WO2014207921A1

WO2014207921A1 - Negative-electrode active substance, method for manufacturing same, and lithium-ion secondary cell

Info

Publication number: WO2014207921A1
Application number: PCT/JP2013/067886
Authority: WO
Inventors: 西村　悦子; 西村　勝憲; 鈴木　修一; 岡井　誠
Original assignee: 株式会社日立製作所
Priority date: 2013-06-28
Filing date: 2013-06-28
Publication date: 2014-12-31

Abstract

　Provided are a negative-electrode active substance (1) capable of making the energy density and capacity of a lithium-ion secondary cell superior to those of the prior art, a method for manufacturing said substance (1), and a lithium-ion secondary cell (100) in which said substance (1) is used. The negative-electrode active substance (1) includes microparticles (10) provided with an electroconductive outer shell (11) and a plurality of silicon nanoparticles (13) held on the inner surface of the outer shell (11) and exposed to the interior space (12) of the outer shell (11). The outer shell (11) has an opening section (11a) interconnecting the exterior space and the interior space (12) of the outer shell (11).

Description

Negative electrode active material, method for producing the same, and lithium ion secondary battery

The present invention relates to a negative electrode active material used for a negative electrode of a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery using the negative electrode active material.

Since lithium ion secondary batteries have a higher energy density than other secondary batteries, they are attracting attention as batteries for electric vehicles and power storage. In particular, in an electric vehicle, a zero-emission electric vehicle that uses a lithium ion secondary battery as a power source without an engine, or a hybrid electric vehicle that includes both an engine and a lithium ion secondary battery, or a lithium ion from a system power source. Plug-in hybrid electric vehicles that charge secondary batteries are in practical use. A lithium ion secondary battery used as a power source for an electric vehicle is desired to have a higher capacity in order to increase the travel distance of the electric vehicle.

Also, the lithium ion secondary battery is expected to be used as a stationary power storage system that supplies power in an emergency when the power system is shut off. In such a power storage system, a smaller system can be provided as the capacity density of the battery is higher.

Furthermore, in consumer applications, the amount of power used by mobile devices such as mobile phones and smartphones is increasing, and the demand for higher capacity for lithium-ion secondary batteries has become very strong.

Under such circumstances, development of materials for the positive and negative electrodes of lithium ion secondary batteries has become active. For example, the negative electrode active material is composed of either silicon or tin and at least one element selected from elements that do not react with lithium, and has pores in both the inner core and the outer periphery of the primary particles. In addition, a negative electrode for a non-aqueous secondary battery has been developed in which a conductive material made of carbon is present on the surface of the active material and inside the pores (see Patent Document 1 below). According to this negative electrode for a non-aqueous secondary battery, the volume change of the negative electrode active material primary particle itself is suppressed, and even if a part of the negative electrode active material primary particle structure collapses due to the stress caused by the volume change, The network is maintained, which improves cycle characteristics.

JP 2012-94463 A

However, although the conventional negative electrode active material has pores in both the inner core and the outer peripheral portion, the electrolyte does not penetrate into the inner core, and there is no conductive material on the surface of the active material particles. Does not contribute to capacity. That is, since the negative electrode active material has a relatively large volume that does not contribute to capacity, there is a limit to the improvement in capacity density of a lithium ion secondary battery using the negative electrode active material for the negative electrode.

The present invention has been made in view of the above problems, and the object of the present invention is to provide a negative electrode active material capable of improving the capacity density of a lithium ion secondary battery as compared with the prior art, and a method for producing the same, Another object is to provide a lithium ion secondary battery using the negative electrode active material.

The negative electrode active material of the present invention is a negative electrode active material for a lithium ion secondary battery, and includes a conductive outer shell and a plurality of silicon held on the inner surface of the outer shell and exposed to the inner space of the outer shell. Nanoparticles are included, and the outer shell has an opening that connects the outer space of the outer shell and the inner space.

According to the negative electrode active material of the present invention, the electrolytic solution penetrates from the outer space of the outer shell into the inner space through the opening of the outer shell, and the electrolytic solution contacts the outer surface and the inner surface of the outer shell. The surface area of the outer shell of the contributing fine particles can be increased than before, the volume of the fine particles not contributing to the capacity can be reduced as compared with the conventional case, and the capacity density of the lithium ion secondary battery can be improved as compared with the conventional case.

The schematic cross section which shows one Embodiment of the negative electrode active material of this invention. The schematic cross section which shows another embodiment of the negative electrode active material of this invention. 1 is a schematic cross-sectional view showing an embodiment of a lithium ion secondary battery of the present invention. The flowchart which shows one Embodiment of the manufacturing method of the negative electrode active material of this invention. The schematic cross section which shows the 1st precursor which concerns on the manufacturing method shown in FIG. The schematic cross section which shows the 2nd precursor which concerns on the manufacturing method shown in FIG. FIG. 4 is a schematic cross-sectional view showing fine particles according to the manufacturing method shown in FIG. 3. The schematic of the module provided with the lithium ion secondary battery shown in FIG. Schematic of a battery system provided with the module shown in FIG. The schematic cross section of the negative electrode active material of a comparative example.

(Negative electrode active material)
Hereinafter, an embodiment of the negative electrode active material of the present invention will be described with reference to the drawings. 1A and 1B are schematic partial cross-sectional views illustrating the configuration of a negative electrode used in a lithium ion secondary battery. The negative electrode 122 includes a current collector 122a and a negative electrode mixture layer 122b formed on the current collector 122a. The negative electrode mixture layer 122b mainly includes the negative electrode active material 1, and includes, for example, polyvinylidene fluoride (PVDF) as a binder.

The negative electrode active material 1 is a material for the negative electrode 122 of the lithium ion secondary battery, and is mainly composed of the fine particles 10. The fine particle 10 includes a conductive outer shell 11 and silicon nanoparticles 13 accommodated in the internal space 12. The silicon nanoparticles 13 are exposed in the internal space 12 defined by the outer shell 11. The outer shell 11 has an opening 11a. The opening 11 a communicates the external space of the outer shell 11 and the internal space 12. The number of openings 11a formed in the outer shell 11 may be one or plural.

The fine particles 10 may include flaky fine particles 10a shown in FIG. 1B. The flaky fine particles 10a are formed by dividing the outer shell of the fine particles 10 into a flaky shape during the manufacturing process. For example, a fine particle 10 having a diameter when the opening 11a approximates a circle is equal to or larger than the average particle diameter of the granular fine particle 10 shown in FIG. 1A can be defined as a flaky fine particle 10a. On the other hand, fine particles 10 having a diameter of the opening 11a smaller than the average particle diameter can be defined as granular fine particles 10b. In the present embodiment, it is essential that the fine particles 10 include granular fine particles 10b.

When the fine particles 10 are granular fine particles 10b, the diameter of the opening 11a is, for example, ½ or less, preferably 1/5 or less, more preferably 1/10 or less of the particle size of the outer shell 11. It is. The diameter of the opening 11a is the maximum value of the opening width of the opening 11a. In the following, unless otherwise specified, the fine particles 10 are assumed to be granular fine particles 10b.

The material constituting the outer shell 11 of the fine particles 10 includes, for example, a material that reduces lithium ions to form an alloy or an intermetallic compound. Among them, silicon or an alloy of silicon and a light element such as aluminum, magnesium, boron, carbon, nitrogen and the like having a high capacity density of 500 mAh / g or more is particularly suitable as the material of the outer shell 11. is there. The material of the outer shell 11 may be a conductive metal material such as copper. The outer shell 11 of the present embodiment is made of a material containing silicon (Si) and occludes or releases lithium ions. The conductivity of the outer shell 11 is realized when the material constituting the outer shell 11 mainly contains silicon or a silicon alloy.

The outer shell 11 may contain, for example, carbon powder, carbon fiber, or carbon nanotube (CNT) in addition to silicon or the like. The outer surface of the outer shell 11 may have a coating layer made of, for example, fluoride, carbide, nitride, or sulfide. Alternatively, a highly conductive material having higher conductivity than the outer shell 11 such as carbon powder, carbon fiber, or CNT may be held on the outer surface of the outer shell 11. Moreover, you may have both a coating layer and a highly conductive material in the outer surface of the outer shell 11. FIG.

The silicon nanoparticles 13 are held on the inner surface of the outer shell 11 and exposed to the inner space 12. The material constituting the silicon nanoparticles 13 includes silicon (Si). In the illustrated example, three silicon nanoparticles 13 are accommodated in the inner space 12 of the outer shell 11, but the number of silicon nanoparticles 13 depends on the volume of the inner space 12 of the outer shell 11 or the inner surface of the outer shell 11. It can be set to an arbitrary number according to the area.

The volume of the silicon nanoparticles 13 increases due to the charging of the lithium ion secondary battery. Therefore, if the filling rate of the silicon nanoparticles 13 in the inner space 12 of the outer shell 11 is too high, the outer shell 11 may be destroyed as the volume of the silicon nanoparticles 13 increases. Therefore, it is necessary to limit the volume occupation rate of the silicon nanoparticles 13 with respect to the volume of the internal space 12 of the outer shell 11.

