JP2009238641A - Anode active material for lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode active material for a lithium ion secondary battery, which can achieve high capacity and excellent cycle characteristics. <P>SOLUTION: The anode active material for a lithium ion secondary battery contains a mixture of Mg<SB>2</SB>A or LaA<SB>2</SB>(A denotes Si or Ge) and an element A (A denotes Si or Ge). An active material layer containing the active material is preferably formed by attaching Mg<SB>2</SB>A or LaA<SB>2</SB>and element A on a surface of a collector by gas deposition method. Alternatively, on the active material layer, at least part of a surface of a particle of Mg<SB>2</SB>A or LaA<SB>2</SB>and a particle of the element A are coated with a metal material with low forming ability of a lithium compound, as well as a gap is preferably formed between the particles coated with the metal material. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a negative electrode active material used for a lithium ion secondary battery. The present invention also relates to a negative electrode using the negative electrode active material.

  Lithium ion batteries are widely used in mobile devices such as mobile phones and notebook personal computers. With the increase in functionality of these portable devices, power consumption has increased remarkably, and large capacity secondary batteries are increasingly required. Until now, carbon materials have been used as negative electrode active materials for lithium ion secondary batteries. However, as long as carbon materials are used, it is expected that it will be difficult to meet the need for higher capacity in the near future.

Therefore, at present, a negative electrode active material made of a Si-based material having a capacity potential 5 to 10 times that of graphite is being actively developed. For example, Patent Document 1 proposes a negative electrode active material of silicon containing lithium represented by Li _x Si. However, the Si-based material has a drawback that the volume change accompanying the occlusion and release of lithium is remarkably caused and the cycle characteristics are not good.

In addition to the Si-based material, the present inventors previously used an alloy of Mg and a group 14 element such as Mg ₂ Ge or an alloy of a rare earth element and a group 14 element such as LaSn ₃ as the negative electrode active material. This was proposed (see Patent Document 2). Although these alloys have good cycle characteristics, there is room for improvement in increasing the capacity.

Japanese Patent Laid-Open No. 7-29602 JP 2006-156248 A

  Accordingly, an object of the present invention is to provide a negative electrode active material for a lithium ion secondary battery and a negative electrode for a lithium ion secondary battery having the active material, which can eliminate the various disadvantages of the above-described prior art.

The present invention includes a mixture of Mg ₂ A or LaA ₂ (A represents Si or Ge) and an element A (A represents Si or Ge), and a negative electrode active for a lithium ion secondary battery, It provides a substance.

  Moreover, this invention provides the negative electrode for lithium ion secondary batteries provided with the active material layer containing the said active material.

  By constituting the negative electrode using the active material of the present invention, the capacity of a battery equipped with the negative electrode can be made higher than before and the cycle characteristics can be improved.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The active material of the present invention is used for a negative electrode of a lithium ion secondary battery. The negative electrode basically has a current collector and a negative electrode active material layer formed on at least one surface of the current collector. The negative electrode active material layer includes a mixture of Mg ₂ A or LaA ₂ (A represents Si or Ge) and element A (A represents Si or Ge). These substances are present in a substantially uniformly dispersed state throughout the active material layer.

LaA ₂ is an alloy of La and element A or an intermetallic compound. La is known to form a stable compound with Si and Ge, which are 14 group elements. Similarly, Mg ₂ A is an alloy of Mg and element A or an intermetallic compound. These materials have the advantage that the cycle characteristics of the battery are improved when they are used as the negative electrode active material of a non-aqueous electrolyte secondary battery.

In the present invention, LaA ₂ and Mg ₂ A may be used alternatively or in combination. In addition, Si and Ge may alternatively be used as the element A, or both may be combined. When Si and Ge are used alternatively as the element A, it is particularly preferable to use Si from the viewpoint of further improving the cycle characteristics.

