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

Abstract

To provide 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 easily manufactured at low cost, a method for manufacturing the same, and an all-solid-state fluoride ion shuttle battery using the positive electrode material.SOLUTION: A positive electrode material includes a positive electrode active material and used in an all-solid-state fluoride ion shuttle battery. The positive electrode active material included in the positive electrode material consists of a carbon material having a lower fluorine content than the stage 1 type fluorine-graphite intercalation compound, and a fluoride catalyst that is at least one of metal fluoride and HF.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a cathode 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 cathode material. [Background technology]

[0002] 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 use 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).

[0003] In addition, 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 (see, for example, Patent Document 1 and Non-Patent Document 2). If fluoride ions can be introduced into these materials up to a saturated composition, they are expected to exhibit a high capacity of 2232 mAh / g. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2009-529222 [Non-patent literature]

[0005] [Non-Patent Document 1] M. Anji Reddy, et al, J. Mater. Chem., 2011, 21, 17059 [Non-patent document 2] Y. Matsuo, et al, Electrochem. Commun., 2020, 110 106626 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when metal fluorides such as CuF2 and BiF3 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 a material that complements these properties must be added to the positive electrode in addition to the active material. Furthermore, the large structural changes that occur during charging and discharging pose problems with power density and cycle characteristics.

[0007] 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 been reported.

[0008] And, C described in Non-Patent Document 2 x F is stage 1 C x Although it can be charged and discharged by adding CuF2, there are problems such as the need for a relatively large amount of CuF2 and the fact that the discharge voltage is significantly lower than the theoretical discharge voltage, so there is room for improvement. x F can be obtained by inserting fluorine into the raw material graphite using fluorine gas at low temperatures in the presence of a catalyst, but the reaction takes a long time and is costly.

[0009] Therefore, the present 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 can be produced easily and at low cost, a method for producing the same, and an all-solid-state fluoride ion shuttle battery using the positive electrode material. [Means for solving the problem]

[0010] In order to solve the above-mentioned 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 having a lower fluorine content than a stage 1 fluorine-graphite intercalation compound, and a fluoride catalyst which is at least one of a metal fluoride and HF.

[0011] Carbon materials are difficult to oxidize, so a high voltage is required to insert fluoride ions. In addition, to generate strong C—F bonds, an insulating fluoride layer is formed on the surface of the carbon material, making it prone to passivation.

[0012] Here, Non-Patent Document 2 states that adding CuF2 can produce stage 1 type C x It is reported that the discharge reaction of F proceeds. The mechanism is thought to be as follows: x Although the above-mentioned fluoride insulating layer is formed on the surface of F, Cu generated by the discharge of CuF2 is x The fluoride insulating layer on the surface of F is removed by reducing and decomposing it, and C x It is believed that the discharge reaction of F proceeds.

[0013] The inventors of the present application have x Based on the mechanism of discharge reaction of F, stage 1 C x We came 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, which have a lower fluorine content (molar ratio) than F. We then discovered that the charging reaction can proceed even in such carbon materials by adding a fluoride catalyst, which is at least one of a metal fluoride and HF.

[0014] That is, according to this configuration, the stage 1 type C xBy using a positive electrode material in which a carbon material having a lower fluorine content than F and the above-mentioned fluoride catalyst are used as the positive electrode active material, it is possible to obtain an FIB that can exhibit higher voltage, higher capacity, and higher cycle characteristics than conventional FIBs that use metal fluorides as the positive electrode active material. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Unlike F, the treatment to introduce fluoride ions has not been applied, or has not been applied strongly, so it is a stage 1 type C. x Compared to F, the amount of fluoride insulating layer formed on the surface is originally small. Therefore, there is no need to remove the fluoride insulating layer on the surface, so Stage 1 type C x Compared to using F as the positive electrode active material, it is possible to reduce the amount of fluoride catalyst added and improve the discharge voltage. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Compared to F, it can be prepared easily and at low cost, which makes FIB manufacturing easier and more cost-effective.

[0015] The positive electrode material is preferably a mixture containing the positive electrode active material and a solid electrolyte.

[0016] By using a mixture containing a solid electrolyte, the fluoride ion conductivity of the positive electrode material is improved.

[0017] The content of the carbon material contained in the mixture is preferably 10% by mass or more and 50% by mass or less, and the content of the fluoride catalyst contained in the mixture is preferably 1% by mass or more and 5% by mass or less.

