TWI480227B - Method of manufacturing a cathode active material for lithium secondary batteries - Google Patents

Description

Method for producing positive electrode active material for lithium secondary battery

The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery.

The present invention claims priority based on Japanese Patent Application No. 2011-199408 filed on Sep. 13, 2011, the entire disclosure of which is incorporated herein.

LiMPO ₄ (M-based Fe, Mn, etc.), which is one of olivine-type lithium metal phosphates, is more expensive than LiCoO _{2 which} is widely used as a positive electrode active material for lithium batteries, and is expected to be a lithium secondary battery in particular. A positive electrode active material for large lithium batteries such as automobiles.

A method for producing LiMPO ₄ is as described in the following prior art documents, such as a solid phase synthesis method, a hydrothermal synthesis method, or a sol-gel method, but in which a pellet can be obtained at a relatively low temperature and in a short period of time. The hydrothermal synthesis method of LiMPO _{4 having a} small diameter is the most excellent.

[Practical Technical Literature] [Patent Document]

[Patent Document 1] International Publication No. 97/040541

[Patent Document 2] International Publication No. 05/051840

[Non-patent literature]

[Non-Patent Document 1] Chemistry Letters 36 (2007) 436

[Non-Patent Document 2] Electrochemical and Solid-State Letters, 9 (2006) A277-A280

However, the conventional hydrothermal synthesis method is, for example, an example in which the pH of the reaction liquid is adjusted as a particle size control means for the product, but it is complicated when the pH is adjusted. Further, since the pH value in the reaction is easily changed, it is difficult to control the particle diameter of the product to be an optimum value.

In view of the above, the present invention is directed to a method for producing a positive electrode active material for a lithium secondary battery having excellent particle size control of LiMPO ₄ in a hydrothermal synthesis reaction of LiMPO ₄ .

In order to solve the above problems, the following configuration is employed.

[1] A method for producing a positive electrode active material for a lithium secondary battery, which comprises a source of Li and a source of M (wherein M is selected from the group consisting of Mg, Ca, Fe, Mn, Ni, Co, Zn, Ge, Cu, Cr) , Ti, Sr, Ba, Sc, Y, Al, Ga, In, Si, B, a group of rare earth elements, one or more elements) and a phosphoric acid source as a raw material, by performing a synthetic olivine type LiMPO ₄ of hydrothermal synthesis reaction produced by the aforementioned lithium secondary battery LiMPO ₄ formed by the method of the positive electrode active material, characterized by comprising the phosphate source to the (RO) (R'O) (R "O) P The organic phosphate represented by =O (wherein R, R', R" each independently represents an alkyl group, an aryl group, an arylalkyl group, a cycloalkyl group or a hydrogen atom).

[2] The method for producing a positive electrode active material for a lithium secondary battery according to the above aspect, wherein the M source is any one or more of a sulfate halide salt, a nitrate, a phosphate, and an organic salt of the M element.

[3] The method for producing a positive electrode active material for a lithium secondary battery according to the above [1], wherein the lithium source is any one or two of LiOH, Li ₂ CO ₃ , CH ₃ COOLi or (COOLi) ₂ . the above.

[4] The production method according to any one of [1] to [3] wherein the reaction temperature of the hydrothermal synthesis reaction is 100 ° C or higher.

[5] A method for producing a positive electrode active material for a lithium secondary battery, characterized in that the LiMPO ₄ and a carbon source obtained by the production method according to any one of [1] to [4] are mixed in an inert gas atmosphere. Heating in a reducing gas atmosphere forms a carbon film on the surface of the aforementioned LiMPO ₄ .

[6] The method for producing a positive electrode active material for a lithium secondary battery according to [5], wherein sucrose, lactose, ascorbic acid, 1,6-hexanediol, polyethylene glycol, polyethylene oxide, carboxymethylcellulose, Any one or more of carbon black and fibrous carbon is used as the carbon source.

According to the present invention, it is possible to provide a method for producing a positive electrode active material for a lithium secondary battery which is excellent in particle size control of LiMPO ₄ in a hydrothermal synthesis reaction of LiMPO ₄ .

[In order to implement the invention]

In the following, a method for producing a positive electrode active material for a lithium secondary battery according to an embodiment of the present invention will be described with reference to the drawings.

In the method for producing a positive electrode active material for a lithium secondary battery of the present embodiment, when a hydrothermal synthesis reaction of a synthetic olivine-type LiMPO _{4 is} carried out using a Li source, an M source, and a phosphoric acid source as a raw material, RO) (R'O)(R"O)P=O is an organic phosphate as a source of phosphoric acid. By using an organic phosphate, the average particle size of LiMPO ₄ is smaller when compared with the conventional one, and by changing The type of organophosphate can control the average particle size of LiMPO ₄ .

