Negative electrode mixture for secondary battery, negative electrode, and secondary battery

Abstract

This negative electrode mixture for a secondary battery comprises a negative electrode active material and a hollow columnar substance (43) as an additive. The negative electrode active material has at least a silicon-containing material. The hollow columnar substance (43) has pores having a diameter of 1 nm or more. When the length of the hollow columnar substance (43) in the long axis direction is L and the diameter of the pores is D, the ratio L / D is D is L / D>5.

Description

Negative electrode mixture for secondary battery, negative electrode, and secondary battery

[0001] The present disclosure relates to a negative electrode mixture for a secondary battery, a negative electrode, and a secondary battery, and particularly to reducing the resistance and improving the capacity retention rate of a secondary battery.

[0002] In secondary batteries, increasing the capacity per unit area of ​​the negative electrode can be achieved by incorporating a large amount of silicon-containing material into the active material in the negative electrode mixture layer. However, significant expansion and contraction of the silicon-containing material during charge and discharge can cause the electrolyte solution that has penetrated the negative electrode mixture layer to be expelled from the negative electrode mixture layer, resulting in so-called liquid squeezing. This results in a non-uniform distribution of the electrolyte solution in the negative electrode mixture layer, resulting in increased resistance. Furthermore, liquid squeezing can cause a decrease in the capacity retention rate during charge and discharge cycles.

[0003] Patent Document 1 describes a secondary battery negative electrode in which a negative electrode mixture layer includes a negative electrode active material and ion exchange particles that adsorb transition metal ions and release predetermined cations, and the ion exchange particles contain at least one of gold and platinum. Patent Document 1 describes the use of zeolite, kaolinite, halloysite, illite, and montmorillonite as examples of the ion exchange particles.

[0004] JP 2019-53860 A

[0005] In a configuration in which the negative electrode active material in the negative electrode mixture contains a silicon-containing material, it is desirable to improve the increase in resistance and the decrease in capacity retention rate of the secondary battery caused by squeezing out the liquid during charging and discharging. On the other hand, in the configuration described in Patent Document 1, the negative electrode active material does not contain a silicon-containing material. Therefore, there is little need to improve the squeezing out of the liquid during charging and discharging.

[0006] A negative electrode mixture for a secondary battery according to one aspect of the present disclosure includes a negative electrode active material and a hollow columnar material as an additive, wherein the negative electrode active material has a silicon-containing material, the hollow columnar material has pores with a diameter of 1 nm or more, and where L is the length of the hollow columnar material in the major axis direction and D is the diameter of the pore, the ratio L / D is L / D>5.

[0007] A negative electrode for a secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector, the negative electrode mixture layer including a negative electrode mixture, and the negative electrode mixture is the negative electrode mixture of the present disclosure.

[0008] A secondary battery according to one embodiment of the present disclosure includes an electrode group having a positive electrode and a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the negative electrode of the present disclosure.

[0009] According to the negative electrode mixture for a secondary battery, the negative electrode, and the secondary battery of the present disclosure, the negative electrode active material in the negative electrode mixture includes a silicon-containing material, and the negative electrode mixture includes a hollow columnar material as an additive. The hollow columnar material has pores with a diameter of 1 nm or more, and the ratio L / D is L / D > 5, where L is the length in the longitudinal direction of the hollow columnar material and D is the diameter of the pore. This allows the hollow columnar material to retain the electrolyte regardless of large expansion and contraction during charge and discharge, thereby suppressing squeezing of the electrolyte due to charge and discharge. Therefore, by mitigating uneven distribution of the electrolyte in the negative electrode mixture layer, the resistance of the secondary battery can be reduced and the capacity retention rate during charge and discharge cycles when the silicon-containing material is present in a large amount in the negative electrode mixture can be improved.

[0010] Fig. 4 is a perspective view of a secondary battery of an embodiment. Fig. 5 is a schematic partial cross-sectional view of the secondary battery of an embodiment, taken along a cross section that passes through the negative electrode tab and includes a direction parallel to the thickness direction of the electrode body and the direction of the winding axis. Fig. 6 is a cross-sectional view showing only one side portion in the thickness direction of a negative electrode constituting a secondary battery of an embodiment. Fig. 7 is a schematic view of halloysite, which is a hollow columnar material contained in the negative electrode mixture layer of the negative electrode shown in Fig. 3. Fig. 8 is a cross-sectional view of a main portion of a secondary battery of another example of an embodiment.

