Lithium-ion battery

Abstract

This lithium secondary battery is provided with: a positive electrode; a negative electrode that faces the positive electrode; a separator that is arranged between the positive electrode and the negative electrode; a nonaqueous electrolyte that has lithium ion conductivity; and a spacer that is arranged between the negative electrode and the separator. Lithium metal deposits on the negative electrode at the time of charging, and the lithium metal is dissolved from the negative electrode at the time of discharging. The spacer has a network structure that is formed of a plurality of linear projections.

Description

[Technical Field]

[0001] This disclosure relates to a lithium secondary battery comprising a lithium-ion conductive non-aqueous electrolyte. [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. In these applications, there is a demand for even higher capacity in non-aqueous electrolyte secondary batteries. Lithium-ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Higher capacity lithium-ion batteries can be achieved by using a combination of alloy active materials, such as graphite and silicon compounds, as the negative electrode active material. However, the capacity of lithium-ion batteries is reaching its limits.

[0003] As a high-capacity non-aqueous electrolyte secondary battery that surpasses lithium-ion batteries, lithium secondary batteries (lithium metal secondary batteries) are promising. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging, and this lithium metal dissolves into the non-aqueous electrolyte during discharge.

[0004] Patent Document 1 proposes providing multiple protrusions on both sides of the negative electrode current collector in a lithium secondary battery in order to suppress the expansion of the negative electrode due to the deposition of lithium metal. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-212603 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The multiple protrusions described in Patent Document 1 are arranged independently and discontinuously. In this case, the pressure applied to the electrode during lithium metal deposition increases locally, which can damage the electrode and degrade its cycle characteristics. [Means for solving the problem]

[0007] One aspect of this disclosure relates to a lithium secondary battery comprising a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte having lithium ion conductivity, and a spacer disposed between the negative electrode and the separator, wherein lithium metal is deposited on the negative electrode during charging, the lithium metal is dissolved from the negative electrode during discharge, and the spacer has a mesh structure formed of a plurality of linear protrusions. [Effects of the Invention]

[0008] According to this disclosure, it is possible to suppress the deterioration of the cycle characteristics of lithium secondary batteries. 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. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic longitudinal cross-sectional view showing an example of a lithium secondary battery according to the embodiment of this disclosure. [Figure 2] This is a schematic cross-sectional view showing the configuration of the positive electrode in Figure 1. [Figure 3] This is a schematic cross-sectional view showing the configuration of the negative electrode unit in Figure 1. [Figure 4] Figure 1 is a schematic top view showing an example of the negative electrode unit of a lithium secondary battery. [Figure 5] Figure 4 is an enlarged top view of a portion of the negative electrode unit. [Figure 6] Figure 5 further shows the position of the spacer provided on the back surface of the negative electrode. [Figure 7] It is a top view schematically showing an example of a negative electrode unit (spacer) provided in a conventional lithium secondary battery. [Figure 8] It is a top view schematically showing another example of a negative electrode unit (spacer) provided in a conventional lithium secondary battery. [Figure 9] It is a top view schematically showing still another example of a negative electrode unit (spacer) provided in a conventional lithium secondary battery.

Embodiments for Carrying out the Invention

[0010] A lithium secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte having lithium ion conductivity, and a spacer disposed between the negative electrode and the separator. The spacer has a mesh structure formed by a plurality of linear convex portions. The lithium secondary battery is a secondary battery of a type in which lithium metal is deposited on the negative electrode during charging and lithium metal is dissolved from the negative electrode during discharging. Hereinafter, the positive electrode and the negative electrode may be collectively referred to as an electrode.

[0011] The spacer secures a space in which lithium metal is deposited on the surface of the negative electrode, and reduces the volume change of the negative electrode accompanying the deposition of lithium metal. When the spacer is disposed between the negative electrode and the separator, the release of lithium ions from the positive electrode during the first charge is smoother than when the spacer is disposed between the positive electrode and the separator, which is advantageous in terms of improving the initial capacity. The spacer may be provided on the surface of the negative electrode or on the surface of the separator facing the negative electrode. Hereinafter, the negative electrode provided with the spacer on the surface is also referred to as a negative electrode unit. Also, the separator provided with the spacer on the surface is also referred to as a composite separator.

[0012] Conventional lithium secondary batteries include spacers such as the spacer 70 shown in Figure 7, the spacer 80 shown in Figure 8, and the spacer 90 shown in Figure 9. The spacer 70 shown in Figure 7 is formed by distributing multiple spot-shaped protrusions 71. The spacer 80 shown in Figure 8 is formed by arranging multiple line-shaped protrusions 81 (linear protrusions) parallel to each other. The spacer 90 shown in Figure 9 is formed by arranging multiple line-shaped protrusions 91 (linear protrusions) alternately with their orientations changed.

