Lithium metal battery comprising electrolyte additive

Abstract

The present disclosure relates to a lithium metal battery including: an anode; a cathode spaced apart from and facing the anode; an electrolyte positioned between the anode and the cathode and including a lithium salt and an additive; and a solid-electrolyte interface (SEI) layer positioned on a surface where the anode and the electrolyte are in contact with each other, wherein the additive contains a fluorine-containing magnesium salt, and the SEI layer includes magnesium having an oxidation number of 0.

Description

Lithium metal battery containing electrolyte additives

[0001] The present disclosure relates to a lithium metal battery comprising an electrolyte additive.

[0002] Lithium (Li) metal has a high theoretical capacity of approximately 3860 mAh / g and a large reduction potential of -3.04 V (vs RHE), which allows it to achieve high energy density when used as an anode in lithium-ion batteries, and is attracting attention as a next-generation anode material. However, lithium metal batteries using lithium metal as an anode have a problem in that lithium metal in a dendrite structure forms on the surface of the anode during repeated charge and discharge cycles. As lithium dendrites grow on the surface of the anode, they impede the movement of lithium ions, forming dead lithium, which continuously consumes the electrolyte and severely reduces battery performance and lifespan. Furthermore, if lithium dendrites come into contact with the separator, a battery short circuit may occur, leading to an explosion or fire, threatening battery stability.

[0003] Furthermore, from a practical perspective, lithium metal batteries with a cathode / anode capacity ratio (N / P) of less than 3 and an electrolyte weight / anode capacity ratio (E / C) of less than 3.0 g / Ah are required to surpass currently commercialized lithium-ion batteries. Reducing the volume of the cathode is also a critical challenge to satisfy these requirements.

[0004] To suppress lithium dendrite formation, research is underway on forming a protective layer by coating the surface of the lithium anode with inorganic or polymeric materials, or introducing a solid electrolyte interface (SEI) layer on the surface of the anode. However, while increasing the thickness of the protective layer and the solid electrolyte interface may suppress lithium dendrite formation, it also increases the resistance of the lithium metal battery, resulting in a decline in rate characteristics.

[0005] Meanwhile, lithium metal batteries containing various electrolyte additives to form a solid electrolyte interfacial layer are being developed. For example, metal salts containing metal ions such as sodium ions, potassium ions, or cesium ions have been attempted as electrolyte additives. However, when forming a solid electrolyte interfacial layer on the negative electrode surface using electrolyte additives containing the metal salts, the metal ions exhibit higher activity than lithium, drastically reducing battery safety. Forming a solid electrolyte interfacial layer as an electrostatic shielding mechanism can accelerate electrolyte consumption during battery operation, reducing Coulombic efficiency.

[0006] Accordingly, there is a need to develop a lithium metal battery that can suppress the growth of lithium dendrites by stably forming a solid electrolyte interfacial layer without lowering the energy density of the battery.

[0007] The purpose of the present disclosure is to solve the problems of the above-mentioned prior art, and to provide a lithium metal battery capable of improving battery safety by suppressing the formation of lithium dendrites on the surface of an anode.

[0008] Another object of the present disclosure is to provide a lithium metal battery having high capacity retention and coulombic efficiency for a long period of time.

[0009] Another object of the present disclosure is to provide a lithium metal battery capable of minimizing energy density degradation while improving battery stability.

[0010] The lithium metal battery of the present disclosure comprises: a negative electrode; a positive electrode spaced apart from and facing the negative electrode; an electrolyte positioned between the negative electrode and the positive electrode, the electrolyte including a lithium salt and an additive; and a solid-electrolyte interface (SEI) layer positioned on a surface where the negative electrode and the electrolyte come into contact; wherein the additive includes a fluorine-containing magnesium salt, and the solid-electrolyte interface (SEI) layer includes magnesium having an oxidation state of 0.

[0011] In one example, the fluorine-containing magnesium salt may include magnesium trifluoromethanesulfonate (MTFMS), magnesium bis(trifluoromethanesulfonimide), Mg(TFSI)2, or a combination thereof.

[0012] In one example, the electrolyte may contain 0.5 to 5 parts by weight of the additive based on 100 parts by weight of the lithium salt.

[0013] In one example, the solid-electrolyte interface (SEI) layer may further include lithium fluoride (LiF).

[0014] In one example, the lithium salt may contain fluorine.

[0015] In one example, the additive may further contain a metal salt of a different type than the fluorine-containing magnesium salt.

[0016] In one example, the metal salt may include a nitrogen-containing metal salt, a fluorine-containing metal salt, or a combination thereof.

[0017] In one example, the nitrogen-containing metal salt may include at least one selected from lithium nitrate (LiNO3), magnesium nitrate (Mg(NO3)2), silver nitrate (AgNO3), copper nitrate (CuNO3), aluminum nitrate (Al(NO3)3), and sodium nitrate (NaNO3).

[0018] In one example, the fluorine-containing metal salt may include at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSi), silver trifluoromethanesulfonate (AgTFMS), copper trifluoromethanesulfonate (CTFMS), aluminum trifluoromethanesulfonate (ATFMS), and sodium trifluoromethanesulfonate (NTFMS).

[0019] In one example, the thickness of the lithium metal layer deposited between the cathode and the solid-electrolyte interface (SEI) layer may be 5 to 50 μm.

[0020] In one example, the ratio (I1 / I0) of the peak intensity (I1) for the Li-F bond in the F 1s XPS spectrum of the solid-electrolyte interface (SEI) layer and the peak intensity (I0) for the Li-F bond in the F 1s XPS spectrum of the negative electrode included in the lithium metal battery in which the electrolyte does not contain an additive may be 1 to 5.

[0021] In one example, the electrolyte may further include an organic solvent.

[0022] In one example, the organic solvent may contain a carbonate-based organic solvent.

[0023] In one example, the organic solvent may contain a cyclic carbonate and a linear carbonate.

[0024] In one example, the organic solvent may contain a cyclic carbonate and a linear carbonate in a volume ratio of 1:1.5 to 9.

[0025] A lithium metal battery according to one embodiment of the present disclosure can improve battery safety by suppressing the formation of lithium dendrites on the surface of an anode.

[0026] In addition, a lithium metal battery according to one embodiment of the present disclosure can have high capacity retention and coulombic efficiency for a long period of time, and can have improved rate characteristics.

[0027] Furthermore, a lithium metal battery according to one embodiment of the present disclosure can have a high energy density by minimizing the volume of the negative electrode while improving the battery life with excellent cycle stability.

[0028] Figures 1(a) and 1(b) show the lithium metal batteries of Examples 1 to 4 and Comparative Example 1, respectively, at 2 mA / cm 2 The graphs show the battery coulombic efficiency and potential profile when driven for one cycle at a current density of 4 mA / cm, and FIG. 1(c) and FIG. 1(d) show the lithium metal batteries of Examples 1 to 4 and Comparative Example 1 at a current density of 4 mA / cm. 2 This graph shows the battery Coulomb efficiency and potential profile when driven for one cycle at a current density of .

[0029] Figures 2(a) and 2(b) show the lithium metal batteries of Comparative Example 1 and Example 3, respectively, at 4 mAh / cm 2This is an image of the surface of the cathode observed after driving for one cycle under a fixed capacity, and Fig. 2(c) is a schematic diagram showing the process of lithium ions being deposited on the surface of a copper current collector when driving a lithium metal battery of Comparative Example 1 and Example 3.

[0030] Figures 3(a) and (b) are 4 mA / cm 2 The potential profiles when the lithium metal batteries according to Examples 5 to 8 and Comparative Example 2 were driven under a current density of 8 mA / cm, and Figs. 3(c) and (d) are potential profiles when the lithium metal batteries according to Examples 5 to 8 and Comparative Example 2 were driven under a current density of 8 mA / cm. 2 This is a potential profile when the lithium metal battery according to Examples 5 to 8 and Comparative Example 2 was driven under a current density of .

[0031] Figures 4(a) to (e) show lithium metal batteries according to Comparative Example 2 and Examples 5 to 8, respectively, at 4 mAh / cm 2 This is a scanning electron microscope image of the surface of the cathode after three cycles of operation under a fixed capacity.

[0032] Figures 5(a) to (e) show lithium metal batteries according to Comparative Example 2 and Examples 5 to 8, respectively, at 4 mAh / cm 2 This is a scanning electron microscope image of the cross-section of the cathode after three cycles of operation under a fixed capacity.

