KR20190083108A - Electrolyte system and lithium metal battery comprising the same - Google Patents

Abstract

The present invention relates to an electrolyte for a lithium secondary battery and a lithium metal secondary battery comprising the same. According to the present invention, an electrolyte system for maintaining energy density and extending battery life of a secondary battery based on a lithium metal negative electrode can be provided. Specifically, the present invention can provide an electrolyte for a lithium secondary battery in which a stable SEI film is formed to stabilize a negative electrode of a lithium metal battery having a high possibility of battery ignition and to suppress lithium dendrite formation and diffusion so that internal short circuit does not occur, and a lithium metal secondary battery comprising the same.

Description

[0001] The present invention relates to an electrolyte for a lithium metal secondary battery and a lithium metal secondary battery comprising the lithium metal secondary battery,

The present invention relates to an electrolyte system for a lithium metal secondary battery and a lithium metal secondary battery including the same.

The first concept of a Li-ion battery (LiB) was presented in 1962, and a LiB secondary battery was proposed by MS Whittingham of Exxon to lead to the invention of a Li-TiS ₂ battery. However, commercialization of battery systems using lithium metal and TiS ₂ as cathodes and anodes, respectively, failed, due to the poor safety of lithium metal batteries (LiM) and the high manufacturing cost of TiS ₂ , which is sensitive to air and water. LiB was successfully commercialized by solving these problems by using graphite and anodic oxide (developed by JO Besenhard), which reversibly intercalate and deintercalate lithium, as cathodes and anodes, respectively. For the first time in 1991, LiB's commercial products were launched by Sony and Asahi, bringing innovative breakthroughs that have led to successful market penetration of portable electronic devices. Since then, LiB has been used explosively and has been meeting the electrical energy needs directly connected with the constant innovation of everyday electrical devices such as cell phones, music players, speakers, drones, automobiles and micro sensors. Many researchers and scientists have been studying new and advanced energy materials, chemistry and physics for fixed or mobile energy storage systems that meet growing energy needs.

In recent years, research and development of new energy storage materials and systems with different forms and compositions are indispensable, as the development of commercial LiB technology has reached saturation, where only gradual improvement in the electrochemical performance of LiB is reported. Therefore, secondary batteries such as lithium-sulfur batteries and lithium-air batteries having a lithium metal anode and a switching anode have high energy densities and are attracting attention as a next-generation battery. Sulfur and carbon-based air anodes are theoretically energy densities of ~ 2,600 Wh / kg and ~ 11,400 Wh / kg respectively and almost ten times the energy density of existing LiB (~ 360 Wh / kg, C / LiCoO ₂ ) High value. Lithium metal, a cathode material, has a very low redox potential (-3.04 V vs. SHE) and a density of 0.59 g / cm ³ with a high theoretical energy density of ~ 3,860 Wh / kg, while the graphite anode material A theoretical energy density of ~ 372 mAh / g, a slightly higher redox potential and density. Therefore, if the graphite cathode is replaced with a lithium cathode, the energy density per weight of the existing LiB can be greatly increased. If lithium-sulfur and lithium-air cells are to be commercialized in the future, lithium metal cathodes and switchable cathode materials will be a promising way to overcome the high energy density requirements.

Despite these advantages, commercialization of lithium-metal-cathode batteries requires some tough challenges to be solved. And the reversibility of the electrodeposition and dissolution of lithium ions is secured at the center thereof. High reactivity and non-uniform electrodeposition of lithium can cause problems such as thermal runaway, electrolyte decomposition, and lithium loss. Uneven electrodeposition of lithium ions during the charging process causes branch-like dendrite growth, which is due to the growth of solid electrolyte interface (SEI) films, side reactions of the electrolyte and the lithium anode, resulting in lower Coulomb efficiency and electrochemical properties of the lithium surface Which causes a problem of sharply lowering. In addition, the short circuit due to the lithium dendrite growth causes a lot of heat and flame, causing a serious safety problem that causes ignition of the electrolyte, which is a flammable organic matter. Therefore, in order to increase the safety, it is necessary to form a stable SEI on the lithium anode for the formation of dendrites and a severe side reaction with the electrolyte. This is possible by appropriately mixing the salts used in the electrolyte and changing the constituent materials of the SEI film . It is important to form an SEI film that is thin and strong and has high lithium ion conductivity on the surface of the lithium anode. Development of an electrolyte system capable of maintaining the characteristics of the desired anode is a key to the development of a lithium metal battery.

Korean Patent No. 10-15380790 Korean Patent No. 10-10747830 Korean Patent Publication No. 10-2015-0004179 U.S. Published Patent Application No. 2014-0127577

High activity and light lithium metal surfaces during electrochemical cycles tend to form dendrites during charging due to local current density differences on irregular surfaces. As the dendrite formation that grows in dendrites is diffused, the surface area of the lithium anode is rapidly increased, thereby continuously forming the SEI film on the surface of the lithium anode. The continuous formation of the thick SEI film leads to electrolyte depletion and lithium ion conductivity of the lithium anode There is a problem of sharply lowering. In order to suppress the formation of unstable SEI by the generation of lithium dendrite, lithium tris (fluorosulfonyl) imide (LiTFSI), lithium bis (fluorosulfonyl) imide (LiFSI), lithium bis (LiBOB), lithium hexafluorophosphate (LiPF ₆ ), lithium fluoride (LiF) and salts in a solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and 1,2-dimethoxyethane We have also developed electrolytes using fluoroethylene carbonate (FEC), vinylene carbonate (VC) and di-2,2,2-trifluoroethyl carbonate (TFEC) as solvent additives. Compatible with various anode materials. As a result, by applying the invented electrolyte system, it is possible to increase the cycle life and the capacity retention rate of the lithium metal anode based battery by generating a thin, firm and stable SEI on the surface of the lithium anode and suppressing the side reaction with the electrolyte.

