JP5721171B2 - Electrode active material and method for producing the same - Google Patents

Description

  The present invention belongs to the technical field of non-aqueous electrolyte secondary batteries, and particularly relates to a novel production method for efficiently producing an electrode active material at low cost and a secondary battery using the same.

  Secondary batteries using non-aqueous electrolytes are actively researched and developed as batteries capable of achieving high energy density at high voltage. In particular, in recent years, application to a power source for a hybrid vehicle or an electric vehicle has been studied, and further development of a large-capacity and economical large battery has been demanded.

Electrode active materials containing fluoride are widely used for the positive electrode of such secondary batteries. As an electrode active material, for example, there is FeF ₃ having a fluoride perovskite type structure, and this material has been attracting attention in recent years because it has an energy density that exceeds that of a conventionally used olivine type LiFePO ₄ positive electrode. Furthermore, since FeF ₃ has a vertex shared skeleton structure, reversible insertion / desorption has been confirmed for sodium (Na) ions whose ion volume is about twice that of lithium (Li). Expected to be useful.
As a conventional electrode active material, there is a positive electrode active material for a non-aqueous electrolyte secondary battery, characterized by comprising a metal fluoride represented by the general formula MF ₃ coated with carbon (M represents a metal element). Yes (see Patent Document 1). In addition, a metal halide is included as a positive electrode active material, an alkali metal or a material capable of occluding and releasing alkali metal is used as a negative electrode active material, and ions of the alkali metal react with the positive electrode active material and the negative electrode active material. There is also a non-aqueous electrolyte battery characterized in that a substance that can be moved to make the electrolyte is an electrolyte substance (see Patent Document 2).

JP 2008-130265 A JP 2010-170865 A JP 2008-243646 A

D. Gocheva, M. Nishijima, T. Doi, S. Okada, J. Yamaki, T. Nishida, J. Power Sources, 187, (2009) 247-252.

However, since the conventional electrode active material does not include lithium element (Li) or sodium element (Na) in FeF ₃ , a carbon negative electrode and an ion battery cannot be formed in this state. As a solution to this problem, a method of mechanochemical synthesis of NaFeF ₃ from FeF ₂ and NaF at room temperature has been proposed (see Patent Document 3). However, at present, the amount of pre-doping of sodium (Na) in mechanochemical synthesis is insufficient, and there is a problem that the initial charge capacity is only about 80 mAh / g (see Non-Patent Document 1).

  The present invention has been proposed to solve the above-described problems, and can provide a manufacturing technique of an electrode active material that can be manufactured at a lower cost than conventional methods and has a sufficient charge-discharge capacity. is there.

  As a result of intensive studies, the present inventors have newly found a method for producing an electrode active material suitable for a secondary battery using a non-aqueous electrolyte. Furthermore, it has been found that a non-aqueous electrolyte secondary battery with high operational stability can be constructed by selecting and combining negative electrode active materials using this electrode active material.

Thus, according to the present invention, M ¹ F ₂ and / or M ¹ F ₃ (M ¹ represents any metal element of Fe, Ti, V, Mn, Co, Ni) is converted to M ² F. And / or M ² HF ₂ (M ² represents an alkali metal element) and a method of producing M ² M ¹ F ₃ for electrode active material, comprising a step of melting and quenching in an inert gas atmosphere Provided.

Furthermore, according to the present invention, there is also provided a method for producing M ² M ¹ F ₃ for an electrode active material, including a step of subjecting M ² M ¹ F ₃ obtained by melting and quenching to a carbon coating treatment under an inert gas atmosphere. Is done. Furthermore, according to the present invention, there is also provided a method for producing M ² M ¹ F ₃ for electrode active materials, including a step of carbothermal treatment of carbon-coated M ² M ¹ F ₃ in an inert gas atmosphere. The

  Furthermore, according to this invention, the non-aqueous electrolyte secondary battery which has a positive electrode containing the manufactured electrode active material is also provided.

