Aging method for lithium metal secondary battery

Abstract

To provide an aging method for a lithium metal secondary battery which includes an electrode stack in which a positive electrode layer, a separator, and a negative electrode layer are stacked in this order, and an electrolyte solution, and in which the negative electrode layer includes a lithium-containing metal layer, and which can form a dense lithium metal layer, has a high yield rate, and can shorten the aging time.SOLUTION: An aging method for a lithium metal secondary battery includes: a wetting step of holding a lithium metal secondary battery in a temperature environment of 20°C or higher and 40°C or lower for a holding time of 24 hours or more and 60 hours or less; and a first charging step of starting charging the lithium metal secondary battery at a charge rate of 0.05C or higher and 0.20C or lower while applying a pressure of 400 KPa or higher and 650 KPa or lower in the stacking direction of the electrode stack in a temperature environment of 20°C or higher and 40°C or lower, and terminating the charging when the voltage reaches a range of 4.1 V or higher and 4.3 V or lower.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for aging a lithium metal secondary battery. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency, ensuring that more people have access to affordable, reliable, sustainable, and advanced energy. Known examples of such secondary batteries include lithium secondary batteries that use a lithium-containing compound as the positive electrode active material in the positive electrode layer, and that transfer lithium ions from the positive electrode active material to the negative electrode layer during charging and from the negative electrode layer to the positive electrode active material during discharging. Since lithium is present in the positive electrode active material immediately after production, lithium secondary batteries with such a configuration typically undergo aging, including a charging process (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2024-500687 Summary of the Invention [Problem to be solved by the invention]

[0004] Increasing capacity is one of the challenges facing secondary battery technology. Lithium metal secondary batteries are known as high-capacity lithium secondary batteries. Lithium metal secondary batteries use a lithium-containing metal layer as a negative electrode active material layer. During charging, lithium ions are deposited on the lithium-containing metal layer to form a lithium metal layer. During discharging, the lithium ions released from the lithium metal layer are absorbed into the positive electrode. However, according to the inventors' studies, when the electrolyte concentration of the electrolyte solution in a lithium metal secondary battery is high, ranging from 1.0 mol / L to 3.0 mol / L, the lithium-containing metal layer does not absorb the electrolyte, making it difficult for the electrolyte to penetrate into the electrode stack. Furthermore, because the lithium-containing metal layer has low wettability with the electrolyte, lithium is less likely to deposit during charging during aging. This causes variations in the location of lithium deposition, making it difficult to form a dense lithium metal layer. As a result, aged lithium metal secondary batteries are prone to charging failure and to defects due to micro-short circuits, such as a sudden drop in voltage in the charged state, resulting in a low aging yield.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a method for aging a lithium metal secondary battery that can form a dense lithium metal layer, has a high yield rate, and shortens the aging time, thereby contributing to improved energy efficiency. [Means for solving the problem]

[0006] The inventors have found that, to address the above-mentioned problems, it is effective to maintain a lithium metal secondary battery at a predetermined temperature for a predetermined time, and then start charging the battery at a predetermined charge rate while pressurizing the battery at a predetermined pressure, and have completed the present invention.

[0007] (1) A method for aging a lithium metal secondary battery having an electrode laminate in which a positive electrode layer, a separator, and a negative electrode layer are laminated in this order, and an electrolyte, the negative electrode layer including a lithium-containing metal layer, the method comprising: a wetting step of holding the lithium metal secondary battery in a temperature environment of 20°C or higher and 40°C or lower for a holding time of 24 hours or higher and 60 hours or lower; and a first charging step of starting charging the lithium metal secondary battery at a charge rate of 0.05C or higher and 0.20C or lower while applying a pressure of 400 kPa or higher and 650 kPa or lower in the stacking direction of the electrode laminate in a temperature environment of 20°C or higher and 40°C or lower, and terminating the charging when the voltage reaches a range of 4.1V or higher and 4.3V or lower.

