Negative Electrode Active Material For Nonaqueous Electrolyte Secondary Batteries And Nonaqueous Electrolyte Secondary Battery Containing Negative Electrode Active Material

Abstract

In a nonaqueous electrolyte secondary battery in which SiO.sub.x is used as a negative electrode active material, initial charge/discharge efficiency and cycle characteristics are improved. Provided is a negative electrode active material, containing particles made of SiO.sub.x (0.5.ltoreq.X.ltoreq.1.5), for nonaqueous electrolyte secondary batteries. In the negative electrode active material, amorphous carbon is stuck on carbon coatings. The particles made of SiO.sub.x preferably have a size of 1 .mu.m to 15 .mu.m. Particles of amorphous carbon preferably have a size of 0.01 .mu.m to 1 .mu.m. One hundred percent of the surface of SiO.sub.x is preferably covered by the carbon coatings.

Description

TECHNICAL FIELD

[0001] The present invention relates to a negative electrode active material for nonaqueous electrolyte secondary batteries and a nonaqueous electrolyte secondary battery containing the negative electrode active material.

BACKGROUND ART

[0002] Silicon (Si) and a silicon oxide represented by SiO.sub.x have higher capacity per unit volume as compared to carbon materials such as graphite and therefore have been investigated for applications in negative electrode active materials. In particular, SiO.sub.x has a smaller volume expansion coefficient as compared to Si when SiO.sub.x stores Li during charge and therefore is expected to be quickly commercialized. For example, Patent Literature 1 discloses SiO.sub.x having a carbon coating formed on the surface.

CITATION LIST

Patent Literature

[0003] PTL 1: Japanese Published Unexamined Patent Application No. 2004-47404

SUMMARY OF INVENTION

Technical Problem

[0004] However, there is a problem in that a nonaqueous electrolyte secondary battery in which SiO.sub.x or the like is used as a negative electrode active material has poorer initial charge/discharge efficiency and a larger reduction in capacity in initial cycles as compared to the case where graphite is used as a negative electrode active material.

Solution to Problem

[0005] A major cause of the problem is that the change in volume of SiO.sub.x or the like during charge and discharge is larger than that of graphite. The large change in volume of an active material probably causes, for example, the reduction in electrical conductivity of an active material layer, leading to the deterioration of initial charge/discharge efficiency or the like.

[0006] In order to solve the problem, a negative electrode active material for nonaqueous electrolyte secondary batteries according to the present invention is a particulate negative electrode active material used in nonaqueous electrolyte secondary batteries. The negative electrode active material includes mother particles made of SiO.sub.x (0.5.ltoreq.X.ltoreq.1.5), carbon coating layers each covering at least one portion of the surface of a corresponding one of the mother particles, and amorphous carbon particles stuck on the carbon coating layers.

[0007] A nonaqueous electrolyte secondary battery according to the present invention includes a negative electrode containing the negative electrode active material, a positive electrode, a separator placed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.

Advantageous Effects of Invention

[0008] According to the present invention, in a nonaqueous electrolyte secondary battery in which SiO.sub.x is used as a negative electrode active material, cycle characteristics and initial charge/discharge efficiency can be improved.

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a sectional view of a negative electrode which is an example of an embodiment of the present invention.

[0010] FIG. 2 is a sectional view of a particle of a negative electrode active material which is an example of an embodiment of the present invention.

[0011] FIG. 3 is a first electron micrograph showing a cross section of a negative electrode active material particle used in Experiment 1.

[0012] FIG. 4 is a second electron micrograph showing a cross section of a negative electrode active material particle used in Experiment 1.

[0013] FIG. 5 is a graph showing results of the laser Raman spectroscopic analysis of negative electrode active material particles used in Experiment 1.

[0014] FIG. 6 is a graph showing results of the laser Raman spectroscopic analysis of negative electrode active material particles used in Experiment 4.

