JP2006253126A - Anode active material for nonaqueous electrolyte secondary cell, anode for nonaqueous electrolyte secondary cell using same, and nonaqueous electrolyte secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell suppressing deterioration of cell characteristics with an expansion of an alloy material and having high capacity and good cycle characteristics, in an anode using both an alloy material and a graphite material. <P>SOLUTION: The anode active material for a nonaqueous electrolyte secondary cell contains an alloy material including Si and at least either Ti or Zr, as well as a graphite material, with a surface of the graphite material covered with the alloy material. An anode including the anode active material is preferred to have a porosity in a discharge state in a range of 10% to 80% inclusive. The nonaqueous electrolyte secondary cell is structured by combining the anode with a cathode including lithium-containing transition metal oxide and nonaqueous electrolyte including lithium salt. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-capacity, long-life non-aqueous electrolyte secondary battery, and more particularly to improvement of a negative electrode for a non-aqueous electrolyte secondary battery.

  Many researches and developments have been made using metallic lithium capable of realizing a high energy density at a high voltage as a negative electrode of a nonaqueous electrolyte secondary battery. Currently, lithium ion batteries that use a graphite material that reversibly occludes and releases lithium and has excellent cycle life and safety as a negative electrode have been put into practical use.

  However, in a battery using a graphite material as a negative electrode, the achieved practical capacity of graphite is about 350 mAh / g, which is quite close to the theoretical capacity of graphite material (372 mAh / g). Therefore, as long as a graphite material is used for the negative electrode, future dramatic capacity improvement cannot be expected. On the other hand, the capacity required for a non-aqueous electrolyte secondary battery serving as an energy source tends to increase as the functionality of portable devices increases. Therefore, in order to realize a further increase in capacity, a negative electrode material having a capacity higher than that of graphite is required.

  Currently, alloy materials containing silicon and alloy materials containing tin are attracting attention as materials that provide high capacity. Metal elements such as silicon and tin can occlude and release lithium ions electrochemically, and can charge and discharge with a very large capacity compared to graphite materials. For example, silicon is known to have a theoretical discharge capacity of 4199 mAh / g, which is 11 times higher than that of graphite.

  On the other hand, the alloy material forms lithium alloys such as lithium-silicon and lithium-tin when occludes lithium. The formation of a lithium alloy involves a very large expansion based on changes in the crystal structure of the alloy. For example, the volume of silicon expands theoretically by a factor of 4.1 by charging lithium to the maximum. Therefore, the alloy material which is an active material is peeled off from the current collector of the negative electrode, electric conduction is lost, and high rate discharge characteristics, charge / discharge cycle characteristics and the like are deteriorated. In the case of graphite, since the intercalation reaction in which lithium is inserted between graphite layers is used, the volume of graphite expands only 1.1 times.

  Many attempts have been made to use a combination of graphite and an alloy material from the viewpoint of alleviating the above expansion and obtaining a high capacity. However, when graphite and alloy material are simply mixed, the expansion of the alloy material is larger than that of the graphite material and expands in a non-uniform direction within the electrode plate, so that the surrounding graphite moves under stress, It leads to peeling. As a result, similarly to the negative electrode using the alloy material alone, the electron conductivity is lowered, and the high rate discharge characteristics and charge / discharge cycle characteristics of the battery are lowered.

  In order to solve the above problems, Patent Document 1 discloses that a granulated body in which crystalline Si fine particles are attached to the surface of graphite particles and a carbon film is coated on at least a part of the graphite particles is activated. Attempts have been made to improve the above problems by using them as substances. Similarly, Patent Document 2 proposes to use, as an active material, a granulated body in which a carbonaceous material containing Si and / or a Si compound is adhered to the surface of graphite particles.

In these proposals, a Si material that repeats large expansion and contraction is placed on graphite that is smaller in expansion and contraction and electrochemically active, and is further covered with carbon. As a result, the Si material is unlikely to be peeled off from the graphite, and a stable current collecting property is imparted to improve the charge / discharge cycle characteristics. However, since the coated carbon is strong, the granulated body is destroyed by the expansion of the Si material, and the characteristics are not sufficiently improved. Further, since the carbon material covers the surface, it is difficult to get wet with the binder, and a sufficient adhesive force cannot be obtained. Therefore, a process for imparting a binder to the active material surface by applying a strong load during kneading becomes indispensable. At that time, it has been found that the coated carbon and Si material have a serious problem that they are peeled off from the graphite surface by the load.
JP 2002-8652 A JP 2004-182512 A

  As described above, the combined use of an alloy material and a graphite material has been widely studied from the viewpoint of using a high-capacity alloy material as a negative electrode material, but both proposals are sufficiently affected by the uneven expansion of the alloy material. It has not been reduced. That is, in the case of the conventional proposal, the disconnection of the electrical continuity between the particles in the negative electrode and the peeling of the alloy material from the current collector or the graphite material occur, and the electronic conductivity of the negative electrode eventually decreases, and the battery characteristics also deteriorate. .

  In view of the above, the present invention suppresses the deterioration of battery characteristics due to the expansion of the alloy material as described above when an alloy material capable of electrochemically inserting and extracting Li and a graphite material are used as active materials. It is intended.

  The negative electrode active material for a non-aqueous electrolyte secondary battery of the present invention is an active material capable of electrochemically occluding and releasing Li, comprising an alloy material containing at least one of Ti and Zr and Si, and a graphite material. And the surface of the graphite material is covered with the alloy material.

