Secondary battery manufacturing method

Abstract

A method for manufacturing a secondary battery that allows a self-discharge test to be properly performed after charging the secondary battery is provided. A method for manufacturing a secondary battery includes charging the secondary battery (S16), and calculating |d, which is the absolute value obtained by second-order differentiation of the voltage V of the secondary battery with respect to time t after the charging of the secondary battery is completed. 2 V / dt 2 | is 0.02mV / h 2 The secondary battery is left uncharged until the following occurs (S171 and S172), and |d 2 V / dt 2 | is 0.02mV / h 2 When the following occurs, a self-discharge test of the secondary battery is performed (S18).

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a secondary battery. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2019-113450 (Patent Document 1) discloses a technique for connecting a power source to a secondary battery to form a circuit, and using the power source to pass a current through the circuit to perform a self-discharge test on the secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-113450 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, Patent Document 1 does not fully consider the drop in battery voltage immediately after charging the secondary battery. If the technology described in Patent Document 1 is used to perform a self-discharge test after charging the secondary battery, there is a risk that the test accuracy will decrease.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to appropriately perform a self-discharge test after charging a secondary battery. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, there is provided a method for manufacturing a secondary battery, the method including charging the secondary battery, and calculating |d 2 V / dt 2 | is 0.02mV / h 2 Do not leave the secondary battery uncharged until it reaches the following level: 2 V / dt 2 | is 0.02mV / h2 and performing a self-discharge test on the secondary battery when: [Effects of the Invention]

[0007] According to the present disclosure, it is possible to appropriately perform a self-discharge test after charging a secondary battery. [Brief explanation of the drawings]

[0008] [Figure 1] 3 is a flowchart showing a method for manufacturing a secondary battery according to an embodiment. [Figure 2] 1 is a cross-sectional view showing a stack that constitutes a secondary battery according to an embodiment of the present invention. [Figure 3] 10A and 10B are diagrams for explaining charging of each cell and self-discharge inspection of the cells after charging in the method for manufacturing a secondary battery according to the present embodiment. [Figure 4] FIG. 1 is a diagram showing data obtained by methods according to an example and a comparative example. [Figure 5] 2 is a flowchart showing a modified example of the processing flow shown in FIG. 1. [Figure 6] 6 is a diagram for explaining the operation and effect of the method according to the modified example shown in FIG. 5. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] Embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, identical or corresponding parts are designated by the same reference numerals, and their description will not be repeated. In each of the drawings used below, the X-axis, Y-axis, and Z-axis are perpendicular to each other, and the Z-axis indicates the thickness direction of the battery. Hereinafter, the directions indicated by the arrows of the X-axis, Y-axis, and Z-axis will be indicated with a "+" and the opposite directions will be indicated with a "-".

[0010] Fig. 1 is a flowchart showing the procedure of the method for manufacturing a secondary battery according to this embodiment. In the method for manufacturing a secondary battery according to this embodiment, first, a stack is formed. Then, according to the process flow shown in Fig. 1, the stack is subjected to injection, charging, aging, self-discharge testing, and the like. Details of the process flow shown in Fig. 1 will be described later.

[0011] In this embodiment, a laminate 10 shown in FIG. 2 is prepared. FIG. 2 is a cross-sectional view showing a laminate constituting a secondary battery according to this embodiment. Referring to FIG. 2, the laminate 10 includes a power storage unit 10a and a sealing unit 3 that seals the power storage unit 10a. The Z direction corresponds to the stacking direction. The power storage unit 10a includes multiple cells C arranged in the Z direction. Each of the multiple cells C includes a negative electrode active material layer 12A, a positive electrode active material layer 12B, and a separator 13. Each of the multiple cells C is configured to be able to store electricity. Each of the multiple cells C functions as a secondary battery. Each of the multiple cells C corresponds to an example of a "power storage cell" according to the present disclosure. In this embodiment, the power storage unit 10a includes 10 or more cells C. However, the number of cells C can be set arbitrarily. The number of cells C included in the power storage unit 10a may be 3 or more but less than 50, or may be 50 or more. The sealing unit 3 is formed to surround the power storage unit 10a.

