Lithium permselective membrane and method for producing the same

Abstract

The present invention provides a lithium permselective membrane and the like that can recover lithium ions without subjecting the lithium permselective membrane to complicated treatment, and that reduces the time and cost required for recovering the lithium ions. [Solution] The lithium permselective membrane 10 of the present invention includes a sintered body of a lithium ion conductor. At least a portion of the outer surface of the lithium permselective membrane 10 is composed of a roughened surface 101. The lithium permselective membrane 10 has a median particle size of 1 μm or more and 60 μm or less. The lithium permselective membrane 10 has a median void size of 1 μm or more and 10 μm or less. The area occupancy of voids in one cross section of the lithium permselective membrane 10 is 1% or more and 5% or less.

Description

[Technical Field]

[0001] The present invention relates to a lithium permselective membrane that selectively allows lithium ions to pass therethrough, and a method for producing the lithium permselective membrane. [Background technology]

[0002] Metals, such as rare metals, are found in smaller quantities in the earth's crust than other substances, and due to the technical difficulties of mining and refining, their circulation is limited. However, they are used as additives to structural materials, electronic and magnetic materials such as light-emitting diodes, batteries, and permanent magnets, and as functional materials such as photocatalysts and new glass, and have been attracting a great deal of attention in recent years.

[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 expanding in recent years. To supply lithium for these uses, technology is needed to produce large quantities of lithium at low cost.

[0004] Rare metals such as lithium can be extracted from ores contained in the earth's crust, but as mentioned above, there are technical difficulties in producing them in large quantities.

[0005] Therefore, in recent years, technology has been developed to extract ions derived from rare metals contained in seawater. Seawater contains multiple ions, and the amount of these ions can be enormous compared to the amount of ions buried underground. It is also known that industrial waste (such as discarded lithium secondary batteries) contains large amounts of rare metals. However, seawater and industrial waste contain not only ions typified by rare metals (such as lithium ions), but also other metal ions such as potassium, sodium, and calcium. Therefore, in order to recover the target ions, membranes (selective permeable membranes) that can separate the target ions from other metal ions are being developed.

[0006] On the other hand, the concentration of rare metal elements as ions contained in seawater or industrial waste (such as discarded lithium secondary batteries) is low, and the ion permeation rate for recovering the target ions through a selectively permeable membrane is also low. Therefore, according to conventional techniques, recovering the target ions from seawater or industrial waste (such as discarded lithium secondary batteries) requires a long time and is costly.

[0007] To solve these problems, Patent Document 1 discloses a technique for extracting lithium ions from salt lake brine or lithium ore with a relatively high lithium concentration, or from industrial waste (such as discarded lithium secondary batteries) or seawater with a relatively low lithium concentration. Patent Document 1 discloses a lithium permselective membrane having a lithium adsorption layer formed on one surface by acid treatment, the lithium adsorption layer being made of a material different from the lithium permselective membrane itself. It has been revealed that such a lithium permselective membrane can recover lithium ions from the raw solution at a high recovery rate.

[0008] In order to solve these problems, the inventors of the present invention have developed a lithium permselective membrane in which at least a portion of the outer surface of the lithium permselective membrane is formed as a surface-roughened surface (Patent Document 2). This lithium permselective membrane allows lithium ions to be recovered without subjecting the lithium permselective membrane to complicated treatment, and also reduces the time and cost required for lithium ion recovery. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2017-131863 A [Patent Document 2] Patent application No. 2023-025133 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the strength of the lithium permselective membrane may be weakened depending on the manufacturing method. Therefore, an object of the present invention is to increase the strength of the lithium permselective membrane while maintaining the above-mentioned advantages. [Means for solving the problem]

[0011] In order to achieve the above object, the lithium permselective membrane of the present invention is characterized by the following invention-specific features.

[0012] The lithium permselective membrane is A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed of a surface-roughened surface; The lithium permselective membrane is characterized in that the median particle size is 1 μm or more and 60 μm or less.

[0013] In addition, the lithium permselective membrane is A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed of a surface-roughened surface; The lithium permselective membrane is characterized in that the median void diameter is 1 μm or more and 10 μm or less.

[0014] In addition, the lithium permselective membrane is A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed of a surface-roughened surface; The lithium permselective membrane is characterized in that the area occupancy of voids in one cross section is 1% or more and 5% or less.

[0015] Generally, when a lithium ion conductor is used as a lithium selective membrane, the lithium ion conductor is often used as a sintered body. Sintered bodies often have different structures or compositions near the outer surface, which is exposed to the firing atmosphere during sintering, and the inner structure or composition, which is not affected by the firing atmosphere during sintering. With such a lithium selective permeable membrane, the portion with the structure or composition near the outer surface that changed during sintering is removed by surface roughening treatment, and the portion with the structure or composition that is not affected by the firing atmosphere during sintering is exposed on the surface of the lithium selective permeable membrane. This allows the structure or composition of the ion conductor to be uniform on the outer surface and inside of the lithium selective permeable membrane.

[0016] Furthermore, the structure or composition of the outer surface of the membrane, which is exposed to the firing atmosphere during sintering, changes during sintering, causing the lithium ion permeation rate in this region to be slower than the original lithium ion permeation rate (the lithium ion permeation rate in a portion not affected by the firing atmosphere during sintering). With this type of lithium permselective membrane, a region having the structure or composition of an ion conductor that is not affected by the firing atmosphere during sintering is exposed on the outer surface of the membrane, preventing a decrease in the lithium ion permeation rate. As described above, with this type of lithium permselective membrane, the time and cost required for lithium ion recovery can be reduced.

[0017] Furthermore, in such a lithium-permeable membrane, at least one of the particle size, void size, and void volume of the lithium-permselective membrane is controlled to the above-mentioned value. Generally, the strength of a sintered body increases when the particle size, void size, and void volume are small. Therefore, in addition to the above-mentioned effects, such a lithium-permselective membrane can increase its strength, thereby broadening the range of use of the lithium-permselective membrane.

