Ferritic stainless steel for current collector of non-aqueous electrolyte lithium secondary battery, method for preparing same, current collector comprising same, and lithium secondary battery

Abstract

According to an embodiment of the present invention, a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery may be provided, wherein the ferritic stainless steel comprises 18.0-32.0 wt% of Cr and has 0.740-6.300 area% of microdefects that are formed on the outermost surface of the ferritic stainless steel.

Description

Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries and a method for manufacturing the same, a current collector including the same, and a lithium secondary battery

[0001] The present invention relates to a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery having low interfacial contact resistance by controlling the composition and surface modification process, a method for manufacturing the same, a current collector including the same, and a lithium secondary battery.

[0002] Rechargeable batteries are one of the core technologies of future industries, utilized in everything from small devices to large equipment such as electric vehicles. Electrolytes are one of the most critical components of rechargeable battery materials; however, the currently most widely used lithium-ion batteries utilize liquid electrolytes, which pose a risk of battery damage due to expansion caused by temperature changes or leakage from external impacts. In contrast, all-solid-state batteries, which use non-aqueous electrolytes, maintain their shape even if the electrolyte is damaged, making them structurally stable. Furthermore, they eliminate the risk of explosion and fire, allowing for a reduction in safety-related components and an increase in capacity. Solid electrolytes can be broadly classified into three types: sulfide-based, oxide-based, and polymer-based. Sulfide-based electrolytes are attracting attention because they possess the highest ionic conductivity, enabling them to be spherical with high energy density. Currently, copper (Cu) is used as the negative electrode current collector in sulfide-based all-solid-state batteries. However, since the formation of CuS corrosion products during use leads to performance degradation, there is a need to switch from conventional copper to stainless steel. Ferritic stainless steel offers excellent corrosion resistance with minimal addition of expensive alloying elements; however, there is a challenge in ensuring sulfur resistance and sufficient adhesion to the electrolyte for use as a negative electrode current collector in sulfide-based all-solid-state batteries.

[0003] The present invention aims to solve the aforementioned problems by optimizing the composition and manufacturing process of stainless steel to improve its surface characteristics, thereby increasing adhesion with sulfide-based solid electrolytes and suppressing reactivity when used as a current collector.

[0004] A ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention contains 18.0 to 32.0 weight% of Cr and contains 0.740 to 6.300% of fine defects formed on the outermost surface of the ferritic stainless steel as an area fraction.

[0005] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may contain 0.100 weight% or less of Cu.

[0006] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may contain Ni in an amount of 3,000 weight% or less.

[0007] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have an average thickness of 0.005 to 0.030 mm.

[0008] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have an Open Circuit Voltage (OCV) value of 0.070 V or less after applying a Circuit Voltage (CV) at a scan rate of 20 mV / sec at room temperature.

[0009] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have an OCV value of 0.040V or less after applying a CV (Circuit Voltage) at a scan rate of 20mV / sec at 90℃.

[0010] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have a real impedance value of 100Ω or less and an imaginary impedance value of 220Ω or less at a frequency of 0.5MHz when measured by Electrochemical Impedance Spectroscopy (EIS) after a Circuit Voltage (CV) cycle of -4V to 4V.

[0011] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have a real impedance value of 600Ω or less at a frequency of 1KHz and an imaginary impedance value of 300Ω or less.

[0012] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may further comprise, in weight%, C: 0.0005 to 0.0200%, N: 0.0010 to 0.0200%, Si: 0.01 to 0.25%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, Ti: 0.01 to 0.50%, Al: 0.001 to 0.500%, Mo: 0.01 to 2.00%, and the remainder being Fe and unavoidable impurities.

[0013] A method for manufacturing a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to another embodiment of the present invention comprises: a step of preparing a ferritic stainless steel material containing 18.0 to 32.0 wt% of Cr in wt%; and a step of immersing the ferritic stainless steel material in an aqueous sulfuric acid solution of 10 to 20 wt% at a temperature of 50 to 75°C to remove a first passivation film formed on the outermost surface of the ferritic stainless steel material. and the step of immersing the ferritic stainless steel in a mixed acid aqueous solution of 10 to 20 weight% nitric acid and 1 to 10 weight% hydrofluoric acid at a temperature of 40 to 60°C to form a second passivation film on the outermost surface of the ferritic stainless steel; wherein, in the step of forming the second passivation film, the outermost surface of the ferritic stainless steel contains 0.740 to 6.300% of fine defects as an area fraction.

[0014] In addition, the method for manufacturing a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may include 0.100 weight% or less of Cu in the ferritic stainless steel material.

[0015] In addition, the method for manufacturing a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may include the ferritic stainless steel material containing 3,000 weight% or less of Ni.

[0016] In addition, a method for manufacturing a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may further include, in weight%, C: 0.0005 to 0.0200%, N: 0.0010 to 0.0200%, Si: 0.01 to 0.25%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, Al: 0.001 to 0.500%, Ti: 0.01 to 0.50%, Mo: 0.01 to 2.00%, and the remainder being Fe and unavoidable impurities.

[0017] A current collector for a non-aqueous electrolyte lithium secondary battery according to another embodiment of the present invention is manufactured from ferritic stainless steel as described above.

[0018] A current collector for a non-aqueous electrolyte lithium secondary battery according to another embodiment of the present invention comprises: a negative electrode including the current collector and a negative electrode active material layer formed on the current collector; a positive electrode disposed opposite to the negative electrode and including a positive electrode plate and a positive electrode active material layer formed on the positive electrode plate; and a solid electrolyte disposed between the negative electrode and the positive electrode.

[0019] According to the present invention, a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery can be provided, which has high adhesion to a sulfide-based solid electrolyte and suppresses reactivity when used as a current collector.

[0020] Figure 1 is a flowchart showing a method for measuring OCV (Open circuit voltage).

[0021] FIG. 2 is a perspective view showing fine scratches formed on the outermost surface of a ferritic stainless steel for a current collector.

[0022] Figure 3 is an enlarged view of area I shown in Figure 2, illustrating a method for measuring the depth of a micro-blemish in the thickness direction (z-axis direction).

