Heat treatment method for lithium ion secondary batteries

Abstract

To provide a method for preventing a thermal runaway caused by ignition of lithium ion secondary batteries at low costs without controlling a heating atmosphere in heat treatment used for preliminary treatment for recovering valuable materials from the lithium ion secondary batteries.SOLUTION: A heat treatment method for lithium ion secondary batteries includes: storing the lithium ion secondary batteries into a plurality of metallic housing means; and heating the plurality of metallic housing means housing the lithium ion secondary batteries using heating means disposed outside the plurality of metallic housing means under an air atmosphere. A thermal runaway by ignition of the lithium ion secondary batteries at the time of heat treatment can be prevented by adjusting temperature rise speed of the housing means housing the lithium ion secondary batteries by performing heating after storing water with the lithium ion secondary batteries in some or all of the plurality of metallic housing means.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a heat treatment method used in pretreatment for recovering valuable materials from lithium ion secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries are lighter, have a higher capacity, and have a higher electromotive force than conventional secondary batteries, and are therefore widely used in electronic devices such as personal computers and mobile phones, as well as in automotive batteries.

[0003] The positive electrode active material of lithium-ion secondary batteries has generally been composed of a composite oxide of lithium and a transition metal. Among them, lithium cobalt oxide (LiCoO2), a composite oxide containing Co, is widely used. Recently, lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), or ternary oxide (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, etc.) and their composite types are also increasingly being used. The electrolyte for lithium-ion secondary batteries is mainly made by dissolving lithium salts such as electrolyte LiPF6 and LiBF4 in a mixed solvent of cyclic carbonate esters such as PC (propylene carbonate) and EC (ethylene carbonate) and chain esters such as DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate), but these organic solvents are flammable and may pose a risk of fire if handled incorrectly.

[0004] In recent years, the use of the lithium-ion secondary batteries has been expanding rapidly, and therefore it is desired to separate and recover valuable metals, such as nickel and cobalt used in the positive electrode active material and aluminum and copper used in the electrode current collector, from used and discarded lithium-ion secondary batteries and from scrap of defective products generated during the manufacturing process of such batteries.

[0005] In this case, for example, lithium ion secondary batteries used in portable electronic devices and the like are becoming smaller and thinner, and it is practically impossible to process independent lithium ion secondary batteries individually, considering the cost, etc. Therefore, in the past, a large number of lithium ion secondary batteries have been stored or transported in an accumulated state in a metal storage means, or the lithium ion secondary batteries have been subjected to separation and recovery processing.

[0006] Discarded lithium-ion secondary batteries contain flammable organic solvents. In addition, it may be difficult to completely discharge and deactivate all the batteries at the source of waste lithium-ion batteries, so lithium-ion batteries that remain in a charged state may be discarded. As mentioned above, lithium-ion secondary batteries are becoming smaller and thinner, and when many lithium-ion secondary batteries are stacked together, they are easily deformed by external forces, which can cause a short circuit inside the battery and cause the battery itself to catch fire during storage or separation and collection of waste lithium-ion secondary batteries.

[0007] In the separation and recovery process, the organic solvent remaining in the lithium ion secondary battery is combusted by heat treatment as a pretreatment. However, during this process, the organic solvent may combust explosively and abnormally, causing thermal runaway in the storage means. This thermal runaway may also occur due to a reaction in which the positive electrode active material of the lithium ion secondary battery thermally decomposes, releasing oxygen and accelerating its own combustion, so that the thermal runaway may not be prevented even if the atmosphere (oxygen concentration) of the heat treatment is adjusted. When abnormal combustion occurs, the temperature of the exhaust gas may rise rapidly, which may damage the furnace wall and flue of the heating furnace. In addition, rapid combustion may cause an oxygen deficiency in the furnace, which may increase the CO concentration in the exhaust gas. Therefore, a method for controlling the combustion speed of the lithium ion secondary battery in the storage means is required.