(B) a second salt selected from LiBOB, LiDFOB, and mixtures thereof; (c) a third salt comprising LiPF ₆ , wherein the second salt is selected from the group consisting of (e) at least one first solvent additive selected from the group consisting of FEC and VC, and (f) at least one additive selected from the group consisting of (D) a mixture of EC and DMC, 2 solvent additive, wherein the first solvent additive is at least one selected from the group consisting of 0.4 to 1.5 wt% FEC and 0.01 to 3.5 wt% VC based on the total weight of the electrolyte, and the second solvent And the concentration of the additive is 2.5 to 4.5 wt% based on the total weight of the electrolyte.

Another aspect of the present invention relates to a lithium metal secondary battery including an electrolyte for a lithium metal secondary battery according to various embodiments of the present invention.

Another aspect of the present invention relates to an electric device including an electrolyte for a lithium metal secondary battery according to various embodiments of the present invention.

It is an object of the present invention to provide an electrolyte system for maintaining energy density and prolonging battery life of a lithium metal anode-based secondary battery. The ultimate application goal of this technology is to use conventional lithium-ion batteries for future unmanned electric vehicles and power grid energy storage systems, as well as various anodes such as transition metal oxides, sulfur, and air poles used in high- Together they use lithium metal. It will also contribute to the development of the field of unmanned aerial vehicles such as the recently emerging drone. It is expected that the present invention can secure world competitiveness of the related secondary battery and electrochemical capacitor industry. Recently, safety studies have been attracting attention as one of the core researches, especially when dealing with materials with high density energy. The reason for this is the lowering of safety due to the implementation of high energy density in commercialization of products. Due to the recent social and technological ups and downs caused by smartphone firing, it is inevitable to secure battery safety with high energy density. In particular, next-generation batteries, which will be coming soon, must be studied and verified for the safety of systems handling batteries and batteries, since the energy density of existing lithium-ion batteries is practically at least two to eight times higher. Accordingly, the present invention is directed to a lithium metal battery electrolyte capable of stabilizing a negative electrode of a lithium metal battery having a high possibility of battery ignition and forming a stable SEI film capable of inhibiting the formation and diffusion of lithium dendrite so that internal short- .

FIGS. 1A and 1B show the molecular structures of salts such as LiFSI, LiTFSI, LiBOB, LiDFOB, LiPF ₆ , and LiF and solvents such as EC, DMC, VC, FEC and TFEC constituting the electrolyte according to the present invention.
FIG. 2 shows the characteristics of a lithium metal battery using ECM DMC (4: 6 weight ratio) in which LiFSI, LiTFSI, LiBOB, LiDFOB and LiPF ₆ salts are mixed and a coin type NCM (6/2/2) anode using DME electrolyte Show.
3 is a graph showing the results of a lithium metal coin using an NCM (6/2/2) anode according to the FEC addition ratio in a 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) Cell battery characteristics.
FIG. 4 is a graph showing the results of a lithium metal coin using an NCM (6/2/2) anode according to VC addition ratio in a 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) Cell battery characteristics.
FIG. 5 is a graphical representation of a lithium metal coin cell using an NCM (6/2/2) anode according to the FEC addition ratio in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) It shows the characteristics of the battery.
6 shows the characteristics of a lithium metal coin cell cell using an NCM (6/2/2) anode according to VC addition ratio in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) Lt; / RTI >
FIG. 7 is a graph showing the results of a lithium ion battery using an NCM (6/2/2) anode coated with FEC and VC addition solvent in a 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: Metal coin cell battery.
8 is a graph showing the results of an NCM (6/2) application with 0.5 wt% FEC and 0.5 wt% FEC + 2 wt% VC added solvent in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: / 2) shows the characteristics of a lithium metal coin cell battery using an anode.
FIG. 9 is a SEM image of a lithium surface after a cycle by applying LiTFSI, LiBOB, and LiPF ₆ salts and electrolytes to which EC and DMC are not added, and electrolytes to which an addition solvent is added.
FIG. 10 is a spectrum obtained by charging and discharging a cell to which an electrolyte including LiTFSI, LiBOB, LiPF ₆ salts and an additive solvent is applied at 1 C for 200 cycles, and then measuring the surface of the lithium negative electrode by XPS.
11 shows the characteristics of a lithium metal coin cell cell using an NCM (8/1/1) anode according to FEC addition ratio in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) Lt; / RTI >
12 is a graph showing the results of an NCM (8/1/2) ratio according to LiF (0.3 to 1 M) concentration in a 1 wt% FEC (weight ratio) electrolyte system of 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC 1) shows the characteristics of a lithium metal coin cell battery using an anode.
FIG. 13 is a graph showing the effect of the addition of a TFEC-added solvent on the electrolyte mixed with 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts in EC: DMC (4: 6 weight ratio), 1 wt% FEC, And a cycle characteristic of a lithium metal coin cell battery using an NCM (8/1/1) anode by adding 0 to 30% by weight in weight ratio.
14 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF and 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 weight% FEC, 3 weight% VC, 0 weight% TFEC electrolyte (C, O, F, B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles.
15 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% (C, O, F, and B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles.
Figure 16 shows the results of a 1 C solution of 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% And XPS narrow scan spectra (C, O, F, B elements) of the surface of the lithium anode separated from the battery.
17 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF and 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% (C, O, F, B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles.
FIG. 18 is a list of narrow scan XPS spectra of the C 1s and F 1s elements of FIGS. 14-17, showing the variation of each peak.

