JP2019145291A - Negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery, which are excellent in cycle characteristics.SOLUTION: A negative electrode for a lithium ion secondary battery includes a negative electrode active material layer on at least one of main surfaces of a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material particle containing silicon and carbon. A cross section of the negative electrode active material particle has a peak A at 450±20 cmin an argon laser Raman spectrum, and a peripheral portion of a cross section of the active material particle has a peak G at 1,575±15 cmand a peak D at 1,350±15 cmin an argon laser Raman spectrum, the intensity ratio G/D of the peak G to the peak D satisfying 0.5<G/D<2.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. In recent years, it has been used as a power source mounted for hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

  Currently, carbon materials such as graphite are often used as negative electrode active materials for lithium ion secondary batteries. In recent years, a large number of alloy-based negative electrode active materials such as silicon (Si) and SiO having a larger discharge capacity than graphite have been studied. However, these alloy-based negative electrode active materials have a larger discharge capacity than graphite, and also have a large volume expansion associated with the charge / discharge reaction, and are more susceptible to particle cracking and isolation than graphite. The decrease was significant.

  Patent Documents 1 and 2 disclose that by adding a metal element to an alloy negative electrode to improve electron conductivity, a charge / discharge reaction can be uniformly performed in an electrode cross section, and cycle characteristics are improved. .

  In Patent Document 3, a lithium silicate layer that does not contribute to the charge / discharge reaction is formed at the interface between the negative electrode current collector and the negative electrode active material layer, whereby sufficient adhesion strength is obtained between the current collector and the active material layer. It is disclosed that the characteristics are improved.

JP 2007-27102 A International Publication No. 2007/046327 International Publication No. 2011/132428

  However, the method disclosed in the above prior art does not provide sufficient cycle characteristics, and further improvement is required.

  The present invention has been developed under such circumstances, and an object thereof is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery excellent in cycle characteristics.

In order to achieve the above object, a negative electrode for a lithium ion secondary battery according to the present invention includes a negative electrode active material layer on at least one of main surfaces of a negative electrode current collector, and the previous negative electrode active material layer contains silicon and carbon. The negative electrode active material particles have a cross section, and the cross section of the negative electrode active material particles has a peak A at 450 ± 20 cm ⁻¹ in the argon laser Raman spectrum, and the periphery of the cross section of the active material particles has an argon laser Raman spectrum. in, 1575 anda peak D in peak G, 1350 ± 15cm ^-1 to ± 15cm ^-1, the peak G, the intensity ratio G / D of the peak D is at 0.5 <G / D <2 It is characterized by being.

Since the cross section of the active material particle has a peak A at 450 ± 20 cm ⁻¹ in the argon laser Raman spectrum, the amorphous portion in the active material particle becomes a good conduction path for Li ⁺ ions, and the cross section of the active material particle in the periphery, 1575 anda peak D in peak G, 1350 ± 15cm ^-1 to ± 15cm ^-1, intensity ratio G / D peak G and peak D is, 0.5 <G / D <2 Therefore, it is presumed that good electrons are transferred to and from the negative electrode active material particles by carbon as a conductive path.

  The average primary particle size of the negative electrode active material particles is preferably 0.1 μm or more and 20 μm or less.

  With such a configuration, cycle characteristics are improved. Although this effect is not necessarily clear, it is presumed that the use of the negative electrode active material particles in the above range can suppress the pulverization and cracking of the active material and further improve the cycle characteristics.

  The negative electrode active material layer preferably contains graphite.

  With such a configuration, cycle characteristics are improved. When the silicon material is mixed with graphite having a small volume expansion during Li occlusion, peeling and collapse of the negative electrode active material layer are alleviated, and cycle characteristics are further improved.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode for lithium ion secondary batteries and the lithium ion secondary battery which were excellent in cycling characteristics can be provided.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Lithium ion secondary battery)
FIG. 1 shows a cross-sectional configuration diagram of a lithium ion secondary battery 100 according to this embodiment. The lithium ion secondary battery 100 includes an outer body 50, a positive electrode 10 and a negative electrode 20 provided inside the outer body, and an electrode body 30 formed by being stacked via a separator 18 disposed therebetween. The separator 18 holds the non-aqueous electrolyte, which is a lithium ion transfer medium between the positive and negative electrodes during charging and discharging. Furthermore, one end is electrically connected to the negative electrode 20 and the other end protrudes outside the exterior body, and one end is electrically connected to the positive electrode 10. The other end is provided with a positive electrode lead 60 protruding outside the exterior body.