  According to the present invention, in a negative electrode using a combination of an alloy material and a graphite material, it is possible to suppress a decrease in battery characteristics due to expansion of the alloy material, thereby realizing a high capacity non-aqueous electrolyte secondary battery with excellent cycle characteristics. it can.

A negative electrode active material according to the present invention is a negative electrode active material for a non-aqueous electrolyte secondary battery capable of electrochemically inserting and extracting Li, an alloy material containing at least one of Ti and Zr and Si, and graphite It includes a material, and the surface of the graphite material is covered with the alloy material.
The shape of the negative electrode active material in one embodiment of the present invention is shown in FIG. The active material according to the present invention can always maintain contact with the graphite material even when the alloy material expands and contracts by covering the surface of the graphite material with the alloy material. As a result, a decrease in the electronic conductivity of the negative electrode is suppressed. Therefore, the negative electrode for a non-aqueous electrolyte secondary battery of the present invention containing this alloy material and graphite gives a battery with high capacity and excellent cycle characteristics.

  In addition, the negative electrode active material according to the present invention has a structure in which the surface of a graphite material that is difficult to wet with water is covered with a highly hydrophilic Ti-Si alloy material or Zr-Si alloy material, so that the surface of the negative electrode active material Can be easily combined with the binder. Therefore, it is possible to prepare a state in which a sufficient amount of the binder is attached to the surface of the negative electrode active material without the need for kneading that applies high stress. As a result, the occurrence of a problem that the graphite material and the alloy material are peeled off during kneading is prevented. Furthermore, the binder adheres to the surface of the active material and exists so as to cover the surface of the active material, thereby suppressing peeling from the graphite material due to expansion and contraction of the alloy material. Further, most of the binders are organic polymer materials or rubber materials. Since such a material is rich in flexibility, it is possible to prevent peeling of the alloy material from the active material by following the expansion and contraction of the alloy material.

  Furthermore, by taking the structure as described above, the high-rate discharge characteristics of the electrode are improved by improving the current collecting property.

  The alloy material includes at least one of Ti and Zr and Si. Ti and Zr have a higher reactivity with oxygen than other transition metal elements, and form a stable oxide on the outermost surface of the particle to play a role in preventing excessive oxidation. In addition, since the oxide is formed on the surface as described above, the wettability is higher than that of other metal particles, and in particular, the wettability with respect to water is increased. Is possible.

The alloy material further has at least _two phases of a phase mainly composed of Si (hereinafter referred to as A phase) and a phase of an intermetallic compound TiSi ₂ or ZrSi ₂ (hereinafter referred to as B phase). In addition, a long-life battery is realized. 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), hydrogen (H), etc., or transition metal elements to the A phase up to about 5% by weight.

The B phase has at least a phase of the intermetallic compound TiSi ₂ or ZrSi ₂ . An intermetallic compound containing Si has a high affinity with the A phase, and is not easily cracked at the crystal interface. In addition, among the intermetallic compounds of transition metal element and Si, the B phase is particularly high in electronic conductivity and hardness as compared with the Si single phase in the intermetallic compound phase composed of Ti or Zr and Si. . Among them, the TiSi ₂ or ZrSi ₂ phase is particularly preferable because it has a high affinity with a phase mainly composed of Si as the A 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.

Further B phase may be present more, TiSi _2, or different intermetallic compounds of phases and composition of ZrSi ₂ TiSi or ZrSi may be present together with the phase of TiSi ₂ or ZrSi _2. For example, TiSi ₂ and TiSi, Ti ₅ Si ₄ , Ti ₃ Si may be present in the alloy particles. Similarly, ZrSi ₂ and ZiSi, Zr ₅ Si ₄ , Zr ₃ Si ₂ and Zi ₂ Si may be present in the alloy particles.

In addition, intermetallic compounds containing different transition metal elements, such as TiSi ₂ and ZrSi ₂ , may be simultaneously present in the alloy particles. Further, transition metal elements other than Ti and Zr may be present as long as they are in a small amount. The amount is preferably 10% by weight or less in the alloy material, excluding Si.

  The A phase and / or the B phase are preferably composed of microcrystalline or amorphous regions. When a microcrystalline or amorphous alloy material is used, cracking of the alloy particles due to expansion associated with the occlusion of Li hardly occurs. This is because as the structure of the alloy material becomes microcrystalline, more crystal grain boundaries exist, and the expansion stress applied to one crystal grain boundary becomes relatively small. In addition, the concept of grain boundary is lost in amorphous, and the expansion stress is dispersed in a very fine region, so that cracks are hardly generated in the same manner.

  Furthermore, in the present invention, it is particularly desirable that the crystal grain size of the phase mainly composed of Si is 50 nm or less. Here, an alloy material having a crystal grain (crystallite) diameter of 50 nm or less is defined as a microcrystal. If the alloy material has a microcrystalline region, one or more peaks that are not sharp but can be obtained with a half-value width are observed in the diffraction spectrum of alloy particles obtained by X-ray diffraction measurement. The The diameter of the crystal grain (crystallite) of the alloy material is obtained by calculating the half width of the peak with the highest intensity in the diffraction spectrum of the alloy particle obtained by X-ray diffraction measurement. It can be calculated from

  On the other hand, when the alloy material has an amorphous region, a broad halo pattern in which the half width cannot be recognized in the range of 2θ = 15 to 40 ° of the diffraction spectrum of the alloy particle obtained by X-ray diffraction measurement. Is observed.

