Negative electrode for lithium secondary battery and method for manufacturing same

Abstract

The present invention provides a coating layer of a novel composition on a lithium metal negative electrode for use in a lithium secondary battery, thereby achieving the effect of improving the coulombic efficiency and lifespan of a lithium secondary battery using the lithium metal negative electrode. A coating layer of a negative electrode for a lithium secondary battery according to an aspect of the present invention is characterized by including fluorine and boron.

Description

Negative electrode for lithium secondary battery and method for manufacturing the same

[0001] The present invention relates to a negative electrode for a lithium secondary battery and a method for manufacturing the same.

[0002] This application claims priority based on Korean Patent Application No. 2024-0185112 filed with the Korean Intellectual Property Office on December 12, 2024, and all contents disclosed in the specification of said application are incorporated into this application.

[0003] With the rapid development of the electronics, telecommunications, and computer industries, the application fields of energy storage technology are expanding to include camcorders, mobile phones, laptops, PCs, and even electric vehicles. Accordingly, the development of high-performance rechargeable batteries that are lightweight, long-lasting, and highly reliable is underway, and in particular, lithium-ion batteries are gaining attention as batteries that meet these demands.

[0004] A lithium secondary battery has a structure in which an electrode assembly comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode is stacked or wound, and the electrode assembly is embedded in a battery case and a non-aqueous electrolyte is injected into the interior. The lithium secondary battery produces electrical energy through oxidation and reduction reactions when lithium ions are inserted into and extracted from the positive and negative electrodes.

[0005] Typically, lithium oxide, transition metal oxide, metal chalcogenide compounds, conductive polymers, and sulfur-sulfur bond-containing compounds are mainly used as active materials for the positive electrode of lithium secondary batteries, while lithium metal, carbon, and silicon are mainly used as active materials for the negative electrode.

[0006] In lithium secondary batteries using lithium metal as the negative electrode, electron density non-uniformity may occur on the lithium metal surface due to various factors during operation. For example, lithium metal forms an oxide film on its surface even when exposed to trace amounts of oxygen and / or moisture. This oxide film hinders the electrodeposition and detachment of the lithium metal negative electrode during charging and discharging, causing non-uniform battery density on the surface and potentially leading to the formation of tree-branch-shaped lithium dendrites. Lithium dendrites not only increase the battery's resistance but, in severe cases, can cause damage to the separator and lead to a short circuit. Consequently, the internal temperature of the battery rises, posing a risk of explosion and fire.

[0007] Furthermore, lithium used in electrodes, particularly lithium electrodes, has high reactivity with electrolyte components. Consequently, when lithium metal comes into contact with electrolyte components, a film referred to as a passivation layer is formed through a spontaneous reaction. Since the passivation layer formed on the lithium surface undergoes repeated destruction and formation during charging and discharging, repeated charging and discharging of the battery leads to an increase in the passivation layer components within the lithium anode and causes the electrolyte to become depleted. Additionally, some reduced substances in the electrolyte cause side reactions with the lithium metal, accelerating the consumption of lithium. As a result, this can lead to a decrease in the battery's lifespan.

[0008] Accordingly, various studies have been conducted to stabilize lithium metal, and as part of these efforts, a method of forming a protective layer at the location in contact with the anode has been proposed. For example, due to its high chemical stability, research is actively underway to apply LiF as a protective layer for lithium anodes.

[0009] To solve the aforementioned problem, the present invention aims to provide a lithium metal film layer of a novel composition capable of improving the electrochemical performance of a lithium secondary battery using a lithium anode, and a method for manufacturing the same.

[0010] In particular, the present invention aims to provide a method for forming a film layer having a novel composition advantageous for the electrodeposition and desorption of lithium while chemically removing an oxide layer formed on the surface of a lithium metal layer. By doing so, the invention aims to provide a negative electrode and a method for manufacturing the same that can reduce resistance caused by the oxide layer on the surface of the lithium metal layer and further improve the lifespan and Cholesterol efficiency of a lithium secondary battery.

[0011] In order to achieve the above objective,

[0012] According to one aspect of the present invention, a negative electrode for a lithium secondary battery of the following embodiments is provided.

[0013] The negative electrode for a lithium secondary battery according to the first embodiment is,

[0014] lithium metal layer, and

[0015] It includes a film layer formed on at least one surface of the lithium metal layer, and

[0016] The above film layer comprises a fluorine element (F) and a boron element (B), and

[0017] The relative concentration ratio of the fluorine element to the boron element is set to be 1 or less.

[0018] According to the second embodiment, in the first embodiment,

[0019] The ratio of the relative concentration of the fluorine element to the boron element may be 0.8 or less.

[0020] According to the third embodiment, in the first embodiment or the second embodiment,

[0021] The ratio of the relative concentration of the fluorine element to the boron element may represent the ratio of the content of the constituent elements obtained by the elemental analysis method.

[0022] According to the fourth embodiment, in any one of the first to third embodiments,

[0023] In addition to the fluorine element (F) and the boron element (B), the above constituent elements include,

[0024] It may further include one or more elements selected from the group consisting of carbon (C), nitrogen (N), oxygen (O), and bromine (Br).

[0025] According to the fifth embodiment, in any one of the first to fourth embodiments,

[0026] The above elemental analysis method may be energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), or a combination thereof.

[0027] According to the 6th embodiment, in any one of the 1st to 5th embodiments,

[0028] The above film layer further contains oxygen elements, and

[0029] The ratio of the relative concentration of the boron element to the oxygen element may be 0.1 to 1.