Hereinafter, various aspects and various embodiments of the present invention will be described in more detail.

(B) a second salt selected from LiBOB, LiDFOB, and mixtures thereof; (c) a third salt comprising LiPF ₆ , wherein the second salt is selected from the group consisting of (e) at least one first solvent additive selected from the group consisting of FEC and VC, and (f) at least one additive selected from the group consisting of (D) a mixture of EC and DMC, 2 solvent additive, wherein the first solvent additive is at least one selected from the group consisting of 0.4 to 1.5 wt% FEC and 0.01 to 3.5 wt% VC based on the total weight of the electrolyte, and the second solvent And the concentration of the additive is 2.5 to 4.5 wt% based on the total weight of the electrolyte.

According to one embodiment, the primary salt comprises 0.2 to 0.4 M of LiFSI and 0.2 to 0.4 M of LiTFSI, or consists only of 0.3 to 1 M of LiTFSI, and the concentration of LiPF ₆ is 0.03 to 0.07 M , The mixing ratio by weight of the EC and the DMC is 3 to 6: 7 to 4.

According to another embodiment, the first salt is composed of only 0.5 to 0.7 M of LiTFSI, the second salt is composed of only 0.3 to 0.5 M of LiBOB, the concentration of LiPF ₆ is 0.04 to 0.06 M, the EC And the mixing ratio by weight of the DMC is 3.75 to 5.25: 6.25 to 4.75.

According to another embodiment, the first solventable additive is 0.5 to 1.3 wt.% FEC based on the total weight of the electrolyte, and optionally 0.01 to 3.5 wt.% VC.

According to another embodiment, the electrolyte for a lithium metal secondary battery further comprises a LiF salt.

According to another embodiment, the concentration of the LiF salt is 0.3 to 0.5 M.

According to another embodiment, the first solventable additive is 0.7 to 1.3 wt.% FEC based on the total weight of the electrolyte.

According to a most preferred embodiment, (a) a first salt selected from LiTFSI, LiFSI, or a mixture thereof, (b) a second salt consisting of a mixture of LiBOB and LiPF ₆ , (c) (D) at least one first solventable additive selected from FEC and VC, (f) further comprising a TFEC as a second solvent additive, wherein the solvent Wherein the first additive is at least one selected from the group consisting of 0.4 to 1.5 wt% of FEC and 0.01 to 3.5 wt% of VC based on the total weight of the electrolyte. (3) the concentration of LiPF ₆ is 0.04 to 0.06 M, (4) the mixing ratio by weight of EC and the DMC is 3.75 to 0.75 M, 5.25: 6.25 4. The method of claim 1, further comprising: (e) adding LiF salt in a concentration of 0.3 to 0.5 M; (6) additionally comprising 0.8 to 1.2 wt.% FEC based on the total weight of the electrolyte as the first solvent additive; To 3.5% by weight, and (7) the concentration of the second solventable additive is 2.5 to 3.5% by weight based on the total weight of the electrolyte.

When the above-mentioned seven conditions are satisfied, a peak for -CF ₂ -CF ₂ and -CF ₂ -CH ₂ is newly formed due to TFEC corrosion through XPS analysis and the physical properties of the LiF-based SEI are changed Respectively. This is achieved by maintaining the dense and rigid nature of SEI, including boron, while at the same time changing the new bonding points from CF ₂ -CF ₂ and -CF ₂ -CH ₂ to more elastic SEI 9), it was confirmed that the electrochemical stability and retention characteristics of SEI were improved and the interface resistance was kept constant.

It was confirmed that the above-mentioned improvement of the effect and the development of the new effect were not observed when any one of the above seven conditions was not satisfied, and this effect was also confirmed by the lithium metal secondary battery or the carbonate using the lithium metal itself as the cathode This phenomenon occurs only when the above electrolyte is applied to a lithium-sulfur secondary battery or a lithium-air secondary battery using a series of electrolytes, and this effect is exhibited for a lithium ion secondary battery used as a cathode together with a carbon material or silicon Respectively. This is because SEI is directly formed on the surface of the lithium metal. On the other hand, the electrode of a lithium ion secondary battery having a mechanism in which lithium ions are inserted or removed, for example, graphite, silicon, Is not expressed.

