Lithium secondary battery

Abstract

The invention relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode, a separator, a negative electrode, and an electrolyte solution. The negative electrode includes a collector sheet. And the thickness of the collector plate is 10-20 microns. The collector sheet has a plurality of recesses. For each recess, the surface diameter is a, the center-to-center distance from another recess closest to the recess is b, the depth is d, the average value of the diameters a is A, the average value of the center-to-center distance b is B, the average value of the depth d is D, and the thickness of the collector sheet is C; the relationship of formula (1), formula (2), and formula (3) is satisfied: 0.05 < = A < = 0.18 (1) B < = 0.18 (2) D / C > = 3 / 5 (3).

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from Japanese Patent Application No. 2023-188451 filed on November 2, 2023, which is hereby incorporated by reference herein in its entirety. Technical Field

[0003] The present disclosure relates to a lithium secondary battery. Background Art

[0004] Japanese Patent Application Laid-Open No. 2019-160776 discloses that a plurality of protrusions are respectively provided on the first surface and the second surface of a negative electrode current collector. Summary of the invention

[0005] In the negative electrode of a lithium secondary battery, a dissolution reaction is used. That is, during charging, lithium metal is precipitated from the electrolyte. During discharge, lithium metal dissolves in the electrolyte. By utilizing the dissolution reaction, it is expected that the energy density will increase. However, there is room for improvement in the cycle durability of lithium secondary batteries.

[0006] An object of the present disclosure is to improve the cycle durability of a lithium secondary battery.

[0007] The following describes the structure and effects of the technology disclosed in the present invention. However, the mechanism of action in this specification includes assumptions. The mechanism of action does not limit the scope of the technology disclosed in the present invention.

[0008] 1. A lithium secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte. The negative electrode comprises a collector. The thickness of the collector is 10 to 20 μm. The collector has a plurality of recesses. For each of the recesses, the diameter at the surface is set to a (mm), the center-to-center distance to another recess closest thereto is set to b (mm), the depth is set to d (μm), and further, the average value of the diameter a (mm) is set to A (mm), the average value of the center-to-center distance b (mm) is set to B (mm), the average value of the depth d (μm) is set to D (μm), and further, the thickness of the collector is set to C (μm), and the relationship between equations (1), (2), and (3) is satisfied:

[0009] 0.05≤A≤0.18(1)

[0010] B≤0.18(2)

[0011] D / C ≥ 3 / 5(3).

[0012] In a lithium secondary battery, when charging, lithium metal sometimes precipitates from multiple starting points on the negative electrode to form multiple precipitates. Each precipitate causes the expansion of the negative electrode, which can be the reason for the reduction of the capacity retention rate. The so-called expansion of the negative electrode refers to the increase in the combined volume of the volume of the negative electrode and the volume of the precipitated lithium metal. In addition, as each precipitate grows larger, a part of it falls off in the electrolyte. The detached lithium metal can become irreversible capacity. Due to the expansion of the negative electrode and the increase in the precipitates of lithium metal that fall off in the electrolyte, the capacity retention rate of the lithium secondary battery is reduced and the cycle durability is reduced.

[0013] For example, in Japanese Patent Laid-Open No. 2019-160776, it is disclosed that a plurality of protrusions are respectively provided on the first surface and the second surface of the negative electrode current collector to control the precipitation of lithium metal. However, the processing of providing a plurality of protrusions on the first surface and the second surface of the negative electrode current collector is complicated. In addition, since the plurality of protrusions are composed of a resin material, the surface of the protrusions no longer becomes the starting point for the precipitation of lithium metal. In the present disclosure, a collector sheet having a plurality of recesses is used to control the precipitation of lithium metal. The inner walls of the plurality of recesses become the starting point for precipitation, and the processing of providing a plurality of recesses is simple.

[0014] In the present disclosure, the cycle durability of lithium secondary batteries can be improved. This can be achieved by having a plurality of recesses in the collector sheet, and arranging the plurality of recesses in a manner that satisfies the above-mentioned formula (1), formula (2) and formula (3). That is, since the collector sheet has a plurality of recesses, lithium metal is precipitated from the inner wall of the plurality of recesses, and the number of starting points for precipitation is moderately increased. If the number of starting points for precipitation is moderately increased, the current density of each precipitate is reduced, and each precipitate precipitated from each starting point can be grown at a high density. Moreover, if the density of each precipitate is high, it is suppressed to fall off. In addition, each precipitate grows at a high density and from a starting point that is moderately close, so that due to the subsequent growth, the expansion of the negative electrode is also suppressed after the precipitate becomes a continuous precipitate of multiple precipitates. It is inferred that the relationship between the above-mentioned formulas (1), (2) and (3) is satisfied by a plurality of recesses, so that the growth of precipitates from a starting point that is moderately close can be achieved.

