ELECTRODE MATERIAL FOR LITHIUM-ION BATTERY AND Si ALLOY COMPOSITE POWDER
20240396016 ยท 2024-11-28
Inventors
Cpc classification
C01P2004/61
CHEMISTRY; METALLURGY
C01P2004/62
CHEMISTRY; METALLURGY
International classification
H01M4/36
ELECTRICITY
Abstract
The present invention relates to an electrode material for lithium ion battery, containing: a graphite powder; and a Si alloy composite powder, in which the Si alloy composite powder has an average particle diameter of 5 m or less and contains Si particles, SiX compound particles (X=Fe, Ni, Cr, Co, Mn, Zr, or Ti), and at least one of SnY compound particles and AlY compound particles (Y=Cu, Fe, Ni, Cr, Co, Mn, Zr, or Ti), a proportion of the Si particles in the Si alloy composite powder is 30 mass % to 95 mass %, and a coverage of the Si alloy composite powder on a surface of graphite particles is 5% or more.
Claims
1. An electrode material for lithium ion battery, comprising: a graphite powder; and a Si alloy composite powder, wherein the Si alloy composite powder has an average particle diameter of 5 m or less and contains Si particles, SiX compound particles, and at least one of SnY compound particles and AlY compound particles, an element X constituting the SiX compound particles is at least one element selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti, an element Y constituting the SnY compound particles and the AlY compound particles is at least one element selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti, a proportion of the Si particles in the Si alloy composite powder is 30 mass % to 95 mass %, and a coverage of the Si alloy composite powder on a surface of graphite particles is 5% or more.
2. The electrode material for lithium ion battery according to claim 1, wherein a proportion of the graphite powder in a mixed powder of the graphite powder and the Si alloy composite powder is 97 mass % to 20 mass %.
3. The electrode material for lithium ion battery according to claim 1, wherein the element X is at least one element selected from the group consisting of Fe, Ni, Cr, and Zr.
4. The electrode material for lithium ion battery according to claim 1, wherein a mass ratio represented by {SiX compound/(total of SnY compound and AlY compound)} is 1 to 39.
5. The electrode material for lithium ion battery according to claim 1, wherein the element Y is Cu.
6. The electrode material for lithium ion battery according to claim 1, wherein the average particle diameter of the Si alloy composite powder is 1 m or less.
7. The electrode material for lithium ion battery according to claim 1, wherein the average particle diameter of the Si alloy composite powder is 0.7 m or less.
8. A Si alloy composite powder, having an average particle diameter of 5 m or less, comprising: Si particles; SiX compound particles; and at least one of SnY compound particles and AlY compound particles, wherein an element X constituting the SiX compound particles is at least one element selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti, an element Y constituting the SnY compound particles and the AlY compound particles is at least one element selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti, and a proportion of the Si particles in the Si alloy composite powder is 30 mass % to 95 mass %.
9. The Si alloy composite powder according to claim 8, which is used for an electrode material together with a graphite powder.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0034]
DESCRIPTION OF EMBODIMENTS
[0035] Next, an electrode material for lithium ion battery according to an embodiment of the present invention and a lithium ion battery using the present electrode material as a negative electrode (hereinafter may be simply referred to as a battery) are specifically described. Note that, to is used to mean that numerical values written before and after it are included as a lower limit value and an upper limit value.
1. Present Electrode Material
[0036]
[0037] The graphite powder has been used as a negative electrode material for a lithium ion battery in the related art. Since graphite hardly expands or contracts due to intercalation and desorption of Li ions, characteristics thereof do not deteriorate even after repeated charging and discharging. However, as described above, graphite has a low theoretical capacity and cannot be expected to have an increased battery capacity. Therefore, in this example, the capacity as a negative electrode material is increased by mixing graphite with the Si alloy composite powder to be described below.
[0038] Here, in this example, a proportion of the graphite powder in the mixed powder of the graphite powder and the Si alloy composite powder is preferably 97 mass % to 20 mass %. This is to balance the capacity (initial discharge capacity) and the cycle characteristics.
[0039] Note that, a particle diameter (average particle diameter) of the graphite powder used in this example can be exemplified as 0.5 m to 50 m.
[0040] On the other hand, the Si alloy composite powder is an alloy powder including each phase of Si alone, a SiX compound, and at least one of a SnY compound and an AlY compound. Here, the element X constituting the SiX compound is at least one element selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti. In addition, the element Y constituting the SnY compound and the AlY compound is at least one element selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti.
[0041] That is, the Si alloy composite powder is made of these main constituent elements, that is, Si, at least one of Sn, and Al, the element X, and the element Y, and does not contain any elements other than these main constituent elements except for inevitable impurities. Examples of the inevitable impurities include nitrogen (N), sulfur(S), and phosphorus (P). The respective upper limits are N0.10 mass %, S0.10 mass %, P0.10 mass %, and O15 mass %.
