Lithium secondary battery

Abstract

To improve the cycle durability of a lithium secondary battery.SOLUTION: A lithium secondary battery includes a positive electrode, a separator, a negative electrode, and an electrolyte. The negative electrode includes a current collector sheet. The current collector sheet has a thickness of 10 to 20 μm. The current collector sheet has a plurality of recesses. When a diameter on the surface is a (mm), a center-to-center distance between the recess and the closest recess is b (mm), a depth is d (μm), the average value of the diameter a (mm) is A (mm), the average value of the center-to-center distance b (mm) is B (mm), the average value of the depth d (μm) is D (μm), and the thickness of the current collector sheet is C (μm) for each recess, the following relationships are satisfied. 0.05≤A≤0.18 (1). B≤0.18 (2). D / C≥3 / 5 (3).SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Japanese Patent Laid-Open Publication No. 2019-160776 (Patent Document 1) discloses providing a plurality of protrusions on each of a first surface and a second surface of a negative electrode current collector. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-160776 A Summary of the Invention [Problem to be solved by the invention]

[0004] The negative electrode of a lithium secondary battery utilizes dissolution and precipitation reactions. That is, during charging, lithium metal precipitates from the electrolyte. During discharging, lithium metal dissolves into the electrolyte. By utilizing dissolution and precipitation reactions, it is expected that the energy density will increase. However, there is room for improvement in the cycle durability of lithium secondary batteries.

[0005] An object of the present disclosure is to improve the cycle durability of lithium secondary batteries. [Means for solving the problem]

[0006] The technical configuration and effects of the present disclosure will be described below. However, the mechanism of action in this specification includes assumptions. The mechanism of action does not limit the technical scope of the present disclosure.

[0007] 1. A lithium secondary battery includes a positive electrode, a separator, a negative electrode, and an electrolyte. The negative electrode includes a current collecting sheet. The current collecting sheet has a thickness of 10 to 20 μm. The current collecting sheet has a plurality of recesses. Each of the recesses has a diameter on the surface of a (mm), a center-to-center distance between the recess and the closest other recess is b (mm), and a depth of d (μm). Furthermore, if the average value of the diameter a (mm) is A (mm), the average value of the center-to-center distance b (mm) is B (mm), the average value of the depth d (μm) is D (μm), and the thickness of the current collecting sheet is C (μm), then Equation (1), Equation (2), and Equation (3): 0.05≦A≦0.18 (1) B≦0.18 (2) D / C≧3 / 5 (3) Satisfy the relationship.

[0008] In a lithium secondary battery, during charging, lithium metal may precipitate from multiple starting points on the negative electrode to form multiple precipitates. Each precipitate may cause the negative electrode to expand, which may lead to a decrease in the capacity retention rate. The expansion of the negative electrode refers to an increase in the total volume of the negative electrode and the volume of the precipitated lithium metal. As each precipitate grows and becomes larger, a part of it falls off into the electrolyte. The fallen lithium metal may become an irreversible capacity. Due to the expansion of the negative electrode and the increase in the amount of lithium metal precipitates that fall off into the electrolyte, the capacity retention rate of the lithium secondary battery decreases and the cycle durability decreases.

[0009] For example, Patent Document 1 discloses that the deposition of lithium metal is controlled by providing multiple protrusions on each of the first and second surfaces of the negative electrode current collector. However, the process of providing multiple protrusions on each of the first and second surfaces of the negative electrode current collector is complicated. In addition, since the multiple protrusions are made of a resin material, the surfaces of the protrusions do not become the deposition starting points of lithium metal. In contrast, in the present disclosure, the deposition of lithium metal is controlled by using a current collecting sheet having multiple recesses. The inner walls of the multiple recesses become the deposition starting points, and the process of providing the multiple recesses is simple.

