JP6831517B2 - How to make electrodes - Google Patents

Description

The present invention relates to electrodes for electrochemical cells such as batteries that use solid electrolytes.

Batteries are becoming more and more important as power sources for mobile devices, automobiles, aircraft, etc., and for storing electric power generated by renewable energy.
In recent years, research on solid electrolytes has been promoted for various applications such as as an electrolyte for an all-solid-state battery having excellent safety, and as an electrolyte layer provided on a separator or a positive electrode side or a negative electrode side in a battery using a liquid electrolyte. ing. In addition, metal-based and alloy-based electrodes typified by Li are expected because they can obtain a larger capacity than conventional electrodes, and development is being promoted as a promising electrode even when a solid electrolyte is used. There is.
When using a metal or alloy-based electrode in a battery using a solid electrolyte, it is important to construct a solid electrolyte / electrode interface. As a conventional technique for constructing such an interface, methods such as crimping, vapor deposition, deposition, heat fusion, and interface modification are used (crimping and vapor deposition: Non-Patent Document 1, heat fusion and interface modification: non-patent). Document 2).

Electrochem. Commun., 22, 177 (2012) Nature Mater. (2016) doi: 10.1038 / nmat4821 Phys.Chme.Chem.Phys., 14, 10008 (2012) J. Mater. Sci., 48, 5846 (2013)

When the electrode is crimped to the molded solid electrolyte, there is a risk that the solid electrolyte will crack. Further, it is difficult to uniformly apply pressure to the adhesive surface, and it is difficult to prepare a homogeneous interface. In addition, this method cannot be used for materials with poor adhesiveness. On the other hand, the thin-film deposition and deposition methods are suitable for producing an interface with high adhesion, but they require vacuum conditions, and the deposition rate is slow and the area that can be deposited is limited, so that time and cost are required. It takes. In addition, these methods are often used in the interface modification method, which also increases the number of work processes. In heat fusion, it is difficult to create an interface with high adhesion because some materials are difficult to fuse (Non-Patent Documents 2 to 4).
An object of the present invention is to provide a method for bringing an electrode into close contact with a solid electrolyte by a simple operation without having the drawbacks of these conventional methods.

As a result of intensive studies for the above purpose, the present inventors have conducted a metal or alloy having high adhesion on the solid electrolyte by simultaneously irradiating ultrasonic waves when the electrode material is thermally fused onto the solid electrolyte. It was found that a system electrode can be produced, the internal resistance of the electrode-solid electrolyte interface is small, and good electrochemical characteristics can be obtained in the electrochemical cell constructed by using the electrode-solid electrolyte structure. It was.

Specifically, the present inventors use Li _{1 + x} Al _y Ge _2-y (PO ₄ ) ₃ (hereinafter referred to as LAGP) and Al doped-Li ₇ La ₃ Zr ₂ O ₁₂ as solid electrolytes. The Li metal is thermally melted using an ultrasonic soldering iron (Fig. 1) using (hereinafter referred to as LLZ), and the Li metal is fused onto the solid electrolyte while applying ultrasonic waves. By forming the electrodes, a uniform Li / solid electrolyte interface with high adhesion is formed (Fig. 3), and as a result, the Li / solid electrolyte is compared with the usual heat fusion in which the Li metal is simply heated and melted. The resistance at the interface can be significantly reduced (Figs. 4 and 5), and the symmetrical cells Li / LLZ / Li produced in this way stably electrochemically dissolve and precipitate Li for a long period of time. It was found that it is possible to do (Fig. 6).
The present invention has been made based on these findings by the present inventors.

That is, the present application provides the following inventions.
<1> A method for producing an electrode / solid electrolyte structure by heat-sealing an electrode material onto a solid electrolyte and forming a metal or alloy-based electrode on the solid electrolyte. The electrode material is placed on the solid electrolyte. A method characterized in that ultrasonic waves are applied when heat-sealing to a metal.
<2> A method for producing an electrode / solid electrolyte structure by the method of <1>, and combining the obtained electrode / solid electrolyte structure with other components to produce an electrochemical cell.
<3> The method according to <2>, wherein the electrochemical cell is a battery.
<4> The method according to <2>, wherein the electrochemical cell is an all-solid-state battery.
<5> The method according to <2>, wherein the electrochemical cell is a metal-air battery.

