Lithium secondary battery

Abstract

This lithium secondary battery includes: a positive electrode (11); a negative electrode (12); and a nonaqueous electrolyte. At the negative electrode (12), lithium metal precipitates during charging, and the lithium metal is dissolved during discharging. A protective layer (40) is disposed on a surface of the negative electrode (12). The protective layer (40) contains a fluorinated polymer and a block copolymer. The block copolymer includes a first polymer portion having a monomer unit A repeating structure, and a second polymer portion having a monomer unit B repeating structure. The fluorinated polymer and the block copolymer are present in the protective layer (40) as separate molecules.

Description

Lithium secondary battery

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

[0002] Lithium ion 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 even higher than lithium ion 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 / 163538) discloses "a lithium secondary battery comprising: a positive electrode that absorbs lithium ions during discharge and releases the lithium ions during charge; a negative electrode from which lithium metal precipitates during charge and from which the lithium metal dissolves during discharge; and a non-aqueous electrolyte having lithium ion conductivity, wherein the surface of the negative electrode is covered with a protective layer, and the protective layer includes a block polymer in which a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B are bonded."

[0004] Claim 1 of Patent Document 2 (U.S. Patent Application Publication No. 2016 / 0329567) discloses "a negative electrode for a lithium battery comprising lithium metal and a protective layer disposed on at least a portion of the lithium metal, the protective layer comprising a block copolymer comprising a structural domain and a hard domain covalently bonded to the structural domain, the structural domain comprising a structural block of the block copolymer, the hard domain comprising a hard block of the block copolymer, the structural domain comprising a plurality of structural repeat units, and the hard block comprising a plurality of olefin repeat units."

[0005] Claim 1 of Patent Document 3 (JP 2017-199678 A) describes a method for manufacturing a lithium metal electrode comprising: ６The document discloses "a negative electrode for a lithium metal battery, comprising one or more particles selected from organic particles, inorganic particles, and organic-inorganic particles, each having a viscosity of 1 Pa or more and a size of more than 1 μm and not more than 100 μm."

[0006] International Publication No. 2022 / 163538 U.S. Patent Application Publication No. 2016 / 0329567 JP 2017-199678 A

[0007] 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.

[0008] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharging, and a protective layer is disposed on a surface of the negative electrode, the protective layer including a fluorinated polymer and a block copolymer, the block copolymer including a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B, and the fluorinated polymer and the block copolymer are present in the protective layer as separate molecules.

[0009] 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.

[0010] 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 cross-sectional view schematically showing a part of another example of an electrode group used in the lithium secondary battery of Embodiment 1.

[0011] 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.

[0012] (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, and a non-aqueous electrolyte. The negative electrode is an electrode on which lithium metal deposits during charging and dissolves during discharging. A protective layer is disposed on the surface of the negative electrode. The protective layer includes a fluorinated polymer and a block copolymer. The block copolymer includes a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B. The fluorinated polymer and the block copolymer exist as separate molecules in the protective layer. In other words, the fluorinated polymer and the block copolymer exist as independent molecules in the protective layer.

[0013] 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.

[0014] However, observations have revealed that protective layers formed solely from fluorinated polymers are prone to fracture due to repeated charge / discharge cycles. Fracture of the protective layer reduces the effectiveness of the protective layer. After further investigation, the present inventors have newly discovered that the capacity retention rate can be significantly increased by using a protective layer containing a fluorinated polymer and a block copolymer as separate molecules. This disclosure is based on this new finding. The reason why the capacity retention rate is significantly increased by using a protective layer containing a fluorinated polymer and a block copolymer is currently unclear. However, because it is easy to increase the shear strength of block copolymers, it is possible to suppress fracture of the protective layer by using a fluorinated polymer and a block copolymer in combination. This is thought to be the result of the significant increase in capacity retention rate.

[0015] The term "monomer unit A (or B)" means a unit formed in a polymer by polymerized monomer A (or B).

[0016] By using a block copolymer in the protective layer, the functions of the first polymer portion and the second polymer portion can be compatible. Compared to when a protective layer is formed using a mixture of a homopolymer corresponding to the first polymer portion and a homopolymer corresponding to the second polymer portion, when a block copolymer is used, the disadvantages of the first polymer portion and the second polymer portion cancel each other out. Therefore, by using a block copolymer, it is possible to obtain high effects.

