JP2002056843A - Negative electrode material for lithium secondary battery and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for lithium secondary battery having a high capacity, a long cycle service life and the high discharging characteristic. SOLUTION: A negative electrode material impregnated with the doping element contained Si into pores of a porous SiC sintered compact is manufactured by dipping the porous SiC sintered compact into the molten Si, in which the doping element (B, P, As or Sb) is added, or dipping the non-sintered compact composed of the SiC powder, C powder, the mixture powder of the Si powder and the C powder, and the mixture powder of the SiC powder and the C powder into the molten Si containing the doping element. This negative electrode material is formed of particles including the Si phase and the SiC phase, and the Si phase contains the doping element and a part of the Si phase is exposed in a particle surface, and other parts of the Si phase except for the part exposed in the particle surface are in contact with the SiC phase.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a negative electrode material thereof and a method for producing the negative electrode material. In the present invention, a lithium secondary battery is a general term for various lithium secondary batteries including a lithium ion secondary battery.

[0002]

2. Description of the Related Art With the spread of portable communication devices and small electronic devices, the demand for lithium secondary batteries is rapidly increasing.

In the current lithium secondary battery, a carbon material is used as a negative electrode material, and its charge / discharge capacity is about 320 mA.
It is around h / g. In the case of carbonaceous anode material, LiC ₆
The theoretical charge / discharge capacity when occluding Li is 372 mAh / g
It is. Since this theoretical capacity is 1/10 or less of the theoretical capacity of the negative electrode material composed of metallic Li, a dramatic increase in capacity of the lithium secondary battery cannot be expected as long as a carbonaceous negative electrode material is used.

Therefore, it has been proposed to use Si or the like having a theoretical capacity much larger than that of the carbonaceous negative electrode material as the negative electrode material. For example, Japanese Patent Application Laid-Open No. 10-83817 proposes a high-capacity negative electrode material containing Si as a main component.
It is disclosed that when the conductivity is improved by doping B or P as a type or n-type impurity, the capacity is further increased.

Since Si can store more Li than C, Si
It is well known that the use of as a negative electrode material increases the capacity of a lithium secondary battery. However, when a lithium secondary battery is actually manufactured using Si as a negative electrode material, there is a problem that the capacity is remarkably reduced as charge / discharge cycles are repeated, resulting in a battery having only a short cycle life. Although the above publication also discloses data on discharge capacity, it does not mention cycle life, and does not solve this problem. As described above, a lithium secondary battery using Si as a negative electrode material cannot provide a cycle life necessary for practical use, and thus cannot be put to practical use.

The reason why the cycle life when Si is used as a negative electrode material of a lithium secondary battery is short is considered as follows. Since Si stores more Li than C,
The expansion and contraction of the negative electrode material during discharge becomes larger than C. For this reason, the negative electrode material cannot follow the volume change that occurs every time charge and discharge are repeated, and the negative electrode material is cracked and pulverized. Since the pulverized negative electrode material is isolated and does not contribute to occlusion and release of Li, as the pulverization of the negative electrode material progresses due to repetition of charge and discharge, the charge and discharge capacity decreases.

Another problem when Si is used as a negative electrode material of a lithium secondary battery is that high-rate discharge characteristics, which are discharge characteristics at a large current, are low. For example, 10 times the amount of electricity when discharged at 0.2 C
If the capacity at the time of discharging at 2 C is extremely low, the use of the lithium secondary battery is limited, and the lithium secondary battery cannot be used for an application requiring a large current such as a power supply for an electric vehicle.

[0008]

SUMMARY OF THE INVENTION The present invention provides a negative electrode material for a lithium secondary battery having a large capacity and excellent not only cycle life but also high-rate discharge characteristics, a method for producing the same, and a negative electrode comprising the negative electrode material. It is an object to propose a rechargeable lithium battery. More specifically, it is to enable the above-described negative electrode material and its production by using Si as an active material.

