WO2012014381A1

WO2012014381A1 - Negative electrode active material for non-aqueous secondary battery, and process for production thereof

Info

Publication number: WO2012014381A1
Application number: PCT/JP2011/003815
Authority: WO
Inventors: 三好　学; 村瀬　仁俊
Original assignee: 株式会社豊田自動織機
Priority date: 2010-07-29
Filing date: 2011-07-04
Publication date: 2012-02-02
Also published as: JP2012033311A; JP5177183B2

Abstract

A negative electrode active material for a non-aqueous secondary battery, which comprises a composite powder complexed by milling a silicon-oxide-based powder containing silicon oxide and an iron-based powder containing iron, and which is characterized in that the composite powder comprises silicon oxide derived from the silicon-oxide-based powder and iron derived from the iron-based powder and/or an iron compound containing iron derived from the iron-based powder. This negative electrode active material for a non-aqueous secondary battery can have both an initial charge-discharge efficiency of a non-aqueous secondary battery and cycle properties in spite of a matter the negative electrode active material contains silicon oxide.

Description

Negative electrode active material for non-aqueous secondary battery and method for producing the same

The present invention relates to a non-aqueous secondary battery such as a lithium ion secondary battery, and particularly relates to an active material for a non-aqueous secondary battery.

Secondary batteries such as lithium ion secondary batteries are small and have a large capacity, so they are used in a wide range of fields such as mobile phones and notebook computers. A lithium ion secondary battery has an active material capable of inserting and removing lithium (Li) in each of a positive electrode and a negative electrode. And it operate | moves because Li ion moves in the electrolyte solution provided between both electrodes.

The performance of the secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. Among them, active research and development of active material forming active material is being actively conducted. For example, the use of silicon monoxide (SiO _n : n is about 0.5 ≦ n ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _n decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. . The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material obtained by decomposing SiO _n into Si and SiO ₂ has excellent cycle characteristics.

However, it is known that when the negative electrode active material contains SiO ₂ , the initial charge / discharge efficiency is deteriorated. This is because, for example, when SiO ₂ occludes lithium ions, a stable compound (Li ₄ SiO ₄ , Li ₂ O, etc.) is formed, and lithium ions are hardly released, resulting in an irreversible capacity.

Patent Document 1 describes a negative electrode active material obtained by milling silicon oxide (SiO or SiO ₂ ), aluminum, and lithium oxide. The negative electrode active material includes a composite composed of fine Si phase particles in which silicon oxide is reduced by aluminum and aluminum oxide surrounding the Si phase particles.

Japanese Patent Laid-Open No. 2007-500421

In Patent Document 1, Si phase is generated by reducing silicon oxide with aluminum. That is, since silicon oxide is not substantially contained in the negative electrode active material, the irreversible capacity caused by the presence of silicon oxide is reduced, but the effect of suppressing the decomposition of the electrolytic solution cannot be expected. As a result, even if the initial charge / discharge efficiency is improved, the cycle characteristics may be deteriorated. That is, in the negative electrode active material containing silicon oxide, the initial charge / discharge efficiency and the cycle characteristics are contradictory, and it is difficult to achieve both.

In view of the above problems, the present invention provides a novel negative electrode active material for a non-aqueous secondary battery that can achieve both initial charge and discharge efficiency and cycle characteristics even if it is a negative electrode active material containing silicon oxide, and a method for producing the same. The purpose is to do.

A negative electrode active material for a non-aqueous secondary battery according to the present invention includes a composite powder obtained by milling a silicon oxide-based powder containing silicon oxide and an iron-based powder containing iron to form a composite powder. A substance,
The composite powder includes silicon oxide derived from the silicon oxide-based powder and iron derived from the iron-based powder and / or an iron compound containing iron derived from the iron-based powder.

The negative electrode active material for a non-aqueous secondary battery according to the present invention only needs to contain at least a composite powder that has been composited through milling, and other processes such as heat treatment before and / or after milling. The process may be performed.

The method for producing a negative electrode active material for a non-aqueous secondary battery according to the present invention is a method for producing the negative electrode active material for a non-aqueous secondary battery according to the present invention,
Including a milling step in which a raw material powder containing silicon oxide-based powder containing silicon oxide and iron-containing powder containing iron is milled in an inert atmosphere to combine the silicon oxide-based powder and the iron-based powder. It is characterized by.

Since the negative electrode active material of the present invention contains silicon oxide, the effect of suppressing the decomposition of the electrolytic solution by silicon oxide is exhibited, and the cycle characteristics are maintained high.

