JP2013211171A - Method of manufacturing sulfide-based solid electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a new sulfide-based solid electrolyte that can make up an S defect and increase conductivity as compared with a conventional solid electrolyte.SOLUTION: There is provided a method of manufacturing sulfide-based solid electrolyte that is characterized by including: mixing sulfide powder comprising one or two or more selected from the group consisting of lithium sulfide (LiS), phosphorus sulfide (PSor PS), silicon sulfide (SiS), germanium sulfide (GeS), boron sulfide (BS), gallium sulfide (GaS), tin sulfide (SnS or SnS), antimony sulfide (SbSor SbS), and aluminum sulfide (AlS); and baking the mixed powder in a hydrogen sulfide gas atmosphere.

Description

  The present invention relates to a method for producing a sulfide-based solid electrolyte that can be suitably used as a solid electrolyte of a lithium ion battery.

  Lithium-ion batteries are secondary batteries that have a structure in which lithium is dissolved as ions from the positive electrode during charging, moves to the negative electrode and is stored, and reversely returns from the negative electrode to the positive electrode during discharging. It has been widely used as a power source for home appliances such as video cameras, portable electronic devices such as notebook computers and mobile phones, and power tools such as power tools. Then, it is applied also to the large sized battery mounted in an electric vehicle (EV), a hybrid electric vehicle (HEV), etc.

  This type of lithium ion battery is composed of a positive electrode, a negative electrode, and an ion conductive layer sandwiched between the two electrodes. The ion conductive layer includes a separator made of a porous film such as polyethylene or polypropylene, and a nonaqueous electrolytic cell. The one filled with liquid is generally used. However, since an organic electrolyte using a flammable organic solvent as a solvent is used as the electrolyte, it was necessary to improve the structure and materials to prevent volatilization and leakage. It was also necessary to improve the structure and materials in order to prevent the occurrence of short circuits by installing safety devices that suppress the temperature rise.

On the other hand, an all-solid-state lithium battery obtained by fully solidifying a battery using a solid electrolyte such as lithium sulfide (Li ₂ S) does not use a flammable organic solvent. In addition to being excellent in manufacturing cost and productivity, it has a feature that it can be stacked in series in a cell to increase the voltage. Further, in this type of solid electrolyte, since only Li ions do not move, side reactions due to the movement of anions do not occur, and it is expected to lead to improvements in safety and durability.

  This type of sulfide-based solid electrolyte is extremely reactive with moisture and oxygen, and the formation of hydroxide and oxide causes a decrease in conductivity. Usually, it is carried out in an inert gas containing no benzene. Patent Document 1 discloses a method of synthesizing a solid electrolyte by heating and melting in an inert gas flow containing oxygen in a range of 1000 ppm or less.

In Patent Document 2, Li ₂ S and P ₂ S ₅ are mixed in a dry nitrogen atmosphere, the mixed powder is put into a quartz tube, vacuum sealed, and fired at 600 to 700 ° C. to cause a solid phase reaction. Thereafter, a method for synthesizing a target ceramic by rapid cooling is disclosed.

In Patent Document 3, Li ₂ S, SiS _{2 and the} like, which are constituent compounds of a solid electrolyte, are mixed in a predetermined stoichiometric ratio, sulfur gas is added to this mixture, and the above-described compound is added in the presence of excess sulfur. A process for heating and melting the mixture is disclosed.

In Non-Patent Document 1, Li ₂ S, GeS ₂ and P ₂ S ₅ which are constituent compounds of a solid electrolyte are added to a predetermined stoichiometric ratio with 10% excess P ₂ S ₅ added. A method of synthesizing Li _3.25 Ge _0.25 P _0.75 S ₄ by placing it in a carbon pot and firing it at 700 ° C. for 2 hours in an Ar gas atmosphere in a quartz tube capable of gas flow. It has been reported.

