Electrolyte for lithium metal battery and lithium metal battery comprising same

Abstract

The present invention relates to a negative electrode for a lithium metal secondary battery, the negative electrode comprising: a conductive metal layer including a first metal; an interface layer formed on the conductive metal layer and including a second metal oxide having a greater work function than the first metal; and a lithium metal layer formed on the interface layer and having a work function greater than 3.0 eV. Due to said configuration, lithium loss due to side reactions with an electrolyte can be reduced, and the lifespan characteristics of the lithium metal secondary battery can be improved.

Description

Anode for lithium metal secondary battery and lithium metal secondary battery containing same

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0092582, filed July 17, 2023, and Korean Patent Application No. 10-2024-0068576, filed May 27, 2024, the entire contents of which are incorporated herein by reference.

[0003] The present invention relates to a negative electrode for a lithium metal secondary battery and a lithium metal secondary battery including the same, which can reduce the loss of lithium due to side reactions with an electrolyte and improve the life characteristics of a lithium metal secondary battery.

[0004] Lithium metal secondary batteries utilize lithium metal (Li-metal) as the anode active material. Compared to conventional batteries using graphite or lithium alloy anodes, they theoretically possess significantly higher energy density and capacity. Therefore, research and development are ongoing to apply lithium metal secondary batteries to batteries requiring high energy densities.

[0005] The anodes of existing lithium metal secondary batteries have not been commercialized due to high irreversibility and poor cycle life characteristics caused by dendritic growth of lithium during the charging process. During the charging process, there are two types of lithium loss pathways: one is the loss of lithium due to electrical short-circuiting of lithium deposited on the anode, and the other is the loss of lithium due to the reaction between lithium and the electrolyte caused by the increased lithium surface area. Dendritic growth of lithium can accelerate these problems. As a result, existing lithium metal secondary batteries have the disadvantage of poor cycle life characteristics due to the continuous loss of lithium, which acts as the anode active material.

[0006] Meanwhile, in the field of research on electrolytes for lithium metal secondary batteries, research has been conducted in the direction of controlling the uniform growth of lithium metal and minimizing lithium that is electrically short-circuited by forming a good oxide film on the surface of the negative electrode through reaction with lithium metal.

[0007] However, recent research has shown that these electrolytes fundamentally form oxide films through their high reactivity with lithium metal, leading to a significant proportion of lithium loss through the second lithium loss pathway, namely, the lithium loss that occurs when lithium reacts with the electrolyte. Therefore, even with recently developed electrolytes, lithium metal secondary batteries still suffer from insufficient cycle life due to the continuous lithium loss, particularly lithium loss caused by side reactions between the electrolyte and lithium.

[0008] Accordingly, the present invention provides a negative electrode for a lithium metal secondary battery that can reduce lithium loss due to side reactions with an electrolyte and improve the life characteristics of a lithium metal secondary battery.

[0009] The present invention also provides a lithium metal secondary battery exhibiting improved life characteristics, including the negative electrode.

[0010] Accordingly, the present invention comprises a conductive metal layer including a first metal;

[0011] An interface layer formed on a conductive metal layer and including a second metal oxide having a work function greater than that of the first metal; and

[0012] A negative electrode for a lithium metal secondary battery is provided, which includes a lithium metal layer formed on the above interfacial layer and having a surface work function exceeding 3.0 eV.

[0013] The present invention also provides a lithium metal secondary battery comprising: the negative electrode; a positive electrode facing the negative electrode; an electrolyte comprising a lithium salt and a non-aqueous organic solvent; and a separator between the positive electrode and the negative electrode.

[0014] The negative electrode for a lithium metal secondary battery of the present invention forms an interface layer including a metal oxide having a large work function on a conductive metal layer acting as a current collector, and forms a lithium metal layer having a relatively thin thickness on this interface layer.

[0015] In these cathodes, it was confirmed that the surface work function of the lithium metal layer can be relatively controlled, resulting in low surface reactivity of the lithium metal layer. Due to this low surface reactivity of the lithium metal layer, its reactivity with the electrolyte is reduced, and a thinner oxide film (solid electrolyte interphase) can be formed on the cathode.

[0016] This can result in reducing the loss of lithium due to the reaction between the electrolyte and the lithium metal layer, and can further improve the life characteristics of the lithium metal secondary battery.

