Negative electrode active material layer, negative electrode, and lithium ion secondary battery

Abstract

To provide a negative electrode active material, a negative electrode, and a lithium-ion secondary battery that are capable of suppressing excessive heat generation.SOLUTION: The negative electrode active material includes silicon particles. X-ray photoelectron spectra measured at any position in the depth direction from the surface when the X-ray photoelectron spectra are measured in the range of binding energy from 678 eV or more and 698 eV or less have a first peak with a binding energy of 687 eV or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material layer , a negative electrode and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptop computers, as well as hybrid cars.

[0003] The capacity of a lithium-ion secondary battery depends mainly on the active material of the electrode. Graphite is generally used as the negative electrode active material, but negative electrode active materials with higher capacity are in demand. Therefore, silicon (Si), which has a theoretical capacity much larger than that of graphite (372 mAh / g), has attracted attention. For example, Patent Document 1 describes a lithium-ion secondary battery with high energy density that uses silicon for the negative electrode.

[0004] The higher the energy density of a lithium-ion secondary battery, the more attention must be paid to safety. For example, if heat is generated inside the lithium-ion secondary battery due to misuse, overcharging, an internal short circuit, etc., problems such as decomposition of the non-aqueous electrolyte and an increase in internal pressure may occur.

[0005] For example, Patent Document 2 describes that internal heat generation in a lithium ion secondary battery can be suppressed by providing an insulating member between a current collector and a composite layer. Also, for example, Patent Document 3 describes that thermal stability can be improved by adding an additive to the electrolyte solution and adjusting the void volume of each component. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-181820 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-198591 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-9532 Summary of the Invention [Problem to be solved by the invention]

[0007] The above-mentioned methods may not be an option in some cases, and therefore there is a need for a configuration that can suppress excessive heat generation using a method other than these methods.

[0008] The present disclosure has been made in view of the above problems, and aims to provide a negative electrode active material, a negative electrode, and a lithium ion secondary battery that can suppress excessive heat generation. [Means for solving the problem]

[0009] In order to solve the above problems, the following means are provided.

[0010] (1) A negative electrode active material according to a first aspect includes silicon particles, and when an X-ray photoelectron spectroscopy spectrum having a binding energy in the range of 678 eV to 698 eV is measured in the depth direction from the surface, the X-ray photoelectron spectroscopy spectrum measured at any position in the depth direction has a first peak having a binding energy of 687 eV or more.

[0011] (2) In the negative electrode active material according to the above aspect, an X-ray photoelectron spectroscopy spectrum measured at a depth position different from the depth position at which the first peak is measured may have a second peak at a position different from the first peak, and an energy difference between the binding energy of the first peak and the binding energy of the second peak may be 1 eV or more.

[0012] (3) The negative electrode according to the second embodiment includes the negative electrode active material according to the above embodiment.

[0013] (4) A lithium ion secondary battery according to a third aspect includes the negative electrode according to the above aspect, a positive electrode, and an electrolyte connecting the positive electrode and the negative electrode. [Effects of the Invention]

[0014] The lithium ion secondary battery according to the above aspect can suppress excessive heat generation. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view of a negative electrode active material according to a first embodiment. [Figure 2] 1 shows an X-ray photoelectron spectroscopy spectrum of the negative electrode active material according to the first embodiment. [Figure 3] 1 shows an X-ray photoelectron spectroscopy spectrum of the negative electrode active material according to the first embodiment. [Figure 4] 1 is a cross-sectional view schematically illustrating a lithium ion secondary battery according to a first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0017] "Negative electrode active material" 1 is a cross-sectional view of a negative electrode active material according to Embodiment 1. The negative electrode active material 1 includes, for example, silicon particles 2, a surface layer 3, and a coating layer 4.

[0018] The silicon particles 2 may be silicon alloys, silicon compounds, silicon composites, or the like, in addition to simple silicon. The silicon particles 2 may be crystalline or amorphous.

