Secondary battery and positive electrode for secondary battery

Abstract

A secondary battery according to an embodiment of the present disclosure comprises an electrode group in which a long positive electrode, a long negative electrode, and a long separator positioned between the positive electrode and the negative electrode are wound in the length direction. The positive electrode comprises a long positive electrode current collector and a long positive electrode mixture layer positioned on at least one surface of the positive electrode current collector. A cavity region extending from the winding start side of the positive electrode toward the winding end side of the positive electrode is formed in the positive electrode mixture layer so as to include one end portion near the winding start of the positive electrode.

Description

Secondary batteries and positive electrodes for secondary batteries

[0001] This invention relates to a secondary battery and a positive electrode for a secondary battery.

[0002] Secondary batteries, such as lithium-ion batteries, are used in a variety of applications due to their characteristics, including high capacity. A secondary battery, for example, comprises an electrode group (wound electrode group) in which a long positive electrode, a long negative electrode, and a long separator placed between the positive and negative electrodes are wound in the longitudinal direction.

[0003] Patent Document 1 discloses a non-aqueous electrolyte secondary battery comprising a wound electrode body in which a positive electrode and a negative electrode having a negative electrode active material layer formed on the surface of a strip-shaped negative electrode current collector are wound in a spiral shape with a separator in between, wherein the negative electrode includes a negative electrode lead joined to the winding start end of the negative electrode current collector and is wound at least once from the inner end in the winding direction without facing the positive electrode via the separator, and includes an insulating tape attached to the negative electrode current collector so as to straddle the surface of the negative electrode lead in the winding direction, with a portion of the insulating tape overlapping the negative electrode active material layer.

[0004] Furthermore, Patent Document 1 discloses that in a non-aqueous electrolyte secondary battery configured as described above, insulating tape is attached to the negative electrode current collector so as to straddle the surface of the negative electrode lead, thereby suppressing electrode plate deformation associated with charge-discharge cycles in a wound-type electrode body in which the negative electrode lead is joined to the winding start end of the negative electrode current collector.

[0005] Patent No. 7035017

[0006] In secondary batteries equipped with wound electrode groups, there is room for further investigation into suppressing electrode plate deformation during charging and discharging, particularly the suppression of deformation of the positive electrode current collector.

[0007] Therefore, this disclosure aims to provide a secondary battery that can suppress deformation of the positive electrode current collector during charging and discharging. Furthermore, this disclosure also aims to provide a positive electrode for a secondary battery that can suppress deformation of the positive electrode current collector during charging and discharging.

[0008] One aspect of the present invention relates to a secondary battery. The secondary battery comprises an electrode group in which a long positive electrode, a long negative electrode, and a long separator arranged between the positive electrode and the negative electrode are wound in the longitudinal direction. The positive electrode comprises a long positive electrode current collector and a long positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode mixture layer has a cavity region formed therein that extends from the starting end of the winding of the positive electrode toward the ending end of the winding of the positive electrode, including one end on the winding starting end of the positive electrode.

[0009] Another aspect of this disclosure relates to a positive electrode for a secondary battery, comprising an elongated positive electrode current collector and an elongated positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode mixture layer has a cavity region formed therein, extending from one end in the longitudinal direction to the other end in the longitudinal direction, so as to include one end of the positive electrode in the longitudinal direction.

[0010] According to this disclosure, it is possible to provide a secondary battery that can suppress deformation of the positive electrode current collector during charging and discharging. Furthermore, according to this disclosure, it is also possible to provide a positive electrode for a secondary battery that can suppress deformation of the positive electrode current collector during charging and discharging.

[0011] This is a plan view showing the positive electrode according to the first aspect of this disclosure as viewed from one side. This is a plan view showing the positive electrode according to the second aspect of this disclosure as viewed from one side. This is a plan view showing the positive electrode according to the third aspect of this disclosure as viewed from one side. This is a plan view showing the positive electrode according to the fourth aspect of this disclosure as viewed from one side. This is a plan view showing the positive electrode according to the fifth aspect of this disclosure as viewed from one side. This is a schematic cross-sectional view showing a secondary battery according to one embodiment of this disclosure.

[0012] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is used, that range includes numerical values ​​A and B.

[0013] In the following explanation, when examples are given for the lower and upper limits of numerical values ​​related to specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.

[0014] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.

[0015] [Secondary Battery] A secondary battery according to the embodiment of this disclosure comprises an electrode group in which a long positive electrode, a long negative electrode, and a long separator arranged between the positive electrode and the negative electrode are wound in the longitudinal direction.

[0016] In the secondary battery according to the embodiment of the present disclosure, the positive electrode comprises an elongated positive electrode current collector and an elongated positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. In the secondary battery according to the embodiment of the present disclosure, a cavity region is formed in the positive electrode mixture layer, extending from the starting end of the positive electrode to the ending end of the positive electrode, including one end on the starting end of the positive electrode.

[0017] In the secondary battery according to the embodiment of this disclosure, it is important that the positive electrode mixture layer has a cavity region that extends from the beginning of the winding of the positive electrode to the end of the winding of the positive electrode, including one end of the winding on the beginning side of the positive electrode. The reason for this is explained below.

[0018] In a secondary battery, the negative electrode typically comprises a negative electrode active material, which is contained, for example, in a negative electrode mixture layer. Such a negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material expands by absorbing lithium ions during charging and contracts by releasing lithium ions during discharge. That is, the negative electrode active material repeatedly expands and contracts with charging and discharging. Furthermore, as described above, when the negative electrode comprises a negative electrode mixture layer, the negative electrode mixture layer also repeatedly expands and contracts in conjunction with the expansion and contraction of the negative electrode active material during charging and discharging.

[0019] When a secondary battery has an electrode group consisting of a long positive electrode, a long negative electrode, and a long separator wound in the longitudinal direction between the positive and negative electrodes, that is, when the secondary battery has a wound electrode group, the positive electrode is pressed radially by the negative electrode mixture layer that expands during charging. This radial pressure can cause the positive electrode to deform. More specifically, the radial pressure caused by the expansion of the negative electrode mixture layer generates stress on the positive electrode current collector, which can deform. When deformation occurs in the positive electrode current collector, variations can occur in the distance between the positive and negative electrodes in the radial direction of the wound electrode group. In this case, parts of the wound electrode group become easier to charge and parts become harder to charge (i.e., charge / discharge unevenness occurs), and the battery capacity of the secondary battery decreases with each repeated charge / discharge cycle. In other words, the cycle maintenance rate of the secondary battery decreases.

[0020] However, in the secondary battery according to the embodiment of this disclosure, a cavity region is formed in the positive electrode mixture layer, extending from the beginning of the winding to the end of the winding, including one end of the positive electrode on the winding start side. In this case, even if stress is generated in the positive electrode current collector due to radial pressure accompanying the expansion of the negative electrode mixture layer, this stress can be released into the cavity region. Therefore, the stress generated in the positive electrode current collector can be made relatively small. This makes it possible to suppress deformation of the positive electrode current collector due to stress. In other words, deformation of the positive electrode current collector accompanying charging and discharging can be suppressed.