[0012] The laminate 10 includes multiple electrodes (one negative terminal electrode 2A, multiple bipolar electrodes 1, and one positive terminal electrode 2B) stacked along the Z direction. A separator 13 is disposed between the electrodes. The bipolar electrode 1 includes a current collector 11, a negative electrode active material layer 12A provided on the +Z side surface of the current collector 11, and a positive electrode active material layer 12B provided on the -Z side surface of the current collector 11. The negative electrode terminal electrode 2A has a configuration in which the positive electrode active material layer 12B has been removed from the bipolar electrode 1. An insulating layer 19A covering the periphery of the current collector 11 is formed on the -Z side surface of the current collector 11 constituting the negative electrode terminal electrode 2A. The positive electrode terminal electrode 2B has a configuration in which the negative electrode active material layer 12A has been removed from the bipolar electrode 1. An insulating layer 19B covering the periphery of current collector 11 is formed on the +Z side surface of current collector 11 that constitutes positive terminal electrode 2B.

[0013] In this embodiment, a metal foil (e.g., aluminum foil) is used as the current collector 11 of each electrode. One or both sides of the metal foil may be subjected to a surface treatment (e.g., plating treatment). A voltage detection terminal 20 is connected to the current collector 11 of each electrode. In this embodiment, the voltage detection terminal 20 comprises stainless steel (e.g., SUS304). Stainless steel has excellent corrosion resistance, heat resistance, and workability. However, the material of the voltage detection terminal 20 can be changed as appropriate. Other metals (e.g., copper) may be used instead of stainless steel.

[0014] The negative electrode active material layer 12A contains a negative electrode active material. The positive electrode active material layer 12B contains a positive electrode active material. In one example, the positive electrode active material is olivine-type lithium iron phosphate (LiFePO4), and the negative electrode active material is a carbon-based material. However, other examples of the negative electrode active material include silicon and tin.

[0015] In the laminate 10, cells C are formed between the stacked current collectors 11. Specifically, a cell C is formed between a certain current collector 11 (first current collector) and a current collector 11 (second current collector) adjacent to the first current collector. Furthermore, a cell C is formed between a second current collector and a current collector 11 (third current collector) adjacent to the second current collector. In this way, the current collectors 11 and the cells C are arranged alternately in the stacking direction of the laminate 10. The sealing portion 3 includes seal layers 14 and 15 arranged around each of the plurality of cells C included in the laminate 10, and the aforementioned insulating layers 19A and 19B. Any sealing material can be used as the material of the sealing portion 3.

[0016] The laminate 10 functions as a bipolar secondary battery. Each of the multiple cells C included in the laminate 10 (particularly, the power storage unit 10a) functions as, for example, an LFP battery (a lithium-ion secondary battery containing lithium iron phosphate as a positive electrode active material). Hereinafter, the first, second, third, and so on cells C from the end of the positive electrode side (+Z side) of the laminate 10 may be referred to as cell C-1, cell C-2, cell C-3, and so on, respectively (see FIG. 3, which will be described later). In the laminate 10, multiple cells are stacked in the Z direction. Adjacent cells have a common electrode. Specifically, the current collector 11 and the voltage detection terminal 20 located between adjacent cells function as a common electrode. This common electrode functions as a common wiring, which will be described later (see FIG. 3).

[0017] The manufacturing system according to this embodiment forms a laminate 10 (FIG. 2) to which the voltage detection terminal 20 is connected, through various processes such as coating, pressing, seal welding, separator welding, cutting, terminal (voltage detection terminal) welding, end face welding, and inlet welding. Although not shown in FIG. 2, a connector 30 (see FIG. 3), which will be described later, is further provided for the voltage detection terminal 20. The laminate 10 may also be restrained by a restraining jig. The laminate 10 may also be sandwiched and pressed between a pair of end plates (restraint plates).

[0018] The manufacturing system according to this embodiment includes a system for preparing the laminate 10 (a system including devices corresponding to each process for forming the laminate 10) and an inspection system for injecting, charging, aging, and inspecting the laminate 10. However, it is not essential that the manufacturing (including inspection) of the power storage device be performed automatically (i.e., all processes related to manufacturing are performed by devices), and some processes may be performed by a person (operator). Furthermore, the structure of the laminate 10 is not limited to the structure shown in FIG. 2 and can be modified as appropriate.

