JP2007311207A - Negative electrode material for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery using it and manufacturing method of negative electrode material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having high capacity and an excellent charge-discharge cycle characteristic. <P>SOLUTION: This negative electrode material for a nonaqueous electrolyte secondary battery comprises composite particles containing active material particles containing an element capable of being alloyed with lithium, and carbon nano-fibers supported to the surfaces of the active material particles. The carbon nano-fibers are burnt down only in a temperature range exceeding 550°C in temperature rise of 20°C/min in the air. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery using the same, and a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery.

  In recent years, in order to increase the energy density of batteries, it has been studied to use Si, Sn, or Pb that can be alloyed with lithium, their oxides and alloys as a negative electrode active material having a high theoretical capacity density. . However, these materials have a very large volume change due to insertion and extraction of lithium, and repeat expansion and contraction during the charge / discharge cycle, so that the active material particles are pulverized and the conductivity between the active material particles is reduced. , There is a drawback that the cycle characteristics are greatly deteriorated.

  On the other hand, for example, in Patent Document 1, composite particles including active material particles containing an element that can be alloyed with lithium and carbon nanofibers supported on the surface of the active material particles are used as the negative electrode material. Thus, it has been proposed to increase the capacity and improve the cycle characteristics of a lithium ion secondary battery.

  The composite particles carry a catalyst on the surface of the active material particles and react with the carbon-containing gas in a high-temperature inert atmosphere, so that carbon nanofibers grow from the catalyst as a starting point. It is obtained by carrying on the surface. The particle size of the particulate catalyst supported on the surface of the active material particles can be reduced by adjusting the amount of the catalyst supported. However, since it is difficult to reduce the number density per unit area, the bulk density of the portion formed by the grown carbon nanofiber tends to be high. The carbon nanofibers obtained above are not uniform, and very fine carbon nanofibers or amorphous carbon such as soot may adhere to the surface of the active material particles.

Therefore, in the above composite particles, the carbon component including the carbon nanofibers existing on the surface of the active material particles is very dense and has a very large specific surface area. When the carbon component including the carbon nanofiber is dense, the flexibility with respect to stress is reduced, so that the electron conductivity between the active material particles is likely to be reduced with the volume change of the active material particles during charge and discharge. In addition, when the specific surface area of the carbon component including the carbon nanofiber is large, the amount of side reactions occurring at the initial stage due to contact with the electrolytic solution increases, and the irreversible capacity and gas generation amount in the negative electrode may increase.
JP 2004-349056 A

  From the above, as the form of the composite particles, it is preferable that the carbon nanofibers uniformly grown on the surface of the active material particles exist without being so dense. However, in the manufacturing method of Patent Document 1, the carbon component including the carbon nanofiber covering the active material surface tends to have a high bulk density and a high specific surface area.

  If the weight ratio of the carbon nanofibers to the active material particles is very low, the above problem is unlikely to occur. However, in order to obtain good battery characteristics, particularly excellent cycle characteristics, the weight ratio of the carbon nanofibers to the active material particles is preferably 10% to 50%, more preferably 15% to 40%. There is a need. In addition, in the production method such as Patent Document 1, it is difficult to produce the composite particles having the above preferable form within the range of the above weight ratio.

  Therefore, in order to solve the above-described conventional problems, the present invention can easily optimize the coating form of the carbon component on the surface of the active material particle in the composite particle in which the carbon component is supported on the surface of the active material particle. It aims at providing the manufacturing method of negative electrode material. It is another object of the present invention to provide a non-aqueous electrolyte secondary battery having a high capacity and excellent charge / discharge cycle characteristics using the negative electrode material obtained by the above production method.

  The present invention is a negative electrode material for a non-aqueous electrolyte secondary battery comprising composite particles including active material particles containing an element that can be alloyed with lithium and carbon nanofibers supported on the surfaces of the active material particles. The carbon nanofibers are burned down only in a temperature range exceeding 550 ° C. at a temperature increase of 20 ° C./min in air.

