JP2007165300A - Nonaqueous electrolyte secondary battery, and method for producing negative electrode material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having an excellent cycle characteristic, and excelling in a capacity recovery rate after storage in an overdischarged state, by improving a negative electrode material. <P>SOLUTION: This nonaqueous electrolyte secondary battery includes a positive electrode capable of reversibly storing and releasing lithium, a negative electrode including an alloy material as an active material, and a nonaqueous electrolyte. The alloy material includes a phase (phase A) containing at least Si and a phase (phase B) containing an intermetallic compound composed of Si and at least one kind selected from the group consisting of Ti, Zr, Ni and Cu; and the alloy material contains 0.0006 to 1.0 wt.% of Fe in a metallic state. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a nonaqueous electrolyte secondary battery, and particularly to an improvement of the negative electrode. More specifically, the present invention relates to an improvement in an alloy material containing Si and a transition metal.

Nonaqueous electrolyte secondary batteries have been actively researched because they can achieve high energy density at high voltage. The positive electrode of a non-aqueous electrolyte secondary battery that has been studied includes transition metal oxides and transition metal chalcogen compounds such as LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , V ₂ O ₅ , Cr ₂ O ₅ , and MnO _2. TiS ₂ , MoS ₂ or the like is used. These have a layered or tunnel-like crystal structure through which lithium ions can enter and exit. On the other hand, for the negative electrode, a carbon material capable of reversibly inserting and extracting lithium and having excellent cycle life and safety is used. A lithium ion battery using a graphite-based carbon material as a negative electrode has been put into practical use.

However, since the graphite material has a theoretical capacity of 372 mAh / g and a theoretical density of 2.2 g / cm ³ , both of which are relatively small, it is expected that a metal material capable of realizing a higher capacity than the graphite material will be used as the negative electrode. Has been. Among these materials, silicon (Si) has a theoretical capacity of 4199 mAh / g (theoretical density 2.33 g / cm ³ ), and is actively researched and developed.

  Si is highly expected as a high-capacity negative electrode material, but has a serious problem in the charge / discharge cycle characteristics of the battery. It is caused by repeated expansion and contraction of Si accompanying the insertion and desorption of lithium during charging and discharging reactions, increasing the contact resistance between particles inside the negative electrode, and degrading the current collecting network. It is a problem. The deterioration of the current collection network is a main factor for shortening the charge / discharge cycle life.

  Many proposals have already been made for the above problem. For example, Patent Document 1 and Patent Document 2 include reversible insertion and extraction of lithium as a negative electrode material, including solid phase A and solid phase B having different compositions, and at least a part of solid phase A is An alloy that is coated with a solid phase B, where the solid phase A contains silicon, tin, zinc, etc., and the solid phase B contains a group 2A element, transition element, group 2B element, group 3B element, group 4B element, etc. Materials have been proposed. Further, it has been proposed that the solid phase A is more preferably in an amorphous or low crystalline state.

  Furthermore, in Patent Document 3, the negative electrode material is composed of at least two different phases, one is an A phase mainly composed of Si, the other is a B phase containing a silicide of a transition metal and Si, and the A phase and It has been proposed that at least one of the B phases is at least one selected from an amorphous state or a low crystalline state, thereby improving the cycle life.

Further, Patent Document 4 proposes that the cycle characteristics can be further improved by including iron in an amount of 0.002% by weight or more as a whole alloy.
US Pat. No. 6,090,505 JP 2004-103340 A JP 2004-335272 A JP 2000-173616 A

  According to these prior arts, cracks at the time of expansion and contraction of the alloy material can be greatly suppressed, and there is a certain effect in that deterioration of the current collecting network, which is a main factor of cycle characteristic deterioration, can be suppressed. However, as a result of detailed examination of the storage characteristics in an overdischarge state, which is one of the important characteristics of the battery, it has become clear that the above technique may not provide sufficient characteristics.

  Patent Document 3 discloses a so-called mechanical synthesis method in which an alloy is mechanically synthesized in order to make the alloy material amorphous or low crystalline for the purpose of suppressing further cycle deterioration. Although it has been proposed, there is no description focusing on the Fe element contained. Furthermore, Patent Document 4 describes a synthesis method by a rapid cooling method, and the obtained alloy material is considered to be crystalline. Moreover, it is a regulation of the content of Fe contained in the alloy as a whole, there is no description about the chemical state of Fe, and the mechanism for improving the cycle characteristics is not clarified in detail. .

  From the viewpoint of obtaining a negative electrode material not only having good cycle characteristics but also having excellent storage characteristics in an overdischarged state, the present inventors have proposed a phase consisting mainly of Si, and a metal of transition metal and Si. With regard to the alloy material containing the B phase having the intermetallic compound, the inventors have intensively studied the content and content of Fe. As a result, the inventors have come to know that optimizing the Fe content in the alloy material and its chemical state is effective in achieving both cycle characteristics and storage characteristics in an overdischarged state. Furthermore, the present inventors have found that the Fe content can be specified by the saturation magnetic susceptibility as a method for measuring the chemical state of Fe.

  The present invention provides a nonaqueous electrolyte secondary battery comprising a negative electrode based on the above knowledge. That is, the non-aqueous electrolyte secondary battery of the present invention includes a positive electrode capable of reversibly inserting and extracting lithium, a negative electrode including an alloy material as an active material, and a non-aqueous electrolyte, and the alloy material includes at least Si. And a phase (B phase) containing an intermetallic compound of Si and at least one selected from the group consisting of Ti, Zr, Ni and Cu, and the alloy material contains Fe in a metallic state. It is characterized by containing 0.0006 to 1.0% by weight.

