Electrode active material and method for manufacturing electrode active material

Abstract

The present invention provides a Si-containing powder material that can be used as an electrode active material for the negative electrode of a lithium-ion secondary battery, which can achieve high cycle characteristics without excessive particle refinement, and a method for manufacturing such an electrode active material. [Solution] The electrode active material comprises a powder material containing a Si phase consisting of Si in which element X is in solid solution, and a SiX compound phase consisting of a compound of Si and element X, wherein the powder material has a cumulative 50% particle size D50 of 0.1 μm or more and 1.0 μm or less, and a cumulative 99% particle size D99 of 3.0 μm or less. Here, element X is at least one selected from Fe, Ti, Cr, Mn, Co, Ni, Zr, B, and P. The method for producing the electrode active material is to carry out an atomization step, a grinding step, a granulation step, and a crushing step in this order.

Description

Technical Field

[0001] The present invention relates to an electrode active material and a method for manufacturing the electrode active material. More specifically, the present invention relates to an electrode active material containing Si and usable as an electrode active material for a negative electrode of a lithium ion secondary battery, and a method for manufacturing such an electrode active material.

Background Art

[0002] Conventionally, graphite has generally been used as an electrode active material constituting a negative electrode of a lithium ion secondary battery. However, since graphite has a low theoretical capacity, there is a limit to achieving high capacity by using graphite as a main component of the electrode active material. Therefore, Si has come to be used as an electrode active material as one of the materials showing a higher theoretical capacity than graphite.

[0003] In a lithium ion secondary battery, the negative electrode stores Li ions during charging and releases the Li ions during discharging. When a negative electrode is constituted using Si as an electrode active material, high capacity can be achieved as described above. However, since significant expansion and contraction occur during the absorption and release of Li ions, the destruction of the electrode active material tends to progress as the charge-discharge cycle is repeated. When the destruction of the electrode active material occurs, the conductive path in the negative electrode is gradually lost, and the characteristics of the lithium ion secondary battery deteriorate.

[0004] As one method for improving the battery characteristic maintainability (cycle characteristics) when repeating the charge-discharge cycle in the electrode active material constituting the negative electrode of a lithium ion secondary battery, there is a method of constituting the electrode active material using fine Si particles. By making the Si particles finer, even when expansion occurs due to the absorption of Li ions, the collapse of the particles due to the contact between the Si particles is suppressed. An electrode active material using fine Si-containing particles for improving cycle characteristics is disclosed in, for example, Patent Document 1.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Special Publication No. 2024-512562 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In electrode active materials for lithium-ion secondary batteries using Si, as described above, by miniaturizing the Si-containing particles, particle breakdown due to repeated charging and discharging can be suppressed, thereby improving cycle characteristics. However, when mechanically producing fine powder, wet grinding equipment is usually used, and aggregation of fine particles occurs during drying after grinding, making it difficult to control particle size. Furthermore, if the powder is too fine, the specific surface area of ​​the powder increases, requiring a large amount of binder when molding the powder with a binder to manufacture electrodes, resulting in a smaller proportion of Si in the overall electrode. In addition, excessively fine powder is difficult to handle. It is desirable to develop electrode active material materials that exhibit high cycle characteristics while avoiding excessive miniaturization of Si-containing particles.

[0007] The problem that the present invention aims to solve is to provide a Si-containing powder material that can be used as an electrode active material for the negative electrode of a lithium-ion secondary battery, which can obtain high cycle characteristics without excessive particle refinement, and a method for producing such an electrode active material. [Means for solving the problem]

[0008] To solve the above problems, the electrode active material and the method for producing the electrode active material of the present invention have the following configuration. [1] The first electrode active material according to the present invention comprises a powder material containing a Si phase consisting of Si in which element X is in solid solution, and a SiX compound phase consisting of a compound of Si and the element X, wherein the powder material has a cumulative 50% particle size D50 of 0.1 μm or more and 1.0 μm or less, and a cumulative 99% particle size D99 of 3.0 μm or less. Here, element X is at least one selected from Fe, Ti, Cr, Mn, Co, Ni, Zr, B, and P.

[0009] [2] In the embodiment of [1] above, the powder material may further include an XY compound phase comprising a compound of element X and element Y, where element Y is at least one of Sn and Al.

[0010] [3] In the embodiments of [1] or [2] above, the powder material may further include a CuY compound phase comprising a compound of Cu and element Y, where element Y is at least one of Sn and Al.

[0011] [4] In any of the embodiments described in [1] to [3] above, the element X may contain Fe.

[0012] [5] In any of the embodiments of [1] to [4] above, the electrode active material may further comprise at least one of a conductive additive mixed with the powder material and a coating of a conductive material covering the particle surface of the powder material.

[0013] [6] In the embodiment described in [5] above, the electrode active material may have a carbon coating on the particle surface of the powder material.