[0019] Silicon alloys include, for example, X nIt is represented by Si. X is a cation. Examples of X include Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, and K. n satisfies the condition 0≦n≦0.5.

[0020] The silicon compound is, for example, SiO x The silicon oxide is expressed as follows: where x satisfies, for example, 0.8≦x≦2. The silicon oxide may consist of only SiO2, or may consist of only SiO, or may be a mixture of SiO and SiO2. The silicon oxide may also have some oxygen deficiencies.

[0021] A silicon composite is, for example, a silicon or silicon compound particle whose surface is at least partially coated with a conductive material. Examples of the conductive material include carbon materials, Al, Ti, Fe, Ni, Cu, Zn, Ag, and Sn. For example, a silicon-carbon composite (Si-C) is an example of the composite.

[0022] The surface layer 3 is formed on at least a portion of the surface of the silicon particle 2. When an X-ray photoelectron spectroscopy (XPS) spectrum with a binding energy range of 678 eV to 698 eV is measured in the depth direction from the surface, the surface layer 3 is a portion where a peak appears at a position where the binding energy is 687 eV or higher. The XPS spectrum is an F1s spectrum. Hereinafter, this peak appearing at a position where the binding energy is 687 eV or higher will be referred to as the first peak. The first peak appears, for example, at a position where the binding energy is 687 eV to 690 eV.

[0023] Silicon and fluorine are bonded to each other in the surface layer 3. The surface layer 3 is formed by previously treating the surface of the silicon particle 2 with fluorine.

[0024] FIG. 2 shows an X-ray photoelectron spectroscopy (XPS) spectrum of the negative electrode active material according to the first embodiment. The XPS spectrum shown in FIG. 2 was obtained by measuring the XPS spectrum in the binding energy range of at least 678 eV to 698 eV in the depth direction from the surface of the negative electrode active material 1 by X-ray photoelectron spectroscopy. FIG. 2 shows the measurement results of the negative electrode active material 1 according to the first embodiment in which the surfaces of the silicon particles 2 were fluorine-treated, as well as the measurement results of the negative electrode active material that was not fluorine-treated. Samples s1 and s2 show the measurement results of the negative electrode active material 1 according to the first embodiment in which the surfaces of the silicon particles 2 were fluorine-treated. Samples s3 and s4 show the measurement results of the negative electrode active material that was not fluorine-treated.

[0025] As shown in Figure 2, the first peak is observed in the XPS spectra of samples s1 and s2. On the other hand, the first peak is not observed in the XPS spectra of samples s3 and s4. The first peak is thought to be a peak derived from the bond between silicon and fluorine resulting from the fluorine treatment of silicon particles 2.

[0026] The coating layer 4 is formed so as to cover at least a portion of the silicon particle 2 or the surface layer 3. When an XPS spectrum in the binding energy range of 678 eV to 698 eV is measured in the depth direction from the surface, the coating layer 4 is a portion in which a peak appears at a position different from the first peak. Hereinafter, this peak will be referred to as the second peak. The second peak appears at a binding energy less than 687 eV, for example, at a binding energy of 685 eV to 686 eV. The energy difference between the binding energy of the first peak and the binding energy of the second peak is, for example, 1 eV or more.

[0027] The coating layer 4 is located outside the surface layer 3 in the negative electrode active material 1. When X-ray photoelectron spectroscopy is performed while etching from the surface of the negative electrode active material 1 in the depth direction, a second peak is detected in the XPS spectrum at a position where etching has progressed to a certain depth, and then a first peak is detected in the XPS spectrum at a position where etching has continued further.

