JP7728006B2 - Positive electrode material, its manufacturing method, and all-solid-state fluoride ion shuttle battery - Google Patents

Description

This disclosure relates to a positive electrode material for an all-solid-state fluoride ion shuttle battery, a method for producing the same, and an all-solid-state fluoride ion shuttle battery using the positive electrode material.

In recent years, various high-energy density secondary batteries that operate by shuttling ions other than lithium between positive and negative electrodes have been investigated. Among these, fluoride ion shuttle batteries (FIB), which operate by shuttling fluoride ions, have attracted attention because they have been reported to exhibit high capacity when all-solid-state batteries using metal fluorides such as _CuF2 and _BiF3 as the positive electrode active material and Ce as the negative electrode active material (see, for example, Non-Patent Document 1).

Furthermore, carbon materials such as graphite, which have electronic conductivity and fluoride ion conductivity, and fluorine-graphite intercalation compounds (C _x F) have also been proposed as high-capacity positive electrode materials for FIB (see, for example, Patent Document 1 and Non-Patent Document 2). These materials are expected to exhibit a high capacity of 2232 mAh/g if fluoride ions can be introduced into them up to a saturated composition.

Special Publication No. 2009-529222

M. Anji Reddy, et al, J. Mater. Chem., 2011, 21, 17059 Y. Matsuo, et al, Electrochem. Commun., 2020, 110 106626

However, when metal fluorides such as CuF ₂ and BiF ₃ described in Non-Patent Document 1 are used as the positive electrode active material, the maximum capacity is 843 mAh/g (in the case of Cu). Furthermore, the fluorides generated by charging often lack electronic conductivity or fluoride ion conductivity, so it is necessary to add a material to the positive electrode that compensates for these properties in addition to the active material. Furthermore, due to the large structural changes that occur during charging and discharging, there are problems with output density and cycle characteristics.

Furthermore, Patent Document 1 only points out the possibility of carbon materials as positive electrode materials, and no examples of actual charging and discharging have yet been reported.

The C _x F described in Non-Patent Document 2 is a stage 1 type C _x F, which can be charged and discharged by adding CuF ₂ , but there are problems such as the need for a relatively large amount of CuF ₂ and the fact that only a discharge voltage significantly lower than the theoretical discharge voltage can be obtained, leaving room for improvement. Furthermore, stage 1 type C _x F can be obtained by inserting fluorine into graphite as a raw material using fluorine gas at a low temperature in the presence of a catalyst, but the reaction takes a long time and there are also problems in terms of cost.

This disclosure provides a positive electrode material for an all-solid-state fluoride ion shuttle battery that can exhibit higher voltage, higher capacity, and higher cycle characteristics, and that can be manufactured easily and at low cost; a method for manufacturing the same; and an all-solid-state fluoride ion shuttle battery that uses the positive electrode material.

In order to solve the above problems, one aspect of the cathode material disclosed herein is a cathode material for use in an all-solid-state fluoride ion shuttle battery, characterized in that the cathode active material contained in the cathode material comprises a carbon material with a lower fluorine content than a stage 1 fluorine-graphite intercalation compound, and a fluoride catalyst that is at least one of a metal fluoride and HF.

Carbon materials are resistant to oxidation, so a high voltage is required to insert fluoride ions. Furthermore, because strong C-F bonds are formed, an insulating fluoride layer is formed on the surface of the carbon material, making it prone to passivation.

Here, Non-Patent Document 2 describes that the addition of _CuF2 advances the discharge reaction of stage ₁ type C _x F. The mechanism behind this is thought to be as follows: Although the above-mentioned fluoride insulating layer is formed on the surface of C _x F, Cu generated by the discharge of CuF2 reduces and decomposes the fluoride insulating layer on the surface of C _x F, thereby removing the fluoride insulating layer and allowing the discharge reaction of C _x F to advance.

Based on the mechanism of the discharge reaction of C _x F _, the present inventors have come to the conclusion that the formation of a fluoride insulating layer may prevent the charging reaction from proceeding even in carbon materials such as graphite that have a lower fluorine content (molar ratio) than Stage 1 C x F. They have also found that the charging reaction can proceed in such carbon materials by adding a fluoride catalyst, which is at least one of a metal fluoride and HF.

