JP4343618B2 - Method for producing polyanion type lithium iron composite oxide and battery using the same - Google Patents

Description

  The present invention relates to a method for producing a polyanion-type lithium iron composite oxide that can be used as a positive electrode active material of a lithium secondary battery, and a battery using the same.

  Lithium secondary batteries are widely used as a power source for information and communication equipment, and in recent years, in view of energy and environmental issues, their application as a power source for automobiles is being studied and put into practical use. It has become. For this reason, it has been attempted to use a lithium iron composite oxide containing, as a constituent element, iron having abundant resources as a positive electrode material instead of cobalt or the like that has been used so far.

As one of the attempts, for example, Japanese Patent Laid-Open No. 9-134725 (Patent Document 1) discloses a lithium secondary battery using LiFePO ₄ , LiFeVO _4, or the like having an olivine structure as a positive electrode active material. These lithium iron composite oxides are collectively referred to as polyanion type, and by introducing elements such as phosphorus, vanadium, and sulfur into the oxygen lattice, an oxidation-reduction potential between +2 and +3 valences of iron. It is said that the high energy density can be achieved (Non-patent Document 1).

The polyanion-type lithium iron composite oxide occludes lithium in a discharged state, and iron has a valence of +2. Conventionally, in order to synthesize this, an iron compound having a valence of +2 such as iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) or iron acetate (Fe (CH ₃ CO ₂ ) ₂ ) is used. It was used as an iron source (Patent Document 2). However, + 2-valent iron compounds have poor stability and are expensive. Further, it is susceptible to oxidation and reduction due to the influence of the atmosphere and coexisting compounds during synthesis. In addition, these iron sources generate a large amount of gas during firing, and the bulk density is reduced accordingly, which makes it difficult to synthesize.

On the other hand, a method of synthesizing a precursor using a + trivalent iron compound as a raw material and then reducing it has been reported, but there is a problem that the process becomes complicated (Non-patent Documents 2 and 3). Although using metallic iron FePO 4 ₍₊₃ valent iron compound) method of synthesizing a (non-patent document 4) have also been reported, a method for direct synthesis of LiFePO ₄ was not before.
"Next-generation cathode materials for lithium-ion batteries" Yasuo Yamada, Abstracts of Switching Power Supply / Battery Systems Symposium (April 2003), F5-1-1 Seung-Taek Myung et al., Electrochemistry and industrial physical chemistry, 71 (3), 177 (2003) Pier Paolo Prosiniet al., J. Electrochem. Soc., 149 (7), A886 (2002) Takafumi Yamamoto, Shigeto Okada, Junichi Yamaki, Abstracts of the 70th Anniversary Conference of the Electrochemical Society of Japan (April 2015), 3A19, p60 JP-A-9-134725 JP 2003-034534 A

  As a result of intensive studies, the present inventors have found that a polyanion-type lithium iron composite oxide can be easily synthesized by controlling an oxidation / reduction state by oxidizing an iron-containing substance having a valence lower than +2. The invention has been reached.

That is, the present invention is at least a lithium-containing material, an iron-containing material having a valence lower than +2 , a polyanion-forming element-containing material in which the polyanion-forming element is phosphorus, and an iron-containing material having a valence higher than +2. an oxidizing agent are mixed, characterized by firing at a temperature range of 300 to 1000 ° C., in particular, the production method as the oxidizing agent, polyanionic lithium-iron composite oxide which comprises using a Fe ₃ O ₄ Is to provide.

  Hereinafter, the present invention will be described in detail.

In the present invention, the polyanion-type lithium iron composite oxide basically has lithium, iron, and a polyanion-forming element in the void of the oxygen lattice, and is desorbed from lithium and oxidized from +2 to +3 by charging. This is a composite oxide in which lithium insertion and iron reduction from +3 to +2 occur due to electric discharge. An example is LiFePO ₄ . An example of the polyanion-forming element is P. Here, “basically” means that a compound in which some of these constituent elements are substituted with other elements is included.

The method for producing a polyanion-type lithium iron composite oxide of the present invention includes at least a lithium-containing material, an iron-containing material having a valence lower than +2 , a polyanion-forming element-containing material in which the polyanion-forming element is phosphorus, It is characterized by comprising a raw material mixing step of mixing an oxidizing agent that is an iron-containing substance having a higher valence and a baking step of baking the mixture. In addition, if an iron-containing substance having a valence higher than +2 is used as the oxidant, the oxidant becomes a part of the iron source, and the target lithium iron composite oxide can be easily manufactured at low cost. Become.