Particularly, LiFSI, LiTFSI, and TFEC have a function of forming a suitable SEI film of lithium fluoride (LiF) and? CF ₂ -CF ₂ and -CF ₂ -CH ₂ on the surface of lithium metal through a corrosive action to increase the conductivity of lithium ion Function. It is also electrochemically stable and has better thermal stability than LiPF ₆ . LiBOB also forms SEI with the elements of boron and Li ₂ SO _x through corrosion, forming a thin and robust SEI film with high lithium ion conductivity. The LiPF ₆ salt improves the electrochemical stability of the lithium negative electrode by controlling the formation and shape of the lithium dendrite by including a polycarbonate component in the SEI on the surface of the lithium negative electrode. Through this, it is possible to obtain a stable SEI film on the surface of the lithium anode to improve the lifetime and energy density retention of the lithium metal battery.

When the solvent is a mixed solvent of EC and DMC and the concentration of each salt and the weight ratio of the mixed solvent satisfy both of the above conditions, aluminum corrosion as a positive electrode current collector by LiFSI and LiTFSI is blocked (FIG. 10) A small amount of dendrite formed by suppressing the formation of the dyes was also confirmed as a stable and elastic SEI type structure (FIG. 9). When the solvent is a mixed solvent of EC and DMC, and the concentration of each of the added solvent and the salt and the weight ratio of the mixed solvent satisfy both of the above conditions, A stable SEI is formed at the lithium-electrolyte interface and exhibits a very excellent lithium metal battery characteristic.

On the other hand, if any of the above numerical ranges deviates, the amount of -F bonds not included due to the changed composition of LiFSI, LiTFSI, LiPF ₆ , LiF salt and FEC, VC, TFEC added solvent is included in SEI, Deteriorates the characteristics and electrochemical characteristics, which makes the interface resistance of the lithium negative electrode large. In addition, if the concentration of the main salts (LiTFSI, LiBOB) is deviated, it exhibits low LiBOB solubility, resulting in the non-stability of the electrolytic solution precipitated in the dissolved lithium salt. The composition thus proposed not only improves the electrochemical characteristics of the lithium metal battery but also improves the stability of the electrolyte solution. In particular, it has been confirmed that when the addition amount of other salts such as LiDFOB is also included in a small amount and the addition ratio of FEC and VC as additional solvents deviate from the above values, the stability of SEI is lowered, battery life is shortened, and the resistance of the cathode increases.

Another aspect of the present invention relates to a lithium metal secondary battery including an electrolyte for a lithium metal secondary battery according to various embodiments of the present invention.

According to one embodiment, the material of the positive electrode in the lithium metal secondary battery is one selected from the group consisting of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt oxide, and lithium iron phosphate.

According to another embodiment, the material of the anode is lithium nickel cobalt manganese oxide.

Another aspect of the present invention relates to an electric device including an electrolyte for a lithium metal secondary battery according to various embodiments of the present invention.

According to one embodiment, the electric device is one selected from a communication device, a transportation device, an energy storage device, and a sound device.

Specific examples of these electrical devices include, but are not limited to, cell phones, music players, speakers, drones, automobiles, sensors, and the like.

Hereinafter, the present invention will be described in more detail with reference to Examples and the like, but the scope and content of the present invention can not be construed to be limited or limited by the following Examples. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. It is natural that it belongs to the claims.

In addition, the experimental results presented below only show representative experimental results of the embodiments and the comparative examples, and the respective effects of various embodiments of the present invention which are not explicitly described below will be specifically described in the corresponding part.

Example

Comparative Example 1: LiFSI, LiTFSI, LiBOB, LiPF ₆  Electrolyte synthesis using salt and EC: DMC solvent

First, the EC was melted at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. The LiBOB salt was dissolved in the mixed solvent at a concentration of 0.4 M for 12 to 24 hours in a water-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.3 M and a concentration of LiFSI and 0.3 M, respectively.

A lithium metal battery was used in this manner (0.3 M LiFSI + 0.3 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in EC: DMC (4: 6 w / w) electrolyte.

Comparative Example 2: LiTFSI, LiBOB, LiPF ₆  Electrolyte synthesis using salt and EC: DMC solvent

First, the EC was melted at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. The LiBOB salt was dissolved in the mixed solvent at a concentration of 0.4 M for 12 to 24 hours in a water-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.6 M.

The thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in EC: DMC (4: 6 w / w) electrolyte electrolyte was used in the lithium metal battery.

Comparative Example 3: LiFSI, LiDFOB, LiPF ₆  Electrolyte synthesis using salt and EC: DMC solvent

First, the EC was melted at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. To the mixed solvent, the LiDFOB salt was dissolved in 0.4 M concentration for 3 to 6 hours in a moisture-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiFSI salt was dissolved to a concentration of 0.6 M.

The thus prepared (0.6 M LiFSI + 0.4 M LiDFOB + 0.05 M LiPF ₆ ) in EC: DMC (4: 6 w / w) electrolyte was used in the lithium metal battery.

Comparative Example 4: LiFSI, LiTFSI, LiBOB, LiPF ₆  Salt and FEC: Electrolyte synthesis using DMC solvent

FEC was mixed with DMC at a weight ratio of 3: 7 for 15 to 30 minutes while stirring in a water-free environment to prepare a mixed solvent. The LiBOB salt was dissolved in the mixed solvent at a concentration of 0.4 M for 12 to 24 hours in a water-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.3 M and a concentration of LiFSI and 0.3 M, respectively.

A lithium metal battery was used in this manner (0.3 M LiFSI + 0.3 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in FEC: DMC (3: 7 w / w) electrolyte.

