JP7219462B2 - zinc secondary battery - Google Patents

Description

The present invention relates to zinc secondary batteries.

Zinc is an abundant resource, has a high theoretical capacity, and can be used in combination with a highly safe aqueous electrolyte. Therefore, it is widely used as a primary battery that achieves both economy and energy density. In recent years, rechargeable zinc secondary batteries have attracted attention. In particular, zinc-air secondary batteries equipped with an air positive electrode that takes in oxygen from the atmosphere and reacts with it are being researched and developed as large-capacity batteries (non-patented). Reference 1).

Electrolytes used in zinc-air secondary batteries are currently limited to alkaline electrolytes such as potassium hydroxide because of compatibility with the positive electrode catalyst material and suppression of hydrogen generation during charging. However, when an alkaline electrolyte is used, formation of zinc dendrite, formation of potassium carbonate in the electrolyte due to reaction with carbon dioxide in the atmosphere, increase in battery resistance due to drying out of the solvent, zinc corrosion during storage, etc. (Non-Patent Document 2). Therefore, in order to improve the performance of the zinc-air secondary battery, it is necessary to improve the electrolytic solution.

Candidates for electrolytes used in zinc-air secondary batteries include, for example, weakly acidic or neutral aqueous solutions with a zinc ion concentration of 1 mol/L or less. There are problems. In addition, studies on continuous zinc deposition and dissolution required for secondary batteries are insufficient, and studies on electrolyte circulation are also necessary (Non-Patent Document 3, Patent Document 1).

Dendrite-free electrodeposition of a high-concentration zinc chloride aqueous solution has been reported (Non-Patent Document 4, Patent Document 1), but the electrolytic solution used (30 mol/L, the molar ratio of water to zinc chloride is 1.8) has a melting point close to room temperature, making it difficult to apply to continuous electrodeposition and secondary batteries.

U.S. Pat. No. 4,220,690

J. Fu et al. Adv. Mater. 2017, 29, 1604685. A.R. Mainar et al. J. Energy Storage 2018, 15, 304. J. Jindra et al. J. Appl. Electrochem. 1973, 3, 297. C. Zhang et al. Chem. Commun. 2018, 54, 14097.

An object of the present disclosure is to realize dendrite-free zinc electrodeposition and to provide a zinc secondary battery that is excellent in charge-discharge efficiency, cycle characteristics, and durability.

The present disclosure is a zinc secondary battery comprising a positive electrode, a negative electrode containing zinc and an electrolyte, wherein the electrolyte has a pH of less than 7 and contains a zinc metal salt and water, wherein the zinc metal salt It relates to a zinc secondary battery in which the molar ratio of water is in the range of 2 or more and 6 or less.

In the zinc secondary battery, the zinc metal salt is preferably at least one selected from the group consisting of zinc chloride, zinc bromide, zinc iodide, and zinc nitrate.

In the zinc secondary battery, the positive electrode preferably contains, as a positive electrode active material, oxygen or a metal oxide, sulfide, or polyanion compound capable of intercalating and deintercalating metal ions.

In the zinc secondary battery, the negative electrode preferably contains zinc, a zinc alloy, or zinc oxide as a negative electrode active material.

In the zinc secondary battery, the electrolyte preferably contains an alkali metal salt or an alkaline earth metal salt.

In the zinc secondary battery, the electrolytic solution preferably contains a non-aqueous solvent.

In the zinc secondary battery, the non-aqueous solvent is preferably an aprotic solvent.

In the zinc secondary battery, the electrolytic solution preferably contains a matrix polymer or inorganic particles and is gel or solid.

After achieving dendrite-free zinc electrodeposition, it is possible to provide a zinc secondary battery that is excellent in charge-discharge efficiency, cycle characteristics, and durability.

