JP2012014866A - Negative electrode active substance for lithium secondary battery, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active substance for lithium secondary battery that compatibly improves both an active substance use rate and cycle characteristics.SOLUTION: The negative electrode active substance 10 for lithium secondary battery has Si alloy 12 having Si crystallites 12a dispersed in a first metal matrix 12b, the Si alloy 12 being dispersed in a second metal matrix 14 having Li activity. It is preferred that the second matrix 14 is Sn or an Sn-based alloy. Furthermore, it is preferred that the second metal matrix 14 contains Sn of 50 atom% or more.

Description

  The present invention relates to a negative electrode active material for a lithium secondary battery and a method for producing the same.

Conventionally, a negative electrode using a carbon material such as graphite as a negative electrode active material, a positive electrode using a lithium-containing compound such as cobalt oxide, and a lithium secondary using a lithium salt such as LiPF ₆ as an electrolyte sandwiched between these positive and negative electrodes Batteries are known. In a lithium secondary battery, lithium ions are occluded in the negative electrode active material when the battery is charged, and lithium ions are released from the negative electrode active material when the battery is discharged.

  In recent years, this type of lithium secondary battery has attracted attention as a power source for electric vehicles. However, the negative electrode active material graphite currently widely used has a theoretical capacity of only 372 mAh / g, and a further increase in capacity is desired. Therefore, recently, metal materials such as Si and Sn that can be expected to have a high capacity have been actively studied as alternative materials for the carbon-based negative electrode active material.

  However, Si and Sn cause large volume expansion / contraction as the lithium ions are occluded / released. For this reason, there has been a problem that the cycle characteristics are liable to be peeled off due to the expansion / contraction stress. Therefore, as a countermeasure, techniques such as composite of negative electrode active material and surface coating have been studied.

  For example, Patent Document 1 discloses a composite powder obtained by alloying Si, Sn, and the like and other elements such as Cu in a predetermined component range as a negative electrode active material for a lithium secondary battery. Further, in this document, the relationship between the component ratio of the Cu element in the Si—Cu alloy and the initial discharge capacity / cycle characteristics is specifically described in comparison with pure Si (Example 14, Comparative Example). 7, Comparative Example 17, Comparative Example 18, etc.).

  Patent Document 2 discloses a negative electrode active material for a lithium secondary battery in which the surface of Si particles is coated with Cu.

Japanese Patent No. 4029291 JP 2008-16198 A

  However, the prior art has room for improvement in the following points. That is, as described in Patent Document 1, when Cu is combined with Si, although the cycle characteristics are improved, the initial discharge capacity is greatly reduced. This is considered to be because Si is inactivated by Cu. The technique described in Patent Document 2 in which the surface of the Si particles is coated with Cu is presumed to have the same problem. Thus, it has been difficult to achieve both improvement of the active material utilization rate (= initial discharge capacity / theoretical capacity of active material × 100) and improvement of cycle characteristics.

  The present invention has been made in view of the above circumstances, and the problem to be solved by the present invention is that a negative electrode active for a lithium secondary battery capable of achieving both an improvement in active material utilization and an improvement in cycle characteristics. To provide a substance.

  In order to solve the above problems, a negative electrode active material for a lithium secondary battery according to the present invention has a Si alloy in which Si crystallites are dispersed in a first metal matrix, and the Si alloy has Li activity. The gist is that it is dispersed in the second metal matrix.

  Here, the second metal matrix is preferably Sn or a Sn-based alloy.

  The second metal matrix preferably contains 50 atomic% or more of Sn.

  The method for producing a negative electrode for a lithium secondary battery according to the present invention includes a step of producing a Si alloy powder in which Si crystallites are dispersed in a first metal matrix, and the obtained Si alloy powder and Li activity. And a step of mixing and compounding the two metal matrix powders to disperse the Si alloy in the second metal matrix.

  The negative electrode active material for a lithium secondary battery according to the present invention includes a Si alloy in which Si crystallites are dispersed in a first metal matrix, and the Si alloy is included in a second metal matrix having Li activity. Is distributed.

  According to the negative electrode active material for a lithium secondary battery according to the present invention, since the second metal matrix has Li activity, lithium can easily reach the Si crystallite inside the Si alloy, and the active material utilization rate is improved. be able to.

