KR20100133231A - Positive active material for rechargeable lithium battery, method of preparing same and rechargeable lithium battery including same - Google Patents

Abstract

PURPOSE: A method for preparing a positive active material for a rechargeable lithium battery is provided to easily prepare a positive active material for a lithium battery in which a carbon layer is uniformly formed on the surface of LiFePO4. CONSTITUTION: A method for preparing a positive active material for a rechargeable lithium battery containing a compound of chemical formula 1: LiFePO4 comprises the steps of: adding a lithium precursor compound to water to prepare a lithium precursor compound; adding an iron precursor to the lithium precursor compound solution to prepare a mixed solution; adding a water-soluble carbon raw material to the mixed solution, milling the mixture, and drying the resultant; and firing the dried product.

Description

A positive electrode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same.

The present disclosure relates to a cathode active material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same.

Recently, with the trend toward miniaturization and light weight of portable electronic devices, the need for high performance and high capacity of batteries used as power sources for these devices is increasing.

A battery generates power by using a material capable of electrochemical reactions at a positive electrode and a negative electrode. A typical example of such a battery is a lithium secondary battery that generates electric energy by a change in chemical potential when lithium ions are intercalated / deintercalated at a positive electrode and a negative electrode.

The lithium secondary battery is manufactured by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

A lithium composite metal compound is used as a positive electrode active material of a lithium secondary battery, and examples thereof include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), and LiMnO ₂ . Composite metal oxides are being studied.

In addition, LiFePO ₄ , which has recently been spotlighted as a cathode active material of a large-capacity battery, has excellent thermal stability, rich reserves, and raw material prices.

As a method of synthesizing the positive electrode active material LiFePO ₄ , there are a coprecipitation method, a solid phase reaction method, a hydrothermal synthesis method and the like. In Japanese Patent Laid-Open No. 2000-294238, iron oxalate (FeC ₂ O ₄ .2H ₂ O) is used as an iron raw material, and ammonium hydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) are synthesized. A method for synthesizing LiFePO ₄ as a raw material is described. US Patent Publication No. 20020004169 discloses the synthesis of LiFePO ₄ using iron acetate 2Fe (CH ₃ COO) ₂ , lithium carbonate (Li ₂ CO ₃ ) and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) as a raw material. The method is presented.

In many other patents, phosphates such as NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , (NH ₄ ) ₃ PO _4, etc., lithium carbonate (Li ₂ CO ₃ ), iron acetate (2Fe (CH ₃ COO) ₂ ) The method of synthesizing LiFePO ₄ using as a raw material is shown.

In addition, since the conductivity of LiFePO ₄ is somewhat low, in order to improve conductivity, a method of carbon coating LiFePO ₄ by mixing carbon raw materials in a raw material mixing step has been attempted. However, due to the difference in density between the raw material and the carbon raw material, as the mixing becomes uneven, the entire LiFePO ₄ particles are not carbon coated, and as a result, the active material capacity is 120 mAh / g or less under the discharge current density of 0.2 mA / cm ² . It was only about. In addition, since a separate mixing facility is required to uniformly coat carbon on the LiFePO ₄ surface, the cost is inevitably high, so this method is also not a suitable method for producing LiFePO ₄ on an industrial scale.

One embodiment of the present invention to provide a positive electrode active material for lithium secondary battery excellent in conductivity.

Another embodiment of the present invention is to provide a method of manufacturing a cathode active material for a lithium secondary battery that can improve conductivity, be easily mass-produced, and have economical efficiency.

Another embodiment of the present invention to provide a lithium secondary battery comprising the positive electrode active material.

A first embodiment of the present invention is a core represented by the formula (1); And it provides a cathode active material for a lithium secondary battery comprising a carbon layer formed surrounding the core.

[Formula 1]

LiFePO ₄

The carbon layer is uniformly formed to completely surround the core.

According to a second embodiment of the present invention, an aqueous lithium precursor compound solution is prepared by adding water to a lithium precursor compound; Preparing a mixed solution by adding an iron precursor compound to the aqueous lithium precursor compound solution; Milling while adding a water-soluble carbon raw material to the mixture and drying it; It provides a method for producing a cathode active material for a lithium secondary battery comprising a compound of formula (1) comprising the step of firing a dry product.

[Formula 1]

LiFePO ₄

A third embodiment of the present invention includes a positive electrode including the positive electrode active material; A negative electrode including a negative electrode active material; And to provide a lithium secondary battery comprising a non-aqueous electrolyte.

Other specific details of embodiments of the present invention are included in the following detailed description.

