Electrolyte for a lithium metal secondary battery and a lithium metal secondary battery including the same

Abstract

The present disclosure relates to an electrolyte for a lithium metal secondary battery. In the electrolyte according to one embodiment of the present disclosure, the lithium salt includes lithium bis(fluorosulfonyl)imide, and the non-aqueous solvent includes: 1,2-(1,1,2,2-tetrafluoroethoxy)ethane; a cyclic fluorinated carbonate solvent; and a solvent containing at least one selected from the group consisting of a chain carbonate, a chain ester, and a chain ether, and 1,2-(1,1,2,2-tetrafluoroethoxy)ethane is included in an amount of 5% by volume to 30% by volume based on the total volume of the non-aqueous solvent.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0047502, filed with the Korean Intellectual Property Office on April 20, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present disclosure relates to an electrolyte for a lithium metal secondary battery, which can be applied to a lithium metal secondary battery using lithium metal as an anode active material to improve battery life characteristics. Background Art

[0004] With the rapid development of the electronics, communication, and computer industries, the application fields of energy storage technologies are expanding to cameras, mobile phones, laptops, PCs, and even electric vehicles. Therefore, the development of high-performance secondary batteries that are lightweight, long-lasting, and highly reliable is underway.

[0005] Among currently available secondary batteries, lithium secondary batteries developed in the early 1990s have attracted attention due to their advantages such as higher working voltage and higher energy density compared to conventional water-based electrolyte batteries (e.g., Ni-MH, Ni-Cd, and lead sulfate batteries).

[0006] Lithium metal, carbon-based materials, silicon, etc. are used as negative electrode active materials for lithium secondary batteries, and lithium metal has the advantage of obtaining the highest energy density, so research is continuously being conducted.

[0007] A lithium electrode using lithium metal as an active material is usually manufactured by using a flat copper foil or nickel foil as a current collector and adhering a lithium foil thereto. Alternatively, using the lithium foil itself as a lithium electrode without a separate current collector, or using only a current collector without a lithium foil to assemble a battery, and then charging and discharging the battery to form a lithium metal layer and using it as a battery method, etc. are known.

[0008] Such lithium batteries are called lithium metal batteries and free lithium batteries, etc., but are generally called lithium metal secondary batteries because lithium metal is mainly used as the negative electrode.

[0009] However, a lithium metal secondary battery including a lithium metal electrode has high reactivity of lithium metal, volume expansion of the negative electrode during battery charge / discharge, and surface non-uniformity that occurs during the process of electrodepositing and stripping lithium metal on the negative electrode. There is a problem that a stable interface is not formed between the electrolyte and the lithium metal electrode, and a continuous decomposition reaction of the electrolyte solution occurs. These side reactions of the electrolyte not only rapidly increase the battery resistance but also deplete the electrolyte and available lithium in the battery, which is the main cause of the decrease in battery life. Summary of the Invention

[0010] [Technical Problem]

[0011] The present invention is designed to solve the problems of the prior art. Therefore, an object of the present invention is to provide a non-aqueous electrolyte having excellent stability to lithium metal, thereby being able to suppress side reactions of the electrolyte and improve life characteristics.

[0012] [Technical Solution]

[0013] According to one embodiment of the present disclosure, there is provided an electrolyte for a lithium metal secondary battery, including: a lithium salt and a non-aqueous solvent, wherein the lithium salt includes lithium bis(fluorosulfonyl)imide, and the non-aqueous solvent includes 1,2-(1,1,2,2-tetrafluoroethoxy)ethane; a cyclic fluorinated carbonate solvent; and a solvent containing at least one selected from the group consisting of a linear carbonate, a linear ester, and a linear ether, and 1,2-(1,1,2,2-tetrafluoroethoxy)ethane is included in an amount of 5% to 30% by volume based on the total volume of the non-aqueous solvent.

[0014] In some embodiments, 1,2-(1,1,2,2-tetrafluoroethoxy)ethane may be included in an amount of 10% to 20% by volume based on the total volume of the non-aqueous solvent.

