WO2013038517A1 - Manganese iron magnesium ammonium phosphate, method for producing same, positive electrode active material for lithium secondary batteries using manganese iron magnesium ammonium phosphate, method for producing same, and lithium secondary battery using said positive electrode active material - Google Patents

Abstract

Provided are: a positive electrode active material for lithium secondary batteries, which is formed of a composite lithium manganese iron magnesium phosphate, which has fine particle diameters, and which exhibits excellent battery characteristics such as high capacity and high energy density in cases when used as a positive electrode active material; and a method for producing a positive electrode active material for lithium secondary batteries, which uses, as a precursor, a manganese iron magnesium ammonium phosphate that has a uniform composition. A method for producing a precursor of a positive electrode active material for lithium secondary batteries, which is a method for producing a manganese iron magnesium ammonium phosphate, said method being characterized by comprising: a mixed solution preparation step wherein a mixed solution of divalent Mn ions, Fe ions, Mg ions and phosphate ions is prepared; and a crystallization step wherein a manganese iron magnesium ammonium phosphate represented by the general formula NH₄Mn_1-a-bFe_aMg_bPO₄∙H₂O (wherein 0.2 ≤ a ≤ 0.4 and 0 < b < 0.1) is obtained by coprecipitation by adding ammonia to the mixed solution so as to adjust the pH thereof to within the range of 7-9.

Description

Ammonium manganese iron magnesium phosphate and its production method, positive electrode active material for lithium secondary battery using said ammonium manganese iron magnesium magnesium, its production method, and lithium secondary battery using said positive electrode active material

The present invention relates to an ammonium manganese iron phosphate magnesium as a precursor of a positive electrode active material for a lithium secondary battery, a method for producing the same, and a positive electrode active material for a lithium secondary battery using the ammonium manganese iron magnesium phosphate as a precursor And a manufacturing method thereof, and a lithium secondary battery using the positive electrode active material using the positive electrode active material.

Lithium secondary batteries are lightweight and have high energy density, so they are widely used in small batteries such as mobile phones, notebook computers, and other IT devices. For these applications, LiCoO ₂ and LiCo _1/3 are mainly used. Layered rock salt compound positive electrode active materials such as Ni _1/3 Mn _1/3 O ₂ and LiNiO ₂ are used.
With the development and popularization of IT equipment, the demand is still growing on a global scale. In addition to these small batteries, industrial large batteries can also be used for hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, power storage, and more. The demand is expected to expand in the direction, and research and development are also actively conducted.

Under such circumstances, high safety, long life, high output, and low cost are required for the positive electrode active material as a problem for full-scale commercialization of large industrial batteries. Among them, as a material exhibiting high safety and excellent cycle performance, an olivine-type positive electrode active material has attracted attention as an alternative positive electrode active material such as LiCoO ₂ or LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ . .
The olivine-type positive electrode active material has a theoretical capacity of about 170 mAh / g and all O is covalently bonded to P. Therefore, even if the battery generates heat, there is a risk of ignition without releasing oxygen. In addition, the structure is stable due to the skeleton of phosphoric acid, and therefore, the electrode is not easily deteriorated even after repeated charge and discharge, and the cycle life is long.

Among the olivine-type positive electrode active materials, lithium manganese phosphate (LiMnPO ₄ ) has a potential of 4.1 V with respect to Li metal, which is the same potential as the layered rock salt compound positive electrode active material, and is expected to have a high energy density. Has attracted the attention of developers around the world.
However, LiMnPO ₄ has a problem that it is difficult to obtain a practical charge / discharge capacity because the electron conductivity and the Li ion conductivity are lower than those of a conventional layered rock salt compound positive electrode active material.
In response to this problem, measures for reducing resistance such as shortening the distance of movement of electrons and lithium ions in the particles by miniaturization of LiMnPO ₄ particles and imparting conductivity by treatment such as coating of graphite have been studied. However, it shows almost no charge / discharge capacity as a secondary battery. Therefore, LiMnPO ₄ has not yet been put into practical use even though much effort is devoted to development.

From the above circumstances, in order to a material suitable for LiMnPO ₄ as a lithium secondary battery positive electrode active material, the LiMnPO _4, by adding different kinds of elements, is an attempt to optimize the crystal structure and electron structure ing.
For example, LiMn _0.6 Fe _0.4 PO ₄ , that is, lithium manganese iron composite phosphate obtained by substituting 60 mol% of Mn of LiMnPO ₄ with Fe is used as the positive electrode active material, so that the initial discharge capacity is about 160 mAh / g. (For example, refer nonpatent literature 1.).
In addition, LiMn _0.96 Mg _0.04 PO ₄ , that is, lithium manganese magnesium composite phosphate in which 4 mol% of Mn of LiMnPO ₄ is substituted with Mg is used as a positive electrode active material, thereby obtaining an initial discharge capacity of 126 mAh / g. It has also been reported that it has been made (for example, see Non-Patent Document 2).
Further, an initial discharge capacity of 132 mAh is obtained using lithium manganese iron magnesium composite phosphate (LiMn _1-ab Fe _a Mg _b PO ₄ ) expressed as LiMn _0.68 Fe _0.29 Mg _0.03 PO _4. It has also been reported that / g was obtained (see, for example, Patent Document 1).

As described above, in order to increase the capacity of LiMnPO ₄ , it has been proposed to replace a part of manganese with a different element such as iron or magnesium. However, in order to put lithium secondary batteries into practical use, it is necessary to increase the energy density and improve the charge / discharge efficiency. Not done.
On the other hand, the olivine-type positive electrode active material is greatly affected by the characteristics of the positive electrode active material depending on the manufacturing method. That is, in order to synthesize an olivine-type positive electrode active material obtained by adding a metal element to LiMnPO _{4 so} that each element in the crystal becomes uniform, it is necessary to devise from the synthesis method. Some elements segregate in the obtained positive electrode active material, and the target composite phosphate is not obtained, and sufficient characteristics cannot be obtained.
There are a hydrothermal synthesis method, a spray pyrolysis method, a sol-gel method, and the like for the synthesis of the olivine-type positive electrode active material. The solid-phase reaction method is generally used. As a method for producing LiMnPO _4, a production method having a mixing step of mixing a plurality of substances serving as synthesis raw materials into a precursor and a heating step of reacting the precursor by heating is proposed (for example, non- (See Patent Document 1). At this time, manganese carbonate is used as a manganese source.

