Electrode materials and Si alloy composite powders for lithium-ion batteries

Abstract

The present invention pertains to an electrode material for lithium-ion batteries, in which: graphite powder and an Si alloy composite powder are mixed; the Si alloy composite powder has an average particle diameter of 5 μm or less, and contains Si particles, Si-X compound particles (X=Fe, Ni, Cr, Co, Mn, Zr, Ti), and at least one of Sn-Y compound particles (Y=Cu, Fe, Ni, Cr, Co, Mn, Zr, Ti) and Al-Y compound particles; the proportion of the Si particles in the Si alloy composite powder is 30-95 mass%; and, the coverage of the surface of the graphite particles by the Si alloy composite powder is 5% or greater.

Description

Technical Field

[0001] This invention relates to an electrode material for a lithium-ion battery and an Si alloy composite powder.

Background Art

[0002] Lithium-ion batteries have the advantages of high capacity, high voltage, and being able to be miniaturized, and are widely used as power sources for mobile phones, notebook computers, etc. In recent years, they have also received great expectations as power sources for power applications such as electric vehicles and hybrid vehicles, and their development has been actively promoted.

[0003] In this lithium-ion battery, lithium ions (hereinafter referred to as Li ions) move between the positive electrode and the negative electrode to perform charging and discharging. On the negative electrode side, Li ions are occluded in the negative electrode active material during charging, and Li ions are released from the negative electrode active material during discharging. Conventionally, generally, lithium cobaltate (LiCoO2) has been used as the active material on the positive electrode side, and graphite has been widely used as the negative electrode active material. However, the theoretical capacity of graphite as the negative electrode active material is only 372 mAh / g, and further increase in capacity has been desired.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As an alternative to carbon-based electrode materials, metallic materials such as silicon (Si) are being considered as they offer the potential for higher capacity. Electrodes in which Si-based material powder is mixed with graphite powder, a conventional electrode material, are being studied. The theoretical capacity of this Si is 4198 mAh / g. However, because Si undergoes alloying reactions with Li (Li) to intercalate Li ions, it experiences significant volume expansion and contraction due to the intercalation and release of Li ions. This can cause Si particles to crack or detach from the current collector, degrading the cycle characteristics, which are the capacity maintenance characteristics during repeated charging and discharging.

[0006] In response to the above, it has been proposed to alloy Si in electrode materials containing Si. For example, in Patent Document 1, a Si compound phase is provided together with the Si phase within Si-based alloy particles. The Si compound phase is effective in suppressing the expansion of the Si phase and improving cycle characteristics. However, the Si compound phase that constrains the Si phase may collapse due to its inability to withstand the expansion stress of the Si phase. Therefore, there was still room for improvement in enhancing battery characteristics, including initial discharge capacity and cycle characteristics, in electrode materials containing Si.

[0007] Against the background described above, the present invention aims to provide an electrode material and a Si alloy composite powder for lithium-ion batteries that can improve battery characteristics, taking into account initial discharge capacity and cycle characteristics. [Means for solving the problem]

[0008] The present invention relates to an electrode material for lithium-ion batteries, which is a mixture of graphite powder and Si alloy composite powder. The Si alloy composite powder has an average particle size of 5 μm or less and contains Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles. The element X constituting the Si-X compound particles is one or more elements selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti. The element Y constituting the Sn-Y compound particles and the Al-Y compound particles is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti. The proportion of the Si particles in the Si alloy composite powder is 30 to 95% by mass. The Si alloy composite powder is characterized by having a surface coverage rate of 5% or more on the graphite particle surface.

[0009] As described above, the electrode material for a lithium-ion battery according to the present invention comprises a Si alloy composite powder constituting the electrode material, which is refined to an average particle size of 5 μm or less, and contains Si particles, Si-X compound particles, and at least one of Sn-Y particles and Al-Y compound particles. By including Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles in the Si alloy composite powder, a space that allows for Si expansion is more easily formed around the Si particles. As a result, the Si particles can expand independently during Li-ion absorption without being constrained by other compound particles. Consequently, the collapse of Si particles during expansion is suppressed. In addition, the expansion amount is reduced by the miniaturization of Si particles, and the collapse of Si particles is suppressed more effectively. Furthermore, the space around these Si particles acts as a buffer region against the expansion of Si, suppressing the collapse of the Si-X compound particles, which play a role as a framework within the electrode.

