Si alloy powder for negative electrode of lithium ion battery

Abstract

To provide Si alloy powder for a negative electrode of a lithium ion battery having a new structure excellent in cycle characteristics.SOLUTION: The Si-alloy powder for the negative electrode of the lithium-ion battery has a Si phase and a Si2Fe compound phase. The amount of Si forming the Si phase is 50 to 80% by mass. In the Si phase, 0.01 to 3.0 mass% of an X element, which is at least one element selected from Ni, Cr, Co, Mn, Ti, V, Nb, Mo, Ta, W, and Hf, is dissolved.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a Si alloy powder for use in the negative electrode of 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 sometimes 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 is absorbed into the negative electrode active material during charging, and Li is released as ions 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 through an alloying reaction with Li, significant volume expansion and contraction occurs as the Li is 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] For this reason, various proposals have been made to alloy Si in negative electrode active materials using Si, as shown in Patent Document 1. In the alloying of Si, the Si compound phase formed around the Si phase acts to relieve stress caused by volume expansion during Li absorption in Si, thereby suppressing cracking and collapse of the Si phase and enabling improvement in cycle characteristics. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-62660 Summary of the Invention [Problem to be solved by the invention]

[0008] However, although the previously proposed alloying of silicon has been found to have a certain effect on improving cycle characteristics, the effect is not sufficient and there is still room for improvement. The present invention has been made in light of the above circumstances, with the object of providing a Si alloy powder for a lithium ion battery negative electrode having a novel structure and excellent cycle characteristics. [Means for solving the problem]

[0009] As a result of various investigations to solve the above problems, the present inventors have found that the cycle characteristics of a lithium-ion battery can be improved by dissolving a predetermined element in the Si phase that absorbs Li. The present invention is based on this finding.

[0010] The Si alloy powder for a lithium ion battery negative electrode of the present invention has the following properties: The alloy has a Si phase and a SiFe compound phase, The amount of Si forming the Si phase is 50 to 80 mass %; The alloy is characterized in that an X element, which is at least one element selected from Ni, Cr, Co, Mn, Ti, V, Nb, Mo, Ta, W, and Hf, is dissolved in the Si phase at 0.01 to 3.0 mass %.

[0011] The Si alloy powder for a lithium-ion battery negative electrode of the present invention may further contain a CuY compound phase, where the Y element is at least one element selected from Sn, Al, Ga, In, and Y, and the proportion of the CuY compound phase in the entire Si alloy powder is 0.1 to 10 mass %.

[0012] In the Si alloy powder for lithium ion battery negative electrodes of the present invention, a part of the Si phase can be replaced with a Ge phase, and the amount of Ge forming the Ge phase is 0.1 to 3.0 mass %. DETAILED DESCRIPTION OF THE INVENTION

[0013] Next, a Si alloy powder for a lithium ion battery negative electrode (hereinafter, sometimes simply referred to as a Si alloy powder for a negative electrode) according to one embodiment of the present invention and a lithium ion battery (hereinafter, sometimes simply referred to as a battery) using this Si alloy powder for a negative electrode in its negative electrode will be specifically described.

[0014] 1.Si alloy powder for negative electrode The Si alloy powder for a negative electrode is composed mainly of Si, Fe, and an X element, where the X element is at least one element selected from the group consisting of Ni, Cr, Co, Mn, Ti, V, Nb, Mo, Ta, W, and Hf.

[0015] The Si alloy powder for a negative electrode contains, as its metal structure, a Si phase, a Si2Fe compound phase, and a SiX compound phase.

[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] In the present Si alloy powder for a negative electrode, if the proportion of the Si phase that absorbs Li is low, the initial discharge capacity decreases. Therefore, in this embodiment, the amount of Si phase (the amount of Si that forms the Si phase) is set to 50 mass% or more, preferably 60 mass% or more, and more preferably 65 mass% or more. However, if the proportion of the Si phase is high, the proportion of the SiFe compound phase will relatively decrease, resulting in poor cycle characteristics. Therefore, in this embodiment, the amount of the Si phase is set to 80 mass% or less, preferably 78 mass% or less, and more preferably 75 mass% or less.