[0017] The polymer forming the protective layer preferably has high lithium ion conductivity. However, polymers with high lithium ion conductivity may have a high affinity with the electrolyte solution and be easily dissolved in the electrolyte solution. Furthermore, using a polymer with high lithium ion conductivity tends to reduce the strength of the protective layer. A low protective layer strength may cause lithium metal dendrites to penetrate the protective layer or cause the protective layer to break due to expansion and contraction of the negative electrode during charge and discharge. As a result, the discharge capacity decreases with an increase in the number of charge and discharge cycles. To suppress such a decrease in discharge capacity, a block copolymer containing a first polymer portion with high mechanical strength and a second polymer portion with high lithium ion conductivity may be used. The use of such a block copolymer makes it possible to achieve both high mechanical strength and high lithium ion conductivity.

[0018] Block copolymers can form a secondary structure in which they aggregate with each other. In this case, the secondary structure can be formed so that portions having the same monomer units are located close to each other within the block copolymer. That is, the secondary structure of the block copolymer can be formed so that first polymer portions of multiple block copolymers aggregate and second polymer portions of multiple block copolymers aggregate. For example, it is possible to control the secondary structure of the block copolymer so that the first polymer portion with high strength is located on the surface side of the protective layer and the second polymer portion with excellent lithium ion conductivity is located inside the protective layer. The secondary structure of an ABA triblock copolymer can be easily controlled by controlling the types of monomer units in the first and second polymer portions and the degree of polymerization (number average molecular weight) of the first and second polymer portions.

[0019] In the protective layer, the ratio Mf / Ms of the mass of the fluorinated polymer Mf to the total mass Ms of the fluorinated polymer and the block copolymer may be 0.20 or more, or 0.50 or more, or 0.95 or less, or 0.80 or less. By setting the ratio Mf / Ms to 0.20 or more (e.g., 0.50 or more), the formation of a LiF layer due to contact between the fluorinated polymer and Li is more likely to occur, and a decrease in capacity retention due to a side reaction with the non-aqueous electrolyte can be particularly suppressed. By setting the ratio Mf / Ms to 0.95 or less (e.g., 0.80 or less), the strength of the protective layer against shear can be particularly increased. As a result, a decrease in capacity retention due to breakage of the protective layer can be particularly suppressed.

[0020] The total proportion of the fluorinated polymer and the block copolymer in the protective layer may be 50% by mass or more, 80% by mass or more, or 90% by mass or more, and may be 100% by mass or less, 90% by mass or less, or 80% by mass or less.

[0021] The negative electrode may include a layer containing LiF (lithium fluoride) on the surface in contact with the protective layer. This configuration suppresses side reactions between the negative electrode and the non-aqueous electrolyte, improving the capacity retention rate. By forming a protective layer containing a fluorinated polymer, it is possible to form a LiF layer.

[0022] (Fluorinated Polymer) The protective layer includes a fluoropolymer. A fluoropolymer is a polymer containing fluorine. The fluoropolymer may be a polymer that is not a block copolymer. Polymers that are not block copolymers include homopolymers and random copolymers. Examples of fluoropolymers include polyvinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride and hexafluoropropylene, and copolymers of vinylidene fluoride and trifluoroethylene.

[0023] The number average molecular weight of the fluoropolymer may be 500,000 or more, or 800,000 or more, or may be 1,500,000 or less, or 1,000,000 or less. The fluoropolymer may be a commercially available fluoropolymer. Alternatively, the fluoropolymer may be synthesized using a known synthesis method.

[0024] (Block Copolymer) The protective layer includes a block copolymer. The block copolymer may be a polymer that does not contain fluorine. By using a fluorinated polymer and a fluorinated block copolymer in combination, the fluorinated polymer and the block copolymer are easily separated within the protective layer. As a result, the block copolymer is easily disposed on the surface side of the protective layer, and the fluorinated polymer is easily disposed on the negative electrode side of the protective layer. This configuration makes it easier to obtain the effect of improving mechanical strength by the block copolymer and the effect of forming a LiF layer by the fluorinated polymer.