[0011] In a negative electrode mixture for a secondary battery in which the negative electrode active material contains a silicon-containing material, it is desirable to improve the increase in resistance and the decrease in capacity retention of the secondary battery due to the squeeze-out of the liquid during charge and discharge, as described above. After extensive research into this issue, the present inventors have found that this issue can be resolved by including hollow columnar materials as additives in the negative electrode mixture and controlling the dimensions and shape of the hollow columnar materials. Specifically, the hollow columnar materials added to the negative electrode mixture have pores with a diameter of 1 nm or more, and the ratio L / D is set to L / D > 5, where L is the longitudinal length of the hollow columnar materials and D is the diameter of the pores. This reduces the resistance of the secondary battery and improves the capacity retention during charge and discharge cycles when the silicon-containing material is present in a large amount in the negative electrode mixture.

[0012] An example of an embodiment of a lithium-ion secondary battery, which is a secondary battery and a nonaqueous electrolyte secondary battery according to the present disclosure, will be described in detail below with reference to Figures 1 to 3. Below, a secondary battery in which a wound electrode assembly is housed in an exterior body made of a laminate sheet including a metal layer and a resin layer, and a secondary battery in which a stacked electrode assembly is housed in the exterior body instead of the wound electrode assembly, will be exemplified, but the electrode may also be a cylindrical battery housed in a cylindrical battery case, and the battery case may be prismatic, coin-shaped, or the like.

[0013] "Configuration of Secondary Battery" Fig. 1 is a perspective view showing the general shape of a secondary battery 10 according to an embodiment. Fig. 2 is a schematic partial cross-sectional view of secondary battery 10 taken along a plane that passes through negative electrode tab 24 and includes a direction parallel to the thickness direction of the electrode body and the direction of winding axis O1.

[0014] The secondary battery 10 has a thin rectangular parallelepiped shape as a whole, and includes an electrode body 60 and an exterior body 12 that houses the electrode body 60 and a non-aqueous electrolyte.

[0015] The outer casing 12 is a laminate sheet outer casing, and is formed by folding a film-like laminate sheet 11 containing an adhesive resin layer in half along the bottom portion 17, and a wound electrode body 60 is housed in a thick rectangular prism-shaped housing portion 13.

[0016] The laminate sheet 11 is a metal layer made of, for example, aluminum or an aluminum alloy, with an inner resin layer, which is an adhesive resin layer, provided on the surface that will be on the inside when the laminate sheet 11 is folded in half and stacked. The inner resin layer is made of, for example, polypropylene. The metal layer and the inner resin layer can be adhered together using, for example, carboxylic acid-modified polypropylene, in which carboxyl groups are added to polypropylene. The electrode body 60 is then wrapped in the folded laminate sheet 11, and the remaining three sides of the laminate sheet 11 are sealed.

[0017] 1 , the exterior body 12 has a top portion 16 on the upper side, a bottom portion 17 on the lower side, a first side portion 18 on the left side, and a second side portion 20 on the right side. The top portion 16 and the first and second side portions 18, 20 are each formed by overlapping the laminate sheet 11, and a sealing portion is formed in each. The first and second side portions 18, 20 are overlapped on the side walls of the storage portion 13 by folding the laminate sheet 11 in the thickness direction. The bottom portion 17 is the folded portion of the laminate sheet 11 in the exterior body 12.

[0018] At the overlapping portion of the outer periphery of the folded laminate sheet 11, both longitudinal ends of the top portion 16 are overlapped, leading out from the top portion 16. The positive electrode tab 22 and the negative electrode tab 24 are connected to the positive electrode and the negative electrode of the electrode body 60, respectively. Molten resins 26, 28 are welded to the base portions of the positive electrode tab 22 and the negative electrode tab 24.

[0019] The secondary battery 10 equipped with such an exterior body 12 is electrically connected to an external load by the positive electrode tab 22 and the negative electrode tab 24 .

[0020] In the top portion 16 and each of the side portions 18, 20, the opposing portions of the folded laminate sheet 11 are overlapped with the inner resin layers in contact and welded by heat and pressure. The housing portion 13 is provided in the exterior body 12 inside the overlapping portion of the folded laminate sheet 11, and houses the electrode body 60.

[0021] The electrode assembly 60 is formed by winding a strip-shaped positive electrode 30 and a strip-shaped negative electrode 40 with a separator 70 interposed therebetween. At this time, molten resins 26, 28 are welded to the positive electrode tab 22 and the negative electrode tab 24, respectively. Next, the positive electrode tab 22 is welded to an exposed portion of a metal positive electrode core that forms the positive electrode 30, and the negative electrode tab 24 is welded to an exposed portion of a metal negative electrode core that forms the negative electrode 40.

