JP2015162349A - Li-AIR SECONDARY BATTERY AND METHOD FOR RESTORING DISCHARGE PERFORMANCE OF THE BATTERY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Li-air secondary battery capable of restoring deterioration in discharge performance due to deposition of LiOH HO and LiCOwithout additionally using a member, such as an anion-exchange membrane, inhibiting a battery reaction, and a method for restoring discharge performance.SOLUTION: A Li-air secondary battery includes oxygen supply restricting means 2 for restricting the supply of oxygen to a positive electrode and/or discharge current increasing means 2A for increasing a discharge current.

Description

  The present invention relates to a Li-air secondary battery and a method for recovering its discharge performance.

  Among metal-air batteries, a Li-air secondary battery using metallic lithium as a negative electrode has a very high theoretical energy density, and has attracted much attention in recent years. In the case of the water-based Li-air secondary battery in the Li-air secondary battery, the Li metal used as the negative electrode has a problem of reacting when it comes into contact with the aqueous electrolyte, but as shown in FIG. By using a conductive lithium ion conductive glass ceramic (hereinafter abbreviated as LATP) as a protective layer, an aqueous electrolyte can be used (see Patent Document 1).

  On the other hand, a technology has been developed in which LiCl, which is more easily ionized and dissolved than LiOH, is allowed to coexist in the electrolyte solution, thereby preventing ionization of LiOH as a discharge product and corrosion of LATP (see Non-Patent Document 1). ).

  In addition, a battery having an air electrode in which a porous carbon dioxide gas removing agent in which an alkali metal hydroxide is adhered to calcium hydroxide is provided in an air suction path connected to the air electrode is disclosed (see Patent Document 2). ).

  Moreover, in an air zinc battery comprising a zinc negative electrode and an air electrode filled with a metal mesh with a positive electrode material mainly composed of metal oxide, graphite, activated carbon and a fluorine-based binder, a carbon dioxide absorbent is provided in the air electrode. Has been disclosed (see Patent Document 3).

In addition, as shown in Fig. 2, an anion exchange membrane is placed between the air electrode (positive electrode) and the alkaline electrolyte, so that the intrusion of CO ₂ from the atmosphere and the discharge product LiOH · H ₂ O A technique for preventing precipitation on the air electrode surface is known (see Patent Document 4).

JP-T-2007-513464 JP 49-49128 A Japanese Patent No. 4277327 Republished WO2009 / 104570

Edited by Tatsumi Ishihara, “Development and Latest Technologies of Metal / Air Secondary Batteries”, Technology Education Publishers, April 2011, p.91-104

For the conventional aqueous Li- air secondary battery, the moving speed of the Li ⁺ generated in the anode reaction, OH generated in the positive electrode reaction ^- is higher as compared to the ion movement speed, Li ⁺ ions in the cell It moves in the direction of the positive electrode and generates LiOH.H ₂ O as a discharge product on the surface of the positive electrode. Therefore, there is a problem that the generated LiOH · H ₂ O covers the surface of the air electrode catalyst and the pores of the gas diffusion layer are blocked, resulting in a decrease in discharge performance as the discharge time increases.

In addition, it is possible to use various acidic, neutral, and alkaline electrolytes as the electrolyte of the water-based Li-air secondary battery, but with any electrolyte, the discharge reaction proceeds. LiOH · H ₂ O is generated, and the electrolyte changes to strong alkalinity. Therefore, like other air batteries, the electrolytic solution reacts with CO ₂ in the atmosphere, and a carbonate that is poorly soluble in water and electrochemically inert is generated in the vicinity of the air electrode. It is also a technical problem to coat the film.

