Method for heat treatment of lithium-ion secondary batteries and control device for heat treatment furnace of lithium-ion secondary batteries

Abstract

To provide a method for heat-treating a lithium ion secondary battery and a control device for a heat treatment furnace for a lithium ion secondary battery, which are capable of separating valuable materials without causing combustion of an electrolyte.SOLUTION: A heat treatment method for lithium ion secondary batteries includes a first heating step of raising the temperature inside a heat treatment furnace into which lithium ion secondary batteries are placed to a drying temperature that is equal to or higher than the opening temperature of the safety valve of the lithium ion secondary batteries and lower than 450°C, 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, and a temperature lowering step of lowering the temperature inside the furnace after the first maintaining step. The heating rate during the first heating step until the temperature inside the furnace reaches an intermediate temperature that is lower than the drying temperature is faster than the heating rate during the period in which the temperature inside the furnace is raised from the intermediate temperature to the drying temperature.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a heat treatment method for a lithium-ion secondary battery and a control device for a heat treatment furnace of a lithium-ion secondary battery.

Background Art

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

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

[0004] Therefore, in recent years, technologies for separating valuable substances from lithium-ion secondary batteries have been studied. For example, a method for separating valuable substances has been proposed that includes a heat treatment step of heat-treating a lithium-ion secondary battery in a heat treatment furnace, a crushing step of crushing the heat-treated product obtained in the heat treatment step, and a classification step of classifying the crushed product obtained in the crushing step (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

[0007] In industrial waste treatment facilities where combustion of treated materials occurs, it is necessary to obtain a permit for installation as an incineration facility, and high standards must be met regarding the facility's structure, maintenance methods, and consideration for the surrounding environment. Therefore, conventional heat treatment methods for lithium-ion secondary batteries, which involve the combustion of electrolyte, have the problem of high costs for the installation and maintenance of treatment facilities.

[0008] The present invention was made to solve the problems of the prior art 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 that can separate valuable materials without causing combustion of the electrolyte. [Means for solving the problem]

[0009] To achieve the above objective, the heat treatment method for lithium-ion secondary batteries according to the present invention includes a first heating step of raising the furnace temperature of a heat treatment furnace into which the lithium-ion secondary battery is placed to a dry temperature which is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C; a first maintenance step of maintaining the furnace temperature at the dry temperature for a predetermined time after the first heating step; and a cooling step of lowering the furnace temperature after the first maintenance step, wherein in the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the dry temperature is faster than the heating rate while the furnace temperature rises from the intermediate temperature to the dry temperature.

[0010] In the present invention configured as described above, in the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate while the furnace temperature rises from the intermediate temperature to the drying temperature. Therefore, when there is no risk of electrolyte ignition or combustion immediately after the furnace temperature starts to rise from room temperature, the furnace temperature is rapidly increased. Furthermore, when the furnace temperature approaches the drying temperature and there is a possibility of electrolyte ignition or combustion if the furnace temperature is rapidly increased, the furnace temperature is slowly increased, allowing the vaporized electrolyte to be released little by little from the lithium-ion secondary batteries that have low safety valve opening pressures due to individual differences among the various lithium-ion secondary batteries placed in the heat treatment furnace. Therefore, it is prevented that the electrolyte will be suddenly ejected into the high-temperature heat treatment furnace and ignite or burn. This makes it possible to shorten the time required for the first heating step and prevent electrolyte ignition and combustion at the same time. In addition, since the furnace temperature is maintained at the drying temperature for a predetermined time, the electrolyte contained in the lithium-ion secondary battery can be thoroughly dried while preventing ignition and combustion.

[0011] In the present invention, the intermediate temperature is preferably 250°C.

[0012] In the present invention, preferably, in the first heating step, the heating rate until the furnace temperature reaches the intermediate temperature is 5°C / min, and the heating rate while the furnace temperature rises from the intermediate temperature to the drying temperature is 1°C / min.

[0013] In the present invention, preferably, 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.

[0014] In the present invention configured in this way, it is possible to more reliably prevent the electrolyte released from the lithium-ion secondary battery from reacting with oxygen and igniting / burning during part or all of the first heating step and the first maintenance step.