[0010] In the present disclosure, the cycle durability of a lithium secondary battery can be improved. This can be achieved by the current collecting sheet having a plurality of recesses, and the plurality of recesses being arranged to satisfy the above formulas (1), (2), and (3). That is, since the current collecting sheet has a plurality of recesses, lithium metal is also precipitated from the inner walls of the plurality of recesses, and the number of precipitation starting points increases moderately. When the number of precipitation starting points increases moderately, the current density of each precipitate decreases, and each precipitate precipitated from each starting point can be grown to a high density. And, when the density of each precipitate is high, the dropout is suppressed. In addition, each precipitate grows from a high density and moderately close starting point, so that the expansion of the negative electrode is suppressed even after the plurality of precipitates become a continuous precipitate by subsequent growth. It is presumed that the growth of the precipitates from the moderately close starting points is possible when the plurality of recesses satisfy the relationship of the above formulas (1), (2), and (3).

[0011] 2. The lithium secondary battery described in the above item "1" may include, for example, the following configuration: The current collecting sheet is a copper foil or a copper alloy foil.

[0012] 3. The lithium secondary battery according to the above item "1" or "2" may include, for example, the following configuration: The current collecting sheet has a surface opening ratio of 20 to 45%.

[0013] It is expected that a surface aperture ratio of 20% or more will increase the number of starting points for lithium metal deposition and reduce the current density of the deposits from each starting point, while a surface aperture ratio of 45% or less will allow lithium metal to deposit from appropriately close starting points.

[0014] 4. The lithium secondary battery according to any one of the above items "1" to "4" is, for example, an anode-free battery. The anode-free battery is expected to contribute to improving the capacity.

[0015] 5. In the lithium secondary battery described in the above item "1," for example, the current collecting sheet is a copper foil or a copper alloy foil, and the opening ratio of the surface is 20 to 45%.

[0016] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present embodiment") will be described. However, the present embodiment and the present embodiment do not limit the technical scope of the present disclosure. The present embodiment and the present embodiment are illustrative in all respects. The present embodiment and the present embodiment are non-restrictive. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the description of the claims. For example, it is originally intended that any configuration may be extracted from the present embodiment and the present embodiment, and that they may be arbitrarily combined. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a conceptual diagram showing a lithium secondary battery according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a conceptual diagram illustrating an example of a current collecting sheet in the present embodiment. [Diagram 3] FIG. 4 is a conceptual diagram showing another example of the current collecting sheet in the present embodiment. [Figure 4] 3 is a conceptual diagram showing a charge / discharge process in the vicinity of a current collecting sheet of the lithium secondary battery according to the present embodiment. FIG. [Diagram 5] Table 1 shows the arrangement pattern of the recesses of the current collecting sheet and the evaluation results. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] <Terminology explanation> The terms used in this specification are explained. Terms not explained here may be explained at each use in this specification.

[0019] Numerical ranges such as "m to n%" include the upper and lower limits unless otherwise specified. That is, "m to n%" indicates a numerical range of "m% or more and n% or less". Also, "m% or more and n% or less" includes "more than m% and less than n%". "More than" and "less than" are represented by an inequality sign with an equal sign "≦". "More than" and "less than" are represented by an inequality sign without an equal sign "<". A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part, table, figure, etc. of this specification.

[0020] All numerical values ​​are modified by the term "about". The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values ​​may be approximations that may vary depending on the application of the disclosed technology. All numerical values ​​may be expressed in significant figures. Measurements may be average values ​​of multiple measurements unless otherwise specified. The number of measurements may be three or more, five or more, or ten or more. In general, the more measurements are made, the more reliable the average value is expected to be. Measurements may be rounded off based on the number of significant figures. Measurements may include errors associated with, for example, the detection limit of the measurement device.

[0021] "SOC (State Of Charge)" indicates the percentage of the battery's charge capacity at that point in time relative to the battery's fully charged capacity.

