TWI727734B - Solid electrolyte - Google Patents

Abstract

The present invention proposes a solid electrolyte which is related to a sulfide-based solid electrolyte containing lithium, phosphorus, sulfur and halogen capable of suppressing the generation of hydrogen sulfide and securing ionic conductivity, as a novel solid electrolyte, is contains Li_7-aPS_6-a (Ha represents halogen, a is 0.2<a ≦ 1.8) having an Argyrodite type crystal structure and Li₃PS₄, in the X-ray diffraction pattern obtained by the X-ray diffraction method (XRD), with respect to the peak intensity appearing at the position of diffraction angle 2 θ =24.9 to 26.3° derived from the Argyrodite type crystal structure, the ratio of peak intensity appearing at position of diffraction angle 2 θ =26.0 to 28.8° derived from Li₃PS₄ is 0.04 to 0.3.

Description

Solid electrolyte

The present invention relates to a solid electrolyte, which can be suitably used as, for example, a solid electrolyte of a lithium secondary battery.

The lithium secondary battery is a secondary battery with a structure in which lithium is eluted from the positive electrode as ions and moves toward the negative electrode during charging, and is encapsulated, and the lithium ions return from the negative electrode to the positive electrode during discharge. Lithium secondary batteries have the characteristics of high energy density and long life, so they are widely used as electrical appliances such as cameras and other home appliances, or laptop personal computers, mobile phones and other mobile electronic devices, power tools, etc. Power supplies for tools, etc., have recently been used in large batteries such as electric vehicles (EV) or hybrid electric vehicles (HEV).

This kind of lithium secondary battery is composed of a positive electrode, a negative electrode, and an ion-conducting layer sandwiched by the two electrodes. From the past, the ion-conducting layer can generally be used in polyethylene, polypropylene, etc. The separation membrane composed of porous membrane meets the requirements of non-aqueous electrolyte. However, in such an ion-conducting layer, an organic electrolyte using a flammable organic solvent as a solvent is used. Therefore, it is necessary to improve the structure/material surface to prevent volatilization or leakage, and it is necessary to improve the suppression of temperature rise during short-circuit. The installation of the safety device or the structure/material surface used to prevent short circuit.

In contrast, if it is _{an all-solid lithium secondary battery using a sulfide-based solid electrolyte such as lithium sulfide (Li 2} S) as a starting material, it does not use a flammable organic solvent, so it can be a safe device It is simplified, and not only can be made to be excellent in manufacturing cost and productivity, but also can be stacked in series in the battery cell to achieve high voltage. Furthermore, since this kind of solid electrolyte does not move due to lithium ions, it is expected that there will be no side reactions caused by the movement of anions, which are related to the improvement of safety or durability.

Regarding this kind of sulfide-based solid electrolyte, for example, Patent Document 1 discloses a crystalline solid electrolyte with a composition formula: Li _x Si _y P _z S _a Ha _w (where Ha contains Br, Any one or more of Cl, I and F. 2.4<(xy)/(y+z)<3.3), and the content of S is 55 to 73 mass%, and the content of Si is 2 to 11 mass %, and the content of Ha element is 0.02% by mass or more.

Patent Document 2 discloses a sulfide-based solid electrolyte for lithium ion batteries, which is cubic crystal and has a crystal structure belonging to the space group F-43m, and contains the composition formula: Li _7-x PS _6-x Ha _x ( Ha is a compound represented by Cl or Br), where x in the aforementioned composition formula is 0.2 to 1.8, and the brightness L* value of the L*a*b* color system is 60.0 or more.

In Patent Document 3, a sulfide-based solid electrolyte compound for lithium ion batteries is disclosed, which contains a cubic crystal phase of Argyrodite type crystal structure, and has a composition formula: Li _7-x+y The _{sulfide-based solid electrolyte compound for lithium ion batteries shown in PS 6-x} Cl _x+y , and the x and y in the aforementioned composition formula satisfy 0.05≦y≦0.9 and -3.0x+1.8≦y≦-3.0x +5.7.

Patent Document 4 discloses a sulfide-based solid electrolyte for lithium-ion batteries, which has a cubic sulfide-germanite crystal structure and contains the composition formula: Li _7-x-2y PS _6-xy Cl _x The compound shown in the foregoing composition formula satisfies 0.8≦x≦1.7, 0<y≦-0.25x+0.5.

[Prior Technical Literature]

[Patent Literature]

[Patent Document 1] WO2015-1818 Publication

[Patent Document 2] WO2015-12042 Publication

[Patent Document 3] WO2016-104702 Publication

[Patent Document 4] WO2016-9768 Publication

Sulfide-based solid electrolytes containing lithium, phosphorus, and sulfur have high ion conductivity. On the other hand, when exposed to moisture in the atmosphere, hydrogen sulfide is generated and the ion conductivity is reduced. In particular, if halogen is contained in the sulfide-based solid electrolyte of this kind, it can be seen that the ion conductivity is further improved, and on the other hand, hydrogen sulfide is further generated.

Therefore, the present invention relates to a sulfide-based solid electrolyte containing not only lithium, phosphorus, and sulfur, but also halogen. It is intended to provide a novel solid electrolyte that can suppress the generation of hydrogen sulfide and can ensure ion conductivity.

