JP2000149951A - Lithium secondary battery, and negative active material for the lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having high initial efficiency and a cycle characteristic, and a negative active material used for it. SOLUTION: A negative active material comprising powder of a silicon material containing boron and having 0.1-50 wt.% of boron content in the silicon material is used as a negative active material, in a lithium secondary battery containing a positive active material, the negative active material and a nonaqueous electrolyte. A 50%-cumulative size of the negative active material powder is 1-100 μm, and a peak intensity ratio (SiB4;021)/I(Si;111) of peak intensity of a diffracted ray from a (021) plane of SiB4 to peak intensity of diffracted ray from a (111) plane of Si in an X-ray wide angle diffraction method for the negative active material powder is preferably 1 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery and a negative electrode active material used for the same. More specifically, the present invention relates to a high-performance lithium secondary battery having a high voltage, a large discharge capacity, and a small capacity loss during charge and discharge, and a negative electrode active material used for the same.

[0002]

2. Description of the Related Art Since lithium secondary batteries have a high energy density, they are used as power sources for mobile communication and portable information terminals, and the market is rapidly expanding with the spread of terminals. Along with this, in order to further pursue the small size and light weight, which are the characteristics of the terminal device, there is a demand for a battery occupying a large volume in the device to further improve the performance of the size and weight reduction.

The negative electrode active material currently used for the secondary battery is mainly a graphite-based carbonaceous material, and is a key material that affects battery performance. However, the amount of lithium that can be inserted and removed reversibly in the material is carbon 6
One atom of lithium is the limit to the atom, and 372 mAh / g in terms of electric capacity is a theoretical limit capacity for charging and discharging of the carbon material. Since current secondary batteries are used at a level close to this limit capacity, a dramatic improvement in performance in the future cannot be expected.

Under such circumstances, search for materials other than carbon, for example, alloys and inorganic compounds having a capacity much larger than 372 mAh / g is gradually being conducted. Among them, it has been found that a crystalline or amorphous oxide material containing tin and silicon exerts a discharge capacity close to 1000 mAh / g (for example, JP-A-7-220721, JP-A-7-20771). 249409, etc.). In addition, when silicon has recently been used as a negative electrode active material, it is 3000 mAh.
/ G initial discharge capacity (about 38th Battery Symposium,
3A16 (1997)), when silicon oxide was used as the negative electrode active material, an initial discharge capacity of around 1500 mAh / g was shown (38th Battery Symposium, 3A17 (1997)).
It has been reported to have a discharge capacity that far exceeds the critical capacity of graphite-based carbonaceous materials. However, any material has a large initial charge capacity relative to its initial discharge capacity,
That is, there was a serious problem that the capacity loss during charge / discharge was very large (about 1000 mAh / g for both materials) and the cycle characteristics were low (the capacity was reduced by half in several cycles after the start of charge / discharge).

On the other hand, as a material containing boron in silicon, a silicon boride alloy structure in which n is 0.1 to 3 in general formula SiB _n (Japanese Patent Application Laid-Open No. 53-136630).
JP), n is a boron compound of silicon which mainly SiB ₄ ranging from 3.2 6.6 powder (JP-A-8-13874
No. 4) is disclosed. Although the above-mentioned alloy structure shows a large discharge capacity far exceeding that of the graphite-based carbonaceous material inherent to silicon, since the electrode is formed by impregnating a current collecting matrix in an alloy bath, the lithium ion has recently been put into practical use. It was difficult to mass-produce compared to an electrode formed by applying a slurry of an active material powder together with a binder on a current collector foil as seen in a battery and forming the slurry. In addition, since it is difficult to reduce the film thickness of this alloy structure, at a large current density, lithium is not sufficiently diffused in the electrode, causing a large polarization resistance, and the absorbed lithium is not sufficiently absorbed. Because you can not withdraw
There has been a major problem in reversibility, such as an increase in capacity loss during charging and discharging. On the other hand, in the latter case, it is difficult to obtain a discharge capacity higher than that of the graphite-based carbonaceous material with the silicon boron compound powder mainly composed of SiB ₄ or the like.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel negative electrode active material for a lithium secondary battery and a lithium secondary battery using the same.

Another object of the present invention is to provide a lithium secondary battery having high initial efficiency and cycle characteristics while realizing a high discharge capacity, and a negative electrode active material used for the same.

The present invention further provides the above-mentioned problems that occur when a boron-containing silicon material is used as a negative electrode active material for a lithium secondary battery, that is, it is unsuitable for mass production and has a large capacity during charging and discharging. A negative active material for a lithium secondary battery, which solves the problems of causing loss and low reversibility and obtaining a discharge capacity lower than that of a graphite-based carbonaceous material, and a lithium secondary battery using the same. It is intended to provide a battery.

