Welding Defect Inspection Method

Abstract

The present technology relates to a method for inspecting welding defects. The method includes: manufacturing an electrode assembly sample by welding an electrode lead to an electrode tab formed on an electrode assembly; measuring the tensile strength, torsional strength, and peel strength of a welded portion between the electrode tab and the electrode lead for the electrode assembly sample; deriving a correlation between the presence or absence of a welding defect and each of the tensile strength, torsional strength, and peel strength; and deriving a reference value for determining the presence or absence of a welding defect for the tensile strength, torsional strength, and peel strength, respectively.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2020-0105340, filed on August 21, 2020, and the entire contents of the Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a method for inspecting welding defects and a method for manufacturing a secondary battery including the method. Background Art

[0003] Recently, secondary batteries capable of charging and discharging have been widely used as an energy source for wireless mobile devices. In addition, secondary batteries have attracted attention as an energy source for electric vehicles, hybrid electric vehicles, etc., and have been proposed as a solution to air pollution of existing gasoline vehicles and diesel vehicles using fossil fuels. Therefore, due to the advantages of secondary batteries, the types of applications using secondary batteries are currently very diverse, and it is expected that secondary batteries will be applied to many fields and products in the future.

[0004] Depending on the composition of the electrode and the electrolyte, such secondary batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, lithium polymer batteries, etc. Among them, the use of lithium-ion polymer batteries, which are less likely to leak electrolytes and are easy to manufacture, is increasing. Generally, secondary batteries are classified into cylindrical batteries and prismatic batteries, in which the electrode assembly is embedded in a cylindrical or rectangular metal can according to the shape of the battery case; and pouch-type batteries, in which the electrode assembly is embedded in a pouch-shaped case of an aluminum laminate. The electrode assembly built into the battery case is composed of a positive electrode, a negative electrode, and a separator inserted between the positive electrode and the negative electrode, and is a power generation element capable of charging and discharging. The electrode assembly is classified into a jelly roll type and a stacked type. In the jelly roll type, a separator inserted between a positive electrode and a negative electrode, which are in the form of long sheets coated with an active material, is wound. In the stacked type, a plurality of positive electrodes and negative electrodes of a predetermined size are sequentially stacked while inserting a separator therebetween.

[0005] The positive electrode and the negative electrode are formed by applying a positive electrode paste containing a positive electrode active material and a negative electrode paste containing a negative electrode active material to a positive electrode current collector and a negative electrode current collector, and then drying and rolling them. At this time, a small amount of binder is added to the positive electrode paste and the negative electrode paste to prevent the active material from coming off the current collector.

[0006] In addition, a positive electrode tab and a negative electrode tab are formed at the positive electrode and the negative electrode for electrical connection, and a positive electrode lead and a negative electrode lead are formed at the positive electrode tab and the negative electrode tab, respectively. At this time, the positive electrode tab can be joined to the positive electrode lead by welding, and the negative electrode tab can also be joined to the negative electrode lead by welding. At this time, since welding defects may occur between the tab and the lead, welding defect inspection is performed to detect welding defects.

[0007] When performing a welding defect inspection, the welding strength of the welded portion between the tab and the lead is usually measured. For this purpose, the tensile strength of the welded portion between the tab and the lead is mainly measured. Specifically, the tensile strength generated when the tab and the lead are tensioned in opposite directions is measured.

[0008] However, it is difficult to detect welding defects in all cases and small welding defects generated intermittently only using the tensile strength.

[0009] Therefore, it is necessary to develop a method for inspecting welding defects that can detect all small welding defects. SUMMARY OF THE INVENTION

[0010] TECHNICAL PROBLEM

[0011] The present invention is proposed to solve the above problems, and an object of the present invention is to provide a method for inspecting welding defects that has improved welding reliability and accuracy by detecting all small welding defects.

[0012] TECHNICAL SOLUTION

[0013] A method for inspecting welding defects according to an embodiment of the present invention includes: manufacturing an electrode assembly sample by welding an electrode lead to an electrode tab formed on an electrode assembly; measuring the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead for the electrode assembly sample; deriving the correlation between the presence or absence of a welding defect and each of the tensile strength, torsional strength, and peel strength; and deriving reference values for determining the presence or absence of a welding defect for the tensile strength, torsional strength, and peel strength, respectively.

