Manufacturing method of secondary batteries

Abstract

To improve the accuracy of short-circuit detection. [Solution] The method for manufacturing a secondary battery includes the steps of: injecting electrolyte (S11); performing initial charging (S12); raising the temperature of the laminate (S13); performing CCCV charging on odd-numbered cells (S14); performing CCCV charging on even-numbered cells (S15); performing CC charging on odd-numbered cells (S16); cooling the laminate (S17); and performing a self-discharge test (S18).

Description

Technical Field

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

Background Art

[0002] Japanese Patent Application Laid-Open No. 2019-021513 (Patent Document 1) discloses a secondary battery including one or more cells including an electrode laminate in which a plurality of bipolar electrodes are laminated via a separator.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As a method for manufacturing a bipolar type secondary battery in which a plurality of bipolar electrodes and separators are laminated to form a plurality of cells as described above, a high-temperature aging process for the purpose of dissolving foreign matter or forming a film is included. This high-temperature aging process includes a process of charging all cells to a specified voltage.

[0005] However, in the process of charging all cells to a specified voltage, when charging is performed individually for the cell group arranged in the even number and the cell group arranged in the odd number, since the charging timings are different, the slopes of the voltage changes at the time of determining the quality of the cells after charging are different, so further improvement in the accuracy of the quality determination is required.

[0006] The present disclosure has been made to solve the above-described problems, and an object thereof is to provide a method for manufacturing a secondary battery that improves the accuracy of determining the quality of cells.

Means for Solving the Problems

[0007] A method for manufacturing a secondary battery according to a certain aspect of the present disclosure is a method for manufacturing a secondary battery comprising a plurality of cells composed of bipolar electrodes and separators. The plurality of cells includes an odd-numbered cell group arranged from one end to an odd-numbered position and an even-numbered cell group arranged from the other end to an even-numbered position. This manufacturing method includes the steps of performing constant voltage charging by alternately changing the target of charging for the odd-numbered cell group and the even-numbered cell group before determining the quality of the cells, and performing constant current charging so that the voltage of the cell group that has been charged with constant voltage first matches the voltage of the other cell group.

[0008] In this way, by performing constant-current charging on one of the cell groups that has already undergone constant-voltage charging to increase its voltage and match it with the voltage of the other cell group, it is possible to perform cell quality judgment at a high voltage for both odd-numbered and even-numbered cell groups. Therefore, it is possible to improve the accuracy of cell quality judgment while shortening the lead time required for judgment.

[0009] In this embodiment, the manufacturing method further includes the step of setting at least one of the execution time of constant current charging and a voltage threshold for terminating constant current charging, using the first rate of voltage decrease of the other cell group and the second rate of voltage decrease of the one cell group after constant current charging.

[0010] In this way, by appropriately setting the execution time and voltage threshold, the voltages of odd-numbered and even-numbered cell groups can be matched before determining whether the cells are good or bad. Therefore, the accuracy of the cell quality determination can be improved. [Effects of the Invention]

[0011] According to this disclosure, it is possible to provide a method for manufacturing secondary batteries that improves the accuracy of determining whether a cell is good or bad. [Brief explanation of the drawing]

[0012] [Figure 1]This flowchart shows the processing procedure for manufacturing a secondary battery according to this embodiment. [Figure 2] This is a cross-sectional view showing a laminate constituting a secondary battery according to this embodiment. [Figure 3] This diagram illustrates the charging process for each cell and the self-discharge test of the cells after charging. [Figure 4] This flowchart shows an example of a secondary battery manufacturing method in a comparative example. [Figure 5] This figure shows an example of the voltage changes in odd-numbered and even-numbered cells in a comparative example. [Figure 6] This figure shows an example of the voltage changes in odd-numbered and even-numbered cells in this embodiment. [Modes for carrying out the invention]

[0013] The embodiments of this disclosure will be described in detail below with reference to the drawings. The same or corresponding parts in the drawings will be denoted by the same reference numerals, and their descriptions will not be repeated. In the drawings used below, of the mutually orthogonal X, Y, and Z axes, the Z axis indicates the thickness direction of the battery. Hereinafter, the direction indicated by the arrows on the X, Y, and Z axes will be denoted by "+", and the opposite direction by "-".

