Selectively permeable lithium membrane and method for manufacturing same

Abstract

The present invention provides a lithium permselective membrane and the like capable of recovering lithium ions without performing complicated treatment on the lithium permselective membrane, and reducing the time required for recovering the lithium ions and the cost required for recovering the lithium ions. This lithium permselective membrane (10) comprises a sintered body of a lithium ion conductor. The lithium permselective membrane (10) is characterized in that at least a partial region of the outer surface of the lithium permselective membrane (10) is composed of a roughened surface (101).

Description

[Technical field]

[0001] The present invention relates to a lithium selective permeable membrane that selectively transmits lithium ions and a method for manufacturing the lithium selective permeable membrane. [Background Technology]

[0002] Metals, such as rare metals, are found in smaller quantities in the Earth's crust than other substances, and their circulation is limited due to the technical difficulties of mining and refining. However, these metals have recently attracted significant attention as additives in structural materials, electronic and magnetic materials for applications such as light-emitting diodes, batteries, and permanent magnets, and functional materials such as photocatalysts and new glass.

[0003] For example, lithium, a rare metal, is used in the production of materials for lithium-ion batteries and fuel for nuclear fusion reactors, and its demand has been growing in recent years. To meet these needs, technology is needed to produce large quantities of lithium at low cost.

[0004] Rare metals such as lithium can also be extracted from ores contained in the earth's crust, but as mentioned above, there are problems in mass production due to technical difficulties.

[0005] Therefore, in recent years, the technology of extracting ions derived from rare metals contained in seawater has been developing. Seawater contains a variety of ions, and their content is relatively large relative to the amount buried underground. In addition, it is known that industrial waste (discarded lithium secondary batteries, etc.) also contains a large amount of various ions. However, seawater and industrial waste contain not only ions represented by rare metals (lithium ions, etc.), but also other metal ions such as potassium, sodium, and calcium. Therefore, in order to recover the target ions, the development of membranes (selective permeable membranes) that separate the target ions from other metal ions has been promoted.

[0006] On the other hand, the concentration of rare metal elements, which are ions contained in seawater or industrial waste (such as discarded lithium secondary batteries), is low, and the ion permeation rate through the selective permeable membrane used to recover the target ions is also low. Therefore, according to the existing technology, recovering the target ions from seawater or industrial waste (such as discarded lithium secondary batteries) is time-consuming and costly.

[0007] To address this issue, Patent Document 1 discloses a technology for extracting lithium ions from salt lake brine or lithium ore with relatively high lithium concentrations, industrial waste (such as discarded lithium secondary batteries) with relatively low lithium concentrations, or seawater. Patent Document 1 discloses a lithium-selective membrane with a lithium adsorption layer formed on one surface through an acidic treatment. This layer is known to recover lithium ions from a raw solution at a high rate.

[0008] [Prior art literature]

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-131863 [Summary of the invention]

[0011] Problems to be solved by the invention

[0012] While the lithium-selective membrane of Patent Document 1 can improve lithium ion recovery rates, it requires acid treatment of one surface of the membrane, complicating its manufacture. Furthermore, even with acid treatment of one surface of the membrane, lithium recovery still requires significant time and cost.

[0013] To solve the above problems, an object of the present invention is to provide a lithium selective permeable membrane and a method for manufacturing the same, which can recover lithium ions without performing complicated treatment on the lithium selective permeable membrane and reduce the time and cost required for the lithium ion recovery.

[0014] Means for solving problems

[0015] In order to achieve the above-mentioned object, the lithium selective permeable membrane of the present invention is characterized by having the following essential technical features.

[0016] (1) The lithium selective permeable membrane of the present invention is:

[0017] A lithium selective permeable membrane comprising a sintered body of a lithium ion conductor, wherein at least a portion of an outer surface of the lithium selective permeable membrane is constituted by a surface roughening process.

[0018] Generally, when a lithium ion conductor is used as a lithium selective membrane, it is often used in the form of a sintered body. In many cases, the structure or composition of a sintered body near the outer surface, which is exposed to the sintering atmosphere during sintering, differs from the structure or composition of the inner portion, which is not affected by the sintering atmosphere during sintering. According to the lithium selective membrane (1), the portion having the structure or composition near the outer surface that changes during sintering is removed by surface roughening treatment, and the portion having the structure and composition that is not affected by the sintering atmosphere during sintering is exposed on the surface of the lithium selective membrane. Therefore, the structure or composition of the ion conductor can be made the same on the outer surface and inside the lithium selective membrane.

[0019] Furthermore, near the outer surface that comes into contact with the sintering atmosphere during sintering, the structure or composition changes during sintering, so the lithium ion permeation rate in this area is slower than the original lithium ion permeation rate (the lithium ion permeation rate of the portion not affected by the sintering atmosphere during sintering). According to the lithium selective membrane (1), the area having the structure or composition of the ion conductor that is not affected by the sintering atmosphere during sintering is exposed on the outer surface of the selective membrane, thereby preventing a decrease in the lithium ion permeation rate. As described above, according to the lithium selective membrane (1), the time required for lithium ion recovery can be reduced and the required cost can be reduced.

[0020] (2) Furthermore, preferably, in the lithium selective permeable membrane of the present invention,

[0021] Satisfy at least one of the following requirements (A), (B), and (C).

[0022] (A) The arithmetic mean height Ra of all or part of the roughened surface is 0.8 μm or more and 20 μm or less.

[0023] (B) The root mean square slope Rdq of all or part of the roughened surface is 10° or more and 80° or less.

[0024] (C) The developed area ratio Sdr of the entire or a portion of the roughened surface is 0.2 or more and 2 or less.

[0025] The lithium-selective membrane according to (2) satisfies at least one of the following requirements: an arithmetic mean height Ra of all or part of the roughened surface is 0.8 μm or greater, a root mean square slope Rdq is 10° or greater, and a developed area ratio Sdr is 0.2 or greater. Therefore, the surface area of ​​the lithium-ion conductor can be substantially increased. As a result, lithium ions enter the lithium-selective membrane through the lithium-ion conductor having a larger surface area, resulting in an increased lithium ion recovery rate.