[0013] In the spacers shown in Figures 7-9, multiple protrusions are arranged independently and discontinuously, resulting in localized areas where the space (distance) between the negative electrode and the separator is small, leading to non-uniformity in the space between the negative electrode and the separator. This is particularly likely to occur in the case of the spot-shaped protrusions shown in Figure 7. This non-uniformity in the space between the negative electrode and the separator can locally increase the pressure applied to the electrode during lithium metal deposition.

[0014] Lithium metal tends to deposit on the periphery of the sides of the protrusions. On the other hand, in the spacers shown in Figures 7-9, multiple protrusions are arranged independently, and gaps exist between adjacent protrusions. For example, in the spacer 90 shown in Figure 9, gaps P exist between adjacent line-shaped protrusions 91. The shortest distance between the line-shaped protrusions 91 in gap P is, for example, 1 mm or more and 5 mm or less. In these gaps, lithium metal is deposited sparsely, which leads to uneven deposition of lithium metal and can cause a localized increase in pressure on the electrodes.

[0015] Furthermore, in the spacer 80 shown in Figure 8, multiple line-shaped protrusions 80 are arranged parallel to each other along the longitudinal direction of the strip-shaped negative electrode 40, and lithium metal tends to precipitate along the longitudinal direction of the negative electrode 40. Therefore, the deposition of lithium metal becomes non-uniform between the longitudinal and width directions of the negative electrode 40, which can lead to a localized increase in pressure on the electrode.

[0016] In contrast, in the lithium secondary battery according to the embodiment of this disclosure, the spacer has a mesh structure formed of multiple linear protrusions, thereby suppressing a localized increase in pressure applied to the electrode during lithium metal deposition and the resulting decrease in cycle characteristics.

[0017] In this disclosure, multiple meshes surrounded by multiple linear protrusions are formed. Therefore, a space is easily formed uniformly and stably between the negative electrode and the separator, and a localized increase in pressure on the electrode during lithium metal deposition due to non-uniformity of the space is suppressed. Note that the formation of a uniform space between the negative electrode and the separator means that a constant distance is maintained between the negative electrode and the separator.

[0018] In this disclosure, multiple linear protrusions are arranged in a continuous manner. That is, adjacent linear protrusions are substantially connected to each other. This forms a mesh structure. Therefore, the non-uniformity of lithium metal deposition that occurs when there is a gap between adjacent linear protrusions (for example, a gap P shown in Figure 9) is suppressed, and the local increase in pressure on the electrode due to the non-uniformity of lithium metal deposition is suppressed. Here, "substantially connected adjacent linear protrusions" includes not only cases where adjacent linear protrusions are actually connected, but also cases where there is a slight gap between adjacent linear protrusions (for example, a slight gap of 0.1 mm or less at the shortest distance). In the case of the above slight gap, there is almost no effect on the morphology of lithium metal deposition.

[0019] In this disclosure, since multiple linear protrusions are arranged in a mesh-like pattern on the negative electrode surface, the non-uniformity of lithium metal deposition that occurs when the spacer has the shape shown in Figure 8 is suppressed, and the localized increase in pressure on the electrode due to the non-uniformity of lithium metal deposition is suppressed.

[0020] The linear protrusions may be straight, curved, or include both straight and curved portions. The spacer may consist of one type of linear protrusion or a combination of two or more types. For example, a polygonal mesh may be formed by multiple straight protrusions. At least some of the multiple linear protrusions may have small gaps with a width of 0.1 mm or less. The presence of these small gaps has little effect on the deposition morphology of lithium metal.

[0021] It is preferable that the multiple linear protrusions are integrally formed. The mesh structure may have connecting parts where the ends of three or more adjacent linear protrusions are connected. It is preferable that the mesh structure is regularly arranged. For example, multiple polygons may be regularly arranged to form a mesh. In the above case, non-uniformity of the space between the negative electrode and the separator, non-uniformity of lithium metal deposition, and non-uniformity of the pressure applied to the electrode during lithium metal deposition are easily suppressed.

[0022] From the viewpoint of improving the liquid flow of the non-aqueous electrolyte on the negative electrode surface, the height of a portion of the linear protrusion may differ from the height of the rest of the linear protrusion, and the heights of adjacent linear protrusions may differ. Multiple linear protrusions may include linear protrusions of height h1 and linear protrusions of height h2 which is smaller than height h1. In this case, the ratio of height h2 to height h1: h2 / h1 may be, for example, 0.8 or more and less than 1.0, or 0.8 or more and 0.95 or less.

