Additives for secondary batteries and lithium metal batteries containing the same

Abstract

The present invention relates to an additive for secondary batteries that can suppress the growth of acicular lithium and induce uniform lithium growth on a thin lithium metal film to improve the performance and life of a lithium metal battery, and a lithium metal battery including the additive.

Description

[Technical Field]

[0001] [Cross-reference of related applications] This application claims priority rights based on Korean Patent Application No. 10-2022-0053361 dated April 29, 2022, and all content disclosed in the said Korean Patent Application is incorporated herein by reference.

[0002] The present invention relates to an additive for secondary batteries and a lithium metal battery containing the same, which can suppress the growth of needle-shaped lithium and induce uniform lithium growth on a lithium metal thin film, thereby improving the performance and lifespan of the lithium metal battery. [Background technology]

[0003] Lithium metal batteries are batteries that use a lithium metal (Li-metal) thin film as the negative electrode active material. Compared to existing secondary batteries that use graphite-based negative electrodes, they have the advantage of theoretically higher energy density and capacity (3860 mAh g-1). Therefore, research and development are continuing to apply lithium metal batteries to secondary batteries that require high energy density.

[0004] However, lithium metal batteries have a problem in that, due to the properties of lithium metal acting as the negative electrode active material, the volume change of the negative electrode is large during the charge / discharge process, and needle-shaped lithium generated during charging grows to form lithium dendrites. If the growth of such lithium dendrites continues, they can penetrate the separator membrane and cause an internal short circuit in the cell, which can result in significant problems with the performance of the secondary battery, as well as safety problems such as ignition, and a significant reduction in the lifespan characteristics of the secondary battery.

[0005] The growth of needle-shaped lithium and lithium dendrites on such lithium thin films is thought to occur because a stronger electric field is concentrated in the lithium chip formation area compared to the flat areas, causing the flow of lithium ions during charging / discharging to concentrate on the protruding parts of the lithium chip.

[0006] In particular, lithium metal acting as the negative electrode active material in lithium metal batteries is highly reactive with the electrolyte, so irreversible reactions can continuously occur during the charge / discharge process. Needle-shaped lithium and lithium dendrites that grow rapidly in such irreversible reactions can break down the solid electrolyte interface (SEI) film on the lithium metal thin film, which can further accelerate the aforementioned irreversible reactions. As a result, continuous irreversible reactions and the growth of needle-shaped lithium can occur during the charge / discharge process of lithium metal batteries, leading to a rapid decrease in the cell's capacity characteristics and performance, and a significant reduction in the lifespan and safety of the lithium metal battery.

[0007] Due to these problems with existing lithium metal batteries, research has been conducted from various angles on technologies to solve problems such as the growth of needle-shaped lithium by forming a protective film on the lithium metal thin film, or more specifically, on the lithium chip or protrusions, or by strengthening the solid electrolyte interface film.

[0008] One such technology involves using an ionic liquid compound, which exists in a liquid state containing cations and anions at the battery's operating temperature, as an additive. The cations of this ionic liquid compound adsorb to the surface of the lithium chip, forming a protective layer, which repels lithium ions from the area surrounding the lithium chip. As a result, the phenomenon of lithium ion flow concentrating around the lithium chip or protrusions can be suppressed, thereby inhibiting the rapid growth of needle-shaped lithium or lithium dendrites and inducing uniform lithium growth. For this reason, it is known that using the ionic liquid compound as an additive can mitigate problems caused by the rapid growth of needle-shaped lithium to some extent.

[0009] However, previously proposed ionic liquid compounds exhibit a strong tendency to self-aggregate around lithium chips due to their high amphiphilicity. As a result, the cations of the ionic liquid compounds cannot completely cover the lithium chips, thus forming an incomplete protective layer, which has been found to still lead to problems with needle-shaped lithium or lithium dendrite growth.

[0010] Therefore, there is a continuing need for the development of additives and other technologies that can further reduce problems in lithium metal batteries, such as the growth of needle-shaped lithium or lithium dendrites. [Overview of the project] [Problems that the invention aims to solve]

[0011] Therefore, the present invention provides an additive for secondary batteries that can improve the performance and lifespan of lithium metal batteries by forming a uniform protective layer around lithium chips and the like, more effectively suppressing the growth of needle-shaped lithium, and inducing uniform lithium growth on a lithium metal thin film.

[0012] Furthermore, the present invention aims to provide a lithium metal battery that contains the aforementioned additive in the electrolyte and exhibits improved lifespan characteristics and safety. [Means for Solving the Problems]

[0013] The present invention provides an additive containing an ionic liquid compound that exhibits a liquid state containing cations and anions at atmospheric pressure and a temperature of 100 °C or lower, wherein the cation has a standard reduction potential lower than that of a lithium cation (Li -11 , 2 , -11 , -1 , 2 , -1 , 2 , -1 , -11 , -1 , -11 ), based on the Standard Hydrogen Electrode (SHE), and the cation has an even number of aliphatic hydrocarbon groups having 3 or more carbon atoms that are identical to each other bonded to its central element and has a symmetric structure with respect to the central element, for use as an additive for a secondary battery.

[0014] In such an additive for a secondary battery, the cation can have a standard reduction potential of -3.7 V to -3.1 V, or -3.65 V to -3.15 V, or -3.6 V to -3.3 V, based on the Standard Hydrogen Electrode. Therefore, the cation is not substantially decomposed even during charging / discharging and operation of a secondary battery such as a lithium metal battery, and may form an anti-lithium protective layer on the lithium metal thin film. Further, a cation having such a low standard reduction potential can preferentially adsorb on the surface of a lithium chip of the lithium metal thin film over lithium ions and can form a selective protective layer on the lithium chip.

