TWI566454B - Sulfide solid electrolte for lithum ion battery and lithum ion battery - Google Patents

Description

Sulfide solid electrolyte for lithium ion battery and lithium ion battery

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

A lithium ion battery is a secondary battery in which lithium is eluted from a positive electrode and moved to a negative electrode and is occluded during charging, and a lithium ion is reversely returned from a negative electrode to a positive electrode during discharge, due to its energy density. Because it is characterized by its large size and long life, it is widely used in power supplies such as portable electronic products such as cameras, notebook personal computers, and mobile phones, such as portable electronic devices and power tools. It has also recently been applied to electric vehicles (EVs). ), a large battery equipped with a hybrid electric vehicle (HEV).

Such a 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 is generally filled with a porous film made of polyethylene or polypropylene. Separator. However, since an organic electrolyte using a flammable organic solvent as a solvent is used as an electrolyte, in addition to the structure/material surface for preventing volatilization and leakage, it is necessary to install a safety device and an improvement for suppressing the temperature rise during short circuit. To prevent short circuit structure / material surface.

On the other hand, a solid electrolyte using lithium sulfide (Li ₂ S) or the like as a starting material is used, and since the solid-state lithium battery in which the battery is fully solidified does not use a combustible organic solvent, the safety device can be simplified. In addition to being excellent in manufacturing cost and productivity, it is also characterized in that a series of layers are stacked in a cell to increase the voltage. Further, in such a solid electrolyte, since it does not move other than Li ions, it is expected that a side reaction due to movement of anions does not occur in association with improvement in safety and durability.

The solid electrolyte used in such a battery requires ion conductivity to be as high as possible and chemically stable, and it is known that, for example, lithium halide, lithium nitride, lithium oxyacid salt, or the like are candidates. material.

With regard to such a solid electrolyte, for example, Patent Document 1 discloses a sulfide-based solid electrolyte in which a high-temperature lithium ion conductive compound including lithium phosphate (Li ₃ PO ₄ ) exists in the general formula Li ₂ SX (only, X represents SiS). _2, GeS _2, B ₂ S ₃ in at least one of sulfide) shown in the lithium ion conductive sulfide glass.

Further, Patent Document 2 discloses a sulfide-based solid electrolyte which is a crystalline material and exhibits a very high ionic conductivity of 6.49 × 10 ^-5 Scm ^-1 as an ion conductivity at room temperature, and is contained as a general formula Li. A lithium ion conductive material of a composite compound represented by ₂ S-GeS ₂ -X (except that X represents at least one of Ga ₂ S ₃ and ZnS).

Patent Document 3 discloses a lithium ion conductive sulfide ceramic as a sulfide ceramic having high lithium ion conductivity and high decomposition voltage, which is mainly composed of Li ₂ S and P ₂ S ₅ and has Li ₂ as a molar %. The composition of S = 82.5 to 92.5 and P ₂ S ₅ = 7.5 to 17.5, and preferably has a composition of a molar ratio of Li ₂ S/P ₂ S ₅ = 7 (composition formula: Li ₇ PS ₆ ).

Patent Document 4 discloses a lithium ion conductive material having a chemical formula: Li ⁺ _(12-nx) B ⁿ⁺ X ^2- _(6-x) Y ^- _x (B ⁿ⁺ is selected from P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb and Ta and at least one, X ^2- is selected from S, Se, Te, and at least one of the, Y ^- is selected from F, Cl, Br, I, At least one of CN, OCN, SCN, and N ₃ , 0≦x≦2), a sulphur-silver-mineral-type crystal structure.

Patent Document 5 discloses a lithium sulphide yttrium ore as a solid compound which is not only highly reactive with lithium ions but which can be monolayer blended, and is of the formula (I) Li ⁺ _(12-nx) B ⁿ⁺ X ^2- _6-x Y ^- _x lithium sulphide ore, wherein B ⁿ⁺ is selected from the group consisting of P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb and Ta, X ^2- It is selected from the group consisting of S, Se and Te, and the Y ^- line is selected from the group consisting of Cl, Br, I, F, CN, OCN, SCN, N ₃ , 0≦x≦2.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 3184517

[Patent Document 2] Japanese Patent No. 3744665

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2001-250580

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2011-96630

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2010-540396

The inventors of the present invention focused on a solid electrolyte material for a lithium ion battery, focusing on a composition having a crystal structure belonging to a cubic F-43m space group: Li _7-x PS _6-x Ha _x (Ha-based Cl or Br) The compound shown.

However, attempts have been made to use this compound as a solid electrolyte for a lithium ion battery, which has revealed problems such as high electron conductivity and inability to improve charge/discharge efficiency or cycle characteristics as expected.