  The composite particle manufacturing method of the present invention includes (1) a step of supporting a catalyst on the surface of an active material particle containing an element that can be alloyed with lithium, and (2) the active material particle supporting the catalyst at a high temperature. Reacting with a carbon-containing gas in an inert atmosphere, and growing and supporting carbon nanofibers on the active material particles with a catalyst as a starting point, to obtain composite particles; and (3) 550 the composite particles in an oxidizing atmosphere. It includes a step of heat-treating the carbon nanofibers on the active material particles in a temperature range of less than or equal to ° C.

  When the composite particles obtained by the step (2) of the above production method are subjected to thermal analysis, in the TG / DTA measurement in the air, two temperatures of about 550 ° C. and about 600 ° C. or higher are observed at a temperature rising rate of 20 ° C./min. It was found that the above exothermic peak exists. Since the active material particles that can be alloyed with lithium have a size on the order of μm, they are thermally stable even in an oxidizing atmosphere such as in the air for a short time, and this exothermic peak is a carbon component including carbon nanofibers. This is thought to be due to the combustion of Since there are two or more exothermic peaks, it can be seen that the composite particles contain two or more types of carbon components having different thermal stability.

  Carbon components with low thermal stability are considered to be amorphous carbon and very fine carbon nanofibers as described above. These thermal stability can be achieved by heat treating the composite particles in an oxidizing atmosphere for a short time. Only low carbon components can be selectively burned and removed. The composite particles produced in this way are considered to remain only uniform carbon nanofibers with high thermal stability. Compared to the composite particles not subjected to the heat treatment in the above oxidizing atmosphere, the carbon nanofibers are more The bulk density and specific surface area of the carbon component contained are small.

  According to the present invention, in the composite particles in which the carbon component is supported on the surface of the active material particle, the coating form of the carbon component on the surface of the active material particle is optimized, the flexibility with respect to stress is improved, and the initial electrolyte solution Side reactions can be suppressed. For this reason, in the nonaqueous electrolyte secondary battery using the composite particles as the negative electrode material, the capacity can be increased by reducing the irreversible capacity, and excellent charge / discharge cycle characteristics can be obtained.

The present invention includes (1) a step of supporting a catalyst on the surface of an active material particle containing an element that can be alloyed with lithium, and (2) a carbon-containing gas in the active material particle supporting the catalyst in a high-temperature inert atmosphere. And a step of growing and supporting carbon nanofibers on the active material particles using the catalyst as a starting point to obtain composite particles, and (3) heat-treating the composite particles in an oxidizing atmosphere at a temperature range of 550 ° C. or lower. In addition, the present invention relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery, comprising a step of sparse carbon nanofibers on active material particles.
Thereby, the coating form of the carbon component on the surface of the active material particles can be easily and reliably improved to a form suitable for the negative electrode material for a non-aqueous electrolyte secondary battery.

  The above-described production method will be described in more detail. First, the composite particles are heated to about 500 ° C. in an inert atmosphere, and then an oxidizing gas such as air is flown into the system to contain carbon components including carbon nanofibers. Among them, the thermally unstable one is selectively burned, and the system is cooled while flowing an inert gas into the system again. What is necessary is just to adjust the temperature, time, etc. of heat processing according to the state of the carbon component containing a carbon nanofiber, the target carrying amount, or the kind and particle size of active material particle.

  In the step (1), for example, the active material particles and the catalyst particles are mixed with a dispersion medium such as water, and then the dispersion medium is removed by drying or the like, thereby supporting the catalyst particles on the surface of the active material particles. Can do. Further, for example, a catalyst can be supported on the active material particles by forming a catalyst layer on the active material particles by an electroless plating method or the like. The layered catalyst agglomerates in the form of particles due to heat at the time of temperature rise in step (2) (carbon nanofiber growth and support step).

