Manufacturing method for all-solid-state batteries

Abstract

To integrate a positive electrode, an electrolyte, and a negative electrode at low temperature.SOLUTION: A flux 4 that is melted at a temperature lower than reaction temperature of positive and negative electrode materials 1, 2 and a solid electrolyte 3 and functions as an adhesive is pre-mixed with a solid electrolytic 3 interposed between the negative electrode material 1 containing a negative electrode active material and the positive electrode material 2 containing a positive electrode active material. The solid electrolyte and flux 4 are adapted to be integrated by single sintering by heating them at sintering temperature of more than melting temperature of the flux, and less than the reaction temperature of the positive and negative electrode materials 1, 2 and the solid electrolyte 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing an all-solid-state battery in which the positive electrode material, negative electrode material, and solid electrolyte material are all composed of solid or powder materials. More specifically, this invention relates to a method for manufacturing an all-solid-state battery in which a bulk-type battery is constructed by sintering. [Background technology]

[0002] Conventional bulk-type all-solid-state batteries consist entirely of solid or powder materials for the positive electrode, negative electrode, and the electrolyte material interposed between them. These materials are sequentially stacked and then solidified all at once through sintering to function as a battery.

[0003] In the manufacture of oxide-based solid-state batteries, high-temperature processes (generally above 1000°C) are required for high-density sintering and good interface formation. However, high-temperature processes can cause chemical reactions between the solid electrolyte and electrode material, potentially leading to the decomposition of the electrode material and preventing the battery from functioning properly. Therefore, there is a need for technology to manufacture solid-state batteries at low temperatures.

[0004] In an oxide-based all-solid-state battery using sodium ions as conductive ions, for example, it is composed of an electrode material NVP (Na3V2(PO4)3) that functions as both a positive and negative electrode, and a solid electrolyte of NZSP (Na3Zr2(SiO4)2PO4), which is press-molded while being heated at 900°C by hot-press firing or spark plasma sintering (SPS) (Non-Patent Literature 1). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] F. Lalere, JB Leriche, M. Courty, S. Boulineau, V. Viallet, C. Masquelier, V. Seznec, J. Power Sources, 247, 975-980, 2014. [Non-Patent Document 2] Y. Noguchi, E. Kobayashi, LS Plashnitsa, S. Okada, J. Yamaki, Electrochemica Acta, 101, 59-65, 2013. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, the manufacturing method described in Non-Patent Document 1, which produces an all-solid-state battery by sintering the solid electrolyte and positive and negative electrode materials in one step, is at a temperature higher than the reaction temperature of many oxides (400-800°C). Therefore, sintering at 900°C may cause the solid electrolyte and positive and negative electrode materials to chemically react or thermally decompose, potentially rendering the battery inoperable. In fact, it is known that sintering above 800°C can cause parts of the electrodes to change into Na4SiO4 or SiP (Non-Patent Document 2), so there is a need for technology to produce all-solid-state batteries at the lowest possible temperature.

[0007] The present invention aims to meet such demands and to provide a method for manufacturing an all-solid-state battery that enables the integration of the positive electrode, electrolyte, and negative electrode at lower temperatures. [Means for solving the problem]

[0008] A method for manufacturing an all-solid-state battery to achieve this objective includes a negative electrode material containing a negative electrode active material Green body and positive electrode material containing positive electrode active material Green body Solid electrolyte interposed between Green body A flux that melts at a temperature below the reaction temperature between the positive and negative electrode materials and the solid electrolyte and functions as an adhesive. 1-20 wt% relative to the solid electrolyte Mix beforehand and under pressure Below 600℃ Above 300℃By heating at the temperature of, once Baked knot The negative electrode material, positive electrode material, and solid electrolyte are integrated.

[0009] Here, in the method for manufacturing an all-solid-state battery of the present invention, it is preferable to mix 5-20 wt% of the above-mentioned flux with the solid electrolyte and sinter at 600 °C.

[0010] More preferably, 10 wt% of the flux is mixed with the solid electrolyte and sintered at a temperature of 600 °C or higher 400℃ above.

[0011] Furthermore, in the method for manufacturing an all-solid-state battery of the present invention, as the active materials of the positive and negative electrodes, NASICON type Na3V2(PO4)3, and as the solid electrolyte, NASICON type Na , Under pressure , , 1-20 wt% of solid electrolyte , , Green body , ,

[0012] , Below 600℃ Above 300℃ , Green body , Green body , [Figure 2] , , [Figure 1] , ,