[0017] Figure 1 is a graph showing the work function and band gap of various metal oxides.

[0018] Figure 2 shows the results of evaluating the surface work function of the lithium metal layer in the negative electrode of the examples, comparative examples, and reference examples.

[0019] Figure 3 shows the results of TEM analysis of the lithium metal layer deposited and formed during the negative electrode manufacturing process of Comparative Example 2 and the solid electrolyte interface film on its surface.

[0020] Figure 4 shows the results of capacity retention rate evaluation of lithium metal secondary batteries (lithium-free secondary batteries) including the negative electrodes of the examples and comparative examples.

[0021] In this specification, when a part is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless specifically stated otherwise.

[0022] The terms "about," "substantially," and the like used throughout this specification are used to mean at or near the numerical value when manufacturing and material tolerances inherent to the meanings stated are presented, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that state precise or absolute values ​​to aid understanding of this specification. The terms "step of doing" or "step of" used throughout this specification do not mean "step for doing."

[0023] In this specification, the term "combination of these" included in the expression in the Makushi format means one or more mixtures or combinations selected from the group consisting of the components described in the expression in the Makushi format, and means including one or more selected from the group consisting of the components.

[0024] In addition, in this specification, “lithium metal secondary battery” can be defined as any secondary battery that utilizes “lithium metal” as an anode active material. The category of such “lithium metal secondary battery” can include, for example, not only a secondary battery including an anode manufactured by forming a lithium metal layer on an anode current collector including a conductive metal layer and a metal oxide-containing interfacial layer, but also a battery referred to as a “lithium-free secondary battery” or “anode-free secondary battery.”

[0025] Such a “lithium-free secondary battery” or “anode-free secondary battery” may refer to a secondary battery in which, before charging and discharging, for example, immediately after manufacturing, a separate lithium metal layer, lithium alloy layer, or other negative electrode active material layer does not exist on the negative electrode current collector including the conductive metal layer and the interface layer. In such a “lithium-free secondary battery” or “anode-free secondary battery,” a lithium metal layer may be electrodeposited and formed on the negative electrode current collector during the charging and discharging process, and such lithium metal layer may function as a negative electrode active material. Therefore, such a “lithium-free secondary battery” or “anode-free secondary battery” may also belong to the category of a “lithium metal secondary battery” that utilizes lithium metal as a negative electrode active material.

[0026]

[0027] Hereinafter, based on the above definitions, implementation examples of the invention will be described in detail. However, these are presented as examples and are not intended to limit the invention. The invention is defined solely by the scope of the claims set forth below.

[0028] According to one embodiment of the invention, a conductive metal layer including a first metal;

[0029] An interface layer formed on a conductive metal layer and including a second metal oxide having a work function greater than that of the first metal; and

[0030] A negative electrode for a lithium metal secondary battery is provided, which includes a lithium metal layer formed on the above interfacial layer and having a surface work function exceeding 3.0 eV.

[0031] In the negative electrode of the above embodiment, an interface layer including a second metal oxide having a higher work function, for example, a metal oxide having a work function of 6.0 eV or more, is formed on a conductive metal layer including a first metal such as copper, which acts as a negative electrode current collector. In addition, a lithium metal layer acting as a negative electrode active material is formed on this interface layer. This lithium metal layer may be formed separately during the manufacturing process of a lithium metal secondary battery, or may be electrodeposited and formed during charging and discharging of the secondary battery.

[0032] At this time, the work function of the metal or metal oxide can be defined as the minimum thermodynamic energy (eV) required to remove one electron from the surface of the solid metal or metal oxide, and can be measured and calculated from the results of analyzing the solid material using UPS (Ultraviolet Photoelectron Spectroscopy).

[0033] The inventors' experimental results confirmed that by forming an interfacial layer containing a metal oxide having a large work function between the conductive metal layer and the lithium metal layer, the surface work function of the lithium metal layer can be relatively controlled to a large extent. In addition, as described in more detail below, by controlling the formation conditions of the lithium metal layer to form a lithium metal layer having a thinner thickness, the surface work function of the lithium metal layer can be further increased.

[0034] This large surface work function means that a large amount of energy is required to remove electrons from the lithium metal layer, and this large work function may suppress the reaction between the lithium metal layer formed on the second metal oxide-containing interfacial layer and the electrolyte. Due to this low surface reactivity of the lithium metal layer, its reactivity with the electrolyte is reduced, and a thinner oxide film (solid electrolyte interphase) may be formed on the negative electrode.

