JP2017103136A - Negative electrode active material for lithium ion secondary batteries, negative electrode for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material with good charge and discharge cycle characteristics, and a lithium ion secondary battery using the negative electrode active material.SOLUTION: A negative electrode active material comprises: rod-like particles formed by a compound including Fe and O. In each rod-like particle, an external region is larger than an internal region in crystallite diameter. The crystallite diameter of the external region is 1.2-13.7 nm. Preferably, the crystal structure of the external region of the rod-like particles is FeO. A method for synthesizing a negative electrode active material for lithium ion secondary batteries, including rod-like particles comprises the step of performing a supercritical hydrothermal synthesis of an iron aqueous solution prepared by dissolving a soluble iron salt, a neutralizer and an additive agent in water with a 30%-HOwater as an oxidizing agent under the condition of a preheating temperature of 180°C and a reaction temperature of 400°C.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a negative electrode active material for lithium ion secondary batteries, a negative electrode for lithium ion secondary batteries, 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.

As an example of such a negative electrode active material, a composition containing iron oxide (an oxide containing iron as a constituent element) such as Fe ₂ O ₃ can be given. These iron oxides can occlude and release lithium ions electrochemically, and can charge and discharge with a very large capacity compared to graphite. In particular, the theoretical discharge capacity of Fe ₂ O ₃ is 1005 mAh / g, which is known to be 2.7 times higher than that of graphite. (Non-Patent Document 1)

JOURNAL OF SOLID STSTE CHRMISTY 55, 280-286 (1984)

  However, even in the negative electrode active material described in Non-Patent Document 1 described above, the charge / discharge cycle characteristics are not sufficient.

  This invention is made | formed in view of the said subject, and provides the negative electrode active material for lithium ion secondary batteries with favorable charging / discharging cycling characteristics, the negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery. Objective.

  In order to achieve the above object, the negative electrode active material for a lithium ion secondary battery according to the present invention is a rod-like particle composed of a compound composed of Fe and O, and the crystallite diameter of the outer region of the rod-like particle is The crystallite diameter of the outer region is larger than the inner region, and the crystallite size of the outer region is 1.2 nm or more and 13.7 nm or less.

  By using the negative electrode active material of the present invention, good charge / discharge cycle characteristics can be obtained. When the crystallite is small, the grain boundary increases and the reaction field for lithium ion conduction increases, which shows excellent battery characteristics, while the expansion associated with charge / discharge increases, but the crystallite diameter in the outer region By making the crystallite diameter larger than the crystallite diameter and the crystallite diameter in the outer region 1.2 nm or more and 13.7 nm or less, the outer region itself suppresses expansion associated with charge / discharge, thereby reducing the pulverization of the negative electrode active material. It is suppressed. For this reason, unnecessary contact surfaces with the electrolytic solution are reduced, and film formation, which is a cause of an increase in resistance, is suppressed. Therefore, it is considered that excellent charge / discharge cycle characteristics are obtained.

  The outer region of the rod-like particles is preferably FeO.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material for lithium ion secondary batteries with favorable charging / discharging cycling characteristics, the negative electrode for lithium ion secondary batteries, 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 SEM image of the rod-shaped particle comprised with the compound which consists of Fe and O which concerns on this embodiment. It is a TEM image of the rod-shaped particle comprised with the compound which consists of Fe and O which concerns on this embodiment. It is illustration of the concept of the internal area | region and external area | region of a rod-shaped particle | grain comprised with the compound which consists of Fe and O which concerns on this embodiment. It is a Fast Fourier Transform (FFT) image using the high-resolution transmission electron microscope (TEM) of the external area | region of the rod-shaped particle | grains which consist of Fe and O 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 view of a lithium ion secondary battery 100 as a lithium ion secondary battery. The lithium ion secondary battery 100 of FIG. 1 is formed by laminating via an outer package 50, a positive electrode 10 and a negative electrode 20 provided inside the outer package, and a separator 18 disposed therebetween. The separator 18 is composed of an electrode body 30 and a non-aqueous electrolyte containing an electrolyte, and the separator 18 holds the non-aqueous electrolyte that is a lithium ion moving medium between the positive and negative electrodes during charging and discharging. 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.

(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.