  Various known compounds are used as the material of the active material particles, and the element that can be alloyed with lithium is at least selected from the group consisting of Al, Si, Zn, Ge, In, Sn, Sb, and Pb. One is preferable, and among these, at least one element of Si and Sn is particularly preferable from the viewpoint of battery characteristics. Moreover, it is preferable that it is an oxide with high thermal stability in the air. As the active material particles, for example, particles having a particle size of 0.1 to 100 μm are used.

Although a catalyst is not specifically limited, For example, elements, such as Mn, Fe, Co, Ni, Cu, or Mo, or the compound containing these elements is used. These may be used alone or in combination of two or more.
When the catalyst is particulate, for example, the particle size is 1-1000 nm. Moreover, when the catalyst is layered, for example, the thickness of the layer is about 1 to 500 nm.

In the step (2), the active material particles carrying the catalyst obtained in the step (1) are reacted with a carbon-containing gas in a high-temperature inert atmosphere around 500 ° C., for example, and the active material particles are based on the catalyst. Carbon nanofibers are grown and supported on the above to obtain composite particles.
The carbon-containing gas is not particularly limited, and examples thereof include methane, ethane, butane, ethylene, acetylene, carbon monoxide, benzene, and toluene. Depending on the catalyst, the carbon-containing gas, and the reaction temperature, the shape of the growing carbon nanofibers differs, but the bulk density and specific surface area can be obtained by selectively burning the carbon component containing the carbon nanofibers in the step (3) described later. The effect of reducing is seen in various combinations. As the inert gas, helium gas, argon gas, or the like is used.

Here, an example of the composite particles obtained in the step (2) is shown in FIG.
The composite particles are composed of active material particles 1 and carbon components such as carbon nanofibers 3 and amorphous carbon 4 supported on the surface of the active material particles 1. The carbon nanofiber 3 includes a sufficiently grown carbon nanofiber 3b (fiber diameter 30 to 200 nm, fiber length 1 to 500 μm) and a fine carbon nanofiber 3a (fiber diameter 1 to 30 nm, fiber length 0) that is insufficiently grown. 0.5 to 100 μm).
The particulate catalyst 2 is initially supported on the surface of the active material particle 1, but a part of the catalyst 2 moves together with the carbon nanofiber as the carbon nanofiber grows. In FIG. 2, only the catalyst 2 located at the tip of the carbon nanofiber is shown for convenience, but the catalyst 2 is also present at, for example, the middle or base point of the carbon nanofiber.

In the step (3), the composite particles obtained in the step (2) are heat-treated in an oxidizing atmosphere at a temperature range of 550 ° C. or lower to denature the carbon nanofibers on the active material particles.
Of the carbon components present on the active material particles by the heat treatment in the oxidizing atmosphere of the composite particles shown in the step (3), amorphous carbon 4 adhering to the surface of the active material particles 1 in the composite particles shown in FIG. In addition, it is possible to selectively burn components having low thermal stability such as very fine carbon nanofibers 3a. As a result, as shown in FIG. 1, composite particles in which only the relatively uniform carbon nanofibers 3b having high thermal stability are supported on the surface of the active material particles 1 are obtained.

The selective combustion of the above-mentioned components having low thermal stability is caused by a plurality of exothermic peaks observed in TG / DTA measurement (differential heat / thermogravimetric simultaneous measurement) at a temperature rising rate of 20 ° C./min in air. It corresponds to an exothermic peak existing in a temperature region of 550 ° C. or lower.
Therefore, the carbon nanofibers in the composite particles that are the negative electrode material for a nonaqueous electrolyte secondary battery obtained by the above production method are not burned out in a temperature range of 550 ° C. or lower at a temperature increase of 20 ° C./min in air. That is, it burns down only in a temperature range exceeding 550 ° C. at a temperature increase of 20 ° C./min in air. In other words, the carbon nanofiber has no exothermic peak in the temperature range of 550 ° C. or lower in TG / DTA measurement at a temperature rising rate of 20 ° C./min in air, that is, 20 ° C. in air. In the TG / DTA measurement at a temperature rising rate of / min, an exothermic peak is present only in the temperature range exceeding 550 ° C.

