JP2017152379A - 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 with excellent rate characteristics, a negative electrode using the same, and a lithium ion secondary battery.SOLUTION: A negative electrode active material is a compound composed of Fe and O, and the content thereof is not less than 0.001 mas% and not more than 1.2 mass% in terms of S.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 an oxide. For example, Fe ₂ O ₃ can occlude and release lithium ions electrochemically, and can charge and discharge with a very large capacity compared to graphite. Fe ₂ O _{3 has} a theoretical discharge capacity of 1005 mAh / g and is known to exhibit a capacity 2.7 times that of graphite. (Patent Documents 1 and 2)

JP 2008-204777 A JP 2011-029139 A

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

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

  In order to achieve the above object, the negative electrode active material according to the present invention is a compound composed of Fe and O, the compound containing S, and the S content is 0.001% by mass or more in terms of S. It is 1.2 mass% or less.

  According to this configuration, when the S content is 0.001% by mass or more and 1.2% by mass or less in terms of S, it is estimated that high ionic conductivity is improved and high rate characteristics are obtained.

  The compound is preferably a rod-like or plate-like particle.

  According to such a configuration, since the particles have a rod-like or plate-like shape, the Li ion diffusion path is controlled along the longitudinal direction of the rod-like particles, so that a Li diffusion path can be secured. It is presumed that the rate characteristic is improved because the Li ion diffusion path can be maintained even by discharge.

  The rod-like or plate-like particles preferably have a ratio B / A of the longest diameter B in the direction intersecting the shortest diameter A of 3 or more.

  According to such a configuration, by controlling the ratio B / A of the longest diameter B in the direction intersecting the shortest diameter A of the rod-like or plate-like particles to 3 or more, the diffusion path of Li ions becomes rod-like particles. Therefore, it is presumed that the rate characteristic is improved because the Li diffusion path can be secured and the Li ion diffusion path can be maintained even by charging and discharging.

The compound composed of Fe and O preferably has a specific surface area of 5 m ² / g or more and 30 m ² / g or less. Thereby, the rate characteristic is further improved.

The compound composed of Fe and O is preferably at least one selected from the group consisting of FeO, α-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , and Fe ₃ O ₄ . According to this configuration, the rate characteristics are further improved.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material excellent in the rate characteristic, 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. It is a schematic diagram of the rod-shaped particle which concerns on this embodiment. It is a schematic diagram of the plate-shaped particle which concerns on this 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 configuration diagram of a lithium ion secondary battery 100 according to this 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 30 formed by being stacked via a separator 18 disposed therebetween. And the separator 18 holds the non-aqueous electrolyte, which is a lithium ion moving 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 the present invention, 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.

<Embodiment 1>
(Negative electrode)
The negative electrode active material layer 24 formed on the negative electrode 20 of Embodiment 1 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 the present embodiment is a compound composed of Fe and O, and the compound contains S, and the S content is 0.001% by mass or more and 1.2% by mass or less in terms of S. It is characterized by being.

  When the S content is 0.001% by mass or more and 1.2% by mass or less in terms of S, the rate characteristics are improved. By controlling this content, it is estimated that high ionic conductivity is improved and high rate characteristics can be obtained.

  Moreover, it is more preferable that content of S which concerns on this embodiment is 0.005 mass% or more and 1.0 mass% or less.

  The compound according to this embodiment is preferably a rod-like or plate-like particle.

  Since the particles have a rod-like or plate-like shape, the Li ion diffusion path is controlled so as to be along the longitudinal direction of the rod-like or plate-like particles, so that a Li diffusion path can be ensured by charging and discharging. It is estimated that the rate characteristic is improved because the diffusion path of Li ions can be maintained.

  The rod-like or plate-like particles preferably have a ratio B / A of the longest diameter B in the direction intersecting the shortest diameter A of 3 or more.

  By controlling the ratio B / A of the longest diameter B in the direction intersecting the shortest diameter A of the rod-shaped or plate-shaped particles to 3 or more, the diffusion path of Li ions is in the longitudinal direction of the rod-shaped particles. Therefore, it is presumed that the rate characteristic is improved because the Li diffusion path can be secured and the Li ion diffusion path can be maintained even by charging and discharging.