It is a graph which shows the precipitation dissolution behavior in a tungsten electrode (test 1). 1 is a graph showing coulombic efficiency (C.E.%) in tungsten electrodes (Test 1). 1 is an electron micrograph showing the morphology of a zinc electrodeposit (Test 2). 4 is a graph showing voltage fluctuations when zinc is continuously deposited and dissolved in various electrolytic solutions (Test 3). 4 is an electron micrograph showing the state of the zinc surface after storage in various electrolytic solutions (Test 4). It is drawing explaining one Example of a zinc secondary battery (Test 5). 4 is a graph showing charge/discharge curves and capacity changes of a zinc secondary battery (Test 5). 4 is a graph showing charge/discharge curves of a zinc secondary battery (Test 6). 4 is a graph showing changes in capacity of a zinc secondary battery (Test 6). 7 is a graph showing the results of X-ray diffraction analysis (XRD) of an air electrode after a charge-discharge test of a zinc secondary battery (test 7). 10 is a graph showing changes over time in the water content of various electrolytic solutions (Test 8). It is drawing which shows the volume change of various electrolyte solutions (Test 9).

[Zinc secondary battery]
A zinc secondary battery of the present disclosure is a zinc secondary battery comprising a positive electrode, a negative electrode containing zinc, and an electrolytic solution, wherein the electrolytic solution has a pH of less than 7 and contains a zinc metal salt and water, and the The molar ratio of the water content to the zinc metal salt is in the range of 2 or more and 6 or less.

(positive electrode)
Electrode configurations known in the art can be employed for the positive electrode of the zinc secondary battery of the present disclosure. For example, it may have a cathode material and a cathode current collector.

The cathode material has a cathode active material. Moreover, the positive electrode material may contain a conductive material, a binder, and the like, if necessary.

A positive electrode active material is a material that directly contributes to the charging reaction and the discharging reaction in the positive electrode. The positive electrode active material is not particularly limited, and positive electrode active materials known in the art can be used. As the positive electrode active material, oxygen; or metal oxides, sulfides, or polyanion compounds capable of intercalating and deintercalating metal ions are preferable. Among these, oxygen is more preferable. The oxygen supply source may be air or oxygen gas.

A conductive material can be added to the cathode material to increase the conductivity of the cathode. The conductive material is not particularly limited as long as it is a material having conductivity, and any conductive material known in the art can be used. Examples include carbon such as carbon black, graphite, and carbon fibers; conductive fibers such as metal fibers; metal powders such as nickel and aluminum; and organic conductive materials such as polyphenylene derivatives. These can be used individually by 1 type or in combination of 2 or more types.

A binder can be added to the positive electrode material to maintain the shape of the positive electrode material and the positive electrode current collector. The binder is not particularly limited, and any binder known in the art can be used. For example, olefin resins such as polyethylene; fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); rubber resins such as styrene-butadiene rubber (SBR rubber); perfluorosulfonic acid polymer and the like. These can be used individually by 1 type or in combination of 2 or more types.

When the positive electrode active material is oxygen, the positive electrode preferably contains an oxygen redox catalyst. Thereby, the oxidation-reduction reaction at the positive electrode can be performed more efficiently. Examples of redox catalysts for oxygen include carbon-based materials having a redox catalytic function such as graphite; metals such as platinum, palladium, nickel, cobalt, iridium, ruthenium, and iron, and alloys containing these metals; manganese dioxide and metal oxides such as tricobalt tetroxide; and perovskite oxides.

As the positive electrode current collector, metals such as stainless steel, nickel, molybdenum, aluminum, copper, titanium, and tungsten, and carbon sheets can be used. As for the shape, a shape having pores such as a net shape or a mesh shape is preferable in order to facilitate the diffusion of oxygen.

The positive electrode can be produced by mixing a positive electrode active material and, if necessary, a redox catalyst, a conductive material, a binder, and the like, and molding a positive electrode mixture. When the positive electrode has a positive electrode current collector, the positive electrode mixture may be applied to the positive electrode current collector, dried, and/or press-molded. Examples of the method for mixing the positive electrode mixture include wet mixing in which solvents are mixed and dry mixing.

(negative electrode containing zinc)
The negative electrode of the zinc secondary battery of the present disclosure is not particularly limited as long as it contains zinc, and an electrode configuration known in the art can be adopted. For example, it may have a negative electrode material and a negative current collector.

The negative electrode material has a negative electrode active material. Moreover, the negative electrode material may contain a conductive material, a binder, and the like, if necessary.

