Method for producing sintered body

Abstract

To provide a method for producing a sintered compact by which a sintered compact having relatively high relative density and relatively high ion conductivity can be produced.SOLUTION: A method for producing a sintered body of the present invention is a method for producing a sintered body containing a lithium lanthanum titanium oxide represented by General Formula (I): La2 / 3-LA (0 <x <0.16), the method including a step of heating and sintering a molded body containing the lithium xLi3xTiO3 titanium oxide, wherein the step of sintering includes a pre-sintering step at a temperature of 1100 °C to 1200 °C for 4 hours to 6 hours and a post-sintering step at a temperature of 1300 °C to 1350 °C for 0.5 hours to 1.5 hours.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a sintered body containing lithium-lanthanum-titanium oxide. [Background technology]

[0002] Lithium lanthanum titanium oxide has high ionic conductivity for lithium ions, and is therefore expected to be used in a variety of applications, including solid electrolytes for all-solid-state lithium ion batteries and lithium-permeable membranes for extracting lithium from salt lake brine or seawater.

[0003] Such applications may require a sintered body containing lithium-lanthanum-titanium oxide. To produce such a sintered body, a raw material powder containing lithium-lanthanum-titanium oxide may be formed into a compact by cold isostatic pressing (CIP) or the like, and the raw material powder in the compact may be sintered by heating the compact.

[0004] As a technique related to this, Patent Document 1 states that "the obtained lithium lanthanum titanium oxide powder is molded into a desired shape using a molding method such as CIP molding, die molding, casting molding, extrusion molding, green sheet casting molding, etc. to obtain a molded body. The molding conditions for die molding include, for example, a molding pressure of 400 to 1500 kg / cm. 2 The resulting molded body is sintered to obtain the lithium-lanthanum-titanium oxide of the present invention. After primary sintering at 1000 to 1200°C for 1 to 4 hours, secondary sintering is carried out at 1200 to 1500°C for 4 to 20 hours to obtain the lithium-lanthanum-titanium oxide." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-120264 Summary of the Invention

Problems to be Solved by the Invention

[0006] In the method described in Patent Document 1, a sintered body with a certain degree of high ionic conductivity can be obtained, but there is still room for further improvement from the viewpoint of increasing the relative density and further increasing the ionic conductivity.

[0007] This invention addresses such issues, and its objective is to provide a method for manufacturing a sintered body that can produce a sintered body with a relatively high relative density and a relatively high ionic conductivity.

Means for Solving the Problems

[0008] In order to improve the ionic conductivity of the sintered body, the inventor conducted intensive studies to increase the density of the sintered body. As a result, it was found that it is effective to perform sintering in two or more stages including pre-sintering and post-sintering during sintering, and to shorten the time of the post-sintering to a certain extent.

[0009] The method for manufacturing a sintered body of this invention is a method for manufacturing a sintered body containing lithium lanthanum titanate represented by the following general formula (I), and has a sintering step of heating and sintering a molded body containing the lithium lanthanum titanate. The sintering step includes a pre-sintering at a temperature of 1100°C to 1200°C for 4 hours to 6 hours and a post-sintering at a temperature of 1300°C to 1350°C for 0.5 hours to 1.5 hours. La 2 / 3-x Li 3x TiO3(I) (In the general formula (I), x satisfies 0 < x < 0.16.)

Advantages of the Invention

[0010] According to the method for manufacturing a sintered body of this invention, a sintered body with a relatively high relative density and a relatively high ionic conductivity can be produced.

Embodiments for Carrying out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail. A method for manufacturing a sintered body according to an embodiment of the present invention is represented by the general formula (I): La 2 / 3-x Li 3x It is for manufacturing a sintered body containing a lithium lanthanum titanate represented by TiO3 (0 < x < 0.16). This manufacturing method includes a sintering step of heating and sintering a molded body containing the above lithium lanthanum titanate.

[0012] In the sintering step, pre-sintering and post-sintering are included. In the pre-sintering, the heating temperature is set to 1100 °C to 1200 °C, and the heating time is set to 4 hours to 6 hours. In the subsequent post-sintering, the heating temperature is set to 1300 °C to 1350 °C, and the heating time is set to 0.5 hours to 1.5 hours. The sintered body obtained through such a sintering step has a high relative density and can achieve excellent ionic conductivity. Due to the high relative density of the sintered body, for example, when collecting lithium from salt lake brine or seawater, it is possible to suppress the penetration of the stock solution while allowing lithium ions to permeate, and suppress the contamination of the stock solution in the collected solution.