[0014] In a specific example, measuring the tensile strength, torsional strength, and peel strength includes: preparing three groups of electrode assembly samples; and measuring any one of the tensile strength, torsional strength, and peel strength for each group.

[0015] The tensile strength, torsional strength, and peel strength are measured using a measuring device including the following: a first gripper configured to fix an end portion of the electrode lead; and a second gripper configured to fix the electrode assembly.

[0016] At this time, the first gripper applies a force to the electrode lead in a predetermined direction, thereby breaking the welded portion.

[0017] In addition, in an embodiment of the present invention, deriving the correlation includes: establishing a database regarding the correlation between the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead and the presence or absence of a welding defect.

[0018] At this time, the reference value is derived from the database.

[0019] In another embodiment of the present invention, the method for inspecting welding defects further includes: measuring the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead by using different welding methods, and for each welding method, deriving the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength.

[0020] At this time, the method for inspecting welding defects according to the present invention further includes, for each welding method, deriving a reference value for determining the presence or absence of welding defects.

[0021] The welding method includes ultrasonic welding or laser welding.

[0022] The method for inspecting welding defects according to the present invention further includes determining the cause of welding defects based on the correlation and establishing a database thereon.

[0023] In addition, the method for inspecting welding defects according to the present invention further includes determining welding defects of the electrode assembly.

[0024] At this time, determining welding defects includes: preparing three sets of electrode assemblies to be inspected by welding the electrode leads to the electrode tabs; measuring any one of the tensile strength, torsional strength, and peel strength for each set; and determining the presence or absence of welding defects by comparing the measured value obtained by the measurement with the reference value.

[0025] At this time, determining the presence or absence of welding defects includes comparing the measured value with the reference value for two of the tensile strength, torsional strength, and peel strength.

[0026] In addition, determining whether it is a welding defect includes comparing the measured value with the reference value for the tensile strength, torsional strength, and peel strength.

[0027] In addition, the present invention provides a method for manufacturing a secondary battery, including the method for inspecting welding defects as described above.

[0028] Advantageous Effects

[0029] According to the welding defect inspection method of the present invention, by checking the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength and standardizing the correlation, this inspection method can be applied to all welding processes.

[0030] In addition, regarding the welding strength, by measuring other strength values in addition to the tensile strength and deriving a reference value therefrom, small welding defects can be detected, and the reliability and accuracy of welding can be improved. Brief Description of the Drawings

[0031] Figure 1 is a flowchart showing the sequence of a method for inspecting welding defects according to an embodiment of the present invention.

[0032] Figure 2 is a schematic diagram showing a method for measuring tensile strength in the method for inspecting welding defects according to the present invention.

[0033] Figure 3 is a schematic diagram showing a method for measuring torsional strength in the method for inspecting welding defects according to the present invention.

[0034] Figure 4 is a schematic diagram showing a method for measuring peel strength in the method for inspecting welding defects according to the present invention.

[0035] Figure 5 is a flowchart showing the sequence of a method for inspecting welding defects according to another embodiment of the present invention. Detailed Description of the Invention

[0036] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The terms and words used in this specification and the claims should not be construed as limited to ordinary or dictionary terms, and the inventor may appropriately define the concept of the terms in order to best describe his invention. The terms and words should be construed as having meanings and concepts consistent with the technical concept of the present invention.

[0037] In this application, it should be understood that terms such as "comprising" or "having" are intended to indicate the presence of the features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and they do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof. In addition, when a part such as a layer, film, region, plate, etc. is referred to as being "on" another part, this includes not only the case where the part is "directly on" the other part, but also the case where another part is inserted between them. On the other hand, when a part such as a layer, film, region, plate, etc. is referred to as being "under" another part, this includes not only the case where the part is "directly under" the other part, but also the case where another part is inserted between them. Additionally, in this application, arranging "on..." can include the cases of being set at the bottom as well as the top.

[0038] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0039] Figure 1 is a flowchart showing the sequence of a method for inspecting welding defects according to an embodiment of the present invention.