[0015] Preferably, the present invention further includes a second heating step, which, after the first maintenance step and before the cooling step, raises the furnace temperature to an aluminum oxidation temperature that oxidizes aluminum to a temperature higher than the drying temperature.

[0016] In the present invention configured in this way, the furnace temperature is raised to an aluminum oxidation temperature higher than the drying temperature, so the oxidation of aluminum contained in the lithium-ion secondary battery can be promoted by the atmosphere with an aluminum oxidation temperature higher than the drying temperature.

[0017] Preferably, the present invention includes a second maintenance step, which is performed after the second heating step and before the cooling step, in which the furnace temperature is maintained at the aluminum oxidation temperature for a predetermined time.

[0018] In the present invention configured in this way, the furnace temperature is maintained at the aluminum oxidation temperature for a predetermined time, which further promotes the oxidation of aluminum contained in lithium-ion secondary batteries.

[0019] In the present invention, preferably, during part or all of the first heating step and the first maintenance step, the oxygen concentration in the heat treatment furnace is reduced to 20% or less, and during the second heating step, the oxygen concentration in the heat treatment furnace is increased to the oxygen concentration in the atmosphere.

[0020] In the present invention configured in this way, it is possible to more reliably prevent the electrolyte released from the lithium-ion secondary battery from reacting with oxygen and igniting / combusting during part or all of the first heating step and the first maintenance step. Furthermore, since the oxygen concentration in the heat treatment furnace is raised to the same level as the oxygen concentration in the atmosphere during the second heating step, the oxidation of aluminum contained in the lithium-ion secondary battery can be promoted after the electrolyte dries.

[0021] In the present invention, preferably, the internal pressure of the heat treatment furnace is kept at a negative pressure at all times during the heat treatment process.

[0022] According to another aspect of the present invention, the control device for a heat treatment furnace for lithium-ion secondary batteries according to the present invention comprises a temperature sensor for measuring the internal temperature of the heat treatment furnace, a heater for heating the inside of the heat treatment furnace, and a controller for controlling the heater, wherein the controller is configured to control the heater to perform a first heating step of raising the internal temperature of the heat treatment furnace into which the lithium-ion secondary battery is placed to a dry temperature which is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C, a first maintenance step of maintaining the internal temperature at the dry temperature for a predetermined time after the first heating step, and a cooling step of lowering the internal temperature after the first maintenance step, wherein in the first heating step, the heating rate until the internal temperature reaches an intermediate temperature lower than the dry temperature is faster than the heating rate while the internal temperature rises from the intermediate temperature to the dry temperature.

[0023] In the present invention configured as described above, the controller controls the heater to raise the internal temperature of the heat treatment furnace to a drying temperature that is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C. In the first heating step, the heating rate until the internal temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate from the intermediate temperature to the drying temperature. Therefore, when there is no risk of electrolyte ignition or combustion immediately after the internal temperature starts to rise from room temperature, the internal temperature is rapidly increased. Furthermore, when the internal temperature approaches the drying temperature and there is a possibility of electrolyte ignition or combustion if the internal temperature is rapidly increased, the internal temperature is slowly increased, allowing the vaporized electrolyte to be released little by little from the lithium-ion secondary batteries with low safety valve opening pressures due to individual differences among the various lithium-ion secondary batteries placed in the heat treatment furnace. Thus, it is prevented that the electrolyte will be suddenly ejected into the high-temperature heat treatment furnace and ignite or burn. This makes it possible to shorten the time required for the first heating step and prevent electrolyte ignition and combustion at the same time. Furthermore, the controller controls the heater to maintain the furnace temperature at a drying temperature for a predetermined time, thereby ensuring that the electrolyte contained in the lithium-ion secondary battery is thoroughly dried while preventing ignition and combustion. [Effects of the Invention]

[0024] According to the heat treatment method of the lithium-ion secondary battery and the control device of the heat treatment furnace of the lithium-ion secondary battery of the present invention, valuable substances can be selected without generating combustion of the electrolytic solution.