[0023] FIG. 4 is a scanning electron microscope (SEM) image of a micro-defect formed on the outermost surface of a ferritic stainless steel for a current collector according to Example 1 of the invention, and shows a method for measuring the area fraction of the micro-defect within any region II.

[0024] FIG. 5 is a scanning electron microscope (SEM) image of a micro-defect formed on the outermost surface of a ferritic stainless steel for a current collector according to Note 8, showing a method for measuring the area fraction of the micro-defect within any III region.

[0025] Figure 6 is a scanning electron microscope (SEM) image of inclusions formed when Al or Ti is added in excess.

[0026] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0027] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0028] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0029] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0030] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0031] First, the stainless steel for the current collector of a non-aqueous electrolyte lithium secondary battery according to the present invention will be described in detail.

[0032] (Stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries)

[0033] A ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention comprises 18.0 to 32.0 weight% of Cr and has a fine defect formed on the outermost surface of the ferritic stainless steel in an area fraction of 0.740 to 6.300%.

[0034] Cr may be 18.0 to 32.0 wt%. Since excellent corrosion resistance is required in the sulfuric acid atmosphere in which the all-solid-state battery operates, and Cr can improve corrosion resistance by forming an oxide film by forming Cr2O3, Cr may be included in an amount of 18.0 wt% or more. However, since there may be a problem with reduced toughness if it exceeds 32.0 wt%, the upper limit can be controlled to 32.0 wt%, preferably 25.0%, and more preferably 20.5 wt%.

[0035] Fine defects formed on the outermost surface of the ferritic stainless steel may be included in an area fraction of 0.740 to 6.300%. Fine defects refer to structures having an average depth of greater than 0 and less than 1 μm in the z-axis direction from the outermost surface of the stainless steel, and having a convex shape as shown in FIG. 2. When viewed in a plan view of the outermost surface, they may be in the form of a point, or when viewed in a perspective view such as FIG. 2, they may be in the form of a straight line or a curved shape such as a U-shape including a convex part. Since such fine defects exhibit an anchoring effect at the interface between the surface of the stainless steel and the sulfide-based solid electrolyte, thereby improving adhesion, it is desirable for them to be included in an area fraction of 0.740% or more. However, if excessive fine defects are present, the reaction between the surface of the stainless steel and the sulfide-based solid electrolyte may occur more easily through the fine defects, leading to a problem of deteriorating interfacial adhesion; therefore, the area fraction of the fine defects is controlled to 6.300% or less. Preferably, it can be 0.790 to 6.250%.

[0036] Figure 3 is a diagram showing a method for measuring the depth of a micro-blemish in the z-axis direction.

[0037] The average depth in the z-axis direction from the outermost surface of the stainless steel with fine defects is measured as follows.

[0038] First, a sample for cross-sectional observation is prepared from ferritic stainless steel using a focused ion beam processing device (FIB, Versa 3D Dual Beam manufactured by FEI).

[0039] Next, the prepared cross-sectional observation sample is observed at 100x magnification in any 5 fields of view using a scanning electron microscope (SEM) to obtain SEM images. For each obtained SEM image, as shown in FIG. 3, the lowest point V (lowest point in the z-axis direction) of the concave part (on both sides) adjacent to one convex part is connected by a straight line b, and the distance h between the straight line b and the vertex T (vertex in the z-axis direction) of this convex part is calculated. The calculated distance h is taken as the height of the convex part, and the average of these is taken as the average depth in the z-axis direction from the outermost surface of the micro-defect. If the micro-defect is in the shape of a straight line, L is the length of the straight line and θ is the angle formed with the z-axis, and L*cosθ is taken as the depth in the z-axis direction.

[0040] FIGS. 4 and 5 are scanning electron microscope (SEM) images of micro-defects formed on the outermost surface of ferritic stainless steel for current collectors, illustrating a method for measuring the area fraction of micro-defects. The method for measuring the area fraction of micro-defects is explained using FIG. 4 as an example.

[0041] After obtaining an SEM image of a prepared cross-sectional observation sample using a scanning electron microscope (SEM) at a magnification of 100x, one arbitrary region (II) of size 1㎛ X 1㎛ in each of the four quadrants (1st to 4th quadrants) is selected, and the area fraction of the micro-defect is measured using an image analyzer and calculated as the average value. In addition, it is possible to further control features related to micro-defects; for example, by controlling the average diameter of the micro-defect (maximum depth in the planar direction of the micro-defect), it is possible to secure electrolyte permeability while maintaining mechanical strength, and it is also possible to control the number density of the micro-defects or the aspect ratio (depth / width) of the micro-defects in terms of securing interfacial contact resistance.

[0042] The ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery of the present invention may have an Open Circuit Voltage (OCV) value of 0.070 V or less, and preferably 0.060 V or less, after applying a Circuit Voltage (CV) at a scan rate of 20 mV / sec at room temperature.

[0043] Current collectors for all-solid-state batteries are exposed to environments where potential differences occur and current flows during use; in this case, changes in current flow and surface resistance occur due to the products generated from reactions at the interface between the current collector and the sulfide-based solid electrolyte.

[0044] Therefore, the reactivity of the current collection material and the sulfide-based solid electrolyte can be evaluated by observing the current flow and subsequent change in surface resistance in an environment where an artificial potential difference is generated using the electrochemical method.

