Lithium secondary battery

Abstract

This lithium secondary battery includes: a positive electrode (11); a negative electrode (12); a separator (13) disposed between the positive electrode (11) and the negative electrode (12); a non-aqueous electrolyte; and a spacer (50) disposed between the separator (13) and either the positive electrode (11) or the negative electrode (12). At the negative electrode (12), lithium metal precipitates during charging, and the lithium metal is dissolved during discharging. The negative electrode (12) contains a lithium alloy containing magnesium. A protective layer (40) containing a fluorinated polymer is disposed on a surface of the negative electrode (12).

Description

Lithium secondary battery

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

[0002] Lithium ion secondary batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Lithium secondary batteries (lithium metal secondary batteries) are promising non-aqueous electrolyte secondary batteries with capacities exceeding those of lithium ion secondary batteries. In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte as lithium ions during discharge.

[0003] Claim 1 of Patent Document 1 (WO 2022 / 181363) discloses "a lithium secondary battery comprising: a positive electrode; a negative electrode facing the positive electrode; a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte having lithium ion conductivity; and a spacer disposed between the negative electrode and the separator, wherein lithium metal precipitates on the negative electrode during charging and the lithium metal dissolves from the negative electrode during discharging, and the spacer has a network structure formed of a plurality of linear protrusions."

[0004] Claim 1 of Patent Document 2 (JP 2016-527679 A) discloses "a lithium electrode comprising: an electrode composite including a porous metal current collector and lithium metal inserted into pores formed in the metal current collector; and a lithium ion conductive protective film formed by coating at least one surface of the electrode composite."

[0005] International Publication No. 2022 / 181363 Special Publication No. 2016-527679

[0006] Currently, there is a demand for lithium secondary batteries with higher capacity retention rates. One of the objects of the present disclosure is to provide a lithium secondary battery with a high 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, a non-aqueous electrolyte, and a spacer disposed between the separator and either one of the positive electrode or the negative electrode, wherein the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharging, the negative electrode includes a lithium alloy containing magnesium, and a protective layer containing a fluoropolymer is disposed on a surface of the negative electrode.

[0008] According to the present disclosure, a lithium secondary battery with a high 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.

[0009] Fig. 1 is a cross-sectional view schematically showing an example of a lithium secondary battery of Embodiment 1. Fig. 2 is a cross-sectional view schematically showing a part of an example of an electrode group used in the lithium secondary battery of Embodiment 1. Fig. 3 is a top view schematically showing an example of the arrangement of spacers. Fig. 4 is a top view schematically showing an example of the arrangement of spacers.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is 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 of the present disclosure can be implemented. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, 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 greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0011] (Lithium secondary battery) The lithium secondary battery according to this embodiment may be referred to as a "lithium secondary battery (B)" below. The lithium secondary battery (B) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte, and a spacer disposed between the separator and either the positive electrode or the negative electrode. The negative electrode is an electrode from which lithium metal precipitates during charging and from which lithium metal dissolves during discharge. The negative electrode includes a lithium alloy containing magnesium. A protective layer containing a fluorinated polymer is disposed on the surface of the negative electrode.

[0012] Forming a protective layer on the surface of the negative electrode can suppress the dendritic deposition of lithium metal. Therefore, forming the protective layer can suppress the expansion of the electrode group during charging. Furthermore, forming the protective layer suppresses contact between the lithium metal and the non-aqueous electrolyte, thereby suppressing side reactions between the lithium metal and the non-aqueous electrolyte. The suppression of dendritic deposition of lithium metal and the suppression of side reactions between the lithium metal and the non-aqueous electrolyte increases the capacity retention rate.

[0013] However, observations have revealed that the protective layer is prone to fracture due to repeated charge and discharge. Fracture of the protective layer reduces the effectiveness of the protective layer. After further investigation, the present inventors have newly discovered that combining a negative electrode containing a magnesium-containing lithium alloy with a protective layer containing a fluoropolymer significantly increases the capacity retention rate. This disclosure is based on this new finding. The reason why the above configuration significantly increases the capacity retention rate is currently unclear. However, it is possible that the magnesium-containing lithium alloy and the fluoropolymer form a magnesium fluoride layer on the surface of the negative electrode, thereby suppressing fracture of the protective layer.