The present invention proposes a solid electrolyte, which contains Li _7-a PS _6-a Ha _a (Ha is halogen, a is 0.2<a≦1.8) composed of Argyrodite crystal structure and Li ₃ PS ₄ , and, in the X-ray diffraction pattern obtained by the X-ray diffraction method (XRD), the diffraction angle 2 θ= The ratio of the intensities of the spectral peaks appearing at the position of 24.9 to 26.3° _{is 0.04 to 0.3 at the position of the diffraction angle 2 θ from Li 3} PS _{4 = 26.0 to 28.8°.}

The solid electrolyte proposed in the present invention can ensure ion conductivity while suppressing the generation of hydrogen sulfide. Therefore, even if it is exposed to dry air in a drying room (typically, the moisture concentration is below 100 ppm, and the dew point is below -45°C), the generation of hydrogen sulfide and the deterioration of quality can be suppressed, so it is easy to use industrially. . In addition, when a battery is manufactured using the solid electrolyte proposed by the present invention, the manufacturing operation can be performed with simpler equipment or protective equipment, so that the safety is high and the mass productivity can be improved.

Figure 1 shows the XRD spectra of the compound powders (samples) obtained in Examples 1 to 3 and Comparative Example 1.

Fig. 2 is a graph showing the composition range (range of x and y) of compound powders (samples) obtained in Examples and Comparative Examples.

FIG. 3 is a graph showing the results of producing all-solid battery cells using the compound powders (samples) obtained in Examples 1 and 3 and performing battery evaluation (initial charge and discharge capacity characteristics).

Figure 4 shows the XRD spectra of the compound powders (samples) obtained in Examples 3 and 8.

Next, the present invention will be explained based on examples of implementation types. However, the present invention is not limited to the embodiments described later.

<This solid electrolyte>

The solid electrolyte (referred to as the "present solid electrolyte"), which is an example of the embodiment of the present invention, contains the composition formula (1) composed of the sulphur-germanite crystal structure: Li _7-a PS _6-a Ha _a (Ha is halogen. a is 0.2<a≦1.8.) and composition formula (2): Li ₃ PS ₄ solid electrolyte.

Here, the above-mentioned "Silvergermanite crystal structure" is derived from the crystal structure of the compound group of minerals represented by the _{chemical formula: Ag 8} GeS _6.

Furthermore, as for the compound represented by the aforementioned "Li ₃ PS _{4 ", α-Li 3} PS ₄ , β-Li ₃ PS ₄ , and γ-Li ₃ PS _{4 are known} . In the present invention, when it is described as "Li ₃ PS ₄ ", as long as there is no special statement, it includes all of these meanings. Therefore, the present solid electrolyte system for the aforementioned Li ₃ PS ₄ may contain only one of α-Li ₃ PS ₄ , β-Li ₃ PS ₄ and γ-Li ₃ PS ₄ , or two of them. It may contain all three of them.

_{In addition, the type of Li 3} PS ₄ contained in the solid electrolyte can be confirmed by, for example, an X-ray diffraction pattern obtained by XRD measurement. _{Specifically, in the X-ray diffraction pattern, the presence of α-Li 3} PS ₄ can be confirmed by the appearance of the peak derived from the α phase, and β can be confirmed by the appearance of the peak derived from the β phase -Li ₃ PS ₄ of the present, can be visualized by gamma] derived from the peak phase of the γ-Li confirmed the presence of ₃ PS _4.

Here, in the present invention, any one of the α phase (α-Li ₃ PS ₄ ), β phase (β-Li ₃ PS ₄ ), and γ phase (γ-Li ₃ PS ₄ ) occupies Li _{When the} ratio of 3 PS ₄ is 65% or more in terms of molar ratio, Li ₃ PS ₄ is this phase, that is, it is estimated to be a single phase that occupies 65 molar% or more.

On the other hand, any one of the α phase (α-Li ₃ PS ₄ ), β phase (β-Li ₃ PS ₄ ), and γ phase (γ-Li ₃ PS ₄ ) occupies the existence of _{Li 3} PS ₄ When the ratio is less than 65% in terms of molar ratio, the Li ₃ PS ₄ system is evaluated as a mixed phase of two or three of α phase, β phase, and γ phase.

In the aforementioned composition formula (1), "a" representing the molar ratio of the halogen element is preferably greater than 0.2 and 1.8 or less.

If "a" is greater than 0.2, the cubic sulphur-germanite crystal structure is stable near room temperature, and high ion conductivity can be ensured. If it is less than 1.8, it is easy to control _{the amount of Li 3} PS ₄ produced and can be It is better to improve the conductivity of lithium ions.

From such a viewpoint, "a" is preferably greater than 0.2 and 1.8 or less, and among them, it is 0.4 or more or 1.7 or less, and among them, it is 0.5 or more or 1.65 or less.

When the halogen (Ha) is a combination of Cl and Br, "a" in the above composition formula (1) is the total value of the molar ratio of each element of Cl and Br.

The solid electrolyte is preferably in the XRD pattern obtained by the X-ray diffraction measurement (XRD) using CuK α rays, relative to the diffraction angle 2 θ=24.9 to 24.9 to which is derived from the pyrite crystal structure The peak intensity displayed at the position of 26.3° is preferably the ratio of the peak intensity displayed at the position of the diffraction angle 2 θ=26.0 to 28.8° _{derived from Li 3} PS _{4 of 0.04 to 0.3.} If the ratio is 0.04 or more, the presence of Li ₃ PS ₄ can reduce the amount of hydrogen sulfide generated, so it is preferable. If the ratio is 0.3 or less, it can ensure practical conductivity, so it is more preferable.

Therefore, from such a viewpoint, the ratio is preferably 0.04 to 0.3, and among them, it is 0.06 or more or 0.2 or less, and among them, it is more preferably 0.065 or more or 0.1 or less.

Here, the so-called "spectral peak intensity" means the value of the measured number of X-ray photons (cps) of the largest peak in the range of the above-mentioned diffraction angle 2θ.