[0023] From the perspective of securing the minimum necessary space between the negative electrode and the separator, the average height h of the linear protrusion may be 0.02 mm or more and 0.09 mm or less, or 0.015 mm or more and 0.01 mm or less, depending on the battery size. The average height h of the linear protrusion can be determined by averaging the measurements of any 10 points.

[0024] The width of the multiple linear protrusions (width W of linear protrusion 51 in Figure 5) may be 1 mm or less, or 0.1 mm or more and 1 mm or less. In this case, a stable space is formed between the negative electrode and the separator, and the impact on battery performance due to the negative electrode surface being covered by the linear protrusions can be minimized.

[0025] From the viewpoint of suppressing the deposition of lithium metal on the surface of the linear protrusions, the multiple linear protrusions may be made of a material with lower conductivity than the negative electrode, or they may be made of a resin material.

[0026] If the mesh shape is polygonal, the polygon may be hexagonal. From the viewpoint of suppressing non-uniformity of the space between the negative electrode and the separator, the interior angles of the polygon may be 120° or less.

[0027] From the viewpoint of ensuring that the space between the negative electrode and the separator is stably and uniformly formed by the spacer, the opening area of ​​the mesh is 3.5 mm per mesh. 2 The following may also be true: 1.0 mm 2 Above, 3.5mm 2 The following is also acceptable: The opening area of ​​the mesh is 1.0 mm per mesh. 2 In the above cases, sufficient space is secured for the deposition of lithium metal.

[0028] The ratio of the area of ​​the negative electrode surface covered by the spacer to the surface area of ​​the negative electrode (hereinafter also referred to as the spacer coverage rate of the negative electrode surface) may be 21% or less, 0.2% or more and 21% or less, or 1% or more and 21% or less. When the spacer coverage rate of the negative electrode surface is 21% or less, the impact on battery performance due to the negative electrode surface being covered by the spacer can be minimized. When the spacer coverage rate of the negative electrode surface is 0.2% or more (or 1% or more), the space between the negative electrode and the separator is more easily formed stably and uniformly.

[0029] The negative electrode may have a first surface and a second surface opposite to the first surface. In this case, the separator may include a first separator positioned on the first surface side and a second separator positioned on the second surface side. The spacer may include a first spacer positioned between the negative electrode and the first separator and a second spacer positioned between the negative electrode and the second separator. The plurality of linear protrusions may include a plurality of first linear protrusions and a plurality of second linear protrusions, the first spacer may have a mesh structure formed by the plurality of first linear protrusions, and the second spacer may have a mesh structure formed by the plurality of second linear protrusions.

[0030] When viewed from the direction normal to the first surface, it is preferable that the ratio of the area of ​​the overlapping portion of the first spacer and the second spacer to the area of ​​the first spacer be small. When viewed from the direction normal to the first surface, it is preferable that the first spacer and the second spacer are provided such that the multiple first linear protrusions and the multiple second linear protrusions intersect each other or do not overlap. When the first linear protrusions and the second linear protrusions intersect as linear protrusions, the angle formed by the intersection of the first linear protrusions and the second linear protrusions (an angle of 90° or less) may be, for example, 20° or more, or 40° or more.

[0031] Lithium metal tends to precipitate on the periphery of the sides of the protrusions. Furthermore, lithium metal can also precipitate on the surface of the protrusions and in the region between the protrusions and the separator. Therefore, the deposition of lithium metal can locally increase the thickness of the negative electrode near the protrusions. Thus, by arranging the first and second spacers such that the area of ​​overlap between them is reduced when viewed from the normal direction of the first surface (for example, so that the first and second linear protrusions intersect or do not overlap), local expansion of the negative electrode can be suppressed.

[0032] In this disclosure, when the electrode group is of the wound type, "when viewed from the direction normal to the surface" means when the surface is stretched into a planar shape and viewed from the direction normal to the surface. Therefore, "the convex portions overlap" does not include cases where the convex portions overlap due to winding.

[0033] A lithium secondary battery may include a stacked electrode group consisting of a positive electrode and a negative electrode stacked with a separator in between, or it may include a wound electrode group consisting of a positive electrode and a negative electrode wound in a spiral shape with a separator in between.

[0034] The following provides a more detailed explanation of each component of a lithium-ion secondary battery. [Negative electrode] The negative electrode is equipped with a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode current collector during charging. More specifically, lithium ions contained in the non-aqueous electrolyte accept electrons on the negative electrode current collector during charging, becoming lithium metal, and depositing on the surface of the negative electrode current collector. The lithium metal deposited on the surface of the negative electrode current collector dissolves as lithium ions in the non-aqueous electrolyte during discharge. The lithium ions contained in the non-aqueous electrolyte may originate from lithium salts added to the non-aqueous electrolyte, or they may be supplied from the positive electrode active material during charging, or both.

