Solid electrolyte, metal air battery, and method for manufacturing solid electrolyte

Abstract

Provided is a secondary battery which has high stability and high power storage performance.　This solid electrolyte contains an amorphous body which is partially composed of elemental vanadium, elemental barium, and an element M (the element M is at least one element that is selected from among Fe, Li, Na, Mg, Al, and Zn). Specifically, the amorphous body is 20BaO∙(10 + x)Fe2O3∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive), 20BaO∙(10 + x)Li2O∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive), 20BaO∙(10 + x)Na2O∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive), 20BaO∙(10 + x)MgO∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive), 20BaO∙(10 + x)Al2O3∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive), or 20BaO∙(10 + x)ZnO∙(70 - x)V2O5 (wherein x is 4 to 19 inclusive).

Description

Solid electrolyte, metal-air battery, and method for manufacturing solid electrolyte

[0001] The present invention relates to a solid electrolyte, a metal-air battery, and a method for manufacturing the same.

[0002] For example, Patent Document 1 discloses Li ２ O, SiO ２ , TiO ２ , P ２ O ５ , BaO, Cs ２ O, V ２ O ５ a step of mixing materials including: melting the mixed materials; rapidly cooling the molten materials at room temperature and compressing the molten materials with a preheated plate to form an electrolyte glass; heating the electrolyte glass to 500 to 600°C to relieve stress; heating the electrolyte glass at a temperature higher than the temperature at which the stress is relieved to crystallize it; and polishing the electrolyte glass to precisely adjust its thickness.

[0003] Patent Document 2 also describes an oxide glass composition containing vanadium, barium, and iron, which has an electrical conductivity of 10 -4 ~10 -1 S.cm. -1 A vanadate glass is disclosed, characterized in that:

[0004] Furthermore, Patent Document 3 discloses an air electrode catalyst used in the air electrode of an air battery that uses oxygen as an active material, which is characterized by containing vanadate glass containing vanadium oxide as a main component.

[0005] JP-T-2017-510936 A JP-A-2003-034548 JP-A-2019-012685

[0006] An object of the present invention is to provide a secondary battery with high stability and high power storage performance.

[0007] The solid electrolyte according to the present invention contains an amorphous body containing vanadium, barium, and element M (element M is at least one element selected from Fe, Li, Na, Mg, Al, and Zn) as some of its constituent elements.

[0008] Preferably, the amorphous body is vanadate glass, and the element M is configured to be an ion conductor within the amorphous body.

[0009] Preferably, the vanadate glass is 20BaO.(10+x)Fe ２ O ３ (70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Li ２ O. (70-x) V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Na ２ O. (70-x) V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)MgO.(70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Al ２ O ３ (70-x)V ２ O ５ (x is 4 or more and 19 or less), or 20BaO.(10+x)ZnO.(70-x)V ２ O ５ (x is 4 or greater and 19 or less).

[0010] Preferably, the vanadate glass is obtained by heat treating a raw material composition containing vanadium element, barium element and the element M at a temperature of 820° C. or higher for one hour or longer.

[0011] Preferably, it further contains polyacrylic acid.

[0012] The metal-air battery according to the present invention comprises the solid electrolyte described above.

[0013] A method for producing a solid electrolyte according to the present invention includes the steps of: mixing a plurality of raw material compositions containing vanadium oxide, barium carbonate, and a metal oxide in a predetermined ratio; and heat-treating the mixed raw material compositions at a temperature of 820°C or higher.

[0014] According to the present invention, a secondary battery with high stability and high power storage performance can be provided.

[0015] 1A and 1B are diagrams illustrating the synthesis procedure for vanadate glass containing iron (20BaO·(10+x)Fe2O3·(70-x)V2O5 (x = 0 to 20)). (a) is an X-ray diffraction pattern of a synthetic sample (vanadate glass) synthesized by treatment at 720°C, and (b) is an X-ray diffraction pattern of a synthetic sample (vanadate glass) synthesized by treatment at 850°C. (b) is an X-ray diffraction pattern of a synthetic sample (vanadate glass) synthesized by treatment at 850°C. (c) shows the results of differential thermal analysis (DTA). (d) shows the results of AC impedance measurements at 100 kHz for each sample. (a) shows the appearance of a prototype solid electrolyte sheet, and (b) shows the appearance of the prototype iron-air battery. (e) shows X-ray diffraction patterns for each example (x = 0, 10, 20) in the second embodiment. (f) shows the starting materials, synthesis process, and synthesis results for vanadate glass containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). 1 is a diagram showing the crystallinity (XRD) evaluation results of vanadate glasses containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn); 2 is a diagram showing the AC impedance measurement results of ionic conductors containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn); 3 is a diagram showing the conductivity of ionic conductors containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn).

