Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery includes an electrode group and a non-aqueous electrolyte. The electrode group includes a positive electrode, a negative electrode, and a separator that is interposed between the positive electrode and the negative electrode. The negative electrode includes a band-shaped negative electrode current collector having a first end and a second end in the short direction. The negative electrode current collector has at least one slit group constituted from: a plurality of slits that extend in a dotted line configuration along the short direction from one of the first end and the second end toward the other end; and respective holes that are formed on the other-end sides of the plurality of slits and have a width larger than the average width of the plurality of slits.

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are used in applications such as ICT (Information and Communication Technology) for personal computers and smartphones, automotive applications, and energy storage. Various improvements are being attempted to enhance the characteristics of non-aqueous electrolyte secondary batteries. For example, improvements to the shape of the current collector have been proposed.

[0003] Patent Document 1 proposes making a metal current collector flexible by providing a slit in it, in the electrode of a non-aqueous electrolyte battery. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2001-266894 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In non-aqueous electrolyte secondary batteries, the expansion and contraction of the negative electrode during charging and discharging is significant. During charging, the expansion of the negative electrode can cause the negative electrode current collector to break if it cannot keep up, resulting in a loss of electrical connection. When the electrical connection in the negative electrode current collector is lost, the cycle characteristics of the non-aqueous electrolyte secondary battery deteriorate. [Means for solving the problem]

[0006] One aspect of this disclosure includes an electrode group and a non-aqueous electrolyte, The electrode group includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a strip-shaped negative electrode current collector having a first end and a second end in the short direction. The present invention relates to a non-aqueous electrolyte secondary battery, wherein the negative electrode current collector has at least one slit group comprising a plurality of slits extending in a dashed line shape along the short direction from one end of the first end and the second end toward the other end, and a hole formed on the other end side of the plurality of slits, having a width greater than the average width of the plurality of slits. [Effects of the Invention]

[0007] This can improve the cycle characteristics of non-aqueous electrolyte secondary batteries. [Brief explanation of the drawing]

[0008] [Figure 1] This is a plan view showing an example of a negative electrode current collector used in a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. [Figure 2] This is a schematic longitudinal cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0009] Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0010] The non-aqueous electrolyte secondary battery of this disclosure includes an electrode group and a non-aqueous electrolyte. The electrode group includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a strip-shaped negative electrode current collector having a first end and a second end in the short direction. The negative electrode current collector has at least one slit group comprising a plurality of slits extending in a dashed line along the short direction from one end of the first end and the second end toward the other end, and a hole formed on the other end side of the plurality of slits and having a width greater than the average width of the plurality of slits.

[0011] In the above non-aqueous electrolyte secondary battery, when stress due to the expansion of the negative electrode during charging is applied to the negative electrode current collector, the negative electrode current collector breaks along a plurality of dashed-line slits. The stress is relaxed by holes formed on the other end side of the plurality of slits and having a width larger than the average width of the slits, and the breakage of the negative electrode current collector terminates at the hole portion. Therefore, although the negative electrode current collector breaks on the one end side, the breakage of the negative electrode current collector is suppressed on the other end side. In other words, by providing the above slit group in the negative electrode current collector, when stress due to the expansion of the negative electrode is applied to the negative electrode current collector, the position where the negative electrode current collector breaks can be controlled. Thus, it is possible to suppress the negative electrode current collector from being completely cut along the short side direction, and the electrical connection of the negative electrode current collector is maintained. As a result, the cycle characteristics can be improved.

[0012] The negative electrode will be described in more detail below.

[0013] (Negative electrode) In one slit group, the plurality of slits are arranged at intervals along the short side direction of the negative electrode current collector. In one slit group, each slit is formed to extend along the short side direction of the negative electrode current collector. The width of the slit is a value obtained by averaging the maximum value of the size of the slit in the longitudinal direction of the negative electrode current collector for each slit in the slit group. The width of the hole is the maximum value of the size of the hole in the longitudinal direction of the negative electrode current collector.

[0014] Of the first end and the second end of the negative electrode current collector, one end may be cut by a slit on one end side of the plurality of slits. Since the strength of one end of the negative electrode current collector is low, when stress is applied to the negative electrode current collector, the breakage smoothly progresses along the plurality of slits. Therefore, it becomes easier to further control the position where the negative electrode current collector breaks.

[0015] The slit on one end refers to the slit closest to one end of the negative electrode current collector in the short direction, among the multiple slits that make up a group of slits. The slit on the other end refers to the slit closest to the other end of the negative electrode current collector in the short direction, among the multiple slits that make up a group of slits.

[0016] The negative electrode current collector may have at least one group of slits, but may also have multiple groups of slits. If the negative electrode current collector has multiple groups of slits, all of the slit groups may be formed to extend along the shorter direction from one end of the negative electrode current collector (e.g., the first end) to the other end (e.g., the second end) (in other words, in the same direction).

[0017] When the negative electrode current collector has multiple slit groups, some of the slit groups may be formed in different orientations from the remaining slit groups. More specifically, the multiple slit groups may include a first slit group consisting of multiple first slits extending along the short direction from the first end to the second end of the negative electrode current collector and a first hole formed on the second end side of the multiple first slits, and a second slit group consisting of multiple second slits extending along the short direction from the second end to the first end and a second hole formed on the first end side of the multiple second slits. In this case, the stress applied to the negative electrode current collector due to the expansion of the negative electrode can be more easily distributed, and the effect of suppressing fracture at locations other than the slit groups can be enhanced, making it easier to maintain the electrical connection in the event of fracture of the negative electrode current collector.