[0033] Figures 6(a) to (d) are 4 mA / cm 2 The surface of the negative electrode was analyzed by EDS when the lithium metal battery according to Example 7 (2 wt% MTFMS) was operated for 3 cycles under a current density of . FIG. 6(a) is an EDS analysis spectrum, and FIGS. 6(b) to (d) are EDS element mapping images showing the distribution of magnesium (Mg), carbon (C), and fluorine (F) on the surface of the negative electrode, respectively.

[0034] Figure 7 shows the X-ray photoelectron spectroscopy (XPS) analysis of the cathode surface after the lithium metal batteries of Comparative Example 2 (without additive) and Example 7 (2 wt% MTFMS) were operated for 3 cycles at a current density of 4 mA / cm2. Figures 7(a) to 7(d) are XPS spectra for Mg 1s, F 1s, Li 1s, and S 2p, respectively.

[0035] Figure 8 is a graph measuring the cycle performance when the lithium metal batteries of Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO) were driven at a charge / discharge rate of 0.5 C.

[0036] Figure 9 is a graph measuring the cycle performance when the lithium metal batteries of Examples 9 to 12 were driven at a charge / discharge rate of 0.5 C.

[0037] Figure 10 is a drawing showing the XPS spectrum of the surface of a lithium metal negative electrode after the lithium metal batteries of Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO) were operated for 3 cycles at a charge / discharge rate of 0.5 C.

[0038] FIG. 11 is a Nyquist plot showing the results of electrochemical impedance spectroscopy (EIS) analysis of lithium metal batteries according to Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO). FIG. 11(a) is a Nyquist plot of the battery before operation, and FIG. 11(b) is a Nyquist plot after the battery has been operated for 100 cycles.

[0039] Figure 12 is a graph showing the rate characteristics of lithium metal batteries according to Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO).

[0040] Figure 13 is a graph measuring the cycle performance when the lithium metal batteries according to Comparative Example 6 (without additive), Comparative Example 7 (LTFMS), Comparative Example 8 (MTBO), and Example 13 (MTFMS) were driven at a charge / discharge rate of 0.5 C.

[0041] FIG. 14 shows the surface and cross-section of the negative electrode after the lithium metal batteries of Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), Comparative Example 5 (MTBO), and Example 11 (MTFMS) were driven at a charge / discharge rate of 0.5 C for 3 cycles, and FIGS. 14(a) to (d) are scanning electron microscope images of the surfaces of the lithium metal batteries of Comparative Example 3, Comparative Example 4, Comparative Example 5, and Example 11, respectively, and FIGS. 14(e) to (f) are scanning electron microscope images of the cross-sections of the lithium metal batteries of Comparative Example 3, Comparative Example 4, Comparative Example 5, and Example 11, respectively.

[0042] Figure 15 is a graph measuring the cycle performance when the lithium metal batteries of Example 11 (MTFMS), Example 14 (LiNO3+MTFMS), Example 15 (LiTFSi+MTFMS), Example 16 (LiNO3+LiTFSi+MTFMS), Comparative Example 3 (without additive), Comparative Example 9 (LiNO3), and Comparative Example 10 (LiTFSi) were driven at a charge / discharge rate of 0.5 C.

[0043] Figure 16 is a graph measuring the cycle performance when the lithium metal batteries of Comparative Example 3 (EC:EMC = 3:7), Comparative Example 11 (EC:EMC = 7:3), Comparative Example 12 (EC:EMC = 5:5), and Comparative Example 13 (EC:EMC = 1:9) were driven at a charge / discharge rate of 0.5 C.

[0044] Figure 17 is a graph measuring the cycle performance when the lithium metal batteries of Example 11 (MTFMS) and Example 18 (Mg(TFSI)2) were driven at a charge / discharge rate of 0.5 C.

[0045] A lithium metal battery comprising the electrolyte additive of the present disclosure is described in detail. The terminology used in this specification has been selected from widely used terms, taking into account the functions of the present disclosure. However, this may vary depending on the intentions of engineers working in the relevant field, precedents, the emergence of new technologies, etc. Unless otherwise defined, the technical and scientific terms used may have the meaning commonly understood by those of ordinary skill in the technical field to which this invention pertains.

[0046] In this specification and the appended claims, the terms “include” or “have” mean that a feature or component described in the specification is present, and unless specifically limited, does not preclude the possibility that one or more other features or components may be added.

[0047] In this specification and the appended claims, the terms first, second, etc. are not used in a limiting sense but are used to distinguish one component from another.

[0048] As used herein and in the appended claims, the singular expression "a" includes the plural expression unless the context clearly dictates otherwise. Furthermore, the plural expression "a" includes the singular expression unless the context clearly dictates otherwise.

[0049] Additionally, the numerical ranges used herein include lower and upper limits and all values ​​within that range, increments logically derived from the shape and width of the defined range, all doubly defined values, and all possible combinations of upper and lower limits of numerical ranges defined in different shapes. Unless otherwise specifically defined in the specification of the present disclosure, values ​​outside the numerical range that may arise due to experimental error or rounding of values ​​are also included in the defined numerical range.

[0050] The term "about" or the like used in this specification and the appended claims is used to encompass the tolerance when an tolerance exists.

[0051] The term “XPS peak intensity” as used in this specification and the appended claims may mean the height of a peak in an XPS spectrum.

[0052] Lithium (Li) metal has a high theoretical capacity of approximately 3860 mAh / g and a large reduction potential of -3.04 V (vs RHE), which allows it to achieve high energy density when used as an anode in lithium-ion batteries, and is attracting attention as a next-generation anode material. However, lithium metal batteries that use lithium metal as an anode have a problem in that lithium metal in a dendrite structure forms on the surface of the anode during repeated charge and discharge cycles. As lithium dendrites grow, they impede the movement of lithium ions, forming dead lithium, which continuously consumes the electrolyte and severely reduces battery performance and lifespan. Furthermore, if lithium dendrites come into contact with the separator, a battery short circuit may occur, leading to an explosion or fire, threatening battery stability.

[0053] In addition, from a practical perspective to surpass the currently commercialized lithium-ion batteries, the cathode / anode capacity ratio (N / P) of 3 or less, the electrolyte weight / anode capacity ratio (E / C) of 3.0 g / Ah or less, and the cathode / anode capacity ratio of 4 mAh / cm 2Lithium metal batteries with high anode capacities are in demand. Reducing the volume of the anode to lower the anode / cathode capacity ratio is also a very important task.

[0054] To suppress lithium dendrite formation, research is underway on forming a protective layer by coating the surface of the lithium anode with inorganic or polymeric materials, or introducing a solid electrolyte interface (SEI) layer on the surface of the anode. However, while increasing the thickness of the protective layer and the solid electrolyte interface may suppress lithium dendrite formation, it also increases the resistance of the lithium metal battery, resulting in a decline in rate characteristics.

[0055] Meanwhile, lithium metal batteries containing various electrolyte additives to form a solid electrolyte interface layer are being developed. For example, Na + , K + or Cs + It was attempted to use metal salts containing Na as electrolyte additives, but + , K + or Cs + When metal ions such as these are plated on the cathode surface, the metal ions exhibit higher activity than lithium, which can rapidly reduce battery safety and accelerate electrolyte consumption, thereby reducing coulombic efficiency.

[0056] Accordingly, the present applicant has developed a lithium metal battery that can improve long-term stability by suppressing the growth of lithium dendrites, while maintaining a high volumetric energy density and thereby improving high-rate characteristics and cycle performance.

[0057] A lithium metal battery according to the present disclosure comprises: a negative electrode; a positive electrode spaced apart from and facing the negative electrode; an electrolyte positioned between the negative electrode and the positive electrode, the electrolyte including a lithium salt and an additive; and a solid-electrolyte interface (SEI) layer positioned at a surface where the negative electrode and the electrolyte come into contact, wherein the additive includes a fluorine-containing magnesium salt, and the solid-electrolyte interface (SEI) layer includes magnesium having an oxidation state of 0.

[0058] By introducing an electrolyte additive containing a fluorine-containing magnesium salt, a solid-electrolyte interface (SEI) containing metallic magnesium is formed on the surface of the negative electrode, thereby effectively suppressing the formation of dendrites on the surface of the negative electrode and alleviating electrolyte consumption due to continuous corrosion of the solid-electrolyte interface layer, thereby improving the long-term stability of the battery.