[0013] , , Zr 2-x Mg [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​This graph shows the relationship between the amount of flux (weight ratio wt%) mixed with the solid electrolyte and the theoretical density ratio of the solid electrolyte. [Figure 3] The diagram shows the appearance of pellets obtained by sintering at a low temperature of 600°C under pressure. (A) is without flux, (B) is mixed with 1 wt%, (C) is mixed with 5 wt%, (D) is mixed with 10 wt%, (E) is mixed with 15 wt%, (F) is mixed with 20 wt%, and (G) is mixed with 30 wt%. [Figure 4] These are cross-sectional SEM images of the solid electrolyte portion of the pellet, with (A) being without flux, (B) being mixed with 1 wt%, (C) being mixed with 5 wt%, (D) being mixed with 10 wt%, (E) being mixed with 15 wt%, (F) being mixed with 20 wt%, and (G) being mixed with 30 wt%. [Figure 5] The charge-discharge curves shown here represent the results of charge-discharge tests at an operating temperature of 60°C for all-solid-state batteries obtained by the manufacturing method of the present invention. (A) represents a 5 wt% mixture, (B) a 10 wt% mixture, (C) a 15 wt% mixture, and (D) a 20 wt% mixture. [Figure 6] The cycle characteristic diagrams show the results of charge-discharge tests at an operating temperature of 60°C for all-solid-state batteries obtained by the manufacturing method of the present invention, with (A) representing a 5 wt% mixture, (B) a 10 wt% mixture, (C) a 15 wt% mixture, and (D) a 20 wt% mixture. [Figure 7] The following shows the results of a charge-discharge test of an all-solid-state battery with a 1 wt% flux mixture obtained by the manufacturing method of the present invention at an operating temperature of 60°C: (A) is the charge-discharge curve, and (B) is the cycle characteristic diagram. [Figure 8] These images show cross-sectional SEM images of the solid electrolyte portion when NVP positive and negative electrode materials, NZMSP solid electrolyte, and 10 wt% borax as a flux are mixed and sintered at a low temperature of 600°C or lower. (A) is at 600°C, (B) at 500°C, (C) at 400°C, and (D) at 300°C. [Figure 9] The diagram shows the appearance of pellets obtained by sintering at low temperatures below 600°C, with (A) at 500°C, (B) at 400°C, and (C) at 300°C. [Figure 10]This graph shows the relationship between the sintering temperature and the theoretical density ratio of the solid electrolyte in an all-solid-state battery fabricated using NVP positive and negative electrode materials and a solid electrolyte of NZMSP mixed with 10 wt% borax as a flux. [Figure 11] This is the charge-discharge curve at an operating temperature of 60°C and a sintering temperature of 500°C for an all-solid-state battery made with NVP positive and negative electrode materials and a solid electrolyte of NZMSP mixed with 10 wt% borax as a flux. [Figure 12] This is the charge-discharge curve of the all-solid-state battery at an operating temperature of 60°C and a sintering temperature of 400°C. [Figure 13] This is the charge-discharge curve of the all-solid-state battery at an operating temperature of 60°C and a sintering temperature of 300°C. [Figure 14] (A) is the charge-discharge curve and (B) is the cycle characteristic diagram for an all-solid-state battery fabricated with NVP positive and negative electrode materials and NZSP solid electrolyte mixed with 10 wt% borax as a flux, at an operating temperature of 200°C and a sintering temperature of 600°C. [Modes for carrying out the invention]

[0014] The configuration of the present invention will be described in detail below based on the embodiments shown in the drawings.

[0015] Figure 1 shows one embodiment of the method for manufacturing an all-solid-state battery according to the present invention. In this method of manufacturing an all-solid-state battery, a flux 4 that melts at a temperature lower than the reaction temperature between the electrode materials 1 and 2 and the solid electrolyte 3 and functions as an adhesive is pre-mixed into a solid electrolyte 3 interposed between a negative electrode material 1 containing a negative electrode active material and a positive electrode material 2 containing a positive electrode active material, before sintering. The mixture is then heated under pressure at a temperature above the melting temperature of the flux 4 and below the reaction temperature between the electrode materials 1 and 2 and the solid electrolyte 3, thereby integrating the materials in a single low-temperature sintering. The method of manufacturing an all-solid-state battery according to the present invention can be implemented regardless of whether it is a sodium-based all-solid-state battery or a lithium-based all-solid-state battery, but in this embodiment, the example of a sodium-based all-solid-state battery will be mainly given below.

[0016] Here, the active material of the electrode material is not limited to a specific crystal structure or material. For example, NASICON-type oxides Na x M(PO4)3 (M is Ti, V, Cr, Mn, Fe, Co, Ni), phosphate-based oxides Na4Ni3(PO4)2P2O7, Na4Co3(PO4)2P2O7, layered rock salt-type oxides Na x MO2 (M is Cr, Mn, Fe, Co, Ni), spinel-type oxides Na3LiTi5O 12 and other oxide materials are used. In the case of this embodiment, for example, NVP is used as the active material of the positive and negative electrodes, but it is not particularly limited thereto and can also be applied to other materials. And the positive electrode member and the negative electrode member are preferably configured as a composite material, that is, a composite electrode member, which is a composite of the electrode active material, the solid electrolyte, and carbon (C) as a conductive aid. In this embodiment, an example in which the same active material, that is, NVP, is used for the positive electrode material and the negative electrode material is given, but it is not particularly limited thereto, and a composite electrode material using different active materials may also be used. For example, NCPP may be used as the positive electrode side electrode member and NVP may be used as the negative electrode side electrode member. A battery using Na4Ni3(PO4)2P2O7 (NNPP) for the positive electrode and Na3V2(PO4)3 (NVP) for the negative electrode, or a battery using Na3V2(PO4)3 (NVP) for the positive electrode and NaTi2(PO4)3 (NTP) for the negative electrode may also be used.