Accordingly, the present invention relates to a compound having a space group F-43m and a cubic crystal structure and having the composition formula: Li _7-x PS _6-x Ha _x (Ha-based Cl or Br), and providing a loanable A novel sulfide-based solid electrolyte for a lithium ion battery which has improved lithium ion conductivity and reduced electron conductivity to improve charge and discharge efficiency and cycle characteristics.

The present invention proposes a sulfide-based solid electrolyte for a lithium ion battery, which has a crystal structure having a cubic crystal F-43m space group and has a composition formula: Li _7-x PS _6-x Ha _x (Ha-based Cl or Br) In the compound shown, x in the above composition formula is 0.2 to 1.8, and the L*a*b* color system has a luminance L ^* value of 60.0 or more.

The present invention further proposes a method for producing a sulfide-based solid electrolyte for a lithium ion battery, which comprises lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl) powder or lithium bromide ( LiBr) powder is mixed, fired at 350 to 500 ° C under an inert gas atmosphere, or fired at 350 to 650 ° C in an atmosphere containing hydrogen sulfide gas.

The sulfide-based solid electrolyte of the present invention is characterized in that it has low lithium ion conductivity and high crystallinity, and has high lithium ion conductivity, low electron conductivity, and high lithium ion transport number. . Therefore, when a solid electrolyte as a lithium ion battery is used, charge and discharge efficiency and cycle characteristics can be improved.

Fig. 1 is a view showing an XRD pattern of an example sample when Ha is Cl.

Fig. 2 is a view showing an XRD pattern of an example sample when Ha is Br.

Fig. 3 is a view showing an XRD pattern of a comparative sample when Ha is Cl.

Fig. 4 is a view showing an XRD pattern of a comparative sample when Ha is Br.

Fig. 5 is a graph showing the charge and discharge characteristics of the all-solid InLi/NCM battery in the first and third cycles at the time of battery evaluation using the sample obtained in Example 5.

Fig. 6 is a graph showing the charge and discharge characteristics of the first and third cycles of the all-solid InLi/NCM battery produced by using the sample obtained in Example 13.

Fig. 7 is a view showing the preparation of an all-solid InLi/NCM battery using the sample obtained in Comparative Example 1, and charging and discharging in the first and third cycles at the time of battery evaluation. A map of the characteristics.

Fig. 8 is a graph showing the charge and discharge characteristics of the all-solid Gr/NCM battery in the first and third cycles at the time of battery evaluation using the sample obtained in Example 5.

Fig. 9 is a graph showing the capacity retention rate at the time of battery evaluation using the sample obtained in Example 5 to prepare an all-solid Gr/NCM battery.

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

The sulfide-based solid electrolyte of the present embodiment (abbreviated as "the present solid electrolyte") contains a crystal structure having a cubic crystal F-43m space group and has a composition formula (1): Li _7-x PS _6-x Ha _x ( A sulfide-based solid electrolyte of a compound represented by Ha-based Cl or Br).

In the above composition formula (1): Li _7-x PS _6-x Ha _x , the _x indicating the content of the Ha element is preferably 0.2 to 1.8. When x is from 0.2 to 1.8, the crystal structure of the cubic crystal system F-43m space group can be obtained, and the formation of an impurity phase can be suppressed, so that lithium ion conductivity can be improved.

From this point of view, x is preferably from 0.2 to 1.8, wherein x is particularly preferably 0.6 or more or 1.6 or less.

In the above composition formula, when Ha is Cl, x is preferably 0.2 to 1.8, particularly preferably 0.4 or more, and particularly x is 0.6 or more or 1.6 or less, and particularly preferably 0.8 or more or 1.2 or less. Further, when Ha is Cl, it has been thought that if Cl is less than 0.4, it will be dominated by orthorhombic crystals. However, the subsequent test results can be confirmed by further carrying out the original In the mixing and mixing step, if x is Cl and x is 0.2 or more, it becomes cubic crystal.

On the other hand, when Ha is Br, x is preferably 0.2 to 1.2, particularly x is 0.4 or more or 1.2 or less, and particularly preferably 0.6 or more or 1.2 or less.

The sulfide-based solid electrolyte is known to have excellent ion conductivity, and it is easy to form an interface with an active material at room temperature, and the interface resistance can be made low. Among these, the solid electrolyte has low sulfur deficiency and high crystallinity, and thus has low electron conductivity and is particularly excellent in lithium ion conductivity.

