Positive electrode for secondary battery, and secondary battery

Abstract

A positive electrode (11) includes: a long positive electrode core (30); and positive electrode mixture layers (31) disposed on both surfaces of the positive electrode core (30). The positive electrode mixture layers (31) include a first region (32) and a thin portion (33) that is a second region thinner than the first region (32). The thickness of the thin portion (33) is 20-90% of the average thickness of the first region (32). The thin portion (33) includes a first thin portion (33) and a second thin portion (34) provided at a predetermined distance from the first thin portion (33).

Description

Positive electrode for secondary battery and secondary battery

[0001] The present disclosure relates to a positive electrode for a secondary battery and a secondary battery, and more particularly to a positive electrode constituting a wound electrode assembly and a secondary battery including the electrode assembly.

[0002] Generally, a positive electrode constituting a wound electrode assembly includes a long positive electrode core and positive electrode mixture layers disposed on both sides of the positive electrode core (see, for example, Patent Document 1). Patent Document 1 discloses an electrode plate having multiple grooves formed on the surface of the mixture layer. In secondary batteries such as lithium-ion batteries, the volume of the electrode assembly changes significantly during charging and discharging. For example, the electrode plate core may expand in the width direction, causing plastic deformation of the core, resulting in a short circuit. To prevent such short circuits, insulation measures are taken, such as by disposing insulating plates between the electrode assembly and the lead, the sealing member, the bottom of the outer can, etc.

[0003] Japanese Patent Application Laid-Open No. 2002-015764

[0004] In secondary batteries such as lithium-ion batteries, it is important to suppress plastic deformation of electrode plates during charging and discharging. Suppressing plastic deformation of electrode plates, for example, can simplify insulation measures, leading to higher capacity and lower costs.

[0005] The positive electrode for a secondary battery according to the present disclosure is a positive electrode for a secondary battery including a long positive electrode core and positive electrode mixture layers arranged on both sides of the positive electrode core, wherein the positive electrode mixture layer includes a first region and a second region thinner than the first region, the thickness of the second region being 20% ​​to 90% of the average thickness of the first region, and the second region including a first thin-walled portion extending from one end of the positive electrode mixture layer in the longitudinal direction to a length of 1% to 3% of the length of the positive electrode mixture layer, and a second thin-walled portion extending from one end of the positive electrode mixture layer in the longitudinal direction to a length of 40% or less of the length of the positive electrode mixture layer with a predetermined gap between the first thin-walled portion and the second thin-walled portion.

[0006] The secondary battery according to the present disclosure is a secondary battery comprising an electrode body including the above-mentioned positive electrode, a negative electrode, and a separator, in which the positive electrode and the negative electrode are wound with the separator interposed therebetween, and the positive electrode is arranged so that the second region is located on the winding core side of the electrode body.

[0007] According to the positive electrode for a secondary battery according to the present disclosure, plastic deformation of the electrode plate caused by charging and discharging the battery can be effectively suppressed.

[0008] Fig. 2 is an axial cross-sectional view of a cylindrical battery that is an example of an embodiment. Fig. 3 is a perspective view of a positive electrode that is an example of an embodiment. Fig. 4 is a cross-sectional view taken along line AA in Fig. 2. Fig. 5 is a cross-sectional view of a positive electrode that is another example of an embodiment.

[0009] Hereinafter, an example of an embodiment of a positive electrode for a secondary battery according to the present disclosure and a secondary battery using the positive electrode will be described in detail with reference to the drawings. Note that the positive electrode and secondary battery according to the present disclosure are not limited to the embodiment described below.

[0010] In the embodiment described below, a cylindrical battery 10 in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is illustrated, but the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include a prismatic battery having a prismatic outer can, a pouch-type battery having an outer can made of a laminate sheet including a metal layer and a resin layer, and the like.

