Negative electrode and lithium secondary battery comprising same

Abstract

A negative electrode according to an aspect of the present invention comprises: a negative electrode active material layer; a first protective layer; and a second protective layer, wherein the negative electrode active material layer includes at least one of lithium or a lithium alloy, the first protective layer includes a polyalkylene oxide, the second protective layer includes a polyvinylidene fluoride-based polymer, and the second protective layer has a porosity of 10% or less. In the negative electrode, the growth of lithium dendrites is suppressed and porosification is prevented. In addition, the negative electrode can have high ion conductivity even when including the protective layers, and can ensure stability and excellent mechanical strength.

Description

Anode and lithium secondary battery containing the same

[0001] The present invention relates to a negative electrode that can be used in an electrochemical device such as a lithium secondary battery and a lithium secondary battery including the same.

[0002] This application claims priority to Korean Application No. 10-2023-0174922, filed December 5, 2023, the entire disclosure of which is incorporated herein by reference.

[0003] Interest in energy storage technology has been growing steadily in recent years. As its application expands to include energy for mobile phones, camcorders, laptops, and even electric vehicles, research and development efforts for electrochemical devices are becoming increasingly concrete.

[0004] Electrochemical devices are receiving the most attention in this regard, and within this field, the development of rechargeable secondary batteries is a major focus. Recently, active research and development is underway on new electrode and battery designs to improve capacity density and specific energy in secondary batteries.

[0005] Among the secondary batteries currently in use, lithium secondary batteries developed in the early 1990s are attracting attention due to their advantages of higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use aqueous electrolytes.

[0006] These secondary batteries can be broadly divided into anode, cathode, separator, and electrolyte. During the first charge, lithium ions from the cathode active material are inserted into the cathode active material, and then, during discharge, the lithium ions are desorbed again and move back and forth between the two electrodes. This process transfers energy, enabling charging and discharging. For example, a lithium secondary battery consists of an electrode assembly comprising a cathode containing a lithium transition metal oxide as an electrode active material, an anode containing a carbon-based active material, and a separator containing a porous polymer substrate, which is impregnated with an electrolyte.

[0007] Many attempts have been made to utilize lithium metal as an anode for secondary batteries to achieve high capacity. When lithium metal is used as an anode in a secondary battery, a redox reaction occurs: lithium ions are reduced on the surface of the lithium metal, deposited as lithium metal on the electrode surface, and then the lithium metal is oxidized and released into the electrolyte as lithium ions. Therefore, controlling the interfacial phenomena between the lithium metal and the electrolyte is crucial.

[0008] Lithium metal anodes can experience porous needle-like or dendrite-like electrodeposition during charging, or “dead lithium” can be generated by repeated charge / discharge reactions, resulting in low charge / discharge efficiency and deterioration of battery life characteristics. Furthermore, the desorption of lithium metal can lead to the formation of pores in the lithium metal anode, weakening the mechanical strength of the anode. Furthermore, continuous dendrite growth can cause short circuits between the anode and cathode, leading to problems such as battery ignition and explosion due to high chemical / electrochemical reactivity.

[0009] In addition, side reactions with the electrolyte can cause electrolyte consumption. Side reactions and electrolyte consumption of the lithium anode can be a bigger issue, especially when the lithium anode is applied as the anode of a lithium-sulfur battery that requires a lean electrolyte condition. Lithium-sulfur batteries use sulfur-based materials containing sulfur-sulfur bonds as cathode active materials, and in order to achieve an appropriate energy density of the battery, the ratio of the volume of the electrolyte to the volume of the cathode active material needs to be maintained below a certain level. Therefore, when a lithium metal anode is used in a lithium-sulfur battery, the electrolyte is quickly depleted due to side reactions between the anode and the electrolyte, resulting in insufficient amount for operation, which can result in a deterioration in the life characteristics of the battery.

[0010] In addition, when lithium metal is used as a negative electrode in a secondary battery, a redox reaction occurs in which lithium ions are reduced on the surface of the lithium metal and deposited as lithium metal on the electrode surface, and in which the lithium metal is oxidized and dissolved into the electrolyte in the form of lithium ions. Therefore, it can be seen that controlling the interfacial phenomenon between the lithium metal and the electrolyte is very important.

[0011] The present invention aims to address the above-mentioned problems and provides a lithium metal anode that prevents side reactions with the electrolyte, lithium dendritic growth, and strength degradation. Furthermore, the present invention aims to provide a lithium secondary battery with reduced electrolyte consumption and improved electrochemical reaction stability and capacity retention.

[0012] The present invention can provide a negative electrode and a lithium secondary battery according to the following embodiments.

[0013] The cathode according to the first embodiment is

[0014] A negative electrode active material layer comprising at least one of lithium or a lithium alloy;

[0015] A first protective layer formed on at least one surface of the negative electrode active material layer; and

[0016] A second protective layer formed on the other surface of the first protective layer;

[0017] The first protective layer comprises polyalkylene oxide,

[0018] The second protective layer includes a polyvinylidene fluoride polymer (PVDF polymer), and the other surface of the first protective layer is the opposite surface of the surface of the first protective layer that faces the negative electrode active material layer.

[0019] According to the second embodiment, in the first embodiment,

[0020] The thickness of the first protective layer is characterized by being 0.1 ㎛ or more and less than 5 ㎛.

[0021] According to the third embodiment, in the first embodiment or the second embodiment,

[0022] The thickness of the second protective layer is characterized by being 0.1 ㎛ or more and less than 5 ㎛.

[0023] According to the fourth embodiment, in any one of the first to third embodiments,

[0024] The above polyalkylene oxide is characterized in that it includes at least one selected from polyethylene oxide, polypropylene oxide, and polybutylene oxide.

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

[0026] The above polyvinylidene fluoride polymer is characterized by including a homopolymer of vinylidene fluoride, a copolymer including vinylidene fluoride, or both.

[0027] According to the sixth embodiment, in any one of the first to fifth embodiments,

[0028] The copolymer containing the above vinylidene fluoride is characterized in that it contains 0.1 wt% to 20 wt% of a monomer that is combined with vinylidene fluoride with respect to a total of 100 wt%.

[0029] According to the seventh embodiment, in any one of the first to sixth embodiments,

[0030] The copolymer comprising the above vinylidene fluoride,

[0031] It is characterized by including polyvinylidene fluoride hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride chlorotrifluoroethylene (PVDF-CTFE), polyvinylidene fluoride tetrafluoroethylene (PVDF-TFE), polyvinylidene fluoride trifluoroethylene (PVDF-TrFE), or a mixture of two or more thereof.

[0032] According to the eighth embodiment, in any one of the first to seventh embodiments,

[0033] The ionic conductivity of the first protective layer is 10 at 25°C. -11 10 inland -6 It is characterized by S / cm.

[0034] According to the ninth embodiment, in any one of the first to eighth embodiments,

[0035] The entire protective layer including the first protective layer and the second protective layer is characterized in that the yield elongation is 2.5% or more.

[0036] According to the tenth embodiment, in any one of the first to ninth embodiments,

[0037] The total thickness of the first protective layer and the second protective layer is characterized by being 0.2 µm or more and less than 10 µm.

[0038] According to the eleventh embodiment, in any one of the first to tenth embodiments,

[0039] The lithium alloy is characterized by being an alloy of lithium (Li) and at least one metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0040] A lithium secondary battery according to the 12th embodiment,

[0041] A cathode; an anode; and a separator disposed between the cathode and the anode,

[0042] The above cathode is characterized as being a cathode according to claim 1.

[0043] The 13th embodiment is, in the 12th embodiment,

[0044] The above positive electrode is characterized by having a positive electrode active material including a sulfur-carbon complex.

[0045] According to one aspect of the present invention, a negative electrode has a protective layer on the surface of lithium metal, thereby suppressing side reactions with an electrolyte and the growth of lithium dendrites. In addition, when lithium is re-electrolyzed on the negative electrode during charging, porosity on the surface of the lithium metal can be prevented. The protective layer is composed of a first protective layer having high adhesion and ionic conductivity and a second protective layer having high strength and shape deformation rate, so that high ionic conductivity can be maintained even by the protective layer, and even when the shape of the lithium metal negative electrode is deformed due to charge and discharge, the protective layer correspondingly deforms, so that the protective layer and the lithium metal do not separate from each other and can be maintained in close contact. In addition, the protective layer has excellent mechanical strength, so that perforation of the protective film is suppressed.

[0046] In addition, the lithium secondary battery according to the present invention has excellent electrochemical reaction stability and capacity retention rate by applying the negative electrode on which the protective film is disposed.

[0047] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the contents of the invention described above, serve to further understand the technical concept of the present invention. Therefore, the present invention should not be interpreted as being limited to the matters described in these drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings included in this specification may be exaggerated to emphasize a clearer explanation.

[0048] Figure 1 is a schematic diagram showing a cross-section of a cathode according to one aspect of the present invention.

[0049] Figure 2 is a schematic diagram showing a cross-section of a cathode according to one aspect of the present invention.

[0050] Figure 3 is a schematic diagram showing a cross-section of a cathode according to one aspect of the present invention.

[0051] Figure 4 illustrates a strain-stress curve of a protective layer according to a manufacturing example of the present invention.

[0052] Figure 5 illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0053] Figure 6a illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment of the present invention.