On the other hand, in the case of an aqueous Li-air secondary battery using LATP as a protective layer for a lithium metal negative electrode, there is a problem that LATP is strongly alkaline and corrodes (Satoshi Hasegawa et al., “Study on lithium / air secondary batteries Stability of NASICON-type lithium ion conducting glass ceramics with water ", Journal of Power Sources, vol. 189 (1), p371-377 (2009)), LiCl, which is more easily ionized and dissolved than LiOH, coexists in the electrolyte. A technique for preventing ionization of LiOH.H ₂ O as a discharge product and preventing the electrolyte from becoming strongly alkaline (see Non-Patent Document 1) is effective. However, in this case, LiOH · H ₂ O deposited on the positive electrode surface by the discharge reaction cannot be re-dissolved in the electrolyte solution, and the coating of the air electrode catalyst surface with the above LiOH · H ₂ O or gas diffusion layer It is in a situation where the pore blockage is likely to occur.

To solve the problem of electrochemically inactive carbonate formation, a material that reacts with CO ₂ such as calcium hydroxide (CO ₂ absorbing material) is placed outside the battery or inside the positive electrode of the battery. There is a technology that prevents CO _{2 from} being mixed into the electrolyte (see Patent Documents 2 and 3), but in these cases, the CO ₂ absorbing material reacts and is consumed, so periodic replacement and regeneration are possible. There is a problem that it is necessary.

In the case of the technique of Patent Document 4, OH ⁻ ions generated on the air electrode surface during discharge pass through the anion exchange membrane, and LiOH · H ₂ O is generated at the anion exchange membrane and the electrolyte solution interface. The conductivity of OH ^- ions in the anion exchange membrane is smaller than that of the liquid electrolyte, and there is a problem that the battery performance is lowered. Furthermore, the anion exchange membrane has a problem that the electrical conductivity of OH 2 ⁻ ions is increasingly lowered by absorbing and reacting with CO ₂ in the atmosphere.

The present invention was made in view of the above problems, and without using a member that hinders the battery reaction, such as an anion exchange membrane, discharge performance by deposition of LiOH.H ₂ O or Li ₂ CO ₃ It is an object of the present invention to provide a Li-air secondary battery capable of recovering the deterioration of the battery and a method of recovering the discharge performance.

  The present invention has been created by examining the discharge reaction of a Li-air secondary battery in detail as follows.

In the case of water-based Li-air secondary batteries, Li ⁺ + 1/2 O ₂ + 3/2 H ₂ O + e- ⇒ LiOH • H ₂ O (E ⁰ = 0.401V vs. SHE)
... (1)
In the negative electrode, Li ⇒ Li + + e- (E ⁰ = -3.040V vs. SHE) (2)
For the whole battery, Li + 1/2 O ₂ + 3/2 H ₂ O => LiOH · H ₂ O (electromotive force: 3.405V) (3)
Accordingly, a discharge reaction occurs, and LiOH.H ₂ O as a discharge product is generated on the surface of the air electrode. Here, E ⁰ is a potential, and SHE is a hydrogen standard potential.

On the other hand, if the oxygen supply of the air electrode is not sufficient for the discharge current,
At the air electrode, Li ⁺ + H ₂ O + e- ⇒ LiOH ・ H ₂ O + H ₂ ↑ (E ⁰ = -0.828V vs. SHE)
···(Four)
In the whole battery, Li + 1/2 O ₂ + 3/2 H ₂ O ⇒ LiOH · H ₂ O (electromotive force: 2.212V) (5)
Reaction occurs.

When the supply of oxygen at the air electrode is not sufficient for the discharge current, gaseous H ₂ is generated on the air electrode surface as a discharge product in addition to the generation of LiOH · H ₂ O. It is considered that deposits on the air electrode surface can be physically removed by this H ₂ . In other words, the LiOH · H ₂ O and Li ₂ CO ₃ deposited during a long discharge operation cause a reaction of the formula (4) for a short time by external operation to cause the deposited LiOH · H 2 _It is considered that the discharge performance can be recovered by removing ₂ O and Li ₂ CO ₃ . The condition for the reaction of the formula (4) (when the oxygen supply to the air electrode is not sufficient for the discharge current) is to suppress the oxygen supply to the air electrode or to reduce the oxygen supply to the air electrode. It is also satisfied by increasing the discharge current without suppressing the supply.

