JP7171487B2 - Method for producing perovskite-type ion-conducting oxide - Google Patents

Description

The present invention relates to a method for producing a perovskite-type ion-conducting oxide.

An all-solid-state secondary battery that uses an ion-conductive oxide as an electrolyte has high heat resistance and is non-flammable, so the cooling mechanism and safety mechanism can be simplified compared to conventional lithium-ion secondary batteries. Therefore, the cells can be easily modularized, and a battery with high energy density can be easily produced.

As ion-conductive oxides that can be applied to all-solid secondary batteries, for example, perovskite-type ion-conductive oxides that are produced from raw materials containing Li, Sr, and Zr elements and have a perovskite-type crystal structure are known. . Since this ion-conductive oxide is chemically stable, it is a suitable material for an oxide solid electrolyte for an all-solid secondary battery.
Li _x/2 Sr _{1-3 x/4} □ _x/4 Ta _x Zr _1-x O ₃
(□ represents an atomic vacancy). It is known that this ion-conducting oxide can be produced with high density by sintering at 1300° C. (for example, Non-Patent Document 1).

Solid State Ionics 261 (2014) 95-99

However, since the ion-conducting oxide having the above composition formula requires sintering at 1300° C., Li tends to volatilize, and the composition of the material deviates, sometimes failing to exhibit desired characteristics. Moreover, if the sintering temperature is lowered in order to suppress the volatilization of Li, it may not be possible to produce a high density.

Therefore, the present invention provides a method for producing a perovskite-type ion-conducting oxide that can produce a high-density perovskite-type ion-conducting oxide while suppressing volatilization of Li.

The method for producing a perovskite-type ion-conducting oxide of the present invention comprises:
A mixing step of mixing raw materials containing Li, Sr and Zr elements to form a mixed powder;
A calcining step of calcining the mixed powder to obtain a calcined powder;
A sintering aid mixing step of mixing the calcined powder with a sintering aid containing at least one element selected from B, Ga, and Bi to obtain a sintering aid mixed powder;
and a sintering step of sintering the molded body of the sintering aid mixed powder to obtain a sintered body of perovskite-type ion-conductive oxide.

In addition, the method for producing a perovskite-type ion-conductive oxide of the present invention comprises:
In the mixing step, wet mixing using an organic solvent as a solvent for dispersing the raw materials is preferably used.

In addition, the method for producing a perovskite-type ion-conductive oxide of the present invention comprises:
In the calcination step, the calcination temperature is preferably 800° C. or higher and 1300° C. or lower.

According to the present invention, it is possible to produce a high-density perovskite-type ion-conductive oxide while suppressing volatilization of Li.

4 is an XRD pattern of the calcined powder of Example 1. FIG. 4 is an XRD pattern of the sintered body of Example 1. FIG. 1 is a cross-sectional SEM photograph of a sintered body of Example 1. FIG. 4 is a cross-sectional SEM photograph of a sintered body of Comparative Example 1. FIG. 15 is an XRD pattern of the calcined powder of Example 15. FIG. 15 is an XRD pattern of the sintered body of Example 15. FIG. 10 is a cross-sectional SEM photograph of the sintered body of Example 15. FIG.

Hereinafter, embodiments of the method for producing a perovskite-type ion-conducting oxide, which is the present invention, will be described. In the following description, "perovskite-type ion-conducting oxide" may be simply referred to as "ion-conducting oxide".

(Weighing process)
First, in the weighing step, raw materials containing the elements Li, Sr, Me, Ta, and Zr are weighed so that the ratio of these elements is the same as the ratio of elements in composition formula (1).
Li _x/2 Sr _1-3x/4 □ _x/4 Me _y Ta _x Zr _1-xy O _3-y/2 (Me=Al, Ga, Yb, Lu) (1)

Here, as raw materials containing Li, Sr, Me, Ta, and Zr, carbonates, oxides, nitrates, alkoxides, and the like of these metal elements are used. is preferably used. Moreover, it is preferable to powder these raw materials in consideration of making a mixed powder in a mixing step to be described later. If the particle size of the raw material powder is made small, the solid-phase reaction in the sintering process can proceed rapidly. For this reason, the particle diameter of the raw material powder is preferably 0.1 μm or more and 10 μm or less in median diameter (D ₅₀ ).