By the way, in order to obtain LiMn _1-ab Fe _a Mg _b PO ₄ , it is conceivable to add iron oxalate as an iron source and magnesium hydroxide as a magnesium source in the mixing step. However, iron oxalate is toxic, unfavorable to the human body and the environment, and expensive, and therefore is unsuitable as a raw material for battery positive electrode active materials that require mass production. Furthermore, in order to synthesize lithium manganese iron magnesium composite phosphate with a uniform composition by solid phase reaction from separate iron source, manganese source and magnesium source, long-time pulverization and mixing before firing, Firing is required. Long pulverization and mixing require a large amount of energy and are also undesirable because of contamination from the pulverizing medium. Further, when firing at a high temperature for a long time, sintering between the particles proceeds, and the LiMn _1-ab Fe _a Mg _b PO ₄ particles become coarse, which causes high resistance as a positive electrode active material. . In order to pulverize this, it must be pulverized again, resulting in a very costly manufacturing process.
In addition, regarding the toxicity of iron oxalate, improvement by using harmless raw materials is also being studied. For example, ferrous ammonium phosphate is produced from ferrous sulfate (FeSO ₄ ), a phosphoric acid source such as ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and ammonia (NH ₄ OH). A stable iron iron phosphate (NH ₄ FePO ₄ ) and phosphoric acid raw material and a lithium raw material such as lithium hydroxide (LiOH) or lithium carbonate (Li ₂ CO ₃ ) are reacted to form lithium iron phosphate as a positive electrode material A method for producing (LiFePO ₄ ) has been proposed (see, for example, Patent Document 2).
NH ₄ FePO ₄ · _{H 2} O is stable in air, non-toxic, since it synthesize an inexpensive iron sulfate as a raw material, useful for synthesis of LiFePO _4. Further, since Fe and P are mixed at the atomic level, it is considered suitable for synthesizing uniform LiFePO ₄ .
However, the synthesis method of Non-Patent Document 1, an iron _material, replaced with _{NH 4 FePO 4 · H 2 O} , any attempt the synthesis of _{LiMn 1-a-b Fe a} Mg b PO 4, a still uniform In order to synthesize LiMn _1-ab Fe _a Mg _b PO ₄ , long pulverization and mixing before firing and a high firing temperature are required.

As described above, in lithium manganese phosphate (LiMnPO ₄ ), which is expected as a positive electrode active material, high capacity has been studied, but high energy density and high efficiency have been sufficiently studied. However, an olivine-type positive electrode active material having good characteristics has not been developed yet. In addition, the conventional production methods have the problems as described above, and the present situation is that a method for producing an olivine-type positive electrode active material having a uniform composition and fineness industrially at low cost has not been developed.

JP 2004-063422 A JP 2006-056754 A JP 2003-292309 A

In view of the above-mentioned problems of the prior art, the object of the present invention is a lithium manganese iron-magnesium composite phosphorus having a fine particle size and a high capacity and high energy density when used as a positive electrode active material. A positive electrode active material for lithium secondary battery comprising an acid salt, and a method for producing a positive electrode active material for lithium secondary battery using ammonium manganese iron phosphate magnesium having a uniform composition as a precursor, and the positive electrode active material It is to provide a lithium secondary battery having good characteristics using

In order to achieve the above object, the present inventors have conducted intensive studies on a lithium manganese iron composite phosphate having a uniform composition and a fine particle size, and as a result, manganese, iron, magnesium, and phosphorus are present in the liquid phase. It has been found that by performing crystallization from a uniformly mixed state, a coprecipitate in which these elements are uniformly mixed at the atomic level can be obtained. The coprecipitate is used as a precursor, mixed with a lithium source, and fired at a low temperature to obtain the above lithium manganese iron magnesium composite phosphate, which not only has a high capacity but also has a high energy density and a high charge / discharge. We have learned that efficiency is achieved. The present invention has been completed based on these findings.

That is, according to the first invention of the present invention, a method for producing a precursor of a positive electrode active material for a lithium secondary battery,
A mixed solution preparation step of preparing a mixed solution of divalent Mn ions, Fe ions and Mg ions and phosphoric acid ions; and adding ammonia to adjust the pH of the mixed solution to a range of 7-9; Phosphoric acid co-precipitated and represented by the general formula: NH ₄ Mn _1-a-b Fe _a Mg _b PO ₄ .H ₂ O (0.2 ≦ a ≦ 0.4, 0 <b <0.1) And a crystallization step of obtaining ammonium manganese iron magnesium. A method for producing ammonium manganese iron magnesium phosphate is provided.

According to the second invention of the present invention, in the first invention, the mixed solution preparation step is selected from sulfate or chloride salt as a feedstock for Mn ions, Fe ions and Mg ions. Provided is a method for producing ammonium manganese iron phosphate magnesium, wherein at least one kind of water-soluble metal salt is used.
Furthermore, according to the third invention of the present invention, in the first invention, in the mixed solution preparation step, at least one selected from phosphoric acid or ammonium dihydrogen phosphate as a feedstock of phosphoric acid ions. There is provided a process for producing ammonium manganese iron phosphate magnesium, characterized in that a water-soluble salt is used.

According to a fourth invention of the present invention, in any one of the first to third inventions, the crystallization step is performed in a non-oxidizing atmosphere. Is provided.
Further, according to a fifth invention of the present invention, in any one of the first to fourth inventions, the crystallization step is characterized in that the liquid temperature of the pH adjusted mixed solution is maintained at 25 to 60 ° C. A method for producing ammonium manganese iron phosphate magnesium is provided.

According to a sixth aspect of the present invention, there is provided a precursor of a positive electrode active material for a lithium secondary battery obtained by the production method according to any one of the first to fifth aspects,
Phosphorus characterized by being represented by the general formula: NH ₄ Mn _1-a-b Fe _a Mg _b PO ₄ .H ₂ O (0.2 ≦ a ≦ 0.4, 0 <b <0.1) Ammonium ammonium manganese iron is provided.

According to the seventh aspect of the present invention, the heat treatment step of mixing the ammonium manganese iron magnesium phosphate according to the sixth aspect and the lithium salt and then heat-treating at 200 to 500 ° C. in an inert or reducing atmosphere. And a compound serving as a carbon source in LiMn _1-а-b Fe _a Mg _b PO ₄ (0.2 ≦ a ≦ 0.4, 0 <b <0.1) obtained by the heat treatment step after firing. A carbon source mixing step of mixing to obtain a carbon source mixture so that the carbon content is 1 to 5% by mass, and firing in which the carbon source mixture is fired at 500 to 800 ° C. in an inert or reducing atmosphere And a process for producing a positive electrode active material for a lithium secondary battery.
Further, according to an eighth invention of the present invention, in the seventh invention, the positive electrode active for a lithium secondary battery, wherein the pulverization is simultaneously performed when the ammonium manganese iron phosphate magnesium and the lithium salt are mixed. A method of manufacturing a material is provided.

