JP2007335198A - Composite active material for nonaqueous secondary battery, and nonaqueous secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite active material that provides a non-aqueous electrolyte secondary battery in which a higher initial discharge capacity and superior cycle characteristics than those in conventional ones can be compatible. <P>SOLUTION: This is the composite active material for the non-aqueous electrolyte secondary battery which contains active material particles capable of storing and releasing lithium, and fibers grown from surfaces of the active material particles, in which the active material particles contain at least silicon, and the fibers contain at least carbon, and which has between the active material particles and the fibers an adhesive phase that enhances connection force of the active material particles and the fibers. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention mainly relates to a composite active material for a non-aqueous electrolyte secondary battery, and more particularly to a composite active material including active material particles containing at least silicon and fibers grown from the surface of the active material particles. The composite active material of the present invention has an adhesive phase between the active material particles and the fiber to increase the bonding force between the active material particles and the fiber.

  The nonaqueous electrolyte secondary battery is small and lightweight and has a high energy density. Therefore, the demand for non-aqueous electrolyte secondary batteries is increasing as equipment becomes more portable and cordless. Currently, carbon materials (natural graphite, artificial graphite, etc.) are mainly used for the negative electrode active material of non-aqueous electrolyte secondary batteries. The theoretical capacity of graphite is 372 mAh / g. The capacity of a negative electrode active material made of a carbon material that is currently in practical use is approaching the theoretical capacity of graphite. Therefore, it is very difficult to realize a further increase in capacity by improving the carbon material.

  On the other hand, the capacity of an element that can be alloyed with lithium, particularly a material containing Si, greatly exceeds the theoretical capacity of graphite. Therefore, a material containing silicon is expected as a next-generation negative electrode active material. However, a material containing silicon has a very large volume change associated with insertion and extraction of lithium. Therefore, when the charge / discharge cycle of the battery is repeated, the negative electrode active material repeatedly expands and contracts, and the conductive network between the active material particles is cut. Therefore, the deterioration accompanying the charge / discharge cycle becomes very large.

  Therefore, in order to prevent cracking of the active material particles, it has been proposed to reduce the crystallite size of the active material to the nano level. It has also been proposed to combine a material containing silicon and a carbon material. Further, it has been studied to alloy silicon with a transition metal. However, it is difficult to obtain practical cycle characteristics.

  Under such circumstances, composite particles including a negative electrode active material containing silicon, a catalytic element that promotes the growth of carbon nanofibers, and carbon nanofibers grown from the surface of the negative electrode active material have been proposed as negative electrode materials. Yes. It has been found that by using such composite particles, high charge / discharge capacity and excellent cycle characteristics can be realized (see Patent Document 1).

In the composite particles of Patent Document 1, the active material particles are chemically bonded to the carbon nanofibers, and the carbon nanofibers are intertwined with each other. For this reason, even if the negative electrode active material repeats expansion and contraction, the electrical connection between the active material particles is maintained through the carbon nanofibers. Therefore, the disconnection of the conductive network between the active material particles is less likely to occur than before. In addition, since the carbon nanofibers secure a space between the active material particles, the stress accompanying the volume change of the active material particles is relieved.
JP 2004-349056 A

  In the case of the composite particles of Patent Document 1, when the active material particles repeatedly occlude and release lithium, the carbon nanofibers may fall off from the particle surface as the active material particles expand and contract. Also, when the bond between the fiber and the active material particles is weak, the composite particles are mixed with a binder to prepare an electrode mixture paste, or the electrode mixture paste is applied to a current collector to form an electrode. In addition, the fiber tends to fall off from the surface of the active material particles. If the dropped fiber is included in the electrode, it is difficult to sufficiently improve the cycle characteristics. Furthermore, when the fiber falls off from the surface of the active material particles, the ratio between the active material particles and the fiber varies. Therefore, it becomes difficult to strictly control the electrode capacity.

  Fiber makes little contribution to battery capacity. Therefore, in order to improve the battery capacity, it is essential to improve the density of the active material particles contained in the negative electrode. That is, from the viewpoint of improving the capacity, it is desirable that the volume ratio of the fiber in the total of the active material particles and the fiber is small. On the other hand, when the ratio of the fiber decreases, the space secured by the fiber becomes small, and the stress accompanying the volume change of the active material particles cannot be sufficiently relaxed.

  In order to sufficiently improve the cycle characteristics, it is important to prevent the fiber from falling off the active material particles, that is, to strongly bond the fiber and the active material particles. In order to achieve both improvement in battery capacity and improvement in cycle characteristics, it is necessary to grow fibers on the surface of the active material particles sparsely within a range where a space between the active material particles can be secured and a conductive network can be secured. is there. In this case, it is particularly important to prevent the fiber from falling off the active material particles.

  In view of the above, the present invention firstly includes an active material particle capable of inserting and extracting lithium, and a fiber grown from the surface of the active material particle. The active material particle includes at least silicon. The present invention relates to a composite active material for a non-aqueous electrolyte secondary battery, which contains at least carbon and has an adhesive phase between the active material particles and the fiber to enhance the bonding force between the active material particles and the fiber.

  Here, the adhesion phase contains a transition metal element, and the transition metal element is preferably at least one selected from the group consisting of Ti, Fe, Co, Ni, Zr, and Hf. The adhesion phase includes, for example, an alloy of silicon and a transition metal element or an oxide of a transition metal element.