[0030] According to the seventh embodiment, in any one of the first to sixth embodiments,

[0031] The above film layer further contains oxygen elements, and

[0032] Based on the total constituent elements of the film layer, the relative concentration of the oxygen element may be 20% to 80%.

[0033] According to the eighth embodiment, in any one of the first to seventh embodiments,

[0034] The above film layer may include lithium fluoride (LiF) and borate.

[0035] According to the ninth embodiment, in any one of the first to eighth embodiments,

[0036] The above lithium metal layer may be a lithium foil or a lithium alloy foil.

[0037]

[0038] According to another aspect of the present invention, a method for manufacturing a negative electrode for a lithium secondary battery according to the following embodiments is provided.

[0039] The method for manufacturing a negative electrode for a lithium secondary battery according to the 10th embodiment is,

[0040] A method for manufacturing a negative electrode for a lithium secondary battery, wherein a film layer comprising a fluorine element (F) and a boron element (B) is formed on a lithium metal layer, and the ratio of the relative concentration of the fluorine element to the boron element is 1 or less.

[0041] (S1) A step of preparing a boron solution by dissolving boron halides in an aprotic organic solvent,

[0042] (S2) A step of reacting the boron solution and the lithium metal layer under an inert atmosphere,

[0043] (S3) After the above reaction, a step of contacting the aprotic organic solvent with the lithium metal layer,

[0044] (S4) After the above contact, the step of reacting the lithium metal layer with a fluorine-based organic solvent under an inert atmosphere may be included.

[0045] According to the 11th embodiment, in the 10th embodiment,

[0046] The contact in step (S3) above can be performed for a period of time of 10 hours or less.

[0047] According to the 12th embodiment, in the 10th embodiment or the 11th embodiment,

[0048] The contact in step (S3) above can be performed for a period of time of 1 hour or less.

[0049] According to the 13th embodiment, in any one of the 10th to 12th embodiments,

[0050] The above aprotic organic solvent may include C4 to C30 aliphatic acyclic compounds.

[0051] According to the 14th embodiment, in any one of the 10th to 13th embodiments,

[0052] The above boron halide may include boron tribromide (BBr3).

[0053] According to the 15th embodiment, in any one of the 10th to 14th embodiments,

[0054] The above fluorine-based organic solvent may include a fluorine-containing carbonate solvent.

[0055] According to the 16th embodiment, in any one of the 10th to 15th embodiments,

[0056] The above step (S4) may involve reacting the lithium metal layer with the electrolyte for a lithium secondary battery containing the above fluorine-based organic solvent and lithium salt.

[0057] According to the 17th embodiment, in any one of the 10th to 16th embodiments,

[0058] The reaction of step (S2) above can be carried out at a temperature of 45°C or higher and 85°C or lower.

[0059]

[0060] According to another embodiment of the present invention, lithium secondary batteries of the following embodiments are provided.

[0061] A lithium secondary battery according to the 18th embodiment is,

[0062] It includes a cathode, an anode, and an electrolyte according to any one of the first to ninth embodiments.

[0063] According to one aspect, the cathode of the present invention can exhibit the effect of suppressing and preventing lithium dendrite growth on the surface of the cathode due to a film layer formed on the cathode for a lithium secondary battery.

[0064] Through this, a lithium secondary battery with the negative electrode of the present invention can exhibit the effect of improving lifespan and Coulomb efficiency.

[0065] In particular, the negative electrode for a lithium secondary battery according to the present invention includes a boron element (B) as a component of the protective layer, thereby having the advantage of contributing to the improvement of battery performance even without containing a large amount of fluorine element (F).

[0066] FIG. 1 is a graph showing the results of a lifespan evaluation of a lithium secondary battery using the cathode according to Example 1, Comparative Example 1, and Comparative Example 2 in this specification.

[0067] FIG. 2 is a graph showing the evaluation results of the Coulomb efficiency of a lithium secondary battery using the cathode according to Example 1, Comparative Example 1, and Comparative Example 2 in this specification.

[0068] The present invention will be described in more detail below.

[0069] Lithium secondary batteries have the characteristic that lithium ions are reduced on the surface of the negative electrode during charging and discharging, and lithium metal accumulates on the negative electrode through repeated charging and discharging.

[0070] According to one aspect of the present invention, a negative electrode for a lithium secondary battery is provided to improve the lifespan and Coulomb efficiency of a lithium secondary battery using the same by suppressing and preventing the formation of dendrites of lithium ions on the surface of a negative electrode comprising a lithium metal layer.

[0071] First, in one embodiment of the present invention, the cathode may be provided as a free-standing film comprising a lithium metal layer and a film layer without a separate support.

[0072] In another embodiment of the present invention, the negative electrode may be provided in a form comprising a lithium metal layer and a film layer on a support. In this case, the support may be a polyolefin porous support used as a current collector or separator in conventional lithium secondary battery electrodes, but is not limited thereto.

[0073] In one embodiment of the present invention, the current collector supports the lithium metal layer and is not particularly limited as long as it has high conductivity without causing chemical changes in the lithium secondary battery using it. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, a copper or stainless steel surface treated with carbon, nickel, silver, etc., or an aluminum-cadmium alloy may be used as the current collector.

[0074] A negative electrode for a lithium secondary battery according to one aspect of the present invention comprises a lithium metal layer and a film layer formed on at least one surface of the lithium metal layer. Here, the relative concentration ratio of the fluorine element to the boron element is 1 or less.

[0075] In one embodiment of the present invention, the lithium metal layer is a configuration generally used as a cathode in a cathode made of lithium metal as the main raw material, and, for example, a lithium foil or a lithium alloy foil may be used.