[0014] [7] The method for producing an electrode active material according to the present invention comprises: an atomization step of producing a Si alloy powder containing Si and element X by an atomization method; a grinding step of grinding the Si alloy powder by a wet grinding method to obtain a slurry in which particles containing a Si phase consisting of Si in which element X is solid-solved and a SiX compound phase consisting of a compound of Si and the element X are dispersed; a granulation step of obtaining a granulated body from a mixture obtained by mixing a binder with the slurry by a spray-drying method; and a crushing step of crushing the granulated body, all of which are carried out in this order to produce an electrode active material containing a powder material containing the Si phase and the SiX compound phase.

[0015] [8] In the embodiment described in [7] above, the method for producing the electrode active material may further include a heat treatment step between the granulation step and the crushing step, in which the granulated body is heat-treated to remove at least a portion of the binder in the granulated body.

[0016] [9] In the embodiments of [7] or [8] above, it is preferable that the particle size distribution of the powder material has a cumulative 50% particle size D50 of 0.1 μm or more and 1.0 μm or less, and a cumulative 99% particle size D99 of 3.0 μm or less.

[0017]

[10] The second electrode active material according to the present invention is manufactured by any of the methods for manufacturing electrode active materials described in [7] to [9] above. [Effects of the Invention]

[0018] The first electrode active material according to the present invention having the configuration described in [1] above includes a powder material containing a Si phase and a Si compound phase, and serves as a constituent material for an electrode active material exhibiting high capacity. Furthermore, the D50 of this powder material is 0.1 μm or more, which suppresses excessive micronization of the powder material. As a result, particle aggregation is suppressed, making it easier to control the particle size to a predetermined level, and eliminating the need to use an excessive amount of binder when forming it into an electrode. The handling properties of the powder material are also improved. On the other hand, by setting the D50 to 1.0 μm or less, when used as a constituent material for the negative electrode of a lithium-ion secondary battery, the density of the active material in the negative electrode can be increased, resulting in excellent battery characteristics, and particle collapse due to expansion associated with Li-ion intercalation is suppressed, resulting in high cycle characteristics. Furthermore, by setting the D99 to 3.0 μm or less, a sharp particle size distribution with a small particle size distribution range is obtained, resulting in an electrode active material that exhibits high cycle characteristics and other battery characteristics with high homogeneity.

[0019] In the embodiment described in [2] above, the inclusion of an XY compound phase in the powder material provides a high degree of effectiveness in suppressing particle disintegration due to expansion associated with the intercalation of Li ions. In addition to the XY compound phase, the powder material may also include a Y phase consisting of element Y in its pure form.

[0020] In the embodiment described in [3] above, the inclusion of a CuY compound phase in the powder material is highly effective in suppressing particle collapse due to expansion associated with the intercalation of Li ions.

[0021] In the embodiment described in [4] above, by including Fe as element X, an electrode active material with excellent cycle characteristics can be obtained while keeping material costs down.

[0022] In the embodiment described in [5] above, the electrode active material comprises at least one of a conductive additive mixed with a powder material and a conductive coating that covers the particle surface of the powder material. By mixing in a conductive additive and forming a conductive coating, the low conductivity of Si is compensated for, and a high degree of improvement in battery characteristics such as cycle characteristics can be obtained.

[0023] In the aspect of [6] above, a carbon coating is formed on the particle surface of the powder material. The carbon coating can be easily formed on the particle surface of the powder material, and particularly high effects can be obtained on the improvement of battery characteristics due to the improvement of conductivity.

[0024] In the manufacturing method of the electrode active material according to the present invention having the configuration of [7] above, after obtaining a granulated body containing a Si phase and a SiX compound phase by an atomizing step, a pulverizing step, and a granulating step, a crushing step is carried out, and by crushing the granulated body, an electrode active material containing a powder material containing a Si phase, a SiX compound phase, and composed of an aggregate of small particles can be manufactured. After the wet pulverizing step and through the granulating step, by performing crushing, the powder material can be reduced in diameter while suppressing the aggregation of small particles. However, in crushing, it is difficult to cause extreme reduction in diameter, and a powder material having a sharp particle size distribution with the particle size distribution suppressed in a narrow range is easily obtained. Therefore, an electrode active material that exhibits high cycle characteristics with high homogeneity can be manufactured while suppressing excessive refinement of the powder material.

[0025] In the aspect of [8] above, between the granulating step and the crushing step, a heat treatment step of removing at least a part of the binder in the granulated body by heat-treating the granulated body is carried out. By removing the binder in the heat treatment step, through the subsequent crushing step, the particles constituting the granulated body are more likely to be separated from each other. Thereby, in the powder material, it becomes easier to achieve reduction in particle diameter and a sharp particle size distribution.

[0026] In the particle size distribution of the powder material in the aspect of [9] above, the cumulative 50% particle diameter D50 is 0.1 μm or more and 1.0 μm or less, and the cumulative 99% particle diameter D99 is 3.0 μm or less. In this case, the manufactured electrode active material is particularly excellent in the effect of exhibiting high cycle characteristics while suppressing excessive refinement of the particles.