[0028] FIG. 3 shows an X-ray photoelectron spectroscopy (XPS) spectrum of the negative electrode active material according to the first embodiment. The XPS spectrum shown in FIG. 3 was obtained by measuring the XPS spectrum in the binding energy range of at least 678 eV to 698 eV in the depth direction from the surface of the negative electrode active material 1 by X-ray photoelectron spectroscopy. FIG. 3 shows the measurement results of the negative electrode active material 1 according to the first embodiment in which the surfaces of the silicon particles 2 were fluorine-treated, as well as the measurement results of the negative electrode active material that was not fluorine-treated. Samples s1 and s2 show the measurement results of the negative electrode active material 1 according to the first embodiment in which the surfaces of the silicon particles 2 were fluorine-treated. Samples s3 and s4 show the measurement results of the negative electrode active material that was not fluorine-treated.

[0029] As shown in FIG. 3, a second peak is observed in the XPS spectra of all samples s1 to s4. The second peak is thought to be a peak derived from fluorine contained in the SEI (Solid Electrolyte Interphase) coating. The SEI coating is a stable coating formed in the early stages of use of a lithium-ion secondary battery. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte, preventing decomposition of the electrolyte.

[0030] The average particle size of the negative electrode active material 1 is, for example, 0.1 μm or more and 10 μm or less, preferably 0.5 μm or more and 8 μm or less, and more preferably 1 μm or more and 7 μm or less.

[0031] When the negative electrode active material 1 is available in the form of particles, the median diameter (D50) can be determined as the average particle diameter using a particle size distribution analyzer (e.g., manufactured by Malvern Panalytical). When using a particle size distribution analyzer, the average particle diameter of 50,000 particles is determined, for example. When the negative electrode active material 1 is present within the electrode and is difficult to separate from the electrode, the average particle diameter can be determined using at least 100 particles of the negative electrode active material 1 confirmed in a cross-sectional image.

[0032] The negative electrode active material 1 according to the first embodiment can be produced by producing silicon particles 2 and then subjecting the surfaces of the silicon particles 2 to a fluorine treatment.

[0033] The silicon particles 2 can be produced by a known method. The silicon particles 2 may also be purchased as a commercially available product.

[0034] Next, a fluorine treatment is performed. For example, the silicon particles 2 are placed in a container and plasma treated in fluorine gas, thereby fluorine treating the silicon particles 2. Alternatively, the silicon particles 2 may be immersed in hydrofluoric acid. By immersing in hydrofluoric acid, the surfaces of the silicon particles 2 are etched and fluorine treated. A surface layer 3 is formed by the fluorine treatment. A coating layer 4 is formed during charge and discharge in the early stages of use of the lithium-ion secondary battery.

[0035] The negative electrode active material 1 according to the first embodiment has a surface that has been pre-fluorinated, and therefore can suppress excessive heat generation in a lithium ion secondary battery.

[0036] For example, when an abnormality such as an internal short circuit occurs in a lithium-ion secondary battery, heat is generated, which promotes the reaction between silicon and fluorine in the electrolyte. The reaction between silicon and fluorine in the electrolyte forms a stable coating, and further heat is generated during the formation of this coating. In other words, the heat generated by the internal short circuit is enhanced by the heat generated during the formation of the coating.

[0037] In contrast, the negative electrode active material 1 according to the first embodiment has a fluorine-treated surface, which reduces the amount of heat generated when silicon reacts with fluorine in the electrolyte, thereby preventing excessive heat generation.

[0038] "Lithium-ion secondary battery" Fig. 4 is a schematic diagram of a lithium ion secondary battery according to the first embodiment. The lithium ion secondary battery 100 shown in Fig. 4 includes a power generating element 40, an exterior body 50, and an electrolyte (for example, a non-aqueous electrolyte solution). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte solution is accommodated in the exterior body 50. Although Fig. 4 illustrates an example in which one power generating element 40 is provided within the exterior body 50, multiple power generating elements 40 may be stacked.

[0039] (power generating element) The power generating element 40 includes a separator 10, a positive electrode 20, and a negative electrode 30. The power generating element 40 may be a laminate in which these are stacked, or a wound body in which a structure in which these are stacked is wound.