[0018] It is preferable that the first slit group and the second slit group are arranged alternately. In this case, it becomes easier to control the position where the negative electrode current collector breaks, and it becomes easier to maintain the electrical connection when the negative electrode current collector breaks. An example of a negative electrode current collector in which the first slit group and the second slit group are arranged alternately is shown in Figure 1.

[0019] Figure 1 is a plan view showing an example of a negative electrode current collector used in a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. In Figure 1, the strip-shaped negative electrode current collector 112 has a plurality of slit groups 113. Each slit group 113 is located in the short direction D of the negative electrode current collector 112. S From one end toward the other end, in the shorter direction D S It is composed of multiple slits 113a extending in a dashed line along the longitudinal direction D and a circular hole 113b. In each slit group 113, the hole 113b is formed on the other end side of the multiple slits 113a. More specifically, the hole 113b is continuous with the slits on the other end side of the multiple slits 113a. Also, in each slit group 113, the width of the hole 113b is greater than the average width of the multiple slits 113a. In Figure 1, the negative electrode current collector 112 has multiple slit groups 113. The multiple slit groups 113 are in the longitudinal direction D of the negative electrode current collector 112. L They are formed at intervals along the direction D. In Figure 1, the multiple slit groups 113 are in the short direction D of the negative electrode current collector 112. S From the first end e1 toward the second end e2, in the short direction D S A plurality of first slit groups 1131 extending along and a shorter direction D from the second end e2 toward the first end e1 S It is composed of multiple second slit groups 1132 extending along the first slit group 1131 and the second slit groups 1132, which are arranged alternately. Note that the following explanation is not limited to the specific example shown in Figure 1.

[0020] In a single slit group, the direction in which the multiple slits constituting this group extend in a dashed line may be parallel to or intersect with the short-side direction of the negative electrode current collector. The acute angle between the direction in which the multiple slits extend in a dashed line and the short-side direction of the negative electrode current collector may be, for example, 30° or less, or 15° or less. Note that the direction in which the multiple slits extend in a dashed line is the average direction in the longitudinal direction of each slit constituting a single slit group.

[0021] The average spacing between adjacent slits in the direction extending in the dashed line is, for example, 0.02 mm or more, preferably 0.1 mm or more, and may be 0.2 mm or more. When the average spacing between adjacent slits is within this range, the breakage of the negative electrode current collector during the negative electrode manufacturing process or the electrode group manufacturing process can be suppressed. The average spacing between adjacent slits in the direction extending in the dashed line is, for example, 3 mm or less, preferably 2.5 mm or less, and more preferably 1.5 mm or less or 1 mm or less. In this case, it is even easier to control the position where the negative electrode current collector breaks due to the expansion of the negative electrode, and it is easier to ensure high current collection performance of the negative electrode current collector after breakage. These lower and upper limits can be combined arbitrarily. Note that the spacing between adjacent slits in the direction extending in the dashed line is the distance between adjacent slits (in other words, the distance between the abutting ends of adjacent slits).

[0022] The length of the slit is, for example, 0.1 mm to 7 mm, and may also be 0.5 mm to 3 mm, or 0.5 mm to 1 mm. When the length of the slit is within this range, it is easier to control the position at which the negative electrode current collector breaks due to the expansion of the negative electrode. The length of the slit is the average value of the lengths of each slit in the direction extending in the dashed line within the slit group. The length of the slit is greater than the width of the slit.

[0023] The width of the slits may be, for example, 0.005 mm or more and 0.1 mm or less, and may also be 0.01 mm or more and 0.05 mm or less, or 0.02 mm or more and 0.04 mm or less. When the width of the slits is within this range, the negative electrode current collector can be broken more smoothly along multiple slits arranged in a dashed line pattern.

[0024] The holes are formed on the other end side of a group of slits. As shown in Figure 1, the holes may be continuous with the slits on the other end side of the group of slits. Alternatively, the holes may be formed with a gap between them and the slits on the other end side. In this case, the holes are formed near the slits on the other end side so that the group of slits and the holes constitute a single group of slits. The gap between the holes and the slits on the other end side is, for example, 3 mm or less, preferably 2.5 mm or less, and more preferably 1.5 mm or less or 1 mm or less. From the viewpoint of further enhancing the effect of suppressing the failure of the negative electrode current collector at an unintended location when the negative electrode expands, it is preferable that the holes are continuous with the slits on the other end side. Note that the gap between the holes and the slits on the other end side is the distance between the holes and the slits on the other end side.

[0025] The width of the hole is, for example, 0.05 mm or more, and may be 0.1 mm or more or 0.2 mm or more. In this case, the effect of relieving the stress applied to the negative electrode current collector due to the expansion of the negative electrode is enhanced, and the effect of suppressing the fracture of the negative electrode current collector at an unintended location can be further enhanced. From the viewpoint of easily ensuring higher current collection performance, the width of the hole is preferably 2 mm or less, and more preferably 1.5 mm or less or 1 mm or less. These lower and upper limits can be arbitrarily combined. In addition, the length of the hole in the short direction of the negative electrode current collector can be selected from the range described for the width of the hole. Note that the length of the hole is the maximum size of the hole in the short direction of the negative electrode current collector.