  Further, the present invention provides a raw material containing at least one transition metal element selected from the group consisting of Si, Fe, and Ti, Zr, Ni, and Cu, a phase containing at least Si (phase A), and the at least one kind A phase (B phase) including a transition metal element and an intermetallic compound of Si, and including a step of manufacturing an alloy material including Fe in a metal state, and in the step, the amount of Fe in the alloy material Provided is a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery, wherein a saturation magnetic susceptibility that changes in response is detected, and the process is terminated when the detected saturation magnetic susceptibility reaches a predetermined value.

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having a high capacity, excellent charge / discharge cycle characteristics, and excellent storage characteristics in an overdischarged state.

  According to the present invention, by appropriately controlling the amount of Fe in a metal state, deterioration of storage characteristics due to dissolution of metal state Fe from the negative electrode can be suppressed during storage in an overdischarged state. This is considered to be due to the improved resistance of the alloy material, which is the negative electrode active material, during storage in the overdischarged state.

The nonaqueous electrolyte secondary battery of the present invention includes a negative electrode containing an alloy material as an active material. The alloy material has a phase containing at least Si (A phase) and a phase containing an intermetallic compound of Si and at least one selected from the group consisting of Ti, Zr, Ni, and Cu (B phase), It contains 0.0006 to 1.0% by weight of Fe in a metal state.
Here, if the metal Fe contained in the alloy material is less than 0.0006% by weight, the cycle characteristics deteriorate, and if it exceeds 1.0% by weight, the storage characteristics in the overdischarge state deteriorate.

  The alloy material as the negative electrode active material according to the present invention includes an A phase mainly composed of Si, and a B phase composed of an intermetallic compound of at least one selected from the group consisting of Ti, Zr, Ni and Cu and Si. Including. This alloy material not only mitigates the influence of expansion, but also suppresses the decrease in electronic conductivity of the negative electrode that accompanies the expansion and contraction. Therefore, the negative electrode containing this alloy material provides a battery with high capacity and excellent cycle characteristics.

  Here, the A phase is a phase responsible for insertion and extraction of Li, and is a phase that can electrochemically react with Li. The A phase may be a phase mainly composed of Si, but is preferably a phase composed of Si alone. When the A phase is composed of Si alone, the amount of Li absorbed and released by the alloy material per unit weight or unit volume can be made extremely large. However, since Si simple substance is a semiconductor, it has poor electronic conductivity. Therefore, it is effective to add a trace amount of additive elements such as phosphorus (P), boron (B), hydrogen (H), etc., or transition metal elements to the A phase up to about 5% by weight.

On the other hand, the B phase is composed of an intermetallic compound of a transition metal and Si. The intermetallic compound containing Si has a high affinity with the A phase, and cracking at the interface between the A phase and the B phase is unlikely to occur even when the alloy volume expands during charging. In addition, the B phase has higher electron conductivity and higher hardness than the Si single phase. Therefore, the B phase works to supplement the low electronic conductivity of the A phase and to maintain the shape of the alloy particles against the expansion stress. A plurality of B phases may be present, and two or more intermetallic compounds having different compositions may be present as the B phase. For example, when the transition metal is represented by M, MSi ₂ and MSi may exist in the alloy particles. Further, intermetallic compounds containing different transition metal elements, for example, M ¹ Si ₂ and M ² Si ₂ (M ¹ ≠ M ² ) may be present in the alloy particles.

  The A phase and / or the B phase are not particularly limited in their crystallinity, but are preferably amorphous or low crystalline. The reason for this is that cracking of alloy particles due to expansion associated with the occlusion of Li hardly occurs.

  When the alloy material constituting the A phase and / or the B phase is amorphous or low crystalline, the crystallite (crystal grain) size is preferably 100 nm or less, and preferably 5 nm or more and 100 nm or less. Is more preferable. When the crystallite size is larger than 100 nm, the grain boundary between the crystallites is reduced, so that the effect of suppressing particle cracking is reduced. Moreover, when the crystallite size is less than 5 nm, the grain boundary between the crystallites increases, and the electron conductivity in the alloy may decrease. When the electronic conductivity of the alloy decreases, the polarization of the negative electrode increases, leading to a decrease in battery capacity.

  The state of the A phase and the B phase constituting the alloy material uses CuKα as an X-ray source, and performs X-ray diffraction measurement within a diffraction angle 2θ of 10 ° to 80 °. , Whether or not there is a peak attributed to the crystal face of the A phase and / or the B phase.

  For example, in the case of the A phase composed of Si, a peak corresponding to the crystal plane (111) is observed at a diffraction angle 2θ = 28.4 °, reflecting the crystal plane of Si, and a crystal plane (220 at 47.3 °). ), A peak corresponding to the crystal plane (311) is observed at 56.1 °, a peak corresponding to the crystal plane (400) is observed at 69.1 °, and a peak corresponding to 76.4 ° is observed. A peak corresponding to the crystal plane (331) is observed. In addition, the peak corresponding to the crystal plane (111) observed at the diffraction angle 2θ = 28.4 ° often has the strongest intensity. However, when the phase is composed of a microcrystalline region, a sharp peak is not observed and a relatively broad peak is observed. On the other hand, when the alloy material is made of an amorphous region, a broad halo pattern in which the half width cannot be recognized is observed in the diffraction spectrum of the alloy particles obtained by the X-ray diffraction measurement.

  The crystallite size can be determined by X-ray diffraction measurement. Specifically, the half-value width of a peak attributed to each phase in the diffraction spectrum of alloy particles obtained by X-ray diffraction measurement can be obtained and calculated from the half-value width and Scherrer's formula. When there are a plurality of peaks attributed to each phase, the half width of the peak with the highest intensity is obtained, and the Scherrer formula is applied thereto.

  According to Scherrer's equation, the crystallite size D is given by equation (1). In Equation (1), λ = X-ray wavelength (nm) (1.5405 nm in the case of CuKα), β = half-value width (rad) of the peak, and θ = half value (rad) of the peak angle 2θ.

