Lithium secondary battery

Abstract

Disclosed is a lithium secondary battery including: a positive electrode (11); a negative electrode (12); a separator (50) disposed between the positive electrode (11) and the negative electrode (12); and a nonaqueous electrolyte having lithium ion conductivity. At the negative electrode (12), lithium metal is deposited during charging, and lithium metal is dissolved during discharging. The separator (50) includes a base material (51) and a spacer (53). The spacer (53) is disposed on a side closer to the positive electrode than the base material (51). The spacer (53) includes linear protruding portions disposed in a mesh-like pattern. The mesh-like pattern includes deficient portions connecting regions within adjacent meshes.

Description

Lithium secondary battery

[0001] The present disclosure relates to lithium secondary batteries.

[0002] Non-aqueous electrolyte secondary batteries are used in applications such as ICT (Information Communication Technology) applications such as personal computers and smartphones, in-vehicle applications, and power storage. In these applications, non-aqueous electrolyte secondary batteries are required to have even higher capacities. Lithium-ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Higher capacities in lithium-ion batteries can be achieved by using, for example, graphite and an alloy active material such as a silicon compound as the negative electrode active material. However, the capacity of lithium-ion batteries is reaching its limit.

[0003] Lithium secondary batteries (lithium metal secondary batteries) are promising non-aqueous electrolyte secondary batteries with higher capacities than lithium ion batteries. In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging, and this lithium metal dissolves in the non-aqueous electrolyte during discharging. Various proposals have been made for lithium secondary batteries.

[0004] Patent Document 1 (WO 2020 / 066254) discloses a lithium secondary battery including: a positive electrode including a positive electrode current collector and a positive electrode composite layer including a positive electrode active material; a negative electrode including a negative electrode current collector facing the positive electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity, wherein the positive electrode active material includes a composite oxide including lithium and a metal M other than lithium, and the metal M includes at least a transition metal, wherein lithium metal precipitates on the negative electrode during charging and dissolves from the negative electrode during discharge, wherein a first length of the positive electrode current collector in a first direction D1 is smaller than a second length in a second direction D2 intersecting with the first direction, wherein a spacer is provided between the positive electrode and the negative electrode to accommodate the lithium metal, and wherein a straight line SL can be drawn along the first direction D1 so as to pass through the spacer at three or more points.

[0005] International Publication No. 2020 / 066254

[0006] Currently, there is a demand for further improvements in the characteristics of lithium secondary batteries. One of the objects of the present disclosure is to provide a lithium secondary battery with improved characteristics (for example, discharge capacity retention rate).

[0007] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharging, the separator including a substrate and a spacer, the spacer being disposed closer to the positive electrode than the substrate, the spacer including linear protrusions arranged in a mesh pattern, and the mesh pattern including defects connecting adjacent mesh regions.

[0008] Another aspect of the present disclosure relates to a lithium secondary battery, the lithium secondary battery including: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity, the negative electrode being an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharge, the separator including a substrate and a spacer, the spacer being disposed closer to the positive electrode than the substrate and including linear protrusions arranged in a mesh pattern, and the separator including a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.

[0009] According to the present disclosure, a lithium secondary battery with higher characteristics (e.g., discharge capacity retention rate) can be obtained. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] 5 is a longitudinal sectional view schematically showing an example of a lithium secondary battery according to an embodiment of the present disclosure; FIG. 6 is a sectional view schematically showing a part of the lithium secondary battery shown in FIG. 1; FIG. 7 is a top view showing an example of a spacer pattern; FIG. 8 is a top view showing another example of a spacer pattern; FIG. 9 is a partial enlarged view of FIG.

[0011] Below, embodiments according to the present disclosure will be described using examples, but the embodiments according to the present disclosure are not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the invention according to the present disclosure can be implemented. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0012] (Lithium secondary battery) A lithium secondary battery according to an embodiment of the present disclosure will be described below. Hereinafter, the lithium secondary battery may be referred to as a "lithium secondary battery (L)." Hereinafter, two lithium secondary batteries will be described as "lithium secondary battery (L)." Hereinafter, these may be referred to as a "lithium secondary battery (L1)" and a "lithium secondary battery (L2)."

[0013] (Lithium secondary battery (L1)) The lithium secondary battery (L1) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. The negative electrode is an electrode from which lithium metal precipitates during charging and from which lithium metal dissolves during discharging. The separator includes a substrate and a spacer. The spacer is disposed closer to the positive electrode than the substrate. The spacer includes linear protrusions arranged in a mesh pattern. The mesh pattern includes defects connecting adjacent mesh regions.

[0014] (Lithium Secondary Battery (L2)) The lithium secondary battery (L2) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. The negative electrode is an electrode from which lithium metal precipitates during charging and from which lithium metal dissolves during discharging. The separator includes a substrate and a spacer. The spacer is disposed closer to the positive electrode than the substrate. The spacer includes linear protrusions arranged in a mesh pattern. The separator includes a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.

[0015] The only difference between the lithium secondary battery (L1) and the lithium secondary battery (L2) is the spacer arrangement pattern (planar shape of the spacer). Therefore, unless otherwise noted or contradictory, the following description can be applied to both the lithium secondary battery (L1) and the lithium secondary battery (L2). Furthermore, the matters described for the lithium secondary battery (L) can be applied to both the lithium secondary battery (L1) and the lithium secondary battery (L2).

[0016] In this specification, the form in which an element B is formed on an element A includes a form in which the element B is formed directly on the element A, and a form in which the element B is formed on the element A via another element.

[0017] Lithium secondary batteries (L) are also called lithium metal secondary batteries. In the negative electrode of this type of battery, lithium metal is deposited during charging and dissolved during discharging. Specifically, the negative electrode has at least a negative electrode current collector, and lithium metal is deposited on the negative electrode current collector.

[0018] In a lithium secondary battery (L), for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charge and discharge is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (or current from another perspective) at the negative electrode during charge and discharge is due to the deposition and dissolution of lithium metal. In other words, the negative electrode according to the present disclosure differs from a negative electrode in which the movement of electrons at the negative electrode during charge and discharge is mainly due to the absorption and release of lithium ions by the negative electrode active material (such as graphite).

[0019] Hereinafter, the positive electrode, negative electrode, and separator may be collectively referred to as an "electrode group." The positive electrode, negative electrode, and separator may be wound so that the separator is disposed between the positive electrode and the negative electrode. When forming a wound electrode group, a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator are used. Alternatively, the positive electrode, negative electrode, and separator may be stacked. For example, a flat positive electrode, a flat negative electrode, and a flat separator may be stacked. That is, the electrode group may be a wound electrode group or a stacked electrode group.

[0020] In the lithium secondary battery (L), a space between the positive electrode and the negative electrode is secured by a spacer. The spacer is disposed on the positive electrode side of the separator, forming a space on the surface of the positive electrode. During charging, lithium metal is deposited on the surface of the negative electrode. The deposited lithium metal causes the spacer-free portion of the separator to move toward the positive electrode. Because the spacer-free portion is empty, the separator (substrate and heat-resistant layer) can move toward the positive electrode. This prevents the electrode group from expanding during repeated charge and discharge. Furthermore, in the lithium secondary battery (L), a substrate or heat-resistant layer is present on the surface of the negative electrode. This prevents the lithium metal from depositing in a coarse state. This prevents the negative electrode from expanding.

