Lithium secondary battery

Abstract

A lithium secondary battery (10) disclosed in the present invention includes: a positive electrode (11); a negative electrode (12); a separator (13) and a spacer (53) disposed between the positive electrode (11) and the negative electrode (12); and a non-aqueous electrolyte. The negative electrode (12) is an electrode in which lithium metal is deposited during charging and dissolved during discharging. The positive electrode (11) includes a positive current collector (11a) and a positive electrode mixture layer (11b) disposed on the positive current collector (11a). A part of the spacer (53) cuts into the positive electrode mixture layer 11b.

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] Claim 1 of Patent Document 1 (JP Patent Publication No. 1998-12279 A) describes, "A metal lithium secondary battery in which a negative electrode using lithium or a lithium alloy as the negative electrode active material and a positive electrode made of a chargeable and dischargeable material are arranged opposite each other with a separator interposed therebetween, characterized in that a buffer space for accommodating lithium deposited on the surface of the negative electrode is provided between the negative electrode or the positive electrode and the separator."

[0005] Claim 1 of Patent Document 2 (WO 2020 / 066254) describes: "A lithium secondary battery comprising: a positive electrode including a positive electrode current collector and a positive electrode mixture 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 deposits on the negative electrode during charging and dissolves from the negative electrode during discharging, 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."

[0006] JP 10 (1998)-12279 A International Publication No. 2020 / 066254

[0007] By disposing a spacer between the positive electrode and the negative electrode, it is possible to suppress battery expansion. However, as a result of investigation, the present inventors have found that forming a spacer can sometimes degrade charge-discharge cycle characteristics. One of the objects of the present disclosure is to provide a lithium secondary battery that exhibits small expansion during charging and good charge-discharge cycle performance.

[0008] One aspect of the present disclosure relates to a lithium secondary battery including: a positive electrode; a negative electrode; a separator and a spacer disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte; the negative electrode is an electrode from which lithium metal deposits during charging and from which the lithium metal dissolves during discharging; the positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector; and a portion of the spacer is embedded in the positive electrode mixture layer.

[0009] According to the present disclosure, a lithium secondary battery with small expansion during charging and good charge / discharge cycle performance 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] Fig. 1 is a longitudinal sectional view schematically showing an example of a lithium secondary battery according to an embodiment of the present disclosure. Fig. 2 is a sectional view schematically showing a portion of an example of an electrode group. Fig. 3 is a sectional view schematically showing a portion of another example of an electrode group. Fig. 4 is a top view schematically showing an example of a spacer arrangement. Fig. 5 is a top view schematically showing another example of a spacer arrangement. Fig. 6 is a top view schematically showing another example of a spacer arrangement.

[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) The lithium secondary battery according to this embodiment may be referred to as a "lithium secondary battery (L)" below. The lithium secondary battery (L) includes a positive electrode, a negative electrode, a separator and a spacer disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode is an electrode from which lithium metal deposits during charging and from which lithium metal dissolves during discharging. The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. A portion of the spacer is embedded in the positive electrode mixture layer. This portion may be referred to as a "portion (Pp)" below.

[0013] In the lithium secondary battery (L), a spacer ensures a space between the positive electrode and the negative electrode. This prevents the electrode group from expanding in the stacking direction due to lithium metal deposited on the negative electrode during charging. However, the present inventors discovered that forming a spacer can degrade charge-discharge cycle performance. After further investigation, the present inventors discovered that the degradation of charge-discharge cycle performance can be suppressed by embedding a portion of the spacer into the positive electrode mixture layer. The present disclosure is based on this new finding.

[0014] Generally, a coating is formed on the surface of a positive electrode material mixture layer of a lithium secondary battery. The formation of this coating is accompanied by decomposition of the non-aqueous electrolyte. If the formed coating is destroyed, a new coating is formed, which accelerates the decomposition of the non-aqueous electrolyte, resulting in a deterioration in charge-discharge cycle characteristics. Therefore, it is necessary to avoid destruction of the coating formed on the surface of the positive electrode material mixture layer.

