KR20070108664A - Method of manufacturing lithium iron phosphate - Google Patents

Abstract

The present invention comprises the steps of (a) mixing a precursor compound of lithium and phosphorus in an aqueous solution of the Fe ²⁺ precursor compound to prepare an aqueous solution or suspension of the mixture; (b) heating and washing the aqueous solution or suspension of the mixture of step (a) to obtain a solid material; And (c) relates to a method for producing LiFePO ₄ comprising the step of heat-treating the solid material of step (b) in an inert atmosphere or inert and reducing atmosphere. The present invention also relates to a LiFePO ₄ produced by the above method and a secondary battery including the same as a cathode active material.

Description

Manufacturing Method of ＬｉＦｅＰ０４ {METHOD OF MANUFACTURING LITHIUM IRON PHOSPHATE}

1 is an X-ray diffraction analysis (XRD) pattern diagram of a LiFePO ₄ powder prepared according to Example 1. FIG.

2 is a SEM photograph of the LiFePO ₄ powder prepared according to Example 1.

3 is a graph showing initial charge and discharge capacity at 0.1C of a battery prepared according to Example 2.

4 is a graph showing initial charge and discharge capacity at 0.1C of a battery prepared according to Comparative Example 2.

5 is a graph showing high-rate discharge characteristics (C-rate capability) of the battery prepared according to Example 2.

6 is a graph showing the high-rate discharge characteristics (C-rate capability) of the battery prepared according to Comparative Example 2.

7 is a graph showing the cycle characteristics of the battery prepared according to Example 2.

The present invention relates to a method for producing LiFePO ₄ that minimizes the reaction to suppress the occurrence of side reactions, and a secondary battery including the LiFePO ₄ produced thereby.

Recently, as miniaturization and light weight of electronic equipment have been realized and the use of portable electronic devices has become common, research on lithium secondary batteries having high energy density has been actively conducted. A lithium secondary battery is prepared by using a material capable of inserting and detaching lithium ions as a negative electrode and a positive electrode, and filling an organic or polymer electrolyte between the positive electrode and the negative electrode, and lithium ions can be inserted and removed from the positive electrode and the negative electrode. Electrical energy is generated by oxidation and reduction reactions.

LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, etc. have been mainly studied as a cathode active material. However, LiCoO ₂ , which is currently widely used, has a high raw material price and causes environmental problems. In addition, LiNiO ₂ , which is studied as a substitute material for LiCoO ₂ , is difficult to manufacture and has low thermal stability, and LiMn ₂ O ₄ has disadvantages of rapid electrode degradation at high temperature and low electrical conductivity.

The new battery material, LiFePO _4, is environmentally friendly and contains iron, which is the most abundant metal, instead of using rare metals such as cobalt. In addition, the discharge voltage is 3.4 V (vs. Li / Li ⁺ ), and the theoretical capacity is 170 mAh / g, and the battery capacity is also excellent.

The LiFePO ₄ powder is prepared by a conventional solid phase reaction method, sol-gel method and the like. US 2002-0004169A1 discloses a method for synthesizing LiFePO ₄ using iron acetate, lithium carbonate and ammonium dihydrogen phosphate as reactants. However, when using the preparation method, toxic by-products such as acetic acid and ammonia are generated, which not only requires a large capacity gas collector but also a problem such as a large total volume of the precursor mixture and a high cost of iron acetate.

WO02 / 099913A1 discloses a method for the synthesis of LiFePO ₄ via solid state reaction of Fe (NO ₃ ) ₃ .9H ₂ O and LiH ₂ PO ₄ in a reducing atmosphere. However, LiH ₂ PO ₄ , one of the reactants, was not commercially accessible and therefore not suitable for mass production processes.

In addition, EP 1391424 A2 proposed a method for producing LiFePO ₄ , but according to this, toxic gases such as carbon monoxide and ammonia were obtained as by-products, and high temperatures in the range of 750 to 800 ° C. were required. In addition, the discharge capacity of the manufactured LiFePO ₄ was only about 121mAh / g under a current density of 0.2mA / cm ² .