[0008] As a method for controlling the burning rate of a lithium-ion secondary battery, for example, Patent Document 1 discloses a technique in which a lithium-ion secondary battery is housed in an oxygen-shielding container, and the oxygen partial pressure in the shielding container is adjusted to 0% to 5% to roast the lithium-ion secondary battery. The use of an oxygen-shielding container reduces the burning rate of the lithium-ion secondary battery, but when multiple lithium-ion secondary batteries are treated, the combustion heat of one lithium-ion secondary battery may heat the other lithium-ion battery, causing a problem that thermal runaway may occur due mainly to the decomposition of the positive electrode active material and the generation of oxygen. In addition, a step of reducing the oxygen partial pressure in the oxygen-shielding container is required, which increases the cost of treatment.

[0009] Patent Document 2 discloses a method for heat treating waste batteries, in which an atmospheric gas containing at least one selected from the group consisting of oxygen, nitrogen, carbon dioxide, and water vapor is passed through a heat treating furnace in which waste lithium ion secondary batteries are placed, to adjust the oxygen partial pressure in the furnace. The oxygen partial pressure in the heat treating furnace is set to 5×10 -4 atm~4×10 -2 It is said that a pressure range of atm is preferable. However, even in the case of the method disclosed in Patent Document 2, when multiple lithium ion secondary batteries are treated, the combustion heat of one lithium ion secondary battery heats the other lithium ion battery, which may cause thermal runaway mainly due to decomposition of the positive electrode active material and oxygen generation. In addition, it is necessary to reduce the oxygen partial pressure in the heat treatment furnace, which increases the treatment cost.

[0010] Patent Document 3 discloses a technique for storing lithium ion secondary batteries in a metal storage means, and for reducing the rate of temperature rise of the storage means storing the lithium ion secondary batteries by providing a flame interruption means between the direct flame burner and the storage means when the storage means is heated from the outside with a direct flame burner. However, while the technique disclosed in Patent Document 3 can control the burning rate of the metal storage means, it has a drawback in that when multiple metal storage means are heated to a high temperature using the technique, if thermal runaway occurs in one storage means, it is not possible to prevent the influence of this on the other storage means. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2016-219402 A [Patent Document 2] JP 2021-163645 A [Patent Document 3] JP 2022-048614 A Summary of the Invention [Problem to be solved by the invention]

[0012] In view of the above problems, the present invention aims to provide a method for preventing thermal runaway caused by ignition of a lithium ion secondary battery at low cost without controlling the heating atmosphere in a heat treatment used in a pretreatment for recovering valuable materials from a lithium ion secondary battery. In this specification, thermal runaway means a combustion state in which a rapid temperature rise behavior is observed even in a temperature range where the heating furnace outlet temperature is 1000°C or higher. The heating furnace outlet temperature is the temperature of the gas at the outlet through which the gas in the furnace flows out to the exhaust passage. [Means for solving the problem]

[0013] In order to achieve the above object, the present specification discloses the following invention. [1] A method for heat treating lithium ion secondary batteries, comprising placing lithium ion secondary batteries in a plurality of metal containing means, and heating the plurality of containing means containing the lithium ion secondary batteries in an atmospheric environment by heating means disposed outside the plurality of metal containing means, wherein water is stored together with the lithium ion secondary batteries in some or all of the plurality of metal containing means, and then heated, thereby adjusting the rate of temperature rise of the containing means containing the lithium ion secondary batteries. [2] The heat treatment method for lithium ion secondary batteries described in [1] above, wherein when a plurality of storage means accommodating the lithium ion secondary batteries are heated by the heating means, storage means storing lithium ion secondary batteries and water and storage means containing only lithium ion secondary batteries are arranged. [3] The method for heat treating a lithium ion secondary battery according to the above [1] or [2], wherein the housing means is made of iron or stainless steel. [4] The heat treatment method for a lithium ion secondary battery according to any one of the above [1] to [3], wherein the storage means for storing water has a lid member having a plurality of openings at its upper portion. Effect of the Invention

[0014] The heat treatment method of the present invention does not require atmospheric control during heat treatment, and allows pretreatment for recovering valuable materials from lithium ion secondary batteries to be performed safely and at low cost. Therefore, the present invention can contribute to reducing the cost of recovering valuable metals from discarded lithium ion secondary batteries. [Brief description of the drawings]