[0009]

Means for Solving the Problems The present inventors diligently studied a silicon material powder containing boron based on the electrochemical characteristics of the silicon material powder itself. As a result, the silicon material powder having a certain appropriate particle size was obtained. Is mixed with low-concentration boron and heat-treated under a certain condition, whereby substantially less silicon boride is present than the amount of silicon boride (SiB ₄ ) thermodynamically estimated from the contained boron amount. It is possible to obtain a boron-containing silicon material powder in a supercooled state, and by using it, the capacity loss is greatly improved while maintaining a large discharge capacity far exceeding the graphite-based carbonaceous material that silicon originally possesses As a result, it has been found that excellent electrode characteristics with good cycle characteristics are exhibited. The present invention has been completed based on such findings.

[0010] The negative electrode active material for a lithium secondary battery of the present invention comprises a boron-containing silicon material powder, wherein the silicon material has a boron content of 0.1 to 50% by weight. It is.

Furthermore, in a preferred embodiment of the present invention, the 50% cumulative diameter (d ₅₀ ) of the negative electrode active material powder is 1 to 5.
A negative electrode active material that is 100 μm is shown. In another preferred embodiment of the present invention, the negative electrode active material powder X
(021) of SiB ₄ with respect to the peak intensity I (Si; 111) of the diffraction line from the (111) plane of Si in the line wide angle diffraction method
The ratio I _{of; (021 SiB 4) (SiB} 4; peak intensity of the diffraction line from the plane I 02
1) A negative electrode active material in which / I (Si; 111) is 1 or less is shown.

According to the present invention, which achieves the above objects, a lithium secondary battery containing a positive electrode active material, a negative electrode active material and a non-aqueous electrolyte contains at least 0.1 to 50% by weight of boron as the negative electrode active material. A lithium secondary battery characterized by using a silicon material powder.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The specific contents of the present invention will be described below.

The negative electrode active material for a lithium secondary battery according to the present invention comprises a powder of a silicon material containing a low concentration of boron.

Such a boron-containing silicon material powder is obtained, for example, by mixing boron in a silicon material at a low concentration, heat-treating and, if necessary, pulverizing and classifying the silicon material. The amount of silicon boride such as SiB ₄ is reduced. Less boron-containing silicon material powder in a substantially supercooled state. As described above, by using a material in which the particle size, the specific surface area, and the amount of silicon boride generated as a coexisting phase are controlled, not only a large-scale production as an electrode is enabled, but also a graphite-based carbonaceous material is used. It has a much larger discharge capacity, has dramatically reduced the capacity loss during charge and discharge, and has succeeded in dramatically improving the cycle characteristics.

The effect of boron which has played a major role in improving the electrode characteristics of silicon alone will be considered as follows.

The reaction of lithium with simple silicon or silicon oxide is tin metal, which is expected to undergo a similar reaction to the present system.
Report on oxides (J. Electrochem. Soc., 144, 6, 2045
(1997)), the following can be inferred. That is, this reaction is basically considered to be an alloying / dealloying reaction between silicon and lithium as hosts. When silicon undergoes an alloying reaction with a large amount of lithium at the time of charging, a number of phase changes occur along with a change in lithium concentration, and a large volume expansion occurs. Conversely, in the case of discharge, a large volume shrinkage occurs with a phase change as lithium is desorbed from the alloy phase with lithium. Although the cause of the irreversible capacity that occurs in silicon alone and silicon oxide is not clear, it is not sufficient that the shape of the electrode collapses due to a large phase change or volume change that occurs during charging and discharging, resulting in a decrease in current collection efficiency. This may cause a large capacity loss at the time of discharging and a decrease in reversibility.

The reason why the irreversible capacity is reduced by using a boron-containing silicon material powder in a substantially supercooled state with a small amount of the boron compound according to the present invention is not yet clear. When the boron-containing silicon material powder of the present invention has a smaller maximum alloy composition with lithium than the simple substance, the charging depth becomes shallow and the expansion of the crystal structure is suppressed. Suppressed to reduce the irreversible capacity, or when boron contains a crystal structure suitable for lithium diffusion, the diffusion of lithium is faster than in pure silicon and the irreversible capacity is reduced. It is conceivable that the irreversible capacity decreases because the electrical conductivity of the material itself is improved by forming the acceptor level by doping with boron.

The material containing boron in silicon is formed into a powder having an appropriate particle size, and is used as a negative electrode active material of a lithium secondary battery. Since it can be used without modifying the electrode production line of the installed lithium-ion battery, it is possible to mass-produce the battery with existing facilities. Furthermore, compared to conventional alloy structures, electrodes coated with powder can form a thin film that is more advantageous for lithium diffusion in the electrodes, and the polarization resistance at high current densities can be kept low. It is possible to improve the efficiency of electricity at the time and reduce the capacity loss.

With respect to the boron-containing silicon material powder according to the present invention, it is necessary that the boron concentration in the silicon material powder satisfy 0.1 to 50% in terms of weight. When the boron concentration is less than 0.1% by weight, the effect of containing boron is not sufficiently exerted, and a large capacity loss is exhibited during charge and discharge as in the case of silicon alone, which is not preferable. on the other hand,
A dramatic improvement in electrode properties was seen by including boron in materials with boron concentrations of 0.1% by weight or more. On the other hand, when the boron concentration exceeds 50% by weight, a large amount of silicon boride such as SiB ₄ does not contribute to the occlusion (alloying) reaction of lithium, so that the discharge capacity per unit weight and energy density are extremely low. It is not preferable because it is lowered. More preferably, the boron concentration is preferably from 10% by weight to 50% by weight, and more preferably from 20% by weight to 40% by weight. When the content of boron contained in the silicon material powder is 1
When the content is 0% by weight or more, the silicon material powder can exhibit higher cycle characteristics. This is due to the phenomenon that expansion and shrinkage of the silicon material powder due to charging and discharging are suppressed by boron addition found in the study of the present invention, and the effect is remarkable when the boron content is 10% by weight or more. I think it will be higher.