[0021] Furthermore, in the lithium secondary battery (L), the separator spacers include a mesh pattern. This prevents the spacers from shifting position and causing undesirable bending of the electrode plates when the electrode group is stacked or wound. Therefore, in the lithium secondary battery (L), the spacers can appropriately form spaces in which lithium metal can be deposited.

[0022] In the separator of the lithium secondary battery (L1), the mesh pattern includes defects. The non-aqueous electrolyte can migrate through the defects. If there were no defects, the fluidity of the non-aqueous electrolyte would decrease, and the battery characteristics would deteriorate. In the lithium secondary battery (L1), the non-aqueous electrolyte can migrate through the defects, so such deterioration in characteristics can be suppressed. As a result, a high discharge capacity retention rate can be achieved.

[0023] The separator of the lithium secondary battery (L2) includes a second region where no mesh pattern is formed. The non-aqueous electrolyte can move freely in the second region. Absence of the second region may result in a decrease in the fluidity of the non-aqueous electrolyte, leading to a deterioration in battery performance. In the lithium secondary battery (L2), the non-aqueous electrolyte can move via the second region, thereby suppressing such deterioration in performance. As a result, a high discharge capacity retention rate can be achieved.

[0024] Furthermore, when a spacer is placed on the positive electrode, the spacer covers the positive electrode surface, partially inhibiting the exchange of lithium ions with the positive electrode active material. This can result in a decrease in cell capacity. In contrast, the lithium secondary battery (L) has a spacer formed on the separator, which avoids the above-mentioned problems. That is, the lithium secondary battery (L) can simultaneously suppress the expansion of the negative electrode and maintain a high cell capacity, and furthermore, can suppress the temperature rise of the battery in abnormal conditions.

[0025] When a spacer is placed on the surface of the negative electrode, lithium metal is freely deposited and dissolved on the surface of the negative electrode during charge and discharge. As a result, short circuits due to dendrites may be more likely to occur, and side reactions may be more likely to occur due to the increased surface area of ​​the negative electrode. The increased side reactions lead to a decrease in capacity retention. On the other hand, in the lithium secondary battery (L), the spacer is placed on the positive electrode side, so no spacer is placed on the surface of the negative electrode. Therefore, the lithium secondary battery (L) can achieve high reliability and a high capacity retention.

[0026] The lithium secondary battery (L2) preferably satisfies at least one of the following conditions (J1) to (J3), and may satisfy two or all of them. For example, the lithium secondary battery (L2) may satisfy the following conditions (J2) and (J3): (J1) First regions having a mesh pattern and second regions not having a mesh pattern are alternately arranged along the longitudinal direction LD of the separator. (J2) The length L1 of the first regions in the longitudinal direction LD of the separator is longer than the length L2 of the second regions in the longitudinal direction LD. The ratio L1 / L2 of the length L1 to the length L2 is greater than 1, and may be 2 or more, or 3 or more, and may be 10 or less, or 5 or less. (J3) The length L2 of the second regions in the longitudinal direction LD of the separator is 1 mm or more and 15 mm or less. The length L2 may be 1 mm or more, 3 mm or more, or 5 mm or more, and may be 15 mm or less, or 10 mm or less.

[0027] The average height Hs of the spacers is greater than the average thickness Tb of the substrate. The ratio Hs / Tb of the average height Hs to the average thickness Tb may be greater than 1, 1.5 or greater, 2 or greater, or 3 or greater. The ratio Hs / Tb may be 10 or less, 8 or less, 5 or less, or 4 or less. By making this ratio 1.5 or greater, the expansion coefficient of the electrode group can be particularly reduced. Furthermore, by making the spacers taller, the strength of the spacers is increased, thereby improving the effect of suppressing contraction of the substrate during overheating. As a result, excessive temperature rise of the electrode group can be particularly suppressed.

[0028] The average height Hs can be measured by the following method. First, a cross section of the separator in the thickness direction of the separator is photographed using an electron microscope to obtain an image of the cross section. Next, 20 arbitrary locations among the spacers are selected in the image, and the heights of the spacers at those locations are measured. Next, the heights measured at the 20 locations are arithmetically averaged, and the obtained average value is defined as the average height Hs. The average thickness Tb and the average thickness Tt of the heat-resistant layer, which will be described later, can also be measured using the same procedure.

[0029] The average thickness Tb of the substrate may be 5 μm or more or 10 μm or more, and may be 30 μm or less, or 20 μm or less. The average thickness Tt of the heat-resistant layer described below may be 1 μm or more or 2 μm or more, and may be 5 μm or less or 3 μm or less. The average height Hs of the spacers may be 10 μm or more or 20 μm or more, and may be 100 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less. These heights and thicknesses vary depending on the configuration of the positive electrode and negative electrode, and may take values ​​outside the ranges exemplified here. Note that the spacers are usually formed to have as constant a height as possible so that the spacing between the electrode plates formed by the spacers is as constant as possible.

[0030] In one example of a defect, no spacers are formed in the defect. However, the defect may be formed so that, when an electrode assembly is constructed, nonaqueous electrolyte present in a space (region) within the mesh formed by the spacers can migrate to adjacent spaces. Therefore, the defect may be formed by reducing the height of a portion of the spacer. For example, the height of a portion of the spacer may be 50% or less (e.g., 30% or less, 20% or less, or 10% or less) of the average height Hs of the spacers. The average height Hd of the defect may be 0% or more, 3% or more, 5% or more, 10% or more, or 20% or more of the average height Hs. Leaving a portion of the spacer as a defect allows nonaqueous electrolyte to migrate between adjacent meshes while maintaining the network-like interconnection pattern. Maintaining the network-like interconnection pattern can particularly enhance the effect of suppressing shrinkage of the substrate when the temperature of the electrode assembly increases. To reduce the height of a portion of the spacer, for example, the amount of slurry (or coating liquid) applied to form the spacer may be varied depending on the location.

[0031] The spacer preferably has a non-porous structure that does not allow lithium ions to pass through. Such a spacer can be realized by forming the spacer under conditions that prevent the spacer from becoming porous. In this specification, "lithium ions do not pass through" means that lithium ions do not pass through in an amount that would affect the characteristics or shape of the battery, and includes cases where lithium ions move within the spacer in an amount that can be considered to be substantially non-permeable.

[0032] The area S1 of the spacer may be 30% or less of the area S0 of the separator. This range ensures sufficient space for lithium metal deposition. The areas S1 and S0 are the areas of the separator when viewed from the spacer side. The ratio S1 / S0 of the area S1 to the area S0 may be 0.20 or less (20% or less) or 0.10 or less, or 0.03 or more (3% or more) or 0.05 or more. By setting this ratio to 0.05 or more (5% or more), the effect of suppressing excessive temperature rise in the electrode group can be enhanced.

[0033] The separator may include at least one heat-resistant layer. The heat-resistant layer may be formed on the main surface of the substrate facing the positive electrode, on the main surface facing the negative electrode, or on each of the two main surfaces. The spacer may be formed on the heat-resistant layer, or may be formed on the substrate without a heat-resistant layer. For example, the separator may have a structure of substrate / heat-resistant layer / spacer, heat-resistant layer / substrate / spacer, or heat-resistant layer / substrate / heat-resistant layer / spacer. In these structures, the spacer is disposed on the positive electrode side. That is, the separator is disposed so that the spacer faces the positive electrode. In one example, the heat-resistant layer is formed on the main surface of the substrate facing the positive electrode, and the spacer is formed on the heat-resistant layer. Forming the spacer on the heat-resistant layer particularly enhances the effect of suppressing thermal shrinkage of the substrate.