[0014] Figure 1 is a flowchart showing the processing procedure for the secondary battery manufacturing method according to this embodiment. In the secondary battery manufacturing method according to this embodiment, first, a laminate is formed. Then, according to the processing flow shown in Figure 1, the laminate is subjected to liquid injection, charging, aging, and self-discharge testing. Details of the processing flow shown in Figure 1 will be described later.

[0015] In this embodiment, a laminate 10 shown in Figure 2 is prepared. Figure 2 is a cross-sectional view showing the laminate 10 constituting the secondary battery according to this embodiment. Referring to Figure 2, the laminate 10 comprises a power storage section 10a and a sealing section 3 that seals the power storage section 10a. The Z direction corresponds to the stacking direction. The power storage section 10a includes a plurality of cells C arranged in the Z direction. Each of the plurality of cells C comprises a negative electrode active material layer 12A, a positive electrode active material layer 12B, and a separator 13. Each of the plurality of cells C is configured to store energy. Each of the plurality of cells C functions as a secondary battery. Each of the plurality of cells C corresponds to an example of a "cell" according to this disclosure. In this embodiment, the power storage section 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 section 10a may be 3 or more but less than 50, or 50 or more. The sealing section 3 is formed to surround the power storage section 10a.

[0016] The laminate 10 includes a plurality of electrodes (one negative terminal electrode 2A, a plurality of bipolar electrodes 1, and one positive terminal electrode 2B) stacked along the Z direction. Separators 13 are placed between the electrodes. The bipolar electrode 1 includes a current collector 11, a negative electrode active material layer 12A provided on the +Z side of the current collector 11, and a positive electrode active material layer 12B provided on the -Z side of the current collector 11. The negative terminal electrode 2A has a configuration in which the positive electrode active material layer 12B is removed from the bipolar electrode 1. An insulating layer 19A covering the periphery of the current collector 11 is formed on the -Z side of the current collector 11 that constitutes the negative terminal electrode 2A. The positive terminal electrode 2B has a configuration in which the negative electrode active material layer 12A is removed from the bipolar electrode 1. An insulating layer 19B is formed on the +Z side surface of the current collector 11 that constitutes the positive terminal electrode 2B, covering the peripheral edge of the current collector 11.

[0017] In this embodiment, a metal foil (for example, an aluminum foil) is employed as the current collector 11 of each electrode. Surface treatment (for example, plating treatment) may be performed on one or both sides of the metal foil. A voltage detection terminal 20 is connected to the current collector 11 of each electrode. In this embodiment, the voltage detection terminal 20 contains stainless steel (for example, SUS304). Stainless steel is excellent in corrosion resistance, heat resistance, and workability. However, the material of the voltage detection terminal 20 can be appropriately changed. Another metal (for example, copper) may be employed instead of stainless steel.

[0018] 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 this embodiment, the positive electrode active material of each cell included in the laminate 10 contains lithium iron phosphate (LiFePO4) having an olivine structure. The negative electrode active material of each cell contains, for example, a carbon-based material. However, as another example of the negative electrode active material, silicon or tin can be mentioned.

[0019] In the laminate 10, a cell C is formed between a plurality of stacked current collectors 11. Specifically, a cell C is formed between a certain current collector 11 (the first current collector) and the current collector 11 (the second current collector) adjacent to the first current collector. Further, a cell C is also formed between the second current collector and the current collector 11 (the third current collector) adjacent to the second current collector. Thus, the current collector 11 and the cell C are alternately arranged in the stacking direction of the laminate 10. The sealing portion 3 includes seal layers 14 and 15 disposed around each of the plurality of cells C included in the laminate 10, and the insulating layers 19A and 19B described above. Any sealing material can be employed as the material of the sealing portion 3.

[0020] The laminate 10 functions as a bipolar secondary battery. Each of the plurality of cells C included in the laminate 10 (particularly, the power storage unit 10a) functions as, for example, an LFP battery (specifically, a lithium-ion secondary battery containing olivine-type lithium iron phosphate as a positive electrode active material). Hereinafter, from the end on the positive electrode side (+Z side) of the laminate 10, the first, second, third, ··· cells C may be denoted as cell C-1, cell C-2, cell C-3, ··· respectively (see FIG. 3 described later). In the laminate 10, a plurality of cells are laminated 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 described later (see FIG. 3).