[0022] The term "lithium secondary battery" refers to a battery in which the negative electrode reaction includes a dissolution and precipitation reaction of lithium metal. For example, the dissolution and precipitation reaction of lithium metal may account for 1 to 100%, 25 to 100%, 50 to 100%, or 75 to 100% of the negative electrode capacity. The negative electrode capacity indicates a reversible capacity. For example, lithium metal may be precipitated on the negative electrode at an SOC of 1 to 100%, 1 to 75%, 1 to 50%, or 1 to 25%. At an SOC of 0% (at full discharge), the lithium metal may be entirely dissolved in the electrolyte. At an SOC of 0%, a part of the lithium metal may remain on the negative electrode.

[0023] In addition, the precipitation of lithium metal in a lithium ion secondary battery can usually result in irreversible capacity. The precipitation of lithium metal in a lithium ion secondary battery is, for example, an unintended reaction. The precipitation of lithium metal in a lithium ion secondary battery can occur, for example, during abnormality or misuse.

[0024] In an "anode-free battery", no lithium metal is present in the negative electrode before the first charge (after assembly and before the first charge). An anode-free battery is assembled in a state where the negative electrode does not contain lithium metal (negative electrode active material). During the first charge, lithium is supplied from the positive electrode to the negative electrode, and lithium metal is then precipitated on the negative electrode. In an anode-free battery, the entire amount of lithium metal may dissolve during full discharge.

[0025] <Lithium secondary battery> 1 is a conceptual diagram showing a lithium secondary battery in this embodiment. A battery 100 includes a power generating element 50 and an electrolyte (not shown).

[0026] <Exterior body> The battery 100 may include an exterior body (not shown). The exterior body may house the power generating element 50 and an electrolyte. The exterior body may have any shape. The exterior body may be, for example, a metal case or a pouch made of a metal foil laminated film. The case may have any shape. The case may be, for example, cylindrical, rectangular, flat, coin-shaped, or the like. The exterior body may include, for example, Al, or the like. The exterior body may house, for example, one power generating element 50, or may house a plurality of power generating elements 50. The plurality of power generating elements 50 may form, for example, a series circuit or a parallel circuit. In the exterior body, the plurality of power generating elements 50 may be stacked in the thickness direction of the battery 100.

[0027] <Power generation elements> The power generating element 50 includes a positive electrode 10, a negative electrode 20, and a separator 30. The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. The power generating element 50 may have any shape. The power generating element 50 may have a bipolar structure or a monopolar structure. The power generating element 50 may be, for example, a laminated type. For example, the power generating element 50 may be formed by alternately laminating the positive electrode 10 and the negative electrode 20 while sandwiching the separator 30 between them. For example, the strip-shaped separator 30 may be zigzag-folded, and the positive electrode 10 and the negative electrode 20 may be alternately disposed each time the separator 30 is folded. The power generating element 50 may be, for example, a wound type. For example, the positive electrode 10, the negative electrode 20, and the separator 30 may all be strip-shaped. For example, the positive electrode 10, the separator 30, and the negative electrode 20 may be laminated in this order to form a laminate. The laminate may be spirally wound to form the power generating element 50. The wound type power generating element 50 may be formed into a flat shape after winding.

[0028] <Negative electrode> Negative electrode 20 includes a current collecting sheet 21. At an SOC greater than 0%, negative electrode 20 further includes a Li metal layer 23 on current collecting sheet 21. As the SOC increases or decreases, the thickness of Li metal layer 23 also increases or decreases. Li metal layer 23 may be composed of a plurality of precipitates, or may be composed of a continuous, integrated precipitate.

[0029] The current collecting sheet 21 has electrical conductivity. The current collecting sheet 21 can function as a current collector, and examples of the current collecting sheet include foil and film. The current collecting sheet 21 may have a thickness of 10 to 20 μm, for example, 12 to 18 μm. The material of the current collecting sheet 21 may be a conductive material such as a metal or an alloy, other than lithium metal and lithium alloy. The conductive material is preferably a material that does not react with lithium. Such a conductive material may include at least one selected from the group consisting of copper (Cu), nickel (Ni), iron (Fe), zinc (Zn), lead (Pb), silver (Ag), and gold (Au). Examples of the alloy include copper alloy and stainless steel (SUS). The current collecting sheet 21 is, for example, a copper foil or a copper alloy foil.