[0027] The second electrode active material according to the present invention having the configuration described in

[10] above is manufactured by the method for manufacturing the electrode active material of the present invention, and in particular, is manufactured by going through a crushing step after a granulation step, thereby exhibiting a high effect in improving cycle characteristics without excessively refining the particles. [Brief explanation of the drawing]

[0028] [Figure 1] These are the X-ray diffraction measurement results for Example 35. [Modes for carrying out the invention]

[0029] Hereinafter, an electrode active material according to an embodiment of the present invention and a method for producing the electrode active material will be described in detail.

[0030] [Electrode active material] First, the configuration of the electrode active material according to one embodiment of the present invention will be described. The electrode active material according to this embodiment can be used as a constituent material of the negative electrode of a lithium-ion secondary battery and functions as an active material in the negative electrode.

[0031] (1) Ingredient composition The electrode active material according to this embodiment comprises a powder material containing Si and other elements as components. The powder material contains at least a Si phase and a SiX compound phase. Here, element X is at least one selected from Fe, Ti, Cr, Mn, Co, Ni, Zr, B, and P.

[0032] The Si phase is composed of Si in which element X is in solid solution. The SiX compound phase is composed of a compound (intermetallic compound) of Si and element X. At least a portion of the Si phase and the SiX compound phase may form oxides.

[0033] Each of the Si phase and SiX compound phase, as well as the XY compound phase and CuY compound phase described later, constitutes particles independently of each other, except for components that inevitably form identical particles. Furthermore, in the electrode active material, the particles made up of each phase are basically dispersed as single particles (primary particles). Some of the particles may be aggregated by aggregation or binding via a binder, but preferably, except for these inevitably formed aggregated particles, they should be dispersed as single particles.

[0034] In electrode active material, the Si phase is the primary active material, but by including a SiX compound phase in addition to the Si phase, particle decay caused by volume changes due to Li ion intercalation is suppressed. This is because the SiX compound phase is less prone to volume changes due to Li ion intercalation. By suppressing particle decay, the electrode active material exhibits superior cycle characteristics, that is, excellent maintenance of battery characteristics (battery capacity, etc.) after repeated charge-discharge cycles.

[0035] As element X, at least one of those listed above can be used without restriction, but from the viewpoint of achieving both cost reduction and improved cycle characteristics, it is preferable that element X contains at least Fe. Furthermore, from the viewpoint of increasing the amount of element X dissolved in the Si phase and improving battery characteristics, it is preferable that element X contains two or more of those listed above. In other words, the most preferable form of electrode active material is one in which element X contains Fe and one or more other elements from those listed above. The element X contained in the Si phase by solid solution and the element X that forms a compound in the SiX compound phase may be the same or different, but are typically the same. The content of element X in the electrode active material is not particularly limited, but a form in which it is 0.1% by mass or more and 30% by mass or less of the total powder material can be exemplified as a preferred form. The ratio of the Si phase to the SiX compound phase is also not particularly limited, but for example, the proportion of the SiX compound in the total powder material may be 0.1% by mass or more and 60% by mass or less.

[0036] The powder material constituting the electrode active material preferably further comprises at least one of an XY compound phase and a CuY compound phase, in addition to the Si phase and SiX compound phase described above. Here, element Y is at least one of Sn and Al. The XY compound phase is composed of a compound of element X and element Y, and the CuY compound phase is composed of a compound of Cu and element Y. At least a portion of the XY compound phase and the CuY compound phase may form oxides.

[0037] The XY compound phase and CuY compound phase exhibit slight volume expansion upon Li-ion intercalation. Therefore, when included in the electrode active material along with the Si phase and SiX compound phase, they provide a buffering effect that mitigates the difference in volume expansion rates between the Si phase and the SiX compound. Thus, in the electrode active material, they play a role in suppressing particle decay associated with Li-ion intercalation and effectively improving cycle characteristics. The electrode active material may contain either the XY compound phase or the CuY compound phase, or both. However, if Cu is present in the composition, at least the CuY compound phase is more likely to form. On the other hand, if Cu is absent or present in small amounts in the composition, the XY compound phase is more likely to form. Furthermore, when the XY compound phase is formed, the Y phase, consisting of elemental Y, may also be present in the electrode active material along with the XY compound phase.

[0038] As described above, element Y can be at least one of Sn and Al, and there are no particular restrictions on its specific composition, but it is preferable that it contains at least Sn. The element Y constituting the XY compound phase and the element Y constituting the CuY compound phase may be the same or different, but are typically the same. The content of element Y in the electrode active material is not particularly limited, but a preferred example is when it is between 0.1% by mass and 5.0% by mass of the total powder material. Similarly, when the CuY compound phase is included, the content of Cu is not particularly limited, but a preferred example is when it is between 0.1% by mass and 3.0% by mass of the total powder material. The proportion of the XY compound phase and the CuY compound phase in the powder material is also not particularly limited, but for example, the combined proportion of the XY compound phase and the CuY compound phase in the total powder material may be between 0.1% by mass and 15% by mass.