[0026] From the perspective of lithium ion recovery speed, larger values ​​for Ra, Rdq, and Sdr are preferred. However, increasing the values ​​of Ra, Rdq, and Sdr increases the risk of damage to the lithium-selective membrane during surface roughening. Furthermore, the time and cost required for surface roughening increase. Therefore, to stably manufacture a lithium-selective membrane comprising a lithium-ion conductor with a large surface area, it is preferable to satisfy at least one of the following requirements: an arithmetic mean height Ra of the roughened surface of the lithium-ion conductor of 20 μm or less, a root mean square slope Rdq of 80° or less, and a developed area ratio Sdr of 2 or less.

[0027] (3) Furthermore, preferably, in the lithium selective permeable membrane of the present invention,

[0028] The sintered body particles in the entire or a portion of the surface roughened surface do not have a step and terrace structure on their surfaces.

[0029] The lithium selective membrane (3) does not have the step and terrace structures formed on the surface of the sintered particles during the heat treatment in the sintering and annealing steps on the surface of the sintered particles that are exposed to all or part of the surface roughening process. Therefore, the surface that is not in direct contact with the firing atmosphere during the heat treatment is fully exposed. Therefore, the lithium selective membrane (3) can reduce the time required for lithium ion recovery and reduce the required costs.

[0030] (4) Furthermore, preferably, in the lithium selective permeable membrane of the present invention,

[0031] The lithium ion conductor is a sintered body of lithium lanthanum titanate.

[0032] According to the lithium-selective membrane of (4), the lithium-ion conductor is a sintered body of lithium lanthanum titanate. As a lithium-ion conductor, lithium lanthanum titanate has an excellent lithium ion conduction rate. Furthermore, lithium ions can be stably permeated in an aqueous solution. Therefore, according to the lithium-selective membrane of (4), the time and cost required for lithium recovery can be reduced.

[0033] (5) Furthermore, preferably, in the lithium selective permeable membrane of the present invention,

[0034] In the lithium permeation test within the temperature range of 22°C to 24°C, the lithium permeation rate V expressed by the following formula (1) was 0.4 mmol / (hr·cm 2 ·(mol / L)) or above.

[0035] V=M / (T·S·C)··(1)

[0036] Where M is the amount of lithium ions permeating the lithium selective membrane (mmol), T is the time required for lithium ions to permeate M (hr), and S is the effective membrane area of ​​the lithium selective membrane (cm 2 ), C is the lithium ion concentration in the stock solution (mol / L).

[0037] According to the lithium selective permeable membrane of (5), the lithium permeation rate V is 0.4 mmol / (hr·cm 2 Therefore, the lithium selective permeable membrane of (5) can reduce the time and cost required for lithium recovery.

[0038] Furthermore, a method for producing a lithium permselective membrane for solving the above-mentioned problems is characterized by having the following essential technical features.

[0039] (6) The method for producing a lithium selective permeable membrane of the present invention is characterized by comprising:

[0040] a sintering step of sintering the compact including the lithium ion conductor; and a surface roughening step of removing at least a portion of the outer surface of the sintered compact after sintering and roughening the surface.

[0041] According to the method for producing a lithium-selective membrane of (6), a portion or all of the portion originally present on the outer surface of the lithium-ion conductor that has changed in structure or composition due to contact with the sintering atmosphere during sintering can be removed by the surface roughening treatment step, thereby making it possible to produce the lithium-selective membranes of (1) to (5). Therefore, the lithium-selective membrane produced by the method of (6) can reduce the time required for lithium ion recovery and reduce the required cost.

[0042] (7) Furthermore, preferably, in the method for producing a lithium selective permeable membrane of the present invention,

[0043] Satisfy at least one of the following requirements (D), (E), and (F).

[0044] (D) The surface roughening step is performed so that the arithmetic mean height Ra of all or part of the surface of the sintered body is 0.8 μm or more and 20 μm or less.

[0045] (E) The surface roughening step is performed so that the root mean square slope Rdq of the entire or a portion of the surface of the sintered body is 10° or more and 80° or less.

[0046] (F) The surface roughening step is performed so that the developed area ratio Sdr of the entire or a portion of the surface of the sintered body is 0.2 or more and 2 or less.

[0047] The lithium selective membrane of (2) can be manufactured by the method for manufacturing a lithium selective membrane of (7). Therefore, the lithium selective membrane manufactured by the method of (7) can reduce the time required for lithium ion recovery and reduce the required cost.

[0048] (8) Furthermore, preferably, in the method for producing a lithium selective permeable membrane of the present invention,

[0049] The surface roughening treatment is a blasting treatment.

[0050] According to the method for manufacturing a lithium-selective membrane of (8), the blasting treatment simultaneously removes and roughens the surface of the sintered body in a short period of time. Furthermore, residual stress can be imparted to the lithium-ion conductor, thereby improving the strength and durability of the lithium-ion conductor and, consequently, the lithium-selective membrane. Therefore, a lithium-selective membrane with excellent durability can be manufactured without complicating the manufacturing process of the lithium-selective membrane.

[0051] [Brief Description of the Drawings]

[0052] Figure 1 This is a diagram schematically showing a lithium ion recovery device using a lithium selective permeable membrane.

[0053] Figure 2 These are the results of powder X-ray diffraction measurements of the sintered body of the lithium ion conductor produced in Example 1, which was pulverized and measured.

[0054] Figure 3 This is a SEM image of the sintered body of the lithium ion conductor produced in Example 1 before surface roughening treatment (sintered state).

[0055] Figure 4 This is an SEM image of the surface roughened surface of the sintered body of the lithium ion conductor produced in Example 1 after the surface roughening treatment.

[0056] Figure 5 This is a current-time curve when a Li recovery test was conducted using the lithium permselective membrane prepared in Example 1.

[0057] Figure 6 This is an SEM image of the surface-roughened surface of the sintered body of the lithium ion conductor produced in Comparative Example 3 after the surface was roughened and then annealed. [Specific implementation method]

[0058] Hereinafter, the present invention will be described in detail based on the embodiments and the accompanying drawings.

