JP7779293B2 - Positive electrode active material for fluoride ion battery, and fluoride ion battery - Google Patents

Description

This disclosure relates to positive electrode active materials for fluoride ion batteries and fluoride ion batteries.

Patent Document 1 discloses a positive electrode active material for use in a fluoride ion battery, which comprises a first active material having a composition represented by Pb _2-x Cu _1+x F ₆ (0≦x<2) and a second active material containing Bi and F elements.

Japanese Patent Application Laid-Open No. 2019-200852

There is a demand for increasing the capacity of fluoride-ion batteries, especially in the noble potential region.

The fluoride ion battery described in Patent Document 1 has a capacity of approximately 150 mAh/g, which can be reversibly used for two or more cycles at a discharge voltage of around 0.6 V.

The present disclosure aims to provide a positive electrode active material for a fluoride ion battery that has a high capacity, particularly in the noble potential region, and a fluoride ion battery that includes such a positive electrode active material.

The present inventors have found that the above problems can be solved by the following means.
<Aspect 1>
Calcium carbide, a positive electrode active material for fluoride-ion batteries.
<Aspect 2>
2. The positive electrode active material according to claim 1, wherein the positive electrode active material has peaks at 27.9°±0.5°, 32.3°±0.5°, 37.4°±0.5°, 46.1°±0.5°, and 54.1°±0.5° in X-ray diffraction measurement using Cuka radiation.
<Aspect 3>
A positive electrode mixture comprising the positive electrode active material according to aspect 1 or 2.
<Aspect 4>
A positive electrode active material layer is provided, and the positive electrode active material layer contains the positive electrode mixture according to aspect 3.
Fluoride-ion battery.

The present disclosure provides a positive electrode active material for a fluoride ion battery that has a large capacity, particularly in the noble potential region, and a fluoride ion battery that includes such a positive electrode active material.

FIG. 1 is a graph showing the results of XRD measurements on calcium carbide. FIG. 2 is a graph showing the charge/discharge curve of the evaluation battery of Example 1. FIG. 3 is a graph showing the discharge capacity down to 0.5 V in the second cycle of the test batteries of each example.

Embodiments of the present disclosure are described in detail below. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosure.

<Positive electrode active material for fluoride ion batteries>
The positive electrode active material for a fluoride ion battery of the present disclosure is calcium carbide.

The present inventors have unexpectedly discovered that calcium carbide (particularly represented by _CaC2 ) can be used as a positive electrode active material for fluoride ion batteries.

In this disclosure, calcium carbide may be a commercially available product or one prepared by conventional methods. Commercially available products include those manufactured by Sigma-Aldrich.

The positive electrode active material of the present disclosure may have peaks at 27.9°±0.5°, 32.3°±0.5°, 37.4°±0.5°, 46.1°±0.5°, and 54.1°±0.5° in X-ray diffraction (XRD) measurement using Cukα radiation. The width of these peak positions may be ±0.3° or ±0.1°.

XRD measurements can be performed, for example, using an Ultima III (Rigaku) CuKα radiation source on the positive electrode active material in an argon (Ar) atmosphere, with a measurement range of 20° to 70°, a scan rate of 2°/min, and a measurement interval of 0.02°.

<Positive electrode mixture>
The cathode mixture of the present disclosure includes the cathode active material of the present disclosure. The cathode mixture of the present disclosure optionally includes a solid electrolyte, a conductive material, and a binder.

In the context of this disclosure, the term "cathode mixture" refers to a composition that can form a cathode active material layer either as is or by further containing other components. Also, in the context of this disclosure, the term "cathode mixture slurry" refers to a slurry that contains a dispersion medium in addition to a "cathode mixture" and that can be applied and dried to form a cathode active material layer.

<Cathode active material>
For the positive electrode active material, reference can be made to the above description regarding the positive electrode active material for a fluoride ion battery of the present disclosure.

From the standpoint of capacity, it is preferable that the positive electrode active material content in the positive electrode mixture be as high as possible. The ratio of the mass of the positive electrode active material to the total mass of the positive electrode mixture may be 10 to 90 mass%, and preferably 20 to 80 mass%.

<Solid electrolyte>
Examples of the solid electrolyte include fluorides of lanthanoid elements such as La and Ce, fluorides of alkali metal elements such as Li, Na, K, Rb, and Cs, and fluorides of alkaline earth elements such as Ca, Sr, and Ba. The solid electrolyte may also be a fluoride containing multiple types of lanthanoid elements, alkali metal elements, and alkaline earth elements.

Specific examples of solid electrolytes include La _{(1-x) BaxF (3-x)} (0≦x≦2), Pb _{(2-x) SnxF4} (0≦x≦2 ₎ , Ca _{(2-x) BaxF4} (0≦x≦2), and Ce _{(1-x) BaxF (3-x)} (0≦x≦2). Each of the _x 's may be greater than 0, 0.3 or greater, 0.5 or greater, or 0.9 or greater. Each of the x's may be less than 1, 0.9 or less, 0.5 or less, or 0.3 or less.

