Method for detecting low voltage battery cells

Abstract

The present technology provides a method for detecting a battery cell having a low voltage defect, comprising: setting a reference time point at which the voltage of a battery is stabilized after transport charging, and measuring a first voltage of the battery cell at the reference time point; measuring a second voltage of the battery cell at a time interval longer than a period during which self-discharge of the battery cell is suppressed; and determining whether the battery cell has a low voltage defect by comparing a difference (ΔOCV) between the first voltage and the second voltage with a reference value, wherein the reference value is +3 sigma (δ) to +6 sigma (δ) in a normal distribution of a voltage drop amount obtained by measuring the first voltage and measuring the second voltage for a plurality of normal sample battery cell groups.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0122552, filed on September 22, 2020, and the entire contents of this Korean Patent Application are incorporated herein by reference.

[0002] The present invention relates to a method of detecting a low voltage defect of a lithium secondary battery, and more particularly to a method of detecting a low voltage defect of a secondary battery that can quickly detect a battery cell having a low voltage defect while maintaining appropriate detection capability. Background Art

[0003] With the development of mobile device technology and the increase in demand for mobile devices, the demand for secondary batteries as energy sources has also increased rapidly. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life and low self-discharge rate, have been commercialized and widely used.

[0004] After the electrode assembly and electrolyte are assembled in a battery case, the lithium secondary battery undergoes an activation process. The activation process stabilizes the battery structure and makes the assembled battery usable by charging, aging, and discharging the battery.

[0005] In lithium secondary batteries, various types of defects may occur due to various reasons during the manufacturing process or use. In particular, some secondary batteries that have been manufactured have a phenomenon in which a voltage drop behavior is exhibited at a self-discharge rate, and this phenomenon is called low voltage.

[0006] The low voltage failure phenomenon of a secondary battery is usually due to foreign metal materials located therein. In particular, when there is a metallic foreign material such as iron or copper in the positive plate of the secondary battery, the metallic foreign material may grow as a dendrite at the negative electrode. In addition, such dendrites cause an internal short circuit of the secondary battery, which may lead to failure or damage to the secondary battery, or in severe cases, cause a fire. Therefore, the process of manufacturing secondary batteries includes a process of selecting battery cells with low voltage defects in order to prevent the delivery of battery cells with low voltage defects.

[0007] In addition, the secondary battery is delivered after the activation process, which is completed by accommodating the electrode assembly in the battery case, injecting the electrolyte solution into the battery case, and sealing the battery case. The activation process of the secondary battery includes a formation process of initially charging the assembled battery cell to a predetermined SOC range, an aging process of stabilizing the battery cell by storing the battery cell under a specific temperature and humidity environment, a degassing process of exhausting the gas inside the battery cell, a full charge and full discharge process, and a selection process of selecting batteries with low voltage defects while aging the battery after transport charging. In addition, the process of selecting batteries with low voltage defects after transport charging includes: measuring the voltage of the transport charged battery cell at predetermined time intervals, recording the measured voltage value, and calculating the voltage drop amount, and determining that the battery cell has a low voltage defect if the voltage drop amount exceeds a predetermined reference value.

[0008] Figure 1 is a schematic diagram illustrating a process of selecting a battery cell having a low voltage defect. Figure 1 The process of selecting a battery cell with a low voltage defect includes comparing the voltage at a time point approximately one day after the room temperature aging time point (OCV 1), the voltage at a time point approximately eight days after the room temperature aging time point (OCV 2), and the voltage at a time point approximately 14 days after the room temperature aging time point (OCV 3). Therefore, selecting a battery cell with a low voltage defect takes approximately 14 days. This is understood to mean that even good battery cells have a voltage drop caused by self-discharge, and in order to clearly distinguish good products from defective products, this takes a considerable amount of time.

[0009] However, about 14 days is a relatively long time, and a technique for reducing the time to select a battery cell having a low-voltage defect while maintaining detection capability is required.