Further, the Li ₇ PS ₆ system having the same skeleton structure as Li _7-x PS _6-x Ha _x has an orthorhombic crystal (space group Pna2 ₁ ) having low lithium ion conductivity and a cubic crystal having high lithium ion conductivity ( The two crystal structures of the space group F-43m) have a phase transition point of about 170 ° C, and the crystal structure of about room temperature is an orthorhombic crystal having low ion conductivity. Therefore, as shown in the aforementioned Patent Document 3, in order to obtain a cubic crystal having high ion conductivity, it is usually quenched after heating to a phase transition point or more. However, in the case of the compound of the above composition formula, since the phase transition point is not present at a temperature above room temperature, the crystal structure can maintain a cubic system having high ion conductivity even at room temperature, and therefore, even if it is not subjected to quenching or the like It is particularly good in this respect to ensure high ionic conductivity.

The solid electrolyte preferably contains substantially no phase including lithium sulfide, lithium chloride or lithium bromide. In the case of a single phase of Li _7-x PS _6-x Ha _x , the charge and discharge efficiency and cycle characteristics are better when the battery is assembled, which is more preferable.

Here, "substantially free of phases including lithium sulfide, lithium chloride and lithium bromide" means that the peak intensity of lithium sulfide, lithium chloride and lithium bromide in the XRD chart is less than Li _7-x PS _6-x Ha _x The case of 3% of the peak intensity.

(Lithium ion migration number)

In the present solid electrolyte, the lithium ion migration number is preferably 90% or more, particularly 95% or more, and particularly preferably 99% or more.

When the lithium ion migration number is 90% or more, the electron conductivity can be further lowered, and the battery characteristics can be further improved.

In the method for making the lithium ion migration number of the solid electrolyte 90% or more, for example, in the above composition formula (1), x is adjusted to 0.4 to 1.8 when Ha is Cl, and Ha is In the case of Br, x is adjusted to 0.2 to 1.2, and lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl) powder or lithium bromide (LiBr) powder is used in a ball mill or the like as will be described later. The mixture is fired at 350 to 500 ° C under an inert gas atmosphere or at 350 to 650 ° C in an atmosphere containing hydrogen sulfide gas. But not limited to this method.

(brightness)

In the solid electrolyte, the L*a*b* color system has a luminance L ^* value of preferably 60.0 or more, particularly 70.0 or more, and particularly preferably 75.0 or more. It is considered that when there are many sulfur defects in the solid electrolyte, the sulfur defect absorbs visible light and the brightness is lowered. Further, when the sulfur in the solid electrolyte is deficient, electrons are generated by the following formula (1), and electron conductivity is exhibited.

(1) Formula...Ss→Vs ^{. .} +2e ^' +1/2S ₂ ↑

Ss: sulfur element, Vs, present in the sulfur position in the solid electrolyte ^{. .} : sulfur position in the solid electrolyte, e ^' : generated electrons, S ₂ ↑: sulfur molecules detached from the solid electrolyte

In the method for suppressing the generation of sulfur deficiency in the solid electrolyte and setting the luminance L ^* value to 60.0 or more, for example, in the above composition formula (1), x is adjusted to 0.4 when Ha is Cl. 1.8, when x is Br, the x is adjusted to 0.2 to 1.2, and lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl) powder or lithium bromide (LiBr) powder is used. The method described later is mixed using a ball mill or the like, fired at 350 to 500 ° C in an inert gas atmosphere, or fired at 350 to 650 ° C in an atmosphere containing hydrogen sulfide gas. But not limited to this method.

(Production method)

Next, an example of the method for producing the solid electrolyte will be described. However, the manufacturing method described here is only one example, and is not limited to this method.

Preferably, the solid electrolyte is, for example, weighed lithium sulfide (Li ₂ S) powder, phosphorus sulfide (P ₂ S ₅ ) powder, lithium chloride (LiCl) powder or lithium bromide (LiBr) powder, and uses a ball mill, a bead mill, A homogenizer or the like is used to pulverize and mix.

In this case, the pulverization and mixing is performed by mechanically pulverizing and mixing, such as mechanical alloying, to reduce the crystallinity of the raw material powder or to make the raw material powder homogeneous, or to homogenize the raw material mixed powder, and the bond between the cation and the sulfur is broken. Sulfur is detached during firing, and sulfur defects are formed, and electron conductivity is exhibited. Therefore, it is preferred to carry out pulverization and mixing to the extent that the crystallinity of the raw material powder can be maintained.

After mixing as described above, it may be dried as needed, and then fired in an inert gas atmosphere or hydrogen sulfide gas (H ₂ S), crushed or pulverized as needed, and classified as necessary.

Further, when the temperature of the sulfide material rises, sulfur is desorbed and sulfur deficiency is likely to occur. Therefore, it has been conventionally baked by a quartz ampoule or the like. However, such a situation can be difficult to manufacture in the industry. Moreover, since the sealed quartz ampoule is sealed, the heating causes the gas contained in the quartz ampoule to expand, so that the pressure inside the quartz ampoule rises and there is a crack. Therefore, it must be as vacuum as possible during sealing. However, in a vacuum state, sulfur in the sulfide material is detached to easily cause sulfur deficiency.