[0034] The average height Hs of the spacers is preferably greater than the sum Tw of the average thickness Tb of the substrate and the average thickness Tt of the heat-resistant layer. The ratio Hs / Tw of the average height Hs to the sum Tw may be greater than 1, 1.5 or greater, 2 or greater, or 3 or greater. The ratio Hs / Tw may be 10 or less, 8 or less, 5 or less, or 4 or less.

[0035] When the separator includes a heat-resistant layer formed on the surface of the substrate, the heat-resistant layer can suppress shrinkage of the substrate when the temperature of the electrode assembly rises excessively. If the substrate shrinks, the positive electrode and the negative electrode are likely to short-circuit, which further increases the temperature of the electrode assembly. By including the heat-resistant layer in the separator, shrinkage of the substrate can be suppressed, thereby suppressing further temperature increases in the electrode assembly.

[0036] Furthermore, the spacer not only ensures a space between the electrode plates, but also, when combined with the heat-resistant layer, can dramatically improve the effect of suppressing excessive temperature rise in the electrode group.

[0037] The reason why the above effect is obtained by combining the heat-resistant layer and the spacer is not clear at present, but it is possible that the effect of the heat-resistant layer in suppressing the thermal shrinkage of the substrate is significantly enhanced by the spacer.

[0038] By disposing the heat-resistant layer on the positive electrode side, it is possible to prevent the substrate from being deteriorated by an oxidation reaction, whereas by disposing the heat-resistant layer on the negative electrode side, it is possible to prevent the substrate from being deteriorated by a reduction reaction.

[0039] (Substrate) A porous sheet having ion permeability and insulating properties is used as the substrate. Examples of porous sheets include porous membranes, woven fabrics, and nonwoven fabrics. The material of the separator is not particularly limited, but may be a polymer material. Examples of polymer materials include polyolefin resins, polyamide resins, and cellulose. Examples of polyolefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The substrate may contain additives as needed. Examples of additives include inorganic fillers. The substrate may be a sheet used as a separator in a lithium secondary battery.

[0040] (Spacer) The spacer may contain a resin (e.g., an insulating resin) or may contain a resin and particles. The spacer may be composed of only a resin, or may be composed of a resin and particles. The proportion of resin in the spacer may be 10% by volume or more, 30% by volume or more, or 50% by volume or more, or may be 100% by volume or less, or 80% by volume or less. The particle content in the spacer may be lower than the particle content in the heat-resistant layer.

[0041] Examples of resin materials include fluorine-containing resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluorine-containing rubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer, styrene-butadiene copolymer or hydrogenated product thereof, acrylonitrile-butadiene copolymer or hydrogenated product thereof, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, and rubbers such as polyvinyl acetate, cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose, acrylic resins such as acrylic acid-methacrylic acid copolymer, polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamides such as wholly aromatic polyamide (aramid), polyamideimide, polyacrylonitrile, polyvinyl alcohol, polyether, polyacrylic acid, polymethacrylic acid, polyester, polyolefin, silicone resin, urethane resin, melamine resin, urea resin, and epoxy resin.

[0042] Among the above resin materials, preferred materials that are impermeable to lithium ions include polyimide, polyvinylidene fluoride, and acrylonitrile-acrylic acid ester copolymers, and polyimide may also be used. A non-porous spacer having a certain height or greater and made of these resin materials is a layer that is impermeable to lithium ions. When the separator includes a heat-resistant layer, the average height of the spacer may be 3 times or more, 5 times or more, or 10 times or more the average thickness of the heat-resistant layer, or may be 100 times or less, 30 times or less, or 20 times or less.

[0043] The particles may be inorganic or organic. Examples of inorganic particles include insulating metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of metal oxides include aluminum oxide (alumina and boehmite), magnesium oxide, titanium oxide (titania), zirconium oxide, and silicon oxide (silica). Examples of metal hydroxides include aluminum hydroxide. Examples of metal nitrides include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of metal carbides include silicon carbide and boron carbide. Examples of metal sulfides include barium sulfate. Minerals such as aluminosilicates, layered silicates, barium titanate, and strontium titanate may also be used. Among these, alumina, silica, and titania are preferred.

[0044] The average particle size of the particles is not particularly limited, but may be 0.1 μm or more, 0.5 μm or more, or 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, a cross-section of the spacer in the thickness direction of the separator is photographed using an electron microscope to obtain an image of the cross-section. Next, the image is subjected to image processing such as binarization to identify the particle portion. Next, the diameter of a circle having the same area as the cross-section area of ​​each particle (equivalent circle diameter) is determined, and the arithmetic mean of the determined equivalent circle diameters can be used as the average particle size. The arithmetic mean can be determined, for example, from 100 or more particles. The average particle size of other particles contained in the electrode plate and separator can also be determined by a similar method.

[0045] When the spacer contains a resin material and particles, the particle content in the spacer is preferably 50% by volume or less, which makes it easier to ensure sufficient strength of the spacer.

[0046] The area S1 of the spacer may be 30% or less of the area S0 of the separator. This range ensures sufficient space for lithium metal deposition. The areas S1 and S0 are the areas of the separator when viewed from the spacer side. The ratio S1 / S0 of the area S1 to the area S0 may be 0.20 or less (20% or less) or 0.10 or less, or 0.03 or more (3% or more) or 0.05 or more. By setting this ratio to 0.05 or more (5% or more), the effect of suppressing excessive temperature rise in the electrode group can be enhanced.

[0047] The spacer may include dot-shaped protrusions in addition to linear protrusions. From one viewpoint, the linear protrusions are ridge-shaped protrusions. In order to enhance the effect of suppressing excessive temperature rise in the electrode group, it is preferable that the spacer include linear protrusions, or the spacer may be composed of only linear protrusions.

[0048] The width of the linear convex portion may be 100 μm or more, or 200 μm or more, and may be 2000 μm or less, or 1000 μm or less.

[0049] The mesh pattern may be a collection of polygons. An example of a mesh pattern includes a shape in which polygons are combined so as to share sides. Polygons include triangles, quadrilaterals, hexagons, etc. Different types of polygons may be combined. The mesh pattern may also be a honeycomb pattern.

[0050] In the lithium secondary battery (L1), the positions of the defects are not particularly limited. The defects may be formed at the positions of the sides of the polygons that make up the mesh pattern. Alternatively, the defects may be formed at the positions of the vertices of the polygons that make up the mesh pattern. Alternatively, the defects may be formed at the positions of the sides and vertices of the polygons. The total length of the defects along the linear protrusions may be 1% or more, 5% or more, or 10% or more of the total length of the linear protrusions, and may be 80% or less, 65% or less, or 50% or less. Increasing this ratio can improve the liquid circulation of the nonaqueous electrolyte. On the other hand, decreasing this ratio can enhance the effect of suppressing the temperature rise of the electrode group in the event of a battery abnormality. Taking these points into consideration, this ratio may be in the range of 10 to 50%. In addition, when the defect portion is a type of defect portion in which no spacer is formed at all, the total length of the defect portion along the linear protrusion may be 1% or more, 5% or more, or 10% or more of the total length of the linear protrusion, or may be 50% or less, 30% or less, or 20% or less.

[0051] In the lithium secondary battery (L2), a missing portion may be formed in a part of the mesh pattern, similar to the spacer of the lithium secondary battery (L1). From another viewpoint, the lithium secondary battery (L1) may include a first region in which a mesh pattern is formed and a second region in which a mesh pattern is not formed.