  There is no restriction | limiting in particular as a shape of a lithium ion secondary battery, For example, any, such as a cylindrical type, a square type, a coin type, a flat type, a laminate film type, may be sufficient. In the following examples, an aluminum laminated film type battery is produced and evaluated.

  The positive electrode 10 includes a positive electrode active material layer 14 containing a positive electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the positive electrode current collector 12. The negative electrode 20 includes a negative electrode active material layer 24 containing a negative electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the negative electrode current collector 22. Yes.

(Negative electrode)
The negative electrode active material layer 24 formed on the negative electrode 20 of the present embodiment contains a negative electrode active material, a binder, and a conductive additive.

  For the negative electrode active material layer 24, a paint containing a negative electrode active material, a binder, a conductive additive and a solvent is applied on the negative electrode current collector 22, and the solvent in the paint applied on the negative electrode current collector 22 is removed. Can be manufactured.

The negative electrode for a lithium ion battery according to this embodiment includes a negative electrode active material layer on at least one of the main surfaces of the negative electrode current collector, and the negative electrode active material layer includes negative electrode active material particles containing silicon and carbon. The cross section of the negative electrode active material particles has a peak A at 450 ± 20 cm ⁻¹ in the argon laser Raman spectrum, and the periphery of the cross section of the active material particles is 1575 ± 15 cm ⁻¹ in the argon laser Raman spectrum. The peak G has a peak D at 1350 ± 15 cm ⁻¹ , and the intensity ratio G / D of the peak G and the peak D is 0.5 <G / D <2.

The cross section of the negative electrode active material particle has a peak A at 450 ± 20 cm ⁻¹ in the argon laser Raman spectrum, and the peripheral portion of the cross section of the negative electrode active material particle has a peak G at 1575 ± 15 cm ⁻¹ in the argon laser Raman spectrum. And a peak D at 1350 ± 15 cm ⁻¹ , and the intensity ratio G / D between the peaks G and D satisfies the relationship of 0.5 <G / D <2, so that the electron path associated with charge / discharge Therefore, the cycle characteristics are improved.

  The peripheral portion in the cross section of the negative electrode active material particle according to the present embodiment is a quarter of the distance from the surface to the center of the negative electrode active material particle in the cross section of the negative electrode active material particle. Let the region be the periphery.

  The average primary particle size of the negative electrode active material particles according to this embodiment is preferably 0.1 μm or more and 20 μm or less.

  Thereby, pulverization and cracking of the active material can be suppressed, and the cycle characteristics are further improved.

The negative electrode active material particles according to the present embodiment preferably include silicon oxide represented by SiO _x (0 <x ≦ 2).

  The negative electrode active material particles according to this embodiment preferably have a surface coated with carbon.

  Although it does not specifically limit as a coating method by carbon, Physical vapor deposition method, chemical vapor deposition method, mechanical milling, molten salt carbon plating method, thermal plasma method etc. are mentioned, and carbon coating is carried out by molten salt carbon plating method and thermal plasma method. It is preferable.

  As a result, excessive reaction with the electrolytic solution on the surface of the negative electrode active material particles is suppressed, cycle characteristics are further improved, and a favorable conductive path can be formed in the negative electrode active material layer.

  The negative electrode active material layer according to this embodiment preferably contains a graphite material.

  As the graphite material, a known carbon material that can be used for a lithium ion secondary battery can be used, and examples thereof include natural graphite, artificial graphite, and mesocarbon microbeads (MCMB). These graphite materials may be used individually by 1 type, and may use 2 or more types together.

  The mixing amount of the graphite material according to the present embodiment is preferably in the range of 0.4 ≦ d1 / d2 ≦ 2.3 when the weight ratio d1 of the negative electrode active material and the weight ratio d2 of the graphite material are set.

  The content of the negative electrode active material in the negative electrode active material layer 24 is preferably 50 to 95% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder, and is 75 to 93% by mass. More preferably. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(Measuring method)
The Raman spectrum in the cross section of the active material particles according to the present embodiment can be measured using an argon ion laser having a wavelength of 532 nm.