[0018] In addition, in any one of the lithium permselective membranes described above, It is preferable that at least one of the following requirements (A), (B), and (C) be satisfied. (A) The arithmetic mean height Ra of the entire or part of the surface roughened is 0.8 μm or more and 20 μm or less. (B) The root mean square gradient Rdq of the entire or part of the roughened surface is 10° or more and 80° or less. (C) The developed area ratio Sdr of the entire or part of the surface roughened is 0.2 or more and 2 or less.

[0019] This lithium permselective membrane satisfies at least one of the following requirements: an arithmetic mean height Ra of 0.8 μm or more on all or part of the roughened surface, a root-mean-square gradient Rdq of 10° or more, and a developed area ratio Sdr of 0.2 or more. This effectively increases the surface area of ​​the lithium ion conductor. Therefore, lithium ions are absorbed into the lithium permselective membrane through the lithium ion conductor with a large surface area, thereby increasing the lithium ion recovery rate.

[0020] From the viewpoint of the lithium ion recovery rate, the larger the values ​​of Ra, Rdq, and Sdr, the better. However, the larger the values ​​of Ra, Rdq, and Sdr, the greater the risk of damage to the lithium permselective membrane during the surface roughening treatment. Furthermore, the time and cost required for the surface roughening treatment also increase. Therefore, to stably produce a lithium permselective membrane containing a lithium ion conductor with a large surface area, it is preferable to satisfy at least one of the following requirements on the roughened surface of the lithium ion conductor: an arithmetic mean height Ra of 20 μm or less, a root-mean-square gradient Rdq of 80° or less, and a developed area ratio Sdr of 2 or less.

[0021] In addition, in any one of the lithium permselective membranes described above, It is preferable that the surface of the sintered body particles in the whole or part of the surface-roughened surface does not have a step-and-terrace structure.

[0022] With such a lithium permselective membrane, the surface of the sintered body particles exposed on all or part of the roughened surface does not have the step-and-terrace structure formed on the surface of the sintered body particles by heat treatment in the sintering or annealing process. Therefore, the surface that is not directly exposed to the sintering atmosphere during the heat treatment is fully exposed. Therefore, such a lithium permselective membrane can reduce the time and cost required for lithium ion recovery.

[0023] In addition, in any one of the lithium permselective membranes described above, The lithium ion conductor is preferably a sintered body of lanthanum lithium titanate.

[0024] In this lithium permselective membrane, the lithium ion conductor is a sintered body of lanthanum lithium titanate. Lanthanum lithium titanate, a lithium ion conductor, has an excellent lithium ion conduction rate. Furthermore, it allows stable lithium permeation in an aqueous solution. Therefore, this lithium permselective membrane can reduce the time and cost required for lithium recovery.

[0025] In addition, in any one of the lithium permselective membranes described above, In a lithium permeation test at a temperature range of 22°C to 24°C, the lithium permeation rate V, expressed by the following formula (1), was 0.050 mmol / (hr cm 2 ) or more is preferable. V = M / (T S) (1) Here, M is the amount of lithium ions (mmol) that permeated the lithium permselective membrane, T is the time (hr) required for lithium ions of permeation amount M to permeate, and S is the effective membrane area (cm) of the lithium permselective membrane. 2 )

[0026] With such a lithium permselective membrane, the lithium permeation rate V is 0.050 mmol / (hr cm 2) As a result, such a lithium permselective membrane can reduce the time and cost required for recovering lithium.

[0027] In addition, in any one of the lithium permselective membranes described above, The three-point bending strength of the lithium permselective membrane is preferably 100 MPa or more and 300 MPa or less.

[0028] Further, a method for producing a lithium permselective membrane for solving the above problems is characterized by the following invention-specific features.

[0029] The method for producing a lithium permselective membrane is as follows: The method is characterized by comprising a sintering step of sintering a compact containing a lithium ion conductor at a temperature of 1100°C to 1400°C for a time of 5 hours to 100 hours, 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.

[0030] According to this method for producing a lithium permselective membrane, any of the above-mentioned lithium permselective membranes can be produced by removing, in part or in whole, the portions originally present on the outer surface of the lithium ion conductor whose structure or composition has changed due to exposure to the firing atmosphere during sintering in the surface roughening treatment step. Therefore, the lithium permselective membrane produced by this method can reduce the time and cost required for lithium ion recovery.

[0031] In addition, in any one of the above-described methods for producing a lithium permselective membrane, It is preferable that at least one of the following requirements (D), (E), and (F) be satisfied. (D) The surface roughening treatment step makes the arithmetic mean height Ra of all or part of the surface of the sintered body 0.8 μm or more and 20 μm or less. (E) The surface roughening treatment step makes the root mean square gradient Rdq of the entire or part of the surface of the sintered body 10° or more and 80° or less. (F) The surface roughening treatment step sets the developed area ratio Sdr of all or part of the surface of the sintered body to 0.2 or more and 2 or less.

[0032] According to this method for producing a lithium permselective membrane, it is possible to produce a lithium permselective membrane with controlled surface roughness as described above, and therefore the lithium permselective membrane produced by this method can reduce the time and cost required for lithium ion recovery.

[0033] In addition, in any one of the above-described methods for producing a lithium permselective membrane, The surface roughening treatment is preferably a blasting treatment.

[0034] According to this method for producing a lithium permselective membrane, the surface of the sintered body is simultaneously removed and roughened in a short time by blasting. Furthermore, residual stress can be applied to the lithium ion conductor, improving the strength and durability of the lithium ion conductor and, in turn, the lithium permselective membrane. Therefore, a lithium permselective membrane with excellent durability can be produced without complicating the manufacturing process of the lithium permselective membrane. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 1 is a diagram showing an outline of a lithium ion recovery device using a lithium permselective membrane. [Figure 2] FIG. 2 shows the results of powder X-ray diffraction measurement of the sintered lithium ion conductor produced in Example 8, which was pulverized. [Figure 3] FIG. 3 is an SEM image of the sintered lithium ion conductor produced in Example 8 before surface roughening treatment (as-sintered). [Figure 4] FIG. 4 is an SEM image of the roughened surface of the sintered lithium ion conductor produced in Example 8 after the surface roughening treatment. DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will be described in detail below with reference to embodiments and the accompanying drawings.