[0015] Further, in such an additive for a secondary battery, the cation utilizes the analysis results of PFG-NMR (Pulsed Field Gradient-NMR) and has a self-diffusivity calculated by the Stejskal-Tanner formula of 15*10 -11 m 2 .s -1 to 30*10 -11 m 2 .s -1 , or 18*10 -11 m 2 .s -1 to 28*10 -11 m 2 .s -1 , or 20*10 -11 m2 .s -1 or 25*10 -11 m 2 .s -1 It can exhibit the characteristic that, by utilizing the PFG-NMR analysis results, the hydrodynamic diameter calculated by the Stockes-Einstein equation is 1.5 to 3.0 nm, 1.8 to 2.8 nm, or 2.0 to 2.5 nm.

[0016] The aforementioned self-diffusivity and hydrodynamic diameter may reflect the fact that the additive's cations have a symmetric structure of multiple identical long-chain aliphatic hydrocarbon groups, such as long-chain linear alkyl groups, resulting in lower amphiphilicity and particularly lower interaction with non-aqueous organic solvents contained in the electrolyte, compared to additives with asymmetric cations. Therefore, the additive's cations have a low tendency to self-aggregate around lithium chips or protrusions and can uniformly adsorb or bind to the lithium chip to form a protective layer that selectively and uniformly surrounds the lithium chip. Thus, by using such an additive, the growth of needle-shaped lithium or lithium dendrites from the lithium chip can be more effectively suppressed.

[0017] To enable the additive to exhibit such effects, the plurality of long-chain aliphatic hydrocarbon groups may be alkyl groups having 3 or more carbon atoms, or 3 to 20 carbon atoms, or 4 to 15 carbon atoms, or 5 to 10 carbon atoms, or more preferably, linear alkyl groups having such numbers of carbon atoms, and an even number of such hydrocarbon groups may be bonded to the central element of the cation so that the cation has a symmetric structure.

[0018] To give a specific example, the ionic liquid compound of the additive may be represented by the following chemical formula 1, or more specifically, by the following chemical formula 2.

[0019] [Chemical formula 1] [ka] In the aforementioned chemical formula 1, [ka] R1 represents a heterocycle containing nitrogen with 3 to 8 carbon atoms, or 4 to 7 carbon atoms, or 5 to 6 carbon atoms, and R1 represents a linear alkyl group with 3 to 20 carbon atoms, or 4 to 15 carbon atoms, or 5 to 10 carbon atoms, A - This indicates an anion.

[0020] [Chemical formula 2] [ka]

[0021] In the aforementioned chemical formula 2, R1 represents a linear alkyl group having 3 to 20 carbon atoms, 4 to 15 carbon atoms, or 5 to 10 carbon atoms, and A - This indicates an anion.

[0022] On the other hand, the present invention also provides a lithium metal battery comprising the aforementioned additive. Such a lithium metal battery may comprise a negative electrode comprising a lithium metal thin film formed on a negative electrode current collector, an electrolyte comprising the aforementioned additive, and a positive electrode comprising a positive electrode active material layer formed on a positive electrode current collector.

[0023] Such a lithium metal battery is formed on the lithium metal thin film and may further include a protective layer containing cations of the additive, wherein the cations of the additive in the protective layer may be adsorbed or bound onto the needle-shaped lithium so as to selectively cover the needle-shaped lithium protruding from the lithium metal thin film. [Effects of the Invention]

[0024] The additive of the present invention is an ionic liquid compound whose cations have a low standard reduction potential, allowing it to adsorb better than lithium ions onto the surface of the lithium metal negative electrode and form a protective layer. In particular, the additive can selectively form a protective layer on the lithium chip formation area where the flow of lithium ions concentrates during charging / discharging.

[0025] Furthermore, the cations of the additive have long-chain aliphatic hydrocarbon groups that exhibit lithium-repulsive (lithiophobic) properties, thereby repelling the lithium ions to the periphery of the lithium chip. As a result, the growth of needle-shaped lithium or lithium dendrites from the lithium chip is suppressed, and lithium can be grown uniformly on the lithium metal anode.

[0026] Furthermore, the amphiphilicity of the additive, which is relaxed by the cation structure, reduces the phenomenon of self-aggregation among the cations. As a result, a uniform protective layer can be selectively formed around the lithium chip, minimizing the phenomenon of the protective layer not being properly formed in some areas around the lithium chip.

[0027] Therefore, when using the additive as an electrolyte additive to provide a secondary battery such as a lithium metal battery, the phenomenon of needle-shaped lithium or lithium dendrites growing on the lithium metal anode can be greatly reduced, and uniform lithium can be grown on the lithium metal anode during the charge / discharge process.

[0028] Therefore, such lithium metal batteries minimize safety issues such as cell short circuits or ignition due to the growth of needle-shaped lithium, and can exhibit significantly improved lifespan characteristics while maintaining the high capacity characteristics unique to lithium metal batteries over the long term. [Brief explanation of the drawing]

[0029] [Figure 1]Figure 1 schematically illustrates the technical principle by which needle-shaped lithium growth is suppressed and uniform lithium growth is achieved when using the additive (Pyr6(6)+) of one embodiment of the invention, comparing it with the case when the additive is not used and when using the comparative example additive (Pyr1(12)+). [Figure 2a] Figure 2a shows electron microscope images illustrating the growth of lithium by applying an electric charge to lithium metal thin films for Comparative Example 1 (no additives used), Comparative Example 2 (using Pyr1(12)FSI additive), and Example 1 (using Pyr6(6)FSI additive), as well as graphs showing the number of lithium chips (protuberances), average protuberance size, and standard deviation calculated for each thin film. [Figure 2b] Figure 2b shows electron microscope images of lithium growth on lithium metal thin films by applying an electric charge for Comparative Example 1 (no additives used), Comparative Example 2 (using Pyr1(12)FSI additive), and Example 1 (using Pyr6(6)FSI additive), as well as graphs showing the number of lithium chips (protuberances), average protuberance size, and standard deviation calculated for each thin film. [Figure 2c] Figure 2c shows electron microscope images illustrating the growth of lithium by applying charge to lithium metal thin films for Comparative Example 1 (no additives used), Comparative Example 2 (using Pyr1(12)FSI additive), and Example 1 (using Pyr6(6)FSI additive), as well as graphs showing the number of lithium chips (protuberances), average protuberance size, and standard deviation calculated for each thin film. [Modes for carrying out the invention]

[0030] In this specification, when a part "includes" a component, this means that, unless otherwise stated, it may further include other components rather than excluding them. Terms of degree used throughout this specification, such as “about,” “substantially,” etc., are used either numerically or in a sense close to numerically when manufacturing and material tolerances specific to the meaning referred to are presented, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that refer to precise or absolute numerical values ​​for the understanding of this application. Terms of degree used throughout this specification, such as “~(to) step” or “~ step,” do not mean “~ step for.”

