WO2011043255A1

WO2011043255A1 - Positive electrode material for lithium ion secondary battery, and process for production thereof

Info

Publication number: WO2011043255A1
Application number: PCT/JP2010/067217
Authority: WO
Inventors: 剛本間; 高行小松; 拓也富樫
Original assignee: 国立大学法人長岡技術科学大学
Priority date: 2009-10-06
Filing date: 2010-10-01
Publication date: 2011-04-14
Also published as: JPWO2011043255A1; JP5446017B2

Abstract

Disclosed are: a positive electrode material for a lithium ion secondary battery, which can be produced by a more simplified production process and in which an electrically conductive auxiliary agent can be attached on the surface of particles of the positive electrode material with high efficiency; and a process for producing the positive electrode material. The process for producing a positive electrode material for a lithium ion secondary battery is characterized by comprising the steps of: (1) preparing a batch which comprises lithium methaphosphate (LiPO₃) powder and a transition metal oxide represented by chemical formula MO, MO₂, M₃O₄ or M₂O₃ (wherein M represents at least one element selected from Fe, Mn, Co and Ni) in such a manner that the molar ratio among lithium, phosphorus and a transition metal is 1:1:(0.8-1.2); (2) adding a reducing agent to prepare a mixed powder; and (3) burning the mixed powder to form crystals of LiMPO₄ (wherein M represents at least one element selected from Fe, Mn, Co and Ni) or crystals composed of a solid solution of the aforementioned LiMPO₄.

Description

Positive electrode material for lithium ion secondary battery and method for producing the same

The present invention relates to a positive electrode material for lithium ion secondary batteries used in portable electronic devices and electric vehicles, and a method for producing the same. More specifically, the present invention relates to a phosphate-based positive electrode material that is inexpensive and effective for mass synthesis, and a method for producing the same, in place of conventional lithium cobalt oxide (LiCoO ₂ ).

Lithium ion secondary batteries have established themselves as high-capacity and light-weight power supplies essential for portable electronic terminals and electric vehicles. In the past, inorganic metal oxides such as lithium cobaltate and lithium manganate (LiMnO ₂ ) have been used as the positive electrode material of the lithium ion secondary battery. With the recent increase in power consumption due to higher performance of electronic devices, further increase in capacity of lithium ion secondary batteries is required. In addition, from the viewpoint of environmental conservation problems and energy problems, there is a demand for switching from materials with a large environmental load such as Co and Mn to more environmentally conscious materials. Further, in recent years, the problem of depletion of cobalt resources has attracted attention, and from such a viewpoint, conversion to an inexpensive positive electrode material replacing LiCoO ₂ is desired.

In recent years, manganese-based spinel, NASICON-type Li ₃ Fe ₂ (PO ₄ ) ₃ and olivine-type LiFePO ₄ crystals have attracted attention among iron-containing lithium compounds because they are advantageous in terms of cost and resources. Various researches and developments are underway (see, for example, Patent Document 1). Among them, the olivine type LiFePO ₄ crystal is superior in temperature stability to LiCoO ₂ and is expected to operate safely at high temperatures. Further, because of the structure having phosphoric acid as a skeleton, it has a feature of excellent resistance to structural deterioration due to charge / discharge reaction.

It is known that the iron sites of manganese spinel type, NASICON type Li ₃ Fe ₂ (PO ₄ ) ₃ and olivine type LiFePO ₄ crystals can be replaced with various transition metal ions. For example, Li ₃ Mn ₂ (PO ₄ ) ₃ , LiMnPO ₄ or the like in which iron is completely substituted with manganese, or partially substituted Li ₃ (Mn _x Fe _1-x ) ₂ (PO ₄ ) ₃ or LiMn _x Fe _{1 -X} PO ₄ (0 <x <1) or the like also has a function as a positive electrode material.

In addition, the characteristics required for the positive electrode material of the lithium ion secondary battery include high ionic conductivity and electronic conductivity. However, the electronic conductivity of the lithium iron phosphate positive electrode material is usually higher than that of LiCoO _2. Low is reported as a problem.

In addition, as a method for synthesizing lithium iron phosphate, a hydrothermal method or a thermal decomposition method for obtaining nanometer-order crystal grains by heat-treating an iron raw material having a divalent valence at a low temperature has been proposed (for example, However, a raw material reagent containing divalent iron is expensive, which leads to an increase in manufacturing cost. Therefore, it is desirable that the production process be simple in order to realize large-scale synthesis of lithium iron phosphate at a low production cost.

In order to solve these problems, the inventors of the present invention have, as far as shown in Patent Document 2, a mixture containing a lithium-based oxide, an iron-based oxide, and a phosphate-based oxide in a reducing atmosphere. A precursor glass body is prepared by melting and quenching in, and a method for producing lithium iron phosphate by applying a heat treatment in the vicinity of the crystallization temperature of the obtained precursor glass is proposed.

