All-solid-state battery and method for manufacturing the same

Abstract

The objective is to leave more graphene behind after thermal firing than in conventional methods in an all-solid-state battery and a method for manufacturing the same. [Solution] The all-solid-state battery according to the present invention contains graphene having a dispersibility index of 50% or less calculated by Lambert-Beer's law. The particle size of the graphene having a dispersibility index of 50% or less may be larger than the particle size of the graphene having a dispersibility index of more than 50%. The functionalization rate of the graphene having a dispersibility index of 50% or less may be lower compared to the graphene having a dispersibility index of more than 50%.

Description

Technical Field

[0001] The present invention relates to all-solid-state batteries and a method for manufacturing the same.

Background Art

[0002] In recent years, research on secondary batteries such as lithium-ion batteries and all-solid-state batteries has been actively conducted. As the active material of the secondary battery, a material with poor electronic conductivity such as a metal oxide is used. Therefore, in order to impart electronic conductivity, an electrode in which a conductive assistant is added in addition to the active material is used.

[0003] As a conductive assistant added to the positive electrode active material and the negative electrode active material of the oxide-based all-solid-state battery, it is widely known to use vapor-grown carbon fiber (VGCF) or carbon nanotube (CNT) which are said to be difficult to burn. However, these conductive assistants have a problem that their dispersibility is very poor and the paste viscosity becomes high.

[0004] In order to suppress the increase in this viscosity, methods such as increasing the dispersant or changing the raw material particle size are generally performed. However, there are problems such as loss of conductivity by increasing the dispersant and further increase in viscosity and deterioration of battery characteristics by changing the raw material particle size.

[0005] In Patent Documents 1 to 3, attempts are made to reduce the viscosity of the paste using graphene. Patent Document 4 discloses a graphene material with high dispersibility.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0007] The manufacturing of all-solid-state batteries requires a process of heat firing at a high temperature of around 600°C to bond the active material and electrolyte. The combustion start temperature of graphene is approximately 500°C.

[0008] The combustion initiation temperature of highly dispersible graphene, as described in Patent Document 4, is lower than that of aggregated graphene. Furthermore, the combustion termination temperature is similar to that of the combustion initiation temperature.

[0009] Therefore, when graphene-containing materials described in Patent Documents 1 to 4 are fired at a high temperature of, for example, around 600°C, a new problem arises: the graphene burns and disappears. Due to this problem, general graphene materials are applied to liquid-based lithium-ion batteries, semi-solid-state batteries, or products that do not involve a heat firing process (see Patent Documents 5 and 6).

[0010] This invention has been made in view of these circumstances, and aims to provide an all-solid-state battery in which more graphene remains after thermal firing than in conventional batteries, and a method for manufacturing the same. [Means for solving the problem]

[0011] To solve the above problems, the all-solid-state battery and its manufacturing method according to the present invention employ the following means.

[0012] The present invention provides an all-solid-state battery containing graphene, in which the dispersion index calculated by the Lambert-Beer law is 50% or less.

[0013] The dispersibility index of conventionally known highly dispersed graphene exceeds 50%. For graphene with a dispersibility index of 50% or less, the combustion start temperature and the combustion end temperature are higher than those of conventional highly dispersed graphene.

[0014] All-solid-state batteries are manufactured by thermal firing. By using graphene with a high combustion start temperature and a high combustion end temperature as a conductive aid, it becomes difficult for the graphene to burn (disappear) during firing. As a result, an all-solid-state battery in which more graphene remains than before can be obtained.

[0015] In one aspect of the above invention, the particle size of the graphene with a dispersibility index of 50% or less may be larger than the particle size of the graphene with a dispersibility index of more than 50%.

[0016] By increasing the particle size of graphene more than before, the combustion start temperature and the combustion end temperature can be increased.

[0017] In one aspect of the above invention, the functionalization rate of the graphene with a dispersibility index of 50% or less may be lower than that of the graphene with a dispersibility index of more than 50%.

[0018] By reducing the amount of functional groups that are the starting point of the combustion of graphene more than before, the combustion start temperature and the combustion end temperature can be increased.

[0019] In one aspect of the above invention, the combustion start temperature of the graphene with a dispersibility index of 50% or less may be 50°C or more higher than the combustion start temperature of the graphene with a dispersibility index of more than 50%.

[0020] The firing start temperature of graphene with a dispersibility index of more than 50% is, for example, 503°C. In that case, the combustion start temperature of graphene with a dispersibility index of 50% or less is 553°C or higher.