In the method of applying ultrasonic waves at the time of heat fusion (hereinafter referred to as ultrasonic-assisted heat fusion method) adopted in the present invention, an electrode / solid electrolyte interface having high adhesion can be obtained in a very short time of several seconds. It is possible to form.
Further, according to the present invention, an electrode / solid electrolyte interface having high adhesion is formed, and the resistance of the interface is reduced, so that the resistance of the entire electrochemical cell constructed by using the interface can be reduced. Therefore, it is possible to improve the rate characteristics.
Further, the electrode / solid electrolyte interface formed by the present invention is electrochemically stable, and by using this, a battery having stable charge / discharge characteristics for a long period of time can be obtained.

A photograph of an ultrasonic soldering iron used in the present invention. Photographs of Li metal fused onto LLZ pellets by (a) heat fusion method using a hot plate and (b) ultrasonic assisted heat fusion method using an ultrasonic soldering iron. A scanning electron microscope with a fracture surface of Li / LLZ fused by (a) a heat fusion method using a hot plate and (b) an ultrasonically assisted heat fusion method using an ultrasonic soldering iron. SEM) Photo. (A) Au / LAGP / Au, (b) Li / LAGP / Au fused by a heat fusion method using a hot plate, and (c) ultrasonically assisted heat fusion using an ultrasonic soldering iron. Impedance plot of Li / LAGP / Au fused by the method. The value at the right end of the arc means the total resistance. Impedance plot of Li / LLZ / Li fused by (a) heat fusion method using a hot plate and (b) ultrasonic assisted heat fusion method using an ultrasonic soldering iron. The results of constant current polarization measurement of Li / LLZ / Li fused by (a) heat fusion method using a hot plate and (b) ultrasonic assisted heat fusion method using an ultrasonic soldering iron. The figure which shows. A current of 0.1 mA / cm ² is flowing in the reverse direction every 30 minutes. Optical photograph of Li metal bonded to a slide glass at each temperature and each ultrasonic output by an ultrasonic-assisted heat fusion method (ultrasonic frequency is fixed at 60 kHz). Optical photograph of Li metal bonded on a slide glass under the condition of ultrasonic output of 240 ° C and 5W.

Examples of the solid electrolyte used in the present invention include a lithium-containing phosphoric acid compound having a NASICON-type structure such as the above-mentioned Li _{1 + x} Al _y Ge _2-y (PO ₄ ) ₃ (LAGP), and the above-mentioned Li. ₇ La ₃ Zr ₂ O ₁₂ (LLZ) and other compounds with a garnet-type structure, LIPON (LiPO _4-x N _x ) in which lithium phosphate is doped with nitrogen and similar compounds, Li _3x La _{2 / 3-x} TiO compounds having a perovskite structure, such as _3, Li ₄ SiO ₄ such as an oxide-based solid electrolyte, and a lithium ion conductor, such as sulfide-based solid electrolyte such as phosphoric lithium sulfide, sodium-containing phosphate having a NASICON-type structure Examples include compounds, β-alumina such as Na ₂ O-11 Al ₂ O ₃ , and sodium ion conductors such as sulfide-based solid electrolytes such as sodium phosphorus sulfide. Further, not only lithium ion and sodium ion, but also Mg. The present invention is also applicable to other anionic conductors such as cations and oxide ions.

Examples of the metal or alloy-based electrode used in the present invention include electrodes made of Li, Na, Al, K, Sn, Pb, In, Li-In alloy, Li-Sn alloy, Li-Al alloy and the like.

In the ultrasonic-assisted heat fusion method used in the present invention, it is preferable to apply ultrasonic waves having a frequency of about 10 to 60 kHz at an output of about 2 to 10 W for about 1 to 3 seconds, and heat fusion. The landing temperature is preferably about 60 to 750 ° C. depending on the melting point of each metal or alloy. In the embodiment of the present invention, an ultrasonic soldering iron is used as a means for realizing such ultrasonic-assisted heat fusion, but the present invention is not limited to this, and the above-mentioned ultrasonic waves can be applied. Any means can be used as long as it can achieve the above heat fusion.