[0040] <Positive electrode> The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0041] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0042] [Cathode active material layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive additive and a binder as necessary.

[0043] The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions with counter anions.

[0044] The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium manganese spinel (LiMnO), and lithium manganese oxides represented by the general formula: LiNi x Co y Mn z M aA compound of O2 (where x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1 in the general formula, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), a lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0045] The positive electrode active material may also be a lithium-free material. Examples of lithium-free materials include FeF3, conjugated polymers containing organic conductive substances, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. The lithium-free material may be used alone or in combination of multiple materials. When the positive electrode active material is a lithium-free material, for example, discharging is performed first. Lithium is inserted into the positive electrode active material by discharging. In addition, the lithium-free material of the positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0046] The conductive assistant enhances the electron conductivity between the positive electrode active materials. Examples of the conductive assistant include carbon powder, carbon nanotubes, carbon materials, metal fine powder, a mixture of carbon materials and metal fine powder, and conductive oxides. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, etc.

[0047] The content rate of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content rate of the conductive assistant with respect to the total mass of the positive electrode active material, the conductive assistant, and the binder is 0.5 mass% or more and 20 mass% or less, preferably 1 mass% or more and 5 mass% or less.

[0048] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. The binder is preferably one that is insoluble in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0049] The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0050] <Negative electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0051] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0052] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer may contain a conductive additive, a dispersion stabilizer, etc. as needed. The negative electrode active material used is the above-mentioned negative electrode active material.

[0053] The conductive additive and binder may be the same as those used in the positive electrode 20. The binder in the negative electrode 30 may be, in addition to those listed for the positive electrode 20, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. The cellulose may be, for example, carboxymethyl cellulose (CMC).

[0054] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents short-circuiting between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0055] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0056] <Electrolyte> The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The electrolytic solution is not limited to a liquid electrolyte, but may be a solid electrolyte. The non-aqueous electrolytic solution contains, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0057] The solvent is not particularly limited as long as it is a solvent generally used in lithium ion secondary batteries. The solvent includes, for example, any of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may include a mixture of these compounds in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, and vinylene carbonate. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Examples of the cyclic ester compound include γ-butyrolactone, and the like. Examples of the chain ester compound include propyl propionate, ethyl propionate, and ethyl acetate, and the like.

[0058] The electrolytic salt is, for example, a lithium salt. Examples of the electrolyte include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. One lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of the degree of ionization, it is preferable that the electrolyte contains LiPF6. The degree of dissociation of the electrolytic salt in a carbonate solvent at room temperature is preferably 10% or more.

[0059] The electrolyte is preferably, for example, LiPF6 dissolved in a carbonate solvent. The concentration of LiPF6 is, for example, 1 mol / L. When the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit charging behavior similar to that of soft carbon. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, lithium can be reacted uniformly with the polyimide. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate, or vinylene carbonate.

[0060] <Exterior body> The exterior body 50 seals the power generating element 40 and the non-aqueous electrolyte solution inside. The exterior body 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0061] 4, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0062] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0063] <Terminal> Terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0064] The lithium ion secondary battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.

[0065] The negative electrode 30 is produced, for example, by sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0066] The slurry preparation process involves mixing a negative electrode active material, a binder, a conductive additive, and a solvent to prepare a slurry. The negative electrode active material is fluorine-treated silicon. Adding a dispersion stabilizer to the slurry can suppress aggregation of the negative electrode active material.

[0067] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material, a binder, a conductive additive, and a solvent, such as water or N-methyl-2-pyrrolidone.

[0068] The electrode coating step is a step of coating the surface of the negative electrode current collector 32 with a slurry. The method of coating the slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry. The slurry is coated at room temperature, for example.

[0069] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere at 80°C to 350°C.

[0070] The rolling step is performed as necessary. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling step is performed using, for example, a roll press device.

