Cathode active material for calcium secondary batteries, method for manufacturing a cathode active material for calcium secondary batteries, cathode for calcium secondary batteries, calcium secondary battery

Abstract

The objective is to provide a new positive electrode active material for calcium secondary batteries. [Solution] A positive electrode active material for a calcium secondary battery containing NaTi2(PO4)3.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for a calcium secondary battery, a method for producing a positive electrode active material for a calcium secondary battery, a positive electrode for a calcium secondary battery, and a calcium secondary battery. [Background technology]

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and laptop computers, there has been a strong demand for the development of small, lightweight rechargeable batteries with high energy density and durability. Furthermore, there is a strong demand for high-output rechargeable batteries for use in power tools and electric vehicles, including hybrid cars. In addition to the above-mentioned required characteristics, there is also a growing need for rechargeable batteries with high durability that do not degrade easily even after repeated use.

[0003] Lithium-ion secondary batteries are known as secondary batteries that meet these requirements. For this reason, various studies have been conducted on lithium-ion secondary batteries and their positive electrode active materials, as disclosed in, for example, Patent Document 1.

[0004] However, lithium-ion rechargeable batteries have the problem of being expensive because they use rare metals in materials such as lithium and the positive electrode active material.

[0005] Therefore, various studies are underway to develop next-generation batteries that can reduce the use of rare metals and have superior energy density. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-134262 [Overview of the project] [Problems that the invention aims to solve]

[0007] One of the next-generation batteries attracting attention is the calcium secondary battery, which uses the abundant resource of calcium metal and, although it has a low operating voltage, is expected to have a high energy density.

[0008] However, with regard to calcium secondary batteries, sufficient research into positive electrode active materials that can be used in calcium secondary batteries has not been conducted, and there has been a need for positive electrode active materials for calcium secondary batteries.

[0009] Therefore, in view of the problems of the above-mentioned prior art, one aspect of the present invention aims to provide a new positive electrode active material for calcium secondary batteries. [Means for solving the problem]

[0010] A positive electrode active material for a calcium secondary battery according to one aspect of this disclosure includes NaTi2(PO4)3. [Effects of the Invention]

[0011] According to one aspect of the present invention, a novel positive electrode active material for calcium secondary batteries can be provided. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is an explanatory diagram of the configuration of the coin-type batteries fabricated in the examples and comparative examples. [Figure 2] Figure 2 is an explanatory diagram of the XRD pattern of the positive electrode active material for calcium secondary batteries obtained in Example 1. [Figure 3] Figure 3 shows the charge-discharge curve of the calcium secondary battery fabricated in Example 1. [Figure 4] Figure 4 shows the charge and discharge curves of the calcium secondary batteries prepared in Example 1 and Example 2. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments for implementing the present invention will be described. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Positive electrode active material for calcium secondary battery] (1) Regarding the composition The positive electrode active material for a calcium secondary battery in this embodiment (hereinafter, also simply referred to as "positive electrode active material") can contain NaTi2(PO4)3 (sodium titanium phosphate). The positive electrode active material of this embodiment can also be composed only of NaTi2(PO4)3, but even in this case, it does not exclude containing inevitable impurities.

[0014] The inventor of the present invention searched for a compound having a crystal structure capable of inserting and desorbing calcium ions with a large ionic radius compared to lithium ions and the like as a material for the positive electrode active material of a calcium secondary battery. In addition, as a material for the positive electrode active material of a calcium secondary battery, a compound was searched for in which the operating voltage when applied to a calcium secondary battery is lower than the voltage at which the electrolyte of the calcium secondary battery decomposes.

[0015] As a result, attention was paid to NaTi2(PO4)3, which has a large space within the three-dimensional crystal structure lattice, stores sodium ions with an ionic radius almost the same as that of calcium ions, and has not been known as a positive electrode active material for conventional calcium secondary batteries.

[0016] As shown in the examples described below, the inventors of the present invention fabricated a calcium secondary battery using a positive electrode containing NaTi2(PO4)3 as a positive electrode active material and performed repeated charge and discharge. As a result, in the charge-discharge curve, a plateau region, which is a region where there is almost no change in potential and it becomes flat, was confirmed. From this, it was understood that the insertion and desorption reactions of calcium into the positive electrode active material were occurring, and it was confirmed that power storage was possible. Also, even when repeated charge and discharge was performed, a charge-discharge curve including a plateau was observed, so it was confirmed that repeated charge and discharge could be performed. That is, it was confirmed that NaTi2(PO4)3 functions as a positive electrode active material for a calcium secondary battery.

