Negative electrode active material for lithium-ion batteries

Abstract

To provide an anode active material for a lithium-ion battery capable of enhancing overall battery performance in terms of cycle performance, initial coulombic efficiency, and discharge rate performance.SOLUTION: An anode active material for a lithium-ion battery is composed of Si phase, Si-Zr compound phase, Si-X compound phase, and Sn-Cu compound phase. An element X is one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Ti, V, and Cr. The proportion of Sn-Cu compound phase in the anode active material for the lithium-ion battery is 0.1 to 18 mass%, and the proportion of the Si phase is 10 to 90 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material for a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries have the advantages of high capacity, high voltage, and the ability to be miniaturized, and are widely used as power sources for mobile phones, laptops, etc. In recent years, they have also attracted great expectations as a power source for electric vehicles, hybrid vehicles, etc., and their development is actively progressing.

[0003] In this lithium-ion battery, lithium ions (hereinafter referred to as Li ions) move between the positive electrode and negative electrode to charge and discharge the battery. On the negative electrode side, Li ions are absorbed into the negative electrode active material during charging, and Li ions are released from the negative electrode active material during discharge.

[0004] Traditionally, lithium cobalt oxide (LiCoO2) has generally been used as the positive electrode active material, and graphite has been widely used as the negative electrode active material. However, the theoretical capacity of graphite as a negative electrode active material is only 372 mAh / g, and further increases in capacity are desired. Therefore, metal materials such as silicon (theoretical capacity of silicon is 4198 mAh / g) that are expected to increase capacity have recently been actively researched as alternatives to carbon-based negative electrode active materials.

[0005] However, because Si absorbs Li ions through an alloying reaction with Li, significant volume expansion and contraction occurs as the Li ions are absorbed and released. Therefore, if the negative electrode active material is made solely of Si, the stress from this expansion and contraction can cause the Si particles to crack or peel off from the current collector, resulting in a deterioration in cycle characteristics, which are the capacity retention characteristics during repeated charge and discharge.

[0006] To solve these problems, it has been proposed to alloy Si in a negative electrode active material using Si. For example, as described in the following patent document, it is known to provide a Si-Zr compound phase as a Si compound phase together with Si. The Si-Zr compound phase is effective in suppressing the expansion of Si and improving cycle characteristics. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-224499 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, the Si-Zr compound phase is effective in improving cycle characteristics. However, in addition to cycle characteristics, other properties required of fuel cells include initial coulombic efficiency and discharge rate characteristics. Therefore, there is still room for improvement in improving the overall battery characteristics that take into account these multiple properties.

[0009] The present invention has been made in light of the above circumstances, with an object to provide a negative electrode active material for a lithium ion battery that can improve overall battery characteristics in consideration of cycle characteristics, initial coulombic efficiency, and discharge rate characteristics. [Means for solving the problem]

[0010] The present invention provides a composite alloy including a Si phase, a Si-Zr compound phase, a Si-X compound phase, and a Sn-Cu compound phase, wherein the element X is one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Ti, V, and Cr, The composition is characterized in that the proportion of the Sn--Cu compound phase in the entire composition is 0.1 to 18 mass %, and the proportion of the Si phase in the entire composition is 10 to 90 mass %.

[0011] As described above, the negative electrode active material for a lithium ion battery of the present invention contains two or more types of Si compound phases. One is a Si-Zr compound phase. The Si-Zr compound phase suppresses the expansion of the Si phase and has the effect of improving the capacity retention characteristics, i.e., the cycle characteristics, during repeated charge and discharge.

[0012] However, if the Si compound phase is composed only of Si-Zr compounds, it is difficult to improve the initial Coulomb efficiency and discharge rate characteristics, so in the present invention, an Si-X compound phase is provided in addition to the Si-Zr compound phase as the Si compound phase, where element X is one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Ti, V, and Cr.

[0013] Since Si-X compounds have different properties from Si-Zr compounds in terms of Li storage capacity, conductivity, etc., by including an Si-X compound phase as the Si compound phase, it is possible to improve the overall battery characteristics, taking into consideration cycle characteristics, initial coulombic efficiency, and discharge rate characteristics. Here, in order to enhance the effect of improving battery characteristics by the Si-X compound phase, it is desirable to set the Zr content, expressed as [Zr] / ([Zr]+[X]) (where [ ] indicates the content of the element in [ ] in at%), to 1 to 30%. DETAILED DESCRIPTION OF THE INVENTION

[0014] Next, a negative electrode active material for a lithium ion battery (hereinafter sometimes simply referred to as a negative electrode active material) according to one embodiment of the present invention and a lithium ion battery (hereinafter sometimes simply referred to as a battery) using this negative electrode active material in the negative electrode will be specifically described.