Further, according to the ninth aspect of the present invention, it is represented by a general formula: LiMn _1-ab Fe _a Mg _b PO ₄ (0.2 ≦ a ≦ 0.4, 0 <b <0.1). A positive electrode active material for a lithium secondary battery comprising a olivine-type lithium manganese iron-magnesium composite phosphate, wherein the crystallite diameter determined from the (131) plane in X-ray diffraction is 55 nm or less, and the BET specific surface area is 15 m ^A positive electrode active material for a lithium secondary battery, characterized by having a carbon content of 1 to 5% by mass and at least ² / g.
Furthermore, according to the tenth aspect of the present invention, in the ninth aspect, when used as the positive electrode active material of a C2023 type coin battery, the initial discharge capacity is 145 mAh / g or more, the average discharge voltage is 3.8 V or more, and Provided is a positive electrode active material for a lithium secondary battery that exhibits battery characteristics with a discharge efficiency of 85% or more.

Further, according to the eleventh aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode composed of a positive electrode active material for a lithium secondary battery according to the ninth or tenth invention.

According to the present invention, a precursor of a positive electrode active material for an olivine-type lithium secondary battery in which Mn, Fe, and Mg are uniformly mixed at an atomic level can be obtained, and a positive electrode active material obtained using the precursor is The lithium secondary battery using the positive electrode active material exhibits excellent battery characteristics such as high capacity, high energy density, and high charge / discharge efficiency. Furthermore, the production method is easy and suitable for industrial scale production without using toxic compounds, and its industrial value is extremely high.

Hereinafter, the present invention will be described in detail.
(1) Method for Producing Ammonium Manganese Iron Magnesium Phosphate A method for producing ammonium manganese iron magnesium phosphate as a precursor of the positive electrode active material for secondary battery of the present invention comprises divalent Mn ion, Fe ion and Mg ion. A mixed solution preparation step for preparing a mixed solution with phosphoric acid ions, and co-precipitation by adding ammonia to adjust the pH of the mixed solution to a range of 7 to 9, and the general formula: NH ₄ Mn _{1-а And a} crystallization step of obtaining ammonium manganese iron phosphate magnesium represented by _-b Fe _a Mg _b PO ₄ .H ₂ O (0.2 ≦ a ≦ 0.4, 0 <b <0.1). It is characterized by that.

In the mixed solution preparation step, a mixed solution of divalent Mn ions, Fe ions, Mg ions and phosphate ions is prepared. Since the composition of manganese iron phosphate obtained in the crystallization process of the next step matches the composition ratio of the mixed solution, the ratio of Mn ions and Fe ions contained in the mixed solution to the phosphate ions is represented by the following general formula ( The divalent Mn salt, Fe salt and phosphorous oxide are dissolved in water so as to achieve the composition ratio of 1).
Here, the total amount of Mn, Fe and Mg and the molar ratio of phosphorous oxide is 1: 1 in terms of stoichiometry, but considering the yield during crystallization, the ratio of Mn and Fe to phosphorous oxide The total molar ratio can range from 0.9 to 1.1. When the molar ratio is 0.90 or less, the yield of phosphate ions deteriorates. On the other hand, when the molar ratio is 1.1 or more, impurities such as Fe ₃ O ₄ and MnO ₂ are likely to be generated. It is preferably dissolved so as to be 0.95 to 1.05. What is necessary is just to let the molar ratio of Mn, Fe, and Mg be the molar ratio in the ammonium manganese iron phosphate magnesium to be obtained.
General formula: NH ₄ Mn _1-a-b Fe _a Mg _b PO _{4 .H 2} O (1)
(In the formula, a is 0.2 ≦ a ≦ 0.4, and b is 0 <b <0.1.)

In the above production method, a mixed solution of divalent Mn ions, Fe ions and Mg ions and phosphoric oxide ions is prepared, and Mn, Fe and phosphorous oxide are co-precipitated simultaneously. A mixed solution of ions and phosphate ions, and a mixed solution of divalent Fe ions and phosphate ions are separately prepared, and the divalent Mg ions are added to the mixed solution containing Mn ions or Fe ions. In the crystallization process, the pH of each mixed solution is individually adjusted and coprecipitated, and mixed to obtain a molar ratio of lithium manganese iron composite phosphate to obtain Mn, Fe and Mg. It is good.
Since ammonium manganese phosphate and iron iron phosphate have similar structures, a lithium manganese iron composite phosphate having a uniform composition can be obtained even by low-temperature heat treatment by thoroughly mixing each of them. On the other hand, since Mg is added in a small amount, it can be co-precipitated with Mn or Fe to be uniformly dispersed in the lithium manganese iron composite phosphate, thereby realizing good battery characteristics.

As the divalent Mn salt, Fe salt and Mg salt, water-soluble salts can be widely used, but divalent inorganic salts are preferred. Specifically, it is preferable to use one or more water-soluble metal salts selected from sulfates and chloride salts as a feedstock for Mn ions and Fe ions.
As the phosphor oxide, a water-soluble one can be used. Specifically, as a feedstock of phosphor oxide ions, one or more water-soluble metal salts selected from phosphoric acid or ammonium dihydrogen phosphate Is preferably used.

In the crystallization step, which is the next step, ammonia is added to the acidic mixed solution, and the pH of the mixed solution is adjusted to a range of 7 to 9 to coprecipitate Mn ions, Fe ions, and phosphate ions. To obtain ammonium manganese iron phosphate magnesium. In the coexistence state in the solution, the phosphate anion and the metal ion are in a state of being completely uniformly mixed, and by coprecipitation, Mn and Fe and phosphoric acid are strictly mixed and uniform. A coprecipitate having a proper composition can be obtained.
Here, it is important to use a pH range to be controlled and ammonia. If the pH is less than 7, ammonia, metal ions, and phosphate ions do not react completely and remain in the mixed solution, resulting in a decrease in yield and compositional deviation. On the other hand, when the pH is more than 9, oxidation of Mn and Fe hardly occurs, impurities such as Fe ₃ O ₄ and MnO ₂ are generated and remain as a different phase even after heat treatment by mixing with lithium salt, thereby deteriorating the characteristics. .
If the purpose is to co-precipitate by controlling the pH to a high pH side, an alkali metal hydroxide or the like can be used. However, if an alkali metal hydroxide is used, the alkali metal remains in the coprecipitate. It becomes an impurity. In particular, when sodium hydroxide is used, sodium is increased as a residual impurity, so that sodium in the finally obtained positive electrode active material for a lithium secondary battery is increased and the characteristics of the positive electrode active material are deteriorated.
By controlling the pH to 7 to 9, it is possible to obtain ammonium manganese iron phosphate magnesium suitable as a precursor of a positive electrode active material for a lithium secondary battery that has no composition deviation and does not contain impurities.