Comparative Examples 5a and 5b: LiTFSI, LiBOB, LiPF ₆  Salt and FEC: Electrolyte synthesis using DMC or VC: DMC solvent

Mixed solvents were prepared by mixing FEC and DMC or VC and DMC in a weight ratio of 3: 7 and 3: 7, respectively, for 15 to 30 minutes while stirring in a moisture-free environment. The LiBOB salt was dissolved in the mixed solvent at a concentration of 0.4 M for 12 to 24 hours in a water-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.6 M.

The thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF 6) in (FEC: DMC (3: 7 w / w) or VC: DMC (3: a 7 w / w) the electrolyte used in the lithium metal battery .

Comparative Example 6: LiPF 6 ₆  Electrolyte synthesis using salt and EC: DMC solvent

First, EC was dissolved at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. The LiPF ₆ salt was dissolved in the mixed solvent so as to have a concentration of 1 M for 12 to 24 hours in a water-free environment with constant stirring.

The thus prepared (1 M LiPF ₆ ) in EC: DMC (4: 6 w / w) electrolyte was used for the lithium metal battery.

Comparative Examples 7a to 7e: LiTFSI, LiBOB, LiPF ₆ , Electrolyte synthesis using LiF salt and EC: DMC solvent

First, the EC was melted at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. The LiBOB salt was dissolved in the mixed solvent at a concentration of 0.4 M for 12 to 24 hours in a water-free environment with constant stirring. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.6 M. In addition, LiF salt was further dissolved so that the concentrations were 0.3 M, 0.4 M, 0.5 M, 0.7 M and 1 M, respectively.

The electrolyte (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ + 0.3 to 1 M LiF) in EC: DMC (4: 6 w / w) electrolyte thus prepared was used in a lithium metal battery.

Examples 1a and 1b: Electrolyte synthesis with FEC solvent added

FEC was added to the electrolyte prepared in Comparative Example 1 in an amount of 0.5 wt% and 5 wt% based on the total weight of the electrolyte, and the mixture was stirred for 1 hour in an environment free from moisture.

The thus prepared (0.3 M LiFSI + 0.3 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF 6) in a (EC:: DMC (4 6 w / w) + 0.5 or 5% by weight FEC) the electrolyte used in the lithium metal battery .

Examples 2a to 2e: Electrolyte synthesis with addition of VC solvent

0.5 wt%, 1 wt%, 2 wt%, 3 wt%, and 5 wt% of VC were added to the electrolyte prepared in Comparative Example 1 based on the total weight of the electrolyte, and stirred for 1 hour in an environment free from moisture Lt; / RTI >

The electrolyte thus prepared (0.3 M LiFSI + 0.3 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in (EC: DMC (4: 6 w / w) + 0.5 to 2 wt% VC) .

Example 3: Electrolyte synthesis with FEC + VC solvent added

The electrolyte prepared in Comparative Example 1 was added with 0.5 wt% of FEC and 2 wt% of VC, based on the total weight of the electrolyte, and stirred for 1 hour in an environment free from moisture.

The electrolyte thus prepared (0.3 M LiFSI + 0.3 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in (EC: DMC (4: 6 w / w) + 0.5 wt% FEC + 2 wt% Lt; / RTI >

Examples 4a to 4i: Electrolyte synthesis with FEC solvent added

The electrolyte prepared in Comparative Example 2 contained 0.3 wt%, 0.5 wt%, 0.7 wt%, 1 wt%, 3 wt%, 5 wt%, 7 wt%, and 10 wt% of FEC, , 20% by weight, and the mixture was stirred for 1 hour in a moisture-free environment.

The electrolyte thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in (EC: DMC (4: 6 w / w) + 0.3 to 20 wt% FEC) electrolyte was used in the lithium metal battery.

Example 5: Electrolyte synthesis with VC solvent added

2% by weight of VC was added to the electrolyte prepared in Comparative Example 2 based on the total weight of the electrolyte, and the mixture was stirred for 1 hour in an environment free from moisture.

The electrolyte thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in (EC: DMC (4: 6 w / w) + 2 wt% VC) electrolyte was used in lithium metal batteries.

Example 6: Electrolyte synthesis with FEC + VC solvent added

To the electrolyte prepared in Comparative Example 2, 0.5 wt% of FEC and 2 wt% of VC were added based on the total weight of the electrolyte, and the mixture was stirred for 1 hour in an environment free from moisture.

The electrolyte thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ ) in (EC: DMC (4: 6 w / w) + 0.5 wt% FEC + 2 wt% VC) electrolyte was used in lithium metal batteries.

Examples 7a to 7e: LiTFSI, LiBOB, LiPF ₆ , Electrolyte synthesis using LiF salt and EC: DMC solvent

LiF salt was further dissolved in the electrolyte prepared in Example 4d such that the concentration was 0.3 M, 0.4 M, 0.5 M, 0.7 M and 1 M, respectively.

The electrolyte thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ + 0.3 to 1 M LiF) in (EC: DMC (4: 6 w / w) + 1 wt% FEC) .