The raw materials containing Li, Sr, Ta, and Zr are weighed so that the range of x is 0.65≦x≦0.75. By setting the content within this range, the finally produced ion-conducting oxide can be a single perovskite phase, and the conductivity can be sufficiently increased. In addition, raw materials containing Me and Zr are weighed at an elemental ratio such that the range of y is 0.01≤y≤0.15. This range is preferable because the substitution element Me can dissolve in the perovskite phase, and the finally produced ion-conducting oxide can have a single perovskite phase.

(Mixing process)
Next, in the mixing step, the raw materials weighed in the weighing step are mixed to form a mixed powder. Mixing may be dry mixing, but a wet mixing method in which the raw materials are dispersed in a solvent and mixed is preferred. By using a wet mixing method, the raw materials after mixing can be recovered at a high yield. It is possible to mix while suppressing the reaction between the raw materials and water. As the wet mixing method, a ball mill, a bead mill, or the like can be freely selected depending on the purpose.

(calcination process)
Next, in the calcining step, the mixed powder obtained in the mixing step is calcined to obtain a calcined powder. In this step, the mixed powder is turned into a calcined powder of oxide by a solid-phase reaction, and the raw material carbonate is thermally decomposed before the sintering step to be described later, and carbon dioxide gas is generated in the sintering step. It is possible to suppress the occurrence of cracks and swelling of the sintered body. Various furnaces such as a stationary batch furnace, a tubular furnace, an elevator furnace, and a conveyor furnace can be used for calcination, and a crucible or setter made of alumina or zirconia can be used to hold the mixed powder.

The calcination temperature is preferably 800° C. or higher in order to thermally decompose the raw material carbonate, but if the temperature is too high, Li contained in the mixed powder may volatilize. Therefore, the calcination temperature is preferably 800° C. or higher and 1300° C. or lower, and more preferably 800° C. or higher and 900° C. or lower in order to further suppress the volatilization of Li. Further, the holding time at the above temperature is preferably 1 hour or more and 24 hours or less depending on the amount of treatment. By setting the calcining temperature and holding time as described above, it is possible to reliably thermally decompose the carbonate contained in the mixed powder while suppressing volatilization of Li contained in the mixed powder.

(Sintering aid mixing step)
Next, in the sintering aid mixing step, the calcined powder obtained in the calcining step is mixed with a sintering aid to obtain a sintering aid mixed powder. As the sintering aid, carbonates, oxides, nitrates, alkoxides and Li salts containing at least one element selected from B, Ga, and Bi can be used. The sintering aid can suppress the volatilization of Li by lowering the sinterable temperature in the sintering process. , the properties as an ion-conducting oxide may deteriorate. Therefore, the amount of the sintering aid to be added is preferably 1 mol % or more and 10 mol % or less with respect to the calcined powder.
It should be noted that, as in the mixing step, wet mixing is preferably used for mixing the calcined powder and the sintering aid from the viewpoint of yield, and the solvent includes alcohols such as ethanol and ethers such as dimethyl ether. , esters such as ethyl acetate, and the like are preferably used. If an organic solvent is used as the solvent, the reaction between the calcined powder and water can be suppressed. As the wet mixing method, a ball mill, a bead mill, or the like can be freely selected depending on the purpose. In addition, in the sintering aid mixing step, the calcined powder may be pulverized when mixing the calcined powder and the sintering aid.

(Molding process)
Next, in the molding step, the sintering aid mixed powder obtained in the sintering aid mixing step is molded. For molding, it is possible to use pressure molding such as uniaxial pressure molding and cold isostatic pressing (CIP), and a sheet molding method in which a slurry obtained by adding a binder to a mixed powder of sintering aids is molded into a green sheet. can.