[0008] According to the aging method for a lithium metal secondary battery (1), the temperature and retention time in the wetting step are within the above ranges, which allows the electrolyte to easily penetrate the entire electrode stack. Furthermore, the applied pressure and charge rate at the start of charging in the first charging step are within the above ranges, which allows the lithium metal layer formed on the surface of the lithium-containing metal layer to be dense, improves the aging yield rate, and shortens the aging time.

[0009] (2) The aging method for a lithium metal secondary battery according to (1), further comprising a discharging step of discharging the lithium metal secondary battery charged in the first charging step until the voltage reaches 2.65 V, and a second charging step of charging the lithium metal secondary battery discharged in the discharging step until the voltage reaches 3.72 V.

[0010] According to the aging method for a lithium metal secondary battery in (2), the discharging step and the first charging step are further carried out, so that each layer of the electrode stack of the lithium metal secondary battery is stabilized and the aging yield rate is improved.

[0011] (3) The method for aging a lithium metal secondary battery according to (1) or (2), wherein the applied pressure is increased continuously or stepwise in the first charging step.

[0012] According to the aging method for a lithium metal secondary battery (3), the density of the deposited lithium in the thickness direction can be adjusted by pressurizing the lithium metal secondary battery according to the state of charge of the lithium metal secondary battery, so that the lithium metal layer formed in the first charging step tends to become denser.

[0013] (4) The method for aging a lithium metal secondary battery according to (3), wherein in the first charging step, the applied pressure at the end of charging is in the range of 650 kPa or more and 900 kPa or less.

[0014] According to the aging method for lithium metal secondary batteries (4), the applied pressure at the end of the first charging step is within the above range, so that the balance between the lithium deposition density and the permeability of the electrolyte is good, and furthermore, the lithium metal secondary battery is unlikely to be pressurized with excessive pressure, causing the electrolyte to be squeezed out. Therefore, the aging yield rate is improved.

[0015] (5) The method for aging a lithium metal secondary battery according to any one of (1) to (4), wherein the charge rate is increased continuously or stepwise in the first charging step.

[0016] According to the aging method for a lithium metal secondary battery (5), charging is started at a low charge rate to facilitate the formation of a dense lithium metal layer in the lithium-containing metal layer, and then the charge rate is increased. This makes it possible to achieve both the formation of a dense lithium metal layer with a uniform interface between the lithium-containing metal layer and the deposited lithium, and the shortening of the aging time. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a method for aging a lithium metal secondary battery that can form a dense lithium metal layer, has a high yield rate, and shortens the aging time. [Brief explanation of the drawings]

[0018] [Figure 1]1 is a cross-sectional view of an example showing the configuration of an electrode stack included in a lithium metal secondary battery to be treated by an aging method according to an embodiment of the present invention. [Figure 2] FIG. 1 is a flow diagram of an aging method for a lithium metal secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments described below are merely examples of the present invention, and the present invention is not limited to the following.

[0020] In this embodiment, the target of the aging treatment is a lithium metal secondary battery. The lithium metal secondary battery has an electrode stack and an electrolyte solution. The electrode stack and the electrolyte solution are housed in an exterior body and sealed.

[0021] Fig. 1 is a cross-sectional view showing an example of the configuration of an electrode laminate included in a lithium metal secondary battery to be treated by an aging method according to one embodiment of the present invention. As shown in Fig. 1, the electrode laminate 1 is a laminate including a positive electrode layer 10, a negative electrode layer 20, and a separator 30 laminated between the positive electrode layer 10 and the negative electrode layer 20.

[0022] The positive electrode layer 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12 laminated on the surface of the positive electrode current collector 11. Examples of materials for the positive electrode current collector 11 include aluminum, aluminum alloy, stainless steel, nickel, iron, and titanium.