[0010] Korean Patent Publication No. 10-2020-0039215 discloses a technology for detecting battery cells with low voltage defects during the formation process of pre-aged battery cells. This method has the advantage of being able to detect low voltage defects earlier, but because the voltage of the battery cell is monitored before a stable SEI layer is fully formed through the formation process, the detection capability may be reduced due to unstable voltage changes, and capacity defects may be detected simultaneously with low voltage defects, which may make it difficult to distinguish between capacity defects and low voltage defects.

[0011] [Prior art literature]

[0012] [Patent Document]

[0013] (Patent Document 1) Korean Patent No. 10-2020-0039215 Summary of the Invention

[0014] [Technical Issues]

[0015] The present invention is made to solve the above-mentioned problem, and an object of the present invention is to reduce the time required to detect a battery cell having a low voltage defect.

[0016] Another object of the present invention is to improve the ability to detect battery cells having low voltage defects by improving the dispersion of voltage drops of good battery cells.

[0017] [Technical solution]

[0018] A method for detecting a battery cell having a low voltage defect according to the present invention includes: setting a reference time point at which the voltage of the battery is stabilized after transport charging, and measuring a first voltage of the battery cell at the reference time point; measuring a second voltage of the battery cell at a time interval longer than a period during which self-discharge of the battery cell is suppressed; and determining whether the battery cell has a low voltage defect by comparing a difference (ΔOCV) between the first voltage and the second voltage with a reference value, wherein the reference value is +3 sigma (δ) to +6 sigma (δ) in a normal distribution of a voltage drop amount obtained by measuring the first voltage and measuring the second voltage for a plurality of normal sample battery cell groups.

[0019] In an embodiment of the present invention, the reference value is +4 sigma (δ) to +5 sigma (δ) in a normal distribution of voltage drop amounts obtained by measuring the first voltage and measuring the second voltage for a plurality of normal sample battery cell groups.

[0020] In an embodiment of the present invention, during the measuring of the first voltage and the measuring of the second voltage, the voltages are measured using a voltage measuring instrument having a resolution of a microvolt unit.

[0021] In an embodiment of the present invention, measuring the first voltage further includes charging a minute current after shipping charging.

[0022] In an embodiment of the present invention, during charging of the fine current, a current of 50 mA to 150 mA is applied for 5 to 15 minutes in a CV charging scheme.

[0023] In an embodiment of the present invention, measuring the second voltage may include measuring the second voltage of the battery cell within 15 to 72 hours from a reference time point.

[0024] In an embodiment of the present invention, measuring the second voltage may include measuring the second voltage of the battery cell within 18 to 45 hours from a reference time point.

[0025] In an embodiment of the present invention, measuring the first voltage and measuring the second voltage are performed at a temperature of 20 to 30°C.

[0026] The method of manufacturing a lithium secondary battery according to the present invention includes: activating assembled battery cells by injecting an electrolyte solution and sealing a battery container; and detecting low voltage defects as above while stabilizing the battery cells at room temperature after transportation charging.

[0027] In an embodiment of the present invention, activating the battery cells includes: forming the battery cells; aging the battery cells at room temperature or high temperature; exhausting gas inside the battery cells; and performing full charging and full discharging.

[0028] [Beneficial Effects]

[0029] According to the method of detecting battery cells with low voltage defects of the present invention, the time required to detect low voltage defects is significantly reduced, and as the voltage drop dispersion of good battery cells becomes smaller, the boundary between good products and defective products becomes clear, thereby improving the detection capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic diagram illustrating an activation process including a process of inspecting a low voltage defect according to a conventional art.

[0031] Figure 2 is a flowchart illustrating a method of detecting a battery cell having a low voltage defect according to an embodiment of the present invention.

[0032] Figure 3 is a flowchart illustrating a method of detecting a battery cell having a low voltage defect according to another embodiment of the present invention.

[0033] Figure 4 is a schematic diagram illustrating an activation process for secondary battery production.

