Heat treatment method for lithium-ion secondary battery

Abstract

[Problem] To provide, for a heat treatment that is used in a pretreatment for recovering valuable materials from a lithium-ion secondary battery, a low-cost method for preventing thermal runaway due to ignition of the lithium-ion secondary battery without controlling the heating atmosphere. [Solution] A heat treatment method for a lithium-ion secondary battery, whereby a lithium-ion secondary battery is inserted into a plurality of housing means made of metal, and the plurality of housing means housing the lithium-ion secondary battery are heated by a heating means arranged outside the plurality of metal housing means under atmospheric conditions. Water is stored and heated together with the lithium-ion secondary battery in some or all of the plurality of metal housing means, and thereby the rate of temperature increase of the housing means in which the lithium-ion secondary battery is housed is adjusted, and as a result, during the heat treatment, thermal runaway due to ignition of the lithium-ion secondary battery can be prevented.

Description

Heat treatment method for lithium-ion secondary battery

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

[0002] Lithium-ion secondary batteries are lighter, have 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 (LiCoO), which is a composite oxide containing Co as a component, has been particularly popular. ２ ) is widely used. Recently, lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMn ２ O ４ ), or ternary (LiNi １／３ Mn １／３ Co １／３ O ２ The use of electrolytes such as LiPF4 and LiPF5 is also increasing. ６ , LiBF ４ The lithium salts, such as those listed above, are dissolved in a mixed solvent of a cyclic carbonate ester, such as PC (propylene carbonate) or EC (ethylene carbonate), and a chain ester, such as DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), or DEC (diethyl carbonate). However, these organic solvents are flammable and may pose a risk of fire depending on how they are handled.

[0004] In recent years, the use of the lithium-ion secondary batteries has been rapidly expanding, 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 scrap of defective products generated during the manufacturing process of such batteries.

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

[0006] Discarded lithium-ion secondary batteries contain flammable organic solvents. Furthermore, it can be difficult to completely discharge and deactivate all batteries at the source of discarded lithium-ion batteries, resulting in discarded lithium-ion batteries that remain in a charged state. As mentioned above, lithium-ion secondary batteries are becoming increasingly smaller and thinner. When multiple lithium-ion secondary batteries are stacked together, they are easily deformed by external forces, which can cause short circuits within the batteries and lead to the risk of the batteries catching fire during storage or during separation and collection.

[0007] In the separation and recovery process, a pretreatment step involves burning the organic solvent remaining in the lithium-ion secondary battery through heat treatment. However, this process can lead to explosive and abnormal combustion of the organic solvent, resulting in thermal runaway within the storage means. This thermal runaway can also occur when the positive electrode active material of the lithium-ion secondary battery thermally decomposes, releasing oxygen and accelerating its own combustion. Therefore, adjusting the atmosphere (oxygen concentration) during the heat treatment may not prevent thermal runaway. Abnormal combustion can cause a sudden rise in the temperature of the exhaust gas, potentially damaging the furnace walls and flue. Furthermore, rapid combustion can lead to an oxygen deficiency within the furnace, potentially increasing the CO concentration in the exhaust gas. Therefore, a method for controlling the combustion rate of the lithium-ion secondary battery within the storage means is needed.

[0008] As a method for controlling the combustion rate of lithium-ion secondary batteries, for example, Patent Document 1 discloses a technique in which lithium-ion secondary batteries are housed in an oxygen-shielding container and roasted while adjusting the oxygen partial pressure within the container to between 0% and 5%. While using an oxygen-shielding container reduces the combustion rate of lithium-ion secondary batteries, there is a problem in that when multiple lithium-ion secondary batteries are processed, the combustion heat from one lithium-ion secondary battery can heat the other lithium-ion batteries, potentially causing thermal runaway due primarily to decomposition of the positive electrode active material and oxygen generation. Furthermore, a step of reducing the oxygen partial pressure within the oxygen-shielding container is required, which increases processing costs.

[0009] Patent Document 2 discloses a method for heat treating battery waste in which an atmospheric gas containing at least one selected from the group consisting of oxygen, nitrogen, carbon dioxide, and water vapor is flowed into a heat treatment furnace in which lithium ion secondary battery waste is placed, and the oxygen partial pressure in the furnace is adjusted to 5×10 －４ atm ~ 4 x 10 －２ It is said that a range of 1000 to 2000 atm is preferable. However, even in the case of the method disclosed in Patent Document 2, when treating multiple lithium-ion secondary batteries, the combustion heat of one lithium-ion secondary battery may heat the other lithium-ion batteries, which may cause thermal runaway due mainly 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 technology in which a lithium ion secondary battery is housed in a metal housing means, and when the housing means is heated from the outside with a direct flame burner, a flame interruption means is provided between the direct flame burner and the housing means to reduce the rate of temperature rise of the housing means housing the lithium ion secondary battery. However, while the technology disclosed in Patent Document 3 can control the combustion rate of the metal housing means, it has the drawback that when multiple metal housing means are heated using this technology to a high temperature, if thermal runaway occurs in one housing means, it is not possible to prevent the other housing means from being affected.