[0015] In one embodiment, the cyclic fluorinated carbonate solvent may include at least one selected from the group consisting of vinylene carbonate fluoride, ethylene carbonate difluoride, and trifluoromethyl carbonate.

[0016] In some embodiments, the cyclic fluorinated carbonate solvent may be included in an amount of 5% to 30% by volume based on the total volume of the non-aqueous solvent, and specifically, may be included in an amount of 10% to 20% by volume based on the total volume of the non-aqueous solvent.

[0017] In some embodiments, the linear carbonate may include at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0018] In some embodiments, the linear ester may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0019] In some embodiments, the linear ether may include at least one selected from the group consisting of methyl ether, ethyl ether, propyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether.

[0020] In some embodiments, at least one solvent selected from the group consisting of a chain carbonate, a chain ester, and a chain ether may include at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, methyl ether, and ethyl ether.

[0021] In some embodiments, lithium bis(fluorosulfonyl)imide may be included in an amount of 20 wt% to 50 wt% of the total weight of the electrolyte, and specifically, may be included in an amount of 25 wt% to 40 wt% of the total weight of the electrolyte.

[0022] According to another embodiment of the present disclosure, there is provided a lithium metal secondary battery including: a positive electrode; a negative electrode made only of a negative electrode current collector, or including lithium metal coated on the negative electrode current collector, or made of lithium metal; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0023] In some embodiments, the positive electrode may include a lithium nickel-cobalt-manganese-based compound or lithium cobalt oxide as a positive electrode active material. Detailed Embodiments

[0024] The terms used herein are for describing exemplary embodiments only and are not intended to limit the present disclosure. Singular expressions include plural expressions unless they have a clearly opposite meaning in the context. It should be understood that the terms "comprising", "including", and "having" used herein are intended to indicate the presence of the described features, quantities, steps, components, or combinations thereof, but it should be understood that they do not exclude the possibility of the presence or addition of one or more other features, quantities, steps, components, or combinations thereof.

[0025] Since the present disclosure can be modified in various forms and can have various embodiments, the following exemplary embodiments are shown and described in detail. However, this is not intended to limit the present disclosure to specific embodiments, and the present disclosure should be construed as including various changes, equivalents, and alternatives within the technical scope and spirit of the present invention.

[0026] An electrolyte for a lithium metal secondary battery according to an embodiment of the present disclosure is an electrolyte for a lithium metal secondary battery, including a lithium salt and a non-aqueous solvent, wherein the lithium salt includes lithium bis(fluorosulfonyl)imide, the non-aqueous solvent includes 1,2-(1,1,2,2-tetrafluoroethoxy)ethane; a cyclic fluorinated carbonate solvent; and a solvent containing at least one selected from the group consisting of a chain carbonate, a chain ester, and a chain ether, and 1,2-(1,1,2,2-tetrafluoroethoxy)ethane is included in an amount of 5 vol% to 30 vol% based on the total volume of the non-aqueous solvent.

[0027] The inventors have conducted in-depth research on the composition of non-aqueous electrolytes suitable for lithium metal secondary batteries containing lithium metal as negative electrode active materials, and found that when an electrolyte solution satisfying the composition of the present disclosure is applied to a lithium metal battery, it exhibits significantly improved stability compared to conventional electrolyte solutions, and improves battery life, high rate charging performance, and high temperature performance, thereby completing the present invention. The above effects of the present disclosure can only be ensured when the combination of lithium salt and solvent is satisfied, and are difficult to achieve when any of the above components is insufficient.

[0028] 1,2-(1,1,2,2-tetrafluoroethoxy)ethane (TFEE) is a material having the following structural formula, and is used as a solvent in the nonaqueous electrolyte solution of the present disclosure.

[0029]

[0030] TFEE can inhibit the side reaction between lithium metal and electrolyte to improve battery life characteristics, and reduce the viscosity of the electrolyte to improve the impregnation performance of electrodes and separators, thereby improving lithium ion conductivity.

