JP6904303B2 - Manufacturing method of positive electrode mixture - Google Patents

Description

The present application discloses a method for producing a positive electrode mixture for an all-solid-state lithium-sulfur battery.

Since sulfur has a high theoretical electrochemical capacity, the development of lithium-sulfur batteries using sulfur as a positive electrode active material is underway.

In Patent Document 1, Li ₂ SP ₂ S ₅ sulfide solid electrolyte, sulfur and / or its discharge product (Li ₂ S, etc.), and a conductive auxiliary material are used as the solid electrolyte, and these are mechanically milled. Disclosed is a positive electrode mixture for an all-solid-state lithium-sulfur battery. Patent Document 2 describes _{a first amorphization step of amorphizing a first mixture containing Li 2} S, P ₂ S ₅ and Li Br, an amorphized first mixture, and a positive electrode active material. A method for producing a positive electrode mixture including a second amorphization step of amorphizing a second mixture containing a simple sulfur and a conductive auxiliary material containing carbon is disclosed.

JP 2015-5452 JP-A-2017-91810

In the method described in Patent Document 1, there are many voids between the positive electrode active material and the solid electrolyte, and there is a risk that the reversible discharge capacity may be suppressed. This is a problem that cannot be solved even if Patent Documents 1 and 2 are combined.

Therefore, it is an object of the present application to provide a method for producing a positive electrode mixture for an all-solid-state lithium-sulfur battery having a high reversible discharge capacity.

As a result of diligent studies, the present inventor has made voids in the positive electrode mixture produced by heat-treating the composite formed by the mechanical milling treatment at 300 ° C. to 500 ° C. under non-oxygen conditions. We have found that the rate can be lowered and the reversible discharge capacity can be improved, and the present invention has been completed.

That is, the present application as one means for solving the above problems, performs a positive electrode active material containing Li 2 _S, and the solid electrolyte, the mechanical milling of the raw material mixture containing a conductive agent, to form a complex Disclosed is a method for producing a positive electrode mixture for an all-solid-state lithium-sulfur battery, which comprises a step and a step of heat-treating the composite at 300 ° C. to 500 ° C. under non-oxygen conditions.

According to the manufacturing method disclosed in the present application, a positive electrode mixture for an all-solid-state lithium-sulfur battery having a high reversible discharge capacity can be manufactured.

It is a flowchart of manufacturing method 10. It is a graph which showed the discharge capacity density of Examples 1 and 2 and Comparative Examples 1 and 3. It is an SEM image of the positive electrode cross section of Example 1 and Comparative Example 1.

In the following, the notation "A to B" for the numerical values A and B shall mean "A or more and B or less". When a unit is attached only to the numerical value B in such a notation, the unit shall be applied to the numerical value A as well.

1. 1. Method for Manufacturing Positive Electrode Mixture The present application discloses a method 10 for manufacturing a positive electrode mixture for an all-solid-state lithium-sulfur battery (hereinafter, may be referred to as "manufacturing method 10"). FIG. 1 shows a flowchart of the manufacturing method 10. As shown in FIG. 1, the manufacturing method 10 includes a step S1 for forming a complex (hereinafter, may be referred to as “step S1”) and a step S2 for heat-treating the complex (hereinafter, “step S2”). There are times when.) And.
The details will be described below.

1.1. Process S1
In step S1, it performs a positive electrode active material containing Li 2 _S, and the solid electrolyte, the mechanical milling of the raw material mixture containing a conductive agent, to form a complex. A "complex" is not simply a mixture of predetermined components, but mechanical and thermal energy is applied to a mixture of predetermined components, and a chemical reaction occurs in all or part of the predetermined components. Say what happened. In step S1, mechanical and thermal energy is applied to the raw material mixture by mechanical milling treatment to form a complex.

1.1.1. Raw Material Mixture Here, the raw material mixture in step S1 will be described. Feed mixture as described above, those containing a positive electrode active material containing Li 2 _S, and a solid electrolyte, and a conductive additive.