  As described above, by the step (3), the carbon nanofibers existing on the surface of the active material particles can be easily made into a form suitable for battery characteristics. That is, the bulk density and specific surface area of the carbon component on the active material particles in the composite particles in the step (2) can be reduced to a range preferable for battery characteristics. In the composite particles obtained in the step (3), the weight ratio of the carbon nanofibers to the active material particles is, for example, 10 to 50%.

Further, in the non-aqueous electrolyte secondary battery using the composite material as the negative electrode material, side reactions caused by contact with the electrolyte in the initial stage are suppressed, and the irreversible capacity is reduced, so that the capacity can be increased. . In addition, the amount of gas generated due to side reactions is reduced. Furthermore, since excellent electronic conductivity between the active material particles is maintained during repeated charge / discharge, excellent charge / discharge cycle characteristics can be obtained.
The binder used for the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, the current collector, or the negative electrode mixture paste applied to the current collector during the preparation of the negative electrode is not particularly limited, and a known one is used. do it. Furthermore, constituent members such as a positive electrode, a separator, and an electrolytic solution used in the nonaqueous electrolyte secondary battery of the present invention are not particularly limited, and a known material may be used.

Examples of the present invention will be described in detail below, but the present invention is not limited to the following examples.
Example 1
(1) Preparation of negative electrode material 100 parts by weight of silicon monoxide (SiO) powder (manufactured by Wako Pure Chemical Industries, Ltd., reagent) pulverized and classified in advance to a particle size of 10 μm or less, nickel nitrate (II) hexahydrate Japanese (Kanto Chemical Co., Ltd.)
(Product made, special grade reagent) 5 parts by weight were mixed in ion-exchanged water. After stirring this mixture for 1 hour, water was removed by an evaporator and dried to obtain particles having nickel (II) nitrate supported on the surface of silicon monoxide particles. As a result of analyzing the particles by SEM, it was confirmed that nickel (II) nitrate was in the form of particles having a particle size of about 100 nm.

The obtained active material particles were put into a ceramic reaction vessel and heated to 500 ° C. in helium gas. The helium gas was replaced with a mixed gas of 50% hydrogen gas and 50% ethylene gas, and maintained at 500 ° C. for 1 hour to reduce nickel (II) nitrate and grow carbon nanofibers. Thereafter, the mixed gas was replaced with helium gas and cooled to room temperature to obtain composite particles having carbon nanofibers supported on the surface of SiO particles.

  The obtained composite particles were put into the reaction vessel again and heated to 500 ° C. in helium gas. The helium gas was replaced with air and held at 500 ° C. for 5 minutes to selectively burn a component having low thermal stability among the carbon components of the composite particles. Thereafter, the air was replaced with helium gas and cooled to room temperature to obtain a negative electrode material.

As a result of analyzing this negative electrode material by SEM, carbon nanofibers with a fiber diameter of about 80 nm and a fiber length of about 100 μm were formed almost uniformly, and no very fine carbon nanofibers with a fiber diameter of about 10 nm were found. . Further, amorphous carbon such as soot was not attached to the surface of the SiO particles. The weight ratio of the grown carbon nanofibers was calculated from the weight increase with respect to the active material particles before and after the synthesis, and was about 25% with respect to the entire composite particles.
TG / DTA measurement was performed on this negative electrode material in air at a temperature rising rate of 20 ° C./min. As a result, no exothermic peak was observed in a temperature range of 550 ° C. or lower. For the TG / DTA measurement, ThermoPlus 2 manufactured by Rigaku was used.

(2) Production of negative electrode 100 parts by weight of the negative electrode material, 10 parts by weight of a styrene butadiene rubber emulsion as a binder in terms of solid content, and carboxymethyl cellulose (Dellogen Kogyo Seiyaku Co., Ltd., Cellogen, 4H as a thickener) 3) parts by weight were mixed well while adding an appropriate amount of ion-exchanged water to obtain a paste. This paste was applied to both sides of a 15 μm thick Cu foil as a current collector, then dried and rolled to obtain a negative electrode.