[0015] FIG. 7 is a graph showing results of the laser Raman spectroscopic analysis of carbonaceous matter prepared by heat-treating citric acid only.

[0016] FIG. 8 is a graph showing I.sub.V/I.sub.G values in FIGS. 5 to 7.

[0017] FIG. 9 is a third electron micrograph showing a cross section of a negative electrode active material particle used in Experiment 4.

DESCRIPTION OF EMBODIMENTS

[0018] Embodiments of the present invention are described below in detail.

[0019] In this specification, the term "approximately **" is intended to include completely the same things and those regarded as substantially the same, as described using the term "approximately the same" as an example. Drawings referred to in the description of the embodiments are those schematically drawn. Dimensional proportions of each component illustrated in the drawings may possibly be different from those of an actual one. Detailed dimensional proportions and the like should be judged in consideration of descriptions below.

[0020] A nonaqueous electrolyte secondary battery which is an example of an embodiment of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a nonaqueous electrolyte containing a nonaqueous solvent, and a separator. An example of the nonaqueous electrolyte secondary battery is a structure in which an electrode assembly formed by winding the positive electrode, the negative electrode, and the separator placed therebetween and the nonaqueous electrolyte are housed in an enclosure.

(Positive Electrode)

[0021] The positive electrode is preferably composed of a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode current collector used is, for example, a conductive thin film, particularly metal or alloy foil which is made of aluminium or the like and which is stable within the potential range of the positive electrode or a film including a metal surface layer made of aluminium or the like. The positive electrode active material layer preferably contains a conductive material and a conductive agent in addition to the positive electrode active material.

[0022] The positive electrode active material is not particularly limited and is preferably a lithium transition metal oxide. The lithium transition metal oxide may contain a non-transition metal such as Mg or Al. Examples of the lithium transition metal oxide include lithium cobaltate, olivine-type lithium phosphates typified by lithium iron phosphate, Ni--Co--Mn, Ni--Mn--Al, and Ni--Co--Al. The positive electrode active material may contain one or more of these compounds.

[0023] For the conductive material, carbon materials such as carbon black, acetylene black, Ketjenblack, and graphite and mixtures of two or more of the carbon materials can be used.

[0024] For the binding agent, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, and mixtures of two or more of these compounds can be used.

(Negative Electrode)

[0025] As exemplified in FIG. 1, the negative electrode 10 preferably includes a negative electrode current collector 11 and a negative electrode active material layer 12 placed on the negative electrode current collector 11. The negative electrode current collector 11 used is, for example, a conductive thin film, particularly metal or alloy foil which is made of copper or the like and which is stable within the potential range of the negative electrode or a film including a metal surface layer made of copper or the like. The negative electrode active material layer 12 preferably contains a binding agent (not shown) in addition to the negative electrode active material 13. The binding agent used may be polytetrafluoroethylene or the like as is the case with the positive electrode and is preferably styrene-butadiene rubber (SBR), polyimide, or the like. The binding agent may be used in combination with a thickening agent such as carboxymethylcellulose.

[0026] As exemplified in FIG. 2, the negative electrode active material 13 contains negative electrode active materials 13a including mother particles 14 made of SiO.sub.x (0.5.ltoreq.X.ltoreq.1.5), carbon coating layers 15 each covering at least one portion of the surface of a corresponding one of the mother particles 14, and amorphous carbon particles 16 stuck to the surfaces of the carbon coating layers 15. The negative electrode active material 13 may contain the negative electrode active materials 13a only and preferably contain negative electrode active materials 13b having a smaller change in volume during charge and discharge than that of the negative electrode active materials 13a in combination with the negative electrode active materials 13a from the viewpoint of achieving both high capacity and enhanced cycle characteristics. The negative electrode active materials 13b are not particularly limited and are preferably a carbonaceous active material such as graphite or hard carbon.