Japanese Patent No. 3125506 JP 2001-250580 A Japanese Patent No. 3284215

Journal of Power Sources, Vol. 174, 632-636 (2007)

Since this type of sulfide material tends to cause S deficiency and S deficiency as the heating temperature rises, as described above, conventionally, the sulfide material was often enclosed in a quartz tube and fired. This method has a problem that it is difficult to manufacture industrially.
In addition, a production method has been proposed in which sulfur powder is added to the raw material mixture and the mixture is heated and melted in the presence of an excess of sulfur. However, since sulfur becomes a gas only in excess, sulfur can be compensated for. Is insufficient. In addition, since sulfur that has become gas escapes out of the system, the partial pressure of sulfur gas in the system greatly changes in the reaction between the sulfur gas and the melt at the initial and final stages of the reaction. For this reason, the produced solid electrolyte has a problem that the amount of S is less than the stoichiometric composition ratio.

  Therefore, the present invention is intended to provide a new method for producing a sulfide-based solid electrolyte that can compensate for S deficiency and can increase the electrical conductivity as compared with a conventional solid electrolyte.

The present invention relates to lithium sulfide (LiS ₂ ), phosphorus sulfide (P ₂ S ₃ or P ₂ S ₅ ), silicon sulfide (SiS ₂ ), germanium sulfide (GeS ₂ ), boron sulfide (B ₂ S ₃ ), sulfide One selected from the group consisting of gallium (Ga ₂ S ₃ ), tin sulfide (SnS or SnS ₂ ), antimony sulfide (Sb ₂ S ₃ or Sb ₂ S ₅ ), and aluminum sulfide (Al ₂ S ₃ ) Alternatively, a method for producing a sulfide-based solid electrolyte is proposed in which two or more sulfide powders are mixed and fired in a hydrogen sulfide gas atmosphere.

  According to the method for manufacturing a sulfide-based solid electrolyte proposed by the present invention, a new sulfide-based solid electrolyte that can compensate for S deficiency and can increase the electrical conductivity compared to the conventional solid electrolyte is manufactured. can do.

  Embodiments of the present invention will be described in detail below, but the scope of the present invention is not limited to the embodiments described below.

<Method for producing the present solid electrolyte>
An example of a method for producing a solid electrolyte according to an embodiment of the present invention (hereinafter referred to as “the present production method”) will be described.

The present production method is a method for producing a sulfide-based solid electrolyte, characterized in that lithium sulfide (LiS ₂ ) and sulfide powder are mixed and fired in a hydrogen sulfide gas atmosphere.

(Raw material powder)
Examples of the sulfide powder mixed with lithium sulfide (LiS ₂ ) include phosphorus sulfide (P ₂ S ₃ or P ₂ S ₅ ), silicon sulfide (SiS ₂ ), germanium sulfide (GeS ₂ ), and boron sulfide (B ₂ S _3). ), Gallium sulfide (Ga ₂ S ₃ ), tin sulfide (SnS or SnS ₂ ), antimony sulfide (Sb ₂ S ₃ or Sb ₂ S ₅ ), and aluminum sulfide (Al ₂ S ₃ ). What is necessary is just the sulfide powder which consists of 1 type or 2 types or more. Among these, it is preferable to use a combination of phosphorus sulfide (P ₂ S ₅ ) powder and silicon sulfide (SiS ₂ ) powder.

The particle size of the raw material is not particularly limited.
However, regarding the raw materials, particularly lithium sulfide (Li ₂ S) powder, those having a smaller particle size are preferred because they have higher reactivity and can make the production of the solid electrolyte easier. Among them, it is preferable to use fine lithium sulfide having a D50 (referred to as “average particle size (D ₅₀ )”) of 20 μm or less based on a volume-based particle size distribution obtained by measurement by a laser diffraction / scattering particle size distribution measurement method.
In order to atomize the lithium sulfide (Li ₂ S) powder, the lithium carbonate powder and a gas containing sulfur (S) are brought into contact with each other in a dry manner, and the lithium carbonate powder is heated by heating the lithium carbonate. In the production method for obtaining the above, by reducing the particle size of the lithium carbonate powder as a raw material, the obtained lithium sulfide powder can be made smaller.
In particular, in order to obtain fine lithium sulfide having an average particle diameter (D ₅₀ ) of 20 μm or less, carbonic acid having an average particle diameter of 1/3 to 2/3 of the average particle diameter (D ₅₀ ) of the target fine lithium sulfide. Lithium may be used, and lithium carbonate having an average particle diameter (D ₅₀ ) of 2/5 to 3/5 is more preferably used. More specifically, for example, if the average particle size _{(D 50)} using lithium carbonate powder 8Myuemu～12myuemu, can mean particle size of the lithium sulfide _{(D 50)} and 20μm or less, an average of lithium carbonate powder if the particle diameter of the _{(D 50)} and 4Myuemu～6myuemu, the average particle size of the lithium sulfide _{(D 50)} can be set to 10μm or less, an average particle size of lithium carbonate powder _{(D 50) 0.8μm~1} If the thickness is 0.2 μm, the average particle diameter (D ₅₀ ) of lithium sulfide can be 2 μm or less.