The ratio of the mass of the solid electrolyte to the total mass of the positive electrode mixture may be 10 to 90 mass%, and preferably 20 to 80 mass%.

<Conductive materials>
The conductive material is not particularly limited as long as it has the desired electronic conductivity, and examples of the conductive material include carbon materials, such as carbon blacks such as acetylene black, ketjen black, furnace black, and thermal black.

The ratio of the mass of the conductive material to the total mass of the positive electrode mixture may be 0.1 to 20 mass%, and preferably 1 to 10 mass%.

<binder>
The binder is not particularly limited as long as it is chemically and electrically stable, and examples thereof include fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

Fluoride-ion battery
The fluoride ion battery of the present disclosure has a positive electrode active material layer, and the positive electrode active material layer contains the positive electrode mixture of the present disclosure.

The fluoride ion battery of the present disclosure may have, in this order, a positive electrode current collector, a positive electrode active material layer containing the positive electrode mixture of the present disclosure, an electrolyte layer, a negative electrode active material layer, and a negative electrode current collector.

The fluoride ion battery of the present disclosure may have a battery case that houses these components.

The fluoride ion battery of the present disclosure may be a liquid-based battery or a solid-state battery. Note that, in the context of the present disclosure, "solid-state battery" refers to a battery that uses at least a solid electrolyte as the electrolyte; therefore, a solid-state battery may use a combination of a solid electrolyte and a liquid electrolyte as the electrolyte. The solid-state battery of the present disclosure may also be an all-solid-state battery, i.e., a battery that uses only a solid electrolyte as the electrolyte.

The fluoride ion battery in this disclosure may be a primary battery or a secondary battery.

The shape of the fluoride ion battery disclosed herein may be, for example, a coin type, a laminate type, a cylindrical type, or a prismatic type.

<Positive electrode current collector>
The material for the positive electrode current collector is not particularly limited as long as it has the desired electronic conductivity and does not undergo a significant change in volume or shape during charging and discharging. Examples of the material include stainless steel (SUS), aluminum, titanium, iron, nickel, copper, silver, platinum, gold, and carbon.

Positive electrode current collectors can be in the form of foil, mesh, pellet, or porous, for example.

<Cathode active material layer>
The positive electrode active material layer contains the positive electrode mixture of the present disclosure.

For information on the positive electrode mixture, please refer to the above description of the positive electrode mixture of this disclosure.

The thickness of the positive electrode active material layer varies greatly depending on the battery configuration and is not particularly limited.

The positive electrode active material layer can be formed, for example, by depositing a positive electrode mixture containing a positive electrode active material and, optionally, a solid electrolyte, a conductive additive, and a binder on a positive electrode current collector as a substrate.

More specifically, for example, a positive electrode active material, an optional solid electrolyte, a conductive additive, and a binder are mixed in a ball mill to produce a powder of the positive electrode composite, and a compact of the powder of the positive electrode composite is then formed on the positive electrode current collector. Alternatively, for example, a positive electrode active material, an optional solid electrolyte, a conductive additive, and a binder may be dispersed in a dispersion medium to produce a slurry, which is then applied to the positive electrode current collector and dried to form the positive electrode.

<Electrolyte layer>
When the fluoride ion battery of the present disclosure is a liquid battery, the electrolyte layer may be composed of, for example, an electrolyte solution and an optional separator.

(electrolyte)
The electrolyte may contain, for example, a fluoride salt and an organic solvent. Examples of the fluoride salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids. An example of the inorganic fluoride salt is XF (X is Li, Na, K, Rb, or Cs). An example of the cation of the organic fluoride salt is an alkylammonium cation such as a tetramethylammonium cation.

The concentration of fluoride salt in the electrolyte solution is, for example, 0.1 mol% or more and 40 mol% or less, and preferably 1 mol% or more and 10 mol% or less.

The organic solvent in the electrolyte is typically a solvent that dissolves fluoride salts. Examples of organic solvents include glymes such as triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether (G4), cyclic carbonates such as ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), propylene carbonate (PC), and butylene carbonate (BC), and chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Ionic liquids may also be used as the organic solvent.

(separator)
The separator is not particularly limited as long as it has a composition that can withstand the range of uses of a fluoride ion battery, and examples of the separator include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and microporous fillers of olefin resins such as polyethylene and polypropylene.

When the fluoride ion battery of the present disclosure is a solid-state battery, the electrolyte layer may be, for example, a layer of a solid electrolyte. The solid electrolyte may be any solid electrolyte that can be used in a fluoride ion battery.

(solid electrolyte)
For the solid electrolyte, reference can be made to the above description regarding the positive electrode mixture of the present disclosure.

<Negative electrode active material layer>
The negative electrode active material layer contains a negative electrode active material, and optionally a solid electrolyte, a conductive material, and a binder.