[0011] JP 2016-219402 A JP 2021-163645 A JP 2022-048614 A

[0012] In view of the above problems, an object of the present invention is 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 during heat treatment used in pretreatment for recovering valuable materials from the lithium-ion secondary battery. In this specification, thermal runaway refers to a combustion state in which a rapid temperature rise is observed even in a temperature range where the outlet temperature of a heating furnace is 1000°C or higher. The outlet temperature of the heating furnace is the temperature of the gas at the outlet through which the gas in the furnace flows out to the exhaust passage.

[0013] To achieve the above-mentioned object, the present specification discloses the following inventions. [1] A method for heat treating lithium ion secondary batteries, which comprises placing lithium ion secondary batteries in a plurality of metal housing means and heating the plurality of housing means housing the lithium ion secondary batteries in an atmospheric environment using a heating means disposed outside the plurality of metal housing means, wherein water is stored together with the lithium ion secondary batteries in some or all of the plurality of metal housing means and then heated, thereby adjusting the rate of temperature rise of the housing means housing the lithium ion secondary batteries. [2] The method for heat treating lithium ion secondary batteries according to [1] above, wherein, when heating the plurality of housing means housing the lithium ion secondary batteries using the heating means, housing means housing means housing lithium ion secondary batteries and water and housing means housing only lithium ion secondary batteries are arranged. [3] The method for heat treating lithium ion secondary batteries according to [1] or [2] above, wherein the housing means are made of iron or stainless steel. [4] The heat treatment method for a lithium ion secondary battery according to any one of [1] to [3] above, wherein the storage means for storing water has a lid member having a plurality of openings at the top thereof.

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

[0015] Graph showing an example of the change over time in the heating furnace outlet temperature and the oxygen concentration in the chimney when only lithium ion secondary batteries are housed and externally heated. A horizontal cross-sectional view of a heating furnace schematically showing the arrangement of DM and other objects to be heated in Example 1. A graph showing the change over time in the heating furnace outlet temperature and the temperature near the mobile phone DM and mixed DM when the heat treatment of Example 1 is performed. A horizontal cross-sectional view of a heating furnace schematically showing the arrangement of the mixed DM and the vehicle-mounted LIB storage means in Example 2. A graph showing the change over time in the heating furnace outlet temperature when the heat treatment of Example 2 is performed. A vertical cross-sectional view of a heating furnace schematically showing the A-A cross section in Figure 2. A horizontal cross-sectional view of a heating furnace schematically showing the arrangement of the vehicle-mounted LIB storage means and the mixed LIB-containing DM in Example 3. A graph showing the change over time in the heating furnace outlet temperature when the heat treatment of Example 3 is performed.

[0016] [Lithium-ion secondary battery] The discarded lithium-ion secondary batteries that are the subject of the heat treatment method of the present invention are not limited to small lithium-ion secondary batteries such as button batteries used in mobile phones and other electronic devices, but may be any lithium-ion secondary battery, such as large lithium-ion secondary batteries for automotive use, etc. Furthermore, 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] [Storage Means] When waste lithium-ion secondary batteries are accumulated and stored in a storage means, the batteries may deform due to their own weight or external forces, potentially resulting in the batteries catching fire. Furthermore, in the heat treatment method for lithium-ion secondary batteries of the present invention, the storage means containing the batteries is heated from the outside, so the storage means must be heat-resistant and impact-resistant. Therefore, the storage means for storing the lithium-ion secondary batteries is made of metal, preferably iron or stainless steel plate. Specifically, from the standpoints of availability and cost, drums specified in JIS Z1600 can be used. Here, a drum is a cylindrical body with a base plate attached to the bottom at one end.

