Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises: an electrode group including a band-like positive electrode, a band-like negative electrode, and a band-like separator disposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte. The negative electrode comprises a band-like negative electrode current collector, and a band-like lithium layer disposed on at least one surface of the negative electrode current collector. The lithium layer includes at least one of lithium metal and a lithium alloy, has lithium layer end surfaces on both end surfaces in the length direction and on both end surfaces in the width direction, and has a recess-protrusion shape formed on at least one of the lithium layer end surfaces positioned on both end surfaces in the length direction and on both end surfaces in the width direction.

Description

Nonaqueous electrolyte secondary battery

[0001] This invention relates to a non-aqueous electrolyte secondary battery.

[0002] A non-aqueous electrolyte secondary battery comprises an electrode group and a non-aqueous electrolyte. The electrode group includes, for example, a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator positioned between the strip-shaped positive electrode and the strip-shaped negative electrode. Examples of non-aqueous electrolyte secondary batteries include lithium-ion batteries and lithium secondary batteries. In a lithium secondary battery, lithium metal is deposited on the strip-shaped negative electrode during charging, and the lithium metal is dissolved in the non-aqueous electrolyte during discharge. The lithium metal dissolved in the non-aqueous electrolyte is then released as lithium ions in the non-aqueous electrolyte. A lithium secondary battery is also called a lithium metal secondary battery, and a lithium metal secondary battery is synonymous with a metallic lithium secondary battery, which will be described later.

[0003] During the charging and discharging of lithium secondary batteries, significant volume changes occur in the electrode group due to the deposition and dissolution of lithium metal as described above. These large volume changes in the electrode group can lead to various problems in lithium secondary batteries.

[0004] Patent Document 1 discloses that a problem occurring in lithium secondary batteries is that a large volume change in the electrode group can cause damage to the separator or abnormal deposition of lithium, leading to an internal short circuit. Furthermore, Patent Document 1 also discloses that an internal short circuit can prevent the battery from achieving a long lifespan.

[0005] Furthermore, Patent Document 1 discloses a metallic lithium secondary battery in which a negative electrode made of lithium or a lithium alloy as the negative electrode active material and a positive electrode made of a rechargeable material are arranged opposite each other with a separator in between, and which has a buffer space between the negative electrode or the positive electrode and the separator for accommodating the lithium deposited on the surface of the negative electrode.

[0006] Japanese Patent Application Publication No. 10-12279

[0007] As described above, various publicly available documents, including Patent Document 1, have examined configurations to address the reduction in cycle life of lithium secondary batteries caused by internal short circuits resulting from at least one of separator damage and abnormal lithium deposition. However, it is difficult to say that sufficient consideration has been given to configurations to address the reduction in cycle life of lithium secondary batteries caused by factors other than those mentioned above. Therefore, there is room for further investigation into suppressing the reduction in cycle life.

[0008] Therefore, the objective of this disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress the reduction in cycle life.

[0009] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery comprises an electrode group including a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector. The lithium layer contains at least one of lithium metal and lithium alloy, and has lithium layer end faces on both end faces in the length direction and both end faces in the width direction. An uneven shape is formed on at least one of the lithium layer end faces located on both end faces in the length direction and both end faces in the width direction.

[0010] According to this disclosure, it is possible to provide a non-aqueous electrolyte secondary battery that can suppress the reduction in cycle life.

[0011] This is a cross-sectional view in the longitudinal direction of the electrode group according to the first embodiment. This is a cross-sectional view in the width direction of the electrode group according to the first embodiment. This is a magnified view of the end face side of the first lithium layer. This is a diagram for explaining the maximum distance Dmax. This is a cross-sectional view in the longitudinal direction of the electrode group according to the second embodiment. This is a cross-sectional view in the width direction of the electrode group according to the second embodiment. This is a schematic cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present 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] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises an electrode group including a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0016] In the non-aqueous electrolyte secondary battery according to the embodiment of this disclosure, the negative electrode comprises a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector.

[0017] In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the lithium layer comprises at least one of lithium metal and lithium alloy, and has lithium layer end faces on both end faces in the longitudinal direction and on both end faces in the width direction. In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the uneven shape is formed on at least one of the lithium layer end faces located on both end faces in the longitudinal direction and on both end faces in the width direction.

[0018] In the secondary battery according to the embodiment of the present disclosure, it is important that (i) the lithium layer has lithium layer end surfaces on both lengthwise end surfaces and both widthwise end surfaces, respectively, and (ii) the uneven shape is formed on at least one of the lithium layer end surfaces located on each of both lengthwise end surfaces and both widthwise end surfaces. The reason therefor will be described below.

[0019] A non-aqueous electrolyte secondary battery comprises an electrode group including a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In a non-aqueous electrolyte secondary battery, the negative electrode may include a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector. In such a non-aqueous electrolyte secondary battery, lithium metal precipitates on the surface of the negative electrode through charging, and the lithium metal precipitated on the surface of the negative electrode dissolves as lithium ions into the non-aqueous electrolyte through discharging.

[0020] In the non-aqueous electrolyte secondary battery as described above, the negative electrode expands due to the lithium metal precipitated on the surface of the negative electrode during charging. When the negative electrode expands, the separator becomes pressed toward the positive electrode, and this pressing may cause deformation of the separator. Then, there is a concern that the displacement of the separator in the electrode group may occur due to the deformation of the separator. When the separator is displaced, an internal short circuit occurs inside the non-aqueous electrolyte secondary battery. As described above, when an internal short circuit occurs inside the non-aqueous electrolyte secondary battery, the cycle life of the non-aqueous electrolyte secondary battery decreases. Further, when the electrode group is a wound-type electrode group formed by winding a stacked body of a positive electrode, a negative electrode, and a separator in the length direction, displacement of the separator is likely to occur during winding of the stacked body in the length direction. In this case also, an internal short circuit occurs inside the non-aqueous electrolyte secondary battery, and this internal short circuit causes a decrease in the cycle life of the non-aqueous electrolyte secondary battery.

[0021] Here, the non-aqueous electrolyte secondary battery according to the embodiment of this disclosure is configured to satisfy (i) and (ii) above. In short, the uneven shape is formed on at least one of the lithium layer end faces located at both ends in the length direction and at both ends in the width direction. Therefore, if the uneven shape is formed on at least one of the lithium layer end faces located at both ends in the length direction, frictional grip can be created between the uneven shape and at least one of the longitudinal end and other end of the separator. Also, if the uneven shape is formed on at least one of the lithium layer end faces located at both ends in the width direction, frictional grip can be created between the uneven shape and at least one of the width direction of the separator. Therefore, it is possible to suppress the displacement of the separator in the electrode group due to the deformation of the separator. Furthermore, if the electrode group is a wound-type electrode group formed by winding a laminate of positive electrode, negative electrode, and separator in the longitudinal direction, it is possible to suppress misalignment of the separator during winding in the longitudinal direction of the laminate. This prevents internal short circuits from occurring inside the non-aqueous electrolyte secondary battery, which can reduce the cycle life of the non-aqueous electrolyte secondary battery.

[0022] As described above, the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure may include an electrode in which lithium metal is deposited as a negative electrode during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge. Such a non-aqueous electrolyte secondary battery is also called a lithium secondary battery (or lithium metal secondary battery). Hereinafter, the configuration of the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure will be specifically described using a lithium secondary battery as an example.