(3) Preparation of positive electrode 100 parts by weight of LiCoO ₂ powder as a positive electrode active material, 10 parts by weight of acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent, and polyvinylidene fluoride as a binder in terms of solid content 8 parts by weight were mixed well while adding an appropriate amount of N-methyl-2-pyrrolidone to obtain a paste. This paste was applied to both sides of a 20 μm thick Al foil as a current collector, then dried and rolled to obtain a positive electrode.

(4) Preparation of battery for evaluation After the positive electrode and the negative electrode obtained above were cut to the required sizes, an Al lead was attached to the end of the positive electrode current collector, and an Ni lead was attached to the end of the negative electrode current collector. Welded. The positive electrode and the negative electrode were stacked as a separator through a porous polyethylene film having a thickness of 20 μm (manufactured by Asahi Kasei Co., Ltd., Hypore), and then the laminate was wound to obtain an electrode group. An insulating plate made of polypropylene was placed on the upper and lower sides of the electrode group, inserted into a battery outer can having a diameter of 18 mm and a height of 65 mm, and then a nonaqueous electrolyte was injected into the battery outer can. As the non-aqueous electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio of 1: 1) containing 1 mol / L LiPF ₆ (manufactured by Mitsubishi Chemical Corporation, Sollite) was used. The outer can was depressurized, the electrode group was impregnated with the electrolytic solution, and sealed with a sealing plate to produce a cylindrical battery. The design capacity of this battery is 2400 mAh.

Example 2
A negative electrode material was obtained in the same manner as in Example 1 except that silicon powder (manufactured by Wako Pure Chemical Industries, Ltd., reagent) was used instead of silicon monoxide.
The particle diameter of nickel (II) nitrate supported on the surface of the Si particles and the fiber diameter, fiber length, and weight ratio of the grown carbon nanofibers were almost the same as those in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the above negative electrode material.

Example 3
A Ti—Si alloy was produced by the following method. 50 parts by weight of titanium powder (manufactured by High Purity Chemical Co., Ltd., reagent 150 μm or less) and 100 parts by weight of silicon powder (manufactured by Wako Pure Chemical Industries, Ltd., reagent) are mixed, and 3.5 kg of the mixture is placed in a vibration mill device. I put it in. A stainless steel ball having a diameter of 2 cm was introduced so as to be 70% of the volume in the apparatus, and mechanical alloying operation was performed in argon gas for 80 hours to obtain a Ti—Si alloy.

As a result of observing the obtained Ti-Si alloy with XRD, TEM, etc., it was confirmed that an amorphous phase, a microcrystalline Si phase of about 10 nm to 20 nm, and a TiSi ₂ phase existed. It was. Assuming that consists of only Si and TiSi _2, approximately Si in a weight ratio: TiSi ₂ = 30: was about 70.
A negative electrode material was obtained in the same manner as in Example 1 except that the Ti—Si alloy obtained above was used instead of silicon monoxide.

The particle diameter of nickel nitrate (II) supported on the surface of the Ti—Si alloy particles, and the fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same values as in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

Example 4
A negative electrode material was obtained in the same manner as in Example 1 except that tin (IV) powder (manufactured by Kanto Chemical Co., Inc., special grade reagent) was used instead of silicon monoxide.
The particle diameter of nickel nitrate (II) supported on the SnO ₂ particle surface, and the fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same as those in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

Example 5
Cobalt nitrate (II) hexahydrate instead of nickel nitrate (II) hexahydrate (Kanto Chemical Co., Ltd.)
A negative electrode material was obtained in the same manner as in Example 1 except that the product made in Japan, special grade reagent) was used.
The particle size of cobalt (II) nitrate supported on the surface of the SiO particles and the fiber diameter, fiber length, and weight ratio of the grown carbon nanofibers were almost the same as those in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