The electrode / solid electrolyte structure produced by the present invention can be used in various electrochemical cells such as primary batteries, secondary batteries, capacitors, and electrochemical measurement cells by appropriately combining with other components. it can.
The battery produced by the present invention includes, for example, an all-solid battery such as a lithium ion battery composed of a Li negative electrode / lithium ion conductive solid electrolyte structure and a positive electrode such as Li _1-x FePO _4, or in the air. Examples thereof include a metal-air battery such as a Li-air battery, which uses the oxygen of the above as a positive electrode active material and is composed of a Li negative electrode / lithium ion conductive solid electrolyte structure and an air electrode such as porous carbon.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Example 1. Adhesion of Li Metal to LAGP Using Li _{1 + x} Al _y Ge _2-y (PO ₄ ) ₃ (LAGP) as a solid electrolyte, a conventional heat fusion method and ultrasonically assisted heat fusion according to the present invention. By the method, a Li metal electrode was formed on the solid electrolyte. Each fusion was performed in a glove box filled with Ar gas by the following procedure.
(1) Normal heat fusion method (fusing with a hot plate)
A Cu mesh with a Φ10 mm Li metal attached was placed on a hot plate heated to 230 ° C., and LAGP pellets sputtered with an Au electrode on one side were placed on the Cu mesh with the Au electrode facing up. The LAGP pellets were pressed with tweezers and held for about 3 minutes to confirm that the Li metal had melted, and then removed from the hot plate for cooling.
(2) Ultrasonic-assisted heat fusion method As a means for performing ultrasonic-assisted heat fusion, an ultrasonic soldering iron (Fig. 1) (Manufacturer name: Kuroda Techno Co., Ltd., Product number: Sanbonda USM-560, Ultrasonic oscillation frequency : 60kHz ± 5kHz, ultrasonic oscillation output: 1 to 12 W, heater temperature setting: 200 to 500 ° C) was used.
Φ10 mm was masked on the LAGP pellets with masking tape. The heater was set to 240 ° C., Li metal was placed on the heated iron tip, and after contacting with LAGP pellets, 60 kHz, 5 W ultrasonic waves were applied for about 1 second to perform adhesion.

Example 2. Adhesion of Li Metal to LLZ Al doped-Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ) is used as the solid electrolyte, and the heat fusion method according to the present invention and the ultrasonic assisted heat fusion method according to the present invention are used. A Li metal electrode was formed on the solid electrolyte. Each fusion was performed in a glove box filled with Ar gas by the following procedure. The doping of Al in LLZ was performed to stabilize the garnet-type structure of LLZ.
(1) Normal heat fusion method (fusing with a hot plate)
LLZ pellets were placed on a hot plate heated to 230 ° C, a disc-shaped Li metal of Φ10 mm was placed on it, and pressed with tweezers from above through the Cu mesh. After holding for about 3 minutes and confirming that the Li metal had melted, it was removed from the hot plate and cooled. Li metals were fused in the same manner on the opposite side to prepare Li / LLZ / Li cells.
(2) Ultrasonic assisted heat fusion method An ultrasonic soldering iron (Fig. 1) was used as a means for performing ultrasonic assisted heat fusion.
Φ10 mm was masked on the LLZ pellets with masking tape. The heater was set to 240 ° C., Li metal was placed on the heated iron tip, and after contacting with the LLZ pellet, 60 kHz, 5 W ultrasonic waves were applied for about 1 second to perform adhesion. Li metals were fused in the same manner on the opposite side to prepare Li / LLZ / Li cells.
FIG. 2 shows a photograph when Li is fused onto the LLZ by a hot plate (a) or an ultrasonically assisted heat fusion method (b).
LLZ is known as a solid electrolyte that does not easily react with Li metal (Non-Patent Documents 3 and 4), and both seem to be fused in FIG. 2, but they were fused on a hot plate. If it is, it can be easily peeled off. On the other hand, when it was fused using the ultrasonic-assisted heat fusion method, it could not be easily peeled off. It is considered that this is because the surface of the solid electrolyte is activated by ultrasonic waves and the reaction between the Li / solid electrolyte is promoted.
FIG. 3 shows an electron micrograph of the fracture surface of Li / LLZ. When fused with a hot plate (a), many voids are found at the Li / LLZ interface. On the other hand, in the case of fusion using the ultrasonic-assisted heat fusion method (b), it can be seen that a uniform interface with high adhesion can be constructed.

