Manufacturing method of garnet-type solid electrolyte single crystal

Abstract

To provide a manufacturing method of a garnet-type solid electrolyte single crystal capable of producing a large-diameter single crystal.SOLUTION: A manufacturing method of a garnet-type solid electrolyte single crystal by lifting and growing a single crystal from a raw material melt in a crucible by the Czochralski method includes the steps of: lifting a garnet-type solid electrolyte single crystal being grown, including a seed crystal, while maintaining the temperature at 1150°C or more; and separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing a garnet-type solid electrolyte single crystal which is expected to be a material for high-performance secondary batteries and which can provide high lithium ion conductivity. [Background technology]

[0002] Garnet-type solid electrolyte single crystals are materials that exhibit high lithium ion conductivity and are expected to be used as materials for high-performance secondary batteries. 12 ), LLZT(Li (7-x) La3Zr (2-x) Ta x O 12 ), LLZN(Li (7-x) La3Zr (2-x) Nb x O 12 ), LLZTN(Li (7-x-y) La3Zr (2-x-y) Ta x Nb y O 12 ), LGaLZ(Li (7-x) Ga c La3Zr2O 12 ), LALZ(Li (7-3x) Al x La3Zr2O 12 ), LLTO(Li5La3Ta2O 12 ), LBLT(LiBaLaTaO 12 ), and it is known that these single crystals can be produced by the floating zone (FZ) method or the Czochralski (CZ) method (see Patent Documents 1 to 6).

[0003] However, the single crystals reported so far are limited to small diameters of around 10 mm at most, which is not suitable for practical use or reducing manufacturing costs, and there is a demand for larger diameter single crystals. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6524089 [Patent Document 2] Patent No. 6120346 [Patent Document 3] Patent No. 6296263 [Patent Document 4] Patent No. 6278433 [Patent Document 5] Patent No. 6667182 [Patent Document 6] Patent No. 7361299 Summary of the Invention [Problem to be solved by the invention]

[0005] The applicant of the present application believed that the CZ method, which is known as a method for producing high-quality single crystals with excellent mass productivity, would be advantageous for producing large-diameter single crystals at low cost, and attempted to grow garnet-type solid electrolyte single crystals with diameters of 25 mm or more by the CZ method. However, there were problems such as the crystals falling during growth, and the grown single crystals being altered and cracked, making it extremely difficult to obtain single crystals with diameters of 25 mm or more.

[0006] As a result of intensive research by the present applicant to solve this problem, it has been revealed through experiments that the causes of the above phenomenon occurring when the diameter of a garnet-type solid electrolyte single crystal is increased depend on the increase in growth time due to the increased diameter and the temperature range and time experienced by the single crystal during growth. Specifically, the present applicant conducted experiments in which garnet-type solid electrolyte single crystals were heat-treated at various temperatures and for various times, and found that specific temperature ranges and time periods cause the single crystal to deteriorate and become embrittled.

[0007] It is obvious that an increase in the growth time is unavoidable when the diameter of a single crystal is increased, but the applicant has discovered that this means that the single crystal undergoes a temperature range that deteriorates during growth for a long period of time, resulting in the aforementioned phenomenon and making it impossible to achieve a larger diameter.

[0008] The present invention has been made under these circumstances, and its object is to provide a method for producing a garnet-type solid electrolyte single crystal that identifies the causes of crystal dropping during growth, deterioration of the grown single crystal, and cracking in the growth of a large-diameter garnet-type solid electrolyte single crystal by the CZ method, and solves these problems, thereby enabling the growth of a large-diameter single crystal. [Means for solving the problem]

[0009] The method for producing a garnet-type solid electrolyte single crystal according to the present invention, which has been made to solve the above-mentioned problems, is a method for producing a garnet-type solid electrolyte single crystal by pulling up and growing a single crystal from a raw material melt in a crucible by the Czochralski method, and is characterized by including a step of pulling up the garnet-type solid electrolyte single crystal including a seed crystal during growth while maintaining the crystal at 1150°C or higher, and a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it.