Regarding the particle size of the boron-containing silicon material powder according to the present invention, the 50% cumulative diameter (d ₅₀ ) of the powder is
Satisfies 1 to 100 μm, more preferably 5 to 50 μm. If d ₅₀ of less than 1μm, the powder of a small particle size often and that handling properties is observed a tendency that deteriorate because it contains the energy density per unit volume required many binders and conductive agent is reduced It is not preferable because there is a possibility. On the other hand, d ₅₀ is 100
If it exceeds μm, it may be difficult for lithium to diffuse into the active material powder, or the electrode thickness of the current lithium ion battery may be about 200 μm or less, making it difficult to prepare an electrode. Therefore, it is not preferable.

The particle size specific surface area of the boron-containing silicon material powder is desirably 100 m ² / g or less.
If the specific surface area exceeds 100 m ² / g, the surface area is large, which may cause a reaction with the electrolyte at the time of initial charging to increase the capacity loss or occlusion in the material during the charge / discharge cycle. The reactivity between the lithium and the solvent in the electrolyte may increase, and the safety may decrease. On the other hand, the lower limit of the specific surface area is not particularly limited. However, it is difficult to diffuse lithium into the inside of the active material powder, and it is difficult to prepare a thin electrode. It is about 0.01 m ² / g.

Furthermore, in the present invention, among the diffraction peaks observed in the wide angle X-ray diffraction pattern measurement of the boron-containing silicon material powder, the Si (111) from surface of the diffraction line and SiB ₄ (021)
Peak intensities I (Si; 111) and I (SiB ₄ ;
021) preferably satisfies the relationship of I (SiB ₄ ; 021) / I (Si; 111) ≦ 1.

When this peak intensity ratio exceeds 1,
It means that coexisting SiB ₄ is present in a considerable amount in the material, and since SiB ₄ itself does not contribute to the occlusion (alloying) reaction of lithium, the discharge capacity per unit weight and energy density are extremely reduced, This is because the cycle characteristics may be lowered.

Here, the wide-angle X-ray diffraction pattern derived from the boron-containing silicon material powder in the negative electrode active material for a lithium ion secondary battery of the present invention mainly belongs to peaks derived from Si and SiB ₄ . However, silicon material powder containing the above-described boron is not a simple mixture of mere Si and SiB _4. From observation by a transmission electron microscope, that a large number of fine silicon boride (SiB ₄ ) is precipitated in each particle of the silicon material powder containing boron, or that it is in the early stage of precipitation,
Further, it is recognized that each particle of the silicon material powder has many very complicated local strains. Although the detailed mechanism is not clear, the present inventors consider that the size and the form of the precipitated SiB ₄ , boron dissolved in the silicon material powder and many local strains, etc., are the activities of the present invention. We believe that it contributes to the excellent performance of the material.

The proportion of SiB ₄ present in the peak intensity ratio is equilibriumly determined by the amount of boron to be added and the reaction temperature (for the phase diagram of boron and silicon, for example, J .Less-Common Met., 71, 195 (1980)), the amount of silicon boride such as SiB ₄ coexisting in the material greatly depends on the form and firing pattern of silicon as a raw material. In particular, when silicon as a raw material is a powder and the temperature drop process in the heat treatment step is fast, the SiB in the material is
It is possible to prepare a substantially supercooled boron-containing silicon material in which the proportion of ₄ is small and therefore the amount of silicon boride is small.

With respect to the boron-containing silicon material powder according to the present invention, the lower limit of the peak intensity ratio I (SiB ₄ ; 021) / I (Si; 111) is not particularly limited. May include a value of zero for reasons such as shown in That is, the boron-containing silicon material powder according to the present invention exerts a dramatic improvement in electrode characteristics as compared with the case of using a silicon material alone, even with a small amount of boron of 0.1% by weight as described above. In the embodiment in which such a small amount is added, boron completely dissolves in silicon and forms S
This is because the peak of iB ₄ is not observed. In the measurement accuracy of the current wide-angle X-ray diffraction method, a detection limit of the peak intensity ratio of about 0.01, in the value lower than this, even in the presence of a diffraction peak derived from SiB ₄ In some cases, it is difficult to distinguish the noise from the background. Therefore, the peak intensity ratio is 0.01
In the following cases, it is difficult to clearly determine whether or not a diffraction peak derived from SiB ₄ was present, but in any case, it is clear that boron is contained in the silicon material particles. As far as possible, the embodiment of the present invention is irrespective of whether boron is completely dissolved in silicon or partly precipitated as silicon boride. The case where the peak intensity ratio is 0.01 or more and 1 or less is, of course, included in the present invention as a preferred embodiment.