[0016] First, we explain the background to the invention. Metal-air batteries, which are expected to offer several times higher capacity than conventional storage batteries, have attracted attention in recent years. Metal-air batteries are energy storage devices that charge and discharge through the oxidation and reduction of metals. They use oxygen (O2) from the air as the reactive material (active material) for the positive electrode and various metals for the negative electrode. In this way, by utilizing O2 from the air, the weight of the positive electrode active material can theoretically be reduced to almost zero, enabling the reduction of battery components and miniaturization. Given these advantages, metal-air batteries are commercially available as primary batteries, such as button batteries for hearing aids. However, they have not yet been put to practical use as reusable secondary batteries. Furthermore, they have issues such as reduced energy efficiency due to the decomposition of the electrolyte to generate hydrogen during charging, and deterioration due to evaporation of the electrolyte.

[0017] Therefore, the inventors of the present invention focused on amorphous vanadate glass materials primarily composed of transition metal oxides or alkali metal oxides. In the first embodiment, an amorphous vanadate glass material primarily composed of iron oxide is described as a specific example of an amorphous body that serves as an ionic conductor. This material is an amorphous vanadate glass containing iron, with a glass transition temperature (Tg) of 265-290°C and a crystallization temperature (TC) of 331-371°C. Heat treatment at temperatures above Tg exhibits high electronic conductivity. Meanwhile, electronic conductivity is frozen below Tg. Furthermore, when immersed in an aqueous solution, this material selectively dissolves iron, one of its constituent elements. Therefore, it is expected that the bonding of iron ions in the solid is weak and the ionic conductivity is high. Therefore, an iron-containing vanadate glass was synthesized and its ionic conductivity was evaluated.

[0018] First Embodiment Iron-air batteries have a theoretical energy density of 1584 Wh / kg, more than double the 662 Wh / kg of lithium-ion batteries. However, their energy density is inferior to that of lithium (11140 Wh / kg) and magnesium (6462 Wh / kg) air batteries, which are attracting attention, and therefore have not been widely studied. On the other hand, iron-air batteries are safe to handle and are abundant resources, making them an inexpensive and sustainable secondary battery. In the first embodiment, we developed a solid electrolyte that can be used in iron-air batteries, aiming to achieve a challenge in metal-air batteries by converting the electrolyte to a solid electrolyte and to realize an economical next-generation secondary battery using iron, an environmentally friendly material.

[0019] (Synthesis Procedure for Solid Electrolyte Material) First, referring to Figure 1, we will explain the synthesis procedure for iron-containing vanadate glass (20BaO·(10+x)Fe2O3·(70-x)V2O5 (x = 0 to 20)). The starting materials (raw material composition) were vanadium oxide (VO5), barium carbonate (BaCO3), and iron oxide (Fe2O3). Next, the starting materials were weighed and mixed to a predetermined ratio, placed in an alumina crucible, and melted at a temperature between 720°C and 850°C for 1 hour. The mixing ratio was adjusted to the base composition of 20BaO·(10+x)Fe2O3·(70-x)V2O5, with x ranging from 0 to 20, to verify the upper limit of Fe solid solubility and ionic conductivity in the amorphous state. The melted sample was poured into a graphite crucible and rapidly cooled for vitrification. The obtained sample was crushed in a mortar or the like to prepare samples for various evaluations.

[0020] Crystal structure evaluation was performed using an X-ray diffractometer (Bruker AXS D8 ADVANCE) to verify the amorphous structure of the glass samples. Tg and TC were evaluated using a differential thermal analyzer (Netsch Japan DIL402). Ionic conductivity was measured in accordance with JIS R 1661, "Measurement Method for Electrical Conductivity of Fine Ceramics Ionic Conductors." The glass powder was press-molded into a 4 x 3 x 40 mm sample, and electrodes were formed using silver paste. An impedance meter (Agilent Technologies 4294A) was used for AC four-terminal measurements.