[0042] The Si alloy composite powder has an average particle diameter of 5 m or less, and contains Si particles, SiX compound particles, and at least one of SnY compound particles and AlY compound particles. The reason why the Si alloy composite powder is specified to have an average particle diameter of 5 m or less is mainly to reduce an absolute amount of expansion of Si alone (Si particles) that occludes Li. The average particle diameter is preferably 3 m or less, more preferably 2 m or less, still more preferably 1 m or less, and particularly preferably 0.7 m or less. The lower limit of the average particle diameter of the Si alloy composite powder is not particularly limited, and is generally 0.05 m or more. The average particle diameter of Si alloy composite particles is preferably smaller than the average particle diameter of the graphite particles.
[0043] Here, the particle diameter refers to the diameter when the area of particles constituting the Si alloy composite powder obtained by analyzing a cross-sectional scanning electron microscope (SEM) image is converted into a circle having the same area, that is, the diameter of an equivalent circle. In addition, the average particle diameter refers to the average value analyzed for 100 particles from a cross-sectional SEM image (magnification: 5000 times) of the Si alloy composite powder.
[0044] The Si particles are particles consisting only of a Si phase, or particles in which 95 mass % or more of the particles consist of a Si phase.
[0045] The proportion of the Si particles in the entire Si alloy composite powder is 30 mass % to 95 mass %, more preferably 45 mass % to 90 mass %, still more preferably 50 mass % to 80 mass %, and particularly preferably 60 mass % to 70 mass %. Here, the proportion of the Si particles is 30 mass % or more, more preferably 45 mass % or more, still more preferably 50 mass % or more, and particularly preferably 60 mass % or more, from the viewpoint of preventing a decrease in initial discharge capacity. In addition, the proportion of the Si particles is 95 mass % or less, more preferably 90 mass % or less, still more preferably 80 mass % or less, and particularly preferably 70 mass % or less, from the viewpoint of preventing a relative decrease in SiX compound particles and a decrease in cycle characteristics.
[0046] The SiX compound particles are particles consisting only of a SiX compound, or particles in which 95 mass % or more of the particles consist of a SiX compound.
[0047] The SiX compound has a poor Li occlusion property and has very little expansion due to a reaction with Li ions. Therefore, the SiX compound particles play the role of a skeleton that maintains a structure of the electrode material. In addition, the SiX compound has high conductivity and is effective in ensuring the conductivity between the Si alloy composite powder and the graphite powder.
[0048] In this example, the SiX compound may have different characteristics such as a Li occlusion property and conductivity depending on which element is selected as the element X. Among the above elements X, the element X is preferably at least one element selected from the group consisting of Fe, Ni, Cr, and Zr, since Fe, Ni, Cr, and Zr are particularly excellent in low expansion property and high conductivity expected of the SiX compound.
[0049] Note that the SiX compound particles can include only one type of compound, and can also include two types of compounds, such as a SiFe compound and a SiNi compound.
[0050] The SnY compound particles are particles consisting only of a SnY compound, or particles in which 95 mass % or more of the particles consist of a SnY compound.
[0051] The SnY compound has a theoretical capacity lower than that of Si and higher than that of the SiX compound. For example, a SiZr compound (SiX compound) has a theoretical capacity of 100 mAh/g, while the SnY compound has a theoretical capacity of 150 mAh/g to 600 mAh/g. Therefore, in this example, a diffusion path for Li ions is easily ensured through the SnY compound particles. On the other hand, since a degree of expansion of the SnY compound due to the reaction with Li ions is smaller than that of Si or Sn alone, which has high reactivity with Li ions, an adverse influence on the cycle characteristics due to formation of the SnY compound can be reduced. In addition, the SnY compound has the effect of increasing the conductivity, similar to the SiX compound.
[0052] Note that, such effects of the SnY compound can also be obtained by using an AlY compound. Therefore, in the Si alloy composite powder in this example, it is also possible to use AlY compound particles instead of all or a part of the SnY compound particles.
[0053] Here, a SnCu compound or an AlCu compound in which Cu is selected as the element Y is preferred because it has excellent conductivity and more hardly causes a decrease in cycle characteristics than other Sn compounds or Al compounds.