The manufacturing method according to an embodiment of the present invention can easily prepare a cathode active material for a lithium secondary battery in which a carbon layer is uniformly formed on the surface of LiFePO ₄ , and this method is easy to mass produce and economical.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, by which the present invention is not limited and the present invention is defined only by the scope of the claims to be described later.

The cathode active material for a rechargeable lithium battery according to the first embodiment of the present invention includes a core represented by the following Chemical Formula 1 and a carbon layer formed surrounding the core.

[Formula 1]

LiFePO ₄

The carbon layer is formed to surround the core substantially completely uniformly, where the thickness of the carbon layer may be 10 to 30 nm. When the thickness of the carbon layer is included in this range, the effect of improving the conductivity of the active material can be obtained.

The carbon layer is formed on the surface of the core by firing a water-soluble carbon raw material which is a water-soluble resin, a water-soluble carbonic acid, or a combination thereof.

According to a second embodiment of the present invention, an aqueous lithium precursor compound solution is prepared by adding water to a lithium precursor compound, an iron precursor compound is added to the aqueous lithium precursor compound solution to prepare a mixed solution, and a water-soluble carbon raw material is added to the mixed solution. It is providing the manufacturing method of the positive electrode active material for lithium secondary batteries containing the process of drying after milling and baking a dry product.

Next, the manufacturing method according to the second embodiment of the present invention will be described in detail for each process.

First, water is added to a lithium precursor compound to prepare an aqueous lithium precursor compound. The amount of water added may be 30 to 70 parts by weight based on 100 parts by weight of the lithium precursor compound. When the added amount of water is included in this range, the lithium precursor compound may be dissolved well, and there may be an advantage in that dispersion of the iron precursor compound added later is easy.

The lithium precursor compound includes both lithium and phosphorus, and a compound that can be dissolved in water may be appropriately used, and representative examples thereof may be Li ₃ PO ₄ , LiH ₂ PO _4, or a combination thereof. In particular, LiH ₂ PO ₄ can be suitably used as the lithium precursor compound.

Subsequently, an iron precursor compound is added to the aqueous lithium precursor compound solution to prepare a mixed solution.

As the iron (Fe) precursor compound, a compound containing Fe ²⁺ may be used, and representative examples thereof include iron oxalate (FeC ₂ O ₄ · 2H ₂ O) and iron acetate (Fe (CH ₃ COO) ₂ ). Or combinations thereof. In particular, iron oxalate (FeC ₂ O ₄ .2H ₂ O) can be suitably used as the iron precursor compound. Since the iron precursor compound is not dissolved in water, it is present in a dispersed state in the mixed solution.

In this case, the mixing ratio of the lithium precursor compound and the iron precursor compound may be 1: 1 to 1.05: 1 mol (mol) ratio. When the molar ratio of the lithium precursor compound and the iron precursor compound is included in the above range, P of the lithium precursor compound is appropriate because it does not exist as an unreacted substance and all participate in the reaction.

The milling process is performed while adding the water-soluble carbon raw material to the mixed liquid. This milling process can be carried out using a ball mill like a planetary ball mill and can be carried out by a wet milling process. In this case, the mixing process may be performed for 30 minutes to 1 hour.

The addition amount of the water-soluble carbon raw material may be 1 to 10 parts by weight based on 100 parts by weight of the mixture of the lithium precursor compound and the iron precursor compound in the mixture. In the case where the water-soluble carbon raw material is used in this range, a uniform carbon coating may be applied to the active material to obtain an effect of improving conductivity appropriately.

At this time, water-soluble resin, water-soluble carbonic acid, or a combination thereof can be used as the water-soluble carbon raw material. Carboxymethyl cellulose, dextrin, glucose, polyvinyl alcohol, etc. can be used as said water-soluble resin, and any water-soluble carbonic acid can be used as said water-soluble carbonic acid.

Subsequently, the obtained product is dried. This drying process can be carried out in a reducing atmosphere or a vacuum atmosphere, and it may be more appropriate to carry out in a vacuum atmosphere in terms of cost and time. When the drying step is carried out in a vacuum atmosphere, it can be carried out at a vacuum degree of about 760mmHg.

The drying process may be carried out at a temperature of 80 to 120 ℃, the drying time may be carried out for 1 to 5 hours. When the product is dried in this range, there is an effect that water can be removed more quickly while preventing the product from being oxidized.

The obtained dry product is calcined.

The firing process may be performed at 600 to 850 ° C. for 8 hours to 10 hours.