[0039] The electrode active material may consist only of the above powder material containing a Si phase and a SiX compound phase, and optionally at least one of an XY compound phase and a CuY compound phase. However, if the above powder material alone does not provide sufficient conductivity to constitute the negative electrode of a lithium-ion secondary battery, a substance to supplement conductivity may be added as appropriate and used to constitute the negative electrode. Suitable substances for this purpose include at least one of a conductive additive mixed with the powder material and a conductive coating (conductive coating) that covers the surface of the particles of the powder material. In particular, a form in which a conductive coating is formed on the particle surface is preferred. Carbon powder can be suitably used as the conductive additive. A carbon coating can be suitably used as the conductive coating. Suitable examples of carbon materials constituting the conductive additive and the conductive coating include graphite, acetylene black, Ketjenblack, carbon nanotubes, carbon nanofibers, and the like. When forming a carbon film, suitable examples of the amount of the film include those in which the total amount of carbon in the electrode active material is 0.3% by mass or more, more specifically 0.8% by mass or more, more specifically 1.0% by mass or more, more specifically 10% by mass or less, more specifically 6% by mass or less, and more specifically 5% by mass or less.

[0040] In addition to conductive additives and conductive coatings, binders can be used as materials that constitute the electrode active material together with the powder material. Binders consist of organic polymers and have the function of binding particles together. Specific examples of binders include polyacrylic acid (PAA), water-soluble polyacrylates, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylidene fluoride (PVDF), and polyimide (PI), but among these, PAA is the most preferred. Since PAA is easily removed by heating, it is highly effective in reducing the particle size through the crushing process in the manufacturing method described later. As for the amount of binder in the electrode active material, a suitable example is a form in which the total amount of carbon, including the carbon in the carbon coating, is 0.3% by mass or more and 10% by mass or less relative to the total amount of the electrode active material.

[0041] (2) Particle size distribution The powder material constituting the electrode active material material according to this embodiment has a predetermined particle size distribution. Specifically, the cumulative 50% particle size D50 is between 0.1 μm and 1.0 μm. Furthermore, the cumulative 99% particle size D99 is 3.0 μm or less. Here, the particle size distribution is defined collectively for all particles constituting the powder material, including the Si phase and SiX compound phase, and also the XY compound phase and CuY compound phase, if present, without distinguishing between particle types. As described above, the particles in the powder material are basically in a dispersed state of primary particles, and the particle size distribution is defined for these primary particles. However, if aggregate particles formed by aggregation or binding via a binder are included, the particle size distribution is defined including the contribution of the particle size (secondary particle size) of the aggregate particles as a whole. In this specification, the particle size distribution is defined as a volume-based cumulative particle size distribution and is preferably evaluated by a laser diffraction-scattering particle size distribution measurement method. Even when a conductive coating is formed on the particle surface, the thickness of the coating is negligible compared to the particle size; therefore, the particle size distribution can be evaluated with the conductive coating in place.

[0042] If the particle size of the powder material is large, the expansion of Si phase particles due to Li ion absorption can cause adjacent particles to come into contact and easily disintegrate. However, by setting the D50 of the powder material to 1.0 μm or less, such particle disintegration can be suppressed, and the cycle characteristics of the electrode active material can be improved. From the viewpoint of enhancing this effect, a D50 of 0.8 μm or less, and even more preferably 0.6 μm or less, is preferable.

[0043] On the other hand, excessively fine-graining the powder material constituting the electrode active material can also cause problems. One such problem is that the specific surface area of ​​the powder material increases, making particle aggregation more likely. Particle aggregation can lead to a decrease in battery characteristics, such as a decrease in cycle characteristics. Another problem with excessive fine-graining is that a large amount of binder is required when forming the electrode active material into electrodes. This reduces the proportion of active material in the electrode, leading to a decrease in battery characteristics. Another problem with excessive fine-graining is a decrease in the handling properties of the electrode active material. However, if the D50 of the powder material constituting the electrode active material is set to 0.1 μm or larger, these problems caused by excessive fine-graining of the particles constituting the electrode active material do not have a practical impact. From the viewpoint of enhancing this effect, a D50 of 0.12 μm or larger, and even 0.15 μm or larger, is preferable.

[0044] A D99 of 3.0 μm or less for the powder material constituting the electrode active material means that even a small amount of large-diameter particles are not included in the powder material, and the particle size distribution of the powder material is sharp and confined to a narrow particle size range. As a result, the electrode active material exhibits excellent battery characteristics, such as excellent cycle characteristics, with high homogeneity. From the viewpoint of enhancing this effect, a D99 of 2.0 μm or less, and even 1.5 μm or less, is more preferable. There is no specific lower limit for D99, but for example, it is good to set it to 0.5 μm or more, and even 0.7 μm or more.

[0045] Thus, by having a D50 of 0.1 μm or more and a D99 of 3.0 μm or less in the powder material constituting the electrode active material, high cycle characteristics can be obtained in the electrode active material without excessive particle refinement. Powder material having such a particle size distribution can be suitably obtained by obtaining a granule by wet crushing of an alloy material and granulation using a binder, as in the method for manufacturing electrode active material according to the embodiment of the present invention described below, and then crushing the granule. It should be noted that, as described above, the electrode active material is not limited to the first electrode active material of the present invention which contains powder material in which D50 is 0.1 μm or more and D99 is 3.0 μm or less, but by employing the method for manufacturing electrode active material according to the embodiment of the present invention described below, it is possible to manufacture an electrode active material (second electrode active material of the present invention) which contains powder material with a sharp particle size distribution while suppressing excessive refinement by reducing the particle diameter.