  The compound particles are primary particles.

(Measurement of shortest diameter and longest diameter)
2 and 3 are schematic views of rod-like or plate-like particles. A SEM (scanning electron microscope) is used to observe rod-like or plate-like particles, the shortest diameter A and the longest diameter B are measured, and the ratio of B / A may be obtained. At this time, the shortest diameter A and the longest diameter B may be calculated by image processing of secondary electron images or reflected electron images observed by the SEM. Specifically, for example, the reflected electron image is binarized, and thereby the shortest diameter A and the longest diameter B are calculated.

  The value of B / A according to the present embodiment was an average value obtained by randomly extracting 50 particles.

  FIG. 2 shows a schematic diagram of rod-like particles. As shown in FIG. 2, A is the shortest diameter when the rod-like particles are viewed from the side. That is, A corresponds to the thickness of the rod-like particle shown in FIG. On the other hand, the direction in which the rod-like particles extend is defined as the longest diameter B. That is, B corresponds to the length of the rod-like particles. FIG. 3 shows a schematic diagram of the plate-like particles. As with the rod-like particles, A is the shortest diameter constituting the side surface other than the main surface. That is, it corresponds to the thickness of the plate-like particle. The longest diameter of the surfaces constituting the main surface is B. That is, it corresponds to the length of the plate-like particle.

  The longest diameter of the rod-like particles is preferably 5 μm or less, and the longest diameter of the plate-like particles is preferably 30 μm or less. Furthermore, the longest diameter of the rod-like particles is 0.5 μm or more and 5 μm or less, and the longest diameter of the plate-like particles is more preferably 3 μm or more and 30 μm or less.

  In the case of a plate-like particle, there are three sides, but B> C> A is specified in the longest order. In the case of C = A, it becomes a rod-like particle.

Furthermore, the negative electrode active material according to this embodiment preferably has a specific surface area of a compound composed of Fe and O of 5 m ² / g or more and 30 m ² / g or less. This improves the rate characteristics.

The specific surface area of the negative electrode active material according to this embodiment is more preferably 7 m ² / g to 25 m ² / g, and still more preferably 10 m ² / g to 25 m ² / g.

  The specific surface area was determined by a gas adsorption method using nitrogen gas using a specific surface area measuring device.

The compound composed of Fe and O is composed of iron (Fe) and oxygen (O), and is not particularly limited as long as it is a compound composed of Fe and O as a main skeleton. Iron oxide or lithium iron oxide is preferred. In particular, the main component is a crystal skeleton of a single metal oxide. Specifically, FeO, α-Fe2O3, γ-Fe2O3, Fe3O4
, LiFeO2, LiFe5O8 is preferably at least one selected from the group consisting of LiFeO2 and LiFe5O8. Furthermore, it is preferably at least one selected from the group consisting of FeO, α-Fe 2 O 3, γ-Fe 2 O 3 and Fe 3 O 4. This improves the rate characteristics.
<Embodiment 2>

  The negative electrode active material layer 24 formed on the negative electrode 20 of Embodiment 2 contains a negative electrode active material, graphite, a binder, and a conductive additive.

  The negative electrode active material layer 24 is formed by applying a paint containing a negative electrode active material, graphite, a binder, a conductive additive, and a solvent onto the negative electrode current collector 22, and removing the solvent in the paint applied on the negative electrode current collector 22. It can manufacture by removing.

  As the graphite according to the present embodiment, natural graphite, artificial graphite (non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, MCF (mesocarbon fiber), MCMB, etc.) and the like can be used.

  Among these, artificial graphite is preferable because it exhibits a favorable negative electrode capacity and cycle characteristics, and from the viewpoint of improving electrode density, it is more preferable to use artificial graphite mixed with natural graphite.

  These graphites preferably have a coating layer of carbon or the like on the surface. This further improves the cycle characteristics.

  The configuration of the negative electrode according to this embodiment is preferably Embodiment 2.

(Method for producing negative electrode active material)
The negative electrode active material according to this embodiment can be produced by a solid phase reaction method or a hydrothermal synthesis method.

