Non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery according to the present disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains a negative electrode active material and an electrically conductive agent. The negative electrode active material contains a silicon-containing material. The silicon-containing material has a single particle fracture strength of at least 50 MPa but less than 400 MPa. The electrically conductive agent contains a fibrous carbon material, the fibrous carbon material having a length of at least 2 μm but less than 10 μm.

Description

Non-aqueous electrolyte secondary battery

[0001] The present invention relates to a non-aqueous electrolyte secondary battery.

[0002] Conventionally, a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte has been known. In such a non-aqueous electrolyte secondary battery, it is known to use a silicon-containing material as a negative electrode active material.

[0003] Patent Document 1 below describes a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a carbon material, a first silicon-containing material, and a second silicon-containing material. The first silicon-containing material includes a silicate phase and a first silicon phase dispersed in the silicate phase, and the second silicon-containing material includes a carbon phase and a second silicon phase dispersed in the carbon phase.

[0004] Patent Document 1 below describes that voids are formed around the first silicon-containing material due to expansion and contraction, or cracks occur in the first silicon-containing material due to stress caused by expansion and contraction, resulting in a decrease in the contact points between a part of the first silicon-containing material and its surroundings, making it easy for the capacity to decrease. And since the carbon phase of the second silicon-containing material has electron conductivity, even if voids are formed around the second silicon-containing material due to expansion and contraction or cracks occur in the second silicon-containing material due to stress caused by expansion and contraction, the contact points between the second silicon-containing material and its surroundings are likely to be maintained. And Patent Document 1 below describes that by replacing a part of the first silicon-containing material with the second silicon-containing material, it becomes easier to maintain the contact points with the surroundings as a whole for the silicon-containing material, so it becomes easier to suppress the capacity decrease when repeating charge and discharge cycles. That is, Patent Document 1 describes that by combining two types of silicon-containing materials with different compositions, the cycle characteristics of the secondary battery can be improved.

[0005] International Publication No. 2021 / 200343

[0006] As described above, Patent Document 1 discusses the improvement of the cycle characteristics of a secondary battery by combining two types of silicon-containing materials with different compositions, but does not fully discuss the sufficient improvement of the initial efficiency. Furthermore, in any publicly known document other than Patent Document 1, there has not yet been sufficient discussion on achieving both a sufficient improvement in the initial efficiency and a sufficient improvement in the cycle characteristics in a secondary battery using a silicon-containing material as the negative electrode active material.

[0007] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that can sufficiently improve the initial efficiency and cycle characteristics.

[0008] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode active material and a conductive agent, the negative electrode active material includes a silicon-containing material, the silicon-containing material having a single particle fracture strength of 50 MPa or more and less than 400 MPa, and the conductive agent includes a fibrous carbon material, the fibrous carbon material having a length of 2 μm or more and less than 10 μm.

[0009] According to the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery that can sufficiently improve the initial efficiency and cycle characteristics.

[0010] 1 is a cross-sectional view schematically showing a lithium secondary battery according to a first embodiment.

[0011] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0012] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0013] The present disclosure encompasses any combination of two or more features arbitrarily selected from the appended claims, i.e., any combination of two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode includes a negative electrode active material and a conductive agent. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode active material includes a silicon-containing material, and the silicon-containing material has a single particle fracture strength of 50 MPa or more and less than 400 MPa. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the conductive agent includes a fibrous carbon material, and the fibrous carbon material has a length of 2 μm or more and less than 10 μm.

[0015] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, it is important that (1) the negative electrode active material contains a silicon-containing material, and the silicon-containing material has a single particle fracture strength of 50 MPa or more and less than 400 MPa, and (2) the conductive agent contains a fibrous carbon material, and the fibrous carbon material has a length of 2 μm or more and less than 10 μm. The reasons for this will be explained below.

[0016] Because silicon-containing materials have a high lithium ion absorption capacity, nonaqueous electrolyte secondary batteries using silicon-containing materials as negative electrode active materials exhibit large battery capacities. On the other hand, silicon-containing materials undergo large expansion and contraction due to the absorption and desorption of lithium ions, which can cause mechanical degradation of the silicon-containing material. For example, the silicon-containing material in the negative electrode may crack or become isolated due to microparticulate formation. In such cases, the conductive network formed in the negative electrode is damaged, increasing the irreversible capacity. As a result, the nonaqueous electrolyte secondary battery often fails to achieve sufficient initial efficiency or sufficient cycle characteristics.

[0017] However, in the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode contains a silicon-containing material as the negative electrode active material, the silicon-containing material having a single particle fracture strength of 50 MPa or more but less than 400 MPa, i.e., a silicon-containing material having a relatively low single particle fracture strength. In other words, in the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode contains a relatively soft silicon-containing material as the negative electrode active material. Therefore, even if the silicon-containing material undergoes significant expansion and contraction due to the absorption and desorption of lithium ions, its soft properties prevent the silicon-containing material from cracking or becoming miniaturized and isolated. This prevents the conductive network formed within the negative electrode from being damaged, resulting in an increase in irreversible capacity. Furthermore, in the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode contains a relatively long fibrous carbon material, the length of which is 2 μm or more but less than 10 μm, as the conductive agent. Therefore, the negative electrode can ensure a sufficient number of contact points between the silicon-containing material and the negative electrode current collector. Therefore, even if cracks occur in the silicon-containing material or the silicon-containing material becomes isolated, the relatively long fibrous carbon material still allows a sufficient conductive network to be maintained in the negative electrode. Furthermore, since the fibrous carbon material does not have an excessive length, the number of fibers per unit mass can be increased. This ensures a sufficient number of contact points between the fibrous carbon material and the silicon-containing material. These effects enable the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure to have sufficiently improved initial efficiency and cycle characteristics.

[0018] In the secondary battery according to the embodiment of the present disclosure, a separator is preferably interposed between the positive electrode and the negative electrode. In addition, in the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte are preferably housed in a battery case. The positive electrode, the negative electrode, the separator, the non-aqueous electrolyte, and the battery case will be described below.

[0019] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a binder, and a conductive additive.

[0020] The positive electrode current collector preferably has a strip shape (long shape) in a plan view. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of materials for the positive electrode current collector include metal materials such as Al, Al alloys, Ti, Ti alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, and even more preferably 10 to 20 μm.

[0021] The positive electrode mixture layer may be formed on both main surfaces of the positive electrode current collector, or on only one main surface. When the positive electrode current collector is a porous conductive substrate as described above, the positive electrode mixture layer may be formed in a state where at least a portion of the positive electrode mixture layer is embedded in the pores of the porous substrate.

[0022] The positive electrode active material is a material that electrochemically absorbs and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered crystal structure and are classified as rock salt type.

[0023] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４, Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４ Examples of the metals include F. In the above composite oxide, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. In the above composite oxide, a, b, and c satisfy 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. The value of a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0024] As the positive electrode active material, it is preferable to use a lithium nickel composite oxide. The lithium nickel composite oxide is, for example, represented by the formula (1): Li ａ Ni ｂ M １－ｂ O ２ In formula (1), M is at least one selected from the group consisting of Mn, Co, and Al, and a and b satisfy 0<a≦1.2 and 0.3≦b<1, respectively. From the viewpoint of increasing capacity, b preferably satisfies 0.85≦b<1. From the viewpoint of stabilizing the crystal structure, the lithium nickel composite oxide contains Co and Al as M and can be represented by formula (2): Li ａ Ni ｂ Co ｃ Al ｄ O ２ In formula (2), a, b, c, and d satisfy the following conditions: 0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, and b+c+d=1.

[0025] The positive electrode active material may have an average particle size of 5 μm or more and 30 μm or less, or 10 μm or more, or 20 μm or less, or 15 μm or less.

[0026] The average particle size of the positive electrode active material is the cumulative 50% particle size (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the positive electrode active material is incorporated into the positive electrode mixture layer.

[0027] The average particle diameter of the positive electrode active material may be measured from a cross section obtained by cutting the laminate of the positive electrode mixture layer and the positive electrode current collector in the thickness direction. The cross section may be formed using a cross-section polisher (CP). In this case, the positive electrode mixture layer may be embedded in a thermosetting resin (e.g., epoxy resin). The average particle diameter from the cross section can be measured using a scanning electron microscope (SEM) image of the cross section. An SEM image can be used in which 10 or more positive electrode active materials are observed. Then, the equivalent circle diameters of the cross sections of 10 or more positive electrode active materials are determined by image processing, and the average value is calculated as the average particle diameter. Here, the equivalent circle diameter refers to the diameter of a circle having the same area as the cross section of the positive electrode active material (the area of ​​the positive electrode active material observed in the cross section of the positive electrode mixture layer). Note that the average particle diameter of the positive electrode active material measured using a particle size distribution analyzer is equivalent to the average particle diameter determined from the cross section.

[0028] Examples of the binder include resin materials. Examples of the resin material include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone and polyvinyl acetate; polyethersulfone, nitrile rubber, etc. That is, it is preferable to use a copolymer as the resin material. Note that the vinyl resin is a resin containing a vinyl group (CH ２ It is a resin obtained by polymerizing a monomer having the formula (=CH-).