In the case of the sheet molding method, first, a solution of, for example, polyvinyl butyral (PVB) is prepared as a binder, and the content of the calcined powder with respect to this solution is, for example, 5% by mass or more and 20% by mass or less. to mix. In addition, this solution may be mixed with a plasticizer (for example, dioctyl phthalate (DOP), etc.). Then, the resulting mixture is sufficiently mixed using a ball mill to obtain a slurry for a green sheet. The viscosity of this slurry may be adjusted by defoaming and partially volatilizing the solvent under reduced pressure. The prepared slurry is coated on a polyethylene terephthalate (PET) film or the like by a blade method, dried as a whole, peeled off from the film after drying, and cut into a desired size and shape to obtain a green sheet.

(Sintering process)
Finally, in the sintering step, the molded body obtained in the molding step is sintered to obtain a sintered body of the ion conductive oxide. Various furnaces such as a stationary batch furnace, a tubular furnace, an elevator furnace, and a conveyor furnace can be used in the sintering process, and a crucible or setter made of alumina or zirconia can be used to hold the compact. The sintering process is usually carried out in the atmosphere, but the atmosphere may be an atmosphere containing no oxygen such as Ar or _N2 , or an atmosphere containing oxygen at a higher concentration than the atmosphere. Whether or not the perovskite phase of the sintered body is also a single phase can be confirmed by a technique such as XRD.
In the present invention, a sintering aid containing at least one element selected from B, Ga, and Bi is added in the sintering aid mixing step. As a result, in the sintering step, neck growth between particles can be started at a lower temperature, and the sintering temperature that can suppress the volatilization of Li, specifically, a sintering temperature of 1300 ° C. or less. Thus, a high-density perovskite-type ion-conductive oxide can be produced while suppressing the volatilization of Li.

Examples are described below. First, Table 1 below shows the composition of each example (the element selected as Me, the values of x and y in the composition formula (1)), the d ₅₀ of the powder before molding, and the addition in the sintering aid mixing step. The composition and amount of the sintering aid added, the sintering start temperature obtained by measurement with a thermomechanical analyzer described later, and the sintering temperature at which the shrinkage rate is equivalent to that of a 1300 ° C. sintered body without a sintering aid, Furthermore, the experimental results of the relative density and electrical conductivity of the sintered bodies obtained in each example described below are listed.

Example 1 was carried out as follows.
In the weighing process, stoichiometric weighing was performed so that x=0.75 and y=0 (no Me) in composition formula (1). At this time, powders of Li ₂ CO ₃ , SrCO ₃ , Ta ₂ O ₅ and ZrO ₂ were used as raw materials. In the mixing step, wet mixing was performed using a ball mill, and the weighed raw materials were mixed together with ethanol and zirconia balls for 20 hours using a ball mill to evaporate the ethanol.
In the calcining step, this raw material mixed powder was placed in an alumina crucible and calcined at 1300° C. for 4 hours. After the calcination, the surface in contact with the alumina crucible was discarded so that the yield was 50%, and the calcination powder was prevented from being mixed with Al from the crucible. FIG. 1 shows the XRD pattern of the calcined powder thus obtained. It can be seen that only the perovskite phase indicated by the arrow is detected.
In the sintering aid mixing step, the calcined powder was pulverized with an automatic mortar for 3 hours, then 5 mol % of Ga ₂ O ₃ powder was added to the calcined powder, and the mixture was further mixed with an automatic mortar for 30 minutes.
In the molding process, pellets were produced by uniaxial pressing with a die of φ14 mm at 9.8 kN·m ⁻² . In the sintering step, the pellets to be sintered were covered with twice the weight of mother powder (calcined powder sintered at 1100° C. for 4 hours) and sintered at 1200° C. for 4 hours. In the sintered body thus obtained, LiTaO ₃ indicated by a triangle is detected in addition to the perovskite phase as shown in the XRD pattern shown in FIG. Moreover, it can be seen from the cross-sectional SEM shown in FIG.