  In the case of an alloy with Zr, the amount of Si contained in the alloy material is preferably at least 38% by weight or more. Similarly, when the elements contained in the alloy material are only Ti and Si, the amount of Si is preferably 54% by weight or more. Conversely, the amount of Si contained in the alloy material is preferably at most 95% by weight. In the composition in these ranges, the A phase and the B phase can be mixed and finely dispersed.

  A more preferable amount of Si is 60% by weight or more and 93% by weight or less. Within the above composition range, the negative electrode active material of the present invention can achieve both high capacity and long life.

  The average particle size of the alloy material is preferably less than 5 μm. Particularly preferably, it is 3 μm or less. The smaller the average particle size of the alloy material, the smaller the amount of displacement per particle due to expansion, and the more difficult the separation from the surface of the graphite material occurs.

  The graphite used in the present invention may be any graphite material as long as it can generally be used for a non-aqueous electrolyte secondary battery. For example, natural graphite or artificial graphite produced by various methods can be used.

  The average particle size of graphite is preferably 5 μm or more and 45 μm or less, and more preferably 7 μm or more and 25 μm or less. If the average particle diameter of graphite becomes too fine, the specific surface area of graphite itself increases. From the viewpoint of suppressing side reactions between graphite and electrolyte, reducing the film formed on the graphite surface, and limiting the irreversible capacity of the negative electrode to a small amount, the average particle size of graphite should be 5 μm or more, and the ratio of graphite itself It is desirable not to increase the surface area too much. Further, when the average particle diameter of graphite is larger than 50 μm, irregularities are easily formed on the negative electrode surface, and voids between the negative electrode active materials are increased, making it difficult to collect current to the alloy inside the negative electrode. From the viewpoint of obtaining a negative electrode having excellent current collecting properties, it is desirable that the average particle size of graphite is 50 μm or less.

Furthermore, the specific surface area of the negative electrode active material of the present invention is preferably 15 m ² / g or more. By satisfying this condition, the charge / discharge reaction of the negative electrode active material can proceed smoothly. However, the specific surface area of the graphite material itself contained in the negative electrode active material is desirably 5 m ² / g or less. By satisfying this condition, excessive generation of a film due to side reaction with an electrolytic solution generated on the graphite surface is suppressed, and good battery characteristics are given.

The negative electrode for a non-aqueous electrolyte secondary battery of the present invention includes an alloy material containing at least one of Ti and Zr and Si, and a graphite material, and a negative electrode active material in which the surface of the graphite material is coated with the alloy material The porosity is 10% or more and 80% or less in a state of discharge, that is, a state where Li is released from the negative electrode active material, and electrochemically having a potential nobler than 0.8 V with respect to Li. It is preferable that The porosity is expressed by the following formula, where D _ideal is the negative density calculated from the mixing ratio and true density of each component included in the negative electrode, and D _real is the density of the negative electrode. .

  The negative electrode active material includes an alloy material having a large expansion / contraction, and stress applied to the expansion / contraction spreads outside each negative electrode active material particle. Therefore, it is necessary to have a sufficient space inside the negative electrode in a discharge state. is there. If the porosity of the negative electrode is less than 10%, the expansion cannot be sufficiently absorbed in the negative electrode due to the above reasons, and the negative electrode mixture may collapse or peel off. Conversely, if the porosity of the negative electrode is greater than 80%, it becomes easy to absorb the expansion stress inside the negative electrode, but the amount of the negative electrode active material becomes extremely small, resulting in a low battery capacity. Preferably, the porosity is 30% or more and 70% or less.

  Further, in the state of charge, that is, the state in which Li is occluded in the negative electrode active material, and the electrochemically has a base potential lower than 0.3 V with respect to Li, the porosity is 5% or more and 50% or less. It is desirable that If the porosity is less than 5%, it is difficult for the nonaqueous electrolyte solution to penetrate into the negative electrode, and there is a negative electrode active material that cannot participate in the reaction. Conversely, if it is greater than 50%, the negative electrode active material is extremely Since the amount is small, the battery capacity is low.

  The negative electrode for a non-aqueous electrolyte secondary battery usually contains not only the negative electrode active material but also a conductive agent and a binder. The conductive agent serves to help form a conductive network between the negative electrode active materials and between the negative electrode active material and the current collector. The conductive agent material is preferably at least one selected from the group consisting of carbon black, graphite, and carbon materials. Moreover, it is preferable that the average particle diameter of a electrically conductive agent is 5 micrometers. By satisfying the above conditions, the expansion of the negative electrode active material can be effectively absorbed, and a good conductive network can be maintained. The conductive agent is preferably 0.1 to 10 parts by weight, particularly preferably 1 to 5 parts by weight, based on 100 parts by weight of the negative electrode active material.

  The negative electrode includes a binder for fixing the negative electrode active materials or the negative electrode active material and the conductive agent to each other and fixing the mixture layer to the current collector. Any material can be used as the binder as long as it has an adhesive ability to the negative electrode active material and the conductive agent. However, the binder is electrochemically insensitive to Li in the range of potential of the negative electrode. A material is selected that is active and has as little influence on other substances as possible. A material having a carboxyl group as a functional group in the binder is preferable. The outermost surface of the alloy material contained in the negative electrode active material is covered with an oxide formed by air oxidation, and the oxide surface and the carboxyl group in the binder obtain a strong bond through a hydrogen bond. It becomes possible. As a result, the negative electrode can withstand the expansion of the negative electrode active material with a very strong force.