Therefore, there is a demand for a new manufacturing method suitable for mass production while having good reproducibility, simple manufacturing process, and low cost in producing LiFePO ₄ .

The present invention aims to provide a method for producing LiFePO ₄ with uniform and excellent physical properties by minimizing the occurrence of side reactions by reducing the type of reactants participating in the reaction and simplifying the reaction.

In addition, the present invention is to provide a LiFePO ₄ produced by the above method and a secondary battery using the same as a cathode active material.

The present invention comprises the steps of (a) mixing a precursor compound of lithium and phosphorus in an aqueous solution of the Fe ²⁺ precursor compound to prepare an aqueous solution or suspension of the mixture; (b) heating and washing the aqueous solution or suspension of the mixture of step (a) to obtain a solid material; And (c) provides a method for producing LiFePO ₄ including the step of heat-treating the solid material of step (b) under an inert atmosphere, or an inert and reducing atmosphere.

In addition, the present invention provides a secondary battery comprising LiFePO ₄ and the cathode active material prepared by the above method.

Hereinafter, the present invention will be described in detail.

The invention causes the supply of the two, i.e., Fe ²⁺ the type of reactants used for the preparation of the LiFePO ₄ precursor compounds Fe ^2+, and Li ⁺ and (PO ₄₎ ^3-, and the lithium supply source comprises at the same time the By using only the prepared precursor compound, the reaction can be simplified as much as possible to suppress the occurrence of side reactions.

In addition, the production method of the present invention is characterized by using a hydrothermal method. Therefore, LiFePO ₄ can be prepared in a faster time than the conventional solid state reaction, and the rate of the primary particles of LiFePO ₄ produced by controlling the temperature and the concentration of the reactants is controlled to a level of 100 nm or less. rate) characteristic can be improved.

In addition, the LiFePO ₄ prepared according to the present method has a primary particle size of about 100 nm and a smaller primary particle size than a material synthesized by a solid phase reaction, and the synthesis temperature (heating temperature in step (b)) is a critical point of water. If higher, the degree of agglomeration between LiFePO ₄ particles is low.

In the production method of the present invention, the aqueous solution or suspension of the mixture of step (a), or the solid material obtained in step (b) may be used by additionally mixing a carbon source.

That is, the aqueous solution or suspension of the mixture of step (a) may be prepared by additionally mixing a carbon source. Specifically, an aqueous solution or suspension of the mixture may be prepared by mixing a precursor compound of lithium and phosphorus in an aqueous solution of the Fe ²⁺ precursor compound and further mixing a carbon source.

In addition, in the production method of the present invention, the solid material obtained in step (b) may be used by additionally mixing a carbon source. Specifically, after heating the aqueous solution or suspension of the mixture of step (a) without mixing the carbon source to synthesize LiFePO ₄ as a solid material of step (b), the carbon source is mixed with LiFePO ₄ in step (c) Carbon coating can also be performed by baking together.

The carbon source is C, H, O; Or C, H, N; Or a water-soluble compound composed of C, H, O, and N elements. The carbon source is preferably not volatile, has a high carbon content, and leaves no residue other than carbon after heat treatment. Specifically, the carbon source may be sucrose, or a water-soluble polymer such as polyvinyl alcohol (PVA), polyethylene glycol (PEG), or oxalic acid, resorcinol, citric acid. And organic compounds such as cellulose acetate. These compounds can be used individually or in mixture of 2 or more types.

When additionally mixing the carbon source with the solid material obtained in step (b), the carbon source is not necessarily water soluble and may vary depending on the solvent used. That is, when water is used as the solvent, a water-soluble carbon source may be used, and when an organic solvent is used, a carbon source that is soluble in an organic solvent may be used. A non-limiting example of the latter is a solution of pitch dissolved in THF.