[0019] 1 again, when the laminate 10 obtained as described above (for example, the laminate 10 in a constrained state) is handed over to an inspection system, the inspection system automatically executes the processing flow shown in Fig. 1 for the laminate 10. Note that "S" in the flowchart denotes a step.

[0020] In S11, the inspection system injects the electrolyte into the laminate 10. This fills the space surrounded by the sealing portion 3 in FIG. 2 with the electrolyte. The separator 13 is impregnated with the electrolyte. The electrolyte is, for example, a non-aqueous electrolyte. However, the electrolyte is not limited to this, and may be an aqueous electrolyte. Alternatively, a gel or solid electrolyte may be used instead of the electrolyte.

[0021] In the following step S12, the inspection system performs an initial charge of the laminate 10. The initial charge is the first charge of the formed laminate 10. For example, the inspection system applies a voltage between the positive and negative terminals of the laminate 10 (for example, the current collectors 11 located at both ends in the Z direction shown in FIG. 2). This charges all of the cells C connected in series.

[0022] In the next step S13, the inspection system increases the temperature of the laminate 10 to an aging temperature higher than room temperature. The inspection system may, for example, set the laminate 10 in a temperature-controllable thermostatic chamber and adjust the temperature of the laminate 10 using the thermostatic chamber. The aging temperature may be 50°C or higher and 85°C or lower. However, the aging temperature can be set arbitrarily.

[0023] In the following step S14, the inspection system performs high-temperature aging on the laminate 10. The inspection system maintains the temperature of the laminate 10 at the aging temperature until a predetermined time (hereinafter referred to as the "aging time") has elapsed. The aging time can be set arbitrarily.

[0024] When high-temperature aging is complete, the inspection system cools the stack 10 to room temperature (for example, about 25°C) in S15. Then, in S16, the inspection system connects the voltage detection terminal 20 of each cell to a power source to charge each cell individually. The processing from S16 onwards will be described below with further reference to FIG. 3.

[0025] FIG. 3 is a diagram illustrating charging of each cell and self-discharge testing of the cells after charging. As shown in FIG. 3, a connector 30 is provided for multiple voltage detection terminals 20 (see FIG. 2) connected to the laminate 10. The voltage detection terminals 20 are welded to the +X-side end of the current collector 11, for example. Examples of welding methods include ultrasonic welding and laser welding. The connector 30 includes a resin part 31 and a housing 32. For example, the housing 32, which aligns the voltage detection terminals 20, is attached to the tip of the voltage detection terminals 20, and the resin part 31, which connects the +X-side end face of the laminate 10 to the housing 32, is formed by injection molding. This forms the connector 30 joined to the laminate 10. The multiple voltage detection terminals 20 are configured to be connectable to an external power source (e.g., a DC power source 41). Each of the multiple voltage detection terminals 20 functions as a pin of the connector 30. The connector 30 is a male connector configured to be able to accommodate a female connector (e.g., a socket).

[0026] The inspection system according to this embodiment includes an inspection device 100. The inspection device 100 includes a power supply unit 110, a connection unit 120, and a control device 150. The control device 150 includes a processor and a storage device. In this embodiment, the processor executes a program stored in the storage device to perform the self-discharge inspection. However, each process related to the self-discharge inspection may be performed only by hardware (electronic circuits) without using software.

[0027] The power supply unit 110 has a plurality of channels (hereinafter referred to as "Ch") for self-discharge inspection. Each Ch has a DC power supply 41, an ammeter 42, a voltmeter 43, and terminals T1 and T2. The output voltage of the DC power supply 41 is variable. The DC power supply 41 is controlled by the control device 150. The ammeter 42 is connected in series to the DC power supply 41, and the voltmeter 43 is connected in parallel to the DC power supply 41. The ammeter 42 detects the current flowing through the cell connected to this Ch. The voltmeter 43 detects the voltage between the terminals T1 and T2.