A negative electrode active material is a material that directly contributes to the charging reaction and the discharging reaction in the negative electrode. Zinc may be included as a negative electrode active material. As such zinc, zinc (zinc metal), zinc alloy, zinc oxide, or the like is preferable. Among these, zinc is more preferable.

As the conductive material and the binder, the same materials as those enumerated above as those that may be contained in the positive electrode material can be employed.

As the negative electrode current collector, metals such as stainless steel, nickel, molybdenum, aluminum, copper, titanium, and tungsten, and carbon sheets can be used. Also, the negative electrode can be produced by mixing a negative electrode active material, and, if necessary, a conductive material, a binder, and the like, similarly to the positive electrode, and forming a negative electrode mixture. When the negative electrode has a negative electrode current collector, the negative electrode mixture may be applied to the negative electrode current collector, dried, and/or press-molded. Examples of the method for mixing the negative electrode mixture include wet mixing in which a solvent is mixed, and dry mixing.

(Electrolyte)
Where the electrolyte of the zinc secondary battery of the present disclosure has a pH of less than 7, the pH may be less than 6, less than 5, or less than 4, and may be 2 or more or 3 or more. If the pH is 7 or more, the resulting zinc secondary battery is inferior in charge/discharge efficiency (coulombic efficiency) and cycle characteristics.

In addition, in the electrolytic solution of the zinc secondary battery of the present disclosure, the molar ratio of water to zinc metal salt is 2 or more and 6 or less. The molar ratio of water to the zinc metal salt is preferably from 2 to 5, more preferably from 2 to 4, even more preferably from 2 to 3, and most preferably from 2 to 2.33. This is because the charging and discharging efficiency (coulomb efficiency) is excellent.

The type of zinc metal salt in the electrolytic solution is not particularly limited. For example, it may be at least one selected from the group consisting of zinc chloride, zinc bromide, zinc iodide, and zinc nitrate. Among these, zinc chloride is preferred because of its high water solubility and low cost.

When the electrolytic solution of the present disclosure is composed of a zinc metal salt and water, the zinc metal salt is in a hydrated state, so the water in the electrolytic solution is difficult to volatilize, and the cycle characteristics and durability of the zinc secondary battery are improved. Excellent. Moreover, since zinc metal salt and water are both inexpensive, it is excellent in terms of cost. In the present disclosure, the excellent durability of the zinc secondary battery can be rephrased as being able to be used stably even after repeated charging and discharging many times.

The electrolyte of the present disclosure may contain optional ingredients other than the zinc metal salt and water. Examples include a component that suppresses volatilization of water in the electrolyte, a component that does not participate in the hydration structure of the zinc metal salt, and a component that suppresses the fluidity of the electrolyte.

Optional components of the electrolytic solution of the present disclosure include, for example, base metal salts other than zinc metal salts that can lower the activity of water as a solvent and suppress volatilization of water in the electrolytic solution. Examples of base metal salts other than zinc metal salts include alkali metal salts and alkaline earth metal salts. By containing a base metal salt other than such a zinc metal salt, the durability of the zinc secondary battery of the present disclosure can be further improved.

The content of the base metal salt other than the zinc metal salt in the electrolytic solution is not particularly limited, but may be, for example, 1 mol/L or more and 20 mol/L or less.

Specific examples of base metal salts other than zinc metal salts are not particularly limited, but examples include chlorides, fluorides, bromides, iodides, amide salts, imide salts, sulfonates, and hexafluoro Phosphate, tetrafluoroborate, perchlorate, chlorate, nitrate, sulfate, phosphate, acetate, carbonate, oxide, hydroxide and the like. These can be used individually by 1 type or in combination of 2 or more types.

Secondly, non-aqueous solvents are mentioned. The non-aqueous solvent is preferably a non-aqueous solvent that lowers the activity of water as the solvent and does not participate in the hydration structure of the zinc metal salt. By containing such a non-aqueous solvent, the vapor pressure of the electrolytic solution can be lowered, the water retention can be further improved, and the durability of the zinc secondary battery of the present disclosure can be improved.