However, the reducing and melting step in the production method described in Patent Document 2 is a complicated process, and the atmosphere control in the step is difficult in practice, so a simpler synthesis method is desired.

In order to solve the above-mentioned problem of Patent Document 2, the inventors of the present application disclosed a mixture containing a lithium-based oxide, an iron-based oxide, and a phosphate-based oxide in the atmosphere, as shown in Patent Document 3. A precursor glass is prepared by melting, and after pulverizing the precursor glass, a carbon compound is mixed, and the precursor glass and the carbon compound are co-fired at around 500 to 700 ° C. to obtain an olivine type LiFePO ₄ , or A method for synthesizing a solid solution of LiMn _x Fe _1-x PO ₄ has been proposed. In this method, the target crystal can be synthesized by heat-treating glass (amorphous) powder with a uniform composition distribution, and the synthesis process is completed at a lower temperature and in a shorter time than conventional synthesis methods. It is a feature.

However, the manufacturing method described in Patent Document 3 does not require the reduction melting step essential to the manufacturing method of Patent Document 2, but the step of subjecting the mixture of lithium, iron, and phosphoric acid oxide to normal atmospheric melting is still performed. is necessary. The melting process of the mixture requires enormous energy and has problems such as corroding a noble metal container (for example, a container made of a platinum-based material) containing the mixture. That is, a melt containing a large amount of a transition metal oxide such as iron corrodes a noble metal, which may significantly reduce the life of a noble metal container. In addition, a step of finely pulverizing a glass sample (precursor glass) once produced by this atmospheric melting step is also required. Therefore, there is room for further simplification of the manufacturing process, and for this purpose, it is necessary to overcome these problems.

JP-A-9-134725 JP 2008-047412 A JP 2009-087933 A

Several methods of synthesizing positive electrode materials for lithium secondary batteries at low temperatures have been proposed in this way, but expensive transition metal compounds are used as starting materials, and the starting materials are subjected to pretreatment such as melting and grinding processes. It is a complicated manufacturing process.

The present invention has been made in view of such a situation, and a lithium ion secondary battery positive electrode material capable of efficiently imparting a conductive additive to the surface of the positive electrode material particles, further simplifying the manufacturing process, and its An object is to provide a manufacturing method.

As a result of studying to solve the above problems, the inventors of the present application prepared lithium metaphosphate (a composite oxide of phosphorus and lithium) and a transition metal compound as raw materials at a predetermined molar ratio, set in a reducing atmosphere, or If a reducing agent is added and calcined in a temperature range of 450 to 700 ° C., crystallized glass powder of olivine type LiMPO ₄ crystal (M is at least one selected from Fe, Mn, Co, Ni) can be obtained. And that the glass powder can be efficiently combined with a conductive additive that improves electronic conductivity, and a positive electrode material for a lithium ion secondary battery with high conductivity can be obtained, and is proposed as the present invention. It is.

That is, according to the first aspect of the present invention, (1) powdered lithium metaphosphate LiPO ₃ and chemical formula MO, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is Fe, Mn, Co, A step of preparing a batch of a transition metal oxide represented by at least one selected from Ni) so that the molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2) And (2) a step of setting the ambient environment of the prepared batch under a reducing atmosphere, and (3) firing the obtained mixed powder, LiMPO ₄ crystal (M is selected from Fe, Mn, Co, Ni) And a step of forming a crystal composed of a solid solution thereof, and a method for producing a positive electrode material for a lithium ion secondary battery.

In addition, electrochemical characteristics change by substituting transition metal ions with various ions. For example, lithium iron phosphate LiFePO ₄ is characterized by abundant iron resources and low raw material costs. In addition, LiMnPO ₄ , LiCoPO ₄ and LiNiPO ₄ substituted with manganese, cobalt, or nickel instead of iron are superior materials from the viewpoint of higher output because they have a higher voltage than LiFePO ₄ .

The second aspect of the present invention is as follows: (1) Powdered lithium metaphosphate LiPO ₃ and chemical formula MO, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is Fe, Mn, Co, A step of preparing a batch of a transition metal oxide represented by at least one selected from Ni) so that the molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2) And (2) a step of adding a reducing agent, and (3) firing the obtained mixed powder, LiMPO ₄ crystals (M is at least one selected from Fe, Mn, Co, Ni) or a solid solution thereof. Forming a crystal composed of: a lithium ion secondary battery positive electrode material manufacturing method.

The difference between the second embodiment and the first embodiment is that the reducing agent is actually added to the prepared batch or the ambient environment of the prepared batch is subjected to a predetermined reducing atmosphere (for example, 7% hydrogen-93 % Argon). The production method of the first aspect is advantageous in that LiMPO ₄ crystals (for example, LiFePO ₄ ) can be formed by heat treatment in a predetermined reducing atmosphere without adding a reducing agent such as glucose. In order to give, the manufacturing method of the 2nd aspect is suitable.

