JP6074394B2 - Aqueous lithium-positive electrode for air secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for an aqueous lithium-air secondary battery using an alkaline electrolyte.

  With the aim of increasing the energy density of secondary batteries, metal-air secondary batteries that use oxygen in the air as a positive electrode active material have attracted attention. Among them, a lithium-air secondary battery using metallic lithium as a negative electrode active material is a secondary battery having the largest weight energy density, and is attracting the most attention. There are two types of lithium-air secondary batteries: non-aqueous lithium-air secondary batteries that use non-aqueous solvents such as organic electrolytes and ionic liquids as electrolytes, and aqueous lithium-air secondary batteries that use aqueous electrolytes. Lithium-air secondary batteries have been proposed.

As shown in FIG. 1, in the water based lithium-air secondary battery, the metal lithium 20 as the negative electrode active material reacts with the water based electrolyte 30, so that the Li _{1 + x} (M, Al, Ga) is connected to the metal lithium negative electrode 20. ) _x (Ge _1-y Ti _y ) _{2 -x} (PO ₄ ) ₃ (where X ≦ 0.8 and 0 ≦ Y ≦ 1.0, and M is Nd, Sm, Eu, Gd, Tb, Dy, Ho , Er, Tm, Yb) and / or Li _{1 + x + y} Q _x Ti _2-x Si _y P _3-y O ₁₂ (where 0 <X ≤ 0.4 and 0 <Y It is performed by stacking and protecting a water-impermeable lithium ion conductive glass ceramic (hereinafter abbreviated as LATP) 50 represented by ≦ 0.6 and Q is Al or Ga) (hereinafter, abbreviated as LATP). (See Patent Document 1).

The battery reaction of the negative and positive electrodes of the water-based lithium-air secondary battery is

The negative ^{electrode: Li ⇔ Li + + e -} (1)
The positive _{^{electrode: Li + + 1/4 O 2 +}} 1/2 H 2 O + e- ⇔ Li + + OH - (2)

When discharging, the battery reaction proceeds from the left term to the right term in equations (1) and (2), and from the right term to the left term during charging. The problem of the alkaline corrosion of LATP50 is mentioned as a problem of the water-based lithium-air secondary battery in which the negative electrode metal lithium 20 is protected by LATP50. In the water-based lithium-air secondary battery, LiOH is generated by the discharge reaction at the positive electrode, as shown by the formula (2). Therefore, as the discharge reaction proceeds, the electrolytic solution shifts to the alkali side. When the pH of the electrolyte is pH> 10, there is a problem that corrosion of LATP50 occurs. As a countermeasure technique, a technique for preventing ionization of LiOH and preventing the electrolyte from becoming strongly alkaline by adding a high concentration of LiCl in the electrolyte in advance is known (Non-Patent Document). 1).

In an electrolyte containing LiOH · H ₂ O and high-concentration LiCl, the discharge reaction at the positive electrode is

The positive electrode (discharge _{^{reaction): Li + + 1/4 O 2}} + 3/2 H 2 O + e - → LiOH · H 2 O ↓ (3)

LiOH generated at the positive electrode (air electrode) by discharge is suppressed on ionization and deposited on the positive electrode (air electrode) 10 in the form of monohydrate (LiOH.H ₂ O). In this case, as shown in FIG. 2, the deposited LiOH.H ₂ O seals the pores of the positive electrode 10, lowers the oxygen diffusibility, and causes a problem of lowering the discharge performance.

As a countermeasure against the problem of the pores of the positive electrode 10 being sealed with this discharge product, a positive electrode as shown in FIG. 3 has been proposed (see Non-Patent Document 2). That is, in the positive electrode 10, an anion exchange membrane 13 having OH ⁻ ion conductivity is disposed on the surface of the positive electrode reaction layer 12, and the LiOH · H ₂ O precipitation surface is exposed from the surface of the positive electrode reaction layer 12 to the anion exchange membrane 13. This prevents the pores of the air electrode 10 from being sealed by LiOH.H ₂ O.

On the other hand, in the electrolyte in which LiOH · H ₂ O high-concentration LiCl coexists, the charge reaction at the positive electrode is also a reaction that shifts from the right term to the left term in formula (2) (OH ⁻ ions are oxidized, O ₂ Is a normal charging reaction. In an electrolytic solution in which a high concentration of LiCl coexists, the OH ⁻ ion concentration is low, and the charging reaction is unlikely to occur. During charging, OH ^- ions are consumed by the reaction, and the OH ^- ion concentration near the positive electrode further decreases. As shown in FIG. 4, when charging at a low current density, LiOH · H ₂ O is ionized and OH ⁻ ions can be supplied to the positive electrode, but when charged at a high current density, LiOH · H ₂ O can be supplied. The ionization of H ₂ O could not catch up, instead of OH ^- ion oxidation,

The positive electrode (charging side _{^{reaction): Cl - → 1/2 Cl 2}} + e - (4)

There is a problem that a charging side reaction represented by the following occurs, Cl 2 ⁻ ions are oxidized, and chlorine gas is generated.