[0030] The current collecting sheet 21 has a plurality of recesses. With respect to each recess, the diameter on the surface is a (mm), the center-to-center distance between the recess and the closest recess is b (mm), and the depth is d (μm), and the average value of the diameter a (mm) is A (mm), the average value of the center-to-center distance b (mm) is B (mm), the average value of the depth d (μm) is D (μm), and further the thickness of the current collecting sheet is C (μm), then the following formulas (1), (2), and (3): 0.05≦A≦0.18 (1) B≦0.18 (2) D / C≧3 / 5 (3) The above average value is the arithmetic average.

[0031] The arrangement pattern of the plurality of recesses is not limited as long as it satisfies the above formula (1), formula (2), and formula (3). The surface shape of the plurality of recesses is not limited, and may be, for example, a circle, an ellipse, or a rectangle. The diameter of the recess is the diameter of the circle when the shape of the recess on the surface is a circle, and the diameter of the smallest circle that encompasses the shape of the surface when the shape of the recess is other than a circle. The center of the recess is the center of the circle when the shape of the recess on the surface is a circle, and the center of the smallest circle that encompasses the shape of the surface when the shape of the recess is other than a circle. The diameter a (mm), center-to-center distance b (mm), and depth d (μm) of the recess can be determined from an image taken by SEM.

[0032] The current collecting sheet 21 is not limited as long as the arrangement pattern of the multiple recesses in the target area satisfies the above formulas (1), (2), and (3). The target area may be the entire area of ​​the surface of the current collecting sheet 21, or may be a part of the area.

[0033] The arrangement pattern of the multiple recesses may be, for example, the following formula (1a) and formula (2a): 0.08≦A≦0.14 (1a) B≦0.17 (2a) The above relationship may be satisfied.

[0034] The plurality of recesses can deposit Li metal at a high density by satisfying the relationship of formula (3) with an average depth D (μm). The plurality of recesses may further satisfy the relationship of formula (4) with an average depth D (μm). D / C≦4 / 5 (4)

[0035] A negative electrode mixture layer may be formed on the surface of the current collector sheet 21. The negative electrode mixture layer is formed, for example, by applying a paste containing a negative electrode active material such as a carbon material such as graphite or a Si material to at least a part of the surface of the negative electrode current collector. In addition, a metal coat may be applied to the surface of the current collector sheet 21. The metal coat may contain at least one selected from the group consisting of magnesium (Mg), aluminum (Al), zinc (Zn), silver (Ag), gold (Au), platinum (Pt), and tin (Sn).

[0036] FIG. 2 is a conceptual diagram showing an example of a current collecting sheet in this embodiment. FIG. 2 is a schematic diagram showing the upper surface (one surface) of the current collecting sheet 21. The current collecting sheet 21 has a plurality of recesses 211. In the arrangement pattern shown in FIG. 2, the recesses 211 have six other recesses 211 closest to each other, except for the recesses 211 arranged on the outermost periphery. The recesses 211 have a circular shape on the upper surface. The plurality of recesses 211 are arranged so that each has a constant diameter a1 (mm) and a constant center-to-center distance b1 (mm) between each of the recesses 211 and the other recesses 211 closest to each other. In the arrangement pattern shown in FIG. 2, the average diameter A (mm) of the recesses 211 is a1 (mm), and the average center-to-center distance B (mm) is b1 (mm).

[0037] FIG. 3 is a conceptual diagram showing an example of the current collecting sheet in this embodiment different from FIG. 2. FIG. 3 is a schematic diagram showing the upper surface (one surface) of the current collecting sheet 21. The current collecting sheet 21 has a plurality of recesses 212. In the arrangement pattern shown in FIG. 3, the recesses 212 have four other recesses 212 closest to each other, except for the recesses 212 arranged on the outermost periphery. The recesses 212 have a circular shape on the upper surface. The plurality of recesses 212 are arranged so that each has a constant diameter a2 (mm) and a constant center-to-center distance b2 (mm) between each of the recesses 212 and the other recesses 212 closest to each other. In the arrangement pattern shown in FIG. 3, the average diameter A (mm) of the recesses 212 is a2 (mm), and the average center-to-center distance B (mm) is b2 (mm).