  For the negative electrode cross section, the battery is disassembled in a dry Ar glove box, the negative electrode is cut out, set in an air non-exposed cross-section milling holder, polished in an air non-exposed state with an Ar milling device, and then in an air non-exposed state. The Raman spectrum of the active material particle cross-section can be measured in a non-exposed state of the atmosphere with a stand-by non-exposed measuring jig that is returned to the Ar glove box and allows the laser to pass through the garnet plate.

  A specific method for measuring the Raman spectrum according to the present embodiment will be described below.

(Production of cross-section sample)
The cross-sectional sample of the active material layer of the negative electrode according to the present embodiment was disassembled from the lithium ion secondary battery in a glove box in a dry Ar atmosphere, the negative electrode was taken out, washed with dimethyl carbonate, vacuum dried, and exposed to the atmosphere. It is prepared by mounting it in an Ar ion milling device.

(Laser Raman spectroscopic analysis)
The method for measuring the peak of the Raman scattering spectrum in the negative electrode active material according to this embodiment is as follows. First, NRS-7100 manufactured by JASCO Corporation is used, and a green laser with a wavelength of 532 nm and Raman spectroscopy are used to adjust and measure to an irradiation intensity of 1 mW or less using a dimmer.

The Raman scattering spectrum is a graph showing the Raman shift wavenumber (cm ⁻¹ ) on the horizontal axis and the Raman scattering intensity obtained on the vertical axis. Using this graph, we determined the point P Raman scattering intensity is minimum, and a point Q Raman scattering intensity in a range of wavenumber 1900～2300Cm ^-1 is minimized within the range of wave numbers 500～900Cm ^-1, A straight line passing through these points P and Q is defined as base-in (BL), and is corrected to a graph obtained by removing peaks below the baseline from the graph of wave numbers 900 to 1900 cm ⁻¹ .

Next, the peak in each range is specified in the corrected graph. ^{Incidentally,} the fact that a peak in the range of 1330cm ^-1 ~1355cm -1, upon subtracting the baseline in the Raman scattering spectrum, the spectrum of the convex exists on for the baseline, half width There it means that the convex spectrum is 5 cm ^-1 or more 150 cm ^-1 or less is comprised in the range described above, the Raman shift position indicating the maximum Raman scattering intensity within the range are included within the range mentioned above Means. Furthermore, when there are a plurality of convex spectra in the range, the convex spectrum having the strongest Raman scattering intensity in each range may be set as the peak of the Raman scattering spectrum in each range.

  The negative electrode cross-section produced by ion milling is returned to the Ar glove box in the non-exposed state of the atmosphere, and the active non-exposed state of the cross-section of the active material particles in the non-exposed state of the atmosphere by a stand-by non-exposed measurement jig that allows the laser to pass through the garnet plate. A Raman spectrum can be measured.

(binder)
The binder binds the negative electrode active materials to the current collector 22 while bonding the negative electrode active materials to each other. A binder will not be specifically limited if the above-mentioned coupling | bonding is possible. For example, a fluororesin such as polyvinylidene fluoride (PVDF), cellulose, styrene / butadiene rubber, polyimide, polyamideimide, polyacrylic acid, polyacrylonitrile, polyalginic acid, or the like can be used.

  The content of the binder in the negative electrode active material layer 24 is preferably 1 to 30% by mass, and preferably 5 to 15% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder. Is more preferable. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(Conductive aid)
The conductive aid is not particularly limited as long as the conductivity of the negative electrode active material layer 24 is improved, and a known conductive aid can be used. Examples thereof include carbon blacks such as acetylene black, furnace black, channel black, and thermal black, vapor grown carbon fibers (VGCF), carbon fibers such as carbon nanotubes, and carbon materials such as graphite. More than seeds can be used.

  The content of the conductive auxiliary in the negative electrode active material layer 24 is not particularly limited, but when added, it is usually 1 to 10% by mass based on the sum of the mass of the negative electrode active material, conductive auxiliary and binder. Preferably there is.

(solvent)
The solvent is not particularly limited as long as it can form the above-described negative electrode active material, conductive additive, and binder. For example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, and the like can be used. .