When the O ₂ gas and / or Cl ₂ gas, which is a charge product, is generated on the surface of the positive electrode reaction layer (charge catalyst surface) 12, as shown in FIG. 4, it is discharged to the electrolyte 30 side. There is. In that case, there is a problem that the contact area between the electrolytic solution 30 and the positive electrode is lowered, and the charging performance is lowered.

As a technique for solving the problem of the positive electrode during charging, it has been proposed that the positive electrode reaction layer 12 is composed of an oxygen generation catalyst layer 12a and a hydrophilic layer 12b as shown in FIG. 5 (see Patent Document 2). .) In this technique, by using IrO ₂ as an oxygen generation catalyst, selective oxidation of OH ^- ions is possible, and charging side reactions that cause oxidation of Cl ^- ions are reduced. Furthermore, since the hydrophilic layer 12b is formed on the oxygen generation catalyst layer 12a, the movement of O ₂ which is a charge product to the electrolyte solution 30 side is inhibited, and it is easy to discharge to the gas diffusion layer 11 side. ing.

  To prevent alkaline corrosion of LATP in a water-based lithium-air secondary battery in which the metallic lithium of the negative electrode is protected by a water-impermeable glass ceramic (LATP), a high concentration of LiCl is added to the electrolyte. It is possible to prevent the alkaline corrosion. However, new problems occur during discharging and charging as described above, and it is necessary to take separate measures for the problems during discharging and charging.

  Separately from the discharge positive electrode adjusted to the specifications suitable for discharge as seen in conventional metal-air secondary batteries, a charge positive electrode (called auxiliary electrode or third electrode) is installed in the cell separately. There is a method of performing (see Patent Document 3). However, in this case, there is a problem that an external electric circuit for switching between the positive electrode for discharging and the positive electrode for charging is required. Further, it is necessary to devise a place where the auxiliary electrode is installed in the battery and to secure a route for discharging oxygen generated from the auxiliary electrode. Furthermore, it is pointed out that a short circuit easily occurs in the battery by installing an auxiliary electrode.

JP-T-2007-513464 Japanese Patent Application No. 2013-35798 Japanese Patent No. 3523506

Edited by Tatsumi Ishihara, “Development and Latest Technologies of Metal / Air Secondary Batteries”, Education Publishers, April 1, 2011, p91-104 P. Stevens et al., "Development of a lithium air rechargeable battery", ECS Transactions, 28 (32) 1-12 (2010)

  Therefore, the creation of a bi-functional positive electrode structure in which two functions of a discharge function and a charge function are simultaneously provided in one air electrode has been awaited.

In the positive electrode of a lithium-air secondary battery using an aqueous electrolyte in which LiCl coexists at a high concentration, the electrode structure is suitable for discharge, and the water-repellent property using a Pt catalyst that is excellent as an oxygen reduction catalyst A structure in which an anion exchange membrane is bonded to the discharge reaction layer as shown in Non-Patent Document 2 (see FIG. 3) is effective. On the other hand, as an electrode structure suitable for charging, as shown in Patent Document 2, a hydrophilic layer is added to a charging catalyst layer using an IrO ₂ catalyst having selective oxidation of OH 2 ⁻ ions (FIG. 5). See) is valid.

  In this way, an electrode structure suitable for discharging / charging is formed in one positive electrode, and as a measure for bifunctionalization, an electrode structure for discharging and an electrode structure for charging are formed on the same surface of the air electrode. The arranged structure shown in FIG. 6 has been proposed by the present inventors (see Patent Document 2). However, in this case, since the discharge reaction occurs only in the electrode structure for discharging and the charge reaction occurs only in the electrode structure for charging, there is a problem that the entire area of the positive electrode cannot be used effectively.

  On the other hand, in order to effectively use the entire area of the positive electrode, a bifunctional positive electrode in which a discharge electrode layer and a charge electrode layer are laminated and integrated in a positive electrode of a zinc-air secondary battery is known (M. Klein and S. Viswanathan, "Zinc / Air Battery R & D, Research and Development of Bifunctional Oxygen Electrode, Tasks I & II Final Report, Lawrence Berkeley National Laboratory, Berkeley, CA (Dec, 1986)), but aqueous lithium-air As a bifunctional positive electrode for a secondary battery, there is no disclosure of an integrated structure in which a discharge electrode layer and a charge electrode layer are laminated.

  As a method of laminating the discharge electrode layer and the charge electrode layer, as shown in FIG. 7, the electrolyte solution 30 / charge electrode layer 12 (hydrophilic layer 12b / oxygen generating catalyst layer 12a) / discharge electrode layer 15 ( The method of laminating the anion exchange membrane 15b / water repellent reaction layer 15a) / gas diffusion layer 11 in this order, and as shown in FIG. 8, the electrolyte 30 / the electrode layer 15 for discharge (anion exchange membrane 15b / water repellent) A method of laminating in order of reaction layer 15a) / charging electrode layer 12 (hydrophilic layer 12b / oxygen generating catalyst layer 12a) / gas diffusion layer 11 is conceivable.