Brief Description of the Drawings

[0025] [Figure 1] It is a flowchart showing the flow of a method for selecting valuable substances from a lithium-ion secondary battery according to an embodiment of the present invention. [Figure 2] It is a schematic longitudinal sectional view showing the configuration of a heat treatment furnace according to an embodiment of the present invention. [Figure 3] It 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] It is a time chart showing the time change of the temperature inside the furnace in the heat treatment method of a lithium-ion secondary battery according to an embodiment of the present invention. [Figure 5] It is a time chart showing the time change of the pressure inside the furnace in the heat treatment method of a lithium-ion secondary battery according to an embodiment of the present invention. [Figure 6] It is a flowchart showing the flow of a heat treatment method for a lithium-ion secondary battery according to a modified example of an embodiment of the present invention. [Figure 7] It is a time chart showing the time change of the temperature inside the furnace in the heat treatment method of a lithium-ion secondary battery according to a modified example of an embodiment of the present invention.

Modes for Carrying Out the Invention

[0026] Hereinafter, a method for manufacturing a motor core and a motor core according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0027] [Selection of Valuable Substances from Lithium-Ion Secondary Batteries] First, referring to FIG. 1, a method for selecting valuable substances from a lithium-ion secondary battery will be described. FIG. 1 is a flowchart showing the flow of a method for selecting valuable substances from a lithium-ion secondary battery according to the present embodiment.

[0028] In this embodiment, any lithium-ion secondary battery can be used as the target for sorting for valuable materials. For example, used lithium-ion secondary batteries that were built into discarded electronic devices or lithium-ion secondary batteries that were discarded as defective products can be targeted. Furthermore, any known type of lithium-ion secondary battery can be targeted, such as lithium polymer, ternary, iron phosphate, manganese, nickel, and cobalt. In addition, any shape of lithium-ion secondary battery can be targeted, such as cylindrical, prismatic, or laminated can. Moreover, a mixture of the above-mentioned types and shapes of lithium-ion secondary batteries may be targeted, or only one type of lithium-ion secondary battery may be targeted.

[0029] As shown in Figure 1, first, in step S1, a drying and aluminum oxidation process is performed to dry the electrolyte contained in the lithium-ion secondary battery and to oxidize the aluminum contained in the housing and other parts of the lithium-ion secondary battery. In this drying and aluminum oxidation process, the lithium-ion secondary battery is placed in a heat treatment furnace and heat treatment is performed according to temperature conditions, including the heating rate and maintenance at a predetermined temperature. Details of this heat treatment will be described later.

[0030] Next, in step S2, a separation process is performed to separate valuable materials from the processed material removed from the heat treatment furnace after the drying and Al oxidation process in step S1. Specifically, first, if the housing of the lithium-ion secondary battery is mainly composed of iron (Fe), the strength of the housing has not sufficiently decreased even after the drying and Al oxidation process, so the iron housing is roughly crushed using a crusher. Next, the processed material is put into a sieving machine and sieved into copper (Cu) and iron (Fe) that remain undissolved in the drying and Al oxidation process, and other granular material. The granular material contains concentrated slag (black mass) including alumina (Al2O3), cobalt (Co), nickel (Ni), manganese (Mn), lithium (Li), etc. These granular materials may be further separated using multiple sieving machines with different sieve sizes, or magnetic materials may be further separated from non-magnetic materials using a magnetic separator. Through the above separation process, valuable materials can be separated from the processed material.

[0031] [Heat treatment furnace] Next, with reference to Figure 2, the heat treatment furnace and control device for performing heat treatment on lithium-ion secondary batteries in the drying and Al oxidation process described above will be explained. Figure 2 is a schematic longitudinal cross-sectional view showing the configuration of the heat treatment furnace according to this embodiment.

[0032] In this embodiment, the heat treatment furnace 1 is a trolley-type heating furnace (batch furnace) in which lithium-ion secondary batteries L to be heat-treated are placed on a trolley and loaded into and out of the furnace from the front (right side in Figure 2). A front cover 2 that can be opened and closed is positioned 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.

[0033] The loading and unloading of lithium-ion secondary batteries / processing material L into and out of the heat treatment furnace 1 is performed using a trolley 6. With the front cover 2 open, the trolley 6 can move both inside and outside the heat treatment furnace 1.