Example 2-1: LiTFSI, LiBOB, LiF, LiPF ₆  Electrolyte synthesis using salt and EC: DMC solvent

First, EC was melted at 60 DEG C and mixed with DMC at a weight ratio of 4: 6 in an environment free from water for 15 to 30 minutes with stirring to prepare a mixed solvent. To the mixed solvent, the LiBOB salt was dissolved by continuous stirring in a moisture-free environment for 12 to 24 hours so as to have a concentration of 0.4 M. Here, the LiPF ₆ salt was dissolved for 6 to 12 hours so as to have a concentration of 0.05 M. In addition, the LiTFSI salt was dissolved to a concentration of 0.6 M. In addition, a LiF salt was further dissolved so as to have a concentration of 0.4 M. Here, 1% by weight and 3% by weight of FEC and VC, respectively, were added based on the total weight of the electrolyte, and the mixture was stirred in a moisture-free environment for 1 hour.

The electrolyte thus prepared (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ + 0.4 M LiF) in (EC: DMC (4: 6 w / w) + 1 wt% FEC + 3 wt% Lt; / RTI >

Examples 2-2a to 2-2i: LiTFSI, LiBOB, LiF, LiPF ₆  Electrolyte synthesis using salt and EC: DMC solvent

1, 3, 5, 7, 10, 15, 20, and 20 weight percent, based on the total weight of the electrolyte, of the electrolyte prepared in Example 2-1. 30% by weight of TFEC was added and stirred for 2 hours in a moisture free environment.

(EC: DMC (4: 6 w / w) + 1% by weight FEC + 3% by weight VC + 0.5 to 30% by weight of (0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF ₆ + TFEC) electrolyte was used for the lithium metal battery.

Electrochemical Analysis of Lithium Metal Cells of Modified Electrolytes

The electrochemical characteristics of the lithium metal battery were measured by conducting constant current charging and discharging tests on the coin cell of the lithium metal secondary battery to which the above prepared electrolytes were applied. Lithium metal (1.5 cm diameter, 150 μm thick disc), 25 μm thick PP membrane (diameter 1.8 cm), NCM anode (3.8 mAh / cm ² loading of 8/1/1 composition, diameter 1.2 cm), and 30 μL of the electrolyte was used to measure the electrochemical characteristics. The applied voltage range was 3 to 4.2 V, and when the voltage reached 4.2 V, the cell was charged to the constant voltage mode until the current value reached 0.05 C. The first charge and discharge of the battery proceeded to 0.1 C in the forming process, and thereafter was continuously charged and discharged at 1 C thereafter.

Hereinafter, the analysis results will be described in detail with reference to the drawings.

FIG. 2 shows the characteristics of a coin type lithium metal battery using an EC: DMC (4: 6 weight ratio) electrolyte containing LiFSI, LiTFSI, LiBOB, LiDFOB and LiPF ₆ salts. _{1 M LiPF 6 EC: DMC (} 4: 6 weight ratio) 1C, while the comparison data is lost, the battery life ended in 80 cycles, 0.3 M LiFSI, 0.3 M LiTFSI , 0.4 M LiBOB, 0.05 M LiPF 6 EC: DMC (4: 6 weight ratio) electrolyte system having a composition of 1C and 0.6M LiTFSI, 0.4M LiBOB, 0.05M LiPF ₆ EC: DMC (4: 6 weight ratio) shows somewhat stable capacity retention and life characteristics, It is possible to confirm that the electrolyte system according to the present invention is significantly lower than the electrolyte system according to the present invention. The anode used here is NCM (6/2/2).

FIG. 3 shows the characteristics of a lithium metal coin cell cell according to FEC addition ratios in 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system. _{1 M LiPF 6 EC: DMC (} 4: 6 weight ratio) 1C is the comparison data, on the other hand, the battery life and ending at 80 cycles, 0.3 M LiFSI, 0.3 M LiTFSI , 0.4 M LiBOB, 0.05 M LiPF 6 EC: DMC (4: 6 weight ratio), the FEC-added 1C coin cell exhibits somewhat improved lifetime characteristics and shows the most stable capacity retention rate when 0.5 wt% of FEC is added. This is because an appropriate amount of LiF-based SEI formed from the FEC is formed on the lithium anode and the SEI of the NCM anode is also stabilized. The anode used here is NCM (6/2/2).

FIG. 4 shows the characteristics of a lithium metal coin cell cell according to VC addition ratio in 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system. _{1 M LiPF 6 EC: DMC (} 4: 6 weight ratio) 1C is the comparison data, on the other hand, the battery life and ending at 80 cycles, 0.3 M LiFSI, 0.3 M LiTFSI , 0.4 M LiBOB, 0.05 M LiPF 6 EC: DMC (4: 6 weight ratio) showed a somewhat improved lifetime characteristics at 1C condition and showed the most stable capacity retention rate when 0.5 wt% or 2 wt% of VC was added. This is because the SEI formed from the VC at the lithium negative electrode and the NCM suppresses the side reaction with the electrolyte and provides the electrochemical surface stabilization. The anode used here is NCM (6/2/2).

FIG. 5 shows the characteristics of a lithium metal coin cell cell according to the FEC addition ratio in the 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system of FIG. 1 M LiPF ₆ EC: DMC (4: 6 weight ratio) 1C is comparative data, while battery life is terminated at 80 cycles, while 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC The coin cell with FEC exhibits somewhat improved lifetime characteristics under 1 C charge and discharge and shows the most stable capacity retention rate when 0.5 wt% of FEC is added. This is because an appropriate amount of LiF-based SEI formed from the FEC is formed on the lithium anode and the SEI of the NCM anode is also stabilized. The anode used here is NCM (6/2/2).

