Lithium metal secondary battery and method for manufacturing same

Abstract

To provide a lithium metal secondary battery capable of improving charge / discharge cycle characteristics by suppressing decomposition of a high-concentration electrolyte while controlling a deposition form of lithium.SOLUTION: The lithium-metal secondary battery 2 includes a positive electrode 22, a negative electrode 20 having a lithium-metal layer 28, a separator 23 disposed between the positive electrode 22 and the negative electrode 20, and an electrolytic solution 26 containing 2 to 6 mol of electrolytes per 1L of solvent and having high oxidation resistance such that a voltage at which a current density is 0.4 mA / cm2 when a counter electrode is lithium and a working electrode is platinum is 5.5 V or more. The lithium metal layer 28 has a relative density of 70 to 95%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a lithium metal secondary battery and a method for manufacturing the same. [Background technology]

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted in order to ensure that more people have access to affordable, reliable, sustainable and advanced energy.

[0003] Lithium metal secondary batteries, which have a lithium metal layer on the negative electrode, are known as a secondary battery technology. Compared to conventional lithium-ion secondary batteries, lithium metal secondary batteries have a significantly higher energy density and are expected to be put to practical use. However, unlike conventional carbon-based negative electrodes, lithium metal secondary batteries do not have a film structure-maintaining material for storing and releasing lithium ions. Instead, lithium metal secondary batteries form a lithium metal layer by depositing lithium metal particles on a negative electrode current collector or lithium foil. Therefore, they have issues with poor charge-discharge efficiency and poor charge-discharge cycle characteristics due to factors such as expansion and contraction of the electrode during the dissolution and deposition of lithium metal during charge and discharge, dendrites formed on the negative electrode, and reactions between lithium metal and the electrolyte.

[0004] In response to this, a technology has been proposed for lithium metal secondary batteries in which a space of 100 to 120% of the thickness at which lithium would theoretically deposit between the negative electrode and the separator is provided (see, for example, Patent Document 1). This technology is said to be able to mitigate the expansion and contraction of lithium metal secondary batteries, thereby improving charge-discharge efficiency and charge-discharge cycle characteristics.

[0005] Furthermore, a technology has been proposed for lithium metal secondary batteries that uses a high-concentration electrolyte solution containing approximately 4 to 6 moles of lithium bis(fluorosulfonyl)imide (LiFSI) as an electrolyte (supporting salt) per liter of organic solvent (see, for example, Patent Document 2). According to this technology, the oxidation-reduction stability of the electrolyte solution is improved by including a high concentration of electrolyte, and it is believed that the charge-discharge cycle life can be extended. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-192628 [Patent Document 2] Special Publication No. 2018-505538 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology of Patent Document 1 provides a space to prevent pressure from being applied to the lithium deposition site, which can mitigate expansion and contraction that occurs during charge and discharge, but this results in uneven lithium deposition and an increased contact area with the electrolyte. In particular, carbonate esters react easily with lithium metal, which increases the amount of decomposition of the electrolyte and deteriorates the charge and discharge cycle characteristics.

[0008] Furthermore, in the technology of Patent Document 2, since ether-based solvents have low oxidation resistance, the oxidative decomposition reaction can be suppressed by dissolving the electrolyte at a high concentration, but the lithium ions near the positive electrode rapidly decrease during the discharge reaction, resulting in a deterioration in charge-discharge cycle characteristics.

[0009] The present invention has been made in view of the above, and aims to provide a lithium metal secondary battery that can control the precipitation form of lithium while suppressing decomposition of a high-concentration electrolyte, thereby improving charge-discharge cycle characteristics. [Means for solving the problem]

[0010] (1) The present invention provides a lithium metal secondary battery (for example, lithium metal secondary battery 1 described below) comprising a positive electrode (for example, positive electrode 12 described below), a negative electrode (for example, negative electrode 10 described below) having a lithium metal layer (for example, lithium metal layer 18 described below), a separator (for example, separator 15 described below) disposed between the positive electrode and the negative electrode, and a highly reduction-resistant electrolyte (for example, electrolyte solution 16 described below) containing 2 to 6 mol of electrolyte per liter of solvent and having a lithium deposition and dissolution efficiency, which is the ratio of the amount of lithium redissolved to the amount of lithium deposited on the copper surface, of 98.5% or more, wherein the lithium metal layer has a relative density of 40 to 85%.