The content of the non-aqueous solvent in the electrolytic solution is not particularly limited, but may be, for example, 1 vol % or more and 50 vol % or less.

Specific examples of non-aqueous solvents include, but are not limited to, aprotic solvents. Examples of aprotic solvents include cyclic carbonates such as ethylene carbonate; cyclic ethers such as tetrahydrofuran; cyclic sulfones such as sulfolane; chain carbonates such as dimethyl carbonate; nitriles such as acetonitrile; ethers such as dimethyl ether; and chain ether carbonates such as dimethoxyethane. In addition, these non-aqueous solvents may be substituted with halides such as fluorine-substituted compounds or sulfur elements. These can be used individually by 1 type or in combination of 2 or more types.

Thirdly, there are matrix polymers or inorganic particles. Since the electrolytic solution can be gelled and/or solidified to suppress fluidity and volatility, the electrolytic solution is less likely to leak from the zinc secondary battery of the present disclosure, and durability can be improved.

The content of the matrix polymer or the inorganic particles in the electrolytic solution is not particularly limited as long as the electrolytic solution can be gelled and/or solidified.

Specific examples of matrix polymers include, but are not limited to, polyether polymers, fluorine polymers, polyacrylic polymers, polyacrylonitrile, polysiloxane, and polyphosphazenes. Specific examples of inorganic particles are not particularly limited, but silica, alumina, zirconia, titanium oxide, and the like can be exemplified. A matrix polymer and an inorganic particle can be used individually by 1 type or in combination of 2 or more types.

(separator)
The zinc secondary battery of the present disclosure may have a separator, and may be composed of a positive electrode and a negative electrode facing each other with the separator interposed therebetween.

The separator is not particularly limited as long as it electrically insulates between the positive electrode and the negative electrode, has ion permeability, and has resistance to oxidation on the positive electrode side and reducibility on the negative electrode side. , any separator known in the art can be employed. Examples of separators include olefin resins such as polyethylene.

(Preparation of electrolytic solution)
Electrolyte A: A zinc chloride aqueous solution was prepared by mixing 1 mol part of zinc (II) chloride and 4 mol parts of water. The pH was 7 or less. This zinc chloride aqueous solution may be referred to as ZnCl ₂ .4H ₂ O.
Electrolyte B: An aqueous solution of zinc chloride was prepared by mixing 1 mol part of zinc (II) chloride and 3 mol parts of water. The pH was 7 or less. This zinc chloride aqueous solution may be referred to as ZnCl ₂ .3H ₂ O.
Electrolyte C: 1 mol part of zinc (II) chloride and 2.33 mol parts of water were mixed to prepare an aqueous solution of zinc chloride. The pH was 7 or less. This zinc chloride aqueous solution may be referred to as ZnCl ₂ ·2.33H ₂ O.
Electrolyte D: A potassium hydroxide-zinc (II) chloride aqueous solution was prepared so as to have a composition of 6 mol/L of potassium hydroxide and 0.2 mol/L of zinc (II) chloride. The pH was 14 or higher. This potassium hydroxide-zinc (II) chloride aqueous solution is sometimes referred to as 6M KOH-0.2M ZnCl ₂ .

(Test 1)
In order to examine the amount of zinc deposited on the tungsten electrode and the retention of the deposited zinc when using electrolytes A, B, C and D, respectively, zinc deposition was performed by cyclic voltammetry under the following conditions.・Remelting behavior was measured. A tungsten electrode was used as the working electrode, a Zn/Zn ²⁺ electrode as the reference electrode, and a zinc electrode as the counter electrode. The measurement cell containing each electrolytic solution was allowed to stand in a constant temperature bath at 30° C. and 1 atm for 0.5 hours, and the atmosphere in the measurement cell was replaced with dry air. Under the conditions of 30° C., dry air, and 1 atm, the scanning potential range is −0.5 to 1.5 V v. s. Five cycles of cyclic voltammetry (CV) measurements were performed with Zn/Zn ²⁺ and a scan rate of 10 mVs ⁻¹ . Cyclic voltammetry (CV) measurement in the case of using electrolyte solution D was performed under the conditions of 30° C., atmospheric air, and 1 atm.