[0059] In addition, by more densely depositing lithium metal between the negative electrode and the solid electrolyte interfacial layer during charge and discharge to minimize the thickness of the lithium metal layer, high stability and high energy density are maintained even at low negative electrode / positive electrode capacity ratios (N / P) and electrolyte weight / positive electrode capacity ratios (E / C), thereby improving cycle performance, including rate characteristics, of the battery.

[0060] Specifically, when magnesium ions and lithium ions come into contact, the magnesium ions can be reduced to produce metallic magnesium with an oxidation state of 0. The metallic magnesium with an oxidation state of 0 can help lithium to be stably deposited on the surface of the negative electrode during battery operation. By depositing lithium metal more densely and with a more uniform thickness on the surface of the negative electrode, it can suppress the formation of lithium dendrites and simultaneously minimize the thickness of the negative electrode, thereby contributing to improving the energy density of the battery.

[0061] In one embodiment, the fluorine-containing magnesium salt may include magnesium trifluoromethanesulfonate (MTFMS), magnesium bis(trifluoromethanesulfonimide, Mg(TFSI)2), or a combination thereof, and preferably, the fluorine-containing magnesium salt may include magnesium trifluoromethanesulfonate (MTFMS).

[0062] In addition, the solid-electrolyte interface (SEI) layer may further include lithium fluoride (LiF). Since the electrolyte additive contains fluorine, when the lithium metal battery is operated, lithium ions and fluorine ions react, allowing the solid-electrolyte interface layer to be enriched with lithium fluoride (LiF). Lithium fluoride has excellent chemical stability and high mechanical strength, while having low solubility in the electrolyte, thereby further improving the stability of the negative electrode during battery operation.

[0063] Specifically, the ratio (I1 / I0) of the peak intensity (I1) for the Li-F bond in the F 1s XPS spectrum of the solid-electrolyte interface (SEI) layer and the peak intensity (I0) for the Li-F bond in the F 1s XPS spectrum of the negative electrode included in the lithium metal battery in which the electrolyte does not contain an additive may be 1 to 5, 1 to 4.5, 1 to 4, 1 to 3.5, or 1 to 3. This is advantageous because the stability and durability of the lithium metal battery can be enhanced within the above range.

[0064] In addition, in the F 1s XPS spectrum of the solid-electrolyte interface (SEI) layer, the peak intensity (I1) for the Li-F bond may be greater than the peak intensity (I2) for Mg on the metal with an oxidation number of 0 in the Mg 1s XPS spectrum of the solid-electrolyte interface (SEI) layer. Accordingly, the stability of the lithium metal battery is improved while the battery performance is improved, so that the battery can be operated stably for a long time without performance degradation.

[0065] The peak for Mg on the metal with the oxidation number of 0 refers to a peak observed at a binding energy of 1303 ± 0.3 eV in the Mg 1s XPS spectrum, and the peak for the Li-F bond refers to a peak observed at a binding energy of 685.5 ± 0.7 eV in the F 1s XPS spectrum.

[0066] In one example, the electrolyte may contain 0.5 to 5 parts by weight, 0.7 to 4 parts by weight, or 1.0 to 3 parts by weight of the additive, based on 100 parts by weight of the lithium salt, and advantageously, 1.0 to 2.5 parts by weight, or 1.5 to 2.5 parts by weight. When the additive is contained in the above range, the most excellent lithium dendrite formation inhibition effect is achieved, and the thickness of the lithium metal layer described below is reduced, thereby improving the volumetric energy density of the lithium metal battery.

[0067] When the lithium metal battery of the present disclosure is operated, a solid electrolyte interface may be formed on the surface of the negative electrode in contact with the electrolyte through a chemical reaction among the lithium metal, lithium salt, organic solvent, and electrolyte additive. Furthermore, when the lithium metal battery is operated, charging and discharging may be performed as lithium is deposited and de-deposited between the solid electrolyte interface and the negative electrode. When the lithium is deposited, a lithium metal layer may be formed between the solid electrolyte interface and the negative electrode.

[0068] At this time, if the solid electrolyte interface layer is formed with an uneven thickness, during the battery charge / discharge process, the area where the solid electrolyte interface layer is thick has difficulty in allowing lithium ions to reach the negative electrode surface, so the solid electrolyte interface layer acts as an interface resistance layer, which reduces battery performance and may thin the lithium metal layer. In the area where the solid electrolyte interface layer is thin, lithium ions and current may be concentrated during charge / discharge, which may increase the thickness of the lithium metal layer.

[0069] Accordingly, if the lithium metal layer includes a large amount of surface irregularities and is deposited thickly with an uneven thickness, the risk of lithium dendrites developing increases significantly, and as the thickness of the lithium metal layer increases, the energy density of the battery decreases and the electrolyte consumption also increases, so there is a risk of rapid performance degradation occurring in a short period of time.

[0070] Accordingly, since a solid electrolyte interface layer in which lithium fluoride (LiF) and magnesium having an oxidation number of 0 are uniformly dispersed, including the electrolyte additive of the present disclosure, is formed on the negative electrode with a uniform thickness, a lithium metal layer can also be thinly deposited with a uniform thickness on the negative electrode surface, thereby minimizing the interface resistance and suppressing the formation of lithium dendrites.

[0071] In one specific example, the thickness of the lithium metal layer formed between the negative electrode and the solid-electrolyte interface (SEI) layer may be 5 to 50 μm, 5 to 40 μm, 5 to 35 μm, or 10 to 30 μm. In addition, the difference between the maximum and minimum thicknesses of the lithium metal layer may be 8 nm or less, 7 nm or less, 6.5 nm or less, or 6 nm or less, and may be, but is not limited to, 0.1 nm or more, 0.05 nm or more, or 0.01 nm or more, and advantageously, there may be no difference between the maximum and minimum thicknesses of the lithium metal layer.

[0072] Since the lithium metal layer is deposited thinly within the above thickness range and the difference between the minimum and maximum thicknesses is small, the lithium metal layer can be deposited with a uniform thickness across the entire negative electrode. Since the lithium metal layer is deposited with a uniform thickness, the formation of dendrites on the surface of the negative electrode can be minimized when repeating charge and discharge cycles. Since the lithium metal is densely deposited and the lithium metal layer has the above thickness range, the decrease in the volumetric energy density of the battery is prevented, so that even if the negative electrode / positive electrode capacity ratio (N / P) and the electrolyte weight / positive electrode capacity ratio (E / C) are reduced, excellent safety can be maintained and battery performance can be improved.

[0073] In one specific example, the cathode / anode capacity ratio (N / P) may be 0.5 to 3.0, 0.8 to 2.5, 1.0 to 2.0, or 1.0 to 1.5, and the electrolyte weight / anode capacity ratio may be 1.0 to 8.0 g / Ah, 1.5 to 6.0 g / Ah, 2.0 to 4.0 g / Ah, or 2.5 to 3.5 g / Ah.

[0074] In one example, the additive may further contain a metal salt of a different type from the fluorine-containing magnesium salt. Specifically, the metal salt may include a nitrogen-containing metal salt, a fluorine-containing metal salt, or a combination thereof. By further containing a metal salt of a different type from the fluorine-containing magnesium salt, the stability of the solid-electrolyte interface (SEI) layer can be significantly improved.

[0075] When the additive contains more fluorine-containing metal salts, the solid-electrolyte interface (SEI) layer may contain more lithium fluoride (LiF), and when the additive contains more nitrogen-containing metal salts, the solid-electrolyte interface (SEI) layer may contain more lithium nitride (Li3N) as well as lithium fluoride (LiF).

[0076] Accordingly, the solid-electrolyte interface (SEI) layer has excellent chemical and mechanical strength as well as high ionic conductivity, which improves battery performance and promotes uniform deposition of lithium metal during charge and discharge, thereby suppressing the formation and growth of lithium dendrites, thereby further improving long-term cycle performance and battery durability.

[0077] For a more advantageous example, the additive may contain a fluorine-containing magnesium salt, a nitrogen-containing metal salt, and a fluorine-containing metal salt. When a lithium metal battery uses these three electrolyte additives, it can operate stably for extended periods of time without degrading battery performance, thereby further enhancing battery performance and lifespan.

[0078] The above nitrogen-containing metal salt may include at least one selected from lithium nitrate (LiNO3), magnesium nitrate (Mg(NO3)2), silver nitrate (AgNO3), copper nitrate (CuNO3), aluminum nitrate (Al(NO3)3), and sodium nitrate (NaNO3), and specifically may include lithium nitrate (LiNO3).

