Heat treatment method of lithium ion secondary battery and control device of heat treatment furnace of lithium ion secondary battery

Abstract

To provide a heat treatment method of a lithium ion secondary battery and a control device of a heat treatment furnace of a lithium ion secondary battery capable of selecting a valuable material without generating burning of an electrolyte.SOLUTION: The heat treatment method of a lithium ion secondary battery includes: a first heating step of raising the temperature inside the heat treatment furnace in which the lithium ion secondary battery is placed to a predetermined drying temperature at a first heating rate; a first maintaining step of maintaining the temperature inside the furnace at the drying temperature for a predetermined period of time after the first heating step; a temperature reducing step of reducing the temperature inside the furnace after the first maintaining step. The first heating rate is 1°C / min or more and 3°C / min or less, and the drying temperature is equal to or higher than the opening temperature of a safety valve of the lithium ion secondary battery and lower than 450°C.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a heat treatment method for lithium ion secondary batteries and a control device for a heat treatment furnace for lithium ion secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries have many advantages, such as high energy density, low self-discharge, and no memory effect, and are therefore used in a wide range of fields, including portable electronic devices, home appliances, electric vehicles, industrial machinery, etc. As the use of lithium-ion secondary batteries increases, the amount of used lithium-ion secondary batteries being discarded is also increasing.

[0003] Lithium-ion secondary batteries contain valuable materials such as cobalt, nickel, manganese, lithium, aluminum, iron, copper, etc. Therefore, from the perspective of effective resource utilization, it is desirable to separate and recover these valuable materials from discarded lithium-ion secondary batteries and recycle them.

[0004] In recent years, therefore, techniques for separating valuable materials from lithium ion secondary batteries have been considered. For example, a method for separating valuable materials has been proposed that includes a heat treatment step of heat-treating lithium ion secondary batteries in a heat treatment furnace, a crushing step of crushing the heat-treated material obtained in the heat treatment step, and a classification step of classifying the crushed material obtained in the crushing step (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2022-182724 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional technology described above, the heat treatment temperature in the heat treatment process is set to 660°C or higher, which is the melting point of aluminum, in order to melt the aluminum contained in the casing of the lithium ion secondary battery and separate it from other parts. As a result, in the heat treatment process, the electrolyte contained in the lithium ion secondary battery suddenly vaporizes and sprays out into the heat treatment furnace, causing ignition and combustion.

[0007] Here, when combustion of materials is generated in an industrial waste treatment facility, it is necessary to obtain permission to install the facility as an incineration facility, and high standards must be met in terms of the structure of the facility, the maintenance method, consideration for the surrounding environment, etc. Therefore, the conventional heat treatment method for lithium-ion secondary batteries, which involves the combustion of the electrolyte, has a problem in that it is costly to install and maintain the treatment facility.

[0008] The present invention has been made to solve the problems of the conventional technology described above, and aims to provide a heat treatment method for lithium ion secondary batteries and a control device for a heat treatment furnace for lithium ion secondary batteries, which are capable of separating valuable materials without causing combustion of the electrolyte. [Means for solving the problem]

[0009] In order to achieve the above object, the heat treatment method for lithium ion secondary batteries according to the present invention includes a first heating step of raising the temperature inside a heat treatment furnace in which a lithium ion secondary battery is placed to a predetermined drying temperature at a first heating rate, a first maintenance step of maintaining the temperature inside the furnace at the drying temperature for a predetermined time after the first heating step, and a temperature decreasing step of decreasing the temperature inside the furnace after the first maintenance step, wherein the first heating rate is 1°C / min or more and 3°C / min or less, and the drying temperature is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery and less than 450°C.

[0010] In the present invention thus configured, the temperature inside the heat treatment furnace is raised to a drying temperature of not less than 450°C and not less than the opening temperature of the safety valve of the lithium ion secondary battery at a first heating rate set to not less than 1°C / min and not more than 3°C / min, so that the vaporized electrolyte can be released little by little from the lithium ion secondary batteries that have a lower opening pressure of the safety valve due to individual differences among the various lithium ion secondary batteries put into the heat treatment furnace. Therefore, it is possible to prevent the electrolyte from suddenly spraying out into the high-temperature heat treatment furnace and igniting or burning. In addition, since the temperature inside the furnace is maintained at the drying temperature for a predetermined time, it is possible to sufficiently dry the electrolyte contained in the lithium ion secondary batteries while preventing ignition or burning.