[0015] 1.This negative electrode active material The negative electrode active material comprises a Si phase, a Si-Zr compound phase, a Si-X compound phase, and a Sn-Cu compound phase, where element X is one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Ti, V, and Cr. No elements other than these main constituent elements (Si, Zr, Sn, Cu, element X) are included, except for unavoidable ones. Here, examples of unavoidable impurity elements include nitrogen (N), sulfur (S), phosphorus (P), etc. The upper limits of each element are N≦0.10 mass%, S≦0.10 mass%, and P≦0.10 mass%.

[0016] The Si phase is a phase that mainly contains Si. From the viewpoint of increasing the amount of Li absorption, it is preferable that the Si phase is a single Si phase. However, the Si phase may contain unavoidable impurities.

[0017] The proportion of the Si phase in the whole is set to 10 to 90 mass%. If the amount of Si that absorbs Li ions is small, the initial discharge capacity decreases, and conversely, if the amount of Si is large, the amount of the Si compound phase decreases relatively, which may result in a decrease in cycle characteristics. A more preferable proportion of the Si phase is 25 to 70 mass%.

[0018] The Si-Zr compound phase is a phase that primarily contains Si2Zr, but may unavoidably contain other Zr silicide phases (Si4Zr, Si3Zr2, Si5Zr4, SiZr, SiZr2, etc.). Because the Si-Zr compound phase expands little when reacting with Li ions, it acts as an aggregate that maintains the particle structure, more effectively suppressing particle collapse and enabling improved capacity retention during repeated charge and discharge, i.e., cycle performance.

[0019] In this embodiment, the Si compound phase further includes an Si-X compound phase. Since the Si-X compound has different properties from the Si-Zr compound in terms of Li storage capacity, conductivity, etc., by configuring the Si compound phase to include both the Si-Zr compound phase and the Si-X compound phase, it is possible to improve the overall battery characteristics in consideration of the cycle characteristics, initial coulombic efficiency, and discharge rate characteristics.

[0020] Here, element X effective for improving initial coulombic efficiency is Ni, Co, Ti, V, or Mn. Element X effective for improving discharge rate characteristics is Ti. Element X effective for further improving cycle characteristics is Fe or Cr. To enhance the effect of improving characteristics due to the Si-X compound phase, it is desirable to set the Zr content, expressed as [Zr] / ([Zr]+[X]) (where [ ] indicates the content of the element in [ ] in at%), to 1 to 30%. The Si—X compound phase may be composed of only one type of compound, or may be composed of two types of compounds, for example, an Si—Fe compound and an Si—Ni compound.

[0021] Next, the Sn-Cu compound phase has a higher Li-ion diffusivity than the Si-Zr compound. Comparing the Li reactivity, the Si-Zr compound has a Li-ion diffusivity of 100 mAh / g, the Sn elemental phase has a Li-ion diffusivity of 930 mAh / g, and the Sn-X compound has a Li-ion diffusivity of 150-600 mAh / g.

[0022] That is, in the negative electrode active material of this example, a diffusion path for Li ions is easily secured through the Sn-Cu compound phase. On the other hand, the degree of expansion due to reaction with Li ions is smaller than that of Sn, which is highly reactive with Li ions, so the adverse effect on cycle performance due to the formation of the Sn-Cu compound can be kept low.

[0023] In this example, the proportion of the Sn-Cu compound phase in the entire battery is set to 0.1 to 18 mass%. The Sn-Cu compound phase expands upon reaction with Li ions, although to a lesser extent than simple Sn. Therefore, if the proportion of the Sn-Cu compound phase is excessively high, there is a risk of deterioration in cycle performance. A more preferable proportion of the Sn-Cu compound phase is 3 to 15.0 mass%. Incidentally, the Sn constituting the Sn-Cu compound may contain non-compound Sn simple substance as an impurity, provided that the proportion of the Sn occupying the entire compound is 5 mass % or less.

