All-solid-state secondary battery

Abstract

Provided is an all-solid-state secondary battery comprising: a positive electrode 11 that contains a cyanide which contains sodium; a negative electrode 13 that contains a substance into / from which sodium can be inserted and desorbed at a potential lower than that of the positive electrode; and a chloride solid electrolyte 12 that contains sodium.

Description

All-solid-state secondary battery

[0001] The present disclosure relates to an all-solid-state secondary battery.

[0002] Conventionally, all-solid-state lithium secondary batteries have been developed as secondary batteries mounted in electric vehicles, mobile devices and the like. An all-solid-state lithium secondary battery is a secondary battery with high safety and high energy density.

[0003] Patent No. 6565724, Patent No. 7281296

[0004] An all-solid-state secondary battery is configured by laminating a solid negative electrode, a solid electrolyte, and a solid positive electrode. Unlike conventional secondary batteries, it does not use a liquid electrolyte containing a flammable organic solvent. Therefore, the all-solid-state secondary battery has a low risk of fire and the like, and can achieve high safety. In addition, since all components of the all-solid-state secondary battery are solid, high energy density of the battery can be achieved by stacking a plurality of battery cells with high efficiency.

[0005] All-solid-state secondary batteries are expected to be widely used in a wide range of applications including large-scale applications such as electric vehicles. When such an all-solid-state secondary battery becomes widespread, it is desirable from the viewpoint of reducing environmental load to recycle and reuse the constituent materials after the battery has been used for a certain period of time and deterioration has progressed.

[0006] In the all-solid-state lithium secondary batteries of Patent Documents 1 and 2, Li x La3Zr2O 12 (6≦x≦8), (Li 3a ,La 2 / 3-a )(Mg b ,W 1-b )3(1 / 6≦a≦1 / 3, 0.4≦b≦0.6) and other metal oxide materials are used as solid electrolytes. In general, chemically stable metal oxides do not dissolve in neutral water in a short time. In addition, Patent Documents 1 and 2 also exemplify manganese dioxide (MnO2), iron oxide and the like as positive electrode active materials. Since these are also stable metal oxides, they do not have solubility in neutral water.

[0007] Thus, conventional all-solid-state rechargeable batteries have the drawback of using lithium, a resource with limited production countries and high resource risk, and undergoing complex recycling processes without consideration for improving recycling efficiency or reducing environmental impact.

[0008] This disclosure is made in view of the above circumstances and aims to provide an all-solid-state secondary battery that reduces resource risk and has excellent recyclability.

[0009] The all-solid-state secondary battery of this disclosure includes a positive electrode containing sodium-containing cyanide, a negative electrode containing a substance that allows sodium to be inserted into and removed from at a potential lower than that of the positive electrode, and a chloride solid electrolyte containing sodium.

[0010] According to this disclosure, it is possible to provide an all-solid-state secondary battery that reduces resource risk and has excellent recyclability.

[0011] Figure 1 is a basic schematic diagram of the all-solid-state secondary battery of this embodiment. Figure 2 is a schematic cross-sectional view showing an example of the structure of the all-solid-state secondary battery. Figure 3 is a diagram showing the method of adjusting the positive electrode of the embodiment. Figure 4 is a diagram showing the method of adjusting the negative electrode of the embodiment. Figure 5 is a graph showing the discharge curves during the first discharge of Embodiments 1 and 3. Figure 6 is a table showing the battery performance of the embodiment and comparative example. Figure 7 is a table showing the solubility evaluation of the embodiment and comparative example.

[0012] Hereinafter, embodiments of the present invention will be described with reference to the figures.

[0013] [Configuration of the secondary battery] Figure 1 is a configuration diagram showing the configuration of an all-solid-state secondary battery in an embodiment of the present invention. The all-solid-state secondary battery in this embodiment is a secondary battery in which a negative electrode 13, a solid electrolyte 12, and a positive electrode 11 are stacked.

[0014] The all-solid-state secondary battery of this embodiment includes a positive electrode 11 containing sodium-containing cyanide, a negative electrode 13 containing a substance that allows for the insertion and removal of sodium at a potential lower than that of the positive electrode 11, and a chloride solid electrolyte 12 containing sodium.

[0015] The positive electrode 11 may contain Na4Fe(CN)6 or NaFeFe(CN)6 as a sodium-containing cyanide.