[0022] A positive electrode mixture layer is formed on both sides of the positive electrode core, and a negative electrode mixture layer is formed on both sides of the negative electrode core. The positive electrode core is made of, for example, aluminum foil, and the negative electrode core is made of, for example, copper foil. The positive electrode mixture layer contains a positive electrode active material such as a lithium transition metal composite oxide. As will be described in detail later, the negative electrode mixture layer contains a negative electrode active material and a hollow columnar material as an additive. The negative electrode active material contains at least a silicon-containing material. The winding direction end of the electrode body 60 is fixed to the outermost peripheral surface of the electrode body 60 with tape.

[0023] The positive electrode 30, the negative electrode 40, the separator 70, and the non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below, particularly the negative electrode mixture layer that constitutes the negative electrode 40 and the negative electrode mixture that forms the negative electrode mixture layer.

[0024] [Positive Electrode] The positive electrode 30 includes a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on the surface. The positive electrode mixture layer includes, for example, a positive electrode active material, a binder, and a conductive material. The positive electrode mixture layer can be formed on only one side of the positive electrode current collector, but is preferably formed on both sides of the positive electrode current collector. The positive electrode 30 can be manufactured, for example, by applying a slurry of a positive electrode mixture containing a positive electrode active material, a binder, a conductive material, etc., onto the positive electrode current collector, drying and rolling the coating, and forming a positive electrode mixture layer on both sides of the positive electrode current collector.

[0025] The positive electrode active material contained in the positive electrode mixture layer is mainly composed of a lithium-containing metal composite oxide. Examples of metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, and Ge. An example of a suitable lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0026] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0027] [Negative Electrode] FIG. 3 is a cross-sectional view showing only one side in the thickness direction of a negative electrode constituting a secondary battery 10. The negative electrode 40 includes a negative electrode current collector 41 and a negative electrode mixture layer 42 formed on the negative electrode current collector 41, i.e., on both sides of the negative electrode current collector 41. In FIG. 3, the negative electrode mixture layer formed on the other side in the thickness direction (the lower side in FIG. 3) is omitted. The negative electrode current collector 41 can be a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. Although the negative electrode mixture layer 42 can be formed on only one side of the negative electrode current collector 41, it is preferable that the negative electrode mixture layer 42 be formed on both sides of the negative electrode current collector 41. The negative electrode 40 can be manufactured by applying a slurry of a negative electrode mixture containing a negative electrode active material, hollow columnar materials 43, and a binder onto a negative electrode current collector 41, drying and rolling the coating, and forming, for example, a negative electrode mixture layer 42 on both sides of the negative electrode current collector 41.

[0028] [Negative Electrode Mixture] At least a silicon-containing material is used as the negative electrode active material contained in the negative electrode mix that forms the negative electrode mix layer 42. The negative electrode active material may also contain a carbon material such as graphite in addition to the silicon-containing material.

[0029] Examples of carbon materials that function as negative electrode active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads, and mixtures thereof.

[0030] The silicon-containing material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and composite materials containing Si. Among these, composite materials containing Si are preferred. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0031] A suitable silicon-containing material (composite material) is a composite particle containing an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The silicon phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase composed of an aggregate of particles finer than the silicon phase. The ratio of the total mass of the silicon layer to the total mass of the silicon-containing material is preferably 30 mass% or more and 60 mass% or less.

[0032] The composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is made of a material with higher conductivity than the ion-conducting layer and forms a good conductive path in the negative electrode mixture layer 42. The conductive layer is, for example, a carbon coating made of a conductive carbon material. Examples of conductive carbon materials that can be used include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity. The thickness of the conductive layer is preferably 1 nm to 200 nm, or 5 nm to 100 nm, taking into consideration ensuring conductivity and the diffusibility of Li ions into the particles. The thickness of the conductive layer can be measured by observing the cross section of the composite material using a SEM or a transmission electron microscope (TEM).

[0033] The ion-conducting phase may contain at least one element selected from the group consisting of Group 1 and Group 2 elements of the periodic table. The ion-conducting layer may be a silicon oxide phase doped with Li. The ion-conducting phase may also contain at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, W, and lanthanides.

[0034] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si is dispersed substantially uniformly in an amorphous silicon oxide phase, and is generally represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The silicon oxide phase may be doped with Li. The oxygen to Si content (x) is, for example, 0.5≦x<2.0, and preferably 0.8≦x≦1.5.

