JP4735982B2 - Lithium-iron-manganese composite oxide - Google Patents

Description

  The present invention relates to a lithium-iron-manganese composite oxide, a production method thereof, and an application thereof.

  At present, most of the secondary batteries installed in portable devices such as mobile phones and notebook computers in Japan are lithium ion secondary batteries having outstanding energy density. Lithium ion secondary batteries are also expected to play an important role in the future as large batteries in various fields such as electric vehicles and power load leveling systems, and their importance is increasing. .

Conventionally, 4V-class lithium secondary battery positive electrode materials that utilize iron (in this specification, described as iron-based positive electrode materials) have reduced battery costs, stabilized material costs, reduced environmental impact, etc. Has been needed from the perspective of The present inventors have already found that a LiFeO ₂ —Li ₂ MnO ₃ solid solution (described in this specification as a lithium-iron-manganese composite oxide) can be a 4V class iron-based positive electrode material. (Patent Documents 1 and 2). It has also been clarified that charge / discharge characteristics can be improved by dissolving nickel ions (Patent Document 3).

  However, as Non-Patent Document 1 reveals, the charge / discharge characteristics of the material represented by the above composition formula are greatly influenced by the iron content in the material or the manufacturing conditions, so quality control during mass production. Is not easy. Therefore, it is extremely important to find out what composition, characteristics, manufacturing method, etc. of the lithium-iron-manganese composite oxide can give stable and good charge / discharge characteristics to the positive electrode material.

  It is also known that by controlling the distribution of Li ions and transition metal ions in the crystal structure of lithium-iron-manganese composite oxide, it is possible to provide a material with stable and good charge / discharge characteristics. (See Patent Document 4).

However, in the lithium-iron-manganese composite oxides described in Patent Documents 1, 2, 4, etc., reduction of the inert capacity reaching 40% of the initial charge capacity is an important issue.
JP 2002-068748 A Japanese Patent Laid-Open No. 2002-121026 JP 2003-048718 JP 2005-154256 A M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, T. Nakamura, M. Kohzaki, A. Hirono and R. Kanno, Journal of The Electrochemical Society, 149, A509-A524, 2002

  The present invention is a lithium-iron-manganese composite that can exhibit stable and good charge / discharge characteristics as a positive electrode material of a lithium ion secondary battery, and that has superior charge / discharge characteristics compared to existing positive electrode materials. The main purpose is to provide oxides.

The present inventor has conducted various studies in view of the present situation of the iron-based positive electrode material, and as a result, a lithium-iron-manganese composite oxide satisfying specific conditions can achieve the above-described object. As a result, the present invention has been completed.

That is, this invention provides the invention which concerns on each following item.
1. Composition formula: Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ (where each subscript is the following numerical value, x = 0.05 to 0.20)
, Y = 0.10-0.75, z = 0.4-0.7. However, it is y> x) and is in a two-phase coexistence state satisfying the conditions shown in the following (1) to (4): (1) first The phase is α-NaFeO ₂ type structure belonging to the space group R-3m, Li ions are present at the 3a site (atomic coordinates: 0,0,0), and the 3b site (atomic coordinates: 0,0,1 / 2) Fe ions, Mn ions and Li
Ion is mixed irregularly,
(2) Among the 6c sites of the first phase (atomic coordinates: 0, 0, z), 5 pm from the oxygen tetrahedral site surrounded by four oxide ions whose atomic coordinates are expressed by 0.33 <z ≦ 0.39 Fe ion is present in the site within, the amount is 1% or more and 12% or less in terms of seat occupancy rate of the 6c site,
(3) The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions and Li ions are irregularly mixed in the 4a site (0,0,0), and the 4b site (atomic coordinates). : 1/2, 1/2, 1/2) oxide ions are present
(4) The abundance ratio of the first phase is 60% or more and 90% or less in terms of molar fraction.
2. Composition formula: Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ (in the formula, each subscript is the following numerical value: x = 0.05 to 0.23, z = 0.4 to 0.7), The lithium-iron described in item 1 above, wherein a composite oxide satisfying the following conditions (1) to (4) is brought into contact with a solution containing an oxidizing substance: -Manufacturing method of manganese composite oxide:
(1) The first phase is α-NaFeO ₂ type structure belonging to the space group R-3m, Li ions are present at the 3a site (atomic coordinates: 0,0,0), and the 3b site (atomic coordinates: 0 , 0,1 / 2), Fe ions, Mn ions and Li ions are mixed irregularly,
(2) Within 5pm from the oxygen tetrahedron site surrounded by four oxide ions whose atomic coordinates are 0.33 <z ≦ 0.39 among the 6c sites of the first phase (atomic coordinates: 0,0, z) In the case where Fe ions are present at the site, the amount is less than 1% in terms of seat occupancy of the 6c site.
(3) The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions and Li ions are irregularly mixed in the 4a site (0,0,0), and the 4b site (atomic coordinates). : 1/2, 1/2, 1/2) Oxide ions are present at the site.
(4) The abundance ratio of the first phase is 60% or more and 90% or less in terms of molar fraction.
3. A positive electrode material for a lithium ion secondary battery, comprising the composite oxide according to Item 1.
4). A lithium ion secondary battery using the material according to item 3 for a positive electrode.