[0046] [Method for manufacturing electrode active material] Next, a method for manufacturing an electrode active material according to an embodiment of the present invention will be described. In the method for manufacturing an electrode active material according to this embodiment, the atomization process, the pulverization process, the granulation process, and the crushing process are carried out in this order. Furthermore, although optional, it is preferable to carry out a heat treatment process between the granulation process and the crushing process, and a coating formation process after the crushing process. Each process will be described in order below.

[0047] (1) Atomization process In the atomization process, Si alloy powder is prepared by atomization. The Si alloy powder should be prepared as an alloy powder containing the desired elemental components in the desired ratios in the overall component composition of the powder material constituting the electrode active material to be manufactured. In other words, an alloy powder containing Si and element X (at least one selected from Fe, Ti, Cr, Mn, Co, Ni, Zr, B, P), and optionally element Y (at least one of Sn and Al), should be prepared. Any atomization method can be used, such as water atomization or gas atomization, but gas atomization is preferred. In the obtained Si alloy powder, a Si phase and a SiX compound phase, and depending on the component composition, an XY compound phase and / or a CuY compound phase crystallize.

[0048] (2) Grinding process In the grinding process, the Si alloy powder obtained in the atomization process is ground using a wet grinding method. The specific grinding method is not limited; wet grinding using an attritor, grinding using a bead mill, etc., can be suitably applied. It is also preferable to combine multiple grinding methods, such as grinding first with an attritor and then with a bead mill. By adjusting the conditions of this grinding process, the particle size of the powder material in the final electrode active material can be effectively controlled. For example, increasing the time the grinding process is performed reduces the overall particle size of the powder material. The solvent (dispersion medium) used in the wet grinding process is not particularly limited; water, alcohol-based solvents, amide-based solvents, etc., can be used.

[0049] The grinding process yields a slurry in which particles containing the Si phase, SiX compound phase, and depending on the component composition, the XY compound phase and / or CuY compound phase are dispersed in a solvent. As described above, in the Si alloy powder obtained in the atomization process, multiple phases crystallize, and multiple phases coexist inside at least some of the particles. However, through the grinding process, these multiple phases are basically separated into independent particles. In other words, a slurry is obtained as a dispersion of a mixture of Si particles, SiX compound particles, and depending on the component composition, XY compound particles and / or CuY compound particles.

[0050] (3) Granulation process In the granulation process, a granular material is formed using the slurry produced in the grinding process. To form the granular material, a spray-drying method is applied to the mixture obtained by mixing the slurry with a binder. In the spray-drying method, the mixture is formed into fine droplets and instantly dried with hot air to remove the solvent, thereby forming a granular material in which multiple particles are bound together by the binder. In the spray-drying method, drying proceeds simultaneously with granulation, and the granular material is obtained in a dry state.

[0051] As described above, various organic polymers can be used as binders, and PAA is particularly suitable. The binder content in the mixture used as a raw material for the spray drying method is preferably 1% by mass or more and 50% by mass or less. If the binder content is 1% by mass or more, the increase in flammability of the electrode active material due to the exposure of small-diameter particles on the surface can be suppressed, thereby improving flame resistance. On the other hand, if the binder content is 50% by mass or less, the binding of particles within the granules by the binder can be released in the crushing process after an appropriate heat treatment process, making it easier to reduce the particle size. More preferably, the binder content is 3% by mass or more and 20% by mass or less.

[0052] (4) Heat treatment process In the heat treatment process, the granules obtained in the granulation process are subjected to heat treatment. The heat treatment is carried out by dry heating of the granules using a heating furnace or the like. Through the heat treatment process, at least a portion of the binder constituting the granules is removed. In particular, when the binder is PAA, the removal of the binder proceeds more easily. Examples of heating temperatures in the heat treatment process include 500°C to 1000°C.

[0053] (5) Crushing process In the crushing process, the granules obtained in the granulation process, which have optionally undergone a heat treatment process, are crushed. The crushing process is carried out dry. For example, by introducing the granules into a jet mill and accelerating them within the container with compressed gas, crushing can be achieved through repeated contact and friction between the granules.

[0054] In granulated materials, multiple particles are bound together and aggregated via a binder. However, by performing a crushing process, this binding is broken, and the particles that made up the granulated material become dispersed as single particles. Alternatively, even if they are not reduced to single particles, the material becomes composed of fewer particles than the original granulated material.