[0017] Also, the inorganic solid electrolyte is not limited to a specific crystal structure or material, and a sodium ion-based inorganic solid electrolyte or a lithium ion-based inorganic solid electrolyte can also be implemented. In the case of a sodium-based all-solid-state battery, in particular, the use of NASICON crystals with excellent sodium ion conductivity is preferred. Examples of NASICON crystals include, for example, Na3Zr2(SiO4)2PO4 (hereinafter referred to as NZSP), Na 3+2x Zr 2-x Mg x (SiO4)2(PO4) (where x = 0, 0.05, or 0 < x < 0.05) (hereinafter referred to as NZMSP), Na1+xZr2SixP3-xO 12(However, x = 0 to 3) etc. can be mentioned, preferably Na3Zr2(SiO4)2PO4 (hereinafter referred to as NZSP), more preferably Na 3+2x Zr 2-x Mg x (SiO4)2(PO4) (where x = 0, 0.05, or 0 < x < 0.05) (hereinafter referred to as NZMSP). Of course, the oxide solid electrolyte is not limited to NASICON crystals such as NZMSP and NZSP. As the solid electrolyte, for example, β-alumina solid electrolyte, garnet-type crystal structure, and silicon-type crystal structure are known. When adhesion is obtained by the amorphization (vitrification) of boron oxide, it is considered applicable to all solid electrolytes. As sodium ion-based inorganic solid electrolytes other than oxides, for example, Na2(B 12 H 12 ) 0.5 (B 10 H 10 ) 0.5 、Na 11 Sn2PS 12 、Na3 + 5xP1 - xS4 (However, x = 0 to 3) etc. can be mentioned.

[0018] The above-mentioned Na 3+2x Zr 2-x Mg x(SiO4)2(PO4) (where x = 0, 0.05, or 0 < x < 0.05) (hereinafter referred to as NZMSP), the assumed operating temperature varies depending on the value of x in the composition formula. One of the major factors affecting the output of the battery is the ionic conductivity of the solid electrolyte (how easily ions flow; the larger the value, the easier the flow). The ionic conductivity tends to increase with increasing temperature. Since the NASICON-type NZMSP has a lower ionic conductivity when x = 0 than when x = 0.05, the battery operates at a higher temperature to compensate for the low ionic conductivity of the material itself. As a general property of the battery, the expected output cannot be obtained when used at a lower temperature. Although the operating temperature and the value of x do not correspond one-to-one, for example, when assuming use at a low temperature, a material with high ionic conductivity (a material with precisely controlled x) is required, but when assuming use at a high temperature, a certain degree of low ionic conductivity is acceptable, so it is not always necessary to set x to the value at which the ionic conductivity is maximized (i.e., x = 0.05), but of course, it can also be set to the maximum x.

[0019] Furthermore, when the positive electrode and negative electrode are configured as a composite electrode, the mixing ratio of the electrode active material, solid electrolyte, and conductive carbon is not limited to a specific ratio, but for example, it is preferable that electrode active material:solid electrolyte:carbon = 20~70:75~20:5~15 wt%. This range of values ​​is a general value in the field of all-solid-state batteries that is empirically expected to yield the desired performance. For example, taking the electrode active material as an example, it is considered that the performance (in this case, capacity) will decrease if the value is smaller than 20 wt% or larger than 70 wt%. In other words, if the proportion of electrode active material is less than 20 wt%, the battery characteristics tend to decrease in terms of energy density characteristics and power density characteristics per unit mass. In addition, the conductive additive should be included in the composite electrode powder at a concentration of 5~15 wt%, preferably 15 wt%. If the content of the conductive additive is too low, it tends to become difficult to achieve high capacity and high rate in the electrode composite material. On the other hand, if the content of the conductive additive is too high, the amount of active material per unit mass of the electrode composite material decreases, which tends to reduce the charge / discharge capacity. In addition, sintering is inhibited, which can break the ion conduction path, leading to a decrease in charge / discharge capacity and discharge voltage. On the other hand, if the proportion of active material is greater than 70 wt%, the network of solid electrolyte within the electrode layer may be interrupted. Therefore, it is preferable that the composite electrode materials for the positive and negative electrodes be appropriately adjusted within the above-mentioned mixing ratio range. For example, the positive and negative electrode materials in this embodiment are a mixture (composite electrode material) in which the active material NVP, the solid electrolyte NZMSP, and the electron conduction additive carbon are mixed in a weight ratio of 25:60:15.