[0023] In a lithium secondary battery, for example, 70% or more of the rated capacity is exhibited through the deposition and dissolution of lithium metal. Electron transfer at the negative electrode during charging and discharging mainly occurs through the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70% to 100% of the electron transfer (current from another perspective) at the negative electrode during charging and discharging occurs through the deposition and dissolution of lithium metal. 80% to 100% of the electron transfer at the negative electrode during charging and discharging may occur through the deposition and dissolution of lithium metal, or 90% to 100% may occur through the deposition and dissolution of lithium metal. Therefore, the negative electrode of a lithium secondary battery is different from the negative electrode of a lithium ion secondary battery, in which electron transfer during charging and discharging mainly occurs through the insertion and extraction of lithium ions in a negative electrode active material (e.g., graphite).

[0024] (Negative Electrode) A lithium secondary battery includes a strip-shaped negative electrode. The negative electrode includes a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode by charging. More specifically, lithium ions contained in a non-aqueous electrolyte receive electrons on the surface of the negative electrode during charging to form lithium metal, which deposits on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves as lithium ions into the non-aqueous electrolyte during discharging. Note that the "surface of the negative electrode" means both the surface of the negative electrode current collector and the surface of the lithium layer. That is, lithium metal is deposited on both the surface of the negative electrode current collector and the surface of the lithium layer during charging.

[0025] The lithium layer comprises at least one of lithium metal and lithium alloy. The lithium layer may be a lithium metal layer, a lithium alloy layer, or a layer containing both lithium metal and lithium alloy. The lithium alloy layer is composed of lithium and other elements. Examples of other elements (e.g., other metallic elements) include magnesium, aluminum, indium, copper, zinc, potassium, calcium, sodium, silver, and gold. The content of other elements may be 5% by mass or less (e.g., 3% by mass or less). Furthermore, because the negative electrode is equipped with a lithium layer, even if lithium ions become inactive during charging and discharging of the non-aqueous electrolyte secondary battery, these inactive lithium ions can be replenished.

[0026] The method for forming the lithium layer is not particularly limited, and various known methods may be applied. For example, the lithium layer may be formed by pressing a lithium foil onto the negative electrode current collector (first example). The lithium layer may be formed by vapor deposition (second example). The lithium layer may be formed using a resin sheet on which a lithium layer has been formed (third example). In the third example, to obtain the lithium layer, first, the resin sheet on which the lithium layer has been formed and the negative electrode current collector are laminated and pressed so that the lithium layer and the negative electrode current collector are in contact. Next, the resin sheet is peeled off to obtain a negative electrode current collector on which a lithium layer has been formed. The method using a resin sheet on which a lithium layer has been formed and the vapor deposition method are suitable for forming a thin lithium layer.

[0027] The lithium layer has lithium layer end faces at both ends in the length direction and at both ends in the width direction. The uneven shape is formed on at least one of the lithium layer end faces located at both ends in the length direction and at both ends in the width direction. The uneven shape means a shape that includes at least one of a recess and a convex portion. Therefore, the uneven shape may include only a recess, only a convex portion, or both a recess and a convex portion.

[0028] An example of a lithium layer will be described with reference to Figures 1A and 1B. Figures 1A and 1B show an example of an electrode group 14 including a positive electrode 11, a negative electrode 12, and a separator 13 disposed between the positive electrode 11 and the negative electrode 12. In the electrode group 14 shown in Figures 1A and 1B, the positive electrode 11 comprises a positive electrode current collector 11A and a positive electrode mixture layer 11B disposed on the surface of the positive electrode current collector 11A, and the negative electrode 12 comprises a negative electrode current collector 12A and a lithium layer 12B disposed on the surface of the negative electrode current collector 12A. Figure 1A shows a cross-section in the longitudinal direction of the electrode group 14, and Figure 1B shows a cross-section in the width direction of the electrode group 14. The electrode group shown in Figures 1A and 1B will also be referred to as the electrode group according to the first embodiment.

[0029] In lithium foil or lithium alloy foil used in lithium layer 12B, the edges of the foil are usually machined or otherwise processed. In this case, the size of Dmax, described later, can be adjusted by attaching adhesive tape to the edge of the lithium layer and then peeling the adhesive tape off the edge of the lithium layer. Lithium metal and lithium alloy have a relatively brittle crystal structure. Therefore, as described above, by peeling the adhesive tape off the edge of the lithium layer, at least a portion of the lithium metal and lithium alloy present on the edge of the lithium layer can be removed by adhering it to the adhesive tape. This makes it possible to reduce the size of Dmax. Specifically, the greater the adhesive strength of the adhesive tape, the more of the lithium metal and lithium alloy present on the edge of the lithium layer can be removed, thus reducing the value of Dmax. Also, the greater the adhesive strength of the adhesive tape, the shallower the depth of the recesses that may be included in the uneven shape R can be. Conversely, the smaller the adhesive strength of the adhesive tape, the larger the value of Dmax can be, and the deeper the recesses can be.

[0030] As shown in Figure 1A, the lithium layer 12B has, for example, a first lithium layer end face LE1 and a second lithium layer end face LE2 on each of its end faces in the length direction L. The uneven shape R is formed on, for example, the first lithium layer end face LE1 and the second lithium layer end face LE2. Also, as shown in Figure 1B, the lithium layer 12B has, for example, a third lithium layer end face LE3 and a fourth lithium layer end face LE4 on each of its end faces in the width direction W. The uneven shape R is formed on, for example, the third lithium layer end face LE3 and the fourth lithium layer end face LE4.

[0031] Figures 1A and 1B show an example in which the uneven shape R is formed on all of the end faces LE1 to LE4 of the first lithium layer, but the examples of how the uneven shape R is formed are not limited to this. The uneven shape R only needs to be formed on at least one of the end faces LE1 to LE4 of the first lithium layer. For example, the uneven shape R may be formed on any one of the end faces LE1 to LE4 of the first lithium layer. Furthermore, the uneven shape R may be formed on the end faces LE1 and LE2 of the first lithium layer, which are located at both ends in the length direction L, or on the end faces LE3 and LE4 of the third lithium layer, which are located at both ends in the width direction W. Furthermore, the uneven shape R may be formed on either the end face LE1 of the first lithium layer or the end face LE2 of the second lithium layer, and on either the end face LE3 of the third lithium layer or the end face LE4 of the fourth lithium layer. In other words, the uneven shape R may be formed on either one end face of the lithium layer in the length direction and either one end face of the lithium layer in the width direction.

[0032] Furthermore, while Figures 1A and 1B show a shape R that includes only convex portions, the uneven shape R is not limited to a shape that includes only convex portions. The uneven shape R may include both convex and concave portions, as shown in Figure 1C. Also, the uneven shape R may be a wavy shape in which the convex and concave portions are continuously connected. In the wavy shape, the convex and concave portions may have rounded ends, such as U-shapes, or pointed ends, such as V-shapes. Moreover, the uneven shape R may be a shape that includes only concave portions. Note that Figure 1C is an enlarged view of the end face LE1 side of the first lithium layer.

[0033] Figures 1A and 1B show an example where the uneven shape R has a wavy shape along the thickness direction. However, the uneven shape R may also have a wavy shape along the width direction or a wavy shape along the length direction. In this case, the uneven shape R may be formed at any one location in the height direction. For example, the uneven shape R may have a wavy shape along the width direction at any one location in the height direction, or it may have a wavy shape along the length direction at any one location in the height direction.