Further, as described above, in the negative electrode active material containing silicon oxide, for example, when silicon oxide occludes lithium ions, stable compounds such as Li ₄ SiO ₄ and Li ₂ O are formed. Therefore, lithium ions are hardly released from the negative electrode active material, and irreversible capacity is generated. However, the negative electrode active material for a non-aqueous secondary battery of the present invention including a composite powder obtained by combining a silicon oxide powder and an iron powder has reduced irreversible capacity due to silicon oxide and improved initial charge / discharge efficiency. I found out that This mechanism is considered as follows. The composite powder contains iron and / or iron compounds. Even if silicon oxide forms a stable compound after occlusion of lithium ions, it is presumed that the release of occluded lithium ions is promoted by the catalytic action of iron and / or iron compounds. Specifically, iron compounds such as iron silicide and iron oxide react with lithium ions in the charging of a lithium secondary battery and decompose into iron (single substance) and lithium silicide, thereby producing simple iron (pure iron). ) Is considered to exhibit a catalytic action. As a result, the stable compound returns to silicon oxide in a state in which lithium ions can be occluded again, so that the irreversible capacity is reduced.

Even if the negative electrode active material for a non-aqueous secondary battery of the present invention is a negative electrode active material containing silicon oxide, the initial charge / discharge efficiency and cycle characteristics of the non-aqueous secondary battery are compatible.

It is a schematic diagram explaining the negative electrode active material for non-aqueous secondary batteries of this invention, and its manufacturing method. The charging / discharging curve of the lithium secondary battery using the negative electrode active material for non-aqueous secondary batteries of an Example and a comparative example is shown. It is a graph which shows the charge capacity in each cycle number of the lithium secondary battery using the negative electrode active material for non-aqueous secondary batteries of an Example and a comparative example. It is a graph which shows the capacity | capacitance maintenance factor in each cycle number of the lithium secondary battery using the negative electrode active material for non-aqueous secondary batteries of an Example and a comparative example. 2 is an X-ray diffraction pattern of heat-treated SiO powder used as a raw material powder in the production of a negative electrode active material for a non-aqueous secondary battery in an example. It is an X-ray diffraction pattern of FeSi ₂ powder used as a raw material powder in the manufacture of the negative active material for a nonaqueous secondary battery of Example. The X-ray diffraction pattern of the composite powder of each Example is shown together with the X-ray diffraction pattern of the heat-treated SiO powder and FeSi ₂ powder.

Hereinafter, the best mode for carrying out the negative electrode active material for non-aqueous secondary batteries of the present invention (hereinafter abbreviated as “negative electrode active material of the present invention”) and the method for producing the same will be described. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y. In addition, the numerical range can be configured by arbitrarily combining the numerical values described in the present specification within the numerical range.

<Negative electrode active material for non-aqueous secondary battery>
The negative electrode active material of the present invention includes a composite powder obtained by milling a silicon oxide-based powder containing silicon oxide and an iron-based powder containing iron.

The silicon oxide-based powder may contain silicon oxide represented by a composition formula SiO _m (0.1 ≦ m ≦ 2). Specific examples include silicon monoxide, silicon dioxide, silicon oxide having a composition slightly deviated from SiO and SiO ₂ . A powder containing particles containing two phases of a silicon phase (Si phase) and a silicon dioxide phase (SiO ₂ phase) obtained by disproportionating the silicon monoxide powder can also be used.

The iron-based powder is not particularly limited as long as it contains iron (Fe). The iron-based powder may be a powder containing one or more of pure iron particles, iron alloy particles, and iron compound particles. Along with these particles, silicon-based powder, cobalt-based powder, manganese-based powder and the like may be further included.

The iron-based powder desirably contains Si together with Fe, and particularly preferably uses an iron-based powder containing an alloy and / or a compound containing Fe and Si. This is because the inclusion of Si in the iron-based powder generates silicon oxide at the interface between the silicon oxide-based powder and the iron-based powder by a milling process described later, and further improves the cycle characteristics.

The alloys and compounds containing Fe and Si may be ternary or higher. Specifically, Fe—Si based compound particles such as FeSi ₂ particles, Fe ₃ Si particles, FeSi particles, Fe ₅ Si ₃ particles and Fe ₂ Si particles, Fe—Si binary alloy particles, TiFeSi particles, TiFeSi ₂ Particles, Fe—Si—Ti compound particles such as TiFe ₂ Si particles, TiFe ₄ Si ₃ particles, Fe—Si—Ti ternary alloy particles, MnFe ₂ Si particles, Mn ₂ Fe ₃ Si ₃ particles, Mn ₃ Fe Fe such as ₂ Si ₃ particles, Mn ₄ FeSi ₃ particles, Fe—Si—Mn compound particles such as MnFe ₃ Si ₃ particles, Fe—Si—Mn ternary alloy particles, Fe ₂ CoSi particles, FeCo ₂ Si particles, etc. -Fe-Ni-Si compound particles such as Co-Si compound particles, Fe-Co-Si ternary alloy particles, FeNiSi particles, Fe-Ni-Si ternary Alloy particles, FeAl ₅ Si particles, FeAl ₂ Si _particles, Fe ₂ _Al 9 Si 2 _particles, Fe 2 _Al 8 Si particles, FeAl-Si-based compound particles, such as FeAl ₃ Si ₂ particles, FeAl-Si three Ternary alloy particles, and the like. One or more of these can be used as the iron alloy particles and the iron compound particles.