[0018] Here, in this Si alloy powder for a negative electrode, an X element is dissolved in the Si phase to improve cycle characteristics. It is presumed that dissolving the X element in the Si phase increases the conductivity of the Si phase, which contributes to uniform expansion of the Si phase during repeated charge and discharge, thereby suppressing cracking and collapse of the Si phase. To achieve this effect, in this embodiment, the X element is dissolved in the Si phase at 0.01% by mass or more. The X element is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more. However, there is a limit to the amount of X element that can be dissolved in the Si phase, and in this embodiment, the amount of X element that is dissolved in the Si phase is set to 3.0 mass % or less. In order to obtain such an effect, the content of the X element in the Si alloy powder is preferably 9.0 mass % or less, and more preferably 5.0 mass % or less.

[0019] It is also possible to replace a portion of the Si phase with a Ge phase that also absorbs Li. In this case, the amount of Ge can be set to 0.1 to 3.0 mass%. Adding a small amount of Ge stabilizes the Si structure, enabling uniform Li absorption and desorption. To achieve this effect, the lower limit of the Ge amount is set to 0.1 mass%. On the other hand, from the viewpoint of manufacturability, the upper limit of the Ge amount is set to 3.0 mass%.

[0020] Next, the Si2Fe compound phase, which is a Si compound phase, is a phase that mainly contains Si2Fe. The Si2Fe compound phase has poor Li absorption and expands very little when reacting with Li ions, and it serves as a skeleton that maintains the structure of the electrode. In addition, Si2Fe compounds have excellent conductivity, making it easier to ensure a diffusion path for Li ions. This is expected to result in a more uniform concentration of Li absorbed in Si. The X element added for the purpose of forming a solid solution in the Si phase is an element that forms a compound with Si, and the added X element that does not form a solid solution in the Si phase forms a SiX compound, which has substantially the same effect as a SiFe compound. Therefore, in this embodiment, in order to obtain the above-mentioned effects of the Si compound, the proportion of the Si compound phase, including the SiFe compound phase and the SiX compound phase, is preferably 10% by mass or more, more preferably 15% by mass or more. However, as the proportion of the Si compound phase increases, the proportion of the Si phase decreases relatively, resulting in a decrease in initial discharge capacity. Therefore, the proportion of the Si compound phase, including the SiFe compound phase and the SiX compound phase, is preferably 50% by mass or less, more preferably 40% by mass or less. In this embodiment, in order to fully obtain the effect of the Si2Fe compound phase, it is preferable that the amount of the Si2Fe compound is greater than the amount of the SiX compound phase.

[0021] The present Si alloy powder for a negative electrode can be configured to have a CuY compound phase as needed, where the Y element is at least one element selected from Sn, Al, Ga, In, and Y (yttrium). CuY compounds absorb Li like Si, but are less likely to form oxides than Si, making them effective in increasing Coulomb efficiency. To achieve this effect, the proportion of the CuY compound phase in the entire Si alloy powder can be set to 0.1 mass% or more. However, because CuY compounds expand upon absorbing Li, an excessively high proportion of the CuY compound phase will degrade cycle performance. For this reason, the proportion of the CuY compound phase must be set to 10 mass% or less.

[0022] The average particle size (median diameter d50) of the present Si alloy powder for anodes thus constructed is preferably 30 μm or less. This is because miniaturization suppresses the expansion of the Si phase and inhibits its collapse. It is more preferably 1 μm or less. However, if the particle size is too small, the specific surface area of ​​the Si alloy powder increases (the area in contact with the electrolyte increases), increasing the amount of irreversible reactions occurring on the surface. Therefore, the average particle size (d50) is preferably 0.1 μm or more. Here, the average particle size (d50) is based on volume and can be measured using a laser diffraction / scattering particle distribution analyzer.

[0023] Next, a method for producing the present Si alloy powder for a negative electrode will be described.

[0024] Each raw material is weighed out so as to obtain a predetermined chemical composition, and the weighed raw materials are melted using a melting means such as an arc furnace, a high-frequency induction furnace, or a heating furnace, and the resulting molten alloy is then quenched using an atomization method to obtain a Si alloy as a quenched alloy.

[0025] In the atomization method, molten alloy is poured into an atomization chamber and flows downward continuously (in a rod-like shape). Gas such as N2, Ar, or He is sprayed at high pressure (for example, 1 to 10 MPa) onto the molten alloy, pulverizing and cooling the molten alloy. The cooled molten alloy, while still semi-molten, falls freely through the atomization chamber, gradually becoming spherical, yielding Si alloy particles. Si phase, Si2Fe compound phase, and SiX compound phase are formed within the structure of the Si alloy particles, and some of the X element dissolves in the Si phase. In the atomization method, high-pressure water may be sprayed instead of gas in order to improve the cooling effect. In some cases, it is also possible to obtain a Si alloy foil by using a roll quenching method instead of the atomization method.