[0035] As a result, the amount of lithium loss due to the reaction between the electrolyte and the lithium metal layer can be significantly reduced, and the life characteristics of the lithium metal secondary battery can be further improved.

[0036] Meanwhile, in the negative electrode for a lithium metal secondary battery according to the above-described embodiment, the conductive metal layer may have a thickness equivalent to that of a general negative electrode current collector, for example, a thickness of 3 to 500 μm, or 5 to 100 μm, or 7 to 30 μm.

[0037] In addition, the first metal forming the conductive metal layer is a metal that has high conductivity while having relatively low reactivity and does not cause chemical changes in the battery, and can be formed using any metal that has been previously known to be usable as a negative electrode current collector.

[0038] Specific examples thereof include metals such as stainless steel, aluminum, nickel, titanium, or copper, or metals surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper, aluminum, or stainless steel. This conductive metal layer can be formed in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric. However, considering the excellent conductivity and light weight of the negative electrode current collector and the negative electrode, and the appropriate work function difference and low reactivity with the second metal oxide, the first metal may include copper (Cu), and more specifically, may be a copper foil.

[0039] Such a conductive metal layer comprising copper may have a work function of, for example, 4.53 to 5.10 eV, or 4.55 to 4.90 eV, or 4.55 to 4.65 eV.

[0040] In addition, an interface layer including a second metal oxide may be formed on at least one surface of the conductive metal layer. As described above, since the second metal oxide has a higher work function than the first metal of the conductive metal layer, it has been confirmed that the surface work function of the lithium metal layer formed on the interface layer can be further increased and the reactivity between the lithium metal layer and the electrolyte can be reduced.

[0041] More specifically, the second metal oxide may have a work function of 6.0 eV or more, or 6.0 to 7.0 eV, or 6.0 to 6.5 eV, and a band gap of 2.7 eV or more, or 2.7 to 3.5 eV. In this case, the band gap may be defined as the minimum amount of energy required for one electron to escape from a bound state in each metal oxide. For reference, FIG. 1 illustrates the work functions and band gaps of various metal oxides.

[0042] By forming an interface layer with a second oxide having the work function and band gap described above, the surface work function of the lithium metal layer can be further increased, the reactivity with the electrolyte can be lowered, and the amount of lithium loss can be further reduced.

[0043] Considering the above-described appropriate work function and band gap range, etc., the second metal oxide may include nickel oxide (NiO), molybdenum oxide (MoO3), or tungsten oxide (WO3), and among these, by using tungsten oxide, side reactions between the lithium metal layer and the electrolyte in a lithium metal secondary battery can be further reduced, and the life characteristics of the secondary battery can be further improved.

[0044] The interfacial layer including the second metal oxide may have a thickness of, for example, 10 nm to 1 μm, or 10 to 500 nm, or 10 to 200 nm, in order to effectively suppress the reactivity of the lithium metal layer while not impairing the excellent conductivity and lightness of the negative electrode.

[0045] In addition, the method for forming the interface layer including the second metal oxide is not particularly limited, and may be formed by any method for coating the metal oxide on the conductive metal layer. In a specific example, the interface layer may be formed by thermally evaporating the second metal oxide on the conductive metal layer, and such thermal evaporation may be formed to an appropriate deposition thickness by monitoring the deposition thickness and deposition rate of the second metal oxide in real time using a quartz crystal microbalance (QCM).

[0046] Meanwhile, in the cathode of the above-described embodiment, a lithium metal layer having a surface work function exceeding 3.0 eV, for example, a surface work function of 3.0 to 4.5 eV, or 3.1 to 4.0 eV, can be formed on the above-described interface layer.

[0047] Since the lithium metal layer is formed on the interface layer containing the second metal oxide having the large work function described above, it can have a large surface work function compared to a lithium metal layer formed directly on a general lithium metal layer or a conductive metal layer such as a copper current collector.

