JP2005285382A - Manufacturing method for active material for lithium secondary battery, active material for the lithium secondary battery and the lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method for a silicon-based active substance material having high stability. <P>SOLUTION: This manufacturing method comprises (1) a process which mixing and heating silicon-based active substance material powder and a matrix raw material, having a carbon skeleton for pyrolyzing the matrix raw material, (2) a process of applying compression or shear force to a thermally decomposed object, and (3) a process of mixing and heating a second matrix raw material for pyrolyzing. A silicon-carbon composite material, with firm connection between the silicon active substance material powder and the matrix, can be obtained by a mechanical action etc. by applying compression or the shear force, after pyrolyzing the matrix raw material mixed with the silicon active substance material powder. The silicon-carbon composite material can be dispersed to the matrix by mixing and pyrolyzing the second matrix raw material. According to above processes, the silicon-based active substance material powder and the matrix can be firmly combined, without an element little directly affecting a cell reaction, such as a binder. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing an active material for a lithium secondary battery using a silicon-based active material and an active material for a lithium secondary battery, and further relates to a lithium secondary battery using the active material for a lithium secondary battery. .

Since the lithium ion secondary battery was commercialized in 1991, its high energy density and operating voltage have attracted a great deal of attention. A commercially available battery has a configuration of an organic electrolyte containing graphite as a negative electrode active material, lithium intercalation oxide as a positive electrode active material, and LiPF ₆ as a salt.

Here, the negative electrode active material made of graphite has a small battery capacity, and instability when lithium is occluded is a problem. Therefore, in order to further improve the battery capacity, the focus is on the search for new negative electrode materials other than graphite. Among them, silicon-based active material materials such as silicon, silicon boride, siliconide, and silicon oxide (SiO _x ) are considered promising. Taking silicon as an example, it is known that silicon forms an Li-Si alloy electrochemically at a low operating voltage (vs. Li) and exhibits a large capacity of 4000 mAh / g.

  However, this system has significant application problems. That is, a very large volume change is accompanied when lithium is inserted into and desorbed from the active material. Although the electrode stability of the Li—Si alloy negative electrode can be improved to some extent by reducing the particle size (nanosize), it is not sufficient.

  By the way, in recent years, attention has been focused on a composite material of a lithium storage alloy and a carbon material. For example, it has been reported that silicon coated with carbon by the CVD method exhibits extremely excellent cycle characteristics at a capacity of 600 mAh / g or more (Non-patent Document 1). Further, it has been reported that a thermal decomposition product of pitch in which silicon and graphite are embedded also stabilizes silicon (Non-patent Document 2). The improvement of the cycle characteristics is considered to be due to the uniform dispersion of Si in the carbon matrix. Furthermore, it is thought that soft carbon can absorb the volume change of Li-Si alloy and can improve the mechanical strength as an electrode. Furthermore, it has been reported that silicon and graphite are mixed and pulverized (Non-patent Document 3).

It has also been reported that a silicon alloy and graphite are simply mixed (Patent Document 1). It has been reported that silicon is supported on carbon particles having a predetermined lattice spacing and further covered with a carbon material (Patent Document 2). Patent Document 2 exemplifies a method in which coating with a further carbon material is performed by thermally decomposing the polymer material after mixing with the polymer material. Reported adoption of Si-O-C-based composites, which are produced by thermal decomposition of composites obtained by condensation / polymerization of organic compounds containing silicon elements and polymer materials used as carbon sources, as negative electrode materials (Patent Document 3).
JP 2003-331826 A JP 2003-263986 A JP 2003-197193 A N. Dimov, K. Fukuda, T. Umeno, S. Kugino, M. Yoshio, Journal of Powder Sources 114 (2003) 88-94 ZSWen, J. Yang, BFWang, K. Wang, Y. Liu, Electrochemistry Communications 5 (2003) 165-168 N. Dimov, T. Umeno, S. Kugino, M. Yoshio, Electrochimica Acta 48 (2003) 1579-1587

  However, even if the techniques listed in the prior art are employed, it cannot be said that the battery performance is sufficient to put a negative electrode active material containing a silicon-based active material into practical use.