[0023] The positive electrode active material layer 12 contains a positive electrode active material. The positive electrode active material is a lithium compound that releases lithium ions during discharge and absorbs lithium ions during charge. Examples of the lithium compound that can be used include layered active materials, spinel-type active materials, and olivine-type active materials. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium-nickel-manganese cobalt oxide (NMC:LiNi p Mnq Co r O2(p+q+r=1)), LiNi p Al q Co r O2 (p+q+r=1), lithium manganese oxide (LiMn2O4), Li 1+x Mn 2-x-y Examples of such an element-substituted Li-Mn spinel are MO4 (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), and lithium metal phosphate (LiMPO4, M=at least one selected from Fe, Mn, Co, and Ni). The positive electrode active material layer 12 may further contain a conductive additive and a binder.

[0024] The negative electrode layer 20 includes a negative electrode current collector 21 and a lithium-containing metal layer 22 laminated on the surface of the negative electrode current collector 21. Examples of materials for the negative electrode current collector 21 include copper, copper alloy, nickel, and stainless steel.

[0025] In the lithium-containing metal layer 22, lithium ions are precipitated during charging to form a lithium metal layer, and lithium from the lithium metal layer is released during discharging. Therefore, the thickness of the negative electrode layer 20 changes with charging and discharging. Lithium and metals that form alloys with lithium can be used as materials for the lithium-containing metal layer 22. Examples of metals that form alloys with lithium include Mg, Si, Au, Ag, In, Ge, Sn, Pb, Al, and Zn.

[0026] For example, a porous sheet or a nonwoven fabric sheet can be used as the separator 30. Examples of materials for the porous sheet include polyolefins such as polyethylene and polypropylene, aramid, polyimide, and fluororesin. Examples of materials for the nonwoven fabric sheet include glass fiber and cellulose fiber.

[0027] The electrolytic solution contains an organic solvent and an electrolyte. Examples of the organic solvent include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, hydrofluoroethers (HFEs), aromatic ethers, sulfones, cyclic esters, chain carboxylic acid esters, and nitriles. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, vinylene carbonate, and fluoroethylene carbonate. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, and 4-methyl-1,3-dioxolane. Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, and diethyl ether. Examples of hydrofluoroethers include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl)ether, and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane. Examples of aromatic ethers include anisole. Examples of sulfones include sulfolane and methylsulfolane. Examples of cyclic esters include γ-butyrolactone. Examples of chain carboxylic acid esters include acetate esters, butyrate esters, and propionate esters. Examples of nitriles include acetonitrile and propionitrile. The organic solvents may be used alone or in combination of two or more.

[0028] The electrolyte is a source of lithium ions, which are a charge transfer medium, and includes a lithium salt. Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), LiN(FSO2)2 (LiFSI), and LiBC4O8. The lithium salts may be used alone or in combination of two or more. The concentration of the electrolyte may be, for example, within a range of 1.0 to 4.0 mol / L, or within a range of 1.0 mol / L to 3.0 mol / L.

[0029] The exterior body is expandable and contractible in accordance with changes in the thickness of the negative electrode layer 20 due to charging and discharging. A laminate film can be used as the material for the exterior body. As the laminate film, a laminated film having a three-layer structure in which an inner resin layer, a metal layer, and an outer resin layer are laminated in this order from the inside can be used. The outer resin layer can be, for example, a polyamide (nylon) layer or a polyethylene terephthalate (PET) layer, the metal layer can be, for example, an aluminum layer, and the inner resin layer can be, for example, a polyethylene layer or a polypropylene layer.

[0030] Next, the aging method for the lithium metal secondary battery of this embodiment will be described.

[0031] 2 is a flow diagram of a lithium metal secondary battery aging method according to one embodiment of the present invention. As shown in FIG. 2, the lithium metal secondary battery aging method of this embodiment includes a wetting step S1, a first charging step S2, a discharging step S3, and a second charging step S4.

