Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery according to the present disclosure comprises: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte. The negative electrode includes a negative electrode active material and a binder, the negative electrode active material containing a first silicon-containing material and a second silicon-containing material. The first silicon-containing material and the second silicon-containing material are both composite materials provided with a carbon phase and a silicon phase dispersed in the carbon phase. In each of the outer surfaces of the first silicon-containing material and the second silicon-containing material, the abundance ratio PSi of a silicon element, the abundance ratio PC of a carbon element, and the abundance ratio PO of an oxygen element satisfy a prescribed relationship. In the negative electrode active material, a content ratio R1 of the first silicon-containing material and a content ratio R2 of the second silicon-containing material satisfy a prescribed relationship.

Description

Non-aqueous electrolyte secondary battery

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

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been known that include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In such non-aqueous electrolyte secondary batteries, it is known to use a silicon-containing material as the negative electrode active material.

[0003] The following Patent Document 1 describes a nonaqueous electrolyte secondary battery including a positive electrode plate having a positive electrode mixture layer containing a positive electrode active material capable of absorbing and releasing lithium ions, a negative electrode plate having a negative electrode mixture layer containing a negative electrode active material capable of absorbing and releasing lithium ions, a separator, and a nonaqueous electrolyte. The following Patent Document 1 also describes that in the nonaqueous electrolyte secondary battery, a negative electrode active material is used that is a mixture of at least one of metallic silicon and silicon oxide represented by SiOx (0.5≦x<1.6) with a graphite material, wherein the graphite material contains 20% by mass or more and 90% by mass or less of a graphite material coated with amorphous carbon, and the content of the metallic silicon and the silicon oxide is 1% by mass or more and 20% by mass or less of the total negative electrode active material.

[0004] The following Patent Document 1 describes that the battery capacity of a non-aqueous electrolyte secondary battery is increased by including at least one of metallic silicon and silicon oxide represented by SiOx as the negative electrode active material. The following Patent Document 1 also describes that the graphite material contains 20% by mass or more and 90% by mass or less of a graphite material coated with amorphous carbon, thereby suppressing decomposition of the non-aqueous electrolyte solution and decomposition of the reduced coating on the negative electrode during a period of storage after initial charging. The following Patent Document 1 also describes that this enables the non-aqueous electrolyte secondary battery to simultaneously suppress battery swelling and deterioration of cycle performance. Furthermore, the following Patent Document 1 describes that the content of metallic silicon and silicon oxide is 1% by mass or more and 20% by mass or less of the total negative electrode active material, thereby suppressing an increase in the rate of decomposition of the reduced coating on the negative electrode during a period of storage after initial charging, thereby suppressing not only battery swelling but also deterioration of cycle performance.

[0005] Patent No. 6030070

[0006] In recent years, there has been an increasing demand for higher capacity and improved cycle characteristics in non-aqueous electrolyte secondary batteries.

[0007] However, in any of the known documents including Patent Document 1, it is difficult to say that sufficient research has been conducted yet into achieving a more satisfactory balance between high capacity and improved cycle characteristics for non-aqueous electrolyte secondary batteries.

[0008] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that can more fully achieve both high capacity and improved cycle characteristics.

[0009] One aspect of the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material and a binder, the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material, the first silicon-containing material and the second silicon-containing material both being composite materials comprising a carbon phase and a silicon phase dispersed within the carbon phase, and the abundance ratio of silicon element is defined as PSi, the abundance ratio of carbon element is defined as PC, and the abundance ratio of oxygen element is defined as PO on the outer surface of the first silicon-containing material and the outer surface of the second silicon-containing material. the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) at the outer surface of the first silicon-containing material is 0.01 or more and 0.10 or less, and the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) at the outer surface of the second silicon-containing material is 0.15 or more and 0.20 or less, and in the negative electrode active material, when a content ratio of the first silicon-containing material is R1 and a content ratio of the second silicon-containing material is R2, the ratio of R2 to R1 + R2 (R2 / (R1 + R2)) is 0.20 or more and 0.80 or less.

[0010] According to the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery that can more fully achieve both high capacity and improved cycle characteristics.