However, as shown in FIG. 7, electrolyte 30 / charging electrode layer 12 (hydrophilic layer 12b / oxygen generating catalyst layer 12a) / discharge electrode layer 15 (anion exchange membrane 15b / water-repellent reaction layer 15a) / gas diffusion When the layers 11 are stacked in this order, O ₂ generated during charging cannot be discharged in the direction of the gas diffusion layer 11 due to the anion exchange membrane 15b installed for discharging, and as a result, discharged to the electrolyte 30 side. Therefore, there is a problem that the generated O ₂ hinders the supply of the electrolyte solution to the charging electrode layer 12. Furthermore, in this case, it is necessary to newly add a mechanism for discharging O ₂ from the electrolytic solution 30 side.

  On the other hand, as shown in FIG. 8, electrolytic solution 30 / discharge electrode layer 15 (anion exchange membrane 15b / water-repellent reaction layer 15a) / charge electrode layer 12 (hydrophilic layer 12b / oxygen generating catalyst layer 12a) / gas diffusion When the layers 11 are stacked in this order, there is a problem that the supply of oxygen from the atmosphere necessary at the time of discharge is hindered by the hydrophilic layer 12b installed for charging. Furthermore, since the reaction layer 15a of the discharge electrode layer 15 is water-repellent, there is a problem that it is difficult to supply the electrolyte solution necessary for charging to the charge electrode layer 12.

When the discharge electrode layer 15 and the charge electrode layer 12 are simply laminated as shown in FIGS. 7 and 8, there are problems as described above. From the structure of FIG. By moving the position of the anion exchange membrane 15b to the surface of the charging electrode layer 12, it can be considered that the discharge function can be provided while avoiding the problem of the structure of FIG. However, as will be described later in [Mode for Carrying Out the Invention], when the structure of FIG. 9 is actually prototyped and evaluated, an anion exchange membrane 15b is installed on the charging electrode layer 12. It was found that the movement speed of OH ^- ions decreased, and the supply of OH ^- ions necessary for the charging reaction was insufficient.

The present invention has been made in view of the above problems, and can supply OH ^- ions necessary for a charging reaction and discharge oxygen generated during charging in the direction of a gas diffusion layer. It is an object of the present invention to provide a bifunctional positive electrode for a water-based lithium-air secondary battery in which a discharge reaction layer and a charge reaction layer are laminated and integrated so that supply of water is not hindered.

  The positive electrode for an aqueous lithium-air secondary battery of the present invention made to solve the problem is a gas diffusion layer, a water-repellent discharge reaction layer laminated on the gas diffusion layer, and a layer laminated on the water-repellent discharge reaction layer A hydrophilic charge reaction layer, wherein the water-repellent discharge reaction layer has air permeability, reduces oxygen in the air during a discharge reaction, and generates oxygen generated in the hydrophilic charge reaction layer during a charge reaction. It moves to a gas diffusion layer direction, It is characterized by the above-mentioned.

  The water-repellent discharge reaction layer has air permeability, reduces oxygen in the air during the discharge reaction, and moves oxygen generated in the hydrophilic charge reaction layer during the charge reaction toward the gas diffusion layer. The supply of electrolyte is not hindered. As a result, a decrease in charging performance can be suppressed. Further, since the water-repellent discharge reaction layer is laminated on the gas diffusion layer, the supply of oxygen to the water-repellent discharge reaction layer is not hindered during the discharge reaction.

  In the positive electrode for an aqueous lithium-air secondary battery, the water-repellent discharge reaction layer preferably contains a water-repellent material and is porous. This makes it easier to move oxygen generated in the hydrophilic charge reaction layer during the charge reaction toward the gas diffusion layer.

  Moreover, it is good to use resin which has anion conductivity as a binder of the material which comprises the said water repellent discharge reaction layer. As a result, the surface of the oxygen reduction catalyst is coated with the resin having anionic conductivity, and it is possible to prevent the discharge product from being directly deposited on the surface of the oxygen reduction catalyst. A decrease in supply amount can be prevented.

  Moreover, the addition amount of the resin binder having anion conductivity is preferably 10 to 70 wt% with respect to the weight of the water-repellent discharge reaction layer.

  When the amount added is increased, the ratio of the pores formed by the oxygen reduction catalyst and the water repellent material being blocked by the resin binder having anionic conductivity increases, and the entry and exit of oxygen is restricted. When the addition amount decreases, the ratio of the oxygen reduction catalyst coated with the resin binder having anionic conductivity decreases, and the discharge product directly deposits on the surface of the oxygen reduction catalyst not coated with the resin binder. When the addition amount of the resin binder is 10 to 70 wt% with respect to the weight of the discharge reaction layer, the entry and exit of oxygen is rarely restricted, and the precipitation of discharge products on the oxygen reduction catalyst surface is also small.