[0059] (Structure of lithium ion recovery device)

[0060] Figure 1 1 is a structural diagram illustrating the structure of a lithium ion recovery device 1 using a lithium selective permeable membrane 10. Figure 1 As shown, the lithium ion recovery device 1 includes a lithium permselective membrane 10, a first electrode 121, and a second electrode 122. The lithium permselective membrane 10 has a structure described below and selectively permeates lithium ions 131 contained in a raw liquid 111, which serves as a first liquid. The lithium permselective membrane 10 selectively permeates lithium ions 131 through the raw liquid 111, which contains multiple ions, thereby transferring lithium ions 131 from the raw liquid 111 to the recovery liquid 112, which serves as a second liquid. In this case, the lithium permselective membrane 10, which has a high permselectivity, is preferably used in conjunction with an electrode or the like having a structure that enhances this permselectivity.

[0061] The first electrode 121 and the second electrode 122 are respectively disposed facing Figure 1One main surface (left surface) side and the other main surface (right surface) side of the lithium selective permeable membrane 10 in the lithium ion recovery device 1. Here, the first electrode 121 and the second electrode 122 are the anode and the cathode in the lithium ion recovery device 1, respectively. The first electrode 121 and the second electrode 122 may be in direct contact with the lithium selective permeable membrane 10 or may not be in contact. With such a structure, the potential difference between one main surface and the other main surface of the lithium selective permeable membrane 10 is maintained at a certain potential difference. As the material of the first electrode 121 and the second electrode 122, it is preferred to appropriately use a conductive material that does not chemically react with the components in the raw solution 111 or the recovered solution 112. At this time, the materials of the first electrode 121 and the second electrode 122 may be the same as each other, or may be different from the perspective of ionization tendency.

[0062] The stock solution 111 is an aqueous solution obtained by treating salt lake brine, lithium ore, industrial waste (abandoned lithium secondary batteries, etc.) or seawater, and is an aqueous solution containing lithium ions 131. The stock solution 111 is supplied from the sea, salt lake or stock solution storage tank to the lithium ion recovery device 1 through a stock solution piping and a pump (not shown). In addition, from the viewpoint of increasing the lithium ion permeation rate and even the lithium ion recovery rate, the pH of the stock solution 111 can also be adjusted. In this case, it is preferred to make the stock solution 111 alkaline as disclosed in Patent Document 1, but it can also be non-alkaline. Furthermore, when seawater is used as the stock solution 111, a solution obtained by separating monovalent ions (lithium ions, sodium ions, etc.) using a cation exchange membrane in advance can also be used as the stock solution 111.

[0063] The recovered liquid 112 is an aqueous solution containing lithium ions 131 that have permeated the raw liquid 111. The recovered liquid 112 is discharged from the lithium ion recovery device 1 via a recovery liquid piping and pump (not shown) and can be stored in a recovered liquid storage tank. Then, by bubbling CO2 in the recovered liquid storage tank, the lithium ions 131 can be recovered as a carbonate precipitate.

[0064] The first electrode 121 and the second electrode 122 are connected via a conductive lead or the like, with a DC power supply 14 interposed between the leads to maintain a constant potential difference between the first electrode 121 and the second electrode 122. The shapes of the first electrode 121 and the second electrode 122 are not particularly limited. For example, the first electrode 121 and the second electrode 122 preferably have a grid shape as disclosed in Patent Document 1 and are fixed relative to the lithium-selective membrane 10. Furthermore, a current collector composed of, for example, a carbon felt sheet may be interposed between each of the grid-shaped first electrode 121 and the second electrode 122 and the lithium-selective membrane 10. Furthermore, the first electrode 121 and the second electrode 122 may each be formed into a substantially plate-like shape, as in conventional Daniel cells.

[0065] Furthermore, the lithium ion recovery device 1 using the lithium-selective permeable membrane 10 of this embodiment can not only recover lithium ions 131 by applying a voltage, but can also be used as a battery that extracts power without applying an external voltage. In this case, during the process of lithium ions 131 moving from the raw solution 111 to the recovered solution 112, a voltage is generated in which the first electrode 121 acts as a negative electrode and the second electrode 122 acts as a positive electrode. Furthermore, when a voltage is applied to the lithium ion recovery device 1, the flow of lithium ions 131 can be increased compared to when no voltage is applied, thereby improving the efficiency of lithium ion 131 recovery.

[0066] The lithium-selective membrane 10 may be composed solely of a lithium-ion conductor, or it may be composed of a composite of a lithium-ion conductor and another material lacking lithium-ion conductivity. To improve flexibility, the lithium-selective membrane 10 may be a composite of lithium-ion conductor particles and a resin. Furthermore, to improve strength, the lithium-selective membrane 10 may be a composite of a thin-film lithium-ion conductor and a porous support. To reduce manufacturing process complexity and production costs, the lithium-selective membrane 10 is preferably composed of a single sintered body of the lithium-ion conductor.

[0067] The lithium selective permeable membrane 10 is configured to separate a raw liquid 111 including a plurality of ions including lithium ions 131 and non-designated ions 132 (ions other than lithium ions 131) from a recovered liquid 112 that is a recovery destination for the lithium ions 131. In this case, the lithium selective permeable membrane 10 needs to be configured so that a surface roughening surface 101 obtained by a surface roughening treatment described later faces the raw liquid 111. In addition, the lithium selective permeable membrane 10 can be formed into a spherical, cylindrical, regular octahedral shape, etc., or at least one concave or convex portion can be provided on a portion of a plate or the shapes listed above. In this case, the lithium selective permeable membrane 10 is not particularly limited in shape as long as it can separate the raw liquid 111 from the recovered liquid 112. In addition, the surface roughening surface 101 described later can also be provided according to the electric field generated in the lithium ion recovery device 1. For example, the surface roughened surface 101 may be provided on a surface facing the raw solution 111 and in a region where the electric flux density perpendicular to the surface is relatively high when a voltage is applied to the lithium ion recovery device 1 .