  The present invention has been made in view of the above circumstances, and in order to contribute to the practical use of a silicon-based active material, the stability of a negative electrode material containing an active material based on a silicon-based active material is improved. PROBLEM TO BE SOLVED: To provide an active material for a lithium secondary battery capable of improving the cycle life associated with insertion / extraction of lithium and a method capable of producing such an active material for lithium secondary battery .

  And providing the lithium secondary battery which employ | adopted said active material for lithium secondary batteries is also made into the problem which should be solved.

(1) A method for producing an active material for a lithium secondary battery according to the present invention that solves the above-mentioned problems is obtained by mixing a silicon-based active material powder capable of removing and inserting lithium ions and a matrix raw material having a carbon skeleton, and then mixing the mixture. And pyrolyzing the matrix raw material by heating
Applying a compression or shear force to the pyrolyzate;
And a step of mixing the second matrix material having a carbon skeleton and then heating to thermally decompose the second matrix material.

  By adopting a process of applying compression or shear force after pyrolyzing (baking) the matrix raw material mixed with the silicon-based active material powder, the silicon-based active material powder is mechanically or mechanochemically applied. As a result, a silicon-carbon composite material having a strong connection with a matrix composed mainly of carbon can be obtained, and the stability of the active material can be improved. Furthermore, by mixing and thermally decomposing the matrix material (second matrix material), the silicon-carbon composite material can be dispersed in a matrix (second matrix) that is amorphous carbon. According to the above operation, the silicon-based active material powder and the matrix can be firmly bonded without interposing an element that has no direct influence on the battery reaction such as a binder.

  That is, by dispersing the silicon-carbon composite material in the second matrix made of amorphous soft carbon, the effect of absorbing the volume change of the Li—Si alloy is maintained, and the silicon-based active material powder and Adhesion between the matrix can be improved. It is considered that there are many portions where there is no clear silicon-carbon interface due to the reaction between silicon and the matrix derived from the matrix raw material, and the interface between the two can be closely bonded. As a result, the strength of the active material can be improved, and the formation of the SEI film due to the reaction between the silicon-based active material and the electrolyte can be suppressed. Therefore, the cycle characteristics can be improved as a result of effectively suppressing the degradation of the cycle characteristics caused by the generation of the SEI film, the falling off of the active material, and the decrease in electrical contact.

  In addition to the effect of firmly bonding the matrix to the silicon-based active material powder by the first thermal decomposition and the step of applying compression or shearing force, the effect of aligning the size of the silicon-carbon composite material particles can be exhibited. . Dispersibility of the silicon-carbon composite material in the second matrix raw material is improved by aligning the particle diameters of the silicon-carbon composite material. Accordingly, the dispersibility of the silicon-based active material powder in the active material for a lithium secondary battery generated after the second thermal decomposition can be improved. As a result, the performance of the active material for a lithium secondary battery, which is the final product, can be improved.

  In addition, the matrix generated through the first thermal decomposition and the step of applying compression or shearing force is a preferable structure that can effectively suppress electrolyte permeation as compared to the matrix generated by the second thermal decomposition (for example, It can be expected to become a more precise organization. As a result, as an incidental effect, the initial irreversible capacity in the silicon-based active material can be eliminated, and the initial efficiency can be improved.

  Here, the step of applying the compression or shearing force is preferably a step of pulverizing with a ball mill. Furthermore, the matrix raw material and / or the second matrix raw material can be mixed by pulverization with a ball mill. By mixing with a ball mill, the silicon-based active material powder can be uniformly dispersed.

The matrix material and the second matrix material are preferably selected from polymer materials. Note that the composition of the matrix material and the second matrix material can be selected independently, and the same material or different materials may be selected. Specifically, both the matrix material and the second matrix material are preferably polyvinyl chloride. Polyvinyl chloride contains elemental chlorine, and this elemental chlorine reacts with silicon by thermal decomposition (for example, Si + 4Cl → SiCl ₄ ) to clean the surface of the silicon-based active material powder. By cleaning the surface of the silicon-based active material powder, the adhesion with the matrix derived from the matrix material can be improved, and the performance as the negative electrode active material can be improved. That is, it is preferable that the matrix raw material and / or the second matrix raw material is subjected to a thermal decomposition step together with the halogen compound.
(2) And the active material for a lithium secondary battery of the present invention that solves the above problems is an active material for a lithium secondary battery that can be obtained by the above-described method for producing an active material for a lithium secondary battery of the present invention. It is characterized by being.