[0045] Specifically, the voltage of the mounted cell (OCV: Open circuit voltage) is measured. Since each electrode is identical, it should theoretically measure as 0V; however, voltage is generated depending on the surface condition. Depending on the connection direction of the electrodes, the voltage measured from the same sample may be positive or negative. Therefore, OCV is compared as an absolute value regardless of whether it is positive or negative. An artificial voltage is applied using the CV method to induce an interfacial reaction between the current collector and the solid electrolyte, and the interfacial reaction between the current collector and the solid electrolyte is evaluated. In CV, the characteristics according to the applied potential can be evaluated in various ways by varying the applied potential, and the scan rate of CV can be fixed at 20mV / sec. When CV is applied at a scan rate of 20 mV / sec at room temperature, if the OCV value exceeds 0.070 V, the reaction between the current collector and the sulfide-based solid electrolyte occurs actively, and an excessive amount of product is formed at the interface; therefore, the OCV value is controlled to be 0.070 V or less. In addition, the ferritic stainless steel for the current collector of the non-aqueous electrolyte lithium secondary battery of the present invention may have an OCV value of 0.040 V or less after applying CV at a scan rate of 20 mV / sec at 90°C. The reactivity between the current collector and the sulfide-based solid electrolyte increases with higher temperatures. However, when CV is applied at a scan rate of 20 mV / sec at 90°C and the OCV value exceeds 0.040 V, the reaction between the current collector and the sulfide-based solid electrolyte becomes active and an excessive amount of product is formed at the interface, so the OCV value is controlled to be 0.040 V or less.

[0046] A ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have a real impedance value of 100Ω or less and an imaginary impedance value of 220Ω or less at a frequency of 0.5MHz when measured by Electrochemical Impedance Spectroscopy (EIS) after a Circuit Voltage (CV) cycle of -4V to 4V.

[0047] As described above, changes in current flow and surface resistance can occur due to the products generated from the reaction at the interface between the current collector and the sulfide-based solid electrolyte; accordingly, it is possible to evaluate the reactivity between the current collector and the sulfide-based solid electrolyte by observing the current flow and subsequent changes in surface resistance in an environment where an artificial potential difference is generated using electrochemical methods.

[0048] At a frequency of 0.5 MHz, the real impedance value may be 100 Ω or less, and the imaginary impedance value may be 220 Ω or less. The 0.5 MHz frequency is considered to be the point at which the electron conduction reaction begins to change to an ion conduction reaction as the frequency changes from a high frequency to a low frequency. Therefore, when used as a current collector material, this frequency band can be viewed as the resistance value for electron conduction between the surface of the current collector and the contact part with the electrolyte. In particular, the real impedance value indicates the degree of energy dissipated due to resistance to electron conduction, and the imaginary impedance value indicates the degree of energy accumulated on the contact surface being dissipated.

[0049] When the real impedance value at a frequency of 0.5 MHz exceeds 100 Ω and the imaginary impedance value exceeds 220 Ω, the reaction between the current collector and the sulfide-based solid electrolyte occurs vigorously, and an excessive amount of product is formed at the interface, resulting in poor electronic conductivity.

[0050] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may have a real impedance value of 600Ω or less at a frequency of 1KHz and an imaginary impedance value of 300Ω or less.

[0051] The 1KHz frequency is considered to be the point at which the reaction begins to change from the adsorption and desorption of small ion species to the movement of large ion species as the frequency changes from high to low. Therefore, when used as a current collector material, this frequency band can be viewed as the resistance value for the movement of ion species between the surface of the current collector and the contact part with the electrolyte. In particular, the real impedance value indicates the degree of exhaustion due to the resistance of the adsorption and desorption of the ion zone, while the imaginary impedance value indicates the degree of energy storage accumulated on the contact surface and subsequently exhausted.

[0052] When the real impedance value at a frequency of 1KHz exceeds 600Ω and the imaginary impedance value exceeds 300Ω, the reaction between the current collection material and the sulfide-based solid electrolyte occurs vigorously, and an excessive amount of product is formed at the interface, resulting in poor conductivity of the adsorbed ion species.

[0053] In addition, the ferritic stainless steel for the current collector of the non-aqueous electrolyte lithium secondary battery of the present invention may have an average thickness of 0.005 to 0.030 mm. The average thickness is calculated as the average value of thicknesses measured by selecting any 10 points along the z-axis direction of the ferritic stainless steel, and the method or device for measuring the thickness is not limited and may be measured by, for example, a mechanical method or an optical method. If the average thickness of the ferritic stainless steel for the current collector is less than 0.005 mm, production efficiency is significantly reduced and manufacturing costs increase significantly, and if the average thickness exceeds 0.030 mm, it results in an increase in the weight of the battery, so the average thickness can be controlled to 0.005 to 0.030 mm.

[0054] Hereinafter, the content of other alloy components of the ferritic stainless steel of the present invention will be described.

[0055] A ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may further comprise, in weight%, C: 0.0005 to 0.0200%, N: 0.0010 to 0.0200%, Si: 0.01 to 0.25%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, Ti: 0.01 to 0.50%, Al: 0.001 to 0.500%, Mo: 0.01 to 2.00%, and the remainder being Fe and unavoidable impurities.

[0056] C: 0.0005~0.0200 wt%

[0057] C may be included in an amount of 0.0005 to 0.0200 wt%. Although C is necessary for strength enhancement, if it exceeds 0.0200 wt%, corrosion resistance deteriorates, so the upper limit can be controlled to 0.0200 wt%. Preferably, it may be 0.0047 to 0.0185 wt%, and more preferably, 0.0050 to 0.0185 wt%.

[0058] N: 0.0010~0.0200 wt%

[0059] N may be included in an amount of 0.0010 to 0.0200 wt%. Although N is necessary to ensure strength, it forms Cr carbonitrides together with C, which impairs the corrosion resistance of the Cr-deficient layer; therefore, the upper limit can be controlled to 0.0200 wt%. Preferably, it may be 0.0010 to 0.0193 wt%, and more preferably, 0.0045 to 0.0193 wt%.

[0060] Si: 0.01~0.25 wt%

[0061] Si may be included in an amount of 0.01 to 0.25 weight%. If the amount of Si is less than 0.01 weight%, there is a problem of high refining costs, and if it exceeds 0.25 weight%, there is a problem of increased impurities leading to reduced formability and adverse effects on the conductivity and hydrophilicity of the product due to SiO2 oxide generated during the annealing process, so it can be controlled to 0.01 to 0.25 weight%. Preferably, it may be 0.13 to 0.25 weight%, and more preferably, 0.14 to 0.21 weight%.

