Electrolyte composition and lithium metal battery containing same

Abstract

The present invention relates to an electrolyte composition that is less corrosive to a lithium metal negative electrode and that allows a uniform coating to be formed on the lithium metal negative electrode, and a lithium metal battery including the same.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0105317, filed August 23, 2022, and Korean Patent Application No. 10-2023-0103596, filed August 8, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention discloses an electrolyte composition and a lithium metal battery containing the same. [Background technology]

[0003] Lithium (Li) metal is the most popular material targeted as a cathode material for next-generation batteries. When Li metal is electrodeposited in a common organic solvent along with a lithium salt, it forms a three-dimensional microstructure with a large surface area, which reacts with the liquid electrolyte to form a solid-electrolyte interphase (SEI). However, recent reports have shown that the plating / stripping process of Li metal induces the continuous breakdown and regeneration of the SEI and the formation of inactive Li, leading to anode expansion and depletion of Li and the electrolyte, resulting in cell failure.

[0004] Essentially, to control the morphology of the lithium metal precipitate in the electrolyte design, strategies are employed to form specific SEI phases by decomposition of electrolyte species, which are believed to be beneficial for lithium reversibility. Existing studies have focused on Li2O, LiF, Li2S, and LiN. x O y It has been known that increasing the content of organic materials with crystallinity or excellent physicochemical properties has a positive effect on the stability of the SEI, and therefore on the reversibility of the Li metal cathode.

[0005] Therefore, many researchers have focused their research on adding additives that react favorably with Li to form a stable SEI, and on solution engineering (highly concentrated electrolytes (HCE), locally concentrated electrolytes (LHCE), and electrolytes using fluorinated solvents) that promote such reactions.

[0006] However, while many of the SEI materials proposed in these studies can extend the life of lithium metal batteries (LMBs), high coulombic efficiency (CE) electrolytes favor the formation of Li rather than inactive Li. + This has the problem of affecting the formation of SEI.

[0007] In fact, when compared to recent solid-state batteries, liquid electrolyte-based LMBs have a technological gap in terms of cycle life, which is likely due to the lower coulombic efficiency of LMBs caused by corrosion between the liquid electrolyte and Li3. Also, lithium metal batteries are significantly lower than existing anode chemistry-based batteries (e.g., graphite or silicon anodes, ~1.5g Ah). -1 ) in much larger amounts of electrolytes (>2.4g Ah -1 ) and the cycle life is limited by the consumption of the electrolyte. In addition, recently developed electrolytes have similar or worse calendar characteristics than existing electrolytes, and their performance is still insufficient for use in electric vehicles that must run for long periods of time, such as 10 years or more.

[0008] Therefore, beyond the morphological uniformity of the electrodeposited lithium, + Forming an SEI while minimizing electrolyte loss has been selected as an urgent issue to be resolved in this technical field. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention suggests that the problem of high-density lithium morphology in lithium metal anodes can be solved by controlling the liquid electrolyte, but the problem of lithium loss due to lithium corrosion with the liquid electrolyte persists. Therefore, the present inventors apply a borate-pyran-based electrolyte to solve the problem of lithium electrolyte corrosion in conventional liquid electrolyte-based LMBs. [Means for solving the problem]

[0010] Hereinafter, an electrolyte composition for a lithium metal battery according to a specific embodiment of the present invention will be described.

[0011] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best describe his or her invention.

[0012] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0013] It should be understood that in this specification, the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0014] electrolyte composition According to one embodiment of the present invention, there is provided an electrolyte composition comprising an ether-based solvent having an ether group, and a boron fluoride-based lithium salt or a non-boron fluoride-based lithium salt dissolved in the ether-based solvent, wherein the boron fluoride-based lithium salt or the non-boron fluoride-based lithium salt forms a complex with the ether-based solvent. The electrolyte composition may be an electrolyte composition for a lithium metal battery.

[0015] Specifically, the ether solvent may be a cyclic ether solvent, and the ether solvent may have one ether group.

[0016] The electrolyte composition may be, for example, a boron fluoride-cyclic ether-based electrolyte, which has low corrosiveness and can finely passivate the lithium-electrolyte interface through the reconstitution of LiF, thereby minimizing the Coulomb inefficiency of lithium SEI formation. Furthermore, the boron fluoride-cyclic ether-based electrolyte can exhibit ideal lithium deposition and high lithium plating / stripping efficiency, meeting the requirements for long-term operation of LMBs at low N / P and E / C ratios.

[0017] In an exemplary embodiment, the boron fluoride-based lithium salt may include a boron fluoride-based lithium salt represented by the following Chemical Formula 1: chemical formula 1

[0018] LiBF x R y where R=-CF3 or -styrene and x+y=4.

[0019] In an exemplary embodiment, the non-fluoride boron-based lithium salt may include an anion-based lithium salt having a Group 13 element as a central atom, or a lithium salt having a boron central atom and composed of a halide-based anion other than fluorine. For example, the anion-based lithium salt having a Group 13 element as a central atom may include LiAlF4, LiGaF4, etc.

[0020] In an exemplary embodiment, the ether solvent may include one or more solvents selected from the group consisting of tetrahydrofuran (THF), tetrahydropyran (THP), furan, pyran, oxirane, oxetane, oxepane, and dioxane. Specifically, tetrahydrofuran (THF) and tetrahydropyran (THP) are cyclic ether solvents in which a functional group is added to a cyclic ether, furan and pyran are unsaturated cyclic ether solvents containing a partial double bond, oxirane, oxetane, and oxepane are triangular, square, and heptagonal cyclic ether solvents, respectively, and dioxane may be a solvent containing multiple ether groups. For example, tetrahydropyran (THP), tetrahydrofuran (THF), and diethyl ether (DEE), which have one ether group and a functional group added to the cyclic ether, may have significantly lower reducing potential than other solvent molecules. However, in the case of linear molecules such as DEE, Li + While THP has a relatively low bond energy and may be disadvantageous in that it may promote coordination of anions, THP has a relatively high boiling point compared to THF and is therefore more chemically stable as a group 6 ring, making it more preferable.

[0021] In an exemplary embodiment, the boron trifluoride-based lithium salt may be LiBF4, and the ethereal solvent may be tetrahydropyran (THP). When the lithium salt and the solvent are combined, a boron trifluoride-ethereal complex may be formed to form a uniform SEI structure.

