KR20110117552A - Process for preparation of lithium iron phosphate having olivine crystal structure - Google Patents

Abstract

The present invention provides a method for preparing lithium iron phosphate having an olivine crystal structure, comprising: preparing a first reaction aqueous solution including a lithium (Li) precursor and an alkylating agent; Preparing a second reaction aqueous solution including an iron (Fe) precursor and a phosphorus (P) precursor; Firstly mixing the first reaction aqueous solution and the second reaction aqueous solution and secondly mixing water under warm and pressurized conditions within 3 seconds to proceed with the reaction of the precursors; Preparing an intermediate powder by adding carbon (C) precursor to the reactant obtained in the step and then removing water; Firing the intermediate powder to produce a lithium iron phosphate powder; is configured to include.

Description

Process for Preparation of Lithium Iron Phosphate Having Olivine Crystal Structure

The present invention relates to a method for producing lithium iron phosphate having an olivine crystal structure, and more particularly, a first mixing of the first reaction aqueous solution and the second reaction aqueous solution is carried out in the secondary mixing for mixing water under heating and pressurized conditions. After proceeding within a very short time, the present invention relates to a production method capable of producing a high purity lithium iron phosphate containing almost no impurities.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among them, lithium secondary batteries with high energy density and voltage, long cycle life, and low self discharge rate It is commercially used and widely used.

As a negative electrode active material of such a lithium secondary battery, a carbon material is mainly used, and use of lithium metal, a sulfur compound, a silicon compound, a tin compound, etc. is also considered. In addition, lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as the positive electrode active material, and lithium-containing manganese oxides such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, and lithium-containing nickel oxide (LiNiO). The use of ₂ ) is also under consideration.

LiCoO ₂ is widely used because of its excellent physical properties such as excellent cycle characteristics, but it is low in safety, and is expensive due to the resource limitation of cobalt as a raw material and has a limitation in using it as a power source in fields such as electric vehicles. LiNiO ₂ is difficult to apply to the actual mass production process at a reasonable cost, due to the characteristics of its manufacturing method, lithium manganese oxides such as LiMnO ₂ , LiMn ₂ O ₄ has the disadvantage that the cycle characteristics are bad.

In recent years, a method of using lithium transition metal phosphate as a cathode active material has been studied. Lithium transition metal phosphate is divided into Li _x M ₂ (PO ₄ ) ₃ , which is a Nasicon crystal structure, and LiMPO ₄ , which is an Olivine crystal structure, and has a higher temperature stability than LiCoO ₂ . Is being studied. Currently, Li ₃ V ₂ (PO ₄ ) ₃ is known as a compound of the nacicon crystal structure, and among the compounds of the olivine crystal structure, LiFePO ₄ and Li (Mn, Fe) PO ₄ have been most widely studied.

Among the olivine crystal structures, in particular, LiFePO ₄ has a ˜3.5 V voltage and a high bulk density of 3.6 g / cm ³ compared to lithium, and has a theoretical capacity of 170 mAh / g, which is excellent in high temperature stability compared to cobalt (Co). Inexpensive Fe is used as a raw material, and therefore it is highly applicable to a cathode active material for lithium secondary batteries in the future.

However, this LiFePO ₄ is low due to the electrical conductivity, there is a problem that the internal resistance of the battery is increased when using the LiFePO ₄ as a positive electrode active material. This increases the polarization potential when the battery circuit is closed, thereby reducing the battery capacity.

Therefore, some prior arts, such as Japanese Patent Application Laid-Open No. 2001-110414, disclose a technique of adding a conductive material to an olivine-type metal phosphate to improve conductivity.

However, LiFePO ₄ is generally produced by the solid state method or the hydrothermal method using Li ₂ CO ₃ or LiOH as a lithium source, Li ₂ CO ₃ in the firing process by the lithium source and a carbon source added to improve conductivity There is a problem that a large amount occurs. Since Li ₂ CO ₃ decomposes during charging or reacts with the electrolyte to generate CO ₂ gas, an excessive amount of gas is generated during storage or cycle. As a result, a swelling phenomenon of the battery is generated and a high temperature safety is deteriorated.