[0015] During charge and discharge, the positive electrode also expands and contracts in a direction perpendicular to the stacking direction. If the spacer does not penetrate into the positive electrode mixture layer, strong friction occurs between the spacer and the positive electrode as the positive electrode expands and contracts. As a result, it is believed that the coating on the surface of the positive electrode mixture layer is destroyed. If the coating is destroyed, as described above, decomposition of the non-aqueous electrolyte progresses. On the other hand, in the lithium secondary battery (L), the spacer penetrates into the positive electrode mixture layer, so destruction of the coating on the surface of the positive electrode mixture layer can be suppressed. Therefore, the lithium secondary battery (L) can maintain good charge and discharge cycle characteristics. As described above, the lithium secondary battery (L) exhibits small expansion during charge and good charge and discharge cycle characteristics.

[0016] The depth Dp (or average depth Dpa) of the portion (Pp) embedded in the positive electrode mixture layer may be 0.5 μm or more, 3.0 μm or more, or 5.0 μm or more. By setting the depth Dp (or average depth Dpa) to 0.5 μm or more, a high effect can be obtained. From the viewpoint of ease of manufacturing, the depth Dp (or average depth Dpa) is preferably 3% or less of the thickness Tp of the positive electrode mixture layer. Here, when the positive electrode mixture layer is formed on both sides of the positive electrode current collector, the thickness Tp of the positive electrode mixture layer is the thickness of one positive electrode mixture layer formed on one side of the positive electrode current collector.

[0017] The depth Dp (or average depth Dpa) may be 10 μm or less, or 5.0 μm or less. By setting the depth Dp (or average depth Dpa) to 5.0 μm or less, damage to the positive electrode can be suppressed. Furthermore, by setting the depth Dp (or average depth Dpa) to 5.0 μm or less, a decrease in output due to the discharge of non-aqueous electrolyte in the positive electrode mixture layer where the spacer is located can be suppressed. When the electrode group is a wound type, setting the depth Dp (or average depth Dpa) to 5.0 μm or less can increase the circularity of the cross section of the electrode group. The depth Dp (or average depth Dpa) may be in the range of 0.5 to 10 μm, the range of 3.0 to 10 μm, or the range of 5.0 to 10 μm. Within these ranges, the upper limit may be 5.0 μm as long as the lower limit is not greater than the upper limit.

[0018] The average depth Dpa can be determined by the following procedure. First, five locations where the portion (Pp) penetrates into the positive electrode mixture layer are arbitrarily selected, and the depth Dp of the portion (Pp) penetrating into each location is measured. The average depth Dpa is determined by arithmetically averaging the depths Dp measured at the five locations. The depth Dp can be measured by a method described in the examples.

[0019] The spacer is disposed between the separator and the positive electrode, so that the spacer abuts against and bites into the positive electrode mixture layer.

[0020] The negative electrode of the lithium secondary battery (L) includes a negative electrode substrate foil. The portion of the separator that the spacer abuts against may or may not be embedded in the negative electrode substrate foil. This portion may be referred to below as the "portion (Pn)." It is preferable that the portion (Pn) not be embedded in the negative electrode substrate foil, since this makes it difficult for the spacer to move as the negative electrode expands. On the other hand, if the portion (Pn) is embedded in the negative electrode substrate foil, it is possible to suppress the deposition of lithium metal between the separator and the negative electrode substrate foil during charging. As a result, it is possible to suppress the expansion of the electrode group during charging.

[0021] The depth Dn (or average depth Dna) of the portion (Pn) embedded in the negative electrode substrate foil may be 0.5 μm or more, 1.0 μm or more, or 2.0 μm or more. By setting the depth Dn (or average depth Dna) to 0.5 μm or more, expansion of the electrode group during charging can be particularly suppressed. The depth Dn and average depth Dna can be determined by the same method as the depth Dp and average depth Dpa, respectively.

[0022] The method for embedding the portion (Pp) into the positive electrode mixture layer and the method for embedding the portion (Pn) into the negative electrode substrate foil are not limited, but the methods exemplified in the manufacturing method (M) described later can be used.