[0024] The contents of the main elements suitable for obtaining the above-described constituent phases are as follows: In the following description, "%" means "mass %" unless otherwise specified.

[0025] If the Si content is low, the initial discharge capacity will be low. However, if the Si content is too high, the cycle characteristics will deteriorate. Therefore, the Si content is preferably in the range of 45.0 to 85.0%, and more preferably in the range of 50.0 to 75.0%.

[0026] If the Zr content is low, the cycle characteristics will be poor. However, if the Zr content is too high, characteristics other than the cycle characteristics will also be poor. Therefore, the Zr content is preferably in the range of 0.1 to 40.0%, and more preferably in the range of 1.0 to 15.0%.

[0027] If the Cu content is low, it will not function as a Li diffusion path for Sn-Cu. However, if the Cu content is too high, the Sn-Cu will expand significantly, resulting in a decrease in cycle performance. Therefore, the Cu content is preferably in the range of 0.03 to 7.2%, more preferably 1.2 to 6.0%.

[0028] If the Sn content is low, it will not function as a Li diffusion path for Sn-Cu. However, if the Sn content is too high, the Sn-Cu will expand significantly, resulting in a decrease in cycle performance. Therefore, the Sn content is preferably in the range of 0.05 to 11.0%, more preferably 1.8 to 9.0%.

[0029] If the content of element X is low, the amount of Zr becomes relatively high, resulting in deterioration of properties other than cycle characteristics. However, if the content is too high, the amount of Zr becomes relatively low, resulting in deterioration of cycle characteristics. For this reason, the content of element X is preferably in the range of 1.0 to 40.0%, and more preferably in the range of 5.0 to 30.0%.

[0030] The form of the negative electrode active material is not particularly limited. Specific examples include flakes and powder. Powder form is preferable from the viewpoint of ease of application to the production of negative electrodes. The negative electrode active material of the present invention may be dispersed in a suitable solvent.

[0031] The negative electrode active material of the present invention can be produced by a method that involves quenching a molten alloy having a predetermined chemical composition to form a quenched alloy. If the obtained quenched alloy is not in powder form or if a smaller particle size is desired, a step of pulverizing the quenched alloy using an appropriate pulverizing means to form a powder may be added. If necessary, a step of classifying the obtained quenched alloy to adjust it to an appropriate particle size may also be added. It is also possible to produce the negative electrode active material of the present invention by separately preparing Si, Si-Zr compounds, Si-X compounds, and Sn-Cu compounds and then mixing them.

[0032] The particle size (average particle size (d50)) of the active material is preferably within the range of 0.1 to 20 μm. The average particle size (d50) in the present invention is based on volume and can be measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3000). Even when a Si alloy is used as the active material, the active material itself expands and contracts during charge-discharge reactions, generating stress in the composite layer (i.e., the conductive film) that binds the negative electrode active material with the binder. If the binder cannot withstand this stress, the binder collapses, resulting in peeling of the conductive film from the current collector, resulting in reduced conductivity within the electrode and reduced charge-discharge cycle performance. However, when the active material has an average particle size of 0.1 to 20 μm, the finer active material increases the contact area with the binder, effectively suppressing binder collapse and resulting in improved cycle performance.

[0033] In the above-described manufacturing method, the molten alloy can be obtained, for example, by weighing out each raw material so as to have a predetermined chemical composition, and melting the weighed raw materials using melting means such as an arc furnace, a high-frequency induction furnace, or a heating furnace.

[0034] Specific examples of methods for quenching the molten alloy include roll quenching (single roll quenching, twin roll quenching, etc.) and liquid quenching methods such as atomization (gas atomization, water atomization, centrifugal atomization, etc.), but it is particularly desirable to use roll quenching, which has a high cooling rate.

[0035] Here, when the negative electrode active material of the present invention is produced using a molten alloy containing Si and Zr, the following method may be specifically employed. Specifically, when using the roll quenching method, the molten alloy is poured into a chamber such as a quenching and recovery chamber and flows downward continuously (in a rod-like shape). The molten alloy is cooled on a rotating roll (made of a material such as Cu or Fe, and the roll surface may be plated) rotating at a peripheral speed of approximately 10 m / s to 100 m / s. The molten alloy is cooled on the roll surface to form a foil or foil flakes of the alloy material. In this case, the alloy material is pulverized using an appropriate pulverizing means such as a ball mill, bead mill, disk mill, coffee mill, or mortar grinder, and then classified or further pulverized as necessary to obtain a powdered negative electrode active material.