[0031] In this specification, the term “these combinations” as used in a Markush expression means one or more mixtures or combinations selected from the group of components described in the Markush expression, and includes one or more selected from the group of components.

[0032] Based on the definitions and accompanying drawings described above, embodiments of the invention will be described in detail. However, these are presented as examples only and do not limit the invention; the invention is defined solely by the scope of the claims described below.

[0033] Figure 1 shows an example of an additive according to one embodiment of the invention (Pyr6(6) + The technical principle by which the growth of needle-shaped lithium is suppressed and more uniform lithium growth is achieved when the additive is used is schematically shown in comparison to the case when the additive is not used and when other additives are used.

[0034] According to one embodiment of the invention, an additive comprising an ionic liquid compound that exhibits a liquid state containing cations and anions at atmospheric pressure and a temperature of 100°C or less, wherein the cations are lithium cations (Li) relative to a standard hydrogen electrode (SHE). + The present invention provides an additive for secondary batteries that has a standard reduction potential lower than that of the given element, and in which the cation has an even number of aliphatic hydrocarbon groups with three or more carbon atoms bonded to its central element, thereby having a symmetrical structure with respect to the central element.

[0035] The additive of the above embodiment is an ionic liquid compound that exhibits a liquid state containing cations and anions at the driving or charge / discharge temperature of the secondary battery, for example, at atmospheric pressure (e.g., 1 atmosphere) and temperatures of 100°C or less, or 0 to 100°C, or 20 to 100°C, and can be effectively included as an additive in the electrolyte of a lithium metal battery, for example, in an electrolyte solution containing a lithium salt and a non-aqueous organic solvent.

[0036] Furthermore, the cation of the additive in one embodiment is a lithium cation (Li) relative to the Standard Hydrogen Electrode (SHE). + It can exhibit a standard reduction potential lower than the reduction potential of (approximately -3.04V). Therefore, as shown in Figure 1, when an electric field is concentrated around the lithium chip or protrusion during charging / discharging, the cations are preferentially reduced by such an electric field, and can bind to or adsorb onto the surface of the lithium metal negative electrode of the lithium chip forming portion. Long-chain aliphatic hydrocarbon groups are assembled by these cations, forming a selective protective layer surrounding the lithium chip. Furthermore, due to the low standard reduction potential, the cations are not decomposed during charging / discharging and operation of secondary batteries such as lithium metal batteries, and can form the protective layer.

[0037] Therefore, the additive selectively forms a protective layer on the lithium chip formation area where lithium ion flow is concentrated during charging / discharging of a lithium metal battery. The lithium-repulsive (lithiophobic) long-chain aliphatic hydrocarbon groups contained in the protective layer repel lithium ions to the area around the lithium chip, thereby suppressing the rapid growth of needle-shaped lithium or lithium dendrites from the lithium chip and enabling uniform lithium growth across the entire lithium metal thin film anode.

[0038] Furthermore, the cations of the additive exhibit a relaxed amphiphilicity because they have a structure in which identical long-chain aliphatic hydrocarbon groups are bonded to each other in a symmetrical structure, and can exhibit low interaction with non-aqueous organic solvents contained in the electrolyte. Therefore, unlike when an additive with an asymmetric structure of cations is used (see the left diagram in the lower row of Figure 1), the phenomenon of self-aggregation among the additive cations is reduced (see the right diagram in the lower row of Figure 1), and as a result, a uniform protective layer can be formed that covers the entire lithium chip formation area. In contrast, when an additive containing the asymmetric structure of cations is used, the protective layer may not be properly formed in some areas around the lithium chip due to self-aggregation among the cations, and the growth of needle-shaped lithium and other structures may still occur in those areas.

[0039] Therefore, when providing a secondary battery such as a lithium metal battery using the additive of the above embodiment as the electrolyte, the phenomenon of needle-shaped lithium or lithium dendrites growing on the lithium metal anode can be minimized, and uniform lithium can be grown on the lithium metal anode during the charge / discharge process.

[0040] Therefore, a lithium metal battery using the additive of one embodiment can minimize problems such as cell short circuits or ignition due to the growth of needle-shaped lithium or lithium dendrites, and can maintain the high capacity characteristics unique to lithium metal batteries over the long term, exhibiting improved lifespan characteristics.

[0041] On the other hand, in the additive of the above embodiment, the cation can have a standard reduction potential of -3.7V to -3.1V, -3.65V to -3.15V, or -3.6V to -3.3V relative to a standard hydrogen electrode. Such a standard reduction potential may be calculated based on the standard hydrogen electrode's reduction potential of 0V. By using an ionic liquid compound additive containing such a standard reduction potential, the additive cation can preferentially bind to the lithium chip forming area over lithium ions during charging / discharging of the lithium metal battery, forming a selective protective layer, thereby more effectively suppressing the growth of needle-shaped lithium and the like. Furthermore, the standard reduction potential can further suppress the decomposition of the cation during the battery's operation.

[0042] Furthermore, in the additive of the above embodiment, the cation's self-diffusivity, calculated using the Stejskal-Tanner equation in Equation 1 below, based on the results of PFG-NMR (Pulsed Field Gradient-NMR) analysis, is 15*10 -11 m 2 .s -1 or 30*10 -11 m 2 .s -1 , or 18*10 -11 m 2 .s -1 or 28*10 -11 m 2 .s -1 , or 20*10 -11 m 2 .s -1 or 25*10 -11 m 2 .s -1 It can exhibit the following characteristics, and by utilizing the PFG-NMR analysis results, it can be shown that the hydrodynamic diameter at an absolute temperature of 298 K, calculated by the Stockes-Einstein equation in Equation 2 below, is 1.5 to 3.0 nm, or 1.8 to 2.8 nm, or 2.0 to 2.5 nm.