  Secondly, the present invention provides (i) at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf on the surface of active material particles containing at least silicon. And (ii) reacting a transition metal element with silicon to form an alloy, and (iii) growing a carbon-containing fiber from the surface of the active material particles supporting the alloy. The present invention relates to a method for producing a composite active material for a nonaqueous electrolyte secondary battery (Method A).

  In Method A, it is preferable that the step (ii) of forming an alloy includes a step of reducing the transition metal element to a metal state and then reacting with silicon in a reducing atmosphere or an inert atmosphere.

  Thirdly, the present invention relates to (i) active material particles containing at least silicon and vapor of a compound containing at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf. (Ii) bringing a vapor of a compound containing silicon into contact with the surface of the active material particle to which the transition metal element has been applied, and bringing the transition metal element into contact with the surface of the active material particle. And (iii) a method for producing a composite active material for a non-aqueous electrolyte secondary battery (Method B), including a step of growing a carbon-containing fiber from the surface of active material particles supporting the alloy. About.

  In the method B, the step (i) of applying the transition metal element includes a step of bringing the vapor of the compound containing the transition metal element into contact with the surface of the active material particle at a temperature at which the compound containing the transition metal element is decomposed. Preferably, the step (ii) of forming an alloy includes a step of bringing the vapor of the compound containing silicon into contact with the surface of the active material particles at a temperature at which the compound containing silicon decomposes.

  Fourthly, the present invention relates to (i) at least one first transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf on the surface of active material particles containing at least silicon. And (ii) the surface of the active material particles carrying the oxide is made of Ti, Fe, Co, Ni, Zr and Hf. A step of applying at least one second transition metal element selected from the group; and (iii) reducing the second transition metal element, and then growing a carbon-containing fiber from the surface of the active material particles. The manufacturing method (method C) of the composite active material for nonaqueous electrolyte secondary batteries including a process.

  The present invention further includes a non-aqueous electrolyte secondary battery comprising a positive electrode capable of inserting and extracting lithium, a negative electrode including the composite active material, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. About.

  By forming an adhesive phase as described above between the active material particles containing at least silicon and the fiber containing carbon, the bond between the active material particles and the fiber is significantly increased. This is presumed to be because the adhesion phase has a high bonding force with respect to both the active material particles containing silicon and the fiber containing carbon.

  According to the present invention, the bond between the active material particles containing at least silicon and the fiber containing carbon is remarkably enhanced. Therefore, in the electrode manufacturing process, dropping of the fibers from the active material particles can be suppressed, and fibers that are not connected to the surface of the active material particles can be prevented from being mixed into the composite active material. In addition, even if the active material particles repeatedly expand and contract during the charging / discharging process of the battery, the fibers are not easily detached from the active material particles.

As described above, according to the present invention, the current collecting property of the electrode is improved, and the initial discharge capacity and cycle characteristics of the nonaqueous electrolyte secondary battery are improved. Moreover, since the fluctuation | variation of the ratio of an active material particle and a fiber can be prevented, an electrode capacity | capacitance can be controlled strictly.
The present invention is particularly useful when both a further improvement in battery capacity and an improvement in cycle characteristics are achieved by growing fibers sparsely on the surface of active material particles.

  The composite active material for nonaqueous electrolyte secondary batteries of the present invention (hereinafter referred to as composite active material) includes active material particles containing at least silicon and fibers containing carbon grown from the surface of the active material particles. Between the active material particles and the fiber, there is an adhesive phase that increases the binding force between the active material particles and the fiber.

  FIG. 1 shows one embodiment of the composite active material. In the composite active material 10 of FIG. 1, an adhesive phase 12 is formed on a part of the surface of the active material particles 11. The fiber 13 containing carbon grows from a region where the adhesion phase 12 on the surface of the active material particle 11 is formed. At the tip (free end) of the fiber 13, particles (catalyst particles) 14 made of a catalyst element are supported. In FIG. 1, the catalyst particles 14 are supported on the free ends of the fibers 13, but the catalyst particles 14 may exist on the surface of the active material particles 11.

  FIG. 2 shows another embodiment of the composite active material. In the composite active material 20 of FIG. 2, the adhesion phase 22 is formed in a layer shape on almost the entire surface of the active material particles 21. In FIG. 2, the catalyst particles 24 are supported on the free ends of the fibers 23, but the catalyst particles 24 may exist on the surface of the active material particles 21.

  The fiber containing carbon plays a role of securing a space between the active material particles. The space formed by the fiber relaxes the stress accompanying the volume change of the active material particles. A conductive network between the active material particles is also ensured by contact and entanglement between the fibers.

  The bond between the active material particle and the fiber is a chemical bond (covalent bond, ionic bond, etc.). That is, the fiber is directly bonded to the adhesive phase on the surface of the active material particles. Therefore, even if the active material repeats large expansion and contraction during charge and discharge, the bond between the fiber and the active material is easily maintained.

  The adhesion phase preferably contains at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf. For example, an adhesive phase containing an alloy of silicon and a transition metal element or an adhesive phase containing a compound of a transition metal element is suitable. Such an adhesive phase is strongly bonded to active material particles containing silicon and fibers containing carbon. Therefore, the adhesion phase plays a role of remarkably enhancing the bond between the active material particles and the fiber.