        D (nm) = 0.9 × λ / (β × cos θ) (1)

  Usually, attention should be paid to the peak with the highest intensity in the range of the diffraction angle 2θ of 10 to 80 °, but it is more preferable to pay attention to the peak with the highest intensity in the range of the diffraction angle 2θ of 20 to 35 °.

  When X-ray diffraction measurement of an alloy material is performed using CuKα ray as a radiation source, the highest intensity observed in the diffraction angle 2θ = 10 to 80 ° or diffraction angle 2θ = 20 to 35 ° of the obtained diffraction spectrum. The half width of a strong diffraction peak is preferably 0.09 ° or more. In this case, it can be determined that the crystallite size is 100 nm or less.

  In addition, the cross-section of the alloy particles can be observed using an AFM (Atomic Force Microscope), TEM (Transmission Electron Microscope), etc., and the crystallite size can be directly measured. The abundance ratio (phase composition) of the A phase and the B phase in the alloy can be measured using EDX (energy dispersive X-ray spectroscopy (EDS)) or the like.

  In the alloy material, the proportion of the A phase in the total weight of the A phase and the B phase is preferably 5% by weight or more and 95% by weight or less. Although the one where the ratio of A phase is large can achieve high capacity effectively, the volume change at the time of charging / discharging increases simultaneously. Therefore, it is important to maintain the current collecting property inside the electrode. From the viewpoint of maintaining the cycle characteristics at a high level, the ratio of the A phase to the total weight of the A phase and the B phase is preferably 80% by weight or less, and more preferably 50% by weight or less.

The transition metal element is at least one selected from the group consisting of Ti, Zr, Ni and Cu, and preferably at least one selected from the group consisting of Ti and Zr. These element silicides have higher electronic conductivity and higher hardness than silicides of other elements. When the transition metal element is Ti, the B phase preferably contains TiSi ₂ .

  In the alloy materials as described above, the transition of the alloy accompanying the expansion of the A phase during occlusion of Li is blocked by the grain boundaries between the crystallites, so that the occurrence of particle cracking is remarkably suppressed. It is done. In this way, by suppressing particle cracking of the alloy material that is the negative electrode active material, it is possible to obtain a non-aqueous electrolyte secondary battery with little deterioration accompanying the charge / discharge cycle.

  Next, Fe contained in the alloy material that plays an important role in the present invention will be described.

  It is essential that Fe is contained in the alloy material in a metallic state by 0.0006 to 1.0% by weight. As a result of detailed studies by the present inventors, it was found that when the Fe in the metal state in the alloy material is less than 0.0006% by weight, the cycle characteristics are deteriorated. It is presumed that Fe in a metal state plays a catalytic role in suppressing the formation of a film on the surface of the alloy material during charging and discharging, and acts as a nucleus that inhibits the formation of a film that becomes an excessive resistance component. In this role, it is considered that 0.0006% by weight or more of Fe in the metal state is effective.

  Moreover, when there is more Fe in a metal state than 1.0 weight%, the capacity | capacitance recovery rate after preserve | saving in an overdischarge state becomes low. The reason is considered as follows. That is, since the negative electrode potential rises (electrochemical oxidation) during storage in an overdischarged state, the dissolution phenomenon of Fe in the metal state increases, leading to an increase in battery impedance and deteriorating storage characteristics. .

  Thus, although Fe is an effective element for improving the cycle characteristics, it has been found that if it is excessively present in the metal state, there is an adverse effect of deteriorating the storage characteristics in the overdischarge state as described above.

  In particular, in order to maintain the cycle characteristics at a high level, it is important to make the A phase and the B phase amorphous or low crystalline. For this purpose, a mechanical synthesis method is used for the production of alloy materials. It is preferable to choose. In the mechanical synthesis method, different metal elements can be made into an amorphous or low crystalline state by causing an alloying reaction using mechanical impact energy. Such a mechanical synthesis method has extremely high reaction homogeneity between the raw material elements as compared with a rapid cooling method which is a conventional method for producing a typical alloy material. In the rapid cooling method, it is considered that the reaction tends to be inhomogeneous or non-equilibrium due to rapid cooling and solidification from the molten state of the alloy. Therefore, the mechanical synthesis method is a synthesis method in which an amorphous or low crystalline state can be easily obtained, and has a different feature that a homogeneous alloying reaction is easily performed.

Further, Fe contained in the alloy material is difficult to be detected by X-ray diffraction measurement or TEM because its content is very small.
Therefore, saturation magnetic susceptibility measurement is performed in order to examine the existence state of the Fe component. In the following examples, a sample vibration type magnetometer (MODEL 7407, manufactured by Toyo Technica Co., Ltd.) was used as a measuring apparatus.

The sample vibration type magnetometer vibrates the sample at a low frequency of about 80 Hz with a slight amplitude of about 0.1 to 0.2 mm, and searches for the change in magnetic flux due to the magnetization of the sample near the magnetic pole of the electromagnet. It is detected as an induced electromotive force induced in the coil. Since the induced electromotive force is proportional to the magnetization, the magnetization can be measured. That is, M (magnetization) is calculated from the following equation to obtain a saturation magnetic susceptibility.
B = μ ₀ H + M
In the formula, B represents the magnetic flux density, H represents the magnetic field, and μ ₀ represents the vacuum permeability.
Thus, the magnetic field (H) against the magnetic flux density (B) generated by the external magnetic field in the sample is plotted.

  In general, when the sample is a diamagnetic material and a ferromagnetic material, the BH curves are as shown in FIGS. 2 and 3, respectively. When the sample is a mixture of a diamagnetic material and a ferromagnetic material, the BH curve is as shown in FIG. In FIG. 4, a point b where the extension of the straight line portion on the left side of the BH curve intersects the B axis is obtained as the saturation magnetic susceptibility.

