JPWO2015029100A1 - Negative electrode active material for lithium ion secondary battery, lithium ion secondary battery, and method for producing negative electrode active material for lithium ion secondary battery - Google Patents

Abstract

A negative electrode active material for a lithium ion secondary battery, wherein the negative electrode active material particles are composed of an aggregate of Si particles and SiO2 particles, and the aggregate is formed by dispersing a plurality of Si particles in SiO2, The Si particles include Si particles having a protrusion part of the surface protruding from the surface of SiO2, a SiO2 layer is formed on the surface of the protrusion, and the thickness of the SiO2 layer is 1 nm or more, and 9 nm or less.

Description

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

  In recent years, a lithium ion secondary battery has attracted attention as a secondary battery having a high energy density, and its research and development and commercialization are rapidly progressing. As a result, lithium ion secondary batteries have become widespread in small consumer devices such as mobile phones and notebook computers. Furthermore, from the viewpoints of global warming, fuel depletion, denuclear power generation, and the like, large-capacity lithium ion secondary batteries with higher capacities than conventional batteries are required as storage batteries for home use, industrial use, and on-vehicle use. As one measure for increasing the capacity of a lithium ion secondary battery, it is known to use Si (silicon) as a negative electrode active material. However, since Si has a volume change due to charging / discharging four times that of graphite, there is a problem that the negative electrode collapses due to charging / discharging, and the electrolytic solution is easily decomposed and cycle characteristics are inferior.

  Patent Document 1 discloses a nonaqueous electrolyte secondary battery using particles having a structure in which Si microcrystals are dispersed in a Si-based compound as a negative electrode active material. Patent Document 2 discloses a negative electrode composed of a granular material made of Si oxide represented by SiOx (0.3 ≦ x ≦ 1.6) and a silicon carbide coating that covers the surface of the granular material. A lithium ion secondary battery using an active material is disclosed.

JP 2004-323284 A JP2012-178269

  However, in a lithium ion secondary battery using particles having the structure disclosed in Patent Document 1 as a negative electrode active material, the capacity retention rate is low because the Si exposed on the particle surface decomposes the electrolyte (cycle characteristics). Is bad). Moreover, in order to produce the negative electrode active material having the configuration disclosed in Patent Document 2, an expensive and large-scale production apparatus such as a CVD apparatus is required, which raises a problem that the production cost increases.

  An object of the present invention is to provide a negative electrode active material for a lithium ion secondary battery having an excellent capacity retention rate at a low manufacturing cost without using an expensive and large-scale manufacturing facility.

  The negative electrode active material for a lithium ion secondary battery of the present invention comprises an aggregate of Si particles and SiO2 particles, and the aggregate is formed by dispersing a plurality of Si particles in SiO2, and the plurality of Si particles are divided into a plurality of Si particles. Includes a Si particle having a protrusion part of the surface protruding from the surface of SiO2, a SiO2 layer is formed on the surface of the protrusion, and the thickness of the SiO2 layer is 1 nm or more and 9 nm or less, It is.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material for lithium ion secondary batteries excellent in capacity | capacitance maintenance factor can be provided at low manufacturing cost, without requiring expensive and large-scale manufacturing equipment.

FIG. 1 is a cross-sectional view schematically showing a negative electrode active material according to the present invention. FIG. 2 is a flowchart showing a manufacturing process of the negative electrode active material according to the present invention. FIG. 3 is a cross-sectional view of a cylindrical lithium ion secondary battery. FIG. 4 is an exploded perspective view of a cylindrical lithium ion secondary battery. FIG. 5 is an exploded cross-sectional perspective view of the power generation element of the cylindrical lithium ion secondary battery. FIG. 6 is a table showing the SiO 2 layer thickness of the negative electrode active material and the capacity retention rate of the lithium secondary battery for the examples and comparative examples. FIG. 7 is a graph showing the relationship between the thickness of the SiO 2 layer and the capacity retention ratio.