[0011] FIG. 1 is a cross-sectional view of a cylindrical battery 10 according to an embodiment. As shown in FIG. 1 , the cylindrical battery 10 includes an electrode assembly 14 having a positive electrode 11, a negative electrode 12, and a separator 13, with the positive electrode 11 and the negative electrode 12 wound with the separator 13 interposed therebetween. The cylindrical battery 10 also includes a cylindrical outer can 16 with a bottom that houses the electrode assembly 14, and a sealing member 17 that closes the opening of the outer can 16. The outer can 16 houses an electrolyte together with the electrode assembly 14. The outer can 16 has a groove 22 formed in its side wall, and the sealing member 17 is supported by the groove 22 and closes the opening of the outer can 16. Hereinafter, for convenience of explanation, the sealing member 17 side of the cylindrical battery 10 will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0012] As will be described in detail later, the positive electrode mixture layer 31 constituting the positive electrode 11 includes a first region 32 and a second region thinner than the first region 32. The second region is preferably located on the winding core side of the electrode body 14 and includes a first thin-walled portion 33 and a second thin-walled portion 34 formed at a predetermined distance between the first thin-walled portion 33 and the first thin-walled portion 34. In the cylindrical battery 10, the use of the positive electrode 11 including the thin-walled portions 33, 34 effectively suppresses plastic deformation of the electrode plate associated with charging and discharging of the battery. The provision of the specific thin-walled portions 33, 34 is thought to effectively alleviate stress acting on the electrode plate during charging and discharging, suppressing plastic deformation of not only the positive electrode 11 but also the negative electrode 12.

[0013] The electrolyte may be an aqueous electrolyte, but in this embodiment, a non-aqueous electrolyte is used. The non-aqueous electrolyte has lithium ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The cylindrical battery 10 is, for example, a non-aqueous electrolyte secondary battery, and preferably a lithium ion battery.

[0014] The liquid electrolyte (electrolytic solution) contains 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. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0015] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. 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. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] The negative electrode 12 may be disposed on the outer peripheral surface of the electrode body 14, and an exposed portion may be provided in which the surface of the negative electrode core 40 constituting the negative electrode 12 is exposed. In this case, the exposed portion may be in contact with the inner peripheral surface of the outer can 16, and the negative electrode 12 and the outer can 16 may be electrically connected.

[0019] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The gasket 28 also functions as an insulating member that prevents electrical contact between the outer can 16 and the sealing body 17. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0020] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0021] [Positive Electrode] The positive electrode 11 includes a positive electrode core 30 and a positive electrode mixture layer 31 disposed on the positive electrode core 30. The positive electrode core 30 can be a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film having such a metal disposed on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core 30. A protective layer containing inorganic particles and a binder may be disposed between the positive electrode core 30 and the positive electrode mixture layer 31, or on the positive electrode mixture layer 31.

[0022] The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it to form a positive electrode mixture layer 31 on both sides of the positive electrode core 30. N-methyl-2-pyrrolidone (NMP), for example, is used as a dispersion medium for the positive electrode mixture slurry. The positive electrode mixture slurry is preferably applied to the entire area of ​​both sides of the positive electrode core 30, excluding the portion to which the positive electrode lead 20 is connected. As will be described in detail later, the thin portion 33 of the positive electrode mixture layer 31 may be formed by controlling the application method of the positive electrode mixture slurry, or may be formed by forming the positive electrode mixture layer 31 and then peeling off a portion of it.

[0023] The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination.

[0024] The lithium transition metal composite oxide has, for example, a layered rock salt structure. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of high capacity and stability of the crystal structure. The content of the positive electrode active material is, for example, 90% to 99% of the mass of the positive electrode mixture layer 31. From the viewpoint of increasing the capacity of the battery, the density of the positive electrode mixture layer 31 is preferably 3.3 g / cc or more, and is, for example, 3.3 cc to 3.8 g / cc.

[0025] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is, for example, 0.1% to 5% of the mass of the positive electrode mixture layer 31.

[0026] Examples of binders contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The binder content is, for example, 0.1% to 5% of the mass of the positive electrode mixture layer 31.

[0027] 2 to 4, the second region (first thin portion 33 and second thin portion 34) of the positive electrode mixture layer 31 will be described in detail. Fig. 2 is a perspective view of the positive electrode 11, and Fig. 3 is a cross-sectional view taken along line AA in Fig. 2.

[0028] 2 and 3 , the positive electrode mixture layer 31 includes a first region 32 and a first thin portion 33 and a second thin portion 34 that are second regions thinner than the first region 32. The average thickness of the thin portions 33, 34 is 20% to 90% of the average thickness T of the first region 32. The thin portions 33, 34 are formed in a portion of the positive electrode mixture layer 31, and from the viewpoint of increasing the capacity of the battery, it is preferable that the area of ​​the thin portions 33, 34 be smaller than that of the first region 32.

[0029] The thickness T of the first region 32 is preferably substantially constant throughout its entire area, but may have a thickness difference of, for example, about 1%. The average thickness T of the first region 32 on one side of the positive electrode core 30 is preferably 10 μm or more and 150 μm or less, and more preferably 50 μm or more and 120 μm or less. Furthermore, the thickness of the first region 32 is preferably substantially the same on both sides of the positive electrode core 30.