[0055] The crushing process is a process that crushes particles more gently than the preceding grinding process, and extremely fine particles are less likely to be formed. In addition, the particle size after grinding is relatively uniform, and a sharp particle size distribution is easily formed. Furthermore, unlike the case in which particles are ground to a desired small diameter in a wet grinding process and then the slurry is removed to obtain powder material, particle aggregation is less likely to occur, thus suppressing the increase in particle size due to aggregation. Thus, in the method of manufacturing electrode active material according to this embodiment, by performing a crushing process on the granulated body that has been formed, the particle size can be reduced while avoiding excessive fineness of the powder material, the disintegration of particles due to Li ion absorption is suppressed, and an electrode active material exhibiting high cycle characteristics can be obtained. Furthermore, by controlling the conditions of the crushing process, such as the pressure of the compressed gas of the jet mill, the material feed rate, and the execution time of the crushing process, the particle size distribution of the obtained powder material, such as specific values ​​of D50 and D99, can be adjusted. Since powder material with a sharp particle size distribution is easily obtained in the crushing process, there is no need to separately classify the powder material in the method of manufacturing electrode active material according to this embodiment.

[0056] (6) Film formation process In the coating formation process, a conductive material coating is formed on the particle surface of the powder material obtained through the crushing process. When the coating is composed of a carbon coating, the coating can be formed by a vapor deposition method such as chemical vapor deposition (CVD).

[0057] (7) Other processes If the electrode active material contains a conductive additive, the conductive additive powder can be mixed with the powder material obtained by each of the above steps. Furthermore, when creating a negative electrode from the manufactured electrode active material, the electrode active material can be molded into a predetermined electrode shape. For molding, for example, the electrode active material powder can be mixed with a binder or solvent to create a paste, which can then be applied to the current collector. Note that the binder used for molding here is distinct from the binder used in the granulation step described above. [Examples]

[0058] The present invention will be described in more detail below using examples. Here, electrode active material was prepared with various configurations and its properties were compared.

[0059] [Sample preparation] Electrode active material materials according to Examples 1-41 and Comparative Examples 1-6 were prepared as samples. For Examples 1-39, samples were prepared using the following basic manufacturing method. For the other samples, they were manufactured using a method that modified part of the basic manufacturing method, as shown below as an alternative manufacturing method.

[0060] (1)Basic manufacturing method As a basic manufacturing method, the atomization process, grinding process, granulation process, heat treatment process, crushing process, and coating formation process were carried out in this order, according to the method for manufacturing electrode active material according to the embodiment of the present invention described above. In the atomization process, alloy powder having the component composition shown in Table 1 was prepared by gas atomization using nitrogen gas. In the grinding process, wet grinding was performed using an attritor and a bead mill in sequence. In this case, the grinding time with the bead mill was set to 3 to 24 hours. In the granulation process, a raw material containing 5% by mass of PAA as a binder was used, and the spray drying method was performed. In the heat treatment process, heating was performed at 800°C. In the crushing process, the granulated material was crushed using a jet mill. The particle size distribution of the powder material produced after the crushing process was adjusted by controlling the grinding time in the grinding process and the crushing conditions in the crushing process. In the coating formation process, a hydrocarbon gas was used as a raw material for forming a conductive coating, and a carbon coating was formed on the particle surface of the powder material by the CVD method. The thickness of the carbon film was adjusted by controlling the CVD temperature and time. The actual amount of carbon film formed was confirmed as the total amount of carbon in the electrode active material, as described in the evaluation method section.

[0061] (2) Other manufacturing methods As a manufacturing method that modifies part of the basic manufacturing method described above, the following manufacturing method was applied to some of the examples and comparative examples. • Extended grinding time (Comparative Example 1) - The grinding time using a bead mill in the grinding process was set to 50 hours. • Reduced grinding time (Comparative Example 2) - The grinding time using the bead mill in the grinding process was set to 2 hours. • Binder reduction (Examples 40, 41) - In the granulation process, the amount of binder added to the raw materials by the spray-drying method was reduced compared to the basic method. The binder content was 0.8% by mass in Example 40 and 0.9% by mass in Example 41. • Binder change (Comparative Examples 3 and 4) - In the granulation process, PVB, which is more likely to remain than PAA, was used as the binder instead of PAA. The binder content was 5% by mass in Comparative Example 3 and 15% by mass in Comparative Example 4. • No heat treatment (Example 41, Comparative Example 5) - The heat treatment in the basic manufacturing method was omitted, and the crushing process was carried out immediately after the granulation process. • No crushing (Comparative Example 6) - The crushing step of the basic manufacturing method was omitted, and the coating formation step was carried out immediately after the heat treatment step.

[0062] [Evaluation Method] <State of electrode active material> First, we evaluated the state of the powdered electrode active material.

[0063] (1) Evaluation of carbon content To evaluate the amount of carbon film formed during the coating process, the total carbon content of the electrode active material in each sample was quantified. The quantification was performed on the prepared electrode active material using a combustion-infrared absorption method with a carbon-sulfur analyzer.

[0064] (2) Evaluation of particle size distribution The particle size distribution of the electrode active material of each sample was evaluated using laser diffraction-scattering particle size distribution analysis. From the obtained particle size distribution, the volume-based cumulative 50% particle size (D50) and cumulative 99% particle size (D99) were acquired.

[0065] (3) Analysis of phase ratio The phase composition of the electrode active material in each sample was analyzed by X-ray diffraction (XRD) measurement. XRD measurements were performed using Co Kα rays and the θ-2θ method, with 2θ in the range of 120° to 20°. The constituent phases present in each electrode active material were identified from the XRD analysis results. Furthermore, the proportion of each phase was quantified for the Si phase, SiX compound phase, and the sum of the XY compound phase and CuY compound phase (XY&CuY).