[0038] 1 shows the depth d (μm) of the recesses 211, 212. The depth d (μm) of each of the recesses 211, 212 is the distance in the thickness direction of the current collecting sheet 21. When the depths of the recesses 211, 212 are not uniform, the maximum value is taken as the depth of the recesses 211, 212.

[0039] In the present embodiment, the opening ratio on one surface of the current collecting sheet 21 is, for example, 20 to 60%, or may be 20 to 45%, or may be 25 to 40%. The opening ratio on the surface means the ratio of the total area of ​​the surface of a plurality of recesses to the area occupied by one surface of the current collecting sheet 21. For example, in the example shown in Fig. 2 and Fig. 3, it means the ratio of the total area of ​​the sizes of the recesses on the surface of the recesses 211, 212 formed on the current collecting sheet 21 to the area occupied by the upper surface of the current collecting sheet 21.

[0040] In the current collecting sheet 21 of this embodiment, the processing method for forming the plurality of recesses is not limited, and they can be formed by electric discharge processing, cutting processing, laser processing, or the like.

[0041] FIG. 4 is a conceptual diagram showing the charge / discharge process near the negative electrode of the lithium secondary battery in this embodiment. FIG. 4(a) shows the initial state. When the lithium secondary battery is charged from the initial state, lithium metal starts to precipitate from a plurality of starting points of the current collector sheet 21 to form precipitates 23a, as shown in FIG. 4(b). The current collector sheet 21 has a plurality of recesses 213, and the inner walls of the plurality of recesses 213 also become starting points for the formation of precipitates 23a. When charging is continued in this state, as shown in FIG. 4(c), the precipitates 23a precipitated and grown from different starting points are integrated to form a Li metal layer 23. Next, when the lithium secondary battery is discharged, the metallic lithium layer 23 is dissolved. The Li metal layer 23 may be formed by integrating precipitates, or may be composed of a plurality of precipitates.

[0042] <Positive electrode> The positive electrode 10 may be, for example, in the form of a sheet. The positive electrode 10 may include, for example, a positive electrode substrate 11 and a positive electrode active material layer 12. The positive electrode substrate 11 has electrical conductivity. The positive electrode substrate 11 can function as a current collector. The positive electrode substrate 11 supports the positive electrode active material layer 12. The positive electrode substrate 11 may be, for example, in the form of a sheet. The positive electrode substrate 11 may have a thickness of, for example, 5 to 50 μm. The positive electrode substrate 11 may include, for example, a metal foil. The positive electrode substrate 11 may include, for example, at least one selected from the group consisting of aluminum (Al), manganese (Mn), titanium (Ti), iron (Fe), and chromium (Cr). The positive electrode substrate 11 may include, for example, an Al foil, an Al alloy foil, a Ti foil, a SUS foil, or the like.

[0043] <Electrolyte> The electrolyte is a liquid electrolyte. The electrolyte contains Li ions. The electrolyte may contain, for example, a solute and a solvent.

[0044] The concentration of the solute may be, for example, 0.5 to 1 mol / L, 1 to 1.5 mol / L, 1.5 to 2 mol / L, 2 to 2.5 mol / L, or 2.5 to 3 mol / L. The solute includes a supporting salt (Li salt). The solute may include, for example, an inorganic acid salt, an imide salt, an oxalate complex, a halide, etc. The solute may include at least one selected from the group consisting of, for example, LiPF6, LiBF4, LiClO4, LiAsF6, LiSbF6, LiN(SO2F)2 (commonly known as LiFSI), LiN(SO2CF3)2 (commonly known as LiTFSI), LiB(C2O4)2 (commonly known as LiBOB), LiBF2(C2O4) (commonly known as LiDFOB), LiPF2(C2O4)2 (commonly known as LiDFOP), LiPO2F2, FSO3Li, LiI, LiBr, and derivatives thereof.