[0022] In an exemplary embodiment, the battery may further include one or more selected from the group consisting of an ion-conductive lithium salt and an additive. Specifically, the ion-conductive lithium salt may include one or more lithium salts of LiPF, LiDFOB, LiBOB, LiTf, LiBETI, LiCTFSI, LiNO, LiFSI, LiAsF, LiClO, and LiSbF. The additive may include one or more lithium salts containing -F, such as LiDFOB and LiFSI, which act as LiF formers, and solvents containing -F, such as FEC, DFEC, TFEC, TTE, TFTFE, TFOFE, BTFE, and OFE. Preferably, the ion-conductive lithium salt is LiTFSI, and the additive may be FEC, which acts as a LiF former.

[0023] In an exemplary embodiment, the anion of the ion-conducting lithium salt has a lower Li than the anion of the boron fluoride-based lithium salt. + This allows the anion of the boron fluoride-based lithium salt and THP to bond with the Li ions present mainly in the primary solvation shell. + While most of the anions of ion-conducting lithium salts and LiF formers can exist as free molecules, most of the anions of ion-conducting lithium salts and LiF formers can exist as free molecules.

[0024] In an exemplary embodiment, the electrolyte composition can have a B-O stretching and B-F bending signal in a SERS analysis, specifically, a potential drop of ∼490 cm -1 and ~1350cm -1A pair of nearby peaks gradually intensifies, which can correspond to the BO stretching and FBF bending signals, respectively, confirming the complexation of BF3 and ether groups on the THP molecules.

[0025] Lithium metal battery In another embodiment of the present invention, there is provided a lithium metal battery including: the above-described electrolyte composition; an anode including a lithium metal thin film formed on an anode current collector; a cathode including a cathode active material layer formed on a cathode current collector; and a separator formed between the anode and the cathode.

[0026] In an exemplary embodiment, the boron fluoride lithium salt or the non-boron fluoride lithium salt and the cyclic ether solvent can form a complex on the surface of the lithium metal thin film, for example, the boron fluoride and the ether solvent can form a boron trifluoride-ether complex.

[0027] In an exemplary embodiment, the lithium metal thin film further includes an SEI layer formed on the lithium metal thin film, and the SEI layer may have only a signal corresponding to the LiF(111) or LiF(220) lattice in a fast Fourier transform (FFT) image. Meanwhile, the SEI layer may be free of S, N, and B elements. In particular, the absence of S, N, and B elements in the SEI layer means that LiF is a BF4 - or TFSI - This may mean that it was not formed by the defluorination reaction of FEC.

[0028] In an exemplary embodiment, the SEI layer may be formed by a reconstitution process, specifically, BF-THP is formed on the surface of the lithium metal thin film, LiF is precipitated on the LiO, BF-THP reacts with the initial LiF particles and is converted back to LiBF, and LiBF precipitates LiF again on the exposed LiO, forming BF-THP. This process is repeated, and the Li surface is passivated with fine LiF crystals. [Effects of the Invention]

[0029] An embodiment of the present invention provides a boron fluoride-cyclic ether-based electrolyte as an electrolyte composition for a lithium metal battery. The boron fluoride-cyclic ether-based electrolyte has high reduction stability and finely passivates the lithium-electrolyte interphase during the LiF reconstitution process, enabling the formation of a dendrite-free lithium morphology and a solid-electrolyte interphase with minimized Coulomb inefficiency.

[0030] In addition, in the embodiment of the present invention, the electrolyte / negative electrode capacity ratio (E / C, 1.82 g Ah -1 , energy density 424Wh kg -1 Boron fluoride-cyclic ether-based liquid electrolyte Li||LiNi with a long cycle life of 500 cycles at 10 ... 0.8 Co 0.1 Mn 0.1 O2 (NCM811) pouch cell provided, 1.18g / Ah -1 (energy density 456Wh kg -1 ) can achieve 250 cycles, which far exceeds the E / C ratios of the latest LMBs reported to date.

[0031] The electrolyte design according to the present invention includes improving the reduction stability at the cathode-electrolyte interface by mitigating the inherent reactivity of the electrolyte with Li (FIG. 1A), and finely passivating Li through dynamic reorganization of LiF crystals in the SEI (FIG. 1B). The core function of the borate-pyran-based electrolyte is that the LiF reorganization process imparts high uniformity and passivation to the SEI, delaying the depletion of Li and the electrolyte, which is achieved by E / C<2g Ah. -1 It is also applicable to LMB under the following conditions. [Brief explanation of the drawings]