[0018] By setting the content of the carbon material and the fluoride catalyst in the mixture within the above ranges, an FIB with higher capacity and higher cycle characteristics can be obtained.

[0019] The metal fluoride is preferably at least one selected from the group consisting of LiF, AgF, AlF3, CuF2, KF, KHF2, and CaF2.

[0020] According to this configuration, the charging reaction of the carbon material can proceed quickly.

[0021] The carbon material preferably contains a layered compound having, as constituent elements, a carbon atom and an oxygen atom covalently bonded to the carbon atom.

[0022] According to this configuration, since the carbon material is a partially oxidized carbon material, the anion insertion potential is lower than that of, for example, graphite, which is composed only of carbon atoms. This makes it easier to insert fluoride ions, and the charging reaction of the carbon material progresses more easily.

[0023] The interlayer distance of the layered compound is preferably larger than the interlayer distance of graphite.

[0024] 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 FIB.

[0025] One aspect of the method for producing a positive electrode material disclosed herein is the 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.

[0026] According to this configuration, by mixing the carbon material and the fluoride catalyst, the contact area between them increases, which is advantageous for the electron conductivity and the fluoride ion conductivity.

[0027] The carbon material is a layered compound having carbon atoms and oxygen atoms covalently bonded to the carbon atoms as constituent elements, and in the preparation step, the layered compound is preferably obtained by oxidizing graphite powder and then subjecting it to a heat treatment.

[0028] 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.

[0029] One aspect 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.

[0030] This configuration provides 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. Furthermore, the FIB can be manufactured more easily and cost-effectively. [Effects of the Invention]

[0031] As described above, according to the present disclosure, stage 1 C x By using a positive electrode material in which a carbon material having a lower fluorine content than F and the above-mentioned fluoride catalyst are used as the positive electrode active material, it is possible to obtain an FIB that can exhibit higher voltage, higher capacity, and higher cycle characteristics than conventional FIBs that use metal fluorides as the positive electrode active material. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Unlike F, it has not been treated to introduce fluoride ions, so it is a stage 1 type C xCompared to F, the amount of fluoride insulating layer formed on the surface is originally small. Therefore, there is no need to remove the fluoride insulating layer on the surface, so Stage 1 type C x Compared to using F as the positive electrode active material, the amount of fluoride catalyst added can be reduced and the discharge voltage can be improved. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Compared to F, it can be manufactured easily and at low cost, so FIB manufacturing is also easy and advantageous in terms of cost. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a diagram showing the configuration of an all-solid-state fluoride ion shuttle battery according to one embodiment. [Figure 2] FIG. 1 is a diagram showing an example of the molecular structure of graphene-like graphite. [Figure 3] 3 is a graph showing charge / discharge curves of the battery cell of Example 1. [Figure 4] 6 is a graph showing charge / discharge curves of the battery cell of Example 2. [Figure 5] 6 is a graph showing charge / discharge curves of the battery cell of Example 3. [Figure 6] 10 is a graph showing charge / discharge curves of the battery cell of Example 4. [Figure 7] 10 is a graph showing charge / discharge curves of the battery cell of Example 5. [Figure 8] 10 is a graph showing charge / discharge curves of the battery cell of Example 6. [Figure 9] 10 is a graph showing charge / discharge curves of the battery cell of Example 7. [Figure 10] 10 is a graph showing charge / discharge curves of the battery cell of Example 8. [Figure 11] 10 is a graph showing charge / discharge curves of the battery cell of Example 9. [Figure 12] 4 is a graph showing charge / discharge curves of a battery cell of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the present disclosure, its applications, or its uses.

[0034] (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.

[0035] The constituent material of the current collector is not particularly limited as long as it is a conductive material. Specific examples of the current collector include aluminum (Al), other metal current collectors used in known secondary batteries, and resin current collectors having a conductive resin layer. Examples of the shape of the current collector include foil, mesh, and porous.

[0036] The FIB 10 may also include a battery case (not shown) that houses the negative electrode current collector 11, the negative electrode active material layer 12, the solid electrolyte layer 13, the positive electrode active material layer 14, and the positive electrode current collector 15. The material of the battery case may be a conventionally known material.

[0037] The FIB in the present disclosure is a secondary battery that can be repeatedly charged and discharged, and is useful as, for example, an in-vehicle battery, a stationary battery, a medical battery, a consumer battery, etc. The shape of the FIB 10 in the present disclosure is not particularly limited, and known shapes such as a coin shape, a laminate shape, a cylindrical shape, and a rectangular shape can be used.