[0016] The materials that allow for the insertion and removal of sodium from the negative electrode 13 also include materials that allow for the alloying and non-alloying of sodium. The negative electrode 13 can be made of metallic materials such as tin, carbon-based materials, etc. For example, the negative electrode 13 may contain metallic tin or hard carbon.

[0017] The chloride solid electrolyte 12 can be a sodium-containing chloride material that has sodium ion conductivity and is water-soluble. The chloride solid electrolyte 12 may also contain zirconium.

[0018] The following describes each of the above-mentioned components of the all-solid-state secondary battery of this embodiment.

[0019] (1) Positive electrode The positive electrode 11 of this embodiment contains at least a positive electrode active material and may also contain the following solid electrolyte powder, conductive additive, binder, and current collector.

[0020] (1-1) Positive electrode active material In this embodiment, the positive electrode active material contains a cyanide containing sodium. The cyanide is cyanide (CN － It is a complex having ). As a sodium-containing cyanide, Na4M1(CN)6 (where M1 is at least one transition metal selected from the group consisting of Mn, Fe, Co, Ni, Cu, and Mo) can be used, but Na4Fe(CN)6 (sodium ferrocyanide) is preferred from the viewpoint of water solubility. Alternatively, NaM1M2(CN)6 (where M1 and M2 are at least one transition metal selected from the group consisting of Mn, Fe, Co, Ni, Cu, and Mo) can also be used, but NaFeFe(CN)6) is preferred from the viewpoint of water solubility. Na4Fe(CN)6 can be used by purchasing a commercially available reagent and performing vacuum drying treatment. NaFeFe(CN)6) can be synthesized based on the following reaction equation.

[0021] Fe4[Fe(CN)6]3 + Na4Fe(CN)6 → 4 NaFeFe(CN)6 The addition ratio of the positive electrode active material is preferably 30-60% (by weight) of the total electrode of the positive electrode 11.

[0022] (1-2) Solid electrolyte powder In order to ensure ion conduction paths in the positive electrode, it is necessary to mix the solid electrolyte powder into the positive electrode. The solid electrolyte powder is the powder of the solid electrolyte described later. It is preferable to use a chloride solid electrolyte as the solid electrolyte.

[0023] The solid electrolyte powder needs to be as fine as possible and uniformly dispersed in the positive electrode. Therefore, the solid electrolyte powder and other positive electrode materials are crushed and mixed using a ball mill or the like. The addition ratio of the solid electrolyte powder is preferably 30-50% (by weight) of the entire positive electrode 11.

[0024] (1-3) Conductive additives For conductive additives, carbon can be used, for example. Specifically, carbon blacks such as Ketjenblack and acetylene black, activated carbon, graphites, and carbon fibers can be used. These carbons can be obtained, for example, as commercially available products or by known synthesis. The additive ratio of the conductive additive is preferably 5 to 30% (by weight) of the entire electrode of the positive electrode 11.

[0025] (1-4) Binding agent The positive electrode 11 may contain a binding agent. Specific examples of binding agents include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, ethylene propylene diene rubber, and natural rubber. The binding agent addition ratio is preferably 5-30% (by weight) of the entire electrode of the positive electrode 11.

[0026] (1-5) Current collector The current collector may be a current collector comprising at least one selected from the group consisting of aluminum, copper, iron, and titanium, or a nonwoven fabric current collector containing carbon.

[0027] (1-6) Method for manufacturing a positive electrode A positive electrode 11 can be manufactured by pressing a mixture obtained by crushing and mixing a positive electrode active material (sodium-containing cyanide powder), a solid electrolyte powder, a conductive additive powder (e.g., carbon powder), and a binder powder. Alternatively, the positive electrode 11 can also be manufactured by rolling the mixture in a roll press, processing it into a sheet, and pressing it onto a current collector.

[0028] Alternatively, the positive electrode 11 can be manufactured by applying a slurry containing dispersed positive electrode active material to a current collector, or by impregnating the current collector with the slurry, and then drying the coated or impregnated current collector. Here, by applying a cold press or hot press to the electrode (positive electrode 11) after drying, the strength of the electrode can be increased, and a more stable positive electrode 11 can be manufactured.

[0029] (1-7) Determination of a recyclable positive electrode material Considering recyclability, it is preferable to avoid using substances other than water-soluble substances as the positive electrode material. Therefore, by optimizing the electrode composition and / or current collection method, the positive electrode 11 could be constructed using only a positive electrode active material, chloride solid electrolyte powder, and a conductive additive (carbon), as will be explained in the examples described later.