Here, the average internal volume of the outer shell 11, that is, the average volume of the internal space 12 is C, the number of silicon nanoparticles 13 accommodated in the internal space 12 is n, and the average particle diameter of the silicon nanoparticles 13 is ½. That is, the radius of the silicon nanoparticles 13 is r, and the volume increase ratio of the silicon nanoparticles 13 after charging the lithium ion secondary battery is ΔV. In addition, a filling rate when the volume of the silicon nanoparticles 13 after the volume increase is closest packed in the internal space 12 is defined as a. At this time, a condition for the silicon nanoparticles 13 having an increased volume to be accommodated in the internal space 12 of the outer shell 11 is expressed by the following formula (1).
^{n · (4πr 3/3)} · ΔV ≦ a · C ... (1)

Here, when the volume increase ratio ΔV of the silicon nanoparticles 13 is 4 and the closest packing ratio a is 0.74, the following equation (2) is obtained.
Vc ≦ 0.74 (2)

Vc is a volume occupation ratio of the silicon nanoparticles 13 with respect to the internal space 12 of the outer shell 11 and is represented by the following formula (3).
^{Vc = n · (4πr 3/} 3) · ΔV / C ... (3)

If the condition of the above formula (2) is satisfied, the outer shell 11 is destroyed even if the volume of the silicon nanoparticles 13 accommodated in the inner space 12 of the outer shell 11 is increased by charging the lithium ion secondary battery. Can be prevented. On the other hand, since the negative electrode capacity density decreases when the volume occupancy Vc of the silicon nanoparticles 13 decreases, the volume occupancy Vc of the silicon nanoparticles 13 is preferably 0.20 or more.

(Lithium ion secondary battery)
FIG. 2 is a partial cross-sectional view of the lithium ion secondary battery 100 of the present embodiment. The lithium ion secondary battery 100 is an electrochemical device that can store or use electrical energy by occlusion / release of lithium ions by electrodes in a non-aqueous electrolyte.

The lithium ion secondary battery 100 includes a battery container 110 and an electrode group 120 accommodated in the battery container 110. The shape of the battery case 110 can be formed in an arbitrary shape such as a cylindrical shape, a flat oval shape, a rectangular shape, or the like according to the shape of the electrode group 120. The material of the battery container 110 is selected from materials that are corrosion resistant to the non-aqueous electrolyte L, such as aluminum, stainless steel, steel, nickel-plated steel, and the like.

The battery container 110 includes a cylindrical part 111 having an opening 111a at the top, a positive battery cover 112a that seals the opening 111a of the cylindrical part 111, and a bottom 112b that seals the bottom of the cylindrical part 111. Have. In the present embodiment, the positive battery cover 112a also serves as a positive external terminal, and the container bottom 112b also serves as a negative external terminal.

The upper part of the cylindrical part 111 has an annular part 111c around the opening 111a. The annular portion 111 c is provided in a ring shape around the opening 111 a, and extends from the side wall end of the cylindrical portion 111 to constitute a part of the upper surface of the battery case 110.

The positive electrode battery lid 112a is arranged so that the peripheral edge thereof overlaps the inside of the annular portion 111c of the cylindrical portion 111 with the gasket 113 interposed therebetween. The positive battery lid 112a is joined to the cylindrical portion 111 by an appropriate method such as caulking, welding, or welding, and seals the opening 111a. A structure similar to that of the positive battery cover 112a may be formed on the bottom surface 112b, and the negative battery cover may be installed.

At the center of the positive battery lid 112a, there is provided an internal pressure release valve 114 that ruptures and releases the internal pressure of the battery container 110 when the internal pressure of the battery container 110 rises above a predetermined value. A positive temperature coefficient (PTC: Positive Temperature Coefficient) resistance element 115 and an inner lid 116 are provided inside the positive battery lid 112a. The PTC resistance element 115 stops the charging / discharging of the lithium ion secondary battery 100 when the temperature inside the battery container 110 becomes high, and protects the lithium ion secondary battery 100. The positive battery cover 112a, the gasket 113, the internal pressure release valve 114, the PTC resistance element 115, and the internal cover 116 are configured as an integrally structured battery cover unit.

The electrode group 120 is configured by laminating a positive electrode 121 and a negative electrode 122 with a separator 123 interposed therebetween. The electrode group 120 may be a wound electrode group in which a positive electrode 121 and a negative electrode 122 that are stacked via a separator 123 are wound around a winding axis. When the electrode group 120 is a wound electrode group, it can be wound into an arbitrary shape such as a flat shape. The electrode group 120 can be formed in various shapes such as a strip shape.

The positive electrode 121 includes, for example, an aluminum foil that is a positive electrode current collector and a positive electrode mixture layer formed on the current collector.

As shown in FIGS. 1A and 1B, the negative electrode 122 includes a negative electrode current collector 122a and a negative electrode mixture layer 122b formed on the current collector 122a. The negative electrode mixture layer 122b constituting the negative electrode 122 mainly contains the negative electrode active material 1 of the present embodiment. As described above, the negative electrode active material 1 constituting the negative electrode mixture layer 122b is held by the conductive outer shell 11, the inner space 12 defined by the outer shell 11, and the inner surface of the outer shell 11, and is contained inside. Fine particles 10 including a plurality of silicon nanoparticles 13 exposed in the space 12 are included. The outer shell 11 has an opening 11 a that communicates the outer space of the outer shell 11 and the inner space 12.

The negative electrode mixture layer 122b may contain a conventional negative electrode active material in addition to the negative electrode active material 1 of the present embodiment. Here, that the negative electrode mixture layer 122b mainly includes the negative electrode active material 1 is that the capacity of the negative electrode active material 1 of the present embodiment with respect to the entire capacity density of the negative electrode active material included in the negative electrode mixture layer 122b. It means that the density is greater than 50%.

In the negative electrode mixture layer 122b, when the filling property of the fine particles 10 constituting the negative electrode active material 1 is increased, the capacity of the negative electrode 122 can be increased. However, when the density of the negative electrode mixture layer 122b is increased by compressing the negative electrode active material 1 in order to increase the filling property of the fine particles 10 in the negative electrode mixture layer 122b, the outer shell 11 of the fine particles 10 constituting the negative electrode active material 1 is obtained. May be destroyed, and the silicon nanoparticles 13 may fall off the outer shell 11. In order to prevent this, assuming that the outer shell 11 of the fine particles 10 of the negative electrode active material 1 is closed, the porosity Va of the negative electrode mixture layer 122b excluding the internal space 12 is defined. It is preferable to adjust so that it may become in the range of (4).
0.2 ≦ Va ≦ 0.5 (4)

The true negative electrode porosity is the porosity of the negative electrode mixture layer 122b in consideration of the internal space 12 of the fine particles 10 of the negative electrode active material 1. That is, the true negative electrode porosity is determined by considering the void ratio Va of the negative electrode mixture layer 122b obtained by considering the outer shell 11 of the fine particles 10 as a closed particle and excluding the internal space 12, and the ratio of the internal space 12 in the fine particles 10. It can be calculated from The negative electrode active material 1 may include flaky fine particles 10a in addition to the granular fine particles 10b as the fine particles 10, and the porosity of the negative electrode mixture layer 122b, that is, the true negative electrode porosity is 0.72 or more and 0.82. The following range is preferable. Furthermore, the negative electrode porosity is more preferably in the range of 0.74 or more and 0.81 or less. If the negative electrode porosity is within the above range, the retention amount of the electrolytic solution L in the negative electrode 122 is prevented from being reduced, and the electrical contact between the fine particles 10 of the negative electrode active material 1 is prevented from being impaired. It is possible to prevent the initial capacity density and capacity retention rate of the ion secondary battery 100 from decreasing.

The separator 123 may be a polyolefin polymer sheet made of polyethylene, polypropylene, or the like, or a multilayer structure sheet in which a polyolefin polymer and a fluorine polymer sheet typified by tetrafluoropolyethylene are welded. Is possible. A mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 123 so that the separator 123 does not shrink when the temperature of the lithium ion secondary battery 100 increases. The separator 123 needs to allow lithium ions to pass therethrough during charge / discharge of the battery, so that it can be used if the pore diameter is generally 0.01 to 10 μm and the porosity is 20 to 90%. In this embodiment, a single-layer separator made of polyethylene having a thickness of 25 μm, a pore diameter of 0.5 μm, and a porosity of 45% is used. Although not shown in FIG. 2, the separator 123 is also disposed between the electrode group 120 and the battery container 110 when the electrode group 120 is accommodated in the battery container 110, and the positive electrode 121 and the negative electrode 122 are connected to the battery container 110. Do not short circuit through.

The positive electrode 121 is connected to the inner lid 116 via the positive electrode current collecting tab 131. The negative electrode 122 is connected to the container bottom surface 112 b through the negative electrode current collecting tab 132. The

current collecting tabs

131 and 132 can take any shape such as a wire shape or a plate shape. The

current collecting tabs

131 and 132 have dimensions, shapes, and structures that can reduce ohmic loss when an electric current is passed, and are made of a material that does not react with the electrolyte L, depending on the structure of the battery case 110. Can be arbitrarily selected. In addition, the positive electrode current collecting tab 131 and the negative electrode current collecting tab 132 are affected by corrosion of the inner lid 116 and the negative electrode battery lid 112b or alloying with lithium ions in the portion in contact with the non-aqueous electrolyte L. Select the lead wire material so that it does not occur.

The battery container 110 is filled with an electrolytic solution L made of an electrolyte and a non-aqueous solvent, and the electrolytic solution L is held on the surfaces of the separator 123, the positive electrode 121, and the negative electrode 122 and inside the pores. As a typical example of the electrolytic solution L that can be used in this embodiment, a solvent in which dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or the like is mixed with ethylene carbonate, lithium hexafluorophosphate (LiPF ₆ ), or borofluoride as an electrolyte is used. A solution in which lithium (LiBF ₄ ) is dissolved may be used. The lithium ion secondary battery 100 of the present embodiment is not limited to the type of solvent, electrolyte, and solvent mixing ratio, and other electrolytes can be used. The electrolyte can also be used in a state of being contained in an ion conductive polymer such as polyvinylidene fluoride (PVDF) or polyethylene oxide. In this case, the separator 123 becomes unnecessary.