  It is preferable to use what was obtained by the hydrothermal synthesis method as the compound comprised from Fe and O concerning this embodiment. A compound composed of Fe and O obtained by hydrothermal synthesis has a small particle size, and a negative electrode active material using the compound can have a small particle size.

  The compound composed of Fe and O according to this embodiment can be produced by a 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 the iron source, 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 (s) or 2 or more types 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. Moreover, the rod-shaped or plate-shaped particle | grain can be obtained by adding excessively.

  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 0.5-4M.

  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 from Fe and O 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 a compound composed of Fe and O.

  Next, the precursor of the compound composed of Fe and O obtained by the hydrothermal synthesis method is filtered and dried. The precursor is fired in the range of 270 to 300 ° C. By such a process, compound particles composed of Fe and O are obtained.

  The compound composed of Fe and O according to the present embodiment may be synthesized using another liquid phase method or solid phase method.

  The amount of S in the negative electrode active material according to the present embodiment can be adjusted by introducing water vapor into the furnace at a predetermined flow rate using air as a carrier gas during firing.

  In addition, S may be substituted in the negative electrode active material, and may be mixed as another particle.

  The S content can be analyzed with an infrared absorption carbon / sulfur analyzer.

  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 include composite metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), and LiMPO ₄ (wherein M represents 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 prevent leakage of the electrolytic solution to the outside or entry of moisture or the like into the lithium ion secondary battery 100 from the outside, such as a metal can, an aluminum laminate film, or the like 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.

  About the produced lithium ion secondary battery, the rate characteristic was evaluated with the following method.

このレート特性が高いほど、特性が良好であることを意味する。実施例および比較例で作製したリチウムイオン二次電池は、上記の条件によってレート特性を評価した。 (Measurement of rate characteristics)
Using a secondary battery charge / discharge test apparatus, the battery was charged at a current value of 0.1 C when the voltage range was 4.0 V to 1.0 V, and 1 C = 1200 mAh / g per negative electrode active material weight. Charging at a current value of 0.5 C with respect to the discharge capacity when discharged at a current value of 1 C, the ratio of the discharge capacity when discharging at a current value of 1 C was defined as a rate characteristic. The rate characteristic is expressed by the following formula (1).

Higher rate characteristics mean better characteristics. The lithium ion secondary batteries fabricated in the examples and comparative examples were evaluated for rate characteristics under the above conditions.

  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.

[Experimental Example 1]
(Preparation of negative electrode active material)
The compounds composed of Fe and O in Table 1 were prepared using a hydrothermal synthesis method.

  The procedure of this hydrothermal synthesis method will be described in detail. First, an iron aqueous solution was prepared such that the iron chloride hydrate was 0.5M, the iron sulfate hydrate was 0.1M, the urea was 1.5M, and the ethylene glycol was 0.5M. Next, this iron aqueous solution was heated at 100 ° C. for 5 hours while applying a pressure of 2 MPa in a sealed reactor to obtain a precursor of a compound composed of Fe and O.

  The obtained precursor of the compound composed of Fe and O was filtered and washed. Next, this precursor was calcined at 300 ° C. for 3 hours to obtain a compound composed of Fe and O. At that time, 3 L / min. Using air (Air) as a carrier gas. Steam was introduced into the furnace at a flow rate of

  The dew point of water vapor is as follows: Comparative Example 1 has a dew point of 95 ° C., Comparative Example 2 has a temperature of 90 ° C., Example 1 has a temperature of 85 ° C., Example 2 has a temperature of 80 ° C., Example 3 has a temperature of 75 ° C., and Example 4 has a temperature of 70 ° C. 5 is 60 ° C, Example 6 is 55 ° C, Example 7 is 50 ° C, Example 8 is 45 ° C, Example 9 is 40 ° C, Comparative Example 3 is 35 ° C, and Comparative Example 4 is 30 ° C. Each S content was adjusted.

The resulting phase of the compound composed of Fe and O was identified by an X-ray diffractometer (tube ball: Cu, tube voltage: 40 kV, tube current: 40 mA). In each of Comparative Examples 1 to 4 and Examples 1 to 5, an α-Fe ₂ O ₃ phase was detected.

  The amount of S in the compound composed of the obtained Fe and O was examined with an infrared absorption carbon / sulfur analyzer. The S contents in Comparative Examples 1 to 4 and Examples 1 to 9 are as shown in Table 1.