[0014] (Fluorinated Polymer) The protective layer contains a fluorinated polymer. A fluorinated polymer is a polymer containing fluorine. Examples of fluorinated polymers include polyvinylidene fluoride, polytetrafluoroethylene, a copolymer of vinylidene fluoride and hexafluoropropylene, and a copolymer of vinylidene fluoride and trifluoroethylene. Polyvinylidene fluoride is preferable because it does not dissolve in non-aqueous electrolytes and has relatively high flexibility and breaking strength.

[0015] The number average molecular weight of the fluorinated polymer may be 500,000 or more, or 800,000 or more, or 1,500,000 or less, or 1,000,000 or less. The number average molecular weight of the fluorinated polymer may be in the range of 500,000 to 1,500,000. Such fluorinated polymers are less soluble in non-aqueous electrolytes and have higher breaking strength. Furthermore, such fluorinated polymers are easily processed by dissolving them in a predetermined organic solvent or the like. Commercially available fluorinated polymers may be used as the fluorinated polymer. Alternatively, the fluorinated polymer may be synthesized using a known synthesis method.

[0016] The content of the fluoropolymer in the protective layer may be 50% by mass or more, 80% by mass or more, or 90% by mass or more, but is 100% by mass or less. The protective layer may contain a resin (e.g., a polymer) other than the fluoropolymer. The protective layer may contain a substance other than the fluoropolymer. Examples of the other substance include a resin (e.g., a polymer) other than the fluoropolymer, inorganic particles, and a lithium salt. Examples of the resin include a polyolefin resin, a silicone resin, and an epoxy resin. Examples of the lithium salt include a lithium salt added to a non-aqueous electrolyte.

[0017] The inorganic particles are particles made of an inorganic material (e.g., metal oxide, metal hydroxide, metal composite oxide, metal nitride, metal carbide, metal fluoride, etc.). The mixture of the polymer and inorganic particles in the protective layer tends to increase the lithium ion conductivity of the protective layer. As a result, lithium ions move smoothly between the negative electrode and the non-aqueous electrolyte through the protective layer during charge and discharge. To increase the strength of the protective layer, the density of the inorganic particles is set to 6 g / cm. ３ It may be more than that.

[0018] The density of the inorganic particles may be 3.5 times or more the density of the region of the protective layer occupied by materials other than the inorganic particles. In this case, the inorganic particles are likely to be deposited thinly and densely when the protective layer is formed. As a result, uneven distribution of the inorganic particles due to aggregation is suppressed, and the thickness and strength of the protective layer are likely to be uniform.

[0019] Examples of inorganic materials constituting the inorganic particles include copper oxide, bismuth oxide, tungsten oxide, indium oxide, and silver oxide. The inorganic particles may contain at least one selected from the group consisting of copper oxide particles and bismuth oxide particles. By using these particles, the density of the inorganic particles can be reduced to 6 g / cm. ３ Furthermore, in this case, it is easy to make the density of the inorganic particles 3.5 times or more higher than the density in the region of the protective layer that is occupied by materials other than the inorganic particles.

[0020] The proportion of inorganic particles in the protective layer may be 50% by mass or less, 35% by mass or less, or 20% by mass or more. The proportion may be 5% by mass or more and 20% by mass or less. When the proportion of inorganic particles in the protective layer is within the above range, the flexibility, strength, and lithium ion conductivity of the protective layer are sufficiently ensured.

[0021] The protective layer may contain a lithium salt. When the protective layer contains a lithium salt, the charge / discharge reaction is facilitated at the stage when the battery is first used. The lithium salt is preferably contained in the protective layer at the stage when the protective layer is formed. That is, the protective layer is preferably formed using a material containing the above-mentioned polymer and lithium salt.

[0022] The lithium salt is LiBF ４ , LiPF ６ , LiClO ４ , LiCF ３ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) ２ , Li(S.O. ２ F) ２ , LiPF ３ (CF ２ CF ３ ) ３ , and LiPF ３ (CF ３ ) ３ The compound may include at least one selected from the group consisting of the above, or may be any one selected from the group.