[0031] However, as a result of experiments by the present inventors, the above-mentioned effect can be exhibited when the content of TFEE is 5% by volume to 30% by volume based on the total volume of the nonaqueous solvent used in the electrolyte.

[0032] When the TFEE content exceeds 30% by volume outside the range and is too much, the life improvement effect is reduced. In addition, even when the TFEE content is too small, less than 5% by volume relative to the total volume of the non-aqueous solvent, the above effect cannot be obtained. Therefore, in order to ensure the effect of improving the stability of the electrolyte and improving the battery life characteristics, TFEE is preferably contained in an amount of 10% by volume or more, 25% by volume or less, or 20% by volume or less based on the total volume of the non-aqueous solvent.

[0033] In the present disclosure, cyclic fluorocarbonate-based solvents are included together with TFEE.

[0034] The cyclic fluorinated carbonate-based solvent is not particularly limited as long as it is a compound in which at least one hydrogen is replaced by fluorine in a cyclic carbonate-based solvent commonly used as a solvent for an electrolyte. Specifically, the cyclic fluorinated carbonate-based solvent may include at least one selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate and trifluoromethylethylene carbonate, preferably fluoroethylene carbonate.

[0035] The cyclic fluorocarbonate solvent is preferably contained in an amount of 5 volume % or more, or 10 volume % or more, and 30 volume % or less, or 20 volume % or less, based on the total volume of the nonaqueous solvent.

[0036] If the content of the cyclic fluorinated carbonate-based solvent is less than 5% by volume, the effect of suppressing side reactions of the electrolyte cannot be ensured. If the content exceeds 30% by volume, the lithium salt may not dissociate sufficiently, and thus the ionic conductivity of the electrolyte may not be ensured.

[0037] Meanwhile, in addition to TFEE and the cyclic fluorinated carbonate-based solvent, the electrolyte of the present disclosure is a non-aqueous solvent and includes at least one solvent selected from the group consisting of linear carbonates, linear esters, and linear ether solvents (hereinafter referred to as linear solvents). An electrolyte solution prepared by mixing such a linear solvent, TFEE, and a cyclic fluorinated carbonate-based solvent with LiFSI salt exhibits excellent stability to lithium metal. However, an electrolyte solution using a combination of a cyclic solvent, TFEE, a cyclic fluorinated carbonate, and LiFSI cannot achieve this effect, which can be confirmed from the results of the examples described later.

[0038] The cyclic solvent can be used in an amount of 50% by volume or more, or 60% by volume or more, based on the total volume of the non-aqueous solvent of the electrolyte solution, and can be used in an amount of 85% by volume or less, and 80% by volume or less.

[0039] Alternatively, the remaining portion except for TFEE and the cyclic fluorinated carbonate-based solvent can be filled with the linear solvent.

[0040] That is, in the electrolyte solution according to an embodiment of the present disclosure, the non-aqueous solvent may be composed of TFEE, a cyclic fluorinated carbonate solvent, and a linear solvent. In other words, except for these, the electrolyte solution may not contain any additional solvent.

[0041] As the linear solvent, the linear solvents generally used in electrolyte solutions for lithium secondary batteries can be used without limitation.

[0042] Specifically, the linear carbonate may include at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0043] The linear ester may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0044] The linear ether may include at least one selected from the group consisting of methyl ether, ethyl ether, propyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether.

[0045] At least one solvent selected from the group consisting of linear carbonates, linear esters, and linear ethers may include at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, methyl ether, and ethyl ether.

[0046] Meanwhile, the electrolyte for a lithium metal secondary battery according to the present disclosure includes lithium bis(fluorosulfonyl)imide (LiFSI) as a lithium salt.

[0047] The content of LiFSI is preferably 20-50% by weight, or 25-40% by weight, or 31-40% by weight of the total weight of the electrolyte. If the content of LiFSI is less than 20% by weight, there are problems of corrosion of the positive electrode current collector (e.g., aluminum foil) and dissolution of the transition metal of the positive electrode active material. If the content of LiFSI exceeds 50% by weight, there may be problems of performance deterioration due to low ionic conductivity and impregnation performance deterioration due to high viscosity, which is not preferred.