[0017] In the above study, when repeated charge and discharge was performed, it is considered that calcium ions were supplied from the calcium metal used for the negative electrode to NaTi2(PO4)3, which is the positive electrode active material, and a part of the sodium in NaTi2(PO4)3 was replaced by calcium.

[0018] Therefore, the positive electrode active material of the present embodiment may also contain (Ca x , <\(0000007\)>, <\(0000008\)>, <\(0000005\)>, <\(0000006\)>, <\(0000101\)>, <\(0000010\)>Na 1-x )Ti2(PO4)3 (0 < x < 1).

[0019] As described above, since a part of the sodium is replaced by calcium, the positive electrode active material of the present embodiment does not contain NaTi2(PO4)3 and may also contain (Ca x [[ID=I8]]Na 1-x )Ti2(PO4)3 (0 < x < Ⅰ).

[0020] <000011O>Note that NaTi2(PO4)3 and (Ca x Na 1-x )Ti2(PO4)3 (0 < x < 1) can also be expressed together as (Ca x Na[[ID=三十一]] 1-x )TiZ(PO4)3 (0 ≤ x ≤ 1). That is, the positive electrode active material of the present embodiment is (Ca x Na 1-x)Ti2(PO4)3(0≦x≦1) can also be included.

[0021] In the general formula for the positive electrode active material of this embodiment, Ca, Na, Ti, and (PO4) are described in theoretical stoichiometric ratios, hence the molar ratio (Ca,Na):Ti:PO4 = 1:2:3. However, the actual content ratio of each element in the positive electrode active material may differ due to errors in compositional analysis or defects.

[0022] Therefore, for example, if the difference in each molar ratio is within ±0.1, it can be considered that the above general formula is satisfied. However, a smaller difference is preferable, and a difference of ±0.05 is even more preferable.

[0023] In other words, for example, the general formula is satisfied when Ti / (Ca,Na) is between 1.9 and 2.1, and when (PO4) / (Ca,Na) is between 2.9 and 3.1. Similarly, for the phosphate group PO4, the general formula is satisfied when, for example, O / P is between 3.9 and 4.1. In this explanation, (Ca,Na) represents the total amount of moles of calcium and sodium, Ti represents the amount of mole of titanium, PO4 represents the amount of mole of the phosphate group, P represents the amount of mole of phosphorus, and O represents the amount of mole of oxygen. (2) Conductive materials The positive electrode active material of this embodiment can be used together with a conductive material when used as the positive electrode of a calcium secondary battery. Therefore, the positive electrode active material of this embodiment can also be used in mixture with a conductive material. The material containing the positive electrode active material of this embodiment and a conductive material can also be referred to as a positive electrode material for calcium secondary batteries or a positive electrode composition for calcium secondary batteries, etc.

[0024] By using the positive electrode active material of this embodiment mixed with a conductive material, a voltage can be applied to the positive electrode active material even when it is far from the current collector, allowing for calcium absorption and release, thereby improving the battery characteristics of the calcium secondary battery.

[0025] The positive electrode active material in this embodiment may be supported on a conductive material. By supporting the positive electrode active material on a conductive material, the contact area between the positive electrode active material and the conductive material can be increased, thereby improving the battery characteristics when used in a calcium secondary battery. Furthermore, by supporting the positive electrode active material on a conductive material, the specific surface area of ​​the positive electrode active material can be increased, which is thought to facilitate the absorption and release of calcium ions, thus improving the battery characteristics when used in a calcium secondary battery from this viewpoint as well.

[0026] The conductive material is not particularly limited; calcium secondary batteries and conductive materials used in various types of secondary batteries can be used.

[0027] The conductive material is not particularly limited, but may include, for example, carbon materials. Examples of carbon materials that can be used as conductive materials include one or more selected from graphite (natural graphite, artificial graphite, and expanded graphite, etc.), carbon black materials such as acetylene black and Ketjenblack (registered trademark), carbon nanotubes, and fullerenes.