[0011] 1 is a cross-sectional view schematically showing a lithium secondary battery according to a first embodiment.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] [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, a separator interposed between the positive electrode and the 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 binder. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode active material includes a first silicon-containing material and a second silicon-containing material, and both the first silicon-containing material and the second silicon-containing material are composite materials including a carbon phase and a silicon phase dispersed within the carbon phase.

[0016] In the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, when the abundance ratio of silicon element is PSi, the abundance ratio of carbon element is PC, and the abundance ratio of oxygen element is PO on the outer surface of the first silicon-containing material and the outer surface of the second silicon-containing material, the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) on the outer surface of the first silicon-containing material is 0.01 or more and 0.10 or less, and the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) on the outer surface of the second silicon-containing material is 0.15 or more and 0.20 or less.

[0017] In the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, when the content ratio of the first silicon-containing material in the negative electrode active material is R1 and the content ratio of the second silicon-containing material is R2, the ratio of R2 to R1+R2 (R2 / (R1+R2)) is 0.20 or more and 0.80 or less.

[0018] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, it is important that (1) when the abundance ratio of silicon, carbon, and oxygen on the outer surfaces of the first and second silicon-containing materials is PSi, PC, and PO, respectively, the ratio PSi / (PSi + PC + PO) is 0.01 to 0.10 on the outer surface of the first silicon-containing material and PSi / (PSi + PC + PO) is 0.15 to 0.20 on the outer surface of the second silicon-containing material, and (2) when the content ratio of the first silicon-containing material is R1 and the content ratio of the second silicon-containing material is R2, the ratio R2 / (R1 + R2) is 0.20 to 0.80 in the negative electrode active material. The reasons for this are explained below.

[0019] In nonaqueous electrolyte secondary batteries, if a silicon-containing material with a high abundance ratio of silicon at its outer surface is used as the negative electrode active material, the silicon present at the outer surface is high, resulting in a high capacity, but the binding strength between the silicon-containing materials and between the silicon-containing material and the negative electrode current collector due to the binder is reduced. This reduced binding strength between the silicon-containing materials or between the silicon-containing material and the negative electrode current collector results in a decrease in cycle performance. On the other hand, if a silicon-containing material with a low abundance ratio of silicon at its outer surface is used as the negative electrode active material, the silicon present at the outer surface is low, resulting in a decrease in the binding strength between the silicon-containing materials and between the silicon-containing material and the negative electrode current collector due to the binder. This reduces the decrease in cycle performance, but results in a decrease in capacity.

[0020] However, in the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, both a first silicon-containing material having a moderately low silicon content at the outer surface, i.e., PSi / (PSi + PC + PO) of 0.01 to 0.10, and a second silicon-containing material having a moderately high silicon content at the outer surface, i.e., PSi / (PSi + PC + PO) of 0.15 to 0.20, are used as the negative electrode active material. Furthermore, the content ratios of the first silicon-containing material and the second silicon-containing material are appropriately balanced so that R2 / (R1 + R2) is 0.20 to 0.80. Therefore, the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure achieves high capacity while suppressing a decrease in the binding strength between the silicon-containing materials and between the silicon-containing material and the negative electrode current collector due to the binder. In other words, high capacity and improved cycle characteristics can be more fully achieved.

[0021] 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.

[0022] (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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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 ４Examples 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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-).

[0032] The binder may be a resin material, and may be used alone or in combination of two or more kinds.

[0033] The binder may have an average particle size of 10 μm to 150 μm or less.

[0034] 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.

[0035] 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).

[0036] 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.

[0037] 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.

[0038] (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 binder. In other words, the negative electrode includes a negative electrode active material and a binder.

[0039] 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.

[0040] 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.

[0041] As described above, the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure includes a first silicon-containing material and a second silicon-containing material as negative electrode active materials. Both the first silicon-containing material and the second silicon-containing material are composite materials including a carbon phase and a silicon phase dispersed within the carbon phase. In the first silicon-containing material and the second silicon-containing material, the carbon phase functions as an ion-conducting phase and has lithium ion conductivity. The carbon phase functioning as an 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, both the first silicon-containing material and the second silicon-containing material may be composite materials having a sea-island structure. In such a composite material, 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 silicon phase preferably contains at least one selected from the group consisting of elemental silicon, silicon alloys, and silicon oxides. Examples of silicon alloys include silicon-tin alloys, silicon-lithium alloys, and silicon-germanium alloys. Examples of silicon oxides include SiOx (where 0<x≦2).

