JP5011518B2 - Method for producing positive electrode material for secondary battery, and secondary battery - Google Patents

Description

The present invention relates to a method for producing a positive electrode material for a secondary battery and a secondary battery having the positive electrode material as a constituent element. More specifically, the negative electrode active material includes an alkali metal such as metallic lithium or an alloy or compound thereof. The present invention relates to a method for producing a positive electrode material (FePO ₄ ) used in a secondary battery such as a metal lithium battery, a lithium ion battery, or a lithium polymer battery, and a secondary battery having a positive electrode material produced by the method.

Secondary batteries such as metallic lithium batteries, lithium ion batteries, and lithium polymer batteries having an alkali metal such as metallic lithium or an alloy or compound thereof as a negative electrode active material have been attracting attention in recent years because of their large capacity. Various materials have been studied as a rare metal-free positive electrode active material from the viewpoints of high functionality, high capacity, low cost, and the like of such secondary batteries. For example, Patent Document 1 describes a positive electrode active material mainly composed of an olivine-type iron phosphate complex represented by the general formula LiFePO ₄ , but this material has a drawback that it cannot be synthesized in the atmosphere and is inactive. It is necessary to fire in an atmosphere or a reducing atmosphere. On the other hand, Patent Document 2 proposes iron phosphate (FePO ₄ ) as a rare metal-free positive electrode that can be synthesized in the atmosphere. As a method for simply synthesizing iron phosphate, Patent Document 3 discloses a manufacturing method at an extremely low raw material cost in which an aqueous solution of iron and phosphorus pentoxide is mixed for one day using a planetary ball mill and then annealed. There is a problem that the mixing process of the planetary ball mill significantly hinders the mass productivity.

Japanese Patent No. 3523397 Japanese Patent No. 3126007 Japanese Patent Application No. 2002-303932

  The subject of this invention is providing the high performance secondary battery which has the positive electrode material obtained by the manufacturing method which can mass-produce iron phosphate suitable for the positive electrode material for secondary batteries easily and simply, and this method.

  In order to solve the above-mentioned problems, the invention of the method for producing a positive electrode material for a secondary battery according to claim 1 leaves the compound free of phosphate ions and an aqueous solution of metallic iron, without crushing and stirring them. It is characterized by reacting only by this, and then firing.

  According to this method for producing a positive electrode material for a secondary battery, iron phosphate as a positive electrode material can be easily and simply synthesized in the air from a raw material having a stoichiometric composition ratio.

  According to this method for producing a positive electrode material for a secondary battery, not only can the risk of an increase in the internal pressure of the mixing vessel of the aqueous solution due to the generation of hydrogen gas in the planetary ball mill process be avoided, but also the contamination by impurities due to ball wear can be prevented. Can improve the mass productivity by reducing the number of processes.

  The invention of the secondary battery according to claim 3 is characterized in that the positive electrode material produced by the method according to claim 1 or 2 is used as a constituent element. A secondary battery having a positive electrode material manufactured by the method of the present invention can exhibit high-performance battery characteristics as a secondary battery because the electrochemical characteristics of the positive electrode material are excellent.

  The method for producing a positive electrode material for a secondary battery of the present invention is characterized in that a compound that liberates phosphate ions and metallic iron in a solution are allowed to stand in an aqueous solution for 2 weeks, followed by firing.

As the positive electrode material in the present invention, iron phosphate represented by the general formula FePO ₄ that can be synthesized by reacting the raw materials and then firing the reaction product in the air is preferable. Iron phosphate has a trigonal P321 space group, and the FeO ₄ tetrahedron and PO ₄ tetrahedron form a vertex-sharing skeleton, so the bottleneck of guest cation diffusion is large, not only lithium ions, It is an ideal rare metal-free positive electrode material that can be expected to function as an intercalation host even for sodium ions having a 2.5 times larger ion volume.