  As the binder having a carboxyl group as a functional group, for example, polyacrylic acid, carboxymethylcellulose, methylcellulose and the like are suitable. These may be used alone or in combination. Further, a system in which, for example, styrene-butylene copolymer rubber, polyethylene, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, or the like is added as an auxiliary agent together with the above binder may be used. The amount of the binder added is preferably as large as possible from the viewpoint of maintaining the structure of the mixture layer, but is preferably as small as possible from the viewpoint of improving battery capacity and improving discharge characteristics.

  The negative electrode in the present invention includes a current collector, and the thickness of the current collector is desirably 8 μm or more and 40 μm or less. When a current collector thinner than 8 μm is used, there is a high possibility that the current collector will be broken or damaged during the production of the negative electrode, making it difficult to use. On the other hand, when a current collector thicker than 40 μm is used, a large volume inside the battery can be taken up by the current collector, so that a sufficient amount of active material or electrolyte cannot be charged, resulting in a low capacity.

  It is desirable to use a copper foil or a copper alloy foil for the current collector. In the case of a copper alloy foil, the Cu content is preferably 90% by weight or more. From the viewpoint of improving the strength or flexibility of the current collector, it is effective to include an element such as P, Ag, or Cr in the current collector. Cu is an element that is inexpensive, foil-like, has sufficient strength, and is very rich in electron conductivity. In the present invention, the electrolytic copper foil preferably has a thickness of 10 μm or more and 25 μm or less.

  The negative electrode of the present invention comprises a mixture of a negative electrode active material, a conductive agent, a binder and the like and a current collector. The thickness of the negative electrode mixture layer is generally 10 μm or more and 100 μm or less, preferably 25 μm or more and 60 μm or less, per one side of the current collector. Although the thickness of the mixture layer may be thinner than 10 μm, it is necessary to consider that the volume ratio of the current collector in the negative electrode does not become too large. Moreover, although the thickness of the mixture layer may be thicker than 100 μm, the high-rate discharge characteristics may be deteriorated because the electrolyte solution hardly penetrates to the vicinity of the current collector.

  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 positive electrode can be used without any particular limitation as long as it is known to be usable for the positive electrode of a nonaqueous electrolyte secondary battery. The manufacturing method of the positive electrode may be performed as usual. For example, a positive electrode active material, a conductive agent such as carbon black, and a binder such as polyvinylidene fluoride are mixed in a liquid phase, and the obtained paste is applied onto a positive electrode current collector made of Al or the like. The positive electrode is obtained by drying and rolling.

As the positive electrode active material, a lithium-containing transition metal compound is preferable. Representative examples of the lithium-containing transition metal compound include, but are not limited to, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and LiMnO ₂ . A compound obtained by substituting a transition metal element of the above compound with a different metal element is also preferably used. For _{example, LiCo 1-x Mg x O} 2, LiNi 1-y Co y O 2, LiNi 1-yz Co y Mn z O 2 (0 <x <1,0 <y <1,0 <z <1) , etc. Is mentioned.

  The non-aqueous electrolyte can be used without particular limitation as long as it is known as an electrolyte for a non-aqueous electrolyte secondary battery. However, an electrolyte composed of a non-aqueous solvent and a lithium salt soluble therein can be used. preferable. As the non-aqueous solvent, a mixed solvent of cyclic carbonates such as ethylene carbonate and propylene carbonate and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate is generally used. Furthermore, γ-butyrolactone, dimethoxyethane, or the like may be mixed in a nonaqueous solvent.

Examples of the lithium salt include inorganic lithium fluoride and a lithium imide compound. Examples of the former include LiPF ₆ and LiBF ₄ , and examples of the latter include LiN (CF ₃ SO ₂ ) ₃ . Furthermore, LiClO ₄ , LiCF ₃ SO _3, or the like may be mixed in the lithium salt. The non-aqueous electrolyte may be a gel electrolyte or a solid electrolyte.

  In order to prevent an internal short circuit between the positive electrode and the negative electrode, a separator is installed between them. As the material of the separator, any material can be used as long as it allows a non-aqueous electrolyte to pass through appropriately and prevents the contact between the positive electrode and the negative electrode. A microporous film made of polyethylene, polypropylene, or the like is generally used for the nonaqueous electrolyte secondary battery, and the thickness is generally 10 μm or more and 30 μm or less.

  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.

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

<< Examples 1 and 2 >>
In this example, negative electrodes and cylindrical batteries were produced in the following manner, and their cycle life and discharge capacity were evaluated.

(1) Preparation of alloy material Metal Ti (purity 99.99%, manufactured by Kojundo Chemical Laboratory Co., Ltd., 100-150 μm product) and metal Si (purity 99.999%, manufactured by Kanto Chemical Co., Inc., 100 ˜150 μm product) were weighed and mixed so that the weight ratio was Ti: Si = 10: 90. Similarly, metal Zr (purity: 99.99%, manufactured by Furuuchi Chemical Co., Ltd., 100-150 μm product) and the above-mentioned metal Si were weighed and mixed so that the weight ratio was Zr: Si = 12: 88. .

  Each of these mixed powders is weighed in an amount of 3.5 kg, put into a vibration mill device (Chuo Kako Co., Ltd., FV-20) prepared for each mixed powder, and further a stainless steel ball (diameter 2 cm). It charged so that it might occupy 70 volume% of the internal volume of a mill apparatus. After the inside of the container was evacuated, Ar (purity 99.999%, Japanese oxygen) was introduced so that the pressure became 1 atm. The frequency of the mill device was 720 Hz. Under these conditions, mechanical alloying operation was performed for 80 hours.

