JP2017152239A - Negative electrode active material, negative electrode using the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material having excellent cycle characteristics, and also to provide a negative electrode using the same, and a lithium ion secondary battery.SOLUTION: Disclosed is a negative electrode active material which contains a first chemical compound composed of Fe and O and a second chemical compound composed of Li, Fe and O. The first chemical compound is FeOand the second chemical compound is LiFeO.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode active material, a negative electrode using the negative electrode active material, and a lithium ion secondary battery.

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

  The capacity of the lithium ion secondary battery mainly depends on the active material of the electrode. In general, graphite is used as the negative electrode active material. However, the theoretical capacity of graphite is 372 mAh / g, and a battery of about 350 mAh / g has already been used in a battery that has been put into practical use. Therefore, in order to obtain a non-aqueous electrolyte secondary battery having a sufficient capacity as an energy source for future high-performance portable devices, it is necessary to further increase the capacity. A negative electrode material is required.

An example of such a negative electrode active material is iron oxide (an oxide containing iron as a constituent element). These iron oxides can occlude and release lithium ions electrochemically, and can charge and discharge with a very large capacity compared to graphite. For example, it is known that the discharge capacity of LiFeO ₂ is twice as high as that of graphite. (Patent Documents 1 to 3)

JP 2012-28264 A JP 2014-2847 A International Publication No. 2011/125202

  However, the iron oxides disclosed in the above prior art cannot be said to have excellent cycle characteristics.

  The present invention was created under such circumstances, and an object thereof is to provide a negative electrode active material having excellent cycle characteristics, a negative electrode using the negative electrode active material, and a lithium ion secondary battery.

In order to achieve the above object, a negative electrode active material according to the present invention contains a first compound represented by Fe ₂ O ₃ and a second compound represented by LiFeO _2. It is a substance.

According to this configuration, cycle characteristics are improved. The presence of LiFeO ₂ , which is a compound containing Li in the negative electrode, is expected to suppress expansion and contraction due to charge / discharge in the entire negative electrode active material and improve cycle characteristics. LiFeO ₂ is composed of Li, Fe, and O, and LiFeO ₂ has the highest Li content among the stable compounds. Therefore, it is presumed that it contributes to the improvement of cycle characteristics.

A second compound represented by LiFeO ₂ is, α-LiFeO _2, is preferably a _gamma-LiFeO 2, at least one selected from the group consisting of orth-LiFeO _2.

  Such a configuration further improves the cycle characteristics.

First compound represented by Fe ₂ O ₃ is preferably a α over _Fe 2 _{O 3.}

  Such a configuration further improves the cycle characteristics.

To the total amount of Fe ₂ O ₃ and _{LiFeO 2, LiFeO 2 / (Fe} 2 O 3 + LiFeO 2) the mass ratio of the content of LiFeO ₂ = x that is represented by 0.007 ≦ x ≦ 0.196 preferable.

It is presumed that by controlling the LiFeO ₂ content within the above range with respect to the total amount of Fe ₂ O ₃ and LiFeO _2, the amount of expansion / contraction during charge / discharge is reduced, and the cycle characteristics are particularly improved.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material excellent in cycling characteristics, the negative electrode using the same, and a lithium ion secondary battery can be provided.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Lithium ion secondary battery)
FIG. 1 shows a cross-sectional view of a configuration of a lithium ion secondary battery 100 according to the present embodiment. The lithium ion secondary battery 100 includes an outer body 50, a positive electrode 10 and a negative electrode 20 provided inside the outer body, and an electrode body formed by being stacked via a separator 18 disposed therebetween. 30 and a non-aqueous electrolyte containing an electrolyte, and the separator 18 holds the non-aqueous electrolyte, which is a lithium ion transfer medium between the positive and negative electrodes during charge and discharge. Furthermore, one end is electrically connected to the negative electrode 20 and the other end protrudes outside the exterior body, and one end is electrically connected to the positive electrode 10. The other end is provided with a positive electrode lead 60 protruding outside the exterior body.

  There is no restriction | limiting in particular as a shape of a lithium ion secondary battery, For example, any, such as a cylindrical type, a square type, a coin type, a flat type, a laminate film type, may be sufficient. In this embodiment, a laminate film is used as the outer package 50, and in the following examples, an aluminum laminate film type battery is produced and evaluated.