[0043] 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 (hard carbon), easily graphitizable carbon (soft carbon), or other. The carbon phase is preferably formed from at least one of hard carbon and soft carbon. Both hard carbon and soft carbon have a microscopic graphite-like structure, and this structure is randomly arranged to form an amorphous structure overall. Therefore, when the carbon phase is at least one of hard carbon and soft carbon, such a carbon phase can exhibit good conductivity due to the graphite-like structure and exhibits the characteristics of small expansion and contraction during charge and discharge. Therefore, when the silicon-containing material contains such a carbon phase, the non-aqueous electrolyte secondary battery exhibits high cycle performance.

[0044] 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.

[0045] The first silicon-containing material and the second silicon-containing material can be obtained, for example, by mixing and crushing the raw carbon source and silicon using a mixer such as a ball mill to obtain a mixture, and then firing the mixture under pressure in an inert gas atmosphere. The composite material may also be obtained by heating the mixture to a predetermined temperature, necking the silicon in the mixture to obtain a sintered body, and then pulverizing the sintered body. When obtaining the first silicon-containing material and the second silicon-containing material, the abundance ratio PSi of silicon element and the abundance ratio PC of carbon element at the outer surface can be adjusted by adjusting the amount of the raw carbon source and silicon. For example, increasing the amount of silicon can increase the abundance ratio PSi of silicon element at the outer surface, and increasing the amount of carbon source can increase the abundance ratio PC of carbon element at the outer surface.

[0046] In the first silicon-containing material and the second silicon-containing material, the carbon phases are preferably both formed of a porous carbon material. The silicon contained in the silicon phase undergoes significant expansion and contraction by absorbing and releasing lithium ions during charging and discharging of the nonaqueous electrolyte secondary battery. Therefore, when the silicon phase is disposed within multiple pores of the porous carbon material, the space within the pores of the porous carbon material can serve as a space that alleviates the expansion of the silicon contained in the silicon phase. Therefore, even if the silicon contained in the silicon phase undergoes significant expansion and contraction during charging and discharging of the nonaqueous electrolyte secondary battery, the application of large stress to the porous carbon material is suppressed. This makes it less likely that damage such as cracks will occur in the porous carbon material, thereby suppressing deterioration of the conductive paths due to such damage. As a result, it is possible to suppress a decrease in capacity and a decrease in cycle characteristics in the nonaqueous electrolyte secondary battery.

[0047] The porous carbon material may be any porous carbon material having a plurality of pores, but is preferably a carbon material having many micropores and mesopores with pore diameters of 1 nm to 10 nm. Micropores and mesopores are suitable for disposing nano-sized silicon particles therein. The more micropores and mesopores a porous carbon material has, the easier it is to dispose more silicon particles in the pores. The volume ratio of micropores and 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 micropores and mesopores to the total pore volume can be determined from the pore size distribution of the porous carbon material.

[0048] 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.

[0049] 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.

[0050] In the step (i), siloxane may be produced from an organosilicon compound within a plurality of pores of the porous carbon material.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 and the abundance ratio PSi of the silicon element at the outer surface of the porous carbon material. For example, the greater the amount of the organosilicon compound, the greater the amount of silicon phase to be contained in the pores, and the higher the abundance ratio PSi of the silicon element at the outer surface of the porous carbon material. Furthermore, the smaller the amount of the organosilicon compound, the smaller the amount of silicon phase to be contained in the pores, and the lower the abundance ratio PSi of the silicon element at the outer surface of the porous carbon material. 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 water molecules sufficiently penetrate into the pores of the porous carbon material.

[0055] 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.

