JP2007335418A - Anode material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode material for a lithium ion secondary battery which has a high discharging capacity and is excellent in cycle characteristics. <P>SOLUTION: The anode material is composed of only an active phase of a plurality of sorts of intermetallic compounds which can occlude/discharge lithium reversibly, or is composed of a non-active phase not occluding lithium additionally. A difference in discharging capacities per volume of quantitatively biggest two sorts of active phases in volume is in a range of more than two times and less than 20 times and moreover the active phase having a bigger discharging capacity occupies less than 80 vol.%. The active phases of quantitatively biggest 2 sorts are preferably intermetallic compounds belonging to the same crystal group and they occupy more preferably 50 vol.% or more of the anode material. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery capable of reversibly occluding and releasing an alkali metal such as Li, and more specifically, a non-aqueous electrolyte secondary battery having a high discharge capacity and excellent cycle characteristics. The present invention relates to a negative electrode material for a battery.

  A typical example of the non-aqueous electrolyte secondary battery is a lithium secondary battery, particularly a lithium ion secondary battery. The negative electrode material of the present invention is suitable for the negative electrode material, but other non-aqueous electrolytes that are expected to be developed in the future. It can also be used as a negative electrode material for electrolyte secondary batteries. In the following, the present invention will be described with respect to a lithium ion secondary battery.

  With the widespread use of portable small electric and electronic devices, high energy density has also been demanded for small secondary batteries that serve as power sources. In particular, for lithium ion secondary batteries having a battery voltage about three times that of Ni-hydrogen batteries, various studies have been made to increase the capacity of positive and negative electrode materials in order to further extend the feature of high energy density. .

  As a negative electrode material of a lithium ion secondary battery, a carbon material, particularly a highly crystalline carbon material, that is, graphite is mainly used. Many of the highly crystalline carbon materials have a capacity per weight of about 330 to 350 mAh / g, and the capacity per volume multiplied by the specific gravity is about 660 to 750 mAh / cc.

  Improvements to this carbon material include attempts to achieve high capacity by using low crystalline carbon such as non-graphitizable carbon, or by attaching or supporting a low crystalline carbon material on the surface of high crystalline carbon. Attempts have been made to achieve high capacity. However, since the specific gravity of each carbon material is small, even if the capacity per unit weight shows a high capacity, the capacity per unit volume becomes small, and it is not satisfactory as an electrode material aiming at high capacity.

In order to solve this, various negative electrode materials that have a large specific gravity and can be expected to have a high volume energy density when a compound is formed with Li have been proposed.
For example, the crystal structure shown in Patent Document 1 belongs to any of CaF ₂ type, ZnS type, and AlLiSi type, and the material composed of a mixed phase of cubic and orthorhombic crystals shown in Patent Document 2 is exemplified. . These are negative electrode materials that have a very large Li intrusion site, can be expected to have a large discharge capacity, and can have a high volumetric energy density because of their high specific gravity.

  Further, metal nitrides such as Ni and Fe have been proposed as negative electrode materials (see Patent Document 3, Patent Document 4, and Patent Document 1). Furthermore, the development of negative electrode materials for lithium ion secondary batteries, such as those using metal oxides such as Sn and those using metal nitrides such as Co and Mn, has been extensively performed.

  As a result of these various developments, negative electrode materials with very large discharge capacities have emerged. However, since the expansion and contraction of the material during charging / discharging is large, the cycle characteristics are remarkably inferior to those of the carbon material, and it is necessary to improve the material of the secondary battery that is forced to be repeatedly used.