  The positive electrode 10 includes a positive electrode active material layer 14 containing a positive electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the positive electrode current collector 12. The negative electrode 20 includes a negative electrode active material layer 24 containing a negative electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the negative electrode current collector 22. Yes.

(Negative electrode)
The negative electrode active material layer 24 formed on the negative electrode 20 of the present embodiment contains a negative electrode active material, a binder, and a conductive additive.

  For the negative electrode active material layer 24, a paint containing a negative electrode active material, a binder, a conductive additive and a solvent is applied on the negative electrode current collector 22, and the solvent in the paint applied on the negative electrode current collector 22 is removed. Can be manufactured.

(Negative electrode active material)
The negative electrode active material according to this embodiment includes a first compound represented by Fe ₂ O ₃ and a second compound represented by LiFeO ₂ .

The presence of LiFeO ₂ , which is a compound containing Li in the negative electrode, is expected to suppress expansion and contraction due to charge / discharge in the entire negative electrode active material and improve cycle characteristics. LiFeO ₂ is composed of Li, Fe, and O, and LiFeO ₂ has the highest Li content among the stable compounds. Therefore, it is presumed that it contributes to the improvement of cycle characteristics.

Furthermore, the second compound according to this embodiment is preferably at least one selected from the group of α-LiFeO ₂ , γ-LiFeO ₂ , and ortho-LiFeO ₂ . This improves the cycle characteristics.

Furthermore, the first compound according to this embodiment is preferably α-Fe ₂ O ₃ . This improves the cycle characteristics.

To the total amount of the _Fe 2 _{O 3} and _{LiFeO 2, LiFeO 2 / (Fe} 2 O 3 + LiFeO 2) the mass ratio of the content of LiFeO ₂ = x that is represented by 0.007 ≦ x ≦ 0.196 Is preferred.

It is presumed that by controlling the LiFeO ₂ content within the above range with respect to the total amount of Fe ₂ O ₃ and LiFeO _2, the amount of expansion / contraction during charge / discharge is reduced, and the cycle characteristics are improved.

(Method for producing negative electrode active material)
The negative electrode active material according to the present embodiment can be produced by producing and mixing the first compound and the second compound, respectively.

(Method for producing first compound)
The first compound according to this embodiment can be synthesized by the following method.

  The first compound can be obtained using various synthesis methods such as a solid phase reaction method and a hydrothermal synthesis method. In particular, it is preferable to use one obtained by a hydrothermal synthesis method.

  The first compound can be produced by the following hydrothermal synthesis method. In the hydrothermal synthesis method, first, an iron source, water, and a neutralizing agent are introduced into a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside to prepare a mixture in which these are dispersed. .

  As an iron source used as a raw material for the first compound, iron chloride, iron sulfate, iron nitrate, iron citrate, iron phosphate, and the like can be used. These hydrates may also be used.

  Furthermore, these 1 type or 2 types or more used for the raw material of a 1st compound can be mixed and used.

  As the neutralizing agent, monoethanolamine, triethanolamine, urea or the like can be used, and it is preferable to add a large excess to the anion of the iron source in order to suppress the generation of acid in the aqueous solution after the reaction.

  At this time, it is desirable to add alcohol. As a result, it is possible to reduce the variation in the obtained particle diameter, and it becomes easy to obtain the targeted average particle diameter.

  The alcohol is preferably a polyol, and more preferably selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, and glycerin. The concentration is preferably 0.1 to 2M.

  In the hydrothermal synthesis method, a hydrothermal reaction is advanced in the mixture by heating the mixture in a sealed reactor while applying pressure. Thereby, the precursor of the compound comprised by Fe and O which is a negative electrode active material in this embodiment is hydrothermally synthesized. In addition, what is necessary is just to adjust suitably the time which heats a mixture, pressurizing according to the quantity of a mixture.

  Thus, in the hydrothermal synthesis method, the mixture is heated at 100 ° C. or higher under pressure. The pressure applied to the mixture is preferably 1.0 to 5.0 MPa. If the pressure applied to the mixture is too low, it will be difficult to obtain a precursor of the first compound.