[0066] (4) Flame resistance evaluation To evaluate the flame resistance of the electrode active material of each sample, a small gas flame ignition test was conducted in accordance with the Class 2 hazardous materials testing standards. In the small gas flame ignition test, a small gas flame was brought into contact with the electrode active material, and samples that did not ignite within 10 seconds were evaluated as having high flame resistance. On the other hand, samples that ignited within 10 seconds were evaluated as having low flame resistance.

[0067] <Battery properties of electrode active material> Furthermore, the battery characteristics were evaluated using electrodes formed from each electrode active material.

[0068] (1) Fabrication of coin-type batteries for charge-discharge testing First, 100 parts by mass of the electrode active material from each sample prepared above, 5 parts by mass of acetylene black (manufactured by Denka Co., Ltd.) as a conductive additive, and 15 parts by mass of polyimide as a binder were mixed together. This mixture was then mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a paste containing the electrode active material.

[0069] Next, a coin-type half-cell was fabricated as follows. For a simplified evaluation, the electrode fabricated using the above-mentioned electrode active material was used as the test electrode, and Li foil was used as the counter electrode. First, the pastes prepared above were applied to the surface of a 20 μm thick stainless steel (SUS) 316L foil, which would serve as the current collector, to a thickness of 30 μm using the doctor blade method, and then dried to form the electrode active material layer. After formation, the electrode active material layer was compacted using a roll press, and then punched out into a disc shape with a diameter of 11 mm to serve as the test electrode.

[0070] Next, a Li foil (500 μm thick) was punched out in approximately the same shape as the test electrode to prepare a counter electrode. A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / l in an equal-volume mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). The test electrode was then placed in the positive electrode container (although the test electrode should be the negative electrode in a lithium secondary battery, when the counter electrode is Li foil, the Li foil becomes the negative electrode and the test electrode becomes the positive electrode), and the counter electrode was placed in the negative electrode container. A polyolefin microporous membrane separator was placed between the test electrode and the counter electrode. Next, the non-aqueous electrolyte was poured into the containers, and the negative electrode container and the positive electrode container were crimped together. This obtained a coin-type battery.

[0071] (2) Charge and discharge test Each fabricated coin-type half-cell was subjected to one cycle of constant current charging and discharging at a current of 0.2 mA. The initial charging capacity and initial discharging capacity (mAh / g) were determined by dividing the charging and discharging capacity (mAh) by the amount of electrode active material (g). The ratio of the initial discharging capacity to the initial charging capacity was calculated as the initial Coulomb efficiency (initial discharging capacity / initial charging capacity × 100%).

[0072] Separately, the theoretical capacity of each electrode active material was calculated based on the phase ratios of the Si phase, SiX compound phase, XY compound phase, and CuY compound phase. Then, the ratio of the initial discharge capacity measured above to this theoretical capacity was determined and used as the active material utilization rate (initial discharge capacity / theoretical capacity × 100%).

[0073] Furthermore, the charge-discharge test performed for one cycle as described above was also performed for the second and subsequent cycles. However, from the second cycle onward, the charge-discharge test was performed at a 1 / 5C rate. Here, regarding the C rate, 1C is defined as the current value required to charge and discharge the electrodes in one hour. At 5C, it takes 12 minutes to charge and discharge, and at 1 / 5C, it takes 5 hours. The charge-discharge cycle was performed 50 times, and the ratio of the discharge capacity at the 50th cycle to the initial discharge capacity at the 2nd cycle was calculated and used as the capacity retention rate (discharge capacity at the 50th cycle / discharge capacity at the 2nd cycle × 100%). If the capacity retention rate is 80% or higher, the cycle characteristics can be considered to be sufficiently high.

[0074] [Test Results] Table 1 below summarizes the component composition and manufacturing method of the powder material constituting the electrode active material for Examples 1-41 and Comparative Examples 1-6, and Table 2 summarizes the evaluation results. Figure 1 shows the XRD measurement results for the electrode active material of Example 35 as a representative example.

[0075] [Table 1]

[0076] [Table 2]

[0077] According to Tables 1 and 2, in all of Examples 1 to 41, the electrode active material contained at least Si and element X, and was manufactured by a process of crushing the granulated material that had been formed. Corresponding to these component compositions and manufacturing methods, electrode active materials were obtained that contained at least a Si phase and a SiX compound phase, with a D50 of 0.1 μm to 1.0 μm and a D99 of 3.0 μm or less. Furthermore, in all of the examples, a capacity retention rate of 80% or more was obtained, indicating that the electrode active material possessed high cycle characteristics.