[0062] Mn: 0.01~1.00 wt%

[0063] Mn may be included in an amount of 0.01 to 1.00 weight%. If the amount of Mn is less than 0.01 weight%, there is a problem that the refining cost becomes high, and if it exceeds 1.00 weight%, there is a problem that impurities increase, resulting in reduced moldability and reduced corrosion resistance due to MnS inclusions, so it can be controlled to 0.01 to 1.00 weight%. Preferably, it may be 0.01 to 0.94 weight%, more preferably 0.55 to 0.94 weight%, and even more preferably 0.78 to 0.92 weight%.

[0064] Ti: 0.01~0.50 wt%

[0065] Ti may be included in an amount of 0.01 to 0.50 weight%. Ti is an element that forms carbonitrides of C and N in steel, and since Ti nitrides are high-melting point inclusions, there is a high possibility of defects caused by inclusions when rolling into a thin film for a negative electrode current collector of an all-solid-state battery. Therefore, in the present invention, the upper limit may be limited to 0.50 weight% or less, preferably 0.01 to 0.36 weight%, and more preferably 0.05 to 0.36 weight%.

[0066] Al: 0.001~0.500 wt%

[0067] When Al is added, high-melting point Al oxides formed by Al deoxidation act as inclusions, which are an adverse factor during ultra-thin rolling, so the amount is managed to be 0.001 to 0.500 wt%. As shown in FIG. 6, TiO2-Al2O3 inclusions of several μm size formed when Ti and Al are added in excess of 0.500 wt% can be observed. Although inclusions of several μm size do not actually pose a problem in general hot-rolled or cold-rolled materials, they reduce rollability and impede productivity when performing 10 μm ultra-thin rolling. Therefore, to suppress this, the upper limit can be controlled to 0.500 wt% or less, preferably 0.005 to 0.500 wt%, and more preferably 0.009 to 0.500 wt%.

[0068] Mo: 0.01~2.00 wt%

[0069] Mo may be included in an amount of 0.01 to 2.00 wt%. Although Mo is an element advantageous for corrosion resistance in the fuel cell operating environment, excessive addition may reduce moldability, so the content can be controlled to 0.01 to 2.00 wt%. Preferably, it may be 0.01 to 1.71 wt%, and more preferably, 0.89 to 1.60 wt%.

[0070] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may contain Cu in an amount of 0.100 wt% or less. Even when added in trace amounts, Cu forms CuS dendrites in a sulfur atmosphere and causes performance degradation of the battery due to corrosion generation, so the room temperature OCV after CV application is 0.10 V or higher, so it can be controlled to a range of 0.100 wt% or less.

[0071] In addition, the ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to one embodiment of the present invention may contain Ni in an amount of 3,000 wt% or less. Since Ni reduces battery efficiency due to reaction with the electrolyte and causes a problem of increased cost when added, it can be controlled within a range of 3,000 wt% or less.

[0072] The remaining component of the composition is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0073] Next, a method for manufacturing stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries is described.

[0074] (Method for manufacturing stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries)

[0075] A method for manufacturing a ferritic stainless steel for a current collector of a non-aqueous electrolyte lithium secondary battery according to another embodiment of the present invention comprises: a step of preparing a ferritic stainless steel material containing 18.0 to 32.0 wt% of Cr in wt%; and a step of immersing the ferritic stainless steel material in an aqueous sulfuric acid solution of 10 to 20 wt% at a temperature of 50 to 75°C to remove a first passivation film formed on the outermost surface of the ferritic stainless steel material. and the step of immersing the ferritic stainless steel in a mixed acid aqueous solution of 10 to 20 weight% nitric acid and 1 to 10 weight% hydrofluoric acid at a temperature of 40 to 60°C to form a second passivation film on the outermost surface of the ferritic stainless steel; wherein, in the step of forming the second passivation film, the outermost surface of the ferritic stainless steel contains 0.740 to 6.300% of fine defects as an area fraction.

[0076] Specifically, the step of preparing a ferritic stainless steel material may involve manufacturing a slab having a compositional system as described above for ferritic stainless steel for current collectors, and performing reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing, and the process conditions may be carried out under normal conditions and are not particularly limited.

[0077] The step of removing the first passivation film can remove the first passivation film formed on the outermost surface of the ferritic stainless steel by immersing the ferritic stainless steel in an aqueous sulfuric acid solution of 10 to 20 weight% at a temperature of 50 to 75°C. If the concentration of the aqueous sulfuric acid solution is less than 10 weight% or the temperature is less than 50°C, the removal of the first passivation film does not occur sufficiently, and if the concentration of the aqueous sulfuric acid solution exceeds 20 weight% or the temperature exceeds 75°C, the ferritic stainless steel may be damaged. Subsequently, a step of washing with water may be included.

[0078] Subsequently, the ferritic stainless steel material from which the first passivation film has been removed can be immersed in a mixed acid aqueous solution of 10 to 20 weight% nitric acid and 1 to 10 weight% hydrofluoric acid at a temperature of 40 to 60°C to form a second passivation film on the outermost surface of the ferritic stainless steel material.

[0079] If the processing temperature is below 40℃, the processing time becomes longer, which hinders productivity, and if the temperature exceeds 60℃, it causes damage to the stainless steel, so it is carried out in a temperature range of 40 to 60℃.

[0080] In addition, if the concentration of nitric acid is less than 10% by weight, the second passivation film may become unstable, and if it exceeds 20% by weight, there is no effect of reducing the resistance value. In particular, if the concentration of nitric acid exceeds 20% by weight, it may cause damage to the stainless steel and increase the area fraction of fine defects, thereby degrading the reactivity and adhesion between the steel and the sulfide-based solid electrolyte; therefore, it is desirable to control the concentration of nitric acid to 10% to 20% by weight.

[0081] In addition, if the concentration of hydrofluoric acid is less than 1 weight%, the second passivation film may become unstable, and if it exceeds 10 weight%, it causes damage to the stainless steel, impairing corrosion resistance and having no effect on reducing the resistance value. In particular, if the concentration of hydrofluoric acid exceeds 10 weight%, the area fraction of fine defects increases, which may lead to a decline in reactivity and adhesion between the steel and the sulfide-based solid electrolyte; therefore, it is desirable to control the concentration of hydrofluoric acid to 1 to 10 weight%.