[0071] The positive electrode 20 can be produced by the same procedure as that for the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available products.

[0072] Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

[0073] Finally, the power generation element 40 is sealed in the exterior body 50. A non-aqueous electrolyte solution is poured into the exterior body 50. After the non-aqueous electrolyte solution is poured, the pressure is reduced, heating, etc. is performed, and the non-aqueous electrolyte solution is impregnated into the power generation element 40. The lithium-ion secondary battery 100 is obtained by sealing the exterior body 50 by applying heat, etc. Note that the power generation element 40 may be impregnated with the electrolyte solution instead of pouring the electrolyte solution into the exterior body 50. After the electrolyte solution is poured into the power generation element, it is preferable to leave it to stand for 24 hours.

[0074] The lithium ion secondary battery 100 according to the first embodiment has a predetermined negative electrode active material, and therefore is highly safe even if an internal short circuit occurs due to an impact or the like.

[0075] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention. [Example]

[0076] "Example 1" One surface of a 15 μm thick aluminum foil was coated with a positive electrode slurry prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0077] The positive electrode active material is Li x CoO2 was used. Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive additive, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The amount of the positive electrode active material supported in the positive electrode active material layer after drying was 25 mg / cm. 2The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer, which was then pressed with a roll press to prepare a positive electrode.

[0078] Next, silicon particles with an average particle size of 3.7 μm were placed in a container and subjected to plasma treatment in fluorine gas. The fluorine treatment time was 60 minutes. Then, a negative electrode slurry was prepared using the fluorine-treated silicon particles. Carbon black was used as the conductive additive. Polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 90 parts by mass of the fluorine-treated silicon particles, 5 parts by mass of the conductive additive, and 5 parts by mass of the binder were mixed with N-methyl-2-pyrrolidone to prepare a negative electrode slurry.

[0079] The negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm and then dried. After drying, the amount of the negative electrode active material carried in the negative electrode active material layer was 2.5 mg / cm. 2 The negative electrode active material layer was pressed with a roll press and then baked in a nitrogen atmosphere at 300° C. or higher for 5 hours.

[0080] Next, an electrolyte solution was prepared. The solvent for the electrolyte solution was fluoroethylene carbonate (FEC):ethylene carbonate (EC):diethyl carbonate (DEC) = 10% by volume:20% by volume:70% by volume. The electrolyte solution was also supplemented with additives for improving output, gas suppression, cycle characteristics improvement, and safety performance improvement. LiPF6 was used as the electrolyte salt. The concentration of LiPF6 was 1 mol / L.

[0081] (Fabrication of lithium-ion secondary batteries for evaluation) The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) interposed between them so that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. This laminate was inserted into an exterior body made of aluminum laminate film and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior body, and the remaining location was heat-sealed while reducing the pressure using a vacuum sealer, to produce a lithium-ion secondary battery. The fabricated lithium-ion secondary battery was left to stand for 24 hours.

[0082] (nail penetration test) A nail penetration test was carried out using the fabricated lithium ion secondary battery. First, the lithium ion secondary battery was charged. Charging was carried out at a constant current charge rate of 1.0C (a current value at which charging is completed in one hour when constant current charging is carried out at 25°C) in an environment of 25°C until the battery voltage reached 4.4V. Then, a nail having a diameter of 2.5mm was penetrated into the charged battery at a speed of 150mm / s to carry out a nail penetration test. Then, the surface temperature of the lithium ion secondary battery after nail penetration was measured. The surface temperature of the lithium ion secondary battery of Example 1 was 27°C.

[0083] (XPS measurement) A lithium-ion secondary battery fabricated under the same conditions was charged at a constant current and constant voltage of 0.5C up to 4.2V, and then discharged at a constant current of 1C down to 2.8V. The negative electrode was then removed and subjected to XPS analysis. XPS analysis was performed while etching the negative electrode active material from the surface in the depth direction. The XPS analysis was performed using a Quantera 2 manufactured by PHI.