[0038] [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.

[0039] 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, such as La, LaF. x , Mg, MgF x , Pb, PbF x , Al, AlF x , Ce, CeF x , Ca, 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.

[0040] 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 having electronic conductivity, and may be, for example, a material generally known in the FIB field, such as acetylene black, carbon black, or graphene. The solid electrolyte may be the same material as the solid electrolyte contained in the solid electrolyte layer 13 described below. The binder is not particularly limited and may be a material generally known in the FIB field, such as a fluorine-based binder, such as polyvinylidene fluoride or polytetrafluoroethylene.

[0041] The contents of the negative electrode active material and the 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.

[0042] [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.

[0043] The solid electrolyte is not particularly limited, and known materials generally 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 La, Ba, Pb, Sn, Ca, and Ce. Specific examples of the solid electrolyte include Ba. 0.03 La 0.97 F 2.97 , PbSnF4, etc. The shape of the solid electrolyte is not particularly limited, but examples thereof include particulate forms.

[0044] The solid electrolyte layer 13 may contain a binder or the like other than the solid electrolyte. In this case, the content of the solid electrolyte in the solid electrolyte layer 13 may be a content generally adopted in secondary batteries, particularly in FIB.

[0045] [Cathode active material layer] The positive electrode active material layer 14 is made of a positive electrode material containing a positive electrode active material.

[0046] In the present disclosure, the positive electrode active material is a stage 1 type C x The catalyst is composed of a carbon material having a lower fluorine content than F, and a fluoride catalyst which is at least one of a metal fluoride and HF.

[0047] -Carbon materials- Carbon materials are host materials for fluoride ions, and stage 1 C x There are no particular limitations on the carbon material as long as it has a lower fluorine content than F.

[0048] C x F is a compound in which fluoride ions are inserted between graphite layers by reacting with F2 at low temperatures in the presence of a catalyst.

[0049] 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.

[0050] That is, "Stage 1 C x "F" means that fluorine layers are contained between all layers of graphite. x F. Also, stage 2 C x In F, every other layer is stage 3 type C x In F, every third layer contains a fluorine layer. x In F, the value of x is approximately 4 or less.

[0051] Therefore, stage 1 C x Carbon materials with a lower fluorine content than F include, for example, carbon materials into which fluoride ions have not been actively introduced by the reaction with F as described above (including carbon materials in the initial state before charge / discharge) and carbon materials with a value of x exceeding 4. x At least one of F.

[0052] Specific examples of carbon materials into which fluoride ions have not been actively introduced by, for example, a reaction with F as described above include graphene-like graphite (also referred to as "GLG" in this specification), a carbon material containing GLG, graphite, acetylene black, hard carbon, ketjen black, carbon black, graphene, and carbon nanotubes, and are preferably at least one selected from the group consisting of GLG, a carbon material containing GLG, graphite, acetylene black, and hard carbon.

[0053] Also, C where x is greater than 4 x F is the C of the stage structure where the value of n is 2 or more, such as stage 2 type, stage 3 type, etc. x F. Such a low fluorine content C x F is stage 1 C xCompared to F, it can be synthesized more easily by reaction with F2 at low pressure or for a short time, or by electrolysis in aqueous HF.

[0054] Fluorine content in the positive electrode material (C x The value of X in 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 with, for example, a 1M LiPF6 solution (solvent: ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1 by volume) as the electrolyte and Li metal as the counter electrode, and constant current discharge is performed to determine the discharge capacity up to 1.5V. This allows the fluorine content (C x It is possible to determine the value of X in F.

[0055] An example of the molecular structure of GLG is shown in Figure 2. 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.

[0056] The carbon material is preferably GLG or a carbon material containing GLG. Specific examples of the carbon material containing GLG include a mixture of GLG and another carbon material, and a layered compound having an outer layer made of GLG and an inner layer made of graphite.

[0057] Because GLG is a partially oxidized carbon material, its anion insertion potential is lower than that of carbon materials such as graphite, which is composed only of carbon atoms. This facilitates the insertion of fluoride ions, facilitating the charging reaction of the carbon material.

[0058] When GLG or a carbon material containing GLG is used as the carbon material, the interlayer distance of GLG is preferably larger than that of graphite. As described above, the interlayer distance of graphite is generally known to be 0.335 nm. Because GLG contains covalent 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.