By obtaining the saturation magnetic susceptibility of a standard sample containing metal Fe in advance, the amount of Fe in the metallic state in the sample can be obtained from the saturation magnetic susceptibility of each sample.
Specifically, since the saturation magnetic susceptibility of metal Fe in this measurement method was 150 emu / g (Fe), assuming that the saturation magnetic susceptibility of each sample is Memu / g (sample), the metal Fe contained in each sample The amount is approximately (M / 150) × 100% by weight.
It is desirable that the saturation magnetic susceptibility per 1 g of the alloy material is 0.001 to 1.7 emu / g.

  The method for producing a negative electrode material for a non-aqueous electrolyte secondary battery according to the present invention detects a saturation magnetic susceptibility that changes according to the amount of Fe element in the negative electrode material during the synthesis, and the detected saturation magnetic susceptibility is predetermined. When the value is reached, it is desirable to finish the synthesis of the negative electrode material. A mechanical alloy method in which a raw material containing Si and a raw material containing a transition metal are mixed and subjected to mechanical alloying is particularly suitable because the state of the crystallites of each phase can be easily controlled.

  Although it does not specifically limit as a raw material of an alloy material and the raw material of Fe contained in an alloy material, For example, a simple substance, an alloy, a solid solution, an intermetallic compound etc. can be used. As a method for producing the Fe raw material, specifically, a mechanical alloying method can be given, but besides this, a casting method, a gas atomizing method, a liquid quenching method, an ion beam sputtering method, a vacuum deposition method, a plating method, Any of gas phase chemical reaction methods may be used.

  The form of the negative electrode active material is not particularly limited as long as a necessary constituent ratio can be realized. For example, the element simple substance which comprises a negative electrode active material can be mixed with the target structural ratio, the alloy which has the target structural ratio, a solid solution, an intermetallic compound, etc. can be used.

  Moreover, before performing a mechanical alloying process, you may perform the process which fuse | melts the mixture of a raw material and quenches and solidifies a molten material. However, in order to efficiently provide the raw material containing Si with the effect of compounding, that is, the refinement of crystallites by mixing different kinds of elements, to the raw material containing Si and the raw material containing the transition metal element from the beginning. A mechanical alloy method in which mixing and mechanical alloying are performed is particularly preferable.

  Among them, a raw material containing Si and a raw material containing at least one element selected from transition metal elements are mixed, and further, a raw material containing Fe to be contained in the alloy material is mixed, and a mechanical alloy method is used for the negative electrode material. When used in the manufacturing process, the state can be easily controlled, which can be said to be a preferable method for manufacturing a negative electrode material. Further, before the step of performing the mechanical alloying process, there may be a step of melting or a step of rapidly cooling and solidifying the molten material to form a solidified product.

The manufacturing method by the above mechanical alloying treatment is a synthesis method in a dry atmosphere, but the particle size distribution after synthesis may have a very large or small width. Therefore, after the synthesis, a pulverization process or a classification process for adjusting the particle size may be performed.
As a pulverizing apparatus, a general apparatus may be used, but apparatuses such as an attritor, a vibration mill, a ball mill, a planetary ball mill, a bead mill, and a jet mill can be used.

  The negative electrode according to the present invention can contain a conductive agent as necessary in addition to the alloy material. Examples of the conductive agent include natural graphite such as flake graphite, graphite such as artificial graphite and expanded graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; carbon Examples thereof include conductive fibers such as fibers, carbon nanotubes, and metal fibers: metal powders such as copper powder and nickel powder: organic conductive materials such as polyphenylene derivatives. These may be used alone or in combination. Among these, it is preferable to use graphite from the viewpoints of density, stability with respect to the electrolytic solution, capacity, and the like.

  When the conductive agent is included in the negative electrode, the amount of the conductive agent is not particularly limited, but is preferably 1 to 50 parts by weight, particularly preferably 1 to 40 parts by weight with respect to 100 parts by weight of the alloy material. However, since the alloy material according to the present invention itself has electronic conductivity, it is possible to obtain a sufficiently functioning negative electrode without using a conductive agent.

  The negative electrode is, for example, an alloy material, if necessary, a conductive agent, a binder, and a dispersion medium are mixed to prepare a negative electrode mixture, which is molded or applied to a current collector, It can be obtained by drying.

  The binder is preferably a material that is electrochemically inactive to Li in the working potential range of the negative electrode and does not affect other substances as much as possible. For example, styrene-butadiene copolymer rubber, polyacrylic acid, polyethylene, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, carboxymethylcellulose, methylcellulose and the like are suitable. The negative electrode used in the present invention has a large volume change at the time of charging, and therefore, a styrene-butadiene copolymer rubber that can respond to the volume change relatively flexibly, and a polyacryl that easily maintains a strong binding state even when the volume changes. An acid or the like is preferable. The amount of the binder added is preferably as large as possible from the viewpoint of maintaining the structure of the negative electrode, but is preferably as small as possible from the viewpoint of improving battery capacity and improving discharge characteristics.

  The non-aqueous electrolyte secondary battery of the present invention includes the above-described negative electrode, a positive electrode capable of electrochemically inserting and extracting Li, and a non-aqueous electrolyte.

  The nonaqueous electrolytic solution may be a gel electrolyte or a solid electrolyte, but in general, an electrolytic solution composed of a nonaqueous solvent and a solute dissolved therein is used. Non-aqueous solvents include, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; methyl formate, methyl acetate Aliphatic carboxylic acid esters such as methyl propionate and ethyl propionate; γ-lactones such as γ-butyrolactone; chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, and ethoxymethoxyethane Cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile Propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate Examples thereof include aprotic organic solvents such as derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, dimethyl sulfoxide, N-methylpyrrolidone, butyl diglyme, and methyl tetraglyme. These are preferably used in combination.

Examples of the solute dissolved in the non-aqueous solvent include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , Examples thereof include LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, lithium tetrachloroborate, lithium tetraphenylborate, and imides. These may be used alone or in combination. The amount of these solutes dissolved in the non-aqueous solvent is not particularly limited, but is preferably 0.2 to 2.0 mol / L, and more preferably 0.5 to 1.5 mol / L.