The first compound precursor is then filtered and dried. The precursor is fired in the range of 270 to 300 ° C. By such a process, Fe ₂ O _{3 as} the first compound is obtained.

(Method for producing second compound)
The second compound according to this embodiment can be synthesized by the following method.

  The second compound can be obtained using various synthesis methods such as a solid phase reaction method and a hydrothermal synthesis method. In particular, it is preferable to use one obtained by a hydrothermal synthesis method.

  The second compound can be produced by the following hydrothermal synthesis method. In the hydrothermal synthesis method, first, a lithium source, an iron source, water, and a neutralizing agent are charged into a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside, and a mixture in which these are dispersed. To prepare.

  As the lithium source used as the raw material for the second compound, lithium carbonate, lithium hydroxide, lithium chloride, lithium sulfate, lithium nitrate, lithium oxalate, or the like can be used.

  As an iron source used as a raw material for the second compound, iron chloride, iron sulfate, iron nitrate, iron citrate, iron phosphate, and the like can be used. These hydrates may also be used.

  Furthermore, the lithium source and the iron source used for the raw material of the second compound can be used alone or in combination of two or more.

  As the neutralizing agent, monoethanolamine, triethanolamine, urea, etc. can be used, and a large excess is added to the anion of the lithium source and the iron source in order to suppress the generation of acid in the aqueous solution after the reaction. Is preferred.

In the hydrothermal synthesis method, a hydrothermal reaction is advanced in the mixture by heating the mixture in a sealed reactor while applying pressure. Thus, precursor of the compound represented by LiFeO ₂ is a negative electrode active material in the present embodiment is hydrothermal synthesis. In addition, what is necessary is just to adjust suitably the time which heats a mixture, pressurizing according to the quantity of a mixture.

  Thus, in the hydrothermal synthesis method, the mixture is heated at 100 ° C. or higher under pressure. The pressure applied to the mixture is preferably 1.0 to 5.0 MPa. If the pressure applied to the mixture is too low, a precursor of a compound composed of Li, Fe and O cannot be obtained.

  Next, the precursor obtained in the hydrothermal synthesis step is filtered and dried. The precursor is fired in the range of 300 to 700 ° C. Such a process yields a second compound.

  Moreover, it is compoundable also by using a 1st compound as an iron source used for the raw material of a 2nd compound.

  As a procedure for synthesizing the second compound, first, the above-described Li source and the first compound are stirred, for example, in water by a ball mill.

  A slurry prepared by stirring the Li source and the first compound with a ball mill is dried and pulverized, and then fired in the range of 300 to 900 ° C. A necessary oxygen concentration may be appropriately selected for the firing atmosphere. By such a process, a compound composed of Li, Fe and O is obtained. In addition, what is necessary is just to select the holding time of baking suitably in the range of 1 to 5 hours.

  The obtained second compound is preferably wet pulverized using a ball mill or the like. Thereby, the particle diameter of the second compound can be adjusted. In addition, what is necessary is just to select a grinding | pulverization time suitably in the range of 1 to 24 hours.

  In addition, you may synthesize | combine the 2nd compound particle concerning this embodiment using another liquid phase method or a solid-phase synthesis method.

  The negative electrode active material according to this embodiment can be obtained by mixing the first compound and the second compound synthesized by the above method.

  The crystal structure of the first compound is preferably a trigonal crystal. The second compound is preferably tetragonal, cubic, or orthorhombic.

  The average particle size of the first compound is preferably 0.05 μm or more and 1 μm or less. The second compound is preferably 0.02 μm or more and 0.2 μm or less.

  The first compound and the second compound are preferably in a mixed state.

To the total amount of LiFeO ₂ is _Fe 2 _{O 3} and the second compound is a first _{compound, LiFeO 2 / (Fe 2 O} 3 + LiFeO 2) the mass ratio of the content of LiFeO ₂ = x 0.007 It is preferable that ≦ x ≦ 0.196.

  The mass ratio is adjusted by weighing the first compound and the second compound so that the desired mass ratio is obtained.

  The mass ratio of the first compound and the second compound of the obtained negative electrode active material is determined by quantitative analysis by X-ray diffraction. A calibration curve is prepared in advance using a mixture in which the mass ratio of the first compound and the second compound is changed. The obtained negative electrode active material is measured, and the mass ratio is obtained from the peak intensity ratio of the first compound and the second compound with reference to a calibration curve.