[0084] In the XPS spectrum of the negative electrode active material according to Example 1 measured near the surface, a peak was observed at a binding energy position of 686 eV. Furthermore, in the XPS spectrum of the negative electrode active material according to Example 1 measured at a position 40 nm etched from the surface, a peak was observed at a binding energy position of 688 eV. That is, when the X-ray photoelectron spectroscopy spectrum in the binding energy range of 678 eV to 698 eV was measured in the depth direction from the surface, two peaks were observed. The energy difference between the two peaks was 2 eV.

[0085] "Examples 2 and 3" Examples 2 and 3 differ from Example 1 in that the conditions for fluorine treatment of the silicon particles were changed. Specifically, the treatment time for fluorine treatment was changed to 30 minutes in Example 2, and 45 minutes in Example 3. The other conditions were the same as in Example 1, and the evaluation was performed.

[0086] Example 4 Example 4 differs from Example 2 in that the conditions for initial charge and discharge were changed. Example 4 differs from Example 2 in that the initial charge and discharge were performed in an environment of 45°C, and the coating structure was changed. The other conditions were the same as in Example 2, and the evaluation was performed.

[0087] "Comparative Example 1, Comparative Example 2" Comparative Examples 1 and 2 differ from Example 1 in that no fluorine treatment was performed. Evaluation was performed under other conditions similar to those of Example 1. Comparative Examples 1 and 2 differed in the initial charge / discharge conditions, and in Comparative Example 2, two different states of fluorine bonds were mixed in the coating layer.

[0088] The results of Examples 1 to 4 and Comparative Examples 1 and 2 are summarized in Table 1 below. "Maximum bond energy" in Table 1 is the peak top position of the peak with the largest bond energy among the peaks observed in the bond energy range of 678 eV to 698 eV. "Number of peaks" is the number of peaks observed in the bond energy range of 678 eV to 698 eV. "Bond energy difference" is the bond energy difference between the maximum energy peak and the minimum energy peak observed in the bond energy range of 678 eV to 698 eV. "Nail penetration test temperature" is the surface temperature of the lithium ion secondary battery after the nail penetration test.

[0089] [Table 1]

[0090] In Examples 1 to 4, the surface temperature after the nail penetration test was lower than in Comparative Examples 1 and 2. This is thought to be because in Examples 1 to 4, the silicon surface was fluorine-treated, which prevented the reaction between the silicon and the fluorine in the electrolyte. [Explanation of symbols]

[0091] 1 Negative electrode active material 2. Silicon particles 3 Surface layer 4 Covering layer 10 Separator 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 terminals 100 Lithium-ion secondary battery

Claims

1. A negative electrode comprising a plurality of negative electrode active materials, each of the plurality of negative electrode active materials includes silicon particles, a surface layer that coats the silicon particles and contains fluorine, and a coating layer that coats the surface layer; When the X-ray photoelectron spectroscopy spectrum in the binding energy range of 678 eV to 698 eV was measured in the depth direction from the surface, an X-ray photoelectron spectroscopy spectrum measured at any position in the depth direction has a first peak having a binding energy of 687 eV or more; an X-ray photoelectron spectroscopy spectrum measured at a depth position different from the depth position at which the first peak is measured has a second peak at a position different from the first peak; a negative electrode active material layer in which the energy difference between the binding energy of the first peak and the binding energy of the second peak is 1 eV or more.

2. The negative electrode active material layer described in claim 1, wherein the energy difference between the binding energy of the first peak and the binding energy of the second peak is 2 eV or more.

3. A negative electrode comprising the negative electrode active material layer according to claim 1 or 2.

4. A lithium ion secondary battery comprising: the negative electrode according to claim 3; a positive electrode facing the negative electrode; and an electrolyte connecting the negative electrode and the positive electrode.

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