[0011] In the lithium metal secondary battery (1), a high-concentration electrolyte with high reduction resistance and susceptibility to oxidative decomposition is used as the electrolyte, and the relative density of the lithium metal layer is set within a range of 40 to 85%. When a high-concentration electrolyte with high reduction resistance and susceptibility to oxidative decomposition (hereinafter also referred to as a high-concentration electrolyte with high reduction resistance) is used, the lithium ion concentration near the positive electrode tends to decrease during discharge. The depletion of lithium ions near the positive electrode eliminates the high-concentration effect, facilitating oxidative decomposition of the electrolyte. In contrast, in the lithium metal secondary battery (1), the relative density of the lithium metal layer is within a low- to medium-density range, which promotes the diffusion of lithium ions redissolved from the lithium metal layer to the positive electrode. This eliminates uneven distribution of lithium ions on the negative electrode side and suppresses the decrease in lithium ion concentration on the positive electrode side, thereby suppressing the oxidative decomposition of the electrolyte on the positive electrode side. Therefore, in the lithium metal secondary battery (1), by controlling the morphology of lithium deposition and adjusting the relative density of the lithium metal layer, decomposition of the high-concentration electrolyte can be suppressed, improving charge-discharge cycle performance.

[0012] (2) The present invention also provides a battery comprising a positive electrode (e.g., a positive electrode 22 described later), a negative electrode (e.g., a negative electrode 20 described later) having a lithium metal layer (e.g., a lithium metal layer 28 described later), a separator (e.g., a separator 25 described later) disposed between the positive electrode and the negative electrode, and a solvent containing 2 to 6 mol of electrolyte per 1 L, and a current of 0.4 mA / cm when the counter electrode is lithium and the working electrode is platinum. 2and a highly oxidation-resistant electrolyte solution (for example, electrolyte solution 26 described later) in which the voltage at which the current density is reached is 5.5 V or higher, and the relative density of the lithium metal layer is 70 to 95%.

[0013] In the lithium metal secondary battery (2), a high-concentration electrolyte (hereinafter referred to as a high-oxidation-resistant high-concentration electrolyte) that is highly oxidation-resistant and reactive with lithium metal is used as the electrolyte, and the relative density of the lithium metal layer is set to a range of 70 to 95%. When a high-concentration electrolyte that is highly oxidation-resistant and reactive with lithium metal is used, the lithium ion concentration near the negative electrode tends to decrease during charging. This depletion of lithium ions near the negative electrode eliminates the high-concentration effect, facilitating reductive decomposition of the electrolyte. In contrast, in the lithium metal secondary battery (2), the relative density of the lithium metal layer is within a medium- to high-density range, which prevents lithium ions from resolving from the lithium metal layer from diffusing to the positive electrode. This allows the lithium ions to remain near the negative electrode, thereby maintaining a high concentration of lithium ions at the negative electrode and reducing the contact area between the electrolyte and lithium metal. Therefore, in the lithium metal secondary battery (2), by controlling the morphology of lithium deposition and adjusting the relative density of the lithium metal layer, decomposition of the high-concentration electrolyte can be suppressed, improving charge-discharge cycle performance.

[0014] (3) The present invention also provides a method for producing a lithium metal secondary battery (e.g., the lithium metal secondary battery 1 described later) including a positive electrode (e.g., the positive electrode 12 described later), a negative electrode (e.g., the negative electrode 10 described later) having a lithium metal layer (e.g., the lithium metal layer 18 described later), a separator (e.g., the separator 15 described later) disposed between the positive electrode and the negative electrode, and an electrolytic solution (e.g., the electrolytic solution 16 described later), the electrolytic solution containing 2 to 6 mol of electrolyte per 1 L of solvent and capable of forming a lithium metal layer on a copper surface. The present invention provides a method for manufacturing a lithium metal secondary battery, which includes a lithium metal layer formation step in which a charge / discharge cycle is repeated two or more times under an applied load of 0.05 to 1.0 MPa, with a charge rate of 0.2 C or less and a discharge rate of 0.2 C or more, in order to form a lithium metal layer (for example, lithium metal layer 18 described below) having a relative density of 40 to 85%, using a highly reduction-resistant electrolyte solution having a lithium deposition / dissolution efficiency of 98.5% or more, which is the ratio of the amount of redissolved lithium to the amount of deposited lithium.

[0015] According to the method for producing a lithium metal secondary battery in (3), a lithium metal layer having a relative density of 40 to 85% can be formed by repeating two or more charge-discharge cycles in which the charge rate is 0.2 C or less and the discharge rate is 0.2 C or more under a load of 0.05 to 1.0 MPa. Therefore, according to the method for producing a lithium metal secondary battery in (3), by adjusting the cell confining pressure to 0.05 to 1.0 MPa, a lithium metal layer having a relative density of 40 to 85%, which is preferable when using a highly reduction-resistant high-concentration electrolyte, can be formed, thereby suppressing decomposition of the high-concentration electrolyte and obtaining a lithium metal secondary battery that can improve the charge-discharge cycle characteristics.