[0023] The method for forming the protective layer is not particularly limited. The protective layer may be formed by the following method. First, a coating liquid for forming the protective layer is prepared. The coating liquid can be prepared by mixing a fluorinated polymer, a liquid medium, and, if necessary, other materials (lithium salt, resin, inorganic particles, etc.). The liquid medium is not particularly limited, and N-methyl-2-pyrrolidone, dimethyl ether, tetrahydrofuran, etc. may be used. Next, the coating liquid is applied to the negative electrode current collector (or negative electrode substrate) and then dried. The application and drying methods are not particularly limited, and known methods may be used. For example, application may be performed using a bar coater, an applicator, a gravure coater, etc. In this manner, the protective layer is formed.

[0024] (Spacer) The lithium secondary battery (B) includes a spacer disposed between either the positive electrode or the negative electrode and the separator. From the viewpoint of improving productivity, the spacer may be made of the same material as the protective layer. When forming the protective layer, the thickness of the protective layer may be partially increased, and the thicker portion may be used as the spacer. That is, the spacer may be integrated with the protective layer. Alternatively, a coating liquid for forming a spacer may be applied linearly onto the protective layer to form linear convex portions (spacers). The coating liquid for forming the protective layer may be used as the coating liquid for forming the spacer.

[0025] The spacers may be formed on the positive electrode, the protective layer, or the separator. The spacers may be disposed between the positive electrode and the separator, or between the protective layer and the separator. The spacers may be composed of linear convex portions and / or dot-shaped convex portions. The spacers may be arranged in a striped pattern, a mesh pattern (e.g., a honeycomb pattern), or other patterns. The height of the spacers may be 10 μm or more and 100 μm or less. The width of the spacers may be 200 μm or more and 2000 μm or less.

[0026] Consider a case where the spacers are arranged in a polygonal shape (for example, a honeycomb shape). In this case, the spacers may have defects connecting the polygonal regions. The defects increase the fluidity of the non-aqueous electrolyte.

[0027] The negative electrode may have a layer containing Mg and F on the surface in contact with the protective layer. It is believed that this layer can particularly suppress breakage of the protective layer. The layer containing Mg and F may be, for example, MgF ２ The layer containing Mg and F can be formed by contacting a negative electrode containing Mg with a protective layer containing a fluorinated polymer.

[0028] From the viewpoint of suppressing contact between lithium metal and the non-aqueous electrolyte and suppressing penetration of dendrites through the protective layer, the average thickness of the protective layer may be 0.1 μm or more and 5 μm or less, or 0.5 μm or more and 2 μm or less. The average thickness of the protective layer can be measured by the following procedure. First, a cross-sectional image of the protective layer is obtained using a scanning electron microscope (SEM). Next, the thickness of any 10 points on the protective layer is measured using the cross-sectional image. The average thickness of the protective layer is determined by arithmetically averaging the thicknesses measured at the 10 points.

[0029] The average height of the spacers may be greater than the average thickness of the separators. This configuration can particularly suppress an increase in internal pressure due to expansion and contraction during charge and discharge. The average height of the spacers is determined by arithmetically averaging the heights of the spacers at 10 arbitrarily selected positions. The average thickness of the separators is determined by arithmetically averaging the thicknesses of the separators at 10 arbitrarily selected positions.

[0030] The spacer may have a non-porous structure that does not allow lithium ions to pass through. This configuration allows lithium to be deposited more uniformly, thereby particularly suppressing an increase in the internal pressure of the battery. The non-porous structure can be achieved by forming the spacer under conditions that do not result in porosity. The method for forming a spacer having a non-porous structure is not particularly limited, and known methods may be used. For example, a spacer having a non-porous structure may be formed by printing the constituent material of the spacer as a coating liquid.

[0031] The area of ​​the spacer may be 3% or more, or 10% or more, or 30% or less, or 20% or less of the area of ​​the separator, where the areas of the spacer and the separator are the areas when viewed in plan.