  Hereinafter, the method for producing the lithium-iron-manganese composite oxide of the present invention and the requirements that the composite oxide should have will be described in detail.

Method for producing lithium-iron-manganese composite oxide
I. Process (1)
Step (1) is a step of obtaining a coprecipitate by adding an alkaline aqueous solution to a mixed solution of an iron salt and a manganese salt.

  As the iron source material and the manganese source material, water-soluble salts (chlorides, nitrates, sulfates, oxalates, acetates, etc.), hydroxides, and the like of the respective metals can be used. These compounds may be either anhydrides or hydrates. These salts and hydroxides may be used alone or in combination of two or more for both metals. As the metal source, for any metal, an aqueous solution in which a metal or metal oxide is dissolved with an acid such as hydrochloric acid may be used.

The Fe / (Fe + Mn) molar ratio in the mixed aqueous solution can be appropriately selected according to the Fe / (Fe + Mn) molar ratio in the target composite metal oxide, but corresponds to the above composition formula Thus, it is more preferable that 0.4 ≦ z ≦ 0.7.

In the present invention, an alkaline aqueous solution is added to a mixed solution of an iron salt and a manganese salt to precipitate an iron-manganese coprecipitate, and the reaction temperature is preferably 20 ° C. or less, more preferably 10 ° C. or less, More preferably, it is kept at about 5 to -15 ° C. By this temperature control, it is possible to suppress the generation of spinel ferrite (MnFe ₂ O ₄ ) as an impurity and prevent a decrease in the amount of iron ions in the target composite oxide. In particular, in order to maintain a low temperature state of 0 ° C. or lower, it is preferable to add an amount of alcohol such as methanol, ethanol, ethylene glycol or the like necessary for exhibiting the function as an antifreeze solution to the mixed solution. The amount of antifreeze added can be about 1 to 50 parts by weight with respect to 100 parts by weight of water in consideration of the set reaction temperature.

As an alkali source for forming an iron-manganese coprecipitate from the above mixed solution, lithium hydroxide, potassium hydroxide, sodium hydroxide, ammonia and the like are in the form of an aqueous solution (usually about 0.1 to 20M, preferably 0.5 to 10M). Degree). The aqueous alkali source solution is added dropwise with stirring until the mixed solution becomes completely alkaline (pH 11 or more). When lithium hydroxide is used as the alkali source, lithium derived from the alkali source also constitutes a part of the lithium component in the target composite oxide.

II. Process (2)
Next, it is about 0 to 150 ° C. (more preferably about 20 to 100 ° C.) for about 2 to 7 days (more preferably 3
Over about 5 days), the coprecipitate is oxidized and aged while blowing air into the reaction solution containing the coprecipitate. The resulting oxide is then washed with distilled water to remove excess alkali components and residual salts, and a coprecipitate separated by filtration is obtained.