[0035] Another example of a suitable Si-containing composite material is a composite particle having a sea-island structure in which fine Si particles are uniformly dispersed in an amorphous silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, a compound represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase contains Li ４ SiO ４ It is preferable that (Z=2) is not included. ４ SiO ４ is an unstable compound and reacts with water to become alkaline, which may cause Si to change and lead to a decrease in charge / discharge capacity. The lithium silicate phase is considered to be a suitable phase for Li, from the viewpoints of stability, productivity, Li ion conductivity, etc. ２ SiO ３ (Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0036] Another example of a suitable composite material containing Si is a composite particle having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains a larger amount of amorphous phase components. The amorphous carbon phase is composed of, for example, a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction. The composite material containing a carbon phase may or may not have a conductive layer separate from the carbon phase.

[0037] It is preferable that the ratio of the total mass of silicon contained in the silicon-containing material to the total mass of the negative electrode mixture is 10 mass% or more, from the viewpoints of increasing the capacity retention rate of the battery capacity during charge / discharge cycles and reducing the electrical resistance.

[0038] It is more preferable that the ratio of the total mass of silicon contained in the silicon-containing material to the total mass of the negative electrode mixture is 30% by mass or more, in order to increase the capacity retention rate and reduce electrical resistance. The ratio of the total mass of silicon contained in the silicon-containing material to the total mass of the negative electrode mixture is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. If the ratio of the total mass of silicon contained in the silicon-containing material to the total mass of the negative electrode mixture exceeds 50% by mass, the volume change of the negative electrode mixture during charge and discharge may become excessively large, and the diffusibility of the non-aqueous electrolyte within the electrode body may decrease. Therefore, the ratio of the total mass of silicon contained in the silicon-containing material to the total mass of the negative electrode mixture is preferably 10% by mass or more and 50% by mass or less, more preferably 30% by mass or more and 50% by mass or less, and even more preferably 30% by mass or more and 40% by mass or less.

[0039] 4 is a schematic diagram of halloysite, which is a hollow columnar material 43 contained in a negative electrode mixture layer 42. The halloysite is tubular and has pores 44 with a diameter of 1 nm or more. When the length of the halloysite in the major axis direction is L and the diameter of the pores 44 is D, the ratio L / D is L / D>5. When halloysite within an appropriate size range is contained in the negative electrode mixture in this way, the pores 44 of the halloysite can easily retain an electrolyte.

[0040] Furthermore, the hollow columnar material 43 is preferably a silicate compound such as halloysite. Silicate compounds have lithophilicity and high lithium diffusivity. This promotes the transport of lithium ions in the negative electrode 40, thereby further reducing the resistance of the secondary battery 10.

[0041] The content of the silicate compound that is the hollow columnar material 43 relative to the total mass of the negative electrode mixture can be set to 1 mass % or more and 10 mass % or less, thereby further reducing the resistance of the secondary battery 10.

[0042] Furthermore, it is more preferable that the content of the silicate compound, which is the hollow columnar material 43, relative to the total mass of the negative electrode mixture be 1 mass % or more and 3 mass % or less, in terms of increasing the capacity retention rate and further reducing the resistance.

[0043] Furthermore, it is more preferable that the discharge capacity per 1.0 g of the negative electrode mixture layer is 0.60 Ah or more in terms of increasing the capacity retention rate and reducing the resistance.

[0044] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 70. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 70 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 70 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 70.

[0045] A filler layer containing an inorganic filler may be formed at the interface between the separator 70 and at least one of the positive electrode 30 and the negative electrode 40. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 30, the negative electrode 40, or the separator 70.

[0046] Examples of the binder contained in the negative electrode mixture layer 42 include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0047] [Non-aqueous Electrolyte] The non-aqueous electrolyte has ion conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (nonaqueous electrolyte solution) and may be a solid electrolyte using a gel-like polymer or the like. For example, lithium salts such as LiBF4 and LiPF6 are used as the electrolyte salt. For example, esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP), ethers, nitriles, amides, and mixed solvents of two or more of these are used. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0048] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP). From the viewpoint of suppressing a decrease in the charge-discharge cycle characteristics of a nonaqueous electrolyte secondary battery or improving input characteristics, the nonaqueous electrolyte preferably contains 5% by mass or more of FEC, and more preferably 5% by mass or more and 15% by mass or less of FEC, relative to the mass of the nonaqueous electrolyte.

[0049] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte includes, for example, 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. 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, halide-based solid electrolytes, etc.).

[0050] According to the negative electrode mixture, negative electrode, and secondary battery 10 of the embodiment, the negative electrode active material in the negative electrode mixture contains a silicon-containing material. The negative electrode mixture contains hollow columnar materials 43 as an additive. The hollow columnar materials 43 have pores with diameters of 1 nm or more. When the length of the hollow columnar materials 43 in the longitudinal direction is L and the diameter of the pores is D, the ratio L / D is L / D > 5. This allows the hollow columnar materials 43 to retain the electrolyte despite large expansion and contraction during charge and discharge, thereby suppressing squeezing of the electrolyte due to charge and discharge. Therefore, by mitigating the non-uniformity of the distribution of the electrolyte in the negative electrode mixture layer 42, the resistance of the secondary battery 10 can be reduced, and the capacity retention rate during charge and discharge cycles when the silicon-containing material is present in a large amount in the negative electrode mixture can be improved.