(Negative electrode active material)
The negative electrode active material may be any active material having a lower potential than the positive electrode active material. Examples of the negative electrode active material include simple metals, alloys, and fluorides of metal oxides. Examples of metal elements contained in the negative electrode active material include La, Ca, Al, Eu, Li, Si, Ge, Sn, In, V, Cd, Cr, Fe, Zn, Ga, Ti, Nb, Mn, Yb, Zr, Sm, Ce, Mg, and Pb. Among these, the negative electrode active material is preferably MgF _x , AlF _x , LaF _x , CeF _x , CaF _x , or PbF _x . Note that x is a real number greater than 0.

From the standpoint of capacity, it is preferable that the content of the negative electrode active material in the negative electrode active material layer is as high as possible. The ratio of the mass of the negative electrode active material to the total mass of the negative electrode active material layer may be 10 to 90 mass%, and preferably 20 to 80 mass%.

(Solid electrolyte, conductive material, and binder)
For the solid electrolyte, the conductive material, and the binder, reference can be made to the above descriptions regarding the positive electrode mixture of the present disclosure.

<Negative electrode current collector>
The material for the negative electrode current collector is not particularly limited as long as it has the desired electronic conductivity and does not undergo a significant change in volume or shape during charging and discharging. Examples of the material include stainless steel (SUS), aluminum, titanium, iron, nickel, copper, silver, platinum, gold, and carbon.

Examples of the shape of the negative electrode current collector include foil, mesh, pellet, and porous.

《Fabrication of fluoride-ion batteries》
Example 1
Calcium carbide (CaC ₂ ) powder (manufactured by Sigma-Aldrich) as the positive electrode active material, solid electrolyte (Ca _0.5 Ba _0.5 F ₂ : synthesized by mixing CaF ₂ (manufactured by Kojundo Chemical) and BaF ₂ (manufactured by Kojundo Chemical) in a ball mill at 600 rpm for 20 hours), and conductive material (VGCF: manufactured by Showa Denko) in a weight ratio of 19:19:2 were mixed using a ball mill (rotation speed: 200 rpm) to obtain a positive electrode composite. The obtained positive electrode composite, solid electrolyte (Ca _0.5 Ba _0.5 F ₂ ) forming the solid electrolyte layer, PbF ₂ and conductive material (acetylene black) were mixed at a ratio of 95:5 to obtain a counter electrode composite, and Pb foil were laminated in this order and pressed into powder molding. This produced the evaluation battery of Example 1.

Comparative Example 1
An evaluation battery for Comparative Example 1 was prepared in the same manner as in Example ₁ , except that lanthanide carbide ( _LaC2 ) powder (manufactured by Rare Metallic), which has the same _MC2 composition (M is a metal element) as _CaC2 , was used as the positive electrode active material instead of CaC2 powder.

"evaluation"
<XRD measurement>
The XRD measurement was carried out using, for example, an Ultima III (manufactured by Rigaku) with a CuKα radiation source, on the positive electrode active material (CaC ₂ ) in an argon (Ar) atmosphere under the conditions of a measurement range of 20° to 70°, a scan rate of 2°/min, and a measurement interval of 0.02°.

<Charge/Discharge Test>
A charge-discharge test was carried out on the prepared evaluation battery under the conditions of a temperature of 200° C., a cut-off potential of the positive electrode of −2.5 V (vs. Pb/PbF ₂ ) to 3 V (vs. Pb/PbF ₂ ), and a current of 50 μA/cm ² .

"result"
<XRD measurement>
The results of XRD measurement for _CaC2 are shown in Figure 1. As shown in Figure 1, XRD measurement of _CaC2 using Cuka radiation showed peaks at 27.9°, 32.3°, 37.4°, 46.1°, and 54.1°.

<Charge/Discharge Test>
The charge-discharge curve of the evaluation battery of Example 1 is shown in Figure 2. As shown in Figure 2, the discharge capacity at the second cycle of the battery of Example 1 containing _CaC2 as the positive electrode active material was equal to the discharge capacity at the first cycle.

The discharge capacity up to 0.5 V of the evaluation batteries in each example at the second cycle is shown in Figure 3. As shown in Figure 3, the discharge capacity of the battery of Example 1 containing _CaC2 as the positive electrode active material was about 4.5 times larger than the discharge capacity of the battery of Comparative Example 1.

Claims (4)

Calcium carbide is a positive electrode active material for fluoride-ion batteries. The positive electrode active material according to claim 1, which has peaks at 27.9°±0.5°, 32.3°±0.5°, 37.4°±0.5°, 46.1°±0.5°, and 54.1°±0.5° in X-ray diffraction measurement using Cukα radiation. A positive electrode mixture containing the positive electrode active material according to claim 1 or 2. A positive electrode active material layer is provided, and the positive electrode active material layer contains the positive electrode mixture according to claim 3.
Fluoride-ion battery.