[0034] Figure 5 Shows the time of low voltage test. Figure 5 (a) shows a low voltage test method according to conventional technology, and Figure 5 (b) shows a low voltage test method according to the present invention.

[0035] Figure 6 is a graph showing a cumulative discharge pattern and an accurate voltage by monitoring the voltage drop amount at intervals of 30 seconds after OCV 1 for a good battery cell.

[0036] Figure 7 is a graph showing the cumulative discharge pattern over time after OCV 1 for a good battery cell.

[0037] Figure 8 An example of a distribution curve of the pressure drop amount of a sample group according to an embodiment of the present invention is shown.

[0038] Figure 9 The voltage drop amount data according to whether the fine current application charging step has been performed for the good battery cells is shown. DETAILED DESCRIPTION

[0039] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The terms and words used in this specification and claims should not be construed as limited to common terms or dictionary terms, and the inventors may appropriately define the concepts of the terms to best describe their invention. The terms and words should be interpreted as having meanings and concepts consistent with the technical concept of the present invention.

[0040] In the present application, it should be understood that terms such as "including" or "having" are intended to indicate that the described features, quantities, steps, operations, components, parts or combinations thereof are present in the present specification, and they do not preclude the possibility of the presence or addition of one or more other features or quantities, 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 the part and the other part. 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 the part and the other part. In addition, to be arranged "on" in the present application may include the case of being arranged at the bottom as well as the top.

[0041] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0042] First, refer to Figure 4 Describe the activation steps in secondary battery production.

[0043] Reference Figure 4 , the activation steps of secondary battery production may include pre-aging process, formation process, aging process, degassing process and transportation-charging process.

[0044] Generally, pre-aging is the process of placing an electrode assembly in a battery container, injecting an electrolyte solution into the battery container, and sealing the battery container to manufacture a battery cell (i.e., a bare cell). Formation is the process of initially charging the pre-aged battery cell under a preset voltage condition (e.g., the voltage at which the SEI film of the negative electrode is formed or higher). Aging is the process of storing the battery cell under a preset voltage condition (e.g., 3.4V to 3.6V) and a preset temperature condition (e.g., 50 degrees to 70 degrees) until the battery cell stabilizes to a specific state. In this article, the pre-aging process, the formation process, and the aging process correspond to the wetting period. The degassing process is the process of removing unnecessary gas from the aged battery cell. For example, in the case where the secondary battery has a round or prismatic shape, the degassing process can be omitted. Transport charging is the process of charging the battery cell under a preset voltage condition (e.g., SOC 20% to 50%) before delivery, and preset characteristic tests (cell resistance, output, and charge / discharge capacity, etc.) can be performed on the battery cell during transport charging. Thereafter, after the transport-charging process, a low voltage test is performed through an open circuit voltage (OCV) tracking scheme.

[0045] Reference Figure 5 , the low voltage test of the present invention is compared with the low voltage test according to the conventional technology. Figure 5 (a) shows a low voltage test method according to conventional technology, and Figure 5 (b) shows the low voltage test method according to the present invention. According to conventional technology, such as Figure 5 As shown in (a), a six-day inspection period is required, which is the time interval between the OCV 2 time point and the OCV 3 time point. However, the present invention is characterized in that, as Figure 5 As shown in (b), the voltage is measured at the time point of OCV 2-1 which is much earlier than the existing time point of OCV 3, and a battery cell having a low voltage defect is selected by comparing the voltage measured at the time point of OCV 2-1 with the voltage measured at the time point of OCV 2.

[0046] [Detailed description of preferred embodiments]

[0047] In the following, reference will be made to Figure 6 and Figure 7 The reason why low voltage detection can be performed at the OCV 2-1 time point in the present invention is described.

[0048] Figure 6 is a graph showing the cumulative discharge pattern and the precise voltage by monitoring the voltage drop amount at 30-second intervals after OCV 1 for a good battery cell. Figure 7 is a graph showing the cumulative discharge pattern over time after OCV 1 for a good battery cell.