[0035] The negative electrode current collector can be any conductive sheet. Examples of conductive sheets include foil and film.

[0036] The surface of the conductive sheet may be smooth. This makes it easier for lithium metal from the positive electrode to deposit evenly on the conductive sheet during charging. Smoothness means that the maximum height roughness Rz of the conductive sheet is 20 μm or less. The maximum height roughness Rz of the conductive sheet may be 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS B 0601:2013.

[0037] The material of the negative electrode current collector (conductive sheet) may be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. It is preferable that the conductive material is one that does not react with lithium. More specifically, it is preferable that the conductive material does not form any alloys or intermetallic compounds 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, copper and / or copper alloys with high conductivity are preferred.

[0038] The thickness of the negative electrode current collector is not particularly limited, but can be, for example, 5 μm or more and 300 μm or less.

[0039] A negative electrode composite layer may be formed on the surface of the negative electrode current collector. The negative electrode composite layer is formed, for example, by applying a paste containing a negative electrode active material such as graphite to at least a portion of the surface of the negative electrode current collector. However, from the viewpoint of achieving a lithium secondary battery with a higher capacity than lithium-ion batteries, the thickness of the negative electrode composite layer is set to be sufficiently thin so that lithium metal can be deposited at the negative electrode.

[0040] [Spacer] The materials constituting the spacer are not particularly limited. The spacer may be made of a conductive material and / or an insulating material. The spacer may be provided on the surface of the negative electrode or on the surface of the separator (the surface facing the negative electrode).

[0041] The conductive material can be appropriately selected from the materials exemplified for the negative electrode current collector. Such a spacer may be provided by forming a protrusion on the negative electrode current collector by press working or the like. Alternatively, conductive paint may be applied to the surface of the negative electrode, or conductive tape may be attached to the surface of the negative electrode.

[0042] Examples of insulating materials include resin materials. Examples of resin materials include polyolefin resins, acrylic resins, polyamide resins, polyimide resins, silicone resins, and fluororesins. Cured products of curable resins such as epoxy resins may also be used. In addition, inorganic fillers may be mixed with these resin materials. A spacer can be formed, for example, by attaching a resin adhesive tape to the surface of the negative electrode. Alternatively, a spacer may be formed by applying a solution or dispersion containing a resin material to the surface of the negative electrode or the surface of the separator facing the negative electrode and drying it. A spacer may also be formed by applying a curable resin in a desired shape to the surface of the negative electrode or the surface of the separator facing the negative electrode and curing it.

[0043] [Positive electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode composite layer supported by the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector or on both sides. The positive electrode can be obtained, for example, by applying a positive electrode composite slurry containing the positive electrode active material, a conductive material, and a binder to both sides of the positive electrode current collector, drying the coating, and then rolling it.

[0044] The positive electrode active material is a material that intercepts and releases lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because they have low manufacturing costs and a high average discharge voltage.

[0045] 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 elements may be Co, Ni, and / or 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. The main group elements may also be Al, etc.

[0046] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni, and / or Mn as transition metal elements, and possibly containing Al as an optional component, and having a layered rock salt-type crystalline structure, are preferred for obtaining high capacity. In this case, in a lithium secondary battery, the molar ratio of the total amount of lithium (mLi) in the positive and negative electrodes to the amount of metal M other than lithium in the positive electrode (mM), : mLi / mM, is set to, for example, 1.1 or less.

[0047] Conductive materials include, for example, carbon materials. Examples of carbon materials include carbon black, acetylene black, Ketjenblack, carbon nanotubes, and graphite.

[0048] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0049] The positive electrode current collector can be any conductive sheet. Examples of conductive sheets include foil and film. The surface of the positive electrode current collector may also be coated with a carbon material.

[0050] Examples of materials for the positive electrode current collector (conductive sheet) include metallic materials containing Al, Ti, Fe, etc. The metallic material may be Al, Al alloy, Ti, Ti alloy, Fe alloy, etc. The Fe alloy may be stainless steel (SUS).

[0051] The thickness of the positive electrode current collector is not particularly limited, but can be, for example, 5 μm or more and 300 μm or less.

[0052] [Separator] A porous sheet having ion permeability and insulating properties is used as the separator. Examples of porous sheets include thin films, woven fabrics, and nonwoven fabrics with microporous properties. The material of the separator is not particularly limited, but it may 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.

[0053] [Non-aqueous electrolytes] A non-aqueous electrolyte having lithium ion conductivity 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.

[0054] Liquid non-aqueous electrolytes are prepared by dissolving lithium salts in a non-aqueous solvent. The dissolution of lithium salts in the non-aqueous solvent generates lithium ions and anions.