[0015] [Figure 1] 6 is a graph showing an example of the change over time in the temperature at the outlet of a heating furnace and the oxygen concentration in a chimney when only lithium ion secondary batteries are housed and externally heated. [Diagram 2] FIG. 2 is a horizontal cross-sectional view of a heating furnace, illustrating the arrangement of a DM and other objects to be heated in the first embodiment. [Diagram 3]Graph showing the change over time in the temperature at the outlet of the heating furnace and the temperature in the vicinity of the mobile phone DM and the mixed DM when the heat treatment of Example 1 was performed. [Figure 4] FIG. 11 is a horizontal cross-sectional view of a heating furnace, showing a schematic arrangement of a mix DM and an on-vehicle LIB accommodation means in Example 2. [Diagram 5] 4 is a graph showing the change in temperature at the outlet of a heating furnace over time when the heat treatment of Example 2 was performed. [Figure 6] 3 is a vertical cross-sectional view of the heating furnace, showing a schematic view of the AA cross section in FIG. 2. [Figure 7] FIG. 11 is a horizontal cross-sectional view of a heating furnace, showing a schematic arrangement of an in-vehicle LIB accommodating means and a mix LIB accommodating DM in the third embodiment. [Figure 8] 1 is a graph showing the change in temperature at the outlet of a heating furnace over time when the heat treatment of Example 3 was performed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] [Lithium-ion secondary battery] The discarded lithium ion secondary batteries that are the subject of the heat treatment method of the present invention may be any lithium ion secondary battery, including not only small lithium ion secondary batteries such as button batteries used in mobile phones and other electronic devices, but also large lithium ion secondary batteries for vehicle use, etc. Also, the present invention includes not only used and discarded lithium ion secondary batteries, but also scrap discarded during the manufacturing process of lithium ion secondary batteries due to poor performance or other reasons.

[0017] [Means of Containment] When waste lithium ion secondary batteries are accumulated and stored in a storage means, the batteries may be deformed due to their own weight or external forces, which may cause the batteries to ignite. In addition, in the heat treatment method for lithium ion secondary batteries of the present invention, the storage means storing the batteries is heated from the outside, so the storage means must have heat resistance and impact resistance. For this reason, the storage means for storing the lithium ion secondary batteries is made of metal, preferably iron or stainless steel plate. Specifically, from the viewpoints of availability and cost, drums and the like as specified in JIS Z1600 can be used. Here, the drum is a cylindrical body plate with a base plate provided at the bottom of one end side.

[0018] The storage means for storing the lithium ion secondary batteries may have a lid member with a plurality of openings at the top. In this case, the opening diameter of the plurality of openings is slightly smaller than the diameter and length of the batteries to be heat-treated. The lid member may be a punching plate, a mesh-like or lattice-like metal member, a metal woven fabric, or the like. The lid member is structured so as to be removable in order to store discarded lithium ion secondary batteries in the storage means. The reason for providing the lid member with the plurality of openings on the storage member is as follows.

[0019] The greatest technical feature of the heat treatment method for lithium ion secondary batteries of the present invention is that, as described below, the rate of temperature rise of the storage means that stores the lithium ion secondary batteries is adjusted by storing water in the storage means and heating it. In this case, it is necessary to adjust the amount of water stored in the storage means.

[0020] When lithium ion secondary batteries are stacked and stored in a storage means, it is common to store the batteries in water in the storage means in which the lithium ion secondary batteries are stored in order to prevent spontaneous combustion of the batteries. In this case, a simple method for adjusting the amount of water is to invert the storage means containing the lithium ion secondary batteries and water by a known mechanical means, and to drain the water through the lid member.

[0021] [Heating means] In the heat treatment method for lithium ion secondary batteries of the present invention, a plurality of storage means for storing discarded lithium ion secondary batteries are heated in an air atmosphere using a heating means disposed outside the plurality of metal storage means. In the case of air heating, equipment for controlling the heating atmosphere is not required, and the treatment cost can be reduced. In the heat treatment method of the present invention, as a heat treatment furnace for external heating, heating by a direct flame burner, a batch type muffle furnace, a continuous type roller hearth kiln, a mesh belt kiln, an electric heating furnace, etc. can be used, but from the viewpoint of energy efficiency, heating by a direct flame burner is preferable. Note that, when a plurality of direct flame burners are used for external heating, it becomes possible to heat any of the plurality of metal storage means.