An olivine-type LiMPO ₄ produced by the production method of the preferred embodiment of the present embodiment, more specifically, a lithium metal phosphate having a composition of, for example, Li _x Mn _y M _z P _w O ₄ . Wherein, x, y, z, w in terms of molar ratio are 0<x<2, 0<y<1.5, 0≦z<1.5, 0.9<w<1.1. In the following, the manufacturing method will be described in detail.

(Li source)

The Li source is preferably a compound which is melted during hydrothermal synthesis, and is, for example, any one or two or more of LiOH, Li ₂ CO ₃ , CH ₃ COOLi or (COOLi) ₂ . Among these, LiOH is preferred.

(M source)

The M source is a compound which melts upon hydrothermal synthesis, and a compound containing an M element is preferred. Here, the M element is, for example, Mg, Ca, Fe, Mn, Ni, Co, Zn, Ge, Cu, Cr, Ti, Sr, Ba, Sc, Y, Al, Ga, In, Si, B, rare earth elements One or more elements. Among these, a divalent transition metal is more preferable, and a divalent transition metal such as any one or two or more of Fe, Mn, Ni or Co is more preferable, for example, Fe and/or Mn. M source such as M element sulfate, halide (chlorinated , fluoride, bromide, iodide), nitrate, phosphate, organic acid salt (such as oxalate or acetate). The M source is preferably a compound which can be easily dissolved in a solvent used in the hydrothermal synthesis reaction. Among these, a divalent transition metal sulfate is preferred, and iron (II) sulfate and/or manganese (II) sulfate and such hydrates are more preferred.

(phosphoric acid source)

The phosphate can be arbitrarily selected as long as it contains a phosphate ion, and is preferably a compound which is easily dissolved in a polar solvent. For example, phosphoric acid (o-phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, hydrogen phosphate, dihydrogen phosphate, ammonium phosphate, anhydrous ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium phosphate, iron phosphate, and the like. Lithium phosphate or a divalent transition metal phosphate can also be used as the Li source or the M source.

Further, it is preferred to include an organic phosphate represented by (RO)(R'O)(R"O)P=O in the phosphoric acid source. R, R', R" each independently represent an alkyl group (carbon chain) The number is 1 to 10), the aryl group (the number of carbon chains is 1 to 3), the aralkyl group (the number of carbon chains is 1 to 3), the cycloalkyl group (the number of carbon chains is 1 to 2), or any of hydrogen atoms. In particular, an alkyl group having 1 to 10 carbon atoms is preferred, and an alkyl group having 1 to 4 carbon atoms is more preferred. However, since all of R, R', and R" are hydrogen atoms, they become phosphoric acid, and these are not included in the organic phosphate. The alkyl group, the aryl group, the aralkyl group, and the cycloalkyl group may be optionally combined with a halogen group ( -Cl, -Br, -I, -F), hydroxy (-OH), amine (-NH ₂ ), imine (=NH), N-oxide (N→O), N-hydrogenamine Base (-NH-OH), nitro (-NO ₂ ), nitroso (-NO), azo (-N=N-), diazo (-N+≡N), azide ( -N ₃ ), an epoxy group such as an epoxy group (-O-), an oxy group (=O), a carbonyl group (-CO-), a thio group (-S-), or an S-oxide group (S→O) The thiol group (=S). R, R', R" may be the same or different.

The larger the size of the substituents R, R', and R" of the organophosphate, the smaller the average particle diameter of LiMPO ₄ is. This is because the size of the substituents R, R', and R" becomes large, and it becomes difficult to carry out. hydrothermal synthesis reaction, it is speculated that would encumber LiMPO ₄ of crystal growth and LiMPO ₄ made of a small particle diameter. For example, when the substituent is an alkyl group, the larger the carbon number, the smaller the particle size of LiMPO ₄ . Further, in the substituents R, R ', R ", the more the number of hydrogen atoms hours, LiMPO ₄ of smaller particle size when the particle size is controlled LiMPO _4, the tendency to follow these, select an organic acid.

Specific examples of the organic phosphate ester, such as methyl phosphate, dimethyl phosphate, trimethyl phosphate, ethyl phosphate, diethyl phosphate, triethyl phosphate, propyl phosphate, dipropyl phosphate, tripropyl phosphate, Butyl phosphate, dibutyl phosphate, tributyl phosphate, methyl ethyl phosphate, triethanolamine phosphate or a mixture of these.

The content of the organic phosphate in the phosphoric acid source is preferably in the range of 5 to 20% by mass, more preferably in the range of 10 to 15% by mass. When the content of the organic phosphoric acid in the phosphoric acid source is 5% by mass or more, the particle diameter can be easily controlled, and when the content of the organic phosphoric acid is 20% by mass or less, the progress of the side reaction can be controlled.