(Negative electrode current collector)
The negative electrode current collector 22 is preferably a conductive plate material having a small thickness, and is preferably a metal foil having a thickness of 8 to 30 μm. The negative electrode current collector 22 is preferably formed of a material that does not alloy with lithium, and a particularly preferable material is copper. Examples of such copper foil include electrolytic copper foil. For example, an electrolytic copper foil is prepared by immersing a metal drum in an electrolytic solution in which copper ions are dissolved, and flowing current while rotating the copper drum, thereby depositing copper on the surface of the drum and peeling it off. It is the obtained copper foil.

  Moreover, the rolled copper foil manufactured by rolling the cast copper lump to desired thickness may be sufficient, and copper is deposited on the surface of the rolled copper foil by an electrolytic method, and the surface is roughened. There may be.

  Thus, there is no restriction | limiting in particular as an application | coating method which apply | coats the coating material containing a negative electrode active material, a binder, a conductive support agent, and a solvent mentioned above, The method employ | adopted when producing an electrode normally can be used. . Examples thereof include a slit die coating method and a doctor blade method.

(Negative electrode active material layer)
The method for removing the solvent in the paint applied on the negative electrode current collector 22 is not particularly limited, and the negative electrode current collector 22 applied with the paint may be dried at, for example, 80 ° C. to 150 ° C.

  Then, the negative electrode 20 on which the negative electrode active material layer 24 is formed in this way may be subsequently subjected to a press treatment using, for example, a roll press device as necessary. The linear pressure of the roll press can be set to 100 to 5000 kgf / cm, for example.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution has an electrolyte dissolved in a nonaqueous solvent, and may contain a cyclic carbonate and a chain carbonate as a nonaqueous solvent.

  The cyclic carbonate is not particularly limited as long as it can solvate the electrolyte, and a known cyclic carbonate can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate is not particularly limited as long as the viscosity of the cyclic carbonate can be reduced, and a known chain carbonate can be used. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, fluorinated ethylene carbonates typified by fluoroethylene carbonate, etc. May be used. From the viewpoint of increasing the oxidation resistance, it is particularly preferable to use fluoroethylene carbonate.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

  Furthermore, you may add various additives in the electrolyte solution of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ , CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ Lithium salts such as CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of conductivity.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

(Positive electrode)
The positive electrode 10 of the present embodiment has a structure in which a positive electrode active material layer 14 containing a positive electrode active material is formed on one surface or both surfaces of a positive electrode current collector 12. The positive electrode active material layer 14 is coated on the positive electrode current collector 12 with a paint containing the positive electrode active material, a binder, a conductive additive, and a solvent in the same process as the negative electrode manufacturing method. It can be produced by removing the solvent in the applied paint.

Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions of the lithium ions (for example, ClO ₄ ⁻ ). Is not particularly limited as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) Examples include composite metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), and LiMPO ₄ (wherein M represents Co, Ni, Mn, Fe, or VO).

  In addition, oxides and sulfides capable of inserting and extracting lithium ions can be used as the positive electrode active material.

  Furthermore, each component (conductive auxiliary agent and binder) other than the positive electrode active material can be the same material as that used in the negative electrode 20.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. For example, a metal foil such as aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used, and an aluminum foil is particularly preferable.

(Separator)
As long as the separator 18 is formed of an insulating porous body, the material, the manufacturing method, and the like are not particularly limited, and a known separator used for a lithium ion secondary battery can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

  The outer package 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside or entry of moisture or the like into the lithium ion secondary battery 100 from the outside, such as a metal can, an aluminum laminate film, or the like Can be used. The aluminum laminate film includes, for example, a structure that is configured as a three-layer structure in which polypropylene, aluminum, and nylon are laminated in this order.

  The negative electrode lead 62 and the positive electrode lead 60 may be formed of a conductive material such as aluminum or nickel.

  As mentioned above, although the example of this invention was described in detail by embodiment, this invention is not limited to the said embodiment, A various deformation | transformation is possible.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

(Example 1)
(Preparation of positive electrode)
96% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ as a positive electrode active material, 2% by weight of carbon black as a conductive additive, 0.5% by weight of graphite, and 1 PVDF as a binder 0.5 wt% and N-methyl-2-pyrrolidone solvent were mixed and dispersed to prepare a paste-like positive electrode slurry.