In the preparation of LiFePO ₄ according to the present invention, when a precursor compound of lithium and phosphorus is mixed with an aqueous solution of a Fe ²⁺ precursor compound and additionally a carbon source is mixed to prepare an aqueous solution or suspension of the mixture, the aqueous solution or suspension of the mixture The solid material obtained by step (b) of heating and washing is a mixture of LiFePO ₄ -carbon sources. In addition, the carbon source may be carbonized to prepare carbon-coated LiFePO ₄ by the step (c) of heat treating the solid material obtained by the step (b).

According to the manufacturing process may be carbon coating so that the carbon is located between the primary particles of LiFePO ₄ having a low electron conductivity, the coated carbon may improve the electrical conductivity between the LiFePO ₄ primary particles as a conductive agent.

In the method for producing LiFePO ₄ of the present invention, the Fe ²⁺ precursor compound is preferably a compound containing Fe ²⁺ . This is because Fe ³⁺ is not easily reduced to Fe ²⁺ . Examples of Fe ²⁺ precursor compounds include FeSO ₄ , FeCl ₂ , Fe (NO ₃ ) ₂ , Fe (OH) ₂ , Fe (OCOCH ₃ ) ₂ , iron oxalate (FeC ₂ O ₄ 2H ₂ O), and the like. These compounds can be used individually or in mixture of 2 or more types.

The precursor compounds of lithium and phosphorus include LiH ₂ PO ₄ , Li ₂ HPO ₄ , Li ₃ PO ₄ , and the like, and these compounds may be used alone or in combination of two or more thereof.

When mixing a precursor compound of lithium and phosphorus in an aqueous solution of the Fe ²⁺ precursor compound in step (a) of the present invention, the molar ratio of the Fe ²⁺ precursor compound: the precursor compound of lithium and phosphorus may be 1: 1. When mixed outside the molar ratio, reaction by-products may occur or unreacted materials may remain.

In addition, when the carbon source is additionally mixed with the aqueous solution or suspension of the mixture of step (a) or the solid material obtained in step (b), the amount of carbon source to be mixed depends on the type of specific carbon source, but (c The content of carbon in the carbon-coated LiFePO ₄ prepared by heat treatment in step) is preferably 0.01 to 20% by weight, preferably 0.01 to 10% by weight. If the carbon content of the produced LiFePO ₄ is less than 0.01% by weight, it is difficult to obtain sufficient electronic conductivity, and if it is more than 20% by weight, the amount of the active material is reduced to decrease the capacity.

The heating in step (b) of obtaining a solid material by heating and washing the aqueous solution or suspension of the mixture prepared in step (a) of the present invention may vary depending on the reactants, and generally may be performed at 100 to 400 ° C. The washing can be done using distilled and de-ionized water, and the solid material thus obtained becomes amorphous or crystalline LiFePO ₄ . As the synthesis temperature increases, the crystallinity of the resulting solid material increases.

In addition, the temperature and time during the heat treatment of step (c) is not particularly limited, but is preferably made in the range of 1 to 40 hours at 500 ~ 800 ℃. If the heat treatment temperature is lower than 500 ℃ hard to form a crystalline material, if higher than 800 ℃ because the size of the particles too large rate characteristics are inferior. In addition, the atmosphere at the time of heat treatment is conventional inert (N ₂ , Ar, He, Ne, Kr, Xe, Rn, etc.), or inert and reducing (H ₂ ) atmosphere is suitable, especially 2% (volume ratio) of hydrogen Mixed nitrogen gas may be used. This is to prevent oxidation of iron (Fe ²⁺ ).

The present invention includes LiFePO ₄ prepared by the method presented above.