[0032] [Figure 1A] The design concept of the borate-pyran-based electrolyte and a schematic of the electrolyte reduction process at the Li anode. [Figure 1B] The BF3 complexation reaction involves the electrodeposition of LiF onto LiO from THP and LiBF4, followed by the reaction with BF3-THP, which dissolves LiF back into THP and LiBF4. The purple-red atoms represent the atoms that participate in the formation of the BF3 adduct in BF4-, and the orange and green F represent the F of LiF reconstructed on the LiO surface and the F of LiF initially formed in the SEI, respectively. [Figure 1C] The reduction potentials and Li+ binding energies of various organic solvents are shown. [Figure 1D] The distribution of Li+ dissolution structures and the most dominant structure for each anion in borate-pyran-based electrolytes are shown. [Figure 1E] The primary reduction potentials are shown for an ethereal electrolyte (4 M LiFSI DME) and a borate-pyran-based electrolyte (0.4 M LiBF4 0.4 M LiTFSI THP + 20% FEC), where the blue square indicates the anion reduction potential, the red square indicates the solvent reduction potential, and the gray square indicates the reduction potential for the additional dissolved structures. [Figure 2A] SEI microstructure by cryo-TEM and XPS is shown, showing cryo-TEM images of the SEI layer of Li metal during aging in a borate-pyran based electrolyte. [Figure 2B] Single-crystalline LiF particles in the SEI of electrodeposited Li and reconstituted LiF in the SEI of Li after aging are shown. [Figure 2C] The figure shows a comparison of the crystal size of inorganic particles in the SEI due to aging. [Figure 2D] The atomic concentrations of oxygen and fluorine in the SEI before (D) and after (E) 12-hour aging and the curve fitting results are shown by XPS spectra. [Figure 2E] The atomic concentrations of oxygen and fluorine in the SEI before (D) and after (E) 12-hour aging and the curve fitting results are shown by XPS spectra. [Figure 3A]In-situ SERS spectra (middle) and galvanostatic voltage profile (left) and Raman intensities corresponding to electrolyte components (right) are shown for a borate-pyran-based electrolyte in a Li||Cu-Ag (SERS substrate) cell during potential drop, demonstrating BF complexation and LiF dissolution in the borate-pyran electrolyte. [Figure 3B] The energy diagram for the BF3 complexation reaction with LiBF4 is shown. [Figure 3C] 11B NMR spectra of BF3-THF with various concentrations of dissolved LiF salt. [Figure 3D] The relative intensities of the 11B NMR spectra are shown, normalized by integrating all peak areas. [Figure 3E] The ionic conductivity of LiF solutions with different concentrations and in various solvents is compared. [Figure 3F] Schematic representation of the SEI reconstruction process in the borate-pyran electrolyte. [Figure 4A] Electrochemical analysis for liquid electrolyte shows the morphology of Li electrodeposited (2 mAh cm-2) on Cu current collector under 290 kPa pressure at a pressurized Li plating setting and a current density of 0.5 mA cm-2. [Figure 4B] The coulombic efficiency of Li plating / stripping in Li||Cu cells with other electrolyte configurations is shown for comparison. [Figure 4C] The volume usage in the modified Auerbach method was quantified by proper gas chromatography. [Figure 4D] Figure 1 shows the interface resistance and normalized resistance (relative to R at t = 0) of Li||Li coin cells in various electrolytes as a function of time, where the dotted lines indicate curves fitted by an empirical power law. [Figure 4E] Coulombic efficiency of Li plating / aging / stripping in a Li||Cu coin cell as a function of time. [Figure 4F]Average corrosion rate of plated Li during aging is shown. [Figure 5A] Schematic showing the construction of a Li||NCM811 bicell for pouch cell performance analysis. [Figure 5B] The results of comparing the electrolyte volume of LMB with that of LIB are shown below. [Figure 5C] The speed performance of LMB is compared and shown, and 0.2C constant current and 4.25V constant voltage modes are applied for all charging processes. [Figure 5D] and [Figure 5E] The cycling performance and coulombic efficiency of the LMB are shown. [Figure 5F] The swelling degree of Li||NCM811 bicells with three electrolytes and the same initial cell thickness is compared, where the data for each cycle was calculated by measuring the thickness at various positions in one pouch cell. [Figure 6] (a) MD snapshot at t = 0, (b) RDF, and (c) N(r) are shown in 0.4M LiBF4, 0.4M LiTFSI, and 20% THP FEC electrolyte, respectively. [Figure 7] MD snapshots showing ion distributions at various times in 0.4M LiBF4, 0.4M LiTFSI, THP20%FEC electrolyte, at (a) t = 0 ns, (b) t = 1.67 ns, (c) t = 3.33 ns, and (d) t = 5.00 ns, respectively. [Figure 8] (a) MD snapshot at t = 0, (b) RDF, and (c) N(r) are shown in 4 M LiFSI DME electrolyte, respectively. [Figure 9] MD snapshots showing the ion distribution at various times in 4 M LiFSI DME electrolyte, respectively at (a) t = 0 ns, (b) t = 1.67 ns, (c) t = 3.33 ns, and (d) t = 5.00 ns. [Figure 10]XPS spectra of the SEI layer, showing (a) F1s, (b) O1s, (c) C1s, (d) N1s, (e) S2p, and (f) B1s spectra for 1M LiPF6 EC / EMC+2%VC (black spectrum) and 0.4M LiBF4 0.4M LiTFSI THP20%FEC (red spectrum). [Figure 11] The atomic percentage of the SEI layer is shown for (a) 1M LiPF6 EC / EMC+2%VC and (b) 0.4M LiBF4 0.4M LiTFSI THP20%FEC. [Figure 12A] 1 is a schematic diagram of an in-situ Raman cell used in the in-situ SERS device of the present invention. [Figure 12B] SEM image of the laboratory-fabricated SERS substrate (AgNPs on a Cu grid) and a planar image of the in-situ Raman cell are shown. [Figure 13A] Figure 1 shows peak assignments obtained by comparing the ex-situ Raman spectrum of a 0.4M LiBF4, 0.4M LiTFSI, and THP20%FEC electrolyte with the spectrum of an in-situ Raman cell. [Figure 13B] 1 shows the in-situ Raman spectrum of 0.8M LiTFSI THP20%FEC electrolyte. [Figure 14A] Figure 1 shows the DFT simulation of BF3 complexation reaction using various solvents. The figure shows the energy diagram of BF3 complexation reaction of Li with organic solvents. [Figure 14B] The optimized molecular structure of the BF3 adduct is shown in the DFT simulation of the BF3 complexation reaction. [Figure 15] Optical images of 1M LiF solutions, showing (a) 1M LiF BF3-THP, (b) 1M LiF DME, (c) 1M LiF THF, and (d) 1M LiF THP solutions, respectively. [Figure 16]XPS spectra of the SEI layer for electrodeposited and aged lithium showing (a) F1s, (b) O1s, (c) C1s, (d) N1s, (e) S2p, and (f) B1s spectra, where the electrodeposited lithium is the black spectrum and the lithium aged for 12 hours is the red spectrum. [Figure 17] Cryo-TEM images of the SEI layer of Li metal in 0.8 M LiTFSI THP20%FEC electrolyte during aging at (a) t = 0 h and (b) t = 12 h, respectively. [Figure 18] Lithium morphology electrodeposited in (a) 1M LiPF6 EC / EMC 2% VC electrolyte, (b) 1.5M LiFSI DME / TTE electrolyte, and (c) 0.4M LiBF4 0.4M LiTFSI THP 20% FEC electrolyte. [Figure 19] The discharge capacity and coulombic efficiency of the Li||NCM523 whole cell are shown, respectively. [Figure 20] The ionic conductivities of representative electrolytes are shown below. [Figure 21] (a) shows the results of measuring the oxidation stability of various electrolytes in Li||Al batteries using LSV, and (b) shows an enlarged view of a part of the results in (a). DETAILED DESCRIPTION OF THE INVENTION

[0033] Although the present invention may be embodied in various different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention is not limited to the embodiments set forth herein.

[0034] In the examples and comparative examples, 1M LiPF6EC / EMC (1:1 v / v, Phanax) 2% VC (99.5%, Sigma-Aldrich) and 1.5M LiFSI (99.9%, Sigma-Aldrich) DME (Sigma-Aldrich) / TTE (SynQuest) Labs. (22:88 v / v) were used as the control electrolyte. THP (99%), LiBF4 (99.99%), and LiTFSI (99.95%) were purchased from Sigma-Aldrich, and FEC (98%) was purchased from TCI. All electrolyte compositions in the examples and comparative examples were prepared in an argon-filled glove box and had the following compositions:

[0035] Example 1: Borate-pyran-based liquid electrolyte A borate-pyran-based liquid electrolyte was prepared by mixing 0.4 M LiBF4 as the lithium boron fluoride salt, 0.4 M LiTFSI as the ionically conductive lithium salt, and THP / FEC 8:2 v / v as the solvent.