[0068] The lithium selective permeable membrane 10 has a surface roughened surface 101 ( Figure 1 The surface roughening surface 101 is provided on a part or the whole of the outer surface of the lithium selective permeable membrane 10. As described above, the surface roughening surface 101 needs to be provided to face the raw liquid 111. Figure 1In the embodiment, the non-processed surface 102 that has not been subjected to the surface roughening treatment is provided on the main surface of the lithium selective permeable membrane 10 facing the recovery liquid 112. However, the present invention is not limited thereto, and in this case, the main surface of the lithium selective permeable membrane 10 facing the recovery liquid 112 may be provided with the surface roughening processed surface 101 on a portion or the entirety thereof, or the non-processed surface 102 that has not been subjected to the surface roughening treatment may be provided on the entirety thereof.

[0069] The surface roughening processing surface 101 is subjected to removal and surface roughening of the sintered body surface layer by any surface roughening treatment method described later. The arithmetic mean height Ra of the surface in all or part of the surface of the surface roughening processing surface 101 is preferably 0.8 μm or more and 20 μm or less, more preferably 1.5 μm or more and 15 μm or less, and further preferably 2.0 μm or more and 10 μm or less. In addition, the surface roughness in this embodiment is expressed by Ra, but is not limited to this, and can also be expressed by the root mean square slope Rdq. In this case, the root mean square slope Rdq of the surface in all or part of the surface of the surface roughening processing surface 101 is preferably 10° or more and 80° or less, more preferably 12° or more and 70° or less, and further preferably 14° or more and 60° or less. These Ra and Rdq can be calculated using a stylus surface roughness meter. The measurement conditions for the entire or a portion of the surface are preferably evaluated according to the degree of surface roughness using the reference length, measurement length, and cutoff value recommended in JIS B 0601-2001 or ISO 4287-1997.

[0070] In addition, the surface roughening process surface 101 of the present embodiment can also be expressed by the surface roughness parameter of the developed area ratio Sdr. In this case, the developed area ratio Sdr is preferably 0.2 to 2, more preferably 0.3 to 1.8, and even more preferably 0.5 to 1.5. In addition, the developed area ratio Sdr as a surface roughness parameter can be calculated using a non-contact surface roughness meter.

[0071] In addition, all or part of the roughened surface 101 in this embodiment preferably does not have a step-and-terrace structure on the surface of the sintered particles. The presence or absence of a step-and-terrace structure in the roughened surface 101 can be determined by, for example, observing an SEM image of the surface.

[0072] The surface roughening surface 101 in this embodiment preferably has a uniform surface to meet the above conditions, but is not limited thereto. For example, it may also include two or more surface roughening surfaces 101 with different structures. In this case, it is preferred that at least one surface roughening surface 101 meets the conditions related to the surface roughness or the presence or absence of step and platform structures. In addition, in this case, the above-mentioned multiple surface roughening surfaces 101 can change intermittently in the horizontal direction of the surface, or can change continuously. The conditions of these surface roughening surfaces 101 can be adjusted according to the design of the lithium ion recovery device 1.

[0073] The material of the lithium selective permeable membrane 10 of this embodiment is not particularly limited as long as it is a material that can selectively transmit lithium ions 131. As the material of the lithium selective permeable membrane 10, from the viewpoint of transmitting lithium ions 131 and suppressing reaction with water and other aqueous solutions, it is preferable to use a super lithium ion conductor, namely, lanthanum titanate, which has high ion conductivity with respect to lithium ions 131 and does not react with components in water and aqueous solutions. As lanthanum titanate, preferably Li x La (2-x) / 3 TiO3 and x < 2 / 3 lithium lanthanum titanate, more preferably Li 0.29 La 0.57 TiO3.

[0074] The lithium-selective membrane 10 may use, in addition to lithium ion conductors having a perovskite-type crystal structure, such as lithium lanthanum titanate, lithium ion conductors having a NASICON-type crystal structure, lithium ion conductors having a garnet-type crystal structure, and the like. Furthermore, the lithium-selective membrane 10 is not limited to the aforementioned crystal structures; any ion conductor having high lithium ion conductivity and capable of stably existing in the raw solution 111 and the recovered solution 112 during lithium ion recovery may be used.

[0075] In addition, from the viewpoint of being able to fully recover lithium ions 131, the lithium ion permeation rate of the lithium selective permeable membrane 10 of this embodiment is preferably 0.4 mmol / (hr·cm 2 ·(mol / L)) or more, more preferably 0.5mmol / (hr·cm 2 ·(mol / L)) or more, more preferably 0.6mmol / (hr·cm 2 The lithium permeation rate is calculated by the following formula (1).

[0076] V=M / (T·S·C)··(1)

[0077] Where M is the amount of lithium ions 131 that permeate the lithium selective membrane 10 (mmol), T is the time required for lithium ions 131 to permeate M (hr), and S is the effective membrane area of ​​the lithium selective membrane 10 (cm 2 ), C is the lithium ion concentration in the original solution 111 (mol / L).

[0078] (Method for Manufacturing Lithium Selective Permeable Membrane)

[0079] Next, the method for manufacturing the lithium-selective membrane 10 of this embodiment will be described. The lithium-selective membrane 10 can be composed solely of a lithium-ion conductor, or it can be composed of a composite of a lithium-ion conductor and another material that lacks lithium-ion conductivity. Since the raw liquid 111 and the recovered liquid 112 need to be separated by the lithium-selective membrane 10, the lithium-selective membrane 10 must be structured without through-pores. There are no particular limitations on the method for manufacturing a lithium-selective membrane 10 without through-pores, but a densified sintered body is preferably used as the lithium-ion conductor. The steps for manufacturing this sintered body are as follows: particles that can form the target lithium-ion conductor are crushed to the desired particle size range, granulated as needed, and then mixed with optional additives such as a sintering aid, dispersant, pore-forming agent, release agent, and binder, and then molded into any shape (suitable for the shape of the lithium ion recovery device 1 to be used). The molding method is not particularly limited; sheet molding, uniaxial pressing, cold isostatic pressing, warm isostatic pressing, etc. can be appropriately selected depending on the shape of the desired molded body. The produced molded body can also be further processed into any shape by machining as needed.