[0028] When a carbon material is used as the conductive material, the carbon material may contain carbon nanotubes or may be composed of carbon nanotubes. Carbon nanotubes have carbon elements arranged in a tubular shape and have an elongated form. Therefore, by supporting the positive electrode active material of this embodiment on carbon nanotubes, the contact area between the positive electrode active material and the carbon nanotubes is increased, and long-distance conductive paths can also be formed. As a result, when the positive electrode active material of this embodiment supported on carbon nanotubes is used in a calcium secondary battery, the proportion of the positive electrode active material that contributes to the absorption and release of calcium can be increased, and the battery characteristics can be particularly improved. [Method for manufacturing positive electrode active material for calcium secondary batteries] A method for producing a positive electrode active material for a calcium secondary battery according to this embodiment will now be described. According to the method for producing a positive electrode active material of this embodiment, a positive electrode active material according to one aspect of the present disclosure can be produced. Therefore, some explanations of matters already described will be omitted. Note that the method for producing a positive electrode active material according to one aspect of the present disclosure is not limited to the following method for producing a positive electrode active material.

[0029] The method for producing the positive electrode active material of this embodiment may include, for example, a dropping step and a heat treatment step.

[0030] In the dropwise addition step, a reaction aqueous solution can be formed by dropwise adding a titanium source to an initial aqueous solution containing a sodium source, phosphoric acid, water, and alcohol.

[0031] In the heat treatment process, the products in the reaction aqueous solution can be heat-treated.

[0032] The following describes each step. (1)Dripping process (Regarding the initial aqueous solution) The sodium source material is not particularly limited, and any sodium-containing compound can be used. While the sodium-containing compound is not particularly limited, one or more selected from, for example, sodium acetate and sodium carbonate can be used.

[0033] The type of alcohol used in the initial aqueous solution is not particularly limited, but one or more types selected from methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, etc., can be used.

[0034] The mixing ratio of the sodium source to the phosphoric acid is not particularly limited, but the mixing ratio can be selected according to the desired composition of the positive electrode active material.

[0035] The mixing ratio of water to alcohol is not particularly limited, but the volume ratio of water to alcohol may be 0.25 or less. By setting the volume ratio of water to alcohol (volume of water / volume of alcohol) to 0.25 or less, the particle size of the resulting NaTi2(PO4)3 can be reduced, and the contact area with conductive materials can be increased. Therefore, the battery performance when used in calcium secondary batteries can be improved.

[0036] The lower limit of the volume ratio of water to alcohol is not particularly limited, but it may be, for example, 0.04 or higher.

[0037] If necessary, an acid may be added to the initial aqueous solution to adjust its pH. The type of acid is not particularly limited, but one or more types selected from, for example, citric acid, oxalic acid, etc., can be used. (Regarding titanium sources) The titanium source material is not particularly limited, and any titanium-containing compound can be used. While not particularly limited, examples of titanium-containing compounds include titanium-containing organic compounds. For example, titanium alkoxides can be used as titanium-containing organic compounds. One or more titanium alkoxides selected from titanium methoxide, titanium ethoxide, titanium butoxide, etc., can be used.

[0038] The titanium source may be mixed with a solvent (dispersion medium) such as alcohol and used in liquid form. (Regarding the dripping conditions) In the dropwise addition step, the titanium source can be added dropwise to the initial aqueous solution while stirring it. By using titanium alkoxide as the titanium source, and by having the initial aqueous solution contain a sodium source, phosphoric acid, water, etc., the hydrolysis reaction of the titanium alkoxide can generate particles containing titanium, phosphoric acid, and sodium in the reaction aqueous solution.

[0039] In this process, the particle size and other properties of the resulting particles can be adjusted by selecting the rate at which the titanium source is added, the size of the droplets, and the stirring rate of the initial aqueous solution or the reaction aqueous solution.

[0040] Therefore, the dropping conditions for the titanium source can be selected according to the desired particle size, etc.

[0041] The atmosphere in which the titanium source is added is not particularly limited and may be an atmospheric environment.