[0022] [Heat treatment] As described above, in the heat treatment method for lithium ion secondary batteries of the invention, a plurality of storage means that store lithium ion secondary batteries are heated from the outside in an air atmosphere, and the greatest technical feature is that, in this case, water is stored together with the lithium ion secondary batteries in some or all of the plurality of metal storage means, and then heated, thereby adjusting the rate of temperature rise of the storage means that store the lithium ion secondary batteries. If the rate of temperature rise of the storage means can be adjusted, even if thermal runaway due to battery ignition occurs in one storage means, the effects of this can be prevented from spreading to other storage means.

[0023] 1 shows an example of the time change in the furnace outlet temperature and the oxygen concentration in the chimney when thermal runaway occurs in the furnace when only lithium ion secondary batteries are stored in all the storage means and external heating is performed (Comparative Example 1 described later). When thermal runaway occurs, rapid combustion of the lithium ion secondary batteries occurs, and the oxygen in the furnace is suddenly consumed, resulting in incomplete combustion, and a rapid rise in the furnace outlet temperature and a drop in the oxygen concentration in the chimney are observed.

[0024] In the first embodiment of the present invention, water is stored in all of the plurality of storage means that store the lithium ion secondary batteries and then heated. In this embodiment, the amount of water stored in each storage means may be the same, but it is preferable to store different amounts for each storage means. If the storage means have different amounts, the time it takes for all of the water stored in the storage means to evaporate when external heating is performed differs depending on the storage means, so even if thermal runaway occurs in one storage means, there will be storage means around it that store water inside, without all of the water evaporating. Even if the amount of water stored in the storage means is the same, the amount of water evaporated in each storage means differs depending on the distance from the heat source in the furnace (for example, a direct flame burner), so it is possible to prevent each storage means from experiencing thermal runaway at the same time.

[0025] The amount of water to be stored in the storage means is adjusted by adjusting the amount of water poured into the storage means storing the waste lithium ion secondary batteries if no water was placed in the storage means when the waste lithium ion secondary batteries were stored, or if the waste lithium ion secondary batteries were stored with water placed in the storage means, the amount of water to be stored is adjusted by adjusting the time for draining the water by turning over the storage means storing the waste lithium ion secondary batteries.

[0026] In a second embodiment of the present invention, water is stored in some of the multiple storage means that store the lithium ion secondary batteries and heated. The storage means for storing water may store an amount of water that fills the storage means. If the storage means contains water and waste lithium ion secondary batteries are stored, the water in the storage means that stores the waste lithium ion secondary batteries can be completely drained to store only waste lithium ion secondary batteries and no water is stored in the storage means.

[0027] In a second embodiment of the present invention, external heating is performed in a state where a storage means that does not store water and a storage means that stores water are arranged in a heating furnace. In this embodiment, the storage means that stores water may be appropriately arranged between the storage means that do not store water, taking into consideration the capacity of the waste lithium ion secondary batteries to be treated and the external heating means. From the viewpoint of preventing thermal runaway, it is preferable that the ratio of the number of storage means that stores water to the total number of storage means that store lithium ion secondary batteries is 30% or more.

[0028] In the second embodiment of the present invention, the process of individually adjusting the amount of water to be stored in the storage means can be omitted, and therefore, using this embodiment is preferable from the viewpoint of omitting steps and reducing processing costs. EXAMPLES

[0029] [Example 1] Four 200L steel open drums (DM) were prepared as storage means for the lithium-ion secondary batteries (LIBs) to be heat-treated. LIBs for mobile phones were placed into two of the DMs until they were nearly full. The mass of the mobile phone LIBs placed in each DM was 200 kg. The remaining two DMs were placed into a mixture of small LIBs used in home appliances until they were nearly full. The mass of the LIB mixture placed in each DM was 150 kg.