[0023] The shape of the lithium secondary battery (L) is not particularly limited, and examples of the shape of the lithium secondary battery (L) include a cylindrical shape, a coin shape, a square shape, a sheet shape, a flat shape, etc. The form of the electrode group is also not particularly limited, and may be a wound type or a laminated type.

[0024] (Method for manufacturing lithium secondary battery) A manufacturing method (M), which is an example of a method for manufacturing a lithium secondary battery (L), will be described below. However, the lithium secondary battery (L) may be formed by a method other than the manufacturing method (M) described below. The matters described for the lithium secondary battery (L) can be applied to the manufacturing method (M). The matters described for the manufacturing method (M) may also be applied to the lithium secondary battery (L).

[0025] The production method (M) includes steps (i), (ii), and (iii) in this order. Each step will be described below.

[0026] Step (i) is a step of forming spacers on the separator. The method for forming the spacers is not particularly limited and may be a known method. For example, the spacers may be formed by applying a spacer material to a predetermined member (separator) and then drying it. The spacer material may be applied using a dispenser, or by known printing methods such as gravure printing, inkjet printing, and screen printing, or by other methods. The spacers may be formed on the negative electrode. In this case, in step (ii), a laminate is formed in which the separator is disposed between the spacer and the positive electrode so that the positive electrode and the separator are in contact with each other. When the spacer is formed on the negative electrode, a portion of the separator that is in contact with the spacer may be embedded in the positive electrode mixture layer. Alternatively, when the spacer is formed on the negative electrode, a portion of the separator that is in contact with the spacer and a portion of the spacer may be embedded in the positive electrode mixture layer. Even in these cases, damage to the coating on the surface of the positive electrode mixture layer can be suppressed.

[0027] Step (ii) is a step of forming a laminate (electrode group) including a positive electrode, a negative electrode, a separator, and a spacer. The separator and spacer are disposed between the positive electrode and the negative electrode (negative electrode substrate foil). The laminate is formed so that the positive electrode and the spacer are in contact with each other. The laminate may be a wound body formed by winding the positive electrode, the negative electrode, and the separator so that the separator and the spacer are disposed between the positive electrode and the negative electrode (negative electrode substrate foil). In the wound body, the positive electrode, the negative electrode, and the separator are stacked in the radial direction of the wound body. The wound body can be formed using a strip-shaped positive electrode, a strip-shaped negative electrode, and a strip-shaped separator.

[0028] The stack may be formed by stacking flat positive electrodes, flat negative electrodes (negative electrode substrate foils), and flat separators in one direction, with the positive electrodes and negative electrodes alternately arranged and the separator disposed between the positive and negative electrodes.

[0029] In step (ii), the laminate is formed so that the portion (Pp) penetrates into the positive electrode mixture layer. For example, when forming the wound body, the electrode plates (positive electrode, negative electrode) and / or separator may be pulled with a tension higher than usual. Alternatively, in step (ii), the wound body may be formed by pressing the wound body being formed around the winding axis with a roller. In this case, by applying a higher-than-usual force to press the wound body with the roller, the portion (Pp) can be penetrated into the positive electrode mixture layer. Alternatively, step (ii) may include step (iiA) of forming a laminate by stacking a positive electrode, a negative electrode (negative electrode substrate foil), and a separator in one direction, and step (iiB) of pressing the laminate to penetrate the portion (Pp) into the positive electrode mixture layer. In step (ii), when the portion (Pp) penetrates into the positive electrode mixture layer, the portion (Pn) can also be penetrated into the negative electrode substrate foil.

[0030] Alternatively, step (ii) may include step (iia) of stacking and pressing a separator with a spacer and a positive electrode to form a laminate sheet with a portion (Pp) embedded in the positive electrode mixture layer, and step (iib) of forming a laminate using the laminate sheet and a negative electrode. In step (iia), separators are disposed on both sides of the positive electrode as needed to form a laminate sheet. In step (iib), the laminate may be formed by winding the laminate sheet and the negative electrode, or by stacking the laminate sheet and the negative electrode in one direction.