  The content of the negative electrode active material in the negative electrode active material layer 24 is preferably 50 to 95% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder, and is 75 to 93% by mass. More preferably. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(binder)
The binder binds the negative electrode active materials to the current collector 22 while bonding the negative electrode active materials to each other. A binder will not be specifically limited if the above-mentioned coupling | bonding is possible. For example, a fluororesin such as polyvinylidene fluoride (PVDF), cellulose, styrene / butadiene rubber, polyimide, polyamideimide, polyacrylic acid, polyacrylonitrile, polyalginic acid, or the like can be used.

  The content of the binder in the negative electrode active material layer 24 is preferably 1 to 30% by mass, and preferably 5 to 15% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder. Is more preferable. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(Conductive aid)
The conductive aid is not particularly limited as long as the conductivity of the negative electrode active material layer 24 is improved, and a known conductive aid can be used. Examples thereof include carbon blacks such as acetylene black, furnace black, channel black, and thermal black, vapor grown carbon fibers (VGCF), carbon fibers such as carbon nanotubes, and carbon materials such as graphite. More than seeds can be used.

  The content of the conductive auxiliary in the negative electrode active material layer 24 is not particularly limited, but when added, it is usually 1 to 10% by mass based on the sum of the mass of the negative electrode active material, conductive auxiliary and binder. Preferably there is.

(solvent)
The solvent is not particularly limited as long as it can form the above-described negative electrode active material, conductive additive, and binder. For example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, and the like can be used. .

(Negative electrode current collector)
The negative electrode current collector 22 is preferably a conductive plate material having a small thickness, and is preferably a metal foil having a thickness of 8 to 30 μm. The negative electrode current collector 22 is preferably formed of a material that does not alloy with lithium, and a particularly preferable material is copper. Examples of such copper foil include electrolytic copper foil. For example, an electrolytic copper foil is prepared by immersing a metal drum in an electrolytic solution in which copper ions are dissolved, and flowing current while rotating the copper drum, thereby depositing copper on the surface of the drum and peeling it off. It is the obtained copper foil.

  Moreover, the rolled copper foil manufactured by rolling the cast copper lump to desired thickness may be sufficient, and copper is deposited on the surface of the rolled copper foil by an electrolytic method, and the surface is roughened. There may be.

  Thus, there is no restriction | limiting in particular as an application | coating method which apply | coats the coating material containing a negative electrode active material, a binder, a conductive support agent, and a solvent mentioned above, The method employ | adopted when producing an electrode normally can be used. . Examples thereof include a slit die coating method and a doctor blade method.

  The method for removing the solvent in the paint applied on the negative electrode current collector 22 is not particularly limited, and the negative electrode current collector 22 applied with the paint may be dried at, for example, 80 ° C. to 150 ° C.

  Then, the negative electrode 20 on which the negative electrode active material layer 24 is formed in this way may be subsequently subjected to a press treatment using, for example, a roll press device as necessary. The linear pressure of the roll press can be set to 100 to 5000 kgf / cm, for example.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution has an electrolyte dissolved in a nonaqueous solvent, and may contain a cyclic carbonate and a chain carbonate as a nonaqueous solvent.

  The cyclic carbonate is not particularly limited as long as it can solvate the electrolyte, and a known cyclic carbonate can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate is not particularly limited as long as the viscosity of the cyclic carbonate can be reduced, and a known chain carbonate can be used. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like may be mixed and used.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ , CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ Lithium salts such as CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of conductivity.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

(Positive electrode)
The positive electrode 10 of the present embodiment has a structure in which a positive electrode active material layer 14 containing a positive electrode active material is formed on one surface or both surfaces of a positive electrode current collector 12. The positive electrode active material layer 14 is coated on the positive electrode current collector 12 with a paint containing the positive electrode active material, a binder, a conductive additive, and a solvent in the same process as the negative electrode manufacturing method. It can be produced by removing the solvent in the applied paint.

Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions of the lithium ions (for example, ClO ₄ ⁻ ). Is not particularly limited as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) Examples thereof include composite metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), and LiMPO ₄ (wherein M represents at least one selected from Co, Ni, Mn, Fe, or VO).