[0054] Figure 6b illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0055] Figure 6c illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0056] Figure 6d shows the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0057] Figure 6e illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0058] Figure 6f shows the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0059] Figure 6g illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0060] Figure 7 illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0061] Figure 8a illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment of the present invention.

[0062] Figure 8b illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0063] Figure 8c illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0064] Figure 8d shows the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0065] Figure 8e illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0066] Figure 8f shows the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0067] Figure 8g illustrates the performance evaluation results of a lithium sulfur battery according to a comparative example of the present invention.

[0068] Figure 9a illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0069] Figure 9b illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0070] Figure 9c illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0071] Figure 10a illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0072] Figure 10b illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0073] Figure 10c illustrates the performance evaluation results of a lithium sulfur battery according to one embodiment and a comparative example of the present invention.

[0074] FIG. 11a is an image of the surface and cross-section of a cathode according to one embodiment of the present invention.

[0075] Figure 11b is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0076] Figure 11c is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0077] Figure 11d is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0078] FIG. 12a is an image of the surface and cross-section of a cathode according to one embodiment of the present invention.

[0079] Figure 12b is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0080] Figure 12c is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0081] Figure 12d is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0082] Figure 12e is an image of the surface and cross-section of a cathode according to a comparative example of the present invention.

[0083] FIG. 12f is an image of the surface of a cathode according to one embodiment of the present invention.

[0084] Figure 13a is an image of a cross-section of a cathode according to one embodiment of the present invention.

[0085] Figure 13b is an image of a cross-section of a cathode according to a comparative example of the present invention.

[0086] Figure 13c is an image of a cross-section of a cathode according to a comparative example of the present invention.

[0087] Figure 13d is an image of a cross-section of a cathode according to a comparative example of the present invention.

[0088] Figure 13e is an image of a cross-section of a cathode according to a comparative example of the present invention.

[0089] Figure 14a is a cross-sectional image of a negative electrode according to charge and discharge of a lithium-sulfur battery according to one embodiment of the present invention.

[0090] Figure 14b is a cross-sectional image of a negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0091] Figure 14c is a cross-sectional image of a negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0092] Figure 14d is a cross-sectional image of a negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0093] Figure 14e is a cross-sectional image of a negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0094] Figure 15a shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0095] Figure 15b shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0096] Figure 15c shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0097] Figure 15d is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0098] Figure 16a is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0099] Figure 16b shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0100] Figure 16c is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0101] Figure 16d is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0102] Figure 17a shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0103] Figure 17b shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0104] Figure 17c shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0105] Figure 17d is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0106] Figure 18a is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0107] Figure 18b shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0108] Figure 18c shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0109] Figure 18d is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to a comparative example of the present invention.

[0110] Figure 19a shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to one embodiment of the present invention.

[0111] Figure 19b shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to one embodiment of the present invention.

[0112] Figure 19c shows the XPS analysis results of the negative electrode according to charge and discharge of a lithium-sulfur battery according to one embodiment of the present invention.

[0113] Figure 19d is the XPS analysis result of the negative electrode according to charge and discharge of a lithium-sulfur battery according to one embodiment of the present invention.

[0114] Figure 20 is a graph showing the results of resistance measurement of a symmetrical cell according to a manufacturing example of the present invention.

[0115] Figure 21a is a surface image of a cathode according to one embodiment of the present invention.

[0116] Figure 21b is a surface image of a cathode according to one embodiment of the present invention.

[0117] Figure 21c is a surface image of a cathode according to one embodiment of the present invention.

[0118] Figure 21d is a cross-sectional image of a cathode according to one embodiment of the present invention.

[0119] Figure 22a is a surface image of a cathode according to a comparative example of the present invention.

[0120] Figure 22b is a surface image of a cathode according to a comparative example of the present invention.

[0121] Figure 22c is a surface image of a cathode according to a comparative example of the present invention.

[0122] Figure 22d is a cross-sectional image of a cathode according to a comparative example of the present invention.

[0123] Figure 23a illustrates the performance evaluation results of one embodiment of the present invention and a comparative example.

[0124] Figure 23b illustrates the performance evaluation results of one embodiment of the present invention and a comparative example.

[0125] Figure 23c illustrates the performance evaluation results of one embodiment of the present invention and a comparative example.

[0126] Figure 23d illustrates the performance evaluation results of one embodiment of the present invention and a comparative example.

[0127] Hereinafter, the present invention will be described in detail with reference to the drawings. Terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention, they should be interpreted in the sense and concept consistent with the technical spirit of the present invention.

[0128] Accordingly, the embodiments described in this specification and the configurations described in the drawings are only the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, it should be understood that there may be various equivalents and modified examples that can replace them at the time of filing this application.

[0129] Additionally, throughout the specification, whenever a part is said to “include,” “comprise,” “have,” or “have” a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0130] In addition, the terms 'about', 'substantially', etc. used throughout this specification are used in the sense of or near to the numerical values ​​when manufacturing and material tolerances inherent to the meanings stated are presented, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosure where exact or absolute values ​​are mentioned to aid understanding of this specification.

[0131] Throughout this specification, references to 'A and / or B' mean 'A or B or both.'

[0132] Unless otherwise specified throughout this specification, temperature refers to Celsius temperature, and the unit is ℃.

[0133] The term "composite" used in this specification refers to a material in which two or more materials are combined to form physically and chemically different phases and exhibit more effective functions.

[0134] The term "polysulfide" as used herein means "polysulfide ion (S x 2- , x - = 8, 6, 4, 2)" and "lithium polysulfide (Li2S) x or Li2S x - = It is a concept that includes all of 8, 6, 4, 2).

[0135]

[0136] The first aspect of the present invention relates to a cathode for an electrochemical device.

[0137] Figure 1 schematically illustrates a cross-section of a cathode according to one aspect of the present invention.

[0138] Referring to FIG. 1, a negative electrode (10) according to one aspect of the present invention is formed by sequentially stacking a negative electrode active material layer (11), a first protective layer (12), and a second protective layer (13), and includes: the negative electrode active material layer (11); the first protective layer (12) formed on at least one surface of the negative electrode active material layer (11); and the second protective layer (13) formed on one surface of the first protective layer (12).

[0139] That is, the first protective layer (12) is positioned on at least one surface of the negative electrode active material layer (11), and the second protective layer (13) is positioned on the other surface of the first protective layer (12), which is the opposite surface of the first protective layer (12) that faces the negative electrode active material layer (11).

[0140] Since the above negative electrode (10) includes the first protective layer and the second protective layer, the interface reaction on the negative electrode surface is controlled, and porosity is prevented when lithium is electrodeposited, thereby suppressing a decrease in the strength of the negative electrode.

[0141]

[0142] The above first protective layer (12) includes polyalkylene oxide. Since the negative electrode includes the first protective layer, high adhesion between the negative electrode active material layer and the protective layer can be maintained.

[0143]

[0144] In one embodiment of the present invention, the polyalkylene oxide included in the first protective layer may include a structure represented by the following chemical formula 1.

[0145] [Chemical Formula 1]

[0146]

[0147] Here, the above a1 and the above a2 may each independently include a silyl group having three aliphatic organic groups having 1 to 10 carbon atoms, an aliphatic organic group having 1 to 10 carbon atoms, or an aromatic organic group having 6 to 20 carbon atoms.

[0148] The above x can be an integer from 1 to 4, and the above n can be an integer from 10 to 100000.

[0149]

[0150] In one embodiment of the present invention, the polyalkylene oxide may include polyethylene oxide, polypropylene oxide, polybutylene oxide, or a mixture thereof.

[0151]

[0152] In one embodiment of the present invention, the first protective layer may be formed to a thickness of 0.1 μm or more and less than 5 μm. Specifically, it may be formed to a thickness of 0.1 μm to 4.9 μm, 0.1 μm to 4.5 μm, 1 μm to 4 μm, or 2 μm to 3 μm. When the thickness of the first protective layer falls within the above range, sufficient adhesion between the protective layer and the negative electrode active material layer can be secured, while also having appropriate ionic conductivity.

[0153]

[0154] In one embodiment of the present invention, the polyalkylene oxide polymer included in the first protective layer may have a weight average molecular weight of 50,000 to 150,000, 60,000 to 140,000, or 70,000 to 130,000.

[0155] At this time, the weight average molecular weight of the polymer was measured under the following gel permeation chromatography (GPC: gel permeation chromatography, PL GPC220, Agilent Technologies) analysis conditions:

[0156] - Column: PL MiniMixed B x 2

[0157] - Solvent: THF

[0158] - Flow rate: 0.3 ml / min

[0159] - Sample concentration: 2.0 mg / ml

[0160] - Injection volume: 10 ㎕

[0161] - Column temperature: 40℃

[0162] - Detector: Agilent RI detector

[0163] - Standard: Polystyrene (corrected with a cubic function)

[0164] - Data processing: ChemStation

[0165]

[0166] In one embodiment of the present invention, the ionic conductivity of the first protective layer is 10 at 25°C. -11 10 inland -6 S / cm, 10 -10 10 inland -7 S / cm or 10 -9 10 inland -8 It can be S / cm. When the ionic conductivity of the first protective layer falls within the above range, sufficient ionic conductivity for battery operation can be secured without increasing the resistance due to the protective layer.