[0079] The above fluorine-containing metal salt may include at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSi), silver trifluoromethanesulfonate (AgTFMS), copper trifluoromethanesulfonate (CTFMS), aluminum trifluoromethanesulfonate (ATFMS), and sodium trifluoromethanesulfonate (NTFMS), and specifically may include lithium bis(trifluoromethanesulfonyl)imide (LiTFSi).

[0080] As a more specific example, the combination of the three types of electrolyte additives may be a combination of Magnesium Trifluoromethanesulfonate (MTFMS), Lithium nitrate (LiNO3), and Lithium bis(trifluoromethanesulfonyl)imide (LiTFSi).

[0081] In one embodiment, when two types of electrolyte additives are included, the fluorine-containing magnesium salt and a different type of metal salt may be included in an amount of 0.1 to 2 parts by weight, 0.1 to 1.7 parts by weight, 0.1 to 1.4 parts by weight, or 0.1 to 1 part by weight, based on 100 parts by weight of lithium salt.

[0082] In another embodiment, when three kinds of electrolyte additives are included, the fluorine-containing magnesium salt and the different kinds of nitrogen-containing metal salt and fluorine-containing metal salt may be independently contained in the electrolyte in an amount of 0.1 to 2 parts by weight, 0.1 to 1.7 parts by weight, 0.1 to 1.4 parts by weight, or 0.1 to 1 part by weight, based on 100 parts by weight of the lithium salt.

[0083] In one example, the electrolyte may further include an organic solvent. The organic solvent may include a carbonate-based compound, a glyme-based compound, a dioxolane-based compound, or an ether-based compound, and preferably may contain a carbonate-based organic solvent. Specifically, the carbonate-based organic solvent may include, for example, a cyclic carbonate-based organic solvent such as ethylene carbonate (EC) or propylene carbonate (PC), and a linear carbonate-based organic solvent such as ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC).

[0084] The electrolyte can be reduced to a metallic magnesium having an oxidation state of 0 by an alloying reaction between the magnesium and lithium contained in the electrolyte additive, which includes a combination of a cyclic carbonate-based organic solvent and a linear carbonate-based organic solvent. Since the solid electrolyte interfacial layer contains metallic magnesium, the formation of lithium dendrites can be suppressed while a thin, uniformly thick lithium metal layer can be deposited, thereby stably maintaining the battery capacity for a long period of time.

[0085] In one specific example, the organic solvent may contain a larger volume of linear carbonate than cyclic carbonate, and more specifically, may contain the cyclic carbonate and linear carbonate in a volume ratio of 1:1.5 to 9, 1:1:1.8 to 9, or 1:2.3 to 9, and advantageously, may contain the cyclic carbonate and linear carbonate in a volume ratio of 1:2 to 3.

[0086] When the electrolyte contains more linear carbonate than cyclic carbonate in the above volume ratio, the reaction between lithium and magnesium is more active, so that metallic magnesium can be uniformly dispersed in the solid electrolyte interfacial layer.

[0087] In one example, the lithium salt may contain fluorine so that the solid electrolyte interfacial layer can contain more lithium fluoride (LiF) due to the synergistic effect with the fluorine-containing magnesium salt. For example, the lithium salt may be LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (where x and y are natural numbers) or a mixture thereof, but this is only an example and any fluorine-containing lithium salt known in the art may be used.

[0088] In one specific example, the positive electrode may include a positive electrode active material, a conductive material, and a binder. For example, the positive electrode active material may be LiCoO2, LiMn x O 2x (x=1, 2), LiNi 1-x Mn x O2(0 <x<1), LiNi 1-x-y Co x Mn yIt may include O2(0≤x≤0.5, 0≤y≤0.5) or LiFePO4, and more specifically, it may include a conventional cathode active material capable of insertion / de-insertion of lithium, such as LiMn2O4, LiCoO2, LiNiO2, LiFeO2, LiFePO4, V2O5, TiS or MoS.

[0089] Carbon black and graphite particles may be used as the conductive material, and the binder may include poly(vinylidene fluoride)-co-hexafluoropropylene, poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), polytetrafluoroethylene (PTFE), and mixtures thereof. Or, it may include styrene-butadiene rubber polymer (SBR), etc., but the present disclosure is not limited to the specific types of the positive electrode active material, conductive material, and binder, and an active material, conductive material, and binder commonly used in a lithium metal battery may be used.

[0090] In one example, the positive electrode including the positive active material, the conductive material, and the binder may be positioned on a positive electrode current collector, and the negative electrode may be positioned on a negative electrode current collector. The negative electrode current collector and the positive electrode current collector may include a metal mesh material, and more specifically, the positive electrode current collector may include an aluminum electrode plate, and the negative electrode current collector may include a copper electrode plate, but the present disclosure is not limited by the specific type of the current collector.

[0091] In one specific example, a separator interposed between the positive and negative electrodes may be further included. The separator may be any material commonly used in lithium metal batteries without limitation. Specifically, a separator having low resistance to ion movement of the electrolyte and excellent moisture retention capacity of the electrolyte is preferred. For example, the separator may include glass fiber, polyester, polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), or a combination thereof.

[0092] Hereinafter, the present invention will be described in more detail through examples.

[0093] (Example 1) Manufacturing of Li / Cu half-cell

[0094] A solution containing 1M LiPF6 dissolved in an organic solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 was prepared. 1 part by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of the LiPF6, and stirred for 3 days to prepare an electrolyte.

[0095] The negative electrode was used by punching out a 14 mm diameter lithium electrode with a thickness of 100 ㎛ placed on a 12 ㎛ thick copper collector, and the positive electrode was used by punching out a 25 ㎛ thick copper electrode plate with a diameter of 16 mm, and a polypropylene separator (Welcos corporation) was used.

[0096] After assembling the positive electrode, negative electrode, and separator inside a glove box filled with argon (Ar) gas and containing less than 0.1 ppm of oxygen (O2), an electrolyte was injected and sealed to manufacture a 2032 type coin cell type lithium metal battery.

[0097] (Example 2) Manufacturing of Li / Cu half-cell

[0098] A lithium metal battery was manufactured in the same manner as in Example 1, except that 1.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0099] (Example 3) Manufacturing of Li / Cu half-cell

[0100] A lithium metal battery was manufactured in the same manner as in Example 1, except that 2 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0101] (Example 4) Manufacturing of Li / Cu half-cell

[0102] A lithium metal battery was manufactured in the same manner as in Example 1, except that 2.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0103] (Example 5) Manufacturing of Li / Li half-cell

[0104] A lithium metal battery was manufactured in the same manner as in Example 1, but a 100 μm thick lithium electrode plate punched to a diameter of 14 mm was used instead of copper for the positive electrode.

[0105] (Example 6) Manufacturing of Li / Li half-cell

[0106] A lithium metal battery was manufactured in the same manner as in Example 5, except that 1.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0107] (Example 7) Manufacturing of Li / Li half-cell

[0108] A lithium metal battery was manufactured in the same manner as in Example 5, except that 2 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0109] (Example 8) Manufacturing of Li / Li half-cell

[0110] A lithium metal battery was manufactured in the same manner as in Example 5, except that 2.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0111] (Example 9) Manufacturing of Li / NMC811 full battery

[0112] A lithium metal battery was manufactured in the same manner as Example 1, but lithium having a thickness of 50 μm and punched to a diameter of 14 mm was used as the negative electrode, the electrolyte content was adjusted so that the electrolyte weight / positive electrode capacity ratio (E / C) was 8.0 g / Ah, and a positive electrode slurry containing NMC811 as a positive electrode active material was applied on an aluminum electrode instead of a copper electrode and used as the positive electrode.

[0113] Specifically, to manufacture the anode, LiNi 0.8 Mn 0.1 Co 0.1O2 (NMC811), super-p, and polyvinylidene fluoride (PVDF) were added to N-methyl-2-pyrrolidinone (NMP) solvent at a weight ratio of 90:5:5 and mixed to prepare a cathode slurry. During mixing, a Thinky mixer was rotated at 2000 rpm to prepare a homogeneously mixed cathode slurry. An aluminum electrode plate with a thickness of 9 μm was used as a cathode current collector. The cathode slurry was coated on the aluminum electrode plate using a doctor blade and dried in an oven at 100°C for 2 hours to prepare a cathode. The cathode was passed through a rolling device to obtain a cathode having an NMC811 content of 20 mg / cm. 2 It was loaded into a mold, adjusted to a thickness of 90 ㎛, and then punched to a diameter of 13 mm to be used as an anode.