[0031] In step (ii), the penetration depth Dp (or average depth Dpa) can be increased by increasing the tension applied to the electrode plate (positive electrode, negative electrode) and / or separator or the pressure applied to press the laminate sheet (or laminate). When the tension applied to the electrode plate and / or separator is used to penetrate a portion (Pp) into the positive electrode mixture layer, the tension applied to the electrode plate and / or separator may be 10 mN / mm or more (e.g., 15 mN / mm or more). This tension may be in the range of 10 to 90 mN / mm (e.g., 20 to 80 mN / mm). The tension applied to achieve a predetermined penetration depth Dp (or average depth Dpa) can be appropriately adjusted depending on the shape and strength of the spacer, the hardness of the positive electrode mixture layer, and the like. Here, tension (mN / mm) refers to the pulling force per 1 mm of width of the electrode plate (or separator).

[0032] In step (iii), the laminate formed in step (ii) and the non-aqueous electrolyte are housed in an outer casing. The method for carrying out step (iii) is not particularly limited, and a known method may be used. In this manner, the lithium secondary battery (L) can be produced.

[0033] Examples of the components of the lithium secondary battery (L) will be specifically described below. Note that the components described below are merely examples, and the components of the lithium secondary battery (L) of this embodiment are not limited to the following components. Known components may be used for components other than those characteristic of this embodiment.

[0034] The lithium secondary battery (L) is also called a lithium metal secondary battery. In the negative electrode of the lithium secondary battery (L), lithium metal precipitates during charging and dissolves during discharging. The negative electrode includes a negative electrode substrate foil. During charging, lithium metal precipitates on the negative electrode substrate foil.

[0035] 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 (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).

[0036] (Separator) The separator is not particularly limited, and separators used in known lithium secondary batteries may be used. A porous sheet having ion permeability and insulating properties may be used as the separator. Examples of porous sheets include porous membranes, woven fabrics, and nonwoven fabrics. The material of the separator is not particularly limited, and 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 separator may contain additives as needed. Examples of additives include inorganic fillers. The separator may include the porous sheet described above and a heat-resistant layer formed on the porous sheet.

[0037] The thickness of the separator is not particularly limited and is selected depending on the configuration of the lithium secondary battery (L). The thickness of the separator may be 6 μm or more, or 9 μm or more, or 40 μm or less, or 20 μm or less.

[0038] (Spacer) The spacer is formed on one or both sides of a predetermined member (positive electrode or separator) depending on the battery configuration. When a spacer is formed on the positive electrode, the spacer is usually formed on both sides of the positive electrode. When a spacer is formed on the separator, the spacer is usually formed on one side of the separator.

[0039] The spacer includes an insulating polymer (e.g., an insulating resin). The spacer may be composed of only the insulating polymer, or may be composed of the insulating polymer and particles (filler). The proportion of the insulating polymer 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.

[0040] The insulating polymer constituting the spacer is not particularly limited. Examples of insulating polymers constituting the spacer include polyolefin, acrylic resin, polyamide, polyimide, polyamideimide, silicone resin, fluorine-containing resin, epoxy resin, cellulose derivative, and rubber. Examples of polyolefin include polyethylene, polypropylene, and copolymers thereof. Examples of fluorine-containing resin include polyvinylidene fluoride and polytetrafluoroethylene. Examples of polyamide include wholly aromatic polyamide (aramid). Examples of rubber include vinylidene fluoride-tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, styrene-butadiene rubber, acrylonitrile rubber, and acrylonitrile-acrylic acid ester copolymer.

[0041] Among the above polymers, polyimide, polyvinylidene fluoride, acrylonitrile-acrylic acid ester copolymer, etc. are preferred because they do not allow lithium ions to pass through. The spacer may contain polyimide.

[0042] The particles (filler) may be insulating particles. The particles may be inorganic particles or organic particles. Examples of inorganic particles include insulating inorganic particles such as metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Specifically, examples of insulating particles include particles of aluminum oxide, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, silicon nitride, aluminum nitride, titanium nitride, and silicon carbide.

[0043] When the spacer contains an insulating polymer and particles, the particle content in the spacer may be 10% by mass or more, or 50% by mass or more, or may be 90% by mass or less, 80% by mass or less, or 50% by mass or less. By making the particle content in the spacer 50% by mass or more, it becomes easier to ensure the strength of the spacer.

