JP4028920B2 - Method for synthesizing lithium ion conductive solid electrolyte - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for synthesizing a lithium ion conductive solid electrolyte mainly composed of sulfide.
[0002]
[Prior art]
Secondary batteries used in personal digital assistants can be used for a long time, and there is a strong demand for compact and lightweight high-energy density batteries. In particular, research and development on increasing the energy density of lithium secondary batteries has become active. ing. However, many of the lithium secondary batteries currently being developed coexist with a flammable organic electrolyte, a positive electrode active material that acts as an oxidizing agent, and a negative electrode active material that acts as a reducing agent. Therefore, for example, when the battery is overcharged, there is a possibility that an unexpected situation may occur in which metallic lithium is deposited on the negative electrode and the positive electrode and the negative electrode are short-circuited. When this happens, the battery will generate heat and, in extreme cases, cause a burst explosion. For these reasons, as the energy density of batteries increases, securing the safety of lithium secondary batteries is now an important issue.
[0003]
As one of the methods for improving the safety of the lithium secondary battery, there is a method of forming an all solid lithium secondary battery using a nonflammable lithium ion conductive solid electrolyte instead of a flammable organic electrolyte, There is extensive research. In connection with the development of such batteries, various inorganic solid electrolytes have been studied so far, most of which have an ionic conductivity of 10 ⁻⁵ to 10 ⁻⁶ S / cm, which is 2% higher than that of organic electrolytes. ~ 3 orders of magnitude lower, not yet in practical use. In contrast, sulfide-based lithium ion conductive solid electrolyte glass has an ionic conductivity of 10 ⁻³ S / cm (about 10 ⁻⁴ S / cm when powdered) in the state of a glass plate, and is an organic electrolyte. It is known to have the same ionic conductivity as the liquid. Therefore, the expectation of the development of a safe all-solid lithium secondary battery using this solid electrolyte glass is increasing, and its practical application is desired.
[0004]
[Problems to be solved by the invention]
In general, synthesis of a lithium ion conductive solid electrolyte glass containing lithium sulfide is performed by putting a mixture (starting material) made of a plurality of raw materials constituting the glass into a carbon crucible, for example, at a temperature of 850 ° C. or higher (usually , Heated in the vicinity of 1000 ° C.) to melt into a glassy state (amorphous state), and then the glassy melt is poured into liquid nitrogen or twin rollers to soften the glass. The lithium ion conductive solid electrolyte glass is synthesized by rapidly cooling to a temperature (for example, around 300 ° C.) or lower.
[0005]
At this time, at the beginning of the synthesis, a high-temperature sulfide gas is required, or equipment having high temperature resistance and corrosion resistance is required as equipment for synthesizing the solid electrolyte. Moreover, since the generated sulfide gas is harmful, it is also necessary to introduce pollution-free equipment for treating it.
Accordingly, an object of the present invention is to provide a method for synthesizing a solid electrolyte having excellent lithium ion conductivity under a lower temperature condition without such disadvantages.
[0006]
[Means for Solving the Problems]
The gist of the present invention is the gist of the constituents described in claim 1, and is particularly characterized by reacting lithium sulfide with a combination mixture of other sulfides or lithium compounds in a relatively low temperature range. .
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention has found that a solid electrolyte having high lithium ion conductivity can be obtained by reacting a mixture comprising lithium sulfide with another sulfide or lithium compound in a solid phase under a certain temperature condition. Is based on that. Furthermore, in the present invention, the lithium sulfide-containing mixture to be reacted is preferably pulverized, for example, to an average particle size of 10 μm or less, and the mixture comprising such fine particles is reacted in a temperature range of 150 ° C. to 300 ° C. It is important to let If the temperature is less than 150 ° C., the reaction time becomes very long, which is not preferable, and if it exceeds 350 ° C., a crystallization reaction occurs, which is inconvenient. The preferred heating and melting temperature range varies somewhat depending on the type of compound to be combined and the mixing amount, but is 150 to 300 ° C, and the particularly preferred temperature is 250 to 300 ° C.
[0008]
In the method of the present invention, the compound that undergoes a mixed solid phase reaction with lithium sulfide is a sulfide other than lithium sulfide and a lithium compound. Examples of the sulfide include silicon sulfide, phosphorus sulfide, and boron sulfide. In addition, typical examples of the lithium compound include lithium halide and lithium phosphate. Each fine particle of the mixture with the above compound mixed with lithium sulfide forms a solid electrolyte by solid-phase reaction at the contact interface, and lithium ions similar to conventional lithium ion conductive solid electrolyte glass (amorphous state) It turns into a conductor.