(Mixing of raw materials)
As the raw material mixing means, any appropriate means can be adopted. For example, means such as a ball mill, a bead mill, and a homogenizer can be adopted. When the wet mixing means is employed, it is preferable to select a solvent that does not dissolve the raw material, such as heptane, which is a nonpolar solvent, and to dry sufficiently after mixing.

(Baking)
Firing is preferably performed at 600 to 800 ° C. under a flow of hydrogen sulfide gas (H ₂ S).
When firing raw material powder composed of a mixture of lithium sulfide (LiS ₂ ) and sulfide powder in a hydrogen sulfide gas atmosphere, if the firing temperature is lower than 600 ° C., S is released during firing, and the sulfide-based solid electrolyte However, when the firing temperature is 600 ° C. or higher, the S content tends to increase as compared with the case of firing at a temperature lower than 600 ° C. In this case, the S content of the raw material powder can be sufficiently recovered.
Therefore, from the viewpoint of increasing the conductivity by suppressing S deficiency in the sulfide, the firing temperature is preferably 600 to 800 ° C., and more preferably 650 ° C. or higher or 750 ° C. or lower.
In particular, when a raw material powder composed of a mixture of lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder and silicon sulfide (SiS ₂ ) powder is fired in a hydrogen sulfide gas atmosphere, it is performed at 600 to 750 ° C. Is more preferable.

  Firing may be performed in a closed furnace, but in consideration of industrial production, it is preferably performed in a fluidized furnace.

  The hydrogen sulfide gas concentration in the firing atmosphere is preferably 10 to 100%, and particularly preferably 50 to 100%.

  Since the raw materials and fired products are extremely unstable and easily oxidize or react with moisture in the atmosphere, the raw materials and fired products are set in the furnace through the glove box replaced with an inert gas atmosphere, or the fired products are removed from the furnace. It is preferable to perform a series of operations such as taking out.

Further, since the unreacted H ₂ S gas is a toxic gas, it is preferable that the exhaust gas is completely burned with a burner or the like, then neutralized with a sodium hydroxide solution and treated as sodium sulfide or the like.

<This solid electrolyte>
The sulfide-based solid electrolyte (referred to as “the present solid electrolyte”) obtained by this production method is Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —SiS ₂ , Li ₂ S—P ₂ S. _5- Al ₂ S ₃ , Li ₂ S—P ₂ S ₅ —GeS _2, Li ₂ S—P ₂ S ₅ —B ₂ S ₃ , Li ₂ S—P ₂ S ₅ —Ga ₂ S ₃ , Li ₂ S -P ₂ has S _5-SNS or _{SnS 2, Li 2 S-P} 2 S 5 -Sb 2 S 3 or features having crystallinity such as _Sb 2 _{S 5.} Since this solid electrolyte is all crystalline and has few lattice defects, it has fewer defects compared to amorphous sulfide-based solid electrolytes, excellent chemical stability, and stable with little variation. Characteristics can be obtained.
In addition, it is known that the present solid electrolyte is excellent in ion conductivity, can easily form an interface with an active material at room temperature, and can reduce interface resistance compared to an oxide. Among these, this solid electrolyte is remarkably excellent in electrical conductivity at room temperature.