[0076] In this specification, the term 'lithium foil' refers to a thin film composed of lithium metal with a purity of 99.5% or higher, specifically 99.9% or higher, in which only trace amounts of impurities are present.

[0077] In this specification, the term 'lithium alloy foil' refers to a thin film composed solely of lithium and a lithium alloy alloyed with a material that forms an alloy with lithium, such as silicon, tin, indium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, aluminum, or two or more of these materials, or composed of a lithium alloy with a purity of 99.5% or higher, specifically 99.9% or higher, in which only trace amounts of impurities are present.

[0078] Research has continued to provide a film layer rich in lithium fluoride (LiF) to improve performance in a conventional negative electrode comprising a lithium metal layer, for example, a film layer having a composition in which the relative concentration of a fluorine element exceeds 50% based on the total content of the constituent elements of the film layer. However, according to one aspect of the present invention, a negative electrode for a lithium secondary battery is provided having a film layer of a novel composition that can achieve excellent performance improvement even without being rich in lithium fluoride.

[0079] According to one embodiment of the present invention, the relative concentration of constituent elements within the film layer can be measured using a conventional elemental analysis method with respect to the composition of the film layer. In this case, the cathode according to one embodiment of the present invention has the characteristic that the relative concentration of fluorine elements within the film layer is equal to or less than the relative concentration of boron elements. More specifically, it can be characterized in that the relative concentration of fluorine elements within the film layer is less than the relative concentration of boron elements.

[0080] Specifically, in one embodiment of the present invention, the negative electrode for a lithium secondary battery may be composed only of a lithium metal layer and a film layer formed on the surface of the lithium metal layer. In this case, a lithium metal foil or a lithium alloy foil may be used as the lithium metal layer, and thus, when analyzing the constituent elements of the negative electrode, elements other than those constituting the lithium metal foil or the lithium alloy foil may be defined as elements originating from the film layer.

[0081] In one embodiment of the present invention, the elemental analysis method may utilize, for example, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), or a combination thereof. For the EDS method, a known EDS analyzer may be used, for example, an EDS instrument combined with Scanning Electron Microscopy (SEM) may be used. For example, Jeol’s SEM-EDS instrument (JSM-7200F) may be used for the EDS method, and Thermo Fisher Scientific’s XPS instrument (K-AlPHA+) may be used for the XPS method, but the present invention is not limited thereto.

[0082] In one embodiment of the present invention, the elemental analysis method can measure the types of constituent elements contained within the film layer and the relative concentrations of the constituent elements. For example, when EDS and / or XPS analysis is performed on the entire cathode, constituent elements and their relative concentrations can be measured according to depth from the surface of the cathode. In particular, when the elemental analysis method is performed on the entire cathode, the boundary between the lithium metal layer and the film layer may be unclear, such as when lithium elements originating from the lithium metal layer are detected in the film layer as well. At this time, according to one embodiment of the present invention, the film layer is defined as the maximum depth from the surface of the cathode where elements originating from the boron element precursor are detected, and the lithium metal layer is distinguished from the depth where elements originating from the boron element precursor are not detected, thereby allowing for the measurement of the relative concentrations of the constituent elements.

[0083] For example, the film layer may be formed using boron tribromide (BBr3) as described below. In this case, the film layer may contain boron element (B) and bromine element (Br). When performing elemental analysis on the cathode formed with the film layer, it is preferable to distinguish the film layer from the surface of the cathode up to a depth where at least one of the boron element (B) and the bromine element (Br) is detected, and to distinguish the lithium metal layer from a depth where neither the boron element nor the bromine element is detected, in order to measure the relative concentration of the constituent elements.

[0084] According to one embodiment of the present invention, when the relative concentrations of constituent elements according to the depth of the film layer are each measured, the ratio of the relative concentrations among the constituent elements can be calculated as the ratio of the sum of the relative concentrations of the corresponding constituent elements at the entire depth of the film layer.

[0085] In one embodiment of the present invention, the ratio of the relative concentration of the fluorine element to the boron element within the film layer is 1 or less, and may be, for example, 0.8 or less. Specifically, the ratio of the relative concentration of the fluorine element to the boron element may be 0.6 or less, 0.5 or less, 0.1 or less, or 0.10 or less. More specifically, the ratio of the relative concentration of the fluorine element to the boron element may be, for example, 0.01 to 1, 0.05 to 0.5, 0.09 to 0.10, or 0.5 to 0.6, but the present invention is not limited thereto.

[0086] As described above, unlike conventional research, the present invention comprises a film layer containing a boron element (B) on the surface of a lithium metal layer, rather than a film layer rich in lithium fluoride. Due to the empty orbital state of the boron element, even if electrons are accepted in the negative electrode having the lithium metal layer during charging and discharging of a lithium secondary battery, the uniformity of electron density on the surface of the lithium metal layer can be improved. This may result in the effect of suppressing the growth of dendrites on the surface of the lithium metal layer, but the mechanism of the present invention is not limited thereto. Furthermore, the negative electrode having a film layer containing a boron element exhibits faster ion conductivity at the interface between the film layer and the lithium metal layer, and thereby may result in the effect of improving the electrochemical performance of a lithium secondary battery using the negative electrode, but the mechanism of the present invention is not limited thereto.

