Nonaqueous electrolyte secondary battery

Abstract

This secondary battery comprises: an electrode body in which a positive electrode (11) and a negative electrode (12) are wound, with a separator (13) being interposed therebetween; an outer can casing which houses the electrode body and an electrolyte solution; and a negative electrode current collector plate (17) which is disposed on the bottom of the outer can casing and is electrically connected to the outer can casing. The negative electrode (12) has a core body and a mixture layer that is provided on the core body, and a negative electrode core body exposure part (41) is joined to the negative electrode current collector plate (17). The mixture layer contains a silicon-containing material as an active material, and 5 mass% or more of the silicon-containing material is contained in a lower region (43) and an upper region (44) which each have a length ratio of 20% or less in the vertical direction. The negative electrode charge capacity is 470 mAh / g or more, and the silicon-containing material includes a low expansion silicon-containing material that has a particle expansion coefficient of 210% or less in the lower region.

Description

Non-aqueous electrolyte secondary battery

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

[0002] In recent years, non-aqueous electrolyte secondary batteries, which perform charging and discharging by transferring lithium ions or the like between a positive electrode and a negative electrode via a non-aqueous electrolyte, have been widely used as high-power, high-energy-density secondary batteries. The secondary battery includes, for example, a wound electrode assembly in which a separator is disposed between a positive electrode having a positive electrode mixture layer disposed on a positive electrode core and a negative electrode having a negative electrode mixture layer disposed on a negative electrode core, and the positive electrode and negative electrode are wound together while insulated by the separator.

[0003] Among secondary batteries equipped with a wound electrode assembly, there is known a battery that employs a negative electrode end surface current collecting structure from the viewpoint of improving battery output. The negative electrode end surface current collecting structure is, for example, a structure in which an exposed portion of a negative electrode core disposed at the lower end, which is one end of the electrode assembly in the winding axis direction, is joined to a current collecting plate, and the current collecting plate is disposed on the bottom of an outer can and electrically connected to the outer can (see Patent Document 1).

[0004] JP 2023-87895 A

[0005] In the negative electrode end-face current collecting structure, a spiral-shaped exposed portion of the negative electrode substrate, located at the lower end, is joined to the upper surface of a current collector plate located on the bottom of the exterior can. This creates a closed spiral space between the electrode body and the bottom of the exterior can. In batteries using a liquid electrolyte, the expansion of the negative electrode mixture layer during charging forces the low-salt electrolyte solution from the negative electrode mixture layer into the spiral space. During discharge, the contraction of the negative electrode mixture layer forces the low-salt electrolyte solution back into the negative electrode mixture layer without mixing with the surrounding electrolyte solution. Therefore, repeated charge / discharge cycles result in a lower salt concentration of the electrolyte solution at the bottom of the negative electrode, increasing the resistance of the electrolyte solution. This leads to poor charge / discharge cycle characteristics.

[0006] On the other hand, it is conceivable that by reducing the proportion of the silicon-containing material in the entire negative electrode active material including the lower part, it is possible to suppress the expansion of the lower part of the negative electrode and to suppress the decrease in the salt concentration of the electrolyte in the lower part of the negative electrode, but in this case, it causes a decrease in the negative electrode charge capacity.

[0007] The present disclosure aims to suppress a decrease in negative electrode charge capacity and improve charge / discharge cycle characteristics in a nonaqueous electrolyte secondary battery having a negative electrode end face current collection structure.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, an outer can containing the electrode assembly and a liquid non-aqueous electrolyte, and a negative electrode current collector plate disposed on the bottom of the outer can and electrically connected to the outer can, wherein the negative electrode includes a core and a mixture layer disposed on the core, and a spiral-shaped negative electrode core exposed portion on which no mixture layer is disposed is provided at a lower end of the electrode assembly on the core, and the lower end of the negative electrode core exposed portion is joined to the negative electrode current collector plate, and the mixture layer is a nonaqueous electrolyte secondary battery comprising a silicon-containing material as an active material, the battery being divided into a lower region in which the ratio of the vertical length from the lower end of the mixture layer to the overall vertical length of the mixture layer is 20% or less, and an upper region above the lower region of the mixture layer, the silicon-containing material comprising 5% by mass or more of the total amount of active material in each of the upper and lower regions, the charge capacity of the negative electrode being 470 mAh / g or more, and the silicon-containing material comprising in the lower region a low-expansion silicon-containing material having a particle expansion coefficient of 210% or less.