[0082] (Lithium-ion battery current collector and lithium-ion battery)

[0083] A current collector for a non-aqueous electrolyte lithium secondary battery according to another embodiment of the present invention comprises: a negative electrode including the current collector and a negative electrode active material layer formed on the current collector; a positive electrode disposed opposite to the negative electrode and including a positive electrode plate and a positive electrode active material layer formed on the positive electrode plate; and a solid electrolyte disposed between the negative electrode and the positive electrode.

[0084] The above-mentioned cathode includes a cathode current collector and a cathode active material layer located on the cathode current collector. The cathode current collector may be manufactured from ferritic stainless steel as described above.

[0085] When stainless steel is used as the negative electrode current collector for a sulfide-based all-solid-state battery, the problem of battery performance degradation caused by the formation of corrosive substances, such as CuS, which occurs when copper is used conventionally, can be resolved. Furthermore, when the ferritic stainless steel according to the present invention is used as the negative electrode current collector, reactivity with the sulfide-based solid electrolyte is suppressed and adhesion is improved, thereby lowering interfacial contact resistance. Additionally, the ferritic stainless steel according to the present invention is not limited to the negative electrode current collector but may also be used as an anode current collector.

[0086] The above-mentioned cathode active material layer optionally includes a binder and a conductive material together with the cathode active material.

[0087] As the above-mentioned cathode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specifically, carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium such as SiOβ (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the above-mentioned metallic compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one or more of these may be used.

[0088] In addition, a metallic lithium thin film may be used as the above-mentioned negative electrode active material. Furthermore, both low-crystallinity carbon and high-crystallinity carbon may be used as carbon materials. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0089] The above conductive material is used to impart conductivity to the electrode, and in the battery being constructed, it may be used without special limitations as long as it possesses electronic conductivity without causing chemical changes. Specifically, examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The above conductive material may typically be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, based on the total weight of the negative electrode active material layer.

[0090] The above binder serves to improve adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector. Specifically, examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, based on the total weight of the negative electrode active material layer.

[0091] The above-mentioned cathode active material layer may be manufactured by, as an example, applying a cathode slurry comprising a cathode active material and optionally a binder and a conductive material onto a cathode current collector and drying it, or by casting the cathode slurry onto a separate support and then laminating the film obtained by peeling it off from the support onto the cathode current collector.

[0092] The lithium secondary battery of the present invention may include a positive electrode disposed opposite to the negative electrode and comprising a positive plate and a positive active material layer formed on the positive plate.

[0093] The above-mentioned anode may include an anode active material, and the anode active material itself may be used as the anode active material, or an anode composite obtained by mixing the anode active material with a binder, an anode composite paste obtained by additionally adding a solvent, or an anode formed by additionally applying this to a current collector may also be applicable.

[0094] The above-mentioned anode may include an anode current collector and an anode active material layer formed on the anode current collector and comprising the anode active material. In the above-mentioned anode, the anode current collector may be manufactured from ferritic stainless steel as described above.

[0095] In addition, the positive active material layer may include a conductive material and a binder along with the positive active material described above. The conductive material is used to impart conductivity to the electrode and can be used without special limitations as long as it possesses electronic conductivity without causing chemical changes in the battery being constructed. Specifically, graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives may be used. The conductive material may typically be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, based on the total weight of the positive active material layer.

[0096] The above binder serves to improve adhesion between positive active material particles and adhesion between the positive active material and the positive current collector. Specifically, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof may be used. The above binder may be included in an amount of 1% to 30% by weight, preferably 1% to 20% by weight, and more preferably 1% to 10% by weight, based on the total weight of the positive active material layer.

[0097] The above-mentioned anode may be manufactured according to a conventional anode manufacturing method. For example, the above-mentioned anode may be manufactured by mixing an anode active material, a binder, and / or a conductive material in a solvent to prepare an anode slurry, applying the anode slurry onto an anode current collector, and then drying and rolling. At this time, the types and contents of the anode active material, binder, and conductive material are as described above.

[0098] The above solvent may be a solvent commonly used in the relevant technical field, and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of the above solvent used is sufficient if it has a viscosity that dissolves or disperses the anode active material, conductive material, and binder, taking into account the coating thickness of the slurry and the manufacturing yield, and subsequently provides excellent thickness uniformity when coated for anode manufacturing.

[0099] Alternatively, the anode may be manufactured by casting the anode slurry onto a separate support and then laminating the film obtained by peeling off from the support onto an anode current collector.

[0100] In addition, the lithium secondary battery according to the present invention may include the above-mentioned positive electrode, negative electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are as described above, and the solid electrolyte may be a sulfide-based, oxide-based, or polymer solid electrolyte, and preferably a sulfide-based solid electrolyte. Specifically, the sulfide-based solid electrolyte may be a solid electrolyte material (Li-AS) composed of Li, A, and S. In this case, A among the sulfide-based solid electrolyte material Li-AS is at least one selected from the group consisting of P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta. Specific examples of such a sulfide-based solid electrolyte material Li-AS include 70Li2S-30P2S5 and LiGe 0.25 P 0.75 Examples include S4, 80Li2S-20P2S5, Li2S-SiS2, etc., and 70Li2S-30P2S5, which has high ionic conductivity, is more preferable. The method for manufacturing the sulfide-based solid electrolyte material used in the present invention is not particularly limited as long as it is a method that can obtain the desired sulfide-based solid electrolyte material.

[0101] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0102] {Example}

[0103] 1. Preparation of Invention Examples 1 to 11

[0104] A ferritic stainless steel satisfying the composition of Table 1 below was manufactured by reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing of a slab under normal conditions, and the first passivation film was removed by immersing it in an aqueous sulfuric acid solution of 10 to 20 wt% at a temperature of 50 to 75°C. Subsequently, as shown in Table 2 below, it was immersed in an aqueous mixed acid solution of 10 to 20 wt% nitric acid and 1 to 10 wt% hydrofluoric acid at a temperature of 40 to 60°C.