[0009]

According to the present invention, there is provided a semiconductor device comprising:
A negative electrode material for a lithium secondary battery comprising particles including a SiC phase, wherein the Si phase includes at least one doping element belonging to Group 3B or 5B of the long-period periodic table, and a part of the Si phase. The above problem is solved by a negative electrode material for a lithium secondary battery, wherein is exposed to the particle surface and is in contact with the SiC phase except for the part of the Si phase that is exposed to the particle surface. it can.

[0010] Preferably, the Si phase contains the above-mentioned doping elements in a total amount of 10 ¹⁸ to 10 ²² atoms / cm ³ . The present invention also relates to a lithium secondary battery including a negative electrode manufactured from the negative electrode material for a lithium secondary battery.

In the negative electrode material for a lithium secondary battery of the present invention, the periphery of the Si phase is surrounded by a solid SiC phase.
The volume change of the Si phase that occurs with each repetition of charging and discharging
Since it is suppressed by the restraint by the SiC phase, the anode material is less likely to crack, and the cycle life is improved.

Further, the Si phase contains at least one doping element belonging to Group 3B or 5B of the long-periodic periodic table.
(Also referred to as "doped"), the Si phase becomes p-type or n-type, and the conductivity is increased. As a result, the high-rate discharge characteristics of the negative electrode material are improved, and the cycle life is improved for an unknown reason.

As the method for producing the above-mentioned negative electrode material for a lithium secondary battery of the present invention, the following two methods are possible. Method 1 is a method in which SiC is made into a porous sintered body and then impregnated with molten Si, and Method 2 is a method in which impregnation with molten Si also serves as formation of a SiC sintered body.

Method 1: A porous SiC sintered body is immersed in a melt of Si containing at least one doping element belonging to Group 3B or 5B of the Long Periodic Periodic Table. A method for producing a negative electrode material for a lithium secondary battery, comprising: a step of impregnating a sintered body; and a step of pulverizing the impregnated sintered body.

Method 2: Raw material powder consisting of SiC powder, C powder, mixed powder of Si powder and C powder, mixed powder of SiC powder and C powder, or mixed powder of SiC powder, C powder and Si powder The green compact obtained by pressure molding is immersed in a melt of Si containing at least one doping element belonging to Group 3B or 5B of the Long Periodic Periodic Table to form a porous SiC sintered body. Forming a porous SiC sintered body at the same time as impregnating the molten material, and pulverizing the porous SiC sintered body impregnated with the molten material. A method for producing a negative electrode material for a battery.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The negative electrode material for a lithium secondary battery according to the present invention comprises a Si phase doped with at least one element belonging to Group 3B or 5B of the periodic table and a SiC phase. It is a powder material composed of particles.

The SiC phase contains the above-mentioned doping element,
It may not be included. Since Li hardly passes through the SiC phase, it is not necessary that the SiC phase contains a doping element, but even if it does, the effect of the present invention is not impaired. SiC has α-type (hexagonal) and β-type (equiaxed)
Although there are two types of crystal forms, any of them may be used in the present invention.

The Si phase is composed of Li and an intermetallic compound (eg, Li _4,4 Si).
Since it can react reversibly through the formation of Li, it can occlude and release Li reversibly like C, and the theoretical capacity in that case is:
It is 10 times or more the theoretical capacity of C. When Si is used as the negative electrode material of a lithium secondary battery, the amount of Li absorbed per unit volume is larger than that of C, so the volume expansion and contraction during charging and discharging is large, and the negative electrode material is rapidly pulverized. However, the capacity decreases significantly as the charge / discharge cycle progresses. That is, even if the capacity is high, the cycle life is very short, so that it is not a practical negative electrode material.