[0020] Furthermore, the flux only needs to melt at a temperature below the reaction temperature between the electrode material and the solid electrolyte and function as an adhesive to the solid electrolyte. For example, in this embodiment, where positive and negative electrode materials containing NVP as the active material and NZMSP as the solid electrolyte, the flux only needs to melt at a temperature lower than the reaction temperature of NZMSP and NVP (800°C) and function as an adhesive to the solid electrolyte. In the case of an all-solid-state battery using a combination of NZMSP and NVP, a temperature lower than 800°C is sufficient, so a flux that melts at, for example, 700°C would be acceptable. However, in order to develop the manufacturing method of the present invention into a general-purpose sintering method applicable to other oxide materials, it is desirable to enable battery fabrication at a temperature lower than the reaction temperature of many oxides (e.g., 300°C-600°C). For this reason, it is desirable that the flux melts at a temperature lower than at least 400°C.

[0021] Examples of such fluxes include sodium tetraborate hydrate (also called decahydrate borax, Na2B4O7·10H2O), boric acid ((H3BO3) or B(OH3)), sodium metaborate (metaborax (NaBO2·4H2O)), oxo salts such as sodium hydrogen phosphate, inorganic sodium salts such as sodium hydroxide, and organic sodium salts such as sodium acetate and sodium stearate. Among these, sodium tetraborate hydrate is preferred as a flux. Sodium tetraborate hydrate melts at temperatures below 100°C, but in the state in which it is manufactured as a battery, it is thought to have changed from the original flux due to dehydration after melting (i.e., formation of anhydrous Na2B4O7) and reactions with solid electrolytes, and the resulting all-solid-state battery will not melt at the operating temperature (i.e., it can maintain its battery shape).

[0022] Even if only 1 wt% of the flux is mixed with the solid electrolyte, a density ratio exceeding that of the case without mixing the flux can be obtained. And the inventors have found through various experiments and studies that if the mixing amount of the flux is increased, the density ratio of the electrolyte increases monotonically, but at a certain point, the characteristics of the battery deteriorate. That is, by mixing 5 wt% or more of the flux with the solid electrolyte, a density ratio sufficient to maintain the battery shape can be achieved, and when mixed at 20 wt%, a density ratio sufficient to maintain the battery shape can be achieved even with sintering at 300 °C. However, when 30 wt% of the flux is mixed, the battery shape cannot be maintained during pressure sintering. From this, the mixing amount of the flux with respect to the solid electrolyte is preferably in the range of 5 wt% to 20 wt% with respect to the solid electrolyte, more preferably about 10 wt%.

[0023] Therefore, in the method for manufacturing an all-solid-state battery of the present embodiment, for example, any material appropriately selected from the above-described electrode material, inorganic solid electrolyte, and flux. For example, as the positive and negative electrode active materials, NASICON-type Na3V2(PO4)3 (hereinafter referred to as NVP), and as the solid electrolyte, NASICON-type Na 3+2x Zr 2-x Mg x (SiO4)2PO4 (where x = 0, 0.05, or 0 < x < 0.05) (hereinafter referred to as NZMSP) are combined, and as the flux, Na2B4O7·10H2O (sodium tetraborate (borax), referred to as NBO) is used. An appropriate amount, for example, 10 wt% of the flux (Na2B4O7·10H2O) is previously mixed in NZMSP. Here, the positive electrode material and the negative electrode material use a composite electrode material in which NVP of each electrode active material, NZMSP of the solid electrolyte, and carbon (C) as a conductive aid are mixed at a weight ratio of 25:60:15. This composite electrode material and the solid electrolyte NZMSP in which the flux (Na2B4O7·10H2O) is previously mixed are formed into a pellet-shaped green body, for example, by pressing.

[0024] Next, a solid electrolyte green body, pre-mixed with a flux, is placed between the negative electrode green body and the positive electrode green body. Under pressure, it is heated at a temperature above the melting point of the flux and below the reaction temperature between the electrode materials and the solid electrolyte, for example, 600°C, to bond them together, thereby sintering and integrating them in one step. In this case, a portion of the molten Na2B4O7-10H2O (sodium tetraborate + hydrate) permeates the negative electrode and positive electrode materials, allowing them to bond with the inorganic solid electrolyte. The solid electrolyte and the molten flux react to irreversibly function as an adhesive and form conductive paths. In other words, the molten flux at the contact points between the positive electrode material, the negative electrode material, and the solid electrolyte promotes these contacts. The flux also contributes as a passage for sodium ions. This ensures ionic conductivity and provides a structure that functions as a battery. It is preferable to perform the sintering of the green bodies using, for example, a spark plasma sintering (SPS) apparatus. Discharge plasma sintering is a type of solid compression sintering method, similar to hot press sintering (HP), in which a graphite sintering mold filled with powder or solid material is heated under pressure. Rapid heating and cooling are possible, and the combination of pressure and rapid heating is expected to produce a dense sintered body with suppressed grain growth.