FIG. 1 shows the relationship between the scanning potential and the current density when sweeping in the negative direction in the first cycle and then sweeping back from -0.5 V in the reverse direction. FIG. 2 is the coulombic efficiency (C.E.%) at the 1st to 5th cycles. The coulombic efficiency indicates the deposition and dissolution efficiency of zinc (in other words, the amount of positive charge/the amount of negative charge, further in other words, the amount of redissolved zinc that has been deposited/the amount of zinc that has been deposited). It was calculated based on the results of voltammetry (CV) measurements.

In electrolyte D (6M KOH-0.2M ZnCl ₂ ), the amount of zinc deposited (the amount of electricity in the reduction reaction) is greater than in electrolytes A, B and C (ZnCl ₂ ·nH ₂ O system) (Fig. 1), only about 30% of the precipitated zinc was redissolved, that is, the coulombic efficiency was about 30% (see the right side of FIG. 2). On the other hand, in the electrolytic solutions A, B and C, 98% or more of the precipitated zinc was redissolved, and the coulombic efficiency was 98% or more (see the left side of FIG. 2).

The reason why the Coulombic efficiency is inferior when electrolyte solution D is used is that zinc is deposited in a resinous form (dendrites are formed) (see FIG. 3 (4)), and zinc is easily peeled off from the electrode; Corrosion is likely to occur when in contact with the electrolyte solution D (see FIG. 5 (3)); hydrogen generation is occurring at the same time.

On the other hand, electrolyte solutions A, B and C (ZnCl ₂ ·nH ₂ O system) were shown to have excellent coulombic efficiency and excellent charge-discharge efficiency. Among these, the electrolytic solution C (ZnCl ₂ ·2.33H ₂ O), which _has the lowest water content, exhibits a _particularly high coulombic efficiency. This composition is used as representative of

(Test 2)
The morphology of zinc electrodeposited on a tungsten electrode was investigated in the case of using electrolytes A, B, C and D, respectively. The conditions for electrodeposition are as follows. The potential for depositing zinc (Deposition potential) was -0.2 V vs. A voltage was applied under the same conditions as in Test 1 except that Zn/Zn ²⁺ was used and the deposition time was set to 1 hour. Then, the morphology of the electrodeposited zinc was observed with an electron microscope. Results are shown in FIG.

In electrolyte D (6M KOH-0.2M ZnCl ₂ ), dendritic zinc was observed. On the other hand, in electrolytes A, B and C (ZnCl ₂ ·nH ₂ O system), larger and smoother blocks of zinc were observed. Therefore, it was shown that the electrolytes A, B and C (ZnCl ₂ ·nH ₂ O system) can suppress dendrites and allow dendrite-free zinc electrodeposition.

(Test 3)
In order to investigate whether the zinc negative electrode can be repeatedly used as a zinc secondary battery, electrolytes C and D were used, respectively, and a zinc symmetrical cell was constructed using zinc metal for both electrodes, and positive and negative were alternately repeated on both zinc electrodes. A current was applied continuously to deposit and dissolve zinc metal, and the voltage fluctuation at that time was measured. The voltage fluctuation was measured at a current density of 1 mAcm ^-2 , an electric quantity of 1 mAhcm ^-2 and a measurement temperature of 30°C. The relationship between time (h) and voltage when electrolytes C and D are used are shown in the top and bottom of FIG. 4, respectively.

As shown in the upper part of FIG. 4, the electrolytic solution C (ZnCl ₂ .2.33H ₂ O) increases only up to about 100 mV even after repeating cycles for 1000 hours (500th cycle). On the other hand, in electrolyte solution D (6M KOH-0.2M ZnCl ₂ ), as shown in the bottom of FIG. 4, 150 hours (about 75 cycles because positive and negative currents alternately flow every hour). The voltage became 1.5V or higher.

The reason for the increase in overvoltage when electrolyte D is used is that zinc tends to corrode quickly when it is in contact with electrolyte D (see FIG. 5 (3)); or a passive film of zinc oxide. is formed to increase the resistance at the interface between the electrolyte and the zinc electrode.