[0028] The connection portion 120 functions as a female connector that can be attached to the connector 30. In S16 of FIG. 1, the inspection device 100 connects the connection portion 120 to the connector 30. The inspection device 100 may include a robot that connects connectors. Each cell included in the stack 10 is connected to the power supply unit 110 of the inspection device 100 by connecting a corresponding terminal (e.g., female terminal) of the connection portion 120 to each terminal (e.g., male terminal) of the connector 30. This forms an inspection circuit for each cell. The inspection circuits of adjacent cells C (energy storage cells) have a common wiring portion. Each inspection circuit is a closed circuit including a cell C, a channel (Ch), and a common wiring. In FIG. 3, Ch1, Ch2, Ch3, Ch4, Ch5, and Ch6 are Chs connected to cells C-1, C-2, C-3, C-4, C-5, and C-6, respectively.

[0029] In the individual charging of each cell (S16), a power source (DC power source 41) connected to the corresponding cell applies a voltage to the corresponding cell through the voltage detection terminal 20. This allows power to be supplied to each cell from the corresponding power source (DC power source 41) through the corresponding voltage detection terminal 20. Using the voltage detection terminal 20 makes it possible to supply power (current) to each cell individually. This charging brings all cells included in the stack 10 to a fully charged state. Charging each cell individually makes it easier to bring the SOC (State of Charge) of each cell closer to 100%. The SOC indicates the ratio of the current amount of charge to the amount of charge in a fully charged state. For example, all cells may be charged simultaneously. However, this is not limited to this, and some cells may be charged preferentially. For example, individual charging may be performed on either the odd-numbered cells or the even-numbered cells, and then the other cells. The odd-numbered cells are the odd-numbered cells C (cells C-1, C-3, ...) counting from the positive electrode end of the stack 10. The even-numbered cells are the even-numbered cells C (cells C-2, C-4, . . . ) from the end of the stack 10 on the positive electrode side.

[0030] 1, the inspection device 100 monitors the state of each cell included in the stack 10. Specifically, the inspection device 100 measures the voltage of each cell, and records the measured value (measured cell voltage) for each cell in the storage device of the control device 150, linking it to the time of acquisition. The inspection device 100 may acquire the cell voltage based on the voltage detection result by the voltmeter 43. Note that, among the multiple cells included in the stack 10, cells for which a self-discharge test has not yet begun are left uncharged in S171.

[0031] In the next step S172, the inspection device 100 calculates |d 2 V / dt 2 Calculate | and calculate |d 2 V / dt 2 | is 0.02mV / h 2 Determine whether or not |d 2 V / dt 2 | corresponds to the second derivative of the cell voltage. More specifically, |d 2 V / dt2 | is the absolute value obtained by second-order differentiation of the cell voltage after charging is completed with respect to time.

[0032] In the laminate 10, |d 2 V / dt 2 | is 0.02mV / h 2 If there is no cell that is determined to be equal to or smaller than the number of cells, the determination in S172 is NO, and the process returns to S171. 2 V / dt 2 | is 0.02mV / h 2 While the charge current is greater than |d, each cell is left in an uncharged state, and steps S171 and S172 are repeated. 2 V / dt 2 | is 0.02mV / h 2 It is determined whether the following occurs:

[0033] On the other hand, |d of any cell 2 V / dt 2 | is 0.02mV / h 2 If the result is YES in S172, the inspection device 100 performs the following in S18: |d 2 V / dt 2 | is 0.02mV / h 2 A self-discharge test is performed on one or more cells that have reached the following conditions. Specifically, a power supply (DC power supply 41) connected to the cell being tested applies a voltage to the cell through the voltage detection terminal 20. In this embodiment, the DC power supply 41 applies a power supply voltage to the cell that is the same magnitude as the cell voltage at the start of the test but in the opposite direction. The control device 150 determines whether the current of the cell being tested has converged, and for a cell for which it is determined that the current has converged, estimates the converged current value as the self-discharge current. The control device 150 may determine that the current has converged when the amount of change in current per unit time is equal to or less than a predetermined value. However, this is not limited to this, and any method for determining convergence may be used. The control device 150 may also determine whether the cell is good or bad based on the estimated self-discharge current of the cell. For example, the control device 150 determines that a cell whose self-discharge current is equal to or greater than a predetermined reference value (hereinafter referred to as "Is") is a short-circuited cell (defective).