[0026] The negative electrode current collector has a pair of main surfaces that occupy most of the surface of the strip-shaped negative electrode current collector. The shape of the hole when the main surface of the negative electrode current collector is viewed from a direction perpendicular to the main surface may be, for example, a polygon (square, pentagon, hexagon, etc.), but it is preferably circular or elliptical. When the shape of the hole is circular or elliptical, the effect of relieving the stress applied to the negative electrode current collector as the negative electrode expands is enhanced, and the effect of suppressing the fracture of the negative electrode current collector at an unintended position with the hole as the starting point is enhanced.

[0027] When the length of the negative electrode current collector in the shorter direction (in other words, the width of the negative electrode current collector) is W, the distance between the hole and the other end of the negative electrode current collector is preferably 0.1W or more and 0.5W or less, more preferably 0.15W or more and 0.4W or less, and even more preferably 0.15W or more and 0.35W or less. When the distance between the hole and the other end of the negative electrode current collector is within this range, the effect of suppressing complete breakage of the negative electrode current collector while easing the stress associated with the expansion of the negative electrode is enhanced. Note that the distance between the hole and the other end of the negative electrode current collector is the shortest distance between the position of the hole closest to the other end of the negative electrode current collector and the other end.

[0028] When the negative electrode current collector has multiple slit groups, the multiple slit groups may be formed at intervals along the longitudinal direction of the negative electrode current collector. The average spacing between adjacent slit groups is greater than the width of the holes. The average spacing between adjacent slit groups is, for example, 0.2 mm or more, preferably 0.5 mm or more, and may be 1.5 mm or more or 2 mm or more. When the average spacing between adjacent slit groups is within this range, even if stress due to the expansion of the negative electrode is applied to the negative electrode current collector, it becomes easier to smoothly fracture the negative electrode current collector along the direction in which the multiple slits extend in a dashed line. There is no particular upper limit to the average spacing between adjacent slit groups. From the viewpoint of further controlling the position where the negative electrode current collector fractures when the negative electrode expands, the average spacing between adjacent slit groups is preferably 10 mm or less, and may be 5 mm or less. These lower and upper limits can be combined arbitrarily.

[0029] In an electrode group, the spacing between adjacent slit groups may be the same. In an electrode group, in areas where stress due to the expansion of the negative electrode is likely to occur, it is preferable to reduce the spacing between adjacent slit groups compared to other areas. This allows for more effective relaxation of the stress due to the expansion of the negative electrode, thereby further enhancing the effect of suppressing fracture of the negative electrode current collector at unintended locations. For example, near a current-collecting tab or lead connected to the negative electrode, it is preferable to reduce the spacing between adjacent slit groups compared to areas further away from the tab or lead.

[0030] When the electrode group is a wound electrode group in which a positive electrode, a negative electrode, and a separator are wound, the stress associated with the expansion of the negative electrode is difficult to be released inside the wound electrode group, so it goes outward. Therefore, it is preferable to make the interval between adjacent slit groups in the outer peripheral side portion of the negative electrode current collector smaller than the interval between adjacent slit groups in the remaining portion. More specifically, when the length in the longitudinal direction of the negative electrode current collector is L, the average interval between adjacent slit groups in the portion from the outer peripheral side end of the negative electrode current collector to L / 4 is preferably smaller than the average interval between adjacent slit groups in the remaining portion. FIG. 1 shows an example in which the interval between adjacent slit groups 113 in the portion from the outer peripheral side end Eo of the negative electrode current collector 112 to L / 4 is smaller than the interval between adjacent slit groups 113 in the remaining portion (specifically, the portion from the inner peripheral side end Ei of the negative electrode current collector 112 to 3L / 4).

[0031] When the average interval between adjacent slit groups in the portion from the outer peripheral side end of the negative electrode current collector to L / 4 is Po and the average interval between adjacent slit groups in the remaining portion is Pi, the ratio Po / Pi is, for example, 0 < Po / Pi < less than 1, preferably 0.1 ≦ Po / Pi ≦ 0.7, and more preferably 0.2 ≦ Po / Pi ≦ 0.5. For example, in FIG. 1, the average interval between adjacent slit groups 113 in the portion from the outer peripheral side end Eo of the negative electrode current collector 112 to L / 4 is obtained by summing and averaging the intervals p1 between adjacent slit groups 113. For the remaining portion as well, the average interval between adjacent slit groups is obtained by summing and averaging the intervals p2 between adjacent slit groups.

[0032] Note that the interval between adjacent slit groups is the distance between two straight lines at the first end when a straight line is drawn from the first end to the second end in the direction in which a plurality of slits constituting each slit group extend in a broken line shape in adjacent slit groups.

[0033] Examples of materials for the negative electrode current collector include metals and alloys. Examples of metals include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements. Examples of alloys include copper alloys and stainless steel (SUS). Among these, copper and copper alloys, which have high conductivity, are preferred.

[0034] The negative electrode current collector is obtained by forming a group of slits in a sheet (foil, etc.) of the above material using known slit and hole formation techniques. Examples of slit and hole formation techniques include laser processing, etching, press working, cutting, and punching.

[0035] The thickness of the negative electrode current collector is, for example, between 5 μm and 300 μm.

[0036] The negative electrode includes a negative electrode active material. The negative electrode may comprise a negative electrode current collector and a negative electrode active material layer. In addition to the negative electrode active material, the negative electrode active material layer may optionally further include at least one selected from the group consisting of binders, conductive materials, thickeners, and additives. The negative electrode active material layer may be formed on only one main surface of the negative electrode current collector, or on both main surfaces.