In the present invention, a negative electrode material in which a SiC phase coexists with a Si phase is used. Since SiC is not reactive with Li, and Li can hardly pass through the SiC phase, the SiC phase is charged and
Substantially no volume change occurs during discharge. Therefore, SiC
The phase can effectively restrain the volume change of the Si phase accompanying the occlusion and release of Li. In other words, when a negative electrode material in which a SiC phase coexists with a Si phase is added, SiC that does not occlude Li reduces the discharge capacity, but still achieves a very high discharge capacity compared to the current carbonaceous negative electrode material. In addition, the cycle life of the Si-based negative electrode material can be improved to a level sufficient for practical use.

In the present invention, the Si phase is partially exposed on the particle surface, and is in contact with the SiC phase except for the part exposed on the particle surface. If part of the Si phase is exposed on the particle surface,
When Li enters the particles during charging, Li can directly enter the Si phase without passing through the SiC phase, and can move to the inside of the Si phase only through the inside of the Si phase. Similarly, when Li is released at the time of discharge, Li in the Si phase moves to the surface only through the Si phase and can go out without passing through the SiC phase. As a result, all the Si phases can be used for charging and discharging, and at the same time, can be bound by the SiC phase.

The number of Si phases in the particles may be one or two or more. Since each Si phase is impregnated, a part thereof is always exposed on the particle surface. The portion not exposed on the surface of the Si phase is in contact with the SiC phase. Thereby, the effect of restraining the volume change of the Si phase by the SiC phase can be maximized.

Actually, the raw material (powder of Si, C, SiC, etc.) used for producing the negative electrode material of the present invention and the material (dispersant, plasticizer, binder, etc.) used for forming the raw material powder described later are used. Contains inevitably trace amounts of various impurities. These impurities form silicides and carbides during sintering and impregnation. For example, silicides and carbides such as WSi ₂ and B ₄ C may be formed and precipitated. Strictly speaking, the Si phase is Si
It comes into contact with the third phase caused by these impurities other than the C phase. However, this third phase is very small, and according to the experience of the present inventor, the ratio of the third phase to the whole is not more than 3% by volume, and the effect of the present invention is not impaired. In the negative electrode material of the present invention, the precipitation of such a small amount of the third phase can be ignored as an impurity. Therefore, the Si phase is in contact with only the SiC phase except for the part exposed on the surface.

When the Si phase contains one or more of the above-mentioned doping elements, the conductivity of Si increases. 3B or 5 for Si
When a group B element is doped, these doping elements become so-called p-type impurities (group 3B elements) or n-type impurities (5B
It is well known in semiconductor theory that it acts as a group element) and increases conductivity.

When this phenomenon is applied to a Si-based negative electrode material,
Due to the increase in conductivity of Si due to the addition of doping elements,
Li easily passes through the inside, and the high rate discharge characteristics are improved. The improvement in the high-rate discharge characteristics is an effect that can be predicted to some extent from the improvement in conductivity by doping. However, surprisingly, the addition of
Reduced capacity reduction due to repeated discharge cycles,
It was found that the cycle characteristics were significantly improved. Since there is no direct relationship between the doping or conductivity and the cycle characteristics, the improvement of the cycle characteristics by doping the Si phase is a completely unexpected effect.

If the Si phase contains a small amount of the doping element, the effect is not so limited, and the content is not particularly limited, but the Si phase contains the doping element in a total of 10 ¹⁸ to 10 ²² atoms / cm 2.
Preferably, it is contained in an amount in the range of ³ , particularly 10 ²⁰ to
When the content is in the range of 10 ²² atoms / cm ³ , both the high-rate discharge characteristics and the cycle life have more remarkable improvement effects, which is more preferable.

[0026] doping atoms is less than 10 ¹⁸ atoms / cm ^3, there is the effect of the present invention becomes small, 10 ²² atoms /
If it exceeds cm ³ , segregation of the doping element may occur, resulting in a reduction in capacity, deterioration in conductivity, and the like.