  As the raw material for iron phosphate, which is the positive electrode material of the present invention, a compound that releases phosphate ions in solution and metallic iron are used. These raw materials are preferably adjusted along the stoichiometric composition ratio so that the molar ratio of P: Fe is 1: 1. The compound that liberates phosphate ions in the solution is not particularly limited, and examples thereof include phosphoric acid, phosphorus pentoxide, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and the like. Among these, phosphoric acid and phosphorus pentoxide are preferred because it is preferable to maintain relatively strong acidic conditions in the process of dissolving iron and to generate less ammonia gas as an extra by-product.

  When phosphoric acid is used as a raw material, since phosphoric acid is usually marketed in the form of an aqueous solution, it is preferably used after its content (purity) is determined by titration or the like. In order to promote the reaction without stirring, the metallic iron is preferably in a fine powder state (particle size of 100 microns or less) which has a small surface area and is easily dissolved in an acid. Since these are used as raw materials in an aqueous solution state for 2 weeks and then water is removed, it is possible to synthesize iron phosphate free from impurities by heating and baking. Since the aqueous solution does not contain a non-volatile element such as sodium in the raw material, not only stirring but also complicated steps such as filtration and fractional distillation are unnecessary.

  Firing can be performed, for example, in a one-step temperature rise and holding process from room temperature to firing completion temperature (100 ° C. to 800 ° C., more preferably 300 ° C. to 650 ° C.). Moreover, it can also divide into two steps, the baking process (temporary baking) in a low temperature range (normal temperature-300 degreeC), and the baking process (main baking) in a high temperature range (300 degreeC-800 degreeC). In the firing, it is preferable to keep the precursor in the temperature range for several hours to about 1 day.

  The positive electrode material according to the present invention obtained as described above can be suitably used as a constituent material of a secondary battery. One nonaqueous electrolyte secondary battery provided by the present invention includes the positive electrode material described above. Moreover, the negative electrode which has the material which occludes and discharges alkali metal and alkaline-earth metal ions, such as lithium ion and sodium ion, is provided. In addition, the secondary battery can include a non-aqueous electrolyte. Since such a secondary battery includes an electrode active material with improved charge / discharge characteristics, the battery performance can be improved.

  The positive electrode active material according to the present invention can function as an electrode active material of a secondary battery by inserting and removing various cations. As cations to be inserted and removed, alkali metal ions such as lithium ion, sodium ion, potassium ion and cesium ion, alkaline earth metal ions such as calcium ion and barium ion, magnesium ion, aluminum ion, silver ion and zinc ion Ammonium ions such as tetrabutylammonium ion, tetraethylammonium ion, tetramethylammonium ion, triethylmethylammonium ion, triethylammonium ion, imidazolium ions such as imidazolium ion, ethylmethylimidazolium ion, pyridinium ion, hydrogen ion Tetraethylphosphonium ion, tetramethylphosphonium ion, tetraphenylphosphonium ion, triphenylsulfonium ion Emissions, triethyl sulfonium ions, and the like. Among these, preferred are alkali metal and alkaline earth metal ions, and lithium ions and sodium ions are particularly preferred.

  When this positive electrode active material is used in a battery, the active material of the negative electrode that is the counter electrode can be a metal such as lithium, sodium, magnesium, calcium, or an alloy thereof, or a carbon material that can occlude and release cations. .

  The electrode having the positive electrode active material according to the present invention can be suitably used as an electrode of a secondary battery having various shapes such as a coin shape, a cylindrical shape, and a square shape. For example, the electrode active material can be compression-molded to form a pellet-shaped electrode. A plate-like or sheet-like electrode can be formed by attaching the electrode active material to a current collector made of a conductive material such as metal. In addition to the electrode active material according to the present invention, such an electrode can contain one or more materials similar to those using a general electrode active material, if necessary. Typical examples of such a material include a conductive material and a binder. A carbon material such as acetylene black can be used as the conductive material. As the binder, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), or the like can be used.