[0037] In the non-aqueous electrolyte secondary battery of this disclosure, since a negative electrode current collector having the above-mentioned slit group is used, even when a negative electrode active material that undergoes a large volume change due to expansion during charging is used, complete severance of the negative electrode current collector due to the expansion of the negative electrode can be suppressed, and high cycle characteristics can be obtained. In a negative electrode containing such a negative electrode active material, the thickness at full charge is significantly larger than the thickness of the negative electrode after the first discharge. The thickness of the negative electrode at full charge is preferably 1.18 times or more, more preferably 1.3 times or more, and may be 2 times or more, than the thickness of the negative electrode after the first discharge. For example, in a lithium-ion secondary battery using graphite as the negative electrode active material, even when a negative electrode active material layer containing graphite is formed on both main surfaces of the negative electrode current collector, the thickness of the negative electrode at full charge is about 1.1 times the thickness of the negative electrode after the first discharge. Thus, the non-aqueous electrolyte secondary battery of this disclosure is particularly effective when using a negative electrode active material that undergoes a larger volume change than graphite.

[0038] The thickness of the negative electrode described above is determined by taking a photograph of the cross-section of the electrode group, measuring the thickness of the negative electrode at multiple locations (for example, 10 locations) in this photograph, and averaging the results. However, the thickness of the negative electrode is measured at any multiple locations in the region where the negative electrode active material is present on both main surfaces of the negative electrode current collector, at least in the charged state.

[0039] The thickness of the negative electrode at full charge is the thickness of the negative electrode in a fully charged non-aqueous electrolyte secondary battery. A fully charged non-aqueous electrolyte secondary battery is a battery that has been charged to a State of Charge (SOC) of 0.98C or higher, where C is the rated capacity of the battery. The thickness of the negative electrode after the first discharge is the thickness of the negative electrode in a non-aqueous electrolyte secondary battery that has been discharged to a completely discharged state during the first discharge. A non-aqueous electrolyte secondary battery that has been discharged to a completely discharged state during the first discharge is a battery that has been assembled, undergone break-in charge-discharge, then charged for the first time, and then discharged for the first time until the SOC is 0.05C or lower. A non-aqueous electrolyte secondary battery that has been discharged to a completely discharged state during the first discharge may, for example, be a non-aqueous electrolyte secondary battery that is commercially available in a charged state and has been discharged for the first time until the SOC is 0.05C or lower. A non-aqueous electrolyte secondary battery that has been discharged to a completely discharged state during the first discharge may, for example, be a battery that has been discharged for the first time at a constant current of 0.05C until the lower limit voltage.

[0040] The negative electrode active material is selected according to the type of non-aqueous electrolyte secondary battery. For example, in a non-aqueous electrolyte secondary battery, if the ions that act as charge carriers are lithium ions, the negative electrode active material can be metallic lithium, a lithium alloy, or a material that can electrochemically intercept and release lithium ions. Examples of lithium alloys include lithium-aluminum alloys or lithium-magnesium alloys. Examples of materials that can electrochemically intercept and release lithium ions include carbonaceous materials and materials containing at least one selected from the group consisting of Si and Sn. The negative electrode may contain one negative electrode active material, or it may contain a combination of two or more.

[0041] Examples of carbonaceous materials include graphite, soft carbon, hard carbon, and amorphous carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Graphite is a carbonaceous material in which a graphite-type crystal structure is well-developed. The interplanar spacing d002 of the (002) plane of graphite, as measured by X-ray diffraction, may be, for example, 0.340 nm or less, or between 0.3354 nm and 0.340 nm.

[0042] Examples of silicon-containing materials include elemental silicon, silicon alloys, and silicon compounds (silicon oxides, silicates, silicon nitrides, etc.). Examples of silicon oxides include SiO2. x Particles are an example. x may be, for example, 0.5 ≤ x < 2, and 0.8 ≤ x ≤ 1.6. As the Si-containing material, a material comprising a lithium silicate phase and silicon particles dispersed within the lithium silicate phase may be used. Examples of Sn-containing materials include elemental Sn, tin alloys, and tin compounds (tin oxides, tin nitrides, etc.). The negative electrode containing the Si-containing material or Sn-containing material undergoes a large volume change due to expansion during charging. In the non-aqueous electrolyte secondary battery of this disclosure, since a negative electrode current collector having the above-mentioned slit group is used, high cycle characteristics can be obtained even when using the Si-containing material or Sn-containing material. The negative electrode may include, as the negative electrode active material, at least one selected from the group consisting of Si-containing material and Sn-containing material, and a carbonaceous material.

[0043] The non-aqueous electrolyte secondary battery of this disclosure is particularly useful as a lithium secondary battery. Lithium secondary batteries are also called lithium metal secondary batteries. The negative electrode active material in a lithium secondary battery is lithium metal. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode current collector during charging, and the lithium metal dissolves during discharging. Therefore, the volume change of the negative electrode during charging and discharging is very large. In addition, in a lithium secondary battery, lithium metal may be deposited on the negative electrode in a dendrite-like manner during charging. When lithium metal is deposited in a dendrite-like manner, the expansion of the negative electrode becomes even larger. In addition, the lithium metal deposited in a dendrite-like manner is harder and bulkier than lithium metal available as a commercially available product. Therefore, when lithium metal is deposited in a dendrite-like manner, a large stress is applied to the negative electrode current collector. Even in a lithium secondary battery in which a large stress is easily applied to the negative electrode current collector during charging, the position at which the negative electrode current collector breaks can be controlled by using a negative electrode current collector having the above-mentioned group of slits. Because the electrical connection of the negative electrode current collector can be maintained, high cycle characteristics can be ensured.