  The Ti—Si alloy and Zr—Si alloy obtained by the above operation were recovered and the particle size distribution was examined. As a result, it was found to have a wide particle size distribution of 0.5 μm to 80 μm. By classifying these alloys with a sieve (under 45 μm), an alloy material having a maximum particle size of 45 μm and an average particle size of 16 μm was obtained. These alloy materials are dispersed in pure water so as to have a solid content concentration of 20% by weight, and using a bead mill (trade name: Dino Mill KD-6, manufactured by Shinmaru Enterprises Co., Ltd.), the paste flow rate is 2 L / min. Then, pulverization was performed for 1 hour using zirconia beads (diameter: Φ1 mm) as a crushing medium. As a result, an alloy material having an average particle size of 1.5 μm and a maximum particle size of 3.5 μm was obtained.

  When these alloy materials were analyzed by X-ray diffraction measurement, the alloy materials produced in this example were all microcrystals, and were calculated from the half-value width of the peak with the highest intensity based on the Scherrer equation. The particle size of the grains (crystallites) was 18 nm.

Furthermore, from the results of X-ray diffraction measurement, each of the Ti-Si alloy and the Zr-Si alloy has a single Si phase (A phase) and a TiSi ₂ phase or a ZrSi ₂ phase (B phase), respectively. Was estimated.

When the above two types of alloy materials were observed with a transmission electron microscope (TEM), an Si region composed of an amorphous region, crystal grains (crystallites) having a particle size of about 10 nm, and a particle size of about 15 to 20 nm. It was found that a TiSi ₂ phase or a ZrSi ₂ phase having crystal grains (crystallites) was present.

  MCMB (manufactured by Osaka Gas Chemical Co., Ltd., average particle size 25 μm) was selected as the graphite material. Each of the above alloy materials and graphite material were mixed at a weight ratio of 20:80, and 200 g of the mixed powder was put into a mechanofusion apparatus (AMS-lab, manufactured by Hosokawa Micron Corporation). The inside of the apparatus was placed under a nitrogen stream and treated at 1000 rpm for 10 minutes to obtain a negative electrode active material. A material using a Ti—Si alloy as an alloy material is referred to as a negative electrode active material 1, and a material using a Zr—Si alloy as a negative electrode active material 2.

  The shape of the negative electrode active material 1 obtained by the above operation was as shown in FIG. 1, and it was found that the alloy material was uniformly attached to the surface of the graphite material.

(2) Production of negative electrode For 100 parts by weight of each negative electrode active material obtained above, 7 parts by weight of polyacrylic acid (molecular weight 150,000, manufactured by Wako Pure Chemical Industries, Ltd.) as a binder and a conductive agent were used. 2 parts by weight of acetylene black (trade name Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) was added, and pure water was added and sufficiently kneaded to obtain negative electrode mixture pastes.

  The negative electrode mixture paste was applied to both sides of a current collector made of an electrolytic copper foil (manufactured by Furukawa Circuit Foil Co., Ltd.) having a thickness of 10 μm, dried and rolled. As a result, a negative electrode comprising a current collector and a negative electrode mixture layer carried on both surfaces thereof was obtained.

When the cross section of the obtained negative electrode was observed with a scanning electron microscope (SEM), a phenomenon that the alloy material was peeled off from the graphite by kneading was not observed. The density of the negative electrode mixture layer was 1.3 to 1.4 g / cm ³ , and the porosity of the negative electrode mixture layer was 40 to 45%.

(3) Production of positive electrode Li ₂ CO ₃ and CoCO ₃ were mixed at a predetermined molar ratio and heated at 950 ° C. to synthesize a positive electrode active material LiCoO ₂ . This was classified to a size of 45 μm or less. To 100 parts by weight of the positive electrode active material, 5 parts by weight of acetylene black as a conductive agent, 4 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone as a dispersion medium are added. To obtain a positive electrode mixture paste.

  The positive electrode mixture paste was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm (manufactured by Showa Denko KK), dried and rolled. As a result, a positive electrode comprising a current collector and a positive electrode mixture layer carried on both surfaces thereof was obtained.

(4) Production of Cylindrical Battery The negative electrode and the positive electrode are cut to a predetermined size, and the positive electrode current collector is made of an aluminum positive electrode lead, and the negative electrode current collector is made of a nickel negative electrode lead. Connected respectively. The negative electrode, the positive electrode, and a separator made of a polyethylene resin microporous film having a thickness of 20 μm inserted between them are wound in a spiral shape to form an electrode plate group, which is incorporated into a cylindrical battery case, and is shown in FIG. A cylindrical lithium ion secondary battery as shown was produced.

In FIG. 2, 11 represents a positive electrode, and 12 represents a negative electrode. The outer surface of the electrode plate group including the positive electrode 11, the negative electrode 12, and the separator 13 wider than both electrode plates is covered with the separator 13. An upper insulating ring 16 and a lower insulating ring 17 are arranged above and below the electrode plate group, respectively. After the electrode plate group is accommodated in the battery case 18, a nonaqueous electrolyte is injected to impregnate the electrode plate group. The opening of the battery case is sealed by a sealing plate 19. The positive electrode lead 11 is welded to a metal fitting connected to the positive electrode terminal 20 provided on the sealing plate 19. The negative electrode lead 15 is welded to the inner bottom surface of the battery case 19.
A battery using the negative electrode active material 1 is referred to as a battery A1, and a battery using the negative electrode active material 2 is referred to as a battery A2. Battery A1 and battery A2 correspond to Example 1 and Example 2, respectively.