[0043] [Formula 1]

number

number

[0044] In equations 1 and 2 above, E represents the signal attenuation ratio, γ represents the gyromagnetic ratio, g represents the gradient strength, δ (ms) represents the pulse duration, Δ (ms) represents the gradient pulse interval, D represents the self-diffusivity, d represents the hydrodynamic diameter, and k represents the hydrodynamic diameter. B θ represents the Boltzmann constant, T represents the absolute temperature (e.g., the measurement temperature of 298 K), and η represents the viscosity of the solvent.

[0045] Such self-diffusivity and hydrodynamic diameter are properties that can be calculated using equations 1 and 2 from the parameters derived from the analysis of the cations of the ionic liquid compound by PFG-NMR. These properties can reflect the interaction, diffusivity, or self-aggregation of the cations with non-aqueous organic solvents.

[0046] Specifically, the additive cations of the above embodiment have a symmetrical structure in which identical long-chain aliphatic hydrocarbon groups, for example, identical long-chain linear alkyl groups, are bonded to each other. This allows them to have a larger degree of self-diffusivity and a smaller hydrodynamic diameter compared to additive cations with an asymmetric structure. This indicates that the additive cations of the above embodiment have relatively low amphiphilicity, for example, low interaction with non-aqueous organic solvents contained in electrolytes.

[0047] Therefore, as shown in Figure 1, an additive of one embodiment having such a symmetrical cation has a low tendency to self-aggregate around the lithium chip or protrusion, and can be uniformly adsorbed or bound onto the lithium chip, forming a protective layer that selectively and uniformly surrounds it. Thus, by using such an additive, the growth of needle-shaped lithium or lithium dendrites from the lithium chip can be suppressed even more effectively.

[0048] To effectively exhibit the aforementioned properties, the long-chain aliphatic hydrocarbon group bonded to the cation may be an alkyl group having 3 or more carbon atoms, or 3 to 20 carbon atoms, or 4 to 15 carbon atoms, or 5 to 10 carbon atoms, or more preferably a linear alkyl group having such a number of carbon atoms. An even number of such hydrocarbon groups can be bonded to the central element of the cation, allowing the cation to have a symmetric structure. Therefore, the growth of needle-shaped lithium and the like from the lithium chip can be suppressed even more effectively.

[0049] On the other hand, to give a specific example, the ionic liquid compound of the additive may be represented by the following chemical formula 1, or more specifically, by the following chemical formula 2.

[0050] [Chemical formula 1] [ka] In the aforementioned chemical formula 1, [ka] R1 represents a heterocycle containing nitrogen with 3 to 8 carbon atoms, or 4 to 7 carbon atoms, or 5 to 6 carbon atoms, and R1 represents a linear alkyl group with 3 to 20 carbon atoms, or 4 to 15 carbon atoms, or 5 to 10 carbon atoms, A - This indicates an anion.

[0051] [Chemical formula 2] [ka] In the aforementioned chemical formula 2, R1 represents a linear alkyl group having 3 to 20 carbon atoms, 4 to 15 carbon atoms, or 5 to 10 carbon atoms, and A - This indicates an anion.

[0052] A specific example of the additive represented by the aforementioned chemical formula 1 or 2 is the 1,1-dihexylpyrrolidium (Pyr6(6)) cation. + ) or 1,1-dipropylpyrrolidium cations, etc., are examples of anion A described later. - Examples include ionic liquid compounds bonded with [a certain compound].

[0053] As can be seen in the following examples, such compounds, by containing a long-chain linear alkyl group symmetrically bonded to a pyrrolidium cation, can form a highly uniform and selective protective layer on the lithium chip formation portion of a lithium metal thin film, effectively suppressing the growth of needle-shaped lithium or lithium dendrites and enabling uniform lithium growth from the lithium metal anode. Among such compounds, the Pyr6(6) compound is used to more effectively suppress the growth of needle-shaped lithium and the like. + As shown above, ionic liquid compounds containing pyrrolidium cations in which linear alkyl groups having 4 or more carbon atoms, or 5 or more carbon atoms, or 5 to 15 carbon atoms are symmetrically bonded can preferably be used.

[0054] On the other hand, the additive compounds of chemical formula 2, etc., can be produced by alkylating pyrrolidine with R1-X (where R1 is as defined in chemical formula 2 and X is a halogen) in the presence of a base such as potassium carbonate to form the compound of chemical formula 2A shown below, and then reacting such a compound of chemical formula 2A with LiA (where A is as defined in chemical formula 2), etc.

[0055] [Chemical formula 2A] [ka]

[0056] In the aforementioned chemical formula 2A, R1 is as defined in chemical formula 2, and X - This indicates a halogen anion.

[0057] In the additive of the aforementioned embodiment, the anion bonded to the cation is not particularly limited as long as the additive can become an ionic liquid compound at atmospheric pressure and below 100°C and can be included as an additive in the electrolyte of a lithium metal battery. However, from the viewpoint of forming a more stable solid electrolyte interface (SEI) film on the lithium metal thin film making up the negative electrode of a lithium metal battery, it is preferable that the anion consists of a fluorine-containing anion, more specifically, a bis(fluorosulfonyl)imide (FSI) anion, a bis((trifluoromethyl)sulfonyl)imide (TFSI) anion, a hexafluorophosphate (PF6) anion, or a difluoro(oxalato)borate (DFOB) anion.

[0058] When an ionic liquid compound having such anions is used as an electrolyte additive, it becomes possible to form a more stable SEI film on the anode through the reaction between the electrolyte and the lithium metal anode, such as an SEI film containing lithium fluoride (LiF) or lithium nitride (Li3N), while maintaining the excellent ionic conductivity of the lithium metal battery. This further effectively suppresses the growth of needle-shaped lithium or lithium dendrites.