Of LaA ₂ and Mg ₂ A, when LaA ₂ is used, the combination of this material and element A includes a combination of LaGe ₂ and Si, a combination of LaSi ₂ and Si, and a combination of LaGe ₂ and Ge. A combination and a combination of LaSi ₂ and Ge are preferred. Of these combinations, it is particularly preferable to use a combination of LaGe ₂ and Si, a combination of LaSi ₂ and Si, and a combination of LaGe ₂ and Ge because the advantages described above become more remarkable. On the other hand, when using Mg ₂ A, a combination of Mg ₂ Ge and Si, a combination of Mg ₂ Ge and Ge, a combination of Mg ₂ Si and Si, and a combination of Mg ₂ Si and Ge are preferable. Among these combinations, it is particularly preferable to use a combination of Mg ₂ Ge and Si since the advantages described above become more remarkable.

LaA ₂ and Mg ₂ A can be produced by a known method. For example, it can be manufactured by a mechanical alloying method (mechanical milling method) or a gas atomizing method. Moreover, you may grind | pulverize what was manufactured by the normal metallurgical method etc. A particularly preferable production method is a mechanical alloying method and / or a mechanical milling method. This is because by using LaA ₂ particles and Mg ₂ A particles produced by these methods, it is possible to provide a secondary battery exhibiting high charge / discharge characteristics and the like.

  The mechanical alloying method and the mechanical milling method can be performed based on known conditions. For example, a starting material prepared so as to become a predetermined powder material may be charged into a ball mill and milling may be performed. A known device such as a planetary ball mill can be used as the ball mill. The milling may be either dry or wet, but it is particularly desirable that the milling be dry. Milling conditions can be appropriately set according to the properties of the desired powder raw material. In general, the rotational speed may be about 100 to 500 rpm at room temperature (especially 0 to 50 ° C.). The milling atmosphere is preferably an inert gas atmosphere such as argon gas or nitrogen gas.

Element A used in combination with LaA ₂ and / or Mg ₂ A is used for the purpose of increasing the capacity of the battery. An appropriate range of the quantitative relationship between LaA ₂ and / or Mg ₂ A and element A in the active material layer is selected according to the type of element A. Specifically, when the element A is Si, the ratio of the total weight of LaA ₂ and Mg ₂ A to the weight of the element A (the former: the latter) is 8: 2 to 2: 8, especially 7 : 3 to 4: 6 is preferable from the viewpoint of the balance between improvement in capacity density and improvement in cycle life or suppression of expansion of the active material layer. From the same viewpoint, when the element A is Ge, the ratio (the former: latter) of the total weight of LaA ₂ and Mg ₂ A to the weight of the element A is 9: 1 to 1: 9, particularly 8 : It is preferable that it is 2-1: 9.

In the present invention, it is possible to significantly increase the capacity of a battery by using LaA ₂ and / or Mg ₂ A in combination with the element A, as compared with the case of using them alone. It became clear as a result of these examinations. The reason is considered that the capacity of the element A is added to the capacity of LaA ₂ and / or Mg ₂ A. Moreover, the cycle characteristics of the battery are also improved by using a combination of both. The reason for this is that since LaA ₂ and Mg ₂ A are softer materials than the element A, the stress due to the volume change caused by the insertion and release of lithium by the element A is expressed as LaA ₂ and / or This is probably because Mg ₂ A absorbs. The stress relaxation is also achieved by the presence of voids between the particles, as will be described later. Thus, according to the negative electrode of the present invention that employs a combination of LaA ₂ and / or Mg ₂ A and element A, the battery can have a high capacity and a long life.

Means for mixing LaA ₂ and / or Mg ₂ A and element A include, for example, means for mixing LaA ₂ and / or Mg ₂ A particles and element A particles for a predetermined time using a mortar and pestle. Can be mentioned. Alternatively, when LaA ₂ and / or Mg ₂ A particles are produced by the mechanical alloying method (mechanical milling method), the element is in excess of the stoichiometric ratio of LaA ₂ and / or Mg ₂ A. The method of using A with La and / or Mg is mentioned. In the powder raw material 2 obtained by this method, the LaA ₂ phase and / or the Mg ₂ A phase exist around the element A phase in one particle. That is, a composite particle in which the LaA ₂ phase and / or the Mg ₂ A phase and the element A phase are combined at the level of one particle. As a result, there is an advantage that the LaA ₂ phase and / or the Mg ₂ A phase acts as a matrix of the phase of the element A, and the stress generated by the volume change accompanying charge / discharge is further relaxed. As a result, by using composite particles, there is an advantage that the ratio in the active material of the element A, which is a substance having a large volume change at the time of charge / discharge, can be increased while improving the cycle characteristics. .