[0013] Before the sintering step, a powder production step of producing a lithium lanthanum titanate powder containing an oxide represented by the general formula (I) and, thereafter, a molding step of molding the lithium lanthanum titanate powder to obtain a molded body can be performed. However, when the lithium lanthanum titanate powder or the molded body can be obtained by purchase or the like, the powder production step and further the molding step may be omitted.

[0014] (Powder production step) When producing a lithium lanthanum titanate powder, for example, preparation of raw materials, first wet pulverization, calcination, and second wet pulverization can be performed in this order. A detailed description of an example is as follows.

[0015] First, lithium compounds such as lithium hydroxide and lithium carbonate are prepared as lithium raw materials, titanium compounds such as titanium oxide, metatitanic acid, and orthotitanic acid are prepared as titanium raw materials, and lanthanum oxide is prepared as lanthanum raw material. If necessary, hydroxide, chloride, and / or carbonate of at least one element selected from the group consisting of Sr, K, Fe, Ga, and Ta are also prepared.

[0016] In the first wet milling step, the raw materials, such as the lithium raw material, titanium raw material, and lanthanum raw material, are mixed and milled in a predetermined molar ratio using a ball mill or a bead mill. Pure water and, if necessary, a dispersion medium such as an organic solvent such as ethanol are added to the ball mill or bead mill, and the raw materials are milled in this state. Note that the lithium raw material may be added in an amount of 0 to 15% by weight more than the amount of lithium in the desired composition, taking into account the volatile content during calcination.

[0017] When grinding using a ball mill, it is desirable to prevent the inclusion of Al2O3, SiO2, and other components of ball mills such as alumina-lined ball mills. Specifically, to shorten the time required for grinding and prevent the inclusion of ball mill components, grinding can be performed for, for example, 1 to 2 hours. Among ball mills, it is preferable to use a urethane-lined ball mill, a nylon ball mill, or a natural rubber-lined ball mill. Zirconia media and alumina media can be used as grinding media.

[0018] In the first wet pulverization, after pulverization, the mixture is dried using a spray dryer, fluidized bed dryer, tumbling granulation dryer, freeze dryer, hot air dryer, etc. When a spray dryer is used, the hot air inlet temperature is set to 200 to 250°C and the exhaust air temperature is set to 90 to 120°C, for example. This produces a first pulverized powder.

[0019] After the first wet pulverization, the first pulverized powder is subjected to calcination. In the calcination, the first pulverized powder can be sieved using a sieve with a specified mesh size as needed, and then heated at 900°C to 1200°C for 1 hour to 12 hours in an oxygen or air atmosphere, or an inert gas atmosphere such as nitrogen. After heating in the calcination, the powder may be sieved using a sieve with a specified mesh size. This produces a calcined powder.

[0020] Next, the calcined powder is wet-pulverized in a second wet pulverization step. In this step, a dispersant such as a surfactant and the calcined powder are added to a solvent such as pure water in a ball mill or a bead mill to form a slurry, and the calcined powder is then pulverized in the slurry. The ball mill or bead mill may have a drum with an inner surface lined with alumina, urethane, or natural rubber, or may be made of nylon. The grinding media may be made of zirconia or alumina.

[0021] In the second wet pulverization, after the pulverization is completed, the mixture can be dried in the same manner as in the first wet pulverization. By undergoing the second wet pulverization, a second pulverized powder is obtained.

[0022] Thereafter, the second pulverized powder may be crushed. When crushing is performed, the second pulverized powder is crushed to have a smaller particle size. For crushing, a dry jet mill, a ball mill, or the like may be used, but among them, a jet mill is preferred in terms of crushing efficiency.

[0023] In this way, lithium lanthanum titanium oxide powder containing the lithium lanthanum titanium oxide represented by the above general formula (I) is obtained.

[0024] (molding process) In the molding step, the lithium-lanthanum-titanium oxide powder is compressed under pressure to be molded into a predetermined arbitrary shape, thereby obtaining a molded body.

[0025] A variety of forming techniques can be used to form the lithium lanthanum titanium oxide powder, such as cold isostatic pressing (CIP), molding including uniaxial pressing, casting, or extrusion.