Conductivity was measured as follows. Both surfaces of the pellet obtained in the sintering process were polished and Au vapor-deposited. This pellet was sandwiched between In foils and placed in an electrochemical cell. The resistance (R) of this cell was measured by the AC impedance method using an impedance analyzer (Solartron 1260). The area (S) of the pellet was calculated from the diameter (r) of the pellet, and the area (S) of the pellet and the thickness (t) of the pellet were used to determine the electrical conductivity (σ) by the following formula.

Relative density was measured as follows. The true density of the calcined powder was determined with a pycnometer (ULTRAPYC 1200e). The pellet density was obtained from the thickness, area, and pellet weight measured as described above, and the ratio to the true density was defined as the relative density, which was calculated as a percentage.

The sintering start temperature and sintering temperature were determined by a thermomechanical analyzer (Thermo plus EVO2). For the thermomechanical analysis, the sintering aid mixed powder was uniaxially pressed with a die of φ5 mm at 9.8 kN·m ⁻² to prepare pellets. The measurement was performed up to 1300°C at a compression load of 49 mN and a heating rate of 10°C/min. Shrinkage was determined by the following formula.

The temperature at which the shrinkage rate becomes 2% was taken as the sintering start temperature, and the temperature at which the same shrinkage rate as that obtained by sintering at 1300° C. in the absence of the sintering aid was obtained was taken as the sintering temperature.
Regarding the determination of whether the sample was good or bad, a sintered body with a relative density exceeding 70% was evaluated as good (o), and a sample with a relative density of less than 70% was evaluated as defective (x).

Example 2 was carried out in the same manner as in Example 1, except that Bi ₂ O ₃ was used as the sintering aid in the sintering aid mixing step and was weighed so as to be 3 mol % with respect to the calcined powder.

Example 3 was carried out in the same manner as in Example 1 except that H ₃ BO ₃ was used as the sintering aid in the sintering aid mixing step and was weighed so as to be 1 mol % with respect to the calcined powder.

In Example 4, in the weighing step, Me=Al in the composition formula (1), Al ₂ O ₃ was used as the Al raw material, and the chemical amounts were adjusted so that x=0.750 and y=0.020. It was carried out in the same manner as in Example 1, except that the raw materials were weighed in stoichiometric proportions.

Example 5 was carried out in the same manner as in Example 4 except that Bi ₂ O ₃ was used as the sintering aid in the sintering aid mixing step and was weighed so as to be 3 mol % with respect to the calcined powder.

Example 6 was carried out in the same manner as in Example 4, except that H ₃ BO ₃ was used as the sintering aid in the sintering aid mixing step, and was weighed so as to be 1 mol % with respect to the calcined powder.

In Example 7, in the weighing step, Me=Ga in the composition formula (1), Ga ₂ O ₃ was used as the raw material for Ga, and the chemical amounts were adjusted so that x=0.750 and y=0.050. It was carried out in the same manner as in Example 2, except that the raw materials were weighed in stoichiometric proportions.

Example 8 was carried out in the same manner as in Example 7, except that H ₃ BO ₃ was used as the sintering aid in the sintering aid mixing step, and was weighed so as to be 1 mol % with respect to the calcined powder.

In Example 9, in the weighing process, Me=Yb in the composition formula (1), Yb ₂ O ₃ was used as the raw material of Yb, and the chemical amounts were adjusted so that x=0.750 and y=0.030. It was carried out in the same manner as in Example 1, except that the raw materials were weighed in stoichiometric proportions.

Example 10 was carried out in the same manner as in Example 9 except that Bi ₂ O ₃ was used as the sintering aid in the sintering aid mixing step and was weighed so as to be 3 mol % with respect to the calcined powder.