[0010] Alternatively, the method for producing a garnet-type solid electrolyte single crystal according to the present invention, which has been made to solve the above-mentioned problems, is a method for producing a garnet-type solid electrolyte single crystal by pulling up and growing a single crystal from a raw material melt in a crucible by the Czochralski method, and is characterized by comprising a pulling step in which the growing garnet-type solid electrolyte single crystal including a seed crystal is exposed to a temperature range of 1000°C to 1100°C within 24 hours, and a step in which the garnet-type solid electrolyte single crystal is separated from the raw material melt and slowly cooled.

[0011] Alternatively, the method for producing a garnet-type solid electrolyte single crystal according to the present invention, which has been made to solve the above-mentioned problems, is a method for producing a garnet-type solid electrolyte single crystal by pulling up and growing a single crystal from a raw material melt in a crucible by the Czochralski method, and is characterized by comprising a step of pulling up the garnet-type solid electrolyte single crystal including a seed crystal during growth within 96 hours, during which the garnet-type solid electrolyte single crystal is exposed to a temperature of 600°C or higher and lower than 1000°C, and a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it.

[0012] Alternatively, a method for producing a garnet-type solid electrolyte single crystal according to the present invention, which has been made to solve the above-mentioned problems, is a method for producing a garnet-type solid electrolyte single crystal by pulling up and growing a single crystal from a raw material melt in a crucible by the Czochralski method, and is characterized by comprising a pulling step in which the garnet-type solid electrolyte single crystal including a seed crystal being grown is exposed to a temperature range of 1000°C or higher and 1100°C or lower for more than 24 hours but less than 48 hours, and a step in which the garnet-type solid electrolyte single crystal is separated from the raw material melt and slowly cooled.

[0013] According to this method for producing a garnet-type solid electrolyte single crystal, when growing a large-diameter garnet-type solid electrolyte single crystal by the CZ method, it is possible to prevent the crystal from falling during growth, and the grown single crystal from deteriorating or cracking, thereby realizing a large-diameter single crystal. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for producing a garnet-type solid electrolyte single crystal that enables the diameter of the single crystal to be increased. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of a single crystal pulling apparatus using the Czochralski method. [Figure 2]FIG. 2 is a flowchart showing a method for producing a garnet-type solid electrolyte single crystal according to the present invention. [Figure 3] FIG. 3 is a flow chart of Experiment 1 of the present embodiment. [Figure 4] FIG. 4 is a graph showing the results of Experiment 1. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the method for producing a garnet-type solid electrolyte single crystal according to the present invention will be described in detail with reference to the drawings, although the present invention is not limited to this embodiment.

[0017] First, a single crystal pulling apparatus used in the method for producing a garnet-type solid electrolyte single crystal of this embodiment will be described.

[0018] Figure 1 is a schematic diagram of a single crystal pulling apparatus using the Czochralski method. The single crystal pulling apparatus shown in Figure 1 has an iridium crucible 3 filled with raw material melt 2 installed in the center of a chamber 1. A high-frequency coil 4 is arranged around the iridium crucible 3 to heat the iridium crucible 3 from the periphery. An afterheater 5 is installed above the iridium crucible 3 to heat the periphery of the single crystal being pulled. Furthermore, a reflector 6 is installed at the top end of the iridium crucible 3 to control the temperature of the raw material melt 2 in the iridium crucible 3 and the single crystal 9 being pulled.

[0019] In a single crystal pulling apparatus using the Czochralski method, a seed crystal 8 held at the lower end of a seed rod 7 is placed on the surface of the raw material melt 2 in an iridium crucible 3, and the seed rod 7 is pulled up while rotating the seed crystal 8, thereby growing a single crystal 9. When rotating the seed crystal 8, the iridium crucible 3 may be rotated in the opposite direction to the rotation of the seed crystal 8, with the center line of the seed rod 7 as the rotation axis.

[0020] <Manufacturing method> (First embodiment) Fig. 2 is a flowchart showing a method for producing a garnet-type solid electrolyte single crystal according to the present invention. As shown in Fig. 2, in step S1, lanthanum oxide (La2O3), zirconium oxide (ZrO2), tantalum oxide (Ta2O5), and lithium carbonate (Li2CO3) are weighed and mixed to obtain a desired composition ratio. Here, it is desirable to measure the adsorbed components (mainly water) in each material in advance and correct the weighed weight. Furthermore, since the Li ratio may decrease in the subsequent thermal process, it is acceptable to have a few percent excess Li in advance.