[0042] The reaction aqueous solution may also contain a conductive material. By including a conductive material in the reaction aqueous solution, the generated particles can be supported on the conductive material. While the conductive material can be added dropwise along with the titanium source, from the viewpoint of uniformly supporting the positive electrode active material on the conductive material, the conductive material can be added to the initial aqueous solution.

[0043] Since conductive materials have already been explained, I will omit further explanation. (2) Heat treatment process In the heat treatment process, the products in the reaction aqueous solution obtained in the dropwise addition process can be subjected to heat treatment.

[0044] In the heat treatment process, the product in the reaction aqueous solution may be heat-treated in a single step, or it may be heat-treated in two or more steps.

[0045] In the heat treatment step, the heat treatment may be performed on the product in the reaction aqueous solution while it is still in its aqueous solution state, or it may be performed on the product after it has been recovered from the reaction aqueous solution, that is, after it has been separated from the solvent (dispersion medium) of the reaction aqueous solution.

[0046] If the heat treatment process has multiple steps, for example, the first heat treatment step may be performed on the product in the reaction aqueous solution while the reaction aqueous solution is still in its aqueous state. Then, the product may be recovered from the reaction aqueous solution after the first heat treatment step, and the second heat treatment step may be performed on the recovered product.

[0047] For example, if the heat treatment process is performed in a single step and applied to the product recovered from the reaction aqueous solution, the method for producing the positive electrode active material of this embodiment may also include a recovery step before the heat treatment step in which the product is separated from the reaction aqueous solution into solid and liquid components. In the recovery step, the solid and liquid separated product may be dried as needed.

[0048] In this case, the heat treatment process can be performed on the product recovered in the recovery process.

[0049] Furthermore, if the heat treatment process is carried out in two steps, the heat treatment process may include, for example, a first heat treatment step in which the products in the reaction aqueous solution are heat-treated, and a second heat treatment step in which the products in the reaction aqueous solution after the first heat treatment step are heat-treated.

[0050] In the first heat treatment step, the product may be subjected to a hydrothermal reaction (hydrothermal treatment) in the state of the reaction aqueous solution, or the first heat treatment step may be carried out under conditions other than hydrothermal reaction.

[0051] In the second heat treatment step, the product may be recovered from the reaction aqueous solution after the completion of the first heat treatment step by solid-liquid separation and the product may be heat-treated, or the product may be heat-treated while it is still in the reaction aqueous solution state.

[0052] The above examples of heat treatment process configurations are merely illustrative; heat treatment conditions can be selected according to the raw materials used to obtain the desired positive electrode active material.

[0053] The following explanation will use the case where the heat treatment process is carried out by a first heat treatment process and a second heat treatment process as an example, but as explained above, the heat treatment process is not limited to the following configuration example. (2-1) First heat treatment process In the first heat treatment step, for example, the reaction aqueous solution can be subjected to a hydrothermal reaction.

[0054] The conditions for the hydrothermal reaction in the first heat treatment step are not particularly limited, but for example, the reaction can be carried out under a pressure of 1 atm to 20 atm, at a temperature of 140°C to 220°C, for 12 hours to 24 hours.

[0055] In the first heat treatment step, heat treatment of the product promotes the polymerization reaction between the titanium source, the sodium source, and phosphoric acid, thereby accelerating the formation of the NaTi2(PO4)3 precursor.

[0056] After the first heat treatment step is completed, the precursor, which is a product contained in the reaction aqueous solution, can be recovered. The method for recovering the precursor is not particularly limited, but for example, solid-liquid separation can be performed by filtration or centrifugation to recover the solid component of the precursor, and washing with water or drying can be performed as needed. (2-2) Second heat treatment process In the second heat treatment step, for example, the precursor recovered after the first heat treatment step can be heat-treated in an inert atmosphere.

[0057] The conditions for the second heat treatment step are not particularly limited, but for example, the heat treatment can be performed at a temperature of 500°C to 900°C for 3 to 6 hours.

[0058] The type of inert atmosphere used in the second heat treatment step is not particularly limited, and one or more selected from nitrogen and noble gases can be used.

[0059] The furnace used in the second heat treatment step is not particularly limited; any furnace capable of heat-treating the precursor in a predetermined atmosphere is acceptable. However, from the viewpoint of maintaining a uniform atmosphere inside the furnace, an electric furnace that does not generate gas may be used, for example. Either a batch-type or continuous-type furnace may be used.

