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

Abstract

To provide a secondary battery which has high stability and high power storage performance.SOLUTION: A solid electrolyte includes an amorphous body that partially comprises a vanadium element, a barium element, and an element M (the element M is at least one selected from Fe, Li, Na, Mg, Al, and Zn). Specifically, the amorphous body is 20BaO (10+x)Fe2O3 (70-x)V2O5 (x is 4 or more and 19 or less), 20BaO (10+x)Li2O (70-x)V2O5 (x is 4 or more and 19 or less), 20BaO (10+x)Na2O (70-x)V2O5 (x is 4 or more and 19 or less), 20BaO (10+x)MgO (70-x)V2O5 (x is 4 or more and 19 or less), 20BaO (10+x)Al2O3 (70-x)V2O5 (x is 4 or more and 19 or less), or 20BaO (10+x)ZnO (70-x)V2O5 (x is 4 or more and 19 or less).SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] For example, Patent Document 1 discloses a method for manufacturing a solid electrolyte composition for a lithium secondary battery, including a step of mixing materials containing Li2O, SiO2, TiO2, P2O5, BaO, Cs2O, and V2O5, a step of melting the mixed materials, a step of rapidly cooling the melted materials at room temperature and compressing the melted materials with a preheated plate to produce an electrolyte glass, a step of raising the temperature of the electrolyte glass to remove stress at 500 to 600 ° C, a step of heating the electrolyte glass in a temperature range higher than the step of removing the stress to crystallize, and a step of polishing the electrolyte glass to precisely adjust the thickness.

[0003] Further, Patent Document 2 discloses a vanadate glass that is an oxide-based glass composition containing vanadium, barium, and iron, and has an electric conductivity at room temperature of 10 -4 ~10 -1 S·cm -1 -1.

[0004] Further, Patent Document 3 discloses a catalyst for an air electrode used in an air electrode of an air battery using oxygen as an active material, which contains a vanadate glass containing vanadium oxide as a main component.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

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

Means for Solving the Problems

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

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

[0009] Preferably, the vanadate glass is 20BaO·(10 + x)Fe2O3·(70 - x)V2O5 (x is 4 or more and 19 or less), 20BaO·(10 + x)Li2O·(70 - x)V2O5 (x is 4 or more and 19 or less), 20BaO·(10 + x)Na2O·(70 - x)V2O5 (x is 4 or more and 19 or less), 20BaO·(10 + x)MgO·(70 - x)V2O5 (x is 4 or more and 19 or less), 20BaO·(10 + x)Al2O3·(70 - x)V2O5 (x is 4 or more and 19 or less), or 20BaO·(10 + x)ZnO·(70 - x)V2O5 (x is 4 or more 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 1 hour or more.

[0011] Preferably, it further contains polyacrylic acid.

[0012] In addition, the metal-air battery according to the present invention has the solid electrolyte according to claim 1 or claim 5.

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

Effects of the Invention

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

Brief Description of the Drawings

[0015]

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Embodiments for Carrying Out the Invention

[0016] First, the background of the present invention will be described. In recent years, metal-air batteries, which are expected to have several times higher capacity than conventional storage batteries, have attracted attention. A metal-air battery is a power storage device that performs charge and discharge by oxidation and reduction of a metal. It uses oxygen (O2) in the air as the reaction material (active material) of the positive electrode and various metals for the negative electrode. In this way, since O2 in the air is utilized, theoretically the weight of the positive electrode active material can be made almost zero, enabling reduction and miniaturization of the battery constituent materials. Due to such advantages, metal-air batteries are commercially available as primary batteries such as button-type batteries for hearing aids, but have not been put into practical use as rechargeable secondary batteries. Also, there are problems such as a decrease in energy efficiency due to 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 an amorphous glass material of vanadate mainly composed of transition metal oxides or alkali metal oxides. In the first embodiment, an amorphous glass material of vanadate mainly composed of iron oxide will be described as a specific example of an amorphous body serving as an ion conductor. This material is an amorphous glass of vanadate containing iron with a glass transition point (Tg) of 265 to 290 °C and a crystallization temperature (TC) of 331 to 371 °C, and high electronic conduction is exhibited by applying heat treatment at a temperature of Tg or higher. On the other hand, electronic conduction is frozen in the temperature range below Tg. Also, when this material is immersed in an aqueous solution, iron among the constituent elements is selectively eluted, so it can be expected that the binding property of iron ions in the solid is weak and the ionic conductivity is high. Therefore, vanadate glass containing iron was synthesized and the ionic conductivity was evaluated.