  In addition, since the first metal matrix surrounds the fine Si crystallites, the collapse of Si is suppressed. Furthermore, since the second metal matrix surrounds the first metal matrix (having a double matrix structure), the collapse of the Si alloy is suppressed and it is difficult for the current collector to fall off. Therefore, cycle characteristics can be improved.

  Here, when the second metal matrix is Sn or an Sn-based alloy, the Li activity is relatively high, so that it is easier for lithium to reach the Si crystallite inside the Si alloy, and the active material utilization rate. It can contribute to improvement. In particular, when the second metal matrix is an Sn-based alloy, the capacity is lower than that of Sn, but the expansion and contraction of the second metal matrix can be reduced, contributing to the improvement of the stability of the negative electrode active material. can do.

  Further, when the second metal matrix contains 50 atomic% or more of Sn, it is easy to contribute to the improvement of the active material utilization rate and the stability of the negative electrode active material.

  According to the method for producing a negative electrode for a lithium secondary battery according to the present invention, it is possible to obtain the above-described negative electrode active material for a lithium secondary battery capable of achieving both an improvement in active material utilization and an improvement in cycle characteristics. it can.

It is the figure which showed typically the negative electrode active material for lithium secondary batteries which concerns on one Embodiment of this invention. It is the figure which showed typically the negative electrode for lithium secondary batteries which concerns on one Embodiment of this invention. It is the figure which showed typically the other negative electrode for lithium secondary batteries which concerns on one Embodiment of this invention. 2 is a reflected electron image of a negative electrode active material according to Example 1 by a scanning electron microscope (SEM). 4 is a reflected electron image of a negative electrode active material according to Comparative Example 1 by a scanning electron microscope (SEM).

  Hereinafter, a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention (hereinafter sometimes referred to as “the present negative electrode active material”), a manufacturing method thereof, and a lithium secondary battery using the present negative electrode active material The negative electrode (hereinafter sometimes referred to as “the present negative electrode”) will be described in detail.

1. First Anode Active Material and Production Method Thereof First, the configuration of the present anode active material will be described. This negative electrode active material is a material used for a negative electrode of a lithium secondary battery such as a lithium ion secondary battery or a lithium polymer secondary battery.

  FIG. 1 is a diagram schematically showing the present negative electrode active material. The negative electrode active material 10 includes a Si alloy 12 and a second metal matrix 14. The Si alloy 12 has a Si crystallite 12a and a first metal matrix 12b.

  The Si alloy 12 has a structure in which a large number of Si crystallites 12a are dispersed in the first metal matrix 12b. That is, in the Si alloy 12, a large number of Si crystallites 12a are surrounded by the surrounding first metal matrix 12b. In the present negative electrode active material 10, a large number of the above-described Si alloys 12 are dispersed in the second metal matrix 14. That is, in the present negative electrode active material 10, a large number of Si alloys 12 are surrounded by the surrounding second metal matrix 14. Therefore, the negative electrode active material 10 has a double matrix structure in which the first metal matrix 12b is arranged around the Si crystallite 12a and the second metal matrix 14 is arranged around the first metal matrix 12b. . The “metal” in the first metal matrix 12b and the second metal matrix 14 includes not only a simple metal but also an alloy.

  In the present negative electrode active material, the Si crystallite is a phase mainly containing Si. From the viewpoint of increasing the amount of lithium occlusion, the Si crystallite is preferably composed of a Si single phase. However, inevitable impurities may be contained in the Si crystallite.

  The shape of the Si crystallite is not particularly limited, and the outer shape may be relatively uniform, or the outer shape may be uneven. Further, individual Si crystallites may be separated from each other, or Si crystallites may be partially connected to each other.

  The upper limit of the size of the Si crystallite is preferably 700 nm or less, more preferably 300 nm or less, and even more preferably 100 nm or less. This is because Si cracking can be easily reduced by reducing the size of Si, thereby contributing to improvement of cycle characteristics.

  Since the smaller Si crystallite is better, the lower limit of the size of the Si crystallite is not particularly limited. However, from the viewpoint of capacity reduction due to oxidation of Si, etc., it is preferably 50 nm or more.

