JP5323410B2 - Method for producing lithium iron phosphorus-based composite oxide carbon composite and method for producing coprecipitate containing lithium, iron and phosphorus - Google Patents

Description

  The present invention relates to a method for producing a lithium iron phosphorus composite oxide carbon composite useful as a positive electrode active material for a lithium secondary battery.

  In recent years, as home appliances have become portable and cordless, lithium ion secondary batteries have been put to practical use as power sources for small electronic devices such as laptop computers, mobile phones, and video cameras. Regarding this lithium ion secondary battery, in 1980, Mizushima et al. Reported that lithium cobalt oxide was useful as a positive electrode active material for lithium ion secondary batteries ("Material Research Bulletin" vol15, P783-789 (1980)). Since then, research and development on lithium cobaltate has been actively promoted, and many proposals have been made so far.

However, since Co is unevenly distributed on the earth and is a rare resource, development of, for example, LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂ , LiFePO _4, etc., is being promoted as new positive electrode active materials replacing lithium cobalt oxide. ing.

LiFePO ₄ has a large volume density of 3.6 g / cm ³ , generates a high potential of 3.4 V, has a large theoretical capacity of 170 mAh / g, and LiFePO ₄ is electrochemically detached in the initial state. Since one Li atom that can be doped is contained per Fe atom, there is a great expectation as a positive electrode active material of a new lithium secondary battery that replaces lithium cobalt oxide.

As a method for producing this LiFePO _4, a method obtained by a solid phase method has been proposed, but in order to obtain single-phase LiFePO ₄ in X-ray diffraction analysis, a uniform mixture in which each raw material is precisely mixed is obtained. It is necessary, and it is difficult to obtain a stable quality industrially.

  Various methods of using a coprecipitation method have been proposed as a method for easily obtaining a uniform mixture of raw materials. For example, Patent Document 1 below proposes a method using a coprecipitate obtained by adding a solution containing lithium hydroxide to a solution containing lithium dihydrogen phosphate and iron sulfate. Patent Document 2 below proposes a method of using a coprecipitate obtained by adding lithium carbonate or lithium hydroxide to a solution containing a compound that releases phosphate ions in a solution and metallic iron. . Patent Document 3 below discloses a coprecipitate of a composite phosphorous oxide of lithium and iron obtained by mixing an alkaline solution with an aqueous phosphoric acid solution containing a lithium salt, an iron salt and a water-soluble reducing agent. A method of using it has been proposed.

Special Table 2004-525059, page 5 International Publication WO 2004/036671 Pamphlet, page 1 JP 2002-117831 A, page 1

However, these coprecipitation methods have problems that it is difficult to adjust the composition of Li, Fe, and P, and that it is difficult to obtain single-phase LiFePO ₄ in X-ray diffraction analysis.

Therefore, an object of the present invention is to easily adjust the composition of Li, Fe and P of the lithium iron phosphorus composite oxide in the lithium iron phosphorus composite oxide carbon composite, and to obtain single-phase LiFePO in X-ray diffraction analysis. ₄ is obtained, and it is providing the manufacturing method of the lithium iron phosphorus type complex oxide carbon composite which can provide the battery performance which was excellent in the lithium secondary battery.

As a result of intensive studies in the above circumstances, the present inventors have determined that a solution containing lithium ion, divalent iron ion and phosphate ion (liquid A) and a solution containing alkali (liquid B) are in a specific range. The reaction is carried out while controlling the pH of the composition to facilitate the composition adjustment of Li, Fe and P in the coprecipitate containing lithium iron phosphorus, and the composition ratio of Li, Fe and P is 1: 1: 1. Since it can be close, the composition adjustment of Li, Fe, and P in the lithium iron phosphorus composite oxide-carbon composite becomes easy, and a coprecipitate can be obtained in high yield. Further, by firing the mixture of the coprecipitate obtained in this way and the conductive carbon material in an inert gas atmosphere, it is possible to obtain a LiFePO ₄ single-phase lithium iron phosphorus-based composite oxide from the viewpoint of X-ray diffraction analysis. Lithium iron phosphorus composite oxide carbon composite in which particles and conductive carbon material are uniformly dispersed is obtained. Furthermore, the lithium secondary battery using the lithium iron phosphorus-based composite oxide-carbon composite thus obtained as a positive electrode active material has been found to have excellent battery performance, and the present invention has been completed. .

That is, the present invention (1) is a solution containing lithium ion, divalent iron ion and phosphate ion (solution A) and a solution containing lithium hydroxide while controlling the pH to 5.5 to 9.5. (Liquid B) is contacted to obtain a first step of obtaining a coprecipitate containing lithium, iron and phosphorus, and a second step of mixing the coprecipitate and a conductive carbon material to obtain a calcined raw material mixture. And a third step of firing the firing raw material mixture in an inert gas atmosphere to obtain a lithium iron phosphorus composite oxide carbon composite, and a lithium iron phosphorus composite oxide carbon composite The manufacturing method of this is provided.

According to the present invention, the composition adjustment of Li, Fe and P of the lithium iron phosphorus composite oxide in the lithium iron phosphorus composite oxide carbon composite is easy, and the composition ratio of Li, Fe and P is 1: A method for producing a lithium iron-phosphorus-based composite oxide carbon composite that can be close to 1: 1, can obtain single-phase LiFePO ₄ in X-ray diffraction analysis, and can impart excellent battery performance to a lithium secondary battery. Can be provided.