[0054] As described above, the SiX compound and the at least one of the SnY compound and the AlY compound play different roles, and the battery characteristics obtained also change depending on the proportions of these compounds. The SnY compound or the AlY compound expands more than the SiX compound due to the reaction with Li ions, although the degree of expansion is not so large. Therefore, the mass ratio represented by {SiX compound/(SnY compound or AlY compound)} is preferably 0.5 to 45, more preferably 1 to 39, still more preferably 1.5 to 39, and particularly preferably 2.5 to 10. Here, the above mass ratio is preferably 0.5 or more, more preferably 1 or more, still more preferably 1.5 or more, and particularly preferably 2.5 or more, from the viewpoint of preventing a decrease in cycle characteristics. On the other hand, the above mass ratio is preferably 45 or less, more preferably 39 or less, and still more preferably 10 or less, from the viewpoint of obtaining a high initial discharge capacity.
[0055] The content of each main element suitable for obtaining the above composition phase in the entire Si alloy composite powder is as follows. Note that in the following description, % means mass % unless otherwise specified.
[0056] The content of Si is preferably 50% to 95%, more preferably 60% to 80%, and still more preferably 71% to 80%. Here, the content of Si is preferably 50% or more, more preferably 60% or more, and still more preferably 71% or more, from the viewpoint of obtaining a high initial discharge capacity. In addition, the content of Si is preferably 95% or less, and more preferably 80% or less, from the viewpoint of obtaining good cycle characteristics.
[0057] The content of the element X is preferably 1% to 30%, and more preferably 5% to 20%. Here, the content of the element X is preferably 1% or more, and more preferably 5% or more, from the viewpoint of obtaining good cycle characteristics. In addition, the content of the element X is preferably 30% or less, and more preferably 20% or less, from the viewpoint of obtaining a high initial discharge capacity.
[0058] The content of each of Sn and Al is preferably 0.1% to 20%, more preferably 1% to 10%, and still more preferably 2% to 9%. Here, the content of each of Sn and Al is preferably 0.1% or more, more preferably 1% or more, and still more preferably 2% or more, from the viewpoint of further obtaining the effect as a Li diffusion path. In addition, the content of each of Sn and Al is preferably 20% or less, more preferably 10% or less, and still more preferably 9% or less, from the viewpoint of preventing a decrease in cycle characteristics due to expansion of the SnY compound or the AlY compound.
[0059] In addition, in the case where Sn and Al are contained together, the total content of Sn and Al is preferably within the above range. Specifically, the total content is preferably 0.1% to 20%, more preferably 1% to 10%, and still more preferably 2% to 9%.
[0060] The content of the element Y is preferably 0.1% to 15%, and more preferably 1% to 10%. Here, the content of the element Y is preferably 0.1% or more, and more preferably 1% or more, from the viewpoint of further obtaining the effect as a Li diffusion path. In addition, the content of the element Y is preferably 15% or less, and more preferably 10% or less, from the viewpoint of preventing a decrease in cycle characteristics due to expansion of the SnY compound or the AlY compound.
[0061] In this example, the Si alloy composite powder configured as described above is mixed with a graphite powder, and the proportion (coverage) of the Si alloy composite powder covering the surface of the graphite particles is 5% or more. Here, the coverage is a value (percentage) obtained by dividing the length of a contact portion between the particles constituting the Si alloy composite powder and the graphite particles by the total circumference length of the graphite particles in cross-sectional observation using an electron microscope. This coverage also serves as an index indicating a degree of dispersion of the Si alloy composite powder in the electrode material. In the case where the coverage is low and the Si alloy composite powder is locally unevenly distributed, the expansion in the unevenly distributed portion is larger than in other areas, increasing a concern of peeling-off or collapse in the unevenly distributed portion.
[0062] According to the results evaluated by the inventors of the present invention, in the case where the Si alloy composite powder is dispersed and mixed such that the coverage is 5% or more, it is possible to increase the initial discharge capacity and prevent a decrease in cycle characteristics. The preferred coverage is 7% or more, and the more preferred coverage is 10% or more.
[0063] Next, a method for producing the present negative electrode material containing the graphite powder and the Si alloy composite powder is described.
[0064] First, an example of a method for producing the Si alloy composite powder is described.
[0065] Respective raw materials are weighed out such that a predetermined chemical composition is obtained, and a molten alloy obtained by melting the weighed raw materials by using a melting device such as an arc furnace, a high frequency induction furnace, or a heating furnace is quenched by using an atomization method, to thereby obtain the Si alloy as a quenched alloy.
[0066] In the atomization method, a gas such as N.sub.2, Ar, or He is sprayed at a high pressure, for example, 1 MPa to 10 MPa, against the molten alloy that is discharged into an atomization chamber and that continuously (rod-like) flows downward, whereby the molten alloy is pulverized and cooled. The cooled molten alloy approaches a spherical shape while free-falling in the atomization chamber in a semi-molten state, and a Si alloy in the form of a powder (see, for example, (A) in
[0067] In some cases, it is also possible to obtain a foiled Si alloy by using a roll quenching method instead of the atomization method.