[0056] When the hydrolysis reaction and the dehydration condensation reaction are carried out in a gas phase, for example, a porous carbon material is exposed to the vapor of an organosilicon compound having a hydrolyzable functional group and water vapor, allowing the molecules of the organosilicon compound and water molecules to penetrate into the pores of the porous carbon material, and then the porous carbon material is heated. This heating promotes the hydrolysis reaction of the functional group of the organosilicon compound in 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. As described above, when the hydrolysis reaction and the dehydration condensation reaction are carried out in a gas phase, the greater the amount of vapor of the organosilicon compound, the greater the amount of silicon phase contained in the pores, and the higher the abundance ratio PSi of the silicon element at the outer surface of the porous carbon material. Furthermore, the smaller the amount of vapor of the organosilicon compound, the smaller the amount of silicon phase contained in the pores, and the lower the abundance ratio PSi of the silicon element at the outer surface of the porous carbon material.

[0057] 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.

[0058] 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).

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 2Mg + -(O-Si-O)- → 2MgO + Si...(1)

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] In the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, when the abundance ratio of silicon element is PSi, the abundance ratio of carbon element is PC, and the abundance ratio of oxygen element is PO at the outer surface of the first silicon-containing material and the outer surface of the second silicon-containing material, PSi / (PSi+PC+PO) is 0.01 or more and 0.10 or less at the outer surface of the first silicon-containing material, and PSi / (PSi+PC+PO) is 0.15 or more and 0.20 or less at the outer surface of the second silicon-containing material. That is, the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure uses, as the negative electrode active material, both a first silicon-containing material having an appropriately low abundance ratio at its outer surface and a second silicon-containing material having an appropriately high abundance ratio at its outer surface.

[0074] Furthermore, in the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, when the content ratio of the first silicon-containing material in the negative electrode active material is R1 and the content ratio of the second silicon-containing material is R2, R2 / (R1+R2) is 0.20 or more and 0.80 or less. That is, in the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, the content ratio of the first silicon-containing material and the content ratio of the second silicon-containing material in the negative electrode active material are appropriately balanced.

[0075] Therefore, in the nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, a high capacity is realized, and a decrease in the binding strength between the silicon-containing materials and between the silicon-containing material and the negative electrode current collector due to the binder can be suppressed. In other words, a high capacity and improved cycle characteristics can be more fully achieved at the same time.

[0076] In the first silicon-containing material and the second silicon-containing material, the silicon phase content ratio CSi is preferably 10% by mass or more and 80% by mass or less. By having both CSi within the above numerical ranges, it becomes easier to adjust the abundance ratio of silicon element on the outer surfaces of the first silicon-containing material and the second silicon-containing material. The silicon phase content ratio CSi may be 30% by mass or more, or 60% by mass or more. The silicon phase content ratio CSi may be 40% by mass or less, or 50% by mass or less.

[0077] In the first silicon-containing material and the second silicon-containing material, the carbon phase content ratios CC are preferably 10% by mass or more and 60% by mass or less. Having CC within the above ranges also makes it easier to adjust the abundance ratio of elemental silicon on the outer surfaces of the first silicon-containing material and the second silicon-containing material. The carbon phase content ratio CC may be 40% by mass or more, or 50% by mass or more. The carbon phase content ratio CC may be 58% by mass or less, or 56% by mass or less.

[0078] The inclusion of a silicon phase and a carbon phase in the first silicon-containing material and the second silicon-containing material can be confirmed by imaging and observing the cross-sections of the first silicon-containing material and the second silicon-containing material with a scanning electron microscope (SEM). Observation of the cross-sections of the first silicon-containing material and the second silicon-containing material can be performed, for example, by forming a cured product of a thermosetting resin filled with particles of the first silicon-containing material and the second silicon-containing material, obtaining a cross-section of this cured product using a cross-section polisher (CP), and then observing this cross-section with an SEM. Furthermore, by performing elemental mapping analysis using energy dispersive X-rays (EDX) on the cross-sections of the particles of the first silicon-containing material and the second silicon-containing material, both quantitative and qualitative analysis can be performed on both the silicon phase and the carbon phase. That is, in the first silicon-containing material and the second silicon-containing material, both the silicon phase content ratio RSi and the carbon phase content ratio RC can be measured.