  The method for producing a lithium iron phosphorus-based composite oxide carbon composite of the present invention is a solution (Liquid A) containing lithium ions, divalent iron ions and phosphate ions while controlling the pH to 5.5 to 9.5. ) And an alkali-containing solution (Liquid B) to obtain a coprecipitate containing lithium, iron and phosphorus, and the coprecipitate and a conductive carbon material are mixed and fired. Lithium iron phosphorus composite oxide carbon having a second step of obtaining a raw material mixture and a third step of firing the firing raw material mixture in an inert gas atmosphere to obtain a lithium iron phosphorus composite oxide carbon composite It is a manufacturing method of a composite_body | complex.

  In the first step according to the method for producing a lithium iron phosphorus-based composite oxide carbon composite of the present invention, while controlling the pH to 5.5 to 9.5, lithium ions, divalent iron ions and phosphate ions are added. A coprecipitate containing lithium, iron, and phosphorus (hereinafter, “coprecipitate”) is abbreviated by reacting the solution containing the solution (A solution) with the solution containing the alkali (solution B). ).

  The liquid A according to the first step is an aqueous solution containing lithium ions, divalent iron ions, and phosphate ions.

  The lithium source of the liquid A is not particularly limited as long as it is a compound having lithium ions and soluble in water. For example, lithium sulfate, lithium nitrate, lithium chloride, lithium acetate, lithium carbonate, lithium hydroxide, oxalic acid Lithium etc. are mentioned, Among these, lithium sulfate is preferable at a low price. The lithium source of these A liquids may be 1 type, or may use 2 or more types together.

  The divalent iron source of the liquid A is not particularly limited as long as it is a compound having a divalent iron ion and dissolved in water. For example, ferrous sulfate (II), iron (II) acetate, oxalic acid Examples thereof include iron (II), ferrous chloride (II), ferrous nitrate (II), etc. Of these, ferrous sulfate is preferable because of its low cost. These divalent iron sources of the liquid A may be one kind or a combination of two or more kinds.

  The phosphoric acid source of the liquid A is not particularly limited as long as it is a compound that has phosphate ions and dissolves in water, and examples thereof include phosphoric acid, ammonium dihydrogen phosphate, sodium hydrogen phosphate, and metaphosphoric acid. Of these, phosphoric acid is preferred because of its low cost. The phosphoric acid source of these A liquids may be 1 type, or may use 2 or more types together. In addition, in this invention, the phosphate ion which concerns on A liquid is a general term for phosphate ions, such as an orthophosphate ion, a metaphosphate ion, a pyrophosphate ion, a triphosphate ion, a tetraphosphate ion.

The ratio of lithium ions, divalent iron ions and phosphate ions in the liquid A is such that the molar ratio of each element in the coprecipitate is close to Li: Fe: PO ₄ = 1: 1: 1. The molar ratio (Li: Fe: P) when converted to a divalent iron atom and phosphorus atom is preferably 0.8 to 1.2: 0.8 to 1.2: 1, particularly preferably 0.8. 95 to 1.05: 0.95 to 1.05: 1. Moreover, content of the lithium ion in A liquid is 0.1-1.0 mol / L in conversion of Li atom, Preferably it is 0.5-1.0 mol / L, The content is 0.1 to 1.0 mol / L, preferably 0.5 to 1.0 mol / L, in terms of divalent iron atoms, and the phosphate ion content is in terms of phosphorus atoms. 0.1 to 1.0 mol / L, preferably 0.5 to 1.0 mol / L. The ratio of lithium ions, divalent iron ions and phosphate ions in the liquid A, and the content of lithium ions, the content of divalent iron ions and the content of phosphate ions are within the above ranges. In preparing the liquid A, the dissolution rate of the lithium source, divalent iron source and phosphoric acid source in the solution does not become too slow, which is industrially efficient, and is preferable in that the waste liquid can be reduced.

  Liquid A is prepared by dissolving a lithium source of liquid A, a divalent iron source of liquid A, and a phosphoric acid source of liquid A in water.

  The pH of the liquid A is preferably 2.5 or less, particularly preferably 0.1 to 1.5. When the pH of the liquid A is within the above range, the composition adjustment of Li, Fe and P in the coprecipitate becomes easy. On the other hand, when pH of A liquid is higher than the said range, it will become difficult to adjust the composition of Li, Fe, and P in a coprecipitate.

  The B liquid which concerns on a 1st process is an aqueous solution containing an alkali, and is adjusted by dissolving an alkali source in water. Examples of the alkali source of the liquid B include lithium hydroxide, ammonia, sodium hydroxide, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, sodium carbonate, potassium carbonate, etc. Among these, sodium hydroxide or hydroxide Lithium is preferable, and lithium hydroxide is particularly preferable because it is composed of lithium which is the same element as the constituent element of the lithium iron phosphorus composite oxide-carbon composite. The alkali source of these B liquids may be 1 type, or may use 2 or more types together.

  Content of the alkali in B liquid is 0.1-10 equivalent / L, Preferably it is 1-10 equivalent / L. When the alkali content in the B liquid is within the above range, the waste liquid can be reduced.