[0029] The binder may be a resin material, and may be used alone or in combination of two or more kinds.

[0030] The binder may have an average particle size of 10 μm to 150 μm or less.

[0031] Like the average particle diameter of the positive electrode active material, the average particle diameter of the binder is the cumulative 50% particle diameter (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the binder is incorporated into the positive electrode mixture layer.

[0032] From the viewpoint of increasing the voltage resistance, the binder preferably contains a fluororesin as a resin material, and among fluororesins, it is preferable that the binder contains polyvinylidene fluoride (PVDF).

[0033] As the conductive aid, for example, a conductive carbonaceous material can be used. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes (CNT), graphite, etc. Examples of carbon black include acetylene black, ketjen black, etc. The conductive aid may be used alone or in combination of two or more.

[0034] The positive electrode can be obtained, for example, by applying a slurry containing the components of the positive electrode mixture layer and a dispersion medium onto a positive electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the positive electrode mixture layer include a positive electrode active material, a binder, and a conductive additive.

[0035] (Negative Electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material and a conductive agent. By including such a negative electrode mixture layer, the negative electrode includes a negative electrode active material and a conductive agent. The negative electrode mixture layer may further include a binder.

[0036] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include metal materials such as Ni, Ni alloys, Cu, Cu alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0037] The negative electrode mixture layer may be formed on both main surfaces of the negative electrode current collector, or may be formed on only one main surface. When the negative electrode current collector is a porous conductive substrate as described above, the negative electrode mixture layer may be formed in a state where at least a portion of the layer is embedded in the pores of the porous conductive substrate.

[0038] In the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the negative electrode includes a negative electrode active material, as described above, and the negative electrode active material includes a silicon-containing material. Since silicon-containing materials have a higher lithium ion absorption capacity than carbon materials such as graphite, the capacity of the non-aqueous electrolyte secondary battery can be increased by including the silicon-containing material in the negative electrode active material.

[0039] The silicon-containing material may be silicon particles. The silicon particles are not particularly limited, but preferably have a particle size of 1 nm or more and 10 μm or less. In particular, nano-sized silicon particles are preferred from the viewpoint of reducing the non-uniformity of the lithium ion concentration and reducing particle collapse. That is, the silicon particles are preferably nanosilicon particles. The particle size of the silicon particles is preferably, for example, 500 nm or less. By having the particle size within the above range, the difference in lithium ion concentration between the surface and interior of the silicon particles is reduced, making it difficult for non-uniform volume expansion to occur in the silicon particles. This significantly reduces particle collapse and significantly suppresses the capacity decrease of the lithium ion battery. The smaller the particle size of the silicon particles, the more preferable, but it is not necessary to reduce it to less than 1 nm; 1 nm or more is sufficient, and it may be 3 nm or more.

[0040] The particle size of silicon particles can be determined by determining the particle sizes of any 100 silicon particles observed in a high-resolution transmission electron microscope (HR-TEM) image and arithmetically averaging them. The silicon particles observed in the HR-TEM image may be primary particles or secondary particles. There is no need to distinguish between primary particles and secondary particles. The particle size of a silicon particle can be calculated as the diameter of an equivalent circle having an area equal to the area of ​​the silicon particle obtained in the HR-TEM image.

[0041] The silicon-containing material is preferably a composite material including an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase. The ion-conducting phase is also referred to as a matrix phase, and the silicon phase is also referred to as a domain phase. That is, the silicon-containing material may be a composite material having a sea-island structure. The ion-conducting phase may be composed of a material having lithium ion conductivity. The ion-conducting phase is preferably at least one selected from the group consisting of a silicate phase, a silicide phase, a carbon phase, and a silicon oxide phase. In such a composite material, the silicon contained in the silicon phase reversibly forms an alloy with lithium. Therefore, such a composite material can reversibly absorb and release lithium ions.

[0042] The silicate phase contains at least one element M selected from the group consisting of Li, Na, K, Mg, Ca, B, and Al. Among these, the silicate phase preferably contains at least Li as the element M. That is, the silicate phase is preferably a lithium silicate phase. If the element M contained in the silicate phase is an element other than Li, the element other than Li may be replaced by Li during charging and discharging of a nonaqueous electrolyte secondary battery including a negative electrode containing the composite material, and Li ions may be irreversibly captured in the silicate phase, resulting in a decrease in capacity. However, when the element M contains Li as described above, such a decrease in capacity during charging and discharging can be suppressed.

[0043] The silicate phase is composed of, for example, the element M, Si, and O. From the viewpoint of reducing irreversible capacity and chemical stability, the silicate phase is composed of, for example, Li４ SiO ４ , Li ２ SiO ３ , and Li ２ Si ２ O ５ Among these, from the viewpoint of reducing the irreversible capacity in the silicate phase, the silicate phase may contain at least one selected from the group consisting of Li ２ Si ２ O ５ It is preferred that the compound contains:

[0044] When the silicate phase is particulate, the average particle diameter of the silicate phase may be 10 nm or more and 100 μm or less, or 100 nm or more and 10 μm or less. The shape of the particles is not particularly limited. Furthermore, when the maximum diameter of the particulate silicate phase is A and the maximum width in the direction perpendicular to the maximum diameter A is B, the ratio of A to B (A / B) may be 1 or more and 20 or less, 1 or more and 10 or less, 1 or more and 5 or less, or 1 or more and 3 or less. A / B may be determined by arbitrarily selecting 10 particulate silicate phases, obtaining A / B for each, and then arithmetically averaging the obtained 10 A / Bs. The smaller the average particle diameter of the silicate phase, the more advantageous it is in terms of alleviating the stress of expansion and contraction during charge and discharge.

[0045] The average particle size of the silicate phase can be measured using a cross-sectional image of the composite particles obtained by SEM. The average particle size of the silicate phase can be determined by measuring the maximum diameter of any 100 silicate phase particles and then arithmetically averaging the obtained maximum diameters of the 100 particles.

[0046] The composition of the silicate phase can be determined by scanning electron microscope (SEM)-energy dispersive X-ray (EDX) analysis. In SEM-EDX analysis, a cross section of the composite material is observed using an SEM, and elemental mapping analysis is performed using EDX. Since the silicate phase has a relatively high degree of crystallinity, the composition of the silicate phase may also be confirmed by X-ray diffraction (XRD).

[0047] The silicide phase is a phase composed of silicide. The silicide phase may contain a mixture of five or more types of silicide. The silicide phase may contain, for example, an intermetallic compound of silicon and a metal element Me other than silicon, and the metal element Me may contain N or more types of constituent elements Mi (where i = 1 to N and 5 ≤ N).

[0048] When the mole fraction of N or more constituent elements Mi is Ci (where ΣCi = 1), the condition 1.5<-ΣCi x lnCi may be satisfied. In this case, the entropy S representing the state of N or more constituent elements Mi is expressed as S = -RΣCi x lnCi, where S > 1.5R. In other words, when the above condition is satisfied, the N or more constituent elements Mi are in a high-entropy state, where R is the gas constant (J / (mol·K)).

[0049] The N or more constituent elements Mi of the metal element Me are in a high-entropy state, which significantly increases the mechanical strength of the silicide phase, thereby significantly suppressing the volume change of the composite material when the silicon phase undergoes a large volume change due to the insertion and desorption of lithium ions.

[0050] The silicon content in the composite material may be, for example, greater than 50 atomic %, 70 atomic % or more, or 75 atomic % or more. The higher the silicon content in the composite material, the greater the capacity. On the other hand, the content of the metal element Me other than silicon in the composite material may be 10 atomic % or more, or even 20 atomic % or more. This allows a significant amount of silicide phase to be present in the composite material, thereby further significantly suppressing volume change of the composite material. When the atomic ratio of Si to Me is Si:Me = 1:1, it is considered that most of the composite material will be in the silicide phase, resulting in a small capacity. On the other hand, when Si:Me = (3-4):1, a sufficient amount of silicon phase is present in the composite material, resulting in a relatively large capacity (e.g., 1380 mAh / g to 1640 mAh / g).

[0051] The average atomic weight (average atomic weight) of the N or more constituent elements Mi is, for example, 60 or less. The smaller the average atomic weight, the higher the capacity density per mass of the composite material, making it useful as a negative electrode active material. If the average atomic weight is 60 or less, the capacity density of the composite material will be higher than, for example, the capacity density of SiO (silicon oxide). On the other hand, from the viewpoint of improving charge / discharge efficiency, the average atomic weight is preferably, for example, 50 or more.

[0052] The average atomic weight is expressed as ΣAi×Ci, where Ai is the atomic weight of N or more constituent elements Mi and Ci is the molar fraction.