In order to improve this point, in Patent Document 5 and Patent Document 6, stress change caused by expansion / contraction of the negative electrode material during charge / discharge is achieved by making the intermetallic compound as an active material amorphous or low-crystallized. Attempts have been made to minimize the effects of this, but the effect is still not sufficient and there remains room for improvement.
JP-A-9-63651 JP 2001-118575 A JP-A-5-159780 JP-A-7-240201 Japanese Patent Laid-Open No. 10-223221 JP-A-11-102699

  Accordingly, an object of the present invention is to provide a negative electrode material for a nonaqueous electrolyte secondary battery having a high discharge capacity and excellent cycle characteristics. A more specific object is to improve the cycle characteristics of a negative electrode material having a high discharge capacity using an intermetallic compound capable of reversibly inserting and extracting Li.

  Most negative electrode materials having a high discharge capacity using an intermetallic compound capable of reversibly inserting and extracting Li have a problem in cycle characteristics. The cause is considered to be that the negative electrode material is repeatedly expanded and contracted repeatedly during charge and discharge, and therefore causes particle destruction. The fragment of the negative electrode material that has caused the particle destruction loses the electron conductivity from the electrode plate and becomes electrically separated, and the fragment is not charged from the next charge, and the charge capacity decreases. It is considered that the cycle characteristics deteriorate due to this repetition, and to improve the cycle characteristics, it is required to suppress the particle destruction of the negative electrode material.

  It is inevitable that the intermetallic compound occupies Li and expands. Therefore, it is inevitable that each crystal grain of the negative electrode material having a large discharge capacity expands greatly. For example, the volume expansion of about 3.2 times for Si and about 2.6 times for Sn can be estimated at least, and the stress experienced by the intermetallic compound during Li storage can be estimated to be several tons. Although the expansion ratio varies depending on the metal element used, it is clear that a negative electrode material having a large discharge capacity repeats a large expansion and contraction and is likely to generate cracks.

  Considering the case where a single intermetallic compound occludes Li, the crystal grain boundary existing in the compound is subjected to expansion and contraction stress. In the case of an intermetallic compound having a large discharge capacity, this grain boundary It is presumed that cracks are likely to occur at the interface.

  Many of negative electrode materials for lithium ion secondary batteries that have been considered so far are composed of two types of active phases that occlude Li and inactive phases that do not occlude Li. The inert phase is present to suppress the volume change of the active phase from the surroundings. Even in this case, it is considered that a large stress is applied to the interface where the active phase and the inactive phase are in contact with each other, and the fracture proceeds, and there is a limit to the effect of improving the cycle characteristics simply by the presence of the inactive phase. is there.

In order to solve this problem, as a result of the intensive studies by the present inventors, as if it is a gradient material,
(1) “Active phase with large discharge capacity capable of inserting and extracting Li” + “Active phase capable of inserting and extracting Li but small discharge capacity”, or (2) “Discharge capacity capable of storing and releasing large amount of Li "Large active phase" + "Active phase that can occlude / release Li but has a small discharge capacity" + "Inactive phase that cannot occlude Li",
It was found that particle breakage can be greatly reduced by the combination.

  In addition, when the crystal structures (crystal systems) of the intermetallic compounds constituting the above two types of active phases are the same, the grain boundary bondability is good with respect to the expansion and contraction of the alloy due to Li occlusion and release, and particle breakage occurs. I also found it difficult. Furthermore, it has also been found that this effect is very large when the particle size of these active phase crystals is small.

  The present invention based on the above findings comprises a mixture of two or more intermetallic compounds, at least two of which are intermetallic compounds forming an active phase capable of reversibly occluding and releasing Li, and these active phases The difference in discharge capacity per volume between the top two types of intermetallic compounds in volume among the intermetallic compounds forming the above is in the range of not less than 2 times and not more than 20 times, and the top two types of the above quantitatively Among the intermetallic compounds, the intermetallic compound having a larger discharge capacity accounts for 80% by volume or less of the negative electrode material. This is a negative electrode material for a non-aqueous electrolyte secondary battery.

  Preferably, at least two of the intermetallic compounds forming the active phase belong to the same crystal system. In that case, it is more preferable that at least two kinds of active phases composed of intermetallic compounds belonging to the same crystal system occupy 50% by volume or more of the negative electrode material.

