixing with air. In a mixed gas of N ₂ (oxygen concentration 1
(5% by volume) at 750 ° C. for 20 hours. Thereafter, LiOH · H ₂ O was added so that the final composition became Li _1.04 Mn _1.96 O ₄ , and the mixture was further added for 55 hours.
The mixture was heated at 0 ° C. to obtain a lithium manganese oxide. The obtained product was found to be Li _1.037 Mn _1.960 O ₄ having a spinel structure by X-ray diffraction and chemical analysis.

Next, after a positive electrode and a XAFS cell were fabricated in the same manner as in Example 1, X-ray absorption fine structure analysis (X
(AFS) method to obtain an X-ray absorption spectrum. 675
X at MnK absorption edge when normalized by absorption coefficient at 0 eV
It was found that the pre-edge peak intensity appearing at around 6540 eV in the ANES spectrum gradually increased with lithium deintercalation. (Pre-edge type R) Subsequently, a battery was produced in the same manner as in Example 1. This battery was charged with a charge / discharge current of 700 mA and a charge end voltage of 4.2.
And charging / discharging at a discharge end voltage of 2.7 V. The discharge capacity at that time was 700 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 92% and 82%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 68%, and the recovery rate was 81%.

Example 3 In the synthesis of the positive electrode active material, LiNO ₃ was used as a lithium compound and γ was used as a manganese compound.
Using MnOOH, mixing at an atomic ratio of Li / Mn = 0.56, 24 hours at 650 ° C. in N ₂ (oxygen concentration 0 vol%).
The heat treatment was performed for a time. The obtained lithium manganese oxide was found to have a spinel structure of Li _1.06 Mn _1.927 O ₄ by X-ray diffraction and chemical analysis. Next, a positive electrode and an XAFS cell were prepared in the same manner as in Example 1, and then measured by an X-ray absorption fine structure analysis (XAFS) to obtain an X-ray absorption spectrum. XANES spectrum 6 at the MnK absorption edge when normalized by the absorption coefficient at 6750 eV
It was found that the pre-edge peak intensity appearing near 540 eV gradually increased with lithium deintercalation. (Pre-edge type R) Subsequently, a battery was produced in the same manner as in Example 1. This battery was charged and discharged with a charge current of 680 mA and a charge end voltage of 4.2.
And charging / discharging at a discharge end voltage of 2.7 V. The discharge capacity at that time was 680 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 95% and 80%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 66%, and the recovery rate was 80%.

Example 4 The synthesis of the positive electrode active material was performed using LiOH.H ₂ O as a lithium compound and chemically synthesized manganese dioxide (CMD) as a manganese compound.
Mixing at an atomic ratio of Mn = 0.53, and mixing in N ₂ (oxygen concentration 0
(% by volume) at 750 ° C. for 20 hours. The obtained lithium manganese oxide was found to have a spinel structure of Li _1.035 Mn _1.961 by X-ray diffraction and chemical analysis.
O ₄ . Next, a positive electrode and an XAFS cell were prepared in the same manner as in Example 1, and then measured by an X-ray absorption fine structure analysis (XAFS) to obtain an X-ray absorption spectrum. 6
It was found that the pre-edge peak intensity appearing near 6540 eV in the XANES spectrum at the MnK absorption edge when normalized by the absorption coefficient at 750 eV gradually increased with lithium deintercalation. (Pre-edge type R) Subsequently, a battery was produced in the same manner as in Example 1. This battery was charged and discharged with a charge current of 680 mA and a charge end voltage of 4.2.
And charging / discharging at a discharge end voltage of 2.7 V. The discharge capacity at that time was 680 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 92% and 82%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 71%, and the recovery rate was 88%.

Example 5 A positive electrode active material was synthesized by using Li ₂ CO ₃ as a lithium compound and electrolytic manganese dioxide (EMD) as a manganese compound.
Mix at an atomic ratio of 0.61 and in air (oxygen concentration 20 vo
1%) by heating at 900 ° C. for 24 hours. The obtained lithium manganese oxide was found to have a spinel structure of Li _1.120 Mn _1.876 O ₄ by X-ray diffraction and chemical analysis.
Met.