  Note that the above “crystallinity” means having a structure in which atoms are regularly and periodically arranged three-dimensionally to the extent that a diffraction phenomenon occurs with respect to light having a wavelength of about X-rays. . For this reason, the crystalline sulfide solid electrolyte has a specific crystal structure depending on the type and proportion of atoms contained therein. Therefore, in an X-ray diffractometer, an X-ray diffraction pattern along each crystal structure is used. Is measured.

Among these, as an example of the present solid electrolyte, Li _{7 + x} P _1-y Si _y S ₆ having a structural skeleton of Li ₇ PS ₆ and partially replacing P with Si (where x is −0.6) A sulfide-based solid electrolyte containing ˜0.6 and y is 0.1 to 0.6) can be mentioned.

In the present solid electrolyte, the amount of S contained in the crystal structure is preferably 95 at% or more of the theoretical amount calculated from the stoichiometric composition, more preferably 97% or more, and more preferably 99% or more.
In order to set the amount of S contained in the crystal structure to 95 at% or more of the theoretical amount calculated from the stoichiometric composition, it is preferable to perform firing under a hydrogen sulfide gas (H ₂ S) flow.

<Uses of this solid electrolyte>
The present solid electrolyte can be used as a solid electrolyte layer of an all-solid lithium secondary battery or an all-solid lithium primary battery, a solid electrolyte mixed with a positive electrode mixture, or the like.
For example, an all-solid lithium secondary battery can be formed by forming a layer made of the above solid electrolyte between the positive electrode, the negative electrode, and the positive electrode and the negative electrode.

Here, the layer made of the solid electrolyte can be prepared by, for example, dropping a slurry onto a substrate and scraping it with a doctor blade, cutting with an air knife after contacting the slurry, or a screen printing method.
As the positive electrode material, a positive electrode material used as a positive electrode active material of a lithium ion battery can be used as appropriate.
As the negative electrode material, a positive electrode material used as a positive electrode active material of a lithium ion battery can be used as appropriate.

<Glossary of terms>
In the present invention, the “solid electrolyte” means any substance that can move ions such as Li ^{+ in} the solid state.
Further, in the present invention, when “X to Y” (X and Y are arbitrary numbers) is described, “X is preferably greater than X” or “ The meaning of “smaller than Y” is also included.
Further, when “X or more” (X is an arbitrary number) or “Y or less” (Y is an arbitrary number), the intention of “preferably larger than X” or “preferably smaller than Y” Is included.

  Hereinafter, the present invention will be described based on examples. However, the present invention is not construed as being limited to these.

<Measurement of conductivity>
Samples obtained in Examples and Comparative Examples were uniaxially pressed at a pressure of 200 MPa in a glove box to produce pellets, and further, carbon paste as an electrode was applied on both upper and lower surfaces of the pellets, and then at 180 ° C. for 30 minutes. Heat treatment was performed to prepare a sample for measuring ionic conductivity. The ionic conductivity was measured by the AC impedance method at room temperature (25 ° C.).

<Measurement of product phase and composition ratio>
About the sample obtained by the Example and the comparative example, the production | generation phase was measured by the X ray diffraction method. Each composition ratio was measured by ICP emission spectrometry.

<Example 1>
Lithium sulfide (Li ₂ S) powder 2.15 g, phosphorus sulfide (P ₂ S ₅ ) powder 1.84 g, and silicon sulfide (SiS ₂ ) powder 1.02 g so as to have the composition formula shown in Table 1. Each was weighed and mixed, and pulverized with a ball mill for 12 hours to prepare a mixed powder. The mixed powder is filled in a carbon container, and this is heated at a temperature rising / lowering rate of 300 ° C./h while flowing 1.0 L / min of hydrogen sulfide gas (H ₂ S, purity 100%) in a tubular electric furnace. Baked at 600 ° C. for 4 hours. Thereafter, the sample was crushed in a mortar and sized with a sieve having an opening of 53 μm to obtain a powdery sample.
At this time, the above weighing, mixing, setting in the electric furnace, taking out from the electric furnace, crushing and sizing operations are all carried out in the glove box substituted with sufficiently dried Ar gas (dew point -60 ° C or higher). It carried out in.