[0044] The average height Hs of the spacers may vary depending on the battery design, but may be in the range of 10 μm to 100 μm (e.g., 20 μm to 70 μm). When the spacers are linear, the width of the linear spacers may be in the range of 50 μm to 1000 μm (e.g., 250 μm to 750 μm).

[0045] The average height Hs of the spacers 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.

[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.

[0047] The spacer may include linear portions and / or dot-shaped portions. In one aspect, the linear portions are ridge-shaped portions. The linear portions may be mesh-shaped portions arranged in a mesh pattern. Alternatively, the spacer may include a plurality of linear portions arranged in a stripe pattern.

[0048] The planar shape of the linear portion may be a shape obtained by combining polygons. An example of a mesh-like shape is a shape obtained by combining polygons so that they share a common edge. Polygons include triangles, quadrilaterals, hexagons, etc. Different types of polygons may be combined. The planar shape of the linear portion may be a honeycomb shape.

[0049] The linear portions (e.g., mesh portions) may be discontinuous or may include missing portions. When the linear portions are discontinuous, the spacer can more easily follow the expansion of the electrode. A plurality of linear portions arranged in a stripe pattern is also preferred because it can more easily follow the expansion of the electrode.

[0050] The spacers may be formed in a pattern other than a closed mesh pattern, i.e., examples of closed mesh patterns include a honeycomb pattern and a lattice pattern.

[0051] The spacer may have a plurality of first linear portions and a plurality of second linear portions. The plurality of first linear portions and the plurality of second linear portions may face each other across the positive electrode or the negative electrode. When the plurality of first linear portions and the plurality of second linear portions are viewed overlapping each other, each of the plurality of first linear portions may intersect with at least a portion of the plurality of second linear portions at multiple locations. By intersecting the plurality of first linear portions and the plurality of second linear portions that face each other across the electrode, it is possible to prevent the spacers from becoming trapped in the gap. As a result, it is easier to ensure sufficient space between the separator and the electrode, and expansion of the electrode group due to charge and discharge can be particularly suppressed.

[0052] (Negative Electrode) The negative electrode includes a negative electrode substrate foil. In the lithium secondary battery (L), lithium metal is deposited on the negative electrode substrate foil during charging. The lithium metal deposited on the negative electrode substrate foil dissolves as lithium ions in the non-aqueous electrolyte during discharge.

[0053] The negative electrode substrate foil includes a negative electrode current collector foil. Metallic materials other than lithium metal and lithium alloys can be used as the material for the negative electrode current collector foil. 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), and alloys containing these metal elements. 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 foil is not particularly limited and may be in the range of 5 to 300 μm (e.g., 10 to 100 μm).

[0054] The negative electrode substrate foil may include a negative electrode current collector foil and a lithium-containing metal layer disposed on the negative electrode current collector foil. The lithium-containing metal layer may be a layer containing lithium as a main component (50 atomic % or more). The lithium-containing metal layer may be at least one layer selected from the group consisting of lithium metal and lithium alloys. Examples of elements other than lithium contained in lithium alloys include aluminum, magnesium, indium, and zinc. By providing the underlayer in advance and depositing lithium metal on it during charging, dendritic deposition can be effectively suppressed. The thickness of the lithium-containing metal layer is not particularly limited, but may be in the range of 5 μm to 25 μm. When the negative electrode substrate foil includes a lithium-containing metal layer, the lithium-containing metal layer is formed on one or both sides of the negative electrode current collector foil depending on the battery configuration.

[0055] The method for forming the lithium-containing metal layer is not particularly limited. The lithium-containing metal layer may be formed by pressing a lithium-containing metal foil (lithium metal foil or lithium alloy foil) onto the negative electrode current collector foil. The lithium-containing metal layer is dense and can be distinguished from lithium metal (generally porous) that precipitates during charging.