[0037] (Configuration of lithium ion recovery device) FIG. 1 is a diagram illustrating the configuration of a lithium ion recovery device 1 using a lithium permselective membrane 10. As shown in FIG. 1, the lithium ion recovery device 1 includes the lithium permselective membrane 10, a first electrode 121, and a second electrode 122. The lithium permselective membrane 10 has a configuration described below and selectively allows lithium ions 131 contained in a stock solution 111, which is a first liquid, to pass therethrough. The lithium permselective membrane 10 selectively allows lithium ions 131 to pass through the stock solution 111, which contains a plurality of ions, thereby transferring the lithium ions 131 in the stock solution 111 to a recovery liquid 112, which is a second liquid. In this case, it is preferable that the highly permselective lithium membrane 10 be used together with electrodes or the like configured to enhance this permeability.

[0038] The first electrode 121 and the second electrode 122 are provided facing one main surface (left surface) and the other main surface (right surface), respectively, of the lithium permselective membrane 10 in FIG. 1 . Here, the first electrode 121 and the second electrode 122 are the anode and cathode, respectively, of the lithium ion recovery device 1. The first electrode 121 and the second electrode 122 may or may not be in direct contact with the lithium permselective membrane 10. With this configuration, the potential difference between the one main surface and the other main surface of the lithium permselective membrane 10 is maintained at a constant potential difference. The first electrode 121 and the second electrode 122 are preferably made of a conductive material that does not chemically react with the components in the raw solution 111 or the recovered solution 112. In this case, the materials of the first electrode 121 and the second electrode 122 may be the same or different from the viewpoint of ionization tendency.

[0039] The raw solution 111 is an aqueous solution containing lithium ions 131, such as brine from a salt lake, lithium ore, industrial waste (such as discarded lithium secondary batteries), or seawater. The raw solution 111 is supplied to the lithium ion recovery device 1 from the sea, a salt lake, or a raw solution storage tank via a raw solution pipe and a pump (not shown). The pH of the raw solution 111 may be adjusted to increase the lithium ion permeation rate and, therefore, the lithium ion recovery rate. In this case, as disclosed in Patent Document 1, the raw solution 111 is preferably made alkaline, but may also be made non-alkaline. Furthermore, when seawater is used as the raw solution 111, a solution from which monovalent ions (such as lithium ions and sodium ions) have been separated in advance using a cation exchange membrane may be used as the raw solution 111.

[0040] The recovered liquid 112 is an aqueous solution containing lithium ions 131 that have permeated from the raw liquid 111. The recovered liquid 112 is discharged from the lithium ion recovery device 1 via a recovery liquid piping and a pump (not shown), and can be stored in a recovered liquid storage tank or the like. Thereafter, CO2 bubbling or the like may be generated in the recovered liquid storage tank or the like, and the lithium ions 131 may be recovered as carbonate precipitates.

[0041] The first electrode 121 and the second electrode 122 are connected via conductive lead wires or the like, and a DC power supply 14 is interposed between the lead wires 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 mesh shape as disclosed in Patent Document 1 and are fixed to the lithium permselective membrane 10. Alternatively, for example, a current collector made of a carbon felt sheet or the like may be interposed between the mesh-shaped first electrode 121 and the second electrode 122 and the lithium permselective membrane 10. Alternatively, the first electrode 121 and the second electrode 122 may each be formed in a substantially plate shape, as in an existing Daniel cell.

[0042] Furthermore, the lithium ion recovery device 1 using the lithium permselective membrane 10 of the present invention can recover lithium ions 131 by applying a voltage, and may also be used as a battery that extracts power without applying an external voltage. In this case, in the process of transferring the lithium ions 131 from the raw solution 111 to the recovery solution 112, a voltage is generated that causes the first electrode 121 to act as a negative electrode and the second electrode 122 to act as a positive electrode. On the other hand, when a voltage is applied to the lithium ion recovery device 1, the flow of the lithium ions 131 can be increased compared to when no voltage is applied, and the recovery efficiency of the lithium ions 131 can be improved.

[0043] The lithium permselective membrane 10 may be composed solely of a lithium ion conductor, or may be composed of a composite of a lithium ion conductor and another material that does not have lithium ion conductivity. The composite may be a composite of lithium ion conductor particles and a resin in order to improve flexibility. Furthermore, in order to improve the strength of the lithium permselective membrane 10, it may be a composite of a thin-film lithium ion conductor and a porous support that supports it. From the viewpoints of the complexity of the manufacturing process and manufacturing costs, it is preferable that the lithium permselective membrane 10 be composed solely of a sintered body of a lithium ion conductor.

[0044] The lithium permselective membrane 10 is provided to separate the raw solution 111 containing a plurality of ions, including lithium ions 131 and non-designated ions 132 (ions other than the lithium ions 131), from the recovered solution 112 into which the lithium ions 131 are recovered. In this case, the lithium permselective membrane 10 needs to be provided so that the roughened surface 101, which is obtained by a surface roughening treatment described below, faces the raw solution 111. The lithium permselective membrane 10 may be formed in a spherical, cylindrical, or regular octahedral shape, or may have at least one recessed or protruding portion formed on a portion of a plate or one of the shapes described above. In this case, the shape of the lithium permselective membrane 10 is not particularly limited as long as it can separate the raw solution 111 from the recovered solution 112. The roughened surface 101, which will be described below, may be provided depending on the electric field generated in the lithium ion recovery device 1. For example, the roughened surface 101 may be provided in a region of the surface facing the stock solution 111 where the electric flux density perpendicular to the surface, which is generated when a voltage is applied to the lithium ion recovery device 1, is relatively large.

