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

Abstract

This negative electrode active material layer comprises a negative electrode active material and a fibrous material. The negative electrode active material contains silicon. The fibrous material contains at least one type of substance selected from the group consisting of titanic oxide, potassium titanate, aluminum oxide, silicon carbide, silicon nitride, and silicon oxide. The fiber length of the fibrous material falls within the range of 20 μm to 150 μm. The value of the layer thickness divided by the fiber length falls within the range of 0.4 to 1.0.

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 laptops, as well as hybrid cars.

[0003] The charge and discharge of lithium-ion secondary batteries is carried out by an active material layer containing an active material. To improve the performance of lithium-ion secondary batteries, the structure of the active material layer has been investigated.

[0004] For example, Patent Documents 1 and 2 describe active material layers containing fillers of a predetermined shape. For example, Patent Documents 3 and 4 describe the use of two types of lithium cobalt oxide with different tap densities as active materials. For example, Patent Document 5 describes an active material layer in which carbon nanofibers are added to a binder. For example, Patent Document 6 describes an active material layer in which fibrous carbon is added to a binder. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-172901 [Patent Document 2] Japanese Patent Application Publication No. 2019-186164 [Patent Document 3] US Patent Application Publication No. 2005 / 0271576 [Patent Document 4] US Patent Application Publication No. 2008 / 0087862 [Patent Document 5] US Patent Application Publication No. 2007 / 0092796 [Patent Document 6] US Patent Application Publication No. 2011 / 0266495 Summary of the Invention [Problem to be solved by the invention]

[0006] Negative electrode active materials containing silicon undergo significant volume expansion during charging. This volume expansion of the negative electrode active material causes a deterioration in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, the conductive paths between the negative electrode active materials are severed, peeling occurs at the interface between the negative electrode active material layer and the current collector, or cracks occur in the SEI (Solid Electrolyte Interphase) coating, causing decomposition of the electrolyte. These factors reduce the cycle characteristics of the battery. Therefore, there is a demand for batteries that are resistant to deterioration in charge / discharge characteristics, even when silicon-based materials are used as the negative electrode active material.

[0007] The present disclosure has been made in view of the above problems, and aims to provide a negative electrode active material layer, a negative electrode, and a lithium ion secondary battery that are less susceptible to deterioration in charge / discharge characteristics. [Means for solving the problem]

[0008] In order to solve the above problems, the following means are provided.

[0009] (1) A negative electrode active material layer according to a first aspect includes a negative electrode active material and a fibrous material. The negative electrode active material includes silicon. The fibrous material includes at least one selected from the group consisting of titanium oxide, potassium titanate, aluminum oxide, silicon carbide, silicon nitride, and silicon oxide. The fiber length of the fibrous material is 20 μm or more and 150 μm or less. The value obtained by dividing the layer thickness of the negative electrode active material layer by the fiber length is 0.4 or more and 1.0 or less.

[0010] (2) In the negative electrode active material layer according to the above embodiment, the average interparticle distance of the negative electrode active material may be 3.0 μm or more and 4.5 μm or less.

[0011] (3) In the negative electrode active material layer according to the above aspect, the fibrous material may contain at least one material selected from the group consisting of titanium oxide, potassium titanate, and aluminum oxide.

[0012] (4) The negative electrode according to the second embodiment includes the negative electrode active material layer according to the above embodiment.

[0013] (5) A lithium ion secondary battery according to a third aspect includes the negative electrode according to the above aspect, a positive electrode facing the negative electrode, and an electrolyte connecting the negative electrode and the positive electrode. [Effects of the Invention]

[0014] The negative electrode active material layer, negative electrode, and lithium ion secondary battery according to the above aspects are less susceptible to deterioration in charge / discharge characteristics. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a lithium-ion secondary battery according to a first embodiment. [Figure 2] FIG. 2 is an enlarged schematic view of a characteristic portion of the negative electrode active material layer according to the 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] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and an electrolyte (e.g., 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 terminals 60, 62 that are connected together. The non-aqueous electrolyte solution is accommodated within the exterior body 50. Although FIG. 1 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.