[0034] As described above, in this embodiment, |d 2 V / dt 2 | is 0.02mV / h 2 The self-discharge test starts in order from the cells that are judged to be below |d 2 V / dt 2 | is 0.02mV / h 2 If a cell (hereinafter referred to as "adjacent cell") located next to a cell (hereinafter referred to as "target cell") that has been determined to be below 100% is currently being tested, the test of the target cell may not be started immediately, but may be started after the test of the adjacent cell is completed. By doing so, the test circuit of the target cell will not be affected by the test circuit of the adjacent cell in the self-discharge test, making it easier to measure the current of the target cell with high accuracy. However, this is not limited to this, and it is also possible to simultaneously test the self-discharge of the target cell and the adjacent cell. All cells' |d 2 V / dt 2 | is 0.02mV / h 2 After this, the self-discharge test for all cells may be started simultaneously.

[0035] In S18, |d 2 V / dt 2 | is 0.02mV / h 2 A self-discharge test is performed on cells that meet the criteria below and for which the self-discharge test has not yet been completed. In the following S19, the test device 100 determines whether the self-discharge test has been completed for all cells included in the stack 10. While the self-discharge test has not been completed for any cell, the determination in S19 is NO, and S171, S172, S18, and S19 are repeated. Then, when the self-discharge test has been completed for all cells (YES in S19), the processing flow shown in FIG. 1 ends.

[0036] The inspection device 100 may determine that the stack 10 is defective if the stack 10 includes at least one short-circuited cell. The inspection system may terminate the processing flow shown in Fig. 1 for the stack 10 when it is determined that the self-discharge current of at least one cell included in the stack 10 is equal to or greater than Is, and may start the processing flow shown in Fig. 1 for the next inspection target (secondary battery). In this way, by terminating the inspection of a certain secondary battery when the pass / fail determination result is known and moving on to the inspection of the next secondary battery, the efficiency of the inspection can be improved.

[0037] Hereinafter, the operation and effect of the method for manufacturing a secondary battery according to this embodiment (method according to the example) will be described in comparison with a method according to a comparative example. In the method according to the example, the above-described processing flow shown in FIG. 1 was adopted. In contrast, in the method according to the comparative example, a processing flow was adopted in which S171 and S172 were omitted from the processing flow shown in FIG. 1. Therefore, in the method according to the comparative example, the |d 2 V / dt 2 | is 0.02mV / h 2 before (i.e., |d 2 V / dt 2 A self-discharge test was initiated (when | was large).

[0038] 4 is a diagram showing data obtained by a method according to an example and data obtained by a method according to a comparative example. Lines L11 to L13 are data related to the method according to the example. Lines L21 to L23 are data related to the method according to the comparative example.

[0039] In both the example and comparative example, after individual charging was completed, the cell voltage dropped as shown by line L1. Line L1 shows the change in cell voltage after individual charging (e.g., S16 in Figure 1) was completed. For line L1, the vertical axis represents cell voltage, and the horizontal axis represents the elapsed time from the end of individual charging. As shown by line L1, there was a large change in the slope of the cell voltage immediately after the end of individual charging. This is presumably due to voltage relaxation immediately after charging.

[0040] In the method according to the comparative example, the self-discharge test was started immediately after the individual charging was completed (hereinafter referred to as "t1"). In contrast, in the method according to the example, the self-discharge test was started later than t1 (hereinafter referred to as "t2"). More specifically, t2 is the time when |d 2 V / dt 2 | is 0.02mV / h 2 Line L2 in FIG. 4 shows the transition of the absolute value (i.e., |dV / dt|) obtained by differentiating the cell voltage shown by line L1 with respect to time. |dV / dt| corresponds to the first-order differential value of the cell voltage. Line L3 in FIG. 4 shows the transition of the absolute value (i.e., |d 2 V / dt 2 At t2, |d 2 V / dt 2 |(second-order differential value of cell voltage) is 0.02mV / h 2 It was as follows.