[0011] In the present invention, the oxygen concentration in the heat treatment furnace is preferably reduced to 20% or less during part or the entire period of the first temperature increasing step and the first maintaining step.

[0012] In the present invention configured in this manner, it is possible to more reliably prevent the electrolyte released from the lithium ion secondary battery from reacting with oxygen and igniting or burning during part or all of the first temperature-raising step and the first maintenance step.

[0013] In the present invention, preferably, after the first maintaining step and before the temperature decreasing step, a second heating step is further included in which the temperature inside the furnace is increased at a second heating rate to an aluminum oxidation temperature at which aluminum is oxidized, the aluminum oxidation temperature being higher than the drying temperature, and the second heating rate being faster than the first heating rate.

[0014] In the present invention configured in this manner, the temperature inside the furnace is raised to an aluminum oxidation temperature higher than the drying temperature at a second heating rate faster than the first heating rate, thereby shortening the time required for the second heating step and promoting the oxidation of aluminum contained in the lithium ion secondary battery by an atmosphere with an aluminum oxidation temperature higher than the drying temperature.

[0015] In the present invention, it is preferable to include a second maintaining step of maintaining the temperature inside the furnace at the aluminum oxidation temperature for a predetermined time after the second temperature increasing step and before the temperature decreasing step.

[0016] In the present invention thus configured, the temperature inside the furnace is maintained at the aluminum oxidation temperature for a predetermined period of time, so that the oxidation of the aluminum contained in the lithium ion secondary battery can be further promoted.

[0017] In the present invention, it is preferable that the oxygen concentration in the heat treatment furnace is reduced to 20% or less during part or all of the first heating step and the first maintenance step, and the oxygen concentration in the heat treatment furnace is increased to the oxygen concentration in the atmosphere during the second heating step.

[0018] In the present invention thus configured, it is possible to more reliably prevent the electrolyte solution discharged from the lithium ion secondary battery from reacting with oxygen and igniting or burning during part or all of the first heating step and the first maintaining step. Also, since the oxygen concentration in the heat treatment furnace is increased to the oxygen concentration in the atmosphere during the second heating step, it is possible to promote the oxidation of aluminum contained in the lithium ion secondary battery after the electrolyte solution is dried.

[0019] According to another aspect of the present invention, a control device for a heat treatment furnace for lithium ion secondary batteries according to the present invention includes a temperature sensor that measures the temperature inside the heat treatment furnace, a heater that heats the inside of the heat treatment furnace, and a controller that controls the heater, and the controller is configured to control the heater to execute a first heating process in which the temperature inside the heat treatment furnace in which a lithium ion secondary battery is placed is raised to a predetermined drying temperature at a first heating rate, a first maintenance process in which the temperature inside the furnace is maintained at the drying temperature for a predetermined time after the first heating process, and a temperature reduction process in which the temperature inside the furnace is reduced after the first maintenance process, wherein the first heating rate is 1°C / min or more and 3°C / min or less, and the drying temperature is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery and less than 450°C.

[0020] In the present invention thus configured, the controller controls the heater to raise the temperature inside the heat treatment furnace to a drying temperature that is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery and lower than 450°C at a first heating rate set to 1°C / min or higher and 3°C / min or lower, so that the vaporized electrolyte can be released little by little from the lithium ion secondary batteries that have a lower opening pressure of the safety valve due to individual differences among the various lithium ion secondary batteries placed in the heat treatment furnace. This prevents the electrolyte from being suddenly sprayed into the high-temperature heat treatment furnace and igniting or burning. In addition, the controller controls the heater to maintain the temperature inside the furnace at the drying temperature for a predetermined time, so that the electrolyte contained in the lithium ion secondary batteries can be sufficiently dried while preventing ignition or burning. Effect of the Invention

[0021] According to the method for heat treating a lithium ion secondary battery and the control device for a heat treatment furnace for a lithium ion secondary battery of the present invention, valuable materials can be separated without causing combustion of the electrolyte. [Brief description of the drawings]