[0034] The negative electrode current collector has current collector end faces on both ends in the length direction and on both ends in the width direction. Referring to Figures 1A and 1B, the strip-shaped negative electrode current collector 12A has a first current collector end face CE1 and a second current collector end face CE2 on both ends in the length direction, and a third current collector end face CE3 and a fourth current collector end face CE4 on both ends in the width direction. As shown in Figure 1A, the first current collector end face CE1 is located on the same side as the first lithium layer end face LE1, and the second current collector end face CE2 is located on the same side as the second lithium layer end face LE2. Also, as shown in Figure 1B, the third current collector end face CE3 is located on the same side as the third lithium layer end face LE3, and the fourth current collector end face CE4 is located on the same side as the fourth lithium layer end face LE4.

[0035] It is preferable that the lithium layer end face on which the uneven shape is formed is positioned outside the current collector end face located on the same side as the lithium layer end face. This increases the contact area between the uneven shape and the separator, thereby further increasing the frictional grip between the uneven shape and the separator. Consequently, the displacement of the separator can be further suppressed. Referring to Figure 1A, when the uneven shape R is formed on the first lithium layer end face LE1, it is preferable that the first lithium layer end face LE1 is positioned outside the first current collector end face CE1, and when the uneven shape R is formed on the second lithium layer end face LE2, it is preferable that the second lithium layer end face LE2 is positioned outside the second current collector end face CE2. Furthermore, referring to Figure 1B, when the uneven shape R is formed on the end face LE3 of the third lithium layer, it is preferable that the end face LE3 of the third lithium layer be positioned outside the end face CE3 of the third current collector, and when the uneven shape R is formed on the end face LE4 of the fourth lithium layer, it is preferable that the end face LE4 of the fourth lithium layer be positioned outside the end face CE4 of the fourth current collector.

[0036] Referring to Figure 1A, we will specifically explain why the lithium layer end face with an uneven shape is positioned outside the current collector end face located on the same side as this lithium layer end face. First, as shown in Figure 1A, the first current collector end face CE1 does not have an uneven shape. Therefore, if at least one convex portion included in the uneven shape R formed on the first lithium layer end face LE1 is located outside the first current collector end face CE1, then the first lithium layer end face LE1 is positioned outside the first current collector end face CE1 located on the same side. The same applies to the relationship between the second lithium layer end face LE2 and the second current collector end face CE2, the relationship between the third lithium layer end face LE3 and the third current collector end face CE3, and the relationship between the fourth lithium layer end face LE4 and the fourth current collector end face CE4.

[0037] When the maximum distance between the lithium layer end face, which has an uneven shape formed on it, and the current collector end face located on the same side as this lithium layer end face, is defined as Dmax (mm), it is preferable that Dmax satisfies the relationship Dmax ≤ 1.0 mm. Referring to Figure 1D, the relationship between the first lithium layer end face LE1 and the first current collector end face CE1 can be explained as follows: the maximum distance Dmax is the distance between the top of the highest convex portion Cmax included in the uneven shape R formed on the first lithium layer end face LE1 and the first current collector end face CE1. The same applies to the relationship between the second lithium layer end face LE2 and the second current collector end face CE2, the third lithium layer end face LE3 and the third current collector end face CE3, and the fourth lithium layer end face LE4 and the fourth current collector end face CE4. By satisfying the above relationship, frictional snagging between the uneven shape and the separator can be suitably adjusted, thereby further suppressing the displacement of the separator. It is more preferable that Dmax satisfies the relationship Dmax ≤ 0.4 mm, and more preferably that Dmax ≤ 0.1 mm.

[0038] The maximum distance Dmax can be measured using a digital microscope. Specifically, five arbitrary locations on the lithium layer end face where the uneven shape is formed are observed using a digital microscope, and the highest convex Cmax within the uneven shape is determined at each of the five arbitrary locations. Then, the distance Dmax is measured between the top of the highest convex Cmax and the end face of the current collector located on the same side as the lithium layer end face where the highest convex Cmax was formed. Note that Dmax can be measured using the scale function of the digital microscope. For example, a digital microscope such as the KEYENCE VHX-7000 can be used.

[0039] The negative electrode may include a lithium ion storage layer supported on the negative electrode current collector. In this case, it is preferable that a lithium layer is arranged on one surface of the negative electrode current collector and the lithium ion storage layer is supported on the other surface of the negative electrode current collector. The lithium ion storage layer is a layer that exhibits capacity through the absorption and release of lithium ions by the negative electrode active material such as graphite. In this case, the open-circuit potential of the negative electrode when fully charged may be 70 mV or less relative to the lithium metal (lithium dissolution and release potential). In this case, lithium metal is present on the surface of the lithium ion storage layer when fully charged. Therefore, even when the negative electrode is configured in this way, capacity is exhibited through the deposition and dissolution of lithium metal.

[0040] Here, "fully charged" refers to the state when the battery is charged to a charge level of, for example, 0.98 × C or higher, where C is the rated capacity of the battery. The open-circuit potential of the negative electrode at full charge can be measured by disassembling the 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 contained in the cell may have the same composition as the non-aqueous electrolyte contained in the disassembled battery.

[0041] The lithium-ion storage layer is formed by creating layers of a negative electrode composite material containing a negative electrode active material. In addition to the negative electrode active material, the negative electrode composite material may also contain binders, thickeners, and conductive agents.

[0042] Examples of negative electrode active materials include carbonaceous materials, Si-containing materials, and Sn-containing materials. The negative electrode may contain one type of negative electrode active material alone, or it may contain a combination of two or more types of negative electrode active materials. Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon).

[0043] The conductive agent is, for example, a carbon material. Examples of carbon materials include carbon black, carbon nanotubes, and graphite. Examples of carbon black include acetylene black and Ketjenblack.

[0044] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF).

[0045] On the other hand, from the viewpoint of ensuring sufficient battery discharge capacity and improving volumetric energy density, it is preferable that the negative electrode does not contain the lithium-ion storage layer mentioned above.

[0046] The negative electrode current collector can be any conductive sheet. Examples of conductive sheets include foil and film.

[0047] The material forming the negative electrode current collector can be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as metals and alloys. It is preferable that the conductive material is one that does not react with lithium. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metallic elements, or graphite in which the basal surface is preferentially exposed. Examples of alloys include copper alloys and stainless steel (SUS). Among these conductive materials, it is preferable to use at least one of copper and copper alloys because they have high conductivity. Alternatively, a laminated sheet may be used as the negative electrode current collector, in which a metal such as copper or nickel, or an alloy such as a copper alloy, nickel alloy, or stainless steel is laminated on the surface of a resin film. The material forming the resin film is not particularly limited and examples include polyester, polyethylene, polypropylene, polyamide, and polyimide. Examples of polyester include polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).

[0048] The thickness of the negative electrode current collector is not particularly limited, but is, for example, 5 μm or more and 300 μm or less.

[0049] (Positive electrode) The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. 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. The positive electrode can be obtained, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to at least one surface of the positive electrode current collector to form a coating, and then drying this coating. The dried coating may be rolled.

[0050] The positive electrode active material is a material capable of intercalating and releasing lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Due to their low manufacturing cost and high average discharge voltage, lithium-containing transition metal oxides are preferred as the positive electrode active material.

[0051] During charging, lithium contained in the lithium-containing transition metal oxide is released from the positive electrode as lithium ions and deposited as lithium metal on the surface of the negative electrode. The surface of the negative electrode also includes the surface of the negative electrode current collector. During discharge, lithium metal dissolves from the negative electrode, releasing lithium ions, which are then absorbed into the lithium-containing transition metal oxide contained in the positive electrode. Therefore, the lithium ions involved in charging and discharging generally originate from the solute in the non-aqueous electrolyte and the positive electrode active material.

