KR20060025842A - Preparation method of lithium iron phosphate - Google Patents

Abstract

The present invention comprises the steps of a) mixing a lithium hydroxide solution and a phosphorus precursor compound; b) adding a precursor compound comprising Fe ²⁺ to the mixture of step a) and evaporating the solvent to obtain a solid mixture; And c) it provides a novel method of producing LiFePO ₄ including the step of heat treating the solid mixture of step b) under an inert or reducing atmosphere. The present invention also provides an electrode using LiFePO ₄ prepared according to the above method as an electrode active material and a lithium secondary battery having the same.

LiFePO ₄ manufacturing method according to the present invention by using a low-cost and high-density precursor compound as a reactant, mass production is easy and economical, and toxic by-products are not discharged can improve safety.

Lithium Iron Composite Oxide, Density, Electrode Active Material, Lithium Secondary Battery

Description

Manufacturing method of ＬｉＦｅＰ０４ {PREPARATION METHOD OF 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 view showing the initial charge and discharge capacity at 0.1C of a lithium secondary battery prepared using LiFePO ₄ prepared according to Example 1 as a positive electrode active material.

3 is a diagram illustrating high-rate discharge characteristics (C-rate properties) of a lithium secondary battery manufactured using LiFePO ₄ prepared according to Example 1 as a cathode active material.

4 is a view showing the charge-discharge capacity after 5 to 11 cycles of the lithium secondary battery prepared by using LiFePO ₄ prepared according to Example 1 as a cathode active material under 0.2C conditions.

The present invention relates to a novel method for producing LiFePO ₄ with improved economics and safety and easy mass production.

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.

LiFePO ₄ , a new battery material, is environmentally friendly, rich in reserves and very low on raw materials. In addition, the discharge voltage is 3.4V (vs. Li / Li ⁺ ), which makes it possible to implement low power and low voltage more easily than the existing materials.

The LiFePO ₄ powder is prepared by a conventional solid phase reaction method, sol-gel method and the like. US 2002-0004169 A1 discloses a method for synthesizing LiFePO ₄ as shown in Scheme 1 using iron acetate, lithium carbonate and ammonium dihydrogen phosphate as reactants.

2Fe (CH ₃ COO) ₂ + Li ₂ CO ₃ + 2NH ₄ H ₂ PO ₄ = 2LiFePO ₄ + CO ₂ + 2NH ₃ + 4CH ₃ COOH + H ₂ O

However, using the above production 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 EP 1391424 A2, the synthesis of LiFePO ₄ was initiated via the same reaction as in Scheme 2 below.

FePO ₄ + 0.5Li ₂ CO ₃ + 0.5C = LiFePO ₄ + 0.5CO ₂ +0.5 CO

0.5Fe ₂ O ₃ + 0.5Li ₂ CO ₃ + (NH ₄ ) ₂ HPO ₄ + 0.5C = LiFePO ₄ + 0.5CO ₂ + 2NH ₃ + 3 / 2H ₂ O + 0.5CO

When using the preparation method, not only toxic gases such as carbon monoxide and ammonia were obtained as by-products, but also 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 .

In view of the above problems, an object of the present invention is to provide a method for producing LiFePO ₄ having no toxic by-products, easy to mass-produce, and economical.

Another object of the present invention is to provide an electrode using LiFePO ₄ produced by the above manufacturing method as an electrode active material and a lithium secondary battery having the electrode.

The present invention comprises the steps of a) mixing a lithium hydroxide solution and a phosphorus precursor compound; b) adding a precursor compound comprising Fe ²⁺ to the mixture of step a) and evaporating the solvent to obtain a solid mixture; And c) it provides a method for producing LiFePO ₄ including the step of heat treating the solid mixture of step b) under an inert or reducing atmosphere.

The present invention also provides an electrode using LiFePO ₄ prepared by the above manufacturing method as an electrode active material and a lithium secondary battery having the electrode.

Hereinafter, the present invention will be described in detail.

The present invention is to provide a novel method for producing LiFePO ₄ in which toxic by-products such as acetic acid and ammonia are not emitted while using a precursor compound having a high density and low cost in consideration of problems occurring in the production of the conventional LiFePO ₄ .