[0036] On the other hand, when the atomization method is applied, a molten alloy is poured into a spray chamber and flows downward continuously (in a rod-like shape). Gas such as N2, Ar, or He is sprayed at high pressure (e.g., 1 to 10 MPa) to crush and cool the molten alloy. The cooled molten alloy, while remaining semi-molten, falls freely within the spray chamber, gradually becoming spherical, yielding a powdered negative electrode active material. To improve the cooling effect, high-pressure water may be sprayed instead of gas.

[0037] 2. This battery The present battery is constructed using a negative electrode containing the present negative electrode active material.

[0038] The negative electrode has a conductive substrate and a conductive film laminated on the surface of the conductive substrate. The conductive film contains at least the present negative electrode active material described above in a binder. The conductive film may also contain a conductive additive as needed. When the conductive additive is contained, it becomes easier to ensure a conductive path for electrons.

[0039] The conductive film may also contain an aggregate, if necessary, which helps to suppress expansion and contraction of the negative electrode during charge and discharge, thereby preventing the negative electrode from collapsing, and thus further improving cycle characteristics.

[0040] The conductive substrate functions as a current collector. Examples of the material include Cu, Cu alloy, Ni, Ni alloy, Fe, and Fe-based alloy. Preferably, the conductive substrate is Cu or a Cu alloy. Specific examples of the conductive substrate shape include a foil shape and a plate shape. Preferably, the conductive substrate is foil-shaped, which allows for a smaller battery volume and improved freedom of shape.

[0041] Suitable materials for the binder include, for example, polyvinylidene fluoride (PVdF) resin, fluororesins such as polytetrafluoroethylene, polyvinyl alcohol resin, polyimide resin, polyamide resin, polyamideimide resin, styrene-butadiene rubber (SBR), and polyacrylic acid. These can be used alone or in combination. Of these, polyimide resin is particularly preferred because it has high mechanical strength, can withstand the volume expansion of the active material, and effectively prevents the conductive film from peeling off from the current collector due to binder destruction.

[0042] Examples of the conductive additive include carbon black such as ketjen black, acetylene black, and furnace black, graphite, carbon nanotubes, and fullerene. These may be used alone or in combination. Of these, ketjen black and acetylene black are preferably used from the viewpoint of easily ensuring electronic conductivity.

[0043] The content of the conductive additive is preferably 0 to 30 parts by mass, more preferably 4 to 13 parts by mass, relative to 100 parts by mass of the negative electrode active material, from the viewpoints of improved conductivity, electrode capacity, etc. The average particle diameter (d50) of the conductive additive is preferably 10 nm to 1 μm, more preferably 20 to 50 nm, from the viewpoints of dispersibility, ease of handling, etc.

[0044] As the aggregate, a material that does not expand or contract during charge and discharge or that expands or contracts very little can be preferably used. Examples include graphite, alumina, calcia, zirconia, and activated carbon. These may be used alone or in combination. Of these, graphite is preferred from the viewpoints of electrical conductivity, Li activity, and the like.

[0045] The content of the aggregate is preferably 10 to 400 parts by mass, more preferably 43 to 100 parts by mass, per 100 parts by mass of the negative electrode active material, from the viewpoint of improving cycle characteristics. The average particle size of the aggregate is preferably 10 to 50 μm, more preferably 20 to 30 μm, from the viewpoint of functionality as the aggregate and control of electrode film thickness. The average particle size of the aggregate is a value measured using a laser diffraction / scattering particle size distribution analyzer.

[0046] The present negative electrode can be produced, for example, by adding the present negative electrode active material, and, if necessary, a conductive additive and an aggregate in required amounts to a binder dissolved in a suitable solvent to form a paste, applying the paste to the surface of a conductive substrate, drying it, and, if necessary, subjecting it to compaction, heat treatment, or the like.

[0047] When a lithium ion battery is constructed using the present negative electrode, the basic components of the battery other than the present negative electrode, such as the positive electrode, electrolyte, and separator, are not particularly limited.

[0048] Specific examples of the positive electrode include those 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.

