JP7613401B2 - Positive electrode active material and fluoride ion battery - Google Patents

Description

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

For example, Li-ion batteries are known as high-voltage, high-energy density batteries. Li-ion batteries are cation-based batteries that use Li ions as carriers. For example, Patent Document 1 discloses that metal fluorides or metal chlorides are used as the positive electrodes for lithium-ion batteries.

On the other hand, as an anion-based battery, a fluoride ion battery using fluoride ions as a carrier is known. Non-Patent Document 1 discloses a fluoride ion battery using _BiF3 , _SnF2 , _CuF2 or _KBiF4 as a positive electrode active material and Ce metal as a negative electrode active material, and discloses that the battery voltage is highest when _CuF2 is used as a positive electrode active material and Ce metal is used as a negative electrode active material.

Special Publication No. 2020-507181

M. Anji et al., “Batteries based on fluoride shuttle”, J. Mater. Chem., 2011, 21, 17059

From the viewpoint of improving the energy density, it is desirable to further improve the battery voltage of fluoride ion batteries. The present invention was made in consideration of the above-mentioned circumstances, and the main object of the present invention is to provide a positive electrode active material with a high charge/discharge potential.

To solve the above problems, the present disclosure provides a positive electrode active material for use in a fluoride ion battery, the positive electrode active material being CuX (X is Cl, Br, or I).

According to the present disclosure, the positive electrode active material used in a fluoride ion battery is a copper halide containing monovalent copper ions, which allows the charge/discharge potential to be increased.

In the above disclosure, the positive electrode active material may contain at least CuCl.

In the above disclosure, the positive electrode active material may contain at least CuBr.

In the above disclosure, the positive electrode active material may contain at least CuI.

The present disclosure also provides a fluoride ion battery having a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer, in which the positive electrode active material layer contains the above-mentioned positive electrode active material.

According to the present disclosure, by using the above-mentioned active material, a fluoride ion battery with a high battery voltage can be obtained.

The present disclosure has the effect of providing a positive electrode active material with a high charge/discharge potential.

FIG. 1 is a schematic cross-sectional view showing an example of an all-solid-state battery according to the present disclosure. 1 is a graph showing the results of a charge/discharge test on the evaluation batteries of Examples 1 to 3 and a comparative example. 1 shows the results of XRD measurement of the evaluation battery of Example 1 before and after charging and discharging.

The positive electrode active material and fluoride ion battery in this disclosure are described in detail below.

A. Positive Electrode Active Material The positive electrode active material in this disclosure is a positive electrode active material for use in fluoride ion batteries and is CuX (X is Cl, Br, or I).

According to the present disclosure, the positive electrode active material used in the fluoride ion battery is a copper halide containing monovalent copper ions, so that the charge/discharge potential can be increased. Here, Non-Patent Document 1 describes that the discharge potential of a battery using CuF ₂ as the positive electrode active material and Ce metal as the negative electrode active material is about 2.7 V. In CuF ₂ , the reaction between zero-valent metallic copper (Cu) and divalent copper ions (Cu ²⁺ ) is utilized. Note that in copper (divalent copper) fluoride, monovalent copper ions (Cu ⁺ ) cannot exist stably.

On the other hand, CuX (X is Cl, Br, or I) in the present disclosure is a copper halide in which monovalent copper ions can exist stably. Therefore, the reaction potential shifts to a more noble potential due to oxidation-reduction between monovalent copper ions (Cu ⁺ ) and divalent copper ions (Cu ²⁺ ), and thus the charge-discharge potential is higher than that of _CuF2 .

The positive electrode active material in the present disclosure may be any one of CuCl, CuBr, and CuI, or may be a mixture of two or more of them. The positive electrode active material preferably contains at least CuCl. In this case, the positive electrode active material may contain only CuCl, or may contain CuCl and at least one of CuBr and CuI. In addition, the positive electrode active material preferably contains at least CuBr. In this case, the positive electrode active material may contain only CuBr, or may contain CuBr and at least one of CuCl and CuI. In addition, the positive electrode active material preferably contains at least CuI. In this case, the positive electrode active material may contain only CuI, or may contain CuI and at least one of CuCl and CuBr. In addition, the positive electrode active material in the present disclosure is usually fluorinated during charging of a fluoride ion battery and defluorinated during discharging.