Specifically, when an active material for a lithium secondary battery obtained by the above-described method for producing an active material for a lithium secondary battery according to the present invention is exemplified, a silicon-based active material material powder capable of removing and inserting lithium ions, and the silicon-based active material An active material for a lithium secondary battery having a carbon-based matrix in which material powder is dispersed,
The silicon-based active material powder is coated with a carbon-based material having a structure different from that of the carbon-based matrix.

  And a primary particle material composed of a silicon-based active material material powder capable of removing and inserting lithium ions, a carbon-based material in which the silicon-based active material material powder is dispersed, and a carbon-based matrix in which the primary particle material is dispersed. The active material for lithium secondary batteries characterized by these can also be illustrated.

Here, it is preferable that the silicon-based active material powder is uniformly dispersed in the carbon-based matrix. Moreover, it is preferable that the carbon-based material has a structure having a density sufficient to prevent the electrolyte from penetrating into the silicon-based active material powder inside when applied to a lithium secondary battery.
(3) Furthermore, the lithium secondary battery of the present invention that solves the above problems is a lithium secondary battery having an electrode having a positive and negative electrode active material capable of inserting or extracting lithium ions, and an electrolyte,
The negative electrode active material is an active material for a lithium secondary battery manufactured by the above-described method for manufacturing an active material for a lithium secondary battery of the present invention or an active material for a lithium secondary battery of the present invention described above. And

[Method for producing active material for lithium secondary battery]
The method for producing an active material for a lithium secondary battery according to the present embodiment has roughly three steps.
(1) The first step is a step of thermally decomposing the matrix material after mixing the silicon-based active material powder and the matrix material. The mixing ratio of the silicon-based active material powder and the matrix raw material is not particularly limited, but is mixed at a mass ratio of about 1:99 to 99: 1, 10:90 to 90:10, and 30:70 to 70:30. be able to. The mass ratio between the silicon-based active material and the other matrix (derived from both the matrix material and the second matrix material) in the lithium secondary battery active material of the present embodiment that is finally manufactured is 30: About 70-50: 50 is preferable and about 40:60 is more preferable.

As the silicon-based active material powder, silicon alone; silicon alloys such as Si—Cu, Si—Ni, Si—Mg, and Si—Ca; carbides and nitrides such as SiC and Si ₃ N ₄ can be used. In particular, silicon alone or a silicon alloy is preferable. P-type silicon, n-type silicon, and the like can also be suitably employed from the viewpoint of conductivity. The particle diameter of the silicon-based active material material powder is not particularly limited, but is preferably about 10 nm to 5 μm. By setting the particle diameter within this range, lithium trapping, oxide generation, and the like can be suppressed while sufficiently maintaining a reaction area necessary for the battery reaction.

  The matrix raw material is a material that can form a matrix mainly composed of a carbon material by pyrolysis described later, and is a compound having a carbon skeleton. A low molecular weight or high molecular weight material may be used, but it is preferably selected from polymer materials. In particular, a polymer material whose main chain is made of carbon can be easily carbonized by thermal decomposition in an inert atmosphere. The matrix raw material is not particularly limited, and may be solid, liquid, gas, etc., but considering the miscibility with the silicon-based active material powder, it is solid and is similar to the silicon-based active material powder. It is preferable to use powder. The method of mixing the matrix raw material with the silicon-based active material powder is not particularly limited. For example, in addition to a general mixing operation such as stirring, a pulverizing operation using a ball mill or the like can be employed. By employing a ball mill for mixing, the adhesion of the matrix material to the silicon-based active material powder can be improved. This mixing is preferably performed in an inert atmosphere. When employing a ball mill, the mixing (grinding) time can be from about 30 minutes to about 4 hours. The rotation speed of the ball mill is 100 rpm when a cylindrical container having an inner diameter of about 20 mm is used, twelve zirconia balls having a diameter of about 4 mm are used, and the filling rate is about 30% (1 g filling) and the operation is performed under grinding conditions. It can be about 600 rpm. Further, the matrix raw material can be mixed with the silicon-based active material powder in a state where the matrix raw material is melted in some solvent or heated and melted. A method of granulating by coating a molten or melted matrix material with a silicon-based active material powder as a core material can also be employed. Further, the thermal decomposition operation described later can be performed while mixing.