The negative electrode active material is
A rod-like particle composed of a compound composed of Fe and O, wherein the crystallite diameter in the outer region of the rod-like particle is larger than the crystallite diameter in the inner region. 2 is an SEM image of rod-shaped particles composed of a compound composed of Fe and O, FIG. 3 is a TEM image of rod-shaped particles composed of a compound composed of Fe and O, and FIG. 4 is composed of a compound composed of Fe and O. The conceptual diagram of the outer area | region and inner area | region of the rod-shaped particle | grains to show.

As the compound composed of Fe and O, FeO, Fe ₂ O ₃ , Fe ₃ O ₄ , Li ₄ FeO ₄ , LiFe ₅ O ₈ , LiFeO _{2 and the} like can be used. Any of the above-described compounds composed of Fe and O can be used for both the outer region and the inner region of the rod-like particle composed of the compound composed of Fe and O of the present invention, but the outer region is preferably FeO for the reasons described above. In addition, as the negative electrode active material, a compound composed of Fe and O may be used alone, or another negative electrode active material may be used in combination. In addition, for example, a metal capable of forming a compound with lithium such as silicon or tin and an amorphous compound mainly composed of an oxide thereof, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), negative electrode active such as graphite You may mix and use a material.

    When the crystallite is small, the grain boundary increases and the reaction field for lithium ion conduction increases, which shows excellent battery characteristics, while the expansion associated with charge / discharge increases, but the crystallite diameter in the outer region By making the crystallite diameter larger than the crystallite diameter and the crystallite diameter in the outer region 1.2 nm or more and 13.7 nm or less, the outer region itself suppresses expansion associated with charge / discharge, thereby reducing the pulverization of the negative electrode active material. It is suppressed. For this reason, unnecessary contact surfaces with the electrolytic solution are reduced, and film formation, which is a cause of an increase in resistance, is suppressed. Therefore, it is considered that excellent charge / discharge cycle characteristics are obtained.

  The external region is a region corresponding to the outer peripheral portion in the cross section in the minor axis direction shown in FIG. 3, and in the case of a columnar particle, the depth direction from the periphery of the circle toward the center of the circle Indicates the area. On the other hand, the inner region means the inside of the outer region. The outer region and the inner region can be determined from the change in contrast in the high-resolution transmission electron microscope (TEM) image and the crystallite size of each region.

  The crystallite size in the inner region is preferably less than 1.2 nm, and the crystallite size in the outer region is preferably 1.2 nm or more and 13.7 nm or less. The crystallite diameter in the outer region is more preferably 1.9 nm or more and 5.2 nm or less. The above contents, that is, the crystallite diameter of the compound composed of Fe and O existing in the inner region is more crystalline than the compound composed of Fe and O in the outer region having a crystallite diameter of 1.2 nm to 13.7 nm. This means that the diameter is small. Therefore, the crystallite of the compound composed of Fe and O existing in the inner region may be an amorphous or microcrystal where the crystallite cannot be measured.

  The crystallite diameters in the inner region and the outer region are observed with a high-resolution TEM. After measuring the crystallite diameters of 20 arbitrary points in the inner region and the outer region from the obtained TEM image, the upper and lower 10% are cut. The trimmed average was calculated as the crystallite size.

  Furthermore, the outer region of the rod-like particles is more preferably FeO.

  The crystal structure of the outer region is preferably FeO. Since FeO has a small amount of occlusion of Li, it is considered that the volume change accompanying charging of FeO itself is small, and pulverization of FeO itself is suppressed and charge / discharge cycle characteristics are further improved.

  In a high-resolution TEM image directly observed at a magnification of 400,000 or more, the outer region of the rod-shaped particles was subjected to Fast Fourier Transform (FFT) on a 10-nm square area, and the constituent phases were identified.

  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.