There is no particular limitation on the form of the active material layer containing LaA ₂ and / or Mg ₂ A and element A. For example, LaA ₂ and / or Mg ₂ A particles, element A particles, a binder and conductive particles are dispersed in a solvent to prepare a negative electrode mixture, and this mixture is applied to the surface of the current collector. The active material layer can be formed by drying. Alternatively, the active material layer thus formed may be fired to sinter the particles. Instead of separately using LaA ₂ and / or Mg ₂ A particles and element A particles, the composite particles described above may be used.

In particular, the active material layer is preferably formed by adhering LaA ₂ and / or Mg ₂ A particles and element A particles to the surface of the current collector by a gas deposition method. Alternatively, it is preferable that the composite particles of LaA ₂ and / or Mg ₂ A and element A are formed by adhering to the surface of the current collector by a gas deposition method. The active material layer having such a form has an advantage that the adhesion between the particles and the current collector is high and the adhesion between the particles is also high. Therefore, even if charging / discharging is repeated, the particles constituting the active material layer are difficult to drop off, thereby improving the cycle characteristics of the battery. Due to the good adhesion between the particles, there is an advantage that the electron conductivity of the active material layer can be increased even when the element A which is a material having low electron conductivity is used. Moreover, since the density in the thickness direction and the surface direction of the active material layer becomes non-uniform by adopting the gas deposition method, it is caused by the volume change caused by the insertion and extraction of lithium by the element A, particularly Si. Stress is easily relaxed. This also improves the cycle characteristics of the battery. Further, adopting the gas deposition method has an advantage that the film forming speed is high.

  The gas deposition method is a method of forming an active material layer by generating an aerosol by using a powder raw material and a carrier gas and injecting the aerosol onto the surface of a current collector. Specifically, as shown in FIG. 1, after the carrier gas 1 having a predetermined initial pressure is aerosolized in the conduit 3 together with the powder raw material 2, the aerosol is maintained in a vacuum state by the decompression device 4. It is carried out by ejecting from the nozzle 7 attached to the tip of the conduit 3 toward the surface of the current collector 6 installed in the chamber 5. In addition, since the particle | grains in the powder raw material 2 deform | transform by the collision with the electrical power collector 6, in many cases, the original shape is not kept. The gas deposition method itself can be carried out according to a known method (apparatus). In the present invention, the gas deposition method is preferably performed under the following conditions.

As the carrier gas, for example, an inert gas such as argon gas or nitrogen gas is preferably used. The pressure difference (difference between the pressure in the apparatus and the gas gauge pressure) is preferably about 3 × 10 ⁵ to 1 × 10 ⁶ Pa. The distance between the current collector and the nozzle is preferably about 5 to 30 mm.

When the gas deposition method is used, the average particle diameter D _{50 of the} LaA ₂ and Mg ₂ A particles used as the powder raw material 2 is preferably 0.1 to ₅₀ μm, particularly preferably 0.1 to 10 μm. There is no particular limitation on the shape of the particles. The average particle diameter D ₅₀ is measured by a laser diffraction / scattering particle size distribution analyzer. In the following description, the average particle diameter D ₅₀ refers to a value measured using this apparatus.