In the crystallization step, it is preferable to coprecipitate in an inert atmosphere. By an inert atmosphere, it is possible to suppress the formation of impurities such as _Fe 3 _{O 4} and MnO ₂ by oxidation. The inert atmosphere is preferably an inert gas atmosphere such as nitrogen gas.
During crystallization, the liquid temperature of the mixed solution is preferably maintained at 25 to 60 ° C. When the liquid temperature is less than 25 ° C., the solubility of metal ions in the mixed solution is low, and the precipitation rate of Mn, Fe, and Mg is different, which may cause a composition shift. Moreover, when the liquid temperature exceeds 60 ° C., the solubility of metal ions in the mixed solution becomes high, and the crystallinity of ammonium manganese iron magnesium phosphate obtained by decreasing the precipitation rate becomes too high. The obtained positive electrode active material may be coarsened.

As an apparatus used for the crystallization step, a reaction vessel with a stirrer is preferable in order to cause the reaction to occur uniformly, and in order to control the atmosphere during crystallization, it is preferable to have a sealed structure. preferable.

After completion of the crystallization reaction, solid-liquid separation is performed by filtration, centrifugation, etc., and in order to remove impurities, the ammonium manganese iron phosphate magnesium obtained in the crystallization step is sufficiently washed and then dried. Here, since the ammonium manganese iron phosphate magnesium has a fine particle structure, impurities such as sodium can be easily removed by washing with water.

The ammonium manganese iron phosphate magnesium obtained in the crystallization step is easily oxidized during drying, and Mn, Fe or Mg may be oxidized to leave a foreign phase as an impurity. For this reason, drying after washing is performed in a non-oxidizing atmosphere. Although it will not specifically limit if it is in a non-oxidizing atmosphere, It is preferable to carry out in an inert atmosphere or a vacuum atmosphere.
Moreover, the drying temperature should just be the range which can suppress oxidation, it is preferable to set it as 250 degrees C or less, and it is more preferable to set it as 150 degrees C or less. On the other hand, if it is less than 60 ° C., it takes time to dry, which is not preferable.

(2) Ammonium Manganese Iron Magnesium Phosphate The ammonium manganese iron magnesium phosphate of the present invention is a precursor of a positive electrode active material for a lithium secondary battery, and is a coprecipitate obtained by the above production method. It is represented by Formula (1).
Moreover, since the ammonium manganese iron magnesium phosphate of the present invention is obtained by the above production method, Mn, Fe and Mg are uniformly mixed at the atomic level. For this reason, after mixing with the lithium salt, the composition can be made uniform by low-temperature heat treatment, and a lithium manganese iron-magnesium composite phosphate with a fine particle size can be obtained.

In addition, the ammonium manganese iron magnesium phosphate of the present invention has a sodium content of 0.01% by mass or less, and sufficient characteristics can be obtained with the obtained positive electrode active material. When the sodium content exceeds 0.01% by mass, the movement of Li ions in the olivine structure is inhibited by Na, so that the positive electrode performance such as capacity and output using the obtained positive electrode active material is deteriorated.

(3) Method for producing positive electrode active material for lithium secondary battery The method for producing a positive electrode active material for lithium secondary battery according to the present invention is a positive electrode active material for lithium secondary battery (hereinafter sometimes simply referred to as a positive electrode active material). A heat treatment step of heat-treating at 200 to 500 ° C. in an inert or reducing atmosphere after mixing the ammonium manganese iron magnesium magnesium phosphate and the lithium precursor, and a carbon content after firing the compound serving as the carbon source A carbon source mixing step of mixing to obtain 1 to 5% by mass to obtain a carbon source mixture, and a baking step of baking the carbon source mixture at 500 to 800 ° C. in an inert or reducing atmosphere. It is characterized by.

(3-1) Heat Treatment Step In the heat treatment step, first, the ammonium manganese iron phosphate magnesium and the lithium salt are mixed. Mixing with a lithium salt mixes ammonium manganese iron magnesium phosphate and a lithium salt so that a lithium manganese iron composite phosphate represented by the following general formula (2) is obtained, and in a molar ratio: The ratio of Li to the sum of Mn and Fe (Me) (Li / Me) is preferably 0.95 to 1.05.
General formula: LiMn _1-ab Fe _a Mg _b PO ₄ (2)
(In the formula, a is 0.2 ≦ a ≦ 0.4, and b is 0 <b <0.1.)

The lithium salt is not particularly limited, and general lithium salts such as lithium hydroxide, lithium carbonate, and lithium acetate can be used.
As a mixing method, a mixer capable of sufficiently mixing ammonium manganese iron magnesium phosphate and lithium salt may be used. Specifically, a shaker mixer or a dry or wet mill using alumina or zirconia spheres may be used. it can. In particular, in order to atomize the finally obtained positive electrode active material, it is preferable to pulverize ammonium manganese iron phosphate magnesium in advance, and it is preferable to pulverize simultaneously with mixing with the lithium salt. In this case, using a mill such as a ball mill, a planetary mill, a vibration mill, a bead mill or the like is preferable because pulverization can be performed simultaneously with mixing.

After mixing with the lithium salt, the mixture is heat-treated at 200 to 500 ° C., preferably 300 to 400 ° C. in an inert or reducing atmosphere.
Since the ammonium manganese iron phosphate magnesium of the present invention is in a state in which Mn, Fe and magnesium are uniformly mixed at the atomic level, the above constituent elements are uniformly mixed even in heat treatment in the above temperature range, which is good Lithium manganese iron composite phosphate (LiMn _1-ab Fe _a Mg _b PO ₄ ) having excellent crystallinity can be obtained. When the heat treatment temperature is less than 200 ° C., lithium carbonate as a reaction raw material may remain. When the heat treatment temperature exceeds 500 ° C., sintering of the particles proceeds and coarse particles are generated, and finally obtained positive electrode The conductivity of the active material is reduced.
The reducing atmosphere is preferably a mixed gas of an inert gas and hydrogen gas in order to suppress contamination of impurities, and the hydrogen gas content in the mixed gas is preferably 1 to 20% by volume.