[0068] Next, the obtained Si alloy is finely pulverized by using an appropriate pulverizing means such as a ball mill, a bead mill, a disk mill, a coffee mill, or a mortar pulverizer to have an average particle diameter of 5 m or less, to obtain a Si alloy composite powder containing Si particles, SiX compound particles, and at least one of SnY compound particles and AlY compound particles, each of which is present independently.
[0069] Next, the obtained Si alloy composite powder and a graphite powder are prepared in accordance with a predetermined blending ratio, and mixed by using a ball mill, a mixer, or the like to prepare the electrode material in this example. At this time, the coverage of the Si alloy composite powder with respect to the graphite powder can be adjusted by appropriately changing conditions such as the mixing time.
2. Present Battery
[0070] The present battery is formed by using a negative electrode containing the present electrode material.
[0071] The negative electrode includes a conductive substrate and a conductive film laminated on a surface of the conductive substrate. The conductive film contains at least the present electrode material described above in a binder.
[0072] The conductive substrate functions as a current collector. Examples of a material thereof include Cu, a Cu alloy, Ni, a Ni alloy, Fe, and an Fe-based alloy. Preferably, it is Cu or a Cu alloy. Examples of a specific form of the conductive substrate include a foil form and a plate form. A foil form is preferred from the viewpoint of reducing the volume of the battery and improving the degree of freedom in form.
[0073] As a material of the binder, for example, a polyvinylidene fluoride (PVdF) resin, a fluorine resin such as polytetrafluoroethylene, a polyvinyl alcohol resin, a polyimide resin, a polyamide resin, a polyamideimide resin, a styrene-butadiene rubber (SBR), or polyacrylic acid can be suitably used. These may be used alone or in combination of two or more thereof. Among these, a polyimide resin is particularly preferred because it has high mechanical strength, can withstand volume expansion of the active material, and effectively prevents the conductive film from peeling off from the current collector due to breakage of the binder.
[0074] The conductive film may also contain a conductive aid, if necessary. In the case where a conductive aid is contained, it is easier to ensure a conductive path for electrons. In addition, the conductive film may contain an aggregate, if necessary. In the case where an aggregate is contained, expansion and contraction of the negative electrode during charging and discharging can be easily prevented, and collapse of the negative electrode can be prevented, so that the cycle characteristics can be further improved.
[0075] The present negative electrode can be produced by, for example, adding necessary amounts of the present negative electrode material, and, if necessary, a conductive aid and an aggregate to a binder dissolved in an appropriate solvent to form a paste, applying the paste to a surface of the conductive substrate, followed by drying, and optionally subjecting it to densification, a heat treatment, or the like.
[0076] In the case of forming a lithium ion battery using the present negative electrode, there are no particular limitations on a positive electrode, an electrolyte, a separator, and the like, which are basic components of the battery other than the present negative electrode.
[0077] Specific examples of the positive electrode include those in which a layer containing a positive electrode active material such as LiCoO.sub.2, LiNiO.sub.2, LiFePO.sub.4, and LiMnO.sub.2 is formed on a surface of a current collector such as an aluminum foil.
[0078] Specific examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent. In addition, it is also possible to use a polymer in which a lithium salt is dissolved, a polymer solid electrolyte in which a polymer is impregnated with the above-described electrolytic solution, and the like.
[0079] Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. These may be used alone or in combination of two or more thereof.
[0080] Specific examples of the lithium salt include LiPF.sub.6, LiBF.sub.4, LiCIO.sub.4, LiCF.sub.3SO.sub.3, and LiAsF.sub.6. These may be used alone or in combination of two or more thereof.
[0081] Other battery components include a separator, a can (battery case), and a gasket. As for these, as long as they are commonly used in a lithium ion battery, any of them can be appropriately combined to form a battery.
[0082] Note that the shape of the battery is not particularly limited, and may be any shape such as a cylindrical shape, a rectangular shape, or a coin shape, and can be appropriately selected according to a specific application.
EXAMPLES
[0083] Hereinafter, the present invention is described more specifically using Examples. Note that % in the alloy composition is mass % unless otherwise specified.
1. Preparation of Electrode Material for Negative Electrode
[0084] Table 1 below shows alloy compositions of Si alloy composite powders, 33 types thereof for Examples and 6 types thereof for Comparative Examples. Respective alloy compositions shown in Table 1 are defined so as to obtain target compositions shown in Tables 2 and 3 below. Note that, in Table 1, the total of all chemical components may be 100.1%, but this is due to rounding to the same significant figure.