Example 6
In the growth process of carbon nanofiber, instead of flowing a mixed gas of 50% hydrogen gas and 50% ethylene gas at 500 ° C., a mixed gas of 50% hydrogen gas and 50% methane gas was flowed at 800 ° C. A negative electrode material was obtained in the same manner as in Example 1.
The fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same as those in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

Example 7
A negative electrode material was obtained in the same manner as in Example 6 except that cobalt nitrate (II) hexahydrate was used instead of nickel nitrate (II) hexahydrate.
The fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same as those in Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, no exothermic peak was observed at 550 ° C. or lower.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

<< Comparative Example 1 >>
After the carbon nanofiber growth step, a negative electrode material was obtained in the same manner as in Example 1 except that it was not heat-treated in air.
As a result of analyzing the composite particles by SEM, the grown carbon nanofibers were approximately uniform with a fiber diameter of about 80 nm and a fiber length of about 100 μm. Fibers and amorphous carbon deposits such as soot were confirmed on the surface of the SiO particles. Further, the weight ratio of the grown carbon nanofibers was calculated from the weight increase with respect to the active material particles before and after the synthesis, and was about 35% with respect to the entire composite particles. Further, as a result of TG / DTA measurement of the negative electrode material, an exothermic peak was observed at around 550 ° C.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

<< Comparative Example 2 >>
After the carbon nanofiber growth step, a negative electrode material was obtained in the same manner as in Example 4 except that the heat treatment was not performed in air.
The fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same values as in Comparative Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, an exothermic peak was observed at around 550 ° C.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

<< Comparative Example 3 >>
A negative electrode material was obtained in the same manner as in Example 5 except that after the carbon nanofiber growth step, no heat treatment was performed in air.
The fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same values as in Comparative Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, an exothermic peak was observed at around 550 ° C.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

<< Comparative Example 4 >>
After the carbon nanofiber growth step, a negative electrode material was obtained in the same manner as in Example 6 except that the heat treatment was not performed in air.
The fiber diameter, fiber length, and weight ratio of the grown carbon nanofiber were almost the same values as in Comparative Example 1. Further, as a result of TG / DTA measurement of the negative electrode material, an exothermic peak was observed at around 550 ° C.
A cylindrical battery was produced in the same manner as in Example 1 using the negative electrode material obtained above.

[Evaluation of battery characteristics]
About each battery produced above, in a 20 degreeC environment, it carries out constant current charging / discharging in the range of 4.2V-2.5V with the electric current value of 480mA (0.2C), and investigates the discharge capacity in 0.2C discharge. It was.
Further, the process of constant current charging to 4.2 V at 1680 mA (0.7 C) in a 20 ° C. environment was repeated, followed by constant current discharging to 2.5 V at 2400 mA (1 C). Then, after 50 cycles, constant current charge / discharge was performed at 480 mA in the range of 4.2 V to 2.5 V, and the discharge capacity at 0.2 C discharge was examined. The ratio of the 0.2C discharge capacity after 50 cycles to the initial 0.2C discharge capacity was determined as the cycle capacity retention rate.

Furthermore, about each battery immediately after battery preparation, after performing initial charging / discharging, the gas in a battery was collected and the total amount was quantified.
The evaluation results are shown in Table 1. In addition, the heat processing in Table 1 means the heat processing performed by the process of selectively burning the carbon nanofiber on the composite particle surface.

  The carbon nanofibers are grown and supported on the surface of the active material particles that can be alloyed with lithium, and the thermally unstable components among the carbon components covering the active material surface by heat treatment in air are selectively burned. In the batteries of Examples 1 to 7 using the negative electrode material made of the removed composite particles, good discharge characteristics and charge / discharge cycle characteristics were obtained. Regardless of the type of active material or catalyst, or the growth conditions of the carbon nanofibers, the same effect was obtained. In particular, in the batteries of Examples 1 and 4 to 7 using the oxide active material which is the best mode of the present invention, charge / discharge superior to the batteries of Examples 2 and 3 in which the active material is not an oxide. Cycle characteristics were obtained. It is considered that the deterioration of the active material due to heat treatment in the air was suppressed because the active material was a thermally stable oxide.