[0009] According to one aspect of the present disclosure, in a nonaqueous electrolyte secondary battery having a negative electrode end surface current collection structure, the upper and lower regions of the negative electrode mixture layer each contain 5% by mass or more of a silicon-containing material relative to the total amount of active material in the negative electrode, and the negative electrode charge capacity is 470 mAh / g or more, thereby suppressing a decrease in negative electrode charge capacity. Furthermore, even though the silicon-containing material in the lower region is 5% by mass or more relative to the total amount of active material, the lower region contains a low-expansion silicon-containing material with a particle expansion coefficient of 210% or less, thereby increasing the porosity of the lower region during both charge and discharge. This facilitates retention of electrolyte in the mixture layer during charge and discharge, thereby suppressing the flow of electrolyte between the portion of the electrode assembly other than the exposed portion of the negative electrode substrate and the space between this portion and the bottom of the outer can. This suppresses a decrease in the salt concentration of the electrolyte in the lower portion of the negative electrode, thereby improving charge-discharge cycle characteristics.

[0010] Fig. 2 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. Fig. 3 is a schematic diagram showing the flow of an electrolytic solution at a lower portion of the nonaqueous electrolyte secondary battery shown in Fig. 1 during charging. Fig. 4 is a schematic diagram showing the configuration of a negative electrode constituting the nonaqueous electrolyte secondary battery of Fig. 1. Fig. 5 is a schematic diagram showing the flow of an electrolytic solution at a lower portion of the nonaqueous electrolyte secondary battery shown in Fig. 1 during discharging. Fig. 6 is a diagram showing experimental results of the capacity retention rate and the negative electrode charge capacity in Examples 1 and 2 and Comparative Examples 1 to 3.

[0011] An example of an embodiment will be described with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiment described below. Furthermore, the drawings referred to in the description of the embodiment are schematic.

[0012] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a wound electrode assembly 14, an electrolytic solution that is a liquid nonaqueous electrolyte, an outer can 15 that houses the electrode assembly 14 and the electrolytic solution, and a sealing member 16 that closes the opening of the outer can 15. For ease of explanation, the sealing member 16 side will be referred to as "top" and the bottom 15a side of the outer can 15 as "bottom." In the following description, the nonaqueous electrolyte secondary battery 10 will be referred to as a secondary battery 10.

[0013] The electrolyte solution has, for example, lithium ion conductivity. The electrolyte solution includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt includes, for example, LiPF ６ Lithium salts such as

[0014] 2 is a schematic diagram showing the flow of the electrolyte solution in the lower part during charging of the secondary battery 10. As shown in Fig. 1 and Fig. 2, the electrode assembly 14 is a wound electrode assembly having a positive electrode 11, a negative electrode 12, and a separator 13 disposed between the positive electrode 11 and the negative electrode 12, and in which the positive electrode 11 and the negative electrode 12 are wound with the separator 13 interposed therebetween.

[0015] As shown in FIG. 2 , the positive electrode 11 has a strip-shaped positive electrode core 30 and positive electrode mixture layers 32 disposed on both sides of the positive electrode core 30 .

[0016] The negative electrode 12 has a strip-shaped negative electrode core 40 and a negative electrode mixture layer 42 disposed on both sides of the negative electrode core 40. The negative electrode mixture layer 42 is not disposed at the lower end, which is one end of the negative electrode core 40 in the winding axis direction of the electrode body 14, and a negative electrode core exposed portion 41 where the negative electrode core 40 is exposed is provided. The negative electrode core exposed portion 41 is provided, for example, from one end to the other end in the longitudinal direction of the negative electrode core 40. In other words, the lower end of the electrode body 14 shown in FIG. 2 is constituted by the negative electrode core exposed portion 41. The width, which is the length in the winding axis direction of the electrode body 14, of the negative electrode core exposed portion 41 is, for example, 2 mm or more and 20 mm or less.

[0017] The positive electrode core 30 constituting the positive electrode 11 may be, for example, a metal foil such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on the surface. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 50 μm or less. The positive electrode mixture layer 32 includes, for example, a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing the positive electrode active material and the like onto the positive electrode core 30, drying the coating, and then rolling it to form the positive electrode mixture layer 32 on the positive electrode core 30. The positive electrode mixture layer 32 may be formed on only one side of the positive electrode core 30 or on both sides. The thickness of the positive electrode mixture layer 32 is, for example, 10 μm or more and 150 μm or less on one side of the positive electrode core 30.