As the non-aqueous electrolyte used for the secondary battery, a non-aqueous solvent and a compound containing a compound (supporting electrolyte) containing a cation that can be inserted into and removed from the electrode active material can be used.
As the non-aqueous solvent constituting the non-aqueous electrolyte, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used without particular limitation. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1, Examples include 3-dioxolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. Only 1 type selected from such a non-aqueous solvent may be used, and 2 or more types may be mixed and used for it.
In addition, as the supporting electrolyte constituting the non-aqueous electrolyte, a compound containing a cation inserted / extracted from the electrode active material, for example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO _{2 in} the case of a lithium ion secondary battery). ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiClO _4, or one or more selected from lithium compounds (lithium salts) can be used.

Hereinafter, the present invention will be described in more detail with reference to experimental examples.
<Example 1: Preparation of FePO ₄ powder sample>
Iron powder (98% Wako Pure Chemical) as the starting material and phosphorus pentoxide (P ₂ O ₅ ) (98% Wako Pure Chemical) as the phosphorus source in the glove box to a total stoichiometric ratio of 25 g Weighed to be. Then, after adding pure water (200 ml) in a fume hood and allowing it to stand for 15 days, it was dried in the atmosphere at 100 ° C. for 24 hours, and then thoroughly agitated using an agate mortar. Annealing treatment was performed at 100 ° C. to 650 ° C. for 12 hours. The calcined sample was identified using a powder X-ray diffractometer (Rigaku RINT 2100HLR / PC). From the X-ray diffraction results shown in FIG. 1, it was confirmed that the positive electrode material obtained by the heat treatment at 650 ° C. for 12 hours was a trigonal P321 iron phosphate (ICDD No. 29-0715) single phase.

<Comparative Example 1: Preparation of FePO ₄ powder sample>
Iron powder (98% Wako Pure Chemical) as the starting material and phosphorus pentoxide (P ₂ O ₅ ) (98% Wako Pure Chemical) as the phosphorus source in the glove box to a total stoichiometric ratio of 25 g Weighed to be. After that, after adding pure water (200 ml) in a fume hood and reacting with stirring in a planetary ball mill for 1 day, it was dried in the atmosphere at 100 ° C. for 24 hours in the same manner as in Example 1. The agate mortar was used, and annealing was performed at 100 ° C to 650 ° C for 12 hours in the atmosphere.

<Example 2: Production of FePO ₄ positive electrode pellet>
Weight of the positive electrode material obtained in Example 1 and Comparative Example 1, acetylene black (Denka Black 50% manufactured by Denki Kagaku Kogyo) as a conductivity-imparting agent, and PTFE (polytetrafluoroethylene) as a binder The ratio was adjusted to 70: 25: 5, mixed and kneaded to form a 0.7 mm sheet, which was punched out to a diameter of 10 mm to obtain a positive electrode pellet.

<Example 3: Production of FePO ₄ positive electrode / lithium negative electrode coin battery>
As the counter electrode of the positive electrode pellet obtained in Example 2, a lithium foil having a diameter of 1.5 mm and a thickness of 0.15 mm was used. As the separator, a porous polyethylene sheet having a diameter of 22 mm and a thickness of 0.02 mm was used. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of about 1 mol / liter in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1: 1 was used. These constituent elements were incorporated into a stainless steel container to produce a coin-type measuring cell shown in FIG. 2 having a thickness of 2 mm and a diameter of 32 mm (2032 type). A series of battery assembly operations were performed in a dry box having a dew point of −90 ° C. or less equipped with an argon purifier.

（表１） 実施例３と実施例４で作製したリチウム及びナトリウムコインセルの構成表
<Example 4: Production of FePO ₄ positive electrode / sodium negative electrode coin battery>
As the counter electrode of the positive electrode pellet obtained in Example 2, a sodium foil having a diameter of 1.5 mm and a thickness of 0.15 mm was used. As the non-aqueous electrolyte, a solution obtained by dissolving LiClO ₄ in a propylene carbonate (PC) single solvent at a concentration of about 1 mol / liter was used. Other constituent elements are the same as those in the third embodiment as shown in Table 1. A series of battery assembly operations were performed in a dry box having a dew point of −90 ° C. or less equipped with an argon purifier.