[0041] In both the example and comparative example, an inspection circuit (a closed circuit including a power supply) is formed for each cell (see FIG. 3). The inspection circuit for each cell has the same configuration as circuit 300 shown in FIG. 4. Circuit 300 includes a battery circuit 310 and a power supply circuit 320. Battery circuit 310 corresponds to an equivalent circuit model of one cell. Between terminals B1 and B2 of battery circuit 310, there are an electromotive element 311, a short-circuit resistor 312 connected in parallel to electromotive element 311, and an internal resistor 313 connected in series to electromotive element 311. The electromotive force of electromotive element 311 (hereinafter referred to as "Vcell") decreases due to cell self-discharge. The cell self-discharge current (hereinafter referred to as "Icell") flows through short-circuit resistor 312. The smaller the resistance value (hereinafter referred to as "Rp") of short-circuit resistor 312, the larger Icell. The terminal voltage (hereinafter referred to as "VB") of the battery circuit 310 corresponds to the potential difference (cell voltage) between the positive and negative electrodes of the cell. The power supply circuit 320 includes a DC power supply 321 and a circuit resistor 322. The DC power supply 321 outputs a voltage (hereinafter referred to as "VS"). The resistance value (hereinafter referred to as "Rext") of the circuit resistor 322 is, for example, the total value of parasitic resistance present in the entire circuit. The parasitic resistance includes wiring resistance (the electrical resistance of each conductor that makes up the circuit) as well as contact resistance. Hereinafter, the current that flows through the circuit 300 due to VS will be referred to as "IB."

[0042] In both the example and the comparative example, during the self-discharge test, the DC power supply 41 of the corresponding channel continued to apply a power supply voltage to the cell that was the same magnitude as the cell voltage at the start of the test but in the opposite direction. That is, the power supply voltage was kept constant during the test (see lines L11 and L21). When the output voltage (VS) of the DC power supply 321 is constant in this way, the formula "IB=(VS-VB) / Rext" holds.

[0043] The self-discharge test begins with the application of the power supply voltage. At the start of the test, the cell voltage (VB) and the power supply voltage (VS) are the same. Therefore, the circuit current (IB) becomes zero. After that, as VB decreases due to cell self-discharge, IB increases. When IB increases and becomes equal to the self-discharge current (Icell), the decrease in VB stops and the cell voltage becomes constant. The increase in IB also stops and IB converges. The converged value of IB indicates the self-discharge current. Based on this principle, the self-discharge current can be obtained from the current value (IB).

[0044] However, in the comparative example, VB (line L22) and IB (line L23) behaved differently from the above principle. The variation in IB is due to the slope change rate of VB (|d 2 V / dt 2 This is presumably due to the large difference in current values. Such variations in current values ​​lead to a decrease in the accuracy of the self-discharge test. In contrast, in the method according to the embodiment, VB (line L12) and IB (line L13) behaved in accordance with the above principle. Therefore, the method according to the embodiment makes it easier to measure the self-discharge current of each cell with high accuracy. Note that for lines L11 to L13 and lines L21 to L23, the horizontal axis indicates the elapsed time from the start of the test.

[0045] As described above, the method for manufacturing a secondary battery according to this embodiment includes the steps shown in Fig. 1. In S16, the inspection system charges the cell C (secondary battery). After charging the cell C, the inspection system performs steps S171 and S172 to determine whether |d 2 V / dt 2 | is 0.02mV / h 2 Leave cell C uncharged until it reaches the value below |d 2 V / dt 2 | is 0.02mV / h 2 If the following condition is met, the inspection system performs a self-discharge inspection on cell C in step S18. This method makes it possible to properly inspect the self-discharge after charging the secondary battery, and also makes it easier to manufacture high-quality secondary batteries with high efficiency.

[0046] The laminate 10 determined to be a non-defective product by the above inspection can function as a bipolar secondary battery by itself. However, a bipolar secondary battery may be manufactured by combining a plurality of modules, each of which is the laminate 10 as one module. The manufactured secondary battery may be mounted on a mobile object. Examples of mobile objects include automobiles (electric vehicles, hybrid vehicles, etc.), vehicles other than automobiles, and mobile machinery (agricultural machinery, construction machinery, etc.). However, the battery may be used for any purpose, and a stationary battery may be manufactured by the above method.