[0018] (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.

[0019] <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.

[0020] [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.

[0021] [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.

[0022] 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.

[0023] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compound (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), 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 also be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0024] The positive electrode active material may be a lithium-free material. The lithium-free material is, for example, FeF3, a conjugated polymer containing an organic conductive substance, a Chevrel phase compound, a transition metal chalcogenide, a vanadium oxide, a niobium oxide, etc. The lithium-free material may use only one of the materials or a combination of multiple materials. When the positive electrode active material is a lithium-free material, for example, discharging is first performed. 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 chemically or electrochemically pre-doped with lithium.

[0025] The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, and iron.

[0026] There are no particular limitations on the content of the conductive additive in the positive electrode active material layer 24. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 5 mass% or less.

[0027] 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.

[0028] 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.

[0029] <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.

[0030] [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.

[0031] [Negative electrode active material layer] 2 is an enlarged schematic view of a characteristic portion of the negative electrode active material layer 34. The negative electrode active material layer 34 includes, for example, a negative electrode active material 1, a fibrous material 2, a conductive additive 3, and a binder (not shown). The negative electrode active material layer 34 may also include other substances such as a dispersion stabilizer.

[0032] The negative electrode active material 1 contains silicon. The negative electrode active material 1 may be any of silicon alone, a silicon alloy, a silicon compound, and a silicon composite. The negative electrode active material 1 may be crystalline or amorphous.

[0033] Silicon alloys include, for example, X n It 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.

[0034] 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.

[0035] 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.

[0036] The average interparticle distance L of the negative electrode active material 1 is, for example, 3.0 μm or more and 4.5 μm or less. The average interparticle distance L can be measured using a scanning electron microscope (SEM). To determine the average interparticle distance L, first, three fields of view are measured using an SEM image magnified 1000 times to 1500 times. Then, in each of the three fields of view, the distance between the geometric centers of adjacent negative electrode active materials 1 is measured at 50 points. The average interparticle distance L is then obtained by averaging the interparticle distances measured at a total of 150 points.

[0037] The average particle diameter of the negative electrode active material 1 is, for example, 0.1 μm to 10 μm, preferably 0.5 μm to 8 μm, and more preferably 1 μm to 7 μm. Like the average interparticle distance L of the negative electrode active material 1, the average particle diameter of the negative electrode active material 1 is also determined as the average value of particle diameters at 150 points measured with a scanning electron microscope.

[0038] The fibrous material 2 is a linear particle having shape anisotropy. The fibrous material 2 includes, for example, one or more selected from the group consisting of titanium oxide, potassium titanate, aluminum oxide, silicon carbide, silicon nitride, and silicon oxide. The fibrous material 2 includes, for example, a material different from the negative electrode active material 1. Because silicon-based materials may expand and contract similarly to the negative electrode active material 1, the fibrous material 2 preferably includes one or more selected from the group consisting of titanium oxide, potassium titanate, and aluminum oxide.

[0039] The fiber length of the fibrous material 2 is, for example, 20 μm or more and 150 μm or less. The fiber length of the fibrous material 2 can be determined by the following procedure. First, the lithium-ion secondary battery 100 is disassembled, and the negative electrode 30 is removed. Next, the negative electrode 30 is immersed for one hour in a solvent capable of dissolving the binder constituting the negative electrode active material layer 34. For example, if the binder is a polyimide-based resin, e-solv 21KZE-100 manufactured by Kaneko Chemical Co., Ltd. is used. Next, the solvent is filtered and washed multiple times, and the powder remaining on the filter paper is dried. The dried powder contains the negative electrode active material 1, the fibrous material 2, and the conductive additive 3. Mapping using energy dispersive X-ray spectroscopy (EDS) can identify the fibrous material 2 from the powder. The size of the identified fibrous material 2 is then measured using a scanning electron microscope. The size of the fibrous material 2, like the average interparticle distance L, is determined as the average value of 150 points of the material measured using a scanning electron microscope.