As a third aspect, the method for producing a positive electrode material for a lithium ion secondary battery according to the present invention is characterized in that the formed LiMPO ₄ crystal is a LiMn _x Fe _1-x PO ₄ crystal (0 <x <1). And

LiMnPO ₄ has a higher voltage than LiFePO ₄ , but since lithium desorption is poor, a solid solution LiMn _X Fe _1-x PO ₄ crystal (0 <x <1) mixed with iron and manganese is formed. Thus, lithium insertion / extraction is improved as compared with LiMnPO _{4, and} a positive electrode material having a higher voltage than LiFePO ₄ can be synthesized.

As a fourth aspect, in the method for producing a lithium ion secondary battery positive electrode material of the present invention, the reducing agent is added in an amount of 0.1 to 50 parts by mass with respect to 100 parts by mass of the mixture prepared in the step (1). It is characterized by being. Thereby, the valence state of the transition metal ion is controlled, and the reduction is effectively promoted.

As a fifth aspect, the method for producing a positive electrode material for a lithium ion secondary battery of the present invention is characterized in that the reducing agent is glucose.

Here, glucose added as a reducing agent for crystal synthesis changes into a carbon-based substance (amorphous carbon) having conductivity during the heat treatment, so that the conductivity of the lithium ion secondary battery positive electrode material is improved. It is valid.

As a sixth aspect, the method for producing a positive electrode material for a lithium ion secondary battery according to the present invention is characterized by further comprising (4) a step of adding a conductive additive containing a carbon-based compound.

Since olivine-type LiMPO ₄ has no electron conductivity in a single crystal, a secondary battery positive electrode excellent in electron conductivity by coating the surface of the crystal with a carbon-containing conductive auxiliary agent for the purpose of supplementing the electron conductivity. The material can be synthesized. As described above, the addition of the reducing agent is effective in improving the conductivity because the predetermined reducing agent also changes to a carbonaceous material having conductivity during the heat treatment. However, in order to increase the electron conductivity, the conductive auxiliary agent May be added separately. Carbon-based materials such as graphite, acetylene black, and amorphous carbon are chemically stable substances, and lithium ions having higher electrical conductivity can be obtained by coating the surface of LiMPO ₄ powder with a carbon-based conductive additive. A secondary battery positive electrode material can be synthesized.

As a seventh aspect, the method for producing a positive electrode material for a lithium ion secondary battery of the present invention is characterized in that the firing temperature is in the range of 600 to 700 ° C.

As an eighth aspect, the method for producing a positive electrode material for a lithium ion secondary battery according to the present invention comprises the step of melting lithium metaphosphate LiPO ₃ at a temperature of 1000 to 1200 ° C. before the preparation step of (1). And a pretreatment step of pulverizing the lithium metaphosphate LiPO ₃ in an amorphous state. By using amorphous lithium metaphosphate as a raw material, firing in a lower temperature range is possible as described below.

As a ninth aspect, the manufacturing method according to the eighth aspect is characterized in that the firing temperature is in a temperature range of 450 to 700 ° C. In other words, by using the amorphous lithium metaphosphate obtained in the pretreatment step described in the eighth aspect as a raw material, phosphorous can be produced at a lower firing temperature than when crystalline lithium metaphosphate is used as the raw material. It becomes possible to synthesize lithium iron oxide.

As a tenth aspect, the present invention relates to a powdered lithium metaphosphate LiPO ₃ and a chemical formula MO, M ₂ O, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is Fe, Mn, Co, A transition metal oxide represented by at least one selected from Ni) is mixed so that the molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2). The present invention relates to a positive electrode material for a lithium ion secondary battery, which is fired in an atmosphere.

As an eleventh aspect, the present invention relates to a powdered lithium metaphosphate LiPO ₃ and a chemical formula MO, M ₂ O, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is Fe, Mn, Co). , At least one selected from Ni) and a molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2), The present invention relates to a positive electrode material for a lithium ion secondary battery, which is fired by further adding a reducing agent.

As a twelfth aspect, the lithium ion secondary battery positive electrode material of the present invention is characterized in that the lithium metaphosphate LiPO _{3 according} to the tenth or eleventh aspect is in an amorphous state.

As a thirteenth aspect, the lithium ion secondary battery positive electrode material of the present invention includes an inorganic oxide composed of phosphoric acid, lithium, and a transition metal in the fired product according to any of the tenth to twelfth aspects. The amorphous phase of the product is contained, and the content is 0.01 to 20 parts by volume.