[0080] Next, the molded body is sintered, but a degreasing process may be performed to decompose or remove organic matter contained in the binder, etc. of the mixture before sintering. The sintering temperature and sintering time in the sintering process are suitable for obtaining a sintered body as the target lithium ion conductor. For example, in order to obtain a sintered body of lithium lanthanum titanate as a lithium ion conductor, it is preferably sintered in a temperature range of 1100°C to 1500°C with a holding time of 5 hours to 100 hours. At a temperature below 1100°C, sintering cannot be performed even if time is spent, so a dense body cannot be obtained. In addition, at a temperature exceeding 1500°C, the molded body and the sintered parts are welded and reacted, making it impossible to obtain a good sintered body. In addition, from the perspective of suppressing changes in the structure or composition of the surface of the molded body caused by the influence of the firing atmosphere during sintering, the molded body can also be sintered on the basis of embedding the molded body in a powder such as a mother powder or a covering powder that is the same as the molded body.

[0081] Next, a surface roughening treatment is performed on part or all of the outer surface of the resulting sintered body (sintered state), thereby providing a roughened surface 101 on the outer surface of the lithium selective permeable membrane 10. At this time, it is preferred that the arithmetic mean height Ra of the roughened surface 101 be 0.8 μm or more and 20 μm or less, or that the root mean square slope Rdq be 10° or more and 80° or less, or that the developed area ratio Sdr be 0.2 or more and 2 or less, but these are not limiting and may be within the numerical ranges described above.

[0082] In addition, the depth of removal of the surface layer in the surface roughening treatment of the outer surface of the obtained sintered body (sintered state) is not particularly limited, and it is preferably removed by more than 5 μm. From the viewpoint of changes in structure and composition caused by the influence of the sintering atmosphere during sintering, the removal depth is preferably changed according to the material type or composition of the ion conductor, or the temperature or time of the sintering process. For example, the removal depth can be adjusted according to the sintering time, or the removal depth can be adjusted according to the sintering temperature and the sintering time. At this time, the removal depth of the surface layer is preferably removed until the surface roughening processing surface 101 becomes the target structure, composition or particle size distribution.

[0083] From the viewpoint of simultaneously removing the surface layer and forming a surface roughening processing surface 101 with a surface roughness of Ra of 0.8 μm or more and 20 μm or a root mean square slope Rdq of 10° or more and 80° or a development area ratio Sdr of 0.2 or more and 2 or less, the method for the above-mentioned surface roughening treatment is preferably based on a blasting treatment for surface roughening treatment, and more preferably based on a sandblasting treatment for surface roughening treatment. In this case, residual stress can be imparted to the sintered body, so the strength and durability of the sintered body are also improved. In addition, the present embodiment is not limited to sandblasting, and in addition to blasting treatments such as wet blasting or shot blasting, dry or wet mechanical grinding, chemical etching treatment, etc. can also be used. The abrasive used in the blasting treatment is not particularly limited and is aluminum oxide abrasive, SiC abrasive, glass abrasive, zircon abrasive, etc.

[0084] When the sintered body is subjected to the above-mentioned surface roughening treatment and then subjected to a high-temperature heat treatment step such as annealing, the surface structure and composition may change due to the influence of the firing atmosphere during the heat treatment, similar to the sintering process. Therefore, it is preferable to use the lithium selective membrane 10 without performing a high-temperature heat treatment step (generally above the Debye temperature of the material) after the surface roughening treatment to enable diffusion of atoms constituting the lithium ion conductor.

[0085] By observing the surface morphology of the lithium-selective membrane 10, it can be determined whether the surface of the lithium-selective membrane 10 has undergone surface roughening after the final heat treatment step or has not undergone surface roughening after the final heat treatment step. For example, in the case of a sintered body of lithium lanthanum titanate, steps are present on the surfaces of the ceramic particles constituting the sintered body at the interfaces (grain boundaries) between the crystal grains after sintering. A striped pattern of step-and-terrace structures is observed on the surfaces of the crystal grains as a sign of grain growth caused by sintering. On the other hand, on the surface-roughened surface 101 after heat treatment, the surface layer is physically removed, so the aforementioned steps along the grain boundaries and the step-and-terrace structures as signs of crystal grain growth are not observed. Instead, a surface morphology corresponding to the surface roughening method is observed. For example, in the case of blasting, a random, rough surface is formed. Furthermore, in mechanical polishing, linear scratches similar to those seen in polishing damage are observed.

[0086] Therefore, it is necessary to reliably remove the portion of the surface roughened surface 101 that is affected by the firing atmosphere during heat treatment. Therefore, it is preferable that the surface roughened surface 101 does not have the step and terrace structures on the surface of the sintered particles formed during heat treatment. As described above, it is easy to determine whether the surface is in a state after heat treatment (sintering step, annealing step) or whether the surface has been fully roughened after the final heat treatment.

[0087] The lithium selective membrane 10 of this embodiment can be obtained by the above method. However, the method for manufacturing the lithium selective membrane 10 is not limited to the above method, and can be modified within the scope of achieving the purpose of this embodiment.

[0088] (Evaluation of lithium permeation rate)

[0089] The lithium selective permeable membrane 10 manufactured by the above method is installed in the lithium ion recovery device 1 having the above structure, and lithium recovery can be performed by applying a voltage to the first electrode 121 and the second electrode 122 .

[0090] At this point, it can be seen that the permeation rate of lithium ions 131 through the lithium selective permeable membrane 10 is mainly determined by the temperature of the lithium selective permeable membrane 10, the pH of the stock solution 111, and the lithium ion concentration in the stock solution 111. Therefore, when evaluating the lithium permeation rate, it is preferable to calculate the lithium ion permeation rate (unit: mmol / (hr.cm)) as the lithium ion permeation rate per unit area of ​​the lithium selective permeable membrane 10, per unit time, and per unit volume of the stock solution 111 at a certain temperature and a certain pH of the stock solution 111. 2 If the lithium permeation rate is as defined above, the lithium permeation rate can be used as the performance of the lithium permselective membrane 10 as a whole for comparative evaluation.