  Hereinafter, the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing a negative electrode active material according to the present invention. As shown in FIG. 1, the negative electrode active material according to the present invention has a structure in which Si particles 100 are dispersed inside SiO 2 particles 200. Most of the Si particles 100 are dispersed inside the SiO 2 particles 200, but some of the Si particles 100 protrude from the surface of the SiO 2 particles 200. A SiO2 layer 300 is provided on the surface of the protruding Si particles 1. The thickness of the SiO2 layer 300 is 1 nm or more and 9 nm or less. Here, the Si particles serve as an active material. If the lithium ion secondary battery is configured in such a state that the Si particles are not covered with the SiO 2 layer and are in direct contact with the electrolytic solution, the electrolytic solution is decomposed on the surface of the Si particles by the electric potential and deposited as a salt. As the electrolytic solution decomposes, the resistance of the lithium ion secondary battery increases, and as a result, the battery capacity decreases. That is, a decrease in capacity maintenance rate (cycle deterioration) occurs. In the negative electrode active material according to the present invention, since the surface of the Si particles 100 protruding from the surface of the SiO2 particles 200 is covered with the SiO2 layer 300, the Si particles and the electrolytic solution are not in direct contact. As a result, it is possible to suppress the electrolytic solution from being deposited as a salt, and to maintain the battery capacity retention rate at a high level. Further, by covering the Si particles with the SiO 2 layer, the volume change of the Si particles during charging / discharging can be suppressed.

  When the thickness of the SiO2 layer 300 covering the surface of the Si particle 100 protruding from the surface of the SiO2 particle 200 is less than 1 nm, the coating is not sufficient, so that the effect of suppressing the decomposition of the electrolytic solution and the volume change of the Si particle are suppressed. The effect to do is small. On the other hand, when the thickness of the SiO2 layer 300 exceeds 9 nm, the effect of suppressing the decomposition of the electrolytic solution is enhanced, but the reaction between lithium ions and Si is inhibited, so that the capacity retention rate of the battery is lowered.

  The particle diameter of the SiO 2 particles 200 is preferably 1 to 30 μm, and more preferably 3 to 10 μm. The particle diameter of the SiO2 particles 200 can be adjusted as appropriate by changing the manufacturing conditions. When the particle diameter of the SiO 2 particles 200 is less than 1 μm, the aggregation of the SiO 2 particles becomes intense and the handling of the particles becomes difficult. Moreover, when the particle diameter of SiO2 particle | grains exceeds 30 micrometers, the distance to the Si particle | grain of the part close | similar to the center of SiO2 particle | grain becomes large. Since the conductivity of SiO2 is low, the Si particles near the center of the SiO2 particles cannot participate in charging / discharging of the battery. As a result, the initial capacity of the battery is reduced.

  The particle size of the Si particles 100 is preferably 1 to 100 nm, and more preferably 3 to 20 nm. Moreover, it is preferable that the ratio of the mass of Si particle with respect to the mass of the whole negative electrode active material is 40-80 mass%. When Si particles are dispersed so as to have such a ratio, the number of Si particles protruding from the surface of the SiO2 particles increases. If the surface of the Si particles is not covered with the SiO2 layer, as already described, the amount of the electrolytic solution to be decomposed increases and the capacity retention rate of the battery decreases. However, in the negative electrode active material of the present invention, since the Si particles protruding from the surface of the SiO2 particles are covered with the SiO2 layer, the decomposition of the electrolytic solution is suppressed, and the capacity retention rate of the battery can be kept high. Since the ratio of Si particles can be increased, the capacity of the battery can be increased.

  FIG. 2 is a flowchart showing a manufacturing process of the negative electrode active material according to the present invention. The manufacturing process of the negative electrode active material according to the present invention includes a mixing step of mixing Si particles and SiO2 particles, a disproportionation reaction step of aggregating SiO2 particles and simultaneously growing Si crystals, and a disproportionation reaction step. It comprises a pulverizing step of pulverizing the SiO2 particles in which Si crystal particles are dispersed, and an oxidation treatment step of oxidizing the surface of Si exposed on the surface of the pulverized particles and covering it with an SiO2 layer.