As the silicon oxide-based powder and the iron-based powder, it is desirable to use general powders obtained by various atomizing methods or grinding methods. These powders will be described in detail later, and may be classified before use in the milling step.

The composite powder is formed by at least milling a silicon oxide powder and an iron powder to form a composite, and includes at least silicon oxide and iron and / or an iron compound. Silicon oxide is derived from a silicon oxide-based powder, and iron and / or iron compounds are derived from an iron-based powder. Milling is said not only to mix the raw material powder, but also to refine the particles and cause chemical atomic diffusion at the solid phase interface. Therefore, the composite powder obtained by milling has a form different from a simple mixed powder. Hereinafter, the composite powder and the manufacturing method thereof will be described with reference to FIG. [1] to [4] in FIG. 1 are schematic views of typical composite particles contained in the composite powder.

It is considered that the iron-based particles are attached to the surface of the silicon oxide-based particles by milling the silicon oxide-based powder and the iron-based powder. Then, it is considered that chemical atomic diffusion occurs between the silicon oxide-based powder and the iron-based powder by mechanical energy due to milling. The composite powder thus obtained contains at least silicon oxide and iron and / or an iron compound, and both initial charge / discharge efficiency and cycle characteristics are compatible.

The composite powder contains silicon oxide (SiO _m : 0.1 ≦ m ≦ 2) derived from silicon oxide-based powder. The composite powder may contain silicon derived from the silicon oxide powder. Silicon may be crystalline or non-crystalline, but a negative electrode active material containing crystalline silicon is preferable because of high initial charge / discharge efficiency.

The composite powder contains iron derived from iron-based powder and / or an iron compound containing iron derived from iron-based powder. Examples of such iron and / or iron compounds include pure iron, iron compounds, iron alloys, pure iron that is produced from iron-based powders and that contains at least part of iron contained in iron-based powders, One or more of the iron compounds may be mentioned. Examples of the iron compound generated from the iron-based powder by atomic diffusion at the solid phase interface by milling include iron oxide.

Furthermore, the composite powder may contain silicon oxide generated as a result of atomic diffusion at the solid phase interface by milling. Such silicon oxide is considered to contain mainly silicon derived from iron-based powder. That is, the composite powder contains iron and silicon.

In addition, although the composite particle shown in FIG. 1 has illustrated the structure of particle | grains simply for description, it is needless to say depending on the particle diameter and mixing ratio of the silicon oxide type powder and iron type powder to be used. It may not be arranged as you did. Moreover, depending on manufacturing conditions, when SiO is contained together with SiO ₂ and Si, the iron-based powder does not remain as it is, and there are cases where a plurality of types of reaction products are included.

Further, it is considered that composite oxides such as Fe ₂ SiO ₄ are not formed after milling. That is, it is considered that the composite powder does not substantially contain a composite oxide such as Fe ₂ SiO ₄ . This is apparent from the fact that the generation energy (ΔH) obtained by the first principle calculation performed for the reaction formula: SiO ₂ + FeSi ₂ → 2.5Si + 0.5Fe ₂ SiO ₄ was 380 kJ / mol · O ₂ . This is because if the generated energy is ΔH <0, a reaction according to the reaction formula occurs, but a reaction satisfying ΔH> 0 does not occur theoretically.

<Method for producing negative electrode active material for non-aqueous secondary battery>
Next, the manufacturing method of the negative electrode active material of this invention containing these composite powders is demonstrated.

The silicon oxide powder and iron powder used for milling are as described above. Prior to milling, these powders may be classified (screened) into silicon oxide powders of 50 μm or less, further 35 μm or less, and iron-based powders of 30 μm or less, further 20 μm or less. Since the silicon oxide powder is classified so as to include particles larger than the iron powder, the iron powder tends to adhere to the surface of the particles so as to cover the silicon oxide particles. Therefore, if expressed in terms of the average particle diameter, the relationship of (average particle diameter of silicon oxide-based powder)> (average particle diameter of iron-based powder) is preferable.