[0048] In addition, as confirmed through the following examples and the like, the lithium metal layer may have a thinner thickness, for example, a thickness of 10 nm to 10 μm, on the interfacial layer. More specifically, the lithium metal layer may be a lithium metal nuclei-containing layer formed to a thickness of 10 nm to 1 μm, or 20 nm to 700 nm, or a lithium metal electrodeposition layer formed to a thickness of 3 μm to 10 μm, or 4 μm to 9 μm. In any case, it was confirmed that as the lithium metal layer is formed to a thinner thickness, it may have a higher work function. Due to this larger surface work function, the lithium metal layer may exhibit low reactivity with the electrolyte, and the thickness of the solid electrolyte interfacial film formed on the negative electrode may be thinner. Therefore, a lithium metal secondary battery including the negative electrode of one embodiment may exhibit excellent life characteristics due to a reduced lithium loss amount.

[0049] In order to form the lithium metal layer with a thinner thickness, the lithium metal layer is, for example, 1.0 to 2.5 mAh / cm 2 , or 1.0 to 2.0 mAh / cm 2 It can be deposited and formed on the interface layer with a low capacity.

[0050] Meanwhile, the lithium metal layer may be formed on the interface layer during the manufacturing process of the negative electrode and lithium metal secondary battery of one embodiment, but may also be formed during the charging and discharging process after manufacturing the lithium metal secondary battery.

[0051] In this configuration, a lithium metal secondary battery including an anode of one embodiment may have the form of a lithium-free secondary battery that does not include a separate lithium metal layer or anode active material layer immediately after manufacturing. However, as charging and discharging of such a lithium metal secondary battery proceeds, lithium ions moved from the anode may be deposited on the interfacial layer of the anode to form a lithium metal layer, and this lithium metal layer may function as anode active material.

[0052] Meanwhile, according to another embodiment of the invention, a lithium metal secondary battery comprising the negative electrode of the above-described embodiment is provided. Such a lithium metal secondary battery may include, for example, the negative electrode of the above-described embodiment; a positive electrode facing the negative electrode; an electrolyte comprising a lithium salt and a non-aqueous organic solvent; and a separator between the positive electrode and the negative electrode.

[0053] These other embodiments of the battery include a lithium metal layer with a high surface work function, thereby reducing lithium loss during charging and discharging and exhibiting excellent cycle life characteristics. Since the negative electrode of the embodiment has been described in detail above, the following description will focus on the remaining components, excluding the negative electrode.

[0054] In the lithium metal secondary battery of the above other embodiment, the positive electrode may include a positive electrode current collector and a positive electrode active material layer positioned on the positive electrode current collector.

[0055] These positive electrodes can be manufactured by mixing an active material and a binder, and in some cases, a conductive material, a filler, etc. in a solvent to prepare a positive electrode slurry composition, and applying the composition to a positive electrode current collector.

[0056] The positive electrode current collector may generally have a thickness of 3 to 500 μm. In addition, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The current collector may also form fine unevenness on its surface to increase the adhesive strength of the positive electrode active material, and various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric are possible.

[0057] And, in the case of the above positive electrode active material, it may include a compound capable of reversible intercalation and deintercalation of lithium, specifically a lithium metal oxide containing one or more metals such as iron, cobalt, manganese, nickel, or aluminum and lithium.

[0058] Specifically, the lithium metal oxide is a lithium-manganese oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt oxide (e.g., LiCoO2, etc.), a lithium-nickel oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese oxide (e.g., LiNi 1-Y Mn Y O2 (here, 0 <Y<1), LiMn 2-Z Ni Z O4 (where 0<Z<2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2 (here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2 (here, 0 <Y2<1), LiMn 2-Z1 Co Z1 O4 (wherein, 0<Z1<2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mnr )O2(wherein, 0<p<1, 0<q<1, 0<r<1, p+q+r=1) or Li(Ni p1 Co q1 Mn r1 )O4 (wherein, 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2) etc.), lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (wherein, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r2 and s2 are atomic fractions of independent elements, respectively, 0<p2<1, 0<q2<1, 0<r2<1, 0<s2<1, p2+q2+r2+s2=1), etc.), or lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (Here, M is at least one selected from Al, Mg, and Ti, X is at least one selected from F, S, and N, -0.5≤a≤+0.5, 0≤x≤0.5, 0≤b≤0.1), and any one or two or more compounds thereof may be included.