  According to the present invention, a high performance lithium ion secondary battery that exhibits cycle characteristics equivalent to those of graphite conventionally used as a negative electrode material for lithium ion secondary batteries and has a discharge capacity per volume that is more than twice that of graphite. A negative electrode material can be provided. Therefore, the present invention contributes to downsizing or high energy density of the lithium ion secondary battery.

  The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery, particularly a lithium ion secondary battery, having a large discharge capacity and excellent cycle characteristics. Hereinafter, unless otherwise specified,% indicates mass%.

  The negative electrode material of the present invention comprises a mixture of two or more intermetallic compounds, at least two of which are capable of reversibly occluding and releasing Li (Li occluding property) and forming an active phase. It is. That is, the negative electrode material of the present invention may be composed of only two or more kinds of Li-occluding intermetallic compounds (active phase), or these two or more kinds of Li-occluding intermetallic compounds (active phase). In addition to the above, an intermetallic compound (inactive phase) that does not substantially occlude one or more kinds of Li may be used.

Examples of the Li-occluding intermetallic compound that forms the active phase include, but are not limited to, the following for each crystal system.
Cubic crystal: AlP, InP, InSb, Mg ₂ Si, FeS ₂ ;
Tetragonal: In ₂ S ₃ , In ₃ Sn, MnSn ₂ , FeSn ₂ , Al ₁₁ Mn ₁₄ , Ni ₃ Sn ₄ ;
Orthorhombic: PdSn, Mn ₃ C, Fe ₃ C, PdSn ₄ , Ag ₃ Sn;
Hexagonal crystals: Cu ₃ P, Cu ₆ Sn ₅ , AuSn, Mn ₃ Sn, Mn ₂ Sn, Al ₁₀ Mn ₃ , Al ₂₃ V ₄ , InSn ₄ , Co ₂ Sn ₂ ;
_{Trigonal: TiS 2, ZrS 2, Cr} 2 S 3.

Examples of the inert intermetallic compound that does not substantially occlude Li are not limited to these, but include the following.
Cubic crystal: CoSi, SiC, TiCo ₂
Tetragonal crystal: Ti ₃ Si, Al ₂ Cu
Orthorhombic: Cu ₃ Sn, Cu ₄ Ti, Pd ₃ Si
Hexagonal crystals: Ti ₆ Sn ₅ , Fe ₂ Ti, Mn ₃ Sn
Trigonal: W ₂ C, Ni ₂ Al ₃
As exemplified above, the intermetallic compound in the present invention includes sulfides, phosphides, carbides, and silicides.

  According to the present invention, the two Li-occluding intermetallic compounds forming the active phase are combined such that the difference in discharge capacity per volume is not less than 2 and not more than 20 times. When there are three or more types of intermetallic compounds in the active phase, the difference in discharge capacity between the upper two types (ie, the maximum amount and the next largest amount) of intermetallic compounds in terms of volume ratio is as described above. If it becomes. However, the amount of discharge capacity of other active phase intermetallic compounds that are smaller in quantity is also more than double or 20 times as described above, compared with the quantity of at least one of the top two intermetallic compounds. It is preferable to be within.

  If the difference between the discharge capacities of the different active phases is less than twice, the difference between expansion and contraction is too small, and there is no significant difference from the case of a single intermetallic compound. Stress relaxation due to expansion and contraction during charge and discharge is eliminated. It is difficult to do. On the other hand, in the combination of the intermetallic compounds having a difference in discharge capacity larger than 20 times, the expansion rate is greatly different, so that the particle breakage easily occurs, and as a result, the cycle characteristics are deteriorated.

  Since it is desirable to use a combination of “phase with large discharge capacity” + “phase with small discharge capacity” like a gradient material, the difference in discharge capacity between active phases is preferably 3 to 8 times, more preferably 3 to 3. 4 times.