[0030] Thickness T of the thin-walled portions 33 and 34 ３３ , T ３４ The thickness T may be substantially constant throughout the entire area, and as shown in FIG. 3, there may be a step where the thickness changes sharply between the first region 32 and the thin-walled portions 33, 34. Alternatively, the thickness T ３３ , T ３４ The thickness of the first region 32 may gradually increase toward the first region 32 on both sides in the longitudinal direction. The regions where the thickness of the thin-walled portions 33, 34 gradually increases may be formed in a stepped shape or a gentle slope without any steps. The densities of the first region 32 and the thin-walled portions 33, 34 may be different from each other, but in this embodiment they are substantially the same.

[0031] The average thickness T of the first thin-walled portion 33 ３３ The average thickness T of the second thin-walled portion 34 is more preferably 40% or more and 90% or less of the average thickness T of the first region 32, and particularly preferably 50% or more and 85% or less. ３４ is the average thickness T of the first thin-walled portion 33 ３３ However, in this embodiment, the average thickness T ３３ , T ３４ If the value is within this range, the plastic deformation of the electrode plate can be efficiently suppressed while maintaining a high capacity.

[0032] In this embodiment, a positive electrode mixture layer 31 is formed over the entire area of ​​both surfaces of the positive electrode core 30 except for the portion to which the positive electrode lead 20 is connected. The positive electrode lead 20 is connected, for example, to the center portion in the longitudinal direction of the positive electrode 11. For this reason, a core exposed portion where the surface of the positive electrode core 30 is exposed is provided in the center portion in the longitudinal direction of the positive electrode 11. The length L of the positive electrode mixture layer 31 means the length of the positive electrode mixture layer 31 along the longitudinal direction of the positive electrode 11. The length L of the positive electrode mixture layer 31 is approximately the same as the length of the positive electrode 11 and the positive electrode core 30, but more precisely, it is the length obtained by subtracting the length of the core exposed portion from the length of the positive electrode core 30.

[0033] The first thin-walled portion 33 has a length L that is 1% or more and 3% or less of the length of the positive electrode mixture layer 31 from a start end 11x that is one end in the length direction of the positive electrode mixture layer 31. ３３ Here, the length L of the thin portion 33 is ３３ means the length of the thin portion 33 along the length direction of the positive electrode 11 (the length L of the second thin portion 34 ３４ The same applies to (1) and (2). The starting end 11x is the longitudinal end of the positive electrode mixture layer 31 located on the winding core side of the electrode body 14. In this embodiment, the winding start end of the positive electrode 11 and the starting end 11x of the positive electrode mixture layer 31 coincide with each other.

[0034] The second thin portion 34 is provided at a predetermined interval from the first thin portion 33 in a range from the starting end 11x of the positive electrode mixture layer 31 to 40% or less of the length L of the positive electrode mixture layer 31. That is, the thin portion 34 is provided at a predetermined length (L ３３ +Lx) as the starting position. In this embodiment, the first region 32 exists between the first thin portion 33 and the second thin portion 34.

[0035] The thin-walled portions 33, 34 are thought to form minute spaces between the positive electrode 11 and the negative electrode 12, absorbing the expansion of the negative electrode 12 and alleviating stress due to the expansion. Providing the thin-walled portions 33, 34 in the positive electrode mixture layer 31 suppresses plastic deformation of not only the positive electrode 11 but also the negative electrode 12. The thin-walled portions 33, 34 also reduce the expansion of the negative electrode 12 during charging to some extent. As a result, the expansion of the electrode plate in the width direction is suppressed, simplifying insulation measures, for example, by eliminating insulating plates disposed above and below the electrode body 14. Providing two thin-walled portions 33, 34 within a predetermined length range from the starting end 11x of the positive electrode mixture layer 31 significantly suppresses plastic deformation of the electrode plate.

[0036] Length L of the first thin-walled portion 33 ３３ If the length L is less than 1% of the length L of the positive electrode mixture layer 31, the effect of suppressing deformation of the electrode plate cannot be obtained. ３３ If the length L of the first thin-walled portion 33 exceeds 3% of the length L of the positive electrode mixture layer 31, the plastic deformation of the electrode plate cannot be suppressed. ３３ However, if the length L of the positive electrode mixture layer 31 is less than 1%, the formation of a space for suppressing plastic deformation and the suppression of the amount of expansion of the negative electrode are insufficient. If it is 3% or more, the mixture region between the second thin portion 34 is narrow, and the space retention is considered to be poor. ３３ When the thickness of the second thin portion 34 is 1% or more and 3% or less, the plastic deformation of the electrode plate is specifically suppressed by a synergistic effect with the second thin portion 34 .