[0016] (4) The present invention also provides a method for producing a lithium metal secondary battery (for example, the lithium metal secondary battery 2 described later) including a positive electrode (for example, the positive electrode 22 described later), a negative electrode (for example, the negative electrode 20 described later) having a lithium metal layer (for example, the lithium metal layer 28 described later), a separator (for example, the separator 25 described later) disposed between the positive electrode and the negative electrode, and an electrolyte (for example, the electrolyte 26 described later), wherein the electrolyte contains 2 to 6 mol of electrolyte per 1 L of solvent and has a current density of 0.4 mA / cm when the counter electrode is lithium and the working electrode is platinum. 2 the lithium metal layer forming step includes repeating two or more charge-discharge cycles in which the charge rate is 0.2 C or less and the discharge rate is 0.2 C or more under an applied load of 0.1 to 3.0 MPa, in order to form the lithium metal layer (for example, lithium metal layer 28 described below) having a relative density of 70 to 95%.

[0017] According to the method for producing a lithium metal secondary battery in (4), a lithium metal layer having a relative density of 70 to 95% can be formed by repeating two or more charge-discharge cycles in which the charge rate is 0.2 C or less and the discharge rate is 0.2 C or more under a load of 0.1 to 3.0 MPa. Therefore, according to the method for producing a lithium metal secondary battery in (4), by adjusting the cell confining pressure to 0.1 to 3.0 MPa, a lithium metal layer having a relative density of 70 to 95%, which is preferable when using a highly oxidation-resistant high-concentration electrolyte, can be formed, thereby suppressing decomposition of the high-concentration electrolyte and providing a lithium metal secondary battery with improved charge-discharge cycle characteristics. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a lithium metal secondary battery that can control the morphology of lithium deposition while suppressing decomposition of a high-concentration electrolyte, thereby improving charge-discharge cycle characteristics, which in turn contributes to energy efficiency. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram showing the configuration of a lithium metal secondary battery according to a first embodiment. [Figure 2] FIG. 4 is a diagram showing the configuration of a lithium metal secondary battery according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description of the second embodiment, the same components as those in the first embodiment will be denoted by the same reference numerals, and the description thereof will be omitted.

[0021] [First embodiment] 1 is a diagram showing the configuration of a lithium metal secondary battery 1 according to the first embodiment. The lithium metal secondary battery 1 according to the first embodiment includes a positive electrode current collector 11, a positive electrode 12, a negative electrode current collector 13, a lithium foil 14, a separator 15, an electrolyte 16, and a negative electrode 10 having a lithium metal layer 18 composed of lithium metal particles 17.

[0022] A conventionally known positive electrode current collector can be used as the positive electrode current collector 11. For example, aluminum foil can be used as the positive electrode current collector 11.

[0023] The positive electrode 12 is composed of a layer containing a positive electrode active material. Examples of the positive electrode active material include a layered active material containing lithium, a spinel-type active material, and an olivine-type active material. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), and LiNi p Mn q Co r O2(p+q+r=1), LiNi p Al q Co rExamples of the positive electrode active material include LiMnO2 (p+q+r=1), lithium manganate (LiMn2O4), a heteroelement-substituted Li-Mn spinel represented by Li1+xMn2-x-yMyO4 (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). Preferably, the positive electrode active material is Li1Ni 0.8 Co 0.1 Mn 0.1 O2 (NCM811) is used, and a pre-dopant may be used in combination.

[0024] The positive electrode 12 may be composed of a layer containing, in addition to the above-mentioned positive electrode active material, a binder, a conductive additive, etc. For example, polyvinylidene fluoride (PVDF) can be used as the binder, and acetylene black (AB) can be used as the conductive additive.

[0025] A conventionally known negative electrode current collector can be used as the negative electrode current collector 13. For example, copper foil can be used as the negative electrode current collector 13.

[0026] The lithium foil 14 is laminated on the negative electrode current collector 13. Although the lithium foil 14 is not an essential component, providing the lithium foil 14 on the negative electrode current collector 13 can promote the deposition of lithium metal particles 17, which will be described later, and can promote the formation of the lithium metal layer 18 in the negative electrode 10. In addition, the adhesion of the lithium metal layer 18 in the negative electrode 10 can be improved.

[0027] The separator 15 is disposed between the positive electrode 12 and the negative electrode 10. A conventionally known separator can be used as the separator 15. For example, the separator 15 can be a microporous film made of polyethylene coated with alumina.

[0028] The electrolytic solution 16 is disposed between the positive electrode 12 and the negative electrode 10. For example, the electrolytic solution 16 is disposed by impregnating the separator 15. The electrolytic solution 16 includes an organic solvent and an electrolyte.

[0029] As the organic solvent, for example, a hydrofluoroether, which is a fluorine-substituted chain hydrocarbon, such as 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane, methyl nonafluoroisobutyl ether, or methyl nonafluorobutyl ether can be used as the first organic solvent. Furthermore, for example, 1,2-dimethoxyethane (DME), ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL), dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC) can be used as the second organic solvent. In the electrolytic solution 16 of this embodiment, the first organic solvent and the second organic solvent can be used in combination.

