JP2019067511A - Solid electrolyte and method for manufacturing the same - Google Patents

Abstract

To provide a solid electrolyte which has at least a satisfying mechanical strength and a lithium ion conductivity for use in a mobile such as a motor vehicle over a long time, and a method for manufacturing the solid electrolyte.SOLUTION: A solid electrolyte satisfies the following formula (I): LiM1M2Ti(PO)(I) (where M1 is one or more element selected from a group consisting of Al, Cu, Co, Fe, Ni, Ga, Crand Sc; M2 is one or more element selected from a group consisting of Si, Ge, Sn, Hfand Zr; and X and Y are each a real number that satisfies X+Y≤1). The solid electrolyte has NASICON type crystal structure, and comprises at least one metal oxide selected from a group consisting of TiO, SiO, ZrO, MgO and AlOat a grain boundary of NASICON type crystal structure.SELECTED DRAWING: None

Description

  The present invention relates to a solid electrolyte and a method of producing a solid electrolyte.

  Due to the widespread use of electric vehicles, air batteries having much higher energy density than lithium ion batteries are expected. An air battery uses oxygen in air as a positive electrode active material. Among them, a lithium air battery using metal lithium, an alloy containing lithium as a main component, or a compound containing lithium as a main component to the negative electrode active material has a high energy density and is required for full-fledged widespread use of electric vehicles. It is expected as a battery that can obtain an energy density of 700 Wh / kg. This energy density is more than three times higher than that of lithium-ion batteries currently in vehicles.

Focusing on the types of electrolytes, lithium-air batteries are roughly classified into two types, those using water-based electrolytes and non-water-based electrolytes.
Non-aqueous electrolyte lithium air batteries are the mainstream of research and development because they can utilize the technology of lithium ion batteries other than the air electrode.
On the other hand, research and development of water-based electrolyte lithium-air batteries are also in progress, though the number is still small. A water-based electrolyte lithium-air battery is less susceptible to moisture in the air than a non-water-based electrolyte lithium-air battery, and has the advantage that the electrolyte is inexpensive and incombustible.
However, metal lithium, which is a negative electrode active material, reacts when coming into direct contact with oxygen or water. Therefore, in a lithium-air battery of a water-based electrolyte, in order to protect metal lithium from the air or an aqueous solution, a protective layer is formed using a solid electrolyte having conductivity of lithium ions.

Such solid electrolyte, NASICON type _{Li 1 + X A X Ti 2} -X (PO 4) 3 lithium ion conductive solid electrolyte (hereinafter, referred to as NASICON type solid electrolyte) is known. Since the NASICON type solid electrolyte is stable to moisture, it can be prepared in open air, and is stable even in contact with a LiCl saturated aqueous solution which is an electrolytic solution. In addition, lithium ion conductivity is also good.

Here, as shown in Non-Patent Document 1, Li _1.4 Al _0.4 Ge _0.2 Ti _1.4 (PO ₄ ) ₃ is a solid electrolyte having high lithium ion conductivity. Non-Patent Document 2 discloses a lithium-air battery using this solid electrolyte. However, in order to actually manufacture an air battery and use it for a long time and in a mobile body such as an automobile, the strength of the solid electrolyte becomes a major issue. One of the strength indexes is 3-point bending strength, but the known 3-point bending strength of the high lithium ion conductive solid electrolyte is only 65 Nmm ^{2 at the} maximum. This bending strength is such that it is easily broken if stressed by hand, and it is completely insufficient as a solid electrolyte for use in the above-mentioned application.

On the other hand, solid electrolytes having improved strength are commercially available from OHARA CORPORATION. Its strength is in practical level of about 140Nmm ^-2, the solid electrolyte is mar output of the air battery have only the lithium ion conductivity of about 1.0 × ¹⁰ -4 ^{Scm -1.}

Further, Non-Patent Document _{3, Li 1 + x Al x} Ge y Ti 2-x-y (PO 4) 3 (LAGTP) composition ratio of Oite Al have been optimized, the highest case of Al = 0.45 It is stated that conductivity is obtained. The conductivity is very high at about 1.0 × 10 ⁻³ Scm ⁻¹ , but the mechanical strength is about 90 N mm ⁻² and is somewhat insufficient.
As mentioned above, in the prior art, the solid electrolyte which can satisfy both mechanical strength and lithium ion conductivity is not obtained.

