JP2014154216A - Lithium ion conductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion conductor capable of being stably manufactured and having high degree of crystallinity after heat treatment and high lithium ion conductivity.SOLUTION: A lithium ion conductor contains POof 25-40%, GeOof 25-50%, LiO of 10-25%, AlOof 0-10% (provided that 0% is not included), and F of 0.01-15% in mol%.

Description

  The present invention mainly relates to a lithium ion conductor suitable as a solid electrolyte for a lithium ion secondary battery.

  With recent downsizing and higher performance of electronic devices, high capacity, small and lightweight batteries are required. Among them, a lithium ion secondary battery can obtain a high voltage (for example, about 4 V) because of a large electrode potential difference. Moreover, since lithium has a small atomic weight, it is easy to achieve a high energy density.

  A lithium ion secondary battery basically includes a positive electrode, a negative electrode, and an electrolyte. An electrolyte made of an organic solvent is usually used as an electrolyte of a lithium ion battery that has been put into practical use. However, there has been a problem that a liquid electrolyte is liable to leak or easily ignite.

  In view of the above-mentioned problems of liquid electrolytes, solidification of electrolytes has been studied. For example, sulfide-based materials and oxide-based materials are known as lithium ion conductive solid electrolytes. Since sulfide-based materials react with moisture in the atmosphere to generate hydrogen sulfide, mass production and handling are difficult, and oxide-based materials with excellent chemical stability are attracting attention.

As the oxide material, a solid phase reaction method or crystallization of NASICON type prepared in glass process _{Li 1-x M x Ti 2} -x (PO 4) 3 or _{Li 1-x Al x Ge 2} -x (PO 4 ) ₃ (M = Al or rare earth element) (see Patent Documents 1 to 3, Non-Patent Document 1) and the like.

Japanese Patent Laid-Open No. 2-162605 Japanese Patent No. 3197246 Japanese Patent No. 4745472

P. Maldonado-Manso, MC Martin-Sedeno, S. Bruque, J. Sanz, ER Losilla, “Unexpected potential distribution in tetrahedral / octahedral sites in nominal Li1-xAlxGe2-x (PO4) 3NASICON series”, Solid State Ionics, 178 , 43-52 (2007)

During the production of an oxide material by a solid phase reaction method, volatilization of Li ₂ O or the like is likely to occur at the time of firing and at the time of melting an oxide material by a crystallized glass method. There was a problem that the lithium ion conductivity of the metal tends to decrease.

  In view of the above circumstances, an object of the present invention is to provide a lithium ion conductor that can be stably produced, has a high degree of crystallinity, and high lithium ion conductivity.

Lithium ion conductor of the present invention, in _{mol%, P 2 O 5 25~40%} , GeO 2 25~50%, Li 2 O 10~25%, Al 2 O 3 0~10% ( provided that 0% And F) 0.01 to 15%.

If the composition, to suppress volatilization of such Li 2 _O, stable to manufacture, and can crystallinity is high lithium ion conductivity to obtain a high lithium ion conductor. In addition, the mechanism which can suppress the fall of lithium ion conductivity if F is added during a composition is unknown at present, and is earnestly investigating now.

  The lithium ion conductor of the present invention is preferably produced by a crystallized glass method.

  With this structure, when a bulk body is produced, a dense structure is easily obtained. When powdered, it is crystallized after being softened as glass, and thus can be sintered densely. As a result, the interfacial resistance is reduced, so that lithium ion conductivity can be improved.

  The lithium ion conductor of the present invention is preferably produced by a solid phase reaction method.

With this structure, since it can be manufactured at a lower temperature, volatilization of Li ₂ O and the like can be suppressed, and lithium ion conductivity can be improved.

In the lithium ion conductor of the present invention, the main crystal phase is preferably Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ crystal (0 <x <2).

  With this configuration, a lithium ion conductor having high lithium ion conductivity can be easily obtained.

  The lithium ion conductor of the present invention is preferably used for a solid electrolyte for a lithium ion secondary battery.

It is a schematic diagram which shows one Embodiment of the lithium ion secondary battery which used the lithium ion conductor of this invention as a solid electrolyte for lithium ion secondary batteries. It is a schematic diagram which shows another embodiment of the lithium ion secondary battery which used the lithium ion conductor of this invention as a solid electrolyte for lithium ion secondary batteries.