[0034] Inside the heat treatment furnace 1, a heater 8 is provided to heat the atmosphere inside the heat treatment furnace 1. Various heating elements can be used for the heater 8, but it is desirable to use a heating element that can heat to above the melting point of aluminum (660°C) and that is not corroded by corrosive gases produced by the vaporization of the electrolyte. For example, a silicon carbide (SiC) heating element is used. In the example shown in Figure 2, the heater 8 is provided on the ceiling of the heat treatment furnace 1, but the heater 8 may also be positioned to extend from the side wall of the heat treatment furnace 1 above the lithium-ion secondary battery / processed material L. The heater 8 is electrically connected to a controller 20, and the amount of heat generated is controlled by the controller 20.

[0035] Furthermore, the heat treatment furnace 1 is equipped with a thermocouple 10 (temperature sensor) for measuring the internal temperature of the heat treatment furnace 1 and a pressure gauge 12 for measuring the internal pressure of the heat treatment furnace 1. These thermocouple 10 and pressure gauge 12 are electrically connected to the controller 20 and output their respective measured values ​​to the controller 20.

[0036] 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 equipped with an exhaust fan 16, a butterfly valve 18, and an exhaust gas treatment device (not shown).

[0037] The exhaust fan 16 maintains a constant negative pressure inside the heat treatment furnace 1 by drawing in the atmosphere inside the furnace 1 and exhausting it to the outside environment during heat treatment. The internal 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. Alternatively, the exhaust fan 16 may be omitted by using a chimney at the outlet of the flue 14 to allow the atmosphere inside the heat treatment furnace 1 to be naturally exhausted by the chimney effect.

[0038] The butterfly valve 18 maintains the internal pressure of the heat treatment furnace 1 at a predetermined negative pressure by adjusting the flow rate of exhaust gases through the flue 14. The butterfly valve 18 may be a self-operating automatic valve that balances the differential pressure across the valve to a predetermined value by the elastic force of a spring inside the valve, or it may be a manually operated automatic valve that adjusts the opening degree by controlling an actuator connected to the valve with a controller 20. For example, while vaporized electrolyte is not being released from the lithium-ion secondary battery L, the opening degree of the butterfly valve 18 is relatively small to prevent the internal pressure from becoming too low. When vaporized electrolyte is released from the lithium-ion secondary battery L and the internal pressure rises, the opening degree of the butterfly valve 18 is relatively large to prevent the internal pressure from becoming positive, that is, to prevent the atmosphere inside the furnace from leaking out of the heat treatment furnace 1 through gaps other than the flue 14.

[0039] The controller 20 controls the output of the heater 8 based on measurements input from the thermocouple 10 and the pressure gauge 12. The controller 20 mainly comprises one or more processors such as CPUs that perform various processes, one or more memories (such as ROM, RAM, or hard disk) that store programs to be executed by the processors and various data necessary for the execution of these programs, an operation unit that receives operation input from the operator, and a display unit that displays the operating status of the heat treatment furnace 1.

[0040] [Heat treatment method] Next, with reference to Figures 3 to 5, a heat treatment method for performing heat treatment on the lithium-ion secondary battery L in the drying and Al oxidation process described above will be explained. Figure 3 is a flowchart showing the flow of the heat treatment method of this embodiment, Figure 4 is a time chart showing the change in furnace temperature over time in the heat treatment method of this embodiment, and Figure 5 is a time chart showing the change in furnace pressure over time in the heat treatment method of this embodiment.

[0041] As shown in Figure 3, first, in step S11, the lithium-ion secondary battery L to be processed is placed on the trolley 6 and put into the furnace of the heat treatment furnace 1, and the front lid 2 is closed.

[0042] Next, in step S12, the controller 20 turns on the output of the heater 8 to start heating the atmosphere of the heat treatment furnace 1, and in step S13, controls the output of the heater 8 so that the furnace temperature rises at a first heating rate (first heating step). The first heating rate is 1°C / min or more and 3°C / min or less, preferably 2°C / min. When the furnace temperature is raised at such a first heating rate, vaporized electrolyte is gradually released from the lithium-ion secondary batteries L that have low safety valve opening pressures due to individual differences among the various lithium-ion secondary batteries L put into the heat treatment furnace 1, thus preventing the electrolyte from being suddenly ejected into the high-temperature heat treatment furnace 1 and causing ignition and combustion. On the other hand, if the heating rate is faster than 3°C / min, for example 5°C / min, the electrolyte may be suddenly ejected into the furnace after the furnace temperature reaches a high temperature (for example 500°C or higher), causing combustion.