  The water-repellent discharge reaction layer has air permeability, reduces oxygen in the air during the discharge reaction, and moves oxygen generated in the hydrophilic charge reaction layer during the charge reaction to the gas diffusion layer. Liquid supply is not hindered. As a result, a decrease in charging performance can be suppressed. Further, since the water-repellent discharge reaction layer is laminated on the gas diffusion layer, oxygen can be easily supplied to the water-repellent discharge reaction layer during the discharge reaction.

FIG. 4 is a cross-sectional view schematically showing a concept of a water-based lithium-air secondary battery according to a conventional technique. It is a figure which shows typically the positive electrode reaction at the time of discharge of the conventional water-system lithium-air secondary battery. It is a figure which shows typically the positive electrode reaction at the time of discharge of another conventional water-system lithium-air secondary battery. FIG. 3 is a diagram schematically showing a positive electrode reaction during charging of the conventional aqueous lithium-air secondary battery shown in FIG. 2. FIG. 4 is a diagram schematically showing a positive electrode reaction during charging of the conventional aqueous lithium-air secondary battery shown in FIG. 3. FIG. 6 is a cross-sectional view schematically showing another water based lithium-air secondary battery according to the prior art. 1 is a schematic cross-sectional view of a positive electrode of an aqueous lithium-air secondary battery in which a discharge reaction layer and a charge reaction layer are laminated. FIG. 8 is a schematic cross-sectional view of a positive electrode for an aqueous lithium-air secondary battery different from FIG. 7 in which a discharge reaction layer and a charge reaction layer are stacked. FIG. 9 is a schematic cross-sectional view of a positive electrode for an aqueous lithium-air secondary battery different from FIGS. 7 and 8 in which a discharge reaction layer and a charge reaction layer are stacked. 1 is a schematic cross-sectional view of a positive electrode for an aqueous lithium-air secondary battery according to an embodiment of the present invention. It is a schematic sectional drawing of the cell which evaluates the electrochemical characteristic of the air electrode for water-system lithium-air secondary batteries. It is a graph which shows the time-dependent change of the electrical potential difference of the reference electrode at the time of discharge. It is a graph which shows the time-dependent change of the electrical potential difference of the reference electrode at the time of charging / discharging. It is a graph which shows the change at the time of charging / discharging repeated connection of the electrical potential difference of the reference electrode at the time of charging / discharging and the positive electrode whose resin binder addition amount is 38 wt%. It is a graph which shows the change at the time of charging / discharging repeated connection of the electrical potential difference of the reference electrode at the time of charging / discharging and the positive electrode whose resin binder addition amount is 52 wt%. It is a graph which shows the change at the time of charging / discharging repeated connection of the electrical potential difference of the reference electrode at the time of charging / discharging and the positive electrode whose resin binder addition amount is 59 wt%. It is a graph which shows the change at the time of charging / discharging repeated transition of the electrical potential difference of the reference electrode at the time of charging / discharging and the positive electrode whose resin binder addition amount is 64 wt%.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the invention will be described in detail with reference to the drawings.

  As shown in FIG. 10, the positive electrode for an aqueous lithium-air secondary battery of the embodiment has a water-repellent discharge reaction layer 16 and a hydrophilic charge reaction layer 12 laminated on a gas diffusion layer 11.

  As the gas diffusion layer 11, for example, a fluororesin (PTFE, FEP, PVF, etc.) filter, a hydrocarbon resin (PE, PP, etc.) filter, or the like can be used. Further, a metal mesh, a metal foam, or a carbon fiber filter that has been subjected to a water repellent treatment with a fluororesin can be used.

  The water repellent discharge reaction layer 16 includes an oxygen reduction catalyst 16b, a water repellent material 16a, and an anion conductive material 16c.

  Examples of the oxygen reduction catalyst 16b include noble metals such as platinum, ruthenium, iridium, vanadium, silver, alloys thereof, metal oxides, cyclic metal complexes, and the like. These may be used alone or in combination with carbon black (CB) or zeolite. It may be supported on a carrier such as.

  As the water repellent material 16a, water repellency such as polytetrafluoroethylene (PTFE), meltable fluororesin chlorotrifluoroethylene, trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether and ethylene, propylene, etc. Resin can be used.

  Water repellent resins include ethylene / tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene / hexafluoropropylene copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), and functional Those obtained by copolymerizing a small amount of a monomer having a group (hydroxyl group, carboxylic acid group, etc.) and those obtained by copolymerizing a small amount of a monomer having a cyclic structure as a comonomer are also included.