  When an inert phase that does not substantially occlude Li is introduced in addition to the two or more active phases described above, particle breakage can be further suppressed and cycle characteristics are improved. At this time, the capacity difference between the inactive phase and the active phase having a small discharge capacity is preferably small. Since the discharge capacity of the inactive phase is substantially 0, the discharge capacity of the active phase having the smaller discharge capacity is preferably 1500 mAh / cc or less so that the capacity difference from the inactive phase becomes small.

  The volume ratio between the “phase with a large discharge capacity” and the “phase with a small discharge capacity” is not particularly limited, but the ratio of the “phase with a large discharge capacity” is 80% by volume or less of the whole negative electrode material. When three or more kinds of active phase intermetallic compounds are present, the intermetallic compound having the larger discharge capacity among the upper two kinds of intermetallic compounds in the same manner as described above is referred to as a “phase with a large discharge capacity”. And the ratio may be 80% by volume or less. In addition to the first two kinds of first and second active phases quantitatively, if there is one or more third active phases having a higher capacity, the capacity of the first and second is larger. The total amount of the active phase and the third active phase is preferably 80% by volume. When the proportion of the phase having a large discharge capacity exceeds 80% by volume, the cycle characteristics are not sufficiently improved. The ratio of the phase having a large discharge capacity is preferably 70% by volume or less.

  When the difference between the discharge capacities of the respective phases is large, the effect of improving the cycle characteristics is enhanced by increasing the number of phases having a small discharge capacity. However, the discharge capacity of the negative electrode material decreases as the number of phases with a small discharge capacity increases. In that sense, a phase with a large discharge capacity (when there are three or more active phases, the total of the phase having a higher discharge capacity and the phase having a higher capacity than this phase among the upper two active phases quantitatively) The proportion is preferably 30% by volume or more, more preferably 40% by volume or more of the entire negative electrode material.

  According to a preferred embodiment of the present invention, it is preferable that at least two of the Li-occluding intermetallic compounds forming the active phase have a crystal structure belonging to the same crystal system. More preferably, the crystal structures of the upper two types of active phase intermetallic compounds quantitatively by volume belong to the same crystal system. Thereby, particle destruction is more effectively suppressed and the effect of improving cycle characteristics is enhanced.

  It is considered that when the compounds are of the same crystal system, the bondability of the crystal grain boundary is high and the strain existing at the grain boundary is small. Therefore, it is easy to relieve stress due to volume expansion that occurs during charge and discharge, and particle breakage during charge and discharge is reduced. It is preferable that two or more active phases belonging to the same crystal system exist in an amount of 50% by volume or more in the whole material. When this volume ratio is less than 50%, the stress relaxation for preventing particle breakage is insufficient, and the effect of improving the cycle characteristics is lowered.

  When there is an inactive phase intermetallic compound that does not substantially occlude Li, if the inactive phase belongs to the same crystal system as the two or more active phases, the effect of improving cycle characteristics by stress relaxation can be obtained. Further increase.

  The active phase composed of two or more intermetallic compounds having Li storage properties preferably has an average crystal grain size of 10 μm or less. As a result, the effect of relaxing the generated stress due to expansion / contraction is further increased. However, it is not necessary to be amorphous or low crystal as proposed in Patent Document 5 (half-width of the strongest peak measured by X-ray diffraction is 0.6 ° or more). A preferable range of the average crystal grain size is in the range of 50 nm to 1 μm.

  The crystal grain size of the active phase of the negative electrode material can be measured by cross-sectional observation using an SEM (scanning electron microscope) or the like together with EDX (energy dispersive characteristic X-ray). Further, the detailed structure of the material can be determined by TEM (transmission electron microscope), and the main generated phase can be determined by X-ray diffraction. The structure of the negative electrode material is determined by the combination of the metal composition and the manufacturing method.