On the other hand, since the solid electrolyte is crystallized at about 200 ° C, it can be synthesized by firing at a lower temperature. Therefore, by firing in an inert gas atmosphere or hydrogen sulfide gas (H ₂ S) at 350 ° C or higher, it is possible to produce a solid electrolyte having a sulfur-free defect and a composition of a chemical composition.

In the case where hydrogen sulfide gas is used during the firing, the sulfur gas generated by the decomposition of hydrogen sulfide during firing can increase the sulfur partial pressure in the vicinity of the fired sample, so that it is difficult to form sulfur even at a high firing temperature. Defects can reduce electron conductivity. Therefore, when firing in an atmosphere containing hydrogen sulfide gas, the firing temperature is preferably from 350 to 650 ° C, especially from 450 ° C to 600 ° C, and particularly preferably from 500 ° C to 550 ° C.

When fired under the flow of hydrogen sulfide gas (H ₂ S), the sulfur in the sulfide can be fired without being damaged by firing at 350 to 650 °C.

On the other hand, when it is fired in an inert gas atmosphere, it is different from the case of hydrogen sulfide gas, and the firing test cannot be improved due to firing. Since the partial pressure of sulfur in the vicinity of the material is high, the sulfur deficiency is likely to occur at the high firing temperature, and the electron conductivity is high. Therefore, when firing in an inert gas atmosphere, the firing temperature is preferably 350 to 500 ° C, especially 350 ° C or more or 450 ° C or less, and particularly preferably 400 ° C or more or 450 ° C or less.

In addition, since the unreacted phase is usually completely reacted and the unreacted phase disappears, it is preferred that the hydrogen sulfide gas is circulated and fired at 500 ° C or higher. However, when a raw material powder having a small particle size and high reactivity is used, Even if the reaction is promoted at a low temperature, it can be fired in an inert gas atmosphere.

Further, since the raw material and the fired material are extremely unstable in the atmosphere and easily react with water and decompose to generate hydrogen sulfide gas or oxidize, it is preferable to carry out the process by replacing the inertia atmosphere into a handle box. The raw material is placed in the furnace and a series of operations of the fired product are taken out from the furnace.

By manufacturing in this way, generation of sulfur defects can be suppressed, and electron conductivity can be lowered. Therefore, when an all-solid lithium ion battery is produced using the solid electrolyte, the charge and discharge characteristics and cycle characteristics of the battery characteristics can be improved.

Further, since the unreacted hydrogen sulfide gas is a toxic gas, it is preferred to treat the exhaust gas completely after combustion using a burner or the like, and then neutralize it with sodium hydroxide solution as sodium sulfide or the like.

<Use of the solid electrolyte>

As the solid electrolyte, a solid electrolyte layer as an all solid lithium secondary battery or an all solid lithium primary battery, a solid electrolyte mixed in a positive electrode/negative electrode mixture, or the like can be used.

For example, by forming a positive electrode, a negative electrode, and between the positive electrode and the negative electrode The layer formed of the solid electrolyte constitutes an all-solid lithium secondary battery.

Here, the layer formed of the solid electrolyte may be dropped onto the substrate by, for example, a slurry including a solid electrolyte and a binder and a solvent, using a doctor blade or the like, and a pneumatic blade is used after the slurry is contacted. It is produced by a cutting method, a screen printing method, or the like. Alternatively, the powder of the solid electrolyte may be produced by a press or the like, and then appropriately processed.

As the positive electrode material, a positive electrode material used as a positive electrode active material of a lithium ion battery can be suitably used.

As the negative electrode material, a negative electrode material used as a negative electrode active material of a lithium ion battery can be suitably used. However, in terms of electrochemical stability, the solid electrolyte can use artificial graphite, natural graphite, and non-graphitizable carbon (hard carbon) which are charged and discharged at a low potential (about 0.1 V vs Li ⁺ /Li) equivalent to lithium metal. (hard carbon)) and other carbon-based materials. Therefore, by using a carbon-based material as the negative electrode material, the energy density of the all-solid lithium secondary battery can be greatly improved. Therefore, for example, a lithium ion battery having the present solid electrolyte and a negative electrode active material containing carbon such as artificial graphite, natural graphite, or non-graphitizable carbon (hard carbon) can be configured.

<Explanation of terms>

In the present invention, "solid electrolyte" means an ion that maintains a solid state, such as Li ⁺ movable.

Further, in the present invention, when "X to Y" (X, Y is an arbitrary number), unless otherwise noted, "X or more and Y or less" and "better than X" are included. Or "better than Y".