[0027] In the case of using the negative electrode active materials 13a and the negative electrode active materials 13b in combination, when the negative electrode active materials 13b are graphite, the mass ratio of the negative electrode active materials 13a to graphite preferably ranges from 1:99 to 20:80. When the mass ratio thereof is within the above range, both high capacity and enhanced cycle characteristics are likely to be achieved. However, when the percentage of the negative electrode active materials 13a with respect to the mass of the negative electrode active material 13 is less than 1% by mass, the merit of increasing the capacity by adding the negative electrode active materials 13a is small.

[0028] In the negative electrode active materials 13a (hereinafter referred to as the negative electrode active material particles 13a), the carbon coating layers 15 are placed on the surfaces of the mother particles 14, which are made of SiO.sub.x (0.5.ltoreq.X.ltoreq.1.5), and the amorphous carbon particles 16 are stuck to the surfaces of the carbon coating layers 15. SiO.sub.x has a structure in which Si is dispersed in an amorphous SiO.sub.2 matrix. The presence of dispersed Si can be confirmed by observation using a transmission electron microscope (TEM).

[0029] In the nonaqueous electrolyte secondary battery, which contains the negative electrode active material particles 13a, the carbon coating layers 15 on the surfaces of the mother particles 14 can improve a disadvantage of SiO.sub.x, which has low electronic conductivity, and the amorphous carbon particles 16, which are stuck to the surfaces of the carbon coating layers 15, improve the binding force between SiO.sub.x and a binder by an anchoring effect. When particles stuck to the surfaces of the carbon coating layers 15 are amorphous carbon particles, initial charge/discharge efficiency and cycle characteristics are particularly improved. The reason for this is as described below. In the case where highly crystalline carbon typified by graphite or fine metal particles are stuck to the surface of SiO.sub.x, a high-temperature treatment step, an electroless plating step, or the like is necessary. Treating SiO.sub.x at high temperature significantly reduces the charge/discharge capacity because of the disproportionation reaction of SiO.sub.x. In the case where the SiO.sub.x surface is electrolessly plated, irregularities are unlikely to be formed on the surfaces of particles and no sufficient anchoring effect is obtained.

[0030] The fact that the amorphous carbon 16 is stuck to the surfaces of the carbon coating layers 15 means that the amorphous carbon 16 is attached to the surfaces of the carbon coating layers 15 even in the case of mixing with a solvent or the like when the negative electrode is prepared. This is different from secondary aggregation.

[0031] The mother particles 14 preferably have an average size of 1 .mu.m to 15 .mu.m and more preferably 4 .mu.m to 10 .mu.m. In this specification, the term "average size" refers to the particle size (volume-average particle size: Dv.sub.50) at a cumulative volume percentage of 50% in the particle size distribution determined by a laser diffraction/scattering method. Dv.sub.50 can be measured using, for example, "LA-750" manufactured by HORIBA. When the average size of the mother particles 14 is too small, the surface area of the particles is too large and the amount of the mother particles 14 reacting with an electrolyte solution is large, hence, the capacity may possibly be reduced. However, when the average size thereof is too large, the influence of the volume expansion of SiO.sub.x during charge is large and therefore charge/discharge characteristics may possibly be reduced.

[0032] The amorphous carbon particles 16 preferably have an average size of 0.01 .mu.m to 1 .mu.m and more preferably 0.05 .mu.m to 0.8 .mu.m. When the average size of the amorphous carbon particles 16 is too small, surface irregularities of the carbon coating layers 15 on the mother particles 14 are small and therefore no sufficient anchoring effect is likely to be obtained. However, when the average size thereof is too large, the number of the amorphous carbon particles 16 stuck on the carbon coating layers 15 is limited and therefore no sufficient anchoring effect is likely to be obtained.

[0033] The amorphous carbon particles 16 are preferably more than 0% to 15% by mass with respect to the mother particles 14 and more preferably 2% to 8% by mass. When the amorphous carbon particles 16 are too few with respect to the mother particles 14, surface irregularities of the carbon coating layers 15 on the mother particles 14 are few and therefore no sufficient anchoring effect is likely to be obtained. However, when the amorphous carbon particles 16 are too many, the fraction of amorphous carbon in the active material is large and the capacity is likely to be reduced.