[0030] The electrolyte is a source of lithium ions, which are a charge transfer medium, and contains a lithium salt. As the lithium salt, at least one selected from the group consisting of LiFSI, LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), LiN(FSO2)2 (LiFSI), and LiBC4O8 can be used. Among them, LiFSI is preferably used as the electrolyte.

[0031] The electrolyte 16 of this embodiment is a highly reduction-resistant, high-concentration electrolyte that is highly reduction-resistant and easily oxidatively decomposed. Here, "high reduction resistance" means that the lithium deposition / dissolution efficiency, which is the ratio of the amount of lithium redissolved to the amount of lithium deposited on the copper surface, is 98.5% or higher. Specifically, "high reduction resistance" means that the lithium deposition / dissolution efficiency, determined based on the reduction resistance evaluation of the electrolyte, which will be described in detail in the Examples below, is 98.5% or higher.

[0032] The high reduction resistance of the electrolyte solution 16 can be achieved by adjusting the combination of the above-mentioned organic solvents and their blending ratio. In particular, by adjusting the combination of the above-mentioned first and second organic solvents and their blending ratio, an electrolyte solution with high reduction resistance can be obtained. In general, an electrolyte solution containing primarily an ether-based organic solvent has high reduction stability and is easily oxidatively decomposed. In addition, an electrolyte solution containing primarily an ester-based organic solvent has high oxidation stability and is easily reactive with lithium metal. In consideration of these characteristics, for example, a combination of hydrofluoroether and 1,2-dimethoxyethane with an adjusted blending ratio can be preferably used as the electrolyte solution 16.

[0033] Furthermore, the electrolytic solution 16 of this embodiment is a high-concentration electrolytic solution containing 2 to 6 mol of electrolyte per 1 L of solvent. The high concentration of the electrolyte in the electrolytic solution 16 improves the oxidation-reduction stability and the charge-discharge cycle characteristics.

[0034] The electrolytic solution 16 may contain additives. Examples of additives include a film-forming material, a dispersant, etc. Specific examples of additives include LiNO, lithium nitrite, LiPOF, Cs-PF, PS, ES, DTD, lithium sulfate, and LiFOB.

[0035] The negative electrode 10 has a lithium metal layer 18 composed of lithium metal particles 17. That is, the lithium metal layer 18 is a lithium metal deposition layer formed by the accumulation of the lithium metal particles 17 deposited on the negative electrode 10. Therefore, the lithium metal layer 18 in this embodiment does not include the lithium foil 14 that is laminated on the negative electrode current collector 13 in advance.

[0036] The relative density of the lithium metal layer 18 is 40 to 85%. That is, the relative density of the lithium metal layer 18 of this embodiment is set to a low to medium density by controlling the deposition form of the lithium metal deposited on the negative electrode 10. Here, the relative density is the actual density relative to the true density, and is expressed as relative density % = (actual density / true density) × 100. That is, this relative density means the packing rate %.

[0037] Next, with reference to FIG. 1, the effects of the lithium metal secondary battery 1 of this embodiment will be described in detail.

[0038] As described above, in the lithium metal secondary battery 1 of this embodiment, a highly reduction-resistant, high-concentration electrolyte that is highly resistant to reduction and susceptible to oxidative decomposition is used as the electrolyte 16, and the relative density of the lithium metal layer 18 is set within a range of 40 to 85%. Conventionally, when a highly reduction-resistant, high-concentration electrolyte that is highly resistant to reduction and susceptible to oxidative decomposition is used, the lithium ion concentration near the positive electrode tends to decrease during discharge, and the high-concentration effect is lost due to depletion of lithium ions near the positive electrode, making it easier for the electrolyte to undergo oxidative decomposition.

[0039] In contrast, in the lithium metal secondary battery 1 of this embodiment, the relative density of the lithium metal layer 18 is set within a range from low density to medium density, which promotes the diffusion of lithium ions redissolved from the lithium metal layer 18 to the positive electrode 12. This eliminates uneven distribution of lithium ions on the negative electrode 10 side and suppresses a decrease in the lithium ion concentration on the positive electrode 12 side, thereby suppressing the progress of oxidative decomposition of the electrolyte solution 16 on the positive electrode 12 side. Therefore, in the lithium metal secondary battery 1 of this embodiment, by controlling the precipitation form of lithium and adjusting the relative density of the lithium metal layer 18, it is possible to suppress the decomposition of a high-concentration electrolyte solution and improve charge-discharge cycle characteristics.