[0021] The components of the secondary battery according to the embodiments of this disclosure will be described below. Specifically, the positive electrode, negative electrode, electrolyte, and separator will be described.

[0022] (Positive electrode) The positive electrode has an elongated shape. The positive electrode comprises an elongated positive electrode current collector and an elongated positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode mixture layer may be disposed on only one side of the positive electrode current collector or on both sides of the positive electrode current collector. A protective layer containing inorganic particles and a binder may be disposed between the positive electrode current collector and the positive electrode mixture layer, or on the positive electrode mixture layer. In the secondary battery according to the embodiment of this disclosure, the positive electrode is wound longitudinally together with an elongated negative electrode with an elongated separator interposed between them. That is, in the secondary battery according to the embodiment of this disclosure, the positive electrode is a component of a wound-type secondary battery.

[0023] The positive electrode will be described below with reference to Figures 1A to 1E. Figures 1A to 1E are all plan views showing the positive electrode from one side. As mentioned above, the positive electrode is a component of a wound-type secondary battery. Therefore, one end of the positive electrode in the longitudinal direction is called the winding start end, and the other end of the positive electrode in the longitudinal direction is called the winding end end.

[0024] As shown in Figure 1A, the positive electrode 11 comprises a positive electrode current collector 11A and a positive electrode mixture layer 11B disposed on at least one surface of the positive electrode current collector 11A. In the positive electrode 11 according to the first embodiment shown in Figure 1A, a cavity region H is formed in the positive electrode mixture layer 11B, extending from the winding start side WS of the positive electrode 11 toward the winding end side WE of the positive electrode 11, including one end PE1 of the winding start side WS of the positive electrode 11. The cavity region H has one end HE1 on the winding start side WS of the positive electrode 11 and the other end HE2 on the winding end side WE of the positive electrode 11.

[0025] As explained above, the winding start side WS of the positive electrode 11 is one end of the longitudinal direction LD of the positive electrode, and the winding end side WE of the positive electrode 11 is the other end of the longitudinal direction LD of the positive electrode. Furthermore, in the positive electrode 11 according to the first embodiment shown in Figure 1A, the cavity region H is formed only in the positive electrode mixture layer 11B. A part of the positive electrode current collector 11A is exposed from the cavity region H.

[0026] On the other hand, in the positive electrode 11 according to the second embodiment shown in Figure 1B, the cavity region H is formed to penetrate both the positive electrode mixture layer 11B and the positive electrode current collector 11A. That is, the cavity region H may be formed only in the positive electrode mixture layer 11B, or it may be formed in both the positive electrode mixture layer 11B and the positive electrode current collector 11A. In the positive electrode 11, if the positive electrode mixture layer 11B is arranged on both sides of the positive electrode current collector 11A, the cavity region H may be formed only in the positive electrode mixture layer 11B arranged on one side, or it may be formed in each of the positive electrode mixture layers 11B arranged on both sides. In this case, it is preferable that the cavity region H is formed in each of the positive electrode mixture layers 11B arranged on both sides. This makes it possible to increase the volume of the escape route for the stress generated in the positive electrode current collector 11A, so that the cavity region H can function even more effectively as an escape route for the stress generated in the positive electrode current collector 11A.

[0027] Even when the cavity region H is formed only in the positive electrode mixture layer 11B, as in the first embodiment shown in Figure 1A, it can adequately function as a stress relief area for the positive electrode current collector 11A. On the other hand, from the viewpoint of increasing the volume of the stress relief area for the positive electrode current collector 11A, it is preferable that the cavity region H is formed in both the positive electrode mixture layer 11B and the positive electrode current collector 11A, as in the second embodiment shown in Figure 1B. This allows the cavity region H to function even more adequately as a stress relief area for the positive electrode current collector 11A.

[0028] In the first embodiment shown in Figure 1A and the second embodiment shown in Figure 1B, the cavity region H is rectangular in shape when viewed from above. However, the shape of the cavity region H when viewed from above is not limited to a rectangle. The shape of the cavity region H when viewed from above may be triangular, trapezoidal, or even rectangular with rounded corners.

[0029] Although the first embodiment shown in Figure 1A and the second embodiment shown in Figure 1B show an example in which the positive electrode 11 has one cavity region H, the number of cavity regions H is not limited to one. The positive electrode 11 may have multiple cavity regions H. In this case, the multiple cavity regions H may be formed to be aligned in the width direction WD of the positive electrode 11.

[0030] The cavity region H may be formed by cutting out a portion of at least one of the positive electrode current collector 11A and the positive electrode mixture layer 11B using a cutter or laser. Specifically, in the first embodiment, the cavity region H can be formed by cutting out a portion of the positive electrode mixture layer 11B using a cutter or laser. In the second embodiment, the cavity region H can be formed by cutting out a portion of both the positive electrode mixture layer 11B and a portion of the positive electrode current collector 11A using a cutter or laser. Furthermore, as in the first embodiment, when the cavity region H is formed only in the positive electrode mixture layer 11B, such a cavity region H may be formed by applying tape (for example, masking tape) to the positive electrode mixture layer 11B and then peeling off a part of the positive electrode mixture layer 11B with the tape, or by impregnating a part of the positive electrode mixture layer 11B with an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) and then removing the part that has been softened by the impregnation of the organic solvent. Moreover, as in the first embodiment, when the cavity region H is formed only in the positive electrode mixture layer 11B, as will be described later, when forming the positive electrode mixture layer 11B using a positive electrode mixture slurry, the cavity region H may be formed by making the area corresponding to the cavity region H an uncoated area. That is, the uncoated area of ​​the positive electrode mixture slurry may be used as the cavity region H.

[0031] As shown in Figures 1A and 1B, the elongated positive electrode 11 according to the first and second embodiments is provided with a positive electrode tab (or positive electrode lead) 20 extending along the width direction WD at one end WE1 in the width direction WD. Although Figures 1A and 1B show an example in which the positive electrode tab 20 is configured to be elongated, the shape of the positive electrode tab 20 is not limited to elongated. The shape of the positive electrode tab 20 may be trapezoidal. Also, the shape of the positive electrode tab 20 may be elongated with rounded corners. Furthermore, although Figures 1A and 1B show an example in which the positive electrode 11 is provided with one positive electrode tab 20 extending along the width direction WD, the number of positive electrode tabs 20 is not limited to one. The positive electrode 11 may have multiple positive electrode tabs 20 spaced apart along the length direction LD at one end in the width direction WD. The positive electrode tab 20 is connected to the positive electrode current collector 11A. Therefore, at the point where the positive electrode tab 20 is connected to the positive electrode 11, the positive electrode current collector 11A is exposed.