  The size of the Si crystallite is an average value of the sizes of the Si crystallites measured for 20 Si crystallites arbitrarily selected from a microstructure photograph (one field of view) of the negative electrode active material.

  In the present negative electrode active material, the first metal matrix is a phase that exists in phase separation with Si crystallites. The first metal matrix may have Li activity or may not have Li activity like a second metal matrix described later. Preferably, the first metal matrix has Li activity. This is because lithium is easily reached by the Si crystallite and contributes to improvement of the active material utilization rate. Specific examples of the material of the first metal matrix include an Al alloy, an Sn alloy, an In alloy, an Ag alloy, a Pb alloy, an Sb alloy, and a Bi alloy. it can. Among these, Al-based alloys, Sn-based alloys, and In-based alloys are preferable from the viewpoint of low melting point, high vapor pressure, cost, and the like.

  Examples of the Al alloy include the Al-Co alloy, Al-Bi alloy, Al-Co-Bi alloy, Al-Bi-Co alloy, and Al-Sn alloy. Examples thereof include Sn—Co, Sn—Ni, Sn—Fe, Sn—Cu, Sn—Ag, Sn—In, Sn—Sb, Sn—Bi, and In—Sb.

  The shape of the Si alloy is not particularly limited, and the outer shape may be relatively uniform, or the outer shape may be uneven. Moreover, each Si alloy may be isolate | separated, respectively, and Si alloys may continue in part.

  The upper limit of the size of the Si alloy is preferably 5 μm or less, more preferably 1 μm or less, and even more preferably 100 nm or less. This is because the smaller the Si alloy, the smaller the expansion / contraction stress of the Si alloy and the better the cycle characteristics.

  On the other hand, the lower limit of the size of the Si alloy is preferably 10 nm or more, more preferably 40 nm or more, and even more preferably 50 nm or more. This is because when the particle size is small, oxidation tends to occur, leading to a decrease in capacity.

  In addition, the magnitude | size of the said Si alloy is an average value of the magnitude | size of the Si alloy measured about 20 Si alloys arbitrarily selected from the micro structure photograph (one visual field) of this negative electrode active material.

  In the present negative electrode active material, the Si alloy is preferably contained in an amount of 25% by mass or more, more preferably 40% by mass or more, and still more preferably 50% by mass or more based on the total mass of the present negative electrode active material. Good to be. If the Si alloy is excessively reduced, the capacity of the negative electrode active material is reduced, and the meaning as an alternative material for graphite is reduced. This is because, if the Si alloy content is within the above range, the balance between the improvement effect of cycle characteristics and the increase in capacity is excellent.

  On the other hand, the Si alloy is preferably contained in an amount of 80% by mass or less, more preferably 60% by mass or less based on the total mass of the negative electrode active material. If the Si alloy is excessively large, the amount of the second metal matrix is relatively decreased, and the effect of suppressing the expansion and contraction of the Si alloy tends to be reduced. This is because if the Si alloy content is in the above range, the balance between the improvement effect of the cycle characteristics and the increase in capacity is excellent.

  In the present negative electrode active material, the second metal matrix has Li activity. Here, “having Li activity” means that the second metal matrix alone reacts with Li to form a Li compound. (The same applies to the case where the first metal matrix has “Li activity”). In the present invention, when the second metal matrix has Li activity, lithium easily reaches the Si crystallite inside the Si alloy.

  Specific examples of the material of the second metal matrix include Sn, Sn alloy, Al, Al alloy, In, In alloy, Bi, Bi alloy, Sb, Sb alloy and the like. be able to. Of these, Sn or an Sn-based alloy is preferable from the viewpoint of having high Li activity. This is because the capacity is lower than that of Si, so that the expansion and contraction of the second metal matrix can be reduced, and the stability of the negative electrode active material can be improved. Particularly preferred is a Sn-based alloy from the viewpoints of reactivity with Li, matrix stability, cost, and the like.

  The second metal matrix preferably contains Sn at 40 atom% or more, more preferably 50 atom% or more. It is because it becomes easy to contribute to the improvement of the active material utilization rate and the stability of the negative electrode active material. However, when Sn exceeds 80 atomic%, the effect of improving the characteristics tends to decrease. Therefore, the second metal matrix preferably contains Sn at 80 atomic% or less, more preferably 75 atomic% or less, and further preferably 70 atomic% or less.