[0025] The positive and negative composite electrode materials and the solid electrolyte are sintered at the temperature and pressure necessary to obtain a sintered body, resulting in a sodium-based all-solid-state battery. For example, the sintering conditions for a composite electrode material using NVP as the polar active material and a solid electrolyte of NZMSP are a sintering temperature of 300°C to 600°C and a pressure of 102 MPa. In this embodiment, the pressure applied for densification during sintering is set to the maximum pressure capacity of the equipment used, but it is not necessarily limited to this; higher pressures may be applied, or in some cases, lower pressures may be used. In other words, the above-mentioned sintering conditions are merely an example and are not particularly limited. What is important is that the sintering conditions for the composite electrode material and the inorganic solid electrolyte allow the sintering temperature under pressure to be lowered to 600°C, and that a sintered body that can maintain its battery shape can be obtained even below 600°C. According to experiments by the inventors, in the case of an all-solid-state sodium battery with the above configuration, it was confirmed that even below 600°C, at 500°C or 400°C, the battery would function regardless of the size of the battery capacity. Furthermore, depending on the combination of electrode material and solid electrolyte, it has become possible to obtain a battery that can function even with low-temperature sintering at 300°C.

[0026] In this embodiment, the all-solid-state battery produced by the above-described method uses composite electrode materials containing the same electrode active material as the positive and negative electrode materials. However, there are no restrictions on the selection or combination of electrode materials, and it goes without saying that the pressure and temperature during sintering should be appropriately changed depending on the electrode material selected, as there are optimal conditions specific to each electrode material. In other words, the all-solid-state battery according to the present invention can also be used when different electrode materials are combined, and it goes without saying that the firing temperature that can be implemented should be appropriately set accordingly.

[0027] The above-described embodiment is merely one example of a preferred embodiment of the present invention, and is not limited thereto. Various modifications can be made without departing from the spirit of the present invention. For example, the above-described embodiment mainly described an example implemented with an all-solid-state sodium battery, but is not particularly limited thereto. It goes without saying that it can be applied to other all-solid-state batteries, such as all-solid-state lithium batteries. In other words, the manufacturing method of the present invention makes a battery functional by pressing and sintering a powdered positive and negative electrode composite electrode material and an inorganic solid electrolyte material that has been pre-mixed with a flux that melts at low temperatures, thereby melting the flux and integrating and joining them together. That is, by using a flux, it is possible to manufacture an all-solid-state battery at low temperatures. Therefore, it goes without saying that the manufacturing method of the present invention is applicable to various combinations of electrode materials, regardless of the electrode material. For example, for a Li-based all-solid-state battery, integration is possible in a single low-temperature sintering by applying Li2B4O7·5H2O, which is a boric acid-based hydrate similar to sodium metaborate.

[0028] Furthermore, while the above embodiments mainly described examples of sintering using a discharge plasma sintering apparatus, the invention is not particularly limited to this, and in some cases, it is also possible to manufacture the product by hot pressing (HP), hot isostatic pressing (HIP), or gas pressure sintering. In addition, the pressure and temperature conditions during sintering required to obtain a sintered body that functions as a battery are inherent to the electrode material, so the pressure and sintering temperature mentioned in the embodiments do not have absolute significance. Since the objective of the present invention is to realize low-temperature sintering, it is preferable to make the temperature as low as possible within the temperature range below the reaction temperature between the electrode material and the solid electrolyte, but this depends on the electrode active material and inorganic solid electrolyte material used. In other words, the numerical values ​​given in this specification are merely examples, and the pressure and temperature during sintering mentioned in the embodiments do not have absolute significance. [Examples]

[0029] (Battery performance test 1) Na3V2(PO4)3 (so-called NVP) is used as the electrode active material for the positive and negative electrode materials, and Na is used as the oxide solid electrolyte.3+2x Zr 2-x Mg x A sodium-based all-solid-state battery was fabricated using an oxide solid electrolyte (NZMSP) represented by (SiO4)2(PO4) (where x=0.05) and sodium tetraborate hydrate (Na2B4O7-10H2O) as a flux, and its battery performance was confirmed.

[0030] NVP was synthesized as follows: First, a solution was prepared by dissolving NaH2PO4·2H2O, V2O5, NaNO3, and C2H2O4·2H2O in deionized water in a molar ratio of 3.0:1.0:0.15:1-30, and then heating and drying it at 80°C. The resulting powder was then calcined in an Ar atmosphere at 700°C for 12 hours, and the resulting calcined product was pulverized to obtain the electrode active material powder (NVP material powder) represented by Na3V2(PO4)3.