[0032] In the first embodiment shown in Figure 1A and the second embodiment shown in Figure 1B, the positive electrode tab 20 is arranged so as not to overlap with the cavity region H in the longitudinal direction LD of the positive electrode 11, but the arrangement of the positive electrode tab 20 is not limited to this. The positive electrode tab 20 may be arranged so as to overlap with the cavity region H in the longitudinal direction LD of the positive electrode 11. On the other hand, it is preferable that the positive electrode tab 20 is arranged so as not to overlap with the cavity region H in the longitudinal direction LD of the positive electrode 11, as in the first and second embodiments.

[0033] In the positive electrode 11, if the region where the cavity region H is formed is designated as the first region, and the region where the cavity region H is not formed is designated as the second region, then the portion of the positive electrode 11 included in the first region is relatively weaker than the portion of the positive electrode 11 included in the second region due to the presence of the cavity region H. Furthermore, when the positive electrode 11 is wound in the longitudinal direction LD, the presence of the cavity region H makes the portion of the positive electrode 11 included in the first region more prone to radial movement than the portion of the positive electrode 11 included in the second region. Therefore, when the positive electrode tab 20 is positioned so as to overlap with the cavity region H in the longitudinal direction LD of the positive electrode 11, that is, when the positive electrode tab 20 is positioned in the first region, the strength is reduced, and the portion of the positive electrode 11 included in the first region is more prone to deformation due to its increased radial movement. Moreover, this increased tendency to deform can cause the portion of the positive electrode 11 included in the first region to break. However, if the positive electrode tab 20 is positioned in the longitudinal direction LD of the positive electrode 11 so as not to overlap with the cavity region H, that is, if the positive electrode tab 20 is positioned in the second region, then when the positive electrode 11 is wound in the longitudinal direction LD, deformation or damage to the portion of the positive electrode 11 where the cavity region H is formed, as described above, can be suppressed. In other words, it is possible to suppress an increase in the winding failure rate of the wound electrode group due to the presence of a positive electrode 11 that has undergone at least one of deformation and / or damage.

[0034] As described above, when the positive electrode tab 20 is positioned on the positive electrode 11 so as not to overlap with the cavity region H, it is preferable that the positive electrode tab 20 is positioned within a separation range of up to 400 mm from the other end HE2 of the cavity region H toward the winding end side WE of the positive electrode 11. This further suppresses deformation or damage to the portion of the positive electrode 11 where the cavity region H is formed. In other words, it further suppresses an increase in the winding defect rate of the wound electrode group.

[0035] Within the spacing range of up to 400 mm as described above, the arrangement position of the positive electrode tab 20 on the positive electrode 11 is arbitrary. For example, as shown in FIGS. 1A and 1B, the positive electrode tab 20 may be arranged at a position relatively close to the other end HE2 of the hollow region H. Alternatively, the positive electrode tab 20 may be arranged at a position relatively far from the other end HE2 of the hollow region H, like the positive electrode 11 according to the third aspect shown in FIG. 1C. Furthermore, the positive electrode tab 20 may be arranged along the other end HE2 of the hollow region H (that is, the separation distance between the other end H2 of the hollow region and the positive electrode tab 20 is 0 mm). Note that the dimension in the length direction of the positive electrode 11 (that is, the total length) is usually 500 mm or more.

[0036] In the length direction LD of the positive electrode 11, the ratio R1 of the length dimension L2 of the hollow region H to the total length L1 of the positive electrode mixture layer 11B is preferably 30% or less. This makes it possible to secure a relatively high capacity while suppressing deformation of the positive electrode current collector 11A during charge and discharge. From the viewpoint of more sufficiently achieving higher capacity, the ratio R1 is more preferably 15% or less. Further, the ratio R1 is preferably 0.5% or more. Even in this case, deformation of the positive electrode current collector 11A during charge and discharge can be suppressed.

[0037] In the length direction LD of the positive electrode 11, the length dimension L2 of the hollow region H is preferably 5 mm or more and 200 mm or less. This makes it possible to secure a relatively high capacity while suppressing deformation of the positive electrode current collector 11A during charge and discharge. From the viewpoint of more sufficiently achieving higher capacity, the upper limit of the length dimension L2 of the hollow region H in the length direction LD of the positive electrode 11 is more preferably 100 mm or less.

[0038] In the width direction WD of the positive electrode 11, the ratio R2 of the width dimension W21 of the hollow region H to the total width WT1 of the positive electrode mixture layer is preferably 15% or less. This makes it possible to secure a relatively high capacity while suppressing deformation of the positive electrode current collector 11A during charge and discharge. From the viewpoint of more sufficiently achieving higher capacity, the ratio R2 is more preferably 10% or less, and even more preferably 7% or less. Further, the ratio R2 is preferably 0.1% or more. Even in this case, deformation of the positive electrode current collector 11A during charge and discharge can be suppressed.

[0039] In the width direction WD of the positive electrode 11, the width dimension W21 of the cavity region H is preferably 0.1 mm or more and 8 mm or less. This makes it possible to secure a relatively high capacity while suppressing deformation of the positive electrode current collector 11A during charging and discharging. From the viewpoint of more sufficiently achieving higher capacity, in the width direction WD of the positive electrode 11, the upper limit of the width dimension W21 of the cavity region H is more preferably 4 mm or less.

[0040] The dimension in the width direction (that is, the total width) of the positive electrode mixture layer 11B is usually 50 mm or more. In this case, it is preferable that the cavity region H is formed such that in the width direction WD of the positive electrode 11, the central portion HC of the cavity region H is included within a separation range of 10 mm from the central portion 11C of the positive electrode mixture layer 11B.

[0041] In a wound electrode group, the positive electrode 11 is wound in the length direction LD. Therefore, the width direction WD of the positive electrode 11 corresponds to the winding axis direction (height direction) of the wound electrode group. Even if stress is generated in the positive electrode current collector 11A due to the radial pressing force accompanying the expansion of the negative electrode mixture layer during charging of the wound electrode group, both end sides in the winding axis direction (height direction) are open, so these both end sides can serve as a release place for the aforementioned stress. On the other hand, since there is no open portion in the central portion of the wound electrode group in the winding axis direction (height direction), there is no release place for stress in this central portion. Therefore, in a wound electrode group, large stress tends to remain in the positive electrode current collector 11A located at the central portion in the winding axis direction (height direction) after charging. Due to this cause, in a wound electrode group, deformation or damage is likely to occur in the positive electrode current collector 11A located at the central portion in the winding axis direction (height direction).