[0056] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, for example, a positive electrode active material and additives (conductive material, binder, etc.). The positive electrode mixture layer is formed on one or both sides of the positive electrode current collector depending on the battery configuration. The positive electrode can be formed by a known method. For example, first, a positive electrode mixture slurry containing a positive electrode active material and additives is prepared. Next, the positive electrode mixture slurry is applied to the positive electrode current collector and then dried to form a coating. Next, the laminate consisting of the positive electrode current collector and the coating is rolled to obtain a positive electrode. The formed positive electrode is cut to a predetermined size as necessary.

[0057] The thickness Tp of the positive electrode mixture layer may be 50 μm or more, or 100 μm or more, and may be 300 μm or less, or 250 μm or less.

[0058] The positive electrode active material is a material capable of reversibly absorbing and releasing 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 manufacturing cost and high average discharge voltage.

[0059] 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 at least one selected from the group consisting of Co, Ni, and 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.

[0060] The conductive material may be a carbon material, etc. Examples of the carbon material include carbon black (acetylene black, ketjen black, etc.), carbon nanotubes, and graphite.

[0061] 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.

[0062] The positive electrode current collector may be a conductive sheet. Examples of the conductive sheet include metal foil. The surface of the positive electrode current collector may be coated with a carbon material.

[0063] 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.

[0064] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity can be used as the non-aqueous electrolyte. The non-aqueous electrolyte may be liquid or gel-like. A liquid non-aqueous electrolyte can be prepared by dissolving a lithium salt in a non-aqueous solvent. When the lithium salt dissolves in the non-aqueous solvent, lithium ions and anions are generated.

[0065] The gel-like non-aqueous electrolyte may contain a lithium salt and a matrix polymer, or may contain a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer may be, 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.

[0066] Known solvents can be used as the nonaqueous solvent. Examples of nonaqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, fluorinated chain ethers, cyclic ethers, and fluorinated cyclic ethers. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, diethylene glycol dimethyl ether, etc. The non-aqueous solvent may be used alone or in combination of two or more.

[0067] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ etc.), lithium halides (LiCl, LiBr, LiI etc.), lithium salts containing oxalate complexes (LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LiPF ２ (C ２ O ４ ) ２ The lithium salts may be used alone or in combination of two or more.

[0068] The concentration of the lithium salt in the nonaqueous electrolyte may be 0.5 mol / L or more, 1 mol / L or more, or 1.5 mol / L or more, and may be 3.5 mol / L or less, 2 mol / L or less, or 1.5 mol / L or less. By setting the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained.

[0069] The non-aqueous electrolyte may contain an additive (for example, a known additive). Examples of the additive include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0070] (Exterior Body) The exterior body accommodates the non-aqueous electrolyte and the electrode group including the spacer. The exterior body is not particularly limited, and a known exterior body can be used. The exterior body may include a battery case, a sealing body that seals the opening of the battery case, and a gasket.

[0071] An example of the lithium secondary battery (L) of this embodiment will be specifically described 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. Furthermore, the components of the example described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the lithium secondary battery described below, components that are not essential for the lithium secondary battery (L) 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.

[0072] (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 an electrode group 14 and a non-aqueous electrolyte (not shown) housed in the battery case. The electrode group 14 includes a positive electrode 11, a negative electrode 12, a separator 13, and a spacer. The spacer is not shown in Fig. 1.

[0073] The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the airtightness 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 winding axis direction. The case body 15 has a step portion 21.

[0074] 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. The lower valve body 23 and the upper valve body 25 are connected at their respective centers. An insulating member 24 is disposed between the peripheral edge of the lower valve body 23 and the peripheral edge of the upper valve body 25. The filter 22 and the lower valve body 23 are connected at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected at their respective peripheral edges. All of the components constituting the sealing body 16, except for the insulating member 24, are electrically connected.

[0075] A vent hole 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 formed in the cap 26.

[0076] The positive electrode 11 is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. 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.

[0077] FIG. 2 is an enlarged view schematically illustrating a portion of an example of an electrode group 14. The electrode group 14 includes a positive electrode 11, a negative electrode 12, a separator 13, and a spacer 53. The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode mixture layer 11b. FIG. 2 illustrates a negative electrode substrate foil 12a on which no lithium metal is deposited as the negative electrode 12. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The spacer 53 is disposed between the positive electrode 11 and the separator 13. As described above, the spacer 53 may be formed on the separator 13. The spacer 53 forms a space 14s between the positive electrode 11 and the negative electrode 12.