[0053] The N or more constituent elements Mi are MiSi and MiSi ２ In other words, the silicide phase may be contained in at least one of MeSi, MeSi ２ It may be composed of an intermetallic compound such as

[0054] It is preferable that substantially all of the N or more constituent elements Mi form intermetallic compounds. In this case, the total content of the N or more constituent elements Mi that do not form intermetallic compounds (in other words, the N or more constituent elements Mi that exist as simple substances) in the composite material is 1 mass% or less or 10,000 ppm or less, and may be substantially 0 mass%. This can be confirmed, for example, by X-ray diffraction analysis of the composite material.

[0055] The N or more constituent elements Mi may all have atomic weights of elements in the fourth period or lower of the long-form periodic table. In this case, the average atomic weights of the N or more constituent elements Mi are relatively small, thereby increasing the capacity density of the composite material.

[0056] The N or more constituent elements Mi may be, for example, at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo. All of the N or more constituent elements Mi may have an atomic weight of 29 (Cu) or less. In this case, the average atomic weight of the N or more constituent elements Mi becomes even smaller, and the capacity density of the composite material can be further increased. All of the N or more constituent elements Mi may be transition metals. The N or more constituent elements Mi may be, for example, at least five selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. In this case, the charge / discharge efficiency of the composite material can be improved.

[0057] The composition of the composite material can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP). For example, a sample of the composite material is completely dissolved in a heated acid solution (e.g., a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid) to obtain a solution, and the residue in the solution is removed by filtration to obtain a filtrate. The filtrate is then analyzed by ICP to measure the spectral intensity of each element. Subsequently, a calibration curve is prepared using commercially available standard solutions of the elements, and the content of each element contained in the composite material is calculated based on this calibration curve.

[0058] The presence of a silicide phase and a silicon phase in a composite material can be confirmed by imaging and observing a cross section of the composite material using a scanning electron microscope (SEM). The cross section of the composite particle can be observed, for example, by forming a cured thermosetting resin filled with particles of the composite material, obtaining a cross section of the cured product using a cross section polisher (CP), and then observing the cross section with an SEM. Furthermore, by performing elemental mapping analysis using energy dispersive X-rays (EDX) on the cross section of the particles of the composite material, both quantitative and qualitative analysis of the silicide phase and the silicon phase can be performed.

[0059] The carbon phase may contain crystalline carbon (graphite) or amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, non-graphitizable carbon, easily graphitizable carbon, or other.

[0060] Examples of carbon sources that can be used include sugars, water-soluble resins, etc. Examples of carbon sources that can be used include carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose.

[0061] The silicon oxide phase contains Si and O. The silicon oxide phase is silicon dioxide (SiO ２ ) phase.

[0062] When the silicon-containing material is the above-mentioned composite material, the composite material may be composed of any one of the following (a) to (d): (a) a silicon phase and silicon dioxide (SiO ) in which the silicon phase is dispersed; ２ (b) a structure including a silicon phase and a lithium silicate phase in which the silicon phase is dispersed (second composite material); (c) a structure including a silicon phase and a carbon phase in which the silicon phase is dispersed (third composite material); and (d) a structure including a silicon phase and a silicide phase in which the silicon phase is dispersed (fourth composite material).

[0063] When the silicon-containing material is the first composite material (case (a) above), there is an advantage that the volume change accompanying the absorption and desorption of lithium ions is small. It is presumed that one of the reasons for this advantage is that the silicon dioxide phase has a relatively large number of sites that irreversibly trap lithium ions, making it difficult for the first composite material to shrink in volume accompanying the desorption of lithium ions.

[0064] The first composite material can be synthesized, for example, by heating silicon oxide, which is a raw material, in a non-oxidizing atmosphere (inert atmosphere) to cause a disproportionation reaction.

[0065] When the silicon-containing material is the second composite material (case (b) above), the advantage of being able to reduce the irreversible capacity is obtained. Therefore, when the second composite material is used as the silicon-containing material, excellent charge / discharge efficiency can be obtained. This effect is particularly noticeable in the early stages of charge / discharge.

[0066] The lithium silicate phase contained in the second composite material may contain elements other than Si, O, and Li. Such elements may be at least one selected from the group consisting of Group 1 elements (other than Li) and Group 2 elements of the long form periodic table. The Group 1 elements and Group 2 elements may be, for example, K, Na, Mg, Ca, Sr, Ba, etc. The lithium silicate phase may also contain Al, B, La, P, Zr, Ti, Fe, Cr, Ni, Mn, Cu, Mo, Zn, etc.

[0067] The ratio of the number of O atoms to the number of Si atoms in the lithium silicate phase (O / Si) is, for example, greater than 2 and less than 4. In this case, in addition to being advantageous in terms of the stability of the lithium silicate phase, it is also advantageous in terms of lithium ion conductivity. The O / Si ratio may be greater than 2 and less than 3. The ratio of the number of Li atoms to the number of Si atoms in the lithium silicate phase (Li / Si) is, for example, greater than 0 and less than 4.

[0068] The lithium silicate, which is the raw material for obtaining the lithium silicate phase, has the formula Li ２ｚ SiO ２＋ｚ (0<z<2). It is preferable that z satisfies 0<z<1. When z is in this range, the stability of the lithium silicate is increased and the lithium silicate is easily produced. Furthermore, when the lithium silicate is made into a lithium silicate phase, the lithium ion conductivity can be increased. It is more preferable that z is 1 / 2.

[0069] The second composite material can be obtained, for example, by mixing and stirring the raw materials, lithium silicate and silicon, while crushing them in a mixer such as a ball mill to obtain a mixture, and then firing the mixture under pressure in an inert atmosphere. Note that the second composite material may also be obtained by heating the mixture to a predetermined temperature, necking at least one of the lithium silicate and silicon in the mixture to obtain a sintered body, and then pulverizing the sintered body.

[0070] Even when the silicon-containing material is the third composite material (case (c) above), the advantage of being able to reduce irreversible capacity is obtained. Furthermore, the carbon phase exhibits capacity through a Faraday reaction with lithium ions, which is advantageous in achieving high capacity. In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, the silicon-containing material is preferably the third composite material. That is, the silicon-containing material is preferably a composite material including a carbon phase and a silicon phase dispersed within the carbon phase.

[0071] The third composite material can be obtained in the same manner as the second composite material, except that a carbon source and silicon are used as raw materials. The carbon source and silicon may be dispersed in an organic solvent such as alcohol to obtain the mixture.

[0072] In the third composite material, the carbon phase may be formed of a porous carbon material. In this case, the third composite material may be a composite material having a carbon phase formed of the porous carbon material and a silicon phase dispersed in the carbon phase.

[0073] The porous carbon material may be any porous carbon material having a plurality of pores, but is preferably a carbon material having many mesopores with a pore diameter of 2 nm to 50 nm. Mesopores are suitable for disposing nano-sized silicon particles therein. The more mesopores a porous carbon material has, the easier it is to dispose more silicon particles in the pores. The volume ratio of mesopores to the total pore volume of the porous carbon material is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more. Pores are classified according to their pore diameter into micropores (<2 nm), mesopores (2 nm to 50 nm), and macropores (>50 nm). The volume ratio of mesopores to the total pore volume can be determined from the pore size distribution of the porous carbon material.

[0074] As the porous carbon material, for example, activated carbon can be used. The porous carbon material may be hard carbon or soft carbon as described above. It is preferable that the porous carbon material has many mesopores. An example of a commercially available porous carbon material having many mesopores is Knobel (registered trademark), a porous carbon manufactured by Toyo Tanso Co., Ltd. It is preferable that the porous carbon material is at least one of hard carbon and soft carbon. In this case, the carbon phase is formed by at least one of hard carbon and soft carbon.

[0075] The composite material having a carbon phase formed by a porous carbon material and a silicon phase dispersed within this carbon phase as described above can be obtained through the steps of (i) preparing a precursor composite including a porous carbon material and siloxane disposed within a plurality of pores of this porous carbon material, and (ii) bringing magnesium vapor into contact with the precursor composite to reduce the siloxane disposed within the pores to silicon.

[0076] In the step (i), siloxane may be produced from an organosilicon compound within a plurality of pores of the porous carbon material.

[0077] For example, a precursor composite containing a porous carbon material and a siloxane can be obtained by mixing a porous carbon material with an organosilicon compound having a hydrolyzable functional group, hydrolyzing the functional group to obtain a hydrolysis product, and then dehydrating and condensing the hydrolysis product. Hydrolysis of the functional group generates hydroxyl groups (—OH). Dehydration and condensation of the hydroxyl groups between molecules of the hydrolysis product forms siloxane bonds (Si—O—Si), resulting in a siloxane compound.

[0078] The conditions for the hydrolysis reaction and dehydration condensation reaction of the organosilicon compound are not particularly limited. The hydrolysis reaction and dehydration condensation reaction may be carried out in a liquid phase or a gas phase.

[0079] When the hydrolysis reaction and dehydration condensation reaction are carried out in a liquid phase, the functional groups of the organosilicon compound can be hydrolyzed, for example, by preparing a dispersion containing a porous carbon material, an organosilicon compound, an organic solvent, and water, and stirring the dispersion at room temperature to 80° C. When preparing the dispersion, it is preferable to mix the porous carbon material, the organic solvent, and water, and allow the mixed liquid of the organic solvent and water to fully penetrate into the pores of the porous carbon material, and then add the organosilicon compound. Thereafter, the organic solvent and the silicon compound remaining in the organic solvent are removed by centrifugation, filtration, or the like, and the solid content is recovered.