[0045] The lithium permselective membrane 10 has a roughened surface 101 (shown by the bold line in FIG. 1 ). The roughened surface 101 is provided on a part or all of the outer surface of the lithium permselective membrane 10. As described above, the roughened surface 101 needs to be provided so as to face the stock solution 111. In FIG. 1 , an untreated surface 102 that has not been subjected to surface roughening is provided on the main surface of the lithium permselective membrane 10 that faces the recovered solution 112, but this is not limited thereto. In this case, the main surface of the lithium permselective membrane 10 that faces the recovered solution 112 may be provided with the roughened surface 101 on part or all of the surface, or the untreated surface 102 that has not been subjected to surface roughening may be provided on the entire surface.

[0046] The surface-roughened surface 101 is subjected to removal of the sintered compact surface layer and surface roughening by any of the surface roughening treatment methods described below. The arithmetic mean height Ra of the surface of all or a portion of the surface-roughened surface 101 is preferably 0.8 μm to 20 μm, more preferably 1.5 μm to 15 μm, and even more preferably 2.0 μm to 10 μm. Furthermore, while the surface roughness in the present invention is expressed in terms of Ra, it is not limited thereto and may also be expressed in terms of the root-mean-square gradient Rdq. In this case, the root-mean-square gradient Rdq of the surface of all or a portion of the surface-roughened surface 101 is preferably 10° to 80°, more preferably 12° to 70°, and even more preferably 14° to 60°. These Ra and Rdq values ​​can be calculated using a stylus-type surface roughness meter. The measurement conditions for the entire or partial surface are preferably evaluated using the reference length, measurement length, and cut-off value recommended in the JIS B 0601-2001 or ISO4287-1997 standards in accordance with the degree of surface roughness.

[0047] Specifically, the surface line roughness is preferably measured using a stylus-type surface roughness meter (Form Talysurf PGI1250A, manufactured by Taylor Hobson) in terms of arithmetic mean height Ra and root mean square gradient Rdq. The evaluation length and cutoff value (λc) are preferably values ​​recommended by JIS B 0601-2001 or ISO 4287-1997, depending on the degree of surface roughness. Measurements are preferably taken at five arbitrary positions on the roughened surface 101 of the ceramic sheet to be measured. The stylus scanning direction is preferably selected arbitrarily each time a measurement is made, and the average value is calculated from the five measurement results.

[0048] Furthermore, the surface of all or part of the surface-roughened surface 101 of the present invention may be expressed by the developed area ratio Sdr, which is a surface roughness parameter. In this case, the developed area ratio Sdr is preferably 0.2 or more and 2 or less, more preferably 0.3 or more and 1.8 or less, and even more preferably 0.5 or more and 1.5 or less. The developed area ratio Sdr, which is a surface roughness parameter, can be calculated using a non-contact surface roughness meter.

[0049] Specifically, the surface roughness is preferably measured using an optical interference type non-contact three-dimensional surface roughness measuring instrument (Talysurf CCI HD-XL manufactured by Taylor Hobson). The measurement is preferably performed in a mode for evaluating a "low reflectance rough surface" using a 20x magnification objective lens (evaluation area 0.82mm x 0.82mm). The measurement is preferably performed at three arbitrary points on the roughened surface 101 to be measured on the ceramic sheet, and an average value is calculated from the measurement results obtained at the three points.

[0050] In addition, it is preferable that all or part of the surface-roughened surface 101 in the present invention 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 on the surface-roughened surface 101 can be determined, for example, by checking an SEM image of the surface.

[0051] The roughened surface 101 in the present invention preferably has a uniform surface that satisfies the above conditions, but is not limited to this. For example, the roughened surface 101 may have two or more different configurations. In this case, it is preferable that at least one of the roughened surfaces 101 satisfies the conditions regarding surface roughness or the presence or absence of a step-and-terrace structure. Furthermore, in this case, the roughened surface 101 may vary intermittently or continuously in the horizontal direction of the surface. These conditions for the roughened surface 101 can be adjusted depending on the design of the lithium ion recovery device 1.

[0052] From the viewpoint of increasing the strength of the lithium permselective membrane 10 while also increasing the lithium ion permeation rate of the lithium permselective membrane 10, the median particle size (D50) of the lithium permselective membrane 10 is preferably 1 μm or more and 60 μm or less, more preferably 2 μm or more and 50 μm or less, and even more preferably 5 μm or more and 25 μm or less. If the particle size of the lithium permselective membrane 10 is larger than any of the above ranges, the strength of the lithium permselective membrane 10 will decrease. If the particle size of the lithium permselective membrane 10 is smaller than any of the above ranges, the lithium ion permeation rate of the lithium permselective membrane 10 will decrease.

[0053] Here, the particle size and particle size distribution are measured by observing particles on a cross section or surface of the lithium permselective membrane 10. Here, from the viewpoint of measuring a minimum statistical distribution, it is preferable to perform a statistical calculation of the circle-equivalent diameter of particles on a plane at a visual field magnification at which 900 to 2000 sintered particles are observed in the visual field.

[0054] Furthermore, from the viewpoint of increasing the lithium ion permeation rate of the lithium permselective membrane 10 while increasing the strength of the lithium permselective membrane 10, the median void diameter of the lithium permselective membrane 10 is preferably 1 μm or more and 10 μm or less, more preferably 1 μm or more and 8 μm or less, and even more preferably 1 μm or more and 5 μm or less. If the void diameter of the lithium permselective membrane 10 is larger than any of the above ranges, the strength of the lithium permselective membrane 10 will decrease.

[0055] Additionally, from the viewpoint of increasing the lithium ion permeation rate of the lithium permselective membrane 10 while increasing the strength of the lithium permselective membrane 10, the area occupancy of voids on one cross section or surface of the lithium permselective membrane 10 is preferably 1% to 5%, more preferably 1% to 4%, and even more preferably 1% to 3%. If the amount of voids in the lithium permselective membrane 10 is larger than any of the above ranges, the strength of the lithium permselective membrane 10 will decrease.