(Method for producing negative electrode active material)
The rod-shaped particle | grains comprised with the compound which consists of Fe and O were synthesize | combined using the 2 liquid flow type supercritical hydrothermal synthesizer. For the synthesis, a step of producing a seed crystal of rod-like particles composed of a compound composed of Fe and O (step 1), a step of growing a seed crystal of rod-shaped particles composed of a compound composed of Fe and O (step) 2) and firing may be performed as necessary (Step 1).
First, a soluble iron salt, a neutralizing agent and an additive were dissolved in water to prepare an iron aqueous solution. Next, the flow rate of the aqueous iron solution is set to 25 to 120 ml / min, and the preheating tube, the reaction tube, and the cooling tube are circulated in this order, the flow rate of 30% hydrogen peroxide water as the oxidizing agent is set to 18 to 36 ml / min, Supercritical hydrothermal synthesis was carried out under the conditions of a preheating temperature of 180 ° C. and a reaction temperature of 400 ° C. in the order of the tubes. The aqueous iron solution and 30% hydrogen peroxide solution were designed to be mixed immediately before the reaction tube. The obtained slurry was filtered and washed with water to obtain a seed crystal of rod-shaped particles composed of a compound composed of Fe and O.
(Process 2)
Furthermore, a slurry containing rod-shaped particle seed crystals composed of a compound composed of Fe and O is mixed with a soluble iron salt, an aqueous iron solution in which a neutralizing agent and an additive are dissolved in water, and a compound composed of Fe and O An aqueous iron slurry containing rod-shaped particle seed crystals composed of The obtained iron aqueous slurry was subjected to supercritical hydrothermal synthesis in the same manner. The obtained slurry was filtered, washed with water, and dried to obtain rod-like particles composed of a compound composed of Fe and O.
Furthermore, rod-like particles made of Fe and O may be mixed with a carbon source or the like as necessary, and fired in air or nitrogen.

  As the iron salt, iron chloride, iron sulfate, iron nitrate, iron citrate, iron phosphate and the like can be used, and iron chloride, iron sulfate, iron nitrate are more preferable from the solubility in water, and hydration thereof. It may be a thing. Furthermore, these 1 type (s) or 2 or more types can be mixed and used. In addition, as an iron source preparation density | concentration, 0.1-1.0M is preferable. When the iron source preparation concentration is less than 0.1M, the particles composed of Fe and O do not sufficiently grow and the rod-like particles are uneven. Moreover, when exceeding 1.0M, the viscosity of the slurry after a synthesis | combination is high, and since possibility of causing piping clogging is high, it is not preferable.

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

  As additives, organic acids such as tartaric acid, pyromellitic acid and formic acid, and fatty acids such as oleic acid, linoleic acid, stearic acid and lauric acid can be used. The appropriate concentration differs depending on the type of organic acid and fatty acid. For example, pyromellitic acid is preferably 0.1 equivalent or more and 0.2 equivalent or less, and lauric acid is 0.02 equivalent or more and 0.05 equivalent or less with respect to the iron ion concentration. preferable.

  As the carbon source, carbon black such as acetylene black, furnace black, channel black, thermal black, ketjen black and the like can be used.

  The filtration method is not particularly limited, and a known filter paper having a diameter capable of collecting the produced rod-like particles composed of Fe and O can be used. For example, a filter paper, a glass filter, a membrane filter, or the like can be used.

  As a drying method, in order to suppress dry aggregation, it is preferable to dry at 150 ° C. or less, and it is more preferable to perform vacuum drying at 60 to 90 ° C. for 6 to 12 hours.

  The firing method is preferably performed at 500 ° C. or lower in order to suppress crystal grain growth, and may be performed at 250 to 450 ° C. in air for 1 to 6 hours.

(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 not particularly limited, but is preferably 1 to 30% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder, and 5 to 15 masses. % Is more preferable.

(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. For example, carbon black such as acetylene black, furnace black, channel black, thermal black, ketjen black, carbon fiber such as vapor grown carbon fiber (VGCF), carbon nanotube, and carbon material such as graphite, etc. 1 type (s) or 2 or more types can be used.

  It is more preferable to use ketjen black from the viewpoint of conductivity. By using ketjen black, a good conductive path is formed between the rod-like particles made of Fe and O, and excellent charge / discharge cycle characteristics can be obtained.

  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.

  There is no restriction | limiting in particular as an application | coating method, Usually, the method employ | adopted when producing an electrode 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 has been formed in this way may be subsequently pressed by a roll press device or the like 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 constituent element (conductive auxiliary agent and binder) other than the positive electrode active material can use the same substances as those 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 60 and the positive electrode lead 62 may be formed of a conductive material such as aluminum or nickel.

  Although the present invention has been described in detail with the embodiments, the present invention is not limited to the above-described embodiments, and various modifications can be made. 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.