Zhang et al. Journal of the Electrochemical Society 162 (7) A 1265-A1271 (2015) “Tape-Cast Water-Stable NASICON-Type High Lithium Ion Conducting Solid Electrolyte Films for Aqueous Lithium-Air Batteries” Takeda, Imanishi, Yamamoto GS Yuasa Technical Report June 2010, Volume 7, No. 1 "Current Status and Challenges of Aqueous Lithium / Air Battery" S. Xuefu, et al. High Lithium-Ion-Conducting NASICON-Type Li1 + xAlxGeyTi2-x-y (PO4) 3Solid Electrolyte. Front. Energy Res. 4:12. (2016)

  In view of the above-described circumstances, it is an object of the present invention to provide a solid electrolyte having at least sufficient mechanical strength and lithium ion conductivity for use in a mobile body such as an automobile for a long time, and a method of manufacturing the same. Do.

In order to solve the above-mentioned problems, the inventors of the present invention have conducted intensive studies, and by adding a metal oxide such as TiO ₂ at the time of production of a solid electrolyte, the conductivity is improved while improving the physical strength. It was conceived to be able to obtain a solid electrolyte that can be used for

That is, in order to achieve the above object, the solid electrolyte according to the present invention is a solid electrolyte satisfying the following formula (I),
_{Li 1 + X M1 X M2 Y} Ti 2-X-Y (PO 4) 3 (I)
(Wherein, M 1 is one or more elements selected from the group consisting of Al ³⁺ , Cu ³⁺ , Co ³⁺ , Fe ³⁺ , Ni ³⁺ , Ga ³⁺ , Cr ³⁺ and Sc ³⁺ , M 2 is Si ⁴⁺ , At least one element selected from the group consisting of Ge ⁴⁺ , Sn ⁴⁺ , Hf ⁴⁺ and Zr ⁴⁺ ,
X and Y are real numbers that satisfy X + Y ≦ 1. )
The solid electrolyte has a NASICON type crystal structure,
A metal oxide is provided at grain boundaries of the NASICON type crystal structure, and the metal oxide is one metal oxide selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ , or TiO. ₂ and a mixture of at least one metal oxide selected from the group consisting of SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ .

The solid electrolyte according to the present invention, in one embodiment, has a lithium ion conductivity of 1.0 × 10 ⁻³ Scm ⁻¹ or more, and a three-point bending strength of 100 N mm ⁻² or more.

Another aspect of the present invention is a method of producing a solid electrolyte, which is a method of producing a solid electrolyte satisfying the following formula (I),
_{Li 1 + X M1 X M2 Y} Ti 2-X-Y (PO 4) 3 (I)
(Wherein, M 1 is one or more elements selected from the group consisting of Al ³⁺ , Cu ³⁺ , Co ³⁺ , Fe ³⁺ , Ni ³⁺ , Ga ³⁺ , Cr ³⁺ and Sc ³⁺ , M 2 is Si ⁴⁺ , At least one element selected from the group consisting of Ge ⁴⁺ , Sn ⁴⁺ , Hf ⁴⁺ and Zr ⁴⁺ ,
X and Y are real numbers that satisfy X + Y ≦ 1. )
A mixing step of obtaining a powder mixture of the solid electrolyte and a metal oxide, wherein the metal oxide is selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ A mixing step of a metal oxide or a mixture of TiO ₂ and at least one metal oxide selected from the group consisting of SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ ;
A forming step of pressure forming the powder mixture to form a green compact;
Annealing the green compact.

  In the method for producing a solid electrolyte according to the present invention, in one embodiment, the mixing amount of the metal oxide is 4 to 8 wt% in the powder mixture.

  According to the present invention, there is provided a solid electrolyte having at least sufficient mechanical strength and lithium ion conductivity, and a method of manufacturing the same, for long-term use in mobile vehicles such as automobiles.