The particles of the element A used as the powder raw material 2 have an average particle diameter D ₅₀ of 0.5 to 6 μm, particularly preferably 1.5 to 3.5 μm. There is no particular limitation on the shape of the particles of the element A. Also relates to the relationship between the average particle diameter D ₅₀ of the particles of the LAA ₂ and Mg ₂ A previously described, (average particle diameter D ₅₀ of the particles of the LAA ₂ and Mg ₂ A) / (average grain particles of the element A The value of the diameter D ₅₀ ) is 0.08 to 12, particularly 0.4 to 2.5, and the volume change caused by the occlusion / release of lithium by the element A, particularly Si particles, is reduced to LaA _2. And / or Mg ₂ A particles are preferable because they can relax.

  When the gas deposition method is performed, the target active material layer may be formed by one injection, but instead, the active material layer may be formed by performing injection a plurality of times. When spraying a plurality of times, an active material layer having a multilayer structure is formed.

As shown in FIG. 2, the active material layer is a metal material in which at least a part of the surface of LaA ₂ and / or Mg ₂ A particles and at least a part of the surface of the element A particles have a low lithium compound forming ability. It is also preferable to have a structure in which voids are formed between the particles coated with the metal material. A negative electrode 10 shown in FIG. 2 includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. 2 shows a state in which the active material layer 12 is formed only on one surface of the current collector 11 for convenience, the active material layer may be formed on both surfaces of the current collector. . “Low lithium compound forming ability” means that lithium does not form an intermetallic compound or solid solution, or even if formed, lithium is in a very small amount or very unstable.

The active material layer 12 shown in FIG. 2 includes LaA ₂ and / or Mg ₂ A particles 12 a and element A particles 12 b. Note that the structure of the active material layer 12 shown in FIG. 2 is schematic for the purpose of easily understanding the present invention, and the shape, size, number, and the like of the particles 12a and the particles 12b are the actual states. Is not necessarily the same. At least a part of the surfaces of the particles 12 a and the particles 12 b is covered with a metal material 13. In FIG. 2, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a and the particle 12b. The particle diameters of the LaA ₂ and / or Mg ₂ A particles 12a and the element A particles 12b may be the same as the particle diameter of the powder raw material 2 used in the gas deposition method described above.

  The metal material 13 is a material different from the constituent material of the particles 12a and the constituent material of the particles 12b. Gaps are formed between the particles 12a coated with the metal material 13, between the particles 12b, and between the particles 12a and 12b. This void serves as a flow path for the non-aqueous electrolyte containing lithium ions. Furthermore, the above-mentioned voids also serve as a space for relieving the stress caused by the volume change of the Si particles 12b during charging and discharging. The increase in the volume of the Si particles 12b whose volume has been increased by charging is absorbed by the voids.

  The metal material 13 has conductivity, and examples thereof include copper, nickel, iron, cobalt, and alloys of these metals. In particular, as the metal material 13, it is preferable to use copper which is a highly ductile material. The metal material 13 is preferably present on the surfaces of the particles 12 a and the particles 12 b over the entire thickness direction of the active material layer 12. The presence of the metal material 13 on the surfaces of the particles 12a and 12b over the entire thickness direction of the active material layer 12 can be confirmed by electron microscope mapping using the metal material 13 as a measurement target.

  The metal material 13 covers the surfaces of the particles 12a and 12b continuously or discontinuously. When the metal material 13 covers the surfaces of the particles 12a and 12b continuously, it is preferable to form fine voids in the coating of the metal material 13 that allow the non-aqueous electrolyte to flow. In the case where the metal material 13 discontinuously coats the surfaces of the particles 12a and 12b, the particles 12a and 12b are not covered with the particles 12a and 12b through the portions of the surfaces of the particles 12a and 12b that are not covered with the metal material 13. A water electrolyte is supplied.

  The average thickness of the metal material 13 covering the surfaces of the particles 12a and 12b is preferably as thin as 0.05 to 2 μm, more preferably 0.1 to 0.25 μm. Here, the “average thickness” is a value calculated based on a portion of the surfaces of the particles 12a and 12b that are actually covered with the metal material 13. Therefore, the part which is not coat | covered with the metal material 13 among the surfaces of the particle | grains 12a and the particle | grains 12b is not used as the basis of calculation of an average value.

  The active material layer 12 of the embodiment shown in FIG. 2 can be obtained, for example, according to the method described in Japanese Patent Application Laid-Open No. 2008-16195 or Japanese Patent Application Laid-Open No. 2008-16198 related to the previous application of the present applicant. Specifically, the active material layer 12 preferably has a predetermined plating applied to a coating film obtained by applying a slurry containing particles 12a, particles 12b and a binder onto the current collector 11 and drying the slurry. It is formed by performing electroplating using a bath and depositing the metal material 13 between the particles 12a, between the particles 12b, and between the particles 12a and 12b. The degree of voids between these particles and the degree of coating of the metal material 13 on the particle surface can be controlled by the degree of deposition of the metal material 13 by electrolytic plating. The conditions for electrolytic plating include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. Regarding the pH of the plating bath, it is preferable to adjust it to more than 7 and 11 or less, particularly 7.1 or more and 11 or less. By setting the pH within this range, the surface of the particles 12b is cleaned while the dissolution of the Si particles 12b is suppressed, and the plating on the particle surface is promoted. At the same time, moderate voids are formed between the particles 12a, between the particles 12b, and between the particles 12a and 12b. The value of pH is measured at the temperature at the time of plating.

For example, when copper is used as the metal material 13 for plating, it is preferable to use a copper pyrophosphate bath. It is preferable to use a copper pyrophosphate bath, even if the active material layer 12 is thick, because the voids can be easily formed over the entire thickness direction of the layer. Further, the metal material 13 is deposited on the surfaces of the particles 12a and 12b, and the metal material 13 is less likely to be deposited between the particles 12a, between the particles 12b, and between the particles 12a and 12b. It is also preferable in that the voids are formed successfully. When using a copper pyrophosphate bath, the bath composition, electrolysis conditions and pH are preferably as follows.
Copper pyrophosphate trihydrate: 85-120 g / 1
Potassium pyrophosphate: 300 to 600 g / 1
-Potassium nitrate: 15-65g / 1
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
PH: Ammonia water and polyphosphoric acid are added to adjust the pH to 7.1 to 9.5.

In the active material layer 12 of the embodiment shown in FIG. 2, the composite particles described above may be used instead of using the LaA ₂ and / or Mg ₂ A particles 12 a and the element A particles 12 b separately. .

  In the negative electrode of the present invention, no matter how the active material layer is produced, the thickness of the active material layer is 0.5 to 40 μm, particularly 0.5 to 30 μm, especially 0.5 to 25 μm. It is preferable that By setting the thickness of the active material layer in this range, the energy density of the battery can be sufficiently improved, the strength of the negative electrode can be sufficient, and the particles can be effectively removed from the active material layer. Can be prevented.

  As the current collector in the negative electrode, the same current collector as that conventionally used as the negative electrode current collector for lithium ion secondary batteries can be used. The current collector is preferably composed of a metal material having a low lithium compound forming ability as described above. Examples of such metal materials are as already described. In particular, it is preferably made of copper, nickel, stainless steel or the like. Also, it is possible to use a copper alloy foil represented by a Corson alloy foil. The thickness of the current collector is preferably 9 to 35 μm considering the balance between maintaining the strength of the negative electrode and improving the energy density. In addition, when using copper foil as a collector, it is preferable to perform the rust prevention process using organic compounds, such as a chromate process and a triazole type compound and an imidazole type compound.

  The negative electrode having the above structure is used with a positive electrode, a separator, and a non-aqueous electrolyte to form a lithium ion secondary battery. As these positive electrode, separator and non-aqueous electrolyte, known ones can be used without particular limitation.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” and “part” mean “% by weight” and “part by weight”, respectively.