[0105] 2. Preparation of Comparative Examples 1 to 7

[0106] The steps of manufacturing ferrite stainless steel and removing the first passivation film were performed in the same manner as in Example 1 of the invention, and in the second passivation film formation step, the mixed acid aqueous solution conditions were immersed differently as shown in Table 2 below.

[0107] Steel Grade C(wt%)N(wt%)Si(wt%)Mn(wt%)Cr(wt%)Ti(wt%)Al(wt%)Mo(wt%)10.01100.00700.210.9119.00.010.0051.5020.00900.00600.190.9018.70.020.0081.4030.01100.00880.250.8818.60.010.0091.6040.00500.00890.24 0.8418.70.010.0071.0050.00550.00450.230.8518.90.010.0051.0560.00600.00610.190.9219.20.050.0061.2270.00690.00500.200.8218.90.040.0041.5080.01100.00480.250.8819.20.020.0091.3090.00510.00890 .200.9918.50.020.0101.50100.07300.00340.090.9218.50.030.0120.95110.01850.01780.180.8719.20.180.0451.08120.01290.00920.200.9518.80.090.0121.12130.00910.01450.140.7819.70.150.0090.89140.015 40.00610.090.9918.30.260.0101.03150.00670.01930.190.8319.10.190.0501.15160.01920.01170.120.9120.00.120.0070.97170.01080.00490.230.8818.90.050.0341.21180.01360.01560.250.8021.40.360.0190.92

[0108] Nitric Acid Concentration (Weight%) Hydrofluoric Acid Concentration (Weight%) 1135214831694187517561537124830692615103525111351230513153141511151251621101722818168

[0109] 3. Preparation of Invention Examples 12 to 17

[0110] A ferritic stainless steel satisfying the composition of Table 3 below was manufactured by reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing of a slab under normal conditions, and the first passivation film was removed by immersing it in an aqueous sulfuric acid solution of 10 to 20 wt% at a temperature of 50 to 75°C. Subsequently, as shown in Table 5 below, it was immersed in an aqueous mixed acid solution of 14 wt% nitric acid and 6 wt% hydrofluoric acid at a temperature of 40 to 60°C.

[0111] 4. Preparation of Comparative Examples 8 to 10

[0112] Except for the Cu content exceeding 0.1 wt%, it was manufactured in the same manner as Invention Example 12, and the composition of the ferrite stainless steel is as shown in Table 3 below.

[0113] 5. Preparation of Comparative Examples 11 to 13

[0114] Except for the Ni content exceeding 3.0 wt%, it was manufactured in the same manner as Invention Example 12, and the composition of the ferrite stainless steel sheet is as shown in Table 3 below.

[0115] Steel Grade Cu (wt%) Ni (wt%) C (wt%) N (wt%) Si (wt%) Mn (wt%) Cr (wt%) Ti (wt%) Al (wt%) Mo (wt%) 190.2000.0010.00600.01000.220.118.00.030.0111.66200.3500.0020.00600.00900.230.6418.90.020.0091 .75210.1300.0010.00500.00500.250.8419.10.010.0081.72220.0070.0020.00470.00560.330.7419.10.050.0091.69230.0100.0020.01000.01100.310.6019.00.010.0101.55240.0010.0010. 01100.00700.130.6318.60.030.0111.40250.0014.4000.00500.00600.220.7618.70.020.0051.50260.0013.1000.00490.01100.250.7420.10.010.0091.60270.0026.0000.00550.00650.250.8 418.80.030.0081.73280.0010.2000.01000.00500.260.9419.60.020.0071.65290.0021.8000.00610.00600.210.5519.50.010.0051.70300.0012.5000.00580.00550.220.9120.50.010.0061.71

[0116] {Experimental Example}

[0117] Using the stainless steel materials of the examples and comparative examples prepared as described above, the reactivity with the solid electrolyte in an atmosphere simulating the battery environment of a sulfide-based all-solid-state battery was evaluated by the following method.

[0118] 1. Fabrication of reactivity evaluation cells

[0119] The stainless steel current collector material sample for evaluation is prepared by processing a 0.5 mm thick plate into a circular shape. Processing contaminants on the surface and sides were cleaned off, and to remove residual moisture, the processed sample was dried in a vacuum oven at 130°C for 24 hours and in an argon glove box for 18 hours.

[0120] The solid electrolyte is Argyrodite-based Li, a representative sulfide-based solid electrolyte. 6-x PS5Cl x A solid electrolyte was used, the particle size of the solid electrolyte used was an average of 1 µm, the measured average ionic conductivity was 1.2 mS / cm, the amount of solid electrolyte used was 200~250 mg, and it was molded by applying pressure of 300 MPa or more.

[0121] First, two current collection material samples (B and T) identical to the press jig cell were prepared. The current collection material (B) was mounted on the prepared press jig, and a pressure-molded solid electrolyte pellet was mounted on top of it. After mounting the identical current collection material (T), the press jig cell was assembled. A pressurizing device and a stator were connected to the assembled jig to ensure that a constant pressure of at least 3 MPa was always applied. Two wires electrically connected to the current collection material were each connected to the atmosphere control chamber. Then, the press jig with the evaluation material mounted on it was mounted to the atmosphere control chamber, and the atmosphere control chamber was assembled. The electrical terminals of the atmosphere control chamber were each connected to the terminals of the evaluation device to proceed with the characteristic evaluation. The atmosphere control chamber was physically isolated from the external environment, so that external air and moisture could not penetrate into it.

[0122] Meanwhile, to evaluate characteristics according to changes in the evaluation environment, the atmosphere control chamber was mounted in an oven with a set temperature to change the temperature, and the oven temperature was changed to 25℃, 45℃, 60℃, and 90℃, respectively. To increase the temperature of the sample, the atmosphere control chamber was mounted in the oven with a set temperature and the evaluation was conducted after 24 hours.