Examples of the iron-containing substance having a lower valence than +2 used in the present invention include metallic iron and Fe (CO) ₅ . In particular, metallic iron is preferable because it is inexpensive and easy to handle.

Examples of the lithium compound used in the present invention include Li ₂ CO ₃ , Li (OH), Li (OH) · H ₂ O, LiNO ₃ , LiH ₂ PO _{4 and the} like.

An example of the polyanion-forming element is phosphorus . When the polyanion-forming element is phosphorus, NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , P ₄ O ₁₀ or the like, or phosphite can be used as a raw material. A compound such as LiH ₂ PO _{4 in} which Li: P is contained at a ratio of 1: 1 can also be used as a compound that serves as both a lithium source and a phosphorus source. Furthermore, a compound that combines a phosphorus source, an iron source, and an oxidizing agent, such as FePO ₄ .2H ₂ O, can also be used .

As the oxidizing agent used in the present _{invention, Fe 3 O 4, Fe 2} O 3, FePO iron compounds such as _4. In particular, when a material stable in air at normal temperature such as Fe ₃ O ₄ , Fe ₂ O ₃ , FePO ₄ is used, the composition and the degree of oxidation can be easily controlled. Since the reaction atmosphere is also affected by the addition of carbonaceous material, the selection of atmospheric gas, and the selection of raw material compounds, the selection of oxidants and their mixing ratio depend on the elemental composition ratio, reactivity, and oxidation state of the reaction atmosphere Decide and adjust accordingly. In particular, when a mixture of metallic iron and Fe ₃ O ₄ is used in an inert gas atmosphere, Fe ₃ O ₄ acts as an oxidizing agent for metallic iron, so the valence of iron in the polyanionic lithium iron composite oxide to be produced Can be controlled to +2.

  Next, the firing conditions of the mixture are preferably in the temperature range of 300 to 1000 ° C, more preferably in the range of 450 to 700 ° C. When the temperature is lower than 300 ° C., the polyanion type complex oxide phase does not grow sufficiently, and when the temperature is higher than 1000 ° C., the crystal grains grow too much to reduce the surface area and the characteristics as the positive electrode active material are deteriorated.

  The particle size of the iron-containing substance is preferably in the range of 0.1 to 1000 μm, more preferably in the range of 1 to 150 μm. When the particle diameter is larger than 1000 μm, the contact area is small, so that the solid phase reaction is difficult to proceed. When the particle diameter is smaller than 0.1 μm, especially in the case of metallic iron powder, it is easy to ignite and difficult to handle. Therefore, it is not preferable.

  According to the production method of the present invention, the lithium iron composite oxide can be produced easily and inexpensively with controllability of valence.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited by this Example.

Example 1
After weighing 0.72 g of metal iron (particle diameter 45 μm), 2.99 g of Fe ₃ O ₄ (particle diameter 5 μm) and 5.37 g of LiH ₂ PO ₄ , adding an appropriate amount of acetone and mixing with a ball mill, The raw material mixture was obtained by drying.

When this raw material mixture was calcined at 400 ° C. for 24 hours under the flow of Ar gas, Fe (JCPDS card 6-696), Fe ₃ O ₄ (JCPDS card 19-629), LiPO ₃ (JCPDS card 26-1177) A diffraction line was observed, and a diffraction line of a phase considered to be LiFePO ₄ was weak and unclear.

Therefore, 2.42 g of the raw material mixture was baked at 600 ° C. for 24 hours under the flow of Ar gas, left to cool, taken out and remixed in a mortar, and further baked at 600 ° C. for 24 hours to obtain 2.15 g of product. . FIG. 1 shows the X-ray diffraction of this product. The product obtained was in good agreement with LiFePO ₄ (JCPDS card 40-1499, Triphylite).