[0087] In one embodiment of the present invention, the relative concentration of the boron element (B) based on the total constituent elements of the film layer may be, for example, 5% or more, 10% or more, 15% or more, 20% or more, or 25% or more. For example, the relative concentration of the boron element (B) based on the total constituent elements of the film layer may be 5% to 30%, 5% to 25%, 10% to 20%, 15% to 20%, 20% to 30%, or 25% to 30%, but the present invention is not limited thereto.

[0088] In one embodiment of the present invention, each of the constituent elements of the film layer may exist in an oxide form. In this case, the film layer may further include an oxygen element (O) as a constituent element.

[0089] In one embodiment of the present invention, the film layer may further include an oxygen element, and the boron element may be contained in an amount such that the ratio of the relative concentration of the boron element to the oxygen element is, for example, 0.1 to 1. For example, the ratio of the relative concentration of the boron element to the oxygen element in the film layer may be 0.1 to 1, 0.1 to 0.8, 0.1 to 0.7, 0.2 to 0.6, 0.2 to 0.5, 0.2 to 0.4, 0.2 to 0.3, 0.5 to 0.9, 0.6 to 0.8, or 0.6 to 0.7, but the present invention is not limited thereto.

[0090] In one embodiment of the present invention, the relative concentration of the oxygen element (O) based on the total constituent elements of the film layer may be, for example, 20% to 90%, 20% to 80%, 30% to 90%, 40% to 80%, 45% to 70%, 50% to 60%, 50% to 55%, or 40% to 45%, but the present invention is not limited thereto.

[0091] In one embodiment of the present invention, the film layer may include fluorine fluoride (LiF) and borate, and, although not limited thereto, the fluorine element may be derived from the fluorine fluoride, and the boron element may be derived from the borate.

[0092] Referring to the manufacturing method described below, the fluorinated fluorine may be formed by the reaction between a lithium metal layer and a fluorine-based organic solvent, and the borate may be formed by the reaction between a lithium metal layer and a boron halide, but the present invention is not limited thereto.

[0093] Next, a method for manufacturing a negative electrode for a lithium secondary battery according to one aspect of the present invention will be described in detail. However, the method for manufacturing the negative electrode for a lithium secondary battery is not limited to the method described below.

[0094]

[0095] A method for manufacturing a negative electrode for a lithium secondary battery according to another aspect of the present invention comprises forming a film layer containing a fluorine element (F) and a boron element (B) on a lithium metal layer, wherein the relative concentration ratio of the fluorine element to the boron element is 1 or less, and the method comprises the following steps.

[0096] (S1) A step of preparing a boron solution by dissolving a boron halide in an aprotic organic solvent,

[0097] (S2) A step of reacting the boron solution and the lithium metal layer under an inert atmosphere,

[0098] (S3) After the above reaction, a step of contacting the aprotic organic solvent with the lithium metal layer,

[0099] (S4) After the above contact, a step of reacting the lithium metal layer with a fluorine-based organic solvent under an inert atmosphere.

[0100] According to one embodiment of the present invention, the lithium metal layer may be used to sequentially react with a boron halide and a fluorine-based organic solvent to form a film layer containing a fluorine element (F) and a boron element (B) on the surface of the lithium metal layer.

[0101] In one embodiment of the present invention, the step (S1) may be a step of preparing a reactant to remove at least a portion of the lithium oxide formed on the surface of the lithium metal layer and to form a compound in the form of borate.

[0102] In this specification, the term 'borate' refers to a boron-oxygen anion comprising a boron element and an oxygen element, such as, for example, as a non-limiting example, orthoborate (BO3 3- ), metaborate (BO2 - ), tetraborate (tetraborate, B4O7 2- The term collectively refers to compounds having a form such as ), as well as salt compounds and ester compounds of the boron-oxygen anions. Examples of salt compounds of the boron-oxygen anions include lithium metaborate (Li2(BO2)), and examples of boron-oxygen ester compounds include trimethylborate (B(OCH3)3), but the present invention is not limited thereto.

[0103] According to one embodiment of the present invention, in step (S1), the 'boron halides' may be used as a precursor for forming the borate, but the present invention is not limited thereto. The boron halides are compounds represented by BX3 (X = F, Br, Cl, I, or a combination of two or more selected from these). The boron halides may be, for example, boron trifluoride (BF3), boron tribromide (BBr3), boron trichloride (BCl3), or a mixture of two or more of these.

[0104] In one embodiment of the present invention, the boron halide may include boron tribromide.

[0105] According to one embodiment of the present invention, the boron halide is a compound having strong basic properties and can perform the role of providing a borate to the film layer by removing the lithium oxide layer formed on the lithium metal layer and substituting the lithium oxide in the form of a borate. Accordingly, it may be preferable for the boron halide to be provided dissolved in an aprotic organic solvent so that the lithium metal layer does not react with additional oxygen when the lithium metal layer and the boron halide react.

[0106] In one embodiment of the present invention, the aprotic organic solvent may preferably be a compound consisting specifically of only aliphatic chains. The compound consisting only of aliphatic chains may be, for example, a C4 to C30 aliphatic acyclic compound, and specifically, a C4 to C20 aliphatic acyclic compound, a C4 to C10 aliphatic acyclic compound, a C4 to C8 aliphatic acyclic compound, or a mixture of two or more selected from these may be used. The aliphatic acyclic compound may be an aliphatic straight-chain type, an aliphatic branched-chain type, or a mixture thereof, but the present invention is not limited thereto.

[0107] In one embodiment of the present invention, the aprotic organic solvent may include hexane, for example, n-hexane, but is not limited to any solvent that can dissolve the boron halide without reacting with the lithium metal layer to form a lithium oxide as described above.