Solvents that can be used for the electrolyte L are propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, Dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphoric acid triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2-oxazolidinone, tetrahydrofuran, There are non-aqueous solvents such as 1,2-diethoxyethane, chloroethylene carbonate, chloropropylene carbonate and the like. Other solvents may be used as long as they do not decompose on the positive electrode 121 or the negative electrode 122 built in the lithium ion secondary battery 100 of the present embodiment.

Further, the electrolyte, the chemical formula _{_{_{LiPF 6, LiBF 4, LiClO 4}}} , LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or multi such imide lithium salts represented by lithium trifluoromethane sulfonimide There are different types of lithium salts. A nonaqueous electrolytic solution obtained by dissolving these salts in the above-described solvent can be used as the battery electrolytic solution L. An electrolyte other than this may be used as long as it does not decompose on the positive electrode 121 or the negative electrode 122 built in the lithium ion secondary battery 100 of the present embodiment.

When a solid polymer electrolyte (polymer electrolyte) is used, an ion conductive polymer such as ethylene oxide, acrylonitrile, polyvinylidene fluoride, methyl methacrylate, or hexafluoropropylene polyethylene oxide can be used as the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 123 can be omitted.

Furthermore, an ionic liquid can be used as the electrolytic solution L. For example, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄ ), lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), mixed complex of triglyme and tetraglyme, cyclic quaternary ammonium cation (N-methyl) -N-propylpyrrolidinium), and a combination that does not decompose at the positive electrode 121 and the negative electrode 122 from imide anions (exemplified by bis (fluorosulfonyl) imide). It can be used for the secondary battery 100.

Instead of the non-aqueous electrolyte L, a solid polymer electrolyte (polymer electrolyte) or a gel electrolyte can be used. As the solid polymer electrolyte, a known polymer electrolyte such as polyethylene oxide or a mixture (gel electrolyte) of polyvinylidene fluoride and a nonaqueous electrolytic solution can be used.

In the present embodiment, an electrolytic solution L in which 1 mol concentration (1M = 1 mol / dm ³ ) of LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate (denoted as EC) and ethyl methyl carbonate (denoted as EMC) is used. . The mixing ratio of EC and EMC can be set to 1: 2 by volume ratio, for example. Further, for example, 1% vinylene carbonate may be added to the electrolytic solution L.

Hereinafter, the operation of the negative electrode active material 1 and the lithium ion secondary battery 100 of the present embodiment will be described. In the negative electrode active material 1 of the present embodiment, the outer shell 11 of the fine particles 10 has conductivity. Since the outer shell 11 has conductivity, the adjacent outer shells 11 are electrically connected to each other, and a conductive network of the entire negative electrode 122 of the lithium ion secondary battery 100 is formed. As a result, the entire negative electrode 122 can uniformly occlude or release lithium ions, and the lithium ion secondary battery 100 can be efficiently charged and discharged.

Further, the outer shell 11 of the fine particles 10 has an opening 11 a that communicates the outer space of the outer shell 11 and the inner space 12. Therefore, the electrolytic solution L of the lithium ion secondary battery 100 penetrates into the internal space 12 through the opening 11a of the outer shell 11, and the internal space 12 of the fine particles 10 is filled with the electrolytic solution L. Thereby, the electrolyte solution L contacts the outer surface and inner surface of the outer shell 11 and the area which contributes to the capacity | capacitance of the fine particle 10 increases from the past. Further, the volume that does not contribute to the capacity of the fine particles 10 is reduced. Therefore, the capacity density of the lithium ion secondary battery 100 using the negative electrode active material 1 mainly containing the fine particles 10 can be improved.

In addition, the electrolytic solution L penetrates into the internal space 12 through the opening 11a of the outer shell 11, and the internal space 12 of the fine particles 10 is filled with the electrolytic solution L, so that lithium ions pass through the outer shell through the opening 11a. 11 freely move between the external space and the internal space 12. As a result, as compared with the fine particles X that do not have the openings 11a in the outer shell 11 shown in FIG. 7, lithium ions are more likely to be occluded in the inner surface of the outer shell 11 and the silicon nanoparticles 13 that are exposed in the internal space 12. The lithium ions occluded in the inner surface of the outer shell 11 and the silicon nanoparticles 13 are easily released. Therefore, as compared with a lithium ion secondary battery using a conventional negative electrode active material as a negative electrode material, a lithium ion secondary battery 100 capable of high rate charge / discharge can be obtained.

Further, since the outer shell 11 of the fine particles 10 included in the negative electrode active material 1 has the opening 11a, the stress at the time of expansion and contraction of the outer shell 11 due to charging / discharging of the lithium ion secondary battery 100 is caused by the opening 11a. Is alleviated by. Therefore, it is possible to prevent the outer shell 11 of the fine particles 10 of the negative electrode active material 1 from being destroyed along with expansion and contraction.

Further, when a highly conductive material having higher conductivity than the outer shell 11 is held on the outer surface of the outer shell 11 such as when carbon nanotubes are formed on the outer surface of the outer shell 11, The electrical conductivity is further improved, and the mechanical strength of the outer shell 11 can be improved.

Moreover, based on the said Formula (2), the volume occupation rate Vc of the silicon nanoparticle 13 with respect to the internal space 12 of the microparticle 10 which comprises the negative electrode active material 1 shall be 0.74 or less, for example, and the outer shell 11 Even if the volume of the silicon nanoparticles 13 accommodated in the internal space 12 is increased by charging the lithium ion secondary battery, the outer shell 11 can be prevented from being broken. Therefore, the conductivity of the silicon nanoparticles 13 is maintained by the outer shell 11, and it is possible to provide a long-life negative electrode 122. In addition, by setting the volume occupancy Vc of the silicon nanoparticles 13 to the internal space 12 of the outer shell 11 to be 0.2 or more, it is possible to prevent a decrease in the negative electrode capacity density and to secure a sufficient negative electrode capacity density. .

Further, when the negative electrode active material 1 includes the flaky fine particles 10a in which the silicon nanoparticles 13 are held on the flaky outer shell 11, the diffusion of lithium ions is further facilitated. That is, the presence of the flaky fine particles 10a can improve the charge / discharge performance of the lithium ion secondary battery 100. Further, the granular fine particles 10b maintaining the structure of the outer shell 11 and the flaky fine particles 10a can coexist, and the high-rate charge / discharge of the fine particles 10 can be performed while forming a channel of the electrolytic solution L of the entire negative electrode 122. In the present embodiment, it is essential that the fine particles 10 of the negative electrode active material 1 include granular fine particles 10b in which the structure of the outer shell 11 is maintained. Due to the presence of the granular fine particles 10b, a channel of the electrolytic solution L from the surface of the negative electrode mixture layer 122b to the current collector 122a is formed, and a low-resistance electron conductive network is maintained in the entire negative electrode mixture layer 122b. Therefore, uniform charge / discharge is possible in the thickness direction of the negative electrode 122.

Further, when the porosity Va of the negative electrode active material 1 is in the range of 0.2 or more and 0.5 or less, the outer shell 11 of the fine particles 10 constituting the negative electrode active material 1 is prevented from being broken. Therefore, it is possible to provide the high capacity negative electrode 122. Moreover, the negative electrode porosity in the negative mix layer 122b exists in the range of 0.72 or more and 0.82 or less, More preferably, it exists in the range of 0.74 or more and 0.81 or less. It is possible to provide a high capacity negative electrode 122 that is well retained by the negative electrode mixture layer 122b.

(Method for producing negative electrode active material)
Next, an example of the manufacturing method of the negative electrode active material 1 of this embodiment is demonstrated. FIG. 3 is a flowchart showing an example of the manufacturing process of the negative electrode active material 1. 4A to 4C are schematic cross-sectional views showing the production process of the fine particles 10 contained in the negative electrode active material 1, FIG. 4A is a cross-sectional view of the first precursor 10A, and FIG. 4B is a view of the second precursor 10B. FIG. 4C is a cross-sectional view of the manufactured fine particles 10. The manufacturing method of the negative electrode active material 1 of the present embodiment includes a first precursor manufacturing step S1, a second precursor manufacturing step S2, a fine particle manufacturing step S3, and a negative electrode active material preparing step S4 shown in FIG. Have.

(Manufacturing process of the first precursor)
In the manufacturing process S1 of the first precursor 10A, first, for example, resin beads B shown in FIG. 4A are prepared as granular materials that are thermally decomposed at a temperature lower than the melting point of the outer shell 11. As the material of the resin beads B, a material that is decomposed or melted in a temperature range in which the current collector 122a of the lithium ion secondary battery 100 is not oxidized and the mechanical strength does not decrease is selected. In the case where the current collector 122a of the negative electrode 122 is made of copper, for example, a material that decomposes or melts at a temperature of 300 ° C. or lower is suitable as the material of the resin beads B. As a material for such resin beads B, for example, a mixture of one or more materials selected from polyethylene, polyurethane, and ethylene-butadiene copolymer can be used.

The shape of the resin beads B is, for example, a sphere. The range of the particle size of the resin beads B is desirably, for example, ½ or less of the thickness of the negative electrode mixture layer 122b, and the range of the average particle size is desirably, for example, 0.01 μm or more and 10 μm or less. In the present embodiment, polyurethane resin beads B having an average particle diameter of 1 μm are used.