The crystallinity of the boron-containing silicon material powder is not particularly limited. However, when the crystallinity of the boron-containing silicon material powder is extremely low (for example, the crystallinity is evaluated by X-ray diffraction. When the crystallite size is 10 nm or less), the potential does not show a flat dependence on the charge / discharge amount at the time of charge / discharge, and a stable voltage independent of the discharge amount when used in a battery is to be ensured. Not desirable from a point of view.

The boron-containing silicon material powder as the negative electrode active material for a lithium secondary battery of the present invention is not particularly limited, but can be prepared, for example, by the following method.
That, d ₅₀ is against 1~100μm of silicon powder,
A mixture of a boron compound powder of 0.1 to 50% by weight in terms of boron is mixed at 1350 ° C. to 1400 under an argon atmosphere.
After the temperature is raised to 0 ° C, the temperature is maintained for 1 to 10 hours, then rapidly cooled to 600 ° C at a rate of about 15 ° C / min, and then cooled to about room temperature at a rate of about 5 ° C / min. The particle size of the powder obtained by firing is adjusted by crushing and classification as needed. The firing atmosphere here may be a non-oxidizing atmosphere, and for example, nitrogen or the like may be used instead of argon.

The silicon powder used as a raw material here is Si powder.
In addition, various elements may be included. For example, carbon,
Oxygen, hydrogen, nitrogen, sulfur, phosphorus, halogen, alkali metal, alkaline earth metal, transition metal, Al, Ga, In,
At least one of Ge, Sn, Pb, Sb, Bi, and the like may be included. The total content of these elements is preferably from 0 to 10% by weight. Further, 0 to 5% by weight is more preferable.

The boron compound powder, which is the other raw material, may be any as long as it can be finally dissolved in silicon in the form of boron in silicon at a boron concentration within the range specified in the present invention. For example, one or more of boron alone, boron oxide, boric acid, boron carbide, boron nitride, and the like can be suitably used.

As for the mixing method, it is sufficient that the samples are sufficiently uniformly mixed. For example, a V-blender, a kneader, a ball mill, etc. can be suitably used, but it is not particularly limited thereto. In addition, for adjusting the particle size and specific surface area of the boron-containing silicon material powder after firing, it is possible to use a method generally used in industry. For example, ball mill, pin mill, disk mill, impeller mill, jet mill, roller mill,
A stamp mill, a cutting mill and the like are preferably used for classification, but an air classifier, a sieve and the like are preferably used, but not particularly limited thereto.

Although the method for preparing the negative electrode active material for a lithium secondary battery of the present invention has been described above by way of example, the negative electrode active material for a lithium secondary battery of the present invention is not limited by the above-described preparation method. . An example of another method for preparing the negative electrode active material for a lithium secondary battery of the present invention is to obtain a boron-containing silicon material powder by performing a plasma treatment using a mixed gas of a predetermined ratio of a silane gas and a diborane gas using an Ar gas as a carrier gas. about
By performing a heat treatment at about 1380 ° C. for about 1 hour, a silicon material powder containing boron that can be used for the negative electrode active material for a lithium secondary battery of the present invention can also be obtained. In another example, silicon powder and boron metal are mixed at a predetermined ratio, mechanically alloyed sufficiently using a ball mill for about 20 hours, and the obtained product is heat-treated at about 1380 ° C for about 1 hour, and is appropriately pulverized. By performing classification, a boron-containing silicon material powder that can be used for the negative electrode active material for a lithium secondary battery of the present invention can also be obtained.

Further, the negative electrode active material for a lithium secondary battery according to the present invention is obtained by further adding another negative electrode active material, for example, a carbon material powder to the above-mentioned boron-containing silicon material powder and mixing them. It can also be a powder.

When such a mixed powder is used, the negative electrode active material according to the present invention has a particularly excellent performance in terms of cycle characteristics and expansion / shrinkage rate during charge / discharge despite having a high discharge capacity. Become. Although the cause is not clear, the mixed powder contains a carbon material powder, typically a carbon material powder with a high degree of graphitization, and such a carbon material powder is generally easily deformable. Therefore, it is considered that buffering a relatively large expansion and contraction caused by charging and discharging of the silicon material powder containing boron is one of the important causes.

The carbon material powder used in the present invention may not substantially contain boron, or may contain boron.

Here, as the carbon material powder, a carbon material powder having a high crystallinity (a so-called high graphitization degree) is desirable from the viewpoint of discharge capacity. High crystallinity can be confirmed by X-ray diffraction or the like. In this case, the carbon material powder also functions as a conductive material and has a capacity of about 300 mAh / g.
It also works as a negative electrode active material having a discharge capacity of.