In addition, the firing process may be carried out by firing at 600 to 850 ° C. for 4 to 6 hours, and firing the plastic product at 600 to 850 ° C. for 6 to 10 hours. Moreover, after cooling a plastic product, the process of milling may further be performed.

In addition, after performing a baking process, you may perform a milling process further.

In the case where the firing step is carried out in the above temperature range and time range, the active material particles having an appropriate size can be formed without decreasing the crystallinity of the active material, and the formation of the impurity phase can be suppressed.

In addition, the firing process may be carried out in an inert or reducing atmosphere. The inert atmosphere may be an N ₂ gas atmosphere, and the reducing atmosphere may be a mixed gas atmosphere of Ar / H ₂ . The content of H ₂ in the mixed gas atmosphere of the Ar / H ₂ may be 2 to 7% by volume. The content of H ₂ gas in the mixed atmosphere of Ar / H ₂ has the advantage of maintaining the reducing atmosphere when included in the range.

After cooling the calcined product, it is preferably cooled slowly and then pulverized again to prepare a positive electrode active material.

According to this firing process, the lithium precursor compound and the iron precursor compound react to form a compound represented by the following Chemical Formula 1, and the water-soluble carbon raw material is changed into a conductive carbon material, thereby completely completing the compound core represented by the following Chemical Formula 1. Or a partially enclosed carbon layer.

[Formula 1]

LiFePO ₄

A carbon layer can be appropriately formed on the core surface of the formula (1), the active material particles of appropriate size can be formed without deteriorating the crystallinity of the active material, and the formation of an impurity phase can be suppressed.

A third embodiment of the present invention is to provide a lithium secondary battery including the cathode active material of the first embodiment. The positive electrode active material of the present invention can be usefully used for the positive electrode of a lithium secondary battery. The lithium secondary battery includes a negative electrode and an electrolyte including a negative electrode active material together with a positive electrode.

The positive electrode is prepared by mixing a positive electrode active material according to the present invention, a conductive material, a binder and a solvent to prepare a positive electrode active material composition, and then coating and drying the aluminum active material directly. Alternatively, the cathode active material composition may be cast on a separate support, and then the film obtained by peeling from the support may be manufactured by laminating on an aluminum current collector.

The conductive material is carbon black, graphite, metal powder, the binder is vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene And mixtures thereof. As the solvent, N-methylpyrrolidone, acetone, tetrahydrofuran, decane and the like are used. In this case, the contents of the positive electrode active material, the conductive material, the binder, and the solvent are used at levels commonly used in lithium secondary batteries.

Like the positive electrode, the negative electrode is mixed with a negative electrode active material, a binder, and a solvent to prepare an anode active material composition, and the negative electrode active material film coated on the copper current collector or cast on a separate support and peeled from the support is coated on the copper current collector. It is prepared by lamination. At this time, the negative electrode active material composition may further contain a conductive material if necessary.

As the negative electrode active material, a material capable of intercalating / deintercalating lithium is used, and for example, lithium metal, lithium alloy, coke, artificial graphite, natural graphite, organic polymer compound combustor, carbon fiber, or the like is used. . In addition, a conductive material, a binder, and a solvent are used similarly to the case of the positive electrode mentioned above.

The separator may be used as long as it is commonly used in lithium secondary batteries. For example, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and a polyethylene / polypropylene two-layer separator, It goes without saying that a mixed multilayer film such as polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / polypropylene three-layer separator and the like can be used.

As the electrolyte to be charged in the lithium secondary battery, a non-aqueous electrolyte or a known solid electrolyte may be used, and a lithium salt is used.

Although the solvent of the said non-aqueous electrolyte is not specifically limited, Cyclic carbonate, such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate; Chain carbonates such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate; Esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and γ-butyrolactone; Ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane and 2-methyltetrahydrofuran; Nitriles such as acetonitrile; Or amides such as dimethylformamide can be used. These can be used individually or in combination of two or more. In particular, a mixed solvent of a cyclic carbonate and a linear carbonate can be preferably used.

As the electrolyte, a gel polymer electrolyte in which an electrolyte solution is impregnated with a polymer electrolyte such as polyethylene oxide or polyacrylonitrile, or an inorganic solid electrolyte such as LiI or Li ₃ N can be used.

The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , One selected from the group consisting of LiCl and LiI is possible.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

Example 1

An aqueous lithium precursor compound solution was prepared by adding water to LiH ₂ PO ₄ as a lithium precursor compound. At this time, the amount of water added was 70 parts by weight based on 100 parts by weight of LiH ₂ PO ₄ .