FIG. 6 shows the characteristics of a lithium metal coin cell cell according to the VC addition ratio in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system. 1 M LiPF ₆ EC: DMC (4: 6 weight ratio) 1C is comparative data, while battery life is terminated at 80 cycles, while 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC The coin cell with VC added exhibits somewhat improved lifetime characteristics under 1C charge and discharge conditions and shows the most stable capacity retention rate when 2 wt% of VC is added. This is because the SEI formed from the VC at the lithium negative electrode and the NCM suppresses the side reaction with the electrolyte and provides the electrochemical surface stabilization. The anode used here is NCM (6/2/2).

FIG. 7 shows the characteristics of a lithium metal coin cell cell using an optimized FEC and VC addition solvent in 0.3 M LiFSI, 0.3 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system. _{1 M LiPF 6 EC: DMC (} 4: 6 weight ratio) 1C is the comparison data, on the other hand, the battery life and ending at 80 cycles, 0.3 M LiFSI, 0.3 M LiTFSI , 0.4 M LiBOB, 0.05 M LiPF 6 EC: DMC (4: 6 weight ratio), the coin cell with 0.5 wt% FEC and 2 wt% VC additive showed the most stable lifetime characteristics even at 1C charge / discharge condition and showed more stable lifetime characteristics than the electrolyte system without added solvent. The anode used here is NCM (6/2/2).

8 is a graph showing the results of a lithium metal coin applied with 0.5 wt% FEC and 0.5 wt% FEC + 2 wt% VC added solvent optimized in 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) Cell battery characteristics. 1 M LiPF ₆ EC: DMC (4: 6 weight ratio) 1C is comparative data. At the end of the battery life at 80 cycles and at the end of the battery life at the end of 400 cycles of the electrolyte without the addition solvent, 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC Coin cell using 0.5 wt% FEC and 2 wt% VC optimized for DMC (4: 6 wt%) or solvent added with FEC shows stable lifetime characteristics even under 1C charge / discharge conditions It shows more stable lifetime characteristics than non-solvent-based electrolyte systems.

FIG. 9 is a SEM image of a lithium surface after a cycle by applying LiTFSI, LiBOB, and LiPF ₆ salts and electrolytes to which EC and DMC are not added, and electrolytes to which an addition solvent is added. The high surface area lithium dendrite formation in the cell (a) in the case where LiTFSI, LiBOB, LiPF ₆ salts and EC: DMC were not added to the electrolyte after the 200 cycles of charging and discharging at 1C were used, (B) containing the solvent and the roughness of the surface was less coarse. This is because a stable and elastic SEI is formed on the surface of the lithium anode, which is formed by proper combination of the salts and the addition solvent.

FIG. 10 is a spectrum obtained by charging and discharging a cell to which an electrolyte including LiTFSI, LiBOB, LiPF ₆ salts and an addition solvent is applied at 1 C for 200 cycles, and then measuring the surface of the lithium negative electrode by XPS. (13.99%), B (3.16%), and C (3.16%) in the SEI formed from the main salt composition, even though the electrolyte system including LiTFSI, LiBOB, LiPF ₆ salts and an additive solvent was used for a long period of time, (26.81%), N (3.41%), O (34%), F (12.41%) and S (5.41%) were detected. This indicates that Al corrosion was suppressed due to the proper combination of the salts mentioned above.

FIG. 11 shows the characteristics of a lithium metal coin cell cell according to FEC addition ratios in a 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) electrolyte system. 1 M LiPF ₆ EC: DMC (4: 6 weight ratio) 1C is comparative data, while battery life is ended at 30 cycles, while 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC The coin cell using the optimized 1 wt% FEC addition solvent shows the most stable lifetime characteristics even under the 1C charge / discharge condition and shows more stable lifetime characteristics than the electrolyte system which does not use the additive solvent. The anode used here is NCM (8/1/1). Unlike the above anode, it is an experiment to find the optimum FEC addition ratio when NCM anode of high energy density is used.

FIG. 12 is a graph showing the change in the concentration of LiF (0.3 to 1 M) in a 1 wt% FEC (weight ratio) electrolyte system of 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC Properties. 0.6 M LiTFSI, 0.4 M LiBOB, 0.05 M LiPF ₆ EC: DMC (4: 6 weight ratio) The LiF concentration optimized for 1 wt% FEC (weight ratio) is 0.4 M. Coin cells using a solvent with optimized LiF salt show the most stable life characteristics even under 1C charge and discharge conditions. The anode used here is NCM (8/1/1). Unlike the above-mentioned anode, it is an experiment to find a ratio of LiF added optimized when a high energy density NCM anode is used.

FIG. 13 is a graph showing the effect of the addition of a TFEC-added solvent on the electrolyte mixed with 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts in EC: DMC (4: 6 weight ratio), 1 wt% FEC, And 0% by weight to 30% by weight of the total amount of the composition. 1 M LiPF ₆ EC: DMC (4: 6 weight ratio) [Conventional] Comparative data shows that 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF and 0.05 M LiPF ₆ salts The electrolyte system with EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% VC, 3 wt% TFEC combination shows very stable capacity retention and life characteristics. The anode used here is NCM (8/1/1), and it is most preferable to add 3 wt% of TFEC in a weight ratio of 0 to 30 wt%.

