A lithium-ion conducting solid electrolyte

Abstract: A lithium-ion conducting solid electrolyte of the present invention comprises a parent material, a lithium-ion conducting sulfide glass represented by the formula Li₂S-X (X is at least one sulfide selected from the group consisting of B₂S₃ , SiS₂, P₂S₅, Al₂S₃ , and GeS₂), and a high-temperature lithium-ion conducting compound (i.e. Li₃PO₄ or Li₂SO₄). The lithium-ion conducting solid electrolyte has higher ionic conductivity and higher decomposition voltage compared to the parent material. By the use of this solid electrolyte for electrical/chemical components such as batteries, condensers, electrochromic displays, and the like, electronic apparatus that includes such elements may have improved performance.

Description: 1. Field of the Invention: The present invention relates to a lithium-ion conducting solid electrolyte. More particularly, it relates to a lithium-ion conducting solid electrolytes that are used in solid-state electrical/chemical elements such as solid-state batteries, solid electrolyte double-layered capacitors, solid electrochromic displays, and the like. 2. Description of the Prior Art: In recent years, the progress in technology of electronic industries is remarkable. Electronic devices such as integrated circuits (IC), large scaled integrated circuits (LSI), and the like are used in various fields to make electronic apparatus smaller and thinner. However, diminishment in size of conventional electrical/chemical elements that include electrolytic solution is limited, because sealing of the elements requires highly advanced techniques and the proportion of sealing materials in them become larger as they are diminished in size. In contrast to the conventional elements, solid-state electrical/chemical elements, which consist of solid components can be diminished in size readily, because in such solid elements, the seal is not needed. Solid electrolytes, solid-state ion conductors that can transport ions, are essential for constructing such solid-state elements and thus new materials for solid electrolytes are now being developed. One example is the application of lithium-ion conducting polyelectrolyte, which can be processed readily and has flexibility, to batteries and the like. Typical example of this is poly(oligooxyethylene methacrylate)-alkaline metal system, although it is still under investigation, because of several problems. The problems are that the maximum ionic conductivity of the system, which is about 10⁻⁵ S/cm, does not reach the level required for practical use and that not only cation (Li⁺), but also anions (e.g. ClO₄⁻) are transported in the system. Further, this polyelectrolyte is reactive with a lithium metal that is used for an anode. As for inorganic lithium-ion conducting solid electrolytes, of which conductivities are about 10⁻³ S/cm, LiI-Li₂S-X (wherein X is at least one sulfide selected from the group consisting of B₂S₃, SiS₂, P₂S₅, Al₂S₃, and GeS₂) and Li₃N system are well known. However, they are not put to practical use, because their decomposition voltages are below 3.0 V. EP-A-0 206 339 discloses a three component solid electrolyte having the general formula Y-bLi₂S-aX, wherein X is P₂S₅ or SiS₂, a is from 0.5 to 2, b is from 0.25 to 2 and Y is an oxygene containing lithium compound selected from Li₂O, Li₂SiO₃, Li₂SO₄ and Li₄SiO₄, said compound having at least a conductivity of 0.75 to 10⁻⁴ S/cm. The lithium-ion conducting solid electrolyte of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a lithium-ion conducting sulfide glass represented by the formula Li₂S-X (wherein X is at least one sulfide selected from the group consisting of B₂S₃, SiS₂, P₂S₅, Al₂S₃, and GeS₂) and Li₃PO₄ as a high-temperature lithium-ion conducting compound, which becomes a lithium-ion conductor at a temperature ranging from 400°C to 900°C. Thus, the invention described herein makes possible the objectives of (1) providing