[0114] (Example 10) Manufacturing of Li / NMC811 full battery

[0115] A lithium metal battery was manufactured in the same manner as in Example 9, except that 1.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0116] (Example 11) Manufacturing of Li / NMC811 full battery

[0117] A lithium metal battery was manufactured in the same manner as in Example 9, except that 2 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0118] (Example 12) Manufacturing of Li / NMC811 full battery

[0119] A lithium metal battery was manufactured in the same manner as in Example 9, except that 2.5 parts by weight of Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS) was added to the solution based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0120] (Example 13) Manufacturing of Li / NMC811 full battery

[0121] A lithium metal battery was manufactured in the same manner as Example 11, except that lithium with a thickness of 20 μm was used as the negative electrode and the electrolyte content was adjusted so that the electrolyte weight / positive electrode capacity ratio (E / C) was 3.0 g / Ah.

[0122] (Example 14)

[0123] A lithium metal battery was manufactured in the same manner as in Example 11, except that 0.5 parts by weight of lithium nitrate (LiNO3) was added based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0124] (Example 15)

[0125] A lithium metal battery was manufactured in the same manner as in Example 11, except that 0.5 parts by weight of LiTFSi (lithium bis(trifluoromethanesulfonyl)imide) was added based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0126] (Example 16)

[0127] A lithium metal battery was manufactured in the same manner as in Example 11, except that 0.5 parts by weight of lithium nitrate (LiNO3) and 0.5 parts by weight of LiTFSi (lithium bis(trifluoromethanesulfonyl)imide) were added based on 100 parts by weight of LiPF6 when manufacturing the electrolyte.

[0128] (Example 17)

[0129] A lithium metal battery was manufactured in the same manner as in Example 11, except that 2 parts by weight of Magnesium bis(trifluoromethanesulfonimide)(Mg(TFSI)2) was used based on 100 parts by weight of LiPF6 instead of Magnesium trifluoromethanesulfonate(Mg(CF3SO3)2, MTFMS) when manufacturing the electrolyte.

[0130] (Comparative Example 1)

[0131] A lithium metal battery was manufactured in the same manner as in Example 1, except that the electrolyte did not contain Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0132] (Comparative Example 2)

[0133] A lithium metal battery was manufactured in the same manner as in Example 5, except that the electrolyte did not contain Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0134] (Comparative Example 3)

[0135] A lithium metal battery was manufactured in the same manner as in Example 9, except that the electrolyte did not contain Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0136] (Comparative Example 4)

[0137] A lithium metal battery was manufactured in the same manner as in Example 9, except that the electrolyte was manufactured by adding 2 parts by weight of lithium trifluoromethanesulfonate (LiCF3SO3, LTFMS) based on 100 parts by weight of LiPF6 instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0138] (Comparative Example 5)

[0139] A lithium metal battery was manufactured in the same manner as in Example 9, except that the electrolyte was manufactured by adding 2 parts by weight of magnesium di-tert-butoxide (MTBO) based on 100 parts by weight of LiPF6 instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0140] (Comparative Example 6)

[0141] A lithium metal battery was manufactured in the same manner as in Example 13, except that the electrolyte did not contain Magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0142] (Comparative Example 7)

[0143] A lithium metal battery was manufactured in the same manner as in Example 13, except that the electrolyte was manufactured by adding 2 parts by weight of lithium trifluoromethanesulfonate (LiCF3SO3, LTFMS) based on 100 parts by weight of LiPF6 instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0144] (Comparative Example 8)

[0145] A lithium metal battery was manufactured in the same manner as in Example 13, except that the electrolyte was manufactured by adding 2 parts by weight of magnesium di-tert-butoxide (MTBO) based on 100 parts by weight of LiPF6 instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0146] (Comparative Example 9)

[0147] A lithium metal battery was manufactured in the same manner as in Example 9, except that 0.5 parts by weight of lithium nitrate (LiNO3) based on 100 parts by weight of LiPF6 was added instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0148] (Comparative Example 10)

[0149] A lithium metal battery was manufactured in the same manner as in Example 9, except that 0.5 parts by weight of LiTFSi (lithium bis(trifluoromethanesulfonyl)imide) based on 100 parts by weight of LiPF6 was added instead of magnesium trifluoromethanesulfonate (Mg(CF3SO3)2, MTFMS).

[0150] (Comparative Example 11)

[0151] A lithium metal battery was manufactured in the same manner as in Comparative Example 3, except that an organic solvent containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 7:3 was used to manufacture the electrolyte.

[0152] (Comparative Example 12)

[0153] A lithium metal battery was manufactured in the same manner as in Comparative Example 3, except that an organic solvent containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 5:5 was used to manufacture the electrolyte.

[0154] (Comparative Example 13)

[0155] A lithium metal battery was manufactured in the same manner as in Comparative Example 3, except that an organic solvent containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 1:9 was used to manufacture the electrolyte.

[0156] The types and contents of the negative electrode, positive electrode, and electrolyte additives used in the above examples and comparative examples are summarized in Table 1 below. In Table 1 below, the electrolyte additive content refers to the weight part of each electrolyte additive added based on 100 weight parts of lithium salt (LiPF6), N / P refers to the ratio of the negative electrode capacity (N) and the positive electrode capacity (P), E / C refers to the ratio of the weight of the electrolyte (E) and the positive electrode capacity (C), and EC:EMC refers to the volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) contained in the electrolyte.

[0157] Cathode Anode N / PE / C(g / Ah)EC:EMC Electrolyte Additive Type Content Example 1 LiCu--3 : 7MTFMS1 Example 2 LiCu--3 : 7MTFMS1.5 Example 3 LiCu--3 : 7MTFMS2 Example 4 LiCu--3 : 7MTFMS2.5 Example 5 LiLi--3 : 7MTFMS1 Example 6 LiLi--3 : 7MTFMS1.5 Example 7 LiLi--3 : 7MTFMS2 Example 8 LiLi--3 : 7MTFMS2.5 Example 9 LiNMC8112.5883 : 7MTFMS1 Example 10 LiNMC8112.5883 : 7MTFMS1.5 Example 11 LiNMC8112.5883 : 7MTFMS2Example 12LiNMC8112.5883: 7MTFMS2.5Example 13LiNMC8111.0333: 7MTFMS2Example 14LiNMC8112.5883: 7MTFMS + LiNO32: 0.5Example 15LiNMC8112.5883: 7MTFMS + LiTFSi2: 0.5Example 16LiNMC8112.5883: 7MTFMS + LiNO3+ LiTFSi2: 0.5: 0.5Example 17LiNMC8112.5883: 7Mg(TFSI)22Comparative Example 1LiCu--3:7--Comparative Example 2LiLi--3:7--Comparative Example 3LiNMC8112.5883 : 7--Comparative Example 4LiNMC8112.5883 : 7LTFMS2Comparative Example 5LiNMC8112.5883 : 7MTBO2Comparative Example 6LiNMC8111.0333 : 7--Comparative Example 7LiNMC8111.0333 : 7LTFMS2Comparative Example 8LiNMC8111.0333 : 7MTBO2Comparative Example 9LiNMC8112.5883 : 7LiNO32Comparative Example 10LiNMC8112.5883 : 7LiTFSi2Comparative Example 11LiNMC8112.5887 : 3--Comparative Example 12LiNMC8112.5885 : 5--Comparative Example 13LiNMC8112.5881 : 9--

[0158] <Measuring device and measuring method> The electrochemical characteristics of all lithium metal batteries were measured using a multichannel battery tester from WonA Tech. Specifically, the performance of the lithium metal batteries was measured using a lithium metal battery of 4 mAh / cm 2 By driving with a fixed capacity of , a solid electrolyte interface layer is plated on the cathode and charged, and 1 V (vs Li / Li) + ) was driven to peel off the solid electrolyte interface layer and discharged, and then evaluated. Performing the above charging and discharging processes once each was defined as one cycle.

[0159] The characteristics of the cathode were evaluated by SEM, XRD, and XPS analyses after washing with dimethyl carbonate (DMC) in an argon (Ar)-filled glove box.