The present invention also provides a secondary battery comprising LiFePO ₄ prepared by the above method as a cathode active material. The secondary battery of the present invention can be produced by a conventional method known in the art, including a positive electrode prepared by using the LiFePO ₄ of the present invention as a positive electrode active material. For example, a porous separator may be inserted between the positive electrode and the negative electrode to add an electrolyte solution. The secondary battery includes a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

In the secondary battery of the present invention, the electrode can be prepared by conventional methods known in the art. For example, using LiFePO ₄ prepared according to the manufacturing method of the present invention as a positive electrode active material, a binder and a solvent, if necessary, a conductive agent and a dispersant are mixed and stirred to prepare a slurry and then applied to a current collector of a metal material And then compressed and dried to prepare a positive electrode.

The binder can be suitably used in an amount of 1 to 10 by weight and the conductive agent in an amount of 1 to 30 by weight based on the electrode active material.

As the negative electrode active material, a carbon material capable of occluding and releasing lithium ions, a lithium metal or an alloy thereof may be used, and other TiO ₂ , SnO ₂ , which may occlude and release lithium, and have a potential for lithium less than ₂ V. Metal oxides such as Li ₄ Ti ₅ O ₁₂ , LiTi ₂ O _4, and the like can be used. In particular, carbon materials, such as graphite, are preferable.

Examples of binders that can be used include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and the like.

Generally, carbon black may be used as the conductive agent, and acetylene black series (such as a Chevron Chemical Company or Gulf Oil Company) may be used as a commercially available product. Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cabot Company) and Super P (MMM) There is this.

The current collector of the metal material is a metal having high conductivity, and any metal can be used as long as the slurry of the electrode active material can be easily adhered and is not reactive in the voltage range of the battery. Representative examples include meshes, foils, and the like, which are manufactured by aluminum, copper, gold, nickel or an aluminum alloy or a combination thereof.

The electrolyte may include a nonaqueous solvent and an electrolyte salt.

The nonaqueous solvent is not particularly limited as long as it is normally used as a nonaqueous solvent for nonaqueous electrolyte, and cyclic carbonate, linear carbonate, lactone, ether, ester, or ketone can be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. Examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC) and dipropyl carbonate. (DPC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. Examples of the lactone include gamma butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, and the like. There is this. In addition, examples of the ester include methyl acetate, ethyl acetate, methyl propionate, methyl pivalate, and the like, and the ketone includes polymethylvinyl ketone. These nonaqueous solvents can be used individually or in mixture of 2 or more types.

The electrolyte salt is not particularly limited as long as it is usually used as an electrolyte salt for nonaqueous electrolyte. Electrolytic salt, non-limiting example, A ⁺ B ^- A salt of the structure, such as, A ⁺ comprises a Li ^+, Na ^+, an alkali metal cation or an ion composed of a combination thereof, such as K ⁺ B ^- is PF ₆ ^{- _{, BF 4 -, Cl -,}} Br -, I -, ClO 4 -, ASF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO 2 ) ₃ ^- and a salt containing the same anion ion or a combination thereof. In particular, lithium salts are preferred. These electrolyte salts can be used individually or in mixture of 2 or more types.

The secondary battery of the present invention may include a separator. The separator that can be used is not particularly limited, but it is preferable to use a porous separator, and non-limiting examples include a polypropylene-based, polyethylene-based, or polyolefin-based porous separator.

The secondary battery of the present invention is not limited in appearance, but may be cylindrical, square, pouch type, or coin type using a can.

Hereinafter, the present invention will be described in detail with reference to the following Examples. However, the following examples are merely to illustrate the present invention and the present invention is not limited by the following examples.

(Example 1)

A suspension of the mixture was prepared by adding Li ₃ PO ₄ (1.74 g) to 0.1 M aqueous FeSO ₄ solution (150 mL) with stirring. The suspension of the mixture was then placed in an autoclave reactor.

The autoclave reactor was sealed and all the oxygen inside the reactor was removed using Ar gas. After the reaction was conducted at 390 ° C. for 1 hour, the autoclave reactor was cooled and the product in the reactor was washed with distilled water, filtered, and dried to obtain a solid material.