The chemical ratio of the ratio of the Li source, the M source, and the phosphoric acid source to the lithium metal phosphate (more specifically, the lithium metal phosphate of the composition formed by Li _x Mn _y M _z P _w O ₄ ) It is decided to make a decision. Thereby, there is no problem that excess Li remains in the composition after hydrothermal synthesis, and the composition after hydrothermal synthesis is not required to be washed, and the procedure can be greatly simplified.

Further, water may be added in addition to the Li source, the M source, and the phosphoric acid source. The water may also be a crystal water contained in each of the Li source, the M source, or the phosphoric acid source. When a compound of the M source or the compound of the Li source contains a sufficient amount of water of crystallization, when the Li source, the M source, and the phosphoric acid source are mixed as a raw material for hydrothermal synthesis, water may not be added.

The amount of water added to the raw material for hydrothermal synthesis is preferably in the range of 10 to 99% by mass, more preferably in the range of 24 to 93% by mass, and most preferably in the range of 55 to 80% by mass. When the amount of water added is within this range, hydrothermal synthesis can be smoothly performed. Further, by adjusting the amount of water added, the particle size of the particles of the lithium metal phosphate can be controlled. When the amount of water added is small, the particle size of the lithium metal phosphate tends to be small, and when the amount of water added is increased, the particle size of the lithium metal phosphate tends to increase, so that the required particle diameter is required. Adjust the amount of water. However, when the amount of water added is too small, the particle size tends to be too large.

Further, in addition to water, a polar solvent which can be hydrothermally synthesized, such as methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, cyclohexanone, 2-methylpyrrolidone, ethyl ketone, can be arbitrarily selected. , 2-ethoxyethanol, propylene carbonate, ethylene carbonate, dimethyl carbonate, dimethylformamide, dimethyl alum, and the like. These solvents may be used alone instead of water, or they may be mixed in water.

The above is the main raw material of the production method of the preferred embodiment of the present embodiment, except that the above-mentioned main raw materials may be added.

A reducing substance such as ascorbic acid can be used as a carbon source and as an anti-inhibition An antioxidant that oxidizes the raw materials in the hydrothermal synthesis. As the antioxidant, in addition to ascorbic acid, tocopherol, dibutylhydroxytoluene, butylhydroxyanisole, propyl gallate or the like can be used.

In the production method of the preferred embodiment of the present embodiment, the Li source is reacted with the M source and the phosphoric acid source at 100 ° C or higher. Here, when the Li source, the M source, and the phosphoric acid source are simultaneously mixed, since the undesired side reaction is performed, it is necessary to control the progress of the reaction.

Therefore, in the present production method, the first raw material liquid containing any one of a lithium source, a phosphoric acid source, and an M source in the solvent and the second raw material liquid formed by containing the solvent other than the solvent are used. The first raw material and the second raw material are mixed. These first and second raw material liquids can be mixed, and the temperature and pressure can be set under specified conditions to start the shift reaction.

Specific examples of the first and second raw material liquids are prepared by preparing a liquid containing a Li source as a first raw material liquid, preparing a liquid containing a source of M and a source of phosphoric acid as a second raw material liquid, and preparing a liquid containing a source of phosphoric acid. In the first raw material liquid, a liquid containing the M source and the Li source is prepared as a second raw material liquid, and a liquid containing the M source is prepared as the first raw material liquid, and a liquid containing a phosphoric acid source and a Li source is prepared as the second raw material. The form of the liquid. When the following reaction is carried out, the first raw material and the second raw material liquid are not brought into contact with each other. Specifically, the first and second raw material liquids may not be mixed. Thus, at less than 100 ° C, substantially no shift reaction is caused.

When modulating each of the Li source, the M source, and the phosphoric acid source, it is preferable that the ratio of the Li ion, the M metal ion, and the phosphate ion to the LiMPO ₄ is about the same ratio.

Further, in the present production method, LiMPO ₄ having one metal M can be produced by using a compound containing any one of Fe, Mn, Ni or Co as an M source. In addition, LiMPO ₄ having two or more kinds of metal M can be produced by using a compound containing two or more elements of Fe, Mn, Ni or Co as an M source.

Next, the first raw material and the second raw material liquid are brought into contact, and the reaction of converting into LiMPO ₄ is started at 100 ° C or higher.