[0010] Furthermore, in the electrode material according to the present invention, the coverage rate of the Si alloy composite powder on the surface of the graphite particles is set to 5% or more. This makes it possible to avoid localized stress concentration caused by the expansion of Si and to improve battery characteristics considering initial discharge capacity and cycle characteristics.

[0011] Considering the balance between initial discharge capacity and cycle characteristics, it is preferable that the proportion of graphite powder in the mixed powder of graphite powder and Si alloy composite powder be 97 to 20% by mass.

[0012] It is preferable that the element X is one or more elements selected from the group consisting of Fe, Ni, Cr, and Zr. This is particularly effective when you want to improve the cycle characteristics.

[0013] It is preferable that the mass ratio expressed as {Si-X compound / (total of Sn-Y compound and Al-Y compound)} be between 1 and 39. By setting the above mass ratio to 39 or less, a high initial discharge capacity is maintained, and by setting the above mass ratio to 1 or more, high cycle characteristics are maintained.

[0014] It is preferable that the aforementioned element Y is a Cu element. This is effective because it has excellent conductivity and can suppress the deterioration of cycle characteristics.

[0015] From the viewpoint of suppressing the expansion of Si, it is preferable to set the average particle size of the Si alloy composite powder to 1 μm or less, and more preferably to 0.7 μm or less.

[0016] Another aspect of the present invention relates to a Si alloy composite powder useful for use in the above-mentioned electrode material. Specifically, the Si alloy composite powder has an average particle size of 5 μm or less and contains Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles, wherein the element X constituting the Si-X compound particles is one or more elements selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti, and the element Y constituting the Sn-Y compound particles and the Al-Y compound particles is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti, and the proportion of the Si particles in the Si alloy composite powder is 30 to 95 mass%.

[0017] The Si alloy composite powder is preferably used together with graphite powder as an electrode material. [Brief explanation of the drawing]

[0018] [Figure 1]FIG. 1 is a schematic diagram showing the structure of an electrode material according to an embodiment of the present invention. (A) in FIG. 1 shows Si alloy particles before fine pulverization, (B) in FIG. 1 shows Si alloy composite powder after fine pulverization, and (C) in FIG. 1 shows a state in which graphite powder and Si alloy composite powder are mixed.

Mode for Carrying Out the Invention

[0019] Next, an electrode material for a lithium ion battery according to an embodiment of the present invention and a lithium ion battery using this electrode material as a negative electrode (hereinafter sometimes simply referred to as a battery) will be specifically described. Note that "~" is used to mean including the numerical values described before and after it as a lower limit value and an upper limit value.

[0020] 1. This electrode material FIG. 1 is a diagram showing the structure of this electrode material. In the figure, 1 is an electrode material, 2 is Si alloy particles before fine pulverization, 3 is Si alloy composite powder after fine pulverization, 3a is Si particles, 3b is Si-X compound particles, 3c is Sn-Y compound particles or Al-Y compound particles, and 4 is graphite particles. As shown in the figure, this electrode material is a mixture of graphite powder and Si alloy composite powder. Specifically, fine Si particles 3a, Si-X compound particles 3b, Sn-Y compound particles or Al-Y compound particles 3c that make up the Si alloy composite powder 3 cover a part of the surface of the graphite particles 4.

[0021] Graphite powder has conventionally been used as a negative electrode material for lithium ion batteries. Since graphite hardly expands or contracts due to the insertion and desorption of Li ions, its characteristics do not deteriorate even when charge and discharge are repeated. However, as described above, graphite has a low theoretical capacity and it is not possible to increase the battery capacity. Therefore, in this example, the capacity as a negative electrode material is increased by mixing with the Si alloy composite powder described below. Here, in this example, the ratio of graphite powder in the mixed powder of graphite powder and Si alloy composite powder is preferably 97 to 20% by mass. This is to balance the capacity (initial discharge capacity) and cycle characteristics. In addition, as the particle size (average particle size) of the graphite powder used in this example, 0.5 to 50 μm can be exemplified.