[0043] Next, in step S14, the controller 20 determines whether the furnace temperature measured by the thermocouple 10 has reached a predetermined drying temperature. The drying temperature is the temperature required to dry the electrolyte of the lithium-ion secondary battery L that has been placed in the heat treatment furnace 1, and is above the opening temperature of the safety valve of the lithium-ion secondary battery L and below 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 processed, but is, for example, 350°C. In this embodiment, the drying temperature is set to 400°C.

[0044] If the result of the determination in step S14 indicates that the furnace temperature has not reached the drying temperature (step S14: NO), the process returns to step S13, and the controller 20 continues to raise the furnace temperature. On the other hand, if the furnace temperature reaches the drying temperature (step S14: YES), the process proceeds to step S15, and the controller 20 controls the output of the heater 8 to maintain the drying temperature for a predetermined time T1 (first maintenance step). The predetermined time T1 is appropriately determined as the time required to sufficiently dry the electrolyte contained in the lithium-ion secondary battery L, depending on the type and amount of lithium-ion secondary battery L introduced into the heat treatment furnace 1, but can be in the range of, for example, 10 minutes to 2 hours, and in this embodiment it is 30 minutes.

[0045] The changes in furnace temperature and pressure during steps S13 to S15 (first heating step and first maintenance step) described above will be explained with reference to Figures 4 and 5. As shown in Figure 4, when the furnace temperature is raised from room temperature at a first heating rate of 2°C / min in step S13, the drying temperature of 400°C is reached in approximately 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 and combustion due to the rapid ejection of the electrolyte into an atmosphere hotter than the drying temperature does not occur.

[0046] On the other hand, as shown in Figure 5, the furnace pressure is maintained at a negative pressure of -0.5 kPa G while the furnace temperature is rising, then rises around the time the furnace temperature reaches 400°C, peaking at -0.1 kPa G 200 minutes after the start of heating, and then decreasing back to -0.5 kPa G. This indicates that as the furnace temperature approaches the drying temperature, the electrolyte of the lithium-ion secondary battery L vaporizes and is gradually released outside the housing, causing the furnace pressure to rise. In response, the opening of the butterfly valve 18 increases, thus maintaining the furnace pressure at a negative pressure. Thus, in the first heating step and first maintenance step of the heat treatment in this embodiment, the furnace temperature does not rise sharply, nor does the furnace pressure rise to a positive pressure.

[0047] Returning to Figure 3, in step S15, the drying temperature is maintained for a predetermined time T1, and then the process proceeds to step S16, in which the controller 20 controls the output of the heater 8 so that the furnace temperature rises at a second heating rate (second heating step). Since the electrolyte contained in the lithium-ion secondary battery L is dried by the time of step S15 and combustion does not occur, the second heating rate can be faster than the first heating rate. For example, in this embodiment, the second heating rate is 5°C / min.

[0048] Next, in step S17, the controller 20 determines whether the furnace temperature measured by the thermocouple 10 has reached the aluminum oxidation temperature (Al oxidation temperature). The Al oxidation temperature is set higher than the drying temperature mentioned above, preferably higher than the melting point of aluminum (660°C) and lower than the melting point of copper (1085°C). This promotes oxidation by dissolving the aluminum contained in the housing of the lithium-ion secondary battery L and increasing its surface area, thereby obtaining granular alumina, while also preventing the dissolution of copper and facilitating subsequent separation. In this embodiment, the Al oxidation temperature is set to 700°C.

