JP2009238687A - Fluoride electrode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel electrode active material for a nonaqueous electrolyte secondary battery which can make the best use of features of alkali-metal-containing fluoride metal. <P>SOLUTION: The electrode active material made by sufficiently kneading the alkali-metal-containing fluoride metal with a carbon material does not emits oxygen at the time of fully charging at high temperature, and consequently, the secondary battery having no problem on heat generation and expansion of the battery can be obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention belongs to the technical field of chargeable / dischargeable nonaqueous electrolyte secondary batteries, and particularly relates to improvements in electrode active materials that significantly improve the safety of nonaqueous electrolyte secondary batteries.

As conventional positive electrode active materials for lithium ion batteries, layered oxide positive electrodes such as LiCoO ₂ and LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ called ternary system have been used. However, in the fully charged state, so-called abnormal electronic valence states such as Co tetravalent and Ni tetravalent appear, and it becomes an oxygen release source in the sealed space of the battery, causing problems such as heat generation of the battery and increase in internal pressure. It was the cause. In order to solve this type of thermal stability and overcharge resistance problems, a polyanion positive electrode in which oxygen is firmly fixed in the lattice by a covalent bond with a hetero element so that oxygen is not easily released is a positive electrode for next-generation lithium batteries. Although it has been attracting attention as an active material, olivine-type LiFePO ₄ is regarded as the most promising positive electrode for electric vehicle batteries. However, as long as oxygen is present, the amount of released oxygen is not 0, and it is exposed to a high temperature of 300 ° C. or higher. In other words, it is inevitable that oxygen is desorbed from the fully charged olivine positive electrode (S. Okada, M. Ueno, Y. Uebou and J. Yamaki, J. Power Sources, 146, 565-569 (2005) (nonpatent literature 1)).
S. Okada, M. Ueno, Y. Uebou and J. Yamaki, J. Power Sources, 146, 565-569 (2005)

The present inventors have devised the use of a metal fluoride as the positive electrode active material of a nonaqueous electrolyte secondary battery, but there are still few other reported examples of fluoride positive electrodes. It was the present state (Japanese Patent Laid-Open No. 9-22698 (Patent Document 1); Japanese Patent Laid-Open No. 9-55201 (Patent Document 2)).
Japanese Patent Laid-Open No. 9-22698 Japanese Patent Laid-Open No. 9-55201

  An object of the present invention is to provide a novel electrode active material for a non-aqueous electrolyte secondary battery that takes advantage of the characteristics of an alkali metal-containing metal fluoride.

  The inventors of the present invention have sufficiently mixed the alkali metal-containing metal fluoride with the carbonaceous material to form a positive electrode active material, so that oxygen is not released even at full charge at high temperatures. The present invention was derived by finding that a secondary battery having no problem of expansion can be obtained.

Thus, the present invention is an alkali metal-containing metal fluoride (wherein of the general formula _{A x + 2 M x N 1} -x F 6 coated with carbon, A is an alkali metal, M is a trivalent metal element, N Represents a tetravalent metal element, and x represents 0 ≦ x ≦ 1, and provides a positive electrode active material for a nonaqueous electrolyte secondary battery and a secondary battery using the same.

In the non-aqueous electrolyte containing an alkali metal fluoride is carbon coated constituting the positive electrode active material for a secondary battery metal _{A x + 2 M x N 1} -x F 6 of the present invention, the metal element represented by A, an alkali metal Any are applicable. Examples of the metal applied to A include Li (lithium) and Na (sodium). As the metal element represented by M, any trivalent metal can be used. Examples of metals applied to M include Fe (iron), Mn (manganese), V (vanadium), Co (cobalt), Ni (nickel), Sc (scandium), Zr (zirconium), Al (aluminum), Bi (bismuth), Ti (titanium), etc. are mentioned. As the metal element represented by N, any tetravalent metal can be applied. Examples of metals applied to N include Ti (titanium), Zr (zirconium), Mn (manganese), V (vanadium), Co (cobalt), Ni (nickel), Mo (molybdenum), Cr (chromium), Sn (tin) etc. are mentioned. Carbon-coated Li ₂ TiF ₆ , Li ₃ FeF ₆ , Li _2.5 Fe _0.5 Ti _0.5 F _{6 and the} like are used as the positive electrode active material, respectively.