[0022] On the other hand, the Si alloy composite powder is an alloy powder including each phase of elemental silicon, a Si-X compound, and at least one of a Sn-Y compound and an Al-Y compound. Here, the element X constituting the Si-X compound is one or more elements selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti. Further, the element Y constituting the Sn-Y compound and the Al-Y compound is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti. That is, the Si alloy composite powder consists of these main constituent elements, namely, at least one of Si, Sn, and Al, the element X, and the element Y, and does not contain elements other than these main constituent elements except for inevitable ones. Examples of inevitable impurities include nitrogen (N), sulfur (S), phosphorus (P), etc. The upper limit of each is N ≤ 0.10 mass%, S ≤ 0.10 mass%, P ≤ 0.10 mass%, and O ≤ 15 mass%.

[0023] The Si alloy composite powder has an average particle size of 5 μm or less and is composed of including Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles. Defining the Si alloy composite powder to have an average particle size of 5 μm or less is mainly for reducing the absolute amount of expansion of elemental silicon (Si particles) that mainly stores Li. The preferable average particle size is 3 μm or less, more preferably 2 μm or less, still more preferably 1 μm or less, and particularly preferably 0.7 μm or less. The lower limit of the average particle size of the Si alloy composite powder is not particularly limited, but is usually 0.05 μm or more. The average particle size of the Si alloy composite particles is preferably smaller than the average particle size of the graphite particles. Here, the "particle size" refers to the diameter, that is, the equivalent circle diameter, obtained by converting the area of the particles constituting the Si alloy composite powder obtained by analyzing a cross-sectional scanning electron microscope (SEM) image into a circle having the same area. The "average particle size" refers to the average value analyzed for 100 particles from the cross-sectional SEM image (magnification 5000 times) of the Si alloy composite powder.

[0024] Si particles are particles consisting solely of the Si phase, or particles in which 95% or more of the particle's mass is composed of the Si phase. The proportion of Si particles in the total Si alloy composite powder is 30 to 95% by mass, more preferably 45 to 90% by mass, even more preferably 50 to 80% by mass, and particularly preferably 60 to 70% by mass. Here, from the viewpoint of preventing a decrease in initial discharge capacity, the proportion of Si particles is 30% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, and particularly preferably 60% by mass or more. Furthermore, from the viewpoint of suppressing a decrease in cycle characteristics due to a relative decrease in Si-X compound particles, the proportion of Si particles is 95% by mass or less, more preferably 90% by mass or less, even more preferably 80% by mass or less, and particularly preferably 70% by mass or less.

[0025] Si-X compound particles are particles consisting solely of Si-X compounds or particles in which 95% or more by mass consists of Si-X compounds. Si-X compounds have poor lithium absorption capabilities and expand very little due to reaction with lithium ions. Therefore, Si-X compound particles play a role as a framework that maintains the structure of the electrode material. Furthermore, Si-X compounds have high conductivity and are effective in ensuring conductivity between Si alloy composite powder and graphite powder.

[0026] In this example, the properties of the Si-X compound, such as Li storage capacity and conductivity, may differ depending on which element X is selected. Among the above elements X, Fe, Ni, Cr, and Zr are particularly excellent in terms of the low expansion and high conductivity expected of Si-X compounds. Therefore, element X is preferably one or more elements selected from the group consisting of Fe, Ni, Cr, and Zr. Furthermore, Si-X compound particles can be composed of a single compound, or they can be composed of two compounds, such as a Si-Fe compound and a Si-Ni compound.

[0027] Sn-Y compound particles are particles consisting solely of Sn-Y compounds or particles in which 95% or more of the particle mass consists of Sn-Y compounds. Sn-Y compounds have a theoretical capacity lower than Si but higher than Si-X compounds. For example, while Si-Zr compounds (Si-X compounds) have a capacity of 100 mAh / g, Sn-Y compounds have a capacity of 150-600 mAh / g. Therefore, in this example, a diffusion path for Li ions is easily secured through Sn-Y compound particles. On the other hand, the degree of expansion due to reaction with Li ions is smaller than that of elemental Si or Sn, which have high reactivity with Li ions, so the adverse effect on cycle characteristics due to the formation of Sn-Y compounds can be kept to a minimum. Furthermore, Sn-Y compounds have the effect of improving conductivity, similar to the Si-X compounds mentioned above.

[0028] Furthermore, such effects in Sn-Y compounds can also be obtained by using Al-Y compounds. Therefore, in the Si alloy composite powder of this example, it is possible to use Al-Y compound particles instead of all or part of the Sn-Y compound particles.