[0051] Furthermore, since the hollow columnar material 43 is made of a silicate compound, the resistance of the secondary battery 10 can be further reduced.

[0052] In the configuration described in Patent Document 1, the negative electrode active material does not contain a silicon-containing material, and at least one of gold and platinum is present in the ion exchange particles, and metal ions other than Li ions are adsorbed. Therefore, in the configuration described in Patent Document 1, the ion exchange particles do not contribute to improving the conductivity of Li ions, and it is thought that adding the ion exchange particles to the negative electrode mixture is likely to lead to an increase in the resistance of the secondary battery.

[0053] 5 is a cross-sectional view of a main portion of a secondary battery 10a according to another embodiment. The secondary battery 10a of this example is a stacked secondary battery including an electrode assembly 60a formed by stacking a positive electrode 30a and a negative electrode 40a with a separator 70a interposed therebetween, and an exterior housing 72. The exterior housing 72 accommodates the electrode assembly 60a and a nonaqueous electrolyte and has openings (not shown) at both ends in a first direction (the left-right direction in FIG. 5 ) perpendicular to the stacking direction of the electrode assembly 60a. The exterior housing 72 is formed by folding a laminate sheet including a metal layer and an adhesive resin layer provided on the inner surface of the metal layer in half, and the overlapping portion around the electrode assembly 60a is sealed by adhesive.

[0054] A positive electrode tab 76 connected to the positive electrode 30a extends from one opening of the exterior body 72 to the outside of the secondary battery 10a, and a negative electrode tab 75 connected to the negative electrode 40a extends from the other opening of the exterior body 72 to the outside of the secondary battery 10a. At one of the openings, a first gasket (not shown) welded to the inner surface of the opening is interposed between the exterior body 72 and the positive electrode tab 76, thereby sealing the one opening. At the other opening, a second gasket (not shown) welded to the inner surface of the opening is interposed between the exterior body 72 and the negative electrode tab 75, thereby sealing the other opening. Note that a gasket may not be used for the opening, and a laminate sheet may be directly welded to the periphery of the opening, sandwiching the tabs 75, 76.

[0055] The positive electrode 30a, negative electrode 40a, and separator 70a are each rectangular, with the negative electrode 40a being slightly larger than the positive electrode 30a. The materials of the positive electrode 30a, negative electrode 40a, and separator 70a are the same as those in the configurations of FIGS. 1 to 4. In particular, the negative electrode mixture layer 42a constituting the negative electrode 40a is formed from a negative electrode mixture containing a negative electrode active material and a hollow columnar material, which is a silicate compound, as an additive. The negative electrode active material contains at least a silicon-containing material. This, as in the configurations of FIGS. 1 to 4, reduces the resistance of the secondary battery 10a and improves the capacity retention rate during charge-discharge cycles when the silicon-containing material content in the negative electrode mixture is high. In this example, the other configurations and functions are the same as those in FIGS. 1 to 4.

[0056] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples. The configuration of the examples is the same as the configuration shown in FIG.

[0057] Example 1 [Positive Electrode] LiNi was used as the positive electrode active material. ０．９ Co ０．０５ Al ０．０５ O ２ A lithium transition metal oxide represented by (NCA) was used. 100 parts by mass of the positive electrode active material, 0.4 parts by mass of carbon nanotubes (CNT), and 0.6 parts by mass of polyvinylidene fluoride (PVDF) were mixed to prepare a positive electrode mixture layer slurry. Next, the positive electrode mixture layer slurry was applied to the aluminum foil positive electrode current collector 31a, leaving the portion where the positive electrode tab 76 was connected, and the coating was dried. The coating was rolled using a roller and then cut to a predetermined electrode size having a 20 mm square positive electrode mixture layer. A positive electrode 30a was fabricated in which a positive electrode mixture layer 32a was formed on one side of the positive electrode current collector 31a.

[0058] [Negative electrode] The negative electrode active material was a silicon-containing material, a composite particle (Si-C) containing a carbon phase and a Si phase dispersed in the carbon phase, graphite (Gr), a lithium salt of polyacrylic acid (PAA), a sodium salt of carboxymethyl cellulose (CMC), a dispersion of styrene-butadiene copolymer (SBR), and halloysite, a hollow columnar material, as an additive. These were mixed in a solid mass ratio of X:(100-X):1:1:5:Y, and an appropriate amount of water was added to prepare a negative electrode mixture layer slurry. In Example 1, X was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 20% by mass. Y was determined so that the ratio of the halloysite content to the total mass of the negative electrode mixture was 1% by mass.