[0021] In one aspect of the above invention, the combustion end temperature of the graphene with a dispersibility index of 50% or less may be 50°C or more higher than the combustion end temperature of the graphene with a dispersibility index of more than 50%.

[0022] The firing end temperature of graphene with a dispersibility index exceeding 50% is, for example, 564°C. In that case, the combustion end temperature of graphene with a dispersibility index of 50% or less is 614°C or higher.

[0023] In one aspect of the above invention, the battery material containing graphene with a dispersibility index of 50% or less may be fired at a temperature of 570°C or higher and 630°C or lower.

[0024] Graphene with a dispersibility index of 50% or less has a higher "combustion start temperature" and "combustion end temperature" than graphene with a dispersibility index exceeding 50%. Therefore, since it is difficult to completely burn in a firing temperature environment of 600°C, more graphene than before can remain.

[0025] The present invention also provides a method for manufacturing an all-solid-state battery, in which graphene having a dispersibility index calculated by Lambert-Beer's law of 50% or less is added to the material of the all-solid-state battery and then fired.

[0026] In one aspect of the above invention, as the graphene with a dispersibility index of 50% or less, graphene having a larger particle size than the particle size of graphene with a dispersibility index exceeding 50% can be used.

[0027] In one aspect of the above invention, as the graphene with a dispersibility index of 50% or less, graphene having a lower functionalization rate than graphene with a dispersibility index exceeding 50% may be used.

[0028] In one aspect of the above invention, the firing temperature may be 570°C or higher and 630°C or lower.

Advantages of the Invention

[0029] By using graphene with a dispersibility index of 50% or less, the combustion start temperature and the combustion end temperature can be raised higher than those of conventional graphene. As a result, an all-solid-state battery in which more graphene remains can be obtained. [Brief explanation of the drawing]

[0030] [Figure 1] This is a schematic cross-sectional view showing the basic structure of an all-solid-state battery according to the first embodiment. [Figure 2] This figure shows the TG curve of a conductive additive (up to 800°C). [Figure 3] This figure shows the TG curve (at 500°C) of a conductive additive. [Figure 4] This is an SEM image showing the fracture surface of the positive electrode layer in Comparative Example 1. Figure 4(a) is an image at a magnification of 1000x, and Figure 4(b) is a magnified image of the area within the red frame in Figure 4(a). [Figure 5] This is an SEM image showing the fracture surface of the positive electrode layer in Example 1. Figure 5(a) is an image at a magnification of 1000x, and Figure 5(b) is a magnified image of the area within the red frame in Figure 5(a). [Figure 6] This is an SEM image showing the fracture surface of the positive electrode layer in Example 2. Figure 6(a) is an image at a magnification of 1000x, and Figure 6(b) is a magnified image of the area within the red frame in Figure 6(a). [Figure 7] This is an SEM image showing the fracture surface of the negative electrode layer in Comparative Example 1. Figure 7(a) is an image at a magnification of 1000x, and Figure 7(b) is a magnified image of the area within the red frame in Figure 7(a). [Figure 8] This is an SEM image showing the fracture surface of the negative electrode layer in Example 1. Figure 8(a) is an image at a magnification of 1000x, and Figure 8(b) is a magnified image of the area within the red frame in Figure 8(a). [Figure 9] This is an SEM image showing the fracture surface of the negative electrode layer in Example 2. Figure 9(a) is an image at a magnification of 1000x, and Figure 9(b) is a magnified image of the area within the red frame in Figure 9(a). [Modes for carrying out the invention]

[0031] An embodiment of the all-solid-state battery and its manufacturing method according to the present invention will be described below with reference to the drawings.

[0032] [First Embodiment] (All-solid-state battery) Figure 1 is a schematic cross-sectional view showing the basic structure of an all-solid-state battery according to this embodiment. The all-solid-state battery 10 has a solid electrolyte layer 11, a positive electrode layer 12, and a negative electrode layer 13. The all-solid-state battery 10 is a sintered body formed by heat firing of the battery material (the material of the solid electrolyte layer 11, the positive electrode layer 12, and the negative electrode layer 13).

[0033] The solid electrolyte layer 11 is sandwiched between the positive electrode layer 12 and the negative electrode layer 13. A current collector (not shown) can be electrically connected to the positive electrode layer 12 and the negative electrode layer 13, respectively.

[0034] The solid electrolyte layer 11 mainly consists of a solid electrolyte having ionic conductivity. The solid electrolyte may be an oxide-based compound having lithium ion conductivity. For example, an oxide-based compound may be Li 1.5 Al 0.5 Ge 1.5 (PO4)3.