The carbon material powder can be prepared, for example, by the following method. First, in the case of carbon material powder containing no boron, coal tar pitch coke (carbonized product)
After pulverizing and classifying, the carbon material powder obtained by preparing particles of 10 μm or less by volume under 325 mesh to be 10% or less under a non-oxidizing atmosphere such as an argon atmosphere is heated to about 2900 ° C. Hold for 1 hour, then cool to near room temperature. The powder obtained by the heat treatment obtains a carbon material powder that can be used for the negative electrode active material for a lithium secondary battery of the present invention by simple crushing. In the case of carbon material powder containing boron, coal tar pitch coke is used.
(Carbonized product) is pulverized and classified, and the carbon material powder obtained by preparing particles having a particle size of 10 μm or less under 325 mesh under 10% by volume is 325mesh under boron alone, boron oxide, boric acid, carbonized. Boron, boron nitride, etc., one or more boron compound powder about 0.5 wt% in terms of boron
About 2900 ° C under argon atmosphere after adding about 10 wt% and mixing
After the temperature rises, the temperature is maintained for about 1 hour, and then it is allowed to cool to around room temperature. In this case, pulverization and classification after heat treatment are usually required, so that this is performed to obtain a carbon material powder that can be used as the negative electrode active material for a lithium secondary battery of the present invention.

The carbon material powder that can be used in the present invention is not particularly limited. In addition to the above-described artificial graphite derived from coal tar, artificial graphite derived from petroleum tar, natural graphite, quiche graphite, etc. Or those containing boron by heat treatment or the like can also be used.

With respect to the carbon material powder containing substantially no boron, of the diffraction peaks measured by the wide-angle X-ray diffraction pattern, two diffraction lines derived from the carbon material powder, that is, from the (101) plane of carbon The peak intensities I (C; 101) and I (C; 100) of the diffraction line from the (100) plane and the diffraction line from the carbon (100) plane, respectively, satisfy I (C; 101) / I (C; 100) ≧ 1. It is desirable to be satisfied. At this time, since the carbon material powder itself has a high crystallinity (degree of graphitization), the discharge capacity becomes large, so that the mixing amount of the boron-containing silicon material powder can be reduced. The expansion rate due to the entire charge can be suppressed to a low level.

When the peak intensity ratio I (C; 101) / I (C; 100) is smaller than 1, the discharge capacity of the carbon material powder itself is small, so that the discharge capacity exceeds the theoretical capacity of graphite. In such a case, it is necessary to mix a large amount of the boron-containing silicon material powder, and the expansion rate due to charging as a negative electrode material increases, which may make it difficult for the mixed powder to exhibit sufficient cycle characteristics. There is.

In addition, since the carbon material powder containing boron generally has a high crystallinity and a large discharge capacity, it is preferable that the carbon material powder itself contains boron in the first place. In particular, the weight content of boron contained in the carbon material powder relative to the carbon material powder is 0.5% or more and 10%
It is desirable that: The boron content of 10% or more hardly contributes to the improvement of the crystallinity of the carbon material powder as is apparent from the wide-angle X-ray diffraction pattern measurement, and only generates electrochemically inactive B ₄ C. In addition, the capacity per unit weight is reduced by the amount of B ₄ C generation, as well as not contributing to the capacity improvement at all. In addition, the boron-containing carbon material powder has a diffraction peak from the (101) plane of carbon and (10
The peak intensities I (C; 101) and I (C; 10) of each of the diffraction lines from the (0) plane
0) preferably satisfies I (C; 101) / I (C; 100) ≧ 2. At this time, the crystallinity of the carbon material powder is further improved, so that a capacity close to the theoretical capacity of graphite is easily exhibited.

As a method for forming a negative electrode using the negative electrode active material for a lithium secondary battery of the present invention, the performance of the negative electrode active material for a lithium secondary battery of the present invention is sufficiently obtained, and the shapeability is high. It is not limited to this as long as it is chemically and electrochemically stable. For example, there is a method in which a conductive agent such as carbon black, a powder of a fluororesin such as polytetrafluoroethylene or a dispersion solution is added to a silicon material powder, followed by mixing and kneading. There is also a method in which a conductive material such as carbon black and a resin powder such as polyethylene and polyvinyl alcohol are added to a silicon material powder, and then a dry mixture is inserted into a mold and molded by hot pressing. Further, a conductive agent such as carbon black, a fluororesin powder such as polyvinylidene fluoride (PVdF) or a water-soluble binder such as carboxymethyl cellulose as a binder is used as a binder for silicon material powder, and N-methylpyrrolidone (NMP) and dimethylformamide are used. Alternatively, a slurry can be prepared by mixing with a solvent such as water or alcohol, and applied to a current collector and dried to form a slurry.

In the embodiment in which the negative electrode active material for a lithium ion secondary battery of the present invention is a mixed powder of a carbon material powder and a boron-containing material powder, the negative electrode active material contains a coexisting carbon material powder or boron. In order to form a negative electrode in order for the carbon material powder to function sufficiently as a conductive material,
Further, it is not particularly necessary to add a conductive material such as carbon black.

The negative electrode active material of the present invention can be appropriately used in combination with a positive electrode active material and a non-aqueous electrolyte (for example, an organic solvent-based electrolyte). These non-aqueous electrolytes (for example, an organic solvent-based electrolyte) can be used. The positive electrode active material is not particularly limited as long as it can be generally used for a lithium secondary battery.