  Examples of the alloy of silicon and a transition metal element include nickel silicide, iron silicide, and cobalt silicide. Examples of the transition metal element compound include titanium oxide, zirconia oxide, and hafnium oxide.

The active material containing silicon is not particularly limited, and examples include silicon alone, silicon oxide, and silicon alloy. For example, SiO _x (0 <x <2, preferably 0.1 <x <1.9) can be used as the silicon oxide. As the silicon alloy, for example, an alloy containing Si and a transition metal element (M-Si alloy) can be used. For example, it is preferable to use a Ni—Si alloy, a Ti—Si alloy, or the like.

The particle size of the active material particles is not particularly limited, but is preferably 0.1 μm to 100 μm, and particularly preferably 1 to 10 μm. When the average particle size is smaller than 0.1 μm, the specific surface area of the active material particles increases, and the irreversible capacity during the first charge / discharge may increase. When the average particle size is larger than 100 μm, the active material particles are easily pulverized by charge / discharge. The average particle diameter of the active material particles can be measured by a laser diffraction particle size distribution measuring apparatus (for example, “Microtrac FRA” manufactured by Nikkiso Co., Ltd.). In this case, the median diameter (D ₅₀ ) in the volume-based particle size distribution is the average particle diameter.

  The fiber containing carbon preferably contains at least one selected from the group consisting of carbon nanofibers and carbon nanotubes. Here, the carbon nanofiber refers to all fibrous carbon materials, and the carbon nanotube refers to a hollow fibrous carbon material. Carbon nanotubes correspond to one form of carbon nanofibers.

The fiber length of the fiber containing carbon is preferably 10 nm to 1000 μm, and more preferably 500 nm to 500 μm. When the fiber length of the fiber is less than 10 nm, the effect of maintaining the conductive network between the active material particles becomes small. When the fiber length exceeds 1000 μm, the active material density of the electrode is lowered, and a high energy density may not be obtained.
The fiber diameter of the fiber is preferably 1 nm to 1000 nm, and more preferably 50 nm to 300 nm. However, a part of the fiber is preferably a fine fiber having a fiber diameter of 1 nm to 40 nm from the viewpoint of improving the electron conductivity of the electrode. For example, it is preferable to simultaneously include a fine fiber having a fiber diameter of 40 nm or less and a large fiber having a fiber diameter of 50 nm or more. It is more preferable to include a fine fiber having a fiber diameter of 20 nm or less and a large fiber having a fiber diameter of 80 nm or more at the same time.

  The amount of the fiber grown on the surface of the active material particles is preferably 5 to 70% by weight, more preferably 10 to 40% by weight, based on the entire composite active material. When the amount of the fiber is less than 5% by weight, the effect of maintaining the conductive network between the active material particles becomes small. When the amount of the fiber exceeds 70% by weight, the active material density of the electrode is lowered and a high energy density may not be obtained.

  The shape of the fiber is not particularly limited, and examples thereof include a tube shape, an accordion shape, a plate shape, and a herring bone shape.

The composite active material can be produced, for example, by the following methods A to C.
[Method A]
Method A includes (i) applying at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr, and Hf to the surface of active material particles containing at least silicon; ii) reacting a transition metal element with silicon to form an alloy; and (iii) growing a carbon-containing fiber from the surface of the active material particles supporting the alloy.

  At least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr, and Hf has a catalytic action that promotes the growth of carbon-containing fibers. An alloy of a transition metal element and silicon constitutes an adhesion phase.

  Step (i) includes, for example, a step of mixing a transition metal compound solution and active material particles. In the solution, the concentration of the transition metal compound is preferably 0.01 to 5% by weight. As the transition metal compound, it is preferable to use, for example, an oxide, a carbide, a nitrate, or the like. For example, nickel nitrate, cobalt nitrate, iron nitrate, or the like can be used. Of these, nickel nitrate and cobalt nitrate are particularly preferable. As the solvent of the solution, for example, water, an organic solvent, a mixture of water and an organic solvent, or the like is used. As the organic solvent, for example, ethanol, isopropyl alcohol, toluene, benzene, hexane, tetrahydrofuran and the like can be used.

Next, the obtained solution and active material particles are mixed. At that time, the temperature of the solution may be room temperature (for example, 10 to 30 ° C.). It is desirable to sufficiently stir the mixture of the active material particles and the solution.
Thereafter, the solvent is removed from the active material particles, and the active material particles carrying the transition metal compound are dried. The drying temperature is preferably 80 to 120 ° C., and the drying time is preferably 15 to 30 minutes. Drying can be performed in air | atmosphere, for example. Thereby, particles of the transition metal compound (hereinafter referred to as catalyst particles) are imparted to the surface of the active material particles. When nickel nitrate is used as the transition metal compound, nickel nitrate particles are formed as catalyst particles.

  The amount of the transition metal element imparted to the active material particles is preferably 0.01 to 10 parts by weight and more preferably 0.5 to 3 parts by weight per 100 parts by weight of the active material particles. If the amount of the transition metal element is less than 0.01 parts by weight, it takes a long time to grow the fiber containing carbon, and the production efficiency of the composite active material is lowered. When the amount of the transition metal element exceeds 10 parts by weight, non-uniform and thick fiber diameter fibers grow due to aggregation of the catalyst particles. For this reason, the conductivity and active material density of the electrode are reduced.