[0052] A preferred example of a spacer satisfies the following condition (J4) and / or (J5). When the following condition is satisfied, a good balance can be achieved between ensuring space for lithium metal deposition and suppressing excessive temperature rise in the electrode group. (J4) The ratio S1 / S0 of the area S1 to the area S0 is 0.30 or less. The ratio S1 / S0 may be within the range described above. (J5) The width of the linear convex portions constituting the spacer is 2000 μm or less. The width of the convex portions may be within the range described above.

[0053] The first resin constituting the spacer preferably has higher heat resistance than the second resin constituting the substrate. Here, "high heat resistance" means that the decomposition temperature or melting point of the first resin is higher than the decomposition temperature or melting point of the second resin. Note that the first resin and the second resin may each contain multiple types of resin.

[0054] (Heat-resistant layer) The heat-resistant layer may contain a polymer (hereinafter sometimes referred to as "polymer (PL)") and inorganic particles. The inorganic particles may include first particles of a lithium-containing phosphate, and may further include second particles other than phosphate. The heat-resistant layer is a layer that allows lithium ions to pass through.

[0055] The phosphate constituting the first particles is lithium phosphate (Li ３ P.O. ４ ), dilithium hydrogen phosphate (Li ２ HPO ４ ), and lithium dihydrogen phosphate (LiH ２ P.O. ４ Among these, lithium phosphate is preferred because it is highly effective in suppressing heat generation in the battery during abnormal conditions.

[0056] The average particle size of the first particles may be in the range of 0.1 μm to 1.0 μm (e.g., 0.1 μm to 0.5 μm, 0.1 μm to 0.2 μm, or 0.1 μm to 0.19 μm). The average particle size of the first particles may be 0.1 μm or more or 0.15 μm or more. The average particle size of the first particles may be 1.0 μm or less, 0.5 μm or less, 0.3 μm or less, or 0.2 μm or less. By setting the average particle size to 0.1 μm or more, sufficient pores necessary for the electrolyte to penetrate can be secured. Setting the average particle size to 1.0 μm or less is preferable from the viewpoint of forming a high-density layer of the first particles.

[0057] The polymer (PL) can be a polymer with higher heat resistance than the main component of the separator substrate. The polymer (PL) preferably includes at least one selected from the group consisting of aromatic polyamides, aromatic polyimides, and aromatic polyamideimides. These are known as polymers (or polymers or resins, from another perspective) with high heat resistance. From the viewpoint of heat resistance, aramids, i.e., meta-aramids (meta-wholly aromatic polyamides) and para-aramids (para-wholly aromatic polyamides), are preferred. A preferred example of the polymer (PL) is meta-aramid. Known aromatic polyamides, aromatic polyimides, and aromatic polyamideimides may also be used for the polymer (PL).

[0058] Examples of aromatic polyamides (polymers (PL)) include polymers formed by condensation polymerization of monomers having an aromatic backbone and containing amide bonds in the repeating units. Examples of aromatic polyamides (e.g., wholly aromatic polyamides) include meta-aromatic polyamides (e.g., meta-aromatic wholly aromatic polyamides) and para-aromatic polyamides (e.g., para-aromatic wholly aromatic polyamides). Wholly aromatic polyamides are also called aramids.

[0059] A preferred example of the second particles (inorganic particles) is a particle made of an insulating inorganic compound that does not melt or decompose when the battery abnormally heats up. The second particles may be inorganic particles commonly used as inorganic fillers. Examples of materials for the second particles include oxides, oxide hydrates, hydroxides, nitrides, carbides, sulfides, etc., which may contain metal elements. The average particle size of the second particles may be 0.2 μm or more and 2 μm or less.

[0060] Examples of oxides and oxide hydrates include aluminum oxide, boehmite, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of carbides include silicon carbide, boron carbide, etc. Examples of sulfides include barium sulfate, etc. Examples of hydroxides include aluminum hydroxide, etc.

[0061] The material of the second particles may be a porous aluminosilicate such as zeolite, a layered silicate such as talc, or barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ From the viewpoint of insulation property and heat resistance, the material of the second particles may be at least one selected from the group consisting of aluminum oxide, boehmite, talc, titanium oxide, and magnesium oxide.

[0062] The average particle size of the second particles may be within the range exemplified for the average particle size of the first particles.

[0063] The inorganic particles may include the first particles and second particles other than phosphate. In this case, the heat-resistant layer may include a first layer including the first particles and a second layer including the second particles. This configuration can particularly enhance the effect of suppressing excessive temperature rise in the electrode group. The heat-resistant layer may be composed of only the first layer or only the second layer.

[0064] The first layer and the second layer may be laminated on the main surface of the substrate facing the positive electrode, on the main surface facing the negative electrode, or on different main surfaces. For example, the separator may have a laminate structure of substrate / first layer / second layer / spacer, substrate / second layer / first layer / spacer, first layer / second layer / substrate / spacer, or second layer / first layer / substrate / spacer. Alternatively, the first layer and the second layer may be arranged on different main surfaces of the substrate. For example, the separator may have a laminate structure of first layer / substrate / second layer / spacer, or second layer / substrate / first layer / spacer. The separator is arranged so that the spacer faces the positive electrode. That is, the spacer is arranged closer to the positive electrode than the substrate and the heat-resistant layer.

[0065] The first layer may contain first particles as a main component. The content of the first particles in the first layer may be in the range of 50% by mass to 99% by mass, 85% by mass to 99% by mass, or 90% by mass to 98% by mass. The content may be 50% by mass or more, 70% by mass or more, 85% by mass or more, or 90% by mass or more. The content may be 99% by mass or less, 98% by mass or less, or 95% by mass or less. Within these ranges, the first particles have a sufficient surface area, making it easy to inactivate lithium at high temperatures.

[0066] The first layer may include solid components other than the first particles, such as a binder, an inorganic substance other than the first particles (e.g., inorganic particles), or a polymer (PL).

[0067] When the first layer contains a binder, the binder content in the first layer may be in the range of 1% by mass to 15% by mass, or in the range of 2% by mass to 10% by mass. The binder content in the first layer may be 1% by mass or more, or 2% by mass or more. The binder content in the first layer may be 15% by mass or less, or 10% by mass or less.

[0068] The binder contained in the first layer is not particularly limited, and examples thereof include polyolefins (polyethylene, polypropylene, copolymers of ethylene and α-olefins, etc.), fluorine-containing resins (polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, etc.), fluorine-containing rubbers (vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, ethylene-tetrafluoroethylene copolymers, etc.), styrene-butadiene copolymers and hydrogenated products thereof, acrylonitrile-butadiene copolymers and hydrogenated products thereof, acrylonitrile-butadiene-styrene copolymers and hydrogenated products thereof, and N-vinylacetamide.

[0069] (Second Layer) As described above, the second layer contains second particles other than the first particles (phosphate particles). Note that the second layer may or may not contain the first particles.

[0070] The second layer preferably contains a polymer (PL). The content of the polymer (PL) in the second layer may be in the range of 50% by mass to 100% by mass (e.g., 80% by mass to 100% by mass or 90% by mass to 100% by mass). The second layer may consist of only the polymer (PL).