[0032] The first resin constituting the spacer may have higher heat resistance than the second resin constituting the separator, where higher 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.

[0033] The method for forming the spacers is not particularly limited. The spacers may be formed by the following method. First, a coating liquid containing the spacer material is prepared. The coating liquid can be prepared by dissolving and / or dispersing the spacer material in a liquid medium. The spacer material includes a polymer and may also include other substances (such as inorganic fillers) as necessary. The spacer material includes materials used for separators and protective layers. The liquid medium is not particularly limited, and N-methyl-2-pyrrolidone, dimethyl ether, tetrahydrofuran, etc. may be used. Next, the spacers can be formed by applying the coating liquid in a predetermined pattern and drying it. The method for applying the coating liquid is not particularly limited, and screen printing or application using a dispenser may be used.

[0034] The negative electrode includes a lithium alloy containing magnesium. The magnesium content in the lithium alloy may be 0.1% by mass or more, or 1% by mass or more, and may be 30% by mass or less, or 10% by mass or less. The magnesium content in the lithium alloy may be in the range of 0.1 to 30% by mass. This configuration further enhances the effect of suppressing dendritic precipitation of lithium metal, and also makes it easier to form an SEI (Solid Electrolyte Interface) on the surface of the negative electrode.

[0035] The non-aqueous electrolyte may contain hydrofluoroether. When the non-aqueous electrolyte contains hydrofluoroether, the capacity retention rate is higher than that expected when the negative electrode contains a lithium alloy containing magnesium and when a non-aqueous electrolyte containing hydrofluoroether is used. This is thought to be because the negative electrode containing a lithium alloy containing magnesium interacts with the non-aqueous electrolyte containing hydrofluoroether to form a solid electrolyte interface (SEI) with excellent film quality, thereby synergistically improving the charge-discharge cycle characteristics.

[0036] Hydrofluoroether is an ether compound having a hydrogen atom and a fluorine atom. The nonaqueous solvent constituting the nonaqueous electrolyte contains at least a hydrofluoroether (first solvent). The nonaqueous solvent may contain a nonaqueous solvent other than a hydrofluoroether (second solvent). The hydrofluoroether may be linear or cyclic. The hydrofluoroether may be a compound having an organic group containing an ether bond (—O—) and a fluorine atom, and also containing a hydrogen atom.

[0037] The hydrofluoroether may include at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, or may be any one of them.

[0038] The method for producing the lithium secondary battery (B) is not particularly limited. In one example of the method for producing the lithium secondary battery (B), a protective layer is formed, a magnesium-containing negative electrode is formed, and a spacer is formed. The lithium secondary battery (B) may be formed by combining the method described in this specification with a known method.

[0039] The method for forming the magnesium-containing negative electrode is not particularly limited, and the magnesium-containing negative electrode may be formed by pressing a magnesium-containing lithium alloy foil onto a negative electrode current collector.

[0040] (Examples of Components) Examples of the components of the lithium secondary battery (B) will be specifically described below. Note that the components described below are merely examples, and the components of the lithium secondary battery (B) of this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment.

[0041] The lithium secondary battery (B) may include an electrode group composed of a positive electrode, a negative electrode, and a separator. The electrode group may be a wound type or a laminated type. A wound type electrode group is formed by winding a positive electrode, a negative electrode, and a separator.

[0042] (Negative Electrode) The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode upon charging, becoming lithium metal and depositing on the surface of the negative electrode. The deposited lithium metal 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 or may be supplied from the positive electrode active material. When lithium metal is not deposited on the surface of the negative electrode current collector, a protective layer covers the surface of the negative electrode current collector. When lithium metal is deposited on the surface of the negative electrode current collector, a protective layer covers the surface of the lithium metal.

[0043] A conductive sheet can be used for the negative electrode current collector. The conductive sheet may be a metal foil. The material of the negative electrode current collector (conductive sheet) may be a conductive material other than lithium metal and lithium alloy. The conductive material may be a metal material. The conductive material may be a material that does not react with lithium. The conductive material may be a material that does not form an alloy with lithium or an intermetallic compound with lithium. 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 copper alloys are preferred materials for the negative electrode current collector because of their high conductivity. The negative electrode current collector may be copper foil, copper alloy foil, or stainless steel foil. The thickness of the negative electrode current collector is not particularly limited and may be 5 μm or more and 300 μm or less.