Lithium ion conductor of the present invention, in _{mol%, P 2 O 5 25~40%} , GeO 2 25~50%, Li 2 O 10~25%, Al 2 O 3 0~10% ( provided that 0% ), And 0.01 to 15% of F. The reason for limiting the ingredients in this way will be described below. In addition, in description of content of the following components,% display points out mol% unless there is particular notice.

P ₂ O ₅ is a constituent component of the lithium ion conductive crystal, and its content is 25 to 40%. The content of P ₂ O ₅ is preferably 30 to 38%. When the content of P ₂ O ₅ is too small, the lithium ion conductive crystal is hardly precipitated. On the other hand, when the content of P ₂ O ₅ is too large, tends to vitrification, since the lithium ion conductivity of the vitreous is very low, the lithium ion conductivity tends to decrease.

GeO ₂ is a constituent component of the lithium ion conductive crystal, and its content is 25 to 50%. The GeO ₂ content is preferably 30 to 45%. When the content of GeO ₂ is too small, the lithium ion conductive crystal is hardly precipitated. On the other hand, when the content of GeO ₂ is too large, crystals other than the lithium ion conductive crystal (GeO ₂ or the like) are likely to precipitate.

Li ₂ O is a component serving as a source of lithium ions, the content thereof is 10% to 25%. The content of Li ₂ O is preferably 15 to 25%. When the Li 2 _O content is too small, the lithium ion conductivity tends to decrease. On the other hand, when the content of Li 2 _O is too large, the weather resistance tends to lower. In addition, the amount of lithium ion conductive crystals deposited decreases, or crystals other than lithium ion conductive crystals tend to precipitate, and the lithium ion conductivity tends to decrease.

Al ₂ O ₃ is a component that improves the lithium ion conductivity of the lithium ion conductive crystal, and its content is 0 to 10% (however, 0% is not included). The content of Al ₂ O ₃ is preferably 3 to 8%. When the content of Al ₂ O ₃ is 0%, it is difficult to obtain the above effect. When the content of Al ₂ O ₃ is too large, crystals other than lithium ion conductive crystals tend to precipitate, and the lithium ion conductivity tends to decrease.

  F is a component that suppresses a decrease in lithium ion conductivity, and its content is 0.01 to 15%. The content of F is preferably 1 to 12%. If the content of F is too small or too large, the above effect is difficult to obtain.

In addition to the above components, SiO ₂ , ZrO ₂ , TiO ₂ , rare earth oxides and the like may be contained as long as the effects of the present invention are not impaired.

  Next, a method for producing the lithium ion conductor of the present invention will be described.

The lithium ion conductor of the present invention is preferably produced by a crystallized glass method or a solid phase reaction method. In the crystallized glass method, first, raw material powders are adjusted and melted to be vitrified so as to have a desired composition. The melting temperature is preferably 1200 to 1450 ° C. If the melting temperature is too low, undissolved raw material powder tends to remain, whereas if the melting temperature is too high, P ₂ O ₅ and Li ₂ O tend to volatilize, making it difficult to obtain a desired glass composition. A lithium ion conductor precursor glass is obtained by molding molten glass. Moreover, a lithium ion conductor can be produced by heat-treating and crystallizing the lithium ion conductor precursor glass. Here, the bulk lithium ion conductor precursor glass may be heat-treated, and the powder obtained by pulverizing the lithium ion conductor precursor glass is heat-treated by press molding or green sheet molding. May be. It is preferable that the heat processing temperature is 650-1000 degreeC. When the heat treatment temperature is too low, crystal precipitation becomes insufficient and the lithium ion conductivity tends to decrease. On the other hand, if the heat treatment temperature is too high, the lithium ion conductivity tends to decrease due to the growth of crystals other than the lithium ion conductive crystal or the melting of a part of the precipitated crystal.

  In the solid phase reaction method, first, a solid raw material such as an oxide is prepared so as to have a desired composition, and this is fired. The raw material is preferably pulverized and mixed before firing. In this way, since the raw materials are mixed in a fine powder state under mechanical shock, the specific surface area of the raw materials is increased and the solid phase reaction can be efficiently advanced. The firing temperature is preferably 900 to 1100 ° C. If the firing temperature is too low, the reaction will be insufficient and the lithium ion conductive crystals will not be sufficiently precipitated. On the other hand, if the firing temperature is too high, the lithium ion conductivity tends to decrease due to the growth of crystals other than the lithium ion conductive crystals and the melting of a part of the precipitated crystals.