Next, after a positive electrode and a XAFS cell were prepared in the same manner as in Example 1, X-ray absorption fine structure analysis (X
(AFS) method to obtain an X-ray absorption spectrum. 675
X at MnK absorption edge when normalized by absorption coefficient at 0 eV
It was found that the pre-edge peak intensity appearing at around 6540 eV in the ANES spectrum gradually increased with lithium deintercalation. (Pre-edge type R) Subsequently, a battery was produced in the same manner as in Example 1. This battery was charged with a charge / discharge current of 600 mA and a charge end voltage of 4.2.
And charging / discharging at a discharge end voltage of 2.7 V. The discharge capacity at that time was 600 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 95% and 85%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 68%, and the recovery rate was 85%.

Comparative Example 1 A positive electrode active material was synthesized using LiNO ₃ as a lithium compound, electrolytic manganese dioxide (EMD) as a manganese compound, and Li / Mn = 0.
Mixed at an atomic ratio of 50 and in air (oxygen concentration 20 vol
%) By performing a heat treatment at 450 ° C. for 24 hours. The obtained lithium manganese oxide was found to have a spinel structure of Li _0.997 Mn _1.996 O ₄ by X-ray diffraction and chemical analysis.
Met. Next, in the same manner as in Example 1, the positive electrode and X
After fabricating the AFS cell, X-ray absorption fine structure analysis (XA
The X-ray absorption spectrum was obtained by measurement by the FS) method. 6750
XA of MnK absorption edge when normalized by absorption coefficient in eV
FIG. 3 shows an enlarged pre-edge portion of the NES spectrum. In FIG. 3, 4 is a pre-edge peak of an uncharged active material, 5 is a pre-edge peak of an active material obtained by deintercalating 0.33 of Li by charging, and 6 is a de-interpolating of 0.91 of Li by charging. The pre-edge peak of the calmed active material is shown. From this, 6540 eV
It was found that the pre-edge peak intensity appearing in the vicinity became smaller than that in the uncharged state once with lithium deintercalation, and the peak intensity increased as lithium was further deintercalated. (This pre-edge behavior associated with lithium deintercalation is defined in Table 1 as pre-edge type S.)

Subsequently, a battery was manufactured in the same manner as in Example 1. This battery was charged and discharged at a charge / discharge current of 570 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 2.7 V. The discharge capacity at that time was 570 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 60% and 25%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 38%, and the recovery rate was 42%. Comparative Example 2 The synthesis of the positive electrode active material was performed using LiOH.H ₂ O as a lithium compound and chemically synthesized manganese dioxide (CMD) as a manganese compound, and Li / Mn = 0.5.
The mixture was mixed at an atomic ratio of 6 and heat-treated at 750 ° C. for 20 hours in a mixed gas of air and O 2 (oxygen concentration: 50 vol%). The obtained lithium manganese oxide is X
Li _1.069 Mn with spinel structure by _X- ray diffraction and chemical analysis
_1.926 O ₄ . Next, a positive electrode and an XAFS cell were prepared in the same manner as in Example 1, and then measured by an X-ray absorption fine structure analysis (XAFS) to obtain an X-ray absorption spectrum.

6540 eV of the XANES spectrum at the MnK absorption edge when normalized by the absorption coefficient at 6750 eV
It was found that the pre-edge peak intensity appearing in the vicinity became smaller than that in the uncharged state once with lithium deintercalation, and the peak intensity increased as lithium was further deintercalated. (Pre-edge type S) Subsequently, a battery was manufactured in the same manner as in Example 1. This battery was charged and discharged at a charge / discharge current of 610 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 2.7 V. The discharge capacity at that time was 610 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 93% and 43%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 40%, and the recovery rate was 45%. (Comparative Example 3) In the synthesis of the positive electrode active material, Li ₂ CO ₃ was used as a lithium compound, Mn ₂ O 3 was used as a manganese compound, and Li / Mn was mixed at an atomic ratio of 0.61, and oxygen was mixed (oxygen concentration 100 vol%). The heat treatment was performed at 700 ° C. for 20 hours. The obtained lithium manganese oxide was found to have a spinel structure of Li by X-ray diffraction and chemical analysis.
_1.112 Mn _1.879 O ₄ .