[0009]
If the solid electrolyte is in an amorphous state, the conduction path for the movement of ions in the amorphous solid electrolyte is disordered and unrestricted, so that high lithium ion conductivity can be obtained, but it becomes a glass state. When the melt is gradually cooled to form a crystalline solid electrolyte, the solid electrolyte in the crystalline state needs a conduction path for ions to move. In other words, a crystalline ion conductor (solid electrolyte) has a conduction path for ions to move. Therefore, in order to increase ion conductivity between crystalline solid electrolytes, ion conduction between crystal grains can be improved. It is necessary to align the route. However, in a solid electrolyte of a crystalline body, it is extremely difficult to align the infinite number of conduction paths existing in each crystal. In general, a crystalline solid electrolyte has lower ionic conductivity than an amorphous solid electrolyte. It will be a thing.
[0010]
Therefore, in order to synthesize lithium ion conductors by the method of the present invention, it is desirable to make the raw material powder mixture fine, which shortens the reaction time for imparting lithium ion conductivity between the particles. The generated electrolyte can be made almost amorphous, and as a result, a lithium ion conductive sulfide-based solid electrolyte having high ionic conductivity can be obtained. The average particle size of the mixture preferably used in the method of the present invention is 10 μm or less. Moreover, in order to advance this reaction more efficiently, it is desirable to increase the contact opportunity of each particle | grain in a mixture, and it is very desirable to heat these mixtures, stirring.
[0011]
The conventional lithium ion conductive glass has a softening point near 300 ° C., and a crystallization reaction occurs when the temperature is about 350 ° C. or higher. When the temperature is near 850 ° C., the glass becomes a glass state and melts. Lithium ion conductivity is formed. Therefore, when the melt is gradually cooled, there is a crystallization temperature region in the middle of the melt, and in order to avoid this, it is necessary to rapidly cool the melt in a glass state below the softening temperature.
Therefore, in the method of the present invention, it is important to heat the mixture at a temperature of 300 ° C. or less, and the powdery reaction product obtained under such temperature conditions is almost kept in an amorphous state and has excellent lithium conductivity. Have sex. As another material used in combination with a lithium sulfide-containing mixture, when silicon sulfide is used, a lithium ion conductive sulfide system having an ionic conductivity with a high decomposition voltage similar to that of conventional lithium ion conductive glass A solid electrolyte can be obtained.
[0012]
Further, in the method of the present invention, as described above, the lithium sulfide-containing powder mixture is mixed with a ball mill, or a container-fixed mixer such as a spiral type, ribbon type, screw type, high-speed fluid type, or muller type. Or a mixing method using a composite mixer such as a cylindrical type, a twin cylindrical type, a horizontal cylindrical type, a V type or a double cone type, or a ball medium mill such as a vibrating ball mill or a planetary type crusher, or a compression pulverizing type, It is preferable to prepare a homogeneous composition as much as possible using a pulverizer such as an impact compression pulverization mold, a shear pulverization mold, and a friction pulverization mold.
[0013]
【Example】
Example 1
In this embodiment, lithium sulfide (Li ₂ S) having an average particle diameter of 35 μm and silicon sulfide (SiS ₂ ) having an average particle diameter of 50 μm are used as a mixture containing at least lithium sulfide, and these mixtures are mixed in a certain mixed state. Below, the correlation between the reaction time and the ionic conductivity of the mixture when the reaction temperature was varied was investigated.
The mixture used here weighed Li ₂ S and SiS ₂ at a weight ratio of 60:40, mixed lightly in advance in a mortar, and then using a ball mill in a thermostatic chamber at 45 revolutions per minute, A reaction for imparting lithium ion conductivity was performed.
In this case, the reaction time is 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, and 10 hours, and the reaction temperatures in the thermostatic bath are 20 ° C, 100 ° C, 150 ° C, 200 ° C, The temperature was 250 ° C, 300 ° C, and 350 ° C.
[0014]
After the reaction treatment, 300 mg of the mixture was weighed and pelletized by pressurizing with a press using a cylindrical insulating tube made of Teflon with φ = 10 mm. A measurement cell was constructed by arranging metallic lithium (Li foil) on both end faces of a pellet obtained by pressure molding.
