Secondary battery manufacturing method

Abstract

A method for manufacturing a secondary battery that allows for appropriate self-discharge testing of the secondary battery is provided. A method for manufacturing a secondary battery includes preparing a secondary battery and starting a self-discharge test of the secondary battery when the |dV / dt| of the secondary battery during self-discharge is greater than a reference value (Th2) (S17, S18, S21). The method for manufacturing a secondary battery may further include adjusting the amount of charge stored in the secondary battery so that the |dV / dQ| of the secondary battery is greater than a threshold value (Th1) (S12, S14, S15).

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, in Patent Document 1, temperature changes during self-discharge testing are not sufficiently considered. With the technology described in Patent Document 1, there is a risk that testing accuracy will decrease if the environmental temperature changes during self-discharge testing.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to appropriately perform a self-discharge test on 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: preparing a secondary battery; and starting a self-discharge test of the secondary battery in a state where |dV / dt|, which is an absolute value obtained by differentiating a voltage V of the secondary battery with respect to time t during self-discharge, is greater than a reference value. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to appropriately perform a self-discharge test on 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] 10 is a flowchart illustrating a method according to a comparative example. [Figure 5] FIG. 1 is a diagram showing data obtained by methods according to an example and a comparative example. [Figure 6] FIG. 10 is a diagram showing data obtained when the environmental temperature is changed in the methods according to the example and the comparative example. [Figure 7] FIG. 2 is a diagram showing a modification of the flowchart shown in 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 olivine-type LFP battery (a lithium-ion secondary battery containing olivine-type 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 next step S14, the test system individually charges each cell while aging the stack 10 at the aging temperature (hereinafter referred to as "high-temperature aging"). Specifically, the test system performs CV (Constant Voltage) charging for each cell until |dV / dQ| becomes greater than a threshold value (hereinafter referred to as "Th1"). |dV / dQ| is the absolute value obtained by differentiating the cell voltage with respect to the amount of charge stored. The constant voltage applied to each cell during CV charging may be 3.7 V or higher. Th1 is set so that |dV / dt| of each cell during self-discharge becomes greater than a reference value (hereinafter referred to as "Th2") after charging of each cell (more specifically, after cooling). |dV / dt| is the absolute value obtained by differentiating the cell voltage with respect to time during self-discharge. For example, Th2 may be determined first, and then Th1 may be determined based on Th2. The CV charging voltage, Th1, and Th2 are set according to the characteristics of cell C.

[0024] The processing from S14 onward will be described below with further reference to FIG. 3. FIG. 3 is a diagram for explaining the charging of each cell and the self-discharge inspection of the charged cell. As shown in FIG. 3, a connector 30 is provided for a plurality of voltage detection terminals 20 (see FIG. 2) connected to the laminate 10. The voltage detection terminals 20 are welded, for example, to the end of the current collector 11 on the +X side. 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 resin part 31 connecting the end face on the +X side of the laminate 10 to the housing 32 is formed by injection molding, with the housing 32 for aligning the voltage detection terminals 20 attached to the tip of the voltage detection terminal 20. This forms the connector 30 joined to the laminate 10. The plurality of voltage detection terminals 20 are configured to be connectable to an external power source (e.g., a DC power source 41). Each of the plurality of 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).

[0025] 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.

[0026] 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.

[0027] The connection portion 120 functions as a female connector that can be attached to the connector 30. In S14 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.

[0028] During the individual charging of each cell during high-temperature aging (S14), a power supply (DC power supply 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 supply (DC power supply 41) through the corresponding voltage detection terminal 20. Using the voltage detection terminal 20 makes it possible to individually supply power (current) to each cell. Charging each cell individually facilitates accurate adjustment of the SOC (State of Charge) of each cell. SOC indicates the amount of stored power, and is, for example, the ratio of the current amount of stored power to the amount of stored power in a fully charged state, expressed as a percentage from 0 to 100%. 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.

[0029] In the next step S15, the control device 150 determines whether the |dV / dQ| of all the cells has become greater than Th1. While the |dV / dQ| of any cell is equal to or less than Th1 (NO in S15), CV charging (S14) is performed on the cells whose |dV / dQ| is equal to or less than Th1. The inspection device 100 stops charging the cells whose |dV / dQ| is greater than Th1. This stops charging before the cells reach a fully charged state, preventing overcharging. Then, when the |dV / dQ| of all the cells becomes greater than Th1 (YES in S15), the inspection system cools the stack 10 in S16. Specifically, the inspection system lowers the ambient temperature to room temperature. This completes high-temperature aging. The device (e.g., a thermostatic chamber) used to keep the stack 10 at a high temperature is removed from the stack 10.