[0108] In one embodiment of the present invention, the concentration of the boron solution is not particularly limited as long as it is sufficient to dissolve the boron halide.

[0109] In one embodiment of the present invention, the boron solution may be a solution in which hexane is used as an aprotic organic solvent and a boron halide is dissolved at a concentration of 0.5 M to 3 M, for example, 1 M to 2 M or 1 M to 1.5 M.

[0110] In one embodiment of the present invention, the lithium metal layer may be provided as a free-standing film comprising lithium metal or a lithium metal alloy without a separate support.

[0111] In another embodiment of the present invention, the lithium metal layer may be provided in a form comprising lithium metal or a lithium metal alloy on a support.

[0112] At this time, the support may be a porous polymer support used as a current collector or separator in conventional lithium secondary battery electrodes, but is not limited thereto.

[0113] In one embodiment of the present invention, the lithium metal layer may comprise a current collector and a lithium metal or lithium metal alloy formed on at least one surface of the current collector. The current collector supports the lithium metal layer and is not particularly limited as long as it has high conductivity without causing chemical changes in the lithium secondary battery using it. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., and aluminum-cadmium alloy may be used.

[0114] In one embodiment of the present invention, the current collector may be a copper foil with a thickness of, for example, 6 to 30 μm or, for example, 10 μm, but the present invention is not limited thereto.

[0115] In one embodiment of the present invention, the thickness of the lithium metal layer may be, for example, 20 μm or more. For example, the thickness of the lithium metal layer may be 20 μm to 100 μm, 20 μm to 80 μm, 20 μm to 70 μm, 20 μm to 60 μm, 20 μm to 55 μm, 25 μm to 55 μm, 30 μm to 50 μm, 35 μm to 45 μm, or 40 μm to 45 μm. When the thickness of the lithium metal layer is within the above-described range, it may exhibit advantageous effects in terms of the packing density of the battery and the energy density of the battery, but the present invention is not limited thereto.

[0116] Next, the above step (S2) is a step of reacting the boron solution prepared above with the lithium metal layer.

[0117] The above step (S2) can also be performed under an inert atmosphere so that the lithium metal layer does not react with external oxygen or moisture to form an additional lithium oxide layer, and so as not to cause surface changes of the lithium metal layer due to external gases. It may be preferable for the inert atmosphere to be formed by, for example, argon (Ar).

[0118] In one embodiment of the present invention, step (S2) can be performed by depositing the lithium metal layer in the boron solution at a temperature above room temperature. For example, step (S2) can be performed at a temperature of 45°C or higher and 85°C or lower, and specifically, it may be performed at a temperature of 50°C to 80°C, 55°C to 75°C, 55°C to 70°C, or 60°C to 65°C.

[0119] According to one embodiment of the present invention, through the reaction of step (S2), a lithium metal or lithium alloy foil may react with a boron halide to form lithium halide (LiX, X = F, Br, Cl, I), or a lithium oxide layer formed on a lithium metal layer may be decomposed and a borate may be formed.

[0120] In one embodiment of the present invention, the step (S2) may be performed by immersing the lithium metal layer in the boron solution or by coating the boron solution on at least one surface of the lithium metal layer according to a conventional coating method, and is not particularly limited to such method.

[0121] The subsequent (S3) step may be intended to allow the previously formed material to be stably distributed on the lithium metal layer by contacting the reaction product of (S2) with an aprotic organic solvent.

[0122] In one embodiment of the present invention, step (S3) may involve washing the lithium metal layer after step (S2) using the aprotic organic solvent.

[0123] The contact in step (S3) above is not specifically limited to a specific time unless the borate formed on the lithium metal layer is left for an excessive amount of time, and can be performed for, for example, less than 10 hours, less than 5 hours, less than 2 hours, or less than 1 hour.

[0124] Subsequently, the above step (S4) may be a step for further stabilizing the film layer when the negative electrode comes into contact with the electrolyte during the manufacture of the electrode assembly by reacting the lithium metal layer with a fluorine-based organic solvent to contain a predetermined fluorine element (F).

[0125] In one embodiment of the present invention, the fluorine-based organic solvent is typically used as an electrolyte for a lithium secondary battery, and any organic solvent containing a fluorine element can be used without particular limitation.

[0126] For example, the above-mentioned fluorine-based organic solvent may be a carbonate-based organic solvent containing a fluorine element, such as fluoroethylene carbonate (FEC), but is not limited thereto.

[0127] In one embodiment of the present invention, it is preferable that step (S4) is also performed under an inert atmosphere, as in step (S2).

[0128] In one embodiment of the present invention, the step (S4) may be performed by reacting the lithium metal layer only with the fluorine-based organic solvent, or by reacting the lithium metal layer with the electrolyte of a lithium secondary battery to which the negative electrode is applied.

[0129] For example, the electrolyte of the lithium secondary battery may contain the fluorine-based organic solvent and the lithium salt. Accordingly, the step (S4) can be performed by reacting the lithium metal layer with the electrolyte for the lithium secondary battery containing the fluorine-based organic solvent and the lithium salt.

[0130] In one embodiment of the present invention, the lithium salt can be used without limitation as long as it performs the role of a lithium ion transport and electron transport medium and can be included as a lithium salt in the electrolyte composition of a lithium secondary battery.

[0131] In one embodiment of the present invention, the electrolyte may further include an additive for additional stabilization of the cathode in addition to a non-aqueous solvent and a lithium salt. The additive may be used without limitation as long as it is capable of being included for the stabilization of lithium as an electrolyte composition of a lithium secondary battery. For example, the additive may be nitrates, but the present invention is not limited thereto.