In this regard, a manufacturing method for coating carbon on LiFePO ₄ using a heated and pressurized water as a reaction system is known. However, the inventors have confirmed through experiments that LiFePO ₄ prepared using the above technique is an impurity. It was confirmed that it can include a large number, greatly reducing the operating performance and safety of the battery.

Therefore, there is an urgent need for a technology that can solve these problems.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

After in-depth research and various experiments, the inventors of the present application surprisingly, when performing the process of mixing the reaction raw materials and mixing the water under heating and pressurization conditions in a very short time, surprisingly, the high purity of almost no impurities It has been found that lithium iron phosphate can be produced and the present invention has been completed.

Therefore, the method for producing lithium iron phosphate having an olivine crystal structure according to the present invention,

(a) preparing a first reaction aqueous solution comprising a lithium (Li) precursor and an alkylating agent;

(b) preparing a second reaction aqueous solution including an iron (Fe) precursor and a phosphorus (P) precursor;

(c) first mixing the first reaction aqueous solution and the second reaction aqueous solution and secondly mixing water under warm and pressurized conditions within 3 seconds to proceed with the reaction of the precursors;

(d) adding the carbon (C) precursor to the reactant obtained in step (c) and then removing the water to prepare an intermediate powder;

(e) calcining the intermediate powder to produce lithium iron phosphate powder;

It is configured to include.

According to the production method of the present invention, the secondary mixing for mixing the water under heating and pressurized conditions is carried out within a very short time after the first mixing of the first reaction aqueous solution and the second reaction aqueous solution, so that it contains almost no impurities. Since lithium iron phosphate can be produced, ultimately high quality lithium iron phosphate can be produced in high yield.

In the step (a), for example, Li ₂ CO _{3 ,} Li (OH), Li (OH) H ₂ O, LiNO _3, etc. may be used as the lithium (Li) precursor. For example, alkali metal hydroxides, alkaline earth metal hydroxides, ammonia compounds and the like can be used.

The alkylating agent may be included in the reaction catalyst amount, and water may be used in an amount of 10 to 1000 times, preferably 100 to 700 times, based on the weight of the lithium (Li) precursor for preparing the first reaction aqueous solution.

In said step (b), as the iron (Fe) precursor, for example, it may be used. FeSO _4, FeC ₂ O ₄ o 2H ₂ O, FeCl _2, such as, phosphorus (P) precursor, e. Examples thereof include H ₃ PO ₄ , NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , and P ₂ O ₅ . As described later, in the case of preparing lithium iron phosphate containing a predetermined amount of sulfur, FeSO ₄ may be preferably used as the iron precursor.

Water may be used in an amount of 3 to 100 times, preferably 4 to 200 times, based on the total weight of the iron (Fe) precursor and the phosphorus (P) precursor for the preparation of the second reaction aqueous solution.

In some cases, in order to provide a reducing reaction atmosphere, a carbon-containing material such as sugar may be added to the second reaction aqueous solution in a range of 2 to 30% based on the weight of the iron (Fe) precursor.

Step (c) may preferably be carried out in a continuous reactor. Continuous reactors have the advantage of controlling the time difference between the primary and secondary mixing and increase the overall efficiency of the process.

Further, in step (c), the water under warming and pressurizing conditions may be supercritical or subcritical water, for example, water in the range of 200 to 700 ° C. under a pressure of 180 to 550 bar.

As described above, the manufacturing method according to the present invention is characterized in that the first mixing of the reaction aqueous solutions including the reaction raw materials (precursors) and the second mixing of the water in the heated and pressurized state in a very short time. The time difference may be within 3 seconds, more preferably within 2 seconds as defined above. In some cases, the primary mixing and the secondary mixing may be performed simultaneously.

The reactant obtained in the step (d) may be in a slurry state, to which the carbon (C) precursor is added to remove the water to prepare an intermediate powder. The carbon (C) precursor is a material that is carbonized in the firing process of step (e) to form a carbon coating on the surface of lithium iron phosphate particles, for example, sucrose and lactose-based sugars, PVDF, PE-based polymers, and coke Although these etc. are mentioned, it is not limited only to these.

Preferably, the sucrose may be dissolved in water and added in step (d), wherein the amount of the sucrose may be determined in the content range of the carbon coating coated on the surface of the lithium iron phosphate particles as described later.

