JP2012096974A - Lithium manganese compound oxide-carbon composite and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new positive electrode material for a lithium ion secondary battery, which is a material indicating a high average discharge voltage in a discharge cycle and having an excellent discharge capacity, has fewer restrictions in terms of resources, is capable of being obtained by using an inexpensive raw material, and is capable of demonstrating more excellent charge/discharge characteristics compared to a well-known positive electrode material of a low cost.SOLUTION: For a lithium manganese compound oxide-carbon composite, lithium manganese compound oxide indicated by the compositional formula: Li(MnTi)O(provided that -1/3<x<1/3, 0.4≤y≤0.6) is bonded through carbon. The average valence of a manganese element in the lithium manganese compound oxide is equal to or smaller than 3.8, and the lithium manganese compound oxide contains the crystal phase of a cubic rock-salt type structure.

Description

  The present invention relates to a lithium manganese composite oxide-carbon composite useful as a positive electrode material for a next-generation low-cost lithium ion secondary battery, a production method thereof, and a lithium ion secondary battery using the composite as a positive electrode material.

  With the development of various devices and systems in recent years, there is an increasing demand for higher performance of storage batteries as a power source. In particular, lithium ion secondary batteries are widely used as secondary batteries that power electronic devices such as portable communication devices and laptop computers, and are also expected to be used as motor drive batteries for automobiles from the viewpoint of reducing environmental impact. Has been. For this reason, the demand for lithium ion secondary batteries is expected to increase further in the future, and the development of high energy density lithium ion secondary batteries corresponding to higher performance of these devices is demanded.

In the current lithium ion secondary battery, lithium cobalt oxide (LiCoO ₂ ) is mainly used as the positive electrode material, but because it contains a large amount of cobalt, which is a rare metal, the material cost of the lithium ion secondary battery is reduced. It has become one of the factors that raise it. In the future, it is considered difficult to respond to the increase in demand accompanying the expansion of applications for in-vehicle use, etc. and the increase in size of batteries, using only a positive electrode material made of LiCoO ₂ .

As cathode materials that can replace LiCoO ₂ , lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), etc. have been researched and developed as cheaper and less resource-constrained elements. Some have been put to practical use as alternative materials. However, LiNiO ₂ has the problem of reducing the safety of the battery during charging, and LiMn ₂ O ₄ elutions trivalent manganese into the electrolyte during high-temperature (about 60 ° C) charging and discharging, which improves battery performance. Substitutions for these materials are less advanced due to the problem of significant degradation.

On the other hand, among lithium manganese oxides, a positive electrode material called Li ₂ MnO ₃ has also been proposed, and has been actively researched and developed in recent years because of its high capacity. The present inventors have shown that lithium manganese oxides (iron-containing Li ₂ MnO ₃ and titanium-containing Li ₂ MnO ₃ ) containing iron and titanium, which are resource-rich and inexpensive, exhibit a high capacity, particularly with a specific chemical composition, It has been found that when it has a transition metal ion distribution, it exhibits excellent discharge characteristics under high current density at room temperature and excellent discharge characteristics at low temperatures (see Patent Documents 1-6 below).

As described above, various lithium manganese-based cathode materials that can be substituted for lithium-cobalt-based cathode materials have been reported. However, since these materials have a high initial charge flat potential of about 4.5 V, they have improved characteristics such as initial efficiency. For this purpose, additive elements having a valence of 3 or less, such as Ni and Co, are essential as activation elements (see Non-Patent Document 1 below). On the other hand, if tetravalent Mn in Li ₂ MnO ₃ is reduced to a trivalent state, it is expected that a positive electrode material exhibiting the same characteristic improvement is obtained. However, in a simple reduction reaction, orthorhombic LiMnO ₂ is formed as a by-product, and this orthorhombic LiMnO ₂ has adverse effects such as gradually changing the charge / discharge curve through cycles, so that this does not appear. Is required.

In addition, since Li ₂ MnO ₃ has low electron conductivity, there is a problem that only charging and discharging can be performed at a relatively low current density. In order to solve this problem, methods such as micronization of active material particles and compounding with carbon as a conductive material have been attempted, but in both cases, the tap density is reduced, so the electrode density is reduced, It has been pointed out that the volumetric energy density of the battery decreases.

JP 2002-68748 A JP 2002-121026 A JP 2005-154256 A JP 2008-63211 A JP 2009-179501 A JP 2009-274940 A

Mitsuharu Tanabe et al., 50th Battery Symposium Abstracts, 1A11, November 2009 (Kyoto)

  The present invention has been made in view of the above-described state of the art, and its main purpose is a material that exhibits a high average discharge voltage in a charge / discharge cycle and has a good discharge capacity, and is resource-intensive. New positive electrode for lithium-ion secondary batteries that can be obtained by using inexpensive raw materials with less restrictions and that can exhibit better charge / discharge characteristics than known low-cost positive electrode materials Is to provide materials.

  The present inventor has intensively studied to achieve the above-described object. As a result, a lithium manganese complex oxide having a specific composition is used as a raw material, and this is filled together with carbon powder into an electron conductive container, and a direct current pulse current is applied to cause a heating reaction in a non-oxidizing atmosphere. According to the method, it has been found that the manganese ions in the composite oxide are reduced to have a low average valence of 3.8 or less and are in a composite state with the carbon powder as the conductive material. The composite obtained in this way has excellent discharge characteristics as compared with a composite oxide having a high Mn valence obtained by a conventional method, and the composite oxide and carbon are closely bonded, It was found that the electron conductivity was improved at high density. In particular, it has been found that when the lithium manganese based composite oxide includes a crystal phase of a cubic rock salt type structure at a specific ratio, the initial discharge capacity is greatly improved and the initial charge / discharge efficiency is also increased. The inventors have found that a lithium-ion secondary battery having excellent performance can be produced by using a lithium-manganese composite oxide-carbon composite having such characteristics as a positive electrode active material for a lithium-ion battery. The present invention has been completed.