  The particle size of the catalyst particles is preferably 1 nm to 1000 nm, more preferably 10 nm to 100 nm. Generation of catalyst particles having a particle size of less than 1 nm is very difficult. When the particle size of the catalyst particles exceeds 1000 nm, the size of the catalyst particles becomes extremely non-uniform and it becomes difficult to grow the fiber.

Step (ii) includes, for example, a step of reducing active material particles carrying a transition metal element (catalyst particles) and then heating to react the catalyst element with silicon.
First, active material particles carrying a transition metal element are introduced into an inert atmosphere containing argon (Ar) or helium (He) and heated at 300 to 400 ° C. for 5 minutes to 1 hour. During this time, the catalyst particles are converted to oxides. For example, nickel nitrate particles change to nickel oxide particles.

  Thereafter, the active material particles carrying the transition metal element are heated at 300 to 600 ° C. for 1 minute to 1 hour, for example, in a reducing atmosphere containing hydrogen gas. During this time, the transition metal element is reduced to the metallic state. For example, nickel oxide particles are reduced to nickel particles in a metallic state.

The diameter and distribution density of the catalyst particles are affected by the concentration of the salt solution containing the catalyst element, the reduction temperature, the surface state of the active material particles, and the like. It is preferable to control the distribution density of the catalyst particles to about 10 ⁹ to ¹⁰ 10 particles / μm ² by controlling these parameters.

After the transition metal element is reduced to the metal state, the reaction between silicon and the transition metal element is allowed to proceed. The reaction temperature at this time depends on the type of active material particles and transition metal element. The reaction between silicon and the transition metal element proceeds by heating the active material particles carrying the catalyst particles in the metal state at 400 to 800 ° C. in a reducing atmosphere. Thereby, an alloy of a transition metal element and silicon is generated. This alloy functions as an adhesive phase. For example, at least part of the nickel particles in the metal state reacts with silicon to form nickel silicide (eg, Ni ₂ Si). Ni ₂ Si has a catalytic action that promotes the growth of carbon-containing fibers. From the viewpoint of obtaining excellent catalytic activity and an adhesive phase, Ni ₂ Si is preferably partially generated. The amount of Ni ₂ Si produced can be controlled by the reaction temperature and reaction time.

  When the reaction between the transition metal element and silicon is not performed, an adhesion phase cannot be formed between the active material particles and the fiber. In this case, the fibers tend to grow densely on the surface of the active material particles, and it becomes difficult to secure a necessary space between the active material particles while maintaining a high capacity.

In the case where the active material particles are made of silicon oxide containing a large amount of oxygen, silicide is hardly generated. In such a case, the silicon oxide is heat-treated in an inert atmosphere or a reducing atmosphere, for example, at a temperature of 600 ° C. or higher. The heat treatment is performed in advance before the catalyst particles are supported on the active material particles. By this heat treatment, a disproportionation reaction occurs, and a domain made of silicon alone in silicon oxide and a domain made of silicon oxide having an oxygen amount close to SiO ₂ are separated. Silicide is easily generated in a domain composed of silicon alone. Fe or Co reacts with silicon relatively easily as compared with Ni to produce silicide, so that it is not necessary to carry out the above disproportionation reaction.

  Step (iii) is a step of growing a fiber containing carbon. The fiber containing carbon proceeds by introducing active material particles carrying a transition metal element in a metallic state and a reactive gas serving as a carbon source into a reaction apparatus set at a predetermined temperature. The fiber grows starting from a transition metal element in a metallic state.

  It is preferable to use a mixed gas of a carbon atom-containing gas and hydrogen gas as the reactive gas. The mixing ratio and total amount of the carbon atom-containing gas and hydrogen gas are not particularly limited. The carbon atom-containing gas is not particularly limited, but methane, ethane, ethylene, butane, carbon monoxide and the like are used. These may be used alone or in combination of two or more. Conditions for growing a fiber containing carbon are not particularly limited. It is desirable that the catalyst particles be in a metallic state during fiber growth.

  If the introduction of the reactive gas is stopped after the fiber is grown for a predetermined time, the growth of the fiber is also stopped. Thereafter, the temperature in the reactor is returned to room temperature, and the composite active material is recovered.

[Method B]
In the method B, (i) a vapor of a compound containing at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf is brought into contact with the surface of active material particles containing at least silicon. And (ii) bringing a vapor of a compound containing silicon into contact with the surface of the active material particle to which the transition metal element has been applied, and forming an alloy of the transition metal element and silicon. And (iii) growing a fiber containing carbon from the surface of the active material particle supporting the alloy.

Steps (i) and (ii) are steps for forming an adhesive phase by CVD (Chemical Vapor Deposition). The CVD method is not particularly limited, but examples are given below.
Step (i) includes, for example, a step of bringing the transition metal compound vapor into contact with the surface of the active material particles at a temperature at which the transition metal compound decomposes. At this time, it is preferable to apply 0.01 to 5 parts by weight of a transition metal element to the active material particles per 100 parts by weight of the active material particles.

As the transition metal compound, for example, an organometallic compound such as Ni (TMOD) ₂ (TMOD: tetramethyloctanedione) can be used.
The transition metal compound is preferably dissolved in a solvent to form a solution and then gasified. As the solvent of the solution, toluene, hexane or the like can be used. In the solution, the concentration of the transition metal compound is preferably 0.05 mol / L to 0.1 mol / L.