As the negative electrode active material used together with the positive electrode active material in the nonaqueous electrolyte secondary battery, an alkali metal or an alkali metal compound is used, and is generally lithium (Li) or sodium (Na). The system in which the carbon-coated alkali metal-containing fluoride metal A _{x + 2} M _x N _1-x F ₆ of the present invention is used as a positive electrode active material not only exhibits reversible charge / discharge characteristics with a large capacity with respect to Li, Similarly, reversible charge / discharge characteristics are exhibited for Na.

For example, when Li ₂ TiF ₆ coated with carbon in a Li battery is used as a positive electrode active material, a reversible capacity (energy density) of 100 mAh / g or more is realized, and not only Li but also an inexpensive positive electrode for a Na battery In addition, a reversible charge / discharge reaction having a large energy density is obtained (see Examples described later).

Another particularly preferable example to which the present invention is applied is a case where Li ₃ FeF ₆ is used as the lithium-containing fluoride metal, and in a lithium battery using a carbon-coated Li ₃ FeF ₆ as a positive electrode active material, There is little difference in discharge characteristics, and a highly efficient secondary battery having a high energy density exceeding 120 mAh / g is realized (see examples described later).

In addition, for example, by using carbon-coated Li _2.5 Fe _0.5 Ti _0.5 F ₆ or the like as the positive electrode active material, a Li battery or Na battery having reversible charge / discharge characteristics and good energy density can be obtained. It is done.

Alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6 which is a carbon-coated according to the present invention, after the metal oxide as a raw material is heated by dissolving the hydrogen fluoride solution, lithium carbonate or sodium carbonate is added Then, after drying, it is obtained by annealing at a temperature lower than the thermal decomposition temperature of fluoride. The obtained alkali metal-containing fluoride metal A _{x + 2} M _x N _1-x F ₆ is carbon coated by dry mixing for several hours or more by a mechanical mixing means in an atmosphere of a carbonaceous material and an inert gas. .

A preferred and generally used mechanical mixing means is a ball mill, especially a planetary ball mill. Planetary ball mill is particularly preferable because it can sufficiently be pulverized and mixed with an alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6 and carbonaceous material by pulverization energy due to the rotation-revolution.
The mixing time is 1 hour or longer, preferably 4 hours or longer, and the mixing is carried out dry in an inert gas atmosphere such as argon gas.

In the above process, as the carbonaceous material mixed with an alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6, but any carbon-based material having conductivity can be applied, for example, Acetylene black, carbon black, activated carbon and the like can be used. The ratio of metal fluoride MF ₃ : carbonaceous material is generally 50:50 to 90:10 on a weight basis, for example 70:25.

According to the above operation, the alkali metal-containing fluoride metal A _{x + 2} M _x N _1-x F ₆ and the carbonaceous material are sufficiently mixed so that the surface of the fluoride metal is uniformly coated with carbon. It has been confirmed by observation using SEM (scanning electron microscope) and EDS (energy dispersive X-ray spectroscopy).

Thus, the present invention is based on alkali metal-containing gold fluoride A _{x + 2} M _x N _1-x F ₆ , and provides both conductivity imparting and suppression of elution into the electrolyte solution by coating the surface of the fine particles with carbon. As a result, a non-aqueous electrolyte secondary battery with extremely high reversible capacity (energy density) has been realized.
Hereinafter, various alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6 a lithium battery that the cathode active material (Li cell) and sodium that are carbon coated to further specifically showing the features of the present invention An example of charge / discharge measurement performed on the battery (Na cell) will be described.

<Preparation of carbon-coated lithium-containing metal fluoride>
Using planetary ball mill (manufactured by Ito Seisakusho, LA-PO4), mixed with a small amount of fluoropolymer (PTFE: polytetrafluoroethylene) at 200 rpm under Ar atmosphere for a predetermined time with metal fluoride and acetylene black as binder. did.