  As the non-aqueous electrolyte, a solution obtained by dissolving lithium hexafluorophosphate at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1 was used.

<< Comparative Example 1 >>
A negative electrode was prepared in the same manner as described above from 100 parts by weight of graphite, 4 parts by weight of polyacrylic acid as a binder, and 2 parts by weight of acetylene black as a conductive agent. A battery B1 was produced in the same manner as the battery A1, except that this negative electrode was used.

<< Comparative Examples 2 and 3 >>
In preparation of the negative electrode, the graphite and the bead mill-treated fine powder alloy were used by simply mixing them with a kneader without passing through the mechanofusion device, and other operations were performed in the same manner as in the batteries A1 and A2, respectively. Battery B2 and Battery B3 of Comparative Example 3 were produced.

  The following evaluation was performed about battery A1, A2, and B1-B3.

(5) Battery evaluation (a) Discharge capacity In a thermostat set at 20 ° C., the cylindrical battery was charged at a constant current of 0.2 C until the battery voltage reached 4.05 V, and then 4.05 V. The battery was charged at a constant voltage until the current value reached 0.01C. Then, it discharged until the battery voltage became 2.5V with the electric current of 0.2C. The discharge capacity at this time is shown in Table 1.

(B) Cycle life In the thermostat set to 20 degreeC, the charging / discharging cycle of the battery after measuring the said discharge capacity was repeated on condition of the following.

  The battery voltage is charged at a constant current of 1 C until the battery voltage reaches 4.05 V, then charged at a constant voltage of 4.05 V until the current value reaches 0.05 C, and then the battery voltage at a current of 2 C corresponding to high rate discharge. The operation of discharging until 2.5V was repeated. And the ratio of the discharge capacity of the 100th cycle with respect to the discharge capacity of the 2nd cycle was calculated | required in percentage, and it was set as the capacity | capacitance maintenance factor (%). The closer the capacity retention rate is to 100%, the better the cycle life. These results are shown in Table 1.

  It can be seen that the batteries A1 and A2 have a particularly improved capacity compared to the battery B1 and an improved cycle life compared to the batteries B2 and B3.

  Moreover, these batteries A1, A2, and B1 to B3 were disassembled in a charged state after 100 cycles, and the degree of expansion was evaluated. As a result, the battery B1 expanded 1.1 times, and the batteries B2 and B3 expanded 1.6 times and 1.7 times, respectively, compared to the negative electrode thickness before charging and discharging. On the other hand, about 1.3 times expansion | swelling was confirmed with battery A1 and A2. That is, it can be seen that the batteries of the examples achieve a long life by suppressing the excessive expansion of the negative electrode and thus maintaining the electronic conductivity of the negative electrode mixture.

<< Examples 3 to 10 >>
At the time of producing the alloy material, the mixing ratio of Ti or Zr and Si was mixed at a ratio shown in Table 2, and a negative electrode active material was synthesized by the same method as that for battery A1. In addition to the B phase consisting of the A phase and the TiSi ₂ phase or the ZrSi ₂ phase, which are all Si single phases, the alloy material produced in this example is, for example, a TiSi phase and a Ti ₅ Si ₄ in the batteries A3 and A4. The phase was confirmed from the X-ray diffraction results. Further, from the batteries A5 and A6, a ZrSi phase and a Zr ₅ Si ₄ phase were similarly confirmed. The crystal grain size of these alloy materials calculated from the X-ray diffraction results was 15 to 35 nm. Moreover, the average particle diameter after the crushing process by a bead mill was 1.8-3.8 micrometers. The graphite used in this example is MCMB as in the battery A1. Except for the above, batteries A3 to A10 were produced in the same manner as battery A1.

  The batteries A3 to A10 were evaluated in the same manner as the battery A1. The results are shown in Table 2.

In the batteries A3 to A6, it has been found that a result that achieves both a high capacity and a long life can be obtained. However, the batteries A7 and A9 had almost the same capacity as the battery B1 described above, but exhibited a high capacity retention rate. This is probably because the aforementioned A phase is hardly generated at the time of synthesis, and only the TiSi ₂ or ZrSi ₂ intermetallic compound phase corresponding to the B phase is generated, so that lithium cannot be occluded / released. It is done. However, it is considered that the alloy material adhering to the graphite surface easily adheres to the binder, thereby exhibiting a higher capacity retention rate than the battery B1. Furthermore, in the batteries A8 and A10, although the capacity is higher than in any of the examples, the cycle performance is lowered. Contrary to the batteries A7 and A9, this factor is generated only in the A phase that absorbs and releases lithium, and as a result, the capacity is deteriorated more than other examples due to excessive expansion and contraction. This is probably because of this.

<< Examples 11 to 15, Comparative Examples 4 and 5 >>
A Ti—Si alloy having a wide particle size distribution of 0.5 μm to 80 μm obtained by the same operation as that of the battery 1 is passed through a first sieve (under 45 μm) to remove particles larger than 45 μm, and then a second sieve ( Particles smaller than 20 μm were removed by passing through (under 20 μm), and an alloy material having a particle size distribution of 20 to 45 μm and an average particle size of 32 μm was obtained.