[0026] During forming, a pressure of, for example, 200 MPa or less can be applied to the lithium lanthanum titanate powder. As a result, La 2 / 3-x Li 3x A molded body containing lithium lanthanum titanate represented by TiO3(0 < x < 0.16) can be obtained.

[0027] (Sintering process) In the sintering process, the molded body containing the oxide represented by the general formula (I) is heated to sinter the lithium lanthanum titanate powder constituting the molded body, thereby obtaining a sintered body.

[0028] Here, at least two-stage sintering including pre-stage sintering and post-stage sintering is performed. By performing post-stage sintering at a higher heating temperature after pre-stage sintering, a sintered body with a high relative density and a relatively high ionic conductivity can be obtained. It is only necessary that post-stage sintering is performed after pre-stage heating, and a sintering process under another condition may be included before pre-stage sintering, between pre-stage sintering and post-stage sintering, and / or after post-stage sintering.

[0029] In the pre-stage sintering, heating is performed at a temperature of 1100°C to 1200°C for 4 to 6 hours. If the heating temperature of the pre-stage sintering is too low, sintering may not proceed, so the relative density may not improve and the ionic conductivity may not be sufficiently exhibited. If it is too high, there is a concern that the growth of crystal grains will proceed and the relative density will decrease. Also, if the heating time of the pre-stage sintering is too short, there is a risk that sintering will not proceed, and if it is too long, there is a concern that productivity will decrease.

[0030] In the second-stage sintering after the first-stage sintering, heating is performed at a temperature of 1300°C to 1350°C for 0.5 to 1.5 hours. If the heating temperature in the second-stage sintering is too high compared to the heating temperature in the first-stage sintering, excessive crystal grain growth may occur, resulting in a decrease in relative density. If the heating temperature is too low, crystal grain growth may not occur, resulting in a decrease in ionic conductivity. Furthermore, if the heating time in the second-stage sintering is too short, crystal grain growth may not occur, resulting in a decrease in ionic conductivity. If the heating time is too long, crystal grain growth may proceed, resulting in a decrease in relative density.

[0031] The heating temperature here means the temperature inside the sintering furnace in which the compact is heated, and can be measured by a temperature sensor such as a thermocouple installed inside the sintering furnace.

[0032] After the sintering step, a sintered body is obtained, which contains the lithium-lanthanum-titanium oxide represented by the general formula (I) above.

[0033] The presence of the oxide represented by general formula (I) in the sintered body can be confirmed by subjecting the sintered body to X-ray diffraction. In X-ray diffraction, the X-ray diffraction pattern of the sintered body obtained using PANalytical's X'pert Pro or a device substantially equivalent thereto is compared with the ICDD database (PANalytical Example Database and PDF-4+ 2019RDB) to identify the oxide represented by general formula (I) contained in the sintered body.

[0034] Here, the X-ray diffraction pattern of the sintered body obtained by X-ray diffraction and from which noise has been removed is compared with the ICDD database, and if it is determined that the X-ray diffraction pattern of the sintered body contains the X-ray diffraction pattern of the oxide represented by general formula (I), it is determined that the sintered body contains the oxide represented by general formula (I).On the other hand, if it is determined that the X-ray diffraction pattern of the sintered body does not contain the X-ray diffraction pattern of the oxide represented by general formula (I) after similar comparison with the ICDD database, it is determined that the sintered body does not contain the oxide represented by general formula (I).

[0035] The oxide of general formula (I) contained in the sintered body may have some of its oxygen atoms substituted with other elements such as F or Cl, or some of its transition metal atoms substituted with other metals such as Fe, Cr, Ti, Nb, W, Mo, Na, K, Mg, or Ca. Furthermore, the oxide of general formula (I) may have an excess or deficiency of Li or oxygen relative to the stoichiometric composition. Furthermore, the crystalline structure of the oxide of general formula (I) contained in the sintered body may be distorted. Even when a constituent element is substituted, deficient, or excessive relative to the stoichiometric composition of the oxide, or when the crystalline structure is distorted, the oxide is still acceptable as the oxide of general formula (I) as long as the properties of the sintered body are not affected.

[0036] When the X-ray diffraction patterns of oxides lacking constituent elements or oxides with excess constituent elements are compared with the ICDD database, there is a possibility that the peaks will shift from the X-ray diffraction pattern of the oxide represented by general formula (I). If such a peak shift is within ±10% of the reference value in the ICDD database, the sintered body is determined to contain the oxide represented by general formula (I).