[0132] As described above, according to one aspect of the present invention, a negative electrode is provided that has a film layer of a novel composition formed thereon, which is used in a lithium secondary battery and has excellent effects in preventing and controlling the degradation of the negative electrode.

[0133]

[0134] According to another aspect of the present invention, a lithium secondary battery having the above-described negative electrode is provided.

[0135] The above lithium secondary battery includes the aforementioned negative electrode, positive electrode, and electrolyte.

[0136] Specifically, a lithium secondary battery is provided comprising an electrode assembly including the aforementioned cathode, a positive electrode, and, if necessary, a separator or a separating layer, an electrolyte, and a battery case housing them.

[0137] According to one embodiment of the present invention, the cathode may further include conventional configurations that can be used in a lithium secondary battery in addition to the configuration described above, and is not particularly limited to the additional configurations provided that they do not impede the purpose of the present invention.

[0138] In one embodiment of the present invention, when the lithium secondary battery includes a separator, it may be preferable to assemble the negative electrode film layer so that it is positioned on a surface facing the separator, but the present invention is not limited thereto.

[0139] In one embodiment of the present invention, the positive electrode can be used without specific limitations on its composition and structure, as long as it comprises a positive electrode active material used in a lithium secondary battery.

[0140] In one embodiment of the present invention, the lithium secondary battery may be a lithium-sulfur battery comprising a sulfur-based compound containing a sulfur(S)-sulfur(S) bond as a positive electrode active material, but the present invention is not limited thereto.

[0141] The term "sulfur-based active material" applicable to the positive electrode of the above lithium-sulfur battery is a term encompassing, for example, inorganic sulfur (S8) and / or sulfur-containing compounds. The sulfur-containing compounds include, for example, Li2Sn (n≥1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers ((C2S x ) n , x=2.5 to 50, n≥2) or a mixture thereof may be included, but the present invention is not limited thereto.

[0142] In one embodiment of the present invention, the positive electrode of the lithium sulfur battery comprises inorganic sulfur and / or a sulfur-containing compound as an active material, and may comprise a carbon material as a conductive material to ensure the electrical conductivity of sulfur. For example, the positive electrode may be provided in a form including a current collector and a positive electrode active material layer provided on at least one surface of the current collector, comprising inorganic sulfur (S8) and a carbon-based conductive material such as carbon black, but the present invention is not limited thereto.

[0143] In one embodiment of the present invention, the sulfur-based active material is an active material comprising sulfur and a carbon material, and may include, for example, a composite in which inorganic sulfur (S8) and / or a sulfur-containing compound is composited with a porous carbon material. For example, a sulfur-carbon composite prepared by mixing the inorganic sulfur (S8) and / or a sulfur-containing compound with a porous carbon material and then heat-treating it at a temperature at which sulfur melts to diffuse sulfur into the pores and / or outer surface of the porous carbon material may be used as an active material, but the present invention is not limited thereto.

[0144] In one embodiment of the present invention, the sulfur-based active material may include a material comprising a carbon-sulfur-carbon (C-Sn-C, n≥1) bond. Here, "carbon-sulfur-carbon (C-Sn-C, n≥1) bond" refers to a chemical bond between [carbon atom]-[n sulfur atoms]-[carbon atoms]. Here, the chemical bond may refer to a covalent bond.

[0145] In one embodiment of the present invention, the sulfur-based active material comprises carbon-sulfur-carbon (C-Sn-C, n≥1) bonds, and may include a material containing sulfur atoms that are covalently bonded to and fixed with carbon in a polymer matrix by, for example, mixing a polymer matrix and sulfur (S8) and heat-treating. The material containing carbon-sulfur-carbon (C-Sn-C, n≥1) bonds is a material characterized by not containing free sulfur and containing sulfur atoms fixed in a polymer matrix. For example, the active material may include a material known as sulfurized polyacrylonitrile (SPAN), but the present invention is not limited thereto.

[0146] In one embodiment of the present invention, the polyacrylonitrile sulfide is a material having a structure in which sulfur (S) is chemically bonded or inserted into an acrylonitrile polymer chain, and can generally be prepared by heat-treating an acrylonitrile polymer with sulfur (e.g., 300 to 600 °C). During the heat treatment process, sulfur can form covalent or quasi-covalent bonds with the nitrogen and carbon skeletons within the polymer chain to form a stable composite structure. The acrylonitrile sulfide has the advantage of having sulfur fixed in the polymer matrix, which suppresses the leaching of sulfur or the dissolution of lithium polysulfide during the charging and discharging process, and provides excellent electrochemical stability.

[0147] More specifically, SPAN (polyacrylonitrile sulfide) can be a polymer composite in which sulfur is strongly covalently bonded to a polyacrylonitrile (PAN) polymer backbone in the form of oligos, or sulfide chains. It is formed by sulfur chemically bonding to the nitrogen atoms of the nitrile group or immediately adjacent carbon atoms of the PAN polymer, thereby covalently bonding sulfur chains in the form of oligos to the polymer chain. On average, the sulfur chain consists of about four sulfur atoms and is fixed in a form that docks to the carbon-nitrogen chain of the PAN polymer; it may include not only CSC bonds but also SS bonds, NS bonds, and NC-S bonds. In this way, within SPAN, sulfur is firmly bound to the polymer backbone through strong covalent bonds with some carbon atoms in the carbon chain or nitrogen atoms in the nitrile group. Consequently, when SPAN is used as an active material, free sulfur elements are not present in the active material, and since sulfur is stably fixed within the polymer, the leaching of polysulfides from the electrode can be effectively suppressed.