The shape of the positive electrode active material in the present disclosure may be, for example, particulate (positive electrode active material particles). Specific shapes of the positive electrode active material particles may be, for example, spherical, elliptical, etc. The average particle diameter (D ₅₀ ) of the positive electrode active material particles is preferably, for example, in the range of 10 nm to 50 μm, and more preferably in the range of 20 nm to 10 μm. The average particle diameter of the positive electrode active material particles can be measured, for example, by observation with a scanning electron microscope (SEM) (for example, n≧20). It can also be calculated from the measured value of the BET specific surface area.

B. Fluoride-ion battery Fig. 1 is a schematic cross-sectional view showing an example of a fluoride-ion battery in the present disclosure. The fluoride-ion battery 10 shown in Fig. 1 has a positive electrode active material layer 1, a negative electrode active material layer 2, an electrolyte layer 3 formed between the positive electrode active material layer 1 and the negative electrode active material layer 2, a positive electrode current collector 4 that collects current from the positive electrode active material layer 1, a negative electrode current collector 5 that collects current from the negative electrode active material layer 2, and a battery case 6 that houses these members. In the present disclosure, the positive electrode active material layer 1 is a fluoride-ion battery having a positive electrode active material layer 2 and a negative electrode active material layer 3 formed therebetween.
The major feature of the present invention is that it contains the positive electrode active material described above.

According to the present disclosure, by using the above-mentioned positive electrode active material, a fluoride ion battery with a high battery voltage can be obtained.

1. Positive electrode active material layer The positive electrode active material layer in the present disclosure is a layer containing at least the positive electrode active material described above. The positive electrode active material is the same as that described in "A. Positive electrode active material" above. The positive electrode active material layer may contain only the positive electrode active material described above, or may also contain other active materials. In the latter case, the ratio of the positive electrode active material described above in the entire active material is, for example, 85% by weight or more, may be 90% by weight or more, may be 95% by weight or more, or may be 99% by weight or more.

The content of the positive electrode active material in the positive electrode active material layer is, for example, 10% by weight or more and 90% by weight or less, or may be 20% by weight or more and 80% by weight or less, or may be 30% by weight or more and 70% by weight or less. If the proportion of the positive electrode active material is too small, the energy density may decrease.

The positive electrode active material layer may further contain at least one of a conductive material, a binder, and an electrolyte, as necessary.

The conductive material is not particularly limited as long as it has the desired electronic conductivity, but examples of the conductive material include carbon materials. Examples of the carbon material include carbon black such as acetylene black, ketjen black, furnace black, and thermal black, carbon fiber, fibrous carbon materials such as carbon nanotubes (CNT) and carbon nanofibers (CNF), graphene, graphite, and fullerene.

The content of the conductive material in the positive electrode active material layer is, for example, 1% by weight or more, and may be 5% by weight or more, or 10% by weight or more. On the other hand, the content of the conductive material is, for example, 20% by weight or less, and may be 15% by weight or less. If the proportion of the conductive material is too small, a good electron conduction path may not be formed, and if the proportion of the conductive material is too high, the proportion of the active material may be relatively small, and the energy density may decrease.

There are no particular limitations on the binder as long as it is chemically and electrically stable, but examples include fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

The electrolyte is the same as that described in "3. Electrolyte Layer" below. The electrolyte content in the positive electrode active material layer is, for example, 5% by weight or more, and may be 10% by weight or more. On the other hand, the electrolyte content is, for example, 70% by weight or less, and may be 40% by weight or less. If the proportion of electrolyte is too small, it may not be possible to diffuse fluoride ions deep into the positive electrode active material layer with low resistance. If the proportion of electrolyte is too high, the proportion of active material will be relatively small, and the energy density may decrease.

The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

The negative electrode active material layer in the present disclosure is a layer containing at least a negative electrode active material. The negative electrode active material layer may further contain at least one of a conductive material, an electrolyte, and a binder, as necessary.

As the negative electrode active material, any active material having a lower potential than the positive electrode active material can be selected. Examples of the negative electrode active material include fluorides of simple metals, fluorides of alloys, and fluorides of metal oxides. Examples of the 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 them, the negative electrode active material is preferably MgFx, AlFx, LaFx, CeFx, CaFx, or PbFx. Note that the above x is a real number greater than 0.

The conductive material and binder may be the same as those described in "1. Positive electrode active material layer" above. The electrolyte is the same as that described in "3. Electrolyte layer", so a description of it here is omitted.