[0040] Furthermore, when the particle size distribution of the dried powder particles is measured using a particle size distribution measuring device (for example, manufactured by Malvern Panalytical), the particle size distribution of the dried powder can be obtained. The particle size distribution of the dried powder has two or more peaks. One of the peaks is due to fibrous material 2 and occurs near the fiber length of fibrous material 2. Another peak is due to negative electrode active material 1 and occurs near the particle size of negative electrode active material 1. Because the particle sizes of negative electrode active material 1 and fibrous material 2 are different, multiple peaks appear in the particle size distribution of the powder. When both fibrous material 2 and negative electrode active material 1 contain silicon, the presence or absence of fibrous material 2 can be confirmed by measuring the particle size distribution of this powder.

[0041] The fibrous material 2 preferably accounts for 20% or less of the total mass of the negative electrode active material layer 34. The fibrous material 2 preferably accounts for 1% or more of the total mass of the negative electrode active material layer 34.

[0042] The thickness t (see FIG. 1) of the negative electrode active material layer 34 is, for example, 8 μm or more and 150 μm or less. The value obtained by dividing the thickness t of the negative electrode active material layer 34 by the fiber length of the fibrous material 2 is, for example, 0.4 or more and 1.0 or less. When the fiber length of the fibrous material 2 is sufficient relative to the thickness t of the negative electrode active material layer 34, the fibrous material 2 functions as a skeleton of the negative electrode active material layer 34, and cracking of the negative electrode active material layer 34 during charge and discharge can be suppressed.

[0043] The conductive additive 3 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).

[0044] <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.

[0045] 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.

[0046] <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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] <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.

[0051] 1, 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).

[0052] 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.

[0053] <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.

[0054] 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.

[0055] 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.

[0056] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material 1, a fibrous material 2, a conductive additive 3, a binder, and a solvent. Adding a dispersion stabilizer to the slurry can suppress aggregation of the negative electrode active material. The solvent is, for example, water or N-methyl-2-pyrrolidone.

[0057] 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.

[0058] 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.

[0059] 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, for example, using a roll press device or the like. By performing the rolling step, the layer thickness of the negative electrode active material layer 34 can be adjusted.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] In the lithium-ion secondary battery 100 according to the first embodiment, the negative electrode active material layer 34 contains the fibrous material 2, which makes the charge-discharge characteristics less likely to deteriorate. This is thought to be because the fibrous material 2 functions as a skeleton of the negative electrode active material layer 34, preventing the negative electrode active material layer 34 from cracking during charge and discharge. Cracks in the negative electrode active material layer 34 interrupt the conductive path within the negative electrode active material layer 34. By making the negative electrode active material layer 34 less likely to crack, the conductive path within the negative electrode active material layer 34 is maintained, and the charge-discharge characteristics of the lithium-ion secondary battery 100 are less likely to deteriorate.

[0064] 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]

[0065] "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.

[0066] The positive electrode active material is Li xCoO2 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. 2 The 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.

[0067] Next, one side of a 10 μm-thick copper foil was coated with a negative electrode slurry prepared by mixing a negative electrode active material, acicular particles, a conductive additive, a binder, and a solvent.

[0068] Silicon particles were used as the negative electrode active material. The silicon particles had an average particle size of 3.0 μm. Titanium oxide was used as the fibrous material. The fibrous material used had a fiber length of 20 μm. Carbon black was used as the conductive additive. Polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 78 parts by mass of silicon particles, 2 parts by mass of acicular particles, 5 parts by mass of the conductive additive, and 15 parts by mass of the binder were mixed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. The amount of negative electrode active material supported in the negative electrode active material layer after drying was 1.5 mg / cm. 2 The negative electrode active material layer was pressed with a roll press and then fired in a nitrogen atmosphere at 300° C. or higher for 5 hours. The thickness of the fired negative electrode active material layer was 8 μm.

[0069] 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.

[0070] (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.

[0071] (Measurement of capacity retention rate after 100 cycles) The cycle characteristics of the lithium ion secondary battery were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation).