The intermediate powder is a form in which the carbon precursor is coated on the particle surface, and preferably may be prepared by spray drying.

In step (e), the intermediate powder is calcined to produce lithium iron phosphate, and the temperature for the firing may be, for example, 600 to 1200 ° C.

In one preferred example, the lithium iron phosphate powder obtained in step (e) may be a powder coated with carbon (C) on the surface of the lithium iron phosphate particles having a composition of the formula (1).

Li _{1 + a} Fe _{1 - x} M _x (PO _{4 -b} ) X _b (1)

Where

M is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y,

X is at least one selected from F, S and N,

−0.5 ≦ a ≦ + 0.5, 0 ≦ x ≦ 0.5, and 0 ≦ b ≦ 0.1.

The lithium iron phosphate may be preferably LiFePO ₄ , but is not limited thereto.

The carbon (C) is preferably coated with 0.01 to 10% by weight based on the total weight of lithium iron phosphate particles. In the case of containing too much carbon, the amount of the active material may be relatively small, leading to a decrease in capacity as well as a problem of decreasing the electrode density. It is not preferable because it is not obtained. More preferred coating amount may be 0.03 to 7% by weight.

In addition, the carbon coating is preferably uniformly coated with a thickness of 2 to 50 nm on the surface of the lithium iron phosphate. Too thick a coating on the surface of lithium iron phosphate may interfere with occlusion and release of lithium ions, while too thin coatings may be difficult to ensure uniform coating and may not provide the desired electrical conductivity. More preferred coating thickness may be 3-10 nm.

In some cases, the lithium iron phosphate particles may contain a predetermined amount of sulfur. The sulfur contained in this way can assist the carbon to be coated on the particle surface of the lithium iron phosphate with a uniform and strong bonding force by a complex action with carbon on the lithium iron phosphate.

Such sulfur (S) is preferably contained in 0.05 to 5% by weight based on the total weight of lithium iron phosphate. When the amount of sulfur is included in an excessive amount, it may cause a decrease in the properties of lithium iron phosphate. On the contrary, when the amount of sulfur is too small, a complex action with carbon may be difficult to be expected. More preferred content may be 0.1 to 2% by weight.

The sulfur, for example, may be derived from a precursor for the production of lithium iron phosphate. When FeSO ₄ is used for the production of lithium iron phosphate, sulfur may remain in the product after the reaction. In general, when sulfur remains in the active material, the washing process is performed several times to completely remove such sulfur, but the washing process may be limited in order to maintain such sulfur.

Moreover, sulfur can also be introduce | transduced by adding a sulfur containing compound to lithium iron phosphate. The sulfur containing compound may be one or more selected from sulfides, sulfites and sulfates.

The present invention also provides lithium iron phosphate prepared by the above method.

The lithium iron phosphate according to the present invention may be in the form of primary particles or in the form of secondary particles. The lithium iron phosphate in the form of secondary particles may be prepared by drying and agglomerating a mixture of primary particles, a binder, and a solvent having a predetermined particle diameter, which may be performed simultaneously with step (e) or afterwards.

In the mixture, the primary particles are 5 to 20 wt% based on the weight of the solvent, the binder is preferably 5 to 20 wt% based on the weight of the solvent. At this time, the internal porosity of the secondary particles can be adjusted by adjusting the ratio of the primary particles and the solvent. Examples of the solvent that can be used in the above process may use both a polar solvent such as water and a non-polar organic solvent. In addition, examples of the binder that may be used in the above process may include saccharides, PVDF, PE-based polymers, coke, and the like, which may be dissolved in a polar solvent, but are not limited thereto.

The drying and the production of the secondary particles may be performed at the same time, for example, various methods known in the art, such as spray drying, fluidized bed drying, vibration drying may be used. In particular, the rotary spray drying method of the spray drying method is preferable because the secondary particles can be produced in a spherical shape and the tap density can be increased. In some cases, a carbon-containing precursor may be added during the drying and the preparation of the secondary particles. In this case, carbon is coated on the surface of the active material in the firing process of the reducing atmosphere, thereby increasing the electrical conductivity of the active material, thereby greatly improving the electrode characteristics. The carbon-containing precursor can be any material as long as it can produce carbon in the firing step of the reducing atmosphere. It may preferably be a carbon containing precursor in the form of a polyol, more preferably sucrose.