[0031] NZMSP was synthesized as follows: First, stoichiometric amounts of NaNO3, ZrO(NO3)2·2H2O, Si(OC2H5)4, (NH4)2HPO4, and Mg(NO3)2·6H2O were dissolved in a mixed solution of water, ethanol, and nitric acid, and heated and dried at 80°C to obtain a powder. This powder was calcined at 800°C for 6 hours to obtain a precursor powder. This precursor powder was formed into pellets using a tablet molder and heated and calcined at 1150°C for 12 hours to obtain Na 3+2x Zr 2-x Mg x An oxide solid electrolyte represented by (SiO4)2(PO4)(NZMSP) (where x=0.05) was obtained. This oxide solid electrolyte pellet was crushed in a mortar to obtain oxide solid electrolyte powder.

[0032] In this embodiment, the positive and negative electrode materials are composed of a composite electrode material that includes a solid electrolyte and conductive graphite in addition to the positive and negative electrode active materials. The positive and negative electrode composite material was prepared by mixing the aforementioned NVP, NZMSP, and carbon in a weight ratio of 25:60:15 and mixing for a predetermined time, for example, 30 minutes, to obtain the composite electrode materials for the positive and negative electrodes, respectively.

[0033] For the battery fabrication, a powder mixture of NZMSP (0.075g) and sodium tetraborate hydrate (Na2B4O7·10H2O) was placed in a cylindrical graphite die with an inner diameter of 10mm and pressed at a pressure of 185MPa to obtain a pellet-shaped molded body (green body). Furthermore, 0.010g of positive electrode composite electrode material and 0.020g of negative electrode composite electrode material were placed on the bottom surfaces of this pellet-shaped solid electrolyte molded body, and pressed at a pressure of 74MPa. This resulted in a green body in which the solid electrolyte was sandwiched between the positive and negative electrode materials.

[0034] Next, using a plasma discharge sintering apparatus (Sumitomo Coal Mining Co., Ltd., model SPS-511ET), the material was heated to 600°C for 10 minutes under a pressure of 102 MPa to sinter it. This melted the flux mixed with the solid electrolyte and reacted with it, and also allowed some of the flux to permeate into the solid electrolyte of the positive and negative electrode composite electrode material, causing it to react and bond together.

[0035] In this example, sodium tetraborate hydrate (Na2B4O7·10H2O), used as a flux, melts at temperatures below 100°C. However, during battery fabrication, dehydration occurs after melting (forming anhydrous Na2B4O7) and reactions with solid electrolytes occur, changing the original flux. Therefore, the resulting all-solid-state battery will not melt at the operating temperature.

[0036] Following the procedure described above, seven types of bulk-type all-solid-state sodium batteries were fabricated with varying amounts of flux mixed with the solid electrolyte: 1 wt% (Example 1), 5 wt% (Example 2), 10 wt% (Example 3), 15 wt% (Example 4), 20 wt% (Example 5), 30 wt% (Example 6), and no flux mixed (0 wt%: Comparative Example). The bulk-type all-solid-state sodium batteries were fabricated with a diameter of 10 mm, a thickness of 0.5 mm, and a weight of approximately 0.1 g.

[0037] Figure 2 shows the relationship between the theoretical density ratio of the solid electrolyte and the weight ratio of the flux mixed with the solid electrolyte for the all-solid-state sodium batteries of Examples 1 to 6, which were manufactured as described above. As is clear from this figure, even mixing just 1 wt% of the flux with the solid electrolyte improved the theoretical density ratio of the solid electrolyte (NZMSP), and this ratio increased monotonically with increasing amounts of flux.

[0038] Figure 3 shows the shapes of the seven types of bulk-type all-solid-state sodium batteries that were fabricated. The batteries without flux (comparative example) and with 1 wt% flux (Example 1) were brittle due to their low density and easily broke, while the batteries with 30 wt% flux (Example 6) were distorted due to uneven pressure during sintering caused by excessive flux. In other words, normal pelletization was not successful. On the other hand, Examples 2-5, in which flux was pre-mixed with the solid electrolyte in the range of 5 wt% to 20 wt%, were successfully pelletized.

[0039] Table 1 shows the relationship between the theoretical density ratio of the solid electrolyte and the weight ratio of the flux mixed with the solid electrolyte at 600°C sintering. The density ratios for the comparative example and each example were 66.2% for the comparative example without flux, 75.2% with 1 wt%, 77.5% with 5 wt%, 81.4% with 10 wt%, 88.3% with 15 wt%, 91.7% with 20 wt%, and NA with 30 wt%. In this experiment, the density ratio was calculated using Equation 1 below, assuming that all the water in the flux had vaporized. The thickness used to calculate the electrolyte density was determined from the cross-sectional SEM image.

[0040] [Table 1]

[0041]

number

[0042] Next, Figure 4 shows cross-sectional SEM images of the seven types of battery electrolytes described above. As is clear from these SEM images, a sintering temperature of 600°C is low for firing oxide solid electrolytes. Therefore, in the case of a solid electrolyte without a flux (Comparative Example 1), the phenomenon of solid electrolyte material particles fusing and bonding at the contact surface did not proceed, and the voids were not filled, leaving particles of various sizes prominently present. In other words, it is clear that the sintering was not successful because the particles of the solid electrolyte material did not stick together at the contact surface, resulting in gaps.