[0085] First, each raw material shown in Table 1 was weighed out. The weighed raw materials were heated and melted by using a high frequency induction furnace to obtain molten alloys. Si alloys in the form of a powder were prepared from the molten alloys by a gas atomization method. Note that, an argon atmosphere was used as an atmosphere during the preparation of the molten alloys and the gas atomization. In addition, during the gas atomization, high-pressure (4 MPa) argon gas was sprayed onto the molten alloys falling like a rod in the atomization chamber.
[0086] Each of the obtained Si alloys was mechanically finely pulverized by using a wet bead mill to obtain a Si alloy composite powder for use in an electrode material for a negative electrode.
[0087] The obtained Si alloy composite powder and a graphite powder were prepared according to the predetermined proportions shown in Tables 2 and 3 below, and these were mixed by using a mixer to prepare an electrode material for a negative electrode. The graphite powder used here has an average particle diameter of 20 m.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Si Sn Cu Fe Ni Cr Co Mn Ti Zr Al Ex. 1 77.7 3.0 2.0 17.3 2 87.6 3.0 2.0 7.4 3 92.5 3.0 2.0 2.5 4 75.2 6.0 4.0 14.9 5 75.3 6.0 4.0 14.7 6 75.6 6.0 4.0 14.4 7 74.6 6.0 4.0 15.4 8 75.2 6.0 4.0 14.8 9 60.3 6.0 4.0 29.7 10 60.7 6.0 4.0 29.3 11 61.2 6.0 4.0 28.8 12 73.4 6.0 4.0 7.4 9.2 13 75.7 6.0 4.0 7.4 6.9 14 74.6 6.0 4.0 12.4 3.1 15 75.3 6.0 4.0 12.4 2.3 16 75.2 6.7 14.9 3.3 17 75.2 8.0 14.9 2.0 18 75.2 8.1 16.8 19 75.2 7.3 14.9 2.7 20 75.2 7.0 14.9 3.0 Ex. 21 75.2 18.9 6.0 22 75.2 14.9 4.2 5.8 23 75.2 14.9 5.3 4.7 24 75.2 14.9 3.7 6.3 25 77.7 3.0 2.0 17.3 26 77.7 3.0 2.0 17.3 27 77.7 3.0 2.0 17.3 28 77.7 3.0 2.0 17.3 29 77.7 3.0 2.0 17.3 30 77.7 3.0 2.0 17.3 31 77.7 3.0 2.0 17.3 32 77.7 3.0 2.0 17.3 33 77.7 3.0 2.0 17.3 Comp. 1 75.2 6.0 4.0 14.9 Ex. 2 75.2 6.0 4.0 14.9 3 80.2 19.8 4 60.0 24.0 16.0 5 50.3 12.0 8.0 29.7 6 98.99 0.03 0.02 0.97
2. Preparation of Coin-Type Battery for Charging and Discharging Test
[0088] First, 100 parts by mass of the prepared electrode material as a negative electrode active material, 6 parts by mass of Ketjen black (manufactured by Lion Corporation) as a conductive aid, and 19 parts by mass of a polyimide (thermoplastic resin) binder as a binder were blended and mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to thereby prepare each paste containing each electrode material. Note that, the kneading time during the paste preparation was 1 hour in Examples 27 to 33. The kneading time was 5 minutes in Comparative Example 1. The kneading time was 15 to 30 minutes in other Examples and Comparative Examples.
[0089] Each coin-type half cell was prepared as follows. Here, for sake of simple evaluation, an electrode prepared by using an electrode material for a negative electrode was used as a test electrode, and a Li foil was used as a counter electrode. First, each paste was applied to a surface of a stainless steel (SUS) 316L foil (thickness: 20 m) as a negative electrode current collector by using a doctor blade method so as to be 50 m, followed by drying to form a negative electrode active material layer. After formation, the negative electrode active material layer was densified by roll pressing. Accordingly, test electrodes made of electrode materials according to Examples and Comparative Examples were prepared.
[0090] Next, each of the test electrodes according to Examples and Comparative Examples was punched into a disc shape having a diameter of 11 mm to obtain a test electrode.
[0091] Next, a Li foil (thickness: 500 m) was punched into substantially the same shape as the test electrode to prepare a counter electrode. In addition, a non-aqueous electrolytic solution was prepared by dissolving LiPF.sub.6 at a concentration of 1 mol/l in an equivalent mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in terms of volume proportion.
[0092] Next, the test electrode was housed in a positive electrode can, and the counter electrode was housed in a negative electrode can, and a separator made of a polyolefin-based microporous film was disposed between the test electrode and the counter electrode. Note that the test electrode should be a negative electrode in a lithium ion battery, but when a Li foil is used as the counter electrode, the Li foil is the negative electrode and the test electrode is the positive electrode.
[0093] Next, the above-described non-aqueous electrolytic solution was injected into both cans, and the negative electrode can and the positive electrode can were crimped and fixed to each other.