[0078] In Examples 15-23, neither Cu nor element Y is present, so neither the XY compound phase nor the CuY compound phase is formed (the phase ratio of "XY&CuY" is zero). However, in Examples 1-14 and 24-41, element Y is present, and at least one of the XY compound phase and the CuY compound phase is formed. Among these Examples 1-14 and 24-41, Cu is not present in Examples 35-39. As a representative example, the XRD chart for Example 35 is shown in the upper part of Figure 1. The lower part shows library data for elemental Si, Fe-Si compounds (SiX compounds), elemental Sn, and Fe-Sn compounds (XY compounds). According to this XRD chart, in addition to the Si phase consisting of Si with solid solution of X and the SiX compound phase consisting of Fe-Si compounds, it can be confirmed that the Fe-Sn compound phase is formed. The Sn phase is also formed. Thus, in Examples 35-39, which did not contain Cu, an XY compound phase was formed in addition to the Si phase and the SiX compound phase. On the other hand, in Examples 1-14, 24-34, and 40-41, which contained both Cu and element Y, at least a CuY compound phase was formed, and in some samples, an XY compound phase was also formed.

[0079] Of Examples 1 to 41, only Examples 40 and 41 use less binder in the granulation process than the other examples. In Examples 40 and 41, the battery characteristics are similar to those of the other examples, but the flame resistance is lower. This is interpreted as being due to the fact that the surface of small-diameter Si-containing particles is more exposed because of the smaller amount of binder, and it can be said that these are preferable for use in applications where high flame resistance is not required. In Example 41, heat treatment was not performed, but in response to the reduced amount of binder, the particle size of the manufactured electrode active material did not become too large, and D50 and D90 within the predetermined range were obtained.

[0080] None of the comparative examples 1 to 6 have a particle size distribution where D50 is between 0.1 μm and 1.0 μm, and D99 is 3.0 μm or less. In comparative example 1, the D50 is small because the wet grinding time in the grinding process is extended. In comparative example 2, both D50 and D99 are large because the wet grinding time in the grinding process is shortened. In comparative examples 3 and 4, D99 is large because a binder that tends to remain is used, making it easier for relatively large diameter particles to remain without being sufficiently crushed. In comparative example 5, both D50 and D99 are large because the binder in the granules remains because a heat treatment process is not performed. In comparative example 6, both D50 and D99 are significantly large because the particle binding within the granules is not resolved because a crushing process is not performed. In the samples of these comparative examples 1 to 6, the volume retention rate is below 80% whether D50 and / or D99 are large or small, indicating that sufficiently high cycle characteristics are not obtained.

[0081] From the above test results, it can be seen that in an electrode active material containing a powder material containing at least a Si phase and a SiX compound phase, and optionally an XY compound phase and / or a CuY compound phase, setting D50 to 0.1 μm or more and D99 to 3.0 μm or less results in an electrode active material exhibiting high cycle characteristics. Furthermore, by adopting a manufacturing method in which granules are first formed and then crushed, an electrode active material having such a controlled particle size distribution can be manufactured.

[0082] The embodiments of the present invention have been described above. The present invention is not particularly limited to these embodiments, and various modifications are possible.

Claims

1. A Si phase consisting of Si in which element X is in solid solution, The powder material contains a SiX compound phase, which is a compound of Si and the element X, The aforementioned powder material is The cumulative 50% particle size D50 is between 0.1 μm and 1.0 μm. An electrode active material in which the cumulative 99% particle size D99 is 3.0 μm or less. Here, element X is at least one selected from Fe, Ti, Cr, Mn, Co, Ni, Zr, B, and P.

2. The electrode active material material according to claim 1, wherein the powder material further comprises an XY compound phase consisting of a compound of element X and element Y. Here, the element Y is at least one of Sn and Al.

3. The electrode active material material according to claim 1 or claim 2, wherein the powder material further comprises a CuY compound phase consisting of a compound of Cu and element Y. Here, the element Y is at least one of Sn and Al.

4. The electrode active material material according to claim 1 or claim 2, wherein the element X comprises Fe.

5. A conductive additive mixed with the aforementioned powder material, and The electrode active material material according to claim 1 or claim 2, further comprising at least one of a coating of a conductive material that covers the particle surface of the powder material.

6. The electrode active material material according to claim 5, wherein the particle surface of the powder material has a carbon coating.

7. The atomization process involves preparing Si alloy powder containing Si and element X using the atomization method, A grinding step of grinding the Si alloy powder by a wet grinding method to obtain a slurry in which particles are dispersed, comprising a Si phase consisting of Si in which element X is solid-solved, and a SiX compound phase consisting of a compound of Si and the element X. A granulation step is to obtain granules from a mixture obtained by mixing a binder with the slurry using a spray-drying method. The crushing step for crushing the granular material is carried out in this order, A method for producing an electrode active material, comprising producing an electrode active material containing the Si phase and the SiX compound phase in a powder material.

8. Between the granulation step and the crushing step, The method for producing an electrode active material according to claim 7, further comprising a heat treatment step of heat-treating the granules to remove at least a portion of the binder in the granules.

9. In the particle size distribution of the aforementioned powder material, The cumulative 50% particle size D50 is set to be between 0.1 μm and 1.0 μm. A method for producing an electrode active material according to claim 7 or claim 8, wherein the cumulative 99% particle size D99 is 3.0 μm or less.

10. An electrode active material manufactured by the method for manufacturing an electrode active material described in claim 7 or claim 8.

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