[0022] [Figure 1] 1 is a flowchart showing the flow of a method for separating valuable materials from a lithium ion secondary battery according to an embodiment of the present invention. [Diagram 2] 1 is a schematic vertical cross-sectional view showing a configuration of a heat treatment furnace according to an embodiment of the present invention. [Diagram 3] 2 is a flowchart showing the flow of a heat treatment method for a lithium-ion secondary battery according to an embodiment of the present invention. [Figure 4] 4 is a time chart showing the change over time of the temperature inside the furnace in the heat treatment method for the lithium ion secondary battery according to the embodiment of the present invention. [Diagram 5] 4 is a time chart showing the change over time of the pressure inside the furnace in the heat treatment method for the lithium ion secondary battery according to the embodiment of the present invention. [Figure 6] 10 is a flowchart showing the flow of a heat treatment method for a lithium ion secondary battery according to a modified embodiment of the present invention. [Figure 7] 10 is a time chart showing the change over time of the temperature inside the furnace in the heat treatment method for the lithium ion secondary battery according to the modified embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a motor core and a manufacturing method thereof according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0024] [Sorting valuable materials from lithium-ion secondary batteries] First, a method for separating valuable materials from lithium ion secondary batteries will be described with reference to Fig. 1. Fig. 1 is a flow chart showing the flow of the method for separating valuable materials from lithium ion secondary batteries according to this embodiment.

[0025] In this embodiment, any lithium ion secondary battery can be used as the target for sorting valuables. For example, the target can be a used lithium ion secondary battery built into a discarded electronic device, or a lithium ion secondary battery discarded as a defective product. The type of lithium ion secondary battery can be any known type of lithium ion secondary battery, such as a lithium polymer type, a ternary type, an iron phosphate type, a manganese type, a nickel type, or a cobalt type. The shape of the lithium ion secondary battery can be any shape, such as a cylindrical shape, a square shape, or a laminated can. The target can be a mixture of the above various types and shapes of lithium ion secondary batteries, or any one type of lithium ion secondary battery.

[0026] As shown in Fig. 1, first, in step S1, a drying / Al oxidation process is performed to dry the electrolyte contained in the lithium ion secondary battery and oxidize the aluminum contained in the casing of the lithium ion secondary battery. In this drying / Al oxidation process, the lithium ion secondary battery is placed in a heat treatment furnace and heat treated according to temperature conditions including a heating rate and maintaining the battery at a predetermined temperature. Details of this heat treatment will be described later.

[0027] Next, in step S2, a separation step is performed to separate valuable materials from the treated material removed from the heat treatment furnace after the drying and Al oxidation step in step S1. Specifically, first, when the casing of the lithium-ion secondary battery is mainly composed of iron (Fe), the strength of the casing is not sufficiently reduced even after the drying and Al oxidation step, so the iron casing is roughly crushed by a crusher. Next, the treated material is put into a sieving machine and sieved into copper (Cu) and iron (Fe) that remain undissolved in the drying and Al oxidation step and other granular materials. The granular materials include alumina (Al2O3), cobalt (Co), nickel (Ni), manganese (Mn), lithium (Li), and other concentrated slag (black mass). These granular materials may be further separated by a plurality of sieving machines with different sieve meshes, and magnetic and non-magnetic materials may be further separated by a magnetic separator. Valuable materials can be separated from the treated material by the above separation steps.

[0028] [Heat treatment furnace] Next, a heat treatment furnace and a control device for performing heat treatment on the lithium ion secondary battery in the above-mentioned drying / Al oxidation step will be described with reference to Fig. 2. Fig. 2 is a schematic vertical cross-sectional view showing the configuration of the heat treatment furnace according to the present embodiment.

[0029] The heat treatment furnace 1 in this embodiment is a cart-type heating furnace (batch furnace) in which the lithium ion secondary batteries L to be heat-treated are placed on a cart and are taken in and out of the furnace from the front (the right side in FIG. 2 ) of the heat treatment furnace 1. An openable and closable front cover 2 is disposed on the front of the heat treatment furnace 1, and the heat treatment furnace 1 can be closed by pressing the front cover 2 against the front opening of the heat treatment furnace 1 with a piston cylinder 4.