The foregoing reaction is carried out in a pressure resistant reactor such as a hot pot. When the first raw material and the second raw material liquid are brought into contact, the first raw material and the second raw material liquid may be heated to about 60 to 100 ° C in advance, or may not be heated. After mixing the first and second reaction liquids in the pressure-resistant reactor, the container is sealed, and then heated directly to the temperature of 100 ° C or higher by a hot pot directly (for example, within 1 to 2 hours). It is preferred to replace the reactor with an inert gas or a reducing gas. An inert gas such as nitrogen, argon or the like.

Next, it is kept at a temperature of 180 to 260 ° C and a pressure of 1.0 to 4.7 MPa for 30 minutes to 30 hours. When the temperature is 180 ° C or higher, the quality of the crystal can be improved. When the temperature is 260 ° C or lower, the energy required for heating does not need to be more than necessary, so that energy is not wasted. In addition, when the time is 30 minutes or longer, the defects of crystallization can be sufficiently repaired, and when it is 30 hours or less, the production efficiency is not lowered. Further, when the pressure is in the range of 1.0 to 4.7 MPa, the reaction can be smoothly carried out.

By this shift reaction, particles formed of LiMPO ₄ can be grown. Thus, a suspension containing the positive electrode active material of the present embodiment can be obtained.

The resulting suspension was cooled to room temperature and subjected to solid-liquid separation. Since the separated liquid contains unreacted lithium ions or the like, a Li source or the like can be recovered from the separated liquid. There are no particular restrictions on the recovery method. For example, a source of alkaline phosphate is added to the separated liquid to precipitate lithium phosphate. The foregoing precipitate can be recovered and reused as a Li source or a phosphoric acid source.

The positive electrode active material separated from the suspension is washed and dried as needed. Drying is preferred to select conditions in which the metal M is not oxidized. The aforementioned drying is preferably carried out using a vacuum drying method.

Further, in order to impart conductivity to the LiMPO ₄ to which the positive electrode active material is imparted, the obtained LiMPO ₄ and the carbon source are mixed, and the mixture is vacuum dried as required, and then, in an inert gas atmosphere or a reducing gas atmosphere, preferably. The firing is carried out at a temperature of from 500 ° C to 800 ° C, and more preferably at a temperature of from 650 ° C to 750 ° C.

When this baking is performed, the positive electrode material which coat|covered the carbon film on the surface of LiMPO ₄ particle can be obtained. The firing is preferably carried out under the condition that the selective element M is not oxidized.

The carbon source usable in the above calcination is arbitrarily selected, and the water solubility of saccharides, ascorbic acid, 1,6-hexanediol, polyethylene glycol, polyethylene oxide, and carboxymethylcellulose exemplified by sucrose or lactose is exemplified. Organic matter is preferred. Further, carbon black or fibrous carbon can also be used.

The LiMPO ₄ thus obtained is of the olivine type Li _x Mn _y (M, M') _z P _w O ₄ (where x, y, z, w in terms of molar ratio, 0 < x2, 0 < The composition formed by y<1.5, 0≦z<1.5, 0.9<w<1.1). The composition of the lithium metal phosphate can be adjusted by changing the mixing ratio of the lithium source, the M source, and the phosphoric acid source.

(Positive active material for lithium battery)

The positive electrode active material for a lithium secondary battery of the present embodiment is an olivine-type LiMPO ₄ produced by the above-described production method. The positive electrode active material is more preferably coated with a film of LiMPO ₄ as a carbon film.

Further, the positive electrode active material is an average particle diameter D ₅₀ of a cumulative 50% diameter based on a volume basis, preferably 0.01 to 1 μm, more preferably 0.05 to 0.5 μm. In the present embodiment, the average particle diameter D _{50 of the} positive electrode active material can be arbitrarily controlled by changing the type of the organic phosphoric acid.

(lithium battery)

The lithium secondary battery of the present embodiment is composed of a positive electrode, a negative electrode, and a nonaqueous electrolyte. In the lithium secondary battery, the positive electrode active material contained in the positive electrode is LiMPO ₄ produced by the above method. By providing the positive electrode active material, the energy density of the lithium secondary battery can be improved. Hereinafter, the positive electrode, the negative electrode, and the nonaqueous electrolyte constituting the lithium secondary battery will be described in order.

(positive electrode)

In the lithium secondary battery of the preferred embodiment of the present invention, a positive electrode material comprising a positive electrode active material, a conductive material and a binder, and a positive electrode current collector can be used. Sheet electrode. Further, a granule type or a sheet-shaped positive electrode formed by forming the above-mentioned positive electrode material into a disk shape may be used as the positive electrode.

As the positive electrode active material, a lithium metal phosphate produced by the above method is used, and the lithium metal phosphate may be mixed with a conventional positive electrode active material.

The adhesive can be arbitrarily selected, for example, polyethylene, polypropylene, ethylene propylene copolymer, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, polytetrafluoroethylene, poly (a) Acrylate, polyfluorinated ethylene oxide, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphorus chloride, polyacrylonitrile, and the like.