That is, according to this configuration, by using a carbon material having a lower fluorine content than Stage 1 _CxF and the above-mentioned fluoride catalyst as a cathode active material, a FIB can be produced that can exhibit higher voltage, higher capacity, and better cycle characteristics than a FIB using a conventional metal fluoride as a cathode active material. Furthermore, a carbon material having a lower fluorine content than Stage 1 _CxF is not subjected to a treatment to introduce fluoride ions, or the treatment is not performed as intensively as in Stage 1 _{CxF ,} and therefore the amount of fluoride insulating layer formed on the surface is originally smaller than that of Stage 1 _CxF . This eliminates the need to remove the surface fluoride insulating layer, which may reduce the amount of fluoride catalyst added and improve the discharge voltage compared to when Stage 1 _CxF is used as the cathode active material. Furthermore, a carbon material having a lower fluorine content than Stage 1 _CxF can be prepared more easily and at lower cost than Stage 1 CxF, which makes the manufacture of the FIB easier and more cost-effective.

The positive electrode material is preferably a mixture consisting of the positive electrode active material and a solid electrolyte.

By using a mixture containing a solid electrolyte, the fluoride ion conductivity of the positive electrode material is improved.

It is preferable that the content of the carbon material contained in the mixture be 10% by mass or more and 50% by mass or less, and the content of the fluoride catalyst contained in the mixture be 1% by mass or more and 5% by mass or less.

By setting the content of carbon material and fluoride catalyst in the mixture within the above ranges, an FIB with higher capacity and better cycle characteristics can be obtained.

The metal fluoride is preferably at least one selected from the group consisting of LiF, AgF, _AlF3 , _CuF2 , KF, _KHF2 and _CaF2 .

This configuration allows the charging reaction of the carbon material to proceed quickly.

The carbon material preferably contains a layered compound whose constituent elements are carbon atoms and oxygen atoms covalently bonded to the carbon atoms.

With this configuration, because the carbon material is partially oxidized, the anion insertion potential is lower than, for example, graphite, which is made up of only carbon atoms. This makes it easier to insert fluoride ions, facilitating the charging reaction of the carbon material.

The interlayer distance of the layered compound is preferably greater than the interlayer distance of graphite.

The interlayer distance of graphite is generally known to be 0.335 nm (J. Inamoto, et al., J. Electrochem. Soc., 2021, 168, 010528). The layered compounds described above contain covalently bonded oxygen atoms, resulting in a larger interlayer distance than graphite. The larger interlayer distance facilitates the insertion and desorption of fluoride ions, potentially improving the cycle characteristics of FIBs.

One aspect of the method for producing a positive electrode material disclosed herein is a method for producing the above-mentioned positive electrode material, characterized by comprising a preparation step of preparing the carbon material, and a mixing step of mixing the carbon material with the fluoride catalyst to obtain the positive electrode active material.

According to this configuration, by mixing the carbon material and the fluoride catalyst, the contact area between the two increases, which is advantageous for electronic conductivity and fluoride ion conductivity.

The carbon material is a layered compound whose constituent elements are carbon atoms and oxygen atoms covalently bonded to the carbon atoms. In the preparation step, it is preferable to obtain the layered compound by oxidizing graphite powder and then subjecting it to heat treatment.

Graphite oxide is obtained by oxidizing graphite powder. When graphite oxide is subjected to heat treatment, some of the oxygen atoms in the graphite oxide are removed, forming nanometer-sized pores in the layers. The layered compound obtained in this way has some of its carbon atoms oxidized, and the layers have pores, which lowers the anion insertion potential compared to graphite and makes it easier to insert fluoride ions. This makes it easier for the charging reaction of carbon materials to proceed.

One embodiment of the all-solid-state fluoride ion shuttle battery disclosed herein comprises a positive electrode, a negative electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains the above-described positive electrode material.

This configuration makes it possible to produce an FIB that can exhibit higher voltage, higher capacity, and longer cycle life than conventional FIBs that use metal fluorides as the positive electrode active material. It also makes the FIB easier to manufacture and is more cost-effective.

As described above, according to the present disclosure, by using a carbon material having a lower fluorine content than Stage 1 _CxF and the above-mentioned fluoride catalyst as a cathode active material, a FIB can be produced that can exhibit higher voltage, higher capacity, and better cycle characteristics than a FIB using a conventional metal fluoride as a cathode active material. Furthermore, a carbon material having a lower fluorine content than Stage 1 _CxF is not treated to introduce fluoride ions like Stage 1 _CxF , and therefore has a smaller amount of fluoride insulating layer formed on the surface than Stage 1 _{CxF .} This eliminates the need to remove the surface fluoride insulating layer, potentially reducing the amount of fluoride catalyst added and improving discharge voltage compared to when Stage 1 CxF is used as the cathode active material. Furthermore, a carbon material having a lower fluorine content than Stage 1 _CxF can be manufactured more easily and at lower cost than Stage 1 _CxF , making the FIB easier to manufacture and more cost-effective.