As will be described later, the electrical conductivity of the positive electrode material of the lithium ion secondary battery of the present invention is 4 to 5 digits relative to the value of the conventional product (usually on the order of 1.0 × 010 ⁻⁹ S · cm ⁻¹ ). Improve with orders. This is presumably because many amorphous phases remain in the final fired product, which is excellent in ionic conductivity (that is, the amorphous phase behaves as a good ionic conductive phase).

As a fourteenth aspect, in addition to the fired product according to any one of the tenth to thirteenth aspects, a conductive additive is further contained.

According to the present invention, since lithium metaphosphate is used as a raw material, the molar ratio of lithium to phosphorus is 1: 1, and a transition metal oxide is added to this to form lithium, phosphorus, transition, which is a composition of olivine crystals. The molar ratio of metals can be easily set in the vicinity of 1: 1: 1. Moreover, since the melting point of lithium metaphosphate is 640 ° C., which is lower than the melting point of lithium orthophosphate, 837 ° C., it is possible to synthesize olivine crystals at a low temperature.

According to the present invention, a pretreatment process for melting and crushing a lithium-iron (transition metal) -phosphoric acid-based mixture is not required in producing a lithium ion secondary battery positive electrode material. Therefore, the manufacturing process can be further simplified, and it is possible to solve the problems of the conventional manufacturing method, such as requiring enormous energy and corroding the container containing the mixture.

In one embodiment of the present invention, in order to obtain amorphous lithium metaphosphate, it is preferable to perform pretreatment by melting and pulverizing lithium metaphosphate itself, but lithium, iron (transition metal), and phosphoric acid. There is no problem as in the conventional pretreatment process in which the mixture of the system oxides is melted and pulverized. A melt containing a large amount of a transition metal oxide such as iron corrodes a noble metal, which may significantly reduce the life of a noble metal container (for example, a container made of a platinum-based material). On the other hand, in the case of the lithium metaphosphate employed in the present invention, the melt containing lithium metaphosphate does not react with the container even if it is melted in a platinum-based container.

Further, according to the above aspect of the present invention, since a high-quality olivine-type crystal can be obtained even when a lithium metaphosphate powder having a relatively large average particle size is used, the time required for the lithium metaphosphate grinding process And less effort.

It is a figure which shows the powder X-ray-diffraction pattern (in the case of reducing agent addition) of the sample produced in Example 1. FIG. It is a figure which shows the powder X-ray-diffraction pattern (in the case of no addition of a reducing agent) of the sample produced in Example 1. 4 is a diagram showing a differential thermal analysis curve of lithium metaphosphate LiPO ₃ glass produced in Example 3. FIG. 6 is a diagram showing a powder X-ray diffraction pattern of a sample produced in Example 3. FIG.

The lithium ion secondary battery positive electrode material of the present invention includes lithium metaphosphate LiPO ₃ and the chemical formula MO, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is at least one selected from Fe, Mn, Co, Ni) ) In which the molar ratio of lithium, phosphorus and transition metal is close to 1: 1: 1 (that is, 0.8 to 1.2). The body is used as a raw material.

Here, when the valence of the transition metal ion M is divalent, the transition metal oxide is represented by the chemical formula MO, and examples thereof include Fe (II) O and NiO. When the valence of the transition metal ion M is trivalent, the transition metal oxide is represented by the chemical formula M ₃ O ₄ or M ₂ O ₃ , for example, hematite Fe ₂ O ₃ , magnetite Fe ₃ O ₄ , Co ₃ O _4. , Mn ₃ O ₄ . When the valence of the transition metal ion M is tetravalent, the transition metal oxide is represented by the chemical formula MO ₂ , for example, MnO ₂ .

Next, the reason for limiting the composition as described above will be described below.

Lithium metaphosphate is a composite oxide having a molar ratio of phosphorus to lithium of 1: 1. The advantage of using lithium metaphosphate as a raw material is that the molar ratio of lithium and phosphorus is 1: 1, and by adding a transition metal oxide to this, the molar ratio of lithium, phosphorus, and transition metal, which is the composition of olivine crystals, is 1 1: 1 is easy. For example, when lithium orthophosphate (Li ₃ PO ₄ ) is used as a raw material for other lithium phosphates, the ratio of phosphorus to lithium is 1: 3. It is necessary to add ammonium dihydrogen phosphate. Moreover, since the melting point of lithium metaphosphate is 640 ° C., which is lower than the melting point of lithium orthophosphate, 837 ° C., it is suitable for low-temperature synthesis.

The lithium metaphosphate is preferably in an amorphous state. Lithium metaphosphate itself easily exhibits an amorphous state (that is, a glass state) through melting and quenching. Since the glass transition temperature of the glassy lithium metaphosphate is as low as 329 ° C., even if it is not heated to 640 ° C., which is the melting point, if it is heated to 329 ° C. or higher, it becomes a supercooled liquid state and exhibits fluidity. For this reason, it can be predicted that the reaction with the transition metal compound occurs at a lower temperature than when crystalline lithium metaphosphate is used as a raw material.