[0018] <First Embodiment> The iron-air battery has an energy density of 1584 Wh / kg, which is more than twice that of a lithium-ion battery with a theoretical energy density of 662 Wh / kg. However, it is inferior to lithium (11140 Wh / kg) and magnesium (6462 Wh / kg) air batteries, which have attracted attention, so extensive studies have not been conducted. On the other hand, since the iron-air battery is safe to handle and abundant in resources, an inexpensive and sustainable secondary battery can be realized. In the first embodiment, aiming to achieve the solid electrolyte conversion of the electrolyte, which is an issue of metal-air batteries, and to realize an economical next-generation secondary battery using iron, an environmentally friendly material, a solid electrolyte applicable to iron-air batteries was developed.

[0019] (Synthesis Procedure of Solid Electrolyte Material) First, with reference to FIG. 1, the synthesis procedure of vanadate glass (20BaO·(10+x)Fe2O3·(70-x)V2O5 (x = 0 to 20)) containing iron will be described. The starting materials (raw material compositions) were vanadium oxide (V2O5), barium carbonate (BaCO3), and iron oxide (Fe2O3). Next, the starting materials were weighed and mixed to a predetermined mixing ratio, placed in an alumina crucible, and melted by holding at 720°C to 850°C for 1 hour. At this time, the mixing ratio was used to synthesize samples in the range of x = 0 to 20 with 20BaO·(10+x)Fe2O3·(70-x)V2O5 as the basic composition in order to verify the solid solution upper limit value of Fe in the amorphous state and the ionic conductivity. The melted sample was poured into a graphite crucible and rapidly cooled to perform vitrification treatment. The obtained sample was pulverized using a mortar or the like to obtain samples for various evaluations.

[0020] An X-ray diffractometer (Bruker AXS·D8 ADVANCE) was used for crystal structure evaluation to verify the amorphous structure of the glass sample. Also, Tg and TC were evaluated using a differential thermal analyzer (Netzsch·Japan Co., Ltd.·DIL402). Ionic conductivity was evaluated by pressing glass powder into a size of 4×3×40 mm according to JIS R 1661 "Method for Measuring Conductivity of Fine Ceramic Ion Conductors" and forming electrodes with silver paste to obtain an evaluation sample. An impedance meter (Agilent Technologies, Inc. 4294A) was used for the four-terminal AC measurement.

[0021] Figure 2(a) shows the X-ray diffraction pattern of the synthesized sample (vanadate glass) treated at 720 °C, and Figure 2(b) shows the X-ray diffraction pattern of the synthesized sample (vanadate glass) treated at 850 °C. In the case of the 720 °C treatment, no peak was observed in the sample with x = 0 and it was in an amorphous state, but diffraction peaks were detected for all samples with x ≥ 5. When the diffraction peaks were identified, FeVO4 was detected at x = 5 and 10, and Fe2O3 was detected at x = 20. FeVO4 was generated during the melting process, and Fe2O3 is considered to be due to the unmelted starting materials. Considering the ionic conductivity of Fe, it is thought that Fe in the sample exists in the amorphous phase, weakening the bonding with adjacent atoms and contributing to ionic conductivity. At x = 5 and 10 for the 800 °C and 850 °C treatments, the peaks of FeVO4 that existed in the 720 °C treatment disappeared, and an amorphous and homogeneous glass (amorphous body) was obtained. Also, it was found that Fe2O3 remained slightly in the samples with x = 15. From these results, the upper limit of Fe solid solution is considered to be x = 15.

[0022] Figure 3 shows the results of differential thermal analysis (DTA). As shown in Fig. 3, for all values of x from 5 to 15, peaks of heat generation and absorption associated with phase changes were observed from around 270°C to 500°C. At x = 5, Tg = 272°C, and at x = 10 and 15, Tg increased to 296°C and 297°C, respectively. The Tc at which electronic conductivity appears increased from 330°C at x = 5 to Tc = 405°C and 404°C at x = 10 and 15, respectively, with a wider increase range. Clear Tg and Tc were also observed from the thermal analysis measurements of the synthesized samples treated at 720°C and 850°C. From these results, it became clear that all the synthesized samples maintained an amorphous state and could be used as solid electrolytes in which ionic conduction is dominant below Tg.

[0023] Fig. 4 shows the AC impedance measurement results at 100 kHz for each sample. As shown in Fig. 4, for the samples treated at 720°C, an increase in conductivity by two orders of magnitude was shown in the range of 10 -6 ~10 -3 , suggesting that the conductivity is derived from the ionic conductivity of Fe. On the other hand, the heat-treated samples at 800°C and 850°C had low dependence on the Fe content and both showed ionic conductivities on the order of 10 -2 . From the above results, it was shown that the vanadate glass containing iron (Fe) retains an amorphous crystal structure and its conductivity is dominated by ionic conduction by iron ions. Also, at a melting temperature of 850°C, it became clear that it shows a high conductivity on the order of 10 -2 S / cm regardless of the Fe content and is high as a solid electrolyte.