[0018] Examples of the positive electrode active material include lithium-containing metal composite oxides. Examples of metal elements contained in the lithium-containing metal composite oxides include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. An example of a preferred lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0019] Examples of conductive agents include carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes (single-walled carbon nanotubes, multi-walled carbon nanotubes, etc.). Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, styrene-butadiene copolymer (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyethylene oxide (PEO).

[0020] FIG. 3 is a schematic diagram showing the configuration of the negative electrode 12. The negative electrode 12 includes a negative electrode core 40 and a negative electrode mixture layer 42 formed on both sides of the negative electrode core 40. FIG. 3 illustrates only one of the negative electrode mixture layers 42 on both sides of the negative electrode core 40. The negative electrode core 40 may be made of a metal foil, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The thickness of the negative electrode core 40 is, for example, 5 μm or more and 50 μm or less. The negative electrode mixture layer 42 contains a negative electrode active material and a binder. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material to the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 42 on the negative electrode core 40. The negative electrode mixture layer 42 may be formed on only one surface or both surfaces of the negative electrode core 40. The thickness of the negative electrode mixture layer 42 is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core 40.

[0021] The binder contained in the negative electrode mixture layer 42 may be the same material as that in the positive electrode 11. The negative electrode mixture layer 42 may contain a conductive agent.

[0022] The negative electrode mixture layer 42 contains a silicon-containing material as a negative electrode active material. The negative electrode mixture layer 42 is divided into a lower region 43 and an upper region 44. The lower region 43 is a region in which the ratio of the vertical length from the lower end of the negative electrode mixture layer 42 to the entire vertical length of the negative electrode mixture layer 42 is 20% or less. The upper region 44 is a region of the negative electrode mixture layer 42 above the lower region 43.

[0023] The upper region 44 and the lower region 43 each contain 5% by mass or more of a silicon-containing material relative to the total amount of the negative electrode active material. The silicon-containing material can occlude more Li ions than carbon-based active materials commonly used as negative electrode active materials, such as graphite. This can suppress a decrease in negative electrode charge capacity. Specifically, the negative electrode charge capacity of the secondary battery 10 of this embodiment is specified to be 470 mAh / g or more.

[0024] The negative electrode charge capacity is determined by the following method. (1) A battery to be evaluated is disassembled, and a negative electrode plate is cut out. A single-electrode cell is fabricated using the negative electrode plate as the working electrode, metallic Li as the counter electrode, and an ionic liquid as the electrolyte. (2) The single-electrode cell is charged at 0.005 C in a temperature environment of 25°C until the cell voltage reaches 5 mV. The charge capacity at this time is divided by the mass of the negative electrode active material contained in the working electrode to determine the negative electrode charge capacity, which is the charge capacity per mass of the negative electrode active material.

[0025] The negative electrode mixture layer 42 may contain substantially only a silicon-containing material as the negative electrode active material, but may also contain a carbon material.

[0026] 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.

[0027] Furthermore, the silicon-containing material includes a low-expansion silicon-containing material having a particle expansion coefficient of 210% or less in the lower region 43 of the negative electrode mixture layer 42. The lower region 43 preferably includes only the low-expansion silicon-containing material as the silicon-containing material. This increases the porosity of the lower region 43, as described below. This makes it easier to retain the electrolyte in the negative electrode mixture layer 42 during charge and discharge, thereby suppressing a decrease in the salt concentration of the electrolyte in the lower part of the negative electrode 12 and improving charge-discharge cycle characteristics.

[0028] More preferably, the particle expansion coefficient of the low-expansion silicon-containing material contained in the lower region 43 is 150% or higher. If the particle expansion coefficient is too small, the negative electrode charge capacity tends to decrease. Therefore, the particle expansion coefficient of the low-expansion silicon-containing material in the lower region 43 is preferably in the range of 150% to 210%. This allows for both high capacity and improved charge-discharge cycle characteristics.