[0040] The lithium metal secondary battery 1 of this embodiment can be manufactured by a manufacturing method characterized by adding a lithium metal layer formation step to the conventional manufacturing method for lithium ion secondary batteries. Specifically, in the lithium metal layer formation step of this manufacturing method, a charge / discharge cycle in which a charge rate of 0.2 C or less and a discharge rate of 0.2 C or more is repeated for two or more cycles while applying a load of 0.05 to 1.0 MPa to the lithium metal secondary battery cell. In this way, by controlling the cell restraining pressure to the range of 0.05 to 1.0 MPa, a lithium metal layer 18 with a relative density of 40 to 85% can be formed. In other words, the relative density of the lithium metal layer 18 can be adjusted in the initial chemical conversion step, thereby reliably achieving the effects described above.

[0041] [Second embodiment] FIG. 2 is a diagram showing the configuration of a lithium metal secondary battery 2 according to the second embodiment. The lithium metal secondary battery 2 according to the second embodiment includes a positive electrode current collector 21, a positive electrode 22, a negative electrode current collector 23, a lithium foil 24, a separator 25, an electrolyte 26, and a negative electrode 20 having a lithium metal layer 28 composed of lithium metal particles 27. The lithium metal secondary battery 2 of this embodiment has the same configuration as the lithium metal secondary battery 1 of the first embodiment, except for the configurations of the electrolyte 26 and the lithium metal layer 28 composed of lithium metal particles 27. Only the configurations and effects that differ from the first embodiment will be described in detail below.

[0042] The electrolytic solution 26 is disposed between the positive electrode 22 and the negative electrode 20. For example, the electrolytic solution 26 is disposed by being impregnated into the separator 25 described above. The electrolytic solution 26 contains an organic solvent and an electrolyte. The organic solvent and the electrolyte can be selected from the same organic solvents and electrolytes as those described in the first embodiment. Furthermore, the electrolytic solution 26 may contain the above-mentioned additives as appropriate, similar to the electrolytic solution 16 of the first embodiment.

[0043] The electrolyte 26 of this embodiment is a highly oxidation-resistant, high-concentration electrolyte that is highly oxidation-resistant and readily reacts with lithium metal. Here, high oxidation resistance means a current of 0.4 mA / cm when the counter electrode is lithium and the working electrode is platinum. 2 This means that the voltage at which the current density is equal to or higher than 5.5 V. Specifically, high oxidation resistance means that the oxidative decomposition potential determined based on the oxidation resistance evaluation of the electrolyte solution, which will be described in detail in the Examples below, is equal to or higher than 5.5 V.

[0044] The high oxidation resistance of the electrolyte solution 26 can be achieved by adjusting the combination of the above-mentioned organic solvents and their blending ratio. In particular, by adjusting the combination of the above-mentioned first and second organic solvents and their blending ratio, an electrolyte solution with high oxidation resistance can be obtained. In general, an electrolyte solution containing primarily an ether-based organic solvent has high reduction stability and is easily oxidatively decomposed. In addition, an electrolyte solution containing primarily an ester-based organic solvent has high oxidation stability and is easily reactive with lithium metal. In consideration of these characteristics, for example, dimethyl carbonate (DMC) can be preferably used as the electrolyte solution 26.

[0045] Similarly to the electrolytic solution 16 of the first embodiment, the electrolytic solution 26 of this embodiment is a high-concentration electrolytic solution containing 2 to 6 mol of electrolyte per 1 L of solvent. The high concentration of the electrolyte in the electrolytic solution 26 improves the oxidation-reduction stability and the charge-discharge cycle characteristics.

[0046] The negative electrode 20 has a lithium metal layer 28 composed of lithium metal particles 27. That is, the lithium metal layer 28 is a lithium metal deposition layer formed by the accumulation of lithium metal particles 27 deposited on the negative electrode 20. Therefore, the lithium metal layer 28 in this embodiment does not include the lithium foil 24 that is pre-laminated on the negative electrode current collector 23. This is the same as in the first embodiment.

[0047] The relative density of the lithium metal layer 28 is 70 to 95%. That is, the relative density of the lithium metal layer 28 of this embodiment is set to a medium to high density by controlling the deposition form of the lithium metal deposited on the negative electrode 20.

[0048] Next, with reference to FIG. 2, the effects of the lithium metal secondary battery 1 of this embodiment will be described in detail.

[0049] As described above, in the lithium metal secondary battery 2 of this embodiment, a highly oxidation-resistant, high-concentration electrolyte that is highly oxidation-resistant and reactive with lithium metal is used as the electrolyte 26, and the relative density of the lithium metal layer 28 is set within the range of 70 to 95%. Conventionally, when a high-concentration electrolyte that is highly oxidation-resistant and reactive with lithium metal is used, the lithium ion concentration near the negative electrode tends to decrease during charging, and the high-concentration effect is lost due to depletion of lithium ions near the negative electrode, making it easier for reductive decomposition of the electrolyte to proceed.