[0160] X-ray diffraction (XRD) patterns were measured using a Mini flex 600 from Horiba Co. at 40 kV and 150 mA using Cu Kα1 radiation (λ = 1.5418 Å) as a light source.

[0161] The surface and cross-section of the cathode were observed using a field emission scanning electron microscope (SU4800, Hitachi Co.) with an EDS detector, and the composition and content of elements contained on the cathode surface were measured.

[0162] To analyze the chemical bonding state of the cathode surface, X-ray photoelectron spectroscopy (XPS) analysis was performed using PA326 from Thermo Fisher Scientific Co.

[0163] (Evaluation Example 1) Li / Cu half-cell performance evaluation

[0164] Li / Cu half-cell at 4 mAh / cm 2The coulombic efficiency and potential profile when driven for one cycle under a fixed capacity were measured and shown in Fig. 1, and the cathode surface after driving the Li / Cu half-cell for one cycle was observed using a scanning electron microscope and shown in Fig. 2.

[0165] Specifically, FIG. 1(a) and FIG. 1(b) show the lithium metal batteries of Examples 1 to 4 and Comparative Example 1, respectively, at 2 mA / cm 2 The graphs show the battery coulombic efficiency and potential profile when driven for one cycle at a current density of 4 mA / cm, and FIG. 1(c) and FIG. 1(d) show the lithium metal batteries of Examples 1 to 4 and Comparative Example 1, respectively. 2 This graph shows the battery Coulomb efficiency and potential profile when driven for one cycle at a current density of .

[0166] Referring to Fig. 1(a), the lithium metal battery of Example 3 exhibited a high Coulombic efficiency of 96.6%, showing a significant performance improvement compared to the lithium metal battery of Comparative Example 1, which exhibited a Coulombic efficiency of 87.1%. It was confirmed that Example 1 (91.3%), Example 2 (94.3%), and Example 4 (94.8%) also had excellent initial performance, with the Coulombic efficiency measured to be over 90%.

[0167] Referring to Fig. 1(b), the lithium metal battery of Example 3 exhibited the lowest overpotential of -95.0 mV, confirming that the lithium nucleus formation step on the copper current collector progressed stably in the initial cycle. On the other hand, the lithium metal battery of Comparative Example 1 exhibited an overpotential of -223.0 mV, indicating that its initial stability was significantly low.

[0168] As shown in Figure 1(c) and Figure 1(d), the current density is 4 mA / cm 2Even when the current density increased, the lithium metal battery of Example 3 showed excellent initial performance, with a Coulombic efficiency of 95.8% and an overpotential of -158.9 mV. However, the lithium metal battery of Comparative Example 1 showed a Coulombic efficiency of 79.3% and an overpotential of -402 mV under high current density, indicating that the battery performance decreased rapidly as the current density increased.

[0169] FIG. 2(a) and FIG. 2(b) are images of the surface of the negative electrode after the lithium metal batteries of Comparative Example 1 and Example 3 were driven by the method of Evaluation Example 1, respectively, and FIG. 2(c) is a schematic diagram illustrating the process of lithium ions being deposited on the surface of a copper current collector when the battery is driven.

[0170] As shown in Figures 2(a) to (c), the lithium metal battery of Comparative Example 1 showed that a large number of irregularities were formed on the surface of the copper current collector, indicating that lithium was unevenly deposited. In the case of Example 3, the lithium metal was uniformly deposited on the copper current collector, and there were almost no irregularities on the surface of the negative electrode.

[0171] (Evaluation Example 2) Li / Li half-cell performance evaluation

[0172] Li / Li half-cell at 4 mAh / cm 2 The potential when driven under a fixed capacity was measured and shown in Fig. 3(a) to Fig. 3(d), and the Li / Li half-cell was 4 mAh / cm 2 , 4 mA / cm 2 The cathode surface and cross-section after driving for 3 cycles under the conditions were observed using a scanning electron microscope and are shown in Figs. 4 and 5.

[0173] Specifically, Figs. 3(a) and (b) show 4 mA / cm 2 Potential profiles of lithium metal batteries according to Examples 5 to 8 and Comparative Example 2 under a current density of 8 mA / cm, and Figs. 3(c) and (d) are 2Potential profiles of lithium metal batteries according to Examples 5 to 8 and Comparative Example 2 under a current density of .

[0174]

[0175] *Referring to FIGS. 3(a) and (b), a short circuit occurred in the lithium metal battery of Comparative Example 2 (without additive) when the battery was operated for about 250 hours. This is because lithium was unevenly deposited on the negative electrode surface, resulting in rapid consumption of active lithium and electrolyte. However, it was confirmed that the lithium metal batteries of Examples 5 to 8 were operated for a longer period of time compared to Comparative Example 2, and that the cycle performance and long-term stability were significantly improved. Specifically, the lithium metal batteries of Examples 5 (1 wt%), 6 (1.5 wt%), and 8 (2.5 wt%) were operated for about 600 hours, about 650 hours, and about 830 hours, respectively. In particular, the lithium metal battery of Example 7 (2 wt%) was stably operated for more than 1,000 hours without any degradation in battery performance, and exhibited the highest buffered overpotential.

[0176] Referring to Figures 3(c) and (d), the current density is 8 mA / cm 2 As the voltage increased, the lithium metal battery of Comparative Example 2 experienced a battery short circuit within about 40 hours, but the lithium metal batteries of Examples 5, 6, and 8 still showed excellent cycle stability as the batteries were operated for about 150 hours, about 180 hours, and about 320 hours, respectively. The lithium metal battery of Example 7 exhibited the highest fully charged overpotential and was operated stably for 400 hours, and no problems such as battery short circuit occurred.

[0177] FIGS. 4(a) to (e) are scanning electron microscope images of the surface of the negative electrode after the lithium metal batteries according to Comparative Example 2 and Examples 5 to 8 were driven for 3 cycles, respectively, and FIGS. 5(a) to (e) are scanning electron microscope images of the cross-section of the negative electrode after the lithium metal batteries according to Comparative Example 2 and Examples 5 to 8 were driven by the method of Evaluation Example 2, respectively.

[0178] As shown in FIGS. 4 and 5, the lithium metal battery of Comparative Example 2 (without additive) showed that a large number of irregularities were formed on the negative electrode surface, forming lithium dendrites. In addition, lithium was deposited in an uneven thickness, and the thickness of the deposited lithium was thick, at about 59.3 μm. On the other hand, it was confirmed that the lithium metal batteries of Examples 5 to 8 formed a solid electrolyte interfacial layer with a uniform thickness on the negative electrode surface, and the surface irregularities were alleviated, effectively suppressing the growth of lithium dendrites. In particular, the lithium metal battery of Example 7 showed the most uniform deposition of lithium on the negative electrode surface, and the thinnest deposition of lithium, at a thickness of 21.9 μm. The thinner the thickness of the lithium deposition, the less electrolyte is consumed, the higher the lithium density, and the smoother the lithium deposition, which can improve long-term stability and cycle performance.

[0179] Figures 6(a) to (d) are 4 mA / cm 2The EDS analysis of the negative electrode surface when the lithium metal battery according to Example 7 was operated for 3 cycles under a current density of , FIG. 6(a) is an EDS analysis spectrum, and FIGS. 6(b) to (d) are EDS element mapping images showing the distribution of magnesium (Mg), carbon (C), and fluorine (F) on the negative electrode surface, respectively. As shown in FIG. 6, magnesium, carbon, and fluorine are uniformly dispersed on the negative electrode surface, and as a result of quantifying the EDS signal to calculate the atomic percentage, it was confirmed that magnesium was contained at 1.1 atom%, carbon at 57.2 wt%, and fluorine at 41.7 atom%. As the MTFMS, lithium salt, and organic solvent contained in the electrolyte react with each other through charge-discharge cycles and are deposited on the surface of the negative electrode, the formation of lithium dendrites is effectively suppressed, and the thickness of lithium uniformly deposited on the surface of the negative electrode is minimized, thereby preventing a decrease in the energy density of the battery, thereby significantly improving long-term stability and cycle performance.

[0180] Figure 7 shows the lithium metal battery of Comparative Example 2 (without additive) and Example 7 (2 wt% MTFMS) at 4 mA / cm 2 XPS spectra of the cathode surface after driving for 3 cycles under a current density of , Figs. 7(a) to 7(d) are XPS spectra for Mg 1s, F 1s, Li 1s, and S 2p, respectively.