[0032] In the wetting step S1, the lithium metal secondary battery is held in a temperature environment of 20°C to 40°C for a holding time of 24 hours to 60 hours. The wetting step S1 can be performed using, for example, a thermostatic bath. The wetting step S1 may be performed without applying pressure to the lithium metal secondary battery. By performing the wetting step S1 under these conditions, the electrolyte solution is uniformly permeated into the electrode stack 1. This makes it easier to form a uniform lithium metal layer on the surface of the lithium-containing metal layer 22 during charging in the next first charging step S2.

[0033] In the first charging step S2, the lithium metal secondary battery, which has been held at a predetermined temperature for a predetermined time in the wetting step, is charged at a charge rate of 0.05 C to 0.20 C in a temperature environment of 20°C to 40°C while applying a pressure of 400 kPa to 650 kPa in the stacking direction of the electrode stack 1. Charging is completed when the voltage reaches 4.1 V.

[0034] Since the temperature and applied pressure at the start of charging in the first charging step S2 are within the above ranges, the interlayer resistance of the electrode laminate 1 can be reduced while retaining the electrolyte solution that has permeated the electrode laminate 1. Furthermore, since the charge rate at the start of charging is within the above ranges, the lithium metal layer formed on the surface of the lithium-containing metal layer 22 by charging becomes dense. The higher the temperature, the lower the viscosity of the electrolyte solution and the higher the wettability. However, temperatures above 45°C cause excessive formation of a coating on the surface of the lithium negative electrode, so a temperature of 40°C or lower is preferred. The amount of applied pressure for the initial charge is preferably at least 0.2 C or lower and 300 kPa or higher.

[0035] The applied pressure after the start of charging may be increased continuously or stepwise. "Continuously" means, for example, increasing the applied pressure in accordance with the state of charge (SOC) of the lithium metal secondary battery or the thickness of the lithium metal layer formed on the surface of the lithium-containing metal layer 22 of the negative electrode layer 20. "Stepwise" means, for example, increasing the applied pressure when the state of charge of the lithium metal secondary battery or the thickness of the lithium metal layer formed on the surface of the lithium-containing metal layer 22 of the negative electrode layer 20 reaches a predetermined value. The increase in the applied pressure is not particularly limited, but may be set so that the applied pressure at the end of charging is within a range of 650 kPa to 900 kPa. The ratio of the applied pressure at the end of charging to the applied pressure after the start of charging may be within a range of 1.1 to 2.0, for example.

[0036] The charge rate after the start of charging may be increased continuously or stepwise. "Continuously" means, for example, increasing the charge rate in accordance with the charge rate of the lithium metal secondary battery or the thickness of the lithium metal layer formed on the surface of the lithium-containing metal layer 22 of the anode layer 20. "Stepwise" means, for example, increasing the charge rate when the charge rate of the lithium metal secondary battery or the thickness of the lithium metal layer formed on the surface of the lithium-containing metal layer 22 of the anode layer 20 reaches a predetermined value. The charge rate may be increased when the charge rate of the lithium metal secondary battery reaches 20% or more. The increase in the charge rate is not particularly limited, but may be set so that the average charge rate is within the range of 0.10 C to 0.30 C. The average charge rate relative to the charge rate at the start of charging may be within the range of 1.1 to 4.0 C.

[0037] In the discharging step S3, the lithium metal secondary battery charged in the first charging step is discharged until the voltage reaches 2.65 V. The discharging is performed, for example, in a temperature environment of 20° C. or higher and 40° C. or lower. The discharge may be performed at a discharge rate of 1C while applying a pressure of 1000 kPa in the stacking direction of the electrode stack 1 of the lithium metal secondary battery.

[0038] In the second charging step S4, the lithium metal secondary battery discharged in the discharging step is charged until the voltage reaches 3.72 V. Charging may be performed, for example, in a temperature environment of 20° C. or higher and 40° C. or lower, while applying a pressure of 1000 kPa in the stacking direction of the electrode stack 1 of the lithium metal secondary battery, at a charge rate of 0.1 C.