[0050] In contrast, in the lithium metal secondary battery 2 of this embodiment, the relative density of the lithium metal layer 28 is set within a range from medium density to high density, which makes it possible to suppress the diffusion of lithium ions redissolved from the lithium metal layer 28 into the positive electrode 22. This allows the lithium ions to be retained in the vicinity of the negative electrode 20, making it possible to maintain a high concentration of lithium ions at the negative electrode 20 at all times, while also reducing the contact area between the electrolyte 26 and lithium metal. Therefore, in the lithium metal secondary battery 2 of this embodiment, by controlling the precipitation form of lithium and adjusting the relative density of the lithium metal layer 28, it is possible to suppress the decomposition of the high-concentration electrolyte and improve the charge-discharge cycle characteristics.

[0051] The lithium metal secondary battery 2 of this embodiment can be manufactured by a manufacturing method characterized by adding a lithium metal layer formation step to the conventional manufacturing method for lithium ion secondary batteries. Specifically, in the lithium metal layer formation step of this embodiment, a charge / discharge cycle in which a charge rate of 0.2 C or less and a discharge rate of 0.2 C or more is repeated for two or more cycles while applying a load of 0.1 to 3.0 MPa to the lithium metal secondary battery cell. In this way, by controlling the cell restraining pressure to the range of 0.1 to 3.0 MPa, a lithium metal layer 28 with a relative density of 70 to 95% can be formed. In other words, the relative density of the lithium metal layer 28 can be adjusted in the initial chemical conversion step, thereby reliably achieving the effects described above.

[0052] The present invention is not limited to the above-described embodiment, and includes modifications and improvements within the scope of achieving the object of the present invention. [Example]

[0053] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0054] <Examples 1 to 5 and Comparative Examples 1 to 5> [Preparation of electrolyte] First, the electrolyte solutions used in each of the Examples and Comparative Examples were prepared. Specifically, the materials shown in Table 1 below were mixed in the mass ratios shown in Table 1 to prepare electrolyte solutions EL-a, EL-b, EL-c, EL-d, and EL-e. The electrolyte concentrations (mol / L) of each electrolyte solution were as shown in Table 2 below.

[0055] [Table 1]

[0056] [Evaluation of electrolyte reduction resistance] Separators made of alumina-coated polyethylene microporous membranes were impregnated with the electrolytes EL-a, EL-b, EL-c, EL-d, and EL-e shown in Table 1. Copper foil with a thickness of 8 μm was punched out to a diameter of 18 mm, and a clad material made of lithium foil with a thickness of 40 μm and copper foil with a thickness of 10 μm was punched out to a diameter of 14 mm. The punched copper foil and clad material were then placed with the electrolyte-impregnated separator sandwiched between them so that the copper and lithium faced each other, to produce coin cells.

[0057] After the coin cell was prepared, it was left at 25° C. for 1 hour. The positive electrode was connected to the copper electrode and the negative electrode was connected to the lithium electrode. Then, the following treatments (1) to (10) were carried out in order. (1) 0.24 mA / cm 2 10 hours, constant current discharge (2) 5 minutes pause (3) 0.24 mA / cm 2 Constant current charging until it reaches 1.0V (4) 0.24mA / cm 2 10 hours, constant current discharge (5) 0.24 mA / cm 2 1 hour constant current charging (6) 5 minute break (7) 0.24mA / cm 2 1 hour, constant current discharge (8) 5 minutes pause (9) (5) to (8) were performed for a total of 10 cycles. (10) 0.24mA / cm 2 Constant current charging until it reaches 1.0V

[0058] After the above treatments (1) to (10) were performed, the lithium deposition / dissolution efficiency, which is the ratio of the amount of re-dissolved lithium to the amount of lithium deposited on the copper surface, was calculated. Specifically, as shown in the following mathematical formula 1, the lithium deposition / dissolution efficiency was calculated from the ratio of the discharge capacity obtained as a result of the constant current discharge in the above (4) to the charge capacity obtained as a result of the constant current charge in the above (10).

[0059] [Number 1] Lithium deposition and dissolution efficiency (%) = {discharge capacity of (4) / charge capacity of (10)} × 100 Formula 1

[0060] [Evaluation of electrolyte oxidation resistance] The evaluation cell used was a microanalysis cell manufactured by EC Frontier Co., Ltd. The working electrode was a platinum wire electrode with a diameter of 3 mm, and the counter electrode was the same lithium and copper clad material used in the reduction resistance evaluation, processed to a size of 10 mm x 5 mm. 2 mL of electrolyte was used. Linear sweep voltammetry (LSV) was used as the evaluation method. The temperature was 25°C, and the voltage range was swept from the open circuit voltage (OCV) to 5.5 V. The sweep rate was 1.0 mV / s, and the current was 0.4 mA / cm. 2 The voltage at which a current value of 100 kJ / cm2 was detected (oxidative decomposition potential) was measured.