[0044] The negative electrode may include a negative electrode current collector and a lithium alloy layer laminated on both sides of the negative electrode current collector. In this case, a protective layer can be formed on the lithium alloy layer. The lithium alloy layer contains magnesium. By using the lithium alloy layer containing magnesium, a negative electrode containing a lithium alloy containing magnesium can be obtained.

[0045] The lithium alloy layer may contain elements other than lithium and magnesium. Examples of such elements include aluminum, indium, and zinc. Forming a lithium alloy layer can suppress the decrease in discharge capacity due to repeated charge and discharge. Furthermore, forming a lithium alloy layer can suppress the dendritic deposition of lithium metal. The method for forming the lithium alloy layer is not particularly limited, and it may be formed by a known method. For example, the lithium alloy layer may be formed by pressing a lithium alloy foil onto a negative electrode current collector. The lithium alloy layer is dense, which makes it distinguishable from lithium metal (which is generally porous) that precipitates during charging.

[0046] (Positive Electrode) The positive electrode includes a positive electrode mixture layer containing a positive electrode active material. The positive electrode may include a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer may include a positive electrode active material and additives (such as a conductive material, a binder, or a thickener). Depending on the form of the electrode group, the positive electrode mixture layer may be formed on only one side of the positive electrode current collector, or on both sides of the positive electrode current collector.

[0047] 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 an additive is prepared. Next, the positive electrode mixture slurry is applied to a positive electrode current collector and then dried to form a coating film. Next, a laminate consisting of the positive electrode current collector and the coating film is rolled to obtain a positive electrode. The formed positive electrode is then cut to a predetermined size as necessary.

[0048] The positive electrode active material can be 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, transition metal sulfides, etc. Lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0049] 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, W, etc. The lithium-containing transition metal oxide may contain only one transition metal element or may contain two or more transition metal elements. The transition metal element may be at least one element selected from the group consisting of Co, Ni, and Mn. The lithium-containing transition metal oxide may contain one or more typical elements. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, B, etc.

[0050] 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.

[0051] 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.

[0052] The thickener may be a cellulose derivative. Examples of the cellulose derivative include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. Examples of modified forms of CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0053] The positive electrode current collector may be a conductive sheet, such as a metal foil, and the surface of the positive electrode current collector may be coated with a carbon material.

[0054] 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 (e.g., stainless steel), etc. The thickness of the positive electrode current collector is not particularly limited and may be in the range of 5 to 300 μm.

[0055] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of porous sheets include microporous 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 olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives (such as inorganic fillers) as needed.

[0056] The thickness of the separator is not particularly limited and may be 5 μm or more and 20 μm or less (e.g., 10 μm or more and 20 μm or less). The separator may include a substrate and a heat-resistant layer formed on the substrate. The substrate may be a known separator used as a separator for lithium secondary batteries.

[0057] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity can be used as the non-aqueous electrolyte. As described above, the non-aqueous electrolyte may contain hydrofluoroether. 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 is dissolved in the non-aqueous solvent, lithium ions and anions are generated.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] The concentration of the lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or more, 1.0 mol / L or more, or 1.5 mol / L or more, and may be 3.5 mol / L or less, 2.0 mol / L or less, or 1.5 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained.

[0062] The non-aqueous electrolyte may contain an additive (for example, a known additive), such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, or fluorobenzene.

[0063] (Exterior Body) The exterior body accommodates the non-aqueous electrolyte and the electrode group. The exterior body is not particularly limited, and a known exterior body can be used. The shape of the exterior body is selected according to the shape of the lithium secondary battery (B). The shape of the lithium secondary battery (B) is not limited, and may be cylindrical, prismatic, or any other shape. The exterior body may include a cylindrical battery case with a bottom, and a sealing body and a gasket that seal the opening of the battery case.

[0064] An example of the lithium secondary battery (B) 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 (B) according to the present disclosure may be omitted.