[0039] According to the aging method for a lithium metal secondary battery of this embodiment configured as described above, the holding temperature and holding time in the wetting step S1 are within the above ranges, so the electrolyte solution easily permeates the entire electrode stack 1. Furthermore, the applied pressure and charge rate at the start of charging in the first charging step S2 are within the above ranges, so the lithium layer formed on the surface of the lithium-containing metal layer 22 becomes dense, the aging yield rate is improved, and the aging time can be shortened.

[0040] According to the aging method for a lithium metal secondary battery of this embodiment, the discharging step S3 and the second charging step S4 are further performed, which stabilizes each layer of the electrode stack 1 of the lithium metal secondary battery and improves the aging yield rate. However, if the lithium metal secondary battery is to be used immediately after the first charging step S2, the discharging step S3 and the second charging step S4 may be omitted.

[0041] In the first charging step of the aging method for a lithium metal secondary battery according to this embodiment, if the applied pressure is increased continuously or stepwise, the density of the deposited lithium in the thickness direction can be adjusted by applying pressure to the lithium metal secondary battery according to the state of charge of the lithium metal secondary battery, and the lithium metal layer formed in the first charging step is likely to become denser. Furthermore, if the applied pressure at the end of charging is within the above range, the balance between the lithium deposition density and the permeability of the electrolyte is good, and further, the lithium metal secondary battery is unlikely to be pressurized with excessive pressure, causing the electrolyte to be squeezed out. Therefore, the aging yield rate is improved.

[0042] In the first charging step of the aging method for a lithium metal secondary battery of this embodiment, when the charge rate is increased continuously or stepwise, charging is started at a low charge rate to facilitate the formation of a dense lithium metal layer in the lithium-containing metal layer, and then the charge rate is increased. This makes it possible to achieve both the formation of a dense lithium metal layer with a uniform interface between the lithium-containing metal layer and the deposited lithium and the shortening of the aging time.

[0043] Although the aging method for a lithium metal secondary battery according to this embodiment has been described above, the present invention is not limited to the above embodiment. For example, a standing step in which the lithium metal secondary battery is allowed to stand may be provided between the first charging step S2 and the discharging step S3. In this standing step, the lithium metal secondary battery may be allowed to stand without pressure in a temperature environment of, for example, 20°C or higher and 40°C or lower. The standing time is, for example, within a range of 1 hour to 50 hours. [Example]

[0044] The present invention will be described with reference to examples, but is not limited to these examples. In these examples, a lithium metal secondary battery prepared as follows was used.

[0045] (Preparation of positive electrode) 2 wt% of acetylene black (AB) as an electron conductive material, 1.5 wt% of polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidone (NMP) as a dispersion solvent were premixed and wet mixed in a planetary mixer to obtain a premixed slurry. 0.8 Co 0.1 Mn 0.1 The obtained premixed slurry was mixed with O2 (NCM811) and a pre-dope material, and dispersed using a planetary mixer to obtain a positive electrode paste. NCM811 had a median diameter of 4 μm. Next, the obtained positive electrode paste was applied to an aluminum positive electrode current collector without a primer layer, dried, and pressed with a roll press, resulting in a positive electrode active material layer with a thickness of 64 μm and a density of 3.3 g / cm. 3The cathode was then dried in a vacuum at 120°C to form a cathode plate having a cathode active material layer. The obtained cathode plate was punched out to a size of 30 mm x 40 mm to form a cathode.

[0046] (Preparing the negative electrode) The negative electrode was made of a clad material consisting of a 10 μm thick copper foil and a 20 μm thick lithium foil, which was punched out to have an electrode area of ​​34 mm × 44 mm.

[0047] (Preparing the separator) An alumina-coated polyethylene microporous film was used as the separator.

[0048] (Preparation of electrolyte) Dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) were mixed in a mass ratio of 60:40 to obtain a mixed solvent, and LiFSI was dissolved in the resulting mixed solvent at a concentration of 2.1 mol / L to prepare an electrolyte solution.