[0049] First, refer to Figure 6 , after the shipping charge, the voltage of the battery cell discharges continuously over time, but both the continuous discharge and the discharge delay / limitation occur in a minute voltage (microvolt) level.

[0050] Reference Figure 7 , after the transport charge, over time, due to the fact that after a certain time (in Figure 7 In the example, after 168 hours (ie, 7 days), the internal resistance is stabilized due to sufficient aging, so the good battery cell becomes to have a discharge limit period (cycle) of 18 to 24 hours (ΔOCV=0).

[0051] Thus, the inventors of the present invention have come up with the following considerations: if the voltage (first voltage) is measured at a time point (OCV 2) after 168 hours, then the voltage (second voltage) is measured at a time point (OCV 2-1) with a time interval including a discharge limit period (cycle) of 18 to 24 hours from the time point of voltage measurement, and the voltage drop amount is calculated, the voltage drop amount of good battery cells is minimized, thereby making it easy to distinguish good battery cells from defective battery cells. Therefore, in the present invention, it is important to set the OCV 2 time point and the OCV 2-1 time point as the voltage measurement time point, which becomes the basis for calculating the voltage drop amount.

[0052] Figure 2 is a flow chart illustrating a method of detecting a battery cell having a low voltage defect according to an embodiment of the present invention. Figure 2 , the method for detecting a battery cell having a low voltage defect of the present invention includes: setting a reference time point when the voltage of the battery is stabilized after transportation charging, and measuring a first voltage of the battery cell at the reference time point (S10); measuring a second voltage of the battery cell at a time interval longer than a period for suppressing self-discharge of the battery cell (S20); and determining whether the battery cell has a low voltage defect by comparing a difference (ΔOCV) between the first voltage and the second voltage with a reference value (S30), wherein the reference value is +3 sigma (δ) to +6 sigma (δ) in a normal distribution of a voltage drop amount obtained by measuring the first voltage and measuring the second voltage for a plurality of normal sample battery cell groups.

[0053] In the first step of measuring the first voltage, it is important to set a reference time point for measuring the first voltage. In this case, it is preferable to measure the first voltage at a time point when the battery cell is stabilized, as described above. The time point when the battery cell is stabilized is as follows: Figure 7 The first time point at which discharge limitation (ΔOCV) begins to occur in a specific period is shown in the graph of FIG. Figure 7In the case of , the reference time point is 168 hours after the start of monitoring, that is, the reference time point of the present invention refers to the time point when discharge restriction starts to occur in a specific period.

[0054] The second step is a process of measuring the second voltage for calculating the voltage drop amount. In the present invention, the second voltage is measured at a time interval greater than the period during which self-discharge is limited from the reference time point. Figure 7 The self-discharge limitation period is 24 hours, and if the second voltage is measured after a time interval greater than this period, the self-discharge limitation section is included. This reduces the voltage drop compared to a battery cell with a low-voltage defect, making it easier to distinguish between good and defective products.

[0055] In one specific example, the second step of measuring the second voltage may be to measure the second voltage of the battery cell within 15 to 72 hours from the reference time point, or within 18 to 45 hours from the reference time point. The numerical ranges are examples only and are not limited thereto. The numerical ranges may vary depending on the battery capacity and material properties. That is, the time point for measuring the second voltage may be a sufficient period from the reference time point for the second voltage to be limited after the battery stabilizes, and is not limited to the numerical ranges described above.

[0056] In one specific example, during the measurement of the first voltage and the measurement of the second voltage, a voltage measuring instrument with a microvolt resolution is used to measure the voltage. Even if a voltage drop occurs due to self-discharge, the voltage drop is very small, making it difficult to sense. Discharge delay / limitation can be detected at a subtle voltage level in the microvolt range.

[0057] In a specific example, the first step of measuring the first voltage and the second step of measuring the second voltage may be performed at a temperature of 20°C to 30°C.

[0058] The third step of determining whether a battery cell has a low voltage defect is to compare the calculated voltage drop with a reference value. In particular, the present invention is characterized in that the reference value is set using a statistical method. The method of setting the reference value of the present invention will be described below.