As a method of doping the Si phase with one or more elements belonging to the 3B group or 5B group, for example, there is a method of adding a doping element alone or a compound thereof to a Si melt. As the doping element, one or more of B, P of the 3B group, P, As, and Sb of the 5B group are preferable.

According to the present invention, a part was exposed on the particle surface
The negative electrode material composed of particles consisting of Si phase and SiC phase, and the part not exposed to the surface of the Si phase is in contact with the SiC phase converts porous SiC sintered body into a Si melt (melt). It can be easily manufactured by using a method of immersing and impregnating the pores of the SiC sintered body with Si. The impregnation is preferably performed under the conditions described below.

The above-mentioned doping element is added to the Si melt to be impregnated in an appropriate amount (so as to obtain a predetermined doping element addition amount in anticipation of evaporation). After the impregnation, the impregnated material is cooled to solidify Si, and then pulverized into particles. Even when pulverized, part of the Si phase is exposed on the particle surface,
The feature that the unexposed portion is bound by the SiC phase remains. Therefore, a part of the impregnated Si phase is always exposed on the particle surface. Further, when the Si phase is manufactured by impregnation, the Si phase comes into contact with the SiC phase except for the part exposed on the surface.

The method for producing the porous SiC sintered body is not particularly limited, but typically, a SiC powder is used as a raw material powder, this is press-molded, and the obtained compact is fired and sintered. It is manufactured by the method of making. Instead of using SiC powder, use C powder, and pyrolytic silicon compound during firing
A porous SiC sintered body can also be produced by introducing (eg, silanes) and reacting with C to form SiC simultaneously with firing.

As is well known, the molding of the raw material powder is usually carried out by appropriately mixing a dispersant, a plasticizer, a binder and the like. The raw material powder may be granulated using a suitable powder granulator before compaction, which generally makes it possible to make the pore diameters small and does not generate a Si phase having a large particle size. If a Si phase having a large particle size is generated, particles of the Si phase alone may be formed by pulverization, which is not preferable. The firing temperature of the SiC sintered body is usually 500 to 2200 ° C. The porosity of the porous SiC sintered body is
It fluctuates depending on the particle size distribution and shape of the raw material powder, the pressure for pressure molding, the firing temperature, and the like.

The porosity of the porous sintered body impregnated with the molten Si determines the amount of Si impregnated, and hence the proportion of the Si phase in the negative electrode material, and the capacity of the negative electrode material changes accordingly. The volume ratio of the Si phase (equal to the porosity of the porous SiC sintered body) in the negative electrode material for a lithium secondary battery of the present invention is 10 to 70 v
ol%.

Instead of using a porous SiC sintered body, an unsintered green body obtained by press-forming raw material powder is
It is also possible to immerse the molded product in a liquid and sinter the molded product by the heat of molten Si to produce a porous sintered body and impregnate Si at the same time. Impregnation can occur simultaneously with or after sintering.
In this method, a firing step for sintering the molded body is not required.

As the raw material powder, SiC powder alone may be used, but powder of a material which reacts during sintering to become SiC can also be used. That is, C powder alone which reacts with molten Si to become SiC, or
A mixed powder of Si powder and C powder, a mixed powder of SiC powder and C powder, and a mixed powder of SiC powder, Si powder and C powder are possible.

The method using SiC powder is the simplest.
On the other hand, since SiC powder is expensive, a mixed powder of Si powder and C powder is effective for cost reduction. When this mixed powder is used, the C powder is combined with the molten Si to form SiC. When SiC is formed by the reaction during sintering in this way, the pore size becomes relatively small, and the diameter of the impregnated Si phase becomes small. Therefore, the constraint of the Si phase by the SiC phase works well, and when SiC powder is used. The cycle life is longer than that. Although the reason is unknown, this effect is particularly remarkable in the case of the mixed powder of SiC powder and C powder, and when the C powder is used alone or when the mixed powder of C powder and Si powder is used. Has a cycle life not so different from that in the case of using SiC powder.