[0059] The interlayer distance of the layered compound is preferably more than 0.335 nm, more preferably 0.340 nm or more, and particularly preferably 0.360 nm or more.

[0060] -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.

[0061] 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, CuF, KF, KHF, and CaF. This configuration allows the charging reaction of the carbon material to proceed quickly.

[0062] When HF is used as the fluoride catalyst, HF derived from water contained in the air may be used, or HF may be actively added. When HF derived from water contained in the air is used, for example, the HF generated by the oxidation of water in the cell may be used by suppressing drying of the electrodes to leave water.

[0063] The fluoride catalyst of the present disclosure is C x Does not contain F. C x Since a fluoride insulating layer is formed on the surface of F, x Even if F is added, it is unlikely to function as a catalyst.

[0064] -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.

[0065] The positive electrode material has improved fluoride ion conductivity due to the use of a mixture containing a solid electrolyte. 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 is preferably the same as the solid electrolyte contained in the mixture.

[0066] The content of the carbon material contained in the mixture is preferably greater than the content of the fluoride catalyst contained in the mixture in terms of mass ratio.

[0067] 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.

[0068] If the content of the carbon material in the mixture is less than the lower limit, it becomes difficult to obtain a sufficient capacity, whereas if the content exceeds the upper limit, the amount of solid electrolyte becomes insufficient, making it difficult to ensure sufficient fluoride ion conductivity.

[0069] The content of the fluoride catalyst 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.

[0070] If the content of the fluoride catalyst contained in the mixture is less than the lower limit, the charging reaction may not proceed, and FIB charging and discharging may not be possible. On the other hand, if the content exceeds the upper limit, the amount of solid electrolyte is insufficient, making it difficult to ensure sufficient fluoride ion conductivity.

[0071] By setting the content of the carbon material and the fluoride catalyst in the mixture within the above ranges, an FIB with higher capacity and higher cycle characteristics can be obtained.

[0072] The mixture may be molded as it is to form the positive electrode active material layer 14, or may be mixed with a conductive additive, a 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 the positive electrode active material layer 14 contains a conductive additive, a binder, etc., the contents thereof may be the same as those used in the negative electrode active material layer 12.

[0073] -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. This forms an insulating fluoride layer on the surface of the carbon material, passivating it. This inhibits the insertion of fluoride ions into the carbon material, preventing the charging reaction from proceeding, making charging and discharging impossible.

[0074] 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, promoting the charging reaction. Furthermore, the presence of the fluoride catalyst suppresses the formation of a fluoride insulating layer on the surface of the carbon material, allowing fluoride ions to be released from the carbon material, enabling overall charging and discharging.

[0075] That is, according to the configuration of the present disclosure, stage 1 type C xBy using a positive electrode material in which a carbon material with a lower fluorine content than F and a fluoride catalyst are used as the positive electrode active material, it is possible to produce an FIB that can exhibit higher voltage, higher capacity, and higher cycle characteristics than conventional FIBs that use metal fluorides as the positive electrode active material. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Unlike F, it has not been treated to introduce fluoride ions, so it is a stage 1 type C x Compared to F, the amount of fluoride insulating layer formed on the surface is originally small. Therefore, there is no need to remove the fluoride insulating layer on the surface, so Stage 1 type C x Compared to using F as the positive electrode active material, the amount of fluoride catalyst added can be reduced and the discharge voltage can be improved. x Carbon materials with a lower fluorine content than F are known as stage 1 C x Compared to F, it can be manufactured easily and at low cost, so FIB manufacturing is also easy and advantageous in terms of cost.

[0076] <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.

[0077] That is, one example of a method for producing 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.

[0078] In the preparation step, a commercially available carbon material is prepared, or if necessary, a carbon material is prepared by synthesis.

[0079] For example, when the carbon material is GLG, in the preparation step, graphite powder is oxidized and then heat-treated to obtain GLG.

[0080] The method for oxidizing graphite is not particularly limited, and known methods can be used. Examples of known methods include the Brodie method using fuming nitric acid and KClO3, the Hummers method using H2SO4, NaNO3, and KMnO4, the Staudenmeier method using H2SO4, HNO3, and KClO3, and the like. Note that the method for oxidizing graphite affects the oxygen content and interlayer distance of GLG.

[0081] The heat treatment temperature of the oxidized graphite also affects the oxygen content and interlayer distance of 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, more preferably 300°C or higher and 500°C or lower.