The obtained positive electrode slurry was uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm using a comma roll coater so that the applied amount of the active material was 22.0 mg / cm ² . Next, a positive electrode active material layer was formed by drying the N-methyl-2-pyrrolidone solvent in the positive electrode active material in an air atmosphere at 110 ° C. in a drying furnace.

In addition, the thickness of the coating film of the positive electrode active material layer apply | coated to both surfaces of the said aluminum foil was adjusted to the substantially same film thickness. The positive electrode on which the positive electrode active material was formed was pressure-bonded to both surfaces of the positive electrode current collector by a roll press to produce a positive electrode so that the density of the positive electrode active material layer was 3.6 g / cm ³ . . Thus, a positive electrode was obtained.

(Preparation of negative electrode)
As the negative electrode active material particles, silicon oxide particles having a molar ratio of oxygen to silicon of 1: 1 were coated with carbon on the surface using molten salt carbon plating. As the molten salt carbon plating bath, a mixture of LiCl and KCl in a molar ratio of 6: 4 was used as a molten salt maintained at 600 ° C. and added with 3 mol% of CaC 2. With a fluidized bed electrode in which silicon oxide particles having a D50 of 3 μm and a molten salt are suspended, 1.5 V vs. The carbon coating was performed by applying current for 5 minutes so as to be Li / Li +. The obtained particles were taken out of the bath, washed with water and dried to obtain negative electrode active material particles of Example 1.

The negative electrode active material particles were mixed and dispersed in 87.9% by weight, acetylene black 2.1% by weight, polyamideimide resin 10% by weight, and N-methyl-2-pyrrolidone solvent. A coating for layer formation was prepared. The obtained paint is applied to one side of a 10 μm thick copper foil so that the amount of active material applied is 3.8 mg / cm ² and dried at 100 ° C. Formed. Thereafter, a negative electrode active material layer was similarly formed on the other surface of the negative electrode current collector.

In addition, the thickness of the coating film of the negative electrode active material layer apply | coated to both surfaces of the said copper foil was adjusted to the substantially same film thickness. The negative electrode on which the negative electrode active material was formed was pressure-bonded to both surfaces of the negative electrode current collector by a roll press, and the negative electrode was prepared so that the density of the negative electrode active material layer was 1.5 g / cm ³ . Thus, a negative electrode was obtained.

The negative electrode is immersed in a 1M LiPF6 solution (solvent: EC / DEC = 3/7 (volume ratio)) in a glove box substituted with dry Ar, and the charge capacity of the active material is obtained using metal Li as the counter electrode. The battery was charged with 0.1 C to 30%, washed with DEC, dried, and partially pre-doped with lithium. Note that the charge capacity of the active material is a half cell manufactured by combining a negative electrode and metal lithium, and 0.02 V vs. 200 mA / g of negative electrode active material weight. It was set as the capacity | capacitance when it was set as charge to Li / Li +.

(Example 2)
As negative electrode active material particles, mixed particles of carbon-coated silicon oxide particles and graphite obtained by subjecting silicon oxide particles having a composition of a molar ratio of oxygen to silicon of 1: 1 with graphite to high-frequency thermal plasma treatment were used. The high-frequency thermal plasma treatment was performed by introducing a sample in which silicon oxide particles having a D50 of 3 μm and graphite having a D50 of 8 μm were mixed at a weight ratio of 20: 1 into a plasma torch. As a plasma gas, Ar is 6 L / min. As the sheath gas, Ar is 30 L / min. And N2 at 3 L / min. , Ar as a carrier gas is 10 L / min. The pressure in the plasma torch is controlled to 27 kPa, the frequency is 2 MHz, the input power is 40 kW, and the powder supply rate is 3 g / min. Went as. At this time, according to the model calculation, the plasma temperature is 10,000 ° C. or higher.

  The negative electrode active material layer of Example 2 was formed in the same manner as in Example 1 except that the negative electrode active material layer was changed to the mixed particles of carbon-coated silicon oxide particles and graphite as negative electrode active material particles when forming the negative electrode active material layer. And a negative electrode was obtained.