[0167] The above ionic conductivity can be calculated by manufacturing a coin cell by placing a protective layer between two sheets of stainless steel (SUS), and using an analysis device (VMP3, Bio logic science instrument) to measure the electrochemical impedance at 25°C under conditions of an amplitude of 10 mV and a scan range of 500 kHz to 20 MHz, and using the measured impedance.

[0168]

[0169] In one embodiment of the present invention, the first protective layer may have a porosity of 10% or less, 8% or less, 5% or less, or 3% or less. The first protective layer may be composed solely of a binder polymer without the addition of separate inorganic particles. Therefore, the first protective layer may have a very small porosity. When the porosity of the first protective layer falls within the above range, sufficient adhesion with the negative electrode active material layer can be secured.

[0170] The above porosity can be calculated and obtained from the density / ratio of inorganic fine particles in the slurry, the density / ratio of binder, and the measured weight (loading amount) and thickness of the sample.

[0171] Alternatively, the porosity can be calculated by the following formula using the ratio of the density obtained by measuring the volume and mass of the first protective layer for a sample coated with the above composition on an electrode of a certain area and the theoretical density of the solid content of the coating composition.

[0172] (Formula) Porosity (%) = {1 - (actual density) / (theoretical density)} × 100.

[0173]

[0174] In one embodiment of the present invention, the second protective layer includes a polyvinylidene fluoride (PVdF)-based polymer. The mechanical strength of the protective layer can be improved by including the second protective layer. In addition, the second protective layer has a high deformation rate, so that its shape can change in response to deformation of the shape of the negative electrode active material layer. For example, even when lithium is desorbed or deposited from the lithium metal, which is the negative electrode active material layer, and the surface of the lithium metal becomes uneven or uneven, the second protective layer can remain in close contact with the surface of the lithium metal.

[0175] Figure 2 is a schematic diagram showing that a PEO layer as a first protective layer and a PVdF-HFP layer (P-H8) as a second protective layer are formed on one surface of a lithium metal, which is a negative electrode active material layer. The surface of the lithium metal becomes curved as lithium is desorbed and deposited through repeated charge and discharge, and the second protective layer has a high shape deformation rate, so that its shape is deformed to correspond to the curved surface of the lithium metal and can maintain a state of being in close contact with the surface of the lithium metal. The above (P-H8) means that 8 wt% of HFP is included with respect to the total 100 wt% of PVDF-HFP.

[0176] Specific examples of the polyvinylidene fluoride-based polymer include, but are not limited to, a homopolymer of vinylidene fluoride and a copolymer containing vinylidene fluoride. The copolymer containing vinylidene fluoride may include, for example, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride-tetrafluoroethylene (PVDF-TFE), polyvinylidene fluoride-chlorofluoroethylene (PVDF-CTFE), or two or more thereof.

[0177] The second protective layer may be formed with a thickness of 0.1 μm or more and less than 5 μm. Specifically, it may be formed with a thickness of 0.1 μm to 4.9 μm, 0.1 μm to 4.5 μm, 1 μm to 4 μm, or 2 μm to 3 μm. When the thickness of the second protective layer falls within the above range, the mechanical strength of the protective layer can be sufficiently secured. In addition, when the thickness of the protective layer excessively exceeds the above range, it may be difficult for lithium ions to pass through the second protective layer, which may cause an overvoltage in the battery and deteriorate the performance of the battery.

[0178]

[0179] In one embodiment of the present invention, the total thickness of the first protective layer and the second protective layer may be 0.2 μm or more and less than 10 μm. Specifically, it may be 0.2 μm to 9.8 μm, 0.2 μm to 9 μm, 2 μm to 8 μm, or 4 μm to 6 μm. When the total thickness of the first protective layer and the second protective layer falls within the above range, the mechanical strength of the negative electrode can be sufficiently secured while suppressing excessive increase in resistance.

[0180]

[0181] In one embodiment of the present invention, the weight average molecular weight of the polyvinylidene fluoride polymer included in the second protective layer may be 10,000 to 500,000, 50,000 to 500,000, 100,000 to 400,000, or 200,000 to 350,000.

[0182]

[0183] In one embodiment of the present invention, the copolymer comprising vinylidene fluoride may contain a monomer bonded with vinylidene fluoride in an amount of 0.1 wt% to 20 wt%, 3 wt% to 20 wt%, 3 wt% to 15 wt%, 5 wt% to 15 wt%, 8 wt% to 20 wt%, or 8 wt% to 15 wt%, based on a total of 100 wt%. When the monomer bonded with vinylidene fluoride is included within the above range, the mechanical rigidity of the negative electrode can be sufficiently improved.

[0184]

[0185] In one embodiment of the present invention, the second protective layer may have a porosity of 10% or less, 8% or less, 5% or less, or 3% or less. The second protective layer may be composed solely of a binder polymer without including separate inorganic particles. Therefore, the porosity may be very small. When the porosity of the second protective layer falls within the above range, the mechanical strength of the protective layer can be sufficiently improved. If the porosity of the second protective layer excessively exceeds the above range, the mechanical strength cannot be secured.

[0186] The porosity measurement of the second protective layer can be measured by the same method as the porosity measurement method of the first protective layer.

[0187]

[0188] In one embodiment of the present invention, the elongation at yield (%) of the entire protective layer including the first protective layer and the second protective layer may be 2.5% or more, 2.7% or more, 3.0% or more, 3.3% or more, 3.5% or more, 3.8% or more, or 3.9% or more. When the elongation at yield (%) of the entire protective layer falls within the above range, sufficient flexibility and elasticity can be secured, thereby improving the mechanical strength of the negative electrode and ensuring adhesion between the protective layer and the negative electrode active material layer. In particular, even if the surface is uneven due to repeated insertion / de-insertion of lithium in the negative electrode active material layer, the protective layer can remain in close contact with the negative electrode active material.

[0189] The above yield elongation is a value that expresses the degree of elongation compared to the initial thickness as a percentage (%). If the yield elongation is high, the protective layer becomes more flexible and can have greater elasticity. Therefore, the maximum value of the yield elongation of the entire protective layer is not limited, and may be, for example, 300% or less, 500% or less, or 1000% or less.

[0190] The above yield elongation can be obtained by measuring the magnitude of the stress applied to the material using a universal material testing machine (NA-05M / NA-10M / NA-20M) from Nanotech Co., Ltd., drawing a stress-strain graph, and then measuring the degree of deformation (elongation) at the yield point on the graph. At this time, the yield point can mean the value when the stress exceeds the elastic limit while increasing the external force applied to the material, and when the permanent strain increases rapidly even though the external force is hardly increased.

[0191]

[0192] According to one embodiment of the present invention, the negative electrode may be a current collector-free type that is composed only of a negative electrode active material layer without including a current collector, or may be a negative electrode active material layer formed on at least one surface of the negative electrode current collector.

[0193] The above-mentioned negative current collector may be any current collector used in the field of lithium secondary battery technology, and is not particularly limited as long as it has high conductivity and does not cause side reactions in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. may be used.

[0194] The thickness of the above negative electrode current collector may be 1 µm to 300 µm, or 1 µm to 100 µm, but is not particularly limited and may be set to an appropriate range in consideration of the mechanical strength of the electrode, productivity, capacity of the battery, etc.

[0195] The thickness of the negative electrode active material layer may be 1 µm to 200 µm, 5 µm to 100 µm, 10 µm to 80 µm, or 20 µm to 70 µm. When the thickness of the negative electrode active material layer falls within the above range, sufficient battery capacity can be secured.

[0196] Figure 3 is a schematic diagram showing a cross-section of a negative electrode (10) including a current collector (14). Referring to this, the negative electrode according to the present invention has a negative electrode active material layer (11), a first protective layer (12), and a second protective layer (13) sequentially laminated on the surface of the current collector (14).

[0197]

[0198] In one embodiment of the present invention, the negative electrode active material may include lithium metal and / or a lithium alloy, and may be in the form of a thin film or powder. For example, the negative electrode active material may be in the form of a lithium metal thin film or lithium metal powder.

[0199] The lithium alloy may be, for example, an alloy of lithium (Li) and one or more metals selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0200]

[0201] A second aspect of the present invention relates to a battery including the negative electrode.

[0202] The battery may be a lithium secondary battery, and specifically, may be a lithium ion battery or a lithium sulfur battery. As described above, a lithium-sulfur battery needs to be operated under a lean electrolyte condition, and when operated under a lean electrolyte condition, electrolyte consumption may be further aggravated. When the negative electrode according to one aspect of the present invention is used as the negative electrode of a lithium-sulfur battery, side reactions between the negative electrode and the electrolyte are suppressed and electrolyte consumption is significantly reduced, so that the capacity retention rate and life characteristics of the battery can be significantly improved.

[0203]

[0204] A battery according to one aspect of the present invention may include: a cathode; an anode; a separator interposed between the cathode and the anode; and an electrolyte. In addition, the cathode may include the cathode described above.

[0205]

[0206] In one embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector. In addition, the positive electrode active material layer includes a positive electrode active material and a binder polymer, and may further include a conductive material and / or additives.

[0207]

[0208] In one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it supports the positive electrode active material and has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., aluminum-cadmium alloy, etc. can be used. The positive electrode current collector can form fine irregularities on its surface to strengthen the bonding strength with the positive electrode active material, and can be used in various forms such as a film, sheet, foil, mesh, net, porous body, foam, non-woven fabric, etc.