[Example 1]
(1) was prepared Mg ₂ Ge powder by Mg ₂ Ge powder of a mechanical alloying (MA) method. First, each powder of Mg and Ge (about 5 g in total amount) was put in a stainless steel ball mill container (capacity 80 mL) together with five balls (diameter 5 mm). The molar ratio of Mg / Ge was 2.0. The weight ratio of the ball to the sample was 15: 1. The operation was performed under an argon atmosphere, and the container used was an O-ring that was kept airtight. Milling was performed at 300 rpm (rpm) for about 25 hours at room temperature using a planetary ball mill. In this way, Mg ₂ Ge powder was obtained. The average particle diameter D ₅₀ of this powder was about 1 μm.

(2) Production of Mg ₂ Ge and Si mixed powder Si powder having an average particle diameter D ₅₀ of 0.5 μm and Mg ₂ Ge powder produced by the above-described method are mixed for 15 minutes using an alumina mortar and pestle. Mixed. The weight ratio of Mg ₂ Ge powder to Si powder was 7: 3. In this way, a mixed powder was obtained.

(3) Production of negative electrode An electrolytic copper foil having a thickness of 20 μm was used as a current collector. Using this copper foil, a negative electrode was produced using the gas deposition method shown in FIG. The thickness of the active material layer in the negative electrode was 2 μm. The conditions for the gas deposition method were as follows.
Carrier gas: helium (6N)
・ Pressure difference: 7.0 × 10 ⁵ Pa
・ Nozzle diameter: 0.8mm
・ Nozzle-current collector distance: 30 mm
・ Atmosphere: Argon at room temperature

[Example 2]
A negative electrode was obtained in the same manner as in Example 1 except that the weight ratio of the Mg ₂ Ge powder and the Si powder was 5: 5.

[Comparative Example 1]
A negative electrode was obtained in the same manner as in Example 1 except that only the Mg ₂ Ge powder was used without using the Si powder.

[Comparative Example 2]
A negative electrode was obtained in the same manner as in Example 1 except that only the Si powder was used without using the Mg ₂ Ge powder.

[Evaluation]
A tripolar cell was constructed using the negative electrodes obtained in Examples 1 and 2 and Comparative Examples 1 and 2, and charge and discharge were performed to obtain charge and discharge curves. Metallic lithium was used for the reference electrode and the counter electrode in the triode cell. As the electrolytic solution, a lithium perchlorate propylene carbonate solution (concentration: 1M) was used. In addition, a test of cycle characteristics and a test of Coulomb efficiency (charge / discharge efficiency) in each cycle (except for Example 2) were performed using a triode cell. These results are shown in FIGS. The charge / discharge conditions were a current density of 0.5 mA / cm ² . The temperature was 30 ° C. and the atmosphere was argon.

  As is apparent from the results shown in FIGS. 3 to 5, it can be seen that the negative electrode of the example has both high capacity and good cycle characteristics. On the other hand, it can be seen that the negative electrode of Comparative Example 1 has a low capacity although it has good cycle characteristics and Coulomb efficiency. In the negative electrode of Comparative Example 2, as shown in FIG. 5, a decrease in Coulomb efficiency is observed around 50 cycles. The reason for this is considered to be that the Si particles dropped from the current collector due to the expansion and contraction of the Si particles due to charge and discharge.

Example 3
It was prepared Lage ₂ powder by mechanical alloying (MA) method. First, each powder of La and Ge (about 5 g in total) was put in a stainless steel ball mill container (capacity 80 mL) together with five balls (diameter 5 mm). The molar ratio of Ge / La was 2.0. The weight ratio of the ball to the sample was 15: 1. The operation was performed under an argon atmosphere, and the container used was an O-ring that was kept airtight. Milling was performed at 300 rpm (rpm) for about 5 hours at room temperature using a planetary ball mill. In this way, LaGe ₂ powder was obtained. The average particle diameter D ₅₀ of this powder was about 1 μm. A negative electrode was obtained in the same manner as in Example 1 except that the LaGe ₂ powder thus obtained was used in place of the Mg ₂ Ge powder used in Example 1.

Example 4
A negative electrode was obtained in the same manner as in Example 3 except that the weight ratio of LaGe ₂ powder and Si powder was 5: 5.