[0021] Next, each material is calcined to obtain a polycrystalline calcined LLZT raw material (step S2), and the obtained calcined LLZT raw material is then filled into an iridium crucible 3 (step S3).

[0022] Next, after placing each component including the iridium crucible 3 filled with the LLZT calcination raw material in the chamber 1 (step S4), the atmosphere inside the chamber 1 is replaced with an inert gas atmosphere before melting the raw material, and thereafter, the inert gas is flowed into the chamber 1 until it is cooled (step S5).

[0023] Next, current is passed through the high frequency coil 4, and the iridium crucible 3 is heated by high frequency induction heating to melt the LLZT calcination raw material (step S6). Also, the seed rod 7 is rotated from the start of heating the iridium crucible 3 until the single crystal is cooled.

[0024] After the raw material in the iridium crucible 3 becomes molten, a seed crystal is lowered from above the melt to contact the melt (step S7). Then, the seed rod 7 is raised while rotating to grow a single crystal (step S8). At this time, growth parameters such as heater power and pulling speed are controlled based on the weight measurement attached to the seed rod so that the desired diameter is achieved.

[0025] In step S8 of this embodiment, the single crystal including the seed crystal being grown is pulled while being maintained at 1150°C or higher. Temperature management is mainly performed by controlling the afterheater 5. As a result, the grown single crystal experiences a temperature range that alters its properties only during the cooling process after growth, and therefore is hardly altered, making it possible to produce a large diameter single crystal.

[0026] Thereafter, when the single crystal has grown to the target size by pulling, the rising speed of the seed rod 7 is increased and the pulled single crystal is separated from the raw material melt (step S9). After the single crystal is separated from the raw material melt, the single crystal is slowly cooled in the chamber 1 (step S10). After the single crystal has cooled sufficiently, it is removed from the chamber (step S11).

[0027] (Second embodiment) The second embodiment differs only in step S8 described above. That is, in step S8 of this embodiment, the single crystal including the seed crystal being grown is pulled up within 24 hours after being exposed to a temperature range of 1000°C to 1100°C. In this embodiment, although some alteration occurs in the single crystal, this can be kept to a minor extent, allowing for a larger diameter.

[0028] (Third embodiment) In step S8 of the third embodiment, the time during which the single crystal including the seed crystal is grown is set to 600°C or higher and lower than 1000°C and pulled up is set to 96 hours or less. In this case, there is no significant difference in the degree of alteration compared to the second embodiment, and temperature control is easier. Note that the processes other than step S8 are the same as those in the first embodiment.

[0029] (Fourth embodiment) In step S8 of the fourth embodiment, the time elapsed until the temperature of the single crystal including the seed crystal being grown reaches a range of 1000°C or higher and 1100°C or lower is set to be more than 24 hours and less than 48 hours, and the single crystal is pulled. In this case, although the degree of alteration becomes wider than in the first to third embodiments, it is possible to increase the diameter of the single crystal. Note that the processes other than step S8 are the same as in the first embodiment. [Example]

[0030] The method for producing a garnet-type solid electrolyte single crystal according to the present invention will be further described with reference to examples. In these examples, the following experiments were carried out based on the above-described embodiment.

[0031] <Experiment 1> In Experiment 1, we investigated how the heating temperature and holding time of the single crystal in the chamber affect the deterioration of the single crystal before and after heating. (7-x) La3Zr (2-x) Ta x O 12 ) x=0.5 and LLZN(Li (7-x) La3Zr (2-x) Nb x O 12 ) x=0.5 was prepared as a sample for heat treatment experiments.

[0032] The experimental flow is shown in Fig. 3. As shown in Fig. 3, in step St1, small pieces of garnet-type single crystals (LLZT and LLZN) were prepared as samples for heat treatment experiments, and their appearance before heating was recorded.

[0033] Next, a heat treatment experiment sample was placed in an iridium crucible in the chamber (step St2). The atmosphere in the chamber was replaced with an inert gas atmosphere, and thereafter, the inert gas was allowed to flow until the sample was cooled (step St3).