(Mixing process)
Si particles and SiO2 particles are put into a ball mill drum at an appropriate ratio. The drum is rotated at a rotational speed of about 150 rpm for a predetermined time, and Si particles and SiO2 particles are mixed to prepare mixed particles.

(Disproportionation reaction process)
The mixed particles are heated to 1000 ° C. in an argon gas atmosphere and then held for 3 hours. As a result, a disproportionation reaction occurs, and Si-dispersed particles in which crystal-grown Si particles are dispersed in aggregated SiO 2 are obtained.

(Crushing process)
The Si-dispersed particles are again put into the drum of the ball mill, and the drum is rotated at a rotation speed of about 500 rpm for a predetermined time. Thereby, the Si dispersed particles are pulverized to obtain pulverized Si dispersed particles having a desired size.

(Oxidation process)
On the surface of the pulverized Si dispersed particles, there are portions where crystallized Si particles protrude and are exposed. The pulverized Si dispersion particles are oxidized in a high-temperature atmosphere to oxidize the surface of the exposed and exposed Si particles to form a SiO2 layer. Thus, the negative electrode active material according to the present invention is manufactured.

  When the composition of the negative electrode active material according to the present invention is represented by SiOx, the value of x is preferably 0.2 ≦ x ≦ 1.2. Note that the thickness of the SiO 2 layer covering the exposed surface of the Si particles can be appropriately adjusted depending on the temperature and time, which are conditions for the oxidation treatment. In this oxidation treatment step, the equipment used in the disproportionation reaction step which is the previous step can be used. Further, no special material for covering the surface of the exposed Si particles is required. Therefore, it is not necessary to introduce a large-scale manufacturing facility for covering the surface of the Si particles, and as a result, the manufacturing cost can be suppressed.

(Preparation of negative electrode)
A negative electrode is produced using the negative electrode active material according to the present invention produced by the above steps. The negative electrode is produced by forming a negative electrode mixture layer by applying a negative electrode mixture comprising a negative electrode active material and a negative electrode binder to the surface of a copper foil as a negative electrode plate. A conductive agent may be added to the negative electrode mixture as necessary for the purpose of increasing the conductivity of the negative electrode mixture layer. As the conductive agent, carbon materials such as carbon black, graphite, carbon fiber, and metal carbide can be used. These materials may be used alone or in combination of two or more.

   As the negative electrode binder, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are preferably used, and the ratio of the negative electrode binder is about 98: 1: 1 in terms of mass ratio of the negative electrode active material, SBR, and CMC. Preferably there is. As the negative electrode binder, any material other than the above SBR and CMC can be used as long as the negative electrode active material and the negative electrode plate are brought into close contact with each other. For example, a homopolymer such as vinylidene fluoride, tetrafluoroethylene, acrylonitrile, ethylene oxide, or a copolymer thereof can be used.

  Next, a lithium ion secondary battery in which the negative electrode active material according to the present invention is applied to the negative electrode will be described with reference to FIGS. 3 is a cross-sectional view showing an embodiment of a cylindrical lithium ion secondary battery, FIG. 4 is an exploded perspective view thereof, and FIG. 5 is an exploded cross-sectional perspective view of the power generation element. A cylindrical lithium ion secondary battery 1 includes a battery container 4 including a cylindrical battery can 2 having an upper opening and a battery lid 3 that seals the upper portion of the battery can 2. Iron or the like is used as the material of the battery can 2. Inside the battery container 4 are housed each component for power generation described below and a non-aqueous electrolyte 5.

  On the upper end side (opening 2b side) of the battery can 2, a groove 2a protruding inward is formed. A power generation element 10 is disposed inside the battery can 2. The power generation element 10 includes a cylindrical shaft core 15 having a hollow portion along the central axis of the battery can 2, and a positive electrode plate and a negative electrode plate wound around the shaft core 15 via a separator. A groove 15 a having a diameter larger than that of the hollow portion is formed in the shaft core 15 on the inner side of the upper end portion in the central axis direction (vertical direction in FIG. 3) of the battery can 2. A thin cylindrical positive current collector plate 27 is press-fitted into the groove 15a. The positive electrode current collector plate 27 includes a disc-shaped base portion 27a, a lower tube portion 27b that protrudes from the base portion 27a toward the shaft core 15 and is press-fitted into the shaft core 15, and an upper tube that protrudes from the base portion 27a toward the battery lid 3 side. Part 27c.