[0036] Comparative Example 1: Conventional low-corrosion electrolyte A low-corrosion electrolyte was prepared by mixing 1M LiPF6 as a lithium boron fluoride salt, EC / EMC (hexafluorophosphate ethylene carbonate / ethyl methyl carbonate) 1:1 v / v as a solvent, and 2% VC as an additive.

[0037] Comparative Example 2: Highly Corrosive Electrolyte A highly corrosive electrolyte was prepared by mixing 1M LiFSI as an ion-conductive lithium salt and DME / TTE (1,2-dimethoxyethane / 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) 22:78 v / v as a solvent.

[0038] Manufacturing example: Pouch cell production A prototype pouch cell was fabricated as follows: a Li anode (40 μm, Honjo Metals) and a double-sided coated LiNi 0.8 Mn 0.1 Co 0.1 O2 cathode (NMC811, provided by LG Energy Solutions, 18.78 mg cm -2 The active material (Super P C65:PVDF = 96:2:2) was punched to 40 mm x 60 mm and 30 mm x 50 mm, respectively. The Cu foil and NMC positive electrode were welded with Ni and Al tabs using an ultrasonic welder. All electrodes and separators were laminated and packaged in an aluminum pouch bag. 200 μL (1.92 g Ah) of 0.4M LiBF4, 0.4M LiTFSI, and THP / FEC from Example 1 were added. -1 ) or 130 μl (1.24 g Ah -1 ) to fabricate Li||NCM811 pouch cells, and all cell assembly steps were carried out in an argon-filled glove box.

[0039] Experimental Method Li morphology and SEI properties SEM analysis was performed using a field emission scanning electron microscope (FE-SEM, Sirion by FEI). Electrodeposited Li (0.5 mA cm) was analyzed by FIB-SEM (Helios Nanolab 450F1, FEI). -2 and 2mAh cm -2 The cross section of the sample was observed, and the surface was cleaned at 5 kV to remove the layer damaged by Ga ions.

[0040] The composition of the SEI was analyzed by X-ray photoelectron spectroscopy (In-Situ X-ray Photoelectron Spectroscopy, Axis-Supra by Kratos) using Al X-rays (hν=1486.7 eV).

[0041] Cryo-TEM Cryo-TEM analysis was performed using a 200 kV transmission electron microscope (TEM, Glacios, Thermo Fisher) to observe the microstructure of the SEI electrodeposited with Li while minimizing reaction with air and beam damage. Lithium was electrodeposited (0.5 mA cm -2 and 1mAh cm -2 ) TEM grids (200 mesh Cu with a lace carbon film) were placed in microtubes (Eppendorf) in an Ar-filled glove box, then removed and immediately placed in liquid nitrogen. The samples were then transferred to the TEM instrument fully immersed in liquid nitrogen using an autoloader capsule and cassette system.

[0042] NMR:400ul DMSO(200ul Ely+1.2ml DME)→400ul solution+1ml DME In-situ SERS An in-situ Raman cell (EQ-STC-RAMAN, MTI) was used to detect BF3 adducts formed on the surface of the positive electrode. An Ag-Cu grid was prepared as follows for the SERS substrate: a 200-mesh Cu grid was immersed in a 5 mmol / L AgNO3 aqueous solution for 2 minutes, followed by galvanic exchange of Cu and Ag. The Cu-Ag grid was then rinsed several times with DI water. After drying under vacuum for 1 day, the in-situ Raman cell was assembled in an argon-filled glove box, and the Raman signal was measured using a 514 nm laser with a 60-second acquisition time.

[0043] Electrochemistry Experiment A 2032 coin cell containing 50 μl of electrolyte was used for the electrochemical measurements.

[0044] The average CE was measured according to the following standard protocol: (1) 0.5 mA cm -2 Cu at a current density of 5 mAh cm -2 (2) 0.5 mA cm -2 Cu at a current density of 5 mAh cm -2of Li was electrodeposited; (3) 0.5 mA cm during 10 cycles -2 at a current density of 1 mAh cm -2 (4) All Li was stripped at 1.0 V. Before CE measurements, the Li||Cu cell was kept at 0 V for the entire 24 h cycle to allow for the formation of an SEI on the Cu substrate.

[0045] LSV, ionic conductivity and R interface was measured using a Solartron 1470E frequency response analyzer (Solartron Analytical).

[0046] EIS was performed in the frequency range of 1 MHz to 0.1 Hz with a perturbation degree of 10 mV.

[0047] Li||NCM523 coin cell test is LiNi 0.5 Mn 0.3 Co 0.2 O2 positive electrode (NCM532, 21.47mg cm -2 The test was carried out using an area loading of 1000 kJ / cm, NCM523:Super P C65:PVDF=94:3:3), a 40 μm Li anode, and a PE separator (Asahi, 19 μm).

[0048] For the operation of the Li||NCM811 pouch cell of the manufacturing example, a clamping torque was applied uniformly using a pressure jig with a 1 mm silicone pad. An initial pressure of 286 kPa was applied. The cell was tested using a WBCS3000L battery tester at 25°C. All cell assembly processes were performed in an argon-filled glove box.

[0049] DFT calculation Density functional theory (DFT) calculations were performed using the Materials Studio program package DMol3 (Accerlys Inc.). Numerical orbitals were used as basis functions in Dmol3, where each orbital corresponds to an atomic orbital. A double-numeric-plus-polarization (DNP) function and a global orbital cutoff of 4 Å were utilized.

[0050] DFT calculations were performed using the Perdew-Bueke-Ernzerhof (PBE) exchange-correlation function with gradient correction (GGA) function. The solvent environment was determined by applying a dmol3-PBE COSMO calculation. The energy, gradient, and displacement convergence tolerances were 0.00001 hartree, 0.002 hartree / Å, and 0.005 Å, respectively. The force tolerance for the self-consistent-field (SCF) cycle was 1.0 × 10 -6 The DFT-D2 method shown in the figure was adopted to account for the van der Waals interactions (vdW), and this method was optimized for various DFT functions.

[0051] Li + The reduction potential of the primary dissolution coating (Ered) was calculated using the following equation 1.

[0052]

number

[0053] where:

number

number

number

number

[0054] MD calculation Molecular mechanics calculations were performed using the Materials Studio Forcite program package (Accerlys Inc.). All components were configured into simulation cells by geometric optimization, and charges were applied by DFT.