3. Evaluation on Electrode Material
3-1. Confirmation of Composition Phase of Electrode Material
[0094] The finely pulverized Si alloy composite powder was analyzed by XRD (X-ray diffraction) to confirm the presence or absence of Si particles, SiX compound particles, SnY compound particles, and AlY compound particles.
3-2. Calculation of Phase Proportion
[0095] A method of calculating a phase proportion (proportion of each phase to the whole) shown in Tables 2 and 3 below is described with reference to Example 1 as an example.
[0096] (1) First, the composition phases in the prepared powder are confirmed. In the case of Example 1, as a result of the above described XRD analysis, Si, Si.sub.2Fe, and SnsCu.sub.6 have been confirmed (see Table 2).
[0097] (2) Si.sub.2Fe is 50.1 [Si]49.9 [Fe] in terms of ratio in mass %. Correspondingly, the amount of Si to be compound is 17.350.1/49.9=17.4 (mass %). Therefore, the proportion of the SiX compound phase (Si.sub.2Fe) is a value of the total of the amount of Si to be compound (17.4 mass %) and the amount of Fe (17.3 mass %) in Table 1, and is thus 35% in this example.
[0098] (4) The proportion of the Si phase is a value obtained by subtracting the amount of Si to be compound (17.4 mass %) from the total amount of Si (77.7 mass %), and is thus 60% in this example.
[0099] Note that, the proportion of the SnY phase (the proportion of the SnY compound phase in the whole) is a value of the total of the amount of Sn and the amount of the element Y (Cu in the case of Example 1) in Table 1, and is thus 5% in this example.
[0100] In Tables 2 and 3, SiX/(SnY or AlY) indicates the mass ratio represented by {SiX compound/(total of SnY compound and AlY compound)}.
TABLE-US-00002 TABLE 2 Phase proportion (mass %) Target composition phase Type of SiX Si SiX SnY AlY Ex. 1 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 2 80[Si]15[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 80 15 5 0 3 90[Si]5[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 90 5 5 0 4 60[Si]30[Si.sub.2Fe]10[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 30 10 0 5 60[Si]30[Si.sub.2Ni]10[Sn.sub.5Cu.sub.6] Si.sub.2Ni 60 30 10 0 6 60[Si]30[Si.sub.2Cr]10[Sn.sub.5Cu.sub.6] Si.sub.2Cr 60 30 10 0 7 60[Si]30[Si.sub.2Co]10[Sn.sub.5Cu.sub.6] Si.sub.2Co 60 30 10 0 8 60[Si]30[Si.sub.2Mn]10[Sn.sub.5Cu.sub.6] Si.sub.2Mn 60 30 10 0 9 30[Si]60[Si.sub.2Fe]10[Sn.sub.5Cu.sub.6] Si.sub.2Fe 30 60 10 0 10 30[Si]60[Si.sub.2Ni]10[Sn.sub.5Cu.sub.6] Si.sub.2Ni 30 60 10 0 11 30[Si]60[Si.sub.2Cr]10[Sn.sub.5Cu.sub.6] Si.sub.2Cr 30 60 10 0 12 60[Si]15[Si.sub.2Fe]15[Si.sub.2Zr]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe/Si.sub.2Zr 60 30 10 0 13 60[Si]15[Si.sub.2Fe]15[Si.sub.2Ti]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe/Si.sub.2Ti 60 30 10 0 14 60[Si]25[Si.sub.2Fe]5[Si.sub.2Zr]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe/Si.sub.2Zr 60 30 10 0 15 60[Si]25[Si.sub.2Fe]5[Si.sub.2Ti]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe/Si.sub.2Ti 60 30 10 0 16 60[Si]30[Si.sub.2Fe]10[Sn.sub.5Ti.sub.6] Si.sub.2Fe 60 30 10 0 17 60[Si]30[Si.sub.2Fe]10[Sn.sub.2Co] Si.sub.2Fe 60 30 10 0 18 60[Si]30[Si.sub.2Fe]10[Sn.sub.2Fe] Si.sub.2Fe 60 30 10 0 19 60[Si]30[Si.sub.2Fe]10[Ni.sub.3Sn.sub.4] Si.sub.2Fe 60 30 10 0 20 60[Si]30[Si.sub.2Fe]10[AlCu] Si.sub.2Fe 60 30 0 10 Discharge Si composite Graphite Initial capacity powder average Graphite powder discharge retention SiX/(SnY particle diameter coverage proportion capacity rate or AlY) (m) (%) (wt %) (mAh/g) (%) Ex. 1 7 4.2 10.8 90 B (475) B (88) 2 3 4.5 11.7 90 B (493) B (84) 3 1 4.1 10.1 90 B (511) B (80) 4 3 4.2 12.5 90 B (487) B (87) 5 3 4.3 10.9 90 B (472) B (85) 6 3 4.9 13.1 90 B (463) B (86) 7 3 4.7 10.4 90 B (480) B (85) 8 3 4.3 10.6 90 B (460) B (86) 9 6 4.5 11.5 90 B (469) B (87) 10 6 4.6 12.1 90 B (468) B (85) 11 6 4.8 10.6 90 B (464) B (87) 12 3 4.2 12.3 90 B (470) B (92) 13 3 4.4 11.3 90 B (465) B (87) 14 3 4.7 11.1 90 B (473) B (90) 15 3 4.1 10.9 90 B (476) B (88) 16 3 4.1 11.4 90 B (472) B (83) 17 3 4.2 10.3 90 B (463) B (81) 18 3 4.2 10.6 90 B (451) B (82) 19 3 4.1 10.7 90 B (469) B (80) 20 3 4.3 11.5 90 B (461) C (78)
TABLE-US-00003 TABLE 3 Phase proportion (mass %) Target composition phase Type of SiX Si SiX SnY AlY Ex. 21 60[Si]30[Si.sub.2Fe]10[Al.sub.3Fe] Si.sub.2Fe 60 30 0 10 22 60[Si]30[Si.sub.2Fe]10[Al.sub.3Ni] Si.sub.2Fe 60 30 0 10 23 60[Si]30[Si.sub.2Fe]10[Al.sub.3Zr] Si.sub.2Fe 60 30 0 10 24 60[Si]30[Si.sub.2Fe]10[Al.sub.3Ti] Si.sub.2Fe 60 30 0 10 25 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 26 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 27 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 28 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 29 60[Si]13[Si.sub.2Fe]27[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 30 60[Si]39[Si.sub.2Fe]1[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 31 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 32 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 33 60[Si]35[Si.sub.2Fe]5[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 35 5 0 Comp. 1 60[Si]30[Si.sub.2Fe]10[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 30 10 0 Ex. 2 60[Si]30[Si.sub.2Fe]10[Sn.sub.5Cu.sub.6] Si.sub.2Fe 60 30 10 0 3 60[Si]40[Si.sub.2Fe] Si.sub.2Fe 60 40 0 0 4 60[Si]40[Sn.sub.5Cu.sub.6] 60 40 0 5 20[Si]60[Si.sub.2Fe]20[Sn.sub.5Cu.sub.6] Si.sub.2Fe 20 60 20 0 6 98[Si]1.95[Si.sub.2Fe]0.05[Sn.sub.5Cu.sub.6] Si.sub.2Fe 98 1.95 0.05 0 Discharge Si composite Graphite Initial capacity powder average Graphite powder discharge retention SiX/(SnY particle diameter coverage proportion capacity rate or AlY) (m) (%) (wt %) (mAh/g) (%) Ex. 21 3 4.5 11.2 90 B (451) C (76) 22 3 4.6 10.8 90 B (472) C (75) 23 3 4.6 10.3 90 B (464) C (77) 24 3 4.2 10.9 90 B (470) C (75) 25 7 0.8 11.1 90 B (481) A (95) 26 7 0.5 10.3 90 B (474) A (98) 27 7 4.4 11.2 95 C (404) A (95) 28 7 4.7 10.7 30 A (1743) C (71) 29 0.5 4.6 10.5 90 B (472) B (85) 30 45 4.9 11.1 90 B (451) B (88) 31 7 4.4 11.4 80 B (592) B (90) 32 7 4.3 11.8 65 A (892) B (81) 33 7 4.4 12.2 50 A (1003) C (75) Comp. 1 3 4.2 2.2 90 B (465) D (68) Ex 2 3 8.1 10.4 90 B (464) D (62) 3 4.5 10.2 90 D (395) B (82) 4 4.4 10.3 90 B (484) D (68) 5 3 3.9 10.7 90 D (381) B (93) 6 39 3.7 11.2 90 B (497) D (59)
3-3. Average Particle Diameter of Si Alloy Composite Powder
[0101] The average value of particle diameters analyzed for 100 particles from a cross-sectional SEM image (magnification: 5000 times) of the Si alloy composite powder was defined as the average particle diameter of the Si alloy composite powder. The results are shown in Tables 2 and 3 as Si composite powder average particle diameter (um).
3-4. Coverage of Si Alloy Composite Powder on Surface of Graphite Particles
[0102] The cross section of the negative electrode active material layer containing the graphite powder and the Si alloy composite powder hardened by the binder was observed by using an electron microscope, the coverages of the Si alloy composite powder on 10 graphite particles were determined, and the average value thereof was taken as the coverage. The results are shown in Tables 2 and 3 as graphite coverage (%).