[0071] The second layer may contain second particles as a main component. The content of the second particles in the second layer may be in the range of 50% by mass to 99% by mass (e.g., in the range of 85% by mass to 99% by mass). In this case, the second layer may also contain a binder. Examples of the binder include the binders exemplified in the description of the first layer. The content of the second particles in the second layer may be 50% by mass or more, 70% by mass or more, 85% by mass or more, or 90% by mass or more. The content may be 99% by mass or less, 98% by mass or less, or 95% by mass or less.

[0072] The thicknesses of the first and second layers may each independently be in the range of 0.2 μm to 10 μm (e.g., 1 μm to 8 μm, 2 μm to 4 μm, or 4 μm to 10 μm). For example, the thickness of the first layer may be in the range of 0.2 μm to 10 μm, and the thickness of the second layer may be in the range of 0.2 μm to 10 μm. The thickness of the first layer may be 0.2 μm or more, 0.3 μm or more, or 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more. The thickness of the first layer may be 10 μm or less, 8 μm or less, 7 μm or less, or 5 μm or less. The thickness of the second layer may be 0.2 μm or more, 0.3 μm or more, or 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more, even more preferably 3 μm or more, and even more preferably 4 μm or more. The thickness of the second layer may be 10 μm or less, 8 μm or less, or 7 μm or less, or 5 μm or less. When the thickness of at least one of the first layer or the second layer is 0.2 μm or more, it is advantageous in terms of suppressing an increase in battery temperature in the event of an abnormality. When the thickness of at least one of the first layer or the second layer is 10 μm or less, it is advantageous in terms of the electrical characteristics of the battery.

[0073] When the heat-resistant layer contains the first particles, it is possible to particularly suppress temperature rise in the battery under abnormal conditions. The mechanism behind this is currently unclear. One possibility is that when the battery temperature rises abnormally, the first particles react with the lithium metal in the negative electrode, reducing the reactivity of the lithium metal surface.

[0074] A preferred example of a separator may satisfy the following condition (K1) and may further satisfy the following condition (K2) and / or (K3). By satisfying the following conditions, excessive temperature rise in the electrode group can be particularly suppressed. (K1) The spacer contains at least one resin selected from the group consisting of polyvinylidene fluoride, acrylonitrile-acrylic acid ester copolymer, and polyimide. (K2) The heat-resistant layer contains at least one resin selected from the group consisting of wholly aromatic polyamide, polyvinylidene fluoride, and N-vinylacetamide. In this case, the heat-resistant layer preferably contains phosphate particles. (K3) The substrate contains polyolefin (polyethylene, polypropylene, etc.) as a main component (content: 50% by mass or more).

[0075] (Method of Forming Separator) There is no particular limitation on the method of producing the separator, and it may be produced by the following method. First, a substrate is prepared. A commercially available substrate may be used. Next, a heat-resistant layer is formed on the substrate.

[0076] The method for forming the heat-resistant layer is not particularly limited, and the layer may be formed by the following method. First, a slurry (or coating liquid) is formed by mixing the components of the heat-resistant layer with liquid components (dispersion medium). Next, the slurry (or coating liquid) is applied to a substrate to form a coating film, and the coating film is then dried. In this manner, the heat-resistant layer can be formed. There are no particular limitations on the liquid components used to form the slurry (or coating liquid). Examples of such liquid components include N-methyl-2-pyrrolidone. When the heat-resistant layer includes a first layer and a second layer, each layer may be formed by the method described above.

[0077] There are no particular limitations on the steps in forming the heat-resistant layer, and known methods can be applied. For example, the application of the slurry (or coating liquid) may be carried out by a known method such as a method using a bar coater. Furthermore, the drying may be carried out by a known method such as drying by heating or natural drying.

[0078] The spacers are formed on the heat-resistant layer or substrate. The spacer formation method is not particularly limited, and may be formed by the following method. First, a slurry or coating liquid is prepared by mixing the spacer components with a liquid component. Next, the slurry or coating liquid is applied to the area where the spacer is to be formed and then dried. In this manner, the spacers can be formed. Examples of liquid components include N-methyl-2-pyrrolidone. The slurry or coating liquid may be applied using a dispenser or by known printing methods such as gravure printing, inkjet printing, and screen printing. Furthermore, drying may be performed by known methods such as heating or natural drying. In this manner, the separator is obtained. The missing portions can be formed, for example, by not applying the slurry (or coating liquid) to certain areas or by applying a smaller amount of the slurry (or coating liquid) to certain areas than to other areas.

[0079] Examples of each component of the lithium secondary battery will be specifically described below. Note that the components described below are merely examples, and the components of the lithium secondary battery of this embodiment are not limited to the following components. Known components may be used for components other than those characteristic of this embodiment. The separator has been described above, so a duplicated description will be omitted.

[0080] [Negative Electrode] The negative electrode includes a negative electrode current collector. In the lithium secondary battery (L), lithium metal is deposited on the negative electrode current collector upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode current collector upon charging, becoming lithium metal, which is then deposited on the negative electrode current collector. The lithium metal deposited on the negative electrode current collector dissolves as lithium ions in the non-aqueous electrolyte upon discharging. The lithium ions contained in the non-aqueous electrolyte may be derived from a lithium salt added to the non-aqueous electrolyte, may be supplied from the positive electrode active material upon charging, or may be both.

[0081] The negative electrode current collector may be a conductive sheet. When the electrode group is a wound type, a strip-shaped conductive sheet is used. Examples of the conductive sheet include a conductive film and a metal foil.

[0082] 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. "Smooth" 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 (Japanese Industrial Standard) B 0601:2013.

[0083] 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. The conductive material is preferably a material that does not react with lithium. More specifically, a material that does not form an alloy or intermetallic compound with lithium is preferred. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), alloys containing these metal elements, and graphite with a preferentially exposed basal surface. Examples of alloys include copper alloys and stainless steel (SUS). Copper and / or copper alloys are preferred due to their high conductivity. The thickness of the negative electrode current collector is not particularly limited and may be in the range of 5 to 300 μm.

[0084] A negative electrode mixture layer may be formed on the surface of the negative electrode current collector. The negative electrode mixture 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 high-capacity lithium secondary battery exceeding that of a lithium ion battery, the thickness of the negative electrode mixture layer is set to be sufficiently thin so that lithium metal can be precipitated on the negative electrode.

[0085] The negative electrode may include a negative electrode current collector and a sheet-like lithium metal or lithium alloy disposed on the negative electrode current collector. That is, the negative electrode current collector may be provided with a base layer (a layer of lithium metal or lithium alloy) containing lithium metal in advance. The lithium alloy may contain elements such as aluminum, magnesium, indium, and zinc in addition to lithium. By providing the base layer in advance and depositing lithium metal on it during charging, dendritic deposition can be more effectively suppressed. The thickness of the base layer is not particularly limited, but may be, for example, in the range of 5 μm to 25 μm.

[0086] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on 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 may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0087] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material 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 of their low production cost and high average discharge voltage.

[0088] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0089] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni, and / or Mn as transition metal elements, and optionally containing Al, and having a layered rock-salt crystal structure are preferred in terms of obtaining high capacity. In this case, in the lithium secondary battery, the molar ratio mLi / mM of the total amount of lithium contained in the positive electrode and negative electrode to the amount mM of metal M other than lithium contained in the positive electrode is set to, for example, 1.1 or less.

[0090] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0091] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0092] The positive electrode current collector may be a conductive sheet. Examples of the conductive sheet include foil and film. The surface of the positive electrode current collector may be coated with a carbon material.

[0093] Examples of materials for the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, etc. The Fe alloy may be stainless steel (SUS). The thickness of the positive electrode current collector is not particularly limited and may be in the range of 5 to 300 μm.