[0030] The lithium ion secondary batteries / processed object L are loaded into and unloaded from the heat treatment furnace 1 using a cart 6. The cart 6 is movable between the inside and outside of the heat treatment furnace 1 with the front lid 2 open.

[0031] A heater 8 for heating the atmosphere in the heat treatment furnace 1 is provided inside the heat treatment furnace 1. Various heating elements can be used as the heater 8, but a heating element that can heat up to 660°C or higher, the melting point of aluminum, and is not corroded by corrosive gas generated by vaporization of the electrolyte is preferable, and for example, a silicon carbide (SiC) heating element is used. In the example of FIG. 2, the heater 8 is provided on the ceiling of the heat treatment furnace 1, but the heater 8 may be arranged so as to extend from the side wall of the heat treatment furnace 1 above the lithium ion secondary battery / processing object L. The heater 8 is electrically connected to a controller 20, and the amount of heat generated is controlled by the controller 20.

[0032] Also provided inside the heat treatment furnace 1 are a thermocouple 10 (temperature sensor) for measuring the temperature inside the heat treatment furnace 1 and a pressure gauge 12 for measuring the pressure inside the heat treatment furnace 1. The thermocouple 10 and the pressure gauge 12 are electrically connected to the controller 20 and output their respective measurement values ​​to the controller 20.

[0033] A flue 14 is connected to the rear of the heat treatment furnace 1 for exhausting the atmosphere inside the heat treatment furnace 1 to the outside environment. The flue 14 is provided with an exhaust fan 16, a butterfly valve 18, and an exhaust gas treatment device (not shown).

[0034] The exhaust fan 16 constantly maintains the pressure inside the heat treatment furnace 1 at a negative pressure by sucking in the atmosphere inside the heat treatment furnace 1 and discharging it to the outside environment during heat treatment. The pressure inside the furnace during heat treatment is, for example, -0.5 kPa (gauge pressure). Note that the heat treatment furnace 1 is not completely airtight, and a small amount of air is introduced into the furnace from the outside in response to the exhaust by the exhaust fan 16. Also, the exhaust fan 16 may be omitted by using the outlet of the flue 14 as a chimney so that the atmosphere inside the heat treatment furnace 1 is naturally exhausted by the chimney effect.

[0035] The butterfly valve 18 maintains the pressure inside the heat treatment furnace 1 at a predetermined negative pressure by adjusting the flow rate of the exhaust gas flowing through the flue 14. The butterfly valve 18 may be a self-acting automatic valve that balances the pressure difference before and after the butterfly valve 18 to a predetermined value by the elastic force of a spring inside the valve, or may be a separately-acting automatic valve that adjusts the opening degree by controlling an actuator connected to the valve with a controller 20. For example, while the vaporized electrolyte is not being released from the lithium ion secondary battery L, the opening degree of the butterfly valve 18 becomes relatively small to prevent the pressure inside the furnace from becoming too low, and when the vaporized electrolyte is released from the lithium ion secondary battery L and the pressure inside the furnace rises, the opening degree of the butterfly valve 18 becomes relatively large to prevent the pressure inside the furnace from becoming positive, i.e., to prevent the atmosphere inside the furnace from leaking out of the heat treatment furnace 1 through gaps other than the flue 14.

[0036] The controller 20 controls the output of the heater 8 based on the measured values ​​input from the thermocouple 10 and the pressure gauge 12. The controller 20 mainly includes one or more processors such as a CPU for executing various processes, one or more memories (such as a ROM, a RAM, or a hard disk) for storing programs to be executed by the processor and various data required for executing the programs, an operation unit for receiving operation input from an operator, and a display unit for displaying the operating status of the heat treatment furnace 1.

[0037] [Heat treatment method] Next, a heat treatment method for performing heat treatment on the lithium ion secondary battery L in the above-mentioned drying / Al oxidation step will be described with reference to Fig. 3 to Fig. 5. Fig. 3 is a flow chart showing the flow of the heat treatment method of this embodiment, Fig. 4 is a time chart showing the change in temperature in the furnace over time in the heat treatment method of this embodiment, and Fig. 5 is a time chart showing the change in pressure in the furnace over time in the heat treatment method of this embodiment.

[0038] As shown in FIG. 3, first, in step S11, the lithium ion secondary battery L to be treated is placed on a cart 6 and inserted into the heat treatment furnace 1, and the front lid 2 is closed.