FIG. 1 is a diagram showing the configuration of an all-solid-state fluoride ion shuttle battery according to one embodiment. FIG. 1 is a diagram showing an example of the molecular structure of graphene-like graphite. 3 is a graph showing charge/discharge curves of the battery cell of Example 1. 6 is a graph showing charge/discharge curves of a battery cell according to Example 2. 10 is a graph showing charge/discharge curves of the battery cell of Example 3. 10 is a graph showing charge/discharge curves of a battery cell according to Example 4. 10 is a graph showing charge/discharge curves of the battery cell of Example 5. 10 is a graph showing charge/discharge curves of the battery cell of Example 6. 10 is a graph showing charge/discharge curves of the battery cell of Example 7. 10 is a graph showing charge/discharge curves of the battery cell of Example 8. 10 is a graph showing charge/discharge curves of the battery cell of Example 9. 10 is a graph showing charge/discharge curves of a battery cell of Comparative Example 1.

Embodiments of the present disclosure are described in detail below with reference to the accompanying drawings. The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the present disclosure, its applications, or its uses.

(Embodiment 1)
<All-solid-state fluoride-ion shuttle battery>
1 , an all-solid-state fluoride ion shuttle battery 10 according to the present disclosure includes an anode active material layer 12 (anode), a solid electrolyte layer 13 (solid electrolyte), and a cathode active material layer 14 (cathode material, cathode). The FIB 10 also includes an anode current collector 11 (anode) that collects current from the anode active material layer 12, and a cathode current collector 15 (cathode) that collects current from the cathode active material layer 14.

The constituent material of the current collector is not particularly limited as long as it is a conductive material. Specific examples of current collectors that can be used include aluminum (Al), other metal current collectors used in known secondary batteries, and resin current collectors with a conductive resin layer. Current collectors can be in the form of, for example, foil, mesh, or porous.

The FIB 10 may also include a battery case (not shown) that houses the anode current collector 11, anode active material layer 12, solid electrolyte layer 13, cathode active material layer 14, and cathode current collector 15. The battery case may be made of a conventionally known material.

The FIB in this disclosure is a secondary battery that can be repeatedly charged and discharged, and is useful, for example, as an automotive battery, a stationary battery, a medical battery, a consumer battery, etc. The shape of the FIB 10 in this disclosure is not particularly limited, and known shapes such as a coin shape, a laminated shape, a cylindrical shape, and a rectangular shape can be used.

[Negative electrode active material layer]
The negative electrode active material layer 12 is made of a negative electrode material containing a negative electrode active material.

The negative electrode active material is not particularly limited, and known materials generally used in FIB can be used. Specific examples of the negative electrode active material include metals, alloys, metal oxides, and fluorides thereof, each of which has a lower potential than the positive electrode active material described below, and preferably includes, for example, La, LaF _x , Mg, MgF _x , Pb, PbF _x , Al, AlF _x , Ce, CeF _x , Ca, and CaF _x (where x is a real number greater than 0). The negative electrode active material may be one of the above materials, or two or more of them.

The negative electrode active material layer 12 may further contain at least one of a conductive additive, a solid electrolyte, and a binder, as needed. The conductive additive is not particularly limited as long as it is a material that has electronic conductivity, and can be a material commonly known in FIB, such as acetylene black, carbon black, or graphene. The solid electrolyte can be the same material as the solid electrolyte contained in the solid electrolyte layer 13, which will be described later. The binder is not particularly limited and can be a material commonly known in FIB, such as a fluorine-based binder such as polyvinylidene fluoride or polytetrafluoroethylene.

The contents of the negative electrode active material and conductive additive in the negative electrode active material layer 12 are not particularly limited and can be amounts generally known in FIB. Specifically, for example, the content of the negative electrode active material can be 10% by mass or more and 90% by mass or less, and the content of the conductive additive can be 20% by mass or less.

[Solid electrolyte layer]
The solid electrolyte layer 13 is disposed between the negative electrode active material layer 12 and the positive electrode active material layer 14. The solid electrolyte layer 13 contains a solid electrolyte having fluoride ion conductivity.

The solid electrolyte is not particularly limited, and known materials commonly used in FIBs can be used. Specific 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 at least one metal element selected from the group consisting of La, Ba, _Pb , _Sn , Ca, and Ce. Specific examples of the solid electrolyte include _{Ba0.03La0.97F2.97} and _PbSnF4 . The shape of the solid electrolyte is not particularly limited, and may be, for example, particulate.

The solid electrolyte layer 13 may contain a binder or other material in addition to the solid electrolyte. In this case, the solid electrolyte content in the solid electrolyte layer 13 may be the content commonly used in secondary batteries, particularly FIBs.

[Cathode active material layer]
The positive electrode active material layer 14 is made of a positive electrode material containing a positive electrode active material.

In the present disclosure, the positive electrode active material is composed of a carbon material having a lower fluorine content than Stage 1 C _x F, and a fluoride catalyst which is at least one of a metal fluoride and HF.