[0029] As described above, the reason why the lower region 43 is a region where the ratio of the vertical length from the lower end of the anode mixture layer 42 to the entire vertical length of the anode mixture layer 42 is 20% or less, and the lower region 43 contains a low-expansion silicon-containing material, is that experiments conducted by the present inventors have shown that when the anode mixture layer does not contain a low-expansion silicon-containing material but contains a silicon-containing material whose particle expansion coefficient exceeds 210%, and when an end-face current collection structure described below is adopted on the anode side, the capacity decreases in the lower 20% region of the anode mixture layer, and this is thought to be due to a particularly low salt concentration of the electrolyte.The present inventors have found that by adopting a configuration in which a low-expansion silicon-containing material is included in the lower region of the lower 20% or less of the anode mixture layer, as in the present embodiment, it is possible to suppress the inflow and outflow of the electrolyte, thereby suppressing a decrease in the salt concentration of the electrolyte and improving the charge-discharge cycle characteristics.

[0030] On the other hand, the silicon-containing material contained in the upper region 44 of the negative electrode mixture layer 42 has a particle expansion coefficient of 150% or more. The silicon-containing material contained in the upper region 44 may have a particle expansion coefficient of more than 210%. The upper region 44 preferably contains a silicon-containing material other than a low-expansion silicon-containing material. This allows for higher negative electrode charge capacity and battery capacity.

[0031] In this specification, the particle expansion coefficient of the silicon-containing material is measured by the following method. (1) A battery to be evaluated is disassembled, and the negative electrode plate is cut out. A single-electrode cell is prepared using metallic Li as the counter electrode and an ionic liquid as the electrolyte, with the particle cross-section of the silicon-containing material exposed. (2) The single-electrode cell is charged at 0.002 C in a temperature environment of 25°C until the cell voltage reaches 5 mV, and after a 10-minute pause, is discharged at 0.05 C until the cell voltage reaches 1.0 V, and the particle cross-section of the silicon-containing material is observed in situ with an SEM. (3) From the change in the particle cross-sectional area of ​​the silicon-containing material, the particle cross-sectional area of ​​the silicon-containing material in the charged state (Spc) and the particle cross-sectional area of ​​the silicon-containing material in the discharged state (Spd) are determined, and the particle expansion coefficient (Sp = (Spc / Spd)) is calculated. １．５ ) is calculated.

[0032] 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. The D50 of silicon-containing materials is generally smaller than the D50 of graphite. The volume-based D50 of silicon-containing materials is, for example, 1 μm or more and 20 μm or less, or 1 μm or more and 15 μm or less. Note that one type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0033] A suitable silicon-containing material (composite material) is a composite particle containing an ion-conducting phase and a Si 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 Si phase is formed by dispersing Si in the form of fine particles, and the ion-conducting phase is a continuous phase formed by an aggregation of particles that are finer than the Si phase.

[0034] 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).

[0035] 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.

[0036] An example of a suitable Si-containing composite material used in the upper region 44 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.

[0037] 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 ４It 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.

[0038] Another example of a suitable composite material containing Si for use in the lower region 43 is composite particles 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.

[0039] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 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 13.

[0040] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. 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 11, the negative electrode 12, or the separator 13.

[0041] [End Surface Current Collector Structure] The secondary battery 10 shown in FIG. 1 has a negative electrode current collector 17 disposed below the electrode assembly 14. The negative electrode current collector 17 is a metal plate made of, for example, nickel or a nickel alloy. A negative electrode substrate exposed portion 41 at the lower end of the electrode assembly 14 is joined to the negative electrode current collector 17 by welding or the like. The negative electrode current collector 17 is disposed on the inner surface of the substantially disc-shaped bottom portion 15a of the outer can 15 and joined to the bottom portion 15a by welding or the like, thereby electrically connecting the negative electrode current collector 17 to the outer can 15. As a result, the negative electrode substrate exposed portion 41 is electrically connected to the outer can 15 via the negative electrode current collector 17, and the outer can 15 serves as a negative electrode terminal. At this time, as shown in FIG. 2 , the lower end of the negative electrode substrate exposed portion 41 is bent radially inward, perpendicular to the winding axis direction, and joined to the negative electrode current collector 17. At this time, the lower end of the negative electrode substrate exposed portion 41 may be bent radially outward and joined to the negative electrode current collector plate 17. In this manner, the secondary battery 10 employs an end face current collecting structure in which the negative electrode substrate exposed portion 41 is joined to the negative electrode current collector plate 17 on the negative electrode 12 side, and current is collected directly from the negative electrode substrate 40.