To the lithium compound aqueous solution of a precursor, FeC ₂ O ₄ · 2H ₂ O a, mol (mol) ratio of LiH ₂ PO ₄ and _{FeC 2 O 4 · 2H 2 O} : was added to be 1, to prepare a mixed solution.

While adding carboxymethylcellulose to the mixture, a milling process of mixing and grinding for 1 hour using a wet planetary ball mill was performed to prepare a mixed product in the form of a slurry. The amount of the carboxymethyl cellulose added was 6 parts by weight based on 100 parts by weight of the mixture of the lithium precursor compound and the iron precursor compound. The mixture product in the form of the obtained slurry was dried for 5 hours at a vacuum atmosphere of 760 mmHg in a vacuum atmosphere and at a temperature of 100 ° C., and then ground.

The obtained powder was Ar / H ₂ (content of H ₂ 5% by volume) The mixture was calcined by holding at 750 ° C. for 10 hours under a reducing atmosphere of the mixed gas. The calcined product was slowly cooled and then milled again to prepare a cathode active material in which a carbon layer was uniformly formed on the LiFePO ₄ core. The thickness of the carbon layer was 15 nm .

Example 2

An aqueous lithium precursor compound solution was prepared by adding water to LiH ₂ PO ₄ as a lithium precursor compound. At this time, the amount of water added was 50 parts by weight based on 100 parts by weight of LiH ₂ PO ₄ .

To the lithium compound aqueous solution of a precursor, FeC ₂ O ₄ · 2H ₂ O a, mol (mol) ratio of LiH ₂ PO ₄ and _{FeC 2 O 4 · 2H 2 O} : was added to be 1, to prepare a mixed solution.

While adding carboxymethyl cellulose to the mixture, mixing and grinding were performed for 1 hour using a wet planetary ball mill to prepare a mixed product in the form of a slurry. The amount of the carboxymethyl cellulose added was 6 parts by weight based on 100 parts by weight of the mixture of the lithium precursor compound and the iron precursor compound. The resulting mixture in the form of a slurry was pulverized after drying for 5 hours at a vacuum atmosphere of 760 mmHg and a temperature of 100 ° C.

Ar / H ₂ (content of H ₂ 5% by volume) of the obtained powder Plasticity was maintained at 750 ° C. for 4 hours in a reducing atmosphere of the mixed gas. After cooling slowly, milling (milling) again gave a mixture.

Ar / H ₂ (content of H ₂ 5% by volume) of the obtained powder The mixture was calcined by holding at 750 ° C. for 6 hours under a reducing atmosphere of the mixed gas. The calcined product was slowly cooled and then ground again to prepare a cathode active material in which a carbon layer was uniformly formed on the LiFePO ₄ core. The carbon layer was 15 nm thick.

Comparative Example 1

The Fe ²⁺ -containing compound was mixed with FeC ₂ O ₄ .2H ₂ O, lithium salt with Li ₂ CO ₃ and phosphate (NH ₄ ) ₂ HPO ₄ in a molar ratio of 1: 0.5: 1.

3 parts by weight of acetylene black was added to the mixture with respect to 100 parts by weight of the mixture, and the mixture was mixed and ground for about 1 to 3 hours using a planetary ball mill to prepare a mixed product.

The resulting mixed product was calcined by holding at 750 ° C. for 10 hours under a reducing atmosphere of an Ar / H ₂ (H ₂ content 5% by volume) mixed gas. The calcined product was cooled slowly and then ground again to prepare a LiFePO ₄ positive electrode active material containing carbon. In the prepared cathode active material, carbon was unevenly present on the surface of LiFePO ₄ .

Test Example 1: XRD Structure Analysis

X-ray diffraction patterns of the olivine-based positive active materials of Examples 1 to 2 and Comparative Example 1 were measured using an X-ray diffraction analyzer (trade name: Rint-2200, company name: Rigaku, Japan) and CuKα rays, and the The results are shown in FIG. As shown in Figure 1, the LiFePO ₄ powder prepared in Examples 1 and 2 and Comparative Example 1 is a single phase, it can be seen that the peak of the standard LiFePO ₄ compound is consistent with the whole range.

Test Example 2: Measurement of Powder Conductivity

The olivine-based positive electrode active materials of Examples 1 to 2 and Comparative Example 1 were measured for conductivity using a powder conductivity measuring device (trade name: MCP-PD51, company name: Mitsubishi chemical, Japan), and the results are shown in FIG. Indicated. As shown in Figure 2, it can be seen that there is a large difference in the conductivity of Examples 1 and 2 and Comparative Example 1.