[0061] The reduction resistance evaluation results and oxidation resistance evaluation results for each of the above-mentioned electrolyte solutions are shown in Table 2 below. Note that electrolyte solutions with a lithium deposition dissolution efficiency of 98.5% or higher obtained in the reduction resistance evaluation were determined to be highly reduction-resistant electrolyte solutions (denoted as "reduction-resistant" in Table 2), and electrolyte solutions with an oxidative decomposition potential of 5.5 V or higher obtained in the oxidation resistance evaluation were determined to be highly oxidation-resistant electrolyte solutions (denoted as "oxidation-resistant" in Table 2). Furthermore, electrolyte solutions that were not determined to be either highly reduction-resistant or highly oxidation-resistant electrolyte solutions were determined to be non-applicable.

[0062] [Table 2]

[0063] [Preparation of positive electrode] Acetylene black (AB) as an electron conductive material, polyvinylidene fluoride (PVDF) as a binder, and polyvinylpyrrolidone (PVP) as a dispersant were premixed with N-methyl-2-pyrrolidone (NMP) as a dispersion solvent, and the mixture was wet mixed in a planetary mixer to obtain a premixed slurry. Subsequently, the premixed slurry and LiNi as a positive electrode active material were mixed. 0.8 Co 0.1 Mn 0.1The O2 (NCM811) and the pre-dope material were mixed and dispersed using a planetary mixer to obtain a positive electrode paste. The median diameter of the NCM811 was 12 μm.

[0064] The resulting positive electrode paste was then applied to an aluminum positive electrode current collector without a primer layer, dried, pressed with a roll press, and dried in a vacuum at 120°C to produce a positive electrode plate with a positive electrode mixture layer. The resulting positive electrode plate was punched out to a size of 30 mm x 40 mm to obtain a positive electrode.

[0065] [Preparation of negative electrode] A clad material consisting of a copper foil with a thickness of 10 μm and a lithium foil with a thickness of 20 μm was punched out to have an electrode area of ​​34 mm×44 mm, thereby obtaining a negative electrode.

[0066] [Separator fabrication] An alumina-coated polyethylene microporous membrane was prepared as a separator.

[0067] [Cell assembly and lithium metal secondary battery fabrication] The lithium metal secondary batteries of each Example and Comparative Example were fabricated by assembling a cell using the above-described positive electrode, negative electrode, separator, and electrolyte solution. Specifically, for each Example and Comparative Example, the lithium metal secondary batteries of each Example and Comparative Example were fabricated by assembling a cell using each electrolyte solution shown in Table 3 below, and setting the cell restraining pressure (surface pressure) during aging as shown in Table 3.

[0068] [Formation of lithium metal layer (deposit layer)] The lithium metal secondary batteries of each Example and Comparative Example fabricated as described above were held at the cell restraining pressure shown in Table 3 and left at a measurement temperature of 25°C for 24 hours. Next, constant current charging was performed at 2.2 mA to 4.3 V, followed by constant voltage charging at a voltage of 4.3 V for 1 hour. After leaving for 30 minutes, constant current discharging was performed at a current value of 8.8 mA to 2.65 V. This was repeated three times to form a lithium metal layer (precipitation layer).

[0069] [Calculation of the relative density of the lithium metal layer (deposit layer)] After forming the lithium metal layer (deposit layer) as described above, constant current charging was performed at 14.7 mA up to 4.3 V, followed by constant voltage charging at 4.3 V for 1 hour. After leaving the cell for 30 minutes, the cell was disassembled and the thickness T1 of the lithium metal layer (deposit layer) was measured. The thickness T1 of the lithium metal layer (deposit layer) was measured by measuring the total thickness of the negative electrode using a microgauge and subtracting the thickness of the lithium foil and copper foil (30 μm) from the measured value.

[0070] In addition, the theoretical thickness T2 of the lithium metal layer (deposit layer) was calculated by the following formula 2, where the opposing area was 12 cm 2 The theoretical capacity density of lithium metal was set to 3860 mAh / g, and the true density of lithium was set to 0.5073 g / cc.

[0071] [Number 2] Theoretical thickness T2 of the lithium metal layer (deposit layer) = Lithium deposition capacity (mAh) ÷ Opposite area ÷ Theoretical capacity density of lithium metal ÷ True density of lithium Formula 2

[0072] Using the thickness T1 of the lithium metal layer (deposit layer) obtained as described above and the theoretical thickness T2 of the lithium metal layer (deposit layer), the relative density of the lithium metal layer (deposit layer) was calculated according to the following formula 3.