As the positive electrode active material, for example, lithium-containing
Transition metal oxide LiM (1)_xO _Two(Where x is 0 ≦
A numerical value in the range of x ≦ 1, where M (1) is a transition metal
And Co, Ni, Mn, Cr, Ti, V, Fe, Z
n, Al, In, Sn)
Or LiM (1)_yM (2)_2-yO_Four(Where y is 0 ≦
A numerical value in the range of y ≦ 1, where M (1), M (2)
Represents a transition metal, Co, Ni, Mn, Cr, Ti,
At least one of V, Fe, Zn, B, Al, In, and Sn
Types), transition metal chalcogenides (TiS_Two,
NbSe_Three, Etc.), vanadium oxide (V_TwoO_Five, V₆
O₁₃, V_TwoO_Four, V_ThreeO₈, Etc.) and its Li compound,
General formula M_xMo₆Ch_8-y(Where x is 0 ≦ x ≦ 4, y
Is a numerical value in the range of 0 ≦ y ≦ 1, where M is a transition metal
And other metals, Ch stands for chalcogen element)
Chevrel phase compound represented, or activated carbon, activated carbon
Fiber or the like can be used.

The organic solvent in the non-aqueous electrolyte (for example, an organic solvent-based electrolyte) is not particularly limited. For example, propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3-dioxolan, anisole , Diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-
Methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, sulfite, dimethyl sulfite, etc., alone or in combination of two or more Can be used.

What is conventionally known as the electrolyte
These can also be used, for example, LiClO_Four, Li
BF_Four, LiPF₆, LiAsF₆, LiB (C
₆H_Five), LiCl, LiBr, LiCF_ThreeSO_Three, L
iCH_ThreeSO_Three, Li (CF_ThreeSO _Two)_TwoN, Li (C
F_ThreeSO_Two)_ThreeC, Li (CF_ThreeCH_TwoOSO_Two)
_TwoN, Li (CF_ThreeCF_TwoCH_TwoOSO_Two)_TwoN, Li
(HCF_TwoCF_TwoCH_TwoOSO_Two)_TwoN, Li ((CF
_Three)_TwoCHOSO_Two)_TwoN, LiB [C₆H_Three(C
F_Three) _Two]_FourEtc. or a mixture of two or more
be able to.

Hereinafter, methods for expressing and measuring various physical values used for defining the negative electrode active material for a lithium secondary battery of the present invention will be described.

(1) Boron Amount Boron was determined by ICP (inductively coupled high frequency plasma spectroscopy).

(2) 50% cumulative diameter (d ₅₀ ) The particle size distribution is analyzed by calculating the diffraction pattern when the dispersed particles are irradiated with parallel rays (Franhofer diffraction), and the cumulative weight is 50%. The particle size at the point where it turned out was determined as a 50% cumulative diameter (d ₅₀ ). Usually, about 0.2 g of each sample was placed in 20 cc of water as a dispersion medium, and a drop of a commercially available surfactant was added thereto, and the particle size was measured by a particle size distribution analyzer LMS-24 manufactured by Seishin Enterprise Co., Ltd.

(3) Specific Surface Area The specific surface area was determined by analyzing using the BET method based on the adsorption curve for each partial pressure of nitrogen when nitrogen was adsorbed on the sample. Usually, it measured by BELSORP-36 made from Japan Bell Co., Ltd. using 1-2 g of each sample.

[0053] _{(4) I (SiB 4;} 021) / I (Si; 111) collimates the monochromatic X-ray parallel beam of Si is irradiated to the sample powder (111) plane, and, of SiB ₄ ( 021) Measure the peak corresponding to the plane. Peak intensity ratio I (SiB ₄ ; 021) / I (Si; 111) from each peak intensity excluding background
Was calculated.

(5) I (C; 101) / I (C; 100) A monochromatic X-ray is collimated into a parallel beam and irradiated to the sample powder, and the diffraction line from the (101) plane of carbon and the (100) ) Measure each peak of the diffraction line from the plane. From each peak intensity excluding the background, the peak intensity ratio I (C; 10
1) / I (C; 100) was calculated.

[0055]

The present invention will be specifically described below with reference to examples.

Example 1 1% by weight of boron powder (purity 99.9%) was added to silicon powder (purity 99.9%, d ₅₀ = 10 μm) and thoroughly mixed using a kneader. Was heated to 1400 ° C. in an argon stream, kept at this temperature for 5 hours, and cooled to room temperature in about 3 hours to obtain a silicon material powder for a negative electrode of a lithium secondary battery. The amount of boron contained in the silicon material powder thus prepared was 0.5% by weight. The obtained boron-containing silicon material powder was crushed by an impeller mill, and the particle size was adjusted using an air classifier to obtain a powder having a 50% cumulative diameter (d ₅₀ ) of 15 μm. The specific surface area of the powder was 5.1 m ² / g. As a result of X-ray diffraction measurement of this material, the peak of SiB ₄ was not observed, and the peak intensity ratio I (SiB ₄ ; 021) / I (Si; 111) became 0, and all boron in the material was dissolved in silicon. I was

With respect to 70% by weight of the boron-containing silicon material powder, 20% by weight of carbon black was used as a conductive agent.
1 polytetrafluoroethylene powder as binder
0% by weight was added and kneaded to form an electrode sheet having a thickness of about 0.1 mm, cut into 1 cm squares (about 21 mg in weight) (about 15 mg in terms of silicon material), and pressed on a Cu mesh as a current collector. Thus, a negative electrode was prepared.