Further, conductive conductive materials such as conductive metal powder such as silver powder; conductive carbon powder such as furnace black, kitchen black, and acetylene black; carbon nanofibers, carbon nanofibers, and gas phase carbon fibers. The conductive aid is preferably a gas phase carbon fiber. The vapor-phase carbon fiber preferably has a fiber diameter of 5 nm or more and 0.2 μm or less, more preferably 10 nm or more and 0.1 μm or less. The ratio of the fiber length/fiber diameter is preferably from 5 to 1,000, more preferably from 100 to 500. The content of the vapor-phase carbon fiber is preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass, based on the dry mass of the positive electrode material.

Further, the positive electrode current collector can be arbitrarily selected, for example, a conductive metal foil, a conductive metal mesh, a perforated metal of a conductive metal, or the like. The conductive metal is preferably aluminum or an aluminum alloy.

(negative electrode)

As the negative electrode, a negative electrode material comprising a negative electrode active material, a binder, and a conductive auxiliary material to be added, and a sheet electrode formed of a negative electrode current collector bonded to a negative electrode material can be used. Moreover, it can also be used to make the above negative The polar material is formed into a pellet form or a sheet-shaped negative electrode formed into a disk shape as a negative electrode.

As the negative electrode active material, a conventional negative electrode active material can be used. For example, a carbon material such as artificial black lead or natural black lead or a metal or semi-metal material such as Sn or Si can be used.

The adhesive can be the same as the adhesive used for the positive electrode.

In addition, conductive materials may or may not be added as needed. For example, conductive carbon powder such as furnace black, kitchen black, acetylene black, carbon nanowire, carbon nanofiber, gas phase carbon fiber, and the like. The conductive auxiliary agent is preferably a gas phase carbon fiber. The vapor-phase carbon fiber preferably has a fiber diameter of 5 nm or more and 0.2 μm or less. The ratio of the fiber length/fiber diameter is preferably from 5 to 1,000. The content of the vapor-phase carbon fiber is preferably 0.1 to 10% by mass based on the dry mass of the negative electrode material.

Further, the negative electrode current collector can be arbitrarily selected, for example, a conductive metal foil, a conductive metal mesh, a perforated metal of a conductive metal, or the like. The conductive metal is preferably copper or a copper alloy.

(non-aqueous electrolyte)

Next, the nonaqueous electrolyte is, for example, a nonaqueous electrolyte formed by dissolving a lithium salt in an aprotic solvent.

The aprotic solvent is optionally selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, and At least one or a mixture of two or more kinds of the group of vinylene carbonate is preferred.

Further, a lithium salt such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li or the like.

Further, a solid electrolyte or a gel electrolyte may also be used as the nonaqueous electrolyte. A solid electrolyte or a gel electrolyte such as a polymer electrolyte such as a styrene-olefin copolymer, a polymer electrolyte using polyethylene oxide and MgClO ₄ , a polymer electrolyte having a trimethyl oxide structure, or the like. The nonaqueous solvent used in the polymer electrolyte may be optionally selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, γ- At least one of the group of butyrolactone and vinylene carbonate is preferred.

Further, the lithium secondary battery of the preferred embodiment of the present invention is not limited to the positive electrode, the negative electrode, and the non-aqueous electrolyte, and may be provided with other members as needed, and for example, a separator having a positive electrode and a negative electrode is provided. It is necessary to provide a separator when the nonaqueous electrolyte is not a polymer electrolyte. For example, non-woven fabric, woven fabric, microporous film, or the like, or a combination thereof. More specifically, a porous polypropylene film, a porous polyethylene film, or the like can be suitably used.

The lithium secondary battery of this embodiment can be used in various fields. Such as word processing computers, typing computers, notebook computers, mobile phones, radio communications, electronic notebooks, electronic dictionaries, PDA (Personal Digital Assistant), electronic computers, electronic keys, electronic tags, power storage devices, power tools, toys, Electric cameras, data cameras, AV machines, cleaning machines, etc.; electric vehicles; hybrid electric vehicles, electric motorcycles, hybrid electric motorcycles, electric bicycles, electric bicycles, railways, airplanes, Vehicles such as ships; the sun Power generation systems such as power generation systems, wind power generation systems, wave power generation systems, geothermal power generation systems, thermal power generation systems, and vibration power generation systems.

As described above, in the method for producing a positive electrode active material for a lithium secondary battery of the present embodiment, when a positive electrode active material for a lithium secondary battery formed of LiMPO ₄ is produced using a Li source, an M source, and a phosphoric acid source as a raw material, The phosphoric acid source contains an organic phosphate represented by (RO)(R'O)(R"O)P=O, and the average particle diameter of LiMPO ₄ is adjusted.