[0034] Then, the iridium crucible was heated to a predetermined temperature by high-frequency induction heating using a high-frequency coil (step St4). At this time, the temperature rise rate was 10°C to 20°C / min, and the predetermined temperature change was within ±5°C. The temperature was measured using a thermocouple installed at the bottom of the iridium crucible.

[0035] Next, after a predetermined time has elapsed at the predetermined temperature, the sample is cooled at a rate of 10 to 20°C / min (step St5), and when the temperature inside the chamber reaches 50°C or below, the sample is removed (step St6). Then, the sample before and after the heat treatment is compared by appearance and touch to evaluate the degree of deterioration of the sample (step St7).

[0036] The results of Experiment 1 are shown in Figure 4. In the graph in Figure 4, the indicators of the degree of deterioration are as follows: double circles indicate no change, white circles indicate discoloration (a slight change in color is observed overall, but the sample is not brittle and embrittlement has not progressed, so this is rated as good), white triangles indicate localized embrittlement (clouding is observed in only a small area, and the sample fractures easily in that area, but embrittlement has not progressed in the rest of the area, so this is rated as good), black triangles indicate widespread embrittlement (clouding is observed over a wide area, and the sample fractures easily in that area, so this is rated as poor), and crosses indicate significant embrittlement (clouding is observed over a wide area, and the sample has become so embrittled that it fractures easily when removed, so this is rated as poor).

[0037] As shown in the graph in Figure 4, it was revealed that there is a time period in which both LLZT and LLZN undergo a significant transformation between 1000°C and 1100°C. In other words, it was found that the longer the single crystal including the seed crystal is exposed to the temperature range of 1000°C to 1100°C during single crystal growth, the more the transformation of the single crystal progresses during growth. Note that the transformation temperature range is different from that of the previously known garnet-type solid electrolyte single crystal (LLZ(Li7La3Zr2O)). 12 ), LLZT(Li (7-x) La3Zr (2-x) Ta x O 12 ), LLZN(Li (7-x) La3Zr (2-x) Nb x O 12 ), LLZTN(Li (7-x-y) La3Zr (2-x-y) Ta x Nb y O 12 ), LGaLZ(Li (7-x) Ga c La3Zr2O 12 ), LALZ(Li(7-3x) Al x La3Zr2O 12 ), LLTO(Li5La3Ta2O 12 ), LBLT(LiBaLaTaO 12 Although it has not been experimentally confirmed whether this is common to all of the above, since they all share Li, La, and O as their main components, it is assumed that they are in roughly the same temperature range.

[0038] Furthermore, from the results shown in Figure 4, it was possible to identify temperature ranges and time periods (ranges marked with black triangles and crosses) that should be avoided during single crystal growth. Temperature ranges and time periods that should be avoided pose risks such as the single crystal falling during growth, or the single crystal breaking due to deterioration and cracking of the grown single crystal (including widespread embrittlement and significant embrittlement).

[0039] <Experiment 2> In Experiment 2, a single crystal was actually grown and evaluated.

[0040] Example 1 In Example 1, an LLZT single crystal with a maximum diameter of 20 mm was grown according to the flow chart in Figure 2. The raw material composition was weighed so that x = 0.5, with 2% more Li added. The temperature inside the chamber on the seed axis was measured in advance, and the conditions inside the chamber were adjusted so that the temperature was 1150°C or higher up to the height of the top end of the afterheater. Specifically, the height of the afterheater was mainly increased, and the high-frequency coil position was set upward, so that the seed crystal position at the end of pulling was below the height of the top end of the afterheater. As a result, it was confirmed that the grown single crystal was free of cracks and alterations, and that a large-diameter single crystal could be obtained.

[0041] Example 2 In Example 2, under the same apparatus conditions as in Example 1, the height of the afterheater was lowered so that the seed crystal position at the end of pulling exceeded the height of the upper end of the afterheater by 20±5 mm. By lowering the height of the afterheater compared to Example 1, the temperature range of 1000°C to 1100°C was changed to a calculated 24 hours by the end of pulling, and an LLZT single crystal was grown. At this time, the pulling rate was also adjusted to adjust the experience time of the single crystal, including the seed crystal, in the temperature range (1000°C to 1100°C). As a result, although slight alteration was observed in the upper part of the grown single crystal, a large-diameter single crystal was obtained.