  The positive electrode tab 16 is welded to the upper cylindrical portion 27 c of the positive electrode current collector plate 27. As a specific welding method, the plurality of positive electrode tabs 16 are brought into close contact with the outer peripheral portion of the upper cylindrical portion 27c of the positive electrode current collector plate 27, and the pressing member 28 is wound around the ring shape in a ring shape and temporarily fixed. In this state, the positive electrode tab 16 is welded to the upper cylindrical portion 27c and the pressing member 28 by ultrasonic welding.

  A step portion 15b having a reduced outer diameter is formed on the outer peripheral portion on the lower end side of the shaft core 15, and a negative electrode current collector plate 21 is press-fitted and fixed to the step portion 15b. The negative electrode current collector plate 21 is formed of, for example, a copper alloy. An opening 21b for press-fitting into the step portion 15b of the shaft core 15 is formed in the disc-shaped base portion 21a, and an outer peripheral cylindrical portion 21c protruding toward the bottom side of the battery can 2 is formed in the outer peripheral portion. ing. In the base 21a, an opening 21d (see FIG. 4) for allowing the non-aqueous electrolyte 5 injected into the hollow shaft of the shaft core 15 to permeate the power generation element 10 is formed.

  A negative electrode tab 17 is welded to the outer peripheral cylindrical portion 21 c of the negative electrode current collector plate 21. As a specific welding method, a plurality of negative electrode tabs 17 are brought into close contact with the outer peripheral portion of the outer peripheral cylindrical portion 21c of the negative electrode current collector plate 21, and the holding member 22 is wound around in a ring shape and temporarily fixed. The negative electrode tab 17 is welded to the outer peripheral member 21c and the pressing member 22 by welding. A connection lead plate 50 is joined to the base portion 21a of the negative electrode current collector plate 21 by ultrasonic welding or the like. The connection lead plate 50 is joined to the bottom 2c of the battery can 2 by resistance welding or the like.

  The plurality of positive electrode tabs 16 are welded to the positive electrode current collector plate 27, and the plurality of negative electrode tabs 17 are welded to the negative electrode current collector plate 21, whereby the positive electrode current collector plate 27, the negative electrode current collector plate 21, and the power generation element 10, a power generation unit 20 that is unitized as one unit is configured. As an example of the non-aqueous electrolyte 5 that is injected into the battery container 4, there is an electric field solution in which a lithium salt is dissolved in a carbonate-based solvent.

  The positive electrode current collector plate 27 is made of, for example, an aluminum metal. As shown in FIG. 4, openings 27 d and 27 e are formed in the base portion 27 a of the positive electrode current collector plate 27. The opening 27d is an opening for releasing gas generated inside the battery, and the opening 27e is used for inserting an electrode rod (not shown) when welding the connection lead plate 50 to the battery can 2. It is an opening. Specifically, the electrode rod is inserted from the opening 27e, the connection lead plate 50 is pressed against the inner surface of the bottom 2c of the battery can 2 through the hollow portion of the shaft core 15, and the connection lead plate 50 is attached to the battery can 2. Resistance welding. The bottom surface of the battery can 2 connected to the negative electrode current collector plate 21 functions as an output terminal when taking out the electric power charged in the power generation element 10 from the battery can 2. A flexible connecting member 33 formed by laminating a plurality of aluminum foils is joined to the upper surface of the base portion 27a of the positive electrode current collector plate 27 by welding one end thereof.

  A battery lid unit 30 is disposed on the upper cylindrical portion 27 c of the positive electrode current collector plate 27. The battery lid unit 30 includes a ring-shaped insulating plate 34 made of an insulating resin material, a connection plate 35 fitted into the opening 34 a of the insulating plate 34, a diaphragm 37 welded to the connection plate 35, and caulking and welding to the diaphragm 37. It is comprised by the battery cover 3 fixed by. The insulating plate 34 has an opening 34a and a side 34b protruding downward. The connection member 33 is joined to the lower surface of the connection plate 35 by welding an end portion opposite to one end welded to the upper surface of the base portion 27 a of the positive electrode current collector plate 27.