3-5. Charging and Discharging Test
[0103] One cycle including constant current charging and discharging at a current value of 0.2 mA was performed by using the prepared coin-type battery. A value obtained by dividing the capacity (mAh) used for releasing Li by the amount (g) of the active material is an initial discharge capacity Co (mAh/g).
[0104] Regarding the measured initial discharge capacity Co, 600 (mAh/g) or more was evaluated as A, 450 or more and less than 600 was evaluated as B, 400 or more and less than 450 was evaluated as C, and less than 400 was evaluated as D. The results are shown in Tables 2 and 3.
[0105] In the second cycle and thereafter, the charging and discharging test was performed at a 1/5C rate. Here, in the C rate, the current value for (charging and) discharging an amount of electricity Co required to (charge and) discharging the electrode in 1 hour is defined as 1C.
[0106] That is, 5C means (charging and) discharging in 12 minutes, and 1/5C means (charging and) discharging in 5 hours. Then, the cycle characteristics were evaluated by performing the charging and discharging cycle 100 times. A capacity retention rate (discharge capacity after 100 cycles/initial discharge capacity (discharge capacity at first cycle)100) was obtained from each of the obtained discharge capacities. Then, the case where the capacity retention rate is 95% or more was evaluated as A, the case of 80% or more and less than 95% was evaluated as B, the case of 70% or more and less than 80% was evaluated as C, and the case of less than 70% was evaluated as D. The results are shown in Tables 2 and 3.
[0107] The results in Tables 2 and 3 obtained as described above show the followings.
[0108] In Comparative Example 1, the coverage of the Si alloy composite powder on the surface of the graphite particles was lower than the lower limit (5%) in the present invention, and it is presumed that the Si alloy composite powder is unevenly distributed. In Comparative Example 1, the cycle characteristics were evaluated as D.
[0109] Comparative Example 2 is an example in which the average particle diameter of the Si alloy composite powder is more than the upper limit (5 m) in the present invention, and the cycle characteristics were evaluated as D.
[0110] Comparative Example 3 had neither a SnY compound phase (SnY compound particles) nor an AlY compound phase (AlY compound particles), and the initial discharge capacity was evaluated as D.
[0111] Comparative Example 4 did not have a SiX compound phase (SiX compound particles) and the cycle characteristics were evaluated as D.
[0112] In Comparative Example 5, the proportion of the Si phase was lower than the lower limit (30%) in the present invention, and the initial discharge capacity was evaluated as D.
[0113] In Comparative Example 6, the proportion of the Si phase was upper than the upper limit (95%) in the present invention, and the cycle characteristics were evaluated as D.
[0114] As described above, in all Comparative Examples, the initial discharge capacity or the cycle characteristics were evaluated as D, and the battery characteristics in consideration of the initial discharge capacity and the cycle characteristics have not yet been sufficiently improved.
[0115] In contrast, in each Example in which the Si alloy composite powder has an average particle diameter of 5 m or less, contains Si particles, SiX compound particles, and at least one of SnY compound particles and AlY compound particles, and has a proportion of the Si particles of 30 mass % to 95 mass % and a coverage of the Si alloy composite powder on the surface of the graphite particles of 5% or more, the battery characteristics are improved compared to the above-described Comparative Examples.
[0116] In all of Examples, there are no evaluation as D in the initial discharge capacity nor the cycle characteristics, and it can be seen that the initial discharge capacity and the cycle characteristics are improved in a well-balanced manner. In particular, in Examples 25 and 26 in which the Si alloy composite powder is refined to an average particle diameter of 1 m or less, excellent cycle characteristics are obtained without impairing the initial discharge capacity.
[0117] Note that, in Example 27 in which the proportion of the graphite powder is increased to 95%, the initial discharge capacity is evaluated as C, but the cycle characteristics are evaluated as A, which is very high. This is particularly suitable in the case where high cycle characteristics are required.
[0118] In addition, in Example 28 in which the proportion of the graphite powder is lowered to 30%, the cycle characteristics are evaluated as C, but the initial discharge capacity is evaluated as A, which is very high. This is particularly suitable in the case where a high initial discharge capacity is required.
[0119] Although the electrode material for lithium ion battery and the lithium ion battery according to the present invention have been described in detail above, the present invention is not limited to the above-described embodiment and Examples. For example, in the above-described embodiment, Si particles, SiX compound particles, and the like are obtained by finely pulverizing Si alloy particles having each phase. However, in some cases, it is also possible to form Si particles, SiX compound particles, and the like directly from a molten metal and mix them to form a Si alloy composite powder. The present invention can be modified in various ways without departing from the spirit thereof.
[0120] The present application is based on Japanese patent application No. 2021-161674 filed on Sep. 30, 2021, and the contents thereof are incorporated herein as reference.