  The method for producing the negative electrode material is not limited. Preferably, a method capable of producing a material having a fine crystal grain size is employed. Examples of such methods include rapid solidification methods such as atomization, roll quenching, and rotating electrode methods, casting methods with increased cooling rates (eg, thin slab casting methods), and solid phase reactions. MA (mechanical alloying) method and MG (mechanical grinding) method.

The melting at this time can be performed by an appropriate method such as arc melting, plasma melting, high-frequency induction heating, or resistance heating in an inert gas or vacuum. Rapid solidification methods with a cooling rate of 100 ° C./sec or more include, for example, gas atomization method, oil atomization method, water atomization method, rotating electrode method, twin roll quenching method, single roll quenching method, casting on a rotating drum, water cooling, etc. It can be carried out by appropriate means such as casting into a mold that brings about the effect of rapid solidification. A method capable of realizing a cooling rate of 100 ° C./sec or more is preferable, but a more preferable cooling rate is 1 × 10 ³ ° C./sec or more.

  The negative electrode material is prepared by mixing powders of constituent elements of the negative electrode material in advance, mechanically mixing, pulverizing, and granulating the mixed powder, and alloying by solid phase reaction. It can also be produced by the MA method to be formed or the MG method in which two or more intermetallic compounds are prepared in advance and the mixture thereof is mechanically mixed, ground and granulated. Examples of devices that can be used in these mechanical methods include a rotary ball mill, a planetary ball mill, a vibration ball mill, and an attritor.

  In the MA method, since alloying is performed in a powder state, a very long processing time is required to obtain a homogeneous alloy structure. In the case of a device with low applied energy, such as a rotary ball mill, generally, the processing time of several days is required, and the productivity is low, so the cost for mass production increases. Therefore, when using the MA method, it is desirable to use a high energy type mixer. The MG method using an intermetallic compound from the beginning is more efficient than the MA method, but it is preferable to use a high energy type mixer even in the MG method because higher productivity can be obtained.

  A negative electrode material produced by a casting method, a rapid solidification method, an MA method, an MG method, or the like can be heat-treated as necessary. In particular, in the case of a material produced by rapid solidification, the quenching is partially effective, and it may be better to apply a weak heat treatment in the sense of homogenizing the material during mass production.

The heat treatment temperature is not particularly limited, but when the rapid solidification method is an atomization method, the temperature is lower than the solidus of each of the two or more active phases present in the material in order to prevent aggregation of the generated powder. It is preferable that When the heat treatment is performed at a temperature higher than this temperature, it is gradually cooled, and the effect of refining the structure obtained by the rapid solidification method, the MA method, the MG method or the like is lost. Thereby, the coarsening of the active phase occurs, and the average particle size may exceed 10 μm. The solidus temperature of the active phase in the negative electrode material can be easily obtained by using a thermal analyzer such as DTA. The heat treatment atmosphere is preferably an inert atmosphere such as Ar or a vacuum of 1.0 × 10 ⁻³ Torr or less.

  Since the negative electrode material for a non-aqueous electrolyte secondary battery is generally used in a powder state, the negative electrode material obtained by the above method is pulverized as necessary, classified in some cases to adjust the particle size, and the negative electrode material in a powder state Get.

  If the powdered negative electrode material produced as described above is subjected to a surface treatment that promotes improvement of electron conductivity, cycle characteristics can be further improved. For example, removal of the surface film by pickling and adhesion of a conductivity imparting material (eg, powder of carbon, Cu, Ni, etc.) by a physical method such as mechanofusion is considered effective.

Manufacture of the negative electrode for lithium ion secondary batteries from the negative electrode material of this invention can be performed by a method well-known to those skilled in the art.
For example, a binder selected from PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), PTFE (polytetrafluoroethylene), SBR (styrene butadiene rubber), etc. is mixed with the powder of the negative electrode material of the present invention, and more fully Usually, carbon material powder selected from natural graphite, artificial graphite, acetylene black and the like is mixed in order to impart conductivity. To this, a solvent selected from NMP (N-methylpyrrolidone), DMF (dimethylformamide), water, etc. is added to dissolve the binder, and if necessary, it is sufficiently stirred using a homogenizer and glass beads to form a slurry. To do. The slurry can be applied to an active material support such as rolled copper foil or electrodeposited copper foil, dried, and then pressed to produce a negative electrode.