[0080] As the organic solvent, for example, toluene can be used. The organic solvent is not particularly limited as long as it has a high affinity with the porous carbon material and can dissolve a small amount of water. In the dispersion, the amount of the organosilicon compound relative to the porous carbon material may be selected depending on the amount of silicon phase to be contained in the pores. In the dispersion, the amount of the organic solvent relative to the porous carbon material is not particularly limited, as long as the surface of the porous carbon material is sufficiently wetted with the organic solvent. In the dispersion, the amount of water relative to the porous carbon material is also not particularly limited, as long as the water molecules sufficiently penetrate into the pores of the porous carbon material.

[0081] The recovered solids are then heated or dried to promote the dehydration condensation reaction of the hydrolysis product to produce siloxanes. For example, by heating the dried solids under reduced pressure, the hydrolysis product is dehydration condensed, thereby accelerating the siloxane production reaction.

[0082] When the hydrolysis reaction and dehydration condensation reaction are carried out in a gas phase, for example, a porous carbon material may be exposed to the vapor of an organosilicon compound having a hydrolyzable functional group and water vapor to allow molecules of the organosilicon compound and water molecules to penetrate into the pores of the porous carbon material, and then the porous carbon material may be heated. This heating promotes the hydrolysis reaction of the functional group of the organosilicon compound within the pores of the porous carbon material to obtain a hydrolysis product, and then the dehydration condensation reaction can be promoted in this hydrolysis product.

[0083] For example, in the gas phase, a porous carbon material is first placed in a reaction chamber under reduced pressure, and the organosilicon compound vapor is brought into contact with the porous carbon material in the reaction chamber, thereby allowing the organosilicon compound to penetrate into the pores of the porous carbon material. Next, the organosilicon compound is evacuated from the reaction chamber, and water vapor is introduced into the reaction chamber under reduced pressure, allowing water molecules to penetrate into the pores of the porous carbon material. By performing this operation one or more times, and optionally heating, a precursor composite containing a porous carbon material and a siloxane can be obtained.

[0084] Although the type of organosilicon compound is not particularly limited, silicon compounds having a hydrolyzable functional group, such as alkoxysilanes and chlorosilanes, are preferred from the viewpoint of ease of reduction to silicon. Among these, alkoxysilanes are preferred because of their high stability and ease of handling. The alkoxysilane may or may not contain a Si-C bond (an alkyl group directly bonded to a silicon atom).

[0085] Examples of alkoxysilanes that do not contain Si-C bonds include tetramethyl orthosilicate (Si(OCH ３ ) ４ ), tetraethyl orthosilicate (Si(OC ２ H ５ ) ４ ), tetrapropyl orthosilicate (Si(OC ３ H ７ ) ４ ), tetrabutyl orthosilicate (Si(OC ４ H ９ ) ４ ) etc.

[0086] Alkoxysilanes containing Si-C bonds include methyltriethoxysilane (CH ３ Si(OC ２ H ５ ) ３ ), 3-aminopropylmethyldiethoxysilane (H ２ N (CH ２ ) ３ Si(CH ３ ) (OC ２ H ５ ) ２) etc.

[0087] Since Si—C bonds are difficult to break at temperatures below 700° C., the Si—C bonds may remain unbroken during the reduction reaction in step (ii). Therefore, it is more preferable that the alkoxysilane does not contain a Si—C bond. By using an alkoxysilane that does not contain a Si—C bond, siloxane that does not contain a Si—C bond can be obtained with high efficiency. Siloxane that does not contain Si—C is easily reduced to silicon, and silicon can be obtained with high efficiency.

[0088] In step (ii) of reducing siloxane to silicon, the precursor composite obtained in step (i) is heated in a non-oxidizing atmosphere containing magnesium vapor. The reduction of siloxane with magnesium vapor produces nano-sized silicon particles (nano-silicon particles). This results in the formation of a silicon phase within the pores of the porous carbon material. Magnesium oxide, silicon oxide, and silicon carbide are also produced. This allows the silicon phase formed by the nano-silicon particles, silicon oxide, and silicon carbide to be arranged within the pores of the porous carbon material.

[0089] The silicon oxide and silicon carbide function to shield the silicon phase from the non-aqueous electrolyte. Even if gaps are formed between the silicon phase and the carbon phase in the pores or the silicon phase is damaged, resulting in the formation of an active surface of the silicon phase, the silicon oxide and silicon carbide prevent the active surface of the silicon phase from coming into contact with the non-aqueous electrolyte. This prevents the formation of SEI, further suppressing the decrease in capacity of the non-aqueous electrolyte secondary battery.

[0090] The silicon oxide may be present in a state of close contact with the silicon phase, or may be present integrally with the silicon phase. The silicon oxide may be in a particulate form or in a matrix form. For example, the silicon phase may be present in a matrix formed by the silicon oxide. From the viewpoint of alleviating stress due to expansion and contraction of the silicon phase, at least a portion of the silicon oxide may be present inside the silicon phase. The silicon carbide may be present in a state of close contact with the porous carbon material or the silicon phase, or may be present integrally with the porous material or the silicon phase. The silicon carbide may be in a particulate form or in the form of a film interposed between the porous carbon material and the silicon phase. The silicon carbide may form a matrix together with the silicon oxide. From the viewpoint of alleviating stress due to expansion and contraction of the silicon phase, at least a portion of the silicon carbide may be present inside the silicon phase.

[0091] The non-oxidizing atmosphere may be a vacuum, a reduced pressure atmosphere, or an inert gas atmosphere. The inert gas may be a rare gas such as argon gas, or nitrogen. In an oxidizing atmosphere, the generated magnesium vapor is oxidized to form magnesium oxide (MgO) or magnesium dioxide (MgO ２ ) is generated, the reduction of siloxane may not proceed sufficiently.

[0092] The melting point of magnesium is 650°C. Magnesium has an extremely high vapor pressure (372 Pa) near its melting point. When the precursor composite obtained in step (i) is heated in the presence of magnesium vapor, a chemical reaction represented by the following formula (1) occurs, reducing siloxane to silicon. Magnesium oxide (MgO) is also produced at this time. The higher the vapor pressure of magnesium, the more efficient the reduction reaction.

[0093] 2Mg + -(O-Si-O)- → 2MgO + Si...(1)

[0094] To reduce the siloxane located in the pores of the porous carbon material with magnesium vapor, the precursor composite obtained in step (i) is mixed with magnesium and the mixture is heated. The mixture of the precursor composite and magnesium is heated in a container made of graphite, stainless steel, or the like. The form of the magnesium mixed with the precursor composite is not particularly limited as long as it can generate magnesium vapor. For example, magnesium in the form of powder, particles, ribbons, rods, pellets, or the like can be used.

[0095] Although the temperature to which the mixture of the precursor composite and magnesium is heated is not particularly limited, the higher the temperature, the higher the vapor pressure of magnesium. However, at temperatures significantly higher than the melting point of magnesium, molten magnesium will aggregate, reducing the surface area of ​​the magnesium that evaporates. The heating temperature is preferably, for example, near the melting point of magnesium.

[0096] The amount of magnesium relative to the precursor composite may be appropriately selected so that most of the magnesium can be vaporized and so that the inclusion of metallic magnesium in the composite material can be reduced.

[0097] In step (ii), it is preferable to remove the magnesium oxide disposed in the pores of the porous carbon material. The magnesium oxide can be removed, for example, by dissolving the magnesium oxide using an aqueous solution containing an acid or an ammonium salt, and then eluting the magnesium oxide from the pores. Magnesium oxide produced near the melting point of magnesium is easily dissolved in an aqueous solution containing an acid or an ammonium salt. By removing the magnesium oxide from the pores, the space within the pores that accommodates the expansion of silicon particles can be increased. This significantly reduces the stress applied to the porous carbon material.

[0098] The acid may be an inorganic acid such as sulfuric acid, boric acid, phosphoric acid, hydrochloric acid, or nitric acid, or an organic acid such as acetic acid, oxalic acid, succinic acid, or malonic acid. The ammonium salt may be ammonium chloride.

[0099] The fourth composite material can be obtained by weighing raw material metal ingots of each element to a predetermined atomic ratio, melting the metal ingots of each element to form a molten metal, and then cooling the molten metal. The melting of the metal ingots of each element is preferably carried out using an arc melting furnace. The arc melting furnace is preferably filled with an inert atmosphere such as argon gas. By cooling the molten metal, a fourth composite material having a desired composition can be obtained as a metal ingot.

[0100] In the metal block of the fourth composite material, the crystallization of the silicide (intermetallic compound) may not have progressed sufficiently. －３ It is preferable to anneal the metal block of the fourth composite material by heating it in a reduced pressure or vacuum atmosphere of 100 Pa or less. The heating temperature and heating time during annealing are not particularly limited. The heating temperature during annealing may be, for example, 600°C or higher and 900°C or lower. The heating time during annealing may be, for example, 10 hours or higher and 120 hours or lower.