The range of the average particle size of the lithium metaphosphate powder is preferably as small as possible (for example, 3 μm or less), but may be a relatively large size of about 20 μm in average particle size. In addition, since lithium metaphosphate becomes a liquid phase before crystallization, a desired crystallized glass can be obtained even if a powder having a relatively large average particle size of about 20 μm is used. Therefore, if lithium metaphosphate having such an average particle diameter is used, it is possible to reduce the labor required for pulverizing lithium metaphosphate.

The transition metal oxide represented by the chemical formula MO, MO ₂ , M ₃ O ₄ , or M ₂ O ₃ (M is at least one selected from Fe, Mn, Co, Ni) is also a main component of the olivine type crystal. It is. Since the valence state of the transition metal ion M represented by the olivine crystal LiMPO ₄ is +2, it is preferable that the valence of the transition metal ion in the raw material is +2 in order to precipitate this crystal. If the valence state becomes +2 in the course of firing due to the effect of the reducing agent, raw materials having other valence states may be used. Since Fe (II) O is expensive if it is iron, it is not suitable for reducing raw material costs, but even if hematite Fe ₂ O ₃ or magnetite Fe ₃ O ₄ is used, the valence state can be easily reduced to +2 by reduction. Therefore, it is preferable to use these as raw materials. From the viewpoint of reducing raw material costs, it is preferable to select MnO _{2 for} other transition metal oxides, Co ₃ O _{4 for} cobalt, NiO for nickel, etc. for cobalt. The smaller the particle size of the transition metal oxide, the richer the reactivity, and the more effective the prevention of coarsening of the resulting olivine-type crystal. Specifically, the average particle size is preferably 5 μm or less, preferably 1 μm or less. More preferably.

In the lithium ion secondary battery positive electrode material of the present invention, the transition metal ion M in the chemical formula LiMPO ₄ forms a compound consisting of any one of Fe, Mn, Co, Ni, or a solid solution consisting of a combination of two or more. The composition is not particularly limited, but lithium iron manganese lithium LiMn _x Fe _1-x PO ₄ crystals (0 <x <1) form a solid solution at a total rate, so the composition of iron and manganese is changed. As a result, the value of x can be changed.

In addition, since the valence of the transition metal ion M in the olivine-type LiMPO ₄ is +2, it is oxidized to +3 due to oxidation-reduction equilibrium when subjected to heat treatment in the open atmosphere, and Li ₃ M ₂ (PO ₄ having a Nasicon-like structure). ₃ ) is easy to form. Therefore, in order to control the valence state, a reducing agent containing carbon such as glucose during heating is added in an amount of 0.1 to 50 parts by mass with respect to 100 parts by mass of the crystal constituent raw materials (lithium metaphosphate and transition metal oxide). It is preferable to add in a range. When the addition amount of the reducing agent is less than 0.1 parts by mass, effective reduction of the transition metal ion does not proceed, and there is a possibility that a heterogeneous phase such as Li ₃ M ₂ (PO ₄ ) ₃ is formed. Moreover, when it exceeds 50 mass parts, there exists a possibility that substantial battery capacity may fall when a battery is formed.

Moreover, it is also preferable to heat-process the crystal | crystallization constituent raw material in the reaction container excellent in the airtightness filled with reducing gas. Furthermore, after the valence state of the transition metal oxide is once controlled, such as by heat treatment in a reducing atmosphere of hydrogen, ammonia, carbon monoxide, etc., the crystal constituent material may be heat-treated at a temperature range of 450 to 700 ° C. Preferably, this makes it possible to produce crystal powder consisting of LiMPO ₄ alone.

Note that when the firing temperature is lower than 450 ° C., reduction of transition metal ions proceeds, but the reaction for forming LiMPO ₄ may not proceed. On the other hand, when the firing temperature exceeds 700 ° C., grain growth and sintering occur to form coarse crystals, which may reduce the specific surface area important for the charge / discharge reaction.

The smaller the particle size of the formed olivine-type LiMPO ₄ crystal powder, the larger the surface area of the positive electrode material as a whole, and the easier exchange of ions and electrons is preferable. Specifically, the average particle size of the crystallized glass powder is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Although it does not specifically limit about a minimum, It is 0.05 micrometer or more actually.

In order to improve ionic conductivity, it is important to eliminate the orientation dependence of ionic conduction in the crystal. Therefore, a positive electrode material in which an amorphous phase of an oxide exhibiting isotropic ion conductivity remains is preferable. Usually, the theoretical density of lithium iron phosphate LiFePO ₄ is 3.5 to 3.6 g · cm ⁻³ , and when an amorphous phase remains to some extent, the density becomes lower than the theoretical density. The preferred amorphous phase content is 0.01 to 20 parts by volume, which is the detection limit. If the amorphous phase exceeds 20 parts by volume, the charge / discharge capacity may be reduced.