[0048] The electrolyte according to the present disclosure may include only LiFSI as a lithium salt, or may further contain other lithium salts in addition to LiFSI. The lithium salt contained in addition to LiFSI is used in an amount of 0.1-3% by weight based on the total weight of the electrolyte solution, which is preferred because the life performance of the lithium metal battery can be ensured.

[0049] The lithium salts that may be included in addition to LiFSI may include LiCl, LiBr, LiI, LiClO 4 , LiBF 4 , LiB 10 Cl 10 , LiPF 6 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , LiSCN, LiC 4 BO 8 , LiCF 3 CO 2 , LiCH 3 SO 3 , LiCF 3 SO 3 , LiN(SO 2 CF 3 ) 2 , LiN(SO 2 C 2 F 5 ) 2 , LiC 4 F 9 SO 3 , LiC(CF 3 SO 2 ) 3 , (CF 3 SO 2 ) 2 NLi and the like.

[0050] In this case, the total concentration of LiFSI and other lithium salts in the electrolyte is preferably 1.8 M (mol / L) or higher. At such a high salt concentration, side reactions between the lithium metal and the electrolyte can be suppressed, and effects of preventing corrosion of the positive electrode current collector and dissolution of transition metals of the positive electrode active material can be ensured. In some embodiments, the concentration of the lithium salt in the electrolyte can be 2.0 M or above, or 2.3 M or above, and 4.0 M or below, or 3.0 M or below. If the concentration of the lithium salt is too high, there may be problems of performance deterioration due to low ionic conductivity and impregnation performance deterioration due to high viscosity. Therefore, the concentration is appropriately adjusted within the above range.

[0051] The electrolyte for a lithium metal secondary battery according to the present disclosure contains TFEE in an amount of 5% to 30% by volume based on the total volume of the non-aqueous solvent, and further includes a cyclic fluorocarbonate and a chain-like solvent, and includes LiFSI as a lithium salt, thereby showing excellent stability to the lithium metal electrode and showing an effect of significantly reducing side reactions during battery operation. Thereby, when the electrolyte solution according to the present disclosure is applied to a lithium metal secondary battery, battery life characteristics, high-rate charging performance, and high-temperature performance can be improved.

[0052] According to another embodiment of the present disclosure, there is provided a lithium metal secondary battery including the electrolyte. Specifically, the lithium metal secondary battery includes: a positive electrode; a negative electrode made only of a negative electrode current collector, or including lithium metal coated on the negative electrode current collector, or made of lithium metal; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0053] Here, the positive electrode includes a positive electrode active material layer coated on a positive electrode current collector. The positive electrode active material layer may include a positive electrode active material, a binder, and optionally a conductive material.

[0054] The positive electrode current collector is not particularly limited as long as the corresponding battery has conductivity and does not cause chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, calcined carbon, and materials formed by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, or the like. At this time, the positive electrode current collector may have fine protrusions and depressions on its surface to increase the adhesion of the positive electrode active material, and can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric structures. As the positive electrode active material, known compounds in the art that are capable of reversibly intercalating and deintercalating lithium can be used.

[0055] For example, lithium manganese-based oxides (e.g., LiMnO 2 、LiMn 2 O 4etc.), lithium cobalt-based oxides (e.g., LiCoO 2 etc.), lithium nickel-based oxides (e.g., LiNiO 2 etc.), lithium nickel manganese-based oxides (e.g., LiNi 1-Y Mn Y O 2 (where 0 < Y < 1), LiMn 2-z Ni z O 4 (where 0 < Z < 2) etc.), lithium nickel cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O 2 (where 0 < Y1 < 1) etc.), lithium manganese cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O 2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O 4 (where 0 < Z1 < 2) etc.), lithium nickel manganese cobalt-based oxides (e.g., Li(Ni p Co q Mn r1 )O 2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O 4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2) etc.), or lithium nickel cobalt transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O 2 (where M is at least one selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of the respective independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1) etc.), and the like, and may include any one or two or more of these compounds.