[0049] (Fabrication of lithium metal secondary batteries) An outer container was fabricated by heat-sealing aluminum laminate for secondary batteries (manufactured by Dai Nippon Printing Co., Ltd.) into a bag shape. After placing a laminate consisting of a positive electrode, a separator, and a negative electrode in that order inside the outer container, 350 μL of electrolyte was poured into the container, and the opening of the outer container was heat-sealed to produce a lithium metal secondary battery.

[0050] [Example 1] The lithium metal secondary battery was placed in a thermostatic chamber adjusted to 25°C and held there for 48 hours (wetting step). Next, the lithium metal secondary battery was charged to 4.1 V at a charge rate of 0.10 C while being pressurized at 500 kPa in the stacking direction of the electrode laminate. The applied pressure was continuously increased so that the pressure at the end of charging was 700 kPa (first charging step). The first charging step was carried out in a thermostatic chamber adjusted to 25°C. Next, the lithium metal secondary battery charged in the first charging step was placed in a thermostatic chamber adjusted to 25°C and held there for 24 hours without pressurization. Thereafter, the lithium metal secondary battery was discharged to 2.65 V at a discharge rate of 1 C while being pressurized at 1000 kPa in the stacking direction of the electrode laminate (discharging step). The discharging step was carried out in a thermostatic chamber at 25°C. Next, the discharged lithium metal secondary battery was charged at a charge rate of 0.1 C to 3.7 V (80% SOC of the lithium metal secondary battery) while being pressurized at 1000 kPa in the stacking direction of the electrode stack (second charging step). The second charging step was carried out in a thermostatic chamber at 25°C. In this manner, the lithium metal secondary battery was aged.

[0051] [Examples 2 to 4, Comparative Examples 1 to 10] The temperature of the thermostatic bath in the wetting step was set to the temperature shown in Table 1 below, and the holding time was set to the holding time shown in Table 1 below. The pressurizing conditions in the first charging step were set to the heating conditions shown in Table 1 below, and the charging conditions in the first charging step were set to the charging conditions shown in Table 1 below. Except for the above, aging of the lithium metal secondary battery was performed in the same manner as in Example 1. For the heating conditions in Table 1, the start pressure is the pressurized pressure at the start of charging, and the end pressure is the pressurized pressure at the end of charging. The pressure difference is the value obtained by subtracting the pressurized pressure at the start of charging from the pressurized pressure at the end of charging. A pressure difference of 0 means that the pressurized pressure was constant from the start of charging to the end of charging. For the charging conditions in Table 1, the start rate is the charge rate at the start of charging. "Yes" in the charge rate switching section means that the charge rate was switched to 1C when the state of charge (SOC) of the lithium metal secondary battery reached 20%. "No" in the charge rate switching section means that the charge rate was not switched. The average charge rate is the average of the charge rates from the start of charging to the end of charging. If charging rate switching is "none", the average charging rate and the starting rate are the same.

[0052] [evaluation] The aged lithium metal secondary batteries were evaluated for yield rate and lithium deposition thickness in the negative electrode layer as described below. The total time required from the wetting step to the second charging step was calculated as the total aging time. The results are shown in Table 1 below.

[0053] (yield rate) The aged lithium metal secondary batteries were discharged and charged / discharged twice under the following conditions. The charged lithium metal secondary batteries were discharged to SOC 50% at a discharge rate of 1 / 3C. The OCV of the lithium metal secondary batteries at SOC 50% was measured over time. If the OCV was 3.9V or less 6 hours after the start of OCV measurement, it was determined to have had a micro-short circuit and was deemed defective, and the yield rate was calculated. A yield rate of 90% or more is preferable.

[0054] (Discharge and charge conditions) The discharge conditions were a constant current discharge at 1 / 3 C down to 2.65 V. The charge conditions were a constant current charge at 1 / 3 C followed by a constant voltage charge at 4.30 V for 1.5 hours.