[0065] (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, and a separator 13. The electrode group 14 is a wound electrode group formed by winding the positive electrode 11, the negative electrode 12, and the separator 13. The separator 13 is disposed between the positive electrode 11 and the negative electrode 12. The positive electrode 11 is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. The negative electrode 12 is electrically connected to a case body 15, which also serves as a negative electrode terminal, via a negative electrode lead 20.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] A partial cross-sectional view of an example of an electrode group 14 is shown schematically in Figure 2. Figure 2 shows an example of a state in which lithium metal is not deposited on the surface of the negative electrode substrate 32. The negative electrode 12 includes the negative electrode substrate 32. The surface of the negative electrode substrate 32 is covered with a protective layer 40. The negative electrode substrate 32 includes a negative electrode current collector and lithium alloy layers formed on both sides of the negative electrode current collector. The lithium alloy layers contain magnesium.

[0070] As shown in Fig. 2, a spacer 50 is disposed between the protective layer 40 and the separator 13. The spacer 50 is composed of linear protrusions arranged along the longitudinal direction of the separator 13. As shown in Fig. 2, when lithium metal is not deposited on the negative electrode substrate 32, a space 51 is formed between the negative electrode 12 and the separator 13 by the spacer 50. Lithium metal deposited on the negative electrode substrate 32 during charging is accommodated in the space 51 between the negative electrode 12 and the separator 13 while being pressed by the separator 13.

[0071] Because the lithium metal is accommodated in the space 51 between the negative electrode 12 and the separator 13, the apparent volume change of the electrode assembly due to the deposition of lithium metal during charge / discharge cycles is reduced. This also reduces stress applied to the negative electrode substrate 32. Furthermore, pressure is applied from the separator 13 to the lithium metal accommodated between the negative electrode 12 and the separator 13. As a result, the deposited lithium metal is less likely to become isolated, and a decrease in charge / discharge efficiency is suppressed.

[0072] 3 and 4 schematically show examples of patterns of the spacers 50 formed on the separator 13. In the example shown in Fig. 3, the spacers 50 are arranged in a honeycomb pattern. In the example shown in Fig. 4, the spacers 50 are arranged in a stripe pattern.

[0073] (Additional Notes) The above disclosure discloses the following. (Technology 1) A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; and a spacer disposed between the separator and either one of the positive electrode or the negative electrode, wherein the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharge, and the negative electrode comprises a lithium alloy containing magnesium, and a protective layer containing a fluorinated polymer is disposed on the surface of the negative electrode. (Technology 2) The lithium secondary battery according to Technology 1, wherein the fluorinated polymer is polyvinylidene fluoride. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the number-average molecular weight of the fluorinated polymer is in the range of 500,000 to 1,500,000. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the negative electrode has a layer containing Mg and F on the surface in contact with the protective layer. (Technology 5) The lithium secondary battery according to any one of Technologies 1 to 4, wherein the average thickness of the protective layer is 0.1 μm or more and 5 μm or less. (Technology 6) The lithium secondary battery according to any one of Technologies 1 to 5, wherein the average height of the spacer is greater than the average thickness of the separator. (Technology 7) The lithium secondary battery according to any one of Technologies 1 to 6, wherein the spacer includes a non-porous structure that is impermeable to lithium ions. (Technology 8) The lithium secondary battery according to any one of Technologies 1 to 7, wherein the area of ​​the spacer is 30% or less of the area of ​​the separator. (Technology 9) The lithium secondary battery according to any one of Technologies 1 to 8, wherein the first resin constituting the spacer has higher heat resistance than the second resin constituting the separator. (Technology 10) The lithium secondary battery according to any one of Technologies 1 to 9, wherein the magnesium content in the lithium alloy is in the range of 0.1 to 30 mass%. (Technology 11) The lithium secondary battery according to any one of Techniques 1 to 10, wherein the nonaqueous electrolyte contains a hydrofluoroether.(Technology 12) The lithium secondary battery according to Technology 11, wherein the hydrofluoroether includes at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0074] The lithium secondary battery according to the present disclosure will be described in more detail based on examples. However, the present disclosure is not limited to the following examples. In these examples, a plurality of lithium secondary batteries with different protective layers were fabricated and evaluated.