  Furthermore, it is preferable to add a halogen compound in addition to the matrix raw material. The halogen compound can react with the silicon-based active material powder during thermal decomposition to clean the surface. The effect of adding a halogen compound can be exhibited by adding it as a material separate from the matrix raw material, such as chloride and bromide, and the same effect can also be achieved by introducing a halogen element into the matrix raw material itself. . For example, polyvinyl chloride can be used as the matrix material.

Pyrolysis aims at carbonizing the matrix raw material. Pyrolysis is preferably performed in an inert atmosphere such as argon or nitrogen in order to prevent oxidation of the silicon-based active material powder and matrix material. The heating temperature for carrying out the thermal decomposition can be about 800 to 1100 ° C. For example, a rate of temperature increase of about 3 ° C./min to 10 ° C./min can be employed. The heating time is preferably long enough for the matrix material to carbonize. For example, it can be performed for about 1 to 3 hours. After completion of the thermal decomposition, it is preferable to leave it in an inert atmosphere until it reaches room temperature.
(2) The second step is a step of applying a compression or shear force to the pyrolyzate.

The step of applying compression or shearing force is preferably performed under an inert atmosphere such as argon or nitrogen. As a specific means for applying compression or shearing force, a ball mill, a vibration ball mill, a combination of a mortar and a pestle, or the like generally called a grinding operation can be employed. A ball mill and a vibrating ball mill are particularly preferable. As a time for pulverization with a ball mill, about 2 to 6 hours can be employed. The rotation speed of the ball mill in that case is when a cylindrical container having an inner diameter of about 20 mm is used, twelve zirconia balls having a diameter of about 4 mm are used, and the filling rate is about 30% (1 g) and the operation is performed under pulverization conditions. It can be set to about 100 rpm to 600 rpm. By applying compression or shearing force, a powder composed of a silicon-carbon composite material having excellent adhesion between the silicon-based active material powder and the matrix can be obtained. A step of applying compression or shearing force can be performed until the particle diameter of the powder is about 5 μm to 25 μm. In addition, after this process, classification operation can be performed and the particle size range of a silicon-carbon composite material can also be made appropriate.
(3) The third step is the same except that the silicon-carbon composite material obtained in the second step is used instead of the silicon-based active material powder, and the second matrix material is used instead of the matrix material. It is the same process as the process. In other words, the silicon-carbon composite material powder is mixed with the second matrix material and then thermally decomposed. Here, the second matrix material is a material that is required to have the same properties as the matrix material, and can be selected based on the same criteria as the matrix material described in the first step. Note that the composition of the second matrix material can be determined independently of the matrix material. Therefore, it is preferable to add a halogen compound in addition to the second matrix material as in the first step, and it is preferable to employ polyvinyl chloride as in the matrix material. The mixing ratio of the silicon-carbon composite material and the second matrix material is not particularly limited, but is mixed at a mass ratio of about 1:99 to 99: 1, 10:90 to 90:10, and 30:70 to 70:30. be able to.

  The form of the second matrix raw material is not particularly limited, but it is preferable that the second matrix raw material be formed in the same powder form in consideration of the mixing property with the powder made of the silicon-carbon composite material. The method of mixing the second matrix raw material with the silicon-based active material powder is not particularly limited, and the description in the first step is just as it is.

The purpose of pyrolysis is to carbonize the second matrix material. Pyrolysis is preferably performed in an inert atmosphere such as argon or nitrogen in order to prevent oxidation of the silicon-carbon composite material and the second matrix material. The heating temperature for carrying out the thermal decomposition can be about 800 to 1100 ° C. For example, a rate of temperature increase of about 3 ° C./min to 10 ° C./min can be employed. The heating time is preferably long enough for the second matrix material to carbonize. For example, it can be performed for about 1 to 3 hours. After completion of the thermal decomposition, it is preferable to leave it in an inert atmosphere until it reaches room temperature.
(Supplement)
The active material for a lithium secondary battery manufactured by the above three steps is processed according to the final usage pattern. For example, when applied to a lithium secondary battery in a powder form, it is pulverized until an appropriate particle size is obtained. In addition, when pulverizing to adjust the form, it is preferable to perform the pulverizing operation under mild conditions so as not to change the structure of the matrix derived from the second matrix raw material. In addition, since the form can be changed relatively freely before the pyrolysis is performed in the third step, the third process is pyrolyzed after the necessary form (powder, plate, etc.) is obtained. be able to.
[Active materials for lithium secondary batteries]
(First embodiment)
The active material for a lithium secondary battery according to the present embodiment is an active material for a lithium secondary battery having a silicon-based active material material powder from which lithium ions can be desorbed and a carbon-based matrix in which the silicon-based active material material powder is dispersed. It is. The active material for a lithium secondary battery may be in a powder form or a shape (for example, a plate shape or a layer shape) when finally applied to a lithium secondary battery. The silicon-based active material powder is coated with a carbon-based material having a structure different from that of the carbon-based matrix. Since the silicon-based active material powder can be selected from the same materials as those described in the above-described manufacturing method, further description is omitted.