If a positive electrode is proposed as a positive electrode of a nonaqueous electrolyte secondary battery, it can be used without limitation. The positive electrode generally includes a positive electrode active material, a conductive agent, and a binder. Any positive electrode active material can be used without particular limitation as long as it is proposed as a positive electrode active material for a non-aqueous electrolyte secondary battery, but a lithium-containing transition metal compound is preferred. Representative examples of the lithium-containing transition metal compound include Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₂ , Li _x Co _y Ni _1-y O ₂ , Li _x Co _y M _1-y O _z , _{Li x Ni 1-y M y} O z, Li x Mn 2 O 4, Li x Mn 2-y M y O 4, LiCo 1-x Mg x O 2, LiNi 1-y Co y O 2, LiNi 1- _Examples thereof include, but are not limited to, _yz Co _y Mn _z O ₂ . In these lithium-containing transition metal compounds, M is selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B. It is at least one type, and x = 0 to 1.2, y = 0 to 0.9, and z = 2.0 to 2.3. The x value increases or decreases as the battery is charged and discharged. Further, transition metal chalcogenides, vanadium oxides and lithium compounds thereof, niobium oxides and lithium compounds thereof, conjugated polymers using organic conductive substances, chevrel phase compounds, and the like can also be used as the positive electrode active material. It is also possible to use a combination of a plurality of active materials.

  As the separator interposed between the positive electrode and the negative electrode, a microporous thin film having high ion permeability, predetermined mechanical strength, and electronic insulation is used. A microporous thin film made of a material such as polypropylene, polyethylene, polyphenylene sulfide, polyethylene terephthalate, polyamide, or polyimide is preferably used because of its excellent resistance to non-aqueous solvents and hydrophobicity. These materials may be used alone or in combination. From the viewpoint of manufacturing cost, it is advantageous to use inexpensive polypropylene or the like. In addition, when imparting reflow resistance to the battery, it is preferable to use polyethylene terephthalate, polyamide, polyimide or the like having a heat distortion temperature of 230 ° C. or higher. Further, a sheet made of glass fiber or the like, a nonwoven fabric, a woven fabric, or the like is also used. The thickness of the separator is generally 10 to 300 μm, and the porosity of the separator is determined according to electron conductivity, ion permeability, material, etc., but is generally 30 to 80%. It is desirable.

  The present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a cylindrical shape, a flat shape, a coin shape, and a square shape, and the shape of the battery is not particularly limited. INDUSTRIAL APPLICABILITY The present invention can be applied to batteries of various sealing forms, including batteries that contain power generation elements such as electrodes and electrolytes in metal battery cans and laminated film cases. Is not particularly limited.

  An embodiment of a nonaqueous electrolyte secondary battery according to the present invention is a coin-type nonaqueous electrolyte battery in which a negative electrode can and a positive electrode can are combined through a gasket, for example. A preferred method for producing this battery will be described. First, a positive electrode pellet made of a positive electrode mixture containing a positive electrode active material capable of occluding and releasing lithium ions is arranged in contact with the positive electrode can, and a negative electrode pellet made of a negative electrode mixture containing an Si-containing alloy material is made into the negative electrode can Arrange it inside. Next, the separator is placed on the positive electrode pellet, and the negative electrode pellet, the positive electrode pellet and the separator are impregnated with a lithium ion conductive non-aqueous electrolyte. Thereafter, the negative electrode can and the positive electrode can are combined with a peripheral edge portion of the positive electrode can via a gasket, and the opening end of the positive electrode can is caulked and sealed to the negative electrode can side via the gasket.

  Next, the present invention will be specifically described based on examples and comparative examples. The following examples illustrate preferred embodiments of the present invention, and the present invention is not limited to the following examples.

Example 1
In this example, the type of transition metal constituting the B phase and the case where Fe that is not in a metal state contained were studied.
A negative electrode and a positive electrode were produced in the following manner, and a coin-type battery was assembled. The battery was evaluated for cycle life, discharge capacity, and storage characteristics in an overdischarged state.

(1) Manufacture of alloy material As a raw material of the transition metal element M, metal Ti, metal Zr, metal Ni, and metal Cu were used. All of these were powders having a purity of 99.9% and a particle size of 100 to 150 μm. Si powder (purity 99.9%, average particle size 3 μm) was used as a raw material for Si.

Assuming that the B phase constitutes MSi ₂ , the transition metal and Si were mixed so that the proportion of the A phase in the total weight of the A phase and the B phase in the alloy material to be produced was 20%.
Metal Fe powder was used as a raw material for Fe element. Its purity was 99.9% and the average particle size was 100 μm.
Fe was mixed with the above transition metal and Si mixture so that the content thereof was 0.001 wt%.

  3.5 kg of each mixture is weighed and put into a vibration mill apparatus (manufactured by Chuo Kako Co., Ltd., model number FV-20), and a stainless steel ball (diameter 2 cm) occupies 70% by volume of the mill apparatus capacity. I put it in. After evacuating the inside of the container, argon gas (purity 99.999%, manufactured by Nippon Oxygen Co., Ltd.) was introduced so that the pressure became 1 atm. Mechanical alloying operation was performed under these conditions. The operating conditions of the mill device were an amplitude of 8 mm and a rotation speed of 1200 rpm. Under these conditions, mechanical alloying operation was performed for 80 hours.

  When the Ti—Si alloy obtained by the above operation was collected and analyzed by X-ray diffraction measurement using CuKα ray as a radiation source, a spectrum showing fine crystals was obtained. Further, in the diffraction spectrum obtained by X-ray diffraction measurement, the half-value width of the strongest diffraction peak observed in the diffraction angle range of 2θ = 10 ° to 80 ° and the alloy calculated based on the Scherrer equation The grain size of the crystal grains (crystallites) was 10 nm.