[0052] 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 transition metal element alone or a combination of two or more transition metal elements. The transition metal element may be at least one selected from the group consisting of Ni, Co, and Mn. Lithium-containing transition metal oxides may optionally contain one or more main group elements. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. Lithium-containing transition metal oxides preferably contain Al as a main group element.

[0053] Among lithium-containing transition metal oxides, it is preferable to use a composite oxide that contains at least one transition metal element selected from the group consisting of Ni, Co, and Mn, has a layered structure, and has a rock salt-type crystalline structure. Such a composite oxide may also contain Al, a typical element, as an optional component. Such a composite oxide is advantageous in that it contributes to increasing the capacity of lithium secondary batteries. In a lithium secondary battery using such a composite oxide as the positive electrode active material, the molar ratio (MLi / mM) of the total amount of lithium in the positive and negative electrodes (MLi) to the amount of metal M other than lithium in the positive electrode (mM) is set to, for example, 1.1 or less.

[0054] The binder and conductive agent can be those exemplified for the negative electrode.

[0055] The positive electrode current collector can be any conductive sheet. Examples of conductive sheets include foil and film.

[0056] Examples of materials for forming the positive electrode current collector (conductive sheet) include metallic materials such as metals and alloys. Examples of metals include Al and Ti, and examples of alloys include Al alloys, Ti alloys, and Fe alloys. Fe alloys may be stainless steel (SUS). Alternatively, a laminated sheet may be used as the positive electrode current collector, in which a metal such as aluminum or titanium, or an alloy such as an aluminum alloy or stainless steel, is laminated on the surface of a resin film. The material for forming the resin film is not particularly limited and examples include polyester, polyethylene, polypropylene, polyamide, and polyimide. Examples of polyesters include polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).

[0057] The thickness of the positive electrode current collector is not particularly limited, but is, for example, 5 μm or more and 30 μm or less.

[0058] (Separator) As described above, in this embodiment, the separator has a strip shape. The separator comprises at least a strip-shaped base material. Preferably, a spacer is placed between the separator and at least one of the positive electrode and the negative electrode. Preferably, the spacer is placed between the separator and the positive electrode. The spacer may be placed as a component of the separator between the separator and at least one of the positive electrode and the negative electrode, or as a component of at least one of the positive electrode and the negative electrode between the separator and at least one of the positive electrode and the negative electrode. When the spacer is a component of the separator, the separator comprises a base material and a spacer placed on one surface of the base material. Below, the case in which the spacer is a component of the separator will be mainly described as an example.

[0059] When the separator comprises a base material and a spacer disposed on one surface of the base material, such a spacer is arranged in the electrode group as shown in Figures 1E and 1F. The electrode group shown in Figures 1E and 1F is also referred to as the electrode group according to the second embodiment. Figure 1E is a longitudinal cross-sectional view of the electrode group according to the second embodiment, and Figure 1F is a widthwise cross-sectional view of the electrode group according to the second embodiment. Specifically, as shown in Figures 1E and 1F, the separator 13 comprises a base material 13A and a spacer 13B disposed on one surface of the base material 13A, and in the electrode group 14, the spacer 13B is arranged on the surface facing the positive electrode 11 (more specifically, the positive electrode mixture layer 11B). By providing the spacer 13B in the separator 13, a space can be formed between the positive electrode mixture layer 11B and the base material 13A. Furthermore, even if the negative electrode expands due to lithium metal deposited on the surface of the negative electrode during charging, this space can suppress the volume change of the electrode group 14. This can improve the cycle characteristics of lithium-ion batteries.

[0060] When the separator 13 comprises a base material 13A and a spacer 13B disposed on one surface of the base material 13A, as shown in Figures 1E and 1F, it is preferable that the lithium layer end face with the uneven shape is positioned outside the spacer. This allows sufficient friction to grip between the base material 13A and the uneven shape. Therefore, it is possible to suppress displacement of the separator 13 in the electrode group due to deformation of the separator 13. In Figures 1E and 1F, the first lithium layer end face LE1 to the fourth lithium layer end face LE4 all have an uneven shape R. Furthermore, the first lithium layer end face LE1 to the fourth lithium layer end face LE4 are all positioned outside the spacer 13B.

[0061] When the uneven shape is formed on the end faces of the lithium layers located at both ends in the longitudinal direction (first lithium layer end face LE1 and second lithium layer end face LE2), if the total length of the strip-shaped separator 13 (or strip-shaped substrate 13A) in the longitudinal direction is L1 and the total length of the strip-shaped lithium layer in the longitudinal direction is L2, the ratio of L1 to L2 (L1 / L2) may be greater than 1.00, 1.05 or greater, 1.10 or greater, or 1.20 or greater. The upper limit of L1 / L2 is, for example, 1.50. Furthermore, when the uneven shape is formed on the end faces of the lithium layers located at both ends in the width direction (the end face of the third lithium layer LE3 and the end face of the fourth lithium layer LE4), if the total length in the width direction of the strip-shaped separator 13 (or strip-shaped base material 13A) is W1 and the total length in the width direction of the strip-shaped lithium layer is W2, the ratio of W1 to W2 (W1 / W2) may be greater than 1.00, 1.05 or more, 1.10 or more, or 1.20 or more. The upper limit of W1 / W2 is, for example, 1.50. Since L1 / L2 and W1 / W2 are within the above numerical ranges, frictional catching can be generated even more sufficiently between the separator 13 (or base material 13A) and the uneven shape.

[0062] As the substrate, a porous sheet having ion permeability and insulating properties is used. Examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. The material constituting the porous sheet is not particularly limited, and examples include polymer materials. 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 substrate may contain additives as needed. Examples of additives include inorganic fillers.

[0063] The thickness of the substrate is not particularly limited, and is, for example, 5 μm or more and 20 μm or less. Preferably, the thickness of the substrate is 10 μm or more and 20 μm or less.

[0064] The substrate may include a porous sheet and a composite material layer (heat-resistant layer). The composite material layer may be located on one surface of the porous sheet or on both surfaces. The composite material layer is a layer that allows lithium ions to pass through. The composite material layer contains inorganic particles. The composite material layer may optionally contain a resin material. The thickness of the composite material layer may be 5% to 50% of the total thickness of the substrate.

[0065] The composite material layer may be placed on the surface facing the positive electrode or on the surface facing the negative electrode in the porous sheet. When the composite material layer is placed on the surface facing the positive electrode, degradation of the porous sheet due to oxidation can be suppressed. When the composite material layer is placed on the surface facing the negative electrode, degradation of the porous sheet due to reduction can be suppressed. As described above, if the separator is equipped with a spacer, the spacer may be placed on the composite material layer. In this case, the effect of suppressing thermal shrinkage of the substrate is enhanced.

[0066] Considering the possibility of abnormal heat generation inside the battery due to a short circuit, it is preferable to use inorganic compound particles that have thermal stability and insulating properties as inorganic particles. Examples of inorganic particles include inorganic oxides, inorganic hydroxides, inorganic nitrides, inorganic carbides, and inorganic sulfides. Examples of inorganic oxides include aluminum oxide, boehmite, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, and zinc oxide. Examples of inorganic nitrides include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of inorganic carbides include silicon carbide and carbon-boron. Examples of inorganic sulfides include barium sulfate. Examples of inorganic hydroxides include aluminum hydroxide. The average particle size of the inorganic particles may be 0.2 to 2.0 μm.