  In the first step, the contact between the liquid A and the liquid B is performed while controlling the pH to 5.5 to 9.5, preferably 5.5 to 8.5. By controlling the contact between the liquid A and the liquid B within the above range, the composition adjustment of Li, Fe and P in the coprecipitate is facilitated, and the yield of the coprecipitate is increased. On the other hand, if the pH at the time of contact between the liquid A and the liquid B is lower than the above range, the lithium component is difficult to precipitate, so the composition ratio of the lithium element in the coprecipitate becomes small, or the reaction solution contains Lithium, iron, or phosphorus elements remain, resulting in a low yield. If the content is higher than the above range, the iron elements in the resulting coprecipitate are easily oxidized.

  The ratio of the amount of liquid A to the amount of liquid B when the liquid A and the liquid B are brought into contact with each other. The number of alkali equivalents in the liquid B relative to the number of moles of phosphorus atoms in the liquid A / Number of moles of phosphorus atoms) is preferably 2.6 to 3.5, and particularly preferably 2.8 to 3.2. When the number of equivalents of alkali in the liquid B with respect to the number of moles of phosphorus atoms in the liquid A is within the above range, the composition ratio of Li, Fe and P in the coprecipitate tends to be close to 1: 1: 1.

  In the first step, the contact temperature when the liquid A and the liquid B are brought into contact is 10 to 100 ° C., preferably 30 to 100 ° C. When the contact temperature between the liquid A and the liquid B is within the above range, the lithium component in the reaction solution is likely to precipitate. On the other hand, if the contact temperature between the liquid A and the liquid B is less than the above range, the lithium component in the solution tends to be difficult to precipitate, and if it exceeds the above range, the solution boils at normal pressure. Liquid phase reaction tends to be difficult.

  The lithium source, divalent iron source, phosphoric acid source, and alkali source used for preparing the liquid A and the liquid B may be hydrated or anhydrous, and high purity lithium iron In order to obtain a phosphorus-based composite oxide carbon composite, those having a small impurity content are preferable.

  In the first step, the liquid A and the liquid B are added by adding the liquid B to the liquid C while adding the liquid A to the water (liquid C) while controlling the pH to 5.5 to 9.5. The method of contacting and reacting to obtain a coprecipitate containing lithium, iron and phosphorus (hereinafter also referred to as contact method A) makes it easy to control the pH when the liquid A and liquid B are contacted. And it is preferable at the point from which the yield of a coprecipitate becomes high. In the present invention, “adding B liquid to C liquid while adding A liquid to C liquid” means adding time of A liquid to C liquid and adding time of B liquid to C liquid. Indicates a complete or partial overlap. And the addition time of the A liquid to the C liquid and the addition time of the B liquid to the C liquid are completely overlapped, that is, the addition start of the A liquid and the addition start of the B liquid are simultaneous and The end of addition of liquid A and the end of addition of liquid B are preferable in terms of easy composition adjustment of Li, Fe and P in the coprecipitate, but as long as the effects of the present invention are not impaired, It does not need to be completely overlapped, and it is sufficient that the liquid B is added at least while the liquid A is added.

  The C liquid which concerns on the contact method A is water, and may contain water-soluble reducing agents, such as ascorbic acid, phenol, and pyroganol.

  In the contact method A, the amount of the liquid C may be an amount that allows the liquid C to be sufficiently stirred in the reaction vessel.

  In the contact method A, the ratio of the addition amount of the A solution and the addition amount of the B solution is the ratio of the number of equivalents of alkali in the B solution to the number of moles of phosphorus atoms in the A solution (number of equivalents of alkali / phosphorus atom The number of moles) is preferably 2.6 to 3.5, and particularly preferably 2.8 to 3.2. When the number of equivalents of alkali in the liquid B with respect to the number of moles of phosphorus atoms in the liquid A is within the above range, the composition ratio of Li, Fe and P in the coprecipitate tends to be close to 1: 1: 1.

  In the contact method A, the temperature of the reaction solution (C solution) when adding the A solution and the B solution to the reaction solution (C solution) is 10 to 100 ° C, preferably 30 to 100 ° C. When the temperature of the reaction solution (C solution) at the time of adding A liquid and B liquid exists in the said range, it will become easy to precipitate the lithium component in a reaction solution. On the other hand, when the temperature of the reaction solution (C solution) when adding the A solution and the B solution to the reaction solution (C solution) is less than the above range, the lithium component in the reaction solution tends to be difficult to precipitate. Moreover, if the above range is exceeded, the solution will boil at normal pressure, so that the liquid phase reaction tends to be difficult.

  In the contact method A, the addition method and addition speed of the liquid A to the reaction solution (liquid C) are not particularly limited, but the liquid A is dropped at a constant speed while stirring the reaction solution (liquid C). It is preferable in that the composition ratio of Li, Fe and P is close to 1: 1: 1 and there is little variation between lots, that is, a stable quality can be obtained. Moreover, although it is the addition method and addition speed of B liquid to a reaction solution (C liquid), dripping speed | velocity | rate using a pH control apparatus etc. so that pH of reaction solution (C liquid) may be maintained at a predetermined value. It is preferable to drop the solution B into the reaction solution (solution C) while controlling the pressure.