[0030] Of the DMs containing the LIBs, tap water was poured into one of the DMs containing LIBs for mobile phones (hereafter referred to as mobile phone DM) and one of the DMs containing a mixture of LIBs (hereafter referred to as mixed DM) until the LIBs in the DM were completely immersed. The four DMs were loaded onto a cart along with 750 kg of other objects to be heated, and then placed in a heating furnace equipped with eight direct flame burners.

[0031] Figure 2 shows the layout of the DM and other objects to be heated. In the figure, the contents of the DM that contains water are labeled "water." "Mixed LIB" in the figure means a mixture of LIBs. The carts are loaded into the heating furnace with the DM at the back and the other objects to be heated at the door side. Thermocouples were installed in each DM to measure the temperature near the LIB during heat treatment. Figure 6 is a schematic diagram showing a cross section of the furnace shown in Figure 2 taken along the line AA. Figure 6 shows the furnace with the door slightly open, and does not show the object to be heated. FIG. 3 shows the change over time in the temperature at the outlet of the heating furnace and the temperature in the vicinity of the mobile phone DM and the mixed DM when the heat treatment in Example 1 was performed.

[0032] In this example, first, burners No. 4 and No. 8 were ignited and the furnace outlet temperature was raised to about 300°C, promoting the combustion of the LIB in the DM where no water was stored. Around 60 min after ignition, the LIB in the DM where no water was stored started to burn, so the burners were extinguished and the LIB was allowed to spontaneously combust. The temperature near the LIB rose due to the spontaneous combustion of the LIB, but thermal runaway did not occur. Spontaneous combustion continued for about 60 min (up to about 120 min from the initial ignition) and was completed.

[0033] From that point on, burners No. 1 and No. 5 were used to heat other objects. As a result, the heat from the combustion of the other objects caused the water in the DM to evaporate, and 180 to 210 minutes after the initial ignition, the start of combustion of the LIB in the DM was confirmed by an in-furnace camera. Even after the combustion of the LIB in the DM was mostly completed, the furnace outlet temperature was kept above 750°C, but no sudden rise in the furnace outlet temperature was observed.

[0034] [Comparative Example 1] The heat treatment of this example was carried out by repeating the same operations as in Example 1, except that tap water was not injected into all DMs. The results are shown in Figure 1. Approximately 30 minutes after ignition, the temperature at the outlet of the heating furnace rose sharply, making it difficult to control the temperature. After that, the temperature at the outlet of the heating furnace rose to 1200°C or higher, and the oxygen concentration was 5% or less, resulting in an oxygen-deficient state in the furnace.

[0035] [Example 2] The LIBs to be heat-treated in this example are a mixed LIB (a mixture of small LIBs) used in household appliances and an in-vehicle LIB pack. The sample preparation method for the mixed LIB used in household appliances is the same as that for the mixed DM containing water described in Example 1, except that the amount of water stored (height inside the DM) was set to about half of the DM. The in-vehicle LIB pack was stored in a dedicated iron storage means (an iron container 2350 mm wide and 1650 mm deep) without storing water, without dismantling a 500 kg item.

[0036] Two storage units containing the vehicle-mounted LIB packs were placed apart on a cart, and two mixed DMs were placed between them. Figure 4 shows the layout of the mixed DM and the storage unit for the vehicle-mounted LIBs. In this case, the left side of the figure is the back side of the heating furnace, and the right side is the door side. FIG. 5 shows the change over time in the temperature at the outlet of the heating furnace when the heat treatment of Example 2 was carried out.

[0037] In this example, first, burners No. 4 and No. 8 were ignited, and after 75 min when the furnace outlet temperature reached about 300°C, the vehicle-mounted LIB pack started to burn, so the burners were extinguished and the vehicle-mounted LIB pack was allowed to spontaneously combust. The spontaneous combustion continued for 60 min (135 min from the first ignition) and was completed. From that point, burners No. 1 and No. 5 were ignited to heat the vehicle-mounted LIB pack on the furnace door side, and the vehicle-mounted LIB pack started to burn immediately after that, so the burners were extinguished and the vehicle-mounted LIB pack was allowed to spontaneously combust. Next, burners No. 2 and No. 6 were ignited to promote the evaporation of water in the mix DM and the combustion of the LIB, and the LIB in the mix DM started to burn 225 min after the first ignition, so the burners were extinguished and the LIB was allowed to spontaneously combust. After the combustion of all the LIBs was completed, the furnace temperature was kept at 750°C for 1 h, but no thermal runaway occurred.