[0055] The gel-like non-aqueous electrolyte comprises a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins.

[0056] As the lithium salt or anion, known ones used in non-aqueous electrolytes for lithium secondary batteries can be used. Specifically, BF4 - ClO4 - PF6 - CF3SO3 - CF3CO2- Examples include anions of imides, anions of oxalate complexes, etc. Examples of anions of imides include N(SO2CF3)2 - , N(C m F 2m+1 SO2) x (C n F 2n+1 SO2) y - (where 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 x + y = 2 is satisfied).) etc. Anions of oxalate complexes may contain boron and / or phosphorus. Examples of anions of oxalate complexes include bisoxalatoborate anion, BF2(C2O4) - , PF4(C2O4) - , PF2(C2O4)2 - etc. The non-aqueous electrolyte may contain these anions alone or in combination of two or more kinds.

[0057] [[ID=

[26] ]From the viewpoint of suppressing the dendritic precipitation of lithium metal, it is preferable that the non-aqueous electrolyte contains at least an anion of an oxalate complex. Due to the interaction between the anion of the oxalate complex and lithium, lithium metal is likely to precipitate uniformly in fine particle form. Therefore, it becomes easy to suppress the local precipitation of lithium metal. The anion of the oxalate complex may be combined with other anions. Other anions may be PF6 - and / or an anion of imides.

[0058] Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, or halogenated derivatives thereof. The non-aqueous electrolyte may contain these non-aqueous solvents alone or in combination of two or more kinds. Examples of the halogenated derivative include fluorides etc.

[0059] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of linear carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 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.

[0060] 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 ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0061] The concentration of lithium salt in the non-aqueous electrolyte is, for example, between 0.5 mol / L and 3.5 mol / L. The concentration of anion in the non-aqueous electrolyte may also be between 0.5 mol / L and 3.5 mol / L. Furthermore, the concentration of anion of the oxalate complex in the non-aqueous electrolyte may be between 0.05 mol / L and 1 mol / L.

[0062] The non-aqueous electrolyte may contain additives. The additives may form a film on the negative electrode. The formation of a film derived from the additive on the negative electrode makes it easier to suppress dendrite formation. Examples of such additives include vinylene carbonate, FEC, vinyl ethyl carbonate (VEC), and the like.

[0063] The configuration of the lithium secondary battery relating to this disclosure will be described below with reference to the drawings, using a cylindrical battery equipped with a wound electrode group as an example. However, this disclosure is not limited to the following configuration.

[0064] Figure 1 is a schematic longitudinal cross-sectional view showing an example of a lithium secondary battery according to the embodiment of this disclosure. Figure 2 is a schematic cross-sectional view showing the configuration of the positive electrode in Figure 1, and is an enlarged view of the portion enclosed by region II in Figure 1. Figure 3 is a schematic cross-sectional view showing the configuration of the negative electrode unit in Figure 1, and is an enlarged view of the portion enclosed by region III in Figure 1. The cross-section of the negative electrode unit in Figure 3 is the cross-section in the III-III direction in Figure 6. Figure 4 is a schematic top view showing an example of a negative electrode unit included in the lithium secondary battery of Figure 1. Figure 5 is an enlarged view of a part of the negative electrode unit in Figure 4. Figure 6 further shows the position of the spacer provided on the back surface of the negative electrode in Figure 5.

[0065] The lithium 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.

[0066] 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.

[0067] 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 components 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 components constituting the sealing body 16 is, for example, disc-shaped or ring-shaped. 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 between 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. In other words, each component except the insulating member 24 is electrically connected to each other.

[0068] 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 disconnects 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.

[0069] The electrode group 14 comprises a positive electrode 11, a negative electrode unit 12, and a separator 13. The positive electrode 11, the negative electrode unit 12, and the separator 13 are all strip-shaped. The positive electrode 11 and the negative electrode unit 12 are wound in a spiral shape with the separator 13 interposed between them, such that the width directions of the strip-shaped positive electrode 11 and the negative electrode unit 12 are 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 unit 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 13 interposed between them. In other words, the longitudinal direction of each electrode is the winding direction, and the width direction of each electrode is the axial direction.

[0070] 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.

[0071] The positive electrode 11 comprises a positive electrode current collector 30 and a positive electrode composite layer 31 (see Figure 2), and is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the longitudinal center of the positive electrode 11. 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 electrode group 14 side of the filter 22.

[0072] As shown in Figures 3 and 4, the negative electrode unit 12 comprises a strip-shaped negative electrode 40, which has a first surface S1 and a second surface S2 opposite to the first surface S1. The negative electrode 40 comprises at least a strip-shaped negative electrode current collector, which may comprise a strip-shaped negative electrode current collector and a negative electrode composite layer formed on both sides of the negative electrode current collector. The negative electrode current collector of the negative electrode 40 is electrically connected to a case body 15, which functions as a 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 current collector of the negative electrode 40, and the other end is welded to the bottom inner surface of the case body 15.