[0059] The additive of the aforementioned embodiment is preferably used as an additive to the electrolyte of a lithium metal battery, for example, an electrolyte solution containing a lithium salt and a non-aqueous organic solvent, and can effectively suppress the growth of needle-shaped lithium or lithium dendrites at the negative electrode of the lithium metal battery, thereby making lithium growth uniform at the negative electrode and greatly improving the safety and lifespan characteristics of the lithium metal battery.

[0060] On the other hand, the category of lithium metal batteries that can use the additive of one embodiment may include not only general lithium metal batteries manufactured by forming a lithium metal thin film on the negative electrode current collector, but also so-called lithium-free batteries that are manufactured without forming a separate lithium metal thin film or negative electrode active material layer on the negative electrode current collector.

[0061] In such lithium-free batteries, the lithium metal that grows on the negative electrode current collector during the charge / discharge process can act as the negative electrode active material. In such lithium-free batteries as well, the additive can form a protective layer on the lithium metal acting as the negative electrode active material, effectively suppressing the uneven growth of needle-shaped lithium or lithium dendrites.

[0062] On the other hand, according to another embodiment of the invention, a lithium metal battery containing the additive is provided. Such a lithium metal battery may include a negative electrode comprising a lithium metal thin film formed on a negative electrode current collector, an electrolyte containing the additive, and a positive electrode comprising a positive electrode active material layer formed on a positive electrode current collector.

[0063] Such a lithium metal battery can further form a protective layer containing the cations of the additive on the lithium metal thin film as the charge / discharge process progresses. In particular, such a protective layer can be formed on the lithium metal thin film so as to selectively cover the lithium chip or protrusion (needle-shaped lithium growth portion) where the electric field and the flow of lithium ions are concentrated during charge / discharge. Within such a protective layer, the cations of the additive can be uniformly adsorbed or bound to the lithium metal thin film of the needle-shaped lithium growth portion (lithium chip formation portion) without self-aggregation.

[0064] In addition, the material may further include an SEI film formed on the lithium metal thin film by the reaction of the anions of the additive with the lithium metal, and such an SEI film may contain lithium fluoride (LiF) and / or lithium nitride (Li3N) as its main components, with the lithium ions bonded to fluorine and / or nitrogen derived from the anions of the additive.

[0065] Such SEI coatings and protective layers can suppress the excessive growth of needle-shaped lithium and other elements from the lithium metal thin film, and the resulting ignition or cell short circuit, thereby enabling lithium metal batteries of other embodiments to exhibit improved safety and lifespan characteristics.

[0066] On the other hand, the lithium metal battery of the other embodiment described above can follow the configuration of a general lithium metal battery, except for the use of the aforementioned electrolyte additive. Additional configurations of such lithium metal batteries will be described below.

[0067] The lithium metal battery of the other embodiment described above includes a lithium metal thin film formed on a negative electrode current collector as the negative electrode. In this case, the negative electrode current collector may be any metal current collector, and is typically a metal current collector such as copper or aluminum.

[0068] Such metal current collectors may generally be formed to a thickness of 3 to 500 μm. Furthermore, the lithium metal thin film formed on such a metal current collector may be formed to a thickness of 1 to 100 μm, 5 to 80 μm, or 10 to 60 μm, according to the typical configuration of a lithium metal battery. The lithium metal thin film may also be formed on the metal current collector by methods widely known in the industry, such as deposition, electroplating, or rolling.

[0069] On the other hand, the electrolyte of the lithium metal battery may be an electrolyte (liquid electrolyte) containing a non-aqueous organic solvent and a lithium salt.

[0070] At this time, the non-aqueous organic solvent acts as a medium that allows ions involved in the electrochemical reaction of the battery to move while dissolving the lithium salt and additives.

[0071] The type of non-aqueous organic solvent is not particularly limited, and ether-based, carbonate-based, ester-based, ketone-based, alcohol-based, or aprotic solvent may be used. As the carbonate-based solvent, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. may be used, and as the ester-based solvent, methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, etc. may be used. As the ether-based solvent, dimethyl ether, 1,2-dimethoxyethane, dibutyl ether, tetraglyme, diglyme, 2-methyltetrahydrofuran, tetrahydrofuran, etc. may be used, and as the ketone-based solvent, cyclohexanone, etc. may be used. As the alcohol-based solvent, ethyl alcohol, isopropyl alcohol, etc. may be used, and as the aprotic solvent, tolyls such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group of C2 to C20, and may include a double-bond oriented ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, etc. Among these, ether-based solvents or carbonate-based solvents can be appropriately used from the viewpoint of improving the lifespan characteristics of lithium metal batteries.

[0072] Furthermore, the non-aqueous organic solvents may be used individually or in combination of one or more. When used in combination of one or more, the mixing ratio can be appropriately adjusted according to the desired battery performance, which should be widely understood by those engaged in this field.

[0073] Furthermore, in the case of the carbonate-based solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate. In this case, mixing the cyclic carbonate and chain carbonate in a volume ratio of approximately 1:1 to approximately 1:9 can produce an electrolyte with excellent performance.

[0074] The non-aqueous organic solvent may further contain the aromatic hydrocarbon organic solvent in addition to the carbonate-based solvent. In this case, the carbonate-based solvent and the aromatic hydrocarbon organic solvent may be mixed in a volume ratio of about 1:1 to about 30:1.

[0075] The aforementioned aromatic hydrocarbon organic solvents include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, and tol You may use xylene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, xylene, or combinations thereof.

[0076] The non-aqueous organic solvent may further contain vinylene carbonate or an ethylene carbonate compound to improve battery life.

[0077] Typical examples of the ethylene carbonate compounds include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, and fluoroethylene carbonate. When vinylene carbonate or the ethylene carbonate compounds are used in addition, the lifespan can be improved by appropriately adjusting the amount used.

[0078] In the electrolyte of the lithium metal battery, the lithium salt dissolves in the organic solvent and acts as a source of lithium ions, enabling the basic operation of the lithium metal battery of the other embodiment and playing a role in promoting the movement of lithium ions between the positive and negative electrodes.