  The weight ratio of the binder to be mixed is preferably about 5 to 10% from the viewpoint of mechanical strength of the negative electrode and battery characteristics. The support is not limited to copper foil, but may be a thin foil of other metal such as stainless steel or nickel, a net-like sheet punching plate, or the like.

  In the production of such a negative electrode, the particle size of the powder of the negative electrode material affects the electrode thickness and electrode density, and thus the electrode capacity. The thinner the electrode, the better, and the total area of the battery active material contained in the battery can be increased. For this reason, the negative electrode material powder preferably has an average particle size of 100 μm or less. The finer the powder, the greater the reaction area and the better the rate characteristics. It has an adverse effect on efficiency. In consideration of these, the average particle size of the preferable powder is 5 to 100 μm, more preferably 10 to 50 μm.

  The lithium ion secondary battery incorporating the negative electrode thus produced is not particularly limited as long as it includes a negative electrode, a positive electrode, a separator, an electrolytic solution or an electrolyte as a basic structure. The shape may be a cylindrical shape, a square shape, a coin shape, a sheet shape, or the like. Further, it can be applied to a battery using a solid electrolyte such as a polymer battery.

In the lithium ion secondary battery using the negative electrode material of the present invention, the positive electrode preferably uses a Li-containing transition metal compound as an active material. Examples of Li-containing transition metal compounds are LiM _1-x M ′ _x O ₂ or LiM _2y M′O ₄ (where 0 ≦ x, y ≦ 1, M and M ′ are Ba, Co, Ni, Mn, respectively) , Cr, Ti, V, Fe, Zn, Al, In, Sn, Sc, and Y).

  However, transition metal chalcogenides; vanadium oxide and its Li compound; niobium oxide and its Li compound; conjugated polymer using organic conductive material; Sheprel phase compound; activated carbon, activated carbon fiber, etc. It is also possible to use it.

The electrolyte solution of a lithium ion secondary battery is generally a non-aqueous electrolyte solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. Examples of lithium salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiB (C ₆ H ₅ ), LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , Li (CF ₂ SO ₂ ) ₂ , LiCl, LiBr, LiI can be mentioned, and one or more can be used. As the organic solvent, carbonates such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate are preferable. However, various other organic solvents including carboxylic acid esters and ethers can also be used. The organic solvent can use 1 type (s) or 2 or more types.

  The separator not only plays a role as an insulator placed between the positive electrode and the negative electrode, but also greatly contributes to the retention of the electrolyte. Usually, a porous body such as polypropylene, polyethylene, a mixed cloth of both, or a glass filter is generally used.

  The discharge capacity and cycle characteristics of the negative electrode materials in the following Reference Examples and Examples were determined by the following methods.

  After 10% of acetylene black as a conductivity-imparting agent and PVDF as a binder were added to the negative electrode material powder (average particle size 20 μm), NMP as a solvent was added to form a slurry. This slurry was applied to a copper foil with a doctor blade and dried. This was punched out to a diameter of 13 mm and pressed to produce a negative electrode.

The performance of the negative electrode was evaluated by a tripolar cell using metal Li for the positive electrode and the reference electrode. And charging to 0V with a constant current of 0.5mA / cm ^2, 0.5mA / cm repeated discharge until 2V vs Li / Li ⁺ at a ^second constant current, the first cycle and 20 th cycle discharge capacity of the negative electrode Was measured. From the measured negative electrode capacity, the discharge capacity per volume of the negative electrode material alone of the present invention was calculated in consideration of the capacity of acetylene black and the amount of binder. The discharge capacity at the first cycle thus determined was recorded as the discharge capacity, and the cycle characteristics were determined as discharge capacity at the 20th cycle / discharge capacity at the first cycle × 100 (%). As for the cycle characteristics, 80% or more was regarded as acceptable.