[0024] (Sheet formation of solid electrolyte) Next, the forming method of the solid electrolyte mainly composed of vanadate glass will be described. In the development of all-solid-state batteries, reducing the interfacial resistance (solid electrolyte / electrode) formed between the solid electrolyte and the electrode is extremely important, and establishing a technology for designing low interfacial resistance has become a crucial issue. In addition, it is difficult to form a sheet from the glass powder alone produced by the melt quenching method. Therefore, an attempt was made to fabricate a solid electrolyte sheet by mixing a 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 crosslinked polyacrylic acid, which is a superabsorbent polymer, and an aqueous KOH solution, and it is expected that the charge transfer reaction at the electrolyte / electrode interface will proceed smoothly. The solid electrolyte sheet was formed through the following procedure. First, a polymer hydrogel was prepared by adding 1 g of polyacrylic acid to 10 ml of an aqueous potassium hydroxide (KOH) solution with a concentration of 4 - 10 M. The obtained polymer hydrogel was kept in a sealed container for 72 hours. Next, an appropriate amount of the powder of the glass electrolyte treated at 850 °C (20BaO·(10+x)Fe2O3·(70-x)V2O5 (x = 5) as an example) was added to this polymer hydrogel, and mixing and defoaming were performed using a rotating and revolving mixer (Shinki Co., Ltd.·AR-250). The thickness adjustment of the sheet forming was carried out by applying the obtained mixture onto a Teflon sheet and using a roll press machine, and the drying process was performed by keeping it in the air for 24 hours.

[0025] (Fabrication of Iron-Air Battery) In the electrode configuration of this embodiment, an iron mesh was used for the negative electrode, and a water-repellent carbon paper with 1.0 mg / cm 2 of platinum supported ((Chemix Co., Ltd.·GP-H-060)) was used for the positive electrode (air electrode). Each electrode and the above solid electrolyte sheet were cut out to a predetermined diameter using an electrode punching machine (Takizawa Co., Ltd.·HS0S-01), and a CR2032 type all-solid-state battery was fabricated. For the battery performance evaluation, a DC voltage / current source / monitor ((ACE Co., Ltd.·6242)) was used. The charge-discharge test was measured in the air under the conditions of a charging current density of 10 mA / cm 2 and a discharging current density of 0.2 mA / cm 2 .

[0026] Fig. 5(a) shows the appearance of the solid electrolyte sheet fabricated under the above conditions, and Fig. 5(b) shows the appearance of the fabricated iron-air battery. As shown in Fig. 5, it is a solid electrolyte sheet fabricated and an all-solid-state battery fabricated by cutting this into φ19 mm. The fabricated battery was in the state of a primary battery (charged state) at the time of assembly, and since a charge-discharge curve was obtained after performing a charge-discharge test again after the discharge test, it was confirmed that the fabricated battery functions as a secondary battery. Also, as a result of the charge-discharge test, the initial discharge capacity estimated from the discharge current, discharge time, and iron weight of the negative electrode was 50 mAh / g. Compared with the initial capacity (200 mAh / g) of a conventional iron-air battery using an 8M aqueous solution of KOH as the electrolyte, the discharge capacity of the all-solid-state battery was 1 / 4. As the main factors for this, a reduction in the reaction area due to the passive film on the iron surface of the negative electrode and power loss due to an increase in the internal resistance are considered.

[0027] In the case of an all-solid-state battery, since an electrochemical reaction occurs at the negative electrode through the mediation of H2O, for an electrochemical reaction by a general solid electrolyte, operating conditions of a temperature of about 80°C and a humidity of about 60% where the ionic conductivity increases due to humidification are required. The reaction at the positive electrode accompanying the discharge of an air battery using iron as the negative electrode material, that is, an iron-air battery, is shown in Equation (1), the reaction at the negative electrode is shown in Equation (2), and the overall reaction is shown 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) From this, it can be seen that Fe in the negative electrode reacts step by step to Fe3O4 through the reaction to Fe(OH)2, and the generated Fe3O4 precipitates in the electrolyte. On the other hand, the battery with the solid electrolyte sheet prototyped in this embodiment was able to operate as a secondary battery in a room temperature environment. The polymer hydrogel has characteristics such as an electrical conductivity (about 0.6 S / cm) close to that of an aqueous KOH solution, an oxygen permeation rate, and a wide potential window, and its applicability as an electrolyte for various batteries such as fuel cells in addition to nickel-hydrogen batteries has been shown. From this, it is suggested that the aqueous KOH solution in the highly water-absorbent polymer hydrogel contributes to the reactions of formulas (1) and (2). From the above results, it became clear that the solid electrolyte sheet using the glass electrolyte and the polymer hydrogel used in this embodiment functions as an electrolyte for an air battery.