[Comparative Example 3]
A negative electrode was obtained in the same manner as in Example 3 except that only the LaGe ₂ powder was used without using the Si powder.

[Evaluation]
For the negative electrodes obtained in Examples 3 and 4 and Comparative Example 3, the cycle characteristics were tested in the same manner as described above. The result is shown in FIG. As is apparent from the results shown in the figure, the negative electrodes of Examples 3 and 4 in which the active material layer includes a combination of LaGe ₂ particles and Si particles are used in Comparative Example 3 in which the active material layer is composed only of LaGe ₂ particles. It can be seen that it has better cycle characteristics than the negative electrode.

Example 5
In Example 3, Ge powder having an average particle diameter D ₅₀ of 0.5 μm was used instead of Si powder, and the weight ratio of LaGe ₂ powder to Ge powder was 1: 9. A negative electrode was produced using the mixed powder of both using the gas deposition method shown in FIG. As the current collector, an electrolytic copper foil having a thickness of 20 μm was used. The thickness of the active material layer in the negative electrode was 2 μm. The conditions for the gas deposition method were as follows.
Carrier gas: helium (4N)
・ Pressure difference: 7.0 × 10 ⁵ Pa
・ Nozzle diameter: 0.8mm
・ Nozzle-current collector distance: 10 mm
・ Atmosphere: Argon at room temperature

[Comparative Example 4]
A negative electrode was obtained in the same manner as in Example 5 except that only the LaGe ₂ powder was used without using the Ge powder.

[Comparative Example 5]
A negative electrode was obtained in the same manner as in Example 5 except that only the Ge powder was used without using the LaGe ₂ powder.

[Evaluation]
With respect to the negative electrodes obtained in Example 5 and Comparative Examples 4 and 5, cycle characteristics were performed in the same manner as described above. The result is shown in FIG. As is clear from the results shown in FIG. 7, the negative electrode of Example 5 in which the active material layer includes a combination of LaGe ₂ particles and Ge particles is about twice the theoretical capacity of the graphite negative electrode even after 300 cycles. It can be seen that it has a high capacity of 726 mAh / g. On the other hand, it can be seen that the negative electrode of Comparative Example 4 in which the active material layer is composed only of LaGe ₂ particles has a low capacity of about 20 mAh / g. Moreover, it turns out that the capacity | capacitance of the negative electrode of the comparative example 5 which an active material layer consists of only a particle | grain of Ge falls rapidly from about 100 cycles.

Example 6
(1) was produced composite powder of LaSi ₂ and Si by LaSi ₂ and Si composite powder of a mechanical alloying (MA) method. First, each powder of La and Si (about 4 g in total) was put in a zirconia ball mill container (capacity 80 mL) together with five balls (diameter 5 mm). The mixing ratio of La and Si was such that the weight ratio of LaSi ₂ and Si was 7: 3. The weight ratio of the ball to the sample was 15: 1. The operation was performed under an argon atmosphere, and the container used was an O-ring that was kept airtight. Milling was performed at 300 rpm (rpm) for about 5 hours at room temperature using a planetary ball mill. Thus, a composite powder of LaSi ₂ and Si was obtained. The average particle diameter D ₅₀ of the composite powder was approximately 1 [mu] m.

(4) Production of negative electrode An electrolytic copper foil having a thickness of 20 μm was used as a current collector. Using this copper foil, a negative electrode was produced using the gas deposition method shown in FIG. The thickness of the active material layer in the negative electrode was 2 μm. The conditions for the gas deposition method were as follows.
Carrier gas: helium (6N)
・ Pressure difference: 7 × 10 ⁵ Pa
・ Nozzle diameter: 0.8mm
・ Nozzle-current collector distance: 10 mm
・ Atmosphere: Argon at room temperature

Example 7
A composite powder of LaSi ₂ and Si was obtained in the same manner as in Example 6 except that the mixing ratio of La and Si was changed so that the weight ratio of LaSi ₂ and Si was 5: 5. Using this composite powder, a negative electrode was obtained in the same manner as in Example 6.