[0034] As a carbon material in the carbon coating layers 15, carbon black, acetylene black, Ketjenblack, graphite, and mixtures of two or more of these materials can be used as is the case with the conductive material in the positive electrode active material layer.

[0035] Each of the carbon coating layers 15 preferably covers 50% to 100% of the surface of a corresponding one of the mother particles 14 and more preferably 100%. In the present invention, the fact that the surfaces of the mother particles 14 are covered by the carbon coating layers 15 means that the surfaces of the mother particles 14 are covered by the carbon coating layers 15 that have a thickness of at least 1 nm in the case where a cross section of each particle is observed with a SEM.

[0036] The carbon coating layers 15 preferably have an average thickness of 1 nm to 200 nm and more preferably 5 nm to 100 nm in view of the ensuring of electrical conductivity and the diffusivity of Li.sup.+ into SiO.sub.x forming the mother particles 14 or the like. The coating layers 15 preferably have substantially a uniform thickness over the entire area thereof. The average thickness of the carbon coating layers 15 can be measured in such a manner that cross sections of the negative electrode active material particles 13a are observed using a scanning electron microscope (SEM), a transmission electron microscope (TEM), or the like. When the thickness of the coating layers 15 is too small, the electrical conductivity is reduced and it is difficult to uniformly cover the mother particles 14. However, when the thickness of the coating layers 15 is too large, the diffusivity of Li.sup.+ into the mother particles 14 is inhibited and the capacity is likely to be reduced. The percentage of the carbon coating layers with respect to SiO.sub.x is preferably 10% by mass or less.

[0037] The carbon coating layers 15 can be formed by, for example, a common process such as a CVD process, a sputtering process, or a plating (electroplating or electroless plating) process. For example, in the case where the coating layers 15 are formed on the surfaces of SiO.sub.x particles by the CVD process using the carbon material, for example, the SiO.sub.x particles and a hydrocarbon gas are heated in a vapor phase and carbon produced by the pyrolysis of the hydrocarbon gas is deposited on the SiO.sub.x particles. The hydrocarbon gas used may be a methane gas or an acetylene gas.

[0038] The negative electrode active materials 13a preferably have a BET specific surface area of 1 m.sup.2/g to 30 m.sup.2/g and more preferably 5 m.sup.2/g to 30 m.sup.2/g. When the BET specific surface area thereof is too small, no sufficient irregularities are formed on the SiO.sub.x particles and no sufficient anchoring effect is likely to be obtained. However, when the BET specific surface area thereof is too large, the amount of a binder attached to the surface of SiO.sub.x is too large, the dispersibility of the binder is reduced, and the adhesion of the negative electrode is likely to be reduced.

[0039] The amorphous carbon particles 16 can be stuck to the carbon coating layers 15 in such a manner that, for example, an aqueous solution containing an organic acid catalyst and the SiO.sub.x particles including the carbon coating layers are mixed together and are subjected to hydrolysis and a polymerization reaction at 80.degree. C. to 120.degree. C., water is evaporated, and heat treatment is then performed at 500.degree. C. to 800.degree. C. The aqueous solution containing the organic acid catalyst and the SiO.sub.x particles including the carbon coating layers may be mixed with a lithium compound. Examples of the organic acid catalyst include citric acid, malic acid, tartaric acid, lactic acid, and glycolic acid. Examples of the lithium compound include LiOH, Li.sub.2CO.sub.3, LiF, and LiCl.

[0040] SiO.sub.x forming the mother particles 14 may contain lithium silicate (such as Li.sub.4SiO.sub.4, Li.sub.2SiO.sub.3, Li.sub.2Si.sub.2O.sub.5, or Li.sub.8SiO.sub.6) in particles.