[0030] Here, chloride solid electrolyte powder exhibits superior deformability compared to oxide solid electrolyte powder. Therefore, chloride solid electrolyte powder has the effect of binding the positive electrode active material and conductive additive, eliminating the need for binders like those used in conventional fluoropolymer resins.

[0031] Furthermore, even if a current collector (current collecting material) is used, if the other materials are water-soluble, the active material will dissolve when the electrodes are immersed in water (neutral), making it easy to recover the current collector, which can then be recycled. Similarly, the carbon used as a conductive additive can also be easily recovered by filtration or centrifugation, and the carbon can be recycled as well.

[0032] (2) Negative electrode The negative electrode 13 of this embodiment contains at least a negative electrode active material and may also contain the following solid electrolyte powder, conductive additive, binder, and current collector.

[0033] (2-1) Negative Electrode Active Material The negative electrode active material of the present embodiment contains a substance capable of intercalating and deintercalating sodium at a lower potential than the positive electrode 11. Examples of the substance that can be used include metallic tin foil that can be alloyed with sodium, and carbon materials that can intercalate and deintercalate sodium. The carbon material is graphite (natural graphite, artificial graphite) used in general secondary batteries, or amorphous hard carbon which is a type of carbon black. The addition ratio of the negative electrode active material which is a carbon material is preferably 30 to 70% by weight relative to the entire electrode of the negative electrode 13.

[0034] (2-2) Solid Electrolyte Powder When a powder material such as a carbon material is used as the negative electrode active material, it is necessary to mix solid electrolyte powder into the negative electrode in order to secure an ion conduction path in the negative electrode. The solid electrolyte powder is a powder of the solid electrolyte described below. It is preferable to use a chloride solid electrolyte as the solid electrolyte.

[0035] The solid electrolyte powder needs to be uniformly dispersed in the negative electrode as fine powder as possible. Therefore, the solid electrolyte powder and other negative electrode materials are pulverized and mixed using a ball mill or the like. The addition ratio of the solid electrolyte powder is preferably 30 to 50% by weight relative to the entire electrode of the negative electrode 13.

[0036] (2-3) Binder The negative electrode 13 may contain a binder. Specific examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, ethylene propylene diene rubber, and natural rubber. The addition ratio is preferably 5 to 30% by weight relative to the entire electrode of the negative electrode 13.

[0037] (2-4) Current Collector The negative electrode 13 may be formed on a current collector containing at least one selected from the group consisting of copper and titanium, or on a non-woven current collector containing carbon. These current collectors can be obtained, for example, as commercially available products.

[0038] (2-5) Method for Producing Negative Electrode When metallic tin is used as the negative electrode active material, the negative electrode 13 can be produced by purchasing a commercially available metallic tin foil and cutting it into a predetermined shape for the battery jig described later.

[0039] When a carbon material is used as the negative electrode active material, the negative electrode 13 can be produced by pressing a mixture obtained by pulverizing and mixing a negative electrode active material (e.g., hard carbon powder), a solid electrolyte powder, and a binder powder. Alternatively, the negative electrode 13 can also be produced by rolling the mixture with a roll press, processing it into a sheet shape, and pressure-bonding the sheet to a current collector.

[0040] Alternatively, the negative electrode 13 can be produced by applying a slurry in which the negative electrode active material is dispersed onto a current collector, or impregnating a current collector with the slurry, and drying the coated or impregnated current collector. Here, by applying cold pressing or hot pressing to the dried electrode (negative electrode 13), the strength of the electrode can be increased, and the negative electrode 13 having more excellent stability can be produced.

[0041] (2-6) Determination of negative electrode material imparting recyclability In consideration of recyclability, it is preferable to avoid using substances other than water-soluble substances as the negative electrode material. Therefore, by optimizing the electrode composition and / or current collection method, when a carbon material is used as the negative electrode active material, as described in the embodiment, the negative electrode 13 can be configured using only the negative electrode active material (e.g., hard carbon) and chloride solid electrolyte powder.

[0042] Here, the chloride solid electrolyte powder is superior in deformability to oxide solid electrolyte powder. Therefore, the chloride solid electrolyte powder has an effect of binding the negative electrode active material, and by using the chloride solid electrolyte powder, the use of a binder such as a conventional fluorine-based resin is unnecessary.