[0101] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, the silicon-containing material has a single particle fracture strength of 50 MPa or more and less than 400 MPa. That is, the silicon-containing material has a relatively low single particle fracture strength. In other words, the silicon-containing material is relatively soft. By having the single particle fracture strength within the above numerical range, i.e., by being relatively soft, even if the silicon-containing material expands and contracts due to the absorption and release of lithium ions in the silicon phase, the silicon-containing material can be prevented from becoming finer due to this expansion and contraction. This prevents the silicon-containing material from becoming isolated in the negative electrode due to the reduction in size of the silicon-containing material, and also prevents the isolation of the fibrous carbon material (e.g., carbon nanotubes) that serves as the conductive agent. Therefore, the initial efficiency and cycle characteristics can be further improved. The single particle fracture strength of the silicon-containing material can be measured using a commercially available measuring device (e.g., a microcompression tester (MCT-W201) manufactured by Shimadzu Corporation) according to the following procedure. In the following, the silicon-containing material will be referred to as a composite material.

[0102] - Procedure: (1) Disperse the composite material on the lower pressure plate of the measuring device. (2) Select a composite particle with a size close to the average particle diameter D50 as a sample while observing with an optical microscope. (3) Use a diamond flat indenter with a diameter of 50 μm as the upper indenter, and ensure that only one particle selected as the sample exists between this upper indenter and the lower pressure plate. (4) Slowly lower the upper indenter to contact the sample, and further lower the upper indenter at a constant acceleration from the time of contact with the sample to apply a load to the sample. Note that the time when the upper indenter contacts the sample can be determined by the change in the descending speed of the upper indenter. Also, the constant acceleration is a displacement speed of 2.7 mN / sec. (5) Obtain the relationship between the load and the deformation amount of the sample, and take the point where the deformation amount of the sample changes rapidly (the inflection point of the load - deformation amount profile) as the fracture point, and calculate the fracture strength based on the following formula (1) from the load and the particle diameter at that time. The fracture strength is obtained by calculating the arithmetic mean of the measured values for 5 samples.

[0103] St = 2.8×P / (π×d ２ ) ······ (1) Here, St is the fracture strength (the unit is MPa or N / mm ２ ), P is the load (the unit is N), and d is the particle diameter (mm).

[0104] In the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the silicon-containing material preferably has an average particle diameter D50 of 4 μm or more and 12 μm or less, and a specific surface area of 2.0 m ２ / g or more and 6.0 m ２ / g or less. When the average particle diameter D50 of the silicon-containing material is within the above numerical range, the stress generated in the silicon-containing material due to the volume change of the silicon-containing material during charge and discharge of the non-aqueous electrolyte secondary battery is easily relaxed. As a result, cracks are less likely to occur in the silicon-containing material, so good cycle characteristics are easily obtained. When the specific surface area of the silicon-containing material is within the above numerical range, the side reaction occurring between the silicon-containing material and the non-aqueous electrolyte can be reduced. As a result, it is possible to suppress the decrease in the capacity of the non-aqueous electrolyte secondary battery caused by this side reaction.

[0105] The average particle diameter D50 of the silicon-containing material can be determined as the cumulative 50% particle diameter (median diameter) in the volume-based particle size distribution using a laser diffraction / scattering particle size distribution analyzer, as in the case of the positive electrode active material. The average particle diameter of the silicon-containing material can also be measured from a cross section cut in the thickness direction of the laminate of the negative electrode mixture layer and the negative electrode current collector, as in the case of the positive electrode active material. Measurement of the average particle diameter from the cross section can be performed using a scanning electron microscope (SEM) image of the cross section. The average particle diameter of the silicon-containing material measured using a particle size distribution analyzer and the average particle diameter determined from the cross section are equivalent values.

[0106] The specific surface area of ​​the silicon-containing material can be measured by a gas adsorption method (BET single-point method). Nitrogen gas is used as the gas. Details of the BET method may be in accordance with JIS R1626.

[0107] In the diffraction spectrum obtained by X-ray diffraction, the silicon-containing material preferably has a peak attributable to the (111) plane of silicon at a diffraction angle 2θ of approximately 28.0°, and the full width at half maximum of this peak is 4.0° or more. X-ray diffraction can be performed using Cu-Kα radiation. Furthermore, as a measuring device, for example, a multipurpose X-ray diffractometer (e.g., a product name "Empyrean" manufactured by Malvern) can be used.

[0108] In the diffraction spectrum obtained by X-ray diffraction, the peak appearing at a diffraction angle 2θ of approximately 28.0° is a peak attributed to the (111) plane of silicon, as described above. That is, the peak appearing at a diffraction angle 2θ of approximately 28.0° corresponds to the crystalline Si contained in the silicon phase of the silicon-containing material. Furthermore, the fact that the full width at half maximum of this first peak is a large value of 4.0° or more, i.e., the first peak is broad, indicates that the size of the Si crystallites is small. Furthermore, if the size of the Si (silicon) crystallites is small, even if the Si contained in the silicon phase of the silicon-containing material repeatedly expands and contracts by absorbing and releasing lithium ions during charging and discharging of the non-aqueous electrolyte secondary battery, excessive miniaturization of the Si is suppressed. This suppresses the deterioration of cycle characteristics in the non-aqueous electrolyte secondary battery caused by excessive miniaturization of the Si.

[0109] The negative electrode mixture layer may contain a carbon-based material as the negative electrode active material in addition to the silicon-containing material. Examples of the carbon-based material include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among the above carbon materials, graphite is preferred because it has excellent charge / discharge stability and can reduce irreversible capacity.

[0110] Graphite is a carbon material having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of graphite as measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more to 300 nm or less, or 10 nm or more to 200 nm or less.

[0111] When a silicon-containing material and graphite are used in combination as the negative electrode active material, the mass ratio MRSi of the silicon-containing material in the negative electrode active material is preferably, for example, 1 mass% or more and 20 mass% or less. Furthermore, the mass ratio MRC of the graphite in the negative electrode active material is preferably, for example, 80 mass% or more and 94 mass% or less. MRC may be 84 mass% or more, or 88 mass% or more. MRC may be 92 mass% or less, or 90 mass% or less. By having MRSi and MRC within the above numerical ranges, improved cycle characteristics and high capacity can be achieved in a well-balanced manner. On the other hand, MRSi may be more than 20 mass%, for example, 50 mass% or more. Furthermore, MRSi may be less than 1 mass%, for example, 0.5 mass% or less.

[0112] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, the negative electrode mixture layer contains a conductive agent, and the conductive agent contains a fibrous carbon material. Examples of the fibrous carbon material include vapor grown carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofibers. From the viewpoint of exhibiting high conductivity, it is preferable to use carbon nanotubes (CNT) as the fibrous carbon material.

[0113] Examples of carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Among these, single-walled carbon nanotubes are preferred because they can further suppress the deterioration of the charge-discharge cycle characteristics of non-aqueous electrolyte secondary batteries. A single-walled carbon nanotube is a cylindrical carbon nanostructure composed of a single graphene sheet. A double-walled carbon nanotube is a cylindrical carbon nanostructure composed of two graphene sheets stacked concentrically. A multi-walled carbon nanotube is a cylindrical carbon nanostructure composed of three or more graphene sheets stacked concentrically. A graphene sheet refers to a layer in which carbon atoms in the sp2 hybrid orbitals constituting a graphite crystal are located at the vertices of a regular hexagon. Carbon nanotubes typically have a cylindrical tube shape. Carbon nanotubes may also have a spirally wound coil shape.

[0114] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, the fibrous carbon material has a length (average length) of 2 μm or more and less than 10 μm, as described above. When the length of the fibrous carbon material is within the above numerical range, sufficient contact between the negative electrode active material and the negative electrode current collector can be ensured. Therefore, even if cracks occur in the silicon-containing material contained in the negative electrode active material or the silicon-containing material becomes isolated, a sufficient conductive network can still be maintained in the negative electrode by using a fibrous carbon material having a length within the above numerical range. Furthermore, when the length of the fibrous carbon material is within the above numerical range, i.e., the fibrous carbon material does not have an excessive length, an increase in the resistance of the fibrous carbon material due to its length can be suppressed. In other words, the conductivity of the fibrous carbon material can be improved. The fibrous carbon material may be present in the negative electrode mixture layer in the form of a bundle of multiple fibrous carbon materials. In such cases, the length and diameter of the fibrous carbon material refer to the length and diameter of a single fibrous carbon material (a single fibrous carbon material) present in the bundle of fibrous carbon materials.

[0115] The average length of the fibrous carbon material is determined by image analysis using a scanning electron microscope (SEM). The average length of the fibrous carbon material is obtained by arbitrarily selecting 100 fibrous carbon materials, measuring their lengths, and calculating the arithmetic mean thereof. Also, the length refers to the length when the fibrous carbon material is extended linearly.