[0030] In the next step S17, the control device 150 determines whether the |dV / dt| of each cell during self-discharge is greater than a reference value (Th2). The control device 150 may acquire the cell voltage based on the voltage detection result of the voltmeter 43 provided in each cell. In this embodiment, the control device 150 determines Th2 so that the voltage of each cell does not increase during the self-discharge test if the room temperature is within a predetermined temperature range. That is, if the |dV / dt| during self-discharge for each cell is greater than Th2 at the start of the self-discharge test (S21), the voltage of each cell will continue to decrease unless the room temperature fluctuates beyond the predetermined temperature range during the test. Such Th2 may be determined in advance by experiment or simulation and stored in the storage device of the control device 150. The predetermined temperature range is set, for example, around 25°C. The predetermined temperature range may be 15°C to 40°C or a range narrower than the range of 15°C to 40°C (any range within the range).

[0031] The control device 150 may store cell information (e.g., a formula or a map) in advance, which indicates the relationship between |dV / dQ| during charging of the cell C and |dV / dt| during self-discharge after charging. This cell information may be obtained in advance by experiment or simulation. Using the cell information stored in the storage device, the control device 150 determines the |dV / dQ| corresponding to Th2 determined as described above as Th1. That is, if the |dV / dQ| of each cell is greater than Th1 as a result of the individual charging of each cell (S14), the calculation will determine YES in S17. However, there is a possibility that the determination in S17 will be NO due to the calculation accuracy of the cell information and Th2 or due to individual differences between the cells C. Therefore, in this embodiment, if the determination in S17 is NO, the process proceeds to S18. In S18, the testing device 100 adjusts the SOC of each cell. Specifically, for cells for which |dV / dt| during self-discharge does not exceed Th2, the testing device 100 performs additional charging so that |dV / dt| during self-discharge exceeds Th2. After that, the process returns to S17, and YES is determined in S17.

[0032] If the determination in S17 is YES, the process proceeds to S21. In S21, the inspection device 100 performs a self-discharge inspection on each cell in a room temperature environment.

[0033] Specifically, a power supply (DC power supply 41) connected to the cell to be inspected 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 inspection but in the opposite direction. The control device 150 determines whether the current of the inspected cell 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 limiting, 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] Next, in S22, the inspection device 100 determines whether the self-discharge test has been completed for all cells included in the stack 10. While the self-discharge test for any cell is incomplete, the determination in S22 is NO, and S21 and S22 are repeated. Then, when the self-discharge test for all cells is completed (YES in S22), the processing flow shown in FIG. 1 ends.

[0035] 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.

[0036] As described above, by measuring the convergence value of a minute leakage current (for example, a current of several μA to several hundred μA) of a secondary battery, the self-discharge current of the secondary battery can be measured with high accuracy in a short time. However, minute leakage currents are prone to fluctuate due to temperature changes. Below, the functions and effects 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. The method according to the example employs the above-described process flow shown in FIG. 1. Meanwhile, FIG. 4 is a flowchart showing a method according to a comparative example. As shown in FIG. 4, the method according to the comparative example employs a process flow in which S15, S17, and S18 are omitted from the process flow shown in FIG. 1.

[0037] 5 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 L15 each represent data related to the method according to an example. Lines L21 to L25 each represent data related to the method according to a comparative example.

[0038] Hereinafter, in a graph showing the charging characteristics of cell C (olivine-type LFP battery), specifically, in a graph with the OCV (open circuit voltage) of cell C on the vertical axis and the SOC of cell C on the horizontal axis, the SOC region above the region lower limit (hereinafter referred to as "P1") and below the region upper limit (hereinafter referred to as "P2") is referred to as "region Rx." In region Rx, |dV / dQ| is below a predetermined value, and the voltage of cell C changes little even when the amount of charge stored in cell C changes. On the other hand, in both the region where the SOC is below P1 and the region where the SOC is above P2, the slope of the graph (|dV / dQ|) is larger than in region Rx. In the region where the SOC is below P1, |dV / dQ| increases as the amount of charge stored decreases. Furthermore, in the region where the SOC is above P2, |dV / dQ| increases as the amount of charge stored increases. |dV / dQ| is the absolute value obtained by differentiating the voltage (e.g., OCV) of cell C by the amount of charge (e.g., SOC). |dV / dQ| corresponds to the ratio of the change in the voltage of cell C to the change in the amount of charge of cell C. |dV / dQ| in region Rx is small.

[0039] The larger the |dV / dQ| of cell C, the larger the |dV / dt| of cell C during self-discharge. Therefore, in the region where the SOC of cell C exceeds P2, the larger the amount of charge stored in cell C, the larger the |dV / dQ| of cell C during self-discharge. |dV / dt| is the absolute value obtained by differentiating the cell voltage with respect to time.