[0040] Reference Figure 1, the method for inspecting welding defects according to an embodiment of the present invention includes: manufacturing an electrode assembly sample by welding an electrode lead to an electrode tab formed on an electrode assembly (S10); measuring the tensile strength, torsional strength, and peel strength of a welded portion between the electrode tab and the electrode lead for the electrode assembly sample (S20); deriving a correlation between the presence or absence of a welding defect and each of the tensile strength, torsional strength, and peel strength (S30); and deriving reference values for determining the presence or absence of a welding defect for the tensile strength, torsional strength, and peel strength, respectively (S40).

[0041] As described above, the tensile strength of the welded portion between the tab and the lead is measured for general welding defect inspection. However, since only the welding strength in one direction can be identified by this tensile strength inspection, it is difficult to detect welding defects in all cases and small welding defects that occur intermittently.

[0042] According to the welding defect inspection method of the present invention, by examining the correlation between the tensile strength, torsional strength, and peel strength as welding strengths and the presence or absence of a welding defect and normalizing the correlation, this inspection method can be applied to all welding processes.

[0043] In addition, regarding the welding strength, by measuring other strength values in addition to the tensile strength and deriving reference values therefrom, small welding defects can be detected, and the reliability and accuracy of welding can be improved.

[0044] Hereinafter, each step of the welding defect inspection method according to the present invention will be described in detail.

[0045] In the specification of the present invention, the welding strength includes the tensile strength, torsional strength, and peel strength as the strength at the welded portion between the electrode tab and the electrode lead.

[0046] <Preparation of Electrode Assembly Sample>

[0047] In the welding defect inspection method according to the present invention, an electrode assembly sample is manufactured by welding an electrode lead to an electrode tab formed on an electrode assembly.

[0048] At this time, the electrode assembly has a structure in which a positive electrode, a separator, and a negative electrode are alternately stacked, and the positive electrode and the negative electrode may each have a structure in which an active material layer is formed by applying an electrode paste containing an electrode active material to a current collector and then through a drying and rolling process.

[0049] The current collector may be a positive current collector or a negative current collector, and the electrode active material may be a positive electrode active material or a negative electrode active material. In addition, in addition to the electrode active material, the electrode paste may further include a conductive material and a binder.

[0050] In the present invention, the positive electrode current collector generally has a thickness of 3 to 500 micrometers. The positive electrode current collector is not particularly limited as long as it has high electrical conductivity without causing chemical changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc. The current collector may have fine irregularities on its surface to increase the adhesion of the positive electrode active material, and various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics are possible.

[0051] The sheet for the negative electrode current collector generally has a thickness of 3 to 500 micrometers. The negative electrode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. In addition, similar to the positive electrode current collector, fine irregularities can be formed on the surface to enhance the binding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics.

[0052] In the present invention, the positive electrode active material is a material capable of causing an electrochemical reaction and a lithium transition metal oxide, and contains two or more transition metals. Examples thereof include: layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) substituted with one or more transition metals; lithium manganese oxide substituted with one or more transition metals; lithium nickel oxide represented by the molecular formula LiNi 1-y M y O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga and contains at least one of the above elements, 0.01 ≤ y ≤ 0.7); lithium nickel cobalt manganese composite oxide represented by the molecular formula Li 1+z Ni b Mn c Co 1-(b+c+d) MdO (2-e) A e such as Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li 1+z Ni 0.4 Mn 0.4 Co 0.2 O2, etc. (where 0.5 ≤ z ≤ 0.5, 0.1 ≤ b ≤ 0.8, 0.1 ≤ c ≤ 0.8, 0 ≤ d ≤ 0.2, 0 ≤ e ≤ 0.2, b + c + d < 1, M = Al, Mg, Cr, Ti, Si or Y, and A = F, P or Cl); lithium represented by the molecular formula Li 1+x M1-y M' y PO 4-z X z Represents olivine-based lithium metal phosphate (where M = transition metal, preferably Fe, Mn, Co or Ni, M' = Al, Mg or Ti, X = F, S or N, and -0.5 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1).

[0053] Examples of the negative electrode active material include carbon such as non-graphitized carbon and graphitic carbon; metal composite oxides such as Li x Fe2O3 (0 ≤ x ≤ 1), Li x WO2 (0 ≤ x ≤ 1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Groups 1, 2 and 3 of the periodic table, halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8); lithium alloys; silicon alloys; tin alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4 and Bi2O5; conductive polymers such as polyacetylene; and Li-Co-Ni-based materials.