  The Li-air secondary battery of the present invention made to solve the problem includes a negative electrode using Li as a negative electrode active material, a positive electrode using oxygen as a positive electrode active material, and an electrolysis interposed between the negative electrode and the positive electrode. A lithium-air secondary battery comprising: a liquid; and an oxygen supply suppressing means for suppressing supply of oxygen to the positive electrode and / or a discharge current increasing means for increasing a discharge current.

By reducing the supply of oxygen to the positive electrode with the oxygen supply suppressing means and / or increasing the discharge current with the discharge current increasing means, the reaction of the formula (4) is caused to generate gaseous H ₂ at the positive electrode. Can do. As a result, LiOH.H ₂ O and Li ₂ CO ₃ deposited during the discharge operation are removed, and the discharge performance is recovered.

In the above Li-air secondary battery, the discharge current increasing means may increase the positive electrode potential to a current that reduces the potential to a hydrogen generation potential or lower. As a result, a large amount of hydrogen can be generated, LiOH.H ₂ O and Li ₂ CO ₃ deposited during the discharge operation are surely removed, and the discharge performance is restored.

  Further, the discharge current raising means may be configured to raise the positive electrode potential to a current that lowers the hydrogen generation potential or less and then make the discharge current zero. As a result, the generated hydrogen can be removed with air, and a mixed potential with the generated hydrogen can be avoided.

  The method of recovering the discharge performance of the Li-air secondary battery of the present invention made to solve the problems includes a negative electrode using Li as a negative electrode active material, a positive electrode using oxygen as a positive electrode active material, the negative electrode and the negative electrode A method for recovering the discharge performance of a Li-air secondary battery having an electrolyte solution interposed between positive electrodes, the oxygen supply suppressing step for suppressing supply of oxygen to the positive electrode and / or increasing the discharge current A discharge current increasing step is provided.

  In the above-described method for recovering the discharge performance of the Li-air secondary battery, the discharge current increasing step may be performed by increasing the positive electrode potential to a current that decreases the hydrogen generation potential or less.

  In the discharge current increasing step, the positive electrode potential may be increased to a current that reduces the potential to a hydrogen generation potential or less, and then the discharge current may be made zero.

By reducing the supply of oxygen to the positive electrode with the oxygen supply control means and / or increasing the discharge current with the discharge current raising means, the reaction of the formula (4) is caused to generate gaseous H ₂ at the positive electrode. Can do. As a result, LiOH.H ₂ O and Li ₂ CO ₃ deposited during the discharge operation are removed, and the discharge performance is recovered.

It is sectional drawing which concerns on a prior art and shows the concept of a water-system Li-air secondary battery typically. It is sectional drawing which concerns on a prior art, and shows the concept of the air secondary battery using an anion exchange membrane typically. It is a schematic block diagram of the Li-air secondary battery which concerns on Embodiment 1 of this invention. It is a schematic block diagram of the Li-air secondary battery which concerns on Embodiment 2 of this invention. It is the schematic of the cell for battery reaction measurement. It is a graph which shows the time change of the discharge current which evaluates the discharge performance of the Li-air secondary battery which concerns on this invention. It is a graph which shows the change over time of the electrode potential at the time of discharge. It is an external appearance photograph of the air electrode after discharge performance evaluation.

(Embodiment 1)
As shown in FIG. 3, the Li-air secondary battery of the present embodiment has an oxygen supply suppression that suppresses the supply of oxygen to the Li-air secondary battery main body 1 and the positive electrode of the Li-air secondary battery main body 1. Means 2 are provided. 3 is a battery load, and 4 is a switch for connecting the battery load 3 to the Li-air secondary battery body 1.

  The oxygen supply suppressing means 2 includes an air cylinder 21 and an inert gas cylinder 22. 21a is an air control valve and 22a is an inert gas control valve.