[0056] Here, the void diameter and the area occupancy of the void amount are measured by observing particles on a cross section or surface of the lithium permselective membrane 10. Here, from the viewpoint of measuring a minimum statistical distribution, it is preferable to perform statistical calculations of the circle-equivalent diameter of the voids on a plane and / or the area occupancy of the voids relative to the area of ​​the observation field at a magnification where 900 to 2000 sintered particles are observed in the observation field.

[0057] The material of the lithium permselective membrane 10 of the present invention is not particularly limited as long as it is a material that can selectively permeate lithium ions 131. However, from the viewpoint of permeating lithium ions 131 while suppressing reactions with water and other aqueous solutions, it is preferable to use lanthanum lithium titanate, which is a super lithium ion conductor that has high ionic conductivity for lithium ions 131 and does not react with water and components in aqueous solutions. As lanthanum lithium titanate, Li x La (2-x) / 3 It is preferable to use lanthanum lithium titanate in TiO3 where x<2 / 3, and more preferably Li 0.29 La 0.57 TiO3 (x=0.29).

[0058] 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, etc. can be used for the lithium permselective membrane 10. Furthermore, the crystal structure is not limited to these, and any material can be used as the lithium permselective membrane 10 as long as it is an ion conductor with high lithium ion conductivity and can exist stably in the raw solution 111 and the recovered solution 112 used in recovering lithium ions.

[0059] In addition, from the viewpoint of being able to sufficiently recover lithium ions 131, the lithium ion permeation rate of the lithium permselective membrane 10 of the present invention is set to 0.050 mmol / (hr cm) in a lithium permeation test in a temperature range of 22°C to 24°C. 2) or more, and 0.055 mmol / (hr cm 2 ) or more, and even more preferably 0.060 mmol / (hr cm 2 The lithium permeation rate is calculated from the following formula (1):

[0060] V = M / (T S) (1)

[0061] Here, M is the amount (mmol) of lithium ions 131 that permeated the lithium permselective membrane 10, T is the time (hr) required for the lithium ions 131 of permeation amount M to permeate, and S is the effective membrane area (cm 2 )

[0062] In order to ensure sufficient strength of the lithium permselective membrane 10 of the present invention, the three-point bending strength is preferably 100 MPa or more, more preferably 110 MPa or more, and even more preferably 150 MPa or more.

[0063] (Method of manufacturing lithium permselective membrane) Next, a method for manufacturing the lithium permselective membrane 10 of the present invention will be described. The lithium permselective membrane 10 may be composed solely of a lithium ion conductor, or may be composed of a composite of the lithium ion conductor and another material that does not have lithium ion conductivity. The lithium permselective membrane 10 must have a structure without through-holes because it is necessary to separate the raw solution 111 and the recovered solution 112 through the lithium permselective membrane 10. While there are no particular limitations on the method for manufacturing the lithium permselective membrane 10 without through-holes, it is preferable to use a densified sintered body as the lithium ion conductor. To manufacture this sintered body, particles that can form the sintered body as the desired lithium ion conductor are crushed to a desired particle size range, granulated as necessary, and then mixed with optional additives such as a sintering aid, dispersant, pore-forming agent, mold release agent, and binder, and molded into a desired shape (a shape compatible with the lithium ion recovery device 1 to be used). The molding method is not particularly limited, and may be appropriately selected depending on the shape of the molded body to be produced, such as sheet molding, uniaxial pressing, cold isostatic pressing, warm isostatic pressing, etc. The produced molded body may be further machined into any shape as needed.

[0064] The compact is then sintered, but a degreasing step may be carried out before sintering in order to decompose or remove organic substances contained in the binder of the mixture, etc. The conditions for the sintering step, such as the sintering temperature and sintering time, are set to those suitable for obtaining a sintered compact as the intended lithium ion conductor.

[0065] For example, to obtain a sintered body of lithium lanthanum titanate as a lithium ion conductor, sintering is preferably performed in a temperature range of 1100°C to 1400°C, more preferably 1150°C to 1380°C, and even more preferably 1180°C to 1330°C, with a holding time of preferably 5 hours to 100 hours, more preferably 7 hours to 72 hours, and even more preferably 10 hours to 65 hours. Sintering at a constant temperature and for a short time outside the above ranges does not proceed even when sintering is performed, and a dense body is not obtained. On the other hand, sintering at a high temperature and for a long time causes the green body and the sintered member to fuse and react, making it impossible to obtain a good sintered body, and also promotes grain growth in the lithium permselective membrane 10, reducing its strength.

[0066] In order to suppress changes in the surface structure or composition of the compact due to the influence of the firing atmosphere during sintering, the compact may be buried in the same powder as the compact, such as a mother powder or a cover powder, and then sintered.

[0067] Next, a part or all of the outer surface of the obtained sintered body (as-sintered body) is subjected to a surface roughening treatment, thereby providing a roughened surface 101 on the outer surface of the lithium permselective membrane 10. At this time, it is preferable that the arithmetic mean height Ra of the roughened surface 101 is 0.8 μm or more and 20 μm or less, or that the root-mean-square gradient Rdq is 10° or more and 80° or less, or that the developed area ratio Sdr is 0.2 or more and 2 or less, but is not limited thereto and may be within the aforementioned numerical ranges.

[0068] Furthermore, the depth of removal of the surface layer in the surface roughening treatment of the outer surface of the obtained sintered body (as-sintered body) is not particularly limited, but removal of 5 μm or more is preferable. Considering that changes in structure and composition may occur due to the influence of the sintering atmosphere during sintering, the removal depth is preferably changed depending on the material type or composition of the ion conductor or the temperature or time of the sintering process. For example, the removal depth may be adjusted depending on the sintering time, or the sintering temperature and sintering time. In this case, the removal depth of the surface layer is preferably removed to the extent that the roughened surface 101 has the desired structure, composition, or particle size distribution.