[0025] The form of the block copolymer is not limited and may be AB type or ABA type. The block copolymer may be an ABA type triblock copolymer in which a first polymer portion (e.g., a linear first polymer portion) is bonded to both ends of a second polymer portion (e.g., a linear second polymer portion). A commercially available block copolymer may be used as the block copolymer. Alternatively, the block copolymer may be synthesized using a known synthesis method.

[0026] The first polymer portion of the block copolymer may be a polymer having high mechanical strength (e.g., tensile strength). The tensile strength of the first polymer portion may be higher than the tensile strength of the second polymer portion. This configuration can prevent a decrease in the mechanical strength of the protective layer even when a second polymer portion having low mechanical strength or a second polymer portion that is easily dissolved in an electrolyte is used.

[0027] The swelling degree of the second polymer portion in the non-aqueous electrolyte may be higher than the swelling degree of the first polymer portion in the non-aqueous electrolyte. By using a second polymer portion with a high swelling degree, a protective layer with high lithium ion conductivity can be formed. The strength of the protective layer tends to decrease as the swelling degree increases, but the strength of the protective layer can be maintained by using a block copolymer containing the first polymer portion and the second polymer portion. Furthermore, even when a second polymer portion made of a polymer that dissolves in the non-aqueous electrolyte alone is used, it can be used in a protective layer by combining the first polymer portion and the second polymer portion.

[0028] The tensile strength and swelling degree of the first and second polymer portions can be determined by the following method. First, the lithium secondary battery is disassembled and the protective layer is removed. Next, the structure of the block copolymer contained in the protective layer is analyzed. By analyzing the structure of the block copolymer, the structures of the first polymer portion and the second polymer portion contained in the block copolymer can be identified. Specifically, the types of monomer units A and B (or the types of monomers A and B) and the molecular weight of the polymer portion can be identified. Next, if necessary, a first polymer that is a homopolymer of the specified monomer A and a second polymer that is a homopolymer of the specified monomer B are synthesized. Then, the tensile strength and swelling degree (swelling degree in a non-aqueous electrolyte) of the first and second polymer portions are measured. In this manner, the tensile strength and swelling degree of the first and second polymer portions can be determined.

[0029] If the tensile strength is listed in a publicly known database, that value can be used. If the tensile strength is not listed in a publicly known database, the tensile strength can be determined in accordance with JIS (Japanese Industrial Standards) K 7161 by forming a sample of a predetermined shape.

[0030] The degree of swelling of a polymer can be determined by the following method. First, a lithium secondary battery is disassembled to remove the nonaqueous electrolyte. Next, the components of the nonaqueous electrolyte are analyzed. Next, a nonaqueous electrolyte with the same components as those obtained by the analysis is prepared, and polymers (first polymer and second polymer) are immersed in the nonaqueous electrolyte in an environment of 40°C. Then, the mass of the polymer before immersion in the nonaqueous electrolyte and the mass of the polymer after immersion in the nonaqueous electrolyte are measured. The rate of increase in the mass of the polymer due to immersion in the nonaqueous electrolyte is defined as the degree of swelling of the polymer.

[0031] Whether a polymer is a copolymer or not can be determined by an end group quantification method using GPC (gel permeation chromatography)-NMR. When a polymer is a block copolymer, its structure (e.g., whether it is a triblock copolymer or a random copolymer) can also be determined by an end group quantification method using GPC (gel permeation chromatography)-NMR.

[0032] The monomer unit A may have a hydrocarbon skeleton. The monomer unit A may be any one of a styrene unit, an ethylene unit, a butylene unit, and a propylene unit. The first polymer portion may be any one of polystyrene, polyethylene, polybutylene, and polypropylene. Polystyrene is preferable because it is stable against non-aqueous electrolytes, has sufficient mechanical strength, and is inexpensive. That is, a preferred example of the monomer A is styrene. Note that 90 mol % or more (e.g., 95 mol % or more) of the monomer units constituting the first polymer portion may be the monomer unit A. That is, 10 mol % or less (e.g., 5 mol % or less) of the monomer units constituting the first polymer portion may be a monomer unit other than the monomer unit A (e.g., the monomer unit B).