[0015] 2. The lithium secondary battery according to the above-mentioned "1" may include the following configurations, for example: The current collector sheet is a copper foil or a copper alloy foil.

[0016] 3. The lithium secondary battery according to the above “1” or “2” may include the following configuration, for example: The surface opening ratio of the current collector sheet is 20 to 45%.

[0017] By making the surface opening ratio 20% or more, it is expected that the number of starting points for lithium metal precipitation is increased and the current density of the precipitate precipitated from each starting point is reduced. In addition, by making the surface opening ratio 45% or less, it is expected that lithium metal is precipitated from starting points that are appropriately close.

[0018] 4. The lithium secondary battery according to the above “1” or “2” is, for example, an anode-free battery. An anode-free battery is expected to contribute to an increase in capacity.

[0019] 5. The lithium secondary battery according to the above “1”, wherein, for example, the collector sheet is a copper foil or a copper alloy foil, and the opening ratio of the surface is 20 to 45%.

[0020] 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 example") are described. However, the present embodiment and the present example do not limit the scope of the technology of the present disclosure. The present embodiment and the present example are illustrative in all aspects. The present embodiment and the present example are non-restrictive. The scope of the technology of the present disclosure includes all variations within the meaning and scope equivalent to the records in the patent claims. For example, it was foreseeable from the beginning that any configuration would be extracted from the present embodiment and the present example and that they would be arbitrarily combined. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Hereinafter, features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and

[0022] in:

[0023] Figure 1 This is a conceptual diagram showing a lithium secondary battery in this embodiment.

[0024] Figure 2 This is a conceptual diagram showing an example of a current collector sheet in this embodiment.

[0025] Figure 3 This is a conceptual diagram showing another example of the current collector sheet in this embodiment.

[0026] Figure 4A This is a conceptual diagram showing the charge and discharge process near the current collector of the lithium secondary battery in this embodiment.

[0027] Figure 4B This is a conceptual diagram showing the charge and discharge process near the current collector of the lithium secondary battery in this embodiment.

[0028] Figure 4C This is a conceptual diagram showing the charge and discharge process near the current collector of the lithium secondary battery in this embodiment.

[0029] Figure 5 Table 1 shows the arrangement pattern of the recessed portions of the current collector sheet and the evaluation results. DETAILED DESCRIPTION

[0030] <Explanation of terms>

[0031] The terms used in this specification are explained. Terms not explained here may be explained each time they are used in this specification.

[0032] Numerical ranges such as “m to n%” include upper and lower limits unless otherwise specified. That is, “m to n%” represents a numerical range of “above m% and below n%”. In addition, “above m% and below n%” includes “more than m% and less than n%”. “Above” and “below” are represented by an inequality sign “≤” with an equal sign. “Exceeding” and “less than” are represented by an inequality sign “<” without an equal sign. A numerical value arbitrarily selected from the numerical range can be set as a new upper limit or lower limit. For example, a new numerical range can be set by arbitrarily combining a numerical value within the numerical range with a numerical value recorded in another part, table, figure, etc. in this specification.

[0033] All numerical values ​​are modified by the term "approximately". The term "approximately" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values ​​may be approximate values ​​that vary according to the form of utilization of the disclosed technology. All numerical values ​​may be represented by significant figures. Unless otherwise specified, the measured value may be an average value from multiple measurements. The number of measurements may be more than 3 times, more than 5 times, or more than 10 times. In general, the more measurements are made, the more reliable the average value can be expected to be. With respect to the measured values, the end number processing may be performed by rounding off based on the number of significant figures. The measured values ​​may include errors such as those associated with the detection limit of the measuring device.

[0034] “SOC (State Of Charge)” indicates the percentage of the charge capacity of the battery at that moment relative to the full charge capacity of the battery.

[0035] "Lithium secondary battery" means a battery in which the negative electrode reaction includes the dissolution and precipitation reaction of lithium metal. For example, the dissolution and precipitation reaction of lithium metal may account for 1-100%, 25-100%, 50-100%, or 75-100% of the negative electrode capacity. The negative electrode capacity represents the reversible capacity. For example, at an SOC of 1-100%, 1-75%, 1-50%, or 1-25%, lithium metal may be precipitated at the negative electrode. At an SOC of 0% (when fully discharged), the lithium metal may be completely dissolved in the electrolyte. At an SOC of 0%, a portion of the lithium metal may remain at the negative electrode.