[0069] The surface roughening treatment described above is preferably performed by blasting, and more preferably by sandblasting, in order to simultaneously remove the surface layer and form a roughened surface 101 having a surface roughness of 0.8 μm to 20 μm, a root-mean-square gradient Rdq of 10° to 80°, or a developed area ratio Sdr of 0.2 to 2. In this case, residual stress can be imparted to the sintered body, thereby improving the strength and durability of the sintered body. Furthermore, the present invention is not limited to sandblasting, and may involve blasting treatments such as wet blasting and shot blasting, as well as dry or wet mechanical polishing and chemical etching. The abrasive grains used in the blasting treatment are not particularly limited, but may include alumina abrasive grains, SiC abrasive grains, glass abrasive grains, or zircon abrasive grains.

[0070] If the sintered body is subjected to the above-described surface roughening treatment and then to a high-temperature heat treatment such as annealing, the surface structure or composition may change due to the influence of the firing atmosphere during the heat treatment, as in the case of sintering. Therefore, after the surface roughening treatment, it is preferable to use the sintered body as the lithium permselective membrane 10 without performing a heat treatment at a high temperature (generally above the Debye temperature of the material) at which the atoms constituting the lithium ion conductor may diffuse.

[0071] Whether the surface of the lithium permselective membrane 10 has been roughened after the final heat treatment step or not can be determined by observing the surface morphology of the lithium permselective membrane 10. For example, in the case of a sintered body of lanthanum lithium titanate, the surface of the sintered body after sintering exhibits steps at the interfaces (grain boundaries) of the crystalline particles of the ceramics that make up the sintered body, and a step-and-terrace structure stripe pattern is observed on the crystalline particle surface as a trace of grain growth due to sintering. On the other hand, on the surface-roughened surface 101 after heat treatment, since the surface layer has been physically removed, the above-mentioned steps along the grain boundaries and the step-and-terrace structure as a trace of crystalline particle growth are not observed, and the surface morphology is observed according to the surface roughening method. For example, blasting produces a rough surface with no regularity. Furthermore, mechanical polishing produces linear scratches similar to those seen in polishing.

[0072] Therefore, specifically, in the present invention, since it is necessary to reliably remove the portions of the roughened surface 101 that were affected by the firing atmosphere during the heat treatment, it is preferable that the roughened surface 101 does not have the step-and-terrace structure of the sintered particle surface that is formed during the heat treatment. As described above, it is possible to easily determine whether the surface is as it is after the heat treatment (sintering step or annealing step) or whether the surface has been sufficiently roughened after the final heat treatment.

[0073] The lithium permselective membrane 10 of the present invention can be obtained by the above-mentioned method, but the method for producing the lithium permselective membrane 10 is not limited to the above-mentioned method and can be modified within the scope that can achieve the object of the present invention.

[0074] (Evaluation of lithium permeation rate) The lithium permselective membrane 10 produced by the above-described method is placed in the lithium ion recovery device 1 configured as described above, and lithium can be recovered by applying a voltage to the first electrode 121 and the second electrode 122.

[0075] At this time, it is known that the permeation rate of lithium ions 131 permeating the lithium permselective membrane 10 mainly depends on the temperature of the lithium permselective 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, the lithium ion permeation amount (unit: mmol / (hr cm)) per unit area of ​​the lithium permselective membrane 10 and per unit time is calculated under a constant temperature and constant pH of the stock solution 111. 2 If the lithium permeation rate is as defined above, the lithium permeation rate can be compared and evaluated as the performance of the lithium permselective membrane 10 itself.

[0076] The amount of lithium ions permeated can be directly determined by evaluating the lithium ion concentrations of the raw solution 111 and the recovered solution 112 before and after the lithium permeation test using ICP analysis, etc. Alternatively, it can also be determined from the value of the current that flows when a voltage is applied to the lithium ion recovery device 1, using the following formula (2).

[0077] M=Itα / nF (2)

[0078] Here, M is the number of moles of lithium ions 131 that permeated the lithium permselective membrane 10, I is the current value flowing through the device, t is time, F is Faraday's constant, and n is the valence (lithium ion is 1). α represents current efficiency, a parameter that indicates the proportion of the current value attributable to the permeation of lithium ions 131 out of the current value flowing through the device, and has a value in the range of 0≦α≦1. For example, the amount of lithium ions 131 that moved can be calculated by multiplying the area enclosed by the current-time curve obtained by measuring and recording the current at regular intervals with a constant voltage applied by the current efficiency and dividing the result by the Faraday's constant.

[0079] The current efficiency α is affected by the configuration of the lithium ion recovery device 1 and the presence or absence of electronic conductivity of the lithium permselective membrane 10. To determine the current efficiency α, the amount of lithium ions 131 contained in the recovery solution 112 is determined by ICP analysis or the like to calculate the actual measured value of the lithium ion permeation amount, and the theoretical value of the lithium ion permeation amount is calculated from the total current value that flows assuming α=1 using equation (2), and the actual measured value of the lithium ion permeation amount is divided by the theoretical value.

[0080] The lithium ion permeation rate defined above can be obtained by dividing the amount of lithium ions permeated calculated by the above method by the total time required for permeation, and then dividing this by the effective membrane area of ​​the lithium permselective membrane 10. [Example]

[0081] Next, the embodiment of the present invention will be described in more detail with reference to experimental results.

[0082] (Preparation of lithium permselective membrane) Lithium lanthanum titanate powder (manufactured by Toho Titanium Co., Ltd.) was ball milled to adjust the particle size, 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 approximately 700 μm thick.

[0083] The obtained green sheet was sandwiched between porous ceramic base plates and subjected to heat treatment at 1000°C for 3 hours in the atmosphere to perform degreasing and calcination.

[0084] Next, the calcined body was sandwiched between dense ceramic plates and heated in the atmosphere according to the method in Table 1. Sintering was carried out by heat treatment under the conditions given above, and Examples 1 to 8 before the surface roughening treatment were obtained.

[0085] [Table 1]

[0086] The relative density of the obtained sintered body was 98% or more, and a dense ceramic sheet without through holes was obtained. The obtained ceramic sheet was pulverized, and silicon powder was added as an internal standard. The powder X-ray diffraction measurement results (CuKα radiation source) in Example 8 were measured and are shown in Figure 2. The results in Figure 2 confirmed that the obtained ceramic sheet consisted of a crystalline phase of lanthanum lithium titanate (the desired ionic conductor).