[0028] As described above, it was found that the solid electrolyte in the first embodiment is a solid electrolyte that can be used in an iron-air battery for the purpose of realizing an all-solid-state battery of a metal-air battery using materials that are cheaper and more readily available than lithium. Specifically, when a glass sample was synthesized by holding a non-crystalline glass material of vanadate mainly composed of transition metal oxide, 20BaO·(10+x)Fe2O3·(70-x)V2O5, at 720 °C to 850 °C for 1 hour and then rapidly cooling it, from the measurement results of the ionic conductivity, the conductivity is derived from the ionic conductivity of Fe, and at the melting temperature of 850 °C, it is -2 It was revealed that it shows high conductivity as a solid electrolyte on the order of 10 S / cm. In addition, a solid electrolyte sheet with a low-resistance interface was fabricated by mixing a polymer hydrogel and a powder of a glass electrolyte containing Fe. By optimizing the mixing ratio of the glass electrolyte, polyacrylic acid, and the aqueous KOH solution and the KOH concentration, the contact resistance was reduced to 0.5 kΩcm 2 (a four-digit reduction). As a result of fabricating an iron-air battery using the solid electrolyte sheet and evaluating the battery performance, the initial capacity was 50 mAh / g, which is 1 / 4 of the capacity compared to the aqueous electrolyte. Also, when charging was attempted after discharging, it was confirmed that it functions as a secondary battery. From the above viewpoints, in this embodiment, it is desirable that x is 4 or more and 19 or less.

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

[0030] In the synthesis procedure of the second embodiment, the starting materials (raw material composition) were barium carbonate (BaCO3), lithium carbonate (Li2CO3), and vanadium oxide (V2O5). Next, the starting materials were weighed and mixed to a predetermined mixing ratio, placed in an alumina crucible, and melted by holding at 720°C to 850°C for 1 hour. The mixing ratio at this time was used to verify the solid solubility limit value of Li and the ionic conductivity in the amorphous state. Samples were synthesized in the range of x = 0 to 20 with 20BaO·(10 + x)LiCO3·(70 - x)V2O5 as the basic composition. The melted sample was poured into a graphite crucible and rapidly cooled to perform vitrification treatment.

[0031] Figure 6 shows the X-ray diffraction patterns of each example (x = 0, 10, 20) in the second embodiment. In the sample with x = 0, peaks of vanadium oxide were detected. In the sample with x = 10, it was amorphous, and peaks of some complex oxides (LiVO3) were detected. In the sample with x = 20, peaks of highly crystalline complex oxides (Li4V 10 O 27 ) were detected. In addition, when the AC impedance of each of the above samples was measured, there was no frequency dependence in the sample with x = 0, but there was frequency dependence in the samples with x = 10 and 20, indicating ionic conductivity. In particular, the conductivity of the ion conductor was on the order of 10 -3 S / cm in the sample with x = 10. From these, it is desirable that x is 4 or more and 19 or less.

[0032] <Other Variations> In the above-described first and second embodiments, a form in which a vanadate glass containing iron or lithium is used as a solid electrolyte has been described, but the present invention is not limited thereto. For example, instead of iron or lithium, a vanadate glass containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) as a part of constituent elements may be used. FIG. 7 is a diagram showing starting materials, a synthesis process, and synthesis results of a vanadate glass containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn). In FIG. 7, the case where x = 10 is illustrated. Vanadate glasses containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) are each produced by a similar synthesis process using sodium carbonate (Na2CO3), magnesium oxide (MgO), aluminum hydroxide (Al(OH)3), or zinc oxide (ZnO) as starting materials (raw material compositions).

[0033] FIG. 8 is a diagram showing the results of evaluating the crystallinity (XRD) of a vanadate glass 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. In addition, the vanadate glass containing sodium (Na) contains an amorphous part together with a composite oxide having high crystallinity.

[0034] FIG. 9 is a diagram showing the results of measuring the alternating current impedance of an ion conductor containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) as constituent elements. All of them have frequency dependence, and it was confirmed that they are ion conductors. FIG. 10 is a diagram showing the conductivity of an ion conductor containing sodium (Na), magnesium (Mg), aluminum (Al), or zinc (Zn) as constituent elements. As shown in FIG. 10, all of them are 10 ―4 ~10 ―5The conductivity of the order was shown.

Claims

1. A solid electrolyte containing an amorphous substance having vanadium element, barium element and element M (element M is at least one selected from Fe, Li, Na, Mg, Al and Zn) as part of constituent elements.

2. The amorphous substance is vanadate glass, and the element M is configured to be an ion conductor in the amorphous substance. The solid electrolyte according to Claim 1.

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

4. 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 1 hour or longer. The solid electrolyte according to Claim 3.

5. Polyacrylic acid The solid electrolyte according to Claim 1, further containing the same.

6. A metal-air battery having the solid electrolyte according to Claim 1 or Claim 5.

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

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