  The connection plate 35 is formed of an aluminum-based metal, and is formed in a dish shape in which the center side is gently bent downward. A central portion of the connection plate 35 is thin, and a projection 35a formed in a convex dome shape is provided at the center, and a plurality of openings 35b are formed surrounding the projection 35a. Has been. The opening 35b has a function of releasing gas generated inside the battery. The protrusion 35 a of the connection plate 35 is joined to the bottom surface of the center portion of the diaphragm 37 by resistance welding or friction stir welding. The diaphragm 37 is formed of an aluminum-based metal, and a circular cut 37a is formed at the center of the diaphragm 37 by crushing the upper surface side into a V shape by pressing. The diaphragm 37 is provided for ensuring the safety of the battery, and when the internal pressure of the battery rises due to the gas generated inside the battery, the notch 37a is cleaved by the pressure to release the internal gas. Have.

  The peripheral edge of the diaphragm 37 is fixed to the peripheral edge of the battery lid 3. In the component stage, the diaphragm 37 has a side wall 37b whose peripheral edge stands vertically toward the battery lid 3 side (upward). The battery lid 3 is accommodated in a space provided above the diaphragm 37 by the side wall 37b, and the battery lid 3 is fixed by bending the side wall 37b to the upper surface side of the battery lid 3 by caulking.

  The battery lid 3 is made of iron such as carbon steel, and nickel plating is applied to both front and back surfaces. The battery lid 3 includes a disc-shaped peripheral edge 3 a that contacts the diaphragm 37 and a cylindrical portion 3 b that protrudes upward from the peripheral edge 3 a. An opening 3c is formed at the center of the cylindrical portion 3b. The opening 3c is for releasing gas to the outside of the battery when the cut 37a of the diaphragm 37 is cleaved by the gas pressure generated inside the battery. The battery lid 3 acts as an output end when taking out the electric power stored in the power generation element 10 from the battery can 2.

  The caulking portion between the diaphragm 37 and the battery lid 3 is covered with a gasket 43 made of an insulating member. In the component stage, the gasket 43 has a shape having an outer peripheral wall portion 43b formed to rise substantially vertically upward at the peripheral side edge of the ring-shaped base portion 43a. In order to cover the caulked portion between the diaphragm 37 and the battery lid 3 with the gasket 43, the outer peripheral wall portion 43b is bent inward together with the battery can 2 by a press or the like, and the diaphragm 37 and the battery are formed by the base portion 43a and the outer peripheral wall portion 43b. The caulking portion of the lid 3 is sandwiched. Thereby, the battery lid unit 30 in which the battery lid 3, the diaphragm 37, the insulating plate 34, and the connection plate 35 are integrated is configured. As a specific material of the gasket 43, rubber is used, but fluorine resin may be used.

  Next, the structure of the power generation element 10 will be described with reference to FIG. The power generation element 10 is configured by winding a positive electrode plate 11, a negative electrode plate 12, a first separator 13, and a second separator 14 around an axis 15. The first separator 13 and the second separator 14 are made of, for example, an insulating polyethylene porous body having a thickness of 40 μm.

  The shaft core 15 is wound several times from the inner periphery in a state where the first separator 13 and the second separator 14 are overlapped. In FIG. 5, for the sake of brevity, a state where only one round is wound is shown. From this position, the second separator 14, the negative electrode plate 12, the first separator 13, and the positive electrode plate 11 are wound in a stacked state several times so that they are in this order. 13, the negative electrode plate 12, the second separator 14, and the positive electrode plate 11 are wound in this order. Therefore, the negative electrode plate 12 is wound one more turn than the positive electrode plate 11. The end of the first separator 13 at the outermost periphery is fixed by an adhesive tape 19.