[0042] However, if the cavity region H is formed such that, in the width direction WD of the positive electrode 11, the center HC of the cavity region H is included within a separation range of 10 mm from the center 11C of the positive electrode mixture layer 11B, then in a wound electrode group, the cavity region H can be positioned in the central part in the winding axis direction (height direction). This allows the stress generated in the positive electrode current collector 11A located in the central part in the winding axis direction (height direction) of the wound electrode group to be released into the cavity region H. Therefore, deformation or damage to the positive electrode current collector 11A located in the central part in the winding axis direction (height direction) can be suppressed.

[0043] The formation of the cavity region H such that its central point HC is included within a 10 mm separation range from the central point 11C of the positive electrode mixture layer 11B in the width direction WD of the positive electrode 11 will be explained below with reference to the drawings. Specifically, as shown in the fourth embodiment in Figure 1D, the central point HC of the cavity region H, indicated by the solid line, may be shifted to one end WE1 in the width direction of the positive electrode mixture layer 11B so that it is included within a 10 mm separation range from the central point 11C of the positive electrode mixture layer 11B in the width direction WD. Alternatively, as shown in the fifth embodiment in Figure 1E, the central point HC of the cavity region H, indicated by the solid line, may be shifted to the other end WE2 in the width direction of the positive electrode mixture layer 11B so that it is included within a 10 mm separation range from the central point 11C of the positive electrode mixture layer 11B in the width direction WD.

[0044] In Figures 1D and 1E, the central HC' of the cavity region H is shown by a dashed line when it is not misaligned with the central 11C in the width direction WD. Therefore, whether or not the central HC of the cavity region H is misaligned with the central 11C of the positive electrode mixture layer 11B toward one end WE1 or the other end WE2 in the width direction WD can be determined by using the imaginary line VL (shown by a dashed line) connecting the central 11C of the positive electrode mixture layer 11B and the central HC' shown by the dashed line as a reference.

[0045] Examples of positive electrode current collectors include foils made of metals that are stable within the positive electrode potential range, such as aluminum, titanium, aluminum alloys, and stainless steel. Alternatively, positive electrode current collectors may include films with a metal surface that is stable within the positive electrode potential range.

[0046] The positive electrode mixture layer contains a positive electrode active material as an essential component. The positive electrode mixture layer may also contain optional components such as a conductive agent and a binder.

[0047] The positive electrode active material is 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 that the lithium transition metal composite oxide contains at least one selected from the group consisting of Ni, Co, Mn, and Al. Suitable lithium transition metal composite oxides include composite oxides containing Ni, Co, and Mn, and composite oxides containing Ni, Co, and Al. The lithium transition metal composite oxide may be used alone or in combination of two or more types.

[0048] Lithium transition metal composite oxides have, for example, a layered rock salt structure. Examples of layered rock salt structures include structures belonging to space group R-3m and structures belonging to space group C2 / m. From the viewpoint of achieving high capacity and exhibiting a high crystalline structure, the layered rock salt structure is preferably a structure belonging to space group R-3m. The content of the positive electrode active material is, for example, 90% to 99% of the mass of the positive electrode mixture layer. From the viewpoint of achieving high capacity, the density of the positive electrode mixture layer is 3.3 g / cm³. ３ The above is also acceptable, or 3.3 g / cm³. ３ 3.8g / cm or more ３ The following is preferable:

[0049] Examples of conductive agents include carbon black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, and conductive whiskers. Examples of carbon black include acetylene black (AB) and Ketjen black (KB). Conductive agents may be used individually or in combination of two or more. The content of the conductive agent is, for example, 0.1% to 5% by mass of the positive electrode mixture layer.

[0050] Examples of binders include fluororesins, olefin resins, and acrylic resins. Examples of fluororesins include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). Examples of olefin resins include polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer. Examples of acrylic resins include ethylene-acrylic acid copolymer. Examples of binders include polyacrylonitrile (PAN), polyimide, and polyamide. The above binders may be used in combination with at least one selected from the group consisting of carboxymethylcellulose (CMC), salts of CMC, and polyethylene oxide (PEO). The binders may be used alone or in combination of two or more. The binder content is, for example, 0.1% to 5% of the mass of the positive electrode mixture layer.

[0051] The positive electrode can be manufactured, for example, by applying a positive electrode slurry containing a positive electrode active material, a conductive agent, and a binder to both sides of a positive electrode current collector to form a coating film on each side, and then drying the coating films formed on both sides. The dried coating film may be rolled if necessary. The positive electrode slurry contains a dispersion medium. As the dispersion medium, for example, N-methyl-2-pyrrolidone (NMP) can be used.

[0052] (Negative electrode) The negative electrode has an elongated shape. The negative electrode contains a negative electrode active material as an essential component. The negative electrode may comprise an elongated negative electrode current collector and an elongated negative electrode mixture layer disposed on at least one surface of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material as an essential component. The negative electrode mixture layer may be disposed on only one side of the negative electrode current collector or on both sides of the negative electrode current collector. A protective layer containing inorganic particles and a binder may be disposed between the negative electrode current collector and the negative electrode mixture layer, or on the negative electrode mixture layer.

[0053] Examples of negative electrode current collectors include foils of metals that are stable in the negative electrode potential range, such as copper, nickel, copper alloys, nickel alloys, and stainless steel. Alternatively, negative electrode current collectors may include films with a metal stable in the negative electrode potential range arranged on their surface.

[0054] The negative electrode mixture layer contains a negative electrode active material as an essential component. The negative electrode mixture layer may also contain optional components such as a conductive agent and a binder.

[0055] The negative electrode active material is a material that reversibly intercalates and releases lithium ions. Examples of such materials include carbon materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Examples of graphite include natural graphite and artificial graphite. Graphite is preferred as the carbon material because it provides excellent stability during charging and discharging and has low irreversible capacity.

[0056] Graphite is a carbonaceous material in which a graphite-type crystal structure is well-developed. The interplanar spacing d002 of the (002) planes of graphite, as measured by X-ray diffraction, may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) can be measured, for example, by the Scherrer method. When the interplanar spacing d002 of the (002) planes of graphite and the crystallite size Lc(002) are within the above ranges, high capacity is easily obtained.

[0057] Examples of negative electrode active materials include elements that alloy with Li, such as Si and Sn, as well as materials containing such elements. Among these, silicon-based materials are preferred. Examples of negative electrode active materials include lithium titanate, which has a higher charge-discharge potential for metallic lithium than carbon materials. The negative electrode active material may be used alone or in combination of two or more types. The content of the negative electrode active material is, for example, 90% to 99.5% of the mass of the negative electrode mixture layer.

[0058] From the viewpoint of increasing capacity, the negative electrode preferably contains a silicon-based material as the negative electrode active material. The content of the silicon-based material may be 6% or more, 8% or more, or 10% or more of the total mass of the negative electrode active material. The upper limit of the silicon-based material content may be 80% or less, 70% or less, or 60% or less of the total mass of the negative electrode active material. When the negative electrode contains a silicon-based material as the negative electrode active material, the degree of expansion and contraction of the negative electrode mixture layer during charging and discharging will be large. However, in the secondary battery according to the embodiment of this disclosure, a cavity region is formed in the positive electrode, so even if the degree of expansion and contraction of the negative electrode mixture layer is large, deformation or damage to the positive electrode current collector can be suppressed.