For example, when Li ₃ PS ₄ is a single phase composed of β phase (β-Li ₃ PS ₄ ) or γ phase (γ-Li ₃ PS ₄ ), the diffraction angle 2 θ=26.0 to 28.8° Among them, it has the largest number of X-ray photons (cps), and the number (cps) derived from the peak of _{β phase (β-Li 3} PS ₄ ) or γ phase (γ-Li ₃ PS _{4) becomes} The spectral peak intensity of Li ₃ PS _4.

On the other hand, when Li ₃ PS ₄ is a mixed phase of β phase (β-Li ₃ PS ₄ ) or γ phase (γ-Li ₃ PS ₄ ), in the range of diffraction angle 2 θ=26.0 to 28.8°, It has the largest number of X-ray photons (cps), and the number (cps) derived from the _{peak of β phase (β-Li 3} PS ₄ ) or γ phase (γ-Li ₃ PS ₄ ) becomes Li ₃ PS ₄ peak intensity.

In addition, in the XRD measurement using CuK α rays, the peaks appearing at the positions of the diffraction angle 2 θ=24.9 to 26.3° are derived from the peaks of the (220) plane of the pyrite crystal structure.

In addition, in the XRD measurement using CuK α rays, the spectrum peak system that appears at the position of the diffraction angle 2 θ=26.0 to 28.8°, for example, may be derived from the (121) plane of _{β-Li 3} PS _{4, (311)} ) Surface, (400) surface peaks, and (210) surface and (020) surface peaks _{derived from γ-Li 3} PS _4. Therefore, when the present solid electrolyte contains β-Li ₃ PS ₄ , in the XRD measurement of the present solid electrolyte, the (121) plane, (311) plane and (400) are displayed at the position of diffraction angle 2 θ=26.0 to 28.8°. ) The spectral peak of the surface. In addition, when the present solid electrolyte contains γ-Li ₃ PS ₄ , in the XRD measurement of the present solid electrolyte, the peaks of the (210) plane and the (020) plane appear at the position of the diffraction angle 2 θ=26.0 to 28.8° .

This solid electrolyte is a sulfide-based solid electrolyte containing lithium, phosphorus, sulfur, and halogen. If it has the above-mentioned characteristics, it can be considered that any one of them can achieve the same effect.

Among them, if a preferable composition example of the solid electrolyte is cited, the composition formula (3) can be cited: Li _7-xy PS _6-x Ha _xy (Ha is a halogen, Cl or Br, or a combination of these two .X and y are the numerical values that meet the predetermined numerical range and relationship.) The compound shown in. However, the present solid electrolyte is not limited to the compound represented by the above composition formula (3).

In addition, the above-mentioned "composition formula (3)" is a composition formula based on the molar ratio of each element obtained by dissolving the solid electrolyte and measuring the amount of each element. For example, the compound represented by _{Li 3} PS _{4 and Li 7- When} the compound shown in a PS _6-a Ha _a is mixed, it can be calculated based on the combined value of the molar ratio of each compound.

The "x" in the above composition formula (3) is preferably 0.2<x≦1.8.

If the "x" is greater than 0.2, high ion conductivity can be ensured, and if it is 1.8 or less, it is preferable _{because it is easy to control the amount of Li 3} PS _{4 produced.}

From such a point of view, the "x" is preferably 0.2<x≦1.8, wherein it is 0.5 or more, and among them, it is 0.6 or more or 1.7 or less, and in particular, it is more preferably 0.8 or more or 1.6 or less.

When the halogen (Ha) alone is chlorine (Cl), the "x" in the above composition formula (3) is preferably 0.65<x≦1.8, and the "y" in the above composition formula (3) satisfies (- It is preferable that x/3+2/3)<y<(-x/3+1.87) and y<x-0.2 is satisfied.

When halogen (Ha) alone is chlorine (Cl), under the so-called y<x-0.2 condition, if the "y" satisfies (-x/3+2/3)<y, the amount of hydrogen sulfide generated can be reduced, and at the same time And maintain conductivity. On the other hand, furthermore, if y<(-x/3+1.87) is satisfied, when an all-solid-state battery is produced using the solid electrolyte, a high discharge capacity can be expressed, which is preferable.

From this point of view, when halogen (Ha) alone is chlorine (Cl), the "y" is under the condition of y<x-0.2 to satisfy (-x/3+2/3)<y<( -x/3+1.87) is better, among them, it is more preferable to satisfy (-x/3+5/6)<y, or to satisfy y<(-x/3+1.8), and among them, to satisfy y< (-x/3+1.7) is even more preferable, and especially, it is even more preferable to satisfy (-x/3+1)<y, or satisfy y<(-x/3+1.6).

On the other hand, when halogen (Ha) alone is bromine (Br), and when Cl and Br are combined, the "y" in the above composition formula (3) satisfies 0<y<(-x/3+1.87) And it is better to satisfy y<x-0.2.

When the halogen (Ha) is Br alone, or in the combination of Cl and Br, under the so-called y<x-0.2 condition, if the "y" satisfies 0<y, the amount of hydrogen sulfide generated can be reduced while maintaining electrical conductivity rate. On the other hand, if y<(-x/3+1.87) is satisfied, when an all-solid-state battery is produced using the solid electrolyte, it is preferable because it can exhibit a high discharge capacity.

From this point of view, when halogen (Ha) alone is Br or a combination of Cl and Br, the "y" is under the condition of 0.2<xy<1.8 to satisfy 0<y<(-x/3+1.87) ) Is better, where it satisfies (-x/3+2/3)<y, where it satisfies (-x/3+5/6)<y, or satisfies y<(-x/3+1.8 ) Is more preferable, and further, among them, it is more preferable to satisfy y<(-x/3+1.7), and among them, especially to satisfy (-x/3+1)<y or satisfy y<(-x/3 +1.6) is better.