[0029] Here, Sn-Cu compounds or Al-Cu compounds in which element Y is selected are preferred because they have excellent conductivity and are less prone to deterioration of cycle characteristics compared to other Sn compounds or Al compounds.

[0030] As described above, the Si-X compound and at least one of the Sn-Y compound and Al-Y compound play different roles, and the resulting battery characteristics change depending on the ratio of these compounds. The Sn-Y compound and Al-Y compound expand more than the Si-X compound due to reaction with Li ions, albeit to a smaller degree. For this reason, the mass ratio expressed as {Si-X compound / (Sn-Y compound or Al-Y compound)} is preferably 0.5 to 45, more preferably 1 to 39, even more preferably 1.5 to 39, and particularly preferably 2.5 to 10. Here, from the viewpoint of suppressing a decrease in cycle characteristics, the above mass ratio is preferably 0.5 or higher, more preferably 1 or higher, even more preferably 1.5 or higher, and particularly preferably 2.5 or higher. On the other hand, from the viewpoint of obtaining a high initial discharge capacity, the above mass ratio is preferably 45 or less, more preferably 39 or less, and even more preferably 10 or less.

[0031] The following are the total content of each major element in the Si alloy composite powder that is suitable for obtaining the above-described constituent phases. In the following explanation, unless otherwise specified, "%" means "mass%".

[0032] The Si content is preferably 50-95%, more preferably 60-80%, and even more preferably 71-80%. Here, from the viewpoint of obtaining a high initial discharge capacity, the Si content is preferably 50% or more, more preferably 60% or more, and even more preferably 71% or more. Furthermore, from the viewpoint of obtaining good cycle characteristics, the Si content is preferably 95% or less, and more preferably 80% or less.

[0033] The content of element X is preferably 1-30%, and more preferably 5-20%. Here, from the viewpoint of obtaining good cycle characteristics, the content of element X is preferably 1% or more, and more preferably 5% or more. Furthermore, from the viewpoint of obtaining a high initial discharge capacity, the content of element X is preferably 30% or less, and more preferably 20% or less.

[0034] The respective contents of Sn and Al are preferably 0.1 to 20%, more preferably 1 to 10%, and even more preferably 2 to 9%. Here, from the viewpoint of obtaining a better effect as a Li diffusion path, the respective contents of Sn and Al are preferably 0.1% or more, more preferably 1% or more, and even more preferably 2% or more. Furthermore, to suppress the decrease in cycle characteristics due to expansion caused by Sn-Y compounds and Al-Y compounds, the respective contents of Sn and Al are preferably 20% or less, more preferably 10% or less, and even more preferably 9% or less. Furthermore, when both Sn and Al are included, it is preferable that the total content of Sn and Al be within the above range, specifically, 0.1 to 20%, more preferably 1 to 10%, and even more preferably 2 to 9%.

[0035] The content of element Y is preferably 0.1 to 15%, and more preferably 1 to 10%. Here, from the viewpoint of obtaining a better effect as a Li diffusion path, the content of element Y is preferably 0.1% or more, and more preferably 1% or more. Furthermore, from the viewpoint of suppressing the decrease in cycle characteristics due to expansion caused by Sn-Y compounds and Al-Y compounds, the content of element Y is preferably 15% or less, and more preferably 10% or less.

[0036] In this example, the Si alloy composite powder configured in this way is mixed with graphite powder, and the percentage of the surface of the graphite particles covered by the Si alloy composite powder (coverage rate) is set to 5% or more. Here, the coverage rate is the value (percentage) obtained by dividing the length of the contact area between the particles constituting the Si alloy composite powder and the graphite particles, as observed in cross-sectional view using an electron microscope, by the total circumference of the graphite particles. This coverage rate is also an indicator of the degree of dispersion of the Si alloy composite powder in the electrode material. If the coverage rate is low and the Si alloy composite powder is locally unevenly distributed, the expansion in these unevenly distributed areas will be greater than in other areas, increasing the risk of delamination or collapse in these areas. According to the inventors' evaluation, by dispersing and mixing Si alloy composite powder so that the coverage rate is 5% or more, it is possible to increase the initial discharge capacity while suppressing the deterioration of cycle characteristics. A preferred coverage rate is 7% or more, and a more preferred coverage rate is 10% or more.