[0059] Among these, the positive electrode active material includes a lithium metal oxide including lithium and two or more metals selected from the group consisting of nickel, manganese, cobalt, and aluminum, and the lithium metal oxide may include nickel in an amount of 50 mol% or more, or 60 to 99 mol%, or 70 to 95 mol%, based on the total metal content excluding lithium. Such a lithium metal oxide may be represented by, for example, the following chemical formula 1:

[0060] [Chemical Formula 1]

[0061] Li x Ni a Co b M1 c M 2 d O2

[0062] In the above chemical formula 1, the M 1 Mn and Al may be one or more selected from or a combination thereof, and M 2 may be at least one selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P and S, and may be 0.90≤x≤1.1, or 0.95≤x≤1.08, or 1.0≤x≤1.08, and may be 0.50≤a<1.0, or 0.60≤a≤0.99, or 0.70≤a≤0.95. In addition, 0 <b≤0.3이고, 0<c≤0.3이고, 0≤d≤0.1일 수 있다.

[0063] By using a lithium metal oxide containing such a high nickel content as a positive electrode active material and combining it with an anode of one embodiment, the output and capacity characteristics, as well as the life characteristics, of a lithium metal secondary battery can be further improved.

[0064] The above-described positive electrode active material may be included in an amount of 60 to 99 wt%, or 70 to 99 wt%, or 80 to 98 wt% based on the total weight of the positive electrode active material layer.

[0065] Meanwhile, the conductive material included in the positive electrode active material layer is a component for further improving the conductivity of the positive electrode active material. The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive nanomaterials such as carbon nanofibers or carbon nanotubes; fluorinated carbon powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. Among these, the conductive material can further lower the resistance of the lithium metal secondary battery and improve the output characteristics, etc. by including a conductive nanomaterial such as carbon nanotubes or carbon nanofibers.

[0066] Typically, the conductive material may be included in an amount of 1 to 20 wt%, or 1 to 15 wt%, or 1 to 10 wt% based on the total weight of the positive electrode active material layer.

[0067] The binder optionally included in the above-described positive electrode active material layer is a component that assists in the bonding of the positive electrode active material and the conductive material, and in the bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, or fluororubber, and a mixture or copolymer of two or more selected from these may also be used.

[0068] Typically, the binder may be included in an amount of 1 to 20 wt%, or 1 to 15 wt%, or 1 to 10 wt% based on the total weight of the positive electrode active material layer.

[0069] Additionally, a filler may be optionally added to the positive electrode as a component that suppresses its expansion. Such filler is not particularly limited as long as it can suppress the expansion of the electrode without causing chemical changes in the battery. Examples of fillers that can be used include olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; and the like.

[0070] The above-described positive electrode can be manufactured by dispersing and mixing the positive electrode active material, binder, and conductive agent in a dispersion medium (solvent) to make a slurry, applying the slurry on a metal current collector, and then drying and rolling. At this time, the dispersion medium may be NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof, but is not necessarily limited thereto.

[0071] Meanwhile, the lithium metal secondary battery of the other embodiment further includes an electrolyte including a non-aqueous organic solvent and a lithium salt.

[0072] The lithium salt included in the above electrolyte is used as a medium for transferring ions within a secondary battery. Lithium salt is, for example, Li as a cation. + Including, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO2- , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - and SCN - It may include an anion selected from the group consisting of .

[0073] Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10, LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl) imide, LiN(SO2CF2CF3)2) and LiTFSI (lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2).

[0074] The concentration of the above lithium salt can be appropriately changed within a commonly usable range, and can be included in the electrolyte at a concentration of 1 M to 6 M, or a concentration of 1.5 M to 5 M.

[0075] In a more specific embodiment, the electrolyte may contain a lithium salt in a relatively low concentration of 0.5 M or more and less than 2 M, or 0.7 M to 1.5 M, but may also contain the lithium salt in a high concentration of 2 M to 6 M, or 2.5 M to 5.5 M. By using an electrolyte containing such a high-concentration lithium salt, the output characteristics of the secondary battery can be further improved. In addition, when the electrolyte containing the high-concentration lithium salt and the negative electrode of one embodiment are used together, the amount of lithium loss due to the reaction between the electrolyte and the lithium metal layer can be reduced. As a result, the output characteristics, etc. improved by the high-concentration lithium salt electrolyte can be effectively exhibited, while the deterioration of the life characteristics due to the increased amount of lithium loss can be effectively suppressed.