In addition, when the halogen (Ha) is a combination of Cl and Br, the "x-y" in the above composition formula (3) is the total value of the molar ratio of each element of Cl and Br.

Furthermore, the solid electrolyte is preferably more than 30% in molar ratio with respect to the entire compound in the solid electrolyte, wherein 40% or more or 95% or less, wherein 50% or more or 90% or less The ratio contains the compound represented by the composition formula (1): Li _7-a PS _6-a Ha _a .

In addition, the solid electrolyte is preferably 3% or more in molar ratio with respect to the entire compound in the solid electrolyte, wherein 5% or more or 60% or less, wherein 10% or more or 50% or less The ratio contains the compound represented by the composition formula (2): Li ₃ PS ₄ .

Furthermore, for _{the compounds represented by Li 3} PS ₄ , it is preferable to contain β-Li ₃ PS _{4 in an} amount of 50 mol% or more, of which 60 mol% or more, of which 70 mol% The above is particularly good.

At this time, the molar ratio (%) of the above compound can be obtained by performing Rietveld analysis on XRD data.

Even if the solid electrolyte contains substances other than the foregoing, such as unavoidable impurities, its content is less than 5 mol% of the solid electrolyte, preferably less than 3 mol%, and particularly preferably less than 1 mol%. From the viewpoint of low impact on performance, it is better.

(Particle size)

The solid electrolyte is preferably particles, and the D50 (called "average particle size (D50)" or "D50") of the volume particle size distribution obtained by the laser diffraction scattering particle size distribution measurement method is preferred. It is 0.1μm to 10μm.

If the D50 of the present solid electrolyte is 0.1 μm or more, the resistance due to the increase in the surface area of the solid electrolyte particles is increased, or it is difficult to mix with the active material, so it is preferable. On the other hand, if the D50 is 10 μm or less, the active material or the solid electrolyte used in combination is likely to be mixed into the solid electrolyte in the gap, and the contact point and contact area become larger, which is preferable.

From this point of view, the average particle size (D50) of the solid electrolyte is preferably 0.1 μm to 10 μm, among which, it is 0.3 μm or more or 7 μm or less, and among them, 0.5 μm or more or 5 μm or less is more preferable. good.

The average particle diameter (D50) when the solid electrolyte is added to the electrode is preferably 1 to 100% of the average particle diameter (D50) of the positive electrode active material or the average particle diameter (D50) of the negative electrode active material.

If the average particle size (D50) of the solid electrolyte is more than 1% of the average particle size (D50) of the positive electrode active material or the average particle size (D50) of the negative electrode active material, it can be buried between the active materials without gaps, so For better. On the other hand, if it is 100% or less, since the filling rate of the electrode can be increased and the active material ratio can be increased at the same time, it is preferable from the viewpoint of increasing the energy density of the battery.

From this point of view, the average particle diameter (D50) of the solid electrolyte is preferably 1 to 100% of the average particle diameter (D50) of the positive electrode active material or the average particle diameter (D50) of the negative electrode active material, wherein 3% or more or 50% or less, among them, 5% or more or 30% or less is more preferable.

<The manufacturing method of the solid electrolyte>

Next, an example of the manufacturing method of the solid electrolyte will be described. However, the manufacturing method described here is just an example, and it is not limited to this method.

An example of a preferable manufacturing method of the solid electrolyte is to first, for example, _{weigh lithium sulfide (Li 2} S) powder, phosphorous sulfide (P ₂ S ₅ ) powder, and halogen compound powder, respectively, and use a pellet mill , Bead grinder, homogenizer, etc. are preferably pulverized and mixed. However, it is not limited to this manufacturing method.

At this time, in the compound represented by the above composition formula (3): Li _7-xy PS _6-x Ha _xy , the raw material powders are adjusted and mixed so that y>0, the above composition formula (1 ): Li _7-a PS _6-a Ha _a shows the phase of the pyrite type crystal structure, and the above composition formula (2): _{The phase of Li 3} PS ₄ is set in a miscible state. Furthermore, by adjusting y to the above-mentioned preferred range, the intensity of the spectral peak appearing at the position of the diffraction angle 2 θ=24.9 to 26.3° derived from the pyrite crystal structure can be compared to The ratio of the intensity of the spectral peaks derived from the position of the diffraction angle 2 θ=26.0 to 28.8° of _{Li 3} PS _{4 is set in the range of 0.04 to 0.3.}

Examples of the halogen compound system include lithium chloride (LiCl) and lithium bromide (LiBr).

After mixing as described above, it is dried as needed, and then _{fired in an inert environment or under the flow of hydrogen sulfide gas (H 2} S). It can be crushed and crushed as needed, and classified as needed. To get.

In addition, if the temperature of the sulfide material rises, sulfur defects are likely to occur, so in the past, it was sintered with a quartz sample or the like. However, it is difficult to manufacture industrially. In addition, the enclosed quartz sample is sealed, so by heating, the gas contained in the quartz sample expands, the pressure in the quartz sample increases, and there is a risk of rupture. Therefore, it is necessary to make it into a vacuum state as much as possible when enclosed. However, in a vacuum state, sulfur defects are easily generated in the sulfide material.

In contrast, the present solid electrolyte is crystallized from about 200 to 300°C in the relatively low temperature range, so it is preheated in the above low temperature range _{in an inert environment or under the circulation of hydrogen sulfide gas (H 2 S).} It is preferable to perform firing at 350°C or higher. In this way, it is possible to more reliably produce the present solid electrolyte of the sulfide of the intended chemical composition that is stable in crystallization and has almost no sulfur defects.