[0116] The fibrous carbon material preferably has a diameter (average diameter) of 1 nm or more and 4 nm or less. That is, the fibrous carbon material preferably has a relatively small diameter. Thereby, it is possible to suppress an increase in the resistance of the fibrous carbon material due to the large diameter. That is, the conductivity of the fibrous carbon material can be further increased. The average diameter of the fibrous carbon material is determined by image analysis using a transmission electron microscope (TEM). The average diameter of the fibrous carbon material can be measured by the following method. First, 100 fibrous carbon materials are arbitrarily selected, and the diameter (outer diameter) at an arbitrary one location of each is measured. Then, the average diameter is obtained by calculating the arithmetic mean of the measured diameters.

[0117] The fibrous carbon material preferably has a ratio (G / D) of the intensity of the G band to the intensity of the D band in the Raman scattering spectrum of 20 or more, having the diameter as described above. The Raman scattering spectrum can be obtained, for example, by microscopic Raman spectroscopy. Microscopic Raman spectroscopy can be carried out, for example, using the product name "NRS-5100" manufactured by JASCO Corporation as a measuring device, using laser light with a wavelength of 532 nm, and under the condition that the laser intensity is 4.7 mW. The wavenumber range is 100 cm －１ ~2000 cm －１ and set as such.

[0118] The D band is a peak recognized in the range of a wavenumber of 1350 cm －１ or more and 1360 cm －１ or less in the Raman scattering spectrum, and is a peak derived from defects in the graphite structure. The G band is a peak in the Raman scattering spectrum at a wavenumber of 1580 cm －１ or more and 1590 cm －１The peaks observed in the range below are peaks derived from the graphite structure. From this, it can be said that the higher the G / D of the fibrous carbon material, the fewer defects in the graphite structure and the higher the crystallinity. Therefore, when the G / D is 20 or more, the fibrous carbon material exhibits sufficiently high electrical conductivity. Therefore, when the fibrous carbon material has a diameter of 1 nm or more and a G / D of 20 or more, as described above, sufficiently high conductivity is achieved. This allows the nonaqueous electrolyte secondary battery to exhibit even better cycle characteristics.

[0119] The negative electrode mixture layer may contain a conductive agent other than the fibrous carbon material. Examples of conductive agents other than the fibrous carbon material include carbon materials such as carbon black (CB), acetylene black (AB), and ketjen black. The conductive agent may contain 50% by mass or more of the fibrous carbon material, 70% by mass or more, or 90% by mass or more. The conductive agent may also contain 100% by mass of the fibrous carbon material. That is, the entire conductive agent may be the fibrous carbon material.

[0120] Examples of binders include resin materials. Examples of resin materials include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0121] The negative electrode mixture layer may contain a conductive additive in addition to the negative electrode active material and binder. As the conductive additive, a conductive carbonaceous material can be used, as described for the positive electrode. Furthermore, in the negative electrode, in addition to the conductive carbonaceous material, metal fibers, metal powder such as aluminum, and the like can also be used. The conductive additive may be used alone or in combination of two or more.

[0122] The negative electrode mixture layer may contain a thickener as needed. Examples of the thickener include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modifications, methyl cellulose, and the like. Examples of modified CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0123] The negative electrode can be obtained, for example, by applying a slurry containing the components of the negative electrode mixture layer and a dispersion medium onto a negative electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the negative electrode mixture layer include a negative electrode active material, a binder, a conductive additive, and a thickener.

[0124] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of the form of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. The separator may be made of a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The separator may contain an additive as needed. Examples of the additive include an inorganic filler.

[0125] The separator may include multiple layers that differ in at least one of form and composition, such as a laminate of polyethylene and polypropylene microporous films, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0126] (Non-aqueous electrolyte) The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) contains a solvent (nonaqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives may be added to the non-aqueous electrolyte.

[0127] As the solvent, various known organic solvents can be used, such as cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, cyclic ethers, fluorinated chain ethers, and fluorinated cyclic ethers.

[0128] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0129] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0130] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0131] Examples of the chain carboxylic acid ester include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0132] Examples of chain ethers include dimethyl ether, ethyl methyl ether, diethyl ether, ethyl propyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, and o-dimethoxybenzene. The chain ether may be a chain ether having two or more ether bonds. Examples of such chain ethers include 1,1-dimethoxymethane, 1,1-diethoxyethane, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol ethyl methyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, and tetraethylene glycol ethyl methyl ether.

[0133] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0134] The fluorinated chain ether has a structure in which one or more hydrogen atoms of the chain ethers described above are substituted with fluorine atoms. Examples of the fluorinated chain ether include bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0135] Fluorinated cyclic ethers have a structure in which one or more hydrogen atoms of the above-mentioned cyclic ethers are substituted with fluorine atoms. An example of the fluorinated cyclic ether is 3,3,4,4-tetrafluorotetrahydrofuran.

[0136] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0137] Examples of the lithium salt include lithium salts of chlorine-containing acids, lithium salts of fluorine-containing acids, lithium salts of fluorine-containing acid imides, lithium halides, and lithium salts containing oxalate complexes. Examples of the lithium salts of chlorine-containing acids include LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ Examples of lithium salts of fluorine-containing acids include LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ Examples of lithium salts of fluorine-containing acid imides include LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Examples of lithium halides include LiCl, LiBr, and LiI. Examples of lithium salts containing oxalate complexes include LiB(C ２ O ４ ) ２, LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LIPF ２ (C ２ O ４ ) ２ The above lithium salts may be used alone or in combination of two or more.

[0138] The concentration of the lithium salt in the liquid nonaqueous electrolyte (nonaqueous electrolytic solution) may be 1 mol / L or more and 5 mol / L or less, or 1 mol / L or more and 3 mol / L or less. By setting the lithium salt concentration within the above range, a liquid nonaqueous electrolyte (nonaqueous electrolytic solution) having excellent ionic conductivity and appropriate viscosity can be obtained.

[0139] The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) may contain various known additives. Examples of such additives include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, ethylene sulfite (ES), etc. Note that cyclic carbonates such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which are exemplified as solvents, may also function as additives.

[0140] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.). Examples of the polymer electrolyte include a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0141] (Battery Case) The battery case includes, for example, a cylindrical case body with a bottom and a sealing body that seals the opening of the case body. The case body may be made of metal. A gasket may be disposed between the case body and the sealing body. Disposing the gasket can ensure the hermeticity of the battery case.

[0142] A specific configuration of a secondary battery according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Note that, hereinafter, the secondary battery according to an embodiment of the present disclosure will be simply referred to as a secondary battery according to a first embodiment.

[0143] FIG. 1 is a longitudinal cross-sectional view schematically illustrating a nonaqueous electrolyte secondary battery (lithium secondary battery 10) according to a first embodiment. The lithium secondary battery 10 is a cylindrical battery. The lithium secondary battery 10 includes a cylindrical battery case (battery can), a wound electrode group 14 housed within the battery case, and a nonaqueous electrolyte (not shown). The battery case includes a cylindrical case body 15 with a bottom and a sealing member 16 that seals the opening of the case body 15. The case body 15 is made of metal. A gasket 27 is disposed between the case body 15 and the sealing member 16. The gasket 27 ensures the hermeticity of the battery case. The case body 15, the sealing member 16, and the gasket 27 form an exterior body. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis.

[0144] The case body 15 has a step portion 21. The step portion 21 supports the sealing body 16. The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The above-mentioned components constituting the sealing body 16 are electrically connected to each other except for the insulating member 24. The cap 26 functions as a positive electrode terminal. The case body 15 functions as a negative electrode terminal.

[0145] The electrode group 14 is a wound electrode group composed of a positive electrode 11, a negative electrode 12, and a separator 13. In the lithium secondary battery 10, the negative electrode 12 is configured as described above. The positive electrode 11, the separator 13, and the non-aqueous electrolyte can be configured as described above.

[0146] The positive electrode 11 is electrically connected to the cap 26 via a positive electrode lead 19. One end of the positive electrode lead 19 is connected to the positive electrode 11. The other end of the positive electrode lead 19 is connected to the sealing body 16 (filter 22). The negative electrode 12 is electrically connected to the case body 15 via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to the negative electrode 12. The other end of the negative electrode lead 20 is connected to the case body 15.

[0147] In the above example, a nonaqueous electrolyte secondary battery has been described in which an electrode group wound into a substantially cylindrical shape is housed in a cylindrical battery case, but the configuration of the nonaqueous electrolyte secondary battery is not limited to this. The nonaqueous electrolyte secondary battery may also be one in which a wound electrode group is housed in a prismatic battery case. Furthermore, the nonaqueous electrolyte secondary battery may also be one in which a stacked electrode group is housed in a battery case such as a film exterior (e.g., a pouch).