  The carbon-based matrix corresponds to the matrix derived from the second matrix raw material in the above-described manufacturing method, and is a material made of amorphous carbon. The carbon material that coats the silicon-based active material powder is a material corresponding to the matrix derived from the matrix raw material in the manufacturing method described above, and the material made of amorphous carbon generated by thermal decomposition was changed by compression or shearing force. Material. For example, the crystallinity is changed or the density is higher than that of the carbon matrix. This difference can be confirmed by EPMA, TEM, scanning probe microscope, or a combination thereof.

(Second Embodiment)
The active material for a lithium secondary battery of the present embodiment includes a primary particle material composed of a silicon-based active material material powder capable of removing and inserting lithium ions, and a carbon-based material in which the silicon-based active material material powder is dispersed, and the primary particle material And a carbon-based matrix in which is dispersed. Here, the silicon-based active material material powder, the carbon material covering the silicon-based active material material powder, and the carbon-based matrix are the same as those described in the first embodiment, so further description is omitted. The primary particles may contain a plurality of silicon-based active material powders therein. The diameter of the primary particles is preferably about 5 μm to 25 μm.

(Supplement)
In any of the active materials for lithium secondary batteries described above, the carbon-based material has a density sufficient to prevent the electrolyte from penetrating into the internal silicon-based active material powder when applied to a lithium secondary battery. An organization is preferred. Furthermore, it is preferable that the silicon-based active material powder and the carbon material are in close contact with each other. The silicon-based active material powder is preferably uniformly dispersed in the carbon-based matrix.

  These active materials for lithium secondary batteries of this embodiment are active materials for lithium secondary batteries that can be obtained by the above-described manufacturing method. In addition, the active material for lithium secondary batteries of the present invention is not limited to those classified exclusively as the active materials for lithium secondary batteries of the first and second embodiments, but is classified across both. Needless to say, there is.

In addition, the active material for a lithium secondary battery according to the present embodiment has a performance capable of exhibiting a very large capacity of 700 mAh / g or more when the configuration of an example described later is taken as an example, but limits the depth of charge and discharge. By doing so (for example, about 600 mAh / g in the examples described later), even better cycle characteristics can be exhibited.
[Lithium secondary battery]
The lithium battery of the present invention can have a known battery structure such as a coin-type battery, a button-type battery, a cylindrical battery, and a square battery. In any case, the positive electrode and the negative electrode are overlapped or wound via a separator to form an electrode body, and the positive electrode terminal and the negative electrode terminal are connected to the positive electrode terminal and the negative electrode terminal. After connecting the electrodes using a current collecting lead or the like, the electrode body can be inserted into a battery case together with a non-aqueous electrolyte, and the battery can be sealed to complete a lithium battery.

  For the positive electrode, a conductive material and a binder are mixed with a positive electrode active material capable of inserting and extracting lithium ions, and an appropriate solvent is added as necessary to form a paste-like positive electrode mixture, such as a metal such as aluminum. It is formed by applying and drying on the surface of the current collector made of foil and then increasing the active material density by pressing.

  As the positive electrode active material, a known positive electrode active material such as a lithium transition metal composite oxide can be used. The lithium-transition metal composite oxide is a preferable material for the present positive electrode active material because of its low electrical resistance, excellent lithium ion diffusion performance, high charge / discharge efficiency, and good charge / discharge cycle characteristics. Examples thereof include lithium nickel oxide, lithium cobalt oxide, lithium manganese oxide, and materials obtained by adding or replacing each of transition metals such as Al and Cr. When these lithium-metal composite oxides are used as the positive electrode active material, they can be used alone or in combination.