  In the contact method A, after completion of the addition of the liquid A and the liquid B, aging may be performed continuously with stirring while maintaining the temperature of the reaction solution (liquid C). By performing this aging, the unreacted element component in the reaction solution phase can be reduced. The pH of the reaction solution (solution C) during aging is preferably 5.5 to 9.5, particularly preferably 5.5 to 8.5. When the pH of the reaction solution (solution C) during aging is within the above range, the precipitated lithium component is difficult to re-elute and the precipitated iron component is difficult to be oxidized. On the other hand, if the pH of the reaction solution (solution C) during aging is less than the above range, the precipitated lithium component is likely to be eluted again, and if it exceeds the above range, the precipitated iron component is likely to be oxidized. Become. The aging temperature at the time of aging is 10 to 100 ° C, preferably 30 to 100 ° C. When the aging temperature is within the above range, an effect of reducing unreacted components in the reaction solution phase is easily obtained. On the other hand, if the aging temperature is less than the above range, the effect of reducing unreacted components in the reaction solution phase tends to be low, and if it exceeds the above range, the solution boils at normal pressure. Phase reaction tends to be difficult.

  In contact method A, liquid A and liquid B are added while injecting an inert gas such as nitrogen gas into the reaction solution (liquid C) when liquid A and liquid B are added to liquid C. Can do. In contact method A, liquids A and B are prepared by coexisting a reducing agent such as ascorbic acid, phenol and pyroganol, preferably ascorbic acid, in liquid A (a solution containing divalent iron ions and phosphate ions). Can be added. At the time of addition of liquid A and liquid B, an inert gas is injected into the reaction solution (liquid C), or a reducing agent is allowed to coexist in liquid A (a solution containing divalent iron ions and phosphate ions). By these, or both of these, the oxidation of Fe in the reaction solution (C solution) can be prevented. The amount of the reducing agent added to the liquid A is preferably 0.1 to 2.0% by mass, particularly preferably 0.5 to 1.5% with respect to the liquid A in that the reaction can be efficiently performed. % By mass.

In the first step, after completion of the contact between the liquid A and the liquid B, solid-liquid separation is performed by a conventional method, and the resulting solid is recovered, and if necessary, washed with water and dried to obtain a coprecipitate. When an alkali source containing sodium or potassium is used, if an alkali metal remains as an impurity, a LiFePO ₄ single-phase lithium iron phosphorus-based composite oxide carbon composite is obtained in X-ray diffraction analysis. Since it will no longer be obtained, it is preferable to sufficiently wash with water until the contents of both sodium and potassium in the coprecipitate are 0.5 mass% or less, preferably 0.1 mass% or less. Moreover, it is preferable that the drying temperature at the time of drying a coprecipitate is 35 to 60 ° C. because the drying efficiency is good and the divalent iron component is hardly oxidized. On the other hand, if the drying temperature of the coprecipitate is less than 35 ° C., it takes too much time to dry, and if it exceeds 60 ° C., divalent iron tends to be oxidized.

  The 2nd process which concerns on the manufacturing method of the lithium iron phosphorus complex oxide carbon composite of this invention is a process which mixes the coprecipitate obtained by the 1st process, and an electroconductive carbon material, and obtains a baking raw material mixture. is there.

  Examples of the conductive carbon material in the second step include natural graphite such as scaly graphite, scaly graphite, and earth graphite, and graphite such as artificial graphite; carbon black, acetylene black, ketjen black, channel black, Carbon blacks such as furnace black, lamp black and thermal black; carbon fibers and the like. In addition, examples of the conductive carbon material according to the second step include organic carbon compounds in which carbon is deposited by firing in the third step. Further, the conductive carbon material may be one kind or a combination of two or more kinds. Of these, carbon black and ketjen black are preferable because fine particles can be easily obtained industrially.

  The average particle size of the conductive carbon material is 1 μm or less, preferably 0.1 μm or less, particularly preferably 0.01 to 0.1 μm. When the conductive carbon material is fibrous, the average fiber diameter of the conductive carbon material is 1 μm or less, preferably 0.1 μm or less, particularly preferably 0.01 to 0.1 μm. When the average particle diameter or the average fiber diameter of the conductive carbon material is within the above range, the conductive carbon material can be easily highly dispersed in the lithium iron phosphorus composite oxide particles. In the present invention, the average particle diameter or the average fiber diameter of the conductive carbon material is an average particle diameter or an average fiber diameter obtained from a scanning electron micrograph (SEM). The average value of the particle diameters of the 20 particles extracted or the fiber diameters of the fibers.

  There is a tendency that the amount of C atoms contained in the conductive carbon material slightly decreases after firing as compared with before firing. Therefore, in the second step, when the blending amount of the conductive carbon material with respect to 100 parts by mass of the coprecipitate is 2 to 15 parts by mass, preferably 5 to 10 parts by mass, the lithium iron phosphorus composite oxide carbon composite The compounding amount of the conductive carbon material with respect to 100 parts by mass of the lithium iron-phosphorus composite oxide is 1 to 12 parts by mass, preferably 3 to 8 parts by mass in terms of C atoms. When the compounding amount of the conductive carbon material with respect to 100 parts by mass of the coprecipitate is within the above range, it is sufficient when the lithium iron phosphorus-based composite oxide carbon composite is used as the positive electrode active material of the lithium secondary battery. Therefore, the internal resistance of the lithium secondary battery can be lowered, and the discharge capacity per mass or volume is increased. On the other hand, when the blending amount of the conductive carbon material with respect to 100 parts by mass of the coprecipitate is less than the above range, it is sufficient when the lithium iron phosphorus composite oxide carbon composite is used as the positive electrode active material of the lithium secondary battery. Thus, the internal resistance of the lithium secondary battery tends to be high, and if it exceeds the above range, the discharge capacity per mass or volume tends to be low.