[0038] From the above results, it was found that by using the heat treatment method of the present invention, thermal runaway due to abnormal combustion in lithium ion secondary batteries can be suppressed during heat treatment as a pretreatment for recovering valuable materials from lithium ion secondary batteries.

[0039] [Example 3] The objects to be heat-treated were an on-board LIB pack and a mixed LIB (a mixture of small LIBs used in mobile phones and household appliances). A 500 kg on-board LIB pack was placed in a dedicated iron container (iron vessel) without being disassembled, without storing water. The mixed LIBs were placed in eight 200 L DMs (iron open drums), each until they were nearly full. The mass of the mixed LIBs contained in each DM was approximately 200 kg. Of these eight DMs, three were filled with water up to 50% of the DM's height when the mixed LIBs were contained, and the remaining five were filled with water up to 80% of the DM's height when the mixed LIBs were contained.

[0040] One of the iron containers containing the vehicle-mounted LIB packs and eight of the DMs containing the mixed LIBs were placed on a cart and loaded into a heating furnace similar to that in Example 1. The layout of the vehicle-mounted LIB storage means and the mixed LIB storage DM are shown in Fig. 7. In Fig. 7, the DM marked with the symbol (A) is filled with water up to 50% height, and the DM marked with the symbol (B) is filled with water up to 80% height. FIG. 8 shows the change over time in the temperature at the outlet of the heating furnace when the heat treatment of Example 2 was carried out.

[0041] In this example, first, burners No. 4 and No. 8 were ignited, and the temperature at the furnace outlet was raised to about 300°C to promote the combustion of the vehicle-mounted LIB pack. 45 min after ignition, the vehicle-mounted LIB pack started to burn, so the burners were turned off and the pack was allowed to spontaneously combust. Spontaneous combustion was completed 75 min later (about 120 min after ignition). Next, burners No. 1 and No. 6, which were directly exposed to the 50% water DM, were ignited to promote the combustion of the mixed LIB in the 50% water DM, and combustion in the 50% water DM started 210 min after the initial ignition. Next, burners No. 2 and No. 5 were ignited to promote the evaporation of water and the combustion of the mixed LIB in the 80% water DM. 270 min after the initial ignition, all the water evaporated, and the remaining mixed LIB started to burn. After that, the flames of burners No. 1, No. 6, No. 2 and No. 5 were continued to be introduced, and the temperature was raised to nearly 1000°C, but the temperature dropped as the combustion of the mixed LIB was completed. After that, the temperature was kept at 750°C or higher for more than 1 hour, and then all the burners were extinguished.

[0042] As in this example, by setting the amount of water to be put into the container, which is the storage means, in multiple stages, it becomes easier to control the timing of combustion of the lithium-ion secondary batteries placed in the furnace to be divided (shifted), preventing unexpected sudden combustion and enabling safer operation.

Claims

1. A method for heat treating lithium ion secondary batteries, comprising placing lithium ion secondary batteries in a plurality of metal containing means, and heating the plurality of containing means containing the lithium ion secondary batteries in the atmosphere by heating means disposed outside the plurality of metal containing means, wherein water is stored together with the lithium ion secondary batteries in some or all of the plurality of metal containing means, and then heated, thereby adjusting the rate of temperature rise of the containing means containing the lithium ion secondary batteries.

2. 2. The heat treatment method for lithium ion secondary batteries according to claim 1, wherein when the plurality of storage means housing the lithium ion secondary batteries are heated by the heating means, storage means housing a lithium ion secondary battery and water and storage means housing only a lithium ion secondary battery are arranged.

3. 2. The method for heat treating a lithium ion secondary battery according to claim 1, wherein the container is made of iron or stainless steel.

4. 2. The method for heat treating a lithium ion secondary battery according to claim 1, wherein the container for storing water has a lid member having a plurality of openings at an upper portion thereof.

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