The positive electrode active material include Li 2 _S, and is not particularly limited as Ere acts as a positive electrode active material. For example, may only Li ₂ S, may be a mixture of Li ₂ S and elemental sulfur (S). Preferably a positive electrode active material composed only of Li 2 _S. The proportion of the positive electrode active material in the raw material mixture is not particularly limited, but it is preferable that the lower limit is 25% by mass or more and the upper limit is 65% by mass or less.

The solid electrolyte is not particularly limited as long as it is a solid electrolyte that can be used in an all-solid-state lithium-sulfur battery. However, an inorganic solid electrolyte is preferable from the viewpoint of having a higher ionic conductivity than an organic polymer electrolyte. Preferred solid electrolytes, _Li 3 oxide solid electrolyte PO _4, etc. or _{Li 2 S-P 2 S 5} , Li 2 S-P 2 S 5 -GeS 2 (Li 3.5 Ge 0.5 P 0.5 In addition to sulfide solid electrolytes such as _{S 4} ), hydride solid electrolytes such as _{LiBH 4 can be exemplified.} From the viewpoint of a high ionic conductivity, etc., sulfide solid electrolyte containing _Li 2 _S-P 2 _{S 5} or _Li 2 _S-P 2 S 5 -GeS ₂ Among these are preferred. Particularly preferably a sulfide solid electrolyte containing _{Li 2 S-P 2 S 5} -GeS 2. The proportion of the solid electrolyte in the raw material mixture is not particularly limited, but it is preferable that the lower limit is 25% by mass or more and the upper limit is 65% by mass or less.

The conductive auxiliary material is not particularly limited as long as it is a conductive auxiliary material that can be used in the positive electrode of an all-solid-state lithium-sulfur battery. For example, carbon materials such as acetylene black, ketjen black, and vapor-grown carbon fiber (VGCF), metal materials such as nickel, aluminum, and stainless steel can be exemplified. The ratio of the conductive auxiliary material in the raw material mixture is not particularly limited, but it is preferable that the lower limit is 10% by mass or more and the upper limit is 20% by mass or less.

Further, a binder such as PVDF (polyvinylidene fluoride) can be added to the raw material mixture as needed.

1.1.2. Mechanical milling process The mechanical milling process in step S1 may be wet mechanical milling or dry mechanical milling. The mechanical milling process can be performed by, for example, a ball mill device, preferably a planetary ball mill device.

Various conditions of mechanical milling are not particularly limited as long as all or part of the raw material mixture can cause a chemical reaction to form a desired complex, and are appropriately set according to the purpose. For example, when using a planetary ball mill device, the rotation speed is preferably in the range of 150 rpm to 600 rpm. The treatment time is preferably in the range of 1 hour to 100 hours. Further, mechanical milling is preferably performed under non-oxygen conditions, and more preferably performed under an inert gas atmosphere (for example, Ar gas atmosphere).

1.2. Process S2
In step S2, the composite produced in step S1 is heat-treated (baked) to prepare a positive electrode mixture. The heat treatment is performed under non-oxygen conditions, preferably under vacuum. Here, "non-oxygen condition" means that the ratio of oxygen in the atmosphere is 1000 ppm or less. The heat treatment temperature is in the range of 300 ° C. to 500 ° C. The heat treatment time is not particularly limited, but it is preferable that the lower limit is 1 hour or more and the upper limit is 8 hours or less.

Here, the reason for heat-treating the complex will be described. The complex is in a state of being mixed by a mechanical milling process. That is, the solid electrolyte and the conductive auxiliary material are in contact with the positive electrode active material, and the Li ion conduction path is secured. However, the mechanical milling treatment alone has many voids in the complex, and there is room for improvement from the viewpoint of securing more Li ion conduction paths. Therefore, the present inventor reduces the porosity of the obtained positive electrode mixture by heat-treating the composite under predetermined conditions (increasing the contact ratio of the positive electrode active material and the solid electrolyte), and Li. We have found that more ion conduction paths can be secured. The more Li ion conduction paths can be secured, the more the reversible discharge capacity can be increased.