[0055] MD was performed using the Forcite module and Forcefield COMPASS III. MD simulations were performed at 298 K and 1 atm in a 0.4M LiBF4 / 0.4M LiTFSI / THP / 20%FEC (16Li + , 8BF4 - , 8TFSI - , 162THP and 57FEC). The system had a 0.001 kcal mol -1 Å -1 The solution was geometrically stabilized using a smart algorithm that uses a convergence tolerance of .

[0056] Next, we equilibrated the 1 ns NPT and 1 ns NVT ensembles using the NOSE algorithm with a Q ratio of 0.1. After equilibration, we collected simulation data by running 5 ns NVT with 1 fs time steps, sampling at 45 fs time intervals.

[0057] The radial distribution function (RDF) was calculated using the following equation 2.

[0058]

number

[0059] The number of coordinates N(r) was calculated using the following formula 3.

[0060]

number

[0061] Experimental Example 1: Electrolyte Design The reduction potential of the electrolyte molecules and dissolved complexes has a significant effect on the reduction stability of the entire electrolyte system. + It has been confirmed that electrolyte molecules bound to ions have a much higher thermodynamic influence in reductive decomposition than free molecules.

[0062] In the present invention, an important goal is to minimize the decrease in Coulomb efficiency due to the reductive decomposition of the electrolyte. For this purpose, highly reactive anions (e.g., FSI) are used in conventional HCE and LHCE. - or DFOB - ) and weakly reactive molecules that are not Li + The reduction potentials of various organic solvents and the Li + The bond energies were investigated. Furthermore, ether solvents, except for 2-methyltetrahydrofuran (MTHF) and 1,3-dioxolane (DOL), showed lower reduction potentials than carbonates. In particular, tetrahydropyran (THP), tetrahydrofuran (THF), and diethyl ether (DEE), which have one ether group, were found to have significantly lower reduction potentials than the other solvent molecules.

[0063] In the case of DEE, Li + The relatively low binding energy (0.32 eV) promotes coordination of anions, which is the opposite of what is pursued in this invention. In contrast, cyclic ethers with one ether group, such as tetrahydrofuran (THF) and tetrahydropyran (THP), can bind Li + and high binding energy (~0.39 eV), and thus Li +Among these, THP has a relatively high boiling point of 88°C compared to THF (66°C), and is chemically stable with a group 6 ring (the ring deformation energy of THP is 0.5 kcal mol -1 , THF is 5.9 kcal mol -1 ), THP was determined as the main solvent.

[0064] Although this LiBF4 / THP electrolyte combination can catalyze the rearrangement reaction in the SEI, the ionic conductivity is too low (2.9 × 10 -6 S cm -1 LiTFSI was added as an auxiliary salt to impart ionic conductivity to the LiBF4 / THP electrolyte because it is relatively stable with respect to Li metal compared to other highly dissociable Li salts. FEC was added as the initial LiF source for the reconstitution reaction.

[0065] Using the procedure described above, a boron fluoride-cyclic ether-based electrolyte (0.4M LiBF4, 0.4M LiTFSI, THP, 20% FEC) was prepared and its properties as an electrolyte for lithium metal batteries were investigated.

[0066] The solution structure of the boron fluoride-cyclic ether-based electrolyte was investigated by molecular dynamics (MD) simulations. The results showed that the ether groups of 3-4 THP molecules in the primary solvation shell were in the Li + TFSI interacts closely with - , BF4 - and other molecules, including FEC, revealed inconsistent average coordination numbers (0.20, 0.72, and 0.0006, respectively, Figures 6 and 7).

[0067] Li + As a result of observing the dissolution structure, BF4 - Only 12.2% of the TFSIs exhibit solvent-separated ion pairs (SSIPs), whereas only 12.2% of the TFSIs exhibit solvent-separated ion pairs (SSIPs). - 77.4% of the samples exhibited SSIP (Fig. 1D). FEC is a weak Li-ion species due to the electron-withdrawing fluorine group.+ Because of its binding energy, Li + did not participate in the primary solvation sheath of

[0068] From these results, THP and BF4 - is mainly Li participating in the primary solvation shell. + TFSI interacts with - It can be seen that most of the FEC exists as free molecules.

[0069] On the other hand, primary Li + The reduction potential of the solvation shell was investigated (Figure 1E). Using the commonly used LiFSI / DME electrolyte as a control, one or two FSI - The formation of contact ion pairs (CIPs) or ion aggregates (AGGs) was shown (Figures 8 and 9).

[0070] The solvation shells are 1.90 V vs Li / Li, respectively, as previously reported. + or 2.21V vs Li / Li + The FSI-rich Li + The dissolved structure is known to have a high reduction potential and rapidly decompose the FSI on the Li surface to form an inorganic-rich SEI.

[0071] Conversely, Li in borate-pyran-based electrolytes + The melting structure is mostly BF4 - (Li + (THP)3(BF4)1) or SSIP(Li + (THP)4), which is a contact ion pair (CIP) with -0.606 V vs Li / Li + or -0.284V vs Li / Li + In addition, free FEC and free TFSI have a reduction potential of - Li + When combined with Li / Li, the voltage is 0.725V. + and 0.402V vs Li / Li+ It has a low reduction potential.

[0072] On the other hand, nitrogen, sulfur, and boron species (>1% atomic percent) were rarely observed in the SEI formed from the borate-pyran-based electrolyte, which suggests that TFSI - and BF4 - It can be seen that almost no reductive decomposition of occurs (FIGS. 10 and 11).

[0073] Experimental Example 2: Analysis of the reconstructed reaction of the coating structure The microstructure of the SEI on the Li metal surface in a borate-pyran-based electrolyte was investigated by cryogenic transmission electron microscopy (cryo-TEM) (Figure 2A). In the fast Fourier transform (FFT) images, only signals corresponding to the LiF(111) or LiF(220) lattice were observed.

[0074] Similar to the previous results, LiF was observed as the most dominant inorganic species in the XPS results for the SEI layer (Figs. 10 and 11). In particular, the absence of S, N, and B elements in the SEI layer is due to the fact that LiF is BF4 - or TFSI - This means that it was not formed by the defluorination reaction of FEC.

[0075] The SEI on the electrodeposited lithium surface has a mosaic structure, consistent with that observed in the pre-existing carbonate electrolyte. As highlighted in blue-green, LiF species crystallize and are scattered at specific locations within the SEI. The large LiF mosaic structure observed on the electrodeposited Li surface gradually disappears, and after 6–12 hours, fine LiF crystals are densely distributed within the SEI. High-magnification TEM and FFT images provide information about the atomic arrangement of LiF species in the SEI (Figure 2B). The initially observed single-crystalline LiF breaks down into glass-like fine LiF after 12 hours. The atomic image shows a short-range structural order of approximately 2 nm (i.e., a few-layer LiF lattice), which is reflected by the LiF(110) signal in the FFT image. Statistical analysis of the crystal size revealed that the average crystal size gradually decreased from 6.5 nm to 2.1 nm (Figure 2C). We also observed that the SEI thickness decreased from 13.3 nm to 9.1 nm with aging time.