In the lithium ion conductor of the present invention, the main crystal phase is preferably Li _{1 + x} Al _x Ge _2−x (PO ₄ ) ₃ crystal (0 <x <2). Since crystals other than lithium ion conductive crystals (for example, GeO ₂ , AlPO _4, etc.) cause a decrease in lithium ion conductivity, the content is preferably as small as possible. Specifically, in the XRD (X-ray diffraction) profile, the maximum peak intensity of crystals other than the lithium ion conductive crystal / the maximum peak intensity of the lithium ion conductive crystal is preferably 0.35 or less. It is more preferably 30 or less, and further preferably 0.20 or less.

The lithium ion conductor of the present invention preferably has a lithium ion conductivity of 1.0 × 10 ⁻⁴ S / cm or more, and more preferably 1.7 × 10 ⁻⁴ S / cm or more.

  Next, an example in which the lithium ion conductor of the present invention is used as a solid electrolyte for a lithium ion secondary battery will be described.

  1 and 2 are schematic views of a lithium ion secondary battery using the lithium ion conductor of the present invention as a solid electrolyte for a lithium ion secondary battery.

  In FIG. 1, a lithium ion secondary battery 1 basically includes a positive electrode 2, a negative electrode 3, and a solid electrolyte layer 4. The solid electrolyte layer 4 is disposed between the positive electrode 2 and the negative electrode 3 so as to be in contact with each electrode.

It does not specifically limit as the positive electrode 2 and the negative electrode 3, A well-known material (electrode active material) can be used. As the positive electrode 2, for example, lithium iron phosphate, lithium cobaltate, lithium nickelate, lithium manganate or the like can be used. Examples of the negative electrode 3 include carbon materials such as graphite, pitch coke, fibrous carbon, and soft carbon, metals such as Si and Sn, or SnO—P ₂ O ₅ glass. For the solid electrolyte layer 4, the lithium ion conductor of the present invention is used.

  When a solid electrolyte is used for the lithium ion secondary battery, the interface resistance between the solid electrolyte layer and the electrode tends to increase. Therefore, in order to reduce the interface resistance as much as possible, as shown in FIG. 2, between the positive electrode 2 and the solid electrolyte layer 4 and / or between the negative electrode 3 and the solid electrolyte layer 4, a solid electrolyte (lithium ion conduction) is used. The intermediate layer 5 containing both the body and the electrode active material is preferably formed. Specifically, the intermediate layer 5 is composed of, for example, a mixture of a powdered electrode active material (a positive electrode active material or a negative electrode active material) in the same manner as a powdered solid electrolyte. The ratio of the solid electrolyte in the intermediate layer 5 is appropriately adjusted, for example, in the range of 50 to 100% by volume. Here, it is preferable that the ratio of the solid electrolyte in the intermediate layer 5 gradually decreases as the solid electrolyte layer 4 approaches the positive electrode 2 or the negative electrode 3.

  The intermediate layer 5 may be a green compact of a mixed powder including a solid electrolyte powder and an electrode active material powder. However, it is preferable that the mixed powder is uniformly kneaded with a vehicle to form a paste because handling becomes easy. . A vehicle typically includes a solvent and a resin. The resin is added for the purpose of adjusting the viscosity of the paste. Moreover, surfactant, a thickener, etc. can also be added as needed. The produced paste is applied to the surface of the solid electrolyte layer 4 using an applicator such as a screen printer.

  Some electrode active materials, particularly negative electrode active materials, may be altered when fired in an air atmosphere. Therefore, the paste containing the solid electrolyte and the electrode active material is preferably fired under an inert atmosphere (nitrogen atmosphere or vacuum atmosphere). In that case, it is necessary to use a vehicle having good decomposability even under an inert atmosphere.

  As the resin, aliphatic polypropylene carbonate, aliphatic polyethylene carbonate, aliphatic polybutylene carbonate and the like are preferable because of their good decomposability in an inert atmosphere. Among these, aliphatic polypropylene carbonate is preferable because of its particularly good thermal decomposability.

  As the solvent, N, N′-dimethylformamide, ethylene glycol, dimethyl sulfoxide, dimethyl carbonate, propylene carbonate, butyrolactone, caprolactone, N-methyl-2-pyrrolidone and the like can be used. In particular, propylene carbonate is preferable because of its good thermal decomposability.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to these Examples.