  Examples of the Sn-based alloy include a Sn—Co alloy, a Sn—Cu alloy, a Sn—Fe alloy, a Sn—Ni alloy, and the like.

  In the present negative electrode active material, the second metal matrix is preferably 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more with respect to the total mass of the present negative electrode active material. Good to be included. If the second metal matrix is excessively reduced, it is difficult to mitigate expansion and contraction of the Si alloy. When the content of the second metal matrix is within the above range, it is easy to contribute to the effect of improving the cycle characteristics.

  On the other hand, the second metal matrix is preferably 90% by mass or less, more preferably 80% by mass or less, with respect to the total mass of the negative electrode active material. If the second metal matrix is excessively large, the Si alloy is relatively lowered, and the contribution to the increase in capacity is reduced. If it is the said range, it is excellent in balance with the improvement effect of cycling characteristics, and high capacity | capacitance.

  The form of the negative electrode active material is not particularly limited. Specifically, forms such as flakes and powders can be exemplified. Preferably, it is in a powder form from the viewpoint of easy application to the production of a negative electrode. Further, the negative electrode active material may be dispersed in an appropriate solvent.

  The upper limit of the size of the negative electrode active material is preferably 50 μm or less, and more preferably 25 μm or less. This is because when the particle size is large, Li hardly diffuses into the active material, and the active material utilization rate tends to decrease.

  On the other hand, the lower limit of the size of the negative electrode active material is preferably 100 nm or more, more preferably 500 nm or more, and even more preferably 1 μm or more. This is because if the particle size becomes too fine, the particles are likely to be oxidized and the capacity is reduced.

  In addition, the magnitude | size of this negative electrode active material can be measured with a laser diffraction / scattering type particle size distribution measuring apparatus.

  Next, the manufacturing method of this negative electrode active material is demonstrated. As a method for producing the present negative electrode active material, a step of producing a Si alloy powder in which Si crystallites are dispersed in a first metal matrix, and the obtained Si alloy powder and a second metal matrix powder having Li activity And the like, and a process of passing through a step of dispersing the Si alloy in the second metal matrix.

  Specifically, the Si alloy powder is, for example, firstly weighed each raw material so as to have a predetermined chemical composition, and each of the weighed raw materials is subjected to melting means such as an arc furnace, a high frequency induction furnace, a heating furnace, etc. It can be suitably manufactured by making a molten Si alloy by, for example, melting it, and quenching the molten Si alloy to obtain a quenched alloy.

Specific examples of methods for rapidly cooling molten alloy include roll quenching methods (single roll quenching method, phase roll quenching method, etc.), atomizing methods (gas atomizing method, water atomizing method, centrifugal atomizing method) and the like. be able to. Preferably, the roll quenching method can be suitably used from the viewpoint of easily reducing the Si crystallite. The maximum rapid cooling rate of the molten alloy is preferably 10 ³ K / second or more, more preferably 10 ⁶ K / second or more.

  In addition, when the obtained quenched alloy is not in a powder form, a step of pulverizing the quenched alloy by an appropriate pulverizing means to form a powder may be added. Moreover, you may add the process of classifying the obtained quenched alloy and adjusting it to a suitable particle size, etc. as needed.

  As a method of mixing and combining the obtained Si alloy powder and the second metal matrix powder having Li activity, for example, an apparatus such as a ball mill, a bead mill, an attritor, a vibration mill, a reiki machine, or a grinding mill is used. Examples thereof include a method of applying a force from the outside to the powder and mixing and compounding. As the above apparatus, a ball mill, an attritor or the like can be preferably used from the viewpoints of productivity, the amount of energy to be applied, and the like.

2. Present Negative Electrode FIG. 2 is a diagram schematically showing a negative electrode for a lithium secondary battery according to an embodiment of the present invention. FIG. 3 is a diagram schematically showing another negative electrode for a lithium secondary battery according to an embodiment of the present invention. As shown in FIGS. 2 and 3, the negative electrode 20 is configured using the negative electrode active material 10.