[0021] The manufacturing system in this embodiment forms the laminate 10 (FIG. 2) to which the aforementioned 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 injection port welding. Although not shown in FIG. 2, a connector 30 (see FIG. 3) described later is further provided for the voltage detection terminal 20. Also, the laminate 10 may be constrained by a restraining jig. The laminate 10 may be sandwiched between one end plate (restraining plate) and pressurized.

[0022] The manufacturing system in 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 performing injection, charging, aging, and inspection on the laminate 10. However, it is not essential that the manufacturing (including inspection) of the power storage device be performed automatically (that is, all processes related to manufacturing are performed by the device), and a person (operator) may perform some of the processes. Also, the structure of the laminate 10 is not limited to the structure shown in FIG. 2 and can be changed as appropriate.

[0023] Referring again to Figure 1, when the laminated body 10 acquired as described above (for example, the laminated body 10 in a constrained state) is passed to the inspection system, the inspection system automatically executes the processing flow shown in Figure 1 for the laminated body 10. Note that "S" in the flowchart represents a step.

[0024] In S11, the inspection system injects the electrolyte into the laminate 10. This fills the space surrounded by the sealing portion 3 in Figure 2 with the electrolyte. The electrolyte impregnates the separator 13. The electrolyte is, for example, a non-aqueous electrolyte. However, it is not limited to this, and the electrolyte may be an aqueous electrolyte. Alternatively, a gel-like or solid electrolyte may be used instead of the electrolyte.

[0025] In the subsequent S12, the inspection system performs the initial charge of the laminate 10. The initial charge is the first time the formed laminate 10 is charged. 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 as shown in Figure 2). This charges all the cells C connected in series.

[0026] In the subsequent S13, the inspection system performs a temperature-raising process to raise 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 constant-temperature bath and adjust the temperature of the laminate 10 using the constant-temperature bath. The aging temperature may be 50°C or higher and 75°C or lower. However, it is not limited to this, and the aging temperature may be lower than 50°C or higher than 75°C. In this embodiment, the aging temperature is set to 65°C.

[0027] In the subsequent S14, the inspection system performs CCCV charging on odd-numbered cells among multiple cells during high-temperature aging of the laminate 10. "Odd-numbered cells" are the odd-numbered cells C (cells C-1, C-3, ...) from one end of the laminate 10 (for example, the positive electrode end). CCCV charging refers to charging using CC (Constant Current) charging, where the charging current is kept constant, and then switching to CV (Constant Voltage) charging when the voltage reaches a set value.

[0028] Specifically, the inspection system maintains the temperature of the laminate 10 at the aging temperature until a predetermined time (hereinafter referred to as "aging time") has elapsed. The aging time may be 5 hours or more and 20 hours or less. However, it is not limited to this, and the aging time may be shorter than 5 hours or longer than 20 hours. In this embodiment, the aging time is set to 10 hours.

[0029] Individual charging of odd-numbered cells in the laminate 10 is performed at the above aging temperature. The inspection system performs CC charging on each of the odd-numbered cells until the voltage reaches a set value, and then performs CV charging on the cells whose voltage has reached the set value. For example, the inspection system performs CV charging when the voltage reaches 3.7V or higher during CC charging. In this embodiment, the voltage applied to each cell during individual charging of odd-numbered cells (hereinafter referred to as "charging voltage") is set as the upper limit voltage of cell C (the LFP battery mentioned above). The upper limit voltage of cell C corresponds to the voltage of cell C in a fully charged state. In this embodiment, the upper limit voltage of cell C is in the range of 3.7V to 3.8V. The inspection system continues CV charging of the odd-numbered cells until a predetermined time (hereinafter referred to as "charging time") has elapsed.

[0030] In the subsequent S15, the inspection system performs CCCV charging on the even-numbered cells among the multiple cells during high-temperature aging of the laminate 10. The "even-numbered cells" are the even-numbered cells C (cells C-2, C-4, ...) from the positive electrode end of the laminate 10. CCCV charging is the same as the individual charging performed on the odd-numbered cells described above, and a detailed explanation will not be repeated. The inspection system performs CC charging on the even-numbered cells and continues CV charging on cells that have reached the set value until the charging time has elapsed.