[0049] Specific examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent, etc. In addition, a lithium salt dissolved in a polymer, a polymer solid electrolyte in which a polymer is impregnated with the electrolytic solution, etc. can also be used.

[0050] Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc. These may be contained alone or in combination of two or more.

[0051] Specific examples of the lithium salt include LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, etc. One or more of these may be contained.

[0052] Other battery components include a separator, a can (battery case), a gasket, etc., and any of these can be appropriately combined to form a battery as long as they are materials that are typically used in lithium-ion batteries.

[0053] The shape of the battery is not particularly limited, and may be any shape such as cylindrical, rectangular, or coin-shaped, and can be selected appropriately according to the specific application. [Example]

[0054] The present invention will be described in more detail below with reference to examples. Note that % in the alloy composition is mass % unless otherwise specified.

[0055] 1. Preparation of negative electrode active material The raw materials shown in Table 1 below were weighed. The weighed raw materials were heated and melted using a high-frequency induction furnace to produce molten alloys. The resulting molten alloys were quenched using a single-roll quenching method to produce quenched alloy ribbons. The roll peripheral speed was 42 m / s and the nozzle distance was 3 mm. The resulting quenched alloy ribbons were mechanically pulverized using a mortar to produce powdered negative electrode active materials. The ratios of the raw materials shown in Table 1 were specified so as to obtain the target compositions shown in Table 2 below.

[0056] [Table 1]

[0057] 2. Confirmation of the constituent phases of the negative electrode active material The negative electrode active materials prepared in each of the examples and comparative examples were analyzed by XRD (X-ray diffraction) and confirmed to contain phases of Si, Si-Zr compounds, Si-X compounds, and Sn-Cu compounds. The XRD analysis was performed using a Co tube over an angle range of 120° to 20°.

[0058] 3. Calculation of Si phase amount The method for calculating the amount of Si phase (the proportion of Si phase in the total) shown in Table 2 below will be explained using Example 1 as an example. (1) First, the constituent phases of the produced powder were confirmed. In the case of Example 1, the XRD analysis confirmed the following phases: Si, Si2Zr, Si2Fe, and Sn5Cu6 (see Table 2). (2) Si2Zr is 38.1 [Si] - 61.9 [Zr] in mass %, which corresponds to the amount of Si that forms the compound: 26.1 × 38.1 / 61.9 = 16.1 (mass %). (3) Si2Fe is 50.1 [Si] - 49.9 [Fe] in mass %, which corresponds to the amount of Si that forms the compound: 9.0 × 50.1 / 49.9 = 9.1 (mass %). (4) Therefore, the amount of Si phase obtained by subtracting the amount of compounded Si from the total amount of Si is calculated as 54.9-25.2=29.7 (mass%). The amount of the Sn—Cu compound phase shown in Table 2 (the proportion of the Sn—Cu compound phase in the total) is the sum of the Sn amount and Cu amount shown in Table 1.

[0059] [Table 2]

[0060] 4. Evaluation of negative electrode active material 4.1 Preparation of coin-type batteries for charge / discharge tests First, 100 parts by mass of each negative electrode active material, 6 parts by mass of Ketjen Black (manufactured by Lion Corporation) as a conductive additive, and 19 parts by mass of a polyimide (thermoplastic resin) binder as a binding agent were blended, and this was mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare pastes containing each negative electrode active material.

[0061] Each coin-shaped half-cell was fabricated as follows. Here, for simple evaluation, an electrode fabricated using the negative electrode active material was used as the test electrode, and Li foil was used as the counter electrode. First, each paste was applied to a 50 μm thickness using a doctor blade method on the surface of SUS316L foil (thickness 20 μm) that would serve as the negative electrode current collector, and then dried to form each negative electrode active material layer. After formation, the negative electrode active material layer was consolidated using a roll press. In this way, test electrodes according to the examples and comparative examples were fabricated.

[0062] Next, the test electrodes according to the example and comparative example were punched out into disks with a diameter of 11 mm to prepare the respective test electrodes.

[0063] Next, Li foil (thickness 500 μm) was punched to approximately the same shape as the test electrode to prepare each counter electrode. Also, LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent of equal amounts of ethylene carbonate (EC) and diethyl carbonate (DEC) to prepare a non-aqueous electrolyte.