Using this product as the positive electrode active material, a lithium secondary battery was prepared. The test cell was produced as follows. 25 parts by weight of acetylene black as a conductive material was mixed with 70 parts by weight of the product powder, and 5 parts by weight of polytetrafluoroethylene (PTFE) was further added and mixed as a binder to obtain a test positive electrode body. When this positive electrode body was extended to a certain thickness, punched out into a circle having a diameter of 10 mm, and vacuum dried at 120 ° C. for 12 hours, the weight was 37 mg. Metal lithium was used for the negative electrode, and a cell was assembled by immersing a 1M-LiPF ₆ / EC + EMC (1: 2) solution as an electrolyte in a polyethylene separator. As a result of the charge / discharge test using this, the current density of the charge / discharge test was 0.2 mA / cm ² , the charge was 4.2 V, and the discharge was 2.5 V (vs. Li / Li +). Was 80 mAh / g based on the product powder.

Example 2
A similar raw material mixture was prepared using ordinary metal iron powder (containing 95% of component of 150 μm or less) in place of the 45 μm metal iron of Example 1. It was baked at 600 ° C. for 24 hours, allowed to cool, taken out, remixed in a mortar, and further baked at 600 ° C. for 24 hours. In X-ray diffraction, in addition to LiFePO ₄ , Fe and Li ₃ Fe ₂ (PO ₄ ) ₃ ( JCPDS card 43-526) has been detected. The same raw material mixture was baked at 700 ° C. for 24 hours, allowed to cool, taken out, remixed in a mortar, and further baked at 700 ° C. for 24 hours. LiFePO ₄ was detected by X-ray diffraction, and the same method as in Example 1 The discharge capacity of the battery prepared in (3) was 32 mAh / g.

Example 3
An aqueous solution containing 0.24 g of sucrose is added to 0.81 g of the raw material mixture of Example 1, and after mixing, baked at 350 ° C. for 24 hours, left to cool, taken out and remixed in a mortar, and further baked at 700 ° C. for 24 hours. As a result, Fe was detected in addition to LiFePO ₄ from X-ray diffraction.

Therefore, reducing the metallic iron, the mixture was supplemented by that amount of Fe content in _Fe 3 _{O 4,} that is, the metallic iron 0.50 g, 2.22 g of _Fe 3 _{O 4,} the _LiH 2 PO ₄ and 3.92g weighed, An appropriate amount of acetone was added and mixed with a ball mill, and then dried to obtain a raw material mixture. An aqueous solution containing 0.24 g of sucrose was added to 0.80 g of this raw material mixture, baked at 350 ° C. for 24 hours after mixing, taken out after standing to cool, remixed in a mortar, and further baked at 700 ° C. for 24 hours. In X-ray diffraction, LiFePO ₄ and Fe were detected, but the diffraction line of Fe was significantly weaker than that of the above-mentioned equivalent mixture of Fe and Fe ₃ O ₄ .

Example 4
0.85 g of metallic iron (particle diameter 45 μm), 2.43 g of Fe ₂ O ₃ and 4.75 g of LiH ₂ PO ₄ were weighed, added with an appropriate amount of acetone, mixed with a ball mill, and then dried to obtain a raw material mixture Got.

This raw material mixture was baked at 600 ° C. for 24 hours under the flow of Ar gas, allowed to cool, taken out, remixed in a mortar, and further baked at 600 ° C. for 24 hours to obtain a product. In addition to LiFePO ₄ , Fe, Fe ₂ O ₃ and LiPO ₃ were detected from X-ray diffraction.

Example 5
0.43 g of metallic iron (particle diameter 45 μm), 2.88 g of FePO ₄ · 2H ₂ O and 0.89 g of Li ₃ PO ₄ were weighed, added with an appropriate amount of acetone, mixed with a ball mill, and then dried. A raw material mixture was obtained.

This raw material mixture was baked at 600 ° C. for 24 hours under the flow of Ar gas, allowed to cool, taken out, remixed in a mortar, and further baked at 600 ° C. for 24 hours to obtain a product. In addition to LiFePO ₄ , Fe and Li ₃ Fe ₂ (PO ₄ ) ₃ were detected from X-ray diffraction.

_{2 is} an X-ray diffraction diagram of LiFePO ₄ produced by the method of Example 1. FIG.

Claims (2)

Mix at least a lithium-containing material, an iron-containing material having a valence lower than +2 , a polyanion-forming element-containing material whose polyanion-forming element is phosphorus, and an oxidizing agent that is an iron-containing material having a valence higher than +2 And baking in a temperature range of 300 to 1000 ° C., a method for producing a polyanion-type lithium iron composite oxide. The method for producing a polyanion-type lithium iron composite oxide according to claim 1, wherein the iron-containing substance having a valence higher than +2 is Fe ₃ O ₄ .

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