[0044] In lithium secondary batteries, for example, 70% or more of the rated capacity is due to the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70-100% (e.g., 80-100% or 90-100%) of the movement of electrons (or current, from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode in lithium secondary batteries differs from a negative electrode where the movement of electrons at the negative electrode during charging and discharging is mainly due to the intercalation and release of lithium ions by the negative electrode active material (such as graphite).

[0045] In batteries such as lithium secondary batteries that deposit lithium metal at the negative electrode during charging, the open circuit voltage (OCV) of the negative electrode at full charge is, for example, 70 mV or less relative to the lithium metal (lithium dissolution potential). The OCV of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The non-aqueous electrolyte of the cell may have the same composition as the non-aqueous electrolyte in the disassembled battery.

[0046] Examples of binders included in the negative electrode active material layer include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0047] Examples of conductive materials included in the negative electrode active material layer include conductive carbonaceous materials. Examples of conductive carbonaceous materials include carbon black and carbon nanotubes. Examples of carbon black include acetylene black and Ketjenblack.

[0048] The negative electrode may be formed, for example, by depositing a negative electrode active material onto the main surface of the negative electrode current collector using a vapor phase method such as electrodeposition or vapor deposition. Alternatively, the negative electrode may be formed by applying a negative electrode slurry containing the components of the negative electrode active material layer and a dispersion medium to the main surface of the negative electrode current collector, and then drying and compressing the coating. Examples of dispersion mediums include at least one selected from the group consisting of water and organic media. In lithium secondary batteries, the negative electrode current collector may be used to fabricate the electrode group. More specifically, in lithium secondary batteries, for example, the electrode group is fabricated by stacking the negative electrode current collector and the positive electrode with a separator in between. In such lithium secondary batteries, lithium ions move from the positive electrode during charging and deposit on the surface of the negative electrode current collector.

[0049] The following describes the configuration of a non-aqueous electrolyte secondary battery, excluding the negative electrode.

[0050] (positive electrode) The positive electrode includes, for example, a strip-shaped positive electrode current collector and a positive electrode composite layer formed on the surface of the positive electrode current collector. The positive electrode current collector may be in the form of a sheet (e.g., foil, film) and may be porous. The sheet-shaped positive electrode current collector may optionally have a group of slits as described for the negative electrode current collector. For details on the group of slits, refer to the description of the positive electrode current collector. The positive electrode composite layer may be formed on one of the main surfaces of the sheet-shaped positive electrode current collector, or on one of the main surfaces. The positive electrode composite layer may also be formed in a state where it is filled into a mesh-shaped positive electrode current collector.

[0051] Examples of materials for the positive electrode current collector include metallic materials containing Al, Ti, Fe, etc. The metallic material may be Al, Al alloy, Ti, Ti alloy, and Fe alloy, etc. The Fe alloy may be stainless steel.

[0052] The positive electrode composite layer contains a positive electrode active material. In addition to the positive electrode active material, the positive electrode composite layer may also contain at least one selected from the group consisting of a binder, a conductive material, and an additive. A conductive carbonaceous material may be placed between the positive electrode current collector and the positive electrode composite layer as needed. Examples of binders include at least one selected from the binders exemplified for the negative electrode active material layer. Examples of conductive materials include at least one selected from the group consisting of the conductive materials exemplified for the negative electrode active material layer and graphite. Examples of conductive carbonaceous materials include at least one selected from the conductive carbonaceous materials exemplified for the conductive material of the negative electrode active material layer.

[0053] The positive electrode active material is selected according to the type of non-aqueous electrolyte secondary battery. For example, in a non-aqueous electrolyte secondary battery, if the ions that act as charge carriers are lithium ions, the positive electrode active material used is a material that electrochemically intercepts and releases lithium ions. Examples of such materials include at least one selected from the group consisting of lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. From the viewpoint of high average discharge voltage and cost advantages, the positive electrode active material may also be a lithium-containing transition metal oxide.

[0054] Examples of transition metal elements included in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides may contain one or more transition metal elements. The transition metal element may be at least one selected from the group consisting of Co, Ni, and Mn. Lithium-containing transition metal oxides may optionally contain one or more typical metal elements. Examples of typical metal elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical metal element may also be Al.

[0055] The positive electrode is obtained, for example, by applying or filling a slurry containing the components of a positive electrode composite layer and a dispersion medium onto a positive electrode current collector, and then drying and compressing the coating. A conductive carbonaceous material may be applied to the surface of the positive electrode current collector as needed. The dispersion medium may be at least one selected from the group consisting of water and organic media.

[0056] (Separator) A porous sheet having ion permeability and insulating properties is used as the separator. Examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. The material of the separator may also be a polymer material. Examples of polymer materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives as needed. Examples of additives include inorganic fillers.