On the other hand, the reason why the overvoltage is less likely to increase when electrolyte C is used is that when zinc is in contact with electrolyte C, corrosion and oxide formation are relatively difficult (see FIG. 5 (2)). One reason for this is that the resistance at the interface between the liquid and the zinc electrode is less likely to increase. Therefore, the zinc secondary battery using the electrolyte solution C (ZnCl ₂ 2.33H ₂ O) has excellent zinc electrode stability, and even after repeated charging and discharging, the overvoltage of the zinc negative electrode is unlikely to increase, and the battery is stable. It was shown that the voltage is less likely to drop, the reversibility of zinc deposition and dissolution is excellent, and the cycle characteristics are excellent.

(Test 4)
In order to evaluate the storage stability of the electrode (negative electrode) with zinc in contact with the electrolyte, zinc was brought into contact with each of the electrolytes C and D at room temperature for 30 days (about one month). The zinc surface was observed with an electron microscope. Results are shown in FIG.

As can be seen from the comparison between FIGS. 5(1) and 5(3), when the electrolyte solution D is used, compared to the state before contact with the electrolyte solution, the hydroxide ions and water in the electrolyte solution contribute to zinc was corroded and covered with a coating of zinc compounds (zinc oxide, zinc hydroxide, etc.), and the coating was torn to reveal the original zinc (crater part).

On the other hand, when electrolyte C is used, as can be seen from the comparison of FIGS. 5(1) and 5(2), there is no significant difference in the state of zinc from before contact with the electrolyte, and the storage stability of the negative electrode is high. This is because the electrolyte solution C (ZnCl ₂ 2.33H ₂ O) has no hydroxide ions in the electrolyte solution, and all the water in the electrolyte solution is coordinated (hydrated) to the zinc ions. , is low in activity (in other words, it has a high melting point and is difficult to volatilize due to the lack of free water molecules), which suppresses the corrosion of zinc metal. Therefore, it can be seen that the zinc secondary battery using the electrolyte solution C can be stably used even after being repeatedly charged and discharged many times, and can be used for a long period of time.

(Test 5)
A zinc plate as the negative electrode, a carbon electrode supporting a platinum catalyst, which is a common air electrode in the field of air batteries and fuel cells as the air positive electrode (the amount of platinum is 50% by weight with respect to the weight of platinum and carbon, and the unit area A zinc secondary battery was assembled using an electrolyte solution C (ZnCl ₂ ^· 2.33H ₂ O) as an electrolyte solution. Details are as shown in FIG.

FIG. 6 is a diagram illustrating an example of a zinc secondary battery. A zinc secondary battery 1 includes a positive electrode 2 and a negative electrode 3 on both sides of an electrolyte chamber 4, and the zinc secondary battery 1 is fixed by an outer frame 5 so as to be integrated. The positive electrode 2 is composed of a positive electrode current collector 6 and a carbon electrode (carbon sheet) supporting a platinum catalyst, and oxygen is used as a positive electrode active material. The positive electrode current collector 6 is made of mesh-like molybdenum. The negative electrode 3 has a negative electrode current collector 7 and uses zinc as a negative electrode active material. The negative electrode current collector 7 is plate-shaped molybdenum. The electrolytic solution chamber 4 is filled with an electrolytic solution, and is sealed by sealing materials 8 provided on both sides of the positive electrode 2 side and the negative electrode 3 side so that the electrolytic solution as the contents does not leak. Separators 9 are provided between the positive electrode 2 and the sealing material 8 and between the negative electrode 3 and the sealing material 8 to separate the positive electrode 2 and the negative electrode 3 from each other. Electricity is supplied to the positive electrode 2 and the negative electrode 3 through the positive electrode current collector 6 and the negative electrode current collector 7 .

A charging/discharging test was repeated 40 times for the assembled zinc secondary battery. As the conditions for the charge/discharge test, a charge/discharge curve was created by flowing a current of 500 mA per unit amount of platinum (g) in an oxygen atmosphere at 30°C. There was an interval of 10 minutes between discharging and charging. The relationship between capacity (mAh·g ⁻¹ :g ⁻¹ represents the unit weight of the catalyst) and voltage is shown on the left side of FIG. Also, the relationship between the number of charge/discharge tests and the capacity (mAh·g ⁻¹ ) is shown on the right side of FIG.