[0054] Based on the total weight of the mixture including the positive electrode active material, the conductive material is usually added in an amount of 1 to 30 wt%. There is no particular limitation on such a conductive material as long as it has conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives, etc.

[0055] Based on the total weight of the mixture containing the positive electrode active material, a binder is added in an amount of 1 to 30 wt% as a component that helps the binding between the active material and the conductive material and the binding to the current collector. Examples of such a binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butene rubber, fluororubber, various copolymers, etc.

[0056] In addition, a separator is inserted between the positive electrode and the negative electrode, and an insulating film with high ion permeability and mechanical strength is used. The pore size of the separator is generally 0.01 - 10 microns, and the thickness is generally 5 - 300 microns. Examples of such separators include olefin-based polymers such as chemically resistant and hydrophobic polypropylene; sheets or non-woven fabrics made of glass fiber, polyethylene, etc.

[0057] In the electrode assembly, electrode tabs are formed on one side of the electrodes, and the electrode tabs can be positive electrode tabs or negative electrode tabs. The positive electrode lead and the negative electrode lead are respectively connected to the positive electrode tab and the negative electrode tab. The positive electrode lead and the negative electrode lead are pulled out to the outside and function as terminals for external electrical connection. At this time, the positive electrode lead and the negative electrode lead can be respectively joined to the positive electrode tab and the negative electrode tab by welding. Known welding methods can be used. For example, ultrasonic welding or laser welding can be used.

[0058] <Measurement of Welding Strength>

[0059] When manufacturing electrode assembly samples, for the electrode assembly samples, the welding strength of the welded part, namely the tensile strength, torsional strength, and peel strength, is measured. In the welding defect inspection method according to the present invention, two or more types of welding strength can be measured according to the direction of the external force.

[0060] That is, according to the welding defect inspection method of the present invention, as an evaluation criterion for determining welding defects, in addition to the tensile strength, all small welding defects can be detected by adding the torsional strength and peel strength measured when an external force is applied in a direction different from the direction of the tensile strength, thereby improving the reliability and accuracy of welding.

[0061] The measurement of the tensile strength, torsional strength, and peel strength includes: preparing three groups of electrode assembly samples; and measuring any one of the tensile strength, torsional strength, and peel strength of each group.

[0062] As described later, the tensile strength, torsional strength, and peel strength are measured by applying an external force to the electrode tab in a constant direction to break the welded part between the electrode tab and the electrode lead. Therefore, it is impossible to measure different types of welding strength using the electrode assembly samples that have been measured for one type of welding strength. Therefore, three groups are prepared, and one of the tensile strength, torsional strength, and peel strength is measured for each group. Here, one group includes at least one electrode assembly sample, and one group can include two or more electrode assembly samples. At this time, if one group includes two or more electrode assembly samples, the average value of the welding strength of the two electrode assembly samples can be determined as the welding strength of this group.

[0063] In addition, the tensile strength, torsional strength, and peel strength can be measured using the same form of measuring device.

[0064] Figure 2 is a schematic diagram of a method for measuring tensile strength in a method for inspecting welding defects according to the present invention, and Figure 3 is a schematic diagram of a method for measuring torsional strength in a method for inspecting welding defects according to the present invention. Figure 4 is a schematic diagram of a method for measuring peel strength in a method for inspecting welding defects according to the present invention.

[0065] Reference Figures 2 to 4 , the electrode assembly 100 has a structure in which a positive electrode 111, a negative electrode 112, and a separator 113 are alternately stacked. Herein, an electrode tab 120, which is one of the positive electrode tab and the negative electrode tab, is formed on one side of the electrode assembly 100, and an electrode lead 130 is joined to the electrode tab by welding.

[0066] In addition, the measuring device 200 includes: a first gripper 210 configured to fix an end portion of the electrode lead; and a second gripper 220 configured to fix the electrode assembly.

[0067] Specifically, the second gripper 220 fixes the electrode assembly 100. The second gripper 220 may have a structure in which a pair of pressing plates are formed on a surface in contact with the electrode assembly 100 to press the electrode assembly 110 from two surfaces. The pair of pressing plates can be fixed by appropriately pressing the electrode assembly through interval adjustment.