[0059] A method for deriving a reference value according to an embodiment of the present invention includes: a data construction step of measuring a minute voltage in a microvolt unit through a sample group and storing the measured values; and a reference value derivation step of deriving a reference value by processing data accumulated by the data construction step with a statistical scheme.

[0060] The data building step includes measuring the voltage of good battery cells forming a sample group according to the first and second steps, calculating the voltage drop (ΔOCV), and accumulating the calculated values ​​as data. In this case, the number of subjects in the sample group can be at least 100,000, preferably 200,000. For reliability reasons, it is preferable to have as many subjects as possible in the sample group.

[0061] In the data construction step, voltage is measured for the sample group objects by using a fine voltage measuring instrument having a resolution of a microvolt (uV) unit.

[0062] The reference value deriving step derives the reference value by processing the data accumulated by the data construction step using a statistical scheme. The statistical processing method according to an embodiment of the present invention obtains a normal distribution curve of the pressure drop amount of the subjects in the sample group and determines +3 sigma (δ) to +6 sigma (δ) as the reference value.

[0063] Specifically, the voltage drop values ​​derived from a large number of good battery cells forming a sample group form a normal distribution curve. In this context, objects with large deviations from the normal distribution curve are considered defective. This predetermined deviation is then established as a baseline value, further improving the reliability of the inspection results.

[0064] Figure 8 An example of a distribution curve of the pressure drop amount of a sample group according to an embodiment of the present invention is shown. Figure 8 , when the sample group shows Figure 8 When a normal distribution curve is drawn, most objects have values ​​close to the mean (u), and the number of objects with values ​​significantly deviating from the mean (u) is small. Therefore, objects with values ​​significantly deviating from the mean can be presumed to be defective. Specifically, the probability of an object with a deviation of 1δ (standard deviation) is approximately 32%, the probability of an object with a deviation of 2δ is approximately 5%, the probability of an object with a deviation of 3δ is approximately 0.3%, the probability of an object with a deviation of 4δ is approximately 0.01%, the probability of an object with a deviation of 5δ is approximately 0.001%, and the probability of an object with a deviation of 6δ is approximately 0.0000001%.

[0065] Thus, in an embodiment of the present invention, the reference value may be set to the sum of the average value of the pressure drop amount and +3 sigma (δ) to +6 sigma (δ), and preferably, the sum of the average value of the pressure drop amount and +4 sigma (δ) to +5 sigma (δ).

[0066] Figure 3 A method for detecting a battery cell having a low voltage defect according to another embodiment of the present invention is shown. Figure 3, in the method of detecting a battery cell having a low voltage defect according to another embodiment of the present invention, the first step further includes a step ( S11 ) of charging the first voltage after the transportation charge.

[0067] If a subtle current is charged to the battery cells after shipping charge, the dispersion of good battery cells is further improved, thereby making the boundary between good products and defective products clearer.

[0068] Figure 9 is a table showing the standard deviation of the voltage drop amount of defective battery cells and the voltage drop amount of good battery cells. In this article, the left side shows the state before performing the fine current application charging step, and the right side shows the state after performing the fine current application charging step. Figure 9 When not performing trickle charging, the standard deviation of the voltage drop for good products is 95 microvolts, but after performing trickle charging, the standard deviation of the voltage drop for good products decreases to 39 microvolts. Furthermore, the asterisked data in the top frame shows the voltage drop for defective cells. When performing trickle charging, the difference between the average voltage drop for good and defective cells is 1180 microvolts, but when not performing trickle charging, the difference is 1131 microvolts. In other words, when performing trickle charging, the difference between the voltage drop for good and defective products increases, and the standard deviation of the voltage drop for good products decreases, thereby making it easier to distinguish between good and defective products and improving detection capabilities.