[0087] The resulting sintered body was cut in one direction and polished to a mirror finish. The cross section was observed under a scanning electron microscope (SEM) at a magnification such that 900 to 2000 sintered body particles were observed in the field of view. The particle size (median equivalent circle diameter (D50)), void size (median equivalent circle diameter (D50) of voids), and void area ratio (total area ratio of voids in the observed field of view) of each sintered body particle were measured using imaging software (WinROOF, manufactured by Mitani Corporation). Table 2 shows the particle size, void diameter, and void area ratio obtained by the measurements according to the firing conditions.

[0088] [Table 2]

[0089] Referring to Table 2, it was found that the higher the firing temperature and the longer the firing time, the greater the tendency for the particle size, void diameter, and void area ratio to increase. It was also revealed that the firing temperature has a greater effect on the particle size, void diameter, and void area ratio than the firing time.

[0090] (Surface roughening treatment) Next, the resulting sintered ceramic sheet was cut into approximately 20 mm x 20 mm pieces, and one side of the cut piece was subjected to a surface roughening treatment by blasting, resulting in Examples 1 to 8 that satisfied at least one of the following requirements (A), (B), and (C). Here, (A) is the requirement that the arithmetic mean height Ra of the entire or partial roughened surface 101 is 0.8 μm or more and 20 μm or less. (B) is the requirement that the root-mean-square gradient Rdq of the entire or partial roughened surface 101 is 10° or more and 80° or less. (C) is the requirement that the developed area ratio Sdr of the entire or partial roughened surface 101 is 0.2 or more and 2 or less. The blasting treatment was performed using an abrasive based on SiC (GC100) to achieve a grinding depth of approximately 10 μm.

[0091] On the other hand, no surface roughening treatment was performed in Comparative Examples 1 to 8. Therefore, in Comparative Examples 1 to 8 in which no surface roughening treatment was performed, the substrates obtained had a surface roughness on the non-processed surface 102 that did not satisfy any of the above requirements (A), (B), and (C).

[0092] FIG. 3 shows an SEM image of the surface of the sintered body in Example 8 before the surface roughening treatment, and FIG. 4 shows an SEM image of the roughened surface 101 of the sintered body after the surface roughening treatment. As shown in FIG. 3, on the surface of the sintered lanthanum lithium titanate sintered body, steps existed along the interfaces (grain boundaries) of the ceramic crystal grains. Furthermore, on the surfaces of some crystal grains, a stepped striped pattern consisting of a step-and-terrace structure was observed as a trace of grain growth during sintering. On the other hand, as shown in FIG. 3, on the roughened surface 101 that had been subjected to the surface roughening treatment by blasting, steps along the grain boundaries and a step-and-terrace structure on the crystal grain surfaces as shown in FIG. 3 were not observed, and a rough surface with no regularity was observed. Furthermore, although not shown, in Examples 1 to 7, steps along the grain boundaries and a step-and-terrace structure on the crystal grain surfaces as shown in FIG. 3 were not observed. On the other hand, although not shown, a step-and-terrace structure was observed in Comparative Examples 1 to 8.

[0093] (Evaluation of lithium permeation rate using a lithium permselective membrane) The ceramic sheets of Examples 1 to 8 and Comparative Examples 1 to 8 obtained by the above method were used as lithium permselective membranes 10 (effective membrane area: Φ12 mm) and installed in a lithium ion recovery device 1 as shown in Figure 1, and lithium recovery tests were conducted. The test was conducted at a temperature ranging from 22°C to 24°C. The raw solution 111 used in the test was a mixed aqueous solution of lithium hydroxide, sodium hydroxide, and potassium hydroxide, prepared so that the lithium ion concentration, sodium ion concentration, and potassium ion concentration were 0.1 mol / L, 0.1 mol / L, and 0.1 mol / L, respectively. A 0.1 mol / L aqueous potassium hydroxide solution was used as the recovery solution 112. The first electrode 121 (anode electrode) and the second electrode 122 (cathode electrode) were each made of carbon felt. The voltage applied to the carbon felt was set to 6 V. The raw solution 111 and the recovery solution 112 were thoroughly stirred during the test to ensure uniform lithium ion concentrations. From the start of voltage application, the current value was recorded at intervals of approximately 30 seconds, and the test was continued for 18 to 33 hours.

[0094] The stock solution 111 before the start of the test and the recovered solution 112 after the test were sampled and subjected to ICP analysis, and the recovery rates (proportions of each ion in the stock solution 111 that were recovered in the recovered solution 112) of lithium ions 131, sodium ions (non-designated ions 132), and potassium ions (non-designated ions 132) during the test period were measured. The recovery rates of sodium ions and potassium ions were 0% in all of Examples 1 to 8 and Comparative Examples 1 to 8. In addition, the theoretical value of the lithium ion permeation amount calculated using equation (1) from the total amount of current that had flowed until the test time elapsed, and the current efficiency α calculated from the actual measured value of the lithium ion permeation amount by ICP analysis were measured.

[0095] In addition, using α obtained by the above method, the amount of lithium ions permeated (unit: mmol) from the start of voltage application until 18 hours had elapsed was calculated from equation (2), and the time required (18 hours) and the effective membrane area (1.13 cm 2) to calculate the lithium ion permeation rate for 18 hours after the start of the test. Table 2 above shows the lithium ion permeation rate, the baking temperature, and the baking time for Examples 1 to 8 and Comparative Examples 1 to 8 based on the above calculation method.

[0096] Referring to Table 2, it is clear that even under the same firing conditions, the lithium ion permeation rate of the lithium permselective membranes 10 prepared in Examples 1 to 8 that had been subjected to blasting was significantly higher than the lithium ion permeation rate of the lithium permselective membranes 10 prepared in Comparative Examples 1 to 8. This indicates that an excellent lithium ion permeation rate can be obtained by roughening the surface of the sintered body.