[0064] Next, each test electrode was housed in a positive electrode can (each test electrode would be a negative electrode in a lithium ion battery, but when the counter electrode was Li foil, the Li foil became the negative electrode and the test electrode became the positive electrode), and a counter electrode was housed in each negative electrode can, and a polyolefin-based microporous membrane separator was placed between each test electrode and each counter electrode.

[0065] Next, the non-aqueous electrolyte solution was poured into each can, and the negative electrode can and the positive electrode can were fixed by crimping.

[0066] 4.2 Evaluation of initial coulombic efficiency and cycle characteristics Each coin-type battery was subjected to one cycle of constant current charging and discharging at a current value of 0.2 mA. The capacity (mAh) used during this Li release was divided by the amount of active material (g) to determine the initial discharge capacity C0 (mAh / g). The ratio of the discharge capacity to the charge capacity in the above charge-discharge cycle was calculated as a percentage of discharge capacity / charge capacity, which was used as the initial coulomb efficiency (%).

[0067] As shown in Table 3 below, the initial coulombic efficiency was evaluated as "A" if it was 90% or more, "B" if it was between 85% and less than 90%, "C" if it was between 80% and less than 85%, "D" if it was between 70% and less than 80%, and "D" if it was less than 70%, and the results are shown in Table 2.

[0068] From the second cycle onward, a charge-discharge test was performed at a 1 / 5C rate (C rate: the current value at which the amount of electricity C0 required to charge and discharge the electrode is charged and discharged in 1 hour is defined as 1C. At 5C, it takes 12 minutes, and at 1 / 5C, it takes 5 hours). The cycle characteristics were evaluated by performing the above charge-discharge cycle 50 times. The capacity retention rate (discharge capacity after 50 cycles / initial discharge capacity (discharge capacity at the first cycle) × 100) was calculated from each discharge capacity obtained. As shown in Table 3 below, a capacity retention rate of 90% or more was evaluated as "A," a capacity retention rate of 80% to less than 90% was evaluated as "B," a capacity retention rate of 70% to less than 80% was evaluated as "C," a capacity retention rate of 60% to less than 70% was evaluated as "D," and a capacity retention rate of less than 60% was evaluated as "E." The results are shown in Table 2.

[0069] [Table 3]

[0070] 4.3 Evaluation of discharge rate characteristics Using each of the coin-type batteries, Li was absorbed to 0.002 V at a current of 0.2 C, and then desorbed to 1.0 V at currents of 0.2 C, 1 C, 2 C, 3 C, 4 C, and 5 C. The discharge capacity at 0.2 C was defined as the reference discharge capacity, and the C-rate at which the discharge capacity was less than half of the reference discharge capacity was determined. As shown in Table 3 above, if the C-rate at which the discharge capacity was less than half of the reference discharge capacity was 5 C or higher, it was evaluated as "A," if it was 4 C, it was "B," if it was 3 C, it was "C," if it was 2 C, it was "D," and if it was less than 2 C, it was "E." The results are shown in Table 2.

[0071] The overall evaluation shown in Table 2 is based on the evaluation results of the initial coulombic efficiency, cycle characteristics, and discharge rate characteristics. Specifically, an evaluation result of "A" was given 5 points, "B" was given 3 points, "C" was given 1 point, "D" was given -1 point, and "E" was given -5 points. For each example and comparative example, if the total score for the above three types of characteristic evaluation was 1 or more, it was considered "pass," and if it was less than 1, it was considered "fail."

[0072] The results in Table 2 obtained as described above reveal the following. Comparative Examples 1 to 3 are all examples that do not have an Si-X compound phase. However, the amount of Si phase differs between Comparative Examples 1 to 3. In Comparative Example 3, in which the amount of Si phase is increased compared to Comparative Example 1, which has a low amount of Si phase, the initial coulomb efficiency is increased, but conversely, the cycle characteristics are reduced. In an overall evaluation that takes into account the three types of characteristics, all of Comparative Examples 1 to 3 are unsatisfactory.

[0073] Comparative Examples 4 and 5 have a Si-Fe compound phase in addition to a Si-Zr compound phase as the Si compound phase. However, Comparative Example 4 does not have a Sn-Cu compound phase, and therefore has higher cycle characteristics but lower initial coulombic efficiency than Comparative Example 1, which has the same amount of Si phase. On the other hand, Comparative Example 5, which has a high Sn—Cu compound phase amount of 20 mass %, has high initial coulomb efficiency but poor cycle characteristics.The overall evaluations of Comparative Examples 4 and 5 are both unacceptable.