[0079] The lithium salt may be any lithium salt that is generally widely used as an electrolyte. For example, lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis((trifluoromethyl)sulfonyl)imide (LiTFSI) may be used, as well as LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C) x F 2x+1 SO2)(C y F 2y+1 SO2) (where x and y are natural numbers), LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalato)borate); LiBOB), or combinations thereof may be used.

[0080] Furthermore, the concentration of the lithium salt in the electrolyte can be controlled within a range of 0.1 to 5.0 M. Within this range, the electrolyte can have appropriate conductivity and viscosity, and lithium ions can move effectively within the lithium metal battery. However, this is merely illustrative and does not limit the invention.

[0081] The electrolyte may be impregnated in a porous separation membrane located between the negative electrode and the positive electrode. Here, the porous separation membrane separates the negative electrode and the positive electrode, providing a passage for lithium ions to move, and any type commonly used in lithium secondary batteries can be used. In other words, an electrolyte with low resistance to ion movement and excellent moisture absorption capacity can be used.

[0082] For example, the material may be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a nonwoven or woven fabric. For example, polyolefin polymer separation membranes such as polyethylene and polypropylene are mainly used, and coated separation membranes containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used for single-layer or multilayer structures.

[0083] Furthermore, the lithium metal battery further includes a positive electrode comprising a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0084] In this case, the positive electrode active material layer may be manufactured by mixing the positive electrode active material and a binder, and optionally a conductive material and filler, in a solvent to produce a slurry-like positive electrode mixture, and then applying this positive electrode mixture to the positive electrode current collector. Since such a positive electrode manufacturing method is widely known in the art, a detailed explanation is omitted in this specification.

[0085] In the case of the positive electrode active material, there are no particular limitations as long as it is a material that allows for the reversible insertion and removal of lithium ions. For example, it may contain one or more composite oxides of a metal such as cobalt, manganese, nickel, iron, aluminum, or a combination of one or more of these, and lithium.

[0086] To give a more specific example, the positive electrode active material may be a compound represented by any one of the following chemical formulas: Lia A 1-b R b D2 (in the above formula, 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5); Li a E 1-b R b O 2-c D c (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE 2-b R b O 4-c D c (in the above formula, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li a Ni 1-b-c Co b R c D α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 1-b-c 2); Li a Ni 1-b-c Co b R c O 2-α Z α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 1-b-c 2); Li a Ni 1-b-c Co b R c O 2-α Z2 (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 1-b-c 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 1-b-c 2); Li a Ni 1-b-c Mn b R c D α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 1-b-c 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 1-b-c 2); Li a Ni 1-b-c Mn b R c O 2-α Z α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 1-b-c 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 1-b-c 2); Li a Ni 1-b-c Mn b R c O 2-αZ2 (where 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 2); Li a Ni b E c G d O2 (wherein the above formula, 0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5 and 0.001≦d≦0.1); Li a Ni b Co c Mn d G e O2 (wherein the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5 and 0 ≤ e ≤ 0.1); Li a NiG b O2 (In the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (In the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (In the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (In the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiTO2; LiNiVO4; Li (3-f) J2(PO4)3(0≦f≦2);Li (3-f) Fe2(PO4)3(0≦f≦2); LiFePO4.

[0087] In the above chemical formula, A is Ni, Co, Mn or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; Z is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu or a combination thereof.

[0088] Among these diverse active materials, cathode active materials that exhibit high capacity characteristics and require a high level of safety include, for example, Li with a high Ni content. a Ni b’ Co c Mn d G e The positive electrode active material, such as O2 (wherein the above formula, 0.90 ≤ a ≤ 1.8, 0.4 ≤ b' ≤ 0.95, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, and 0 ≤ e ≤ 0.1, and G is Al) or LiFePO4, can be preferably applied to lithium metal batteries of different embodiments.

[0089] As the positive electrode active material mentioned above, compounds having a coating layer on their surface may be used, or a mixture of the aforementioned compounds and compounds having a coating layer may be used. The coating layer is a coating element compound and may contain oxides, hydroxides, oxyhydroxys, oxycarbonates, or hydroxycarbonates of the coating element. These compounds forming the coating layer may be amorphous or crystalline. The coating elements included in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The coating layer formation step may use any coating method (e.g., spray coating, immersion method, etc.) that does not adversely affect the physical properties of the positive electrode active material using such elements in the compound, and since this is something that will be well understood by those engaged in this field, a detailed explanation will be omitted.

[0090] On the other hand, the positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The current collector can have fine irregularities formed on its surface to increase the adhesion strength of the positive electrode active material, and can take various forms such as film, sheet, foil, net, porous body, foam, and nonwoven fabric.

[0091] The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and may be used for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

[0092] The lithium metal battery of the above embodiment may be used not only as a unit cell used as a power source for small devices, but also as a unit battery in a medium-to-large battery module containing a large number of battery cells. Furthermore, a battery pack including the battery module may be constructed.

[0093] The following describes preferred embodiments of the invention, comparative examples, and experimental examples for evaluating them. However, the following embodiments are merely preferred embodiments of the invention, and the present invention is not limited to these embodiments.

[0094] Example 1: Electrolyte additive (1,1-dihexylpyrrolidium bis(fluorosulfonyl)imide, Pyr6(6) + FSI - ) synthesis 50 mL of acetonitrile, pyrrolidine (3.56 g, 50 mmol), and potassium carbonate (7.60 g, 55 mmol) base were added to a round-bottom flask. 1-bromohexane (24.76 g, 150 mmol) was slowly added to this mixture, and the mixture was reacted at 65°C for 8 hours with stirring. The solution was then filtered, and the solvent was removed from the filtrate under reduced pressure. The residue was washed with hexane to obtain 1,1-dihexylpyrrolidium bromide [Pyr6(6)Br] in a yellow solid state.