Also, it is described as "X or more" (X is an arbitrary number) or "Y or less" (Y is an arbitrary number), and the meaning of "better than X" or "better than Y" is included.

(Example)

Hereinafter, the present invention will be described based on examples. However, the invention is not limited to the explanations.

(Example 1)

2.14 g of lithium sulfide (Li ₂ S) powder, 2.07 g of phosphorus sulfide (P ₂ S ₅ ) powder, and 0.79 g of lithium chloride (LiCl) powder were respectively weighed in a composition formula shown in Table 1, and pulverized and mixed using a ball mill. The mixed powder was prepared for 15 hours. The mixed powder was filled in a carbon container, and the hydrogen sulfide gas (H ₂ S, purity 100%) was passed through a tubular electric furnace while flowing at a temperature of 200 ° C / hour and 400 ° C at a temperature rise and fall of 4 ° / min. hour. Then, the sample was crushed using a mortar, and a particle size fractionation was carried out using a mesh sieve having a pore size of 53 μm to obtain a powdery sample.

At this time, the weighing, mixing, setting in the electric furnace, taking out from the electric furnace, crushing, and particle size classification were all carried out in a hand-made work box replaced with a sufficiently dry Ar gas (dew point - 60 ° C or higher).

The obtained powdery sample (compound) was analyzed by X-ray diffraction (XRD) and confirmed to have a cubic crystal system having a space group of F-43 m, and was represented by a composition formula: Li ₆ PS ₅ Cl. Compound.

(Example 2)

Lithium sulfide (Li ₂ S) powder 1.84 g, phosphorus sulfide (P ₂ S ₅ ) powder 1.78 g, and lithium bromide (LiBr) powder 1.39 g were weighed and mixed in a ball mill for 15 hours, as shown in the composition formula shown in Table 1. , to prepare a mixed powder. The mixed powder was filled in a carbon container, and it was fired at a temperature of 200 ° C / hour at 500 ° C at a temperature rise and fall of 200 ° C / hour while flowing 1.0 L / min of hydrogen sulfide gas (H ₂ S, purity 100%) in a tubular electric furnace. hour. Then, the sample was crushed using a mortar, and a particle size fractionation was carried out using a mesh sieve having a pore size of 53 μm to obtain a powdery sample.

At this time, the weighing, mixing, setting in the electric furnace, taking out from the electric furnace, crushing, and particle size classification were all carried out in a hand-made work box replaced with a sufficiently dry Ar gas (dew point - 60 ° C or higher).

The powdery sample (compound) obtained by XRD analysis was confirmed to have a crystal structure of a cubic crystal system having a space group of F-43 m, and was a compound represented by a composition formula: Li ₆ PS ₅ Br.

(Examples 3 to 15)

A sample was prepared in the same manner as in Example 1 except that the amount of each raw material was changed so as to have a composition formula shown in Table 1, and the firing environment and the firing temperature were changed to the temperatures shown in Table 1.

The powdery sample (compound) obtained by XRD analysis was confirmed to have a crystal structure of a cubic crystal system having a space group of F-43 m, and each was a compound represented by the composition formula shown in Table 1.

(Comparative Examples 1 and 2)

Each raw material was weighed separately in the form shown in Table 1, and pulverized and mixed for 15 hours using a ball mill to prepare a mixed powder. The mixed powder was pressure-molded at a pressure of 200 MPa to form a pellet, and the sheet was placed in a one-side sealed quartz tube.

Furthermore, the inside of the quartz tube is coated with carbon spray so that it does not react with the sample. reaction. Then, the quartz tube was rotated while being evacuated by using a glass processing lathe, and the unsealed one side portion of the quartz tube was sealed by heating with a burner to prepare a quartz sealing tube containing the sample.

Then, the quartz sealed tube containing the sample was placed in an electric furnace (box furnace), and fired at 550 ° C for 6 days at a temperature rise and fall of 100 ° C / hour. Then, the sample was crushed using a mortar, and a particle size fractionation was carried out using a mesh sieve having a pore size of 53 μm to obtain a powdery sample.

The powdery sample (compound) obtained by XRD analysis was confirmed to be a compound represented by the composition formula shown in Table 1.

(Comparative Examples 3 to 9)

A sample was prepared in the same manner as in Example 1 except that the amount of each raw material was changed so as to have a composition formula shown in Table 1, and the firing environment and the firing temperature were changed to the temperatures shown in Table 1.

The results of analysis of the obtained powdery sample (compound) were confirmed to be the compounds represented by the composition formulas shown in Table 1.