  The method for gasifying the solution is not particularly limited, and is performed by heating the solution to 150 to 250 ° C. or applying ultrasonic waves to the solution with an ultrasonic vibrator. When the vapor of the solution is brought into contact with the surface of the active material particles, the inside of the chamber is preferably an atmosphere such as argon or nitrogen. In the chamber, the active material particles are preferably heated to 300 to 600 ° C. Thereafter, if necessary, hydrogen gas is introduced into the CVD chamber, and the transition metal element is reduced to a metallic state in a reducing atmosphere.

  Step (ii) includes, for example, a step of bringing the vapor of a compound containing silicon into contact with the surface of the active material particles at a temperature at which the compound containing silicon is decomposed. At this time, it is preferable to apply 0.01 to 5 parts by weight of silicon to the active material particles per 100 parts by weight of the active material particles.

As the compound containing silicon, various compounds can be used. For example, silane gas (SiH ₄ ), disilane gas (Si ₂ H ₆ ), or the like can be used. Silane gas reacts with a transition metal element in a metallic state to generate silicide. When the silane gas is brought into contact with the surface of the active material particles, the inside of the chamber is preferably an atmosphere such as argon or nitrogen. In the chamber, the active material particles are preferably heated to 400 to 600 ° C.

Step (iii) of Method B is the same as Step (iii) of Method A.
[Method C]
Method C includes (i) a compound containing at least one first transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr, and Hf on the surface of active material particles containing at least silicon. And (ii) the surface of the active material particles supporting the oxide is selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf. A step of providing at least one second transition metal element; and (iii) a step of growing a fiber containing carbon from the surface of the active material particle after reducing the second transition metal element. . In this case, the oxide derived from the first transition metal element constitutes the adhesion phase.

  Step (i) includes, for example, a step of mixing a solution of a compound containing a first transition metal element (hereinafter referred to as a first transition metal compound) and active material particles. In the solution, the concentration of the first transition metal is preferably 0.01 to 5% by weight with respect to the weight of the active material particles. Although various compounds can be used for the first transition metal compound, for example, an organic metal compound such as water-soluble titanium lactate is preferably used. As the solvent of the solution, for example, water, an organic solvent, a mixture of water and an organic solvent, or the like is used.

Next, the obtained solution and active material particles are mixed. At that time, the temperature of the solution may be room temperature (for example, 10 to 30 ° C.). It is desirable to sufficiently stir the mixture of the active material particles and the solution.
Thereafter, the solvent is removed from the active material particles, and the active material particles carrying the first transition metal compound are dried. The drying temperature is preferably 80 to 120 ° C., and the drying time is preferably 10 to 60 minutes. Drying can be performed in air | atmosphere, for example. Thereby, a 1st transition metal compound is provided to the surface of active material particle.

  Next, the active material particles carrying the first transition metal compound are heated at 300 to 600 ° C. for 1 minute to 1 hour in an oxidizing atmosphere containing air or oxygen. Thereby, a 1st transition metal compound changes to an oxide. The oxide of the first transition metal element is preferably 0.01 to 5 parts by weight per 100 parts by weight of the active material particles.

Step (ii) and step (iii) of method C are the same as step (i) and step (iii) of method A.
Method A is suitable for producing a composite active material 10 having an adhesive phase 12 only in the vicinity of the joint between the active material particles 11 and the fiber 13 as shown in FIG. Methods B and C are suitable for producing a composite active material 20 having a layered adhesive phase 22 as shown in FIG.

  Next, an example of a process for growing carbon nanofibers as carbon-containing fibers from the surface of the active material particles will be described in more detail. However, the method for growing the fiber is not limited to the following. When other fibers such as carbon nanotubes are grown, the catalyst, reaction raw material, etc. may be changed as appropriate.

  Since the composite active material manufactured by the above method has an adhesive phase between the active material particles and the fiber, the active material particles and the fiber are strongly bonded. For example, even when the composite active material is dispersed in water and ultrasonic energy is applied to the obtained dispersion, the fiber is hardly dropped. In the case of a composite active material in which fibers are grown on active material particles that do not have an adhesive phase, it is observed that the fibers that have fallen off the surface of the dispersion float.

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

  In this example, a composite active material as shown in FIG. This will be described with reference to the flowchart of FIG.

Process (i)
First, 1 g of nickel nitrate hexahydrate (special grade) manufactured by Kanto Chemical Co., Ltd. was dissolved in 100 g of ion-exchanged water. The obtained solution was mixed with silicon oxide (SiO) manufactured by Kojundo Chemical Laboratory Co., Ltd. having an average particle diameter of 8 μm and stirred at room temperature (RT) for 1 hour (S31). Then, the water | moisture content was removed from the mixture with the evaporator apparatus, and it was made to dry for 30 minutes at 120 degreeC (S32). As a result, a silicon oxide carrying nickel nitrate particles was obtained. The amount of nickel element in the nickel nitrate particles supported on the silicon oxide was 0.5 parts by weight per 100 parts by weight of the silicon oxide. When the average particle diameter of the nickel nitrate particles was observed by SEM, it was about 50 nm.

Step (ii)
Next, silicon oxide carrying nickel nitrate was put into a ceramic reaction vessel, and the temperature in the reaction vessel was raised to 400 ° C. in the presence of helium gas. The temperature in the reaction vessel was kept at 400 ° C. for 1 hour, and nickel nitrate was denitrated (S33). As a result, nickel nitrate was changed to nickel oxide.