Here, in [0036] of Patent Document 1, in order to strengthen the interfacial contact of the positive electrode mixture and reduce the interfacial resistance, heat treatment may be performed after mixing each component of the positive electrode mixture. Are listed. Further, as the conditions of the heat treatment, it is described that the heat treatment is carried out in an atmosphere of argon, nitrogen, air or the like at 80 ° C. to 250 ° C. for 1 second to 10 hours.
However, Patent Document 1 does not describe an example in which heat treatment is actually performed. Further, it is not disclosed that the porosity of the complex is reduced by the heat treatment. Further, the heat treatment of the present application is performed in the temperature range of 300 ° C. to 500 ° C. as described above, and the heat treatment described in Patent Document 1 is in the temperature range of 80 ° C. to 250 ° C. As can be seen from the examples described later, by performing the heat treatment temperature in the range of 300 ° C. to 500 ° C., the porosity of the positive electrode mixture can be significantly reduced and the reversible discharge capacity can be significantly increased.
Therefore, the effect of lowering the porosity and remarkably increasing the reversible discharge capacity by setting the heat treatment temperature in the range of 300 ° C. to 500 ° C. is an effect that cannot be easily expected from Patent Document 1 which is a prior art. It can be said that.

The porosity of the positive electrode mixture obtained through step S2 is preferably 11% or less.

As described above, according to the manufacturing method 10, a positive electrode mixture for an all-solid-state lithium-sulfur battery having a high reversible discharge capacity can be manufactured.

2. Manufacturing method of all-solid-state lithium-sulfur battery The positive electrode mixture manufactured by the above manufacturing method is used for the positive electrode layer of the all-solid-state lithium-sulfur battery. Therefore, the present application is a method for manufacturing an all-solid-state lithium-sulfur battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer formed between the positive electrode layer and the negative electrode layer, and the positive electrode layer is described above. It is also possible to provide a manufacturing method having a step of forming using a positive electrode mixture.

The step of forming the positive electrode layer using the above-mentioned positive electrode mixture can be performed by a known method. For example, the positive electrode mixture may be pressed to form a positive electrode layer, or the positive electrode mixture may be dissolved in an arbitrary solvent to form a slurry, and the slurry may be applied and dried to form a positive electrode layer.

The solid electrolyte layer is not particularly limited as long as it is a layer containing a solid electrolyte having lithium ion conductivity, and can be produced, for example, by using the above-mentioned solid electrolyte. The negative electrode layer is not particularly limited as long as it includes a negative electrode active material capable of storing and releasing lithium ions, and examples thereof include metallic lithium and lithium alloys (for example, Li-In alloys). The all-solid-state lithium-sulfur battery may also include a positive electrode current collector that collects electricity from the positive electrode layer and a negative electrode current collector that collects electricity from the negative electrode layer.

Here, the all-solid-state lithium-sulfur battery may be a primary battery or a secondary battery. A secondary battery is preferable. This is because it can be charged and discharged repeatedly and is useful as an in-vehicle battery, for example. The primary battery also includes the use of a secondary battery as a primary battery (use for the purpose of discharging only once after charging).

The method for producing the positive electrode mixture of the present application will be described in detail with reference to Examples and Comparative Examples.

1. 1. Production of Positive Electrode Mixture The method for producing a positive electrode mixture according to Examples 1 and 2 and Comparative Examples 1 to 3 will be described below. Unless otherwise specified, the following operations are performed in an Ar gas atmosphere.