[0079] In both the first silicon-containing material and the second silicon-containing material, the average particle diameter D50 of the silicon phase is preferably 1 nm or more and 30 nm or less. That is, it is preferable that the silicon phase has a sufficiently small average particle diameter D50. By having the silicon phase have a sufficiently small average particle diameter D50, excessive expansion and contraction of the silicon phase due to the absorption and desorption of lithium ions can be suppressed. This suppresses miniaturization of the silicon phase. As a result, in non-aqueous electrolyte secondary batteries, both high capacity and improved cycle characteristics can be more fully achieved. The average particle diameter D50 of the silicon phase can be measured using cross-sectional images of particles of the first silicon-containing material and the second silicon-containing material obtained by TEM. The average particle diameter D50 of the silicon phase is determined by determining the maximum diameter for any 100 silicon phases, and then using this maximum diameter to obtain a volume-based particle size distribution, and the cumulative particle diameter (median diameter) is the particle diameter at 50% of the total.

[0080] Both the first silicon-containing material and the second silicon-containing material preferably have a single particle fracture strength of 100 MPa or more and 600 MPa or less. That is, the first silicon-containing material and the second silicon-containing material preferably have a relatively small single particle fracture strength, in other words, are relatively soft. By having the single particle fracture strength within the above numerical range, i.e., by being relatively soft, even if the first silicon-containing material and the second silicon-containing material expand and contract due to the absorption and release of lithium ions in the silicon phase, the first silicon-containing material and the second silicon-containing material can be prevented from becoming finer due to this expansion and contraction. This makes it possible to further fully achieve both high capacity and improved cycle characteristics. The single particle fracture strength of the first silicon-containing material and the second silicon-containing material can be measured according to the following procedure using a commercially available measuring device (e.g., a microcompression tester (MCT-W201) manufactured by Shimadzu Corporation). Hereinafter, the first silicon-containing material and the second silicon-containing material will be referred to as a composite material.

[0081] Procedure: (1) Spread the composite material on the lower pressure plate of the measuring device. (2) While observing with an optical microscope, select a composite material with a size close to the average particle diameter D50 as the sample. (3) Use a 50 μm diameter diamond flat indenter as the upper indenter, ensuring that only one particle selected as the sample is present between the upper indenter and the lower pressure plate. (4) Slowly lower the upper indenter until it contacts the sample. From the point of contact, further lower the upper indenter at a constant acceleration to apply a load to the sample. The point at which the indenter contacts the sample can be determined by the change in the descent speed of the upper indenter. The constant acceleration is a displacement rate of 2.7 mN / sec. (5) Determine the relationship between the load and the deformation of the sample. The point at which the deformation of the sample suddenly changes (the inflection point of the load-deformation profile) is defined as the breaking point. Calculate the breaking strength based on the load and particle diameter at that time using the following formula (1). The breaking strength is obtained by arithmetically averaging the measurements for five samples.

[0082] St=2.8×P / (π×d ２) (1) St is the breaking strength (unit: MPa or N / mm ２ ), P is the load (unit: N), and d is the particle diameter (mm).

[0083] The negative electrode mixture layer may contain a carbon-based material in addition to the first silicon-containing material and the second 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.

[0084] 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.

[0085] When the first silicon-containing material and the second silicon-containing material are used in combination with graphite as the negative electrode active material, the proportion of the first silicon-containing material and the second silicon-containing material in the negative electrode active material is, for example, 5% by mass to 70% by mass. This proportion may be 10% by mass to 60% by mass, 20% by mass to 50% by mass, or 25% by mass to 40% by mass. By having this proportion in the above range, improved cycle characteristics and high capacity can be achieved in a balanced manner.

[0086] 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.

[0087] The negative electrode mixture layer preferably contains a hydrophobic binder as a binder. Silicon atoms present on the outer surface of a silicon-containing material often have hydrophilic functional groups such as hydroxyl groups. Therefore, if the silicon atom abundance ratio on the outer surface of the silicon-containing material is high, many hydrophilic functional groups will be present on the outer surface of the silicon-containing material. Therefore, when a hydrophobic binder is included as a binder, the binding strength of the hydrophobic binder to the silicon-containing material may be excessively reduced. However, the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes, as negative electrode active materials, both a first silicon-containing material having a moderately low silicon atom abundance ratio on the outer surface and a second silicon-containing material having a moderately high silicon atom abundance ratio on the outer surface, with the content ratios of the two materials being appropriately balanced. Therefore, even when the negative electrode mixture layer contains a hydrophobic binder as a binder, excessive reduction in the binding strength of the hydrophobic binder to the silicon-containing material can be suppressed. That is, it is possible to suppress a deterioration in cycle characteristics caused by an excessive decrease in the binding ability of the hydrophobic binder to the silicon-containing material.