[0059] Examples of silicon-based materials include silicon alloys, silicon compounds, and silicon-containing composite materials (hereinafter also referred to as silicon-containing materials). Among these, silicon-containing materials are preferred. Silicon-containing materials are composite particles comprising an ionic conduction phase and a Si phase dispersed in the ionic conduction phase. The Si phase is composed of Si dispersed in fine particulate matter. The ionic conduction phase is, for example, at least one selected from the group consisting of silicate phase, carbon phase, silicide phase, and silicon oxide phase. The ionic conduction phase may contain at least one element selected from Group 1 and Group 2 of the periodic table. The ionic conduction phase is a continuous phase composed of an aggregate of particles finer than the Si phase.

[0060] The surface of the silicon-containing material may be covered with a conductive layer. An ion conductive phase is mainly present on the surface of the silicon-containing material. Therefore, the conductive layer may cover the ion conductive phase. Since the conductive layer is formed of a material having higher conductivity than the ion conductive phase, forming the conductive layer on the silicon-containing material allows a favorable conductive path to be formed in the negative electrode mixture layer. The conductive layer is, for example, a carbon coating formed of a conductive carbon material. Examples of the conductive carbon material that can be used include carbon black (e.g., acetylene black, Ketjenblack, etc.), graphite, and amorphous carbon with low crystallinity. In consideration of ensuring conductivity and diffusivity of Li ions into the particles, the thickness of the conductive layer is preferably 1 nm or more and 200 nm or less, and more preferably 5 nm or more and 100 nm or less.

[0061] A preferred example of the silicon-containing material has a sea-island structure in which fine Si is substantially uniformly dispersed in an amorphous silicon oxide phase, and as a whole, is represented by the general formula: SiO ｘ which is a composite particle represented by (0 < x ≤ 2). A main component of the silicon oxide may be silicon dioxide. Further, the silicon oxide phase may be doped with Li. The content ratio Y of oxygen to Si may satisfy, for example, 0.5 ≤ Y < 2.0, or may satisfy 0.8 ≤ Y ≤ 1.5.

[0062] Another preferred example of the silicon-containing material is a composite particle having a sea-island structure in which fine Si is substantially uniformly dispersed in an amorphous silicate phase. The silicate phase is preferably a lithium silicate phase containing Li. The lithium silicate phase is, for example, represented by the general formula: Li ２ｚ SiO （２＋ｚ） which is a phase of a composite oxide represented by (0 < z < 2). In view of excellent stability, enabling efficient production, excellent Li ion conductivity, and other advantages, the lithium silicate phase contains Li ２ SiO ３ (z = 1) or Li ２ Si ２ O ５ (z = 1 / 2) as a main component.

[0063] Another preferred example of a silicon-containing material is a composite particle having a sea-island structure in which fine Si particles are almost uniformly dispersed in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain crystalline components, but it is preferable that it contains more amorphous components. The amorphous carbon phase is composed of a carbon material in which the average interplanar spacing of (002) planes, as measured by X-ray diffraction, exceeds 0.34 nm.

[0064] As the conductive agent, those exemplified for the positive electrode can be used. The conductive agent may be used alone or in combination of two or more types. The content of the conductive agent is, for example, 0.1% to 5% of the mass of the negative electrode mixture layer. As the binder, in addition to those exemplified for the positive electrode, polyvinyl acetate, styrene-butadiene copolymer rubber (SBR), etc., may be used. The binder may be used alone or in combination of two or more types. The content of the binder is, for example, 0.1% to 5% by mass of the mass of the negative electrode mixture layer. The negative electrode mixture layer preferably contains CMC, a salt of CMC, polyacrylic acid, a salt of polyacrylic acid, or polyvinyl alcohol (PVA).

[0065] The negative electrode can be manufactured, for example, by applying a negative electrode slurry containing a negative electrode active material, a conductive agent, and a binder to both sides of a negative electrode current collector to form coatings on each side, and then drying the coatings formed on both sides. The dried coatings may be rolled as needed. The negative electrode slurry contains a dispersion medium. As the dispersion medium, for example, N-methyl-2-pyrrolidone (NMP) can be used.

[0066] (Electrolyte) The electrolyte may be an aqueous electrolyte or a non-aqueous electrolyte. A non-aqueous electrolyte is preferred. The non-aqueous electrolyte has lithium ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0067] A liquid electrolyte (electrolyte solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, and amides. Specific examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The non-aqueous solvent may be a mixed solvent obtained by combining two or more of the above solvents. The non-aqueous solvent may be a halogen-substituted solvent in which at least some of the hydrogen atoms are replaced by halogen elements such as fluorine. Examples of such non-aqueous solvents include fluoroethylene carbonate. Examples of electrolyte salts include LiPF ６ Lithium salts such as the following are used. The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.5 mol / L or more and 2.0 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above range.

[0068] Examples of solid electrolytes include solid or gel-like polymer electrolytes and inorganic solid electrolytes. Polymer electrolytes may include, 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 the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins. As the inorganic solid electrolyte, materials known from all-solid-state lithium-ion secondary batteries can be used. Examples of such materials include oxide-based solid electrolytes, sulfide-based solid electrolytes, and halogen-based solid electrolytes.

[0069] (Separator) The separator has a long shape. As the separator, a porous sheet having ion permeability and insulating properties can be used. As the porous sheet, for example, a thin film, woven fabric, and nonwoven fabric having microporous properties can be used. The material constituting the separator is not particularly limited, and for example, polymer materials can be used. Examples of polymer materials include polyolefin resin, polyamide resin, and cellulose. Examples of polyolefin resins include polyethylene resin, polypropylene resin, and copolymers of ethylene and propylene. The separator may contain additives (such as inorganic fillers) as needed. The thickness of the separator is not particularly limited and may be 10 μm or more, or 15 μm or more. The thickness of the separator may be 30 μm or less, or 20 μm or less.

[0070] A filler layer containing an inorganic filler may be disposed between the separator and at least one of the positive electrode and the negative electrode. Examples of inorganic fillers include oxides or phosphoric acid compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the inorganic filler to the surface of the positive electrode, the negative electrode, or the separator. A heat-resistant resin layer (heat-resistant layer), such as an aramid resin, may also be disposed on the surface of the separator. The separator may have, for example, a substrate made of a porous sheet and a filler layer or heat-resistant layer disposed on the substrate.

[0071] Hereinafter, an example of a secondary battery according to the embodiments of this disclosure will be described with reference to the drawings. Note that the secondary battery according to the embodiments of this disclosure is not limited to the embodiments described below.