[0181] In the lithium metal battery of Comparative Example 2, no peaks were observed, but in the lithium metal battery of Example 7, a strong peak was observed at 1303.3 eV, confirming that it contained Mg with an oxidation number of 0, i.e., metallic Mg. In addition, in Example 7, the intensity of the peak derived from the PF bond or -CF3 group at 687.8 eV and the intensity of the peak corresponding to the Li-F bond at 685.6 eV were significantly improved compared to Comparative Example 2. In the Li 1s XPS spectra, peaks corresponding to LiF (55.8 eV), Li2CO3 (55.3 eV), and Li2O (54.7 eV) were observed in both Comparative Example 2 and Example 7, but it was found that the intensity of the peak corresponding to LiF (55.8 eV) was significantly increased in Example 7. Meanwhile, in the case of the S 2p XPS spectrum, no peak was observed in both Comparative Example 2 and Example 7, confirming that sulfur (S) did not participate in the reaction.

[0182] (Evaluation Example 3) Li / NMC811 full battery performance evaluation

[0183] The battery performance when the charge / discharge cycle of the Li / NMC811 full battery was repeated was measured and shown in Figs. 8 to 12. At this time, 4 mAh / cm 2 The charge / discharge rate was defined as 1C.

[0184] Figure 8 is a graph measuring the cycle performance when the lithium metal batteries of Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO) were driven at a charge / discharge rate of 0.5 C.

[0185] The lithium metal battery of Example 11 exhibited a discharge capacity of 185.0 mAh / g in the first cycle, a high capacity retention rate of 96.9% over 200 cycles, and the best capacity retention rate of 89.4% over 500 cycles. The lithium metal battery of Comparative Example 3 exhibited a discharge capacity of 179.5 mAh / g in the first cycle, and a battery short-circuit occurred in only 122 cycles. The lithium metal battery of Comparative Example 4 containing LiTFMS as an electrolyte additive showed somewhat improved cycle stability compared to Comparative Example 3, but the battery failed within 450 cycles, showing lower life characteristics than Example 11. In the case of Comparative Example 5 containing MTBO as an electrolyte additive, the performance was improved compared to Comparative Example 4 because the electrolyte additive contained magnesium, but the performance began to deteriorate before 400 cycles because it did not contain fluorine.

[0186] Figure 9 is a graph measuring the cycle performance of the lithium metal batteries of Examples 9 to 12 when driven at a charge / discharge rate of 0.5 C. It was confirmed that the lithium metal batteries of Examples 9 (1 wt% MTFMS), 10 (1.5 wt% MTFMS), 11 (2 wt% MTFMS), and 12 (2.5 wt% MTFMS) were all stably driven for 200 cycles without capacity degradation. Among them, the lithium metal battery of Example 11 containing 2 wt% of the electrolyte additive showed the highest stability.

[0187] After the lithium metal batteries of Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO) were operated for 3 cycles at a charge / discharge rate of 0.5 C, the surface of the lithium metal negative electrode was analyzed by X-ray photoelectron spectroscopy (XPS), and the results are shown in Fig. 10 below. Figs. 10(a) to 10(c) are XPS spectra for Mg 1s, F 1s, and Li 1s, respectively.

[0188] As shown in Fig. 10(a), in Comparative Examples 3 and 4, since the electrolyte additive did not contain magnesium, no peaks related to magnesium were detected in the XPS spectra. In the case of Example 11 and Comparative Example 5, since the electrolyte additive contained a magnesium component, peaks for magnesium with an oxidation number of 0, i.e., metallic magnesium, were observed. In addition, referring to Figs. 10(b) and 10(c), it was confirmed that the intensity of the peak corresponding to lithium fluoride (LiF) was high in the order of Comparative Example 4, Example 11, Comparative Example 5, and Comparative Example 3. Accordingly, it was found that the lithium metal battery of Example 11 containing MTFMS as an electrolyte additive detected a peak for lithium fluoride (LiF) with a strong intensity as well as a peak for Mg on the metal, indicating that the metal-electrolyte interface (SEI) layer not only has an effect of suppressing dendrite formation but also improves chemical and mechanical stability, thereby enhancing battery durability and improving battery performance.

[0189] Figure 11 is a Nyquist plot showing the electrochemical impedance spectroscopy (EIS) analysis results of lithium metal batteries according to Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO). Figure 11(a) is the Nyquist plot of the battery before operation, and Figure 11(b) is the Nyquist plot after the battery has been operated for 100 cycles.

[0190] As illustrated in FIG. 11, before battery operation, the lithium metal batteries of Example 11, Comparative Example 4, and Comparative Example 5 including the electrolyte additive exhibited lower equivalent series resistance (ESR) than the lithium metal battery of Comparative Example 3, and after 100 cycles, the lithium metal battery of Example 11 had the highest equivalent series resistance value. After 100 cycles, the lithium metal battery of Comparative Example 3 had an equivalent series resistance that was about twice as low as that of the other lithium metal batteries.

[0191] As the battery undergoes repeated charge-discharge cycles, the surface area of ​​the lithium metal anode increases due to the loss of active lithium, which can lead to a decrease in battery resistance. Therefore, the lithium metal battery of Example 11, which exhibited the lowest rate of decrease in battery resistance, exhibited the lowest loss of active lithium over 100 cycles. This indicates that the long-term cycle stability of the lithium metal battery of Example 11 was significantly improved.

[0192] Figure 12 is a graph showing the rate characteristics of lithium metal batteries according to Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO). Specifically, the change in discharge capacity was observed while changing the charge / discharge rate from 0.1 C to 20 C every 5 cycles. Referring to Figure 12, it was confirmed that the lithium metal battery of Example 11 consistently maintained the highest capacity characteristics regardless of the change in charge / discharge rate, thereby producing a high-rate lithium metal battery with excellent rate performance.

[0193] FIG. 13 is a graph showing the cycle performance of the lithium metal batteries of Example 13 (MTFMS), Comparative Example 6 (without additive), Comparative Example 7 (LTFMS), and Comparative Example 8 (MTBO) when driven at a charge / discharge rate of 0.5 C. As shown in FIG. 13, the lithium metal battery of Comparative Example 6, which did not include an electrolyte additive, experienced battery failure within 25 cycles, and the lithium metal battery of Comparative Example 7, which included LTFMS as an electrolyte additive, experienced battery failure after only 217 cycles. The electrolyte additive of Comparative Example 8, which included MTBO as an electrolyte additive, did not experience battery failure, but a noticeable decrease in capacity occurred after 150 cycles, and only 69.9% of the initial capacity was maintained after 300 cycles. On the other hand, the lithium metal battery of Example 13 was confirmed to have the best cycle performance, maintaining 72.7% of the initial capacity even after 300 cycles.

[0194] As the capacity ratio of the negative and positive electrodes (N / P) and the electrolyte weight / capacity ratio of the positive electrode (E / C) decrease, the energy density of the battery increases. However, as the amount of lithium metal and electrolyte replenished during the charging and discharging process of the battery decreases, the lithium metal batteries of Comparative Examples 6 and 7 experienced rapid performance degradation in a short period of time, and the lithium metal battery of Comparative Example 8 also showed lower cycle performance than that of Example 13. However, the lithium metal battery of Example 13 showed remarkably excellent cycle stability and capacity retention despite the very low N / P and E / C values ​​of 1.03 and 3.0 g / Ah, respectively, confirming that both battery energy density and long-term cycle characteristics can be improved simultaneously.

[0195] Fig. 14 is a scanning electron microscope image of the negative electrode surface and cross-section of the lithium metal batteries of Example 11 (MTFMS), Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), and Comparative Example 5 (MTBO) after operating at a charge / discharge rate of 0.5 C for 3 cycles. Specifically, Figs. 14(a) to (d) are scanning electron microscope images of the surfaces of the lithium metal batteries of Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), Comparative Example 5 (MTBO), and Example 11 (MTFMS), respectively, and Figs. 14(e) to (h) are scanning electron microscope images of the cross-sections of the lithium metal batteries of Comparative Example 3 (without additive), Comparative Example 4 (LTFMS), Comparative Example 5 (MTBO), and Example 11 (MTFMS), respectively.