[0031] In the subsequent S16, the inspection system performs CC charging on the odd-numbered cells during high-temperature aging of the laminate 10. The inspection system may, for example, perform CC charging on the odd-numbered cells until the elapsed time exceeds a threshold, or until the voltage of each odd-numbered cell exceeds a threshold, or until the elapsed time exceeds a threshold AND the voltage exceeds a threshold. The threshold values ​​for elapsed time and voltage may be determined in advance by experimentation or simulation, for example, so that the dV / dt and voltage of each odd-numbered cell at the start of the subsequent self-discharge test match the dV / dt and voltage of each even-numbered cell (for example, so that the magnitude of the difference between the maximum and minimum values ​​is less than or equal to the threshold), and stored in the memory of the control device 150.

[0032] In this embodiment, the inspection system continues high-temperature aging until the CCCV charging of odd-numbered cells, the CCCV charging of even-numbered cells, and the CC charging of odd-numbered cells are completed, and terminates the high-temperature aging upon completion of the CC charging of odd-numbered cells. However, it is not limited to this, and high-temperature aging may be terminated before starting the CCCV charging of odd-numbered cells, and high-temperature aging may be performed on the uncharged laminate 10 after the CC charging of odd-numbered cells is completed.

[0033] In the following step S17, the inspection system cools the laminate 10 to a predetermined temperature (hereinafter referred to as the "inspection temperature"). The inspection temperature may be 20°C or higher and 40°C or lower. The inspection temperature may also be room temperature. In this embodiment, the inspection temperature is set to 25°C.

[0034] In the subsequent S18, the inspection system performs a self-discharge test on each cell contained in the laminate 10.

[0035] The processes from S13 to S18 will be explained below with further use of Figure 3. Figure 3 is a diagram illustrating the charging of each cell and the self-discharge test of the cells after charging. Prior to S13, the test system connects the voltage detection terminal 20 of each cell to the power supply, as shown in Figure 3.

[0036] More specifically, a connector 30 is provided for each of the multiple voltage detection terminals 20 (see Figure 2) connected to the laminate 10. The voltage detection terminals 20 are welded, for example, to the +X side end of the current collector 11. Examples of welding methods include ultrasonic welding or laser welding. The connector 30 includes a resin part 31 and a housing 32. For example, with the housing 32, which aligns the voltage detection terminals 20, attached to the ends of the voltage detection terminals 20, the resin part 31 connecting the +X side end face of the laminate 10 and the housing 32 is formed by injection molding. This forms a connector 30 connected 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 and is configured to be able to accommodate a female connector (e.g., a socket).

[0037] The inspection system in 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 memory device. In this embodiment, the self-discharge test is performed by the processor executing a program stored in the memory device. However, each process related to the self-discharge test may be performed by hardware (electronic circuits) alone without using software.

[0038] The power supply unit 110 is equipped with multiple channels (hereinafter referred to as "Ch") for self-discharge testing. Each Ch is equipped with 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 with the DC power supply 41, and the voltmeter 43 is connected in parallel. The ammeter 42 detects the current flowing through the cell connected to this Ch. The voltmeter 43 detects the voltage between terminals T1 and T2.

[0039] The connection part 120 functions as a female connector that can be attached to the connector 30. The inspection device 100 connects the voltage detection terminals 20 of each cell to a power supply (DC power supply 41) by connecting the connection part 120 to the connector 30. The inspection device 100 may include a robot for connecting the connectors. Each cell included in the laminate 10 is connected to the power supply unit 110 of the inspection device 100 by connecting the corresponding terminals (e.g., female terminals) of the connection part 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 section. Each inspection circuit is a closed circuit including the cell C, channel (Ch), and common wiring. In Figure 3, Ch1, Ch2, Ch3, Ch4, Ch5, and Ch6 are channels connected to cells C-1, C-2, C-3, C-4, C-5, and C-6, respectively.

[0040] In the processing flow shown in Figure 1, once the heating of the laminate 10 (S13) is complete, in the subsequent S14, a power supply (DC power supply 41) connected to the odd-numbered cells applies voltage to the odd-numbered cells through the voltage detection terminal 20. As a result, power is supplied to each of the odd-numbered cells from the corresponding power supply (DC power supply 41) through the corresponding voltage detection terminal 20. By using the voltage detection terminal 20 corresponding to the odd-numbered cells, it becomes possible to supply power (current) to the odd-numbered cells individually. As mentioned above, the inspection device 100 performs CCCV charging on each of the odd-numbered cells during high-temperature aging. This charging brings each of the odd-numbered cells in the laminate 10 to a fully charged state. By charging each of the odd-numbered cells individually, it becomes easier to bring the State of Charge (SOC) of each odd-numbered cell closer to 100%. SOC represents the ratio of the current amount of stored energy to the amount of stored energy in a fully charged state. At this time, the even-numbered cells are in an uncharged state.