Among them, when hydrogen sulfide gas is used during firing, the sulfur gas generated by decomposition of hydrogen sulfide during firing can increase the sulfur partial pressure near the firing sample, so it is difficult to generate even at high firing temperatures. Sulfur defects can reduce electronic conductivity. Therefore, when firing in an environment containing hydrogen sulfide gas, the firing temperature is preferably 350 to 650°C, among which 450°C or more or 600°C or less, and 500°C or more or 550°C or less is preferred. Especially good.

In this way, when _{firing is performed under the flow of hydrogen sulfide gas (H 2} S), the sulfur in the sulfide can be fired without defects by firing at 350 to 650°C.

On the other hand, when firing in an inert environment, it is different from the case of hydrogen sulfide gas. During firing, the sulfur partial pressure near the firing sample cannot be increased. Therefore, it is easy to generate at a high firing temperature. Sulfur defects increase electron conductivity. Therefore, when firing in an inert environment, the firing temperature Preferably, it is set to 350 to 500°C, and among them, it is set to 350°C or higher or 450°C or lower, and among them, it is particularly preferably set to be 400°C or higher or 450°C or lower.

In addition, the raw material powder is usually completely reacted to make the unreacted phase disappear. Therefore, it is preferable to sinter the hydrogen sulfide gas at 500°C or higher. When the raw material powder with small particle size and high reactivity is used, even at low temperature It also promotes the reaction, so it can be fired in an inert environment.

In addition, the above-mentioned raw materials are extremely unstable in the atmosphere, and react with moisture to decompose and generate hydrogen sulfide gas or oxidize. Therefore, the raw materials are placed in the furnace through a substituted glove box under an inert gas environment. It is better to perform firing.

By manufacturing in this way, the generation of sulfur defects can be suppressed, and the electronic conductivity can be reduced. Therefore, if an all-solid-state lithium secondary battery is produced using the solid electrolyte, the charge-discharge characteristics or cycle characteristics, which are battery characteristics, can be made good.

<Use of this solid electrolyte>

The solid electrolyte is a solid that allows ions such as Li ions to pass through. Because of its high chemical stability, it can be slurried using polar solvents such as N-methyl-2-pyrrolidone (NMP), acetone, and DMF. Moreover, the electrical conductivity after immersing in these solvents can be maintained at a high level. Specifically, the conductivity after immersion in NMP can be set to 1×10 ^-5 S/cm or more.

The solid electrolyte can be used as a solid electrolyte layer of an all-solid lithium secondary battery, or a solid electrolyte mixed with a positive electrode/negative electrode composite.

The shape of the battery includes, for example, a laminated type, a cylindrical type, and an angular type.

For example, by forming a layer containing the solid electrolyte between the positive electrode and the negative electrode, an all-solid lithium secondary battery can be constructed.

In this case, the present solid electrolyte system is excellent in moisture resistance, and even if it is operated in dry air, there is little degradation of characteristics. Therefore, for example, an all-solid-type lithium secondary battery can be assembled in a dry room.

Here, the layer containing the solid electrolyte is produced by the following method: for example, the slurry composed of the solid electrolyte, the binder and the solvent is dropped onto the substrate, and the method is rubbed and cut with a spatula; after the substrate is in contact with the slurry, The method of air knife cutting; the method of forming a coating film by screen printing, etc., and then heating and drying to remove the solvent, etc. In addition, the powder of the present solid electrolyte may be formed into a compact by a press or the like, and then appropriately processed and manufactured.

From the viewpoint of improving lithium ion conductivity, the layer containing the solid electrolyte preferably has a porosity of 50% or less, and among them, 30% or less, and more preferably 20% or less. Therefore, it is preferable to manufacture by extruding the powder of the present solid electrolyte at a pressure of 20 MPa or more.

Here, the porosity is calculated, for example, from the true density and apparent density of the layer including the solid electrolyte obtained by the liquid phase method (Archimedean method), and calculated by the relational expression shown below.

Porosity = (true density-apparent density) ÷ true density × 100

In addition, the thickness of the layer containing the solid electrolyte is preferably from 5 to 300 μm in terms of short-circuit prevention and capacity balance. Among them, 10 μm or more or 100 μm or less is more preferable.

Moreover, it can also be used as a solid electrolyte layer in which this solid electrolyte and other solid electrolytes are mixed. Specifically, the system includes Li ₂ SP ₂ S ₅ system, Li ₄ P ₂ S ₆ , Li ₇ P ₃ S ₁₁ and the like.

As the positive electrode material, a positive electrode material used as a positive electrode active material of a lithium secondary battery can be suitably used. For example, a positive electrode active material containing lithium, specifically, a spinel-type lithium transition metal compound or a lithium metal oxide having a layered structure can be cited. The energy density can be improved by using high-voltage cathode materials.

In addition to the positive electrode active material, the positive electrode material may also contain a conductive material or other materials.

Regarding the negative electrode material system, a negative electrode material used as a negative electrode active material of a lithium secondary battery can be suitably used. For example, the solid electrolyte is electrochemically stable, so graphite, artificial graphite, and natural stone can be used for charging and discharging with lithium metal or the electronegative potential (about 0.1V vs. Li ^{+ /Li) that rivals lithium metal.} Carbon-based materials such as ink, hard-graphitizable carbon (hard carbon), etc. Therefore, the energy density of the all-solid-state lithium secondary battery can be greatly improved. In addition, silicon or tin, which is promising as a high-capacity material, can also be used as the active material. A lithium secondary battery using a general electrolyte is accompanied by charging and discharging, and the electrolyte reacts with the active material, causing corrosion on the surface of the active material, so the battery characteristics are significantly degraded. The solid electrolyte is used as the electrolyte of the lithium secondary battery, and if silicon or tin is used in the negative electrode, since such corrosion reaction does not occur, the durability of the battery can be improved.