[0039] Next, in step S12, the controller 20 turns on the output of the heater 8 to start heating the atmosphere in the heat treatment furnace 1, and in step S13, controls the output of the heater 8 so that the temperature inside the furnace rises at a first temperature rise rate (first temperature rise step). The first temperature rise rate is 1°C / min or more and 3°C / min or less, and is preferably 2°C / min. When the temperature inside the furnace is raised at such a first temperature rise rate, the vaporized electrolyte is released little by little from those of the various lithium ion secondary batteries L put into the heat treatment furnace 1 whose safety valve opening pressure is low due to individual differences, so that the electrolyte is prevented from suddenly spraying into the high-temperature heat treatment furnace 1 and igniting and burning. On the other hand, when the temperature rise rate is faster than 3°C / min, for example, 5°C / min, the electrolyte may suddenly spray into the furnace after the temperature inside the furnace reaches a high temperature (for example, 500°C or more), causing combustion.

[0040] Next, in step S14, the controller 20 judges whether the temperature inside the furnace measured by the thermocouple 10 has reached a predetermined drying temperature. The drying temperature is a temperature for drying the electrolyte of the lithium ion secondary battery L put into the heat treatment furnace 1, and is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery L and lower than 450°C. The opening temperature of the safety valve of the lithium ion secondary battery L can be set according to the lithium ion secondary battery L to be treated, and is, for example, 350°C. In this embodiment, the drying temperature is 400°C.

[0041] If the result of the determination in step S14 is that the temperature inside the furnace has not reached the drying temperature (step S14: NO), the process returns to step S13, and the controller 20 continues to increase the temperature inside the furnace. On the other hand, if the temperature inside the furnace has reached the drying temperature (step S14: YES), the process proceeds to step S15, and the controller 20 controls the output of the heater 8 so as to maintain the drying temperature for a predetermined time T1 (first maintenance step). The predetermined time T1 is appropriately determined as a time required to sufficiently dry the electrolyte contained in the lithium ion secondary batteries L according to the type and amount of the lithium ion secondary batteries L put into the heat treatment furnace 1, and can be within a range of, for example, 10 minutes to 2 hours, and is 30 minutes in this embodiment.

[0042] The changes in the temperature and pressure inside the furnace in the above-mentioned steps S13 to S15 (first heating step and first maintaining step) will be described with reference to Figures 4 and 5. As shown in Figure 4, when the temperature inside the furnace is raised from room temperature at a first heating rate of 2°C / min in step S13, the drying temperature reaches 400°C in about 190 minutes from the start of heating, and then in step S15, the drying temperature of 400°C is maintained for a predetermined time T1 = 30 minutes. During this time, the vaporized electrolyte is gradually released, but ignition or combustion due to the electrolyte suddenly ejecting into an atmosphere with a higher temperature than the drying temperature does not occur.

[0043] On the other hand, as shown in FIG. 5, the pressure inside the furnace is maintained at a negative pressure of -0.5 kPa G while the temperature inside the furnace is rising, and it starts to rise around the time when the temperature inside the furnace reaches 400° C., peaks at -0.1 kPa G 200 minutes after the start of the temperature rise, and then drops back to -0.5 kPa G. This shows that when the temperature inside the furnace approaches the drying temperature, the electrolyte of the lithium ion secondary battery L is vaporized and gradually released outside the case, causing the pressure inside the furnace to rise, but the opening of the butterfly valve 18 increases accordingly, so that the pressure inside the furnace is kept at a negative pressure. Thus, in the first temperature rise step and the first maintenance step of the heat treatment of this embodiment, the temperature inside the furnace does not rise suddenly, and the pressure inside the furnace does not rise to a positive pressure.

[0044] Returning to Fig. 3, after maintaining the drying temperature for a predetermined time T1 in step S15, the process proceeds to step S16, where the controller 20 controls the output of the heater 8 so that the temperature inside the furnace increases at a second heating rate (second heating step). Since the electrolyte contained in the lithium ion secondary battery L has been dried by the time of step S15 and no combustion occurs, the second heating rate can be made faster than the first heating rate. For example, in this embodiment, the second heating rate is 5°C / min.