The impregnation temperature of molten Si is 1410 which is the melting point of Si.
A temperature range from 50 to 300 ° C. above is suitable and the impregnation time is usually from 1 to 30 hours. The impregnation atmosphere is preferably a vacuum or a non-oxidizing atmosphere such as an inert gas atmosphere such as Ar or He gas or a hydrogen-containing gas atmosphere. Above all, a vacuum in which the progress of impregnation is fast is most preferable. Si (in the present invention, Si containing a doping element) penetrates and fills the pores of the porous SiC sintered body due to the impregnation.

When the Si phase is formed by impregnation, since there is no isolated Si phase inside the particles, all the Si phases can contribute to Li occlusion, and the capacity of the negative electrode material increases. The impregnated sintered body of SiC—Si obtained by the impregnation treatment is pulverized to obtain a negative electrode material. The pulverization can be performed by a known method. For example, a mortar, a ball mill, a vibration mill, a satellite ball mill, a tube mill, a rod mill, a jet mill, a hammer mill and the like are exemplified. Classify as necessary,
Make the desired particle size configuration. As the classifier, any of a sieve vibrator, a sonic sieve, a cyclone, a centrifugal classifier, an inertial classifier, an electromagnetic sieve, and the like can be used.

When the negative electrode material of the present invention is used, a lithium secondary battery having a high capacity, excellent cycle life and high rate discharge characteristics can be manufactured. The method for producing the battery is not limited, but is exemplified below. The form of the lithium secondary battery may be any of coins, buttons, sheets, cylinders, flat, polygonal, and the like.

(Negative Electrode) To the negative electrode material of the present invention, a commonly used conductive agent, a binder, a filler, a dispersant, an ionic conductive agent, a pressure enhancing agent and the like are appropriately added, and a solvent such as water is added. Mix to form a slurry or paste. The slurry or paste is applied to an electrode support substrate, consolidated using a rolling roll or the like, and dried to obtain a negative electrode.

(Positive Electrode) For the positive electrode, one or more kinds of commonly used composite oxides and sulfides of transition metals containing Li are used as an active material. In addition, V oxides, organic conductive materials such as conjugated polymers, Chevrel phase compounds, and the like can also be used as the positive electrode active material.

(Separator) A commonly used porous material such as a porous polymer film made of a polyolefin such as polypropylene and / or polyethylene or a glass filter nonwoven fabric is appropriately used.

(Electrolyte) A commonly used organic solvent
Any of a nonaqueous electrolyte solution in which a Li salt is dissolved, a polymer electrolyte, an inorganic solid electrolyte, and a composite system of a polymer and an inorganic solid electrolyte can be used.

[0043]

EXAMPLES (Example 1) This example illustrates the production of a negative electrode material according to the present invention by a method of impregnating a porous SiC sintered body with molten Si.

Water, a dispersant (ammonium carboxylate) and a binder were added to a commercially available α-type SiC powder having an average particle size of 0.7 μm.
(Acrylic resin), lubricant (stearic acid), plasticizer
(Polyethylene glycol) to prepare a slurry having a viscosity of about 500 ps. This slurry was put into a rotary disk type spray granulator to produce granulated powder having a particle size of 150 μm or less. After pressing the obtained granulated powder under pressure of 30 MPa,
Further, a hydrostatic pressure of 190 MPa is applied using CIP, and the diameter is 100 m.
A disc-shaped green compact having a thickness of 5 mm and a thickness of 5 mm was produced.

The molded body of this SiC powder was placed in a vacuum at 700 ° C.
For 3 hours to produce a porous SiC sintered body. When produced by such a method, the porosity of the porous SiC sintered body (apparent porosity determined by JIS R 2205, the same applies hereinafter) is usually 50% by volume or less, and in the case of this embodiment,
48% by volume.