[0148] (Additional Notes) The above description discloses the following technologies. (Technology 1) A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material and a conductive agent, wherein the negative electrode active material comprises a silicon-containing material, wherein the silicon-containing material has a single particle fracture strength of 50 MPa or more and less than 400 MPa, and the conductive agent comprises a fibrous carbon material, wherein the fibrous carbon material has a length of 2 μm or more and less than 10 μm. (Technology 2) The non-aqueous electrolyte secondary battery according to Technology 1, wherein the fibrous carbon material has a diameter of 1 nm or more and 4 nm or less, and wherein the ratio of the G band intensity to the D band intensity (G / D) in a Raman scattering spectrum is 20 or more. (Technology 3) The non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the silicon-containing material is a composite material comprising a carbon phase and a silicon phase dispersed within the carbon phase. (Technology 4) The silicon-containing material has an average particle diameter D50 of 4 μm or more and 12 μm or less, and ２ / g or more 6.0m ２The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 4, wherein the silicon-containing material has a peak at a diffraction angle 2θ of about 28.0°, which is attributed to a silicon (111) plane, in a diffraction spectrum obtained by X-ray diffraction, and the full width at half maximum of the peak is 4.0° or more.

[0149] While the present invention has been described in terms of presently preferred embodiments, such disclosure should not be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0150] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0151] [Example 1] (1) Preparation of Positive Electrode A positive electrode active material, a conductive additive, and a binder were mixed in a mass ratio of 98:1.4:0.6 to obtain a positive electrode mixture. An appropriate amount of N-methyl-2-pyrrolidone (dispersion medium) was added to this positive electrode mixture and stirred to obtain a positive electrode mixture slurry. As the positive electrode active material, lithium nickel composite oxide (Li １．０５ Ni ０．８０ Co ０．１５ O ２ ) was used, carbon black was used as the conductive additive, and polyvinylidene fluoride (PVDF) was used as the binder.

[0152] The positive electrode mixture slurry according to Example 1 was applied to both sides of an aluminum foil (positive electrode current collector, thickness 15 μm) to form a coating film, and the coating film was dried. The dried coating film was then compressed in the thickness direction using a roller to form a positive electrode mixture layer on the positive electrode current collector. Then, a laminate (positive electrode laminate) of the positive electrode current collector and the positive electrode mixture layer was cut to a predetermined size. In this way, a positive electrode according to Example 1 was obtained.

[0153] (2) Preparation of Negative Electrode A negative electrode mixture was obtained by mixing a negative electrode active material, a conductive agent, a binder, and a thickener. The negative electrode active material, the binder, and the thickener were mixed in a mass ratio of 98:1:1, and 0.020 parts by mass of the conductive agent was added per 100 parts by mass of the negative electrode active material. An appropriate amount of water (dispersion medium) was added to this negative electrode mixture and stirred to obtain a negative electrode mixture slurry. A mixture of graphite (Gr) and a silicon-containing material (SiC) was used as the negative electrode active material. SiC is a composite material having a carbon phase and a silicon phase dispersed within the carbon phase, and is the third composite material described above. In the negative electrode active material, the mass ratio of Gr to SiC was Gr:SiC = 90:10. Carbon nanotubes (CNTs), a fibrous carbon material, were used as the conductive agent. Styrene-butadiene copolymer rubber (SBR) was used as the binder, and carboxymethyl cellulose (CMC) was used as the thickener.

[0154] For the SiC of Example 1, the value of the single particle fracture strength (unit: MPa), the value of the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° (the peak attributable to the (111) plane of silicon) in the diffraction spectrum obtained by X-ray diffraction, the value of the average particle diameter D50 (unit: μm), and the value of the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Example 1 is also referred to as first SiC. For the CNTs of Example 1, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below. The measured values ​​in Tables 1 and 2 were obtained according to the method described in the embodiment section above.

[0155] (3) Preparation of Electrolyte Solution Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:3 (EC:DMC), and vinylene carbonate (VC) was added to obtain a non-aqueous mixed solvent. LiPF was then added to the non-aqueous mixed solvent. ６ The VC content in the non-aqueous electrolyte was 5 mass %. ６The concentration of VC was set to 1.5 mol / L. VC functions as a non-aqueous solvent and also functions as an additive for forming a good SEI coating on the surface of Gr contained in the negative electrode active material.

[0156] (4) Fabrication of a Non-Aqueous Electrolyte Secondary Battery One end of an aluminum positive electrode lead was attached to a positive electrode current collector (aluminum foil) by welding. One end of a nickel negative electrode lead was attached to a negative electrode current collector (copper foil) by welding. Then, in an inert gas atmosphere, the positive electrode and negative electrode were stacked with a separator interposed therebetween to obtain a laminate (electrode laminate), which was then wound to obtain a wound electrode group. A polyethylene microporous film was used as the separator.

[0157] Next, as shown in Fig. 1 , a wound electrode group (electrode group 14) and a nonaqueous electrolyte (nonaqueous electrolytic solution) were housed in a cylindrical battery case to complete a nonaqueous electrolyte secondary battery. As shown in Fig. 1 , in the wound electrode group (electrode group 14), the positive electrode 11 was connected to the sealing body 16 via a positive electrode lead 19, and the negative electrode 12 was connected to the case body 15 via a negative electrode lead 20. The other end of the positive electrode lead 19 was connected to the sealing body 16, and the other end of the negative electrode lead 20 was connected to the case body 15. As described in the embodiment section, the cap 26 provided on the sealing body 16 functioned as a positive electrode terminal, and the case body 15 functioned as a negative electrode terminal.

[0158] [Example 2] A nonaqueous electrolyte secondary battery according to Example 2 was completed in the same manner as in Example 1, except that the content ratio of the conductive additive (fibrous carbon material) in the negative electrode mixture was changed to 0.010 parts by mass relative to 100 parts by mass of the negative electrode active material. The SiC of Example 2 was measured for its single particle fracture strength (unit: MPa), the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the average particle diameter D50 (unit: μm), and the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Example 2 was also the first SiC. For the CNTs of Example 2, the length (unit: μm), diameter (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0159] [Example 3] A nonaqueous electrolyte secondary battery according to Example 3 was completed in the same manner as in Example 1, except that the content ratio of the conductive additive (fibrous carbon material) in the negative electrode mixture was changed to 0.015 parts by mass relative to 100 parts by mass of the negative electrode active material. The SiC of Example 3 was measured for its single particle fracture strength (unit: MPa), the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the average particle diameter D50 (unit: μm), and the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Example 3 was also the first SiC. For the CNTs of Example 3, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0160] [Example 4] A nonaqueous electrolyte secondary battery according to Example 4 was completed in the same manner as in Example 1, except that the content ratio of the conductive additive (fibrous carbon material) per 100 parts by mass of the negative electrode active material in the negative electrode mixture was changed to 0.022 parts by mass. The SiC of Example 4 was measured for the single particle fracture strength (unit: MPa), the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the average particle diameter D50 (unit: μm), and the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Example 4 was also the first SiC. For the CNTs of Example 4, the length (unit: μm), diameter (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0161] [Example 5] A nonaqueous electrolyte secondary battery according to Example 5 was completed in the same manner as in Example 1, except that the content ratio of the conductive additive (fibrous carbon material) in the negative electrode mixture was changed to 0.025 parts by mass relative to 100 parts by mass of the negative electrode active material. The SiC of Example 5 was measured for the single particle fracture strength (unit: MPa), the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the average particle diameter D50 (unit: μm), and the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Example 5 was also the first SiC. For the CNTs of Example 5, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0162] Example 6 A nonaqueous electrolyte secondary battery according to Example 6 was completed in the same manner as in Example 1, except that SiC was used as the silicon-containing material in the negative electrode mixture, the single particle fracture strength, the full width at half maximum of the peak at a diffraction angle 2θ of approximately 28.0° (attributed to the (111) plane of silicon), the average particle diameter D50, and the specific surface area exhibiting the values ​​shown in Table 1 below. The SiC used in Example 6 is also referred to as second SiC. For the CNTs of Example 6, the length (unit: μm), diameter (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added (unit: parts by mass) per 100 parts by mass of the negative electrode active material are shown in Table 2 below.

[0163] Example 7 A nonaqueous electrolyte secondary battery according to Example 7 was completed in the same manner as in Example 2, except that SiC was used as the silicon-containing material in the negative electrode mixture, the single particle fracture strength, the full width at half maximum of the peak at a diffraction angle 2θ of approximately 28.0° (attributed to the (111) plane of silicon), the average particle diameter D50, and the specific surface area exhibiting the values ​​shown in Table 1 below. The SiC used in Example 7 was also the second SiC. For the CNTs of Example 7, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added (unit: parts by mass) per 100 parts by mass of the negative electrode active material are shown in Table 2 below.

[0164] Example 8 A nonaqueous electrolyte secondary battery according to Example 8 was completed in the same manner as in Example 5, except that SiC was used as the silicon-containing material in the negative electrode mixture, the single particle fracture strength, the full width at half maximum of the peak at a diffraction angle 2θ of approximately 28.0° (the peak attributable to the (111) plane of silicon), the average particle diameter D50, and the specific surface area exhibiting the values ​​shown in Table 1 below. The SiC used in Example 8 was also the second SiC. For the CNTs of Example 8, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added (unit: parts by mass) per 100 parts by mass of the negative electrode active material are shown in Table 2 below.