a lithium-ion conducting solid electrolyte having high ionic conductivity; (2) providing a lithium-ion conducting solid electrolyte having ionic conductivity that is stable with time; and (3) providing a lithium-ion conducting solid electrolyte having high decomposition voltage. DESCRIPTION OF THE PREFERRED EMBODIMENTS A lithium-ion conducting solid electrolyte of the present invention comprises a lithium-ion conducting sulfide glass represented by the formula Li₂S-X (wherein X is at least one sulfide selected from the group consisting of B₂S₃, SiS₂, P₂S₅, Al₂S₃, and GeS₂) and a high-temperature lithium-ion conducting compound, wherein Li₃PO₄ is used as the compound. The atomic arrangements in Li₃PO₄ are disrupted at high temperature and thus these compounds have high ionic conductivities. When the compounds are cooled to room temperature, their ionic conductivities decrease, because phase transition occurs and their crystal structures are restored. However, Li₃PO₄ may have high conductivity even at room temperature, when it is amorphous. In an amorphous body, atomic arrangements are also disrupted. Therefore, in the present invention, a glass parent material represented by the formula Li₂S-X, is first made to be amorphous and then Li₃PO₄ is added to this amorphous material, which results in an amorphous Li₃PO₄. This is accomplished by first heating the mixture of Li₂S and X, and then quenching (or cooling rapidly) it. The molar ratio of Li₂S to X depends on what X is. Preferably, Li₂S is added in a large amount as long as no sediment is deposited. To this amorphous mixture, Li₃PO₄ is added and then the mixture heated again. This is followed by quenching. Preferably the amount of Li₃PO₄ is 1-5 molar %. The resulting lithium-ion conducting solid electrolyte has higher ionic conductivity and higher decomposition voltage compared to the parent material. Examples Below, the present invention is explained in detail. To compare a lithium-ion conducting solid electrolyte of the present invention with a corresponding parent material (i.e. sulfide glass) and LiI-Li₂S-X (wherein X is the same sulfide as in the lithium-ion conducting solid electrolyte), electrical/chemical characteristics of all these compositions were evaluated. Conductivity was measured by alternating impedance technique at room temperature. Each of the conductivities and decomposition voltages of LiI-Li₂S-X mentioned herein is the maximum value of the system. A parent material (i.e. sulfide glass), parent material-forming substances, and a solid electrolyte synthesized in the present invention are decomposed readily in air. Therefore, all of the procedures described herein were done in dry boxes that are filled with argon gas. Li₃PO₄ is used after drying at 400°C for 6 hours under reduced pressure. Example 1 First, Li₂S and SiS₂, which were in powder form, were mixed in a molar ratio of 3:2, put in a glass-like carbon crucible, baked at 950°C for 1.5 hours in the stream of argon gas, and quenched in liquid nitrogen to obtain amorphous 0.6Li₂S-0.4SiS₂. Then Li₃PO₄ and amorphous 0.6Li₂S-0.4SiS₂ so obtained and crushed were mixed in a molar ratio of 3:97. The mixture was put in a glass-like carbon crucible, baked at 950°C for 1.5 hours in the stream of argon gas, and quenched in liquid nitrogen to produce 0.03Li₃PO₄-0.58Li₂S-0.39SiS₂. The ionic conductivities of 0.03Li₃PO₄-0.58Li₂S-0.39SiS₂ and 0.6Li₂S-0.4SiS₂ were 7.0x10⁻⁴ S/cm and 5.3x10⁻⁴ S/cm, respectively, when they were measured immediately after being synthesized. These values did not change even one month later. Both of the decomposition voltages of 0.03Li₃PO₄-0.58Li₂S-0.39SiS₂ and 0.6Li₂S-0.4SiS₂ were about 4.0 V. The results can be compared with those of 0.30LiI-0.35Li₂S-0.35SiS₂, which were about 2.9 V. Example 2 First, Li₂S and GeS₂, which were in powder form, were mixed in a molar ratio of 3:2, put in a glass-like carbon crucible, baked at 950°C for 1.5 hours in the stream