[0043] Even if a current collector (current collecting material) is used, for example, if other materials are water-soluble, the active material dissolves when the electrode is immersed in neutral water, so the current collector can be easily recovered, and the recovered current collector can be recycled.

[0044] (3) Solid Electrolyte A chloride solid electrolyte containing sodium is used for the solid electrolyte 12. Specifically, a sodium-containing chloride material that has sodium ion conductivity and is water-soluble can be used. For example, Na3YCl6, Na2ZrCl6, NaNbCl6, and NaTaCl6 can be used for the solid electrolyte 12. From the viewpoint of conductivity and stability, it is preferable that the solid electrolyte 12 contains zirconium. Therefore, it is desirable to use Na2ZrCl6 as the solid electrolyte 12.

[0045] (3-1) Method for preparing solid electrolytes The solid electrolyte 12 can be obtained using metal chlorides as raw materials by a mechanochemical synthesis method using a planetary ball mill apparatus.

[0046] The solid electrolyte powder can be used to fill all-solid-state secondary batteries by uniformly spreading a predetermined amount of fixed electrolyte powder on the negative electrode 13 or positive electrode 11 and pressing it.

[0047] (4) Other battery components In addition to the above components, the all-solid-state secondary battery of this embodiment may include structural components such as a battery case and other elements required for a secondary battery. Conventional known components can be used for these.

[0048] (5) Method for manufacturing an all-solid-state secondary battery As described above, the all-solid-state secondary battery of this embodiment includes at least a positive electrode 11, a negative electrode 13, and a solid electrolyte 12, and as illustrated in Figure 1, the solid electrolyte 12 is arranged between the positive electrode 11 and the negative electrode 13 so as to be in contact with the positive electrode 11 and the negative electrode 13.

[0049] Figure 2 is a schematic cross-sectional view showing an example of the structure of an all-solid-state secondary battery according to this embodiment. The illustrated all-solid-state secondary battery can be manufactured by stacking a negative electrode 13, a solid electrolyte 12, and a positive electrode 11 in order using a dedicated jig. The dedicated jig includes a metal jig 21 for the positive electrode side, a metal jig 22 for the negative electrode side, and a PTFE (polytetrafluoroethylene) jig 23 for fixing the all-solid-state secondary battery from both sides. A positive electrode terminal 24 is installed on the metal jig 21, and a negative electrode terminal 25 is installed on the metal jig 22.

[0050] Furthermore, the all-solid-state secondary battery disclosed herein is not limited to the secondary battery shown in Figure 3, and may be modified as appropriate without altering the purpose and scope of this disclosure.

[0051] (6) Battery Recycling Method The all-solid-state secondary battery of this embodiment can be disassembled and broken down, placed in deionized water or the like, and water-soluble metal resources can be recovered by neutralization and precipitation or filtration and centrifugation. Known methods can be used for the extraction process of these metal resources.

[0052] Furthermore, for insoluble current collectors (current collecting materials), if the other materials are water-soluble, the positive electrode active material and negative electrode active material will dissolve when the electrodes are immersed in water (neutral), making it easy to recover the current collector and recycle it. Similarly, carbon can also be easily recovered and recycled by filtration or centrifugation.

[0053] [Examples 1-4] Examples of the all-solid-state secondary battery according to this embodiment will be described in detail below. In this embodiment, the all-solid-state secondary battery was manufactured using the dedicated jig shown in Figure 2.

[0054] In Examples 1 and 2, Na4Fe(CN)6 was used as the positive electrode (positive electrode active material), while in Examples 3 and 4, NaFeFe(CN)6 was used. Na2ZrCl6 was used as the solid electrolyte. In Examples 1 and 3, metallic tin was used as the negative electrode (negative electrode active material), while in Examples 2 and 4, hard carbon was used as the negative electrode (negative electrode active material).

[0055] The battery configurations of Examples 1-4 are shown in Figure 6. However, this disclosure is not limited to those shown in the following examples, and can be modified as appropriate without altering its essence.

[0056] (Preparation of Solid Electrolyte) The solid electrolyte powder Na2ZrCl6 was synthesized using a planetary ball mill (PL-7 manufactured by Fritsch). Specifically, commercially available NaCl powder and commercially available ZrCl4 powder were vacuum-dried at 100°C in a glove box under an argon atmosphere, then mixed in a molar ratio of 2:1, packed into a dedicated pot with zirconia balls (10 mm in diameter), and subjected to ball milling at 600 rpm for 12 hours to synthesize the solid electrolyte powder Na2ZrCl6.