14 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF and 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 weight% FEC, 3 weight% VC, 0 weight% TFEC electrolyte (C, O, F, B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles. The anode used here is NCM (8/1/1).

15 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% (C, O, F, and B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles. The anode used here is NCM (8/1/1).

Figure 16 shows the results of a 1 C solution of 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF, 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% And XPS narrow scan spectra (C, O, F, B elements) of the surface of the lithium anode separated from the battery. The anode used here is NCM (8/1/1).

17 is a graph showing the results of a comparison of 1 C (1 C) using 0.6 M LiTFSI, 0.4 M LiBOB, 0.4 M LiF and 0.05 M LiPF ₆ salts using EC: DMC (4: 6 weight ratio), 1 wt% FEC, 3 wt% (C, O, F, B elements) on the surface of the lithium anode separated from the battery after 20 charge / discharge cycles. The anode used here is NCM (8/1/1).

FIG. 18 is a list of narrow scan XPS spectra of the C 1s and F 1s elements of FIGS. 14-17, showing the variation of each peak.

As the weight ratio of TFEC is increased, the peak ratio to LiF is changed, a peak for -CF ₂ -CF ₂ and -CF ₂ -CH ₂ is newly formed, and the peak ratio of F 1s containing 3 wt% TFEC is the most desirable. In the C 1s peaks, the peaks of [CF, C═O, C (O) F, CO ₃ ], ie, the combined F and C elements, increase as the weight ratio of TFEC increases, A peak ratio of 1s is most preferable.

The reason for this is that proper LiF forms a stable SEI layer but deteriorates ion conductivity, so that when LiF is excessively generated, the ionic conductivity of the SEI layer lowers and becomes too hard, thereby deteriorating physical properties.

On the other hand, since the SEI layer with the appropriate combination of [CF, C = O, C (O) F, CO ₃ ] has some flexibility, the formation of a SEI layer with a proper ratio of C and F bonds results in lithium metal Thereby making a great contribution to the improvement of the characteristics of the secondary battery.

Claims (11)

(b) a second salt selected from LiBOB, LiDFOB, and mixtures thereof; (c) a third salt comprising LiPF ₆ ; (d) a second salt selected from the group consisting of EC and And a solvent composed of a mixture of DMC,
(e) at least one first solventable additive selected from FEC and VC,
(f) further comprising TFEC as a second solvent additive,
Wherein the first solventable additive is at least one selected from the group consisting of 0.4 to 1.5 wt% of FEC and 0.01 to 3.5 wt% of VC based on the total weight of the electrolyte,
Wherein the concentration of the second solventable additive is 2.5 to 4.5 wt% based on the total weight of the electrolyte. The method of claim 1, wherein the first salt comprises 0.2 to 0.4 M LiFSI and 0.2 to 0.4 M LiTFSI, or 0.3 to 1 M LiTFSI only,
The concentration of LiPF ₆ is 0.03 to 0.07 M,
Wherein the EC and the DMC are mixed in a weight ratio of 3 to 6: 7 to 4. 6. The electrolyte for a lithium metal secondary battery according to claim 1, 3. The method of claim 2, wherein the first salt comprises only 0.5 to 0.7 M LiTFSI,
The second salt is composed of only 0.3 to 0.5 M of LiBOB,
The concentration of LiPF ₆ is 0.04 to 0.06 M,
Wherein the mixed weight ratio of the EC and the DMC is 3.75 to 5.25: 6.25 to 4.75. 4. The lithium secondary battery of claim 3, wherein the first solventable additive comprises 0.5 to 1.3% by weight of FEC, based on the total weight of the electrolyte, and optionally 0.01 to 3.5% by weight of VC. Electrolyte. [6] The electrolyte of claim 4, wherein the electrolyte for the lithium metal secondary battery further comprises a LiF salt. The electrolyte for a lithium metal secondary battery according to claim 5, wherein the concentration of the LiF salt is 0.3 to 0.5 M. 7. The lithium secondary battery of claim 6, wherein the first solventable additive comprises 0.7 to 1.2% by weight of FEC based on the total weight of the electrolyte, and optionally 0.01 to 3.5% by weight of VC. Electrolyte. 2. The method of claim 1, wherein the first salt is comprised of only 0.5 to 0.7 M LiTFSI,
The second salt is composed of only 0.35 to 0.45 M LiBOB,
The concentration of LiPF ₆ is 0.04 to 0.06 M,
The mixed weight ratio of the EC and the DMC is 3.75 to 5.25: 6.25 to 4.75,
Further comprising a 0.3 to 0.5 M concentration of LiF salt,
Wherein the first solventable additive comprises from 0.8 to 1.2% by weight of FEC, based on the total weight of the electrolyte, and optionally from 0.01 to 3.5% by weight of VC,
Wherein the concentration of the second solventable additive is 2.5 to 3.5 wt% based on the total weight of the electrolyte. A lithium metal secondary battery comprising an electrolyte for a lithium metal secondary battery according to any one of claims 1 to 8.
Wherein the material of the positive electrode in the lithium metal secondary battery is one selected from the group consisting of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt oxide, and lithium iron phosphate.
Wherein the material of the anode is lithium nickel cobalt manganese oxide. An electric device comprising the lithium metal secondary battery according to claim 9. 11. The electrical device of claim 10, wherein the electrical device is selected from a communication device, a transport device, an energy storage device, and a sound device.

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