It is a conceptual diagram explaining a NASICON type | mold crystal structure. It is a figure which shows the XRD measurement result of the obtained LAGTP about Example 1 of the solid electrolyte which concerns on this invention. For Example 1 of the solid electrolyte of the present invention, it is a graph showing the relationship between the added amount and LAGTP conductivity of TiO _2. For Example 1 of the solid electrolyte of the present invention, it is a graph showing the relationship between the maximum stress of the addition amount of TiO ₂ and LAGTP. For Example 1 of the solid electrolyte of the present invention, it is a graph showing the relationship between the relative density of the additive amount of TiO ₂ and LAGTP.

  Hereinafter, preferred embodiments of the solid electrolyte according to the present invention and the method for producing the same will be described in more detail.

The solid electrolyte according to the present invention is a solid electrolyte satisfying the following formula (I).
_{Li 1 + X M1 X M2 Y} Ti 2-X-Y (PO 4) 3 (I)
In the formula (I), M1 is a metal element, and one or more metal elements selected from the group consisting of Al ³⁺ , Cu ³⁺ , Co ³⁺ , Fe ³⁺ , Ni ³⁺ , Ga ³⁺ , Cr ³⁺ and Sc ³⁺ It is. Here, the assigned valences are the valences that the metal element M1 bears when forming the target NASICON-type solid electrolyte. Of these, Al ³⁺ is particularly preferred.
M2 is a metal element and is one or more elements selected from the group consisting of Si ⁴⁺ , Ge ⁴⁺ , Sn ⁴⁺ , Hf ⁴⁺ , and Zr ⁴⁺ . Here, the assigned valences are the valences that the metal element M2 bears when forming the target NASICON-type solid electrolyte. Of these, Ge ⁴⁺ is particularly preferred.

  In formula (I), X and Y are real numbers satisfying X + Y ≦ 1. If X + Y ≦ 1, then the strength of the solid electrolyte and the lithium ion conductivity can be improved. Further, it is preferable that 0 <Y <X. By including a trivalent metal atom (M1), carrier ions in the solid electrolyte can be increased, and lithium ion conductivity can be improved. Then, by setting Y <X, the crystal lattice constant of the solid electrolyte can be set appropriately. As a result, the relative density of the solid electrolyte can also be increased.

The first solid electrolyte according to the present invention has a NASICON type crystal structure. More specifically, the solid electrolyte according to the present invention has a rhombohedral (hexagonal) structure represented by space group R-3c. The crystal structure is shown in FIG. The crystal structure of FIG. 1 is simplified to LiM ₂ (PO ₄ ) ₃ .
What is shown by Li (1) in FIG. 1 indicates fixed Li unrelated to ion conduction, and Li (2) is mobile Li related to ion conduction.
In addition, "R" shows that it is a rhombohedral structure among R-3c which is description of space group. “−3” indicates a symmetric operation that reverses 120 degrees (adds − to xyz). c indicates that c / 2 is moved (shifted) in the c axis direction.
In the figure, the length of the a axis is a (in the figure, x axis, y axis direction), and the length of the c axis is c (z axis direction in the figure).

The solid electrolyte according to the present invention comprises a metal oxide at grain boundaries of the NASICON type crystal structure. Here, in the present specification and claims, “a metal oxide is provided at grain boundaries of NASICON type crystal structure” means that an interface between particles constituting the NASICON type crystal structure and particles are gathered. The state in which metal oxide exists in the gap.
The metal oxide is one metal oxide selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ , or TiO ₂ and SiO ₂ , ZrO ₂ , MgO and Al ₂ O A mixture with at least one metal oxide selected from the group consisting of ₃ .

The solid electrolyte according to the present invention preferably has a lithium ion conductivity of 1.0 × 10 ⁻³ Scm ⁻¹ or more and a three-point bending strength of 100 N mm ⁻² or more. With these characteristics, it can be used for a long time in a mobile such as a car.

  The solid electrolyte according to the present invention comprises: a mixing step of obtaining a powder mixture of the solid electrolyte described above and the metal oxide described above; a forming step of pressure forming the powder mixture to form a green compact; It can manufacture by implementing the process of annealing the said green compact. That is, the method for producing a solid electrolyte according to the present invention at least includes these steps.