[0045] Next, in step S17, the controller 20 judges whether the temperature inside the furnace measured by the thermocouple 10 has reached the aluminum oxidation temperature (Al oxidation temperature). The Al oxidation temperature is set to be higher than the drying temperature described above, and preferably higher than the melting point of aluminum (660°C) and lower than the melting point of copper (1085°C). This makes it possible to dissolve the aluminum contained in the housing of the lithium ion secondary battery L and the like to increase the surface area, thereby promoting oxidation and obtaining granular alumina, while preventing the copper from dissolving and facilitating the subsequent separation. In this embodiment, the Al oxidation temperature is set to 700°C.

[0046] If the result of the determination in step S17 is that the temperature inside the furnace has not reached the drying temperature (step S17: NO), the process returns to step S16, and the controller 20 continues to increase the temperature inside the furnace. On the other hand, if the temperature inside the furnace has reached the Al oxidation temperature (step S17: YES), the process proceeds to step S18, and the controller 20 controls the output of the heater 8 so as to maintain the Al oxidation temperature for a predetermined time T2 (second maintenance step). The predetermined time T2 is appropriately determined as a time required to sufficiently oxidize the aluminum contained in the lithium ion secondary batteries L according to the type and amount of the lithium ion secondary batteries L put into the heat treatment furnace 1, and can be within a range of, for example, 10 minutes to 2 hours, and is 30 minutes in this embodiment.

[0047] Next, in step S19, the controller 20 turns off the output of the heater 8 to stop heating the atmosphere in the heat treatment furnace 1, and lowers the temperature inside the furnace (temperature lowering step).

[0048] The changes in the temperature and pressure in the furnace during the above-mentioned steps S16 to S18 (Al oxidation step) and the temperature drop step S19 will be described with reference to Figs. 4 and 5. As shown in Fig. 4, when the temperature in the furnace is raised from the drying temperature of 400°C at a second heating rate of 5°C / min, which is faster than the first heating rate, in step S16, the Al oxidation temperature of 700°C is reached in 60 minutes from the start of the temperature rise, and then in step S18, the Al oxidation temperature of 700°C is maintained for a predetermined time T2=30 minutes. Since the electrolyte of the lithium ion secondary battery L has already been dried in the first heating step and the first maintaining step, ignition and combustion of the electrolyte do not occur. After that, when the heater 8 is turned off in step S19, the temperature in the furnace gradually drops and returns to room temperature.

[0049] 5, the pressure inside the furnace is maintained at a negative pressure of −0.5 kPa G during the Al oxidation step and the temperature decreasing step. Thus, even during the Al oxidation step and the temperature decreasing step of the heat treatment of this embodiment, the temperature inside the furnace does not rise suddenly, and the pressure inside the furnace does not rise to a positive pressure.

[0050] 3, after the temperature inside the furnace has sufficiently dropped to near room temperature, in step S20, the front cover 2 is opened, the dolly 6 is pulled out from the heat treatment furnace 1, and the heat-treated material L is transferred to a sieving machine. This ends the heat treatment.

[0051] <Modification> Next, a modified example of the embodiment of the present invention will be described.

[0052] In the above-described embodiment, air is introduced into the heat treatment furnace 1, but an inert gas such as nitrogen or helium may be introduced during part or all of the first temperature increase step and the first maintenance step to reduce the oxygen concentration in the heat treatment furnace 1 to 20% or less. This makes it possible to more reliably prevent the electrolyte discharged from the lithium ion secondary battery L from reacting with oxygen and igniting or burning. In this case, the heat treatment furnace 1 is provided with an inert gas supply device for introducing the inert gas into the furnace and an O2 sensor for measuring the oxygen concentration in the furnace (not shown). The inert gas supply device and the O2 sensor are electrically connected to the controller 20, and the controller 20 controls the introduction of the inert gas by the inert gas supply device so that the atmosphere in the furnace has a predetermined oxygen concentration. In this modification, nitrogen (N2) is used as the inert gas.

[0053] A heat treatment method for performing heat treatment on a lithium ion secondary battery L according to this modified example will be described with reference to Fig. 6. Fig. 6 is a flowchart showing the flow of the heat treatment method according to this modified example. Note that steps S21, S23 to S26, and S28 to S32 in the flowchart shown in Fig. 6 are similar to steps S11 to S20 in the flowchart shown in Fig. 3, and therefore will not be described as appropriate.