[Examples] (Example 1) 1. Hydrothermal synthesis step

In an argon-filled kit, argon was bubbled in distilled water for 15 hours, and carbonic acid gas or oxygen dissolved in distilled water was driven out. 0.12 g of L(+)-ascorbic acid (special grade manufactured by Kanto Chemical Co., Ltd.) was dissolved in 60 ml of the distilled water, and then 12.66 g of MnSO ₄ ̇5H ₂ O (manufactured by Kanto Chemical Co., Ltd.) and 4.87 g of FeSO ₄ ̇7H ₂ O were dissolved ( And light pure medicine system special grade). In addition, 7.26 g of H ₃ PO ₄ (85.0% of special grade made by Kanto Chemical Co., Ltd.) and 0.83 g of methyl phosphate (mixture of monomethyl phosphate and dimethyl phosphate) (manufactured by Tokyo Chemical Industry Co., Ltd.) were further dissolved. The content of the methyl phosphate of the phosphoric acid source was 10% by mass. Use this as the A liquid.

Next, 8.81 g of LiOH ̇H ₂ O (manufactured by Kanto Chemical Co., Ltd.) was dissolved in 40 ml of the same pulverized distilled water as described above. Take this as liquid B. The pH of solution B is 14.

Mix A and B in an argon-filled kit, stir for 10 minutes, add the mixture to a 100 ml PTFE sample container, and place the container in a pressure-resistant stainless steel outer tube (HUS-100). And close the lid.

Then, the outer tube made of pressure-resistant stainless steel to which the raw material for hydrothermal synthesis was added was placed in a autoclave, and the temperature was raised to 200 ° C for 1 hour in a temperature rising time, and maintained at 200 ° C for 7 hours to carry out a hydrothermal synthesis reaction. After 7 hours, the heating was stopped and cooled to room temperature.

After cooling to room temperature, the suspension was taken out from the autoclave, and the suspension was subjected to solid-liquid separation by a centrifugal separator. By discarding the generated supernatant liquid, adding new distilled water, stirring the solid material to redisperse, and then centrifuging the redispersion, discarding the operation of the upper layer clear liquid, and repeating the conductivity to the upper layer clear liquid is 1 ×10 ^-4 S/cm or less. Then, drying was carried out in a vacuum dryer controlled to 90 °C. LiFe _m1 Mn _m2 PO ₄ was thus obtained.

2. Carbon film formation step

After drying, 5.0 g of the obtained LiFe _m1 Mn _m2 PO _{4 was} added, 0.5 g of sucrose was added, and 2.5 ml of distilled water was added thereto for mixing, followed by drying in a vacuum dryer controlled at 90 °C. The dried product was placed in an alumina pot and fixed in a tubular furnace having a quartz tube having a diameter of 80 mm as a core tube. Nitrogen gas was passed through at a flow rate of 1 L/min, and the temperature was raised at a rate of 100 ° C / hour, and maintained at 400 ° C for 1 hour to discharge the decomposition gas of sucrose to the outside of the system. Then, the temperature was raised to 700 ° C at a rate of 100 ° C / hour, and nitrogen gas was passed through for 4 hours. After the completion of the completion, nitrogen gas was passed through and cooled to 100 ° C or lower, and the fired product was taken out from the tubular furnace as a positive electrode active material.

3. Battery evaluation

Each weighing 1.5 g of the positive active material, 0.43 g of acetylene black (HS-100 manufactured by Electric Chemical Industry Co., Ltd.) as a conductive auxiliary material, and 0.21 g of a polyvinylidene fluoride as an adhesive (KF Polymer W# manufactured by Kureha Co., Ltd.) 1300). After the mixture was sufficiently mixed, 3.0 g of N-methyl-2-pyrrolidone (manufactured by Kishida Chemical Co., Ltd.) was slowly added as a coating liquid. The coating liquid was applied to an Al foil having a thickness of 20 μm with a gap-adjusted blade. After drying the N-methyl-2-pyrrolidone from the obtained coating film, it was cut into a circular shape having a diameter of 15 mm. Then, the cut coating film was pressed at 3 MPa for 20 seconds, and when the thickness was measured, the average film thickness was 49 μm. Further, the weight of the coating film was 8.7 mg. The positive electrode was fabricated in this manner.

The obtained positive electrode was introduced into a toolbox controlled to have an argon-filled dew point of -75 ° C or lower. The positive electrode was placed on a cover of a 2320 type coil type battery (manufactured by Takara), and an electrolytic solution (1 M LiPF ₆ EC: MEC = 40: 60) was added. Then, a separator (CELGUARD 2400) cut into a diameter of 20 mm was cut in this order, and cut into a metal lithium foil having a diameter of 17.5 mm. A coil type battery having a diameter of 2 mm and a thickness of 23 mm was produced by forming a gap in which a gasket was formed.