Next, after a positive electrode and a XAFS cell were manufactured in the same manner as in Example 1, X-ray absorption fine structure analysis (X
(AFS) method to obtain an X-ray absorption spectrum. 675
X at MnK absorption edge when normalized by absorption coefficient at 0 eV
The pre-edge peak intensity appearing in the vicinity of 6540 eV of the ANES spectrum becomes smaller than the uncharged state once with lithium deintercalation, and the peak intensity becomes larger as lithium is further deintercalated. all right. (Pre-edge type S) Subsequently, a battery was manufactured in the same manner as in Example 1. This battery was charged and discharged at a charge / discharge current of 610 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 2.7 V. The discharge capacity at that time was 610 mAh. This battery was repeatedly charged and discharged under the above-mentioned charge and discharge conditions at temperature conditions of 20 ° C. and 60 ° C., respectively. The capacity retention rates after 100 cycles (the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle) were 95% and 48%, respectively. When stored at 85 ° C. in the same manner as in Example 1, the capacity retention rate was 42%, and the recovery rate was 50%.

[0024]

[Table 1]

[0025]

The positive electrode active material of the present invention is a lithium manganese oxide having a spinel structure and is specified by an absorption coefficient at 6750 eV in an X-ray absorption spectrum measured by X-ray absorption fine structure analysis (XAFS). M when converted
It is characterized in that the pre-edge peak intensity, which appears near 6540 eV in the X-ray absorption near edge structure (XANES) spectrum at the nK absorption edge, gradually increases with lithium deintercalation. A battery using this is extremely useful industrially because of its excellent properties at high temperatures.

[Brief description of the drawings]

FIG. 1 is an X-ray absorption near edge structure (XANES) spectrum of a MnK absorption edge obtained from an X-ray absorption spectrum of a lithium manganese oxide which is an example of an embodiment of the present invention.

FIG. 2 shows the MnK absorption edge near the X-ray absorption edge structure (XANES) spectrum obtained from the X-ray absorption spectrum of the lithium manganese oxide of Example 1 of the present invention.
This is a pre-edge peak that appears near 40 eV.

FIG. 3 shows 6540 eV in a structure near the X-ray absorption edge (XANES) spectrum of the MnK absorption edge obtained from the X-ray absorption spectrum of the lithium manganese oxide of Comparative Example 1.
It is a pre-edge peak that appears in the vicinity.

[Explanation of symbols]

1 Pre-edge peak of uncharged active material 2 Pre-edge peak of active material de-intercalated 0.25 Li by charging 3 Pre-edge peak of active material de-intercalated 0.67 Li by charging 4 Pre-edge peak of uncharged active material 5 Pre-edge peak of active material having 0.33 Li deintercalated by charging 6 Pre-edge peak of active material having 0.91 Li de-intercalated by charging

Claims (1)

[Claims] 1. A secondary battery comprising lithium or a substance capable of inserting and extracting lithium as a negative electrode, a non-aqueous electrolyte and a positive electrode, wherein the positive electrode active material is lithium manganese oxide having a spinel structure, and X-ray absorption. In the X-ray absorption spectrum measured by the fine structure analysis (XAFS) method, MnK when normalized by the absorption coefficient at 6750 eV
A non-aqueous secondary battery characterized in that a pre-edge peak intensity appearing at around 6540 eV in an X-ray absorption edge near-edge structure (XANES) spectrum at an absorption edge gradually increases with lithium deintercalation.