The mixing ratio of the silicon oxide-based powder and the iron-based powder may be mixed using a predetermined stoichiometric ratio according to the type of the silicon oxide-based powder and the iron-based powder as a guide. However, in anticipation that silicon oxide remains in the composite powder after milling, it is preferable to mix so that Si contained in the silicon oxide-based powder is larger in atomic ratio than Fe contained in the iron-based powder. Specifically, the atomic ratio between Fe contained in the iron-based powder and Si contained in the silicon oxide-based powder is Fe: Si = 1: 3.5 to 1:20, 1: 5 to 1:10 Is preferably 1: 6.5 to 1: 7.5.

When using a powder containing silicon monoxide particles, the powder containing silicon monoxide particles may be used for milling as it is, or using a powder containing silicon monoxide particles as a raw material silicon oxide powder, SiO ₂ phase and Si You may manufacture the silicon oxide type powder containing two phases with a phase. That is, the negative electrode active material manufacturing method of the present invention is performed before the milling step, and silicon monoxide, which is a raw material silicon oxide powder containing silicon monoxide powder, is disproportionated into a SiO ₂ phase and a Si phase to produce silicon oxide. A disproportionation step for obtaining a system powder may be included. In the disproportionation step, silicon monoxide (SiO _n : n is 0.5 ≦ n ≦ 1.5), which is a homogeneous solid having an atomic ratio of Si and O of approximately 1: 1, is converted into Si by reaction inside the solid. The disproportionation reaction that separates into two phases of the phase and the SiO ₂ phase proceeds. That is, the silicon oxide-based powder obtained after the disproportionation step includes silicon oxide-based particles including a Si phase and a SiO ₂ phase. In general, when oxygen is turned off, it is said that almost all silicon monoxide is disproportionated and separated into two phases at 800 ° C. or higher. Specifically, the raw material silicon oxide powder containing amorphous silicon monoxide powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as vacuum or in an inert gas. Thus, a silicon oxide-based powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

The method for producing a negative electrode active material of the present invention includes a milling step of milling a raw material powder containing a silicon oxide powder and an iron powder in an inert atmosphere. The silicon oxide powder and the iron powder are refined by applying mechanical energy by milling, and in some cases, crystals contained in the raw material powder become amorphous. Even if the silicon oxide-based powder contains a crystalline Si phase, a part of the mechanical energy of the milling contributes to the amorphization of the Si phase. As a result, the crystalline Si phase becomes amorphous after milling. Presumed to be of quality. Further, part of the mechanical energy of milling contributes to chemical atomic diffusion at the solid phase interface between the silicon oxide powder and the iron powder, and generates an iron compound, and in some cases, a silicon compound. That is, the composite powder containing the composite particles shown in [1] or [4] in FIG. 1 is obtained through a milling step and, if necessary, a disproportionation step before the milling step.

Milling is performed in an inert atmosphere such as in argon gas in order to suppress oxidation of raw material powder and unexpected reactions. Moreover, although it is thought that diffusion is accelerated | stimulated by heating the raw material powder in milling, it is not necessary to heat in particular and milling should just be performed at room temperature.

In the milling process, a ball mill, attritor, jet mill or the like may be used. If a ball milling device is used, the balls introduced together with the raw material powder are preferably made of zirconia, and may be substantially spherical with a diameter of 3 to 20 mm. The milling conditions should be appropriately selected according to the amount and type of raw material powder to be milled. However, if the degree of milling is intentionally defined, when a silicon oxide powder containing a crystalline Si phase produced by a disproportionation reaction is measured by X-ray diffraction, at least a clear diffraction peak of crystalline Si is present. It is desirable to perform milling until it becomes amorphous to such an extent that it cannot be detected. If specifically defined, the rotation speed of the container of the ball milling device is preferably 500 rpm or more, more preferably 700 to 800 rpm, and the mixing time is 10 to 50 hours.

When it is desired to obtain a composite powder containing a crystalline Si phase (for example, FIGS. 1 [2] and [3]), it is necessary to further perform a heat treatment or the like after the milling step. For example, it may further include a crystallization step of crystallizing an amorphous Si phase. The crystallization step may be a step of performing heat treatment at 800 to 1200 ° C. for 1 to 5 hours. By performing a heat treatment at 800 ° C. or higher, the amorphous Si phase is easily crystallized. The crystallization process is preferably performed in an inert atmosphere such as vacuum or argon gas in order to suppress the oxidation of the composite powder and an unexpected reaction.