Example 3. Impedance measurement Li / LAGP / Au and Li / LLZ / Li cells produced by the heat fusion method and the ultrasonic-assisted heat fusion method described in Examples 1 and 2 are placed in a plastic film and a closed bottle in a glove box, respectively. The film was sealed, taken out of the glove box, and the impedance was measured in a constant temperature bath heated to 30 ° C. The AC voltage was 10 mV and the frequency range was 100 mHz to 1 MHz.
In FIG. 4, Au / LAGP / Au in which Au electrodes are sputtered on both sides of LAGP, Li / LAGP / Au in which Li is fused on a hot plate, and Li / LAGP / in which Li is fused by an ultrasonic-assisted fusion method. The impedance plot of Au is shown. The value at the right end of the arc in the impedance plot means the resistance of the entire cell.
In Au / LAGP / Au (a), the resistance of the LAGP solid electrolyte itself was observed, and a resistance of about 930 ohms was observed. In Li / LAGP / Au, the resistance at the Li / LAGP interface was added to this, and when fused on a hot plate (b), the resistance was about 4300 ohms. On the other hand, when fused by the ultrasonic-assisted thermal fusion method (c), the resistance was about 1250 ohms, indicating that the resistance at the Li / LAGP interface could be significantly reduced.
FIG. 5 shows an impedance plot of Li / LLZ / Li fused by a hot plate (a) or by an ultrasonically assisted thermal fusion method (b).
It can be seen that the resistance at the Li / LLZ interface could be significantly reduced by using the ultrasonic-assisted heat fusion method as in LAGP.

Example 4. Constant current polarization measurement The Li / LLZ / Li cells produced by the heat fusion method and the ultrasonic-assisted heat fusion method described in Example 2 are sealed in a plastic film and a closed bottle in a glove box, respectively, and in the glove box. The constant current polarization was measured in a constant temperature bath warmed to 30 ° C. The measurement was performed by passing a current of 0.1 mA / cm ^{2 in} the reverse direction every 30 minutes and observing the change in voltage for 100 hours.
FIG. 6 shows the results obtained in each Li / LLZ / Li cell.
In the case of fusion using a hot plate (a), the voltage dropped sharply immediately after the start of measurement. It is considered that this is because Li was concentrated in a part and precipitated, resulting in a partial short circuit. On the other hand, in the case of fusion using the ultrasonic-assisted heat fusion method (b), such behavior is not observed, and even if the current reversal is repeated 100 times or more for 100 hours, the fluctuation width is constant. I was able to observe the voltage.

Example 5.
Li metal was ultrasonically assisted and heat-sealed on a slide glass by changing the heat melting temperature and the output of ultrasonic waves that could be applied. The ultrasonic frequency was fixed at 60 kHz. The result is shown in FIG.
From FIG. 7, it can be seen that when the temperature and the ultrasonic output are low, the Li metal is difficult to fuse, it starts to adhere cleanly from about 220 ° C. and 1 W, and it can be sufficiently adhered at about 240 ° C. and 5 W.
As shown in FIG. 8, under the conditions of 240 ° C. and 5 W, it is possible to form a complicated shape such as a letter by spreading Li metal on a slide glass.

Claims (5)

A method for producing an electrode / solid electrolyte structure by heat-sealing an electrode material onto a solid electrolyte and forming a metal or alloy-based electrode on the solid electrolyte. The electrode material is heat-fused onto the solid electrolyte. A method characterized in that the electrode material is thermally melted and ultrasonic waves are applied at the time of wearing. A method for producing an electrode / solid electrolyte structure by the method according to claim 1, and combining the obtained electrode / solid electrolyte structure with other components to produce an electrochemical cell. The method according to claim 2, wherein the electrochemical cell is a battery. The method according to claim 2, wherein the electrochemical cell is an all-solid-state battery. The method of claim 2, wherein the electrochemical cell is a metal-air battery.

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