The drying temperature may be preferably 100 ~ 200 ℃, it may be carried out in an inert gas atmosphere, such as Ar, N ₂ .

The present invention also provides a positive electrode mixture containing the lithium iron phosphate as a positive electrode active material. The positive electrode mixture may optionally include a conductive material, a binder, a filler, etc. in addition to the positive electrode active material.

The conductive material is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component which assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, and various copolymers.

The filler is optionally used as a component for suppressing the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing any chemical change in the battery. Examples of the filler include olefin-based polymerizers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

On the other hand, the positive electrode active material may be composed of only the olivine-type lithium iron phosphate according to the invention, in some cases may be configured with other lithium-containing transition metal oxide.

Examples of the lithium-containing transition metal oxides include layered compounds such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), or compounds substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y} Mn _2-y O ₄ (where y is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site-type lithium nickel oxide represented by the formula LiNi _1-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and y = 0.01 to 0.3; Formula LiMn _2-y M _y O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta and y = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The present invention provides a cathode in which the cathode mixture is coated on a current collector.

The positive electrode for a secondary battery may be manufactured by, for example, applying a slurry made by mixing the positive electrode mixture with a solvent such as NMP onto a positive electrode current collector, followed by drying and rolling.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode collector is not particularly limited as long as it has conductivity without causing chemical change to the battery, and may be formed on the surface of stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel Carbon, nickel, titanium, silver, or the like may be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The present invention provides a lithium secondary battery comprising the positive electrode, the negative electrode, the separator, and a lithium salt-containing nonaqueous electrolyte.

The negative electrode is prepared, for example, by applying a negative electrode mixture containing a negative electrode active material on a negative electrode collector and then drying the same. The negative electrode mixture may contain a conductive material, a binder, a filler, May be included.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

As said negative electrode active material, For example, carbon and graphite materials, such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pt, Ti which can be alloyed with lithium, and compounds containing these elements; Composites of metals and compounds thereof with carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, tin-based active material, silicon-based active material, or silicon-carbon-based active material is more preferable, and these may be used alone or in combination of two or more.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymerizers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, in the electrolyte solution, for the purpose of improving the charge and discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride and the like may be added. . In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics, and FEC (Fluoro-Ethylene) may be further included. carbonate), PRS (propene sultone), FEC (Fluoro-Ethlene carbonate) and the like may be further included.

In particular, the secondary battery according to the present invention may be used as a unit cell of a battery module which is a power source for medium and large devices requiring high temperature stability, long cycle characteristics, high rate characteristics, and the like.

Preferably, the medium to large device comprises, for example, a power tool that is powered by an electric motor; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); Electric two-wheeled vehicles including E-bikes and E-scooters; Electric golf carts, and the like, but are not limited thereto.

As described above, the method for producing olivine-type lithium iron phosphate according to the present invention is a very short time after the first mixing of the first reaction aqueous solution and the second reaction aqueous solution by mixing the secondary mixture of mixing water under heating and pressurized conditions Proceed to the inside, it is possible to manufacture a high purity lithium iron phosphate containing almost no impurities, there is an advantage that ultimately high quality lithium iron phosphate can be produced in a high yield.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

21.5 g of LiOH-H ₂ O, 14.7 g of ammonia water (˜29 wt%), and 963.9 g of distilled water were mixed and dissolved with each other to form an aqueous solution A. In the same manner as above, 70.65 g of FeSO ₄ -7H ₂ O, 7.065 g of sucrose, 28.85 g of phosphoric acid (85 wt%) and 893.435 g of distilled water were mixed and dissolved with each other to form an aqueous solution B.

In a continuous tubular reactor, supercritical water of 250 bar and 450 ° C. was flowed at 100 g / min under warm and pressurized conditions, and the aqueous solution A and aqueous solution B were flowed at 15 g / min, respectively, to meet the supercritical water for several seconds. The reaction was carried out. In this process, the aqueous solution A and the aqueous solution B were allowed to meet the supercritical water within 1 second after forming the slurry.