  In the second step, it is preferable that the coprecipitate and the conductive carbon material are sufficiently mixed in a dry manner so that they are uniformly mixed. In the second step, the apparatus used for mixing the coprecipitate and the conductive carbon material is not particularly limited as long as a uniform firing raw material mixture can be obtained. For example, a high-speed mixer, Examples include a super mixer, a turbo sphere mixer, a Henschel mixer, a nauter mixer, and a ribbon blender. The uniform mixing operation of the coprecipitate and the conductive carbon material is not limited to the exemplified mechanical means.

  The third step is a step in which the firing raw material mixture obtained in the second step is fired in an inert gas atmosphere to obtain a lithium iron phosphorus composite oxide carbon composite.

  In the third step, the firing raw material mixture is fired in an inert gas atmosphere such as nitrogen or argon in order to prevent oxidation of the Fe element.

In the third step, the firing temperature when firing the firing raw material mixture is 500 to 800 ° C, preferably 550 to 750 ° C. When the firing temperature of the firing raw material mixture is within the above range, the LiFePO ₄ crystallinity is increased, so that the discharge capacity is increased, and the particle size growth is difficult to proceed, and the discharge capacity is increased. On the other hand, if the firing temperature of the firing raw material mixture is less than the above range, the LiFePO ₄ crystallinity is low and the discharge capacity tends to be low, and if it exceeds the above range, the particle size growth proceeds and the discharge capacity becomes low. Tend. Moreover, the baking time of a baking raw material mixture is 1 hour or more, Preferably it is 2 to 10 hours. In the third step, if desired, the firing may be performed twice or more, and the fired once may be pulverized and then refired for the purpose of making the powder characteristics uniform.

  In the third step, after the firing raw material mixture is fired, the fired product is appropriately cooled, and pulverized or classified as necessary to obtain a lithium iron phosphorus composite oxide-carbon composite. In order to prevent oxidation of the Fe element, it is preferable to cool the fired product in an inert gas atmosphere. In addition, although the pulverization of the fired product is performed as necessary, the calcination of the fired product is appropriately performed when the lithium iron phosphorus composite oxide-carbon composite obtained by firing is crumbly block-like. Do.

In the lithium iron phosphorus composite oxide carbon composite obtained by performing the method for producing a lithium iron phosphorus composite oxide carbon composite of the present invention, LiFePO ₄ particles and fine conductive carbon materials are uniformly dispersed. . Moreover, the lithium iron phosphorus composite oxide in the lithium iron phosphorus composite oxide carbon composite obtained by carrying out the method for producing the lithium iron phosphorus composite oxide carbon composite of the present invention is a single element in X-ray diffraction analysis. The phase is LiFePO ₄ . Further, the lithium iron phosphorus composite oxide carbon composite obtained by performing the method for producing the lithium iron phosphorus composite oxide carbon composite of the present invention comprises lithium iron phosphorus composite oxide particles, fine conductive carbon material, Although it is a homogeneous mixture of the lithium iron phosphorus composite oxide particles and the conductive carbon material can be visually distinguished by scanning electron microscope observation (SEM), the lithium iron phosphorus system required from the SEM photograph The average particle diameter of the composite oxide particles itself is 0.05 to 1 μm, preferably 0.1 to 0.5 μm. In the present invention, the average particle size of the lithium iron phosphorus composite oxide in the lithium iron phosphorus composite oxide carbon composite is an average particle size determined from a scanning electron micrograph (SEM). It is the average value of the particle diameters of 20 particles arbitrarily extracted from the electron micrograph.

  And in the manufacturing method of the lithium iron phosphorus complex oxide carbon composite of the present invention, composition adjustment of the lithium iron phosphorus complex oxide in the lithium iron phosphorus complex oxide carbon complex is easy.

  The lithium iron phosphorus composite oxide carbon composite obtained by carrying out the method for producing a lithium iron phosphorus composite oxide carbon composite of the present invention comprises a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte comprising a lithium salt. It is suitably used as a positive electrode active material for secondary batteries. In addition, since the lithium iron phosphorus composite oxide carbon composite has hygroscopicity, when the water content is 2000 ppm or more, before using the lithium iron phosphorus composite oxide as the positive electrode active material, vacuum It is preferable to perform operations such as drying so that the water content of the lithium iron phosphorus composite oxide is 2000 ppm or less, preferably 1500 ppm or less.