[0148] As such, since SPAN does not generate glassy sulfur or long-length, highly solid-soluble lithium polysulfides, polysulfide leaching from the electrode is significantly reduced. This can substantially mitigate the problems of capacity degradation and shortened lifespan in lithium secondary batteries (specifically, lithium-sulfur batteries) caused by the shuttle phenomenon. Consequently, it enables more stable and longer charge-discharge cycles and offers the advantage of greatly mitigating side reactions within the electrode and electrolyte caused by the dissolution of lithium polysulfides.

[0149] In one embodiment of the present invention, the separator separates or insulates the anode and the cathode from each other and enables lithium ion transport between the anode and the cathode; it may be made of a porous, non-conductive, or insulating material and may be used without special limitations as long as it is commonly used as a separator in a lithium secondary battery. Such a separator may be an independent component such as a film, or it may be a coating layer added to the anode and / or cathode. When the separator is provided as a coating layer added to the electrode, it may also be referred to as a separating layer, but the present invention is not limited thereto.

[0150] In one embodiment of the present invention, the electrolyte may include a lithium salt as a medium through which ions involved in the electrochemical reaction of a lithium secondary battery can move, and may further include a non-aqueous solvent as needed depending on the configuration of the battery.

[0151] The above electrolyte is not particularly limited as long as it has a composition that can be used in a lithium secondary battery.

[0152] In one embodiment of the present invention, the lithium secondary battery may have various shapes, for example, coin type, pouch type, or cylindrical type, but is not limited thereto.

[0153]

[0154] Hereinafter, the present invention will be described in more detail with reference to examples to aid in understanding the invention. The following examples are merely illustrative of the invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the invention, and that such variations and modifications fall within the scope of the appended claims.

[0155] [Formation of a film layer]

[0156] Comparative Example 1

[0157] A lithium foil with a thickness of 45 μm was prepared as a lithium metal layer without a separate support.

[0158]

[0159] Comparative Example 2

[0160] A 1M BBr3 in hexane solution was prepared as a boron solution by dissolving 1M boron tribromide (BBr3) in n-hexane inside a glove box under an argon atmosphere. While maintaining an argon atmosphere, the lithium metal layer prepared in Comparative Example 1 was immersed in the boron solution, and a first reaction was carried out at a temperature of 60°C for 3 hours.

[0161] Afterward, the lithium metal layer was immersed in n-hexane for 24 hours, removed, and then immersed in an electrolyte containing 1 M LiPF6 dissolved in a mixed solvent (50 / 50 v / v) of ethylene carbonate (EC) and diethyl carbonate (DEC) as a non-aqueous solvent and 10 wt% fluoroethylene carbonate (FEC) to perform a secondary reaction for 1 hour.

[0162]

[0163] Comparative Example 3

[0164] The lithium metal layer prepared in Comparative Example 1 above was immersed in fluoroethylene carbonate (FEC) for 1 hour.

[0165]

[0166] Example 1

[0167] A 1M BBr3 in hexane solution was prepared as a boron solution by dissolving 1M boron tribromide (BBr3) in n-hexane inside a glove box under an argon atmosphere. While maintaining an argon atmosphere, the lithium metal layer prepared in Comparative Example 1 was immersed in the boron solution, and a first reaction was carried out at a temperature of 60°C for 3 hours.

[0168] Afterward, the lithium metal layer was immersed in n-hexane for 1 hour, then removed and immersed in fluoroethylene carbonate (FEC) to perform a secondary reaction for 1 hour.

[0169]

[0170] [Analysis of the composition of the film layer]

[0171] The lithium metal layer and the film layer in the cathodes of Comparative Example 1, Comparative Example 2, and Example 1 prepared above were subjected to component analysis using a Jeol SEM-EDS analyzer (JSM-7200F) and are shown in Tables 1 and 2 below.

[0172] Elements, wt% Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Boron (B)---29.77 Carbon (C) 33.23 2.94 3.20 2.23 Nitrogen (N)---0.37 Oxygen (O) 67.13.65 6.5.80 4.2.63 Fluorine (F)-58.38 -2.74 Bromine (Br)-1.50 -1.26 Silicon (Si)-1.32 -- Phosphorus (P)-1.21 -- Total 100 100.00 100.00 100.00

[0173] Element Concentration Ratio Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Fluorine (F) / Boron (B)---0.09 Boron (B) / Oxygen (O)---0.70

[0174] [Performance Evaluation of Lithium Secondary Batteries]

[0175] Using the cathode prepared above, a lithium secondary battery was manufactured by the following method and its performance was evaluated.

[0176] An anode slurry composition is prepared by mixing 96 wt% of a sulfur-carbon composite (S870 wt%) prepared by mixing and heat-treating sulfur (S8) and carbon nanotubes (CNT) as an anode, and 4 wt% of polyacrylate (PAA) as a binder. After applying the anode slurry composition to an aluminum current collector and drying it, a current of 2.6 mAh / cm² is obtained. 2An anode with a loading amount was manufactured. As a separator, a polyethylene separator with a thickness of 11 μm and a porosity of 46 vol% was prepared and interposed between the anode and cathode prepared above. As an electrolyte, an electrolyte solution composed of 1M LiFSI in DME / 2-MeF (4:1 v / v) and 6 wt% LiNO3 was prepared. An assembly assembled in the order of anode / separator / cathode was placed in a pouch-type case, the electrolyte was injected, and the lid was closed to seal it (El / S ratio = 2.4 g / g).