1 shows a schematic diagram of a high-frequency induction heating / single roll quenching apparatus, a pellet electrode, and a coating electrode according to the present invention. It shows the XRD pattern results of the electrode active material NaFeF ₃ produced by the present invention. The test result of the initial charge / discharge of the electrode active material NaFeF ₃ produced according to the present invention and FeF ₃ as a control is shown.

M ¹ F ₂ and / or M ¹ F ₃ (M ¹ represents any metal element of Fe, Ti, V, Mn, Co, and Ni), M ² F and / or M ² HF ₂ ( M ² represents an alkali metal element) and M ² M ¹ F ₃ for electrode active material, which includes a step of melting and quenching in an inert gas atmosphere, can be produced.

Here, the raw material M ² F and M ² HF ₂ are not particularly limited as long as the alkali metal element exists as M ² . As satisfying this condition, the alkali metal element M ² can be selected from lithium, sodium, potassium, rubidium or cesium, and among these, lithium and sodium having high charge / discharge characteristics are particularly preferable as the secondary battery, for example, You can choose sodium. That is, for example, NaHF ₂ can be used as a raw material.

In addition, M ¹ F ₂ and M ¹ F ₃ as raw materials can use M ¹ as a metal element of any ^one of Fe, Ti, V, Mn, Co, and Ni. It is preferable to use it.

According to the present invention, the target positive electrode active material is produced by melt quenching (method). That is, M ¹ F ₂ and / or M ¹ F ₃ (M ¹ represents any metal element of Fe, Ti, V, Mn, Co, and Ni) as a raw material, M ² F and / or From the state in which M ² HF ₂ is melted and mixed, the target M ² M ¹ F ₃ (solid is obtained by cooling as quickly as possible so that the constituent components (particularly fluorine) are not volatilized. : Crystalline or amorphous). For such melting and quenching, conventionally known various melting (heating) means and quenching means can be used.

  For example, as the melting means, a known melting method such as a high frequency induction heating method or an arc melting method can be used. As the quenching means, a known quenching method such as a single roll quenching method, a twin roll quenching method, an atomizing quenching method, or a splat quenching method can be used. Among these, it is particularly preferable to use high-frequency induction heating / single roll quenching to achieve the object of the present invention. In addition, the above-described known melting method and quenching method may be combined. Good.

  For example, when using high-frequency induction heating / single roll quenching, the raw material metal previously charged in the reaction vessel is melted with an induction coil, and then the molten raw metal is injected from the melting nozzle onto the surface of the single roll. The product can be obtained by rapid cooling at From the viewpoint of increasing the purity of the product, the melting and quenching treatment according to the present invention is cooled as quickly as possible to suppress desorption due to vaporization of fluorine from the raw material metal.

  As an example, the high-frequency induction heating / single roll quenching apparatus used in the present invention includes a quartz tube 1 and a crucible 2 with a pilot hole placed inside the quartz tube 1 as shown in FIG. A copper tube coil 3 made of copper wound around the quartz tube 1 and a copper roll 4 as a solid cooling medium can be used.

  A raw material is put into a crucible 2 with a prepared hole placed inside a quartz tube 1, and a raw material metal falling from the prepared hole of the crucible with a prepared hole is induction-heated by a copper tube coil 3 (indicated by A in the figure). And the induction-heated raw material metal becomes a glass ribbon-like sample B and is rapidly cooled by coming into contact with a copper roll 4 as a metal water-cooled roll.

  The melt quenching process according to the present invention is generally performed in an inert gas atmosphere such as nitrogen gas or argon gas, but it is preferable to use argon gas.

  The conditions of the melt quenching process, such as the processing time, the rotation speed of the metal water cooling roll (copper roll), the induction heating speed, the cooling speed, etc., are analyzed and confirmed by XRD etc. What is necessary is just to determine so that the crystal | crystallization of the target electrode active material can produce many.