[0067] The average particle size of inorganic particles can be determined as the median diameter (D50) in a volume-based particle size distribution. The median diameter in a volume-based particle size distribution of inorganic particles can be measured, for example, using a laser diffraction / scattering particle size distribution analyzer (e.g., Microtrac manufactured by Nikkiso Co., Ltd.). Alternatively, the average particle size of inorganic particles may be determined using images taken with a transmission electron microscope (TEM). For example, a cross-section of a substrate can be observed with a transmission electron microscope (TEM) to take a TEM image, the area (area of ​​the portion enclosed by the contour) of any 100 inorganic particles in the image can be calculated, the diameter of an equivalent circle (true circle) having the same area as each of the calculated areas can be determined, and these can be arithmetically averaged to obtain the average particle diameter of the inorganic particles.

[0068] Examples of resin materials included in the composite material layer (heat-resistant layer) include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; fluororubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene polymer; styrene-butadiene copolymer or its hydride, acrylonitrile-butadiene copolymer or its hydride, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate. Examples include rubbers; cellulose derivatives such as ethylcellulose, methylcellulose, hydroxyethylcellulose, and carboxymethylcellulose; acrylic resins such as acrylic acid-methacrylic acid copolymers; polyphenylene ethers; polysulfones; polyethersulfones; polyphenylene sulfides; polyetherimides; polyimides; polyamides such as fully aromatic polyamides (aramids); polyimides; polyacrylonitriles; polyvinyl alcohols; polyethers; polyacrylic acid; polymethacrylic acid; polyesters; polyolefins; silicone resins; urethane resins; melamine resins; urea resins; epoxy resins, etc.

[0069] It is preferable to use a polymer material as the resin material included in the composite material layer (heat-resistant layer). It is preferable that such a polymer material has higher heat resistance than the polymer material constituting the porous sheet. It is preferable that such a polymer material includes at least one selected from the group consisting of aromatic polyamide, aromatic polyimide, and aromatic polyamide-imide. All of these have high heat resistance. From the viewpoint of heat resistance, it is preferable to use aramid as the polymer material. In other words, it is preferable to use meta-aramid (meta-total aromatic polyamide) or para-aramid (para-total aromatic polyamide) as the polymer material.

[0070] The inorganic particle content in the composite material layer may be in the range of 50% to 99% by mass, or in the range of 85% to 99% by mass.

[0071] A composite material layer can be obtained, for example, by applying a coating solution containing inorganic particles, a resin material, and a liquid component (dispersion medium) to a porous sheet to form a coating film, and then drying this coating film. Examples of liquid components include N-methyl-2-pyrrolidone.

[0072] The spacer is placed, for example, on one surface of the substrate. It is preferable that the substrate and the spacer are integrated, as this facilitates the fabrication of the electrode group. The spacer may be placed on the surface of the substrate facing the positive electrode, on the surface facing the negative electrode, or on both the surface facing the positive electrode and the surface facing the negative electrode. Alternatively, the spacer may be placed on the surface facing the separator at the positive electrode, or on the surface facing the separator at the negative electrode.

[0073] When a spacer is placed on the substrate surface facing the positive electrode, compared to when the spacer is placed on the substrate surface facing the negative electrode, Li precipitates between the spacers in a way that stretches the substrate towards the positive electrode. This creates compressive stress on the precipitated Li, making it easier for the Li to precipitate densely. When Li precipitates densely in this way, the discharge efficiency and cycle characteristics of the lithium secondary battery are improved. Therefore, from the viewpoint of improving the discharge efficiency and cycle characteristics of the lithium secondary battery, it is preferable to place the spacer on the substrate surface facing the substrate.

[0074] When a spacer is placed on the surface of the substrate facing the negative electrode, a space is pre-formed between the substrate and the negative electrode, which reduces the tensile load on the substrate that occurs as Li precipitates. This makes it easier to maintain the insulating properties of the substrate and also easier to maintain its short-circuit resistance.

[0075] In lithium-ion batteries, the main role of the spacer is to create a space for the deposition of lithium metal. By housing the lithium metal within the space provided by the spacer, the expansion of the negative electrode during charging can be suppressed.

[0076] The spacer may contain a resin material (for example, an insulating resin), or it may contain a resin material and particles. The proportion of resin material in the spacer may be 10% by volume or more, 30% by volume or more, or 50% by volume or more. Alternatively, the proportion of resin material in the spacer may be 100% by volume or less, or 80% by volume or less.

[0077] The resin material included in the spacer can be the same as the resin material included in the composite material layer.

[0078] It is preferable to use a resin material that does not allow lithium ions to permeate the resin material contained in the spacer. Suitable resin materials include, for example, polyimide, polyvinylidene fluoride, and acrylonitrile-acrylic acid ester copolymers, with polyimide being particularly preferred. A non-porous spacer formed using these resin materials to have a certain height or higher is a layer that does not allow lithium ions to permeate. Because the spacer is formed in a non-porous manner as described above, it is possible to suppress an increase in the gas generation reaction rate when an internal short circuit occurs in a lithium secondary battery.

[0079] The particles may be inorganic or organic. Inorganic particles are preferred, and among inorganic particles, inorganic oxides, inorganic hydroxides, inorganic nitrides, inorganic carbides, and inorganic sulfides are preferred. Examples of inorganic oxides include aluminum oxide (alumina and boehmite), magnesium oxide, titanium oxide (titania), zirconium oxide, and silicon oxide (silica). Examples of inorganic hydroxides include aluminum hydroxide. Examples of inorganic nitrides include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of inorganic carbides include silicon carbide and boron carbide. Examples of inorganic sulfides include barium sulfate. Furthermore, minerals such as aluminosilicate, layered silicate, barium titanate, and strontium titanate may be used as inorganic particles. Among the above inorganic particles, alumina, silica, and titania are preferred.

[0080] The average particle size is not particularly limited and may be 0.1 μm or more, or 0.5 μm or more. The average particle size may be 10 μm or less, 5 μm or less, or 2 μm or less. The average particle size can be measured by the following method. First, the cross-section of the spacer in the thickness direction of the separator is photographed with an electron microscope to obtain an image of the cross-section. Next, image processing such as binarization is performed on this image to identify the particle portion. Next, for any number of particles (for example, 100 or more), the diameter of a circle having the same area as the cross-sectional area (equivalent circle diameter) is calculated for each particle, and the average particle size can be measured by taking the arithmetic mean of each of the calculated equivalent circle diameters.

[0081] When the spacer contains resin material and particles, it is preferable that the particle content in the spacer be 50% by volume or less. This makes it possible to obtain a spacer with sufficient strength.

[0082] The spacer includes protrusions. The spacer may include at least one of linear protrusions and dot-shaped protrusions. In one view, the linear protrusions are ridge-shaped protrusions. The linear protrusions may be intermittently arranged or continuously arranged. The linear protrusions may be straight or curved.

[0083] The width of the linear protrusion may be 100 μm or more, or 200 μm or more. The width of the linear protrusion may be 2000 μm or less, or 1000 μm or less.

[0084] The spacer preferably has a predetermined repeating pattern. In other words, it is preferable that the protrusions are arranged in a predetermined repeating pattern. Linear protrusions may be arranged in a stripe pattern or in a mesh pattern. The mesh pattern may be an aggregate of polygons. The mesh pattern may include, for example, shapes in which polygons are combined so as to share sides. Polygons include triangles, quadrilaterals, and hexagons. A mesh pattern may be formed by combining polygons having different shapes. The mesh pattern may be honeycomb-shaped. In addition, dot-shaped protrusions may be arranged in a predetermined repeating pattern.