The lithium secondary battery positive electrode material of the present invention preferably contains a conductive assistant having high electron conductivity and stability in order to improve conductivity with respect to the crystallized glass powder. Regarding the conductive assistant, as described later in the following examples, the reducing agent added before firing the crystal constituent raw material may serve as a conductive assistant after firing, or may be reduced. You may give the conductive support agent different from an agent before baking or after baking.

Further, the conductive aid is preferably coated on the crystal powder interface. Examples of the conductive assistant include carbon-based conductive assistants such as graphite, acetylene black, and amorphous carbon, and metallic conductive assistants such as metal powder. As the amorphous carbon, those in which a CO bond peak and a CH bond peak causing a decrease in conductivity of the positive electrode material are not substantially detected in the FTIR analysis are preferable.

The smaller the particle size of the conductive additive, the more uniformly it can be dispersed at each crystal powder particle interface. Specifically, the particle diameter of the conductive assistant is preferably 50 μm or less, and more preferably 30 μm or less. When the particle diameter of the conductive auxiliary agent is larger than 50 μm, it tends to be difficult to uniformly disperse at each crystallized glass powder particle interface. The lower limit is not particularly limited, but is actually 0.05 μm or more.

The content of the conductive assistant is preferably 0.1 to 50 parts by weight, preferably 2 to 40 parts by weight, and 3 to 30 parts by weight with respect to 100 parts by weight of the crystal powder. Is more preferable. When the content of the conductive assistant is less than 0.1 parts by mass, there is a tendency that the effect of imparting conductivity to the crystal cannot be sufficiently obtained. When the content of the conductive additive exceeds 50 parts by mass, the potential difference between the positive electrode and the negative electrode in the lithium ion secondary battery becomes small, and a desired electromotive force may not be obtained.

The electrical conductivity of the positive electrode material for a lithium ion secondary battery of the present invention is 1.0 × 10 ⁻⁸ S · cm ⁻¹ or more, and is 1.0 × 10 ⁻⁶ S · cm ⁻¹ or more. Preferably, it is 1.0 × 10 ⁻⁴ S · cm ⁻¹ or more.

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited to such examples.

(Example 1: Synthesis 1 of lithium iron phosphate LiFePO ₄₎
Lithium metaphosphate LiPO _3, and a hematite _Fe 2 _{O 3} having an average particle diameter of 1 [mu] m, lithium, molar ratio of phosphorus and hematite 1: 1: 1 so as to 10g (i.e., lithium metaphosphate 5.1831G, Hematite is weighed and mixed (4.8169 g), 10 parts by mass (ie, 1 g) of glucose as a reducing agent is added to 100 parts by mass of the mixed powder, alcohol is added and 5 minutes in an alumina mortar. Mixed. After drying, it was transferred to an alumina boat, introduced into a tubular furnace filled with 7% hydrogen-93% argon, and heat-treated at a predetermined temperature condition for 3 hours. Here, it heat-processed at each temperature of 500 degreeC, 600 degreeC, and 640 degreeC for the comparison of baking temperature.

In addition, the present inventors do not add glucose, which is a reducing agent, to the mixed powder, but simply put the surrounding environment of the mixed powder under the reducing atmosphere (7% hydrogen-93% argon). Whether the synthesis of lithium iron phosphate LiFePO ₄ was possible or not was also tested.

FIG. 1 shows an X-ray diffraction pattern (in the case of addition of a reducing agent) of the synthetic powder after adding 10 parts by mass of glucose and heat treatment. As shown in FIG. 1, the diffraction pattern of LiPO ₃ and Fe ₃ O ₄ was confirmed in the sample heat-treated at 500 ° C. Since the diffraction peaks of Fe ₂ O _{3 as the} raw material and LiFePO _{4 as} the target crystal are not confirmed, iron reduction proceeds at 500 ° C., but no reaction with lithium metaphosphate has occurred. It is done. On the other hand, the diffraction peak derived from LiFePO ₄ was confirmed in the samples heat-treated at 600 ° C. and 640 ° C.

FIG. 2 shows an X-ray diffraction pattern (in the case of no addition of a reducing agent) of the crystal powder obtained without addition of glucose. Similar to the result of FIG. 1, a diffraction pattern derived from LiFePO ₄ was confirmed at 600 ° C. or higher.

The density of each of the fired products formed in the case of adding glucose in FIG. 1 and in the case of adding no glucose in FIG. 2 was measured and found to be 3.42 g · cm ⁻³ . This is a density that is about 3% lower than the theoretical density (3.5 to 3.6 g · cm ⁻³ ) shown in the conventional literature on LiFePO ₄ .