[0088] In this specification, a hydrophobic binder refers to a resin that dissolves in 100 g of water in an amount of less than 1.0 g at a liquid temperature of 25° C. The amount of resin that dissolves in 100 g of water in an amount of 25° C. is preferably less than 0.5 g, and more preferably less than 0.25 g. Of the various binders exemplified above, examples of hydrophobic binders include rubber-like materials such as styrene-butadiene copolymer rubber (SBR).

[0089] The negative electrode mixture layer preferably contains 0.5% by mass or more and 4.0% by mass or less of the binder. By containing the binder within the above numerical range, the negative electrode mixture layer can exhibit good binding properties with respect to the first silicon-containing material and the second silicon-containing material. The negative electrode mixture layer may contain 0.7% by mass or more of the binder, or 0.8% by mass or more of the binder. The negative electrode mixture layer may contain 2.0% by mass or less of the binder, or 1.5% by mass or less of the binder.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] (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.

[0094] The separator may include multiple layers differing in at least one of form and composition, such as a laminate of a polyethylene microporous film and a polypropylene microporous film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0095] (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.

[0096] 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.

[0097] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0098] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0099] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0100] 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).

[0101] 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.

[0102] 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.

[0103] 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.

[0104] Fluorinated cyclic ethers are those in which one or more hydrogen atoms of the above-mentioned cyclic ethers have been substituted with fluorine atoms. Examples of fluorinated cyclic ethers include 3,3,4,4-tetrafluorotetrahydrofuran.

[0105] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] (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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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).

[0117] (Additional Notes) The above description discloses the following technology: (Technology 1) A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material and a binder, the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material, the first silicon-containing material and the second silicon-containing material are both composite materials comprising a carbon phase and a silicon phase dispersed in the carbon phase, and when the abundance ratio of silicon element is PSi, the abundance ratio of carbon element is PC, and the abundance ratio of oxygen element is PO at the outer surface of the first silicon-containing material and the outer surface of the second silicon-containing material, the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) at the outer surface of the first silicon-containing material is 0.01 or more and 0.10 or less, A nonaqueous electrolyte secondary battery, wherein on the outer surface of the second silicon-containing material, the ratio of PSi to PSi+PC+PO (PSi / (PSi+PC+PO)) is 0.15 or more and 0.20 or less, and wherein, in the negative electrode active material, when a content ratio of the first silicon-containing material is R1 and a content ratio of the second silicon-containing material is R2, the ratio of R2 to R1+R2 (R2 / (R1+R2)) is 0.20 or more and 0.80 or less. (Technology 2) The nonaqueous electrolyte secondary battery according to Technology 1, wherein, in the first silicon-containing material and the second silicon-containing material, the carbon phases are both formed of a porous carbon material. (Technology 3) The nonaqueous electrolyte secondary battery according to Technology 1 or 2, wherein, in the first silicon-containing material and the second silicon-containing material, the content ratios CSi of the silicon phases are both 10% by mass or more and 80% by mass or less. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the average particle diameter D50 of the silicon phase in the first silicon-containing material and the second silicon-containing material is 1 nm or more and 30 nm or less. (Technology 5) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 4, wherein the content ratio CC of the carbon phase in the first silicon-containing material and the second silicon-containing material is 10 mass % or more and 60 mass % or less.(Technology 6) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 5, wherein in the first silicon-containing material and the second silicon-containing material, the carbon phases are both formed of at least one of hard carbon and soft carbon. (Technology 7) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 6, wherein the first silicon-containing material and the second silicon-containing material both have particle fracture strengths of 100 MPa or more and 600 MPa or less. (Technology 8) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 7, wherein the negative electrode contains 0.5 mass % or more and 4.0 mass % or less of the binder. (Technology 9) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 8, wherein the binder contains a hydrophobic binder.