[0037] Next, we will describe a method for producing this negative electrode material containing graphite powder and Si alloy composite powder.

[0038] First, we will explain an example of a method for producing Si alloy composite powder. Each raw material is weighed out to obtain a predetermined chemical composition, and the weighed raw materials are melted using melting methods such as an arc furnace, high-frequency induction furnace, or heating furnace to obtain a molten alloy, which is then rapidly cooled using the atomization method to obtain a Si alloy as a rapidly cooled alloy.

[0039] In the atomization method, molten alloy is dispensed into a spray chamber and flows continuously (in a rod shape) downwards. A gas containing N2, Ar, He, etc., is sprayed at high pressure, for example, 1 to 10 MPa, to cool the molten metal while simultaneously crushing it. The cooled molten metal, while still semi-molten, free-falls within the spray chamber, gradually becoming spherical, and a powdered Si alloy is obtained (see, for example, Figure 1(A)). Alternatively, high-pressure water may be sprayed instead of gas to improve the cooling effect. In some cases, it is also possible to obtain Si alloy in foil form by using the roll quenching method instead of the atomization method.

[0040] Next, the obtained Si alloy is finely ground using a suitable grinding method such as a ball mill, bead mill, disc mill, coffee mill, or mortar and pestle grinder to reduce its average particle size to 5 μm or less, and is obtained as a Si alloy composite powder containing independently existing Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles.

[0041] Next, the obtained Si alloy composite powder and graphite powder are prepared according to a predetermined mixing ratio, and these are mixed using a ball mill or mixer to produce the electrode material of this example. At this time, the coverage of the graphite powder by the Si alloy composite powder can be adjusted by appropriately changing conditions such as the mixing time.

[0042] 2. This battery This battery is constructed using a negative electrode containing this electrode material.

[0043] The negative electrode comprises a conductive substrate and a conductive film laminated on the surface of the conductive substrate. The conductive film contains at least the aforementioned electrode material in a binder.

[0044] The conductive substrate described above functions as a current collector. Examples of its material include Cu, Cu alloy, Ni, Ni alloy, Fe, and Fe-based alloy. Preferably, it is Cu or a Cu alloy. Examples of specific forms of the conductive substrate include foil or plate. Preferably, it is foil-shaped, from the viewpoint of reducing the volume of the battery and improving the degree of freedom in shape.

[0045] Suitable materials for the binder include, for example, polyvinylidene fluoride (PVdF) resin, fluororesins such as polytetrafluoroethylene, polyvinyl alcohol resin, polyimide resin, polyamide resin, polyamide-imide resin, styrene-butadiene rubber (SBR), and polyacrylic acid. These can be used individually or in combination of two or more. Of these, polyimide resin is particularly preferred because it has high mechanical strength, can withstand the volume expansion of the active material well, and effectively prevents the conductive film from peeling off the current collector due to binder failure.

[0046] The conductive film may also contain conductive additives as needed. When conductive additives are included, it becomes easier to secure electron conduction paths. Furthermore, the conductive film may also contain aggregates as needed. When aggregates are included, it becomes easier to suppress the expansion and contraction of the negative electrode during charging and discharging, thereby suppressing the collapse of the negative electrode and further improving cycle characteristics.

[0047] This negative electrode can be manufactured, for example, by adding the necessary amounts of the negative electrode material, and optionally conductive additives and aggregates, to a binder dissolved in a suitable solvent to form a paste, coating this paste onto the surface of a conductive substrate, drying it, and then compacting or heat treating it as needed.

[0048] When constructing a lithium-ion battery using this negative electrode, the other basic components of the battery, such as the positive electrode, electrolyte, and separator, are not particularly limited.

[0049] Specific examples of the above-mentioned positive electrode include, for instance, one in which a layer containing a positive electrode active material such as LiCoO2, LiNiO2, LiFePO4, or LiMnO2 is formed on the surface of a current collector such as aluminum foil.

[0050] Specifically, examples of the electrolytes mentioned above include an electrolyte solution obtained by dissolving a lithium salt in a non-aqueous solvent. Other examples include a polymer in which a lithium salt is dissolved, or a polymer solid electrolyte obtained by impregnating a polymer with the above-mentioned electrolyte.

[0051] Examples of the above-mentioned non-aqueous solvents include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. These may be present individually or in combination of two or more.