[0042] Example 3 In Example 3, the height of the afterheater was further reduced compared to the conditions in Example 2, so that the seed crystal position at the end of pulling exceeded the height of the upper end of the afterheater by 40±5 mm. By lowering the height of the afterheater compared to Example 2, the LLZT single crystal was grown in a temperature range of 1000°C or higher and 1100°C or lower for a calculated time of 45 hours until the end of pulling. At this time, the pulling rate was also adjusted to adjust the experience time of the single crystal, including the seed crystal, in the temperature range (1000°C or higher and 1100°C or lower). As a result, although slight alteration was observed in the upper part of the grown single crystal, a large-diameter single crystal was obtained.

[0043] (Comparative Example 1) In Comparative Example 1, the height of the afterheater was further reduced compared to the conditions of Example 3, so that the seed crystal position at the end of pulling exceeded the height of the upper end of the afterheater by 60±5 mm. By lowering the height of the afterheater compared to Example 3, the LLZT single crystal was grown in a temperature range of 1000°C to 1100°C for a calculated 72 hours until the end of pulling. The pulling rate was also adjusted to adjust the time the single crystal, including the seed crystal, experienced in the temperature range (1000°C to 1100°C). As a result, the upper part of the crystal broke and fell during the cooling process after pulling was completed, but the grown single crystal was observable because the raw material melt had solidified. Clear alteration and cracks were observed in the upper part of the single crystal.

[0044] (Comparative Example 2) In Comparative Example 2, the conditions of Comparative Example 1 were used except that the afterheater was omitted and the temperature range of 1000°C to 1100°C was changed so that the LLZT single crystal was grown for a calculated time of 120 hours by the end of pulling. At this time, the pulling rate was also adjusted to adjust the time that the single crystal including the seed crystal experienced the temperature range (1000°C to 1100°C). As a result, the upper part of the crystal broke during growth, and the crystal fell into the raw material melt.

[0045] From the results of the above examples, it was confirmed that the present invention makes it possible to produce a large-diameter garnet-type solid electrolyte single crystal. [Explanation of symbols]

[0046] 1 chamber 2 Raw material melt 3 Iridium crucible 4 High frequency coil 5 Afterheater 6 Reflector 7 Seed Sticks 8 seed crystals 9 Single crystal

Claims

1. A method for producing a garnet-type solid electrolyte single crystal by pulling and growing a single crystal from a raw material melt in a crucible by the Czochralski method, a step of pulling up the garnet-type solid electrolyte single crystal including the seed crystal while maintaining the temperature at 1150°C or higher; a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it; 1. A method for producing a garnet-type solid electrolyte single crystal, comprising:

2. A method for producing a garnet-type solid electrolyte single crystal by pulling and growing a single crystal from a raw material melt in a crucible by the Czochralski method, a step of pulling up the garnet-type solid electrolyte single crystal including the seed crystal during growth within 24 hours at a temperature in the range of 1000°C or more and 1100°C or less; a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it; 1. A method for producing a garnet-type solid electrolyte single crystal, comprising:

3. A method for producing a garnet-type solid electrolyte single crystal by pulling and growing a single crystal from a raw material melt in a crucible by the Czochralski method, a step of pulling up the garnet-type solid electrolyte single crystal including the seed crystal during growth within 96 hours after the crystal has been exposed to a temperature of 600°C or higher but lower than 1000°C; a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it; 1. A method for producing a garnet-type solid electrolyte single crystal, comprising:

4. A method for producing a garnet-type solid electrolyte single crystal by pulling and growing a single crystal from a raw material melt in a crucible by the Czochralski method, a step of pulling up the garnet-type solid electrolyte single crystal including the seed crystal during growth while exposing it to a temperature in the range of 1000°C or higher and 1100°C or lower for more than 24 hours and less than 48 hours; a step of separating the garnet-type solid electrolyte single crystal from the raw material melt and slowly cooling it; 1. A method for producing a garnet-type solid electrolyte single crystal, comprising:

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