[0042] [Other Structures of the Secondary Battery] The exterior can 15 is, for example, a cylindrical metal container with a bottom. A gasket 27 is provided between the exterior can 15 and the sealing body 16 to ensure airtightness inside the battery. The exterior can 15 has, for example, a grooved portion 21 that protrudes inward from a portion of the side surface and supports the sealing body 16. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior can 15, and supports the sealing body 16 on its upper surface.

[0043] The sealing body 16 shown in FIG. 1 has a structure in which, in order from the electrode body 14 side, a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 are stacked. Each component constituting the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and the insulating member 24 is interposed between their respective peripheral edges. If the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 23 may deform and rupture, pushing the upper valve body 25 toward the cap 26, thereby interrupting the current path between the lower valve body 23 and the upper valve body 25. If the internal pressure further increases, the upper valve body 25 may rupture, and gas may be discharged through the through-hole 26a of the cap 26.

[0044] The secondary battery 10 shown in FIG. 1 also includes an annular insulating plate 19 and a connection lead 20 disposed above the electrode assembly 14. A positive electrode core exposed portion (not shown) where the positive electrode core 30 ( FIG. 2 ) is exposed is formed in the middle of the positive electrode 11 in the longitudinal direction of the electrode plate, and the lower end of the connection lead 20 is joined to the positive electrode core exposed portion by welding or the like. The upper portion of the connection lead 20 is led out from the upper end of the electrode assembly 14 and extends toward the sealing body 16 using a through hole in the insulating plate 19. The upper end of the connection lead 20 is joined to the lower surface of the filter 22 of the sealing body 16 by welding or the like. As a result, the positive electrode core 30 is electrically connected to the filter 22 via the connection lead 20. In other words, the cap 26 electrically connected to the filter 22 serves as the positive electrode terminal.

[0045] 1 has a positive electrode 11 and a negative electrode 12, and in the negative electrode 12, an end surface current collecting structure is adopted in which a negative electrode substrate exposed portion 41 located at the end of the electrode body 14 in the winding axis direction is joined to a negative electrode current collector plate 17, and current is collected directly from the negative electrode substrate 40. On the other hand, the secondary battery 10 of this embodiment may also have an end surface current collecting structure in which a substrate exposed portion located at the end of the electrode body 14 in the winding axis direction is joined to a current collector plate, and current is collected directly from the substrate, in both the positive electrode and the negative electrode.

[0046] [Use state of secondary battery] Secondary battery 10 is preferably used as a desktop or stationary power source installed indoors or outdoors, or as a power source installed in a vehicle such as an electric car. Secondary battery 10 used for such purposes is, for example, installed on a fixed part such as a mounting base or case, and used in a state fixed to the fixed part, so that the up-down orientation does not change significantly during use.

[0047] [Effects of the Secondary Battery] According to the secondary battery 10 described above, in a configuration having a negative electrode end surface current collecting structure, the upper region 44 and the lower region 43 of the negative electrode mixture layer 42 each contain 5% by mass or more of silicon-containing material relative to the total amount of negative electrode active material. The charge capacity of the negative electrode 12 is 470 mAh / g or more, thereby suppressing a decrease in the negative electrode charge capacity. Furthermore, even though the silicon-containing material in the lower region 43 is 5% by mass or more relative to the total amount of active material, the lower region 43 contains a low-expansion silicon-containing material with a particle expansion coefficient of 210% or less, thereby increasing the porosity of the lower region 43 during both charge and discharge. Specifically, during charging of the secondary battery 10, the negative electrode active material expands, and the silicon-containing material in particular expands significantly. However, by including a low-expansion silicon-containing material in the lower region, the amount of expansion of the silicon-containing material in the lower region can be suppressed, thereby increasing the porosity of the lower region. Furthermore, the silicon-containing material shrinks during discharge after charging, causing the lower region to shrink as well. However, some of the voids present during charging remain, which is thought to allow for a larger porosity during discharge in the lower region than when a silicon-containing material with a larger expansion amount is used. This makes it easier to retain electrolyte in the negative electrode mixture layer 42 during charging and discharging, thereby suppressing the flow of electrolyte between the portion of the electrode assembly 14 other than the negative electrode substrate exposed portion 41 and the space S between this portion and the bottom 15a of the outer can 15. In FIG. 2 , dashed arrow α indicates the electrolyte being pushed out from the lower region into space S during charging, and in FIG. 3 , dashed arrow β indicates the electrolyte being returned from space S to the lower region during discharge. In this embodiment, the flow of electrolyte along these dashed arrows α and β can be reduced.