[0035] The positive electrode layer 12 includes a positive electrode active material 21, a solid electrolyte 22, and a conductive additive 23. The negative electrode layer 13 includes a negative electrode active material 31, a solid electrolyte 32, and a conductive additive 33.

[0036] The positive electrode layer 12 and the negative electrode layer 13 function as electrodes by comprising electrode active materials (21, 31). The positive electrode active material 21 is, for example, Li2CoP2O7. The negative electrode active material 31 is, for example, anatase-type titanium oxide.

[0037] The solid electrolytes (22,32) contained in the positive electrode layer 12 and the negative electrode layer 13 may be the same as those in the solid electrolyte layer 11. The solid electrolytes (22,32) impart ionic conductivity to the positive electrode layer 12 and the negative electrode layer 13.

[0038] The conductive additive (23,33) is graphene with a dispersibility index of 50% or less. This graphene has a dispersibility index of over 0.1% and is highly dispersible. The "dispersibility index" is calculated according to the Lambert-Beer law. The conductive additive imparts conductivity to the positive electrode layer and the negative electrode layer.

[0039] Graphene is a thin-layer material in which carbon atoms are arranged in a hexagonal lattice. Its thickness (measured with an atomic force microscope) is approximately 0.6 to 0.9 nm. Graphene is modified with functional groups. It is preferable for the functional groups to modify the edges of the graphene. Graphene modified with functional groups is less likely to aggregate due to the repulsive force between the functional groups. Therefore, graphene modified with functional groups has improved solvent dispersibility.

[0040] The particle size of graphene with a dispersibility index of 50% or less is larger than that of graphene with a dispersibility index of more than 50%. Particle size can be measured using a particle size analyzer. The particle size of graphene can be adjusted by factors such as the grinding time of the raw material.

[0041] The combustion start temperature of graphene with a dispersion index of 50% or less is more than 50°C higher than that of graphene with a dispersion index of more than 50%.

[0042] The combustion termination temperature of graphene with a dispersion index of 50% or less is more than 50°C higher than that of graphene with a dispersion index of more than 50%.

[0043] The positive electrode layer 12 and the negative electrode layer 13 may contain conductive materials other than "graphene having a dispersibility index of 50% or less".

[0044] The solid electrolyte layer 11, the positive electrode layer 12, and the negative electrode layer 13 may contain a binder, a plasticizer, a dispersant, a diluent, etc. The binder is, for example, polyvinyl butyral. The plasticizer is, for example, bis(2-ethylhexanoic acid)triethylene glycol. The dispersant is, for example, HIPLAAD ED350 ("HIPLAAD" is a registered trademark of Kusumoto Chemical Co., Ltd.). The diluent is, for example, terpineol.

[0045] (Manufacturing method for all-solid-state batteries) The method for manufacturing an all-solid-state battery according to this embodiment includes the following steps (S1) to (S4). (S1) Preparation of solid electrolyte layer sheet The raw materials for the solid electrolyte layer material are mixed in a ball mill. This results in a paste-like solid electrolyte layer material in which the raw materials are uniformly mixed.

[0046] After degassing a paste-like solid electrolyte layer material in a vacuum, the solid electrolyte layer material is coated onto a PET film using a doctor blade method to obtain a sheet-like solid electrolyte layer material. To adjust the solid electrolyte layer sheet to the desired thickness, a predetermined number of sheets of the solid electrolyte layer material obtained in a single coating are stacked, and the stacked sheets are then pressed together to obtain a solid electrolyte layer sheet made of green sheets.

[0047] (S2) Cathode layer sheet fabrication The raw materials for the cathode layer material are mixed in a ball mill. This results in a paste-like cathode layer material in which the raw materials are uniformly mixed. Using this paste-like cathode layer material, a cathode layer sheet consisting of a green sheet is obtained using the same procedure as for the solid electrolyte layer sheet.

[0048] (S3) Fabrication of negative electrode layer sheet The raw materials for the negative electrode layer material are mixed in a ball mill. This results in a paste-like negative electrode layer material in which the raw materials are uniformly mixed. Using this paste-like negative electrode layer material, a negative electrode layer sheet consisting of a green sheet is obtained using the same procedure as for the solid electrolyte layer sheet.

[0049] (S4) Firing A solid electrolyte layer sheet is sandwiched between a positive electrode layer sheet and a negative electrode layer sheet and pressed together to obtain a laminate. The obtained laminate is then fired. The firing temperature may be between 570°C and 630°C.