  A negative electrode active material was synthesized by carrying out mechanofusion treatment and crushing treatment in the same manner as the battery A1, except that the above alloy material was used, and a battery A11 was produced.

  A Ti—Si alloy having a wide particle size distribution of 0.5 μm to 80 μm obtained by the same operation as that of the battery A1 was passed through the first sieve (under 20 μm) and then through the second sieve (under 5 μm). An alloy material having a particle size distribution of ˜20 μm and an average particle size of 9 μm was obtained.

  A negative electrode active material was synthesized by mechanofusion treatment and crushing treatment in the same manner as the battery A1, except that the above alloy material was used, and a battery A12 was produced.

The Ti and Si mixed powder mixed with the same composition as the battery A1 is changed to 5 hours, 20 hours, and 40 hours by using the same vibration mill apparatus as the battery A1, thereby obtaining an alloy material under each condition. It was. First, alloy particles of 100 μm or more were removed from these alloy materials with a 100 μm-under sieve, and then crushed by a bead mill in the same manner as the battery A1, thereby obtaining alloy materials having an average particle diameter of 2 μm. The alloy materials synthesized with an operation time of 5 hours, 20 hours, and 40 hours are referred to as alloy a, alloy b, and alloy c, respectively. When the crystallinity and phase structure of these alloy materials were evaluated by X-ray diffraction, Ti and Si single crystalline peaks were observed in alloy a. From this result, it was found that the alloy a was not alloyed and was a mixture of Ti and Si. Regarding the alloy b, although the A phase (Si single phase) and the B phase (TiSi ₂ phase) are formed, the XRD peak is crystalline, and the crystal grain size is calculated from the Scherrer equation using the half width. Was calculated to be as large as 1.5 μm. Regarding alloy c, the formation of A phase and B phase was confirmed as in alloy b, but the crystal grain size was 80 nm. Further, as in Example 1, when the phase of the alloy material was observed by TEM, it was found that the crystal grain size was almost the same.

  A negative electrode active material was synthesized by subjecting these alloy materials and graphite material (MCMB) to mechanofusion treatment in the same manner as in Example 1, and batteries A13 to A15 were produced in the same manner as battery A1.

An alloy material was obtained under the same conditions as in Battery A1, using Ni or Fe powder (both manufactured by Kojundo Chemical Laboratory Co., Ltd., particle size: 100 to 150 μm) instead of Ti powder. At this time, the mixing ratio of Ni—Si alloy is Ni: Si = 30: 70, and the mixing ratio of Fe—Si alloy is Fe: Si = 25: 75 (both are weight ratios). It was found that the alloy material after the crushing treatment by the bead mill had an average particle diameter of 1.6 μm. When these alloy materials were analyzed by X-ray diffraction measurement, they were microcrystals similar to the battery A1, and the crystal grains (crystallites) calculated from the half-value width of the strongest peak based on the Scherrer equation The diameter was 23 nm. Further, from the results of X-ray diffraction measurement, it was found that there existed a single Si phase (A phase) and a NiSi ₂ phase or a FeSi ₂ phase (B phase), respectively.

  A negative electrode active material was synthesized by subjecting each of the above alloy materials and graphite material (MCMB) to mechanofusion treatment in the same manner as in battery A1, and a battery was fabricated in the same manner as battery A1. The battery using the Ni—Si alloy is designated as B4, and the battery using the Fe—Si alloy is designated as B5.

  These batteries A11 to A15, B4, and B5 were evaluated in the same manner as in Example 1. The results are shown in Table 3.

  The batteries A11 to A15 had a high capacity and a long life. Here, although the capacity retention rate was lower than that of the batteries A1 and A2, this was due to the peeling of the mixture when the batteries A11 and A12 were disassembled after 100 cycles and the negative electrode was observed. Furthermore, the current collector was observed to be wrinkled and cracked at the ends, and further, the peeled mixture was observed by SEM, and it was found that the particle size of the alloy material was further pulverized.

  Similarly, also in the batteries A13 to A15, the negative electrode after 100 cycles had a reduced degree of peeling with respect to the length of operation time of the vibration mill device. From these results, it is preferable that the alloy material used in the present invention has a fine average grain size and a fine crystal grain size.

  In addition, the batteries B4 and B5 were found to have a lower capacity and lower cycle characteristics than the batteries A1 and A2. When these negative electrodes were evaluated using lithium metal as a counter electrode, the irreversible capacity was as large as 30 to 40%, and the amounts of oxygen contained in the Ni—Si alloy and the Fe—Si alloy were 5.6% by weight and 7%, respectively. It was as large as 1% by weight. As inferred from these results, most of the above alloys generate oxides such as SiO, and the amount of electricity is consumed because the oxides are reduced during the first charge. As a result, it is estimated that the reversible capacity is consumed and the battery has a low capacity. Moreover, since the lithium oxide produced | generated by the reduction | restoration of the said oxide is a nonconductor, it becomes a factor which reduces the electronic conductivity of negative electrode active material itself, and it is thought that it is a factor which reduces cycling characteristics.

<< Example 16 to Example 21 >>
About the negative electrode of battery A1, it rolled so that it might become a density and a porosity as shown in Table 4, and the evaluation was performed. The density and porosity are all measured values before battery assembly. Furthermore, the change of the negative electrode thickness measured when these batteries were decomposed in a charged state after 100 cycles was described as the expansion coefficient. This is 100 (%) with respect to the thickness before battery assembly if the thickness does not change after the cycle.