That is, the present invention provides the following lithium manganese composite oxide-carbon composite, a method for producing the same, and a lithium ion secondary battery including the composite.
1. Composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (where -1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) In which the average valence of manganese element in the lithium manganese composite oxide is 3.8 or less, and the lithium manganese composite oxide is a crystal having a cubic rock salt structure. A lithium manganese based composite oxide-carbon composite containing a phase.
2. The ratio of the cubic rock salt structure crystal phase to the layered rock salt structure crystal phase in the lithium manganese composite oxide is cubic rock salt structure crystal phase: layered rock salt structure crystal phase = 60: 40 to 85:15 Item 2. The lithium manganese composite oxide-carbon composite according to Item 1.
3. 3. The lithium manganese composite oxide-carbon composite according to item 1 or 2, wherein the carbon ratio is 0.001 to 30% by weight based on the total amount of the lithium manganese composite oxide and carbon.
4). Composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) Lithium manganese complex oxide and carbon The lithium manganese based composite oxide according to any one of the above items 1 to 3, wherein the powder is filled in a conductive container, and is heated and reacted by applying a direct current pulse current in a non-oxidizing atmosphere. -Manufacturing method of carbon composite.
5. Item 5. The method according to Item 4, wherein a DC pulse current is applied under pressure.
6). The positive electrode active material for lithium ion secondary batteries which consists of a lithium manganese type complex oxide-carbon composite in any one of said claim | item 1-3.
7). 7. A lithium ion secondary battery comprising the positive electrode active material according to item 6 as a constituent element.

  Hereinafter, the lithium manganese composite oxide-carbon composite of the present invention and the production method thereof will be specifically described.

(1) Lithium Manganese Composite Oxide-Carbon Composite The lithium manganese composite oxide-carbon composite of the present invention is firmly bonded to the lithium manganese composite oxide having a specific composition via carbon, which is a conductive material. It is not only a state in which the lithium manganese composite oxide and the carbon powder are mixed, but in a state in which carbon is attached to a part or the whole of the surface of the composite oxide. It is in a joined state.

  At this time, the composite was joined by placing 0.5 g of the composite and 50 ml of a water / ethanol mixed solution (volume ratio 1: 1) in a 100 ml beaker, and placing a rotor having a length of 3 cm and a central cross-sectional diameter of 5 mm. It is defined by the fact that the carbon material does not separate even when stirred at 600 rpm for 5 minutes. In this respect, it is clearly distinguished from a mixture of the composite oxide and carbon powder.

  In the composite, the ratio of the lithium manganese composite oxide and carbon is about 0.001 to 30% by weight, preferably 0.01 to 20% by weight, based on the total amount of the composite. %.

The lithium manganese composite oxide in the composite has a composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (where −1/3 <x <1/3, 0.4 ≦ y ≦ 0.6), a cubic rock salt structure similar to the well-known substance a-LiFeO ₂ based on the rock salt structure that is the general crystal structure of oxides

The crystal phase of other rock salt structures with different cation distribution, for example, a monoclinic layered rock salt structure similar to Li ₂ MnO ₃ which is a known substance

Crystal phase, known hexagonal layered rock-salt structure similar to LiCoO ₂ and LiNiO ₂

A mixed phase including a crystal phase of According to the production method described later, the lithium manganese composite oxide in the obtained lithium manganese composite oxide-carbon composite usually has a cubic rock salt structure crystal phase as the main crystal phase, and other monoclinic structures. The crystal phase of the layered rock salt type structure such as the crystal layered rock salt type structure or the hexagonal layered rock salt type structure may be included. The range is about 50 to 100: 0. Such a lithium manganese composite oxide-carbon composite has a high average discharge voltage, a high discharge capacity, and a positive electrode material having a high energy density. In particular, when the cubic rock salt structure crystal phase: layered rock salt structure crystal phase (weight ratio) is in the range of about 60:40 to 85:15, the initial discharge efficiency is further improved and the discharge capacity is remarkably high. Indicates the value.

  The lithium manganese composite oxide in the lithium manganese composite oxide-carbon composite of the present invention is represented by the above composition formula, and the average valence of the manganese element is 3.8 or less. It is a feature.

In the present invention, the average valence of the manganese element is determined by measuring the average valence of manganese and titanium by the iodine titration method as shown in the examples described later, and the average content of manganese and titanium and the average of titanium measured separately. It is a value calculated by the following formula based on the valence.
Average valence of manganese element = (average valence of manganese and titanium−average valence of titanium × titanium content y value) / manganese content (1-y)
In the lithium manganese based composite oxide known conventionally, manganese element is basically a tetravalent element and exists as a component constituting Li ₂ MnO ₃ having a monoclinic layered rock salt type structure. Regarding Li ₂ MnO ₃ , reference 1 (CS Johnson, JS Kim, C. Lefief, JT Vaughey, and MM Thackeray, Electrochemistry Communication, vol. 6, p.1085-1091 (2004).) A potential flat part due to Li ₂ O desorption appears in the vicinity of 4.5 V, but the charge capacity up to 5.0 V is 383 mAh / g, whereas the capacity to reach 4.5 V is reported to be 20 mAh / g. Has been. Further, Reference 1 describes that the discharge capacity of Li ₂ MnO ₃ is 208 mAh / g, and the initial charge / discharge efficiency is a low value of 54%.