III. Process (3)
Next, the purified coprecipitate is mixed with distilled water in a container such as a polytetrafluoroethylene beaker, and this is mixed with lithium such as lithium hydroxide (anhydride and / or hydrate), lithium chloride, and lithium nitrate. The compound is added to a concentration of about 0.1 to 10M (more preferably about 1 to 8M) and stirred, and further an oxidizing agent such as potassium chlorate is about 0.1 to 10M (more preferably a concentration of about 0.5 to 5M). Then, the vessel is allowed to stand in a hydrothermal reactor (for example, an autoclave), and the mixture in the vessel is subjected to a hydrothermal reaction. This hydrothermal reaction treatment mixture may be prepared by preparing a liquid containing a lithium salt and an oxidizing agent in advance, and then adding the product coprecipitate to this and stirring.

  Hydrothermal reaction conditions are not particularly limited, but are usually about 100 to 400 ° C. for about 0.1 to 150 hours, more preferably about 150 to 300 ° C. for about 1 to 100 hours. .

After allowing the reaction solution after completion of the hydrothermal reaction treatment to cool, the hydrothermal reaction treatment product is washed with water, water-alcohol, acetone, etc. and filtered to remove the remaining lithium salt and the like. Thus, a purified layered rock salt type lithium-iron-manganese composite oxide (low crystalline sample) can be obtained.

IV. Process (4)
Next, in order to further improve the crystallinity of the layered rock salt type lithium-iron-manganese composite oxide, the composite oxide obtained above is pulverized and Li salt (lithium hydroxide, lithium chloride, It is selected from lithium nitrate, etc .: The amount added varies depending on the alkali species, but in the case of lithium hydroxide and potassium hydroxide, a lithium salt of about 0.3-1.2 times mol of the charged molar amount is required. In this case, it is necessary to mix about 0.3-1.7 times the molar amount of lithium salt charged), dry in the atmosphere at about 50 to 200 ° C (more preferably about 70 to 150 ° C), and then pulverize Firing in air, in an oxidizing atmosphere, in an inert atmosphere or in a reducing atmosphere at about 525 to 775 ° C. (more preferably about 550 to 750 ° C.) for about 1 to 200 hours (more preferably about 10 to 100 hours) May be. Lithium salt is used to ensure reaction uniformity.
More preferably, it is used in the form of an aqueous solution. Further, after the firing, in order to remove excess lithium salt, the fired product is subjected to a water washing treatment or a solvent washing treatment, filtered, and then dried by heating at a temperature of 80 ° C. or higher (more preferably, a temperature of about 100 ° C.). May be.

Furthermore, by repeating a series of operations of pulverizing, firing, washing, and drying this heat-dried product, excellent characteristics of the lithium-iron-manganese composite oxide (as a positive electrode material for a lithium ion secondary battery) The stable charge / discharge characteristics, high capacity, etc. in the operating voltage range can be further improved.

The lithium-iron-manganese composite oxide synthesized through the above steps (1) to (4) is in a two-phase coexistence state.

The first phase is an α-NaFeO ₂ type structure belonging to the space group R-3m, with Li ions present at the Na site (ie, 3a site, atomic coordinates: 0,0,0), and Fe sites (ie, 3b site). Atomic coordinates: 0, 0, 1/2), Fe ions, Mn ions, and Li ions are irregularly mixed. FIG. 1 shows sites 3a and 3b and sites occupied by oxide ions.

Further, among the 6c sites (atomic coordinates: 0,0, z), the atomic coordinates are expressed as 0.33 <z ≦ 0.39, and the sites are within 5pm from the oxygen tetrahedral site surrounded by four oxide ions. When Fe ions are present, the amount is less than 1% in terms of seat occupancy at the 6c site. There are 18h and 36i sites around the 6c site. The 18h site has 3 times the 6c site and the 36i site has 6 times the 6c site, so the same amount of iron ions as the 6c site. , The 18h site will have 1/3 times the 6c site, and the 36i site will have 1/6 times the 6c site. Therefore, the seat occupancy conversion value for the 6c site mentioned here is gc for the seat occupancy for the 6c site, gh for the seat occupancy for the 18h site,
If the seat occupancy rate of the site is gi, it is expressed as gc + 3gh + 6gi, which means that gc + 3gh + 6gi will be less than 1%. Further, less than 1% is an area that is difficult to distinguish from 0, and includes 0%.