From the result of the X-ray diffraction measurement, it was estimated that the Si single phase (A phase) and the TiSi ₂ phase (B phase) exist in the Ti—Si alloy. Assuming that these two phases are mainly present and calculating the ratio of the Si simple phase and the TiSi ₂ phase, it was found that Si: TiSi ₂ = 20: 80 (weight ratio).

  The other three types of alloys were also subjected to X-ray diffraction measurement, and the crystallite size and the weight ratio between the A phase and the B phase were obtained. The same results as above were obtained.

When a cross section of the Ti—Si alloy was observed with a transmission electron microscope (TEM), an Si region composed of an amorphous region, crystal grains (crystallites) with a particle size of about 10 nm, and crystals with a particle size of about 15 to 20 nm. It has been found that a TiSi ₂ phase having grain crystallites exists. When the same measurement was performed on the other three types of alloys, the same results as described above were obtained.

  Further, in order to specify the amount of metallic Fe in the alloy material, the amount of Fe in the metallic state in the alloy material was determined by measuring the saturation magnetic susceptibility. In the present Example, it turned out that 0.001 weight% Fe of a metal state contains in each alloy material.

(2) Production of negative electrode Using the alloy obtained as described above, graphite and binder, a negative electrode was produced in the following manner.

Alloy, graphite (manufactured by Nippon Graphite Industry Co., Ltd., SP-5030) and binder polyacrylic acid (manufactured by Wako Pure Chemical Industries, Ltd., average molecular weight 150,000) in a weight ratio of 70.5: The mixture was mixed at a ratio of 21.5: 7 to obtain a negative electrode mixture. This negative electrode mixture was formed into pellets having a diameter of 4 mm and a thickness of 0.3 mm, and then the pellets were dried at 200 ° C. for 12 hours. The pellet thickness after drying was 300 μm, the porosity was 26.6%, and the density was 1.721 g / cm ³ .

(3) Preparation of positive electrode Manganese dioxide and lithium hydroxide were mixed at a molar ratio of 2: 1, and the mixture was fired in air at 400 ° C. for 12 hours to obtain lithium manganate.

  The obtained lithium manganate, carbon black as a conductive agent, and polytetrafluoroethylene as a binder were mixed in a weight ratio of 88: 6: 6 to obtain a positive electrode mixture. The binder was used in the form of an aqueous dispersion. This positive electrode mixture was formed into pellets having a diameter of 4 mm and a thickness of 1.0 mm, and then the pellets were dried at 250 ° C. for 12 hours.

(4) Production of coin-type battery Using the above-mentioned negative electrode mixture pellet and positive electrode mixture pellet, a coin-type non-aqueous electrolyte 2 having an outer diameter of 6.8 mm and a thickness of 2.1 mm as shown in FIG. The next battery was assembled.

  The positive electrode can 2 also serves as a positive electrode terminal and is made of stainless steel having excellent corrosion resistance. The positive electrode mixture pellet 1 is disposed in the center of the positive electrode can 2. The negative electrode can 6 also serves as a negative electrode terminal and is made of the same stainless steel as the positive electrode can 2. A negative electrode mixture pellet 4 is disposed at the center of the negative electrode can 6. A gasket 5 insulates the positive electrode can 2 and the negative electrode can 6 and is made of polypropylene. A pitch is applied to the surface where the positive electrode can 2 and the negative electrode can 6 and the gasket 5 are in contact with each other.

A separator 3 made of a non-woven fabric made of polyethylene is inserted between the positive electrode mixture pellet 1 and the negative electrode mixture pellet 4. In the electrolytic solution, 1 mol / l LiN (CF ₃ SO ₂ ) ₂ was dissolved in a solvent in which propylene carbonate, ethylene carbonate, and 1,2-dimethoxyethane were mixed at a volume ratio of 1: 1: 1. A thing was used.

<< Comparative Example 1 >>
The same raw material transition metal, Si, and Fe mixture as described above is put in a dissolution tank, dissolved in an inert atmosphere at 1400 ° C., and the melt is rapidly cooled by a roll quenching method to obtain a solidified product. It was. The coagulated product was heat-treated at 500 ° C. for 20 hours under an inert atmosphere, and the heat-treated product was pulverized with a ball mill and classified with a sieve to obtain particles of 45 μm or less. In this way, a negative electrode material was obtained.

About these negative electrode materials, it is confirmed by X-ray diffraction and TEM observation that the crystallite size is composed of crystalline Si single phase and MSi ₂ phase (M = Ti, Zr, Ni, or Cu). It was done.

Further, when the saturation magnetic susceptibility was respectively determined in order to specify the amount of metallic Fe in the alloy material, metallic Fe was not detected. On the other hand, the same amount of Fe as that of the charged raw material was detected by ICP emission analysis. Therefore, it was found that Fe that was not in a metal state was mixed in the alloy synthesized by the rapid cooling method. Furthermore, as a result of X-ray diffraction measurement and TEM observation, the crystallite size of the Si phase and the MSi ₂ phase was about 100 nm, indicating a high crystallinity state.

(5) Battery Evaluation In a thermostat set at 20 ° C., the batteries of Example 1 and Comparative Example 1 were charged with a constant current of charge current 2C (1C is 1 hour rate current), discharge current 0.2C. Then, 200 cycles were repeated in the battery voltage range of 2.0V to 3.3V.

  At that time, the discharge capacity at the second cycle was determined as the initial discharge capacity. In addition, the ratio of the discharge capacity at the 200th cycle to the discharge capacity at the second cycle was obtained as a percentage (%), and was used as the capacity maintenance rate. The closer the capacity retention rate is to 100 (%), the better the cycle life.