For the negative electrode material, in addition to the negative electrode active material, it can also contain conductive material or other materials.

<Explanation of terms>

In the present invention, the so-called "solid electrolyte" refers to all substances that can ^{directly move in solid state ions such as Li +.}

Furthermore, in the present invention, when it is described as "X to Y" (X and Y are arbitrary numbers), unless otherwise stated, it also includes the meaning of "more than X and less than Y" and "preferably greater than X" Or the meaning of "preferably less than Y".

In addition, when it is described as "X or more" or "X≦" (X is any number), it includes the intention of "more than X is better", and it is described as "Y or less" or "Y≧" (Y is any number). When the number), it includes the intention of "less than Y is better".

Example

Hereinafter, the present invention will be described in more detail based on the following examples and comparative examples.

<Example 1>

The composition of the compound with the cubic pyrite-type crystal structure is Li _5.0 PS _4.4 Cl _1.2 , and the total amount is 5 g, respectively. Weigh lithium sulfide (Li ₂ S) powder and phosphorus pentasulfide (P ₂ S). ₅ ) The powder and lithium chloride (LiCl) powder are pulverized and mixed with a pellet mill for 15 hours. The obtained mixed powder was filled in a container made of carbon, and then heated at 300°C for 4 hours while circulating hydrogen sulfide gas at 1.0 L/min in a tubular electric furnace, and then heated at 500°C for 4 hours. The temperature rise and fall rate is set to 200°C/hour. After that, the sample was crushed in a mortar, and sieved with a 53 μm mesh to obtain a powdery sample. At this time, the aforementioned weighing, mixing, and installation of the electric furnace, taking out from the electric furnace, crushing and screening operations are all carried out in a glove box replaced with fully dried Ar gas (dew point below -60°C) to obtain a composition Formula: Li _5.0 PS _4.4 Cl _1.2 (ie, "x=1.6, y=0.4" in _{Li 7-xy} PS _6-a Ha _{xy) compound powder (sample).}

<Examples 2, 3 and Comparative Examples 1, 4>

The above-mentioned lithium sulfide (Li ₂ S) powder, phosphorus pentasulfide (P ₂ S ₅ ) powder, and lithium chloride (LiCl) powder were weighed and mixed in the composition shown in Table 1, and the rest was the same as in Example 1. Method to obtain compound powder (sample).

<Examples 4 to 6 and Comparative Example 2>

Except that the aforementioned lithium sulfide (Li ₂ S) powder, phosphorus pentasulfide (P ₂ S ₅ ) powder, and lithium chloride (LiCl) powder were weighed and mixed in the composition shown in Table 2, the rest was the same as in Example 1. Method to obtain compound powder (sample).

<Example 7, Example 9, and Comparative Example 3>

The aforementioned lithium sulfide (Li ₂ S) powder, phosphorus pentasulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl), and lithium bromide (LiBr) powder were weighed and mixed in the composition shown in Table 3, and the rest were In the same manner as in Example 1, compound powder (sample) was obtained.

<Example 8>

The aforementioned lithium sulfide (Li ₂ S) powder, phosphorus pentasulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl), and lithium bromide (LiBr) powder were weighed and mixed in the composition shown in Table 3, and then fired Except for heating at 400°C for 4 hours, the rest was the same as in Example 1 to obtain compound powder (sample).

<Comparative Examples 5 to 7>

Except that the aforementioned lithium sulfide (Li ₂ S) powder and phosphorus pentasulfide (P ₂ S ₅ ) powder were weighed and mixed with the composition shown in Table 4, the rest was the same as in Example 1 to obtain compound powder (sample) .

<Determination of Element Composition>

All the compound powders (samples) obtained in the Examples/Comparative Examples were dissolved, and the elemental composition was measured by the ICP emission analysis method. Confirm the composition formula shown in Tables 1 to 4.

The composition ranges (ranges of x and y) of the compound powders (samples) obtained in Examples 1 to 9 and Comparative Examples 1 to 7 are shown in FIG. 2.

<X-ray diffraction measurement>

The compound powder (sample) obtained in the embodiment/comparative example was analyzed by X-ray diffraction method (XRD, Cu line source) to obtain the X-ray diffraction pattern, and the peak intensity (cps) in each position was measured ).

The XRD spectra of the compound powders (samples) obtained in Examples 1 to 3 and Comparative Example 1 are shown in Fig. 1, and the XRD spectra of the compound powders (samples) obtained in Examples 3 and 8 are shown in Fig. 4 in.

In addition, using the XRD device "Smart Lab" manufactured by RIGAKU, it was performed under the conditions of scanning axis: 2 θ/θ, scanning range: 10 to 140 deg, step width 0.01 deg, and scanning speed 1 deg/min. Compared with the intensity of the peaks appearing at the position of the diffraction angle 2 θ=24.9 to 26.3° derived from the pyrite crystal structure, the diffraction angle 2 θ=26.0 to 28.8° _{derived from the Li 3} PS _{4 The ratio (Int(Li 3} PS ₄ )/Int(Li _7-a PS _6-a Ha _a )) of the peak intensities appearing at the position is shown in Tables 1 to 4.

In addition, when the above ratio is less than 0.04, compared to Li _7-a PS _6-a Ha _a , _{the phase of Li 3} PS ₄ is considered to be substantially non-existent. In the table, "Phases other than the sulfide-silver-germanite phase" The column is expressed as "None".