From the viewpoint of capacity, the content of the negative electrode active material in the negative electrode active material layer is preferably higher, for example, 30% by weight or more, preferably 50% by weight or more, and more preferably 70% by weight or more. The thickness of the negative electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

3. Electrolyte Layer The electrolyte layer in the present disclosure is a layer formed between the positive electrode active material layer and the negative electrode active material layer. The electrolyte constituting the electrolyte layer may be a liquid electrolyte (electrolytic solution), a polymer electrolyte, or an inorganic solid electrolyte.

The electrolyte contains, for example, a fluoride salt and a solvent. Examples of the fluoride salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids. Examples of the inorganic fluoride salt include XF (X is Li, Na, K, Rb, or Cs). Examples of the cation of the organic fluoride salt include alkyl ammonium cations such as tetramethylammonium cations. The concentration of the fluoride salt in the electrolyte is, for example, 0.1 mol/L or more, and may be 0.3 mol/L or more, or may be 0.5 mol/L or more. On the other hand, the concentration of the fluoride salt is, for example, 6 mol/L or less, and may be 3 mol/L or less.

Examples of the solvent include glymes represented by R ¹ -O(CH ₂ CH ₂ O) _n -R ² (R ¹ and R ² are each independently an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, and n is in the range of 2 to 10), 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). The electrolyte may contain an alkali metal amide salt. The polymer electrolyte can be obtained, for example, by adding a polymer to a liquid electrolyte and gelling it.

On the other hand, examples of inorganic solid electrolytes include fluorides having lanthanoid elements such as La and Ce, fluorides having alkali metal elements such as Li, Na, K, Rb, and Cs, and fluorides having alkaline earth elements such as Ca, Sr, and Ba. In addition, the inorganic solid electrolyte is preferably a fluoride having at least one metal element of La, Ba, Pb, Sn, Ca, and Ce. The inorganic solid electrolyte may have only one of the above metal elements, or may have two or more of them. Specific examples of inorganic solid electrolytes include La _1-x Ba _x F _3-x (0≦x≦1), Pb _2-x Sn _x F ₄ (0≦x≦2), Ca _2-x Ba _x F ₄ (0≦x≦2), and Ce _1-x Ba _x F _3-x (0≦x≦1). Each of the x's may be greater than 0, greater than or equal to 0.3, greater than or equal to 0.5, or greater than or equal to 0.9. Each of the x's may be smaller than 1, less than or equal to 0.9, less than or equal to 0.5, or less than or equal to 0.3. The shape of the inorganic solid electrolyte is not particularly limited, but may be, for example, a particulate shape.

4. Other Configurations The fluoride ion battery in the present disclosure has at least the above-mentioned positive electrode active material layer, negative electrode active material layer, and electrolyte layer. In addition, it usually has a positive electrode current collector that collects the current of the positive electrode active material layer, a negative electrode current collector that collects the current of the negative electrode active material layer, and a battery case that houses the above-mentioned members. The materials of the positive electrode current collector, the negative electrode current collector, and the battery case can be conventionally known materials. The shape of the current collector can be, for example, a foil shape, a mesh shape, or a porous shape. The fluoride ion battery may also have a separator between the positive electrode active material layer and the negative electrode active material layer. By providing a separator, a safer battery can be obtained.

5. Fluoride ion battery The fluoride ion battery in the present disclosure may be a liquid battery or an all-solid-state battery, but is preferably an all-solid-state battery. The fluoride ion battery in the present disclosure may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because the battery can be repeatedly charged and discharged, and is useful, for example, as an in-vehicle battery. The shape of the fluoride ion battery in the present disclosure may be, for example, a coin type, a laminate type, a cylindrical type, or a square type.

This disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and anything that has substantially the same configuration as the technical ideas described in the claims of this disclosure and has similar effects is included within the technical scope of this disclosure.

[Example 1]
(Preparation of Evaluation Battery)
_{CuCl (manufactured by Aldrich), solid electrolyte La0.9Ba0.1F2.9 ( LaF3 and BaF2} (both manufactured by Kojundo Chemical) were mixed in a ball mill and then fired at 600°C), and a conductive material were mixed in a weight ratio of 30:60:10 in a ball mill (rotation speed 100 rpm) to obtain a positive electrode composite. The obtained composite (working electrode), a counter electrode formed by mixing a solid electrolyte ( _{La0.9Ba0.1F2.9} ) _that forms _a solid electrolyte layer, _PbF2 (manufactured by Kojundo Chemical) and acetylene black HS-100 (manufactured by Denka) in a weight ratio of 95:5, and Pb foil were pressed into powder to obtain a battery for evaluation.