[0123] Meanwhile, the EIS method was performed using the P-EIS method, which applies an AC voltage signal. The potential was applied with an amplitude of 10 mV relative to 0 V, and the measurement frequency was 7 MHz to 0.1 MHz.

[0124] 2. Reactivity Evaluation Method

[0125] Current collector materials for all-solid-state batteries are exposed to environments with potential differences and current flow during use. Therefore, the reactivity of the current collector material and the sulfide-based solid electrolyte was evaluated by observing the current flow and subsequent changes in surface resistance in an environment where an artificial potential difference was generated using electrochemical methods.

[0126] The voltage of the mounted cell (OCV: Open circuit voltage) was measured. Specifically, as shown in Figure 1, the OCV and EIS for the initial state were measured, and then an artificial voltage was applied using the CV method to induce a surface reaction between the current collector and the solid electrolyte. After the reaction, the change in surface resistance was measured using the EIS method to evaluate the interfacial reaction between the current collector and the solid electrolyte. The scan rate of the CV was fixed at 20 mV / sec.

[0127] 3. Reactivity evaluation based on area fraction control

[0128] Invention Examples 1 to 11 and Comparative Examples 1 to 7 prepared above were used to obtain SEM images at a magnification of 100x using a scanning electron microscope (SEM). Then, based on four quadrants (1st to 4th quadrants), one arbitrary region (II) of size 1㎛ X 1㎛ in each quadrant was selected, and the area fraction of the fine defects was measured using an image analyzer. The average value of these values ​​was calculated and is shown in Table 4 below.

[0129] In addition, OCV was measured at room temperature and 90℃ at a scan speed of 20mV / sec, and the magnitude of the impedance value (absolute value of imaginary impedance, value of real impedance) in the 0.5MHz frequency band was determined from the EIS curve measured after a -4 to 4V CV cycle. Real impedance values ​​were evaluated as O for 100Ω or less, X for 120Ω or more, and △ for greater than 100Ω and less than 120Ω, while imaginary impedance values ​​were evaluated as O for 220Ω or less, X for 300Ω or more, and △ for greater than 220Ω and less than 300Ω. For the 1KHz frequency band, real impedance values ​​were evaluated as O for 600Ω or less, X for 800Ω or more, and △ for greater than 600Ω and less than 800Ω, and imaginary impedance values ​​were evaluated as O for 300Ω or less, X for 500Ω or more, and △ for greater than 300Ω and less than 500Ω, as shown in Table 4 below.

[0130] Classification Steel Grade Area Fraction (%) Room Temperature OCV (V) 90℃ OCV (V) 0.5MHz 1KHz CV Is it Before CV Is it After CV Is it Before CV Is it After Real Impedance Imaginary Impedance Real Impedance Imaginary Impedance Invention Example 1 10.954 -0.018 0.002 0.023 O△OO Invention Example 2 21.004 0.022 0.062 0.017 0.033 O△OO Invention Example 3 30.990 0.004 0.058 0.018 0.036 O△OO Invention Example 4 40.944 0.003 0.045 0.016 0.029 O△OO Invention Example 5 51.116 0.0100. 0670.0100.028O△OO Invention Example 661.2830.0210.0530.0080.022OOOO Invention Example 771.6100.0050.0220.0110.030O△OO Comparative Example 180.3110.0320.0830.0130.066XXXX Comparative Example 290.3950.0060.1000.0120.042XOXX Comparative Example 3106.7000.0030.2200.0100.710XXXX Invention Example 8110.790-0.0660.0040.035O△OO Comparative Example 4120.6800.0020.0910.0080.050XOXX Invention Example 9136.2300.0050.0510.0110.039O△OO Comparative Example 5146.470-0.0880. 0280.043XXXXInvention Example 10156.2000.0100.0380.0100.038OOOOComparative Example 6166.6000.0050.0750.0190.060XXXXComparative Example 7176.3600.0220.1600.0060.064XOXXInvention Example 11186.2500.0150.0610.0110.025OOOO

[0131] According to Table 4 above, Invention Examples 1 to 11 satisfy the composition of the present invention and have an area fraction of 0.740 to 6.300%, satisfy an OCV of 0.070 V or less at room temperature and an OCV of 0.040 V or less at 90℃, and show a real / imaginary impedance of △ or greater at 0.5 MHz and 1 KHz, confirming that the reaction of the ferritic stainless steel material of the current collector with the sulfide-based solid electrolyte is suppressed. On the other hand, Comparative Examples 1 to 7, in which the nitric acid concentration and / or hydrofluoric acid concentration ranges did not satisfy the range of the present invention during the second passivation film formation step, did not satisfy the fine defect area fraction of 0.740 to 6.300%, and the OCV at room temperature exceeded 0.070 V, the OCV at 90℃ exceeded 0.040 V, and the real and / or imaginary impedance at 0.5 MHz and 1 K Hz was X, confirming that the ferritic stainless steel material of the current collector did not secure sufficient sulfid resistance.

[0132] This is judged to be because the resistance increased more in Comparative Examples 1 to 7 as a greater amount of material was generated from the reaction between the stainless steel and the sulfide-based solid electrolyte compared to the Inventive Example.

[0133] 4. Evaluation of reactivity according to Cu and Ni content control

[0134] For the above-described Invention Examples 12 to 17 and Comparative Examples 8 to 13, the OCV was measured at room temperature and 90°C at a scan speed of 20 Mv / sec, and the magnitude of the impedance value (absolute value of imaginary impedance, value of real impedance) in the 0.5 MHz frequency band was determined from the EIS curve measured after a -4 to 4 V CV cycle. The real impedance value was evaluated as O for 100 Ω or less, X for 120 Ω or more, and △ for greater than 100 and less than 120 Ω; the imaginary impedance value was evaluated as O for 22 Ω or less, X for 30 Ω or more, and △ for greater than 22 and less than 30 Ω. For the 1KHz frequency band, real impedance values ​​were evaluated as O for 600Ω or less, X for 800Ω or more, and △ for greater than 600Ω and less than 800Ω, and imaginary impedance values ​​were evaluated as O for 300Ω or less, X for 500Ω or more, and △ for greater than 300Ω and less than 500Ω, as shown in Table 5 below.