[0073] As shown in Figures 3 and 4, the negative electrode unit 12 includes spacers 50 (linear protrusions 51) provided on the first surface S1 and the second surface S2 of the negative electrode 40, respectively. As shown in Figures 4 to 6, the spacer 50 has a mesh structure (honeycomb structure) formed by multiple linear protrusions 51. A hexagonal mesh is formed by the connection of six linear protrusions 51 (straight protrusions). As shown in Figure 3, the presence of multiple linear protrusions 51 creates spaces 35 between the first surface S1 and the separator 13 and between the second surface S2 and the separator 13.

[0074] In the lithium secondary battery 10, during charging, lithium metal is deposited in the space 35 on the negative electrode 40, and during discharge, the deposited lithium metal dissolves in the non-aqueous electrolyte. As the lithium metal deposited on the surface of the negative electrode is contained within the space 35, the pressure on the electrode is reduced due to the deposition of lithium metal, and electrode damage due to increased pressure is suppressed.

[0075] As shown in Figure 6, when viewed from the direction normal to the first surface S1, the spacer 50 provided on the second surface S2 of the negative electrode 40 (shown by the dashed line in Figure 6) is offset from the spacer 50 provided on the first surface S1 of the negative electrode 40 (shown by the solid line in Figure 6). When viewed from the direction normal to the first surface S1, some of the multiple line-shaped protrusions 51 provided on the first surface S1 (two of the six line-shaped protrusions 51 that form the hexagonal mesh) do not overlap with the line-shaped protrusions 51 provided on the second surface S2. The remaining parts of the multiple line-shaped protrusions 51 provided on the first surface S1 (four of the six line-shaped protrusions 51 that form the hexagonal mesh) intersect with the line-shaped protrusions 51 provided on the second surface S2. The overlap between the area covered by the spacer on the first surface S1 and the area covered by the spacer on the second surface S2 may be 30% or less of the area covered by the spacer on the first surface S1 (or the area covered by the spacer on the second surface S2), or it may be 15% or less.

[0076] The illustrated example describes a cylindrical lithium secondary battery equipped with a wound electrode group, but this embodiment is applicable to other types of batteries as well. Depending on the application, the shape of the lithium secondary battery can be appropriately selected from various shapes other than cylindrical, such as coin-shaped, prismatic, sheet-shaped, and flat-shaped batteries. The shape of the electrode group is also not particularly limited and may be stacked. Furthermore, known components other than the electrode group and non-aqueous electrolyte of the lithium secondary battery can be used without particular restriction.

[0077] [Examples] The lithium secondary batteries relating to this disclosure will be described in detail below based on examples and comparative examples. This disclosure is not limited to the following examples.

[0078] Example 1 (1) Preparation of the positive electrode A rock salt-type lithium-containing transition metal oxide (NCA; positive electrode active material) having a layered structure and containing Li, Ni, Co, and Al (with a molar ratio of Li to the total of Ni, Co, and Al being 1.0), acetylene black (AB; conductive material), and polyvinylidene fluoride (PVdF; binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was then added and the mixture was stirred to prepare a positive electrode slurry. Next, the obtained positive electrode slurry was applied to both sides of an Al foil (positive electrode current collector), dried, and the coating of the positive electrode slurry was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode slurry was cut to a predetermined electrode size to produce a positive electrode with positive electrode slurry layers on both sides of the positive electrode current collector.

[0079] (2) Fabrication of the negative electrode unit A rectangular electrolytic copper foil (12 μm thick) was prepared as the negative electrode (negative electrode current collector). Polyimide ink was dispensed onto one surface of the electrolytic copper foil using a dispenser, and then vacuum-dried to obtain a polyimide resin spacer with the shape (honeycomb structure) shown in Figure 4.

[0080] Subsequently, the same spacers as described above were obtained on the other surface of the electrolytic copper foil in the same manner as described above. At that time, the positions of the 0.8 mm diameter holes formed at the four corners of the electrolytic copper foil were confirmed with a CCD camera, and the positions of the spacers were adjusted so that the spacers formed on both sides of the negative electrode were in the positional relationship shown in Figure 6. Next, the electrolytic copper foil was cut to a predetermined electrode size. In this way, a negative electrode unit was obtained comprising a strip-shaped negative electrode and spacers having a mesh structure arranged on both sides of the negative electrode. The width of the electrolytic copper foil cut to the electrode size was 65 mm, and its length in the longitudinal direction was 1000 mm.