  Specifically, as shown in FIGS. 2 and 3, the negative electrode 20 includes a conductive substrate 22 and a conductive film 24 laminated on the surface of the conductive substrate 22. The conductive film 24 contains at least the negative electrode active material 10 described above in a binder 26. As shown in FIGS. 2 and 3, the conductive film 24 may further contain a conductive additive 28 as necessary. When the conductive auxiliary material 28 is contained, it is easy to secure a conductive path for electrons.

  Moreover, as shown in FIG. 3, the electrically conductive film 24 may contain the aggregate 30 as needed. When the aggregate 30 is contained, the expansion and contraction of the negative electrode 10 during charge / discharge can be easily suppressed, and the collapse of the negative electrode 10 can be suppressed. Therefore, cycle characteristics can be further improved.

  The film thickness of the conductive film is preferably in the range of 10 to 100 μm, more preferably 20 to 50 μm, from the viewpoint of increasing the capacity per electrode area and decreasing the battery volume.

  The conductive substrate functions as a current collector. Examples of the material include Cu, Cu alloy, Ni, Ni alloy, Fe, and Fe-based alloy. Preferably, Cu or Cu alloy is preferable. Moreover, as a form of a specific electroconductive base material, foil shape, plate shape, etc. can be illustrated. A foil shape is preferable from the viewpoint of reducing the volume of the battery and improving the degree of freedom in shape.

  Examples of the binder material include polyvinylidene fluoride (PVdF) resin, fluorine resin such as polytetrafluoroethylene, polyvinyl alcohol resin, polyimide resin, polyamide resin, polyamideimide resin, styrene butadiene rubber (SBR), and polyacrylic acid. Etc. can be used suitably. These can be used alone or in combination of two or more. Of these, polyvinylidene fluoride (PVdF) resin is preferable from the viewpoints of stability in electrochemical reaction, solubility in a solvent, and the like.

  Examples of the conductive aid include carbon black such as ketjen black, acetylene black and furnace black, graphite, carbon nanotube, fullerene and the like. One or more of these may be used in combination. Of these, ketjen black, acetylene black, and the like can be preferably used from the viewpoint of easily ensuring electron conductivity.

  The content of the conductive additive is preferably 0 to 30 parts by mass, more preferably 6 to 13 parts by mass with respect to 100 parts by mass of the active material from the viewpoint of the degree of improvement in conductivity, electrode capacity, and the like. It should be within the range. The average particle diameter of the conductive aid is preferably 10 nm to 1 μm, more preferably 20 to 50 nm, from the viewpoints of dispersibility and ease of handling.

  As the above-mentioned aggregate, a material that does not expand or contract during charging / discharging or that has a very small expansion / contraction can be suitably used. For example, graphite, alumina, calcia, zirconia, activated carbon and the like can be exemplified. One or more of these may be used in combination. Of these, graphite and the like can be preferably used from the viewpoints of conductivity, Li activity, and the like.

  The content of the aggregate is preferably 10 to 400 parts by mass, more preferably 43 to 100 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoint of improving cycle characteristics and the like. And good. The average particle diameter of the aggregate is preferably 10 to 50 μm, more preferably 20 to 30 μm, from the viewpoints of functionality as an aggregate and control of the electrode film thickness. The average particle diameter of the aggregate is a value measured using a laser diffraction / scattering particle size distribution measuring apparatus.

  For example, the negative electrode is made into a paste by adding a necessary amount of the negative electrode active material, and, if necessary, a conductive additive and an aggregate to a binder dissolved in a suitable solvent. It can be produced by coating, drying, and applying consolidation or heat treatment as necessary.

  When a lithium secondary battery is configured using the present negative electrode, the positive electrode, electrolyte, separator, and the like, which are basic components of the battery other than the main negative electrode, are not particularly limited.

Specific examples of the positive electrode include, for example, a material in which a layer containing a positive electrode active material such as LiCoO ₂ , LiNiO ₂ , LiFePO ₄ , LiMnO ₂ is formed on the surface of a current collector such as an aluminum foil. Can do.

  Specific examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent (lithium ion secondary battery). In addition, a polymer in which a lithium salt is dissolved in a polymer, a polymer solid electrolyte in which a polymer is impregnated with the electrolytic solution, and the like can be used (lithium polymer secondary battery).

  Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. These may be contained alone or in combination of two or more.

Specific examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiA ₃ F _{3, and the} like. These may be contained alone or in combination of two or more.