  The conductive material is for ensuring the electrical conductivity of the positive electrode, and a mixture of one or two or more carbon material powders such as carbon black, acetylene black, and graphite can be used. The binder plays a role of connecting the active material particles and the conductive material particles, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder. Some or all of these conductive materials may be coated with the oxygen removing material and / or the heat storage material described above.

  The negative electrode is not particularly limited except that the above-described active material for a lithium secondary battery according to this embodiment is used, and other configurations can be used. As the active material for a lithium secondary battery used as the negative electrode active material, a material obtained by applying a negative electrode mixture obtained by mixing a binder as described in the positive electrode to a current collector as necessary is used. Is preferred.

  The nonaqueous electrolytic solution is obtained by dissolving an electrolyte in an organic solvent.

  An organic solvent will not be specifically limited if it is an organic solvent normally used for the non-aqueous electrolyte of a lithium battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds and the like can be used. In particular, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran and the like, and mixed solvents thereof are suitable. For example, a mixed organic solvent of a main solvent having a high dielectric constant such as ethylene carbonate or propylene carbonate and a low-viscosity auxiliary solvent such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate is preferable. Further, dimethoxyethane, tetrahydrofuran, butyl lactone, or the like may be used as a co-solvent.

The type of the electrolyte is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ , an organic salt selected from LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and derivatives of the organic salts It is preferable that there is at least one.

  By using these electrolytes, the battery performance can be further improved, and the battery performance can be maintained even higher in a temperature range other than room temperature. The concentration of the electrolyte is not particularly limited, and it is preferable to appropriately select the electrolyte and the organic solvent in consideration of the use.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used. Note that the separator is preferably larger than the positive electrode and the negative electrode in order to ensure insulation between the positive electrode and the negative electrode.

  The case is not particularly limited and can be made of a known material and form.

  The gasket secures electrical insulation between the case and both the positive and negative terminal portions and airtightness in the case. For example, it can be composed of a polymer such as polypropylene that is chemically and electrically stable to the electrolyte.

(Example 1)
<Manufacture of active materials for lithium secondary batteries>
Silicon powder (particle diameter: 1 μm) as a silicon-based active material material powder and polyvinyl chloride (PVC) as a matrix material were mixed at a mass ratio of 7: 3. Mixing was performed in a ball mill for 30 minutes under an argon atmosphere. The ball milling was performed using a cylindrical container having an inner diameter of 20 mm, twelve zirconia balls having a diameter of 4 mm, a sample filling rate of about 30% (1 g filling), and a rotation speed of 500 rpm. The mixture was heated to 900 ° C. at a heating rate of 5 ° C./min in an argon atmosphere and pyrolyzed (baked) at 900 ° C. for 1 hour (first step).

  The obtained product was pulverized with a ball mill under an argon atmosphere for 2 hours. The grinding condition by the ball mill was performed using a cylindrical container having an inner diameter of 20 mm, twelve zirconia balls having a diameter of 4 mm, a sample filling rate of about 30% (1 g filling), and a rotation speed of 600 rpm (second step). ).

  The pulverized silicon-carbon composite material powder was further subjected to thermal decomposition by mixing PVC so as to have a mass ratio of 7: 3. Mixing and pyrolysis were performed in the same manner as in the first step. The pyrolyzed sample was crushed under mild conditions using a mortar. The product was used as a test sample as an active material for a lithium secondary battery of this example.

  After the first step, the state of silicon dispersion in the product after the second step and after the third step was verified by EPMA (FIG. 1: after the first step, FIG. 2: after the second step, FIG. 3: third) After the process). Then, the crystallinity of the constituent components (silicon-based active material powder and matrix) is analyzed by X-ray diffraction for the product after the third step (test sample of this example, lightly crushed in a mortar and powdered). (FIG. 4). Moreover, the external appearance was observed by SEM about the test sample of a present Example (FIG. 5).