Further, when the silicon oxide powder in the raw material powder contains silicon monoxide powder, a disproportionation step for disproportionating silicon monoxide into a SiO ₂ phase and a crystalline Si phase is performed after the milling step. Good. The disproportionation step may be performed in the same manner as the disproportionation step before the milling step. However, by performing heat treatment at 800 to 1100 ° C. for 1 to 5 hours in an inert atmosphere, a crystalline Si phase is obtained. Is desirable.

In the crystallization process and the disproportionation process, it is only necessary to perform a heat treatment only for the purpose of generating a crystalline Si phase. It may be performed in parallel with the processing. For example, a CVD process for forming a carbon-based film on the surface of the composite particle may be performed after the milling step. The formation of the carbon film is expected to improve conductivity. The formation of the carbon-based film by the CVD process is performed in an atmosphere in which the oxygen concentration is reduced and the composite powder is heated to a certain high temperature during the process. Therefore, the above crystallization process or disproportionation process is performed simultaneously with the CVD process. It becomes possible.

In addition, since the composite powder obtained after the crystallization step and the disproportionation step may be sintered and hardened, it may be used after being pulverized. By pulverizing, the particle size becomes suitable for producing a negative electrode for a non-aqueous secondary battery. The composite powder may be classified to 20 μm or less, further 5 μm or less, and then used for the production of a negative electrode.

<Negative electrode for non-aqueous secondary battery>
A negative electrode for a non-aqueous secondary battery is produced using the negative electrode active material. The negative electrode for a non-aqueous secondary battery mainly includes a negative electrode active material, a conductive additive, and a binder that binds the negative electrode active material and the conductive additive.

The negative electrode active material is the above-described negative electrode active material for non-aqueous secondary batteries. In addition, after making said negative electrode active material for non-aqueous secondary batteries into the main active material material, you may add and use another already known negative electrode active material (for example, graphite, Sn, Si, etc.).

As the conductive aid, a material generally used for an electrode of a lithium secondary battery may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides these carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary agent may be used. One of these may be used alone or in combination of two or more. The blending ratio of the conductive additive is, in mass ratio, negative electrode active material: conductive additive = 1: 0.01 to 1: 0.5. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the moldability of the electrode is deteriorated and the energy density of the electrode is lowered.

The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used. The mixing ratio of the binder is preferably negative electrode active material: binder = 1: 0.05 to 1: 0.5 in terms of mass ratio. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

The negative electrode active material is generally used in a state in which the negative electrode is pressed onto a current collector as an active material layer. A metal mesh or metal foil can be used for the current collector. For example, a current collector made of copper or copper alloy may be used.

The method for producing the negative electrode is not particularly limited, and may be performed in accordance with a generally practiced method for producing an electrode for a non-aqueous secondary battery. For example, the conductive additive and the binder are mixed with the negative electrode active material, and an appropriate amount of an organic solvent is added as necessary to obtain a paste-like electrode mixture. The electrode mixture is applied to the surface of the current collector, dried, and then pressed and pressed as necessary. According to this manufacturing method, the produced electrode becomes a sheet-like electrode. This sheet-like electrode may be cut into dimensions according to the specifications of the non-aqueous secondary battery to be produced.

<Non-aqueous secondary battery>
A non-aqueous secondary battery is composed of the positive electrode, the negative electrode for a non-aqueous secondary battery, and a non-aqueous electrolyte solution in which an electrolyte material is dissolved in an organic solvent. This non-aqueous secondary battery includes a separator and a non-aqueous electrolyte sandwiched between a positive electrode and a negative electrode in addition to a positive electrode and a negative electrode, as in a general secondary battery.

The separator separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

The nonaqueous electrolytic solution is obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent. There is no limitation in particular in the kind of nonaqueous electrolyte solution used with a nonaqueous secondary battery provided with said negative electrode for nonaqueous secondary batteries. As the non-aqueous electrolyte, one or more selected from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like. Can be used. As the electrolyte to be dissolved, an alkali metal salt that is soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , NaPF ₆ , NaBF ₄ , NaAsF ₆ , LiBOB can be used.

The negative electrode is as described above. The positive electrode includes a positive electrode active material into which alkali metal ions can be inserted and removed, and a binder that binds the positive electrode active material. Further, a conductive aid may be included. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the nonaqueous secondary battery. Specifically, examples of the positive electrode active material include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₂ , and S. The current collector may be any material that is generally used for the positive electrode of a non-aqueous secondary battery, such as aluminum, nickel, and stainless steel.

The shape of the non-aqueous secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be adopted. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. After connecting using a lead or the like, the electrode body is sealed in a battery case together with a non-aqueous electrolyte to form a battery.

As mentioned above, although embodiment of the negative electrode active material for non-aqueous secondary batteries of this invention and its manufacturing method was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

Hereinafter, the present invention will be specifically described with reference to examples of the negative electrode active material for a non-aqueous secondary battery and a method for producing the same according to the present invention.