This porous SiC sintered body was immersed in a Si melt containing a doping element in a vacuum to impregnate the doped Si. B is contained in various amounts as a doping element,
Alternatively, P, As, or Sb was used in place of B. The Si melt used for the impregnation is obtained by adding a simple element of the doping element to the molten Si in an amount in consideration of the amount of evaporation, and then once cooled and solidified, and then melted again. The temperature of the Si melt during impregnation is
1500 ° C., the impregnation time was 3 hours. Since the pores of the porous SiC sintered body were substantially completely impregnated with Si, the impregnation amount of Si was 48% by volume. After the impregnation, the mixture was allowed to cool in a vacuum, and then the Si attached to the surface was removed by sandblasting.

Thereafter, the impregnated material was pulverized with a ball mill in an Ar atmosphere to obtain a negative electrode material of the present invention comprising a doped Si phase and a SiC phase. The doping amount of the negative electrode material was measured using an ICP emission spectrometer. Using this negative electrode material, the results of evaluating the electrode performance by the following method are shown in Table 1 together with the types and amounts of the doping elements.

For comparison, a negative electrode material in which molten Si containing no doping element was impregnated into a porous SiC sintered body was prepared in the same manner as above. In addition, the doped Si melt was solidified as it was and pulverized to produce a negative electrode material consisting only of the Si phase. The electrode performance of these negative electrode materials was similarly evaluated.
Shows the results.

(Evaluation of Electrode Performance) The powder of the negative electrode material of the test cell was classified and the particle size was adjusted so that the average particle size became 10 μm. To 88.3 parts by mass of this powder, 8.8 parts by mass of acetylene black as a conductive agent and 2.9 parts by mass of polyvinylidene fluoride as a binder were added, and mixed in N-methylpyrrolidone as a solvent to obtain a slurry. Apply this slurry to a thickness of 20
μm copper foil by doctor blade method, and after preliminary drying,
Rolled to consolidate, dried in vacuum at 120 ° C for 16 hours, punched into a disk of 16 mm diameter (area 2 cm ² )
(Working electrode).

As a counter electrode, a disk obtained by punching a Li metal foil to a diameter of 16 mm was used, and a porous polypropylene film was used as a separator. As the electrolytic solution, a solution obtained by dissolving LiClO ₄ at a concentration of 1 M in a mixed solution of ethylene carbonate and dimethoxyethane at a volume ratio of 1: 1 was used. The separator was impregnated with the electrolytic solution, the separator was sandwiched between the working electrode and the counter electrode, housed in a stainless steel case, and the case was hermetically sealed and sealed to form a test cell for electrode evaluation.

Battery test charge: At a current of 2 mA, the potential of the counter electrode with respect to the working electrode is 0 V
Charge and discharge until the current becomes 2 mA, and the potential of the counter electrode with respect to the working electrode is -1.
Discharge until 0V. In this test cell, the Li metal at the counter electrode is lower (that is,
(Negative electrode), charging and discharging are opposite to the above, but since this test is for evaluating the product of the example as a negative electrode material, the one that releases Li was defined as `` discharge '' .

It is considered that charging and discharging once under these conditions is one of the followings.
The discharge capacity was measured at the first cycle and at the 500th cycle. Also, to evaluate the cycle life, 1
A capacity retention ratio (= [500 cycle discharge capacity / 1 cycle discharge capacity] × 100), which is a ratio (%) of the discharge capacity at the 500th cycle to the discharge capacity at the cycle, was calculated.

In order to evaluate the high-rate discharge characteristics, the discharge capacity in the first cycle when the discharge was performed by increasing the discharge current to 20 mA, which is 10 times the above, was measured. The ratio to the discharge capacity was calculated. The closer the discharge capacity ratio is to 1, the better the high-rate discharge characteristics are.

[0054]

[Table 1] As can be seen from Table 1, the negative electrode material of the present invention has an initial discharge capacity of 800 mAh / g or more, which is much higher than the initial discharge capacity of current carbon-based negative electrode materials (generally around 320 mAh / g). Moreover, the capacity retention rate after 500 cycles is 70% or more,
It has a cycle life sufficient for practical use and also has good high rate discharge characteristics.