[0068] The first gripper 210 may have a structure in which a space for accommodating the electrode lead 130 is formed to fix the end portion of the electrode lead 130. For example, a structure in which a pair of pressing plates are formed on a surface in contact with the electrode lead. In this case, the interval between the pressing plates can be appropriately adjusted to fix the electrode lead. In the present invention, the first gripper 210 and the second gripper 220 can fix the electrode assembly and the electrode lead, and there is no specific limitation on their forms.

[0069] At this time, the first gripper 210 applies a force to the electrode lead in a predetermined direction, thereby disconnecting the welded portion.

[0070] Reference Figure 2 , the first gripper 210 can apply a tensile force to the welded portion between the electrode lead 130 and the electrode tab 120 by pulling the electrode lead 130 parallel to the pulling direction. The first gripper 210 can pull the electrode lead 130 until the welded portion is disconnected, and the force applied to the welded portion at the time of disconnection can be defined as the tensile strength of the welded portion. A known measuring device or the like can be used to measure the force applied to the welded portion at the time of disconnection.

[0071] Reference Figure 3, the first gripper 210 can rotate by using the central portion in the width direction of the electrode lead as a rotation axis to apply a torsional force to the welded portion between the electrode lead 130 and the electrode tab 120. The first gripper 210 rotates until the welded portion breaks, and the force applied to the welded portion at the break can be defined as the torsional strength of the welded portion. Similarly, the force applied to the welded portion at the break can be measured using known measuring devices or the like.

[0072] Reference Figure 4 , the first gripper 210 can apply a force to peel the electrode lead 130 from the electrode tab 120 while applying a force in a direction perpendicular to the direction in which the electrode tab 120 or the electrode lead 130 is pulled out. The first gripper 210 can apply a force until the welded portion breaks, and the force applied to the welded portion at the break can be defined as the peel strength of the welded portion. Similarly, the force applied to the welded portion at the break can be measured using known measuring devices or the like.

[0073] The welding strength can be measured for each of a plurality of electrode assembly samples, and as described later, the welding strength can be measured for electrode assembly samples by different welding methods.

[0074] <Derive the correlation with the presence or absence of welding defects>

[0075] When measuring the tensile strength, torsional strength, and peel strength for an electrode assembly sample, the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength is derived. Here, the correlation refers to the trend indicated by the presence or absence of welding defects based on the numerical values of the tensile strength, torsional strength, and peel strength.

[0076] Specifically, deriving the correlation includes establishing a database on the correlation between the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead and the presence or absence of welding defects. This is to recognize the trend of the presence or absence of welding defects for the tensile strength, torsional strength, and peel strength. To this end, after manufacturing a plurality of electrode assembly samples, the tensile strength, torsional strength, and peel strength are measured separately, and the measured information can be stored in a storage system such as a memory. In addition, such a database can be recorded in visual data such as a table or a graph. At this time, for the accuracy of the measurement, it is preferable to measure the tensile strength, torsional strength, and peel strength of a large number of electrode assembly samples.

[0077] Here, the presence or absence of welding defects indicates whether there is actually a defect at the welded portion between the electrode lead and the electrode tab. For example, it can be determined by observing the appearance of the welded portion or the surface state of the welded portion.

[0078] <Derive reference values>

[0079] After deriving the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength, a reference value for determining the presence or absence of welding defects is derived therefrom. The reference value is derived from a database. Herein, a reference value is derived for each of the tensile strength, torsional strength, and peel strength.

[0080] Specifically, in the database, the measured value information of the tensile strength, torsional strength, and peel strength of the electrode assembly samples determined to have welding defects, and the measured value information of the tensile strength, torsional strength, and peel strength of the electrode assembly samples determined to have no welding defects are stored. Herein, for example, the minimum value among the measured values of the tensile strength, torsional strength, and peel strength of the electrode assembly samples determined to have no welding defects can be determined as the reference value.

[0081] That is, according to the welding defect inspection method of the present invention, by checking the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength and normalizing the correlation, this inspection method can be applied to all welding processes.

[0082] In addition, regarding the welding strength, by measuring other strength values in addition to the tensile strength and deriving a reference value therefrom, small welding defects can be detected, and the reliability and accuracy of welding can be improved.

[0083] In addition, in another embodiment of the present invention, the method for inspecting welding defects includes measuring the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead using different welding methods, and for each welding method, deriving the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength. In addition, the welding defect inspection method further includes, for each welding method, deriving a reference value for determining the presence or absence of welding defects.