The crystal powder obtained by the method of Example 1 was molded into a pellet shape so as to have a diameter of 13 mmφ and a thickness of 0.5 mm by uniaxial pressure molding, and the electrical conductivity at room temperature was measured by an AC impedance method. As a result, the sample without glucose showed 5 × 10 ⁻⁵ S · cm ⁻¹ , while the sample added with only 10 parts by mass of glucose showed 4.8 × 10 ⁻⁴ S · cm ⁻¹ . This measured value is much higher than the reported conductivity of LiFePO ₄ (approximately 1.0 × 10 ⁻⁹ S · cm ⁻¹ ).

(Example 2: Synthesis 2 of the lithium iron phosphate LiFePO ₄₎
Example 2 is the same as the process of Example 1 except that the iron reagent used in Example 1 is magnetite Fe ₃ O ₄ having an average particle diameter of 1 μm instead of hematite Fe ₂ O ₃ . Even when the synthetic powder was produced in the process of Example 2, the sample in which the formation of LiFePO ₄ was confirmed by the X-ray diffraction method was a sample heat-treated at a firing temperature of 600 ° C. or higher.

(Example 3: Synthesis method 3 of lithium iron phosphate LiFePO ₄ using lithium metaphosphate LiPO _{3 in} an amorphous state)
Lithium metaphosphate LiPO ₃ was placed in a platinum crucible and melted in an electric furnace heated to 1100 ° C. for 10 minutes. The molten melt was poured onto an iron plate to obtain a glass body (amorphous lithium metaphosphate). The glass body was pulverized into a powder having an average particle size of 20 μm. The result of differential thermal analysis of this glass body is shown in FIG. The glass transition temperature Tg of the glass body was 329 ° C., the crystallization start temperature Tx was 440 ° C., the crystallization peak temperature Tp was 482 ° C., and the melting point of the crystal was 640 ° C. The formed crystal was confirmed to be LiPO ₃ by X-ray diffraction.

Thereafter, 10 g of pulverized amorphous lithium metaphosphate powder and hematite Fe ₂ O ₃ having an average particle diameter of 1 μm are mixed so that the molar ratio of lithium, phosphorus and hematite is 1: 1: 1 (that is, metalin Weigh and mix lithium acid (5.1831 g, hematite 4.8169 g), add 10 parts by mass (ie, 1 g) of glucose to 100 parts by mass of the mixed powder, add alcohol, and add to the alumina mortar. And mixed for 5 minutes. After drying, it was transferred to an alumina boat, introduced into a tubular furnace filled with 7% hydrogen-93% argon, set to a predetermined temperature condition, and heat-treated for 3 hours. In order to examine the influence of the firing temperature, the set temperature conditions are 340 ° C., 500 ° C., and 640 ° C., respectively.

FIG. 4 shows an X-ray diffraction pattern of the synthetic powder after the heat treatment. In the sample heat-treated at 340 ° C., only the diffraction peak derived from Fe ₂ O ₃ added as a raw material and the diffraction peak derived from reduced Fe ₃ O ₄ were confirmed. This shows that amorphous lithium metaphosphate remains in a glass state. In the sample heat-treated at 500 ° C., diffraction peaks derived from LiFePO ₄ and LiPO ₃ were confirmed. Compared to the case where the crystalline LiPO ₃ shown in FIG. 1 was used as a raw material, it was confirmed that LiFePO ₄ could be synthesized at a low temperature. Although the mechanism that causes this phenomenon (result) is not completely clarified, the glassy LiPO ₃ exhibits fluidity when heated above the glass transition temperature, and covers the reduced Fe ₃ O ₄ particle surface, It is considered that LiFePO ₄ is formed by the reaction between the supercooled liquid LiPO ₃ and Fe ₃ O ₄ particles. In the sample subjected to heat treatment at 500 ° C., a part of LiPO ₃ was crystallized, but in the vicinity of 640 ° C., which is the melting point of LiPO ₃ , a liquid phase was obtained again, and completely single-phase LiFePO ₄ was obtained.

(Example 4: Production of lithium iron phosphate LiFePO ₄ pellets)
In the same manner as in Example 3, LiPO ₃ glass powder having an average particle diameter of 20 μm and hematite Fe ₂ O ₃ having an average particle diameter of 1 μm are adjusted so that the molar ratio of lithium, phosphorus and hematite is 1: 1: 1. 10 g (that is, lithium metaphosphate 5.1831 g, hematite 4.8169 g) is weighed and mixed, 10 parts by mass (that is, 1 g) of glucose is added to 100 parts by mass of the mixed powder, and alcohol is added. In addition, it was mixed for 5 minutes in an alumina mortar. After the mixed powder was dried, the mixed powder was formed into pellets so as to have a diameter of 13 mmφ and a thickness of 0.5 mm by uniaxial pressing. Thereafter, heat treatment was performed at 640 ° C. for 3 hours to obtain a pellet-shaped sintered body sample.