[0036] It should be noted that, generally, the deposition of lithium metal in a lithium ion secondary battery may become irreversible capacity. The deposition of lithium metal in a lithium ion secondary battery is, for example, an unintended reaction. The deposition of lithium metal in a lithium ion secondary battery may occur, for example, in abnormal conditions or when used improperly.

[0037] In an "anode-free battery", lithium metal is not present at 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 metal is first precipitated at the negative electrode by supplying lithium from the positive electrode to the negative electrode. In an anode-free battery, all lithium metal can be dissolved during full discharge.

[0038] <Lithium secondary battery>

[0039] Figure 1 This is a conceptual diagram showing a lithium secondary battery in this embodiment. The battery 100 includes a power generation element 50 and an electrolyte solution (not shown).

[0040] <Outer packaging>

[0041] The battery 100 may include an outer package (not shown). The outer package may contain a power generation element 50 and an electrolyte. The outer package may have any form. The outer package may be, for example, a metal shell, a bag made of a metal foil laminated film, or the like. The shell may have any shape. The shell may be, for example, cylindrical, square, flat, coin-shaped, or the like. The outer package may contain, for example, Al, or the like. The outer package may contain, for example, one power generation element 50 or a plurality of power generation elements 50. The plurality of power generation elements 50 may form, for example, a series circuit or a parallel circuit. In the outer package, a plurality of power generation elements 50 may be stacked in the thickness direction of the battery 100.

[0042] <Power Generation Factors>

[0043] The power generation element 50 includes a positive electrode 10, a negative electrode 20 and a separator 30. The separator 30 is arranged between the positive electrode 10 and the negative electrode 20. The power generation element 50 may have any form. The power generation element 50 may be a bipolar structure or a monopolar structure. The power generation element 50 may be, for example, a laminated type. For example, the power generation element 50 may be formed by alternately laminating the positive electrode 10 and the negative electrode 20 while sandwiching the separator 30 between the positive electrode 10 and the negative electrode 20. For example, the strip-shaped separator 30 may also be set to a zigzag shape, and the positive electrode 10 and the negative electrode 20 are alternately arranged each time the separator 30 turns back. The power generation element 50 may be, for example, a winding type. For example, the positive electrode 10, the negative electrode 20 and the separator 30 may all be strip-shaped. For example, a laminated body may be formed by sequentially laminating the positive electrode 10, the separator 30 and the negative electrode 20. The power generation element 50 is formed by winding the laminated body into a spiral shape. The wound type power generation element 50 can be formed into a flat shape after being wound.

[0044] <Negative electrode>

[0045] The negative electrode 20 includes a current collector sheet 21. At an SOC greater than 0%, the negative electrode 20 further includes a Li metal layer 23 on the current collector sheet 21. The thickness of the Li metal layer 23 increases or decreases with the increase or decrease of the SOC. The Li metal layer 23 may be composed of a plurality of precipitates, or may be a structure in which precipitates are continuously integrated.

[0046] The collector sheet 21 has conductivity. The collector sheet 21 can function as a collector, for example, foil, film, etc. can be listed. The thickness of the collector sheet 21 is 10 to 20 μm, for example, 12 to 18 μm. The material of the collector sheet 21 can be any conductive material such as metal, alloy, etc., 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, for example, include at least one selected from copper (Cu), nickel (Ni), iron (Fe), zinc (Zn), lead (Pb), silver (Ag), and gold (Au). As alloys, copper alloys, stainless steel (SUS), etc. can be listed. The collector sheet 21 is, for example, copper foil or copper alloy foil.

[0047] The collector sheet 21 has a plurality of recesses. For each recess, the diameter at the surface is set to a (mm), the center-to-center distance to the closest recess is set to b (mm), the depth is set to d (μm), the average value of the diameter a (mm) is set to A (mm), the average value of the center-to-center distance b (mm) is set to B (mm), the average value of the depth d (μm) is set to D (μm), and the thickness of the collector sheet is set to C (μm), and the relationship between equations (1), (2) and (3) is satisfied. In the above, the average value is set to the arithmetic mean.