  In addition, after 5 cycles under the above charge / discharge cycle conditions, the battery was discharged at a discharge current of 0.2 C to a battery voltage of 0 V, and the battery terminals were short-circuited in a thermostat set at 60 ° C. for 100 days. saved. Thereafter, charging and discharging were repeated 100 cycles in a range of battery voltage 2.0V to 3.3V with a charging current of 0.2C and a discharging current of 0.2C in a 2 ° C thermostat, and the discharge capacity was measured. The percentage of the capacity after storage with respect to the capacity before storage is defined as a capacity recovery rate.

  Table 1 shows the initial discharge capacity, capacity maintenance rate, and capacity recovery rate. In Table 1, Sample No. 1-1 to 1-4 are Example 1, and sample Nos. 11-1 to 11-4 is Comparative Example 1.

  As is clear from Table 1, the batteries of Example 1 all have a higher capacity retention rate at the 200th cycle than the battery of Comparative Example 1, and the capacity recovery rate after storage in the overdischarged state is at the same level. there were.

Although the details of the mechanism by which Fe in the metal state improves the cycle characteristics are unclear, Fe has a catalytic function that suppresses the formation of a film on the surface of the alloy material during charging, and excessive impedance during cycling. It is estimated that the rise is suppressed.
It was also found that the amount of Fe in the metallic state in this example had no adverse effect on the capacity recovery rate after storage in the overdischarged state.

Example 2
In this example, the effect of including Fe in the metal state when the type and crystallinity of the transition metal were changed was examined.

In the same manner as in Example 1, an alloy material (sample No. 2-1 to 2-4) composed of Si phase and MSi ₂ phase (M = Ti, Zr, Ni, or Cu) and Fe content of 0.001 wt%. ) Was produced. The crystallite sizes of these A and B phases were about 10 nm and 15 nm, respectively. An alloy material having high crystallinity, that is, an alloy material having a large crystallite size was obtained by shortening the synthesis time of the mechanical alloy method. Specifically, the synthesis time was 40 hours. The crystallite size of the obtained alloy material (Sample No. 2-5) was 100 nm for both the Si phase (A phase) and the TiSi ₂ phase (B phase).

<< Comparative Example 2 >>
A low crystalline alloy material (sample Nos. 12-1 to 12-4) having a Si phase (A phase) and a TiSi ₂ phase (B phase) in the same manner as in Example 2 except that Fe is not mixed in the raw material. ) And a highly crystalline alloy material (Sample No. 12-5).

  Negative electrodes were produced using the alloy materials of Example 2 and Comparative Example 2, and batteries were assembled and evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3.

The batteries of Example 2 all had a higher capacity retention rate at the 200th cycle than the battery of Comparative Example 2, and the recovery rate after storage in the overdischarged state was at the same level. Further, as is apparent from Table 3, it was found that even in an alloy having high crystallinity, it is effective to improve the cycle characteristics to contain Fe in a metal state.
It was also found that the amount of Fe in the metal state in this example had no adverse effect on the capacity recovery rate after storage.

Example 3
The amount of metallic Fe contained in the alloy material and the saturation magnetic susceptibility of the alloy material were investigated.
Except for changing the amount of Fe added to the raw material, the weight ratio of the A phase (Si phase) to the B phase was 20:80 and Fe was 0.0003 to 2.0% by weight in the same manner as in Example 1. Various alloy materials were produced, including in the range. The obtained alloy material particles were classified into a maximum particle size of 8 μm and an average particle size of 5 μm.

In the same manner as in Example 1, the crystallite size of the A phase was calculated from the half-value width of the peak with the largest intensity obtained by X-ray diffraction measurement of the alloy material and the Scherrer equation.
For example, in a Ti—Si alloy material, it has been found that there is a Si single phase composed of crystal grains (crystallites) of about 10 nm and a TiSi ₂ phase having crystal grains (crystallites) of about 15 nm to 20 nm. The same was true for the other three types of alloy materials.

  Further, the amount of Fe in the metal state was determined from the saturation magnetic susceptibility, as in Example 1. Using each alloy material, a negative electrode was produced in the same manner as in Example 1, and a battery was assembled. Table 4 shows the evaluation results of the battery.

  As is clear from Table 4, the capacity retention rate and the capacity recovery rate after storage are excellent when the amount of Fe in the metal state contained in the alloy material is in the range of 0.0006 to 1.0% by weight. all right.

  That is, when the Fe in the metal state in the alloy material is less than 0.0006% by weight, the cycle characteristics deteriorated. It is presumed that Fe in a metal state plays a catalytic role in suppressing the formation of a film on the surface of the alloy material during charging and discharging, and acts as a nucleus that inhibits the formation of a film that becomes an excessive resistance component. In this role, it is considered that 0.0006% by weight or more of Fe in the metal state is effective. Further, when the amount of Fe in the metal state was more than 1.0% by weight, the capacity recovery rate after storage was deteriorated in the overdischarged state. This is because the negative electrode potential rises (electrochemical oxidation) during overdischarge, so that the dissolution of Fe in the metallic state increases due to long-term storage at that potential, resulting in an increase in battery impedance. It is considered that the storage characteristics are deteriorated.

Example 4
In this example, the amount of the phase containing Si was examined.
As shown in Table 5, except that the weight ratio of the A phase to the B phase was 1:99 to 98: 2, as shown in Table 5, Ti—Si alloy, Zr—Si alloy, Ni -Si alloy and Cu-Si alloy were synthesized. The amount of added Fe was 0.001% by weight. The obtained alloy material was classified into a maximum particle size of 8 μm and an average particle size of 5 μm.

In the same manner as in Example 1, the crystallite sizes of the A phase and the B phase were calculated from the half-value width of the peak with the highest intensity obtained by X-ray diffraction measurement of the alloy material and the Scherrer equation. It was found that there was a Si single phase composed of crystal grains (crystallites) of about 10 nm and an MSi ₂ phase having crystal grains (crystallites) of about 15 nm to 20 nm.
Moreover, as a result of measuring the amount of Fe in a metal state, all were 0.001% by weight.