[0021] Figure 2(a) shows the X-ray diffraction pattern of a vanadate glass synthesized at 720°C, and Figure 2(b) shows the X-ray diffraction pattern of a vanadate glass synthesized at 850°C. For the 720°C sample, no peaks were observed for x = 0, indicating an amorphous state, but diffraction peaks were detected for x = 5 and above. Identification of the diffraction peaks revealed FeVO4 for x = 5 and 10, and Fe2O3 for x = 20. FeVO4 is thought to be generated during the melting process, while Fe2O3 is thought to originate from unmelted starting materials. Considering the ionic conductivity of Fe, the presence of Fe in the amorphous state weakens the bonding between neighboring atoms, contributing to ionic conductivity. For x = 5 and 10 samples synthesized at 800°C and 850°C, the FeVO4 peaks present at 720°C disappeared, resulting in amorphous, homogeneous glass (amorphous material). Furthermore, it was found that a small amount of Fe2O3 remained in all samples with x = 15. From these results, it is thought that the upper limit of Fe solid solubility is x = 15.

[0022] Figure 3 shows the results of differential thermal analysis (DTA). As shown in Figure 3, for all samples with x = 5 to 15, exothermic and endothermic peaks associated with the phase change were observed between approximately 270 °C and 500 °C. The Tg was 272 °C for x = 5, and increased to 296 °C and 297 °C for x = 10 and 15, respectively. The Tc at which electronic conductivity appears was 330 °C for x = 5, but increased to 405 °C and 404 °C for x = 10 and 15, respectively. Furthermore, clear Tg and Tc were also observed in thermal analysis measurements of the synthesized samples treated at 720 °C and 850 °C. These results demonstrate that all synthesized samples maintain an amorphous state and can be used as solid electrolytes in which ionic conduction is dominant below the Tg.

[0023] Fig. 4 shows the results of measuring the AC impedance of each sample at 100 kHz. As shown in Fig. 4, in the sample treated at 720°C, the impedance increased by 10 -6 ~10 -3The conductivity increased by two orders of magnitude in the range of 100°C, suggesting that the conductivity was due to the ionic conductivity of Fe. On the other hand, the samples heat-treated at 800°C and 850°C showed little dependence on the Fe content, and both showed a 10 -2 These results indicate that vanadate glasses containing iron (Fe) maintain an amorphous crystalline structure, and that their electrical conductivity is dominated by ionic conductivity due to iron ions. Furthermore, at a melting temperature of 850°C, the electrical conductivity was 10 -2 It was found that the conductivity was high for a solid electrolyte, on the order of S / cm.

[0024] (Forming Solid Electrolyte Sheets) Next, we will explain a method for forming solid electrolytes primarily composed of vanadate glass. Reducing the interfacial resistance (solid electrolyte / electrode) between the solid electrolyte and the electrode is crucial in the development of all-solid-state batteries, and establishing a technology for designing low interfacial resistance is a key challenge. Furthermore, glass powder prepared by the melt-quenching method alone is difficult to fabricate into sheets. Therefore, we attempted to fabricate solid electrolyte sheets by mixing powder of vanadate glass containing Fe (20BaO·(10+x)Fe2O3·(70-x)V2O5) with a polymer hydrogel. The polymer hydrogel used in this embodiment is composed of cross-linked polyacrylic acid, a highly absorbent polymer, and a KOH aqueous solution, and is expected to facilitate charge transfer reactions at the electrolyte / electrode interface. The solid electrolyte sheets were fabricated using the following procedure. First, a polymer hydrogel was prepared by adding 1 g of polyacrylic acid to 10 ml of a 4-10 M potassium hydroxide (KOH) aqueous solution. The resulting polymer hydrogel was stored in a sealed container for 72 hours. Next, an appropriate amount of glass electrolyte powder (20BaO·(10+x)Fe2O3·(70-x)V2O5 (x=5) in this example) treated at 850°C was added to the polymer hydrogel, and the mixture was mixed and degassed using a planetary mixer (Thinky Corporation, AR-250). The thickness of the sheet was adjusted by applying the resulting mixture to a Teflon sheet using a roll press, and the mixture was then dried in the atmosphere for 24 hours.

[0025] (Fabrication of Iron-Air Battery) In the electrode configuration of this embodiment, an iron mesh is used for the negative electrode and a 1.0 mg / cm 2 A platinum-supported water-repellent carbon paper (GP-H-060 manufactured by Chemix Corporation) was used. Each electrode and the above solid electrolyte sheet were cut to a specified diameter using an electrode punching machine (HS0S-01 manufactured by Hosen Corporation) to fabricate a CR2032-type all-solid-state battery. A DC voltage and current source / monitor (6242 manufactured by ADC Corporation) was used to evaluate the battery performance. The charge / discharge test was performed at a charge current density of 10 mA / cm. 2 ,Discharge current density 0.2mA / cm 2 Measurements were carried out in the atmosphere under the following conditions.