[0037] The sintered body preferably contains 99.0% by mass or more of the oxide represented by the general formula (I), and even more preferably 99.5% by mass or more. The higher the oxide content, the more desirable it is. There is no particular upper limit, but it may be, for example, 99.999% by mass or less, typically 99.99% by mass or less. The oxide content is measured by ICP (Inductively Coupled Plasma).

[0038] The sintered body may contain, in addition to the oxide represented by the general formula (I), at least one impurity selected from the group consisting of Si, Al, and Fe. The content of impurities in the sintered body is preferably 1.0% by mass or less, more preferably 0.5% by mass or less. [Example]

[0039] Next, the manufacturing method of the sintered body of this invention was experimentally implemented and its effects were confirmed, and thus will be described below. However, the description here is for the purpose of mere exemplification and is not intended to be limited thereto.

[0040] A green compact made from lithium lanthanum titanate powder was heated and sintered to produce a sintered body. In Comparative Examples 1 to 4, one-step sintering was performed, and in Examples 1 to 3 and Comparative Example 5, two-step sintering of pre-sintering and post-sintering was performed. The respective temperatures and times are shown in Table 1.

[0041] Regarding the sintered bodies obtained after sintering in Comparative Examples 1 to 5 and Examples 1 to 3, when the components were confirmed by the method described above, La 2 / 3-x Li 3x They contained lithium lanthanum titanate represented by TiO3 (0 < x < 0.16). Further, the relative density and ionic conductivity (lithium ion conductivity) of each sintered body were measured according to the method described below. The results thereof are also shown in Table 1.

[0042] The relative density was obtained by dividing the weight of the sintered body by the volume to obtain the measured density, and dividing this by the theoretical density and expressing it as a percentage. The volume of the sintered body was calculated from the measured lengths of each side. As the theoretical density, La 2 / 3-x Li 3x The density of TiO3 (0 < x < 0.16), which is 5.23 g / cm 3 was used.

[0043] In the measurement of ionic conductivity, Pt was deposited on both surfaces of the plate (Φ12 mm) of the sintered body under the conditions of 15 mA and 30 seconds using an ion sputter MC1000 (manufactured by Hitachi High-Technologies Corporation). First, a Nyquist plot was measured under the conditions of an impedance analyzer E4990A (manufactured by Keysight Technologies) and a measurement frequency of 20 Hz to 20 MHz. By fitting the obtained Nyquist plot with Zveiw, the respective resistance values of the intragranular and grain boundary were read, and the ionic conductivity was obtained from the following calculation formula. Bulk ionic conductivity (S / cm) = 1 / Rb × (L / S) Grain boundary ionic conductivity (S / cm) = 1 / Rgb × (L / S) Total ionic conductivity (S / cm) = 1 / Rtotal × (L / S) Rb: Resistance value within the grain (Ω) Rgb: Resistance of grain boundary (Ω) Rtotal (=Rb+Rgb): Total resistance (Ω) L: thickness of the plate-shaped lithium lanthanum titanium oxide (cm) S: electrode area (cm 2 )

[0044] [Table 1]

[0045] In Comparative Examples 1 to 4, sintering was performed in only one stage, resulting in a low relative density or low ionic conductivity of the sintered body. In Comparative Example 5, the temperature and time of the second sintering were high and long, respectively, resulting in a sintered body with low relative density. In contrast, in Examples 1 to 3, the temperatures and times of the first and second sintering stages were appropriate, resulting in sintered bodies with high relative density and high ionic conductivity.

[0046] From the above, it has been found that according to the present invention, a sintered body having a relatively high relative density and a relatively high ionic conductivity can be produced.

Claims

[Claim 1] A method for producing a sintered body containing lithium lanthanum titanium oxide represented by the following general formula (I): a sintering step of heating and sintering the compact containing the lithium lanthanum titanium oxide, The sintering step Pre-sintering at a temperature of 1100°C to 1200°C for 4 to 6 hours; Post-sintering at a temperature of 1300°C to 1350°C for 0.5 to 1.5 hours; A method for producing a sintered body, comprising: Lạ 2 / 3-x Li 3x TiO 3 (I) (In general formula (I), x satisfies 0<x<0.16.)

Images