About the comprised cell, the ionic conductivity of the solid electrolyte was measured using the alternating current impedance method, and the temperature condition range in which excellent ionic conductivity was obtained was obtained.
The results are shown in FIG. When reacted at 20 ° C., the mixture showed little ionic conductivity regardless of the reaction time.
On the other hand, when the reaction is performed at 100 ° C., the mixture shows low ionic conductivity, but in order to obtain high ionic conductivity, it is necessary to lengthen the reaction time extremely, which has a problem in practicality. I understood. As for this reaction temperature, it was found that the higher the temperature of the reaction, the shorter the ion conductivity, and the higher the ion conductivity. Particularly at 300 ° C., an ionic conductivity of 0.024 × 10 ⁻³ mS / cm 2 was obtained.
However, it has been found that when the temperature is 350 ° C. or higher, the ionic conductivity rapidly decreases. This is probably because the mixture reacted in this temperature range cannot maintain an amorphous state, and the crystallization reaction proceeds.
As described above, it has been found that the reaction temperature is preferably 150 ° C. to 350 ° C. or less.
[0015]
(Example 2)
In this example, Li ₂ S and SiS ₂ were preliminarily pulverized as raw materials constituting the mixture, and the average particle diameter was classified into 4 μm, 8 μm, 10 μm, 12 μm, 16 μm, 20 μm, and 30 μm or less, and classified. After these mixtures were reacted at 300 ° C. for 6 hours, the ionic conductivity was measured in the same manner as in Example 1.
The results are shown in FIG. As can be seen from the figure, it has been found that the ionic conductivity increases rapidly when the average particle size is 10 μm or less. In particular, it was found that a high ionic conductivity of 0.24 × 10 ⁻³ mS / cm 2 can be obtained at 300 ° C. (6 hours treatment). This tendency is similar to the mixture having other compositions. For example, when the composition of Example 2 is treated with a powder having an average particle size of 10 μm under the same processing conditions as these conditions, the ratio is 0. An ionic conductivity of 27 × 10 ⁻³ mS / cm 2 is 0.25 × 10 ⁻³ mS / cm 2 for the composition of Example 3, and 0.18 × for the composition of Example 4. The sample having a composition of 10 ⁻³ mS / cm 2 and Example 5 showed a value of 0.20 × 10 ⁻³ mS / cm 2, and the ionic conductivity was found to be about 1 digit higher.
[0016]
(Example 3)
In this example, Li ₂ S, SiS ₂ and Li ₃ PO ₄ added to the mixture used in Example 1 were used, and the composition of each mixture was 63: 36: 1 by weight. A test similar to Example 1 was performed except that it was used.
As a result, almost the same result as in Example 1 was given.
That is, when reacted at 20 ° C., the mixture showed almost no ionic conductivity regardless of the reaction time. At 100 ° C., the mixture showed low ionic conductivity. It was also found that the higher the temperature, the higher the ionic conductivity, and the higher the ionic conductivity.
In particular, it was found that an ionic conductivity of 0.036 × 10 ⁻³ mS / cm 2 can be obtained at 300 ° C. (6 hours treatment).
[0017]
Example 4
In this example, the same test was performed using a mixture obtained by adding lithium iodide (LiI) to the mixture (Li ₂ S, SiS ₂ ) used in Example 1. The composition condition of the mixture at that time was 36:24:40 by weight.
As a result, almost the same result as in Example 1 was shown. That is, when reacted at 20 ° C., the mixture showed almost no ionic conductivity regardless of the reaction time. However, it was found that when the reaction was carried out at 100 ° C. or higher, the mixture showed ionic conductivity. When compared with the results of Example 1, it was found that high ionic conductivity was obtained in a shorter time. In particular, an ion conductivity of 0.031 × 10 ⁻³ mS / cm 2 was obtained at 300 ° C. (5 hours treatment).
[0018]
(Example 5)
In this example, instead of the mixture (Li ₂ S, SiS ₂ ) used in Example 1, a mixture of lithium sulfide (Li ₂ S) and phosphorus sulfide (P ₂ S ₅ ) (weight ratio 67:33) The test was performed in substantially the same manner as in Example 1 except that was used. As a result, almost the same result as in Example 1 was shown. That is, when reacted at 20 ° C., the ionic conductivity of the mixture was hardly shown regardless of the reaction time. Furthermore, when it was made to react at 100 degreeC or more, it turned out that a mixture comes to show ionic conductivity. Moreover, it turned out that high ion conductivity is obtained in a shorter time compared with Example 1. In particular, it was found that an ionic conductivity of 0.028 × 10 ⁻³ mS / cm 2 can be obtained at 300 ° C. (4 hours treatment).