  Next, the helium gas in the reaction vessel was switched to a mixed gas of 20% by volume of hydrogen gas and 80% by volume of helium gas, and the temperature in the reaction vessel was maintained at 400 ° C. for 30 minutes (S34). During this time, nickel oxide was reduced to metallic nickel.

Thereafter, the temperature in the reaction vessel was raised to 700 ° C., and then the temperature in the reaction vessel was maintained at 700 ° C. for 1 hour (S35). During this time, silicon in the silicon oxide and nickel reacted to produce an alloy (Ni ₂ Si) exhibiting catalytic activity. After cooling the reaction vessel to room temperature, silicon oxide particles were taken out and subjected to X-ray diffraction analysis. As a result, it was confirmed that Ni ₂ Si was formed.

Step (iii)
Silicon oxide particles carrying Ni ₂ Si were put into the reaction vessel, and the temperature in the reaction vessel was raised to 400 ° C. in the presence of helium gas. Next, the helium gas was replaced with a mixed gas of 20% by volume of hydrogen gas and 80% by volume of ethylene gas (total flow rate was 4 L / min). Thereafter, the temperature in the reaction vessel was held at 400 ° C. for 1 hour to grow carbon nanofibers (S36), and composite active material A was obtained. Thereafter, the mixed gas was replaced with helium gas and cooled to room temperature.

  In the composite active material A, the carbon nanofibers had a fiber diameter of 80 nm and a fiber length of 20 μm. The amount of carbon nanofibers was 25 parts by weight per 100 parts by weight of silicon oxide particles (20% by weight of the total composite active material was carbon nanofibers). The amount of carbon nanofibers was measured from the change in weight of the silicon oxide particles before and after the carbon nanofibers were grown.

Step (iv)
100 parts by weight of the composite active material A, 7 parts by weight of polyvinylidene fluoride resin (PVDF), and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed to prepare an electrode mixture slurry. The obtained slurry was applied to one side of a copper foil having a thickness of 15 μm, and after drying, the electrode mixture was rolled to obtain an electrode A.

In this example, a composite active material as shown in FIG.
Process (i)
Silicon oxide (SiO) manufactured by Kojundo Chemical Laboratory Co., Ltd. having an average particle diameter of 8 μm was set in a CVD chamber and heated to 500 ° C. On the other hand, a solution in which Ni (TMOD) ₂ was dissolved in toluene was prepared. The concentration of Ni (TMOD) _{2 in} the solution was 0.1 mol / L.

  The resulting solution was gasified at 200 ° C. and introduced into the CVD chamber. The inside of the chamber was an argon atmosphere. As a result, a NiO film was formed on the surface of the silicon oxide particles. Next, hydrogen gas was introduced into the CVD chamber, and the silicon oxide particles having the NiO film were heated to 500 ° C. to reduce the NiO film to a metallic Ni film. The amount of Ni film formed was 1 part by weight per 100 parts by weight of silicon oxide.

Step (ii)
Thereafter, silane gas was introduced into the CVD chamber, and the silane gas was brought into contact with silicon oxide particles having a Ni film heated to 500 ° C. As a result, silane reacted with the Ni film to produce Ni ₂ Si. The formation of Ni ₂ Si was confirmed by X-ray diffraction analysis. Due to the reaction with the silane gas, the weight of the silicon oxide having the Ni film increased by 0.1% by weight.

Step (iii)
Carbon nanofibers were grown on the surface of silicon oxide particles supporting Ni ₂ Si by the same operation as in step (iii) of Example 1, and composite active material B was obtained. The fiber diameter, fiber length, and generation amount of the carbon nanofiber were the same as in Example 1.

Step (iv)
An electrode B was produced in the same manner as in Example 1 except that the composite active material B was used.

In this example, a composite active material as shown in FIG.
Process (i)
Titanium lactate (TiC ₆ H ₁₀ O ₇ ) manufactured by Matsumoto Pharmaceutical Co., Ltd. was dissolved in water to prepare an aqueous solution having a titanium concentration of 1% by weight with respect to the weight of the active material particles. The obtained solution was mixed with silicon oxide (SiO) manufactured by Kojundo Chemical Laboratory Co., Ltd. having an average particle size of 8 μm. After stirring this mixture for 1 hour, water was removed with an evaporator device, and titanium lactate was supported on the surface of the silicon oxide.

  Silicon oxide carrying titanium lactate was put into a ceramic reaction vessel, and the temperature in the reaction vessel was raised to 400 ° C. in the atmosphere. By maintaining the temperature in the reaction vessel at 400 ° C. for 1 hour, the titanium lactate was changed to titanium oxide (TiO). The amount of titanium supported on silicon oxide was 1 part by weight per 100 parts by weight of silicon oxide.

Step (ii)
1 g of nickel nitrate hexahydrate (special grade) manufactured by Kanto Chemical Co., Ltd. was dissolved in 100 g of ion-exchanged water. The obtained solution was mixed with silicon oxide particles supporting titanium oxide, and the mixture was stirred for 1 hour. Then, the water | moisture content was removed from the mixture with the evaporator apparatus, and it was made to dry. As a result, nickel nitrate particles were further supported on the surface of the silicon oxide supporting titanium oxide. The amount of nickel element in the nickel nitrate particles applied to the silicon oxide was 1 part by weight per 100 parts by weight of the silicon oxide. Further, when the average particle diameter of the nickel nitrate particles was observed by SEM, it was about 50 nm.