[0026] Next, the Si alloy particles are finely pulverized using a wet pulverization method to obtain the present Si alloy powder for the negative electrode. As the wet milling method, a wet milling method using a bead mill or a planetary ball mill can be used. In the wet milling method, a solvent is used together with the Si alloy particles to be milled. Examples of the solvent that can be used include ethanol, methanol, isopropyl alcohol, and naphthesol. It is also possible to add a dispersing agent. After wet pulverization, the solvent is removed by passing an inert gas such as argon through the pulverized material or by vacuum drying, and the present finely pulverized Si alloy powder for anode can be obtained.

[0027] 2.Battery Next, a battery constructed using a negative electrode containing the present Si alloy powder for a negative electrode will be described.

[0028] The negative electrode has a conductive base material and a conductive film laminated on the surface of the conductive base material. The conductive film contains at least the present Si alloy powder for a negative electrode 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.

[0029] 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.

[0030] 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.

[0031] 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), polyacrylic acid, and polyurethane. These materials can be used alone or in combination. Among 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.

[0032] 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.

[0033] 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 present Si alloy powder for a negative electrode, from the viewpoints of improved conductivity, electrode capacity, etc. The average particle size (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.

[0034] 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.

[0035] 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 present Si alloy powder for a negative electrode, from the viewpoint of improving cycle characteristics, etc. The average particle size of the aggregate is preferably 2 to 50 μm, more preferably 5 to 20 μm, from the viewpoint of functionality as the aggregate and control of electrode film thickness, etc. The average particle size of the aggregate is a value measured using a laser diffraction / scattering particle size distribution analyzer.

[0036] The present negative electrode can be produced, for example, by adding the present Si alloy powder for the negative electrode, and, if necessary, a conductive additive and an aggregate in required amounts to a binder dissolved in a suitable solvent to form a paste, which is then applied to the surface of a conductive substrate, dried, and, if necessary, subjected to compaction, heat treatment, etc.

[0037] 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.

[0038] Specific examples of the positive electrode include those in which a layer containing a positive electrode active material such as LiCoO2, LiNiO2, LiFePO4, LiMnO2, LiNi8Mn1Co1O2, LiNi6Mn2Co2O2, and LiNi1Mn1Co1O2 is formed on the surface of a current collector such as aluminum foil.

[0039] 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.

[0040] 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.

[0041] Specific examples of the lithium salt include LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, etc. One or more of these may be contained.

[0042] 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.

[0043] 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]

[0044] 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.

[0045] 1. Preparation of Si alloy powder for negative electrode Table 1 below shows the alloy compositions of 17 types of Si alloy powders for negative electrodes in Examples and 4 types of Si alloy powders for Comparative Examples.

[0046] First, each raw material shown in Table 1 was weighed. The weighed raw materials were heated and melted using a high-frequency induction furnace to prepare a molten alloy. Si alloy particles were produced from the molten alloy by gas atomization. The atmosphere during the preparation of the molten alloy and gas atomization was an argon atmosphere. During gas atomization, argon gas at high pressure (4 MPa) was sprayed onto the molten alloy falling in a rod-like shape within the atomization chamber. The obtained Si alloy particles were mechanically pulverized using a wet bead mill to produce Si alloy powder for negative electrodes with an average particle size (d50) of 0.2 to 1.0 μm.

[0047] [Table 1]

[0048] 2. Preparation of coin-cell batteries for charge-discharge tests 100 parts by mass of the prepared negative electrode Si alloy powder as the 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 polyimide (thermoplastic resin) binder as a binding agent were blended and mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare pastes containing each negative electrode Si alloy powder.

[0049] Next, each coin-shaped half-cell was fabricated as follows. Here, for simple evaluation, an electrode fabricated using a Si alloy powder for the negative electrode was used as the test electrode, and Li foil was used as the counter electrode. First, each paste was applied to a surface of SUS316L foil (thickness 20 μm) serving as the negative electrode current collector using a doctor blade method to a thickness of 50 μm, 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 3. Evaluation of Si alloy powder for negative electrodes 3-1. Calculation of the phase ratios of Si phase, Si compound phase, and CuY compound phase The ratios of the Si phase, Si compound phase (SiFe compound phase and SiX compound phase), and CuY compound phase formed in the metal structure of the obtained Si alloy powder were calculated from the raw material compositions shown in Table 1 and the amount of the X element dissolved in solid solution determined by measurement. The results are shown in Table 2 below. The amount of dissolved elements was determined by first measuring the weight, then eluting precipitates other than Si with hydrofluoric acid, and dividing the element ratio obtained from the remaining Si by the initial weight using inductively coupled plasma emission spectroscopy (ICP).