[0047] The processing flow shown in Fig. 1 can be modified as appropriate. For example, the order of processing may be changed, or the content of any of the processing may be changed, depending on the purpose. Fig. 5 is a flowchart showing a modification of the processing flow shown in Fig. 1. The processing flow shown in Fig. 5 is the same as the processing flow shown in Fig. 1, except that S16 (Fig. 1) is omitted and S14A is adopted instead of S14 (Fig. 1).

[0048] Referring to FIG. 5, in S14A, the test system individually charges each cell during high-temperature aging. The conditions for high-temperature aging and individual charging in S14A may be the same as those in S14 and S16 in FIG. 1, respectively. In this way, by individually charging each cell during high-temperature aging, the test system can reduce the time from the end of individual charging to |d 2 V / dt 2 | is 0.02mV / h 2 This can reduce the time it takes to:

[0049] FIG. 6 is a diagram for explaining the operation and effect of the method according to the modified example shown in FIG. 5. In the graph shown in FIG. 6, the horizontal axis indicates the start timing of the self-discharge test, and the vertical axis indicates the current value (IB) six hours after the start of the self-discharge test. Data D1 shows an example of data when each cell is individually charged during high-temperature aging. Data D2 shows an example of data when each cell is individually charged at room temperature. From the end of charging |d 2 V / dt 2 | is 0.02mV / h 2The time required to reach the values ​​below was shorter when the cells were charged individually during high-temperature aging than when the cells were charged individually at room temperature.

[0050] In the above embodiment, the self-discharge current of the secondary battery is estimated based on the convergence value of the current after charging the secondary battery. However, the method for measuring the self-discharge current in the self-discharge test is not limited to this method. For example, the amount of voltage drop caused by self-discharge of the secondary battery may be measured, and the self-discharge current of the secondary battery may be estimated based on the measured amount of voltage drop. The type of secondary battery to be tested is not limited to an LFP battery, and any type may be used. The secondary battery to be tested may be a monopolar secondary battery.

[0051] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0052] 1 Bipolar electrode, 10 Laminate, 11 Current collector, 20 Voltage detection terminal, 30 Connector, 41 DC power supply, 42 Ammeter, 43 Voltmeter, 100 Inspection device, 110 Power supply unit, 120 Connection unit, 150 Control device, C Cell.

Claims

1. Charging the secondary battery; After the charging of the secondary battery is completed, the voltage V of the secondary battery is second-order differentiated with respect to time t, and |d ２ V / dt ２ | is 0.02 mV / h ２ leaving the secondary battery without charging it until: |d ２ V / dt ２ | is 0.02 mV / h ２ performing a self-discharge test on the secondary battery when: A method for manufacturing a secondary battery, comprising:

2. the secondary battery is a bipolar secondary battery including a plurality of storage cells, While the secondary battery is left unused, |d ２ V / dt ２ | is 0.02 mV / h ２ Determine whether or not |d ２ V / dt ２ | is 0.02 mV / h ２ The method for manufacturing a secondary battery according to claim 1 , wherein a self-discharge test is initiated for a storage cell determined to be below the threshold.

3. 3. The method for manufacturing a secondary battery according to claim 2, wherein in the self-discharge test of the secondary battery, it is determined whether or not a current has converged for each of the plurality of storage cells, and for a storage cell for which it is determined that the current has converged, the converged current value is estimated to be the self-discharge current.

4. Each of the plurality of storage cells has a voltage detection terminal configured to be connectable to an external power supply, the method for manufacturing a secondary battery further includes connecting a power source to the voltage detection terminal of each of the plurality of storage cells; 4. The method for manufacturing a secondary battery according to claim 3, wherein, in each of the charging and self-discharge tests of the secondary battery, the power source connected to each of the plurality of storage cells applies a voltage to the corresponding storage cell through the voltage detection terminal.

5. 5. The method for manufacturing a secondary battery according to claim 4, wherein in the self-discharge test of the secondary battery, a power supply voltage having the same magnitude as the voltage of the storage cell at the start of the test but in a reverse direction is applied to the storage cell.

Images