(3-2) Carbon Source Mixing Step The carbon source mixing step is a compound (hereinafter referred to as a carbon source) for imparting conductivity to LiMn _1-ab Fe _a Mg _b PO ₄ obtained by the heat treatment step. This is a step of mixing so that the carbon content is 1 to 5% by mass after firing.
The carbon source is not particularly limited as long as it is graphitized by firing to become a conductive carbonaceous material. Graphite such as natural graphite and artificial graphite, carbon black such as acetylene black and ketjen black , Carbon fibers, common hydrocarbons such as sucrose, ascorbic acid, and other organic compounds that generate carbonaceous matter by decomposition can be widely used.
Further, the amount of carbon atoms contained in the carbon source tends to be smaller than that of the carbon source by firing. For this reason, the blending amount of the carbon source is preferably increased by 40 to 120%, more preferably by 50 to 120% by mass ratio with respect to the amount of carbon contained after firing.
The above mixing is sufficiently performed using a shaker mixer or a dry or wet mill using alumina or zirconia spheres so that the LiMn _1-ab Fe _a Mg _b PO ₄ and the carbon source are uniformly mixed. Preferably it is done.
Also in the carbon source mixing step, LiMn _1-ab Fe _a Mg _b PO ₄ obtained in the heat treatment step is pulverized to atomize the positive electrode active material finally obtained and uniformly conduct the particles. Since it can be coated with a carbonaceous material, it is preferable to grind at the same time as mixing. Therefore, it is preferable to use a mill such as a ball mill, a planetary mill, a vibration mill, or a bead mill.

(3-3) Firing step LiMn _1-ab Fe _a Mg _b PO ₄ mixed with a carbon source in the carbon source mixing step is 600 to 800 ° C., preferably 600 to 700 ° C. in an inert or reducing atmosphere. By firing at, it is possible to obtain LiMn _1-ab Fe _a Mg _b PO ₄ that is complex with the carbonaceous material and has good conductivity, that is, a positive electrode active material for a lithium secondary battery.
When the firing temperature is less than 600 ° C., graphitization of carbon does not proceed, and sufficient conductivity cannot be obtained for the positive electrode active material. On the other hand, when the firing temperature exceeds 800 ° C., the sintering of the particles proceeds and becomes coarse, and the conductivity of the positive electrode active material is lowered.
As a furnace in the heat treatment step and the firing step, for example, a general heat treatment furnace / firing furnace such as a batch furnace, a roller hearth kiln, a pusher furnace, a rotary kiln, or a fluidized bed furnace can be used. During the heat treatment, in order to suppress oxidation of iron, manganese, and magnesium, a reducing atmosphere of an inert gas such as nitrogen or argon or a mixed gas of nitrogen and hydrogen is used. preferable.
The positive electrode active material is composed of uniform fine primary particles, which can be used after being pulverized and classified according to the necessity of the battery electrode manufacturing process.

(4) Positive electrode active material for lithium secondary battery The positive electrode active material of the present invention is composed of primary particles having a uniform composition and uniform fine particles that are combined with a carbonaceous material. The olivine type lithium manganese iron magnesium composite phosphate represented.
In the above general formula (2), a is 0.2 ≦ a ≦ 0.4, but when a is less than 0.2, the amount of manganese in the composite phosphate increases, and the positive electrode material has high resistance. It becomes. Further, when a is more than 0.4, the ratio of Fe ²⁺ / Fe ³⁺ oxidation-reduction with a Li potential of about 3.45 occupies the battery reaction, and the average potential when a lithium secondary battery is formed decreases. Thus, the charge / discharge efficiency decreases.
On the other hand, in the positive electrode active material of the present invention, magnesium is compositionally uniform in the positive electrode active material, increasing the discharge voltage and realizing high energy and high charge / discharge efficiency.
However, in the above general formula (2), b is 0 <b <0.1, but when b is 0.1 or more, magnesium that does not contribute to redox in the battery reaction is included excessively. Since the crystallite grows and becomes too large at the time of firing, the capacity when lithium secondary is used is reduced. Magnesium can achieve the above effect with a small amount of addition, but in order to obtain a sufficient effect, b is preferably 0.005 or more. Therefore, the range of b is preferably 0.005 to 0.08, and more preferably 0.01 to 0.07.

The measurement of the primary particle size is limited to means such as measurement from the appearance by scanning electron microscope observation, and it is difficult to obtain an accurate value.
Therefore, in the present invention, the primary particle diameter was evaluated using the crystallite diameter determined by the Scherrer formula from the half-width of the crystal plane peak of the X-ray diffraction profile that can be quantitatively evaluated.
The crystallite size is a structural unit of particles composed of a single crystal, and the primary particles may be aggregates thereof, and thus are not necessarily equal, but if the primary particles are sufficiently fine, The child diameter and the primary particle diameter are considered to have a proportional relationship, and the primary particle diameter can be evaluated.
That is, the crystallite diameter determined from the (131) plane in the X-ray diffraction of the positive electrode active material of the present invention is 55 nm or less. By setting the crystallite diameter to 55 nm or less, sufficient conductivity can be obtained when the lithium manganese iron magnesium composite phosphate is used as a positive electrode active material. When the crystallite diameter is larger than 55 nm, the distance that lithium ions and electrons move inside the lithium manganese iron-magnesium composite phosphate particles having a low resistance and a high resistance of lithium ions and electrons during the battery reaction increases. The reaction rate of the battery becomes extremely slow, the battery resistance increases, and sufficient conductivity cannot be obtained. For this reason, when it uses for the positive electrode of a battery, battery capacity and an output characteristic fall.
The crystallite diameter is preferably 10 nm or more. When the crystallite diameter is less than 10 nm, the bulk density of the positive electrode active material decreases, and the battery capacity per unit volume when configured as a battery may not be sufficiently obtained.

Further, the positive electrode active material of the present invention has a BET specific surface area of 15 m ² / g or more and a carbon content of 1 to 5% by mass, and has good battery characteristics when used as a positive electrode active material for a battery. can get. When the BET specific surface area is less than 15 m ² / g, when the positive electrode of the battery is constructed, sufficient contact with the electrolytic solution cannot be obtained, resulting in an increase in battery resistance and a decrease in conductivity. The upper limit of the BET specific surface area is not particularly limited, but is preferably 40 m ² / g or less. If it exceeds 40 m ² / g, the bulk density will be low, and the battery capacity per unit volume may be too low.
Moreover, when the carbon content is less than 1% by mass, sufficient conductivity of the positive electrode active material cannot be obtained. On the other hand, when the carbon content exceeds 5% by mass, the lithium manganese iron magnesium composite phosphate in the positive electrode active material is reduced. Battery capacity is reduced.
When the positive electrode active material of the present invention is used, for example, as a positive electrode active material of a C2023 type coin battery, good battery characteristics with an initial discharge capacity of 145 mAh / g or more, an average discharge potential of 3.8 V or more, and a charge / discharge efficiency of 85% or more. It is suitable for a lithium secondary battery.