The method for producing LiFePO ₄ according to the present invention may use a conventional solid state reaction known in the art. Solid state reaction is the most basic method for synthesizing powder

Hereinafter, for one embodiment of the production method according to the present invention, 1) First, lithium hydroxide as a lithium precursor is dissolved in a suitable solvent, for example distilled water, to make a lithium hydroxide solution, and polyphosphoric acid as a phosphorus precursor is mixed therein. To prepare a mixed solution.

Lithium (Li) precursor compound that can be used in the production of LiFePO ₄ according to the present invention is a lithium-containing salt that can be ionized, lithium hydroxide (LiOH) is preferred. Lithium hydroxide is water soluble and thus not only easy to ionize, but also has high density (1.51 g / cm ² ), which is advantageous for mass production. In addition, the solubility in water is slightly lower than that of lithium hydroxide described above, but lithium carbonate (Li ₂ CO ₃ ) which is soluble by adding water to an acid such as polyphosphoric acid and using an acid-base reaction is also within the scope of the present invention.

The lithium hydroxide is used as an aqueous solution dissolved in distilled water, wherein the concentration range of the lithium hydroxide aqueous solution is suitably 1 to 3M.

In addition, as a phosphorus (P) precursor compound that may be used in the preparation of LiFePO ₄ according to the present invention, H ₃ PO ₄ , P ₂ O ₅ , polyphosphoric acid or a mixture thereof may be used. desirable. This is because polyphosphoric acid has low cost and high density (2.05 g / cm ² ).

When the lithium hydroxide and the phosphorus precursor compound are mixed, their equivalent ratio is preferably 1: 1.

2) After the iron precursor compound is added to the prepared mixed solution of lithium hydroxide and phosphorus precursor compound, the solvent is evaporated to prepare a solid mixture containing lithium, phosphorus, and iron precursors of LiFePO ₄ .

As the iron (Fe) precursor compound that can be used in the production of LiFePO ₄ according to the present invention, a compound containing Fe ²⁺ is preferable. This is because Fe (III) is not easily reduced to Fe (II). As the compound containing Fe ²⁺ , iron oxalate (FeC ₂ O ₄ 2H ₂ O) or iron acetate (Fe (CH ₃ COO) ₂ ) is preferable.

When preparing a solid mixture by mixing the above-described lithium, phosphorus, iron components, the equivalent ratio of the precursor compound containing lithium hydroxide: phosphorus precursor compound: Fe ²⁺ is preferably 1: 1: 1, the reaction is out of the mixing ratio By-products are generated or unreacted materials remain.

Conventional solvent evaporation is used to remove the solvent contained in the mixture after the iron precursor compound is added.                     

3) LiFePO ₄ can be obtained by heat treating the solid mixture in an inert or reducing atmosphere.

Before heat-treating the solid mixture containing lithium, phosphorus and iron in an inert or reducing atmosphere, it is preferable to add, mix, pulverize and dry the conductive carbon material to the solid mixture.

Conductive carbon materials that can be used are carbon black, acetylene black series (Chevron Chemical Company or Gulf Oil Company), and ketjen black EC series. (Armak company), Vulcan XC-72 (manufactured by Cabot company) and Super-P (manufactured by MMM).

Iron generally has the property of being easily oxidized because Fe ³⁺ is more stable than Fe ²⁺ . Therefore, in order to reduce the iron compound in the production of LiFePO ₄ , such a conductive carbon material is used. The conductive carbon material serves to reduce iron ions, and since a small amount of carbon material remaining after the reaction exists in an initial carbon state, it serves as a conductive component in the final product. The content of the conductive carbon material is preferably 1 to 10% by weight per 100% by weight of the total solid mixture after solvent removal.

It is preferable to grind | pulverize a solid mixture after adding a conductive carbon material to a solid mixture. At this time, the crushing time and the particle size of the crushed solid mixture is not particularly limited, but 1 to 48 hours and 1nm to 100㎛ are preferred, respectively. As the crushing method, a conventional method can be used, and a ball mill method is particularly preferable.

As described above, since iron (Fe ²⁺ ) contained in the solid mixture is easily oxidizable, in order to prevent this, it is preferable to carry out in a non-aqueous solvent such as an inert atmosphere or alcohol during crushing. Moreover, it is preferable to carry out in a vacuum state also at the time of drying.

Prior to heat treatment, the dried solid mixture is preferably pressed into a pellet, but this process may be omitted.