[0043] On the other hand, when a flux was pre-mixed with the solid electrolyte, improvement was observed even with a 1 wt% mixture, and the smoother and denser the surface, the more flux was added. In other words, the cross-section was clean and well-sintered. The molten flux acted like a flux, assisting the sintering process. This revealed that densification could be achieved even at lower sintering temperatures by mixing in a flux.

[0044] Of the all-solid-state sodium batteries manufactured as described above, charge-discharge tests were conducted on four types of batteries that maintained their shape well: 5wt% (Example 2), 10wt% (Example 3), 15wt% (Example 4), and 20wt% (Example 5), as well as on 1wt% (Example 1), which had difficulty maintaining its shape. The charge-discharge tests were performed using a charge-discharge test apparatus (Hokuto Denko, Model HJ1010mSM8A).

[0045] The results of the charge-discharge tests are shown in Figures 5 and 6. As is clear from the charge-discharge curve in Figure 5, in the case of all-solid-state sodium batteries in which 5 wt% to 20 wt% of the flux was mixed with the solid electrolyte, it was confirmed that all of them functioned as batteries, although the battery capacity varied. However, the charge-discharge curve for 5 wt% (Example 2) showed a lower capacity compared to the theoretical capacity and is not sufficient for practical use. It is possible that the conduction path on the electrode side was insufficient due to the small amount of flux. Also, in the case of 20 wt% (Example 5), the capacity was small. The density increased due to the large amount of flux, but it is thought that the passage for sodium ions decreased, leading to a tendency for poor ionic conductivity. Similarly, the capacity was also small in the case of 15 wt% (Example 4). In this case as well, the density increased due to the large amount of flux, but it is thought that the passage for sodium ions decreased. This is thought to be due to a decrease in conduction paths or active material due to side reactions. On the other hand, in the case of 10 wt% (Example 3), the charge-discharge was close to the theoretical value. This is thought to be because a suitable amount of flux allowed many electrodes to sinter together, connecting the conduction paths. Therefore, in the sodium-based all-solid-state battery of this embodiment, the amount of flux mixed is preferably in the range of 5 wt% to 20 wt%, but more preferably about 10 wt% in order for it to function as a battery.

[0046] Furthermore, the charge-discharge cycle characteristics are shown in Figure 6. As a result, in the case of 10 wt% (Example 3), the discharge capacity is high. The discharge capacity does not fluctuate significantly even when the number of cycles is greatly increased. In contrast, in the case of 5 wt% (Example 2), there is less flux and it is not sintered, so there are many gaps between the solid electrolyte particles, resulting in low ionic conductivity and possibly insufficient conduction paths. Also, in the case of 15 wt% (Example 5), the capacity decreased sharply after a few cycles due to a decrease in conduction paths or active material due to side reactions. Furthermore, in the case of 20 wt% (Example 5), since there are fewer conduction paths to begin with, the effects of side reactions and volume changes of the active material are small, and the cycle characteristics were better than those of 15 wt% (Example 5).

[0047] On the other hand, in the case of the 1wt% battery characteristics (Example 1), a flat charging potential appeared around 1.2V, as shown in Figure 7, but the capacity decreased sharply after a few cycles. This is thought to be because the poor contact could not keep up with the volume change of the active material, causing the conduction path to be broken. At 2C, the polarization was too large, and the battery did not function.

[0048] From the above experiments, it was confirmed that the all-solid-state batteries described in Examples 2 to 5 above (i.e., sodium-based all-solid-state batteries in which 5 wt% to 20 wt% of flux was mixed with the solid electrolyte) fabricated by low-temperature sintering at 600°C by mixing in a flux functioned as batteries. In particular, in the case of the all-solid-state sodium battery with a flux content of 10 wt% in Example 3, the charge and discharge rates were close to the theoretical values, and good cycle characteristics were obtained in which the discharge capacity did not fluctuate significantly even when the number of cycles was greatly increased. Specifically, it operated at 60°C and showed a discharge rate of 78 mAh / g.

[0049] (Battery performance test 2) Furthermore, we conducted experiments to investigate the effect of sintering temperature on the flux mixture amount of 10 wt% (Example 3), which was deemed optimal based on the above experiments.

[0050] The experiment involved sequentially switching between three sintering temperatures—500°C, 400°C, and 300°C—to determine how low the temperature could be reduced compared to the optimal sintering temperature of 600°C obtained in the aforementioned experiment. All other conditions were the same as in the previous experiment.

[0051] The experimental results showed that the pellet shape, i.e., the battery shape, could be maintained at any sintering temperature between 300 and 500°C (see Figure 9). However, below 500°C, the theoretical density ratio decreased (see Figure 10), and the SEM image of the solid electrolyte showed that the solid electrolyte particles were granular and had many gaps (see Figure 8). From this, it can be concluded that the flux's action is weak at low temperatures below 500°C.