<Example 1>
(Synthesis of negative electrode active material)
Heat-treated SiO powder and FeSi ₂ powder (Fukuda Metal Foil Powder Co., Ltd.) were prepared. The heat-treated SiO powder was obtained by heat-treating amorphous SiO powder in vacuum at 1100 ° C. for 5 hours.

After classifying the heat-treated SiO powder to 31 μm or less and the FeSi ₂ powder to 15 μm or less, 3.67 g of the heat-treated SiO powder and 1.33 g of FeSi ₂ powder were weighed, and the heat-treated SiO powder and FeSi ₂ powder were 7: 1 ( A raw material powder contained in a molar ratio) was obtained.

Raw material powder 5g the ZrO ₂ balls of φ12mm are 100 in entered the ZrO ₂ made of container (capacity: 45cc) was added to, and milled using a planetary ball mill (Fritsch Japan Co., Ltd. P-7), A composite powder was obtained. Milling was carried out in an argon gas atmosphere for 10 hours at 700 rpm of the container.

(Preparation of negative electrode for lithium secondary battery)
An electrode (negative electrode) using the composite powder obtained by milling as a negative electrode active material was produced.

A negative electrode active material (composite powder) and ketjen black (KB) as a conductive additive were mixed to obtain a mixed powder. Polyamideimide-silica hybrid resin as a binder to N-methylpyrrolidone (NMP) (Arakawa Chemical Industries, solvent composition: NMP / xylene = 4/1, cured residue 30.0%, cured residue) Silica: 2% (all proportions are mass ratios, viscosity 8700 mPa · S / 25 ° C.) were dissolved. This solution was mixed with the mixed powder of the composite powder and KB to prepare a slurry. The compounding ratio of the negative electrode active material, KB, and binder (solid content) was 80.75: 4.25: 15 by mass ratio.
The prepared slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 18 μm using a doctor blade, and a negative electrode active material layer was formed on the copper foil. Then, it dried at 80 degreeC for 20 minute (s), and NMP was volatilized and removed from the negative electrode active material layer. After drying, the current collector and the negative electrode active material layer were firmly and closely joined with a roll press.

This was heat-cured at 200 ° C. for 2 hours to obtain an electrode having an active material layer thickness of about 15 μm.

(Production of lithium secondary battery)
A lithium secondary battery (half cell) was produced using the electrode produced by the above procedure as an evaluation electrode.
The counter electrode was a metal lithium foil (thickness 500 μm). The counter electrode was cut to φ13 mm, the evaluation electrode was cut to φ11 mm, and a separator (Hoechst Celanese glass filter and celgard 2400) was sandwiched between them to form an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032 coin cell manufactured by Hosen Co., Ltd.). In addition, a non-aqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1 M was injected into the battery case in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a ratio of 1: 1 (volume ratio). The battery case was sealed to obtain a lithium secondary battery.

<Example 2>
The composite powder (negative electrode active material) synthesized in Example 1 was heat-treated. The heat treatment was performed in an argon gas atmosphere and held at 1100 ° C. for 5 hours.

<Comparative Example 1>
A lithium secondary battery was produced in the same manner as in Example 1, except that the heat treated SiO powder was used as the negative electrode active material instead of the composite powder of Example 1.

<Comparative example 2>
Only the heat treated SiO powder was milled under the same milling conditions as in Example 1.
Using the milled powder as the negative electrode active material, a lithium secondary battery was produced in the same manner as in Example 1.

<Evaluation>
(Charge / discharge characteristics of lithium secondary battery)
A charge / discharge test was performed on the three types of lithium secondary batteries produced, and initial charge / discharge efficiency and cycle characteristics were evaluated.

The charge / discharge test was conducted at a constant current of 0.05 mA up to a charge end voltage of 2 V after charging at a constant current of 0.05 mA to a discharge end voltage of 0.01 V on a metal Li basis in a temperature environment of 25 ° C. Discharge was performed. “Charge” is the direction in which the active material of the evaluation electrode occludes Li, and “discharge” is the direction in which the active material of the evaluation electrode releases Li.

The charge / discharge curve is shown in FIG. From FIG. 2, the initial charge capacity, the initial discharge capacity at 1V, and the initial discharge capacity at 2V were read, and the initial charge / discharge efficiency was calculated. The initial charge / discharge efficiency is a value determined by a percentage ((initial charge capacity) / (initial discharge capacity) × 100) obtained by dividing the initial charge capacity by the initial discharge capacity. The read initial charge capacity and initial discharge capacity, and the calculated initial charge / discharge efficiency are shown in Table 1.