[0055] [Table 2]

[0056] 3-2. Charge / discharge test Each coin-type battery was subjected to one cycle of constant current charging and discharging at a current value of 0.2 mA. The initial discharge capacity C0 (mAh / g) was calculated by dividing the capacity (mAh) used during this Li release by the amount of active material (g). The results are shown in Table 2.

[0057] From the second cycle onwards, the charge-discharge test was carried out 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 1C. At 5C, it takes 12 minutes, and at 1 / 5C, it takes 5 hours). The cycle characteristics were then evaluated by repeating the above charge-discharge cycle 50 times. The discharge 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. The results are shown in Table 2.

[0058] The results in Tables 1 and 2 obtained as described above reveal the following. Comparative Examples 1 to 4 are all examples in which no X element is added and no solid solution of the X element is formed in the Si phase. Comparative Example 1 has a Si phase and a Si2Fe compound phase, but the discharge capacity retention rate is low at 68%. Comparative Example 2 is an example that does not have a Si2Fe compound phase. The discharge capacity retention rate is 49%, which is even lower than that of Comparative Example 1. In Comparative Example 3, the ratio of the Si phase is below the lower limit of 50 mass % of this embodiment, and the initial discharge amount is extremely low. Comparative Example 4 is an example in which the proportion of the Si phase exceeds 80 mass %, which is the upper limit of this embodiment. Since the proportion of the Si compound phase (here, the SiFe compound phase) is relatively small, the discharge capacity retention rate is even lower than that of Comparative Example 1. As described above, in Comparative Examples 1 to 4, either the initial discharge capacity or the discharge capacity retention rate is low, and the battery characteristics in consideration of the initial discharge capacity and cycle characteristics are not sufficiently improved.

[0059] In contrast, Examples 1 to 17 have a Si phase with a phase ratio (amount of Si forming the Si phase) of 50 to 80 mass % and a SiFe compound phase, and the X element is dissolved in the Si phase. Each Example shows good results in both initial discharge capacity and discharge capacity retention, and it is clear that the battery characteristics considering the initial discharge capacity and cycle characteristics are improved. Comparing Comparative Example 1 and Example 2, which have similar phase ratios, it is clear that Example 2, in which the X element (Cr) is dissolved in the Si phase, has a higher discharge capacity retention rate. Furthermore, when Example 2 and Example 10 are compared, it can be seen that in Example 10, in which part of the Si phase is replaced with the Ge phase (the Ge phase in Example 10 corresponds to part of the Si phase in Example 10 in Table 2), both the initial discharge capacity and the discharge capacity retention rate are higher.

[0060] Although the Si alloy powder for lithium-ion battery anodes and the lithium-ion batteries of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments and examples. For example, the Si alloy powder for anodes of the present invention can be applied to anode materials for all-solid-state lithium-ion batteries in addition to anode materials for liquid lithium-ion batteries as in the above-described embodiments. Various modifications can be made to the present invention without departing from the spirit and scope of the present invention.

Claims

1. Si phase and Si 2 and an Fe compound phase, the amount of Si forming the Si phase is 50 to 80 mass %, The Si alloy powder for a lithium-ion battery negative electrode comprises an X element, which is at least one element selected from Ni, Cr, Co, Mn, Ti, V, Nb, Mo, Ta, W, and Hf, and is solid-solved in the Si phase in an amount of 0.01 to 3.0 mass %.

2. Further, it has a CuY compound phase, The Y element is at least one element selected from Sn, Al, Ga, In, and Y, 2. The Si alloy powder for a lithium ion battery negative electrode according to claim 1, wherein the proportion of the CuY compound phase in the entire Si alloy powder is 0.1 to 10 mass %.

3. 3. The Si alloy powder for a lithium ion battery negative electrode according to claim 1, wherein a part of the Si phase is replaced with a Ge phase, and the amount of Ge forming the Ge phase is 0.1 to 3.0 mass%.

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