- Carbon materials -
The carbon material is not particularly limited as long as it is a host material for fluoride ions and has a lower fluorine content than stage 1 C _x F.

C _x F is a compound in which fluoride ions are inserted between graphite layers by reacting with F ₂ at low temperatures in the presence of a catalyst.

According to the Carbon Glossary, the stage structure of graphite intercalation compounds is defined as "a structure in which intercalated chemical species (intercalates) are regularly distributed in the c-axis direction according to their concentration, and is expressed by the number of stages n," where n represents the number of carbon layers sandwiched between intercalated layers.

That is, "Stage 1 C _x F" means C _x F containing a fluorine layer between every layer of graphite. Stage 2 C _x F contains a fluorine layer between every other layer, and Stage 3 C _x F contains a fluorine layer between every third layer. In Stage 1 C _x F, the value of x is approximately 4 or less.

Therefore, a carbon material having a lower fluorine content than Stage 1 C _x F refers to, for example, at least one of a carbon material into which fluoride ions have not been actively introduced by the reaction with F ₂ as described above (including a carbon material in its initial state before charging/discharging) and C _x F in which the value of x exceeds 4.

Specific examples of the carbon material into which fluoride ions have not been actively introduced by a reaction with _F2 or the like include graphene-like graphite (also referred to as "GLG" in this specification) described below, a carbon material containing GLG, graphite, acetylene black, hard carbon, ketjen black, carbon black, graphene, and carbon nanotubes, and is preferably at least one selected from the group consisting of GLG, a carbon material containing GLG, graphite, acetylene black, and hard carbon.

Furthermore, C _x F in which the value of x exceeds 4 is C _x F of a stage structure in which the value of n is 2 or more, such as stage 2 type or stage 3 type. Such C _x F with a low fluorine content can be synthesized more easily than stage 1 type C _x F by reaction with _F2 at low pressure or in a short time or by electrolysis in an aqueous HF solution.

The fluorine content in the positive electrode material (the value of X in C _x F) can be determined by the following method. The positive electrode material is separated from the solid electrolyte, pressed onto a Ni mesh, and dried. This is placed in a two-electrode cell using, for example, a 1 M LiPF ₆ solution (the solvent is a 1:1 volume ratio of ethylene carbonate (EC):dimethyl carbonate (DMC)) as the electrolyte and Li metal as the counter electrode, and constant current discharge is performed to determine the discharge capacity up to 1.5 V. This makes it possible to determine the fluorine content in the positive electrode material (the value of X in C _x F).

Figure 2 shows an example of the molecular structure of GLG. As shown in Figure 2, GLG, designated by the reference numeral 20, is a layered compound whose constituent elements are carbon atoms 21 and oxygen atoms 22 covalently bonded to the carbon atoms 21, and which has nanometer-order pores 23 formed in the layer. GLG can be prepared by subjecting graphite oxide to heat treatment. It is believed that oxidation of the graphite replaces some of the carbon atoms with oxygen atoms, and heat treatment removes some of the oxygen atoms, forming the pores 23.

The carbon material is preferably GLG or a carbon material containing GLG. Specific examples of carbon materials containing GLG include mixtures of GLG with other carbon materials, and layered compounds with an outer layer made of GLG and an inner layer made of graphite.

Because GLG is a partially oxidized carbon material, its anion insertion potential is lower than that of carbon materials such as graphite, which are made up of only carbon atoms. This makes it easier for fluoride ions to be inserted, facilitating the charging reaction of the carbon material.

When GLG or a carbon material containing GLG is used as the carbon material, the interlayer distance of the GLG is preferably larger than the interlayer distance of graphite. As mentioned above, the interlayer distance of graphite is generally known to be 0.335 nm. Because GLG contains covalently bonded oxygen atoms, the interlayer distance tends to be larger than that of graphite. The larger interlayer distance facilitates the insertion and desorption of fluoride ions, which can improve the cycle characteristics of the FIB.

The interlayer distance of the layered compound is preferably greater than 0.335 nm, more preferably 0.340 nm or greater, and particularly preferably 0.360 nm or greater.

- Fluoride catalyst -
The fluoride catalyst has the function of suppressing the formation of a fluoride insulating layer on the surface of the carbon material during charging and discharging of the FIB, and is at least one of a metal fluoride and HF.

The metal fluoride is not particularly limited as long as it has fluoride ion acceptability, but is preferably at least one selected from the group consisting of LiF, AgF, AlF ₃ , CuF ₂ , KF, KHF ₂ and CaF _2. According to this configuration, the charging reaction of the carbon material can proceed quickly.