[0095] After drying the Pyr6(6)Br under vacuum at 80°C for 10 hours, the same moles of Pyr6(6)Br and LiFSI were dissolved in 20 mL of acetonitrile and stirred at room temperature for 5 hours. Next, the solution was evaporated under reduced pressure, and 10 mL of dichloromethane was added to the concentrated product. The mixture was then filtered and extracted with water, and excess salt was removed from the filtrate. The lower organic phase was collected and rotated to remove the solvent. This sample was left under vacuum at 80°C for 10 hours to allow the Pyr6(6) to mature. + FSI - The additive was finally produced as a birch-colored liquid.

[0096] Comparative Example 1: In the following explanation, the case without the use of electrolyte additives is referred to as Comparative Example 1.

[0097] Comparative Example 2: Electrolyte additive (1-dodecyl-1-methylpyrrolidium bis(fluorosulfonyl)imide, Pyr1(12) + FSI - ) synthesis 50 mL of acetonitrile and 1-methylpyrrolidine (4.26 g, 50 mmol) were added to a round-bottom flask. 1-bromododecane (14.95 g, 60 mmol) was slowly added to this mixture, and the mixture was reacted at 65°C for 8 hours with stirring. Next, the solution was filtered, and the solvent was removed from the filtrate under reduced pressure. The residue was washed with hexane to obtain 1-dodecyl-1-methylpyrrolidium bromide [Pyr1(12)Br] in a yellow solid state.

[0098] After drying the Pyr1(12)Br under vacuum at 80°C for 10 hours, the same moles of Pyr1(12)Br and LiFSI were dissolved in 20 mL of acetonitrile and stirred at room temperature for 5 hours. Next, the solution was evaporated under reduced pressure, and 10 mL of dichloromethane was added to the concentrated product. The mixture was then filtered and extracted with water, and excess salt was removed from the filtrate. The lower organic phase was collected and rotated to remove the solvent. This sample was left under vacuum at 80°C for 10 hours to allow the Pyr1(12) to mature. + FSI - The additive was finally manufactured into a clear liquid.

[0099] Experimental Example 1: Measurement of the self-diffusion rate and hydrodynamic diameter of an additive Using a liquid (400MHz) NMR analyzer (Bruker), the additives of Example 1 and Comparative Example 2 were analyzed. 1 H - , 7 Li - , and 19 F-NMR analysis was performed. Based on these analysis results, the self-diffusivity of the cations of the additives was calculated by applying the Stejskal-Tanner equation shown in Equation 1 below.

[0100] Furthermore, based on the NMR analysis results and the self-diffusivity calculation results, the hydrodynamic diameters of the additive cations at an absolute temperature of 298 K were calculated by applying the Stockes-Einstein equation shown in Equation 2 below.

[0101] [Formula 1]

number

number

[0102] In equations 1 and 2 above, E represents the signal attenuation ratio, γ represents the gyromagnetic ratio, g represents the gradient strength, δ (ms) represents the pulse duration, Δ (ms) represents the gradient pulse interval, D represents the self-diffusivity, d represents the hydrodynamic diameter, and k represents the hydrodynamic diameter. B θ represents the Boltzmann constant, T represents the absolute temperature (=298K), and η represents the viscosity of the solvent.

[0103] For your reference, based on the NMR analysis results mentioned above, 1 For H-NMR, δ(ms) = 3.6 and Δ(ms) = 80-150. 7 For Li-NMR, δ(ms) = 4.0 and Δ(ms) = 300-400. 19 For F-NMR, the values ​​were δ(ms) = 2.5-3.2 and Δ(ms) = 300-500.

[0104] The physical properties analyzed and calculated using the method described above are shown in Table 1 below.

[0105] [Table 1]

[0106] Referring to Table 1 above, it was confirmed that although the additives in Example 1 and Comparative Example 2 contain cations with similar molecular sizes, the additive cation in Example 1 exhibits a higher degree of self-diffusion and a hydrodynamic diameter approximately 1.6 times lower than that in Comparative Example 2.

[0107] Based on this, it was confirmed that the additive cation of Example 1 exhibits relaxed amphiphilicity and a low tendency to self-aggregate, making it advantageous for forming a uniform protective layer surrounding the lithium chip.

[0108] Experimental Example 2: Evaluation of Surface Properties of a Lithium Metal Thin Film After Lithium Metal Deposition An electrolyte was prepared by dissolving the lithium salt of lithium bis(trifluoromethyl)sulfonyl)imide (LiTFSI) at a concentration of 1 M in a mixed solvent of 1,3-dioxolane / 1,2-dimethoxyethane (DOL / DME-1:1 (v / v%)). This additive-free electrolyte was used as the electrolyte for Comparative Example 1.

[0109] Furthermore, the electrolytes of Example 1 and Comparative Example 2 were prepared by dissolving the additive of Example 1 or the additive of Comparative Example 2 in the electrolyte at a concentration of 1 M, respectively.

[0110] Using the electrolytes of Example 1, Comparative Example 1, and Comparative Example 2, respectively, 0.1 mAh cm⁻¹ -2 Under an applied current, electroplating was performed to deposit a thin film of lithium metal onto each copper current collector (thickness: 18 μm).

[0111] After such deposition, the surface condition of each lithium metal thin film was examined using an electron microscope for Comparative Example 1 (no additive used; Figure 2a), Comparative Example 2 (using Pyr1(12)FSI additive; Figure 2b), and Example 1 (using Pyr6(6)FSI additive; Figure 2c), and is shown in Figures 2a to 2c, respectively.

[0112] Furthermore, from the surface analysis results of these lithium metal thin films, the number and size of lithium chips (protrusions) having a size of approximately 0.25 μm or larger were confirmed. From these confirmation results, the average protrusion size and standard deviation of the lithium chips were calculated and shown as graphs in the lower part of Figures 2a to 2c.

[0113] Referring to Figures 2a to 2c, it was confirmed that when the additive of Example 1 was used, the number and size of protrusions on the lithium metal thin film decreased, and a lithium metal thin film with a uniform surface over the entire area of ​​the current collector was formed.

[0114] This is presumably because the additive in Example 1, as shown in Figure 1, induced uniform deposition and growth of lithium by forming a uniform protective layer surrounding the protrusions during lithium deposition.