[0056] However, when using the electrolyte according to the present disclosure, specifically, when the positive electrode active material contains a lithium nickel cobalt manganese compound or a lithium cobalt oxide, the effects of preventing side reactions of the electrolyte and improving life characteristics according to the present disclosure are achieved, which is more preferable.

[0057] Binders are used for binding electrode active materials and conductive materials as well as for binding to current collectors. Non-limiting examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethylacrylonitrile, polyimide (PI), alginic acid, alginate, chitosan, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, various copolymers thereof, and the like.

[0058] Conductive materials are used to further improve the conductivity of electrode active materials. There is no particular limitation on such conductive materials as long as they have conductivity and do not cause chemical changes in the battery. For example, graphite such as natural graphite and artificial graphite can be used; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0059] There is no particular limitation on the method for manufacturing the positive electrode. For example, the positive electrode can be manufactured through the following steps: mixing the active material, binder, and optionally the conductive material in an organic solvent to prepare an active material slurry, then coating the prepared active material slurry onto a current collector and drying it, and optionally, performing compression molding on the current collector.

[0060] As the organic solvent, those organic solvents in which the active material, binder, and conductive agent can be uniformly dispersed and easily evaporated are preferred. Specifically, N-methylpyrrolidone, acetonitrile, methanol, ethanol, tetrahydrofuran, water, and isopropyl alcohol can be exemplified, but are not limited thereto.

[0061] The negative electrode is a lithium metal negative electrode using lithium metal as the negative electrode active material. The lithium metal negative electrode used when assembling a lithium metal secondary battery can be formed only by a negative electrode current collector, can be in a form including lithium metal coated on the negative electrode current collector, or can be made of lithium metal.

[0062] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or materials formed by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, or the like, aluminum cadmium alloy, and the like can be used. In addition, various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc. can be used as the forms of films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics, with or without fine irregularities formed on the surface. For example, copper foil can be used as the negative electrode current collector, but it is not limited thereto.

[0063] The thickness of the current collector is not particularly limited, and it is preferably 5 - 100 μm, more preferably 5 - 50 μm. If the thickness of the current collector is less than 5 μm, it will be difficult to handle in the process. If the thickness exceeds 100 μm, the thickness and weight of the battery will increase unnecessarily, and the energy density may decrease, which may affect the battery performance. Therefore, the above range is desirable.

[0064] If an electrode made only of the current collector is used as the negative electrode when assembling the battery, lithium ions transferred from the positive electrode through initial charging and discharging are irreversibly plated on the negative electrode current collector to form a lithium metal layer after assembling the battery, and then the lithium metal layer can be used as the negative electrode active material layer.

[0065] Alternatively, a negative electrode containing lithium metal as an active material can be used from the battery components, and the method of coating lithium metal onto the negative electrode current collector is not particularly limited. As an example, a method of laminating a lithium metal thin film onto the current collector and then roll-pressing it, a method of electroplating or electroless plating lithium metal onto the current collector, or a similar method can be used. At this time, the thickness of the lithium metal layer of the negative electrode is not particularly limited, but it can be 10 μm or more, or 20 μm or more, and 50 μm or less, or 40 μm or less.

[0066] Meanwhile, in the case of a lithium metal negative electrode made only of lithium metal, the thickness is not particularly limited, but it can be 10 μm or more, or 20 μm or more, and 50 μm or less, or 40 μm or less.

[0067] The separator separates the positive electrode and the negative electrode and provides a channel for the movement of lithium ions, and any separator can be used without particular limitation as long as it is commonly used as a separator in lithium secondary batteries. That is, a separator having excellent electrolyte moisturizing ability and low resistance to the movement of electrolyte ions can be used.