The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions, and Li ions are randomly mixed at Na sites (ie, 4a sites, atomic coordinates: 0,0,0), and Cl Site (i.e. 4b
Oxide ions are present at the site, atomic coordinates: 1/2, 1/2, 1/2). The abundance ratio of the first phase to the second phase is 60% or more and 90% or less in the first phase in terms of molar fraction. This abundance ratio is desirably 60% or more and 85% or less, more desirably 60% or more and 80% or less, and further desirably 60% or more and 75% or less.

The total composition of both phases is as follows: Composition formula: Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ (wherein each subscript is the following numerical value: x = 0.05 to 0.23, z = 0.4 to 0.7).

V. Process (5):
Next, the composite oxide obtained in the step (4) is brought into contact with a solution containing an oxidizing substance. Through this step, chemical oxidation of the composite oxide obtained in step (4) proceeds,
The average valence of the transition metal component ions contained in the composite oxide is increased, and a part of Li is desorbed.

Compared with the composite oxide obtained in the above step (4), the composite oxide obtained by desorbing a part of Li by this method is in the operating voltage range as a positive electrode material for a lithium ion secondary battery. Stable charge / discharge characteristics, high capacity, and the like are further improved.

In the solution containing the oxidizing substance used in the step (5), as the oxidizing substance, for example, MoF ₆ , W
Hexafluorides such as F ₆ ; nitrogen oxides (NO _x , 1 ≦ x ≦ 2); NO ₂ PF ₆ ; peroxides such as hydrogen peroxide and sodium peroxide; permanganates such as potassium permanganate Halogens such as bromine; oxidizing acids such as sulfuric acid can be used.

In the solution, water, a polar organic solvent, or the like can be used as the solvent. In particular, an organic solvent that easily vaporizes at room temperature is preferable, and an organic solvent having a boiling point of 27 ° C. or less is particularly preferable. A specific example of such an organic solvent is acetonitrile. When such a low-boiling solvent is used, since the composite oxide obtained in the step (4) can be easily removed after contacting the solution, the solvent is adsorbed on the surface of the composite oxide. The performance as a Li battery material is not deteriorated.

The method of bringing the composite oxide obtained in the step (4) into contact with a solution containing an oxidizing substance is not particularly limited, but usually the composite oxide to be treated can be uniformly dispersed in the solution. That's fine.

  The concentration and amount of the solution and the amount of sample to be mixed are not particularly limited, but the amount of Li desorbed is determined by the chemical equilibrium before and after this treatment, so the Li content of the target sample and the reaction The optimum value can be determined from the solubility of the solute at the temperature used.

For example, when acetonitrile is used as the solvent, as the oxidizing substance, hexafluorides such as MoF ₆ and WF ₆ , halogens such as bromine, NO ₂ PF _{6 and the} like can be suitably used. Taking this case as an example, when bromine is used, Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ (x = 0.05 to 0.23, z = 0.4 to 0.7), x to 30-200 g / dm ³ (preferably 60-170 g / dm ³ , more preferably 90-150 g / dm) with bromine dissolved in a molar ratio of 30 times (preferably 1-20 times, more preferably 10-18 times) ³ ) An acetonitrile solution having a bromine concentration of about can be used. in this case,
Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ + (y / 2) Br ₂ → Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ + yLiBr
This reaction occurs. In the above reaction, bromine as the oxidant and the substance on the right side coexist and reach chemical equilibrium, so the y value is determined by the relationship between the molar ratio of bromine and the sample and the oxidation potential of the reaction.