As shown in FIG. 7, the zinc secondary battery using electrolyte solution C (ZnCl ₂ .2.33H ₂ O) has a capacity of 1000 mAh. It was confirmed that charging and discharging were repeated 40 times at g ⁻¹ (g ⁻¹ represents the unit weight of the catalyst) while maintaining a discharge voltage of 1 V or higher. Therefore, electrolyte solution C (ZnCl ₂ .2.33H ₂ O) can be used as an electrolyte solution for zinc secondary batteries, and zinc secondary batteries using this have excellent charge/discharge efficiency, cycle characteristics and durability. It was shown to have sex.

(Test 6)
In order to examine the charge-discharge cycle stability of the electrolyte, zinc secondary batteries using each of the electrolytes C and D were assembled in the same manner as in Test 5, and a charge-discharge test was performed in dry air. FIG. 8 shows the relationship between capacity (mAh·g ⁻¹ : g ⁻¹ represents the unit weight of the catalyst) and voltage. FIG. 9 shows the relationship between the number of charge/discharge tests and the capacity (mAh·g ⁻¹ ).

As shown on the right side of FIGS. 8 and 9, the voltage of electrolytic solution D (6M KOH-0.2M ZnCl ₂ ) drops significantly in the 4th or 5th cycle, and drops to 200 mAh·g ⁻¹ or less after the 5th cycle. became. The cause of the voltage drop in a small number of cycles when electrolyte solution D is used is that, as described in Test 2, zinc was deposited in a resinous form (dendrites were formed) at the negative electrode (see FIG. 3 (4)). ); As described in Test 4, the zinc corroded and was covered with a zinc compound film, which increased the resistance and increased the overvoltage; The air electrode (positive electrode) deteriorated due to absorption (see FIG. 10), and the electrolytic solution dried due to air circulation (see FIGS. 11 and 12).

On the other hand, as shown on the left side of FIGS. 8 and 9, when electrolyte solution C (ZnCl _{2.2.33H 2} O) was used, even at the 10th cycle, similar to the result of Test 5 (FIG. 7) stable. According to the electrolyte C, as described in Test 2, dendrites were suppressed (Fig. 3 (3)); as described in Test 4, corrosion of zinc metal was suppressed (Fig. 5 ( 2)); as will be described later, the deterioration of the air electrode (positive electrode) is suppressed;

(Test 7)
X-ray diffraction analysis (XRD) to examine the state of the air electrode after the charge-discharge test (after discharge) of the zinc secondary battery using electrolyte solution D (6M KOH-0.2M ZnCl ₂ ) performed in test 6 did A powder method using Cu-Kα rays as X-rays was used. Results are shown in FIG. In FIG. 10, * is a peak indicating molybdenum, and Δ is an unknown peak.

As shown in FIG. 10, in addition to KOH (potassium hydroxide) and ZnCl ₂ (zinc chloride) contained in the electrolytic solution D from the beginning of the test, K ₂ CO ₃ (potassium carbonate) and KHCO ₃ (hydrogen carbonate potassium) was detected. This is presumably because the electrolytic solution D absorbed carbon dioxide in the air, and the resulting potassium carbonate and the like adhered to the air electrode (positive electrode). Therefore, it can be said that the oxygen reaction of the air electrode (positive electrode) was inhibited, and the capacity deteriorated in a small number of cycles.

(Test 8)
In order to examine the water retention of the electrolytes, nitrogen gas was flowed at 21° C. and a flow rate of 200 mlmin ⁻¹ to 1 mL of electrolytes C and D, respectively, and changes in the weight of the electrolytes over time were measured. The water content was calculated based on the weight of the electrolyte. FIG. 11 shows the relationship between time (h) and water content (% by weight).