  Other battery components include separators, cans (battery cases), gaskets, etc., and any of these may be used as long as they are usually employed in lithium secondary batteries. A battery can be configured by appropriately combining.

  The battery shape is not particularly limited, and may be any shape such as a cylindrical shape, a square shape, or a coin shape, and can be appropriately selected according to the specific application.

  Hereinafter, the present invention will be described more specifically with reference to examples. Note that% of the alloy composition and the alloy mixing ratio is mass% unless otherwise specified.

1. Preparation of negative electrode active material (Example 1)
Each raw material was weighed so as to have an alloy composition shown in Table 1 (68% Si-17% Al-8% Co-7% Bi). Each weighed raw material was heated and melted to obtain a molten alloy. The obtained molten alloy was rapidly cooled using a liquid single roll ultra-quenching method to obtain a quenched alloy ribbon. The roll peripheral speed was 25 m / s and the nozzle distance was 3 mm. The obtained quenched alloy ribbon was mechanically pulverized using a mortar to obtain Si alloy powder.

  Next, the obtained Si alloy powder and an Sn-based alloy powder (produced by gas atomization method) having the alloy composition shown in Table 1 (80% Sn-20% Co is Sn 67 atom% and Co 33 atom%) Were weighed so as to have the mixing ratio shown in Table 1 (Si alloy powder: Sn-based alloy powder = 25%: 75%). Each weighed powder is placed in a planetary ball mill (pot material: SUS304, ball material: SUJ2, ball diameter: 6.4 mm), sealed with Ar gas, mixed and combined under the conditions of a rotation speed of 300 rpm and a mixing time of 30 h. Turned into. Thereby, the negative electrode active material according to Example 1 was obtained.

(Examples 2 to 14)
In the same manner as in Example 1, each Si alloy powder having the alloy composition shown in Table 1, each Sn-based alloy powder (67% Sn-33% Co is Sn 50 atomic%, Co 50 atomic%, 75% Sn-25%. (Co is Sn 60 atomic%, Co 40 atomic%, 80% Sn-20% Co is Sn 67 atomic%, Co 33 atomic%). Subsequently, each negative electrode active material according to Examples 2 to 14 was obtained in the same manner except that the prepared alloy powders were mixed and compounded at the mixing ratios shown in Table 1 and combined.

(Comparative Examples 1 and 2)
In Example 1, pure Si powder (manufactured by gas atomization method) was prepared instead of Si alloy powder, and this pure Si powder and Sn-based alloy powder were mixed and mixed at the mixing ratio shown in Table 1. The negative electrode active materials according to Comparative Examples 1 and 2 were obtained in the same manner.

(Comparative Examples 3 and 4)
The alloy composition shown in Table 1 (17% Si-14% Al-13% Bi-40% Sn-16% Co, 17% Si-4% Al-2% Bi-60% Sn-17% Co) Each raw material was weighed, put in the above-mentioned planetary ball mill apparatus, sealed with Ar gas, mixed and compounded under the conditions of a rotation speed of 300 rpm and a mixing time of 30 h. Thereby, each negative electrode active material according to Comparative Examples 3 and 4 was obtained.

(Comparative Examples 5 and 6)
In Example 1, instead of Sn-based alloy powder, pure Cu powder (Fukuda Metal Foil Powder Co., Ltd., d50 = 5 μm) was prepared, and the Si alloy powder having the alloy composition shown in Table 1 and this pure Cu powder were prepared. Each negative electrode active material according to Comparative Examples 5 and 6 was obtained in the same manner except that it was used and mixed and mixed at the mixing ratio shown in Table 1.

(Comparative Example 7)
In Example 1, pure Si powder (manufactured by gas atomization method) instead of Si alloy powder, pure Cu powder (manufactured by Fukuda Metal Foil Powder Co., Ltd., d50 = 5 μm) was prepared instead of Sn alloy powder, and these A negative electrode active material according to Comparative Example 7 was obtained in the same manner except that pure Si powder and pure Cu powder were mixed and combined at a mixing ratio shown in Table 1.

2. Observation of Structure of Negative Electrode Active Material, etc. FIG. 4 shows, as a representative example, a reflected electron image of the negative electrode active material according to Example 1 by a scanning electron microscope (SEM). In FIG. 5, the reflected electron image by the scanning electron microscope (SEM) of the negative electrode active material which concerns on the comparative example 1 is shown. Elemental analysis was also performed by energy dispersive X-ray spectroscopy (EDX).