(Embodiment 16)

The blending amount of each raw material was changed so as to become the composition formula of Comparative Example 8 shown in Table 1 (Ha: Cl, x = 0.2), and a planetary ball mill which can be more uniformly mixed by the rotation/revolving motion was used. The mixture was pulverized and mixed at a rotation speed of 100 rpm for 10 hours instead of being pulverized and mixed for 15 hours using a ball mill. A sample was prepared in the same manner as in Comparative Example 8, except the above.

The results of analysis of the obtained powdery sample (compound) were confirmed to be the compounds represented by the composition formulas shown in Table 1.

<Measurement of composition ratio>

The composition ratios of the samples obtained in the examples/comparative examples were measured by ICP emission spectrometry.

<Measurement of lithium ion migration number>

For the samples obtained in the examples/comparative examples, a sheet having a diameter of 10 mm and a thickness of 2 to 5 mm was formed by press-molding in a sleeve working box at a pressure of 200 MPa. The upper and lower sides of the sheet were 9 mm in outer diameter and thickness. The 0.6 mm lithium foil was sandwiched, placed in an Al laminated bag, and molded at a pressure of 200 to 250 MPa CIP to form a sheet in which the upper and lower sides were adhered to each other with lithium foil. Then, after sandwiching the sheet with a small vise for about 1 day, DC voltage measurement was performed at a voltage of 0.5 V applied externally, and the AC impedance measurement was performed before and after the measurement, and the obtained value was substituted into the following Evans type. In order to obtain the lithium ion migration number (t _Li ⁺ ).

t _Li ⁺ =I _S (ΔVI ₀ R ₀ )/I ₀ (ΔVI _S R _S )

I ₀ and I _S : initial value and steady state current value in DC polarization measurement, R ₀ , R _S : initial and steady state charge transfer resistance values in AC impedance measurement

<Measurement of Luminance L ^* >

For the samples obtained in the examples and the comparative examples, the sample was filled in a glass holder for XRD measurement using a spectrophotometer (manufactured by Konica Minolta, CM-2600d). Further, a D65 light source is used as a light source.

<Measurement of ionic conductivity>

The sample obtained in the example/comparative example was placed in a handle box. A sheet having a diameter of 10 mm and a thickness of 2 to 5 mm was formed by pressure-molding at a pressure of 200 MPa, and a carbon paste as an electrode was applied to the upper and lower surfaces of the sheet, and then heat-treated at 180 ° C for 30 minutes to prepare an ion conductivity. Sample. The ionic conductivity measurement was carried out by an alternating current impedance method at room temperature (25 ° C).

"x" in Table 1 represents x of Li _7-x PS _6-x Ha _x (Ha-based Cl or Br), and "migration number" represents the number of lithium ion migration (t _Li ⁺ ).

In addition, in Table 1, "LiCl (micro)", "LiBr (micro)", and "Li ₂ S (micro)" mean the peaks of LiCl, LiBr, and Li ₂ S detected in the XRD chart. The strength is less than 3% of the peak intensity of Li _7-x PS _6-x Ha _x .

As shown in Table 1, it was found that the samples of Examples 1 to 16 mainly formed Li _7-x PS _6-x Ha _x having a crystal structure belonging to a cubic crystal F-43 m space group, and unreacted Li ₂ S LiCl and LiBr are not left or only remain a little.

Further, the luminance L ^* value is 60.0 or more, and the lithium ion migration number is 90% or more. The sample having the ionic conductivity was also 10 to ⁴ S/cm or more, which was extremely high.

On the other hand, in the samples of Comparative Examples 1 and 2 shown in Table 1, the main generation phase was Li _7-x PS _6-x Ha _x having a crystal structure belonging to the cubic F-43 m space group, but the luminance L ^{* The} value is less than 60.0, and the number of lithium ion migration is less than 90%. The ionic conductivity is about 10 ^-4 S/cm or more, which is extremely high. This is considered to be due to high electron conductivity.

The samples of Comparative Examples 3 and 8 were not added with Ha or added but only a small amount, and thus the crystal structure having a low lithium ion conductivity of the orthorhombic Pna2 ₁ space group was formed.

However, as in the case of Example 16, even if the same amount of Cl as in Comparative Example 8 was sufficiently pulverized and mixed, it was confirmed that Li _7-x PS _{6- which has a} crystal structure belonging to the cubic F-43m space group. _x Ha _x .

Although the sample of Comparative Example 4 was added with Br but had a large amount of addition, Li _7-x PS _6-x Ha _x which has a crystal structure belonging to a cubic crystal F-43 m space group was mainly formed, but the other was not introduced. A large amount of LiBr remains in the raw material of the main phase. Since these can suppress lithium ion conductivity, the ionic conductivity is too low to be measured.