  As shown in FIG. 1, in the sample after the first step (a sample obtained by mixing silicon-based active material powder and PVC and then thermally decomposing), there is a bias in the dispersion of the silicon-based active material material powder. Presumed to be insufficient. As shown in FIG. 2, it was found that the silicon-based active material powder was uniformly dispersed in the sample after the second step (sample obtained by pulverizing the sample after the first step with a ball mill). As shown in FIG. 3, in the sample after the third step (the test sample of this example), the matrix (derived from the second matrix raw material) is maintained while the silicon-based active material powder is uniformly dispersed. It turns out that it is wrapped in. Therefore, the test sample of this example has high adhesion to the matrix (derived from the matrix raw material) and is encased in the matrix (derived from the second matrix raw material) in a state where the silicon-based active material powder is uniformly dispersed. The silicon-based active material powder can sufficiently absorb the volume change due to lithium insertion and removal while maintaining high adhesion to the matrix.

  As is clear from FIG. 4, it was found that the crystallinity of silicon is high and does not change depending on the pulverization operation in the first to third steps (particularly the second step). Note that the matrix encapsulating or enveloping the silicon-based active material powder did not show a peak derived from carbon with high crystallinity, and only amorphous carbon was observed. From the SEM photograph shown in FIG. 5, it was found that the particle size of the test sample was about 15 to 30 μm.

<Manufacture of lithium secondary batteries>
88 parts by mass of the obtained active material for lithium secondary battery (active material), 4 parts by mass of acetylene black (AB) and 8 parts by mass of PVDF were dissolved or suspended in NMP to obtain a negative electrode mixture. Specifically, AB and the active material were suspended in a PVDF / NMP solution having a concentration of 0.02 g / mL. Thereafter, the negative electrode mixture was cast on a Cu thin film having a thickness of 20 μm, and dried at 120 ° C. for 2 hours to dry and remove NMP. An electrode layer of 50-60 μm was formed on the Cu thin film.

A 2025 coin cell was prepared with an electrode area of 0.55 cm ^{2 and used} as a test battery of this example. A concentration of 1 mol / L LiPF ₆ / EC + DEC (volume ratio 1: 1) was used as the electrolytic solution, and a lithium metal sheet was used as the positive electrode. In order to maintain mechanical contact, nickel foam was interposed on the negative electrode side. A polyethylene porous sheet as a separator was interposed between the positive and negative electrodes.

(Comparative Examples 1-3)
In the active material for a lithium secondary battery of Example 1, between the first step and the second step (Comparative Example 1), the second step is omitted (Comparative Example 2), and the second step and the third step. (Comparative Example 3) was used as the active material for the lithium secondary battery in each Comparative Example. In Comparative Example 2, the sample after the first step was mixed with PVC by grinding with PVC in a mortar, and then pyrolyzed in the same manner as the pyrolysis in the third step.

  Using these active materials for lithium secondary batteries, lithium secondary batteries were produced in the same manner as in Example 1 to obtain test batteries for the respective comparative examples.

(Cycle test)
In the cycle test, charging / discharging at a current density of 0.3 mA · cm ⁻² between 1500 mV and 50 mV was performed as one cycle. An interval of 1 minute was provided between charging and discharging. The charge capacity and terminal voltage were continuously measured during charging and discharging.

(result)
The results are shown in FIG. The test battery of Example 1 had a stable discharge capacity at an earlier cycle than the test batteries of Comparative Examples 1 to 3, and the subsequent capacity decrease was small. It was also found that the irreversible capacity was relatively small and the initial efficiency was high. Although the test battery of each comparative example had an initial discharge capacity higher than that of the test battery of Example 1, the subsequent decrease rate was large, and was lower than that of the test battery of Example 1 at about 25 to 30 cycles.

  Moreover, about the test battery of Example 1, the relationship of the voltage-charge / discharge capacity in the 1st, 2nd and 29th cycles is shown in FIG. A charge / discharge curve of a typical silicon-based active material powder is shown. The average voltage was about 0.3V. It can be inferred that the flat portion in the discharge curve of the first cycle shows a gentle slope after the second cycle because the crystallinity of the carbon material of the matrix has changed. Although a slight decrease in charge / discharge capacity was observed even when proceeding from the 1st cycle to the 29th cycle, no significant change was observed, and the 29th and subsequent cycles showed substantially the same charge / discharge curve.

(Cycle test: Capacity regulation)
For the test batteries of Example 1 and Comparative Example 2, the discharge capacity was limited to 600 mAh / g, and the charge / discharge capacity was measured. The charge / discharge conditions were the same as those in the cycle test described above except that the discharge capacity was regulated.