[0040] In the comparative example, each cell was individually charged without considering the |dV / dt| of each cell during self-discharge. Specifically, in the comparative example, the individual charging was terminated before the SOC of each cell reached P2 to prevent overcharging of each cell. Then, the self-discharge test was started when the SOC of each cell was in region Rx.

[0041] In contrast, in the method according to the embodiment, each cell was individually charged so that the SOC of each cell exceeded P2 and the |dV / dQ| of each cell was greater than Th1. However, the individual charging of each cell was stopped before the corresponding cell reached a fully charged state. This prevented overcharging of each cell. In the method according to the embodiment, after the individual charging of each cell, the self-discharge test was started when the |dV / dt| of each cell during self-discharge was greater than Th2.

[0042] 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. 5. 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."

[0043] In both the self-discharge tests of the example and the comparative example, the DC power supply 41 (FIG. 3) of the corresponding channel continuously applied 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). The DC power supply 41 shown in FIG. 3 functions as the DC power supply 321 in the circuit 300. When the output voltage (VS) of the DC power supply 321 is constant, the equation "IB=(VS-VB) / Rext" holds.

[0044] 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).

[0045] However, in both the examples and comparative examples, the self-discharge test was performed in a room temperature environment. The stack 10 connected to the power supply unit 110 of the test device 100 was placed in a room temperature environment without temperature control by an external device (e.g., a thermostatic chamber). This reduces the cost of temperature control. However, the room temperature may fluctuate during the test. If the room temperature (environmental temperature) fluctuates, the IB may behave differently from the above principle.

[0046] For lines L11 to L13 and lines L21 to L23, the horizontal axis indicates the elapsed time from the start of the test. In the comparative example, |dV / dt| of each cell at the start of the test was approximately 0.3 mV / day (the change in cell voltage per day). When the ambient temperature did not change, VB (line L22) and IB (line L23) exhibited behavior in accordance with the above principle. If the ambient temperature did not change, |dV / dt| would likely remain almost unchanged from its value at the start of the test. On the other hand, when the ambient temperature changed, the behavior of VB (line L24) and IB (line L25) changed significantly. As shown by line L25, IB exhibited behavior that differed from the above principle. It is presumed that this variation in IB is due to fluctuations in VB caused by temperature changes. Variations in current values ​​reduce the accuracy of self-discharge tests. In the comparative example, |dV / dt| was small at the start of the test, which likely made IB more susceptible to temperature fluctuations.

[0047] In contrast, with the method according to the embodiment, the |dV / dt| of each cell at the start of testing was approximately 9 mV / day (the amount of change in cell voltage per day). If |dV / dt| is sufficiently large when there is no temperature change during self-discharge, the change in VB due to temperature change becomes relatively small and is considered to be negligible. With the method according to the embodiment, both VB (line L12) and IB (line L13) when the ambient temperature does not change, and VB (line L14) and IB (line L15) when the ambient temperature changes, exhibited behavior in accordance with the above principle. The method according to the embodiment makes it easier to measure the self-discharge current of each cell with high accuracy.

[0048] FIG. 6 is a diagram showing data obtained when the environmental temperature is changed in the methods according to the example and the comparative example.

[0049] For lines L31, L32 and lines L41, L42, the horizontal axis represents the elapsed time from the start of the test. Line L41 represents the transition of the amount of change in VB when the room temperature (environmental temperature) changes as shown by line L42 in the method of the comparative example. In the method of the comparative example, the change in VB (line L42) changed from a decrease to an increase during the test due to the change in the environmental temperature. In contrast, line L31 represents the transition of the amount of change in VB when the room temperature (environmental temperature) changes as shown by line L32 in the method of the example. In the method of the example, VB continued to decrease (line L32) even when the environmental temperature changed during the test.

[0050] For lines L33 to L35 and lines L43 to L45, the horizontal axis indicates the elapsed time from the timing when the room temperature (ambient temperature) started to change. Lines L43 and L44 respectively indicate the transition of the IB of a good cell and a shorted cell (defective) when the room temperature (ambient temperature) changed as shown by line L45 in the method of the comparative example. In the method of the comparative example, when the ambient temperature fluctuated, the IB of the shorted cell (line L44) became smaller than the IB of the good cell (line L43), albeit temporarily. This IB behavior reduces the accuracy of the self-discharge test. In contrast, lines L33 and L34 respectively indicate the transition of the IB of a good cell and a shorted cell (defective) when the room temperature (ambient temperature) changed as shown by line L35 in the method of the example. In the method of the example, the IB of the shorted cell (line L34) never became smaller than the IB of the good cell (line L33), even during periods when the ambient temperature fluctuated. This method makes it possible to measure the self-discharge current of each cell with high accuracy.