In order to evaluate the monopolar electrode characteristics of the molded electrode, a three-electrode cell using lithium metal for the counter electrode and the reference electrode was used. In the electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 mixture by volume ratio)
Was used in which LiPF ₆ was dissolved at a rate of 1 mol / l. Regarding the charge / discharge test, both charging and discharging were performed at a constant current (0.5 mA / cm ² ) under regulation of potential. The potential range was 0 V to 2.0 V (based on lithium metal).
As a result of the evaluation under these conditions, the boron-containing silicon material powder has an initial charge capacity of 2500 mAh / g, an initial discharge capacity of 2000 mAh / g, and an initial capacity loss of 500 mA.
h / g, and the capacity loss was hardly observed after the second time. In addition, even in the second and subsequent charging and discharging, the discharge capacity hardly changes and shows excellent cycle characteristics.
It had very high electrode performance.

Example 2 A material was prepared under the same conditions as in Example 1 except that 10% by weight of boron was added. The amount of boron contained in the obtained material was 9.0% by weight. After crushing the obtained boron-containing silicon material powder by a jet mill,
By adjusting the particle size using an air classifier, a powder having a 50% cumulative diameter (d ₅₀ ) of 20 μm was obtained. The specific surface area of the powder was 3.5 m ² / g. As a result of X-ray diffraction measurement of this material, a very small peak corresponding to SiB ₄ was observed together with a peak corresponding to silicon, and the peak intensity ratio I (SiB ₄ ; 021) / I (Si; 111) was 0. 05, indicating that most of the boron in the material was in solid solution with silicon.

The electrode of the boron-containing silicon material powder thus prepared was evaluated under the same conditions as in Example 1. As a result, this boron-containing silicon material powder has an initial charge capacity of 2400 mAh / g and an initial discharge capacity of 2000 mAh / g.
g, the initial capacity loss was as small as 400 mAh / g, and almost no capacity loss was observed after the second time. Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 3 A material was prepared under the same conditions as in Example 1 except that 50% by weight of boron was added. The amount of boron contained in the obtained material was 48.9% by weight. The obtained boron-containing silicon material powder was crushed by a pin mill, and the particle size was adjusted using an air classifier to obtain a powder having a 50% cumulative diameter (d ₅₀ ) of 5 μm. The specific surface area of the powder was 25.3 m ² / g. As a result of X-ray diffraction measurement of this material, a peak corresponding to SiB ₄ was observed together with a peak corresponding to silicon, and a peak intensity ratio I (SiB ₄ ; 02
1) / I (Si; 111) was 0.88, indicating that a part of boron was dissolved in silicon and a part of SiB ₄ was formed.

The electrode of the thus prepared boron-containing silicon material powder was evaluated under the same conditions as in Example 1. As a result, the boron-containing silicon material powder had an initial charge capacity of 800 mAh / g, an initial discharge capacity of 500 mAh / g, an initial capacity loss of 300 mAh / g, and a small capacity loss after the second time. Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Comparative Example 1 Silicon powder (purity 99.9%) was used as a negative electrode active material for a lithium secondary battery. The 50% cumulative diameter (d ₅₀ ) of the powder is 1
0 μm, and the specific surface area was 8.7 m ² / g. This material was subjected to electrode evaluation under the same conditions as in Example 1. As a result, the initial discharge capacity of this negative electrode active material was 2000 mA.
h / g, but the initial charge capacity is 3300 mAh
/ G, the initial capacity loss was as large as 1300 mAh / g. With further repetition of charging and discharging, the capacity loss was as large as 800 mAh / g even in the second time, and then gradually decreased to 0 mAh / g at the seventh time. For this reason, the total capacity loss was very large. In addition, the discharge capacity rapidly decreased with the progress of the cycle and dropped to 200 mAh / g at the tenth time, and was not practical for lithium secondary batteries.

Comparative Example 2 Same as Example 1 except that 56% by weight of boron was added.
The materials were prepared under the same conditions. Boronation also obtained
To disintegrate silicon material powder with impeller mill
From the 50% cumulative diameter (d₅₀) Was obtained with a powder of 20 μm.
The specific surface area of the powder is 2.1 m^Two/ G. Obtained
The amount of boron contained in the mixed material is 54.5% by weight.
Was. X-ray diffraction measurement of this material showed that SiB_Four
And SiB ₆Peak corresponding to silicon and peak corresponding to silicon
The peak intensity ratio I (SiB_Four; 021) / I (Si; 111)
Is 0.96, and boron having a high concentration of boron
When a solid solution phase of silicon is formed,
Element is SiB_FourCoexist and produce even higher boron
Silicon boride (SiB)₆) Was produced in a small amount.