[0078] 2, a portion of the spacer 53 is embedded in the positive electrode mixture layer 11b. The depth Dp of the portion of the spacer 53 embedded in the positive electrode mixture layer 11b is shown in FIG.

[0079] As shown in Fig. 2, the separator 13 does not bite into the negative electrode substrate foil 12a. However, as shown in Fig. 3, a part of the separator 13 that is in contact with the spacer 53 may bite into the negative electrode substrate foil 12a. In this case, a part of the separator 13 and a part of the spacer 53 may bite into the negative electrode substrate foil 12a.

[0080] 2 and 3, a portion of the spacer 53 is embedded in the positive electrode mixture layer 11b, which prevents the coating formed on the surface of the positive electrode mixture layer 11b from being destroyed, thereby preventing a decrease in the charge-discharge cycle characteristics.

[0081] An example of the planar shape of the spacer 53 formed on the separator 13 is shown in FIG. 4. The spacer 53 shown in FIG. 4 is composed of linear portions 53a. The linear portions 53a shown in FIG. 4 are arranged in a mesh pattern. More specifically, the linear portions 53a are formed in a honeycomb pattern. The honeycomb pattern is a pattern in which a plurality of hexagons are arranged so as to share sides with each other.

[0082] Another example of the planar shape of the spacer 53 formed on the separator 13 is shown in FIG. 5. The spacer 53 shown in FIG. 5 includes a plurality of linear portions 53a that are spaced apart from one another. A gap P exists between the linear portions. Another example of the planar shape of the spacer 53 formed on the separator 13 is shown in FIG. 6. The spacer 53 in FIG. 6 includes a plurality of linear portions 53a arranged in a stripe pattern. In the examples shown in FIGS. 4 to 6, the areas where the linear portions 53a are not formed constitute the spaces 14s shown in FIG. 2.

[0083] (Additional Notes) The above description discloses the following technologies. (Technology 1) A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator and a spacer disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte; the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharge; the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector; and a portion of the spacer penetrates into the positive electrode mixture layer. (Technology 2) The lithium secondary battery according to Technology 1, wherein the average depth to which the portion of the spacer penetrates into the positive electrode mixture layer is 0.5 μm or more. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the average depth to which the portion of the spacer penetrates into the positive electrode mixture layer is 5.0 μm or less. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the spacer is formed in a pattern that is not a closed mesh pattern.

[0084] The lithium secondary battery (L) according to this embodiment will be described in more detail with reference to Examples. In these Examples, a plurality of lithium secondary batteries were fabricated and evaluated.

[0085] (Battery A1) A lithium secondary battery A1 was fabricated in the following manner.

[0086] (1) Preparation of Positive Electrode A 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. The lithium-containing transition metal oxide used was a rock salt-type lithium-containing transition metal oxide having a layered structure containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0).

[0087] Next, the obtained 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 laminate including a positive electrode mixture layer. Next, the laminate was rolled using a roller. Finally, the rolled laminate was cut to a predetermined electrode size. In this way, a positive electrode including a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector was fabricated. The thickness Tp of the positive electrode mixture layer in the fabricated positive electrode was 161 μm.

[0088] (2) Formation of Negative Electrode Substrate Foil A rolled lithium metal foil (thickness: 25 μm) was pressure-bonded to both sides of a strip of copper foil (thickness: 12 μm) to form a negative electrode substrate foil.

[0089] (3) Formation of Spacers A strip-shaped porous polyethylene film (thickness: 16 μm) was prepared as a separator. Next, a coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was dispensed onto the separator using a dispenser in the pattern shown in FIG. 6 . The coating liquid was then vacuum-dried. In this way, stripe-shaped spacers as shown in FIG. 6 were formed on one side of the separator. The height of the spacers was 40 μm.

[0090] (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 The concentration of LiBF becomes 1 mol / L.２ (C ２ O ４ ) were dissolved in water so that the concentration of the non-aqueous electrolyte was 0.1 mol / L to prepare a liquid non-aqueous electrolyte.