  An anion ionomer can be mentioned as the anion conductive material 16c. Resin having anion conductivity having counterion to ion bond, one obtained by amination of chloromethylated product of copolymer of aromatic polyethersulfone and aromatic polythioethersulfone, or block having chloromethylated styrene A polymer resin obtained by quaternizing a copolymer with an amine is called an anion ionomer. Aonomer is a general term for copolymers such as ethylene containing a group capable of dissociating as an ion such as a carboxyl group.

  As an anion ionomer, AS-4 of Tokuyama Corporation can be illustrated, for example.

  Next, an example of the manufacturing method of the water repellent discharge reaction layer 16 of this embodiment is demonstrated.

  First, an oxygen reduction catalyst, a water repellent material, an anion ionomer, a conductive aid, a pore former, pure water, and ethanol are mixed at a predetermined ratio to form a paste (hereinafter referred to as “discharge reaction paste”). .).

  Next, a predetermined amount of the [discharge reaction paste] is taken out with a pipette, and dropped and extended onto a hydrophilic holder to be uniformly impregnated.

  Next, the holding body uniformly impregnated with [discharge reaction paste] is dried at room temperature for a whole day and night. Then, the water repellent discharge reaction layer 16 of this embodiment is obtained.

  The water-repellent discharge reaction layer 16 manufactured by the above-described manufacturing method has a microstructure as shown enlarged in the circle of FIG. That is, the water repellent discharge reaction layer 16 is formed by binding the oxygen reduction catalyst 16b and the water repellent material 16a with the binder 16c which is an anion ionomer. By appropriately adding the anion ionomer 16c, the oxygen reduction catalyst 16b is partially covered with the anion ionomer 16c in the water-repellent discharge reaction layer 16.

There is an effective range for the amount of anion ionomer added to the [discharge reaction paste]. As shown in the enlarged view of FIG. 10, when the addition amount of the anion ionomer 16c is small, the surface of the oxygen reduction catalyst 16b cannot be completely covered with the anion ionomer 16c, and the anion ionomer 16c. There is a possibility that LiOH.H ₂ O as a discharge product may be deposited on the surface of the oxygen reduction catalyst 16b that is not coated with. Conversely, when the amount of the anion ionomer 16c added is large, the oxygen supply / discharge path in the water-repellent discharge reaction layer 16 may be lowered. Therefore, the amount of anion ionomer added to [discharge reaction paste] has an effective range. The optimum addition amount of the anion ionomer varies depending on the composition of the aqueous electrolyte, the operating temperature, the supply amount of air, the supply method, etc., and thus cannot be specified accurately, but the water repellent discharge reaction layer 16 Preferably it is 10-70 wt% with respect to a weight, More preferably, it is 25-50 wt%.

  Next, an example of the manufacturing method of the hydrophilic charge reaction layer 12 of this embodiment is demonstrated.

  First, an oxygen reduction catalyst, a conductive additive, a binder, pure water, and ethanol are mixed at a predetermined ratio to prepare a paste solution (hereinafter, a paste having this composition is referred to as “charging reaction paste”). To do.

  Next, a predetermined amount of the [charge reaction paste] is taken with a pipette, and dropped onto the water-repellent discharge reaction layer 16. Then, the oxygen generation catalyst layer 12a is formed by drying at room temperature all day and night.

  Next, a hydrophilic layer is formed on the oxygen generation catalyst layer 12a to prevent oxygen generated during charging from moving to the electrolyte side. A conductive assistant, a binder, pure water, and ethanol are mixed at a predetermined ratio to prepare a paste solution (hereinafter, a paste having this composition is referred to as a “hydrophilic layer paste”).

  Finally, a predetermined amount of this [hydrophilic layer paste] is taken with a pipette, dropped onto the oxygen generating catalyst layer 12a and dried at room temperature for a whole day and night to form the hydrophilic layer 12b.

  Next, a comparative evaluation test of charge / discharge characteristics of the positive electrode of the present invention (Examples 1 to 5) and the conventional positive electrode (Comparative Examples 1 to 3) will be described.

Example 1
The positive electrode of Example 1 specialized in the discharge reaction without the charge reaction layer was produced as follows.

  A hydrophilic carbon fiber paper (CP) (TGP H120; manufactured by Toray, 350 μm) was cut out to a size of 40 mm and used as a holder for impregnating and holding a paste for forming a water-repellent discharge reaction layer.

  In order to form a water-repellent discharge reaction layer, a paste was prepared according to the following procedure. Platinum-supported carbon as an oxygen reduction catalyst (TEC10EA50E; Tanaka Kikinzoku), PTFE dispersion (D-210C; manufactured by Daikin, 60 wt%) as a water repellent, vapor phase growth method as a conductive additive and pore former Carbon fiber (VGCF; Showa Denko), pure water, and ethanol are mixed in a ratio of 1.9: 1.3: 1.0: 13: 13, and uniformly dispersed with an ultrasonic homogenizer to form a paste (hereinafter referred to as a paste having this composition) Was called [Paste for Discharge Reaction of Example]. In this [paste for discharge reaction of Example], as a binder, an anion ionomer (AS-4; manufactured by Tokuyama Co., Ltd.) after drying contains 52 wt% with respect to the weight of the water-repellent discharge reaction layer. % [Discharge reaction paste of Example 1] was prepared.