[0054] After closing the front cover 2 of the heat treatment furnace 1 in step S21, in step S22, the controller 20 controls the inert gas supply device to introduce N2 into the heat treatment furnace 1 and to make the oxygen concentration of the atmosphere inside the furnace 20% or less (e.g., 10%).

[0055] After maintaining the drying temperature for a predetermined time T1 in step S26, in step S27, the controller 20 stops the introduction of N2 into the heat treatment furnace 1. Since a small amount of air is introduced into the heat treatment furnace 1 from the outside in response to exhaust by the exhaust fan 16, the oxygen concentration gradually increases to the oxygen concentration in the air after the introduction of N2 is stopped.

[0056] In this manner, in this modified example, N2 is introduced into the heat treatment furnace 1 to reduce the oxygen concentration to 20% or less during the entire first heating step and first maintenance step. However, N2 may be introduced into the heat treatment furnace 1 during part of the first heating step and first maintenance step, for example, during the latter half of the first heating step and the first maintenance step, or N2 may be introduced into the heat treatment furnace 1 only during the first maintenance step.

[0057] In the above embodiment, the separation step is performed using a sieving machine, but the separation step can also be performed manually. In this case, among the first heating step, the first maintaining step, the second heating step, and the second maintaining step included in the heat treatment, the second heating step and the second maintaining step may be omitted and only the first heating step and the first maintaining step may be performed. Specifically, steps S16 to S18 in the heat treatment shown in FIG. 3 or steps S28 to S30 in the heat treatment shown in FIG. 6 may be omitted.

[0058] The change in the temperature inside the furnace over time in this case will be described with reference to Fig. 7. As in the embodiment shown in Fig. 4, when the temperature inside the furnace is raised from room temperature at a first heating rate of 2°C / min in step S13 or S24 (first heating step), the drying temperature reaches 400°C in about 190 minutes from the start of heating, and then in step S15 or S26, the drying temperature of 400°C is maintained for a predetermined time T1 = 30 minutes (first maintenance step). After that, when the second heating step and the second maintenance step are omitted and the heater 8 is turned off in step S19 or S31, the temperature inside the furnace gradually decreases and returns to room temperature. During this time, the temperature inside the furnace does not rise suddenly, and the pressure inside the furnace does not rise to a positive pressure.

[0059] In the above-mentioned embodiment, the first and second heating rates are constant, but may be changed. For example, in step S13 or S24 (first heating step) of the heat treatment, the heating rate may be set relatively fast (e.g., 5°C / min) from the start of heating until the furnace temperature reaches an intermediate temperature (e.g., 250°C) lower than the drying temperature, and the heating rate may be set relatively slow (e.g., 1°C / min) while the furnace temperature is raised from the intermediate temperature to the drying temperature. In this way, the furnace temperature is raised quickly when there is no risk of ignition or combustion of the electrolyte immediately after the furnace temperature starts to rise from room temperature, and the furnace temperature is raised slowly when the furnace temperature approaches the drying temperature and there is a possibility that ignition or combustion of the electrolyte may occur if the furnace temperature is raised rapidly, thereby shortening the time required for the first heating step and preventing ignition or combustion of the electrolyte.

[0060] <Action and effect> Next, the effects of the heat treatment method for the lithium ion secondary battery L and the control device for the heat treatment furnace 1 for the lithium ion secondary battery L according to the above-described embodiment and modified example will be described.

[0061] First, according to the heat treatment method and the control device of the heat treatment furnace 1 of the above-mentioned embodiment and modification, in the first heating step, the temperature inside the heat treatment furnace 1 is raised to a drying temperature that is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery L and lower than 450°C at a first heating rate set to 1°C / min or higher and 3°C / min or lower. Therefore, the vaporized electrolyte can be released little by little from the lithium ion secondary battery L that has a lower opening pressure of the safety valve due to individual differences among various lithium ion secondary batteries L placed in the heat treatment furnace 1. Therefore, it is possible to prevent the electrolyte from being suddenly sprayed into the high-temperature heat treatment furnace 1 and igniting or burning. In addition, in the first maintaining step, the temperature inside the furnace is maintained at the drying temperature for a predetermined time, so that the electrolyte contained in the lithium ion secondary battery L can be sufficiently dried while preventing ignition or burning.