<Battery structure and composition>
The coin type battery (made by Hosen, R2032) shown in FIG. 1 was used as an example of the battery. In FIG. 1, 1 is a positive electrode, 2 is a negative electrode, 3 is a positive electrode container, 4 is a negative electrode lid, 5 is a separator and electrolyte solution.
The positive electrode has a diameter of 1.0 cm and is composed of the active material (lithium-containing metal fluoride): acetylene black: PTFE = 70: 25: 5 prepared as described above. Acetylene black was made by Denki Kagaku Kogyo, and PTFE was made by Daikin.
The negative electrode had a diameter of 1.5 cm, and Li metal (made by Honjo Metal) was used in the case of the Li cell, and Na metal (made by Aldrich) was used in the case of the Na cell.
In the case of a Li cell, the electrolyte is 1M LiPF ₆ / EC: DMC (1: 1 vol%) (manufactured by Toyama Pharmaceutical Co., Ltd.), 1M LiClO ₄ / EC: DMC (1: 1 vol%) (manufactured by Toyama Pharmaceutical Co., Ltd.), It is 1M LiPF ₆ / MFA (manufactured by Daikin), and in the case of a Na cell, it is 1M NaClO ₄ / PC (manufactured by Toyama Pharmaceutical). The separator is a polypropylene microporous material (manufactured by Celgard).

<Charge / discharge measurement>
The apparatus used for the measurement is NAGANO BTS-2004 (manufactured by Nagano Co., Ltd.). The measurement temperature is 25 ° C., the voltage range is 2.0 to 4.5 V (Li cell), 1.5 to 4.0 V (Na cell), and the current density is generally 0.2 mA / cm ² . .

Li ₂ TiF ₆
Lithium-containing metal fluoride Li ₂ TiF ₆ which is one of the electrode active materials of the present invention was synthesized by the following method. First, rutile TiO ₂ well dried at 100 ° C. is dissolved in 46% hydrofluoric acid to make an aqueous solution of H ₂ TiF ₆ . To this, equimolar lithium carbonate is added and reacted. Ethanol was added thereto to precipitate a hydrated fluoride, which was vacuum-dried at 150 ° C. and then heat-treated at 350 ° C. to obtain Li ₂ TiF ₆ . FIG. 2 shows the powder X-ray diffraction pattern. The obtained powder sample is found to be Li ₂ TiF ₆ having a space group P42 / mnm trigonal tritylyl structure.

FIG. 3 shows a charge / discharge profile of a Li cell (lithium battery) using Li ₂ TiF ₆ coated with carbon at a 25% weight mixing ratio by a planetary mill as a positive electrode active material. Since Ti is tetravalent in the initial state, the initial charge is hardly possible, but the subsequent cycle characteristics are good. The ionic conductivity of Li ₂ TiF ₆ is a small value of 2 × 10 ⁻⁴ S / cm, but by carbon coating, a reversible capacity of 100 mAh / g or more was obtained at an average discharge voltage of about 2.5 V. .

Li ₃ FeF ₆
Lithium-containing metal fluoride Li ₃ FeF ₆ which is one of the electrode active materials of the present invention was synthesized by the following method. First, a stoichiometric ratio of lithium carbonate and iron chloride FeCl ₃ were reacted in hydrofluoric acid, washed with an organic solvent, and then vacuum dried at 150 ° C. to obtain Li ₃ FeF ₆ . FIG. 4 shows the powder X-ray diffraction pattern. It can be seen that the obtained sample is a space group Pna2 ₁ orthorhombic α-phase Li ₃ FeF ₆ .

FIG. 5 shows the electrolyte dependency of the charge / discharge profile of a Li cell (lithium battery) using Li ₃ FeF ₆ carbon-coated by a planetary mill at a 25% weight mixing ratio as a positive electrode active material. In either case, a reversible capacity of 100 mAh / g or more was obtained at an average discharge voltage of about 2.5V. FIG. 6 shows the cycle characteristics. Since Fe is trivalent in the initial state, the initial charge is almost impossible, but the subsequent cycle is good. Moreover, it turns out that a substantially constant reversible characteristic is shown irrespective of electrolyte solution.

FIG. 7 shows a deep charge / discharge profile up to a discharge end voltage of 1 V of a Li cell (lithium battery) using Li ₃ FeF ₆ carbon-coated with a planetary mill at a 25% weight mixing ratio as a positive electrode active material. It can be seen that Li ₃ FeF ₆ is a positive electrode system capable of a conversion reaction that maintains reversibility even during deep charge / discharge of approximately 1.6 electrons.