[0026] Figure 5(a) shows the appearance of the solid electrolyte sheet fabricated under the above conditions, and Figure 5(b) shows the appearance of the prototype iron-air battery. Figure 5 shows the prototype solid electrolyte sheet and the all-solid-state battery fabricated by cutting it into a 19 mm diameter piece. The prototype battery was in a primary battery state (charged state) upon assembly. After discharge testing, a charge-discharge test was conducted again to obtain a discharge curve, confirming that the prototype battery functions as a secondary battery. Furthermore, the initial discharge capacity estimated from the discharge current, discharge time, and iron weight of the anode was 50 mAh / g. Compared to the initial capacity of a conventional iron-air battery (200 mAh / g) using an 8 M KOH aqueous solution as the electrolyte, the discharge capacity of the all-solid-state battery was one-quarter of that of the previous one. This is thought to be due to a reduced reactive surface area due to a passivation film on the iron anode surface and power loss due to increased internal resistance.

[0027] In the case of all-solid-state batteries, the electrochemical reaction at the negative electrode occurs via H2O, so the electrochemical reaction with a typical solid electrolyte requires operating conditions of 80°C and humidity of about 60%, at which ionic conductivity increases with humidification. The reaction at the positive electrode accompanying discharge of an iron-air battery, which uses iron as the negative electrode material, is shown in equation (1), the reaction at the negative electrode in equation (2), and the total reaction in equation (3). Positive electrode: O2 + 2H2O + 4e - →4OH - (1) Negative electrode: 3 / 2Fe+4OH - →1 / 2Fe3O4+2H2O+4e- (2) Overall reaction: 3 / 2Fe + O2 → 1 / 2Fe3O4 (3) This indicates that the Fe at the anode reacts stepwise to form Fe(OH)2 and then Fe3O4, and the resulting Fe3O4 precipitates in the electrolyte. In contrast, the solid electrolyte sheet battery prototyped in this embodiment was able to operate as a secondary battery at room temperature. The polymer hydrogel possesses properties similar to those of a KOH aqueous solution, such as electrical conductivity (approximately 0.6 S / cm), oxygen permeation rate, and a wide potential window, demonstrating its potential as an electrolyte for various batteries, including nickel-metal hydride batteries and fuel cells. This suggests that the KOH aqueous solution in the highly water-absorbent polymer hydrogel contributes to the reactions represented by equations (1) and (2). These results demonstrate that the solid electrolyte sheet using the glass electrolyte and polymer hydrogel used in this embodiment can function as an electrolyte for air batteries.

[0028] As explained above, the solid electrolyte in the first embodiment was found to be a solid electrolyte that can be used in iron-air batteries, with the aim of realizing an all-solid-state metal-air battery using a material that is cheaper and more readily available than lithium. Specifically, a glass sample was synthesized by holding an amorphous glass material of vanadate, 20BaO.(10+x)Fe2O3.(70-x)V2O5, containing transition metal oxides as the main component, at 720°C to 850°C for one hour and then rapidly cooling it. Measurement of the ionic conductivity revealed that the conductivity was due to the ionic conductivity of Fe, and that at the melting temperature of 850°C, the ionic conductivity of 10 -2 It was revealed that the solid electrolyte exhibited high conductivity, on the order of S / cm. Furthermore, a solid electrolyte sheet with a low-resistance interface was fabricated by mixing polymer hydrogel with glass electrolyte powder containing Fe. By optimizing the mixing ratio of the glass electrolyte, polyacrylic acid, and KOH aqueous solution, as well as the KOH concentration, the contact resistance was reduced to 0.5 kΩcm. 2(four-digit reduction). An iron-air battery using the solid electrolyte sheet was fabricated and its battery performance was evaluated. The initial capacity was 50 mAh / g, which is one-quarter of the capacity of the aqueous electrolyte. Furthermore, when charging was attempted after discharge, it was confirmed that the battery functioned as a secondary battery. From the above viewpoints, in this embodiment, it is desirable that x be 4 or more and 19 or less.