[0033] The monomer unit B may contain oxygen. The monomer unit B may be any one of a methyl methacrylate unit, an ethyl methacrylate unit, a propyl methacrylate unit, an ethylene glycol unit, a propylene glycol unit, a butylene glycol unit, and a methacrylic acid unit. The monomer B may be a monomer that becomes these units. The second polymer portion may be a polymer obtained by homopolymerizing the monomer B. The monomer B is preferably methyl methacrylate. That is, the second polymer portion is preferably polymethyl methacrylate (PMMA). Note that 90 mol % or more (e.g., 95 mol % or more) of the monomer units constituting the second polymer portion may be the monomer unit B. That is, 10 mol % or less (e.g., 5 mol % or less) of the monomer units constituting the second polymer portion may be a monomer unit other than the monomer unit B (e.g., the monomer unit A).

[0034] The block copolymer may satisfy the following conditions (1) and / or (2): (1) Monomer unit A is any one of a styrene unit, an ethylene unit, a butylene unit, and a propylene unit; and (2) Monomer unit B is any one of a methyl methacrylate unit, an ethyl methacrylate unit, a propyl methacrylate unit, an ethylene glycol unit, a propylene glycol unit, a butylene glycol unit, and a methacrylic acid unit.

[0035] The block copolymer may satisfy the following condition (1a) and / or (2a): Furthermore, the block copolymer may satisfy the following condition (1c): (1a) Monomer unit A is a styrene unit; (2a) Monomer unit B is a methyl methacrylate unit; (1c) The block copolymer is an ABA triblock copolymer in which second polymer segments are bonded to both ends of a first polymer segment.

[0036] The number average molecular weight of the first polymer portion may be smaller than the number average molecular weight of the second polymer portion. This configuration facilitates preparation of a coating liquid for forming the protective layer.

[0037] The block copolymer may satisfy at least one of the following conditions (3) to (5). For example, the block copolymer may satisfy condition (3) and further satisfy conditions (4) and / or (5). (3) The number average molecular weight of the first polymer portion is smaller than the number average molecular weight of the second polymer portion. (4) The number average molecular weight of the first polymer portion is 1,000 or more and 100,000 or less. (5) The number average molecular weight of the second polymer portion is 10,000 or more and 500,000 or less.

[0038] When conditions (3) and (4) are satisfied, a protective layer with good flexibility is easily obtained. A highly flexible protective layer easily conforms to the negative electrode when the negative electrode is wound during the battery manufacturing process. Therefore, adhesion between the negative electrode and the protective layer is easily maintained. The number average molecular weight of the first polymer portion may be 1,000 or more, or 10,000 or more, and may be 100,000 or less, or 50,000 or less. The number average molecular weight of the second polymer portion may be 10,000 or more, or 100,000 or more, and may be 500,000 or less, or 200,000 or less.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] The protective layer may contain materials other than fluorinated polymers and block copolymers. Examples of such materials include resins other than fluorinated polymers and block copolymers, and inorganic particles. Examples of such resins include polyolefin resins, silicone resins, and epoxy resins.

[0043] 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.

[0044] 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.

[0045] 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 kind selected from the group consisting of copper oxide particles and bismuth oxide particles. In this case, the density of the inorganic particles is set 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.

[0046] When the protective layer contains a resin material other than a fluorinated polymer and a block copolymer, the proportion of the block copolymer in the total of the fluorinated polymer, the block copolymer, and the resin material may be 80% or more (e.g., 90% or more, or 95% or more) by mass.

[0047] 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.

[0048] 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 block copolymer, a liquid component, and, as necessary, other materials (lithium salt, resin, inorganic particles, etc.). The liquid component 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.

[0049] The fluorinated polymer and the block copolymer may be present in the protective layer in the form of particles. However, if the fluorinated polymer and the block copolymer are present in the protective layer in the form of particles, their respective properties will not be fully exhibited. Therefore, it is preferable that the fluorinated polymer and the block copolymer are not present in the protective layer in the form of particles. In other words, it is preferable that the fluorinated polymer and the block copolymer are present in the protective layer in a non-particulate state. Therefore, it is preferable to form the protective layer using a coating liquid containing the fluorinated polymer and the block copolymer in a non-particulate state.