[0045] The electrolyte may contain, for example, an ether-based solvent, which may include at least one selected from the group consisting of tetrahydrofuran (THF), 1,4-dioxane (DOX), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), ethylglyme, triglyme, tetraglyme, hydrofluoroether (HFE), and derivatives thereof.

[0046] The battery 100 may include a gel electrolyte. The gel electrolyte includes an electrolytic solution and a polymer material. The polymer material may form a polymer matrix. The polymer material may include at least one selected from the group consisting of, for example, PVDF, PVDF-HFP, PAN, PVDF-PAN, polyethylene oxide (PEO), polyethylene glycol (PEG), and derivatives thereof.

[0047] <separator> The separator 30 has electrical insulation properties. The separator 30 may include, for example, at least one selected from the group consisting of a resin film, an inorganic particle layer, and an organic particle layer. The separator 30 may include, for example, a resin film and an inorganic particle layer.

[0048] The resin film is porous. The resin film may include, for example, a microporous film, a nonwoven fabric, etc. The resin film includes a resin skeleton. The resin skeleton may be continuous, for example, in a network form. Pores are formed in the gaps of the resin skeleton. The resin film can transmit an electrolytic solution. The resin film may have an average pore size of, for example, 1 μm or less. The resin film may have an average pore size of, for example, 0.01 to 1 μm, or 0.1 to 0.5 μm. The "average pore size" can be measured by mercury intrusion porosimetry. The resin film may have a pore size of, for example, 50 to 250 s / 100 cm. 3 The "Gurley value" may be measured by the Gurley test method.

[0049] The resin film may contain at least one selected from the group consisting of, for example, olefin resin, urethane resin, polyamide resin, cellulose resin, polyether resin, acrylic resin, and polyester resin. The resin film may contain at least one selected from the group consisting of, for example, polyethylene (PE), polypropylene (PP), polyamide (PA), polyamideimide (PAI), polyimide (PI), aromatic polyamide (aramid), polyphenylene ether (PPE), and derivatives thereof. The resin film may be formed by, for example, a stretching method, a phase separation method, or the like. The resin film may have a thickness of, for example, 5 to 50 μm, or 10 to 25 μm.

[0050] In this embodiment, as long as the current collecting sheet 21 has the configuration shown in the above-mentioned embodiment having multiple recesses, and the negative electrode 20 utilizes the dissolution and precipitation reaction of lithium metal, other configurations (such as the combination of the positive electrode, separator, and electrolyte) are arbitrary. EXAMPLES

[0051] <Production of test batteries> Test batteries (anode-free batteries) according to Examples 1 and 2 and Comparative Examples 1 to 7 were manufactured according to the following procedure. Hereinafter, for example, a "test battery according to Example 1" may be simply referred to as "Example 1" or the like.

[0052] Comparative Example 1 NCM was mixed as an active material, PVdF as a binder, and a conductive additive, and the resulting mixture was transferred to a container and stirred (2000 rpm, 1 minute) with a THINKY masher 3 to 4 times. After that, N-methylpyrrolidone (NMP) was added while checking the viscosity, and the mixture was stirred until it became uniform (2000 rpm, 5 minutes). If necessary, additional NMP was added and stirred (2000 rpm, 2 minutes). In this way, a slurry was obtained.

[0053] An aluminum foil (thickness: 16 μm) was prepared as a positive electrode substrate. The slurry obtained above was applied onto the aluminum foil (thickness: 16 μm) using a doctor blade. Doctor blades with gaps of 350, 375, and 400 μm were used. The coating weight after the solvent was dried was taken as the basis weight, and the basis weight was about 23 mg / cm. 2 The density was then adjusted to 2.4 to 2.9 g / cc using a roll press. 2 ) or Lamicelle (approx. 27 cm 2 ) to prepare the positive electrode.