[0074] Comparative Examples 6 and 7 have a Si-Zr compound phase and a Si-Fe compound phase as Si compound phases, and further have 10 mass% of a Sn-Cu compound phase. However, Comparative Example 6 has a low Si phase amount of less than 10 mass%, and has high cycle characteristics but low initial coulombic efficiency. On the other hand, Comparative Example 7 has a high Si phase amount of more than 90 mass%, and has high initial coulombic efficiency but low cycle characteristics. The overall evaluation of Comparative Examples 6 and 7 is "fail." As described above, all of the comparative examples are "failed" in the overall evaluation.

[0075] In contrast, each example that includes a Si phase, a Si-Zr compound phase, a Si-X compound phase, and a Sn-Cu compound phase, and in which the amount of Sn-Cu compound phase is specified to be 0.1 to 18 mass % and the amount of Si phase is specified to be 10 to 90 mass %, all received a score of 1 or more in the overall evaluation and were "passed." It can be seen that the overall battery characteristics, including cycle characteristics, initial coulombic efficiency, and discharge rate characteristics, were improved.

[0076] Looking at each example in detail, Examples 1 to 11 have a Si-Fe compound phase as the Si-X compound phase. Comparing Examples 1 and 2, which have the same Si phase amount as Comparative Example 1, and Example 6, which has the same Si phase amount as Comparative Example 2, with these comparative examples, the initial coulombic efficiency and discharge rate characteristics are substantially the same, but the cycle characteristics are improved, demonstrating that the Si-Fe compound phase contributes to the improvement of the cycle characteristics. In particular, Examples 4, 5, 8, and 9, in which the Zr ratio expressed by [Zr] / ([Zr]+[X]) is low at 30% or less (i.e., the Fe ratio is high), show a significant effect in improving the cycle characteristics.

[0077] Examples 12 and 13 are examples in which the Si-Ni compound phase is included as the Si-X compound phase, and compared to Comparative Example 1 or Comparative Example 2 in which the amount of Si phase is the same, the effect of improving the initial coulomb efficiency is observed. The same applies to Examples 14 and 15, which include a Si-Co compound phase as the Si-X compound phase.

[0078] Examples 16 and 17 are examples in which the Si-Ti compound phase is included as the Si-X compound phase, and compared to Comparative Example 1 or Comparative Example 2 in which the amount of Si phase is the same, the effect of improving the discharge rate characteristics is observed.

[0079] Examples 18 and 19 are examples that have a Si-Cr compound phase as the Si-X compound phase, and compared to Comparative Example 1 or Comparative Example 2, which have the same amount of Si phase, they show an effect of improving cycle characteristics.

[0080] Examples 20 and 21 are examples that have a Si-V compound phase as the Si-X compound phase, and compared to Comparative Example 1 or Comparative Example 2, which have the same amount of Si phase, the effect of improving the initial Coulomb efficiency is observed. The same applies to Examples 22 and 23, which include a Si-Mn compound phase as the Si-X compound phase.

[0081] As described above, in the examples, depending on the type of element added as element X, it is possible to improve any one of the cycle characteristics, initial coulombic efficiency, and discharge rate characteristics.

[0082] The negative electrode active material for a lithium ion battery and the lithium ion battery of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments and examples, and various modifications are possible within the scope of the present invention.

Claims

1. The alloy is composed of a Si phase, a Si-Zr compound phase, a Si-X compound phase, and a Sn-Cu compound phase, and the element X is one element selected from the group consisting of Fe, Ni, Co, Mn, Ti, V, and Cr, A negative electrode active material for a lithium ion battery, characterized in that the proportion of the Sn-Cu compound phase in the entire negative electrode active material is 0.1 to 18 mass % and the proportion of the Si phase in the entire negative electrode active material is 10 to 90 mass %.

2. 2. The negative electrode active material for a lithium ion battery according to claim 1, wherein the proportion of the Si phase is 25 to 70 mass %.

3. 3. The negative electrode active material for a lithium ion battery according to claim 1, wherein a proportion of Zr, expressed as [Zr] / ([Zr]+[X]) (where [ ] indicates the content of the element in [ ] in at%), is 1 to 30%.

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