[0209]

[0210] In one embodiment of the present invention, the positive electrode active material may include a sulfur-carbon complex.

[0211] The above sulfur-carbon composite may include a porous carbon material; and a sulfur-based material supported on at least one of the interior of the pores of the porous carbon material and the exterior surface of the porous carbon material.

[0212] In the case of the sulfur-based material acting as the positive electrode active material, since it has no or little electrical conductivity alone, it is used in combination with a conductive material such as the porous carbon material, and the porous carbon material can be used to support the sulfur-based material. In addition, the sulfur-based material added as the positive electrode active material may be sulfur (S8) and / or a sulfur-based compound. The sulfur-based compound may be, for example, lithium sulfide (Li2S), lithium polysulfide (Li2Sx, an integer of 2 ≤ x ≤ 8), a disulfide compound, a carbon-sulfur polymer ((C2S) y ) n , y = 2.5 to 50, n≥2), lithium sulfide (Li2S), or two or more thereof. Preferably, the sulfur-based material may be inorganic sulfur (S8).

[0213] The above porous carbon material is intended to support the sulfur-based material as a positive electrode active material, provide a framework in which the sulfur-based material can be uniformly and stably fixed, and improve the conductivity of the positive electrode. Any porous carbon material can be used without particular limitation. The porous carbon material can generally be manufactured by carbonizing a precursor of various carbon materials. The porous carbon material includes pores therein that are not regular, and the average diameter of the pores ranges from 1 to 200 nm, and the porosity can range from 10 to 90 vol% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if it exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it undesirable for application to the electrode manufacturing process.

[0214] The above 'average pore diameter' can be measured according to a method known in the art for measuring the pore diameter of a porous material, and the measurement method is not particularly limited. For example, the pore diameter may be measured according to a scanning electron microscope (SEM), a field emission electron microscope (laser diffraction method), a laser diffraction method, or the BET (Brunauer-Emmett-Teller) method. The measurement using the laser diffraction method may be performed using, for example, a commercially available laser diffraction particle size measuring device (for example, Microtrac MT 3000). In addition, the measurement according to the BET method may be performed using, for example, an analyzer of the BELSORP series of BEL Japan Co., Ltd., but is not limited thereto.

[0215] The above 'porosity' refers to the ratio of the volume occupied by pores to the total volume of a structure, and uses % as its unit, and can be used interchangeably with terms such as porosity and porosity. In the present invention, the measurement of the porosity is not particularly limited, and according to one embodiment of the present invention, for example, it can be measured according to the BET method using nitrogen gas or the mercury penetration method (Hg porosimeter) and ASTM D2873.

[0216] The shape of the above porous carbon material may be spherical, rod-shaped, needle-shaped, plate-shaped, tubular or bulk-shaped, and can be used without limitation as long as it is a shape commonly used in lithium-sulfur batteries.

[0217] The porous carbon material may be any material commonly used in the art that has a porous structure or a high specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphite and activated carbon such as natural graphite, artificial graphite, and expanded graphite, but is not limited thereto. Preferably, the porous carbon material may be a carbon nanotube.

[0218] The method for manufacturing the above sulfur-carbon composite is not particularly limited in the present invention, and any method commonly used in the art may be used. For example, a method may be used in which the sulfur and porous carbon material are simply mixed and then heat-treated to form a composite.

[0219] In addition to the composition described above, the positive electrode active material may further include one or more selected from among a transition metal element, a group ⅢA element, a group ⅣA element, a sulfur compound of these elements, and an alloy of these elements and sulfur.

[0220] The above transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, etc., the above group ⅢA elements include Al, Ga, In, Ti, etc., and the above group ⅣA elements may include Ge, Sn, Pb, etc.

[0221] In one embodiment of the present invention, the sulfur-carbon complex may be at least 50 wt% based on the total weight of the positive electrode. Specifically, the sulfur-carbon complex may be at least 80 wt%, at least 90 wt%, or at least 95 wt% based on the total weight of the positive electrode active material layer. Specifically, the sulfur-carbon complex may be included in an amount of 80 wt% to 100 wt%, more specifically 85 wt% to 99 wt%, 90 wt% to 99 wt%, 95 wt% to 98 wt%, or 95 wt% to 97 wt%, or 96 wt% based on the total weight of the positive electrode active material layer. When the sulfur-carbon complex is included in the above range, a high-capacity and high-energy-density battery can be realized while sufficiently maintaining the physical properties of the electrode.

[0222]

[0223]

[0224] In one embodiment of the present invention, the binder polymer maintains the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to further increase the bonding strength between them, and any binder polymer known in the art can be used.

[0225] For example, the binder polymer may be a fluororesin binder including polyvinylidene fluoride (PVdF), a polyvinylidene fluoride polymer including at least one vinylidene fluoride as a repeating unit, polytetrafluoroethylene (PTFE), or a mixture of two or more thereof; a rubber binder including styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, or styrene-isoprene rubber; an acrylic binder; a cellulose binder including carboxyl methyl cellulose (CMC), starch, hydroxy propyl cellulose, or regenerated cellulose; a polyalcohol binder; a polyolefin binder including polyethylene or polypropylene; a polyimide binder; a polyester binder; And a silane-based binder; one, two or more kinds of mixtures or copolymers selected from the group consisting of may be used. Preferably, it is PAA (poly acrylic acid).

[0226] The content of the above binder polymer may be 1 to 20 wt%, or 1 to 10 wt%, based on the total weight of the positive electrode active material layer. When the content of the binder polymer is within the above range, the positive electrode active materials or the positive electrode active material and the current collector can be well bonded together, while sufficient battery capacity can be secured.

[0227]

[0228] The conductive material is a material that electrically connects the electrolyte and the positive electrode active material and acts as a path for electrons to move from the current collector to the positive electrode active material. Any conductive material that is physically distinct from the carbon contained in the sulfur-carbon complex and is a component of the electrode can be used without limitation. For example, the conductive material may be carbon black such as Super-P, Denka Black, acetylene black, Ketjen Black, channel black, furnace black, lamp black, summer black, carbon black; carbon derivatives such as carbon nanotubes or fullerene; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole, which can be used alone or in combination.

[0229] The content of the conductive agent may be 1 to 10 wt% based on the total weight of the positive electrode active material layer. If the content of the conductive agent is less than the above range, electron transfer between the positive electrode active material and the current collector is not easy, resulting in a decrease in voltage and capacity. Conversely, if the content exceeds the above range, the proportion of the positive electrode active material may relatively decrease, thereby reducing the total energy (charge) of the battery. Therefore, it is preferable to determine an appropriate content within the above-mentioned range.

[0230]

[0231] In the present invention, the method for manufacturing the positive electrode for the lithium secondary battery is not particularly limited, and various methods known to those skilled in the art or modified methods thereof can be used.

[0232] For example, the positive electrode for the lithium secondary battery may be manufactured by forming the positive electrode active material layer by preparing a positive electrode slurry composition including the composition described above and then applying the same to at least one surface of the positive electrode current collector.

[0233] The above positive electrode slurry composition includes the positive electrode active material described above, and may further include a binder polymer, a conductive material, and a solvent.

[0234] The above solvent is one that can uniformly disperse the positive electrode active material. Water is most preferably an aqueous solvent, and the water may be distilled or deionized water. However, this is not necessarily limited to this, and if necessary, a lower alcohol that is easily mixed with water may be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol, and preferably, these can be mixed with water and used.

[0235] The content of the above solvent may be contained at a level that has a concentration that can facilitate coating, and the specific content varies depending on the application method and device.

[0236] The above-mentioned positive electrode slurry composition may additionally contain, as necessary, substances commonly used in the relevant technical field for purposes such as improving its function. Examples thereof include viscosity modifiers, fluidizing agents, and fillers.

[0237] The method for applying the positive electrode slurry composition is not particularly limited in the present invention, and examples thereof include methods such as doctor blade, die casting, comma coating, and screen printing. In addition, the positive electrode slurry may be applied onto the positive electrode current collector by molding it on a separate substrate and then pressing or lamination.

[0238] After the above application, a drying process for solvent removal can be performed. The drying process is performed at a temperature and time that can sufficiently remove the solvent, and the conditions may vary depending on the type of solvent and are therefore not particularly limited to the present invention. Examples include drying using warm air, hot air, low-humidity air, vacuum drying, and drying using irradiation with (far) infrared rays and electron beams. The drying speed is usually adjusted to remove the solvent as quickly as possible within a speed range that does not cause cracks in the positive electrode active material layer due to stress concentration or cause the positive electrode active material layer to peel off from the positive electrode current collector.

[0239] Additionally, the density of the positive electrode active material within the positive electrode can be increased by pressing the entire body after drying. Pressing methods include mold pressing and roll pressing.

[0240] The porosity of the positive electrode manufactured by the composition and manufacturing method described above, specifically, the positive electrode active material layer, may be 50 to 80% by volume, specifically, 60 to 75% by volume. If the porosity of the positive electrode is less than 50% by volume, the filling degree of the positive electrode slurry composition including the positive electrode active material, the conductive material, and the binder polymer becomes excessively high, so that sufficient electrolyte capable of exhibiting ionic and / or electrical conductivity cannot be maintained between the positive electrode active materials, which may result in a deterioration in the output characteristics or cycle characteristics of the battery, and a serious problem of a decrease in overvoltage and discharge capacity of the battery. On the other hand, if the porosity of the positive electrode exceeds 80% by volume and has excessively high porosity, there is a problem in that the physical and electrical connection with the current collector is lowered, resulting in a decrease in adhesive strength and difficulty in reaction. In addition, the increased porosity may be filled with electrolyte, which may result in a problem in that the energy density of the battery may be lowered, and therefore, the porosity is appropriately controlled within the above range.

[0241]

[0242] In one embodiment of the present invention, the separator is formed of a porous non-conductive or insulating material that separates or insulates the positive electrode and the negative electrode from each other and enables lithium ion transport between the positive electrode and the negative electrode. If it is commonly used as a separator in a lithium secondary battery, it can be used without special limitation. The separator may be an independent member such as a film, or may be a coating layer added to the positive electrode and / or the negative electrode.

[0243] It is preferable that the above separator have low resistance to ion movement of the electrolyte and excellent moisture absorption capacity for the electrolyte.

[0244] In one embodiment of the present invention, the separator may include a porous substrate, and any porous substrate typically used in secondary batteries may be used as the porous substrate, and a porous polymer film may be used alone or in a laminated manner. For example, a nonwoven fabric or a polyolefin porous film made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto.

[0245] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate may be a polyolefin such as polyethylene, polypropylene, etc., a polyester such as polyethyleneterephthalate, polybutyleneterephthalate, polyamide, polyacetal, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, It may include at least one material selected from the group consisting of poly(p-phenylene benzobisoxazole) and polyarylate.

[0246] In one embodiment of the present invention, the thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. The thickness range of the porous substrate is not limited to the above-mentioned range, but if the thickness is excessively thinner than the above-mentioned lower limit, the mechanical properties may deteriorate, and the separator may be easily damaged during battery use.

[0247] In one embodiment of the present invention, the average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 to 50 ㎛ and 10 to 95 volume%, respectively.

[0248] In one embodiment of the present invention, the separator may further include a porous coating layer formed on at least one surface of the porous substrate and including inorganic particles and a binder polymer.

[0249] In one embodiment of the present invention, the inorganic particles and binder polymer included in the porous coating layer may be used without particular limitation as long as they are commonly used in the porous coating layer of a separation membrane, and the manufacturing method thereof is also not particularly limited.

[0250]

[0251] In one embodiment of the present invention, the electrolyte comprises a non-aqueous solvent as a medium through which ions involved in the electrochemical reaction of a lithium secondary battery can move, and a lithium salt as an electrolyte. The electrolyte is not particularly limited as long as it has a composition that can be used in a lithium secondary battery, specifically a lithium-sulfur battery.

[0252] In one embodiment of the present invention, the non-aqueous solvent may be used without particular limitation as long as it is of a type that can be used in a lithium-sulfur battery, and for example, ether solvents, esters, amides, linear carbonates, and cyclic carbonates may be used.

[0253] In one embodiment of the present invention, the ester may be, for example, one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0254] In one embodiment of the present invention, the linear carbonate may be, for example, one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate, or a mixture of two or more thereof, but is not limited thereto.

[0255] In one embodiment of the present invention, the cyclic carbonate is, for example, one selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halogenides thereof, or a mixture of two or more thereof. Examples of the halogenides thereof include, but are not limited to, fluoroethylene carbonate.

[0256] In one embodiment of the present invention, the non-aqueous solvent may include an ether solvent.

[0257] In one embodiment of the present invention, the ether solvent may be included in an amount of 80% by volume or more, for example, 85% by volume to 100% by volume, 90% by volume to 100% by volume, 95% by volume to 100% by volume, 98% by volume to 100% by volume, 90% by volume to 98% by volume, or 90% by volume to 95% by volume, based on the total volume of the non-aqueous solvent. When the amount of the ether solvent is in the above-described range based on the total volume of the non-aqueous solvent, an advantageous effect may be exhibited in terms of the solubility of the electrolyte composition such as a lithium salt, but the present invention is not limited thereto.

[0258] In one embodiment of the present invention, the ether solvent may include an acyclic ether, a cyclic ether, or a mixture thereof.

[0259] In one embodiment of the present invention, the acyclic ether is selected from the group consisting of, for example, dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethylpropyl ether, ethyl tertbutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethylmethyl ether, diethylene glycol isopropylmethyl ether, diethylene glycol butylmethyl ether, diethylene glycol tertbutylethyl ether, and ethylene glycol ethylmethyl ether. It may include one or more kinds. Preferably, it may include one or more kinds selected from the group consisting of dimethyl ether, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and more preferably, it may include dimethoxyethane.

[0260] In one embodiment of the present invention, the cyclic ether is, for example, 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, It may include at least one selected from the group consisting of 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. Preferably, it may include at least one selected from the group consisting of 2-methylfuran, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran, and more preferably, it may include 2-methylfuran.

[0261] In one embodiment of the present invention, the non-aqueous solvent may include a mixture of an acyclic ether and a cyclic ether.

[0262] In one embodiment of the present invention, the non-aqueous solvent may include dimethoxyethane (DME) and 2-methylfuran (2-MeF).

[0263] In one embodiment of the present invention, the volume ratio of the acyclic ether and the cyclic ether may be 5:95 to 95:5 (v / v), specifically 95:5 to 50:50, more specifically 90:10 to 70:30, 85:15 to 75:25, or 80:20. In the present invention, the volume ratio corresponds to the ratio of "volume % of acyclic ether": "volume % of cyclic ether" in the ether solvent.

[0264] In one embodiment of the present invention, the non-aqueous solvent may not contain a carbonate solvent in terms of the solubility of the electrolyte. Alternatively, the non-aqueous solvent may contain a very small amount of a carbonate solvent such that the carbonate solvent does not affect the solubility of the lithium salt. For example, when the non-aqueous solvent contains the carbonate solvent, the content of the carbonate solvent may be 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, or 0 wt% (i.e., not contained at all) based on the total weight of the electrolyte for a lithium secondary battery.

[0265] In one embodiment of the present invention, the lithium salt may be used without particular limitation as long as it can be used as an electrolyte of a lithium secondary battery. The lithium salt may be, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF2SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate, lithium imide, or two or more thereof.

[0266] In one embodiment of the present invention, the concentration of the lithium salt may be appropriately determined in consideration of ionic conductivity, solubility, etc., and may be, for example, 0.1 to 4 M, preferably 0.5 to 2 M. When the concentration of the lithium salt is within the above-described range, it is advantageous to secure ionic conductivity suitable for battery operation, or to exhibit an advantageous effect in terms of improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself by exhibiting an appropriate viscosity of the electrolyte, but the present invention is not limited thereto.

[0267] In one embodiment of the present invention, the electrolyte may further include a nitrogen compound in addition to a lithium salt to improve the electrical conductivity of the electrolyte and extend the life of the lithium-sulfur battery. Specifically, the nitrogen compound may, for example, inhibit the reduction reaction of polysulfides occurring during the charge / discharge process of the lithium-sulfur battery, thereby preventing irreversible depletion of polysulfides, thereby improving the performance of the lithium-sulfur battery, although its efficacy is not limited thereto.

[0268] In one embodiment of the present invention, the nitrogen compound may be used without particular limitation as long as it forms a stable film on a lithium metal electrode, which is a negative electrode of a lithium secondary battery, specifically a lithium-sulfur battery, and exhibits an effect of improving charge / discharge efficiency. For example, the nitrogen compound may be a nitric acid compound, a nitrite compound, or a mixture thereof.

[0269] In one embodiment of the present invention, the nitrogen compound is, for example, an inorganic nitric or nitrous compound such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), ammonium nitrite (NH4NO2); an organic nitric or nitrous compound such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; It may be selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof, and preferably may include lithium nitrate.

[0270] In one embodiment of the present invention, the nitrogen compound may be included in an amount of, for example, 1 wt% to 10 wt%, 2 wt% to 10 wt%, or 3 wt% to 10 wt%, specifically 3 wt% to 8 wt%, 3 wt% to 6 wt%, or 3 wt% to 5 wt%, based on the total weight of the electrolyte for a lithium secondary battery, but is not limited thereto. When the nitrogen compound is included in the above-described amount, it may exhibit a more advantageous effect in terms of improving the electrical conductivity of the electrolyte by the nitrogen compound and suppressing the reduction of polysulfide when used in a lithium-sulfur battery, but the present invention is not limited thereto.

[0271]

[0272] In one embodiment of the present invention, the lithium secondary battery may have various shapes, for example, a coin shape, a pouch shape, or a cylindrical shape, but is not limited thereto.

[0273]

[0274] Hereinafter, the present invention will be described in more detail through examples, but the following examples are intended to illustrate the present invention, and the scope of the present invention is not limited to these examples.

[0275]

[0276] Example 1

[0277] <Manufacturing of the cathode>

[0278] A 40 ㎛ thick lithium metal film was laminated on a 10 ㎛ thick copper film and rolled using a roll press.

[0279] Afterwards, a solution of polyethylene oxide dissolved in DMAc (Dimethylacetamide) was prepared, and doctor blade casting was performed on the lithium metal thin film to form a first protective layer having a thickness of 2.5 μm. Afterwards, drying was performed at 60°C under atmospheric pressure for 1 hour, and a solution of PVdF-HFP dissolved in DMAc was prepared, and doctor blade casting was performed on the first protective layer to form a second protective layer having a thickness of 2.5 μm. Afterwards, drying was performed at 60°C under atmospheric pressure for 1 hour. At this time, HFP was made to be 8 wt% in the PVDF-HFP polymer.

[0280] Figure 11a shows an actual image of the surface of the negative electrode, an SEM image, a cross-sectional SEM image, and EDS analysis data. Referring to these, it was confirmed that a first protective layer (inner layer) and a second protective layer (outer layer) were formed on the lithium metal surface. The left side of the EDS analysis data confirmed the presence of carbon, the center the presence of fluorine, and the right the presence of oxygen. In addition, it was confirmed that the first and second protective layers had few pores.

[0281]

[0282] <Manufacturing of Lithium Sulfur Batteries>

[0283] A sulfur-carbon composite (S / C weight ratio = 2.45) was prepared by mixing inorganic sulfur (S8) and carbon nanotubes (CNT) as positive electrode active materials, and a positive electrode slurry composition was prepared by mixing 96 wt% of the prepared sulfur-carbon composite and 4 wt% of PAA (acrylic acid) as a binder polymer. The positive electrode slurry composition was applied to an aluminum current collector and then dried to prepare a positive electrode. The loading of the prepared positive electrode was 2.6 mAh / cm 2 It was.

[0284] An electrode assembly was prepared by positioning the positive and negative electrodes so that they face each other and interposing a polyethylene separator having a thickness of 16 ㎛ and a porosity of 46 vol% between them.

[0285] The prepared electrode assembly was stored in a pouch-type case, and a lithium-sulfur battery was manufactured by injecting an electrolyte solution containing 1 M lithium salt (LiTFSI) and 2 wt% lithium nitrate (LiNO3) dissolved in a solvent mixed with 1,3-dioxolane (DOL) and dimethoxyethane (DME) in a 1:1 volume ratio.

[0286] At this time, 70 μl of the electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 20 μl / mg.

[0287]

[0288] Comparative Example 1

[0289] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a lithium metal film with a thickness of 40 μm was used as the cathode.

[0290]

[0291] Comparative Example 2

[0292] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a PVdF layer with a thickness of 3 μm was formed on one side of a lithium metal film with a thickness of 40 μm as the cathode. Figure 11b shows an actual image and an SEM image of the surface of the cathode, and an SEM image of a cross-section.

[0293]

[0294] Comparative Example 3

[0295] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a PVDF-HFP layer having a thickness of 3 μm was formed on one side of a lithium metal thin film without forming a polyethylene oxide layer as a cathode, and that HFP was made to be 8 wt% in the PVDF-HFP polymer.

[0296] Figure 11c shows an actual image and an SEM image of the surface of the cathode, and an SEM image of a cross-section.

[0297]

[0298] Comparative Example 4

[0299] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that the PVDF-HFP layer was not formed on the cathode and the first protective layer, PEO, was formed to a thickness of 3 μm.

[0300] Figure 11d shows an actual image and an SEM image of the surface of the cathode, and an SEM image of a cross-section.

[0301]

[0302] Comparative Example 5

[0303] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that the thickness of the first protective layer was formed to be 5 μm and the thickness of the second protective layer was formed to be 5 μm in the negative electrode.

[0304]

[0305] Comparative Example 6

[0306] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a solution of PVdF-HFP dissolved in DMAc was used to form the first protective layer, and a solution of polyethylene oxide dissolved in DMAc was used to form the second protective layer.

[0307]

[0308] Example 2

[0309] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0310]

[0311] Comparative Example 7

[0312] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 1, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0313]

[0314] Comparative Example 8

[0315] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 2, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0316]

[0317] Comparative Example 9

[0318] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 3, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0319]

[0320] Comparative Example 10

[0321] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 4, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0322]

[0323] Comparative Example 11

[0324] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 5, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0325]

[0326] Comparative Example 2

[0327]

[0328] Comparative Example 12

[0329] A lithium-sulfur battery was manufactured in the same manner as in Comparative Example 6, except that 18 μl of electrolyte was injected so that the electrolyte (μl) / sulfur (S) (mg) was 5 μl / mg.

[0330]

[0331] Experimental Example 1: Battery Performance Evaluation

[0332] For the lithium-sulfur batteries according to Example 1 and Comparative Examples 1 to 6, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C were performed at 25°C using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 220 times. At this time, both charging and discharging were performed in cc mode.

[0333] Fig. 5 is a graph showing the results of testing the discharge capacity and coulombic efficiency of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4. Figs. 6a to 6g are charge / discharge profile graphs of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6.

[0334] Referring to FIGS. 5 to 6g, it can be confirmed that the discharge capacity of the lithium-sulfur battery according to Example 1 is maintained satisfactorily even after 200 cycles. On the other hand, it can be confirmed that the discharge capacity of the lithium-sulfur batteries of Comparative Examples 2 to 6 decreases.

[0335]

[0336] Experimental Example 2: Battery Performance Evaluation

[0337] For the lithium-sulfur batteries according to Example 2 and Comparative Examples 7 to 12, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C were performed at 25°C using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 100 times. At this time, both charging and discharging were performed in cc mode.

[0338] Figure 7 shows the results of testing the discharge capacity and coulombic efficiency of Example 2, Comparative Example 7, Comparative Example 8, Comparative Example 9, and Comparative Example 10. Figures 8a to 8g are charge / discharge profile graphs of Example 2, Comparative Example 7, Comparative Example 8, Comparative Example 9, Comparative Example 10, Comparative Example 11, and Comparative Example 12.

[0339] Referring to FIGS. 7 to 8g, it can be confirmed that the lithium-sulfur battery according to Example 2 maintains excellent discharge capacity even after 90 cycles, and that the overvoltage is also stable. On the other hand, it can be confirmed that the lithium-sulfur battery according to Comparative Examples 7 to 12 has a significantly reduced discharge capacity and a very large overvoltage as the cycle is performed.

[0340]

[0341] Experimental Example 3: Observation of the cathode surface and cross-section

[0342] For the lithium-sulfur batteries according to Example 1 and Comparative Examples 1 to 4, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C were performed at 25°C using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 160 times. At this time, both charging and discharging were performed in cc mode. Afterwards, each lithium-sulfur battery was disassembled to extract the negative electrode, and SEM images were taken of the surface and cross-section of the extracted negative electrode.

[0343] FIGS. 12a to 12e are surface and cross-section views of the negative electrode after 25 cycles of operation of the lithium-sulfur battery according to Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, and FIG. 12f is a surface view of the lithium metal layer after the negative electrode is extracted and the protective layer is removed after 25 cycles of operation of the lithium-sulfur battery according to Example 1.

[0344] Referring to FIGS. 12a to 12e, it was confirmed that the negative electrode according to Example 1 suppressed the formation of pores in the protective layer even as the cycle progressed, and maintained adhesion even when the surface of the lithium metal became uneven. On the other hand, in the case of Comparative Example 1, pores were formed in the lithium metal because the protective layer was absent. In the case of Comparative Example 2, the protective layer was removed from the negative electrode, and the surface of the lithium metal layer was photographed, and it was confirmed that the lithium metal layer reacted significantly with the electrolyte, resulting in porosity. In the case of Comparative Example 3, it was confirmed that lithium dendrites penetrated the protective layer, and in the case of Comparative Example 4, it was confirmed that the protective layer dissolved in the electrolyte.

[0345] Figure 12f shows SEM images of the surface of the lithium metal layer after the lithium-sulfur battery according to Example 1 was operated for 25 cycles, and the negative electrode was extracted and the protective layer was removed. The left and right images of Figure 12f are SEM images of different spots on the lithium metal layer. Unreacted lithium was observed in the right image of Figure 12f. This is because the protective film well protected the lithium metal layer. That is, it means that no pores were formed in the protective film, and the lithium metal layer maintained good adhesion to the protective layer despite changes in its surface structure. In addition, both the left and right images of Figure 12f show that there is little porosity in the reacted lithium portion. This means that the protective layer suppresses the lithium metal layer from reacting with the electrolyte.

[0346]

[0347] Experimental Example 4: Observation of Cathode Expansion

[0348] For the lithium-sulfur batteries according to Example 2 and Comparative Examples 7 to 10, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C were performed at 25°C using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 30 times. At this time, both charging and discharging were performed in cc mode. Afterwards, each lithium-sulfur battery was disassembled to extract the negative electrode, and SEM images were taken of the cross-sections of the extracted negative electrodes, and the results are shown in FIGS. 13a to 13f.

[0349] Referring to FIGS. 13a to 13f, it was confirmed that the negative electrode of Comparative Example 7 had lithium porosity and that the lithium layer was greatly expanded. In the negative electrode of Comparative Example 8, the protective layer was partially detached and the electrolyte was introduced, which aggravated the deterioration of the negative electrode and caused the porous negative electrode to greatly expand. In the negative electrode of Comparative Example 9, the protective layer allowed the electrolyte to pass through, and the electrolyte reacted with the lithium layer of the negative electrode, causing the lithium layer to become porous and expand in volume. In the negative electrode of Example 2, it was confirmed that the protective layer suppressed the inflow of the electrolyte, thereby suppressing lithium porosity and volume expansion.

[0350]

[0351] Experimental Example 5: Observation of the Cathode Surface

[0352] Three lithium-sulfur batteries were manufactured according to Example 1 and Comparative Examples 1 to 4, one of which was first discharged at 0.1 C at 25°C, another was first discharged at 0.1 C at 25°C and then charged at 0.1 C, and the remaining one was first discharged at 0.1 C at 25°C, then charged at 0.1 C and discharged at 0.1 C. Afterwards, each lithium-sulfur battery was disassembled to extract the negative electrode, and the surface thereof was photographed by SEM, and the respective images are shown in FIGS. 14a to 14e. At this time, both charging and discharging were performed in cc mode.

[0353] The negative electrode of Comparative Example 1 was extremely porous, and a Cu current collector was observed due to uneven lithium deposition. The negative electrode of Comparative Example 2 was confirmed to have partially detached the protective layer during the first discharge, resulting in uneven lithium deposition on the lithium metal layer. The negative electrode of Comparative Example 3 was confirmed to have the protective layer penetrated by lithium dendrites. The negative electrode of Comparative Example 4 was observed to have the protective layer dissolved in the electrolyte. On the other hand, the negative electrode of Example 1 was confirmed to have no detachment of the protective layer and no perforations formed.

[0354]

[0355] Experimental Example 6: XPS Test

[0356] For the lithium-sulfur batteries of Example 1 and Comparative Examples 1 to 4, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C were performed at 25°C using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 100 times. At this time, both charging and discharging were performed in cc mode. Afterwards, each battery was disassembled to extract the negative electrode, and XPS (X-ray photoelectron spectroscopy) was performed on the extracted negative electrode to compare the composition of lithium surface side reaction products and observe which components in the electrolyte dominantly reacted with them. The results are shown in FIGS. 15a to 19d.

[0357] Specifically, FIGS. 15a to 15d are for Comparative Example 1, FIGS. 16a to 16d are for Comparative Example 2, FIGS. 17a to 17d are for Comparative Example 3, FIGS. 18a to 18d are for Comparative Example 4, and FIGS. 19a to 19d are for Example 1.

[0358] In Figures 15a to 19d, the law data are indicated by thick lines, and the data obtained by fitting the law data are indicated by thin lines. The detection of a specific substance can be confirmed by the occurrence of a peak at a specific binding energy in the XPS data.

[0359] For example, in the XPS data for C 1s, C-F3 at 293 eV, Li2CO3 at 290 eV, C=O at 288.2 eV, CO at 286 eV, and CC(SP) at 284.8 eV 2 ), C-Li can be detected at 283 eV. For F 1s, CF can be identified at 588.8 eV, and Li-F at 684.5 eV. For N 1s, NxOy can be identified at 407 eV, CN and NH at 400 eV, and Li3N at 398 eV. For S 2p, TFSI can be identified at 170 eV. - , SO3 at 167~168 eV 2- and S2O3 2- , LiPS at 160-165 eV, Li2S(S) at 160.2 eV 2- ), Li2S2 at 161.8 eV, and Li2Sx at 163.1 eV can be confirmed.

[0360] Referring to FIGS. 15a to 19d, in the case of Comparative Examples 2 and 3, peaks were observed in the C 1s CF 3 (293 eV), F 1s CF (688.8 eV), N 1s CN, Li 3 N, S 2p high BE region, and the 166-170 eV region, indicating that the product reacted with the electrolyte and decomposed into salts significantly. In the case of Comparative Example 4, the reaction with the electrolyte was measured to be insignificant, which is because the protective layer of Comparative Example 4 was detached or dissolved into the electrolyte and disappeared. It was confirmed that the protective layer of Example 1 suppressed the reaction with the electrolyte, and thus the reaction with polysulfide and the reaction with the electrolyte (LiTFSI) were suppressed.

[0361]

[0362] Experimental Example 7: Protective Layer Yield Elongation Test

[0363] To perform a yield elongation test on the protective layer, each protective layer was prepared separately as follows.

[0364]

[0365] Manufacturing Example 1

[0366] A solution of polyethylene oxide dissolved in DMAc was prepared, and the solution was doctor blade casted onto a release paper made of polyethylene terephthalate to form a first protective layer having a thickness of 100 μm. Then, drying was performed at 60°C for 1 hour, and a solution of PVDF-HFP dissolved in DMAc was prepared, and doctor blade casted onto the first protective layer to form a second protective layer having a thickness of 100 μm. Then, drying was performed at 60°C for 1 hour. At this time, HFP was made to be 8 wt% in the PVDF-HFP polymer. Thereafter, the release paper was removed to obtain a protective layer comprising a PEO layer as the first protective layer and a PVDF-HFP (8 wt%) layer as the second protective layer.

[0367]

[0368] Manufacturing Example 2

[0369] A protective layer consisting of a PVDF-HFP (8 wt%) single layer was obtained in the same manner as in Manufacturing Example 1, except that only the second protective layer was formed without forming the first protective layer.

[0370]

[0371] Manufacturing Example 3

[0372] A protective layer consisting of a PVDF-HFP (15 wt%) single layer was obtained in the same manner as in Manufacturing Example 1, except that only the second protective layer was formed without forming the first protective layer and the substitution rate of HFP in the PVDF-HFP polymer was set to 15%.

[0373]

[0374] Manufacturing Example 4

[0375] A protective layer consisting of a single PVDF layer was obtained in the same manner as in Manufacturing Example 1, except that the first protective layer was not formed and PVDF was used instead of PVdF-HFP.

[0376]

[0377] Manufacturing Example 5

[0378] A protective layer consisting of a PEO single layer was obtained in the same manner as in Manufacturing Example 1, except that the second protective layer was not formed.

[0379]

[0380] For the protective layers according to Manufacturing Examples 1 to 5, the magnitude of the stress applied to the protective layers was measured using a universal testing machine (UTM), and a stress-strain graph was drawn. Then, the degree of deformation (elongation) at the yield point shown on the graph was measured to obtain the yield elongation value. The measured yield elongation values ​​are shown in Table 1 and Fig. 4.

[0381] Yield Elongation (%) Manufacturing Example 13.93 Manufacturing Example 22.27 Manufacturing Example 31.62 Manufacturing Example 41.2 Manufacturing Example 52.14

[0382] Referring to Table 1 and FIG. 4 above, it was confirmed that Manufacturing Example 1, which includes a configuration corresponding to an embodiment of the present invention, had a high elongation at the yield point. This shows that the protective layer of Manufacturing Example 1 is remarkably flexible and elastic compared to Manufacturing Examples 2 to 5, which were manufactured as a single layer.

[0383] Meanwhile, in this experimental example, a protective layer with a thickness of 100 μm was manufactured and the yield elongation was tested. However, since yield elongation is expressed as a percentage (%) of the initial thickness, the effect of thickness is extremely minimal. Therefore, it is obvious to those skilled in the art that even if the protective layer is manufactured with a different thickness, results equivalent to the experimental results in Table 1 above will be obtained.

[0384]

[0385] Experimental Example 8: Resistance Test for Protective Film Thickness

[0386] To confirm the interface resistance according to the thickness and composition of the protective layer, a separate symmetrical cell was manufactured as follows.

[0387]

[0388] Manufacturing Example 6

[0389] Two negative electrodes identical to those manufactured in Example 1 were prepared, and a porous polyethylene (PE) separator having a thickness of 16 μm and a porosity of 46 vol% was interposed between them, and then 70 μl / mg of electrolyte (μl) was injected to manufacture a symmetrical cell.

[0390]

[0391] Manufacturing Example 7

[0392] A symmetrical cell was manufactured in the same manner as in Manufacturing Example 6, except that the same negative electrode manufactured in Comparative Example 1 was used instead of the same negative electrode manufactured in Example 1.

[0393]

[0394] Manufacturing Example 8

[0395] A symmetrical cell was manufactured in the same manner as in Manufacturing Example 6, except that the same negative electrode manufactured in Comparative Example 5 was used instead of the same negative electrode manufactured in Example 1.

[0396]

[0397] Manufacturing Example 9

[0398] A symmetrical cell was manufactured in the same manner as in Manufacturing Example 6, except that the same negative electrode manufactured in Comparative Example 6 was used instead of the same negative electrode manufactured in Example 1.

[0399]

[0400] The bulk resistance, SEI resistance, and interface resistance of the symmetrical cells manufactured in Manufacturing Examples 6 to 9 were measured, and the results are shown in Table 2 below. The resistance measurement results are also shown in Fig. 20.

[0401] The above resistance measurement was performed under conditions of 25℃ and 1 atm. Specifically, the resistance measurement was

[0402] Impedance was measured using electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 Hz to 1 MHz. At this time, the bulk resistance was determined as the x-intercept value of the graph according to Fig. 20, the SEI resistance was determined as the x-axis value corresponding to 29 Hz, and the interfacial resistance was determined as the x-axis value corresponding to 0.05 Hz minus the bulk resistance.

[0403]

[0404] Bulk resistance (R b ) (Ω)SEI resistance (R SEI ) (Ω)Interface resistance (R) I ) (Ω) Manufacturing example 61.05214.57279.71 Manufacturing example 70.91152.09192.09 Manufacturing example 81.3217.91326.03 Manufacturing example 91.01213.54279.22

[0405] Referring to Table 2 and Figure 20 above, it was confirmed that as the thickness of the protective film increases, the bulk resistance, SEI resistance, and interface resistance all increase.