  The positive electrode plate 11 is formed by applying a positive electrode mixture on both surfaces of an elongated positive electrode metal foil 11a made of aluminum to form a positive electrode processing portion 11b. A positive electrode mixture untreated portion 11c in which the positive electrode metal foil 11a is exposed without being coated with the positive electrode mixture is provided on the upper side edge of the positive electrode metal foil 11a. In the positive electrode mixture untreated portion 11c, a plurality of positive electrode tabs 16 for protruding upward along the axis of the shaft core 15 are formed integrally with the positive electrode mixture untreated portion 11c at equal intervals.

  The positive electrode mixture is composed of a positive electrode active material, a positive electrode conductive material, and a positive electrode binder. Examples of the positive electrode active material include lithium oxides of transition metals such as cobalt, manganese, and nickel. Examples of the positive electrode binder include polyvinylidene fluoride (PVDF) and fluororubber.

  The positive electrode mixture slurry prepared by dispersing the positive electrode mixture in a solvent was applied to both sides of an aluminum foil having a thickness of 20 μm as the positive electrode metal foil 11a to a uniform thickness, and the solvent was dried. Cut to desired shape. The coating thickness of the positive electrode mixture is, for example, about 40 μm on one side. When the positive electrode metal foil 11a is cut by a press, a plurality of positive electrode tabs 16 are also formed at the same time. The shapes of the plurality of positive electrode tabs 16 are substantially the same. In addition, as a method of apply | coating a positive mix to the positive electrode metal foil 11a, a roll coating method, a slit die coating method, etc. are mentioned.

  The negative electrode plate 12 is formed by applying a negative electrode mixture on both surfaces of a long negative electrode metal foil 12a made of copper foil to form a negative electrode processing portion 12b. A negative electrode mixture untreated portion 12c in which the negative electrode metal foil 12a is exposed without being coated with the negative electrode mixture is provided on the lower side edge of the negative electrode metal foil 12a. In this negative electrode mixture untreated portion 12c, a plurality of negative electrode tabs 17 for protruding downward in the direction opposite to the positive electrode tab 16 along the axis of the shaft core 15 are integrated with the negative electrode mixture untreated portion 12c at equal intervals. Is formed.

  The negative electrode mixture is composed of a negative electrode active material, a negative electrode binder, and a thickener. An example of the negative electrode active material is graphite carbon. In order to produce a negative electrode, a negative electrode mixture slurry prepared by dispersing a negative electrode mixture in a solvent is applied to both surfaces of a rolled copper foil having a thickness of 10 μm as the negative electrode metal foil 12a, and the solvent is added. After drying, it is cut into a desired shape by a press. The coating thickness of the negative electrode mixture is, for example, about 40 μm on one side. When the negative electrode metal foil 12a is cut by pressing, a plurality of negative electrode tabs 17 are also formed at the same time. The plurality of negative electrode tabs 17 have substantially the same shape. Examples of the method of applying the negative electrode mixture to the negative electrode metal foil 12a include a roll coating method and a slit die coating method.

  The widths of the first separator 13 and the second separator 14 are both larger than the width of the positive electrode plate 11 and the width of the negative electrode plate 12. Further, the width and length of the negative electrode processing portion 12b of the negative electrode plate 12 are larger than the width and length of the positive electrode processing portion 11b of the positive electrode plate 11, respectively. That is, the entire region of the negative electrode metal foil 12a corresponding to the positive electrode processing portion 11b is covered with the negative electrode processing portion 12b. The reason for this configuration is as follows. In a lithium ion secondary battery, lithium which is a positive electrode active material is ionized, penetrates the separator, and is occluded by the negative electrode active material. At this time, when the negative electrode metal foil 12a is exposed without forming the negative electrode active material in the region of the negative electrode metal foil 12a corresponding to the region of the positive electrode active material, lithium is deposited on the negative electrode metal foil 12a, causing an internal short circuit. Cause it to occur. As described above, at least the entire region of the negative electrode metal foil 12a corresponding to the positive electrode processing portion 11b is covered with the negative electrode processing portion 12b, thereby preventing an internal short circuit due to lithium deposition. Next, examples of the lithium ion secondary battery 20 of the present embodiment will be described.