Next, the silicon nanoparticles 13 are added to the powder made of the resin beads B and mixed. For example, silicon nanoparticles 13 having an average particle diameter of 100 nm can be suitably used. The composition ratio of the silicon nanoparticles 13 to be mixed and the resin beads B can be set to, for example, 10: 1 (volume ratio). The obtained mixture is processed by a known mechanical fusion device to hold the plurality of silicon nanoparticles 13 on the surface of the resin beads B. Thereby, the 1st precursor 10A in which the some silicon nanoparticle 13 adhered to the surface of the resin bead B is obtained.

The number n of silicon nanoparticles 13 deposited on the surface of the resin beads B of the first precursor 10A can be measured, for example, by the following procedure. First, the number of adhered silicon nanoparticles 13 per unit surface area is measured based on a photomicrograph taken with a scanning electron microscope. Next, the number of adhesion of the silicon nanoparticles 13 per unit surface area is converted into the number n of adhesion on the surface area of the resin beads B. In this embodiment, the average value n of the number n of silicon nanoparticles 13 on the surface of the resin beads B of the first precursor 10A is, for example, in the range of about 200 to about 250, for example, about 230. .

(Manufacturing process of the second precursor)
In the manufacturing process S2 of the second precursor 10B, as shown in FIG. 4B, the outer shell 11 is formed by forming a layer made of a material constituting the outer shell 11 on the surface of the first precursor 10A. 2 precursor 10B is manufactured. In the present embodiment, the outer shell 11 is formed by forming a layer made of silicon (Si) over the entire surface of the first precursor 10A.

First, the powder composed of the first precursor 10A is accommodated in an apparatus capable of performing heat treatment and plasma CVD (chemical vapor deposition) on the powder, such as a rotary kiln furnace. Then, a layer made of silicon (Si) is formed on the entire surface of the first precursor 10A by the plasma CVD method, and the outer shell 11 is formed. The thickness of the layer made of silicon, that is, the thickness of the outer shell 11 can be set to 100 ± 10 nm, for example. Thereby, the 2nd precursor 10B by which the some silicon nanoparticle 13 adhered to the surface of the resin bead B and was hold | maintained at the inner surface of the outer shell 11 is obtained.

(Production process of fine particles)
In the manufacturing step S3 of the fine particles 10, the second precursor 10B is heated to manufacture the fine particles 10 shown in FIG. 4C. First, the second precursor 10B is heated in a heat-treatable apparatus such as a rotary kiln furnace to thermally decompose the resin beads B. The second precursor 10B is heated to 300 ° C. in a vacuum atmosphere, for example, and is subjected to heat treatment for about 3 hours, for example. By this heat treatment, the resin beads B are thermally decomposed and gasified, whereby the internal pressure of the outer shell 11 is increased, and a part of the outer shell 11 is broken to form the opening 11a. And the gas which is the decomposition product produced | generated by the thermal decomposition of the resin bead B is discharged | emitted from the formed opening part 11a to external space, and the resin bead B lose | disappears. As a result, an internal space 12 is formed in the outer shell 11, and the destroyed portion of the outer shell 11 becomes an opening 11 a. In addition, the silicon nanoparticles 13 are held on the inner surface of the outer shell 11 and are exposed to the internal space 12.

As described above, the fine particles 10 including the conductive outer shell 11 and the plurality of silicon nanoparticles 13 held on the inner surface of the outer shell 11 and exposed to the inner space 12 are obtained. Further, the outer shell 11 of the fine particle 10 is formed with an opening 11 a that communicates the outer space of the outer shell 11 and the inner space 12.

For example, depending on conditions such as the size and composition of the resin beads B, the thickness and strength of the outer shell 11, the temperature and time of the heat treatment, etc., the resin beads B are thermally decomposed while destroying a part of the outer shell 11. It disappears and not only the granular fine particles 10b having one or more openings 11a are formed, but also the openings 11a extend over the entire outer shell 11 so that the granular fine particles 10b are split to form flaky particles 10a. Is done.

(Preparation of negative electrode active material)
In the negative electrode active material preparation step S4, the negative electrode active material 1 is prepared by assembling the fine particles 10 or the like, or mixing the fine particles 10 or the like with other materials such as a conventional negative electrode active material. . Thereby, the negative electrode active material 1 mainly containing the fine particles 10 is obtained. Note that a high conductive material forming step S4a or a coating layer forming step S4b may be included before the negative electrode active material preparing step S4.

In the high conductive material forming step S4a, a high conductive material having higher conductivity than the outer shell 11 is formed on the outer surface of the outer shell 11 of the fine particles 10. For example, powder that is an aggregate of the fine particles 10 is accommodated in a known vapor deposition apparatus, and several tens of ppm of iron catalyst is vapor-deposited on the surface of the outer shell 11 of the fine particles 10. Thereafter, propane is supplied to grow carbon nanotubes (CNT) on the surface of the outer shell 11 of the fine particles 10. The amount of CNTs relative to the weight of the powder that is an aggregate of the fine particles 10 can be set to 1% by weight, for example.

On the other hand, in the coating layer forming step S4b, a coating layer is formed on the outer surface of the outer shell 11 of the fine particles 10. For example, a powder as an aggregate of the fine particles 10 is accommodated in a known rotary annular furnace, and the fine particles 10 are brought into contact with the fluorine gas to form a fluoride layer as a coating layer on the surface of the outer shell 11. Further, the fine particle 10 accommodated in the rotary annular furnace is brought into contact with, for example, propane gas, and subjected to heat treatment at, for example, about 900 ° C. or more and about 1000 ° C. or less, thereby forming a carbide layer (carbide) as a coating layer on the surface of the outer shell 11. Form. Further, for example, ammonia gas is brought into contact with the fine particles 10 accommodated in the rotary annular furnace to form a nitride layer as a coating layer on the surface of the outer shell 11. Further, for example, propane sultone is added to the electrolytic solution L of the lithium ion secondary battery 100 having the negative electrode 122 using the negative electrode active material 1, and charging and discharging of the lithium ion secondary battery 100 are performed. A sulfide layer may be formed on the shell 11 as a coating layer. The thickness of the coating layer can be about 5 nm, for example.

In addition, the lithium ion secondary battery 100 can be disassembled after the initial charge / discharge, and the surface composition of the outer shell 11 of the fine particles 10 can be analyzed by X-ray photoelectron spectroscopy. Moreover, the surface composition of the silicon nanoparticle 13 can be analyzed by analyzing the silicon nanoparticle 13 of the flaky fine particle 10a. The thickness of the coating layer can be measured by performing argon etching.

The specific surface area of the negative electrode active material 1 manufactured by the manufacturing method of the present embodiment is, for example, about 190 m ² / g. The specific surface area can be measured, for example, by the BET (Brunauer-Emmett-Teller) method using nitrogen gas. The specific surface area of the negative electrode active material 1 is preferably, for example, 50 m ² / g or more and 230 m ² / g or less.

The coating layer forming step S4b is performed after the high conductive material forming step S4a, or the coating layer forming step S4b is performed after the high conductive material forming step S4a, so that the high conductivity material is formed on the outer surface of the outer shell 11. And both coating layers may be formed.

(Relationship between outer shell of fine particles and silicon nanoparticles)
Next, the relationship between the outer shell 11 and the silicon nanoparticles 13 in the fine particles 10 will be described based on the relationship between the resin beads B and the silicon nanoparticles 13.

As shown in FIG. 4A, it is assumed that the resin beads B and the silicon nanoparticles 13 are spheres having a radius R and a radius r, respectively. Then, consider the relationship between the radius R of the resin beads B, the radius r of the silicon nanoparticles 13, and the number n of the silicon nanoparticles 13 attached to the surface of the resin beads B. The number n of the silicon nanoparticles 13 is maximized when the silicon nanoparticles 13 are arranged on the surface of the resin beads B in a close-packed manner. In order to obtain the maximum number n of silicon nanoparticles 13, the maximum number n that can be arranged on the surface of a sphere having a radius (R + r) without overlapping a circle having a radius r may be obtained. Since the surface area of the sphere having the radius (R + r) is 4π · (R + r) ² and the area of the circle having the radius r is πr ² , assuming that the maximum occupancy ratio of the circle on the surface is 0.91, the following formula (5 ) Is obtained.
n · πr ² ≦ 0.91 · 4π · (R + r) ² (5)

From the equation (5), the following equation (6) is obtained.
Sc ≦ 0.91 (6)

Note that Sc is an area occupancy ratio of the silicon nanoparticles 13 with respect to the surface of the resin beads B, that is, an area occupancy ratio of the silicon nanoparticles 13 with respect to the inner surface of the outer shell 11 and is represented by the following formula (7).
Sc = n · r ² / {4 · (R + r) ² } (7)

Moreover, the distance from the inner surface of the outer shell 11 shown in FIG. 4B to the center of the outer shell 11, that is, the inner radius Ri of the outer shell 11 is expressed by the following equation (8).
Ri = R + 2r (8)

From the formulas (7) and (8), the area occupation ratio Sc of the silicon nanoparticles 13 uses the number n of the silicon nanoparticles 13, the radius r of the silicon nanoparticles 13, and the inner radius Ri of the outer shell 11. Is represented by the following equation (9).
Sc = n · r ² / {4 · (Ri−r) ² } (9)

The equation (9) is substantially the same as the equation (7), but if this equation is used, the inner radius Ri of the outer shell 11, the radius r of the silicon nanoparticles 13, and the number n of the silicon nanoparticles 13 will be described. From the above, the area occupation ratio Sc of the silicon nanoparticles 13 with respect to the inner surface of the outer shell 11 can be calculated. Further, the inner radius Ri of the outer shell 11, the radius r of the silicon nanoparticles 13, and the number n of the silicon nanoparticles 13 can be determined based on the predetermined area occupation ratio Sc.