[0050] [Second Embodiment] The all-solid-state battery according to this embodiment differs from the first embodiment in the conductive additive contained in the positive electrode layer and / or negative electrode layer. The configuration other than the conductive additive is the same as that of the first embodiment.

[0051] The conductive additive is graphene with a dispersibility index of 50% or less. Furthermore, graphene is highly dispersible. Here, "highly dispersible" means that the dispersibility index is greater than 0.1%.

[0052] Graphene with a dispersibility index of 50% or less has a lower functionalization rate compared to graphene with a dispersibility index of more than 50%. The functionalization rate of graphene can be adjusted, for example, by the amount of functional group material added during graphene synthesis.

[0053] The combustion start temperature of graphene with a dispersion index of 50% or less is more than 50°C higher than that of graphene with a dispersion index of more than 50%.

[0054] The combustion termination temperature of "graphene with a dispersion index of 50% or less" is 50°C or more, preferably 60°C or more, more preferably 70°C or more, and most preferably 80°C or more, higher than that of "graphene with a dispersion index of more than 50%".

[0055] The positive electrode layer and the negative electrode layer may contain conductive materials other than "graphene with a dispersibility index of 50% or less" as a conductive additive.

[0056] [Rationale for selecting conductive additives] The following explains the rationale for selecting the conductive additives used in the first and second embodiments described above.

[0057] Samples 1-3 were used as conductive additives, and their particle size, specific surface area, dispersibility index, combustion start temperature (Air atmosphere), and combustion end temperature (Air atmosphere) were evaluated.

[0058] Sample 1 is a graphene powder (manufactured by Kyocera Corporation) with a dispersibility index of over 50%. Sample 2 is a graphene powder based on the graphene of Sample 1, with significantly improved particle size. Sample 3 is a graphene powder based on the graphene of Sample 1, but with a reduced functionalization rate.

[0059] (Particle size and specific surface area) The particle size (median diameter D50) was measured using a particle size analyzer. The specific surface area was measured using a specific surface area measuring device employing the gas adsorption method by constant volume.

[0060] (Dispersion index) A dispersion of water and isopropyl alcohol (IPA) in a ratio of 6:4 was prepared. 0.3 g of the sample powder was added to 100 mL of the dispersion and dispersed using an ultrasonic homogenizer for 3 minutes. Then, the mixture was centrifuged (1000 rpm, 10 minutes), and the absorbance of the supernatant (660 nm) was measured.

[0061] The concentration of the dispersion was quantified from the absorbance of the dispersion, and the dispersibility index was calculated using the following formula (Lambert-Beer's Law). Dispersion concentration (g / L) = absorbance / (absorbance coefficient × cell thickness) Dispersibility index (wt%) = dispersion liquid concentration / initial concentration × 100 Absorbance coefficient: 3,200 Quartz cell thickness: 0.001 m

[0062] (Combustion start temperature and combustion end temperature) Samples 1-3 were subjected to a differential thermal and thermogravimetric simultaneous measurement device. Measurement conditions (1): Heating rate 10°C / min, heating from room temperature to 800°C (Air atmosphere) Measurement conditions (2): Heating rate 10°C / min, heating from room temperature to 500°C, then holding at 500°C for 2 hours (Air atmosphere).

[0063] In the TG curve obtained under measurement condition (1), the temperature at the intersection of the tangent line before weight loss begins and the tangent line at the point where the weight loss rate (angle) is maximum was calculated as the "combustion start temperature." Furthermore, the temperature at the intersection of the tangent line at the point where the weight loss rate (angle) is maximum and the tangent line after weight loss has ended was calculated as the "combustion end temperature."

[0064] Table 1 shows the evaluation results. The combustion start temperature and combustion end temperature in Table 1 are the results for measurement condition (1).

[0065] [Table 1]

[0066] Figures 2 and 3 show the TG curves obtained from measurements under measurement conditions (1) and (2). Figure 2 is the TG curve for measurement condition (1). In this figure, the horizontal axis is temperature (°C), and the vertical axis is the thermogravimetric rate TG (%). Figure 3 is the TG curve for measurement condition (2). In this figure, the horizontal axis is time (h), the left vertical axis is the thermogravimetric rate TG (%), and the right vertical axis is temperature (°C).

[0067] According to Figure 2 and Table 1, it was confirmed that under an air atmosphere, the combustion start temperature and combustion end temperature of samples 2 and 3 were higher than those of sample 1. The combustion start temperature of both samples 2 and 3 was 540°C or higher.