[0097] Table 2 above shows the three-point bending strengths and firing conditions for Examples 1 to 4 and Comparative Examples 1 to 4. The measured three-point bending strengths are those for Comparative Examples 1 to 4. Each of Examples 1 to 4 is a lithium permselective membrane obtained by surface roughening the corresponding one of Comparative Examples 1 to 4, and the firing conditions are the same, so it is considered that they have equivalent strengths. Therefore, it is considered that the strength data for each of Examples 1 to 4 is substantially the same as the strength data for each of Comparative Examples 1 to 4. Therefore, it is assumed that the three-point bending strengths for each of Examples 1 to 4 are substantially the same as the three-point bending strengths for each of Comparative Examples 1 to 4, and in Table 2, the three-point bending strengths for each of Examples 1 to 4 are considered to be the three-point bending strengths for each of Comparative Examples 1 to 4. Here, the three-point bending strengths were measured using the following method. The three-point bending strengths were measured using rectangular test pieces measuring 50 mm x 4 mm each formed from each of the substrates of Comparative Examples 1 to 4, using a method conforming to JIS R1601 (Testing Method for Room-Temperature Bending Strength of Fine Ceramics). 7, it was confirmed that the higher the firing temperature and the longer the firing time of the lithium permselective membrane 10, the smaller the three-point bending strength. This is because the higher the firing temperature and the longer the firing time, the more grain growth progresses, and the grain size, void diameter, and void volume increase, resulting in a decrease in strength.

[0098] The above results indicate that, in order to obtain a lithium permselective membrane 10 having an excellent lithium permeation rate, the surface layer portion that is affected by the firing atmosphere during heat treatment such as sintering or annealing must be removed by surface roughening treatment; that is, the surface roughening treatment must be reliably removed so that the step-and-terrace structure that forms as growth marks on the surface of the sintered body particles during heat treatment does not appear on the surface 101.

[0099] In the above examples, the surface roughened by blasting was used, but similar results were obtained when the surface was roughened by the disclosed method, depending on the embodiment.

[0100] The present invention has been described above based on examples. From this description, it can be seen that the lithium permselective membrane 10 of the present invention can reduce the time and cost required for lithium ion recovery, while also improving strength. Therefore, the lithium permselective membrane 10 of the present invention can be a suitable component for recovering lithium ions 131 from seawater or industrial waste (such as discarded lithium secondary batteries). Furthermore, the membrane has a high degree of freedom in specifications, making it suitable for use in a variety of industrial fields.

[0101] The lithium permselective membrane 10 and the method for manufacturing the lithium permselective membrane 10 of the present invention have been described based on embodiments and examples, but the present invention can be used within the scope of the object and technical scope of the present invention in addition to the above-mentioned examples and use examples. Furthermore, the examples and use examples of the present invention can be easily modified and altered by those skilled in the art. [Explanation of symbols]

[0102] 1...Lithium ion recovery device, 10...Lithium selective permeable membrane, 101...Roughened surface, 102...Untreated surface, 111...First liquid (raw solution), 112...Second liquid (recovered liquid), 121...First electrode (anode), 122...Second electrode (cathode), 131...Lithium ions, 132...Unspecified ions.

Claims

1. A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed from a surface-roughened surface; A lithium permselective membrane, characterized in that the median particle size of the lithium permselective membrane is 1 μm or more and 10 μm or less.

2. A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed from a surface-roughened surface; A lithium permselective membrane, characterized in that the median void diameter of the lithium permselective membrane is 1 μm or more and 5 μm or less.

3. A lithium permselective membrane comprising a sintered body of a lithium ion conductor, wherein at least a part of an outer surface of the lithium permselective membrane is formed from a surface-roughened surface; A lithium permselective membrane, wherein the area occupancy of voids in one cross section of the lithium permselective membrane is 1% or more and 5% or less.

4. 4. The lithium permselective membrane according to claim 1, wherein the lithium permselective membrane satisfies at least one of the following requirements (A), (B), and (C): (A) The arithmetic mean height Ra of the entire or part of the roughened surface is 0.8 μm or more and 20 μm or less. (B) The root mean square gradient Rdq of the entire or a part of the roughened surface is 10° or more and 80° or less. (C) The developed area ratio Sdr of the entire or part of the roughened surface is 0.2 or more and 2 or less.

5. The lithium permselective membrane according to any one of claims 1 to 3, A lithium permselective membrane characterized in that the surface of the sintered body particles in the entirety or part of the surface-roughened surface does not have a step-and-terrace structure.

6. The lithium permselective membrane according to any one of claims 1 to 3, A lithium permselective membrane characterized in that the lithium ion conductor is a sintered body of lanthanum lithium titanate.

7. The lithium permselective membrane according to any one of claims 1 to 3, In a lithium permeation test in a temperature range of 22°C to 24°C, the lithium permeation rate V represented by the following formula (1) was 0.050 mmol / (hr cm 2 ) or more. V = M / (T・S)...(1) Here, M is the amount of lithium ions (mmol) that permeated the lithium permselective membrane, T is the time (hr) required for lithium ions of permeation amount M to permeate, and S is the effective membrane area (cm 2 )

8. A method for producing a lithium permselective membrane comprising a sintered body of a lithium ion conductor, the method comprising: a sintering step of sintering a compact comprising the lithium ion conductor at a temperature of 1100°C or higher and 1400°C or lower for a time of 5 hours or higher and 100 hours or lower; and a surface roughening treatment step of removing a surface layer from at least a portion of an outer surface of the sintered body after sintering and roughening the surface.

9. 9. The method for producing a lithium permselective membrane according to claim 8, wherein at least one of the following requirements (D), (E), and (F) is satisfied: (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 gradient Rdq of all or part 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 all or part of the surface of the sintered body processed by the surface roughening treatment step is 0.2 or more and 2 or less.

10. A method for producing the lithium permselective membrane according to claim 8 or 9, 4. A method for producing a lithium permselective membrane, wherein the surface roughening step is carried out by blasting.

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