[0091] The lithium ion permeation rate can be directly determined by evaluating the lithium ion concentrations in the raw solution 111 and the recovered solution 112 before and after the lithium permeation test using ICP analysis or the like. Alternatively, it can be determined using the following formula (2) based on the current value flowing when a voltage is applied to the lithium ion recovery device 1 .

[0092] M=Itα / nF··(2)

[0093] In the formula, M is the number of moles of lithium ions 131 that permeate the lithium-selective membrane 10, I is the current flowing through the device, t is time, F is the Faraday constant, and n is the valence (1 for lithium ions). Furthermore, α represents the current efficiency, a parameter that indicates the proportion of the current flowing through the device that is due to the permeation of lithium ions 131, and takes a value between 0 and 1. For example, the amount of lithium ions 131 that migrated can be determined by multiplying the area enclosed by a current-time curve obtained by measuring and recording the current at regular intervals under application of a constant voltage by the current efficiency and dividing the result by the Faraday constant.

[0094] The current efficiency α is affected by the structure of the lithium ion recovery device 1 and the presence or absence of electronic conductivity of the lithium permselective membrane 10. The current efficiency α can be determined as follows: the amount of lithium ions 131 contained in the recovery liquid 112 is determined by ICP analysis or the like, and the measured value of the lithium ion permeation amount is calculated. Furthermore, assuming α = 1, the theoretical value of the lithium ion permeation amount is calculated based on the total current value using formula (2), and the measured value of the lithium ion permeation amount is divided by the theoretical value.

[0095] The lithium ion permeation rate defined above can be obtained by dividing the lithium ion permeation amount calculated by the above method by the total permeation time, further divided by the effective membrane area of ​​the lithium selective permeable membrane 10 , and further divided by the lithium ion concentration in the stock solution 111 .

[0096] Example

[0097] Next, examples of the present embodiment will be described with reference to experimental results. The experimental results described in each example and comparative example are summarized in Table 1.

[0098] (Fabrication of Lithium Selective Permeable Membrane)

[0099] The particle size of lithium lanthanum titanate powder (manufactured by Toho Titanium Co., Ltd.) was adjusted by ball milling, and a binder, dispersant, plasticizer, and solvent were added to prepare a slurry. The resulting slurry was formed into a sheet and dried to obtain a green sheet with a thickness of approximately 700 μm.

[0100] The obtained green sheet is clamped with a porous ceramic backing plate and subjected to a heat treatment of 1000°C for 3 hours in the atmosphere to perform degreasing and calcining. At this time, for comparison, in Comparative Example 3 described later, uniaxial pressing is performed while the two sides of the green sheet are clamped with #200 nylon mesh and then clamped with metal plates on both sides. Thus, the surface is embossed by forming #200 mesh marks on both sides of the green sheet. After the embossing of the green sheet, degreasing and calcining are performed by heat treatment of 1000°C for 3 hours in the atmosphere.

[0101] The calcined body was then sandwiched between dense ceramic backing plates and heat-treated at 1230°C for 72 hours in air to achieve sintering. The resulting sintered body had a relative density of 98% or higher, resulting in a dense ceramic sheet without through-holes. Figure 2 The figure shows the powder X-ray diffraction measurement results (CuKα radiation source) obtained by pulverizing the obtained ceramic sheet and adding silicon powder as an internal standard. Figure 2 As a result, it was confirmed that the obtained ceramic sheet was composed of a crystal phase of lithium lanthanum titanate (target ion conductor).

[0102] (Implementation of surface roughening treatment)

[0103] Next, the resulting sintered ceramic sheet was cut into approximately 50 mm x 50 mm pieces, and a 40 mm x 40 mm area in the central portion of both surfaces was subjected to surface roughening treatment according to the method described in Table 1, resulting in Examples 1 to 4 and Comparative Examples 1 to 4. The depth of surface layer removal was calculated by measuring the difference in thickness of the ceramic sheet before and after the blasting treatment. Furthermore, in Comparative Example 3, after the surface roughening treatment, the surface-roughened ceramic sheet was further annealed by heat treatment at 1230°C for 72 hours. Figure 3 This is a surface SEM image of the sintered body before surface roughening treatment in Example 1. Figure 4 101 is a SEM image of the surface roughened surface 101 of the sintered body after surface roughening treatment. Figure 3 It can be seen that on the surface of the sintered lithium lanthanum titanate sintered body, there are steps along the interface (grain boundary) of the ceramic crystal particles. In addition, on the surface of some crystal particles, as a trace of grain growth during sintering, a stair-like stripe pattern consisting of steps and terrace structures is observed. Figure 4 It can be seen that on the surface roughened surface 101 subjected to the surface roughening treatment by the blasting treatment, no Figure 3In addition, although not shown in the figure, for each of Examples 2 to 4 and Comparative Examples 1 to 2, no irregularities such as those observed on the surface roughened surface 101 after the surface roughening process were observed. Figure 3 Visible steps along the grain boundaries, steps on the surface of crystal particles and platform structures.

[0104] Figure 6 This is an SEM image of the roughened surface in Comparative Example 3, after annealing by heat treatment at 1230°C for 72 hours. Even in the roughened surface 101 treated by blasting, the subsequent annealing causes atomic rearrangement, resulting in the observation of steps along grain boundaries and a step-and-terrace structure representing growth traces of crystalline grains, similar to the sintered surface. Although not shown, the same step-and-terrace structure was observed in Comparative Example 4.