[0075] (Battery A1) Battery A1 was fabricated using the following procedure. (1) Fabrication of Positive Electrode A positive electrode active material, acetylene black (AB, conductive material), and polyvinylidene fluoride (PVdF, binder) were mixed in a mass ratio of positive electrode active material:AB:PVdF=95:2.5:2.5, and N-methyl-2-pyrrolidone (dispersion medium) was added and stirred to prepare a positive electrode mixture slurry. The positive electrode active material used was a layered rock salt lithium-containing transition metal oxide containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0).

[0076] The obtained positive electrode mixture slurry was applied to both sides of an Al foil (positive electrode current collector), dried, and the coating of the positive electrode mixture was rolled using a roller. Finally, the obtained laminate of the positive electrode current collector and the positive electrode mixture 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 formed.

[0077] (2) Preparation of a negative electrode with a protective layer formed on its surface: Lithium alloy foil (thickness: 25 μm) was attached to both sides of a negative electrode current collector. A strip of electrolytic copper foil (thickness: 15 μm) was used as the negative electrode current collector. The lithium alloy foil was a lithium-magnesium alloy foil with a magnesium content of 1 mass%.

[0078] Next, a coating liquid for forming a protective layer was prepared by mixing a fluorinated polymer, a lithium salt, and N-methyl-2-pyrrolidone (dispersion medium). Polyvinylidene fluoride (PVdF) was used as the fluorinated polymer. Lithium bis(fluorosulfonyl)imide was used as the lithium salt. The coating liquid was applied to both sides of the negative electrode and dried to form a protective layer (thickness: 2 μm).

[0079] (3) Spacer Formation The spacers were formed using the following procedure. 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 a second layer and a first layer in this order on the substrate. 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. The mixture was heated to approximately 80°C to completely dissolve the calcium chloride. The solution was then returned to room temperature, and 2200 g was collected. 0.6 mol of paraphenylenediamine (PPD) was then added to completely dissolve the calcium chloride. While maintaining the 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 polymerization solution. Next, 100 g of this polymerization solution was mixed with an N-methyl-2-pyrrolidone solution containing 5.8% by mass of calcium chloride dissolved therein, thereby obtaining a solution (coating solution) containing 2% by mass of paraphenylene terephthalamide (PPTA), which is an aromatic polyamide (aramid).

[0080] 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.

[0081] The first layer was formed as follows: First, lithium phosphate (Li ３ P.O. ４Particles 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 liquid) 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 this manner, a separator was obtained, including a substrate and heat-resistant layers (first and second layers) formed on the substrate.

[0082] Next, a coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was dispensed onto the heat-resistant layer using a dispenser in a substantially honeycomb pattern. The coating liquid was then vacuum-dried. In this manner, a non-porous spacer having a substantially honeycomb pattern was formed.

[0083] 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.

[0084] (4) Preparation of non-aqueous electrolyte: A non-aqueous solvent was prepared by mixing 1,2-dimethoxyethane (DME) and hydrofluoroether (HFE) in a volume ratio of DME:HFE = 30:70. 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether was used as the hydrofluoroether. LiPF ６ The concentration of LiBF becomes 1 mol / L. ２ (C ２ O ４ ) to a concentration of 0.1 mol / L. ６ and LiBF ２ (C ２ O ４ ) was dissolved in a non-aqueous solvent to prepare a non-aqueous electrolyte (non-aqueous electrolytic solution).

[0085] (5) Battery Assembly One end of an aluminum positive electrode lead was attached to the positive electrode current collector by welding. One end of a nickel negative electrode lead was attached to the negative electrode obtained above by welding. An electrode group was produced by winding the positive electrode, negative electrode, and separator in an inert gas atmosphere so that the separator was disposed between the positive electrode and negative electrode. A polyethylene microporous membrane was used as the separator.

[0086] The electrode group was housed in a bag-shaped exterior body. At this time, the end of the positive electrode lead and the end of the negative electrode lead were exposed to the outside of the exterior body. After the nonaqueous electrolyte was injected into the exterior body, the opening of the exterior body was sealed. In this way, Battery A1 (lithium secondary battery) was produced.