The heat treatment time and temperature of the solid mixture containing lithium, phosphorus and iron are not particularly limited, but are preferably in the range of 5 to 40 hours and 700 to 800 ° C. If it is lower than 700 ℃ it becomes difficult to form a crystalline material. Moreover, the atmosphere at the time of heat processing is suitable with a normal inert or reducing atmosphere, and nitrogen gas mixed with 2% (volume ratio) of hydrogen is especially preferable. This is to prevent oxidation of iron (Fe ²⁺ ).

Novel method of producing LiFePO ₄ according to the invention using a dense reagent such as aqueous lithium hydroxide solution (1.51g / cm ^2), polyphosphoric acid (2.05g / cm ^2), and iron oxalate (2.28g / cm ²⁾ By doing so, there is an advantage that mass production is possible with small volume during scale-up. In addition, as shown in Scheme 3, as a by-product of the reaction, there is an advantage that the toxic substances such as acetic acid or ammonia are not discharged and are safe and easy to react. In addition, conventional reactants are expensive, while the reactants of the present invention, in particular, polyphosphoric acid, are low in cost, thereby increasing economic efficiency.

LiFePO ₄ prepared as described above may be used as an electrode active material, preferably a cathode active material of a lithium secondary battery, and after mixing the LiFePO ₄ with a binder according to a conventional method to prepare an electrode slurry, the prepared electrode slurry is a current An electrode can be manufactured by coating on an electrical power collector. As a manufacturing method of such an electrode, the conventional method known in the art can be used, and is not particularly limited.

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

The current collector is a highly conductive metal, and there is no limitation in use as long as it is a metal that can be easily adhered to the paste of the material. The method of applying the slurry to the current collector is also not particularly limited.

In addition, the present invention a) a positive electrode comprising LiFePO ₄ prepared according to the production method; b) a negative electrode comprising a negative electrode active material capable of occluding and releasing lithium; It provides a lithium secondary battery comprising c) a separator and d) an electrolyte. In this case, the lithium secondary battery includes a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

The lithium secondary battery of the present invention may be prepared by inserting a porous separator between a positive electrode and a negative electrode by a conventional method known in the art and adding an electrolyte.

The negative electrode active material is preferably carbon, lithium metal or an alloy thereof. In addition, other lithium may be occluded and released, and metal oxides such as TiO ₂ , SnO _2, and the like having a potential of less than ₂ V may be used. The negative electrode can be prepared by conventional methods known in the art.

The electrolyte of the present invention is a non-aqueous electrolyte containing a lithium salt and an electrolyte compound, and the lithium salt includes LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ . At least one compound selected from is preferred. Electrolyte compounds also include ethylene carbonate (EC), propylene carbonate (PC), and gamma butyrolactone (GBL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and methyl propyl carbonate ( At least one selected from the group consisting of MPC) is preferred.

In manufacturing the battery of the present invention, it is preferable to use a porous separator as a separator, and non-limiting examples include a polypropylene-based, polyethylene-based or polyolefin-based porous separator.

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

The invention is explained in more detail based on the following examples and experimental examples. However, Examples and Experimental Examples are for illustrating the present invention and are not limited to these.

Example 1 Preparation of Cathode Active Material

Polyphosphoric acid was added to a 2 M lithium hydroxide (LiOH) aqueous solution while stirring, followed by cooling, and FeC ₂ O _{4 ·} 2H ₂ O was mixed. At this time, the ratio of Li: P: Fe was adjusted to 1: 1: 1. The resulting mixture was dried and then ketjen black was added 8.3% by weight relative to the final material, and the precursor mixture was then placed in a stainless steel vessel with stainless steel balls and ground using a planetary ball miller. . The ball milling operation proceeded for 24 hours in immersion in ethanol and the resulting mixture was dried under vacuum. The ground precursor mixture was placed in a ceramic crucible and heat treated for 12 hours in an electric furnace at 750 ° C., wherein the heat treatment was performed under a reducing atmosphere of N ₂ /2% H ₂ mixed gas. LiFePO ₄ was finally obtained through the above process, and the average particle diameter thereof was 3.7 μm.

Example 2 Preparation of Nonaqueous Electrolyte Battery

Anode manufacturing

80% by weight of the LiFePO ₄ active material prepared in Example 1, 5% by weight of carbon black as a conductive agent, and 15% by weight of polyvinyridine difluoride (PVdF) as a binder were mixed to form a positive electrode slurry. The slurry was coated on an aluminum foil current collector having a thickness of 44 μm, dried, and then compressed by a roll press.