[0068] Specifically, a porous polymer membrane can be used, for example, a porous polymer membrane made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or a laminated structure having two or more layers thereof. In addition, conventional porous non-woven fabrics can also be used, such as non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, or the like. In addition, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can be used, and optionally, a single-layer or multi-layer structure can be used. For example, the separator is selected from glass fiber, polyester, polytetrafluoroethylene, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and can be a non-woven fabric or a fabric.

[0069] For example, a polyolefin-based polymer separator such as polyethylene and polypropylene can be used, or a separator including a coating containing a ceramic component or a polymer material to ensure heat resistance or mechanical strength can be used. These separators can be used in a single-layer or multi-layer structure. In one embodiment, as the separator, a separator prepared by coating a ceramic coating material containing ceramic particles and an ionic binder polymer on both surfaces of a polyolefin-based polymer substrate can be used.

[0070] The method for manufacturing a lithium secondary battery according to the present disclosure is not particularly limited. In one example, a lithium secondary battery can be manufactured by the following steps: stacking a positive electrode, a separator, and a negative electrode in sequence to prepare an electrode assembly, placing the electrode assembly in a battery case, injecting an electrolyte solution into the upper part of the case, and sealing it with a cover plate and a gasket.

[0071] The shape of the lithium secondary battery as described above is not particularly limited, and for example, it can be a wound type, a stacked type, a stacked-folded type (including a stacked-Z-folded type), or a laminated-stacked type.

[0072] The lithium metal secondary battery manufactured in this way exhibits significantly fewer side reactions between the lithium metal electrode and the electrolyte, and thus can exhibit excellent battery life characteristics and high rate charging performance.

[0073] Hereinafter, preferred embodiments will be provided to help understand the present disclosure, but the following embodiments are provided for illustrative purposes only. It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the present invention. Therefore, the present disclosure is intended to cover modifications and variations of the present invention as long as they fall within the scope of the appended claims and their equivalents.

[0074] [Examples]

[0075] Examples 1 to 3 and Comparative Examples 1 to 3: Manufacture of Lithium Metal Secondary Batteries

[0076] (1) Fabrication of the positive electrode

[0077] Use NCM 811 (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ) as the positive electrode active material, carbon black as the conductive material, polyvinylidene fluoride (PVdF) as the binder, and N-methylpyrrolidone as the solvent. Thus, prepare a positive electrode active material slurry with a weight ratio of active material:conductive material:binder of 96:2:2. Then, coat both sides of an aluminum foil with a thickness of 12 μm with the positive electrode active material slurry, and perform rolling and drying to fabricate a positive electrode with a loading of 3.8 mAh / cm 2 .

[0078] (2) Fabrication of the negative electrode

[0079] Laminating a lithium metal foil with a thickness of 20 μm on one surface of a copper foil with a thickness of 8 μm and performing rolling to fabricate a lithium metal negative electrode.

[0080] (3) Fabrication of the electrolyte

[0081] Prepare the electrolytes of the examples and comparative examples according to the composition shown in Table 1 below. In the table below, DMC is dimethyl carbonate (linear carbonate), FEC is fluoroethylene carbonate, and TFEE is 1,2-(1,1,2,2-tetrafluoroethoxy)ethane. In addition, in the table below, the content of the lithium salt is in weight % based on the total weight of the electrolyte solution, and the content of each solvent is in volume % of each solvent based on the total volume of the non-aqueous solvents.

[0082]

Table 1

[0083]

[0084]

[0085] (4) Assembly of the battery

[0086] Laminating the separator and the negative electrode of (2) on two surfaces of the positive electrode of (1) to prepare a pouch-type dual battery with a capacity of 125 mAh, where the negative electrode / separator / positive electrode / separator / negative electrode are laminated in this order. At this time, use a separator coated with alumina with a thickness of 2.5 μm on both sides of a polyethylene fabric with a thickness of 7 μm.

[0087] Inject 200 μl (1.6 μl / mAh) of each electrolyte solution prepared in (3) into the pouch to fabricate the batteries of Examples 1 and 2 and Comparative Examples 1 to 3.