The electrode evaluation was performed on the boron-containing silicon material powder thus prepared under the same conditions as in Example 1. As a result, the initial charge capacity of the boron-containing silicon material powder is suppressed to a low level of 370 mAh / g, and the initial discharge capacity is 170 mAh / g because silicon boride (SiB ₆ ) having a higher concentration coexists in addition to SiB _4. It was very small. In addition, the charge / discharge was repeated, and the discharge capacity was further reduced after the second time, and became 140 mAh / g at the tenth time. Thus, the battery was not practical for a lithium secondary battery.

Example 4 A pitch coke powder obtained from coal tar pitch was pulverized using a ball mill and then sieved with 325 mesh.
I took out the bottom of the sieve. Add boron powder (purity 9
9.9%, 325 mesh under) with boron concentration of 2
The mixture obtained by adding wt% and mixing well was sealed in a cylindrical graphite crucible, and sealed with a lid with a screw. The temperature of the sealed crucible was raised to 2900 ° C. at a rate of about 12 ° C./min while flowing Ar gas at 10 liter / min by an electric furnace, and maintained at this temperature for 1 hour, followed by cooling to room temperature. The carbon material powder taken out of the cylindrical graphite crucible was pulverized and classified using an impeller mill to obtain a graphitized carbon material powder. From the X-ray diffraction pattern of this graphitized powder, the peak intensity I (C; 101) of the diffraction line from the (101) plane of carbon and the peak I (C; 100) of the diffraction line from the (100) plane of carbon were obtained. As a result of measuring the ratio I (C; 101) / I (C; 100), I (C; 101) / I (C; 100) = 2.6.
Further, the boron content was 1.2% by weight.

A mixture of the graphitized carbon material powder thus obtained and the boron-containing silicon material powder obtained in Example 2 (graphitized carbon material powder: boron-containing silicon material powder = 80: 20)
(Weight ratio)) 500 g and about 1 kg of ethanol were charged into a large beaker, stirred at room temperature for about 1 hour, and then suction-filtered using a filter paper.
Vacuum drying was performed for 4 hours to obtain a mixed powder.

50% of the mixed powder thus prepared
The cumulative diameter (d ₅₀ ) is 28 μm, and the specific surface area is 3.2 m ² /
g, peak intensity ratio I (SiB ₄ ; 021) / I determined by X-ray diffraction measurement
(Si; 111) was 0.05.

Using polyvinylidene fluoride (PVdF) as a binder for the mixed powder thus obtained, 1-
A coating solution was prepared using methylpyrrolidone (NMP) as a solvent, applied to a Cu sheet, and pressed to a pressure of about 0.1 μm.
An electrode sheet having a thickness of 1 mm was prepared and cut into 1 cm × 1 cm squares to prepare a negative electrode. Application density is about 1.3g / c
It was in m ^3.

In order to evaluate the electrode characteristics of a single electrode of the above-mentioned negative electrode, a three-electrode cell using lithium metal as a counter electrode and a reference electrode was used. In the electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 mixture by volume ratio)
Was used in which LiCl ₄ was dissolved at a rate of 1 mol / l. Regarding the charge / discharge test, both charging and discharging were performed at a constant current (0.5 mA / cm ² ) under regulation of potential. The potential range was 0 V to 2.0 V (based on lithium metal), and charging and discharging were repeated 100 times.

According to this electrode evaluation test, the maximum discharge capacity per gram of the mixed powder evaluated as the discharge capacity,
Charge / discharge efficiency in the first charge / discharge, so-called initial efficiency, ratio of discharge capacity per 100 g of mixed powder to maximum discharge capacity per 1 g of mixed powder (cycle characteristics)
Was measured. As a result, the maximum discharge capacity is 744 mA.
h / g, initial efficiency 90.5%, cycle characteristics 86%
And had a very high electrode capability.

[0072]

As is clear from the above description, the negative electrode active material for a lithium secondary battery of the present invention dramatically reduces the capacity loss while maintaining the high discharge capacity of silicon metal and oxide. Accordingly, a high energy density lithium secondary battery having excellent reversibility can be provided.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Taro Kono 2-6-3 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Steel Corporation (72) Inventor Tsutomu Sugiura 20-1 Shintomi, Futtsu-shi, Chiba Japan-made Inside the Technology Development Division, Steel Corporation

Claims (4)

[Claims] 1. A silicon material powder containing boron, wherein the silicon material powder has a boron content of 0.1 to 1.0.
50% by weight of a negative electrode active material for a lithium secondary battery. 2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the 50% cumulative diameter (d ₅₀ ) of the negative electrode active material is 1 to 100 μm. 3. The method according to claim 1, wherein the negative electrode active material has a peak intensity I (Si; 1) of a diffraction line from a (111) plane of Si in an X-ray wide angle diffraction method.
The ratio I (SiB ₄ ; 021) / I (Si; 111) of the peak intensity I (SiB ₄ ; 021) of the diffraction line from the (021) plane of SiB _{4 to} 11) is 1 or less. The negative electrode active material for a lithium secondary battery according to claim 1. 4. A lithium secondary battery containing a positive electrode active material, a negative electrode active material and a non-aqueous electrolyte, wherein at least the negative electrode active material according to claim 1 is used as the negative electrode active material. A rechargeable lithium battery.