[0052] Examples of the lithium salts mentioned above include LiPF6, LiBF4, LiClO4, LiCF3SO3, and LiAsF6. These may be present individually or in combination of two or more types.

[0053] Other battery components include separators, casings (battery cases), gaskets, etc. However, any of these components commonly used in lithium-ion batteries can be combined as appropriate to construct the battery.

[0054] The battery shape is not particularly limited and can be cylindrical, rectangular, coin-shaped, or any other shape, and can be selected as appropriate according to its specific application. [Examples]

[0055] The present invention will be described in more detail below using examples. Unless otherwise specified, the percentages of alloy composition are given in mass percent.

[0056] 1. Fabrication of negative electrode material Table 1 below shows the alloy compositions of Si alloy composite powders for 33 examples and 6 comparative examples. Each alloy composition shown in Table 1 is specified to obtain the target configurations described in Tables 2 and 3 below. Note that in Table 1, some chemical compositions total 100.1%, but this is due to rounding to ensure accurate significant figures. First, each of the raw materials shown in Table 1 was weighed. Each weighed raw material was heated and melted in a high-frequency induction furnace to produce molten alloy. Powdered Si alloy was prepared from the molten alloy using the gas atomization method. The atmosphere during molten alloy preparation and gas atomization was an argon atmosphere. During gas atomization, high-pressure (4 MPa) argon gas was sprayed onto the molten alloy falling in a rod shape within the spray chamber. Each of the obtained Si alloys was mechanically ground using a wet bead mill to obtain Si alloy composite powders for use as negative electrode materials.

[0057] The obtained Si alloy composite powder and graphite powder were prepared according to the predetermined ratios shown in Tables 2 and 3 below, and these were mixed in a mixer to produce an electrode material for the negative electrode. The graphite powder used here had an average particle size of 20 μm.

[0058] [Table 1]

[0059] 2. Fabrication of coin-type batteries for charge / discharge testing First, 100 parts by mass of the electrode material prepared as the negative electrode active material, 6 parts by mass of Ketjenblack (manufactured by Lion Corporation) as a conductive additive, and 19 parts by mass of polyimide (thermoplastic resin) binder as a binder were mixed together. This mixture was then mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare pastes containing each electrode material. The mixing time for paste preparation was 1 hour for Examples 27 to 33. For Comparative Example 1, the mixing time was 5 minutes. For the other examples and comparative examples, the mixing time was 15 to 30 minutes.

[0060] Coin-type half-cells were fabricated as follows. For a simplified evaluation, electrodes fabricated using negative electrode material were used as test electrodes, and Li foil was used as the counter electrode. First, each paste was applied to the surface of a stainless steel (SUS) 316L foil (thickness 20 μm), which would serve as the negative electrode current collector, to a thickness of 50 μm using the doctor blade method, and then dried to form a negative electrode active material layer. After formation, the negative electrode active material layer was compacted using a roll press. This produced test electrodes made from the electrode materials of the examples and comparative examples.

[0061] Next, the test electrodes for the examples and comparative examples were punched out into circular disc shapes with a diameter of 11 mm to form each test electrode.

[0062] Next, Li foil (500 μm thick) was punched out in approximately the same shape as the test electrode to prepare each counter electrode. In addition, a non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / l in an equal volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0063] Next, each test electrode was placed in a positive electrode container, and the counter electrode was placed in a negative electrode container, with a polyolefin microporous membrane separator placed between each test electrode and each counter electrode. Note that each test electrode would normally be the negative electrode in a lithium-ion battery; however, when the counter electrode is a lithium foil, the lithium foil becomes the negative electrode and the test electrode becomes the positive electrode.

[0064] Next, the non-aqueous electrolyte was injected into each can, and each negative electrode can and each positive electrode can were crimped and fixed in place.

[0065] 3. Evaluation of electrode materials 3-1. Confirmation of the constituent phases of the electrode material XRD (X-ray diffraction) analysis was performed on the finely ground Si alloy composite powder to confirm the presence or absence of Si particles, Si-X compound particles, Sn-Y compound particles, and Al-Y compound particles.