[Comparative Example 6]
A negative electrode was obtained in the same manner as in Example 6 except that only Si was used without using LaSi ₂ .

[Comparative Example 7]
A negative electrode was obtained in the same manner as in Example 6 except that only LaSi ₂ was used without using Si.

[Evaluation]
For the negative electrodes obtained in Examples 6 and 7 and Comparative Examples 6 and 7, a cycle characteristic test and a Coulomb efficiency (charge / discharge efficiency) test in each cycle were performed in the same manner as described above (however, Comparative Example 7 Excluded.) The results are shown in FIGS. As is apparent from the results shown in FIG. 8, the negative electrodes of Examples 6 and 7 in which the active material layer includes composite particles of LaSi ₂ and Si are the negative electrode of Comparative Example 6 in which the active material layer is composed of only Si particles. Compared with the negative electrode of Comparative Example 7 in which the active material layer is composed only of LaSi ₂ particles, it can be seen that a high capacity is maintained even after 1000 cycles of charge and discharge. Moreover, as is clear from the results shown in FIG. 9, the negative electrodes obtained in Examples 6 and 7 have good Coulomb efficiency, whereas the negative electrode obtained in Comparative Example 6 is charged and discharged. A decrease in Coulomb efficiency, which is thought to be caused by the falling of the particles from the current collector due to the expansion and contraction of the Si particles, is observed in the vicinity of 50 cycles.

It is a schematic diagram which shows the apparatus for implementing a gas deposition method. It is a schematic diagram which shows the longitudinal cross-section of one Embodiment of the negative electrode of this invention. It is a graph which shows the charging / discharging curve of the negative electrode obtained in Examples 1 and 2 and Comparative Examples 1 and 2. It is a graph which shows the cycling characteristics of the negative electrode obtained in Examples 1 and 2 and Comparative Examples 1 and 2. It is a graph which shows the Coulomb efficiency of the negative electrode obtained in Examples 1 and 2 and Comparative Examples 1 and 2. 6 is a graph showing cycle characteristics of negative electrodes obtained in Examples 3 and 4 and Comparative Example 3. It is a graph which shows the cycling characteristics of the negative electrode obtained in Example 5 and Comparative Examples 4 and 5. It is a graph which shows the cycling characteristics of the negative electrode obtained in Examples 6 and 7 and Comparative Examples 6 and 7. 6 is a graph showing the Coulomb efficiency of the negative electrodes obtained in Examples 6 and 7 and Comparative Example 6.

Explanation of symbols

1 Carrier gas 2 powder raw material 3 conduit 4 decompressor 5 chamber 6 current collector 7 nozzle 10 negative electrode 11 current collector 12 active material layer 12a Mg ₂ Ge and / or particles 13 metal material Lage ₂ particles 12b Si

Claims (8)

A negative electrode active material for a lithium ion secondary battery, comprising a mixture of Mg ₂ A or LaA ₂ (A represents Si or Ge) and an element A (A represents Si or Ge). Element A is Si, and the weight ratio of Mg ₂ A or LaA ₂ : element A is 8: 2 to 2: 8,
_{2. The} negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the element A is Ge and the weight ratio of Mg ₂ A or LaA ₂ : element A is 9: 1 to 1: 9. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising a mixture of LaGe ₂ and Si or Ge. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising a mixture of LaSi ₂ and Si. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising a mixture of Mg ₂ Ge and Si.   A negative electrode for a lithium ion secondary battery comprising an active material layer containing the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5. The negative electrode for a lithium ion secondary battery according to claim 6, wherein the active material layer is formed by adhering Mg ₂ A or LaA ₂ and the element A to the surface of the current collector by a gas deposition method. In the active material layer, at least part of the surfaces of the Mg ₂ A or LaA ₂ particles and the element A particles are coated with a metal material having a low lithium compound forming ability, and are coated with the metal material. The negative electrode for a lithium ion secondary battery according to claim 6, wherein voids are formed between the particles.

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