[0059] Next, the negative electrode mixture layer slurry was applied to one surface of the negative electrode current collector 41a made of copper foil, leaving a portion to be connected to the negative electrode tab 75, and the coating was dried to form the negative electrode mixture layer 42a on one surface of the negative electrode current collector 41a. The coating was then rolled using a roller and cut to a predetermined electrode size, thereby producing the negative electrode 40a in which the negative electrode mixture layer 42a was formed on one surface of the negative electrode current collector 41a.

[0060] [Non-aqueous electrolyte] A mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 was added with 4 mass % of vinylene carbonate (VC), and LiPF ６ was dissolved in a proportion of 1.4 mol / L to prepare a non-aqueous electrolyte.

[0061] [Test Cell] An electrode assembly 60a was fabricated by attaching an anode tab 75 and a cathode tab 76 to the anode 40a and the cathode 30a, respectively, and stacking the anode 40 and the cathode 30a with a separator 70a interposed therebetween. A three-layer polypropylene separator was used as the separator 70a. The fabricated electrode assembly 60a was inserted into an exterior body made of an aluminum laminate sheet, and the opening of the exterior body was sealed to fabricate a test cell (laminate cell).

[0062] Example 2 The other conditions of Example 2 were the same as those of Example 1. X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 10 mass%.

[0063] In preparing the negative electrode mixture slurry, X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 30 mass%. Other conditions in Example 3 were the same as those in Example 1.

[0064] In preparing the negative electrode mixture slurry, X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 40 mass%. Other conditions in Example 4 were the same as those in Example 1.

[0065] In preparing the negative electrode mixture slurry, X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 50 mass%. Other conditions in Example 5 were the same as those in Example 1.

[0066] Example 6 In preparing the negative electrode mixture slurry, Y in Example 1 was determined so that the proportion of the halloysite content relative to the total mass of the negative electrode mixture was 10 mass %. The other conditions of Example 6 were the same as those of Example 1.

[0067] Example 7 In preparing the negative electrode mixture slurry, Y in Example 1 was determined so that the proportion of the halloysite content relative to the total mass of the negative electrode mixture was 5 mass %. The other conditions of Example 7 were the same as those of Example 1.

[0068] Example 8 In preparing the negative electrode mixture slurry, Y in Example 1 was determined so that the proportion of the halloysite content relative to the total mass of the negative electrode mixture was 3 mass%. The other conditions of Example 8 were the same as those of Example 1.

[0069] <Comparative Example 1> In the preparation of the negative electrode mixture slurry, Y in Example 1 was changed to 0%. That is, in Comparative Example 1, the negative electrode mixture did not contain halloysite. The other conditions of Comparative Example 1 were the same as those of Example 1.

[0070] <Comparative Example 2> In preparing the negative electrode mixture slurry, X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 8 mass%. Also, Y in Example 1 was set to 0%. That is, in Comparative Example 2, similar to Comparative Example 1, the negative electrode mixture did not contain halloysite. The other conditions of Comparative Example 2 were the same as those of Example 1.

[0071] <Comparative Example 3> In preparing the negative electrode mixture slurry, X in Example 1 was determined so that the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture was 8 mass%. Furthermore, Y in Example 1 was set to 0%, but instead of halloysite, montmorillonite, a silicate compound that is not a hollow columnar substance but has a flat layered structure, was added as an additive. At this time, the ratio of the montmorillonite content to the total mass of the negative electrode mixture was set to 1 mass%. The other conditions of Comparative Example 3 were the same as those of Example 1.

[0072] <Comparative Example 4> In preparing the negative electrode mixture slurry, X in Example 1 was changed to 0. That is, in Comparative Example 4, no silicon-containing material was contained in the negative electrode mixture. The other conditions of Comparative Example 4 were the same as those of Example 1.

[0073] Comparative Example 5 In the preparation of the negative electrode mixture slurry, Y in Example 1 was set to 0%, but zeolite, which is a hollow material but not a hollow columnar material, was added as an additive instead of halloysite. In this case, the content of zeolite relative to the total mass of the negative electrode mixture was set to 1 mass%. The other conditions of Comparative Example 5 were the same as those of Example 1.

[0074] Table 1 shows the ratio of the total mass of silicon element in the silicon-containing material to the total mass of the negative electrode mixture in the test cells of Examples 1-8 and Comparative Examples 1-5, and the ratio of the content of halloysite to the total mass of the negative electrode mixture, in mass%.