[0050] A space in which lithium metal is deposited is preferably formed between the negative electrode and the positive electrode. The space can be formed by disposing a spacer between the negative electrode and the positive electrode. 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 spacers 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 spacers.

[0051] The spacer may be formed on the positive electrode, the protective layer, or the separator. The spacer may be disposed between the positive electrode and the separator, or between the protective layer and the separator. The spacer may be composed of linear convex portions and / or dot-shaped convex portions. The linear convex portions may be arranged in a stripe pattern or a mesh pattern (e.g., a honeycomb pattern). The height of the spacer may be 10 μm or more and 100 μm or less. The width of the spacer may be 200 μm or more and 2000 μm or less.

[0052] Except for the formation of the protective layer, the method for producing the lithium secondary battery (B) is not particularly limited. The lithium secondary battery (B) may be produced by combining the method described in this specification with a known method.

[0053] (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.

[0054] 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.

[0055] (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.

[0056] 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 alloys. 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), alloys containing these metal elements, and graphite with a preferentially exposed basal surface. Examples of alloys include copper alloys and stainless steel (SUS). Copper and 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.

[0057] The negative electrode may include a negative electrode current collector and a lithium-containing metal layer laminated on both sides of the negative electrode current collector. In this case, a protective layer can be formed on the lithium-containing metal layer. The lithium-containing metal layer laminated on the negative electrode current collector is a lithium metal layer or a lithium alloy layer. The lithium alloy layer contains trace amounts of elements other than lithium (10 atomic % or less). Examples of elements other than lithium contained in the lithium alloy include aluminum, magnesium, indium, and zinc. Forming a lithium-containing metal layer can suppress the decrease in discharge capacity due to repeated charge and discharge. Furthermore, forming a lithium-containing metal layer can suppress the dendritic deposition of lithium metal. The method for forming the lithium-containing metal layer is not particularly limited, and it may be formed by a known method. For example, the lithium-containing metal layer may be formed by pressing a lithium metal foil or a lithium alloy foil to the negative electrode current collector. The lithium-containing metal layer is dense, which distinguishes it from lithium metal (which is generally porous) that precipitates during charging.

[0058] (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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] (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.

[0068] The thickness of the separator is not particularly limited, and may be 5 μm or more and 20 μm or less (for example, 10 μm or more and 20 μm or less).

[0069] (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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] The non-aqueous electrolyte may contain an additive (for example, a known additive), such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, or fluorobenzene.

[0075] (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.

[0076] 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.

[0077] (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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] A partial cross-sectional view of an example of an electrode group 14 is shown schematically in Fig. 2. Fig. 2 shows an example of a state in which lithium metal is not deposited on the surface of the negative electrode current collector. The negative electrode 12 includes a negative electrode current collector 32. The surface of the negative electrode current collector 32 is covered with a protective layer 40. The protective layer 40 includes a fluorinated polymer and a block copolymer.

[0082] A partial cross-sectional view of an example of an electrode group 14 is shown schematically in Figure 3. Figure 3 shows an example of a state in which lithium metal is not deposited on the surface of the negative electrode current collector. The example electrode group 14 shown in Figure 3 differs from the electrode group 14 shown in Figure 2 in that it includes a spacer 50. Duplicate explanations of matters explained in Figure 2 may be omitted.

[0083] The surface of the negative electrode current collector 32 (negative electrode 12) is covered with a protective layer 40. The protective layer 40 contains a fluorinated polymer and a block copolymer.

[0084] As shown in Fig. 3, 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. 3, when lithium metal is not deposited on the negative electrode current collector 32, a space 51 is formed between the negative electrode 12 and the separator 13 by the spacer 50. The lithium metal deposited on the negative electrode current collector 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.

[0085] 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 current collector 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.