[0048] 0.05≤A≤0.18(1)

[0049] B≤0.18(2)

[0050] D / C ≥ 3 / 5 (3)

[0051] The arrangement pattern of the plurality of recesses is not limited as long as it satisfies the above-mentioned formula (1), the above-mentioned formula (2) and the above-mentioned formula (3). The shape of the surface of the plurality of recesses is not limited, and may be, for example, circular, elliptical or rectangular. The diameter of the recess is set to the diameter of the circle when the shape of the recess at the surface is circular, and is set to the diameter of the smallest circle that includes the shape of the surface when the shape of the recess at the surface is other than circular. In addition, the center of the recess is set to the center of the circle when the shape of the recess at the surface is circular, and is set to the center of the smallest circle that includes the shape of the surface when the shape of the recess at the surface is other than circular. For the recess, the diameter a (mm), the center-to-center distance b (mm) and the depth d (μm) can be obtained from an image taken using an SEM.

[0052] The target region of the collector sheet 21 is not limited as long as the arrangement pattern of the plurality of recesses satisfies the above equations (1), (2), and (3). The target region may be the entire surface of the collector sheet 21 or a portion thereof.

[0053] The arrangement pattern of the plurality of recesses may, for example, satisfy the relationship between equations (1a) and (2a):

[0054] 0.08≤A≤0.14(1a)

[0055] B≤0.17(2a).

[0056] The average value D (μm) of the depths of the plurality of recesses satisfies the relationship of formula (3), thereby enabling Li metal to be deposited at a high density. The average value D (μm) of the depths of the plurality of recesses may further satisfy the relationship of formula (4).

[0057] D / C≤4 / 5(4)

[0058] A negative electrode composite layer may be formed on the surface of the current collector sheet 21. The negative electrode composite layer is formed, for example, by applying a paste containing a carbon material such as graphite, a negative electrode active material such as a Si material, to at least a portion of the surface of the negative electrode collector. In addition, a metal coating may be applied to the surface of the current collector sheet 21. The metal coating may include at least one selected from magnesium (Mg), aluminum (Al), zinc (Zn), silver (Ag), gold (Au), platinum (Pt), and tin (Sn).

[0059] Figure 2 This is a conceptual diagram showing an example of a current collector sheet in this embodiment. Figure 2 The upper surface (one surface) of the current collector sheet 21 is schematically shown. The current collector sheet 21 has a plurality of recesses 211. Figure 2 In the arrangement pattern shown, in addition to the concave portion 211 arranged at the outermost periphery, there are 6 other concave portions 211 closest to each other. The shape of the top of the concave portion 211 is circular. The plurality of concave portions 211 are arranged so that each has a constant diameter a1 (mm) and a constant distance b1 (mm) between the centers of the concave portions 211 closest to each other. Figure 2 In the arrangement pattern shown, the average value A (mm) of the diameter of the recessed portions 211 is a1 (mm), and the average value B (mm) of the center-to-center distance is b1 (mm).

[0060] Figure 3 To illustrate the current collector sheet in this embodiment Figure 2 Concept image of a different example. Figure 3 The upper surface (one surface) of the current collector sheet 21 is schematically shown. The current collector sheet 21 has a plurality of recesses 212. Figure 3In the arrangement pattern shown, in addition to the concave portion 212 arranged at the outermost periphery, there are four other concave portions 212 closest to each other. The shape of the top of the concave portion 212 is circular. The plurality of concave portions 212 are arranged so that each has a constant diameter a2 (mm) and a constant distance b2 (mm) between the centers of the concave portions 212 closest to each other. Figure 3 In the arrangement pattern shown, the average value A (mm) of the diameter of the recessed portions 212 is a2 (mm), and the average value B (mm) of the center-to-center distance is b2 (mm).

[0061] exist Figure 1 2 shows the depth d (μm) of the recesses 211 and 212. The depth d (μm) of each recess 211 and 212 is the distance in the thickness direction of the current collector sheet 21. When the depth of each recess 211 and 212 is not constant, the maximum value is set as the depth of the recess 211 and 212.

[0062] In the collector sheet 21 of the present embodiment, the opening ratio in one surface is, for example, 20 to 60%, 20 to 45%, or 25 to 40%. The opening ratio of the surface means the ratio of the total surface area of ​​the plurality of recesses to the area occupied by one surface of the collector sheet 21. Figure 2 and Figure 3 In the example shown, it refers to the ratio of the total area of ​​the sizes of the recesses 211 and 212 formed in the current collector sheet 21 to the area occupied by the upper surface of the current collector sheet 21 .