[0029] Second Embodiment As a second embodiment, a configuration in which lithium is added instead of iron will be described. That is, vanadate glass (20BaO.xMO ｙ -(70-x)V2O5) where M is lithium Li.

[0030] In the synthesis procedure of the second embodiment, the starting materials (raw material composition) were barium carbonate (BaCO), lithium carbonate (LiCO), and vanadium oxide (VO). The starting materials were then weighed and mixed to a predetermined ratio, placed in an alumina crucible, and melted by holding at 720°C to 850°C for one hour. The mixing ratio was determined to verify the upper limit of Li solid solubility and ionic conductivity in the amorphous state. Samples were synthesized with a basic composition of 20BaO·(10+x)LiCO3·(70-x)VO, with x ranging from 0 to 20. The melted sample was poured into a graphite crucible and rapidly cooled for vitrification.

[0031] 6 shows the X-ray diffraction patterns of each example (x=0, 10, 20) in the second embodiment. In the sample with x=0, a vanadium oxide peak was detected, but in the sample with x=10, the sample was amorphous and contained a small amount of composite oxide (LiVO ３ ) was detected. In the sample with x=20, a highly crystalline composite oxide (Li4V 10 O 27 ) peak was detected. In addition, when the AC impedance of each of the above samples was measured, the sample with x=0 showed no frequency dependence, but the samples with x=10 and 20 showed frequency dependence, indicating ionic conductivity. In particular, the conductivity of the ionic conductor was 10 -3 From these results, it is desirable that x is 4 or more and 19 or less.

[0032] <Other Modifications> In the first and second embodiments, vanadate glass containing iron or lithium is used as a solid electrolyte, but the present invention is not limited to these. For example, vanadate glass may contain sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) as one of the constituent elements instead of iron or lithium. FIG. 7 is a diagram showing starting materials, synthesis steps, and synthesis results of vanadate glass containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). Note that FIG. 7 illustrates an example where x = 10. Vanadate glass containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) can be prepared by adding sodium carbonate (Na), magnesium (Mg), aluminum (Al), or zinc (Zn), respectively. ２ CO ３ ), magnesium oxide (MgO), aluminum hydroxide (Al(OH) ３ ), or zinc oxide (ZnO) as a starting material (raw material composition) by a similar synthesis process.

[0033] 8 shows the results of crystallinity (XRD) evaluation of vanadate glasses containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). As shown in FIG. 8, the vanadate glasses containing magnesium (Mg), aluminum (Al), or zinc (Zn) synthesized as described above are amorphous. Furthermore, the vanadate glasses containing sodium (Na) contain amorphous portions in addition to highly crystalline composite oxides.

[0034] Fig. 9 shows the results of measuring the AC impedance of ionic conductors whose constituent elements are sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). All of them have frequency dependence, and it was confirmed that they are ionic conductors. Fig. 10 shows the conductivity of ionic conductors whose constituent elements are sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). As shown in Fig. 10, all of them are ionic conductors with a 10 ―４ ~10 ―５It showed conductivity of the order of magnitude.

Claims

1. A solid electrolyte comprising an amorphous body having vanadium, barium and element M (element M being at least one element selected from Fe, Li, Na, Mg, Al and Zn) as some of its constituent elements.

2. The solid electrolyte according to claim 1, wherein the amorphous body is vanadate glass, and the element M is configured to be an ion conductor within the amorphous body.

3. The vanadate glass is 20BaO.(10+x)Fe ２ O ３ (70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Li ２ O.(70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Na ２ O.(70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)MgO.(70-x)V ２ O ５ (x is 4 or more and 19 or less), 20BaO.(10+x)Al ２ O ３ (70-x)V ２ O ５ (x is 4 or more and 19 or less), or 20BaO.(10+x)ZnO.(70-x)V ２ O ５ (x is 4 or more and 19 or less), The solid electrolyte according to claim 2.

4. The solid electrolyte according to claim 3, wherein the vanadate glass is obtained by heat-treating a raw material composition containing vanadium element, barium element and the element M at a temperature of 820° C. or higher for one hour or more.

5. The solid electrolyte according to claim 1, further comprising polyacrylic acid.

6. A metal-air battery comprising the solid electrolyte according to claim 1 or 5.

7. A method for producing a solid electrolyte comprising the steps of: mixing a plurality of raw material compositions containing vanadium oxide, barium carbonate, and a metal oxide in a predetermined ratio; and heat-treating the mixed raw material compositions at a temperature of 820°C or higher.

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