[0105] Table 1

[0106]

[0107] (Evaluation of Surface Roughness of Lithium Selective Membrane)

[0108] The linear roughness of the surface (surface roughening surface 101) of the ceramic sheet of Examples 1 to 4 and Comparative Examples 1 to 4 prepared by the above method was measured. At this time, in Comparative Example 1 and Comparative Example 4 where surface roughening treatment was not implemented, the linear roughness of the non-machined surface 102 was measured. When obtaining the linear roughness, the arithmetic mean height Ra and the root mean square slope Rdq were measured using a stylus surface roughness meter (Form Talysurf PGI 1250A manufactured by Taylor Hobson). The evaluation length and cutoff value (λc) were based on the values ​​recommended in the standards of JIS B 0601-2001 or ISO4287-1997 according to the degree of surface roughness. The measurement was performed at the positions of any five points on the surface roughening surface 101 as the measurement object on the ceramic sheet. At this time, the direction of the stylus scanning was also selected in any direction for measurement each time. The average value was calculated based on the measurement results of the five points obtained, and the average value was used as the value of the linear roughness for evaluation.

[0109] Next, the surface roughness of the surfaces (all faces) of the ceramic sheets of Examples 1 to 4 and Comparative Examples 1 to 4 made by the above method was measured. At this time, in Comparative Example 1 and Comparative Example 4, in which surface roughening treatment was not implemented, the surface roughness of the non-processed surface 102 was measured. Surface roughness was measured using an optical interference type non-contact three-dimensional surface roughness measuring machine (Talysurf CCIHD-XL manufactured by Taylor Hobson). The measurement was implemented in a mode of evaluating a "rough surface with low reflectivity" using an objective lens (evaluation area 0.82 mm × 0.82 mm) with a magnification of 20 times. The measurement was performed at the position of any three points on the surface roughening processing surface 101 as the measurement object on the ceramic sheet. An average value was calculated based on the measurement results of the three points obtained, and the average value was evaluated as surface roughness. Table 2 shows the line roughness and surface roughness in the obtained Examples 1 to 4 and Comparative Examples 1 to 4.

[0110] Table 2

[0111]

[0112] (A "*" next to a numerical value indicates a deviation from the numerical range of the present invention.)

[0113] (Evaluation of Lithium Permeation Rate Using a Lithium Selective Permeable Membrane)

[0114] The ceramic sheets of Examples 1 to 4 and Comparative Examples 1 to 4 obtained by the above method were used as lithium selective permeable membranes 10 (effective membrane area: 40 mm×40 mm). Figure 1 The lithium ion recovery device 1 is set up as described above, and a lithium recovery test is carried out. The temperature during the test is within the range of 22°C to 24°C. The raw liquid 111 used in the test is a mixed aqueous solution composed of lithium hydroxide, sodium hydroxide, and potassium hydroxide, with the lithium ion concentration, sodium ion concentration, and potassium ion concentration adjusted to 0.1 mol / L, 0.1 mol / L, and 0.1 mol / L, respectively. The recovery liquid 112 is pure water. In addition, the first electrode 121 (the electrode on the anode side) and the second electrode 122 (the electrode on the cathode side) are metal electrodes that are not easily corroded by the raw liquid 111 and the recovery liquid 112, respectively. At this time, the test is carried out in a state where the first electrode 121 and the second electrode 122 are each electrically connected to the lithium selective membrane 10 through carbon felt. The applied voltage to the lithium selective membrane 10 is set to 5V. During the test, the test is carried out while stirring thoroughly to make the lithium ion concentration in the raw liquid 111 and the recovery liquid 112 uniform. The current value was recorded at intervals of approximately 30 seconds from the start of voltage application, and the test was continued for approximately 4 days (times shown in Table 3 described later). Figure 5 The current value recorded in Example 1 is divided by the effective membrane area to obtain the value (current density) and the result of plotting it. Figure 5As shown in FIG. 1 , the current density decreases monotonically from the start of voltage application. This is because the lithium ion concentration in the raw solution 111 decreases as the lithium ions 131 permeate.

[0115] The stock solution 111 before the start of the test and the recovered solution 112 after the test time described in Table 3 below were sampled and subjected to ICP analysis to measure the recovery rates of lithium ions 131, sodium ions (non-specified ions 132), and potassium ions (non-specified ions 132) after the test time described in Table 3 below (the ratio of each ion amount in the stock solution 111 to the amount recovered in the recovered solution 112). At this time, the recovery rates of sodium ions and potassium ions were all 0% in Examples 1 to 4 and Comparative Examples 1 to 4. In addition, the current efficiency α was measured, wherein the current efficiency α was calculated based on the total amount of current flowing until the test time and the theoretical value of the lithium ion permeation amount calculated using Formula (1) and the actual value of the lithium ion permeation amount based on the ICP analysis.

[0116] Furthermore, the lithium ion permeation amount (unit: mmol) from the start of voltage application to the time when 1 hour has passed was calculated according to formula (2) using α obtained by the above method, and then the result was divided by the required time (1 hour) and the effective membrane area (16 cm 2 ), the lithium ion concentration in the stock solution 111 (approximately 0.1 mol / L immediately after the start of the test), and the lithium ion permeation rate at the initial stage of the test were calculated from this. Table 3 shows the lithium ion recovery rate, theoretical lithium permeation amount, measured lithium permeation amount, current efficiency, and lithium permeation rate for Examples 1 to 4 and Comparative Examples 1 to 4, respectively, based on the above calculation method.

[0117] Table 3

[0118]

[0119] (A "*" next to a numerical value indicates a deviation from the numerical range of the present invention.)

[0120] Referring to Table 3, it is clear that the lithium ion permeation rate of the lithium selective permeable membrane 10 prepared in Comparative Example 1 (0.31 mmol / (hr·cm 2 ·(mol / L))), the lithium ion permeation rate of the lithium selective permeable membrane 10 prepared in Examples 1 to 4 is 0.47 to 1.42 mmol / (hr·cm 2 (mol / L))) is significantly higher. This indicates that surface roughening of the sintered body can achieve an excellent lithium ion permeation rate. Furthermore, referring to Tables 2 and 3, it can be seen that the lithium ion permeation rate increases with a greater arithmetic mean height Ra, a greater root mean square slope Rdq, or a greater developed area ratio Sdr of the surface roughened.