[0118] 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.

[0119] 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.

[0120] [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.

[0121] 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.

[0122] (2) Preparation of Negative Electrode A negative electrode active material, a binder, and a thickener were mixed in a mass ratio of 98:1:1 to obtain a negative electrode mixture. 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), a first silicon-containing material (hereinafter also referred to as SiC1), and a second silicon-containing material (hereinafter also referred to as SiC2) was used as the negative electrode active material. Both SiC1 and SiC2 are composite materials comprising a carbon phase and a silicon phase dispersed within the carbon phase. SiC1 and SiC2 were added so that their total mass was 10% by mass, assuming the total mass of the negative electrode active material (Gr + SiC1 + SiC2) to be 100% by mass. Furthermore, the SiC1 and SiC2 content ratios R1 and R2 of SiC2 satisfied the relationship R2 / (R1 + R2) = 0.2. Furthermore, the abundance ratio of silicon element PSi, the abundance ratio of carbon element PC, and the abundance ratio of oxygen element PO were measured on the outer surfaces of SiC1 and SiC2, respectively, and PSi / (PSi + PC + PO) was calculated to be 0.05 for SiC1 and 0.20 for SiC2. Furthermore, styrene-butadiene copolymer rubber (SBR) was used as the binder, and carboxymethyl cellulose (CMC) was used as the thickener.

[0123] For Example 1, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0124] The negative electrode mixture slurry according to Example 1 was applied to both sides of a copper foil (negative electrode current collector, thickness 15 μm) to form a coating film, which was then dried. The dried coating film was then compressed in the thickness direction using a roller to form a negative electrode mixture layer on the negative electrode current collector. The laminate (negative electrode laminate) of the negative electrode current collector and the negative electrode mixture layer was then cut to a predetermined size. In this manner, the negative electrode according to Example 1 was obtained. The negative electrode was fabricated to have dimensions slightly larger than the positive electrode.

[0125] (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 %. ６ The 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.

[0126] (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.

[0127] 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.

[0128] [Example 2] A nonaqueous electrolyte secondary battery according to Example 2 was completed in the same manner as in Example 1, except that SiC1 and SiC2 were made to satisfy the relationship R2 / (R1+R2) = 0.5. For Example 2, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0129] [Example 3] A nonaqueous electrolyte secondary battery according to Example 3 was completed in the same manner as Example 1, except that SiC1 and SiC2 were made to satisfy the relationship R2 / (R1+R2) = 0.8. For Example 3, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0130] Example 4 A nonaqueous electrolyte secondary battery according to Example 4 was completed in the same manner as in Example 2, except that a composite material having a PSi / (PSi+PC+PO) ratio of 0.10 was used as SiC1. For Example 4, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0131] Example 5 A nonaqueous electrolyte secondary battery according to Example 5 was completed in the same manner as in Example 2, except that a composite material having a PSi / (PSi+PC+PO) ratio of 0.01 was used as SiC1. For Example 5, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0132] Example 6 A nonaqueous electrolyte secondary battery according to Example 6 was completed in the same manner as in Example 2, except that a composite material having a PSi / (PSi+PC+PO) ratio of 0.15 was used as SiC2. For Example 6, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0133] [Comparative Example 1] Except for using only SiC1 as the silicon-containing material, a nonaqueous electrolyte secondary battery according to Comparative Example 1 was completed in the same manner as in Example 1. For Comparative Example 1, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0134] [Comparative Example 2] Except for using only SiC2 as the silicon-containing material, a nonaqueous electrolyte secondary battery according to Comparative Example 2 was completed in the same manner as in Example 1. For Comparative Example 2, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0135] Comparative Example 3 A nonaqueous electrolyte secondary battery according to Comparative Example 3 was completed in the same manner as in Example 2, except that a composite material having a PSi / (PSi+PC+PO) ratio of 0.15 was used as SiC1. For Comparative Example 3, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0136] Comparative Example 4 A nonaqueous electrolyte secondary battery according to Comparative Example 4 was completed in the same manner as in Example 2, except that a composite material having a PSi / (PSi+PC+PO) ratio of 0.30 was used as SiC2. For Comparative Example 4, the value of R2 / (R1+R2), the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC1, and the calculated value of PSi / (PSi+PC+PO) on the outer surface of SiC2 are shown in Table 1 below.