[0048] The space S is a closed space surrounded by the spiral-shaped negative electrode substrate exposed portion 41 and the lower ends of the negative electrode current collector plate 17, the negative electrode mixture layer 42 of the electrode body 14, and the positive electrode mixture layer 32. Unlike the embodiment, when the silicon-containing material in the lower region does not contain a low-expansion silicon-containing material, the porosity in the lower region is small, and the flow of electrolyte indicated by the dashed arrows α and β increases during charging and discharging. For example, in FIG. 2 , when the flow of electrolyte indicated by the arrow α increases during charging, a large amount of low-salt concentration electrolyte is pushed into the space S.

[0049] 4 , if the flow of the electrolyte solution indicated by the arrow β increases during discharge, the salt concentration of the electrolyte solution absorbed in the lower region 43 will further decrease with each charge-discharge cycle, leading to a deterioration in charge-discharge cycle characteristics. According to the embodiment, by including a low-expansion silicon-containing material in the lower region 43, such a disadvantage can be suppressed, and a decrease in the salt concentration of the electrolyte solution below the negative electrode 12 can be suppressed. This improves the charge-discharge cycle characteristics.

[0050] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0051] Example 1 Preparation of Positive Electrode 100 parts by mass of a positive electrode active material, 1 part by mass of acetylene black (AB), and 1 part by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. ０．８８ Co ０．０９ Al ０．０３ O ２ An aluminum-containing lithium nickel cobalt oxide represented by the formula (1) was used. Next, the positive electrode mixture slurry was applied to both sides of the aluminum foil. This coating was dried, rolled, and cut to a predetermined electrode plate size to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. At this time, a positive electrode core exposed portion where no coating was formed was provided in the longitudinal middle portion of the aluminum foil, and an aluminum connection lead (positive electrode tab) was welded to the positive electrode core exposed portion.

[0052] [Preparation of negative electrode] 95 parts by mass of natural graphite, 5 parts by mass of Si oxide (SiOx), 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the upper region. Also, 94.75 parts by mass of natural graphite, 5.25 parts by mass of a low-expansion silicon-containing material, composite particles containing a carbon phase and a Si phase dispersed in the carbon phase, 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the lower region.

[0053] Next, the negative electrode mixture slurry for the upper region was applied to both sides of the copper foil from the upper end to the upper region, and the negative electrode mixture slurry for the lower region was applied to both sides of the copper foil in an area adjacent to the upper region to form the lower region, so that the negative electrode core exposed portion described above was formed on the copper foil. In this state, the ratio of the applied area of ​​the upper region to the lower region was 80:20 in the vertical length direction. The applied coating film was dried, rolled, and cut to a predetermined electrode plate size to produce a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode core.

[0054] [Preparation of Electrolyte Solution] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:7, and lithium hexafluorophosphate (LiPF ６ An electrolyte solution was prepared by dissolving 1.3 mol / liter of ammonium hydroxide in water.

[0055] [Preparation of Test Cell] A wound electrode assembly was prepared by spirally winding a positive electrode and a negative electrode with a polyethylene microporous membrane separator interposed therebetween. A negative electrode current collector was placed under the electrode assembly, and the exposed portion of the negative electrode core at the bottom of the electrode assembly was bent inward and then welded to the negative electrode current collector. The electrode assembly was then housed in a bottomed cylindrical outer can, and the negative electrode current collector was welded to the bottom of the bottomed cylindrical outer can, with the connection lead connected to a seal. After pouring a nonaqueous electrolyte into the outer can, the opening of the outer can was sealed with a seal via a gasket to prepare a cylindrical test cell.

[0056] <Example 2> When preparing the negative electrode, 90 parts by mass of natural graphite, 10 parts by mass of Si oxide (SiOx), 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the upper region. Also, 89.5 parts by mass of natural graphite, 10.5 parts by mass of a low-expansion silicon-containing material, composite particles containing a carbon phase and a Si phase dispersed in the carbon phase, 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the lower region. Otherwise, a test cell was prepared in the same manner as in Example 1.

[0057] Comparative Example 1 A test cell was fabricated in the same manner as in Example 1, except that the negative electrode mixture slurry for the upper region in Example 1 was used in common for the entire negative electrode mixture slurry and applied to both sides of a copper foil so that the above-described negative electrode substrate exposed portion was formed on the copper foil.