[0072] In the following, a cylindrical battery 10 in which a wound electrode group is housed in a bottomed cylindrical outer casing is given as an example, but the battery casing is not limited to a cylindrical casing. Other embodiments of the secondary battery according to the embodiments of this disclosure include a rectangular battery having a rectangular outer casing, and a pouch-type battery having an outer casing made of a laminate sheet including a metal layer and a resin layer.

[0073] Figure 2 is a cross-sectional view of a cylindrical battery 10 according to one embodiment of the present disclosure. As shown in Figure 2, the cylindrical battery 10 has a positive electrode 11, a negative electrode 12, and a separator 13, and comprises an electrode group 14 in which the positive electrode 11 and the negative electrode 12 are wound around the separator 13. The cylindrical battery 10 also comprises a bottomed cylindrical outer casing 16 that houses the electrode group 14, and a sealing body 17 that closes the opening of the outer casing 16. In addition to the electrode group 14, the outer casing 16 houses an electrolyte (not shown). The outer casing 16 has a stepped portion 22 formed on its side wall, and the sealing body 17 is supported by the stepped portion 22 and closes the opening of the outer casing 16. The stepped portion 22 is formed, for example, by partially pressing the side wall of the outer casing 16 from the outside. In the following, for convenience of explanation, the side of the cylindrical battery 10 with the sealing body 17 will be referred to as the upper side, and the bottom side of the outer casing 16 will be referred to as the lower side.

[0074] The electrode group 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. Insulating plates 18 and 19 are arranged above and below the electrode group 14. In the example shown in Figure 2, the positive electrode lead 20 extends towards the sealing body 17 through a through hole in the insulating plate 18, and the negative electrode lead 21 extends towards the bottom of the outer can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 becomes the negative electrode terminal.

[0075] A negative electrode 12 may be arranged on the outer circumferential surface of the electrode group 14. In this case, the negative electrode 12 may have an exposed portion of the negative electrode current collector. If the negative electrode 12 has an exposed portion of the negative electrode current collector, the negative electrode 12 and the outer casing 16 can be electrically connected by bringing the exposed portion of the negative electrode current collector into contact with the inner circumferential surface of the outer casing 16.

[0076] A gasket 28 is provided between the outer casing 16 and the sealing body 17, ensuring airtightness inside the battery. The gasket 28 also functions as an insulating member to prevent electrical contact between the outer casing 16 and the sealing body 17. The outer casing 16 has a stepped portion 22 formed on its side wall, which protrudes inward. As described above, the stepped portion 22 supports the sealing body 17. Preferably, the stepped portion 22 is formed in an annular shape along the circumferential direction of the outer casing 16. The sealing body 17 is fixed to the upper part of the outer casing 16 by the stepped portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0077] The sealing body 17 has a structure in which 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 in order from the electrode group 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except 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, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 upward toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure of the battery rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0078] (Note) The following technologies are disclosed by the above description. (Technology 1) A secondary battery comprising an electrode group in which a long positive electrode, a long negative electrode, and a long separator disposed between the positive electrode and the negative electrode are wound in the longitudinal direction, wherein the positive electrode comprises a long positive electrode current collector and a long positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, and the positive electrode mixture layer has a cavity region formed therein that extends from the beginning of winding to the end of winding of the positive electrode, including one end of the positive electrode on the winding starting side. (Technology 2) The secondary battery according to Technology 1, wherein the cavity region is formed to penetrate both the positive electrode mixture layer and the positive electrode current collector. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the positive electrode has a positive electrode tab extending along the width direction at one end in the width direction, and the positive electrode tab is arranged in the longitudinal direction of the positive electrode so as not to overlap with the cavity region. (Technology 4) The secondary battery according to Technology 3, wherein the cavity region has one end on the winding start side of the positive electrode and the other end on the winding end side of the positive electrode, and the positive electrode tab is positioned on the positive electrode within a separation range of up to 400 mm from the other end of the cavity region toward the winding end side of the positive electrode. (Technology 5) The secondary battery according to any one of Technology 1 to 4, wherein in the longitudinal direction of the positive electrode, the ratio of the length dimension of the cavity region to the total length of the positive electrode mixture layer is 30% or less. (Technology 6) The secondary battery according to Technology 5, wherein in the longitudinal direction of the positive electrode, the length dimension of the cavity region is 5 mm or more and 200 mm or less. (Technology 7) The secondary battery according to Technology 6, wherein in the longitudinal direction of the positive electrode, the length dimension of the cavity region is 100 mm or less. (Technology 8) A secondary battery according to any one of Technology 1 to 7, wherein in the width direction of the positive electrode, the ratio of the width dimension of the cavity region to the total width of the positive electrode mixture layer is 15% or less. (Technology 9) A secondary battery according to Technology 8, wherein in the width direction of the positive electrode, the width dimension of the cavity region is 0.1 mm or more and 8 mm or less. (Technology 10) A secondary battery according to Technology 9, wherein in the width direction of the positive electrode, the width dimension of the cavity region is 4 mm or less.(Technical 11) The positive electrode mixture layer has a total width of 50 mm or more, and the cavity region is formed such that, in the width direction of the positive electrode, the center of the cavity region is within a separation range of 10 mm from the center of the positive electrode mixture layer, as described in any one of Technical 1 to 10. (Technical 12) A positive electrode for a secondary battery, comprising an elongated positive electrode current collector and an elongated positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode mixture layer has a cavity region formed therein that extends from one end in the length direction toward the other end in the length direction, so as to include one end of the positive electrode in the length direction.

[0079] The present disclosure will be described below in detail based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0080] (Example 1) (1) Preparation of the positive electrode As the positive electrode active material, lithium nickelate containing cobalt and aluminum (composition formula is LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ A positive electrode mixture slurry was prepared by mixing the positive electrode active material, acetylene black, and polyvinylidene fluoride in a solid content mass ratio of 98:1:1, and then adding N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied to both sides of a long aluminum foil (700 mm in length x 60 mm in width) to form a coating film, and then the coating films formed on both sides were dried and the dried coating films were rolled. As a result, a positive electrode mixture layer was formed on both sides of the aluminum foil (thickness of 90 μm on one side, density of 3.6 g / cm³). ３ A positive electrode was obtained in which a curve was formed. That is, the positive electrode had a long shape. The application of the above positive electrode mixture slurry was carried out so that an exposed portion of the positive electrode current collector for connecting the positive electrode lead was provided at a point 400 mm away from the other end in the length direction of the rectangular cavity region described later toward the winding end side of the long positive electrode. Note that one end in the length direction of the positive electrode corresponds to the winding start side when manufacturing a wound electrode group, and the other end in the length direction of the positive electrode corresponds to the winding end side when manufacturing a wound electrode group.