When using HF as the fluoride catalyst, HF derived from water contained in the air may be used, or HF may be actively added. When using HF derived from water contained in the air, for example, the electrodes may be prevented from drying out, leaving water behind, and the HF produced when water is oxidized in the cell may be used.

It should be noted that the fluoride catalyst of the present disclosure does not contain C _x F. Since a fluoride insulating layer is formed on the surface of C _x F, it is considered unlikely that C _x F will function as a catalyst even if added.

-Combination drug-
The positive electrode material is preferably a mixture containing a positive electrode active material and a solid electrolyte, and more preferably a mixture consisting of a positive electrode active material and a solid electrolyte.

By including a solid electrolyte in the mixture, the fluoride ion conductivity of the positive electrode material is improved. The solid electrolyte contained in the mixture may be the same as or different from the solid electrolyte contained in the solid electrolyte layer 13, but it is preferable that they are the same.

It is preferable that the content of the carbon material contained in the mixture be greater, by mass, than the content of the fluoride catalyst contained in the mixture.

Specifically, the content of the carbon material in the mixture is preferably 10% by mass or more and 50% by mass or less, more preferably 12% by mass or more and 48% by mass or less, and particularly preferably 15% by mass or more and 45% by mass or less.

If the content of carbon material in the mixture is below the lower limit, it will be difficult to obtain sufficient capacity. On the other hand, if the content exceeds the upper limit, the amount of solid electrolyte will be insufficient, making it difficult to ensure sufficient fluoride ion conductivity.

Furthermore, the content of the fluoride catalyst contained in the mixture is preferably 1% by mass or more and 5% by mass or less, more preferably 1.5% by mass or more and 4.5% by mass or less, and particularly preferably 2% by mass or more and 4% by mass or less.

If the content of the fluoride catalyst in the mixture is below the lower limit, the charging reaction may not proceed, making it impossible to charge and discharge the FIB. On the other hand, if the content exceeds the upper limit, the amount of solid electrolyte will be insufficient, making it difficult to ensure sufficient fluoride ion conductivity.

By setting the content of carbon material and fluoride catalyst in the mixture within the above ranges, an FIB with higher capacity and better cycle characteristics can be obtained.

The mixture may be molded as is to form the positive electrode active material layer 14, or may be mixed with a conductive additive, binder, etc. to form the positive electrode active material layer 14. The conductive additive, binder, etc. may be the same materials as those used in the negative electrode active material layer 12. When a conductive additive, binder, etc. is blended into the positive electrode active material layer 14, the content of these may be the same as that of the negative electrode active material layer 12.

-Action and effect-
Carbon materials are resistant to oxidation, so a high voltage is required to insert fluoride ions. However, when a high voltage is applied to insert fluoride ions, a strong C—F bond is formed between the carbon atoms on the surface of the carbon material and the fluoride ions. Then, a fluoride insulating layer is formed on the surface of the carbon material, causing passivation. This inhibits the insertion of fluoride ions into the carbon material, preventing the charging reaction from proceeding, making charging and discharging impossible.

On the other hand, when a fluoride catalyst is added to a carbon material, the carbon atoms of the carbon material and the cations of the fluoride catalyst compete for fluoride ions, suppressing the formation of strong C-F bonds between the carbon atoms on the surface of the carbon material and the fluoride ions. This suppresses the formation of a fluoride insulating layer on the surface of the carbon material, allowing fluoride ions to be inserted into the carbon material, allowing the charging reaction to proceed. Furthermore, the presence of the fluoride catalyst suppresses the formation of a fluoride insulating layer on the surface of the carbon material, enabling the fluoride ions to be released from the carbon material, making charging and discharging possible overall.

That is, according to the configuration of the present disclosure, by using a cathode material containing a carbon material with a lower fluorine content than Stage 1 _CxF and a fluoride catalyst as cathode active materials, a FIB can be produced that can exhibit higher voltage, higher capacity, and better cycle characteristics than FIBs that use conventional metal fluorides as cathode active materials. Furthermore, a carbon material with a lower fluorine content than Stage 1 _CxF is not treated to introduce fluoride ions like Stage 1 _CxF , and therefore the amount of fluoride insulating layer formed on the surface is originally smaller than that of Stage 1 _{CxF .} This eliminates the need to remove the surface fluoride insulating layer, which may reduce the amount of fluoride catalyst added and improve discharge voltage compared to when Stage 1 _CxF is used as the cathode active material. Furthermore, a carbon material with a lower fluorine content than Stage 1 _CxF can be manufactured more easily and at lower cost than Stage 1 CxF, which makes FIB manufacturing easier and more cost-effective.

<Method of manufacturing positive electrode material>
The positive electrode material can be produced, for example, by the following procedure, although this is not intended to be limiting.