When the pellet-like sintered body sample of Example 4 was evaluated by the X-ray diffraction method, it was confirmed that it was composed of single-phase LiFePO ₄ . Further, when the Raman scattering spectrum of this pellet sample was measured, the presence of amorphous carbon was confirmed. Further, a gold electrode was formed on this pellet sample, and the electrical conductivity at room temperature was measured by the AC impedance method. As a result, it was 3.3 × 10 ⁻³ S · cm ^−1, and the electrical conductivity of the sample of Example 1 Improved by an order of magnitude. From this, in this example, glucose added as a reducing agent is further changed to amorphous carbon, which is more effective not only for reducing Fe ₂ O ₃ but also for improving electrical conductivity as a conductive assistant. I found out.

The positive electrode material for a lithium ion secondary battery of the present invention is suitable for portable electronic devices such as notebook computers and mobile phones, and electric vehicles.

Claims

(1) Represented by powdered lithium metaphosphate LiPO 3 and chemical formula MO, MO 2 , M 3 O 4 , or M 2 O 3 (M is at least one selected from Fe, Mn, Co, Ni) Preparing a batch of a transition metal oxide with a molar ratio of lithium, phosphorus and transition metal of 1: 1: (0.8 to 1.2), and (2) surrounding the prepared batch A step of setting the environment in a reducing atmosphere; and (3) the obtained mixed powder is fired, and is composed of LiMPO 4 crystals (M is at least one selected from Fe, Mn, Co, Ni) or a solid solution thereof. Forming a crystal to be produced, and a method for producing a positive electrode material for a lithium ion secondary battery.
(1) Represented by powdered lithium metaphosphate LiPO 3 and chemical formula MO, MO 2 , M 3 O 4 , or M 2 O 3 (M is at least one selected from Fe, Mn, Co, Ni) A step of preparing a batch of a transition metal oxide so that the molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2); and (2) a step of adding a reducing agent. And (3) firing the obtained mixed powder to form a LiMPO 4 crystal (M is at least one selected from Fe, Mn, Co, Ni) or a crystal composed of a solid solution thereof, The manufacturing method of the lithium ion secondary battery positive electrode material characterized by including.
3. The method for producing a positive electrode material for a lithium ion secondary battery according to claim 1, wherein the formed LiMPO 4 crystal is a LiMn x Fe 1-x PO 4 crystal (0 <x <1). 4.
The positive electrode for a lithium ion secondary battery according to claim 2 or 3, wherein the addition amount of the reducing agent is 0.1 to 50 parts by mass with respect to 100 parts by mass of the mixture prepared in the step (1). Material manufacturing method.
The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 2 to 4, wherein the reducing agent is glucose.
(4) The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, further comprising a step of adding a conductive additive containing a carbon-based compound.
The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the firing temperature is in the range of 600 to 700 ° C.
Before the blending step of (1), a pretreatment step of melting lithium metaphosphate LiPO 3 at a temperature of 1000 to 1200 ° C. to make it amorphous and further crushing amorphous lithium metaphosphate LiPO 3 The method for producing a positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 7, further comprising:
The method for producing a positive electrode material for a lithium ion secondary battery according to claim 8, wherein the firing temperature is in a temperature range of 450 to 700 ° C.
Powdered lithium metaphosphate LiPO 3 and the chemical formula MO, M 2 O, MO 2 , M 3 O 4 , or M 2 O 3 (M is at least one selected from Fe, Mn, Co, Ni) The transition metal oxide is mixed so that the molar ratio of lithium, phosphorus and transition metal is 1: 1: (0.8 to 1.2), and is fired in a reducing atmosphere. Lithium ion secondary battery positive electrode material.
Powdered lithium metaphosphate LiPO 3 and the chemical formula MO, M 2 O, MO 2 , M 3 O 4 , or M 2 O 3 (M is at least one selected from Fe, Mn, Co, Ni) The transition metal oxide to be mixed was mixed so that the molar ratio of lithium, phosphorus and transition metal was 1: 1: (0.8 to 1.2), and a reducing agent was further added and calcined. Lithium ion secondary battery positive electrode material characterized by the above.
A lithium ion secondary battery positive electrode material, wherein the lithium metaphosphate LiPO 3 according to claim 10 or 11 is in an amorphous state.
The fired product according to any one of claims 10 to 12 includes an amorphous phase of an inorganic oxide composed of phosphoric acid, lithium, and a transition metal, and the content thereof is 0.01 to 20 parts by volume. A lithium ion secondary battery positive electrode material, characterized in that
14. A positive electrode material for a lithium ion secondary battery, further comprising a conductive additive in addition to the fired product according to any one of claims 10 to 13.