[0072] The battery was charged at a constant current of 0.5 C (the current value at which charging is completed in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2 V, and then discharged at a constant current of 1.0 C until the battery voltage reached 2.5 V. The discharge capacity after charging and discharging was detected to determine the battery capacity Q1 before the cycle test.

[0073] The battery whose battery capacity Q1 was calculated above was again charged using a secondary battery charge / discharge tester at a constant current charge rate of 0.5 C until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was counted as one cycle, and 100 charge / discharge cycles were performed. The discharge capacity after 100 charge / discharge cycles was then measured, and the battery capacity Q2 after 100 cycles was calculated.

[0074] The capacity retention rate after 100 cycles was calculated from the capacities Q1 and Q2 calculated above. The capacity retention rate E was calculated by E=Q2 / Q1×100. The capacity retention rate of Example 1 was 90%.

[0075] The lithium ion secondary battery was then disassembled after charging and discharging to determine the rate of change in thickness of the negative electrode active material layer. The rate of change in thickness of the negative electrode active material layer was calculated by "(thickness after charging) - (thickness after discharging)" / (thickness after discharging) x 100. The rate of change in Example 1 was 55%. Furthermore, the cross section of the negative electrode active material layer after disassembly was measured to determine the distance between the negative electrode active materials. The average interparticle distance between the negative electrode active materials in Example 1 was 2.9 μm.

[0076] "Examples 2 to 44, Comparative Examples 1 to 16" Examples 2 to 44 and Comparative Examples 1 to 16 differ from Example 1 in that any one of the type of material constituting the fibrous material, the fiber length of the fibrous material, the layer thickness of the negative electrode active material layer, and the average interparticle distance between negative electrode active material particles was changed. The changes are summarized in Tables 1 to 3. The other conditions were the same as in Example 1, and the capacity retention rate and the like were measured for Examples 2 to 44 and Comparative Examples 1 to 16.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] The lithium ion secondary batteries of Examples 1-44 all have higher capacity retention rates than the lithium ion secondary batteries of Comparative Examples 1-16.

[0081] The capacity retention rates of Comparative Examples 1 to 4 and Comparative Examples 13 to 16, which used carbon fiber, were lower than those of the lithium ion secondary batteries of Examples 1 to 44. This is thought to be because carbon fiber is softer than inorganic materials such as ceramics, and the effect of reinforcing the negative electrode active material layer 34 was not sufficiently obtained.

[0082] The capacity retention rates of the lithium-ion secondary batteries of Comparative Examples 5 to 8, which used Cu whiskers, were lower than those of the lithium-ion secondary batteries of Examples 1 to 44. This is thought to be because the metal material is ductile and easily stretched, and slippage easily occurs in the crystal structure, so the effect of reinforcing the negative electrode active material layer 34 was not sufficiently obtained.

[0083] The lithium-ion secondary batteries of Comparative Examples 9 to 12, in which the size of the fibrous material was inappropriate, had lower capacity retention rates than the lithium-ion secondary batteries of Examples 1 to 44. This is thought to be because the fibrous material was unable to sufficiently function as a skeleton reinforcing the negative electrode active material layer 34. [Explanation of symbols]

[0084] 1 Negative electrode active material 2. Fibrous materials 3 Conductive additives 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 active material and a fibrous material, the negative electrode active material contains silicon, the fibrous material contains at least one selected from the group consisting of titanium oxide, potassium titanate, aluminum oxide, silicon carbide, silicon nitride, and silicon oxide; The fiber length of the fibrous material is 20 μm or more and 150 μm or less, a value obtained by dividing the layer thickness by the fiber length is 0.4 or more and 1.0 or less;

2. The negative electrode active material layer according to claim 1 , wherein the average interparticle distance of the negative electrode active material is 3.0 μm or more and 4.5 μm or less.

3. The negative electrode active material layer according to claim 1 , wherein the fibrous material comprises at least one material selected from the group consisting of titanium oxide, potassium titanate, and aluminum oxide.

4. A negative electrode comprising the negative electrode active material layer according to claim 1 .

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

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