That is, one example of a method for manufacturing a positive electrode material includes a preparation step of preparing a carbon material, and a mixing step of mixing the carbon material with a fluoride catalyst to obtain a positive electrode active material.

In the preparation process, a commercially available carbon material is prepared, or if necessary, a carbon material is synthesized.

For example, if the carbon material is GLG, in the preparation process, graphite powder is oxidized and then heat-treated to obtain GLG.

The method for oxidizing graphite is not particularly limited, and known methods can be used, such as the Brodie method using fuming nitric acid and _KClO3 , the Hummers method using _H2SO4 , _NaNO3 , _{and KMnO4} , and the Staudenmeier method using _H2SO4 , _HNO3 , and _KClO3 . The graphite oxidation method affects the oxygen content and interlayer distance of _the GLG.

The heat treatment temperature of graphite oxide also affects the oxygen content and interlayer distance of the GLG. The heat treatment temperature is, for example, 100°C or higher and 1000°C or lower, preferably 200°C or higher and 900°C or lower, and more preferably 300°C or higher and 500°C or lower.

The heat treatment time for graphite oxide is not intended to be limited, but can be, for example, 2 hours or more, preferably 3 hours or more and 10 hours or less, and more preferably 4 hours or more and 8 hours or less.

The heat treatment of graphite oxide is preferably carried out under vacuum or in an inert gas atmosphere to prevent further oxidation.

In the mixing step, the carbon material and the fluoride catalyst are mixed. Preferably, in the mixing step, the carbon material, the fluoride catalyst, and the solid electrolyte are mixed to obtain a mixture. This configuration increases the contact area between the carbon material, the fluoride catalyst, and, if added, the solid electrolyte, which is advantageous for electronic conductivity and fluoride ion conductivity. The mixing method is not particularly limited, and known methods such as manual mixing using a mortar or a mixer can be used.

<FIB manufacturing method>
The FIB manufacturing method is not particularly limited as long as the above-mentioned cathode material, solid electrolyte, and anode material are used, and any known method can be used.

Next, we will explain specific examples that were implemented.

Table 1 shows the composition of the positive electrode active materials for Examples 1 to 9 and Comparative Example 1.

Example 1
The positive electrode active material used was GLG as a carbon material, lithium fluoride (LiF) as a fluoride catalyst, and barium lanthanum fluoride (Ba _0.03 La _0.97 F _2.97 , hereinafter also referred to as “BLF”) as a solid electrolyte of the positive electrode material.

Pb and PbSnF ₄ were used as the negative electrode, and BLF was used as the solid electrolyte of the solid electrolyte layer.

[Material preparation]
-GLG-
10 g of graphite powder (Z-5F manufactured by Ito Graphite Industries Co., Ltd., average particle size 5 μm) was placed in a 500 mL beaker, 200 mL of fuming nitric acid was added, and the mixture was heated to 60°C. 80 g of potassium chlorate was then slowly added with stirring, and the mixture was maintained for 3 hours. The reaction solution was then transferred to 2 L of water, suction filtered, and washed with pure water until the pH reached 5 or higher, to obtain graphite oxide (Brodie method).

The resulting graphite oxide was dried overnight at 60°C. This graphite oxide powder was placed in an alumina container and heated to a heat treatment temperature of 300°C at a rate of 1°C/min under vacuum, then held for 5 hours to obtain GLG.

The GLG was identified and the interlayer distance was measured using X-ray diffraction (Bruker D2 Phaser, CuKα).

The oxygen content in the GLG was determined by elemental analysis (Flash Smart, manufactured by ThermoFischer Scientific).

-LiF-
Commercially available LiF (manufactured by Nacalai Tesque Co., Ltd.) was used.

-BLF-
BaF ₂ (Apollo Science) and LaF ₃ (Kanto Chemical) were mixed in a ratio of 7:93, placed in a zirconia cup (45 mL) together with 60 g of 1 mm diameter zirconia balls, and ball milled at 500 rpm for 5 minutes using a Fritsch Premiumline P7. This cycle of 5 minutes of rest was repeated 72 times to obtain a BLF.

_{-PbSnF4-
PbSnF4} was prepared by heat treating _PbF2 and _SnF2 at 200 °C under argon.

[Battery manufacturing]
GLG, LiF, and BLF were mixed in a mortar in a mass ratio of 8:1:30 under atmospheric pressure. 1.5 _g of PbSnF4 and 0.3 g of BLF were placed in a mold (diameter 20 mm), in that order, and the mixture of GLG, LiF, and BLF was then added and pressed at a pressure of 240 MPa for 10 minutes to obtain a three-layer pellet. This three-layer pellet was installed in a cell (manufactured by Nippon Tomcell Co., Ltd.) containing a Pb plate as a negative electrode current collector, with the Pb plate positioned on the _PbSnF4 side.