[0037] Length L of the second thin-walled portion 34 ３４ is, for example, 20% or less of the length L of the positive electrode mixture layer 31, more preferably 15% or less, and particularly preferably 8% or less. ３４ is preferably 3% or more of the length L of the positive electrode mixture layer 31, more preferably 4% or more, and particularly preferably 6% or more. ３４ If the length L of the thin-walled portion 34 is too long, the effect of suppressing the deformation of the electrode plate will reach a plateau, while the capacity will decrease significantly. ３４ is particularly preferably 8% or less of the length L, and may be 6% or less.

[0038] Length L of the second thin-walled portion 34 ３４An example of a suitable range of the length L of the positive electrode mixture layer 31 is 3% or more and 15% or less, or 4% or more and 8% or less, or 6% or more and 8% or less. ３４ Within this range, it is believed that providing the two thin-walled portions 33, 34 within a predetermined length range from the starting end 11x of the positive electrode mixture layer 31 makes it easier to form a space between the negative electrode 12 and the thin-walled portions 33, 34 to absorb expansion of the negative electrode 12, and therefore plastic deformation of the electrode plate can be sufficiently suppressed even if the total length of the thin-walled portions 33, 34 is short.

[0039] Length L of the first thin-walled portion 33 ３３ The length L of the second thin-walled portion 34 is, for example, 5 mm or more and 30 mm or less, and more preferably 10 mm or more and 20 mm or less. ３４ is, for example, 25 mm or more and 75 mm or less, more preferably 30 mm or more and 60 mm or less. In this case, the effect of providing the thin portions 33, 34 becomes more pronounced. Note that, for example, the length L of the positive electrode mixture layer 31 is 600 mm or more and 900 mm or less.

[0040] The thinned portions 33 and 34 may be formed in a part of the width direction of the positive electrode 11, but are preferably formed over the entire width of the positive electrode 11. However, the L of the thinned portions 33 and 34 along the length direction of the positive electrode 11 is ３３ , L ３４ is preferably longer than the length (width) of the thin portions 33, 34 along the width direction of the positive electrode 11. An example of the width of the positive electrode 11 is 50 mm or more and 65 mm or less.

[0041] As described above, the second thin portion 34 is provided in a length range from the start end 11x of the positive electrode mixture layer 31 to 40% or less of the length L of the positive electrode mixture layer 31. The start position of the second thin portion 34 close to the start end 11x is a predetermined length (L ３３ +Lx). ３３+Lx) is preferably 3% to 33% of the length L of the positive electrode mixture layer 31, more preferably 4% to 30% and may be 8% to 20%. The end position of the second thin-walled portion 34 is a position away from the starting end 11x by a length equivalent to 40% of the length L of the positive electrode mixture layer 31, and may be a position away by a length equivalent to 25% of the length L.

[0042] The distance (predetermined length Lx) between the first thin portion 33 and the second thin portion 34 is preferably 3% to 30% of the length L of the positive electrode mixture layer 31, more preferably 5% to 27%, and particularly preferably 8% to 15%. In this case, a thick region of the predetermined length Lx is formed between the two thin portions 33, 34. As a result, it is considered that a space that absorbs expansion of the negative electrode 12 is easily formed between the thin portions 33, 34 and the negative electrode 12, and plastic deformation of the electrode plate can be more effectively suppressed.

[0043] In the examples shown in Figures 2 and 3, the first thin portion 33 and the second thin portion 34 are formed on only one side of the positive electrode 11. When the thin portions 33, 34 are formed on only one side of the positive electrode 11, the thin portions 33, 34 are preferably formed only on the outer surface of the wound positive electrode 11 facing the outside of the electrode body 14. The mixture layer on the inner side of the wound negative electrode 12, which faces the outer surface of the wound positive electrode 11, is thought to be more likely to be densified than the mixture layer on the outer side of the wound negative electrode 12, and therefore undergoes a larger volume change during charge and discharge. For this reason, when the thin portions 33, 34 are formed only on the positive electrode mixture layer 31 on the outer side of the wound positive electrode 11, electrode plate deformation can be efficiently suppressed while maintaining high capacity.