  8 ml of [Paste for discharge reaction of Example 1] was taken with a micropipette and dropped onto the CP impregnation holding body, and uniformly impregnated and held. The film was dried at room temperature for a whole day and night to prepare a water-repellent discharge reaction layer containing 52 wt% of an anion ionomer component.

  CP (TGP-H-120; manufactured by Toray) treated with water repellency was used as the gas diffusion layer, and it was integrated with the above water-repellent discharge reaction layer by hot pressing (150 ° C x 8 MPa x 1 min). .

(Example 2)
The positive electrode of Example 2 having a charge reaction layer was produced as follows.

  A hydrophilic charge reaction layer was formed on the water-repellent discharge reaction layer of the positive electrode of Example 1 by the following procedure to obtain a (bifunctional) positive electrode of Example 2 as shown in FIG.

Iridium oxide catalyst (IrO _x · (H ₂ O) _y (SA = 50m ² / g); Tanaka Kikinzoku) and conductive additive to form oxygen generation catalyst layer of hydrophilic charge reaction layer Conductive carbon black (Ketjen Black, ECP600JD, manufactured by Lion) and ethylene-vinyl alcohol copolymer (Soarnol, D2908, manufactured by Nippon Synthetic Chemical) as a binder with a 10 wt% solution, pure water and ethanol The mixture was mixed at a ratio of 2: 2: 21: 12: 12, and uniformly dispersed with an ultrasonic homogenizer, to prepare a paste solution (hereinafter, the paste having this composition is referred to as “charging reaction paste”). 8 ml of this [paste for charging reaction] was taken with a micropipette, and dropped onto the water-repellent discharge reaction layer containing 52 wt% of the anion ionomer component. Thereafter, it was dried overnight at room temperature to form an oxygen generating catalyst layer.

  Next, a hydrophilic layer was formed on the oxygen generating catalyst layer in order to prevent oxygen generated during charging from moving to the electrolyte side. Conductive carbon black (Ketjen Black, ECP600JD, manufactured by Lion) and ethylene-vinyl alcohol copolymer (Soarnol, D2908, manufactured by Nippon Synthetic Chemical) as a binder at a 10 wt% solution, pure water and ethanol 1:10 The mixture was mixed at a ratio of 12:12 and dispersed uniformly with an ultrasonic homogenizer to prepare a paste solution (hereinafter, a paste having this composition is referred to as a “hydrophilic layer paste”). 2 ml of this [Hydrophilic layer paste] was taken with a micropipette, dropped onto the oxygen-generating catalyst layer, and dried at room temperature for a whole day and night to form a hydrophilic layer.

(Example 3)
In the positive electrode of this example, the amount of anion ionomer added was 52 wt% [the discharge reaction layer paste of Example 1] and the amount of anion ionomer added was 38 wt% [the discharge reaction layer of Example 3 The same as the positive electrode of Example 2 except that [Paste] was used.

Example 4
In the positive electrode of this example, the amount of anion ionomer added was 52 wt% [the discharge reaction layer paste of Example 1], and the amount of anion ionomer added was 59 wt% [the discharge reaction layer of Example 4 The same as the positive electrode of Example 2 except that [Paste] was used.

(Example 5)
In the positive electrode of this example, the amount of anion ionomer added was 52 wt% [the discharge reaction layer paste of Example 1], and the amount of anion ionomer added was 64 wt% [the discharge reaction layer of Example 5]. The same as the positive electrode of Example 2 except that [Paste] was used.

(Comparative Example 1)
As with the positive electrode of Example 1, hydrophilic carbon fiber paper (CP) (TGP H120; manufactured by Toray, 350 μm) is cut out to a size of 40 mm and impregnated with a paste for forming a water-repellent discharge reaction layer. Used as a holding body.

[Electric discharge reaction paste of Example] 10 wt% ethylene-vinyl alcohol copolymer (Soarnol, D2908, manufactured by Nippon Synthetic Chemical) having no OH ^- ion conductivity as a binder binder for water-repellent discharge A paste added so as to be 52 wt% based on the weight of the electrode layer was prepared, and a water repellent discharge reaction layer was prepared by impregnating and holding the CP holding body in the same manner as in Example 1.

  As in Example 1, a water-repellent CP (TGP-H-120; manufactured by Toray Industries, Inc.) was used as the gas diffusion layer, and the above water repellency was obtained by hot pressing (150 ° C. × 8 MPa × 1 min). Integrated with the discharge reaction layer.

(Comparative Example 2)
The positive electrode of Comparative Example 2 has the structure shown in FIG. That is, an anion exchange membrane (A201; manufactured by Tokuyama) was joined on the water-repellent discharge reaction layer of the positive electrode of Comparative Example 1 by hot pressing (140 ° C. × 8 MPa × 1 min). did.