(Preparation of negative electrode)
A compound composed of Fe and O prepared as described above was mixed with 85 parts by mass as a negative electrode active material, 5 parts by mass of ketjen black as a conductive auxiliary agent, and 10 parts by mass of polyamideimide as a binder. It was. 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.

(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 whose thickness is 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 the rate characteristic was evaluated.

[Experiment 2]
(Production of negative electrode active materials having different B / A ratios)
The compounds composed of Fe and O in Table 2 were prepared using a hydrothermal synthesis method.

  The procedure of this hydrothermal synthesis method will be described in detail. The amount of ethylene glycol was 0.5M in Example 10, 1.0M in Example 11, 1.3M in Example 12, 2.0M in Example 13, 2.5M in Example 14, and 15 in Example 15. A precursor of a compound composed of Fe and O was prepared in the same manner as in Experimental Example 1 except that 3.0M and Example 16 were changed to 3.5M.

  The obtained precursor of the compound composed of Fe and O was filtered and washed. Next, this precursor was calcined at 300 ° C. for 3 hours to obtain a compound composed of Fe and O. At that time, 3 L / min. Using air (Air) as a carrier gas. Water vapor with a dew point of 55 ° C. was introduced into the furnace at a flow rate of

  The amount of S of the compound composed of Fe and O obtained was 0.5% by mass when examined by an infrared absorption carbon / sulfur analyzer. Further, the ratio B / A of the longest diameter B / shortest diameter A of the compound composed of Fe and O was measured using SEM. The B / A ratios of Examples 10 to 16 are as shown in Table 2.

The resulting phase of the compound composed of Fe and O was identified by an X-ray diffractometer (tube ball: Cu, tube voltage: 40 kV, tube current: 40 mA). In each of Examples 10 to 16, an α-Fe ₂ O ₃ phase was detected.

  A lithium ion secondary battery was produced in the same manner as in Experimental Example 1 except that the obtained compound composed of Fe and O was used, and the rate characteristics of this lithium ion secondary battery were evaluated. Table 2 shows the value of the rate characteristic of each sample.

[Experiment 3]
(Preparation of negative electrode active materials with different specific surface areas)
After firing, a compound composed of Fe and O of Experimental Example 3 was obtained in the same manner as in Example 6 except that the specific surface area was adjusted by pulverization. The grinding conditions were 0 hours for Example 17, 2 hours for Example 18, 3 hours for Example 19, 4 hours for Example 20, 5 hours for Example 21, 6 hours for Example 22, and 23 hours for Example 23. 8 hours, Example 24 was 12 hours, Example 25 was 16 hours, Example 26 was 22 hours, Example 27 was 24 hours, Example 28 was 28 hours, and Example 29 was 32 hours.

  Table 3 shows the S content and specific surface area of Examples 17 to 29.

The resulting phase of the compound composed of Fe and O was identified by an X-ray diffractometer (tube ball: Cu, tube voltage: 40 kV, tube current: 40 mA). In Examples 17 to 29, an α-Fe ₂ O ₃ phase was detected.

  A lithium ion secondary battery was produced in the same manner as in Experimental Example 1 except that the obtained compound composed of Fe and O was used, and the rate characteristics of this lithium ion secondary battery were evaluated. Table 3 shows the value of the rate characteristic of each sample.

[Experimental Example 4]
(Preparation of negative electrode active material containing Li)
The compounds composed of Fe and O containing Li in Table 4 were prepared using a solid phase reaction method.

The procedure of this solid phase reaction method will be described in detail. First, α-Fe ₂ O ₃ obtained by lithium carbonate and the same synthesis method as in Experimental Example 1 was weighed so as to be equimolar. The S content was 2% by mass of iron sulfate with respect to the total mass of lithium carbonate and iron oxide. Water was added to the mixture, wet-mixed with a ball mill for 6 hours, and then dried to remove the water to obtain a dried product.