The cycle characteristics are as follows: 1st to 5th cycles under a temperature environment of 25 ° C., after charging with a constant current of 0.05 mA to a discharge end voltage of 0.01 V on the basis of metallic Li, 0 to a discharge end voltage of 2 V Charging / discharging was performed repeatedly at a constant current of 0.05 mA. Subsequently, charging / discharging was repeatedly performed at 0.1 mA for the 6th to 10th cycles and 0.2 mA for the 11th to 15th cycles. The final charge / discharge voltage was 0.01 to 2 V in all cycles.

The charge capacity in each cycle is shown in FIG. 3, and the capacity maintenance rate in each cycle is shown in FIG. The capacity retention rate is a value obtained by a percentage ((N-cycle discharge capacity) / (first-cycle discharge capacity) × 100) obtained by dividing the N-cycle charge capacity by the initial charge capacity. is there. N is an integer of 1 to 15.

The lithium secondary battery of Example 1 was excellent in both initial charge / discharge efficiency and cycle characteristics. Moreover, the initial charge / discharge efficiency and cycle characteristics of the lithium secondary battery of Comparative Example 1 were superior to the lithium secondary battery of Comparative Example 2. In other words, it was found that when the heat-treated SiO powder was milled, the initial charge / discharge efficiency and cycle characteristics were lowered by the energy generated by milling affecting the structure of the Si phase related to the insertion and release of Li.

(X-ray diffraction measurement)
The composite powders of Example 1 and Example 2 were subjected to XRD measurement using CuKα. Moreover, in order to compare with the raw material powder before milling, the same measurement was performed on the heat-treated SiO powder and FeSi ₂ powder used as raw materials. The results are shown in FIGS. 5 and 6 indicate the peak positions of Si and SiO ₂ , FeSi ₂ and FeSi calculated from the interplanar spacing d described in the ASTM card.

From FIG. 5, it was confirmed that the heat-treated SiO powder of Comparative Example 1 was decomposed into two phases of an amorphous SiO ₂ phase and a fine crystalline Si phase by a disproportionation reaction. Further, from FIG. 6, it was found that the FeSi ₂ powder used contained FeSi together with FeSi ₂ .

FIG. 7 shows the XRD measurement results of the composite powders of Example 1 and Example 2. The lower two are diffraction patterns of the heat-treated SiO powder and FeSi ₂ powder used as the raw material powder. In the heat-treated SiO powder and the composite powder of Example 2, a diffraction peak from the Si phase was observed in the vicinity of 2θ = 56 °, but not in the composite powder of Example 1. This is because the Si phase is amorphized in the composite powder of Example 1 by milling the raw material powder, and the Si phase is crystallized in the composite powder of Example 2 by further heat treatment. On the other hand, in the composite powder of Example 2, 2 [Theta] = 21.7 ° around broad diffraction lines of the amorphous SiO ₂ phase is observed, after the heat treatment is SiO ₂ phases remained amorphous. Moreover, in Example 1, the crystal | crystallization was refined | miniaturized by the influence of milling, and the half value width of the diffraction peak was wide as a whole.

The following was found from the above XRD measurement results. The heat-treated SiO powder of Comparative Example 1 was composed of an amorphous SiO ₂ phase and a fine crystalline Si phase. In the composite powder of Example 1 obtained by milling the heat treated SiO powder and the Fe—Si based powder, the crystalline Si phase in the heat treated SiO powder was present in an amorphous state. The composite powder of Example 2 obtained by heat-treating the composite powder of Example 1 obtained by milling had a crystalline Si phase in which an amorphous Si phase was crystallized.

(X-ray photoelectron spectroscopy (ESCA))
The surface analysis of the composite powder of Example 1 was performed. In the surface analysis, the photoelectron spectra of Fe _2p and Si _2p were measured using ESCA, and Fe and Si were quantified from the photoelectron intensity. Moreover, in order to compare with the raw material powder before milling, the same measurement was performed on the heat-treated SiO powder and FeSi ₂ powder used as raw materials. The results are shown in Table 2.

The quantitative result of Fe is shown as the abundance ratio of Fe in each chemical bond state when the Fe contained in the sample is 100 atomic%. The quantitative results of Si were normalized with respect to Si in a chemical bond state represented by Si ²⁺ and indicated as the abundance ratio of Si in each chemical bond state. As for the raw material powder, it was assumed that the heat-treated SiO powder and the FeSi ₂ powder were simply mixed, and the calculated values were calculated from the respective heat treatment SiO powder (not shown) and FeSi ₂ powder.