of argon gas, and quenched in liquid nitrogen to obtain amorphous 0.6Li₂S-0.4GeS₂. Then Li₃PO₄ and the amorphous 0.6Li₂S-0.4GeS₂ so obtained and crushed were mixed in a molar ratio of 3:97. The mixture was put in a glass-like carbon crucible, baked at 950°C for 1.5 hours in the stream of argon gas, and quenched in liquid nitrogen to produce 0.03Li₃PO₄-0.58Li₂S-0.39GeS₂. The ionic conductivities of 0.03Li₃PO₄-0.58Li₂S-0.39GeS₂, 0.6Li₂S-0.4GeS₂, and 0.30LiI-0.35Li₂S-0.35GeS₂ were 3.0x10⁻⁴ S/cm, 2.0x10⁻⁴ S/cm, and 4x10⁻⁴ S/cm, respectively, when they were measured immediately after being synthesized. Four hours later, the ionic conductivity of 0.30LiI-0.35Li₂S-0.35GeS₂ decreased by 5%, but those of 0.03Li₃PO₄ -0.58Li₂S-0.39GeS₂ and 0.6Li₂S-0.4GeS₂ did not change even one month later. The decomposition voltages of 0.03Li₃PO₄-0.58Li₂S-0.39GeS₂, 0.6Li₂S-0.4GeS₂, and 0.30LiI-0.35Li₂S-0-35GeS₂ were about 4.0 V, 4.0 V, and 2.9 V, respectively. Example 3 First, Li₂S and P₂S₅, which were in powder form, were mixed in a molar ratio of 2:1, put in a quartz glass-tube and sealed under reduced pressure, baked at 500°C for 12 hours and then at 800°C for 3 hours in the stream of argon gas, quenched in liquid nitrogen to obtain amorphous 0.67Li₂S-0.33P₂S₅. Then Li₃PO₄ and the amorphous 0.67Li₂S-0.33P₂S₅ so obtained and crushed were mixed in a molar ratio of 3:97. The mixture was put in a quartz glass-tube and sealed under reduced pressure, baked at 900°C for 1 hour in the stream of argon gas, and quenched in liquid nitrogen to produce 0.03Li₃PO₄-0.65Li₂S-0.32P₂S₅. The ionic conductivities of 0.03Li₃PO₄-0.65Li₂S-0.32P₂S₅ and 0.67Li₂S-0.33P₂S₅ were 4.2x10⁻⁴ S/cm and 3.0x10⁻⁴ S/cm, respectively, when they were measured immediately after being synthesized. The values did not change even one month later. Both of the decomposition voltages of 0.03Li₃PO₄-0.65Li₂S-0.32P₂S₅ and 0.67Li₂S-0.33P₂S₅ were about 4.0 V. The results can be compared with those of 0.45LiI-0.37Li₂S-0.18P₂S₅, which were about 2.9 V. Example 4 First, Li₂S and B₂S₃, which were in powder form, were mixed in a molar ratio of 1:1, put in a quartz glass-tube and sealed under reduced pressure, baked at 500°C for 12 hours and then at 800°C for 3 hours in the stream of argon gas, and quenched in liquid nitrogen to obtain amorphous 0.5Li₂S-0.5B₂S₃. Then Li₃PO₄ and 0.5Li₂S-0.5B₂S₃ so obtained and crushed up were mixed in a molar ratio of 4:96. The mixture was put in a quartz glass-tube and sealed under reduced pressure, baked at 800°C for 3 hours in the stream of argon gas, and quenched in liquid nitrogen to produce 0.04Li₃PO₄-0.48Li₂S-0.48B₂S₃. The ionic conductivities of 0.04Li₃PO₄-0.48Li₂S-0.48B₂S₃ and 0.5Li₂S-0.5B₂S₃ were 3.0x10⁻⁴ S/cm and 2.0x10⁻⁴ S/cm, respectively, when they were measured immediately after being synthesized. The values did not change even one month later. Both of the decomposition voltages of 0.04Li₃PO₄-0.48Li₂S-0.48B₂S₃ and 0.5Li₂S-0.5B₂S₃ were about 4.0 V. The results can be compared with those of 0.4LiI-0.3Li₂S-0.3B₂S₃, which were about 2.9 V. As described in the examples above, by adding high-temperature a lithium-ion conducting compound (i.e. Li₃PO₄) to a lithium-ion conducting sulfide glass, we could obtain a solid electrolyte that has high ionic conductivity and high decomposition voltage. Beside the examples, Al₂S₃ can be used as a sulfide glass. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims: A lithium-ion conducting solid electrolyte comprising Li₃PO₄ as a lithium-ion conductor and a lithium-ion conducting sulfide glass represented by the formula Li₂S-X (wherein X is at least one sulfide selected from the group consisting of B₂S₃, SiS₂, P₂S₅, Al₂S₃, and GeS₂).

Inventors: HASEGAWA MASAKI, KONDO SHIGEO, TAKADA KAZUNORI, YAMAMURA YASUHARU, HASEGAWA, MASAKI, KONDO, SHIGEO, TAKADA, KAZUNORI, YAMAMURA, YASUHARU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1991
Main classification: H01M6
IPC codes: C03C3, C03C4, H01M6