1.1. Preparation of solid electrolyte (Li _3.5 Ge _0.5 P _0.5 S _{4 ) Li 2} S (manufactured by Nippon Kagaku Kogyo Co., Ltd.) 0.80 g, P ₂ S ₅ (manufactured by Aldrich) in a glove box (Ar gas atmosphere) (Manufactured by) 0.56 g and GeS ₂ (manufactured by High Purity Chemical Laboratory) 0.68 g were weighed, placed in a dairy pot and mixed for 15 minutes. The mixed material was put into a planetary ball mill pot (45 mL, _{made by ZrO 2 ), 10 more ZrO 2} balls (φ = 10 mm) were put into it, and the planetary ball mill pot was put into an overpot (made by SUS) and completely sealed. .. Next, the overpot was taken out of the glove box and attached to a planetary ball mill device (manufactured by Fruche). The rotation speed of the planetary ball mill device was set to 370 rpm, and a 60-hour mechanical milling process was performed in a cycle of "1 hour processing, 15 minutes stop, 1 hour processing in reverse rotation, 15 minutes stop". Of the obtained samples, 1 g was pelletized in a glove box and vacuum-sealed in a carbon-coated quartz tube at 30 Pa. Then, it was calcined at a temperature of 500 ° C. to obtain a _{solid electrolyte (Li 3.5} Ge _0.5 P _0.5 S _4).

1.2. Preparation of positive electrode mixture Solid electrolyte (Li _3.5 Ge _0.5 P _0.5 S ₄ , 0.303 g) _{prepared in a glove box (Ar gas atmosphere), Li 2} S (manufactured by Nippon Kagaku Kogyo Co., Ltd., 0.188 g) and VGCF (manufactured by Showa Denko, 0.115 g) were weighed, placed in a dairy pot and mixed for 15 minutes. The resulting mixture planetary ball mill pot (45 mL, ZrO Ltd. ₂₎ were charged, and further ZrO ₂ balls (phi = 5 mm) was put 160, completely sealed putting planetary ball mill pot over pot (manufactured by SUS) did. Next, the overpot was taken out of the glove box and attached to a planetary ball mill device (manufactured by Fruche). The rotation speed of the planetary ball mill device was set to 500 rpm, and mechanical milling processing was performed for 24 hours. The obtained complex was pelletized in a glove box and vacuum-sealed in a carbon-coated quartz tube at 30 Pa. Then, it was fired at the temperature shown in Table 1 for 5 hours, and then naturally allowed to cool to prepare positive electrode mixture materials according to Examples 1 and 2, and Comparative Examples 1 to 3, respectively.

2. Fabrication of All-Solid-State Lithium-Sulfur Battery Next, a method for producing an all-solid-state lithium-sulfur battery according to Examples 1 and 2 and Comparative Examples 1 to 3 will be described. Unless otherwise specified, the following operations are performed in an Ar gas atmosphere.

2.1. Solid electrolytes _{(75Li 2 S · 25P 2 S} 5) Preparation of a glove box (Ar gas) in a _Li 2 S (Nippon Chemical Industrial Co., _{Ltd., 1.1g), P 2 S 5} ( Aldrich, 1.78 g ) Weighed, placed in a mortar and mixed for 15 minutes. The mixed material was put into a planetary ball mill pot (45 mL, _{made by ZrO 2 ), 10 more ZrO 2} balls (φ = 10 mm) were put into it, and the planetary ball mill pot was put into an overpot (made by SUS) and completely sealed. .. Next, the overpot was taken out of the glove box and attached to a planetary ball mill device (manufactured by Fruche). The rotation speed of the planetary ball mill device was set to 510 rpm, and a 10-hour mechanical milling process was performed in a cycle of "1 hour processing, 15 minutes stop, 1 hour processing in reverse rotation, 15 minutes stop". This gave a solid electrolyte _{(75Li 2 S · 25P 2 S} 5).

2.2. By addition of solid electrolyte prepared bottom was manufactured in a mold of 1 cm ² of an all-solid-state lithium-sulfur battery _{(75Li 2 S · 25P 2 S} 5) were pressed at 6t / cm ^2, to prepare a solid electrolyte layer. Using 9 mg of the positive electrode mixture prepared as described above as the positive electrode, the mixture was added to the mold so as to be in contact with one surface of the solid electrolyte layer, and the whole was pressed at ^{1 t / cm 2.} Further, using a lithium foil as a negative electrode, it was added to the die so as to be in contact with the other surface of the solid electrolyte layer, and pressed at ^{1 t / cm 2.} As a result, all-solid-state lithium-sulfur batteries according to Examples 1 and 2 and Comparative Examples 1 to 3 were produced.