In the sample of Comparative Example 5, although Cl was added but the amount of addition was too large, LiCl, Li ₂ S, and Li ₃ PS _{4 of the} phase raw material were mainly produced. Since these can suppress lithium ion conductivity, the ionic conductivity is too low to be measured.

The samples of Comparative Examples 6 and 7 were too high in firing temperature. Therefore, the sample was melted and firmly adhered to the fired container, and the evaluation could not be performed.

The samples of Comparative Examples 9 and 10 were fired in Ar gas in an inert gas atmosphere, but since the firing temperature was too high, Li _7-x PS ₆ having a crystal structure belonging to a cubic F-43 m space group was mainly formed. _-x Ha _x , but the luminance L ^* value is less than 60.0, and the lithium ion migration number is less than 90%.

<Manufacture and evaluation of all solid lithium batteries (1)>

Using the samples obtained in Examples 5 and 13 and Comparative Example 1 as a solid electrolyte, a positive electrode mixture material was prepared, and an all solid lithium battery (all solid InLi/NCM battery) was prepared, and battery characteristics (evaluation of charge and discharge efficiency and cycle characteristics) were performed.

(material)

A powder coated with a ZrO ₂ film in a ternary layered compound of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) was used as a positive electrode active material, and an indium-lithium (InLi) foil was used as a negative electrode active material, and examples and examples were used. The sample (compound) obtained in the comparative example was used as a solid electrolyte powder.

The positive electrode mixture powder was prepared by mixing a positive electrode active material powder, a solid electrolyte powder, and a conductive auxiliary agent (acetylene black) powder in a ratio of 40:54:6 using a ball mill.

(Production of all solid InLi/NCM battery)

The sample (solid electrolyte powder) obtained in the example/comparative example was filled in an insulating cylinder of a sealed battery, and was formed by molding at 120 MPa. After one side of the solid electrolyte sheet of about 13 mm × 0.5 mmt was filled with 25 mg of the positive electrode mixture powder, the positive electrode layer was formed by forming a positive electrode layer together with the solid electrolyte sheet at 240 MPa. Place A 12 mm × 1 mmt indium-lithium alloy foil was mounted on the opposite side of the positive electrode layer with a compression screw at a torque of 2 N‧ m to produce an all-solid InLi/NCM battery.

(Battery characteristics measurement)

The battery characteristics were measured by placing an all-solid battery cell in an environmental tester maintained at 25 ° C and connected to a charge and discharge measuring device. At this time, the CC-CV method in which the upper limit voltage was set to 3.7 V was charged, and the discharge was performed by a CC method in which the lower limit voltage was set to 2.0 V. Charging and discharging were repeated at the current density of 0.066 (0.05 C) mA/cm from the first cycle to the fifth cycle, and the sixth cycle was charged and discharged at 0.133 (0.1 C) mA/cm, and further at the 7th cycle. The cycle life characteristics were evaluated by repeating charging and discharging at a current density of 0.066 (0.05 C) mA/cm until the 10th cycle. The results are shown in Table 2.

It is known that when a positive electrode active material having a layered structure such as NCM is used, since the irreversible capacity is obtained, the charge and discharge efficiency in the first cycle is lowered. However, the all-solid battery using the InLi/NCM battery of the samples of Examples 5 and 13 can reduce the irreversible capacity of the first cycle, so that the charge and discharge efficiency is high, and the capacity retention rate of the display cycle characteristic of the third cycle is also High results. It is considered that this is because the electron conductivity is low and the ionic conductivity is high, and the battery characteristics also exhibit high performance.

On the other hand, in the all-solid battery of the InLi/NCM battery produced in the sample of Comparative Example 1, the charging voltage of the first cycle was kept lower than that of the all-solid battery produced in the samples of Examples 5 and 13, and the display was performed. Charging capacity up to 3.7V. It is considered that the sample of Comparative Example 1 has high electron conductivity, and is different from the electrochemical reaction in which lithium is removed from the positive electrode active material during charging and lithium is stored in the negative electrode, and a small amount is generated in the solid electrolyte layer. The leakage current requires additional power in addition to the amount of lithium that is removed/occluded, thus indicating a high charging capacity. On the other hand, it is considered that at the time of discharge, due to a decrease in the discharge capacity accompanying the irreversible reaction during charging and a small amount of leakage current due to electron conductivity of the solid electrolyte, an additional current corresponding to the leakage current portion is required. The amount of electricity required for density discharge, thus causing a voltage drop, reaching the lower limit voltage earlier, resulting in only a low discharge capacity. In addition, in the subsequent charge and discharge, the charge/discharge capacity is not obtained due to the influence of the leakage current and the irreversible side reaction, and the capacity retention rate of the display cycle characteristics is also lowered.