[0069] In one specific example, during the fine current charging, a current of 50 to 150 mA may be applied for 5 to 15 minutes in a CV charging scheme. In the present invention, when applying a fine voltage, a constant voltage charging scheme is adopted because, in the case of a defective battery cell, the cumulative charging current increases due to leakage current in the constant voltage mode while the voltage is maintained, and the voltage drop increases during the rest period due to the leakage current, thereby making it easy to distinguish between good products and defective products.

[0070] Hereinafter, a method for manufacturing a lithium secondary battery according to the present invention will be described. The method for manufacturing a lithium secondary battery according to the present invention includes: activating a battery cell assembled by injecting an electrolyte solution and sealing a battery container; and detecting a low voltage defect as described above while stabilizing the battery cell at room temperature after transportation and charging.

[0071] In a specific example, activating the battery cells includes: forming the battery cells; aging the battery cells at room temperature or high temperature; exhausting gas inside the battery cells; and performing full charging and full discharging.

[0072] The formation step is a step of forming a coating of the SEI (solid electrolyte interface) of the negative electrode, and is a step of charging the assembled secondary battery to a predetermined capacity (SOC). The SOC% can be 15-70%, preferably 30-65%, and more preferably 45-60%. In order to improve dispersion by reducing the voltage drop of good battery cells, the SEI film of the negative electrode should be formed uniformly and stably, which can be easily achieved when the volume of the negative electrode expands to the maximum during the formation process. During the formation process, when charging is performed, lithium ions are inserted into the layer structure of the negative electrode, and stage 4 is stabilized to stage 1. In the initial charging step, a stable SEI layer is formed only when charging to the time point when stage 2 is completed. Here, the charging point at which stage 2 is completed varies depending on the type of negative electrode active material, but is generally at an SOC level of 45% to 65%.

[0073] The charging conditions of the formation step can be performed according to methods known in the art. Specifically, charging can be performed with a charging voltage of 3.0 to 4.0 V and a C rate of 1.3 C or less. However, the charging voltage and charging speed may vary depending on the type or characteristics of the secondary battery, but are not limited thereto.

[0074] In a preferred embodiment of the present invention, in order to prevent a gas trap phenomenon and lithium plating in which gas generated during charging in the formation process is trapped between the electrode and the separator, the secondary battery is preferably pressurized while charging in the formation step.

[0075] As described above, by pressurizing the secondary battery in the formation step, the SEI film is uniformly formed on the negative electrode, which has the advantage of maximizing the performance of the battery such as capacity and resistance, and has the effect of shortening the charge and discharge time. Pressing can be performed using a jig or the like, but it is not limited as long as it is a means capable of pressing the secondary battery.

[0076] After that, an aging step is performed to stabilize the formed secondary battery. The aging step is a step to further stabilize the battery by maintaining it at a constant temperature and humidity.

[0077] The aging step may include a high-temperature aging step for aging in a high-temperature environment of 60°C or higher and / or a normal-temperature aging step for stabilizing the secondary battery at a temperature of 20°C to 30°C.

[0078] The high-temperature aging step is a step for stabilizing the SEI film formed in the above-mentioned formation step. It has the advantage that the stabilization of the SEI film is further accelerated when the formed battery is aged at high temperature rather than at room temperature. For the purpose of stabilizing the SEI film to reduce the performance variation of the secondary battery in the present invention, high-temperature aging is preferably performed after the formation process.

[0079] In the present invention, the high-temperature aging step is performed at 60°C or higher, preferably 65°C to 75°C, thereby accelerating the stabilization of the SEI film of good products and reducing the self-discharge of good products to improve low-voltage detection. When high-temperature aging is performed at a temperature below 60°C, it is difficult to achieve the purpose of the present invention, and when the temperature is too high, there is a problem that battery performance such as capacity and life is deteriorated, which is undesirable.

[0080] In one embodiment of the present invention, the high temperature aging step can be performed for 18 to 36 hours, more preferably 21 to 24 hours. If the high temperature aging time is less than 18 hours, the stabilization of the SEI film may not be sufficient to achieve the purpose of the present invention, and when the high temperature aging time exceeds 36 hours, the aging time is prolonged, which is undesirable in terms of productivity.