(Table 1) Composition table of lithium and sodium coin cells prepared in Example 3 and Example 4

<Example 5: Dependence of charge / discharge characteristics of FePO ₄ positive electrode on each heat treatment temperature>
The charge / discharge profile of the first cycle of the FePO ₄ positive electrode of Example 1 obtained at each heating and firing temperature of 100 ° C. to 650 ° C. is shown in FIGS. Rather, it can be seen that the reversible capacity is highest in the 350 ° C. heat-fired FePO ₄ positive electrode that has not progressed in crystallization.

<Example 6: Heat treatment temperature dependency of rate characteristics of heat-fired FePO ₄ positive electrode>
FIG. 8 shows the discharge profile at each discharge current density (1.0, 2.0, 3.0, 4.0, 5.0 mA / cm ^{2 in} order from the right) of the FePO ₄ positive electrode of Example 1 obtained at the heating and firing temperatures of 100 ° C. and 200 ° C. And in FIG. Further, FIG. 10 shows a comparison of the rate characteristic results of the FePO ₄ positive electrode of Example 1 that was allowed to stand for 2 weeks and the FePO ₄ positive electrode of Comparative Example 1 that was reacted with stirring in a planetary ball mill for one day. It can be seen that the FePO ₄ positive electrode allowed to stand for 2 weeks is superior in rate characteristics.

<Example 7: Each heat treatment temperature dependence of rate characteristics of 200 ° C. heat-fired FePO ₄ positive electrode>
The charge / discharge curves of the initial cycle of the sodium battery produced in Example 4 using the FePO ₄ positive electrode of Example 1 obtained at the heating and firing temperatures of 100 ° C. and 200 ° C. are shown in FIGS.
It was confirmed that the FePO ₄ positive electrode composed only of the vertex shared skeleton can function as a positive electrode material excellent in reversibility not only for lithium ions but also for sodium ions as intended.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In addition, the technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Moreover, the technique illustrated in this specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

X-ray profile of the synthesized sample at each heating and baking temperature produced in Example 1 The block diagram of the coin cell produced in Example 3 Initial charge / discharge profile of 100 ° C. heat-fired FePO ₄ positive electrode produced in Example 1 Initial charge / discharge profile of 200 ° C heat-fired FePO ₄ positive electrode produced in Example 1 Initial charge / discharge profile of 350 ° C heat-fired FePO ₄ positive electrode prepared in Example 1 First charge / discharge profile of FePO ₄ positive electrode with 500 ° C. heat-fired heat treatment prepared in Example 1 Initial charge / discharge profile of 650 ° C. heat-fired FePO ₄ positive electrode produced in Example 1 Rate characteristics of 100 ° C. heat-fired FePO ₄ positive electrode produced in Example 1 (1.0, 2.0, 3.0, 4.0, 5.0 mA / cm ^{2 in} order from the right) Rate characteristics of the 200 ° C. heat-fired FePO ₄ positive electrode prepared in Example 1 (1.0, 2.0, 3.0, 4.0, 5.0 mA / cm ^{2 in} order from the right) Rate characteristics of heat-fired FePO ₄ positive electrode produced in Example 1 (1.0, 2.0, 3.0, 4.0, 5.0 mA / cm ^{2 in} order from the right) First charge / discharge curves of lithium battery and sodium battery of FePO ₄ positive electrode heated and baked at 100 ° C. prepared in Example 1 First charge / discharge curves of lithium battery and sodium battery of FePO ₄ positive electrode heated and baked at 200 ° C. prepared in Example 1

Claims (3)

A positive electrode for a secondary battery, characterized in that an aqueous solution of a compound that liberates phosphate ions in a solution and an aqueous solution of metallic iron are crushed and left to stand without reacting and then fired. Production method of material FePO ₄ . 2. The positive electrode material for a secondary battery according to claim 1, wherein the compound that liberates phosphate ions in the solution is phosphoric acid, phosphorus pentoxide, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate. Manufacturing method of FePO ₄ . A secondary battery comprising a positive electrode having a positive electrode active material FePO ₄ produced by the method according to claim 1 or 2 as a constituent element.

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