[0063] In the current collector sheet 21 of the present embodiment, the processing method for forming the plurality of recessed portions is not limited, and the recessed portions can be formed by electrical discharge processing, cutting processing, laser processing, or the like.

[0064] Figure 4A , 4B 4C is a conceptual diagram showing the charge and discharge process near the negative electrode of the lithium secondary battery in this embodiment. Figure 4A If the lithium secondary battery is charged from the initial state, Figure 4B As shown in FIG. 1 , lithium metal begins to precipitate from multiple starting points of the collector sheet 21 to form precipitates 23a. The collector sheet 21 has multiple recesses 213, and the inner walls of the multiple recesses 213 also become starting points for the formation of precipitates 23a. If charging is continued in this way, as shown in FIG. Figure 4C As shown, a Li metal layer 23 is formed in which precipitates 23a grown from different starting points are integrated. Next, when the lithium secondary battery is discharged, the Li metal layer 23 is dissolved. The Li metal layer 23 may be formed in which the precipitates are integrated, or may be composed of a plurality of precipitates.

[0065] <Positive electrode>

[0066] The positive electrode 10 may be, for example, in a sheet shape. 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 conductivity. The positive electrode substrate 11 may 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 a sheet shape. The positive electrode substrate 11 may have, for example, a thickness of 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 aluminum (Al), manganese (Mn), titanium (Ti), iron (Fe) and chromium (Cr). The positive electrode substrate 11 may include, for example, Al foil, Al alloy foil, Ti foil, SUS foil, etc.

[0067] <Electrolyte>

[0068] The electrolyte is a liquid electrolyte. The electrolyte contains Li ions. The electrolyte may contain a solute and a solvent, for example.

[0069] 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 oxalic acid complex, a halide, and the like. The solute may include, for example, at least one selected from 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, Li I, LiBr, and derivatives thereof.

[0070] The electrolyte may include, for example, an ether solvent. The solvent may include, for example, at least one selected from tetrahydrofuran (THF), 1,4-dioxane (DOX), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), ethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, hydrofluoroether (HFE) and derivatives thereof.

[0071] The battery 100 may include a gel electrolyte. The gel electrolyte includes an electrolyte and a polymer material. The polymer material may form a polymer matrix. The polymer material may include, for example, at least one selected from PVDF, PVDF-HFP, PAN, PVDF-PAN, polyethylene oxide (PEO), polyethylene glycol (PEG), and derivatives thereof.

[0072] <Separator>

[0073] The separator 30 has electrical insulation. The separator 30 may include, for example, at least one selected from 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.

[0074] 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 in a mesh shape, for example. Pores are formed in the gaps of the resin skeleton. The resin film allows the electrolyte to pass through. The resin film may have, for example, an average pore diameter of less than 1 μm. The resin film may have, for example, an average pore diameter of 0.01 to 1 μm or 0.1 to 0.5 μm. The "average pore diameter" may be measured by mercury intrusion. The resin film may have, for example, a pore diameter of 50 to 250 s / 100 cm 3 Gray value. The "Gray value" can be determined by the Gray test method.

[0075] The resin film may include, for example, at least one selected from olefin resins, polyurethane resins, polyamide resins, cellulose resins, polyether resins, acrylic resins, and polyester resins. The resin film may include, for example, at least one selected from polyethylene (PE), polypropylene (PP), polyamide (PA), polyamideimide (PAI), polyimide (PI), aromatic polyamide (aramid), polyphenylene ether (PPE), and their derivatives. The resin film may be formed, for example, by a stretching method, a phase separation method, etc. The resin film may have a thickness of, for example, 5 to 50 μm, or 10 to 25 μm.

[0076] In this embodiment, as long as the current collector 21 has a plurality of recessed portions as shown in the above embodiment and the negative electrode 20 utilizes the dissolution reaction of lithium metal, other configurations (combination of the positive electrode, separator, and electrolyte, etc.) are arbitrary.

[0077] <Manufacturing of test batteries>

[0078] The test cells (anode-free cells) according to Examples 1 and 2 and Comparative Examples 1 to 7 were manufactured by the following procedure. Hereinafter, for example, "the test cell according to Example 1" may be abbreviated as "Example 1" or the like.

[0079] Comparative Example 1

[0080] NCM as an active material, PVdF as a binder and a conductive additive are mixed, and the resulting mixture is transferred to a container, and stirred 3 to 4 times using a degassing stirring device (Thinky) (2000 rpm, 1 minute). Then, while confirming the viscosity, N-methylpyrrolidone (NMP) is added and stirred until it becomes uniform (2000 rpm for 5 minutes). If necessary, additional NMP is added and stirred (2000 rpm for 2 minutes). Thus, a slurry is obtained.