[0081] The mesh shape of the spacer was a regular hexagon. The height of the linear protrusions was 0.03 mm. The length of the linear protrusions forming one side of the regular hexagonal mesh was 1.156 mm. The width of the linear protrusions was 0.25 mm. The ratio of the area of ​​the negative electrode surface (one side) covered by the spacer to the area of ​​the negative electrode surface (one side) (coverage rate of the negative electrode surface by the spacer) was 21%. The opening area of ​​each mesh was 3.46 mm². 2 That was the case.

[0082] (3) Preparation of non-aqueous electrolytes EC and DMC were mixed in a volume ratio of EC:DMC = 30:70. LiPF6 was dissolved in the resulting mixed solvent at a concentration of 1 mol / L and LiBF2(C2O4) at a concentration of 0.1 mol / L to prepare liquid non-aqueous electrolytes.

[0083] (4) Making a battery An aluminum tab was attached to the positive electrode obtained above. A nickel tab was attached to the negative electrode current collector of the negative electrode unit obtained above. In an inert gas atmosphere, the positive and negative electrodes were wound in a spiral shape with a separator in between to create a wound electrode group. Since all the lithium contained in the electrode group originates from the positive electrode, the molar ratio of the total amount of lithium (mLi) in the positive and negative electrodes to the amount of metal M (here Ni, Co, and Al) (mM) in the positive electrode (mLi / mM) is 1.0. A microporous polyethylene membrane was used as the separator. The obtained electrode group was housed in a bag-shaped outer casing made of a laminate sheet with an Al layer, the non-aqueous electrolyte was injected into the outer casing containing the electrode group, and then the outer casing was sealed to create a lithium secondary battery A1.

[0084] Example 2 In the fabrication of the negative electrode unit described in (2) above, the length of the line-shaped protrusions forming one side of the hexagonal mesh was set to 2.887 mm. The width of the line-shaped protrusions was set to 1 mm. Lithium secondary battery A2 was fabricated in the same manner as in Example 1, except as described above.

[0085] The ratio of the area of ​​the negative electrode surface (one side) covered by the spacer to the total surface area of ​​the negative electrode surface (spacer coverage rate) was 30.6%. The opening area of ​​each mesh was 21.65 mm². 2 That was the case.

[0086] Comparative Example 1 In the fabrication of the negative electrode unit described in (2) above, spacers of the shape shown in Figure 8 were provided on both sides of the negative electrode. The spacers were formed by applying polyimide ink along the longitudinal direction of the strip-shaped negative electrode to both sides of the negative electrode, and then drying it with hot air to form six parallel linear protrusions. The height of the linear protrusions was 0.03 mm, the width of the linear protrusions was 1 mm, and the spacing between adjacent linear protrusions was 5 mm. When viewed from the direction normal to one surface of the negative electrode, the spacers provided on both sides of the negative electrode were arranged so as not to overlap. Except as described above, a lithium secondary battery was fabricated in the same manner as in Example 1.

[0087] Comparative Example 2 In the fabrication of the negative electrode unit described in (2) above, spacers with the shape shown in Figure 7 were provided on both sides of the negative electrode. The spacers were formed by applying polyimide ink in a spot pattern to both sides of the negative electrode, and then drying it with hot air to form spot-shaped protrusions. The height of the protrusions was 0.03 mm, the diameter of the protrusions was 1 mm, and the spacing between adjacent protrusions was 2.5 mm. When viewed from the direction normal to one surface of the negative electrode, the spacers provided on both sides of the negative electrode were arranged so as not to overlap with each other. Except as described above, a lithium secondary battery was fabricated in the same manner as in Example 1.

[0088] [Rating 1] Charge and discharge tests were performed on each of the obtained batteries. In the charge and discharge tests, the batteries were charged in a constant temperature chamber at 25°C under the following conditions, then left to rest for 20 minutes, and then discharged under the following conditions.

[0089] (charging) Constant current charging was performed at a current of 2.15 mA per unit area (square centimeter) of the electrodes until the battery voltage reached 4.1 V. Then, constant voltage charging was performed at a voltage of 4.1 V until the current value per unit area of ​​the electrodes reached 0.54 mA.

[0090] (discharge) Constant current discharge was performed at a current of 2.15 mA per unit area of ​​the electrodes until the battery voltage reached 3.75 V.

[0091] The above charging and discharging process was considered one cycle, and charging and discharging was performed for 50 cycles. The ratio (%) of the discharge capacity at cycle 50 to the discharge capacity at cycle 1 was calculated as the capacity retention rate. The discharge capacity at cycle 1 was also calculated as the initial capacity.

[0092] The evaluation results are shown in Table 1. In Table 1, the initial capacity is expressed as a relative value with the initial capacity of battery B1 set to 100.