[0044] In this embodiment, the entire area of ​​the positive electrode mixture layer 31 on the inner winding surface of the positive electrode 11 facing inward of the electrode body 14 is a first region 32 having a substantially constant thickness, and thin-walled portions 33, 34 are formed only within a predetermined length range from the starting end 11x of the positive electrode mixture layer 31 on the outer winding surface.

[0045] FIG. 4 is a cross-sectional view showing another example of the embodiment. As shown in FIG. 4 , the positive electrode 11 may have a first thin-walled portion 33 and a second thin-walled portion 34 on both sides of the positive electrode substrate 30. The first thin-walled portion 33 is formed on both the outer and inner surfaces of the winding in a length range from the starting end 11x of the positive electrode mixture layer 31 corresponding to 1% to 3% of the length L of the positive electrode mixture layer 31, and the second thin-walled portion 34 is formed between each first thin-walled portion 33 and the first thin-walled portion 33, with a first region 32 separating them. According to the embodiment illustrated in FIG. 4 , the capacity is lower than when the thin-walled portions 33, 34 are provided only on the outer surface of the winding of the positive electrode 11 (the embodiment illustrated in FIGS. 2 and 3 ), but the same or greater effect of suppressing electrode plate deformation can be obtained.

[0046] Thickness T of the first thin-walled portion 33 ３３ and length L ３３ may be substantially the same on both sides of the positive electrode 11 or may be different. ３３ and length L ３３ When the thickness T of the thin-walled portion 33 located on the outer side of the winding is different, ３３ and length L ３３ The thickness T of the thin-walled portion 33 located on the inside of the winding ３３ and length L ３３ In this case, it is possible to efficiently suppress deformation of the electrode plate while maintaining a high capacity. ３４ and length L ３４ The thickness T of the thin-walled portion 34 located on the outer side of the winding may be substantially the same or different on both sides of the positive electrode 11. ３４ and length L ３４ is the thickness T of the thin-walled portion 34 located on the inside of the winding. ３４ and length L ３４ It may be larger than

[0047] The thin portions 33, 34 of the positive electrode 11 may be formed, for example, by applying a smaller amount of positive electrode mixture slurry to the regions where the thin portions 33, 34 are to be disposed than to other regions. In this case, the thicknesses of the first region 32 and the thin portions 33, 34 can be controlled to a desired range by adjusting the amount of positive electrode mixture slurry applied. Alternatively, the thin portions 33, 34 may be formed by applying a constant amount of positive electrode mixture slurry over the entire length of the positive electrode core 30 and then peeling off part of the coating within a predetermined length range.

[0048] When producing the electrode assembly 14, the positive electrode 11 is positioned so that the thin portions 33, 34 are located on the winding core side of the electrode assembly 14, and the electrode plate and separator 13 are wound. As a result, the thin portion 33 is formed from the starting end 11x on the winding start side of the positive electrode 11. When the thin portions 33, 34 are present on only one side of the positive electrode 11, it is preferable to position the positive electrode 11 so that the thin portions 33, 34 are located on the outer winding surface.

[0049] It is preferable that a first region 32 exists between the first thin-walled portion 33 and the second thin-walled portion 34. However, it is possible to form a thin-walled portion between each of the thin-walled portions 33, 34 as long as the object of the present disclosure is not impaired. For example, a thin-walled portion that is thicker than the adjacent thin-walled portion 33, 34 may be formed between each of the thin-walled portions 33, 34. A specific example is where a thin-walled portion having a thickness exceeding 90% of the average thickness of the first region 32 is formed between each of the thin-walled portions 33, 34, and the thin-walled portions 33, 34 have a thickness of 85% or less of the average thickness of the first region 32. Alternatively, the thin-walled portions 33, 34 may be provided on the outer surface of the positive electrode 11, and only one of the thin-walled portions 33, 34 may be provided on the inner surface of the positive electrode 11.

[0050] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the portion where the negative electrode lead 21 is connected. A protective layer containing inorganic particles and a binder may be disposed between the negative electrode core and the negative electrode mixture layer, or on the negative electrode mixture layer. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0051] As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. A single binder may be used, or multiple binders may be used in combination. The negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. The binder content is, for example, 0.1% or more and 5% or less of the mass of the negative electrode mixture layer. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[0052] Generally, a carbon material that reversibly absorbs and releases lithium ions is used as the negative electrode active material. Examples of carbon materials that function as the negative electrode active material include natural graphite, artificial graphite, and mixtures thereof. Elements that alloy with Li, such as Si and Sn, or materials containing such elements may be used as the negative electrode active material. Among these, silicon-containing materials containing Si are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination. The content of the negative electrode active material is, for example, 90% to 99.5% of the mass of the negative electrode mixture layer.