[0094] [Non-aqueous electrolyte] The non-aqueous electrolyte having lithium ion conductivity contains, for example, a non-aqueous solvent and lithium ions and anions dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in a liquid state or a gel state.

[0095] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0096] The gel-like non-aqueous electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0097] As the lithium salt or anion, any known material used in non-aqueous electrolytes for lithium secondary batteries can be used. Specifically, BF ４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. 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 － (m and n are each independently an integer of 0 or 1 or more, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2.) The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion, BF ２ (C ２ O ４ ) － , P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ －The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0098] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains at least an anion of an oxalate complex. The interaction between the anion of the oxalate complex and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This facilitates the suppression of localized deposition of lithium metal. The anion of the oxalate complex may be combined with another anion. The other anion may be PF ６ － and / or an anion of an imide.

[0099] The non-aqueous electrolyte contains LiBF as a solute (lithium salt). ２ (C ２ O ４ ) (lithium difluorooxalatoborate).

[0100] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. Examples of halogen-substituted derivatives include fluorides.

[0101] 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 chain 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 chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0102] Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0103] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0104] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode makes it easier to suppress the formation of dendrites. Examples of such additives include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).

[0105] An example of a lithium secondary battery according to the present embodiment will be described in detail below with reference to the drawings. The components described above can be applied to the components of the lithium secondary battery of the example described below. The components of the example described below can be modified based on the above description. The matters described below may also be applied to the above embodiment. In the lithium secondary battery described below, components that are not essential for the lithium secondary battery according to the present disclosure may be omitted. Note that the scale of the components has been changed in the following drawings to facilitate understanding.

[0106] (Embodiment 1) Fig. 1 is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to Embodiment 1. The cylindrical lithium secondary battery 10 shown in Fig. 1 includes a cylindrical battery case, and a wound electrode group 14 and a nonaqueous electrolyte (not shown) housed in the battery case. The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing member 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing member 16. The gasket 27 ensures the hermeticity of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis, respectively.

[0107] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0108] 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. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. 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, all components except the insulating member 24 are electrically connected to each other.

[0109] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0110] 2 is an enlarged view of a portion of the electrode group 14. FIG. 2 includes a portion near the positive electrode surrounded by region II in FIG. 1 and a portion near the negative electrode surrounded by region III in FIG.

[0111] The electrode group 14 has a positive electrode 11, a negative electrode 12, and a separator 50. The positive electrode 11, the negative electrode 12, and the separator 50 are all strip-shaped. The positive electrode 11 and the negative electrode 12 are spirally wound with the separator 50 interposed therebetween so that the width directions of the strip-shaped positive electrode 11 and the negative electrode 12 are parallel to the winding axis. In a cross section perpendicular to the winding axis of the electrode group 14, the positive electrodes 11 and the negative electrodes 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 50 interposed therebetween. In other words, the longitudinal direction of each electrode is the winding direction, and the width direction of each electrode is the axial direction.

[0112] The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode composite layer 11b. The positive electrode current collector 11a is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. In FIG. 2, the negative electrode 12 is shown as a negative electrode (negative electrode current collector) on which no lithium metal is deposited. The negative electrode 12 is electrically connected to a case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20.

[0113] 2 , the separator 50 includes a substrate 51, a heat-resistant layer 52, and a spacer 53. In the first embodiment, the heat-resistant layer 52 is formed on the main surface 51a of the two main surfaces 51a and 51b of the substrate 51, the main surface 51a being on the positive electrode 11 side. The heat-resistant layer 52 is preferably formed so as to cover at least a region of at least one main surface of the substrate 51 that is sandwiched between the positive electrode 11 and the negative electrode 12. The heat-resistant layer 52 may be formed so as to cover the entirety of one side of the substrate 51, or may be formed so as to cover the entirety of both sides of the substrate 51.

[0114] The spacers 53 are formed on the heat-resistant layer 52. In the electrode group 14, the spacers 53 are in contact with the negative electrode 12. The spacers 53 form a space 14s on the surface of the positive electrode 11 (between the heat-resistant layer 52 and the positive electrode 11). On the other hand, a substrate 51 is present on the surface of the negative electrode 12. FIG. 2 shows the height h of the spacers 53. The heat-resistant layer 52 is an optional component.

[0115] In the lithium secondary battery 10, lithium metal is deposited on the negative electrode 12 during charging. The lithium metal deposited on the negative electrode 12 pushes the separator 50 (the substrate 51 and the heat-resistant layer 52) toward the positive electrode 11 in the portion where the spacer 53 is not present. As a result, the lithium metal is deposited in the space 14s. The deposited lithium metal dissolves in the non-aqueous electrolyte during discharge. By accommodating the lithium metal deposited on the surface of the negative electrode 12 within the space 14s, the volume change of the electrode group 14 associated with the deposition of lithium metal is reduced, improving the cycle characteristics.

[0116] If the spacer 53 is permeable to lithium ions, the lithium ions pass through the spacer 53 during charging. The lithium ions that have passed through the spacer 53 are precipitated between the substrate 51 and the negative electrode 12 in the portion where the spacer 53 is formed. As a result, the thickness of the electrode group 14 in the stacking direction increases. For this reason, it is preferable that the spacer 53 is not permeable to lithium ions. In the case of a wound electrode group, the stacking direction means the radial direction of the wound electrode group.

[0117] An example of the planar shape of the spacer 53 used in the lithium secondary battery (L1) is shown in Fig. 3. Fig. 3 shows the longitudinal direction LD of the separator 50. Note that the longitudinal direction LD of the separator 50 may also be shown in other figures.

[0118] The spacer 53 is composed of linear protrusions 53a. The linear protrusions 53a are arranged in a mesh pattern, more specifically, uniformly formed in a honeycomb pattern. The honeycomb pattern is a pattern in which multiple hexagons are arranged so that they share sides. Areas where the linear protrusions 53a are not formed form spaces 14s.

[0119] The honeycomb pattern (mesh pattern) shown in Figure 3 has missing portions 53b. Figure 3 shows an example in which missing portions 53b are formed on the sides of a polygon (a hexagon in Figure 3). Note that in the first embodiment, an example is shown in which no spacers 53 are formed in the missing portions 53b. However, as described above, at least a portion of the missing portions 53b may be a portion in which the height of a portion of the spacer 53 is reduced. The missing portions 53b may be formed in a portion other than the sides. Figure 4 shows an example of the planar shape of a spacer 53 in which missing portions 53b are formed at the vertices of a polygon.

[0120] The spacer 53 in Fig. 4 has a honeycomb pattern, and the honeycomb pattern has cutouts 53b. In the example shown in Fig. 4, the cutouts 53b are formed at the vertices of a hexagon.

[0121] The defects 53b connect adjacent mesh regions (spaces 14s). As a result, nonaqueous electrolyte can move between adjacent spaces 14s. Note that FIG. 3 shows an example in which defects 53b are formed on all sides, and FIG. 4 shows an example in which defects 53b are formed on all vertices. However, some edges or vertices may not have defects 53b. For example, some mesh regions (spaces 14s) may not be connected to adjacent mesh regions (spaces 14s) via defects 53b. The proportion (number ratio) of all mesh regions (spaces 14s) that are connected to adjacent mesh regions (spaces 14s) via defects 53b is preferably 50% or more, more preferably 70% or more, 80% or more, or 90% or more, but not more than 100%. In a preferred example, all mesh regions (spaces 14s) are connected to adjacent mesh regions (spaces 14s) via defects 53b.