[0084] Figure 5 is a flowchart showing the sequence of a method for inspecting welding defects according to another embodiment of the present invention.

[0085] That is, referring to Figure 5 , the method for inspecting welding defects includes: manufacturing an electrode assembly sample by welding an electrode lead to an electrode tab formed on an electrode assembly (S20); measuring the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead of the electrode assembly sample using different welding methods (S21); deriving the correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength (S22); and for each welding method, deriving a reference value for determining the presence or absence of welding defects (S23).

[0086] In this text, the welding methods may include laser welding and ultrasonic welding.

[0087] That is, the present invention can reflect the deviation of the welding strength that can vary according to the welding method by differentiating the reference value according to the welding method in the process of deriving the reference value for determining the presence of welding defects.

[0088] In this text, each step in the welding defect inspection method is the same as above. Specifically, three sets of electrode assembly samples are prepared using one welding method, and then for each set, one of the tensile strength, torsional strength, and peel strength is measured. In addition, the same process is repeated using different welding methods.

[0089] At this time, the measuring device may include a first gripper configured to fix the end of the electrode lead and a second gripper configured to fix the electrode assembly.

[0090] Meanwhile, when measuring the welding strength, the correlation between the presence or absence of welding defects derived from the measured welding strength and each of the tensile strength, torsional strength, and peel strength of each welding method is determined. At this time, the step of determining the cause of the welding defect based on the tensile strength, torsional strength, and peel strength according to the welding method and establishing a database based on this. To this end, after manufacturing a plurality of electrode assembly samples using different welding methods, the tensile strength, torsional strength, and peel strength are measured respectively, and the measured information can be stored in a storage system such as a memory. In addition, such a database can be recorded in visual data such as a table or a graph. In addition, a reference value can be derived from such a database. The specific process for deriving the reference value can be the same as above.

[0091] In addition, the method for inspecting welding defects according to the present invention further includes determining the cause of the welding defect based on the correlation and establishing a database based on this. When different welding methods are used, the causes of the welding defects are also different. Therefore, in the present invention, the tensile strength, torsional strength, and peel strength according to the welding method and the cause of the welding defect can be established as a database, and this database can be used as reference data for finding the cause of the welding defect when such a welding defect occurs.

[0092] Similarly, when confirming the reference value for the presence or absence of welding defects, the step of determining the presence or absence of welding defects in the electrode assembly is performed based on this.

[0093] The step of determining the presence or absence of welding defects includes: preparing three sets of electrode assemblies to be inspected by welding the electrode leads to the electrode tabs, measuring one of the tensile strength, torsional strength, and peel strength for each set, and comparing the measured value with the reference value to determine the presence or absence of welding defects.

[0094] Specifically, the method of manufacturing the electrode assembly and the methods of measuring the tensile strength, torsional strength, and peel strength are the same as those described above. The tensile strength, torsional strength, and peel strength can be measured by the above measuring device, and different welding strengths can be measured for each group.

[0095] When measuring the tensile strength, torsional strength, and peel strength, they are compared with reference values to determine whether there are welding defects. At this time, determining whether there are welding defects includes comparing the measured values with the reference values for any two of the tensile strength, torsional strength, and peel strength. In this case, for example, the measured value is compared with the reference values for any two of the tensile strength, torsional strength, and peel strength, and if the measured value meets the reference value requirements of both, the electrode assembly can be determined to be a good product. In this case, the electrode assembly determined to be a good product can be an electrode assembly manufactured on the same production line.

[0096] In addition, in another example, determining whether there are welding defects includes comparing the measured value with the reference values for the tensile strength, torsional strength, and peel strength. In this case, for example, the measured value is compared with the reference values for all of the tensile strength, torsional strength, and peel strength, and if the measured value meets the reference value requirements for all of the tensile strength, torsional strength, and peel strength, the electrode assembly can be determined to be a good product. As described above, the electrode assembly determined to be a good product can be an electrode assembly manufactured on the same production line.

[0097] In addition, the present invention provides a method of manufacturing a secondary battery, including the method of inspecting welding defects as described above.