(5) Lithium Secondary Battery The lithium secondary battery according to the present invention is composed of the same constituent elements as a general lithium secondary battery, such as a positive electrode, a negative electrode, and a nonaqueous electrolyte.
Hereinafter, the embodiment of the lithium secondary battery of the present invention will be described in detail by dividing into its components, uses, etc., but the following embodiments are merely examples, and the lithium secondary battery of the present invention is In addition to the embodiments described in the present specification, various modifications and improvements can be made based on the knowledge of those skilled in the art.

(A) Positive electrode A positive electrode is formed from the positive electrode compound material containing the positive electrode active material of this invention, the electrically conductive material, and the binder.
Specifically, a powdered positive electrode active material and a conductive material are mixed, a binder is added thereto, and if necessary, a solvent for viscosity adjustment is further added to adjust the positive electrode mixture paste, For example, the positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil, dried, and pressurized as necessary to produce a sheet-like positive electrode.
The conductive material is for ensuring the electrical conductivity of the positive electrode, and for example, a material obtained by mixing one or more carbon material powders such as carbon black, acetylene black, and graphite can be used. .
The binder plays a role of anchoring the active material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluorine rubber, a thermoplastic resin such as polypropylene and polyethylene, and other suitable materials. Can be used. An organic solvent such as N-methyl-2-pyrrolidone is used as a solvent added to the positive electrode mixture as needed, that is, as a solvent for dispersing the active material, conductive material, and activated carbon and dissolving the binder. it can. Activated carbon can also be added to increase the electric double layer capacity.
Such a positive electrode active material, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent are added and kneaded to prepare a positive electrode mixture paste.
Each mixing ratio in the positive electrode mixture can also be an important factor that determines the performance of the lithium ion secondary battery. When the total solid content (meaning excluding solvent) of the positive electrode mixture is 100% by mass, the positive electrode active material is 60 to 95% by mass, the conductive material is 1 to It is desirable that the content is 20% by mass and the binder is 1 to 20% by mass.
For example, the above-mentioned positive electrode mixture paste sufficiently kneaded is applied to the surface of a metal foil current collector such as aluminum, dried to disperse the solvent, and then, if necessary, a roll to increase the electrode density By compressing with a press or the like, the positive electrode can be formed into a sheet. The sheet-like positive electrode can be cut into an appropriate size according to the intended battery and used for battery production.

(B) Negative electrode For the negative electrode, metallic lithium, lithium alloy, or the like, and a negative electrode mixture made by mixing a binder with a negative electrode active material capable of inserting and extracting lithium ions and adding a suitable solvent to form a paste. , And applied to the surface of a current collector of a metal foil such as copper, dried, and compressed to increase the electrode density as necessary. At this time, as the negative electrode active material, for example, a fired organic compound such as natural graphite, artificial graphite, or a phenol resin, or a powdery carbon material such as coke can be used. In this case, the negative electrode binder is a fluorine-containing resin such as polyvinylidene fluoride as in the case of the positive electrode, and the negative electrode active material and the binder are dispersed in an organic material such as N-methyl-2-pyrrolidone. A solvent can be used.

(C) Separator A separator is sandwiched and loaded between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and retains the electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

(D) Nonaqueous electrolyte The nonaqueous electrolyte is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, tetrahydrofuran, 2- One kind selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, phosphorous compounds such as triethyl phosphate, triethyl phosphate and trioctyl phosphate alone, or two or more kinds It can be used by mixing.
As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiASF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and complex salts thereof can be used. Further, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

The lithium secondary battery of the present invention configured as described above can have various shapes such as a cylindrical type and a stacked type. Even if any shape is adopted, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal, A current collecting lead or the like is used for connection, the electrode body is impregnated with the nonaqueous electrolyte, and the battery case is sealed to complete the battery.

The lithium secondary battery of the present invention includes a positive electrode using the positive electrode active material for a lithium secondary battery of the present invention as a positive electrode material, and is charged and discharged at a potential of 3.0 to 4.5 V, It is possible to industrially realize a lithium secondary battery that is extremely safer than conventional lithium metal composite oxides and also has a high capacity.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, the chemical analysis method of the metal used in the Example, the measurement method of X-ray diffraction and a specific surface area, and the evaluation method of battery capacity are as follows.
(1) Analysis of metals:
An ICP emission analysis method (Varian, 725ES) was used for ICP emission analysis.
(2) X-ray diffraction:
Using a powder X-ray diffractometer (manufactured by PANalytical, X′Prt PRO), the obtained positive electrode active material was measured by powder X-ray diffraction using Cu—Kα rays.
(3) Measurement of specific surface area:
Using a BET method measuring machine (manufactured by Yuasa Ionix Co., Ltd., Kantasorb QS-10), the BET method was performed by nitrogen gas adsorption.

(4) Battery capacity evaluation:
About the obtained positive electrode active material, the coin type battery was produced in the following procedures, and the charge / discharge capacity of the battery was measured and evaluated.
A positive electrode active material is mixed with 33% by weight of acetylene black as a conductive material, 17% by weight of polyvinylidene fluoride (PVDF) as a binder, and an N-methylpyrrolidone (NMP) solution. -A PVDF 17 wt% mixture was obtained.
This mixture was applied onto an aluminum foil, dried at 80 ° C., punched to an electrode size of 11 mmφ, and pressed at a press pressure of 98 MPa (1.0 tonf / cm ² ) to produce an electrode.
Using this electrode as a positive electrode, a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DEC) containing metal Li as a negative electrode and 1 mol / L of electrolyte LiPF ₆ as an electrolytic solution in a glove box (EC: DEC = volume ratio) 7: 3) A C2023 coin battery was produced using a glass separator as the separator.
The battery was charged and discharged under the conditions of charge 0.2 mA / cm ² , 4.5 V, rest 60 minutes, discharge 0.2 mA / cm ² , 2.0 V, 25 ° C., and the discharge capacity at the first cycle was While used as an evaluation value, the initial charge / discharge efficiency (discharge capacity / charge capacity) was determined. Moreover, the average discharge voltage was calculated | required from the voltage curve measured at the time of charging / discharging.

[Example 1]
Iron sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 0.368 mol (102.2 g) and manganese sulfate n-hydrate (Chuo Electric Works: 99.9% by mass) 1.103 mol (188.1 g) and magnesium sulfate heptahydrate (special grade by Wako Co., Ltd .: purity 99.5% by mass) 0.030 mol (7.4 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) 1.5 mol (172.9 g) was made up to 3 L with distilled water and stirred with a stirrer for 1 hour to obtain a raw material solution. Moreover, 25 mass% ammonia aqueous solution was used as pH adjustment solution.
1 L of pure water was put into a 5 L separable flask equipped with a stirrer and stirred for 30 minutes while the inside was replaced with nitrogen. The raw material solution was added at a rate of 10 mL per minute while the pH adjustment solution was connected to a pH controller and the pH was controlled at 8.0 to 8.2. After completion of dropping, stirring was continued for 30 minutes while replacing the separable flask with nitrogen to complete the coprecipitation reaction. The slurry after the reaction was subjected to solid-liquid separation by suction filtration, and then washed with pure water twice with pure water.
After washing with water and drying under vacuum at 120 ° C. for 24 hours, ammonium manganese iron phosphate magnesium was obtained.