[0062] In addition, during part or all of the first heating step and the first maintenance step, the oxygen concentration in the heat treatment furnace 1 is reduced to 20% or less, which more reliably prevents the electrolyte released from the lithium ion secondary battery L from reacting with oxygen and igniting or burning.

[0063] In addition, in the second heating process, the temperature inside the furnace is raised to an aluminum oxidation temperature higher than the drying temperature at a second heating rate faster than the first heating rate, so that the time required for the second heating process can be shortened and the oxidation of aluminum contained in the lithium ion secondary battery L can be promoted by the atmosphere with an aluminum oxidation temperature higher than the drying temperature.

[0064] In addition, after the second temperature increasing step and before the temperature decreasing step, the temperature inside the furnace is maintained at the aluminum oxidation temperature for a predetermined time in the second maintaining step, so that the oxidation of the aluminum contained in the lithium ion secondary batteries L can be further promoted.

[0065] In addition, during part or all of the first heating step and the first maintenance step, the oxygen concentration in the heat treatment furnace 1 is reduced to 20% or less, and during the second heating step, the oxygen concentration in the heat treatment furnace 1 is increased to the oxygen concentration in the atmosphere, thereby promoting the oxidation of aluminum contained in the lithium ion secondary battery L after the electrolyte is dried. [Explanation of symbols]

[0066] 1 Heat treatment furnace 2 Front lid 4 Piston Cylinder 6 Carts 8 Heater 10 Thermocouples 12 Pressure gauge 14 Flue 16 Exhaust fan 18 Butterfly valve 20 Controller L Lithium-ion secondary battery (processed material)

Claims

1. A method for heat treating a lithium ion secondary battery, comprising the steps of: a first heating step of raising the temperature inside the heat treatment furnace in which the lithium ion secondary battery is placed to a predetermined drying temperature at a first heating rate; a first maintaining step of maintaining the temperature inside the furnace at the drying temperature for a predetermined time after the first temperature increasing step; a temperature reducing step of reducing the temperature inside the furnace after the first maintaining step; Including, The first heating rate is 1° C. / min or more and 3° C. / min or less, The drying temperature is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery and lower than 450° C. A method for heat treatment of a lithium-ion secondary battery.

2. During a part or the entire period of the first heating step and the first maintaining step, the oxygen concentration in the heat treatment furnace is reduced to 20% or less. The method for heat treating the lithium ion secondary battery according to claim 1 .

3. The method further includes a second heating step of increasing the temperature in the furnace at a second heating rate to an aluminum oxidation temperature at which aluminum is oxidized after the first maintaining step and before the temperature decreasing step, the aluminum oxidation temperature is higher than the drying temperature; The second heating rate is faster than the first heating rate. The method for heat treating the lithium ion secondary battery according to claim 1 .

4. A second maintaining step of maintaining the temperature in the furnace at the aluminum oxidation temperature for a predetermined time after the second temperature increasing step and before the temperature decreasing step is included. The method for heat treating the lithium ion secondary battery according to claim 3 .

5. During a part or the entire period of the first temperature increasing step and the first maintaining step, the oxygen concentration in the heat treatment furnace is reduced to 20% or less; During the second heating step, the oxygen concentration in the heat treatment furnace is increased to the oxygen concentration in the atmosphere. A method for heat treating the lithium ion secondary battery according to claim 3 or 4.

6. A control device for a heat treatment furnace for a lithium ion secondary battery, A temperature sensor for measuring an internal temperature of the heat treatment furnace; a heater for heating the inside of the heat treatment furnace; A controller for controlling the heater, The controller: a first heating step of raising the temperature inside the heat treatment furnace in which the lithium ion secondary battery is placed to a predetermined drying temperature at a first heating rate; a first maintaining step of maintaining the temperature inside the furnace at the drying temperature for a predetermined time after the first temperature increasing step; a temperature reducing step of reducing the temperature inside the furnace after the first maintaining step; and configured to control the heater to perform The first heating rate is 1° C. / min or more and 3° C. / min or less, The drying temperature is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary battery and lower than 450° C. A control device for a heat treatment furnace for lithium-ion secondary batteries.

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