  In addition, oxides and sulfides capable of inserting and extracting lithium ions can be used as the positive electrode active material.

  Furthermore, each component (conductive auxiliary agent and binder) other than the positive electrode active material can be the same material as that used in the negative electrode 20.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. For example, a metal foil such as aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used, and an aluminum foil is particularly preferable.

(Separator)
As long as the separator 18 is formed of an insulating porous body, the material, the manufacturing method, and the like are not particularly limited, and a known separator used for a lithium ion secondary battery can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

  The outer package 50 is not particularly limited as long as it can suppress leakage of the nonaqueous electrolyte solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. A film etc. can be used. The aluminum laminate film has a three-layer structure in which, for example, polypropylene, aluminum, and nylon are laminated in this order.

  The negative electrode lead 62 and the positive electrode lead 60 may be formed of a conductive material such as aluminum or nickel.

  As mentioned above, although the example of this invention was described in detail by embodiment, this invention is not limited to the said embodiment, A various deformation | transformation is possible. For example, in the above embodiment, a lithium ion secondary battery having a laminate film structure has been described. However, the present invention also applies to a lithium ion secondary battery having a structure in which a positive electrode and a negative electrode are folded or stacked. Can be applied. Furthermore, the present invention can also be suitably applied to lithium ion secondary batteries such as coin type, square type, and flat type.

  The produced lithium ion secondary battery was evaluated by the following method.

上記の条件によって充放電を繰り返し、１００サイクル後の容量維持率によって充放電サイクル特性を評価した。 (Measurement of cycle characteristics)
Using a secondary battery charge / discharge measurement device, charge / discharge cycle characteristics were evaluated under the conditions of charging at a current value of 0.5C and discharging at a current value of 0.5C. The charge / discharge cycle characteristics are evaluated as capacity retention rate (%), and the capacity retention rate (%) is the ratio of the discharge capacity at each cycle number to the initial discharge capacity, with the discharge capacity at the first cycle being the initial discharge capacity. Yes, expressed by the following formula (1).

Charging / discharging was repeated under the above conditions, and charging / discharging cycle characteristics were evaluated based on a capacity retention rate after 100 cycles.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

(Comparative Example 1)
(Preparation of the first compound)
The first compound represented by Fe ₂ O ₃ was produced by a hydrothermal synthesis method. The production procedure will be described in detail. First, an iron aqueous solution was prepared so that iron chloride hydrate was 0.4 M, urea was 1.5 M, and ethylene glycol was 0.5 M. Next, this iron aqueous solution was heated at 100 ° C. for 5 hours while applying a pressure of 2 MPa in a sealed reactor.

The obtained precursor of the compound represented by Fe ₂ O ₃ was filtered and washed. Next, this precursor was calcined at 300 ° C. for 3 hours to obtain iron oxide having an average particle size of 0.1 μm.

When the resulting iron oxide product phase was identified by an X-ray diffractometer (tube: Cu, tube voltage: 40 kV, tube current: 40 mA), an α-Fe ₂ O ₃ phase was detected.

(Preparation of second compound)
A second compound represented by LiFe ₅ O ₈ was prepared using a solid phase reaction method.

The procedure of this solid phase reaction method will be described in detail. First, lithium carbonate and α-Fe ₂ O ₃ obtained by the above hydrothermal synthesis are weighed so as to be 1: 5 mol, water is added to the raw material mixture, wet-mixed with a ball mill for 6 hours, dried, and then water Was removed.

Next, the dried product was fired at 900 ° C. for 3 hours in the air. Then, it grind | pulverized for 4 hours and obtained the compound comprised from Li, Fe, and O made into the target.

When the resulting phase of the compound composed of Li, Fe, and O was identified by an X-ray diffractometer (tube: Cu, tube voltage: 40 kV, tube current: 40 mA), a LiFe ₅ O ₈ phase was detected. .

(Preparation of negative electrode active material)
The obtained first compound α-Fe ₂ O ₃ and the second compound LiFe ₅ O ₈ were mixed so that the mass ratio was x = 0.007.