Hereinafter, the operation of the method for producing the negative electrode active material 1 of the present embodiment will be described.

In this embodiment, resin beads B are used as granular materials that are thermally decomposed at a temperature lower than the melting point of the outer shell 11 of the fine particles 10 constituting the negative electrode active material 1. Therefore, by forming the first precursor 10A in which the silicon nanoparticles 13 are arranged around the resin beads B as a nucleus, and forming the thin outer shell 11 covering the surface of the first precursor 10A, the outer shell 11 is formed. The silicon nanoparticles 13 can be held on the inner surface of the substrate.

Further, by heating the second precursor 10B having a plurality of silicon nanoparticles 13 attached to the surface of the resin beads B and held on the inner surface of the outer shell 11, the inner space 12 and the opening 11a are formed in the outer shell 11. And the fine particles 10 can be manufactured. Therefore, the manufacture of the fine particles 10 is facilitated, and the productivity can be improved.

Further, by adjusting the particle size of the resin beads B, it is possible to freely adjust the particle size of the manufactured fine particles 10. Further, by adjusting the particle size of the resin beads B, the average volume C of the internal space 12 of the outer shell 11 can be adjusted. Therefore, by adjusting the average volume C of the outer shell of the fine particles 10 based on the above formulas (1) to (3), even if the silicon nanoparticles 13 expand when the lithium ion secondary battery 100 is charged, The silicon nanoparticles 13 can be accommodated in the internal space 12. Therefore, the outer shell 11 of the fine particles 10 constituting the negative electrode active material 1 is prevented from being destroyed when the lithium ion secondary battery 100 is charged, and the life of the lithium ion secondary battery 100 can be extended as compared with the conventional case. .

Moreover, the specific surface area of the negative electrode active material 1 can be made sufficiently large by setting the average particle diameter of the resin beads B in the range of 0.01 μm or more and 10 μm or less, for example. Therefore, in the lithium ion secondary battery 100 using the negative electrode active material 1 of the present embodiment as the negative electrode material, the contact area between the negative electrode active material 1 and the electrolyte L can be increased as compared with the conventional case, and the lithium ion secondary battery 100 high rate charge / discharge becomes possible.

Further, by holding a material (conductive material) having a conductivity higher than that of the outer shell 11 on the outer surface of the outer shell 11 of the fine particles 10 included in the negative electrode active material 1, the conductivity between the fine particles 10 of the negative electrode active material 1. The initial capacity and capacity retention rate of the lithium ion secondary battery 100 can be improved.

Further, by forming a coating layer that does not react with the electrolytic solution L on at least the outer surface of the outer shell 11 of the fine particles 10 contained in the negative electrode active material 1, decomposition of the electrolytic solution L is suppressed, and the lithium ion secondary battery 100 The capacity maintenance rate can be improved. Silicon compounds such as silicon nitride, silicon fluoride, silicon carbide, and silicon sulfide exhibit a lower catalytic activity than non-aqueous solvents and electrolytes for reductive decomposition, and thus can be suitable coating layers. In this embodiment, the coating layer is formed on the outer side of the outer shell 11. However, if the coating layer is formed on the inner side of the outer shell 11, the decomposition amount of the electrolytic solution can be reduced, which is more desirable.

The aforementioned nitride or the like can be formed by decomposing the reaction gas on the surface of the silicon outer shell. As the reaction gas, low molecular hydrocarbons such as ammonia, fluorine, and propane, hydrogen sulfide, and the like can be used. When these gases are brought into contact with silicon particles and the reaction gas is decomposed on the silicon surface, the above-described coating layer is formed. The reaction gas is preferably diluted with an inert gas such as nitrogen or argon to a concentration of 1 to 10% and brought into contact with silicon. Depending on the type of reaction gas and the thickness of the coating layer, the reaction temperature can be adjusted in the range of 300 to 900 ° C.

Further, by obtaining the area occupancy Sc of the silicon nanoparticles 13 by the above formulas (5) to (9), the area occupancy Sc can be reduced to a predetermined value or less, and due to the expansion of the silicon nanoparticles 13 It becomes possible to prevent the outer shell 11 from being broken.

(Method for producing lithium ion secondary battery)
Next, the manufacturing method of the lithium ion secondary battery 100 of this embodiment is demonstrated. In the manufacturing process of the negative electrode 122 used in the lithium ion secondary battery 100, first, a negative electrode slurry in which the negative electrode active material 1, the binder, and the organic solvent are mixed is manufactured. For example, a negative electrode material containing 95% by weight of negative electrode active material 1 and 5% by weight of PVDF (polyvinylidene fluoride) binder is kneaded using NMP (1-methyl-2-pyrrolidone) as a solvent to produce a negative electrode slurry. To do. For the kneading of the negative electrode material, for example, a known kneader such as a planetary mixer or a disperser can be used. The solvent is not particularly limited as long as it dissolves the binder, and can be appropriately selected according to the material of the binder. In this embodiment, NMP is used as a solvent in order to dissolve the PVDF binder.

Next, a metal foil for negative electrode that is a current collector 122a is prepared. As the negative electrode metal foil, for example, a copper foil having a thickness of 10 to 100 μm, a copper perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, etc. can be used. As a material other than copper, for example, stainless steel, titanium, nickel and the like are also applicable. As long as the current collector 122a for the negative electrode can be used as the metal foil for the negative electrode without changing such as dissolution and oxidation during use of the lithium ion secondary battery, the material, shape, and manufacturing method are: There is no particular limitation. In the present embodiment, a rolled copper foil having a thickness of 10 μm is used.

Next, for example, a negative electrode slurry is applied on the current collector 122a by a doctor blade method, a dipping method, a spray method, or the like, and is dried at a temperature of about 120 ° C., for example. Thereafter, a layer made of the negative electrode active material 1 formed by drying the negative electrode slurry is compressed by, for example, a roll press and subjected to pressure molding to form the negative electrode mixture layer 122b having a predetermined porosity. Thereby, the negative electrode 122 for the lithium ion secondary battery 100 in which the negative electrode mixture layer 122b is formed on the current collector 122a is obtained.

Here, the thickness of the negative electrode slurry applied on the current collector 122a can be, for example, about 10 μm. The porosity of the negative electrode mixture layer 122b can be set to 72%, for example. The density of the negative electrode mixture layer 122b can be set to, for example, 0.6 g / cm ³ . In the drying process, the current collector 122a coated with the negative electrode slurry is accommodated in a vacuum drying apparatus, and the NMP can be completely removed by heating at a temperature of 80 ° C., for example. In the present embodiment, the application of the negative electrode slurry onto the current collector 122a is performed only once by the doctor blade method. However, the application from the negative electrode slurry to the drying is performed a plurality of times to thereby apply the negative electrode slurry onto the current collector 122a. It is also possible to form a multilayer mixture layer.

Further, in the manufacturing process of the negative electrode active material 1 shown in FIG. 3, the step S3 is omitted, and the negative electrode active material particles (second precursor) in which the resin beads B are not removed and the openings 11a are not formed are used. Step S3 may be performed after the mixture layer is formed on the current collector 122a. By carrying out step S3 after the formation of the negative electrode mixture layer, the residual amount of the solvent of the negative electrode slurry in the internal space 12 is reduced, which is effective for extending the life of the negative electrode.

In the manufacturing process of the positive electrode 121 used in the lithium ion secondary battery 100, as in the manufacturing process of the negative electrode 122, first, a positive electrode slurry in which a positive electrode active material, a binder, and an organic solvent are mixed is manufactured.

Typical examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . Other examples include LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _{2 -X} M _X O ₂ (where M = Co, Ni, Fe, Cr, Zn, Ta, X = 0.01-0.2), Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, Ni, Cu, Zn), Li _1-X A _X Mn ₂ O ₄ (where A = Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, X = 0.01 to 0.1), LiNi _1-X M _X O ₂ (where M = Co, Fe, Ga, X = 0.01 to 0.2), LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , LiCo _1-X M _X O ₂ (where M = Ni, Fe, Mn, X = 0.01 to 0.2), LiNi _1-X M _X O ₂ (M = Mn, Fe, Co, Al, Ga, Ca, Mg, X = 0 .01-0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO _{4 and the} like. In addition, the positive electrode active material used for the positive electrode of this embodiment is not limited to these materials. In this embodiment, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ is used as the positive electrode active material.

The particle size of the positive electrode active material is specified to be equal to or less than the thickness of the positive electrode mixture layer. When the positive electrode active material powder has coarse particles having a particle size equal to or larger than the thickness of the positive electrode mixture layer, the coarse particles are previously removed by sieving classification, wind classification, etc. Sort particles of diameter.

Also, the positive electrode active material is made of an oxide-based material and has a high electric resistance. Therefore, a conductive additive made of carbon powder that supplements the electrical conductivity of the positive electrode active material is used. As the conductive assistant, for example, a carbon material such as acetylene black, carbon black, graphite, and amorphous carbon can be used. In order to form an electronic network inside the positive electrode mixture layer, the particle diameter of the conductive auxiliary agent is preferably smaller than the average particle diameter of the positive electrode active material and not more than 1/10 of the average particle diameter. Since the positive electrode active material and the conductive additive are both powders, a binder is mixed with the powders to bond the powders together.

For example, a positive electrode slurry containing 89% by weight of the positive electrode active material, 4% by weight of acetylene black and 7% by weight of PVDF binder is kneaded using NMP as a solvent to produce a positive electrode slurry. For the kneading of the positive electrode material, for example, a planetary mixer can be used as in the kneading of the negative electrode material.