  Using the above alloy material, a negative electrode was produced in the same manner as in Example 1, and a battery was assembled. Table 5 shows the characteristics of each battery.

As shown in Table 5, it was found that the capacity retention rate and the capacity recovery rate were high when the Si phase ratio was in the range of 5 to 95% by weight.
When the proportion of the Si phase was lower than 5% by weight, the capacity was greatly deteriorated as the charge / discharge cycle progressed. The reason seems to be that the current density to the Si phase during the charge / discharge cycle is excessively high because the amount of Si contributing to the capacity is too small. In addition, during storage in an overdischarged state, even if the Fe content in the metal state is the same, the amount of Si phase is small, so that it is relatively susceptible to the adverse effects of dissolved Fe. Therefore, the capacity recovery rate seems to have been lowered.

  On the other hand, when the proportion of the Si phase is higher than 95% by weight, the capacity is increased, but the Si phase having a low electronic conductivity is relatively increased in the alloy material. Therefore, the electron conductivity of the whole alloy material becomes low, and it becomes a disadvantageous situation for securing current collection during the charge / discharge cycle. Thus, since the impedance of the battery is essentially large, there is little room for an increase in impedance during overdischarge, and the capacity recovery rate is thought to deteriorate.

Example 5
In this example, a method for producing an alloy material according to the present invention will be described.
In the synthesis method of the present invention, the saturation magnetic susceptibility that changes corresponding to the amount of Fe in the metallic state contained in the alloy material is detected, and the synthesis is terminated when the saturation magnetic susceptibility falls within a predetermined range value. It is a feature.

As a raw material of the transition metal element M, metal Ti (purity 99.9%, particle size 100 to 150 μm) was used. Si powder (purity 99.9%, average particle size 3 μm) was used as a raw material for Si. Assuming that the B phase constitutes TiSi ₂ , Ti and Si were mixed so that the proportion of the A phase in the total weight of the A phase and the B phase in the alloy material to be produced was 20%.
Furthermore, metal Fe powder was used as a raw material for the Fe element. Its purity was 99.9% and the average particle size was 100 μm.

The mixture of Si and Ti and Fe were mixed so that the Fe content was 2.0% by weight.
As in Example 1, 3.5 kg of this raw material mixture was weighed and placed in a vibration mill device (manufactured by Chuo Kako Co., Ltd., model number FV-20), and a stainless steel ball (diameter 2 cm) was further milled. It charged so that it might occupy 70 volume% of internal volume. After evacuating the inside of the container, Ar (purity 99.999%, manufactured by Nippon Oxygen Co., Ltd.) was introduced so that the pressure became 1 atm. Mechanical alloying operation was performed under these conditions. The operating conditions of the mill device were an amplitude of 8 mm and a rotation speed of 1200 rpm.

  Under these conditions, mechanical alloying operation was performed for 80 hours. Samples were withdrawn every 10 hours from the start of synthesis to determine the saturation magnetic susceptibility. The results are shown in Table 6.

  In 30 hours from the start of synthesis, the saturation magnetic susceptibility reached 1.7 emu / g. Based on the results of Example 3, this value falls within the range where the capacity recovery rate after storage in the overdischarged state is good. Furthermore, the saturation magnetic susceptibility decreased as the synthesis time increased and reached 0.001 emu / g in 50 hours. Based on the result of Example 3, when the saturation magnetic susceptibility is lower than this, the cycle characteristics deteriorate. Considering these, the proper synthesis time is 30 to 50 hours.

  As described above, by detecting the saturation magnetic susceptibility during synthesis, it is possible to synthesize a material excellent in cycle characteristics and storage characteristics in an overdischarged state.

  In particular, the present invention provides a non-aqueous electrolyte secondary battery that is optimal as a main power source and a memory backup power source for various electronic devices such as mobile phones and digital cameras. The non-aqueous electrolyte secondary battery according to the present invention is further applicable to applications that require high electric capacity and excellent cycle characteristics even when charging and discharging. Also in use, the capacity recovery rate after storage in an overdischarged state is excellent.

FIG. 2 is a longitudinal sectional view of a coin-type battery that is an example of the nonaqueous electrolyte secondary battery of the present invention. It is a figure which shows the example of the HB curve of a diamagnetic material. It is a figure which shows the example of the HB curve of a ferromagnetic material. It is a figure which shows the example of the HB curve of a ferromagnetic material.

Claims (6)

  A nonaqueous electrolyte secondary battery comprising a positive electrode capable of reversibly inserting and extracting lithium, a negative electrode including an alloy material as an active material, and a nonaqueous electrolyte, wherein the alloy material includes a phase containing at least Si (A Phase) and a phase (B phase) containing an intermetallic compound of Si and at least one selected from the group consisting of Ti, Zr, Ni and Cu. A non-aqueous electrolyte secondary battery comprising 0006 to 1.0% by weight.   The nonaqueous electrolyte secondary battery according to claim 1, wherein at least one of the A phase and the B phase is amorphous or low crystalline.   The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of the A phase in the total amount of the A phase and the B phase in the alloy material is 5 to 95% by weight.   The nonaqueous electrolyte secondary battery according to claim 1, wherein a saturation magnetic susceptibility per 1 g of the alloy material is 0.001 to 1.7 emu / g.   From a raw material containing at least one transition metal element selected from the group consisting of Si, Fe and Ti, Zr, Ni and Cu, a phase containing at least Si (A phase), the at least one transition metal element and Si, And a phase (B phase) containing an intermetallic compound, and a step of producing an alloy material containing Fe in a metallic state, and during the step, saturation magnetization that changes according to the amount of Fe in the alloy material The manufacturing method of the negative electrode material for nonaqueous electrolyte secondary batteries which detects a rate and complete | finishes the said process when the detected saturation magnetic susceptibility becomes a predetermined value.   The manufacturing method of the negative electrode material for nonaqueous electrolyte secondary batteries of Claim 5 in which the said process includes a mechanical alloying process.

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