(Nonaqueous Electrolyte)

[0041] The following salts can be used as an electrolyte salt in the nonaqueous electrolyte: for example, LiClO.sub.4, LiBF.sub.4, LiPF.sub.6, LiAlCl.sub.4, LiSbF.sub.6, LiSCN, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiB.sub.10Cl.sub.10, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, chloroborane lithium, borates, imide salts, and the like. In particular, LiPF.sub.6 is preferably used from the viewpoint of ionic conductivity and electrochemical stability. Electrolyte salts may be used alone or in combination. In 1 L of the nonaqueous electrolyte, 0.8 mol to 1.5 mol of the electrolyte salt is preferably contained.

[0042] For example, a cyclic carbonate, a linear carbonate, a cyclic carboxylate, or the like is used as a solvent in the nonaqueous electrolyte. Examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC). Examples of the linear carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylate include 7-butyrolactone (GBL) and .gamma.-valerolactone (GVL). Nonaqueous solvents may be used alone or in combination.

(Separator)

[0043] The separator used is a porous sheet having ionic permeability and insulating properties. Examples of the porous sheet include microporous thin films, woven fabrics, and nonwoven fabrics. The separator is preferably made of a polyolefin such as polyethylene or polypropylene.

EXAMPLES

[0044] The present invention is further described below with reference to examples. The present invention is not limited to the examples.

Examples

Experiment 1

(Preparation of Negative Electrode)

[0045] SiO.sub.x (X=0.93, an average primary particle size of 5.0 .mu.m) surface-coated with carbon was prepared. Coating was performed by a CVD process. The percentage of carbon with respect to SiO.sub.x was 10% by mass. The carbon coverage of the surface of SiO.sub.x was 100%. The carbon coverage of the SiO.sub.x surface was confirmed by a method below. A cross section of each SiO.sub.x particle was exposed using an ion milling system (ex. IM4000) manufactured by Hitachi High-Technologies Corporation and was checked using a SEM and a backscattered electron image. The interface between a carbon coating layer and SiO.sub.x in the particle cross section was identified from the backscattered electron image. The percentage of carbon coatings, having a thickness of 1 nm or more, present on the surface of each SiO.sub.x particle was calculated from the ratio of the sum of the lengths of the interfaces between the carbon coatings having a thickness of 1 nm or more and SiO.sub.x to the perimeter of SiO.sub.x in the particle cross section. The average of the percentages of the carbon coatings on the surfaces of 30 of the SiO.sub.x particles was calculated as a carbon coverage.

[0046] To 1,000 g of water, 0.5 moles of Li.sub.2CO.sub.3 was added, followed by adding 0.2 moles of citric acid, whereby an aqueous solution in which Li.sub.2CO.sub.3 was completely dissolved was prepared. To the aqueous solution, 1 mole of the above SiO.sub.x was added, followed by mixing. The mixed solution was subjected to a dehydrocondensation reaction at 80.degree. C., followed by drying at 120.degree. C., whereby an intermediate was obtained. The intermediate was heat-treated at 600.degree. C. for 5 hours in an Ar atmosphere. The SiO.sub.x was washed with pure water. The BET specific surface area of the heat-treated and water-washed SiO.sub.x particles was measured using Tristar II 3020 (manufactured by Shimadzu Corporation), resulting in that the BET specific surface area was 20 m.sup.2/g. FIG. 3 shows a SEM image of the heat-treated and water-washed SiO.sub.x particles. In FIG. 3, it is observed that amorphous carbon is finely attached to a carbon coating layer 15 or the surface of a crystalline carbon particle.

[0047] The fact that amorphous carbon was stuck to the surface of SiO.sub.x surface-coated with carbon was confirmed by a method below. FIG. 4 shows a SEM image of the heat-treated and water-washed SiO.sub.x particles that were atomized and dispersed in a solvent using TK FILMIX (manufactured by PRIMIX Corporation). Since amorphous carbon was present on the surface of a carbon coating film after SiO.sub.x surface-coated with carbon was atomized and was dispersed, the amorphous carbon was judged to be stuck on the surface of the carbon coating film without being secondarily aggregated or simply attached.