In addition, when using hexafluoride such as MoF ₆ and WF ₆ and NO ₂ PF ₆ as an oxidizing substance, Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ (x = 0.05 to 0.23, z = 0.4 to 0.7), NO ₂ PF ₆ , MoF ₆ , WF ₆ etc. in a molar ratio of x to 0.4 times (preferably x to 0.3 times, more preferably x to x + 0.05 times) A dissolved acetonitrile solution can be used. in this case,
Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ + yNO ₂ PF ₆ → Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ + yNO ₂ + LiPF ₆
Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ + yMoF ₆ → Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ + yLiMoF ₆
Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ + yWF ₆ → Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ + yLiWF ₆
This reaction occurs. Since the above reaction proceeds to the right, the y value is determined by the molar ratio of oxidant to sample when chemical equilibrium is reached. In the acetonitrile solution in this case, the concentration of the oxidizing substance can be, for example, about 10 to 200 g / dm ³ .

The time required for the reaction is not particularly limited because it varies depending on the volume of the solution, the amount and particle size of the sample powder, and the reaction temperature, and it is only necessary to achieve chemical equilibrium by mixing. In order to sufficiently advance this reaction, it is possible to shorten the reaction time by carrying out homogeneous stirring using a magnetic stirrer or the like during mixing. For example, stirring for about 4 hours is usually sufficient, but it may take about twice as long depending on the particle size of the sample.

After completion of the reaction, it is possible to remove excess solute and products other than the dissolved target product by repeating separation such as washing with a solvent and filtration. The solvent can also be removed by drying at a temperature corresponding to the type of the solvent. For example, 100 ° C for acetonitrile
For 4 hours or longer, preferably 6 hours or longer, more preferably 8 hours or longer.

Lithium-iron-manganese composite oxide The lithium-iron-manganese composite oxide of the present invention can be obtained by the method described above.

The composite oxide after the treatment in the step (5) is in a two-phase coexistence state as before the treatment. The first phase is an α-NaFeO ₂ type structure belonging to the space group R-3m, with Li ions present at the Na site (ie, 3a site, atomic coordinates: 0,0,0), and Fe sites (ie, 3b site). , Atomic coordinates: 0,0,1 / 2)
Ions, Mn ions, and Li ions are mixed irregularly. Furthermore, among the 6c sites (atomic coordinates: 0,0, z), the atomic coordinates are expressed as 0.33 <z ≦ 0.39, and the site is within 5pm from the oxygen tetrahedral site surrounded by four oxide ions. Fe ions are present, and the amount is 1% or more and 12% or less in terms of seat occupancy at the 6c site. The seat occupancy rate of the 6c site is preferably 2% to 10%, more preferably 3% to 8%. Here, the seat occupancy ratio of the 6c site is the same as the sample obtained in the above step (4), the seat occupancy ratio of the 6c site is gc, the seat occupancy ratio of the 18h site is gh, and the seat occupancy ratio of the 36i site is If g is gi, it is expressed as gc + 3gh + 6gi. FIG. 1 schematically shows the structure of the first phase, and the sites occupied by Fe ions are represented by 6c sites.

The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions, and Li ions are irregularly distributed at the Na site (ie, 4a site, atomic coordinates: 0, 0, 0). Oxide ions are present at Cl sites (ie, 4b sites, atomic coordinates: 1/2, 1/2, 1/2). The abundance ratio of the first phase and the second phase in the composite oxide of the present invention is such that the first phase accounts for 60% or more and 90% or less in terms of molar fraction. This abundance ratio is desirably 61% or more and 85% or less, and more desirably 62% or more and 82% or less. The mole fraction of this first phase is compared to the sample obtained in the fourth step,
It is the same or larger value.

The total composition of both phases is represented by the general formula Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ (where each subscript is the following numerical value: x = 0.05 to 0.20, y = 0.10 to 0.75, z = 0.4 to 0.7, where y ≧ x).

  The above-described crystal structure of the composite oxide of the present invention can be easily confirmed by an analysis method called a Rietveld method in which a pattern obtained by a powder X-ray diffraction method is fitted by a least square method. The data in this specification is “RIETAN-2000” (F. Izumi and T. Ikeda, Mater. Sci. Forum, 321-324 (2000) 198-203.) Yes.

  The lithium-iron-manganese composite oxide produced through the step (5) is compared with the composite oxide produced in the above step (4) in the operating voltage region as a positive electrode material for a lithium ion secondary battery. Stable charge / discharge characteristics, high capacity, etc. are further improved.

The lithium ion secondary battery using the lithium-iron-manganese composite oxide according to the present invention can be manufactured by a known method. That is, the composite oxide obtained by the method of the present invention is used as the positive electrode material, and a known metal lithium, lithium alloy, carbon-based material (activated carbon, graphite, non-graphitizable carbon, etc.) is used as the negative electrode material. As the electrolytic solution, a solution obtained by dissolving a lithium salt such as lithium perchlorate or LiPF _{6 in} a known non-aqueous solvent such as ethylene carbonate or dimethyl carbonate, and other known battery components are used. The lithium ion secondary battery can be assembled according to a conventional method.

The lithium-iron-manganese composite oxide according to the present invention can be stably charged and discharged in an operating voltage range (about 4 V) equivalent to that of an existing lithium cobalt oxide-based positive electrode material. It is a new material that shows a high capacity exceeding 200mAh / g by using 3V.

  The lithium-iron-manganese composite oxide according to the present invention is a material in which the inactive capacity of initial charge / discharge is suppressed and the charge / discharge efficiency and capacity are higher than those of the existing lithium-iron-manganese composite oxide .

  Further, the lithium-iron-manganese composite oxide according to the present invention can be synthesized using inexpensive raw materials.

  The lithium-iron-manganese composite oxide according to the present invention can exhibit excellent performance as a positive electrode material even in a high temperature environment. For example, it has a higher capacity when compared with lithium manganese spinel, which is a typical low-cost cathode material that has been put into practical use under high-temperature charge / discharge conditions (for example, about 60 ° C.). Therefore, the lithium-iron-manganese composite oxide of the present invention exhibits excellent performance as a positive electrode for a lithium ion secondary battery that may be used in a high temperature environment such as an in-vehicle secondary battery. can do.

  The lithium-iron-manganese composite oxide according to the method of the present invention is extremely useful as a positive electrode material for lithium ion secondary batteries with low cycle deterioration and low cost.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Examples 1 and 2
In a glass beaker, weigh 50.50 g of iron (III) nitrate nonahydrate equivalent to 0.125 mol and 24.72 g of manganese (II) chloride tetrahydrate equivalent to 0.125 mol, add 300 cm ³ of distilled water and stir well. And completely dissolved. The beaker containing the obtained iron-manganese mixed aqueous solution was kept at room temperature or lower (5 ° C.) in an ice-cooled tank.

In another glass beaker, 50 g of lithium hydroxide monohydrate was weighed, 300 cm ³ of distilled water was added and stirred to completely dissolve. The obtained lithium hydroxide aqueous solution was gradually dropped into an iron-manganese mixed aqueous solution kept at 5 ° C. to prepare an iron-manganese coprecipitate.

The reaction solution containing the coprecipitate was taken out of the ice-cooled bath, and the coprecipitate was air oxidized for 2 days while blowing air at room temperature. Distilled water (1000 cm ³⁾ was added to the air-oxidized coprecipitate and stirred well, followed by filtration to separate and remove excess lithium hydroxide, water-soluble salts and the like.

In a polytetrafluoroethylene beaker with a capacity of 1000 cm ³ , weighed 309 g of potassium hydroxide, 50 g of lithium hydroxide monohydrate and 50 g of potassium chlorate (oxidant), added 500 cm ³ of distilled water, and stirred well, The filtrate obtained above was added and dispersed by stirring. The polytetrafluoroethylene beaker was left in an autoclave and hydrothermally treated at 220 ° C. for 8 hours. After natural cooling of the treatment liquid after completion of the hydrothermal reaction, the product was taken out from the autoclave, washed with distilled water 5 times, washed, filtered, and transferred to a polytetrafluoroethylene petri dish. . A lithium hydroxide aqueous solution in which 0.25 mol (10.49 g) of lithium hydroxide monohydrate was dissolved in 100 cm ³ of distilled water was added to the petri dish, and the mixture was stirred well and then dried at 100 ° C.

Next, after the dried product was pulverized well in an agate mortar, the obtained powder was thinly spread in an alumina firing container and fired at 750 ° C. for 20 hours in the atmosphere using an electric furnace. Example 2
Is a temperature of 700 ° C. ) After calcination, the product was cooled to room temperature over 24 hours, pulverized, then decanted with distilled water 5 times to remove excess lithium hydroxide, lithium carbonate, etc., filtered, and 100 ° C Dry for 12 hours. The sample obtained in this step is referred to as a post-baking sample.

Next, 2.00 g of the obtained calcined sample was weighed and stirred for about 4 hours in a solution of acetonitrile (CH ₃ CN) 200 cm ³ and bromine (Br) 25 g, and the dispersed powder sample was separated by filtration. Washing with acetonitrile and filtration were repeated three times, followed by drying at 100 ° C. for 8 hours.

  As a result, a target lithium-iron-manganese composite oxide was obtained. This composite oxide is referred to as a final sample.

Table 1 shows the conditions for the firing step and Table 2 shows the metal composition obtained by inductively coupled plasma (ICP) analysis for the post-fired sample and the final sample obtained by the above method. Furthermore, the crystal structure data confirmed by the Rietveld method are shown in Tables 3-5.

According to the ICP method analysis, the molar fraction of the fired sample in Example 1 is Li: Fe: Mn = 1.20: 0.40: 0.40, and the molar fraction of the final sample is Li: Fe: Mn = 0.99: 0.40: 0.40. Was confirmed. Therefore, as shown in Table 2, the 1 + xy and z values in the formula Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ were estimated to be 0.99 and 0.50, respectively.

  From the above, comparing the calcined sample and the final sample, it can be seen that the molar ratio of Fe and Mn is 0.50, no change and only Li is decreased. That is, it was confirmed that a part of Li was desorbed by treatment with a solution containing an oxidizing substance. Since the x value was estimated to be 0.20, y was estimated to be 0.21, and the z value was within the range of 0.4 to 0.7, it was found that the substance having the target composition was obtained (see Table 2).

X-ray diffraction pattern data for analysis of the Rietveld method for crystal structure evaluation was measured in 2θ range 15-120 °, 0.02 ° step, and 10s integration for each step. FIG. 2 shows the result of fitting by the Rietveld method for the final sample obtained in Example 1. Α-NaFeO type ₂ structure with R-3m space group (
It is clear that this is a two-phase mixed state of a NaCl-type structural phase having a phase group and a Fm-3m space group. In FIG. 2, in the two-stage markers indicated by vertical bars, the upper stage indicates the peak position of the first phase, and the lower stage indicates the peak position of the second phase. The broken line shown as the residual indicates the difference between the measured value and the calculated value.

  In Example 1, there was no noticeable change in the lattice constant of each phase before and after Li desorption (Tables 3 and 4), and the Fe ions at the 6c site increased from less than 1% to 6.7% with Li desorption. I understand that. The molar fraction of the first phase has also increased from 65% to 69%.

Evaluation of charge / discharge characteristics Using 20 mg each of the sample after firing and the final sample in Example 1 described above, 5 mg of acetylene black and 0.5 mg of polytetrafluoroethylene powder were added thereto, mixed in a mortar, and collected with metal aluminum. Crimped to electrical body. The obtained positive electrode mixture was vacuum-dried at 120 ° C., and then introduced into the glove box. In the glove box, the electrolyte composed of the supporting salt LiPF ₆ and ethylene carbonate / diethyl carbonate mixed solvent and the metal lithium negative electrode were used. A coin-type lithium secondary battery was produced.

  This battery was connected to a charge / discharge device, and charge / discharge characteristics were evaluated at the start of charge at a test temperature of 60 ° C., a charge / discharge potential range of 2.5-4.5 V, and a charge / discharge current density of 42 mA / g. FIG. 3 shows a charge / discharge curve. The results are shown in Table 6 below.

From FIG. 3 and Table 6, the discharge capacity is 200mAh / g or more, the 4V flat region appears at the initial discharge, the initial discharge average voltage is 3.0V or more, and the initial charge / discharge efficiency is 85% or more. it is obvious. From this, it was confirmed that the composite oxide (final sample) according to the present invention has good characteristics as a 3V class lithium secondary battery material.

The reason why the composite oxide of the present invention has the above-described excellent characteristics is not necessarily clear, but the movement of Fe ions is caused by the Li desorption process, and the defects in the crystal structure caused thereby, It can be considered that it works to activate the capacity that has been inactive until now. If it is assumed that the second phase is relatively inactive, it is considered that the capacity is increased due to the increase in the first phase. In any case, the molar fraction of the first phase is 60% or more, and the charge / discharge characteristics are considered to be due to the improvement of the characteristics of the first phase.

  From the above results, it can be seen that the battery using the material of Example 1 (final sample) can be cycled and discharged at an average discharge voltage of 3.4 V and a discharge capacity of 200-220 mAh / g. In particular, the improvement of the initial charge / discharge efficiency is notable, and facilitates the study of the material configuration when designing the battery.

It is a structural model of the first phase of the lithium-iron-manganese composite oxide of the present invention. 2 shows an X-ray diffraction pattern of the lithium-iron-manganese composite oxide powder obtained in Example 1 and its Rietveld analysis result. 2 is a charge / discharge curve at 60 ° C. of the coin-type lithium secondary battery produced in Example 1. FIG.

Claims (4)

Composition formula: Li _{1 + xy} (Fe _z Mn _1-z ) _1-x O ₂ (in the formula, each subscript is the following numerical value: x = 0.05 to 0.20, y = 0.10 to 0.75, z = 0.4 to 0.7) However, the lithium-iron-manganese composite oxide is characterized in that it is in a two-phase coexistence state that satisfies the following conditions (1) to (4):
(1) The first phase is α-NaFeO ₂ type structure belonging to the space group R-3m, Li ions are present at the 3a site (atomic coordinates: 0,0,0), and the 3b site (atomic coordinates: 0 , 0,1 / 2) include Fe ions, Mn ions and Li
Ion is mixed irregularly,
(2) Among the 6c sites of the first phase (atomic coordinates: 0, 0, z), 5 pm from the oxygen tetrahedral site surrounded by four oxide ions whose atomic coordinates are expressed by 0.33 <z ≦ 0.39 Fe ion is present in the site within, the amount is 1% or more and 12% or less in terms of seat occupancy rate of the 6c site,
(3) The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions and Li ions are irregularly mixed in the 4a site (0,0,0), and the 4b site (atomic coordinates). : 1/2, 1/2, 1/2) oxide ions are present
(4) The abundance ratio of the first phase is 60% or more and 90% or less in terms of molar fraction. Composition formula: Li _{1 + x} (Fe _z Mn _1-z ) _1-x O ₂ (in the formula, each subscript is the following numerical value: x = 0.05 to 0.23, z = 0.4 to 0.7), 2. The lithium-iron according to claim 1, wherein a composite oxide satisfying the following conditions (1) to (4) is brought into contact with a solution containing an oxidizing substance: -Manufacturing method of manganese composite oxide:
(1) The first phase is α-NaFeO ₂ type structure belonging to the space group R-3m, Li ions are present at the 3a site (atomic coordinates: 0,0,0), and the 3b site (atomic coordinates: 0 , 0,1 / 2), Fe ions, Mn ions and Li ions are mixed irregularly,
(2) Within 5pm from the oxygen tetrahedron site surrounded by four oxide ions whose atomic coordinates are 0.33 <z ≦ 0.39 among the 6c sites of the first phase (atomic coordinates: 0,0, z) In the case where Fe ions are present at the site, the amount is less than 1% in terms of seat occupancy of the 6c site.
(3) The second phase is a NaCl-type structure belonging to the space group Fm-3m. Fe ions, Mn ions and Li ions are irregularly mixed in the 4a site (0,0,0), and the 4b site (atomic coordinates). : 1/2, 1/2, 1/2) Oxide ions are present at the site.
(4) The abundance ratio of the first phase is 60% or more and 90% or less in terms of molar fraction. A positive electrode material for a lithium ion secondary battery, comprising the composite oxide according to claim 1. The lithium ion secondary battery which used the material of Claim 3 for the positive electrode.

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