According to the present invention, as an example, when the raw materials M ¹ F ₂ and M ² HF ₂ are FeF ₂ and NaHF ₂ , an electrode active material NaFeF ₃ for a nonaqueous electrolyte secondary battery is generated by the following reaction. It is considered to be done.
[Chemical formula 1]
FeF ₂ + NaHF ₂ → NaFeF ₃ + HF
As the positive electrode of the nonaqueous electrolyte secondary battery, the above electrode active material may be used as it is, but in order to improve the rate characteristics of the electrode, a composite with a known conductive material may be formed.

That is, according to the present invention, from the viewpoint of improving rate characteristics, M ² M ¹ F ₃ for electrode active material obtained by the above-described melting and quenching treatment is pulverized and mixed together with carbon fine particles in an inert gas atmosphere. Thus, carbon coating can be performed. As the carbon fine particles, furnace black, channel black, acetylene black, thermal black, and the like can be used, but acetylene black is preferred because of its high conductivity when used as an electrode. As the inert gas, nitrogen gas, argon gas, or the like can be used. For example, argon gas can be used.

Specific means applied to the pulverization / mixing at the time of carbon coating are not particularly limited, and various means conventionally used for the purpose of pulverization / mixing of solid substances can be applied, A ball mill is preferable, and among these, a planetary ball milling is preferably used because the raw materials can be sufficiently pulverized and mixed.

Furthermore, according to the present invention, from the viewpoint of improving rate characteristics, the carbon-coated M ² M ¹ F ₃ for electrode active material can be subjected to carbothermal treatment in an inert gas atmosphere. In general, carbothermal treatment refers to heat treatment by mixing fine carbon powder, but in the present invention, carbon is added to M ² M ¹ F ₃ for electrode active material by the carbon coating treatment in the previous step. Is already in a coated state, and thus substantially refers to a process of heating the carbon-coated M ² M ¹ F ₃ .

  The heating temperature in the carbothermal treatment is preferably 200 to 500 ° C, more preferably 300 to 400 ° C, and particularly preferably 350 ° C. Moreover, preferable heating time is 1 to 5 hours, for example, can be made into 2 hours.

According to the present invention, a positive electrode active material of a nonaqueous electrolyte secondary battery comprising M ² M ¹ F ₃ for electrode active material obtained as described above, a secondary battery positive electrode including the positive electrode active material, and A secondary battery in which a negative electrode is combined with a negative electrode is provided.

  When the positive electrode according to the present invention is manufactured, a known electrode manufacturing method may be followed except that the positive electrode active material described above is used. For example, the active material powder is mixed with a known binder such as polyethylene, if necessary, and a known conductive material such as acetylene black, if necessary, and the resulting mixed powder is made of stainless steel or the like. It can be pressure-molded on the support or filled into a metal container. An example of such a positive electrode is a pellet electrode. As a pellet electrode, as shown in FIG.1 (b), it can be comprised from the pellet electrode 10a, the spacer 11a, the coin cell container (lower lid) 12, and the titanium mesh 13 made from titanium, for example. The pellet electrode 10a can have a thickness of 10 mm, for example. The spacer 11 a mounts the titanium mesh 13 and mounts the pellet electrode 10 a on the titanium mesh 13.

  In addition, for example, the positive electrode of the present invention can also be produced by a method such as applying a slurry obtained by mixing the mixed powder with an organic solvent such as toluene onto a metal substrate such as aluminum, nickel, stainless steel, or copper. Can do. An example of such a positive electrode is a coated electrode. As the application electrode, for example, as shown in FIG. 1 (c), the application electrode 10 b, a spacer 11 b, and a coin cell container (lower lid) 12 can be configured. The coating electrode 10b can have an electrode diameter of 10 mm, for example. As for the spacer 11b, the coating electrode 10b is spot-welded in the center part of the upper surface.