[0086] (Additional Notes) The above description discloses the following technologies. (Technology 1) A lithium secondary battery comprising: a positive electrode; a negative electrode; and a non-aqueous electrolyte, wherein the negative electrode is an electrode from which lithium metal precipitates during charging and from which the lithium metal dissolves during discharge, and a protective layer is disposed on a surface of the negative electrode, and the protective layer comprises a fluoropolymer and a block copolymer, and the block copolymer comprises a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B, and the fluoropolymer and the block copolymer are present in the protective layer as separate molecules. (Technology 2) The lithium secondary battery according to Technology 1, wherein in the protective layer, a ratio Mf / Ms of the mass of the fluoropolymer Mf to the sum Ms of the mass of the fluoropolymer and the mass of the block copolymer is in the range of 0.20 to 0.95. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the negative electrode includes a layer containing LiF on a surface in contact with the protective layer. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the fluorinated polymer is polyvinylidene fluoride. (Technology 5) The lithium secondary battery according to any one of Technology 1 to 4, wherein the block copolymer does not contain fluorine. (Technology 6) The lithium secondary battery according to any one of Technology 1 to 5, wherein the block copolymer is an ABA triblock copolymer in which the first polymer portion is bonded to both ends of the second polymer portion. (Technology 7) The lithium secondary battery according to any one of Technology 1 to 6, wherein the monomer unit A is any one of a styrene unit, an ethylene unit, a butylene unit, and a propylene unit, and the monomer unit B is any one of a methyl methacrylate unit, an ethyl methacrylate unit, a propyl methacrylate unit, an ethylene glycol unit, a propylene glycol unit, a butylene glycol unit, and a methacrylic acid unit. (Technology 8) The lithium secondary battery according to Technology 7, wherein the monomer unit A is a styrene unit.(Technology 9) The lithium secondary battery according to Technology 7 or 8, wherein the monomer unit B is a methyl methacrylate unit. (Technology 10) The lithium secondary battery according to any one of Technology 1 to 9, wherein the number average molecular weight of the first polymer portion is smaller than the number average molecular weight of the second polymer portion. (Technology 11) The lithium secondary battery according to Technology 10, wherein the number average molecular weight of the first polymer portion is 1,000 or more and 100,000 or less, and the number average molecular weight of the second polymer portion is 10,000 or more and 500,000 or less. (Technology 12) The lithium secondary battery according to any one of Technology 1 to 11, wherein the average thickness of the protective layer is 0.1 μm or more and 5 μm or less. (Technology 13) The lithium secondary battery according to any one of Technology 1 to 12, wherein the protective layer contains a lithium salt. (Technology 14) 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 lithium secondary battery according to claim 13, comprising at least one selected from the group consisting of:

[0087] The lithium secondary battery according to the present disclosure will be described in more detail below based on examples. However, the present disclosure is not limited to the following examples. In these examples, multiple lithium secondary batteries with different protective layers were fabricated and evaluated.

[0088] (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).

[0089] 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.

[0090] (2) Preparation of a Negative Electrode with a Protective Layer Formed on the Surface Li foil (thickness: 25 μm) was attached to both sides of the negative electrode current collector. A strip of electrolytic copper foil (thickness: 15 μm) was used as the negative electrode current collector. Next, a coating solution for forming a protective layer was prepared by mixing a fluorinated polymer, a block copolymer, a lithium salt, and N-methyl-2-pyrrolidone (dispersion medium). Polyvinylidene fluoride (PVdF) was used as the fluorinated polymer. The ratio Mf / Ms of the mass of the fluorinated polymer (Mf) to the total mass Ms of the fluorinated polymer and the block copolymer was set to the value shown in Table 1. The amount of lithium salt added was 1 / 10 of the sum of Mf and Ms. The block copolymer used was an ABA triblock copolymer (PS-PMMA-PS) in which the first polymer moiety, polystyrene (PS, number-average molecular weight: 9,000), was bonded to both ends of the second polymer moiety, polymethyl methacrylate (PMMA, number-average molecular weight: 19,000). The lithium salt used was lithium bis(fluorosulfonyl)imide. The coating solution was applied to both sides of the negative electrode and dried to form a protective layer (thickness: 2 μm).

[0091] (3) Preparation of non-aqueous electrolyte: Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:DMC = 30:70 to prepare a non-aqueous electrolyte. 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).

[0092] (4) 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.

[0093] 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.

[0094] (Evaluation) A charge-discharge cycle test was conducted 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 300 times.

[0095] (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 4.3 V, and then charged at a constant voltage of 4.3 V until the current value per unit area of ​​the electrode reached 1 mA.