Battery manufacturing

Anode prepared above; Lithium metal used as negative electrode (or counter electrode) and coin-type cell using 1 M LiPF ₆ / ethylene carbonate (EC): ethyl methyl carbonate (EMC) (volume ratio 1: 2) as electrolyte Was prepared.

Experimental Example 1. LiFePO ₄ Analysis of

In order to analyze LiFePO ₄ prepared according to the present invention, the following experiment was performed.

LiFePO ₄ prepared in Example 1 was used as a sample.

As a result of X-ray diffraction analysis (XRD), it was confirmed that the LiFePO ₄ powder prepared according to the present invention was a single phase and was consistent with all peaks of the standard LiFePO ₄ compound (FIG. 1).

Experimental Example 2. Performance Evaluation of Lithium Secondary Battery

Performance evaluation experiment of the lithium secondary battery using the LiFePO ₄ powder prepared according to the novel manufacturing method of the present invention as a cathode active material was performed as follows.

2-1. Initial capacity evaluation of the battery

The lithium secondary battery of Example 2 using LiFePO ₄ prepared in Example 1 as a cathode active material was used. The current density was 0.1C, and the charge / discharge region was experimented with 0.0 to 4.2 V ( vs. Li / Li ⁺ ).

As a result, the initial charge / discharge efficiency of the battery using LiFePO ₄ prepared according to the present invention as the positive electrode active material showed an efficiency of 90.7%, which is equivalent to the initial charge and discharge efficiency of the battery using the existing positive electrode active material (see FIG. 2). .

2-2. C-rate evaluation of battery

The lithium secondary battery of Example 2 using LiFePO ₄ prepared in Example 1 as a cathode active material was used, and the battery was cycled at discharge rates of 0.1C, 0.2C, 0.5C, and 1C.

As a result, the battery using LiFePO ₄ prepared according to the present invention as a cathode active material showed a high-rate discharge (C-rate) characteristics by showing a discharge rate comparable to the battery using a conventional cathode active material (see Fig. 3).

2-3. Cycle characteristic evaluation

The lithium secondary battery of Example 2 using LiFePO ₄ prepared in Example 1 as a cathode active material was used. The current density was 0.2C, and the charge and discharge regions were experimented with 0.0 to 4.2 V ( vs. Li / Li ⁺ ).

As a result, the battery using LiFePO ₄ as the positive electrode active material according to the present invention had almost no difference in charge and discharge capacity after 5 and 11 cycles, and it was confirmed that the battery had long life characteristics due to its reversibility. 4).

LiFePO ₄ manufacturing method according to the present invention by using a low-cost and high-density precursor compound as a reactant is not only easy to mass production and economical, but also toxic by-products are not discharged can improve the safety.

Claims (9)

a) mixing a lithium hydroxide solution and a phosphorus precursor compound; b) adding a precursor compound comprising Fe ²⁺ to the mixture of step a) and evaporating the solvent to obtain a solid mixture; c) heat treating the solid mixture of step b) under inert or reducing atmosphere Method for producing LiFePO ₄ comprising a. The method according to claim 1, comprising adding, grinding and drying the conductive carbon material to the solid mixture before step c). The method of claim 1, wherein the phosphorus precursor compound is at least one selected from the group consisting of P ₂ O ₅ , H ₃ PO _4, and polyphosphoric acid. The method of claim 1, wherein the precursor compound including Fe ²⁺ is iron acetate (Fe (CH ₃ COO) ₂ ) or iron oxalate (FeC ₂ O ₄ 2 H ₂ O). The method according to claim 1, wherein the equivalent ratio of the precursor compound containing lithium hydroxide: phosphorus precursor compound: Fe ²⁺ is 1: 1: 1. The method of claim 1, wherein the conductive carbon material is carbon black, ketjen black, acetylene black or carbon super P. The method of claim 1, wherein the heat treatment temperature is in the range of 700 to 800 ℃. A lithium secondary battery electrode comprising LiFePO ₄ prepared by the method of claim 1 as an electrode active material. a) a positive electrode comprising LiFePO ₄ prepared by the method of claim 1 as a positive electrode active material; b) a negative electrode comprising a negative electrode active material capable of occluding and releasing lithium; c) a separator existing between the anode and the cathode; And d) nonaqueous electrolyte; Lithium secondary battery comprising a.

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