[0196] As shown in Fig. 14, the lithium metal battery of Comparative Example 3 was observed to have a rough surface due to a large number of unevennesses after being operated for 3 cycles. This is because lithium was unevenly deposited on the surface of the anode during battery operation, causing lithium dendrites to grow. In addition, lithium was deposited thickly with a thickness of 57.9 ㎛. In the lithium metal batteries of Comparative Examples 4 and 5, the surface roughness of the lithium deposited on the surface of the anode was slightly reduced, which had the effect of somewhat suppressing the growth of lithium dendrites, but the anodes still had rough surfaces and unevennesses, and lithium was deposited thickly with a thickness of 46.8 ㎛ and 32.0 ㎛ on the surface of the anode, respectively. However, in the case of Example 11, it was confirmed that lithium was deposited very uniformly on the surface of the anode, and the surface unevenness was also significantly reduced, effectively suppressing the growth of lithium dendrites. In addition, lithium was deposited very densely with a thickness of 22.9 ㎛ on the anode. Accordingly, it was confirmed that when the lithium metal battery of Example 11 is operated, lithium can be very stably deposited on the surface of the negative electrode, thereby suppressing the formation and growth of dendrites, and at the same time, the thickness of lithium deposited on the surface of the negative electrode can be minimized, thereby improving the energy density of the battery.

[0197] Figure 15 is a graph measuring the cycle performance when the lithium metal batteries according to Example 11 (MTFMS), Example 14 (LiNO3+MTFMS), Example 15 (LiTFSi+MTFMS), Example 16 (LiNO3+LiTFSi+MTFMS), Comparative Example 3 (without additive), Comparative Example 9 (LiNO3), and Comparative Example 10 (LiTFSI) were driven at a charge / discharge rate of 0.5 C.

[0198] As illustrated in Fig. 15, the lithium metal battery of Comparative Example 3, which did not include an additive, exhibited the worst cycle performance, with a battery short circuit occurring within 150 cycles. In addition, the lithium metal battery of Comparative Example 9, which included lithium nitrate (LiNO3) as the electrolyte additive, and the lithium metal battery of Comparative Example 10, which included lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) as the electrolyte additive, exhibited battery short circuits within 200 and 250 cycles, respectively. Accordingly, it was confirmed that the long-term cycle stability of the lithium metal battery was severely reduced when the electrolyte additive did not include a fluorine-containing magnesium salt.

[0199] However, the lithium metal batteries of Examples 11 and 14 to 16, in which the electrolyte additive included a fluorine-containing magnesium salt, were stably operated without capacity degradation for more than 600 cycles. Specifically, the lithium metal battery of Example 11 using a single additive MTFMS was stably operated for about 620 cycles, the lithium metal battery of Example 14 including MTFMS and LiNO3 as electrolyte additives was stably operated for about 670 cycles, and the lithium metal battery of Example 15 including MTFMS and LiTFSi as electrolyte additives was stably operated for more than about 750 cycles, indicating that the lithium metal batteries of Examples 14 and 15, which used two types of additives in combination, could further improve cycle performance and stability compared to Example 11 using a single additive. In addition, the lithium metal battery of Example 16 including MTFMS, LiNO3, and LiTFSi as additives showed the best cycle performance with almost no performance degradation for about 750 cycles.

[0200] Therefore, it was confirmed that when the electrolyte additive contains MTFMS, which is a fluorine-containing magnesium salt, the cycle performance of a lithium metal battery can be significantly improved, and when it further contains a metal salt of a different type from the MTFMS, the cycle performance of the battery can be further improved than when a single additive is used.

[0201] Fig. 16 is a graph showing the cycle performance of lithium metal batteries according to Comparative Example 3 (EC:EMC = 3:7), Comparative Example 11 (EC:EMC = 7:3), Comparative Example 12 (EC:EMC = 5:5), and Comparative Example 13 (EC:EMC = 1:9) when driven at a charge / discharge rate of 0.5C. Referring to Fig. 16, the lithium metal batteries of Comparative Examples 3 and 13, in which the electrolyte contains a larger volume of ethyl methyl carbonate (EMC), a linear carbonate solvent, than of ethylene carbonate (EC), a cyclic carbonate solvent, exhibited better cycle performance than those of Comparative Examples 11 and 12. In particular, the lithium metal battery of Comparative Example 3 containing EC and EMC in a volume ratio of 3:7 exhibited the best cycle performance, confirming that the volume ratio is an optimal ratio for improving the cycle performance of the battery.

[0202] Fig. 17 is a graph measuring the cycle performance of lithium metal batteries according to Example 11 (MTFMS) and Example 17 (Mg(TFSI)2) when driven at a charge / discharge rate of 0.5 C. As shown in Fig. 17, the lithium metal batteries of Examples 11 and 18 containing a fluorine-containing magnesium salt as an electrolyte additive were stably driven for a long period of time. The lithium metal battery of Example 18 experienced battery failure due to rapid performance degradation around the 400th cycle, but the lithium metal battery of Example 11 was stably driven while maintaining the initial capacity without performance degradation for more than 500 cycles.

[0203] As described above, the present invention has been described with specific details and limited examples and drawings, but these are provided only to help a more general understanding of the present invention, and are not limited to the above examples, and those skilled in the art to which the present invention pertains can make various modifications and variations based on this description.

[0204] Therefore, the idea of ​​the present invention should not be limited to the described embodiments, and all things that are equivalent or equivalent to the claims described below as well as the claims are considered to fall within the scope of the idea of ​​the present invention.

Claims

1. Cathode; An anode spaced apart from the cathode; An electrolyte positioned between the cathode and the anode, and containing a lithium salt and an additive; and A solid-electrolyte interface (SEI) layer located at the surface where the cathode and the electrolyte come into contact; The above additive contains a fluorine-containing magnesium salt, A lithium metal battery, wherein the solid-electrolyte interface (SEI) layer includes magnesium having an oxidation number of 0.

2. In paragraph 1, A lithium metal battery, wherein the fluorine-containing magnesium salt comprises magnesium trifluoromethanesulfonate (MTFMS), magnesium bis(trifluoromethanesulfonimide), Mg(TFSI)2, or a combination thereof.

3. In paragraph 1, A lithium metal battery, wherein the electrolyte contains 0.5 to 5 parts by weight of the additive based on 100 parts by weight of the lithium salt.

4. In paragraph 1, A lithium metal battery, wherein the solid-electrolyte interface (SEI) layer further comprises lithium fluoride (LiF).

5. In paragraph 1, A lithium metal battery, wherein the lithium salt contains fluorine.

6. In paragraph 1, A lithium metal battery, wherein the additive further contains a fluorine-containing magnesium salt and a different type of metal salt.

7. In paragraph 6, A lithium metal battery, wherein the metal salt comprises a nitrogen-containing metal salt, a fluorine-containing metal salt, or a combination thereof.

8. In paragraph 7, A lithium metal battery, wherein the nitrogen-containing metal salt comprises at least one selected from lithium nitrate (LiNO3), magnesium nitrate (Mg(NO3)2), silver nitrate (AgNO3), copper nitrate (CuNO3), aluminum nitrate (Al(NO3)3), and sodium nitrate (NaNO3).

9. In paragraph 7, A lithium metal battery, wherein the fluorine-containing metal salt comprises at least one selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSi), silver trifluoromethanesulfonate (AgTFMS), copper trifluoromethanesulfonate (CTFMS), aluminum trifluoromethanesulfonate (ATFMS), and sodium trifluoromethanesulfonate (NTFMS).

10. In paragraph 1, A lithium metal battery, wherein the thickness of the lithium metal layer deposited between the negative electrode and the solid-electrolyte interface (SEI) layer is 5 to 50 μm.

11. In paragraph 1, A lithium metal battery, wherein the ratio (I1 / I0) of the peak intensity (I1) for Li-F bonding in the F 1s XPS spectrum of the solid-electrolyte interface (SEI) layer and the peak intensity (I0) for Li-F bonding in the F 1s XPS spectrum of the negative electrode included in the lithium metal battery in which the electrolyte does not contain an additive is 1 to 5.

12. In paragraph 1, A lithium metal battery, wherein the electrolyte further comprises an organic solvent.

13. In paragraph 11, A lithium metal battery, wherein the organic solvent contains a carbonate-based organic solvent.

14. In paragraph 13, A lithium metal battery, wherein the organic solvent contains a cyclic carbonate and a linear carbonate.

15. In paragraph 14, A lithium metal battery, wherein the organic solvent contains a cyclic carbonate and a linear carbonate in a volume ratio of 1:1.5 to 9.

Images