  As the negative electrode (negative electrode active material) used in combination with the above positive electrode, sodium or lithium, a compound or alloy of an alkali metal thereof, or the like can be used, and a carbonaceous material can also be used. As the carbonaceous material used for the negative electrode according to the present invention, a graphite (graphite-based) carbon body is preferable. In addition, hard carbon obtained by firing various polymers is also used, but is not limited thereto. It is not a thing. Furthermore, these carbonaceous materials may be used in combination of two or more.

  The negative electrode may be manufactured by a known method. For example, the negative electrode can be manufactured in the same manner as described above in relation to the positive electrode. That is, for example, if necessary, the negative electrode active material powder is mixed with the known binder described above and, if necessary, the known conductive material described above, and then the mixed powder is formed into a sheet shape. Then, this may be pressure-bonded to a conductor network (current collector) such as stainless steel or copper. Moreover, for example, it can also be produced by applying a slurry obtained by mixing the above mixed powder with the above-mentioned known organic solvent on a metal substrate such as copper.

  As another component, what is used for a well-known nonaqueous electrolyte secondary battery can be used as a component. For example, the following can be illustrated.

  The electrolytic solution usually includes an electrolyte and a solvent. The solvent of the electrolytic solution is not particularly limited as long as it is non-aqueous, for example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, Esters, amides, phosphate ester compounds and the like can be used. Examples of these include 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethylene carbonate, vinylene carbonate, methyl formate, dimethyl sulfoxide, propylene carbonate, acetonitrile, γ-butyrolactone, Dimethylformamide, dimethyl carbonate, diethyl carbonate, sulfolane, ethyl methyl carbonate, 1,4-dioxane, 4-methyl-2-pentanone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, Examples include methylsulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, and triethyl phosphate. . These can be used by 1 type (s) or 2 or more types, for example, can use dimethyl carbonate and ethylene carbonate.

As an electrolyte contained in the electrolytic solution, an electrolyte capable of performing migration for causing an alkali metal ion in the negative electrode active material to electrochemically react with the positive electrode active material or the positive electrode active material and the negative electrode active material in the solvent. Substances such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ³ Li, CF ₃ SO ₃ Li, LiN (SO _{2 CF 3) 2, LiN (SO} 2 C 2 F 5) 2, LiC (SO 2 CF 3) 3, LiN (SO 3 CF 3) 2, NaClO 4, NaPF 6, NaBF 4, NaCF 3 SO 3, NaAsF 6 , NaB (C ₆ H ₅ ) ₄ , NaCl, NaBr, CH ₃ SO ³ Na, CF ₃ SO ₃ Na, NaN (SO ₂ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) ₂ , NaC (SO ₂ CF _{3 3, NaN (SO 3 CF 3} ) can be used ₂ or the like, for example, may be used sodium perchlorate (NaClO _4).

  In the nonaqueous electrolyte secondary battery according to the present invention, various conventionally known materials can be used for elements such as a separator, a battery case, and other structural materials, and there is no particular limitation. What is necessary is just to assemble the non-aqueous electrolyte secondary battery which concerns on this invention according to a well-known method using said battery element. In this case, the shape of the battery is not particularly limited, and various shapes and sizes such as a cylindrical shape, a square shape, and a coin shape can be appropriately employed.