In contrast, the lithium manganese composite oxide-carbon composite of the present invention can be produced by the production method described later, and the manganese of the lithium manganese composite oxide in the composite has an average valence of 3.8. It is a novel compound having a low valence of the following and containing a cubic rock salt structure as a main crystal phase. The lithium manganese composite oxide-carbon composite of the present invention having such characteristics includes a conventionally known tetravalent manganese and a Li ₂ MnO having a monoclinic layered rock salt structure as a main crystal phase. Compared with the composite electrode of lithium manganese oxide represented by ₃ , it has excellent charge / discharge characteristics, has a high average discharge voltage, and has a high discharge capacity. In particular, when a cubic rock salt type structure crystal phase in a specific range is included, the initial discharge efficiency is further increased and the initial discharge capacity is remarkably increased. Although the reason for this is not clear, a lithium manganese composite oxide-carbon composite containing manganese having an average valence of 3.8 or less can be desorbed from Li at a low voltage of 4.5 V or less, which It is considered that the charge / discharge efficiency and the discharge capacity are improved.

In the present invention, the manganese element in the compound represented by the composition formula Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) The average valence needs to be 3.8 or less, preferably 3.78 or less. In addition, the average valence of manganese decreases, and the Li content in the composition formula decreases, so in order to ensure sufficient charge / discharge capacity, the average valence of manganese element is 2.5 or more. Is desirable, and is more preferably 2.7 or more.

  The lithium manganese composite oxide in the lithium manganese composite oxide-carbon composite of the present invention is an oxide containing Li and Mn as essential elements as shown in the above composition formula, and further containing Ti as an additive element As a solid solution.

  The amount of Ti to be dissolved (y value: Ti / (Ti + Mn)) appears to be present in the rock salt structure by substituting Mn and Li. Ti exists in a tetravalent state, and is considered to contribute to the suppression of Li deficiency and the change in powder characteristics as described in Patent Document 4. In the lithium manganese complex oxide in the lithium manganese complex oxide-carbon complex of the present invention, the use of a large amount of Ti that is more difficult to be involved in charge / discharge than Mn leads to a decrease in charge / discharge capacity. Therefore, it is not preferable. On the other hand, when the solid solution amount of Ti is too small, effects such as suppression of Li deficiency are not sufficiently exhibited. For this reason, the amount of Ti in the lithium manganese composite oxide is about 60 mol% or less and 40 mol% or more (0.4 ≦ y ≦ 0.6) of the amount of metal other than Li.

In the lithium manganese complex oxide in the lithium manganese complex oxide-carbon complex of the present invention, as long as the cubic and layered rock salt type crystal structure can be maintained, Li _{1 + x} (Mn _1-y Ti _y ) ₁ The x value of _-x O ₂ can take a value between -1/3 and 1/3 depending on the average valence of the transition metal.

  Further, the lithium manganese composite oxide-carbon composite of the present invention is lithium hydroxide, lithium carbonate, titanium compound, iron compound, manganese compound in a range that does not significantly affect the charge / discharge characteristics (up to about 10% by weight). An impurity phase such as (including hydrates and composite oxides thereof) may also be included.

(2) Method for Producing Lithium Manganese Composite Oxide-Carbon Composite The lithium manganese composite oxide-carbon composite of the present invention has a composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) Lithium manganese complex oxide and carbon powder are filled in a conductive container and DC pulse is applied in a non-oxidizing atmosphere. It can be obtained by a method in which an electric current is applied to cause a heating reaction. Hereinafter, this method will be specifically described.

(I) The lithium manganese composite oxide used as the lithium manganese composite oxide raw material is an oxide having the same composition as the lithium manganese composite oxide in the target lithium manganese composite oxide-carbon composite, , Composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) Use. However, the average valence of manganese in the lithium manganese composite oxide used as a raw material is tetravalent. Further, the crystal structure of the composite oxide is not particularly limited, and a composite oxide containing a crystal phase of a cubic rock salt structure, a crystal phase of a monoclinic layered rock salt structure, etc. in an arbitrary ratio may be used. it can. In particular, when forming a composite in which the ratio of the cubic rock salt type structure crystal phase in the lithium manganese composite oxide constituting the composite is in the range of about 60 to 85% by weight, the lithium manganese composite used as a raw material It is preferable to use the oxide having a cubic rock salt structure crystal phase ratio of about 10% by weight or less.

  The particle size of the lithium manganese composite oxide is not particularly limited, but it is usually preferable to use a powdery one having an average particle size of about 1 to 50 μm. In the present specification, the average particle size is a particle size at which the cumulative frequency distribution is 50% in the particle size distribution measurement by a dry laser diffraction / scattering method.

  There are no particular limitations on the method for producing such a lithium manganese composite oxide, and it can be produced by a hydrothermal reaction method, a solid phase reaction method, or the like, which is a common composite oxide synthesis method. In particular, a production method utilizing a hydrothermal reaction is preferable because a composite oxide having excellent charge / discharge performance can be easily formed.

  An example of a production method using a hydrothermal reaction is as follows. First, a precipitate is formed by making a mixed solution in which a metal compound, which is a source of manganese ions and titanium ions, dissolved in water, a water / alcohol mixture or the like is alkaline. . Subsequently, an oxidizing agent and a water-soluble lithium compound are added thereto, and hydrothermal treatment is performed under alkaline conditions, whereby a lithium manganese composite oxide can be obtained. Next, a lithium compound is added to the obtained lithium manganese composite oxide and fired. At this time, by adjusting the addition amount of the lithium compound and the firing conditions, the target lithium manganese-based composite oxide can be obtained by controlling the powder characteristics such as the particle size, the Li content, and the like. Hereinafter, this manufacturing method will be specifically described.