  Specifically, one embodiment of a method for preparing a solid electrolyte according to the present invention by, for example, a sol-gel method will be described. The case where M1 in the formula (I) is Al and M2 is Ge will be described. However, even if M1 and M2 are replaced with other elements, the following explanation holds.

Dissolve alkoxides of Ti and Ge in a predetermined aqueous acid solution, and after heating and stirring, add a solvent such as LiNO ₃ , Al (NO ₃ ) ₃ , NH ₄ H ₂ PO ₄ and ethylene glycol, and further sufficiently stir Thus, a gel containing the raw material salt is obtained. The ratio of the respective elements is prepared as an amount which corresponds to the NASICON type _{Li 1 + X Al X Ge Y} Ti 2-X-Y (PO 4) 3 lithium ion conductive solid electrolyte of interest. Moreover, it is preferable that each raw material be a chemical reagent grade.
The obtained gel is heated at 500 to 600 ° C. for 3 hours or more, and then it is further calcined at 750 ° C. to 850 ° C. for 3 to 5 hours. The calcination is optimally performed at 800 ° C. for 4 hours.

A powder of metal oxide is mixed with the calcined product, a solvent such as isopropanol is added, and the mixture is pulverized by a pulverizing means such as a planetary ball mill. Thereby, a powder mixture of a solid electrolyte and a metal oxide is obtained (mixing step). The metal oxide is one metal oxide selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ , or TiO ₂ and SiO ₂ , ZrO ₂ , MgO and Al ₂ O _The powder is a mixture with at least one metal oxide selected from the group consisting of _3. Thereafter, the powder is compacted (forming process) and fired (annealing process) to obtain a target solid electrolyte.

The solid electrolyte according to the present invention can also be obtained as follows. Here also, the case where M1 in the formula (I) is Al and M2 is Ge will be described. The following explanation is valid even if M1 and M2 are replaced with other elements.
NASICON-type _{Li 1 + X Al X Ge Y} Ti 2-X-Y (PO 4) ₃ The amount of chemical reagent grade corresponding to the lithium ion conductive solid electrolyte _{Li 2 CO 3, TiO 2,} GeO 2, Al 2 O 3, And NH ₄ H ₂ PO ₄ are ball milled with zirconia balls in a zirconia container to obtain a mixed powder.

The mixed powder is then compression molded into pellets and calcination is performed at a relatively low temperature (500-800 ° C., eg 600 ° C.).
Next, metal oxide powder is added to the calcined pellets, reground, and ball milling again (mixing step). Here, the metal oxide is one metal oxide selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ , or TiO ₂ and SiO ₂ , ZrO ₂ , The obtained powder, which is a mixture with at least one metal oxide selected from the group consisting of MgO and Al ₂ O ₃ , is compression-molded into pellets by means of hydrostatic pressure (forming of green compact, molding step).
Thereafter, sintering is further performed at 900 to 1200 ° C. (annealing step).

Example 1
After dissolving alkoxides of Ti and Ge [Ti (OC ₄ H ₉ ) ₄ and Ge (OC ₂ H ₅ ) ₄ ] in a citric acid aqueous solution and heating and stirring, LiNO ₃ , Al (NO ₃ ) ₃ and NH ₄ H By adding ₂ PO ₄ and ethylene glycol and further stirring sufficiently, a gel containing a raw material salt was obtained. The ratio of each constituent _{element, Li 1 + x Al x Ge} y Ti 2-x-y (PO 4) 3 ( herein, also LAGTP referred to finger.) In, Al = 0.45, Ge = 0.2 It adjusted so that it might become the composition ratio of.

The obtained gel was heated at 580 ° C. for 4 hours, and then calcined at 800 ° C. for 4 hours. The pre-calcined product was mixed with TiO ₂ powder, isopropanol was added, and sufficiently ground in a planetary ball mill for 6 hours at 300 rpm. Thereafter, the powder was press-molded and fired at 950 ° C. for 7 hours to produce the LAGTP.

  The XRD measurement result of LAGTP obtained by FIG. 2 is shown. By determining the XRD pattern of the obtained LAGTP, it was confirmed that the target solid electrolyte of NASICON type structure was obtained. In addition, what was shown by wt% in FIG. 2 is the addition amount of Ti, Ref. Shows the standard data of LAGTP.