[0041] In the subsequent S15, a power supply (DC power supply 41) connected to the even-numbered cells applies a voltage to the even-numbered cells through the voltage detection terminal 20. As a result, each even-numbered cell is supplied with power from the corresponding power supply (DC power supply 41) through the corresponding voltage detection terminal 20. By using the voltage detection terminal 20 corresponding to the even-numbered cells, it becomes possible to supply power (current) to the even-numbered cells individually. As mentioned above, the inspection device 100 performs CCCV charging on each even-numbered cell during high-temperature aging. This charging brings each even-numbered cell in the laminate 10 to a fully charged state. By charging each even-numbered cell individually, it becomes easier to bring the State of Charge (SOC) of each even-numbered cell closer to 100%. At this time, the odd-numbered cells are in an uncharged state.

[0042] In the subsequent S16, a power supply (DC power supply 41) connected to the odd-numbered cells applies a voltage to the odd-numbered cells through the voltage detection terminal 20. As described above, the inspection device 100 performs CC charging on each of the odd-numbered cells during high-temperature aging. At this time, the even-numbered cells are in an uncharged state.

[0043] Subsequently, once the cooling of the laminate 10 (S17) is complete, the inspection device 100 performs a self-discharge test on each cell contained in the laminate 10. Specifically, a power supply (DC power supply 41) connected to the cell to be inspected applies a voltage to that 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 inspection but in the reverse direction. The control device 150 determines whether the current has converged for the cell to be inspected, and for cells where the current has converged, it estimates the converged current value as the self-discharge current. The control device 150 may also determine that the current has converged when the amount of change in current per unit time falls below a predetermined value. However, it is not limited to this, and the method for determining convergence is arbitrary. Once the self-discharge test is completed for all cells, the processing flow shown in Figure 1 is completed.

[0044] The control device 150 determines whether a cell is good or bad based on its estimated self-discharge current. For example, the control device 150 determines a cell to be a short-circuited cell (defective) if the difference in self-discharge current between a good cell and a defective cell after a predetermined time has elapsed since the start of inspection is greater than or equal to a threshold. The self-discharge current of a good cell may be determined by, for example, experimentation or simulation and stored in the memory device of the control device 150. The control device 150 may also determine that the stack 10 is a defective product if it contains at least one short-circuited cell. When the inspection system determines that at least one cell in the stack 10 is defective, it may terminate the processing flow shown in Figure 1 for the stack 10 and start the processing flow shown in Figure 1 for the next inspection target (secondary battery). In this way, the efficiency of inspection can be improved by terminating the inspection as soon as the good or bad result of a secondary battery is known and moving on to the inspection of the next secondary battery.

[0045] The operation and effects of the secondary battery manufacturing method according to this embodiment (the method according to the example) will be described below in comparison with the method according to the comparative example. Figure 4 is a flowchart showing an example of a secondary battery manufacturing method in the comparative example. In the method according to the example, the inspection was performed according to the processing flow shown in Figure 1. In contrast, in the method according to the comparative example, the inspection was performed according to the processing flow shown in Figure 4.

[0046] The process shown in Figure 4 differs in that it omits step S16; otherwise, it is the same as the process shown in Figure 1. Therefore, a detailed explanation of those processes will not be repeated.

[0047] In both the examples and comparative examples, after CCCV charging is completed for the even-numbered cells, the cell voltage of the odd-numbered cells, which were charged using CCCV earlier, decreases. On the other hand, in the process shown in Figure 1, CC charging is then performed on the odd-numbered cells, causing the voltage of the odd-numbered cells to rise and become similar to that of the even-numbered cells.

[0048] Figure 5 shows an example of the voltage changes in odd-numbered and even-numbered cells in the comparative example. The vertical axis in Figure 5 represents voltage. The horizontal axis in Figure 5 represents time. LN1 in Figure 5 shows the time change in voltage of one of the odd-numbered cells. LN2 in Figure 5 shows the time change in voltage of one of the even-numbered cells. This assumes that the processing flow in the comparative example shown in Figure 4 is performed.