[0058] Comparative Example 2 A test cell was fabricated in the same manner as in Example 2, except that the negative electrode mixture slurry for the upper region in Example 2 was used in common for the entire negative electrode mixture slurry and applied to both sides of a copper foil so that the above-described negative electrode substrate exposed portion was formed on the copper foil.

[0059] <Comparative Example 3> In Comparative Example 1, when preparing the negative electrode, 97.5 parts by mass of natural graphite, 2.5 parts by mass of silicon oxide (SiOx), 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of copper foil so that the aforementioned negative electrode substrate exposed portion was formed on the copper foil. Otherwise, a test cell was prepared in the same manner as in Comparative Example 1.

[0060] [Test Method] Using the secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3, a constant current charge of 0.3 C was performed at 25°C until the battery voltage reached 4.2 V, followed by a constant voltage charge of 0.01 C at 4.2 V. The battery was then discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This cycle constituted one charge-discharge cycle. After the above charge-discharge cycle, the battery was disassembled and examined for the occurrence of Li deposition. Furthermore, the above charge-discharge cycle was repeated 500 times, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = (discharge capacity at 500th cycle / discharge capacity at 1st cycle) × 100. Furthermore, the negative electrode charge capacity of the secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 was calculated using the method described above.

[0061] [Test Results] Figure 5 shows the experimental results of the capacity retention rate and negative electrode charge capacity for Examples 1 and 2 and Comparative Examples 1 to 3. The experimental results shown in Figure 5 reveal that in Comparative Examples 1 and 2, the proportion of silicon-containing material in the negative electrode active material was 5 mass% or more, and the negative electrode charge capacity was high, at 470 mAh / g or more. However, the capacity retention rate was low, at 60% or less. This is thought to be due to a decrease in the salt concentration of the electrolyte at the bottom of the negative electrode, which increased the internal resistance.

[0062] On the other hand, in Comparative Example 3, the proportion of silicon-containing material in the negative electrode active material was 2.5 mass%, and the capacity retention rate was high at 80%. However, the negative electrode charge capacity was low at 425 mAh / g. The reason for this is thought to be that the low proportion of silicon-containing material in the negative electrode active material suppressed the amount of expansion of the negative electrode active material, thereby suppressing the decrease in the salt concentration of the electrolyte at the bottom of the negative electrode, but the low proportion of silicon-containing material resulted in the low negative electrode charge capacity.

[0063] On the other hand, in Examples 1 and 2, the capacity retention rate was increased to 80%, and the negative electrode charge capacity was also increased to 470 mAh / g or more. This is thought to be because the proportion of the silicon-containing material in the negative electrode active material was increased to 5 mass% or more, thereby increasing the negative electrode charge capacity, and furthermore, the inclusion of a low-expansion silicon-containing material in the lower region suppressed the decrease in salt concentration, thereby increasing the capacity retention rate.

[0064] REFERENCE SIGNS LIST 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 outer can, 16 sealing body, 17 negative electrode current collector plate, 18 positive electrode current collector plate, 19 insulating plate, 20 connection lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a through hole, 27 gasket, 30 positive electrode core, 32 positive electrode mixture layer, 40 negative electrode core, 41 negative electrode core exposed portion, 42 negative electrode mixture layer, 43 lower region, 44 upper region.

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween; an outer can containing the electrode assembly and a liquid non-aqueous electrolyte; and a negative electrode current collector plate disposed on the bottom of the outer can and electrically connected to the outer can, wherein the negative electrode includes a core and a mixture layer provided on the core; a spiral negative electrode core exposed portion on which the mixture layer is not disposed is provided at a lower end of the electrode assembly on the core; and a lower end of the negative electrode core exposed portion is joined to the negative electrode current collector, the mixture layer includes a silicon-containing material as an active material, and is divided into a lower region in which the ratio of the vertical length from the lower end of the mixture layer to the entire vertical length of the mixture layer is 20% or less, and an upper region above the lower region of the mixture layer, and the silicon-containing material is contained in an amount of 5% by mass or more relative to the total amount of the active material in each of the upper region and the lower region, a charge capacity of the negative electrode being 470 mAh / g or more; and the silicon-containing material including, in the lower region, a low-expansion silicon-containing material having a particle expansion coefficient of 210% or less.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material comprises an ion-conducting phase and an Si phase dispersed in the ion-conducting phase, and 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.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the particle expansion coefficient of the low-expansion silicon-containing material is 150% or more.

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