Regarding Fe, the FeSi ₂ powder was combined with the heat-treated SiO powder by milling, so that the ratio of Fe ¹⁺ / Fe ²⁺ decreased and the ratio of Fe ³⁺ increased. That is, it can be said that FeSi ₂ and FeSi reacted on the surface of the heat-treated SiO particles to produce Fe ₂ O ₃ . Moreover, regarding Si, since the ratio of Si ⁴⁺ in Example 1 was very high, it was found that the composite powder contains more SiO ₂ than the raw material powder that is a simple mixture. That is, it was found that SiO ₂ was generated along with the generation of Fe ₂ O _{3 on} the particle surface of the composite powder.

(Relationship between composite powder components and battery characteristics)
In the lithium secondary battery of Example 1, FeSi ₂ powder derived from FeSi ₂ and FeSi together with amorphous Si phase and amorphous SiO ₂ phase derived from heat-treated SiO powder, derived from FeSi ₂ powder produced by milling on the surface layer A composite powder containing composite particles containing Fe ₂ O ₃ and SiO ₂ was used. The lithium secondary battery of Example 1 was found to be excellent in both initial charge / discharge efficiency and cycle characteristics. It is presumed that the cycle characteristics of the lithium secondary battery of Example 1 were greatly improved as compared with Comparative Example 1 and Comparative Example 2 due to SiO ₂ generated on the particle surface of the composite powder.

The lithium secondary battery of Comparative Example 1 containing a crystalline Si phase had a higher initial discharge efficiency than Comparative Example 2 containing an amorphous Si phase. From this, it is considered that the lithium secondary battery of Example 2 containing a crystalline Si phase exhibits initial discharge efficiency superior to that of Example 1 while maintaining the cycle characteristics shown in Example 1.

Claims

A negative electrode active material for a non-aqueous secondary battery comprising a composite powder obtained by milling a silicon oxide-based powder containing silicon oxide and an iron-based powder containing iron,
The composite powder includes silicon oxide derived from the silicon oxide-based powder and iron derived from the iron-based powder and / or an iron compound containing iron derived from the iron-based powder. Negative electrode active material for batteries.
The negative electrode active material for a non-aqueous secondary battery according to claim 1, wherein the composite powder contains an amorphous silicon phase.
2. The negative electrode active material for a non-aqueous secondary battery according to claim 1, wherein the composite powder contains a crystalline silicon phase.
The non-aqueous secondary battery negative electrode active material according to claim 1, wherein the iron-based powder includes an alloy and / or a compound containing iron and silicon.
The iron-based powder includes an Fe—Si-based compound powder made of iron and silicon,
2. The negative electrode for a non-aqueous secondary battery according to claim 1, wherein the composite powder includes iron oxide containing iron derived from Fe—Si compound powder and silicon dioxide containing silicon derived from Fe—Si compound powder on a particle surface thereof. Active material.
It is a manufacturing method of the negative electrode active material for non-aqueous secondary batteries of Claim 1, Comprising:
Including a milling step in which a raw material powder containing silicon oxide-based powder containing silicon oxide and iron-containing powder containing iron is milled in an inert atmosphere to combine the silicon oxide-based powder and the iron-based powder. A method for producing a negative electrode active material for a non-aqueous secondary battery.
A disproportionation step, which is performed before the milling step, includes a disproportionation step of disproportionating silicon monoxide raw material silicon oxide powder containing silicon monoxide powder into a silicon dioxide phase and a silicon phase to obtain the silicon oxide-based powder. Item 7. A method for producing a negative electrode active material for a non-aqueous secondary battery according to Item 6.
The silicon oxide-based powder includes a silicon phase,
The method for producing a negative electrode active material for a non-aqueous secondary battery according to claim 6, further comprising a crystallization step that is performed after the milling step and crystallizes the silicon phase that has been amorphized in the milling step.
The method for producing a negative electrode active material for a non-aqueous secondary battery according to claim 7, further comprising a crystallization step that is performed after the milling step and crystallizes the silicon phase that has been amorphized in the milling step.
The silicon oxide powder includes silicon monoxide powder,
The manufacturing method of the negative electrode active material for non-aqueous secondary batteries of Claim 6 including the disproportionation process performed after the said milling process and disproportionating a silicon monoxide into a silicon dioxide phase and a crystalline silicon phase. .
The method for producing a negative electrode active material for a non-aqueous secondary battery according to claim 6, wherein the iron-based powder contains an alloy and / or a compound containing iron and silicon.
A negative electrode active material for a non-aqueous secondary battery obtained by the production method according to claim 6.
A non-aqueous secondary battery comprising: a positive electrode; a negative electrode including the negative electrode active material for a non-aqueous secondary battery according to claim 1; and a non-aqueous electrolyte.