[0048] The fact that carbon stuck on the SiO.sub.x surface-coated with carbon was amorphous carbon was confirmed by a method below. FIG. 5 shows the laser Raman spectroscopic analysis of carbonaceous matter (hereinafter referred to as a), measured using the Raman spectrometer ARAMIS (manufactured by Shimadzu Corporation), present on the surfaces of the heat-treated and water-washed SiO.sub.x particles. A Raman spectrum supposed to be a mixture of two or more types was observed. For spectrum interpretation, FIG. 6 shows observation results of carbonaceous matter (hereinafter referred to as .beta.) on the surfaces of untreated SiO.sub.x particles and FIG. 7 shows observation results of carbonaceous matter (hereinafter referred to as .gamma.) prepared by heat-treating citric acid only. The untreated SiO.sub.x particles used were a material used in Experiment 4 below. From the ratio R (=I.sub.D/I.sub.G) of the intensity I.sub.D of a D band (a peak appearing at 1,360 cm.sup.-1) used to evaluate carbon materials to the intensity I of a G band (a peak appearing at 1,600 cm.sup.-1), it can be confirmed that .beta. is carbonaceous matter with high crystallinity and .gamma. is carbonaceous matter, such as soot, having low crystallinity.

[0049] Next, for FIGS. 5 to 7, the intensity of a saddle portion (minimum) between the G band and the D band was defined as I.sub.V, I.sub.V/I.sub.G values were compared, and spectrum interpretation was performed for a. Smoothing was appropriately performed. A base line was linearly approximated at 800 cm.sup.-1 to 1,900 cm.sup.-1. FIG. 8 graphically shows the I.sub.V/I.sub.G values. As is clear from FIG. 8, a is a mixed component of .beta. and .gamma.. Thus, it can be confirmed that carbon stuck on SiO.sub.x surface-coated with carbon is amorphous carbon with low crystallinity.

[0050] SiO.sub.x and PAN (polyacrylonitrile) serving as a binder were mixed at a mass ratio of 95:5, followed by adding NMP (N-methyl-2-pyrrolidone) serving as a dilution solvent. This was stirred using a mixer (ROBOMIX manufactured by PRIMIX Corporation), whereby negative electrode mix slurry was prepared. The negative electrode mix slurry was applied to a surface of copper foil such that the mass per 1 m.sup.2 of a negative electrode mix layer was 25 g/m.sup.2. Next, this was dried at 105.degree. C. in air and was rolled, whereby a negative electrode was prepared. The packing density of the negative electrode mix layer was 1.50 g/ml.

(Preparation of Nonaqueous Electrolyte Solution)

[0051] To a solvent mixture prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3:7, 1.0 mole per liter of lithium hexafluorophosphate (LiPF.sub.6) was added, whereby a nonaqueous electrolyte solution was prepared.

(Assembly of Battery)

[0052] An electrode assembly was prepared in an inert atmosphere using the negative electrode equipped with a Ni tab at the outer periphery thereof, lithium metal foil, and a polyethylene separator placed between the negative electrode and the lithium metal foil. The electrode assembly was put in a battery enclosure composed of an aluminium laminate. Furthermore, the nonaqueous electrolyte solution was poured into the battery enclosure. Thereafter, the battery enclosure was sealed, whereby Battery A1 was prepared.

Experiment 2

[0053] Battery A2 was prepared in substantially the same manner as that described in Experiment 1 except that the amount of added citric acid was 0.18 moles. The BET specific surface area of heat-treated and water-washed SiO.sub.x particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 15 m.sup.2/g.

Experiment 3

[0054] Battery A2 was prepared in substantially the same manner as that described in Experiment 1 except that the amount of added citric acid was 0.25 moles. The BET specific surface area of heat-treated and water-washed SiO.sub.x particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 30 m.sup.2/g.