[0115] Experimental Example 3: Evaluation of Life Cycle Characteristics of Lithium Metal Batteries LiRing 0.6 Co 0.2 Mn 0.2 O2 (NCM622) was used as the positive electrode active material. This positive electrode active material, carbon black conductive material (super-P), and PVdF binder (Mw=455000; Sigma-Aldrich) were mixed in an N-methylpyrrolidone solvent in a weight ratio of 90:5:5 to produce a positive electrode mixture. This mixture was then applied to one surface of an aluminum current collector (thickness: 20 μm), and the positive electrode was produced by drying and rolling it in a vacuum oven at 110°C.

[0116] For the negative electrode, a lithium metal thin film (thickness: 40 μm) was formed on one surface of a copper current collector (thickness: 18 μm).

[0117] An electrode assembly was manufactured by interposing a porous polypropylene separation membrane (Celguard 2325; thickness: 25 μm) between the positive and negative electrodes manufactured as described above. After positioning the electrode assembly inside a case, an electrolyte solution was injected into the case to manufacture a lithium metal battery.

[0118] The electrolyte was prepared by dissolving the lithium salt of lithium bis(trifluoromethyl)sulfonyl)imide (LiTFSI) at a concentration of 1 M in a mixed solvent of 1,3-dioxolane / 1,2-dimethoxyethane (DOL / DME-1:1 (v / v%)). Furthermore, the additive of Example 1 or the additive of Comparative Example 2 was added to this electrolyte at a concentration of 50 mM, and lithium metal batteries of Example 1 and Comparative Example 2 were prepared depending on the type of additive in the electrolyte.

[0119] For each of these lithium metal batteries, the capacity retention rate and Coulomb efficiency were measured after 250 charge-discharge cycles in CCCV mode at 25°C, where the battery was charged at 0.5C until it reached 4.2V, and then discharged at a constant current of 0.5C until it reached 3.0V. The results of these capacity retention rate and Coulomb efficiency measurements are summarized in Table 2 below.

[0120] [Table 2]

[0121] Referring to Table 2 above, it was confirmed that the lithium metal battery using the symmetrical cation-containing additive of Example 1 exhibited high capacity retention and Coulomb efficiency even after 250 charge / discharge cycles, demonstrating excellent life characteristics.

[0122] However, the lithium metal battery using the asymmetric cation-containing additive in Comparative Example 2 was found to exhibit inferior capacity retention and lifespan characteristics compared to the example. This is presumably because, when using the additive in Comparative Example 2, the growth of needle-shaped lithium or lithium dendrites was not sufficiently suppressed, resulting in non-uniform growth of the lithium thin film during charging / discharging.

Claims

1. An additive comprising an ionic liquid compound that exhibits a liquid state containing cations and anions at atmospheric pressure and temperatures below 100°C, The aforementioned cation is a lithium cation (Li) relative to a standard hydrogen electrode (SHE). ＋ ) has a lower standard reduction potential, The aforementioned cation has a symmetrical structure with respect to the central element, in which an even number of aliphatic hydrocarbon groups with three or more carbon atoms each are bonded to the central element. The aforementioned ionic liquid compound is an additive for secondary batteries, represented by the following chemical formula 2: [Chemical formula 2] 【Chemistry 1】 In the aforementioned chemical formula 2, R1 represents a linear alkyl group having 6 to 20 carbon atoms, and A- represents an anion.

2. The additive for secondary batteries according to claim 1, wherein the cation has a standard reduction potential of -3.7V to -3.1V with respect to a standard hydrogen electrode.

3. The aforementioned cation has a self-diffusivity calculated by the Stejskal-Tanner equation of 15 * 10 －１１ I understand ２ . s －１ or 30*10 －１１ I understand ２ . s －１ The additive for secondary batteries according to claim 1.

4. The additive for secondary batteries according to claim 1, wherein the cation has a hydrodynamic diameter of 1.5 to 3.0 nm, calculated by the Stockes-Einstein equation at an absolute temperature of 298 K.

5. The additive for secondary batteries according to claim 1, wherein the anion is selected from the group consisting of fluorine-containing anions.

6. The aforementioned anions are bis(fluorosulfonyl)imide (FSI) anions, bis((trifluoromethyl)sulfonyl)imide (TFSI) anions, and hexafluorophosphate (PF). ６ The additive for secondary batteries according to claim 1, comprising an anion or a difluoro(oxalate) borate (DFOB) anion.

7. An additive for secondary batteries according to claim 1, used in the electrolyte of a lithium metal battery.

8. A negative electrode including a lithium metal thin film formed on a negative electrode current collector, An electrolyte comprising the additive described in any one of claims 1 to 6, A lithium metal battery comprising a positive electrode including a positive electrode active material layer formed on a positive electrode current collector.

9. The lithium metal battery according to claim 8, further comprising a protective layer formed on the lithium metal thin film and containing the cations of the additive.

10. The lithium metal battery according to claim 9, wherein the cations of the additive selectively cover needle-shaped lithium protruding from the lithium metal thin film.

11. The lithium metal battery according to claim 8, further comprising a solid electrolyte interface (SEI) film formed on the lithium metal thin film.

12. The solid electrolyte interface film contains lithium fluoride (LiF) or lithium nitride (Li ３ N), and the lithium metal battery according to claim 11.

13. The lithium metal battery according to claim 8, wherein the positive electrode active material layer comprises a metal, such as cobalt, manganese, nickel, iron, aluminum, or a combination of one or more of these, and a lithium composite oxide.

14. The electrolytes are lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis((trifluoromethyl)sulfonyl)imide (LiTFSI), and LiPF ６ LiBF ４ LiSbF ６ LiAsF ６ LiC ４ F ９ SO ３ LiClO ４ LiAlO ２ LiAlCl ４ ,LiN(C ｘ F ２ｘ＋１ SO ２ ) (C ｙ F ２ｙ＋１ SO ２ (x and y are natural numbers, independently of each other), LiCl, LiI and LiB (C ２ O ４ ) ２ One or more lithium salts selected from the group consisting of, A lithium metal battery according to claim 10, further comprising a non-aqueous organic solvent.

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