[0085] The thickness of the spacer (height of the protrusion) TC may be greater than the thickness of the base material T. The ratio of height TC to thickness T, TC / T, may be greater than 1, 1.5 or more, or 2 or more. TC / T may be 5 or less, 4 or less, or 3 or less. From the viewpoint of suppressing the expansion of the electrode group, TC / T may be, for example, greater than 1 and 3 or less. Also, TC / T may be between 1.5 and 3.

[0086] The spacer thickness (height of the protrusion) TC can be measured, for example, by photographing the spacer with a laser microscope to obtain a height profile, and then using this height profile. Specifically, TC can be determined by selecting six arbitrary points in the height profile, measuring the thickness (height of the protrusion) of the spacer at these six points, and then taking the arithmetic mean of the obtained measurements. For example, a hybrid laser microscope manufactured by Lasertec can be used as the laser microscope.

[0087] The thickness T of the substrate can be measured using a contact-type thickness measuring device. Specifically, after selecting any six locations on the substrate, the thickness of each location is measured using the contact-type thickness measuring device, and the thickness can be calculated by taking the arithmetic mean of the obtained measurements. As a contact-type thickness measuring device, for example, the thickness measuring device (PEACOCK) manufactured by Ozaki Seisakusho Co., Ltd. can be used.

[0088] Spacers can be formed, for example, by applying a coating solution containing the spacer components and liquid components to a predetermined location on a substrate to obtain a coating film, and then drying this coating film. For example, N-methyl-2-pyrrolidone can be used as the liquid component. Coating may be carried out using a dispenser or by various known printing methods such as gravure printing, inkjet printing, and screen printing. The coating film may be heat-dried or air-dried. The thickness of the spacer can be adjusted by adjusting the amount of coating solution applied or by adjusting the viscosity of the coating solution. Spacers can also be formed on the surface of the positive electrode (the surface facing the separator) and on the surface of the negative electrode (the surface facing the separator) in the same manner as described above.

[0089] (Non-aqueous electrolytes) Non-aqueous electrolytes are ionic conductive (e.g., lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes), gel electrolytes, or solid electrolytes. A liquid electrolyte is, for example, an electrolyte containing a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte may contain various known additives.

[0090] The gel-like electrolyte comprises a lithium salt and a matrix polymer, or a lithium salt and a matrix polymer plus a non-aqueous solvent. 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, polyether resins, and polyethylene oxide.

[0091] Examples of solid electrolytes include various known materials used in all-solid-state lithium-ion secondary batteries. Examples of such solid electrolytes include oxide-based solid electrolytes, sulfide-based solid electrolytes, and halide-based solid electrolytes.

[0092] Liquid non-aqueous electrolytes are prepared by dissolving lithium salts in a non-aqueous solvent. The dissolution of lithium salts in a non-aqueous solvent generates lithium ions and anions.

[0093] Examples of the anions include BF ４ － , ClO ４ － , PF ６ － , CF ３ SO ３ － , CF ３ CO ２ － , anions of imides, and anions of oxalate complexes. Examples of the anions of imides include N(SO ２ CF ３ ) ２ － , N(C ｍ F ２ｍ＋１ SO ２ ) ｘ (C ｎ F ２ｎ＋１ SO ２ ) ｙ － , and the like. Here, m and n are each independently an integer of 0 or 1 or more, x and y are each independently 0, 1 or 2, and satisfy x+y=2. The anion of the oxalate complex may contain at least one of boron and phosphorus. Examples of the anions of oxalate complexes include bis(oxalate)borate anions, difluoro(oxalate)borate anions (BF ２ (C ２ O ４ ) ― ), PF ４ (C ２ O ４ ) － , and PF ２ (C ２ O ４ ) ２ － , and the like. The liquid non-aqueous electrolyte may contain any one of these anions alone, or may contain two or more thereof in combination.

[0094] From the viewpoint of suppressing the dendritic deposition of lithium metal, the liquid non-aqueous electrolyte preferably contains at least an oxalate complex anion. Furthermore, the oxalate complex anion preferably contains fluorine. The interaction between the fluorine-containing oxalate complex anion and lithium makes it easier to uniformly precipitate lithium metal in fine particulate form. Therefore, localized deposition of lithium metal can be suppressed. The fluorine-containing oxalate complex anion may be combined with other anions. The other anions are PF ６ － And at least one of the anions of the imide group.

[0095] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The liquid non-aqueous electrolyte may contain one non-aqueous solvent alone or a combination of two or more. Examples of halogen-substituted derivatives include fluorides.

[0096] Examples of esters include carbonate esters and carboxylic acid esters. Examples of carbonate esters include cyclic carbonate esters and linear carbonate esters. Examples of cyclic carbonate esters include ethylene carbonate (EC), propylene carbonate (PC), and fluoroethylene carbonate (FEC). FEC has the function of forming a film on the surface of the negative electrode, as will be described later. Examples of linear carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of carboxylic acid esters include cyclic carboxylic acid esters and linear carboxylic acid esters. 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.

[0097] Examples of 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 diethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0098] The concentration of lithium salt in the liquid non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. In the liquid non-aqueous electrolyte, the concentration of the anion may be 0.5 mol / L or more and 3.5 mol / L or less. In addition, in the liquid non-aqueous electrolyte, the concentration of the oxalate complex anion may be 0.5 mol / L or more and 3.5 mol / L or less.

[0099] The liquid non-aqueous electrolyte may contain additives. The additives may form a film on the surface of the negative electrode. The formation of a film derived from the additive on the surface of the negative electrode makes it easier to suppress dendrite formation. Examples of such additives include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).

[0100] Below, an example of a non-aqueous electrolyte secondary battery of this disclosure will be described in detail with reference to the drawings. The components of the example non-aqueous electrolyte secondary battery described below can be the components described above. Furthermore, the components of the example described below can be modified based on the above. In addition, in the non-aqueous electrolyte secondary battery described below, components that are not essential to the non-aqueous electrolyte secondary battery of this disclosure may be omitted. Note that the scale of the components in the following figures has been changed to facilitate understanding.

[0101] (Embodiment 1) Figure 2 is a schematic longitudinal cross-sectional view showing a lithium secondary battery as an example of a non-aqueous electrolyte secondary battery according to Embodiment 1. The cylindrical lithium secondary battery 10 shown in Figure 2 includes a cylindrical battery case, a wound electrode group 14 housed inside the battery case, and a non-aqueous electrolyte (not shown). The battery case includes a case body 15 which is a bottomed cylindrical metal container, and a sealing body 16 which seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The gasket 27 ensures 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 winding axis direction, respectively.

[0102] The case body 15 has, for example, a stepped portion 21 formed 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 along the circumferential direction of the case body 15 on the side wall of the case body 15. In this case, the sealing body 16 is supported on the opening side surface of the stepped portion 21.

[0103] The sealing body 16 includes 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 has, for example, a disc shape or a ring shape. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed at their respective peripheries. The filter 22 and the lower valve body 23 are connected to each other at their respective centers. The upper valve body 25 and the cap 26 are connected to each other at their respective centers. Therefore, each member except the insulating member 24 is electrically connected to each other.

[0104] The lower valve body 23 has a ventilation hole (not shown) formed therein. Therefore, if the internal pressure of the battery case rises due to abnormal heat generation or the like, the upper valve body 25 bulges towards the cap 26 and separates from the lower valve body 23. This interrupts the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further, the upper valve body 25 ruptures, and gas is released from an opening (not shown) formed in the cap 26.