Example 11 was carried out in the same manner as in Example 9, except that H ₃ BO ₃ was used as the sintering aid in the sintering aid mixing step, and was weighed so as to be 1 mol % with respect to the calcined powder.

Example 12 was carried out in the same manner as in Example 1, except that in the weighing step, the raw materials were weighed in a stoichiometric ratio so that x=0.650 and y=0 in composition formula (1).

Example 13 was carried out in the same manner as in Example 12 except that H ₃ BO ₃ was used as the sintering aid in the sintering aid mixing step and was weighed so as to be 1 mol % with respect to the calcined powder.

Comparative Example 1 was carried out in the same manner as in Example 1 except that no sintering aid was added in the sintering aid mixing step. That is, it is the case where the sintering aid is not included. A cross-sectional SEM image of the sintered body thus produced is shown in FIG.

Comparative Example 2 was carried out in the same manner as in Example 4, except that no sintering aid was added in the sintering aid mixing step. That is, it is the case where the sintering aid is not included.

Comparative Example 3 was carried out in the same manner as in Example 9, except that no sintering aid was added in the sintering aid mixing step. That is, it is the case where the sintering aid is not included.

Comparative Example 4 was carried out in the same manner as in Example 12 except that no sintering aid was added in the sintering aid mixing step. That is, it is the case where the sintering aid is not included.

From Examples 1 to 13, it was found that the relative density exceeded 70% by sintering at 1200° C. by applying a sintering aid regardless of the composition of the calcined powder, and sintering proceeded. The sintering start temperature and sintering temperature determined from TMA were particularly low when H ₃ BO ₃ was used as the sintering aid. rice field. On the other hand, when the sintering aid was not contained as shown in Comparative Examples 1 to 5, the relative density was less than 70%, indicating almost no sintering.
From the above, it was shown that the sintering temperature can be lowered by applying a sintering aid.

Table 2 below lists the results of the same items as in Table (1).

Example 14 was carried out in the same manner as in Example 1, except that in the sintering aid mixing step, the calcined powder was pulverized for 4 hours with a wet ball mill using butyl acetate as a solvent.

Example 15 was carried out in the same manner as in Example 12, except that the sintering temperature was set to 1100°C in the calcination step. FIG. 5 shows the XRD of the calcined powder thus prepared. In this case also, most of the crystals could be attributed to the perovskite phase, and LiTaO3 indicated by triangles was also observed. 6 and 7 show XRD and cross-sectional SEM images of the sintered body. It can be seen that a single perovskite phase is obtained and the compact is a dense sintered body.

In Examples 14 and 15, the particle size of the calcined powder was reduced by pulverizing the calcined powder or lowering the calcining temperature. In any case, the relative density exceeds 90%, and it can be seen that the electrolytes are denser than those of Examples 1 and 12, respectively. From the above, it was shown that the sintering temperature can be lowered and the relative density can be increased by reducing the particle size in addition to adding a sintering aid. Moreover, in the case of Example 15, it was found that the formation of a heterogeneous phase in the sintered body was suppressed by setting x=0.65.

Claims (3)

A mixing step of mixing raw materials containing Li, Sr and Zr elements to form a mixed powder;
A calcining step of calcining the mixed powder to obtain a calcined powder;
A sintering aid mixing step of mixing the calcined powder with a sintering aid containing at least one element selected from B, Ga, and Bi to obtain a sintering aid mixed powder;
a sintering step of sintering the molded body of the sintering aid mixed powder to obtain a sintered body of the perovskite ion conductive oxide. . 2. The method for producing a perovskite-type ion-conductive oxide according to claim 1, wherein in said mixing step, wet mixing using an organic solvent is used as a solvent for dispersing said raw materials. 3. The method for producing a perovskite-type ion-conductive oxide according to claim 1, wherein the calcination temperature is 800[deg.] C. or more and 1300[deg.] C. or less in the calcination step.

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