The resulting material was cooled and filtered at the end of the tubular reactor to obtain a slurry comprising the intermediate. Then, the concentration of water was adjusted to make a slurry having a solid content of 20 wt%, and dissolved in a sucrose at 10 wt% based on the solid content. The slurry thus obtained was spray dried to obtain a powder coated with sucrose. This powder was heat-treated at about 700 ° C. for 10 hours in a nitrogen atmosphere to prepare a carbon-coated LiFePO ₄ powder. As a result of XRD-Rietveld analysis on the LiFePO ₄ powder, it was confirmed that the LiFePO ₄ crystal without a defect structure.

[Example 2]

LiFePO ₄ powder was prepared in the same manner as in Example 1, except that the aqueous solution A, the aqueous solution B, and the supercritical water were met at one time. XRD phase analysis revealed little other impurities than LiFePO ₄ .

Comparative Example 1

LiFePO ₄ powder was prepared in the same manner as in Example 1, except that the aqueous solution A and the aqueous solution B met to form a slurry, and then to meet the supercritical water after 6 seconds.

Experimental Example 1

XRD phase analysis was performed on the LiFePO ₄ powders prepared in Examples 1 and 2 and Comparative Example 1, respectively. As a result, it was found that the LiFePO ₄ powders of Examples 1 and 2 consisted of pure LiFePO ₄ having almost no other impurities. On the other hand, the LiFePO ₄ powder of Comparative Example 1 was found to contain 3 wt% of impurities such as Fe ₃ (PO ₄ ) _{2 in} addition to LiFePO ₄ .

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (15)

As a method for producing lithium iron phosphate having an olivine crystal structure,
(a) preparing a first reaction aqueous solution comprising a lithium (Li) precursor and an alkylating agent;
(b) preparing a second reaction aqueous solution including an iron (Fe) precursor and a phosphorus (P) precursor;
(c) first mixing the first reaction aqueous solution and the second reaction aqueous solution and secondly mixing water under warm and pressurized conditions within 3 seconds to proceed with the reaction of the precursors;
(d) adding the carbon (C) precursor to the reactant obtained in step (c) and then removing the water to prepare an intermediate powder;
(e) calcining the intermediate powder to produce lithium iron phosphate powder;
Method for producing lithium iron phosphate comprising a. The method of claim 1, wherein step (c) is carried out in a continuous reactor. The method of claim 1, wherein the water under warm and pressurized conditions in step (c) is supercritical or subcritical water. The method of claim 1, wherein the secondary mixing in step (c) is carried out within 2 seconds. The method of claim 1, wherein the primary mixing and the secondary mixing in the step (c) is carried out simultaneously. The method of claim 1, wherein the lithium iron phosphate powder obtained in step (e) is coated with carbon (C) on the surface of lithium iron phosphate particles having the composition of Formula 1 below:
Li_{One + a}Fe_{One - x}M_x(PO_{4 -b}) X_b (One)
Where
M is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y,
X is at least one selected from F, S and N,
−0.5 ≦ a ≦ + 0.5, 0 ≦ x ≦ 0.5, and 0 ≦ b ≦ 0.1. The method of claim 6, wherein the lithium iron phosphate is LiFePO ₄ . The method of claim 6, wherein the carbon (C) is coated with 0.01 to 10% by weight based on the total weight of the lithium iron phosphate particles. The method of claim 6, wherein the carbon coating is coated on the surface of the lithium iron phosphate with a thickness of 2 to 50 nm. The method of claim 6, wherein the lithium iron phosphate particles contain 0.05 to 5% by weight of sulfur based on the total weight of lithium iron phosphate. Lithium iron phosphate prepared by the method according to any one of claims 1 to 10. A cathode mixture comprising the lithium iron phosphate according to claim 11 as a cathode active material. A lithium secondary battery comprising an electrode on which a positive electrode mixture according to claim 12 is coated on a current collector. The lithium secondary battery of claim 13, wherein the lithium secondary battery is used as a unit cell of a battery module which is a power source of a medium and large device. 15. The lithium secondary battery of claim 14, wherein the medium-to-large device is a power tool, an electric vehicle, an electric motorcycle, or an electric golf cart.