この容量維持率が高いほど、充放電サイクル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 charge / discharge cycle characteristics)
Using a secondary battery charge / discharge test apparatus, charging is performed at a current value of 0.5 C when the voltage range is 4.0 V to 1.0 V and 1 C = 800 mAh / g per weight of the negative electrode active material. It discharges with the electric current value at 0 C, evaluates charging / discharging cycle characteristics, and shows it as a discharge capacity maintenance rate (it may abbreviate as a capacity maintenance rate hereafter). The capacity retention rate (%) is the ratio of the discharge capacity at each cycle number with respect to the initial discharge capacity, where the discharge capacity at the first cycle is the initial discharge capacity, and is expressed by the following equation (1). Note that 1C is a current value at which charging / discharging is completed in exactly one hour after a constant current charge or constant current discharge is performed on a battery cell having a nominal capacity value.

A higher capacity retention rate means better charge / discharge cycle characteristics. The lithium ion secondary batteries produced in the examples and comparative examples were repeatedly charged and discharged under the above conditions, and the charge / discharge cycle characteristics were evaluated based on the 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.

  Lithium ion secondary batteries of Examples 1 to 16 and Comparative Examples 1 to 4 were produced by the following procedure.

[Example 1]
(Preparation of negative electrode active material)
0.5M iron sulfate hydrate _{(78% as Fe 2 (SO} 4) 3), iron hexahydrate _(FeCl 3 · _6H 2 O) of 0.2M chloride, triethanolamine 2.4M, pyromellitic An aqueous iron solution was prepared so that the acid would be 0.11M. Next, the flow rate of the aqueous iron solution is 20 ml / min, the preheating tube, the reaction tube, and the cooling tube are circulated in this order. went. The obtained slurry was filtered and washed with water to obtain a seed crystal of rod-shaped particles composed of Fe and O in Example 1. Iron sulfate hydrate (78% as Fe ₂ (SO ₄ ) ₃ ) was 0.5M, triethanolamine was 2.8M, and oxalic acid was 0.3M. A slurry was prepared so that the precursor seed crystals of rod-shaped particles made of 15 wt%. Next, the flow rate of the slurry is 20 ml / min, the preheating tube, the reaction tube, and the cooling tube are circulated in this order, the flow rate of 30% hydrogen peroxide water is 30 ml / min, and supercritical hydrothermal synthesis is performed at a reaction temperature of 400 ° C. It was. The obtained slurry was filtered, washed with water, washed with ethanol, and dried to obtain rod-like particles composed of Fe and O as the negative electrode active material of Example 1. An SEM image is shown in FIG.
(Preparation of negative electrode)
85 parts by mass of the rod-like particles made of Fe and O of Example 1 prepared above as the negative electrode active material, 5 parts by mass of ketjen black as the conductive auxiliary agent, and 10 parts by mass of polyamideimide as the binder were mixed to form the negative electrode A mixture was prepared. 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 1000 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 produced negative electrode and positive electrode were laminated via a polypropylene separator having a thickness of 16 μm to produce a laminate. Three negative electrodes and two positive electrodes were laminated via four separators so that the negative and positive electrodes were alternately laminated. Further, in the negative electrode of the electrode body, a nickel negative electrode lead is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer, while the positive electrode of the laminated body is provided with the aluminum not provided with 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.

[Examples 2 to 10]
The concentration, additive type, and concentration of the iron raw material in step 2 in the method for producing a negative electrode active material were changed to the conditions shown in Table 1, and negative electrode active materials of Examples 2 to 10 shown in Table 1 were obtained. The lithium ion secondary battery of Examples 2-10 was produced using the negative electrode active material of Examples 2-10.

[Example 11]
10 g of the negative electrode active material of Example 1 and 0.3 g of acetylene black were mixed by a pot mill, and heat-treated at 400 ° C. for 2 hours in a nitrogen atmosphere to obtain the negative electrode active material of Example 11. A lithium ion secondary battery of Example 11 was produced in the same manner as Example 1 using the negative electrode active material of Example 10.

[Example 12]
A negative electrode active material of Example 12 was obtained in the same manner as Example 11 except that the negative electrode active material of Example 2 was used. Using the negative electrode active material of Example 12, a lithium ion secondary battery of Example 12 was produced in the same manner as Example 1.