[0105] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The separator 13 has a base material. A spacer may be placed on one surface of the base material of the separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The base material of the separator 13 also has a strip shape. The electrode group 14 is formed by winding the strip-shaped positive electrode 11, the strip-shaped negative electrode 12, and the strip-shaped separator 13 in the longitudinal direction so that the strip-shaped separator 13 is placed between the strip-shaped positive electrode 11 and the strip-shaped negative electrode. If the strip-shaped separator 13 comprises a strip-shaped base material and a spacer placed on one surface of the strip-shaped base material, it is preferable that the spacer is placed on the surface of the strip-shaped base material facing the strip-shaped positive electrode 11.

[0106] (Note) The following technologies are disclosed by the above description. (Technology 1) A non-aqueous electrolyte secondary battery comprising: an electrode group including a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the negative electrode comprises a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector, the lithium layer comprising at least one of lithium metal and lithium alloy, and having lithium layer end faces on both end faces in the length direction and both end faces in the width direction, and the uneven shape is formed on at least one of the lithium layer end faces located on both end faces in the length direction and both end faces in the width direction. (Technology 2) The non-aqueous electrolyte secondary battery according to Technology 1, wherein the negative electrode current collector has current collector end faces on both end faces in the length direction and both end faces in the width direction, and the lithium layer end face on which the uneven shape is formed is located outside the current collector end face located on the same side as the lithium layer end face. (Technology 3) A non-aqueous electrolyte secondary battery according to Technology 2, wherein when the maximum distance between the lithium layer end face on which the uneven shape is formed and the current collector end face located on the same side as the lithium layer end face, Dmax satisfies the relationship Dmax ≤ 1.0 mm. (Technology 4) A non-aqueous electrolyte secondary battery according to Technology 3, wherein Dmax satisfies the relationship Dmax ≤ 0.4 mm. (Technology 5) A non-aqueous electrolyte secondary battery according to Technology 3 or 4, wherein Dmax satisfies the relationship Dmax ≤ 0.1 mm. (Technology 6) A non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the electrode group is a wound electrode group formed by winding the positive electrode, the negative electrode, and the separator in the longitudinal direction, and the uneven shape is formed on the lithium layer end face located on each of the two end faces in the width direction of the lithium layer. (Technology 7) A non-aqueous electrolyte secondary battery according to any one of Techniques 1 to 6, wherein a spacer is disposed between the separator and at least one of the positive electrode and the negative electrode. (Technology 8) A non-aqueous electrolyte secondary battery according to Technique 7, wherein the lithium layer end face on which the uneven shape is formed is disposed outside the spacer.(Technology 9) A non-aqueous electrolyte secondary battery according to Technology 7 or 8, wherein the spacer is disposed between the separator and the positive electrode. (Technology 10) A non-aqueous electrolyte secondary battery according to any one of Technology 1 to 9, wherein the negative electrode is an electrode in which lithium metal is deposited during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge.

[0107] 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.

[0108] (Example 1) (1) Preparation of the positive electrode A positive electrode slurry was prepared by mixing a positive electrode active material, acetylene black (AB, conductive agent), polyvinylidene fluoride (PVdF, binder), and an appropriate amount of N-methyl-2-pyrrolidone (NMP). A rock salt type lithium-containing transition metal oxide (NMC) having a layered structure and containing Li, Ni, Co, and Al was used as the positive electrode active material. In the NMC, the molar ratio of Li to the total of Ni, Co, and Al was 1.0. In the positive electrode slurry, the mass ratio of NMC, AB, and PVdF was NMC:AB:PVdF = 95:2.5:2.5. The positive electrode slurry was applied to both sides of a strip-shaped Al foil (positive electrode current collector) to obtain a coating film, and this coating film was dried. Then, the dried coating film was rolled to form positive electrode mixture layers on both sides of the positive electrode current collector. The positive electrode current collector, with positive electrode mixture layers formed on both sides, was cut to a predetermined size to obtain the positive electrode according to Example 1.

[0109] (2) Fabrication of the negative electrode As the negative electrode according to Example 1, a strip of copper foil (negative electrode current collector) with a strip of lithium layer formed on one side was prepared. The lithium layer was formed by pressing and bonding the lithium foil to the copper foil. The strip of lithium layer was laminated on the strip of copper foil so that the length and width directions coincided. In the negative electrode according to Example 1, an uneven shape was provided on the lithium layer end faces located at each of the two ends in the width direction of the lithium layer. The uneven shape was provided on each lithium layer end face using adhesive tape (Kapton tape manufactured by Teraoka Seisakusho) as described in the previous section on embodiments. After firmly pressing the adhesive tape onto each lithium layer end face with a finger, the adhesive tape was peeled off from each lithium layer end face. The maximum distance Dmax between the lithium layer end face with the uneven shape and the current collector end face (width direction end face of the negative electrode current collector) located on the same side as this lithium layer end face was 0.05 mm. Dmax was measured according to the method described in the above Embodiments section.

[0110] (3) Preparation of the substrate A microporous thin film (substrate) made of polyethylene with a thickness of 10 μm was prepared. When the total length of the substrate in the longitudinal direction was L1 mm and the width in the longitudinal direction was W1 mm, and the total length of the lithium layer in the longitudinal direction was L2 mm and the width in the longitudinal direction was W2 mm, the ratio of L1 to L2 (L1 / L2) and the ratio of W1 to W2 (W1 / W2) were both greater than 1.00.

[0111] A coating film was formed by applying a coating solution containing paraphenylene terephthalamide (aromatic polyamide) and alumina (inorganic particles) as resin materials to one surface of a microporous thin film. N-methyl-2-pyrrolidone containing 5.8% by mass of calcium chloride was used as the solvent for the coating solution. The coating solution was also formulated to contain 2% by mass of aromatic polyamide and 4% by mass of alumina. The substrate with the coated film was left for 1 hour in an atmosphere of 25°C and 70% relative humidity to precipitate the aromatic polyamide on the surface of the substrate. Next, the surface of the substrate was washed with water to remove NMP and calcium chloride from the coating film. Then, the coating film was dried at 60°C for 5 minutes to form a composite material layer (heat-resistant layer) on one surface of the substrate. This resulted in obtaining a substrate with a composite material layer.

[0112] A coating solution containing polyvinylidene fluoride and alumina particles (inorganic filler) was applied to the surface of the composite material layer on the substrate to form a coating film, which was then dried. This formed a honeycomb-shaped spacer on the composite material layer of the substrate. In this way, a separator was obtained in which the composite material layer and spacer were formed in this order on the surface of the substrate.

[0113] In the spacer, the width of the linear protrusion was set to 0.25 mm (250 μm), and the thickness (height) TC of the spacer was set to 30 μm.

[0114] (4) Preparation of non-aqueous electrolytes 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CHF ２ (CF ２ OCH ２ ) CF ３ A mixed solvent containing ) and in a volume ratio of 1:2 was prepared. In addition to dissolving lithium bissulfonyliimide (LiFSI) at a concentration of 1 mol / L in this mixed solvent, LiBF was added. ２ (C ２ O ４ A liquid non-aqueous electrolyte was prepared by dissolving ) at a concentration of 0.1 mol / L.

[0115] (5) Fabrication of a non-aqueous electrolyte secondary battery An electrode group was fabricated by spirally winding a strip-shaped positive electrode and a strip-shaped negative electrode in an inert gas atmosphere with a strip-shaped separator in between. The separator was positioned so that the side with the spacer formed on it faced the positive electrode.