[0049] After the liquid is injected into the laminate 10 (S11), the initial charge is performed (S12), and as shown in LN1 and LN2 of Figure 5, the voltage of each cell increases. Then, between time t(0) and time t(1) a heating process is performed (S13), and when the temperature rises to the aging temperature, CCCV charging is performed on the odd-numbered cells at time t(1) (S14). When CCCV charging is performed on the odd-numbered cells, as shown in LN1 of Figure 5, CV charging is performed after CC charging, so the voltage of each odd-numbered cell remains constant while charging. On the other hand, the even-numbered cells are in an uncharged state, so as shown in LN2 of Figure 5, the voltage decreases as time passes after time t(1).

[0050] At time t(2), once the charging time has elapsed, the odd-numbered cells become uncharged, and CCCV charging is performed on the even-numbered cells (S15). When CCCV charging is performed on the even-numbered cells, as shown in LN2 of Figure 5, CV charging is performed after CC charging, so the voltage of each even-numbered cell rises to 3.8V and then maintains a constant state while being charged. On the other hand, the odd-numbered cells become uncharged, so as shown in LN1 of Figure 5, the voltage decreases as time progresses from time t(2) onwards.

[0051] At time t(3), after the charging time has elapsed, both odd-numbered and even-numbered cells become uncharged. At this point, from time t(3) onward, the voltage of both odd-numbered and even-numbered cells decreases over time. At time t(3), cooling of the laminate 10 begins (S17), and at time t(4), a self-discharge test is performed on each cell (S18).

[0052] Figure 5(A) shows a magnified view of the voltage change within the dashed frame. The vertical axis in Figure 5(A) represents voltage. The horizontal axis in Figure 5(A) represents time. LN3 in Figure 5(A) shows the time change in voltage of the defective cell among the odd-numbered cells. LN4 in Figure 5 shows the time change in voltage of the defective cell among the even-numbered cells.

[0053] As shown in LN1 of Figure 5(A), the voltage of odd-numbered cells is lower than the voltage of even-numbered cells when the self-discharge test begins. Therefore, the voltage slope (dV / dt) after the self-discharge test begins is smaller than the voltage slope of even-numbered cells shown in LN2 of Figure 5(A). Consequently, the voltage difference required to distinguish between good and defective cells (short-circuit detection) in odd-numbered cells, as shown in LN1 and LN3 of Figure 5(A), is smaller than the voltage difference required to distinguish between good and defective cells in even-numbered cells, as shown in LN2 and LN4 of Figure 5(A). As a result, the accuracy of the detection of good and defective cells in odd-numbered cells is lower than that of even-numbered cells.

[0054] Figure 6 shows an example of the voltage changes in odd-numbered and even-numbered cells in this embodiment. The vertical axis in Figure 6 represents voltage. The horizontal axis in Figure 6 represents time. LN5 in Figure 6 shows the time change of voltage in one of the odd-numbered cells. LN6 in Figure 6 shows the time change of voltage in one of the even-numbered cells. At this time, we assume that the processing flow in this embodiment shown in Figure 1 is performed.

[0055] After the liquid is injected into the laminate 10 (S11), the initial charge is performed (S12), and as shown in LN5 and LN6 of Figure 6, the voltage of each cell increases. Then, between time t(10) and time t(11), a temperature increase process is performed (S13), and when the temperature rises to the aging temperature, CCCV charging is performed on the odd-numbered cells at time t(11) (S14). When CCCV charging is performed on the odd-numbered cells, as shown in LN5 of Figure 6, CV charging is performed after CC charging, so the voltage of each odd-numbered cell remains constant while charging. On the other hand, the even-numbered cells are in an uncharged state, so as shown in LN6 of Figure 6, the voltage decreases as time passes after time t(11).

[0056] At time t(12), once the charging time has elapsed, the odd-numbered cells become uncharged, and CCCV charging is performed on the even-numbered cells (S15). When CCCV charging is performed on the even-numbered cells, as shown in LN6 of Figure 6, CV charging is performed after CC charging, so the voltage of each even-numbered cell rises to 3.8V and then maintains a constant state while being charged. On the other hand, the odd-numbered cells become uncharged, so as shown in LN5 of Figure 6, the voltage decreases as time progresses after time t(12).

[0057] At time t(13), once the charging time has elapsed, CC charging is performed on the odd-numbered cells (S16). Figure 6(A) shows an enlarged view of the dashed frame. As shown in LN5 of Figure 6(A), when CC charging is performed on the odd-numbered cells at time t(13), the voltage of the odd-numbered cells increases. At time t(14), when CC charging is completed, the voltage of the odd-numbered cells becomes approximately the same as the voltage of the even-numbered cells. At time t(14), cooling of the laminate 10 is started (S17), and at time t(15), a self-discharge test is performed on each cell (S18).