Step (iii)
Carbon nanofibers were grown on the surface of silicon oxide particles supporting titanium oxide and nickel nitrate by the same operation as in step (iii) of Example 1 to obtain composite active material C. The fiber diameter, fiber length, and generation amount of the carbon nanofiber were the same as in Example 1.

Step (iv)
An electrode C was produced in the same manner as in Example 1 except that the composite active material C was used.

<< Comparative Example 1 >>
In this comparative example, a composite active material having no adhesive phase was produced. This will be described with reference to the flowchart of FIG.
Similarly to Example 1, nickel nitrate was supported on the surface of the silicon oxide (S41 to 42). Next, silicon oxide carrying nickel nitrate was put into a ceramic reaction vessel, and the temperature in the reaction vessel was raised to 400 ° C. in the presence of helium gas. The temperature in the reaction vessel was kept at 400 ° C. for 1 hour, and nickel nitrate was denitrated (S43). As a result, nickel nitrate was changed to nickel oxide.

  Next, the helium gas in the reaction vessel was switched to a mixed gas of hydrogen gas and helium gas, and the temperature in the reaction vessel was maintained at 400 ° C. for 30 minutes (S44). During this time, nickel oxide was reduced to metallic nickel.

  Next, carbon nanofibers were grown on the surface of silicon oxide particles carrying metallic nickel by the same operation as in step (iii) of Example 1 (S45), and composite active material D was obtained.

  In this comparative example, it is considered that a composite active material 50 as shown in FIG. 5 was obtained. The composite active material 50 includes an active material particle 51 (silicon oxide), a fiber 53 (carbon nanofiber) directly bonded to the surface of the active material particle 51, and a catalyst particle 54 (metallic nickel) supported on the tip of the fiber 53. Particles).

An electrode D was produced in the same manner as in Example 1 except that the composite active material D was used.
The mixture density of the electrodes prepared in Examples 1 to 3 and Comparative Example 1 was 0.8 to 1.4 g / cm ³ .

[Evaluation]
A coin-type lithium ion secondary battery 60 as shown in FIG. 6 was produced in the following manner. The electrodes prepared in Examples 1 to 3 and Comparative Example 1 were sufficiently dried in an oven at 120 ° C., and then used as the working electrode 64. A lithium metal foil 65 was used for the counter electrode. Battery capacity was regulated by working electrode 64.

First, the working electrode 64 was placed on the battery can 61, and a separator 66 made of a non-woven fabric made of polyethylene was placed thereon. A predetermined amount of non-aqueous electrolyte was injected from above the separator 66. As the non-aqueous electrolyte, a solution in which LiPF ₆ was dissolved at a concentration of 1.0 mol / L in a 1: 1 mixed solvent of ethylene carbonate and diethyl carbonate was used. Next, a lithium metal foil 65 was pressure-bonded to the inner surface of the sealing plate 62, and the opening of the battery can 61 was closed with the sealing plate 62. A gasket 63 was interposed between the battery can 61 and the sealing plate 62.

(Initial discharge capacity)
Each battery was charged to 0 V at a charge rate of 0.2 C, and then discharged to 1.5 V at a discharge rate of 0.2 C to obtain an initial discharge capacity (0.2 C discharge capacity). . The discharge capacity per active material weight is shown in Table 1. The discharge capacity per active material weight was determined by subtracting the weight of the copper foil, PVDF and carbon nanofiber from the weight of the working electrode and dividing the discharge capacity by the weight of the silicon oxide obtained.

(Discharge efficiency)
Each battery was charged to 0V at a charge rate of 0.2C, and then discharged to 1.5V at a discharge rate of 3.0C to obtain a 3.0C discharge capacity. Table 1 shows the ratio (discharge efficiency) of the 3.0 C discharge capacity to the 0.2 C discharge capacity as a percentage value.

(Cycle characteristics)
Each battery was charged to 0 V at a charging rate of 0.2 C, and then discharged 200 cycles at a discharging rate of 0.2 C until 1.5 V was repeated. Table 1 shows the ratio (capacity maintenance ratio) of the discharge capacity at the 200th cycle to the initial discharge capacity as a percentage value.

(Peeling degree)
5 g of each composite active material was dispersed in 100 ml of water. An ultrasonic wave was applied to the obtained dispersion at an output of 300 W for 30 minutes, and the amount of carbon nanofibers peeled off from the silicon oxide particles was measured. After the application of the ultrasonic wave, the sample was left standing for 24 hours, and the carbon nanofibers contained in the supernatant of the solution were collected and the weight was measured. However, the carbon fiber that was not originally bonded to the active material particles was collected before the ultrasonic treatment. Table 1 shows the ratio (peeling degree) of the weight of the peeled carbon nanofiber to the weight of the carbon nanofiber contained in 5 g of the composite active material as a percentage value. The degree of peeling is considered to be related to the bonding force between the active material particles and the carbon nanofibers.