[0076] In the LiBF4-free electrolyte (0.8M LiTFSI THP + 20% FEC), no noticeable changes in the crystal size or SEI microstructure were observed during the aging period, and the SEI thickness increased (Fig. 12), suggesting that LiBF4 is responsible for the time-dependence of the SEI microstructure.

[0077] There was little change in the atomic concentration or bonding state ratio (especially the amount of LiF, Figure 2D and E) in the SEI of the Li surface electrodeposited and aged for 12 h, suggesting that the reduction in LiF crystal size in the borate-pyran-based electrolyte does not occur due to irreversible dissolution of LiF onto the electrolyte.

[0078] The decrease in size of LiF crystallites and the decrease in SEI thickness observed by cryo-TEM cannot be explained by the typical SEI growth with aging time observed in typical electrolyte systems, and on this basis, reorganization of LiF can be expected to occur in boron fluoride-cyclic ether-based electrolytes.

[0079] To confirm the time-dependent evolution of the SEI microstructure in borate-pyran-based electrolytes, the chemical properties of THP for LiBF4 and Li were investigated.

[0080] As can be seen from the Raman spectroscopy (SERS) experiments in Figures 3A and 13, THP, FEC, and LiTFSI (817 cm, respectively) were observed at the cathode / electrolyte interface under open circuit conditions. -1 , 732cm -1 and 741 cm -1 , Figure 14A) signal was clearly observed. - The Raman scattering signal of BF4 - The LiBF4 signal is difficult to detect because it can be blocked by the fluorescent signal of

[0081] During the potential drop (e.g., gradual equipotential with the Li surface), a new pair of peaks (~490 cm -1 and ~1350cm -1 ) gradually strengthened, which can be considered to correspond to FBF bending in the BF3 and BO stretching modes. The FBF bending in the BF3 and BO stretching modes during the potential drop indicates that BF3 and ether groups were complexed with the THP molecules on the positive electrode surface.

[0082] The two peaks were not observed in the electrolyte solution containing no LiBF4 (Fig. 14B). - This confirms that the signal is derived from the BF3-THP complexation reaction with THP. Subsequently, when Li is electrodeposited on Ag at potentials above 0 V, the surface signal of BF3-THP at the electrode decreases, while the signal of the bulk electrolyte (THP) increases due to a decrease in SERS activity. Furthermore, during lithium plating, the BF3-THP signal is persistently observed on the surface of the lithium deposit. While it is generally known that LiBF4 is converted to BF3 by thermal decomposition, the SERS results indicate that BF3-THP can be generated even at room temperature on the electrode surface where the potential of the pyran solvent is low.

[0083] (Figure 3B) performed DFT calculations to investigate the formation of BF3-THP on the LiO surface in pyran solvent. For THP and THF molecules, the activation barriers for the BF3 complexation reaction were 1.98 eV and 1.92 eV, respectively, and the reaction enthalpies were 1.02 eV and 0.96 eV, respectively.

[0084] In the case of ethylene carbonate (EC), BF3 was unable to form stable adducts and existed as free BF3 in an sp2 hybridized structure with a high reaction enthalpy of 1.76 eV. This result suggests that the BF3 complexation reaction is more likely to occur in ether solvents because the cationic bond between the boron and the ether group of BF3 can stabilize the energy of BF3 (Figure 15).

[0085] However, although THP has a lower endothermic reaction enthalpy than carbonate solvents, the predicted equilibrium constant (

number

[0086] The density of states (DOS) shows that the F-2p orbital, which appears near the Fermi level in bulk THP, shifts below the Fermi level at the LiO surface (Figure 16). This indicates that the F-2p orbital accepts electrons from LiO. This indicates that electropositive LiO donates electrons to the electronegative F atom, lowering the activation barrier and reaction enthalpy. Unlike bulk LiBF4 / THP-based electrolytes, the application of THP is expected to play an important role in promoting the BF3 complexation reaction on the LiO surface due to the energy stabilization of F- at the LiO surface.

[0087] BF3 adducts are generally known to react with fluorine salts, but in this study, we investigated the effect of BF3 adducts on LiF crystals. While a 1M solution of LiF in BF3-THF exhibited a completely transparent yellow color, LiF was not completely soluble in common organic solvents (Figure 17).

[0088] 11 Observation of the state of boron in a LiF solution of BF3-THF by B NMR confirmed that the amount of BF3-THF decreased and the amount of LiBF4 increased with increasing LiF concentration (Fig. 3C and D). This indicates that BF3-THF reacts with LiF to convert it to LiBF4.

[0089] LiF solution in BF3-THF is 10 -1 ~10 -2 mS cm -2 In contrast to the ionic conductivity of 1 M LiF (Figure 3E), DME, THF, and THP showed approximately two-fold lower ionic conductivities, indicating that BF3-THP and LiF were converted to dissociated LiBF4.

[0090] In the energy diagram of Figure 3B, the reaction BF3 + LiF → LiBF4 is more exothermic at the bulk THP than at the LiO surface, and therefore it can be expected that LiF in the external SEI is more thermodynamically likely to dissolve near the LiO surface.

[0091] Based on the formation of BF3-THP on LiO and the regeneration of LiBF4, the reorganization of LiF in borate-pyran-based electrolytes can be understood by the mechanism shown in Figure 3F. Specifically, an initial SEI containing LiF particles is formed on LiO. Initially, Li passivation is not yet complete, allowing LiBF4 and THP to come into contact with the LiO surface. BF3-THP forms on the LiO surface, precipitating LiF on the LiO. BF3-THP reacts with the initial LiF particles and converts them back to LiBF4. LiBF4 then precipitates LiF on exposed LiO surfaces, forming BF3-THP. By repeating this process, the initial LiF particles in the SEI are gradually reorganized in a manner that blocks new LiO surfaces, as seen in cryo-TEM observations, ultimately passivating the Li surface with fine LiF crystals.

[0092] Experimental Example 3: Li morphology and coulombic efficiency analysis The boron fluoride-cyclic ether-based electrolyte was compared with an existing carbonate electrolyte (1M LiPF6EC / EMC 1:1v / v + 2%VC) and a locally high concentration electrolyte (LHCE, 1.5M LiFSI DME / TTE, 22:78v / v) to investigate the morphology of Li precipitates, coulombic efficiency / inefficiency, and corrosion properties.