[0018] The storage means for storing the lithium-ion secondary batteries may have a lid member with multiple openings at its top. In this case, the diameter of the multiple openings is slightly smaller than the diameter and length of the batteries to be heat-treated. The lid member may be a punched plate, a mesh or lattice-shaped metal member, or a metal woven fabric. The lid member is designed to be removable so that discarded lithium-ion secondary batteries can be stored in the storage means. The reason for providing the lid member with the multiple 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 will be described later, the rate of temperature rise of the storage means that stores the lithium ion secondary batteries is adjusted by storing and heating water in the storage means. 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 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 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 atmospheric atmosphere using heating means disposed outside the plurality of metal storage means. Heating in the atmosphere eliminates the need for equipment to control the heating atmosphere, thereby reducing processing costs. In the heat treatment method of the present invention, a heat treatment furnace for external heating can be used, such as a direct flame burner, a batch-type muffle furnace, a continuous roller hearth kiln, a mesh belt kiln, or an electric heating furnace. However, from the viewpoint of energy efficiency, heating by a direct flame burner is preferred. Using a plurality of direct flame burners for external heating makes it 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 present invention, a plurality of housing means each housing a lithium ion secondary battery is heated from the outside in an atmospheric environment, and the greatest technical feature is that the rate of temperature rise of the housing means housing the lithium ion secondary batteries is adjusted by storing and heating water together with the lithium ion secondary batteries in some or all of the plurality of metal housing means. If the rate of temperature rise of the housing means can be adjusted, even if thermal runaway due to battery ignition occurs in one housing means, the effects of this can be prevented from spreading to other housing means.

[0023] 1 shows an example of the time changes 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 housed in all housing means and external heating is performed (Comparative Example 1 described below). 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 a first embodiment of the present invention, water is stored in all of the plurality of storage means containing the lithium-ion secondary batteries and 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 of water for each storage means. If the storage means have different amounts of water, the time it takes for all of the water stored in the storage means to evaporate when external heating is applied will differ depending on the storage means. Therefore, even if thermal runaway occurs in one storage means, there will be storage means around it that still have water stored inside. Even if the amount of water stored in each storage means is the same, the amount of water evaporated in each storage means will differ depending on the distance from the heat source (e.g., an open flame burner) in the furnace, preventing simultaneous thermal runaway in each storage means.

[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 that stores the used lithium-ion secondary batteries if no water was placed in the storage means when the used lithium-ion secondary batteries were stored.If the used lithium-ion secondary batteries were stored with water 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 that stores the used lithium-ion secondary batteries.

[0026] In a second embodiment of the present invention, water is stored in some of the plurality of storage means that store the lithium ion secondary batteries and heated. The storage means for storing water may store an amount of water sufficient to fill the storage means. If used lithium ion secondary batteries have been stored in the storage means with water, the water can be completely drained from the storage means that stores the used lithium ion secondary batteries, leaving the storage means in a state in which only used lithium ion secondary batteries are stored and no water is stored.

[0027] In a second embodiment of the present invention, external heating is performed with storage means that do not store water and storage means that store water arranged in a heating furnace. In this embodiment, the storage means that store water may be appropriately arranged between 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 store water to the total number of storage means that store lithium-ion secondary batteries be 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.

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

[0030] Of the DMs containing the LIBs, one containing LIBs for mobile phones (hereinafter referred to as "mobile phone DM") and one containing a mixture of LIBs (hereinafter referred to as "mixed DM") were filled with tap water until the LIBs in the DM were completely submerged. 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 arrangement of the DM and other objects to be heated. In the figure, the contents of the DM containing water are labeled "water." "Mixed LIB" in the figure refers to a mixture of LIB. The carts are loaded into the heating furnace with the DM at the back and the other objects to be heated at the door. Thermocouples were installed in each DM to measure the temperature near the LIB during heat treatment. Figure 6 is a schematic diagram of the A-A cross section of the furnace shown in Figure 2. Figure 6 shows the state with the door slightly open, and the objects to be heated are omitted. Figure 3 shows the changes over time in the temperature at the heating furnace outlet and the temperatures near the mobile phone DM and mixed DM when the heat treatment of 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, thereby promoting the combustion of the LIB in the DM without water storage. Approximately 60 minutes after ignition, the LIB in the DM without water storage began to burn, so the burners were extinguished and the LIB was allowed to spontaneously combust. The spontaneous combustion of the LIB caused the temperature near the LIB to rise, but thermal runaway did not occur. Spontaneous combustion continued for about 60 minutes (from the initial ignition to about 120 minutes) and was completed.

[0033] From that point on, other heating objects were heated using burners No. 1 and No. 5. As a result, the combustion heat from the other heating 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 largely completed, the heating furnace outlet temperature was maintained at 750°C or higher, but no sudden rise in the heating furnace outlet temperature was observed.