[0076] Meanwhile, the type of non-aqueous organic solvent that can be included in the electrolyte is not particularly limited, and any organic solvent that has been previously known to be applicable to electrolytes of lithium ion batteries, etc., can be used. Examples of such organic solvents include at least one selected from the group consisting of carbonate solvents, ether solvents, nitrile solvents, phosphate solvents, and sulfone solvents. However, considering the stability of the lithium metal layer, etc., it is preferable that the non-aqueous organic solvent include an ether solvent.

[0077] More specifically, as the carbonate solvent, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate or methyl (2,2,2-trifluoroethyl) carbonate can be used, and as the phosphate solvent, trimethyl phosphate, triethyl phosphate or 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphosphorane 2-oxide can be used.

[0078] In addition, as the ether solvent, dibutyl ether, tetraglyme, diglyme, dimethoxy ethane, or tetrahydrofuran derivatives such as 2-methyl tetrahydrofuran can be used, and as the nitrile solvent, succinonitrile, adiponitrile, sebaconitrile, acetonitrile, or propionitrile can be used. In addition, as the sulfone solvent, dimethyl sulfone, ethylmethyl sulfone, or sulforane can be used.

[0079] Meanwhile, the lithium metal secondary battery of another embodiment described above may further include a porous separator interposed between the positive electrode and the negative electrode.

[0080] These porous membranes can be made of olefin polymers such as polyethylene and polypropylene, glass fibers, etc. in the form of sheets, multi-membranes, microporous films, woven fabrics, and non-woven fabrics, but are not necessarily limited thereto. However, it may be preferable to use porous polyethylene or porous glass fiber non-woven fabric (glass filter) as the membrane, and it may be more preferable to use porous glass filter (glass fiber non-woven fabric) as the membrane. The membrane may be an insulating thin film having high ion permeability and mechanical strength, and the pore diameter of the membrane may generally be in the range of 0.01 to 10 ㎛, and the thickness may generally be in the range of 5 to 300 ㎛, but is not limited thereto.

[0081] In addition, in another example of the lithium metal secondary battery, the separator may be integrated with the electrolyte and interposed between the positive and negative electrodes in the form of an electrolyte membrane or electrolyte film. In this case, the electrolyte membrane or electrolyte film may be in the form of a polymer matrix containing the lithium salt and a non-aqueous organic solvent described above, and a well-known polymer-based solid electrolyte, etc. may be used as the polymer matrix. In this case, the lithium metal secondary battery of the other embodiment may be a semi-solid battery that uses a liquid electrolyte and a solid electrolyte in combination.

[0082] Meanwhile, the lithium metal secondary battery of the above-described other embodiment can be manufactured according to a conventional method in the art. For example, the electrode assembly including the positive electrode, negative electrode, and separator can be housed in a case and manufactured by injecting and impregnating the electrolyte described above.

[0083] These lithium metal secondary batteries can be applied to battery cells used as power sources for small devices, and are particularly suitable for use as unit cells in battery modules that serve as power sources for medium- to large-sized devices.

[0084]

[0085] Below, preferred embodiments of the invention, comparative examples to compare with them, and experimental examples for evaluating them are described. However, the following examples are merely preferred embodiments of the invention, and the present invention is not limited to the following examples.

[0086]

[0087] Comparative Example 1: Anode for lithium metal secondary battery (lithium-free secondary battery)

[0088] Copper foil itself having a thickness of 10㎛ was used as the negative electrode for the lithium metal secondary battery of Comparative Example 1.

[0089]

[0090] Comparative Examples 2 to 4: Negative electrode for lithium metal secondary battery

[0091] A half-cell was fabricated using a 10 μm thick copper foil as one electrode and a 150 μm thick lithium foil as the other electrode. This half-cell was fabricated to include a porous polypropylene separator interposed between the two electrodes and an electrolyte in which a lithium salt of LiFSI was dissolved in dimethoxyethane (DME) at a concentration of 4 M.

[0092] 1.0mA / cm for the above half-cell 2 Under the application of current, each 1 mAh / cm 2 Capacity (comparative example 2), 3 mAh / cm 2 Capacity (comparative example 3), 4 mAh / cm 2 While charging and discharging were performed at a capacity of (Comparative Example 4), a lithium metal layer was electrodeposited on the copper foil layer to manufacture negative electrodes of Comparative Examples 2 to 4. The electrodeposition and formation thickness of each lithium metal layer was confirmed to be 5 µm (Comparative Example 2), 15 µm (Comparative Example 3), and 20 µm (Comparative Example 4), respectively.