  In the batteries of Comparative Examples 1 to 4 that were not heat-treated in air after growing the carbon nanofibers, not only the initial capacity was slightly smaller than the battery design, but also the cycle capacity retention rate was heat-treated in air. A slightly lower value was obtained as compared with the batteries of Examples 1 to 7. This is because the bulk density and specific surface area of carbon components including carbon nanofibers are higher than those in Examples 1 to 7, and the irreversible capacity due to side reactions with the electrolyte in the initial stage is large, and carbon nanofibers. This is considered to be due to a decrease in the electronic conductivity between the active material particles due to a decrease in flexibility of the carbon component containing s.

  Moreover, it turned out that the amount of gas generation is very much in the battery of Comparative Examples 1-4 compared with the battery of Examples 1-7. As described above, this is considered to be due to the fact that the side reaction with the electrolytic solution in the initial stage became large. Since the capacity of the active material itself is large, there is no significant difference in terms of irreversible capacity, but from the aspect of gas generation, the effect of carbon nanofiber desensitization by heat treatment in air was clearly seen.

  From these results, the bulk density and specific surface area of carbon components including carbon nanofibers can be reduced by growing carbon nanofibers on active material particles that can be alloyed with lithium and then performing heat treatment in air. Can do. And it turned out that a high capacity | capacitance and the outstanding cycling characteristics are acquired by using this composite particle for negative electrode material.

  The non-aqueous electrolyte secondary battery of the present invention has a high capacity and excellent cycle characteristics, and is suitably used as a power source for portable electronic devices such as mobile phones.

It is a schematic sectional drawing of the negative electrode material for nonaqueous electrolyte secondary batteries of this invention. It is a schematic sectional drawing of the composite material produced at the process (2) in the manufacturing method of the negative electrode material for nonaqueous electrolyte secondary batteries of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Active material particle 2 Catalyst 3, 3a, 3b Carbon nanofiber 4 Amorphous carbon

Claims (7)

A negative electrode material for a nonaqueous electrolyte secondary battery comprising active material particles containing an element that can be alloyed with lithium, and composite particles containing carbon nanofibers supported on the surface of the active material particles,
The negative electrode material for a non-aqueous electrolyte secondary battery, wherein the carbon nanofiber is burned out only in a temperature range exceeding 550 ° C. at a temperature increase of 20 ° C./min in air. A negative electrode material for a nonaqueous electrolyte secondary battery comprising active material particles containing an element that can be alloyed with lithium, and composite particles containing carbon nanofibers supported on the surface of the active material particles,
2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon nanofiber has a heat generation peak only in a temperature range exceeding 550 ° C. in TG / DTA measurement at a temperature rising rate of 20 ° C./min in air. Negative electrode material.   The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the element that can be alloyed with lithium is at least one of Si and Sn.   The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the active material particles are made of an oxide containing an element that can be alloyed with lithium. (1) a step of supporting a catalyst on the surface of active material particles containing an element that can be alloyed with lithium;
(2) The active material particles supporting the catalyst are reacted with a carbon-containing gas in a high-temperature inert atmosphere, and carbon nanofibers are grown and supported on the active material particles using the catalyst as a starting point to obtain composite particles. And (3) heat treating the composite particles in an oxidizing atmosphere in a temperature range of 550 ° C. or less to loosen the carbon nanofibers on the active material particles. A method for producing a negative electrode material for a secondary battery.   A negative electrode material for a non-aqueous electrolyte secondary battery obtained by the method for producing a negative electrode material according to claim 5.   The nonaqueous electrolyte secondary battery using the negative electrode material in any one of Claims 1-4 and 6 as a negative electrode active material.

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