[0051] As described above, the method for manufacturing a secondary battery according to this embodiment includes the processes shown in FIG. 1. Specifically, the manufacturing system prepares a cell C (secondary battery) (see FIGS. 2 and 3). The manufacturing system adjusts the amount of charge stored in cell C so that |dV / dQ| of cell C is greater than a threshold value (Th1) (S12, S14, S15). After adjusting the amount of charge stored in cell C, the manufacturing system starts a self-discharge test for cell C when |dV / dt| of cell C during self-discharge is greater than a reference value (Th2) (S17, S18, S21). This method makes it possible to measure the self-discharge current of each cell with high accuracy (see FIGS. 5 and 6). This method also eliminates the need to cover the secondary battery with a thermal insulator or wait until the temperature of the secondary battery stabilizes. This facilitates the efficient manufacturing of high-quality secondary batteries.

[0052] 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.

[0053] The processing flow shown in Fig. 1 can be modified as needed. For example, the order of processing may be changed or the content of any of the processing may be changed depending on the purpose.

[0054] The inspection system may execute the process flow shown in FIG. 7 instead of the process flow shown in FIG. 1. FIG. 7 is a diagram showing a modification of the flowchart shown in FIG. 1. In the process flow shown in FIG. 7, S15 shown in FIG. 1 is omitted, and S14A is used instead of S14 (FIG. 1). In S14A, high-temperature aging of the stack 10 is performed, similar to S14. However, individual charging does not have to be performed in S14A. The process of adjusting the amount of charge stored in each cell (e.g., charging or discharging each cell) may be performed in at least one of steps S13, S14A, and S16. In the process flow shown in FIG. 7, if the |dV / dt| of each cell during self-discharge is not greater than the reference value (Th2) (NO in S17), the process of adjusting the amount of charge stored in each cell (e.g., charging or discharging each cell) is performed in S18. In addition, in a case where the amount of stored power can be reliably adjusted by prior adjustment of the amount of stored power (at least one of S12, S13, S14A, and S16), the determination process (S17) may be omitted. The process of adjusting the amount of stored power in each cell may be performed after each cell has cooled (before the self-discharge test).

[0055] In the above embodiment, the amount of charge stored in each cell is adjusted by charging so that the |dV / dQ| of each cell is greater than Th1. However, this is not limiting, and the amount of charge stored in each cell may be adjusted by discharging so that the |dV / dQ| of each cell is greater than Th1. For example, for cell C having the aforementioned region Rx, a self-discharge test may be performed in a region where the SOC is higher than 0% and lower than P1.

[0056] In the above embodiment, the self-discharge current of the secondary battery is estimated based on the convergence value of the current of the secondary battery during self-discharge. 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.

[0057] 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]

[0058] 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. providing a secondary battery; starting a self-discharge inspection of the secondary battery in a state where |dV / dt|, which is an absolute value obtained by differentiating a voltage V of the secondary battery during self-discharge with respect to time t, is greater than a reference value; A method for manufacturing a secondary battery, comprising:

2. 2. The method for manufacturing a secondary battery according to claim 1, further comprising: adjusting the amount of charge stored in the secondary battery so that |dV / dQ|, which is an absolute value obtained by differentiating a voltage V of the secondary battery with a stored amount Q, is greater than a threshold value before starting the self-discharge test.

3. 3. The method for manufacturing a secondary battery according to claim 2, wherein the adjustment of the stored charge amount of the secondary battery involves charging the secondary battery so that |dV / dQ| of the secondary battery is greater than the threshold value, and stopping the charging before the secondary battery reaches a fully charged state.

4. The self-discharge test of the secondary battery is performed in a room temperature environment, the reference value is determined so that the voltage of the secondary battery does not increase during a self-discharge test if the room temperature is within a predetermined temperature range; The method for manufacturing a secondary battery according to claim 2 , wherein the threshold value is determined based on the reference value.

5. the secondary battery is a bipolar secondary battery including a plurality of storage cells, 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; In adjusting the amount of stored power in the secondary battery, the power source connected to each of the plurality of power storage cells applies a voltage to the corresponding power storage cell through the voltage detection terminal, thereby charging the corresponding power storage cell; The self-discharge inspection of the secondary battery includes: the power supply connected to each of the plurality of storage cells applies a power supply voltage having the same magnitude as a voltage of the corresponding storage cell at the start of inspection but in a reverse direction to the corresponding storage cell through the voltage detection terminal; determining whether or not a current has converged for each of the plurality of power storage cells; For the storage cell whose current has been determined to have converged, estimating the converged current value as a self-discharge current; The method for producing the secondary battery according to any one of claims 2 to 4, comprising:

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