[0055] (Thickness of lithium metal layer in the negative electrode layer) The lithium metal secondary battery that had been subjected to the charge-discharge cycle for the yield evaluation was charged at a constant current of 1 / 3 C, followed by CCCV charging, which involved constant voltage charging at 4.30 V for 1.5 hours, until the SOC reached 100%. The charged lithium metal secondary battery was disassembled, the negative electrode layer was removed, and the total thickness of the lithium foil of the negative electrode layer and the lithium layer deposited on its surface was measured using a micrometer. It is preferable that this total lithium metal layer thickness be less than 49 μm.

[0056] [Table 1]

[0057] The results in Table 1 show that under the conditions of Examples 1 to 4, in which the temperature and holding time in the wetting step, and the applied pressure and charge rate at the start of charge in the first charge step are within the ranges of the present invention, the lithium metal layer formed during charge is thin and dense, the yield rate is low, and the aging time can be shortened. In particular, under the conditions of Examples 2 to 4, in which the charge rate is switched in the first charge step, the aging time can be further shortened.

[0058] In contrast, under the conditions of Comparative Examples 1 and 2, in which the applied pressure at the start of charging in the first charging step is lower than the range of the present invention, the lithium metal layer formed by charging becomes thicker and the density of the lithium metal layer decreases. Under the conditions of Comparative Examples 3 to 8, in which the applied pressure at the start of charging in the first charging step is lower than the range of the present invention, the yield rate decreases.

[0059] In Comparative Example 9, in which the retention time in the wetting step was shorter than the range of the present invention and the pressurizing conditions and charging conditions in the first charging step were the same as those in Comparative Example 8, the yield rate was lower than that of Comparative Example 8. This was because the electrolyte did not sufficiently penetrate into the electrode stack during the wetting step.

[0060] In Comparative Example 10, where the temperature in the wetting step was higher than the range of the present invention, the holding time was shorter than the range of the present invention, and the pressure conditions and charging conditions in the first charging step were the same as those in Comparative Example 8, the yield rate was lower than in Comparative Example 8, and the lithium metal layer formed by charging was thicker. This is because excessive by-products were produced by the reaction between the negative electrode and the electrolyte in the wetting step, reducing the wettability of the lithium surface. [Explanation of symbols]

[0061] 1 Electrode laminate 10 Positive electrode layer 11 Positive electrode current collector 12 Cathode active material layer 20 negative electrode layer 21 Negative electrode current collector 22 Lithium-containing metal layer 30 Separator

Claims

1. A method for aging a lithium metal secondary battery having an electrode stack in which a positive electrode layer, a separator, and a negative electrode layer are stacked in this order, and an electrolyte solution, wherein the negative electrode layer includes a lithium-containing metal layer, the method comprising: a wetting step of holding the lithium metal secondary battery in a temperature environment of 20°C or higher and 40°C or lower for a holding time of 24 hours or higher and 60 hours or lower; a first charging step of initiating charging of the lithium metal secondary battery at a charge rate of 0.05 C or more and 0.20 C or less while applying a pressure of 400 kPa or more and 650 kPa or less in the stacking direction of the electrode stack in a temperature environment of 20°C or more and 40°C or less, and terminating the charging when the voltage reaches a range of 4.1 V or more and 4.3 V or less.

2. a discharging step of discharging the lithium metal secondary battery charged in the first charging step until the voltage reaches 2.65 V; 2. The aging method for a lithium metal secondary battery according to claim 1, further comprising a second charging step of charging the lithium metal secondary battery discharged in the discharging step until the voltage reaches 3.72 V.

3. 3. The method for aging a lithium metal secondary battery according to claim 1, wherein the applied pressure is increased continuously or stepwise in the first charging step.

4. 4. The method for aging a lithium metal secondary battery according to claim 3, wherein the applied pressure at the end of charging in the first charging step is in the range of 650 kPa or more and 900 kPa or less.

5. 3. The method for aging a lithium metal secondary battery according to claim 1, wherein the charge rate is increased continuously or stepwise in the first charging step.

Images