Experiment 4

[0055] Battery Z was prepared in substantially the same manner as that described in Experiment 1 except that untreated SiO.sub.x was used as a negative electrode active material (that is, SiO.sub.x having no amorphous carbon particles on a carbon coating layer). The BET specific surface area of SiO.sub.x particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 5 m.sup.2/g. FIG. 9 shows a cross-sectional SEM image of the SiO.sub.x particles. In FIG. 9, small particulates are carbon particles with high crystallinity and are those remaining without forming a layer when carbon coating layers 15 were formed.

Experiments

[0056] The above batteries were charged and discharged under conditions below, followed by investigating the initial charge/discharge efficiency given by Equation (1) below and the tenth-cycle capacity retention given by Equation (2) below. The results are shown in Table 1.

(Charge and Discharge Conditions)

[0057] After constant-current charge was performed at a current of 0.2 lt (4 mA) until the voltage reached 0 V, constant-current charge was performed at a current of 0.05 lt (1 mA) until the voltage reached 0 V. Next, after a rest was taken for 10 minutes, constant-current discharge was performed at a current of 0.2 lt (4 mA) until the voltage reached 1.0 V.

(Equation for Calculating Initial Charge/Discharge Efficiency)

[0058] Initial charge/discharge efficiency (%)=(first-cycle discharge capacity/first-cycle charge capacity).times.100 (1)

(Equation for Calculating Tenth-Cycle Capacity Retention)

[0059] Tenth-cycle capacity retention (%)=(tenth-cycle discharge capacity/first-cycle discharge capacity).times.100 (2)

TABLE-US-00001 TABLE 1 Amorphous Amount carbon BET Initial of particles specific charge/ citric on carbon surface discharge acid coating area efficiency Capacity Batteries (mol) layers (m.sup.2/g) (%) retention A1 0.2 Observed 20 74 52 A2 0.18 Observed 15 89 21 A3 0.25 Observed 30 71 Not measured Z -- Not observed 5 67 7

[0060] In Battery Z, in which no amorphous carbon particles are placed on carbon coatings on the surfaces of SiO particles, it is conceivable that no sufficient anchoring effect is obtained between active material particles and a binder and the adhesion between active materials is reduced.

[0061] However, in Batteries A1 to A3, since carbon coatings are placed on the surfaces of SiO.sub.R particles and amorphous carbon particles are stuck on the carbon coatings, it is conceivable that the surfaces of the particles have irregularities sufficient to obtain an anchoring effect between active material particles and a binder and the adhesion between active materials is improved.

REFERENCE SIGNS LIST

[0062] 10 Negative electrode [0063] 11 Negative electrode current collector [0064] 12 Negative electrode active material layer [0065] 13, 13a, 13b Negative electrode active material [0066] 14 Mother particles [0067] 15 Carbon coating layers [0068] 16 Amorphous carbon particles

Claims

1. A negative electrode active material for nonaqueous electrolyte secondary batteries being a particulate negative electrode active material used in nonaqueous electrolyte secondary batteries, the negative electrode active material comprising: mother particles made of SiO.sub.x (0.5.ltoreq.X.ltoreq.1.5); carbon coating layers each covering at least one portion of the surface of a corresponding one of the mother particles; and amorphous carbon particles stuck on the carbon coating layers.

2. The negative electrode active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein the mother particles have an average size of 1 .mu.m to 15 .mu.m and the amorphous carbon particles have an average size of 0.01 .mu.m to 1 .mu.m.

3. The negative electrode active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein 100% of the surface of each of the mother particles is covered by a corresponding one of the carbon coating layers.

4. A nonaqueous electrolyte secondary battery comprising: a negative electrode containing the negative electrode active material according to claim 1; a positive electrode containing a positive electrode active material; a separator placed between the positive electrode and the negative electrode; and a nonaqueous electrolyte.