Example 1
The negative electrode active material according to the present invention was prepared by the following procedure. Si particles having a mean diameter of 4 μm and SiO2 particles are charged into a drum of a ball mill so as to have a molar ratio of 1: 1, and the drum is rotated at a rotation speed of 150 rpm to mix Si particles and SiO2 particles. Mixed particles were prepared. The mixed particles are heated to 1000 ° C. in an argon gas atmosphere and maintained in that state for 3 hours to generate a disproportionation reaction. In the aggregated SiO 2, Si dispersed particles in which crystal-grown Si particles are dispersed are obtained. Prepared. The Si-dispersed particles were again put into a drum of a ball mill, and the drum was rotated at a rotation speed of 500 rpm to pulverize the Si-dispersed particles to prepare pulverized Si-dispersed particles. Next, oxidation treatment is performed by holding the pulverized Si dispersed particles in the atmosphere at a temperature of 300 ° C. for 15 minutes, and the surface of the Si particles protruding and exposed on the surface of the Si dispersed particles is oxidized to form a SiO 2 layer. Thus, the negative electrode active material of Example 1 was obtained. It was 1 nm when the thickness of the said SiO2 layer was measured. The thickness of the SiO2 layer is measured by cutting the oxidized Si dispersed particles by FIB (focused ion beam) to form a cross section, and observing the cross section with a STEM (scanning transmission electron microscope). It went by.

(Example 2)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 300 ° C. for 1 hour. The measurement result of the SiO2 layer thickness was 2 nm.

(Example 3)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 400 ° C. for 1 hour. The measurement result of the SiO2 layer thickness was 3 nm.

Example 4
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 400 ° C. for 3 hours. The measurement result of the SiO2 layer thickness was 4 nm.

(Example 5)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 400 ° C. for 5 hours. The measurement result of the SiO2 layer thickness was 5 nm.

(Example 6)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 500 ° C. for 1 hour. The measurement result of the SiO2 layer thickness was 4 nm.

(Comparative Example 1)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was not performed. The measurement result of the SiO2 layer thickness was 0.3 nm.

(Comparative Example 2)
A negative electrode active material was prepared in the same procedure as in Example 1 except that the oxidation treatment in the atmosphere was held at 1000 ° C. for 3 hours. The measurement result of the SiO2 layer thickness was 10 nm.

(Production of lithium secondary battery)
A negative electrode was produced using the negative electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 and 2, and a lithium secondary battery having a structure shown in FIG. 3 was produced using the negative electrode.

(Measurement of cycle characteristics)
The capacity retention rate of the lithium ion secondary battery was measured by the following procedure. First, the lithium secondary battery was repeatedly charged and discharged at a temperature of 25 ° C. The charging conditions were constant current and constant voltage charging for 4 hours with a charging current equivalent to 0.5 C and an upper limit voltage of 4.2 V. The discharge conditions were a discharge current corresponding to 0.5 C and a constant current discharge with a lower limit voltage of 2.5 V. In the charge / discharge, the discharge capacity at the first cycle was measured, and the ratio of the discharge capacity at the 20th cycle to the discharge capacity (discharge capacity at the 20th cycle / discharge capacity at the 1st cycle) was defined as the capacity retention rate. The cycle characteristics were evaluated based on the capacity retention rate.

  In each of the negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 and 2, the thickness of the SiO2 layer covering the surface of the Si particles protruding and exposed on the surface and the capacity retention rate of each lithium secondary battery are shown in FIG. Shown in the table. FIG. 7 is a graph plotting the relationship between the thickness of the SiO 2 layer shown in FIG. 6 and the capacity retention rate of the battery. In lithium ion secondary batteries using the negative electrode active materials of Examples 1 to 6 and Comparative Example 2 manufactured through the oxidation treatment step as the negative electrode, the thickness of the SiO2 layer coated with Si particles was 1 nm. The capacity retention rate was 55%, the highest value. The capacity retention rate gradually decreased as the thickness of the SiO 2 layer increased, and decreased to 20% in Comparative Example 2 where the thickness of the SiO 2 layer was 10 nm. From these results, it can be seen that when the thickness of the SiO2 layer is 1 to 9 nm, the capacity retention rate of the battery can be 25% or more, which is preferable. In addition, when the thickness of the SiO2 layer is 1 to 5 nm, it can be understood that the capacity retention rate of the battery can be 40% or more, which is more preferable. On the other hand, in the lithium ion secondary battery using the negative electrode active material of Comparative Example 1 that has not undergone the oxidation treatment step as the negative electrode, the capacity maintenance rate is 23%, and the capacity maintenance of the lithium ion secondary batteries of Examples 1 to 6 is maintained. It can be seen that it is significantly lower than the rate.