30 g of the above-mentioned ammonium manganese iron magnesium phosphate, 5.94 g of lithium carbonate (Kanto Chemical Co., Ltd .: 99.0% by mass) and 60 ml of ethanol are placed in a 500 ml zirconia pot having an internal volume of 650 g of φ1.0 mm zirconia balls. The mixture was mixed and pulverized with a planetary ball mill (manufactured by Fritsch Japan) at 350 rpm for 10 minutes. After the treatment, the slurry and zirconia balls were sieved and dried at room temperature under vacuum for 24 hours to remove ethanol.
The mixture obtained by using an electric furnace was heated at 10 ° C./min while purging a mixed gas of 98 vol% nitrogen and 2 vol% hydrogen into the furnace at a flow rate of 1 L / min, and then 350 ° C. for 5 hours. Baked. When the fired product was analyzed by X-ray diffraction, it was identified as an olivine-type lithium manganese iron composite phosphate single phase, and lithium manganese iron magnesium composite phosphate in which magnesium was dissolved in lithium manganese iron composite phosphate was obtained. It was confirmed.

30 g of the above lithium manganese iron magnesium composite phosphate and 3.00 g of sucrose were placed in a 500 ml zirconia pot containing 350 g of φ5 mm zirconia balls, and mixed for 10 minutes at 250 rpm with a planetary ball mill (manufactured by Fritsch Japan). After the treatment, the zirconia balls were sieved, and then the mixture was heated at a rate of temperature increase of 10 ° C./hour while purging the mixed gas of 98 volume% nitrogen + 2 volume% hydrogen into the furnace at a flow rate of 1 L / min using an electric furnace. The temperature was raised to 190 ° C. in minutes, held for 2 hours, further heated and baked at 650 ° C. for 5 hours to obtain a positive electrode active material.

The composition of Li: Mn: Fe: Mg: P of this positive electrode active material is 1.00: 0.74: 0.24: 0.02: 1.00 in molar ratio, and the carbon content is 2.2 mass. %Met.
From the X-ray diffraction analysis, it was confirmed that it was an olivine type lithium manganese iron composite phosphate single phase, and the crystallite size of the (131) plane was found from the X-ray diffraction analysis profile using the Scherrer equation, and it was 50 nm. . When observed with a scanning electron microscope (SEM), the primary particle size was 100 to 200 nm. The specific surface area of the positive electrode active material determined by the BET method was 26.5 m ² / g.
When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 157 mAh / g, the initial discharge capacity was 151 mAh / g, the average discharge voltage was 3.81 V, and the initial efficiency was 96%.

[Example 2]
Iron sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 0.356 mol (99.0 g) and manganese sulfate n-hydrate (Chuo Denko: 99.9% by mass) 1.069 mol (182.3 g) and magnesium sulfate heptahydrate (special grade by Wako Co., Ltd .: purity 99.5% by mass) 0.075 mol (18.5 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) A positive electrode active material was obtained in the same manner as in Example 1, except that 1.5 mol (172.9 g) was made up to 3 L with distilled water and stirred for 1 hour with a stirrer to obtain a raw material solution.
The composition of Li: Mn: Fe: Mg: P of this positive electrode active material is 1.00: 0.71: 0.24: 0.05: 1.00 in terms of molar ratio, and the carbon content is 2.1 mass. %Met.
From X-ray diffraction analysis, it was identified as a single phase of olivine type lithium manganese iron composite phosphate, and it was confirmed that it was lithium manganese iron magnesium composite phosphate in which magnesium was dissolved in lithium manganese iron composite phosphate. The crystallite diameter of the (131) plane was determined from the line diffraction analysis profile using the Scherrer equation and found to be 54 nm. When observed with a scanning electron microscope (SEM), the primary particle size was 100 to 200 nm. The specific surface area of the positive electrode active material determined by the BET method was 26.3 m ² / g.
When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 156 mAh / g, the initial discharge capacity was 149 mAh / g, the average discharge voltage was 3.84 V, and the initial efficiency was 96%.

[Comparative Example 1]
Iron sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 0.338 mol (93.8 g) and manganese sulfate n-hydrate (manufactured by Chuo Denko: 99.9% by mass) 1.013 mol (172.7 g) and magnesium sulfate heptahydrate (reagent special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 0.150 mol (37.0 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) A positive electrode active material was obtained in the same manner as in Example 1 except that 1.5 mol (172.9 g) was made up to 3 L with distilled water and stirred for 1 hour with a stirrer to obtain a raw material solution.
The composition of Li: Mn: Fe: Mg: P of this positive electrode active material is 1.00: 0.67: 0.23: 0.10: 1.00 in terms of molar ratio, and the carbon content is 2.0 mass. %Met.
From X-ray diffraction analysis, it was identified as a single phase of olivine type lithium manganese iron composite phosphate, and it was confirmed that it was lithium manganese iron magnesium composite phosphate in which magnesium was dissolved in lithium manganese iron composite phosphate. The crystallite diameter of the (131) plane was determined from the line diffraction analysis profile using the Scherrer equation and found to be 58 nm. When observed with a scanning electron microscope (SEM), the primary particle size was 100 to 200 nm. The specific surface area of the positive electrode active material determined by the BET method was 27.0 m ² / g.
When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 153 mAh / g, the initial discharge capacity was 143 mAh / g, the average discharge voltage was 3.84 V, and the initial efficiency was 94%.

[Comparative Example 2]
The crystallization step was performed in the same manner as in Example 1 except that the pH was controlled to 6 to 6.2 in the crystallization step. When the amounts of Fe, Mn, and P in the filtrate that was solid-liquid separated after the crystallization reaction were measured, the reaction raw materials of 10% or more in terms of Fe, Mn, Mg, and P had remained unreacted. The rest of the process was interrupted.

[Comparative Example 3]
The crystallization step was performed in the same manner as in Example 1 except that the pH was controlled to 9.1 to 9.3 in the crystallization step. When the magnesium manganese iron phosphate ammonium obtained in the crystallization process is analyzed by X-ray diffraction, many different phases in which the valence of metals such as Fe ₃ O ₄ , Fe ₂ O ₃ , and MnO ₂ are more than 2 are detected. Therefore, the heat treatment process and subsequent steps were interrupted.