[Charge/discharge measurement]
The cell was charged using a potentiostat/galvanostat (Hokuto Denko Corporation) by voltage scanning from the open circuit voltage to 2.6 V at 75°C at 3 mV/min, followed by holding at 2.6 V for 10 hours. Charge and discharge operations were then performed at a constant current of 50 μA during discharge and 100 μA during charge. The cutoff voltage had a lower limit of 0.0 V and an upper limit of 2.4 V. The charge and discharge curves are shown in Figure 3.

Example 2
A battery was fabricated and subjected to charge-discharge measurements under the same conditions and procedures as in Example 1, except that the heat treatment temperature during preparation of GLG was set to 400° C. and the upper limit of the cutoff voltage during charge-discharge measurements was set to 2.0 V. The charge-discharge curves are shown in FIG.

Example 3
A battery was fabricated and subjected to charge-discharge measurements under the same conditions and procedures as in Example 1, except that the heat treatment temperature during preparation of GLG was set to 500° C. and the upper limit of the cutoff voltage during charge-discharge measurements was set to 2.0 V. The charge-discharge curves are shown in FIG.

Example 4
Graphite oxide was obtained by the Hummers method, in which 30 g of potassium permanganate was added to a solution of 5 g of graphite, 2.5 g of sodium nitrate, and 115 mL of sulfuric acid to oxidize the graphite, and a battery was fabricated and subjected to charge-discharge measurements under the same conditions and procedures as in Example 1, except that the upper limit of the cutoff voltage during charge-discharge measurements was set to 2.0 V. The charge-discharge curves are shown in FIG.

Example 5
A battery was fabricated under the same conditions and procedures as in Example 1, except that AgF was used as the fluoride catalyst instead of LiF, and charge/discharge measurements were carried out. The charge/discharge curves are shown in FIG.

Example 6
A battery was fabricated under the same conditions and procedures as in Example 1, except that _KHF2 was used as the fluoride catalyst instead of LiF, and charge/discharge measurements were carried out. The charge/discharge curves are shown in Figure 8.

Example 7
A battery was fabricated under the same conditions and procedures as in Example 1, except that commercially available acetylene black (manufactured by Denka Corporation) was used as the carbon material instead of GLG, and charge/discharge measurements were carried out. The charge/discharge curves are shown in FIG.

Example 8
A battery was fabricated under the same conditions and procedures as in Example 1, except that graphite powder, which is a raw material for GLG, was used as the carbon material instead of GLG, and charge/discharge measurements were carried out. The charge/discharge curves are shown in FIG.

Example 9
A battery was fabricated under the same conditions and procedures as in Example 1, except that commercially available hard carbon (manufactured by Kureha Corporation) was used as the carbon material instead of GLG, and charge/discharge measurements were carried out. The charge/discharge curves are shown in FIG.

<Comparative Example 1>
A battery was fabricated under the same conditions and procedures as in Example 1, except that the fluoride catalyst LiF was not added, and charge/discharge measurements were carried out. The charge/discharge curves are shown in FIG.

<Charge/discharge characteristics>
As shown in FIG. 12, in the cell of Comparative Example 1 in which no fluoride catalyst LiF was added, the capacity was almost zero even after charge and discharge operations.

On the other hand, as shown in Figure 3, in the cell of Example 1, which contained the fluoride catalyst LiF, the voltage during discharge in the first cycle began to decrease gradually from around 0.9 V, then decreased somewhat more rapidly from around 0.6 V, and the discharge capacity reached 93.3 mAh/g. Meanwhile, during charge, the voltage increased gradually from around 0.8 V and then increased sharply above 1.7 V. The charge capacity at this point was 170 mAh/g. From the second cycle onwards, the discharge curve was almost the same as that of the first cycle, and even at the 10th cycle, the discharge capacity was 90.7 mAh/g, maintaining 97.2% of the initial capacity. Furthermore, the voltage decreased slightly at the beginning of charge, and the capacity above 1.5 V gradually decreased with each cycle.

Comparing the results of Comparative Example 1 and Example 1, it was found that when GLG was used alone as the positive electrode active material, it was difficult to insert fluoride ions into GLG. However, adding a fluoride catalyst (LiF) to GLG made it possible to insert fluoride ions into GLG. This is believed to be due to the following mechanism. Specifically, when GLG alone in Comparative Example 1 was used, charging caused strong C-F bonds to form between the carbon atoms on the GLG surface and the fluoride ions. This formed a fluoride insulating layer on the GLG surface, which is thought to inhibit the insertion of fluoride ions between the GLG layers. On the other hand, when LiF was added to GLG in Example 1, the carbon atoms of GLG and the lithium ions of LiF competed for fluoride ions, inhibiting the formation of strong C-F bonds between the carbon atoms on the GLG surface and the fluoride ions. It is believed that this suppresses the formation of a fluoride insulating layer on the surface of the GLG, and allows fluoride ions to be inserted between the layers inside the GLG.