<Manufacture and Evaluation of All Solid Lithium Batteries (Part 2)>

Using the sample obtained in Example 5 as a solid electrolyte to prepare a positive electrode The mixed material/negative electrode mixture was used to prepare an all-solid lithium battery (all solid Gr/NCM battery), and battery characteristics were evaluated (charge and discharge efficiency and cycle characteristics).

(material)

A powder coated with a ZrO ₂ film in a ternary layered compound of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) was used as a positive electrode active material, and graphite (Gr) powder was used as a negative electrode active material, and examples and comparative examples were used. The obtained sample (compound) was used as a solid electrolyte powder.

The positive electrode mixture powder was prepared by mixing a positive electrode active material powder, a solid electrolyte powder, and a conductive auxiliary agent (acetylene black) powder in a ratio of 40:54:6 using a ball mill.

On the other hand, the negative electrode (Gr) mixed material powder was prepared by mixing a Gr powder and a solid electrolyte powder in a ratio of 40:60 using a ball mill.

(Production of all solid Gr/NCM battery)

The sample (solid electrolyte powder) obtained in Example 5 was filled in an insulating cylinder of a sealed battery, and was formed by molding at 120 MPa. After one side of the solid electrolyte sheet of about 13 mm × 0.5 mmt was filled with 25 mg of the positive electrode mixture powder, the positive electrode layer was formed by forming a positive electrode layer together with the solid electrolyte sheet at 240 MPa. After filling 13 mg of the negative electrode mixture material powder on the opposite side of the positive electrode layer, the solid electrolyte sheet and the positive electrode layer formed thereafter were again formed by forming a negative electrode layer at 240 MPa, and then using a press screw to 3.5 N ‧ m The torque is locked to make a full solid Gr/NCM battery.

(Battery characteristics measurement)

Battery characteristics are measured by placing an all-solid battery cell at 25 ° C The environmental testing machine was connected and connected to the charge and discharge measuring device for evaluation. At this time, the CC-CV method in which the upper limit voltage was set to 4.2 V was charged, and the discharge was performed in a CC mode in which the lower limit voltage was set to 2.5 V, and the cycle life characteristics from the first cycle to the 50th cycle were evaluated.

Furthermore, in this evaluation, the sixth cycle, the 16th cycle, and the 26th cycle were repeatedly charged and discharged at a current density of 0.133 (0.1 C) mA/cm, and the cycle was 0.066 (0.05 C) mA/ Cm for charging and discharging. The results are shown in Table 3.

The Gr/NCM battery all-solid battery using the sample of Example 5 was similar to the all-solid battery of the InLi/NCM battery, and the irreversible capacity at the first cycle was reduced, the charge and discharge efficiency was high, and the third cycle was performed. The result shows that the capacity retention rate of the cycle characteristics is also high. It was found that even if the cycle charge/discharge was repeated 50 times, the capacity retention rate was about 75%, showing good cycle characteristics. Further, even if the current density is raised from 0.05 C to 0.10 C, the decrease in the capacity retention ratio is suppressed. Therefore, in the present solid electrolyte, a carbon-based material can be used as the negative electrode active material, and the energy density of the all-solid lithium secondary battery can be greatly improved.

Claims (7)

A sulfide-based solid electrolyte for a lithium ion battery, which comprises a crystal structure having a cubic F-43m space group and having a composition formula: Li _7-x PS _6-x Ha _x (Ha-based Cl or Br) In the compound, x in the above composition formula is 0.2 to 1.8, and the L*a*b* color system has a luminance L ^* value of 60.0 or more. The sulfide-based solid electrolyte for a lithium ion battery according to the first aspect of the invention, wherein the lithium ion migration number is 90% or more. The sulfide-based solid electrolyte for a lithium ion battery according to the above formula, wherein, when Ha is Cl, x is 0.2 to 1.8. The sulfide-based solid electrolyte for a lithium ion battery according to the first or second aspect of the invention, wherein, in the composition formula, when Ha is Br, x is 0.2 to 1.2. The sulfide-based solid electrolyte for a lithium ion battery according to the first or second aspect of the invention, which is characterized in that the lithium sulfide (Li ₂ S) powder, the phosphorus sulfide (P ₂ S ₅ ) powder, and the chlorination are used. Lithium (LiCl) powder or lithium bromide (LiBr) powder is mixed, fired at 350 to 500 ° C under an inert gas atmosphere, or fired at 350 to 650 ° C in an atmosphere containing hydrogen sulfide gas. A lithium ion battery comprising a sulfide-based solid electrolyte for a lithium ion battery according to any one of claims 1 to 5. A lithium ion battery, which has a sulfide-based solid electrolyte for a lithium ion battery according to any one of claims 1 to 5, and a negative electrode active material containing carbon.