[0081] A rectangular cavity region measuring 5 mm in length and 4 mm in width was formed in the positive electrode obtained as described above. The cavity region was formed by using a cutter to cut out both the positive electrode mixture layer and the positive electrode current collector from one end to the other in the longitudinal direction of the positive electrode. The cavity region was formed so that it included one end in the longitudinal direction of the positive electrode, and the center of the cavity region coincided with the center of the positive electrode mixture layer. That is, the cavity region was formed as shown in Figure 1B.

[0082] (2) Preparation of the negative electrode As the negative electrode active material, a mixture of graphite powder and silicon-containing material was used in a mass ratio of 90:10. The negative electrode active material, styrene-butadiene rubber dispersion, and carboxymethylcellulose sodium were mixed in a solid content mass ratio of 98:1:1, and then water was added as a dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a long copper foil (length 700 mm x width 60 mm) to form a coating film, and then the coating films formed on both sides were dried, and the dried coating films were rolled. As a result, a negative electrode mixture layer was formed on both sides of the copper foil (thickness of one side 94 μm, density 1.6 g / cm³). ３ A negative electrode was obtained in which a ) was formed. That is, the negative electrode had a long shape. The application of the negative electrode mixture slurry was carried out so that an exposed portion of the negative electrode current collector for connecting the negative electrode lead was provided on one end of the negative electrode current collector in the longitudinal direction.

[0083] (3) Preparation of non-aqueous electrolyte: 100 parts by mass of a mixed solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3 (25°C), to which 5 parts by mass of vinylene carbonate (VC) is added, and then LiPF ６ It was dissolved at a concentration of 1.5 mol / L. This prepared a non-aqueous electrolyte.

[0084] (4) Fabrication of a non-aqueous electrolyte secondary battery In the positive electrode obtained as described above, an Al positive electrode lead was attached to the exposed portion of the positive electrode current collector. In the negative electrode obtained as described above, a Ni negative electrode lead was attached to the exposed portion of the negative electrode current collector. The positive electrode lead and the negative electrode lead were attached by ultrasonic welding.

[0085] A positive electrode with a positive lead attached, a negative electrode with a negative lead attached, and a long separator (700 mm long x 60 mm wide) were wound in a spiral shape using a cylindrical winding core member. Then, winding stopper tape was attached to both axial ends of the outermost surface to create a wound electrode group. At this time, the long positive electrode was wound with one end in the length direction (the side where the cavity region is formed) as the starting point and the other end in the length direction as the ending point. After creating the wound electrode group, the winding core member was removed to obtain a wound electrode group with a cavity formed in the removed portion.

[0086] After placing insulating plates above and below the wound electrode group (above and below in the direction of the winding axis), the negative electrode lead was welded to the inner surface of the bottom of a bottomed cylindrical outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing body, thereby housing the wound electrode group inside the outer can. Next, a non-aqueous electrolyte was injected into the outer can using a reduced pressure method, and then the opening of the outer can was sealed with a sealing body via a gasket. This resulted in a cylindrical non-aqueous electrolyte secondary battery.

[0087] (Example 2) A non-aqueous electrolyte secondary battery according to Example 2 was obtained in the same manner as in Example 1, except that a rectangular cavity region with a length of 100 mm and a width of 4 mm was formed in the positive electrode.

[0088] (Example 3) A non-aqueous electrolyte secondary battery according to Example 3 was obtained in the same manner as in Example 1, except that a rectangular cavity region with a length of 200 mm and a width of 4 mm was formed in the positive electrode.

[0089] (Example 4) A non-aqueous electrolyte secondary battery according to Example 4 was obtained in the same manner as in Example 1, except that a rectangular cavity region with a length of 100 mm and a width of 0.1 mm was formed in the positive electrode.

[0090] (Example 5) A non-aqueous electrolyte secondary battery according to Example 5 was obtained in the same manner as in Example 1, except that a rectangular cavity region with a length of 100 mm and a width of 8 mm was formed in the positive electrode.

[0091] (Example 6) A non-aqueous electrolyte secondary battery according to Example 6 was obtained in the same manner as in Example 2, except that the center of the cavity region was formed so that it was shifted by 10 mm from the center of the positive electrode mixture layer to the other end (the lower end of the wound electrode group) in the width direction of the positive electrode. That is, in the non-aqueous electrolyte secondary battery according to Example 6, the cavity region was formed as shown in Figure 1E.

[0092] (Example 7) A non-aqueous electrolyte secondary battery according to Example 7 was obtained in the same manner as in Example 2, except that the center of the cavity region was formed so that it was shifted by 10 mm from the center of the positive electrode mixture layer toward one end (the upper end of the wound electrode group) in the width direction of the positive electrode. That is, in the non-aqueous electrolyte secondary battery according to Example 7, the cavity region was formed as shown in Figure 1D.

[0093] (Example 8) A non-aqueous electrolyte secondary battery according to Example 8 was obtained in the same manner as in Example 2, except that the application of the positive electrode mixture slurry was carried out in such a way that the exposed portion of the positive electrode current collector for connecting the positive electrode lead was located 50 mm closer from the other end to one end in the longitudinal direction of the rectangular cavity region. That is, in the non-aqueous electrolyte secondary battery according to Example 8, the exposed portion of the positive electrode current collector for connecting the positive electrode lead and the rectangular cavity region were in an overlapping positional relationship in the longitudinal direction of the positive electrode.

[0094] (Example 9) A non-aqueous electrolyte secondary battery according to Example 9 was obtained in the same manner as in Example 2, except that the application of the positive electrode mixture slurry was carried out in such a way that the exposed portion of the positive electrode current collector for connecting the positive electrode lead was located 20 mm closer from the other end to one end in the longitudinal direction of the rectangular cavity region. That is, in the non-aqueous electrolyte secondary battery according to Example 9, although the exposed portion of the positive electrode current collector for connecting the positive electrode lead and the rectangular cavity region overlap in the longitudinal direction of the positive electrode, the degree of overlap was smaller compared to Example 8.

[0095] (Example 10) A non-aqueous electrolyte secondary battery according to Example 10 was obtained in the same manner as in Example 2, except that the application of the positive electrode mixture slurry was carried out in such a way that an exposed portion of the positive electrode current collector for connecting the positive electrode lead was provided at a location where the other end of the rectangular cavity region was brought 0 mm closer to one end in the longitudinal direction (i.e., the distance between the other end of the cavity region and the positive electrode tab was 0 mm). In other words, in the non-aqueous electrolyte secondary battery according to Example 10, the exposed portion of the positive electrode current collector was provided along the other end of the rectangular cavity region in the longitudinal direction.

[0096] (Comparative Example 1) A non-aqueous electrolyte secondary battery according to Comparative Example 1 was obtained in the same manner as in Example 1, except that a cavity region was not provided in the positive electrode.