Next, a positive electrode metal foil as a current collector is prepared. Examples of the positive electrode metal foil include an aluminum foil having a thickness of 10 to 100 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a hole diameter of 0.11 to 10 mm, an expanded metal, a foam metal plate, and the like. Can be used. As a material other than aluminum, for example, stainless steel, titanium, and the like are also applicable. As long as the current collector for the positive electrode can be used as a metal foil for the positive electrode without changing such as dissolution and oxidation during use of the lithium ion secondary battery, the material, shape, and manufacturing method are as follows. There is no particular limitation. In this embodiment, a 20 μm thick aluminum rolled foil is used.

Thereafter, in the same manner as in the manufacturing process of the negative electrode 122, the positive electrode slurry is applied on the current collector for the positive electrode 121, dried, compressed and pressure-molded to form a positive electrode mixture layer. Thereby, the positive electrode 121 for a lithium ion secondary battery in which the positive electrode mixture layer is formed on the current collector is obtained. In this embodiment, the application of the positive electrode slurry on the current collector is performed only once by the doctor blade method, but the multilayer mixture layer is formed on the current collector by performing a plurality of times from the application of the positive electrode slurry to the drying. It is also possible to form

The electrode group 120 is manufactured by laminating the positive electrode 121 and the negative electrode 122 manufactured by the above-described process with a separator 123 as an insulator interposed therebetween. When the electrode group 120 is a wound electrode group, the electrode group 120 is manufactured by winding the positive electrode 121 and the negative electrode 122 laminated with the separator 123 interposed therebetween around the winding axis.

Also, the positive battery cover 112a, the gasket 113, the internal pressure release valve 114, the PTC resistance element 115, and the inner cover 116 are assembled as an integrally structured battery cover unit. In addition, the electrode group 120 is accommodated in the cylindrical portion 111 of the battery container 110 with the separator 123 disposed around the electrode group 120. In addition, the positive electrode 121 is connected to the positive battery cover 112a by the positive current collecting tab 131, the negative electrode 122 is connected to the negative battery cover 112b, and the opening 111a of the cylindrical portion 111 is sealed by the positive battery cover 112a and the negative battery cover 112b. Stop.

Finally, the electrolyte L is injected into the battery container 110. The electrolytic solution L is injected from above the opening 111a of the cylindrical portion 111 before attaching the positive battery cover 112a. Thereafter, the positive battery cover 112a is attached, and the cylindrical portion 111 is sealed. The electrolyte may be supplied from a liquid inlet provided in advance in the battery lid 112a, and the liquid inlet may be sealed by laser welding or the like. As described above, the lithium ion secondary battery 100 of the present embodiment is obtained.

(Module / battery)
FIG. 5 is a schematic diagram of a module (assembled battery) 200 in which eight lithium ion secondary batteries 100 shown in FIG. 2 are connected in series. A positive external terminal 201 and a negative external terminal 202 are connected to the module 200, and the positive external terminal 201 and the negative external terminal 202 are connected to the charge / discharge circuit 301 via a power line 304. The charge / discharge circuit 301 is connected to an arithmetic control unit 302 via a signal line 305 and to a power supply load power source 303 via an external power cable 306.

The power supply load power supply 303 has both power supply and consumption functions, and is installed in place of an external power supply or an external load in order to confirm the effectiveness of the module 200. That is, the power supply load power supply 303 receives electric power from the module 200 via the charging / discharging circuit 301 and the power cable 306, or an electric vehicle such as an electric vehicle that supplies power to the module 200, a machine tool, or a distributed power storage system. And a backup power supply system.

The arithmetic control unit 302 switches the charge / discharge circuit 301 to the charging mode when the power supply load power supply 303 supplies power to the module 200, and discharges the charge / discharge circuit 301 when the module 200 supplies power to the power supply load power supply 303. Switch to mode.

The charge / discharge circuit 301 supplies the charging current from the power supply load power supply 303 to the module 200 in the charging mode, and supplies the discharge current from the module 200 to the power supply load power supply 303 in the discharge mode.

The module 200 of the present embodiment includes the above-described lithium ion secondary battery 100. Therefore, the capacity density of the module 200 can be increased. Further, even when the module 200 repeatedly performs charging / discharging in relation to the power supply load power supply 303, a decrease in the capacity maintenance rate of the module 200 is suppressed, and the cycle life of the module 200 can be improved.

(Battery system)
FIG. 6 is a schematic diagram of a battery system 400 using the module 200 shown in FIG. The battery system 400 includes two

modules

200A and 200B connected in series and a charge / discharge controller 410.

Modules

200A and 200B have the same configuration as module 200 shown in FIG. The negative external terminal 202A of the module 200A is connected to the negative input terminal of the charge / discharge controller 410 by the power cable 401. The positive external terminal 201A of the battery module 200A is connected to the negative external terminal 202B of the battery module 200B via the power cable 402. The positive external terminal 201B of the battery module 200B is connected to the positive input terminal of the charge / discharge controller 410 by the power cable 403. With such a wiring configuration, the two

battery modules

200A and 200B can be charged or discharged.

The charge / discharge controller 410 exchanges power with the external device 500 via the

power cables

404 and 405. The external device 500 includes an external power source for supplying power to the charge / discharge controller 410 and various electric devices such as a regenerative motor, and an inverter, a converter, and a load that supply power from the system. An inverter or the like can be provided in accordance with the types of AC and DC that the external device 500 supports. As these devices, known devices can be arbitrarily applied.

Also, a power generation device 420 that simulates the operating conditions of a wind power generator is installed as a device that generates renewable energy, and is connected to the charge / discharge controller 410 via

power cables

406 and 407. When the power generation device 420 generates power, the charge / discharge controller 410 shifts to the charge mode, supplies power to the external device 500 and charges the

battery modules

200A and 200B with surplus power. The charge / discharge controller 410 operates to discharge the battery modules 200 </ b> A and 200 </ b> B when the power generation amount of the power generation apparatus 420 simulating a wind power generator is smaller than the required power of the external device 500. The power generation device 420 can be replaced with another power generation device, that is, any device such as a solar cell, a geothermal power generation device, a fuel cell, or a gas turbine generator. The charge / discharge controller 410 stores a program that can be automatically operated so as to perform the above-described operation.

The battery system 400 of the present embodiment includes the lithium ion secondary battery 100 described above. Therefore, the capacity density of the battery system 400 can be increased as compared with the conventional case. In addition, even when the battery system 400 repeatedly performs charge and discharge in relation to the power generation device 420 and the external device 500, a decrease in the capacity maintenance rate of the battery system 400 is suppressed, and the cycle life of the battery system 400 can be improved. . In addition, the number of batteries, the number of series, and the number of parallel are not limited to this embodiment, According to the electric energy required by the consumer side, it is possible to increase / decrease the number of series and the number of parallel.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications are included. The above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described.

The battery container of a lithium ion secondary battery may have a bottomed cylindrical or boxed battery can, and a battery container of any shape can be used. Moreover, the manufacturing method of a lithium ion secondary battery is not limited to the above-mentioned embodiment. For example, the compositions of the negative electrode slurry and the positive electrode slurry are changed according to the type of material, specific surface area, particle size distribution, and the like, and are not limited to the exemplified compositions.

The battery module can be used as a power source for consumer electronics such as portable electronic devices, mobile phones, and electric tools, as well as electric vehicles, trains, storage batteries for storing renewable energy, unmanned mobile vehicles, nursing care devices, etc. . Furthermore, the lithium ion secondary battery of the present embodiment can be applied to a power supply of a logistics train for searching for the moon, Mars, and the like. In addition, the lithium ion secondary battery of this embodiment is a space suit, a space station, a building or living space on earth or other celestial body (regardless of sealed or open state), a spacecraft for interplanetary movement. It can be used for various power sources such as air conditioning, temperature control, purification of sewage and air, and power for planetary rover, underwater or underwater sealed space, submarine, fish observation equipment.

Hereinafter, examples of the present invention based on the above-described embodiment will be described.

(Example 1)
First, the negative electrode active materials of Samples 1 to 4 shown in Table 1 were manufactured by the manufacturing method described in the above embodiment.

In Samples 1 to 4, the thickness of the outer shell of the fine particles contained in the negative electrode active material is all 100 nm, and the average particle diameter of the silicon nanoparticles is different in the range of 100 nm to 30 nm. Moreover, a highly conductive material and a coating layer were not formed on the surface of the outer shell.

In each sample, the average particle diameter and the average number of silicon nanoparticles, and the thickness of the outer shell were determined by embedding the negative electrode active material fine particles in a thermosetting resin, and then cutting out the cross section of the resin using a microtome. Estimated from electron micrographs. The average particle size of the fine particles can be estimated in the same manner. Further, based on the expressions (3) and (7) in the above-described embodiment, the volume occupancy Vc of the silicon nanoparticles with respect to the inner space of the outer shell and the area occupancy ratio Sc of the silicon nanoparticles with respect to the inner surface of the outer shell are calculated. Calculated. The porosity Va is obtained from the average particle size of the second precursor in FIG. 4B, and the unit volume of the particle is obtained, and the difference (the difference corresponds to the void volume) with the sum of the average values of the volumes of the resin, silicon particles, and outer shell. )). Each volume can be estimated from an image of a cross-sectional photograph in which the particles of the second precursor are embedded in a thermosetting resin. The specific surface area of the negative electrode active material was measured by the BET method using nitrogen gas.