(Comparative Example 3)
The positive electrode of Comparative Example 3 has the structure shown in FIG. That is, the positive electrode of Comparative Example 3 in which an anion exchange membrane was disposed on the hydrophilic charge reaction layer was produced by the following procedure.

First, in the same manner as in Comparative Example 1, the [Paste for discharge reaction of Example] was used as a binder with an ethylene-vinyl alcohol copolymer having no OH ^- ion conductivity (Soarnol, D2908, manufactured by Nippon Synthetic Chemical Co., Ltd.). ) A paste in which a 10 wt% solution was added to 52 wt% with respect to the weight of the water repellent discharge reaction layer was prepared and impregnated and held in a CP holding body to prepare a water repellent discharge reaction layer.

  Next, an oxygen generation catalyst layer was formed using [Charging reaction paste] of Example 2, and then a hydrophilic layer was formed thereon using [Hydrophilic layer paste] of Example 2.

  Finally, an anion exchange membrane (A201; manufactured by Tokuyama) was joined by hot pressing (140 ° C. × 8 MPa × 1 min) to obtain a positive electrode of Comparative Example 3.

<Evaluation test>
The positive electrode of Examples 1-5 and the positive electrode of Comparative Examples 1-3 were installed in the battery reaction evaluation apparatus as shown in FIG. 11, and the charge / discharge characteristics were examined.

In the battery reaction evaluation apparatus, both ends of a cylindrical acrylic container are closed with a PTFE sheet and PTFE filter paper, and an electrolytic solution is placed therein, and humidified air is supplied from the PTFE filter paper side.
10mol / L LiCl + 4.3mol / L LiOH is used as the electrolyte, 200cc / min. Of atmospheric air is passed through 1M LiOH as the supply gas, and de-CO ₂ treated air flows to the back of the positive electrode The charge / discharge characteristics were evaluated in a constant temperature bath at 28 ° C.

  The counter electrode (CE), the positive electrode (WE), and the reference electrode (RE) were connected to an electrochemical evaluation apparatus (potentio galvanostat: SI-1260 / 1287, manufactured by Solartron), and the following characteristics were measured.

(A) Discharge characteristics (change in discharge voltage over time)
(B) Charging / discharging cycle characteristics (change in charging voltage and discharging voltage over time)
First, (A) discharge characteristics (change in discharge voltage over time) were measured as follows. The discharge current density was kept constant at 1 mA / cm ² and the change with time of the discharge voltage of the positive electrode (WE) with respect to the reference electrode (RE) was recorded. The result is shown in FIG.

Next, (B) charge / discharge cycle characteristics (change in charging voltage and discharge voltage over time) were measured as follows. Discharge current density 1 mA / cm ^2, and the charge current density -1 mA / cm ² constant was measured changes with time of the discharge voltage and the charging voltage of the positive electrode (WE) relative to the reference electrode (RE). The results are shown in FIGS.

<Evaluation test results>
FIG. 12 shows side results when the positive electrode is the positive electrode of Example 1 and the positive electrodes of Comparative Examples 1 and 2. In the figure, curve i (◯) is the positive electrode of Example 1, curve B (Δ) is the positive electrode of Comparative Example 1, and curve C (▽) is the positive electrode of Comparative Example 2.

From FIG. 12, it can be seen that the positive electrode of Comparative Example 1 has a high initial discharge voltage, but the discharge potential suddenly decreases around 10 hours (exceeding the discharge capacity of 10 mAh / cm ² ). The surface of the positive electrode of Comparative Example 1 after the evaluation test was covered with white crystals, and it is considered that the discharge reaction was inhibited by the deposition of LiOH.H ₂ O as a discharge living organism.

On the other hand, the positive electrode of Comparative Example 2 and the positive electrode of Example 1 showed almost the same performance and behavior, and even when the discharge operation was performed at least at a capacity of 20 mAh / cm ² (even if the discharge was continued for at least 20 hours). It can be seen that the discharge potential does not change and stable discharge can be performed. For this reason, it is not always necessary to provide an anion exchange membrane on the entire surface of the positive electrode as in the prior art as a discharge electrode, and as in the positive electrode of Example 1, only OH ^- ions are present on the surface of the oxygen reduction catalyst. It can be seen that if a conductive binder film is formed, the LiOH · H ₂ O deposition prevention function works.

  FIG. 13 is a graph showing charge / discharge cycle characteristics when the positive electrode is the positive electrode of Comparative Example 3. In the figure, curve 1d shows the change over time of the discharge voltage at the first cycle, curve 2d shows the change over time of the discharge voltage at the second cycle, and curve 3d shows the change over time of the discharge voltage at the third cycle. In the figure, curve 1c shows the change with time of the charge voltage in the first cycle, curve 2c shows the change with time of the charge voltage in the second cycle, and curve 3c shows the change with time of the charge voltage in the third cycle.