<X-ray Ritterwold analysis>

Using the XRD data of the compound powder (sample) obtained in each example, the Rittwald analysis shown below was carried out, and the whole compound obtained in each example was calculated from the sulfide-silver-germanium ore As a result, it was confirmed that the molar ratio of the compound composed of the type crystal structure was 30 molar% or more.

In addition, using the XRD data of the compound powder (sample) obtained in each Example 1, the following Ritterwald analysis was performed in the same manner to quantify one of the compounds composed of the pyrite type crystal structure As a result, the composition formula was Li _5.55 PS _4.51 Cl _1.53 . This value is fully integrated into the composition formula calculated from the blending ratio of the filling raw material compounds: Li _5.5 PS _4.5 Cl _1.5 (that is, "a=1.5" _{in Li 7-a} PS _6-a Ha _a). Therefore, Tables 1 to 4 are calculated from the compounding ratios of the filling raw material compounds of the compound powders (samples) obtained in the examples/comparative examples, and show the composition of the compound composed of the pyrite type crystal structure: Li _{7 -a} PS _6-a The value of "a" in Ha _a.

The Ritterwold analysis is performed using the XRD data measured under the above conditions with the analysis software "RIETAN-FP v2.8.3". At this time, the appropriateness index is set as R _wp <10 and S<2.0.

<Measurement of the amount of hydrogen sulfide (H _{2 S) produced>}

The compound powders (samples) obtained in the Examples/Comparative Examples were weighed in a glove box replaced with fully dried Ar gas (dew point below -60°C), respectively, weighed 50 mg, and placed in a bag sealed with a laminated film. After that, by mixing the dry air gas and the atmosphere with the adjusted dew point of -30°C and maintaining at room temperature (25°C) in a constant temperature and humidity tank, placed in a glass separable flask with a ^{capacity of 1500 cm 3 to separate} After the inside of the flask is kept the same as the environment in the constant temperature and humidity tank, the sealed bag containing the sample is opened in the constant temperature and humidity tank, and the sample is quickly placed in the separate flask. The sample was placed in a separate flask, and the hydrogen sulfide concentration was measured with a hydrogen sulfide sensor (GX-2009 manufactured by Riken Keiki Co., Ltd.) with respect to the hydrogen sulfide generated until 60 minutes passed after the sealing. Next, the volume of hydrogen sulfide was calculated from the hydrogen sulfide concentration after 60 minutes to obtain the amount of hydrogen sulfide generated.

In Tables 1 to 3, in the composition formula of the pyrite-type crystal structure: Li _7-xy PS _6-x Ha _xy , the amount of hydrogen sulfide generated as a composition of "y=0" is used as a reference. The ratio of the amount of hydrogen sulfide generated when the composition is shifted (that is, y≠0) is displayed (in the table, it is described as the "ratio of the amount of hydrogen sulfide generated with respect to the standard composition of sulphur-germanite"). When y>0, it can be confirmed that the amount of hydrogen sulfide produced has decreased.

<Measurement of ion conductivity>

The compound powders (samples) obtained in the Examples/Comparative Examples were subjected to uniaxial compression molding in a glove box replaced by fully dried Ar gas (dew point below -60°C), and further, CIP (cold room, etc.) Pressurizing device) to produce pellets with a diameter of 10 mm and a thickness of about 4 to 5 mm at 200 MPa. Furthermore, after coating the upper and lower surfaces of the particles with carbon paste as electrodes, they were heat-treated at 180°C for 30 minutes to prepare samples for ion conductivity measurement.

The ion conductivity measurement was performed at room temperature (25°C) using SOLARTRON 1255B manufactured by Toyo Technic, and the ion conductivity (S/cm) was measured by the AC impedance method under the conditions of a measurement frequency of 0.1 Hz to 1 MHz. The results are shown in Tables 1 to 4.

<Production and Evaluation of All Solid State Battery Units>

The compound powders (samples) obtained in Examples 1 and 3 were used as solid electrolytes to prepare a positive electrode compound material and a negative electrode compound material to produce an all-solid battery, and the battery characteristics (initial charge and discharge capacity) were evaluated.

(material)

As for the positive electrode active material, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) powder (D50=6.7μm) is used as the layered compound, and graphite (D50=20μm) is used as the negative electrode active material, which is used in the examples. The obtained sample was used as solid electrolyte powder.

(Combined materials)

The positive electrode compound powder is prepared by mixing the positive electrode active material powder, solid electrolyte powder, and conductive auxiliary agent (acetylene black) powder in a mass ratio of 60:37:3 in a mortar, and performing uniaxial extrusion molding at 20 MPa. The positive electrode compound material particles are obtained.

The negative electrode compound powder is prepared by mixing graphite powder and solid electrolyte powder with a mass ratio of 64:36 in a mortar.

(Production of all solid-state battery cells)

The bottom opening of a polypropylene cylinder (opening diameter: 10.5 mm, height: 18 mm) opened up and down was blocked with a positive electrode (made of SUS), and the positive electrode compound pellets were placed on the positive electrode. The powder solid electrolyte obtained in the example was placed thereon, and uniaxial extrusion was performed at 180 MPa to form a positive electrode compound material and a solid electrolyte layer. After the negative electrode compound powder is placed on it, the negative electrode (made by SUS) is blocked and uniaxially molded at 550 MPa to produce a positive electrode compound with a thickness of approximately 100 μm, a solid electrolyte layer with a thickness of approximately 300 μm, and a negative compound with a thickness of approximately 20 μm. All solid-state battery cells composed of a three-layer structure of materials. At this time, in the production of the above-mentioned all-solid battery cell, it is carried out in a glove box replaced by dry air with an average dew point of -45°C.