[0087] (Evaluation) A charge-discharge cycle test was performed on Battery A1. In the charge-discharge cycle test, the battery A1 was charged in a thermostatic chamber at 25° C. under the following conditions, followed by a 20-minute break and then discharged under the following conditions, and this cycle was repeated 500 times.

[0088] (Charging) The battery was charged at a constant current of 10 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value per unit area of ​​the electrode reached 1 mA.

[0089] (Discharge) The battery was discharged at a constant current of 10 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 3 V.

[0090] The discharge capacity C1 at the first cycle and the discharge capacity C2 after a predetermined number of cycles were recorded. Then, the capacity retention rate after a predetermined number of cycles was calculated based on the following formula: Capacity retention rate (%) = (C2 / C1) x 100

[0091] (Batteries C1 to C6) Batteries C1 to C6 were fabricated using the same method and conditions as Battery A1, except that the presence or absence of a protective layer, the type of metal foil attached to the negative electrode current collector, and the presence or absence of a spacer were changed as shown in Table 1. The protective layer and spacer were the same as those used in Battery A1. The negative electrodes of Batteries C2, C4, and C5 were formed with lithium foil (thickness: 25 μm) attached to both sides of the negative electrode current collector. The protective layers of Batteries C2 and C4 were formed on the surface of the negative electrode using the same method and conditions as those used for Battery A1. For Battery C5, no protective layer was formed on the surface of the negative electrode. The capacity retention rates of the fabricated Batteries C1 to C6 were measured in the same manner as for Battery A1.

[0092] Some of the manufacturing conditions and the evaluation results are shown in Table 1. In Table 1, "negative electrode metal foil" refers to the metal foil attached to the negative electrode current collector. A high capacity retention rate is preferable. Because the capacity retention rates of batteries C1, C4, and C6 after 100 cycles were low, their capacity retention rates after 500 cycles were not evaluated.

[0093]

[0094] Battery A1 is the lithium secondary battery (B) according to the present disclosure. Batteries C1 to C6 are comparative examples. As shown in Table 1, Battery A1 had a significantly higher capacity retention rate than Batteries C1 to C6.

[0095] 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.

[0096] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 40: Protective layer 50: Spacer 51: Space

Claims

1. A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; and a spacer disposed between the separator and either the positive electrode or the negative electrode; wherein the negative electrode is an electrode from which lithium metal deposits during charging and from which lithium metal dissolves during discharging; the negative electrode comprises a lithium alloy containing magnesium; and a protective layer containing a fluoropolymer is disposed on a surface of the negative electrode.

2. The lithium secondary battery according to claim 1, wherein the fluorinated polymer is polyvinylidene fluoride.

3. The lithium secondary battery according to claim 1, wherein the number average molecular weight of the fluorinated polymer is in the range of 500,000 to 1,500,000.

4. The lithium secondary battery according to claim 1, wherein the negative electrode has a layer containing Mg and F on the surface in contact with the protective layer.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein the average thickness of the protective layer is 0.1 μm or more and 5 μm or less.

6. The lithium secondary battery according to any one of claims 1 to 4, wherein the average height of the spacers is greater than the average thickness of the separator.

7. The lithium secondary battery according to any one of claims 1 to 4, wherein the spacer includes a non-porous structure that is impermeable to lithium ions.

8. The lithium secondary battery according to any one of claims 1 to 4, wherein the area of ​​the spacer is 30% or less of the area of ​​the separator.

9. The lithium secondary battery according to any one of claims 1 to 4, wherein the first resin constituting the spacer has higher heat resistance than the second resin constituting the separator.

10. The lithium secondary battery according to any one of claims 1 to 4, wherein the magnesium content in the lithium alloy is in the range of 0.1 to 30 mass %.

11. The lithium secondary battery according to any one of claims 1 to 4, wherein the non-aqueous electrolyte contains a hydrofluoroether.

12. The lithium secondary battery according to claim 11, wherein the hydrofluoroether includes at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

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