The results of measurement of the conductivity and physical strength (maximum stress) of the obtained LAGTP are shown in FIGS. 3 and 4, respectively.
For lithium ion conductivity, sintered phased pellets (about 12 mm in diameter and 1 mm thick) with gold sputtered electrodes, impedance phase analyzer (in a frequency range of 0.1 Hz to 1 MHz and a bias voltage of 10 mV) It measured using Solartron 1260).
The maximum stress was measured by a three-point bending test as defined by JIS.

From FIG. 3, the conductivity of LAGTP was 1.0 × 10 ⁻³ Scm ⁻¹ or more when the additive amount of TiO ₂ was 3 ≦ x ≦ 8 wt%. On the other hand, according to FIG. 4, the maximum stress of LAGTP was about 100 N mm ⁻² or more when the addition amount of TiO ₂ was 4 wt% or more, and tended to increase as the addition amount of TiO ₂ was large. From the above results, it was found that a solid electrolyte having both conductivity and mechanical strength can be obtained by adding 4 ≦ x ≦ 8 wt% of TiO ₂ to LAGTP.

The result of having calculated | required the relative density of the obtained LAGTP in FIG. 5 is shown.
For relative density, the relative density of the sintered sample was estimated from the ratio of the density calculated from the lattice constant to the density calculated from the volume and weight of the sintered body.
It was found from FIG. 5 that a sufficient relative density can be obtained by adding 3 wt% or more of TiO _2, and a suitable water barrier can be obtained.

  The solid electrolyte according to the present invention has at least sufficient mechanical strength and lithium ion conductivity, and can be used for a long time in a mobile vehicle such as an automobile.

Claims (4)

A solid electrolyte satisfying the following formula (I),
_{Li 1 + X M1 X M2 Y} Ti 2-X-Y (PO 4) 3 (I)
(Wherein, M 1 is one or more elements selected from the group consisting of Al ³⁺ , Cu ³⁺ , Co ³⁺ , Fe ³⁺ , Ni ³⁺ , Ga ³⁺ , Cr ³⁺ and Sc ³⁺ , M 2 is Si ⁴⁺ , At least one element selected from the group consisting of Ge ⁴⁺ , Sn ⁴⁺ , Hf ⁴⁺ and Zr ⁴⁺ ,
X and Y are real numbers that satisfy X + Y ≦ 1. )
The solid electrolyte has a NASICON type crystal structure,
One metal oxide selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ provided with metal oxides at grain boundaries of NASICON type crystal structure, or TiO ₂ And a mixture of at least one metal oxide selected from the group consisting of SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ ,
Solid electrolyte. The solid electrolyte according to claim 1, having a lithium ion conductivity of 1.0 × 10 ^-3 Scm ^-1 or more and a three-point bending strength of 100 N mm ² or more. A method for producing a solid electrolyte satisfying the following formula (I),
_{Li 1 + X M1 X M2 Y} Ti 2-X-Y (PO 4) 3 (I)
(Wherein, M 1 is one or more elements selected from the group consisting of Al ³⁺ , Cu ³⁺ , Co ³⁺ , Fe ³⁺ , Ni ³⁺ , Ga ³⁺ , Cr ³⁺ and Sc ³⁺ , M 2 is Si ⁴⁺ , At least one element selected from the group consisting of Ge ⁴⁺ , Sn ⁴⁺ , Hf ⁴⁺ and Zr ⁴⁺ ,
X and Y are real numbers that satisfy X + Y ≦ 1. )
A mixing step of obtaining a powder mixture of the solid electrolyte and a metal oxide, wherein the metal oxide is selected from the group consisting of TiO ₂ , SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ A mixing step of a metal oxide or a mixture of TiO ₂ and at least one metal oxide selected from the group consisting of SiO ₂ , ZrO ₂ , MgO and Al ₂ O ₃ ;
A forming step of pressure forming the powder mixture to form a green compact;
Annealing the green compact.
Method of producing a solid electrolyte   The method for producing a solid electrolyte according to claim 3, wherein a mixed amount of the metal oxide is 4 to 8 wt% in the powder mixture.

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