Na ₃ FeF ₆
Sodium-containing metal fluoride Na ₃ FeF ₆ which is one of the electrode active materials of the present invention was synthesized by the following method. A stoichiometric ratio of iron hydroxide Fe (OH) ₃ and NaF was added to hydrofluoric acid, and a hydrogen peroxide solution was added thereto, and the resulting precipitate was vacuum-dried to obtain Na ₃ FeF ₆ . FIG. 8 shows the powder X-ray diffraction pattern. It can be seen that the obtained sample is a space group P2 ₁ / n monoclinic Na ₃ FeF ₆ .

FIG. 9 shows a charge / discharge profile of a Na cell having the same carbon-coated sodium-containing metal fluoride metal Na ₃ FeF ₆ as a positive electrode. Although the discharge voltage and the reversible capacity are inferior to those of the Li cell, it can be seen that a high reversible discharge capacity is obtained. Here, 1M NaClO ₄ / PC is used, but it is considered that even higher reversible capacity can be realized by using a corresponding electrolyte.

  The present invention enables high reversible charge / discharge capacity (energy density) for nonaqueous electrolyte secondary batteries including inexpensive Na batteries, and the positive electrode active material of the present invention is a battery in various fields of industry, For example, it is expected to be used for a large-scale load leveling power source that requires parallel economy, safety and capacity, and a positive electrode material for electric vehicle batteries.

A battery in which the positive electrode active material of the present invention is used is exemplified. It illustrates the powder X-ray diffraction pattern of the positive electrode active material _Li 2 TiF ₆ of the present invention. The initial charge / discharge profile of the Li cell using the positive electrode active material Li ₂ TiF ₆ of the present invention is illustrated. It illustrates the powder X-ray diffraction pattern of the positive electrode active material _Li 3 FeF ₆ of the present invention. The initial charge / discharge profile of the Li cell using the positive electrode active material Li ₃ FeF ₆ of the present invention is illustrated. The cycle characteristic of the Li cell using the positive electrode active material Li ₃ FeF ₆ of the present invention is illustrated. The initial deep charge / discharge profile of the Li cell using the positive electrode active material Li ₃ FeF ₆ of the present invention is illustrated. It illustrates the powder X-ray diffraction pattern of the positive electrode active material _Na 3 FeF ₆ of the present invention. Illustrate the charging and discharging profile of the positive electrode active Na cell material _Na 3 FeF ₆ of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Positive electrode container 4 Negative electrode lid 5 Separator and electrolyte solution

Claims (8)

Represented by the general formula A _{x + 2} M _x N _1-x F ₆ (wherein A represents an alkali metal, M represents a trivalent metal element, N represents a tetravalent metal element, x represents 0 ≦ x ≦ 1) An electrode active material for a nonaqueous electrolyte secondary battery, comprising an alkali metal-containing metal fluoride. A is at least one alkali metal composed of Li and Na, M is at least one trivalent metal element group composed of Fe, Mn, V, Co, Ni, Sc, Zr, Al, Bi and Ti, and N is 2. The electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the electrode is selected from at least one of a tetravalent metal element group consisting of Ti, Zr, Mn, V, Co, Ni, Mo, Cr, and Sn. Active material. _{2. The} electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the alkali metal-containing metal fluoride A _{x + 2} M _x N _1-x F ₆ is Li _2.5 Fe _0.5 Ti _0.5 F ₆ . The non-aqueous electrolyte secondary battery electrode active material according to claim 1, wherein the alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6 is Li ₂ TiF _6. The non-aqueous electrolyte secondary battery electrode active material according to claim 1, wherein the alkali metal-containing metal fluoride _{A x + 2 M x N 1} -x F 6 is Li ₃ FeF _6. A nonaqueous electrolyte secondary battery electrode comprising the electrode active material according to claim 1. A nonaqueous electrolyte secondary battery comprising the electrode according to claim 6. A nonaqueous electrolyte secondary battery using the electrode according to claim 6 as a positive electrode.

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