  According to the results, as shown in FIG. 4, the negative electrode active material according to Example 1 has a Si alloy in which a large number of Si crystallites are dispersed in the first metal matrix composed of the Al—Co—Bi phase. Thus, it was confirmed that this Si alloy had a structure that was dispersed in a large amount in an Sn-based alloy (second metal matrix having Li activity). That is, in the negative electrode active material according to Example 1, the periphery of the fine Si crystallite is surrounded by the first metal matrix, and the periphery of the first metal matrix is the second having the Li activity. It was confirmed to have a double matrix structure surrounded by a metal matrix. From this result, it can be seen that Si is present alone in the Si alloy, and other elements (Al, Co, Bi) and Si are phase-separated.

  In contrast, the negative electrode active material according to Comparative Example 1 was confirmed to have a large number of Si crystallites dispersed in the Sn-based alloy (matrix) as shown in FIG. That is, it was confirmed that the negative electrode active material according to Comparative Example 1 had a single matrix structure in which the periphery of the fine Si crystallite was surrounded by the metal matrix.

  Moreover, the magnitude | size of Si crystallite was measured about each negative electrode active material. The size of the Si crystallite is an average value of the sizes of the Si crystallites measured for 20 arbitrary Si crystallites in the SEM image (one field of view).

  Table 1 summarizes the details of the prepared negative electrode active materials.

3. 3. Evaluation of negative electrode active material 3.1 Production of coin-type battery for charge / discharge test First, each negative electrode active material adjusted to 25 μm or less by classification, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., d50). = 36 nm), graphite as an aggregate to be added as required (“SG-BH” manufactured by Ito Graphite Industries Co., Ltd.), and polyvinylidene fluoride (PVdF) as a binder are shown in Table 2. It mix | blended by the mixture ratio, this was mixed with N-methyl-2-pyrrolidone (NMP) as a solvent, and each paste containing each negative electrode active material was produced.

  Each coin type half battery was produced as follows. Here, for simple evaluation, an electrode produced using a negative electrode active material was used as a test electrode, and a Li foil was used as a counter electrode. First, each paste was applied to a surface of a copper foil (thickness: 18 μm) serving as a negative electrode current collector to a thickness of 50 μm using a doctor blade and dried to form each negative electrode active material layer. After the formation, the negative electrode active material layer was consolidated by a roll press. This produced the test pole which concerns on an Example and a comparative example.

  Next, the test electrodes according to Examples and Comparative Examples were punched into a disk shape having a diameter of 11 mm to obtain test electrodes.

Next, a Li foil (thickness: 500 μm) was punched out in substantially the same shape as the above test electrode, and each positive electrode was produced. In addition, LiPF ₆ was dissolved at a concentration of 1 mol / l in an equal mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) to prepare a nonaqueous electrolytic solution.

  Next, each test electrode was accommodated in each positive electrode can, and the counter electrode was accommodated in each negative electrode can, and a polyolefin microporous membrane separator was disposed between each test electrode and each counter electrode.

  Next, the non-aqueous electrolyte was poured into each can, and each negative electrode can and each positive electrode can were fixed by caulking.

3.2 with each coin type half cell charge-discharge test, the constant current charge-discharge current value 0.1mA performed one cycle, and the discharge capacity and the initial capacity C _0. The second and subsequent cycles, the charge and discharge test was performed at 1 / 5C rate (C rate: the electrode (charging) the electric quantity C ₀ required to discharge in 1 hour (charge) and 1C a current value for discharging (If it is 5C, it will discharge (charge) in 20 minutes, and if it is 1 / 5C, it will be charged and discharged in 5 hours.) A value obtained by dividing the capacity (mAh) used at the time of discharge by the amount of active material (g) was defined as each discharge capacity (mAh / g).

  In this example, the cycle characteristics were evaluated by performing the charge / discharge cycle 50 times.

  Then, from each of the obtained discharge capacities, the capacity retention ratio (discharge capacity after 2 cycles / initial discharge capacity (discharge capacity at the first cycle) × 100, discharge capacity after 50 cycles / initial discharge capacity (charge at the first cycle) Capacity) × 100). The results are shown in Table 3.