[0057] The separator may comprise multiple layers having at least one different form and composition. Such a separator may be, for example, a laminate of a polyethylene microporous film and a polypropylene microporous film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0058] (Non-aqueous electrolytes) A non-aqueous electrolyte includes, for example, a non-aqueous solvent, ions that act as charge carriers, and, optionally, counterions of those ions. For example, in a non-aqueous electrolyte secondary battery, if the ions that act as charge carriers are lithium ions, the non-aqueous electrolyte has lithium-ion conductivity. A lithium-ion conductive non-aqueous electrolyte includes, for example, a non-aqueous solvent and lithium ions and anions dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in liquid or gel form.

[0059] Liquid non-aqueous electrolytes are prepared, for example, by dissolving a lithium salt in a non-aqueous solvent. The dissolution of the lithium salt in the non-aqueous solvent generates lithium ions and anions. As the lithium salt, a salt of lithium ions and anions is used.

[0060] The gel-like non-aqueous electrolyte contains, for example, a liquid non-aqueous electrolyte and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of such a polymer material include at least one selected from the group consisting of a fluororesin, an acrylic resin, and a polyether resin.

[0061] As the lithium salt or anion, known components used in the non-aqueous electrolyte of a lithium secondary battery can be used. As the anion, BF4 - , ClO4 - , PF6 - , CF3SO3 - , CF3CO2 - , an anion of an imide compound, an anion of an oxalate compound, etc. are exemplified. As the anion of the imide compound, N(SO2C m F 2m+1 )(SO2C n F 2n+1 ) - (m and n are each independently an integer of 0 or more.) etc. are exemplified. m and n may each be 0 to 3, or may each be 0, 1, or 2. The anion of the imide compound is N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2F)2 - . The anion of the oxalate compound may contain boron and / or phosphorus. The anion of the oxalate compound may be an anion of an oxalate complex. Examples of the anion of the oxalate compound include a bisoxalate borate anion, BF2(C2O4) - , PF4(C2O4) - , PF2(C2O4)2 - etc. The non-aqueous electrolyte may contain one of these anions or two or more thereof.

[0062] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, or halogen-substituted compounds thereof. The non-aqueous electrolyte may contain one of these non-aqueous solvents or two or more. Examples of halogen-substituted compounds include fluorides.

[0063] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. Examples of linear carbonate esters include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of linear carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0064] Examples of the above ethers include cyclic ethers and linear ethers. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of linear ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0065] Examples of nitriles include acetonitrile, propionitrile, and benzonitrile. Examples of amides include dimethylformamide and dimethylacetamide.

[0066] The concentration of lithium salt in the non-aqueous electrolyte is, for example, between 0.5 mol / L and 3.5 mol / L. Here, the lithium salt concentration is the sum of the concentration of dissociated lithium salt and the concentration of undissociated lithium salt. The concentration of anion in the non-aqueous electrolyte may also be between 0.5 mol / L and 3.5 mol / L.

[0067] Non-aqueous electrolytes may contain additives. Examples of additives include vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate. Additives may be used individually or in combination of two or more.

[0068] (others) A non-aqueous electrolyte secondary battery is manufactured, for example, by housing an electrode group and a non-aqueous electrolyte in a battery case. The electrode group is manufactured, for example, by winding a positive electrode and a negative electrode with a separator interposed between them. The shape of the end face in the winding axis direction of the wound electrode group may be circular, elliptical, or oblong. For components other than the electrode group and non-aqueous electrolyte of the non-aqueous electrolyte secondary battery, known configurations can be used without particular limitation.

[0069] Figure 2 is a schematic longitudinal cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The non-aqueous electrolyte secondary battery 10 is a cylindrical battery comprising a cylindrical battery case, a wound electrode group 14 housed within the battery case, and a non-aqueous electrolyte (not shown). The battery case consists of a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16, thereby ensuring the airtightness of the battery case. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively.

[0070] The case body 15 has a stepped portion 21 formed, for example, by partially pressing the side wall of the case body 15 from the outside. The stepped portion 21 may be formed in an annular shape on the side wall of the case body 15 along the circumferential direction of the case body 15. In this case, the sealing body 16 is supported on the opening side of the stepped portion 21.

[0071] The sealing body 16 comprises a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. In the sealing body 16, these members are stacked in this order. The sealing body 16 is fitted into the opening of the case body 15 such that the cap 26 is located on the outside of the case body 15 and the filter 22 is located on the inside of the case body 15. Each of the above-mentioned members constituting the sealing body 16 is, for example, disc-shaped or ring-shaped. Each member, except for the insulating member 24, is electrically connected to one another.

[0072] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The negative electrode 12 includes, for example, a negative electrode current collector 112 as shown in Figure 1. The positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed between them, such that the width direction of the strip-shaped positive electrode 11 and the negative electrode 12 is parallel to the winding axis. In a cross-section perpendicular to the winding axis of the electrode group 14, the positive electrode 11 and the negative electrode 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 13 interposed between them.

[0073] The positive electrode 11 is electrically connected to the cap 26, which also serves as the positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction. The positive electrode lead 19 extending from the positive electrode 11 extends to the filter 22 through a through hole (not shown) formed in the insulating plate 17. The other end of the positive electrode lead 19 is welded to the side of the filter 22 facing the electrode group 14.

[0074] The negative electrode 12 is electrically connected to the case body 15, which also serves as the negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected, for example, to the longitudinal end of the negative electrode 12, and the other end is welded to the inner bottom surface of the case body 15.