[0049] If the result of the determination in step S17 indicates that the furnace temperature has not reached the drying temperature (step S17: NO), the process returns to step S16, and the controller 20 continues to raise the furnace temperature. On the other hand, if the furnace temperature reaches the Al oxidation temperature (step S17: YES), the process proceeds to step S18, and the controller 20 controls the output of the heater 8 to maintain the Al oxidation temperature for a predetermined time T2 (second maintenance step). The predetermined time T2 is appropriately determined as the time required to sufficiently oxidize the aluminum contained in the lithium-ion secondary battery L, depending on the type and amount of lithium-ion secondary battery L introduced into the heat treatment furnace 1, but can be in the range of, for example, 10 minutes to 2 hours, and in this embodiment it is 30 minutes.

[0050] 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 lower the temperature inside the furnace (cooling process).

[0051] The changes in furnace temperature and pressure during the steps S16 to S18 (Al oxidation step) and the cooling step S19 described above will be explained with reference to Figures 4 and 5. As shown in Figure 4, in step S16, when the furnace temperature is raised from a drying temperature of 400°C at a second heating rate of 5°C / min, which is faster than the first heating rate, the Al oxidation temperature of 700°C is reached 60 minutes from the start of heating, 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 maintenance step, ignition and combustion of the electrolyte do not occur. After that, when the heater 8 is turned OFF in step S19, the furnace temperature gradually decreases and returns to room temperature.

[0052] Furthermore, as shown in Figure 5, the furnace pressure is maintained at a negative pressure of -0.5 kPa G during the Al oxidation process and the cooling process. In this way, even during the Al oxidation process and the cooling process of the heat treatment in this embodiment, the furnace temperature does not rise sharply, nor does the furnace pressure rise to a positive pressure.

[0053] Returning to Figure 3, after the temperature inside the furnace has sufficiently decreased to near room temperature, in step S20, the front cover 2 is opened, the trolley 6 is pulled out of the heat treatment furnace 1, and the heat-treated material L is transferred to the sieving machine. This completes the heat treatment.

[0054] <Variation> Next, a modified example of the embodiment of the present invention will be described.

[0055] In the embodiment described above, air is introduced into the heat treatment furnace 1. However, during part or all of the first heating step and the first maintenance step, an inert gas such as nitrogen or helium may be introduced 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 released 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 a controller 20, and the controller 20 controls the introduction of the inert gas by the inert gas supply device so that the atmosphere inside the furnace has a predetermined oxygen concentration. In this modified example, nitrogen (N2) is used as the inert gas.

[0056] Referring to Figure 6, a heat treatment method for heat-treating a lithium-ion secondary battery L using this modified example will be explained. Figure 6 is a flowchart showing the flow of the heat treatment method of this modified example. Steps S21, S23 to S26, and S28 to S32 in the flowchart of Figure 6 are the same as steps S11 to S20 in the flowchart of Figure 3, so their explanation will be omitted as appropriate.

[0057] In step S21, after closing the front lid 2 of the heat treatment furnace 1, in step S22, the controller 20 introduces N2 into the heat treatment furnace 1 and controls the inert gas supply device so that the oxygen concentration of the atmosphere inside the furnace is 20% or less (for example, 10%).

[0058] Furthermore, after maintaining the drying temperature for a predetermined time T1 in step S26, in step S27, the controller 20 stops introducing 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 rises to the oxygen concentration in the atmosphere after the introduction of N2 is stopped.

[0059] Thus, in this modified example, N2 is introduced into the heat treatment furnace 1 to reduce the oxygen concentration to 20% or less for the entire duration of the first heating step and the first maintenance step. However, N2 may be introduced into the heat treatment furnace 1 for only a portion of the duration of the first heating step and the first maintenance step, for example, in 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.

[0060] Furthermore, although the above-described embodiment explains that the separation process is performed using a sieving machine, the separation process can also be performed manually. In this case, of the first heating process, first maintenance process, second heating process, and second maintenance process included in the heat treatment, the second heating process and the second maintenance process may be omitted, and only the first heating process and the first maintenance process may be performed. Specifically, steps S16 to S18 in the heat treatment shown in Figure 3, or steps S28 to S30 in the heat treatment shown in Figure 6, can be omitted.