[0096] (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.

[0097] The discharge capacity C1 at the first cycle and the discharge capacity C2 at the 300th cycle were recorded. Then, the capacity retention rate at the 300th cycle was calculated based on the following formula: Capacity retention rate (%) = (C2 / C1) x 100

[0098] (Batteries A2, A3, C2, and C3) Batteries A2, A3, C2, and C3 were produced by the same method and under the same conditions as those for producing Battery A1, except that when forming the protective layer, the ratio Mf / Ms of the mass of the fluoropolymer Mf to the total mass Ms of the fluoropolymer and the block copolymer was changed as shown in Table 1. No block copolymer was added to the protective layer of Battery C2. No fluoropolymer was added to the protective layer of Battery C3.

[0099] (Battery C1) Battery C1 was produced in the same manner and under the same conditions as those for producing battery A1, except that no protective layer was formed.

[0100] The capacity retention rates of the fabricated batteries A2 to A3 and C1 to C3 were measured in the same manner as for battery A1. Some of the manufacturing conditions and the evaluation results are shown in Table 1. In Table 1, Mf / Ms indicates the ratio of the mass of the fluorinated polymer Mf to the total mass Ms of the fluorinated polymer and the block copolymer. A high capacity retention rate is preferable.

[0101]

[0102] Batteries A1 to A3 are lithium secondary batteries (B) according to the present disclosure. Batteries C1 to C3 are comparative examples. As shown in Table 1, Batteries A1 to A3 had significantly higher capacity retention rates than Batteries C1 to C3. Although the present invention has been described with respect to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and alterations within the true spirit and scope of the present invention.

[0103] The present disclosure can be used in lithium secondary batteries.

[0104] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group 32: Negative electrode current collector 40: Protective layer 50: Spacer 51: Space

Claims

1. A lithium secondary battery comprising: a positive electrode; a negative electrode; and a non-aqueous electrolyte, wherein the negative electrode is an electrode on which lithium metal precipitates during charging and from which the lithium metal dissolves during discharging, and wherein a protective layer is disposed on a surface of the negative electrode, and wherein the protective layer comprises a fluorinated polymer and a block copolymer, and the block copolymer comprises a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B, and wherein the fluorinated polymer and the block copolymer are present in the protective layer as separate molecules.

2. The lithium secondary battery according to claim 1, wherein in the protective layer, the ratio Mf / Ms of the mass of the fluoropolymer Mf to the total mass Ms of the fluoropolymer and the block copolymer is in the range of 0.20 to 0.

95.

3. The lithium secondary battery according to claim 1, wherein the negative electrode includes a layer containing LiF on the surface in contact with the protective layer.

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

5. The lithium secondary battery according to claim 1, wherein the block copolymer does not contain fluorine.

6. A lithium secondary battery according to any one of claims 1 to 5, wherein the block copolymer is an ABA triblock copolymer in which the first polymer portion is bonded to both ends of the second polymer portion.

7. The lithium secondary battery according to any one of claims 1 to 5, wherein the monomer unit A is any one of a styrene unit, an ethylene unit, a butylene unit, and a propylene unit, and the monomer unit B is any one of a methyl methacrylate unit, an ethyl methacrylate unit, a propyl methacrylate unit, an ethylene glycol unit, a propylene glycol unit, a butylene glycol unit, and a methacrylic acid unit.

8. The lithium secondary battery according to claim 7, wherein the monomer unit A is a styrene unit.

9. The lithium secondary battery according to claim 7, wherein the monomer unit B is a methyl methacrylate unit.

10. The lithium secondary battery according to any one of claims 1 to 5, wherein the number average molecular weight of the first polymer portion is smaller than the number average molecular weight of the second polymer portion.

11. The lithium secondary battery according to claim 10, wherein the number average molecular weight of the first polymer portion is 1,000 or more and 100,000 or less, and the number average molecular weight of the second polymer portion is 10,000 or more and 500,000 or less.

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

13. The lithium secondary battery according to any one of claims 1 to 5, wherein the protective layer contains a lithium salt.

14. 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 lithium secondary battery according to claim 13, comprising at least one selected from the group consisting of:

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