[0034] [Comparative Example 1] The heat treatment of this example was carried out by repeating the same procedure as in Example 1, except that tap water was not injected into any of the 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 temperature control difficult. 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 inside the furnace.

[0035] [Example 2] The LIBs subjected to heat treatment in this example were mixed LIBs (a mixture of small LIBs) used in household appliances and LIB packs for vehicles. The sample preparation method for mixed LIBs used in household appliances was the same as that for the mixed DM containing water described in Example 1, except that the amount of water stored (height in the DM) was set to about half of the DM. A 500 kg LIB pack for vehicles was stored without disassembly in a dedicated iron storage means (an iron container with a width of 2,350 mm and a depth of 1,650 mm) without storing water.

[0036] Two storage units containing the on-board LIB packs were placed apart on a cart, and two of the mixed DM bottles were placed between them. Figure 4 shows the arrangement of the storage units for the mixed DM and on-board LIB. 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. Figure 5 shows the change over time in the temperature at the outlet of the heating furnace when the heat treatment of Example 2 was performed.

[0037] In this example, burners No. 4 and No. 8 were first ignited. 75 minutes after the furnace outlet temperature reached approximately 300°C, the onboard LIB pack began to burn. The burners were then extinguished, allowing the onboard LIB pack to spontaneously combust. Spontaneous combustion continued for another 60 minutes (135 minutes from initial ignition) before completing. From that point, burners No. 1 and No. 5 were ignited to heat the onboard LIB pack on the furnace door side. Immediately afterward, the onboard LIB pack began to combust. The burners were then extinguished, allowing the onboard LIB pack to spontaneously combust. Burners No. 2 and No. 6 were then ignited, promoting the evaporation of water in the mixed DM and the combustion of the LIB. 225 minutes after initial ignition, the LIB in the mixed DM began to combust. The burners were then extinguished, allowing the LIB to spontaneously combust. After the combustion of all the LIBs was completed, the furnace temperature was maintained 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: An in-vehicle LIB pack and mixed LIB (a mixture of small LIBs used in mobile phones and household appliances) were prepared as heat treatment targets. A 500 kg in-vehicle LIB pack was placed in a dedicated iron container (iron container) without disassembly, without storing water. The mixed LIB was then placed in eight 200 L DMs (open iron drums), each until nearly full. The mass of mixed LIB contained in each DM was approximately 200 kg. Of these eight DMs, three were filled with water up to 50% of the DM height while the mixed LIB was still contained, and the remaining five were filled with water up to 80% of the DM height while the mixed LIB was still contained.

[0040] One iron container containing the on-board LIB pack and eight DMs containing mixed LIB were placed on a cart and loaded into a heating furnace similar to that of Example 1. Figure 7 shows a schematic diagram of the arrangement of the on-board LIB storage means and the mixed LIB storage DM. In Figure 7, the DM marked with symbol (A) was filled with water to 50% height, and the DM marked with symbol (B) was filled with water to 80% height. Figure 8 shows the change in the temperature at the outlet of the heating furnace over time when the heat treatment of Example 2 was performed.

[0041] In this example, burners No. 4 and No. 8 were first ignited, and the furnace outlet temperature was raised to approximately 300°C to promote combustion of the vehicle-mounted LIB pack. 45 minutes after ignition, combustion of the vehicle-mounted LIB pack began, so the burners were extinguished and spontaneous combustion was allowed. Spontaneous combustion completed 75 minutes later (approximately 120 minutes after ignition). Next, burners No. 1 and No. 6, which directly flamed the 50% water DM, were ignited to promote combustion of the mixed LIB in the 50% water DM. Combustion began in the 50% water DM at 210 minutes after initial ignition. Next, burners No. 2 and No. 5 were ignited to promote water evaporation and combustion of the mixed LIB in the 80% water DM. All of the water evaporated at 270 minutes after initial ignition, and combustion of the remaining mixed LIB began. The flames of burners No. 1, No. 6, No. 2, and No. 5 were continued to be introduced, and the temperature rose to nearly 1000°C, but the temperature dropped as the combustion of the mixed LIB was completed. After that, the temperature was kept at over 750°C for over 1 hour, and then all 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, 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 at which the temperature of the containing means containing the lithium ion secondary batteries rises.

2. A method for heat treatment of lithium ion secondary batteries as described in claim 1, wherein when a 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. The method for heat treating a lithium ion secondary battery according to claim 1, wherein the container means is made of iron or stainless steel.

4. The method for heat treating a lithium ion secondary battery according to claim 1, wherein the water storage means has a lid member with a plurality of openings at its upper portion.

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