[0061] The time change of the furnace temperature in this case will be explained with reference to Figure 7. Similar to the embodiment shown in Figure 4, when the furnace temperature 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 of 400°C is reached 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, the second heating step and the second maintenance step are omitted, and when the heater 8 is turned OFF in step S19 or S31, the furnace temperature gradually decreases and returns to room temperature. During this time, the furnace temperature does not rise sharply, and the furnace pressure does not rise to positive pressure.

[0062] Furthermore, in the embodiments described above, the first and second heating rates were set to constant values, but they may be made to vary. For example, in heat treatment step S13 or S24 (first heating step), the heating rate may be set to a relatively fast rate (e.g., 5°C / min) from the start of heating until the furnace temperature reaches an intermediate temperature lower than the drying temperature (e.g., 250°C), and the heating rate may be set to a relatively slow rate (e.g., 1°C / min) while raising the furnace temperature from the intermediate temperature to the drying temperature. In this way, the furnace temperature can be rapidly increased when there is no risk of electrolyte ignition or combustion immediately after the furnace temperature starts to rise from room temperature, and the furnace temperature can be slowly increased when the furnace temperature approaches the drying temperature and there is a possibility of electrolyte ignition or combustion if the furnace temperature is raised rapidly, thereby achieving both a reduction in the time required for the first heating step and prevention of electrolyte ignition and combustion.

[0063] <Effects and Effects> Next, the operation and effects of the heat treatment method for lithium-ion secondary battery L and the control device for the heat treatment furnace 1 for lithium-ion secondary battery L according to the above-described embodiment and modification will be explained.

[0064] First, according to the heat treatment method and control device for the heat treatment furnace 1 of the above-described embodiment and modified example, in the first heating step, the internal temperature of the heat treatment furnace 1 is raised to a drying temperature of 450°C or higher than the opening temperature of the safety valve of the lithium-ion secondary battery L at a first heating rate set to 1°C / min or higher and 3°C / min or lower. This allows the vaporized electrolyte to be released little by little from the lithium-ion secondary batteries L with lower safety valve opening pressures due to individual differences among the various lithium-ion secondary batteries L placed in the heat treatment furnace 1. Therefore, it is prevented from the electrolyte being ejected all at once into the high-temperature heat treatment furnace 1 and igniting and burning. Furthermore, in the first maintenance step, the internal temperature of 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 thoroughly dried while preventing ignition and combustion.

[0065] Furthermore, since the oxygen concentration inside the heat treatment furnace 1 is reduced to 20% or less during part or all of the first heating step and the first maintenance step, it is possible to more reliably prevent the electrolyte released from the lithium-ion secondary battery L from reacting with oxygen and igniting or burning.

[0066] Furthermore, in the second heating step, the furnace temperature is raised to an aluminum oxidation temperature higher than the drying temperature at a second heating rate faster than the first heating rate. This shortens the time required for the second heating step and promotes the oxidation of aluminum contained in the lithium-ion secondary battery L by creating an atmosphere with an aluminum oxidation temperature higher than the drying temperature.

[0067] Furthermore, after the second heating step and before the cooling step, the furnace temperature is maintained at the aluminum oxidation temperature for a predetermined time in the second maintenance step, which further promotes the oxidation of aluminum contained in the lithium-ion secondary battery L.

[0068] Furthermore, during part or all of the first heating step and the first maintenance step, the oxygen concentration inside the heat treatment furnace 1 is reduced to 20% or less, and during the second heating step, the oxygen concentration inside the heat treatment furnace 1 is increased to the same level as the oxygen concentration in the atmosphere. This promotes the oxidation of aluminum contained in the lithium-ion secondary battery L after the electrolyte has dried. [Explanation of symbols]

[0069] 1. Heat treatment furnace 2 Front lid 4 Piston Cylinder 6 carts 8 Heaters 10 Thermocouples 12 Pressure gauges 14 Flue 16 Exhaust fan 18 Butterfly valve 20 controllers L Lithium-ion secondary battery (processed material)

Claims

1. A method for heat treatment of lithium-ion secondary batteries, A first heating step involves raising the internal temperature of the heat treatment furnace into which the lithium-ion secondary battery is placed to a drying temperature that is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C. After the first heating step, a first maintenance step is performed in which the temperature inside the furnace is maintained at the drying temperature for a predetermined time, After the first maintenance step, a cooling step is performed to lower the temperature inside the furnace, Includes, In the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate during the period from the intermediate temperature to the drying temperature. The aforementioned intermediate temperature is 250°C. A method for heat treatment of lithium-ion secondary batteries.