[0058] Figure 6(B) shows the voltage changes between odd-numbered and even-numbered cells from the start of the self-discharge test t(15). The vertical axis of Figure 6(B) represents voltage. The horizontal axis of Figure 6(B) represents time. LN7 in Figure 6(B) shows the time change in voltage of the defective cells among the odd-numbered cells. LN8 in Figure 6(B) shows the time change in voltage of the defective cells among the even-numbered cells.

[0059] As shown in LN7 of Figure 6(B), the voltage of odd-numbered cells is approximately the same as the voltage of even-numbered cells when the self-discharge test begins. Therefore, the voltage slope (dV / dt) after the start of the self-discharge test is approximately the same as the voltage slope of even-numbered cells shown in LN8 of Figure 6(B). Consequently, the magnitude of the voltage difference required to distinguish between good and defective cells in odd-numbered cells, as shown in LN5 and LN7 of Figure 6(B), is approximately the same as the voltage difference required to distinguish between good and defective cells in even-numbered cells, as shown in LN6 and LN8 of Figure 6(B). This allows for improved accuracy in determining good and defective cells in odd-numbered cells compared to the comparative example.

[0060] As described above, according to the secondary battery manufacturing method of this embodiment, by performing CC charging on the odd-numbered cells that have undergone CCCV charging first to increase their voltage and match the voltage of the even-numbered cells, a self-discharge test can be performed at a high voltage on both the odd-numbered and even-numbered cells. Therefore, it is possible to improve the accuracy of determining whether a product is good or defective (short circuit detection) while shortening the lead time required for the determination. Thus, it is possible to provide a secondary battery manufacturing method that improves the accuracy of short-circuit detection.

[0061] Furthermore, by appropriately setting the execution time of CC charging or the voltage threshold for terminating CC charging, the voltages of odd-numbered and even-numbered cells can be made to match before determining whether the cells are good or bad. This improves the accuracy of determining whether the cells are good or bad. Note that at least one of the execution time and voltage threshold may be set using the first voltage decrease rate (dV / dt) of the even-numbered cells and the second voltage decrease rate of the odd-numbered cells after CC charging. For example, the execution time and voltage threshold may be set so that the first and second voltage decrease rates match.

[0062] In this embodiment, we have described as an example the case in which CCCV charging is performed on odd-numbered cells, then CCCV charging is performed on even-numbered cells, and then CC charging is performed on odd-numbered cells. However, for example, CCCV charging may be performed on even-numbered cells, then CCCV charging is performed on odd-numbered cells, and then CC charging is performed on even-numbered cells.

[0063] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0064] 1 Bipolar electrode, 2A Negative terminal electrode, 2B Positive terminal electrode, 3 Sealing part, 10 Laminate, 10a Energy storage part, 11 Current collector, 12A Negative active material layer, 12B Positive active material layer, 13 Separator, 14,15 Seal layer, 19A,19B Insulating layer, 20 Voltage detection terminal, 30 Connector, 31 Resin part, 32 Housing, 41 DC power supply, 42 Ammeter, 43 Voltmeter, 100 Inspection device, 110 Power supply unit, 120 Connection part, 150 Control device, C Cell, T1,T2 Terminals.

Claims

1. A method for manufacturing a secondary battery comprising a plurality of cells composed of bipolar electrodes and separators, wherein the plurality of cells include a group of odd-numbered cells arranged in odd-numbered positions from one end and a group of even-numbered cells arranged in even-numbered positions from the one end. Before determining the quality of the cells, the step of performing constant voltage charging by alternately changing the target of charging for the odd-numbered cell group and the even-numbered cell group, A method for manufacturing a secondary battery, comprising the step of performing constant current charging so that the voltage of one of the odd-numbered cell group and the even-numbered cell group, which has been subjected to constant voltage charging first, matches the voltage of the other cell group.

2. The method for manufacturing a secondary battery according to claim 1, further comprising the step of setting at least one of the execution time of constant current charging and a voltage threshold for terminating constant current charging, using the first rate of voltage decrease of the other cell group and the second rate of voltage decrease of the one cell group after constant current charging.

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