  Next, the operation | movement which recovers the discharge performance of the Li-air secondary battery of this embodiment from the state which closed valve | bulb 21a, 22a is demonstrated.

First, the valve 21a is opened to supply sufficient air to the positive electrode of the Li-air secondary battery body 1. Next, the switch 4 is turned on to supply current to the battery load 3 to discharge the Li-air secondary battery main body 1 to cause the reaction of the formula (1). Next, when the positive electrode potential drops to a predetermined value due to discharge, the valve 21a is throttled to reduce the supply amount of air, thereby causing the reaction of the formula (4) to generate gaseous H ₂ at the positive electrode. To restore the discharge performance.

  In the above operation, the discharge performance is recovered by narrowing the bulb 21a. However, the inert gas control bulb 22a may be opened while the bulb 21a remains open.

(Embodiment 2)
As shown in FIG. 4, the Li-air secondary battery of the present embodiment is greatly different in that it includes a discharge current increasing means 2 </ b> A instead of the oxygen supply suppressing means 2 of the first embodiment. The same components as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

Discharge current rise means 2A includes a switch 2A ₂ for connecting the control load 2A ₁ to Li- air secondary cell body 1 and the control load 2A ₁ low impedance for increasing the discharge current during discharge at discharge ing.

Control load 2A ₁ has an impedance that increases until the current that reduces the potential of the positive electrode to below the hydrogen evolution potential ^{(E 0 = -0.828V vs. SHE)} .

  Next, an operation for recovering the discharge performance of the Li-air secondary battery of this embodiment from the state in which the switch 4 is off will be described.

First, in a state where air is supplied to the positive electrode of the Li-air secondary battery body 1, the switch 4 is turned on to supply current to the battery load 3 to discharge the Li-air secondary battery body 1 (1 ) The reaction of the formula is caused. Next, when the positive electrode potential decreases to a predetermined value due to discharge, the switch 2A ₂ is turned on to supply current to the control load 2A ₁ to raise the discharge current and cause the reaction of the formula (4). The discharge performance is recovered by generating gaseous H ₂ at the positive electrode. Next, the switch 4 may be turned off. H ₂ near the positive electrode can be removed with air.

<Change in discharge performance over time in an example in which the method for recovering the discharge performance of the Li-air secondary battery of the present invention is applied and in a comparative example in which the method is not applied>
Next, the results of comparative evaluation of changes over time in the discharge performance of the example in which the method for recovering the discharge performance of the Li-air secondary battery of the present invention and the comparative example in which the method is not applied will be described.

1) Preparation of air electrode A catalyst paste is prepared using platinum-supported carbon PtC (Tanaka Kikinzoku) as the air electrode catalyst and fluororesin PTFE (Daikin Kogyo) as the binder resin (PtC: PTFE = 1.0: 1.0 wt%) and carbon paper (TGP-120 manufactured by Toray Industries, Inc.) were applied as a catalyst layer holder, and an appropriate amount was applied and dried overnight at room temperature to prepare an air electrode catalyst layer. The prepared catalyst layer was bonded to a carbon paper (gas diffusion layer) subjected to water repellent treatment by heating and pressing to form an air electrode.

2) Evaluation of the air electrode A potentio galvanostat was connected to a battery reaction measurement cell as shown in FIG.

The battery reaction measurement cell is a cylindrical acrylic container with both ends sealed with a PTFE sheet and an air electrode, and an electrolyte is placed inside. The humidified air is supplied from the gas diffusion layer side of the air electrode. It has become. As the electrolytic solution, 5.3 mol / L-LiOH + 10 mol / L-LiCl was used. A Pt mesh was used as the counter electrode, and an Hg / HgO electrode was used as the reference electrode. Evaluation was performed while air in the atmosphere of 200 cc / min. Was passed through 1 mol / L-LiOH as the supply gas, and de-CO ₂ air was allowed to flow to the back of the positive electrode. The battery reaction measurement cell was placed in a thermostatic chamber maintained at 28 ° C., and discharge evaluation was performed using a potentio galvanostat.