[0093] [Table 1]

[0094] Batteries A1 and A2 achieved a higher capacity retention rate than batteries B1 and B2.

[0095] Comparative Example 3 In the fabrication of the secondary battery (electrode group) described in (3) above, instead of using a negative electrode unit and separator, a negative electrode current collector and a composite separator were used. When constructing the electrode group, the surface of the composite separator with the spacer was placed facing the positive electrode, and a spacer was placed between the positive electrode and the separator.

[0096] The composite separator was fabricated by placing the same spacers (spacers with the shape shown in Figure 8) as in Comparative Example 1 on the surface of the separator facing the positive electrode. Specifically, polyimide ink was applied to the surface of the separator facing the positive electrode along the longitudinal direction of the strip-shaped separator, and then hot-air dried to form six parallel linear protrusions. The height, width, and spacing between adjacent protrusions were the same as in Comparative Example 1. When viewed from the direction normal to one surface of the positive electrode, the spacers provided on both sides of the positive electrode were arranged so as not to overlap. Secondary battery B3 was manufactured using the same method as in the examples, except as described above.

[0097] [Rating 2] Batteries B1 and B3 underwent the following charging and discharging procedures in a constant temperature bath at 25°C.

[0098] (charging) Constant current charging was performed at a current of 2.15 mA per unit area (square centimeter) of the electrodes until the battery voltage reached 4.1 V. Then, constant voltage charging was performed at a voltage of 4.1 V until the current value per unit area of ​​the electrodes reached 0.54 mA.

[0099] (discharge) After a 20-minute rest period, constant current discharge was performed at a current of 2.15 mA per unit area of ​​the electrodes until the battery voltage reached 3.0 V. The discharge capacity at this time was determined as the initial capacity.

[0100] The evaluation results are shown in Table 2. In Table 2, the initial capacity is expressed as a relative value with the initial capacity of battery B1 set to 100.

[0101] [Table 2]

[0102] In battery B3, which has a spacer between the positive electrode and the separator, the initial capacity was lower than in battery B1, which has a spacer between the negative electrode and the separator, because lithium ions were not released as smoothly from the positive electrode during the initial charge. [Industrial applicability]

[0103] The lithium secondary battery of this disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet devices, electric vehicles including hybrid and plug-in hybrid vehicles, and home battery storage systems combined with solar cells. 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. [Explanation of Symbols]

[0104] 10 Lithium-ion rechargeable batteries 11 Positive electrode 12 Negative electrode units 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 40 negative electrode 50 Spacers 51 Linear protrusions

Claims

1. Positive electrode and, A negative electrode opposite the positive electrode, A separator is placed between the positive electrode and the negative electrode, A non-aqueous electrolyte having lithium ion conductivity, A spacer is placed between the negative electrode and the separator, Equipped with, During charging, lithium metal is deposited on the negative electrode, and during discharge, the lithium metal dissolves from the negative electrode. The spacer has a mesh structure formed by a plurality of linear protrusions, A lithium secondary battery in which the height of a portion of the linear protrusion differs from the height of the rest of the linear protrusion.

2. The lithium secondary battery according to claim 1, having a connecting portion where the ends of three or more adjacent line-shaped protrusions are connected to each other.

3. The lithium secondary battery according to claim 1 or 2, wherein the plurality of linear protrusions are integrally formed.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein the mesh structure is arranged in a regular manner.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein the width of the plurality of line-shaped protrusions is 1 mm or less.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein the plurality of linear protrusions are made of a resin material.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein the shape of the mesh is polygonal.

8. The lithium secondary battery according to claim 7, wherein the polygon is a hexagon.

9. The lithium secondary battery according to claim 7 or 8, wherein the interior angle of the polygon is 120° or less.

10. The opening area of ​​each mesh is 3.5 mm². ２ The lithium secondary battery according to any one of claims 1 to 9, which is as follows:

11. A lithium secondary battery according to any one of claims 1 to 10, wherein the ratio of the area of ​​the surface of the negative electrode covered by the spacer to the surface area of ​​the negative electrode is 21% or less.

12. The negative electrode has a first surface and a second surface opposite to the first surface. The separator includes a first separator disposed on the first surface side and a second separator disposed on the second surface side. The spacer includes a first spacer disposed between the negative electrode and the first separator, and a second spacer disposed between the negative electrode and the second separator. The plurality of linear protrusions include a plurality of first linear protrusions and a plurality of second linear protrusions. The first spacer has a mesh structure formed by the plurality of first line-shaped protrusions, The lithium secondary battery according to any one of claims 1 to 11, wherein the second spacer has a mesh structure formed by the plurality of second line-shaped protrusions.

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