(Evaluation of battery characteristics (initial charge and discharge capacity))

The battery characteristics are measured in an environmental testing machine maintained at 25°C, with all solid-state battery cells built in and connected to a charge-discharge measuring device for evaluation. Use 1mA as 1C to charge and discharge the battery. Charge to 4.5V at 0.1C and CC-CV to obtain the initial charge capacity. The discharge system was carried out at 0.1C and in CC mode to 2.5V, and the initial discharge capacity was obtained.

Figure 3 shows the results of the initial charge and discharge capacity characteristics. The discharge capacity when discharged at 0.1C to 2.5V is above 160mAh/g. Since the solid electrolyte ensures practical ionic conductivity, it can be considered that it can exhibit a high discharge capacity.

(Survey)

The XRD data of the compounds (samples) obtained in Examples 1 to 9 were subjected to Ritterwald analysis. As a result, it was confirmed that they contained Li _7-a PS _{6-a composed of a pyrite type crystal structure.} Ha _a (Ha is a halogen. a is 0.2<a≦1.8.) and Li ₃ PS ₄ , and contains a compound composed of a pyrite type crystal structure of 30 mol% or more.

From the results of the XRD data obtained in the above-mentioned Examples/Comparative Examples, the compound powders obtained in Examples 1, 2, 5, 7, and 9 contained Li ₃ PS ₄ in the β phase occupied by _{Li 3} PS _{4 and} The ratio of the γ phase is less than 65% in terms of molar ratio, and it is a mixed phase _{of β-Li 3} PS ₄ and γ-Li ₃ PS _4.

On the other hand, the compound powders obtained in Examples 3, 4, and 6 contained Li ₃ PS ₄ and _{the ratio of the γ phase occupied by Li 3} PS ₄ was 65% or more in molar ratio, and was γ-Li ₃ PS ₄ single phase (γ phase).

_{In addition, the Li 3} PS ₄ contained in the compound powder obtained in Example 8 has a ratio of β-phase occupied by Li ₃ PS ₄ of 65% or more in molar ratio, and it is a single component _{of β-Li 3} PS ₄ Phase (β-phase).

From the above-mentioned examples/comparative examples and the experimental results performed by the inventors so far, it can be known that Li _7-x PS _6-x Ha _x (Ha is halogen. x is 0.2< x≦1.8.) The deviation composition can include Li _7-a PS _6-a Ha _a (Ha is a halogen. a is 0.2<a≦1.8.) and Li ₃ PS _{4. In} this case, by adding Li ₃ PS _{The content of 4} is adjusted to a predetermined range to ensure ion conductivity while suppressing the generation of hydrogen sulfide.

From the above point of view, it can be seen that Li _7-a PS _6-a Ha _a (Ha system represents halogen. A system 0.2<a≦1.8.) and Li ₃ PS _{4 constituted by a pyrite type crystal structure} , And in the X-ray diffraction pattern measured by X-ray diffraction (XRD), relative to the diffraction angle 2 θ=24.9 to 26.3° derived from the pyrite crystal structure As long as the peak intensity displayed at the position of Li ₃ PS ₄ is a solid electrolyte with the ratio of the peak intensity at the position derived from the diffraction angle 2 θ=26.0 to 28.8° of Li 3 PS 4 of 0.04 to 0.3, the ion conductivity can be ensured At the same time, it inhibits the production of hydrogen sulfide.

In addition, from the results of the battery test described above, it was also confirmed that Li _7-a PS _6-a Ha _a composed of a sulphur-germanite crystal structure was contained (Ha series represents halogen. a series 0.2<a≤1.8.) And Li ₃ PS ₄ , and in the X-ray diffraction pattern obtained by the X-ray diffraction method (XRD) measurement, relative to the diffraction angle 2 θ of the aforementioned pyrite-derived crystal structure =24.9 to 26.3°, as long as it is a solid electrolyte with the ratio of the peak intensity at the position _{derived from the diffraction angle of Li 3} PS _{4 2 θ=26.0 to 28.8° of 0.04 to 0.3,} It can be effectively used as a solid electrolyte for lithium secondary batteries.

Claims (5)

A solid electrolyte containing Li _7-a PS _6-a Ha _a and Li ₃ PS ₄ composed of Argyrodite crystal structure, Ha is halogen, a is 0.2<a≦1.8, and , In the X-ray diffraction pattern obtained by the X-ray diffraction method, that is, XRD, relative to the position where the diffraction angle 2θ=24.9 to 26.3° derived from the pyrite crystal structure The ratio of the apparent peak intensity at the position of the diffraction angle 2θ=26.0 to 28.8° derived from Li ₃ PS _{4 is 0.04 to 0.1.} The solid electrolyte according to claim 1, which is composed of a compound represented by the composition formula: Li _7-xy PS _6-x Ha _xy , wherein Ha represents a halogen, Cl or Br, or both When Ha alone is Cl, x is 0.65<x≦1.8, and y meets -x/3+2/3<y<-x/3+1.87, and meets y<x-0.2; Ha alone is Br When and the combination of Cl and Br, x is 0.2<x≦1.8, y is 0<y<-x/3+1.87, and y<x-0.2 is satisfied. A lithium secondary battery provided with the solid electrolyte described in claim 1 or 2. A lithium secondary battery having the solid electrolyte described in claim 1 or 2 and a negative electrode active material containing carbon or silicon. A lithium secondary battery having the solid electrolyte described in claim 1 or 2 and a positive active material containing lithium.

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