[0121] The experimental results of Comparative Example 2 support the above findings. The lithium-selective membrane 10 produced in Comparative Example 2, like the lithium-selective membranes produced in Examples 1 to 4, had a roughened surface 101 in which the surface layer had been removed by approximately 10 μm through a surface roughening treatment after sintering. However, the lithium ion permeation rate of the lithium-selective membrane 10 produced in Comparative Example 2 was significantly lower than that of the lithium-selective membranes 10 produced in Examples 1 to 4. This indicates that even with a roughened surface 101 in which the surface layer during sintering has been removed through a surface roughening treatment, the excellent lithium ion permeation rate achieved in Examples 1 to 4 cannot be achieved without a sufficiently large surface arithmetic mean height Ra (0.8 μm or greater), root mean square slope Rdq (10° or greater), or developed area ratio Sdr (0.2 or greater). This demonstrates the importance of the surface roughness of the roughened surface 101.

[0122] Furthermore, the lithium-selective membrane 10 produced in Comparative Example 3 was surface-roughened using the same method as in Example 3. However, annealing was performed after the surface-roughening treatment, resulting in a crystal grain surface with a step-and-terrace structure on the surface-treated surface, which was affected by the calcining atmosphere during the heat treatment. Consequently, the lithium permeation rate in Comparative Example 3 was significantly lower than that in Example 3. This indicates that in order to achieve an excellent lithium permeation rate, it is not preferable to leave the surface of the sintered body contained in the lithium-selective membrane 10 in an as-sintered or annealed state, and that the surface layer affected by the calcining atmosphere must be removed during the heat treatment.

[0123] The lithium-selective membrane 10 produced in Comparative Example 4 achieved a sintered surface with extremely large arithmetic mean height Ra, root mean square slope Rdq, and developed area ratio Sdr by roughening the green sheet. However, as in Comparative Example 3, the membrane had an as-sintered surface state without undergoing surface roughening treatment after sintering, and therefore could not achieve an excellent lithium permeation rate.

[0124] The above results demonstrate that in order to obtain a lithium-selective membrane 10 with an excellent lithium permeation rate, it is necessary to perform surface roughening to remove the surface layer affected by the firing atmosphere during heat treatments such as sintering and annealing. Specifically, these results demonstrate that the step and terrace structures formed as growth marks on the grain surfaces of the sintered body during heat treatment must be reliably removed from the surface-roughened surface 101, and that the surface-roughened surface 101 must have a relatively large arithmetic mean height Ra, root mean square slope Rdq, or developed area ratio Sdr.

[0125] The present embodiment has been described above based on examples. As can be seen from this description, the lithium-selective membrane 10 of this embodiment can reduce the time and cost required for lithium ion recovery. Therefore, the lithium-selective membrane 10 of this embodiment can be a component suitable for recovering lithium ions 131 from seawater or industrial waste (such as discarded lithium secondary batteries).

[0126] While the lithium-selective membrane 10 and the method for manufacturing the lithium-selective membrane 10 of this embodiment have been described based on the embodiments and examples, this embodiment can be used in addition to the embodiments and use examples described above within the scope of the objectives of this embodiment and within the technical scope thereof. Furthermore, it will be apparent to those skilled in the art that the embodiments and use examples of this embodiment can be modified and altered within a range that allows for easy modification and alteration.

[0127] [Explanation of symbols]

[0128] 11: Lithium ion recovery device; 10: Lithium selective permeable membrane; 101: Surface roughening processing surface; 102: Non-processed surface; 111: First liquid (original liquid); 112: Second liquid (recovery liquid); 121: First electrode (anode); 122: Second electrode (cathode); 131: Lithium ions; 132: Non-specified ions.

Claims

1. A lithium selective permeable membrane comprising a sintered body of a lithium ion conductor, characterized in that: At least a portion of the outer surface of the lithium permselective membrane is formed of a roughened surface.

2. The lithium selective permeable membrane according to claim 1, characterized in that Satisfy at least one of the following requirements (A), (B), and (C): (A) the arithmetic mean height Ra of all or part of the roughened surface is 0.8 μm or more and 20 μm or less, (B) the root mean square slope Rdq of all or part of the roughened surface is 10° or more and 80° or less, (C) The developed area ratio Sdr of the entire or a portion of the roughened surface is 0.2 or more and 2 or less.

3. The lithium selective permeable membrane according to claim 1, characterized in that The sintered body particles in the entire or a portion of the surface roughened surface do not have a step and terrace structure on their surfaces.

4. The lithium selective permeable membrane according to claim 1, characterized in that The lithium ion conductor is a sintered body of lithium lanthanum titanate.

5. The lithium selective permeable membrane according to any one of claims 1 to 4, characterized in that In the lithium permeation test within the temperature range of 22°C to 24°C, the lithium permeation rate V expressed by the following formula (1) was 0.4 mmol / (hr·cm 2 (mol / L)) or more, V=M / (T·S·C)··(1) Where M is the amount of lithium ions permeating the lithium selective membrane (mmol), T is the time required for lithium ions to permeate M (hr), and S is the effective membrane area of ​​the lithium selective membrane (cm 2 ), C is the lithium ion concentration in the stock solution (mol / L).

6. A method for producing a lithium selective permeable membrane comprising a sintered body of a lithium ion conductor, characterized in that: include: a sintering step of sintering the molded body including the lithium ion conductor; and a surface roughening treatment step of removing a surface layer from at least a portion of the outer surface of the sintered body after sintering and roughening the surface.

7. The method for manufacturing a lithium selective permeable membrane according to claim 6, wherein: Satisfy at least one of the following requirements (D), (E), and (F): (D) the arithmetic mean height Ra of all or part of the surface of the sintered body processed by the surface roughening treatment step is 0.8 μm or more and 20 μm or less, (E) the root mean square slope Rdq of the entire or a portion of the surface of the sintered body processed by the surface roughening treatment step is 10° or more and 80° or less, (F) The developed area ratio Sdr of the entire or a portion of the surface of the sintered body processed in the surface roughening treatment step is 0.2 or more and 2 or less.

8. The method for producing a lithium selective permeable membrane according to claim 6 or 7, wherein: The surface roughening step is performed by blasting.

Images