[0093]

[0094] Reference Example 1: Negative electrode for lithium metal secondary battery (lithium-free secondary battery)

[0095] First, on a copper foil having a thickness of 10 μm, WO3 was thermally deposited while monitoring the deposition thickness and deposition rate in real time using a quartz crystal microbalance (QCM) to form a 30 nm thick tungsten oxide layer. This was used as the cathode of Reference Example 1.

[0096]

[0097] Example 1, Reference Examples 2 and 3: Negative electrode for lithium metal secondary battery

[0098] A half-cell was manufactured using the negative electrode of the above-mentioned Reference Example 1 as one electrode and a 150 μm lithium foil as the other electrode. This half-cell was manufactured to include a porous polypropylene separator interposed between the two electrodes and an electrolyte in which a lithium salt of LiFSI is dissolved in dimethoxyethane (DME) at a concentration of 4 M.

[0099] 1.0mA / cm for the above half-cell 2 Under the application of current, each 1 mAh / cm 2 Capacity (Example 1), 3 mAh / cm 2 Capacity (Reference Example 2), 4 mAh / cm 2 While charging and discharging with a capacity of (Reference Example 3), a lithium metal layer was electrodeposited on the copper foil layer to manufacture negative electrodes of Examples 1, 2, and 3, respectively. The electrodeposition and formation thickness of each lithium metal layer was confirmed to be 5 µm (Example 1), 15 µm (Reference Example 2), and 20 µm (Reference Example 3), respectively.

[0100]

[0101] Experimental Example 1: Surface Work Function Evaluation

[0102] The cathodes of Comparative Examples 1 to 4, Reference Examples 1 to 3, and Example 1 were analyzed by UPS (Ultraviolet Photoelectron Spectroscopy) to measure the surface work function (eV), and the measurement results are shown in Fig. 2. For Comparative Example 1 and Reference Example 1, the work function of the surface of the copper foil and the tungsten oxide layer, respectively, was measured. In addition, for Comparative Examples 2 to 4, Reference Examples 2 and 3, and Example 1, the lithium deposited on the copper foil or the tungsten oxide layer was collected and washed, and then analyzed by UPS to measure the surface work function of the lithium metal layer.

[0103] Referring to Fig. 2, it was confirmed that the surface work function of the lithium metal layer was the largest in Comparative Example 2, which had the smallest electrodeposition capacity and the thinnest thickness among Comparative Examples 2 to 4. In addition, as the thickness of the lithium metal layer increased, the surface work function of the lithium metal layer decreased, and this trend was also confirmed in Example 1, Reference Examples 2 and 3, in which a tungsten oxide layer was additionally formed.

[0104] Additionally, referring to Example 1, Reference Examples 2 and 3, it was confirmed that the surface work function of the lithium metal layer further increased as a tungsten oxide layer (interfacial layer) was additionally formed between the copper foil and the lithium metal layer. As a result, it was confirmed that Example 1 exhibited the largest surface work function of the lithium metal layer and the highest remaining reactivity.

[0105]

[0106] Experimental Example 2: Evaluation of the Thickness of the Solid Interfacial Electrolyte Layer

[0107] In the process of manufacturing the negative electrode of Comparative Example 2, a TEM Cu grid was inserted onto a copper foil having a thickness of 10 μm, and electrodeposition and formation of a lithium metal layer were performed. As a result, a lithium metal layer was also formed on the Cu grid, and after collecting and washing this sample, it was placed in liquid nitrogen and transferred to a TEM analysis device, and the lithium metal layer formed on the Cu grid and the solid electrolyte interface film on its surface were analyzed using cryo-TEM. For reference, the TEM image is shown in Fig. 3.

[0108] Referring to Fig. 3, the formation of a thinner solid electrolyte interface film was confirmed on the surface of the lithium metal layer close to the Cu grid (i.e., the surface of the lithium metal layer with a thin thickness), and the formation of a thicker solid electrolyte interface film was confirmed on the surface of the lithium metal layer far from the Cu grid (i.e., the surface of the lithium metal layer with a large thickness).

[0109] Through this, it was confirmed that the lithium metal layer having a thinner thickness and a large surface work function had relatively low reactivity, resulting in the formation of a thinner solid electrolyte interfacial film.