Further, by using the lithium iron phosphorus composite oxide obtained by performing the method for producing the lithium iron phosphorus composite oxide carbon composite of the present invention in combination with other known lithium transition metal composite oxides, The safety of the lithium secondary battery using the lithium transition metal composite oxide can be further improved. The lithium transition metal oxide that can be used in combination with the lithium iron phosphorus composite oxide obtained by carrying out the method for producing the lithium iron phosphorus composite oxide carbon composite of the present invention includes the following general formula (1);
Li _a M _{1- b} AbO _c (1)
(Wherein M is at least one transition metal element selected from Co and Ni, A is at least one transition metal element selected from Mg, Al, Mn, Ti, Zr, Fe, Cu, Zn, Sn, In) A metal element, a is 0.9 ≦ a ≦ 1.1, b is 0 ≦ b ≦ 0.5, and c is 1.8 ≦ c ≦ 2.2.) Things. An example of the type of lithium transition metal composite oxide represented by the general formula (1) is LiCoO ₂ , LiNiO ₂ , LiNi _0.8 Co _0.2 O ₂ , LiNi _0.8 Co _0.1 Mn _{0. .1} O ₂ , LiNi _0.4 Co _0.3 Mn _0.3 O _{2 and the} like. These lithium transition metal composite oxides may be one kind or two or more kinds. The physical properties of the lithium transition metal composite oxide used in combination with the lithium iron phosphorus composite oxide obtained by carrying out the method for producing the lithium iron phosphorus composite oxide carbon composite of the present invention are not particularly limited. The average particle size is preferably 1 to 20 μm, particularly preferably 1 to 15 μm, more preferably 2 to 10 μm, and the BET specific surface area is preferably 0.1 to 2.0 m ² / g, particularly preferably 0.2. It is -1.5m < ² > / g, More preferably, it is 0.3-1.0m < ² > / g.

  The method for producing a coprecipitate containing lithium, iron and phosphorus according to the present invention comprises a solution containing lithium ion, divalent iron ion and phosphate ion (solution A) while controlling the pH to 5.5 to 9.5. ) And an alkali-containing solution (solution B) to obtain a coprecipitate containing lithium, iron and phosphorus, and a method for producing a coprecipitate containing lithium, iron and phosphorus.

  That is, the method for producing a coprecipitate containing lithium, iron and phosphorus according to the present invention is the same as the first step according to the method for producing a lithium iron phosphorous composite oxide-carbon composite according to the present invention. And the manufacturing method of the coprecipitate containing lithium, iron, and phosphorus of this invention is the solution (A liquid) containing a lithium ion, a bivalent iron ion, and a phosphate ion, The solution (B liquid) containing an alkali, Can be made to contact while controlling at pH 5.5 to 9.5 to facilitate the composition adjustment of Li, Fe and P in the coprecipitate containing lithium, iron and phosphorus. The composition ratio of Fe and P can be close to 1: 1: 1, and the coprecipitate can be obtained in high yield.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.
Example 1
(First step)
<Preparation of liquid A>
9.7 g of lithium sulfate (0.075 mol, 0.15 mol in terms of Li atom), 39.7 g of ferrous sulfate heptahydrate (0.15 mol, 0.15 mol in terms of divalent Fe atom), and 19.6 g of 75% by weight phosphoric acid (0.15 mol, 0.15 mol in terms of P atom) was dissolved in 231 ml of pure water to prepare A1 solution.
<Preparation of liquid B>
Lithium hydroxide monohydrate 19.1 g (0.45 mol, 0.45 equivalent) was dissolved in 131 ml of pure water to prepare solution B1.
<Contact between liquid A and liquid B>
A reaction vessel was charged with 250 ml of pure water (solution C) and heated to 70 ° C. The pH of the reaction solution (C solution) was controlled at 7 and the temperature was controlled at 70 ° C. While stirring the reaction solution, all of A solution and B solution were added dropwise to the reaction vessel simultaneously over 41 minutes. Thereafter, solid-liquid separation was performed by a conventional method, and the solid was dried at 50 ° C. for 10 hours to obtain 28 g of a precipitate.
When XRD measurement and ICP measurement were performed on the obtained precipitate, the obtained precipitate contained ferrous phosphate containing lithium, iron, and phosphorus in a molar ratio of 0.8: 1: 1. It was a coprecipitate of octahydrate and lithium phosphate.

(Second step)
Next, 10 g of the obtained coprecipitate and 0.8 g of carbon black (average particle size 0.05 μm) were sufficiently mixed by a dry method to obtain a uniform mixture.

(Third step)
Next, the obtained uniform mixture was baked in a nitrogen atmosphere at 600 ° C. for 5 hours. Next, it was cooled as it was in a nitrogen atmosphere to obtain a lithium iron phosphorus composite oxide carbon composite.

(Example 2)
(First step)
<Preparation of liquid A>
A1 solution was prepared in the same manner as in Example 1.
<Preparation of liquid B>
Lithium hydroxide monohydrate 19.1 g (0.45 mol, 0.45 equivalent) was dissolved in 131 ml of pure water to prepare solution B1.
<Contact between liquid A and liquid B>
Precipitation was carried out in the same manner as in Example 1 except that the pH of the reaction solution was controlled to 7 and the temperature was controlled to 70 ° C., except that the pH of the reaction solution was controlled to 5.5 and the temperature was controlled to 98 ° C. 27 g of product was obtained.
When XRD measurement and ICP measurement were performed on the obtained precipitate, the obtained precipitate contained ferrous phosphate containing lithium, iron, and phosphorus in a molar ratio of 0.9: 1: 1. It was a coprecipitate of octahydrate and lithium phosphate.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