[0081] Prepare aluminum foil (thickness: 16 μm) as the positive electrode substrate. Use a scraper to apply the slurry obtained above on aluminum foil (thickness: 16 μm). Use scrapers with gaps of 350, 375, and 400 μm. Make the coating weight after the solvent is dried into a unit area mass, so that the unit area mass is about 23 mg / cm 2 Then, the density was adjusted to 2.4 to 2.9 g / cc using a roller press. Then, the cells were cut into coin cells (approximately 1.5 cm 2 ) or laminated battery cell (about 27cm 2 ) size, prepare the positive electrode.

[0082] As a separator, a resin film (thickness: 20 μm) was prepared. The resin film contained PP / PE.

[0083] As a negative electrode, a copper foil (thickness: 15 μm) was prepared. The copper foil was not processed to form recesses, but was metal-coated and used as a current collector.

[0084] The positive electrode, the separator, and the negative electrode are stacked in order to form a power generation element. The power generation element is housed in an outer package. An electrolyte is injected into the outer package. The composition of the electrolyte is as follows.

[0085] Electrolyte composition

[0086] Solute: LiTFSI (1 mol / L)

[0087] Solvent: PC / FEC=7 / 3 (volume ratio)

[0088] After the electrolyte solution was injected, the outer package was sealed. A test battery was manufactured in the above manner.

[0089] Examples 1, 2, and Comparative Examples 2 to 7

[0090] As the negative electrode, a copper foil (thickness: 15 μm) was prepared. The copper foil was processed by laser to Figure 2 The arrangement pattern shown is formed with a plurality of recesses, which serve as current collectors. Figure 5 Table 1 shows the measured values ​​related to the arrangement pattern of the plurality of recesses and the shapes of the recesses. Figure 5 The measured values ​​shown in Table 1 are values ​​calculated by measuring the SEM image of one surface of the current collector sheet. A test cell was produced in the same manner as in Comparative Example 1 except that a copper foil having a plurality of recesses was used as the current collector sheet.

[0091] <Evaluation>

[0092] The cycle test was carried out under the following conditions.

[0093] 1. Pause: 60 minutes

[0094] 2.CCCV charging: (1 / 4)C (termination current (1 / 100)C)

[0095] 3. Pause: 5 minutes

[0096] 4.CC discharge: (1 / 4)C3.0V

[0097] 5. Pause: 5 minutes

[0098] Number of cycles of 2. to 5. above: 20

[0099] The 20-cycle discharge capacity retention rate was calculated by dividing the 20th cycle discharge capacity by the 1st cycle discharge capacity. The 20-cycle discharge capacity retention rate was expressed as a percentage.

[0100] <Results>

[0101] In Table 1 ( Figure 5 ) shows the 20-cycle discharge capacity retention rate. It is believed that the higher the 20-cycle discharge capacity retention rate, the better the cycle durability. For the negative electrode collector, Examples 1 and 2 having a plurality of recesses and the recesses satisfying the relationship of the above formulas (1) to (3) have improved cycle durability compared with Comparative Examples 1 to 7.

Claims

1. A lithium secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, wherein: The negative electrode comprises a current collector. The collector sheet has a thickness of 10 to 20 μm and has a plurality of recesses. For each of the concave portions, when the diameter at the surface is set to a (mm), the center-to-center distance to the other concave portion closest thereto is set to b (mm), the depth is set to d (μm), the average value of the diameter a (mm) is set to A (mm), the average value of the center-to-center distance b (mm) is set to B (mm), the average value of the depth d (μm) is set to D (μm), and the thickness of the collector sheet is set to C (μm), the relationship between equations (1), (2) and (3) is satisfied: 0.05≤A≤0.18(1) B≤0.18(2) D / C ≥ 3 / 5(3).

2. The lithium secondary battery according to claim 1, wherein The current collector is copper foil or copper alloy foil.

3. The lithium secondary battery according to claim 1 or 2, wherein: The opening ratio of the surface of the collector sheet is 20 to 45%. The lithium secondary battery according to claim 1 or 2, which is an anode-free battery.

5. The lithium secondary battery according to claim 1, wherein The collector sheet is copper foil or copper alloy foil, and the opening rate of the surface is 20-45%.

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