Taking the original water content of each as 100% by weight, electrolyte D (6M KOH-0.2M ZnCl ₂ ) decreases to 40% by weight after 4 hours, while electrolyte C (ZnCl ₂ 2.33 H ₂ O) maintained at least 85% by weight for 15 hours. Therefore, it was shown that the electrolytic solution C is difficult to dry and has excellent water retention.

(Test 9)
In order to examine the water holding capacity of the electrolytic solutions, dry air was bubbled through each of 1 mL of electrolytic solutions C and D, and the change in volume was measured. The amount of the electrolytic solution at the start of bubbling was 1 ml in each case, and the bubbling was carried out in a dry chamber at room temperature and at a flow rate of 20 mlmin ⁻¹ . Results are shown in FIG.

Electrolyte D (6M KOH-0.2M ZnCl ₂ ) had a volume of about 0.7 ml due to the evaporation of moisture, while electrolyte C (ZnCl ₂ 2.33H ₂ O) hardly changed in volume. There is no Therefore, it is shown that the zinc secondary battery using the electrolyte solution C has excellent storage stability and a long life because the electrolyte solution is less likely to be depleted even if air circulation is continued for a long time due to charging and discharging, and it has excellent water retention. was done.

Elemental analysis of electrolyte solution C and electrolyte solution D after bubbling revealed that electrolyte solution C contained less than 0.10% by weight of carbon and electrolyte solution D contained approximately 0.61% by weight of carbon. It was confirmed that This is because electrolyte solution D absorbed carbon dioxide in bubbling dry air and was oxidized by this carbon dioxide to produce carbonate, whereas electrolyte solution C hardly absorbed carbon dioxide. It depends.

(Test 10)
In the same manner as in the preparation of electrolyte solutions A, B, C and D, zinc chloride (II ) and water to prepare an aqueous solution of zinc chloride. In FIG. 11 they are referred to as ZnCl ₂ .50H ₂ O, ZnCl ₂ .10H ₂ O, and ZnCl ₂ .1.80H ₂ O, respectively.

The melting points (in other words, the temperature at which they become completely liquid) of electrolytes A, B, C and D, and ZnCl ₂ .50H ₂ O, ZnCl ₂ .10H ₂ O, and ZnCl ₂ .1.80H ₂ O are Measured by differential scanning calorimetry. Results are shown in Table 1. In addition, n in Table 1 is the value _of n of _ZnCl2.nH2O .

ZnCl ₂ .1.8H ₂ O has a high concentration of zinc chloride and is excellent in water retention as an electrolytic solution, but since it has a melting point of 28.2° C., it is difficult to handle it at room temperature while always maintaining its liquid state. On the other hand, electrolyte C (ZnCl ₂ 2.33H ₂ O) has a high concentration of zinc chloride, is excellent in water retention as an electrolyte, and can maintain a liquid state at room temperature or lower, making it suitable for use as an electrolyte. .

Claims (7)

A zinc secondary battery comprising a positive electrode, a negative electrode containing zinc, and an electrolytic solution,
the electrolyte has a pH of less than 7 and contains a zinc metal salt and water;
The content of the water to the zinc metal salt is in the range of 2 or more and 6 or less in molar ratio,
the zinc metal salt is zinc chloride;
A zinc secondary battery, wherein the positive electrode contains, as a positive electrode active material, oxygen or a metal oxide, sulfide, or polyanion compound capable of intercalating and deintercalating metal ions. The zinc secondary battery according to claim 1 , wherein the negative electrode contains zinc, a zinc alloy, or zinc oxide as a negative electrode active material. The zinc secondary battery according to claim 1 or 2 , wherein the content of said water in said electrolytic solution is in the range of 2 or more and 4 or less in molar ratio with respect to said zinc metal salt. The zinc secondary battery according to any one of claims 1 to 3 , wherein the electrolyte contains an alkali metal salt or an alkaline earth metal salt. The zinc secondary battery according to any one of claims 1 to 4 , wherein the electrolyte contains a non-aqueous solvent. The zinc secondary battery according to claim 5 , wherein said non-aqueous solvent is an aprotic solvent. the electrolytic solution contains a matrix polymer or inorganic particles,
The zinc secondary battery according to any one of claims 1 to 6 , which is gel or solid.

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