In addition, as shown in Table 1, the oxygen content of the GLG in Examples 2 to 4 differs from that in Example 1. As shown in Figures 4 to 6, charge/discharge curves similar to those in Figure 3 for Example 1 were obtained in Examples 2 to 4. The oxygen content of the GLG increased in the order of Example 3, Example 2, Example 1, and Example 4, and as shown in Figures 3 to 6, it was found that the capacity tended to increase as the oxygen content of the GLG increased.

Although Examples 5 and 6 differ from Example 1 in the fluoride catalyst, when AgF (Example 5) or KHF ₂ (Example 6) was used as the fluoride catalyst instead of LiF, charge-discharge curves similar to those in Figure 3 of Example 1 were obtained, as shown in Figures 7 and 8. Note that in both Examples 5 and 6, the discharge capacity at the first cycle was small, but the discharge capacity increased from the second cycle onwards, and the cycle characteristics were found to be relatively good.

In Example 7, acetylene black was used as the carbon material. As shown in Figure 9, the discharge capacity in the first cycle was small at 30 mAh/g, but long plateaus were observed at 0.5 V and 1.6 V during charging, followed by a sharp rise in voltage. Discharging in the second cycle was not possible. The results of Example 7 demonstrate that even when acetylene black is used as the carbon material, charge and discharge are possible by adding a fluoride catalyst. Note that, although the cycle characteristics were insufficient under the experimental conditions in Example 7, there is thought to be room for improvement in the cycle characteristics by adjusting the experimental conditions.

Examples 8 and 9 use graphite and hard carbon, respectively, as the carbon material. As shown in Figures 10 and 11, when graphite (Example 8) or hard carbon (Example 9) is used, charge and discharge are possible by adding a fluoride catalyst. As with Example 7, there is likely room for improvement in cycle characteristics by adjusting the experimental conditions.

This disclosure is extremely useful because it provides a positive electrode material for an all-solid-state fluoride ion shuttle battery that can exhibit higher voltage, higher capacity, and higher cycle characteristics and can be manufactured easily and at low cost, a manufacturing method for the same, and an all-solid-state fluoride ion shuttle battery that uses the positive electrode material.

10 All-solid-state fluoride ion shuttle battery 11 Negative electrode current collector (negative electrode)
12 Negative electrode active material layer (negative electrode)
13 Solid electrolyte layer (solid electrolyte)
14 Positive electrode active material layer (positive electrode material, positive electrode)
15 Positive electrode current collector (positive electrode)
20. Graphene-like graphite (layered compound)
21 Carbon atom 22 Oxygen atom 23 Hole

Claims (9)

A cathode material for use in an all-solid-state fluoride ion shuttle battery, comprising:
The positive electrode material is characterized in that the positive electrode active material contained therein comprises a carbon material having a lower fluorine content than a stage 1 type fluorine-graphite intercalation compound, and a fluoride catalyst which is at least one of a metal fluoride and HF. 2. The positive electrode material according to claim 1, wherein the positive electrode material is a mixture containing the positive electrode active material and a solid electrolyte. the content of the carbon material contained in the mixture is 10% by mass or more and 50% by mass or less,
The cathode material according to claim 2 , wherein the content of the fluoride catalyst contained in the mixture is 1% by mass or more and 5% by mass or less. The positive electrode material according to any one of claims 1 to 3, characterized in that the metal fluoride is at least one selected from the group consisting of LiF, AgF, AlF ₃ , CuF ₂ , KF, KHF ₂ and CaF ₂ . The positive electrode material according to any one of claims 1 to 4, characterized in that the carbon material contains a layered compound having, as constituent elements, carbon atoms and oxygen atoms covalently bonded to the carbon atoms. The positive electrode material according to claim 5 , wherein the interlayer distance of the layered compound is greater than the interlayer distance of graphite. A method for producing the positive electrode material according to any one of claims 1 to 6,
a preparation step of preparing the carbon material;
a mixing step of mixing the carbon material with the fluoride catalyst to obtain the positive electrode active material. the carbon material is a layered compound containing carbon atoms and oxygen atoms covalently bonded to the carbon atoms as constituent elements,
8. The method for producing a positive electrode material according to claim 7, wherein in the preparation step, graphite powder is oxidized and then heat-treated to obtain the layered compound. a positive electrode, a negative electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode;
An all-solid-state fluoride ion shuttle battery, characterized in that the positive electrode comprises the positive electrode material according to any one of claims 1 to 6.