(Example 3)
(First step)
<Preparation of liquid A>
A1 solution was prepared in the same manner as in Example 1.
<Preparation of liquid B>
Lithium hydroxide monohydrate 19.7 g (0.47 mol, 0.47 equivalent) was dissolved in 136 ml of pure water to prepare B3 solution.
<Contact between liquid A and liquid B>
Instead of B1 solution, B3 solution is used, and instead of controlling the pH of the reaction solution to 7 and the temperature to 70 ° C, the pH of the reaction solution is controlled to 8.5 and the temperature is controlled to 50 ° C. Except for this, the same procedure as in Example 1 was carried out to obtain 29 g of a precipitate.
When XRD measurement and ICP measurement were performed on the obtained precipitate, the obtained precipitate contained ferrous phosphate containing lithium, iron, and phosphorus in a molar ratio of 1.1: 1: 1. It was a coprecipitate of octahydrate and lithium phosphate.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

Example 4
<Preparation of liquid A>
9.7 g of lithium sulfate, 39.7 g of ferrous sulfate heptahydrate, and 19.6 g of 75% by weight phosphoric acid were dissolved in 231 ml of pure water, and 3 g of L-ascorbic acid was added as a reducing agent. A2 liquid was prepared.
<Preparation of liquid B>
B1 solution was prepared in the same manner as in Example 1.
<Contact between liquid A and liquid B>
A reaction vessel was charged with 250 ml of pure water (solution C) and heated to 70 ° C. The pH of the reaction solution (liquid C) was controlled at 7 and the temperature was controlled at 70 ° C. While blowing the nitrogen gas into the reaction system and stirring the reaction solution, liquid A and liquid B were simultaneously applied to the reaction vessel for 41 minutes. The whole amount was dropped. Thereafter, solid-liquid separation was performed by a conventional method, and the solid was dried at 50 ° C. for 10 hours to obtain 28 g of a precipitate.
When XRD measurement and ICP measurement were performed on the obtained precipitate, the obtained precipitate contained ferrous phosphate containing lithium, iron, and phosphorus in a molar ratio of 0.9: 1: 1. It was a coprecipitate of octahydrate and lithium phosphate.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

(Comparative Example 1)
(First step)
<Preparation of liquid A and preparation of liquid B>
In the same manner as in Example 1, A1 solution and B1 solution were prepared.
<Contact between liquid A and liquid B>
The A1 solution was charged into the reaction vessel, and the entire amount of the B1 solution was added dropwise to the reaction vessel at a constant rate over 37 minutes while stirring at 70 ° C. At this time, the pH of the A1 solution before dropping the B1 solution was 1, and the pH of the reaction solution after the dropping of the B1 solution was 7. After completion of the dropwise addition of the B1 liquid, solid-liquid separation was performed by a conventional method, and the solid was dried at 50 ° C. for 10 hours to obtain 27 g of a precipitate.
When ICP measurement and XRD measurement were performed on the obtained precipitate, ferrous phosphate octahydrate and lithium phosphate containing lithium, iron, and phosphorus in a molar ratio of 0.7: 1: 1. And co-precipitate.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

(Comparative Example 2)
(First step)
<Preparation of liquid A>
A1 solution was prepared in the same manner as in Example 1.
<Preparation of liquid B>
Lithium hydroxide monohydrate 16.9 g (0.4 mol, 0.4 equivalent) was dissolved in 117 ml of pure water to prepare a B4 solution.
<Contact between liquid A and liquid B>
Instead of controlling the pH of the reaction solution to 7 and the temperature to 70 ° C. instead of controlling the B1 solution to the B4 solution, and controlling the pH of the reaction solution to 5 and the temperature to 98 ° C. Was performed in the same manner as in Example 1 to obtain 24 g of a precipitate.
When the XRD measurement and the ICP measurement were performed on the obtained precipitate, the obtained precipitate contained phosphoric acid salt containing lithium, iron, and phosphorus in a molar ratio of 0.4: 1.1: 1. It was a coprecipitate of monoiron octahydrate and lithium phosphate.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

(Comparative Example 3)
(First step)
<Preparation of liquid A>
A1 solution was prepared in the same manner as in Example 1.
<Preparation of liquid B>
Lithium hydroxide monohydrate 30.6 g (0.73 mol, 0.73 equivalent) was dissolved in 212 ml of pure water to prepare B5 solution.
<Contact between liquid A and liquid B>
Instead of controlling the pH of the reaction solution to 7 and the temperature to 70 ° C. instead of controlling the B1 solution to the B5 solution, and controlling the pH of the reaction solution to 10 and the temperature to 70 ° C. Was performed in the same manner as in Example 1 to obtain 33 g of a precipitate.
When the ICP measurement was performed on the obtained precipitate, it contained lithium, iron and phosphorus in a molar ratio of 2.8: 1: 1. From the XRD measurement, only the lithium phosphate peak was observed.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

(Comparative Example 4)
(First step)
<Preparation of liquid A>
A1 solution was prepared in the same manner as in Example 1.
<Preparation of liquid B>
25% by mass sodium hydroxide 65.2 g (0.41 mol, 0.41 equivalent) was dissolved in 37 ml of pure water to prepare B6 solution.
<Contact between liquid A and liquid B>
The A1 solution was charged into the reaction vessel, and the entire amount of the B6 solution was dropped into the reaction vessel at a constant rate over 27 minutes while stirring at 70 ° C. At this time, the pH of the A1 solution before dropping the B6 solution was 1, and the pH of the reaction solution after the dropping of the B6 solution was 7. After completion of the dropwise addition of the B6 liquid, solid-liquid separation was performed by a conventional method, and the solid was dried at 50 ° C. for 10 hours to obtain 25 g of a precipitate.
When the ICP measurement and the XRD measurement were performed on the obtained precipitate, ferrous phosphate octahydrate and phosphorus containing lithium, iron, and phosphorus at a molar ratio of 0.4: 1.2: 1. It was a coprecipitate with lithium acid.