(Example 2)

In addition to using 0.99g of methyl phosphate (monomethyl phosphate and dimethyl A coil type battery was produced under the same conditions as in Example 1 except that a mixture of phosphate esters (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 3)

A coil type battery was produced under the same conditions as in Example 1 except that 1.30 g of triethyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 4)

A coil type battery was produced under the same conditions as in Example 1 except that 1.73 g of triethanolamine phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 5)

A coil type battery was produced under the same conditions as in Example 1 except that 1.50 g of butyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 6)

A coil type battery was produced under the same conditions as in Example 1 except that 1.90 g of tributyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 7)

A coil type battery was produced under the same conditions as in Example 1 except that 3.10 g of trioctyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 8)

A coil type battery was produced under the same conditions as in Example 1 except that 2.70 g of bisphosphonyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 9)

A coil type battery was produced under the same conditions as in Example 1 except that 1.98 g of diphenylmethyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Embodiment 10)

A coil type battery was produced under the same conditions as in Example 1 except that 2.30 g of bis-2-ethylhexyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of the methyl phosphate, and a charge and discharge cycle test was performed.

(Example 11)

A coil type battery was produced under the same conditions as in Example 1 except that 2.59 g of 2-hexyldiphenyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of the methyl phosphate, and a charge and discharge cycle test was performed.

(Embodiment 12)

A coil type battery was produced under the same conditions as in Example 1 except that 1.78 g of diphenyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 13)

A coil type battery was fabricated under the same conditions as in Example 1 except that 2.11 g of 2-methylpropenyloxy 2-methylammonium phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and charging and discharging were performed. Cycle test.

(Example 14)

A coil type battery was produced under the same conditions as in Example 1 except that 1.83 g of 4-methylumbelliferyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Example 15)

A coil type battery was produced under the same conditions as in Example 1 except that 1.56 g of triallyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Embodiment 16)

Coil type electricity was produced under the same conditions as in Example 1 except that 2.20 g of tripentyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate. The pool was subjected to a charge and discharge cycle test.

(Example 17)

A coil type battery was produced under the same conditions as in Example 1 except that 2.63 g of tricresyl phosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was used instead of methyl phosphate, and a charge and discharge cycle test was performed.

(Comparative Example 1)

A coil type battery was produced under the same conditions as in Example 1 except that 8.84 g of H ₃ PO ₄ (Special Grade 85.0% manufactured by Kanto Chemical Co., Ltd.) was used as a phosphoric acid source, and a charge and discharge cycle test was performed.

(material evaluation)

The positive electrode active materials of Examples 1 to 4 and Comparative Example 1 were measured by X-ray diffraction using a CuKα line (X'Pert Powder manufactured by Panalytical), and as shown in Fig. 1, it was confirmed that LiFe _m1 Mn was formed. _M2 PO ₄ (m1=0.25, m2=0.75). The ray (2θ) of LiFePO _{4 is} shown on the lower side of Fig. 1 . The ray obtained by the positive electrode active materials obtained in Examples 1 to 4 and Comparative Example 1 coincided with the pattern of the ray ray. Further, the composition was confirmed to be LiFe _m1 Mn _m2 PO ₄ (m1 = 0.25, m2 = 0.75) for all the samples on the Vegard side.

Further, the scanning electron microscope (SEM) images of the positive electrode active materials obtained in Examples 1 to 4 and Comparative Example 1 are shown in FIGS. 2 to 6 . As can be seen from Fig. 2 to Fig. 6, the particle diameter of the positive electrode active material depending on the type of the organic phosphate can be compared. When compared with Example 1, the particle size of Example 2 was small, and when compared with Example 2, the particle size of Example 3 was small. Further, it is understood that the positive electrode active material having a shape close to a square shape can be obtained in the embodiment 4. From the results of the above, it was judged that the particle size and particle diameter of LiMPO _{4 were} controlled by adjusting the type of the organic phosphate. The same results were obtained for Examples 5 to 17.

Further, the carbon contents of the positive electrode active materials of Examples 1 to 17 and Comparative Example 1 before the formation of the carbon film are shown in Table 2 below. The carbon content is determined by a combustion oxidation method. From this result, it is understood that the positive electrode active material before the formation of the carbon film contains a small amount of carbon by using the organic phosphate.

Further, in the case of hydrothermal synthesis, when an organic phosphate is added, the growth of crystals which are crystallized during the reaction of the organic phosphate is easily stopped. Therefore, when there is an unreacted organic phosphate ester, there is a large amount of carbon present on the surface of the positive electrode active material.