[0082] The heat treatment time of the oxidized graphite is not intended to be limited, but can be, for example, 2 hours or longer, preferably 3 hours or longer and 10 hours or shorter, more preferably 4 hours or longer and 8 hours or shorter.

[0083] The heat treatment of the oxidized graphite is preferably carried out under vacuum or in an inert gas atmosphere from the viewpoint of suppressing further oxidation.

[0084] In the mixing step, the carbon material and the fluoride catalyst are mixed. In the mixing step, it is preferable to mix the carbon material, the fluoride catalyst, and the solid electrolyte to obtain a mixture. With this configuration, the contact area between each other in the carbon material, the fluoride catalyst, and the solid electrolyte when added increases, which is advantageous for electron conductivity and fluoride ion conductivity. The mixing method is not particularly limited, and known methods such as manual mixing using a mortar or the like can be adopted.

[0085] <Method for manufacturing FIB> The method for manufacturing FIB is not particularly limited as long as the above-mentioned positive electrode material, solid electrolyte, and negative electrode material are used, and known methods can be used.

Examples

[0086] Next, specific examples will be described.

[0087] Table 1 shows the configurations of the positive electrode active materials of Examples 1 to 9 and Comparative Example 1.

[0088] [Table 1]

[0089] Example 1 For the positive electrode active material, GLG was used as the carbon material and lithium fluoride (LiF) was used as the fluoride catalyst. In addition, barium lanthanum fluoride (Ba 0.03 La 0.97 F 2.97 , hereinafter referred to as "BLF") was used.

[0090] Pb and PbSnF4 were used as the negative electrode, and BLF was used as the solid electrolyte of the solid electrolyte layer.

[0091] [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 while 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).

[0092] The obtained graphite oxide was dried overnight at 60° C. This graphite oxide powder was placed in an alumina container, and the temperature was increased to a heat treatment temperature of 300° C. at a rate of 1° C. / min under vacuum, and then maintained at that temperature for 5 hours to obtain GLG.

[0093] The identification of GLG and the measurement of the interlayer distance were carried out using an X-ray diffraction method (D2 Phaser, CuKα, manufactured by Bruker).

[0094] The oxygen content in the GLG was determined by elemental analysis (Flash Smart, manufactured by ThermoFischer Scientific).

[0095] -LiF- Commercially available LiF (manufactured by Nacalai Tesque Co., Ltd.) was used.

[0096] -BLF- BaF2 (Apollo Science) and LaF3 (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, followed by a 5-minute rest cycle 72 times to obtain BLF.

[0097] -PbSnF4- PbSnF4 was prepared by heat treating PbF2 and SnF2 at 200°C under argon.

[0098] [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 (20 mm diameter), in that order, and the mixture of GLG, LiF, and BLF was then added. The mixture was pressed at a pressure of 240 MPa for 10 minutes to obtain a three-layer pellet. This three-layer pellet was then placed in a cell (manufactured by Nippon Tomcell Co., Ltd.) containing a Pb plate as a negative electrode current collector, with the Pb plate facing the PbSnF4 side.

[0099] [Charge / discharge measurement] The cell was charged using a potentiostat / galvanostat (Hokuto Denko Corporation) at 75°C, scanning voltage at 3 mV / min from the open circuit voltage to 2.6 V, and then held at 2.6 V for 10 hours. Charge and discharge operations were then performed at a constant current of 50 μA for discharge and 100 μA for charge. The cutoff voltage was set to 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.

[0100] <Example 2> A battery was fabricated and charge / discharge measurements were carried out 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 Figure 4.

[0101] Example 3 A battery was fabricated and charge / discharge measurements were carried out 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 Figure 5.

[0102] 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. A battery was fabricated 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, and charge-discharge measurements were carried out. The charge-discharge curves are shown in Figure 6.

[0103] <Example 5> Except for using AgF instead of LiF as the fluoride catalyst, a battery was fabricated under the same conditions and procedures as in Example 1, and charge / discharge measurements were carried out. The charge / discharge curves are shown in FIG.

[0104] 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 FIG.

[0105] Example 7 A battery was fabricated and subjected to charge-discharge measurements 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. The charge-discharge curves are shown in FIG.

[0106] Example 8 A battery was fabricated and subjected to charge-discharge measurements under the same conditions and procedures as in Example 1, except that graphite powder, which is the raw material for GLG, was used instead of GLG as the carbon material. The charge-discharge curves are shown in FIG.