[0053] From the viewpoint of increasing the capacity of the battery, the negative electrode 12 preferably contains a silicon-containing material as the negative electrode active material. The content of the silicon-containing material is preferably 6% or more, more preferably 8% or more, and particularly preferably 10% or more, of the total mass of the negative electrode active material. The configuration of the cylindrical battery 10 including the positive electrode 11 including the thin-walled portion 33 is particularly suitable when using a negative electrode 12 that contains a silicon-containing material and exhibits a large volume change during charge and discharge. From the viewpoint of the cycle characteristics of the battery, the upper limit of the content of the silicon-containing material is preferably 80% of the total mass of the negative electrode active material, more preferably 70%, and particularly preferably 60%.

[0054] The silicon-containing material functioning as the negative electrode active material may be any material containing Si, and examples include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. 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 Si phase is composed of Si dispersed in the form of fine particles. The ion-conducting phase is, for example, at least one selected from a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase, and may contain at least one element selected from Groups 1 and 2 of the periodic table. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the Si phase.

[0055] The composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is composed of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer. The conductive layer is, for example, a carbon coating composed 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.

[0056] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si is dispersed almost uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘThe 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.

[0057] 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 ２ zSiO （２＋ｚ） (0<z<2). The lithium silicate phase is a composite oxide phase represented by the formula: Li 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.

[0058] 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 component, but preferably contains a larger amount of amorphous components. The composite material may contain particles whose ion-conducting phase is an amorphous carbon phase. The amorphous carbon phase is, for example, composed of 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.

[0059] [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. The separator 13 may have, for example, a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0060] A filler layer containing an inorganic filler may be disposed 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 containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. 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. In addition, a highly heat-resistant resin layer (heat-resistant layer) such as an aramid resin may be disposed on the surface of the separator 13. The separator 13 may have, for example, a substrate made of a porous sheet and a filler layer or heat-resistant layer disposed on the substrate.

[0061] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0062] Example 1 [Fabrication of Positive Electrode] As a positive electrode active material, lithium nickel oxide (LiNi) containing cobalt and aluminum was used. ０．８８ Co ０．０９ Al ０．０３ O ２ The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 98:1:1, and N-methylpyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. The slurry was applied to both sides of a positive electrode core made of a long aluminum foil with a thickness of 15 μm, and the coating was dried and compressed to form a positive electrode mixture layer (average thickness on one side: 90 μm, density: 3.6 g / cm) on both sides of the positive electrode core. ３A positive electrode mixture layer was formed on the entire positive electrode core except for an exposed portion of the positive electrode core to which a positive electrode lead was connected, which was left at the center in the length direction of the positive electrode core.

[0063] A portion of one of the positive electrode mixture layers was peeled off in a length range of 1% (approximately 7 mm) of the positive electrode mixture layer from one longitudinal end of the positive electrode mixture layer, and in a length range of 6% (approximately 40 mm) of the positive electrode mixture layer from a position 4% (approximately 30 mm) of the positive electrode mixture layer from one longitudinal end of the positive electrode mixture layer. This resulted in a positive electrode having first and second thin-walled portions with thicknesses 85% of the average thickness of the remaining region (first region) of the positive electrode mixture layer. That is, the start position of the first thin-walled portion was at one longitudinal end of the positive electrode mixture layer, and the start position of the second thin-walled portion was at a position 4% of the length of the positive electrode mixture layer from one longitudinal end of the positive electrode mixture layer. The thickness of the thin-walled portions was approximately constant, 76.5 μm ± 0.5 μm. An aluminum positive electrode lead was ultrasonically welded to the exposed portion of the positive electrode core.

[0064] [Fabrication of Negative Electrode] A mixture of graphite powder and a Si-containing material in a mass ratio of 90:10 was used as the negative electrode active material. The negative electrode active material, a dispersion of styrene butadiene rubber, and sodium carboxymethyl cellulose were mixed in a solid mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. The slurry was applied to both sides of a negative electrode core made of a long copper foil with a thickness of 8 μm, and the coating was dried and compressed to form a negative electrode mixture layer (average thickness on one side: 94 μm, density: 1.6 g / cm) on both sides of the negative electrode core. ３ A negative electrode was obtained in which a negative electrode mixture layer was formed. A core exposed portion was left at the end of the negative electrode core in the longitudinal direction, and a negative electrode mixture layer was formed over the entire negative electrode core except for the exposed portion. A nickel negative electrode lead was ultrasonically welded to the core exposed portion.