Comparative Example in which Si does not contain a doping element (N
As can be seen from comparison with o.1), when the Si phase contains a doping element, not only the high-rate discharge characteristics (discharge capacity ratio) are improved, but also the cycle life (capacity retention ratio) is significantly improved. This effect is more remarkable especially when the doping amount is 10 ²⁰ atoms / cm ³ or more, and the capacity ratio after 500 cycles is 90%, and the capacity ratio of the discharge current 20 mA / 2 mA is 0.9 or more.
Both cycle life and high rate discharge characteristics are very good.
On the other hand, in the negative electrode material (No. 10) composed of only the doped Si phase without the SiC phase, although the initial discharge capacity is higher than the negative electrode material of the present invention, the discharge capacity at the 500th cycle is 0, It has a very short life and cannot be used for practical batteries.

As described above, constraining the volume change of the Si phase due to the coexistence of the SiC phase is extremely effective for improving the cycle life, and the high-rate discharge characteristics are improved by including the doping element in the Si phase. Not only that, the cycle life is unexpectedly significantly improved.

(Example 2) This example illustrates a method for producing a negative electrode material of the present invention in which sintering of a compact and impregnation of Si are simultaneously performed by immersion in molten Si. The raw material powders (all commercially available) used in this example are as follows.

SiC powder (β-type SiC) average particle size 0.7 μm, Si powder average particle size 3 μm, C powder (carbon black) average particle size 0.1 μm.

Using one or two of the above raw material powders, an unsintered disk-shaped compact was produced by the same molding method as described in Example 1. Using this unsintered compact, it was immersed in a Si melt containing a doping element under the same conditions as described in Example 1, and the compact was sintered and C powder was used. In this case, the formation of SiC and the impregnation of the pores of the sintered body with Si were simultaneously achieved by the reaction with Si. As a Si melt, doping element B is contained in Si at 1 × 10 ²⁰ atoms / cm ^3.
What had been added (after impregnation) was used.

The Si content of the negative electrode material was determined by measuring the density of the impregnated body before pulverization and considering the densities of SiC and Si. The results of evaluating the electrode performance of this negative electrode material in the same manner as in Example 1 are shown in Table 2 below together with the configuration of the raw material powder.

[0061]

[Table 2] Even when sintering and impregnation of Si were performed simultaneously by immersing the green compact of SiC in a doped Si melt, a negative electrode material having electrode performance comparable to that of Example 1 was obtained. Raw material powder is SiC10
Contain C powder instead of 0%,
When converted to SiC (No. 2-3), the raw material powder is SiC
A negative electrode material having a lower Si content than 100% (No. 1). Therefore, the discharge capacity was lower than that of Example 1, but still higher discharge capacity than the current carbon-based negative electrode material was obtained. In particular, when a compact is produced by mixing C powder with SiC powder, the capacity retention rate after 500 cycles is 95%.
% Or more, a negative electrode material having extremely excellent cycle life was obtained.

Example 3 This example illustrates a lithium secondary battery incorporating a negative electrode manufactured from the negative electrode material manufactured in Example 1.

As a negative electrode, Table 1 prepared in Example 1 was used.
No.3, No.3, No.3 and No.10 for comparison
Negative electrode material (does not contain SiC phase, only doped Si phase)
The electrode having a diameter of 16 mm produced for the electrode evaluation test was used. Each negative electrode had the same coating thickness.

Separately, using a carbon material (graphite powder), an electrode having a diameter of 16 mm was prepared in the same manner as in the electrode evaluation test in Example 1.
(However, the conductive agent was not mixed), and a negative electrode was used. When this graphite electrode was evaluated in the same manner as in Example 1, the discharge capacity was 320 mAh / g.