[0177] Next, the lithium secondary battery manufactured as described above was aged at room temperature (23℃) for 24 hours, and then 0.1C discharge and 0.1C charge were repeated twice, followed by 0.3C discharge and 0.2C charge as one cycle. The charge-discharge cycle was repeated, and the performance of the battery was evaluated as follows, and the results are shown in FIGS. 1 and 2.

[0178]

[0179] Lifespan assessment

[0180] The capacity retention rate according to cycle repetition was evaluated based on the discharge capacity measured at the first discharge after activation as 100%.

[0181]

[0182] Coulomb efficiency

[0183] In the charge-discharge cycle after activation, the Coulomb efficiency was measured by evaluating the ratio of discharge capacity to charge capacity within one cycle according to the following equation.

[0184] Coulomb Efficiency (%) = (Discharge Capacity / Charge Capacity) X 100

[0185]

[0186] First, referring to Tables 1 and 2 above, it was confirmed that while the cathode with a lithium foil laminated to a copper current collector contains oxides such as Li2CO3 and LiOH due to the oxidation of the lithium foil, according to Comparative Example 2 and Example 1, in which a film layer is formed on a lithium metal layer, some of the lithium oxide on the surface of the lithium metal layer can be removed. In addition, it was confirmed that Example 1 and Comparative Example 1 differ significantly in the content of fluorine elements based on the presence of boron elements and the total weight of the film layer. Furthermore, according to Comparative Example 3, it was confirmed that a film layer containing fluorine elements is not formed by merely immersing the lithium metal layer in fluoroethylene carbonate (FEC). This was inferred to be because an oxide film had already been formed on the lithium metal layer.

[0187] Next, referring to FIGS. 1 and 2, it was confirmed that in the case of Comparative Example 2, in which a lithium film layer was formed but contains a large amount of fluorine elements, the operating stability of the lithium secondary battery was poor and both the lifespan and Coulomb efficiency were inferior even compared to the untreated lithium metal layer (Comparative Example 1). On the other hand, in the case of Example 1 according to one embodiment of the present invention, it was confirmed that the lifespan characteristics and Coulomb efficiency were stably improved compared to Comparative Example 1 and Comparative Example 2.

Claims

1. Lithium metal layer, and It includes a film layer formed on at least one surface of the lithium metal layer, and The above film layer comprises a fluorine element (F) and a boron element (B), and A negative electrode for a lithium secondary battery, wherein the relative concentration ratio of the fluorine element to the boron element is 1 or less.

2. In Claim 1, A negative electrode for a lithium secondary battery, wherein the ratio of the relative concentration of the fluorine element to the boron element is 0.8 or less.

3. In Claim 1, A negative electrode for a lithium secondary battery, wherein the ratio of the relative concentration of the fluorine element to the boron element represents the ratio of the content of the constituent elements obtained by an elemental analysis method.

4. In Claim 3, In addition to the fluorine element (F) and the boron element (B), the above constituent elements include, A negative electrode for a lithium secondary battery, further comprising one or more elements selected from the group consisting of carbon (C), nitrogen (N), oxygen (O), and bromine (Br).

5. In Claim 3, The above elemental analysis method is energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), or a combination thereof, for a negative electrode for a lithium secondary battery.

6. In Claim 4, The above film layer further contains oxygen elements, and A negative electrode for a lithium secondary battery, wherein the ratio of the relative concentration of the boron element to the oxygen element is 0.1 to 1.

7. In Claim 6, A negative electrode for a lithium secondary battery, wherein the relative concentration of the oxygen element is 20% to 80% based on the total constituent elements of the film layer.

8. In Claim 1, The above film layer comprises lithium fluoride (LiF) and borate, a negative electrode for a lithium secondary battery.

9. In Claim 1, The above lithium metal layer is a lithium foil or a lithium alloy foil, a negative electrode for a lithium secondary battery.

10. A film layer comprising a fluorine element (F) and a boron element (B) is formed on a lithium metal layer, and A method for manufacturing a negative electrode for a lithium secondary battery in which the ratio of the relative concentration of the fluorine element to the boron element is 1 or less, (S1) A step of preparing a boron solution by dissolving boron halides in an aprotic organic solvent, (S2) A step of reacting the boron solution and the lithium metal layer under an inert atmosphere, (S3) After the above reaction, a step of contacting the aprotic organic solvent with the lithium metal layer, (S4) A method for manufacturing a negative electrode for a lithium secondary battery, comprising the step of reacting the lithium metal layer with a fluorine-based organic solvent under an inert atmosphere after the above contact.

11. In Claim 10, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the contact in step (S3) above is performed for a period of 10 hours or less.

12. In Claim 11, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the contact in step (S3) above is performed for a period of time of 1 hour or less.

13. In Claim 10, The above aprotic organic solvent is C4 to C 30 A method for manufacturing a negative electrode for a lithium secondary battery comprising an aliphatic acyclic compound.

14. In Claim 10, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the boron halide comprises boron tribromide (BBr3).

15. In Claim 10, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the above-mentioned fluorine-based organic solvent comprises a fluorine-containing carbonate solvent.

16. In Claim 10, The above step (S4) is a method for manufacturing a negative electrode for a lithium secondary battery, wherein the lithium metal layer is reacted with the electrolyte for a lithium secondary battery containing the above fluorine-based organic solvent and lithium salt.

17. In Claim 10, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the reaction of step (S2) above is performed at a temperature of 45°C or higher and 85°C or lower.

18. A lithium secondary battery comprising a negative electrode, a positive electrode, and an electrolyte according to any one of claims 1 to 9.

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