  As shown in Table 1, the batteries using the electrodes of Examples 1 to 3 were superior in the initial discharge capacity, discharge efficiency, and capacity retention rate to the results of Comparative Example 1. In Comparative Example 1, since the degree of peeling is 50%, it is assumed that many of the carbon nanofibers are easily peeled even in the slurry prepared using the composite active material D. In the slurry of Comparative Example 1, there were many large particles that seemed to be agglomerated only by the carbon nanofibers. Therefore, unless the slurry was filtered, it could not be applied to the copper foil. In Comparative Example 1, it is considered that the discharge capacity, the discharge efficiency, and the capacity retention rate were lowered due to the deterioration of the current collecting property due to the peeling of the carbon nanofibers.

  INDUSTRIAL APPLICABILITY The present invention is useful in realizing a non-aqueous electrolyte secondary battery that requires both a higher initial discharge capacity and excellent cycle characteristics than before. The nonaqueous electrolyte secondary battery to which the present invention can be applied is not particularly limited, and may be a battery of any shape such as a cylindrical shape, a flat shape, a square shape, a coin shape, a button shape, and a sheet shape. The form of the electrode plate group composed of the positive electrode, the negative electrode, and the separator may be a wound type or a laminated type. The size of the battery may be small for a small portable device or the like, or large for an electric vehicle or the like. The nonaqueous electrolyte secondary battery of the present invention can be used as a power source for, for example, a portable information terminal, a portable electronic device, a small electric power storage device for home use, a motorcycle, an electric vehicle, and a hybrid electric vehicle.

It is a conceptual diagram which shows one form of the composite active material of this invention. It is a conceptual diagram which shows another one form of the composite active material of this invention. It is a flowchart which shows an example of the manufacturing process of the composite active material of this invention. It is a flowchart which shows an example of the manufacturing process of the conventional composite active material. It is a conceptual diagram which shows one form of the conventional composite active material. It is a longitudinal cross-sectional view of the coin-type lithium ion secondary battery which concerns on an Example.

Explanation of symbols

10, 20, 50 Composite active material 11, 21, 51 Active material particle 12, 22 Adhesive phase 13, 23, 53 Fiber 14, 24, 54 Catalyst particle 60 Coin type lithium ion secondary battery 61 Battery can 62 Sealing plate 63 Gasket 64 working electrode 65 lithium metal foil 66 separator

Claims (13)

Active material particles capable of occluding and releasing lithium, and fibers grown from the surface of the active material particles,
The active material particles include at least silicon,
The fiber includes at least carbon;
A composite active material for a non-aqueous electrolyte secondary battery, which has an adhesive phase between the active material particles and the fiber to enhance the binding force between the active material particles and the fiber.   The non-aqueous electrolyte 2 according to claim 1, wherein the adhesion phase includes a transition metal element, and the transition metal element is at least one selected from the group consisting of Ti, Fe, Co, Ni, Zr, and Hf. Composite active material for secondary batteries.   The composite active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the adhesion phase includes an alloy of silicon and the transition metal element.   The composite active material for a nonaqueous electrolyte secondary battery according to claim 2, wherein the adhesion phase includes an oxide of the transition metal element.   The composite active material for a non-aqueous electrolyte secondary battery according to claim 3, wherein the alloy is nickel silicide.   The composite active material for a nonaqueous electrolyte secondary battery according to claim 4, wherein the oxide is a titanium oxide.   The composite active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the fiber includes at least one selected from the group consisting of carbon nanofibers and carbon nanotubes. (I) providing at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf on the surface of active material particles containing at least silicon;
(Ii) reacting the transition metal element with silicon to form an alloy;
(Iii) growing a fiber containing carbon from the surface of the active material particles carrying the alloy, and a method for producing a composite active material for a non-aqueous electrolyte secondary battery.   The non-aqueous electrolyte secondary according to claim 8, wherein the step (ii) of forming the alloy includes a step of reducing the transition metal element to a metal state and then reacting with silicon in a reducing atmosphere or an inert atmosphere. A method for producing a composite active material for a battery. (I) contacting a vapor of a compound containing at least one transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf with the surface of active material particles containing at least silicon, Adding the transition metal element to the material particles;
(Ii) contacting a vapor of a compound containing silicon with the surface of the active material particles provided with the transition metal element to form an alloy of the transition metal element and silicon;
(Iii) A method for producing a composite active material for a non-aqueous electrolyte secondary battery, comprising a step of growing a fiber containing carbon from the surface of the active material particles supporting the alloy.   The step (i) of applying the transition metal element includes a step of bringing the vapor of the compound containing the transition metal element into contact with the surface of the active material particles at a temperature at which the compound containing the transition metal element is decomposed. The step (ii) of generating the alloy includes the step of bringing the vapor of the compound containing silicon into contact with the surface of the active material particles at a temperature at which the compound containing silicon decomposes. A method for producing a composite active material for a water electrolyte secondary battery. (I) providing a compound containing at least one first transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf on the surface of active material particles containing at least silicon; Changing the first transition metal element to an oxide;
(Ii) providing at least one second transition metal element selected from the group consisting of Ti, Fe, Co, Ni, Zr and Hf on the surface of the active material particles supporting the oxide;
(Iii) A method for producing a composite active material for a non-aqueous electrolyte secondary battery, comprising reducing the second transition metal element and then growing a carbon-containing fiber from the surface of the active material particles.   A nonaqueous electrolyte secondary battery comprising: a positive electrode capable of inserting and extracting lithium; a negative electrode including the composite active material according to claim 1; a separator interposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte.

Images