Next, using the negative electrode active materials of Samples 1 to 4, five lithium ion secondary batteries were manufactured for each sample by the manufacturing method described in the above embodiment. Here, the negative electrode porosity of the negative electrode mixture layer was measured by a BET specific surface area method using nitrogen gas. The results are shown in Table 2 below. The rated capacity (calculated value) of the manufactured lithium ion secondary battery was 3.5 Ah.

For these batteries, the following initial aging treatment was performed. First, charging of the lithium ion secondary battery was started from the open circuit state. The current was set to 1.75 A, and the voltage was maintained when the voltage reached 4.2 V, and charging was continued until the current reached 0.05 A. Thereafter, a 30-minute rest period was provided, and discharging was started at 3.5 A. When the voltage of the battery reached 3.0V, the discharge was stopped and a 30-minute pause was performed. Similarly, charging and discharging were repeated 5 times to complete the initial aging process of the battery. The value of the discharge capacity in the last cycle (5th cycle) was divided by the weight (10 ± 0.1 g) of the negative electrode active material to calculate the initial capacity density. Moreover, charging / discharging of 1 hour rate was repeated, the capacity density at the time of 100-cycle progress was calculated, and the capacity | capacitance maintenance factor was computed from the obtained capacity density and initial stage capacity density. The results are shown in Table 2 below.

As shown in Table 1 and Table 2, when the particle diameter of the silicon nanoparticles is in the range of 100 nm to 30 nm, the initial capacity density gradually increases from 2700 Ah / kg to 2910 Ah / kg as the particle diameter decreases, and the capacity is maintained. The rate gradually increased from 85% to 90%.

(Example 2)
Next, the conditions for the initial aging treatment of the lithium ion secondary battery using the negative electrode active material of Sample 3 manufactured in Example 1 were changed. Specifically, the charging current of the battery was set to 0.4 A, the voltage was maintained when the voltage reached 4.2 V, and charging was continued until the current reached 0.01 A. Thereafter, a 30-minute rest period was provided, and discharging was started at 0.4 A. When the battery voltage reached 3.0 V, the discharge was stopped and a 30-minute pause was performed. Similarly, charging and discharging were repeated 5 times to complete the initial aging process of the battery. Thereafter, in the same manner as in Example 1, the initial capacity density and the capacity retention ratio were calculated. The results are shown in Table 3 below.

Since the charge / discharge current value was reduced, the amount of Li occlusion in the negative electrode increased, and the capacity density increased compared to Example 1. In addition, since the current density was reduced, the negative electrode active material was uniformly charged and discharged, and the stress between the particles was relieved. Therefore, the capacity retention rate was also improved from the result of Example 1.

(Example 3)
Next, according to the manufacturing method described in the above-described embodiment, the average particle diameter and the average number of silicon nanoparticles and the thickness of the outer shell are set to be equal to those in Example 1, and the negative electrodes of Sample 5 to Sample 9 shown in Table 4 An active material was produced.

In sample 5, 1% by weight of carbon nanotubes (CNT) was grown as a highly conductive material on the surface of the outer shell of the negative electrode active material. In Samples 6 to 9, a fluoride layer, a carbide layer, a nitride layer, and a sulfide layer were formed as a coating layer on the surface of the outer shell of the negative electrode active material with a thickness of 5 nm, respectively. The highly conductive material and each coating layer were formed by the method described in the above embodiment.

Thereafter, in the same manner as in Example 1, lithium ion secondary batteries using the negative electrode active materials of Samples 5 to 9 were manufactured, subjected to initial aging treatment, and initial capacity density and capacity retention ratio were calculated. The results are shown in Table 5.

Compared with the lithium ion secondary battery using the negative electrode active material of sample 3 (Table 2), the lithium ion secondary battery using the negative electrode active material of sample 5 is made of the negative electrode active material by CNT which is a highly conductive material. The conductivity between the fine particles was improved, and the initial capacity density and capacity retention were further improved.

Compared with the lithium ion secondary battery using the negative electrode active material of sample 3 (Table 2), the lithium ion secondary battery using the negative electrode active material of samples 6 to 9 has a smaller lithium charge / discharge capacity. Because of the presence of the layers, the initial capacity density was reduced to the same extent or slightly, but the capacity retention was improved because the decomposition of the electrolyte was effectively suppressed.

Example 4
Next, the average particle diameter and the average number of silicon nanoparticles and the thickness of the outer shell were made equal to those of the negative electrode active material of Sample 2 of Example 1, and the porosity Va was changed as shown in Table 6 below. The negative electrode active materials of Samples 10 to 13 were manufactured. Since the pressure at the time of pressure-molding the negative electrode mixture layer and the negative electrode mixture density showed a monotonous relationship, Va could be changed by adjusting the pressure. Here, the monotonous relationship means a relationship in which, for example, the density increases as the pressure increases, so that the porosity decreases.

Thereafter, in the same manner as in Example 1, lithium ion secondary batteries using the negative electrode active materials of Samples 10 to 13 were manufactured. In the battery using each sample, the negative electrode porosity was 0.70 or more and 0.81 or less. Thereafter, in the same manner as in Example 1, each battery was subjected to initial aging treatment, and the initial capacity density and the capacity retention rate were measured. The results are shown in Table 7.

By using the negative electrode active material of Samples 10 to 13 having a porosity Va of 0.2 or more and 0.5 or less, and by setting the negative electrode porosity to a range of 0.70 or more and 0.81 or less, the initial capacity density and The capacity retention rate was equal to or greater than that of the lithium ion secondary battery using the negative electrode active material of Document 2 of Example 1. That is, the amount of electrolyte retained in the negative electrode of the lithium ion secondary battery was not reduced, and the electrical contact between the fine particles of the negative electrode active material was not impaired.

(Example 5)
Next, a negative electrode active material in which the material of the outer shell of the fine particles was changed to copper was manufactured under the same conditions as the negative electrode active material of Sample 3 manufactured in Example 1. That is, a second precursor is manufactured by copper plating using a barrel sputtering apparatus with respect to the first precursor in the method for manufacturing a negative electrode active material described in the above-described embodiment, and the second precursor The negative electrode active material of the sample 14 shown in the following Table 8 was manufactured by heat-processing.

Thereafter, in the same manner as in Example 1, a lithium ion secondary battery using the negative electrode active material of Sample 14 was manufactured, and initial aging was performed to calculate an initial capacity density and a capacity retention rate. The results are shown in Table 9.

From the results in Table 9, it was demonstrated that even if the outer shell material was changed, the initial capacity density and capacity retention rate of the lithium ion secondary battery were relatively high if the outer shell had conductivity.

(Example 6)
Next, as shown in Table 10 below, the negative electrode active material of Sample 15 in which the particle size of the silicon nanoparticles was increased to 200 nm and the thickness of the outer shell were set to 200 nm by the manufacturing method described in the above embodiment. The negative electrode active material of Sample 16 increased in size was manufactured.

Thereafter, similarly to Example 1, lithium ion secondary batteries using the negative electrode active materials of Sample 15 and Sample 16 were manufactured, and initial aging was performed to calculate initial capacity density and capacity retention rate. The results are shown in Table 11.

Compared to the lithium ion secondary battery using the negative electrode active material of Sample 1 of Example 1, even if the average particle diameter of the silicon nanoparticles or the thickness of the outer shell is increased, the initial capacity density is substantially equivalent. It was proved that the capacity retention rate was obtained.

(Example 7)
Next, negative electrode active materials of Samples 17 to 22 shown in Table 12 below were manufactured by the manufacturing method described in the above embodiment.

In the negative electrode active material of Sample 17, as in Sample 1 of Example 1, the thickness of the outer shell of the fine particles was 100 nm, and the volume occupancy Vc of the silicon nanoparticles with respect to the inner space of the outer shell was increased to 1.20. . Similarly, in the negative electrode active material of Sample 18, the thickness of the outer shell of the fine particles was 100 nm, and the area occupation ratio Sc of the silicon nanoparticles with respect to the inner surface of the outer shell was 0.95, which was larger than 0.91 of Sample 17. .

The negative electrode active material of Sample 19 and Sample 20 is the same as the negative electrode active material of Sample 2 of Example 1.

The negative electrode active material of Sample 21 was the same as the negative electrode active material of Sample 15 of Example 6 except that the average particle size of the silicon nanoparticles was increased to 400 nm. The negative electrode active material of Sample 22 was the same as the negative electrode active material of Sample 16 of Example 6 except that the thickness of the outer shell of the fine particles was increased to 400 nm.

Then, similarly to Example 1, lithium ion secondary batteries were manufactured using the negative electrode active materials of Sample 17 to Sample 22, respectively. Here, the negative electrode porosity of the battery using the negative electrode active material of Sample 2 of Example 1 was 0.74, but the negative electrode porosity of the battery using the negative electrode active material of Sample 19 was set to 0.66. In the battery using the negative electrode active material of Sample 20, the negative electrode porosity was increased to 0.85. Then, as in Example 1, the manufactured lithium ion secondary battery was subjected to the initial aging treatment, and the initial capacity density and the capacity retention rate were calculated. The results are shown in Table 13.

In the battery using the negative electrode active material of Sample 17, when the fine silicon nanoparticles constituting the negative electrode active material expand during charging, the outer shell of many fine particles is destroyed, and Sample 1 of Example 1 is used. As compared with the battery using the negative electrode active material, the initial capacity density and the capacity retention rate were lowered. Therefore, it is demonstrated that the volume occupancy Vc of the silicon nanoparticles with respect to the inner space of the outer shell of the fine particles constituting the negative electrode active material is preferably smaller than 1.20, and more preferably 0.74 or less. It was done.