(Charge/Discharge Evaluation)
A charge/discharge test was carried out on the evaluation battery obtained in Example 1. The charge/discharge test was carried out at a temperature of 140° C., a cut-off potential of 1.5 (vs. Pb/PbF ₂ ) to 0.3 V (vs. Pb/PbF ₂ ), and a current of 50 μA. The results are shown in FIG. 2(a). The average discharge potential was 0.90 V (vs. Pb/PbF ₂ ).

(XRD Measurement)
X-ray diffraction (XRD) measurements using CuKa radiation were performed on the working electrode of the evaluation battery obtained in Example 1 before and after charging and discharging using an Ultima IV (manufactured by Rigaku). The measurement conditions were an Ar atmosphere, 2θ = 10 to 80°, a scanning speed of 2°/min, and a measurement interval of 0.02°. The results are shown in FIG.

Before charging and discharging, the peak of the active material CuCl and the peak of the electrolyte La _0.9 Ba _0.1 F _2.9 can be confirmed. When the positive electrode active material is fluorinated by charging, a peak appears at 2θ = 15 ° that can be assigned to a structure similar to CuCl ₂ (after charging at 1.5 V). It is presumed that CuCl is fluorinated by charging to produce CuClF, but this material has not been reported in the database. Presumably, CuClF and CuCl ₂ have the same crystal structure, and the peak at 2θ = 15 ° is thought to be derived from CuClF. It was also confirmed that after discharging, it was defluorinated and reversibly returned to the CuCl structure (after charging at 1.5 V and discharging at 0.3 V). Therefore, it is presumed that the charge and discharge reaction of CuCl is a reaction between CuCl (monovalent copper ion) and CuClF (divalent copper ion).

[Example 2]
A battery for evaluation was prepared in the same manner as in Example 1, except that the positive electrode active material in Example 1 was changed to CuBr (manufactured by Aldrich), and charge/discharge evaluation was performed. The results are shown in Fig. 2(b). The average discharge potential was 0.48 V (vs. Pb/ _PbF2 ).

[Example 3]
A battery for evaluation was prepared in the same manner as in Example 1, except that the positive electrode active material in Example 1 was changed to CuI (manufactured by Aldrich), and the charge/discharge evaluation was performed. The results are shown in Fig. 2(c). The average discharge potential was 0.57 V (vs. Pb/ _PbF2 ).

[Comparative Example]
A battery for evaluation was prepared in the same manner as in Example 1, except that the positive electrode active material in Example 1 was changed to CuF ₂ (manufactured by Strem Chemicals), and charge/discharge evaluation was performed. The results are shown in Figure 2(d). The average discharge potential was 0.33 V (vs. Pb/PbF ₂ ).

From the results of Figures 2 (a) to (d), it was confirmed that the evaluation batteries of Examples 1 to 3 had a higher initial discharge potential than the evaluation battery of the comparative example. In addition, it was confirmed that the evaluation batteries of Examples 1 to 3 also had a higher charge potential than the evaluation battery of the comparative example. That is, it was confirmed that the positive electrode active materials (CuCl, CuBr and CuI) used in Examples 1 to 3 had a higher charge/discharge potential than the positive electrode active material (CuF ₂ ) used in the comparative example. Since the positive electrode active materials used in Examples 1 to 3 are copper halides in which monovalent copper ions can exist stably, it is presumed that the reaction potential shifted to a more noble potential due to oxidation-reduction between monovalent copper ions (Cu ⁺ ) and divalent copper ions (Cu ²⁺ ).

Reference Signs List 1 positive electrode active material layer 2 negative electrode active material layer 3 electrolyte layer 4 positive electrode current collector 5 negative electrode current collector 6 battery case 10 fluoride ion battery

Claims (4)

A positive electrode active material for use in a fluoride ion battery, comprising:
The positive electrode active material is a copper halide containing a monovalent copper ion,
The copper halide is CuX (X is Br or I). The positive electrode active material according to claim 1 , wherein the positive electrode active material contains at least CuBr. The positive electrode active material according to claim 1 or 2 , wherein the positive electrode active material contains at least CuI. A fluoride ion battery having a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer,
A fluoride ion battery, wherein the positive electrode active material layer contains the positive electrode active material according to any one of claims 1 to 3 .