[0137]

[0138] <Charge-Discharge Test> A charge-discharge cycle test was conducted on the nonaqueous electrolyte secondary batteries according to each example (Examples 1 to 6 and Comparative Examples 1 to 4). The charge-discharge cycle test was conducted by charging the nonaqueous electrolyte secondary battery according to each example under the following charge-discharge conditions, resting it for 20 minutes, and then discharging it under the following discharge conditions, and repeating this cycle 1,000 times. The charge-discharge cycle test was conducted while the nonaqueous electrolyte secondary battery according to each example was left in a thermostatic chamber at 25°C.

[0139] ・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 discharging is performed at a current of 0.2 C until the battery voltage reaches 3.0 V.

[0140] (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 (cycle retention rate) = E1 / E0 × 100

[0141] For the nonaqueous electrolyte secondary batteries according to each example, the initial battery capacity E0 and the cycle retention rate are shown in Table 2 below.

[0142]

[0143] Table 2 shows that the nonaqueous electrolyte secondary batteries according to Examples 1 to 6 all exhibit a high battery capacity E0 of 5,400 mAh / g or more and a high cycle retention rate of 90% or more. On the other hand, the nonaqueous electrolyte secondary battery according to Comparative Example 1 exhibits a high cycle retention rate of 91%, but its battery capacity E0 is reduced to 5,000 mAh / g. Furthermore, the nonaqueous electrolyte secondary batteries according to Comparative Examples 2 to 4 all exhibit a high battery capacity E0 of 5,600 mAh / g or more, but their cycle retention rate is reduced to 77% or less. These results demonstrate that the nonaqueous electrolyte secondary batteries according to Examples 1 to 6 are able to more fully achieve both high capacity and improved cycle characteristics.

[0144] The nonaqueous electrolyte secondary battery according to the present disclosure can be used in applications where both high capacity and improved cycle characteristics are required to be more fully achieved at the same time.

[0145] 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, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material and a binder, the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material, the first silicon-containing material and the second silicon-containing material are both composite materials comprising a carbon phase and a silicon phase dispersed within the carbon phase, and when the abundance ratio of silicon element is PSi, the abundance ratio of carbon element is PC, and the abundance ratio of oxygen element is PO at the outer surface of the first silicon-containing material and the outer surface of the second silicon-containing material, the ratio of PSi to PSi + PC + PO (PSi / (PSi + PC + PO)) at the outer surface of the first silicon-containing material is 0.01 or more and 0.10 or less, a ratio of PSi to PSi+PC+PO (PSi / (PSi+PC+PO)) on the outer surface of the second silicon-containing material is 0.15 or more and 0.20 or less; and in the negative electrode active material, when a content ratio of the first silicon-containing material is R1 and a content ratio of the second silicon-containing material is R2, a ratio of R2 to R1+R2 (R2 / (R1+R2)) is 0.20 or more and 0.80 or less.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon phases in the first silicon-containing material and the second silicon-containing material are both formed of porous carbon materials.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the silicon phase content CSi of the first silicon-containing material and the second silicon-containing material is both 10 mass % or more and 80 mass % or less.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the average particle diameter D50 of the silicon phase in each of the first silicon-containing material and the second silicon-containing material is 1 nm or more and 30 nm or less.

5. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the carbon phase content ratio CC in each of the first silicon-containing material and the second silicon-containing material is 10 mass % or more and 60 mass % or less.

6. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein in the first silicon-containing material and the second silicon-containing material, the carbon phase is formed of at least one of hard carbon and soft carbon.

7. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the first silicon-containing material and the second silicon-containing material both have a single particle fracture strength of 100 MPa or more and 600 MPa or less.

8. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode contains 0.5% by mass or more and 4.0% by mass or less of the binder.

9. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the binder includes a hydrophobic binder.

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