Test Example 3 Evaluation of Electrochemical Properties

The initial discharge capacity and cycle life characteristics of the olivine-based positive electrode active materials of Examples 1 to 2 and Comparative Example 1 were measured, and the results are shown in FIG. 3.

The cycle life characteristics shown in FIG. 3 show that the lithium half battery containing the positive electrode active materials of Examples 1 to 2 and Comparative Example 1 was charged and discharged at 0.5 C for 30 times, and the discharge capacity thereof. The lithium half cell was manufactured by a conventional method using an electrolyte including a positive electrode including the positive electrode active material, a lithium metal counter electrode, and an ethylene carbonate / ethylmethyl carbonate (1/2 vol. Ratio) non-aqueous organic solvent in which 1M LiPF ₆ was dissolved. .

As shown in FIG. 3, the battery including the cathode active materials of Examples 1 to 2 has an initial capacity similar to that of Comparative Example 1, but shows a higher cycle characteristic than that of Comparative Example 1 as the charge and discharge cycle is repeated. have.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the claims and the detailed description of the invention and the accompanying drawings. Naturally, it belongs to the scope of the invention.

1 is an XRD structure analysis graph of the positive electrode active material prepared according to Examples 1 to 2, and Comparative Example 1 of the present invention.

2 is a graph measuring powder conductivity of positive electrode active materials according to Examples 1 to 2 and Comparative Example 1 of the present invention.

3 is a graph showing battery characteristics of the positive electrode active material according to Examples 1 to 2 and Comparative Example 1 of the present invention.

Claims (16)

A core represented by Formula 1; And A carbon layer formed surrounding the core A cathode active material for a lithium secondary battery comprising a. [Formula 1] LiFePO ₄ The method of claim 1, The carbon layer is a positive active material for a lithium secondary battery having a thickness of 10 to 30 nm. The method of claim 1, The carbon layer is a cathode active material for a lithium secondary battery, wherein a water-soluble resin, a water-soluble carbonic acid, or a combination thereof is calcined. Water was added to the lithium precursor compound to prepare an aqueous lithium precursor compound solution; Preparing a mixed solution by adding an iron precursor compound to the aqueous lithium precursor compound solution; Milling and drying while adding a water-soluble carbon raw material to the mixed solution; Method for producing a positive electrode active material for a lithium secondary battery comprising a compound of formula 1 including the step of firing the dried product. [Formula 1] LiFePO ₄ The method of claim 4, wherein The addition amount of the water is an amount of 30 to 70 parts by weight based on 100 parts by weight of the lithium precursor compound manufacturing method of a positive electrode active material for lithium secondary batteries. The method of claim 4, wherein The amount of the water-soluble carbon raw material is 1 to 10 parts by weight based on 100 parts by weight of the mixture of the lithium precursor compound and the iron precursor compound positive electrode active material for lithium secondary batteries. The method of claim 4, wherein The said water-soluble carbon raw material is a manufacturing method of the positive electrode active material for lithium secondary batteries which is water-soluble resin, water-soluble carbonic acid, or a combination thereof. The method of claim 4, wherein The lithium precursor compound is Li ₃ PO ₄ , LiH ₂ PO ₄ or a combination thereof. The method of claim 4, wherein The iron precursor compound is iron oxalate (FeC ₂ O ₄ · 2H ₂ O), iron acetate (Fe (CH ₃ COO) ₂ ) or a combination thereof manufacturing method of a positive electrode active material for a lithium secondary battery. The method of claim 4, wherein The mixing ratio of the lithium precursor compound and the iron precursor compound is 1: 1 to 1.05: 1 mol (mol) method of producing a positive electrode active material for lithium secondary batteries. The method of claim 4, wherein The firing process is a method for producing a positive electrode active material for a lithium secondary battery that is carried out at 600 to 850 ℃ for 8 to 10 hours. The method of claim 4, wherein The firing process is plasticized at 600 to 850 ° C. for 4 to 6 hours; Method for producing a positive electrode active material for a lithium secondary battery which is carried out by firing the plastic product at 600 to 850 ℃ for 6 to 10 hours. The method of claim 12, The manufacturing method of the positive electrode active material for lithium secondary batteries which further performs the process of milling the said plastic product. The method of claim 4, wherein The milling process is a method for producing a positive electrode active material for a lithium secondary battery that is carried out by wet milling. The method of claim 4, wherein The firing step is a method for producing a positive electrode active material for a lithium secondary battery that is carried out in an inert atmosphere or a reducing atmosphere. A positive electrode comprising the positive electrode active material of any one of claims 1 to 3; A negative electrode including a negative electrode active material; And Electrolyte Lithium secondary battery comprising a.

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