(Reference Example 1)
First, the capacity of each intermetallic compound was measured.
Cu ₆ Sn ₅ , Co ₃ Sn ₂ , FeSn ₂ , Ni ₃ Sn ₄ , MnSn ₂ , AuSn, and Al ₂₃ V ₄ were casted from a melted raw material by a roll quenching method and pulverized to a powder obtained in 500% in Ar. After homogenizing by performing a heat treatment of ° C. × 24 hr, one that was confirmed to be a single phase by XRD (X-ray diffraction method) was used.

_InP, Cu 3 P, AlP, the _Mg 2 Si was used as the purchased reagent.
Table 1 summarizes the discharge capacity and cycle characteristics of each of these intermetallic compounds.

As shown in Table 1, the values of discharge capacity per volume (mAh / cc) are Cu ₆ Sn ₅ = 3000, Co ₃ Sn ₂ = 1000, FeSn ₂ = 4800, MnSn ₂ = 3800, Ni ₃ Sn ₄ = 5300. AuSn = 3500, Al ₂₃ V ₄ = 690, InP = 4300, Cu ₃ P = 1500, AlP = 760, Mg ₂ Si = 160. Most of the cycle characteristics of these single intermetallic compounds were below the target value (80%), but those with cycle characteristics exceeding the target value have a difference in discharge capacity from graphite, which is a conventional negative electrode material. There was little or less capacity, and the cycle characteristics were lower than graphite. Accordingly, no negative electrode material made of a single intermetallic compound has a comprehensive evaluation of cycle characteristics and discharge capacity that exceeds that of graphite.

Although not shown in Table 1, when Cu ₃ Sn and Ti ₆ Sn ₅ were tested using materials that were confirmed to be single phase as described above, the discharge capacity values were both 0. It was found to be an inert phase intermetallic compound.

Example 1
As shown in Table 2, it consists of a mixture of (active phase 1-2), (active phase 1-3), (active phase 1-2 + inactive phase), or (active phase 1-3 + inactive phase). A negative electrode material having a composition was produced. Here, the active phases 1 and 2 are phases having a large quantity, the active phase 1 is a phase having a large discharge capacity, and the active phase 2 is a phase having a small discharge capacity. The active phase 3 is a higher-capacity phase than the active phase 1 but is quantitatively a small amount of phase.

For the material containing InP, Mg ₂ Si, or AlP, the anode material was produced by preparing each intermetallic compound alone, weighing it to a predetermined ratio, and then treating it by the MG method. For the other negative electrode materials, the melt raw material was weighed, and a sample was produced by a roll quenching method. In addition, in order to confirm whether or not there is a difference in the negative electrode material between the MG method and the roll quenching method, only the material having two phases of Cu ₆ Sn ₅ and Co ₃ Sn ₂ was used for sample preparation ( Test No. 6 and 7).

  The MG method was performed by placing a stainless steel ball having a diameter of 10 mm into a stainless steel pot having a diameter of 150 mm and a height of 200 mm five times with respect to the powder, followed by a planetary ball mill for 240 hours. In the roll quenching method, the negative electrode performance was evaluated by casting by a single roll method with a peripheral speed of about 600 m / min, and pulverizing and classifying without performing heat treatment.

  When an XRD diagram was prepared for each sample shown in Table 2, the full width at half maximum of the strongest peak was 0.6 ° or less. Further, among the crystal grain sizes of the active phase observed using TEM, the average of 10 crystal grain sizes was determined with the major axis as the grain size, and it was about 50 nm for both the roll quenching method and the MG method. It was. The discharge capacity and cycle characteristics of these negative electrode material samples are shown in Table 2 together with the value of graphite, which is the current negative electrode material.