  Next, the dried product was fired at 900 ° C. for 3 hours in the air. At that time, 3 L / min. Using air (Air) as a carrier gas. Steam was introduced into the furnace at a flow rate of The dew point of water vapor is as follows: Comparative Example 5 has a dew point of 95 ° C, Comparative Example 6 has a temperature of 90 ° C, Example 30 has a temperature of 85 ° C, Example 31 has a temperature of 80 ° C, Example 32 has a temperature of 75 ° C, Example 33 has a temperature of 70 ° C. 34 is 60 ° C, Example 35 is 55 ° C, Example 36 is 50 ° C, Example 37 is 45 ° C, Example 38 is 40 ° C, Comparative Example 7 is 35 ° C, and Comparative Example 8 is 30 ° C. Each S content was adjusted.

The produced phase of the compound composed of Fe and O containing Li was identified by an X-ray diffractometer (tube ball: Cu, tube voltage: 40 kV, tube current: 40 mA). In each of Comparative Examples 5 to 8 and Examples 30 to 38, an α-LiFeO ₂ phase was detected. Table 4 shows the S content.
[Experimental Example 5]
(Preparation of a negative electrode containing a compound composed of Fe and O and graphite)
Α-Fe ₂ O ₃ was obtained in the same manner as in Example 6. This was mixed with graphite in an amount of 0.5% by mass in Example 39, 1.0% by mass in Example 40, 5.0% by mass in Example 41, and 10.0% by mass in Example 42. The negative electrode containing the compound comprised and graphite was obtained.

  A lithium ion secondary battery was obtained in the same manner as in Experimental Example 1 using the negative electrode containing the compound composed of Fe and O obtained in Experimental Example 5 and graphite. In addition, the lithium ion secondary battery was evaluated in the same manner as in Experimental Example 1. Table 5 shows the ratio with the rate characteristics of each sample when the rate characteristics when the iron oxide content is 0% by mass is 100.

  The effect of the present invention is clear from the results shown in Table 1. That is, it is composed of a compound composed of Fe and O, and exhibits an effect of excellent rate characteristics when the S content is 0.001% by mass to 1.2% by mass in terms of S.

  The effect of the present invention is clear from the results shown in Table 2. That is, it is composed of a compound composed of Fe and O, and exhibits an effect of excellent rate characteristics when the particles are rod-shaped or plate-shaped. Further, the rod-like or plate-like particles exhibit an effect of excellent rate characteristics when the ratio B / A of the longest diameter B in the direction intersecting the shortest diameter A is 3 or more.

Further, as apparent from 3, it was possible to obtain more excellent rate characteristics when the specific surface area of the compound composed of Fe and O was 5 m ² / g or more and 30 m ² / g or less.

Further, as apparent from Table 4, even when Li is contained in the compound composed of Fe and O, the effect of excellent rate characteristics when the S content is 0.001% by mass to 1.2% by mass in terms of S Is expressed.

  As apparent from Table 5, even when a compound composed of Fe and O and graphite are included, the effect of excellent rate characteristics is exhibited.

  Thus, it is composed of a compound composed of Fe and O, and it is found that the compound has excellent rate characteristics when the S content is 0.001% by mass or more and 1.2% by mass or less in terms of S. It was.

  According to the present invention, a negative electrode active material having excellent rate characteristics, a negative electrode using the negative electrode active material, and a lithium ion secondary battery can be provided.

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 (8)

  A negative electrode active material comprising Fe and O, wherein the compound contains S, and the S content is 0.001% by mass or more and 1.2% by mass or less in terms of S.   The negative electrode active material according to claim 1, wherein the compound composed of Fe and O is a rod-like or plate-like particle.   3. The negative electrode active material according to claim 2, wherein the rod-shaped or plate-shaped particles have a ratio B / A of a longest diameter B in a direction intersecting the shortest diameter A to 3 or more. The negative electrode active material according to claim 1, wherein the compound composed of Fe and O has a specific surface area of 5 m ² / g or more and 30 m ² / g or less. The compound composed of Fe and O is at least one selected from the group consisting of FeO, α-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , and Fe ₃ O ₄ . The negative electrode active material according to one item.   The negative electrode using the negative electrode active material as described in any one of Claims 1-5.   The negative electrode according to claim 6, wherein the negative electrode contains graphite. A lithium ion secondary battery comprising the negative electrode according to claim 6, a positive electrode, and a nonaqueous electrolytic solution.

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