[0065] [Fabrication of Electrode Assembly] The positive electrode, the negative electrode, and a polyethylene separator were spirally wound around a cylindrical core member, and stop tapes were attached to both axial ends of the outermost peripheral surface to obtain a wound electrode assembly. At this time, the positive electrode was positioned so that the thin portion of the positive electrode mixture layer was located on the core side of the electrode assembly and faced the outside of the electrode assembly. After forming the wound structure of the electrode assembly, the core member was removed to obtain a wound electrode assembly with a cavity formed in the core portion.

[0066] [Preparation of non-aqueous electrolyte] 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 1:3 (25°C), and LiPF ６ was dissolved in a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte solution.

[0067] [Fabrication of Cylindrical Battery] After placing insulating plates above and below the electrode assembly, the negative electrode lead was welded to the inner bottom surface of a cylindrical outer can with a bottom, and the positive electrode lead was welded to the internal terminal plate of a sealing member, and the electrode assembly was housed in the outer can. Thereafter, a nonaqueous electrolyte solution was injected into the outer can under reduced pressure, and the opening of the outer can was sealed with a sealing member via a gasket, thereby obtaining a cylindrical battery.

[0068] Examples 2 and 3 Positive electrodes and cylindrical batteries were fabricated in the same manner as in Example 1, except that the start position of the second thin-walled portion of the positive electrode mixture layer was changed to a position that was 9% and 15% of the length of the positive electrode mixture layer away from one end in the longitudinal direction of the positive electrode mixture layer, respectively.

[0069] Examples 4 and 5 Positive electrodes and cylindrical batteries were fabricated in the same manner as in Example 1, except that the length of each of the first thin portions of the positive electrode mixture layer was changed to 3% of the length of the positive electrode mixture layer, and the start positions of the second thin portions were changed to positions that were 6% and 30% of the length of the positive electrode mixture layer away from one end of the positive electrode mixture layer in the longitudinal direction, respectively.

[0070] Examples 6 to 8 Positive electrodes and cylindrical batteries were fabricated in the same manner as in Example 1, except that the thickness of the thin portion of the positive electrode mixture layer was changed to 60%, 40%, and 20% of the average thickness of the first region, respectively.

[0071] Comparative Example 1 A positive electrode and a cylindrical battery were fabricated in the same manner as in Example 1, except that the first and second thin portions of the positive electrode mixture layer were not formed.

[0072] Comparative Example 2 A positive electrode and a cylindrical battery were produced in the same manner as in Example 1, except that a thin-walled portion was formed from one end of the positive electrode mixture layer in the longitudinal direction to a length that was 7% of the length of the positive electrode mixture layer.

[0073] Comparative Examples 3 and 4 Positive electrodes and cylindrical batteries were fabricated in the same manner as in Example 1, except that the length of each of the first thin portions of the positive electrode mixture layer was changed to 4% of the length of the positive electrode mixture layer, the start positions of the second thin portions were changed to positions that were 7% and 6% of the length of the positive electrode mixture layer away from one end of the positive electrode mixture layer in the length direction, and the lengths of the second thin portions were changed to 3% of the length of the positive electrode mixture layer.

[0074] Comparative Example 5 A positive electrode and a cylindrical battery were fabricated in the same manner as in Example 1, except that the thicknesses of the first and second thin portions of the positive electrode mixture layer were each changed to 95% of the average thickness of the first region.

[0075] Comparative Example 6 A positive electrode and a cylindrical battery were produced in the same manner as in Example 4, except that the first thin portion of the positive electrode mixture layer was not formed, and the starting position of the second thin portion was changed to a position away from one end of the positive electrode mixture layer in the longitudinal direction by a length that was 3% of the length of the positive electrode mixture layer.

[0076] [Evaluation of Plastic Deformation of Electrode Plates (Electrode Plate Elongation)] Each battery of the Examples and Comparative Examples that underwent the cycle test described below was disassembled to remove the positive electrode. The width of the positive electrode was measured to determine the maximum value, and the ratio (elongation rate) of the positive electrode width after the cycle test to the positive electrode width before the cycle test was calculated. The evaluation results are shown in Table 1. The evaluation results shown in Table 1 are relative values ​​when the elongation rate of the positive electrode of Comparative Example 1 is set to 100, and a smaller value means that plastic deformation of the positive electrode is more suppressed.