2. The heat treatment method for a lithium-ion secondary battery according to claim 1, wherein in the first heating step, the heating rate until the furnace temperature reaches the intermediate temperature is 5°C / min, and the heating rate while the furnace temperature rises from the intermediate temperature to the drying temperature is 1°C / min.

3. During part or all of the first heating step and the first maintenance step, the oxygen concentration inside the heat treatment furnace is reduced to 20% or less. A method for heat treatment of a lithium-ion secondary battery according to claim 1.

4. A method for heat treatment of a lithium-ion secondary battery, A first heating step involves raising the internal temperature of the heat treatment furnace into which the lithium-ion secondary battery is placed to a drying temperature that is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C. After the first heating step, a first maintenance step is performed in which the temperature inside the furnace is maintained at the drying temperature for a predetermined time, After the first maintenance step, a cooling step is performed to lower the temperature inside the furnace, Includes, In the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate during the period from the intermediate temperature to the drying temperature. After the first maintenance step and before the cooling step, the process further includes a second heating step in which the furnace temperature is raised to an aluminum oxidation temperature that oxidizes aluminum to a temperature higher than the drying temperature. A method for heat treatment of lithium-ion secondary batteries.

5. After the second heating step and before the cooling step, the process includes a second maintenance step in which the furnace temperature is maintained at the aluminum oxidation temperature for a predetermined time. A method for heat treatment of a lithium-ion secondary battery according to claim 4.

6. During part or all of the first heating step and the first maintenance step, the oxygen concentration in the heat treatment furnace is reduced to 20% or less. During the second heating step, the oxygen concentration inside the heat treatment furnace is raised to the same level as the oxygen concentration in the atmosphere. A method for heat treatment of a lithium-ion secondary battery according to claim 4.

7. A method for heat treatment of a lithium-ion secondary battery according to claim 1 or 4, wherein the internal pressure of the heat treatment furnace is constantly maintained at a negative pressure during the heat treatment process.

8. A control device for a heat treatment furnace for lithium-ion secondary batteries, A temperature sensor for measuring the internal temperature of the heat treatment furnace, A heater for heating the inside of the heat treatment furnace, The system includes a controller for controlling the heater, The aforementioned controller, A first heating step involves raising the internal temperature of the heat treatment furnace into which the lithium-ion secondary battery is placed to a drying temperature that is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C. After the first heating step, a first maintenance step is performed in which the temperature inside the furnace is maintained at the drying temperature for a predetermined time, After the first maintenance step, a cooling step is performed to lower the temperature inside the furnace, The heater is configured to perform the following: In the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate during the period from the intermediate temperature to the drying temperature. The aforementioned intermediate temperature is 250°C. Control device for a heat treatment furnace for lithium-ion secondary batteries.

9. A control device for a heat treatment furnace for lithium-ion secondary batteries, A temperature sensor for measuring the internal temperature of the heat treatment furnace, A heater for heating the inside of the heat treatment furnace, The system includes a controller for controlling the heater, The aforementioned controller, A first heating step involves raising the internal temperature of the heat treatment furnace into which the lithium-ion secondary battery is placed to a drying temperature that is above the opening temperature of the safety valve of the lithium-ion secondary battery and below 450°C. After the first heating step, a first maintenance step is performed in which the temperature inside the furnace is maintained at the drying temperature for a predetermined time, After the first maintenance step, a cooling step is performed to lower the temperature inside the furnace, After the first maintenance step and before the cooling step, a second heating step is performed to raise the furnace temperature to an aluminum oxidation temperature that oxidizes aluminum to a temperature higher than the drying temperature, The heater is configured to perform the following: In the first heating step, the heating rate until the furnace temperature reaches an intermediate temperature lower than the drying temperature is faster than the heating rate while the furnace temperature rises from the intermediate temperature to the drying temperature. Control device for a heat treatment furnace for lithium-ion secondary batteries.

Images