  Test No. in Table 2 Nos. 1 to 9 change the magnification of the difference in discharge capacity per volume (ratio of larger discharge capacity / smaller discharge capacity) for the negative electrode material composed of a mixture of two kinds of active phases having different discharge capacities. The result of the case is shown. As shown here, when two kinds of intermetallic compounds were combined, it was confirmed that the cycle characteristics did not satisfy the target value when the discharge capacity ratio was less than 2 times or more than 20 times. Among these examples, the present invention examples in which the crystal systems of the active phase 1 and the active phase 2 are the same, ie, No. 6 and 7 were also found to have particularly high cycle characteristics. No. 6 and 7 are examples in which negative electrode materials having the same composition were produced by different methods, but negative electrode materials having substantially the same performance were obtained by either the MG method or the roll quenching method.

  Test No. in Table 2 10-15 are test Nos. The result at the time of increasing the ratio (volume%) of the intermetallic compound (active phase 1) with a large discharge capacity with respect to the negative electrode material from which the manufacturing method shown in 5 and 7 differs is shown. Test No. 5 and test no. 10-11 comparison and test no. 7 and test no. As can be seen from the comparison with 12 to 15, when the ratio of the active phase 1 having a large discharge capacity is increased, the discharge capacity of the negative electrode material naturally increases. However, it was confirmed by both the MG method and the roll quenching method that the cycle characteristics become lower than the target value when the ratio exceeds 80%.

  Test No. in Table 2 16-18 shows the result at the time of introduce | transducing into the negative electrode material the inert phase which does not occlude Li. In both the roll quenching method and the MG method, it was confirmed that when the inert phase was introduced, the discharge capacity decreased, but the cycle characteristics could be remarkably increased to 95% or more. As a result, it is possible to obtain a negative electrode material having a discharge capacity per volume that is at least twice as high as that of graphite, and having a cycle characteristic equivalent to or higher than that of graphite.

  Test No. in Table 2 Nos. 19 to 21 are test numbers of Table 2 consisting of two active phases and an inert phase of the same crystal system. The results are shown in the case where a part of the active phase 1 having a high discharge capacity is replaced with a higher capacity active phase of a different crystal system in 16 negative electrode materials. Although the discharge capacity was further increased by introducing a higher capacity active phase 3, the cycle characteristics were slightly deteriorated because the crystal system of the active phase 3 was different from the other active phases.

(Example 2)
In Example 1, Table 2 shows Test No. The negative electrode material shown as 7 was heat-treated in Ar under the conditions shown in Table 3. The average crystal grain size of the active phase of the negative electrode material after the heat treatment was determined by TEM observation in the same manner as described in Example 1. The results are shown in Table 3 together with the results of the negative electrode characteristics.

  As can be seen from Table 3, the average crystal grain size of the active phase was increased by the heat treatment, and the increase in the crystal grain size was increased as the heat treatment temperature was increased or the heat treatment time was increased. Although the discharge capacity did not change significantly due to the growth of the crystal grains by the heat treatment, the cycle characteristics deteriorated. However, the cycle characteristics exceeded the target value until the average crystal grain size was about 10 μm.

Claims (3)

  Consists of a mixture of two or more intermetallic compounds, at least two of which are intermetallic compounds that form an active phase capable of reversibly occluding and releasing Li, and the volume of the intermetallic compounds that form the active phase And the difference in discharge capacity per volume of the top two types of intermetallic compounds is in the range of not less than 2 times and not more than 20 times, and the discharge capacity of the top two types of intermetallic compounds is A negative electrode material for a nonaqueous electrolyte secondary battery, wherein the larger intermetallic compound occupies 80% by volume or less of the negative electrode material.   The negative electrode material for a nonaqueous electrolyte secondary battery according to claim 1, wherein at least two of the intermetallic compounds forming the active phase belong to the same crystal system.   The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 2, wherein at least two active phases composed of intermetallic compounds belonging to the same crystal system occupy 50% by volume or more of the negative electrode material.