[0107] Example 9 A battery was fabricated and charge / discharge measurements were carried out 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. The charge / discharge curves are shown in FIG.

[0108] <Comparative Example 1> A battery was fabricated under the same conditions and procedures as in Example 1, except that LiF, a fluoride catalyst, was not added, and charge / discharge measurements were carried out. The charge / discharge curves are shown in FIG.

[0109] <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.

[0110] On the other hand, as shown in FIG. 3, in the cell of Example 1 containing the fluoride catalyst LiF, the voltage during discharge in the first cycle began to decrease gradually from around 0.9 V, then decreased somewhat 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 time 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 at the beginning of charge decreased slightly, and the capacity above 1.5 V gradually decreased with each cycle.

[0111] 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 enabled the insertion of fluoride ions into GLG. This is believed to be due to the following mechanism. Specifically, when GLG was used alone in Comparative Example 1, a strong C—F bond was formed between the carbon atoms on the GLG surface and the fluoride ions during charging. This is believed to form a fluoride insulating layer on the GLG surface, thereby inhibiting the insertion of fluoride ions between the GLG layers. On the other hand, when LiF was added to the GLG in Example 1, the carbon atoms of GLG and the lithium ions of LiF competed for fluoride ions, inhibiting the formation of a strong C—F bond between the carbon atoms on the GLG surface and the fluoride ions. This is believed to inhibit the formation of a fluoride insulating layer on the GLG surface, allowing fluoride ions to be inserted between the layers inside the GLG.

[0112] Furthermore, in Examples 2 to 4, the oxygen content of the GLG was different from that of Example 1, as shown in Table 1. In Examples 2 to 4, charge / discharge curves similar to those in Figure 3 of Example 1 were obtained, as shown in Figures 4 to 6. 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.

[0113] Although Examples 5 and 6 differ from Example 1 in the fluoride catalyst, when AgF (Example 5) or KHF2 (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.

[0114] In Example 7, acetylene black was used as the carbon material. As shown in FIG. 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 increase 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 of Example 7, it is believed that there is room for improvement in the cycle characteristics by adjusting the experimental conditions.

[0115] Examples 8 and 9 are cases where graphite and hard carbon were used as the carbon material, respectively. As shown in Figures 10 and 11, it was shown that when graphite (Example 8) or hard carbon (Example 9) was used, charge and discharge were possible by adding a fluoride catalyst. As with Example 7, it is believed that there is room for improvement in the cycle characteristics by adjusting the experimental conditions. [Industrial Applicability]

[0116] The present disclosure is extremely useful because it can provide a cathode material for an all-solid-state fluoride ion shuttle battery that can exhibit higher voltage, higher capacity, and higher cycle characteristics and can be produced easily and at low cost, a method for producing the same, and an all-solid-state fluoride ion shuttle battery using the cathode material. [Explanation of symbols]

[0117] 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 atoms 22 oxygen atoms 23 holes

Claims

1. A cathode material for use in an all-solid-state fluoride ion shuttle battery, comprising: The positive electrode active material contained in the positive electrode material is composed of 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. A positive electrode material characterized by:

2. The positive electrode material is a mixture containing the positive electrode active material and a solid electrolyte. The positive electrode material according to claim 1 .

3. the content of the carbon material contained in the mixture is 10% by mass or more and 50% by mass or less, 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 claim 2 .

4. The metal fluorides include LiF, AgF, and AlF ３ , CuF ２ , KF, KHF ２ and CaF ２ At least one selected from the group consisting of The positive electrode material according to any one of claims 1 to 3.

5. The carbon material includes a layered compound having carbon atoms and oxygen atoms covalently bonded to the carbon atoms as constituent elements. The positive electrode material according to any one of claims 1 to 4.

6. The interlayer distance of the layered compound is larger than the interlayer distance of graphite. The positive electrode material according to claim 5 .

7. 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. A method for producing a positive electrode material.

8. the carbon material is a layered compound containing carbon atoms and oxygen atoms covalently bonded to the carbon atoms as constituent elements, In the preparation step, graphite powder is oxidized and then heat-treated to obtain the layered compound. The method for producing a positive electrode material according to claim 7.

9. a positive electrode, a negative electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode; The positive electrode comprises the positive electrode material according to any one of claims 1 to 6. An all-solid-state fluoride ion shuttle battery characterized by:

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