[0122] (Embodiment 2) In Embodiment 2, an example of a lithium secondary battery (L2) will be described. Since Embodiment 2 differs from the lithium secondary battery of Embodiment 1 only in the arrangement pattern of the spacers, a duplicated description will be omitted.

[0123] Fig. 5 shows a part of the arrangement pattern of the spacers 53 in the separator 50 of the lithium secondary battery of embodiment 2. Fig. 6 shows an enlarged view of a part of the pattern in Fig. 5. Fig. 6 shows the width W of the linear protrusions 53a.

[0124] The separator 50 in Fig. 5 includes linear protrusions 53a arranged in a mesh pattern. The separator 50 includes a first region 50a in which the mesh pattern is formed and a second region 50b in which the mesh pattern is not formed. The first region 50a and the second region 50b are alternately arranged along the longitudinal direction LD of the separator 50. The length L1 of the first region 50a in the longitudinal direction LD is longer than the length L2 of the second region 50b in the longitudinal direction LD.

[0125] In the first and second embodiments, a cylindrical lithium secondary battery having a wound electrode group has been described. However, the lithium secondary battery of this embodiment is not limited to the form of the first embodiment, and can be applied to other forms. The shape of the lithium secondary battery can be appropriately selected from various shapes such as a cylindrical shape, a coin shape, a square shape, a sheet shape, and a flat shape depending on the application. The shape of the electrode group is also not particularly limited, and may be a stacked type.

[0126] [Examples] The lithium secondary battery according to this embodiment will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0127] (Battery A1) A lithium secondary battery A1 was fabricated according to the following procedure. (1) Fabrication of Positive Electrode A rock-salt lithium-containing transition metal oxide (NCA; positive electrode active material) containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0) and having a layered structure was prepared. This lithium-containing transition metal oxide (NCA), 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, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry. Next, the resulting positive electrode mixture slurry was applied to both sides of a strip-shaped Al foil (positive electrode current collector) and then dried to form a coating of the positive electrode mixture. Next, the coating film was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode composite was cut to a predetermined electrode size to prepare a positive electrode having a positive electrode composite layer on both sides of the positive electrode current collector.

[0128] (2) Preparation of Negative Electrode A negative electrode was prepared by providing a rolled lithium metal layer (thickness: 25 μm) on both sides of a strip of copper foil (thickness: 12 μm).

[0129] (3) Preparation of Separator First, a strip-shaped porous polyethylene film (average thickness: 10 μm) was prepared as a substrate. Next, a heat-resistant layer (average thickness: 2 μm) was formed on one side of the substrate. The heat-resistant layer was formed by forming the second layer and the first layer in this order on the substrate.

[0130] The second layer was formed as follows. First, N-methyl-2-pyrrolidone (NMP) and calcium chloride were mixed in a mass ratio of 94.2:5.8. This mixture was heated to approximately 80°C to completely dissolve the calcium chloride. Then, this solution was returned to room temperature, and 2200 g of it was collected. 0.6 mol of paraphenylenediamine (PPD) was added to completely dissolve it. While maintaining this solution at approximately 20°C, 0.6 mol of terephthalic acid dichloride (TPC) was added in small portions. The resulting solution was aged at approximately 20°C for 1 hour to obtain a polymerized solution. Next, 100 g of this polymerized solution was mixed with an N-methyl-2-pyrrolidone solution in which 5.8 mass% of calcium chloride was dissolved, to obtain a solution (coating solution) containing 2 mass% of paraphenylene terephthalamide (PPTA), an aromatic polyamide (aramid).

[0131] The coating solution was then applied to a substrate using a slot die method to form a coating film. The substrate on which the coating film was formed was then left to stand for 1 hour in an atmosphere at a temperature of 25°C and a relative humidity of 70% to precipitate the aromatic polyamide. The NMP and calcium chloride in the coating film were then removed by rinsing with water. The coating film was then dried at 60°C for 5 minutes to form a second layer.

[0132] The first layer was formed as follows: First, lithium phosphate (Li ３ P.O. ４ Particles of lithium phosphate (LiPO4) and poly(N-vinylacetamide) (PNVA) were mixed in a mass ratio of 100:8 to obtain a mixture. The lithium phosphate particles used had a volume-based median diameter of 0.19 μm. Water (ion-exchanged water) was added to the resulting mixture and stirred to prepare a slurry (coating solution) with a solids concentration of 12 mass%. The slurry was then applied to the second layer by microgravure coating to form a coating film. The coating film was then dried in a drying oven attached to the coating machine. In this manner, the first layer was formed. In this manner, the heat-resistant layer was formed.

[0133] Next, a coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was dispensed onto the heat-resistant layer using a dispenser in the pattern shown in Fig. 3. The coating liquid was then vacuum-dried, thereby forming the honeycomb-shaped non-porous spacer shown in Fig. 3.

[0134] The mesh shape of the spacer was a regular hexagon including missing portions. The height of the linear protrusions was set to 30 μm (average height: 30 μm). The distance between two opposing sides of the regular hexagonal mesh was approximately 2.25 mm. The width of the linear protrusions was 0.25 mm. The planar shape of the missing portions was 0.25 mm × 0.25 mm. The area S1 of the spacer was 16.7% of the area S0 of the separator.

[0135] (4) Preparation of non-aqueous electrolyte Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:DMC=30:70. LiPF ６ 1 mol / L and LiBF ２ (C ２ O ４ ) were dissolved in water to a concentration of 0.1 mol / L to prepare a liquid non-aqueous electrolyte.

[0136] (5) Battery Fabrication An Al tab was attached to the positive electrode obtained above. A Ni tab was attached to the negative electrode. The positive electrode and negative electrode were spirally wound with a separator in an inert gas atmosphere to prepare a wound electrode group. At this time, the separator was positioned so that the spacer faced the positive electrode.

[0137] The obtained electrode group was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the above-described nonaqueous electrolyte was poured into the exterior body housing the electrode group, and then the exterior body was sealed to produce a lithium secondary battery A1.

[0138] (Battery A2) Battery A2 was fabricated using the same method and conditions as Battery A1, except for the change in the spacer pattern. For Battery A2, spacers were formed with the arrangement pattern shown in Figure 4. The mesh shape of the spacer was a regular hexagon including missing portions. The size of the regular hexagon and the height and width of the linear protrusions were the same as those of the separator of Battery A1. The size of the missing portions was approximately the same as that of the spacer of Battery A1. Therefore, the ratio of the spacer area S1 to the separator area S0 was also approximately the same as that of the separator of Battery A1.

[0139] (Battery A3) Battery A3 was fabricated using the same method and conditions as Battery A1, except for the change in the spacer pattern. In Battery A3, spacers were formed with the arrangement pattern shown in Figure 5. The mesh shape of the spacer was a regular hexagon. The size of the regular hexagon and the height and width of the linear protrusions were the same as those of the separator in Battery A1. The length L1 of the first region in the longitudinal direction LD was 90 mm, and the length L2 of the second region in the longitudinal direction LD was 10 mm. The area S1 of the spacer was 18% of the area S0 of the separator.

[0140] (Battery C1) Battery C1 was fabricated using the same method and conditions as Battery A1, except for the change in the spacer pattern. Battery C1 used a spacer consisting of multiple linear protrusions arranged in a stripe pattern. Each linear protrusion was arranged along the width direction of the separator (the direction perpendicular to the longitudinal direction LD). The spacing between adjacent linear protrusions was 2.25 mm. The height and width of the linear protrusions were the same as those of the separator of Battery A1. The area S1 of the spacer was 20% of the area S0 of the separator.