[0060] By performing the second heat treatment step, low-boiling-point components such as alcohol contained in the precursor obtained in the first heat treatment step can be removed, and the desired positive electrode active material can be produced. [Positive electrode for calcium secondary batteries, calcium secondary batteries] The positive electrode for the calcium secondary battery of this embodiment (hereinafter also referred to as "positive electrode") may include a positive electrode active material according to one aspect of the present disclosure. For this reason, the positive electrode for the calcium secondary battery of this embodiment may include NaTi2(PO4)3.

[0061] Furthermore, the calcium secondary battery of this embodiment (hereinafter also referred to as "secondary battery") may include a positive electrode, a negative electrode, and an electrolyte. The positive electrode may include a positive electrode active material according to one aspect of this disclosure. For this reason, the positive electrode of the secondary battery for the calcium secondary battery of this embodiment may include NaTi2(PO4)3.

[0062] The following describes an example configuration of the positive electrode and secondary battery of this embodiment, with each component explained separately. Note that the embodiment described below is merely illustrative, and the positive electrode and calcium secondary battery of this embodiment can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, including the embodiment described below. Furthermore, the secondary battery is not particularly limited in its application. (positive electrode) The positive electrode of the secondary battery of this embodiment may include a positive electrode active material according to one aspect of the present disclosure.

[0063] An example of a method for manufacturing a positive electrode is described below. First, the positive electrode active material (in powder form), conductive material (conductive additive), and binder are mixed to form a positive electrode mixture. Then, activated carbon and solvents for purposes such as viscosity adjustment are added as needed, and this mixture is kneaded to produce a positive electrode mixture paste.

[0064] The mixing ratio of each material in the positive electrode composite is a factor that determines the performance of the calcium secondary battery, and therefore can be adjusted according to the application. For example, if the total mass of the solid content of the positive electrode composite excluding the solvent is taken as 100% by mass, the positive electrode active material may be contained in a ratio of 50% to 95% by mass, the conductive material in a ratio of 1% to 30% by mass, and the binder in a ratio of 1% to 20% by mass.

[0065] The resulting positive electrode composite paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to remove the solvent, thereby producing a sheet-like positive electrode. If necessary, it can be pressurized using a roll press or the like to increase the electrode density. The sheet-like positive electrode thus obtained can be cut to an appropriate size according to the intended battery and used in the manufacture of the battery.

[0066] The conductive materials have already been explained in the description of the positive electrode active material for calcium secondary batteries, so the explanation will be omitted here. When the positive electrode active material is supported on a conductive material, the conductive material supporting the positive electrode active material and the conductive material added during the manufacturing of the positive electrode may be the same or different.

[0067] The binder plays a role in holding the active material particles together, and can be one or more selected from, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, and polyacrylic acid.

[0068] If necessary, a solvent (dispersion medium) for dispersing the positive electrode active material, conductive material, etc., can be added to the positive electrode mixture. The solvent may also dissolve the binder. Specifically, one or more solvents selected from organic solvents such as N-methyl-2-pyrrolidone and water can be used. In addition, activated carbon can be added to the positive electrode mixture to increase the electrical double layer capacitance.

[0069] The method for manufacturing the positive electrode is not limited to the examples given above, and other methods may be used. For example, it can be manufactured by press-molding the positive electrode composite material and then drying it in a vacuum atmosphere. (Negative electrode) The negative electrode may contain calcium metal, or may consist solely of calcium metal. (Separator) A separator can be placed between the positive and negative electrodes as needed. The separator separates the positive and negative electrodes and holds the electrolyte. Known materials can be used, such as thin films of polyethylene or polypropylene with numerous microscopic pores. In addition to the polymer materials mentioned above, woven or nonwoven fabrics made of inorganic fibers such as glass fibers can also be used as separators. (electrolyte) As the electrolyte, a material capable of transporting calcium ions can be used.

[0070] However, in calcium secondary batteries, when metallic calcium is used for the negative electrode, depending on the type of electrolyte, a film that inhibits the diffusion of calcium ions may form on the surface of the negative electrode.