[0088] Test Example: Evaluation of Life Characteristics at Room Temperature (25 °C) and High Temperature (45 °C)

[0089] For each battery of the examples and comparative examples, repeated charging and discharging were carried out at 25 °C with a standard charge / discharge current density of 0.2C / 2.0C, a charge termination voltage of 4.25V, and a discharge termination voltage of 2.5V. At this time, the cycles when the capacity retention rate represented by the following formula 1 became 80% were recorded and shown in Table 2 below. In addition, the same experiment was carried out on the batteries of Example 2 and Comparative Example 1 at 45 °C, and the results are shown in Table 2 below.

[0090] [Formula 1]

[0091] Capacity retention rate (%) = (discharge capacity of the nth cycle / discharge capacity of the first cycle) * 100

[0092]

Table 2

[0093]

[0094] Referring to Table 2, it can be confirmed that, compared with Comparative Example 1, the electrolyte solution of the example including TFEE exhibits significantly improved room temperature and high temperature life characteristics. However, it can be seen from Comparative Example 2 that when the content of TFEE is too high, the life improvement effect cannot be fully ensured. In addition, it can be seen from Comparative Example 3 that even without FEC, the effects of the present invention cannot be achieved. On the other hand, comparing Example 1 with Examples 2 and 3, it can be seen that when the electrolyte containing TFEE does not contain a more appropriate amount of FEC and LiFSI, the effects of the present disclosure cannot be achieved.

[0095] [Industrial Applicability]

[0096] The electrolyte for a lithium metal secondary battery of the present disclosure has excellent stability to lithium metal, so there are fewer side reactions of the electrolyte. Therefore, it can be applied to lithium metal secondary batteries to improve battery life characteristics, high-rate charging performance, and high-temperature performance.

Claims

1. An electrolyte for a lithium metal secondary battery, comprising: a lithium salt and a non-aqueous solvent, wherein the lithium salt includes lithium bis(fluorosulfonyl)imide, the non-aqueous solvent includes 1,2-(1,1,2,2-tetrafluoroethoxy)ethane; a cyclic fluorinated carbonate solvent; and a solvent containing at least one selected from the group consisting of a linear carbonate, a linear ester, and a linear ether, the 1,2-(1,1,2,2-tetrafluoroethoxy)ethane is included in an amount of 5% to 30% by volume based on the total volume of the non-aqueous solvent, the cyclic fluorinated carbonate solvent is included in an amount of 5% to 30% by volume based on the total volume of the non-aqueous solvent, the linear ester is at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate, and the lithium bis(fluorosulfonyl)imide is included in an amount of 20% to 50% by weight based on the total weight of the electrolyte.

2. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the 1,2-(1,1,2,2-tetrafluoroethoxy)ethane is included in an amount of 10% to 20% by volume based on the total volume of the non-aqueous solvent.

3. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the cyclic fluorinated carbonate solvent includes at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and trifluoromethyl vinylene carbonate.

4. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the cyclic fluorinated carbonate solvent is included in an amount of 10% to 20% by volume based on the total volume of the non-aqueous solvent.

5. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the linear carbonate includes at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

6. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the linear ether includes at least one selected from the group consisting of methyl ether, ethyl ether, propyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether.

7. The electrolyte for a lithium metal secondary battery according to claim 1, wherein at least one solvent selected from the group consisting of the linear carbonate, the linear ester, and the linear ether includes at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, methyl ether, and ethyl ether.

8. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the lithium bis(fluorosulfonyl)imide is included in an amount of 25% to 40% by weight based on the total weight of the electrolyte.

9. A lithium metal secondary battery, comprising: a positive electrode; a negative electrode made only of a negative electrode current collector, or including lithium metal coated on the negative electrode current collector, or made of lithium metal; a separator interposed between the positive electrode and the negative electrode; and the electrolyte according to any one of claims 1 to 8.

10. The lithium metal secondary battery according to claim 9, Wherein the positive electrode includes a lithium nickel-cobalt-manganese-based compound or a lithium cobalt oxide as a positive electrode active material.

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