(Example 3)
In the glove box substituted with dry Ar, the positive electrode produced in Example 1 and the negative electrode before Li pre-doping were sandwiched between them, and a separator made of a polyethylene microporous film was sandwiched between them, and placed in an aluminum laminate pack. A 1M LiPF ₆ solution (solvent: EC / DEC = 3/7 (volume ratio)) as an electrolytic solution was poured into the laminate pack and then vacuum-sealed to prepare a lithium ion secondary battery. After charging, the battery was discharged to 3.0V at 0.1C. Batteries for cross-sectional observation and cycle characteristic evaluation were prepared. The negative electrode for cross-sectional observation was decomposed in a glove box substituted with dry Ar, washed with DEC, and dried.

Example 4
The same operation as in Example 3 was performed except that the negative electrode before Li pre-doping produced in the same manner as in Example 2 was used.

  (Example 5) The high-frequency thermal plasma treatment of the negative electrode active material particles was performed in the same manner as in Example 2 except that silicon oxide particles having a D50 of 0.5 µm were used.

(Example 6)
The negative electrode active material particles were subjected to high-frequency thermal plasma treatment in the same manner as in Example 4 except that silicon oxide particles having a D50 of 8 μm were used.

(Example 7)
The same procedure as in Example 1 was performed except that silicon oxide particles having a D50 of 8.0 μm were used for molten salt carbon plating of the negative electrode active material particles.

(Example 8)
The same procedure as in Example 1 was performed except that silicon oxide particles having a D50 of 18.0 μm were used for molten salt carbon plating of the negative electrode active material particles.

Example 9
The same procedure as in Example 7 was performed except that silicon oxide particles having a composition with a molar ratio of oxygen to silicon of 1: 0.1 were used for molten salt carbon plating of the negative electrode active material particles.

(Example 10)
The negative electrode active material particles were subjected to high-frequency thermal plasma treatment in the same manner as in Example 2 except that silicon oxide particles having a composition with a molar ratio of oxygen to silicon of 1: 2 were used.

(Example 11)
When preparing a coating material for forming an active material layer, instead of silicon oxide plated with molten salt carbon, a mixture of silicon oxide plated with molten salt carbon and graphite having a D50 of 8 μm in a weight ratio of 7: 3 is used. The procedure was the same as in Example 9 except that.

(Example 12)
The high-frequency thermal plasma treatment of the negative electrode active material particles was performed in the same manner as in Example 5 except that silicon oxide particles and graphite were mixed at a weight ratio of 3: 7.

(Comparative Example 1)
Silicon oxide particles having a composition of D50 of 18 μm and a molar ratio of oxygen to silicon of 1: 2 are 87.9% by weight as negative electrode active material particles, 2.1% by weight of acetylene black, 10% by weight of polyamideimide resin, A coating material for forming an active material layer was prepared by mixing and dispersing a solvent of N-methyl-2-pyrrolidone. The obtained paint is applied to one side of a 10 μm thick copper foil so that the amount of active material applied is 3.8 mg / cm ² and dried at 100 ° C. Formed. Thereafter, a negative electrode active material layer was similarly formed on the other surface of the negative electrode current collector.

In addition, the thickness of the coating film of the negative electrode active material layer apply | coated to both surfaces of the said copper foil was adjusted to the substantially same film thickness. The negative electrode on which the negative electrode active material was formed was pressure-bonded to both surfaces of the negative electrode current collector by a roll press, and the negative electrode was prepared so that the density of the negative electrode active material layer was 1.5 g / cm ³ . Thus, a negative electrode sheet was obtained.

(Comparative Example 2)
The same operation as in Comparative Example 1 was conducted except that untreated silicon containing D50 of 3 μm and not containing oxygen was used as the negative electrode active material particles.

  The Raman spectrum of the cross section of the active material particles in the obtained negative electrode active material layer is Nm-7100 manufactured by JASCO Corporation, using a green laser with a wavelength of 532 nm and Raman spectroscopy, and 1 mW or less using a dimmer. It adjusted and measured so that it might become the irradiation intensity of. The negative electrode was sampled for ion milling in a dry Ar glove box, the cross section was prepared by ion milling without exposure to the atmosphere, and a stand-by non-exposure measuring jig that allowed the laser to pass through the garnet plate, In the state of exposure, the Raman spectrum of the active material particle cross section was measured.