(Example)
As shown in the lower part of FIG. 6, after the start of the discharge reaction of a constant current condition of -1 mA / cm ^2, at the time of 80min course, discharged at a current of -10 mA / cm ² positive electrode potential of hydrogen generation by raising up The potential (-0.921 V vs. Hg / HgO) was lowered to below and held for 30 seconds, and the product (LiOH.H ₂ O) deposited on the air electrode surface was removed. If the hydrogen gas generated during the treatment remains in the vicinity of the electrode, the reduction of the hydrogen gas and the reduction of oxygen in the air occur simultaneously, indicating a hybrid electrode potential and the degree of recovery of the air electrode is difficult to evaluate. Therefore, after leaving for 30 seconds at an open circuit voltage, the generated hydrogen gas was sufficiently removed with air, and then the discharge reaction was continued again at -1 mA / cm ² for 70 minutes.

(Comparative example)
As shown in the upper part of FIG. 6, the discharge reaction under a constant current condition of -1 mA / cm ² was continued for 150 minutes.

3) Evaluation results FIG. 7 shows the change over time in the discharge performance, and FIG. 8 shows the appearance of the air electrode after the discharge is continued for 80 minutes. As can be seen from FIG. 7, the discharge potential decreases with the lapse of the discharge time after the start of the discharge reaction under the constant current condition, and in the case of the comparative example, the discharge potential decreases even after exceeding 80 minutes after the start of the discharge. doing. This is considered to be because LiOH.H ₂ O was deposited on the air electrode surface according to the formula (1), and the effective surface area of the air electrode was reduced. On the other hand, in the case of the example, it can be seen that at 80 min after the start of discharge, the lowered discharge potential rises again and the electrode performance is recovered.

From FIG. 8, in the air electrode of the comparative example, white LiOH.H ₂ O deposits can be confirmed on the electrode surface, whereas in the air electrode of the example, almost no deposits can be confirmed. Then, it can be seen that the deposits of LiOH and H ₂ O are peeled off and removed by the hydrogen gas generated on the air electrode surface.

1 .... Li-air secondary battery body 2 .... Oxygen supply suppression means 2A ... Discharge current increasing means

Claims (6)

A Li-air secondary battery having a negative electrode using Li as a negative electrode active material, a positive electrode using oxygen as a positive electrode active material, and an electrolyte solution interposed between the negative electrode and the positive electrode,
An Li-air secondary battery comprising oxygen supply suppression means for suppressing supply of oxygen to the positive electrode and / or discharge current increase means for increasing discharge current.   The Li-air secondary battery according to claim 1, wherein the discharge current increasing means increases the positive electrode potential to a current that decreases the potential to a hydrogen generation potential or less.   3. The Li-air secondary battery according to claim 2, wherein the discharge current raising means raises the positive electrode potential to a current that lowers the hydrogen generation potential to be equal to or lower than a hydrogen generation potential, and then sets the discharge current to zero. A method for recovering the discharge performance of a Li-air secondary battery comprising: a negative electrode using Li as a negative electrode active material; a positive electrode using oxygen as a positive electrode active material; and an electrolyte solution interposed between the negative electrode and the positive electrode. There,
A method for recovering the discharge performance of a Li-air secondary battery, comprising an oxygen supply suppression step for suppressing supply of oxygen to the positive electrode and / or a discharge current increase step for increasing the discharge current.   5. The method for recovering the discharge performance of the Li-air secondary battery according to claim 4, wherein in the discharge current increasing step, the positive electrode potential is increased to a current that decreases the hydrogen generation potential or less.   The method for recovering the discharge performance of the Li-air secondary battery according to claim 5, wherein in the discharge current increasing step, the positive electrode potential is increased to a current that decreases to a hydrogen generation potential or less and then the discharge current is made zero.