[0077] [Cycle Test] Each battery of the Example and Comparative Examples was charged at a constant current of 0.3 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V. Thereafter, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 100 times.

[0078]

[0079] As shown in Table 1, the elongation rate of the electrode plate (positive electrode) after the cycle test was smaller in the batteries of the examples than in the batteries of the comparative examples, and plastic deformation of the electrode plate was suppressed. Even if a thin portion is provided in the positive electrode mixture layer, if only the first thin portion or the second thin portion is present, or if the thin portion does not satisfy the predetermined conditions, the effect of the thin portion cannot be obtained (see comparative examples 2 to 6).

[0080] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode for a secondary battery including a long positive electrode core and positive electrode mixture layers disposed on both sides of the positive electrode core, wherein the positive electrode mixture layer includes a first region and a second region thinner than the first region, the thickness of the second region being 20% ​​to 90% of the average thickness of the first region, and the second region including a first thin-walled portion extending from one end of the positive electrode mixture layer in the longitudinal direction thereof to a length of 1% to 3% of the length of the positive electrode mixture layer, and a second thin-walled portion extending from one end of the positive electrode mixture layer in the longitudinal direction thereof to a length of 40% or less of the length of the positive electrode mixture layer, with a predetermined gap between the first thin-walled portion and the second thin-walled portion. Configuration 2: The positive electrode for a secondary battery according to Configuration 1, wherein the length of the second thin-walled portion is 4% to 8% of the length of the positive electrode mixture layer. Configuration 3: The positive electrode for a secondary battery according to Configuration 1 or 2, wherein the length of the first thin portion is 10 mm or more and 20 mm or less.Configuration 4: The positive electrode for a secondary battery according to any one of Configurations 1 to 3, wherein the length of the second thin portion is 30 mm or more and 60 mm or less.Configuration 5: A secondary battery comprising an electrode assembly including the positive electrode for a secondary battery according to any one of Configurations 1 to 4, a negative electrode, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween, wherein the positive electrode is provided so that the second region is located on the winding core side of the electrode assembly.Configuration 6: The secondary battery according to Configuration 5, wherein the second region of the positive electrode is formed only on a surface facing outward from the electrode assembly.Configuration 7: The secondary battery according to Configuration 5 or 6, wherein the negative electrode contains a silicon-containing material in an amount of 6% or more of the total mass of the negative electrode active material.

[0081] REFERENCE SIGNS LIST 10 Cylindrical battery, 11 Positive electrode, 11x Starting end, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18 Upper insulating plate, 19 Lower insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Bottom plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 32 First region, 33 First thin-walled portion, 34 Second thin-walled portion

Claims

1. A positive electrode for a secondary battery comprising a long positive electrode core and positive electrode mixture layers arranged on both sides of the positive electrode core, wherein the positive electrode mixture layer includes a first region and a second region having a thickness thinner than the first region, the thickness of the second region being 20% ​​to 90% of the average thickness of the first region, and the second region including a first thin-walled portion provided from one end of the positive electrode mixture layer in the longitudinal direction thereof to a length of 1% to 3% of the length of the positive electrode mixture layer, and a second thin-walled portion provided in a range of 40% or less of the length of the positive electrode mixture layer from one end of the longitudinal direction of the positive electrode mixture layer with a predetermined gap between the first thin-walled portion and the first thin-walled portion.

2. The positive electrode for a secondary battery according to claim 1, wherein the length of said second thin portion is 4% or more and 8% or less of the length of said positive electrode mixture layer.

3. The positive electrode for a secondary battery according to claim 1, wherein the length of the first thin portion is 10 mm or more and 20 mm or less.

4. The positive electrode for a secondary battery according to claim 1, wherein the length of the second thin portion is 30 mm or more and 60 mm or less.

5. A secondary battery comprising an electrode assembly including the positive electrode for a secondary battery according to any one of claims 1 to 4, a negative electrode, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween, wherein the positive electrode is provided so that the second region is located on the winding core side of the electrode assembly.

6. The secondary battery according to claim 5, wherein the second region of the positive electrode is formed only on the outer wound surface facing the outside of the electrode body.

7. The secondary battery according to claim 5, wherein the negative electrode contains a silicon-containing material in an amount of 6% or more of the total mass of the negative electrode active material.