[0093] We confirmed the morphology of the lithium precipitates by applying 290 kPa pressure to the bolt clamps in a custom-fabricated pressure jig (Figure 4A and Figure 18). The lithium deposited in these three electrolytes exhibited a similar, shimmering silver color to the naked eye, but SEM observations revealed a mossy morphology relative to the lithium deposited in the existing carbonate electrolyte. In the other two electrolytes (LHCE and the borate-pyran-based electrolyte), dendrite-free Li particles with diameters of approximately 3 μm were observed. Notably, LHCE and the borate-pyran-based electrolyte formed dense, uniform cross-sectional lithium morphologies, a dense morphology that could not be observed in the LiTFSI THP:FEC electrolyte (Figure 19).

[0094] The Coulombic efficiency of lithium plating / de-bonding on copper and lithium metal substrates was measured. Figure 4B shows the Coulombic efficiency of the first cycle (0.5 mA cm) for each electrolyte. -2 and 5mAh cm -2 The Coulombic efficiencies of the carbonate electrolyte, LHCE, and borate-pyran-based electrolyte were approximately 98.8%, 96.9%, and 99.3%, respectively. In particular, the Coulombic efficiencies of the carbonate electrolyte per cycle were higher than those of the LHCE, presumably because the efficiency of the carbonate electrolyte, which has relatively poor Li morphology, was significantly enhanced under uniform pressure.

[0095] The boron fluoride-cyclic ether-based electrolyte demonstrated Li cycling efficiencies of 99.33% and 99.42% for the first and second cycles on a Cu substrate, respectively. Subsequently, the average efficiencies on Li substrates measured by the modified Auerbach method were 88.14% and 99.62% for carbonate and LHCE electrolytes, respectively. For the LHCE electrolyte, the Li cycling efficiencies were similar to the recently reported Li cycling efficiencies (99.5%-99.6%) measured by the Auerbach method for LHCE or fluorinated solvent electrolytes. The average efficiency of the boron fluoride-cyclic ether-based electrolyte was 99.85%, one of the highest values ​​for any non-aqueous electrolyte known to date.

[0096] To understand the source of coulombic inefficiency during lithium plating / stripping, lithium loss pathways were quantified by thermochemical gas chromatography (TGC, Figure 20).

[0097] In Figure 4C, the total accumulated capacity, the sum of accumulated losses, dead-LiO and SEI Li + The amount of SEI was plotted using the Auerbach method followed by TGC. The carbonate electrolyte and LHCE formed SEI at 6.65% and 0.71% of the total cumulative capacity, respectively, using the Li source.

[0098] Unlike the other two electrolytes, the borate-pyran-based electrolyte lost less Li (0.23% of the total accumulated capacity) due to the formation of SEI than due to the formation of dead LiO, indicating that the borate-pyran-based electrolyte has a lower Coulombic inefficiency for the formation of SEI than existing carbonate electrolytes and LHCE systems.

[0099] Time-dependent interfacial resistance due to electrolyte (R interface ) was measured to confirm the corrosion properties of the electrolytes. Figure 4D shows that the initial interface resistance and the change in response to aging time differ for each electrolyte. The existing carbonate electrolyte had the highest R interface (100-250Ωcm -2 ), which indicates that the SEI component formed in the carbonate electrolyte is more resistive than that in the ether electrolyte.

[0100] The LHCE was fabricated using a borate-pyran based electrolyte (20 Ω cm -2 ) initially 40Ωcm -2 High R interface After aging for 5 days, the resistance was 245 Ωcm -2 while the boron fluoride-cyclic ether-based electrolyte showed a resistance of 31 Ωcm even after 5 days of aging. -2 It was nothing more than that.

[0101] In particular, such low interfacial resistance was not observed in the absence of LiBF4. The extent of long-term corrosion of lithium and electrolyte due to aging can be clearly compared by observing the normalized resistance.

[0102] The highly corrosive LHCE system was aged for 5 days. interface The R increased by 6-fold, showing the fastest SEI growth, which is approximately twice that of the conventional carbonate electrolyte or LiTFSI THP:FEC electrolyte. In particular, the borate-pyran-based electrolyte showed a significant increase in R during 5 days of aging. interfaceThe increase in SEI was only 23%. The electrolyte without LiBF4 (LiTFSI THP:FEC) did not show such a low increase in SEI, indicating that the reorganization of LiF with LiBF4 can reduce the increase in SEI in boron fluoride-cyclic ether-based electrolytes.

[0103] The corrosion rate of electrodeposited Li in the electrolyte was quantified by the time-dependent CE loss, which is influenced not only by the intrinsic corrosivity of the electrolyte but also by the surface area (i.e., morphology, Figure 4E) of the electrodeposited Li. After initial lithium plating and lithium stripping after 5 min of aging, a CE of 96.5% was observed for the LHCE, 96.9% for the borate pyran electrolyte, and 85% for the existing carbonate electrolyte in a coin cell configuration.

[0104] During the 2-day aging period, the conventional carbonate electrolyte and LHCE exhibited CE losses of 10.7% and 4.70%, respectively, indicating that both low-CE and high-CE electrolytes experience significant lithium corrosion during aging. The boron fluoride-cyclic ether-based electrolyte exhibited only a 1.02% CE loss during 2 days of aging, with long-term CE losses of 3.86% after 10 days, 6.56% after 21 days, and 14.9% after 3 months, far less than the other electrolyte systems (Figure 21). When the electrolyte did not contain LiBF4, no such CE loss mitigation was observed.

[0105] The average corrosion rate for each period is shown in Figure 4F. Carbonate electrolytes were >3.5 μAh h for up to 24 hours. -1 The EIS results show that LHCE is more corrosive than carbonate electrolytes, but the corrosion rate is 0.8 μAh h, which is lower than carbonate electrolytes due to the lower surface area of ​​the deposited Li. -1 The corrosion rate was shown.

[0106] The boron fluoride-cyclic ether-based electrolyte achieved approximately 0.1 μAh h after 120 h. -1The low corrosion rate of Li deposited in the LHCE and the boron fluoride-cyclic ether-based electrolyte showed almost identical dense morphologies in the SEM images, which indicates that the boron fluoride-cyclic ether-based electrolyte has high passivation.

[0107] Therefore, boron fluoride-cyclic ether-based electrolytes are expected to minimize the loss of electrolyte molecules and lithium due to the SEI formation on electrodeposited lithium or the self-corrosion of dead LiO that gradually develops over the life of the cell.