[0141] (Battery C2) Battery C2 was fabricated using the same method and conditions as Battery A1, except that the spacer pattern was changed. Battery C2 used the same spacers as Battery A1, except that no missing portions were formed. That is, in Battery C2, the spacers were arranged in a honeycomb pattern. The area S1 of the spacers was 21% of the area S0 of the separator.

[0142] Battery C3: In Battery C3, spacers were formed on the positive electrode instead of the separator. The spacers were formed using the same method as in Battery A1. The spacers had the same arrangement pattern and size as those in Battery C1.

[0143] Battery C4: In Battery C4, spacers were formed on the positive electrode instead of the separator. The spacers were formed using the same method as in Battery A1. The spacers had the same arrangement pattern and size as those in Battery C2.

[0144] The batteries thus fabricated were evaluated by the following method. (Charge / Discharge Test) A charge / discharge test was conducted on each of the obtained batteries. In the charge / discharge test, the battery was charged in a thermostatic chamber at 25°C under the following conditions, then rested for 20 minutes, and discharged under the following conditions.

[0145] (Charging) The battery was charged at a constant current of 2.15 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current value per unit area of ​​the electrode reached 0.54 mA.

[0146] (Discharge) Constant current discharge was carried out at a current of 2.15 mA per unit area of ​​the electrode until the battery voltage reached 3.75 V.

[0147] (Expansion Coefficient of Electrode Group) The above charge and discharge constituted one cycle, and the fabricated battery was subjected to a second charge cycle. The battery before charge and discharge and the battery that had been subjected to a second charge cycle were disassembled, and a laminate of a positive electrode, a negative electrode, and two separators located on either side of the negative electrode was removed. Disassembly was performed in an inert gas atmosphere. The removed laminate of the positive electrode, negative electrode, and separator was washed with dimethyl carbonate, dried, and the thickness of the laminate was measured. The thickness of the laminate was measured using a Peacock Digital Thickness Gauge G2-205M. The thickness was measured at five arbitrary points within the laminate, and the arithmetic mean of the five measurements was used as the average thickness of the laminate. Next, the average thickness X was calculated by subtracting the thicknesses of the two substrates and the two heat-resistant layers from this average thickness. The ratio (%) of the average thickness X after the second cycle to the average thickness X before charge and discharge was used as the expansion coefficient of the electrode group. That is, the expansion rate (%) of the electrode group is the ratio of the average thickness X after the second cycle to the average thickness X before charge and discharge, which is taken as 100%.

[0148] (Discharge Capacity Retention Rate) The above charge and discharge cycle was counted as one cycle, and charge and discharge were repeated up to 50 cycles. The ratio (%) of the discharge capacity at the 50th cycle to the discharge capacity at the first cycle was calculated as the discharge capacity retention rate. The discharge capacity at the first cycle was defined as the initial discharge capacity.

[0149] Some of the battery fabrication conditions and evaluation results are shown in Table 1. In Table 1, the initial discharge capacity is expressed as a relative value when the initial discharge capacity of Battery C3 is set to 100. It is preferable that the initial discharge capacity and discharge capacity retention rate are high. It is preferable that the expansion rate of the electrode group is low.

[0150]

[0151] Batteries A1 to A3 are batteries according to the present disclosure, and batteries C1 to C4 are comparative examples. Batteries A1 to A3 had high initial discharge capacities and discharge capacity retention rates, and low electrode assembly expansion rates. In contrast, battery C1, in which the spacers were formed in a striped pattern, had a high electrode assembly expansion rate. This is thought to be because the spacers were misaligned when the electrode plates and separator were wound, preventing sufficient space from being formed between the electrode plates. Battery C2, which used a spacer without any defects, had a low discharge capacity retention rate. This is thought to be due to the effect of the spacers preventing the movement of the nonaqueous electrolyte. Batteries C3 and C4, in which spacers were formed on the positive electrode, had low initial discharge capacities. This is thought to be because the spacers prevented effective use of part of the positive electrode active material.

[0152] The present disclosure is applicable to lithium secondary batteries. For example, lithium secondary batteries according to the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet computers, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

[0153] REFERENCE SIGNS LIST 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 14: Electrode group 14s: Space 50: Separator 50a: First region 50b: Second region 51: Substrate 51a: Main surface 52: Heat-resistant layer 53: Spacer 53a: Convex portion 53b: Defective portion LD: Longitudinal direction

Claims

1. A lithium secondary battery, a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity; the negative electrode is an electrode in which lithium metal is deposited during charging and in which the lithium metal dissolves during discharging, the separator includes a substrate and a spacer, the spacer is disposed closer to the positive electrode than the substrate, the spacer includes linear protrusions arranged in a mesh pattern; The rechargeable lithium battery has a mesh pattern including defects that connect adjacent mesh regions.

2. A lithium secondary battery, a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity; the negative electrode is an electrode in which lithium metal is deposited during charging and in which the lithium metal dissolves during discharging, the separator includes a substrate and a spacer, the spacer is disposed closer to the positive electrode than the substrate, the spacer includes linear protrusions arranged in a mesh pattern; The separator includes a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.

3. the first regions and the second regions are alternately arranged along the longitudinal direction of the separator, a length L1 of the first region in the longitudinal direction is longer than a length L2 of the second region in the longitudinal direction; 3. The lithium secondary battery according to claim 2, wherein the length L2 is 1 mm or more and 15 mm or less.

4. 3. The lithium secondary battery according to claim 1, wherein the spacers have an average height greater than an average thickness of the substrate.

5. 3. The lithium secondary battery according to claim 1, wherein the spacer has a non-porous structure that is impermeable to lithium ions.

6. 3. The lithium secondary battery according to claim 1, wherein the area of ​​the spacer is 30% or less of the area of ​​the separator.

7. 3. The lithium secondary battery according to claim 1, wherein the first resin constituting the spacer has higher heat resistance than the second resin constituting the substrate.

8. 3. The lithium secondary battery according to claim 1, wherein the mesh pattern is an aggregate of polygons.

9. 3. The lithium secondary battery according to claim 1, wherein the mesh pattern is a honeycomb pattern.

10. the separator further includes a heat-resistant layer disposed on the substrate; The lithium secondary battery according to claim 1 , wherein the spacer is disposed closer to the positive electrode than the substrate and the heat-resistant layer.

11. the heat-resistant layer is formed on one of the two main surfaces of the base material, the main surface on the positive electrode side; The lithium secondary battery according to claim 10 , wherein the spacer is formed on the heat-resistant layer.

12. The lithium secondary battery according to claim 10 , wherein the average height of the spacers is greater than the sum of the average thickness of the substrate and the average thickness of the heat-resistant layer.

13. The lithium secondary battery according to claim 10 , wherein the heat-resistant layer comprises a polymer and inorganic particles.

14. 14. The lithium secondary battery according to claim 13, wherein the inorganic particles include first particles of a phosphate containing lithium.

15. the inorganic particles include the first particles and second particles other than phosphate particles, The lithium secondary battery according to claim 14 , wherein the heat-resistant layer includes a first layer including the first particles and a second layer including the second particles.

16. The non-aqueous electrolyte is LiBF ２ (C ２ O ４ 3. The lithium secondary battery according to claim 1, comprising: