Negative electrode and solid-state secondary battery including the same

Abstract

Provided are a negative electrode and a solid-state secondary battery including the same. The negative electrode includes a negative electrode current collector and a first negative electrode active material layer disposed on the negative electrode current collector. The negative electrode current collector includes a first metal substrate and a coating layer located on the first metal substrate and containing a second metal, and the second metal has a Mohs hardness greater than that of the first metal.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10 - 2020 - 0019993, filed on February 18, 2020, with the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a negative electrode and a solid - state secondary battery. Background Art

[0004] Recently, in response to industrial demands, efforts have been increased to develop batteries with high energy density and improved safety. For example, lithium - ion batteries have been put into practical use in automobiles, information - related devices, and communication equipment. In the automotive field, safety is particularly emphasized because a malfunction can endanger lives.

[0005] Since currently available lithium - ion batteries include an electrolyte containing a flammable organic solvent, there is a possibility of overheating or catching fire when a short - circuit occurs. Therefore, there is still a need for solid - state batteries using a solid electrolyte instead of a liquid electrolyte.

[0006] Solid - state batteries do not include flammable organic solvents, and thus even when a short - circuit occurs, the chance of causing a fire or explosion can be significantly reduced. Therefore, solid - state batteries can provide higher safety than lithium - ion batteries using a liquid electrolyte. Summary of the Invention

[0007] A negative electrode having a novel structure is provided.

[0008] A solid - state secondary battery including a negative electrode having a novel structure is provided.

[0009] Additional aspects will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the presented embodiments of the present disclosure.

[0010] According to an aspect of an embodiment,

[0011] The negative electrode includes a negative - electrode current collector and a first negative - electrode active - material layer disposed on the negative - electrode current collector,

[0012] wherein the negative - electrode current collector includes: a first metal substrate; and a coating layer disposed on the first metal substrate and containing a second metal,

[0013] The second metal has a Mohs hardness greater than that of the first metal.

[0014] According to another aspect of an embodiment,

[0015] The solid-state secondary battery includes:

[0016] a positive electrode;

[0017] a negative electrode according to the above; and

[0018] a solid electrolyte layer positioned between the positive electrode and the negative electrode,

[0019] wherein the solid electrolyte layer includes a sulfide-based solid electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other aspects, features, and advantages of some embodiments of the present disclosure will become clearer from the following description taken in conjunction with the accompanying drawings, in which:

[0021] Figure 1 is a cross-sectional view of a negative electrode according to an example embodiment;

[0022] Figure 2 is a cross-sectional view of a negative electrode according to another example embodiment;

[0023] Figure 3 is a cross-sectional view of a solid-state secondary battery according to an example embodiment;

[0024] Figure 4A and 4B is a cross-sectional view of a dual-cell type solid-state secondary battery according to an example embodiment;

[0025] Figure 5 is a schematic view of a positive electrode layer of a solid-state secondary battery according to an example embodiment; and

[0026] Figure 6 is a schematic view partially showing the interior of a solid-state secondary battery according to an example embodiment. DETAILED DESCRIPTION

[0027] Embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings, where like reference numerals always denote like elements. In this regard, the embodiments may have different forms and should not be construed as limited to the description set forth herein. Accordingly, the embodiments are described below only to illustrate aspects of the description by referring to the drawings. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one (each) of" modify the entire list of elements when preceding or following the list of elements, rather than modifying individual elements of the list.

[0028] During the charging / discharging of a solid-state secondary battery, a lithium-containing metal layer is plated (e.g., deposited) on the negative electrode current collector, and the plated lithium-containing metal layer is dissolved as lithium ions in the solid electrolyte layer. As the charging / discharging of the solid-state secondary battery is repeated, the lithium-containing metal layer plated on the negative electrode current collector includes impurities retained in the electrode, decomposition products of the solid electrolyte, etc. Therefore, due to the inclusion of these impurities, the lithium-containing metal layer eventually has a rough and hard surface. The lithium-containing metal layer having a rough surface forms irregularities (irregularities) such as scratches (scratches) on the negative electrode current collector, and these irregularities such as scratches on the negative electrode current collector cause the lithium-containing metal layer to be plated more unevenly. The uneven plating of the lithium-containing metal layer causes uneven pressure, etc. applied to the solid electrolyte layer in contact with the negative electrode, thereby causing microcracks in the solid electrolyte layer, and these cracks in the solid electrolyte layer grow during repeated charging and discharging processes. These cracks allow lithium to penetrate, and thus cause a short circuit between the positive electrode and the negative electrode.

[0029] The negative electrode according to aspects of one or more embodiments having a novel structure prevents degradation of the negative electrode current collector. Further, the occurrence of a short circuit in the solid-state secondary battery is suppressed by using the negative electrode, and the solid-state secondary battery using the negative electrode shows improved cycle characteristics.

[0030] The inventive concept described below can be changed in various forms and has many embodiments, and the specific embodiments are shown in the drawings and described in detail in the detailed description. However, the inventive concept should not be construed as limited to the specific embodiments, but should be understood to cover all changes, equivalents, or substitutions included within the technical scope of the inventive concept.

[0031] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the inventive concept. The singular forms also include the plural forms unless the context clearly indicates otherwise. It will be further understood that the terms "comprising" or "including" when used herein indicate the presence of the stated features, numbers, steps, operations, elements, components, components, materials, and / or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, components, materials, or combinations thereof. The " / " used hereinafter can be interpreted as "and" or "or" depending on the circumstances.

[0032] In the drawings, for clarity of illustration, the thicknesses of layers and regions are enlarged or reduced. The same reference numerals are always assigned to similar elements. When a layer, film, region, plate, etc. is referred to as being "on" another component, it can be directly on the other component or an intermediate component can be present. Terms such as "first", "second", etc. can always be used to describe various elements, but the elements are not limited by the terms. The terms are only used to distinguish one element from another. In this specification and the drawings, elements having substantially the same functional configuration are denoted by the same reference numerals and their description is not repeated.

[0033] Hereinafter, the negative electrode and the solid-state secondary battery according to the exemplary embodiments will be described in detail.

[0034] [Negative Electrode]

[0035] The negative electrode according to an embodiment includes a negative electrode current collector and a first negative electrode active material layer provided on the negative electrode current collector, wherein the negative electrode current collector includes a first metal substrate and a coating layer provided on the first metal substrate and containing a second metal, and the second metal has a Mohs hardness greater than that of the first metal. When the coating layer containing the second metal that is harder due to having a Mohs hardness higher than that of the first metal is located on the first metal substrate included in the negative electrode current collector, irregularities such as scratches on the first metal substrate due to the lithium-containing metal layer deposited during charging / discharging are prevented. Therefore, the lithium-containing metal layer formed on the negative electrode current collector during charging / discharging has improved non-uniformity. Accordingly, cracks caused in the solid electrolyte layer in contact with the negative electrode during charging / discharging are prevented, and thus short-circuiting of the solid-state secondary battery using the negative electrode is prevented. As a result, the solid-state secondary battery using the negative electrode has improved cycle characteristics.

[0036] Refer to Figure 1 and 2 , the negative electrode 20 includes a negative electrode current collector 21 and a first negative electrode active material layer 24 provided on the negative electrode current collector 21, wherein the negative electrode current collector 21 includes a first metal substrate 22 and a coating layer 23 provided on the first metal substrate 22 and containing a second metal, and the second metal has a Mohs hardness greater than that of the first metal.

[0037] [Negative Electrode: Negative Electrode Current Collector]

[0038] Refer to Figure 1, the negative electrode 20 includes a negative electrode current collector 21. The negative electrode current collector 21 includes a first metal substrate 22 and a coating layer 23 provided on the first metal substrate 22 and containing the second metal. The second metal has a Mohs hardness greater than that of the first metal. That is, the coating layer containing the second metal is harder than the substrate containing the first metal, thereby preventing deterioration of the first metal substrate.

[0039] The first metal substrate 22 contains the first metal as a main component or consists of the first metal. For example, the first metal is included in the first metal substrate 22 in an amount of 90 wt% or more, 95 wt% or more, 99 wt% or more, or 99.9 wt% or more based on the total weight of the first metal substrate 22. The first metal substrate 22 may be composed of a material that does not react with lithium, that is, a material that does not form an alloy and / or a compound with lithium. The first metal substrate 22 may be a material that is inert to lithium. The material constituting the first metal substrate 22 has a Mohs hardness of 5.5 or less. For example, the first metal has a Mohs hardness of 5.5 or less, 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, or 3.0 or less. The first metal may have a Mohs hardness of, for example, about 2.0 - about 5.5, about 2.0 - about 5.0, about 2.0 - about 4.5, about 2.0 - about 4.0, about 2.0 - about 3.5, or about 2.0 - about 3.0.

[0040] The first metal is, for example, copper (Cu), nickel (Ni), stainless steel (SUS), iron (Fe), and cobalt (Co), but is not limited thereto, and any one that is used as a current collector in the art and has a Mohs hardness of 5.5 or less can be used. The first metal substrate 22 may be composed of, for example, one of the above metals, or may be composed of an alloy of two or more metals. The first metal substrate 22 is in the form of, for example, a sheet or a foil.

[0041] The coating layer 23 contains the second metal. The coating layer 23 contains, for example, the second metal as a main component, consists essentially of the second metal, or consists of the second metal. For example, the second metal is included in the coating layer 23 in an amount of 90 wt% or more, 95 wt% or more, 99 wt% or more, or 99.9 wt% or more relative to the total weight of the coating layer 23. The coating layer 23 may be composed of a material that does not react with lithium, that is, a material that does not form an alloy and / or compound with lithium. The material forming the coating layer 23 may be a material inert to lithium. The material forming the coating layer 23 has a Mohs hardness of 6.0 or greater. For example, the second metal has a Mohs hardness of 6.0 or greater, 6.5 or greater, 7.0 or greater, 7.5 or greater, 8.0 or greater, 8.5 or greater, or 9.0 or greater. The second metal may have, for example, a Mohs hardness of about 6.0 - about 12, about 6.5 - about 12, about 7.0 - about 12, about 7.5 - about 12, about 8.0 - about 12, about 8.5 - about 12, or about 9.0 - about 12. The second metal may have, for example, a Mohs hardness of about 6.0 - about 10, about 6.5 - about 10, about 7.0 - about 10, about 7.5 - about 10, about 8.0 - about 10, about 8.5 - about 10, or about 9.0 - about 10. When the second metal has too low a Mohs hardness, it may be difficult to prevent deterioration of the negative electrode current collector. When the second metal has too high a Mohs hardness, processing may not be easy.

[0042] The second metal is, for example, at least one selected from the following: titanium (Ti), manganese (Mn), niobium (Nb), tantalum (Ta), iridium (Ir), vanadium (V), rhenium (Re), osmium (Os), tungsten (W), chromium (Cr), boron (B), ruthenium (Ru), and rhodium (Rh). The coating layer 23 may be composed of, for example, one of the above metals, or may be composed of an alloy of two or more metals.

[0043] The difference in Mohs hardness between the first metal included in the first metal substrate 22 and the second metal included in the coating layer 23 may be, for example, 2 or greater, 2.5 or greater, 3 or greater, 3.5 or greater, or 4 or greater. The difference in Mohs hardness between the first metal and the second metal allows for more effective prevention of deterioration of the negative electrode current collector 21.

[0044] The coating layer 23 may have a single-layer structure or a multi-layer structure of two or more layers. The coating layer 23 may have, for example, a two-layer structure having a first coating layer and a second coating layer. The coating layer 23 may have, for example, a three-layer structure having a first coating layer, a second coating layer, and a third coating layer.

[0045] The coating layer 23 has a thickness of, for example, 1 μm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, or 150 nm or less. The coating layer 23 has a thickness of, for example, about 10 nm - about 1 μm, about 20 nm - about 900 nm, about 30 nm - about 800 nm, about 40 nm - about 700 nm, about 50 nm - about 600 nm, about 50 nm - about 500 nm, about 50 nm - about 300 nm, about 50 nm - about 200 nm, or about 50 nm - about 150 nm. When the coating layer 23 is too thin in thickness, it may be difficult to suppress the non-uniform growth of the lithium-containing metal layer. When the coating layer 23 has a relatively large thickness, the solid-state secondary battery has improved cycling characteristics. However, when the coating layer 23 is too thick in thickness, the solid-state secondary battery has a reduced energy density, and the coating layer 23 may thus not be easily formed. The coating layer 23 can be provided on the first metal substrate 22 by, for example, vacuum vapor deposition, sputtering, or plating, but the implementation is not limited thereto, and any method capable of forming the coating layer 23 in the art can be used.

[0046] The coating layer 23 can be, for example, inert to the solid electrolyte. The coating layer 23 can be, for example, inert to the sulfide-based solid electrolyte. Since the coating layer 23 is inert to the sulfide-based solid electrolyte, the first metal substrate 22, which is reactive with the sulfide-based solid electrolyte, can be protected from the influence of the sulfide-based solid electrolyte. That is, the coating layer 23 can act as a protective layer for the first metal substrate 22. For example, in a solid-state secondary battery including a negative electrode 20 containing copper (Cu) as the first metal substrate 22 and a sulfide-based solid electrolyte as the solid electrolyte, a coating layer 23 that is inert to the sulfide-based solid electrolyte is provided on the first metal substrate 22, and thus deterioration of the copper (Cu) substrate (which is the first metal substrate 22), such as corrosion, caused by the sulfide-based solid electrolyte can be effectively prevented.

[0047] Referring to Figure 2 , the coating layer 23 can be additionally provided on the third surface A3, which is a side surface, between the first surface A1 of the first metal substrate 22 facing the coating layer 23 and the second surface A2 opposite to the first surface A1. When the coating layer 23 is provided on the first surface A1 and the third surface A3 of the first metal substrate 22 together, the deterioration of the first metal substrate 22 can be more effectively prevented. The coating layer 23 can be provided on the third surface A3 of the first metal substrate 22 as the entire side surface.

[0048] The negative electrode current collector 21 including the first metal substrate 22 and the coating layer 23 may have a thickness of, for example, about 5 μm to about 50 μm, about 10 μm to about 50 μm, about 10 μm to about 40 μm, or about 10 μm to about 30 μm, but the thickness is not necessarily limited thereto and may be selected according to the required characteristics of the solid-state secondary battery.

[0049] [Negative electrode: Negative electrode active material]

[0050] Reference Figure 1-4B Referring to FIGS. 5 and 6, the negative electrodes 20, 20a, and 20b include first negative electrode active material layers 24, 24a, and 24b provided on the negative electrode current collectors 21, 21a, and 21b. The first negative electrode active material layers 24, 24a, and 24b include, for example, a negative electrode active material and a binder.

[0051] The negative electrode active material included in the first negative electrode active material layers 24, 24a, and 24b is, for example, in the form of particles. The negative electrode active material in the form of particles has an average particle diameter of, for example, 4 μm or less, 3 μm or less, 2 μm or less, 1 μm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, or 100 nm or less. The negative electrode active material in the form of particles has an average particle diameter of, for example, about 10 nm to about 4 μm, about 10 nm to about 3 μm, about 10 nm to about 2 μm, about 10 nm to about 1 μm, about 10 nm to about 900 nm, about 10 nm to about 800 nm, about 10 nm to about 700 nm, about 10 nm to about 600 nm, about 10 nm to about 500 nm, about 10 nm to about 400 nm, about 10 nm to about 300 nm, about 10 nm to about 200 nm, about 10 nm to about 100 nm, or about 20 nm to about 80 nm. When the negative electrode active material has an average particle diameter within these ranges, the reversible absorption and / or desorption of lithium during charge / discharge can be further promoted. The average particle diameter of the negative electrode active material may be, for example, the median particle diameter (D50) measured by using a laser type particle size distribution meter.

[0052] The negative electrode active material included in the first negative electrode active material layers 24, 24a, and 24b includes, for example, at least one selected from the following: carbon-based negative electrode active materials and metal or metalloid negative electrode active materials.

[0053] The carbon-based negative electrode active material is particularly amorphous carbon. Examples of the amorphous carbon may include carbon black (CB), acetylene black (AB), furnace black (FB), Ketjen black (KB), or graphene, but embodiments are not limited thereto, and any amorphous carbon available in the art may be used. The amorphous carbon refers to carbon that does not have crystallinity or has very low crystallinity, which can be distinguished from crystalline carbon or graphite-based carbon.

[0054] The metal or metalloid negative electrode active material may include at least one selected from the following: gold (Au), platinum (Pt), palladium (Pd), silicon (Si), silver (Ag), aluminum (Al), bismuth (Bi), tin (Sn), and zinc (Zn), but embodiments are not limited thereto, and any material available in the art that can be used as a metal negative electrode active material or a metalloid negative electrode active material that forms an alloy or a compound with lithium may be used. For example, nickel (Ni) does not form an alloy with lithium and is thus not a metal negative electrode active material in this specification.

[0055] The first negative electrode active material layers 24, 24a, and 24b may include one type of negative electrode active material among these negative electrode active materials or multiple different types of negative electrode active materials. For example, the first negative electrode active material layers 24, 24a, and 24b may include only amorphous carbon or at least one selected from the following: gold (Au), platinum (Pt), palladium (Pd), silicon (Si), silver (Ag), aluminum (Al), bismuth (Bi), tin (Sn), and zinc (Zn). Alternatively, the first negative electrode active material layers 24, 24a, and 24b may include a mixture of amorphous carbon and at least one selected from the following: gold (Au), platinum (Pt), palladium (Pd), silicon (Si), silver (Ag), aluminum (Al), bismuth (Bi), tin (Sn), and zinc (Zn). The mixing ratio of amorphous carbon to gold or the like may be a weight ratio in the range of about 10:1 - about 1:2, about 10:1 - about 1:1, about 7:1 - about 1:1, about 5:1 - about 1:1, or about 4:1 - about 2:1, but is not necessarily limited thereto, and is selected according to the required characteristics of the solid-state secondary battery 1. When the negative electrode active material has the above composition, the cycle characteristics of the solid-state secondary battery 1 can be further improved.

[0056] The negative electrode active material included in the first negative electrode active material layers 24, 24a, and 24b includes, for example, a mixture of first particles composed of amorphous carbon and second particles composed of a metal or a metalloid. The metal or metalloid may include, for example, gold (Au), platinum (Pt), palladium (Pd), silicon (Si), silver (Ag), aluminum (Al), bismuth (Bi), tin (Sn), and zinc (Zn). The metalloid is additionally a semiconductor. The second particles are included in an amount of about 8 wt% - about 60 wt%, about 10 wt% - about 50 wt%, about 15 wt% - about 40 wt%, or about 20 wt% - about 30 wt% relative to the total weight of the mixture. When the second particles are included in an amount within these ranges, the cycle characteristics of the solid-state secondary battery 1 can be further improved.

[0057] [Negative electrode: Binder]

[0058] Examples of the binder included in the first negative electrode active material layers 24, 24a, and 24b may include styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile, polymethyl methacrylate, but the embodiments are not limited thereto, and any binder available in the art may be used. The binder may be composed of a single binder or a plurality of different binders.

[0059] When the first negative electrode active material layers 24, 24a, and 24b contain a binder, the first negative electrode active material layers 24, 24a, and 24b are stabilized on the negative electrode current collectors 21, 21a, and 21b. Additionally, although there are changes in the volume and / or relative position of the first negative electrode active material layers 24, 24a, and 24b during charge / discharge, cracks in the first negative electrode active material layers 24, 24a, and 24b are prevented. For example, when the first negative electrode active material layers 24, 24a, and 24b do not contain a binder, the first negative electrode active material layers 24, 24a, and 24b can be easily separated from the negative electrode current collectors 21, 21a, and 21b. At portions where the negative electrode current collectors 21, 21a, and 21b are exposed when the first negative electrode active material layers 24, 24a, and 24b are separated from the negative electrode current collectors 21, 21a, and 21b, the negative electrode current collectors 21, 21a, and 21b are in direct contact with the solid electrolyte layers 30, 30a, and 30b, thereby having a higher chance of causing a short circuit. The first negative electrode active material layers 24, 24a, and 24b are prepared, for example, by applying a slurry in which materials constituting the first negative electrode active material layers 24, 24a, and 24b are dispersed onto the negative electrode current collectors 21, 21a, and 21b and drying. When the binder is included in the first negative electrode active material layers 24, 24a, and 24b, stable dispersion of the negative electrode active material in the slurry can be achieved. For example, when the slurry is applied to the negative electrode current collectors 21, 21a, and 21b by screen printing, clogging of the screen (e.g., clogging caused by aggregation of the negative electrode active material) can be prevented.

[0060] The binder included in the first negative electrode active material layers 24, 24a, and 24b may be in an amount of about 1 wt% - about 20 wt%, about 2 wt% - about 15 wt%, or about 3 wt% - about 10 wt% relative to the total weight of the first negative electrode active material layers 24, 24a, and 24b, but the amount is not necessarily limited thereto and can be selected according to the characteristics of the solid-state secondary battery.

[0061] [Negative electrode: Other additives]

[0062] The first negative electrode active material layers 24, 24a, and 24b may further include additives such as fillers, coating agents, dispersants, and ion conductors used in conventional solid-state secondary batteries 1 within a range that maintains or improves the physical properties of the solid-state secondary battery 1.

[0063] The filler, coating agent, dispersant, and ion conductor included in the first negative electrode active material layers 24, 24a, and 24b may be materials commonly used in the negative electrode of a solid-state secondary battery. The filler, coating agent, dispersant, or ion conductor included in the first negative electrode active material layers 24, 24a, and 24b may be in an amount of, for example, about 0.1 wt% - about 5 wt%, about 0.1 wt% - about 3 wt%, or about 0.1 wt% - about 1 wt%.

[0064] [Negative electrode: First negative electrode active material layer]

[0065] The first negative electrode active material layers 24, 24a, and 24b have a thickness of, for example, about 1 μm - about 20 μm, about 1 μm - about 15 μm, about 2 μm - about 10 μm, or about 3 μm - about 7 μm. The thickness of the first negative electrode active material layers 24, 24a, and 24b is, for example, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, or 5% or less of the thickness of the positive electrode active material layer. When the first negative electrode active material layers 24, 24a, and 24b are too thin in thickness, the lithium dendrites formed between the first negative electrode active material layers 24, 24a, and 24b and the negative electrode current collectors 21, 21a, and 21b may damage the first negative electrode active material layers 24, 24a, and 24b, thus hardly improving the cycle characteristics of the solid-state secondary battery. When the first negative electrode active material layers 24, 24a, and 24b are too thick in thickness, the energy density of the solid-state secondary battery using the negative electrode 20 decreases and the internal resistance of the solid-state secondary battery increases due to the first negative electrode active material layers 24, 24a, and 24b, and thus the cycle characteristics of the solid-state secondary battery may hardly be improved.

[0066] When the thicknesses of the first negative electrode active material layers 24, 24a, and 24b are decreased, for example, the first negative electrode active material layers 24, 24a, and 24b also have a reduced charge capacity. The charge capacity of the first negative electrode active material layers 24, 24a, and 24b is, for example, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, 5% or less, or 2% or less of the charge capacity of the positive electrode active material layer. The charge capacity of the first negative electrode active material layers 24, 24a, and 24b is, for example, about 0.1% - about 50%, about 0.1% - about 40%, about 0.1% - about 30%, about 0.1% - about 20%, about 0.1% - about 10%, about 0.1% - about 5%, or about 0.1% - about 2% of the charge capacity of the positive electrode active material layer. When the charge capacity of the first negative electrode active material layers 24, 24a, and 24b is too small, the first negative electrode active material layers 24, 24a, and 24b become too thin in thickness, and during charge / discharge, lithium dendrites formed between the first negative electrode active material layers 24, 24a, and 24b and the negative electrode current collectors 21, 21a, and 21b can cause damage to the first negative electrode active material layers 24, 24a, and 24b, and thus it is difficult to improve the cycle characteristics of the solid-state secondary battery. When the charge capacity of the first negative electrode active material layers 24, 24a, and 24b is too large, the energy density of the solid-state secondary battery decreases and the internal resistance of the solid-state secondary battery increases due to the first negative electrode active material layers 24, 24a, and 24b, and thus it is difficult to improve the cycle characteristics of the solid-state secondary battery.

[0067] The charge capacity of the positive electrode active material layer is obtained by multiplying the charge capacity density (mAh / g) of the positive electrode active material by the weight of the positive electrode active material in the positive electrode active material layer. When different types of positive electrode active materials are used, the value of the charge capacity density × weight for each positive electrode active material is calculated, and the sum of the values is regarded as the charge capacity of the positive electrode active material layer. The charge capacity of the first negative electrode active material layers 24, 24a, and 24b is also calculated in the same manner. That is, the charge capacity of the first negative electrode active material layers 24, 24a, and 24b is obtained by multiplying the charge capacity density (mAh / g) of the negative electrode active material by the weight of the negative electrode active material in the first negative electrode active material layers 24, 24a, and 24b. When different types of negative electrode active materials are used, the value of the charge capacity density × mass for each negative electrode active material is calculated, and the sum of the values is regarded as the charge capacity of the first negative electrode active material layers 24, 24a, and 24b. In this case, the charge capacity density of the positive electrode active material and the negative electrode active material refers to the capacity evaluated by using a solid-state half-cell with a lithium metal as the counter electrode. Based on the charge capacity measurement using the solid-state half-cell, the charge capacities of the positive electrode active material layer and the first negative electrode active material layers 24, 24a, and 24b are directly measured. When the charge capacities so measured are divided by the weight of each active material, the charge capacity density is calculated. Alternatively, the charge capacities of the positive electrode active material layer and the first negative electrode active material layers 24, 24a, and 24b may be the initial charge capacities measured in the first charge cycle.

[0068] [Solid-state secondary battery]

[0069] The solid-state secondary battery according to the embodiment includes a positive electrode, the above-mentioned negative electrode, and a solid electrolyte layer provided between the positive electrode and the negative electrode, wherein the solid electrolyte layer includes a sulfide-based solid electrolyte. When the first metal substrate of the negative electrode current collector included in the negative electrode is coated with a coating layer containing a second metal having a high hardness, deterioration of the first metal substrate, particularly a Cu substrate, due to the sulfide-based solid electrolyte can be prevented. In addition, when the lithium metal-containing layer plated on the negative electrode current collector during charge / discharge has improved uniformity, microcracks in the solid electrolyte layer due to the non-uniform lithium metal-containing layer can be prevented. Therefore, short circuits are prevented during charge / discharge of the solid-state secondary battery. As a result, the solid-state secondary battery can have improved cycle characteristics.

[0070] Referring to Figure 3 、 4A 、4B and 6, the solid-state secondary battery 1 includes a positive electrode 10, the above-mentioned negative electrodes 20, 20a, and 20b, and a solid electrolyte layer 40 provided between the positive electrode 10 and the negative electrodes 20, 20a, and 20b, wherein the solid electrolyte layer 40 contains a sulfide-based solid electrolyte.

[0071] Refer to Figure 3 , the solid-state secondary battery 1 may have, for example, a single unit cell structure having a positive electrode active material layer 12, a solid electrolyte layer 30, and a negative electrode 20 disposed on one side of a positive electrode current collector 11.

[0072] Refer to Figure 4A and 4B , the solid-state secondary battery 1 may have, for example, a dual unit cell structure having positive electrode active material layers 12a and 12b, solid electrolyte layers 30a and 30b, and negative electrodes 20a and 20b disposed on both sides of a positive electrode current collector 11. In the solid-state secondary battery 1 having a dual unit cell structure, pressure is applied symmetrically in two directions of the solid-state secondary battery 1 during the manufacture of the battery, and cracks that may be caused when pressure is applied in one direction during the manufacture of the solid-state secondary battery 1 can be more effectively prevented.

[0073] [Positive electrode]

[0074] [Positive electrode: Positive electrode active material]

[0075] Refer to Figure 3-6 , the positive electrode 10 includes a positive electrode current collector 11 and positive electrode active material layers 12, 12a, and 12b disposed on the positive electrode current collector. The positive electrode active material layers 12, 12a, and 12b contain a positive electrode active material.

[0076] The positive electrode active material is a positive electrode active material capable of reversibly absorbing and desorbing lithium ions. The positive electrode active material may be, for example, a lithium transition metal oxide such as lithium cobalt oxide (LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminum oxide (NCA), lithium nickel cobalt manganese oxide (NCM), lithium manganate, or lithium iron phosphate; nickel sulfide; copper sulfide; lithium sulfide; iron oxide; or vanadium oxide, but the embodiments are not limited thereto, and any material that can be used as a positive electrode active material in the art may be used. Examples of the positive electrode active material may be used alone or in a mixture of at least two selected therefrom.

[0077] The positive electrode active material may be, for example, a compound represented by any of the following formulas: Li a A 1-b B b D2 (where 0.90 ≤ a ≤ 1 and 0 ≤ b ≤ 0.5); Li a E 1-b B b O 2-c D c (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE 2- b B b O4-c D c (where 0 ≤ b ≤ 0.5 and 0 ≤ c ≤ 0.05); Li a Ni 1-b-c Co b B c D α (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2); Li a Ni 1-b-c Co b B c O 2-α F α (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Co b B c O 2-α F2 (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b- c Mn b B c D α (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2); Li a Ni 1-b-c Mn b B c O 2-α F α (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b B c O 2-α F2 (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni b E c G d O2 (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, and 0.001 ≤ d ≤ 0.1); Li a Ni b Co c Mn d G e O2 (where 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, and 0.001 ≤ e ≤ 0.1); Li a NiGb O2 (where 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (where 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (where 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.10); Li a Mn2G b O4 (where 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiIO2; LiNiVO4; Li (3-f) J2(PO4)3 (where 0 ≤ f ≤ 2); Li (3-f) Fe2(PO4)3 (where 0 ≤ f ≤ 2); and LiFePO4. In the compounds, A is Ni, Co, Mn, or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; F is F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q is Ti, Mo, Mn, or a combination thereof; I is Cr, V, Fe, Sc, Y, or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof. Compounds that can be used to add to the surface of the compounds, and mixtures of the compounds with the compounds to which the coating layer is added, can be used. The coating layer added to the surface of the compounds includes, for example, coating element compounds such as oxides of coating elements, hydroxides of coating elements, hydroxyoxides of coating elements, carbonate oxides of coating elements, or hydroxycarbonates of coating elements. The compounds forming the coating layer can be amorphous or crystalline. The coating elements included in the coating layer are Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer is formed using any method that does not adversely affect the physical properties of the positive electrode active material. For example, coating methods include spraying, dipping, etc. Specific coating methods can be well understood by those of ordinary skill in the art and will thus omit their detailed description.

[0078] The positive electrode active material may include, for example, a lithium salt of a transition metal oxide having a layered rock salt-type structure among examples of lithium transition metal oxides. For example, the "layered rock salt-type structure" refers to a structure in which oxygen atom layers and metal atom layers are alternately and regularly arranged in the <111> direction in a cubic rock salt-type structure, where each atom layer forms a two-dimensional plane. The "cubic rock salt-type structure" refers to a sodium chloride (NaCl)-type structure as one of crystal structures, particularly a structure in which face-centered cubic (fcc) lattices formed by anions and cations respectively are shifted by only half of the ridges of each unit lattice. Examples of the lithium transition metal oxide having a layered rock salt-type structure may include those represented as LiNi x Co y Al z O2 (NCA) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1), LiNi x Co y Mn z O2 (NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1), LiNi x Co y Al v Mn w O2 (NCAM) (where 0 < x < 1, 0 < y < 1, 0 < v < 1, 0 < w < 1, and x + y + v + w = 1), LiNi a Co b Al c O2 (0.5 ≤ a < 1, 0 < b < 0.5, 0 < c < 0.5, a + b + c = 1), LiNi a Co b Mn c O2 (where 0.5 ≤ a < 1, 0 < b < 0.5, 0 < c < 0.5, a + b + c = 1), or LiNi a Co b Al d Mn e O2 (0.5 ≤ a < 1, 0 < b < 0.5, 0 < d < 0.5, 0 < e < 0.5, a + b + d + e = 1) ternary / quaternary lithium transition metal oxides. When the positive electrode active material includes a ternary / quaternary transition metal oxide having a layered rock salt-type structure, the energy density and thermal stability of the solid-state secondary battery 1 can be improved.

[0079] The positive electrode active material may be covered with a coating layer as described above. The coating layer is any material that can be used as a coating layer for the positive electrode active material of a solid-state secondary battery in the art. The coating layer may be, for example, Li2O-ZrO2 (LZO) or the like.

[0080] When the positive electrode active material includes nickel (Ni) as a ternary / quaternary lithium transition metal oxide such as NCA, NCM, or NCAM, the capacity density of the solid-state secondary battery 1 increases, and thus elution of metal from the positive electrode active material in a charged state can be reduced. As a result, the solid-state secondary battery 1 can have improved cycle characteristics.

[0081] The shape of the positive electrode active material may be, for example, a particulate shape such as a true spherical shape, an oval shape, or a spherical shape. The particle size of the positive electrode active material is not particularly limited and may be within the range of positive electrode active materials applicable to conventional solid-state secondary batteries. The amount of the positive electrode active material in the positive electrode layer 10 is not particularly limited and may be within the range of positive electrode layers applicable to conventional solid-state secondary batteries.

[0082] The positive electrode active material included in the positive electrode active material layers 12, 12a, and 12b may be included in an amount of about 80 wt% - about 99 wt%, about 80 wt% - about 95 wt%, or about 80 wt% - about 90 wt% of the total weight of the positive electrode active material layers 12, 12a, and 12b.

[0083] [Positive electrode: Solid electrolyte]

[0084] In addition to the positive electrode active material, the positive electrode active material layers 12, 12a, and 12b may further include a solid electrolyte. The solid electrolyte included in the positive electrode 10 may be the same as or different from the solid electrolyte included in the solid electrolyte layers 30, 30a, and 30b. For details regarding the solid electrolyte included in the positive electrode 10, refer to the solid electrolyte layers 30, 30a, and 30b.

[0085] The solid electrolyte used in the positive electrode active material layers 12, 12a, and 12b may have a smaller D50 average particle size than the solid electrolyte used in the solid electrolyte layers 30, 30a, and 30b. For example, the D50 average particle size of the solid electrolyte used in the positive electrode active material layers 12, 12a, and 12b may be 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, 30% or less, or 20% or less of the D50 average particle size of the solid electrolyte used in the solid electrolyte layers 30, 30a, and 30b. The solid electrolyte used in the positive electrode active material layers 12, 12a, and 12b may have a D50 average particle size of, for example, about 0.1 μm - about 2 μm, about 0.2 μm - about 1.5 μm, or about 0.3 μm - about 1.0 μm.

[0086] The solid electrolyte included in the positive electrode active material layers 12, 12a, and 12b may be included in an amount of about 1 wt% - about 20 wt%, about 5 wt% - about 20 wt%, about 10 wt% - about 20 wt%, about 1 wt% - about 15 wt%, about 5 wt% - about 15 wt%, or about 8.0 wt% - about 12.0 wt% of the total weight of the positive electrode active material layers 12, 12a, and 12b.

[0087] [Positive electrode: Binder]

[0088] The positive electrode active material layers 12, 12a, and 12b may include a binder. Examples of the binder may include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, but the embodiments are not limited thereto, and any binder available in the art may be used.

[0089] The binder included in the positive electrode active material layers 12, 12a, and 12b may be included, for example, in an amount of about 0.1 wt% - about 5 wt%, about 0.5 wt% - about 3 wt%, or about 1.0 wt% - about 2.0 wt% of the total weight of the positive electrode active material layers 12, 12a, and 12b.

[0090] [Positive electrode: Conductive agent]

[0091] The positive electrode active material layers 12, 12a, and 12b may include a conductive agent. The conductive agent may be, for example, a carbon-based conductive agent or a metallic conductive agent. Examples of the conductive agent may include graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, metal powder, etc., but are not necessarily limited thereto, and any material available in the art as a conductive agent may be used.

[0092] The conductive agent included in the positive electrode active material layers 12, 12a, and 12b may be included, for example, in an amount of about 0.1 wt% - about 10 wt%, about 0.5 wt% - about 5 wt%, or about 1.0 wt% - about 4.0 wt% of the total weight of the positive electrode active material layers 12, 12a, and 12b.

[0093] [Positive electrode: Other additives]

[0094] In addition to the positive electrode active material, solid electrolyte, binder, and conductive agent, the positive electrode active material layers 12, 12a, and 12b may further include additives such as fillers, coating agents, dispersants, and ion conductors.

[0095] The filler, coating agent, dispersant, and ion conductor that may be included in the positive electrode active material layers 12, 12a, and 12b may be materials commonly used in the positive electrode of a solid-state secondary battery. The filler, coating agent, dispersant, or ion conductor included in the first positive electrode active material layers 12, 12a, and 12b may be in an amount of about 0.1 wt% - about 5 wt%, about 0.1 wt% - about 3 wt%, or about 0.1 wt% - about 1 wt%, respectively.

[0096] [Positive electrode: Positive electrode current collector]

[0097] The positive electrode current collector 11 may be, for example, a plate or foil formed of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or an alloy thereof. The positive electrode current collector 11 may be omitted.

[0098] The positive electrode current collector 11 may further include a carbon layer provided on one or both surfaces of the metal substrate. Disposing the carbon layer on the metal substrate additionally can prevent the metal of the metal substrate from being corroded by the solid electrolyte included in the positive electrode layer and reduce the interfacial resistance between the positive electrode active material layers 12, 12a, and 12b and the positive electrode current collector 11. The carbon layer may have a thickness of, for example, about 0.1 μm - about 5 μm, about 0.5 μm - about 5 μm, about 1 μm - about 5 μm, about 1 μm - about 4 μm, or about 1 μm - about 3 μm. When the carbon layer is too thin in thickness, it may be difficult to completely prevent contact between the metal substrate and the solid electrolyte. When the carbon layer is too thick in thickness, the solid-state secondary battery may have a reduced energy density. The carbon layer may include amorphous carbon, crystalline carbon, etc.

[0099] The positive electrode current collector 11 including the metal substrate and optionally including the carbon layer may have a thickness of, for example, about 10 μm - about 50 μm, about 10 μm - about 40 μm, or about 10 μm - about 30 μm, but the thickness is not limited thereto and may be selected according to the required characteristics of the solid-state secondary battery.

[0100] [Positive electrode: Inactive member]

[0101] Reference Figure 3-6 , the positive electrode 10 may include a positive electrode current collector 11, positive electrode active material layers 12, 12a, and 12b provided on the positive electrode current collector, and further include an inactive member 40 provided on one side surface of the positive electrode active material layer.

[0102] The inactive member 40 is a member that does not contain an electrochemically active material such as an electrode active material. The electrode active material is a material that absorbs / desorbs lithium. The inactive member is a material different from the electrode active material and is formed of a material used in the art.

[0103] The inactive member 40 can surround the side surfaces of the positive electrode active material layers 12, 12a, and 12b and contact the solid electrolyte layers 30, 30a, and 30b. When the inactive member 40 surrounds the side surfaces of the positive electrode active material layers 12, 12a, and 12b and contacts the solid electrolyte layers 30, 30a, and 30b, cracks in the solid electrolyte layers 30, 30a, and 30b caused by a pressure difference during pressing can be effectively prevented in the solid electrolyte layers 30, 30a, and 30b that do not contact the positive electrode active material layers 12, 12a, and 12b.

[0104] The inactive member 40 can extend to the ends of the solid electrolyte layers 30, 30a, and 30b. When the inactive member 40 extends to the ends of the solid electrolyte layers 30, 30a, and 30b, cracks caused at the ends of the solid electrolyte layers 30, 30a, and 30b can be prevented. The ends of the solid electrolyte layers 30, 30a, and 30b are the outermost portions of the solid electrolyte layers 30, 30a, and 30b that contact the side surfaces. That is, the inactive member 40 can extend to the outermost portions of the solid electrolyte layers 30, 30a, and 30b that contact the side surfaces.

[0105] The area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b is smaller than the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b that is in contact with the positive electrode active material layers 12, 12a, and 12b, and the inactive member 40 is disposed to surround the side surfaces of the positive electrode active material layers 12, 12a, and 12b to offset the area deviation between the positive electrode active material layers 12, 12a, and 12b and the solid electrolyte layers 30, 30a, and 30b. When the area (e.g., surface area) S3 of the inactive member 40 offsets the area deviation between the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b and the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b, cracks in the solid electrolyte layers 30, 30a, and 30b caused by the pressure difference during pressing can be effectively prevented. For example, the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b can be less than 100%, 99% or less, 98% or less, 97% or less, 96% or less, 95% or less, or 93% or less of the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b. For example, the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b can be about 50% - less than about 100%, about 50% - about 99%, about 55% - about 98%, about 60% - about 97%, about 70% - about 96%, about 80% - about 95%, or about 85% - about 95% of the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b. When the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b is equal to or greater than the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b, a short circuit can be caused by the physical contact between the positive electrode active material layers 12, 12a, and 12b and the negative electrode active material layers 22, 22a, and 22b, or the chance of causing a short circuit can increase due to overcharging of lithium.

[0106] The area (e.g., surface area) S3 of the inactive member 40 can be 50% or less, 40% or less, 30% or less, 20% or less, or 10% or less of the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b. For example, the area (e.g., surface area) S3 of the inactive member 40 can be about 1% - about 50%, about 5% - about 40%, about 5% - about 30%, about 5% - about 20%, or about 5% - about 15% of the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b. The sum of the area (e.g., surface area) S3 of the inactive member 40 and the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b can be the same as the area (e.g., surface area) S2 of the solid electrolyte layers 30, 30a, and 30b.

[0107] The inactive member 40 may be disposed between the positive electrode current collector 11 and the solid electrolyte layers 30, 30a, and 30b facing each other, or between two solid electrolyte layers 30, 30a, and 30b facing each other. The inactive member 40 may serve as a filler for filling the space between the positive electrode current collector 11 and the solid electrolyte layers 30, 30a, and 30b facing each other, or the space between two solid electrolyte layers 30, 30a, and 30b facing each other.

[0108] The area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b may be smaller than the area (e.g., surface area) S4 of the negative electrode current collectors 21, 21a, and 21b. For example, the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b may be less than 100%, 99% or less, 98% or less, 97% or less, 96% or less, 95% or less, or 93% or less of the area (e.g., surface area) S4 of the negative electrode current collectors 21, 21a, and 21b. For example, the area (e.g., surface area) S1 of the positive electrode active material layers 12, 12a, and 12b may be about 50% - about less than 100%, about 50% - about 99%, about 55% - about 98%, about 60% - about 97%, about 70% - about 96%, about 80% - about 95%, or about 85% - about 95% of the area (e.g., surface area) S4 of the negative electrode current collectors 21, 21a, and 21b.

[0109] The area (e.g., surface area) S4 and / or the shape of the negative electrode current collectors 21, 21a, and 21b may be the same as the area (e.g., surface area) S5 and / or the shape of the positive electrode current collector 11. For example, the area (e.g., surface area) S4 of the negative electrode current collectors 21, 21a, and 21b may be within the range of 100 ± 1% or 100 ± 0.5% of the area (e.g., surface area) S5 of the positive electrode current collector 11. The area (e.g., surface area) S5 and / or the shape of the positive electrode current collector 11 may be the same as the area (e.g., surface area) S2 and / or the shape of the solid electrolyte layers 30, 30a, and 30b. For example, the area S5 of the positive electrode current collector 11 may be within the range of 100 ± 1% or 100 ± 0.5% of the area S2 of the solid electrolyte layers 30, 30a, and 30b. In this specification, "the same" area and / or shape includes all cases having "substantially the same" area and / or shape, except for cases where the area and / or shape are intended to be different.

[0110] The inactive member 40 may include at least one selected from a lithium ion insulator or a lithium ion conductor. The inactive member 40 may be an electronic insulator. That is, the inactive member 40 may not be an electronic conductor.

[0111] The inactive member 40 can be an organic material, an inorganic material, or an organic-inorganic composite material. The organic material can be, for example, a polymer. The inorganic material can be, for example, a ceramic such as a metal oxide. The organic-inorganic composite material can be a composite of a polymer and a metal oxide. The inactive member 40 can include, for example, at least one selected from the following: an insulating polymer, an ion-conductive polymer, an insulating inorganic material, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte. The inactive member 40 can be, for example, an olefin-based polymer such as polypropylene (PP) or polyethylene (PE).

[0112] The density of the inactive member 40 can be, for example, about 10% - about 200%, about 10% - about 150%, about 10% - about 140%, about 10% - about 130%, or about 10% - about 120% of the density of the positive electrode active material included in the positive electrode active material layers 12, 12a, and 12b. The density of the inactive member 40 can be, for example, about 90% - about 110% of the density of the positive electrode active material included in the positive electrode active material layers 12, 12a, and 12b. The density of the inactive member 40 can be, for example, substantially the same as the density of the positive electrode active material included in the positive electrode active material layers 12, 12a, and 12b.

[0113] The inactive member 40 can be, for example, a gasket. When the gasket is used as the inactive member 40, cracks in the solid electrolyte layers 30, 30a, and 30b caused by a pressure difference during pressing can be effectively prevented.

[0114] The thickness of the inactive member 40 can be the same as or similar to the thickness of the positive electrode active material layers 12, 12a, and 12b.

[0115] [Solid electrolyte layer]

[0116] [Solid electrolyte layer: Solid electrolyte]

[0117] Refer to Figure 3-6 , the solid electrolyte layers 30, 30a, and 30b are disposed between the positive electrode 10 and the negative electrodes 20, 20a, and 20b, and include a sulfide-based solid electrolyte.

[0118] The sulfide-based solid electrolyte may include, for example, at least one selected from the following: Li2S-P2S5, Li2S-P2S5-LiX (where X is a halogen element), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (where m and n are positive numbers, and Z is one of Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (where p and q are positive numbers, and M is one of P, Si, Ge, B, Al, Ga, or In), Li 7-x PS 6-x Cl x (where 0 ≤ x ≤ 2), Li 7-x PS 6-x Br x (where 0 ≤ x ≤ 2), and Li 7-x PS 6-x I x (where 0 ≤ x ≤ 2). For example, the sulfide-based solid electrolyte material can be prepared by treating starting materials (such as Li2S or P2S5) using a melt quenching method or a mechanical grinding method. Subsequently, heat treatment can be performed on the obtained product. The sulfide-based solid electrolyte can be in an amorphous, crystalline, or mixed form. Moreover, the solid electrolyte may include sulfur (S), phosphorus (P), and lithium (Li) as at least constituent elements of the above sulfide-based solid electrolyte material. For example, the solid electrolyte can be a material including Li2S-P2S5. When Li2S-P2S5 is used as the sulfide-based solid electrolyte material for forming the solid electrolyte, the mixing molar ratio of Li2S and P2S5 (Li2S:P2S5) can be, for example, in the range of about 50:50 to about 90:10.

[0119] The sulfide-based solid electrolyte may include, for example, a thiogermanate-type solid electrolyte represented by the following Formula 1:

[0120] <Formula 1>

[0121] Li + 12-n-x A n+ X 2- 6-x Y -x

[0122] Wherein in the above formula, A is P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb or Ta, X is S, Se or Te, Y is Cl, Br, I, F, CN, OCN, SCN, or N3, and 1≤n≤5 and 0≤x≤2 are satisfied. The sulfide-based solid electrolyte may be, for example, a thiogermanate compound including at least one selected from the following: Li 7-x PS 6-x Cl x (where 0≤x≤2), Li 7-x PS 6-x Br x (where 0≤x≤2), and Li 7-x PS 6-x I x (where 0≤x≤2). The sulfide solid electrolyte may be, for example, a thiogermanate compound including at least one selected from Li6PS5Cl, Li6PS5Br, and Li6PS5I.

[0123] The thiogermanate solid electrolyte may have a density of about 1.5 g / cc - about 2.0 g / cc. When the thiogermanate solid electrolyte has a density of at least 1.5 g / cc, the internal resistance of the solid-state secondary battery can be reduced, and the solid electrolyte can be effectively prevented from being penetrated by lithium.

[0124] The elastic modulus, i.e., Young's modulus, of the sulfide-based solid electrolyte may be, for example, 35 GPa or less, 30 GPa or less, 27 GPa or less, 25 GPa or less, or 23 GPa or less. The elastic modulus, i.e., Young's modulus, of the sulfide-based solid electrolyte may be, for example, about 10 GPa - about 35 GPa, about 10 GPa - about 30 GPa, about 10 GPa - about 27 GPa, about 10 GPa - about 25 GPa, or about 10 GPa - about 23 GPa. When the sulfide-based solid electrolyte has an elastic modulus within these ranges, the temperature and / or pressure required for sintering, etc. are reduced, and thus the sintering of the solid electrolyte can be more easily performed.

[0125] The solid electrolyte used in the solid electrolyte layers 30, 30a and 30b may have, for example, a D50 average particle size of about 1 μm - about 10 μm, about 1.5 μm - about 7 μm, or about 2 μm - about 5 μm.

[0126] The sulfide-based solid electrolyte included in the solid electrolyte layers 30, 30a, and 30b may be included, for example, in an amount of about 97 wt% - about 100 wt%, about 98 wt% - about 99.9 wt%, or about 98.5 wt% - about 99.0 wt% of the total weight of the solid electrolyte layers 30, 30a, and 30b.

[0127] [Solid electrolyte layer: Binder]

[0128] The solid electrolyte layers 30, 30a, and 30b may further include, for example, a binder. Examples of the binder included in the solid electrolyte layers 30, 30a, and 30b may be styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, or polyethylene, but the embodiments are not limited thereto, and any material available as a binder in the art may be used. The binder of the solid electrolyte layers 30, 30a, and 30b may be the same as or different from the binder included in the positive electrode active material layers 12, 12a, and 12b and the negative electrode active material layers 22, 22a, and 22b.

[0129] The binder included in the solid electrolyte layers 30, 30a, and 30b may be included, for example, in an amount of about 0.1 wt% - about 3 wt%, about 0.5 wt% - about 2 wt%, or about 1.0 wt% - about 2.0 wt% of the total weight of the solid electrolyte layers 30, 30a, and 30b.

[0130] [Negative electrode: Second negative electrode active material layer (coated or uncoated)]

[0131] Refer to Figure 3 、 4A 、4B and 6, the solid-state secondary battery 1 includes a positive electrode 10, the negative electrodes 20, and the solid electrolyte layers 30, 30a, and 30b disposed between the positive electrode 10 and the negative electrodes 20, 20a, and 20b.

[0132] The solid-state secondary battery 1 may further include, for example, a second negative electrode active material layer (not shown) provided between the negative electrode current collectors 21, 21a, and 21b and the first negative electrode active material layers 24, 24a, and 24b by charging. Additionally, the solid-state secondary battery 1 may further include, for example, a second negative electrode active material layer (not shown) provided between the solid electrolyte layers 30, 30a, and 30b and the first negative electrode active material layers 24, 24a, and 24b by charging. The second negative electrode active material layer is a metal layer containing lithium or a lithium alloy. The metal layer contains lithium or a lithium alloy. Thus, as a metal layer containing lithium, the second negative electrode active material layer serves as, for example, a lithium reservoir. Examples of the lithium alloy may include Li-Al alloy, Li-Sn alloy, Li-In alloy, Li-Ag alloy, Li-Au alloy, Li-Zn alloy, Li-Ge alloy, and Li-Si alloy, but the embodiments are not limited thereto, and any lithium alloy available in the art may be used. The second negative electrode active material layer may be composed of one of the alloys or lithium, or may be composed of a plurality of alloys.

[0133] The thickness of the second negative electrode active material layer is not particularly limited and is, for example, about 1 μm - about 1000 μm, about 1 μm - about 500 μm, about 1 μm - about 200 μm, about 1 μm - about 150 μm, about 1 μm - about 100 μm, or about 1 μm - about 50 μm. When the second negative electrode active material layer is too thin in thickness, it is difficult for the second negative electrode active material layer to serve as a lithium reservoir. When the second negative electrode active material layer is too thick in thickness, the solid-state secondary battery 1 may have a larger mass and volume and may have considerably deteriorated cycle characteristics.

[0134] In the solid-state secondary battery 1, the second negative electrode active material layer can be plated, for example, between the negative electrode current collectors 21, 21a, and 21b and the first negative electrode active material layers 24, 24a, and 24b by charging after assembling the solid-state secondary battery 1. When the second negative electrode active material layer is plated by charging after assembling the solid-state secondary battery 1, the solid-state secondary battery 1 does not include the second negative electrode active material layer when assembling the solid-state secondary battery, and thus the energy density of the solid-state secondary battery 1 increases. For example, when the solid-state secondary battery 1 is charged, the solid-state secondary battery 1 is charged by exceeding the charging capacity of the first negative electrode active material layers 24, 24a, and 24b. That is, the first negative electrode active material layers 24, 24a, and 24b are overcharged. At the start of charging, lithium is absorbed into the first negative electrode active material layers 24, 24a, and 24b. The negative electrode active material included in the first negative electrode active material layers 24, 24a, and 24b forms an alloy or a compound with the lithium ions transferred from the positive electrode layer 10. When the first negative electrode active material layers 24, 24a, and 24b are overcharged, for example, lithium is plated on the rear surfaces of the first negative electrode active material layers 24, 24a, and 24b, that is, between the negative electrode current collectors 21, 21a, and 21b and the first negative electrode active material layers 24, 24a, and 24b, and the plated lithium allows the formation of a metal layer corresponding to the second negative electrode active material layer. The second negative electrode active material layer is a metal layer mainly composed of lithium (for example, metallic lithium). For example, when the negative electrode active material included in the first negative electrode active material layers 24, 24a, and 24b is composed of a material that forms an alloy or a compound with lithium, the above result is obtained. When discharging, the lithium ions in the first negative electrode active material layers 24, 24a, and 24b and the second negative electrode active material layer (i.e., the metal layer) are ionized (i.e., dissolved) and move toward the positive electrode layer 10. Therefore, lithium can be used as the negative electrode active material in the solid-state secondary battery 1. In addition, since the first negative electrode active material layers 24, 24a, and 24b cover the second negative electrode active material layer, the first negative electrode active material layers 24, 24a, and 24b serve as a protective layer for the second negative electrode active material layer (i.e., the metal layer) and also prevent the precipitation growth of lithium dendrites. Therefore, short circuits and capacity degradation of the solid-state secondary battery 1 are prevented, and as a result, the cycle characteristics of the solid-state secondary battery 1 are improved. In addition, when the second negative electrode active material layer is provided by charging after assembling the solid-state secondary battery, the negative electrode current collectors 21, 21a, and 21b, the first negative electrode active material layers 24, 24a, and 24b, and the region therebetween are, for example, lithium-free regions that do not include lithium in the initial state of the solid-state secondary battery or after discharging the solid-state secondary battery.

[0135] Alternatively, in the solid-state secondary battery 1, the second negative electrode active material layer may be disposed between the negative electrode current collectors 21, 21a, and 21b and the first negative electrode active material layers 24, 24a, and 24b before assembling the solid-state secondary battery, or may be disposed between the solid electrolyte layer 30 and the first negative electrode active material layers 24, 24a, and 24b. That is, as a non-precipitation layer, the second negative electrode active material layer may be disposed between the negative electrode current collectors 21, 21a, and 21b and the first negative electrode active material layers 24, 24a, and 24b before assembling the battery, or may be disposed between the solid electrolyte layer 30 and the first negative electrode active material layers 24, 24a, and 24b. When the second negative electrode active material layer is provided as a lithium reservoir before assembling the solid-state secondary battery 1, lithium loss caused by side reactions due to charging / discharging of the solid-state secondary battery 1 can be compensated. The second negative electrode active material layer may be, for example, a lithium coating or a lithium metal foil having a thickness within the above range. A lithium metal coating can be coated by sputtering or the like.

[0136] The inventive concept will be described in more detail by the following examples and comparative examples. However, the examples are for illustrative purposes only and are not intended to limit the scope of the inventive concept.

[0137] Example 1: Cu substrate / Ti coating layer having a thickness of 100 nm, first active material layer (metal-carbon layer) having a thickness of 7 μm

[0138] (Preparation of negative electrode layer)

[0139] As a first metal substrate, a Cu foil having a thickness of 10 μm was prepared. A coating layer in the form of a Ti thin film having a thickness of 100 nm was coated on the Cu foil by DC magnetron sputtering to prepare a negative electrode current collector. Cu has a Mohs hardness of 3.0, and Ti has a Mohs hardness of 6.0.

[0140] The DC magnetron sputtering conditions were a cathode power of 125 W, a sputtering pressure of 1.1 Pa, and a vacuum of 4×10 -4 Pa.

[0141] As a negative electrode active material, carbon black (CB) having a primary particle size of about 30 nm and silver (Ag) particles having an average particle size of about 60 nm were prepared.

[0142] NMP solvent and PVDF-HFP binder (#9300 manufactured by Kureha, Mw: 1,200,000 g / mol) were added to a container and stirred for 5 minutes at 1300 rpm using a Thinky mixer to prepare an NMP solution containing the PVDF-HFP binder. Silver (Ag) particles and carbon black (CB) were sequentially added to the NMP solution and stirred for 5 minutes at 1300 rpm using a Thinky mixer to prepare a slurry. The carbon black (CB) and silver (Ag) particles had a weight ratio of 3:1 and contained the PVDF-HFP binder in an amount of 6 wt% relative to the solid content of the slurry.

[0143] The prepared slurry was applied to the Ti-coated layer of the negative electrode current collector using a bar coater and dried in a convection oven at 80 °C for 10 minutes to obtain a laminate. Subsequently, the obtained laminate was vacuum dried at 40 °C for 10 hours to obtain a vacuum-dried laminate. The vacuum-dried laminate was roll-pressed at a pressure of 300 MPa for 10 ms to flatten the surface of the first negative electrode active material layer of the laminate. The negative electrode was prepared through the above process. The thickness of the first negative electrode active material layer included in the negative electrode was approximately 7 μm.

[0144] (Preparation of the positive electrode layer)

[0145] Li2O-ZrO2 (LZO)-coated LiNi 0.8 Co 0.15 Mn 0.05 O2 (NCM) was prepared as the positive electrode active material. The LZO-coated positive electrode active material was prepared according to the method disclosed in Korean Patent Publication No. 10-2016-0064942. As the solid electrolyte, the argyrodite-type crystal Li6PS5Cl (D50 = 0.5 μm, crystalline) was prepared. As the binder, polytetrafluoroethylene (PTFE) binder (Teflon binder manufactured by DuPont) was prepared. As the conductive agent, carbon nanofibers (CNF) were prepared. These materials and p-xylene solvent were formed into a sheet in a mixture with a weight ratio of positive electrode active material: solid electrolyte: conductive agent: binder = 84:11.5:3:1.5, and then the obtained product was vacuum dried at 40 °C for 10 hours to prepare a positive electrode sheet. A washer made of polypropylene (PP) surrounding the positive electrode sheet was arranged around the positive electrode sheet. The positive electrode sheet surrounded by the washer was pressed onto one side of a positive electrode current collector formed of 18-μm-thick carbon-coated aluminum foil to prepare a positive electrode. The positive electrode sheet and the washer were roll-pressed at a pressure of 450 MPa for 100 ms.

[0146] The thickness of the positive electrode active material layer and the washer included in the positive electrode was approximately 100 μm.

[0147] The positive electrode active material layer is disposed at the center of the positive electrode current collector, and the gasket surrounds the positive electrode active material layer and is disposed at the end of the positive electrode current collector. The area of the positive electrode active material layer is about 90% of the area of the positive electrode current collector, and the gasket is disposed to surround the positive electrode active material layer over the entire area of the remaining 10% of the positive electrode current collector where the positive electrode active material layer is not provided.

[0148] (Preparation of Solid Electrolyte Layer)

[0149] 1.5 parts by weight of an acrylic binder relative to 98.5 parts by weight of a solid electrolyte was added to a Li6PS5Cl solid electrolyte (D50 = 3.0 μm, crystalline) which is a thioargentogermanate-type crystal to prepare a mixture. Octyl acetate was added to the prepared mixture and stirred to prepare a slurry. The prepared slurry was applied to a nonwoven fabric using a bar coater, and dried in air at 80 °C for 10 minutes to obtain a sheet-like solid. The obtained solid was dried under vacuum at 40 °C for 10 hours. The solid electrolyte layer was prepared through the above process.

[0150] (Preparation of Solid-State Secondary Battery)

[0151] The solid electrolyte layer was disposed on the positive electrode active material layer of the positive electrode, and the negative electrode was disposed on the solid electrolyte layer such that the first negative electrode active material layer contacted the solid electrolyte layer to prepare a laminate.

[0152] The laminate was placed in a bag and vacuum-sealed, and then subjected to warm isostatic pressing (WIP) at 90 °C under a pressure of 500 MPa for 30 minutes to prepare a solid-state secondary battery.

[0153] Through the pressure treatment, the solid electrolyte layer was sintered, and thus the battery characteristics were improved. The thickness of the sintered solid electrolyte layer was 60 μm.

[0154] Example 2: Cu substrate / Ti coating layer with a thickness of 200 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0155] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: the thickness of the coating layer in the form of a Ti thin film provided on the Cu foil was changed to 200 nm.

[0156] Example 3: Cu substrate / Ti coating layer with a thickness of 300 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0157] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that the thickness of the coating layer in the form of a Ti thin film provided on the Cu foil was changed to 300 nm.

[0158] Example 4: Cu substrate / V coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0159] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that the Ti thin film coating layer provided on the Cu foil was changed to a V thin film coating layer. Cu has a Mohs hardness of 3.0, and V has a Mohs hardness of 7.0.

[0160] Example 5: Cu substrate / W coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0161] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that the Ti thin film coating layer provided on the Cu foil was changed to a W thin film coating layer. Cu has a Mohs hardness of 3.0, and W has a Mohs hardness of 7.5.

[0162] Example 6: Cu substrate / Cr coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0163] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that the Ti thin film coating layer provided on the Cu foil was changed to a Cr thin film coating layer. Cu has a Mohs hardness of 3.0, and Cr has a Mohs hardness of 8.5.

[0164] Example 7: Ni substrate / Cr coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0165] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 6, except that a 10-μm-thick Ni foil was used instead of the Cu foil as the first metal substrate. Ni has a Mohs hardness of 4.0, and Cr has a Mohs hardness of 8.5.

[0166] Example 8: SUS substrate / Cr coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 7 μm

[0167] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 6, except that a 10-μm-thick stainless steel (SUS 304) sheet was used instead of the Cu foil as the first metal substrate.

[0168] The stainless steel has a Mohs hardness of 5.5, and Cr has a Mohs hardness of 8.5.

[0169] Example 9: Cu substrate / Ti coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 3 μm

[0170] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: the thickness of the first negative electrode active material layer was changed to 3 μm.

[0171] Example 10: Cu substrate / Ti coating layer with a thickness of 100 nm, first active material layer (metal-carbon layer) with a thickness of 15 μm

[0172] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: the thickness of the first negative electrode active material layer was changed to 15 μm.

[0173] Example 11: Dual-cell battery

[0174] The solid-state secondary battery was prepared in the same manner as in Example 1, except that: the positive electrode active material layer and the gasket were each arranged on both sides of the positive electrode current collector to prepare a positive electrode layer, and the solid electrolyte layer and the negative electrode layer were sequentially arranged on both sides of the positive electrode layer to prepare the battery.

[0175] Comparative Example 1: Cu substrate / first active material layer (metal-carbon layer) with a thickness of 7 μm, without Ti coating layer

[0176] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: a Cu foil with a thickness of 10 μm was used as the negative electrode current collector. Therefore, the Ti thin film coating layer was omitted.

[0177] Comparative Example 2: Ni substrate / first active material layer (metal-carbon layer) with a thickness of 7 μm, without Ti coating layer

[0178] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: a Ni foil with a thickness of 10 μm was used as the negative electrode current collector. Therefore, the Ti thin film coating layer was omitted.

[0179] Comparative Example 3: SUS substrate / first active material layer (metal-carbon layer) with a thickness of 7 μm, without Ti coating layer

[0180] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: a SUS sheet with a thickness of 10 μm was used as the negative electrode current collector. Therefore, the Ti thin film coating layer was omitted.

[0181] Reference Example 1: Without washer member

[0182] The negative electrode and the solid-state secondary battery were prepared in the same manner as in Example 1, except that: a washer was not used when preparing the positive electrode layer.

[0183] Evaluation Example 1: Charge / discharge test

[0184] The charge / discharge characteristics of the solid-state secondary batteries prepared in Examples 1-11 and Comparative Examples 1-3 were evaluated by the following charge / discharge test. The charge / discharge test was carried out by placing the solid-state secondary battery in a constant temperature chamber at 60 °C.

[0185] In the first cycle, the battery was charged at a constant current of 0.6 mA / cm 2 for 12.5 hours until the battery voltage reached 3.9 V to 4.25 V. Subsequently, the battery was discharged at a constant current of 0.6 mA / cm 2 for 12.5 hours until the battery voltage reached 2.5 V.

[0186] From the second cycle onwards, the battery was charged and discharged under the same conditions as in the first cycle.

[0187] The capacity retention rate was evaluated by the number of cycles having a discharge capacity of 95% or more corresponding to the discharge capacity (standard capacity) in the first cycle.

[0188] Some of the measurement results are shown in Table 1 below.

[0189] [Table 1]

[0190]

[0191]

[0192] The solid-state secondary batteries of Examples 1-8 and 10 had more than 70 cycles with a discharge capacity of 95% or more retained. Therefore, the solid-state secondary batteries of Examples 1-8 and 10 showed an improved capacity retention rate.

[0193] In addition, as shown in Examples 1-3, when the thickness of the second coating layer increased, the capacity retention rate was further improved.

[0194] In addition, as shown in Examples 4-8, when the Mohs hardness of the second coating layer metal increased, the capacity retention rate was further improved.

[0195] Compared with the solid-state secondary battery of Example 1, the solid-state secondary battery of Example 10 had an improved capacity retention rate, but due to the increase in irreversible capacity, the total capacity of the solid-state secondary battery was relatively reduced compared with Example 1.

[0196] It is understood that an improved lifespan is achieved due to the following: deterioration of the first metal substrate is prevented by coating a second metal layer with high hardness on the first metal substrate.

[0197] On the other hand, the solid-state secondary batteries of Comparative Examples 1 and 2 using a negative electrode that does not include a second metal layer with high hardness have less than 40 cycles of maintaining a discharge capacity of 95% or higher. Therefore, the solid-state secondary batteries of Comparative Examples 1 and 2 show a reduced capacity retention rate.

[0198] In particular, the lifespan of the solid-state secondary battery of Comparative Example 1 using a Cu current collector with a low Mohs hardness and a side reaction with sulfide as the negative electrode is the lowest.

[0199] After charging in the first cycle of the solid-state secondary batteries of Examples 1-10 was completed, SEM images of the cross-sections of these batteries were measured, and it was confirmed that a lithium metal layer corresponding to the second negative electrode active material layer was formed between the first negative electrode active material layer and the negative electrode current collector.

[0200] It was confirmed that the solid-state secondary battery of Example 11 also showed excellent cycle characteristics.

[0201] Evaluation Example 2: Observation of cracks after preparing the solid-state battery

[0202] The cross-sections of the solid-state secondary batteries prepared in Examples 1-11, Comparative Examples 1-3, and Reference Example 1 were observed using a scanning electron microscope (SEM) to observe whether cracks penetrating the solid electrolyte layer were caused.

[0203] In the solid-state secondary batteries prepared in Examples 1-11 and Comparative Examples 1-3, no cracks were found in the solid electrolyte layer.

[0204] On the contrary, in the solid-state secondary battery of Reference Example 1, many cracks were found in the solid electrolyte layer at the corners of the positive electrode layer.

[0205] In addition, in the solid-state secondary battery of Reference Example 1, under the conditions of Evaluation Example 1, a short circuit was caused in less than 10 cycles during charge / discharge, resulting in poor lifespan characteristics.

[0206] Therefore, it was confirmed that in the solid-state secondary batteries of Examples 1-11, when an inactive member is used for the positive electrode, cracks in the solid electrolyte layer are prevented during battery manufacturing.

[0207] According to one or more embodiments, by using a negative electrode with a novel structure, deterioration of the negative electrode current collector can be prevented and a solid-state secondary battery with improved cycle characteristics can be provided.

[0208] It should be understood that the embodiments described herein are to be considered only in the descriptive sense and not for purposes of limitation. The description of a feature or aspect within an embodiment should typically be considered applicable to other similar features or aspects in other embodiments. Although one or more embodiments have been described with reference to the accompanying drawings, those of ordinary skill in the art will understand that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

1. The negative electrode comprises: A negative electrode current collector; A first negative electrode active material layer provided on the negative electrode current collector; And a second negative electrode active material layer located between the negative electrode current collector and the first negative electrode active material layer, Wherein the negative electrode current collector comprises: a first metal substrate; And a coating layer provided on the first metal substrate and containing a second metal, The second metal has a Mohs hardness greater than that of the first metal, Wherein the second metal has a Mohs hardness of 6.0 or greater, Wherein the second negative electrode active material layer is a metal layer containing lithium or a lithium alloy.

2. The negative electrode according to claim 1, wherein the first metal has a Mohs hardness of 5.5 or less.

3. The negative electrode according to claim 1, wherein the first metal is at least one selected from the following: copper (Cu), nickel (Ni), stainless steel (SUS), iron (Fe), and cobalt (Co).

4. The negative electrode according to claim 1, wherein the second metal is at least one selected from the following: titanium (Ti), manganese (Mn), niobium (Nb), tantalum (Ta), iridium (Ir), vanadium (V), rhenium (Re), osmium (Os), tungsten (W), chromium (Cr), boron (B), ruthenium (Ru), and rhodium (Rh).

5. The negative electrode according to claim 1, wherein the coating layer has a thickness of 1 μm or less.

6. The negative electrode according to claim 1, wherein the coating layer is inert to the sulfide-based solid electrolyte.

7. The negative electrode according to claim 1, wherein the first negative electrode active material layer comprises a negative electrode active material and a binder, the negative electrode active material is in particulate form and has an average particle diameter of 4 μm or less.

8. The negative electrode according to claim 7, wherein the negative electrode active material comprises at least one selected from the following: carbon-based negative electrode active materials and metal or metalloid negative electrode active materials.

9. The negative electrode according to claim 8, wherein the carbon-based negative electrode active materials comprise at least one selected from the following: amorphous carbon and crystalline carbon.

10. The negative electrode according to claim 8, wherein the metal or metalloid negative electrode active materials comprise at least one selected from the following: gold (Au), platinum (Pt), palladium (Pd), silicon (Si), silver (Ag), aluminum (Al), bismuth (Bi), tin (Sn), and zinc (Zn).

11. The negative electrode according to claim 1, wherein the negative electrode active material comprises a mixture of first particles composed of amorphous carbon and second particles composed of a metal or metalloid, and the second particles are contained in an amount of 8 wt% - 60 wt% relative to the total weight of the mixture.

12. The negative electrode according to claim 1, wherein the first negative electrode active material layer has a thickness of 1 μm - 20 μm.

13. A solid-state secondary battery comprises: A positive electrode; The negative electrode according to any one of claims 1 - 12; and A solid electrolyte layer located between the positive electrode and the negative electrode, Wherein the solid electrolyte layer comprises a sulfide-based solid electrolyte.

14. The solid secondary battery according to claim 13, wherein the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and includes an inactive member that surrounds a side surface of the positive electrode active material layer, is located at a peripheral portion of the positive electrode current collector, and extends to an end portion of the positive electrode current collector.

15. The solid secondary battery according to claim 13, wherein the sulfide-based solid electrolyte includes at least one of the following: Li2S-P2S5, Li2S-P2S5-LiX where X is a halogen element, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z where m and n are positive numbers and Z is one of Ge, Zn, or Ga m S n , Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li p MO q .

16. The solid secondary battery according to claim 13, wherein the sulfide-based solid electrolyte includes at least one selected from the following: Li7P3S 11 , Li7PS6, Li4P2S6, Li3PS6, Li3PS4, and Li2P2S6.

17. The solid secondary battery according to claim 13, wherein the sulfide-based solid electrolyte includes a thiogermanate-type solid electrolyte represented by the following Formula 1: <Formula 1> Li + 12-n-x A n+ X 2- 6-x Y - x wherein in the above Formula 1, A is P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, or Ta, and X is S, Se, or Te, Y is Cl, Br, I, F, CN, OCN, SCN, or N3, and 1 ≤ n ≤ 5 and 0 ≤ x ≤ 2 are satisfied.

18. The solid-state secondary battery according to claim 17, wherein the argyrodite-type solid electrolyte is at least one selected from the following: Li where 0 ≤ x ≤ 2 7-x PS 6-x Cl x , Li where 0 ≤ x ≤ 2 7-x PS 6-x Br x , and Li where 0 ≤ x ≤ 2 7-x PS 6-x I x .

19. The solid secondary battery according to claim 13, further including a second negative electrode active material layer between the solid electrolyte layer and the first negative electrode active material layer, wherein the second negative electrode active material layer is a metal layer containing lithium or a lithium alloy.

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