[0116] After housing the electrode group in a bottomed cylindrical case body, a non-aqueous electrolyte was injected into the case body. Next, a sealing body was placed at the opening of the case body via a gasket, sealing the electrode group and non-aqueous electrolyte in the battery case. In this way, a non-aqueous electrolyte secondary battery (lithium secondary battery) with the structure shown in Figure 2 was completed.

[0117] (Example 2) In the negative electrode, the non-aqueous electrolyte secondary battery according to Example 2 was completed in the same manner as in Example 1, except that the maximum distance Dmax between the lithium layer end face with an uneven shape formed thereon and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face was set to 0.10 mm. Dmax was measured according to the method described in the section on the above embodiments. As in Example 1, Kapton tape manufactured by Teraoka Seisakusho was used as the adhesive tape. Then, the adhesive tape was applied to the lithium layer end face by pressing it with a finger so that a lighter force than in Example 1 was applied, and then the adhesive tape was peeled off from each of the lithium layer end faces.

[0118] (Example 3) In the negative electrode, the non-aqueous electrolyte secondary battery according to Example 3 was completed in the same manner as in Example 1, except that the maximum distance Dmax between the lithium layer end face with an uneven shape formed thereon and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face was set to 0.40 mm. Dmax was measured according to the method described in the above embodiment section. As in Example 1, Kapton tape manufactured by Teraoka Seisakusho was used as the adhesive tape. Then, the adhesive tape was applied to the lithium layer end face by pressing it with a finger so that a lighter force than in Example 2 was applied, and then the adhesive tape was peeled off from each of the lithium layer end faces.

[0119] (Example 4) A non-aqueous electrolyte secondary battery according to Example 4 was completed in the same manner as in Example 1, except that the maximum distance Dmax between the lithium layer end face with an uneven shape formed on it and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face was set to 1.00 mm. Dmax was measured according to the method described in the section on the above embodiments. Nitto Denko polyester adhesive tape was used as the adhesive tape. The adhesive tape was then applied to the lithium layer end face by pressing it with a finger so that a force of about the same magnitude as in Example 1 was applied, and then the adhesive tape was peeled off from each of the lithium layer end faces.

[0120] (Example 5) A non-aqueous electrolyte secondary battery according to Example 5 was completed in the same manner as in Example 1, except that the maximum distance Dmax between the lithium layer end face with an uneven shape formed on it and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face was set to 2.00 mm. Dmax was measured according to the method described in the section on the embodiments above. As in Example 4, Nitto Denko polyester adhesive tape was used as the adhesive tape. The adhesive tape was then applied to the lithium layer end face by pressing it with a finger so that a force similar to that in Example 2 was applied, and then the adhesive tape was peeled off from each of the lithium layer end faces.

[0121] (Comparative Example 1) A non-aqueous electrolyte secondary battery according to Comparative Example 1 was completed in the same manner as in Example 1, except that the maximum distance Dmax between the lithium layer end face, which has an uneven shape formed on it, and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face was set to 0.00 mm. Therefore, in the negative electrode of Comparative Example 1, the lithium layer end face and the current collector end face (the end face in the width direction of the negative electrode current collector) located on the same side as the lithium layer end face were flush. As in Example 1, Kapton tape manufactured by Teraoka Seisakusho was used as the adhesive tape. The adhesive tape was then applied to the lithium layer end face by pressing it with a finger so that a stronger force than in Example 1 was applied, and then the adhesive tape was peeled off from each of the lithium layer end faces.

[0122] [Evaluation] <Number of cycles when abnormal charging occurs> Charge-discharge tests were conducted using the non-aqueous electrolyte secondary batteries related to each example (Examples 1 to 5 and Comparative Example 1). Specifically, 10 mA / cm ２ After performing constant current charging up to a voltage of 4.1V with the specified current, 1mA / cm² was applied at a voltage of 4.1V. ２ Constant voltage charging is performed until the current reaches 10 mA / cm². ２A charge-discharge test was conducted on each example of non-aqueous electrolyte secondary battery, with one cycle defined as constant current discharge until the voltage reached 3.0V. For each example of non-aqueous electrolyte secondary battery, if the charge capacity in one cycle was 1% or more greater than the charge capacity in the cycle immediately preceding it, it was evaluated that an abnormal charge had occurred due to an internal short circuit in that cycle. The number of cycles up to the first cycle was evaluated as the number of cycles when the abnormal charge occurred. The results are shown in Table 1 below. Note that "No abnormality" in Table 1 means that even after conducting the charge-discharge test until the discharge capacity of the first cycle was 90%, the phenomenon of the charge capacity in one cycle being 1% or more greater than the charge capacity in the cycle immediately preceding it did not occur. The Dmax (mm) for each example is also shown in Table 1.

[0123]

[0124] Table 1 shows that in the non-aqueous electrolyte secondary batteries of each example (Examples 1 to 5), the evaluation of the number of cycles when abnormal charging occurs is either "no abnormality" or 100 cycles or more, whereas in the non-aqueous electrolyte secondary battery of Comparative Example 1, the evaluation of the number of cycles when abnormal charging occurs is less than 100 cycles. Furthermore, Table 1 shows that when Dmax is 0.40 mm or less, the evaluation of the number of cycles when abnormal charging occurs is even better, and when Dmax is 0.10 mm or less, the evaluation of the number of cycles when abnormal charging occurs is particularly good (no abnormality).

[0125] 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.

[0126] The non-aqueous electrolyte secondary battery described herein can be used in applications where it is required to suppress a decrease in cycle life.

[0127] 10: Non-aqueous electrolyte secondary battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 14: Electrode group

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode group including a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the negative electrode comprises a strip-shaped negative electrode current collector and a strip-shaped lithium layer disposed on at least one surface of the negative electrode current collector; the lithium layer comprises at least one of lithium metal and lithium alloy, and has lithium layer end faces on both end faces in the longitudinal direction and both end faces in the width direction, and an uneven shape is formed on at least one of the lithium layer end faces located on both end faces in the longitudinal direction and both end faces in the width direction.

2. The negative electrode current collector has current collector end faces on both ends in the length direction and on both ends in the width direction, and the lithium layer end face on which the uneven shape is formed is positioned outside the current collector end face located on the same side as the lithium layer end face, as described in claim 1.

3. The non-aqueous electrolyte secondary battery according to claim 2, wherein when the maximum distance between the lithium layer end face on which the uneven shape is formed and the current collector end face located on the same side as the lithium layer end face is Dmax (mm), Dmax satisfies the relationship Dmax ≤ 1.0 mm.

4. The non-aqueous electrolyte secondary battery according to claim 3, wherein Dmax satisfies the relationship Dmax ≤ 0.4 mm.

5. The non-aqueous electrolyte secondary battery according to claim 3, wherein Dmax satisfies the relationship Dmax ≤ 0.1 mm.

6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrode group is a wound electrode group formed by winding the positive electrode, the negative electrode, and the separator in the longitudinal direction, and the uneven shape is formed on the end faces of the lithium layer located at each of the end faces in the width direction of the lithium layer.

7. The non-aqueous electrolyte secondary battery according to claim 1, wherein a spacer is disposed between the separator and at least one of the positive electrode and the negative electrode.

8. The non-aqueous electrolyte secondary battery according to claim 7, wherein the lithium layer end face on which the uneven shape is formed is positioned outside the spacer.

9. The non-aqueous electrolyte secondary battery according to claim 7, wherein the spacer is disposed between the separator and the positive electrode.

10. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is an electrode in which lithium metal is deposited during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge.

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