[0071] Therefore, from the viewpoint of preventing the formation of the above-mentioned film on the surface of the negative electrode, a hydride electrolyte can be used as the electrolyte.

[0072] Examples of hydride electrolytes include Ca(CB) as a supporting salt. 11 H 12 A solution of )2 dissolved in an organic solvent can be used. Therefore, the electrolyte is Ca(CB 11 H 12 It may also include 2.

[0073] As the organic solvent, one of the following may be used alone or in combination with two or more others: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran (THF), 2-methyltetrahydrofuran, dimethyl ether (DME), and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. (Shape and composition of secondary batteries) As described above, the calcium secondary battery of this embodiment can be made into various shapes, such as cylindrical or stacked. Regardless of the shape, the secondary battery of this embodiment can be made into an electrode body by stacking the positive electrode and negative electrode with a separator in between, and impregnating the resulting electrode body with an electrolyte. The positive electrode current collector and the positive electrode terminal that is open to the outside, and the negative electrode current collector and the negative electrode terminal that is open to the outside can be connected using current collecting leads, and the battery can be sealed in a battery case. [Examples]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples.

[0075] First, we will explain the evaluation method for the positive electrode active material and secondary battery obtained in the following examples. (1) Evaluation method (1-1) Evaluation method for positive electrode active material The following evaluations were performed on the obtained positive electrode active material.

[0076] (a) Measurement of XRD pattern The positive electrode active materials prepared in the following examples were subjected to XRD (X-ray Diode) pattern analysis using an X-ray diffractometer (BRUKER, D8 DISCOVER) with monochromatized CuKα1 as the X-ray source. Phase identification was performed based on the obtained XRD patterns. Note that the XRD patterns of the positive electrode active materials before being used in secondary batteries were measured under atmospheric conditions.

[0077] Furthermore, after charging and discharging the secondary battery, the battery was disassembled, and the recovered positive electrode active material was subjected to XRD pattern measurement under an argon atmosphere. Phase identification was then performed from the obtained XRD patterns. (1-2) Method for evaluating battery characteristics The charge-discharge characteristics were evaluated using the coin-type battery shown in Figure 1, which was fabricated in the following example. (a) Charge / discharge curve After fabricating the coin-type batteries described in the following examples, they were left for approximately 12 hours to allow the open-circuit voltage (OCV) to stabilize. Then, constant-current discharge (CC discharge) was performed in a 25°C constant-temperature bath at a current density of 5 mA / g. Discharge was carried out until the cutoff voltage reached 1.6 V, and the discharge curve was measured during this process.

[0078] Furthermore, the charging curve was measured after discharging, a 1-hour rest period, and charging up to a cutoff voltage of 2.6V.

[0079] In Example 1, charging and discharging were repeated five times within the above cutoff voltage range. (2) Manufacturing conditions for positive electrode active material and secondary battery The manufacturing conditions and evaluation results for the positive electrode active material, etc., in the examples and comparative examples are described below. [Example 1] (1) Manufacturing of positive electrode active material The positive electrode active material was manufactured using the following procedure. (1-1) Dripping process An initial aqueous solution was prepared by mixing a sodium source, phosphoric acid, water, alcohol, and a conductive material.

[0080] 0.238 g of sodium acetate was used as the sodium source, and 0.125 g of Ketjenbrak was used as the conductive material. In addition, 0.238 g of citric acid was added to the initial aqueous solution to control the pH value within the range of 1.8 to 2.2.

[0081] Anhydrous ethanol is used as the alcohol, and 12 mL of alcohol and 0.5 mL of water are added to the initial aqueous solution so that the volume ratio of water to alcohol in the initial aqueous solution is 0.04.

[0082] The initial aqueous solution was stirred with a stirrer at a rotational speed of 700 rpm while the titanium source was added dropwise to form the reaction aqueous solution.

[0083] As the titanium source, an ethanol solution prepared by adding 0.843 g of titanium butoxide (Ti[O(CH2)3CH3]4) to 1.25 mL of anhydrous ethanol was used.

[0084] The titanium source was added dropwise over a period of 30 minutes. (1-2) Heat treatment process (1-2-1) First heat treatment process In the first heat treatment step, the reaction aqueous solution obtained in the dropwise addition step was subjected to a hydrothermal reaction in an autoclave. The hydrothermal reaction was carried out at 180°C, at a pressure of 1 atm to 10 atm for 12 hours.