In the lithium ion secondary battery using the negative electrode active material of sample 18, in addition to the destruction of the outer shell due to the expansion of the silicon nanoparticles during charging, the electrical contact between the fine particles is impaired, and the negative electrode active material of sample 17 The initial capacity density and the capacity retention rate were reduced compared to the lithium ion secondary battery using the battery. Therefore, the area occupation ratio Sc of the silicon nanoparticles with respect to the inner surface of the outer shell of the fine particles constituting the negative electrode active material is preferably smaller than 0.95.

In the lithium ion secondary battery in which the negative electrode active material of sample 19 equivalent to the negative electrode active material of sample 2 of Example 1 was used and the negative electrode porosity was reduced to 0.66, In addition to the destruction of the shell and the removal of the silicon nanoparticles, the amount of electrolyte retained decreased, so that the initial capacity density decreased, and in particular, the capacity retention rate decreased significantly. Therefore, the negative electrode porosity is preferably larger than 0.66.

In the lithium ion secondary battery in which the negative electrode active material of sample 20 equivalent to the negative electrode active material of sample 2 of Example 1 was used and the negative electrode porosity was increased to 0.78, the negative electrode of sample 19 was increased due to the increased porosity. Instead of causing the problem as in the case of using the active material, the electrical contact between the fine particles of the negative electrode active material was impaired, the initial capacity density was reduced, and the capacity retention rate was particularly lowered. Accordingly, the negative electrode porosity is preferably smaller than 0.78.

In the lithium ion secondary battery using the negative electrode active material of Sample 21, silicon nanoparticles were destroyed in the process of charge and discharge, and compared with the lithium ion secondary battery using the negative electrode active material of Sample 15 of Example 6. The initial capacity density and capacity retention rate decreased. Therefore, the average particle diameter of the silicon nanoparticles is preferably smaller than 400 nm.

In the lithium ion secondary battery using the negative electrode active material of sample 22, the outer shell was destroyed during the charge and discharge process and the conductivity deteriorated. Therefore, the lithium ion secondary battery using the negative electrode active material of sample 16 of Example 6 was used. Compared with the secondary battery, the initial capacity density and the capacity retention rate were lowered. Accordingly, the thickness of the outer shell is preferably smaller than 400 nm.

(Example 8)
Next, eight lithium ion secondary batteries 100 using the negative electrode active material of sample 12 of Example 4 were manufactured, and the module 200 (assembled battery) having the configuration shown in FIG. 5 described in the above embodiment was assembled. A charge test and a discharge test were conducted.

In the charging test, the charging / discharging circuit 301 supplies a charging current having a current value (3.5 A) corresponding to an hour rate to the positive external terminal 201 and the negative external terminal 202, and charging is performed for 1 hour at a constant voltage of 33.6V. Went. The constant voltage value set here is eight times the constant voltage value 4.2 V of the lithium ion secondary battery in this example. Electric power necessary for charging / discharging the module 200 was supplied from a power supply load power source 303.

In the discharge test, a reverse current was passed from the positive external terminal 201 and the negative external terminal 202 to the charge / discharge circuit 301, and power was consumed by the power supply load power source 303. The discharge current was 1 hour rate (discharge current: 3.5 A), and discharging was performed until the voltage between the positive external terminal 201 and the negative external terminal 202 reached 24V.

Under such charge / discharge test conditions, the module 200 obtained initial performance with a charge capacity of 3.5 Ah and a discharge capacity of 3.4 to 3.5 Ah. Further, when a charge / discharge cycle test of 300 cycles was performed, the capacity maintenance rate of the module 200 was 94 to 95%. By using the lithium ion secondary battery 100 of the present embodiment, the initial capacity of the module 200 is increased compared to the conventional case, the decrease in the capacity retention rate due to repeated charge and discharge is suppressed, and the cycle life of the module 200 is improved.

Example 9
Next, two

battery modules

200A and 200B assembled in Example 8 were connected in series, and the battery system 400 having the configuration shown in FIG. 6 described in the above embodiment was manufactured.

First, the

battery modules

200A and 200B are charged normally to obtain a rated capacity. Here, constant voltage charging of 4.2 V was performed for 0.5 hour at a charging current of 1 hour rate. The charging conditions are determined by the design of the material type, amount of use, etc. of the lithium ion secondary battery. Therefore, the charging conditions are optimal for each battery specification.

After charging the lithium ion secondary battery, the charge / discharge controller 410 was switched to the discharge mode to discharge each battery, and the discharge was stopped when a certain lower limit voltage was reached. Furthermore, when charging the lithium ion secondary battery, power is supplied from the external device 500 to the lithium ion secondary battery, and when discharging the lithium ion secondary battery, power is supplied from the lithium ion secondary battery to the external device 500. Power was consumed. In this example, charging was performed at a rate of 2 hours, discharging was performed at a rate of 1 hour, and the initial discharge capacity was obtained. As a result, a capacity of 99.1 to 99.6% of the design capacity 3.5 Ah of each battery module was obtained.

Thereafter, the following charge / discharge cycle test was performed under the condition of an environmental temperature of 20-30 ° C. First, when charging is performed at a current of 2 hours (1.75 A) and the depth of charge reaches 50% (a state in which 1.75 Ah is charged), a pulse of 5 seconds is charged in the charging direction and 5 seconds in the discharging direction. Was applied to the battery modules 200 </ b> A and 200 </ b> B, and a pulse test for simulating acceptance of power from the power generation device 420 and power supply to the external device 500 was performed. The magnitude of the current pulse was 150A for both. Subsequently, the remaining capacity of 1.75 Ah is charged with a current (1.75 A) at a rate of 2 hours until the voltage of each battery reaches 4.2 V, and after constant voltage charging for 1 hour is continued at that voltage, Was terminated. Then, it discharged until the voltage of each battery was set to 3.0V with the electric current (3.5A) of 1 hour rate. When such a series of charge / discharge cycle tests were repeated 500 times, a capacity of 88 to 89% of the initial discharge capacity was obtained. It has been found that the performance of the system 400 is hardly degraded when the battery is subjected to power acceptance and power supply current pulses. That is, since the battery system 400 includes the lithium ion secondary battery of this example, a decrease in capacity retention rate is suppressed even when charging and discharging are repeated, and the cycle life of the battery system 400 is improved.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material 10 Fine particle 10a Flaky fine particle 10A 1st precursor 10B 2nd precursor 11 Outer shell 11a Opening part 12 Internal space 13 Silicon nanoparticle 100 Lithium ion secondary battery B Resin beads (granular material)
S1 Process for producing the first precursor S2 Process for producing the second precursor S3 Process for producing the fine particles

Claims

A negative electrode active material for a lithium ion secondary battery,
Including a conductive outer shell and a plurality of silicon nanoparticles held on the inner surface of the outer shell and exposed to the inner space of the outer shell,
The negative electrode active material, wherein the outer shell has an opening communicating the outer space of the outer shell and the inner space.
2. The negative electrode active material according to claim 1, wherein the material constituting the outer shell contains silicon.
3. The negative electrode active material according to claim 2, wherein a highly conductive material having higher conductivity than the outer shell is held on the outer surface of the outer shell.
The negative electrode active material according to claim 2, further comprising a coating layer on an outer surface of the outer shell.
2. The negative electrode active material according to claim 1, wherein the fine particles include flaky fine particles formed by dividing the outer shell.
2. The negative electrode active material according to claim 1, wherein the volume of the outer shell and the number and size of the silicon nanoparticles satisfy the relationship of the following formula (1).
n · (4πr 3/3) · ΔV ≦ a · C ... (1)
However, n is the number of the silicon nanoparticles, r is 1/2 of the average particle diameter of the silicon nanoparticles, ΔV is a volume increase ratio of the silicon nanoparticles during charging of the lithium ion secondary battery, and a is the above The filling rate when the silicon nanoparticles after the volume increase at the time of charging are closely packed in the internal space, C is the volume of the internal space of the outer shell
A negative electrode active material for a lithium ion secondary battery, comprising: a fine particle comprising a conductive outer shell and a plurality of silicon nanoparticles held on the inner surface of the outer shell and exposed to the inner space of the outer shell. A manufacturing method comprising:
Producing a first precursor in which a plurality of the silicon nanoparticles are attached to the surface of a granular material thermally decomposed at a temperature lower than the melting point of the outer shell;
By forming a layer made of a material constituting the outer shell on the surface of the first precursor to form the outer shell, a plurality of the silicon nanoparticles are attached to the surface of the granular material and Producing a second precursor held on the inner surface;
The second precursor is heated to thermally decompose the granular material to form an opening in the outer shell, and the decomposition product of the granular material is discharged from the opening to an external space, Forming and producing the microparticles;
A method for producing a negative electrode active material, comprising:
The method for producing a negative electrode active material according to claim 7, further comprising a step of forming a highly conductive material having higher conductivity than the outer shell on the outer surface of the outer shell after the step of manufacturing the fine particles. .
The method for producing a negative electrode active material according to claim 7, further comprising a step of forming a coating layer on an outer surface of the outer shell after the step of producing the fine particles.
A lithium ion secondary battery comprising an electrode group comprising a negative electrode and a positive electrode, a battery container containing the electrode group, and an electrolyte solution contained in the battery container,
The negative electrode active material constituting the negative electrode includes a fine particle comprising a conductive outer shell and a plurality of silicon nanoparticles held on the inner surface of the outer shell and exposed to the inner space of the outer shell,
The lithium ion secondary battery, wherein the outer shell has an opening that communicates the outer space of the outer shell and the inner space.