(battery evaluation)

The coil type batteries of Examples 1 to 4 and Comparative Example 1 were subjected to constant current charging to 4.5 V at a temperature of 0.1 C at a temperature of 25 ° C, and then charged at a constant voltage of 4.5 V to 0.01 C. . Then, the cycle operation of constant current discharge to 2.5 V was repeated 15 times. The discharge capacity and the discharge capacity retention ratio are shown in Table 2 below. The discharge capacity is the mass discharge capacity per positive active material. Further, the discharge capacity retention rate is a percentage of the discharge capacity at the 15th cycle with respect to the discharge capacity of the first cycle.

From the results, it was confirmed that the initial cycle characteristics were good in Examples 1 to 17. Since the organic phosphate is used as a starting material and has a thin carbon layer which is strongly bonded to LiMPO ₄ on the surface, it has resistance to deterioration from an electrolyte or the like.

[industrial use value]

According to the present invention, a method for producing a positive electrode active material for a lithium secondary battery excellent in particle size control of LiMPO ₄ in a hydrothermal synthesis reaction of LiMPO ₄ can be provided.

[Fig. 1] Fig. 1 is an X-ray diffraction pattern of the positive electrode active materials of Examples 1 to 4 and Comparative Example 1.

[Fig. 2] Fig. 2 is an SEM photograph of the positive electrode active material of Example 1. sheet.

[Fig. 3] Fig. 3 is a SEM photograph of the positive electrode active material of Example 2.

[Fig. 4] Fig. 4 is a SEM photograph of the positive electrode active material of Example 3.

[Fig. 5] Fig. 5 is a SEM photograph of the positive electrode active material of Example 4.

[Fig. 6] Fig. 6 is a SEM photograph of the positive electrode active material of Comparative Example 1.

Claims (9)

A method for producing a positive electrode active material for a lithium secondary battery, which is produced by the above-described LiMPO ₄ by using a Li source, a M source, and a phosphoric acid source as a raw material by performing a hydrothermal synthesis reaction of a synthetic olivine-type LiMPO ₄ The method for using a positive active material for a lithium secondary battery; wherein the M source is selected from the group consisting of Mg, Ca, Fe, Mn, Ni, Co, Zn, Ge, Cu, Cr, Ti, Sr, Ba, Al, Ga, In, Si And B or a rare earth element group of one or more elements; wherein the phosphoric acid source contains an organic phosphate represented by (RO)(R'O)(R"O)P=O; , R, R', R" each independently represent an alkyl group, an aryl group, an arylalkyl group, a cycloalkyl group or a hydrogen atom, and there is no case where R, R', and R" are each a hydrogen atom. . The method for producing a positive electrode active material for a lithium secondary battery according to the first aspect of the invention, wherein the M source is any one or more of a sulfate, a halogenated salt, a nitrate, a phosphate, and an organic salt of the M element. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the lithium source is any one or more of LiOH, Li ₂ CO ₃ , CH ₃ COOLi or (COOLi) ₂ . The production method according to claim 1 or 2, wherein the reaction temperature of the hydrothermal synthesis reaction is 100 ° C or higher. A method for producing a positive electrode active material for a lithium secondary battery, characterized by mixing the aforementioned LiMPO ₄ and a carbon source obtained by the production method according to any one of claims 1 to 4, in an inert gas atmosphere or a reducing gas Heating in the environment forms a carbon film on the surface of the aforementioned LiMPO ₄ . A method for producing a positive electrode active material for a lithium secondary battery according to claim 5, wherein sucrose, lactose, ascorbic acid, 1,6-hexanediol, polyethylene glycol, polyethylene oxide, carboxymethyl cellulose, carbon are used. Any one or more of black or fibrous carbon is used as the aforementioned carbon source. The method for producing a positive electrode active material for a lithium secondary battery according to the first aspect of the invention, wherein the organic phosphate ester is selected from the group consisting of methyl phosphate, dimethyl phosphate, trimethyl phosphate, ethyl phosphate, diethyl phosphate, and phosphoric acid. Triethyl ester, propyl phosphate, dipropyl phosphate, tripropyl phosphate, butyl phosphate, dibutyl phosphate, tributyl phosphate, methyl ethyl phosphate, triethanolamine phosphate or a mixture thereof At least one. The method for producing a positive electrode active material for a lithium secondary battery according to the first aspect of the invention, wherein the content of the organic phosphate in the phosphoric acid source is in the range of 5 to 20% by mass. The method for producing a positive electrode active material for a lithium secondary battery according to the first aspect of the invention, wherein the amount of water added to all of the raw materials for hydrothermal synthesis is in the range of 10 to 99% by mass.