EXAMPLES Examples will be described below to more specifically illustrate the features of the present invention, but the present invention is not limited to the following examples.
Example 1
As the starting materials for the synthesis of the electrode active material NaFeF ₃ by the melt quenching method , FeF ₃ (Soekawa Riken) was used as the iron source, and NaHF ₂ (Soekawa Riken) was used as the sodium source. The starting material was weighed in a glove box so that the iron source: sodium source had a stoichiometric ratio of 1: 1, and thoroughly mixed in an agate mortar. The mixed precursor starting material is placed in a platinum crucible with a pilot hole and then set in a single roll melting and quenching apparatus. After induction heating at an output of 10 A and 110 V, a glass ribbon sample is struck on a copper roll rotating at 2000 rpm. Obtained. In order to avoid exposure of the sample to the atmosphere, the obtained sample was powdered using an agate mortar in a glove box to obtain a positive electrode active material. In addition, exposure to the atmosphere in the synthesis of the sample was performed only during the installation of the platinum crucible in the single roll quenching apparatus and the subsequent sample recovery.
(XRD measurement of synthetic sample)
The synthesized electrode active material NaFeF ₃ was measured by XRD. The XRD pattern after the melt quench is shown as the melt quench sample in FIG. Samples were mixed in a composition ratio of NaFeF ₃ was assigned by NAMF ₃ (M is a metal element) Pnma space group as reported in (perovskite structure). From this result, it was found that a single phase of NaFeF ₃ was synthesized by the melt quenching method.
(Carbon coat treatment)
The electrode active material NaFeF ₃ obtained by melting and quenching was pulverized in an agate mortar and weighed at a weight ratio of 70:25 to acetylene black (others were binders), and the rotational speed was 450 rpn using a planetary ball mill. The carbon coating treatment was performed for 5 hours. The XRD pattern of NaFeF ₃ after carbon coating treatment is shown as the carbon coating sample in FIG.
(Carbothermal treatment)
Using an atmosphere-controlled horizontal tubular furnace (manufactured by Koyo), the NaFeF ₃ after the carbon coating treatment was performed in an argon atmosphere at a heating rate of 200 ° C./h, processing temperatures of 200 ° C., 300 ° C., 350 ° C., 500 Carbothermal treatment was performed at 2 ° C. for 2 hours. All samples were identified using a powder X-ray diffractometer (Rigaku RINT2100HLR / PC). The measurement was performed in the range of 20 ° to 60 ° at 1 ° / min.
(Preparation of pellet electrode)
The NaFeF ₃ / C composite obtained above and the binder (PTFE) were weighed at a weight mixing ratio of 95: 5, put into a standard bottle, and molded into a pellet as a positive electrode active material. A charge / discharge test was conducted using sodium metal as the negative electrode and 1 mol dm ^-3 sodium perchlorate / ethylene carbonate + dimethyl carbonate [NaClO ₄ / EC + DMC (1: 1)] as the electrolyte. The charge / discharge measurement mode was CCV mode. The measurement conditions were 1Na desorption with a theoretical capacity of 197 mAh / g in 1 hour, with a current density of 1C, 1/100 C (2 mA / g), 1/10 C (20 mA / g), The 1C rate (200 mA / g) was performed at a current density of 0.068 mA / cm ² and a voltage range of 4.0 V-1.5 V.
(Preparation of coated electrode)
The NaFeF ₃ / C composite obtained by carbon coating treatment is NMP (1-methyl-2-pyrrolidine) at a weight ratio of 95: 5 with binder (KF polymer L 1320 #, containing PVDF manufactured by Kureha Chemical Industries). ) And stirred with a spatula to be uniform and mixed for 1 hour to form a slurry, which was then applied to an aluminum foil (manufactured by Thunk Metal) to form a positive electrode. In an argon dry box, this is sodium metal for the negative electrode, 1 M sodium perchlorate / ethylene carbonate + dimethyl carbonate [NaClO ₄ / EC + DMC (1: 1)] for the electrolyte, and polypropylene microporous material for the separator. Used after making coin cell. The cell was evaluated using a constant current charging / discharging device in a constant temperature bath at 25 ° C.

As shown in FIG. 2, it was confirmed from XRD that not only the melt-quenched sample but also the sample subjected to carbothermal treatment after carbon coating treatment maintained the NaFeF ₃ perovskite structure (Pnma). From the XRD results, a certain level of crystal quality was confirmed in the carbothermal treatment at 200 ° C to 500 ° C. When the heating temperature is increased to 500 ° C., iron oxide begins to be mixed in part, and thus the heating temperature is preferably lower than 500 ° C. In particular, in the case of 350 ° C., it was found that the NaFeF ₃ single phase is preferable as the temperature for performing the carbothermal treatment because it exhibits the same crystallinity as the sample before the ball mill.