[0108] Example 4: Performance of Li||NCM811 under severe conditions Boron fluoride-cyclic ether-based electrolytes are used for Li / Li + The oxidation stability is about 5 V relative to the electrolyte, which corresponds to the requirement for an LMB based on a negative electrode of >4 V class.

[0109] Linear scan voltammogram (LSV) measurements of various electrolytes confirmed the general trend that the presence of FEC along with LiBF4 can increase the oxidative stability of ether solvent-based electrolytes.

[0110] Double-sided coated NCM811 (18.78 mg cm -2 , 3.89mAh cm -2 ) was paired with a Li positive electrode (40 μm, N / P ratio 2.08) in a pouch-type full cell (Figure 5A), and two volume factors of injected liquid electrolyte were used, as shown in Figure 5B. 1.82 g Ah -1 Although this value is a lower E / C ratio than the latest LMBs reported so far, it is significant in that the volume of injected liquid is much larger than in conventional LIBs, even after filling the cell pores.

[0111] Reducing the corrosiveness of the electrolyte allows for very dilute electrolytes LMB (E / C approx. 1 g Ah -1) was possible, the excess electrolyte volume of the LIB was 1.18 g Ah -1 The LMB with an E / C ratio of 1.82 g / Ah was also evaluated. Based on these cell parameters, the estimated energy density was 1.82 g / Ah. -1 424Wh kg -1 and E / C 1.18g / Ah -1 456Wh kg -1 (Table 1).

[0112] [Table 1]

[0113] The voltage profiles of pouch cells prepared with the three electrolytes were investigated according to the cell capacity and specific capacity (Figure 5C). The boron fluoride-cyclic ether-based electrolytes showed specific capacities during 1C, 2C, and 3C discharge, respectively. The boron fluoride-cyclic ether-based electrolytes exhibited high ionic conductivity (0.35 Ω cm, including the separator). -2 ) and low interfacial resistance, which contributes to high rate capability.

[0114] In conclusion, this paper provides a boron fluoride-cyclic ether-based electrolyte for LMBs. The boron fluoride-cyclic ether-based electrolyte exhibits low corrosivity and finely passivates the lithium-electrolyte interface through LiF reconstitution, minimizing the Coulombic inefficiency of lithium SEI formation. Furthermore, the boron fluoride-cyclic ether-based electrolyte exhibits ideal lithium deposition morphology and high lithium plating / stripping efficiency (average CE of 99.86%), meeting the requirements for long-term operation of LMBs at low N / P and E / C ratios.

[0115] In particular, the nickel-rich NCM811 anode (3.83 mAh cm -2 The prototype pouch cell containing a thin Li cathode (N / P2) had an E / C of 1.82 g Ah -1 (424Wh kg -1 ) for over 500 cycles and 1.18g Ah-1 (456Wh kg -1 ) exhibited excellent cycle life of over 250 cycles and high energy density, which was achieved by mitigating irreversibility caused by the corrosive nature of the electrolyte. -1 This makes it possible to achieve long-cycle LMB under conditions below.

Claims

1. ether solvents having an ether group; and a boron fluoride-based lithium salt or a non-boron fluoride-based lithium salt dissolved in the ether-based solvent; The electrolyte composition, wherein the boron fluoride-based lithium salt or the non-boron fluoride-based lithium salt and the ether-based solvent form a complex.

2. 2. The electrolyte composition according to claim 1, wherein the boron fluoride-based lithium salt comprises a boron fluoride lithium salt represented by the following Chemical Formula 1: Chemical formula 1 LiBF ｘ R ｙ Here, R = -CF ３ or -styrene, where x+y=4.

3. The electrolyte composition according to claim 1 , wherein the ether-based solvent is a cyclic ether-based solvent.

4. The electrolyte composition according to claim 1 , wherein the ether-based solvent has one ether group.

5. 2. The electrolyte composition according to claim 1, wherein the boron fluoride-based lithium salt and the ether-based solvent form a boron trifluoride-ether complex.

6. 2. The electrolyte composition according to claim 1, wherein the non-boron fluoride-based lithium salt comprises a lithium salt based on an anion having a Group 13 element as a central atom or a lithium salt having a boron central atom and composed of a halide-based anion other than fluorine.

7. 2. The electrolyte composition of claim 1, wherein the ether-based solvent comprises one or more solvents selected from the group consisting of tetrahydrofuran (THF), tetrahydropyran (THP), furan, pyran, oxirane, oxetane, oxepane, and dioxane.

8. The boron fluoride-based lithium salt is LiBF ４ 3. The electrolyte composition according to claim 2, wherein the ether solvent is tetrahydropyran (THP).

9. 10. The electrolyte composition of claim 1, further comprising one or more selected from the group consisting of ion-conducting lithium salts and additives.

10. The ion-conductive lithium salt is LiTFSI, LiFSI, LiBETI, LiAsF ６ , LiPF ６ , LiSbF ６ , LiDFOB, and LiBOB; 10. The electrolyte composition of claim 9, wherein the additive comprises one or more additives selected from the group consisting of FEC, DFEC, TFEC, TFTFE, TTE, BTFE, OFE, and TFOFE.

11. 11. The electrolyte composition of claim 10, wherein the ion-conducting lithium salt is LiTFSI and the additive is FEC, which acts as a LiF-forming agent.

12. The anion of the ion-conductive lithium salt has a lower Li than the anion of the boron fluoride-based lithium salt. ＋ The electrolyte composition of claim 9 having a binding energy.

13. 3. The electrolyte composition of claim 2, wherein the electrolyte composition has B-O stretching and F-B-F bending signals in SERS analysis.

14. The electrolyte composition according to any one of claims 1 to 13; a negative electrode including a lithium metal thin film formed on a negative electrode current collector; a positive electrode including a positive electrode active material layer formed on a positive electrode current collector; and a separator formed between the negative electrode and the positive electrode.

15. 15. The lithium metal battery according to claim 14, wherein the boron fluoride-based lithium salt and the ether-based solvent form a boron trifluoride-ether complex on the surface of the lithium metal thin film.

16. The lithium metal thin film further includes an SEI layer formed on the lithium metal thin film, 15. The lithium metal battery of claim 14, wherein the SEI layer has only signals for the LiF(111) or LiF(220) lattice in a Fast Fourier Transform (FFT) image.

17. The SEI layer is formed by depositing BF on the lithium metal thin film. ３ 15. The lithium metal battery of claim 14, wherein the THP is formed and passivated with LiF crystals by a reconstitution process that precipitates LiF on LiO.

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