  Tables 1 and 2 show Examples (Sample Nos. 1 to 5) and Comparative Examples (Sample Nos. 6 to 10) of the present invention.

Sample No. 1-3, 5-7, 9, 10 were produced by the crystallized glass method. First, so as to have the composition in the table, aluminum metaphosphate (Al (PO ₃ ) ₃ ), lithium metaphosphate (LiPO ₃ ), germanium oxide (GeO ₂ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) Each raw material powder of lithium fluoride (LiF) was prepared to prepare a raw material batch. Next, the raw material batch was put in a platinum crucible and melted at 1250 ° C. for 30 minutes, and then the obtained molten glass was poured out between a pair of molding rollers to form a film shape while cooling. The obtained glass film was pulverized with a ball mill to obtain a glass powder having an average particle diameter D ₅₀ = 5 μm. The obtained glass powder was press-molded (green compact) at a pressure of 2.5 MPa, then fired at the firing temperature shown in the table for 30 minutes and crystallized to obtain a lithium ion conductor.

Sample No. 4 and 8 were prepared by a solid phase reaction method. In order to obtain the composition in the table, aluminum metaphosphate (Al (PO ₃ ) ₃ ), lithium metaphosphate (LiPO ₃ ), germanium oxide (GeO ₂ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), Each raw material powder of lithium fluoride (LiF) was prepared to prepare a raw material batch. The prepared batch was mixed in a mortar, press-molded (green compact) at a pressure of 2.5 MPa, and fired at the firing temperature shown in the table for 6 hours. The fired body was pulverized in a mortar, pressed again with a pressure of 2.5 MPa, and fired at the firing temperature shown in the table for an additional 6 hours to obtain a lithium ion conductor.

As a result of identifying the crystal phase by XRD measurement, the main crystal phase is Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ crystal (x is about 0.2 to 1), and the sub-crystal phase (lithium ion conductive crystal) It was confirmed that the other crystals were mainly GeO ₂ , LiPO ₃ , and AlPO ₄ .

In the table, Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ crystal is represented as “LAGP”. The sub-crystal phase / LAGP in the table is the maximum peak intensity ratio of each crystal in the XRD measurement, and the larger the ratio, the greater the ratio of the sub-crystal phase. XRD measurement is performed with a powder X-ray diffractometer (RINT2100 manufactured by Rigaku), using X-rays generated by a Cu target at a voltage of 40 KV and a current value of 40 mA, in a range of 2θ = 10 to 60 °, 1 ° / min. It was measured. When the peak intensity of the crystal is 100 cps or less, it can be determined that the peak is noise.

  The lithium ion conductor was processed to a thickness of 1 mm, and the lithium ion conductivity was measured.

The lithium ion conductivity was determined as follows. Measurement was performed in the range of 10 ³ to 10 ⁷ Hz by the AC impedance method, the resistance value of the sample was obtained from the Cole-Cole plot, and the lithium ion conductivity was calculated from the obtained resistance value.

As is apparent from Tables 1 and 2, No. 1 as an example. 1 to 5 lithium ion conductors showed high lithium ion conductivity of 1.9 to 2.6 × 10 ⁻⁴ S / cm. On the other hand, No. which is a comparative example. The lithium ion conductor of No. 6 has too much F. Since the lithium ion conductors Nos. 7 and 8 did not contain F, the lithium ion conductivity was low. No. Although the lithium ion conductors Nos. 9 and 10 contain F, no. No. 9 has too much GeO ₂ , so no. Since No. 10 does not contain Al ₂ O ₃ , the lithium ion conductivity was low.

Claims (5)

In _{mol%, P 2 O 5 25~40%} , GeO 2 25~50%, Li 2 O 10~25%, Al 2 O 3 0~10% ( however, not including 0%), F 0.01 Lithium ion conductor characterized by containing -15%.   2. The lithium ion conductor according to claim 1, wherein the lithium ion conductor is produced by a crystallized glass method.   The lithium ion conductor according to claim 1, wherein the lithium ion conductor is produced by a solid phase reaction method. The lithium ion conductor according to claim 1, wherein the main crystal phase is Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ crystal (0 <x <2). The lithium ion conductor according to claim 1, wherein the lithium ion conductor is used for a solid electrolyte for a lithium ion secondary battery.