[0057] (Preparation of the positive electrode) For the positive electrode active material, Na4Fe(CN)6, a commercially available reagent (Na4Fe(CN)6・10H2O) was used, which was heat-treated in a vacuum dryer (120°C, 5 hours).

[0058] The positive electrode active material, NaFeFe(CN)6, was prepared by heat-treating commercially available reagents, Prussian blue (Fe4[Fe(CN)6]3) and Na4Fe(CN)6·10H2O powder, in a vacuum dryer at 120°C and 90°C for 12 hours, respectively, to remove moisture. Using these moisture-removed materials as starting materials, Fe4[Fe(CN)6]3 and Na4Fe(CN)6 were mixed in a stoichiometric molar ratio of 1:1 to produce a powder, which was then mixed and ground for 6 hours in a planetary ball mill (PL-7, Fritsch) equipped with a 250 ml agate container. The resulting NaFeFe(CN)6 was further heat-treated in a vacuum dryer (120°C, 5 hours) to promote crystallization and obtain NaFeFe(CN)6 powder.

[0059] Figure 3 shows the method for fabricating a positive electrode using Na4Fe(CN)6 as the positive electrode active material, and a positive electrode using NaFeFe(CN)6 as the positive electrode active material.

[0060] This section describes a positive electrode using Na4Fe(CN)6 as the positive electrode active material. The positive electrode mixture was obtained by adjusting the final composition of Na4Fe(CN)6 powder (positive electrode active material), acetylene black (DenkaBlack, manufactured by Denka) as a conductive additive, and solid electrolyte powder (Na2ZrCl6) to a weight ratio of 40:10:50.

[0061] Specifically, Na4Fe(CN)6 powder and acetylene black (AB) were ground and mixed using a ball mill (S11). The ball mill was rotated at 400 rpm for 1 hour. Solid electrolyte powder (Na2ZrCl6) was added to this mixture (Na4Fe(CN)6-AB) and mixed in a mortar (S12). This yielded a positive electrode mixture (Na4Fe(CN)6-AB-Na2ZrCl6).

[0062] Similarly, for the positive electrode using NaFeFe(CN)6 as the positive electrode active material, a positive electrode mixture (NaFeFe(CN)6-AB-Na2ZrCl6) was obtained using the same manufacturing method shown in Figure 2.

[0063] (Preparation of the negative electrode) In Examples 1 and 3, the negative electrode was prepared by cutting out a circle with a diameter of 10 mm from a metal tin foil (made by Niraco, 100 μm thick) to fit a special jig.

[0064] In Examples 2 and 4, the negative electrode was prepared using the method shown in Figure 4. Specifically, a commercially available hard carbon (Carbotron P type: S (F), manufactured by Kureha Battery Materials Japan), which is the negative electrode active material, and solid electrolyte powder (Na2ZrCl6) were mixed so that the final composition was 50:50 by weight. More precisely, the hard carbon and solid electrolyte powder (Na2ZrCl6) were crushed and mixed using a ball mill (S21) to obtain a negative electrode mixture (hard carbon-Na2ZrCl6). The ball mill was rotated at 400 rpm for 1 hour.

[0065] (Fabrication of secondary batteries) All-solid-state secondary batteries were fabricated using the dedicated jig shown in Figure 2. First, the negative electrode metal or negative electrode mixture was filled into the space in the center of the dedicated jig (a cavity and opening for housing the positive electrode, electrolyte, and negative electrode). In the case of metallic tin foil, a circular foil was placed in the opening. In the case of hard carbon, 40 mg of the negative electrode mixture was uniformly filled into the opening and pressed at 200 MPa.

[0066] Next, 100 mg of solid electrolyte powder (Na2ZrCl6) was uniformly packed onto the negative electrode and pressed at 500 MPa. Finally, 40 mg of positive electrode compound was uniformly packed onto the solid electrolyte and pressed at 200 MPa. This allowed for the formation of an all-solid-state secondary battery within the dedicated jig shown in Figure 2.