[0091] (5) Preparation of Electrode Assembly An Al tab was attached to the positive electrode obtained above. A Ni tab was attached to the negative electrode substrate foil. A wound body (electrode assembly) was prepared by spirally winding the positive electrode and negative electrode (negative electrode substrate foil) with a separator interposed therebetween in an inert gas atmosphere. Specifically, first, one end of the electrode plate and the separator were sandwiched between a winding shaft. At this time, the separator was positioned so that the spacer was in contact with the positive electrode. Next, a wound body was formed by winding the winding shaft while applying the tension shown in Table 1 to the separator.

[0092] (6) Fabrication of Battery 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 fabricate Battery A1 (lithium secondary battery).

[0093] (Batteries A2 to A4 and C1) Batteries A2 to A4 and C1 were fabricated in the same manner and under the same conditions as those for the battery A1, except that the tension of the separator during the wound body fabrication was changed as shown in Table 1.

[0094] (1) Measurement of Penetration Depth Dp For Batteries A1 to A4 and Battery C1, the depth Dp at which a portion of the spacer penetrated into the positive electrode mixture layer was measured. Specifically, the wound electrode group was first disassembled and the positive electrode was removed. Next, the penetration depth Dp was measured at five arbitrary locations using a one-shot 3D shape measuring instrument manufactured by Keyence Corporation. Next, the average depth Dpa was calculated by arithmetically averaging the measured depths Dp at the five locations.

[0095] (2) Evaluation of charge-discharge cycle characteristics A charge-discharge test was performed on each of the produced batteries. In the charge-discharge test, the batteries were charged in a thermostatic chamber at 25°C under the following conditions, then rested for 20 minutes, and discharged under the following conditions.

[0096] (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.

[0097] (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.

[0098] (Calculation of Capacity Retention Rate) In the above charge-discharge test, the discharge capacity DC(1) at the first cycle and the discharge capacity DC(200) at the 200th cycle were measured. The capacity retention rate was calculated using the following formula: Capacity retention rate (%) = 100 × DC(200) / DC(1).

[0099] (3) Evaluation of Discharge Characteristics The discharge capacity of each battery at 0.5 C DC ０．５Ｃ and discharge capacity at 0.2C DC ０．２Ｃ Here, 1C is the hourly rate current, which is calculated by 1C (A) = rated capacity (Ah) / 1 (h). Then, the ratio R of these was calculated based on the following formula: Ratio R (%) = 100 × DC ０．５Ｃ / DC ０．２Ｃ

[0100] Some of the battery fabrication conditions and evaluation results are shown in Table 1. A capacity retention rate is preferable. A high ratio R is preferable. A high ratio R indicates good high-rate discharge characteristics.

[0101]

[0102] Batteries A1 to A4 are batteries according to the present disclosure, and Battery C1 is a comparative example. Batteries A1 to A4 had higher capacity retention rates than Battery C1. This is believed to be due to the suppression of breakdown of the coating of the positive electrode mixture layer. As shown in Table 1, by setting the average depth Dpa to 5.0 μm or less, the deterioration of discharge characteristics at high rates was suppressed.

[0103] The present disclosure can be used for lithium secondary batteries. 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 cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0104] 10: Lithium secondary battery 11: Positive electrode 11a: Positive electrode current collector 11b: Positive electrode mixture layer 12: Negative electrode 12a: Negative electrode substrate foil 13: Separator 14: Electrode group 14s: Space 53: Spacer

Claims

1. A lithium secondary battery comprising: a positive electrode, a negative electrode, a separator and a spacer disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte; the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharging; the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, and a portion of the spacer is embedded in the positive electrode mixture layer.

2. The lithium secondary battery according to claim 1, wherein the average depth to which the portion of the spacer is embedded in the positive electrode mixture layer is 0.5 μm or more.

3. The lithium secondary battery according to claim 1 or 2, wherein the average depth to which the portion of the spacer is embedded in the positive electrode mixture layer is 5.0 μm or less.

4. The lithium secondary battery according to claim 1 or 2, wherein the spacers are formed in a pattern other than a closed mesh pattern.

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