  As a manganese compound and a titanium compound, if it is a component which can form the mixed aqueous solution containing these compounds, it can use without limitation. Usually, a water-soluble compound may be used. Specific examples of such water-soluble compounds include water-soluble salts such as chlorides, nitrates, sulfates, oxalates and acetates, hydroxides and the like. These water-soluble compounds may be either anhydrides or hydrates. Further, even a water-insoluble compound such as an oxide can be used as an aqueous solution after being dissolved using an acid such as hydrochloric acid. Each of these raw material compounds may be used alone or in combination of two or more for each metal source.

  What is necessary is just to make it the mixing ratio of the manganese compound and titanium compound in this mixed aqueous solution become the same element ratio as each element ratio in the target complex oxide.

  The concentration of each compound in the mixed aqueous solution is not particularly limited, and may be determined as appropriate so that a uniform mixed aqueous solution can be formed and a coprecipitate can be smoothly formed. Usually, the total concentration of the manganese compound and the titanium compound may be about 0.01 to 5 mol / l, preferably about 0.1 to 2 mol / l.

  As a solvent for the mixed aqueous solution, water may be used alone, or a water-alcohol mixed solvent containing a water-soluble alcohol such as methanol or ethanol may be used. In particular, by using a water-alcohol mixed solvent, it is possible to form a precipitate at a temperature lower than 0 ° C. The amount of alcohol used may be appropriately determined according to the target precipitation temperature, but it is usually appropriate to use about 50 parts by weight or less with respect to 100 parts by weight of water.

  In order to generate a precipitate (coprecipitate) from the mixed aqueous solution, the mixed aqueous solution may be made alkaline. Conditions for forming a good precipitate vary depending on the type and concentration of each compound contained in the mixed aqueous solution, and thus cannot be defined unconditionally. However, it is usually preferable to set the pH to about 8 or more, and to about pH 11 or more. More preferred.

  The method for making the mixed aqueous solution alkaline is not particularly limited, and usually an alkali or an aqueous solution containing an alkali may be added to the mixed aqueous solution. Moreover, a coprecipitate can be formed also by the method of adding this mixed aqueous solution to the aqueous solution containing an alkali.

  Examples of the alkali used to make the mixed aqueous solution alkaline include alkali metal hydroxides such as potassium hydroxide, sodium hydroxide, and lithium hydroxide, ammonia, and the like. When these alkalis are used as an aqueous solution, for example, they can be used as an aqueous solution having a concentration of about 0.1 to 20 mol / l, preferably about 0.3 to 10 mol / l. Further, the alkali may be dissolved in a water-alcohol mixed solvent containing a water-soluble alcohol, similarly to the mixed aqueous solution of the metal compound described above.

  During precipitation, the temperature of the mixed aqueous solution is set to about -50 ° C to + 15 ° C, preferably about -40 ° C to + 10 ° C. Things are easily formed.

  After making the mixed aqueous solution alkaline, the reaction solution is further blown with air at about 0 to 150 ° C. (preferably about 10 to 100 ° C.) for about 1 to 7 days (preferably about 2 to 4 days). In addition, it is preferable to oxidize and age the precipitate.

  The precipitate can be purified by washing the resulting precipitate with distilled water or the like to remove excess alkali components, residual raw materials, and the like, followed by filtration.

  By calcining the precipitate obtained above together with the lithium compound, a lithium manganese composite oxide used as a raw material can be obtained.

  The lithium compound can be used without particular limitation as long as it contains a lithium element. Specific examples include lithium salts such as lithium chloride, lithium nitrate, and lithium acetate, lithium hydroxide, and hydrates thereof. it can. The usage-amount of a lithium compound should just be about 0.01-2 mol with respect to 1 mol of lithium manganese complex oxide obtained by the hydrothermal method.

  Usually, in order to improve the reactivity, it is preferable to add a lithium compound to the lithium manganese based composite oxide obtained by the hydrothermal method, pulverize and mix, and then fire. As for the degree of pulverization, it is sufficient that coarse particles are not included and the mixture has a uniform color tone.

  The lithium compound can be used in the form of a powder, an aqueous solution, or the like, but is preferably used in the form of an aqueous solution in order to ensure the uniformity of the reaction. In this case, the concentration of the aqueous solution is usually about 0.1 to 10 mol / l.

  The firing atmosphere is not particularly limited, and any atmosphere such as the air, an oxidizing atmosphere, an inert atmosphere, or a reducing atmosphere can be selected. The firing temperature is preferably about 200 to 1000 ° C, more preferably about 300 to 900 ° C. The firing time is preferably about 0.1 to 100 hours including the time to reach the firing temperature, and more preferably about 0.5 to 60 hours.

  After completion of the firing, the fired product is usually subjected to a water washing treatment, a solvent washing treatment or the like in order to remove excess lithium compound. Thereafter, filtration may be performed and, for example, heat drying may be performed at a temperature of 80 ° C. or higher, preferably about 100 ° C.

  Furthermore, if necessary, a series of operations of pulverizing the heat-dried product, adding a lithium compound, firing, washing, and drying may be repeated.

(Ii) Carbon powder The carbon powder used in the production of the lithium manganese composite oxide-carbon composite of the present invention is not particularly limited. For example, the powder obtained by thermally decomposing acetylene at a high temperature, obtained by a so-called explosion method. Known acetylene black powder, ketjen black, etc. can be used.

  The average particle size of the carbon powder is not particularly limited, but is usually about 0.005 to 10 μm, preferably about 0.01 to 1 μm.

(Iii) Method for producing composite In the method for producing a lithium manganese composite oxide-carbon composite of the present invention, first, the composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x A container having electron conductivity after sufficiently mixing a starting material composed of a lithium manganese based composite oxide represented by O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) and carbon powder. In a non-oxidizing atmosphere, the mixture is pressurized, and the mixture is pressurized and energized with a DC pulse current called a discharge plasma sintering method, a pulsed current sintering method, a plasma activated sintering method, etc. The raw material mixture is sintered by the method. Thereby, the target lithium manganese composite oxide-carbon composite can be obtained.

  Specifically, by filling a container having electron conductivity with a mixture of a composite oxide and a carbon material as raw materials, and applying a pulsed ON-OFF direct current while applying pressure in a non-oxidizing atmosphere, Electric current sintering can be performed.

The electric current sintering is performed in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N ₂ or in a reducing atmosphere such as H ₂ . Further, a reduced pressure state in which the oxygen concentration is sufficiently low, for example, a reduced pressure state in which the oxygen partial pressure is about 20 Pa or less may be used.

  When a container that can ensure a sufficiently sealed state is used as a container having electron conductivity, the inside of the container may be a non-oxidizing atmosphere. Further, the container having electron conductivity may not be completely sealed. When an incompletely sealed container is used, the container is accommodated in the reaction chamber, and the reaction chamber is filled with an inert gas atmosphere. A non-oxidizing atmosphere such as a reducing atmosphere may be used. As a result, the reaction between the lithium manganese composite oxide and the carbon powder can be performed in a non-oxidizing atmosphere. In this case, for example, the inside of the reaction chamber is preferably an inert gas atmosphere or a reducing gas atmosphere of about 0.1 MPa or more.

  The mixing ratio of the lithium manganese composite oxide and the carbon powder may be about 0.001 to 30% by weight, particularly about 0.01 to 20% by weight, based on the total amount of both. . If the amount of carbon powder is less than 0.001% by weight, the reduction reaction during current sintering is insufficient, the manganese valence does not decrease to the desired value, and the electronic conductivity of the lithium manganese composite oxide is insufficiently improved. Thus, good charge / discharge characteristics may not be obtained. On the other hand, at 30% by weight or more, when used as a positive electrode active material, the weight output density and the volume output density of the battery decrease as the weight ratio and volume ratio of the composite oxide in the formed composite decrease. Therefore, it is not preferable.

  The container having electron conductivity is not particularly limited as long as it has electron conductivity. Carbon, iron, iron oxide, copper, aluminum, tungsten carbide, carbon and / or iron oxide mixed with silicon nitride. What is formed from etc. can be used conveniently.

  By applying a direct current pulse current in a state in which the mixed powder of the composite oxide and the carbon material is filled in such an electron conductive container, a discharge phenomenon that occurs in the particle gap of the filled mixed powder is used to discharge. Particle oxide purification activation by plasma, discharge shock pressure, etc., electric field diffusion effect caused by electric field, thermal diffusion effect by Joule heat, plastic deformation pressure by pressurization, etc. act as driving force for particle bonding, Joined via a carbon material. At the same time, an electric field gradient is applied to the lithium-manganese composite oxide by the applied DC pulse current, whereby a part of Li is desorbed from the oxide and reduced, and these occur in parallel. It is considered that the lithium manganese composite oxide having a reduced valence is firmly bonded via the carbon powder. At this time, a lithium manganese composite oxide containing a cubic rock salt structure as a main crystal phase is formed by heating at the time of electric current sintering.

  As an apparatus for carrying out current sintering, there is a particular limitation as long as it is possible to heat, cool, pressurize, etc. a mixed powder of lithium manganese composite oxide and carbon powder and to apply a current necessary for discharge. Not. For example, a commercially available electric current sintering apparatus (discharge plasma sintering apparatus) can be used. Such an electric current sintering apparatus and its principle are disclosed in, for example, Japanese Patent Laid-Open No. 10-251070.

  A specific example of the method for producing a lithium manganese composite oxide-carbon composite of the present invention will be described below with reference to FIG. 1 showing a schematic diagram of an electric current sintering apparatus.

  The electric sintering apparatus 1 includes a die (electron conductive container) 3 on which a sample 2 is loaded and a pair of upper and lower punches 4 and 5. The punches 4 and 5 are supported by punch electrodes 6 and 7, respectively, and a pulse current is supplied through the punch electrodes 6 and 7 while applying pressure to the sample 2 loaded in the die 3 as necessary. Can do. The material of the die 3 is not limited, and examples thereof include a carbon material such as graphite.

  In the apparatus shown in FIG. 1, the energization part including the above-described container 3 having electron conductivity, energization punches 4 and 5, and punch electrodes 6 and 7 is accommodated in a water-cooled vacuum chamber 8. It can be adjusted to a predetermined atmosphere by the control mechanism 15. Therefore, the atmosphere control mechanism 15 may be used to adjust the inside of the chamber to a non-oxidizing atmosphere.

  The control device 12 drives and controls the pressurization mechanism 13, the pulse power source 11, the atmosphere control mechanism 15, the water cooling mechanisms 16 and 10, and the temperature measurement device 17. The control device 12 is configured to drive the pressurizing mechanism 13 so that the punch electrodes 6 and 7 pressurize the raw material mixture at a predetermined pressure.

  Regarding the condition of the energization treatment, a composite in which the average valence of manganese is 3.8 or less in the composite oxide in the target composite and the composite oxide and the carbon material are intimately bonded is formed. The conditions may be set. The temperature (heating temperature) of the die (electron conductive container) 3 during specific energization treatment can be appropriately selected according to the type of lithium manganese composite oxide and carbon powder, the particle size, and the like. Usually, the temperature may be about 200 to 1000 ° C., preferably about 300 to 800 ° C. When the heating temperature is less than 200 ° C., the reduction reaction of the lithium manganese composite oxide is insufficient, the manganese valence does not decrease to a desired value, and the bonding with the carbon powder may be insufficient. On the other hand, when the heating temperature exceeds 1000 ° C., the reduction of the lithium manganese composite oxide due to the reduction effect of the carbon powder or the electron conductive container proceeds excessively, causing decomposition and the like, which is not preferable. Accordingly, a heating temperature of about 300 to 800 ° C. is suitable.

  The pulse current applied for heating can be, for example, a pulsed ON-OFF direct current having a pulse width of about 2 to 3 milliseconds and a period of about 3 Hz to 300 kHz. Although the specific current value varies depending on the type and size of the electron conductive container, the specific current value may be determined so as to be in the temperature range described above. For example, when a graphite mold with an inner diameter of 15 mm is used, about 200 to 1000 A is preferable, and when a mold with an inner diameter of 100 mm is used, about 1000 to 8000 A is preferable. During processing, the current value may be controlled so that a predetermined temperature can be managed by increasing or decreasing the current value while monitoring the mold material temperature.

  The current sintering is preferably performed in a state in which a raw material powder made of a lithium manganese composite oxide and a carbon powder is pressurized. As a specific method, for example, the raw material powder filled in the electron conductive container 3 may be pressurized through the punch electrodes 6 and 7. The pressure at the time of pressurizing the raw material powder is, for example, about 5 to 60 MPa, preferably about 10 to 50 MPa. When the pressure is less than 5 MPa, the bonding between the lithium manganese composite oxide and the carbon powder becomes insufficient, and when the pressure exceeds 60 MPa, the decomposition of the lithium manganese composite oxide is promoted. Usually, a pressure of about 10 to 50 MPa is suitable.

  The sintering time by electric current sintering varies depending on the amount of raw materials used, the sintering temperature, etc., and thus cannot be specified in general, but it is usually sufficient to heat until reaching the heating temperature range described above, and the temperature range described above. If it reaches | attains, you may cool immediately, or you may hold | maintain in this temperature range, for example to about 2 hours.

  After conducting the electric current sintering treatment at a predetermined temperature by the method described above, the electron conductive container is cooled, the formed composite is taken out from the container, and lightly pulverized with a mortar or the like as necessary. It is possible to recover the lithium manganese composite oxide-carbon composite. When a large amount of current sintering treatment is performed, a large mold material is used, and the above process may be scaled up.

(3) Use of lithium manganese composite oxide-carbon composite In the lithium manganese composite oxide-carbon composite obtained by the method described above, the composite oxide constituting the composite has an average valence of 3.8. It contains Mn element with a low valence of the following, and a cubic rock salt structure as the main crystal phase. This is a high Mn valence obtained by the conventional method, and has excellent discharge characteristics compared to a composite oxide having a monoclinic layered rock salt structure as the main crystal phase. The voltage shows a high value. Further, the composite is a composite oxide and carbon that are intimately bonded, and has a high density and improved electronic conductivity. Therefore, by using the composite as a positive electrode material for a lithium ion secondary battery, a lithium ion secondary battery having a high discharge capacity and a high energy density can be obtained. In particular, when the lithium manganese composite oxide constituting the composite contains about 60 to 85% by weight of the cubic rock salt type structure crystal phase, the initial discharge capacity and the initial discharge efficiency are remarkably high. By using a lithium manganese composite oxide-carbon composite having such characteristics as a positive electrode active material for a lithium ion secondary battery, the amount of negative electrode material required for initial charging can be reduced. In addition, a battery with high energy density can be manufactured.

A lithium ion secondary battery using the composite as a positive electrode active material can be produced by a known method. That is, except that the composite obtained by the method of the present invention is used as a positive electrode active material, a known metal lithium, a carbon-based material (activated carbon, graphite) or the like is used as a negative electrode material, and a known electrolyte is used as an electrolyte solution. Using a solution obtained by dissolving lithium perchlorate, lithium salt such as LiPF _{6 in} a solvent such as ethylene carbonate or dimethyl carbonate, and using other known battery components, a lithium ion secondary according to a conventional method What is necessary is just to assemble a battery.

  The lithium manganese composite oxide-carbon composite of the present invention is a composite in which a lithium manganese composite oxide and a carbon material are firmly and densely joined, and has a positive electrode active material having good electronic conductivity and high capacity. It can be used effectively as a substance. Further, when the composite is used as a positive electrode material for a lithium ion secondary battery, the initial discharge efficiency is high and the initial discharge capacity is remarkably high.

  For this reason, the lithium manganese composite oxide-carbon composite of the present invention is a highly useful substance as a positive electrode active material of a lithium ion secondary battery.

  Moreover, according to the production method of the present invention, a composite having such excellent performance can be produced relatively easily.

Schematic of an example of an electric current sintering apparatus. 2 is an X-ray diffraction pattern of samples obtained in Example 1 and Comparative Example 1. FIG. 2 is an XANES spectrum of Ti of samples obtained in Example 1 and Comparative Example 1. FIG. It is a graph which shows the charging / discharging characteristic of the lithium ion secondary battery which uses the sample obtained in Example 1 and Comparative Example 1 as a positive electrode active material. 2 is an X-ray diffraction pattern of samples obtained in Example 2 and Comparative Example 2. FIG. 2 is an XANES spectrum of Ti of samples obtained in Example 2 and Comparative Example 2. FIG. It is a graph which shows the charging / discharging characteristic of the lithium ion secondary battery which uses the sample obtained in Example 2 and Comparative Example 2 as a positive electrode active material.

1 Electric current sintering equipment 2 Sample 3 Die (conductive container)
4, 5 Punch 6, 7 Punch electrode 8 Water-cooled vacuum chamber 9 Cooling water channel 10, 16 Water cooling mechanism 11 Power source for sintering 12 Controller 13 Pressurizing mechanism 14 Position measuring mechanism 15 Atmosphere controlling mechanism 17 Temperature measuring apparatus

  The present invention will be specifically described below with reference to examples and comparative examples.

Example 1
Chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ lithium manganese complex oxide (average particle size 0.63 μm) and acetylene black (average particle size 0.02 μm) in a weight ratio of 98 : 2 was weighed, placed in a zirconia pot, and mixed with a planetary ball mill at 200 rpm for 30 minutes. This was filled in a graphite container having an inner diameter of 15 mm, set in a chamber of an electric sintering machine SPS-515S (manufactured by SPS Syntex Co., Ltd.), and then the pressure in the chamber was reduced to about 20 Pa.

  Thereafter, the inside of the chamber was filled with nitrogen gas to atmospheric pressure, and the raw material powder filled in the graphite container was pressurized at about 30 MPa. Further, a DC pulse current of about 650 A (pulse width 2.5 milliseconds, period 30 Hz) was applied to the graphite jig, and the vicinity of the sample was heated at about 10 ° C./min. After the temperature reached 650 ° C., the temperature was maintained for 5 minutes, and then the applied current and pressurization were stopped, and the graphite container was allowed to cool naturally.

  After cooling to near room temperature, the sample was taken out from the graphite container, ground in a mortar, stirred in distilled water for about 2 hours, filtered and dried to obtain a lithium manganese composite oxide / acetylene black composite.

The lithium manganese composite oxide represented by the chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ used as a raw material was prepared as follows.

  First, a mixed aqueous solution in which manganese chloride (II) and titanium sulfate (III) were completely dissolved was prepared. Separately, an aqueous ethanol solution of lithium hydroxide was prepared, placed in a titanium beaker, cooled to −10 ° C. and stirred. The metal salt aqueous solution was gradually added dropwise to the lithium hydroxide ethanol aqueous solution over several hours to form a Ti-Mn precipitate. After confirming that the reaction solution was completely alkaline (pH 11 or more), the reaction solution containing the coprecipitate was blown with air at room temperature for 1 day under stirring to ripen the precipitate.

The resulting precipitate was washed with distilled water and filtered, and lithium hydroxide was added to the precipitated product and stirred well. This solution was dried at 100 ° C., and the obtained powder was pulverized, followed by firing at 850 ° C. in the atmosphere for 20 hours. Thereafter, the fired powder is washed with distilled water, filtered, and dried at 100 ° C., and then a lithium manganese composite represented by the chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ An oxide was obtained.

Comparative Example 1
The lithium manganese-based composite oxide represented by the chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ and acetylene black (AB) used in Example 1 were weighed at a weight ratio of 98: 2, and this was measured. Was placed in a zirconia pot and mixed with a planetary ball mill at 200 rpm for 30 minutes to obtain a sample.

< Measurement of crystal phase ratio and lattice constant >
FIG. 2 shows X-ray diffraction patterns of the samples obtained in Example 1 and Comparative Example 1. The sample obtained in Comparative Example 1 has a monoclinic layered rock salt type structure, whereas the sample obtained in Example 1 has a Li-Mn mixed layer remarkably observed at around 20-30 ^° C. The superlattice peak derived from the regular arrangement disappeared, and the intensity of the peak derived only from the monoclinic layered rock salt structure decreased.

Rietveld analysis was performed on these X-ray diffraction patterns by the analysis program RIETAN-2000 (F. Izumi and T. Ikeda, Mater. Sci. Forum, vol. 321-224 p.198-203 (2000).) The crystallographic parameters shown in Table 1 were calculated. In the composite of Example 1, the lattice constant was larger than that of the original Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ , suggesting that the reduction was progressing. In Comparative Example 1, it was found that about 98% of the crystal phase of the layered rock salt type Li ₂ MnO ₃ was present.

< Measurement of chemical composition and valence of transition metal cation >
The chemical composition of the sample was determined using ICP emission analysis. In addition, the average valence of Mn and Ti was determined by the iodometric titration method, and the valence of Ti was determined from the rise of the XANES spectrum. Furthermore, the average valence of Mn was calculated from these results. The results are shown in Table 2 and FIG. As is apparent from these results, it can be seen that in the sample of Example 1, the amount of Li was reduced and Mn was reduced by the electric current sintering treatment. It can also be seen that the Ti valence remains +4 with little change.

< Measurement of charge / discharge characteristics >
A charge / discharge test was performed on each sample obtained in Example 1 and Comparative Example 1. The battery configuration and charge / discharge test conditions are as follows.

A sample obtained by mixing 5 mg of the sample obtained in Example 1 or Comparative Example 1 with 5 mg of acetylene black and 0.5 mg of PTFE and press-bonding it on an Al mesh was used as a positive electrode material. As the negative electrode material, metallic lithium was used, and as the electrolytic solution, LiPF ₆ dissolved in EC + DMC solvent was used.

  The test temperature was 30 ° C., the current density (per active material) was 40 mA / g, the potential range was 1.5 to 4.8 V, and charging was constant current-constant voltage charging (until it dropped to 10 mA / g).

  The results are shown in FIG. As is clear from this result, the sample obtained in Example 1 has a higher initial charge capacity, initial discharge capacity, initial charge / discharge efficiency, and initial average discharge voltage than the sample obtained in Comparative Example 1. It was confirmed that the charge / discharge characteristics were excellent.

  As apparent from the above results, the sample obtained in Example 1 had a high initial discharge capacity and initial discharge efficiency, and a high average discharge voltage.

< Measurement of bonding strength >
For each sample obtained in Example 1 and Comparative Example 1, the bonding state with carbon was examined by the following method. First, 0.5 g of each sample powder is placed in a 100 mL beaker with 50 mL of a water / ethanol mixed solution (volume ratio 1: 1), and a rotor with a length of 3 cm and a central cross-sectional diameter of 5 mm is rotated at 600 rpm for 5 minutes. And then allowed to stand. The samples of Example 1 all settled, no suspended matter was observed, and it was found that the composite was firmly joined. On the other hand, about the sample of the comparative example 1, the black floating thing was recognized and it turned out that joining of carbon is inadequate.

Example 2
In the method for producing a lithium manganese composite oxide represented by the chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ used as a raw material in Example 1, firing in the atmosphere is performed at 850 ° C., 1 A lithium manganese composite oxide was produced in the same manner as in Example 1 except that the time was changed to minutes (average particle size: 22.3 μm). Using this composite oxide, current sintering was performed in the same manner as in Example 1 to obtain a composite of lithium manganese composite oxide and acetylene black.

Comparative Example 2
Weight ratio of lithium manganese based composite oxide represented by the chemical formula: Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ and acetylene black (average particle size 0.02 μm) prepared in the same manner as in Example 2. The sample was weighed 98: 2 and placed in a zirconia pot and mixed with a planetary ball mill at 200 rpm for 30 minutes to obtain a sample.

< Crystal phase ratio and lattice constant >
FIG. 5 shows X-ray diffraction patterns of the samples obtained in Example 2 and Comparative Example 2. In the sample obtained in Example 2, only a crystal phase of cubic rock salt type α-LiFeO ₂ and a peak derived from carbon were observed, and a composite of lithium manganese composite oxide having a cubic rock salt crystal structure and acetylene black It can be seen that On the other hand, the sample obtained in Comparative Example 1 has a peak (▼) derived only from the crystal phase of the layered rock salt type Li ₂ MnO ₃ and coexists with the crystal phase of the cubic rock salt type α-LiFeO ₂ . It is clear.

Rietveld analysis was performed on these X-ray diffraction patterns by the analysis program RIETAN-2000 (F. Izumi and T. Ikeda, Mater. Sci. Forum, vol. 321-224 p.198-203 (2000).) The crystallographic parameters shown in Table 4 were calculated. In the sample of Example 2, the lattice constant increased from that of the original Li _{1 + x} (Mn _0.5 Ti _0.5 ) _1-x O ₂ , suggesting that the reduction was progressing. In the sample of Comparative Example 2, it was found that about 79% of the layered rock salt type crystal phase was present.

< Measurement of chemical composition and valence of transition metal cation >
The chemical composition of the sample was determined using ICP emission analysis. In addition, the average valence of Mn and Ti was determined by the iodometric titration method, and the valence of Ti was determined from the rise of the XANES spectrum. Furthermore, the average valence of Mn was calculated from these results. The results are shown in Table 5 and FIG. As is clear from these results, it can be seen that the sample obtained in Example 2 was reduced in the amount of Li and reduced in Mn by the electric current sintering treatment. It can also be seen that the Ti valence remains +4 with little change.

< Measurement of charge / discharge characteristics >
Using the samples obtained in Example 2 and Comparative Example 2, a charge / discharge test was performed in the same manner as in Example 1 and Comparative Example 1. The test temperature is 30 ℃, the current density (per active material) is 40mA / g, the potential range is 1.5-4.8V, and the initial charge is constant current-constant voltage charge (up to 10mA / g) up to 5.0V. Otherwise, constant current charging was used.

  The results are shown in FIG. As is clear from these results, the sample obtained in Example 2 has both an initial average discharge voltage and an average discharge voltage after 20 cycles higher than those of the sample obtained in Comparative Example 2, and has a high discharge capacity. I understand that. Moreover, although the sample obtained in Example 2 has a lower initial charge / discharge capacity than that of the sample obtained in Comparative Example 2, the initial discharge efficiency is high, and the discharge capacity retention rate and the average discharge voltage after 20 cycles. Were higher than the sample obtained in Comparative Example 2.

< Measurement of bonding strength >
Each sample obtained in Example 2 and Comparative Example 2 was examined for a bonding state with carbon. 0.5g of each sample powder is put into a 100mL beaker with 50mL of water / ethanol mixed solution (volume ratio 1: 1), and a rotor with a length of 3cm and a central cross-sectional diameter of 5mm is rotated at 600 rpm and stirred for 5 minutes. Then, it was allowed to stand. All the samples of Example 2 settled, no suspended matter was observed, and it was found that the composite was firmly joined. On the other hand, about the sample of the comparative example 2, the black floating thing was recognized and it turned out that joining of carbon is inadequate.

Claims (7)

Composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (where -1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) In which the average valence of manganese element in the lithium manganese composite oxide is 3.8 or less, and the lithium manganese composite oxide is a crystal having a cubic rock salt structure. A lithium manganese based composite oxide-carbon composite containing a phase. The ratio of the cubic rock salt structure crystal phase to the layered rock salt structure crystal phase in the lithium manganese composite oxide is cubic rock salt structure crystal phase: layered rock salt structure crystal phase = 60: 40 to 85:15 Item 2. The lithium manganese composite oxide-carbon composite according to Item 1. 3. The lithium manganese composite oxide-carbon composite according to claim 1, wherein the carbon ratio is 0.001 to 30 wt% based on the total amount of the lithium manganese composite oxide and carbon. Composition formula: Li _{1 + x} (Mn _1-y Ti _y ) _1-x O ₂ (-1/3 <x <1/3, 0.4 ≦ y ≦ 0.6) Lithium manganese complex oxide and carbon The lithium manganese based composite oxide according to any one of claims 1 to 3, wherein the powder is filled in a conductive container, and a direct current pulse current is applied to cause a heating reaction in a non-oxidizing atmosphere. -Manufacturing method of carbon composite. The method according to claim 4, wherein a direct current pulse current is applied under pressure. The positive electrode active material for lithium ion secondary batteries which consists of a lithium manganese type complex oxide-carbon composite in any one of Claims 1-3. A lithium ion secondary battery comprising the positive electrode active material according to claim 6 as a constituent element.

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