[0075]

[0076] [Evaluation of Capacity Retention Rate] The test cells of Examples 1-8 and Comparative Examples 1-5 were charged at a constant current of 0.3 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 0.02 C at 4.2 V. Thereafter, constant current discharge was performed at a constant current of 0.5 C until the battery voltage reached 2.85 V. This charge / discharge cycle was repeated 50 times, and the capacity retention rate in the charge / discharge cycles was calculated based on the following formula: Capacity retention rate = (discharge capacity at 50th cycle / discharge capacity at 1st cycle) × 100

[0077] [Evaluation of Resistance] Before the charge-discharge cycle test, the test cells of Examples 1-8 and Comparative Examples 1-5 were charged until the open circuit voltage reached 3.4 V, and then discharged at a current value of 0.5 C for 30 seconds, after which the voltage was measured. The cell resistance was calculated from the voltage decrease during 30 seconds of discharge and the current value applied, using the following formula for calculating cell resistance: Cell resistance = (voltage decrease during 30 seconds of discharge / current value (0.5 C))

[0078] The test results for the capacity retention rate and cell resistance value (electrical resistance value) of the test cells of Examples 1 to 8 and Comparative Examples 1 to 5 are shown in Table 1. In Table 1, for each of the capacity retention rate and electrical resistance value, the test result of Comparative Example 1 is used as a reference value, and the test results of the other test cells are shown as relative values ​​to that reference value.

[0079] From the results shown in Table 1, in Comparative Example 1-2, halloysite was not added to the negative electrode mixture, so the electrical resistance value was higher than that of Examples 1-8 described below, and the capacity retention rate was lower than that of Examples 1 and 3-5. In Comparative Example 3, montmorillonite was contained in the negative electrode mixture instead of halloysite. However, although montmorillonite is a silicate compound, it is not a hollow columnar substance but a layered silicate mineral. As a result, in Comparative Example 3, the electrical resistance value was lower than in Comparative Example 1, but the capacity retention rate after 50 cycles was lower than that of Comparative Example 1.

[0080] In addition, in Comparative Example 4, the negative electrode mixture contained halloysite but no silicon-containing material, so the capacity retention rate after 50 cycles was even lower than in Comparative Example 3, and the electrical resistance value was significantly higher than in Comparative Example 1.

[0081] In Comparative Example 5, zeolite was included in the negative electrode mixture instead of halloysite. Zeolite is hollow, but it is not a hollow columnar material. As a result, in Comparative Example 5, the capacity retention rate after 50 cycles was higher than in Comparative Example 1, but the electrical resistance value was also higher.

[0082] On the other hand, in Examples 1-8, halloysite was contained in the negative electrode mixture, and therefore the electrical resistance value was able to be reduced regardless of whether the negative electrode active material contained a silicon-containing material. Furthermore, from the results of Examples 1-5, it was confirmed that the higher the total mass of silicon element in the silicon-containing material, the higher the capacity retention rate and the lower the electrical resistance value. Furthermore, from a comparison of the test results of Examples 1, 6-8 with the test results of Comparative Example 1, it was confirmed that the electrical resistance value could be reduced when the content of the silicate compound relative to the total mass of the negative electrode mixture was 1% by mass or more and 10% by mass or less. Furthermore, from a comparison of the test results of Examples 1 and 8 with the test results of Comparative Example 1, it was confirmed that the capacity retention rate could be increased and the resistance could be further reduced when the content of the silicate compound relative to the total mass of the negative electrode mixture was 1% by mass or more and 3% by mass or less. This confirmed the effects of the embodiment shown in FIG. 5 . It is also believed that the effects of the configurations of FIGS. 1 to 4 can be similarly confirmed.