The solid material was placed in a ceramic crucible, and 15% of sucrose, based on the weight ratio of LiFePO ₄ , was mixed well with an aqueous solution dissolved in water, and then dried in a vacuum oven. The dried material was heat treated in an electric furnace at 650 ° C. for 2 hours under an N ₂ gas atmosphere to finally prepare a carbon coated LiFePO ₄ . The average particle size of the produced LiFePO ₄ was 100nm as the primary particle basis, was prepared LiFePO ₄ carbon content was 4.5 wt%.

(Example 2)

LiFePO ₄ prepared in Example 1 was used as a cathode active material. After mixing 88 parts by weight of LiFePO _{4 with} 6 parts by weight of acetylene black as a conductive agent and 6 parts by weight of PVDF with a binder, and added to NMP (N-methyl-2-pyrrolidone) to prepare a positive electrode slurry, this is an aluminum (Al) current collector It was applied to the layer and dried to prepare a positive electrode.

Using a lithium foil as a negative electrode, a coin half cell was constructed together with the positive electrode prepared by the above method.

1 M LiPF ₆ / ethylene carbonate (EC): ethyl methyl carbonate (EMC) (v: v = 1: 2) was used as an electrolyte, and the electrolyte was prepared by interposing a polyolefin-based separator between the prepared anode and cathode. The battery was prepared by injection.

(Comparative Example 1)

Fe ²⁺ containing compound, Fe ₂ O ₃ , Li salt, Li ₂ CO ₃ and Phosphate (NH ₄ ) ₂ HPO ₄ in a molar ratio of 1: 1: 2, soaked in sucrose solution and vacuum oven Dried. After the drying process, the mixture was placed in a stainless steel container having stainless steel balls and uniformly mixed using a planetary ball miller to prepare a mixture.

The mixture was placed in a ceramic crucible and pretreated for 5 hours in an electric furnace at 350 ° C. and then calcined at 750 ° C. for 8 hours, at which time firing was performed under a reducing atmosphere of N ₂ /2% H ₂ mixed gas. LiFePO ₄ coated with carbon was obtained through the firing process, and the carbon content thereof was 9 wt%.

(Comparative Example 2)

A battery was manufactured in the same manner as in Example 2, except that LiFePO ₄ prepared in Comparative Example 1 was used as the cathode active material.

Experiment 1: Analysis of LiFePO ₄

In order to analyze the LiFePO ₄ prepared in Example 1 was carried out the following experiment. LiFePO ₄ prepared in Example 1 was used as a sample.

As a result of X-ray diffraction analysis (XRD: X-ray Diffraction), the LiFePO ₄ powder prepared in Example 1 was found to be a single phase and was consistent with the peaks of the standard LiFePO ₄ compound in all ranges (Fig. 1).

As a result of elemental analysis of LiFePO ₄ prepared in Example 1 using inductively coupled plasma (ICP), it was confirmed that Li: Fe: P = 1: 1.01: 0.99 is almost consistent with the elemental content of LiFePO ₄ . .

As a result of analyzing the shape of the particles by scanning electron microscope (SEM), it was confirmed that the secondary particles are aggregated with the primary particles of 100nm level, and the degree of aggregation becomes very weak when the synthesis temperature exceeds the supercritical temperature of water. Could be (see FIG. 2).

(Experiment 2: Evaluation of Initial Capacity of Battery)

Experiments were conducted for the initial capacity evaluation of the batteries using the batteries prepared in Example 2 and Comparative Example 2.

The current density was 0.1C, and the charge and discharge region was experimented with 2.7 to 4.2 V ( vs. Li / Li ⁺ ).

As a result of the experiment, the initial charge-discharge efficiency of the battery prepared in Example 2 showed an efficiency equivalent to the initial charge-discharge efficiency of the battery prepared in Comparative Example 2 (see FIGS. 3 and 4).