[0135] Division Room Temperature OCV (V) 90℃ OCV (V) 0.5MHz 1KHz Nitric Acid Concentration (WH%) Is it CV? Before CV? After CV? Before CV? After Real Impedance Imaginary Impedance Real Impedance Imaginary Impedance 190.0200.0890.0090.061XXXX14200.0040.1220.0110.059XXXX14 School 1210.0110.0910.0270.086XXXX146 People 1220.0120.0560.0150.031OOOO146 People 1230.0230.0240.0170.027OOO△146 People 1240.0380.0180.0240.021OOOO146 School 1250.0150.2130.0070.048XXXX146gyo1260.0090.1820.0210.051XXXX146gyo1270.0130.3010.0180.070XXXX146myeong1280.0060.0410.0080.028OOOO146myeong1290.0110.0390.0140.0310OOOX146myeong1300.0240.0680.0080.035OOOO146

[0136] According to Table 5 above, Invention Examples 12 to 17, which satisfy a Cu content of 0.100 wt% or less and a Ni content of 3.000 wt%, satisfy an OCV of 0.070 V or less at room temperature and an OCV of 0.040 V or less at 90°C, and a real / imaginary impedance of △ or greater at 0.5 MHz and 1 KHz, confirming that the reaction of the ferritic stainless steel material of the current collector with the sulfide-based solid electrolyte is suppressed.

[0137] On the other hand, Comparative Examples 8 to 10, in which the Cu content exceeded 0.100 wt%, showed an OCV at room temperature exceeding 0.070 V, an OCV at 90°C exceeding 0.040 V, or real and / or imaginary impedances of 0.5 MHz and 1 KHz of X, confirming that the ferritic stainless steel material of the current collector did not secure sufficient sulfid resistance.

[0138] In addition, Comparative Examples 11 to 13, in which the Ni content exceeded 3.000 wt%, showed an OCV at room temperature exceeding 0.070 V, an OCV at 90°C exceeding 0.040 V, or real and / or imaginary impedances of 0.5 MHz and 1 KHz of X, confirming that the ferritic stainless steel material of the current collector did not secure sufficient sulfid resistance.

[0139] This is judged to be because the resistance increased more in Comparative Examples 8 to 13 as a greater amount of material was generated from the reaction between the stainless steel and the sulfide-based solid electrolyte compared to the Inventive Example.

[0140] However, it should be noted that the examples of invention are intended merely to illustrate and embody the present invention, and are not intended to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0141] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

In ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries, The above ferritic stainless steel contains 18.0 to 32.0 weight% of Cr, and A ferritic stainless steel comprising 0.740 to 6.300% of fine defects formed on the outermost surface of the ferritic stainless steel, in terms of area fraction. Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, The above ferritic stainless steel contains 0.100 weight% or less of Cu, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, The above ferritic stainless steel contains 3,000 weight% or less of Ni, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, The above ferritic stainless steel has an average thickness of 0.005 to 0.030 mm, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, After applying CV (Circuit Voltage) at a scan rate of 20 mV / sec at room temperature, the OCV (Open Circuit Voltage) value is 0.070 V or less, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, After applying CV (Circuit Voltage) at a scan rate of 20mV / sec at 90℃, an OCV value of 0.040V or less, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, When measuring by Electrochemical Impedance Spectroscopy (EIS) after a Circuit Voltage (CV) cycle of -4V to 4V, the real impedance value at a frequency of 0.5MHz is 100Ω or less, and the imaginary impedance value is 220Ω or less, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 1, A real impedance value of 600Ω or less at a frequency of 1KHz and an imaginary impedance value of 300Ω or less, Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In any one of claims 1 to 3, The above ferritic stainless steel comprises, in weight%, C: 0.0005 to 0.0200%, N: 0.0010 to 0.0200%, Si: 0.01 to 0.25%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, Ti: 0.01 to 0.50%, Al: 0.001 to 0.500%, Mo: 0.01 to 2.00%, and the remainder being Fe and unavoidable impurities. Ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. A step of preparing a ferritic stainless steel material containing 18.0 to 32.0 weight% of Cr in weight%; A step of immersing the ferritic stainless steel in a 10 to 20 weight% aqueous sulfuric acid solution at a temperature of 50 to 75°C to remove a first passivation film formed on the outermost surface of the ferritic stainless steel; and The method comprises the step of immersing the ferritic stainless steel in a mixed aqueous acid solution of 10 to 20 weight% nitric acid and 1 to 10 weight% hydrofluoric acid at a temperature of 40 to 60°C to form a second passivation film on the outermost surface of the ferritic stainless steel. In the step of forming the second passivation film, the area fraction of fine defects on the outermost surface of the ferritic stainless steel is 0.740 to 6.300%. Method for manufacturing ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 10, The above ferritic stainless steel contains 0.100 weight% or less of Cu, Method for manufacturing ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In claim 10, The above ferritic stainless steel contains 3,000 weight% or less of Ni, Method for manufacturing ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. In any one of claims 10 to 12, The above ferritic stainless steel comprises, in weight%, C: 0.0005 to 0.0200%, N: 0.0010 to 0.0200%, Si: 0.01 to 0.25%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Ti: 0.01 to 0.50%, Al: 0.001 to 0.500%, Mo: 0.01 to 2.00%, and the remainder being Fe and unavoidable impurities. Method for manufacturing ferritic stainless steel for current collectors of non-aqueous electrolyte lithium secondary batteries. A non-aqueous electrolyte lithium secondary battery current collector made of ferritic stainless steel according to any one of claims 1 to 8. A cathode comprising a current collector according to claim 14 and a cathode active material layer formed on the current collector; An anode disposed opposite to the above cathode and comprising an anode plate and an anode active material layer formed on the anode plate; and A lithium secondary battery comprising a solid electrolyte disposed between the above-mentioned negative electrode and the above-mentioned positive electrode.

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