[0081] The secondary battery in which the SEI film is stabilized by high temperature aging can be subjected to room temperature aging to stabilize at room temperature. The room temperature aging step can be performed at 20°C to 30°C, specifically 22°C to 28°C, more specifically 23°C to 27°C, and even more specifically 25°C to 27°C.

[0082] The degassing process is used to remove side reaction gases generated within the secondary battery during the formation and aging steps. Various degassing techniques known at the time of filing this invention can be employed. For example, the degassing process can be performed by cutting an extended portion and sealing the cut portion in a pouch-type secondary battery extending on one side. However, since such degassing techniques are well known to those skilled in the art, a more detailed description thereof will be omitted herein.

[0083] The full charge and discharge process fully charges and discharges the batteries to activate them and identify defective cells. The transport charge step is performed after the full charge and discharge process. The transport charge step charges the batteries for transport after they are fully discharged. Low voltage defects are detected in secondary batteries that have completed transport charging by measuring changes in voltage. The method for detecting low voltage defects of the present invention can be applied in this context.

[0084] In the method of manufacturing a secondary battery according to an embodiment of the present invention, before performing the formation process, a pre-aging step of aging the assembled secondary battery under constant temperature and humidity conditions may be performed.

[0085] First, in a pre-aging step, an electrode mix including an electrode active material and a binder is applied to an electrode current collector to prepare a positive electrode and a negative electrode, respectively, and then an electrode assembly is prepared by interposing a separator between the positive electrode and the negative electrode.

[0086] After the thus prepared electrode assembly is housed in a battery case, an electrolyte is injected, and the battery case is sealed to manufacture a battery.

[0087] The steps of manufacturing such a battery are not particularly limited and can be performed according to known methods.

[0088] In addition, the electrode assembly is not particularly limited as long as it is a structure including a positive electrode, a negative electrode, and a separator interposed therebetween, and may be, for example, a jelly roll type, a stack type, or a stack / folding type.

[0089] The battery case is not particularly limited as long as it serves as an exterior material for encapsulating a battery, and a cylindrical, square, or pouch type may be used.

[0090] The electrolyte includes an organic solvent and a lithium salt, and may optionally further include additives.

[0091] The organic solvent is not limited as long as it can minimize decomposition caused by oxidation reaction during charge and discharge of the battery, and may be, for example, a cyclic carbonate, a linear carbonate, an ester, an ether, or a ketone. These may be used alone, or two or more thereof may be used in combination.

[0092] Among organic solvents, carbonate organic solvents can be preferably used. Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). Linear carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC).

[0093] As for lithium salts, lithium salts commonly used in electrolytes for lithium secondary batteries, such as LiPF6, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiBF4, LiBF6, LiSbF6, LiN(C2F5SO2)2, LiAlO4, LiAlCl4, LiSO3CF3 and LiClO4, etc., can be used without restriction, and these can be used alone, or two or more can be used in combination.

[0094] In addition, the electrolyte may optionally further include an additive. Any one or a mixture of two or more selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, cyclic sulfites, saturated sultones, unsaturated sultones, acyclic sulfones, lithium oxalyldifluoroborate (LiODFB) and derivatives thereof may be used as an additive to stably form a SEI film, but is not limited thereto.

[0095] Cyclic sulfite can include ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethyl propylene sulfite, 4,5-diethyl propylene sulfite, 4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite, 1,3-butylene glycol sulfite, etc. Saturated sultone can include 1,3-propane sultone and 1,4-butane sultone, etc. Unsaturated sultone can include ethylene sultone, 1,3-propylene sultone, 1,4-butene sultone and 1-methyl-1,3-propylene sultone. Acyclic sulfone can include divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone and methyl vinyl sulfone.

[0096] These additives are added to the electrolyte to improve low-temperature output characteristics by forming a solid SEI film on the anode, as well as to inhibit decomposition of the cathode surface and prevent oxidation reactions of the electrolyte during high-temperature cycling operations.