(Preparation of negative electrode)
85 parts by mass of the negative electrode active material prepared as described above, 5 parts by mass of ketjen black as a conductive auxiliary agent, and 10 parts by mass of polyamideimide as a binder were mixed to obtain a negative electrode mixture. Subsequently, the negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture paint. This paint was applied to one surface of an electrolytic copper foil having a thickness of 10 μm so that the amount of the negative electrode active material applied was 2.5 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. Then, it pressure-molded with the roller press with the linear pressure of 2000 kgf / cm, and heat-processed in vacuum at 350 degreeC for 3 hours, and produced the negative electrode with a thickness of 70 micrometers.

  From the obtained negative electrode, when the mass ratio of the first compound and the second compound was analyzed, it was confirmed to be equal to the charging ratio.

(Preparation of positive electrode)
A positive electrode mixture was prepared by mixing 90 parts by mass of LiCoO ₂ as a positive electrode active material, 5 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of polyvinylidene fluoride as a binder. Subsequently, the positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to obtain a paste-like positive electrode mixture paint. This paint was applied to one surface of an aluminum foil having a thickness of 20 μm so that the applied amount of the positive electrode active material was 18.4 mg / cm ² and dried at 100 ° C. to form a positive electrode active material layer. Then, it pressure-molded with the roll press and produced the positive electrode with a thickness of 132 micrometers.

(Production of evaluation lithium-ion secondary battery)
The prepared negative electrode and positive electrode are laminated via a polypropylene separator having a thickness of 16 μm, and three negative electrodes and two positive electrodes are laminated via four separators so that the negative electrode and the positive electrode are alternately laminated. Thus, a laminate was produced. Furthermore, in the negative electrode of the laminated body, a negative electrode lead made of nickel is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer. On the other hand, in the positive electrode of the laminated body, aluminum without the positive electrode active material layer An aluminum positive electrode lead was attached to the protruding end of the foil by an ultrasonic welding machine. Then, this laminated body is inserted into an aluminum laminate film outer package and heat sealed except for one place around it to form a closed portion, and EC / DEC is blended in a ratio of 3: 7 in the outer package. After injecting a non-aqueous electrolyte solution to which 1M (mol / L) LiPF ₆ as a lithium salt was added into the solvent, the remaining one part was sealed with a heat seal while reducing the pressure with a vacuum sealer. The lithium ion secondary battery which concerns on No. 1 was produced, and cycling characteristics were evaluated. The results are shown in Table 1.

(Comparative Example 2)
(Preparation of the first compound)
Commercially available Fe ₃ O ₄ (manufactured by SIGMA-ALDRICH) was used.

(Preparation of second compound)
A second compound represented by LiFeO ₂ was prepared using a solid phase reaction method.

The procedure of this solid phase reaction method will be described in detail. First, lithium carbonate and α-Fe ₂ O ₃ obtained by the hydrothermal synthesis were weighed so as to be equimolar, water was added to the raw material mixture, wet-mixed with a ball mill for 6 hours, and then dried to remove water. .

  Next, the dried product was fired at 900 ° C. for 3 hours in the air. Then, it grind | pulverized for 4 hours and obtained the compound comprised from Li, Fe, and O made into the target.

When the product phase of the compound composed of Li, Fe, and O obtained was identified by an X-ray diffractometer (tube: Cu, tube voltage: 40 kV, tube current: 40 mA), α-LiFeO ₂ phase was detected. .

A negative electrode was produced in the same manner as in Comparative Example 1 using a mixture of the obtained first compound Fe ₃ O ₄ and the second compound α-LiFeO _{2 so as to have} a mass ratio x = 0.007. .

  A lithium ion secondary battery of Comparative Example 2 was produced in the same manner as Comparative Example 1 except that the obtained negative electrode was used.

Example 1
Similar to Comparative Example 1 using a mixture of the first compound obtained in Comparative Example 1 and the second compound obtained in Comparative Example 2 so as to have a mass ratio x = 0.007. A negative electrode was prepared.

  A lithium ion secondary battery of Example 1 was produced in the same manner as Comparative Example 1 except that the obtained negative electrode was used.

(Example 2)
(Preparation of second compound)
The procedure of this solid phase reaction method will be described in detail. First, lithium carbonate and α-Fe ₂ O ₃ obtained by the hydrothermal synthesis were weighed so as to be equimolar, water was added to the raw material mixture, wet-mixed with a ball mill for 6 hours, and then dried to remove water. .