(result)
In the test battery of the example, the discharge amount was 600 mAh / g even after 100 cycles, and it was found that the discharge capacity of 600 mAh / g or more was maintained (FIG. 8). On the other hand, in the test battery of Comparative Example 2, a significant decrease in the discharge capacity was observed even after charging and discharging for about 10 to 20 cycles (FIG. 9). That is, it was found that the test battery of this example can realize high stability of the structure.

(Example 2)
In the first step and the second step, the mixing ratio of the silicon-based active material powder and PVC was 8.5: 1.5 in terms of mass ratio, the thermal decomposition temperature was 1000 ° C., and the heating rate was 10 ° C./min. In addition, a test sample was produced under the same conditions as in Example 1 except that the ball milling time in the second step was 3 hours, and a test battery was produced. As a result of the cycle test, there was no significant change in the test battery of Example 1.

It is the figure which showed the dispersion | distribution state of the silicon element measured by EPMA in each process of manufacturing the test sample of an Example (after the 1st process). It is the figure which showed the dispersion state of the silicon element measured by EPMA in each process of manufacturing the test sample of an Example (after the 2nd process). It is the figure which showed the dispersion state of the silicon element measured by EPMA in each process of manufacturing the test sample of an Example (after the 3rd process). It is the figure which showed the X-ray-diffraction spectrum of the test sample of an Example. It is a SEM photograph of the test sample of an Example. It is the graph which showed the result of the cycle test of the test battery of an Example and a comparative example. 2 is a graph showing the results of a cycle test of the test battery of Example 1. FIG. 2 is a graph showing the results of a cycle test of the test battery of Example 1. FIG. 6 is a graph showing the results of a cycle test of a test battery of Comparative Example 2.

Claims (12)

A step of mixing a silicon-based active material material powder capable of removing and inserting lithium ions and a matrix material having a carbon skeleton, and then thermally decomposing the matrix material by heating the mixture;
Applying a compression or shear force to the pyrolyzate;
A method for producing an active material for a lithium secondary battery, further comprising: mixing a second matrix material having a carbon skeleton, and then thermally decomposing the second matrix material.   The method for producing an active material for a lithium secondary battery according to claim 1, wherein the step of applying a compressive or shearing force is a step of pulverizing with a ball mill.   The active material for a lithium secondary battery according to claim 1 or 2, wherein the mixing of the matrix material and / or the second matrix material is performed by pulverization with a ball mill.   The method for producing an active material for a lithium secondary battery according to claim 1, wherein the matrix material and the second matrix material are selected from polymer materials.   The method for producing an active material for a lithium secondary battery according to claim 4, wherein the matrix material and the second matrix material are polyvinyl chloride.   The method for producing an active material for a lithium secondary battery according to claim 1, wherein the matrix material and / or the second matrix material is subjected to a thermal decomposition step together with a halogen compound.   An active material for a lithium secondary battery, which can be obtained by the production method according to claim 1. An active material for a lithium secondary battery, comprising: a silicon-based active material material powder capable of removing and inserting lithium ions; and a carbon-based matrix in which the silicon-based active material material powder is dispersed,
The active material for a lithium secondary battery, wherein the silicon-based active material powder is coated with a carbon-based material having a structure different from that of the carbon-based matrix.   It has a primary particle material composed of a silicon-based active material powder capable of desorbing lithium ions, a carbon-based material in which the silicon-based active material powder is dispersed, and a carbon matrix in which the primary particle material is dispersed. An active material for a lithium secondary battery.   The active material for a lithium secondary battery according to claim 8 or 9, wherein the silicon-based active material material powder is uniformly dispersed in the carbon-based matrix.   The said carbonaceous material is a structure | tissue provided with the density | concentration of the grade which an electrolyte does not osmose | permeate to the said silicon | silicone type | system | group active material material powder when it applies to a lithium secondary battery. Active material for lithium secondary battery. A lithium secondary battery having an electrode having positive and negative electrode active materials capable of inserting or extracting lithium ions, and an electrolyte,
The said negative electrode active material is an active material for lithium secondary batteries manufactured by the manufacturing method of the active material for lithium secondary batteries in any one of Claims 1-5, or in any one of Claims 6-11. A lithium secondary battery characterized by being an active material for a lithium secondary battery.

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