[0097] When the battery case is a pouch type, an aluminum laminate bag including an aluminum layer may be used. After the electrolyte is injected, the opening portion of the aluminum laminate bag may be sealed by heat welding.

[0098] During the pre-aging step, wetting of the cell is carried out by the injected electrolyte.

[0099] More specifically, when a secondary battery is charged, electrons migrate to the negative electrode and charge is applied, causing lithium ions to intercalate into the negative electrode to achieve electrical neutrality. While lithium ions can be occluded in areas immersed in the electrolyte—that is, areas where ion migration paths are maintained (wetting regions)—they are relatively difficult to occlude in areas not wetted by the electrolyte.

[0100] Therefore, through the pre-aging step, the battery can be aged at room temperature and atmospheric pressure for 0.5 to 72 hours so that the electrolyte can penetrate into the positive electrode and the negative electrode. For example, the pre-aging step can be performed at 20°C to 30°C, specifically 22°C to 28°C, more specifically 23°C to 27°C, and even more specifically 25°C to 27°C.

[0101] The above description is only used to illustrate the technical ideas of the present invention, and those skilled in the art to which the present invention belongs may make various modifications and changes without departing from the essential features of the present invention. Therefore, the drawings disclosed in the present invention are not intended to limit the technical ideas of the present invention, but are used to describe the present invention, and the scope of the technical ideas of the present invention is not limited by these drawings. The scope of protection of the present invention should be interpreted by the attached claims, and all technical ideas within the scope equivalent to them should be interpreted as included within the scope of the present invention.

Claims

1. A method for detecting a battery cell having a low voltage defect, the method comprising: setting a reference time point at which the voltage of the battery is stabilized after transport charging, and measuring a first voltage of the battery cell at the reference time point; measuring a second voltage of the battery cell after a time interval from the reference time point, wherein the time interval is longer than a period during which self-discharge of the battery cell is suppressed; and determining whether the battery cell has a low voltage defect by comparing a difference (ΔOCV) between the first voltage and the second voltage with a reference value, The reference value is +3 sigma (δ) to +6 sigma (δ) in a normal distribution of voltage drop amounts obtained by measuring the first voltage and the second voltage for a plurality of normal sample battery cell groups. The reference time point is a time point at which self-discharge limitation starts to occur in a regular cycle.

2. The method according to claim 1, wherein The reference value is +4 sigma (δ) to +5 sigma (δ) in a normal distribution of voltage drop amounts obtained by measuring the first voltage and measuring the second voltage for a plurality of normal sample battery cell groups.

3. The method according to claim 1, wherein During the measuring of the first voltage and the measuring of the second voltage, the voltages are measured using a voltage measuring instrument having a resolution of a microvolt unit.

4. The method according to claim 1, wherein Measuring the first voltage further includes charging a minute current after the transport charging.

5. The method according to claim 4, wherein During the charging fine current, a current of 50 mA to 150 mA is applied for 5 to 15 minutes in a CV charging scheme.

6. The method according to claim 1, wherein Measuring the second voltage includes measuring the second voltage of the battery cell within 15 to 72 hours from the reference time point.

7. The method according to claim 6, wherein: Measuring the second voltage includes measuring the second voltage of the battery cell within 18 to 45 hours from the reference time point.

8. The method according to claim 1, wherein Measuring the first voltage and measuring the second voltage are performed at a temperature of 20 to 30°C.

9. A method for manufacturing a lithium secondary battery, the method comprising: activating the assembled battery cells by injecting an electrolyte solution and sealing the battery container; as well as The low voltage defect is detected according to the method of claim 1 while the battery cell is stabilized at room temperature after shipping charging.

10. The method according to claim 9, wherein: Activating the battery cell includes: forming the battery cells; Aging the battery cells at room temperature or elevated temperature; exhausting gas from inside the battery cell; and Perform a full charge and a full discharge.

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