(Second step and third step)
It carried out like Example 1 and obtained lithium iron phosphorus system complex oxide carbon composite.

１）表１中の収率はＡ液中の成分量から計算される共沈体の質量に対する実際に得られた沈殿物の質量の百分率として求めた。
２）添加前ｐＨ１から添加終了後ｐＨ７に変化

1) The yield in Table 1 was determined as a percentage of the mass of the precipitate actually obtained with respect to the mass of the coprecipitate calculated from the amount of components in the liquid A.
2) Change from pH 1 before addition to pH 7 after completion of addition

<Evaluation of physical properties of lithium iron phosphorus composite oxide carbon composite>
About the lithium iron phosphorus complex oxide carbon composite obtained in Examples 1-4 and Comparative Examples 1-4, the average particle diameter of the lithium iron phosphorus complex oxide in the lithium iron phosphorus complex oxide carbon complex And the content of the conductive carbon material was measured, and X-ray diffraction analysis was performed. The obtained results are shown in Table 2. Moreover, the X-ray-diffraction figure of the lithium iron phosphorus type complex oxide carbon composite obtained in Example 1 and Comparative Example 1 is shown in FIG. 1 (Example 1) and FIG. 2 (Comparative Example 1). The average particle diameter is the average value of the particle diameters of 20 arbitrarily extracted lithium iron phosphorus composite oxides in the lithium iron phosphorus composite oxide carbon composite by a scanning electron microscope (SEM). It is. The content of the conductive carbon material is the content of C atoms.

<Evaluation of battery performance>
<Battery performance test>
(I) Production of lithium secondary battery;
A positive electrode agent prepared by mixing 91% by mass of the lithium iron phosphorus composite oxide carbon composites of Examples 1 to 4 and Comparative Examples 1 to 4 manufactured as described above, 6% by mass of graphite powder, and 3% by mass of polyvinylidene fluoride. And this was dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The obtained kneaded paste was applied to an aluminum foil, dried, pressed and punched into a disk having a diameter of 15 mm to obtain a positive electrode plate.
Using this positive electrode plate, a lithium secondary battery was manufactured using each member such as a separator, a negative electrode, a positive electrode, a current collector plate, a mounting bracket, an external terminal, and an electrolytic solution. Among these, a metal lithium foil was used for the negative electrode, and 1 mol of LiPF ₆ was dissolved in 1 liter of a 1: 1 kneaded solution of ethylene carbonate and methyl ethyl carbonate as the electrolyte.
(II) Battery performance evaluation The fabricated lithium secondary battery was operated at room temperature, and the discharge capacity was measured. Further, the ratio of LiFePO _{4 to} the theoretical discharge capacity (170 mAH / g) was calculated by the following formula (2). The results are shown in Table 3.
Ratio to theoretical discharge capacity = {discharge capacity / theoretical discharge capacity of LiFePO ₄ (170 mAH / g)} × 100 (2)

1 is an X-ray diffraction pattern of a lithium iron phosphorus composite oxide carbon composite obtained in Example 1. FIG. 2 is an X-ray diffraction diagram of a lithium iron phosphorus composite oxide carbon composite obtained in Comparative Example 1. FIG.

Claims (4)

While controlling the pH to 5.5 to 9.5, a solution (Liquid A) containing lithium ions, divalent iron ions and phosphate ions is brought into contact with a solution containing Lithium hydroxide (Liquid B). A first step of obtaining a coprecipitate containing lithium, iron and phosphorus, a second step of mixing the coprecipitate and a conductive carbon material to obtain a calcined raw material mixture, and inerting the calcined raw material mixture And a third step of firing in a gas atmosphere to obtain a lithium iron phosphorus composite oxide carbon composite, and a method for producing a lithium iron phosphorus composite oxide carbon composite. While the pH is controlled to 5.5 to 9.5, the first step is performed by adding the B solution to the C solution while adding the A solution to water (C solution). the process according to claim 1 Symbol placement lithium-iron-phosphorus compound oxide carbon composite material, characterized in that. In the third step, the firing temperature of the firing material mixture The process according to claim 1 or 2 any one lithium-iron-phosphorus compound oxide carbon composite material wherein it is 500 to 800 ° C. . While controlling the pH to 5.5 to 9.5, a solution (Liquid A) containing lithium ions, divalent iron ions and phosphate ions is brought into contact with a solution containing Lithium hydroxide (Liquid B). And a method for producing a coprecipitate comprising lithium, iron and phosphorus, comprising the step of obtaining a coprecipitate comprising lithium, iron and phosphorus.

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