[0066] 3-2. Calculation of the phase ratio The method for calculating the phase ratio (the proportion of each phase in the whole) shown in Tables 2 and 3 below will be explained using Example 1 as an example. (1) First, the constituent phases of the prepared powder are identified. In the case of Example 1, the above XRD analysis confirmed the presence of Si, Si2Fe, and Sn5Cu6 (see Table 2). (2) The mass percentage ratio of Si2Fe is 50.1[Si]-49.9[Fe]. Correspondingly, the amount of Si that is compounded is 17.3 × 50.1 / 49.9 = 17.4 (mass%). Therefore, the Si-X compound phase (Si2Fe) ratio is the sum of the compounded Si amount (17.4 mass%) and the Fe amount in Table 1 (17.3 mass%), which in this example is 35%. (4) The Si phase ratio is the value obtained by subtracting the amount of compounded Si (17.4% by mass) from the total amount of Si (77.7% by mass), which is 60% in this example. The Sn-Y phase ratio (the proportion of the Sn-Y compound phase in the whole) is the sum of the amount of Sn and the amount of element Y (Cu in the case of Example 1) shown in Table 1, which is 5% in this example. In Tables 2 and 3, SiX / (SnY or AlY) refers to the mass ratio expressed as {Si-X compound / (total of Sn-Y compound and Al-Y compound)}.

[0067] [Table 2]

[0068] [Table 3]

[0069] 3-3. Average particle size of Si alloy composite powder The average particle size of the Si alloy composite powder was determined by analyzing the particle size of 100 particles from cross-sectional SEM images (magnification 5000x) of the Si alloy composite powder. The results are shown in Tables 2 and 3, which show the average particle size (μm) of the Si composite powder.

[0070] 3-4. Coverage rate of graphite particle surfaces by Si alloy composite powder The negative electrode active material layer, containing graphite powder solidified by a binder and Si alloy composite powder, was observed in cross-section using an electron microscope. The coverage rate by Si alloy composite powder was determined for 10 graphite particles, and the average value was taken as the coverage rate. The results are shown in Tables 2 and 3 as graphite coverage rate (%).

[0071] 3-5. Charge / Discharge Test Each of the fabricated coin-type batteries was subjected to one constant current charge-discharge cycle at a current of 0.2 mA. The initial discharge capacity C0 (mAh / g) was calculated by dividing the capacity used during this lithium discharge (mAh) by the amount of active material (g).

[0072] The initial discharge capacity C0 measured above was evaluated as follows: 600 (mAh / g) or more was marked "◎", 450 or more but less than 600 was marked "〇", 400 or more but less than 450 was marked "△", and less than 400 was marked "×". The results are shown in Tables 2 and 3.

[0073] From the second cycle onward, charge-discharge tests were conducted at a 1 / 5C rate. Here, the C rate is defined as the current value required to charge and discharge the electrode in one hour, where C0 is the amount of electricity needed to charge and discharge the electrode. That is, 5C means charging and discharging in 12 minutes, and 1 / 5C means charging and discharging in 5 hours. The cycle characteristics were evaluated by performing the above charge-discharge cycle 100 times. From the obtained discharge capacities, the capacity retention rate (discharge capacity after 100 cycles / initial discharge capacity (discharge capacity in the first cycle) × 100) was calculated. A capacity retention rate of 95% or more was evaluated as "◎", 80% or more but less than 95% as "〇", 70% or more but less than 80% as "△", and less than 70% as "×". The results are shown in Tables 2 and 3.

[0074] From the results obtained in Tables 2 and 3 as described above, the following can be seen. In Comparative Example 1, the coverage rate of the graphite particle surface by the Si alloy composite powder was below the lower limit of the present invention (5%), suggesting that the Si alloy composite powder was unevenly distributed. In Comparative Example 1, the evaluation of the cycle characteristics was unsatisfactory. Comparative Example 2 is an example where the average particle size of the Si alloy composite powder exceeds the upper limit of the present invention (5 μm), and the evaluation of the cycle characteristics was unsuccessful.

[0075] Comparative Example 3 lacked both a Sn-Y compound phase (Sn-Y compound particles) and an Al-Y compound phase (Al-Y compound particles), and therefore its evaluation of initial discharge capacity was unsuccessful. Comparative Example 4 lacked a Si-X compound phase (Si-X compound particles), and its evaluation of cycle characteristics was unsuccessful.