[0110]

[0111] Experimental Example 3: Evaluation of the Lifetime Characteristics of Lithium Metal Secondary Batteries (Lithium-Free Secondary Batteries)

[0112] First, LiNi is used as the positive electrode active material. 0.5 Co 0.2 Mn 0.3 O2 was used, carbon black was used as a conductive material, and PVDF polymer was used as a binder. The positive electrode active material: conductive material: binder were mixed in a weight ratio of 90:5:5 in an N-methylpyrrolidone solvent to prepare a slurry composition, which was then applied to an aluminum current collector, and dried and rolled in a vacuum oven at 110°C to prepare a positive electrode.

[0113] An electrode assembly was manufactured by interposing a porous polypropylene separator between the positive electrode and the negative electrode manufactured in Reference Example 1 or Comparative Example 1, and after positioning this inside the case, an electrolyte in which a lithium salt of LiFSI was dissolved in dimethoxyethane (DME) at a concentration of 4 M was injected to manufacture a full cell of a lithium metal secondary battery (N / P: 2.08, electrolyte / positive electrode ratio: 2.0 g / Ah).

[0114] For these lithium metal secondary batteries, a charge and discharge test was performed by charging to 4.35 V at 25°C in CC mode and 0.2 C, and discharging to 3.0 V at a constant current of 1.0 C. The charge and discharge test was performed for 100 cycles, and the capacity retention rate for each cycle was measured, and the measurement results are shown in Fig. 4. As the charge and discharge proceeded, a lithium metal layer was electrodeposited and formed on the negative electrode, and a lithium metal layer having the same shape as that of Example 1 and Comparative Example 2 was formed.

[0115] Referring to FIG. 4, it was confirmed that a lithium metal secondary battery (lithium-free secondary battery) including the negative electrode of Reference Example 1 (Example 1) exhibits superior life characteristics compared to a battery including the negative electrode of the Comparative Example.

Claims

1. A conductive metal layer including a first metal; An interface layer formed on a conductive metal layer and including a second metal oxide having a work function greater than that of the first metal; and A negative electrode for a lithium metal secondary battery, comprising a lithium metal layer formed on the above interfacial layer and having a surface work function exceeding 3.0 eV.

2. In the first paragraph, the first metal is a negative electrode for a lithium metal secondary battery containing copper (Cu).

3. A negative electrode for a lithium metal secondary battery, wherein the second metal oxide in the first paragraph has a work function of 6.0 eV or more and a band gap of 2.7 eV or more.

4. A negative electrode for a lithium metal secondary battery according to claim 1, wherein the second metal oxide comprises nickel oxide (NiO), molybdenum oxide (MoO3), or tungsten oxide (WO3).

5. A negative electrode for a lithium metal secondary battery, wherein the interface layer has a thickness of 10 nm to 1 μm in the first paragraph.

6. A negative electrode for a lithium metal secondary battery, wherein the lithium metal layer has a thickness of 10 nm to 10 μm in the first paragraph.

7. A negative electrode for a lithium metal secondary battery, wherein the lithium metal layer is deposited and formed on the interface layer according to charging and discharging of the secondary battery.

8. In the 7th paragraph, the lithium metal layer has a density of 1 to 2.5 mAh / cm 2 A negative electrode for a lithium metal secondary battery, which is electrodeposited and formed on the above interfacial layer with a capacity of .

9. The cathode of any one of paragraphs 1 to 8; An anode facing the cathode; An electrolyte comprising a lithium salt and a non-aqueous organic solvent; and A lithium metal secondary battery comprising a separator between the positive and negative electrodes.

10. In the 9th paragraph, the positive electrode comprises a lithium metal oxide including lithium and two or more metals selected from the group consisting of nickel, manganese, cobalt and aluminum, A lithium metal secondary battery, wherein the lithium metal oxide contains nickel in an amount of 50 mol% or more based on the total metal content excluding lithium.

11. A lithium metal secondary battery according to claim 9, wherein the lithium salt is included in the electrolyte at a concentration of 1 M to 6 M.

12. A lithium metal secondary battery according to claim 9, wherein the non-aqueous organic solvent comprises at least one selected from the group consisting of carbonate solvents, ether solvents, nitrile solvents, phosphate solvents, and sulfone solvents.

Images