[0052] Furthermore, in the charge-discharge test for 500°C sintering, as shown in Figure 11, the low density (many gaps) resulted in fewer conduction paths and a considerably low capacity.

[0053] Furthermore, in the case of sintering at 400°C, as shown in Figure 12, the density is low (there are many gaps), resulting in fewer conduction paths and a considerably low capacitance.

[0054] Furthermore, in the case of 300°C sintering, the density is low (there are many gaps), resulting in fewer conduction paths. As shown in Figure 13, it did not function as a battery even at a low current density of 1 / 20C. Due to the large polarization, the cutoff voltage was reached immediately when current was applied, and no potential difference was created between the positive and negative electrodes, resulting in a negative potential during discharge. In other words, it did not function as a battery during 60°C charge / discharge.

[0055] Based on these findings, while the sintering (firing) temperature in Example 3, with a flux content of 10 wt%, can maintain the pellet shape, i.e., the battery shape, at any temperature between 500°C and 300°C, it is desirable to lower the temperature to 400°C in order to obtain a product that can barely function as a battery.

[0056] (Battery performance test 3) Furthermore, in the case of a 20 wt% flux mixture (Example 5) in which the smoothest surface was obtained in the cross-sectional SEM image of the solid electrolyte, the solid electrolyte Na 3+2x Zr 2-x Mg x We investigated whether (SiO4)2(PO4)(NZMSP(x=0.05)) could be replaced with Na3Zr2(SiO4)2(PO4) (i.e., NZSP(x=0)) to determine if it would function as a battery.

[0057] The experimental conditions were a mixing ratio of 20 wt% of flux (Na2B4O7·10H2O) to solid electrolyte (Na3Zr2(SiO4)2(PO4)(NZSP)), a sintering temperature of 600°C, and a battery operating temperature of 200°C. Here, when x=0 for the solid electrolyte (i.e., NZSP without substitution with a small amount of Mg), the ionic conductivity of the electrolyte is lower than when x=0.05 (battery operation at 60°C), so the battery will operate at a higher temperature (200°C).

[0058] The experimental results confirmed that it could function as a battery. As is clear from Figure 14, which shows the charge-discharge curve and cycles, it was found that although the capacity was low, it could function as a battery and also exhibited good cycle characteristics. Furthermore, the solid electrolyte was changed to Na 3+2x Zr 2-x Mg x Compared to the case using (SiO4)2(PO4)(NZMSP, x=0.05), the results were better because charging and discharging were performed at a higher operating temperature of 200°C to compensate for the low ionic conductivity of the solid electrolyte and flux. In other words, even when using a solid electrolyte of x=0.05 (i.e., NZMSP), it is likely that better results will be obtained if charging and discharging are performed at 200°C.

[0059] Furthermore, compared to an example where NZMSP was used as the solid electrolyte and mixed with 20 wt% flux and sintered at 600°C, the result showed that using NZSP as the solid electrolyte resulted in a battery with almost twice the capacity and good cycle characteristics. From this, it was found that even with low-temperature sintering at 300°C, if the battery shape can be maintained, it is possible to obtain a battery that functions depending on the combination of electrode material and solid electrolyte. [Explanation of Symbols]

[0060] 1. Negative electrode material (composite electrode material) 2 Positive electrode material (composite electrode material) 3 solid electrolyte 4. Flux

Claims

1. A flux that melts at a temperature below the reaction temperature between the positive and negative electrode materials and the solid electrolyte and functions as an adhesive is pre-mixed in an amount of 1-20 wt% relative to the solid electrolyte, to the green body of the negative electrode material containing the negative electrode active material and the green body of the positive electrode material containing the positive electrode active material. By heating the green bodies of the negative electrode material, the green bodies of the positive electrode material, and the green bodies of the solid electrolyte under pressure at a sintering temperature of 300°C or higher and between 600°C, the negative electrode material, the positive electrode material, and the solid electrolyte are integrated in a single sintering process. A method for manufacturing an all-solid-state battery, characterized by the above.

2. A method for manufacturing an all-solid-state battery according to claim 1, characterized by mixing 5-20 wt% of the flux with the solid electrolyte and sintering at 600°C.

3. A method for manufacturing an all-solid-state battery according to claim 1, characterized by mixing 10 wt% of the flux with the solid electrolyte and performing sintering at a temperature of 400°C or less but less than or equal to 600°C.

4. As the active materials of the positive and negative electrodes, NASICON type Na ３ V ２ (PO ４ ) ３ , as the solid electrolyte, NASICON type Na ３＋２ｘ Zr ２－ｘ Mg ｘ (SiO ４ ) ２ PO ４ (where x = 0, 0.05, or 0 < x < 0.05), as the flux, Na ２ B ４ O ７ ·10H ２ O, The method for manufacturing an all-solid-state battery according to any one of claims 1 to 3, characterized in that it is O.

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