[0076] Comparative Example 5 had a Si phase ratio below the lower limit of the present invention (30%), and therefore the evaluation of the initial discharge capacity was unsuccessful. Comparative Example 6 had a Si phase proportion exceeding the upper limit of the present invention (95%), and its cycle characteristics were evaluated as unsatisfactory (fail). As described above, all of the comparative examples failed to evaluate either the initial discharge capacity or the cycle characteristics, indicating that the battery characteristics, considering both initial discharge capacity and cycle characteristics, have not yet been sufficiently improved.

[0077] In contrast, each example in which the Si alloy composite powder has an average particle size of 5 μm or less, is composed of Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles, has a Si particle ratio of 30 to 95% by mass, and the coverage rate of the graphite particle surface by the Si alloy composite powder is 5% or more, exhibits improved battery characteristics compared to the comparative example above. In all of the examples, there were no negative ratings for initial discharge capacity or cycle characteristics, indicating that both initial discharge capacity and cycle characteristics were improved in a well-balanced manner. In particular, in Examples 25 and 26, in which the Si alloy composite powder was refined to an average particle size of 1 μm or less, excellent cycle characteristics were obtained without compromising initial discharge capacity.

[0078] In Example 27, where the proportion of graphite powder was increased to 95%, the initial discharge capacity was evaluated as △, but the cycle characteristics were evaluated as ◎, which is very high. It is particularly suitable when high cycle characteristics are required. Furthermore, in Example 28, where the proportion of graphite powder was reduced to 30%, the cycle characteristics were evaluated as △, but the initial discharge capacity was evaluated as ◎, which is very high, making it particularly suitable when a high initial discharge capacity is required.

[0079] Although the electrode material for lithium-ion batteries and lithium-ion batteries of the present invention have been described in detail above, the present invention is not limited to the above embodiments and examples. For example, in the above embodiments, Si particles and Si-X compound particles are obtained by finely grinding Si alloy particles having each phase, but in some cases, it is also possible to form Si particles and Si-X compound particles directly from molten metal and mix them to make a Si alloy composite powder. In short, the present invention can be modified in various ways without departing from its spirit.

[0080] This application is based on Japanese Patent Application No. 2021-161674 filed on September 30, 2021, the contents of which are incorporated herein by reference.

Claims

1. An electrode material for lithium-ion batteries, comprising a mixture of graphite powder and Si alloy composite powder, The Si alloy composite powder has an average particle size of 5 μm or less and contains Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles. The element X constituting the Si-X compound particles is one or more elements selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti. The element Y constituting the Sn-Y compound particles and the Al-Y compound particles is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti. The proportion of the Si particles in the Si alloy composite powder is 30 to 95% by mass. An electrode material for a lithium-ion battery, wherein the coverage rate of the graphite particle surface by the Si alloy composite powder is 5% or more.

2. The electrode material for a lithium-ion battery according to claim 1, wherein the proportion of graphite powder in the mixed powder of the graphite powder and the Si alloy composite powder is 97 to 20% by mass.

3. The electrode material for a lithium-ion battery according to claim 1 or 2, wherein the element X is one or more elements selected from the group consisting of Fe, Ni, Cr, and Zr.

4. An electrode material for a lithium-ion battery according to claim 1 or 2, wherein the mass ratio expressed as {Si-X compound / (total of Sn-Y compound and Al-Y compound)} is 1 to 39.

5. The electrode material for a lithium-ion battery according to claim 1 or 2, wherein the element Y is Cu.

6. The electrode material for a lithium-ion battery according to claim 1 or 2, wherein the average particle size of the Si alloy composite powder is 1 μm or less.

7. The electrode material for a lithium-ion battery according to claim 1 or 2, wherein the average particle size of the Si alloy composite powder is 0.7 μm or less.

8. The average particle size is 0.7 μm or less, and it contains Si particles, Si-X compound particles, and at least one of Sn-Y compound particles and Al-Y compound particles. The element X constituting the Si-X compound particles is one or more elements selected from the group consisting of Fe, Ni, Cr, Co, Mn, Zr, and Ti. The element Y constituting the Sn-Y compound particles and the Al-Y compound particles is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti. A Si alloy composite powder in which the proportion of Si particles in the Si alloy composite powder is 30 to 95% by mass.

9. The Si alloy composite powder according to claim 8, which is used as an electrode material together with graphite powder.

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