  Next, the dried product was fired at 600 ° C. for 3 hours in the air. Then, it grind | pulverized for 4 hours and obtained the compound comprised from Li, Fe, and O made into the target.

When the resulting phase of the compound composed of Li, Fe and O was identified by an X-ray diffractometer (tube: Cu, tube voltage: 40 kV, tube current: 40 mA), a γ-LiFeO ₂ phase was detected. .

The same as in Comparative Example 1 except that the first compound obtained in Comparative Example 1 and the obtained second compound γ-LiFeO ₂ were mixed so that the mass ratio x = 0.007 was used as the negative electrode active material. A negative electrode was prepared.

  A lithium ion secondary battery of Example 2 was produced in the same manner as Comparative Example 1 except that the obtained negative electrode was used.

(Example 3)
(Preparation of second compound)
The procedure of this solid phase reaction method will be described in detail. First, sodium carbonate and α-Fe ₂ O ₃ obtained by hydrothermal synthesis were weighed so as to be equimolar, water was added to the raw material mixture, wet mixed with a ball mill for 6 hours, and then dried to remove water. .

Next, the dried product was calcined at 800 ° C. for 3 hours in the air. Thereafter, milled for 16 hours to give the alpha-NaFeO ₂ as a target.

After the α-NaFeO ₂ and LiNO ₃ were dry-mixed for 5 hours, the mixed powder was fired at 300 ° C. for 5 hours in the atmosphere to perform ion exchange. The fired product was washed with water and dried, and then pulverized for 4 hours to obtain a target compound composed of Li, Fe, and O.

The resulting phase of the compound composed of Li, Fe and O was identified by an X-ray diffractometer (tube: Cu, tube voltage: 40 kV, tube current: 40 mA), and ortho-LiFeO ₂ phase was detected. .

As in Comparative Example 1, the first compound obtained in Comparative Example 1 and the obtained second compound ortho-LiFeO ₂ were mixed so that the mass ratio x = 0.007. A negative electrode was produced.

  A lithium ion secondary battery of Example 3 was produced in the same manner as Comparative Example 1 except that the obtained negative electrode was used.

(Examples 4 to 13)
A mixture obtained by mixing the first compound obtained in Comparative Example 1 and the second compound obtained in Example 2 so that the mass ratio x is the value of x in Examples 4 to 13 in Table 1. A negative electrode was produced in the same manner as Comparative Example 1 as the negative electrode active material.

  Lithium ion secondary batteries of Examples 4 to 13 were produced in the same manner as Comparative Example 1 except that the obtained negative electrode was used.

  Similarly to Comparative Example 1, from the negative electrodes obtained in Comparative Example 1 and Examples 1 to 13, when the mass ratio of the first compound and the second compound was analyzed, it was confirmed that it was equivalent to the charging ratio.

The effect of the present invention is clear from the results shown in Table 1. That express effect excellent in cycle characteristics by containing a second compound represented by the first compound and LiFeO ₂ represented by Fe ₂ O _3.

  The present invention can provide a negative electrode active material having excellent cycle characteristics, a negative electrode using the negative electrode active material, and a lithium ion secondary battery.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 30 ... Laminated body, 50 ... Outer body, 60 ... Positive electrode lead, 62 ... Negative electrode lead, 100 ... Lithium ion secondary battery

Claims (6)

A negative electrode active material comprising a first compound represented by Fe ₂ O ₃ and a second compound represented by LiFeO ₂ . _2. The negative electrode active material according to claim 1, wherein the second compound is at least one selected from the group of α-LiFeO ₂ , γ-LiFeO ₂ , and ortho-LiFeO ₂ . The negative electrode active material according to claim 1, wherein the first compound is α-Fe ₂ O ₃ . To the total amount of the _Fe 2 _{O 3} and _{LiFeO 2, LiFeO 2 / (Fe} 2 O 3 + LiFeO 2) the mass ratio of the content of LiFeO ₂ = x that is represented by 0.007 ≦ x ≦ 0.196 The negative electrode active material according to claim 1, wherein:   The negative electrode using the negative electrode active material as described in any one of Claims 1-4.   A lithium ion secondary battery using the negative electrode according to claim 5.

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