[0406]

[0407] Experimental Example 9: Test for Protective Film Thickness

[0408] For the lithium-sulfur batteries according to Example 1, Comparative Example 1, Comparative Example 5, and Comparative Example 6, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C at 25°C were performed using a Landt cycler, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 250 times. At this time, both charging and discharging were performed in cc mode. The voltage profile for the first charge-discharge during the 0.2 C charge and discharge process is shown in Fig. 9a, and the voltage profile at the 200th charge-discharge is shown in Fig. 9c. In addition, the discharge capacity and coulombic efficiency are shown in Fig. 9b.

[0409] For the lithium-sulfur batteries according to Example 2, Comparative Example 7, Comparative Example 11, and Comparative Example 12, 2.5 cycles (discharge, charge, discharge, charge, and discharge) of discharging and charging at 0.1 C through a Landt cycler at 25°C were performed, and then 3 cycles of charging and discharging at 0.2 C were performed. Then, charging at 0.3 C and discharging at 0.5 C were repeated 100 times. The voltage profile for the first charge-discharge during the 0.2 C charge and discharge process is shown in Fig. 10a, and the voltage profile for the 100th charge-discharge is shown in Fig. 10c. In addition, the discharge capacity and coulombic efficiency are shown in Fig. 10b.

[0410] Referring to FIGS. 9a to 9c, it can be confirmed that the discharge capacity of Comparative Example 5 is significantly reduced due to the thick thickness of the protective layer.

[0411] In addition, referring to FIGS. 10a to 10c, it can be confirmed that the discharge capacity of Comparative Example 10 is significantly reduced due to the thick thickness of the protective layer.

[0412]

[0413] Example 3

[0414] <Manufacturing of the cathode>

[0415] A 40 ㎛ thick lithium metal film was laminated on a 10 ㎛ thick copper film and rolled using a roll press.

[0416] Afterwards, a solution of polyethylene oxide dissolved in DMAc (Dimethylacetamide) was prepared, and doctor blade casting was performed on the lithium metal thin film to form a first protective layer having a thickness of 2.5 μm. Afterwards, drying was performed at 60°C under atmospheric pressure for 1 hour, and a solution of PVdF-HFP dissolved in DMAc was prepared, and doctor blade casting was performed on the first protective layer to form a second protective layer having a thickness of 2.5 μm. Afterwards, drying was performed at 60°C under atmospheric pressure for 2 hours. At this time, HFP was made to be 8 wt% in the PVDF-HFP polymer.

[0417]

[0418] <Manufacturing of Lithium Sulfur Batteries>

[0419] A sulfur-carbon composite (S / C weight ratio = 2.45) was prepared by mixing inorganic sulfur (S8) and carbon nanotubes (CNT) as positive electrode active materials, and a positive electrode slurry composition was prepared by mixing 96 wt% of the prepared sulfur-carbon composite and 4 wt% of PAA (acrylic acid) as a binder polymer. The positive electrode slurry composition was applied to an aluminum current collector and then dried to prepare a positive electrode. The loading of the prepared positive electrode was 3.6 mAh / cm 2 It was.

[0420] An electrode assembly was prepared by positioning the positive and negative electrodes so that they face each other and interposing a polyethylene separator having a thickness of 16 ㎛ and a porosity of 46 vol% between them.

[0421] The prepared electrode assembly was stored in a pouch-type case, and an electrolyte solution containing 1 M lithium salt (LiTFSI) and 2 wt% lithium nitrate (LiNO3) dissolved in a solvent mixed with 1,3-dioxolane (DOL) and dimethoxyethane (DME) in a 1:1 volume ratio was injected to manufacture a lithium-sulfur battery.

[0422] At this time, the electrolyte was adjusted to have an electrolyte (㎕) / sulfur (S) (mg) ratio of 10 ㎕ / mg.

[0423]

[0424] Comparative Example 13

[0425] A lithium-sulfur battery was manufactured in the same manner as in Example 3, except that a PVdF layer with a thickness of 3 μm was formed on one side of a lithium metal film with a thickness of 40 μm as the cathode.

[0426]

[0427] Comparative Example 14

[0428] A lithium-sulfur battery was manufactured in the same manner as in Example 3, except that after forming the second protective layer, drying was performed at 60°C in a vacuum atmosphere for 1 hour.

[0429]

[0430] Experimental Example 10: Porosity Comparison

[0431] The surface and cross-section of the cathode manufactured in Example 3 were photographed by SEM and are shown in Figs. 21a to 21d. Referring to Figs. 21a to 21d, it can be confirmed that there are almost no pores.

[0432] The surface and cross-section of the cathode manufactured in Comparative Example 14 were photographed by SEM and are shown in Figures 22a to 22d. Referring to Figures 22a to 22d, it can be confirmed that numerous pores were formed.

[0433] The volume of the second protective layer in the cathode of Example 3 and Comparative Example 14 was measured, and the second protective layer was obtained and measured for weight. And the density (1.75 g / cm) of PVdF-HFP applied to the second protective layer 3 ) and the results of calculating the porosity according to the equation below are shown in Table 3 below.

[0434] (Formula) Porosity (%) = (volume - (mass / density)) / volume × 100

[0435]

[0436] Second protective layer weight (g) Second protective layer volume (mm) 3 )Porosity (%)Example 32.361.39.2.93Comparative example 1421.7634.9

[0437] Referring to Table 3 above, it was confirmed that the second protective layer of the negative electrode manufactured in Example 3 had a very small porosity.

[0438]

[0439] Experimental Example 11: Battery Performance Evaluation

[0440] For the lithium-sulfur batteries manufactured in Example 3, Comparative Examples 13 and 14, 2.5 cycles (activation stage) of discharging and charging at 0.1 C were performed using a Landt cycler at 25°C, followed by 3 cycles of charging and discharging at 0.2 C. Then, charging at 0.3 C and discharging at 0.5 C were repeated 180 times. At this time, both charging and discharging were performed in cc mode.

[0441] Figure 23a is a charge / discharge profile graph for the first cycle after activation, Figure 23b is a charge / discharge profile graph for 50 cycles after activation, and Figure 23c is a charge / discharge profile graph for 100 cycles after activation. Figure 23d is a graph showing the results of testing discharge capacity and coulombic efficiency.

[0442] The lithium-sulfur battery of Comparative Example 13 was confirmed to have a rapid increase in overvoltage after 40 cycles. The lithium-sulfur battery of Comparative Example 14 was confirmed to have a smaller discharge capacity and a larger overvoltage than the lithium-sulfur battery of Example 3.

[0443]

[0444] As described above, although the present invention has been described by limited embodiments and drawings, the present invention is not limited thereto, and it is obvious that various modifications and variations are possible within the scope of the technical idea of ​​the present invention and the equivalent scope of the patent claims to be described below by a person having ordinary skill in the art to which the present invention pertains.

Claims

1. A negative electrode active material layer; a first protective layer formed on at least one surface of the negative electrode active material layer; and a second protective layer formed on one surface of the first protective layer; comprising: The above negative active material layer comprises at least one of lithium or a lithium alloy, The above first protective layer comprises polyalkylene oxide, The second protective layer comprises a polyvinylidene fluoride polymer (PVDF polymer), A cathode, characterized in that the second protective layer has a porosity of 10% or less.

2. In paragraph 1, A cathode, characterized in that the thickness of the first protective layer is 0.1 ㎛ or more and less than 5 ㎛.

3. In paragraph 1, A cathode, characterized in that the thickness of the second protective layer is 0.1 ㎛ or more and less than 5 ㎛.

4. In paragraph 1, A cathode characterized in that the polyalkylene oxide comprises at least one selected from polyethylene oxide, polypropylene oxide, and polybutylene oxide.

5. In paragraph 1, A cathode characterized in that the above polyvinylidene fluoride-based polymer comprises a homopolymer of vinylidene fluoride, a copolymer comprising vinylidene fluoride, or both.

6. In paragraph 5, A negative electrode characterized in that the copolymer containing the vinylidene fluoride contains 0.1 wt% to 20 wt% of a monomer that is bonded to vinylidene fluoride with respect to a total of 100 wt%.

7. In paragraph 5, The copolymer comprising the above vinylidene fluoride, A cathode characterized by comprising polyvinylidene fluoride hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride chlorotrifluoroethylene (PVDF-CTFE), polyvinylidene fluoride tetrafluoroethylene (PVDF-TFE), polyvinylidene fluoride trifluoroethylene (PVDF-TrFE) or a mixture of two or more thereof.

8. In paragraph 1, The ionic conductivity of the first protective layer is 10 at 25°C. -11 10 inland -6 A cathode characterized by having a S / cm.

9. In paragraph 1, A cathode characterized in that the entire protective layer including the first protective layer and the second protective layer has a yield elongation of 2.5% or more.

10. In paragraph 1, A cathode, characterized in that the total thickness of the first protective layer and the second protective layer is 0.2 ㎛ or more and less than 10 ㎛.

11. In paragraph 1, A lithium alloy is a negative electrode characterized in that it is an alloy of lithium (Li) and at least one metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

12. In paragraph 1, A cathode, characterized in that the first porous layer has a porosity of 10% or less.

13. In paragraph 1, A cathode, characterized in that the second porous layer has a porosity of 8% or less.

14. Including a cathode; an anode; and a separator interposed between the cathode and the anode, A lithium secondary battery, characterized in that the negative electrode is a negative electrode according to any one of claims 1 to 13.

15. In paragraph 14, A lithium secondary battery, characterized in that the positive electrode comprises a positive electrode active material including a sulfur-carbon complex.

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