[0097] Table 1 below shows the configuration of the positive electrode for each example (Examples 1 to 10 and Comparative Example 1) of the non-aqueous electrolyte secondary battery. In Table 1 below, the ratio of the length dimension of the cavity region is the ratio of the length dimension of the cavity region to the total length of the elongated positive electrode mixture layer, and the ratio of the width dimension of the cavity region is the ratio of the width dimension of the cavity region to the total width of the elongated positive electrode mixture layer. Furthermore, the positional displacement of the center of the cavity region is a value that indicates how much the center of the cavity region is displaced in the width direction of the elongated positive electrode, with the center of the positive electrode mixture layer as the reference point. Note that a negative value means that the displacement is toward the lower end of the wound electrode group, and a positive value means that the displacement is toward the upper end of the wound electrode group.

[0098]

[0099] [Evaluation] <Initial Capacity> Charge and discharge tests were conducted using the non-aqueous electrolyte secondary batteries according to each example (Examples 1 to 7 and Comparative Example 1) as follows. Constant current charging was performed with a current of 0.3C until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current reached 0.02C. Next, constant current discharge was performed with a current of 0.5C until the voltage reached 2.5V. The rest period between charging and discharging was 60 minutes. The charging and discharging were performed in an environment of 25°C. The discharge capacity during the first discharge performed in this manner was determined as the initial capacity. The results for Examples 1 to 7 and Comparative Example 1 are shown in Table 2 below. Table 2 below shows the ratio of the initial capacity of the secondary batteries according to Examples 1 to 7 to the initial capacity of the secondary battery according to Comparative Example 1.

[0100] <Presence or absence of deformation of the positive electrode current collector> For each example (Examples 1 to 10 and Comparative Example 1), after repeating the above charge-discharge cycle for 50 cycles, the wound electrode group of each example secondary battery was X-ray CT scanned. The presence or absence of deformation of the positive electrode current collector was then evaluated using the CT images. Specifically, the central part in the winding axis direction of the wound electrode group was visually evaluated to see if deformation had occurred in the positive electrode current collector. X-ray CT scanning was performed on 10 secondary batteries for each example (Examples 1 to 10 and Comparative Example 1), and if deformation of the positive electrode current collector was confirmed in even one of the 10 secondary batteries, it was evaluated as "deformation present". If no deformation of the positive electrode current collector was confirmed in any of the 10 secondary batteries, it was evaluated as "no deformation". The results are shown in Table 2 below.

[0101] <Winding Defect Rate> For the secondary batteries of Example 2 and Examples 8-10, the wound electrode groups were X-ray CT scanned. The winding defect rate of the wound electrode groups was then evaluated using the CT images. Specifically, winding was deemed defective if the positive electrode current collector was tilted relative to the winding axis or if damage occurred to the positive electrode current collector. The ratio of the number of secondary batteries with defective winding to the total number evaluated was then calculated. X-ray CT scanning was performed on 100 secondary batteries from each example (Example 2 and Examples 8-10). Therefore, in this evaluation, the ratio of the number of secondary batteries with defective winding to 100 secondary batteries was calculated. The results are shown in Table 2 below.

[0102]

[0103] Table 2 shows that no deformation was observed in the positive electrode current collector of the secondary batteries in each of the examples (Examples 1 to 10), whereas deformation was observed in the positive electrode current collector of the secondary battery in Comparative Example 1. Furthermore, a slight decrease in initial capacity was observed in the secondary battery in Example 3. This is thought to be due to the large proportion of the cavity area in the positive electrode. In addition, winding defects were observed in the secondary batteries in Examples 8 and 9, with winding defect rates of 10% and 7%, respectively. This suggests that the greater the overlap between the exposed portion of the positive electrode current collector for connecting the positive electrode lead and the cavity area in the longitudinal direction of the positive electrode, the more likely winding defects are to occur.

[0104] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0105] The secondary battery and positive electrode for the secondary battery relating to this disclosure can be used in applications where it is required to suppress deformation of the positive electrode current collector during charging and discharging.

[0106] 10: Cylindrical battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 14: Electrode group, 16: Outer casing, 17: Sealing body, 18: Upper insulating plate, 19: Lower insulating plate, 20: Positive electrode lead, 21: Negative electrode lead, 22: Step section, 23: Bottom plate, 24: Lower valve body, 25: Insulating member, 26: Upper valve body, 27: Cap, 28: Gasket

Claims

1. A secondary battery comprising an electrode group in which a long positive electrode, a long negative electrode, and a long separator disposed between the positive electrode and the negative electrode are wound in the longitudinal direction, wherein the positive electrode comprises a long positive electrode current collector and a long positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, and the positive electrode mixture layer has a cavity region formed therein that extends from the beginning of the winding of the positive electrode toward the end of the winding of the positive electrode, including one end of the positive electrode toward the beginning of the winding.

2. The secondary battery according to claim 1, wherein the cavity region is formed to penetrate both the positive electrode mixture layer and the positive electrode current collector.

3. The secondary battery according to claim 1 or 2, wherein the positive electrode has a positive electrode tab extending along the width direction at one end in the width direction, and the positive electrode tab is arranged in the longitudinal direction of the positive electrode so as not to overlap with the cavity region.

4. The secondary battery according to claim 3, wherein the cavity region has one end on the winding start side of the positive electrode and the other end on the winding end side of the positive electrode, and the positive electrode tab is positioned on the positive electrode within a separation range of up to 400 mm from the other end of the cavity region toward the winding end side of the positive electrode.

5. The secondary battery according to claim 1 or 2, wherein, in the longitudinal direction of the positive electrode, the ratio of the length dimension of the cavity region to the total length of the positive electrode mixture layer is 30% or less.

6. The secondary battery according to claim 5, wherein the length of the cavity region in the longitudinal direction of the positive electrode is 5 mm or more and 200 mm or less.

7. The secondary battery according to claim 6, wherein the length dimension of the cavity region in the longitudinal direction of the positive electrode is 100 mm or less.

8. The secondary battery according to claim 1 or 2, wherein in the width direction of the positive electrode, the ratio of the width dimension of the cavity region to the total width of the positive electrode mixture layer is 15% or less.

9. The secondary battery according to claim 8, wherein the width dimension of the cavity region in the width direction of the positive electrode is 0.1 mm or more and 8 mm or less.

10. The secondary battery according to claim 9, wherein the width dimension of the cavity region in the width direction of the positive electrode is 4 mm or less.

11. The secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer has a total width of 50 mm or more, and the cavity region is formed such that, in the width direction of the positive electrode, the center of the cavity region is within a separation range of 10 mm from the center of the positive electrode mixture layer.

12. A positive electrode for a secondary battery, comprising an elongated positive electrode current collector and an elongated positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode mixture layer has a cavity region formed therein that extends from one end in the longitudinal direction toward the other end in the longitudinal direction, so as to include one end of the positive electrode in the longitudinal direction.

Images