[0085] After the first heat treatment process was completed, solid-liquid separation was performed, and the recovered precursor was repeatedly washed with distilled water and ethanol, and then dried at 80°C under a vacuum atmosphere. (1-2-2) Second heat treatment process The precursor obtained in the first heat treatment step was heat-treated in an inert atmosphere at 750°C for 4 hours to obtain the positive electrode active material. The XRD pattern of the obtained positive electrode active material is shown as XRD pattern 21 in Figures 2(A) and 2(B). For reference, Figures 2(A) and 2(B) also show XRD patterns 201 of NaTi2(PO4)3 and 202 of Na3Ti2(PO4)3, which are included in the ICSD (Inorganic Crystal Structure Database). (2) Manufacturing of secondary batteries A coin-type battery, a calcium secondary battery with the structure shown in Figure 1, was fabricated using the following procedure. The charge-discharge curve of the obtained coin-type battery was also evaluated. The evaluation results are shown in Figure 3. (Coin cell battery) As shown in Figure 1, the coin-type battery 10 has a case 11 and electrodes 12 housed inside the case 11.

[0086] Case 11 includes a hollow positive electrode can 111 with one end open, and a negative electrode can 112 positioned at the opening of the positive electrode can 111. When the negative electrode can 112 is positioned at the opening of the positive electrode can 111, the case 11 is shaped such that a space for housing the electrode 12 is formed between the negative electrode can 112 and the positive electrode can 111.

[0087] The electrode 12 has a positive electrode 121, a separator 122, and a negative electrode 123, which are stacked in this order, and is housed in the case 11 such that the positive electrode 121 is in contact with the inner surface of the positive electrode can 111 and the negative electrode 123 is in contact with the inner surface of the negative electrode can 112.

[0088] Case 11 is equipped with a gasket 113, which restricts relative movement between the positive electrode can 111 and the negative electrode can 112, maintaining a non-contact state, i.e., an electrically insulated state, and thus fixing them in place. The gasket 113 also has the function of sealing the gap between the positive electrode can 111 and the negative electrode can 112, thereby creating an airtight and liquid-tight seal between the inside and outside of case 11.

[0089] This coin-type battery 10 was fabricated using the following procedure. First, the positive electrode active material supported on the obtained conductive material was mixed in a mass ratio of 8, the conductive material, and the binder in a ratio of 1. In this example, acetylene black was used as the conductive material added when fabricating the positive electrode of the coin-type battery. The binder contained PVDF.

[0090] A solvent was added to the resulting positive electrode mixture, and the resulting paste was kneaded. This paste was then applied to the surface of an aluminum foil current collector, dried, and punched out into a disc shape with a diameter of 7.9 mm to form the positive electrode 121.

[0091] Using this positive electrode 121, negative electrode 123, separator 122, and electrolyte, a coin-type battery 10 was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -60°C.

[0092] The negative electrode 123 used calcium metal punched out in the shape of a 10 mm diameter disc.

[0093] For the separator 122, a non-woven fabric made of glass fibers with a thickness of 0.26 mm was used.

[0094] For the electrolyte, Ca(CB 11 H 12 )2 was used as the supporting electrolyte, and 150 μL of an electrolyte solution in which the supporting electrolyte was dissolved in an organic solvent was used. As the organic solvent, a mixed solution of tetrahydrofuran (THF) and dimethyl ether (DME) mixed at a volume ratio of 1:1 was used.

[0095] The evaluation results are shown in FIG. 3.

[0096] Also, the XRD pattern of the positive electrode active material after the first discharge is shown as XRD pattern 22 in FIGS. 2(A) and 2(B), and the XRD pattern of the positive electrode active material after the first charge is shown as XRD pattern 23 in FIGS. 2(A) and 2(B).

[0097] As shown in FIGS. 2(A) and 2(B), the positive electrode active material of this embodiment obtained after the heat treatment step had a peak at the peak position corresponding to NaTi2(PO4)3, and it was confirmed that it contained NaTi2(PO4)3. This can be confirmed from FIG. 2(B) which shows an enlarged view of the range where 2θ is from 23° to 25° in FIG. 2(A), where XRD pattern 21 has a peak 20A corresponding to NaTi2(PO4)3.