[0098] Specifically, the secondary battery is manufactured to have an electrode assembly with a structure in which a positive electrode, a separator, and a negative electrode are alternately stacked, and the electrode assembly is housed in a battery case. At this time, the same positive electrode, negative electrode, and separator as the above electrode assembly sample can be used as the positive electrode, negative electrode, and separator. In addition, a product determined to be a good product when inspected according to the welding defect inspection method as described above can be used as the electrode assembly.

[0099] In addition, the battery case is not particularly limited as long as it serves as an external material for encapsulating the battery, and a cylindrical, square, or pouch type can be used, and specifically, a pouch type battery case can be used. Similarly, the details of the battery case are known to those of ordinary skill in the art, so the detailed description thereof is omitted herein.

[0100] When the electrode assembly is housed in the battery case, an electrolyte solution is injected and the battery case is sealed, and then a forming process is performed to manufacture the secondary battery.

[0101] The above description is only an illustration of the technical concept of the present invention, and those skilled in the art to which the present invention pertains can make various improvements and changes without departing from the basic features of the present invention. Therefore, the attached drawings disclosed in the present invention are not intended to limit the technical concept of the present invention, but to describe the present invention, and the scope of the technical concept of the present invention is not limited by these drawings. The protection scope of the present invention should be interpreted by the appended claims, and all technical concepts within the scope equivalent thereto should be interpreted as being included within the scope of the present invention.

[0102] On the other hand, in this specification, terms indicating directions such as up, down, left, right, before, and after are used, but obviously, these terms are only for convenience of description and can be changed according to the position of the object or the position of the observer.

[0103] [Description of Reference Numerals]

[0104] 100: Electrode assembly

[0105] 111: Positive electrode

[0106] 112: Negative electrode

[0107] 113: Separator

[0108] 120: Electrode tab

[0109] 130: Electrode lead

[0110] 200: Measuring device

[0111] 210: First gripper

[0112] 220: Second gripper

Claims

1. A method for inspecting welding defects, the method comprising: Manufacturing an electrode assembly sample by welding an electrode lead on an electrode tab formed on an electrode assembly using different welding methods; Measuring the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead for the electrode assembly sample; Deriving a correlation between the presence or absence of welding defects and each of the tensile strength, torsional strength, and peel strength for each welding method; And Deriving reference values for determining the presence or absence of welding defects for the tensile strength, torsional strength, and peel strength, respectively, and Determining the cause of the welding defect based on the correlation and establishing a database therewith.

2. The method according to claim 1, wherein Measuring the tensile strength, torsional strength, and peel strength includes: Preparing three groups of electrode assembly samples; and Measuring any one of the tensile strength, torsional strength, and peel strength for each group.

3. The method according to claim 1, wherein Measuring the tensile strength, torsional strength, and peel strength using a measuring device including: A first gripper configured to fix an end of the electrode lead; and A second gripper configured to fix the electrode assembly.

4. The method according to claim 3, wherein, The first gripper applies a force to the electrode lead in a predetermined direction to break the welded portion.

5. The method according to claim 1, wherein, Deriving the correlation includes: establishing a database on the correlation between the tensile strength, torsional strength, and peel strength of the welded portion between the electrode tab and the electrode lead and the presence or absence of welding defects.

6. The method according to claim 5, wherein, Deriving the reference values from the database of the correlation.

7. The method according to claim 1, further comprising deriving reference values for determining the presence or absence of welding defects for each welding method.

8. The method according to claim 7, wherein The welding method includes ultrasonic welding or laser welding.

9. The method according to claim 1, further comprising determining the welding defects of the electrode assembly.

10. The method according to claim 9, wherein, Determining the welding defects includes: Preparing three groups of electrode assemblies to be inspected by welding electrode leads on electrode tabs; Measuring any one of the tensile strength, torsional strength, and peel strength for each group; and Determining the presence or absence of welding defects by comparing the measured values obtained by measurement with the reference values.

11. The method according to claim 10, wherein, Determining the presence or absence of welding defects includes comparing the measured values with the reference values for two of the tensile strength, torsional strength, and peel strength.

12. The method according to claim 10, wherein, Determining whether it is a welding defect includes comparing the measured values with the reference values for the tensile strength, torsional strength, and peel strength.

13. A method for manufacturing a secondary battery, comprising the method for inspecting welding defects according to claim 1.

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