[0019]
(Example 6)
In this embodiment, the mixture used in Example _{1 (Li 2 S, SiS 2} ) in place of lithium sulfide (Li ₂ S), a mixture of phosphorus sulfide (B ₂ S ₃₎ (50:50 weight ratio) The test was performed in substantially the same manner as in Example 1 except that was used. As a result, almost the same result as in Example 1 was shown. That is, when reacted at 20 ° C., the mixture did not exhibit ionic conductivity regardless of the reaction time. However, it was found that when the reaction was carried out at 100 ° C. or higher, the mixture showed low ion conductivity. Compared with the results of Example 1, it was found that high ionic conductivity was obtained in a shorter time. In particular, at 300 ° C. (4 hours treatment), an ionic conductivity of 0.030 × 10 ⁻³ mS / cm 2 was obtained.
[0020]
As described above, in the embodiment of the present invention, the mixture containing sulfide is 0.6Li ₂ S-0.4SiS ₂ , 0.01Li ₃ PO ₄ -0.63Li ₂ S- 0.36SiS ₂ , 0.4LiI. -0.36Li ₂ S- 0.24 SiS ₂ , 0.67Li ₂ S- 0.33P ₂ S ₅ , 0.5Li ₂ S- 0.5B ₂ S ₃ were explained, but the mixing ratio of these mixtures As materials to be mixed with lithium sulfide, such as different materials of Li ₂ S and GeS ₂ , materials not described in the examples, such as lithium chloride (LiCl), lithium bromide (LiBr), etc. other and lithium halide, a _{LiI- Li 2 S- SiS 2 -P 2} S 5, LiI- Li 3 PO 4 -Li 2 S- SiS 2 mixture comprising 4 or more different raw materials powders such as using But the same The results obtained are about to readily understood by those of skill in the art, to be included in the scope of the present invention is naturally not limited to that has been described in the Examples.
[0021]
In this example, a normal ball mill was used as a mixing method for efficiently carrying out the reaction, but other mixing means such as other ball medium mills such as a planetary ball mill and a vibrating ball mill, or a container-fixed mixing type. Needless to say, similar results can be obtained in other mixing methods not described in the examples, such as a mixer, a composite mixer, etc., and these are one means for carrying out the present invention. This category is included.
[0022]
【The invention's effect】
By reacting a powder mixture containing lithium sulfide at a temperature of 150 ° C. or higher and 300 ° C. or lower, an inorganic solid electrolyte having high lithium ion conductivity can be easily and efficiently produced. As a result, since there is almost no generation of sulfide gas, it is not particularly necessary to synthesize the synthesis apparatus with a material having a high corrosion resistance.
Moreover, an inorganic solid electrolyte having higher ionic conductivity can be synthesized by using silicon sulfide or a starting material having a small average particle diameter.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between ion conductivity, reaction temperature, and reaction time.
Mixture used: {Li ₂ S: SiS ₂ (composition ratio = 60: 40)}
Reaction temperature: (20 ° C, 100 ° C, 150 ° C, 200 ° C, 250 ° C, 300 ° C, 350 ° C)
FIG. 2 is a graph showing the relationship between ion conductivity and particle size.
Mixture used: {Li ₂ S: SiS ₂ (composition ratio = 60: 40)}
Reaction conditions: Reaction at 300 ° C. for 6 hours

Claims (3)

A method of synthesizing a lithium ion conductive solid electrolyte, comprising reacting a mixture of lithium sulfide with at least one selected from other sulfides and lithium compounds at a temperature of 150 ° C or higher and 300 ° C or lower. The method for synthesizing a lithium ion conductive solid electrolyte according to claim 1, wherein the other sulfide is selected from silicon sulfide, phosphorus sulfide, and boron sulfide, and the lithium compound is selected from lithium halide and lithium phosphate. . The method for synthesizing a lithium ion conductive solid electrolyte according to claim 1 or 2, wherein the mixture is adjusted to a powder having an average particle size of 10 µm or less.

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