[0073] [Number 3] Relative density of lithium metal layer (deposit layer) = T1 ÷ T2 × 100 (%) Formula 3

[0074] [Initial performance evaluation (discharge capacity)] For each of the lithium metal secondary batteries of the Examples and Comparative Examples, a holding pressure of 0.8 MPa was applied and the battery was left at a measurement temperature of 25°C for 1 hour, after which a constant current charge of 14.7 mA was performed to 4.3 V. Subsequently, a constant voltage charge at 4.3 V was performed for 1 hour, and the battery was left for 30 minutes. The discharge capacity at this time was measured as the initial discharge capacity (mAh). The current value at which discharge could be completed in 1 hour relative to the obtained discharge capacity was defined as 1C. The results are shown in Table 3.

[0075] [Initial resistivity] The lithium metal secondary battery cell after measuring the initial capacity was left at a measurement temperature of 25°C for 1 hour, then constant-current charged at 0.2C, adjusted to a 50% state of charge (SOC), and left for 10 minutes. The voltage was then measured after 10 seconds of pulse discharge at 0.5C. The horizontal axis represents the current, and the vertical axis represents the voltage after 10 seconds of pulse discharge at 0.5C. The lithium metal secondary battery cell was then left for 10 minutes, supplemented, and the SOC was returned to 50%, after which it was left for another 10 minutes. This procedure was repeated at C rates of 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, and plotted in the same manner as above. The slope of the approximated line obtained from the plot was calculated using the least-squares method, and this was used to determine the initial resistance of the lithium metal secondary battery cell. The initial specific resistance was calculated by multiplying the initial resistance by the electrode facing area. The results are shown in Table 3.

[0076] [Post-durability performance evaluation (discharge capacity)] A charge-discharge cycle durability test was conducted on the lithium metal secondary batteries of each Example and Comparative Example. Specifically, in a thermostatic chamber at 45°C, a constant current charge was performed to 4.2 V at a charge rate of 1 C, followed by a constant current discharge to 2.65 V at a discharge rate of 2 C. This cycle was repeated 20 times. After 20 cycles, the thermostatic chamber was changed to 25°C and the battery was left for 24 hours, after which a constant current charge was performed to 4.2 V at 0.33 C. Subsequently, a constant voltage charge was performed at 4.2 V for 1 hour, and after leaving the battery for 5 minutes, a constant current discharge was performed to 2.5 V at a discharge rate of 0.33 C. The discharge capacity (mAh) after the durability test was measured. The results are shown in Table 3.

[0077] [Capacity retention rate after durability] The ratio of the discharge capacity (mAh) after the endurance test to the initial discharge capacity (mAh) was calculated, and the capacity retention rate (%) after the endurance test was calculated. The results are shown in Table 3.

[0078] [Table 3]

[0079] As shown in Table 3, it was confirmed that all of the lithium metal secondary batteries of this example had a higher capacity retention rate after endurance testing than the lithium metal secondary batteries of the comparative examples. Therefore, it was confirmed that with the lithium metal secondary batteries of this example, decomposition of the electrolyte on the electrode surface can be suppressed and charge / discharge cycle characteristics can be improved by adjusting the relative density of the lithium metal layer by adjusting the cell confining pressure in accordance with the characteristics of each electrolyte. [Explanation of symbols]

[0080] 1,2 Lithium metal secondary battery 10,20 negative electrode 11,21 Positive electrode current collector 12,22 Positive electrode 13,23 Negative electrode current collector 14,24 Lithium foil 15,25 separator 16,26 Electrolyte 17,27 Lithium metal particles 18,28 Lithium metal layer

Claims

1. A positive electrode and a negative electrode having a lithium metal layer; a separator disposed between the positive electrode and the negative electrode; When 2 to 6 mol of electrolyte is contained per 1 L of solvent, the counter electrode is lithium and the working electrode is platinum, the current is 0.4 mA / cm ２ and a highly oxidation-resistant electrolyte solution having a voltage of 5.5 V or more at which a current density of A lithium metal secondary battery, wherein the relative density of the lithium metal layer is 70 to 95%.

2. A method for manufacturing a lithium metal secondary battery comprising: a positive electrode; a negative electrode having a lithium metal layer; a separator disposed between the positive electrode and the negative electrode; and an electrolyte solution, The electrolytic solution contains 2 to 6 mol of electrolyte per 1 L of solvent, and has a current density of 0.4 mA / cm when the counter electrode is lithium and the working electrode is platinum. ２ The electrolyte solution has a high oxidation resistance and a voltage at which a current density of 5.5 V or more is obtained. A method for manufacturing a lithium metal secondary battery, comprising: a lithium metal layer forming step of repeating two or more charge / discharge cycles in which a charge rate is 0.2 C or less and a discharge rate is 0.2 C or more under an applied load of 0.1 to 3.0 MPa, in order to form the lithium metal layer having a relative density of 70 to 95%.

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