(Experiment 3: C-rate evaluation of battery)

The cells prepared in Example 2 and Comparative Example 2 were used, and the cells were cycled at discharge rates of 0.1C, 0.2C, 0.5C, and 1C.

As a result of the experiment, the battery prepared in Example 2 showed a high rate discharge (C-rate) characteristics by showing a discharge rate comparable to the battery prepared in Comparative Example 2 (see FIGS. 5 and 6).

(Experiment 4: Cycle Characteristic Evaluation)

The batteries prepared in Example 2 and Comparative Example 2 were used. The current density was 0.5C and the charge / discharge region was experimented with 2.7 to 4.2 V ( vs. Li / Li ⁺ ).

As a result of the experiment, the battery prepared in Example 2 had no difference in charge and discharge capacity of the battery after 90 or more cycles, and it was confirmed that the battery had long life characteristics due to reversibility.

LiFePO ₄ manufacturing method according to the present invention can be produced LiFePO ₄ faster than the conventional solid-state reaction by using a hydrothermal method, it is possible to control the size of the primary particles to improve the rate characteristics . In addition, mass production is easy and economical, as well as minimizing the reactants for producing LiFePO ₄ to suppress the occurrence of side reactions as much as possible, it is possible to perform the carbon coating of LiFePO ₄ more easily and efficiently.

Claims (12)

(a) mixing a precursor compound of lithium and phosphorus with an aqueous solution of a Fe ²⁺ precursor compound to prepare an aqueous solution or suspension of the mixture; (b) heating and washing the aqueous solution or suspension of the mixture of step (a) to obtain a solid material; And (C) a method for producing LiFePO ₄ comprising the step of heat-treating the solid material of step (b) in an inert atmosphere, or inert and reducing atmosphere. In the (a) process for producing a phase mixture of an aqueous solution or suspension, or LiFePO ₄ as the solid material obtained in the step (b) is characterized in that a mixture of an additional carbon source in claim 1. The method of claim 2, wherein the carbon source is selected from the group consisting of C, H, O; Or C, H, N; Or a method for producing LiFePO _4, characterized in that the water-soluble compound consisting of C, H, O, N elements. The method of claim 2, wherein the carbon source is sucrose, polyvinyl alcohol (PVA), polyethylene glycol (PEG), oxalic acid, resorcinol, citric acid and cellulose acetate. Method for producing LiFePO ₄ , characterized in that at least one selected from the group consisting of. The method of claim 1, wherein the Fe ²⁺ precursor compound is FeSO ₄ , FeCl ₂ , Fe (NO ₃ ) ₂ , Fe (OH) ₂ , Fe (OCOCH ₃ ) ₂ and iron oxalate (FeC ₂ O ₄ 2H ₂ O Method for producing LiFePO ₄ , characterized in that at least one selected from the group consisting of. The method of claim 1, wherein the lithium and phosphorus precursor compound is LiH ₂ The method of PO _4, Li ₂ HPO _4, and Li ₃ PO ₄ LiFePO, characterized in that at least one member selected from the group consisting of _4. The method of claim 1, wherein the Fe ²⁺ precursor compound the molar ratio of lithium and phosphorus precursor compound is 1: Preparation of LiFePO _4, characterized in that one. Claim 2 The method for producing LiFePO _4, characterized in that the content of the LiFePO ₄ carbon of the carbon coating is produced by heat treatment of the step (c) is from 0.01 to 20% by weight to. The method of claim 1, wherein the heating of the step (b) is made of LiFePO ₄ , characterized in that at 100 ~ 400 ℃. The method of claim 1, wherein the heat treatment in step (c) is made of LiFePO ₄ , characterized in that at 500 ~ 800 ℃. LiFePO prepared by the method of claim ₁ . A secondary battery comprising LiFePO ₄ prepared by the method of claim ₁ as a cathode active material.

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