[0075] [Examples] The non-aqueous electrolyte secondary batteries of this disclosure will be described in detail below based on examples and comparative examples, but the non-aqueous electrolyte secondary batteries of this disclosure are not limited to the following examples.

[0076] Examples 1-7 and Comparative Examples 1-2 A cylindrical lithium-ion secondary battery was fabricated using the following procedure. (1) Preparation of the positive electrode A positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were mixed in a mass ratio of 95:2.5:2.5. A positive electrode slurry was prepared by adding an appropriate amount of N-methyl-2-pyrrolidone as a dispersion medium to the mixture and stirring. A lithium-containing transition metal oxide containing Ni, Co, and Al was used as the positive electrode active material.

[0077] A positive electrode slurry was applied to both sides of an aluminum foil, which served as the positive electrode current collector, and dried. The dried material was compressed in the thickness direction using a roller. The resulting laminate was cut to a predetermined electrode size to produce a positive electrode with positive electrode composite layers on both sides of the positive electrode current collector. In addition, an exposed portion of the positive electrode current collector without the positive electrode composite layer was formed in a part of the positive electrode. One end of an aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector by welding.

[0078] (2) Preparation of non-aqueous electrolytes A non-aqueous electrolyte was prepared by dissolving lithium hexafluoride phosphate (LiPF6) and then lithium difluorooxalate borate (LiFOB) in a mixture of a non-aqueous solvent containing propylene carbonate and 1,2-dimethoxyethane in a 1:2 volume ratio. The concentrations of LiPF6 and LiFOB in the non-aqueous electrolyte were 1 mol / L and 100 mmol / L, respectively.

[0079] (3) Battery assembly One end of a nickel negative electrode lead was welded to the negative electrode current collector. In an inert gas atmosphere, the negative electrode current collector and the positive electrode were wound in a spiral shape with a polyethylene separator (microporous membrane) between them to create an electrode group. Since all the lithium in the electrode group originates from the positive electrode, the molar ratio of the total amount of lithium yLi in the positive and negative electrodes to the amount of metal M (here Ni, Co, and Al) yM in the positive electrode, yLi / yM, is 1.0.

[0080] The negative electrode current collector was fabricated by forming a group of slits in a strip of copper foil (10 μm thick) using laser processing. In the negative electrode current collector, multiple groups of slits were formed, extending parallel to the short side from one end to the other end of the first and second ends. More specifically, in the negative electrode current collector, a first group of slits extending parallel to the short side from the first end to the second end and a second group of slits extending parallel to the short side from the second end to the first end were formed alternately along the longitudinal direction. In each group of slits in the negative electrode current collector used in the example, as shown in Figure 1, a circular hole was formed in a continuous state with the slit on the other end side of the multiple slits constituting each slit group. The width of the circular hole (in other words, the diameter of the circle) was the value shown in Table 1. In the negative electrode current collector used in the comparative example, the slit group consisted only of multiple slits and no hole was formed. Table 1 shows the length and width of the slits, the spacing between adjacent slits, and the average spacing between adjacent slit groups in the negative electrode current collector used in the examples or comparative examples. In the negative electrode current collector used in Example 7 and Comparative Example 2, as shown in Figure 1, the average spacing Po between adjacent slit groups in the portion L / 4 from the outer edge was made smaller than the average spacing Pi between adjacent slit groups in the remaining portion. Each slit group was formed at a position 2W / 3 from the first or second end.

[0081] The electrode group was housed in a bag-like outer casing made of a laminate sheet with an Al layer, a non-aqueous electrolyte was injected, and then the outer casing was sealed to fabricate a lithium secondary battery. When housing the electrode group in the outer casing, the other ends of the positive electrode lead and the other end of the negative electrode lead were left exposed to the outside of the outer casing.

[0082] The thickness of the negative electrode after the initial discharge, as determined by the procedure described above, corresponds to the thickness of the copper foil and was 10 μm. Furthermore, at full charge, lithium was deposited to a thickness of approximately 10 μm on both main surfaces of the copper foil, resulting in a negative electrode thickness of approximately 30 μm. The thickness of the negative electrode at full charge, as determined by the procedure described above, was approximately three times the thickness of the negative electrode after the initial discharge. In lithium secondary batteries, unlike cases where a negative electrode active material layer containing graphite or similar material is formed, the negative electrode after the initial discharge contains almost no negative electrode active material. Therefore, in lithium secondary batteries, the ratio of the negative electrode thickness at full charge to the negative electrode thickness after the initial discharge is significantly larger than in cases where the negative electrode contains a negative electrode active material layer containing graphite or similar material.

[0083] (4) Evaluation The following evaluations were performed using the fabricated lithium secondary battery. (a) Cycle characteristics Lithium secondary batteries were charged in a constant temperature bath at 25°C under the following charging conditions, paused for 20 minutes, and then discharged under the following discharge conditions. These charging, pausing, and discharging cycles constituted one cycle, and 10 charge-discharge tests were performed. The ratio of the discharge capacity at the 10th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. The cycle characteristics of each lithium secondary battery were evaluated as a percentage (%) relative to the capacity retention rate of the lithium secondary battery of Comparative Example 1, which was set to 100.

[0084] (charging) Constant current charging is performed at a current of 10mA per unit area (square centimeter) of the electrodes until the battery voltage reaches 4.3V. Then, constant voltage charging is performed at a voltage of 4.3V until the current value per unit area (square centimeter) of the electrodes reaches 1mA.