[0083] (Additional Notes) The present disclosure will be further described by the following embodiments. Configuration 1: A negative electrode mixture for a secondary battery, comprising: a negative electrode active material; and a hollow columnar material as an additive; wherein the negative electrode active material contains a silicon-containing material; and the hollow columnar material has pores with a diameter of 1 nm or more; and wherein, where L is the length of the hollow columnar material in the major axis direction and D is the diameter of the pore, the ratio L / D is L / D>5. Configuration 2: The negative electrode mixture according to Configuration 1, wherein the hollow columnar material is a silicate compound. Configuration 3: The negative electrode mixture according to Configuration 2, wherein the content of the silicate compound is 1% by mass or more and 10% by mass or less relative to the total mass. Configuration 4: The negative electrode mixture according to Configuration 3, wherein the content of the silicate compound is 1% by mass or more and 3% by mass or less relative to the total mass. Configuration 5: The anode mix according to any one of Configurations 1 to 4, wherein the ratio of the total mass of elemental silicon contained in the silicon-containing material to the total mass is 10 mass% or more. Configuration 6: The anode mix according to Configuration 5, wherein the ratio of the total mass of elemental silicon contained in the silicon-containing material to the total mass is 30 mass% or more. Configuration 7: The anode mix according to Configuration 5 or 6, wherein the ratio of the total mass of elemental silicon contained in the silicon-containing material to the total mass is 50 mass% or less. Configuration 8: The anode mix according to any one of Configurations 1 to 7, wherein the silicon-containing material includes an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. Configuration 9: The anode mix according to Configuration 8, wherein the ion-conducting phase is at least one phase selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. Configuration 10: The anode mixture according to Configuration 8 or Configuration 9, wherein a ratio of the total mass of the silicon phase to the total mass of the silicon-containing material is 30 mass% or more and 60 mass% or less.Configuration 11: The anode mixture according to any one of Configurations 1 to 10, which does not contain either gold or platinum.Configuration 12: A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode mixture layer formed on the negative electrode current collector, wherein the negative electrode mixture layer contains a negative electrode mixture, and the negative electrode mixture is the anode mixture according to any one of Configurations 1 to 11.Aspect 13: A secondary battery comprising: an electrode group having a positive electrode and a negative electrode; and a nonaqueous electrolyte, wherein the negative electrode is the negative electrode according to Aspect 12. Aspect 14: The secondary battery according to Aspect 13, wherein the discharge capacity per 1.0 g of the negative electrode mixture layer is 0.60 Ah or more.

[0084] REFERENCE SIGNS LIST 10, 10a Secondary battery, 11 Laminate sheet, 12 Exterior body, 13 Storage section, 16 Top section, 17 Bottom section, 18 First side section, 20 Second side section, 22 Positive electrode tab, 24 Negative electrode tab, 26, 28 Welding resin, 30, 30a Positive electrode, 31a Positive electrode current collector, 32a Positive electrode mixture layer, 40, 40a Negative electrode, 41, 41a Negative electrode current collector, 42, 42a Negative electrode mixture layer, 43 Hollow columnar material, 52 Holes, 60, 60a Electrode body, 70, 70a Separator, 72 Exterior body, 75 Negative electrode tab, 76 Positive electrode tab.

Claims

1. A negative electrode mixture for a secondary battery, comprising: a negative electrode active material; and a hollow columnar material as an additive; wherein the negative electrode active material contains a silicon-containing material; the hollow columnar material has pores with a diameter of 1 nm or more; and wherein, where L is the length of the hollow columnar material in the major axis direction and D is the diameter of the pores, a ratio L / D is greater than 5.

2. The negative electrode mixture according to claim 1, wherein the hollow columnar material is a silicate compound.

3. The negative electrode mixture according to claim 2, wherein the content of the silicate compound is 1 mass % or more and 10 mass % or less relative to the total mass.

4. The negative electrode mixture according to claim 3, wherein the content of the silicate compound relative to the total mass is 1 mass % or more and 3 mass % or less.

5. The negative electrode mixture according to any one of claims 1 to 4, wherein the ratio of the total mass of silicon contained in the silicon-containing material to the total mass is 10 mass% or more.

6. The negative electrode mixture according to claim 5, wherein the ratio of the total mass of silicon contained in the silicon-containing material to the total mass is 30 mass % or more.

7. The negative electrode mixture according to claim 5, wherein the ratio of the total mass of silicon contained in the silicon-containing material to the total mass is 50 mass % or less.

8. The negative electrode mixture according to any one of claims 1 to 4, wherein the silicon-containing material comprises an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase.

9. The negative electrode mixture according to claim 8, wherein the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

10. The negative electrode mixture according to claim 8, wherein the ratio of the total mass of the silicon phase to the total mass of the silicon-containing material is 30 mass % or more and 60 mass % or less.

11. The negative electrode mixture according to any one of claims 1 to 4, which contains neither gold nor platinum.

12. A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode mixture layer formed on the negative electrode current collector, wherein the negative electrode mixture layer contains a negative electrode mixture, and the negative electrode mixture is the negative electrode mixture defined in any one of claims 1 to 4.

13. A secondary battery comprising: an electrode group having a positive electrode and a negative electrode; and a non-aqueous electrolyte, wherein the negative electrode is the negative electrode according to claim 12.

14. The secondary battery according to claim 13, wherein the discharge capacity per 1.0 g of the negative electrode mixture layer is 0.60 Ah or more.

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