[0098] In the XRD pattern 22 after discharge, as a result of calcium metal of the negative electrode being inserted into the positive electrode active material, for example, as shown in FIG. 2(B), the peak 20A corresponding to NaTi2(PO4)3 became lower, and instead, a new peak 20B was observed on the low-angle side. It was confirmed that peak 20B is considered to be a peak corresponding to (Ca x Na 1-x )Ti2(PO_{4})_{3}(0 < x < 1).

[0099] In the XRD pattern 23 after further charging following discharge, as shown in Figure 2(B), the peak 20B that was observed during discharge was almost completely gone, and the peak 20A corresponding to NaTi2(PO4)3 was observed again.

[0100] From the XRD pattern results above, we were able to confirm that the reaction during charging and discharging is reversible and that calcium is inserted into the positive electrode active material during discharge.

[0101] In the charge-discharge curves shown in Figure 3, 1st represents the first charge-discharge curve, 2nd represents the second charge-discharge curve, and 5th represents the fifth charge-discharge curve.

[0102] As shown in Figure 3, a plateau region was observed in the charge-discharge curve. Furthermore, according to the charge-discharge curve shown in Figure 3, although a tendency for the battery capacity to decrease with repeated charging and discharging was observed, repeated charging and discharging was possible, confirming that the positive electrode active material according to one embodiment of this disclosure can function as a secondary battery. [Example 2] Carbon nanotubes were used as the conductive material in the dropping process when manufacturing the positive electrode active material. Carbon nanotubes were also used as the conductive material added during the fabrication of the positive electrode of the coin-type battery. Except for the points mentioned above, the positive electrode active material and secondary battery were fabricated under the same conditions as in Example 1.

[0103] The charge-discharge curves of the obtained secondary battery are shown in Figure 4. Figure 4 also shows the charge-discharge curve for the first cycle in Example 1.

[0104] As shown in Figure 4, we confirmed that using carbon nanotubes as the conductive material supporting the positive electrode active material resulted in a wider plateau region and a larger charge / discharge capacity compared to using Ketjenblack as the conductive material supporting the positive electrode active material. [Explanation of symbols]

[0105] 10 coin-cell batteries 11 cases 111 Positive electrode can 112 Negative electrode can 113 Gasket 12 electrodes 121 Positive electrode 122 Separator 123 Negative electrode 21 XRD Patterns 22 XRD patterns 23 XRD Pattern 201 XRD Pattern 202 XRD pattern 20A peak 20B Peak

Claims

1. NaTi ２ (PO ４ ) ３ A positive electrode active material for calcium secondary batteries, including the above.

2. A positive electrode active material for a calcium secondary battery according to claim 1, supported on a conductive material.

3. The positive electrode active material for a calcium secondary battery according to claim 2, wherein the conductive material comprises a carbon material, and the carbon material comprises carbon nanotubes.

4. (Ca ｘ Na １－ｘ ) Ti ２ (PO ４ ) ３ A positive electrode active material for a calcium secondary battery according to any one of claims 1 to 3, including (0 < x < 1).

5. A dropping step to form a reaction aqueous solution by dropping a titanium source into an initial aqueous solution containing a sodium source, phosphoric acid, water, and alcohol, A method for producing a positive electrode active material for a calcium secondary battery, comprising a heat treatment step of heat-treating the product in the reaction aqueous solution.

6. The method for producing a positive electrode active material for a calcium secondary battery according to claim 5, wherein the reaction aqueous solution contains a conductive material.

7. A positive electrode for a calcium secondary battery, comprising the positive electrode active material for a calcium secondary battery described in any one of claims 1 to 3.

8. It includes a positive electrode, a negative electrode, and an electrolyte, A calcium secondary battery comprising the positive electrode positive electrode active material for a calcium secondary battery according to any one of claims 1 to 3.

9. The calcium secondary battery according to claim 8, wherein the negative electrode contains calcium metal.

10. The electrolyte contains Ca(CB １１ H １２ ), and the calcium secondary battery according to claim 8. ２ ​

Images