[0085] (discharge) A constant current discharge is performed at a current of 10mA per unit area (in square centimeters) of the electrode until the battery voltage reaches 2.5V.

[0086] (b) Rupture of the negative electrode current collector After the cycle test, the lithium secondary battery was disassembled, the negative electrode was removed, and the condition of the fracture of the negative electrode current collector was visually observed and evaluated according to the following criteria. A: The negative electrode current collector is fractured along the slit group, but no fracture is observed in areas other than the slit group. B: Fractures are observed in the negative electrode current collector not only in the slit area but also in areas other than the slit area.

[0087] The evaluation results are shown in Table 1. E1 to E7 are examples, and C1 to C2 are comparative examples.

[0088] [Table 1]

[0089] As shown in Table 1, in the comparative example using a negative electrode current collector having a slit group composed of only multiple slits, the negative electrode current collector fractured not only in the slit group but also in parts other than the slit group, resulting in poor cycle characteristics. In contrast, in the embodiment, fracture of the negative electrode current collector in parts other than the slit group was suppressed, and high cycle characteristics were obtained. This is thought to be because, in the embodiment, as the negative electrode expands during charging, the negative electrode current collector fractures along the slit group, and the stress applied to the negative electrode current collector is relieved by the holes, causing the fracture to stop at the holes.

[0090] Furthermore, the cycle characteristics can be further improved by making the average spacing Po between adjacent slit groups in the L / 4 portion from the outer edge of the negative electrode current collector smaller than the average spacing Pi between adjacent slit groups in the remaining portion.

[0091] 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. [Industrial applicability]

[0092] The non-aqueous electrolyte secondary battery described herein can achieve high cycle characteristics even when the expansion and contraction of the negative electrode during charging and discharging are significant. Furthermore, it can increase the capacity of the non-aqueous electrolyte secondary battery. Therefore, the non-aqueous electrolyte secondary battery described herein can be applied to a variety of applications requiring high cycle characteristics or high capacity. Such applications include various electronic devices (e.g., mobile phones, smartphones, tablet devices, wearable devices), electric vehicles including hybrid and plug-in hybrids, and home energy storage systems combined with solar cells. However, the applications of the non-aqueous electrolyte secondary battery are not limited to these. [Explanation of Symbols]

[0093] 112 Negative electrode current collector 113 Slit Group 113a Multiple slits 113b hole 1131 First Slit Group 1132 Second Slit Group D S Short direction of the negative electrode current collector e1 First end of the negative electrode current collector in the short direction e2 Second end of the negative electrode current collector in the short direction D L Longitudinal direction of the negative electrode current collector Eo Outer circumference end of the winding of the negative electrode current collector Ei: Inner end of the winding of the negative electrode current collector. L is the longitudinal length of the negative electrode current collector. W: Length of the negative electrode current collector in the short direction. p1, p2 Spacing between adjacent slit groups 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 electrode group 15 Case body 16 Sealing body 17, 18 Insulating board 19 Positive lead 20 Negative lead 21 Stepped section 22 filters 23 Lower valve body 24 Insulating material 25 Upper valve body 26 caps 27 Gasket 30 Positive electrode current collector 31. Positive electrode composite layer

Claims

1. It includes a cylindrical case, a group of electrodes and a non-aqueous electrolyte housed within the case, The electrode group is a wound electrode group in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are wound together. The negative electrode includes a strip-shaped negative electrode current collector having a first end and a second end in the short direction. The negative electrode current collector has a plurality of slit groups formed at intervals along the longitudinal direction of the negative electrode current collector, The slit group comprises a plurality of slits extending in a dashed line along the shorter direction from one end of the first end and the second end toward the other end, and a hole formed on the other end side of the plurality of slits, having a width greater than the average width of the plurality of slits. A non-aqueous electrolyte secondary battery, wherein, when the longitudinal length of the negative electrode current collector is L, the average spacing between adjacent slit groups in a portion L / 4 from the outer peripheral end of the negative electrode current collector is smaller than the average spacing between adjacent slit groups in the remaining portion.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the hole is continuous with the slit on the other end side of the plurality of slits.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the thickness of the negative electrode when fully charged is 1.18 times or more the thickness of the negative electrode after the first discharge.

4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein one of the aforementioned ends is cut by the slit on the side of the plurality of slits that is on the side of the one end.

5. The plurality of slit groups are, A first slit group comprising a plurality of first slits extending along the short direction from the first end toward the second end, and a first hole formed on the second end side of the plurality of first slits, A second slit group comprising a plurality of second slits extending along the short direction from the second end toward the first end, and a second hole formed on the first end side of the plurality of second slits, A non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, comprising:

6. The non-aqueous electrolyte secondary battery according to claim 5, wherein the first slit group and the second slit group are arranged alternately.

7. A non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the average distance between adjacent slits in the direction extending in a dashed line is 0.02 mm or more and 3 mm or less.

8. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, wherein the width of the hole is 0.05 mm or more and 2 mm or less.

9. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 8, wherein the hole is circular or elliptical.

10. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein the distance between the hole and the other end is 0.1W or more and 0.5W or less, when W is the length of the negative electrode current collector in the shorter direction.

11. A non-aqueous electrolyte secondary battery according to any one of claims 1 to 10, wherein lithium metal is deposited on the surface of the negative electrode current collector during charging, and the lithium metal dissolves during discharge.

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