[0067] (Battery Performance Measurement) The all-solid-state secondary batteries prepared according to the above procedure were subjected to battery performance measurements in a constant temperature chamber maintained at 30°C. The battery cycle test was performed using a charge / discharge measurement system (VMP-3, Bio Logic), with a current density of 20 μA / cm² per effective area of ​​the battery. ２ The charging termination voltage was set to 4.0V and the discharging termination voltage to 2.0V, and the test was performed under constant current charging and discharging conditions.

[0068] The charge / discharge capacity was expressed as a value per unit weight (mAh / g) of the positive electrode active material (Na4Fe(CN)6 or NaFeFe(CN)6).

[0069] (Battery Performance) Figure 5 shows the discharge curves during the first discharge for Examples 1 and 3. Figure 6 shows the initial average discharge voltage and initial discharge capacity, and the discharge capacity at 10 and 30 cycles for Examples 1 to 4. The average discharge voltage was defined as the battery voltage at the midpoint of the discharge capacity.

[0070] In Example 1, as shown in Figures 5 and 6, the initial average discharge voltage was 3.4V and the initial discharge capacity was 45mAh / g. The discharge capacity after 10 and 30 cycles was 40mAh / g, and although a decrease of approximately 10% in discharge capacity was observed after 30 cycles, the all-solid-state secondary battery in Example 1 was confirmed to function as a secondary battery capable of charge-discharge cycles.

[0071] In Example 3, the discharge curve during the initial discharge (Figure 5) shows a flat discharge region around 2.7V, which was not present in Example 1, and the discharge capacity increased significantly to 140 mAh / g. Furthermore, the discharge capacity after 30 cycles was 135 mAh / g, indicating a decrease of approximately 4% in discharge capacity after 30 cycles. Example 3 was confirmed to have superior battery performance compared to Example 1.

[0072] Examples 2 and 4, in which the negative electrode is hard carbon, also exhibited a voltage drop, and showed the same tendency as when the negative electrode is metallic tin.

[0073] Therefore, it was confirmed that the all-solid-state secondary batteries according to Examples 1 to 4 operate as secondary batteries capable of charge-discharge cycling.

[0074] [Comparative Examples 1 and 2] FIG. 6 shows the battery configurations of Comparative Examples 1 and 2. In the present comparative examples, lithium all-solid-state secondary batteries were produced using an oxide solid electrolyte instead of a chloride solid electrolyte.

[0075] Specifically, a commercially available oxide solid electrolyte LICGC disc (LICGC: Li₂O-Al₂O₃-SiO₂-P₂O₅-TiO₂-GeO₂-based solid electrolyte, SP-01 manufactured by Ohara Corporation, thickness 180 µm) was used as the solid electrolyte.

[0076] For the positive electrode, commercially available positive electrode active material LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O₂ (manufactured by Sigma-Aldrich) and LICGC powder (PW-01 manufactured by Ohara Corporation, average particle diameter 0.4 µm) were used. Specifically, the positive electrode was produced by the same method as in Examples 1 to 4, wherein LiNi which is a metal oxide as the positive electrode active material 1 / 3 Mn 1 / 3 Co 1 / 3 O₂ powder, acetylene black as a conductive aid (DenkaBlack manufactured by Denka Company Limited), and LICGC powder as an oxide solid electrolyte were adjusted such that the final composition had a weight ratio of 40:10:50, thereby obtaining a positive electrode mixture.

[0077] For the negative electrode, a metal indium foil capable of being alloyed with lithium was used in Comparative Example 1, and hard carbon powder capable of inserting and desorbing lithium was used in Comparative Example 2. The hard carbon powder is the same as the hard carbon powder in Examples 2 and 4. The hard carbon powder was mixed with LICGC powder in the same manner as in Examples 2 and 4 to obtain a negative electrode mixture. Specifically, hard carbon powder as the negative electrode active material and LICGC powder as the oxide solid electrolyte powder were mixed such that the final composition had a weight ratio of 50:50, thereby obtaining a negative electrode mixture.

[0078] The fabrication and evaluation methods for the all-solid-state secondary battery in this comparative example were carried out in the same manner as in Examples 1 to 4. However, the charge and discharge termination voltages were set to 3.7V and 2.4V, respectively, in Comparative Example 1, and to 4.3V and 3.0V, respectively, in Comparative Example 2.

[0079] Figure 6 shows the initial average discharge voltage and initial discharge capacity of the all-solid-state secondary battery in this comparative example, as well as the discharge capacity after 10 and 30 charge-discharge cycles. In Comparative Examples 1 and 2, a capacity decrease of approximately 60% and 63%, respectively, was observed after 30 discharge cycles, indicating lower battery performance stability in charge-discharge cycles compared to Examples 1 to 4.