[0165] Comparative Example 1 A nonaqueous electrolyte secondary battery according to Comparative Example 1 was completed in the same manner as in Example 1, except that SiO was used as the silicon-containing material in the negative electrode mixture. SiO was silicon dioxide (SiO ２ The SiO of Comparative Example 1 is a composite material comprising a silicon dioxide phase and a silicon phase dispersed in the silicon dioxide phase, and is the first composite material described above. The SiO of Comparative Example 1 is a composite material having a single particle fracture strength (unit: MPa), a full width at half maximum (unit: °) of a peak at a diffraction angle 2θ of approximately 28.0° in a diffraction spectrum obtained by X-ray diffraction, an average particle diameter D50 (unit: μm), and a specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 1, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0166] Comparative Example 2 A nonaqueous electrolyte secondary battery according to Comparative Example 2 was completed in the same manner as in Example 2, except that SiO was used as the silicon-containing material in the negative electrode mixture. For the SiO of Comparative Example 2, the value of single particle fracture strength (unit: MPa), the value of full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the value of average particle diameter D50 (unit: μm), and the value of specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 2, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity (G / D) in the Raman scattering spectrum, and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0167] Comparative Example 3 A nonaqueous electrolyte secondary battery according to Comparative Example 3 was completed in the same manner as in Example 3, except that SiO was used as the silicon-containing material in the negative electrode mixture. For the SiO of Comparative Example 3, the value of single particle fracture strength (unit: MPa), the value of full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the value of average particle diameter D50 (unit: μm), and the value of specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 3, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0168] Comparative Example 4 A nonaqueous electrolyte secondary battery according to Comparative Example 4 was completed in the same manner as in Example 4, except that SiO was used as the silicon-containing material in the negative electrode mixture. For the SiO of Comparative Example 4, the value of single particle fracture strength (unit: MPa), the value of full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the value of average particle diameter D50 (unit: μm), and the value of specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 4, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity (G / D) in the Raman scattering spectrum, and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0169] Comparative Example 5 A nonaqueous electrolyte secondary battery according to Comparative Example 5 was completed in the same manner as in Example 5, except that SiO was used as the silicon-containing material in the negative electrode mixture. For the SiO of Comparative Example 5, the value of single particle fracture strength (unit: MPa), the value of full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the value of average particle diameter D50 (unit: μm), and the value of specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 5, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity (G / D) in the Raman scattering spectrum, and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0170] [Comparative Example 6] A nonaqueous electrolyte secondary battery according to Comparative Example 6 was completed in the same manner as in Comparative Example 1, except that carbon nanotubes (CNTs) having a length of 1.8 μm, a diameter of 1.1 nm, and a G / D ratio of 81 were used as the conductive additive in the negative electrode mixture. For the SiO of Comparative Example 6, the value of single particle fracture strength (unit: MPa), the value of full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the value of average particle diameter D50 (unit: μm), and the value of specific surface area (unit: m ２ / g) are shown in Table 1 below. For the CNTs of Comparative Example 6, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity (G / D) in the Raman scattering spectrum, and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0171] Comparative Example 7 A nonaqueous electrolyte secondary battery according to Comparative Example 7 was completed in the same manner as in Comparative Example 6, except that SiC was used as the silicon-containing material in the negative electrode mixture. The SiC of Comparative Example 7 was measured for its single particle fracture strength (unit: MPa), the full width at half maximum (unit: °) of the peak at a diffraction angle 2θ of approximately 28.0° in the diffraction spectrum obtained by X-ray diffraction, the average particle diameter D50 (unit: μm), and the specific surface area (unit: m ２ / g) are shown in Table 1 below. The SiC used in Comparative Example 7 was SiC No. 1. For the CNTs of Comparative Example 7, the length value (unit: μm), diameter value (nm), ratio of the G band intensity to the D band intensity in the Raman scattering spectrum (G / D), and the amount of CNT added per 100 parts by mass of the negative electrode active material (unit: parts by mass) are shown in Table 2 below.

[0172]

[0173]

[0174] <Charge-Discharge Test> A charge-discharge cycle test was conducted on the nonaqueous electrolyte secondary batteries according to each example (Examples 1 to 8 and Comparative Examples 1 to 7). The charge-discharge cycle test was conducted by repeating 1,000 cycles, where one cycle consisted of charging the nonaqueous electrolyte secondary battery according to each example under the charging conditions below, resting for 20 minutes, and then discharging under the discharging conditions below. The charge-discharge cycle test was conducted while the nonaqueous electrolyte secondary battery according to each example was left in a constant temperature bath at 25°C.

[0175] ・Charging: Constant current charging is performed at a current of 0.2 C until the battery voltage reaches 4.1 V, and then constant voltage charging is performed at a voltage of 4.1 V until the charging current reaches 0.02 C. ・Discharging: Constant current charging is performed at a current of 0.2 C until the battery voltage reaches 3.0 V.

[0176] (Initial Efficiency) The initial efficiency of the nonaqueous electrolyte secondary battery according to each example was evaluated after one cycle of charge / discharge testing. Specifically, the initial efficiency of the nonaqueous electrolyte secondary battery according to each example was calculated using the following formula. In the formula, A is the charge capacity of the nonaqueous electrolyte secondary battery at the first cycle, and B is the discharge capacity of the nonaqueous electrolyte secondary battery at the first cycle. Initial efficiency (%) = B / A × 100

[0177] (Cycle Retention Rate) The cycle retention rate of the nonaqueous electrolyte secondary battery according to each example subjected to the charge / discharge test was evaluated. Specifically, the capacity retention rate of the nonaqueous electrolyte secondary battery according to each example was calculated using the following formula, and the calculated value was taken as the cycle retention rate. In the formula, E0 is the battery capacity of the nonaqueous electrolyte secondary battery measured before the charge / discharge cycle test, and E1 is the battery capacity of the nonaqueous electrolyte secondary battery measured after the charge / discharge cycle test (after 1000 cycles). Capacity retention rate (%) = E1 / E0 × 100

[0178] Table 3 below shows the ratios RIEa (unit: %) of the initial efficiencies of the nonaqueous electrolyte secondary batteries according to Examples 1 to 8 and Comparative Example 7 to the initial efficiency of the nonaqueous electrolyte secondary battery according to Example 1, and the ratios RCEa (unit: %) of the cycle retention ratios of the nonaqueous electrolyte secondary batteries according to Examples 1 to 8 and Comparative Example 7 to the cycle retention ratio of the nonaqueous electrolyte secondary battery according to Example 1. Table 4 below shows the ratios RIEb (unit: %) of the initial efficiencies of the nonaqueous electrolyte secondary batteries according to Comparative Example 1 to the nonaqueous electrolyte secondary battery according to Comparative Example 1, and the ratios RCEb (unit: %) of the cycle retention ratios of the nonaqueous electrolyte secondary batteries according to Comparative Example 1 to the nonaqueous electrolyte secondary battery according to Comparative Example 1.

[0179]

[0180]

[0181] Table 3 shows that no decrease in the initial efficiency ratio RIEa is observed in Examples 2 to 5. Similarly, it is also seen that no decrease in the initial efficiency ratio RIEa is observed in Examples 6 to 8. In contrast, Table 3 shows that a decrease in the initial efficiency ratio RIEa is observed in Comparative Example 7, and Table 4 shows that a decrease in the initial efficiency ratio RIEb is observed in Comparative Examples 2 to 6. Furthermore, Tables 3 and 4 show that the cycle retention ratio RCEa is generally high in Examples 2 to 5, and furthermore, the cycle retention ratio RCEa is also generally high in Examples 6 to 8, whereas the cycle retention ratio RCEa of Comparative Example 7 and the cycle retention ratio RCEb of Comparative Examples 2 to 6 are generally low.

[0182] The non-aqueous electrolyte secondary battery according to the present disclosure can be used in applications that require sufficient improvement in initial efficiency and cycle characteristics.

[0183] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group 15: Case body 16: Sealing body 27: Gasket

Claims

1. A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, and a non-aqueous electrolyte; the negative electrode comprises a negative electrode active material and a conductive agent; the negative electrode active material comprises a silicon-containing material; the silicon-containing material has a single particle fracture strength of 50 MPa or more and less than 400 MPa; the conductive agent comprises a fibrous carbon material; and the fibrous carbon material has a length of 2 μm or more and less than 10 μm.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon material has a diameter of 1 nm or more and 4 nm or less, and a ratio of the G band intensity to the D band intensity (G / D) in a Raman scattering spectrum of 20 or more.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the silicon-containing material is a composite material comprising a carbon phase and a silicon phase dispersed within the carbon phase.

4. The silicon-containing material has an average particle diameter D50 of 4 μm or more and 12 μm or less, and ２ / g or more 6.0m ２ The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the nonaqueous electrolyte secondary battery has a specific surface area of ​​0.15 to 0.25 μm / g or less.

5. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the silicon-containing material has a peak attributable to the (111) plane of silicon at a diffraction angle 2θ of approximately 28.0° in a diffraction spectrum obtained by X-ray diffraction, and the full width at half maximum of the peak is 4.0° or more.

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