  Batteries A16 to A21 were produced in the same manner as the battery A1, and the same evaluation as the battery A1 was performed. The results are shown in Table 4.

  From the results in Table 4, the higher the density of the negative electrode, that is, the lower the porosity, the higher the capacity, but the cycle characteristics tended to deteriorate. In addition, as a factor thereof, an excessively high density, for example, battery A20, increases the expansion coefficient of the electrode, thereby causing wrinkles in the negative electrode or non-uniform distance between the positive electrode and the negative electrode. The battery reacts unevenly. As a result, it is considered that the cycle characteristics deteriorated.

<< Examples 22 to 25 >>
Instead of acetylene black, which is a conductive agent used in the battery A1, fine graphite (trade name: KS4 manufactured by Timcal, average particle size: 3 μm), vapor grown carbon fiber (trade name: VGCF, Showa Denko Co., Ltd., fiber diameter: 0) .15 μm), or scaly graphite (trade name: SGP Co., Ltd., average particle size: 8 μm) using the same parts by weight as acetylene black, or using a mixture of 1 part by weight of acetylene black and 1 part by weight of VGCF. Batteries A22 to A25 were produced in the same manner as A1 and evaluated in the same manner as the battery A1. The results are shown in Table 5.

  As shown in Table 5, the battery A24 showed good characteristics as in the case of the battery A1, but in the battery A24, when the negative electrode was observed after being decomposed after 100 cycles, peeling of the mixture was observed. Batteries A22, A23, and A25 maintain a smooth state even after 100 cycles, and it is considered that the use of a minute conductive agent suppresses deformation of the negative electrode and exhibits stable charge / discharge cycle characteristics.

<< Examples 26 and 27 >>
Instead of polyacrylic acid, which is the binder used in Battery A1, 5 parts by weight of carboxymethylcellulose (trade name 1290 manufactured by Daicel Chemical Industries) and styrene-butadiene copolymer rubber (trade name 0589 manufactured by JSR Corporation) A negative electrode was prepared using a mixture with 3 parts by weight. In addition, a negative electrode was prepared using 10 parts by weight of polyvinylidene fluoride (trade name: 1320 manufactured by Kureha Chemical Co., Ltd.) as a binder and N-methyl-2-pyrrolidone (manufactured by Kanto Chemical Co., Ltd.) as a solvent. . Batteries A26 and A27 were produced using these negative electrodes in the same manner as the battery A1.

  The batteries A26 and A27 were evaluated in the same manner as in Example 1. The results are shown in Table 6.

  From Table 6, good characteristics were shown in the same manner as the battery A1. However, when the battery A27 was decomposed after 100 cycles and the negative electrode was observed, peeling of the mixture was observed. On the other hand, the battery A26 maintained a smooth state even after 100 cycles. This is because the battery A26 uses a binder having a carboxyl group in the same manner as the battery A1, and particularly the carboxyl group and the surface of the alloy material are firmly bonded to prevent the mixture from being peeled off. As a result, it is considered that the charge / discharge cycle characteristics were good.

  The negative electrode for a non-aqueous electrolyte secondary battery of the present invention provides an excellent non-aqueous electrolyte secondary battery having both high capacity and good charge / discharge cycle characteristics. The present invention is applicable to all forms of non-aqueous electrolyte secondary batteries. For example, the present invention has not only the cylindrical shape mentioned in the embodiment but also a coin shape, a square shape, a flat shape, and the like, and a wound type. The present invention is also applicable to a battery having an electrode plate group structure such as a stacked type. The nonaqueous electrolyte secondary battery of the present invention is useful as a main power source for mobile communication devices, portable electronic devices and the like.

It is an electron micrograph of the cross section of an example of the negative electrode in the Example of this invention. It is a longitudinal cross-sectional view of the cylindrical battery produced in the Example of this invention.

Claims (10)

  A negative electrode active material for a non-aqueous electrolyte secondary battery capable of electrochemically occluding and releasing Li, comprising an alloy material containing at least one of Ti and Zr and Si, and a graphite material, and a surface of the graphite material Is coated with the alloy material. A negative electrode active material for a non-aqueous electrolyte secondary battery. 2. The alloy material according to claim 1, wherein the alloy material includes at least a phase mainly composed of Si and a phase of an intermetallic compound TiSi ₂ or ZrSi ₂ , and at least one of the phases is formed of a microcrystalline or amorphous region. Negative electrode active material for non-aqueous electrolyte secondary battery.   The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the crystal grain size of the Si-based phase is 50 nm or less.   The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of Si in the alloy material is 38 wt% or more and 95 wt% or less.   The negative electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the alloy material has an average particle size of less than 5 μm.   An active material capable of electrochemically occluding and releasing Li, comprising an alloy material containing at least one of Ti and Zr and Si, and a graphite material, and a surface of the graphite material is coated with the alloy material A negative electrode for a non-aqueous electrolyte secondary battery, wherein the negative electrode active material has a porosity of 10% to 80% in a discharged state.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 6, comprising a conductive agent and a binder.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 7, wherein the conductive agent is at least one selected from the group consisting of carbon black, graphite, and a carbon material.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 7, wherein the binder has a carboxyl group.   A nonaqueous electrolyte secondary battery comprising the negative electrode according to claim 6, a positive electrode containing a lithium-containing transition metal oxide, and a nonaqueous electrolyte containing a lithium salt.