[0080] This indicates that LICGC, a metal oxide, has a lower bonding ability with the active material (positive electrode active material, negative electrode active material) than chloride solid electrolytes, and that repeated charge-discharge cycles cause the active material to expand and contract, reducing its adhesion to the solid electrolyte and thus degrading battery performance.

[0081] [Solubility Evaluation] After performance evaluation of the all-solid-state secondary batteries of Examples 1-4 and Comparative Examples 1 and 2, the batteries were removed from the dedicated jig, placed in 100 ml of deionized water (neutral), and stirred for 1 hour using a magnetic stirrer. The solubility of each component was then evaluated.

[0082] Figure 7 shows the evaluation results for Examples 1-4 and Comparative Examples 1-2. Here, solubility was evaluated and determined by observing the presence or absence of precipitate and by evaluating the ionic species contained in the aqueous solution by ICP emission spectrometry.

[0083] In all of Examples 1-4 and Comparative Examples 1 and 2, it was found that the tin and hard carbon in the negative electrode, and the acetylene black, a conductive additive in the positive electrode, did not dissolve. However, since these substances can be recovered relatively easily by filtration or centrifugation, this is not considered to be a major obstacle to battery recycling.

[0084] In Comparative Examples 1 and 2, the oxide-based cathodes using metal oxides and oxide solid electrolytes, and the oxide solid electrolytes, were insoluble in water and remained unchanged. In contrast, in Examples 1-4, it was confirmed that the cyanide-based cathodes containing chloride solid electrolytes and the chloride solid electrolytes were completely soluble.

[0085] Therefore, considering the ease of battery recycling, it is clear that the battery configurations of Examples 1-4, which use chloride solid electrolytes, are more advantageous than Comparative Examples 1 and 2, which use oxide solid electrolytes.

[0086] As described above, the all-solid-state secondary battery of this embodiment is a secondary battery that is easy to recycle and has excellent recyclability. Specifically, in this embodiment, water-soluble materials (sodium-containing cyanide material, sodium-containing chloride material) are used for the positive electrode active material and the solid electrolyte. This eliminates the need for a heat treatment process, and allows for the recovery of resources from used all-solid-state secondary batteries using a milder, near-neutral aqueous solution rather than a strong acid such as sulfuric acid. Although the negative electrode and the conductive additive of the positive electrode do not dissolve in the aqueous solution, these substances can be recovered relatively easily by filtration or centrifugation. As a result, the all-solid-state secondary battery of this embodiment can contribute to reducing environmental impact or costs.

[0087] Conventional lithium secondary battery recycling methods involve complex processes, such as recovering black mass obtained by heat-treating used batteries in an electric furnace as cobalt sulfate by dissolving and extracting it in sulfuric acid, or recovering it as lithium carbonate using a carbonate precipitation reaction.

[0088] Furthermore, the all-solid-state secondary battery of this embodiment does not use lithium. This reduces resource risk.

[0089] Furthermore, the all-solid-state secondary battery of this embodiment exhibits excellent charge-discharge characteristics (cycle characteristics).

[0090] The all-solid-state secondary battery of this embodiment can be effectively used as a power source for various electronic devices such as small devices, sensors, and mobile devices, or as a power source for large devices such as electric vehicles.

[0091] This disclosure is not limited to the embodiments described above, and various modifications and combinations are possible within the technical concept of this disclosure.

[0092] 11: Positive electrode 12: Solid electrolyte 13: Negative electrode 21: Positive electrode side metal jig 22: Negative electrode side metal jig 23: PTFE jig 24: Positive electrode terminal 25: Negative electrode terminal

Claims

1. An all-solid-state secondary battery comprising: a positive electrode containing a sodium-containing cyanide; a negative electrode containing a substance that allows for the insertion and removal of sodium at a potential lower than that of the positive electrode; and a sodium-containing chloride solid electrolyte.

2. The all-solid-state secondary battery according to claim 1, wherein the chloride solid electrolyte contains zirconium.

3. The all-solid-state secondary battery according to claim 1, wherein the positive electrode comprises Na4Fe(CN)6 or NaFeFe(CN)6.

4. The all-solid-state secondary battery according to claim 1, wherein the negative electrode comprises metallic tin or hard carbon.

Images