[0068] In sample 2, where the graphene particle size was larger than in the conventional sample (sample 1), the combustion start temperature was more than 50°C higher than in sample 1, and the combustion end temperature was also more than 50°C higher than in sample 1.

[0069] In sample 3, where the functionalization rate of graphene was lower than in the conventional sample (sample 1), the combustion start temperature was more than 70°C higher than in sample 1, and the combustion end temperature was also more than 80°C higher than in sample 1.

[0070] As shown in Figure 3, the combustion initiation temperatures at 500°C for samples 2 and 3 tended to be later than those for sample 1.

[0071] According to Figures 2 and 3, the thermogravimetric change rate of sample 1 reached 0% at the end of the measurement. On the other hand, samples 2 and 3 showed less thermogravimetric change than sample 1, and their thermogravimetric change rates did not reach 0% at the end of the measurement.

[0072] The results above suggest that samples 2 and 3 are conductive additives that exhibit high dispersibility indices of 30-45% and can remain in greater quantities in the positive and negative electrode layers after firing than conventional dispersible graphene.

[0073] (Graphene residue after firing) Solid-state batteries (Comparative Example 1, Examples 1 and 2) were manufactured according to the above embodiments. Comparative Example 1 and Examples 1 and 2 differ in the conductive additives added to the positive and negative electrode layers. Comparative Example 1 used Sample 1 as the conductive additive. Example 1 used Sample 2 as the conductive additive. Example 2 used Sample 3 as the conductive additive. The materials other than the conductive additive and the manufacturing method were the same. Sintering was performed at 600°C.

[0074] Figures 4-9 show cross-sectional images (SEM, ×1000) of Comparative Example 1 and Examples 1 and 2. Figure 4 is the fracture surface of the positive electrode layer of Comparative Example 1. Figure 5 is the fracture surface of the positive electrode layer of Example 1. Figure 6 is the fracture surface of the positive electrode layer of Example 2. Figure 7 is the fracture surface of the negative electrode layer of Comparative Example 1. Figure 8 is the fracture surface of the negative electrode layer of Example 1. Figure 9 is the fracture surface of the negative electrode layer of Example 2. Table 2 shows the graphene count in each fracture surface.

[0075] [Table 2]

[0076] In Examples 1 and 2, a tendency was observed for more conductive additive (graphene) to remain in the sintered body compared to Comparative Example 1. [Explanation of Symbols]

[0077] 10 All-solid-state battery 11 Solid electrolyte layer 12 Positive electrode layer 13. Negative electrode layer 21 Cathode active material (electrode active material) 22,32 solid electrolyte 23,33 Conductive additives 31 Negative electrode active material (electrode active material)

Claims

1. All-solid-state batteries containing graphene with a dispersion index of 50% or less, calculated according to the Lambert-Beer law.

2. The all-solid-state battery according to claim 1, wherein the particle size of graphene having a dispersibility index of 50% or less is larger than the particle size of graphene having a dispersibility index of more than 50%.

3. The all-solid-state battery according to claim 1, wherein the functionalization rate of graphene having a dispersibility index of 50% or less is lower compared to graphene having a dispersibility index of more than 50%.

4. The all-solid-state battery according to claim 1, wherein the combustion start temperature of graphene having a dispersibility index of 50% or less is 50°C or higher than the combustion start temperature of graphene having a dispersibility index of more than 50%.

5. The all-solid-state battery according to claim 1, wherein the combustion termination temperature of graphene having a dispersibility index of 50% or less is 50°C or higher than the combustion termination temperature of graphene having a dispersibility index of more than 50%.

6. The all-solid-state battery according to claim 1, wherein the battery material containing graphene having a dispersibility index of 50% or less is fired at 570°C or higher and 630°C or lower.

7. A method for manufacturing an all-solid-state battery, comprising adding graphene, which has a dispersibility index of 50% or less calculated according to the Lambert-Beer law, to the all-solid-state battery material, and then firing it.

8. The method for manufacturing an all-solid-state battery according to claim 7, wherein the graphene having a dispersion index of 50% or less is graphene with a particle size larger than that of graphene having a dispersion index of more than 50%.

9. The method for manufacturing an all-solid-state battery according to claim 7, wherein the graphene having a dispersibility index of 50% or less is graphene with a lower functionalization rate than graphene having a dispersibility index of more than 50%.

10. The method for manufacturing an all-solid-state battery according to claim 7, wherein the firing temperature is 570°C or higher and 630°C or lower.

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