As shown in FIG. 3, the results of an initial charge / discharge test of NaFeF _{3 under} various conditions and FeF ₃ (Soekawa Riken) as a control are shown. Compared to the mechanochemically synthesized NaFeF ₃ sample, the melt-quenched sample improved Na desorption from the initial charge capacity from 0.40 to 0.69, and the initial charge / discharge efficiency was 96%. From this, it was found that the pre-doping amount of Na can be improved by using the melt quenching method. When the end-of-charge voltage of the melt-quenched sample was increased from 4.0 V to 4.6 V, the discharge capacity increased to about 170 mAh / g, compared to the theoretical capacity of NaFeF ₃ of 197 mAh / g.

As the charge / discharge current density increases, the rate improvement effect of carbothermal treatment becomes more prominent, especially in the high rate region of 0.33 mA / cm ² or more, the carbothermal treatment sample shows better charge / discharge capacity than the carbon coat treatment sample. It was.

In the above description, NaHF ₂ is used as the sodium source. However, it is also possible to produce the electrode active material NaFeF ₃ using NaF ₂ instead of NaHF ₂ . However, since NaF ₂ has a higher melting point than NaHF ₂ , the cost required for melting is higher than NaHF ₂ . Furthermore, the present inventors have also the fact that the high crystallinity electrode active material NaFeF ₃ when using NaHF ₂ than NaF ₂ has been confirmed that the obtained sodium source, the source of sodium NaF ₂ it is preferred to use NaHF ₂ than.
(Example 2)
Synthesis of electrode active material LiFeF ₃ by melt quenching method According to the same procedure as in Example 1 above, a lithium source was used instead of a sodium source, and electrode active material LiFeF ₃ was synthesized. That is, the synthesis was performed using FeF ₃ (Soekawa Rika) as the iron source and LiF (Soekawa Rika) as the lithium source as starting materials. The starting material was weighed in a glove box so that the iron source: lithium source had a stoichiometric ratio of 1: 1, and thoroughly mixed in an agate mortar. A sample was obtained. When the synthesized electrode active material LiFeF ₃ was measured by XRD, it was confirmed that the main peak of the sample mixed at the composition ratio of LiFeF ₃ was the precursor sample FeF ₂ and LiF, and the LiF crystal peak was relatively since I was very small, LiFeF ₃ as an amorphous body is presumed to have been formed.

1 quartz tube 2 crucible with pilot hole 3 copper tube coil 4 copper roll 10a pellet electrode 10b coating electrode 11a spacer 11b spacer 12 coin cell container (lower lid)
13 Titanium mesh

Claims (6)

M ¹ F ₂ and / or M ¹ F ₃ (M ¹ represents any metal element of Fe, Ti, V, Mn, Co, and Ni), M ² F and / or M ² HF ₂ ( M ² represents lithium or sodium) and a method for producing M ² M ¹ F ₃ for electrode active materials, comprising a step of melting and quenching in an inert gas atmosphere. ^2. The production of M ² M ¹ F ₃ for electrode active materials according to claim 1, comprising a step of carbon coating treatment of M ² M ¹ F ₃ obtained by melting and quenching in an inert gas atmosphere. Method. The method for producing M ² M ¹ F ₃ for an electrode active material according to claim 2, comprising a step of subjecting the carbon-coated M ² M ¹ F ₃ to a carbothermal treatment in an inert gas atmosphere. . M ¹ F ₂ is FeF ₂ , M ² HF ₂ is NaHF ₂ , M ² M ¹ F ₃ for electrode active material is NaFeF ₃ , or M ¹ F ₃ is FeF ₃ , M ² F ₂ is LiF, an electrode active material for M ² M ¹ F ₃ claims claim 1 to 3 or the electrode active material M ² M ¹ F ₃ according, characterized in that the LiFeF ₃ Production method.   The electrode active material manufactured by the method in any one of Claims 1-4.   A nonaqueous electrolyte secondary battery having a positive electrode containing the electrode active material according to claim 5.

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