To prepare a positive electrode, 2 parts by mass of LiCoO ₂ , 6 parts by mass of acetylene black as a conductive agent, and 2 parts by mass of polyvinylidene fluoride as a binder were mixed in N-methylpyrrolidone as a solvent. The obtained slurry is coated with a 20 μm thick Al
It was applied on a foil by a doctor blade method, temporarily dried, rolled, dried in a vacuum at 120 ° C. for 16 hours, and punched into a diameter of 16 mm to obtain a positive electrode.

In a coin battery case made of stainless steel, a negative electrode and a positive electrode, and a polypropylene porous film as a separator between them were arranged. LiClO ₄ was added to a mixed solution of ethylene carbonate and dimethoxyethane (volume ratio 1: 1).
Was impregnated with an electrolyte solution having a concentration of 1M. Finally, the case was hermetically sealed by caulking to produce a coin battery. The battery is operated at a voltage of 3.0 to 4.2 V in the range of 2 mA and
The charge / discharge test was repeated at a current of 20 mA. Table 3 shows the obtained results.

[0067]

[Table 3] From Table 3, it can be seen that in a battery test in which a lithium secondary battery was actually produced, the same results as those in the electrode performance test in Table 1 were obtained. That is, the lithium secondary battery using the negative electrode material according to the present invention has a slightly lower initial discharge capacity than the battery using the negative electrode material composed of Si alone, but has a higher capacity retention rate after 500 cycles and a higher rate discharge. Excellent characteristics. In particular, when the doping amount is 10 ²⁰ atoms / cm ³ or more, the capacity retention ratio is extremely high at 90%, the high rate discharge characteristics are further improved, and the value is comparable to that of graphite. Since the initial capacity is much higher than graphite, the capacity after 500 cycles and the capacity at high rate discharge are much higher than graphite electrodes. On the other hand, in the case of a negative electrode material composed of Si alone, the discharge capacity after 500 cycles becomes 0, and thus a lithium secondary battery having no practical use is not obtained.

[0068]

According to the present invention, there is provided a negative electrode material for a lithium secondary battery having a higher capacity, an excellent cycle life, and an excellent high-rate discharge characteristic than existing carbon-based negative electrode materials.
Therefore, the present invention contributes to the improvement of the performance of the lithium secondary battery, and can further improve the various uses using the lithium secondary battery.

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Claims (5)

[Claims] 1. A negative electrode material for a lithium secondary battery comprising particles containing a Si phase and a SiC phase, wherein the Si phase is at least one doping element belonging to Group 3B or 5B of the long period periodic table. A negative electrode material for a lithium secondary battery, wherein a part of the Si phase is exposed on the surface of the particles and the part other than the part of the Si phase that is exposed on the surface of the particles is in contact with the SiC phase. 2. The Si phase comprises a total of 10 ¹⁸ to 10
^The negative electrode material for a lithium secondary battery according to claim 1, which is contained in an amount in the range of ²² atoms / cm ³ . 3. A porous SiC sintered body is immersed in a melt of Si containing at least one doping element belonging to Group 3B or 5B of the Long Periodic Periodic Table, and the melt is sintered. A method for producing a negative electrode material for a lithium secondary battery, comprising: a step of impregnating a sintered body; and a step of pulverizing the impregnated sintered body. 4. SiC powder, C powder, mixed powder of Si powder and C powder, mixed powder of SiC powder and C powder, or SiC powder
A green compact obtained by pressing a raw material powder composed of a mixed powder of a powder, a C powder and a Si powder is subjected to one or more doping elements belonging to Group 3B or 5B of the long-period type periodic table. A step of forming a porous SiC sintered body by impregnating the molten material with the molten Si, and simultaneously impregnating the porous SiC sintered body with the molten material; And pulverizing the negative electrode material for a lithium secondary battery. 5. A lithium secondary battery comprising a negative electrode made from the negative electrode material according to claim 1.