[0054] A resin film (thickness: 20 μm) was prepared as a separator. The resin film contained PP / PE.

[0055] Copper foil (thickness: 15 μm) was prepared as a negative electrode. The copper foil was coated with a metal without being processed to form recesses, and used as a current collecting sheet.

[0056] The positive electrode, the separator, and the negative electrode were laminated in this order to form a power generating element. The power generating element was housed in an exterior body. An electrolyte was injected into the exterior body. The composition of the electrolyte was as follows.

[0057] Electrolyte composition Solute: LiTFSI (1mol / L) Solvent: PC / FEC=7 / 3 (volume ratio)

[0058] After the electrolyte was injected, the exterior was sealed, and thus a test battery was produced.

[0059] Examples 1 and 2, Comparative Examples 2 to 7 Copper foil (thickness: 15 μm) was prepared as the negative electrode. A plurality of recesses were formed in the copper foil by laser processing in the arrangement pattern shown in FIG. 2, and this was used as a current collector sheet. Table 1 in FIG. 5 shows the arrangement pattern of the plurality of recesses and the measured values ​​related to the shape of the recesses. The measured values ​​shown in Table 1 in FIG. 5 were calculated by measuring an SEM image of one surface of the current collector sheet. A test battery was produced in the same manner as in Comparative Example 1, except that copper foil having a plurality of recesses was used as the current collector sheet.

[0060] <Evaluation> A cycle test was carried out under the following conditions: 1. Rest: 60 min 2.CCCV charging: (1 / 4)C (ending current (1 / 100)C) 3. Rest: 5 minutes 4.CC discharge: (1 / 4)C 3.0V 5. Rest: 5 min Number of cycles from 2. to 5. above: 20

[0061] The 20th cycle discharge capacity retention rate was calculated by dividing the 20th cycle discharge capacity by the 1st cycle discharge capacity, and is expressed as a percentage.

[0062] <Result> Table 1 (FIG. 5) shows the 20-cycle discharge capacity retention rate. It is considered that the higher the 20-cycle discharge capacity retention rate, the better the cycle durability. In Examples 1 and 2, in which the negative electrode current collecting sheet had a plurality of recesses and the recesses satisfied the above formulas (1) to (3), the cycle durability was improved compared to Comparative Examples 1 to 7. [Explanation of symbols]

[0063] 11 positive electrode substrate, 12 positive electrode active material layer, 20 negative electrode, 21 current collecting sheet, 23 Li metal layer, 23a deposit, 30 separator, 50 power generating element, 100 lithium secondary battery, 211, 212, 213 recesses.

Claims

1. A positive electrode, a separator, a negative electrode, and an electrolyte solution, The negative electrode includes a current collecting sheet, The current collecting sheet is The thickness is 10 to 20 μm, A plurality of recesses are provided. Assuming that the diameter of each of the recesses on the surface is a (mm), the center-to-center distance between the recesses and the closest other recesses is b (mm), and the depth is d (μm), the average value of the diameters a (mm) is A (mm), the average value of the center-to-center distances b (mm) is B (mm), the average value of the depths d (μm) is D (μm), and the thickness of the current collecting sheet is C (μm), then Equations (1), (2), and (3): 0.05≦A≦0.18 (1) B≦0.18 (2) D / C≧3 / 5 (3) Satisfy the relationship Lithium secondary battery.

2. 2. The lithium secondary battery according to claim 1, wherein the current collecting sheet is a copper foil or a copper alloy foil.

3. 3. The lithium secondary battery according to claim 1, wherein the current collecting sheet has an opening ratio of the surface of 20 to 45%.

4. It is an anode-free battery. The lithium secondary battery according to claim 1 or 2.

5. 2. The lithium secondary battery according to claim 1, wherein the current collecting sheet is a copper foil or a copper alloy foil, and the opening ratio of the surface is 20 to 45%.

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