From FIG. 13, the first discharge and charge and the second discharge were performed without any problem, but at the time of the ^second charge, in the vicinity where the charge capacity of 8 mAh / cm ² was exceeded (around 8 hours passed). It can be seen that the charging voltage rises above 0.9V vs. Hg / HgO. At a potential of 0.9 V vs. Hg / Hg or higher, generation of chlorine gas was confirmed as a charge product, and a part of the oxidation reaction of Cl ^- ion, which is the charge side reaction shown in the formula (4), has started. Shows things. Therefore, when an anion exchange membrane is installed in a hydrophilic charge reaction layer like the positive electrode of Comparative Example 3, the OH ^- ion transfer rate is lower than that without an exchange membrane, which is necessary for the charge reaction. such OH ^- supply of ions was considered a problem occurs to the shortage.

  14 to 17 are graphs showing charge / discharge cycle characteristics when the positive electrode is the positive electrode of Examples 2 to 5. 14 shows the case of the positive electrode of Example 3 when the addition amount of anion ionomer is 38 wt%, FIG. 15 shows the case of the positive electrode of Example 2 when the addition amount of anion ionomer is 52 wt%, and FIG. 16 shows the case of the anion ionomer. In the case of the positive electrode of Example 4 when the addition amount is 59 wt%, FIG. 17 shows the charge / discharge cycle characteristics in the case of the positive electrode of Example 5 when the addition amount of the anion ionomer is 62 wt%.

In this cycle characterization, discharging is continued for 5 hours at a constant current density of 1 mA / cm ² , then the direction of the current is reversed, and charging is continued for 5 hours at a constant current density of 1 mA / cm ^2. Was repeated.

  14-17, compared with the positive electrode of Comparative Example 3 (see FIG. 13), the positive electrodes of Examples 2-5 can perform charge / discharge operations with a larger number of charge / discharge cycles and function as bifunctional electrodes. You can see that

Moreover, it turns out that the cycle characteristics at the time of a charge reaction and a discharge reaction differ by the difference in the addition amount of the anion ionomer in the positive electrode of the Example which concerns on this invention. In the charging reaction, the amount of anion ionomer added is relatively small. In the case of FIG. 14 (38 wt%), FIG. 15 (52 wt%), and FIG. 16 (59 wt%), the charging potential during the charging operation is low. It can be seen that only the oxidation reaction of OH ^- ions occurs, and the change in the charging potential is small and stable as the number of cycles increases.

On the other hand, in the case of FIG. 17 (64 wt%) where the addition amount of the anion ionomer is the largest, it can be seen that the charging potential tends to increase within one charging operation time. This is presumably because the addition of a large amount of anion ionomer blocked the pores of the discharge reaction layer and inhibited the discharge of oxygen generated during discharge. In the discharge reaction, the amount of anion ionomer added was 38 wt%. In the case of FIG. 14, although the initial cycle stage showed a stable discharge potential, the discharge potential dropped rapidly during the ninth cycle. . This is presumably because LiOH.H ₂ O was precipitated on the surface of the oxygen reduction catalyst that was not coated with the anion ionomer. In the case of FIG. 16 where the addition amount of the anion ionomer was 59 wt%, a rapid decrease in the discharge potential was not observed, but the discharge potential tended to gradually decrease as the number of cycles increased. In the case of FIG. 15 where the addition amount of the anion ionomer is 52 wt%, the discharge potential decreased in the 17th and subsequent cycles, but more cycles than in the case of FIG. 14 of 38 wt%. It was found that charging / discharging was possible.

11... Gas diffusion layer 12... Hydrophilic charge reaction layer 12 a... Oxygen reduction catalyst layer 12 b... Hydrophilic layer 16. 16a ... Water repellent material 16b ... Oxygen reduction catalyst 16c ... Anion conductive material

Claims (3)

A gas diffusion layer;
A water-repellent discharge reaction layer laminated on the gas diffusion layer;
A hydrophilic charge reaction layer laminated on the water repellent discharge reaction layer,
The water repellent discharge reaction layer is porous, and is formed by binding an oxygen reduction catalyst and a water repellent material with a resin binder having anion conductivity, and reduces oxygen in the air during a discharge reaction and the charge reaction during the charge reaction. A positive electrode for an aqueous lithium-air secondary battery, wherein oxygen generated in a hydrophilic charge reaction layer is moved in the direction of the gas diffusion layer. The additive amount of the resin binder having anionic conductivity, the water-repellent 38 wt% greater than the weight of the discharge reaction layer, aqueous lithium according to claim 1 is less than 64 wt% - positive air secondary battery. 2. The positive electrode for an aqueous lithium-air secondary battery according to claim 1, wherein the amount of the resin binder having anion conductivity is 52 to 59 wt% with respect to the weight of the water-repellent discharge reaction layer.

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