3. 3. Evaluation of Discharge Capacity The all-solid-state lithium-sulfur batteries according to Examples 1 and 2 and Comparative Examples 1 to 3 prepared above were attached to a charge / discharge test device (manufactured by Hokuto Denko), and the following procedures (1) to (7) were performed. The discharge capacity density was measured while performing the above. In the following, 1C = 4.56mA / cm ² .
(1) OCV measurement.
(2) After discharging to 1.5V at C / 10, pause for 10 minutes.
(3) After charging to 3.1V with C / 10, rest for 10 minutes. After that, it was discharged to 1.5V at C / 10 and paused for 10 minutes. These were performed for a total of 5 cycles.
(4) After charging to 3.1V with C / 10, rest for 10 minutes. After that, it was discharged to 1.5V at C / 3, and the rest was 10 minutes. After that, it was discharged to 1.5V at C / 10 and paused for 10 minutes.
(5) After charging to 3.1V with C / 10, rest for 10 minutes. After that, it was discharged to 1.5V at 1C, and paused for 10 minutes. After that, it was discharged to 1.5V at C / 10 and paused for 10 minutes.
(6) After charging to 3.1V with C / 10, rest for 10 minutes. After that, it was discharged to 1.5V at 2C, and paused for 10 minutes. After that, it was discharged to 1.5V at C / 10 and paused for 10 minutes.
(7) After charging to 3.1V with C / 10, rest for 10 minutes. After that, it was discharged to 1.5V at C / 10 and paused for 10 minutes.

Among the obtained measurement results, the discharge capacity density (mAh / g) when discharging to 1.5V at C / 10 in the second cycle of (3), and discharging to 1.5V at C / 3 of (4). The discharge capacity densities (mAh / g) at the time of the discharge are shown in Table 1, respectively. These results are also shown in FIG.

4. Evaluation of void ratio After cross-sectional processing the positive electrode of the all-solid-state lithium-sulfur battery according to Examples 1 and 2 and Comparative Examples 1 to 3 produced above by CP (manufactured by Hitachi High-Technologies Corporation, IM4000), SEM (FE-SEM) , The positive electrode cross section processed by the acceleration voltage 1.0 kV) was observed. Then, the SEM image was analyzed as follows to determine the porosity of the positive electrode.
(1) Fuji ImageJ, by Weka Machine Learning, divides the SEM image _Li 2 S, a solid electrolyte, the three regions of the air gap.
(2) The area of each region was calculated by Matlab, and the porosity (porosity area / observation area area) was obtained.

The obtained porosity is shown in Table 1. Moreover, the SEM images of Example 1 and Comparative Example 1 are shown in FIG.

5. Results As is clear from Tables 1 and 2, it was found that the discharge capacity densities of the batteries were significantly higher in Examples 1 and 2 than in Comparative Examples 1 and 2. It is considered that this is because the porosity of Examples 1 and 2 is lower than that of Comparative Examples 1 and 2. By porosity is lowered, since the Li 2 S _(positive electrode active material) and the solid electrolyte contact ratio is increased, Li ion conductivity path can be more secured, it believed discharge capacity density is increased. Further, in Comparative Example 3, although the porosity was low, it is considered that the discharge capacity density was lowered because the solid electrolyte was decomposed and the ionic conductivity was lowered. Be compared SEM images of Example 1 and Comparative Example 1 shown in FIG. 3, Example 1 clearly no voids, that Li 2 S _(positive electrode active material) and the solid electrolyte is closely contact I understand.
From the above results, it was found that the discharge capacity density of the battery is improved by performing the heat treatment of the composite under predetermined conditions after the mechanical milling treatment at the time of producing the positive electrode mixture.

Claims (1)

A positive electrode active material containing li 2 _S, and the solid electrolyte, performs mechanical milling of the raw material mixture containing a conductive agent, a step of forming a complex,
A step of heat-treating the complex under non-oxygen conditions in the range of 300 ° C. to 500 ° C.
A method for manufacturing a positive electrode mixture for an all-solid-state lithium-sulfur battery.

Images