[Example 13]
The negative electrode active material of Example 1 was heat-treated in air at 350 ° C. for 3 hours to obtain a negative electrode active material of Example 13. Using the negative electrode active material of Example 13, a lithium ion secondary battery of Example 13 was produced in the same manner as Example 1.

[Example 14]
A negative electrode active material of Example 14 was obtained in the same manner as Example 13 except that the negative electrode active material of Example 2 was used. A lithium ion secondary battery of Example 14 was produced in the same manner as Example 1 using the negative electrode active material of Example 14.

[Comparative Example 1]
10 g of α-FeOOH (high purity chemistry) and 0.3 g of acetylene black were mixed by a pot mill and heat-treated at 400 ° C. for 3 hours in a nitrogen atmosphere to obtain a negative electrode active material of Comparative Example 1. The negative electrode active material at that time was not a rod-like particle having an outer region and an inner region by TEM observation, but just a rod-like particle. That is, the crystallite size in the particles is a rod-like particle having a substantially uniform size.
Using the negative electrode active material of Comparative Example 1, a lithium ion secondary battery of Comparative Example 2 was produced in the same manner as Example 1.

[Comparative Example 2]
α-FeOOH (high purity chemistry) was heat-treated in air at 400 ° C. for 3 hours to obtain a negative electrode active material of Comparative Example 1. The negative electrode active material at that time was also a rod-shaped particle, not a rod-shaped particle having an outer region and an inner region, as observed by TEM.
Using the negative electrode active material of Comparative Example 2, a lithium ion secondary battery of Comparative Example 2 was produced in the same manner as Example 1.

(Measurement of crystallite size and identification of constituent phases in the external region)
The obtained negative electrode active material of Example 1 was observed using a high-resolution TEM. As shown in FIG. From the obtained TEM image, it was found that the negative electrode active material of Example 1 had different crystallite diameters in the outer region and the inner region. The length was measured by the measurement method described above. The crystallite size in the inner region was less than 1 nm, and the crystallite size in the outer region was 1.9 nm. Further, as a result of performing Fast Fourier Transform on an area of 10 nm square with respect to the outer region, a spot corresponding to the lattice image was obtained as shown in FIG. 5, and the distance from the center position to the spot coincided with the surface interval of FeO.
Similarly, in Examples 2 to 14, the crystallite diameters in the inner region and the outer region were measured and the constituent phases in the outer region were identified. The results are shown in Table 1. The crystallite diameter in the inner region was less than 1 nm.

  The produced lithium ion secondary batteries of Examples 1 to 14 and Comparative Examples 1 and 2 were measured by the above-described measurement method for charge / discharge cycle characteristics, and the capacity retention rate after 100 cycles was evaluated. The results are shown in Table 1.

  As is clear from Table 1, the rod-like particles composed of the compound consisting of Fe and O obtained in the examples are particles having an outer region and an inner region as shown in FIG. The crystallite diameter of the outer region of the rod-shaped particles is larger than the crystallite diameter of the inner region, and the crystallite diameter of the outer region is 1.2 nm or more and 13.7 nm or less, The region itself suppresses expansion associated with charging / discharging, and pulverization of the negative electrode active material is suppressed. For this reason, unnecessary contact surfaces with the electrolytic solution are reduced, and film formation, which is a cause of an increase in resistance, is suppressed. Therefore, it is considered that excellent charge / discharge cycle characteristics are obtained.

  Further, as is clear from Examples 10 to 14, the charge / discharge cycle characteristics are improved because the crystal structure of the outer region of the rod-like particles is FeO. This is presumably because FeO has a small amount of occlusion of Li, so that the volume change accompanying the charge of FeO itself is small, so that pulverization of FeO itself is suppressed and charge / discharge cycle characteristics are further improved.

  According to the present invention, a lithium ion secondary battery having excellent charge / discharge cycle characteristics 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, 62 ... Positive electrode lead, 60 ... Negative electrode lead, 100 ... Lithium ion secondary battery

Claims (4)

A rod-like particle composed of a compound comprising Fe and O, wherein the crystallite diameter of the outer region of the rod-like particle is larger than the crystallite size of the inner region, and the crystallite size of the outer region is 1.2 nm. As described above, a negative electrode active material for a lithium ion secondary battery, which is 13.7 nm or less. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the crystal structure of the outer region of the rod-like particles is FeO. A negative electrode for a lithium ion secondary battery comprising the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 2. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 3 and a positive electrode.

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