(Production of evaluation lithium-ion secondary battery)
Using the positive electrode and the negative electrode prepared above, a separator made of a polyethylene microporous film is sandwiched between them and placed in an aluminum laminate pack. In this aluminum laminate pack, a 1M LiPF ₆ solution (solvent: EC) is used as an electrolytic solution. / DEC = 3/7 (volume ratio)) was injected, and then vacuum-sealed to produce a lithium ion secondary battery for evaluation.

  About the produced lithium ion secondary battery, cycling characteristics were evaluated with the following method.

(Measurement of charge / discharge cycle characteristics)
The cycle characteristics were measured using a secondary battery charge / discharge test apparatus. The voltage range was 4.3V to 3.0V. Only the first charge was performed by constant current charging at 0.1 C, and discharging was performed at 0.5 C. Charging was performed at a constant current and a constant voltage. The battery was charged at a current value of 0.5 C, and after reaching 4.3 V, charging was terminated when the current value reached 5% of the 0.5 C current value. Thereafter, the cycle characteristics were measured under the condition of discharging at a current value of 0.5C. The charge / discharge cycle characteristics were evaluated as capacity retention rate (%). The capacity retention rate (%) is the ratio of the discharge capacity at each cycle number to the initial discharge capacity, where the discharge capacity at the first cycle is the initial discharge capacity. The capacity retention rate (%) is expressed by the following mathematical formula (1).
Capacity maintenance rate (%) = (“discharge capacity at the first cycle” / “discharge capacity at each cycle number”) × 100 (1)
A higher capacity retention rate means better charge / discharge cycle characteristics. The lithium ion secondary batteries produced in the examples and comparative examples were repeatedly charged and discharged under the above conditions, and the capacity retention rate after 100 cycles was evaluated as charge / discharge cycle characteristics.

  The cycle characteristics of the lithium ion secondary batteries produced in Examples 1 to 12 and Comparative Examples 1 and 2 were evaluated. Table 1 shows the capacity retention ratio values after 100 cycles in each lithium ion secondary battery.

Moreover, the peak position of 450 ± 20 cm ⁻¹ obtained from the Raman spectrum of the negative electrode active material particles in the cross section of the prepared negative electrode active material particles, and the intensity of peaks G and D of 1575 ± 15 cm ⁻¹ and 1350 ± 15 cm ^−1. The values of the ratio G / D are shown in Table 1 together with the results calculated for the negative electrode active material layers prepared in Examples 1 to 12 and Comparative Examples 1 and 2, respectively.

From the results of Examples 1 to 12 and Comparative Examples 1 and 2, the cross section of the negative electrode active material particles having the negative electrode active material layer having the negative electrode active material particles has a peak A at 450 ± 20 cm ⁻¹ in the argon laser Raman spectrum. It has a peripheral portion of the cross-section of the active material particles, argon laser and D to 1575 ± 15cm ^-1 peak G and 1350 ± 15cm ^-1 in the Raman spectrum has a strength ratio G / D peak G and D However, when 0.5 <G / D <2 was satisfied, it was confirmed that the cycle characteristics were particularly excellent. It was also confirmed that the cycle characteristics were excellent by optimizing the particle size of the negative electrode active material particles. Furthermore, it was confirmed that the cycle characteristics were excellent by mixing graphite in the negative electrode active material layer.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode for lithium ion secondary batteries and lithium ion secondary battery which are excellent in cycling characteristics can be provided.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 30 ... Laminated body, 50 ... Outer body, 60 ... Positive electrode lead, 62 ... Negative electrode lead, 100 ... Lithium ion secondary battery

Claims (4)

A negative electrode active material layer is provided on at least one of the main surfaces of the negative electrode current collector,
The negative electrode active material layer has negative electrode active material particles containing silicon and carbon,
The cross-section of the negative electrode active material particles has a peak A at 450 ± 20 cm ⁻¹ in an argon laser Raman spectrum,
Periphery of the cross section of the active material particles in an argon laser Raman spectrum, has a 1575 ± 15cm ^-1 and peak D in peak G, 1350 ± 15cm ^-1, and,
The negative electrode for a lithium ion secondary battery, wherein the intensity ratio G / D between the peak G and the peak D is 0.5 <G / D <2.   2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein an average primary particle size of the negative electrode active material particles is from 0.1 μm to 20 μm.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material layer contains graphite. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, a positive electrode, and an electrolyte.

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