<Example 2-10>
A sample was prepared in the same manner as in Example 1 except that the blending amount of each raw material was changed so that the composition formula shown in Table 1 was obtained, and the firing temperature was changed to the temperature shown in Table 1.

<Comparative Example 1-8>
A sample was prepared in the same manner as in Example 1 except that the blending amount of each raw material was changed so that the composition formula shown in Table 1 was obtained, and the firing temperature was changed to the temperature shown in Table 1.

<Comparative Example 9>
Lithium sulfide (Li ₂ S) powder 3.11 g, phosphorus sulfide (P ₂ S ₅ ) powder 1.22 g, and silicon sulfide (SiS ₂ ) powder so as to have the same composition formula as Example 1 shown in Table 1. 0.68 g was weighed and mixed, and pulverized with a ball mill for 12 hours to prepare a mixed powder. The mixed powder was put into a quartz tube having one end closed, and the quartz tube was evacuated with a vacuum pump, and the opening with the quartz tube was melted and sealed with a burner. In addition, in order to suppress reaction with the mixed powder, the quartz tube was coated with carbon spray and then heat-treated to perform carbon coating. This quartz tube was put into a box-type electric furnace and baked at 700 ° C. for 4 hours at a temperature rising / falling rate of 300 ° C./h. Thereafter, the sample was crushed in a mortar and sized with a sieve having an opening of 53 μm to obtain a powdery sample. At this time, the above weighing, mixing, crushing, and sizing operations were all performed in a glove box that was replaced with sufficiently dried Ar gas (dew point -60 ° C. or higher).

In Table 1, “Li ₂ S (fine)” means that the peak of lithium sulfide (Li ₂ S) is detected in the XRD chart, but the peak intensity of Li ₂ S is ₃ % of the peak intensity of the main product phase. %. “Unknown” refers to a case where an unidentifiable peak is detected.
Note that when an impurity phase other than the main product phase is generated, an accurate amount of S cannot be calculated. Therefore, the amount of S was calculated only when the peak of lithium sulfide was not detected and only the peak of the main product phase was detected in the XRD measurement.
“C-Li ₇ PS ₆ ” has a cubic crystal structure, and “o-Li ₇ PS ₆ ” has an orthorhombic crystal structure.

(Discussion)
As shown in Table 1, in the samples of Examples 1 to 10, the main product phase was generated, and unreacted Li ₂ S did not remain or remained only slightly. I understood.
Further, the amount of S contained in the main product phase was 96 at% or more. Even in the case where a slight amount of Li ₂ S remains, the amount of S contained in the main product phase is considered to be 95 at% or more. From this, it was found that S deficiency can be suppressed by firing at 600 to 800 ° C. in a hydrogen sulfide gas atmosphere.
The conductivity was 10 ^-4 S / cm or more in all samples except Example 7 in which the o-Li ₇ PS ₆ structure was formed, which was an extremely high value.
Note that the skeletal structure of Li ₇ PS ₆ has two crystal structures, an orthorhombic crystal having a low ion conductivity and a cubic crystal having a high ion conductivity, a phase transition point around 170 ° C., and a crystal structure around room temperature. Was orthorhombic with low ionic conductivity. Therefore, in Example 7, it was found that a cubic crystal having a high conductivity was obtained by heating once above the phase transition point and then performing a rapid cooling treatment.
Further, lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, and silicon sulfide (SiS ₂ ) powder are mixed and fired at 600 to 750 ° C. in a hydrogen sulfide gas atmosphere. It was found that the obtained samples can significantly increase the conductivity.

On the other hand, the samples up to Comparative Examples 1 to 9 shown in Table 1 have a large number of unreacted Li ₂ S and unidentifiable phases in addition to the main product phase, or the samples having only the main product phase. The amount of S contained in the product phase was also less than 95 at%. As a result, the conductivity was less than 10 ⁻⁴ S / cm and a low value.

Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —SiS ₂ , and Li ₂ S—P ₂ S ₅ —Al ₂ S ₃ based solid electrolytes described in the above examples are lithium sulfide (Li ₂ Since sulfides other than S) form a skeletal structure in the product phase and have a reasonably large diffusion path for Li ions, high conductivity can be obtained. Further, if the amount of S contained in the main product phase is 95 at% or more of the theoretical amount calculated from the stoichiometric composition, the amount of S constituting the skeletal structure is contained without excess or deficiency. It is considered that the disturbance is less and higher conductivity can be obtained.
As in the above example, lithium sulfide (Li ₂ S) and phosphorus sulfide (P ₂ S ₅ ), germanium sulfide (GeS ₂ ), boron sulfide (B ₂ S ₃ ), gallium sulfide (Ga ₂ S ₃ ), Li ₂ S—P ₂ S ₅ —GeS _2, Li ₂ S—P ₂ S ₅ —B ₂ formed of tin sulfide (SnS or SnS ₂ ) and antimony sulfide (Sb ₂ S ₃ or Sb ₂ S ₅ ). S ₃ , Li ₂ S—P ₂ S ₅ —Ga ₂ S ₃ , Li ₂ S—P ₂ S ₅ —SnS or SnS ₂ , Li ₂ S—P ₂ S ₅ —Sb ₂ S ₃ or Sb ₂ S ₅ etc. As for the solid electrolyte having the above crystallinity, a high conductivity can be obtained if a skeletal structure having a moderately large diffusion path for Li ions is formed and the amount of S constituting the skeletal structure is included in excess or deficiency. it is conceivable that.
Therefore, lithium sulfide (LiS ₂ ) powder, phosphorus sulfide (P ₂ S ₃ or P ₂ S ₅ ), silicon sulfide (SiS ₂ ), germanium sulfide (GeS ₂ ), boron sulfide (B ₂ S ₃ ), gallium sulfide One or two selected from the group consisting of (Ga ₂ S ₃ ), tin sulfide (SnS or SnS ₂ ), antimony sulfide (Sb ₂ S ₃ or Sb ₂ S ₅ ), and aluminum sulfide (Al ₂ S ₃ ) It is considered that a sulfide-based solid electrolyte having similar characteristics can be produced by mixing the above-described sulfide powder and firing in a hydrogen sulfide gas atmosphere.

Claims (8)

Lithium sulfide (LiS ₂ ) powder, phosphorus sulfide (P ₂ S ₃ or P ₂ S ₅ ), silicon sulfide (SiS ₂ ), germanium sulfide (GeS ₂ ), boron sulfide (B ₂ S ₃ ), gallium sulfide (Ga) ₂ S ₃ ), tin sulfide (SnS or SnS ₂ ), antimony sulfide (Sb ₂ S ₃ or Sb ₂ S ₅ ), and one or more selected from the group consisting of aluminum sulfide (Al ₂ S ₃ ) A sulfide-based solid electrolyte manufacturing method, comprising mixing a sulfide powder and firing in a hydrogen sulfide gas atmosphere.   The method for producing a sulfide-based solid electrolyte according to claim 1, wherein the temperature for firing in a hydrogen sulfide atmosphere is 600 to 800 ° C. Lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, and silicon sulfide (SiS ₂ ) powder are mixed and fired at 600 to 750 ° C. in a hydrogen sulfide gas atmosphere. A method for producing a sulfide-based solid electrolyte. According to claim 1, D50 due to volume-based particle size distribution obtained by measuring by a laser diffraction scattering particle size distribution measuring method is characterized by using less of lithium sulfide (Li ₂ S) powder 20μm A method for producing a sulfide-based solid electrolyte.   A sulfide-based solid electrolyte obtained by the production method according to claim 1, wherein the sulfide-based solid electrolyte has crystallinity.   The sulfide-based solid electrolyte obtained by the production method according to claim 1, wherein the amount of S contained in the crystal structure is 95 at% or more of the theoretical amount calculated from the stoichiometric composition. A sulfide-based solid electrolyte characterized by that.   The sulfide-based solid electrolyte according to claim 5 or 6, wherein the sulfide-based solid electrolyte is used as an electrolyte of a lithium ion battery. The lithium ion battery provided with the sulfide type solid electrolyte in any one of Claims 5-7.