  As described above, according to the present invention, it is possible to provide a negative electrode active material for a lithium ion secondary battery having an excellent capacity retention rate at a low manufacturing cost without using an expensive and large-scale manufacturing facility.

  As described above, various embodiments and modifications have been described, but the present invention is not limited to these contents. Other embodiments conceivable within the scope of the technical idea of the present invention are also included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Battery can 3 Battery cover 4 Battery container 5 Non-aqueous electrolyte 11 Positive electrode plate 12 Negative electrode plate 13 First separator 14 Second separator 15 Axle core 16 Positive electrode tab 17 Negative electrode tab 20 Power generation unit 21 Negative electrode Current collector plate 27 Positive electrode current collector plate 30 Battery cover unit 100 Si particle 200 SiO2 particle 300 SiO2 layer

Claims (7)

A negative electrode active material for a lithium ion secondary battery,
The negative electrode active material comprises an aggregate of Si particles and SiO2 particles,
The aggregate is composed of a plurality of Si particles dispersed in SiO2.
The plurality of Si particles include Si particles having a protruding portion in which a part of the surface protrudes from the surface of the SiO2,
A SiO2 layer is formed on the surface of the protrusion,
The thickness of the said SiO2 layer is 1 nm or more and 9 nm or less, The negative electrode active material for lithium ion secondary batteries characterized by the above-mentioned. The negative electrode active material for a lithium ion secondary battery according to claim 1,
The thickness of the said SiO2 layer is 1 nm or more and 5 nm or less, The negative electrode active material for lithium ion secondary batteries characterized by the above-mentioned. The negative electrode active material for a lithium ion secondary battery according to claim 1 or 2,
The composition of the negative electrode active material is represented by SiOx (0.2 ≦ x ≦ 1.2), the negative electrode active material for a lithium ion secondary battery.   The lithium ion secondary battery which used the negative electrode active material for lithium ion secondary batteries of any one of Claims 1-3 for the negative electrode. It is a manufacturing method of the negative electrode active material for lithium ion secondary batteries of any one of Claims 1-3,
A mixing step of mixing Si particles and SiO2 particles to create mixed particles;
A disproportionation reaction step of aggregating the SiO2 particles in the mixed particles and simultaneously growing Si crystals in the Si particles to produce agglomerated SiO2 particles;
A pulverizing step of pulverizing the aggregated SiO2 particles to produce pulverized SiO2 particles;
An oxidation treatment step of oxidizing the surface of the protruding portion of the Si particles protruding from the surface of the pulverized SiO2 particles and covering the surface of the protruding portion with a SiO2 layer, for a lithium ion secondary battery, Method for producing negative electrode active material. In the manufacturing method of the negative electrode active material for lithium ion secondary batteries of Claim 5,
In the disproportionation reaction step, the mixed particles are heated in an inert gas atmosphere,
In the oxidation treatment step, the pulverized SiO 2 particles are maintained at a predetermined temperature for a predetermined time in the air or in an oxygen atmosphere, and a method for producing a negative electrode active material for a lithium ion secondary battery. In the manufacturing method of the negative electrode active material for lithium ion secondary batteries of Claim 6,
The said predetermined temperature is 300 degreeC or more and 500 degrees C or less, The said predetermined time is 15 minutes or more and 5 hours or less, The manufacturing method of the negative electrode active material for lithium ion secondary batteries characterized by the above-mentioned.

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