  According to Table 3, the following can be understood. That is, the negative electrode active material according to Comparative Examples 1 and 2 is a composite of pure Si and a Sn-based alloy. For this reason, Si does not collapse rapidly due to volume expansion / contraction associated with insertion / extraction of lithium ions without taking a double matrix structure, and a battery using a negative electrode made of these negative electrode active materials has insufficient cycle characteristics.

  The negative electrode active materials according to Comparative Examples 3 and 4 are not rapidly cooled at the time of manufacture, and are combined after mixing all the elements. Therefore, a battery using a negative electrode made of these negative electrode active materials without taking a matrix structure has insufficient cycle characteristics.

  The negative electrode active material according to Comparative Examples 5 and 6 is a composite of Si alloy and Cu. Since Cu having no Li activity becomes the second metal matrix, it is difficult for lithium to reach inside the Si alloy. Therefore, batteries using negative electrodes made of these negative electrode active materials have a low initial discharge capacity and a low active material utilization rate.

  The negative electrode active material according to Comparative Example 7 is also a composite of Si and Cu. Since Cu having no Li activity becomes the second metal matrix, it is difficult for lithium ions to reach Si. Therefore, batteries using negative electrodes made of these negative electrode active materials have a low initial discharge capacity and a low active material utilization rate. In addition, since the double matrix structure is not adopted, the battery using the negative electrode made of these negative electrode active materials has insufficient cycle characteristics.

  On the other hand, it was confirmed that both the negative electrode active material and the negative electrode according to Examples 1 to 12 were able to achieve both improvement of the active material utilization rate and improvement of cycle characteristics. Further, when Examples 1 to 6 are compared, in the Sn—Co alloy, Examples 5 and 6 in which the Sn content is 75% are the same as those in Examples 1 and 2 and Sn in which the Sn content is 80%. The cycle characteristics are better than those of Examples 3 and 4 where the content is 67%. That is, when the second metal matrix is a Sn—Co alloy, it was speculated that the cycle characteristics would be extremely improved when the Sn content was 70 to 70 mass%.

  Further, when Example 13 is compared with Example 1, the Si crystallite diameter of Example 13 is larger than the Si crystallite diameter of Example 1. Therefore, the cycle characteristics of Example 13 are slightly inferior to those of Example 1. This is because, even in the same alloy series, Bi was added to the first metal matrix of Example 1 and Si crystallites were precipitated more finely, so that the cycle characteristics of Example 1 were further improved. It is guessed. Accordingly, it can be said that Bi is preferably contained in the first metal matrix from the viewpoint of contributing to improvement of cycle characteristics.

  Moreover, as shown in Examples 7 to 12, it was confirmed that the cycle characteristics were further improved by adding graphite as an aggregate to the negative electrode. This is considered to be because by adding aggregate to the negative electrode, the expansion and contraction of the negative electrode during charge / discharge can be easily suppressed, and the collapse of the negative electrode can be suppressed.

  As described above, the negative electrode active material for a lithium secondary battery and the method for producing the same according to the present invention have been described. Various modifications are possible.

DESCRIPTION OF SYMBOLS 10 Negative electrode active material 12 for lithium secondary batteries Si alloy 12a Si crystallite 12b 1st metal matrix 14 2nd metal matrix 20 Negative electrode 22 for lithium secondary batteries Conductive base material 24 Conductive film 26 Binder 28 Conductive auxiliary material 30 aggregate

Claims (4)

Having a Si alloy in which Si crystallites are dispersed in a first metal matrix;
The negative active material for a lithium secondary battery, wherein the Si alloy is dispersed in a second metal matrix having Li activity.   The negative electrode active material for a lithium secondary battery according to claim 1, wherein the second metal matrix is Sn or an Sn-based alloy.   3. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the second metal matrix contains 50 atomic% or more of Sn. 4. Producing a Si alloy powder in which Si crystallites are dispersed in a first metal matrix;
Mixing and compounding the obtained Si alloy powder and a second metal matrix powder having Li activity, and dispersing the Si alloy in the second metal matrix;
The manufacturing method of the negative electrode active material for lithium secondary batteries characterized by having.