[Comparative Example 4]
Instead of the ammonium manganese iron phosphate magnesium obtained in the crystallization step, FeC ₂ O ₄ , MnCO ₂ , Mg (OH) ₂ and (NH ₄ ) ₂ HPO ₄ were used as raw materials in the same manner as in Example 1. A positive electrode active material was obtained.
When the obtained positive electrode active material was analyzed by X-ray diffraction, different phases such as Li ₃ PO ₄ , MnO and Fe ₃ O ₄ were observed in addition to LiMn _0.75 Fe _0.25 PO ₄ . These different phases remained up to the positive electrode active material, but the composition of Li: Mn: Fe: Mg: P was 1.00: 0.74: 0.24: 0.02: 1.00 in molar ratio. The carbon content was 2.4% by mass. The primary particle diameter was 100 to 200 nm, and the BET specific surface area was 25.1 m ² / g.
According to battery evaluation, the initial charge capacity was 141 mAh / g, the initial discharge capacity was 133 mAh / g, the average discharge voltage was 3.79 V, and the initial efficiency was 88%.

[Comparative Example 5]
Iron sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 0.375 mol (104.8 g) and manganese sulfate n-hydrate (Chuo Electric Works: 99.9% by mass) 1.125 mol (191.9 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) 1.5 mol (172.9 g) were made up to 3 L with distilled water, stirred with a stirrer for 1 hour, A positive electrode active material was obtained in the same manner as in Example 1 except that.
The composition of Li: Mn: Fe: P of this positive electrode active material was 1.00: 0.75: 0.25: 1.00 in terms of molar ratio, and the carbon content was 2.2 mass%.
From X-ray diffraction analysis, it was identified as a single phase of olivine type lithium manganese iron composite phosphate, and it was confirmed that it was lithium manganese iron magnesium composite phosphate in which magnesium was dissolved in lithium manganese iron composite phosphate. The crystallite size of the (131) plane was found from the line diffraction analysis profile using the Scherrer equation to be 49 nm. When observed with a scanning electron microscope (SEM), the primary particle size was 100 to 200 nm. The specific surface area of the positive electrode active material determined by the BET method was 24.3 m ² / g.
When the battery evaluation of the positive electrode active material was performed, the initial charge capacity was 155 mAh / g, the initial discharge capacity was 148 mAh / g, the average discharge voltage was 3.71 V, and the initial efficiency was 96%.

As is clear from the above, the method for producing ammonium manganese iron-magnesium phosphate of the present invention is a method for producing a precursor of a positive electrode active material for a lithium secondary battery, and Mn, Fe, and Mg are uniformly distributed at an atomic level. A mixed positive electrode active material precursor for an olivine type lithium secondary battery can be obtained, and the positive electrode active material obtained using the precursor has a fine particle size and is uniform in composition, A lithium secondary battery using a substance exhibits excellent battery characteristics such as high capacity, high energy density, and high charge / discharge efficiency, and its industrial applicability is extremely large.

Claims (11)

1. A method for producing a precursor of a positive electrode active material for a lithium secondary battery, comprising:
   A mixed solution preparation step of preparing a mixed solution of divalent Mn ions, Fe ions and Mg ions and phosphoric acid ions; and adding ammonia to adjust the pH of the mixed solution to a range of 7-9; Phosphoric acid co-precipitated and represented by the general formula: NH ₄ Mn _1-a-b Fe _a Mg _b PO ₄ .H ₂ O (0.2 ≦ a ≦ 0.4, 0 <b <0.1) And a crystallization step of obtaining ammonium manganese iron magnesium.
2. The mixed solution preparation step uses at least one water-soluble metal salt selected from sulfates or chloride salts as a feedstock for Mn ions, Fe ions, and Mg ions. The manufacturing method of ammonium manganese iron phosphate magnesium of description.
3. 2. The phosphorus according to claim 1, wherein in the mixed solution preparation step, at least one water-soluble salt selected from phosphoric acid or ammonium dihydrogen phosphate is used as a feedstock for the phosphate oxide ions. Method for producing ammonium manganese iron magnesium oxide.
4. 4. The method for producing ammonium manganese iron phosphate magnesium according to claim 1, wherein the crystallization step is performed in a non-oxidizing atmosphere.
5. 5. The method for producing ammonium manganese iron-magnesium phosphate according to claim 1, wherein in the crystallization step, the liquid temperature of the pH-adjusted mixed solution is maintained at 25 to 60 ° C. .
6. A precursor of a positive electrode active material for a lithium secondary battery obtained by the production method according to any one of claims 1 to 5,
   Phosphorus characterized by being represented by the general formula: NH ₄ Mn _1-a-b Fe _a Mg _b PO ₄ .H ₂ O (0.2 ≦ a ≦ 0.4, 0 <b <0.1) Acid ammonium manganese iron magnesium.
7. A heat treatment step in which the ammonium manganese iron phosphate magnesium according to claim 6 is mixed with a lithium salt and then heat-treated at 200 to 500 ° C in an inert or reducing atmosphere, and LiMn _1-а- obtained by the heat treatment step. _{In b} Fe _a Mg _b PO ₄ (0.2 ≦ a ≦ 0.4, 0 <b <0.1), the carbon content is set to 1 to 5% by mass after firing. A carbon source mixing step of mixing to obtain a carbon source mixture, and a firing step of firing the carbon source mixture at 500 to 800 ° C. in an inert or reducing atmosphere. A method for producing a positive electrode active material for a battery.
8. The method for producing a positive electrode active material for a lithium secondary battery according to claim 7, wherein pulverization is simultaneously performed when mixing the ammonium manganese iron magnesium phosphate and the lithium salt.
9. General formula: LiMn _1-ab Fe _a Mg _b PO ₄ (0.2 ≦ a ≦ 0.4, 0 <b <0.1) olivine type lithium manganese iron magnesium composite phosphate represented by A positive electrode active material for a lithium secondary battery,
   The crystallite diameter determined from the (131) plane in X-ray diffraction is 55 nm or less, the BET specific surface area is 15 m ² / g or more, and the carbon content is 1 to 5 mass%. Positive electrode active material for secondary battery.
10. 10. When used as a positive electrode active material of a C2023 type coin battery, the battery has an initial discharge capacity of 145 mAh / g or more, an average discharge voltage of 3.8 V or more, and a charge / discharge efficiency of 85% or more. The positive electrode active material for lithium secondary batteries as described.
11. A lithium secondary battery comprising a positive electrode comprising the positive electrode active material for a lithium secondary battery according to claim 9 or 10.